88th Anniversary

Special Issue;
Products for Industrial Machinery
November 2006

ISSN 0915-0528 OSAKA,JAPANMessage on our 88th anniversary
Bearings are vital components for carrying the rotating loads of all types of machinery,from automobiles, aircraft, trains and other vehicles to construction machinery, machine
tools and electronic equipment.
NTN was founded in 1918 dedicated to the manufacture of bearings, and we are now celebrating
our 88th anniversary thanks to the continued support of numerous NTN product users.
Throughout our history, we have taken on diverse challenges, including activities in overseas
markets for many years and expansion of our business line into the development andmanufacture of constant velocity joints. Now, we have grown into a global bearing
manufacturer with bases in 19 countries worldwide.
In 1989, as an expression of our intention to make even more positive contributions to
the world, we changed our corporate name from Toyo Bearing Mfg. Co. Ltd., the name
we had used for many years, to NTN Corporation. We continue to remain focusedon the new challenges that we face in the global market.

88th Anniversary

R

TECHNICAL REVIEW
No.74

Special Issue

corporation

Products for
Industrial Machinery

CONTENTS
Kenji OKADA

Preface

Contribution

Trends in Recent Machine Tool Technologies
Dr. Toshimichi MORIWAKI Professor Department of Mechanical Engineering KOBE University

1

2● Special Issue for Mchine Tool
Development of a New Grease Lubrication System for Machine Tool Main Spindles
Sun-Woo LEE and Masatsugu MORI

High-Speed Tapered Roller Bearing for Machine Tool Main Spindle
Masatsugu MORI and Takuji KOBAYASHI

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine Tool
Mineo KOYAMA

dmn170x104 Sealed High-Speed Angular Contact Bearing "New BNS Type"
Futoshi KOSUGIDevelopment of Aerostatic Bearing Spindle for Precision Machine Tools
Teruyoshi HORIUCHI, Kazuyuki AONO and Hiroaki HIYOSHI

Our Line of New products for Machine Tool

8
16
24
28
34
38

● Technical Papers Technical Articles New Products
Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes
Chikara OHKI

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steel
Hiroshi HAMADA and Yukio MATSUBARATechnical Trends in Bearings for Office Equipment

44
54
62

Chikara KATAGIRIUnit Products for Office Equipment

68

Shoji ITOMI and Atsushi MOROOKA

Honing Sludge and Electro Furnace Dust Briquetter

74

Shouzo GOTOH and Kanji NAKAMURA

Ultra-Clean Bearing for Clean Environments

80

Ryouichi NAKAJIMA and Masaaki HONDA

Development of New RCT Bearing for Axleboxes and Insulated Bearing with Shields
Ryutaro OKA

High Load Capacity Cylindrical Roller Bearings

84
90

Takuya OZU

The Rust Guard Bearings of Highly Corrosion-resistant Bearings
Takashi YAGI"SSB" of Sliding Aseismic Isolution Unit Multi-ply Rubber

96
100

Norio ITOH and Takuya ISHII●Our Line of Award Winning Products"The Third Mono-Zukuri Products Creation Awards" Innovative Product Award
Toshio OGUSU

Monodrive Two-Way Feeder

104"The Japanese Society of Tribologists 2005" Technical Award"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

108

Masaki EGAMI

Our Line of New Products

116

NTN TECHNICAL REVIEW No.74（2006）[ Preface ]Introduction to this Special Feature Issue –NTN Products
for Industrial Machinery
Kenji OKADAManaging Director

The current trend in the Japanese economy can be characterized as a steady recovery led by domestic
demand and supported by the active investments of Japanese manufacturers in manufacturing plants andfacilities. These are fueled by the recovering business performance of Japanese enterprises and the strong
personal consumption of consumers. However, at the same time, we face anxieties that threaten our everyday
lifestyles, including unprecedented high petroleum prices and environmental pollution problems that are difficult
to resolve. In short, we are experiencing an era that is increasing in complication.
In this age of dramatic change, NTN is now celebrating its 88th anniversary thanks to support from the users of
NTN products. I would like to take this opportunity to express my deepest gratitude to them.
NTN took part in the 23rd Japan International Machine Tool Fair (JIMTOF2006), which ran from November 1 to
8, 2006. To mark this occasion, we issued the Japanese edition of this publication, the NTN Technical Review
No. 74 (2006). This year’s edition features topics about bearing products for industrial machinery, detailing how
these products boast longer lifespans, lower running torques and compact sizes, which are merits that contributeto the realization of a society that promotes energy conservation and environmental protection. This publication
describes NTN’s achievements in developing our latest technologies and novel products. The review begins witha contribution titled “Trends in Recent Machine Tool Technologies” by Kobe University Professor Toshimichi
Moriwaki who is an international authority in the fields of production engineering and precision machining. Nine
topics about NTN products for industrial machinery follow, including five topics about products for machine tools,
three theses related to material and heat treatments, and two reports about NTN products that won awards in
2005— the Mono-Zukuri Component Grand Prize given for creation and manufacture of a novel product and theEngineering Award of the Japanese Society of Tribologists.
In recent years, the environmental consciousness of the public has been growing steadily, and, as a result, theresponsibilities of manufacturers to achieve a harmonious balance with the global environment have also
become greater. Given this reality, we must remain deeply committed to efforts for reducing CO2 emissions to
help control global warming and to efforts for realizing a recycling-oriented society.
In consideration of these issues, many NTN products directly and indirectly support energy conservation andenvironmental protection. These include our rolling bearings and constant velocity joints, as well as ourhydrostatic bearings, repair devices and other products for precision machinery. Rolling bearings are essentially
products that help reduce rolling friction, so they can be considered to be products that conserve energy. Ourother energy-saving products include NTN constant velocity joints and rolling guideways, which boast much
improved transmission efficiency and NVH characteristics (noise, vibration and riding comfort) throughincorporation of rolling elements into their sliding sections, as well as products for precision equipment thatfeature non-contact bearings as supporting mechanisms.
Our commitments are not limited to energy-saving features for these fundamental products. We are fullycommitted to the development of new products in the fields of energy conservation and eco-friendly equipmentthat help reduce environmental impacts.
For 88 years since our foundation, in the belief that “quality is the source of NTN brand power,” we have always
conducted strict self-evaluations as we have developed novel products with the goal of “realizing quality that
fulfills customer satisfaction.” With the motto, “For New Technology Networks ,” our philosophy is to contribute tointernational society by using NTN engineering abilities to provide novel, high-quality products that are relevant tofulfilling ever-diversifying market needs. In keeping with this, we promise to continue our commitment to realizing
the confidence and satisfaction of our customers.

-1-

NTN TECHNICAL REVIEW No.74（2006）[ Contribution ]Trends in Recent Machine Tool Technologies

Dr. Toshimichi MORIWAKI
Professor Department of Mechanical Engineering KOBE University

Recent trends in the machine tool technologies are surveyed from the view
points of high speed and high performance machine tools, combined multifunctional machine tools, ultraprecision machine tools and advanced andintelligent control technologies. The machine tools are bases ofmanufacturing industries and they are strategically important products forJapan. The views of the author towards the technical developments in both
hardware and software are introduced together with the world wide trends inthe relevant fields.

In this section, I will focus on the avoidance ofchatter vibration, which is one outstanding advantage
of high-speed, high-efficiency machine tools, as otherengineering topics are discussed in papers by otherresearchers.
In the period of the 1960’s and 1970’s, there wereresearch efforts worldwide on the chatter vibration ofmachine tools. As a result, the underlying principles
behind so-called regenerative chatter vibration andforced chatter vibration were clarified, and basic
solutions were proposed.
Unfortunately, however, examples of further
systematic research efforts have been rare.
In recent years, avoidance of chatter vibration has
been posing a new challenge in the machining of

1. Introduction
At present, the machine tool industry worldwide is
enjoying unprecedented demand, and the industry’s
output is apparently even failing to satisfy current
demand. Japan’s machine tool industry, in particular,
has boasted the greatest share in the world since
1982, and its share has been exceptionally high in thelast two years. In order for Japan’s machine toolindustry to maintain this share, I believe that theenterprises involved in it must remain committed not
only to expansion and advancement of their productfacilities, but also to steady research and development
efforts. They must continue to add more value to theirproducts in order to cope with future needs andmaintain competitiveness compared to machine tool
manufacturers in other nations.
In this paper, I intend to report on current topics
about recent machine tools and trends in the researchand development commitments of the Japanesemachine tool industry, highlighting the efforts of The
International Academy for Production Engineering(CIRP), an organization of which I am a member.

Table 1 Background, supporting technologies andadvantages of high-speed, high-efficient machine tools
High-speed, high-efficiency machine tools(Background, supporting technologies)¡Need for highly efficient machining and decreased costs¡Development of high-speed main spindles and high-speed feed means (linear motors, etc.)

2. High-Speed, High-Efficiency Machine Tools¡Advances including the development of high-speed

machining capable tools and progress in machining
techniques

It is well known that demands are mounting for
greater maximum main spindle speeds and feed
speeds—in other words, that machine tools of higher
speed and higher efficiency are much needed. 1)
Background information about high-speed machine
tools and supporting technologies, and their resultant
advantages are summarized in Table 1.

(Advantages)¡Decreased processing times (improved efficiency)¡Improved machining accuracy and better quality offinished surfaces¡Prevention of chatter vibration-2-

Trends in Recent Machine Tool Technologies

chip thickness. If such a condition is realized, chattervibration will not occur even with greater cut depths.
Utilizing this principle, high-speed, high-efficiency
cutting has been implemented for aircraft components
made of aluminum and other materials. Given this,
there has been mounting interest in the dynamic
characteristics of a main spindle system that includes
a main spindle, chuck and tools. As a result, theinterrelation of bearings and other design factors withthe dynamic characteristics of the main spindle andmain spindle system has been clarified boththeoretically and experimentally, and this achievement
has been applied to the design of main spindles.
Recently, various software packages are also being
used frequently for analysis and design. The
theoretical study of main spindle designs will become
increasingly important.

Real Part [μm/N]

hard-to-cut materials as well as in high-speed, highefficiency machining of aluminum materials foraviation purposes. Generally, chatter vibration is
avoided by reducing the depths of cuts and cutting
speeds (low-speed stability), but it is possible to avoid
chatter vibration by increasing spindle speed. This fact
was already known from research done in the 1960’s.
Since high-speed spindles boasting this ability were
not available in those days, however, this fact was
regarded simply as a theoretical possibility. In themathematical field, it was also difficult theoretically tohandle chatter vibration in milling processes, including
end milling. Notwithstanding, Prof. Y. Altintas et. al.
obtained results for stability graphs such as the one
shown in Fig. 1. This graph shows that chattervibration does not occur in the region of depths cut
below the stability lobes relative to spindle speeds on
the horizontal axis. Though detailed discussion of thisfinding 2) is omitted in this paper, the underlying
principle of chatter vibration can be understood fromthe expressions and diagrams given to the right of thisgraph. Variations in chip thickness are caused bydifferences between the roughness of the finishedsurface generated by the immediately previous
revolution of the main spindle or by an immediately
previous cutting edge and the roughness of thefinished surface currently generated by the current
cutting edge. This variation in chip thickness
contributes to variation in the cutting force andcontributes to continuing vibration. If we can run themain spindle at a higher speed that is equivalent tothe vibration frequency, then the difference between
the phase of vibration resulting from the immediately
previous revolution and the phase of vibration derivingfrom the current revolution can be effectively
controlled, thereby eliminating variations caused by

3. Combined Multifunctional Machine Tools
In addition to high-speed, high-efficiency, cuttingcapable machine tools, research on machine tools iscurrently focused on combined multifunctional
machine tools, including 5-axis machining centers andcombined multifunctional turning centers. The
background and advantages of combined
multifunctional machine tools are summarized inTable 2. Combined multifunctional machine tools canbe roughly categorized into turning centers (TC) thathave been developed from lathes and machining
centers (MC) that started as milling machines.
A portion of the investigations into machining
applications with combined machine tools based onlathes is illustrated in Fig. 2. In addition to machiningof bores, outer circumferences and end faces, certain
applications are executed for slope machining and

2kζ=Dynamic Stiffness
Ktc=Cutting Coefficient2
1

n=60/T

T=

2Kπ＋ε
ωc

0
-1

〜 -1
Gmin＝
4kζ

-2
0

500

1000

1500

ε=3π＋2tan-1
2000

2500

3000

3500

lm [φ
（jωc）]
Re [φ
（ωc）]

4000

Axial depth of cut [mm]Frequency[Hz]Chip

5Stabiliy Lobes
4
180˚

3

alim =

2
1
0

acrit =
0

2000

4000

6000

8000

10000

12000 14000

16000

-1
2KtcG(ω)-1
2kζ
=
2Ktc -1
Ktc
4kζ

Spindle speed [rev/min]Fig. 1 Stability chart of regenerative chatter vibration (Y. Altintas)-3-

NTN TECHNICAL REVIEW No.74（2006）

Many different 5-axis machining centers have beendeveloped. In particular, in addition to orthogonal 3axis vertical and horizontal machining centers, many
simultaneous 5-axis control machining center productsthat have work tables with two additional axes forrotation and oscillation are used widely. Most recently,
some machining centers have a work table driven by a
DD motor and a high-speed, high-power rotary tablecapable of high-speed indexing, and they feature thefunctions of vertical turning centers. As mentioned
above, deriving from either lathes or milling machines,
combined multifunctional machining tools may evolve
into novel machine tools that incorporate features ofboth turning centers and milling machines.
Combined multifunctional machine tools haveadvantages that include the following. They arecapable of machining complex forms that require
simultaneous control of five axes. Loss in machining
accuracy from dismounting and remounting theworkpiece is prevented because once a workpiecehas been mounted to the chuck, all machining
processes are executed without need for rechucking
the workpiece. As the needs for function-intensive
parts and components increase, advanced combined
multifunctional machine tools are capable ofmachining these workpieces at higher precision andhigher efficiency. As superior machine tools, thedemand for combined multifunctional machine tools
will increase further in the future. To meet thisdemand, the researchers and engineers in this fieldmust develop the hardware that helps realize
sophisticated functions as well as the supporting
software (CAM) to enable advanced control
techniques and application technology.
Incidentally, within the next 2 years, the STC-M(Scientific Technical Committee: Machines) of CIRP(The International Academy for Production
Engineering) will issue a keynote paper that covers
current and future trends in combined multifunctional
machine tool technology.

hobbing. Recently, a main spindle mounted to an area
equivalent to a turret is capable of not only auxiliary
cutting processes, such as end milling, but also tomore demanding milling processes. In addition, lathebased machine tools that resemble milling machines
have been developed. One example of such a
combined machine tool is shown in Fig. 3 (Mori Seiki
Co. NT5400DCG).

