Special Issue;
Elemental technologies
October 2008

ISSN 0915-0528 OSAKA, JAPANNTN’s Elemental Technological Research & Development Center will becommissioned in January 2009 to enhance development functions forelemental technologies and bearings for industrial machinery.

To enhance NTN’s R&D framework in the Kuwana area, our “Elemental Technological Research & Development
Center” is now under construction in the Kuwana Business Research Park. This facility will be commissioned in
January 2009.
Situated in the Kuwana Business Research Park at Hidamari-no-oka, Kuwana-shi, Mie Prefecture, the R&D
center boasts a total floor area of 12,000 m2 consisting of two buildings : a five-story seismically isolated main
research building and a separate single-story laboratory building. Incorporating a seismically isolating structure
comprising a unique sliding bearing (SSB*1) system from NTN Engineering Plastics Corp., the main building not
only can withstand a big earthquake as intense as the notorious Hanshin-Awaji Great Earthquake but also willfunction as NTN’s disaster relief center in NTN’s Kuwana area of operation such that it is capable of emergency
control in the event of disaster. The Main Research Building incorporates solar power generation panels, a wind
power generator and a rooftop garden to realize both energy savings and eco-friendliness. Also, striving forharmony with nature, this building has a unique arrangement to prevent the adverse effect of lighting toward theeast because a local volunteer group is cultivating fireflies in the green space conservation park No. 2 situateddirectly east of the building.
To Elemental Technological Research & Development Center, Element Technology Laboratory currently
situated in present NTN Kuwana R&D Center (it will be renamed Industrial Machinery Engineering Development
Center) and a portion of experiment functions from Industrial Machinery Product Headquarters will be transferred.
Through improved R&D environments, elemental technologies, which are the very source of NTN’scompetitiveness, will be more sophisticated. These improvements include development of eco-friendly heat
treatment technique and materials, research into higher mechanical strength of materials through nano-levelresearch and surface generation, development of next-generation lubricants, and more strenuous cultivation ofanalysis technique which can help accelerate development work. Additionally, in cooperation with the current
Kuwana Research & Development Center, the Elemental Technological R&D Center will enhance NTN’s capability
in assessment tests for large and extra-large bearings for wind power generators, large building construction
machines, medical equipment, railway rolling stocks and aircraft in order for NTN to strengthen its R&D functions
and ability to satisfy customer needs in the industrial machinery product segment which will grow significantly inthe near future.
［ *1：Super Sliding Bearing］【Features of the Elemental Technological Research & Development Center】
Site address : 5-105 Hidamari-no-oka, Kuwana-shi, Mie Prefecture, Japan (in Kuwana Business Research Park): approximately 30,000 m2
Total floor area : Five-story Main Building, single-story Laboratory Building: total floor area approximately 12,000 m2
Structure: Seismically isolated building (Main Building) incorporating “elastic sliding bearing” from NTN Engineering Plastics Corp.
Equipment: Solar power generation, wind power generation, rooftop garden, eco-friendly air-conditioning with cool pit

R

TECHNICAL REVIEW
No.76

corporation

Special Issue:
Elemental technologies

NTN TECHNICAL REVIEW No.76
CONTENTS
Kenji OKADA

Preface
Contribution

Industry-University Partnerships in R&D of Machine Elements
Yoshitsugu KIMURA Professor Emeritus The University of Tokyo and Kagawa University

1

2● Elemental Technology
Trends in Materials and Heat Treatments for Rolling Bearings
Hirokazu NAKASHIMA

Improvement of Carburized Steel Wear Resistance by Heat Treatment
Nobuyuki MOURI and Kazuhiko TAGUCHI

Deep Nitrided 32CrMoV13 Steel for Aerospace Bearings Applications
Daniel GIRODIN

Test Design and Result Interpretation Methods for Evaluating Rolling Contact Fatigue Life
to Maintain Both Efficiency and Reliability

10
18
24
32

Takumi FUJITA

Research on the Lubrication Mechanism of Grease for High Speed Bearings
Takayuki KAWAMURA

Friction Properties of Polymer Gel with High Mechanical Strength
Eiichirou SHIMAZU, Masaki EGAMI, Takayuki KUROKAWA and Jian Ping Gong

Study on the Effect of Physical Properties on Wear Resistances of TiN Films Coated
by Arc-plasma Ion Plating

40
46

52

Hideyuki TSUTSUIDevelopment of an Antenna Material Based on Rubber that has Flexibility and High Impact Resistance
Kouya OOHIRA

Development of Sensing System for Cerebral Aneurysm Treatment
Yoshitaka NAGANO and Hideo FUJIMOTO

Development of a Method for Monitoring Lubricant Deterioration
Kentaro NISHIKAWA and Tomomi ISHIKAWA

Heat Transfer Analysis of Machine Tool Main Spindles
Yoshimitsu HIRASAWA and Yukimitsu YAMAMOTO

Development of High-Speed Cylindrical Roller Bearings for Machine Tools
Masatsugu MORI and Takuji KOBAYASHI● Products for Industrial Machinery
Angular Contact Ball Bearings that Require a Minimum Quantity of Grease Supply for Lubrication
Sun-Woo LEE and Tadaaki MAEDA

Minimum Quantity and Cooling Jet Lubrication Single-row Cylindrical Roller Bearings
Futoshi KOSUGI and Susumu NOJIMAIntroduction of 3D Electronic Catalog for Precision Machine Tool Bearings
Takehiko UMEMOTOULTAGE Series Large, Long-lasting Double-row Cylindrical Roller Bearings
for the Main Spindles of Large Turning Machines

58
64
69
75
82

90
96
101
109

Naota YAMAMOTO

Technical Trends in Wind Turbine Bearings
Souichi YAGI and Nobuyuki NINOYUNTN Sensor Units for Construction Machinery
Shoji ITOMI and Hiroyoshi ITO

Duplexing Gear Unit for Office Equipment
Tsutomu MAIWA

113
121
129

● Precision Apparatus Products
Large Gantry Table for the 10th Generation LCD Substrates
Katsuyoshi SUZUKI and Naoshi SUZUKI

Development of a Linear Changer with a Built-in Pasting Unit for a Flat Panel Defect Repair System
Akihiro YAMANAKA and Akira MATSUSHIMA

134
138

● Our Line of Award Winning Products
2007 JSPE Technology Award

Development and Practical Application of MQCJ-Lubricated Angular Contact Ball Bearings
for Machine Tool Main Spindles

144

Masatsugu MORI, Yoshinobu AKAMATSU, Mineo KOYAMA, Futoshi KOSUGI, and Hiroshi TAKIUCHI

Our Line of New Products

146

NTN TECHNICAL REVIEW No.76（2008）[ Preface ]– Introduction to this Special Feature Issue –To the new issue featuring
elemental technologies
Kenji OKADA
Executive Director

Global warming, currently occurring on this 4.6 billion years old earth, is a very critical issue to be addressed by
the modern society that has been enjoying economical growth thanks to the consumption of fossil energies.
Since the Industrial Revolution in Great Britain, much carbon dioxide (CO2) has been emitted as a result of the
combustion of petroleum and coal. In the past 200 years, the carbon dioxide concentration in the atmosphere
has increased by as much as 25%, and now the entire earth is, so to speak, situated in a greenhouse.
In this situation, NTN has remained committed to global environment conservation as its most important
challenge not only in its production activities but also in its R&D activities. In the industrial machinery field, we arecommitted to the development of bearing technologies for natural energy utilizing equipment such as wind power
generation plants and for next-generation transportation equipment while “modal shift” is in progress. We arepromoting product development activities to help realize low fuel consumption and electrification in the car
industry. We are attempting to more strenuously cultivate and expand “elemental technologies” that constitutethe basis for these technologies. More specifically, we are committed to new R&D efforts that include use of
bioplastic materials, use of novel materials which can reduce use of rare metals, eco-friendly heat treatment
techniques, research into nano-structure level metal materials, surface generation for higher mechanical strengthand lower friction, and use of higher-polymer gel (a next-generation material) for decreased friction, as well as
development of more sophisticated analysis techniques.
For distribution at JIMTOF 2008 (24th Japan International Machine Tool Fair) held October 30 to November 4,
2008, we published the Japanese language version of NTN TECHNICAL REVIEW No.76 (2008) featuring topics
about NTN’s latest elemental technologies that support engineering aspects of eco-friendly products boasting
compact size, light weight, lower running torque (lower friction) and longer service life. This issue begins with acontribution titled “Industry-University Partnerships in R&D of Machine Elements” written by Yoshitsugu KIMURA,
Professor Emeritus, The University of Tokyo and Kagawa University, a world’s authority in the field of tribology.
This topic is followed by presentations of our recent new technologies and developments for new products suchas elemental technologies including those for improving wear resistance by surface generation and advanced
sensor technologies applied in the medical field; industrial machinery products and precision equipment products
such as wind power generator bearings; and “Development and Commercialization of Minimum Quantity andCooling Jet (MQCJ) Lubrication Angular Ball Bearings for Machine Tool Spindles” which won the 2007
Engineering Award from The Japan Society for Precision Engineering.
Since the beginning of its research and manufacturing of ball bearings in 1918 at the then Kuwana-cho,
Kuwana-gun (present-day Kuwana City, Mie Prefecture, Japan), NTN has been attempting for 90 years todevelop new products always aiming at “realization of quality that can satisfy customers’ needs”. Strictly adhering
to our philosophy “For New Technology Network - We contribute to the international society through creation of
new technologies and development of new products”, we will further remain committed to promote, based on theachievements obtained through more strenuous cultivation of our elemental technologies, our R&D activities tosatisfy requirements for the ecology, safety and comfort from a global standpoint, thereby we will contribute tosociety.

-1-

NTN TECHNICAL REVIEW No.76（2008）[ Contribution ]Industry-University Partnerships
in R&D of Machine Elements
Yoshitsugu KIMURA
Professor Emeritus The University of Tokyo and Kagawa University

Significance of matured technology, is discussed, exemplified by the technology ofmachine elements. Discussed are the roles of universities in handing down
technologies between generations and in clearing the hurdles facing class leaders,along with potential ways to develop fruitful partnerships between industry anduniversities.

1. Introduction

2. What is meant by advanced technology?

After changing trains at Nagoya from the
Shinkansen line to the Kintetsu or JR Kansai line andcrossing bridges over the Kiso, Nagara and Ibi Rivers,
one finds Kuwana City. It is said that in 1918, thefounder of NTN started researching andmanufacturing ball bearings in the town of Kuwana in
Kuwana County. Thus, the firm reached the 90th
anniversary of its foundation this year. As a person
who has been given opportunities to be involved withthe company, I would first like to express my sincere
feelings of congratulations.
With a history of 90 years, younger readers will
surely feel the foundation of NTN is an event long ago.
Having lived for 80% of that period, however, I havedifferent feelings. For this reason, I would like to begin
this article with my personal recollections.
Having finished a postgraduate course in 1966,
when the term “tribology” was made known to theworld, I was employed as an assistant by Professor
Norimune Soda’s laboratory at the Institute of Spaceand Aeronautical Science University of Tokyo. Fromthen on, I have been destined to follow a career as atribologist. In 1997, I retired from the University ofTokyo and moved to Kagawa University to help create
its Faculty of Engineering. Though the period that I
worked for the Faculty of Engineering was short, I
became involved in the management of that
university. Here I was given the opportunity to look at
tribology, a field in which I had been engaged for along time, from a somewhat different perspective.
From such a standpoint, I would like to talk aboutmachine elements and how cooperation between
industry and academia contributes to the developmentof machine element technology.

As a toothed gear is used as the symbol of theJapan Society of Mechanical Engineers, which
recently celebrated the 110th anniversary of its
foundation, machine elements are symbols ofengineering.
There was an era in which smoke from factory
smokestacks, seen only as a cause of air pollution atpresent, was included in school songs as a symbol ofindustrial development. Perceptions are oftenchanged as time passes, and there is probably no one
now who sees machine elements as products offrontier or high technologies. Tribology is no
exception. Someone wrote sarcastically, “Is tribology
high technology, low technology, or no technology?”
What does frontier technology mean anyway? Thisis a term that has come to be used without being
clearly defined, but I prefer the definition 2) given by
Mr. Hiroyuki Yoshikawa, a former president of the
University of Tokyo, some 20 years ago.
In the first place, all technologies are destined tocontinue evolving and in that sense, must be regarded

Fig. 1 Example of “high-technology” : AFM probe 1)-2-

Industry-University Partnerships in R&D of Machine Elements

will not become exports that support Japan’s future
economy.
More importantly than this, according to Mr. Becker,
you have technologies indispensable to the world in
the near future, such as technologies to handle naturalenergy sources including the sun, geothermal heat,wind and the sea, as well as organic agriculture,
recycling, and the afore mentioned drinking water.
For example, try to determine who is the leader inthe field of natural energy technology, and you will findthat Germany leads solar energy utilization
technology, with Australia in second place and Japanjust managing to be within a group of competitors. In
the field of geothermal power generation, Icelandstands first with Italy following. In the field of wind
power generation, Denmark stands first, while in thefield of ocean thermal energy conversion, the state of
Hawaii is ahead of Japan.
The capacity of wind power generation is shown in
Fig. 2. 4) This figure shows that Germany is first, but
Denmark dominates in terms of per-capita capacity.
Either way, the fact remains unchanged that Japan islagging behind.
Mr. Becker places his expectations on Japan,
saying, “If only Japan had the desire, it would neverlose to Australia in the field of solar thermal power
generation.” He criticizes Japan severely, saying,“Japan took the wrong end of the stick in giving priority
only to large-size digital screens and neglecting theneeds of the world in the near future.”
Of course, he does not say that it is wrong to pursue
frontier technologies. It is not wrong, but pursuing only
frontier technologies casts a shadow on the future ofJapan in the world. “For example, a country capable ofstably supplying solar energy at a substantiallyreduced price for export will experience a second
growth stage. There is no reason for not focusing on
this with the concerted efforts of the nation,” insists
Mr. Becker.
However, one may guess that, regardless ofwhether we take on natural energy, organic
agriculture, drinking water, or recycling, we can handle
all of them through the application of the maturetechnologies on which they are founded. Moreover, I

as being at the outset. Why, then, is the “frontier
technology” designation limited to certain
technologies? Mr. Yoshikawa started with thisquestion. What is the concept that is opposite that offrontier technologies? Is the opposite of the frontiercompleted? No. Mr. Yoshikawa says that they are
mature technologies. He took up water-related
technologies as an example for this. The market forbrand-name water has now grown large as we areentering an age in which water shortages havebecome a global issue.
What Mr. Yoshikawa wanted to say is that the
production of water, for example, has its own
sophisticated technology, but it has been involved
deeply in conditions of human society and, thus,
reached a stage of maturity. This makes groundbreaking changes inconceivable. Let us define suchtechnologies as mature ones for the time being.
Moreover, as a result of the increase in the number ofsuch mature technologies and in the importance of
their influence, the degree that a technology has
advanced has become relative, with the notion of afrontier technology being created as a result, says Mr.
Yoshikawa.
Mr. Yoshikawa points out that mature technologies
are indispensable for supporting society, and therefore
should be considered to be very important
technologies. However, it may be inevitable that theattention of society in general is attracted to frontier
technologies because the nature of journalism is tointentionally focus on “groundbreaking changes.”

3. If we only chase after frontiers
Is it good for us to chase after only frontier
technologies? I know one person who has raised thisquestion. 3) Carl Becker, a professor at the Universityof Kyoto who hails from the University of Hawaii, has avery interesting opinion related to this subject.
For example, one object of frontier technology thatJapan has prioritized is large liquid crystal (LCD)
television sets. With sales opportunities like the
Beijing Olympics held this year and the switch toterrestrial digital broadcasting only a few years ahead,
it is natural that manufacturers place high priority on
LCD TVs.
Mr. Becker says, however, “If you consider theworld’s needs, you will find that there are few
countries that desire large LCD TVs.”
Basically, countries for which terrestrial digital
broadcasting is more advantageous are limited tothose, like Japan, with limited territories andparticularly high population densities. When you
receive satellite broadcasting in China or in the UnitedStates, signals deteriorate significantly before they
arrive at television sets. Therefore, even if you use alarge, 50-inch screen, you will not have images thatare as beautiful as you would have in Japan. These
countries do not need the clarity that is one of the
major features of large LCD televisions. For thisreason, even if they are successful inside Japan, theyJapan 1400
Denmark
3132

Others
20666

Spain
15415

China 6000
India
8000

The United States
16818
Germany22247

Unit: MW; data as of the end of 2007

Fig. 2 Capacity of aerogeneration 4)-3-

NTN TECHNICAL REVIEW No.76（2008）

many springs manufactured with the same method,
and, moreover, that such a shift occurs only at a
certain level of vibration that is about the same level
as that of actual flight conditions. If you only look at a
drawing, you will overlook this detail: the coiled springis neither axis-symmetric nor plane-symmetric. For
this reason, the asymmetric motion of “shifting” in a
direction is a phenomenon that can occur.
What was made clear in the course of analyzing
potential causes was the fact that experts on coiled
springs, which are very common machine elements,
could not be found, at least not in Japan. As a result,nobody was able to foresee the possibility that thedestiny of an engineering test satellite costing several
tens of billion yen could be determined by thecapricious behavior of a spring.
This coiled spring is one example, but have we notignored the transmission of expertise related tocommonplace machine elements that seem remotefrom frontier technologies?

am certain that many of the readers would think thatthe application of technologies is a field at which
Japan, proud of its skillfulness, is good. Unfortunately,
however, it is not that simple.

4. Why did the Kiku VI engineering
test satellite fail?
Fourteen years ago there was an incident in which
an engineering test satellite named “Kiku VI” was not
successfully put into orbit. 5) It left an impression in mymind because I joined in the investigation.
Kiku VI was a two-ton class satellite, shown in Fig.
3, 6) that was launched from Tanegashima Island on
August 28, 1994 and put in an elliptical orbit with theearth set as one focus. Everything went wellconcerning this. The next step in the launch program
was to change orbits through the use of a small
rocket, called an apogee engine, at the apogee inorder to put the satellite into geostationary orbit.
Things started going wrong when a command to start
the apogee engine was sent two days after thelaunch. The pressure in the combustion chamber did
not rise beyond one-tenth of the planned value. As
many measures as possible were taken in vain, but
the satellite remained orbiting in the original ellipticalorbit.
Although it was impossible to investigate the cause
or recover the actual satellite for examination, avariety of analysis suggested a problem with thecoiled springs as the most probable cause.
The engine was provided with a mechanism that
controlled the hydrazine, the fuel, and an N2O4 oxidant
with a two-propellant valve to feed it into the
combustion chamber. The return spring of this valve
was thought to have shifted in a lateral direction by thevibration produced by the launch, and was caught in a
gap between the valve piston and casing. This would
cause the valve shaft to become bent, thereby making
the planned control impossible.
Needless to say, preliminary vibration tests had
been performed. The ex post facto analysis, however,revealed that such a shift occurs only in a few of the

5. Elemental technologies in wind
power generation
Another example Mr. Becker mentioned is wind
power generation.
Wind power generation is one topic that is currently
attracting public attention. 7,8) Among so-called newenergy sources, wind power generation exhibits aworldwide growth rate of 15 to 20% because of its
cost-effectiveness, efficiency1 and environmentalfriendliness, as well as the low ratio of construction
cost to power generation cost. Each year sees larger
and larger wind power generators, and it is said that a
6-MW unit was tested in 2007.
Among other things, the huge rotors used in wind
power generation are impressive. The energy that canbe extracted is proportional to the square of the rotor
diameter, while the weight is proportional to the cubeof the diameter. It is impossible, therefore, to say
simply that “the larger a rotor is, the better it is,” andtechnologies for increasing generator efficiency andreducing generator weight are sought.
As shown in the example in Fig. 4, 7) various
mechanisms are packed in the nacelle. The casing
contains the bearing for the rotor shaft a generator,
and a gearbox that increases the 10-to-30-rpm
rotation of the rotor to a speed of 1500 to 2000 rpm.
These are the essential pieces of equipment, to which
components specific to a windmill are added such asa pitch drive mechanism that changes the blade pitch,a swivel ring that changes the orientation of the rotor
in response to the wind direction, a yaw drive
mechanism, and a lubrication device. In comparison,
the casing of an electric fan contains only a motor. In
all, the number of bearings per nacelle is upwards of100, which can be compared to about 150 bearings
per automobile. Even this fact alone could convince usthat wind turbines are quite amazing.
Operating conditions for wind turbines are also
particularly severe. To support a rotor that is several

Fig. 3 Engineering test satellite-^ “KIKU No.6” 6)-4-

Industry-University Partnerships in R&D of Machine Elements

tens of meters in diameter, the bearing of the main
shaft sometimes exceeds three meters in outside
diameter, and it must withstand randomly occurringhigh loads caused by wind gusts. In addition, thebearings of the pitch drive mechanism and swivel
rings face the demanding condition of intermittent
swing. Furthermore, installing a wind turbine on the
top of a tower as high as 100 meters makes measuresagainst lightning crucial, with measures against salt
damage and low temperatures also necessary
depending on the location of installation. Combine allthis with the high reliability that is required to provide astable electricity supply.
Even simply enumerating the requirements for wind
power generation should have convinced us that it is
not something that can be realized by merely
extending conventional basic technologies. In fact, asurvey of the actual circumstances 9) in Hokkaido,
which has 192 wind turbines and accounts for 24% of
the total wind-generated electric energy in Japan,
revealed a total of 240 problems that have occurred ina total of 270 units at 44 power stations during thetwo-year period from 2004 to 2005. Under thesecircumstances, control devices caused problems mostfrequently, 67 times, followed by drive devices, 63
times, combining for 26% of the total number ofproblems.
Such being the case, the difficulties with wind power
generation are that the wind does not blow asplanned, that wind turbines do not exhibit expected
performance, and that they are shut down frequently
because of problems. These problems are mentioned
as challenges to wind power generation. What mattersthen is the fact that 92% of the 192 wind turbines, 176
units, are foreign-made. As a result, obtaining parts
takes a long time, which in turn lengthens the downtime.
As is seen in this case, the technologies of machineelements are crucial in the “technologies that will be
indispensable for the world in the near future” that Mr.
Becker mentions. The fact that Japan is falling behindin this crucial area has already become apparent.

Rotor

Rotor hub

6. Two aspects of the year 2007 problem
I have explained that chasing after only frontier
technologies invites an interruption in the transmissionof elemental technologies, which will be both crucial
and indispensable in the near future.
I expect that, based on what I have written, many
readers will think that the crisis in handing down
technologies is not limited to elemental technologies.
In Japan, people talk much about the “year 2007
problem.” The year 2007 problem is defined as when
all the baby-boomers retire from working life at thesame time, taking with them all their expertise. How
can the expertise they possess and have accumulated
be transmitted to their successors without problems?
This problem will linger far beyond 2007, and the
aspects of “quantity” and “quality” cannot be ignored. 10)
Our country’s age distribution is no longer apyramid. Estimates provided by the National Instituteof Population and Social Security Research 11) give us
pictures of Japan’s five-year-interval population
pyramids for 2030 and 2055. In the former, thepopulation peak moves into the 55-to-59-year-old
bracket, while in the latter the peak shifts to the 80-to84-year-old bracket, with the shape changing from apyramid to something like a bucket. The quantity
aspect of the problem becomes that if you want totransmit expertise, you no longer can find younger
people to whom you can pass on your skills.
People of the first generation, namely people older
than those of the baby-boomer generation, went
through their lives almost in step with the higheconomic growth of Japan. They are people of a hardworking generation who built Japan into a major
manufacturing country, starting with the introduction offoreign technologies. They put their experience into
the form of manuals and created systems in which any
person could work.
The second generation followed in the latter half ofthe 1970s when Japan had ended the age of highgrowth and had established such systems.
As a logical consequence, they carried out theiractivities according to the manuals they inherited fromtheir predecessors, and, in this sense, the transfer ofskills can be said to have been smooth. However, it
was difficult for the second generation of people toperceive the thought that the previous generation had
poured into the manuals, and it is said that thebackground of thought was difficult to transmit.
The problem lies in the days after the first
generation had retired. Needless to say, social needschanged as time passed from the era of higheconomic growth to the bubble economy to its
collapse, while technologies progressed and people’s
senses of values also changed. As a logicalconsequence, the age changed into one in which
traditional manuals could no longer be used without
modifications. This leads to the quality aspect of the
problem; how would the second generation modify
manuals to hand them down to the next generation if
the background hadn’t been transmitted successfully?

Heat
NacelleOil cooler Speedexchanger
increasing gear
Control panelPowertransmission
Coupling
shaft

Pitch drive

Bearing
bracketAnti-vibration
deviceYaw drive

Generator

Fig. 4 Example of aerogenerator 7)-5-

NTN TECHNICAL REVIEW No.76（2008）
Year 2055,
Age

Male

120 100

80

60

40

20

0

105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0

on the leading edge of high technologies. However,the point is that one third is enough for this task, andnot more.
The next third according to this theory areresearchers that should be “jealous” of the first thirdthat is “enjoying good sales,” because they believethat they are doing comparable research work. The
vicissitudes of the world dictate that one field of
research will not always continue to enjoy good sales.
Then, the “jealous” third will be in the spotlight insteadof the first third.
What is the remaining third doing? They are those
who regard their “sales” as irrelevant and are
dedicated to the retention and evolution of systems oflearning. This theory insists that these people, one
third of all researchers, are necessary. They are
directly involved in the transmission of knowledge,
and, in the field of engineering, this is the transmissionof expertise and skill. If technological expertise ispositioned somewhere in the system of learning, it willbe transferred more efficiently. Mach reputedly once
said, “Science is the economy of thought.” I think thatthe role of academia also includes the transmission ofexpertise and that this is a reason why hopes areplaced on the results of cooperation between industry
and academia.

Female

0

20

40

60

80

100 120

Population (in ten thousands)

Fig. 5 Population “bucket” in 2055 11)

7. Role of academia in transmitting
expertise
The author thinks that one of the roles to be borneby academia should be the transmission oftechnologies, which has become difficult for thereasons described above. The author believes this fora couple of reasons.
The first reason is the change in the management
policies of Japanese enterprises. 12) It has been said
that, in the past, the enterprises of Japan conductedlong-term-oriented management and were eager tocultivate manpower, while the management of US
enterprises was short-term-oriented and treatedhuman resources as disposable. For the past 10
years, however, the shareholders’ voice has become
stronger, and the managements of companies withlisted stock have tended to prioritize policies that
achieve results in a short period to create favorable
effects on stock prices. As a result, a number ofenterprises have come to follow a policy of pushingahead with the reduction of labor costs, tending toprefer to meet labor demand by employing manpower
trained by other enterprises, part-timers, and contract
workers rather than spend time, money and effort tocultivate their own people.
In this sense, Japanese enterprises have become
similar to American ones. We must say that it has
become extremely difficult to transmit expertise fromone generation to the next in such an environment.
There is another reason why transferring expertiseis one of the key roles of academia. Education is not
merely the transmission of skills, this is too simple aviewpoint.
Regarding researchers in academia, there is atheory of “1/3-1/3-1/3” which means there are threeequally sized groups of reasearhers. As an example, if
we consider a somewhat large group of researchers
such as a university faculty or laboratory, one third ofthem should be those who “enjoy good sales.” In ourcontext, this means that one third of them are working

8. Cooperation between industry andacademia as viewed by the ministryof economy, trade and industry
I have repeatedly used the expression “one of theroles” in the previous section because the roles ofacademia in cooperation with industry involve muchmore. Transmitting knowledge is the transfer of skill
and expertise that already exist. This is the starting
point, of course, with much more beyond it.
Now, let us change our focus. The Ministry of
Economy, Trade, and Industry have high hopes for thecooperation between industry and academia.
Four years ago, a lecture meeting titled “ResearchIntegration and Cooperation between Industry andAcademia” was held at the Institute of Industrial
Science in the University of Tokyo, where I had
worked for some time. At the lecture meeting, Mr.
Kubota, Chief of the Research and Development
Section of the Industrial Science and Technology
Policy and Environment Bureau, made a speech withthe title of “Critical Challenges in Industrial Technology
Development and Expectations Placed on the
Academia.” 13).
In this speech, Mr. Kubota mentioned the following
three points as increasingly prominent trends
observed among enterprises. First are the strong
demands of investors, who are asserting themselveswith increasingly louder voices, due to theglobalization of value standards. As a result of this,enterprises are forced to compress fundamental andinfrastructural research. Second in this environment is
the thought that buying technologies from otherenterprises is simpler and quicker than carrying out-6-Industry-University Partnerships in R&D of Machine Elements

will discuss the current state of national universities.
“Eagle eats no carrion.” Being as proud as eagle,
national universities did not disclose their internalaffairs. Under these circumstances, Mr. Akito Arima, aformer president of the University of Tokyo, started
publicizing the poverty of Japanese national
universities. A dedicated physicist, he disclosed, in a
brutally blunt manner, the miserable state of university
research facilities, particularly in science andengineering faculties, by citing actual examples close
to us. 15) Thanks to his enlightening activities, I thinkthat society’s recognition of this issue has been
improved considerably, but how has subsequent
follow-up been conducted? The state is somewhat
similar to Japan’s recent business situation.
For example, let me cite the example of how the
Grant-in-Aid for Scientific Research funds, which arecompetitive funds provided by the Ministry of
Education, Culture, Sports, Science and Technology,
exceeded 50 billion yen in fiscal 1989, 100 billion yen
in fiscal 1996, and reached 191.3 billion yen in fiscal
2008. Proud of this great increase, this Ministry holds
its head high. 16)
True, the expenditure as a whole has increased, but
as with the business trend, there is a gap here, too.
Nearly 90% of the Grant-in-Aid for Scientific Research
is distributed to universities, but among over 700
universities 40% or more of the total is distributed toformer Imperial Universities. Universities naturally
differ in their scale from one another, and the funds
are said to be concentrated investments, but there are
a number of weaker universities that cannot conduct
research properly and are on the brink of death. In
fact, if managed improperly, incorporation brings thepossibility of collapse to national universities.
I think that the incorporation of Japanese public
universities was a change difficult to understand.

research and development work independently. These
two observations lead no doubt to the first reason that
I presented in the previous section. The third point is
the concern of having reached a place from which one
can see the “hurdle” to be surmounted, as a result of
having become a technological front-runner.
Under these circumstances, what should industry
expect of academia? There are two types of
expectations, says Mr. Kubota. The first of them isfrom the viewpoint of breaking through limits. One
finds seeds of academic research in industrial
technologies and pushes ahead with solid academic
research in order to provide solutions to performanceor structural limits backed by scientific knowledge andtheories. This is the “progressive research.” Thesecond is from the viewpoint of carrying out new
development. One extracts seeds from academic
research from which industrial technologies will growin order to start up university-launched ventures, forexample, that take future business development into
consideration. This is the “developmental research.”
For elemental technologies, I think the significanceof the “progressive research” is predominant.
Regarding this, Mr. Kubota made a special request of
universities. He states that universities should have afirm grip on what industrial technology is, follow its
trends, see where its limits are, and respond to thisseriously. If a university loses sight of this duty, themeaning of its existence will be questioned. He points
out that the enterprise side would trouble them,though, if they were to “bring individual themes andseek their solutions,” instead of presenting major
trends that can guide research activities andtransmitting industrial strategies and visions properly.
He is very severe in saying, “Before being critical ofpresent-day universities, saying that they are notuseful to industry at all or that they know nothing ofindustry, please think carefully how much information
you provide them.”
To borrow the words of Mr. Kubota, elemental
technologies are the very fields in which “the hurdles
are visible,” and it will be extremely difficult for industry
to overcome those hurdles without external
assistance. I think that his remarks are very valuable
in supporting the significance of cooperation between
industry and academia.

For example, the University of Tokyo is still anational university whose basic expenditures are paid
as operational grants by the government. What, then,has been changed?
National universities under the former system were
national organizations established by the Ministry of
Education, Culture, Sports, Science and Technology
and this Ministry was accountable for explaining themeaning of their existence collectively. In contrast, aseparate corporation was created for each national
university and, in turn, each corporation reestablished
its associated national university. For example, theUniversity of Tokyo has been reestablished by the
National University Corporation, the University ofTokyo. The true character of incorporation is that each
university must bear the accountability for its own
existence. The system has changed to allow auniversity that loses purpose for its existence to perish
as it will. In fact, a one percent reduction per year isapplied to operational grants as an efficiency
promotion factor, accelerating the difficulty in running
universities.
Now, recall the amount of research expenditures I

9. Current state of universities
Cooperation between industry and academia takes
various forms. The results 14) of a survey conducted by
the Ministry of Education, Culture, Sports, Scienceand Technology on the state of joint research withindustry and academia and contract research activities
shows that both the number of cases and the amountof money increase at a pace of over one percent a
year. The number of cases in 2006 was 32,000 with
an amount reaching 180 billion yen. Since national
universities, the Inter-University Research InstituteCorporation and national colleges of technology
account for 70% of the cases and 79% of the money, I-7-

Grant-in-Aid for Scientific Researchin 100 million yen

NTN TECHNICAL REVIEW No.76（2008）

society, and the keywords for this are social value,
humanity and rationality.
At the first glance, one may take these as
substitutes for the three models of basic, applicationand development research, but this is not so. The
concept of classification is different. For example,
research that does not aim at practical applications
and would rank as basic in the traditional classification
can be classified as an extension model if its concept
and methodology are conventional and it ischaracterized by precise verification testing. On thecontrary, research that does not aim at practicalapplications may be classified into a creative model if
it proposes a new method with no prior example.
And as is shown in Fig. 7, 17) there is certainly acourse of evolution from the creative model to theextension model to the unified model, but evolution is
not limited to that course. Unified model research
gives birth to creative model research, and extension
model research gives birth to creative model research.
Through repeated cycling between these three modelsof research, truth is approached and creativity ofacademic research is enhanced.
If you look for examples in the field of engineering,
you will find a number of them in which an encounter
with a technological problem raised a question about
an existing theory and this lead to the expansion of
the theory. Machine elements provide a good
example. In these cases, it will be the role of industry
to provide triggers and the role of academia to extend
new theories. We see there the possibility ofcooperation between industry and academia bringing
about essential developments in science.

2500
2000
1500
1000
500
0

1994

1996

1998

2000

2002

2004

2006

2008

Year

Fig. 6 Increase in Grants-in-Aid for Scientific Research 16)

mentioned above. The amount of expenditures
provided by “industry” for joint research and entrusted
research efforts nearly equals the Grant-in-Aid for
Scientific Research amount, which are research funds
from the government. I think that university staff
members, in their hearts, are crying out for researchfunds through cooperation with industry.

10. Cooperation between industry andacademia for science
There are, no doubt, problems with the researchfunds mentioned above, but cooperation between
industry and academia has a more essential
significance to academia.
A common view is that research can be divided into
basic, application, and development research.
Pursuing basic research gives birth to application
research, which in turn leads linearly to development
research, thereby propelling technology.
About 10 years ago, the Science Council of Japanraised a question about this way of seeing research,and proposed “the concept of model research.” 17)
A survey that the author made on examples ofvarious innovations in science and technology showedthat greater progress in the field of applications ismade when research is first done on an actual
problem then basic phenomena associated with thisproblem are addressed. On the basis of this finding, I
propose that instead of classifying research into thebasic, application and development phases, weshould classify it as “creative model research,”“extension model research” and “unified model
research.” This method of classification follows the
motion of researchers’ minds as research advances.
“Creative model research” is proposing and verifying ahypothesis, that is, a subjective phase in which one
comes to know and find something new. “Extension
model research” that follows is standardization andlearning, which is the phase of objective conception
and creation. One of the most easily understandable
examples of this is generalization by means ofmathematical equations. Finally, “unified model
research” is the phase of harmonization with actual

CultureScience

Extension(secondary)model

Creative(primary)model

Society
Unified(tertiary)model

Fig. 7 Concept of “Model research” 17)-8-

Industry-University Partnerships in R&D of Machine Elements

8) H. Imai, Windmill Tribology, Japan Society of
Tribologists 19th Conference Proceedings, P31-34(2008)
9) Hokkaido Bureau of Economy, Trade and Industry,
Status and issues related to wind power generation in
Hokkaido,
www.hkd.meti.go.jp/hokns/wp_enquete/wp_text.pdf(2005)
10) Y. Kimura et al., The Study of Maintenance forManagement, Japan Institute of Plant Management,
P14-16 (2006)
11) National Institute of Population and Social Security
Research, Collection of Population Statistics
Documentation 2008,
www.ipss.go.jp/syoushika/tohkei/Popular/Popular2008.asp?chap=0 (2008)12) H. Fujimura, Personnel in Japanese Enterprises [1],Tokyo Shimbun, June 24 edition (2008)
13) A. Kubota, Important Issues in the Development ofIndustrial Technology and Expectations for Learning,Monthly Journal of the Institute of Industrial Science,
Vol.56, No.4, P328-335 (2004)
14) Ministry of Education, Culture, Sports, Science andTechnology, About the Status of Cooperation between
Industry and Universities and other Academic
Institutions in 2006,
www.mext.go.jp/a_menu/shinkou/sangaku/sangakub/07083106.htm (2008)
15) For example, A. Arima, Story of University Poverty,
University of Tokyo Press (1996)
16) Japan Society for the Promotion of Science,
Research conducted with scientific research grants-inaid, Overview, www.jsps.go.jp/jgrantsinaid/01_seido/01_shumoku/index.html#suii(2008)
17) Science Council of Japan 3rd Standing Committee,
Seeking Renewed Research Concepts,
www.scj.go.jp/ja/info/kohyo/17htm/1708z.html (1999)

11. Concluding remarks
Ten years ago, I was given the honor of contributing
a congratulatory article on the occasion of the 80th
anniversary of NTN’s foundation. I wrote, “Thematuration of technology and advances ininternational standardization are probably the reasons
why roller bearings have become highly reliable andwidespread. However, I should say something to theeffect that, therefore, the roller bearing field seems tohave lost technological frontiers,” and “the way out of
this impasse is to be found in the exploration oftechnologies by which NTN can create products noother firm can imitate.” This belief remains unchanged,
and I place great expectations on fruitful partnerships
between industry and academia for the technologies
that support society, as well as for the future ofJapanese universities.

References
1) According to Nobuo Ohmae (2008)2) H. Yoshikawa, Recent Technology and People, Sekai,
No.509, P19-34 (1988)
3) Carl Becker, What Can Japanese Do about Threats to
Life? Gakushikai Magazine, No.864, P24-37 (2007)
4) According to data in the NEDO Overseas Report,
No.1021 (2008)
5) Y. Kimura, About the Defects that Occurred in theEngineering Test Satellite Kiku VI, Journal ofJapanese Society of Tribologists, Vol.40, No.11, P889892
6) JAXA, Engineering Test Satellite VI "Kiku 6 (ETS-VI),"
www.jaxa.jp/projects/sat/ets6/index_j.html
7) K. Kobayashi, Bearings and Bearing Steels for Wind
Turbines, Sanyo Technical Report, Vol.13, No.1, P7376 (2006)

Introduction of the author

Yoshitsugu KIMURA
Professor Emeritus, the University of Tokyo, Kagawa University (Doctor of Engineering)
1959: Graduated from the Mechanical Engineering Department, Faculty of Engineering, the University of Tokyo
1966: Finished the doctorate course, the Graduate School of Engineering, the University of Tokyo
1971: Assistant Professor, the University of Tokyo
1979: Professor, the University of Tokyo
1997: Retired from the University of Tokyo; appointed: Professor, Kagawa University
2002: Retired from Kagawa University
2003: President, Kagawa University
2003-2005: Member, the Science Council of Japan
2005: Retired as President of Kagawa University
AwardsFiscal 1984: Best Paper Award, the Japanese Society of Tribologist
Fiscal 1991: Best Paper Award, the Japan Society of Mechanical Engineers
Fiscal 1991: Achievement Award, the Japan Society of Mechanical Engineers Machine Design &Tribology Division
Fiscal 2003: The 2003 Tribology Gold Medal-9-

NTN TECHNICAL REVIEW No.76（2008）[ Perspective ]Trends in Materials and Heat Treatments
for Rolling Bearings
Hirokazu NAKASHIMA*For New Technology Network

R

Compatibility withecology and safety

Advanced sophisticationand evolution of various
elemental technologieslCo

The focus of this paper is the review of past,current, and future trends regarding steel used forrolling bearings, steel making, and heat treatment
technologies. The improvement of steel purity and thedevelopment of new steels have been conducted
mainly with steel manufacturers so far.
Considering the issues related to globalization, theenvironment, raw materials such as rare metals, andsafety, the future focus will be on surface modification
technologies and the coordination of each elemental
technology.

・Least use of rare metals
・Global procurementbolaMaterial ・Full utilization of general-purpose steel
・Processing technologytiora・Energy saving technologynHeat ・CO2 reduction
heat treatment
treatment ・Vacuum
・Oil-free quenching・Simulation technology
modification
Surface ・Mechanical・PVD
modification・Powder technology

・Minimal quantity lubrication
・GTL oil

Lubrication ・Ionic liquid

・Biodegradable oil

1. Introduction

2. Past trend in life-extending techniquesthrough improvement in materials

Rolling element bearings are used in various
industrial machines and equipment with operating
conditions which are increasingly more demanding forthe bearing. In addition to the more demanding
conditions, end users continue to desire longer lifeand lower operating torque. Steel is the most common
material used in rolling element bearings, but othermaterials such as non-ferrous metal materials
including copper alloys as well as resin and ceramicmaterials are also used. The focus of this paper islimited to steel materials that are typically used for the
inner rings, outer rings and rolling elements of NTN's
rolling bearings.
Any rolling bearing is a machine element that must
be able to run smoothly while its small contact surface
carries a large load. Therefore, the material of every
rolling bearing is essentially required to satisfy highdegrees of fatigue strength (rolling contact fatigue life),
wear resistance, dimensional stability, etc.
The most typical material for rolling bearings is 1%
Cr-1.5% Cr high carbon chromium bearing steel. This
steel, developed as early as in 1905 1), has been used
for more than one century without modification to its
major components. This paper briefly summarizes thehistory having taking place during this period aboutbearing materials and heat treatment techniques forthem. Then, based on this knowledge, the author
wants to determine the challenges NTN should toaddress.

2.1 Cleaner -steel technology
The history of evolution of high carbon chromium
bearing steel is not presented in this paper as it isdescribed in detail in a paper within the library 1) of The
Iron and Steel Institute of Japan. This history may be
defined as a series of attempts to eliminate inclusions
in bearing steel. It is known that oxygen and hardoxide inclusions (alumina) adversely affects the rolling
contact fatigue life of bearings. In order to reduce theamount of oxide-type inclusions, various steel making
techniques have been developed which include
vacuum degassing process, ladle furnace process,and continuous casting process.
In Fig. 1 2), the trend of oxygen content in bearing
steel is plotted. Since 1964 when vacuum degassing
process was introduced in Japan (in this year, theworld's first high-speed railway system, shinkansen,began operation in Japan), oxygen content in steel
has been dramatically decreased as compared withsteel materials melted in atmosphere. In the 1980's,
ladle furnace process and continuous casting process
were established, and as a result, the oxygen content
in steel was further reduced. The oxygen content
continued to decrease and is now as low as 5 ppm.
The effect of these technologies will be apparent fromthe result of rolling contact fatigue test with specimens
plotted in Fig. 2 which shows correlation between theyears of steel manufacture and rolling contact fatigue*Former General Manager, Elemental Technological R&D Center-10-Trends in Materials and Heat Treatments for Rolling Bearings

30
Bearing steels
Low alloy carburizing steels
25

Double porous blowing at LF

Oxygen content ppm

CC with complate shrouding
20

EBT at EF

Combined refining

Bottom blowing at EF
11.9ppm

15

10

SNRP operation

8.5ppm

7.2ppm

RH degassing
5

6.7ppm
8.3ppmLD degassing

5.8ppm
0
1965

1970

1975

1980

4.7ppm

5.0ppm
1985

1990

1995

2000

2005
Year

10% life (total umber of loading sequences, ×104)

Fig. 1

The trend of oxygen content in steel 2)

100000FA treatment

10000

1000

100
1960

Ladle furnace process
Continuous casting process
SUJ-2 (vacuum degassing)
SUJ-2 (atmospheric melting)
Carburizing steel
1965

1970

1975

1980

1985

1990

1995

2000

2005

Business year of manufacture of material (Western Calendar)

Fig. 2 The trend of rolling contact fatigue life of 12mm in diameter specimen

steels have shown significant improvements. Now, let
us think of assessment technologies for evaluating
effects of non-metallic inclusions. The most common
assessment techniques for non-metallic inclusions
among conventional methods are the point countmethod per JIS (Japanese Industrial Standards) and
ASTM-A method for indicating worst visual field.
However, owing to dramatic advance in steel
making technologies, these assessment techniques
are no longer capable of accurately differentiating
between steel materials. Since the latter half of the
1980's, new assessment technique have been
considered which focus into the numbers, sizes anddistributions of inclusions. Typical examples of suchtechniques include extreme value statistics 3), electron
beam melting process 4), ultrasonic method 5), and amethod where material is electrolyzed, resultant
solution is subjected to filtration and the filtration

lives of test steel specimens.
The lives of the present-day specimens are morethan 10 times longer than those from 1964. However,no significantly novel steel-making technologies havebeen introduced since 2000, and the increase inrolling contact fatigue life of bearings has alreadyleveled out. Incidentally, in 1964, the rolling contact
fatigue life of carburizing steel (SCr420) was about
twice as long as that of bearing steel (SUJ2).The
fatigue life difference continued to decrease and by
1990 the carburizing grade material no longer had an
advantage for fatigue life.

2.2 Advance in assessment technologies for
effects of non-metallic inclusions
As mentioned above, the history of bearing steel
has been a history of efforts for decreasing inclusions,and as a result, technologies for making highly clean-11-

NTN TECHNICAL REVIEW No.76（2008）

residue is analyzed 6). Despite these efforts, we arestill not able to estimate the rolling contact fatigue life ofa given steel material based on an assessment of the
inclusions contained in it.
Utilizing its unique knowledge about image analysis,
NTN has successfully developed a unique system that
automatically determines the sizes and types of all the
inclusions in a given steel material. By using thissystem, NTN has been developing technologies forquantifying the physical properties of inclusions
contained in a given rolling bearing material andestimating the rolling contact fatigue life of thismaterial 7) 8).
Typical examples of the interrelation between rolling
contact fatigue lives estimated based on measured
inclusion ratings and actual rolling contact fatigue lives
is graphically plotted in Fig. 3. Thanks to these newassessment techniques, it is now possible to adopt a
simplified rolling contact life test for a steel material
and promptly assess the quality of the material inquestion.
Furthermore, there is a recent attempt 9) to analyze
inclusions occurring on the fisheye fractureappearance on specimens undergone test using an
ultrasonic fatigue tester that is capable of high speed
at 20 kHz (fatigue test is completed in a very short test
run as short as about 10 minutes) in order to assess
the quality of specimens. It is said that the volume of
specimen possibly treated with this technique can belarger compared with that with microscopic
examination and this technique is capable ofexamining much larger inclusions. However, owing tovariation in the state of bonding between inclusions
and the base surface, the inclusions detected with thistechnique somewhat differs from specimen to
specimen. Also, for this technique, correlation

between inclusions and rolling contact fatigue life ofsteel material will need to be clarified in the future.

2. 3 Rolling contact fatigue life and chemicalcomponents
Various researches 10) 11) are so far available as tothe effects of alloy elements onto rolling contact
fatigue lives of steels. Efforts for these studies werevery active in a period from the latter half of the 1980's
to 1990's. This is because during this period it became
possible to further clarify the effects of alloy elements
in highly clean steels. At the same time, various
manufacturers strenuously developed bearing
materials each optimized for a particular application
while bearing operating conditions were becoming
more demanding.
The alloy elements possibly affecting rolling contact
life of bearing include Si, Mn, Ni, Cr, Mo and V.
However, for convenience, discussion here is madetaking Si as an example because this elemental
additive is relatively cheap and is very effective.
As summarized in Table 1, the rolling contact
fatigue life is longer with a greater Si content. This isbecause addition of Si improves the resistance totemper softening for the steel material. Using thisTable 1 Relationship between rolling contact fatigue lifeand Si content
Chemical component (%)
Steel

Si

Mn

Cr

SUJ 2

0.97

0.31

0.45

1.46

6570

A

1.02

1.01

0.41

1.43

6990

B

0.96

1.30

0.40

1.45

10800

C

1.00

1.42

0.40

1.41

12600

(a) Weighted regression
L10 est. =5.73×108・EAl2O3,1-1.055(b) Extream value theory
L10 est. =1.08×1010 ( area MAX・Al2O3)-2.21
100000

Actual life L10 exp（×104）

Actual life L10 exp（×104）

100000

10000

1000

Multiple correlation
coefficient=0.795
100
100

10% life

C

1000

10000

10000

1000

Multiple correlation
coefficient=0.698
100
100

100000Estimated life L10 est（×104）

1000

10000

Estimated life L10 est（×104）

Fig. 3 The comparison of rolling contact fatigue life and estimated life based on inclusion rating-12-

100000Trends in Materials and Heat Treatments for Rolling Bearings

advantage of Si, NTJ2 material 12) as semi-hightemperature material and STJ2 13) were developed.
Other bearing manufacturers developed similar steel
materials 14) 15) having a higher Si content; however,these materials have Mo additive which is an
expensive alloy element.
Novel longer life application specific bearing
materials have been developed. At the same time,novel materials featuring optimal machinability havealso been developed. Consequently, NTN has
successfully commercialized NKJ65 material 16) basedon medium carbon steel and utilizing the abovementioned advantage of utilizing Si.
Global demand for raw materials has been
accelerating since the beginning of the 21st century
resulting in skyrocketing prices and reduced
availability of resources including coke and rare
metals.

as lead to greater deformation from aging (in otherwords, poorer dimensional stability). Therefore, people
believed that a lower retained austenite content isfavorable.
In contrast, ever since it was learned that retained
austenite can help extend rolling contact fatigue livesof bearing steel materials 17) 18), heat-treatment
techniques positively utilizing retained austenite havebeen adopted in applications aiming at longer bearing
life, in particular, applications intended to extend
bearing lives in a contaminated oil-lubrication
environment, thereby NTN has developed the TMB
bearing products 19) by utilizing these techniques.
As discussed earlier, the rolling contact fatigue life
with highly inclusion-free carburizing steel is roughly
equivalent to that with highly inclusion-free bearing
steel (Fig. 2), and, therefore, the effect of retained
austenite is less apparent. Nevertheless, if bearings
are used under contaminated oil lubrication, the effectof retained austenite is still apparent as shown in Fig.
4 20). To summarize, retained austenite is an importantmaterial factor for longer bearing life.

3. Life-extending efforts through improvement in heat-treatment techniques
The basic requirements for bearing materials are
longer lives, good wear resistance and higher
dimensional stability. Various heat-treatment
techniques have been developed to obtain bearing
materials that satisfy these requirements.

3.2 Carbonitriding process
In carbonitriding process, propane or butane is fired
to generate reformed carburizing gas into which 515% of ammonia gas is added, thereby in the soformed atmosphere, C and N are allowed tosimultaneously enter and get diffused in a bearing
steel material.
Nitrogen diffused in the surface layer stabilizes
retained austenite. Consequently, the amount ofretained austenite is greater after quenching, and, atthe same time, the solution of nitrogen helps enhance
resistance to temper softening, thereby the rolling
contact fatigue life of bearing steel is extended. This
process is identical to the multi-stressing process 21)

3.1 Effects of retained austenite
A quenched high-carbon chromium bearing steel
contains retained austenite in a range of several % to15%, though this percentage can vary depending on
the heating temperature before quenching. Because
retained austenite is essentially a soft, unstablestructure, it has been believed that too much retained
austenite in a bearing steel material will cause thematerial to be less hard and more wear-prone as well3

Ratio of 10% lives

Ratio of 10% lives

3

2

1

0

2

1

0
0

2

4

6

8

10

12

14

0

10

20

30

Retained austenite %b) Contaminated oil lubrication conditions;specimens include SUJ2 subjected to special
heat-treatment process

Retained austenite %a) Clean oil lubrication conditions; SUJ2

Fig. 4 Relationship between retained austenite and rolling contact fatigue life

-13-

NTN TECHNICAL REVIEW No.76（2008）

disclosed in USA in 1964, and has been positively
employed as a special heat treatment technique by
various Japanese bearing manufacturers since thebeginning of the 1990's.
In Fig. 5 22), rolling contact fatigue lives of various
bearing materials, to which this process was applied,
are graphically plotted, wherein each steel species
exhibits extended life owing to carbonitriding process.
NTN calls this process the AS (Austenite
Strengthening) treatment, and has been adopted forlong-life bearings 23) 24). NTN believes that a
carbonitriding process will remain used to extend
bearing life.

longer bearing life-endowing material factors, a newtechnology has been developed since the year 2000,
wherein the new technology features adoption of anovel longer bearing life-endowing factor that is basedon crystal grain refining technique. This new techniqueis the NTN's unique FA treatment (Fine AS treatment)
25). Though researches for obtaining higher-strength
structural steel materials through enhanced crystal
grain refinement were encouraged by the Japan's
national super steel material research project 26), NTN
is the very first bearing manufacturer who positively
adopted increased crystal grain refinement forbearings that are highly hard mechanical members.
This FA treatment is a complex heat-treatment
technique comprising grain refinement featuring
crystal grain size less than 1/2 (see Fig. 6) comparedwith that obtained from conventional quenching
technique as well as a carbonitriding process.
By combining two techniques, NTN has attempted

3.3 Crystal grain refinement process
As described above, the technologies for longer
bearing life so far attempted use effects of either
retained austenite or nitrogen-derived enhanced
resistance to temper softening. In addition to thesea) Clean oil lubrication conditionsb) Contaminated oil lubrication conditions
7

5

with carbonitriding

without carbonitriding

with carbonitriding

without carbonitriding

6

Ratio of 10% life

Ratio of 10% life

4

3

2

5
4
3
2

1
1
0

A

B

C

D

0

E

A

B

Steel
A: SUJ2
B: Si-Cr steel
C: High Si-Cr steel
D: Si-Mn steel
E: High Cr steel

C

D

E

Steel
A: SUJ2
B: Si-Cr steel
C: High Si-Cr steel
D: Si-Mn steel
E: High Cr steel

Test bearing : 6206
Test load : 12.25kN
Rev. speed : 2000rpm

Test bearing : 6206
Test load : 6.86kN
Rev. speed : 2000rpm
Dust : hardened steel beads
104~177μm.10mg/25ml

Fig. 5 Improvement of rolling contact fatigue life of various steels with special heat treatment

0.05mm

0.05mmSpecimen subjected to conventional quenching

Specimen subjected to FA treatment

Fig. 6 The comparison of prior austenite grain size between conventional heat treatment and FA treatment-14-

Trends in Materials and Heat Treatments for Rolling Bearings

to further extend rolling contact fatigue life of its
bearing (see Fig. 2). This heat-treatment technique issuitable for attaining longer rolling contact fatigue life
with general-purpose bearing steel. Such a heattreatment technique will be more commonly used.