Table 2 Background, supporting technologies andadvantages of combined multi-functional machine tools
Simultaneous 5-axis control machining center(e.g.: three orthogonal axes and two rotational axes)
Combined machining turning center(e.g.: lathe and 2nd main spindle, B axis, Y axis, etc.)(Background, supporting technologies)¡Need for high-precision, high-efficiency machining to makemore advanced (complicated) parts and components¡Increased sophistication of supporting software (CAM)¡Development of high-precision, high-efficiency machineelements (e.g.: DD-motor driven tables, etc.)(Advantages)¡Improved machining accuracy, reduction in machining time(one-chuck process)¡Machining of complicated shapes

Presence/absence ofworkpiece transfer
Hobbing

External circumferencemachining (turning)
External circumference80
machining (milling)
External circumference60
machining (thread cutting)40

Polishing
Slope machining(drilling)

200External circumferencemachining (drilling)sBore machining(turning)
Bore machining(thread cutting)Slope machining(milling)
End face machining(drilling)
End face machining(milling)

Bore machining(milling)
End face machining(turning)

Fig. 2 Survey of machining examples by combined multi-axis
machine tools based on turning machines

Fig. 3 An example of combined multi-axis machine tool (Mori Seiki Co. NT5400DCG)-4-

Trends in Recent Machine Tool Technologies

value of Japanese machine tools.
In the field of ultraprecision-machining and micromachining, typically for the machining of optical partsand components, the requirements for form accuracy
and finished surface roughness have always been
demanding, and the forms of machined parts andcomponents have become increasingly complicatedas summarized in Table 4. Also, the process for
preparing optical lenses has changed from injection
molding with plastic materials to a hot-pressing
process with glass. To cope with this trend, anincreasingly larger number of dies are being made ofmaterials that are extremely difficult to machine, suchas tungsten carbide and ceramics, and these diesmust undergo many machining processes including
grinding and polishing. These techniques in die
machining processes and glass press-forming
processes are contributing positively to the
manufacture of the lenses on camera cell phones anddigital cameras, which are both increasingly common.
Looking more closely at the lenses used in thesetypes of digital equipment, machining processes thatare more demanding are necessary to realize theirunique optical arrangements. For example, this isneeded to create a combination of a Fresnel lens andan aspherical lens, which enhances the opticalcharacteristics of the related optical systems. At thesame time, laser printers and other optical equipmentthat utilize lasers need more sophisticated opticalelements that involve nonaxisymmetric or free-curved
surfaces. Therefore, needs are growing increasingly
for more advanced ultraprecision machining
techniques. To this end, I believe that the importanceof ultraprecision machine tools needed for
ultraprecision-machining and micro-machining will befurther highlighted because they are indispensable inmachining highly sophisticated parts and components
with high added value.
One example of a 3-axis FTS (Fast Tool Servo) 3)
device, which is capable of forming a free-curved
surface with ultra high precision and higher efficiency,
is illustrated schematically in Fig. 4. This device, as
shown in the diagram, machines a workpiece fixed ona running ultraprecision rotary table while shifting in
the radial direction a tool that has its cutting depth in
three axial directions controlled at high-speeds andhigh-response frequencies. If the axial infeed iscontrolled while the tool is fed in a radial direction,then theoretically any intended shape could be formedon the surface of the workpiece.
In reality, however, it is impossible to form a stepsquare to the cutting direction of a machine tool evenat the maximum possible response frequency andresponse speed of the tool. Therefore, by

4. Ultraprecision Machine Tools
Other than high speed and high efficiency, the mostcritical requirement for machine tools is high precision.
Recently, various ultraprecision machine tools havebeen developed that are significantly more evolved
than earlier high-precision machine tools. Previously,
the industrial fields that required ultraprecision
machine tools were limited and the market scale forultraprecision machine tools was relatively small. In
contrast, needs have been increasingly mounting forultraprecision and micro-machined parts andcomponents, such as dies for optical parts andcomponents. In response to this trend, development isin progress for various ultraprecision machine tools.
The background for the needs for ultraprecision
machine tools, supporting technologies for theirrealization and their advantages are summarized inTable 3. Progress in the component technologies forultraprecision machinery, such as air hydrostatic
bearings and guides, is remarkable. I believe that
advances in hardware technologies for thesemechanical elements are contributing to the higher

Table 3 Background, supporting technologies andadvantages of ultraprecision machine tools
Ultraprecision machine tools(ultraprecision cutting/grinding machines)(Background, supporting technologies)¡Growing needs for ultraprecision and micro-machined partsand components (Optical parts and components, dies, etc.)¡Sophistication of ultraprecision machine elements (mainspindles, guideways, feed drive systems, etc.)¡High-precision control technologies(Advantages)¡Development of new market segments through ultraprecision-machining and micro-machining (Satisfaction of needs)(Optical parts and components, micro parts, mechatronic parts, etc.)

Table 4 Trends in ultraprecision micro machining
1. Form accuracy, finished surface roughness:¡Microns→nanometers

2. Form (in the case of optical parts)¡Spherical surfaces→aspherical surfaces→nonaxisymmetricsurfaces→free-curved surfaces

3. Workpiece materials¡Soft metals (aluminum, copper, etc.)¡Hard metals (nickel, hardened steel, etc.)¡Resin materials, anisotropic materials (lithium niobate, fluorite, etc.)¡Brittle materials (tungsten carbide, ceramic materials, etc.)¡Other materials (plastics, etc.)-5-

NTN TECHNICAL REVIEW No.74（2006）

example, let us think of a control technique forcontrolling thermal deformation, which is the mostcritical factor adversely affecting the machining
accuracy of machine tools. A much advanced control
technique is now commercially used in which themagnitude of the current thermal deformation isestimated in real-time based on information about themachine tool and temperatures at various spots on thetool. 4) Using this information, the motion of themachine tool is controlled so that higher machining
accuracy is ensured under any operating condition.
It is also possible to simulate the motion of themachine tool in real-time based on information about
the motion control applied to the machine tool. Utilizing
this idea, Okuma Co. has developed an anti-crush
system that predicts events such as the crushing of atool with a chuck and stops the machine as necessary
by simulating the motion somewhat in advance of theactual motion of the machine tool. The schematic
diagram for this system is illustrated in Fig. 5.

YZ3-axis driven PZTWorkpiece

X

Tool

Feed

Fig. 4 Schematic illustration of ultraprecision cutting by 3-axix FTS

simultaneously controlling the tool along three axes,
the freedom of the machined forms can be increased
and the trajectory of motion of the tool being controlled
can be simplified. Use of such FTS helps greatly
reduce the necessary cutting time on free-curved
surfaces.

Table 5 Background, supporting technologies and advantagesof advanced and intelligent control technologies
Advanced and intelligent control

5. Advanced and Intelligent Control(Background, supporting technologies)

The increasing sophistication of machine tools is
supported by progress in not only hardware but also insoftware. The background, supporting technologies
and advantages of advanced and intelligent control
technologies are summarized in Table 5. Recently,
many advanced (intelligent) control techniques areavailable that reflect an understanding of machine tool
characteristics and machining processes. For¡Need for low-cost controllers with advanced functions¡High-speed, high-precision interpolation systems¡Real-time thermal deformation compensation¡Anti-crush systems that also utilizes simulations¡More advances in computer and IT technologies(Advantages)¡Greater added value for machine tools¡Evolution of knowledge into established technologies

Machine model

Simple modeling

Virtual machine
program settings
Jigs, tools,
materials, etc.

Virtual machine
control

Feed forward
motion command

Interference
detection

Real-time simulation

Fullsynchronization
Machine stop

Any operation

MotioncommandMachine control

Fig. 5 Conceptual diagram of anti-crush system (Okuma Co.)

-6-

Trends in Recent Machine Tool Technologies

machine tool industry in the ever-demanding
challenge to achieve the ultra-high-speed and ultrahigh-precision required of the main spindle bearings
that are critical components of many machine tools.

In an effort to further advance this idea, under thetitle of Virtual Machine Tools, the previously
mentioned STC-M in CIRP is currently attempting toperform complete simulations that cover machining
processes, dynamic characteristics and control
characteristics for machine tools.
This simulation scheme is, for example, capable ofcomplete computer simulation of a machine and its
processes, allowing it to determine how various
components of a machine tool will react when amotion command is given to the machine. It can also
determine how the tool and workpiece will interact witheach other to machine the workpiece and how theresultant cutting force will affect the machine and tool.
The architecture of one example of such a simulation
scheme, the virtual CNC system, 5) is schematicallyshown in Fig. 6.
In order to make virtual machine tools become areality, further research and study efforts need to bemade on many issues, including machining
processes, and the dynamics and motion
characteristics of machine tools. I want to point out
that such research and studies are steadily making
progress to this end.

References
1) Y. Kakino: Latest Trend of Main Spindle for NC Machine
Tool2) Y. Altintas, M. Weck: Chatter Stability of Metal Cutting
and Grinding, Annals of the CIRP, 53/2 (2004) p619.
3) Wada et. al.: Development of Three-axis Fast Tool
Servo for Ultraprecision Machining, Proc. 6th Int. Conf.
of euspen (2006) p115.
4) H. Senda et al.: Main spindle thermal deformation
estimation with the goal of mass production (2nd report),
Japan Society of Mechanical Engineers Journal Series
C, 71-709 (2005), p. 2813. (In Japanese)5) K. Erkorkmaz et. al. : Virtual Computer Numerical
Control system, Annals of the CIRP, 55/1 (2006) p399.
About the author
Toshimichi Moriwaki, Dr. of Engineering
Professor, Kobe University Faculty of Engineering
1966
1968

6. Conclusion

1974

Based on my own experience, I have describedseveral examples of recent trends in machine tooltechnologies. The machine tool industry constitutes
the backbone of Japan’s machinery industry, and forthis reason, endeavors to achieve higher speed,
higher efficiency and ultraprecision will continue withincreasing commitment. In concluding this paper, I
want to express my wish that the staff members of
NTN continue their efforts so that NTN technologies
and products lead their counterparts in the global
CAD Model

1975
1985

Specialties: production systems, machine tools, ergonomic engineering
Select prizes and awards (not all listed)
International Academy for Production Engineering (CIRP) F.W.
Taylor medal 1977
Machine Tool Engineering Foundation Encouragement Prizes
1991, 1994, 1995, 1998, 2001
Iue Cultural Prize (Science and Technology Division) (Iue Memorial
Foundation) 1998
Japan Society for Precision Engineering Prize 2002, 2003
Hyogo Prefecture Science Prize (Hyogo Prefecture) 2004

CL FileCAD/CAM
SystemGraduates Kyoto University Faculty of Engineering, Departmentof Precision Engineering
Receives Masters degree from Kyoto University Graduate
School of Engineering, Department of Precision Engineering
Becomes Assistant Professor of Kobe University Faculty ofEngineering
Becomes Assistant Professor of McMaster University (Canada)
Becomes Professor of Kobe University Faculty of Engineering(2000〜2004 Dean of Faculty of Engineering)…

LOADTL/2
FEDRAT/10000.00,MMPM
RAPID
GOTO/0.000,0.000,50.000

CL FileInterpretationyTool Path Geometryy Pi+1s Pi+1

Pi

x

Pi
x

Pc

Trajectory
Generation
Axis Servo Loop
Feed Drive
Servo Dynamics

DisplacementSr (t), r (t), r (t)Axis Motion
Tracking Controlt
FeedrateSt
Acceleration
t
Jerk
t

Axis
Commands S

Feedback

S

Feed Profiling

Actual Axis Position
Actual Tool Position with Servo Errors

Tracking and Contour Error Estimation

Fig. 6 System architecture of virtual CNC 4)-7-

NTN TECHNICAL REVIEW No.74（2006）[ Technical Paper ]Development of a New Grease Lubrication System
for Machine Tool Main Spindles

Sun-Woo LEE*
Masatsugu MORI*
NTN has developed a new grease lubrication system formachine tools main spindles which features heat-induced baseoil delivery from a grease chamber installed next to the bearing.
The lubrication system does not require any external additional
lubrication device. Experimental result demonstrates high-speed
rotation of an angular contact ball bearing with a 100 mm bore
diameter, that has a 1.8 million dmn value for definite position
preloading and 2.0 million dmn value for constant pressure
preloading. NTN’s new grease lubrication system can thus be analternative to the air-oil lubrication technique while keeping themaintenance-free advantage of the conventional greaselubrication method, in addition, it will support higher performancemachine tools as well as improve customer’s work environmentand reduce environmental burden.

growing for grease lubrication systems for machine
tool main spindles.
One typical example of the authors’ achievement isthat the above-mentioned “grease lubricated sealed
angular contact ball bearing (BNS type)” has achieved
a bearing life of more than 20,000 hours at a runningspeed equivalent to dmn of 1.4 million.
The authors developed a “new grease lubrication
system for machine tool main spindles” that is capableof greater bearing speed and longer bearing life withgrease lubrication that does not need maintenance.
This novel grease lubrication system is comprised of aconventional internal design but has a greasechamber in the front side of the bearing from which
only the base oil in the grease is extracted to lubricate
the bearing. This novel grease lubrication system wasincorporated into an angular contact ball bearing witha bore diameter of 100 mm, making the bearing
capable of high-speed operation and a long bearing
life. The dmn value reached 2 million with constantpressure preloading and 1.8 million with definite
position preloading. In this paper, the authors report
the achievements of their research into a novel greaselubrication system that is completely maintenancefree.

1. Introduction
The use of higher speed main spindles has been
increased used on machine tools, including machining
centers, in order to achieve better machining
efficiency and machining accuracy. To cope with thechallenges associated with high-speed machine tool
main spindles, NTN has been developing andimproving various main spindle bearings and currently
markets these bearings under the “ULTAGE Series”
brand name. 1) In recent years, people around theglobe have been becoming increasingly concerned
about the environment, so manufacturers worldwide
must remain committed to eco-friendly technologies.
In response to the technological challenges related tothis, NTN has been developing unique bearing
products through investigations into bearing
lubrication techniques. The resultant bearing productlines include “eco-friendly jet lubrication angularcontact ball bearings,” 2) “eco-friendly air-oil lubricatedangular contact ball bearings (HSL type)” 1) and“grease lubricated sealed angular contact ballbearings (BNS type).” 1) Grease lubrication systems, inparticular, have smaller impacts on the environmentand can be maintenance-free. As a result, demand is*Elemental Technological R&D Center-8-

Development of a New Grease Lubrication System for Machine Tool Main Spindles

2. Structure of the New Grease
Lubrication System

3. Experiment Using a Model Test Rig
To investigate the characteristics of base oil delivery
from the grease chamber and verify the base oildelivery mechanism described in Sec. 2, the authors
used a model in which this base oil delivery
mechanism was applied to an angular contact ballbearing with a 100 mm bore diameter. The findings
from the experiment are provided below.

To achieve high speed and long life with a bearing
by using grease lubrication, it is necessary to not only
provide a sufficient amount of lubricant for a bearing
while running at a high speed but also to maintain the
supply of grease base oil from a limited amount ofprefilled grease. With the authors’ newly developed
grease lubrication system, the base oil in the grease isactively separated while the bearing is running, andthis separated base oil is steadily supplied to the
raceway surface of the bearing. The bearing structureof the authors’ new grease lubrication system is
illustrated schematically in Fig. 1. As a part of theouter ring spacer, a cavity is formed in front of the
bearing and the grease prefills this space. At the sametime, this provides a shouldered surface near thecontact point between the rolling elements and theouter ring raceway. In addition, a thin axial flow path isformed between the shouldered surface and the end
point of the grease chamber, thus connecting thegrease chamber to the raceway surface.
For this structure, the authors considered a base oildelivery mechanism that works as follows. The
running bearing generates heat, causing thetemperature of the grease in the grease chamber toincrease and the base oil in the grease to separate
from the thickener. As the temperature increases, thepressure in the grease chamber also increases,
causing the base oil from the above-mentioned axial
flow path to flow through the clearance on theshouldered surface, resulting in delivery to the outerring raceway surface. As a result, the bearing is
actively lubricated by the base oil delivered to theouter ring raceway surface.

3.1 Model test rig and test method
The authors’ model test rig for base oil delivery isschematically illustrated in Fig. 2. The appearance ofthe test rig is shown in Photo 1. The radial clearancein Fig. 2 is indicated on a radius basis. The propertiesof the two types of grease developed for machine tool
main spindle bearings that were used are summarizedin Table 1. Grease was filled into the grease chamber
via four grease prefilling holes provided on the outercircumferential surface until the entire space of thegrease chamber was filled. The amount of prefilledgrease was approximately 47 g.
The following characteristics were tested since they
were considered to affect base oil delivery:• Temperature-dependent variation in the greasechamber• Size of radial clearance at the end point• Type of grease used

Table. 1 Grease properties
Characteristic

GreaseGrease-A

Grease-B

Urea

Urea

Thickener
Base oil

PAO

PAO

Polyol ester

Diester
Polyol ester

Clearance on
shouldered surface

Outer ringAmount of thickener wt%
Base oil kinematic
viscosity mm/s2 40˚C
Worked penetration,
60 strokes 60W 25˚C
Spreading consistency

Outer ring
spacer

11.2

11.7

40.6

40.6

243

280

230

260

Enlargedview
Radial clearance
GreaseGrease prefilling hole40

Grease chamber

Fig. 1 Schematic construction of new lubrication system

φ150

Fig. 2 Sketch of model test rig for base oil delivery-9-

NTN TECHNICAL REVIEW No.74（2006）

clearance was situated vertically, and the oil content
delivered to the clearance end point was removed witha piece of blotting paper once every 50 to 100 hours.
The increases in the weight of the pieces of blotting
paper were totaled and the resultant sum was taken
as the amount of oil delivery.
Immediately after grease prefilling, delivery of not
only the base oil but also of the grease itself wasobserved. This portion was wiped away and was notincluded in the amount of delivery. The state of baseoil delivery at the end point of clearance is shown in
Photo 2. When the model test rig was not subject tothe heat cycle, no delivery was observed after 100
hours elapsed though a minor delivery occurred at theinitial stage of testing. In contrast, when the model test
rig was subject to the heat cycle, the delivery
continued and the amount of delivered base oil was
proportional to the time elapsed. In other words, when
the heat cycle is applied to the grease chamber, thebase oil in the grease is repeatedly separated from thegrease and delivered through the clearance tofunction as a lubricant.
Next, the effect of temperature differences in eachheat cycle on the amount of base oil delivery wasstudied using the combination of Grease-A and aradial clearance of 0.15 mm. For comparison, the test
was performed with temperature differences in eachheat cycle of 15˚C and 3˚C. The heat cycle patternused for this test is shown in Fig. 4(a), and the result

3.1.1 Variation in the amount of base oil delivery
depending on the heat cycle
Grease-A in Table 1 was used to investigate thebase oil delivery due to heat increase. The amount ofbase oil delivery in a model test rig subjected torepeated temperature increase and decrease(hereinafter referred to as the “heat cycle”) was
compared with a model test rig in which thetemperature was increased and then maintained that
increased. The temperature was automaticallycontrolled by a constant temperature chamber and thepattern of the heat cycle was as illustrated in Fig. 3(a).
The temperature variation in each heat cycle (15˚C)
was equivalent to the difference between thetemperature in the grease chamber when the bearing
was running (41˚C) and when the bearing was at astandstill (room temperature: 26˚C).
The results of the amounts of base oil delivery
relative to the radial clearance of 0.15 mm aresummarized in Fig. 3(b). The model test rig (Photo 1)
was set so that the flow path composed of the radial

Temperature

41˚C

41˚C

2h

26˚C
48 h

2h

48 h
Time h(a) Heat cycle pattern

0.4
Amount of base oil delivery gWith heat cycle
Without heat cycle
0.3

Photo 1 Model test rig for base oil delivery
0.2

0.1

0
0

100

200

300

400

Time hEvidence ofbase oil delivery(b) Base oil delivery versus elapsed time
Fig. 3 Base oil delivery with/without heat cycle

Photo 2 Delivered base oil-10-

Development of a New Grease Lubrication System for Machine Tool Main Spindlesof this test is graphically plotted in Fig. 4(b). The
amount of base oil delivery is greater with a greatertemperature difference in each heat cycle. However,base oil delivery is still possible with a temperature
difference of about 3˚C.
Thus, the minimum temperature differencenecessary for base oil delivery through a clearance
seems to be 3˚C. Assuming that a machine tool main
spindle is continuously run for an extended period
virtually without stoppage, the authors’ lubrication
system can be applied even though, for example, only
periodic temperature variation due to control over thejacket cooling temperature is available.

bearings (41˚C) were 3˚C (between 41˚C and 38˚C)
and 15˚C (between 41˚C and 26˚C). The heat cycle
patterns shown in Fig. 4(a) are the same as mentioned
previously. After 250 test hours, the measured
amounts of base oil delivery were graphically plotted inFig. 5. Grease-B was used in this test and theproperties, are summarized in Table 1.
The absolute value of the amount of base oildelivery with a temperature difference of 3˚C differsfrom that with a temperature difference of 15˚C.
Nevertheless, under each of these different
temperature conditions, a smaller radial bearing
clearance led to a greater amount of base oil delivery.

3.1.2 Size of radial clearance and associatedamount of base oil delivery

3.1.3 Effects of different grease types on theamount of base oil delivery

The authors studied the interrelation between thesize of radial clearance at the end point through which
the base oil is delivered and the amount of base oildelivery. The sizes of the radial clearances adopted
were 0.05 mm, 0.15 mm, 0.2 mm and 0.45 mm. The
differences in temperature in the heat cycles, relative
to the temperatures on the outer rings of the running

The authors studied the effect of the type of grease
prefilled in the grease chamber on the amount of baseoil delivery by using the two types of greases
summarized in Table 1. The temperature difference ineach adopted heat cycle was 15˚C, and the size of theradial clearance used was 0.05 mm. The heat cycle
pattern adopted was the same as that described

Amount of base oil delivery g

1

Temperature

41˚C
38˚C

2h 2h

41˚C

26˚C

2h 2h
Time h(a) Heat cycle pattern

15˚C temperature difference
3˚C temperature difference

0.8

0.6
0.4

0.2
0

0.05

0.15

0.2

0.45

Radial clearance mm

Fig. 5 Base oil delivery versus radial gap
15˚C temperature difference
3˚C temperature difference

0.8

0.7

Amount of base oil delivery gAmount of base oil delivery g

1

0.6
0.4
0.2

0

0

50

100

150

200

250

300

350

Time h

0.6
0.5
0.4
0.3
0.2
0.1
0

(b) Base oil delivery versus elapsed time

Grease-A

Grease-B
GreaseFig. 4. Relation between temperature difference andbase oil delivery

Fig. 6 Base oil delivery versus grease property-11-

NTN TECHNICAL REVIEW No.74（2006）previously and illustrated in Fig.3(a). After 250 test
hours, the measured amounts of base oil delivered aregraphically plotted in Fig. 6.
It can be understood that the amount of base oildelivery varies greatly depending on the type ofgrease used. Compared with Grease-A, the amount ofbase oil delivery with Grease-B was approximately 2times more than Grease-A. In other words, comparedwith Grease-A, Grease-B is more prone to base oil
separation. The choice of a grease that is prone tobase oil separation is one important consideration for
the authors’ new grease lubrication system.

was used, the pressure in the grease chamber wasmeasured with a distortion gage pressure sensor andthe grease temperature was determined with athermocouple. The sizes of radial clearances adopted
were 0.05 mm, 0.15 mm, 0.2 mm and 0.45 mm.
Grease-B was prefilled in the grease chamber.
The results of the grease chamber pressure measurements are summarized in Fig. 7(a). In addition,
the temperature readings in the grease chamber
obtained at the same time using the model with aradial clearance of 0.05 mm are also indicated. In theheat cycle pattern adopted, the lowest temperature
was 38˚C and the highest temperature was 41˚C(temperature difference of 3˚C), and the cycle wasrepeated once every 2 hours. The pressure in thegrease chamber kept at 38°C at the start of the test
was taken as 0 kPa. Fig. 7(b) is a partial view of themeasurement record shown in Fig. 7(a), with the time
axis enlarged.
From these diagrams, the following observations,which fulfill expectations, can be made about thegrease chamber. First, the pressure increases as thetemperature increases and the pressure drops as thetemperature drops. Second, the pressure during
temperature increase also correlates to the size of theradial clearance at the end point of the greasechamber. The pressure was greatest when theclearance was the smallest at 0.05 mm anddecreased with greater clearance. Furthermore, themaximum pressure relative to the clearance of 0.05
mm is approximately 10 kPa. The authors think thatthe pressure in the grease chamber readily increases
with the smaller radial clearance due to better sealingof the interior of the grease chamber. They also
conclude that the grease in the grease chamber
expands as the temperature increases, causing thebase oil and thickener in the grease to separate. Since
the fluidity of the separated base oil is higher than the
thickener, as the pressure in the grease chamberincreases, the separated base oil is pushed out
through the clearance. It appears that when delivery of
the separated base oil through the clearance begins,
the pressure in the grease chamber gradually
decreases as shown in Fig. 7(b).
Next, when the temperature of the grease drops,
the grease begins to shrink, and as a result, thepressure in the grease chamber drops. While thepressure is dropping, the rate of base oil delivery fromthe clearance gradually decreases and a portion of thedelivered base oil is drawn back into the clearance.
Then, the interior of the grease chamber seems to besealed due to the surface tension of the oil. This
theory is further supported by the fact that theminimum pressure value is somewhat negative. The

3.2 Base oil delivery mechanism
As mentioned above, the factors that allow theauthors’ novel grease lubrication system to be capableof base oil delivery include the heat cycle, optimized
radial clearance and the choice of grease. The
authors studied how these factors affect base oildelivery by measuring the pressure in the greasechamber in the base oil delivery mechanism. For the
test, the base oil delivery model illustrated in Fig. 2

0.05 mm clearance

0.15 mm clearance

0.45 mm clearance

temperature

0.2 mm clearance
50

50
30

40

20
10

35

0
-10

0

10

20

30

40

50

60

70

80
30

-20

Temperature ˚CPressure kPa

40

25

-30
-40

20

-50

Time h0.05 mm clearance

0.15 mm clearance

0.45 mm clearance

temperature

0.2 mm clearance

16

45

12

40
35

8

30
4
25
0
24

26

28

-4

20
30

Temperature ˚CPressure kPa(a) Pressure change history in grease chamber

15
10

-8

Time h(b)

Detailed pressure change

Fig. 7 Pressure change history in grease chamber-12-

Development of a New Grease Lubrication System for Machine Tool Main Spindles

authors believes that slight increases in the pressurethat follow are due to the gradual shrinking of themodel itself. This pattern is further repeated when thetemperature increases and decreases again due tothe repeated heat cycle.
Thus, existence of the base oil delivery mechanism
described above has been confirmed.

4.1 Temperature increase confirmation test
To investigate the operating characteristics of theauthors’ new grease lubrication system, theymeasured the temperature increase on both constantpressure preloaded bearings and definite position
preloaded bearings. A critical requirement in order forthe grease lubrication system to be used industrially is
the prevention of a significant increase in bearing
temperature. Fig. 9(a) and 9(b) illustrate spindle test
rigs for both preloading systems. In each test rig, twobearings being tested were arranged in a back-toback duplex configuration (DB set). The test
conditions used are summarized in Table 3. Amongthe test conditions, the axial load value wasdetermined by considering the maximum allowableload for the contact surface pressure of the rolling
elements. For the definite position preloading test, thepreload was set to the minimum level necessary forpractical machine tool operation (0 initial clearance onmounted bearing).

4. Spindle Test
The authors applied their new grease lubrication
system to an angular contact ball bearing that carriesa spindle and executed a running test, simulating
operation with a machine tool. The bearing used wasan angular ball contact bearing with a 100 mm bore
diameter. The technical data for the bearing issummarized in Table 2, and a cross-sectional view ofthe bearing is shown in Fig. 8.
Incidentally, the lubricant feed rate adjustment
function provided by the “radial clearance (see Fig. 2)”
in Sec. 3. “Experiment Using a Model Test Rig” isborne by the “shouldered surface clearance (see Fig.
1)” with actual bearings such as those used in theauthors’ spindle test.

Table. 2 Bearing specifications for spindle test
Tested bearing

φ100 × φ150 × 24 mm

Contact angle

20˚
Si3N4

Rolling element material
Cage material

Laminated phenol resin

Prefilled grease in bearing

Prefilled Grease-A, 6 g
Bearings tested(a) Schematic construction of test spindle supportedwith constant pressure preloaded bearings

20˚

φ100

φ150

24

Bearings tested(b) Schematic construction of test spindle supportedwith definite position preloaded bearings
Fig. 8 Section view of test bearing

Fig. 9 Spindle test rig-13-

NTN TECHNICAL REVIEW No.74（2006）

The results of bearing temperature measurementswith the constant pressure preloading system aregraphically plotted in Fig. 10(a), and the results withthe definite pressure preloading system are provided
in Fig. 10(b). With the definite preloading system thatkeeps the axial load to the bearings at a near-constantlevel, the test rig was run at dmn of 2 million (15000
min-1). As a result, the authors confirmed that thetemperature on the bearings at various runningspeeds were stable and the spindle can operate
without any problems.
At the same time, as can be understood from Fig.
10, with the definite position preloading system, thetemperatures on the inner ring and outer ring at dmn of
1.8 million (14000 min-1) were 46˚C and 37˚C,respectively, which are roughly the same as in thecase of the constant pressure preloading system thatruns at the same speed. However, with the definite
position preloading system at dmn of 2 million (15000
min-1), the temperatures on the inner ring and outerring were 53˚C and 43˚C, respectively. These values
are higher than those of the constant pressure
preloading system. Since the running bearings wereoverloaded (preloading) under this situation, theauthors judged that the bearings were not capable ofoperating normally under these conditions.
Assuming that the limiting speed at a particularbearing temperature in the definite position preloading
system is the same as the bearing speed of the same
bearing with the constant pressure preloading system,
and based on the test result described above, thelimiting speed of the definite preloading system
matches dmn of 1.8 million (14000 min-1).
Thus, by using the authors’ new grease lubrication
system, it is possible to operate machine tool spindles
at higher speed ranges than had been previously
possible with air-oil lubrication systems.

Table. 3 Temperature rise test conditions
Item

Details

Shouldered surface clearance
Grease chamber
Prefilled grease in bearing
technical data

0.08 mmHeat cycleNone

47 g Grease-A prefilled

Constant pressure preloading test 2.2 kN

Bearing
preloading

Definite position preloading test Clearance after mounting 0

Jacket cooling oil temperature

Room temperature ±1˚C

60
Inner ring temperature
Outer ring temperature

Temperature ˚C

50

40

30
20

4.2 Endurance test

10

0
0

4000

8000

12000

The durability of bearings lubricated with theauthors’ new grease lubrication system wasevaluated. Durability tests were performed with both adefinite position preloading system and a constantpressure preloading system. The test rigs used areillustrated in Figs. 9(a) and 9(b). The constitution of
the grease chamber and the operating conditions
used are summarized in Table 4. The bearing speedof both the constant pressure preloading system andthe definite position preloading system were at dmn of2 million (15000 min-1) to emphasize the runningspeed. With the definite position preloading system,
overloading would occur as judged from the results
obtained in the previous Sec. 4.1. Therefore, the initialclearance of the bearing was adjusted so that an
appropriate preload (axial load 2.2 kN) was attained at
the speed of 2 million dmn (15000 min-1). In the
adopted heat cycle pattern, as illustrated in Fig. 11(a),each repeated cycle consists of 50 hours of operationand 1 hour of rest. The temperature difference in eachcycle was approximately 15˚C. With the definite
position preloading system, the spindle was also
tested in both vertical and horizontal positions to

16000

Running speed of inner ring min-1(a)Constant pressure preloading

60
Inner ring temperature
Outer ring temperature

Temperature ˚C

50

40

30
20

10

0
0

4000

8000

12000

16000

Running speed of inner ring min-1(b) Definite position preloading
Fig. 10 Temperature rise versus rotational speed-14-

Development of a New Grease Lubrication System for Machine Tool Main Spindles

examine the effect of the attitude of the main spindle
on the bearing life.
The test results are graphically illustrated in Fig.
11(b). The endurance test results of bearings
lubricated with the authors’ new grease lubrication
system are presented along with the results for aconventional grease lubricated angular contact ballbearing with seals on both sides that was tested under
the same conditions. When operated at dmn of 2
million, the conventional grease lubricated bearing
seized after approximately 500 hours had elapsed.
It is clear that the authors’ new grease lubrication
system is effective at providing a sufficient amount of
lubricant and an uninterrupted lubricant supply. A
certain bearing lubricated with the authors’ newlubrication system has successfully achieved 18000
hours of continuous operation and is still running.
Incidentally, the endurance test results reported
here are all obtained from bearings lubricated with
Grease-A. Currently, a bearing endurance test is inprogress with the definite position preloading systemusing Grease-B. The bearings being tested in bothhorizontal and vertical attitudes have already achieved
2000 operating hours and are still operating smoothly.