4. Future challenges
4.1 Development of novel materials
As a material assessing technique develops, theperformance of subject materials will further improve.
This is also true in the field of bearing technology.
Much progress has been made in the various
assessment and analysis techniques related tobearings. Typical examples of such techniques
include microspace assessment technology, surface
analysis technology, analysis technique, andsimulation technique. NTN expects that these novel
tools help us attain new knowledge and technologies
such that we can now view nano-structures where in
the past the minimum- possible were micro-structures.
In the field of steel-making, the oxygen content in steel
has already reached lowest practical level, and further
bearing life extension in this aspect seems to be
virtually impossible.
Despite this, we hope that further improvement inbearing life is realized through researches into
bonding between inclusions and base surface and into
techniques to turn inclusions harmless by modificationof inclusions.
In the aspect of bearing material development, there
will remain two long-lasting approaches: one approachis an attempt to seek highly functional materials andthe other approach is placement of heaviest
importance onto cost-performance. NTN believes thatthe latter approach in particular requires morestrenuous utilization of surface modification
technology. More specifically, in an extreme case, it isimportant with a rolling bearing to modify the surface
area in a depth of 1 to 2 mm (can vary depending on
the bearing size) where Hertzian contact pressure isapplied, and it is not necessary to enhance the holebulk of the bearing.
Though details are not presented in this reportowing to space limitation, flaking (brittle flaking)accompanying a white etching structure such as one
shown in Fig. 7 can still occur in limited situations.
Previously this brittle flaking was explained by thestress field theory 27), hydrogen embrittlement theory 28)29) or vibration theory 30). However, recently, the mostwidely accepted explanation is tribochemical reactionderived hydrogen embrittlement. These flaking modes
previously occurred only under operating conditions
associated with severe lubricating conditions such as

50μm

Fig. 7 Cross sectional view of the flaking

those in alternator bearing applications. Bearing
operating conditions are becoming increasingly
severe, and at the same time, the requirements for
lubrication become more demanding. There are also
now examples of brittle flaking occurring in lessdemanding applications where low viscosity oils areused for reduced torque.
Therefore, though attempts have been so far madeto prevent this problem by using improved lubricants,
we will implement comprehensive countermeasuresagainst this problem, and such countermeasures will
include improvements in heat-treatment technique,surface modification technique and bearing design.

4.2 About heat-treatment technologies
Commonly applied heat-treatment technologies forbearing steel are atmospheric heat hardening, gas
carburizing hardening and induction hardening.
Recently, the price of crude oil has been soaring and,at the same time, environmental issues (such as
reduction in CO2 emission to help prevent global
warming) as well as problems associated with work
environment and labor safety have been increasingly
highlighted. These issues are important challenges
closely related with heat-treatment process.
In this context, we expect particular carburizing
process will be more commonly utilized. Examples ofsuch process include carburizing process based on
nitrogen gas, where reforming gas generated frompropane or butane and used for atmospheric heating
and carburizing is not used; vacuum carburizing
process; plasma carbonitriding; high-temperature
carburizing for reducing process time. Table 2,
presented for providing additional information,summarizes features of various carburizing processes.
Incidentally, use of quenching oil (coolant) is
disadvantageous in terms of safety and work
environment of hardening operation. NTN believes
that a positive attempt will be made to supersede oil-15-

NTN TECHNICAL REVIEW No.76（2008）

Table. 2 The comparison of various carburizing process
Item

Quality

Environment

Plasma carburizing Vacuum carburizing Gas carburizing

Presence/absence of abnormal layer

AbsentAbsentCleanliness of surface

High

High

Present
Low

Surface hardness

High

High

MediumCarburizing on porous area

Good

Good

Difficult

Operating environment

Excellent

ExcellentMediumWorking energy

Low

Low

High

Excellent

Amount of gas consumed

Low

Low

High

Moderate

Maintenance cost

Low

MediumHigh

Inferior

hardening, which is currently employed, with air
hardening, cooling with pressurized gas or watersoluble coolant.
Induction heating technique is said to beadvantageous in reducing CO2 emission, and an ultrarapid short-time induction hardening technique 31) has
been sometimes used on a commercial basis in
manufacturing gears. Induction hardening techniques
are very often used in manufacturing automotive
bearing components, and this trend will be morecommon. Simulation techniques for induction
hardening have shown much progress. In addition, a
unique control technique for induction hardening
process, known as feedback control-based induction
hardening technique 32) has been developed.
According to this technique, a product is heated
while its temperature is being monitored; the austenitic
state of the material in quenching is calculated,
thereby the quenching timing as well as the tempering
hardness are calculated, estimated and controlled by
the computer. These new techniques will beeventually adopted.
Furthermore, dramatic advancements in sensor
technologies, analysis techniques and simulation
techniques 33) 34) applicable to heat-treatment process
have recently developed. NTN believes that a heattreatment process, more positively utilizing thesetechniques and capable of more active control, willshow much more advance.

5. Conclusion
The achievements so far attained in the field ofmaterials for rolling bearing and associated heattreatment techniques have been described above.
These achievements are primarily contributable todevelopments in material engineering and material
assessment techniques supported by steel
manufacturers. Typical results of the efforts by steel
manufacturers include development of highly clean
steel materials containing minimal non-metallic
inclusions as well as development of special steel
material with varying alloy components. The futurechallenges in our research and development efforts
will fall in two fields: development of ultimate material
and research R&D efforts thoroughly centered onto
cost-performance. In particular, in the costperformance aspect, material engineers at any
bearing manufacturer will need to fulfill required
bearing functions even when using a general-purpose
material. To this end, it will be increasingly more
important, not only to develop more sophisticatedelemental technologies including materials, heattreatment techniques, surface modification techniques(by heat treatment as well as by processing, powder
technology and surface coating) and lubrication
techniques but also to attempt to realize integrationand collaboration with individual technologies.

-16-

Trends in Materials and Heat Treatments for Rolling Bearings17) H. Muro, Y. Sadaoka, S. Ito and N. Tsushima: Proc.
12th Japan Congress on Materials Research- Metallic
Materials, 3 (1969) 74
18) E. Yajima, T. Miyazaki, T. Sugiyama, H. Terashima:
Journal of The Japan Institute of Metals, 36 (1972)
711
19) N. Tsushima: Bearing Engineer 47 (1980) 33
20) K. Maeda: Mechanical Design, Vol.39, No.13 (1995)
48
21) D. P. Koistinen: Trans. ASM, 57 (1964) 581
22) K. Maeda, H. Nakashima and N. Tsushima: ITC
YOKOHAMA (1995) 1387
23) H. Nakashima, K. Maeda and H. Kashimura: NTN
Technical Review No.63 (1994) 13
24) K. Maeda, H. Nakashima, and H. Kashimura: NTN
Technical Review No.65 (1994) 17
25) C. Ohki, K. Maeda and H. Nakashima: NTN
Technical Review No.71 (2003) 2
26) A. Sato: Materia, Vol.37, 10 (1998) 888
27) Y. Murakami, H. Takemura, M. Naka, T. Ogawa, T.
Momono, A. Iwamoto, S. Ishihara: NSK Technical
Journal, 656 (1993) 1
28) K. Tamada and H. Tanaka: Wear, 199 (1996) 245
29) N. Kino, K. Otani: CAMP-ISIJ, Vol.15 (2002) 1016
30) M. Shibata, M. Goto, N. Oguma, T. Mikami: Koyo
Engineering Journal, 150 (1996) 16
31) K. Kawasaki: Ferrum, Vol.10, 7 (2005) 27
32) T. Fujita, N. Suzuki: The Japan Society for Heat
Treatment Conference Proceedings (2005) 21
33) C. Ohki: NTN Technical Review, No.69 (2001) 61
34) C. Ohki: Tetsu-to-Hagane, Vol.93, 3 (2007) 20

References
1) K. Seto: The Flow of Steel Technologies Series Vol.9:
Steel for Bearings, The Iron and Steel Institute ofJapan2) K. Kawakami: Sanyo Technical Report, Vol.14, 1(2007) 22
3) Y. Murakami: Metal Fatigue: Effects of Small Defects
and Nonmetallic Inclusions, Yokendo (1993)
4) Y. Nuri, S. Umezawa: Tetsu-to-Hagane, 75 (1989)
1897
5) N. Ishikawa, T. Fujimori: Tetsu-to-Hagane, 71 (1985)
893
6) Y. Nuri, S. Kitano: patent publication 2001-159627(2001)
7) H. Murakami: NTN Technical Review No.68 (2000) 58
8) K. Yagita, T. Fujita, CAMP-ISIJ, Vol.20 (2007) 1353
9) Y. Furuya, S. Matsuoka, T. Abe: Tetsu-to-Hagane,
Vol.88, 10 (2002) 643
10) T. Yamamoto: Journal of The Japan Institute of
Metals 11 (1972) 419
11) N. Tsushima: J. Japan Inst. Metals 23 (1984) 50
12) K. Maeda, H. Nakashima: NTN Technical Review
No.63 (1994) 83
13) H. Tanaka, Y. Fujii and K. Maeda: NTN Technical
Review No.68 (2000) 51
14) T. Hoshino, K. Amano, Y. Yamamoto, A. Ohta and M.
Goto: Materia Japan Vol.37, 6 (1998) 516
15) M. Imajo: Special Steel Association of Japan, Vol.41,
3 (1992) 19
16) K. Maeda, H. Kashimura and K. Sahashi: NTN
Technical Review, No.65 (1996) 23

Photo of author

Hirokazu NAKASHIMA
Former General Manager,
Elemental Technological
R&D Center-17-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Improvement of Carburized Steel Wear Resistanceby Heat Treatment

Nobuyuki MOURI*
Kazuhiko TAGUCHI*
The bearings for the guide rolls of continuous casting machines used in steel
manufacturing have wear on the outer ring because the bearings are used at low
rotational speeds and high loads. Therefore, it is necessary that the outer rings ofguide roll bearings have high wear resistance. Carbonitriding is known as a commonand effective treatment for improving wear resistance. In fact, carbonitrided bearings
are used widely for the guide rolls of continuous casting machines. In addition, it ispossible to improve wear resistance more by using special steel that includes Cr andV as elements that form nitrides. However, the use of special steel has problems that include the difficulty of acquisitionand high material costs. The purpose of this study is to define a method to improve the wear resistance of standard
carburized steel. We have performed an investigation to quantitatively confirm that the wear resistance of bearing
materials is improved with increased nitrogen concentration at the wear surface. In addition, we believe that carbonitrided
bearing steels transition from severe to mild wear quickly, such that the overall wear resistance is improved.

1. Foreword

2. Carbonitriding conditions for realizing
excellent wear resistanceA typical bearing application that requires excellent
wear resistance is guide roll bearings for continuous
casting machines in steel-making plants. Because thistype of bearing is used at an extremely low speed with
inner ring rotation and very high loads, problems
including difficulty in oil film formation and deviatedwear on the outer ring raceway surface often occur.
One measure known to improve the wear resistanceof bearing components is nitriding 1). Currently many
bearings for continuous casting machines arecarbonitrided 2). Furthermore, by using a special steel
which contains large amounts of nitride forming
elements such as Chromium and Vanadium, thebearing can feature much improved wear resistance 3).
However, use of high alloy steel is not desirable
because of soaring prices and depletion of scarce
resources for alloying elements. In the present
research, we have studied heat-treatment techniques
for improving the wear resistance of common
carburizing steels (SCM420, SNCM420) andattempted to determine the reasons for excellent wear
resistance in nitrided steels.

Measures to improve wear resistance of a steel
material are increased surface hardness andincreased quantities of precipitants with highhardness. However, these two measures do notalways hold as reasons for why carbonitrided
carburizing steels excel in wear resistance (refer to
Section 4). In the present research, we have attempted
to quantitatively determine the relationship between
surface nitrogen concentration and wear-resistance insteel materials, focusing specifically on the quantity ofnitrogen in the form of solid solution, with the goal offinding a heat treatment process that provides amaterial with excellent wear resistance.
It is known that the quantity of interstitial nitrogen ina steel material as a result of a carbonitriding process
is dependent on the concentration of undecomposed
NH3 gas present in the furnace atmosphere 4). In otherwords, in order to increase the nitrogen concentrationon the surface of a given product, it is necessary tocontrol the atmosphere in the furnace such that theconcentration of undecomposed NH3 gas in the
furnace is sufficiently high. Incidentally, when thetemperature inside the furnace is increased, a higher
proportion of NH3 gas will be decomposed into N2 gasand H2 gas immediately after addition into the furnace.

*Elemental Technological R&D Center-18-

Improvement of Carburized Steel Wear Resistance by Heat Treatment

have checked that this technique will improve thewear resistance of the material specimens.

In an attempt to achieve higher carbon penetration
velocity, a carbonitriding process is often performed at
a higher temperature, and in this situation, it is difficult
to increase surface nitrogen concentration.
Fig. 1 presents examples of interrelation between
NH3 gas addition percent and undecomposed NH3
concentration percent at 940°C and 850°C in a 120L
capacity batch furnace. A temperature of 940°C is
assumed to be an appropriate carbonitriding
temperature for carburizing steel, and a temperatureof 850°C is assumed to be an appropriate
carbonitriding temperature for SUJ2. The “NH3 gas
addition percent” is the proportion of NH3 gas to thetotal gas addition including the generated endothermicgas required for carburizing. Fig. 1 shows a result
obtained by setting the flow rate of generated
endothermic gas at a constant level and varying theflow rate of NH3 gas. The concentration ofundecomposed NH3 gas in the batch furnace wasmeasured using gas chromatography.
From Fig. 1, it is apparent that at the same NH3 gas
addition percent, the concentration of undecomposed
NH3 gas in the furnace is lower at the furnace
temperature of 940°C than that at 850°C. In otherwords, to obtain a particular concentration ofundecomposed NH3, carbonitriding processes athigher temperatures require a greater NH3 flow rate. If
a carbonitriding process is executed while a higher
undecomposed NH3 concentration is maintained in the
furnace, the quantity of penetrated nitrogen will
increase and surface nitrogen concentration will behigher even with SCM420 and SNCM420, eachcontaining only negligible quantities of elements whichfeature good nitrogen reactivity (such elements
include V, Cr, Al and Ti). Therefore, we have executed
a carbonitriding process with the undecomposed NH3
concentration regulated at a higher level in order toincrease the surface nitrogen concentration, and we

3. Carbonitriding process with steel
specimens
The specimens representing carburizing steel forthis study were SCM420 and SNCM420, while thebehavior of SUJ2 was evaluated for comparison
purposes. The composition of each steel species issummarized in Table 1. The heat patterns applied tothese steel species for carbonitriding are shown in
Figs. 2 and 3. The shape of the specimens used is aring (φ50×φ64×18).
The carburizing steel specimens (SCM420,
SNCM420) were subjected to carbonitriding at 940°C,
and the specimens of SUJ2 were subjected tocarbonitriding at 850°C. The flow rate of NH3 wasvaried during the process to control the surface
nitrogen concentration on each steel specimen. Also,
the atmosphere in the furnace was regulated tomaintain constant CP (carbon potential) in the furnace
so that the carbon concentration distribution isconsistent for all of the carburizing steel specimens.
Examples of carbon-nitrogen concentration
distribution for SCM420, SNCM420, and SUJ2 arepresented in Figs. 4, 5, and 6 respectively. This
carbon-nitrogen distribution data has been obtained
from steel specimens carbonitrided in an atmosphere

Table 1 Component table of test sample mass %Steel species

C

Si

Mn

Ni

Cr

Mo

Cu

SCM420

0.22

0.32

0.83

0.09

1.18

0.15

0.13

SNCM420

0.20

0.25

0.61

1.62

0.47

0.20

0.14

SUJ2

1.00

0.17

0.39

0.08

1.40

0.03

0.10

Undecomposed NH3 concentration percent
vol%

Carbonitriding
940˚C
NH3 gas0.6

1st. quenching
2nd. quenching

0.5

Tempering

0.4

Fig. 2 Heat pattern of SCM420 and SNCM420

0.3

Carbonitriding
850˚C

0.2

940˚C
0.1

NH3 gas

850˚C
0

0

10

20

30

40

50

TemperingNH3 gas addition percent vol%Fig. 1 Relationship between addition rate of NH3 gas
and undecomposed NH3 gas

Fig. 3 Heat pattern of SUJ2

-19-

NTN TECHNICAL REVIEW No.76（2008）

C and N concentrations mass%

where undecomposed NH3 concentration was
regulated to about 0.2 vol%. It should be understoodthat a higher nitrogen concentration may be achieved
even in a standard carburizing steel by regulating theundecomposed NH3 concentration in the furnace
atmosphere at a higher level. When specimens of
SCM420 and SNCM420 were subjected to acarbonitriding process in the same batch, the surface
nitrogen concentration on the SCM420 specimens
was found to be higher than that on the SNCM420

specimens.
The reason for this seems to be the higher Cr
content in SCM420 steel material. In contrast, thenitrogen concentration on the surface layer is higher
with SUJ2 as shown in Fig. 6 though the nitriding
depth is shallower when compared with the examples
in Figs. 4 and 5. The reason for this seems to be thatthe heat-treatment temperature is relatively low andthe nitrogen penetration velocity is low with SUJ2.
Incidentally, In order to realize a higher nitrogenconcentration on the surface of the final bearing
product, it is important to increase the nitrogenconcentration at the depth of the grinding allowance.

1.2

SCM420: nitrogen concentration
SCM420: carbon concentration

1.0

4. Wear test under oil lubrication

0.8

To be able to quantitatively verify wear the inhibition
effects of nitrogen in the form of solid solution in steel
material, we have executed a test that determines theinterrelation between surface nitrogen concentration
and wear resistance of various steel species. For
assessment of wear resistance, a Savin wear test rig
was used. The test rig is schematically illustrated inFig. 7, and the test conditions are summarized inTable 2.
By running the counterpart at a lower speed andusing low viscosity lubricant, wear resistance of steel
material under lubricating conditions involving metalto-meal contact can be evaluated. Wear resistance ofa given test piece is evaluated by measuring the minor
axis width (parallel with running direction) on the wear
mark (oval shape) occurring on the test piece andthen calculating the wear volume.

0.6
0.4
0.2
0
0

0.2

0.4 0.6 0.8

1

1.2

1.4 1.6 1.82

2.2 2.4

Depth mm

C and N concentrations mass%

Fig. 4 Distribution of carbon and nitrogen concentration in
SCM4201.2

SCM420
窒素濃度
SNCM420:
nitrogen concentration
SCM420炭素濃度
SNCM420:
carbon concentration

1.0
0.8
0.6
0.4
0.2
0
0

0.2

0.4 0.6 0.81

1.2 1.4 1.6

1.8

2

Load

2.2 2.4

Depth mmFig. 5 Distribution of carbon and nitrogen concentration in
SNCM420

Test piece

C and N concentrations mass%Counterpart
Felt pad

1.2

SUJ2: nitrogen concentration
SUJ2: carbon concentration

1.0

Fig. 7 Schematic of wear test rig

0.8
0.6

Table 2 Test condition of wear test

0.4
0.2
0
0

0.2

0.4 0.6 0.8

1

1.2

1.4 1.6 1.82

2.2 2.4

Depth mmFig. 6 Distribution of carbon and nitrogen concentration
in SUJ2

Counterpart

Material: SUJ2, through-hardened (60HRC)
Size: diameter of rotary member; φ40, curvature; R60
Surface roughness: 0.01μmRa

Load

50 N (contact pressure Pmax=0.49 GPa)

Velocity

0.05m/s（24min-1）

Duration

60 min. (sliding distance; approx. 180 m)

Lubrication system Oil lubrication (lubrication via felt pad, VG2)

-20-

Improvement of Carburized Steel Wear Resistance by Heat Treatment

Specific wear amount ×10-10mm 3/Nm

Test results of surface nitrogen concentrations andwear resistance (specific wear amount) of the steel
species being evaluated are plotted in Fig. 8.
The “specific wear amount” presented along thevertical axis of the graph is a unit for representing thewear volume per unit load and unit distance. The
specific wear amount is obtained from dividing thevolume of wear by the applied load multiplied by thesliding distance. To determine the surface nitrogenconcentration on each test piece, the nitrogenconcentration distribution on the ring cross section
was analyzed in advance, and then the surface
nitrogen concentration of that test piece was read at
its grinding allowance region.
From the results of this test, it has been learned thatwear resistance significantly improves increases insurface nitrogen concentration, virtually independentof steel species. In other words, while factors including
high surface hardness and increased quantities ofprecipitants with high hardness have contributed to
improved wear resistance, increases in surface
nitrogen concentration have been shown to exhibit aneven greater effect. To provide supporting evidence,
Table 3 summarizes the hardness and precipitation
area percent information for sample 1, 2 and 3, andFigs. 9, 10 and 11 show SEM photos of precipitantsfor samples 1, 2 and 3.
Sample 3 is through-hardened SUJ2 and its
surface hardness is higher compared with sample 1(carbonitrided SCM420) and sample 2 (carbonitrided
SNCM420). It should be understood from these300

microstructural photos that the quantity of precipitantsis larger in sample 3than in sample 2. Even so, thecarbonitrided samples 1 and 2 boast better wear
resistance. We believe that this is due to nitrogen in
the form of solid solution in steel material greatly
contributing to improvements in the wear resistance ofthe steel material. Additionally, with SNCM420 and
SCM420, excellent wear resistance has beenachieved by increasing the surface nitrogenconcentration.

10μm

Fig. 9 1 Microstructure of Carbonitrided SCM420 (SEM)

SCM420
SNCM420
SUJ2

250
200

3
150

10μm

Fig. 10 2 Microstructure of Carbonitried SNCM420 (SEM)

100

2

50

1
0
0

0.1

0.2

0.3

0.4

0.5

0.6

Surface nitrogen concentration mass%

Fig. 8 Relationship between nitrogen concentration of
surface and specific wear amount under oil lubrication
Table 3 Hardness and Area rate of precipitates
SampleSurfacehardness
HV

Precipitation
area percent
Area %

1SCM420710

6.3

2SNCM420

710

2.7

3SUJ2

740

6.7

10μm

Fig. 11 3 Microstructure of SUJ2 (SEM)-21-

NTN TECHNICAL REVIEW No.76（2008）

carbonitrided SCM420 specimens appears to be
inhibition of wear by synergy between an increased
quantity of nitrogen in the form of solid solution and anincreased quantity of precipitants with high hardness.

5. Wear test under water lubrication

2000
1800

6. Reasons for why carbonitrided steel
materials boast excellent wear resistance
We have considered why carbonitrided SUJ2, which
is not a carburizing steel, boasts excellent wear
resistance. Fig. 13 illustrates change in wear amountover time of through hardened SUJ2 and carburized
SCM420 as well as that of carbonitrided SUJ2 andSCM420, with each test piece being tested on the
Savin wear test rig. The test conditions applied aresame as those summarized in Table 2. Wearprogresses with through-hardened SUJ2 andcarburized SCM420 in proportion with time elapsed
until about 90 minutes after start of the test, and thenprogress in wear decelerates. In contrast, progress inwear with both carbonitrided steel species decelerates
in a short time span, after elapse of 15 to 30 minutes. It
may seem possible that with the Savin wear test rig, thecontact area increases as wear progresses, and, as aresult, the contact pressure decreases and the wear
amount over a given time frame also decreases.
However, since the progression of wear decelerates at
an early stage with nitrided specimens that feature asmaller wear amount and a negligible decrease incontact pressure, decelerated wear progression withthese specimens must not result from increased contact
area. In other words, it is estimated that the
carbonitrided specimens are capable of a “severe-tomild wear mode transition” 5), from severe wear mode
where wear occurs rapidly to mild wear mode wherelimited wear occurs, and that this transition takes placeat an earlier stage with these specimens. We consider
that the main reason for excellent wear resistance withthe carbonitrided specimens in this test is earlier
occurrence of “severe-to-mild wear mode transition”,

40

SCM420
SNCM420
SUJ2

1600
1400

Wear amount ×10-5 mm3Specific wear amount ×10-10 mm3/Nm

In a continuous casting line in a steel making plant,a large quantity of water is sprayed onto hot steel
material, and not only dust but also water and steam
can enter the bearings. The ingress of water into abearing will accelerate wear of the bearing.
We have assessed wear resistance of specimens
under a water lubrication environment using the Sabinwear resistance test rig that is identical to the oneused in the wear resistance test under oil lubrication
environment. However, because oil and water are notreadily mixed homogeneously, we have executed thewear test using pure water in order to simulate alubrication system contaminated with water. For
lubrication, a felt pad moistened with pure water wasused, while other test conditions were kept the sameas those summarized in Table 2.
Fig. 12 summarizes the interrelation between
surface nitrogen concentration and specific wear
amount with specimens subjected to wear test underwater lubrication. Compared with the test results in
Fig. 8 obtained from oil lubrication, the wear amounts
with water lubrication are much greater; nevertheless,the trend of increasing wear resistance with higher
nitrogen concentration remains same. The wear
amount of carbonitrided SNCM420 tends to be greaterthan that of carbonitrided SCM420. As can beunderstood from Figs. 9 and 10, the quantity of
precipitants in the surface layer of carbonitrided
SNCM420 is smaller compared with carbonitrided
SCM420, though the surface nitrogen concentration isroughly same with both materials. The abovementioned difference in the quantities of precipitantsin the surface layer of the SNCM4200 and SCM420
samples seems to lead to the difference in wear
resistance. The wear mode under water lubrication
appears to be severe wear. The wear amount is muchgreater when compared with oil lubrication. The
reason for the excellent wear resistance of

1200
1000
800
600
400
200
0

0

0.1

0.2

0.3

0.4

0.5

35
30
25
20
15

SCM420 carburized
SCM420 carbonitrided
SUJ2 through-hardened
SUJ2 carbonitrided

10
5
0

0.6

0

Surface nitrogen concentration mass%

50

100

150

200

Test duration min

Fig. 12 Relationship between nitrogen concentration andspecific wear amount under water lubrication

Fig. 13 Change with time of wear amount under oil
lubrication-22-

Improvement of Carburized Steel Wear Resistance by Heat Treatment

Reason 1 "accelerated transition to mild wear mode"
appears to be an effect of nitrogen in the form of solid
solution. As to reason 2, we believe that higher
nitrogen concentration is combined with increased
quantities of hard precipitants and synergy from thesefactors helps improve wear resistance of associatedsteel materials.

and believe that the carbonitriding process improves
the wear resistance of these specimens in anenvironment where transition to mild wear mode canoccur.
In a similar manner, we have investigated the wear
progress with specimens under water lubrication. For
this purpose, we have subjected carbonitrided SCM420
specimens and through-hardened SUJ2 specimens totesting on a Savin wear test rig; therefore, one group of
specimens has been subjected to testing under oil
lubrication while the other group has been subjected totesting under water lubrication. Fig. 14 exhibits theresulting time-dependent wear behavior of these
specimens. Though capable of quick transition to mildwear mode under oil lubrication, wear amounts seen in
the carbonitrided SCM420 specimens under water
lubrication are greater per unit time, and we have notdetected a transition point to mild wear mode.
Likewise, through-hardened SUJ2 has not exhibited
transition to mild wear mode. When wear progress inthrough-hardened SUJ2 under water lubrication iscompared with that in carbonitrided SCM420 underwater lubrication, it becomes apparent that the
carbonitrided steel features excellent wear resistance.
In a situation where transition to mild wear mode does
not take place, such as in the case of the water
lubrication wear test, the amount severe wear that
occurs as time elapses greatly affects the wear
resistance of a steel sample. By increasing the surface
nitrogen concentration of a steel material, the wear
resistance of the material in severe wear mode is
improved.
From the above-mentioned findings, it seems that
improvement in wear resistance of carbonitrided
products is contributable the following two major
reasons:
1 Transition from severe to mild wear mode isaccelerated.
2 Wear resistance in severe wear mode isenhanced.

7. Conclusion
We have quantitatively assessed the interrelation
between surface nitrogen concentration and wear
resistance with various steel species, focusing oncarbonitriding technique as a method for improving
wear resistance of ordinary carburizing steel. As aresult, we have verified that by maintaining a furnace
atmosphere with a higher concentration ofundecomposed NH3 gas in order to increase thesurface nitrogen concentration of a steel product, theperformance of that product in situations requiringhigh wear resistance is much improved. Also, we havestudied reasons for improved wear resistance ofcarbonitrided steel species, and have determined thattransition from severe wear mode to mild wear mode
occurs more quickly with carbonitrided steel materials,
and that carbonitrided steel materials boast good wear
resistance even in severe wear mode under water
lubrication.
We will further clarify the wear resistance
improvement mechanism found through continued
testing, and this new knowledge will be reflected in
new NTN bearing products.

References
1) Iron and Steel Materials Handbook, Bulletin of The Iron andSteel Institute of Japan, 177
2) S. Nasu: Applications of AS Series Bearings for Steel andGeneral Industry, NTN Technical Review, No.67, 65 (1998)
3) For example, K. Yamamura, M. Oohori: Development of High
Wear Resistant "SWRTMBearing," NSK Technical Journal,
No.671, 30 (2001)
4) C. Ooki: Effect of Carbon Activity and Base Gas
Compositions for Carbonitriding of SUJ2 Steel, Tetsu-toHagane, Vol.93 No.1, 44 (2007)
5) T. Sasada: Wear (8), Science of Machine, Vol.57, No.8 (2005)

Wear amount ×10-5 mm3160
SCM420 carbonitrided (oil lubrication)

140

SCM420 carbonitrided (water lubrication)

120

SUJ2 through-hardened (oil lubrication)

Photos of authors

SUJ2 through-hardened (water lubrication)

100
80
60
40
0
0
0

50

100

150

200

Test duration min

Fig. 14 Change with time of wear amount under oil andwater lubrication-23-

Nobuyuki MOURI

Kazuhiko TAGUCHI

Elemental Technological
R&D Center

Elemental Technological
R&D Center

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Deep Nitrided 32CrMoV13 Steelfor Aerospace Bearings Applications

Daniel GIRODIN*
Bearings for aerospace applications such as main shafts of jet engines need to
be extremely reliable in order to reduce the In Flight Shut Down (IFSD) ratio andextend the frequency of maintenance operations.
These functional and economic issues require for the material theimplementation of appropriate and reliable metallurgical solutions.
This report describes the developments of the nitriding 32CrMoV13 steel (AMS6481) to optimize its properties in order to overcome the limits of classical steels
used for aeronautical applications, such as resistance to surface damage andtoughness. The deep nitriding provides a case deeper than 600 µm, a highsurface hardness in the diffusion zone due to a semi-coherent precipitation of
nano-metric chromium nitrides. This precipitation also generate compressive
residual stresses, while maintaining a notably high tough core. These properties offer considerable advantages inextending both the surface resistance to contamination and the ability to design bearings with integrated design and thinsections that consequently reduce the weight of components.

and toughness mainly, the nitriding of the 32CrMoV13
steel grade has been optimized.

1. Introduction
Bearings for aerospace applications operate under
severe service conditions and have been a continuous
challenge from the material point of view over theyears. Bearings for aircraft turbine engines are
required to operate at high speed and temperature,
while bearings for helicopter transmission gearboxes
require high load capacity. These conditions are
combined with a demanding operating environment
such as contamination, deformation and vibration of
the surrounding structure.
In the meantime, the demand for safety is
increased, asking for extended fatigue life andreliability. Modern bearings are also asked to belighter and cheaper both for supply and maintenance.
These requirements induce bearing designed withthin sections, with integrated functions such asflanges, squirrel cages, or integrated bearing
raceways.
The economic aspects require a process with theminimum amount of distortion and grinding and themaintenance costs require raceways more tolerant tosurface damages.
To offset the deficiencies of through hardened andcarburised steels with regard to surface resistance

2. Material requirements of aerospace
bearings
The history of aircraft turbine engine bearings
shows great improvements in reliability andperformance due to the simultaneous progress insteels both in composition and quality to sustain thedevelopment of new engines 1). The through hardened
M50 steel (80MoCrV42-16) developed during the late
50's to fulfill the increase in operating temperatureshows, as other through-hardened steels, limited highspeed capabilities. Indeed, the high tensile hoop
stresses resulting from ring centrifugal stresses at highrotational speed and press fit of the bearing ring, areliable to cause the following problems :¡added to the rolling contact stresses, they increase
the overall stressing and may reduce the bearing
fatigue life.
¡over approximately 2.4 millions dn (bearing bore inmm×shaft speed in min-1), they increase the stressintensity so that fatigue spalling which may arise
can lead to fracture of rings 2).
High temperature case hardening steels such as*SNR Elemental Bearing Technology-24-Deep Nitrided 32CrMoV13 Steel for Aerospace Bearings Applications

M50 NiL (13MoCrNi42-16-14) and X20WCr10 usedlater on, brought improvements, not only in relation
with their tough core but also due to the achievementof compressive residual stresses in the case 3). The
residual stresses influence the equivalent stress
distribution and consequently the initiation andpropagation of cracks, thus retarding the fatigue
process.
Nevertheless, the fracture toughness of these steels
remains poor. They are also subjected to the common
disadvantage of carburizing steels regarding theimportant distortion induced by oil quenching during
the heat treatment process that requires expensive
grinding and impacts manufacturing costs.
Moreover, under high speed conditions, thelubrication is not permanently fully flooded (EHD) but
may be mixed or starved (at least during the transientperiods) so that surface fatigue is experienced 4-5). In
these conditions, both through hardened steels andcarburized steels show limitations.

3.1 The base steel
The chemical composition of the 32CrMoV13 Steelgrade, that has been standardized as AMS 6481 isgiven in Table 1.
Table 1 Chemical composition of 32CrMoV13 as per
AMS6481
Eléments （％）
C
Cr
Mo
V
Mn
Si
32CrMoV13 0.29〜 2.80〜 0.70〜 0.15〜 0.40〜 0.10〜
AMS 6481
0.36
3.30
1.20
0.35
0.70
0.40
0.11〜 4.00〜 4.00〜 1.13〜 0.15〜 0.10〜
M50NiL
0.15
4.25
4.50
1.33
0.35
0.25
NOTE: Other than the components listed above, M50Nil contains 3.20
to 3.60% of Ni.

A VIM + VAR (Vacuum Induction Melting + Vacuum
Arc Remelting) melting process is performed, in orderto achieve a good level of micro-inclusion cleanliness
required to obtain both high and reliable rolling contact
fatigue properties and structural fatigue resistance.
The case hardness after a nitriding treatment
depends on the hardness of the base metal(measured conventionally at the depth = Hv corehardness + 100 Hv), determined by the heat
treatment, and on the composition of the steel,particularly the amount of elements promoting nitride
precipitates (Cr, Mo, V, Al).
The 32CrMoV13 steel is a good compromise, inorder to achieve surface hardness higher than that of
carburized steels, with a rather good diffusivity ofnitrogen for acceptable nitriding duration (deep case≧
0.6 mm in 100h max). It should be made without Al fora better nitrided layer toughness, without Ni and with aclose control of residual elements such as P (avoid
temper embrittlement) for a higher core toughness.

3. The development of nitriding
32CrMoV13 steel
The operating conditions, design and manufacturing
aspects of the aeronautical bearings previously
described require a hard surface and a tough core, ie
a surface hardened steel.
Surface modification processes such as nitriding
have an advantage over carburising because theyusually provide higher hardness, impart higher
compressive residual stresses in the subsurface, bothproperties known to improve indentation damage andsliding wear resistance. The alloy elements (Cr, Mo, V,
Mn and Al) added in nitriding steel to improve thesolubility in ferrite conditions and to promote fineprecipitation that strengthen the diffusion layer, arealso favorable for fracture toughness.
Contrary to carburising, the nitriding treatment
performed at relatively low temperature (500 - 550˚ C),
on pre-treated material, without subsequent
quenching, leads to low distortions, while maintaining
a structural and mechanical stability during
temperature holding.
According to these features, nitriding of 32CrMoV13
grade, already used for other mechanical applications,offers a good solution to the complex and numerous
problems of modern bearings, provided the depth ofthe case will be deep enough to be compatible withHertzian stresses. This particular point implies thedevelopment of a deep nitriding treatment.

3.2 Nitriding treatment
Prior heat treatment 6)
Nitriding process is performed on parts, after priorheat treatment, in a more advanced stage ofmachining. Although distortion during nitriding issmaller than for carburized and quenched parts, it isnecessary to induce a distortion as low as possible
and to control it.
One major cause of part distortion during nitriding is
the softening of the base metal, due to complementarytempering effects.
The effect of different nitriding treatments (from 25 h
to 100 h at 555˚C) on the softening of the base metal
for different initial hardness values (from 400 to 480Hv) is shown in Fig. 1 7).
In order to restrict distortion after a treatment aslong as 100 h, needed to obtain the required casedepth, the initial hardness should be close to 380 -25-

NTN TECHNICAL REVIEW No.76（2008）

555˚C/50h
555˚C/50h
Depth 0.7mm Depth 0.5mm

surface of the parts, a uniform white layer made ofnitrides γ' (Fe4N) and ε(Fe2-3N), by setting thenitriding potential of the atmosphere to a high value 8).
A stable white layer, guarantees a permanent sourceof nitrogen to the diffusion layer, thus avoiding any
nitriding defects (de-nitriding or heterogeneous
nitriding) of the surface.
The nitriding depth is controlled by solid state
diffusion and precipitation, mainly depending on theamount of nitrogen introduced in the subsurface andon the time-temperature cycle. The conventional
depth of the diffusion layer, as a function of time andtemperature, was measured on samples nitrided in anindustrial equipment (Fig. 2).
In order to obtain a depth of more than 0.6 mm
required by applications, in less than 100 h, nitriding
should be performed in the range 525 - 550˚ C. The
temperature (class 5 vertical furnace) and the furnace
atmosphere are controlled in order to maintain astable thickness for the white layer that guarantee anhomogeneous treatment.

555˚C/25h
Depth 0.35mm
520˚C

480
460

575˚C

440
420

620˚C

400

635˚C

Tempering temperature

HV 10 before nitriding

500

380
360

380

400

420

440

460

480

HV 10 Vickers hardness after nit

Fig. 1 Hardness of 32CrMoV13 base metal,
before and after different nitriding treatments

420 HV, which corresponds to a tempering
temperature in the range 625 - 650˚C.
The typical mechanical properties obtained after
prior heat treatment compared to AMS 6481
requirements are given in Table 2.
A comparison between fracture toughness androtative bending endurance limit of 32CrMoV13 andM50NiL carburizing steel is given Table 3. The better
toughness of 32CrMoV13 can be explained by both aslightly lower strength, along with a composition and a
microstructure less sensitive to temper embrittlement.
The fatigue limit is comparable or higher than that of
M50NiL. It may be due to the detrimental effect of thecoarser grain size of M50NiL.

0.8

Nitrided depth mm

0.7
0.6
0.5
0.4

510˚C

0.3
0.2

525˚C

0.1

550˚C

0

The deep nitriding process0

The basic principle of the process, is to build up,
right from the beginning of the cycle, on the outermost

20

40

60

80

100

120

Time hFig. 2 Conventional depth of nitriding vsnitriding time for various temperatures

Table 2 Mechanical properties of 32 CrMoV13 comparedto the requirements of AMS 6481
Hardness U.T.S(Brinell) (MPa)Yield
Rp 0.2
(MPa)

A
(%)

AMS
6481 352〜388 ≧1137 ＞951 ≧13
Typicalvalues 370〜380 1250 1060 16.5

4. Properties of the deep nitrided layer

Z KV Impact K1C(%) strength Toughness(J)(MPa√m)

4.1 Microstructural characterization
Structure after prior heat treatment

≧68
70

133

Before nitriding, the steel is quenched andtempered at 625-650˚ C corresponding to the 3rdstage of martensite tempering. The ferrite structure
have retained the size of the early martensite laths
that ranges between about 0.1 and 1 µm, whiletempering carbides precipitate at the lath boundaries
and inside the laths. The intragranular carbides are
rodlike shaped, about 100 nm in length and arranged
in two families oriented 65 degrees from each other(Fig 3). These carbides have been identified as
cementite Fe3C.

154

Table 3 Comparison of main mechanical properties of
32CrMoV13 and M50NiL carburizing steel
UTS
Rm
(MPa)

Yield
Rp0.2
(MPa)

A
(%)

Endurance Limit
K1C 20˚C（107 cycles）
MPa√m
（MPa）

32CrMoV13

951

1137

13

130

770

M50NiL

1060

1250

16.5

55

750

-26-

Deep Nitrided 32CrMoV13 Steel for Aerospace Bearings Applications

The diffusion layer is divided in 2 zones (Fig. 5a):
¡ A zone, beneath the white layer, about 100 µm
deep, relatively depleted in precipitates.
¡ A zone, toward the core, where "angel hair"precipitates are present, along the grainboundaries, in the direction parallel to the surface.
These precipitates have been identified as
cementite carbides.
The lack of carbide precipitation in the first zone
could be explained by the dissolution of carbides andthe precipitation of more stable nitrides. It results thatthe carbon is pushed inward by the nitrogen diffusion,leading to the precipitation of "angel hair" carbides.
After the nitriding process, the microstructure of the
diffusion layer retains the tempered martensite
morphology as observed after prior heat treatment.
Inter and intragranular carbides are still observed,
except in the sub-surface zone where most of them
have disappeared being replaced by smaller nitride orcarbonitride precipitates.
A fine nano-metric precipitation of disk-shaped
nitrides also occurs inside the needles in the whole
diffusion zone. Diffraction and high resolution T.E.M
pictures (Fig. 6a and Fig. 6b) of these semi-coherent
precipitates show they are of CrN type, only a few
interatomic distances wide, while their diameter isabout 10 nm 8).

0.4 µmFig. 3 T.E.M micrography of the structure
after prior heat treatment

Structure after nitriding
Nitrogen and carbon concentration profiles havebeen made through the diffusion layer using anelectron microprobe (Fig. 5b). The amount of nitrogen
introduced into the surface in the ferrite condition isabove 1% Wt in the main part of the diffusion layerand decreases rapidly beyond 500µm in depth. In themeantime, the carbon content is well beneath thecontent of the base material, then increases to reach amaximum at about a depth of 600 - 700 µm thatcoincides with the maximum of the diffusion front of
nitrogen. Some local carbon peaks reveal thepresence of carbide precipitates.
It could be noticed that the hardness profile varies
accordingly to the nitrogen profile.
The white layer (compound layer), less than 30 µm
deep, combine porous Fe2-3N-εnitride on the outerside with compact Fe4N-γ' nitride on the inner side.
Some branchings may grow along grain boundaries(Fig. 4). The white layer is systematically removed bygrinding, on the rolling races and working surfaces(typically 70-80 mm).

b)
White layer
Subsurface
carbide -depleted
zone

1.8
Vickers Hardness
%C
%N

800

Diffusion
layer

Hardness HV

Zone
Enriched in
carbides

900

700

1.6
1.4
1.2

600

1

500

0.8
0.6

400

0.4
300

0.2

200

0.1mm

0

200

400

600

800

1000

Depth µm

Fig. 5 a) Optical micrography of the nitrided case (nital etching)b) Vickers micro-hardness, weight %C, weight %N profiles of the nitrided layer

-27-

0
1200

Element content %a)Fig. 4 Microstructure of the nitrided case (optical)

NTN TECHNICAL REVIEW No.76（2008）

the "white layer" (75 µm removed in average bygrinding). A typical hardness profile is shown Fig.7 incomparison with carburised M50NiL. The surface
hardness of the nitrided 32CrMoV13 steel is higher
than that of the carburized M50NiL steel, but the casedepth is shallow.
The hardness and other mechanical properties aremaintained after 100h ageing at 450˚ C for both
nitrided case and core (Table 4). On the other hand,
the hot hardness of the nitrided layer (surface
hardness) is comparable to that of M50 steel (Fig. 5).

Fig. 6 a) High resolution image of semi-coherent precipitates.
b) Inverse Fourier Transform showing the very thin width(〜〜3-4 atomic planes)- Compressive residual stresses.
The formation of nano-metric nitrides induce an
expansion of the ferrite matrix which, according to theprinciple of the mechanical balance, builds up theobserved stresses (Fig. 8). The residual stresses were
measured using X-ray diffraction in the circumferential
direction on a ball bearing race ground and polished.
The high level of compressive stresses observed in
the first 10 µm is due to the work hardening producedby the grinding process, while the rise between thedepth of 100 µm and 200 µm is probably linked to thegrowth of precipitates in this zone.

Two other types of precipitates have been observed :¡ Large (Fe,Cr)3C carbides, probably residualcarbides in which small globular precipitates areformed. These precipitates are chromium nitrides,
partially substituted with vanadium andmolybdenium¡ Intergranular precipitates, probably resulting fromformer intergranular carbides : A few isolated
globular precipitates (about 100 nm in depth),
identified as vanadium carbonitrides with partial
substitution in chromium and molybdenium andother precipitates, smaller than 50 nm which are
chromium nitrides partially substituted withvanadium and molybdenium.

900
800

4.2 Mechanical properties

700
600

M50NiL

500

32CrMoV13

400
300
200

The nitriding process induces metallurgicaltransformations that result in changes of themacroscopic mechanical characteristics :Herzian
area

100
0
0

0.5

0.65

Vickers hardness

The structure of the core does not evolve
significantly after nitriding, only the mechanicalproperties slightly decrease (Table 4).

1

1.5

2

2.5

Nitiding depth mm

Fig. 7 Hardness profile of the nitrided layer- A significant hardening of the surface layer.
The surface hardness of the parts ranges between
730 and 830 HV30 with a case depth between 0.55
and 0.75 mm. This value is achieved after removal of0

Residual stress MPa-100

Table 4 Modification of mechanical properties after ageing
Surface Case
Condition

Core

Vickers Nitriding Vickers
UTS Y.S
hardness depth hardness

EL

Charpyimpact

HV10

mm

HV50

MPa MPa%

J/cm2

As treated

375

-

375

1245 1050 15

140

Nitrided

815

0.65

373

1200 995 15.5

125After 450˚C/100H
ageing 550˚C/100H

822

0.70

373

1195 985

16

145

741

0.80

356

1150 945

16

155

-200
-311
-300
-400
-500
-510
-600
-700

-600

0

200

400

Depth

600

µm

Fig. 8 Residual stress profile

-28-

800

Deep Nitrided 32CrMoV13 Steel for Aerospace Bearings Applications

upper curve is the local maximum shear stress taking
into account the local stress concentration due tocarbides. The lower curve is representative of thenitrided steel, including the compressive residual
stresses that reduce the local value of the maximum
shear stress.
Therefore, the M50 steel may develop fatigue
subsurface cracks up to 600 µm in depth contrary to
the nitrided steel which is theoretically free from local
structural modifications in the area of maximum
Hertzian shear stress and thus not subjected todamage except for depth over 550 µm, where thestress is relatively low and the risk unlikely to happen.
Several types of rolling contact fatigue tests were
performed, under various lubrication and operating
conditions, in order to qualify the behavior of thenitrided case.

With a maximum peak value of about -550 Mpa, thecompressive residual stresses are much higher thanthat of case hardened M50NiL (≈ -200 Mpa), and ofcourse higher than that of through hardened steels (noresidual stresses except the grinding one).

4.3 Rolling contact fatigue properties)
The ability to resist rolling bearing fatigue is a prime
requirement for bearing steels, both concerning
subsurface initiated fatigue and surface initiated fatigue.
Subsurface fatigue, experienced under EHD
lubrication conditions is linked to the micro-inclusioncleanliness but is not assumed to be the criticaldamaging process for remelted steels.
According to the Hertzian theory, rolling contact
under EHD lubrication conditions is characterized by a
sub-surface stress field whose maximum is located at
a depth related to the contact loading. The case depthof nitrided parts (as well as for carburised one) has tobe fitted to the Hertzian stress profile in such a waythat the profile of micro-yield stress limit τf (z) isabove the equivalent stress curve τd (z) related to theloading conditions.
In this way, a method for determining the profile ofcompressive micro-yield stress in the nitrided case,
based on nano-indentation measurements, has beendeveloped 10-11). The comparison of the evolution of
the micro-limit profile with the loading stresses canthen provide an evaluation of the potential risk ofsubsurface damage.
Fig. 9 shows the local equivalent stress profile
representative of the loading conditions (Hertzianstress = 3100MPa) for NJ212 cylindrical roller
bearings compared to the micro-yield shear stresslimit of M50 and nitrided steel (the local micro-yieldstress is taken at 20×10-6 plastic strain).
The middle curve represents the maximum shear
stress for the M50 steel without any stress raiser (suchas for example oxide inclusions, carbides...). The

Fatigue life under EHD lubrication conditions
Rolling contact fatigue tests under fully flooded oil
lubrication were performed for different type ofcontacts (testing conditions and results are given
Table 5):¡ flat washer (trust bearing) on the SNR FB2 test
machine¡ 6309 deep groove ball bearing on SNR standard"S" machine¡ NJ212 roller bearing on SNR "FC" bench
For all these tests, the rolling elements were madefrom through hardened steel.
Both for point and linear contacts, the endurance
tests show a fatigue life better for the deep nitrided
32CrMoV13 steel than for trough hardened or
carburized steels. On deep groove ball bearings andtapered roller bearings, the life is about 2 to 3 times
the life of M50 steel.

Test conditions

Table 5 Comparison of rolling contact fatigue of test
samples and actual bearings

1400
1200
1000

M50

800

L10 Life (h)Shear stress MPa

Max shear stress (MPa) M50
Max shear stress (MPa) 32CrMoV13
Max shear stress (MPa) M50 + carbides
Shear elastic limit M50（MPa）
Shear elastic limit 32CrMoV13（MPa）

600
400

32CrMoV13

200
0
0

200

400

600

Depth

µm

800

1000

Fig. 9 Comparison of local maximum shear stressfor NJ212 bearings (PHz=3100MPa)-29-

Bearing type

Flat washer

6309

NJ212

Axial load (daN)

1075

530

300

Radial load (daN)

―

1600

1200

Hertzian stress (MPa)

4200

3200

3100

Speed (min-1)

1500

2200

1200

Lubrication

ISO 46 oil

ISO 46 oil

ISO 46 oil

Temperature (˚C)

40

Room Temp.

Room Temp.

32CrMoV13

≧1300

3863

1240

M50

≧1300

942

560

M50 Nil

850

―

980

X20WCr10

990

―

―

NTN TECHNICAL REVIEW No.76（2008）

Fatigue life under boundary lubrication

5. Industrial statistics and products

Tests have been made on FB2 machine using
NATO 0156 oil at room temperature under a 2500
MPa contact stress. In these conditions, the lubricant
oil film parameterλ= oil film thickness/ (Ra2raceway+Ra2ball)1/2 ≈1.5. Comparative results for M50NiL, M50
and 32CrMoV 13 are given in table 6.
Considering the experimental results, the behaviorof the deep nitrided 32CrMoV13 steel thatexperienced no failures is much better than carburised
M50NiL that shows peeling damage.

When the nitriding conditions described previously
are applied, after proper prior heat treatment for
hardness 380 - 420 Hv, the statistical results for
hardness, conventional depth and distortion measured
on more than 70 batches of a 150 mm outer ring, aregiven in the table 7. These results confirm the great
reproducibility of the process.
Bearings with different shapes and sizes are
manufactured by SNR Roulements using deep
nitrided 32CrMoV13. It concerns deep groove ballbearing, roller bearing with thin sections and/or
complex integrated shapes such as bearing for main
shaft of aircraft turbine engines or rotor mast ofhelicopters (Fig. 10).

Table 6 Rolling contact fatigue under boundary
lubrication and of dented raceways (flat washer)
M50NiL

M50

32CrMoV13 DN

Boundary lubrication L10（h）

218

＞900

＞900

L10（h）

123

503

＞900

Dented raceways

Table 7 Metallurgical and dimensional properties ofdeep nitrided parts

Simulation of contaminated lubrication

Specification Average value

Under contaminated lubrication conditions, the lifeof bearings decreases and surface initiated spalling
occurs on dents produced by foreign particles going
through the contact. The effect of the shape and sizeof artificial dents on the bearing life provides useful
information with regard to the fatigue life undercontaminated lubrication.
For this purpose, tests have been performed on the
SNR FB2 machine. Four Vickers dents, 280 mm indiagonal, had been printed on the race before testing.
Rings were run under a 2500 MPa Hertzian stressusing NATO 0156 oil lubrication.
Deep nitrided 32CrMoV13 steel shows a longer life
compared to M50 and M50NiL steels (Table 6). Tests
were interrupted after 900 hours without any failure
having been observed, contrary to M50 andparticularly M50NiL that experience early failure.
This favorable behavior could be explain by the
combined effects of large surface hardness and highcompressive residual stresses.