5. Conclusion
To cope with the mounting needs for higher-speed
machine tool main spindles, the authors havedeveloped a unique grease lubrication system thatallows the bearings being lubricated to be completely
maintenance-free. By using this lubrication systemthat features heat cycle-driven base oil delivery fromthe grease chamber, an angular contact ball bearing
with a 100 mm bore diameter can run at dmn of 1.8
million with a definite position preloading system andat dmn of 2 million with a constant pressure preloading
system. In summary, the authors’ new greaselubrication system is a unique lubrication scheme thathelps achieve better machine tool performance andmitigate environmental impacts because it possesses
a completely maintenance-free operation of thegrease lubrication system and is also capable ofhigher speed operation not possible with any
conventional grease lubrication system.

Table. 4 Endurance test conditions
Item

Details
Shouldered surface clearance 0.08 mm

Grease chamber
Prefilled grease in bearing Grease-A
technical data
Amount of prefilled grease 47 g
Spindle attitude andnumber of spindles tested Vertical (2 units), horizontal (1 unit)

Operating conditions

Definiteposition
preloading

PreloadInitial clearance that coincides with
2.2 kN axial load when the bearing
is running at dmn 2 million

Running speed

dmn 2 million

Spindle attitude andHorizontal (1 unit)
number of spindles tested

Constant
pressure
preloading

Preload

2.2 kN

Running speed

dmn 2 million
Room temperature ±1˚C

Jacket cooling oil temperature

Temperature

References
1) NTN Precision Rolling Bearings, Cat. No. 2260/E
2) Y. Akamatsu, M. Mori: Development of Eco-friendly Oil
Jet Lubricated Angular Contact Ball Bearings for
Machine Tool, NTN Technical Review No. 72 (2004) pp.
6-10.

41˚C

26˚C

1h
50h

50h

Time h(a) Heat cycle patternContinuous operating time h
0

5000

10000

15000

20000

25000

30000

Photos of authors

Definiteposition

Horizontal

Definiteposition

VerticalDefiniteposition

VerticalConventional
greaselubrication

New greaselubrication

Constant Horipressure zontal

Definiteposition

VerticalDefiniteposition

Horizontal(b)

Bearing endurance of new and conventional greaselubrication system

SeizureFig. 11 Endurance test results-15-

Sun-Woo LEE

Masatsugu MORI

Elemental Technological
R&D Center

Elemental Technological
R&D Center

NTN TECHNICAL REVIEW No.74（2006）[ Technical Paper ]High-Speed Tapered Roller Bearing
for Machine Tool Main Spindles

Masatsugu MORI*
Takuji KOBAYASHI*

Tapered roller bearings feature high stiffness and are used tosupport the main spindles of machine tools, but require newgeometrical and air-oil lubrication systems to overcome theirdifficulty in high-speed operation. Power loss minimization of the
bearings are attained by fully lubricating the rib-roller end contacts
as well as by reducing lubricant supply to the roller-race contacts toenable high-speed rotation. The mechanism is specified by aribbed cup, multiple nozzle holes penetrating the rib for air-oil
supply and a resin cage riding on outer-ring spacers. Based on thisconcept NTN has experimentally produced an air-oil lubricated 100
mm bore tapered roller bearing which successfully operates with amaximum dmn value of 1.25 million which is beyond previously
reported records within the category of air-oil lubrication.

comprised of a double row cylindrical roller bearing
and an axial load carrying angular contact ballbearing.
However, with this spindle-bearing configuration,
the contact angle of the axial load carrying angularcontact ball bearing exceeds 30˚ and slipping occurs
on the bearing due to a gyro-moment when it runs at ahigher speed. Thus, the high speed running
performance of this configuration is not satisfactory.
Modern machine tools are required to haveenhanced machining efficiency which can be achievedby using. Tapered roller bearings which feature very
high rigidity in both radial and axial directions. Due totheir greater rigidity, tapered roller bearings are
employed on certain large lathes and turning centers.
However, since they do not excel in the high-speed
running performance required of recent machine tools,
they are used less frequently. Therefore, NTN
attempted to develop a unique tapered roller bearingthat not only features the high rigidity associated withtapered roller bearings but also boasts a high-speed
performance that is better than a combination of adouble row cylindrical roller bearing and an axial load
carrying angular contact ball bearing.

1. Introduction
High-speed running performance and rigidity, which
are generally required for machine tool main spindles,
are factors that greatly depend on the performance ofthe bearings that support the main spindles. The typesof main spindle bearings used include rolling bearings,
dynamic pressure and hydrostatic bearings lubricatedby oil and air, and magnetic bearings. Rolling bearings
are often used for this purpose because they feature
an excellent total balance of all the important considerations, including maintainability and cost-effectiveness. When rolling bearings are incorporated intomachine tools such as machining centers that are
required to run at higher speeds, four rows of highspeed angular contact ball bearings are provided on
the main spindle front side. In this arrangement, highspeed operation is possible with a definite position
preloading system. However, the rigidity of thisconfiguration is not always high enough, as the main
spindles in combined multifunctional machine tools,
such as lathe main spindles, need higher rigidity.
Therefore, the front-side bearing arrangement on
combined multifunctional machine tools is often*Elemental Technological R&D Center-16-

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles

There have been efforts to develop high-speed
tapered roller bearings for machine tool main spindles,
and an example1) has been reported in which 1.25
million dmn was achieved with a tapered roller bearing
equivalent to the 32020 model. In this example,
however, a forced circulation lubrication system was
employed with a lubricating oil flow rate that exceeds
4 liters per minute. Unfortunately, this example is notpractical as a replacement for a grease- or air-oillubricated combination of a double row cylindricalroller bearing and an axial load carrying angularcontact ball bearing because of a greater lubrication
cost and a greater power loss from the entire bearing
system that includes this lubricating oil circulation
system. The authors have conducted research,
prototyping and experiments on high-speed
application with an air-oil lubricated tapered roller
bearing and have achieved an unprecedented 1.25
million dmn. This paper reports on this achievement.

Fig. 1 Section view of standard tapered roller bearing

take place and the elastic hysteresis loss is usuallysmall.3) Given this, typical factors to be considered fora high-speed tapered roller bearing include:
1) EHL (elasto-hydrostatic lubrication) rollingviscous friction between rollers and thebearing ring2) Sliding friction between the roller large-end
face and the rib face3) Sliding friction between the rollers and cage
pockets
4) Stirring friction with oil and air in the bearing
First, the EHL (elasto-hydrostatic lubrication) rollingviscous friction between rollers and the bearing ring in
1) can be determined with the following expression4):

2. Requirements for Higher Bearing Speed
A challenge that must be addressed to achieve
higher speed with a rolling bearing is limiting thetemperature increase that results from higher bearing
speed. If the temperature on a rolling bearing is
excessively high, the preload varies greatly due tothermal expansion, and this in turn further promotes
temperature increase. This problem is particularly
apparent with definite position preloading systems,
which are often used on bearing systems that musthave very high rigidity.
Arrangements for inhibiting temperature increases
on a rolling bearing include a forced circulation
lubrication system and a jet lubrication system inwhich a large amount of lubricant is fed to a rolling
bearing to remove heat from the bearing in an amountgreater than the heat generated on the bearing that
results from the viscosity of the lubricating oil.
However, the authors did not employ either of thesearrangements. In the case of air-oil lubrication
systems in which small amounts of lubricating oil are
fed to the rolling bearing with compressed air, the key
to high-speed operation is minimization of the heatoccurring from the running rolling bearing.
Now, let us consider the heat generating factor, thatis, the torque generation factor for the tapered roller
bearing illustrated in Fig. 1. On a tapered roller
bearing, the generatrix of the rollers intersects theprolonged line from the inner-outer ring raceway at apoint on the rotation axis. Therefore, sliding friction
from spinning between rollers and raceway does notoccur.2) Also, since the mode of roller-bearing ring
contact is linear contact, differential sliding does not

fEHL＝φs, EHLφt

0.648
29.2R
（GU）
W 0.246αl ……………(1)where, φs, EHL is an EHL starvation coefficient that
represents the degree that the amount of oil is
insufficient, and can take a value in the range from 0
to 1; φt represents a correction coefficient derived
from shearing heat generation occurring at the EHL
inlet and is determined based on the load, bearing
speed and lubricating oil physical properties; α is theviscosity-pressure coefficient of the lubricating oil
used; l stands for the contact length of the rollers: G, U
and W are non-dimensional material, velocity and load
parameters, respectively, and are uniquely determinedby the geometrical shape and running speed of the
bearing, the load, and the physical properties of the
lubricating oil. Therefore, if the geometrical shape ofthe bearing, operating conditions and the lubricating
oil are given, the rolling viscous friction can be
decreased by gradually decreasing φs, EHL to zero byallowing starvation to occur whenever possible by-17-

NTN TECHNICAL REVIEW No.74（2006）

decreasing the oil fed between the rollers and bearing
ring while maintaining the EHL oil film. Consequently,
at the same time, the shearing heat generation at the
inlet decreases and φt becomes smaller.
Next, let us consider the sliding friction occurring
between the roller large-end face and the rib facementioned in 2) above. The contact surface pressure
at this area is low, and the fluid lubrication mode
occurring there is an equivalent viscosity-rigid body orchanging viscosity-elastic body (EHL) mode.5) Thus,
the authors executed numerical analysis in theequivalent viscosity-rigid body mode in order toevaluate quantitatively the effect of starvation on thefrictional resistance between the roller large-end faceand rib face.
More specifically, the authors determined thereaction force on the oil film and the pressure center
position from the oil film pressure distribution, which
was obtained by solving the Reynolds equation for thetwo-dimensional flow on a polar coordinate system inaccordance with a relaxation method. They repeatedcalculation with the Newton-Raphson formula until theconditional expression, where the reaction force on
the oil film was equal to the applied load and thepressure center position passed the roller center
position, was satisfied. The cavitation boundary
conditions applied to the Reynolds equation were theReynolds conditions. From the floatation and skew
angle on the roller large-end face finally obtained, theviscosity-friction torque on the rib can be calculated.
Accordingly, in Fig. 2, which simulates the interference region in terms of fluid lubrication between theroller large-end face and inner-ring rib, the hatchedarea is the region subjected to fluid lubrication analysis
for a case where no starvation is present (θ=θmin).
Assuming that the oil film pressure generation starts at

the angleθ, the rib starvation coefficient can be
defined with the following expression:
φs, rib＝

θ−θmin…………………………(2)θmax−θmin

When this rib starvation coefficient is used and therib viscosity-friction torques for all the 29 rollers aretotaled, the result illustrated in Fig. 3 is obtained.
Since most of the pressure occurs whenθis π/2 orlower, the authors assumed that φs, rib was less than
50%. From Fig. 3, it should be understood that whenφs, rib is at 45%, the torque is approximately twice as
great. This is because, despite the narrower pressure
generation region (that is, frictional torque generation
region), the rib needs to carry the same load, and as aresult, the oil film thickness on the rib is smaller andthe increase in the torque is greater than the effect of
reduction in the pressure generation region. Assumingthat in reality the starvation occurs not only in thecircumferential direction but also in the radial direction,
more increase in the torque can be expected.
In summary, to reduce the rolling viscosity
resistance, it is necessary to minimize the amount of
lubricating oil. On the other hand, to reduce thefrictional resistance on the rib, it is necessary toprovide a sufficient amount of lubricating oil. The
authors believe that by reducing the amount of oil on
the raceway surface, the torque resulting from 3) and4) described above can also be reduced.
To determine the actual relation between amountsof air-oil lubrication and torques, the authors used thestandard 32020 tapered roller bearing, which has
specifications as summarized in Table 1, andmeasured the total torque and rib torque. As usual,lubricating oil was supplied from the back side (innerring smaller rib side) by using eccentric pump action.

Relief on end face
Direction of roller rotation

0.6
0.5

θmax

Torque N･mRollerlarge-end
face
Inner-ringrib
Direction of
inner-ringrotation

0.4
0.3
0.2
0.1
0.0

θ

0

θmin

10

20

30

40

50

Rib starvation coefficient %Fig. 2 Sketch of interaction between roller large end face
and inner-ring rib

Fig. 3 Numerically evaluated viscous torque at inner-ring
rib of standard tapered roller bearing 32020-18-

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles

The resultant measurements are graphically plotted inFig. 4.
In Fig. 4 the difference between the total torque andrib torque equals the sum of 1), 3) and 4) describedabove. Since the effect of 4) is sufficiently small in therelatively low speed range of less than 4000 min-1 and
1) is predominant over 3) with roller bearings,3) the
difference between the total torque and rib torque isessentially equivalent to 1), that is, the EHL rollingviscosity friction on the raceway surface. As can beunderstood from expression (1), when there is noeffect of starvation and heat generation, the rollingviscosity resistance increases in proportion of the
0.65th power of the rotation speed. However, the total
torque starts to decrease at around 300 min-1 whenthe effect of heat generation seems to be minimal.
This appears to be due to the significant effect of
starvation.
Also, it will be understood that as the amount of oil
is reduced, the effect of starvation is more apparent
and the rolling viscosity resistance decreases. At thesame time, because the rib is under fluid lubrication
conditions, the rib torque is proportional to the bearing
speed, and increases with a decreased amount of
lubricating oil. This fact further supports the analysis
results described above.
In summary, in order for a tapered roller bearing to
be able to run at a higher speed, it is necessary toinvent a mechanism that minimizes the supply of
lubricating oil to the raceway surface while supplying
plenty of lubricating oil to the rib.

3. Structure and Performance of the
Authors’ Bearing
3.1 Basic structure
Based on the approach described above, theauthors developed and evaluated various prototype
bearings with unique lubrication schemes and internal
constructions. Consequently, the authors finallydeveloped the unique tapered roller bearing with theconstruction illustrated in Fig. 5. Now, referring to Fig.
5, let us describe its features. First, the authors’tapered roller bearing lacks an ordinary inner-ring rib,and instead, the rib is situated on the outer-ring side.
This arrangement prevents the lubricating oil fromflying away due to the centrifugal force of the rotating
inner-ring as well as resultant insufficiency in theamount of lubricating oil. At the same time, toeffectively lubricate the rib, multiple direct-injection
nozzles are provided on the rib surface that supply airoil lubricating oil. Incidentally, the usual oil supply fromthe back side is not provided. Instead, the lubricating
oil from the direct-injection nozzles is directed to the
raceway surface through the spaces between therollers. Because the pockets and rollers interfere witheach other at higher speeds on roller guiding cages
such as the standard type, the authors have adopted

Table 1 Specifications of torque test bearing
and test conditions

Bearing

Description: 32020XUP4
Size: ID 100 x OD 150 x 32 mm total width
Contact angle: 17˚
Rollers: 29 steel rollers
Cage: Pressed steel cage, rolling element guideTestconditions

Bearing load: 1 kN axial load
Bearing lubrication: Air-oil
Oil supply from both sides of bearing
Nozzle dia. φ1.2×1 (per side)
ISO VG32 oil used
Jacket cooling: None

21.6cc/h total torque7.2cc/h total torque3.6cc/h total torque1.8cc/h total torque
0.9cc/h total torque21.6cc/h rib torque7.2cc/h rib torque3.6cc/h rib torque1.8cc/h rib torque
0.9cc/h rib torque

1.8

Decrease inoil amount

Torque N･m

1.6
1.4

Enlarged view

Direct-injection
nozzleRib

1.2

Total torque

1.0
0.8

Rib torque

0.6
0.4
0.2

Decrease inoil amount

0
0

500

1000

1500

2000

Running speed

2500

3000

3500

Outer-ring spacer
guided resin cage

4000

min-1

Fig. 4 Measured torque values of standard tapered roller
bearing 32020

Ceramic roller

Fig. 5 Section view of developed tapered roller bearing-19-

NTN TECHNICAL REVIEW No.74（2006）

a guide arrangement on which the outer-ring rib
guides the rollers. In addition, the authors have also
adopted a lightweight oil-retaining phenol resin as thecage material. The rollers used are made of alightweight ceramic material of a lower thermal
expansion coefficient.

In testing the authors’ bearing, the temperatures
and torques on the inner and outer rings relative tovarious spindle (inner ring) speeds were measured.
For this purpose, the spindle running speed was set toeach intended speed setting by controlling theinverter-regulated motor, and the temperatures andtorques on the inner and outer rings were measured
and recorded only when they became stable.
The torque was measured with a torque meter
mounted to the spindle. Each torque measurement
includes the torques on the two pieces of the authors’
bearing and the windage on the rotating sections. The
air-oil feed rate to each piece of the authors’ bearing
was 1.8 cc/h.
The measured temperatures on the inner and outerrings are plotted in Fig. 7(a), and the obtained torquesin Fig. 7(b). Note that the torque values in Fig. 7(b)are those converted for one bearing. As can beunderstood from these diagrams, the authors’ bearing
does not exhibit a rapid increase in temperature andtorque up to an inner ring speed of 1000 min-1, andcan attain 1.25 million dmn with air-oil lubrication.
In addition, the authors checked the running speedof the cage at various bearing speeds using a
stroboscope. As a result, no slipping was found,

3.2 Test and bearing performance
The construction of the test spindle is illustrated inFig. 6. The spindle was driven by an external inverterregulated motor that was connected to the spindle bya coupling. Among the bearings used, only theauthors’ bearing was arranged in a two-row back-toback duplex configuration. The boundary dimensionsof the authors’ bearing were the same as those of the32020, with a bore diameter of 100 mm. Since thewidth of the pocket bars is greater with the authors’
bearing than the pressed steel cages on standard
bearings, the number of rollers on the authors’ bearing
is 23 which is less than standard bearings, which have
29. The housing incorporated an arrangement forjacket cooling. The preloading system used was adefinite position preloading system with zero initialclearance. The major test conditions used are assummarized in Table 2.

90
Outer-ringtemperature
Inner-ringtemperature

Temperature ˚C

80
70
60
50
40
30
20
10
0

0

2000

4000

6000

8000

10000

12000

Bearing speed min-1(a)Measured temperature of inner and outer ring

Fig. 6 Schematic construction of test spindle

2.0

Table 2 Specifications of spindle test bearing
and test conditions

Torque N･mBearing

1.6

Description: Equivalent to 32020XUP4 (modified rib construction)
Size: ID 100 x OD 150 x 34 mm total width
Contact angle: 17˚
Rollers: 23 Si3N4 rollers
Cage: Phenol resin machined cage, outer-ring guide

1.2
0.8
0.4

Bearing clearance: Definite position preloading with aclearance of zero as assembled
Bearing lubrication: Air-oil
Test
Oil supply only to the rib surface
conditions
Nozzle dia. φ0.6×12 (one bearing row)
ISO VG32 oil used
0.01 cc/20 s oil feed rate (one bearing row)
Jacket cooling oil temperature: Room temperature ±1˚C

1.0
0

2000

4000

6000

8000

10000

12000

Bearing speed min-1(b)

Measured torque per bearing

Fig. 7 Test results of developed tapered roller bearing-20-

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles

thereby it was confirmed that the rollers were rollingcorrectly. The authors also inspected the bearing
elements that underwent the test and detected nodamage such as discoloration and wear. Incidentally,
the authors did not attempt to use a low viscosity
lubricating oil for the purpose of preventing disruptionof the oil film on the raceway surface that could result
from a decrease in the amount of oil. Instead, they
adopted ISO VG 32, a standard lubricating oil typecommonly used for machine tools. However, theauthors think that use of a lower-viscosity lubricating
oil could further limit the increase in torque andtemperature.
The authors also tested the standard 32020
bearing, and learned that even when a maximum
bearing speed was attained by varying the viscosityand feed rate of lubricating oil, the bearing
temperature dramatically increased at 6000 min-1, andfurther operation was impossible.
As previously mentioned in the first section of thisreport, the authors’ bearing is expected to satisfy

requirements for not only high-speed performance but
also rigidity. Therefore, the authors prepared two
bearing arrangements, wherein one bearing
arrangement was comprised of the authors’ bearing
type and the other bearing arrangement consisted of adouble row cylindrical roller bearing (which was used
as a front-side bearing on a conventional bearing
arrangement for machine tool spindles) and an axialload carrying angular contact ball bearing. Then,assuming that these bearing arrangements were eachincorporated into a machine tool spindle, the authors
calculated the deflections of the spindles that carried
the loads, and compared the calculation results. The
arrangement of the authors’ bearing is illustrated inFig. 8(a), and the arrangement consisting of a double
row cylindrical roller bearing and an axial-load carrying
angular contact ball bearing is shown in Fig. 8(b). The
specifications for the authors’ bearing are identical tothose summarized in Table 2.
The double row cylindrical roller bearing used was astandard NN3020, the axial-load carrying angular

Front

Rear

(a) Arrangement of developed tapered roller bearing(b)Conventional bearing arrangement

Fig. 8 Bearing arrangements for machine tool main spindle-21-

NTN TECHNICAL REVIEW No.74（2006）

As mentioned in the first section of this report, thehigh-speed rotation performance of the axial-load
carrying angular contact ball bearing within theconventional bearing arrangement shown in Fig. 8(b)is insufficient, and this bearing arrangement is capableof only 1 million dmn with air-oil lubrication.
In contrast, in terms of both high-speed
performance and rigidity, a spindle supported by theauthors’ tapered roller bearing excels over spindles
supported by a conventional bearing arrangement thatconsists of a double row cylindrical roller bearing andan axial-load carrying angular contact ball bearing.

contact ball bearing used was a standard
HTA020ADB, and the bearing used on the rear side
was the N1016 single row cylindrical roller bearing.
Deflections occurring on the front end of the spindle
when pure axial loads and pure radial loads areapplied to the front end are plotted in Figs. 9(a) and(b). For example, when an axial load of 4 kN is
applied to the front end, the axial deflection on theauthors’ bearing is approximately 50% less than withthe conventional bearing arrangement. When a radialload of 4kN is applied to the front end, the radial
deflection on the authors’ bearing is approximately
80% less than the conventional bearing arrangement
even though the front side bearing span with theauthors’ tapered roller bearing is approximately 30%
smaller than the conventional bearing arrangement.
More specifically, it has been confirmed that theauthors’ bearing has high rigidity against both axial
and radial loads.

4. Conclusion
By using their unique tapered roller bearing thatboasts greater rigidity for machine tool main spindles,
the authors have invented a novel bearing
construction and a novel air-oil lubrication mechanism
to address the issue of unsatisfactory high-speed
performance, which is a drawback of conventional
tapered roller bearings. As a result of testing theauthors’ tapered roller bearing with an inner bore
diameter of 100 mm, this bearing achieved
unprecedentedly high dmn of 1.25 million with air-oil
lubrication. For bearings that support machine tool
main spindles, higher running speed and greaterrigidity are challenges to be addressed in achieving
better surface finish quality for workpieces andimproved machining efficiency. NTN will remaincommitted to addressing these technical challenges
by pursuing further advances in its bearing
technology.

16
Conventional bearing arrangement

Bearing deflection μm

14

Authors’ tapered roller bearing

12
10
8
6
4
2
0
0

1

2

3

4

5

6

Axial load kN(a)Calculated axial deflection

References
1) T. Miki: Performance and Application of Ultra-High Speed
HA-Type Tapered Roller Bearings, Mechanical Design
26, 10 (1982) pp. 109-114. (In Japanese)
2) N. Soda: Bearings, Iwanami Shoten (1964). (InJapanese)
3) H. Fujiwara, K. Fujii: Rolling bearing torque in oil bathlubrication, Japan Society for Precision Engineering
Spring Conference Academic Lecture Proceedings(2002), p. 219. (In Japanese)
4) Zhou, R. S. and Hoeprich, M. R.: Torque of Tapered
Roller Bearings, ASME J. of Tribology, 113, 3 (1991)
590-597.
5) Gadallah, N. and Dalmaz, G.: Hydrodynamic Lubricationof the Rib-Roller End Contact of a Tapered Roller
Bearing, ASME J. of Tribology, 106, 2 (1984) 265-274.

5
Conventional bearing arrangement

Bearing deflection μm

4.5

Authors’ tapered roller bearing

4
3.5
3
2.5
2
1.5
1
0.5
0
0

1

2

3

4

5

6

Radial load kN(b)Calculated radial deflection

Fig. 9 Comparison of spindle end deflections between
developed bearing and conventional bearing arrangements-22-

High-Speed Tapered Roller Bearing for Machine Tool Main Spindles

Photos of authors

Masatsugu MORI

Takuji KOBAYASHI

Elemental Technological
R&D Center

Elemental Technological
R&D Center-23-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Minimum Quantity and Cooling Jet LubricatedAngular Contact Ball Bearings for Machine Tool

Mineo KOYAMA*
Minimum Quantity and Cooling Jet (MQCJ) angular contact
bearings feature a unique jet lubrication system. The design
incorporates a special inner ring design, with a cooling groove
combined with special geometry for directing lubricant to allow an
optimally small amount of lubricant to reach the raceway, in orderto minimize power losses from friction torque. It was developed in
response to market demands for machine tool spindles withincreased speed capability coupled with high rigidity. Its practicaluse has been demonstrated by having achieved a dmn 3.6 million
with fixed-position preloading. This technical review paper isintended as an overview of its development and design.

1. Introduction

2. Features of the MQCJ Lubrication
Mechanism

Needs have been mounting for faster machine tool
main spindles. This trend is even more apparent withthe main spindles of machining centers for machining
dies because of the desire to realize machine toolsthat are capable of providing higher quality finishedsurfaces of workpieces and to improve machining
efficiency. To address these needs, attempts todevelop higher speed main spindles by using air-oil
lubrication and oil-mist lubrication have been
undertaken. In 2004, NTN developed a novel jet
lubrication mechanism that shoots lubricating oil to ascoop formed on the inner ring end face, realizing
both high-speed operation at 5 million dmn with aconstant pressure preloading system for angularcontact ball bearings and low power loss that iscomparable to the power loss with air-oil lubrication.
To satisfy market needs for higher bearing speed, we
have recently developed angular contact ball bearings
based on a fixed-position preloading system, and the
newly developed bearings have realized high speed
operation of 3.6 million dmn while ensuring preloadingat the initial stage of bearing operation. This report
describes the features of these bearings, and theresults of evaluation testing.
The author has named this novel jet lubrication
system MQCJ (Minimum Quantity and Cooling Jet)
lubrication.

2.1 Jet lubrication for the inner ring
The construction of an MQCJ-lubricated angularcontact ball bearing is illustrated in Fig. 1. When themain spindle runs at a higher speed, the preload
increases because of expansion on the inner ring due
to centrifugal force as well as because of temperature
differences across the inner ring and outer ring due toheat generation on the bearing. If jacket cooling isprovided, the outer-ring side is actively cooled.
However, because the inner-ring side isdisadvantageous in terms of heat radiation comparedto the outer-ring side, the temperature on the innerring side is higher than that on the outer-ring side, andthe thermal expansion on the inner-ring side becomes
greater. As a result, the preload on the bearing
increases unavoidably. The author’s lubrication
mechanism is characterized in that lubricating oil isfirst shot to the scoop formed on the inner-ring end
face to actively cool the inner ring in order to inhibit
preload buildup.

2.2 Application of the minimum quantity of
lubrication mechanism to the rolling
contact surface
Conventional jet lubrication arrangements have thedrawback that their power loss is greater due to***Industrial Engineering Dept. Industrial Sales headquarters-24-

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine ToolJet cooling targeted at
the inner ring scoop

Application of the minimum quantity mechanism leads toa very small amount of lubricating oil in the bearing.

Fig. 1 Bearing and spacer design

greater stirring resistance when a bearing runs at ahigher speed because a greater amount of lubricating
oil flows inside the bearing. With the author’s
lubricating system, the lubricating oil shot at the scoop
deposits on the scoop inner surface, then centrifugal
force and surface tension caused it to move to theconical surface on the inner-ring outer circumference,
thereby only a minimal amount of lubricating oil is fed
into the bearing.
Because the amount of lubricating oil entering thebearing is controlled by the gap between the conicalsurface and the outer ring spacer, most of the
lubricating oil contributes to cooling the inner ring
while a very limited amount of it flows through thebearing. Thus, the author’s bearing arrangement
boasts lower power loss.

lubrication mechanism, one common oil supply system
feeds oil to both the bearing section and the jacket
cooling section, and the used oil is recovered by ascavenge pump. Since the jacket cooling oil also
serves as the bearing lubricating oil, the necessary
accessory equipment can be of simpler construction.

3. Test Result

The test conditions used are summarized in Table
1, while the construction of the test rig is illustrated inFig. 3. Because of dimensional limitations, only thefront side bearing (lower section) in the test rig
incorporated the MQCJ lubrication mechanism (therear side is provided with air-oil lubrication). The
measurements of temperature increase on the outerring at varied lubricating oil feed rates are graphicallyplotted in Fig. 4, and the resultant power losses are
2.3 Bearing lubricating oil serves as jacket
given in Fig. 5. When the spindle was arranged
cooling oil
vertically and the as-mounted preload was set to 0 N
The author’s lubricating system is schematically(zero axial clearance), the author’s bearing was
illustrated in Fig. 2. In many air-oil lubrication and oilcapable of stable operation at 3.6 million dmn andmist lubrication systems for machine tool main
spindles, the bearing is lubricated by a lubricating oil
higher, the outer ring temperature was lower than
supply system and the jacket is cooled by a cooling oil
50˚C and the resultant power loss was not greater
supply system. In contrast, with the author’s
than 8 kW.
The outer ring temperature increases measured atinitial preloads of 0 N and 150 N areplotted in Fig. 6. Though theFiltertemperature increase with the initialpreload of 150 N is somewhat
Ordinary
jacketcoolingsystemsignificant compared to that with the
Cooling oil
supply device
preload of 0 N, when the dmn
exceeded
3.6 million, the outer ring
Main spindle
Jacket coolingtemperature was less than 50˚C.
bearing
Therefore, the author’s bearing
Oil recovery tankarrangement seems to be capable ofpractical use. The measurements ofScavenge pumptemperature increases on the outerrings of vertically and horizontallyarranged spindles are illustrated inFig. 2 MQCJ lubrication system-25-

NTN TECHNICAL REVIEW No.74（2006）

Fig. 7. Regardless of the spindle attitude, thetemperature increase profiles are virtually same, so
the author’s bearing arrangement can be used forpractical applications in both vertical and horizontal
arrangements.

Support bearing

Jacket coolingon the motor section

Table 1 Test condition
Test bearing

ID 70 x OD 110 x 20 mm width

Contact angle

20˚

Preload system

Definite position preloading

Initial preload

0，150 N

Spindle attitude

Vertical, horizontal

Lubricating oil/cooling oil

ISO VG1.5

Rate of oil feed to bearing

0.8，1.5，3.0 L／min

Jacket cooling

Test bearing
lubricatedby MQCJ

Only for the motor section

Jacket cooling oil temperature

20˚C

Bearing ring material

Fig. 3 Schematic construction of test spindle

Special cemented steel

Rolling element material

Ceramic Si3N4Cage material

Outer ring temperature increase ˚CPhenol resin60

3.0L/min1.5L/min
0.8L/min50
40

Initial preload: 0 N

30
20
10
0
15

20

25

30

35

Running speed ×103

40

300

200

dmn value

45

min-1
400

×104

Fig. 4 Affect of amount of lubricant on outer race temperature

10

3.0L/min1.5L/min
0.8L/minPower loss kW

8

Initial preload: 0 N

6
4
2
0
15

20

25

30

35

Running speed ×103

40

300

200

dmn value

×104

Fig. 5 Affect of amount of lubricant on power loss

-26-

45

min-1
400

Outer ring temperature increase ˚C

Minimum Quantity and Cooling Jet Lubricated Angular Contact Ball Bearings for Machine Tool60

0 N preload
150 N preload50

Oil feed rate: 1.5 L/min

40
30
20
10
0
15

20

25

30

35

Running speed ×103
200

40

300

dmn value

45

min-1
400

×104

Outer ring temperature increase ˚C

Fig. 6 Initial preload after mounted with DB set on outer race temperature

60

Vertically arranged spindle
Horizontally arranged spindle

50

Initial preload: 0 N
Oil feed rate to bearing: 1.5 L/min

40
30
20
10
0
15

20

25

30

35

Running speed ×103

40

300

200

dmn value

×104

Fig. 7 Main spindle orientation on outer race temperature

4. Conclusion
The author has demonstrated that a combination ofan angular contact ball bearing and a definite position
preloading system is capable of high-speed operation
at 3.6 million dmn by adopting MQCJ lubrication. The
author believes that this novel technique will greatly
contribute to higher speed, greater rigidity and higher
precision with machine tool main spindle bearings.