As nitrided depth (mm)Range

0.63〜0.75

0.67

0.62〜0.72

Depth on finish parts (mm)

≧0.55

0.60

0.55〜0.65

White layer (µm)

≦35

25

20〜30

Surface Hardness (HV0.5)

750〜850795

760〜830

Core surface (HV0.5)

360〜420400

380〜420

Diameter expansion ratio(µm/mm)―0.8

0.7〜0.9

Out of roundness (µm)

―

25

10〜40

Fig. 10 Example of aerospace bearings-30-

Deep Nitrided 32CrMoV13 Steel for Aerospace Bearings Applications

References

6. Conclusion

1）Pearson, P.K., "The history and future of aircraft
turbine engine bearings, Bearing steels: Into the 21st
century", J.J.C. Hoo Ed, ASTM STP 1327, p335353,1998.
2）Spitzer, R. "New case-hardening steel provides
greater fracture toughness", Ball Bearing J., SKF,
234, 6-11, 1989.
3）Böhmer, H.J., "Residual stresses and material
behavior of M50NiL and RBD", Bearing Steels, J.J.C.
Hoo Ed, ASTM STP 1195, p34-48, 1993.
4）Averbach, B.L. and Bamberger, E.N., "Analysis ofbearing incidents in aicraft gas turbine mainshaft
bearings", Tribology Transaction, 34 ; 2, pp 241-247,
1991.
5）Averbach, B.L, Van Pelt, S.G., Pearson, P.K. andBamberger, E.N., "Surface initiated spalling fatigue in
M50 and M50 NiL bearings", Lubrication engineering,
vol.47,10, pp 837-843, 1991.
6）Joly, P. and Daniel Girodin. "Deep Nitriding of the
32CrMoV13 Steel (AMS 6481 0.32% C-3% Cr-1%
Mo-0.2% V) and its Application for Aerospace
Bearings," Toronto, May 2005.
7）Joly, P., FREY, J.and LARREUR, B.,"Développements récents dans les aciers denitruration et le contrôle des cycles de nitruration pourl'obtention de couches nitrurées très profondes,"NITRURATION, Journées franco-allemandes", J.
Grosch et E.J. Mittermeijer, April 20028）Dulcy, J., Torchane, L., Gantois, M., "Mécanismes deformation et cinétique de croissance des couches
nitrurées, "NITRURATION, Journées francoallemandes", J. Grosch et E.J. Mittermeijer , April
2002.
9）M. Sennour, PhD Thesis, Lyon INSA, 200210）El Ghazal, H., " Study of microstructural andmechanical properties of case hardened 16NiCrMo13
and nitrided 32CrMoV13 steels -- Application to theestimation of the rolling fatigue life", PhD Thesis, Lyon
INSA, 1999.
11）Vincent, A., Elghazal, H., Lormand, G., Hamel, A.,
Girodin, D., "Local elasto-plastic properties ofbearings determined by nano-indentation
measurements", Bearing steel technology--Advancesand state of the art in bearing steel quality assurance,
J. M. Beswick Ed, ASTM STP 1419, p427-442, 2002

The deep nitriding process applied to 32CrMoV13
steel (AMS 6481) solves in a convenient way thedifficult compromise concerning the incompatible
properties required for aerospace bearings.
Indeed, these bearings may operate under severe
conditions such as high-speed, high temperature,
starved or contaminated lubrication. Furthermore, theirdesign may be complex, integrating multiple functions,and requires thin sections for weight reduction.
The Deep Nitriding of 32CrMV13 steel offers abetter compromise than classical high temperature
bearing steels (through hardened or carburized) tomeet these requirements.
The core material has excellent toughness forstructural functions and high rotational speeds. Due tothe semi-coherent precipitation of nano-metric nitrides,
the nitrided layer features high hardness, highcompressive residual stresses and superior rolling
contact properties.
The nitriding technology makes easier the
production of parts with complex shapes, with alimited amount of finish grinding because of the limited
distortions resulting from low temperature cycle. The
manufacturing process is under control and properties
show small dispersion.
All these features make the deep nitrided
32CrMoV13 an excellent solution for high reliability
and safety aerospace components.
Acknowledgment : Author would like to thank the Aubert &Duval Company and the Mateis laboratory of Lyon INSA
for their collaboration to this work.

Photo of author

Daniel GIRODINSNR Elemental
Bearing Technology-31-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Test Design and Result Interpretation Methods
for Evaluating Rolling Contact Fatigue Life
to Maintain Both Efficiency and Reliability

Takumi FUJITA*In this report, several methods for rolling contact fatigue (RCF) life test design
and result interpretation are introduced. These methods generate results using
random numbers following a Weibull distribution. The first method illustrates therelationship between the minimum number of test pieces and the suspension timein a fixed time test required at L10 and L50 life with an arbitrary reliability. This
relationship is useful to maintain qualitative reliability and avoid excessive
quantitative testing. The second method can clarify the relationship between agiven number of test pieces and a resultant significant difference in L10 and L50 lifein accelerated testing with an arbitrary reliability. This relationship is also useful toestimate the necessary number of test pieces based on statistical logic. Of note, applying calculations allows estimation of
the test results. The third method enables the calculation of a range of L10 and L50 lives with significant reliability even if thenumber of test pieces is too small to estimate L10 or L50 lives employing Weibull plots. The fourth method can determine thesignificant difference of L10 and L50 life between two lots and allow the quantitative estimation of the minimum difference
between their lives from data obtained experimentally.
These methods provide techniques that are easier to understand compared to recent mathematical models, and they show
enough flexibility to apply to almost all types of testing. These new systems will therefore eliminate the need for special
experience in statistical design and result interpretation for RCF life tests.

the life of a bearing based on Weibull plots and we
can evaluate the bearing performance based on the
resultant estimated life (hereinafter this type of test isreferred to as "accelerated test").
With a fixed time test, guaranteed bearing life is intheory longer when a large number of test pieces ofthe bearing in question do not develop a failure in alonger time span: however, it has been difficult toselect the necessary number of test pieces (herein
after referred to “test pieces”) and suspension time fora fixed time test based on statistical data. Attempting
to solve this issue, there have been proposed
techniques intended to solve bearing life based on therelation between test pieces and suspension time 3)-5).
However, because these methods are mathematical
approaches, there have been difficulties in handlingmathematical expressions to cope with situations andinterpreting the resultant solutions.
Incidentally, in accelerated testing, the difference inlife between lots can be more reliably determined if amore test pieces are tested and a greater difference inlife is attained: however, it has been difficult to set up
the difference in life that allows positive judgment of aparticular number of test pieces and significant

1. Introduction
Rolling contact fatigue life testing is one of the tests
that must be performed to assess bearing
performance. Generally, the rolling contact fatigue lifeof a given bearing is compatible with Weibull
distribution 1, 2), and variation in estimated rolling
contact fatigue life is significantly large. Therefore, it is
an important practice in rolling contact fatigue life
testing of bearings to assess the bearing life in aconservative manner using high reliability in order toavoid problems in the market with the bearing productin question. Rolling contact fatigue life of a bearing
can be roughly categorized into a test to be performed
under conditions that simulate the operating
conditions of a bearing installed onto machinery, andanother test to be performed in relatively severe
conditions. In the former test scheme, we may judge
that the bearing in question has sufficient life if the test
successfully continues to a target timing without
bearing failure on the bearing (hereinafter this type oftest is referred to as “fixed time test”).
With the latter test practice, a test piece develops
failure in a relatively short run, and we can estimate*Elemental Technological R&D Center-32-

Test Design and Result Interpretation Methods for Evaluating Rolling Contact Fatigue Life to Maintain Both Efficiency and Reliability

difference. To address this issue, Johnson 6) has
developed a plotting method that helps the user toapproximate the relationship between test pieces anddifference in L10 lives for determining significant
difference. This technique, however, has been lessoften used because it does not offer a graphicalplotting arrangement that can be used under anarbitrary set of conditions.
Thus, conventional rolling contact fatigue life testinghas several drawbacks as to design of test and test
result interpretation. This report presents a novel
method for objectively and quantitatively designing
rolling contact fatigue life tests and interpreting test
results. The author's method is a technique thatutilizes calculation based on Weibull random numbers.

g(x)=

e
α

( x−αγ )e−1( x−αγ ) …………(1)

exp −

e

e : shape parameter, α: scale parameter,
γ: location parameter
Because Weibull distribution involves threeparameters, three conditions about these parametersneed to be defined, wherein the author's method
utilizes three sets of conditions for defining Weibull
distribution, and they are 1 a relation 11) between L10life and location parameter determined from the past
vast accumulation of test results 7)-10), 2 life values
that are governed by the purpose of calculation (L10lives, L50 lives, etc.) and 3 shape parameter assumed
from past test performances (a group of shapeparameters is hereinafter referred to as "Weibull
slope"). Incidentally, the condition set 1 isrepresented by the following expression 11):2. Items calculable with the present method
and technique for generating Weibull
random numbers used in calculation

γ=0.05・L10 ………………………………………(2)
L10：L10 life

By using Weibull random numbers, the author's
method can solve all the problems in Table 1. Thougha variety of contents are presented in Table 1, only
one common calculation approach exists there. Thatis, "Weibull random numbers are generated and thena rolling contact fatigue life test is simulated, and theobtained result is applied to design for rolling contact
fatigue life test and interpretation for test result".
To be able to generate Weibull random numbers, itis necessary to define three parameters of Weibull
distribution. Probability density function of Weibull
distribution is give below:

Now, let us assume that the L10 life among Weibull
random numbers to be generated is 100 for
convenience of calculation and that the Weibull slope
is 10/9 which is an ordinary value of life distribution of
bearing 1), 2), 12). Then, in conjunction with relation in
expression (2), the Weibull distribution can be defined
as follows:
g(x)=

10/9
720

x−5
exp −(
( x−5
)
720
720 )
1/9

10/9

…(3)

Table 1 Applicable contents of this method
Type of test Classification

Contents (problem)

Item

Design 1

How many test pieces are needed to be able to complete the fixed time test before the deadline(delivery date)?

3. 1

Design 2

If the number of test pieces or test rigs is fixed, how many hours should be selected as thesuspension time prior to failure?

−

Design 3

If a failure prematurely occurs in an apparently short time span, then how many hours the test piece in question has
run at maximum without developing failure so that we are able to judge that the bearing in question does not fulfill thetargeted bearing life (what are the criteria for judging that the test run may be suspended)?

−

Design 4

If a failure occurs before the fixed time has not yet been reached, then how many hours must be
selected to set suspension time with the other test pieces?

−

Fixed time
test

Interpretation 1 How many hours are at least assured a bearing life that is based on the data from 100% non-failed test pieces? 3. 2
Interpretation 2

When several bearing products have failed, how wide is the range of life with bearing products inquestion that can be judged from non-failed test pieces data and failed test pieces data?

−

Design 1

How many test pieces are necessary to realize a bearing life with a particular degree of reliability? Or, if thenumber of test pieces used has been predetermined, then how great difference is needed between lots sothat we are able to judge that there is significant difference between the lots?

4. 1

Accelerated
test
Interpretation 1 Is there true significant difference between two lives?

4. 2Interpretation 2 If there is significant difference between two lives, then how many times long is the longer life compared with the shorter life? 4. 2

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NTN TECHNICAL REVIEW No.76（2008）

Incidentally, the scale parameter was calculatedwith the relation given below. This relation is obtainedby substituting expression (2) for expression (5) which
is a cumulative distribution function of Weibull
distribution, and by solving the resultant expression
about:
0.95・L10
α(L10) =1/e｛ln (10/9)｝
G (x) = 1− exp −(suspending time for a test intended to guarantee that
the L10 lives of these test pieces are at least 500
hours.
Fig. 2 provides an example Weibull probability
distribution for e =10/9 where L10 life is 500 hours.
First, six Weibull random numbers satisfying Fig. 2 are
generated; this stands for a practice for obtaining
bearing life data where six test pieces are sampledfrom bearing products that are characterized by life
distribution of e =10/9 and L10=500.
Next, the time group (classified by how many hours
elapsed) all the so-obtained six random numbers
belonged to was determined, and this step was
repeated 1000 times, thereby the case (how many
hours elapsed) which provides higher probability wasdetermined through an effort to develop the
cumulative probability distribution shown in Fig. 3.
When the cumulative probability is 0.9, the time
elapsed coincides with 1499 h point, and it should beunderstood that certain numbers among the six
Weibull random numbers are not longer than 1499
hours at 90% probability. In other words, when thedata for suspension time longer than 1499 hours for…………………………(4)

x−γ e
α

)

…………………(5)According to the author's method, the threeparameters of Weibull distribution are determined by
the above-mentioned technique, thereby Weibull
random numbers are generated. Generation of
Weibull random numbers can be executed by applying
either rejection method or inverse function method touniform random numbers. Either of these random
number generation techniques can generate Weibull
random numbers: however, it is necessary to use
quality random numbers for uniform random numbers.
Therefore, the author's method has adopted theMersenne twister method 13) as a technique forgenerating uniform random numbers. Incidentally,
since calculation with an infinite number of random
numbers is impossible even with the author's method,
we should remain aware that there can be maximum
probable error of about 5% with the result of theauthor's method.
Of course, all the contents in Table 1 can becalculated. However, the present report only describes
the underlined contents in Table 1 which are the mostbasic contents.

1 Generate i Weibull random numbers (i is an arbitrary number, and e isan actual value from that experiment) satisfying target L10 lives.
2 Determine how long are the minimum lengths of all the random
numbers.
3 Repeat steps 1 and 2 a plural times (may be arbitrarily set: 1000
times are recommended), thereby by referring to cumulative probability
distribution, determine how long the minimum time, among all the
random numbers (hours), can probably last.
4 Read the timing where the cumulative probability in the cumulative
probability distribution in 3 reaches P% (may be arbitrarily set: 90%
is recommended).

3. Fixed time test

5 Decide the test number (number of test pieces) that ensures completionof test within the lead time by repeating calculations 1 through 4
while increasing the quantity of random numbers generated.

3.1 Settings for number of test pieces andsuspension time for a fixed time test

Fig. 1 Flowchart to design test number in fixed time test

Fig. 1 provides the author's design flowchart for
determining the number of test pieces for a fixed time
test. For simplification of explanation below, aconcrete example is now provided. Suppose that thelead time before delivery (deadline) of a particularbearing product type is 3 months (about 2100 hours),
and a test needs to be executed to guarantee that the
L10 life of the product is longer than 500 hours(targeted quality). For calculation, a Weibull slope(hereinafter "e") for determining the shape of life
distribution needs to be assumed: let us assume thevalue of this Weibull slope to be 10/9 which is an
ordinary value for bearing life distribution1, 2, 12).
Supposing that the appropriate test number is six (thatis, six test pieces are tested), let us calculate the0.0012

Probability

0.001
0.00080.00060.00040.0002
0

0

1000

2000

3000

4000

Time (h)Fig. 2 Weibull probability density distribution for
L10=500h and e=10/9-34-

Test Design and Result Interpretation Methods for Evaluating Rolling Contact Fatigue Life to Maintain Both Efficiency and Reliability

shorter than the suspension time for 100% non-failed
test pieces, thereby whether or not the so-obtained six
random numbers are longer than 3000 hours isdetermined. This process is repeated 1000 times,
thereby how many times among 1000 times the six
test pieces withstand more than 3000 operating hours
is determined. This procedure corresponds with theprevious investigation work where the lives of six test
pieces whose Weibull slope was 10/9 and L10 life was
300 hours were tested to determine the probability at
which all the six test pieces remained non-failed for3000 hours. Furthermore, this investigation was
repeated for Weibull distributions whose L10 lives were
gradually extended, thereby Fig. 5 was developed,
wherein the axis of abscissa represents the L10 lives
and the axis of ordinate stands for the probabilities atCumulative probability

1.2
1.0
0.8
0.6
0.4
0.2

1499h
0.0

0

1000

2000

3000

4000

Time (h)Fig. 3 Cumulative probability distribution to estimate
suspending time prior to failure(Required L10 life=500h, e=10/9, Test number=6)

suspending the test prior to failure has been obtained,
the L10 life of that bearing product is 500 hours or
longer at 90% probability, and 1499 hours marks thesuspension time for 100% non-failed test pieces under
this situation. Because the lead time before delivery is
as long as 2100 hours, the suspending time at the1499 hours point does not pose a problem in terms ofdelivery of bearing product in question, and the test
number can be six pieces. If the suspending time forsuspending test prior to failure comes after thecompletion of lead time, it is necessary to find acondition according to which the quantity of random
numbers generated (number of test pieces) is
increased so that the suspending time for suspending
test prior to failure arrives earlier than the completionof lead time (delivery date). The target quality for thepresent calculation has been set in terms of the L10
lives: however, the author's method is capable ofsetting all the items in Table 1 relative to an arbitrary
failure probability.

1 Generate i Weibull random numbers (i is an arbitrary number, and e
is an actual value from that experiment) having target L10 lives (L10lives are arbitrarily set: 1/10 the suspending time for 100% non-failed
test pieces is recommended) shorter than the suspension time for100% non-failed test pieces.

2 Determine the minimum length of all the random numbers.
3 Repeat steps 1 and 2 a set number of times (may be arbitrarily set:
1000 times are recommended), and determine the probability where
all the random numbers are longer than the suspension time for 100%
non-failed test pieces.

4 Gradually increase L10 lives preset in step 1 (degree of gradualincrease in L10 lives may be arbitrarily set: 1/10 the suspension timefor 100% non-failed test pieces is recommended). Plot the relationshipbetween these L10 lives and the probability at which all the random
numbers are longer than the suspension time for 100% non-failed test
pieces.

5 Terminate the calculation when the probability P% (may be arbitrarily
set: 10% is recommended) at which all the random numbers in step

4 are longer than the suspending time for 100% non-failed test
pieces, and then read the values corresponding with P%.

Fig. 4 Flowchart to estimate L10 life from suspendingtime prior to failure

3.2 Estimation of life judging from result of test
suspended prior to failure
Fig. 4 provides a flowchart for estimating L10 life
from a result of test suspended prior to failure. For
simplicity of explanation, a concrete example ispresented below. Let us think of a situation where atest is being performed to guarantee L10 life of 500
hours given in Sec. 3.1. In this test, all the six test
pieces did not develop failure during a time span of1499 hours, and then the test was further continued.
As a result, all the test pieces did not failed during atime span of 3000 hours. The L10 life estimated fromthis situation is hereunder calculated.
First, six random numbers are generated from the
Weibull distribution whose L10 lives (1/10 thesuspension time for 100% non-failed test pieces isrecommended) are shorter than 3000 hours which is

Cumulative probability

0.12
0.10
0.08
0.06
0.04
0.02

975h
0.00
0

200

400

600

800

1000

1200

L10 Life (h)Fig. 5 Cumulative probability distribution to estimate L10life from the suspending time prior to failure( e=10/9, Test number=6)-35-

NTN TECHNICAL REVIEW No.76（2008）

method 14) for determining the regression linear line
per Weibull plotting. Next, three random numbers
compatible with Weibull distributions having a same
L10 life were generated, and from the data of theserandom numbers, the L10 lives were estimated basedon Weibull plotting, and from L10 lives of both
members, the L10 life ration was determined. This
process was repeated 1000 times, thereby the
cumulative probability distribution for 1000 sets of L10life ratio in Fig. 7 was generated. From this chart, it
should be understood that even though test pieces
were sampled from a same lot, the life ratios were notalways 1 and dispersed in a range of 0.042 to 21.8
times (values in a range of 5% and 95% of cumulative
probability distribution).
This means that with a quantity of test pieces as
small as three (even though belonging to same lot),
difference in life in a range of 0.042 to 21.8 times canoccur at 90% probability. In other words, if only threetest pieces are available, it is possible to judge that
there is significant difference between the lives of two

which all the test pieces remain non-failed for 3000
hours (to improve precision of calculation result, theprobabilities at which all the test pieces remain nonfailed for 3000 hours are indicated in Fig. 5 for L10
lives for every 50 hours rather than every 300 hours).
The life distributions whose L10 life is 975 hours
provide the data about 100% non-failed test pieces(not failed for 3000 hours or longer) only at 10%
probability. Accordingly, if data is available whichshows all the test pieces which remain non-failed for3000 hours, the test pieces in question highly probably
have L10 lives of 975 hours or longer.

4. Accelerated test
4.1 Settings for number of test pieces foraccelerated test
Fig. 6 provides a flowchart for designing the test
number for accelerated test. For simplicity of
explanation, a concrete example is presented below.
In most accelerated tests, several lots of test pieces
are subjected to life test, and the result of a given lot iscompared with that of another lot. Suppose that twolots, each lot consisting of three test pieces, arecompared with each other, and let us think of how
much difference in life is necessary to constitutesignificant difference between the lives of the two lots.
Now let us assume that e of life distribution is 10/9
(refer to the probability density distribution in Fig. 2).
Three random numbers were generated from
Weibull distribution having a given L10 life, and fromdata of these three random numbers, the L10 life wasestimated by Weibull plotting. The methods used toestimate the L10 life were the Jonson's method 5) for
calculating the median rank and a correlation function

Cumulative probability

1.2(x, y)=(21.8, 0.95)

1.0
0.8
0.6
0.4
0.2

(x, y)=(0.042, 0.05)
0.0

0.042

21.8
10

0

20

30

40

50

L10 life ratio

Fig. 7 Cumulative probability distribution of L10 life ratio( e=10/9, Test number=3)

1 Generate i random numbers compatible with Weibull distribution (i=3
for first calculation run: L10 lives are arbitrarily set: e is the actual value
from the experiment), and estimate L10 lives by Weibull plotting.

25

Required significant difference of
L10 and L50 life ratios

2 Reperform step 1, and determine the L10 life ratio from the two L10
lives.

3 Repeat steps 1 and 2 plural times (may be arbitrarily selected:
1000 times are recommended) to generate cumulative probability
distribution of L10 lives, and examine dispersion of L10 life ratios(may be arbitrarily selected: a range of 5% to 95% is recommended).

4 Perform steps 1 through 3 while varying i to develop the relation
between test number and dispersion of L10 life ratios.

L10
20

15

10

L50
5

0

5 From the chart, read the test number with which the dispersion of L10
lives is below j times (may be arbitrarily selected: two times arerecommended) (L10 lives can be similarly calculated).

0

10

20

30

40

50

Test number

Fig. 8 Relationship between test number and required
significant difference of L10 and L50 life ratios ( e=10/9)

Fig. 6 Flowchart to determine test numbers on
accelerated test

-36-

Test Design and Result Interpretation Methods for Evaluating Rolling Contact Fatigue Life to Maintain Both Efficiency and Reliability

lots only when the difference in life is greater than
21.8 times or smaller than 0.042 times.
Next, the quantity of random numbers (test number)
was increased, the dispersion of life ratios wascalculated in a manner as described above, and therelation between test numbers and life ratios wasplotted as shown in Fig. 8. When judging thesignificant difference in lives between lots, based on
the difference in lives of more than two times, thequantity of necessary test pieces in a test where e is
10/9 is 45 in the case of evaluation in terms of L10 lives
and 15 in the case of evaluation in terms of L50 lives.

1 Generate i random numbers compatible with Weibull distribution (i ispredetermined number of test pieces: L10 lives are arbitrarily set: e isthe actual value from the experiment), and estimate L10 lives by
Weibull plotting.

2 Reperform step 1, and determine the L10 life ratio from the two L10
lives.

3 Repeat steps 1 and 2 plural times (may be arbitrarily selected: 1000
times are recommended) to generate cumulative probability
distribution of L10 lives, and examine dispersion of L10 life ratios(may be arbitrarily selected: a range of 5% to 95% is recommended).

4 Perform steps 1 through 3 for combinations of Weibull distributions
having difference in L10 lives (difference may be arbitrarily set, andshould be typically 1.1 times onwards in increment of 0.1 time) todevelop the relationship between test number and dispersion of L10life ratios.

4.2 Estimation of difference in lives from resultof accelerated test
Verification of significant difference for the result ofan accelerated test can be achieved from the result
presented in Fig. 8 in Sec. 4.1. For example, when ten
test pieces are available, and if a difference of fivetimes is present in L10 lives, it can be judged that there
is significant difference. However, it is not possible
with this chart to determine how much difference inlives is guaranteed based on the magnitude insignificant difference. In this section, the calculation
method for difference in lives when there is significant
difference is described.
Fig. 9 presents a flowchart for estimating differencein lives from result of accelerated test. For
simplification of explanation below, a concrete
example is now provided. Suppose that life
comparison has been attempted with ten test pieces
and difference in lives of ten times was obtained interms of L10 lives. As is apparent from Fig. 8, it can be
judged that there is significant difference between
lives of these two lots. Difference in lives under thissituation is hereunder calculated.
The earlier part of the procedure is identical to thatup to generation of Fig. 7 in Sec. 4.1 except in that ten
random numbers are generated. Fig. 10 provides
cumulative probability distribution of L10 life ratio withten test pieces. From Fig. 10, it should be understoodthat the magnitude of dispersion in L10 life ratio issmaller as compared with that in Fig. 7 because theplotting in Fig. 10 involves a greater number of test
pieces. Next, in conjunction with Weibull distribution
having a different L10 life, plotting similar to one in Fig.
10 is generated. More specifically, distributions of L10life ratios are plotted wherein the differences in L10 life
in combinations of distributions are1.1 times, 1.2 times… 3 times.
As an example, Fig. 11 shows cumulative
probability distribution of L10 life ratio calculated fromcombinations of distributions that involve difference of
2.1 times in L10 lives. From this chart, it should be

5 Read the difference in L10 life from a point where dispersion ofdifference in L10 lives in the chart generated in step 4 coincides withthe difference in life obtained from the experiment.

Fig. 9 Flowchart to estimate the minimum life differencewith respect to accelerated test results of 2 lots

1.2
Cumulative probability(x, y)=(4.90, 0.95)
1.0
0.8
0.6
0.4
0.2

(x, y)=(0.181, 0.05)

0.0
0.181 4.90
0
10

20
30
L10 life ratio

40

50

Fig. 10 Cumulative probability distribution of L10 life ratio( e=10/9, Test number=10)

1.2

Cumulative probability(x, y)=(10, 0.95)
1.0
0.8
0.6
0.4
0.2

(x, y)=(0.41, 0.05)

0.0

0.41

10

0

10

20

30

40

50

L10 life ratio

Fig. 11 Cumulative probability distribution of L10 life ratio( e=10/9, Test number=10, Difference of L10 life=2.1)-37-

NTN TECHNICAL REVIEW No.76（2008）

under high contact pressure (point contact, Pmax=5.88
GPa). e in this test was 2.88 and is greater than 10/9
or 9/8, each often used in ordinary rolling contact
fatigue tests 1), 2) 12). Thus, e can greatly varydepending on test conditions: therefore, an assumed eneeds to be correctly determined for each set of test
conditions in question. Fig. 14 presents relationshipbetween test numbers and dispersion of e (possible
values in a range between the lower limit (5%) and theupper limit (95%) in the cumulative density
distribution). This result has been obtained bygenerating Weibull random numbers from life
distribution where e is 10/9 and repeating this process1000 times. It should be understood that thedispersion in e approaches 10/9 in either the upper
limit region or lower limit region as the test number(number of test pieces) increases. Also, it should beapparent that the variation in dispersion is significantly
great in a segment of plotting up to test number ten.
Fig. 15 provides a result derived from the lower limit
and upper limit in Fig. 14, wherein the lower andupper limit values have been differentiated with the

3.5

L50

L10

3.0
2.5

2.1

Cumulative probability of failure (%)

Minimum differences of L10 and L50 lives

understood that a difference in life of ten times at
maximum (at a point coinciding with 95% cumulative
probability) is present with which the difference in L10
lives is 2.1 times as large. Finally, dispersion in L10 life
ratios relative to these differences in L10 lives has
been determined, thereby a diagram such as thatshown in Fig. 12 has been developed. The plotted
diagram in Fig. 12 means that if the data for ten test
pieces are sampled from the lots between which there
is difference in lives along the axis of ordinate, thenthe probability where the difference in lives falls in arange of values along the axis of abscissa is 90%.
Consequently, from Fig. 12, it should be understoodthat difference in L10 lives of ten times can occur only
between lots between which difference in L10 lives is
2.1 times or greater and rarely occurs in other cases.
Accordingly, the difference in L10 lives conservatively
estimated under this situation is 2.1 times.

2.0
1.5
1.0
0.5
0.0
0

5

10

15

20

L10 and L50 life ratio

Fig. 12 Diagram to estimate the minimum differences of L10
and L50 lives with respect to accelerated test results of 2 lots
( e=10/9, Test number=10)

99
80
50

e=2.88

20
10
5

1
106

107

108

109

Life (Loading Cycle)

Fig. 13 Example of RCF life test result(Point contact, Pmax=5.88GPa)

Dispersion limits of Weibull slope

5. Discussion (method for determining
assumed e from result of experiment)
According to the author's method, the result greatly
varies depending on the magnitude of e: therefore, it
is very important whether or not the assumed e iscorrect. The considerations for determining anappropriate value for e from an experiment aredescribed below.
Generally, e is regarded as a parameter that isdependent on change in the state of contact stress on
the rolling surface as well as variation in the fracturemode 12), and is considered to vary depending on thetest conditions. To provide one example, Fig. 13
provides a result of rolling contact fatigue life test

6

lower limit
upper limit

5
4
3
2
1
0
0

10

20

30

40

50

Test number

Fig. 14 Relationship between test number anddispersion of Weibull slope(The result is conducted as e=10/9 of life distribution)-38-

Normalized differential value ofdispersion limits of Weibull slope

Normalized differential value ofdispersion limits of Weibull slope

Test Design and Result Interpretation Methods for Evaluating Rolling Contact Fatigue Life to Maintain Both Efficiency and Reliability

1.2

lower limit
upper limit

1.0
0.8
0.6
0.4
0.2
0
0

20

40

60

80

100

Test number(a) e=10/9

1.2lower limit
upper limit

1.0
0.8
0.6
0.4
0.2
0
0

20

40

60

80

100

Test number(b) e=3

Fig. 15 Normalized differential value of dispersion limits of Weibull slope depending on test number (e=10/9 and 3)

test number and the resultant values have beennormalized with the maximum differential. Regardlessof e, the differential drops to 0.2 when the test number
is ten and approximately 0.1 when the test number is
20. To sum up, in order to be able to estimate an
appropriate e from an experiment, the minimum
recommended number of test pieces is ten, and thepreferable number of test pieces is 20.

References
1) G. Lundberg and A. Palmgren: IVA Handlinger, (1947)
1962) G. Lundberg and A. Palmgren: IVA Handlinger, (1952)
2103) R.B. Abernethy: The New Weilbull Handbook 5thedition, (2006) chapter 64) Reliability Engineering Association of Japan: Reliability
Handbook, Nihon Giren Publishing, (1997) 209
5) S.N. Luko: SAE Technical Paper Series, No.012859,(1999)
6) L.G. Johnson: The Statistical Treatment of Fatigue
Experiments, Elsevier, (1964) 16
7) T.E. Tallian: ASLE Trans., 5 (1962) 183
8) B. Snare: The Ball Bearing Journal (SKF), (1970) 162
9) M. Kuroda: Lubrication, 17, 9 (1972) 569
10) J. Okamato, T. Yoshioka, T. Fujiwara, K. Fujita, T.
Kitahara, S. Koizumi: Lubrication, 22, 5 (1976) 307
11) ISO/TS16281 (2006)12) S. Shimizu: Introduction to Reliability Design for
Machinery, Suurikougakusha, (2006) 36
13) M. Matsumoto and T. Nishimura: ACM Trans. on
Modeling and Computer Simulation, 6 (1996) 99
14) T. Hoshide, T. Sakai, A. Sakata: Science of Machine,
41-2 (1989) 320

6. Conclusion
This report has described a design method forrealizing both reliability and speediness of rolling
contact fatigue life test and a method for interpreting a
result from rolling contact fatigue life test. The author's
method is capable of intuitive understanding,compared with techniques using mathematicalmodels, and is a flexible technique capable of mostsituations experienced in rolling contact fatigue life
test. By following the author's method, a person, evennot experienced in rolling contact fatigue life test, will
be able to attempt to design a test and reliablyinterpret the test result while realizing reliability andspeediness in executing a series of tests.

Photo of author

Takumi FUJITA
Elemental Technological
R&D Center-39-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Research on the Lubrication Mechanism of Greasefor High Speed Bearings

Takayuki KAWAMURA*
Machine tools bearings are operated at high speeds and temperatures. They
are widely used with oil lubrication, but also with grease lubrication.
In the case of grease lubrication, it is necessary to lubricate for tens of
thousands of hours without supplying new grease. However, excellent antioxidation grease does not necessarily lubricate for a longer time.
This report investigates used grease from operation at high speed andtemperature. It was found that the ability to supply oil to the rolling contact area
was more important than oxidation stability.
Moreover, it was also found that grease with easy oil separation and stable
thickener structure had longer life.

lubrication, we had to (a) define the mechanism that
controls grease life under an ordinary-temperature
high-speed environment and (b) develop a long-life
bearing based on the resultant finding.
This technical paper clarifies the characteristics
needed for long-life grease in machine tool bearings,
and presents the information about "SE-1" greasethat is used on NTN sealed angular contact ballbearings that can operated with dmn=1.7 million.

1. Foreword
To help shorten processing time and improvemachining precision, machine tools bearings require
higher rigidity, higher running speed and limited heatrise. Also, to mitigate environmental impacts, reduced
lubricant consumption and more common adoption ofgrease lubrication have been increasingly needed. In
particular, the most wanted technology in the bearing
market is adoption of a grease lubrication system thatis eco-friendly and is easy to handle and maintain. A
grease lubricated bearing system is required towithstand operation of 20,000 hours without
relubrication; therefore, grease boasting long lasting
durability is much needed.
Generally, durability of a given grease is considered
to be greatly affected by its heat resistance feature(resistance against oxidation-deterioration 1)-6).
Accordingly, NTN's grease-lubricated bearings havebeen subjected to continuous test at a hightemperature in excess of 150˚C to check their
durability 7)-9). However, on machine tool bearings thatare used under ordinary-temperature high-speed
environment, it has been known from experience thatgreases with high-temperature durability do notalways achieve longer life. Therefore, greases for
actual machine tool bearings have been evaluatedand selected through durability tests lasting for tens of
thousands of hours 10)-13).
Consequently, in order for machine tools bearings
to efficiently achieve longer life under grease

2. Grease life under ordinary-temperature
high-speed environment
Three different grease types, Greases A, B and SE1, each having unique composition, have been
subjected to ordinary-temperature high-speed
durability test (test!) and high-temperature durability
test (Test@). Table 1 summarizes grease
compositions, Table 2 provides test conditions andFigs. 1 and 2 illustrate the test results obtained. The
grease lives are indicated relative to the life of Grease
A that is taken as 1. In the high-temperature durability
test (Test@), Grease B boasts a longest life, while in
the ordinary-temperature high-speed test (Test!),SE-1 exhibits a longest life.
From these results, it should be understood that all
the greases excelling in high-temperature durability
are not always excellent in durability under an
ordinary-temperature high-speed environment.
To determine whether this variation is attributed totemperature or running speed, ordinary-temperature*Elemental Technological R&D Center-40-Research on the Lubrication Mechanism of Grease for High Speed Bearings

durability test (Test#) was performed. Test# wasexecuted using a test rig identical to that used in
Test@. However, the temperature conditions werevaried (amount of prefilled grease was also varied).
The obtained result is graphically plotted in Fig. 3. In
Test@, the grease life was longer in the order ofGrease A<SE-1<Grease B. In Test#, the grease life
was longer in the order of Grease A<Grease B<SE-1,
and this trend was the same as that with Test!,
which also performed at room temperature. In otherwords, it has become apparent that grease that boasts
a longest life can vary depending on the bearing
operating temperature.

3. Observation of lubrication state on
rolling contact surface
Using an infrared microscope spectroscope, theauthor inspected the state of residual grease on theouter ring rolling contact surface and circumferential
areas on the bearing specimens lubricated with
Grease A, Grease B and SE-1, each specimen having
undergone Test III. The resultant findings are shownin Figs. 4 through 6 and summarized in Table 3.
In the case of Grease A whose life was shortest,
although a sufficient amount of oil was present in thevicinity of rolling contact surface (Fig. 4-2) and the
periphery of rolling contact surface (Fig. 4-3), virtually
no grease was present on the rolling contact surface
itself (Fig. 4-1). This means that virtually no thickener
is present on the rolling surface and supply of oil fromgrease is insufficient in the vicinity of the rolling
contact surface to the rolling contact surface.

Table 1 Tested greases
GreaseGrease A

Grease B

SE-1

Thickener

Metal soap

Urea

Urea

Base oil

Ester

Synthetic oil

Synthetic oil

Kinematic viscosity mm2/s（40˚C）

23

40

22

Consistency

280

230

280

Table 3 Grease distribution in tested bearings
GreaseTable 2 Test conditions
Test!
Test@
Test#
Ordinary-temperature High-speed Ordinary-temperature
high-speed durability durability
durability

Test

7020

6204

6204

Temperature

Room temperature

150˚C

Room temperature

Running speed
min-1

7500

10000

10000

1 million

1 million

0.34 million

1.62

1.54

1.54

Bearingdmn
Contact surfacepressure GPa

Test !

×◎◎

Grease B

○

×

◎

SE-1

○

○

◎

◎: Sufficiently present, ○: present, ×: depleted

2Vicinity of rolling contact surface
3Periphery
1Rolling contact surface

Test @

Test #
60

20

Grease life ratio

Grease life ratio

40

40

20

A

B

SE-1

Fig. 1
Grease life at high speed
and room temperature

40

20

0

0

0

3
Periphery

Grease A

60

60

Grease life ratio

1
2
Rolling contact Vicinity of rollingsurfacecontact surface

A

B

SE-1

Fig. 2
Grease life at high temperature

-41-

A

B

SE-1

Fig. 3
Grease life at room temperature

NTN TECHNICAL REVIEW No.76（2008）

It has been found that, in the case of Grease B
whose life was second shortest next to Grease B,
though the grease in the vicinity of rolling contact
surface has been depleted (Fig. 5-2), the rolling
contact surface (Fig. 5-1) still holds grease and a
sufficient amount of oil was present in the grease in
the periphery (Fig. 5-3). More specifically, it isapparent that though the oil in the grease the vicinityof rolling contact surface is supplied to the rolling
contact surface, the amount of oil transferred from thegrease in the periphery to the vicinity of rolling contact
surface is insufficient.
In the case of SE-1, which boasts the longest life, it
has been verified that a sufficient amount of oil ispresent in the grease in the periphery (Fig. 6-3) andat the same time grease is present in the vicinity ofrolling contact surface (Fig. 6-2) and on the rolling
contact surface itself (Fig. 6-1).
To sum up, grease is positively present on therolling contact surface and the oil in grease in the
periphery is supplied to the rolling contact surface.
As can be understood from Table 3, the mode of oil
supply to the rolling contact surface varies fromgrease to grease. The capability for supplying oil tothe rolling contact surface is the highest with SE-1,
followed by Grease B and Grease A. In an ordinarytemperature high-speed environment, oil supply to therolling contact surface is the most importantconsideration, and it is believed that even grease
excelling in heat resistance, such as Grease B, will be
deteriorated in a short run if its ability to supply oil tothe rolling contact surface is inferior.

1 Grease on rolling
contact surface

Deriving frombase oil

2 Grease in the vicinity×○

Derivingfrombase oil

Deriving fromthickener

3 Grease in the periphery

Deposited grease
Transfer of grease oil

Derivingfrombase oil○: Readily transferrable
×: Not readily transferrable

Deriving fromthickener

Fig. 4 Distribution of tested grease A in a bearing

1 Grease on rolling
contact surface

Deriving frombase oil

Deriving fromthickener

2 Grease in the vicinity○

Derivingfrombase oil×

Deriving fromthickener

3 Grease in the periphery

Deposited grease
Transfer of grease oil

Derivingfrombase oil○: Readily transferrable

Deriving fromthickener

×: Not readily transferrable

Fig. 5 Distribution of tested grease B in a bearing-42-

Research on the Lubrication Mechanism of Grease for High Speed Bearings

1 Grease on rolling
contact surface

Deriving fromthickener
Deriving frombase oil
2 Grease in the vicinity○△

Derivingfrombase oil

Deriving fromthickener

3 Grease in the periphery

Deposited grease
Derivingfrombase oilTransfer of grease oil○: Readily transferrable

Deriving fromthickener△: Somewhat transferrable

Fig. 6 Distribution of tested SE-1 in a bearing

also tested in a situation where thin film of base oilalone was formed between the paraffin paper and thesloped plane surface (provision of base oil film).

4. Experiment for oil transferability
with simulation model
To be able to verify that SE-1 excels Greases A and
B in terms of oil supply performance, a basic
experiment was performed with a model that was
intended to simulate transfer of the oil from the grease
deposited on the shoulder of bearing outer ring bore to
the rolling contact surface (Fig. 7). The reason for why
the author has attempted to develop a model ofgrease deposited on the bore surface of bearing outerring is because the author believes that, on a bearing
running at a higher speed, a lump of grease
accumulates on the bore surface of bearing outer ring
due to centrifugal force.
In this experiment, transferability of oil from thegrease to a strip of paraffin paper has been evaluated,
wherein the amount of oil from the grease on a level
plane to the strip of paraffin paper situated in a sloped
plane was measured as the amount of oil transferred.
Under a standard set of conditions, a thin layer ofgrease was intentionally formed between the strip ofparaffin paper and the sloped plane (provision ofgrease film). For Grease A only, the specimen was

Test pieces have been provided with thin grease
film. The results of test with these test pieces aregraphically plotted in Fig. 8. The degree of positive oiltransferability to the sloped plane is in the order ofGrease B<SE-1<Grease A. Grease A boasts best oiltransferability while Grease B lacks in goodtransferability. From these findings, it is consideredthat with Grease B, the amount of oil supplied from thevicinity of rolling contact surface to the rolling contact
surface itself is limited, and Grease B exhibitedshorter life in Tests I and II because of poor lubrication
ability.

Grease A
Amount of oil transferred wt%

Cross-sectionalview of outer ring

4.1 Oil transferability modeling the situation where
grease is present on rolling contact surface

GreaseGreaseProvision ofgrease film Sloped
Level plane
plane
θ
θ

Modeling

SE-1

5
4
3
2
1
0
0

Strip ofparaffinpaper

Grease B

250

500

750

1000

Standing time hFig. 7 Oil flow model on a raceway of bearing outer ring

Fig. 8 Oil flow with grease film treatment on a slope plane-43-

NTN TECHNICAL REVIEW No.76（2008）

associated with grease on a bearing running at higher
speed causes the thickener to be destroyed oreliminated from the rolling contact surface of the
bearing. As a result, the oil from grease in the
periphery of rolling contact surface is not supplied to
the rolling contact surface any more, and the grease
has exhibited shorter life (see Fig. 10).

4.2 Oil transferability modeling the situation where
grease is not present on rolling contact surface
When grease was present on the sloped surface,
the oil transferability was better in the order of SE1<Grease A. Since no grease (thickener) was
observed on the rolling contact surface lubricated with
Grease A (short life) after the grease durability test, it
was expected that presence/absence of a thickener ingrease can affect transferability of oil in the grease inquestion. Therefore, the author has assessed the oiltransferability of Grease A in a state where thethickener was not present on the slope plane. It has
been learned that in a model experiment (treatment toform film of base oil) (Fig. 9), where absence ofgrease on the rolling contact surface was assumed,
the oil supply capability of Grease A was significantly
jeopardized and the amount of supplied oil wasinsufficient as compared to the oil supply with Grease
B and SE-1 described in Sec. 4.1.
A thickener in grease forms a fibrous structure and oil
is held among the fibers. Accordingly, the gaps in the
fibrous structure in thickener functions as capillaries
that allow base oil of grease to be transferred.
In the case of Grease A, it appears that shearing

5. Conclusion
From the findings in the series of experiments, it
has been learned that to be able to achieve longer
grease life under an ordinary-temperature high-speed
environment, the oil in grease in the periphery ofrolling contact surface needs to be quickly supplied to
the rolling contact surface by the capillary
phenomenon unique to a thickener. The importantconsiderations are 1 use of thickener with highresistance to shearing and 2 use of grease whose
oil component is readily separated (see Fig. 11).
Being an excellent bearing grease that has resilient

Grease A, B
Grease in the vicinity ofrolling contact surface isconsumed.

Insufficiency in oil
supply to the rolling
contact surface

Shorter life

Grease A (Treatment for forming grease film)
Problems
1Elimination of thickener from the rolling contact surface
2Insufficient oil separation with grease in the periphery of
the rolling contact surface

Amount of oil transferred wt%

Grease A (Treatment for forming base oil film)
5
4

Measure for longer
life with grease

SE-1

3

Grease in the vicinity ofrolling contact surface isconsumed.

2

Quick oil supply tothe rolling contact
surface

Longer life

1
Improvements
1Use more resilient thickener.
2Design grease that is capable of prompt oil separation.

0
0

500

250

750

1000

Standing time hFig. 11 Improvement of grease endurance ability
for high speed bearing at room temperature

Fig. 9 Oil flow with oil film treatment on a slope plane

Thickener fibers are not linked together.
Good base oil transferability

Poor base oil transferability

Thickener
Thickener○
Base oil○Base oil
Shearing

Transfer of oil
Rolling contact
surface

Transfer of oil

Vicinity of rolling
contact surface

Rolling contact
surface

Vicinity of rolling
contact surface

Fig. 10 Effect of grease thickener to oil flow toward a rolling contact area

-44-Research on the Lubrication Mechanism of Grease for High Speed Bearings

0

10000h20000hBearing bore dia.: φ100 mm MP-1
5S-2LA-BNS020LLB
Ceramic balls
SE-1
Contact angle: 20˚11000 min-1 continuous (dmn: 1.40 million)
Constant pressure preloading: 3.2 kN
Spindle attitude: horizontal (belt-driven)
No jacket cooling

Bearing bore dia.: φ50 mm
2LA-BNS010LLB
Steel balls
Contact angle: 20˚

MP-1

17000 min-1 continuous (dmn: 1.1 million)
Constant pressure preloading: 1.4 kN
Spindle attitude: horizontal(driven by directly coupled motor)
No jacket cooling

Bearing bore dia.: φ30 mm
5S-7006ADLLB
Ceramic balls
Contact angle: 25˚

MP-1

SE-1

27000 min-1 continuous (dmn: 1.15 million)
Constant pressure preloading: 0.53 kN
Spindle attitude: horizontal(driven by built-in motor)
No jacket cooling

SE-1

Fig. 12 Endurance ability of SE-1 grease with machine tools bearings

Bearings," NTN Technical Review No.72 (2004) 20-25
8) M. Egami, M. Asao and T. Goto: "Hybrid Grease
NA204F for Automotive Electrical Instruments and
Auxiliary Device,"NTN Technical Review No.73 (2005)
66-71
9) T. Kawamura and H. Hikami: Development of Long
Life Grease NA103A for Automotive Components,
NTN Technical Review No.75 (2007) 116-123
10) H. Takiuchi: "Environmental Response Technologies
for Roller Bearings in the Machine Tools Field," The
Tribology, No.174, 2 (2002) P24-25
11) K. Ueda:"Technology Trends in Bearings for Machine
Tools: Environmental Response Technologies," The
Tribology, No.188, 4 (2003) P19-21
12) F. Kosugi: "ULTAGE" Series Precision Bearings for
Machine Tools, NTN Technical Review No.74 (2003)
26-29
13) F. Kosugi: dmn 170×104 Sealed High-Speed Angular
Contact Bearing "New BNS Type," NTN Technical
Review No.74 (2006) 28-33

thickener and boasts good oil supply capability, SE-1
Grease appears to exhibit excellent grease durability
when used on machine tools bearings operated in an
ordinary-temperature high-speed environment (seeFig. 12 13)). Much higher speed will be needed formachine tools bearings, and we will need to develop
grease that boasts better oil supply capability. In thiscontext, NTN will remain committed to development ofgrease that boasts much longer life.

References
1) E.R. Booser (1974), "Grease Life Forecast for Ball
Bearings," Lubrication Engineering, 30, P536-541
2) E.R. Booser and A.E. Baker (1976), "Evaporation-A
Factor in Ball Bearing Grease Life," NLGI Spokesman,
40, P60-65
3) W.W. Bailey and S. Pratt (1982), "Dynamic Oxidation
Stability of Lubricating Greases," NLGI Spokesman,46, P15-18
4) T. Kawamura, M. Minami and M. Hirata (2001),"Grease Life Predication for Sealed Ball Bearings,"
Tribology Transactions, 44, 2, P256-262
5) T. Kawamura, M. Minami and M. Hirata: "Grease Life
Predication for Sealed Ball Bearings," NTN Technical
Review No.69 (2001) 76-81
6) H. Mikami: "Latest Trends in Lifespan Prediction for
Lubrication Grease and Grease," Hydraulics &Pneumatics (Japan) 576, Vol.46, No.11, (2007) P4246
7) H. Mikami: "Development of Long Life Grease forHigh-Speed Application "ME-1" Grease for Motor

Photo of author

Takayuki KAWAMURA
Elemental Technological
R&D Center-45-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Friction Properties of Polymer Gel
with High Mechanical Strength

Eiichirou SHIMAZU**
Masaki EGAMI**
Takayuki KUROKAWA**
Jian Ping Gong**
A polymer gel is composed of a three-dimensional network polymer and alarge amount of liquid. There is the potential to apply it as a low friction
material because it has large amount of liquid internally, compared with theoil-impregnated resin currently used as a sliding material. In this paper, we
report on the friction properties of high strength polymer gel, called double
network gel, when used in conjunction with a steel under various conditions.

feature significantly low friction coefficients and uniquefriction behavior. Their unique friction mechanism 4)
has been increasingly elucidated. However, many
examples thus far reported on friction properties ofpolymer gels have been associated with combinationsof water-solvent polymer gels and smooth-surfaced
glass plates.
Now, let us consider utilizing the excellent lowfriction features of polymer gels in order to reduce
friction loss that occurs between machinecomponents. In this case, the solvent for gel isneeded, for example, not to corrode nearby metalparts, solidify, evaporate, or boil in a wider
temperature range.
In this technical paper, we hereafter report thefriction properties of water solvent-free double networkgel (refer to Fig. 1; hereinafter referred to as DN gel) 1)
used in conjunction with a steel (bearing steel: SUJ2)
under various conditions.

1. Introduction
Polymer gels each comprise a polymer skeleton ofchemically bonded three-dimensional network
structure and a significant liquid component (solvent).
Compared with other polymer materials such as
plastic and rubber, conventional polymer gels haveoften been brittle materials featuring lower mechanicalstrength and lower elastic modulus. This may bereadily understood when considering examples offamiliar polymer gels such as jelly and tofu soy bean
curd.
The swelling degree of a polymer gel will vary,
owing to external stimuli such as temperature,pressure, hydrogen ion index (pH value), light, andelectric field, whereby the volume of polymer gel isaltered. Focusing on this feature, applications ofpolymer gels as functional and intelligent materials
have been researched and commercially adopted forsensors and actuators.
Recent developments regarding polymer gels
include high-strength gel types known as double
network gels 1) and nano-composite gels 2) as well as
high-tenacity gel called topological gel 3) , whereby
brittleness-induced quality issues with polymer gels
resulting from poor handling have been much
alleviated. Consequently, the scope of utilization ofpolymer gels in industrial applications seems to
expand.
Incidentally, though being solid matter, polymer gelsconsist primarily of liquid: as such it is known that they

2. Overview of the DN gel
A polymer constituting a skeletal component in apolymer gel has a three-dimensional network structure
when situated in a solvent. A polymer gel in which thebonds of polymer chains forming the network (that is,cross-links) are covalent is known as chemical gel; if
the bonds are not covalent bonds, the polymer gel iscalled physical gel.
Because cross-links in a chemical gel are formed bycovalent bonds, the network structure once formed is**Elemental Technological R&D Center**Hokkaido University, Faculty of Sciense-46-

Friction Properties of Polymer Gel with High Mechanical Strengtha) Ordinary polymer gel

chemical gel, such fracture of polymer chainscontinuously occurs, and, consequently, chemical gel
is a very brittle, low-strength material.
It is estimated that the DN gel has a uniquestructure such as a one shown in Fig. 3, known as thesea-island structure. Incidentally, Fig. 3 provides aschematic diagram, and actual DN gel possesses
nonuniformity in density of cross-links.
The sea phase, which is a matrix, consists of amixed phase comprising a fine network (high crosslinking density) and a coarse network (low crosslinking density), while the island phase virtuallycomprises soft gel alone. The most outstanding
feature of DN gel is a ratio of hard gel to soft gel: more
specifically, the composition of DN gel is designed
such that the volume of soft gel is about 20 times as
great as that of hard gel.
Having such a unique structure, the DN gel has
compressive strength in a range of 10 to 50 MPa. This
strength is 10 to 100 times as great as that of each gelconstituting the DN gel 1). Furthermore, as shown in
Fig. 1, even when compressed by about 90%, the DN
gel does not fail: when the load is removed, it restoresits original shape like a rubber material.
The high strength mechanism of the DN gel cannot
be explained by using a conventional fracture modelthat can reflect elasticity alone: this mechanism seemsb) DN gelFig. 1 Compression property of DN gel 1)
J. P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada,"Double Network Hydrogels with Extremely High
Mechanical Strength" , Advanced Materials, (2003),
15(14), 1155-1158.
Copyright Wiley-VCH Verlag GmbH & Co. KGaA.
Reproduced with permission.

very stable, being insoluble and infusible. In contrast,a physical gel is characterized in that its cross-links
are liberated by external factors including
temperature, thereby reversibly changing from liquid
to solid or vice versa. Our DN gel is a type of polymer
gel that belongs to a chemical gel category.
For a given chemical gel, a uniform cross-linking
reaction is difficult to achieve, and as a result, acertain degree of nonuniformity in density with crosslinks occurs in a process of cross-linking reaction (seeFig. 2, red circles represent higher density crosslinks).
A network structure involving nonuniform densitywith cross-links will exhibit differences in the degree of"elongation (tensile force)" on the individual polymer
chains across cross-links. Also, there will be difference
in degree of transformation (elastic modulus) between
the higher cross-linking density area and the lower
cross-linking density area. Because of these reasons,when an external force is applied to a gel having
nonuniformity in density of cross-links, the force is notevenly exerted onto the polymer links, and fracture will
occur at a particular polymer link where a greatest
tension takes place.
Any gel material consists primarily of liquid, andcontains only a small amount of skeleton component
that is responsible for mechanical strength, andaccordingly, it is essentially a low-strength material. In
addition, while an external force is being applied to a

Fig. 2 Schematic drawing of polymer structure of chemical gel
Sea phase: mixed phase consisting of hard gel and soft gel
Island phase: phase primarily consisting of soft gel

Fig. 3 Schematic drawing of DN gel-47-

NTN TECHNICAL REVIEW No.76（2008）

to derive from the unique structure of the DN gelcomprising both hard gel and soft gel rather than frommechanical properties of each material 5).

of water and a boiling point higher than that of water.
Table 1 summarizes the composition of the DN gel
specimen used, and Fig. 4 shows the chemical
structures of the raw materials used.
The compression properties of the DN gel specimen
using ethylene glycol solvent are provided in Table 2.