Photo of author

References
1) Y. Akamatsu, M. Mori: Development of Eco-friendly Oil
Jet Lubricated Angular Contact Ball Bearings for Machine
Tool, NTN Technical Review No. 72 (2004) p. 6.

Mineo KOYAMA
Industrial Engineering Dept.
Industrial Sales headquarters

-27-

45

min-1
400

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]dmn170x104 Sealed High-Speed Angular Contact Bearing“New BNS Type”

Futoshi KOSUGI*

The limiting dmn value of the BNS type sealed high-speed
angular contact bearing is dmn 140×104.
A new limiting speed of dmn 170×104 (the worlds highest speed
level) was enabled by optimized internal design and a new
grease. New grease improves the running-in operation for main
spindle bearings. It corresponds to the small machining centerand tapping machine by making it to the series up to the small
diameter size of φ20mm bore.

sealed angular contact ball bearing product line.
This report describes NTN’s product development
efforts for the “New BNS Type” sealed angular contact
ball bearing product, which is capable of the world’s
highest dmn limiting speed, at 170 x 104, with a definite
position preloading system. Based on theconventional BNS type, this product has beendeveloped through optimization of the internal design,
introduction of the new SE-1 grease and employmentof a special carburized steel for the inner and outerrings.

1. Introduction
Four years have already passed since the ULTAGE
Series of products were presented at JIMTOF 2002.
The main purpose for developing the ULTAGE Series
was to provide bearing products that can handle higher
speeds and advanced functions in machine tool main
spindles. The need for higher main spindle speeds has
posed new challenges for bearings that support
machine tool main spindles. That is, higher main
spindle speeds have led to higher bearing speeds, andmain spindle bearings must handle the increased heatgeneration that results from the adoption of built-in
spindle motors capable of rapid acceleration anddeceleration. Main spindle bearings must also
accommodate shorter bearing lengths in order to help
increase critical bearing speeds. Greater centrifugal
force, higher ambient temperatures, and shorter
bearing spans have led to increased internal bearing
preloads, making operating conditions for main spindle
bearings much more demanding. To address thisproblem, NTN has developed and marketed the
ULTAGE Series of standard angular contact ballbearings (HSE type) that are capable of high-speed
operation under high bearing stress conditions.
The ULTAGE Series consists of 12 product types
capable of meeting various customer needs for higher
speeds and difficult environmental considerations. It
includes an eco-friendly bearing product line and a

2. Development of the "New BNS Type"
Sealed Super High-Speed Angular
Contact Ball Bearing Featuring theWorld’s Highest dmn of 170 x 104
2.1 Highlights of development
For machine tool main spindle bearings, higher
speed and greater stiffness are required to shorten
machining time, while inhibition of excessive
temperature increases and higher running accuracy
are also needed to improve machining accuracy.
Furthermore, reduction in the amount of lubricating oil
and employment of grease lubrication are necessary
to ensure eco-friendliness. Among theserequirements, the most outstanding market need isthe adoption of a grease lubrication system becauseof its friendliness to the environment and ease of*Industrial Engineering Dept. Industrial Sales headquarters-28-

×104 Sealed High-speed Angular Contact Bearing “New BNS Type”
dmn170×

BNS type bearings, the New BNS Type products
feature an improved internal design optimized forhigher speed, thereby achieving stable operation at
the world’s highest dmn, 170 x 104, a 20% increaseover conventional BNS type products. Furthermore,temperature increases on main spindles are limited tothe same level as conventional BNS type bearings.
The author believes that adoption of the new grease
leads to simplification of bearing running-in operationand inhibition of adverse effects from heat generated
by built-in motors. Thus, the author’s new bearing
product line can withstand long periods of operation,
in excess of 20,000 hours.
The New BNS Type product series will also include
bearings with bore diameters measuring 20 to 100
mm so that the products can be used on small-sizedmachining centers and tapping centers.

2.2 Optimized design
Beginning with the conventional grease-lubricated
BNS type, the author first redesigned the internal
structure to achieve a higher bearing speed while
maintaining a grease-lubrication arrangement. The
author reached a design with an optimal combinationof diameter, number of balls, groove curvature, andother factors. This combination helped prevent heatbuildup in the bearing, inhibit increases in internalbearing pressure and minimize loss in bearing rigidity(Fig. 1). In addition, the author adopted grease
pockets identical to those used on conventional BNS
type products so that grease is kept nearest to therolling elements running at higher speeds in order to
optimize the lubricating performance of the grease(Fig. 2).
Furthermore, since hoop stress can pose a problem

Axial displacement μm

Axial displacement μm

handling. In this context, the ULTAGE Series greaselubricated, sealed angular contact ball bearing
products are highly favored in the market.
However, demand is strong for higher speeds inexcess of the 140 x 104 dmn limit of conventional
grease-lubricated, sealed angular contact ballbearings. The proposed arrangements for coping withthis trend are a starvation lubrication unit that is
intended to extend lubrication life, as well as a greaselubrication system for bearing spacers (NTN’s new
grease lubrication system).
The most critical performance characteristic
associated with employment of grease lubrication islong-term durability, and a bearing life of longer than
20,000 hours is usually required. To achieve long-term
durability for grease-lubricated main spindle bearings,
it is necessary to inhibit excessive heat increases withthe bearings, retain the grease and supply the base oilaround the rolling surface. It is also necessary tomaintain the durability of the bearings against heatbuildup from built-in motors situated near the
bearings. In addition, the reduction of main spindle
costs is also an important consideration.
To address these challenges and satisfy the needfor super high speed, the author has developed the
New BNS Type, which is capable of the world’s fastest
level of running speed. This new product line retains
the bore diameters, single outside diameters andwidths of conventional grease-lubricated, sealed
angular contact ball bearings. The New BNS Typebearing products allow high-speed main spindles,
which previously needed oil lubrication arrangements
including air-oil lubrication, to be actively lubricatedwith grease. This advantage can help decrease costs
for main spindles. Compared with the conventional

50
Dia. 100 New BNS Type
Dia. 100 conventional BNS

40
30
20
10
0

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

50
Dia. 30 New BNS Type
Dia. 30 conventional BNS

40
30
20
10
0
0

500

20
Dia. 100 New BNS Type
Dia. 100 conventional BNS

15
10
5
0
0

1000

2000

3000

4000

5000

6000

1000

1500

2000

2500

3000

2000

2500

3000

Axial load N
Radial displacement μm

Radial displacement μm

Axial load N

7000

8000

9000

10000

15
Dia. 30 New BNS Type
Dia. 30 conventional BNS

10
5
0
0

Radial load N

500

1000

1500

Radial load N

Fig. 1 Rigidity graph-29-

NTN TECHNICAL REVIEW No.74（2006）

increase in the 100 mm bore diameter bearings differ
from those of the 30 mm bore diameter bearings. Thisis because of the difference in the construction of thetest rigs (Fig. 4).

Grease pockets

2.3 Grease life (lubrication life)
One important consideration in determining the
lubrication specifications for a main spindle bearing isgrease life (lubrication life). The useful life of any
grease greatly varies depending on its operating
environment. The typical factors that greatly affectgrease life include the operating temperature, bearing
load (preload) and ingress of foreign matter (chips,etc.).
Unlike the main spindles used on the test rigs, main
spindles used on real machine tools feature better
sealing performance in order to avoid ingress of
coolant during machining operation. This means thatheat buildup tends to occur inside machine tool main
spindles.
In addition, main spindles that are more compact
mean smaller bearing-to-bearing distances, whichhinders heat dissipation. Poor heat dissipation causes

Fig. 2 New design

when a bearing is used at a higher speed range, thebearing ring of the author’s bearing is made of a
special carburized steel that has an optimized alloy
composition.
In bearing performance test, the newly developed
bearing was run at a maximum speed of 190 x 104
dmn (150 x 104 dn), exceeding the target of thedevelopment effort of 170 x 104 dmn (135 x 104 dn).
Thus, the author proved that the newly developed
bearing design features limited temperature increase incomparison with the conventional BNS type (Fig. 3).
Note that, at same dmn values, the temperature30
Dia. 100 New BNS Type

12

Heat increase ˚CHeat increase ˚C

14

Dia. 100 conventional BNS

10
8
6
4

Dia. 30 New BNS Type
20

Dia. 30 conventional BNS

15
10
5

2
0

0
0

5000

10000

15000

0

10000

Running speed min-1

20000

30000

40000

50000

Running speed min-1

Fig. 3 Temperature rise test results

Dia. 100 test bearing
Dia. 30 test bearing

Fig. 4 Test machine-30-

×104 Sealed High-speed Angular Contact Bearing “New BNS Type”
dmn170×

accelerated heat buildup in main spindles.
Furthermore, when a machining load works on a main
spindle, heat buildup is further promoted, leading toshorter grease life. This trend is more apparent withmachine tool main spindle bearings that lack theprovision of jacket cooling.
Provision of a cooling arrangement on a main
spindle such as jacket cooling greatly affects thetemperature profile of a given machine tool main
spindle, and significantly influences the useful life of
the grease used on the bearings on that main spindle.
The newly developed bearing employs a novel
grease. In developing this grease, a test rig
arrangement was developed that incorporated a main
spindle configuration without jacket cooling to simulate
operation on real machine tool. Thus, the author
attempted to obtain more reliable grease endurance
data.
Note that the main role of the seals on a sealed
bearing is to retain grease, and due to the limitationsof their function, these seals are not intended toprevent coolant ingress.
Ingress of foreign matter (chips, etc.) into main
spindles on actual machine tools rarely occurs.
However, coolant entering a bearing on a main
spindle bearing can cause the consistency of grease
in it to vary, possibly allowing the grease to flow out or
to become hardened. If this situation occurs, thegrease will fail to develop its designed performanceand the bearing will no longer run smoothly.
Therefore, this situation must be actively avoided.

prefilled MP-1 (urea-based thickener and synthetic oil)
as a standard grease. This grease had been
performing well in high-speed electric motors and in
the CRC grease high-temperature life test conductedaccording to ASTM D 3336, exhibiting longer life. For
this reason, this grease was applied to the BNS type.
In addition, it has been performing very well as agrease for machine tool main spindles.
Immediately after adoption by the author, the MP-1
was subjected to a durability test with an angularcontact ball bearing. It exhibited a high degree of
lubrication performance as it withstood 20,000 hoursof operation at 140 x 104 dmn (bore diameter 100 x
11000 min-1, with jacket cooling) (Fig. 5).
The downside, however, is that the viscosity of thebase oil in MP-1 is relatively high (40.6 mm2/s at 40°C)
compared with general-purpose greases for main
spindle bearings that are commonly used by machine
tool manufacturers (Fig. 6). When jacket cooling is notprovided, the outer ring temperature sometimes(though rarely) exceeds 60°C during the running-in
period. In other words, MP-1 needs improvement toachieve easy running-in operation.
As previously mentioned, the durability test was
executed at a high-speed condition of 140 x 104 dmn,
and therefore, jacket cooling was provided. However,there has been demand for main spindles that performreliably even when no jacket cooling is provided. A
portion of the data from durability test is given in Fig.
7.
SE-1 grease has been developed to address twochallenges: easy running-in operation and durability
without jacket cooling.
Employing diurea as a thickener of excellent shear

2.4 Development of the new SE-1 grease
The conventional BNS type is lubricated with

Test conditions
Bearing type

10000hAttitude

20000hVerticalDiscontinued

Horizontal

Discontinued

BNS

BNS type
5S-2LA-BNS020LLB
・Grease: MP-1
・Running speed: 1,1000 min-1
・Preload: 0 N as mounted(definite position preloading)・Jacket cooling: Yes

Fig. 5 Durability test results of BNS type

Grease brand

General purpose grease
for main spindle bearing

Thickener
Base oil

MP-1

SE-1

Barium complex soap

Urea-based

Urea-based

Ester oil

Synthetic oil

Ester oilBase oil viscosity (40˚C mm2/S)

20

40.6

22

Operating temperature (˚C)
range (for reference)−60〜＋130−40〜＋150−50〜＋120

Fig. 6 Grease properties-31-

NTN TECHNICAL REVIEW No.74（2006）

0

10000h20000hBearing bore dia.: 100 mm
MP-1
5S-2LA-BNS020LLB
Ceramic balls
SE-1
Contact angle: 20˚11000 min-1 continuous (1.4 million dmn)
Constant pressure preloading: 3.2 kN
Spindle attitude: horizontal (driven by belt)
No jacket cooling

Bearing bore dia.: 50 mm
2LA-BNS010LLB
Steel balls
Contact angle: 20˚

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

Bearing bore dia.: 30 mm
5S-7006ADLLB
Ceramic balls
Contact angle: 25˚

MP-1
SE-1

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

MP-1
SE-1

Fig. 7 Durability test results (no jacket cooling)

Front-side bearing temperature
Rear-side bearing temperature
Base temperature
Running temperature

20000

Temperature ˚C

100

10000

80

0
Temporary stop

60-1000040-2000020-30000Running speed min-1

120

MP-1
0

-40000
0

1

2

3

4

5

6

7

8

9

10 11

Operating time h100

Temperature ˚C

30000

Front-side bearing temperature
Rear-side bearing temperature
Base temperature
Running temperature

20000

80

10000

60

0

40
20

Running speed min-1

120

General-purpose grease for main spindle bearing
0

0

1

2

3

4

5

6

7

8

9

10

Operating time h
30000

Front-side bearing temperature
Rear-side bearing temperature
Base temperature
Running temperature

100

20000

80

10000

60

0

40
20

SE-1
0
0

1

2

3

4

5

6

Operating time hFig. 8 Running in operation

-32-

7

8

9

10

Running speed min-1

120

Temperature ˚C<Test conditions>
5S-2LA-BNS010LLBDB
0-20000 min-1
Preload as mounted: 98 N
Jacket cooling: No

×104 Sealed High-speed Angular Contact Bearing “New BNS Type”
dmn170×

Test conditions
Bearing type

Attitude

φ100mmHorizontal 1

Bearing type

Attitude

φ30mm

VerticalStill running after 2922h hours

φ30mm

Horizontal

Still running after 2491 hours

10000hStill running after 9719 hours

Dia. 100 externally driven test rig
・Grease: SE-1
・Running speed: 13500 min-1
・Preload: 0 N as mounted(definite position preloading)・Jacket cooling: Yes

Test conditions
5000h

Dia. 30 built-in motor test rig
・Grease: SE-1
・Running speed: 40000 min-1
・Preload: 0 N as mounted(definite position preloading)・Jacket cooling: Yes

Fig. 9 Durability test results

high-speed range and requires the provision of jacket
cooling. Therefore, the author’s durability test rigs
incorporate jacket cooling. During testing, the bearing
temperatures have been stable.

stability, SE-1 is capable of lubrication in a wide speed
range, from very low speeds to higher speeds. Thebase oil used in SE-1 is an ester oil that excels in heat
resistance and oxidation stability, helping extend
grease life.
The viscosity of the base oil is 22 mm2/s (40˚C),which contributes to achieving easy running-in
operation (Fig. 8).
Currently, the author is performing high-speed
durability testing with the newly developed bearing
type that is configured in a back-to-back duplex
arrangement (DB arrangement), with each of thepaired bearings being prefilled with SE-1 (Fig. 9). The
running speed is 13,500 min-1 for the 100 mm bore
diameter bearing and 40,000 min-1 for the 30 mm bore
diameter bearing. The main spindle speed
corresponding with 170 x 104 dmn falls in a sufficiently

3. Conclusion
The process for developing the New BNS Typebearing has been described above. The mostimportant considerations in developing a high-speed
lubrication arrangement are how to inhibit heatincrease in the bearing and how to feed lubricant to
the rolling surface.
The author is now conducting a durability test at 170
x 104 dmn, and will remain committed to thedevelopment of a grease that is capable of muchgreater bearing speed.