3. DN gel specimen
As previously mentioned, the DN gel comprises twopolymer gel types, each having a polymer network
structure independent of the other: since much of thesoft gel exists in a mixed phase comprising hard gel
and soft gel, the DN gel boasts greater mechanicalstrength.
The specimen used is the DN gel whose hard gel ispoly (2-acrylamido-2-methylpropanesulfonate) gel(hereafter simply referred to as PAMPS gel) andwhose soft gel is polyacryl amide gel (hereafter simply
referred to as PAAm gel), wherein the cross-linking
agent used is methylene bis-acryl amide (hereafter
simply referred to as MBAAm).
Having such composition, the DN gel is hydrophilic
and the solvent usually used is water. For thistechnical paper, ethylene glycol was used as thesolvent that features a solidifying point lower than that

4. Friction properties of DN gel
4.1 Test conditions
The test rig adopted for the sliding test isschematically illustrated in Fig. 5. The block portion inthis diagram represents the DN gel test piece, and thedisk portion stands for a metal mating material. The
substance of the metal mating material used is SUJ2.
The load was applied onto the test piece, and themating material was turned, thereby the sliding test
was performed. The friction coefficient was calculatedby dividing the friction force detected with a load cellby the test load.
The test conditions applied are summarized inTable 3, which gives the parameters about friction
properties for the DN gel and SUJ2 material. The
parameters include the load, sliding velocity, surface
roughness of the mating material, and test
temperatures. This technical report hereafter
describes the effects of these parameters.

Table 1 Composition of DN gel
Polymer composition ratio
Hard gel

PAMPS gel

1

Soft gel

PAAm gel20

wt %Approx. 10

Table 3 Test condition
Approx. 90

Ethylene glycol

Solvent

Test piece dimensions mm 5×5×t5.6※Boiling point of ethylene glycol: 198˚C, solidifying point: -13˚C

Gel solvent
Load

Table 1 Compression property of DN gel
Compressive strength
Compressive modulus of elasticity

MPa
MPa

Compression failure skew%

Ethylene glycol (viscosity at 25˚C: 19 mPa-s)
N

Sliding velocity

20m/min 0.5, 1.0, 1.4, 3, 4, 6, 7, 9, 10

Mating material

0.5

2.0, 4.9, 9.8, 24.5
SUJ2（0.005 µmRa, 0.05 µmRa, 0.4 µmRa）

Sliding track dia. mm φ23

95

min 5-30 (per time setting)Time※Test piece dimensions: □10 mm×t5 mm

˚C

Temperature
Atmosphere

Application of
ethylene glycol

Ethylene glycol is applied, as necessary,to the mating material in air.

Load Wa) Main monomer of PAMPS gel

25, 50, 80

DN gel test piece
SUJ2 mating materialb) Main monomer of PAAm gelc) Crosslinking agent

Fig. 4 Chemical structures of the materials

Fig. 5 Sliding test method-48-

Friction Properties of Polymer Gel with High Mechanical Strengthvery smooth surface equivalent to that of a rolling
contact bearing raceway surface to the mating
material surface of a sliding bearing.
In any case, the kinetic friction coefficient µk is lessthan about 0.01, and under the test conditions, the DN
boasts very low friction coefficient. However, when thesurface roughness of mating material is as high as 0.4µmRa, the kinetic friction coefficient of the DN gel issomewhat higher.
When an ordinary plastic material is allowed to slide

4.2 Test result (part 1)-Effects of surface roughness of mating
material, load, and velocityFig. 6 shows the effects of surface roughness ofmating material, load, and velocity on the friction
behavior of the DN gel having ethylene glycol solvent(temperature 25˚C).
For the test, several mating material types, eachhaving a unique degree of surface roughness wereused, whereby the surface roughness ranged from a

Load

0.08

µk

※Test temperature: 25˚C

2.0N
4.9N
9.8N
24.5N

0.07
0.06
0.05

Kinetic friction coefficient

Kinetic friction coefficient μk

0.08

0.04
0.03
0.02
0.01
0.00
0

2

4

6

8

10

0.07

0.05

9.8N
24.5N

0.04
0.03
0.02
0.01
2

0.08

0.05

※Test temperature: 25˚C

Kinetic friction coefficient µkµk
Kinetic friction coefficient

0.06

2.0N
4.9N
9.8N
24.5N

0.04
0.03
0.02
0.01
0.00
0

2

4

6

8

10

0.07

0.05

2.0N
4.9N
9.8N
24.5N

0.03
0.02
0.01
4

6

8

10

2.0N
4.9N
9.8N
24.5N

0.04
0.03
0.02
0.01

0.08

2

12

Load

0.05

※Test temperature: 25˚C

0.04

0.00
0

10

2

4

6

8

10

12

Sliding velocity m/minb) Test temperature 50˚C

Kinetic friction coefficient µkµk
Kinetic friction coefficient

0.06

8

0.06

0.00
0

12

0.08
Load

6

※Surface roughness of mating material:
0.05 µmRa

Sliding velocity m/minb) Surface roughness of mating material 0.05 µmRa

0.07

4

Sliding velocity m/mina) Test temperature 25˚C

0.08
0.07

2.0N
4.9N

Sliding velocity m/mina) Surface roughness of mating material 0.005 µmRa

Load

Load

0.06

0.00
0

12

※Surface roughness of mating material:
0.05 µmRa

0.07

Sliding velocity m/minc) Surface roughness of mating material 0.4 µmRa

Load

2.0N
4.9N
9.8N
24.5N

0.06
0.05
0.04
0.03
0.02
0.01
0.00
0

12

※Surface roughness of mating material:
0.05 µmRa

2

4

6

8

10

Sliding velocity m/minc) Test temperature 80˚C

Fig. 6 Friction properties of DN gel(Part.1)

Fig. 7 Friction properties of DN gel (Part. 2)

-49-

12

NTN TECHNICAL REVIEW No.76（2008）

mentioned above, the DN gel takes a hydrodynamic
lubrication state by the so-called wedge effect and thatthe DN gel in this state can be expressed with themodel in Fig. 8.
With this model, the DN gel test piece is assumed to
be a fixed piece and the mating material (SUJ2) isassumed to move to the right in the model shown inthis page at a sliding velocity U. A liquid film of
ethylene glycol is assumed to be present between the
DN gel test piece and the mating material (SUJ2),
whereby the thickness of the liquid film at the inlet side
is taken as h1 and that at the outlet side as h2(h1>h2>0).
The pressure P working on the DN gel test piece inthis model (the so-called bearing load capacity) canbe expressed by expression 1 derived from the
Reynolds equation. Where,ηis the viscosity of
ethylene glycol, and α is a coefficient determined by
the bearing surface length B and h1 and h2:

relative to SUJ2 under similar conditions, a lubrication
state known as boundary lubrication usually occurs
and the resultant kinetic friction coefficient µk is
approximately 0.1. From this fact, it should beunderstood that the DN gel having ethylene glycol
solvent features unconventional low friction properties
when sliding relative to SUJ2.
Incidentally, the DN gel having ethylene glycol
solvent exhibits unique friction behavior, that is, slow
increase in kinetic friction coefficient µk. Usually, thistrend is experienced in the hydrodynamic lubrication
state.

4.3 Test result (part 2)-Effects of temperature, load, and velocityFig. 7 illustrates the effects of load and sliding
velocity onto the friction behavior of the DN gel having
ethylene glycol solvent with several atmospherictemperature settings (25˚C, 50˚C and 80˚C) (surface
roughness of mating material: 0.05 µmRa).
At 25˚C, the DN gel exhibits excellent friction
coefficients, that is, kinetic friction coefficient µk as low
as 0.01 or smaller in a very wide range of load andsliding velocity. However, at the higher temperature
range, the kinetic friction coefficient µk of the DN gel
tends to significantly increase at low-speed, high-loadcondition settings.
Though the kinetic friction coefficient of the DN gel,
µk, is high at a limited region in low sliding velocity
range, the µk slowly increases as the sliding velocity
increases. Under any test temperature setting in theregions where the kinetic friction coefficient of the DN
gel, µk, increases, the DN gel boasts excellent low
friction quality, that is, a kinetic friction coefficient µk of
0.01 or smaller.

P=

ηUα

………………………………………1h22

B
DN gel test piece

h1
h2
U

Mating material (SUJ2)

Fig. 8 Hydrodynamic lubrication model of DN gel

At the same time, the friction force F working on the
DN gel test piece can be defined by expression 2,
where A is the area of the sliding surface of the DN gel
test piece,βis a coefficient determined by the bearing
surface length B, h1, and h2:

5. Discussion
The DN gel having ethylene glycol solvent wasallowed to slide against a mating material (SUJ2), andthe kinetic friction coefficient, µk, was thereby
determined. At the same time, with a similar test rig, avariety of polymer materials including conventionallyused plastic materials were tested to determine theirkinetic friction coefficients. Consequently, it has beenlearned that the kinetic friction coefficient, µk, of the
DN gel is very low; about 1/10 as low as that of theother examined polymer materials. Beginning with thisfinding, we have attempted to estimate a possible
lubrication state that can occur with the DN gel having
ethylene glycol solvent under the test conditions
adopted for the present research. The method for
estimation is described below.
Let us assume that under the test conditions

F=

ηUA
h2

β ………………………………………2

When the test load is W, the contact surface pressure
P=W/A, and kinetic friction coefficient, µk=F/W, thenfrom expressions 1 and 2, the kinetic friction
coefficient µk can be expressed by expression 3,
whereγis a coefficient determined by h1 and h2:
µk =

-50-

ηU

(P )

1
2γ

…………………………………3

Friction Properties of Polymer Gel with High Mechanical Strength

Consequently, from expression 3, it appears thatthe kinetic friction coefficient µk in the case of fluid
lubrication state is proportional to ηU/P to the 1/2
power.
The results of Figs. 6 and 7 have been reorganized,
takingηU/P along the X axis. The result of
reorganization is plotted in Fig. 9. Incidentally, forcomparison purpose, this diagram also includes theplotting of friction coefficient values obtained from aseries of measurements that used polyether
etherketone (PEEK) resin (a super-engineering
plastic) in place of the DN gel.
In this diagram, two broken lines having inclinationof the 1/2 power are indicated. In a region where hU/P
is greater than about 1×10-9 [m], the inclination of η
U/P is nearly proportional to the "1/2 power" brokenlines: in this situation, the test pieces are under alubrication state equivalent to fluid lubrication.
Incidentally, a fluid lubrication state never occurs witha conventional material under such load-velocity
conditions: this finding is supported by the friction

behavior of PEEK resin test pieces that were
subjected to test conditions identical to those for ourDN gel test pieces. In summary, we believe that ourDN gel is a very unique material that is capable of lowfriction sliding in a fluid lubrication state, beginning at
an unconventionally lowηU/P region, whereby this gel
material boasts a high degree of physical strength
which conventional gel materials lack.

6. Conclusion
We have proven that our DN gel, having ethylene
glycol solvent, exhibits extremely low friction
coefficients under various conditions when used inconjunction with a steel species which is commonlyused as a mating material for a sliding member.
In addition, it has been suggested that, comparedwith other polymer materials, the DN gel enables easyachievement of fluid lubrication state by controlling
contact surface pressure, sliding velocity and solvent
viscosity.
We wish to utilize this technology to help further
reduce friction loss occurring on machinecomponents.

Kinetic friction coefficient µk

1
Mixed lubrication
0.1

Fluid lubrication

References
1) J. P. Gong, Y. Katsuyama, T. Kurokawa, Y. Osada,
Advanced Materials, 15(14), 1155 (2003).
2) K.Haraguchi, T.Takehisa, Advanced Materials, 14,1120(2002).
3) Y.Okumura, K.Ito, Advanced Materials, 13, 485 (2001).
4) J.P.Gong, G.Kagata, Y.Iwasaki, Y.Osada, Wear, 251,
1183(2001).
5) M.Huang, H.Furukawa, Y.Tanaka, T. Nakajima,
Y.Osada, J.P.Gong, Macromolecules, 40(18),6658(2007).

0.01

0.001

25˚C
50˚C
80˚C
PEEK

y=110x0.5
y=22x0.5

0.0001
1.0E-101.0E-9

1.0E-8

η・U/P

1.0E-7m

Fig. 9 Presumption of sliding condition

Photo of authors

Eiichirou SHIMAZU

Masaki EGAMI

Takayuki KUROKAWA

Jian Ping Gong

Elemental Technological
R&D Center

Elemental Technological
R&D Center

Hokkaido University,
Faculty of Sciense

Hokkaido University,
Faculty of Sciense-51-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Study on the Effect of Physical Properties on Wear Resistancesof TiN Films Coated by Arc-plasma Ion Plating

Hideyuki TSUTSUI*
Relationships between the physical properties and wear
resistance of TiN films coated by Arc-plasma Ion Plating in different
conditions were studied. From the result of multiple regression
analysis of these properties, it was clarified that the orientation of
the crystal face is a dominant factor in the wear resistance of TiN
films. TiN films that have high orientation of the (111) crystal faceand low orientation of the (220) crystal face show good wear
resistance.

plasma causes the solid pure titanium target toevaporate. The evaporated titanium ion and thenitrogen atom in plasma state chemically react witheach other to form titanium nitride (TiN). By negative
voltage (bias) applied to a base material, TiN coating
particles are attracted to and accumulate on the basematerial to form a film.
For the present study, eight types of test pieces
were prepared by varying the coating parameters, thatis, coating pressure, arc current and bias voltage. The
thickness of the resultant films fall in a range of 2 to 3
µm.
Each test piece (base material) is a disk measuring
48 mm in outside diameter and 7 mm in thickness.
The material used was SUS440C hardened stainlesssteel with a surface roughness of 0.005 µm Ra on theflat area where the TiN film was formed.

1. Introduction
Nitride based hard films, including TiN films, featurea higher degree of wear resistance compared with thatof steels. Such hard nitride films are commonly
applied to machine components that must satisfy
higher level of requirements for wear resistance.
Examples of such machine components include
cutting tools and dies. Nitride based hard films canhave a significant variation in physical properties,
even if composed of the same material, depending on
the coating technique and coating conditions.
Therefore, these techniques and conditions need tobe carefully selected. Present research studies theeffects of varying physical properties of TiN films
formed with varying coating conditions on wear
resistance. This paper reports the resultant findings.

2. Coating conditions and test pieces used
The test pieces were prepared with an arc-plasma
ion plating (AIP) apparatus.
The operating principle of the AIP apparatus used isschematically illustrated in Fig. 1. Compared with ahollow cathode ion plating system or sputteringsystem, the AIP apparatus is advantageous in that itprovides a higher ionization rate and is capable ofquickly forming a high-density film with goodadhesion.
When a TiN film is formed with an AIP apparatus,
nitrogen atoms are converted into plasma by arc
within a vacuum chamber. The energy of the resultant

Anode

N2 reactiongasElectronBase material

Ti target

Ti ions

TiN

Bias power
supplyPlasmaVacuum chamber

Film

Fig. 1 Principle of Arc-plasma Ion Plating apparatus*Elemental Technological R&D Center-52-

Study on the Effect of Physical Properties on Wear Resistances of TiN Films Coated by Arc-plasma Ion Plating

Table 1 Conditions of TiN film coating

TiN-6Pa

6

150

30

TiN-100A

4

100

30

TiN-200A

4

200

30

TiN-10V

4

150

10

TiN-100V

4

150

100

TiN-300V

4

150

300

3. Evaluation of wear resistance
3.1 Evaluation method
The NTN Savin type wear test rig was used toevaluate the wear resistance of the test pieces. The
test rig is schematically illustrated in Fig. 2, and theapplied test conditions are summarized in Table 2.
On the test rig, the flat surface of the test piece(where the TiN film being evaluated is formed) slides
on the circumferential surface of the disk-shaped
mating piece. The mating piece is made from quenchtempered SUJ2 material with a hardness of HV784.
The mating section has a curvature of R60 mm and asurface roughness of 0.01 µm Ra.
The NTN Savin type wear test rig is identical to thestandard Savin type wear test rig except for themating piece. In a standard Savin type wear test rig
the outer circumferential surface of the mating piece is

3.2 Evaluation result

Specific wear rate ×10-10mm3/（N・m）

The specific wear rates and friction coefficientsobtained with the NTN Savin type wear test rig areshown in Fig. 3.
It is apparent that specific wear rates with TiN films
can vary depending on the coating conditions. The
maximum specific wear rate can be about five times
as great as the minimum specific wear rate.
The friction coefficients of the TiN films at the end of
the test fall in a range of roughly 0.5 to 0.6 with noclear-cut variation between test pieces.

WeightLoad cell

Test piece
Matingmaterial

Fig. 2 Schematic drawing of NTN Savin type wear test rig

Table 2 Conditions of Savin wear test
Matingmaterial

Quenched-tempered SUJ2, curvature R60 mm
hardness Hv784, surface roughness 0.01 µmRa

Load

50 N (max. contact surface pressure 0.5 GPa)）

Velocity

0.05m/sTime

3 min. (sliding distance 9 m)

Atmosphere

Blowing of dry air, humidity 0-20%RH

N number

2

10000

1.0

9000

0.9

8000

0.8

7000

0.7

6000

0.6

5000

0.5

4000

0.4

3000

0.3

2000

0.2

1000

0.1

0

0

Friction coefficient at the end of the test

30

TiN-300V30

150

TiN-100V

150

2

TiN-10V

4

TiN-2Pa

TiN-200A

TiN-standard

TiN-100A

Bias voltage
V

TiN-6Pa

Arc current
A

TiN-2Pa

Coating pressurePa

TiN-standard

Test piecedescription

flat—the load is applied at an edge and greater
variation in test result can occur. In contrast, theNTN’s wear test rig employs a uniquely shaped mating
piece whose outer circumferential surface has acurvature to avoid possible partial contact. Therefore,
the NTN test rig has a higher evaluation accuracy.
In order to determine wear resistance of the TiNfilms, the test duration was set so that the TiN film
wear does not reach the base material of the test
piece. Furthermore, a base material was selected
whose surface roughness value is small so that anadhesion wear configuration is established.
The specific wear rates were determined by firstmeasuring the widths of wear marks after the end oftest run and then subjecting the measurements to a
calculation method. Additionally, friction coefficientswere calculated from friction force measurements
obtained with a load cell.

Fig. 3 Specific wear rates and friction of TiN films-53-

NTN TECHNICAL REVIEW No.76（2008）

peak strength and wear resistance about the (200)crystal face.
These possible factors, the peak strength of (111)

4. Relationship between hardness andwear resistance
Generally, the hardness of a TiN film is associatedwith its wear resistance. Therefore, the effects ofhardness of the TiN film were investigated. To
determine the hardness, a nano-indenter was used toobtain indentation hardness values using a continuous
rigidity measurement technique 1).
The relationship between the indentation hardness
values and specific wear rates is shown in Fig 4. The
diagram includes data for contribution factors.
Note that the contribution factor of indentation
hardness relative to specific wear rate is low; thismeans that indentation hardness is least associatedwith specific wear rate.

(111)
TiN-6Pa

Strength

TiN-100V

(200)

36

37

38

39

2θ

10000

40

41

42

43

44

45

degreeFig. 5 X-ray diffraction (XRD) patterns of TiN films

Contribution factor R2=0.283

9000
8000

Specific wear rate ×10-10mm3/（N・m）

Specific wear rate ×10-10mm3/（N・m）

35

7000
6000
5000
4000
3000
2000
1000
0
20

25

30

35

Indentation hardness GPa

Fig. 4 Relationship between hardness and specific wear rate

10000

Contribution factor R2=0.432

9000
8000
7000
6000
5000
4000
3000
2000
1000
0

0

20

40

60

80

100

Peak strength of (111) crystal face %Fig. 6 Relationship between peak strength of (111) crystal
face in XRD pattern and specific wear rate of TiN film

5. Relationship between orientation of TiNcrystals and wear resistance of TiN film

Specific wear rate ×10-10mm3/（N・m）

Some recent research reports conclude that thewear resistance properties of nitride-based hard films
such as TiN films are affected by the orientation of thenitride crystals 2)-5).
X-ray diffraction analysis was used to determine thecrystal orientation state of the test pieces used in ourstudy. As a result, the pattern in Fig. 5 was obtained.
This pattern shows there is a significant variation in
the state of crystal orientation between test pieces.
Because of this, the relation between TiN crystal
orientation and wear resistance about the main peaksof TiN films was studied, where main peaks meansthe (111) crystal face peak and (200) crystal facepeak. Fig. 6 shows the relationship between the peakstrength and wear resistance about the (111) crystal
face. Fig. 7 provides the relationship between the

10000

Contribution factor R2=0.429

9000
8000
7000
6000
5000
4000
3000
2000
1000
0

0

20

40

60

80

100

Peak strength of (200) crystal face %Fig. 7 Relationship between peak strength of (200) crystal
face in XRD pattern and specific wear rate of TiN film

-54-

Study on the Effect of Physical Properties on Wear Resistances of TiN Films Coated by Arc-plasma Ion Plating

crystal face (contribution factor of 0.432) and peakstrength of (200) crystal face (contribution factor of
0.429), do not provide enough information for
explaining the specific wear rates for the test pieces
used. However, the values from contribution factors
are relatively high. This shows that wear resistance of
TiN films is more strongly affected by their specificwear rates than their indentation hardness values.

6. Relationship between various TiN film
physical properties and wear resistance
As mentioned in Section 5, it has been learned thatcrystal orientation states are relatively closely related
to wear resistance. Therefore, in order to verify thisrelationship more precisely, the effects of all the film
physical properties that can affect wear resistance of
TiN films were evaluated. This was done using a
multiple linear regression analysis technique. The film
physical properties evaluated are summarized inTable 3.
Compared with other hard coating techniques, our
AIP technique involves a relatively large proportion ofparticles known as droplets (the particles ejected fromthe target to undergo chemical reaction and get
deposited on the base material; see Fig. 1) that are
significantly larger than ordinary coating particles. Fig.
8 provides a SEM photo of a TiN film. The droplets
appear as foreign matter on the film surface andmeasure several microns in size. These droplets arepresent not only on the film surface but also in the film
itself. Therefore, to be able to determine the effect of
the coating particle size, we have attempted toanalyze the surface roughness of TiN film.
To determine the crystal orientation state, all the
peak strength ratios in the X-ray diffraction patterns
obtained from the TiN film were used (seemingly

60μm

Fig. 8 SEM image of TiN film surface

equivalent crystal faces which include (111) crystal
face and (200) crystal face have also been
incorporated).
The crystal sizes (maximum sizes of those regarded
as single crystals) were determined by the Scherrer
method, using the values for the (111) crystal facesand (200) crystal faces which correspond with themain peaks.
By the multiple linear regression analysis method,
the specific wear rates were used as target variables
in combination with the film physical properties as
explanatory variables. In the field of multiple linear
regression analysis, variables subjected toinvestigation into the relation with target variables arecalled explanatory variables. The combinations thatexhibited strongest relation with the target variables
were evaluated.
As a result of multiple linear regression analysis, it
was determined that the TiN film physical properties
that are most closely related with the wear resistanceof TiN film are the peak strength of (111) crystal faceand peak strength of (220) crystal face. Like the resultof single linear regression analysis, the effects ofhardness are also small in the multiple linear
regression analysis (see Section 4). The standard
deviation regression coefficients of the explanatory
variables selected are summarized in Table 4. These
coefficients are parameters that represent themagnitude of the effect of the explanatory variables on
the target variables; the larger the absolute value, thegreater the effect. A positive or negative sign indicatesthe direction of effect. The relation between estimatedspecific wear rates derived from the multiple
regression analysis and actual measurements ofspecific wear rates is graphically shown in Fig. 9.
From the result of multiple linear regression
analysis, it is apparent that the wear resistance of TiN
film is greater with a higher peak strength of (111)

Table 3 Physical properties of TiN film investigated in
multiple regression analysis
Film physical properties

Unit

Measuring instrument

Surface roughness Raµm

Surface roughness Rzµm

Surface roughness
measuring instrument
Surface roughness
measuring instrument

Indentation hardness

GPa

Nano-indenter

Indentation hardness elastic modulus

GPa

Nano-indenter

Peak strength of (111) crystal face%

X-ray diffraction analyzer

Peak strength of (200) crystal face%

X-ray diffraction analyzer

Peak strength of (220) crystal face%

X-ray diffraction analyzer

Peak strength of (311) crystal face%

X-ray diffraction analyzer

Peak strength of (222) crystal face%

X-ray diffraction analyzer

Peak strength of (400) crystal face%

X-ray diffraction analyzer(111) crystal face crystallite size

nm

X-ray diffraction analyzer(200) crystal face crystallite size

nm

X-ray diffraction analyzer-55-

NTN TECHNICAL REVIEW No.76（2008）

Table 4 Dominant physical properties on wear
resistance of TiN film in multiple regression analysis
Standard deviation regression coefficient−0.703

Peak strength of (200) crystal face

＋0.479Actual measurement of specific wear rate
×10-10mm3/（N・m）

Explanatory variable
Peak strength of (111) crystal faceby taking the peak strength of (111) crystal face that is
known to most greatly (though indirectly) affect thewear resistance of TiN film as target variables.
The standard deviation regression coefficients of
selected explanatory variables are summarized inTable 6. The relation between estimated values andactual measurements is graphically plotted in Fig. 10.
As a result of the multiple linear analysis, a multiple
linear regression equation of relatively high reliability(contribution factor = 0.668) was generated. According
to this multiple linear regression equation, it waslearned that the peak strength of (111) crystal face of
TiN film tends to be greater with a lower coatingpressure and higher arc current and bias voltage withbias voltage being the predominant factor.

10000
9000

Contribution factor R2=0.694

8000
7000
6000
5000
4000
3000
2000
1000
0

0

2000

4000

6000

Table 6 Dominant parameters of coating conditions on peakstrength of (111) crystal face in XRD pattern of TiN film

8000

Estimated specific wear rate ×10-10mm3/（N・m）

Fig. 9 Relationship between estimated data by multiple linear
regression and experimental data of specific wear rate of TiN film

7. Relation between coating conditions
and wear resistanceTo determine whether or not the wear resistance of
TiN films can be controlled by adjusting the coating
conditions, multiple linear regression analysis was
performed by taking specific wear rates as target
variables and coating conditions as explanatory
variables. The coating conditions investigated aresummarized in Table 5.
As a result of the multiple linear regression analysis,
it has been learned that because the highest
contribution factor is as low as 0.411, coating
conditions do not completely explain the degree ofwear resistance of TiN film.
Therefore, the coating conditions were studied
using a multiple linear regression analysis technique

Unit

Coating pressurePa

Arc current

A

Bias voltage

V

Coating pressure−0.341

Arc current＋0.281

Bias voltage＋0.692120

Contribution factor R2=0.668

100
80

60

40
20

0

0

20

40

60

80

100

120

Estimated peak strength of (111) crystal face %Fig. 10 Relationship between estimated data by multiple
linear regression and experimental data of peak strengthof (111) crystal face in XRD pattern of TiN film

8. Relationship between coating
conditions and hardness
It was learned that in a wear resistance test wherean adhesive wear mode is the predominant mode, an
indentation hardness technique does not explain thewear resistance of TiN film. However, if an aggressive
wear mode is valid for some reason, such aspresence of hard foreign matters on the sliding
surface, then the indentation hardness becomes a

Table 5 Parameters in coating conditions of TiN film
investigated in multiple regression analysis
Coating conditions

Standard deviation regression coefficient

Actual measurement of peak strengthof (111) crystal face %crystal face and a smaller peak strength of (220)crystal face. Furthermore, the peak strength of (111)crystal face significantly affects the wear resistance of
TiN film.

Explanatory variable-56-

Study on the Effect of Physical Properties on Wear Resistances of TiN Films Coated by Arc-plasma Ion Plating

predominant factor.
Accordingly, in order to determine the coating
conditions that can control the indentation hardness of
TiN film, multiple linear regressive analysis was
performed taking indentation hardness as the target
variable (the explanation variables studied are sameas those summarized in Table 5).
Table 7 summarizes the explanatory variables andthe standard deviation regression variables used forthe present multiple linear regression analysis. The
relationship between the estimated values and actual
measurements obtained from the present multiple
linear regression analysis operation is graphicallyplotted in Fig. 11.
From this multiple linear regression equation, it was
found that the coating conditions that help establish aharder TiN film have a tendency similar to that withpeak strength of (111) crystal face (see Section 7).
However, in contrast with peak strength of (111)crystal face, the magnitude of the effect of the coatingpressure is roughly equivalent to that of the bias
voltage.

9. Conclusion
Multiple linear regression analysis was used toinvestigate the effects of various TiN film physicalproperties on the wear resistance of various TiN films
each featuring a unique set of coating conditions. The
resultant findings are summarized below:
1) Crystal orientation state is a significant factor
for wear resistance of the TiN film, and the wear
resistance is higher with a greater peakstrength of (111) crystal face and a lower peakstrength of (220) crystal face. In addition, theeffect of peak strength of (111) crystal face on
wear resistance is significantly higher.
2) Coating conditions alone cannot explain thewear resistance of the TiN film. However, theadjustment of coating conditions can control
the peak strength of (111) crystal face, which
affects the wear resistance of TiN film mostsignificantly.
3) The hardness of TiN film can be controlled
through the adjustment of coating conditions.
We will attempt to explain the reason why higher
peak strength of (111) crystal faces leads to greater
wear resistance of TiN film and remain committed tothe development of high performance hard films.

Table 7 Dominant parameters of coating conditions
on hardness of TiN film
Standard deviation regression coefficient

Coating pressure−0.560

References

Arc current＋0.316

Bias voltage＋0.480

1) W.C. Oliver and G.M. Pharr: An improved technique fordetermining hardness and elastic modules using load anddisplacement sensing indentation experiments, J. Mater.
Res., Vol.7 No.6, 64, 1992
2) Azushima et al.: Proceedings of the 2006 Japan Society of
Mechanical Engineers Annual Conference Proceedings,
No.06-1 (2006) 417
3) Tanno et al.: Wear characteristics under drying conditions of
Ti-B-N hard capsules that have crystal orientation control
through AIP, Tribology Conference Proceedings, Saga
(2007-9) 147
4) Tanno et al.: Wear mechanism under drying conditions of TiN
hard capsules that have orientation control through AIP,
Tribology Conference Proceedings, Tokyo (2008-5) 187
5) Ishii et al.: Improvement of capsule characteristics in titanium
nitride compound surface reform, Tribology Conference
Proceedings, Tokyo (2008-5) 219

Actual measurement of indentation hardness
GPa

Explanatory variable

30

Contribution factor R2=0.634
28

26

24

22

20

20

22

24

26

28

30

Photo of author

Estimated indentation hardness GPa

Fig. 11 Relationship between estimated data by multiple linear
regression and experimental data of hardness of TiN film

Hideyuki TSUTSUIElemental Technological
R&D Center-57-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Development of an Antenna Material Based on Rubberthat has Flexibility and High Impact Resistance

Kouya OOHIRA*NTN has developed an antenna material based on rubber that
displays superior flexibility and high impact resistance compared toceramic and polymer antenna materials. The developed material
also has fire retardancy. The developed material is suitable fordielectric antennas that are used for wireless LAN, radio frequency
identification, ultra wide band and other applications. This reportintroduces the characteristics of the developed material and test
results.

lack in flexibility and are vulnerable to impact.
To address these challenges, NTN has newly
developed "rubber-based antenna materials " 2)-5).
Consisting of a rubber (EPDM) base material, thisseries of rubber materials is capable of arbitrary setup
of relative dielectric constant by appropriately
adjusting the content of highly dielectric ceramic
powder. Being flexible, the NTN's newly developed
rubber material boasts excellent shock resistance andcan be attached to a curved surface: furthermore,since being formed by a heat-compression technique,the rubber material can be formed into a cylinder or
prism, or even into a large-sized sheet that is notformed with a ceramic or resin material. This feature isan outstanding advantage of this material.
Conventional rubber materials have a drawback ofinferior flame retardance. In contrast, the NTN's new
rubber material boasts excellent flame retardance
equivalent to UL94 V0 by adoption of optimallyblended flame retarding agent.

1. Introduction
Recently, the markets have been dramatically
expanding in the field of equipment for high-frequency
wireless LAN, RFID (radio frequency identification),
UWB, GPS, terrestrial digital broadcasting and home
network, etc. In this context, a diversity of dielectricantennas are currently used. A material of dielectricantenna used for these applications is preferably
needed to feature a higher dielectric constant and alower dielectric loss tangent. By using a highly
dielectric material for an antenna, a compact antenna
can be designed thanks to frequency shortening effectof wave that is transmitted in the dielectric material.
Incidentally, use of a material of low dielectric loss
tangent will help suppress heat generation-induced
energy loss and can improve antenna performance.
Conventional antennas have often been consistedof a ceramic material, and the relative dielectric
constant for a ceramic material cannot be arbitrarily
selected. Consequently, degree of freedom indesigning antenna is low. As an antenna material
capable of arbitrary selection of relative dielectric
constant, resin type 1) (composite material containing
ceramic powder) is available. However, thesematerials have involved drawbacks that include 1
surface treatment is needed to improve adhesivenessfor electrode strips, and 2 injection-forming with ahigh dielectric material having a larger portion ofceramic powder is difficult. Furthermore, theseconventional antenna materials are hard and brittle,

2. Advantages of new rubber antennamaterial
The typical features of the NTN's new rubber
antenna material are summarized as follows:
1 Advantages in functionality
・Relative dielectric constant can be arbitrarily
selected (4-20).
・Low dielectric loss tangent (0.01 or lower)
・High degree of flexibility*Elemental Technological R&D Center-58-

Development of an Antenna Material Based on Rubber that has Flexibility and High Impact Resistance

The prototype antenna shown in Fig. 1 comprises a
sheet of NTN's new rubber antenna material: to eachof its faces, a piece of copper foil electrode is
attached, and this electrode is designed to thewavelength of intended radio frequency wave beingtransmitted and received. Fig. 2 shows a situation
where a piece of the new rubber antenna material is
attached to a curved surface. Since the base materialof the new material is flexible rubber, such a piece canbe readily attached to a curved surface. A
conventional ceramic antenna or a resin antenna
containing ceramic powder cannot be readily glued toa curved surface such as a one shown in Fig. 2.
Fig. 3 provides a view of cylindrical moldings
formed by heating-compression technique using a 4gate press die. As necessary, the new material iscapable of insert molding with electrodes embedded.

・High degree of shock resistance・Flame retardance (HB, V1, V0)・Easy machining works including cutting andgrinding
2 Advantage in forming (molding)・Forming into cylinder or prism is possible.
・Forming into a larger sized sheet is possible.
Sheet dimensions: 150 mm×150 mm to 230 mm×230 mm
Sheet thickness: 1 to 3 mm

Fig. 1 shows an RFID patch antenna consisting of
the NTN's new rubber material (100 mm×100 mm×t6mm), Fig. 2 provides a view of the new rubbermaterial attached to a curved surface, and Fig. 3
presents a view of cylindrical moldings of the new
rubber material.

3. Dielectric properties
3.1 Amounts of ceramic powder andcorresponding dielectric properties
EPDM has been used as a base material, and theamount of ceramic powder was varied in a range of 0
to 1200 weight parts (amount of ceramic powder has
been varied relative to 100 weight parts of EPDM),
thereby rubber sheets (each measuring 150 mm×150
mm×t2 mm) have been prepared. Each test piece ofa specified shape has been cut from each rubber
sheet, thereby the relative dielectric constant anddielectric loss tangent with each test piece have beenmeasured with cavity resonance method. The test
conditions used are summarized in Table 1, and thetest results are graphically plotted in Figs. 4 and 5.
With a greater ceramic powder content, the relative
dielectric constant is greater and the dielectric loss
tangent is smaller. With a higher test frequency, both
the relative dielectric constant and dielectric loss
tangent tend to be greater.
With NTN's new rubber antenna material, it ispossible to obtain a desired relative dielectric constantby arbitrarily adjusting the ceramic powder content in
it. Because NTN's new material is capable ofarbitrarily setting a relative dielectric constant, theantenna can be more freely designed: thus, thematerial is advantageous as an antenna material.

Fig. 1 Patch antenna of developed material for RFID

Fig. 2 The developed material sticked on curved surface

Table 1 Test

conditions
Contents

Fig. 3 Example of cylinder molding of the developed material-59-

Measuring method

Cavity resonance method

Test piece shape

Cylinder (1.5mm×1.5mm×70mm)

Frequency MHz

1000，3000

Temperature ˚C

25

NTN TECHNICAL REVIEW No.76（2008）

designing better rubber sheet. As a result, it has beenfound that a composite rubber sheet comprising aplurality of rubber sheets each featuring a different
material can provide a solution to this problem.
Fig. 6 also includes information about thetemperature-dependency of the so-obtained hybrid
sheet. Featuring relative dielectric constant lessdependent on temperature, the hybrid sheet exhibits
the relative dielectric constant nearly constant in atemperature range from -40˚C to +120˚C, thereby thismaterial can be used in a wider temperature range.

25

Relative dielectric constant1000 MHz
20

3000 MHz

15

10

5

0
0

300

600

900

1200

1500

Table 2 Test

Ceramic powder content, weight parts

conditions
Contents

Fig. 4 Relationship between content of ceramic powderand dielectric constant
0.010

Measuring method

Capacitor method

Test piece shape

Plate (30mm×30mm×t1mm)

Frequency MHz

400

Temperature ˚C-40，-10，30，70，100，1201000 MHz
3000 MHz
20
0.006

18

Relative dielectric constantDielectric loss tangent

0.008

0.004

0.002

0.000
0

300

600

900

1200

1500

Ceramic powder content, weight parts

16
14
12
10
8
6

Rubber sheet proper4

Hybrid sheet

2

Fig. 5 Relationship between content of ceramic powder
and tanδ

0
-50

0

50

100

150

Temperature ˚CFig. 6 Relationship between temperature and dielectric
constant

3.2 Temperature-dependency of relative
dielectric constant
A rubber sheet was prepared which was blended sothat its relative dielectric constant at a room
temperature (25˚C) is 16.5, and then its relative
dielectric constant at various temperatures wasmeasured by capacitor method. The test conditions
are summarized in Table 2, and the test result isplotted in Fig. 6.
The relative dielectric constant of a rubber sheet
alone decreases as the temperature increases.
Greater variation in relative dielectric constant is notpreferable because the resonance frequency can
deviate depending on the temperature. In particular,when the rubber sheet alone is used at a higher
frequency band, the deviation in resonant frequency
can pose greater adverse effects.
In order to limit temperature-dependency of relative
dielectric constant, trial and error were repeated in

3.3 Long-term aging quality
Test pieces (relative dielectric constant: 18,
measured at 25˚C) were obtained from the rubber
sheet made of NTN's new rubber antenna material,
and long-term aging quality of their relative dielectric
constant has been evaluated. The aging conditions
are summarized in Table 3, the measuring conditions
for post-aging dielectric constant are listed in Table 4,
and the test result is graphically plotted in Fig. 7.
From Fig. 7, it should be understood that therelative dielectric constants of the test pieces, eachallowed to stand for 1000 hours at -40˚C, +80˚C or+120˚C, remained virtually unchanged, and that NTN's
newly developed rubber antenna material issatisfactorily stable.

-60-

Development of an Antenna Material Based on Rubber that has Flexibility and High Impact Resistance

Table 3 Condition of aging test

4.2 Immunity against molten solder
When used in a commercial application, the new
rubber antenna material is needed to withstand theheat of molten solder because the power lines
between power feed points and electrodes and theconnector mounts are subjected to heat of molten
solder. This is always a case with dielectric antennas.
Table 6 summarizes the result of solder bathimmersion test for NTN's newly developed rubber
antenna material (per JIS C6471: test pieces areplaced on the surface of solder bath kept at a
specified temperature; they are removed after elapseof a specified time, and their appearance isinspected). At each solder bath temperature, thesurface of each test piece did not exhibit a blister orcrack. This fact means that NTN's new rubber antennamaterial boasts good heat resistance, positively
withstanding the temperature of solder bath.
In reality, when fabricating a prototype rubber
antenna for evaluation, NTN used molten solder for
attaching the connector and grounding electrode andsoldering the power feed point, and found that thematerial does not pose a practical problem.

Contents
Test piece shape

Plate (30mm×30mm×t2mm)

Temperature ˚C-40，80，120

Process time h

0，250，500，1000

Table 4 Test conditions
Contents
Measuring method

Capacitor method

Test piece shape

Plate (30mm×30mm×t2mm)

Frequency MHz

400

Temperature ˚C

25

25

Relative dielectric constant

20

15

-40˚C
80˚C

10

120˚C
5

0

Table 6 Result of dipping test in melted solder

0

200

400

600

800

1000

260˚C
280˚C

Time hFig. 7 Relationship between aging time and dielectric constant

Test duration s
510
No irregularity No irregularity↑↑

4. General physical properties

5. Antenna characteristics

4.1 Mechanical properties

5.1 Resonant frequency, voltage standing wave
ratio (VSWR) and gain

Ceramic powder contents and mechanicalproperties of the test pieces are summarized in Table
5. With a greater ceramic powder content, the specificgravity, tensile strength and hardness increase andelongation decreases. Note that no problem will occur
with a specimen having 1200 weight parts of ceramic
powder.

From a material featuring relative dielectric constantof 18 (25˚C), two 2450 MHz patch antenna pieces (40mm×40 mm×t2 mm) were obtained. The initialcharacteristics of both antennas are summarized inTable 7. It is apparent that both antenna pieces are
performing well as antenna in terms of both VSWR
(Voltage Standing Wave Ratio: a ratio of maximum
value to a minimum value of waves that occur fromsuperposing of incoming wave and reflected wave)
and gain (ratio of the received power with the antenna

Table 5 Mechanical properties of test specimen
Ceramic
Specific
powdergravitycontent,
weight parts JIS K6268
0
600
900
1200

1.0
2.8
3.2
3.6

TensilestrengthMPa
JIS K6251
4.5
6.1
6.5
7.8

Elongation%

Hardness
JIS-A

JIS K6251
290
150
110
75

JIS K6253
47
90
92
94

Table 7 Antenna characteristics

-61-

No.

Resonance frequency MHz

VSWR

Gain dBi

1
2

2475
2446

1.23
1.19

2.1
2.1

Target value

2450±50

2 or less 1.5 or higher

NTN TECHNICAL REVIEW No.76（2008）

being tested to that with the reference antenna,
wherein the received power is measured when bothantennas are directed in their maximum electric field
while a same level of electricity is applied to bothantennas).

6. Flame retardance grade
6.1 Dielectric characteristics
EPDM used for rubber antenna is readily
combustible. However, antenna materials used onhome electronic equipment are sometimes needed to
be flame-retardant.
Rubber sheet samples were prepared by blending
specific amounts of metal hydroxide (capable ofretarding flame) and bromine-based flame retarding
agent (outside the scope of RoHS directive), and byvarying the content of ceramic powder. Test pieces ofa particular shape were cut out from each rubber
sheet sample, and the relative dielectric constant anddielectric loss tangent of each test piece have beenmeasured. The test conditions are summarized inTable 11, the test conditions are graphically plotted in
Figs. 9 and 10.
With increase in the ceramic powder content, therelative dielectric constant increases. The dielectricloss tangent of test pieces having the flame retarding
agent is as high as 0.005 while that of test pieces
lacking the flame retardant is as low as 0.003; thismeans an increase of about 0.002 (Fig. 10). Currently,
it is expected that NTN will soon be able to establish atechnique to limit the increase in the dielectric loss

5.2 Heat cycle test
The antenna prototypes were subjected to heat
cycle test under the test condition shown in Fig. 8.
Various antenna characteristics values (resonancefrequency, VSWR, gain) with the antennas before andafter the heat cycle test are summarized in Tables 8through 10.
The test pieces having undergone the heat cycle
test do not exhibit loosening of electrodes or blistering.
At the same time, the variations in resonancefrequency, VSWR and gain are very small (below
target limits) and apparently do not pose problems inpractical use of the antenna.

High temperature:
80˚C

Low temperature:
-30˚C
2h

2h

2h

2h

1 cycle (8 h): a total of 30 cycles

Table 11 Test conditions

Fig. 8 Condition of heat cycle test

Contents
Measuring method

Capacitor method

Table 8 Changing rate of resonate frequency

Test piece shape

Plate (20mm×20mm×t2mm)

Resonance frequency MHz

Frequency MHz

400

Temperature ˚C

25

Variation %Before test

After test

1
2

2475
2446

2465
2434

−0.4
−0.5

Target value

―

―

±2.0

20
Ceramic powder
18
Ceramic powder + metal hydroxide

Table 9 Changing rate of VSWR
Relative dielectric constant

VSWR

16

Variation %Before test

After test

1
2

1.23
1.19

1.31
1.26

6.1
5.6

Target value

―

―

±10

Table 10 Changing rate of gain

Ceramic powder + metal hydroxide+ bromine-based fire retardant

14

Ceramic powder + metal hydroxide (optimized
substance) + bromine-based fire retardant

12
10
8
6
4
2

Gain dBi

Variation %Before test

After test

1
2

2.1
2.1

2.1
2.1

0
0

Target value

―

―

±10

0

0

100

200

300

400

Ceramic powder content, weight parts

Fig. 9 Relationship between content of ceramic powderand dielectric constant-62-

Development of an Antenna Material Based on Rubber that has Flexibility and High Impact Resistance7. Conclusion

0.012
Ceramic powder

NTN's newly developed rubber antenna material
comprises rubber as a base material, and is capableof arbitrary setup of relative dielectric constant (in arange of 4 to 20) by adequately adjusting the content
of highly dielectric ceramic powder contained in it.
Being flexible, this novel material excels in shock
resistance, does not develop cracks even when
dropped, and can be readily attached to a curved
surface. Additionally, when designed to be more
flame-retardant, this material can have a flame
retardance rank of V0. Furthermore, the material canbe formed into a large-sized sheet (150 mm×150 mm
to 230 mm×230 mm) that is not available with aceramic-only or resin-only material.
To sum up, NTN's newly developed rubber antennamaterial has unique advantages many conventional
materials do not offer. Thus this new material may be
used in applications with which conventional ceramicor resin antennas cannot cope.

Ceramic powder + metal hydroxide

0.010

Dielectric loss tangent

Ceramic powder + metal hydroxide+ bromine-based fire retardant
0.008

Ceramic powder + metal hydroxide (optimized
substance) + bromine-based fire retardant

0.006

0.004

0.002

0.000

0

100

200

300

400

Ceramic powder content, weight parts

Fig. 10 Relationship between content of ceramic powder
and tanδtangent to approximately 0.001 through optimization of
the metal hydroxide added. As reference, thedielectric characteristics data for a rubber sheet that
contains 330 weight parts of ceramic powder areprovided in Figs. 9 and 10.

References
1) J.W. Schulz, J.S. Colton, and C.K. Berkowitz: 45th
International SAMPE Symposium, (2000) 1863-1873
2) Nakashima, Ohira: The Society of Rubber Industry,
Japan 2004 Annual Conference Research Report
Abstracts, 43 (2004)
3) Nakashima, Ohira, Sakai: The Society of Rubber
Industry, Japan 2006 Annual Conference Research
Report Abstracts, 70 (2006)
4) Egami, Ohira: The Society of Rubber Industry, Japan
2008 Annual Conference Research Report Abstracts,
119 (2008)5) Ohira: Development of "rubber antenna material" that
can have its relative dielectric constant set at will, Jido
Ninshiki Monthly, Vol.1, 54-57 (2007)

6.2 Flame retardance
The flame retardance of each test piece has been
evaluated through the flammability test per UL94
standard, and the result is summarized in Table 12. A
test piece with low ceramic powder content burns
violently. Test pieces containing not only ceramic
powder but also metal hydroxide and bromine-based
flame retarding agent exhibit good flame retardance ofrank HB. In particular, the test piece containing metalhydroxide, bromine-based flame retarding agent andan increased proportion of ceramic power boasts very
excellent flame retardance of rank V0.

Table 12 Fire retardancy of test specimen

Ceramic powder
content,
weight parts

Ceramic powder

Ceramic powder
Ceramic powder + metal hydroxide+ metal hydroxide + bromine-based
fire retardant

0

×

HB

HB

100

HB

HB

HB

200

HB

HB

V1

300

HB

HB

V0

Photo of author(Definitions of flammability)
HB (slow-burning): combustion velocity 40 mm/min or lower (not self-extinguishing)
V1 (flame-retardant): combustion time 30 sec. or shorter (self-extinguishing)
V0 (flame-retardant): combustion time 10 sec. or shorter (self-extinguishing)

Kouya OOHIRA
Elemental Technological
R&D Center-63-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Development of Sensing System
for Cerebral Aneurysm Treatment

Yoshitaka NAGANO**
Hideo FUJIMOTO**
In recent years, endovascular coil embolization has become a standard
treatment option for people with cerebral aneurysms. Since coilembolization requires an advanced coil insertion technique, an analysis of
the technique and the development of a surgical system for assistance andtraining are desirable. We have developed a novel optical force sensor forthe coil delivery wire. This paper presents the sensor and the results of anevaluation of the sensor by using a silicone dummy aneurysm.

in the aneurysm and prevent it from rupturing. This
technique is known as coil embolization 1)-3). In thissurgical operation, a catheter is inserted into the
cerebral aneurysm in question and through thiscatheter, platinum coils are delivered into the ruptureprone aneurysm: therefore, the coil insertion force
needs to be finely controlled.
To help support the brain surgeon during the coilembolization operation for cerebral aneurysms, athree-dimensional cerebral artery model has beendeveloped to assist in pre-operation simulation andsurgical training 4). However, no instrument is currently
available which is dedicated to support coilembolization for cerebral aneurysms. If a coil insertion
force input sensor used during this operation can be
developed, an excessive coil insertion force will be

1. Foreword
Minimally invasive therapy has become increasingly
attractive in modern medicine, particularly in the field of
surgical applications. "Minimally invasive therapy" is atherapeutic method that minimizes the damage andpain to the patient when receiving a surgical operation.
A typical example of such a surgical method iscatheter surgery, in which a thin tube known as"catheter" is inserted into an appropriate human bloodvessel and through which a variety of instruments
and/or medicines can be delivered to an affected area
to help treat the possible ailment. In particular, asurgical operation executed by inserting a catheter
into a problem blood vessel is known as endovasculartherapy or surgery. For this operation, the surgeon
involved must have a higher level of skill in manualtechnique to be able to prevent endovascular damage.
As the age of society increases, the number ofpatients with subarachnoid hemorrhages (SAH) has
been increasing. In this context, endovascular therapy
has been applied to prevent rupture of a cerebral
aneurysm, which is the major cause of SAH. As
schematically illustrated in Fig. 1, a cerebral aneurysm
occurs when a weak portion of vascular wall inflates
like a balloon due to a factor such as blood pressure.
The vascular wall of a cerebral aneurysm is vulnerable
because it is thin, and it can rupture easily. In a typicalendovascular therapy for cerebral aneurysms, aplurality of platinum coils are packed into the cerebral
aneurysm in order to intentionally develop a thrombus

Cerebral aneurysm
・Inflates like a balloon
・Thin and brittle vascular wall

Blood pressure

Cerebral artery

Fig. 1 Cerebral aneurysm**Mechatronics Research Dept. New Product Development R&D Center**Professor, Nagoya Institute of Technology-64-Development of Sensing System for Cerebral Aneurysm Treatment

reaching the cerebral aneurysm, the platinum coilrestores its three-dimensional spiral structure(shape memory feature).
(3) The surgeon determines the state of cerebral
aneurysm and coil based on an X-ray image, andmanipulates the delivery wire to insert the coil into
the aneurysm. Once the entire coil has been
inserted into the aneurysm, the coil is cut off fromthe delivery wire.
(4) Steps (2) and (3) are repeated to pack a plurality ofcoils into the aneurysm at a high density.
(5) The cerebral aneurysm packed with a plurality ofcoils turns into thrombus to prevent the aneurysm
from rupturing.

avoided and rupturing of cerebral aneurysm will beliminated.
In addressing this challenge, NTN has undertaken
research in cooperation with Professor Hideo
Fujimoto, Graduate School of Engineering, Nagoya
Institute of Technology to develop a unique sensor
that allows the operating surgeon to sense a delicate
force (insertion force with the coil) at their fingertip
while manipulating the coil 5)-7).
This paper reports the result of a coil insertion testin which the NTN-developed sensor was inserted into
the dummy cerebral aneurysm made of a silicone
resin.

2. Coil embolization technique for
cerebral aneurysms

3. Detecting principle and structure of
the sensor

As shown in Fig. 2, in a coil embolization operation
for cerebral aneurysm, a surgeon will insert the coil
into the artery at the femoral region via a surgicalinstrument known as the Y-connector. Fig. 3schematically illustrates the steps (1) through (5) for
inserting coils into the cerebral aneurysm. The detailsof these steps are as described below.
(1) The surgeon directs the catheter into the cerebral
aneurysm.
(2) The surgeon inserts the delivery wire whose end
point has the platinum coil into the catheter. Once

The diameter of the catheter used for endovascularcoil embolization of cerebral aneurysms measuresapproximately 1 mm, and that of delivery wire is as
small as 0.3 mm. Consequently, it is very difficult to
directly mount a sensor piece to these thin members.
Therefore, in the development of our new sensor
system, we have attempted to maintain compatibility
with conventional surgical instruments.
Fig. 4 shows the appearance of the newly
developed sensor. This sensor is unique in that it isCatheter

Platinum coil

Delivery wire
SurgeonFemoral region
Cerebral aneurysm

Y-connector

Fig. 2 Endovascular coil embolization of cerebral aneurysm
Catheter Aneurysm
Bloodvessel（1）

Delivery
wire

Coil

Coil is cut off.

（2）

（3）

Coils aretightly packed.

Thrombus forms
in aneurysm.

（4）

（5）

Fig. 3 Treatment by embolization coil-65-

NTN TECHNICAL REVIEW No.76（2008）

configured around the Y-connector that has been
conventionally used in surgical operations of cerebral
arteries. The Y-connector used for inserting thedelivery wire into the catheter comprises a junction of
two insertion ports for inserting the delivery wire andintroducing physiological salt solution. While holding
the Y-connector with the left hand, the surgeon inserts
and manipulates the coil with right hand.
Our insertion force sensor has been built into the
middle portion of the Y-connector.
Fig. 5 shows the internal structure of the Yconnector. As illustrated in this diagram, an opening issituated at the middle of the Y-connector, and the wire
flexes at this opening according to the insertion force
acting on it. The newly developed sensor opticallydetects the resultant deflection amount to estimate theinsertion force acting on the wire.
As with any medical equipment, the member directly
delivered into a human body must be maintained atsterilized state. With our newly developed sensing

technique, the deflection amount of the wire is
optically measured on a non-contact basis, and no is
problem with sterilization requirements.
Fig. 6 schematically illustrates the opticalconfiguration of the newly developed sensor. The
sensing optical system comprises an LED, a lensarray and a photo line sensor, wherein the sensor
body is made of a transparent resin to enabletransmission of LED light. Each optical part isembedded in the resin and is watertight, thereby thesensor can be rinsed with water.
Fig. 7 schematically illustrates the principle of wire
position detection for measuring the deflection amountof the wire. This diagram shows a cross-sectional planof the sensor body taken along the wire deflection
direction. The light from the LED passes through thesensor body, and then the light path is altered by alens array consisting of SELFOC lens and directed tothe photo line sensor. The wire within the sensor body
blocks a portion of the LED light, thereby the wireLED
Sensor body(transparent resin)

Physiological salt
solution insertion port
CatheterjunctionLens array

Delivery wire

Delivery wire
insertion port

Insertion force sensor

Unit : mm

Photo line sensor

Fig. 4 Y-connector with built-in force sensor

Sensor bodyFig. 6 Optical system of the force sensor

Opening

LED
LED light
Wire

Delivery wire

Lens array

Photoline sensor
Deflection
amountPicture element oflowest luminance pd

Wire flexes.

Fig. 7 Conception diagram of wire position detection

Insertion
force
Optical sensor(photo line sensor)

Fig. 5 Structure of the Y-connector with built-in force
sensor-66-

Development of Sensing System for Cerebral Aneurysm Treatment

location is represented as the identification pd of thepicture element exhibiting the lowest luminance within
the photo line sensor. The photo line sensor consistsof 128 picture elements and features resolution of 400
dpi (approximately 63 µm). Weighting has been
attempted based on the locations and luminance ofthe picture element pd and nearby picture elements,
thereby the deflection amount of the wire has beenmeasured at precisions greater than the resolution of
the photo line sensor.

insertion force estimated from the sensor output andthe actual wire insertion force. It has been learned thatthe maximum detection error falls in a range of ±0.05

4.2 Evaluation using dummy cerebral aneurysm
Using the test system shown in Fig. 10, a coilembolization test has been executed with a dummy
cerebral aneurysm. The dummy cerebral aneurysm
was fabricated with a transparent silicone resin so thatthe effect of coil embolization can be visuallymonitored, wherein the diameter of dummy aneurysm
measured 4 mm and that of cerebral artery measured
3 mm. While progress of coil embolization was viewed
with the monitor display connected to the CCD
camera, the delivery wire insertion force wasmeasured with the newly developed sensor.
Fig. 11 provides a view of the dummy cerebral
aneurysm filled with a plurality of compacted coils.
Fig. 12 shows the resultant measurements for delivery
wire insertion force. This diagram illustrates
intermittent variation in insertion force occurring as
time elapsed. This variation occurred because the4. Performance evaluation
4.1 Detection accuracy of insertion force
Using a delivery wire for coil embolization, therelation between the insertion force and deflection
amount of the wire has been measured, and Fig. 8graphically plots the result. Based on this result, thesensor output has been calibrated.
Fig. 9 presents the relation between the wire

Deflection amountmm

4

Monitor display

3
CCD camera
Control box

Microscope
2

1
Y-connector withbuilt-in sensor

0
0.0

0.2

0.4
0.6
Insertion force N

0.8

1.0

Catheter
Dummy cerebra
aneurysm

Estimated insertion force obtained by sensor N

Fig. 8 Input force and bending value

Fig. 10 Photograph of experimental system for coilembolization

1.2

1.0
Coil

Catheter

0.8

0.6

0.4

0.2

0.0

0.0

0.2

0.4
0.6
0.8
Insertion force N

1.0

Dummy cerebral aneurysm(φ4 mm)1.2

Cerebral artery(φ3 mm)Fig. 11 Photograph of coil embolization

Fig. 9 Calibrated sensor output-67-

NTN TECHNICAL REVIEW No.76（2008）

Insertion force

N

0.4

0.3

0.2

0.1

0.0
0

1

2

3

4

5

6

7

Time

s

8

9

10

11

12

Fig. 12 Measurement result of insertion force

4) S. Ikeda, F. Arai, T. Fukuda, K. Irie and M. Negoro:"Three Dimensional Photoelastic Stress Analysis on
Patient-Tailored Anatomical Model of Cerebral Artery,"Proc. 2004 Int. Symp. on Micro-Nanomechatronics
and Human Science, 145-150 (2004)
5) Y. Nagano, A. Sano, M. Sakaguchi, H. Fujimoto,"Development of a super fine and long linear sensor
for physiological applications," 44(3), Transactions of
the Society of Instrument and Control Engineers,(2008) 278-284
6) Y. Nagano, A. Sano, M. Sakaguchi, H. Fujimoto, S.
Miyachi, T. Oshima, N. Matsubara, O. Hososhima, J.
Yoshida: "Development of Sensing System for
Cerebral Aneurysm Treatment," 16th Japan Society ofComputer Aided Surgery Conference, (2007) 61-62
7) O. Hososhima, S. Miyachi, T. Oshima, T. Izumi, N.
Matsubara, A. Tsurumi, J. Yoshida, Y. Nagano, M.
Sakaguchi, H. Fujimoto: "Development of a coilinsertion force measurement device using opticalsensors," 1 (2), 23rd Annual Meeting of the JapaneseSociety for Neuroendovascular Therapy, (2007) 156(Bronze medal poster in the doctors division)

wire was pinched with fingers and progressively
inserted in order to simulate an actual coilembolization operation. It has been learned that as thedummy cerebral aneurysm was progressively filledwith coils, the insertion force gradually increased. At
the final phase of coil insertion operation, an insertion
force of approximately 0.3 N was measured.

5. Conclusion
We have developed a novel sensor that canmeasure coil insertion force exerted by a surgeon in acoil embolization operation for cerebral aneurysms. So
that conventional catheter, coil and delivery wire canbe used without any modification, we haveincorporated an insertion force sensor into a Yconnector to be able to maintain compatibility withconventional coil embolization techniques.
We will accumulate additional test data andevaluate the accumulated data to help realize moreadvanced coil embolization practices for cerebral
aneurysms.

Photos of authors

References
1) H. Kikuchi: "State of the Art Medicine Series 29
Neurosurgery―Recent Neurosurgery Treatment
Developments," PET Journal (2004)
2) S. Miyachi: "Treatment Inside Cerebrovascular
Vessels," Gendai Igaku 52 (1), (2004) 73-84
3) A. Hyodo, S. Nemoto: "Cerebral Aneurysm Surgery
Inside the Artery Using GDC," Igaku Shoin, (1999)-68-

Yoshitaka NAGANO

Hideo FUJIMOTO

Mechatronics Research Dept.
New Product Development
R&D Center

Professor,
Nagoya Institute ofTechnology

NTN TECHNICAL REVIEW No.76（2008）[ Technical Article ]Development of a Method
for Monitoring Lubricant Deterioration

Light emitting
element

GreaseOptical fiberLight receiving
element

Optical fiber

#1
#2
#3
#4

Kentaro NISHIKAWA*
Tomomi ISHIKAWA*

The monitoring of debris and wear particles in lubricating oil andgrease is desired for early prediction of the failure of machinerycomponents. The state of lubrication can be estimated by monitoring
the amount and size of the particles in the lubricating material. This
paper describes the configuration and principle of the detection, andshows experimental results.

electrostatic sensor 2) that detects electrostatic buildup
on ceramic debris immediately after their peel-off froma mother material. However, these techniques haveproblems to be solved in terms of detection accuracy
and reliability.
Incidentally, large-sized bearings prelubricated withgrease are usually maintained at regular intervals
based on data about run hours elapsed. For example,
grease is sampled from bearings for railway rollingstocks, thereby its acid number, consistency, oilseparation ratio, constituents, water content and otherfactors of the grease are determined 3).
Quantification of amount of worn metal in bearing
poses the most important challenge in lubrication
management 4). However, there has been no reportabout a system that enables full-time monitoring ofamount of metal in bearing.
This paper describes NTN's newly developed
lubrication-related technologies: they are a technology
for detecting ceramic powder in lubricating oil andanother technology for sensing state of grease inbearing.

1. Introduction
Because it is the goal of all bearings to satisfy ahigher degree of reliability, a technique is muchneeded to be able to sense the state of the internal
condition of bearings while in operation. If detection of
the degree of wear of each bearing member ispossible, failure prediction for the bearing in question
will be also possible. As a result, the maintainability
and reliability of the associated machine will be much
improved.
For oil-lubricated bearings, a device capable ofsensing the state inside a bearing has beencommercially used wherein wear particles(ferromagnetic material) occurring from the bearing inquestion are captured by a permanent magnet piecearranged in the oil circulating line and the captured
magnetic particles are monitored with a magnetic
sensor or electric-resistance detection sensor 1).
Recently, bearings have been increasingly used inhigh-speed applications, typically on machine tool
main spindles, and, in this context, hybrid bearings
having ceramic rolling elements have been
increasingly adopted. However, because ceramicmaterial is non-magnetic and non-conductive, ameasuring system capable of detecting metal wear
particles cannot be used for analyzing ceramic wear
debris. There have been reports about measuring anamount of ceramic wear debris, and the examples of
the reported measuring technique include a dielectric
factor method based on a fact that the dielectric factor
measured with ceramic debris is several times as highas that with lubricating oil; and a method using an

2. Ceramic debris detection sensor 5)
2.1 Detection principle and overview of sensing
apparatus
Fig. 1 schematically illustrates our newly developed
sensing apparatus. Two electrode plates are arranged
in an opposing relation within a lubricating oil system,
wherein one electrode plate is forced to the other by
linear motion of an electromagnetic solenoid. If foreign
matters are present in the lubricating oil, a portion of*Mechatronics Research Dept. New Product Development R&D Center-69-

NTN TECHNICAL REVIEW No.76（2008）

24V

SpringSlide shaft

LCR meter

1Hz

Electromagnetic solenoid

PC
Solenoid
Oil

Electrode plates
SensorLubricantMagnet Pump

Fig. 2 Test equipment

Fig. 1 Sensing apparatus for ceramic debris

In the test, oil (Nissan Motor Oil Strong Save X 0W20, 1500 cc) was circulated at a flow rate of 5 L/min
with a magnet pump, and ceramic powder was mixed
into the circulating oil.
The electromagnetic solenoid inserted into thecirculating system was turned ON/OFF once asecond, allowing the capacitance to be measured withan LCR meter. By statistically treating the obtained
capacitance measurements, the sizes and quantity ofcaptured ceramic particles were estimated.

these foreign matters is forced into the space between
both electrode plates, and the distance between theelectrode plates will fluctuate. The distance between
electrode plates is determined by measuring
capacitance across electrode plates, thereby the
maximum size of foreign matters passing between
both electrode plates is estimated. Our sensing
apparatus is unique in that the surface of theelectrodes is coated with insulation, so thatcapacitance measurement is possible without
occurrence of short-circuit, even when conductive
foreign matters such as metal debris are pinchedbetween the electrodes.
The electrodes used on the prototype apparatus
were flat disks measuring 15 mm in diameter. The
surface area of the electrodes is sufficiently large
relative to the sizes of foreign matters, and even when
foreign matters are pinched between the electrodes,
the capacitance is governed only by the distance
between the electrodes.
Consequently, the maximum size of foreign matterspinched between the electrodes can be estimated bymeasuring the capacitance across the electrodes.
A spring installed to the slide shaft on theelectromagnetic solenoid applies a preload between
both electrodes, and when the electromagnetic
solenoid is powered up, the distance between bothelectrodes is adjusted to 5 mm at maximum. In
addition, one electrode is made of aluminum, and its
surface is electrically insulated by anodizing.

2.3 Result of evaluation test for ceramic debris
detection sensor
Fig. 3 provides a graphical plotting of timedependent change in the particle sizes of captured
ceramic debris.
In this test, 1300 seconds or 3000 seconds after thestart of measuring operation, 0.01 g of silicon nitride
featuring mean particle size of 30 µm was mixed into
the oil. In a time span between 4100 seconds and5400 seconds after the start of test, an oil filter that
captures particles larger than 20 µm was inserted in
the piping to remove the ceramic particles from the oil.
Then, the filter was taken out, and alumina powder

2.2 Method for evaluation testµm

80Particle Size

100

60

Si3N4

MixingTime span where
filter remained
inserted
Al2O2

MixingMixing40
200Fig. 2 schematically illustrates the lubricating oil
circulation system used for the evaluation test of oursensor.

Si3N4

0

1000

2000

3000

4000

Time

s

5000

Fig. 3 Experimental result

-70-

6000

7000

Development of a Method for Monitoring Lubricant Deterioration

50μm or higher
40〜50μm
30〜40μm
20〜30μm

#A
#B

#B

#C

#C

#D

#D

#E

#E
0

5

10

15

20

25

80μm or higher
50〜80μm
40〜50μm
30〜40μm
30μm or less

#A

0

30

100

200

300

400

500

600

700

Accumulated frequency %Number of particles(a) Sensor data analysis result(b) Counting result (10cc, with 10μm filter)

Fig. 4 Analysis result of detected data and counting result of sampling oil

measuring 75 µm in mean particle size was mixed into
the oil.
From Fig. 3, it should be understood that after
ceramic powder is mixed, a number of occasions for
capturing foreign matters increases and that virtually
no foreign matters are detected during a time span
where the filter remains inserted in the oil line.
Note also that even when the electrodes are inclose vicinity, the distance between them does notdecrease to zero: in other words, the minimum
detectable particle size is limited by the thickness ofoil film (approximately 10 mm in Fig. 3) formed
between the electrodes. This is because the pressure
for forcing the electrodes together (preload) is as low
as 0.5 N. It will be possible to detect much smaller
particles by adopting a greater preload or providing amechanism to eliminate the oil film between theelectrodes.
Fig. 4 (a) provides the particle size measurements
obtained from 5-minutes measurements (number ofmeasurements: 300 = once/s×300 sec.) in the timespan between time points A through E in Fig. 3.
Also, 10-cc samples A through E taken at thetimings A through E were each allowed to pass a 10
μm-mesh filter, thereby the residual debris on filter
samples were counted and classified, and Fig. 4 (b)provides the actual counts grouped by particle sizes.
The trend in size distribution obtained from detection
result with the sensor shown in Fig. 4 coincides fairly
well with the size distribution of the particles captured
with the filter shown in Fig. 4. Thus we have learned
that the result of detection by sensor reflects thereality. It has been proved that even in a measuring
operation with a very low sampling rate, that is, once asecond, severity of contamination with foreign matterscan be determined by subjecting a plurality ofmeasurement results to a statistical process.

3. Grease state detection sensor 6)
3.1 Detection principle
Our newly developed sensor optically detects theamount of wear particles in grease occurring from aworn bearing. As shown in Fig 5, the sensor
comprises a light emitting element, a light receiving
element and a length of optical fiber. The light fromthe light emitting element is directed to the grease,and based on the intensity of light reaching the lightreceiving element, the quantity of wear debris in thegrease from the bearing is determined. By inserting alength of a flexible optical fiber between the lightemitting element and the light-receiving element, wehave attempted to improve freedom of layout withthese elements.
Fig. 6 is a graphical plotting of the relation between
the quantity of fine iron oxide powder (Fe2O3, red) of
mean particle size 1.5 µm mixed into grease and thelight transmittance of the grease. The lighttransmittance has been defined as a ratio of theoutput from light receiving element when no foreign
matters are mixed in the grease to the output fromlight receiving element when iron oxide is mixed in thegrease. The logarithmic values of iron oxygen content
are linearly proportional to those of light transmittance.

Measurement gap

Light emitting
element

Detection point

GreaseOptical fiberLight receiving
element

Fig. 5 Principle of optical sensor-71-

NTN TECHNICAL REVIEW No.76（2008）

Grease samples were taken from a plurality ofbearings used in two applications, and Fig. 7graphically plots the relation between the iron contents
in greases and light transmittances. It should beunderstood that the logarithmic values of iron content
in grease are linearly proportion to those of lighttransmittance as in the case of Fig. 6 and that the
proportional constant between two sets of logarithmicvalues can vary depending on where the bearing isused. This seems to be because the size and color ofwear debris can vary; therefore, an applicationspecific calibration curve needs to be developed sothat the resultant measurements can be correctly
calibrated.

We have attempted to detect grease being released
from the rolling surface by situating the sensor
detection point in the vicinity of the raceway surface
on the inner ring outer circumference.
Fig. 9 schematically illustrates the construction of
our prototype sensor unit that is intended forincorporation into the test bearing. The detection point
alone is projected in this sensor unit, and the otherportion is enclosed in a sensor cover.

3.3 Evaluation test
A contaminated grease was prepared by mixing fineiron oxide powder (Fe2O3, red) of mean particle size
1.5 µm. The running test was performed while thiscontaminated grease was injected between the rolling
elements, and the test results are plotted in Figs. 10
and 11. Fig. 10 graphically plots the estimated amountof iron oxide detected by our sensor system for 10hours after the start of bearing operation test and Fig.
11 provides a plotting of similar data for 500 hours. In
advance, we prepared grease containing 5 wt% ironoxide. At 3.5 hours (T2 timing) and 100 hours (T5

3.2 Configuration for test sensor unit
Fig. 8 shows a cross-sectional view of a bearing
with a built-in sensor unit. The grease in the bearing isreleased from the raceway surface and is trapped
between the rolling element end faces and the cage,
and then flows outward to the circumferential direction
as the bearing runs 7).

Light transmittance1

Iron oxide
injection point
Sensorunit
Outer ring
Detection
point

Flange

0.1

0.01

0.001

0

0.5

1

1.5

2

Iron oxide content Fe2O3

2.5

φ120

Inner ring
0.0001

3

wt%

Fig. 6 Optical transparency of test grease

Fig. 8 Test bearing with sensor unit

Sensor cover1

Light transmittance

Application A
Application B
0.1

Measuring gap = 0.4 mm

AA
Detection point
Measuring gap: 0.4 mm

0.01

Flange0.001

Light emitting
element

0.00010

0.5

Iron content Fe

1

1.5

wt%

Light receiving
element
View along A-A

Fig. 9 Sensor unit

Fig. 7 Optical transparency of used grease-72-

Development of a Method for Monitoring Lubricant Deterioration

T1: Start of operation

T2: Addition of T3: Rotation speed is T4: Acceleration fromiron oxide
reversed every 2 hours. 800 rpm to 1800 rpm

Detection point

Detected iron oxide content wt%

0.2

0.15

0.1

0.05

(a) Before Examination

0
0

2

4

6

8

10

Time hFig. 10 Experimental result (T1~T4)
Detection point

Detected iron oxide content wt%

T4

T5: Addition of iron oxide

0.2

0.18

0.16

(b) Condition of the attached grease to detection point

0.14

0.12
0

100

200

300

400

500

Detection point

Time hFig. 11 Experimental Result (T1~T5)

timing) after the start of bearing operation, 1 g of thecontaminated grease at a time was added into thebearing. At the T3 timing onwards, the direction ofrotation was reversed once every 2 hours. The
bearing speed in the time span T1 through T4 was 800
rpm, and in each section after T4 was 1800 rpm.
From the test result, it is apparent that until 1 hour
has elapsed after the start of bearing operation, thesensor output level gradually decreases and stabilizes
at around 0.04 wt%. Immediately after iron oxide
powder has been added, the iron oxide content
increases. The change in sensor output level isobserved when the grease flow greatly varies because
rotation direction on bearing is reversed.
Fig. 12 shows various states of grease deposition
around the detection point of the sensor unit during
the test. As the test has progressed, the color ofgrease has changed, indicating that the iron oxide
concentration in the grease around the detection point
has apparently increased. In the situation immediately
after the start of bearing rotation as shown in Fig. 12(c) Condition of the attached grease to detection point
after 100 hours

Detection point(d) After examination

Fig. 12 State of grease around detector-73-

NTN TECHNICAL REVIEW No.76（2008）(b) (a state before the timing T2 in Fig. 10 in whichiron oxide powder was added, the grease has not yet
been thoroughly stirred and grease deposition is not
uniform around the detection point. At the time point
100 hours after the start of bearing operation asshown in Fig. 12 (c) (a state before the timing T5 in
Fig. 11 in which the iron oxide powder was added), a
fresh grease layer appearing reddish in color owing tocontained iron oxide has been formed on the existinglayer of previously released grease. Thus, it should beunderstood that the grease injected at the earlier
phase of the test remains around the sensor cover.
Furthermore, when 500 hours have elapsed after thestart of bearing operation as shown in Fig. 12 (d), thegrease has been more thoroughly stirred comparedwith the state in Fig. 12 (c). However, even at thisstate, the grease has not yet been homogenized. We
have obtained an iron oxide content throughconversion from the sensor output taken on thedetection point having undergone 500 house of test.
Thereby, we have learned that the resultant iron oxide
content level is twice as high compared with an ironoxide content in grease that simulates uniformly
distributed iron oxide powder.

References
1) S. Itomi, H. Ito: Sensor Units for Construction
Machines, Construction Machinery and Equipment,(2008) 9
2) S. Morris, H.E.G. Powrie et al., Electrostatic charge
monitoring of unlubricated sliding wear of a bearing
steel, Wear, 255, (2003) 430-443
3) Railway Technical Research Institute, Organization of
application methods for grease management
standards, Rolling stock diagnostic technologies
research group, Oil analysis subcommittee materials,
6 (2004)
4) Y. Suzuki: Deterioration of grease in use and methodsof determining it, Lubrication, Vol.15, No.7 (1970) 439453
5) T. Takahashi, T. Ishikawa: Detection of ceramic
fragments in lubrication oil using a direct drive
actuator, Japan Society of Mechanical Engineers,
D&D 2007 Lecture No.735, 2007
6) K. Nishikawa, T. Takahashi, 2008 Spring Tribology
Conference Proceedings, Tokyo 2008-5, (2008) 133134
7) Y. Suzuki, S. Sugiyama, Deterioration of rolling stock
axle grease, Lubrication, Vol.19, No.4 (1974) P341346

4. Conclusion
We have developed a unique sensor product thatcan measure the size and quantity of wear debris andforeign matters in a lubricant. This sensing technology
can be used to detect the state of mechanicalcomponents. This newly developed technology can besummarized as follows:(1) We have developed a unique technology
according to which ceramic debris in lubricating oil
are mechanically pinched and their sizes aremeasured. Furthermore, we have established atechnique that can help estimate the amount ofceramic powder mixed into a lubricating oil
through statistical processing of obtained sensing
data.
(2) We have established a technique that can help
calculate the amount of foreign matters mixed into
lubricating grease by an optical sensing techniquefor measuring the resultant light transmittance.
Also, we have incorporated our newly developed
sensor into a bearing, and we have verified that
this arrangement can help estimate the amount ofwear in the bearing.

Photos of authors-74-

Kentaro NISHIKAWA

Tomomi ISHIKAWA

Mechatronics Research Dept.
New Product Development
R&D Center

Mechatronics Research Dept.
New Product Development
R&D Center

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Heat Transfer Analysis of Machine Tool Main Spindle

oshimitsu HIRASAWA*
Yukimitsu YAMAMOTO*

CAE analysis is very useful for shortening development time andreducing the need for development trials. To use CAE effectively, theanalysis results must be accurate. This report describes the use of the
Finite Element Method to estimate the temperature distribution of machine
tool main spindle bearings and the test machine during steady state heattransfer.

"analysis technique for determining the temperature
distribution on an object in a situation where thequantity of heat generated on the object is inequilibration with the quantity of heat dissipated fromthe object (the object is in steady state)".
For example, when a bearing is run under acombination of given load and speed, the temperatureof the bearing will increase for a certain duration after
the start of test. As time elapses, the rate of
temperature rise decreases and finally levels off at aparticular temperature (see Fig. 1). This state isdefined as steady state: where the quantity of heatgenerated by the bearing is equal to the quantity ofheat dissipated from the bearing. Steady state thermal
analysis is an analysis practice for determining thetemperature distribution of an object in steady state.

1. Introduction
Higher running speed has been increasingly needed
for main spindles of machine tools such as machining
centers in order to help achieve improved machining
efficiency. When a main spindle runs at higher
speeds, heat is generated in the bearings that support
it, causing the difference in temperature between the
inner ring and outer ring or the inner ring spacer andouter ring spacer to be greater than expected. If sucha situation occurs, an ideal preload setup will not be
maintained. To address this problem, the effects ofspacer width and cooling efficiency on high-speed
machine tool main spindles need to be more diligently
studied. Previously, an appropriate preload setup
reflecting these considerations has been determinedon the basis of trial and error or past experience. If an
appropriate preload setup can be simulated withhigher accuracy through CAE analysis, bearing
development work will be dramatically accelerated
and the number of test runs necessary can besignificantly reduced. In this paper, we will present ourstudy of finite element method based steady state
thermal analysis for estimating the temperature
distribution of the machine tool main spindle bearingsin our test machine including a comparison of theseresults to measured temperature data.

Steady state

Temperature

Transient state

2. Overview of steady state thermal analysis
Time

2.1 Steady state thermal analysis definedFig. 1 Transient state and steady state

Steady state thermal analysis can be defined as*Product Design Dept. Industrial Sales headquarters-75-

NTN TECHNICAL REVIEW No.76（2008）

temperature, the definitions of which are given in
Table 2.
When considering the actual physical phenomenaof heat transfer, most cases involve emission andabsorption of heat on the surface of an object;
therefore, a heat transfer coefficient needs to be set
for the surfaces of the objects being analyzed. The
specific value for the heat transfer coefficient can bedifficult to determine. It is not a material physicalproperty, and can vary depending on various factors
such as the shape and size of the object as well as
the physical properties, flow velocities, andtemperatures of external fluid (air, oil, etc.). How toestimate heat transfer coefficients is an important but
challenging point in steady state thermal analysis.

2.2 Comparison of stress analysis with steadystate thermal analysis
Table 1 compares features of CAE and finite
element analysis used for stress analysis with thoseused for steady state thermal analysis.
In CAE, an input is a factor that alters the state ofan object. The input in stress analysis is a load orforced displacement, while that in steady state thermal
analysis the input is heat generation (more strictly,
quantity of heat generated per unit time) or a fixed
temperature.
A boundary condition for stress analysis is aconstraint that inhibits the motion of an object.
Boundary conditions considered for steady state
thermal analysis include heat generation, heatinsulation, heat transfer coefficient and fixed
temperature. The most important boundary conditionin steady state thermal analysis is heat transfer
coefficient, which will be described in detail later.
The material physical properties required for stressanalysis include Young’s modulus and Poisson’s ratio,
while the physical property used for steady state
thermal analysis is thermal conductivity, which is ameasure of the ability to transmit heat within thematerial. Note that thermal conductivity is different inconcept from the above-mentioned heat transfer
coefficient though both appear somewhat similar.
The output obtained as a result of stress analysis isa stress distribution while the output obtained as aresult of steady state thermal analysis is thetemperature distribution of an object.

Table 2 Boundary condition of heat transfer analysis

Steady thermal analysis

Load,forced displacement

Heat generation,
fixed temperature

Boundary
conditions

Constraint

Heat generation, heat insulation,
heat transfer coefficient,fixed temperature

Physicalproperty
valuesYoung's modulus,
Poisson's ratio

Heat conductivity

OutputStress distribution

Temperature distribution

Input

Condition where a particular quantity of heat per unit time isbeing generated at the surface of an object; or where thesurface is absorbing a particular quantity of externallygenerated heat per unit time. This condition is used toestablish the heat source.

Heat
insulation

Condition where no heat is generated or absorbed on thesurface of an object. This condition is used when thetemperature at the surface of an object and the temperatureof the surroundings are the same. It is also used where in aphysically symmetrical system the temperatures are thesame on each side of the plane of symmetry.

Heat
transfercoefficient

Condition where heat is transferred through the surface ofan object. The heat transfers from a zone of hightemperature to a zone of low temperature, and the quantityof heat transferred is determined by the temperature
difference between the surface of the object and theexterior of the object and the heat transfer coefficient.

Condition where the temperature on the surface of an
object is fixed regardless of whether or not heat generation/
absorption takes place on that surface. This condition is
Fixed
temperature applied to a surface where the object being analyzed is incontact with an object having very high heat capacity
relative to the object being analyzed.

Table 1 Comparison between stress analysis and heattransfer analysis
Stress analysis

Heatgeneration

2.4 Method for estimating heat transfer coefficient
As mentioned above, the heat transfer coefficient
for an object is difficult to determine. One possible
method for estimating the heat transfer coefficient iscalculation based on a theory from heat transfer
engineering.
A typical sequence of heat transfer coefficient
calculation based on this heat transfer engineering
theory is presented in Figs. 2 and Table 3, andExpressions 1 through 7 1). This sequence describes
calculation of the heat transfer coefficient of the side
face of the rotating inner ring of a bearing.
In the practice of heat transfer engineering,attention is first placed on the presence of flowing fluid(in this case, air) over the surface of an object. Fluid
flow caused by forced movement of fluid is called“forced convection” while flow caused only by

2.3 Challenges to be addressed for steadythermal analysis
Unlike stress analysis, steady state thermal analysis
requires that “certain boundary conditions need to be
set for the entire surface of an object”. As mentioned
earlier, for steady state thermal analysis, there are
four boundary conditions, that is, heat generation,
heat insulation, heat transfer coefficient and fixed-76-Heat Transfer Analysis of Machine Tool Main Spindle

temperature variation of the fluid in the vicinity of thesurface of an object is called “natural convection”.

Next, attention is placed on the velocity (morestrictly, Reynolds number) of the convection system
under consideration. If the velocity is smaller than athreshold value per heat transfer engineering, the fluid
flows in a smooth orderly fashion (laminar flow); if thevelocity is greater than the threshold value, the fluid
flow develops irregular vortices (turbulent flow).
If we can establish demarcation between these twoflow modes, we will be able to calculate the heattransfer coefficient on the surface of a given object
based on the physical properties of air and theobject’s physical parameters including its dimensions.
Let us apply this theory to the rotating ring of agiven bearing. The surfaces on the rotating ring are
moving relative to the surrounding air. This situation isanalytically the same as a system where thecomponent is at a standstill while the air surrounding it
is in motion. Thus, this can be regarded as a system
in a state of “forced convection” with air. Likewise, theair flow occurring over a fixed ring can be assumed to
be in a state of “natural convection”. For components
under natural convection, we need to consider theorientation of the bearing (horizontal or vertical)
because heated air in natural convection flows
vertically upward.
In addition, whether the flow is “laminar flow” or“turbulent flow” is decided based on considerations
including the rotational velocity of the rotating ring.
In Expressions 1 through 7, we have only
presented a calculation process for the side face ofthe rotating ring. Using a similar procedure, we canperform these calculations for the inner and outer
diameter of the rotating ring and for the surfaces of the
fixed ring.
Incidentally, consideration of heat radiation from thesurface of an object is necessary, although thispractice is not explained in this paper.
This method for estimating the heat transfer
coefficient is very refined; however, when this value isactually used for steady state thermal analysis, andthe resulting temperature distribution is compared withactual measured results, acceptable levels of error are
not always attained. The reasons for this may include
error caused by incorrect assumptions made whencalculating the heat transfer coefficient (for example,
when considering natural convection, the surface
temperature on an object must be assumed and thisassumed value should be close to the actual value) oran error related to the environment where actual
measurement is taken (for example, in heat transfer
engineering calculations, the size of the laboratory
room is assumed to be infinite; however, effects fromthe floor, ceiling and walls are unavoidable in an
actual test).

Outer ring
(fixed ring)
Rolling element(ball)
Cage

Revolution

Inner ring(rotating ring)Side face: The heat transfer coefficient on this surface isdetermined by Expressions 1 through 7.

Fig. 2 Cross section of bearing

Table 3 Values for heat transfer coefficient calculation
SymbolDescription

Unit

ρ

Density of air

kg/m3

cp

Constant pressure specific heat of air

J / (kg・˚C)μViscosity coefficient of air

kg / (m・s)λThermal conductivity of air

W / (m・˚C)ωRotational speed of rotating ringrad / sr

Radius of side facem

Expressions describing heat transfer
for side face of rotating inner ring
Dynamic viscosity of air

ν＝

μ
ρ

(m2/s) ………1

Thermal conductivity of air

λ(m2/s) ………2
ρ
・cp
ν
Pr＝
…………………3κ
κ＝Prandtl number

Rotating Reynolds number

Reω＝

ωr 2
………………4νCase where Reω≦3×105 (laminar flow):
Nusselt number Nur＝0.399Reω0.5 Pr 0.43 ………5
Case where Reω＞3×105 (turbulent flow):
Nusselt number Nur＝0.0238Reω0.8 Pr 0.6 ………6
Heat transfer coefficient

H＝

Nur・λ
2r

( W/(m2・˚C)) …7-77-

NTN TECHNICAL REVIEW No.76（2008）

Therefore, in order to further improve the precisionof steady state thermal analysis, we will perform
analysis for a system for which the temperature
distribution has already been measured. Morespecifically, actual temperature measurements on
various areas of the system are prepared in advance,
analysis is performed using a heat transfer coefficientthat was obtained based on the theory of heat transfer
engineering, and the temperature distribution obtained
from actual measurements is compared with thatobtained from theoretical analysis. If the temperature
distribution of the actual measurements does notmatch that of the theoretical analysis, then it will benecessary to vary the heat transfer coefficient suchthat the analysis results more closely match themeasurement results in order to improve the accuracyof the analysis. This process is referred to as “tuning”.

ring caused by heat generation in the bearing, andvariation in preload due to variation in the spacer
width. Using conventional methods, these factors
have been determined based on accumulated test
data, performance data and experience provided by
the customer, or measurements obtained fromdevelopment testing. We have attempted to estimate
these factors using steady state thermal analysis inorder to improve efficiency in performing simulation for
prototype bearings.

3.2 Test rig configuration
Figures. 3-1 and 3-2. illustrate the configuration ofa test rig used for the testing required to performtuning for steady state thermal analysis. The main
spindle is attached to the motor via a coupling. The
positional relation between two angular contact ballbearings in a back-to-back arrangement can be
adjusted by varying the spacer width. Cooling oil flows
through a spiral groove formed circumferentially on
the outer diameter of the housing to absorb heat fromthe bearings. The bearing lubrication system is air-oil
lubrication.

2.5 Significance of steady state thermal analysis
To perform steady state thermal analysis on a new
bearing application, it is necessary to obtain actual
temperature measurements (in other words, we must
perform a series of test runs), and the man-hours
required for testing a new bearing product will not bedecreased. It should be noted that steady state
thermal analysis can be conveniently utilized
downstream of this stage of testing. Once “tuning” hasbeen performed to better define a heat transfer
coefficient for a given bearing application, further
analysis including changes to the system such as
material changes or changes in the quantity of heatgenerated may be performed with greater accuracy,
and without requiring additional tuning. Thus, theadvantage of steady state thermal analysis may besummarized as follows: though tuning of heat transfer
coefficients to better match test results must be
performed first, once this has been completed,analysis can be performed at high precision without
performing additional testing even if new conditions
are adopted.

Fig. 3-1 Test machine
Housing
Jacket cooling groove

3. Application of steady state thermal
analysis to spindle test

Outer ring spacer

3.1 Objective of application to spindle test

Inner ring spacer

At NTN, the design specification of a spindle
bearing is studied in the following manner:
1 According to the required rigidity, accuracy andboundary dimensions, an appropriate bearing partnumber is selected.
2 Relative to an initial preload setting, the operating
preload is estimated.
The factors required for the estimation of 2 are thetemperature gradient across the inner ring and outer

Coupling
Bearings

Fig. 3-2 Cross section of test machine-78-

Heat Transfer Analysis of Machine Tool Main Spindleof the air in the bearings was determined assumingthat the air rotates at a velocity equal to that of therolling elements and cage, and that its temperaturecorresponds to an average of the inner ringtemperature and outer ring temperature. The heattransfer coefficient of the cooling jacket groove wasdetermined by calculating the quantity of heatremoved by the cooling oil flowing through the test rig
and then using tuning to determine the heat transfer
coefficient that matched this amount of heat removal.

The parameters measured in this test are torque,surface temperatures at various locations, the cooling
oil inlet and outlet temperatures, and the room
temperature.

3.3 Analysis modelFig. 4 illustrates a CAE finite element analysis
model that represents our test configuration. The
results of temperature measurements obtained fromthis test showed no significant temperature difference
at locations in the circumferential direction; therefore,
we have developed a 2D axisymmetric model thatsimulates the bearings and peripheral components.
The heat source in this test was defined as the rolling
elements (balls), and the quantity of heat generated
was determined based on actual torquemeasurements performed on the spindle. The
surfaces of the test rig were divided into finite
elements, and based on the size and location of eachelement and the principals of heat transfer
engineering, the heat transfer coefficient for eachelement was determined. The heat transfer coefficient

3.4 Analysis result
Figure 5-1 presents analysis results prior to tuning,
Fig. 5-2 presents post-tuning analysis results, andFig. 5-3 and Table 4 offer comparison of analysis
results with actual test measurements. When the pretuning analysis temperature values are compared withactual measurements, the average temperature
difference of all measuring points is 5.4°C while themaximum temperature difference is 10.7°C. During
the actual test, the average temperature of the test rig
increased from room temperature (28.7°C for the
purpose of present analysis) to 36.0°C; thus, the error
in average temperature increase between the actual
data and the analysis data, is 5.4/ (36.0 – 28.7) =74%. On the other hand, comparing post-tuning
analysis temperatures with actual measured
temperatures, the average temperature difference is
0.8°C, the maximum temperature difference is 1.4°C,
and the error in average temperature increase
between actual data and analysis data is 0.8/ (36.0 –28.7) = 11%. This indicates that the accuracy of theanalysis has been much improved by tuning. Because
the error is very small, we believe the analysis data
correlates to the actual measurement data very well.
Additionally, though not summarized in Table 4, theaverage temperature of the inner ring is approximately
50°C and that of the outer ring is approximately 39°C.
Thus, the temperature difference between the innerring and outer ring is approximately 11°C. With thisHeat transfer coefficientwith jacket cooling
Outer ring side: fixed(heat transfer coefficient withnatural convection)

Rolling elements:
source of heat generation
Inner ring: rotating(heat transfer coefficient with forced convection)

Fig. 4 Analysis model

(˚C)

(˚C)

60

60

50

50

40

40

30

30

Fig. 5-1 Analysis result of pre-tuning

Fig. 5-2 Analysis result of post-tuning-79-

NTN TECHNICAL REVIEW No.76（2008）

Table 4 Comparison between test data and analysis data
ActualAnalysis value Analysis value
Measuring measurement (pre-tuning)(post- tuning)
point
<a>
<b>
<c>

<b>-<a><c>-<a>

A

28.4

32.8

27.4

4.4

−1.0

B

28.5

32.0

27.2

3.5

−1.3

C

27.7

31.3

27.0

3.6

−0.7

D

27.5

32.2

27.4

4.7

−0.1

E

29.6

34.5

28.4

4.9

−1.2

F

39.9

42.3

39.1

2.4

−0.8

G

40.3

44.2

40.0

3.9

−0.3

H

50.3

55.8

50.2

5.5

−0.1

I

50.7

55.8

50.0

5.1

−0.7

J

36.0

46.7

37.4

10.7

1.4

K

36.6

46.8

37.4

10.2

0.8

Average

36.0

41.3

35.6
5.4

0.8

Error inincrease of
averagetemperature

J (Outer ring outside diameter)AEGF H(Inner ring inside
diameter)
I (Inner ring inside diameter)

K※Error in increase of average temperature＝｜Actualmeasurement−Analysis value｜ΣA

Fig. 5-3 Compared points between test data
and analysis data

11

information it is now possible to estimate the reduction
in radial clearance (increase in preload) within abearing based on the bearing diameter and the linear
expansion coefficient of the bearing material.
Likewise, if we obtain the data about the inner ring
spacer temperature and shaft temperature and theouter ring spacer temperature, then we will be able toestimate the increase in the set axial clearance(decrease in preload) based on the linear expansion
coefficients of materials of various assemblycomponents.
Furthermore, by adopting these post-tuningboundary conditions, it is now possible to perform an
analysis operation with changes in initial preload,
bearing speed, spacer width, etc. Fig. 6 and Table 5
provide a comparison between actual test
measurements and analysis values for the same
system but with wider spacers (no tuning under thisset of conditions). It is apparent that the actual values
match the analysis values fairly well.

C K (Outer ring outside
diameter)
B

D

(˚C)
60
50
40
30

Fig. 6 Analysis result with wide spacer

Table 5 Comparison between test data and analysis
data with wide spacer
Measuring
ActualAnalysis Actualmeasurement−
point
measurement value
Analysis value
A

27.3

27.3

0.0

B

27.3

26.9

−0.4

C

26.5

26.6

0.1

D

26.6

27.2

0.6

E

29.2

28.7

−0.5

F

35.8

34.5

−1.3

G

35.4

35.2

−0.2

H

38.1

38.3

0.2

I

38.4

38.0

−0.4

J

31.8

31.3

−0.5

K

30.8

31.2

0.4

Average

31.6

31.4

Error inincrease of
averagetemperature

-80-

0.4

Heat Transfer Analysis of Machine Tool Main Spindle

4. Conclusion
To be able to estimate temperature distributions on
a machine tool main spindle bearing and a test rig, wehave committed to steady state thermal analysis
through finite element method. We have established atechnique that combines a theory of heat transfer
engineering with a parameter tuning technique. As aresult, we are now able to set up an appropriate initialpreload more efficiently for a given machine tool main
spindle bearing.
We will apply this unique technique to otherindustrial fields and gain experience from performing
analysis work in various applications to further
improve efficiency in bearing design.

References
1) Y. Katto: Introduction to Heat Transfer, Yokendo,(1964) P165

Photos of authors

Yoshimitsu HIRASAWA

Yukimitsu YAMAMOTO

Product Design Dept.
Industrial Sales
headquarters

Product Design Dept.
Industrial Sales
headquarters-81-

NTN TECHNICAL REVIEW No.76（2008）[ Technical Paper ]Development of High-Speed Cylindrical Roller Bearings
for Machine Tools

Masatsugu MORI*
Takuji KOBAYASHI*
A newly designed cylindrical roller bearing has been developed sothat the cylindrical roller bearing can operate as fast as the highspeed angular contact ball bearings used for machine tool main
spindles under the condition of air-oil lubrication. The cage riding
structure improves lubrication of the cage-guiding surface at highspeed operation by guiding the cage bore on the air-oil nozzle outside
surfaces. The inner ring made of ceramic prevents excessive bearing
preloading in the radial direction due to thermal and centrifugal
expansion in order to enable high-speed rotation. The ceramic inner ring is compressed radially byring spacers that also work as ribs to reduce the hoop stress generated in the inner ring. The
prototyped bearing with an inner diameter of 70 mm successfully operates at a speed of 3.25 million
dmn, which compares with air-oil lubricated, ultrahigh-speed, angular contact ball bearings formachine tool spindles.

contact ball bearings is about 30% better comparedwith that of cylindrical roller bearings.
Usually, a bearing arrangement for high-speed
machine tool main spindle comprise four rows ofangular contact ball bearing on the front side (greater
shaft diameter) and one to two rows of angular contact
ball bearings or a single cylindrical roller bearing on
the rear side (smaller shaft diameter). In higher-speed
applications with definite position preload systems(these systems boast higher rigidity) there is increased
demand for cylindrical roller bearings on the smaller
diameter rear side (this side is free side). Furthermore,
there have been increasing cases of hybrid machine
tools whose milling main spindles have, on their front
side, not only angular contact ball bearings but also
cylindrical roller bearings in order to enhance rigidity ofthe main spindles.
Therefore, a new cylindrical roller bearing formachine tool main spindle needs to be developed,
and this cylindrical roller bearing needs to feature
high-speed performance coming close to that ofangular contact ball bearings.
Previously there have been many numericalanalysis-based studies in the area of cylindrical roller

1. Introduction
Any machine tool main spindle usually needs to becapable of higher speed and rigidity in order toimprove machining efficiency and machining
accuracy. Efficiency and accuracy in machining are
principally governed by performance of the bearing
arrangement supporting the machine tool main
spindle. A variety of rolling bearing arrangements forsupporting machine tool main spindles are available1),and combinations comprising angular contact ballbearings and cylindrical roller bearings are often used.
Angular contact ball bearings boast high-speed
capability (this capability may depend on the contact
angle) and can carry a load in both radial and axial
directions. In contrast, a cylindrical roller bearing,which carries a load in the radial direction in a linear
contact system, excels in rigidity; however, thisbearing type is inferior to angular contact ball bearingsin the areas of running torque and component
precision. In terms of dmn value (bearing pitch
diameter mm×bearing speed min-1) which is an index
representing high-speed running performance of agiven rolling bearing, the performance of angular*Elemental Technological R&D Center-82-

Development of High-Speed Cylindrical Roller Bearings for Machine Tools

bearings that are run at higher speed. Examples ofthese studies include an investigation of cage to roller
sliding based on 2D static analysis incorporatingknowledge about elastohydrodynamic lubrication 2);
static analysis that also reflects thermal analysis 3); and2D dynamic modeling and analysis efforts4-7). Also,
there has been an experimental study to observe theactual behavior of the rollers and cage of high-speed
cylindrical roller bearings.8). However, we did not know
any previous example of research that has achieved
super high-speed operation with dmn = 3 million orover with air-oil lubrication while limiting the inner ringtemperature to 70°C or lower which is said to be an
upper limit of inner ring temperature for bearings incommercial applications.
We have prototyped and tested a high-speed
cylindrical roller bearing that is lubricated with an airoil lubrication system, and achieved an
unprecedentedly high speed of dmn = 3.25 million.
This report presents this achievement. This speed iscomparable with air-oil lubricated super high-speed
angular contact ball bearing for machine tool main
spindle 9).

To address this problem, we have invented a novel
cage guiding type shown in Fig. 1 (a), based on thehigh-speed cylindrical roller bearing shown in Fig. 1(b) that is suitable for air-oil lubrication, wherein air-oil
mixture ejected from the air-oil nozzle hits the rotating
inner ring ramp and is then directed into the bearing

Air-oil nozzle GuideCage

Oil feedAir-oil nozzle
ring spacer

Oil drain(a) Nozzle outside-surface riding

2. Researches of elemental technologies
for higher bearing speed
2.1 Cage
Currently, an increasingly used material for the cageon high-speed bearings is PEEK (PolyEtherEtherKetone) which is a light-weight resin material boasting
excellent rigidity, heat resistance and hydrolysis
resistance. As to cage guiding types, a raceway ringguiding system is preferable to a roller guiding systembecause of better stability with cage motion in a highspeed bearing operation situation. Further, an outerring land riding system is more advantageous than an
inner ring land riding system because of improved
centrifugal force-induced behavior of lubricating oil.
However, with a bearing having an outer ring landriding system, there is a possibility of direct contact
between guide surfaces, since the roller axial gap can
get excessively small because that the linear
expansion coefficient of a PEEK-made cage is twiceas large as the steel outer ring and additionally theroller axial gap can be affected by centrifugal force.
Assuming the outer ring guide surface can be
regarded as an oil-lubricated small-width dynamic
pressure journal bearing, and in the case of a bearing
whose bore diameter falls in a range of 50 to 100 mm,
we will wish to set the roller axial gap of this bearing
group to 0.1 to 0.2 mm. Nevertheless, there areabove-mentioned issues, and it is difficult to attain an
optimal gap design.

(b) Roller riding(c) Inner ring land riding(d) Outer ring land riding

Fig. 1 Cage guiding types-83-

NTN TECHNICAL REVIEW No.76（2008）

temperature on the inner ring was measured with atelemeter. Table 1 summarizes the basic
specifications for the test bearings and operation
conditions. The tests and experiments presented
below within this paper have been performed withbearings equivalent to cylindrical roller bearing N1014,
and the spindle test rig features the construction
shown in Fig. 2.
The results of high-speed operation test arepresented in Fig. 3. High-speed operation quality ofeach bearing sample is evaluated based on therunning speed of its inner ring at a time point when the
inner ring temperature has reached 60˚C. Fig. 3
illustrates that high-speed operating quality can beranked in the following order (from worst to best):roller riding type, inner ring land riding type, outer ringland riding type and nozzle outside-surface riding
type. In Fig. 3, the difference in the maximum innerring speed reached is relatively small between theouter ring land riding type and nozzle outside-surface

interior along the ramp. Due to the centrifugal force
and surface tension acting on the oil, the oil finallysplashes to the outer circumferential direction at the
top ridge of the ramp, contributing to lubrication.
Compared with a conventional arrangement in which
air-oil jet is directed to the gap between the inner ringand cage, our new arrangement is effective in
improving quietness and reducing air-oil consumption.
The construction shown in Fig. 1 (a) maintains theseadvantages as well as adopts a novel arrangement
where the outside surface of the air-oil nozzle guides
the bore surface of the cage.
Generally, the slip sliding areas of a rolling bearing
are supplied with a sufficient amount of oil and aminimum necessary quantity of oil is fed to the rolling
contact areas of the bearing in order to reduce rollingviscous resistance; this arrangement is important to
reduce torque loss and heat generation within the
bearing to help the bearing to run at a higher speed.
The guide way needs to be supplied with plenty of oil
to constitute a sliding bearing; with our bearing
construction, a portion of oil splashed at the upperedge of the inner ring ramp moves by centrifugal force
from the bearing interior through the roller axial gap tothe bearing end side together with the air ejected fromthe nozzle, thereby the oil is smoothly supplied anddrained (see Fig. 1 (a)). In other words, the guide way
is always lubricated with fresh oil of relatively low
temperature, and hot oil does not remain trapped.
Because the torque loss power is in proportion to the
cube of guide section radius 10), our arrangement ismore advantageous than an outer ring land riding
system. Furthermore, with our arrangement, the roller
axial gap between the steel nozzle and the PEEKmade cage does not decrease to zero even duringhigh-speed bearing operation and the nozzle andcage do not come into contact with each other.
We have actually compared high-speed operating
performance of our prototype bearing having theconstruction shown in Fig. 1 (a) (hereinafter referred
to as "nozzle outside-surface riding type" ) with that ofbearings of other guiding types. The construction in
Fig. 1 (b) shows a roller riding type, Fig. 1 (c) an innerring land riding type and Fig. 1 (d) an outer ring landriding type. Because of the reasons previously
mentioned, the roller axial gap of the outer ring landriding type is greater than that of other guiding types.
Fig. 2 schematically illustrates the construction of ourspindle test rig in which a single-row test bearing andtwo rows of back-to-back angular contact ball bearings(5S-BNT008) support the rotating shaft that is driven
with a built-in motor (an arrangement identical to thatof a high-speed machine tool main spindle). A jacket
cooling arrangement was provided, and theTest bearing

Supporting bearings

Rear side

Front side

Telemeter

Built-in motor

Fig. 2 Section view of spindle test rig

Table 1 Bearing specifications and test conditions inhigh-speed operation test with various cage guiding types-84-

Bearing

Description
Size
Pitch diameter
Raceway ring
Rollers
Cage

Equivalent to N1014HSL
ID φ70×ODφ110×W20
93 mm
SUJ2
φ7×7, Si3N4, 22 rollers
PEEK + CF30 %Testconditions

Initial radaial clearance 0〜2 μm
Bearing lubrication Air-oil lubrication system
ISO VG32
Oil feed from both ends of bearing
0.01cm3/10 min×2
Jacket cooling oil Room temperature ±1˚C
temperature

Development of High-Speed Cylindrical Roller Bearings for Machine Tools

result, the inner ring can fracture.
On our newly developed high-speed cylindricalroller bearing, we have employed a novel technique:ring spacers are press-fitted onto both ends of aceramic inner ring raceway ring to apply in advance aradial compressive stress to the inner ring so thattensile hoop stress during high-speed operation is
mitigated. This can be comparable with a case wherea shrink-fit cylinder can withstand a higher internalpressure compared with an independent cylinder ofsame size. Fig. 4 schematically illustrates theconstruction of our new development: ring spacers arepress-fitted over both shoulders of the ceramic innerring: the fitting mode between the inner ring and theshaft is interference fit and that between the ring
spacers and the shaft is clearance fit. The ring
spacers not only exert radial compressive stress to theinner ring but also function as ribs. Because thermal
expansion and physical density of ceramic are smaller
compared with steel, the initial fitting allowance of theinner ring relative to the shaft can be smaller.
When a ceramic inner ring having a bore diameterof about 50 to 100 mm is run at high speed with dmn =2 million or higher, the recommended fitting allowanceof the inner ring to the shaft should be about 5 µm or
less. In the case of a ceramic inner ring, thenecessary fitting allowance to the shaft will be in arange of 20 to 30 µm to cope with heat-induced andcentrifugal force-induced expansion during high-speed
operation. In other words, use of a ceramic inner ring
will help simplify press-fitting work during the spindle
assembly process. With most steel inner rings inordinary cylindrical roller bearings, their bore istapered as shown in Fig. 1, and the inner ring ispress-fitted onto the shaft, thereby utilizing theresultant elastic deformation, the fitting of inner ring to
the shaft is adjusted by measuring the initial radial
gap. This fitting technique is not valid for a ceramic
inner ring because the Young's modulus of a typicalceramic material is approximately 1.5 times as high asthat of a typical steel material. As shown in Fig. 4, thebore of the inner ring in our bearing design is aNozzle outsidesurface riding
Outer ring landridingInner ring landridingRoller riding
0

5000

10000

15000

20000

Inner ring speed

25000

30000

min-1

Fig. 3 High-speed operation test results with various
cage guiding types

riding type: however, considering the possibility ofcontact between the guide ways and ease of drainingof lubricating oil, the nozzle outside-surface riding type
is more advantageous in higher speed applications.