Photo of author

Futoshi KOSUGIIndustrial Engineering Dept.
Industrial Sales headquarters-33-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Development of Aerostatic Bearing Spindle
for Precision Machine Tools

Teruyoshi HORIUCHI*
Kazuyuki AONO*
Hiroaki HIYOSHI*The main reasons for aerostatic bearing spindles inprecision machining are the increase in stiffness and reductionof thermal displacement of the work. To meet such demands,
NTN has developed a new aerostatic bearing spindle for
precision machining by reviewing the bearing design, coolingmechanism, and material selection. The results of theperformance evaluation test of the prototype air bearing
spindle are described here.

direction, a driving AC servomotor, and a rotary
encoder for detecting the running speed of thespindle, as illustrated in Fig. 1. The rotary shaft is
supported by the bearings without contacting them byusing the pressure of an air layer that is several
micrometers thick. The motor and the rotary encoder
are also non-contact types. Since the rotary shaft is
supported without contacting fixed members, thefrictional resistance on the rotary shaft is limited,allowing the rotary motion of the rotary shaft to becontrolled very accurately.
In addition to the fundamental structure describedabove, as with any other aerostatic bearing spindle for
precision machine tools, the spindle needs toincorporate a bearing arrangement for high spindle
rigidity, a high-torque motor that provides the torquenecessary for the intended machining operation, and avacuum chuck mechanism to hold workpieces inposition with vacuum force. Also, measures against
thermal deformation resulting from heat generation on
the motor and bearings will be necessary in the formof a cooling mechanism. Furthermore, since theenvironments where the aerostatic bearing spindles
for precision machine tools are situated differ from theenvironments in which aerostatic bearing spindles forinspecting devices are used, a sealing mechanism
must be provided for the former in order to prevent
ingress of chips and coolant mist.

1. Introduction
As the demand for personal digital equipment,
including mobile phones and DVD recorders, has
increased, there has been a growing need to improve
productivity the machining precision dies for asphericlenses and other precision components. Ultraprecision machine tools for machining very small
precision dies for these components often employ
precision aerostatic spindles that incorporatehydrostatic air bearings. To improve the productivity ofthese machine tools, their aerostatic bearing spindles
must be capable of both higher running speed andgreater precision.
NTN has long been providing high-precision
aerostatic bearings that are used on original recording
devices for optical disks, as well as on magnetic disk
inspection devices. By further advancing ouraerostatic bearing spindle technology, we havesuccessfully developed a high-speed, high-precision
aerostatic bearing product for precision machine tools.

2. Challenges for Aerostatic Bearing
Spindles for Machine Tools
Generally, an aerostatic bearing spindle iscomprised of two journal bearings that support therotary shaft in the radial direction, a pair of thrustbearings that support the rotary shaft in the axial**Product Engineering Department Precision Equipment Division-34-

Development of Aerostatic Bearing Spindle for Precision Machine Tools

Journal bearings

Thrust bearings

Rotary encoder

AC servo motor

Rotary shaft

Non-contact seal
for vacuum chuck

Thrust plate

Fig. 1 Structure of air spindle

journal bearings have been optimized in order toinhibit high order vibration that would otherwise occur
due to the interaction between the form errors on therotary shaft and the layout of the air feed holes, aswell as to improve the dynamic rigidity of the bearings.

3. Specifications of Prototype Spindle
The specifications (target values) for NTN’s
prototype aerostatic bearing spindle for machine tools
are summarized in Table 1.
Previously, the maximum running speed with thistype of spindle was approximately 10,000 r/min. In
contrast, the maximum targeted running speed with
NTN’s spindle is 20,000 r/min. To ensure goodmachining accuracy, it has been determined that itshould have an NRRO (non-repetitive run-out)
comparable to that of an aerostatic spindle forinspection devices and the axial displacement of thechuck face during steady running of the spindle should
be limited to 1 μm or smaller.

2) Realization of higher torqueTo attain both a higher torque on the driving section
and a smaller spindle outside diameter, two motors
are coupled in tandem, thereby the authors’ aerostatic
bearing spindle features a torque twice as highcompared to ordinary aerostatic bearing spindle
designs.
3) Control of chips and coolant
For the vacuum chuck mechanism that secures
workpieces, to prevent the coolant entering the chuckfrom reaching the bearing section, the suction holes
are situated further apart from the bearings compared
to conventional aerostatic bearing spindle
arrangements. To avoid ingress of chips and coolant
into the bearings, a non-contact air-seal has been
incorporated and a narrow gap is provided between
the rotary shaft and the fixed section (housing),
thereby compressed air is allowed to gush out throughthis gap to prevent chips and coolant from reaching
the bearings.

Table 1 Specifications
Max. running speed

20,000 r/min

Air feed pressure to bearing 0.49 MPa
Radial rigidity

45 N/μm

Axial rigidity

227 N/μm

NRRO

Radial

0.010μm or less

0.010μm or less
Axial
Displacement on chuck face 1μm or less during steady running

4. Structure of the Aerostatic Spindle
for Machine Tools

4) Control of heat buildup
Aerostatic bearings are known to generate lessheat. However, when any aerostatic bearing is run ata higher speed, the heat buildup resulting from theshearing of gaseous matter in the bearing clearance
cannot be ignored. In addition, the magnitude of heatgenerated from the motor arrangement of NTN’s
prototype spindle was expected to be greater than thatof similar conventional arrangements. To solve this1) Realization of greater rigidity and higher accuracy
The structure of the authors’ aerostatic bearing
spindle is illustrated in Fig. 2. A total of four journalbearings are placed in the front and rear sides of thethrust bearings to enhance the rigidity of the aerostatic
bearing spindle. Furthermore, seeking higher
accuracy, the layout for the air feed holes on the-35-

NTN TECHNICAL REVIEW No.74（2006）

Water drain outlet

Feed-water inlet

Waterway for cooling
Rotary shaft

Rotary table

Air seal
Workpiece

Chuck surface
Air seal
Air journal bearing Motor
Air thrust bearing

Rotary encoder

Air supply opening for air seal

Fig. 2 Structure of the air spindle which we developed

For evaluation of the control measure against
thermal deformation, the aerostatic bearing spindle
was started from a standstill in a room temperatureenvironment and allowed to run constantly at 20,000
r/min, and displacement in the position of theworkpiece was measured. This measurement result isgraphically plotted in Fig. 4. Immediately after the startof operation, the shaft developed an elongation ofproblem, the housing on the motor and bearing
sections is water-cooled and the associatedcomponents are composed of materials with low
thermal-expansion in order to inhibit their thermal
deformation and the displacement of the chuck face.
Furthermore, air feed holes are provided in the bore
sides of the motor stators for feeding compressed air
to cool the motors.

5. Spindle Evaluation Test

Radialrun-out

A dummy workpiece simulating an actual workpiece
was mounted to the prototype spindle, and then thespindle was subjected to an accuracy evaluation test.
Run-outs were measured at the locations shown in
Fig. 3 with a capacitance type non-contact
displacement gage. Jitter (variation in rotation
frequency) was determined by measuring variation in
the frequency of Z-phase signal (1 pulse/revolution) of
the rotary encoder with a pulse jitter counter. The
measurement result for non-repetitive run-out (NRRO)
is summarized in Table 2 and that for jitter (variation inrotation frequency) is given in Table 3. As can beunderstood from the data in these tables, NTN’s
aerostatic bearing spindle performed very well fromlow speed to the maximum speed.

Shaft
elongation
Axial run-out
Dummy workpiece

Fig. 3 A measurement position
Table 2 A measurement result of NRRO
Running speed
r/min
1000

NRRO，nmRadial3.75

NRRO，nm
Axial
3.75

5000

3.75

3.75

10000

3.75

3.75

15000

3.75

3.75

20000

3.75

6.25

Table 3 A measurement result of jitter (A dummy))
Jitter（nsec）

Running speed（r/min）

Accuracy
（%）

500

213.0

180.1

190.9

205.5

195.0

0.00018

1800

82.1

72.7

68.5

84.3

67.6

0.00025

5400

33.5

28.2

27.9

31.8

26.5

0.00031

10000

20.8

21.2

17.4

21.2

18.8

0.00035

15000

14.1

12.2

11.5

13.6

14.3

0.00036

20000

9.3

10.3

10.9

10.8

10.2

0.00036-36-

Development of Aerostatic Bearing Spindle for Precision Machine Tools

Elongation of shaft μm

4Motor cooling air: 20 L/min
Motor-bearing cooling water: 2.5 L/min (regulated to 23˚C)
20000 r/min: continual

3

1μm
2
1
0
0

10

20

30

40

50

60

-1
-2

Time elapsed min.

Fig. 4 The measurement of a work position)

approximately 2.5 μm (to the left in Fig. 3). After anelapse of 5 to 10 minutes, the displacement of theworkpiece was approximately 0.6 μm.

7. Conclusion
NTN has successfully developed an aerostatic
bearing spindle for machine tools that features ahigher-torque motor arrangement, coolant mist control
and thermal deformation control, and havedemonstrated its usefulness through an evaluation
test on the spindle proper and a machining test with amachine tool that incorporated NTN’s aerostatic
bearing spindle.
Aerostatic bearings are capable of super highprecision running at a nanometer order, and NTN has
been employing these types of bearings in inspecting
devices and semiconductor device manufacturingsystems that are used in clean environments. NTN
believes that their newly developed aerostatic bearing
spindle has helped expand the applicability ofaerostatic bearings to the field of machine tools, which
is a new field for NTN. NTN will make further efforts toachieve higher precision and higher speed to perfecttheir newly developed spindle product and satisfy therequirements for further improved machining accuracy
and productivity in preparation for commercialization.

6. Machining Test
For a test cut, NTN’s aerostatic bearing spindle wastemporarily installed on a super-high-precision turning
center, and the end face of a workpiece was surfacemachined, and then the resultant surface roughness
was evaluated. The authors proved that theiraerostatic bearing spindle can help achieve
nanometer-class surface roughness.

Table 4 Precision of cut side
Machining
conditions

Mean surface
Mean surface roughness
roughness (Ra) on one cross-section (Ra)1000 r/min −Ａ

3.127 nm1.675 nm

20000r/min −Ａ

2.672 nm0.819 nm

20000r/min −Ｂ

2.386 nm2.671 nmA : Feed rate 2.0 mm/min
B : Feed rate 4.0 mm/min

Photos of authors

Teruyoshi HORIUCHI

Kazuyuki AONO

Hiroaki HIYOSHI

Product Engineering Department
Precision Equipment Division

Product Engineering Department
Precision Equipment Division

Product Engineering Department
Precision Equipment Division-37-

NTN TECHNICAL REVIEW No.74（2006）[ Introduction of New Products for Machine Tools ]79U/70U Type Standard Angular Contact Ball Bearings
Standard series features high-speed performance, long bearing life,and high rigidity, which are among the best in the world

Features¡High-speed
Maximum dmn value with air-oil lubricationis 1.5 million
Maximum dmn value with grease lubrication
is 0.95 million¡Longer life: 10% increase in bearing life

Temperature increase on the outer ring ˚C〜3 times¡Maximum allowable axial load: 2〜
greater

ULTAGE (phenol resin cage)ULTAGE (newly designed polyamide resin cage)
Previous bearing (polyamide resin cage)
50
40
30

＊

Without coolingarrangement on
the jacket

Structure

20

0

0

2000

4000

6000

8000

New design for molded resin cage

With coolingarrangement on
the jacket

＊

10

10000

12000 14000

Bearing speed min−1
50

0

100

150

dmn value ×104

＜Tapered bore form＞
Air-oil lubrication: Helps to circulate and redistributelubricating oil
Grease lubrication: Helps with prefilling of grease【Test conditions】
Bearing
description【ULTAGE】7020UCDB【Previous bearing】 7020CDB
φ100mm×φ150mm×24mm×two rows
Contact angle15˚［］Fig. 2 (a) New resin cage (section of the ball pocket)

Preload as mounted 150 N (definite position preloading system)
Lubrication system Air-oil lubrication
Air supply rate
40 NL/min
Oil supply rate
0.03 mL/5 min/shot
Oil grade
ISO VG32
Cooling arrangement
Availableon the jacket＜Relieving on four cornersof each ball pocket＞
Air-oil lubrication:
promotion of dischargeof lubricating oil
Grease lubrication:
improved grease retention

Fig. 1 High-speed test (air-oil lubrication)

Applications¡Machine tool main spindles, precision machinery, etc.

Fig. 2 (b) New resin cage (appearance)-38-

Introduction of New Products for Machine Tools

HTA U Type Axial Load Carrying Angular Contact Ball Bearings
Designed to have the highest speed, load capacity, and rigidity
among thrust carrying angular contact ball bearings worldwide

Features

Temperature increase on the outer ring ˚C¡High-speed
Maximum dmn value of 1.25 million with
air-oil lubrication(25% improvement over previous NTN design)
Maximum dmn value of 1 million withgrease lubrication(60% improvement over previous NTN design)

40
HTA020UA Without cooling arrangement

35

HTA020UA With cooling arrangement
HTA020A Without cooling arrangement30

HTA020A With cooling arrangement25
20Structure

1510¡Adoption of internal design that inhibits
temperature increases at high-speeds¡Improved design of inner and outer rings forbetter oil discharge with oil-lubrication
systems¡Improved design of contact areas between
polyamide resin molded cage and rolling
elements for improved lubrication efficiency
for both grease and air-oil lubrication

5
0
0

2500

5000

7500

10000

Bearing speed min−1
0

25

50

75

100

125

dmn value ×104
【Test conditions】

Air supply rate

HTA020UADBHTA020ADB(φ100×φ150×22.5 mm×two rows)〜10000min-1
880N
Air-oil lubrication
0.03 mL/shot
5 min oil shot intervals
40NL/minCooling arrangementon the jacket

Available

Bearing [α:30˚]Bearing speed
Preload as mounted
Lubrication system
Oil supply rate

Lubrication
oil flow

Fig. 1 Results of high-speed test (air-oil lubrication)(α:30˚)Applications
Greater clearance¡Machine tool main spindles, etc.

Fig. 2 Flow of lubricating oil-39-

NTN TECHNICAL REVIEW No.74（2006）

Super High-Speed, Long-Life Double Row Cylindrical Roller Bearing
A precision double row cylindrical roller bearing that boasts the highest speed in the world,
while maintaining high load carrying capacity and high rigidity compared to previous design

Features

Temperature increase on the outer ring ˚C¡High-speed
Maximum dmn value of 1.75 million with
air-oil lubrication(50% improvement over previous NTN design)
Maximum dmn value of 1 million withgrease lubrication(20% improvement over previous NTN design)30

New design: HSRT6 type
(PEEK cage)
Previous design:
high-strength brass cage

25
20
15

Structure

10¡Optimized internal structure design withmuch improved cage

5
0
4,000

6,000

8,000

10,00012,00014,00016,000175

200

Bearing speed min−1
50

75

100

125

dmn value

150

×104

【Test conditions】[New design: HSRT6 type (PEEK cage)][Previous design: high-strength brass cage]¡Bearing size
: NN3020K（φ100×φ150×37 mm）¡Bearing speed : 〜15,000min-1（dmn=1.9 million）¡As-mounted clearance: 0μm¡Oil supply rate : 0.02mL/20min（VG32）¡Air supply rate : 30NL/min¡Cooling arrangement on the jacket: Available

Previous design

New design

Fig. 2 Conventional design and new design (HSRT6 Type)

Lubrication pockets

Fig. 1 Temperature rise test results (air-oil lubrication)

Applications¡Machine tool main spindles, etc.