2.2 Inner ring
Machine tool main spindles require cooling in order
to inhibit thermal deflection. Normally refrigerant iscirculated in a jacket (usually located on the staticside) in order to cool rolling bearings and a built-in
motors on machine tools. If such an arrangement is
employed, the bearing inner ring will be hotter
compared with the outer ring owing partly to heatgeneration from the motor and effects from the heatradiation structure; consequently, in the case of acylindrical roller bearing, its preload in the radial
direction will be greater. Also the effect of centrifugal
force-induced expansion of the inner ring cannot beignored for high-speed operation; as a result, preload
can further increase, and operation may become
impossible due to excessive heat generation. In otherwords, inhibition of excessive preload can greatly
contribute to higher speed operation with a bearing.
Therefore, let us think of a bearing having a steel
outer ring and a ceramic inner ring, the ceramicmaterial features the linear expansion coefficientapproximately 30% and density approximately 40%
compared with a steel material. Use of a ceramicmaterial for rolling bearing elements has long been
proposed 11), and a fairly large number of ceramicrolling elements have already been used. However,when an inner ring is made of ceramic material greatpressure may be applied the inner ring bore due tothermal and centrifugal expansion of the mating shaft
outer surface. This cause the hoop stress which is thevertical stress component along the circumferential
direction on the inner ring to be excessive, and, as a

Ceramic
inner ring
Ring spacer

Fig. 4 Ceramic inner ring and its ring spacers-85-

NTN TECHNICAL REVIEW No.76（2008）

straight hole: thereby the initial radial gap isdetermined by adjusting the bore diameter of the innerring and the outside diameter of the shaft throughmachining. Also, featuring a higher Young's modulus,
ceramic materials are favored as they satisfy higher
rigidity required for machine tool bearings.
Now, let us schematically illustrate how muchdifference can take place on the expansion owing toheat generation and centrifugal force between ceramic(Si3N4) inner rings and steel inner rings. Suppose theinner ring (ceramic or steel) on the cylindrical roller
bearing equivalent to N1014 in Fig. 1 is conformably
fitted over the rotating shaft, thereby the deflection on
inner ring outside diameter (expansion) and maximum
hoop stress occurring on the bore surface of ceramic
inner ring have been calculated, and the calculated
values are plotted in Fig. 5, where the inner ring
speed is set at targeted 35,000 min-1 (equivalent to
dmn =3.25 million) and is plotted as a function of thetemperatures of inner ring and shaft. The maximum
permissible temperatures for the inner ring and shaft
in practical applications seem to fall in a range of 60 to
70˚C. Now, let us assume that this temperature is
60˚C: then the deflection in the outside surface of
ceramic inner ring is about 27 µm smaller than that will
steel inner ring: use of a ceramic inner ring appears to
be positively effective in inhibiting excessively large
preload working onto the inner ring.
The resultant calculated hoop stress is about 160
MPa: we have actually prepared a system illustrated
in Fig. 6 and thermally applied a hoop stress onto theceramic ring that simulates an inner ring toexperimentally determine whether or not the ceramic
ring could withstand fracture. We have adjusted theinitial (room temperature 20˚C) fitting allowance to 5,
10 and 19 µm and heated the ceramic ring and shaft

to 150˚C in a constant temperature chamber. The
ordinates in Fig. 7 represent calculated maximum
hoop stress values relative to the respective
temperatures. As a result, no fracture was observed atleast up to a temperature of 150˚C. From Fig. 7, it
should be understood that the hoop stress at 150˚C
exceeds 350 MPa while the maximum hoop stress of
160 MPa in Fig. 5, where actual operation issimulated, is about a half of the value in Fig. 7. In an
application in Fig. 4 where the ring spacer exerts aradial compressive stress to the inner ring, themaximum hoop stress shifts to a much safer side.
Remember, however, to be aware of the "dimension
effect" when intending to employ a greater bearing
size.

19.4

Hoop stress on ceramic ring MPa

Deflection on ceramic inner ringDeflection on steel inner ring
Stress on ceramic inner ring

120

280
240

100

200

80

160

60

120

40

80

20

40

0

0
120

0

20

40

60

80

100

Inner ring hoop stress MPa

320

140

φ70

Fig. 6 Imitated ceramic inner ring and its fitting to shaft
for hoop stress resistance test

µm
Deflection on inner ring raceway diameter

160

φ40

φ78

Ceramic ring

400

Fitting allowance 5 µm
Fitting allowance 10 µm
Fitting allowance 19 µm

350
300
250
200
150
100
50
0
0

Temperature on inner ring and shaft ˚C

50

100

150

200

Temperature on ceramic ring and shaft ˚C

Fig. 7 Calculated maximum hoop stress of ceramic ringassociated with Fig. 6

Fig. 5 Calculated radial deflection and maximum hoop
stress of inner ring due to heat and centrifugal force
Object bearing: Ref. Fig. 1, Rotational speed: 35000 min-1-86-Development of High-Speed Cylindrical Roller Bearings for Machine Tools

We have evaluated lubricity on the ribs bymeasuring the temperatures on the inner ring andouter ring while the inner ring was running at 25000
min-1. In particular, this bearing is not carrying an axialload, and the test conditions are basically identical tothose summarized in Table 1. Fig. 9 plots theevaluation results with “roller axial gap ratio” meaning
the ratio of this bearing’s roller axial gap to that of thestandard bearing design and “rib angle ratio” standing
for the ratio this bearing’s rib angle to that of thestandard bearing design.
Fig. 9 (a) shows the relation between roller axial
gap ratio and temperatures on the inner ring and outerring where the ring angle ratio is taken as 1.0. There is
no apparent interrelation between roller axial gap andtemperature. Fig. 9 (b) illustrates the relation between
rib angle ratio and temperatures on the inner ring andouter ring, wherein the roller axial gap ratio was variedin five levels in a range of 0.4 to 2.32. Even when the
rib angle is altered, there is no apparent change in thetemperature on the inner ring and outer ring.
Additionally, when the roller axial gap ratio is alteredby approximately 2.0, the resultant variation intemperature on the inner ring and outer ring isapproximately 2 to 3˚C.
To sum up, it is apparent that the effect of roller
axial gap and ribs is small. This is because the area of
each actual roller, from the chamfer to the flat sideplane, is machined into a smooth shape, causing theedge load to be mitigated; at the same time, fluid film
lubrication effect takes place at this area, helping
prevent occurrence of solid contact that can lead tosurface damage. In other words, regardless of thegeometrical conditions of roller axial gap and ribs, thefluid film lubrication effect appears to be dominant.

2.3 Rib
When attempting to realize high speed operatingwith a cylindrical roller bearing, it will be necessary toconsider interference between the roller side planes
and ribs. Therefore, we have experimentallyinvestigated interrelation between the roller-to-innerring rib axial gap and lubricity or that between rib
angle and lubricity as shown in Fig. 8.

Roller axial gap

Rib angle

RollerRing spacer

Fig. 8 Exaggerated view of roller axial gap and rib angle inFig. 4Temperature on inner ringand outer ring ˚C

55
50

Inner ring

45
40

Outer ring

30
0
0

0.5

1

1.5

Roller axial gap ratio(a) Inner and outer ring temperatures vs. roller axial gap ratio□Gap ratio 0.40

○Gap ratio 0.80

◇Gap ratio 1.88

×Gap ratio 2.32

3. Performance verification for our
newly developed bearing△Gap ratio 1.12

Remaining aware of the elemental technology forhigher speed operation described in Section 2, wehave developed a prototype bearing shown in Fig. 10.
As shown in Fig. 4, this bearing comprises a nozzle
outside-surface riding type cage and a ceramic innerring with a ring spacer, wherein the cage is made of
PEEK and the rollers are made of steel or ceramic.
Table 2 summarizes the specifications for our newly
developed bearing and a benchmark bearing having
the structure shown in Fig. 1 (a); as well as the test
conditions applied. The benchmark bearing comprises
a roller riding type PEEK cage and steel inner ring androllers.
Fig. 11 provides the results of measurements fortemperatures taken on the inner ring and outer ringTemperature on inner ringand outer ring ˚C

55
50

Inner ring

45
40

Outer ring

30
0

0

2.0

4.0

6.0

Rib angle ratio(b) Inner and outer ring temperatures vs. rib angle ratio

Fig. 9 Inner and outer ring temperatures vs. roller axial
gap and rib angle-87-

NTN TECHNICAL REVIEW No.76（2008）

relative to inner ring rotating speeds. Fig. 11 (a)provides the resultant temperature measurements on
the inner ring and outer ring of our newly developed
bearings with steel rollers and a benchmark bearing
with steel rollers. The temperature on the inner ringand outer ring of the benchmark bearing samples
began to increase rapidly at a bearing speed of about
22000 min-1 and further operation was interrupted. In
contrast, our newly developed bearing did not exhibit
rapid temperature increase even when operated at a
speed of 35000 min-1 which is equivalent to dmn = 3.25
million and operation at higher speed seemed
possible: nevertheless, since the temperature on the
inner ring exceeded 70˚C (a temporary threshold), thetest run was then aborted. Fig. 11 (b) shows the result
obtained from our newly developed bearing having
ceramic rollers in place of steel rollers. In this casetoo, temperature increased gently: however, because
the temperature on the inner ring exceeded 70˚C at a
speed of 35000 min-1 operation was interrupted.
When the result in Fig. 11 (a) is compared with thatin Fig. 11 (b), it should be understood thattemperature gradient is somewhat gentler with our(a) Overall view of bearing assembly(b) Bearing with outer ring detachedFig. 10 Prototype of developed bearing

Temperature on inner ringand outer ring ˚C

80

Table 2 Specifications of developed and benchmark
bearings and test conditions

Description
Size
Pitch diameter
Benchmark
Inner ring andbearingouter ring
RollersCage

Test
conditions

Equivalent to N1014HSL
ID φ70×OD φ110×W20
93 mm
Si3N4 (spacer ring: SUJ29)
SUJ2
φ7×7, SUJ2 or Si3N4, 22 rollers
PEEK + CF30%, nozzle outsidesurface riding type

60
50
40
30
20

Inner ring of our newly
developed bearing
Inner ring of
benchmark bearing

1000Equivalent to N1014HSL
ID φ70×OD φ110×W20
93 mm

10000

20000

Outer ring of our newly
developed bearing
Outer ring of
benchmark bearing

30000

40000

Inner ring speed min-1(a) Inner and outer ring temperatures of developed bearing
with steel rollers and benchmark bearing

SUJ2
φ7×7, SUJ2 or Si3N4, 22 rollers
PEEK + CF30%, roller riding type80

Temperature on inner ringand outer ring ˚C

Newlydeveloped
bearing

Description
Size
Pitch diameter
Inner ring
Outer ring
Rollers
Cage

70

Initial radial gap
0〜3 μmBearing lubrication Air-oil lubrication system
ISO VG32
Oil feed from both sides of bearing
0.01 cm3/5min×2
Jacket cooling oil
Room temperature ±1˚C
temperature

Temperature on inner ring70

Temperature on outer ring
60
50
40
30
20
10
0
0

10000

20000

30000

40000

Inner ring speed min-1(b) Inner and outer ring temperatures of developed bearing
with ceramic rollers

Fig. 11 Inner and outer ring temperatures vs. rotational speed-88-Development of High-Speed Cylindrical Roller Bearings for Machine Tools

newly developed bearing having ceramic rollers
compared with our bearing having steel rollers, andthat the temperature on the inner ring and outer ringon the former bearing type at 35000 min-1 is 2 to 3˚Clower compared with that of the latter bearing type.
This trend appears to stem from the effect of ceramicrollers: better high speed performance is in principle
derived from nozzle riding cage and ceramic inner ring
having side spacer.
We have investigated various interior areas of ourbearings after testing and have not found any damageon the raceway surfaces on the inner ring and outer
ring, rolling surfaces on rollers, pockets and guide way
on cage, and roller end planes and ribs.
As mentioned above, it has been learned that our
newly developed bearing under air-oil lubrication
condition is capable of dmn = 3.25 million which iscomparable with performance of air-oil lubricatedsuper high-speed angular contact ball bearing.

References
1) NTN Catalogue Precision Rolling Bearings, CAT.
No.2260-II/J (2008) 22-23
2) Poplawski, J.V.: "Slip and Cage Forces in a HighSpeed Roller Bearing," ASME J. of Lubrication
Technology, 94, 2, (1972) 143-152
3) Rumberger, J.H. Filetti, E.G. and Gubernick, D., "Gas
Turbine Engine Mainshaft Roller Bearing-System
Analysis," ASME J. of Lubrication Technology, 95, 4,(1973) 401-416
4) Conry, T.F.: "Transient Dynamic Analysis of HighSpeed Lightly Loaded Cylindrical Roller Bearings, IAnalysis, "NASA Contractor Report 3334, (1981)
5) Conry, T. F.: "Transient Dynamic Analysis of HighSpeed Lightly Loaded Cylindrical Roller Bearings, IIComputer Program and Results", NASA Contractor
Report 3335, (1981)
6) Chang, L.: "Analysis of High-Speed Cylindrical Roller
Bearings Using a Full Elastohydrodynamic Lubrication
Model, Part 1: Formulation," STLE Tribology Trans.,
33, 2 (1990) 274-284
7) Chang, L.: "Analysis of High-Speed Cylindrical Roller
Bearings Using a Full Elastohydrodynamic Lubrication
Model, Part 2: Results," STLE Tribology Trans., 33, 2(1990) 285-291
8) Markho, P.H., Smith, B.V. and Lalor, M.J.: "An
Advanced Apparatus for the Study of Roller and Cage
Slip in High-Speed Roller Bearings," ASME J. of
Lubrication Technology, 103, 1, (1981) 46-54
9) NTN Catalogue Precision Rolling Bearings, CAT.
No.2260-II/J (2008) 160-161
10) N. Soda, Bearing, Vol.3, Iwanami, 35 (1968)
11) K. Rokkaku, K. Nishida, Ceramic roller bearings,
Japan Society for Precision Engineering, Vol.54, No.7,(1988) 1240-1244

4. Conclusion
To improve high-speed performance of air-oil
lubricated cylindrical roller bearings for machine tool
main spindles to a level equivalent to that of angularcontact ball bearings, we have developed uniquecylindrical roller bearings having a novel construction.
With our newly developed bearings equivalent toN1014, we have succeeded in operating the bearing
at dmn = 3.25 million by adopting air-oil nozzle outsidesurface riding type resin cage and ceramic inner ring
having rib side spacer. We believe that realization ofthis high-speed performance is mainly due toimproved lubricity on cage guide way and inhibition ofexcessive radial preload.
NTN will remain committed to technologicalimprovement of machine tools that support
competitiveness of manufacturing through further
sophistication in bearing technology and tribology
technology.

Photo of authors

Masatsugu MORI

Takuji KOBAYASHI

Elemental Technological
R&D Center

Elemental Technological
R&D Center-89-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Angular Contact Ball Bearings that Require a
Minimum Quantity of Grease Supply for Lubrication

Sun-Woo LEE**
Tadaaki MAEDA**

A new grease lubrication system named Minimum Quantity ofGrease Supply (MQGS) lubrication was developed. MQGS
lubrication angular contact ball bearings have minimum quantity
reservoir mechanisms for grease lubrication installed in spacers
in the outer ring. It was developed so that high speed and longlife can coexist in machine tool applications, and it has
succeeded in practical use with a high-speed drive of dmn 1.9×
104 in a fixed-position. The outline of development is introducedin this text.

"BNFS Type sealed super high-speed angular contact
ball bearing (dmn 1.7×104)" 4).
We have recently realized higher speed of dmn
1.9×104 and longer life by combining "new greaselubrication system for machine tool main spindle" 5)that is a maintenance-free grease lubrication system,a sample of which was exhibited in JIMTOF 2006 in
the paper titled "BNFS Type" 4) featuring optimized
internal design; and new grease "SE-1". This paper
describes the advantages of our new greaselubricated angular contact ball bearings and illustrates
results of evaluation tests performed on them.
This new grease lubrication system has come to be
known as "MQGS lubrication" after its acronym(Minimum Quantity base oil of Grease Supply).

1. Foreword
Higher speed is needed for main spindle systems
on machining centers and hybrid machine tools inorder to improve machining efficiency and surface
quality of finished work pieces. To cope with thistrend, NTN has been developing new technologies,
examples of which include "ULTAGE Series precision
bearings for machine tools" 1) that boast advanced
functionality. Two varieties include "ecological air-oil
lubricated bearings for machine tools" 2) dedicated toair-oil lubrication arrangement and "jet-lubricated
MQCJ having inner race cooling arrangement andoptimal minimum lubrication mechanism for rollingsurface" 3) which is a novel jet lubrication system.
Incidentally, an excellent lubrication system has been
more strongly needed that is suitable for improving
work environment and mitigating environmental
impacts. At the same time, there has been a strong
need for high-speed bearing operation and longer
bearing life with a grease lubrication system that canhelp reduce occurrence of oil mist and decrease thequantity of energy consumed by the outside device.
To satisfy these needs, we have previously
developed improved grease-lubricated bearing
systems that include the "BNS Type sealed angularcontact ball bearing (dmn 1.4×104)" and the faster2. Features of MQGS lubrication
mechanism
2.1 Optimized minimum quantity lubrication
mechanism and longer life
To be able to achieve higher speed and longer life
with grease lubrication, it is necessary not only toensure lubricating performance in high-speed
operation, but also to allow the quantity of grease filledin the bearing to supply oil for a prolonged duration
with the lubrication mechanism.

**Application Engineering Dept. Industrial Sales headquarters**Product Design Dept. Industrial Sales headquarters-90-

Angular Contact Ball Bearings that Require a Minimum Quantity of Grease Supply for Lubrication

With our MQGS lubrication mechanism, a greaseprefilled spacer is situated at the bearing front faceand a minimum amount of lubricating oil is supplied to
the raceway surface of the bearing owing totemperature variation on the running bearing, thereby
an outside lubrication oil supply device is eliminatedand the bearing can operate without additional
maintenance work.

Our newly developed MQGS lubrication mechanism
features a unique construction where the base oil
within a quantity of grease in a grease-prefilled spacer
is supplied to the raceway surface of the bearing
during operation. Fig. 1 schematically illustrates atypical construction of our MQGS lubrication angularcontact ball bearing that comprises a bearing, outer
race-fixed spacer and a grease-prefilled spacer.
Additionally, a restrictive lubrication gap is provided
on the outer circumference, in the vicinity of contact
points between the rolling elements and outer race
raceway surface. It has been already learned thatwhen the restrictive lubrication gap falls within a rangeof 0.05 to 0.1 mm, the base oil delivered from thegrease is smoothly supplied to the raceway surface ofthe bearing 5). Therefore, we have adopted a uniqueconfiguration in which the grease-prefilled spacer hasa step on its leading edge so that a restrictive
lubrication gap of 0.05 to 0.1 mm is reliably
maintained. The restrictive lubrication gap isconnected to the grease chamber in the greaseprefilled spacer to allow a necessary quantity(minimum amount) of lubricating oil to be supplied to
the running bearing, thereby realizing a longer bearing
life of dmn 1.90×104 (dn = 1.50×104).

3. Lubrication life with MQGS
lubrication
3.1 Quantity of base oil delivered and run time
To determine the interrelation between the quantityof base oil delivered from grease and the bearing runtime, we have performed base oil delivery confirmation
testing with MQGS lubrication at speed of dmn 1.90×
104 that is the development target for a definite
position preloaded bearing system. Next, we havecalculated the quantity of base oil delivered bycomparing the difference in weight of the greaseprefilled spacer before and after the test. Table 1
summarizes the test conditions applied, and Fig. 2schematically illustrates the construction of the test
spindle. The test bearings used are angular contact
ball bearings with a bore diameter of 100 mm,
installed in a back-to-back duplex configuration (DBset) and then subjected to the test. Table 2
summarizes the bearing specifications, and Fig. 3
illustrates a cross-sectional view of the test bearing.
The heat cycle patterns for the operation test havebeen such that the temperature differences on theouter race of the running bearing were 3˚C (41˚C⇔
38˚C) and 15˚C (41˚C⇔26˚C). The temperature
difference of 3˚C simulates a temperature difference
deriving from control of the jacket cooling temperatureduring continuous bearing operation, and thetemperature difference of 15˚C represents thedifference in bearing temperature between the running
condition and when the bearing is at a standstill.

2.2 Realization of maintenance-free feature
Needs for grease-lubricated machine tool main
spindle bearings have been ever mounting as end
users want higher bearing speed in order to reducemachining time as well as mitigate environmental
impacts related to the bearing. Incidentally, there has
been a conventional approach where a necessary
quantity of grease is force-fed from a device outside
the bearing; however, with this arrangement, supply
lines and auxiliary equipment need to be incorporated,and energy sources for driving these additions areneeded; in other words, this arrangement does nothave benefits such as maintenance-free feature anddecreased system cost.

Outer race fixed spacer

Bearing
Restrictive
ubrication gap

Enlargedview

Grease-prefilled spacer

Grease chamber

Fig. 1 Schematic construction of MQGS lubrication-91-

Radial gap

NTN TECHNICAL REVIEW No.76（2008）

Fig. 4 (a), (b) shows the heat cycle patterns
applied, and Fig. 5 illustrates the interrelation between
the quantity of base oil delivered and the run time. The
absolute value of the quantity of base oil delivered
under a temperature difference of 3˚C differs from that
under a temperature difference of 15˚C,demonstrating that the quantity of base oil delivered
tends to be greater with a greater temperature

difference and a longer run time.
Incidentally, when grease lubrication is adopted formachine tool main spindle bearings, durability life of
20,000 hours or longer is typically required.
To satisfy this requirement, 47 g of grease has been
filled into the grease-prefilled spacer (It is known frombase oil separation test that approximately 60% of theamount of grease is separated. From this fact, weTable 1 MQGS grease life test conditions

Table 2 Bearing specifications for test

Item

Content

Shaft attitude andDefinitenumber of test spindle Horizontal (1 unit)
position
Operating
PreloadPost-assembly gap 0 µm
conditions preloading
15000min-1（dmn1.90×104）
Bearing speed
Jacket cooling temperature

Room temperature ±1˚C

Test bearing

φ100×φ150×24 mm

Contact angle

20˚

Rolling element material

Si3N4

Cage material

Laminated phenol resin

Grease filled in bearing

SE-1, 8 g filled

Grease filled in grease-prefilled spacer

SE-1, 47 g filled

Restrictive lubrication gap

0.07 mm

MQGS lubricated test bearings
44

φ100

φ150

24
20˚

Fig. 2 Schematic construction of test spindle
Fig. 3 Section view of test bearing
4h

1 cycle1215000min-1

Quantity of base oil delivered g

Temperature
difference 3˚C14000min-1
2h

2h

Run time h(a) Temperature rise 3˚C48h1 cycle15000min-1
Temperature
difference 15˚C

0min-11h

1hTemperature difference 15˚C
10Temperature difference 3˚C

8

6

4

2

1h

0

Run time h(b) Temperature rise 15˚C

0

5000

10000

15000

20000

Run time hFig. 4 Heat cycle patternFig. 5 Driving time and amount of base oil delivery-92-

Angular Contact Ball Bearings that Require a Minimum Quantity of Grease Supply for Lubrication

have selected 47 g as a standard amount for abearing whose bore diameter measures 100 mm sothat the grease-prefilled spacer can continue to deliverbase oil for 50,000 hours even at a temperature
difference as great as 15˚C).
Thus, we believe that by adopting the MQGS
lubrication mechanism, we can satisfy application
durability requirements. To determine lubrication life,
we are now continuing life test with bearing samples,
one sample group is being tested at a temperature
difference of 3˚C while the other sample group is
being tested at a temperature difference of 15˚C.

diameters of 100 mm, 70 mm and 50 mm. Fig. 6schematically illustrates the base oil delivery model
test rig used, and Table 3 summarize5s the test
conditions applied.
The radial gap at the leading edge where the baseoil is delivered was adjusted to 0.07 mm, and thegrease-prefilled spacer was filled with 47 g of grease ,thereby allowing determination of the interrelation
between bearing size and quantity of base oildelivered. Based on the outer race temperature of arunning bearing, one heat cycle consisted of 4 hours
at 41°C followed by 4 hours at 38°C (temperature
difference of 3°C ) and this cycle was repeated. 200
hours later, the quantity of base oil delivered wasmeasured, and the result is plotted in Fig. 7.
It is apparent that the absolute value of the quantityof base oil delivered differs between bearing bore
diameters of 100 mm, 70 mm and 50 mm, and further
that the quantity of base oil delivered is greater with alarger bearing bore diameter, nearly in proportion to
the bearing size. From this result, we can determine
the optimal quantity of grease to be filled in thegrease-prefilled spacer for a given bearing size. For
example, a bearing with bore diameter 50 mm has asize 50% as large as that of a bearing having bore
diameter 100 mm, and the quantity of grease in thegrease-prefilled spacer of the former should be 23.5 g(47 g×0.5).

3.2 Bearing size and quantity of base oil delivered
To determine bearing size-specific base oil delivery
quality, we performed a test using bearing sizespecific spacer models suitable for bearing bore

Radial gap
Grease sealing hole

Grease chamber

Fig. 6 Sketch of model test rig for base oil delivery

Table 3 Test conditions of model test rig for base oil delivery
Item

4. High-speed operation test

Contents

0.5

Using our newly developed MQGS lubrication
angular contact ball bearings, we have performed an
operation test simulating bearing operation on an
actual machine tool main spindle. The type of bearing
used is an angular contact ball bearing with a bore
diameter measuring 100 mm. The bearing
specifications, test spindle construction and bearing
cross-sectional view are identical to those presented
in Table 2, Fig. 2 and Fig. 3 respectively,

0.4

4.1 Test results

SpacerSpacers compatible with bearing bore diameters of
φ100 mm, φ70 mm and φ50 mm

GreaseSE-1, 47 g filled

Test equipment

Constant temperature chamber

Temperature change 3˚C（38˚C／2h ⇔ 41˚C／2h）

Quantity of base oil delivered g

Test duration

200 h

To investigate operation characteristics of theminimum amount lubrication bearing, we havemeasured temperature increase on the MQGS
lubrication angular contact ball bearings in a definite
position preloaded arrangement, and then we havecompared the resultant temperature increase with thatof an "HSE Type high-speed angular contact ballbearing" 6). Table 4 summarizes the test conditions
applied Fig. 8 (a) schematically illustrates the HSE
Type, and Fig. 8 (b) graphically plots the comparative
results of HSE and MQGS. We have run the bearings
in a definite position preloaded arrangement and

0.3

0.2

0.1

0

Bearing bore
diameter
φ100mmBearing bore
diameter
φ70mm

Bearing bore
diameter
φ50mm

Fig. 7 Bore diameter and amount of base oil delivery-93-

NTN TECHNICAL REVIEW No.76（2008）

Table 4 High speed test conditions
Item

verified that our bearing with MQGS is stable atvarious speeds up to dmn 1.90×104 without posing
any problem. The temperature increase with ourbearing at dmn 1.90×104, which is the development
target for MQGS, is virtually at the same levelcompared with HSE. Thus, we have proven thatangular contact ball bearings lubricated with MQGS
can run at a high-speed range which was previously
possible only with an air-oil lubrication system.

Contents

Minimum amount0.07mmlubrication gap
Grease-prefilled
Prefilled grease SE-1
spacer data
Quantity of
MQGS
prefilled grease 47g
type
position
Bearing preload Definite
Post-assembly gap 0 µm
preloading
Jacket cooling
Room temperature ±1˚C
temperature
Lubricating oil

ISO VG 32

Oil feed rate

0.03 mL/shot (oil shot intervals; 5 min.)

4.2 Life test

HSE Air flow rate
40NL／mintype
Definite position
Bearing preload preloading
Jacket coolingtemperature

We have subjected the MQGS lubrication angularcontact ball bearings to life testing which has been
executed in the definite position preloading mode anddefinite pressure preloading mode as shown in Table
5. As illustrated in Fig. 9, the heat cycle temperature
difference for the definite position preloaded mode has
been set to 3˚C and one heat cycle in the run patternconsists of 14,000 min-1/2 hrs. and 15,000 min-1/48
hrs. To be able to evaluate the effect of main spindle
attitude, the test has been performed with the twomain spindle attitude modes―vertical attitude modeand horizontal attitude mode.
In case of the definite pressure preloaded system,
the heat cycle temperature was set to 15˚C and oneheat cycle within the run pattern consists, as shown in

Post-assembly gap 0 µm

Room temperature ±1˚C

Air oil

1 cycle48h

15000min-1
Temperature
difference 3˚C14000min-1

Fig. 8 (a) Section view of HSE type1h

20

Run time hFig. 9 Heat cycle pattern

MQGS Type
Temperature increase ˚C

1h2h

HSE Type

15

Table 5 Life test conditions
Item

10

Minimum amountlubrication gap

Grease-prefilled
Prefilled grease
spacer data
Quantity ofprefilled grease

5

0
5

Speed

10

15

Operating conditions0

min-1×104

Fig. 8 (b) Temperature rise between MQGS and HSE typeDefiniteposition
preloading

Definitepressure
preloading*Contents
0.07mmSE-1
47g

Spindle attitude and Vertical (one spindle),
horizontal (two spindles)
number of tests
PreloadPost-assembly gap 0 µm

Speed15000 min-1 (dmn 1.90×104 )Spindle attitude andHorizontal (one spindle)
number of tests
Preload

2.2 kN

Speed

15000 min-1 (dmn 0.90×104 )Jacket cooling temperature

Room temperature ±1˚C＊: Life test has been in progress since JIMTOF 2004.

-94-

Angular Contact Ball Bearings that Require a Minimum Quantity of Grease Supply for Lubrication

2000h

MQGS
(definite position preloading, attitude: horizontal)

MQGS(definite position preloading, attitude: vertical)

MQGS(definite position preloading, attitude: horizontal)

4000h

Currently still running
Currently still running
Currently still running
20000h

MQGS

40000hCurrently still running(constant pressure preloading, attitude: horizontal)

Fig. 10 Life test results

Fig. 4 (b), of 15,000 min-1/48 hrs. and stopping to zero
speed in 1 hr., thereby this cycle has been periodicallyrepeated.
Fig. 10 summarizes the test results. Currently, thelife tests are still in progress. However, we believe that
we can achieve problem-free high-speed operation of
dmn 1.90×104 (possibly the world's highest level witha definite position preloaded system) and longer
bearing life.

References
1) H. Takiuchi and F. Kosugi: "ULTAGE" Series Precision
Bearings for Machine Tools, NTN Technical Review
No.72 (2004) 26-29
2) Y. Akamatsu and M. Mori: Development of Eco-friendly
Oil Jet Lubricated Angular Contact Ball Bearings for
Machine Tools, NTN Technical Review No.72 (2004)
6-103) M. Koyama: Minimum Quantity and Cooling Jet
Lubricated Angular Contact Ball Bearings for Machine
Tools, NTN Technical Review No.74 (2006) 22-25
4) F. Kosugi: dmn 170×104 Sealed High-Speed Angular
Contact Bearing "New BNS Type," NTN Technical
Review No.74 (2006) 22-25
5) Sun-Woo Lee and M. Mori: Development of a New
Grease Lubrication System Machine Tool Main
Spindles, NTN Technical Review No.74 (2006) 8-15
6) NTN Catalogue Precision Rolling Bearings, CAT.
No.2260/J. 74

5. Conclusion
We believe that by adoption of our MQGS
lubrication angular contact ball bearings, machine tool
main spindles can run at higher speeds not possible
with conventional grease-lubricated bearings, and that
we can positively contribute to higher performance,
lower energy consumption and mitigatedenvironmental impacts on machine tools thatincorporate NTN bearings.

Photo of authors

Sun-Woo Lee

Tadaaki MAEDA

Application Engineering Dept.
Industrial Sales
headquarters

Product Design Dept.
Industrial Sales
headquarters-95-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Minimum Quantity and Cooling Jet Lubrication
Single-row Cylindrical Roller Bearings

Futoshi KOSUGI**
Susumu NOJIMA**NTN has developed an angular contact ball bearing withminimum quantity and cooling jet (MQCJ) lubrication. A new jet
lubrication method with an inner ring cooling system and an
optimally minimized quantity lubrication mechanism for the
raceway were adopted. A new product that has applied MQCJ
lubrication to a single row cylindrical roller bearings wasdeveloped. We introduce this single row cylindrical roller bearing
with the new jet (MQCJ) lubrication method.

lubrication angular contact ball bearings can beapplied. Fig. 1 schematically illustrates such alubrication system 1), 2).
With this lubrication mechanism, one common oil
supply device supplies oil to the bearing section andthe jacket cooling section, and the used oil isrecovered with a scavenge pump. Since the jacket
cooling oil also functions as the bearing lubricating oil,
auxiliary equipment to the bearing system is simplified.

1. Introduction
Compared with conventional jet lubrication systems,
our MQCJ lubrication system boasts much decreased
power loss. In the 23rd Japan International Machine
Tool Fair held in 2006 (JIMTOF 2006), an actual
example of a MQCJ lubrication angular contact ballbearing was installed on a machining center main
spindle and its performance was demonstrated inmachining operation. Recently, we have adopted the
MQCJ lubrication system to cylindrical roller bearings,
thereby realizing super high-speed operation withmaximum dmn of 3.35 million (practical dmn of 3
million) with oil lubrication. By utilizing MQCJ
lubrication, a cylindrical roller bearing can beincorporated into the rear side (free side) of a main
spindle, helping simplify the main spindle structure.
This paper hereunder presents the features of the"Minimum Quantity and Cooling Jet (MQCJ)
lubrication single-row cylindrical roller bearings" andthe result of evaluation testing for these bearings.

2.2 Specifications of MQCJ lubrication
cylindrical roller bearings
The MQCJ lubrication system is a jet lubrication
system that is capable of cooling the inner race andhas a minimum quantity lubrication mechanism for the
rolling surface. On our MQCJ lubrication angularcontact ball bearings (Fig. 2), a jet of lubricating oil isdirected to the inner race scoop and lubricating oil
adheres to the inside surface of scoop, and is thentransferred to the conical surface outside of the innerrace by centrifugal force and surface tension, with theresult that only a small quantity of lubricating oil is fed
into the bearing.
Since the lubrication space defined by the conicalsurface and the outer race spacer limits the quantity of
lubricating oil fed into the bearing, a larger portion of
lubricating oil takes part in cooling of the inner raceand only a small portion of lubricating oil flows through2. Features of MQCJ lubrication
2.1 System overview
To our Minimum Quantity and Cooling Jet (MQCJ)
lubrication single-row cylindrical roller bearings, alubrication system identical to that for MQCJ**Application Engineering Dept. Industrial Sales headquarters**Product Design Dept. Industrial Sales headquarters-96-

Minimum Quantity and Cooling Jet Lubrication Single-row Cylindrical Roller Bearings

Filter
Cooling oil supply Ordinary jacket cooling system
deviceJacketrefrigeration
Oil recovery tank

Scavenge pump

Fig. 1 MQCJ lubrication system

spacer, the lubrication space can vary while thebearing runs: as a result, the lubrication space can get
narrower (Fig. 3) or wider (Fig. 4) owing to elongationon the shaft.
Because variation in the size of lubrication space
can affect the quantity of oil passing through thespace, we have invented a horizontal space design(Fig. 5) to prevent variation in the size of lubrication
space owing to elongation of the shaft. By adopting
the horizontal space design, we can avoid adverse
effects from variation in the size of lubrication spacethat results from bidirectional shaft elongation.
Though based on ULTAGE Series single-row
cylindrical roller bearings (roller guided cage), our
MQCJ lubrication single-row cylindrical roller bearings
adopt an inner race guided cage to stabilize rotation of
the cage in order to run at a higher speed range. Witha conventional design, lubricating oil was supplied
from one side only; as a result, lubrication on the roller
end faces and inner race ribs on the side opposite tooil supply can be unstable. In contrast, with our new
bearings, lubricating oil is supplied from both sides: asa result, all the areas between the roller end faces andLubricating
oil

Optimal lubricating
oil flow control

Scoop

Fig. 2 MQCJ lubrication angular contact ball bearing

the bearing, thereby resultant power loss will be
reduced 2).
Incidentally, our cylindrical roller bearings arebasically intended for use on the rear side: therefore,
we should remember that when a main spindle isrunning, the shaft elongates to the rear side (free side)owing to heat generated.
Lubrication space may be defined by the inner race
conical surface and the outer race spacer (as in caseswith angular contact ball bearings). However,depending on the installation orientation of the nozzle

Gap between inner race and spacer will shrink.

Gap between inner race and spacer will expand.

Direction of shaft elongation

Direction of shaft elongation

Fig. 3 change of lubrication space 1

Fig. 4 change of lubrication space 2-97-

NTN TECHNICAL REVIEW No.76（2008）

Lubrication
space
ScoopsFig. 5 MQCJ lubrication cylindrical roller bearings

minimum quantity of lubricating oil into the bearing.
Compared with the single-sided lubrication nozzle
configuration, the double-sided lubrication nozzle
configuration is capable of much higher bearing speed(maximum dmn 3.35 million).
Incidentally, in tests with air-oil lubrication and asingle-sided lubrication nozzle configuration,
lubricating oil (VG32) was fed at a rate of 0.02 mL/10
min, thereby it has been learned that both temperature
increase and power loss are more significant with
MQCJ lubrication. This is because the quantity of oil
fed into both bearings is greater with MQCJ
lubrication, and the effect of stirring resistance in the
bearings is greater.
Fig. 11 graphically plots the results of passing oil
flow measurements on bearings with conventional jet
lubrication and MQCJ lubrication, Fig. 12 illustrates
measurement positions for passing flow and nonpassing flow relative to quantity of lubricating oil
supplied. With conventional jet lubrication scheme, thehorizontally situated lubrication nozzle aims at alocation between the cage and bearing ring, withapproximately 50% of lubricating oil passing throughthe bearing. On the other hand, with MQCJ
lubrication, lubricating oil is injected to the scoops on
the inner race and the horizontal gap is regulated by
the spacer and stepped inner race, thereby thequantity of passing lubricating oil at higher bearing
speed is as low as approximately 10%.
Results for air-oil lubrication with a double-sided

inner race ribs are smoothly lubricated and the innerrace is more effectively cooled, thereby increase of
preload is inhibited and stable high-speed operation is
ensured.

2.3 Test data
Table 1 summarizes the test conditions applied, andFig. 6 schematically illustrates the test spindle used.
The test bearing was based on the N1011HSR(bore diameter 55 mm) belonging to the ULTAGE
Series high-speed cylindrical roller bearing family. This
bearing size was based on the assumption that thefront main spindle bearings were comprised of dia. 70
mm MQCJ lubrication angular contact ball bearings in
definite position preloading configuration. The test
spindle used was a vertical test rig driven by a built-in
motor.
The support bearing unit consisted of a pair ofangular contact ball bearings in DB configuration and
was air-oil lubricated. Figs. 7 and 8 plot the test
results based on samples with a single-sided
lubrication nozzle, and Figs. 9 and 10 are plotted basedon a double-sided lubrication nozzle configuration. On
both single-sided and double-sided lubrication nozzle
configurations, temperature increase is small andpower loss is low (including power loss at rearbearing) with MQCJ lubrication compared withconventional jet lubrication. This is due to theeffectiveness of the MQCJ lubrication system that
positively cools the bearing (inner race) and supplies a

Test bearing

Table 1 Test conditions
Item

Content

Test bearing

N1011HSR basis (φ55 mm×φ90 mm×18 mm)

Support bearing

5S-2LA-HSL007DB (air-oil lubrication)

Main spindle attitude Vertical
Target assembly gap 0 µm
Spacer nozzle

Nozzle diameter φ1 mm, two positions (180-degrees spacing)

Oil flow rate

1〜5L／minLubricating oil

VG1.5（VG32）

Temperature of
lubricating oil supplied 23〜24˚C (setting: 20˚C)
Jacket coolingSupport bearing

Fig. 6 Schematic constructionof test spindle

Yes (5 L/min)-98-

Minimum Quantity and Cooling Jet Lubrication Single-row Cylindrical Roller Bearings

This appears to be because that the rollers and cage
generate an air curtain, and, as a result, a proportionof lubricating oil not fed into the bearing increases andthe centrifugal force helps increase the quantity of
lubricating oil released from inside the bearing,
thereby the stirring resistance decreases.
Note also that the temperature at lower speed range
is low with MQCJ lubrication and jet lubrication, eachinvolving greater oil feed rate compared with air-oil
lubrication. This appears to be because a large
quantity of oil cooled by MQCJ lubrication or jet[Single-sided lubrication nozzle configuration]
Power loss kW[Single-sided lubrication nozzle configuration]
Temperature increase on outer race ˚Cnozzle configuration are shown in Figs. 9 and 10 ;wherein like in the case of MQCJ lubrication, the test
was performed while lubricating oil (VG 1.5) was fed
into the bearing at a rate of 0.5 mL/min (maximum
feed rate for air-oil lubrication unit).
The oil feed rate with air-oil lubrication issignificantly low compared with MQCJ lubrication:
however, temperature increase at lower speed range
is very high with air-oil lubrication. This seems to result
from stirring of lubricating oil. Note, additionally, that
there is temperature drop at the middle-speed range.

40
35
30
25
20
15
10
MQCJ (2.5 L/min)
Conventional jet (2.5 L/min)
Air-oil (0.02 mL/10 min)

5
0
5

10

15

20

25

Speed

30

35

40

45

50

8
MQCJ (2.5 L/min)7

Conventional jet (2.5 L/min)
Air-oil (0.02 mL/10 min)

6
5
4
3
2
1
0
5

10

[Double-sided lubrication nozzle configuration]
Power loss kW[Double-sided lubrication nozzle configuration]
Temperature increase on outer race ˚C

MQCJ (5 L/min)
Conventional jet (5 L/min)
Air-oil (0.5 mL/10 min)

25
20
15
10
5
0
5

10

15

20

25

30

35

40

45

50

Speed ×103 min-1

30

35

40

45

50

40

45

50

8
MQCJ
（5L/min）

7

Conventional jet (5L/min）
Air-oil (0.5mL/10min）

6
5
4
3
2
1
0
5

10

15

20

25

30

35

Speed ×103 min-1

Fig. 9 Outer race temperature

Quantities of passing oil and non-passing oil
ml/min25

Fig. 8 Power loss

40

30

20

Speed ×103 min-1

Fig. 7 Outer race temperature

35

15×103 min-1

Fig. 10 Power loss
Lubricating oil (one side only)

4000
3500

MQCJ (quantity of non-passing oil)
MQCJ (quantity of passing oil)

3000

Conventional jet (quantity of non-passing oil)
Conventional jet (quantity of passing oil)

2500

[Single-sided lubrication
nozzle configuration]
Oil feed rate 2.5 L/min

2000
1500
1000
500
0

5

10

15

20

25

30

35

40

Speed ×103 min-1

Quantity of
non-passing oil

Fig. 11 Amount of oil that passes in bearing

Quantity ofpassing oil

Fig. 12 Measurement position-99-

NTN TECHNICAL REVIEW No.76（2008）

Temperature increase on outer race ˚Clubrication removes heat energy that is greater thanheat derived from stirring of the oil.
The graphical plotting in Fig. 13 illustrates the effectof varied oil feed rate on temperature rise at the outer
race for a double-sided lubrication nozzle with MQCJ
lubrication. When the feed rate of lubricating oil isincreased, the temperature increase on the outer raceis mitigated: the cooling effect of the lubricating oil isgreater with higher bearing speed.
Also, the graphical plotting in Fig. 14 allows us todetermine variation in the power loss. It should beunderstood that when the feed rate of lubricating oil isincreased, the power loss increases. (In both Figs. 13
and 14, values obtained from air-oil lubrication aresuperposed and circled in red for comparison.)From these findings, it is apparent that with MQCJ
lubrication, the relation between oil feed rate andpower loss conflicts with the relation between oil feed
rate and bearing cooling effect. Therefore, we need todefine lubrication conditions that best suit the intended
bearing operating conditions.

3. Conclusion
Our MQCJ lubrication single-row cylindrical roller
bearings can be used as a rear-side bearing on
machine tool main spindles whose front-side bearing
comprises a MQCJ lubrication angular contact ballbearing in definite position preloading mode.
NTN has already developed an angular contact
bearing arrangement that achieves high-speed
operation of 40,000 min-1 with MQCJ lubrication, using
bearings with bore diameter of 70 mm -which arearranged in DB set configuration and are provided withdefinite position preload. Since our newly developed
bearings are intended for use on the rear-side of thisbearing arrangement, the maximum speed of our new
bearing design is as high as 40,000 min-1; thus our new
bearing design is capable of commercial high-speed
operation of dmn 3 million.
Note that our MQCJ lubrication system is essentiallya jet-lubricated system, and that main spindle
construction involving an MQCJ lubrication system
requires a much enhanced labyrinthine structure in
addition to the system configuration described
previously in the system overview section.
Notwithstanding, we believe that our new development
helps increase possible options for selecting bearing
and lubrication arrangement for bearings that support
super high-speed machine tool main spindles. We hopethat NTN's new lubrication system helps assist in
realizing further sophistication of machine tool main
spindles.

50
dmn 3.35 million
dmn 2.98 million
dmn 2.24 million
dmn 1.86 million
dmn 1.49 million

45
40
35
30
25
20

References

15101) Y. Akamatsu and M. Mori: Development of Eco-friendly
Oil Jet Lubricated Angular Contact Ball Bearings for
Machine Tools, NTN Technical Review No.72 (2004) 6
2) M. Koyama: Minimum Quantity and Cooling Jet
Lubricated Angular Contact Ball Bearings for Machine
Tools, NTN Technical Review No.74 (2006) 22

Air-oil

5
0
0

1

2

3

4

5

Oil feed rate (double-sided lubrication nozzle configuration)
L/min

Fig. 13 Outer race temperature6
dmn 3.35 million
dmn 2.98 million
dmn 2.24 million
dmn 1.86 million
dmn 1.49 million

Power loss

kW

5
4

執筆者近影

3
2
1

Air-oil
0
0

1

2

3

4

5

Oil feed rate (double-sided lubrication nozzle configuration)
L/min

Fig. 14 Power loss

-100-

Futoshi KOSUGISusumu NOJIMA

Application Engineering Dept.
Industrial Sales
headquarters

Product Design Dept.
Industrial Sales
headquarters

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Introduction of 3D Electronic Catalog
for Precision Machine Tool Bearings

Takehiko UMEMOTO*
Machine tool makers use 3D CAD data in CAE to reduce
development time and conduct advanced analysis, which
may include thermal analysis and/or structural analysis forhigh precision applications.
This document introduces 3D CAD data (i.e. electronic
catalog) for precision bearings and shows examples of 3D
CAD analysis .

thereby resulting in a decrease in the expected
number of prototype bearings required.
This paper presents the configuration of NTN’s 3Delectronic catalog that was recently released, andprovides example applications for using the 3D data.

1. Introduction
Having been developed as precision bearings formachine tools, NTN ULTAGE Series bearing products
are used in various areas on main spindles and feedsystems of machine tools. Their high precision andadvanced functionality have been highly appreciatedby machine tool manufacturers. The term “ULTAGE”
was created from a combination of “ULTIMATE”(meaning last in progression) and “STAGE”. This
name is meant to convey NTN’s goal that this series of
products are unsurpassed in a range of applications.
In order to meet this target, the ULTAGE Series
bearing products are being produced in a wide rangeof sizes.
Recently, NTN developed a 3D Electronic Catalog of
ULTAGE Series Bearing Products for Machine Tools(3D CAD Data) and has started providing it to machine
tool manufacturers (reported in the Nikkei Sangyo
Shimbun on p. 15 of the April 22, 2008 issue).
3D CAD helps customers clearly understandbearing design features. This allows design,
manufacturing and sales to easily communicate andcollaborate during the development phase. For thisreason, 3D CAD systems are being adopted by anincreasing number of manufacturers.
Additionally, 3D CAD data can be used to provide
information for thermal analysis, stress analysis,
structural analysis and other types of analysis. 3D
CAD data also allows designers to analyze various
areas on main spindles and associated structures,

2. 3D Electronic catalog interface
The 3D electronic catalog is provided to users in theform of a CD-ROM that includes: 1 2D and 3D CAD
data, 2 a user’s guide for the CAD data, 3 aprecision rolling bearings catalog (in PDF format), 4
access to “mt-tecnos” (a web site dedicated toprecision bearings technology for machine tools), 5 auser’s guide for “mt-tecnos” and 6 access to NTN’s
home page. Fig. 1 below shows the starting screenthat appears on the computer screen after installationof the CD-ROM.

Fig. 1 Activation screen for CD-ROM*Product Design Dept. Industrial Sales headquarters-101-NTN TECHNICAL REVIEW No.76（2008）

For detailed information about these features, refer
to user’s guides 2 and 5 on the CD-ROM.

Figs 3, 4 and 5 show examples of screens displaying
different levels of the CAD data file structure.
By clicking on the icon for a particular bearing
series, the user can access the corresponding 3D
CAD data.

(1) Structure of 3D CAD data
The “3D Electronic Catalog for Precision Machine
Tool Bearing Products ” contains data for angularcontact ball bearings and cylindrical roller bearings
used on main spindles and feed systems of machine
tools. The CD-ROM has 3D CAD data for a total of1329 bearing base sizes that include 1039 angularcontact ball bearings (including those with ceramicballs), 212 cylindrical roller bearings and 78 ball screw
support bearings.
Table 1 summarizes all bearing series stored on the
CD-ROM.
The CAD data is displayed on the computer screenin the file structure format shown in Fig. 2.

Click.

Table 1 Bearing series stored on CD-ROM

Fig. 3 Top screen

Angular contact ball bearings: 1039 base sizes
7805C〜7834C， 7900U〜7934U

Standard

7000U〜7040U， 7200C〜7226C

High-speed

HSE910〜HSE934，HSE010〜034

Axial load carrying
For grinding machines
Sealed standard
Sealed high-speed

HTA920U〜HTA964U
HTA010U〜HTA064U
BNT900〜BNT913，BNT000〜BNT014
BNT200〜BNT216
7900〜7910LLB

Click.

7000〜7010LLB
BNS910〜BNS920LLB

BNS009〜BNS020LLB
Ball screw support bearings: 78 base sizes

Ball screw
support bearings

BST17x47〜BST60x120

7000HT〜7002HT，7203HT〜7208HT
Cylindrical roller bearings: 212 base sizes

Standard double-row

NN3005〜NN3036

Standard single-row

N1006〜N1032

High-speed double-row

NN30O5HS〜NN3026HS

High-speed single-row

N1011HSRT6〜N1020HSRT6

Fig. 4 Precision bearing screen

Select a bearing
type, and thenclick.

Fig. 2 CAD data file structure
Fig. 5 Precision bearings icons-102-Introduction of 3D Electronic Catalog for Precision Machine Tool Bearings

After clicking on the appropriate icons, the monitor will
eventually display the bearing data as shown in Fig. 9.
Once a bearing base sizes is selected, then thescreen will show:
1 Bearing boundary dimensions (section A)
2 3D CAD data (section B)
3 Bearing bill of materials (section C)
The 3D CAD data shown in item 2 is saved to the
CD-ROM so it can be downloaded to the user’s
computer. The different formats available on the 3D
Electronic Catalog are listed in Fig. 10.
The available file formats (SAT, STEP, IGES, DXF,
etc.) are used to import the data in to various CAD
systems (Solidworks, Inventor, OneSpaceModeling,
Solid Edge, etc.).
When saving these files, the user will obtain the
CAD data complete with constraint conditions, i.e. therelationships between parts and assemblies aremaintained. The user should be sure to select the fileformat that works best with the CAD software currently
being used.

(2) Accessing 3D CAD data
After selecting the appropriate bearing icon,
corresponding bearing part numbers can be selectedto access their 3D CAD data. An example of thisprocess is illustrated in Fig. 6 through Fig. 10.
For example, suppose data is required for the
standard angular contact ball bearing base sizes
7015US (IDφ75 mm×ODφ115 mm×W 20 mm).

Click.

Fig. 6 Angular contact ball bearing icon
C
A

B

Click.

Fig. 9 Example of standard angular contact ball bearing
p/n 7015UC
Fig. 7 Standard angular contact ball bearing icon

Copying
3D CAD
data

Click.

Fig. 8 70U/70 series icon

Fig. 10 CAD data formats available-103-

NTN TECHNICAL REVIEW No.76（2008）

Also, for information about the software in a specificlanguage, NTN Engineering should be contacted fortechnical assistance.

Oil hole

Shoulder height

3. Typical applications
Nozzle target
Position

A 3D CAD image helps everyone easily visualize
and understand the bearing design. Thus, 3D CAD
imaging is expected to reduce the design time for a
new product as well as the workload for the wholedevelopment process through commercialization of
the product (see items 1 ~ 4 below);
1 Design Confirmation
2 Application to Analysis
3 Reduction in Prototype Cost
4 Data Management
It is getting more common to check for possible
interference and perform other various analysis to
optimize the design prior to prototype fabrication. As aresult, both time and cost have been reduced for
prototypes and testing.
Examples of activities NTN is undertaking
associated with items 1 and 2 are described below.

Shoulder heightFig. 11 Angular contact ball bearing with spacer

Design Confirmation
1 Confirmation that the oil nozzle
Oil holeon the spacer lines up with the
給油穴
oil feed/drain piping
in the肩高さhousing
2 Ensure proper location of
spacer positioning pins
3 Verification of oil feed/drain
piping in the housing
4 Detection of interference between
the bearing, spacer and adjacentcomponents
ノズル狙い位置
5 Determination of bearing andspacer width surface contact length

肩高さ(1) Design Confirmation
One major objective of 3D CAD is to assist thedesigner in detecting potential interference between
adjacent components in order to prevent a deficientdesign.
By reviewing the 3D CAD data provided on the CDROM, the dimensional issues described below can bechecked when the bearing is being specified for an
application:
1 Verification of appropriate shoulder height forbearing inner and outer ring when compared to thedimensions of the shaft and spacer.
2 Confirmation that the oil nozzle on the spacer lines
up with the oil feed/drain piping in the housing.
3 Ensure proper bearing position with respect to the
lubricating oil jet ejected by the nozzle.
Fig. 11 shows a typical 2D arrangement of anangular contact ball bearing and a bearing spacer withan oil feed nozzle. A 3D model allows the user to view
the bearing from various angles and confirm theproper position of the nozzle tip with respect to the oilhole/drain hole (see Fig.12).

Fig. 12 3D validation

by loading graphic data into analysis softwareNOTE), it
is possible to predict potential issues and develop
countermeasures to increase bearing life.
NOTE): Analysis software is not included on this electronic
catalog CD-ROM.

■Example of Mode Analysis
An example of the simplest analysis using 3Dgraphic data for a bearing is calculation of its natural
frequencies.
Fig. 13 illustrates deep groove ball bearing partnumber 6208 (IDφ40×ODφ80×W 18 mm). Table 3
summarizes the natural frequencies of this bearing.
Table 3 shows the fundamental frequencies for the
inner and outer rings from the first harmonic throughthe fifth harmonic. When reviewing the calculated
values only, it is extremely difficult to imagine themode diagram for a particular harmonic. In contrast,with a 3D image for a given harmonic, it dramaticallyhelps visualization of the mode image.
For example, 6208 mode diagrams for the first(2) Application to Analysis
Higher precision bearings are being made to endurewith applications running at higher speeds. Also, toreduce the number of sample test runs, thermal
analysis, stress analysis and structural analysis are
now being performed during the design phase.
By performing stress analysis and thermal analysis-104-

Introduction of 3D Electronic Catalog for Precision Machine Tool Bearings

harmonic to third harmonic of the inner ring are shownin Table 4. By reviewing these diagrams, it is clearthat the pattern of the first harmonic is a simple oval
shape; the pattern of the second harmonic is wavy inInner ring

the axial direction; and the pattern of the third
harmonic exhibits a basic triangular shape.

■Structural Analysis Example
Structural 3D analysis is often used to investigate
the stress and displacement that occur when an
external load and rotation-induced centrifugal force
are applied to a bearing. An example of this analysis
is described below, referencing the structure shown in
Fig. 14. This structure is comprised of a rhomboidal
bearing housing with a self-aligning roller bearing
installed into its bore. For simplicity, the diagramshows the bearing outer ring only (light blue portion).
An axial load Fa is applied to the bearing andbearing housing via the main spindle, and a radialload Fr is applied to the upper portion of the housing.
As a result, the load is transferred to the bearing
housing via the main spindle, inner ring, rolling
elements and outer ring.
The stress analysis of this bearing and housing
assembly is described below.
First of all, it should be mentioned that for the loads
acting on the outer ring and housing, the loading ofeach rolling element (spherical rollers: 30 rollers/row×2 rows) was calculated in advance. Using thesecalculated loads, structural analysis for the outer ring
was performed.
(When planning to perform a detailed investigation
as shown in this example, NTN should be contacted
for the calculation results on rolling element loading
and the spring constant of the rolling elements.)

Outer ring

View from A(axial direction)
View from B(radial direction)

Cage

Fig. 13 6208 Deep groove ball bearing
Table 2 Inner ring and outer ring properties
Young' modulus

Poisson’s ratio

Specific gravity

208000MPa

0.3

7.8

Table 3 Fundamental frequencies
Harmonic

Inner ring

Outer ring

1

6727

2526

2

9140

4475

3

18205

7042

4

23313

12994

5

33011

13217

Table 4 Inner ring mode chart
Harmonic

View from A(axial direction)

View from B(radial direction)

Bearing housing
Outer ring

Fr

1

Fa

2

The bearing housing isbolted down to a baseplate.

Fig. 14 Analysis example

3

-105-

NTN TECHNICAL REVIEW No.76（2008）

Fig. 15 illustrates the stress distribution of the
bearing housing.
The red portions of this diagram indicate the highest
stresses. By optimizing the shape and wall thicknessof the housing flange sections, the calculated stresslevels of these portions is reduced.
Fig. 16 shows a view of the housing fromunderneath: this diagram reveals the stress caused byindividual rollers (see arrows) on the housing bore
surface.
Figs. 17 and 18 exhibit the amount of deformationof the bearing and housing.
The amount of bearing and housing deformation
does not pose a problem. However, the stress
distribution does need to be studied carefully for an
extreme case. When a sleeve is mounted to the innerring, it can be deformed and displaced by the applied
load and centrifugal force, causing it to come incontact with the housing bore surface.

■Thermal Analysis Example
Finally, an example of thermal analysis is presented
below.
When the temperature distribution on and around a
shaft is relatively constant, 2D thermal analysis isoften used.
For the following example, 2D thermal analysis was
performed on an NTN high-speed test rig, and theanalysis result was been compared with the test datafrom an NTN bearing.
(For more detail, refer to the “Heat Transfer
Analysis of Machine Tool Main Spindle” section in thisTechnical Review Manual.)
The horizontal bearing test rig shown in Fig. 19
features the internal construction show Fig. 20.
The test rig features a simple structure: both ends ofthe main spindle have an angular contact ball bearing,a spacer is situated in the middle of the rig and thebearing housing is surrounded by a cooling jacket.
In order to obtain an array of bearing test data, thisFig. 15 Housing stress distribution

Fig. 17 Deformation of housing from overhead

Fig. 16 Housing stress distribution

Fig. 18 Deformation of housing from underneath-106-Introduction of 3D Electronic Catalog for Precision Machine Tool Bearings

rig is capable of testing a range of bearing and spacer
dimensions and can vary its operating conditions (eg.
running speed, lubrication conditions and jacket
cooling capacity).
Fig. 21 shows an example of thermal analysis.
When a bearing is running at a high speed, thermal
analysis demonstrates how the heat is transmitted andhow the resultant heat gradient appears.
During an early stage of this test, the results of thetest rig thermal analysis were compared with theactual test data (temperature data from 14 test points
shown in Fig. 20). In certain cases, the difference
between these two results exceeded 5˚C.
To minimize this difference, a tuning technique canbe utilized to reduce the average error. After thistuning technique is applied, the average difference
between the thermal analysis and actual test data falls
in a range of ±1˚C. (Tuning for a case in Fig. 21 isprimarily intended for thermal conductivity.)
The acquisition of basic data using a test rig asshown in Fig. 19 involves a lot of equipment set-up
time, and the corresponding data acquisition also
requires many man-hours.
Currently, it is difficult to achieve perfectly accurate
temperature estimation by thermal analysis only (i.e.
without performing an actual test). Nevertheless, NTN
will further accumulate theoretical data and improve its
thermal analysis technique to achieve better
approximation of the estimated temperature anddevelop improved techniques with other test rigs.

Main test spindle

Torque meter

Fig. 19 Test equipment

Cooling jacket…Temperature
measurement
test points

BearingSpacerBearing

Fig. 20 Test equipment layout

Cooling jacket (oil-cooling)

Test bearings

Fig. 21 Thermal analysis (estimated temperature distribution)-107-

NTN TECHNICAL REVIEW No.76（2008）

4. Conclusion
One of recent trends of machine tool manufacturers
is the increased use of 3D CAD-based design anddevelopment. For this reason, there have been
increasing requests for 3D CAD data for bearings
used in machine tool applications (including main
spindles, feed axis and table axis).
In response to numerous inquiries from machine
tool manufacturers, NTN has started supplying 3D
CAD data (in the form of an electronic catalog) for themost popular precision bearings from the ULTAGE
Series. Included in this new electronic catalog are
angular contact ball bearings and cylindrical roller
bearings (a total of 1329 bearing base part numbers).
By using 3D CAD data, all customer personnel canmore easily interpret the information listed on bearing
drawings. Thus, every detail of the bearing design canbe confirmed and be clearly understood. Furthermore,by importing the 3D CAD data into stress analysis andthermal analysis, the application can be analyzed
even further to ensure that the bearing design is
optimized.

This report presented some examples of NTN’s
analysis techniques. Incidentally, machine tool
manufacturers have been utilizing various analysis
methods to facilitate higher precision requirements,
more severe application conditions and a shorter
development period for new machine tool bearings.
Consequently, NTN believes that machine tool
manufacturers will ultimately develop new machinesthat make use of these advanced analysis techniques.
NTN hopes that its new 3D CAD Data Electronic
Catalog will help develop the machine tools of the
future.

Photo of author

Takehiko UMEMOTO
Product Design Dept.
Industrial Sales
headquarters-108-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]ULTAGE Series
Large, Long-lasting Double-row Cylindrical Roller Bearings
for the Main Spindles of Large Turning Machines
Naota YAMAMOTO*
NTN has responded to the need for improved efficiency,
reliability, quality and environmental responsibility for machine
tools by developing the ULTAGE Series of super-high-speed
precision bearings. NTN has developed a new long-life doublerow cylindrical roller bearing for large turning machines as one
part of the ULTAGE Series product line. Long life is accomplishedby optimization of the internal design and adoption of alightweight, high-strength modified PPS resin cage.
Furthermore, grease reservoirs inside the cage pockets extend
the lubrication life to more than 5 times that of the current bearing(including high strength brass cage). This paper outlines thedevelopment process.

Machining centers in particular are often operated in
the super high-speed range; therefore, their main
spindle bearings use PEEK cages which haveexcellent rigidity. Large sized bearings in contrast,
being mainly used in turning centers often employ
cages made of a special PPS resin that is readily
injection-molded and boasts lower material cost.

1. Introduction
Double-row cylindrical roller bearings are widelyused on machine tool main spindles that must satisfy
requirements for high rigidity and high accuracy, andtypical examples of such machine tools are turning
centers and machining centers. As machine tools are
increasingly sophisticated and cutting operation has tobe performed more efficiently, their main spindles
need to have longer life. Recently, in particular,
demands for large machine tools have been mounting;consequently, the needs for higher speed, advanced
sophistication and longer life have been everincreasing for use as main spindle bearings on large
turning centers.
Our recently developed NN30xxHSRT7 Series large
size double-row cylindrical roller bearings are intended
for use as main spindle bearings on large turning
centers.

(2) Optimized internal design
1 Grease reservoirs in cage bars
In many cases, large size bearings for machine
tools are grease-lubricated; to help extend grease lifein the bearing, we have incorporated two types ofgrease reservoirs into the cage bars (see Figs. 1 and 2).
Grease reservoirs in the outside surface of the cage
supply grease to the crowned area of each roller,
while the grease reservoirs in the bore side not only
retain grease, but also help prevent the leading edgesof each cage bar from coming into contact with therollers during high-speed operation of the bearing.

2. Features(1) Adoption of special PPS resin cage

2 Thicker cage ring and increased inner ring
center rib width

NTN has already been marketing highly rigid PEEK
resin cages for bearings up to medium sizes. The
primary applications for medium sized bearings in themarket are turning centers and machining centers.

A cage ring of increased wall thickness has been
adopted to increase rigidity of the cage, mitigating
deformation of the cage bars during high-speed
bearing operation.

*Product Design Dept. Industrial Sales headquarters-109-

NTN TECHNICAL REVIEW No.76（2008）

In addition, adoption of a wider rib on the inner ringhelps increase the space between rows in the middleof the bearing and an increased quantity of grease
can be retained, leading to extended grease life (seeFigs. 1 and 2).
Fig. 3 shows an example of result of FEM analysisof the cage design, and Fig. 4 graphically plots theamount of radial displacement based on FEM analysis
results at dmn 75×104. With the old design in these
diagrams, the ratio of ring thickness to roller length

falls in a range of approximately 20% to 30%, andrigidity of the cage is low in high-speed operation,
resulting in a high degree of cage deformation. In
contrast, with our new bearing design, the ratio of ring
thickness to roller length falls in a range ofapproximately 30% to 40%, thereby rigidity of the cageis increased in high-speed operation and resultant
deformation of the cage is small.
The amount of deformation of our new cage design
is approximately half that of the old design; thus, it

Grease reservoirs
Dotted line: after deformation
Solid line: before deformation

Narrowerrib
Old design(high strength brass cage)

Wider
rib
New design(special PPS resin cage)

Fig. 1 Old bearing design and new bearing design(HSRT7 Type)Old design: larger deformation
Dotted line: after deformation
Solid line: before deformation

Old designsign
New deof ring
thickness
Increased

Cage ring
Grease reservoirs

New design: smaller deformation

Proportion of radial deformationδ (radius)
relative to roller diameter%

Fig. 2 Drawing of the cage

1.4%

Fig. 3 FEM analysis of deformation (NN3028K, dmn75×104)

NN3028K bearing size:φ140×φ210×53 Inner ring speed: 4300min-1 (dmn75×104)
NN3032K bearing size:φ160×φ240×60 Inner ring speed: 3800min-1 (dmn75×104)

1.2%
1.0%
Amount of deformation δ

Ring thickness

0.8%
Broken line: after deformation
Solid line: before deformation

0.6%
0.4%
0.2%
0.0%
15%

Old design

20%

25%

New design HSRT7

30%

35%

40%

45%

Proportion of ring thickness to roller length %Fig. 4 Relation between width of ring and deformation (FEM analysis of deformation)-110-

ULTAGE Series Large, Long-lasting Double-row Cylindrical Roller Bearings for the Main Spindles of Large Turning Machines

should be understood that our new design can morepositively limit heat generation resulting from contact
between rollers and cage in high-speed operation.

post-assembly radial internal gap: -5 µm, grease:urea-based grease, jacket cooling: synchronized withroom temperature).
The appearance inside the cage after completingthe test is positively acceptable; grease remains in thegrease reservoirs in the cage bars and the bearing
samples of our new design are judged capable offurther continuous operation (Photo 1).

3. Results of grease lubrication test
(1) Performance test
Fig. 5 graphically plots the results of temperature
increase at the bearing outer ring. Temperature
increase at the outer ring of our new bearing design isless apparent compared with that of the benchmark
bearing having a high strength brass cage. With our
new design, the bearing speed that corresponds withtemperature increase of approximately 20˚C on theouter ring is 6,500 min-1 (dmn = 114×104); with the olddesign having high strength brass cage, the speed
corresponding with a similar temperature increase is
4,500 min-1 (dmn = 79×104). In other words, themaximum permissible running speed of our new
design is 45% greater compared with the old design.

Remaininggrease(2) 2,000 hours continuous endurance test

Temperature increase on outer ring ˚C

Two sizes of our new design (bore diameters φ140mm and φ160 mm) have been subjected to a 2,000hour endurance test, with samples of each sizes
successfully completing the test (test conditions
applied are as follows: running speed dmn = 75×104,

Photo. 1 Condition of grease in the reservoirs
after 2000 (h) of endurance test30
New design: HSRT7 (special PPS cage)
Benchmark design: high strength brass cage

《Test conditions》

20[New design: HSRT7 (special PPS cage)][Benchmark design: high strength brass cage]

10･Bearing size: NN3028K (φ140×φ210×53)･Running speed: up to 6,500 min-1 (dmn = 114×104)･Post-assembly radial internal gap: -5 µm･Grease: urea-based grease･Jacket cooling: synchronized with room temperature

0

0

2,000

4,000

6,000

8,000

106

140

Speed min-1
0

35

70

dmn value ×104

Fig. 5 Temperature raise test results at grease lubrication-111-

NTN TECHNICAL REVIEW No.76（2008）(3) Rapid acceleration/deceleration test
Fig. 6 presents rapid acceleration/deceleration test
conditions. One cycle consists of a rapid acceleration
phase, constant speed operation phase, rapid
deceleration phase and standstill phase. This cycle,
simulating actual operation pattern of the bearing, wasrepeated one hundred thousand times in order toverify 100-thousand cycle endurance of the cage.
After completion of the rapid acceleration/deceleration
test, the bearing performance test was conductedagain to verify that temperature increase behavior of
the samples exhibited no problems and that thebearing and cage of our new design having undergone
repeated tests are free from any problems (Photo 2).

Photo. 2 Condition of cage after 2000 (h) of
rapid acceleration and slowdown test

4. Conclusion

Running speed min-1/dmn

Our newly developed ULTAGE Series double-row
cylindrical roller bearing NN30xxHSRT7 products,
which are grease-lubricated, have achieved longer life
in continuous endurance test at dmn 75×104. NTN
believes that this new development will positively fulfill
the market needs for much faster main spindles inlarge turning center applications.

dm n
75×104

min-1

4300

《Test conditions》[New design: HSRT7 (special PPS cage)]･Bearing size: NN3028K (φ140×φ210×53)･Running speed: up to 4,300 min-1 (dmn = 114×104)･Post-assembly radial internal gap: -10 μm･Grease: urea-based grease･Jacket cooling: synchronized with room temperature
1.5 sec

1.0 sec

1.5 sec

1.0 sec

One cycle 5 sec.
Run time

Fig. 6 Rapid acceleration and slowdown test condition at grease lubrication

Photo of author

Naota YAMAMOTO
Product Design Dept.
Industrial Sales
headquarters-112-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Technical Trends in Wind Turbine Bearings

Souich YAGI**
Nobuyuki NINOYU**
The ecologically friendly wind power industry, which emits nocarbon dioxide and utilizes the renewable power of wind, is growing
remarkably. The industry is in a new stage of size increases to theturbine and performance enhancements as large wind farms begin to
be placed offshore.
Because they help reduce friction and power losses in the drivetrain
bearings are an important component in wind turbines. This paper will
cover the evolving technology and special features found in bearings
destined for wind turbines.

Rotor main bearing

1. Introduction

GearboxGenerator

For many years NTN has been supplying bearings
to the wind turbine industry. This paper will describe
the features, drawbacks and technical trends for thesebearings. Specifically this paper will focus on the
bearings found on the main shaft, in the gearbox, inthe generator and in the yaw drive.

Hub

2. Increases in wind turbine size

Yaw driver

Blade

The wind turbine industry offers a variety of turbines
configurations with both horizontal and vertical shaftalignments as well as models that do not include agearbox. In general though the vast majority ofindustrial scale wind turbines (generally speaking
machines that generate more than 500 kW of power)
use a three bladed rotor on horizontal shaft connected
to a gearbox that drives a generator. Fig. 1 illustrates
the nacelle from a typical wind turbine. Inside thenacelle are the mainshaft support bearings, gearbox,
induction generator and the yaw drive system. With
this mechanism, the energy in the wind used to turnthe rotor. This rotation is passed through a gearbox
that increases the shaft speed and then passed into a
generator where the electricity is produced.
With any given wind turbine, the amount of electrical
power generated is proportional to the square of theFig. 1 Nacelleblade length. Accordingly large wind turbines feature
increasing blade lengths. This increase in length waspossible due to improvements in blade materials
which allow the blades to remain a one-piececomponent.
Fig. 21) graphically presents the trend in maximum
output and rotor diameter for wind turbines from 1991
to 2007.
The largest wind turbine currently in existence is a 5
MW unit featuring a rotor diameter of 126 m.
According to statistic data by BTM Consult ApS, aDanish consulting firm, the average rated output of thewind turbines installed around the world stood at 1.28
MW in 2005, 1.42 MW in 2006 and 1.49 MW in 2007.

**Industrial Engineering Dept.
**Application Engineering Dept. Industrial Sales headquarters-113-

NTN TECHNICAL REVIEW No.76（2008）

Maximum rated output

5

160

4
120

Rotor diameter

3

80
2
40

1
0

1991

1993 1995

1997 1999 2001 2003 2005 2007

Rotor diameter m

Maximum rated output MW

Bearing life is calculated using the following
equation.

200

6

6

10
C p
Lnmh = aISO×――― ――
60n P

（ ）

aISO: life correction coefficient
n:
C:
P:
p:

0

year

rotating speed (min-1)
basic load rating [N]dynamic equivalent load [N]p = 3 for ball bearing, p = 10/3 for roller bearing

Fig. 2 Upsizing of wind turbines 1)

ISO 281, the international standard for calculating
the life of bearings, was revised in 2007. The revision
introduced a coefficient, aISO , which accounts forvarious factors that influence bearing life including
fatigue limit load, lubrication conditions andcontamination in the lubricating oil. Based on this new
coefficient, NTN is performing bearing life calculations
with a special program that considers thedisplacement of the shaft, rigidity of the housing,
bearing internal clearance and fatigue limit load of the
shaft system. This process results in a more reliablecalculation for the main shaft bearings.
The load working on a wind turbine can randomly
fluctuate depending on local wind conditions. To allow
analysis of the wind turbine system the loads arebroken down into a Load Duration Distribution (LDD).
The LDD takes the loads that the turbine encountersand divides them into a number of segments (usually
between 100 and 300). The amount of time spent at
each load segment is also determined. Each segment
is referred to as a “Bin”. The bearing life is thencalculated for each Bin, after which a statisticalprocess is used to determine the overall bearing life.
Fig. 4 illustrates the results of a deformation
analysis performed on the housing and bearing outer

4.0

Rated output MW

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0

200

400

600

800

1000

1200

Shaft diameter mm

Fig. 3 Wind turbine capacity and bearing size

As larger wind turbines have been adopted, larger
sized main bearings are needed to support the larger
rotors. Fig. 3 graphically plots the relation between
rated output of a wind turbine and main bearing size.

3. Main shaft bearing
3.1 Selection of the main shaft bearing

Generatorside
1250 mm

In a wind turbine the weight and rotation of the rotor
blades applies a load to the main shaft bearing. The
wind turbine manufacturer considers the normal
operating conditions for the wind turbine and assumes
certain equipment failure modes when calculating theloads applied to the main shaft bearing and presents
the results to the bearing manufacturer. Note that a
special program has been developed for this processaccording to the guidelines set up by the International
Electrotechnical Commission (IEC) 2) andGermanischer Lloyd (GL) 3).
International standards, including ISO, IEC and GL,
have settled on a minimum acceptable calculated life
for the main shaft bearings of 175,000 (20 years).
Additionally a safely factor of at least 2.0 is required(safety factor = static load rating/static equivalentload).

Rotor
side

1900 mmFig. 4 Analysis example of main bearing-114-

Technical Trends in Wind Turbine Bearings

ring for a main shaft using a self-aligning roller
bearing. By investigating the effects that deformationin the housing has on radial clearance, the load
distribution of the rolling elements and consequently
bearing life the design of the bearing can be
optimized. Furthermore, NTN has expanded the scopeof analysis to include the base where the housing andgearbox are installed in order to develop a morereliable design specification.

limiting the axial play in order to improve reliability. At
the same time, the NTN self-aligning roller bearing is a
design that tolerates high dynamic and static mounting
errors. These errors come from such factors as
dynamic deflections of the nacelle frame and shaft
due to the wind load as well as vibration caused by
mounting misalignment between the main shaft andgearbox.

<Type B>
3.2 Main shaft configurations and associatedproblems

This configuration mounts two bearings on themainshaft. This configuration is more common with thehigher loads seen in larger (2 MW+) wind turbines.
When a double-row tapered roller bearing is used as
the blade-side bearing, the bearing is preloaded tolimit the axial play in the shaft as well as fretting within
the bearing. However the design cannot tolerate thehigh mounting errors that the self-aligning roller
bearing can, requiring a higher degree of mounting
accuracy.

Various main shaft configurations are available forwind turbines. Table 1 summarizes three typical main
shaft configurations.

<Type A>This is the most common configuration and uses
two bearings. The main shaft uses a self-aligningroller bearing while the inside of the gearbox uses afull complement cylindrical roller bearing. To preventthe bearing mounted in the gearbox from receiving
axial load from wind, the axial play in the self-aligningroller bearing needs to be smaller than that of the
gearbox bearing. Furthermore, the effect thattemperature has on bearing internal clearance andbearing-shaft fit needs to be limited in order toimprove reliability.
NTN is supplying a unique self-aligning roller
bearing for the blade side bearing where the internal
clearance has been reduced to a special smallerclearance range.
NTN design practice aims to maintain the internal
clearance on a bearing at an appropriate level while<Type C>This configuration is used in direct drive windturbines that eliminate the gearbox. A generator ismounted on the outer surface of the bearing and theouter ring is rotated (compared to the inner ring in
Types A and B) directly driving the generator. The
rotor is supported with two bearings. When thisbearing configuration is grease-lubricated, a special
lubricating scheme is adopted and a special cage isused to allow grease to readily enter the bearing.
When tapered roller bearings are used, temperature
differences between the inner ring and outer ring canaffect the internal clearance of the bearing. This could

Table 1 Construction of the main shaft
Drivetrain layout

Type B

Type A

Blade

Blade-side bearing

Generator-side bearing

Features

SRB

CRB
FCCRB
（NFV type）

SRB
TRB
DTRB

SRB
TRB
CRB

・Induction generator type with gearbox
・Two bearings support main shaft.

TRB
DTRB

TRB
CRB

・Synchronous generator without gearbox
・Rotating outer ring

Bearing

Generator

GearboxGearboxGenerator

・Induction generator with gearbox
・Generator-side bearing also functions as input
bearing for gearbox.

Type C

Generator

SRB: Self-aligning Roller Bearing
CRB: Cylindrical Roller Bearing
FCCRB: Full Complement Cylindrical Roller Bearing
TRB: Single-row Tapered Roller Bearing
DTRB: Double-row Tapered Roller Bearing-115-

NTN TECHNICAL REVIEW No.76（2008）

When compared to a bearing that uses symmetricalrollers, the NTN design exhibits lower torque for bothwind velocity conditions. As a result of this analysis, it
has been verified that the NTN design offers a stable
guiding arrangement for the rollers by limiting tilt andskew in the rollers (factors that would increase
torque). With the lower torque in the NTN design
comes lower heat generation when compared with thesymmetrical roller design. Because a main bearing on
a wind turbine has to carry an axial load from therotor, the NTN design is capable of stable operation
with lower torque and limited heat generation.

either result in excessive preload, which can shorten
bearing life or excessive internal clearance which can
trigger fretting inside the bearing. To avoid these
problems, careful considerations need to be madeincluding a heat transmission analysis for the bearing
housing and peripheral members.

3.3 Features of self-aligning roller bearings forthe main shaft
Currently many wind turbines use a self-aligningroller bearing for supporting the main shaft. The NTN
self-aligning roller bearing (Fig. 5: Asymmetrical roller
design) has a fixed rib at the center of the inner ring toreliably guide the rollers, even in fluctuating windconditions. Competing self-aligning roller bearing
designs (Fig. 5: Symmetrical roller design) use acentral floating guide ring.
Fig. 6 illustrates the results of a torque analysis ona self-aligning roller bearing commonly found in 1.5
MW turbines (NTN part number 240/600B, boundary
dimensions:φ600×φ870×272 mm). The bearing
operated under wind velocities of 8 m/s and 16 m/s.

Cage

4. Gearbox
The gearbox increases the shaft speed of the rotor(10-30 min-1) to a higher speed for the induction
generator (usually, 1200-1800 min-1) by using a seriesof gears.
A gearbox is a critical component in many windturbine designs, and standardization for thiscomponent has long been attempted. The current
guideline for gearboxes was formulated by theAmerican Gear Manufacturers Association (AGMA) 4)
Recently the need for the development of aninternational standard for gearboxes has been
mounting. Therefore, a joint working group consistingof the IEC and ISO has been developing a new
standard.
Fig. 7 illustrates the layout of a typical gearbox.
Rotation from the rotor (input shaft in the figure) drives
the carrier of the planetary gears, which then drives thelow-speed shaft then the intermediate shaft and finallythe high-speed shaft. At each step the rotational speed
is increased resulting in a higher output speed. Table 2
summarizes the bearing types used in various
locations of the gearbox.

Outer ring
RollerInner rib

Inner ring

Asymmetrical roller design(NTN Design)

Guide ring
Symmetrical roller design

Fig. 5 Spherical roller bearing

kN・mm

300

Gearcase
Output200

High-speed
shaft

TorqueCarrierIntermediate
shaft

100

Input shaft
0

Asymmetricalroller design

Symmetricalroller design

Wind velocity: 8m/s
Rotating speed: 16min-1
Radial load: 720kN
Axial load: 128kN

Asymmetricalroller design

Symmetricalroller design

Low-speed
shaft

Input

Wind velocity: 16m/s
Rotating speed: 19min-1
Radial load: 850kN
Axial load: 163kN

Planetary gear

Fig. 6 Torque of the bearing and wind velocity

Bearing

Fig. 7 Construction of the gearbox-116-

Technical Trends in Wind Turbine Bearings

Table 2 Bearing type for gearbox 4)
Bearing type
Fixed-side

SRB, CRB, TRB, BB, 4PCBB

Free-side

SRB, CRB, BB

Intermediate
shaft

Fixed-side

SRB, CRB, TRB, 4PCBB

Free-side

SRB, CRB

Low-speed
shaft

Fixed-side

SRB, TRB

Free-side

SRB, CRB, FCCRBHigh-speed
shaft

Planetary gear

SRB, CRB, FCCRB, TRB

CarrierFCCRB, SRB, TRB

φ500 mmArea

SRB: Self-aligning Roller Bearing CRB: Cylindrical Roller Bearing
FCCRB: Full Complement Cylindrical Roller Bearing TRB: Tapered Roller Bearing
BB: Deep Groove Ball Bearing 4PCVBB: Four Point Contact Ball Bearing

Fig. 8 Analysis example of planet gear

Recently, the input torque on gearboxes has been
increasing as a result of larger wind turbine designs.
To keep pace the size of the carrier and planetary
bearings have also been increasing. In an effort to
reduce weight in the gearbox the outer ring on certain
planetary designs has been eliminated with the gear
bore surface being used as the outer raceway surface.
Other designs are using two stage planetary gearing
to reduce weight.

Load｛kgf｝
1st row
2nd row
3rd row
4th row

4.1 Gearbox bearing selection
Table 3 summarizes the design criteria that go into
selecting gearbox bearings. The two design criteria
are contact stress and required life. The actual design
criteria vary depending upon the location within the
gearbox.
To support planetary gearing it is common to use
two rows of bearings, usually cylindrical roller
bearings, in the planet gears. To select the proper
bearings it is necessary to consider deformation in theplanetary gearing as a result of the loads transmitted
between the sun gear and ring gear.
To illustrate this process the analysis performed ona planet gear from a 1.5 MW turbine is presented. The
planet gear used two double row cylindrical roller
bearings (4 rows total). The analysis examined thedeformation in the gears and bearings as well as how
well the load is distributed in each row.
Fig. 8 illustrates the deformation on the planet gearand bearing under maximum torque conditions. Fig. 9shows the load distribution in each row after
accounting for the deformation found in Fig. 8.

Rolling element No.

Fig. 9 Load distribution of each 4-row

4.2 Common problems in gearbox bearings
Because wind turbine gearboxes are located high inthe nacelle the main gearbox requirements are highreliability, light weight and small size. The bearings on
the planetary shaft, medium-speed shaft and highspeed shaft all see a complex series of loads
consisting of both high axial and radial loads.
Compared with a bearing for a low-speed shaft, thesebearings have to feature not only greater load carrying
capacity but also smooth high-speed operation.
To address these issues, NTN has developed a
new bearing product, the separator style cage 5) (Fig.
10). The bearing uses small separators for the cage
and these separators hold the rollers in the bearing.
The small separator allows for larger and/or morerollers, similar to a full complement bearing. The
increased roller size and quantity allow the bearing toachieved a rated life more than 1.5 times greater thana conventional bearing with a standard cage (theenvelope dimensions remain the same for bothbearings). Additionally since there is a cage the rollers
do not come in contact with each other. The lack ofroller to roller contact means that the new bearing

Table 3 Maximum contact stress and required life 4)
PositionMax. contact stress (MPa) Required life L10 (hours)

High-speed shaft

1300

30,000Intermediate shaft

1650

40,000Low-speed shaft

1650

80,000

Planetary gear

1450

100,000

Not specified

100,000

Carrier-117-

NTN TECHNICAL REVIEW No.76（2008）

avoids a major drawback of a full complement
bearing—limitations in high-speed performance—andachieves maximum allowable speeds comparable tothat of a standard caged bearing. Fig. 11 provides acomparison of the new NTN design with a standard
caged bearing and a full complement bearing.

striped pattern develops (Fig. 12) on the raceway androlling surfaces. This can create severe vibration andthe bearing will fail to function properly.
To address this issue NTN has developed an
insulated bearing (Fig. 13). The outer diameter andboth side faces of the outer ring undergo a thermal
spray treatment, which applies a special ceramic
coating. The resulting bearing will have a resistance of
100 MΩ or greater and a dielectric breakdown voltageof 2 kV or higher. Fig. 14 shows a cross-sectionalview of the insulated bearing.

Fig. 10 High load capacity cylindrical roller bearings

Fig. 12 Example of electrical pitting

Size and quantity of rollers
cannot be increased because
the strength of cage must be
ensured.

New
designThermal sprayedceramic coating

Cage

Full
complement

Rollers rotating in reverse directions come into contact witheach other, leading to greater frictional resistance.

Fig. 11 Comparison of structure

Fig. 13 Insulated bearing

5. Generator bearings
Ceramic layer

A grease filled, deep groove ball bearing is usuallyused to support the shaft in a generator. For a 1.5 MW
size generator, the shaft diameter can exceed 150
mm.
Electrical pitting is a serious concern for any bearing
used in a generator. Electrical pitting is damage that
occurs on the raceway and rolling surfaces of the
bearing due to the passage of electric current throughthe bearing. The current will create a spark at the
point of contact between the rolling elements andraceway surface. When electrical pitting progresses, aFig. 14 Schematic of insulated bearing-118-

Technical Trends in Wind Turbine Bearings

compact, lightweight design. This approach is very
important for wind turbines of 2 MW or greater.
The demands for higher performance and smaller
size have seen the development of a novel power
train layout. The new hybrid wind turbine design
combines a synchronous generator and a gearbox
consisting of a single-stage planetary gear to achieveboth higher performance and lighter weight.
Another wind turbine design employs a system ofmultiple generators to improve generator efficiency in
low wind conditions. In this design the output from thegenerator is adjusted by operating only the needed
generators. The main bearing in such a system is alarge size tapered roller bearings.

6. Yaw drive bearing
In order to produce power the nacelle needs toalways point into the wind. In order for the nacelle torotate it rests upon a four-point contact ball bearing
and a sliding bearing. These bearings carry the weightof the nacelle and rotor as well as any loads created by
the wind. To power the nacelle’s rotation a yaw drive isused. The yaw drive consists of an electric motor and a
gearbox to reduce the motor’s rotational speed. The
gearbox in a yaw drive must be compact and capableof transmitting great torque. The bearings in the
gearbox need to feature a low-profile design andhigher rigidity to handle the moment loads applied fromthe pinion.
The angular contact ball bearing used for the main
shaft in the yaw drive features a special design: theinner and outer rings have raceway surfaces withdeeper grooves so that the bearing can support agreater axial load. Fig. 15 shows a cross-sectionalview of this special angular contact ball bearing. Note
this design is capable of handling higher axial loads
when compared to a standard angular contact ballbearings.

<Longer life>
For both main shaft and gearbox bearings
maximization of load capacity is a key consideration
for long bearing life. For generator bearings control ofelectrical pitting is important in realizing longer bearing
life.
Offshore sites have recently attracted attention as
locations for wind turbines. Compared to sites on land
offshore sites boast better wind characteristics. In
Japan, a nation not possessing a continental shelf,
expectation is mounting for development of floating
offshore wind turbines. Bearings used on a floating
offshore wind turbine are subjected not only to windloads but also to loads caused by oscillation of thefloating structure. Therefore, these bearings musthave a larger basic load rating when compared withbearing used on current land-based wind turbines.
Wind turbines are being designed to lower theamount of load that gets applied to the bearings topromote longer life. New wind turbine designs havebeen commercialized that cope with variations intorque when the wind turbine is braked or extreme
wind conditions occur (such as those from a typhoon).
The new designs may have a novel mechanism thatswitches from upwind mode to downwind mode tominimize the effects of wind load. A downwind styleturbine may also be used.
To achieve longer bearing life, operating conditions
also need to be considered. In addition to fatigue life
other problems can occur to the main shaft bearings
such as lightening-induced damage or seawater
ingress from a faulty seal section. Improved bearing
maintenance technique need to be implemented whileat the same time the bearing design must allow foreasy component replacement.
We believe that as the scale of wind farms has
expanded it will become increasingly necessary toacquire the current operating condition of individual
wind turbines. This will be achieved through the use of

Fig. 15 Schematic of angular contact ball bearing

7. Latest trends in wind power technology<Lighter weight>
A lighter nacelle design allows for a thinner wall
thickness in the tower while still ensuring necessary
design strength. This reduces costs for construction,
installation and transportation of the wind turbine
components. To aid in this effort the following industry
efforts are profiled.
1) Improvements to main shaft design such as
elimination of main shaft itself, adoption of a hollow
main shaft, and lightweight design for the housing,among others2) Incorporation of two-stage planetary gearing andhelical gearing into the gearbox allowing for a-119-

NTN TECHNICAL REVIEW No.76（2008）

automatic monitoring and fault detection systems. This
will allow for the prompt detection of problems andimmediate repairs.

References
1) 2007 2nd wind power generation system technology
course materials: Japan Wind Energy Association
2) IEC 614000-1: Wind Turbines―Part 1: Design
requirements, edition 3 (2005)
3) Germanischer Lloyd: Guideline for the Certification ofWind Turbines Edition 2003
4) ANSI/AGMA/AWEA 6006-A03: Standard for Design
and Specification of Gearboxes for Wind Turbines5) T. Ozu: High Load Capacity Cylindrical Roller
Bearings, NTN Technical Review No.74 (2006)

8. Conclusion
The power output from wind turbines has been
increasing year by year. The demands for highly
reliable wind turbine products with longer life are
increasing. Additionally the turbines must be able towithstand severe natural environments such as thosefound on offshore wind farms.
In the technical field of bearings the needs aremounting for not only standard designs but also
special designs dedicated to wind turbines.
Through development and stable supply of its’
products, NTN wishes to contribute to thedevelopment of wind power generation technology
and to the prevention of global warming.

Photo of authors

Souichi YAGI

Nobuyuki NINOYU

Industrial Engineering Dept.

Application Engineering Dept.
Industrial Sales
headquarters-120-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]NTN Sensor Units for Construction Machinery

Shoji ITOMI*
Hiroyoshi ITO*
Recent construction machines use a lot of electronic controls and it isexpected that in the near future an even greater variety of controls withmore advanced technology will be used in construction machines.
NTN currently supplies a wide variety of sensor units for construction
machine applications that support operation accuracy, efficiency andease of maintenance.
This report introduces four types of sensor units that NTN has
developed.

Foreword

1. Angle sensor unit

NTN has been developing various sensor units
based on it's processing technologies andexperiences in precision bearings. These sensor units
have been commercialized and are used in the areas
summarized in Table 1. These provide higher
precision, better efficiency and improved
maintainability for construction machines.
This paper provides information about the following
four types of sensor units.

Angle sensor unit

Detection of operating angle of boom
and arm of hydraulic shovel andwheel loader arm, etc.

An angle sensor unit for construction machines
detects the operating angle of a joint of a boom, arm,
etc. and monitors the position of bucket: then basedon the detected joint angle, bucket position iscontrolled so that accurate excavation work is
attained.
In 1993, NTN developed and commercialized anangle sensor unit for construction machines thatprovides the following advantages:
1. Excellent vibration and shock resistance
performance
2. Appropriate signal precision
3. Excellent environment resistance (immunity
against temperature change, water resistance,
dust resistance)
4. Excellent durability

Rotation sensorintegrated bearing

Detection of rotation speed of driving
hydraulic motor and electric motor

1.1 Structure and features

Table 1 NTN sensor units
NTN sensor

Subject being detected

Table 2 summarizes product specifications of angle
sensor units for construction machines.
The conventional product in Table 2 was available
before 1993, and is comprised of a shaft supported bya needle roller bearing. The shaft is coupled to therotation shaft of a market-available potentiomete.
The disadvantages of this product may besummarized as follows:

Detection of high-performance
High-resolution rotation
sensor-integrated bearing rotation speed of driving hydraulicmotor and electric motor
Magnetically operatingiron particle sensor unit

Detection of quantity of iron particles
in engine oil, transmission oil andaxle oil*Application Engineering Dept. Industrial Sales headquarters-121-

NTN TECHNICAL REVIEW No.76（2008）

1 A needle roller bearing supports the shaft: thisdesign cannot support an axial load.
2 The rotation shaft of market-available
potentiometer is supported by a simple sliding
bearing: consequently, owing to the initialclearance on the sliding bearing and increase in
the clearance resulting from wear, an excessively
large load is applied to the brushes. When thissensor is installed to a construction machine and issubjected to severe vibration and/or shock, itsbrushes are repeatedly deformed and caneventually fracture.
3 Two shafts are coupled with a coupling:
consequently, backlash occurs between the two
shafts, and delay in response of potentiometer
output for operating angle is unavoidable.

The new design in Table 2 is a non-contact angle
sensor unit developed by NTN in 2007 (Photo 2).
On this sensor unit, a combination of a magnetic
sensor and a magnet detects a rotation angle. In placeof brushes, the magnet is directly mounted to theshaft; and like with the current sensor unit, two
preloaded ball bearings support the shaft in order tomaintain rigidity of the shaft.

The "current product" in Table 2 means a product
NTN is currently mass-producing (Photo 1).
The brushes of the potentiometer are directly
mounted to the shaft to eliminate backlash. In
addition, the shaft is supported by a preloaded ballbearing to eliminate the axial clearance on the bearing.
This provides a structure of higher bearing rigidity,
thereby minimizing shaft displacement resulting fromload, vibration and shock. As a result, sensor
response to operating angle is improved, variation inbrush pressure becomes smaller, positively preventing
brush fracture and deterioration in precision of output
signal.
The housing of the angle sensor unit consists of acasting to ensure mechanical strength, while thesensor shaft is provided with an oil seal to prevent
ingress of water and dust. An O-ring is inserted
between the plate at the bottom of unit and the
housing to prevent ingress of water.

Photo 1 NTN angle sensor unit (Analog type)

Photo 2 NTN angle sensor unit (Digital type)

Table 2 Specifications of angle sensor units

Needle roller
bearing

φ40
φ83

Potentiometer

Oil
seal

Ball bearing
O-ringBrush

45

Coupling

New design

Oil seal

Ball bearing

71

Structure

Current sensor unit (NTN)

Resistor

φ83
φ110

O-ring45

Conventional sensor unit

φ83
φ110

Sensor type

Potentiometer (contact)

Potentiometer (contact)

Magnetic sensor (non-contact)

Output signal

AnalogAnalogAnalog or digitalMax. allowableload on shaft

Axial load is not carried.

Fr：1kN，Fa：1kN

Fr：1kN，Fa：1kN

Signal accuracy＊(individual linearity)

2% max.

0.5% max.

0.5%以下＊：Linearity of output signal relative to shaft rotation angle-122-

NTN Sensor Units for Construction Machinery

The advantages of this new design are as follows:
1 The sensor works on a non-contact basis.
Consequently, loss in life and detection precisionowing to worn brushes and substrate does notoccur even during long operation, and problem
occurring from fractured brush(es) is positively
avoided.
2 Either analog or digital (PWM or serial protocol)
output signal can be used.
3 Rotation angle of up to 360˚ can be detected.
NTN is going to incorporate this non-contact angle
sensor unit as a next generation angle sensor unit into
many construction machines including hydraulicshovel and wheel loader.

2. Rotation sensor-integrated bearings
Rotation sensors are used for rotation speed controlof AC/DC servo motors and hydraulic motors.
Previously, conventional sensors including magnetic
gear speed sensor have been often used for driving
AC/DC servo motors on battery forklift trucks.
Recently, rotation sensor-integrated bearings, eachbeing a unitized product comprising a deep grooveball bearing and a rotation sensor, have been morecommonly adopted.
In the field of construction machinery, there havebeen needs for satisfying requirements of emission
control and improved fuel economy. In an attempt tosatisfy these needs, a development effort is inprogress for a hybrid system that consists of anelectric motor and a diesel engine. In this context, thedemand for electric motors for construction machinerywill increase, and the needs for rotation sensorintegrated bearings for this purpose will also increase.

1.2 Typical application
When installed to a hydraulic shovel to monitor thecurrent bucket position, the new angle sensor helps
the hydraulic shovel achieve safe and accurate
excavation work (Fig. 1).
Delicate work such as excavation on slope, which
was previously dependent on the skill of operator, canbe executed by a less experienced operator throughcontrol of the bucket position.
Furthermore, use of digital sensor output signal
helps limit the error in bucket position caused by noiseinterference, leading to more accurate excavationwork.

2.1 Structure and features
Our rotation sensor-integrated bearing features aunique structure (see Fig. 2 and Photo 3). The innerring side of a deep groove bearing is permanently
fixed to a magnetic pulser ring (multi-pole permanent
magnet) having alternately arranged N poles and S
poles along its circumference. The outer ring is fixed
to a magnetic sensor such as a Hall's IC*.
Magnetic sensor (Hall’s IC)

Boom

Angle sensor unit

Magnetic pulser ring
(multi-pole permanent
magnet)Fig. 2 Sectional view of integrated sensor bearing

Arm

Lever
When the arm swings,
the rotation angle isdetected based on theposition of the lever fixed
to the arm.

Fig. 1 The example of attachment of an angle sensor

Photo 3 Cut model of integrated sensor bearing-123-

NTN TECHNICAL REVIEW No.76（2008）

When the magnetic pulser ring rotates together with
the inner ring and passes the vicinity of the magnetic
sensor fixed to the bearing outer ring, the magnetic
sensor detects variation in the magnetic field on themagnetic pulser ring (for example, change from N pole
to S pole) and outputs a pulsed electric signal
corresponding to the current bearing speed (repetitionof High voltage and Low voltage levels) (see Fig. 3).
By arithmetically processing this cycle, the bearing
rotation speed can be determined.
The number of High and Low voltage signals
repeated per rotation of the magnetic pulser ringcoincides with the number of magnetic poles (N poles
and S poles) on the magnetic pulser ring. As a result,with a larger number of the magnetic poles, thenumber of High and Low voltage signal pulses per
rotation is greater, contributing to more accurate
rotation speed control.
To be able to judge rotation direction of the shaft,two magnetic sensors used, one for phase A output
and the other one for phase B output. These sensors
are fixed such that the phase of electric signal output
from one sensor is deviated by 90 degrees from thatof the other sensor (see Fig. 3). By detecting therising timings with pulses of these electric signals, it ispossible to judge the rotation direction of the shaft.
Integration of a rotation sensor with a bearing
provides the following advantages compared withconventional rotation sensors:

2.2 Typical application
A typical example of an application of an ourrotation sensor-integrated bearing is the travel motor
on a battery forklift truck is presented below. Fig. 4schematically presents the control system for thetravel servo motor, and Fig. 5 schematically illustrates
a cross-sectional view of the servo motor completewith our rotation sensor-integrated bearing.
Let us describe the control scheme for the servo
motor. The operator depresses the accel pedal, anelectric signal corresponding to the amount of
actuation with the accel pedal is transmitted to themotor controller, and then the motor controller sends
an electric signal representing a command forcontrolling the rotation speed to the servo motor: at
the same time, the rotation sensor mounted to theservo motor always correctly detects the rotation
speed of the motor shaft with good response speed
and sends a relevant electric signal to the motor
controller: the motor controller compares the electric
signal from the accel pedal with that from the rotation
sensor, and controls the speed of servo motor.

Accel pedal
CommandMotor controller

1 Sensor gap adjustment and alignment
adjustment are not needed. The unit can bereadily assembled into equipment.
2 A bearing and a rotation sensor are unitized.
Thus, axial space can be reduced.
3 The unit incorporating a magnetic sensor isimmune to outside disturbances including dew
condensation.

Rotation speedcommandServo motor
TravelPhase B
outputRotation sensor
Rotation speed detection

Fig. 4 Servo system of servo motor

Hall's IC*: Hall's sensor having a built-in output transistor.

Phase AoutputFeedback

Rotation sensorintegrated bearing

Servo motor

90
degrees180
degrees
360 degrees(Cycle)Fig. 3 Output signal of sensor

Fig. 5 Sectional view of servo motor-124-

NTN Sensor Units for Construction Machinery

2.3 Series of NTN rotation sensor-integrated
bearing products

3. High-resolution rotation sensorintegrated bearings

Table 3 summarizes the specifications of standard
types of NTN rotation sensor-integrated bearing.
Other than the standard type, a magnetic field
immune type is less prone to malfunction even if built
into an electric motor, which generates a strong
magnetic field, such as a high-torque motor and ahigh-temperature type that can be used at a higher
temperature for example 150˚C.

SNR, one of NTN group companies, has developed
a unique magnetic sensor. Using this sensor, we havedeveloped a "high-resolution rotation sensorintegrated bearing" whose rotation detection maximum
resolution is 40 times higher than conventional
sensors.
This sensor element comprises an IC on which
several sensors are arranged, wherein the electric
signal output from each magnetic sensor isappropriately processed in order to achieve higher
resolution.
NTN's experiences in sensor mounting technology
and conventional sensors produced a new sensor
element with reduced dimensions.
Fig. 6 illustrates a cross-sectional view of deep
groove ball bearing 6202 (ID φ15 mm, OD φ35 mm).
The bore diameter and outside diameter of our new
sensor are same as those of conventional deep
groove ball bearing 6202: however, the new sensor iscapable of 1280 pulses per rotation. This is 40 times
as many compared with the conventional sensor (32pulses per rotation) (see Fig. 7).

Table 3 Specification of standard type of
integrated sensor bearing
Bearing number
6202

6204

6206

6208

6209

Number of output pulses

32

48

64

80

80

φA

15

20

30

40

45

φB

35

47

62

80

85

C

11

14

16

18

19

φD

18

29.5

40.6

53

57.3

φE

32

46.6

58

75

77

Bearing

Sensor

7.5

F

Bearing outer ring (fixed ring)C

Magnetic sensor
Magnetic pulser ring

Magnetic sensor (SNR)

φ18
φ32

φE
φ35
φ15

φB

F

φD
φA
Bearing inner ring (rotating ring)11

9.5

Magnetic pulser ring
(multi-pole permanent magnet)

Fig. 6 Sectional view of integrated
high resolution sensor bearing

Photo 4 External view of integrated
high resolution sensor bearing-125-

NTN TECHNICAL REVIEW No.76（2008）

Magnetic poles of magnetized encoder

Rotation signal of standard sensor

Rotation signal of high-resolution sensor
0.28 degrees (1280 pulses)
11.25 degrees (32 pulses)

Fig. 7 Comparison of rotational signal of standard sensor and high resolution sensor

The NTN high-resolution rotation sensor-integrated
bearing has the following advantages:
1 More precise rotation speed control is possible,
compared with the conventional rotation sensorintegrated bearing.
2 Our high-resolution sensor is a magnetic sensor.
Thus, it boasts resolution as high as that obtainedwith an optical rotation sensor: as it is notadversely affected in a severe environment where
mist or dust is present (an optical rotation sensor
may not be used in such an environment).

thereby the sensor unit detects the quantity of ironparticles in the transmission oil.
Fig. 9 illustrates an example of output pattern andassociated quantities of deposited iron particles.

Cover

Mounting thread

4. Magnetically operating iron particle
sensor unit
NTN has developed a magnetically operating ironparticle sensor unit that can detect quantity of ion
wear particles in six levels (Photo 5). The iron wear
particles occur from operation of transmissions andaxles on construction machines. Certain machines
used in construction sites or mines are operated 24
hours a day: therefore, use of this sensor unit may
allows these machines to be maintained as necessary
and improve their operation efficiency.

Cable

4.1 Structure and features
Fig. 8 schematically illustrates the structure of the
new NTN magnetically operating iron particle sensor
unit.
This unit is essentially comprised of a central
electrode having an internal magnet and six rod
electrodes arranged around the central electrode:when iron powder deposits between these electrodes,
and if the electric resistance across electrodes drops
below a particular level, the sensor unit outputs asignal. The level of output signal will vary in six
settings depending on the quantity of iron particles
deposited on the electrodes. In other words, thissensor unit attracts iron particles from thetransmission oil and outputs a signal whose level is
proportional to the quantity of attracted iron particles,

SensorPhoto 5 Iron particle sensor (fixed type)-126-

NTN Sensor Units for Construction Machinery

4.2 Typical application

Central electrodeNTN has so far developed two types of magneticallyoperating iron particle sensor unit: a fixed type (Photo5), in which the sensor section is fixed to a vessel
such as an oil pan, and a coupler type (Photo 6), in
which the mount is independent of the sensor section
and the sensor section can be coupled and uncoupledwithout causing oil leakage.
The coupler type allows the user to determine thequantity of iron particles deposition on the sensor
section either based on sensor signal or throughvisual check. Also, the user can visually determine thesize of wear particles. Fig. 10 illustrates a scheme formounting the coupler type.

(with built-in magnet inside)

Sensor section
Rod electrodes(six rods)

Power supply

CircuitOutputs the number ofshort-circuits between thecentral electrode and rod
electrodes.

OutputFig. 8 Structure

54Output (V)Appearance of
deposited iron particles

32

1
Appearance before test

Appearance of
deposited iron particles

0

Quantity of ion particles

Smaller

GreaterFig. 9 Output signal and condition of iron particle

Cover

Prevention of oil leakage

Mounting thread
Oil panSensorMount
Coupler
Coupling/uncoupling
Coupler

Sensor

Cable

Fig. 10 Schematic view of coupler type

Photo 6 Coupler type-127-

NTN TECHNICAL REVIEW No.76（2008）

5. Conclusion
This paper has offered information about four typesof NTN's unique sensor units.
We believe that construction machines will
increasingly employ electrical control schemes to help
achieve more accurate work, higher efficiency andmuch improved maintainability. To help fulfill thesegoals, NTN remains committed to improve
performance of conventional sensor units and develop
new and improved sensor units.

Photo of authors

Shoji ITOMI

Hiroyoshi ITO

Application Engineering Dept.
Industrial Sales
headquarters

Application Engineering Dept.
Industrial Sales
headquarters-128-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Duplexing Gear Unit for Office Equipment

Tsutomu MAIWA*
Duplex printing is widely used in current copy machines andprinters as a measure to protect the environment. Those copy
machines and printers generally use reversal mechanisms toreverse paper by using forward and reverse rotation of the paper
feed roller.
NTN has developed a “Duplexing Gear Unit for Office Equipment”
without using an electric motor for forward and reverse rotation.
This new unit only uses one-way rotation, which is created byanother drive system.
This report introduces the structure and function of this product.

1. Foreword

2. Explanation of two-input duplexing
gear unit

Duplex printing-photocopying machine and printermodels are increasingly marketed for their ability tohelp reduce paper consumption as well as contributeto environmental conservation. For duplex printing, areversal mechanism is necessary for reversing thepaper stock. With a typical reversal mechanism, apaper stock having undergone printing on its front sideis switched back by reversing the running direction of
the drive motor to the paper feed roller and thenprinting is resumed on the back side of paper stock.
NTN has developed a unique Duplexing Gear Unit
for office equipment that would replace a motor
dedicated to the reversal mechanism and a drive
motor component such as a fusing section used tomechanically switch the direction of rotation reducing
energy consumption and cost.
The Duplexing Gear Unit is available in two types,
each having unique input scheme. They are two-input
one-output type (hereinafter “two-input duplexing gearunit”) and one-input one-output type (hereinafter “oneinput duplexing gear unit”) (Table 1).

2.1 Description of operation
1) First input gear forward rotation mode
When the first input gear is turned in the forward
direction, the spring clutch locks the first input gear,
causing the output gear engaged with the spring clutch
to turn in the forward direction. During this course, thesecond reverse input gear turns in the forward
direction together with the output gear (Fig. 1).

2) Second input gear reverse rotation mode
When the second input gear is turned in the reverse
direction, it comes into contact with the unlock hook of
the spring clutch, causing the diameter of spring clutch
to reduce. As a result, the spring clutch is unlocked;
thereby interrupting drive power transmission from thefirst input gear to the output gear.
When the second input gear is further turned, itcomes into contact with the output gear while reducing
the diameter of the spring clutch and unlocking thespring clutch, causing the output gear to turn in thereverse direction (Fig. 2).

*Application Engineering Dept. Industrial Sales headquarters-129-

NTN TECHNICAL REVIEW No.76（2008）

Table 1 Summary of direction change unit
Two-input Duplexing Gear Unit
(two-input one-output type)First input gear[forward rotation]

Output gearFirst input gear[forward rotation]

Control gear

Output gear

Appearance

Second input gear
[reverse rotation]

One-input Duplexing Gear Unit
(one-input one-output type)Features

Dimensions

Gear ODΦ26×overall length 28 mm

Gear ODΦ30×overall length 44 mm

This type consists of three gears―first input gear,
output gear and second input gear.
When the first input gear turns in the forward direction,
the output gear turns in the direction same as that of
the first input gear [forward rotation].
Next, when the second input gear turns in the reverse
direction, the output gear can turn in the direction
same as that of the second input gear, regardless of
the rotation direction of the first input gear
[reverse rotation].

This type consists of three gears―(first) input gear,
output gear and control gear.
When the control gear is allowed to freely rotate andthe input gear is turned in the forward direction, thecontrol gear turns in the direction same as that of theoutput gear [forward rotation].
Incidentally, when the control gear is braked to stop
its rotation (locked state), the output gear turns in thereverse direction [reverse rotation].
Note that the control gear is braked with a solenoid orelectromagnetic clutch.

Rotation direction
Engagement point of spring C of first input gear
clutch with output gear
D
Output gear(engaged with spring
clutch, and turns in
the forward direction)

First input gearforward rotation modeFirst input gear
Second input gear

Output gearA

Spring clutch

A
C―O―D

A―A

Fig. 1 Forward rotation of two input type direction change unit

Second input gear
reverse rotation mode

DC

Rotation directionof first input gear

(Spring clutch reduces
its diameter.)A

Rotation directionof second input
gear

Unlock hook Output gear
(spring clutch) (comes into contact
with second input
gear, and turns)

Unlock and torquetransmission section(second input gear)First input gear
Second input gear

Spring clutch
A―A

Second input gear

Fig. 2 Reverse rotation of two input type direction change unit

-130-

A
C―O―D

Output gearDuplexing Gear Unit for Office Equipment

2) Input gear forward rotation mode(control gear in locked state)

3. Explanation of one-input duplexing
gear unit

When the control gear is braked (locked state) by abraking pawl (*1), the unlock hook on the spring clutch
comes into contact with the control gear, reducing thediameter of spring clutch: consequently, the spring
clutch is unlocked blocking the transmission of drive
power from the input gear to the output gear.
When the input gear is further turned while thecontrol gear remains in the braked state, theintermediate shaft stops rotation. As a result, the bevel
gears rotate, causing the output gear to rotate in thereverse direction (Fig. 4).

3.1 Description of operation
1) Input gear forward rotation mode(control gear in free state)
When the input gear is turned in the forward
direction, the spring clutch locks the input gear,
causing the intermediate shaft engaged with thespring clutch to turn in the forward direction and theoutput gear to turn in the forward direction as well.
Note, however, that the braking means for the control
gear is in the OFF state (free state) (Fig. 3).

NOTE: Braking pawl (*1) is not included in the
Duplexing Gear Unit proper. An actual
braking pawl requires use of a solenoid orelectromagnetic clutch.

Input gear forward rotation mode(control gear in free state)
Engagement point of spring
clutch with output gearD

Rotation directionof input gear

C

Control gear

Input gear

A

Output gear

Output gear (engaged with spring
clutch, and turns in the forward direction)

Spring clutch

A

Intermediate shaft
Bevel gear

C―O―D

A―A
Braking pawl（※１）

(Control gear in free state)

Fig. 3 Forward rotation of one input type direction change unit

Input gear forward rotation mode(control gear in locked state)
Unlock hook(spring clutch)

Rotation directionof input gear

Control gear
A

Input gear

Output gearC
Rotation of Dintermediate
shaft stops.

Spring clutch

Unlock section(control gear)(Spring clutch reduces its diameter.)

A―A

Braking pawl（※１）

Intermediate
shaft Bevel gear

A

Bevel gears
rotate.
C―O―D

Brake ON
(control gear in locked state)

Fig. 4 Reverse rotation of one input type direction change unit

-131-

NTN TECHNICAL REVIEW No.76（2008）

Endurance test rigs for Duplexing Gear Unit are
shown in Figs. 5 and 6 (Fig. 5 for two-input type, andFig. 6 for one-input type).
The endurance test rig for two-input Duplexing Gear
Unit in Fig. 5 uses two motorsﾑthe first input gear
driving motor and the second input gear driving motor.
Reverse rotation cycles are provided bydisengagement and engagement with anelectromagnetic clutch.
The endurance test rig for one-input Duplexing Gear
Unit in Fig. 6 uses only one motor--the input gear
driving motor. The brake for control gear is applied
and released by an electromagnetic brake.

4. Endurance test
Endurance of each Duplexing Gear Unit has beenchecked under NTN endurance test conditions (Table
2).
Table 2 Test condition of Endurance test
(Test condition of NTN)
Condition 1 (two-input) Condition 2 (one-input)
300min-1

300min-1

Ambient temperature

100˚C

Ordinary temperature

Load torque0.1N･m0.1N･mRotating speed

One cycle

3 sec. FWD⇒BWD 3 sec. 3 sec. FWD⇒BWD 3 sec.

Two-input
Duplexing Gear Unit

Output-side load limiterFirst input gear
driving motor(forward rotation)
Second input gear
driving motor(reverse rotation)Reverse input gear
electromagnetic
clutchFig. 5 Endurance test machine of two input type direction change unit

One-input
Duplexing Gear Unit
Output-side load limiter

Input gear
driving motor(forward rotation)

Control gear
electromagnetic
brake

Fig. 6 Endurance test machine of one input type direction change unit-132-

Duplexing Gear Unit for Office Equipment

4.1 Test result

5. Conclusion

Table 3 summarizes the results of endurance test.
NTN studied the operation appearance quality ofsamples that underwent 300 thousands cycles(endurance requirement for ordinary printers), andlearned that all the gears are free from any damage or
fracture and function normally.

Society will increasingly become eco-conscienceoriented. In this context, duplex printing function will
be increasingly adopted even for less expensive photocopying machines and printers as this function canhelp reduce paper consumption. Consequently, there
will be growing needs for cost reduction with the costsof components used for producing this function.
The Duplexing Gear Unit for office equipment NTN
has recently developed is a unique product intended
to serve for ecological conservation and value
addition, and is characterized in that an electric motor
dedicated to duplex printing is eliminated, paper feed
direction can be mechanically switched over between
forward direction and reverse direction, thereby duplex
printing mechanism is available at a lower cost. NTN
will further reduce cost, improve performance andpropose new applications; thereby NTN will contributeto improve the environment and value.

Appearance after test

Table 3 Test

results

Two-input

One-input

Gearing has certain degree offouling, but each gear poses noproblem.

Gearing has certain degree offouling, but each gear poses noproblem.

Light contact has occurred on theouter circumference:no deformation at all.

Light contact has occurred on theouter circumference:no deformation at all.

No problem with both forward
rotation and reverse rotation

No problem with both forward
rotation and reverse rotation

Exterior state

Spring clutch

Operation quality

Photo of author

Tsutomu MAIWA
Application Engineering Dept.
Industrial Sales
headquarters-133-

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Large Gantry Table for the 10th Generation LCD Substrates

Katsuyoshi SUZUKI*
Naoshi SUZUKI*NTN has deeply cultivated precision positioning technology andapplied it to various types of positioning systems. One of theirmajor fields of application is LCD manufacturing equipment, andNTN provides large, long-stroke XY tables suitable for various
types of equipment, including NTN LCD repair systems.
This paper describes problems arising from the growing size of
XY tables and gives an outline of a prototype gantry-type XY tablethat meets the upcoming super-sized (about 3-m2 G10) glass
substrates.

1. Foreword

2. History of upsizing of LCD glass
substrate

The demands for flat panel displays, typically flatscreen television sets, have been increasing at a rapid
pace. To address this trend, flat panel manufacturers
have been making investments for production of
displays that are capable of providing images ofhigher definition and quality, by adoption of larger
sized glass substrate in order to improve production
efficiency and enhance value of display products. In
this context, longer stroke and higher precision areneeded for XY tables that constitute the core ofequipment for manufacturing and inspecting LCD
substrate.
NTN has been improving and enhancing its XY
table products through lighter, larger more rigid
designs, and has been incorporating its improved XY
tables into LCD repair equipment, color filter repair
equipment and plasma display repair equipment.
Building upon this accumulated experience, NTN has
also been actively developing large-sized tables. We
have recently developed a large XY table to be able tohandle the generation 10 LCD mother glass substrate(the world's largest size). This paper hereunder
describes the engineering challenges, and the resultsof the technical evaluation for this product.

As the applications of LCD expand from personal
computers to television sets, the need for larger
screens continues to increase rapidly. To cope withthis trend, facilities and technologies for producing
LCD panels have been improving. Consequently, thesize of LCD mother glass substrate has been
dramatically increasing.
Table 1 summarizes history of upsizing of LCD
glass substrate sizes. The generation 10 motherglass, whose production has started recently,
measures about 3,000 mm in one side. The stroke ofan XY table, on which such large LCD mother glass
substrate is subjected to processing such as laser cut,must also exceed 3,000 mm, and the size of the tableneeds to be accordingly as large.
Table 1 LCD mother glass size
Generation 2

270×360mm（1987）

Generation 3

360×465mm（1994）

Generation 4

550×650mm（1997）

Generation 5

680×880mm（2000）

Generation 6

1100×1250mm（2002）

Generation 7

1500×1800mm（2004）

Generation 8

1870×2200mm（2005）

Generation 92160×2460mm（2006）

Generation 10

2850×3050mm（2009）*Actual year of realization may differ with a given LCD panel manufacturer.

*Product Engineering Department Precision Equipment Division-134-

Large Gantry Table for the 10th Generation LCD Substrates

transportation as separate units, and the machinemust be able to be quickly reassembled and adjusted
at the site where the machine is installed. Morespecifically, the block structure must clear thefollowing issues:

3. Restrictions about upsizing of table
So far, in response to the need for handling larger
glass substrate, LCD panel manufacturers have beenachieving compact table structure by optimizing theshape of the base on an XY table, and improving
layout for the linear guide, linear motor, and cablechain. This achievement helps reduce the necessary
floor area of the XY table in a clean room andcontributes to prevention of occurrence of problems in
manufacture and transportation of XY table. In thecase of the tables capable of handling generation 10
LCD glass substrate, the minimum table width
exceeds 3,500 mm. Consequently, the following two
problems have to be addressed:

1 Accuracy of parts mounting surfaces andjoint faces
The mounting surfaces on the base for mounting
the linear guide, linear scale, linear motor, etc. areimportant factors that determine the characteristics of
XY table. Therefore, higher levels of precision inflatness, straightness, etc. must be satisfied.
When a plurality of bases are assembled into one
entity, accuracy of table operation can be jeopardizedor the effective life of the linear guide can become
shorter if the joints have a problem such as height
difference or bend. To avoid this problem, machining
precision for each mounting surface needs to be
improved, and at the same time, incorporation of an
adjusting mechanism will be necessary.

1) Problem arising from allowable work piecesize for machine tool
A relatively high number of machine tools canmachine work pieces whose width measures less than
3,000 mm. In contrast, a smaller number of machine
tools are capable of processing work pieces with width
over 3,000 mm, and machine tool users may fail tosatisfy the customer's request for lead time.

2 Easy disassembly, reassembly and adjustment
To help simplify disassembly before shipment, andreassembly and adjustment after delivery at site, theXY table has to consist of a minimum number ofcomponents. The XY table needs to be designed suchthat the functional units mounted can be removed in
the form of single block. Additionally the XY table mustallow handling with accessible equipment, such as
hydraulic jack, through adoption of light-weight, highly
rigid members.

2) Problem about maximum allowabledimensions for transportation
With respect to transportation, a larger XY tableposes a problem. A relevant law or regulation in Japan(Law of Road Transport Vehicles etc.) stipulates thatland transportation with an ordinary vehicle is not
permitted if the cargo width exceeds 3,500 mm. A
transportation agent may acquire special roadoccupation permission from a relevant authority andtransport the large XY table on a public road during
night time. If the machine is fabricated in a sea-side
plant, it can be transported on a maritime vessel.
Considering these additional requirements, an XY
table capable of handling work pieces exceeding
3,500 mm in width poses a large challenge in fulfilling
requirements for shorter lead time, and timely
fabrication and delivery.

3 Accuracy measurement at site
After joining and reassembly of the base, it isnecessary to confirm restoration of accuracy of thebase. Note, however, the utilities including electric
power supply and compressed air may not yet beavailable at site. Therefore, a simple accuracy
confirmation method has to be established inadvance.

5. Specifications of super-large table
Table 2 summarizes the specifications of the NTN
gantry table that is capable of generation 10 (G10)
LCD glass substrate, and Fig. 1 shows appearance ofthe gantry table.
The Y-axis base can be split into two pieces at its
near-middle location, with the line of separation being
parallel with the X-axis gantry beam. Therefore, thelocation of the base joint coincides with the location of
the linear guide joint (Fig. 2).
Because of availability and reasonable cost, canstructures have been adopted for the base and beam,
each a major member of the XY table.
Compared with stone material, steel material boasts

4. Considerations about split structure
The above-mentioned problems can be solved byadopting a split-structured design for the base of XY
table. Because dimensions of individual units are
smaller, and the scope of available machine tools isgreater, engineering considerations about the shapeand precision of work piece can be less demanding.
At the same time, considerations about limitations withtransportation are not necessary.
In developing a block structure, certain requirements
need to be satisfied. The structure must feature adesign that excels in restoration of accuracy after-135-

NTN TECHNICAL REVIEW No.76（2008）

disassembly and reassembly work while addressing
technical challenges associated with disassembly andreassembly procedure.
High performance linear motors have been
incorporated into the drive system of the gantry table
to cope with the longer stroke and higher speed withthe larger XY table. A high thrust cored linear motor(Fig. 3) has been adopted for the Y-axis to be able todrive the heavy gantry beam with sufficient
acceleration, and a coreless linear motor (Fig. 4)
featuring smaller space and constant speed has been
adopted for the X-axis.
Upsizing of the LCD glass substrate is intended for
improved productivity. A longer process time inproportion with the glass substrate size cannot bejustified. Therefore, the table must be faster. The tablespeed has been increased to 1,500 mm/s byimproving thrust for the linear motor and incorporating
a lighter table design. Additionally, these enhancements
have realized acceleration/deceleration rates of 0.3 G
with the table (ability to accelerate to 1,500 mm/s inapproximately 0.5 sec.).
To ensure easy disassembly and reassembly of thetable, a detachable linear scale that boasts goodworkability and reliable restoration for mounting
accuracy has been selected.
The drivers to be used in conjunction with the linear
motors are types featuring high-speed response,
helping achieve improved resolution for scale signals:
this feature leads to improved controllability that helps
inhibit speed variation.

higher rigidity and this feature helps achieve a lighter
structure in spite of somewhat inferior vibration
damping quality. Steel material also features higher
degree of freedom in a shape-forming process. In
designing the base and beam, NTN has attempted torealize simplicity and restoration of accuracy inTable 2 Specification of table for G10 LCD glass substrate
X-axisY-axisStrokeTable specifications

3,200mm3,650mmMax. velocity

1,500mm/s

1,500mm/s

Max. deceleration/acceleration
Resolution

0.3G

0.3G

0.1 µm/pulse

0.1 µm/pulse

Positioning accuracy

30 µm

30 µm

Repeated positioning accuracy±0.5 µm±0.5 µm

Horizontal straightness

40 µm

40 µm

Vertical straightness

20 µm

20 µm

Linear motor

Coreless

Cored

Linear scale

Permanently glued

Removable

Gantry constitution

Upper axis beam

Lower axis baseNone

Two pieces

SplittingMaterial

Steel (can structure)

External dimensions

W3,900×D5,200×H2,500 mm

Total mass

15,000 kg

X-axisBeam
Y-axisBase
Cable chain

Lower frame
Linear motor

Linear guideFig. 1 Appearance of the super-sized table

Fig. 3 Linear Servomotor with iron core

Joint on linear guideLinear scale
Joint on baseFig. 2 Division part of the Y-axis baseFig. 4 Coreless Linear Servomotor-136-Large Gantry Table for the 10th Generation LCD Substrates

the present evaluation, our gantry table has notexhibited any adverse effects on various accuracy
criteria including static accuracy, positioning accuracy,
and straightness. In order to fully develop XY tableperformance, a vibration damping mechanism will beneeded which can inhibit transmission of floor vibration
to the XY table and promptly dampen residual vibration
occurring from transportation of a heavy object. Various
vibration damping units and combinations are available,
including vibration damping rubber material, vibration
damping damper, passive vibration isolation table, andactive vibration isolation table. To support a greatertable mass, it is necessary to select vibration isolatingparts that are resilient, boasting larger load carrying
capacity and higher performance. As an XY tablemanufacturer, NTN will address difficult challenges as
to optimum selection and proposal of an XY table typethat best suits the intended application andperformance requirement, while satisfying the costperformance need.

6. Evaluation test
A sample temporarily assembled (state before
disassembly) and another sample in disassembled
state were transported on a truck for about 10 km.
Then, the samples were reassembled and adjusted.
Next, these samples were compared with each otherin terms of resultant positioning accuracy, repeatedpositioning accuracy, and horizontal/verticalstraightness in order to confirm effect of reassemblyon original accuracy.
As summarized in Table 3, we have been able toconfirm restoration of accuracy to the specified levels
after the Y-axis base was disassembled andreassembled. It has also been verified thatdisassembly/reassembly is completed in the expected
time span.
Fig. 5 plots the difference in actual measurement of
positioning accuracy with Y-axis between before andafter disassembly of the Y-axis base. Similarly, Fig. 6
plots the difference in terms of horizontal straightness.
Before the evaluation test, the impact of vibration andinstallation site factors (such as floor structure) on theperformance of the NTN gantry table were unknown. In

7. Conclusion
Performance of a large XY table directly governs theperformance of LCD manufacturing equipment. In thecontext of upsizing and achieving higher precision with
LCD glass substrate, the requirements have been
increasingly complex for the XY table used in LCD
manufacturing equipment, and such complex
requirements include shape, functions, controllability,and performance.
Based on its accumulated technologies andexperiences, NTN will address the challenges at higher
levels for development, including higher precision,
higher speed, and enhanced performance. NTN as an
XY table manufacturer, will further remain committed to
improve productivity and quality of flat panel displays.

Table 3 Result of evaluation test（Y-axis）
Evaluation criteria
Positioning accuracy

Variation after reassembly
5.0 µm max.

Repeated positioning accuracy

0.1 µm max.

Horizontal straightness

7.0 µm max.

Vertical straightness

0.1 µm max.

（µm） 1050-5References-101) Electronic Journal supplement 2007, LCD fab/equipment/facilities, Electronic Journal, 2007 LCD Equipment Data Book

Measurement range (full stroke)

Fig. 5 Difference of positioning accuracy (Y-axis)
before and after disassembly and transportation

Photo of authors（µm） 10
5
0
-5

-10

Measurement range (full stroke)

Fig. 6 Difference of horizontal straightness (Y-axis)
before and after disassembly and transportation-137-

Katsuyoshi SUZUKI

Naoshi SUZUKI

Product Engineering Department
Precision Equipment Division

Product Engineering Department
Precision Equipment Division

NTN TECHNICAL REVIEW No.76（2008）[ New Product ]Development of a Linear Changer with a Built-in Pasting Unit
for a Flat Panel Defect Repair System
Akihiro YAMANAKA*
Akira MATSUSHIMA*

The NTN color filter repair system fixes defects in color filters, which are theprimary components of liquid crystal displays.
The most important feature of this system is the function of pasting ink to acolorless spot, commonly referred to as a "white defect", on the color filter.
The pasting unit carries out the ink pasting process using a pasting needle.
The system moves the pasting needle to touch the defect in the color filter andpastes a slight amount of ink from the tip of the pasting needle to the defect.
The pasting needle system can stably carry out the pasting process because
there is no possibility of clogging as in a dispenser system. It has received
favorable comments from many customers.
This paper explains the outline of the pasting mechanism and the linearchanger with built-in pasting units.

1. Foreword

2. Possible defects on color filter (CF)of LCD and repair method

In the past several years, flat panel displays (FPD)
such as liquid crystal displays (LCD) and plasma
displays (PDP) have been widely used for home
television sets, and their screen size has been everincreasing.
Since 1990, NTN has been developing andmarketing pattern repair systems that repair defects
occurring in production processes for these FPD
products. When used to repair defects and reduce
defective substrates being scrapped, the NTN repair
systems have been contributing to mitigation ofenvironmental impacts and reduction in FPD
production cost. In recent years, a repair process has
been widely recognized as mandatory in FPD
manufacturing while larger, more finely defined
screens are increasingly demanded.
To help improve production efficiency for LCD, thesize of LCD mother glass substrate has been rapidly
increasing, and production of the generation 10 size
(2850×3050 mm, t = 0.7 mm) is going to start verysoon.
Using the NTN ink pasting unit used for repairingcolor filter (CF) for LCD as an example, this report
describes our newly developed linear changer withbuilt-in pasting unit.

As illustrated in Fig. 1, an LCD comprises a TFT
substrate and a CF substrate which are glued
together. Both substrates are separated with pillars
called photo spacers. The space between bothsubstrates is filled with liquid crystal and a back light issituated on the back face of LCD.
A color filter (CF) is a component that adds color
information to light that is turned ON/OFF by the liquid
crystal. The CF consists of a glass substrate on which

Eye
Color filter(CF) layer

Polarizing
plate

Photo spacer

Commonelectrode［ITO］

CF substrate[glass]
Seal

Seal

TFT substrate[glass]Polarizing plate Liquid crystal TFT
layer
Light

Pixel electrode

Back light

Fig. 1 LCD structure figure*Product Engineering Department Precision Equipment Division-138-

Development of a Linear Changer with a Built-in Pasting Unit for a Flat Panel Defect Repair System

R (red), G (green) and B (blue) pixels are arranged ina stripe or matrix pattern. Light passing the CF isprovided with color information corresponding to the
pixels involved. As a result of this addition of color
information by the CF, the LCD exhibits a color image.
A color filter (CF) repair system, repairs a defectoccurring in a color filter (CF) during CF panel
production process.
A white defect is defined as a defect of a pixel which
has lost its color, and can be repaired with a pasting
unit by applying ink whose color is same as that of thedefective pixel.
A black defect is defined as a defect of a pixel towhich another color has been mixed or dust or
contaminant has been discarded. This type of defectcan be repaired first by removing a defective area witha laser to convert the black defect into a white defect,
and then applying ink of the same color as that of theoriginal pixel to the white defect.

needle. The tip of the pasting needle has a uniqueshape so that ink can be very stably applied to a verysmall area about 30μm to 300μm in diameter.
Consequently, this system is highly appreciated by theusers. Unlike an ink pasting operation with adispenser where ink is dispensed through a fine hole,
the pasting operation with a pasting needle has anadvantage because the pasting needle is free fromclogging and can handle paste of a wider scope ofviscosity.
The ink pasting unit NTN initially developed isschematically illustrated in Fig. 4.
NTN's initial ink pasting unit was comprised of an
actuator that moves the pasting needle up/down, arotary table that carries ink cassettes (four ink
cassettes for repairing color filter, that is, R (red), G(green), B (blue) and Bk (black)), the rinsing
mechanism that is mounted on the rotary table inorder to rinse the pasting needle, and the air purge
mechanism that dries the rinse liquid deposited on the
tip of pasting needle.
In the ink pasting operation, ink is allowed to adhere
to the tip of the pasting needle, and then the tip of thepasting needle is brought into contact with a white
defect to transfer ink on the needle tip to the defect.

Black
defectWhite
defectPasting unit

Observation
optical system

Fig. 2 The defect of CF panel

CF substrate

Fig. 3 Color filter repair system

3. History of pasting units
A color filter (CF) repair system, shown schematicallyin Fig. 3, has a pasting unit mounted beside anobservation optical system that can be shifted to anarbitrary location on a color filter substrate that is to be
repaired.
During a defect repairing process, the observation
optical system is shifted to the location of the defect,
the defect is observed, and then repaired by applying
ink to the defect with the pasting unit.

Actuator

Ink cassette
Pasting needle

3.1 Initial model pasting unit
To apply ink to a white defect on a color filter (CF),NTN has adopted a system using a the pastingRinsing mechanism

Air purge mechanism

Fig. 4 Initial model pasting unit

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NTN TECHNICAL REVIEW No.76（2008）

The pasting needle ismounted to the end of an
actuator drive shaft for
driving the pasting needle
via a vertically movableslide mechanism, whereinconstant contact pressure
Flat faceis maintained between the
tip of the pasting needle
and the substrate during
Fig. 5 Needle tip shape
the ink pasting process.
The end of the pasting needle is tapered as shown in
Fig. 5, and the needle tip is machined to have a flatface that is square to the axis of the needle.
When ink is allowed to adhere to the tip of thepasting needle, the ink is drawn up along the pastingneedle by surface tension. By transferring ink on theflat face of the needle tip to a defect, very delicate ink
application is stably executed. The dimension of theflat face on the needle tip can be varied to adjust thepasting size. Consequently, pasting with a diametersize of 30μm to 300 µm is possible as previously
mentioned.
The initial model of the pasting unit was equippedwith only one or two pasting needles. Therefore, when
ink color had to be changed, the pasting needles
needed to be rinsed in order to avoid mixing colors.
The pasting needles were rinsed with the rinsing
mechanism and air purge mechanism shown in Fig. 4.
The rinsing mechanism rinses a pasting needle bystirring rinsing liquid in the rinsing vessel with apropeller mounted to the motor shaft. Then, the air
purge mechanism blows air around the pasting needle
to dry any rinsing liquid remaining at the tip of needle
to complete the rinsing process.

In the improved pasting unit, a drive mechanism
dedicated to pasting has been developed to shorten
the time needed for travelling the offset distance,
wherein the pasting needle is deployed into theobservation location beneath the objective lens toapply ink to a defect. By adoption of this arrangement,
the pasting needle can be quickly deployed to thepasting position.

Z-axis
X1-axis
X2-axisObservation
optical system
Pasting
needleY-axis(Appearance of pasting unit)

Ink cassetteFig. 6 Improved pasting unit

Observation
optical system
Pasting unit

Observation
location
OffsetPasting needle

Fig. 7 Arrangement of observation optical system andInitial model pasting unit

3.2 Improved pasting unit
As larger screen LCD television sets have been
increasingly marketed, pixel sizes have become
increasingly larger, and larger defects are occurring
more frequently. To shorten repair tact time for suchlarger defects, NTN has developed the improved
pasting unit shown in Fig. 6.
The improved pasting unit features the following
major improvements.

(2) Adoption of pasting needles each dedicatedto specific repair ink
As previously mentioned, the initial model pasting
unit had only one or two pasting needles. Because ofthis, the pasting needle needed to be rinsed before adifferent color ink could be used. To eliminate thisrinsing process to shorten the pasting tact time, theimproved pasting unit has four color-specific pastingneedles―R (red), G (green), B (blue) and Bk (black).
Thanks to this arrangement, the repair tact time has
been much reduced.

(1) Adoption of drive mechanism dedicated topasting
On the initial model pasting unit, the optical systemfor observing defects was offset from the location of
pasting needle as shown in Fig. 7. Therefore, in orderfor pasting to be possible, the pasting needle had tobe shifted to the observation location by moving thetable on the defect repair system.
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Development of a Linear Changer with a Built-in Pasting Unit for a Flat Panel Defect Repair System

As a result of adopting a support construction thatunites the pasting needle and the ink cassette, repair
ink is fed to the pasting needle by simply stroking thepasting needle into and out of the ink cassette. As
illustrated in Fig. 9, and compared with the initialmodel pasting unit, the improved pasting unit iscapable of completing repairing for repeated repairing
ink applications with a plurality of defects.
Thanks to the above-mentioned improvements, theimproved pasting unit has reduced the repair tact timeby 70% compared to the initial model pasting unit.

(3) Integrated support structure comprising
pasting needle and ink cassette
Because the improved pasting unit has four pastingneedles as mentioned above, the arrangement for ink
cassettes has been reconsidered. Therefore as shownin Fig. 8, an ink cassette of a particular color is
assigned to a pasting needle dedicated to that colorand individual units each consisting of a pastingneedle and ink cassette are independently supported.

4. Linear changer with built-in pastingunit

Pasting needle

As described above, NTN has been improving its
pasting unit. To further reduce repair tact time andimprove positioning accuracy for pasting, NTN has
recently developed a linear changer with a built-in
pasting unit.

Ink cassette
Ink

Fig. 8 United support construction of the Needle
and Ink cassette

4.1 Linear changer(new objective lens switch-over mechanism)
On the improved pasting unit, the pasting needle isinserted into the observation location beneath theobjective lens, and then pasting action is performed.
However, this system has a drawback--because oflimitation in the work distance with the objective lens
used for observation (distance between the subject
being observed and the objective lens), the pastingneedle can be inserted into the observation location
only when an objective lens of a lower magnification
power is used. Consequently, if ink is to be applied toa defect that is observed with a high magnification
power objective lens, a low magnification objective
lens needs to be deployed and only when pasting is
executed. Therefore, time needed for switching-overof objective lenses poses a challenge in reduction of
repair tact time.
Previously, the revolver unit shown in Fig. 10 wasused to switch over between objective lenses. This
unit holds several objective lenses and indexes these

Each ink cassette has holes on its top and bottom
so that a pasting needle can penetrate it. The pastingneedle is inserted downward through the upper hole.
The tip of the pasting needle always remains
immersed in ink in the ink cassette so that ink on the
tip of the needle does not dry. In an ink pastingprocess, the tip of the pasting needle protrudes
through the lower hole of the ink cassette, and comes
into contact with a defect to apply ink to the defect. As
mentioned previously, the pasting needle during thisprocess remains secured to the drive shaft of thepasting needle driving actuator via the verticallymovable slide mechanism, thereby maintaining
contact pressure between the pasting needle and thesubstrate.

Pasting action

Resupply of ink
a. Standby for pastingb. Needle moves up.

c. Pasting in progress(1) Initial model pasting unit
Pasting action

Resupplyof inkb. Pasting in progressa. Standby for pasting(2) Improved pasting unit

Fig. 9 Ink pasting process comparison

Fig. 10 Revolver unit

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NTN TECHNICAL REVIEW No.76（2008）

Starting with the improved pasting unit, NTN
attempted to design a more compact pasting unit.
Consequently, NTN has succeeded in developing avery compact pasting unit that can be built into thelinear changer.
Remember that on the improved pasting unit, thepasting needle is inserted in the visual field being
observed. However, on the linear changer thatincorporates the pasting unit, the pasting needle does
not interfere with the objective lens, and the linearchanger can hold a greater number of pasting needles
compared with the improved pasting unit.
On the pasting unit that is built into the linearchanger, the space occupied by one objective lens
can accommodate two pasting needles in place of one
objective lens, thereby enabling the pasting units tohold a maximum of eight pasting needles in place ofobjective lenses.
Recently in the flat panel manufacturing industry,
screens of different sizes are obtained from single
mother substrate. In this case, it may be necessary tosimultaneously repair pixels of different sizes. To copewith such a situation, the new pasting unit cansimultaneously mount a variety of pasting needles ofdifferent sizes for differently sized pixels. Also, in the
recent LCD industry, development of a novel color
filter has been disclosed. This color filter is capable offive colors, including yellow and cyan in addition to
RGB colors already available with conventional color
filters, in order to improve image quality attained with
LCD. NTN's new pasting unit is capable of repairingsuch multi-color color filter.

lenses to move a necessary lens to the observation
location. To reduce the lens switch-over tact time, thelenses need to be turned at a higher speed.
Therefore, NTN has been improving this unit to reducelens switch-over time. However, since this mechanism
mechanically stops each objective lens at an
observation location, if the revolver is turned at ahigher speed, it is difficult to stop an intended
objective lens exactly at an observation location. In
other words, further reduction in lens switch-over timewith this system appears to be nearly impossible.
To address this problem, NTN has developed adirectly acting (linear system) objective lens switchover unit (linear changer).
This unit has several objective lenses, eachobjective lens assigned to a laser of unique frequency
for cutting a defect. Depending on the intended
application, the unit may be equipped with a laser
capable of emitting a variety of frequencies.
Accordingly, the linear changer has been designed tohold a maximum of ten objective lenses.
If ten objective lenses are arranged in one line andone lens is selected from this line, the length of thelinear changer will be excessively large, and the linearchanger will not be easily built into the pasting system.
To avoid this problem, a configuration of five lenses in
two rows has been adopted. With a two-row layout,
the objective lenses need to be shifted in two
directions (X and Y directions). When a laser beam isallowed to enter a given objective lens, there islimitation in the distance between the lens barrel andthe objective lens pupil plane owing to the relationshipwith the imaging lens. That is, if the thickness of thelinear changer is large, the distance between theobjective lens pupil plane (lens plane at the upper
portion of objective lens) and lens barrel is greater andthe laser power density on the objective lens pupil
plane is greater, which will damage the objective lens.
To avoid this situation, NTN has invented an improved
XY feed mechanism that allows the distance between
the objective lens and lens barrel to fall within an
appropriate range.
A linear motor and a linear encoder (resolution of
0.1 µm) have been adopted as a means for actuating
the XY feed mechanism in order to realize higher
speed for actuation and improved positioning
accuracy.

4.3 Features of linear changer with built-in
pasting unit
The newly developed linear changer with a built-in
pasting unit is schematically illustrated in Fig. 11.

(1) Improved positioning accuracy for pasting
The improved pasting unit described in Sec. 3.2 has
improved positioning accuracy by eliminating thenecessity for offset compensation, which waspreviously accomplished by turning the table in thepasting unit. However, when a variety of defects arepresent in the visual field being observed, the table yetneeds to be turned accordingly. This is because, on
the improved pasting unit, when the pasting needle isinserted in the visual field being observed and theneedle applies ink to a defect, the needle can interferewith a nearby objective lens, and at the same time, notattain a sufficient travel distance for a pasting needle
in the visual field being observed. Since travel of thetable is unavoidable, a longer repair tact time isaccordingly necessary if a variety of pixels in the

4.2 Pasting unit built into linear changer
In conjunction with the development of the linearchanger, NTN has reconsidered the arrangement of
the ink pasting unit. The integrated support structure
comprising the pasting needle and ink cassette,
adopted for the improved pasting unit, is very compact.
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Development of a Linear Changer with a Built-in Pasting Unit for a Flat Panel Defect Repair System[Conventional][Conventional]

Lens barrel

RevolverPasting unit

Actuation in Y-axis

Objective lensObjective lensPasting unit

Pasting unit

Actuation in X-axis

Fig. 11 Linear Changer with built-in pasting unit

production continues (generation 10 glass substrate
will be processed soon), reduction in tact time andsaving in labor saving on a given flat panel production
equipment are critical considerations for improving
productivity. NTN wishes to develop novel equipmentthat helps improve productivity for plat panels.

visual field being observed have to be repaired.
Remember that the revolver type objective lensswitch-over system mechanically positions theintended objective lens. As a result, positioning
accuracy for objective lens will somewhat deteriorate
as the objective lens switch-over system is repeatedly
actuated.
On NTN's newly developed linear changer withbuilt-in pasting unit, travel of the objective lens andpasting needle to the intended observation location isachieved with the XY feed mechanism on the pasting
unit alone, thereby enabling execution of highly
accurate ink pasting at high speed.

References
1) Yamanaka, Monthly Display, Dec. 2004
2) Yamanaka, New material developments andmanufacturing processes in color filters compilation,
Jul. 2006 issue
3) Yamanaka, Monthly Display, Dec. 2006
4) Saruta: NTN Technical Review No.72 (2004)
5) Yamanaka and Matsushima: NTN Technical Review
No.72 (2004)(2) Reduction in repair tact time
Unlike the improved pasting unit, the NTN linearchanger with built-in pasting unit is free frominterference between a pasting needle and an
objective lens, and can immediately start pasting
regardless of the magnification power of the objective
lens currently used for observation. Also, the linearchanger boasts increased objective lens switch-overspeed and greater pasting needle shift speed.
Through the above-mentioned improvements, thelinear changer with built-in pasting unit has achieved
reduction of approximately 20% in repair tact timecompared with the improved pasting unit.

Photo of authors

5. Conclusion
This report has described the NTN linear changer
with built-in pasting unit. NTN has been committed to
not only development of this product but also to
improvement in repair quality and reduction in repair
tact time through improvement of the pasting needle
for the pasting unit. Also, NTN has been helping
realize labor saving through development of automatic
defect repair software.
As upsizing of mother glass substrates used for LCD-143-

Akihiro YAMANAKA

Akira MATSUSHITA

Product Engineering Department
Precision Equipment Division

Product Engineering Department
Precision Equipment Division

NTN TECHNICAL REVIEW No. 76（2008）

Presentation of award-winning articles
2007 Engineering Award, The Japan Society for Precision Engineering

Development and Practical Application of MQCJ-LubricatedAngular Contact Ball Bearings for Machine Tool Main Spindles
Masatsugu MORI, Yoshinobu AKAMATSU, Mineo KOYAMA, Futoshi KOSUGI, and Hiroshi TAKIUCHI

1. Features of MQCJ-lubricatedangular contact ball bearings 1)

3. Test results 2)
The temperature difference across the inner ringand outer ring as well as power loss were analyzed foran angular contact ball bearing configured withconstant pressure preloading and a bore diametermeasuring 70 mm. The bearing was lubricated with
MQCJ lubrication, jet lubrication and air-oil lubrication.
Then, the analysis results obtained from theselubrication systems have been compared with eachother.
Fig. 2 graphically plots temperature differences
across the inner ring and outer ring and power losses
obtained from these lubrication systems. From thesecharts, it should be understood that the bearing
cooling performance of MQCJ lubrication is equivalent
to that of jet lubrication, and fairly excels that of air-oil
lubrication; and that power loss with MQCJ lubrication
is equivalent to that with air-oil lubrication and is muchsmaller than that with jet lubrication.

Our newly developed angular contact ball bearings
incorporate newly developed MQCJ lubrication
mechanism and boast not only bearing coolingcapability of jet lubrication but also low power lossfeature of air-oil lubrication.
The MQCJ lubrication system can inhibit
overheating of a bearing even under high-speed
running conditions and contribute to super high-speed
operation with constant pressure preloading,increased preload (higher rigidity) by definite position
preloading, and mitigation of environmental impacts.

2. Structure of MQCJ-lubricatedangular contact ball bearings

Temperature difference acrossinner ring and outer ring
˚C

Fig. 1 schematically illustrates the structure of an
MQCJ-lubricated angular contact ball bearing set andthe associated lubricant flow path. Lubricant fed
through the oil hole on the outer ring spacer issqueezed by the nozzle, and a high-speed jet of
lubricant hits the scoop formed around the entire
circumference of the inner ring side plane tothoroughly cool down the inner ring. A large portion of
the lubricant received by the scoop is allowed to drainaway; the remaining portion of lubricant adheres to theramp on the inner ring, and moves toward the bearing
interior along the ramp due to centrifugal force andsurface tension and lubricates the bearing.

25

Temperature difference acrossinner ring and outer ring20

Air-oil
Jet
MQCJ

15
10
5
0
-5

0

10000

20000

30000

40000

50000

Bearing speed min-1
5Power loss
Outer ring spacer

Power loss kW

Oil hole

43

Air-oil
MQCJ
Jet

2
1
0
0

Ramp

Scoop

10000

20000

30000

Bearing speed

Nozzle

40000

50000

min-1

Fig. 2 Temperature difference between inner and outerrings and comparison of power loss among lubrication types

Fig. 1 Structure and lubricant flow

-144-

Presentation of award-winning articles

4. Conclusion
The newly developed MQCJ-lubricated angularcontact ball bearings satisfy requirements for highspeed performance, higher rigidity and low
environmental impact quality, which are therequirements recent machine tool main spindle
bearings must satisfy, and will help machine tools
reach a higher level of engineering excellence.

References
1) M. Koyama: Minimum Quantity and Cooling Jet
Lubricated Angular Contact Ball Bearings for Machine
Tools, NTN Technical Review No.74 (2006) 22-25
2) Mori, Akamatsu, Koyama, Kosugi, Takiuchi andFurubayashi: Development and Practical Realizationof Minimum Quantity and Cooling Jet LubricatedAngular Contact Ball Bearings for Machine Tools,
Journal of JSPE, Vol.74 No.6 (2008) 539-542

Photo of authors(Leader)

Masatsugu MORI
Elemental Technological
R&D Center-145-

NTN TECHNICAL REVIEW No.76（2008）[ New Products Information ]V-series Hub Joint
A hub joint is integrated with a constant-velocity joint
to achieve a compact, light-weight hub joint design!

Features

Construction [means for realization]¡Adoption of novel joining technique—Press-cut
joining(1) Weight: 12% weight reduction(compared to conventional design)(2) High performance: No play between hub
bearing and constant velocity joint.
Prevents stick-sliding noise.
(3) Simpler assembly work: Number of work
hours at the user’s site is decreased asa hub joint is integrated with a constantvelocity joint.

Hub ring
Press-cut joint
Constant-velocity joint
outer ring

¡Two arrangements are available—Consolidated
type (hub bearing is consolidated with constantvelocity joint) and separable type (hub hearing canbe separated from and joined again to constantvelocity joint).

Applications¡Hub bearing and driveshaft for
automotive applications resistance.

[Consolidated hub joint]-146-[Separable hub joint]

New Products Information

V-series Angular Unit
Need for light-weight design and decreased material use are simultaneously
achieved while necessary mechanical strength is realized!!

Features (comparison to conventional design)

Construction¡Light-weight shape: Precision metalforming technique is applied to the innerring and outer ring.

(1) Weight: 12% lighter(2) Amount of material used: 30% reduction(3) Simplicity of assembling: Can beassembled with peripheral components
in an unmodified conventional process【Conventional design】

Applications¡Automotive hub bearing-147-【New design】

NTN TECHNICAL REVIEW No.76（2008）

Hub Bearing w/ Built-in Load Sensor
The sensor is effective even at a vehicle speed of 0 km/h. It detects a cornering
force working on a running vehicle on a real-time basis in order to help assist
control in side skidding of the vehicle!

Features

Construction(1) Sensor installed on each wheel detects
the cornering force; the magnitude of
detected cornering force is important incontrolling the skidding behavior of thevehicle in question.
(2) Improved response in detecting
cornering force(3) Even when a particular wheel is locked(zero wheel velocity), a cornering force
working on it can be detected.
(4) Load detection range: -4 kN to 10 kN(5) Detection accuracy: ±3%FS (full scale)Acceleration equivalent to that on a car
in 0.6G turnCornering force

Strain sensor

Vehicle turns.
(cornering force)

Applications

Detection by strain sensor¡Hub bearings for passenger cars
Cornering force is detected.

-148-

New Products Information

High-Efficiency Compact Constant-Velocity Joint(ELJ)
Compact, light-weight, highly efficient drive shaft
constant-velocity joint

Features (comparison with conventional design)

Construction¡Cross groove type constant-velocity joint
with ten balls (conventionally, six)(1) Compact, light-weight
Outer ring outside diameter: 4% size
reduction
Weight: 18% reduction(2) Higher efficiency
Torque loss rate: 50% reduction(3) Lower slide resistanceSlide resistance: 50% reduction

Applications
Inner ring

¡Automotive driveshaft

-149-

Balls

Cage

Outer ring

NTN TECHNICAL REVIEW No.76（2008）

Planetary Gear Unit
for Automotive Transmission
Realization of both higher speed and compact size!!

Cage
High-speed capable
cage design throughstatic analysis anddynamic analysis(50% reduction instress generated)

Rollers and
shaft

Gear
Longer life from adoption of
special surface treatment.

High-speed
capable cage

Conventional
cage↑Special surface
treatment (image)

Application of long-life
treatment
Rollers and shaft:
FA treatment↑ FA-treated material
Conventional austenite
crystal boundary

Features

Applications(1) High-speed capability:
Capable of 25,000 rpm (47% increaseover conventional design)(2) Compact design:
20% size reduction in gear width(when conventional gear unit life ismaintained)¡Automotive transmissions(AT, CVT, hybrid system)-150-

New Products Information

Oilless Chain Tensioner
Oil supply from the engine is not needed!

Features

Construction(1) Oil supply from the engine is not needed.
(2) Provision of oil piping in the engine is
not needed.
(3) Efficiency of a chain tension adjustment
function is equivalent with that of aconventional hydraulic chain tensioner.
(4) Complete with no-stage no-backfunction

Spring damper↓
Absorbs vibration.
Lever guide<No-stage no-back function>
Conventionally, when the engine is shut off while thechain remains tensioned, then the rod may remain in thepushed-in position: then if, in this situation, the engine isrestarted and the chain gets loosened, noise will occur atthe mesh points with the camshaft and crankshaft. In
contrast, the rod in our new product can be retained (noback) at an arbitrary (no-stage) projected position; noiseoccurrence is prevented.

Rod

Special screw(serrated-screwed thread)↓No-stage no-back function

Applications¡Auto-tensioner for timing chain on fourand two-wheeled vehicles-151-

NTN TECHNICAL REVIEW No.76（2008）

High-Temperature Resin Pulley Unit
for Automotive Auxiliaries
Light weight—its life is equivalent to or better than that with conventional
iron pulley unit!

Features

Construction [means for realization]
Adoption of resin pulley↓Lighter weight(1) Weight: 65% reduction[Compared to conventional iron pulley unit]

Increased heat radiating
surface area↓
Improved bearing life thanks
to mitigation of bearing
temperature increase(2) Bearing life: At least twice as long(3) Creep resistance: At least twice
as strong[Compared to conventional resin pulley unit]

Adoption of long-life grease↓
Longer bearing life

Applications

Increased resin volume↓
Improved creep resistance¡Idler pulley for belt on automotive
auxiliary-152-

New Products Information

Super-High Load Carrying
Cylindrical Roller Bearing
The number and length of rollers have been maximized to help realize abearing design featuring a much smaller size and lighter weight!

Features

Construction [means for realization][Comparison to bearing of same bearing life and rigidity]

1 Increased number of rollers
Clearance between outer ring andcage (smaller)(1) Lighter weight: 58% weight
reduction(2) Compacter size: 17% reduction in
outside diameter and 25%
reduction in width

Width of cage bar(equivalent)[Comparison to bearing of same size]

Roller-to-roller
clearance(smaller)(3) Higher rigidity: 16% increase inrigidity(4) Longer life: five times as long (inclean oil-lubrication environment)

2 Longer roller

Roller length ismaximized to the rollingsurface width limit.

Applications
Cage:
thinner wall at smaller rib section
Inner ring: no smaller flange¡Differential gearing in automotivetransmission-153-

Prevention ofdisintegration by incorporation of cage claw

NTN TECHNICAL REVIEW No.76（2008）

Low-Friction Capable Needle Roller Bearing
for Automotive Engine
Approximately 3% improvement in fuel economy (10.15 mode)
Helps promote introduction of compacter, lighter oil pump and starter!!

Features

Construction [means for realization](1) Reduction in friction
Reduction in bearing running torque:approx. 50% reduction(2) Compacter starter may be adopted.
Reduction in bearing start torque:approx. 90% reduction(3) Compacter oil pump may be adopted.
Necessary quantity of lubricating oil canbe decreased to 1/10.

●Crankshaft support
・Thanks to newly developed split outer ring
design, the bearing can have a much lower
profile: outer ring wall can be as thin as 2 mm.
・Cage design is optimized for application toengine, boasting improved seizure resistanceand enhanced mechanical strength.

●Camshaft support
・Outer ring is formed by pressing process, andthe bearing can have a much lower profile:outer ring wall can be as thin as 0.6 mm.
・Shape of the outer ring is optimized throughutilization of FEM analysis, and the outer ringboasts enhanced mechanical strength.

(1) Improved fuel economy for cars
⇒Improvement of approx. 3% in
10.15 mode (desk study)(2) A total package can be adopted in
VA/VE applications.

Applications¡Automotive engine(for supporting crankshaft and camshaft)-154-

New Products Information

MnS-Blended Stainless Steel
Sintered Oil-impregnated Bearing
NTN has developed and commercialized a low-friction bearing that excels
in corrosion resistance, seizure resistance and wear resistance!!

Features

Construction [means for realization](1) Corrosion resistance
Uses stainless steel as base metal.
(2) Seizure resistance and wear resistance
Blended MnS (manganese sulfide) helps
form a sulfur-based lubricant film,
achieving excellent seizure resistanceand wear resistance.

Challenges of conventional technology to beaddressed(1) Graphite (C) and molybdenum disulfide (MoS2), etc. are
blended into stainless steel base metal.
⇒ Chemical compound is formed as result of sintering, andthis compound is harder than the base metal, attacking
the mating shaft.
(2) Lead (Pb), which is highly sliding material, is added as
lubricant.
⇒ Use of lead is regulated or banned because it ishazardous to human health.

Applications¡IT (information technology) equipment,
automotive fuel pump, medical
equipment, food machinery, etc.

Newly developed technology that has solved
previous challenges※ Appropriate amount of manganese sulfide is added
to stainless steel base metal.
⇒ Sulfur deriving from manganese sulfide helps forma highly lubricating film boasting excellent extreme
pressure performance.
The newly developed bearing product boasts excellent
corrosion resistance, seizure resistance and wear
resistance.

This product has been developed in cooperation with
Nagoya Municipal Industrial Research Institute.

-155-