Fig. 3 Drawing of the retainer-40-Introduction of New Products for Machine Tools

Next-Generation Deep-Groove Ball Bearing for High-Speed Servo Motors
New design of high speed deep-groove ball bearings
with prefilled grease

Structure

Sealed deep-groove ball bearings¡Optimized design that ensures reliablelubrication
Newly designed resin cage for high-speedwith long-life grease

62XX and 63XX series

Capable of dmn 95 x 104
Newly designed resin cage forhigh-speed with long-life grease¡Maintenance-free
No grease refilling needed
No cleaning needed
More than three times as durable

High-speed temperature increase test

High-speed and high-temperature endurance test
6000

Newly designed cage
Cap-type cage

80

5000

Test time hTemperature increaseon the outer ring ˚C

100

60
40
20

4000
3000
2000
1000

0
3000

6000

9000

Bearing speed min-1

12000

［

15000
dn=60 million
dmn=95 million

0］

Sheet steel wave-typecage with MP-1 grease

Newly designed cagewith ME-1 grease

Bearing
description

6308 dual-side non-contact seal
（φ40mm×φ90mm×23mm）

Bearing
description

GreaseME-1 grease common to both cage designs

Bearing speed Blue: n =11,111min-1，red: n =13,333min-1

6209 dual-side non-contact seal
（φ45mm×φ85mm×19mm）

130˚C (on outer circumference of the bearing)
Bearing
temperature-41-

NTN TECHNICAL REVIEW No.74（2006）“Ecologically Packaged Precision Bearing”
that is Easy on People and the Environment
This bearing can be mounted immediately after unpacking with no needto remove the rust preventative oil

Conventional
rust preventative oil

Features

Bearing oil usedon new product

Bearings that can use this oil¡Precision angular contact ball bearings with 50 to 100
mm bore diameters (oil-lubricated type)"Ecological bearing that is easy on people
and the environment""¡No rinsing is needed to remove the rustpreventative oil, therefore reducing labortime.
¡No need to manage the cleanliness of thewashing device and cleaning oil¡No need to manage waste cleaning oil

Applications¡Machine tool main spindles, etc.

Structure¡The bearing surface is coated with spindle
oil that contains an additive featuring
excellent rust preventative characteristics¡The bearings are shipped specially packed
in a volatile corrosion inhibitor film
¡Guaranteed rustproofing for one year

-42-

Introduction of New Products for Machine Tools

Super High-Speed AT Spindle
Hydrostatic air bearing spindle that can be mounted toa machining center main spindle

Features¡Attachment type can be mounted to a machining center main spindle (no special machine tool is needed)¡Hydrostatic air bearing helps achieve a super high speed of 150,000 rpm and high-accuracy running¡High-accuracy, high-rigidity chucking is possible with "a chuck that is shrink-fitted to the shaft"¡Low-vibration, quiet, and long-life

Specifications
Maximum speed

150,000 min-1
Motor
Axial：40 N

Rating: 0.6 kW
3-phase induction motor: 200 VAC
Water-cooled: 1-2 L/min

Load capacity
Radial：20 N

Applicable tool diameter

1 mm or lessHeat-shrink method (dia. 6 mm)

Axial：1.8 N /μm

Tool grip system

Radial：1.0 N/ μｍ

Bearing air supply pressure

0.49 MPa

5 kg (not including shank)

Bearing air consumption

90 L/min (A.N.R.)

Static rigidity

Spindle mass

*The data for radial load capacity and rigidity were taken at a point 14 mm away from the spindle end.

-43-

NTN TECHNICAL REVIEW No.74（2006）[ Technical Paper ]Atmospheric Control Method for JIS-SUJ2
Carbonitriding Processes

Chikara OHKI*

NTN has been investigating the mechanism of nitrogen
penetration into JIS-SUJ2 bearing steel during carbonitriding
processes. In the course of carbonitriding experiments undervarious conditions and Electron Probe Micro Analysis (EPMA)
analysis, the influences of carbon activity and base gas
components on nitrogen penetration behavior were examined. As
a result, the nitrogen penetration content of steel increased withan increase in undecomposed NH3 partial pressure and with andecrease in carbon activity, and H2 partial pressure.
Assuming that the penetration mechanism of nitrogen is similar
to that of carbon, a calculation method was developed in order toestimate nitrogen concentration distribution profiles. It was
confirmed that the nitrogen concentration distribution profiles
predicted by NTN's method agreed well with the experimentresults under various conditions. Hence, a development of a new
atmosphere control method for JIS-SUJ2 carbonitriding
processes was achieved.

1. Introduction

2. Test Methods

Carbonitriding processes are heat treatment
techniques that are often used to improve the fatigue
strength of steel materials. There are many
applications for using carbonitriding techniques on
bearing steels. In particular, it has been verified thatcarbonitriding bearing steel materials contribute tolonger bearing life under contaminated lubrication
conditions. 1)
The carbon penetration mechanism that takes place
during carburizing is understood. In contrast, although
there has been active research on the nitrogen
penetration mechanism in carbonitriding processes,2)
there has been little research on nitrogen penetrationin carbonitriding processes for high carbon steels(including bearing steel). To address this, NTN has
systematically performed a series of carbonitriding
experiments for JIS-SUJ2 bearing steel in an attempt
to clarify the nitrogen penetration mechanism.3) 4)

2.1 Experimental furnace and furnace
atmosphere measuring method
For the series of experiments performed, a batchtype heat treatment furnace (capacity 120 L) wasused. During carbonitriding processes, theatmosphere was analyzed by the equipment
summarized in Table 1. The furnace itself isschematically illustrated in Fig. 1 and the base gas
feeder is shown in Fig. 2.

2.2 Material tested and method for measuringnitrogen concentration
The material used for the test was a JIS-SUJ2 highcarbon chromium bearing steel. The chemicalcomponents of the material are summarized in Table 2.
The test pieces were ring-shaped with dimensions of 30
ID x 38 OD x 10 T (mm).
Nitrogen concentration distributions of the test

*Elemental Technological R&D Center-44-

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

Table 1 Equipment used for atmospheric measurement
Equipment

Measurement

Measurement technique

CO/CO2 concentration

Non-dispersion IR absorption

Mirror-surface cooling dew-point recorder

H2O concentration

Optical dew point

Zirconia sensor

O2 concentration

Zirconia oxygen partial pressure analyzer

Thermal conductivity gas analyzer

H2 concentration

Thermal conductivity

Undecomposed NH3 concentration

Gas chromatograph (carrier: helium)

IR gas concentration analyzer

Thermal conductivity detector

Heating
chamber

Cooling blast air
chamber

VestibuleA
Heating
chamberB

Oilquenchingtank

CFig. 1 Heat treatment furnace

Fig. 2 Base gas feeder(A: Supply from pure gas tank, B: mass flow controller,
C: gas mixer)

pieses were determined by electron beam analysis (2μm spot diameter, 2μm intervals between measuring
points) with an Electron Probe Micro Analyzer(EPMA). The test pieces were placed in the center ofthe experimental furnace for carbonitriding prior to thismeasurement. A sample piece was cut, as illustrated
in Fig. 3, from each test piece that underwent
carbonitriding (after quenching). Then, the nitrogenconcentration distribution on each sample piece wasmeasured on the cut surface, beginning at the
midpoint at the outer diameter and proceeding toward
the bore.
To determine the amount of nitrogen penetration ofeach test piece, the nitrogen concentration distribution
obtained from analysis were integrated in the direction
to where the nitrogen concentration approached zero(Fig. 4). The total amount of nitrogen penetration wasconverted into a weight, assuming that the specificgravity of the original material was 7.85 x 10-3 g/mm3.

Table 2 Chemical composition of JIS-SUJ2 used (wt%)
C

Si

Mn

0.99

0.26

0.44

P

S

0.012 0.006

Ni

Cr

Mo

Cu

0.08

1.46

0.03

0.15

10mm

Outsidediameter

φ30

φ38

5mm

Area analyzed by EPMA

Insidediameter

Cross-sectionof test piece

Fig. 3 Test specimen dimensions and scanning position
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
-0.1

2.3 Definition of carbon activity
One very important factor that can influence theatmosphere for carbonitriding is the carbon activity,
which determines carbon penetration. The author
employed the Boudouard reaction for calculating
carbon activity. The equilibrium reaction is defined byexpressions (1), (2) and (3) below:

0.78wt%

0

0.2

0.4

0.6

Depth from surface mm
Amount of nitrogen penetration（g/mm2）=…＋7.85×10-3（g/mm3）×0.002（mm）×0.78（wt%）÷100＋…

Fig. 4 Definition of nitrogen penetration content-45-

NTN TECHNICAL REVIEW No.74（2006）

temperature was maintained at 850˚C, the process
duration was set at 9,000 seconds, and the furnacepressure was kept at 1.03 atm. At the same time, thetotal surface area of the respective test pieces was thesame.
The flow rates of each component in the base gas
and the partial pressure of undecomposed NH3 arelisted in Table 3. The H2 flow rate was set to a range
from 1.15 to 6.9 L/min, N2 flow rate from 0 to 9.2 L/min
and the CO flow rate from 1.15 to 4.6 L/min. The flow
rates of these gases were adjusted so that the total
flow rate of the mixture of gases was 11.5 L/min ineach set of conditions. This adjustment was achieved
with the base gas feeder shown in Fig. 2. Two levelsof partial pressure of undecomposed NH3 were used:
0.0010 atm and 0.0021 atm.

〈C〉
＋CO2⇔2CO………Boudouard reaction
ac＝

PCO

2

K1・PCO2（

K1＝exp…………………………（1）0−ΔGT………………………（2）
RT

）

0

ΔGT＝172500−175.6・T ……………（3）where ac = carbon activity; K1 = equilibrium
constant; R = gas constant (8.31441 J/(mol-K)); T =absolute temperature (˚K); ΔG** = standard freeenergy of formation;5) PCO = CO partial pressure and
PCO2 = CO2 partial pressure (atm).
Because of the definition of carbon activity, ac＞1 is
not possible. However, as shown by expression (1), a
calculated value of ac can be greater than 1 in certain
situations, and a carbonitriding process can takeplace.6) To investigate the effect of such an
atmosphere, expression (1) was redefined as (1’), andthis expression was used as a parameter for theexperiment.

Table 3 Conditions for experiments to investigate theinfluence of base gas components
Undecomposed
N2 flow rate CO flow rate
NH3 volumetric H2 flow rate
(L/min)1(L/min)1(L/min)1
partial pressure

2

Total
(L/min)10.0010

6.9

3.45

1.15

11.5

0.0010

6.9

2.30

2.3

11.5

0.0021

6.9

2.30

2.3

11.5

0.0010

6.9

0

4.6

11.5

0.0021

6.9

0

4.6

11.5

0.0010

1.15

9.2

1.15

11.5

0.0021

1.15

9.2

1.15

11.5

2.4 Experimental Parameters2.4.1 Effects of partial pressure ofundecomposed NH3 and carbon activity
on carbonitriding processes

0.0010

1.15

8.05

2.3

11.5

0.0021

1.15

8.05

2.3

11.5

0.0010

1.15

5.75

4.6

11.5

0.0021

1.15

5.75

4.6

11.5

Within every set of parameters in these experiments,
temperature was maintained at 850˚C and furnacepressure was held at 1.03 atm (1 atm = 1.013 x 105
Pa). At the same time, the total surface area of thetest pieces was the same. Four types of test
parameters were varied: the carbon activity ac*,carbonitriding process duration, base gas flow rates
and partial pressure of undecomposed NH3. The ac*
was held at one of six levels ranging between 0.78
and 1.08 and the carbonitriding process duration
varied from 1,800 to 18,000 seconds. The base gas
flow rate ranged from 6 to 20 L/min and theundecomposed NH3 varied from 0.36 to 3.4 x 10-3 atm.
Incidentally, the base gas feeder shown in Fig. 2 was
not used for these experiments. Instead, the base gas
used was an ordinary endothermic modified gas
derived from propane.

0.0010

3.45

5.75

2.3

11.5

0.0021

3.45

5.75

2.3

11.5

0.0010

3.45

6.9

1.15

11.5

0.0021

3.45

6.9

1.15

11.5

0.0010

3.45

3.45

4.6

11.5

0.0021

3.45

3.45

4.6

11.5

ac*＝

PCO

K1・PCO2…………………………（1'）

where, when ac*≦1, ac* = ac (carbon activity), andwhen ac*＞1, ac* is a variant expressed by expression(1’).

1) Converted to values under ordinary temperature and atmospheric pressure

2.4.2 Effects of base gas component partialpressures on carbonitriding processes
For this series of experiments, the heating-46-

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

Figs. 5 through 7 plot the results of theseexperiments.
It was learned that there is strong relationshipbetween the partial pressure of undecomposed NH3
and nitrogen penetration amounts, regardless of thebase gas flow rate.
The relationship between carbonitriding process
duration, partial pressure of undecomposed NH3 andnitrogen penetration amounts is graphically illustrated
in Fig. 8. The profile shown in Fig. 8 is anapproximation of the actual measurement values, andthe black dots represent actual measurement values.
It was learned that with a partial pressure ofundecomposed NH3 of 0.002 atm or higher, thenitrogen penetration amount is nearly maximized andthe maximum amount of nitrogen penetration
increases as the process duration is lengthened.

3. Results of Experiments
The actual measurements of nitrogen concentration
are non-continuous, and their reliability is low due tothe effects of deposits and measurement errors. Thus,
it would be inappropriate to use the measurements topredict a general trend. Therefore, the results of theexperiments were adjusted by the amount of nitrogen
penetration, allowing the effects of deposits andmeasurement errors to be averaged and errors in thedata to be minimized.

3.1 Effects of NH3 on amounts of nitrogen
penetration
To clarify the effect of base gas flow rates on therelationship between NH3 addition flow rates andnitrogen penetration amounts, nitrogen penetration
amounts were measured in a series of experiments
performed with ac* = 1.03 and a carbonitriding
process time of 9,000 seconds.

10

Amount of nitrogen penetration(10-6g/mm2 )Amount of nitrogen penetration(10-6g/mm2 )

10
9
8
7
6
5
4

Base gas flow rate: 11.5 L/min
Base gas flow rate: 11.5 L/min (retest)
Base gas flow rate: 6 L/min
Base gas flow rate: 8.5 L/min
Base gas flow rate: 20 L/min

3
2
1
0
0

0.2

0.4

0.6

0.8

1.0

9
8
7
6
5
4
Base gas flow rate: 11.5 L/min
Base gas flow rate: 11.5 L/min (retest)
Base gas flow rate: 6 L/min
Base gas flow rate: 8.5 L/min
Base gas flow rate: 20 L/min

3
2
1
0

1.2

0

5

NH3 addition flow rate (L/min)

10

15

20

NH3 addition ratio (%)

Fig. 5 Relationship between the flow rate of
NH3 and nitrogen penetration content

Fig. 6 Relationship between the addition ratio of
NH3 and nitrogen penetration content

Amount of nitrogen penetration(10-6g/mm2 )Amount of nitrogen penetration(10-6g/mm2 )

10
9
8
7
6
5
4

Base gas flow rate: 11.5 L/min
Base gas flow rate: 11.5 L/min (retest)
Base gas flow rate: 6 L/min
Base gas flow rate: 8.5 L/min
Base gas flow rate: 20 L/min

3
2
1
0
0

0.001

0.002

0.003

Partial pressureof undecomposed NH3
(atm)

0.004

Partial pressure of undecomposed NH3 (atm)Fig. 7 Relationship between partial pressureof undecomposed NH3 and nitrogen penetration content

Process duration (s)Fig. 8 Relationship among duration times andundecomposed NH3, and nitrogen penetration content-47-

NTN TECHNICAL REVIEW No.74（2006）

3.2 Effects of carbon activities on nitrogen
penetration amounts［ASN］＝−1.0549・ac*＋1.8771……………（4）ac*＝0.78

ac*＝0.98

ac*＝0.89
ac*＝0.94

ac*＝1.02
ac*＝1.08

where, ASN is the saturation value (wt%) of surface
nitrogen concentration.

3.3 Effects of base gas component partialpressures on nitrogen penetration amounts
Multiple regression analysis was performed bytaking the nitrogen penetration amounts obtained fromexperiments summarized in Table 3 as target
variables, and using partial pressures of CO, N2,
undecomposed NH3 and H2 as variables. As a result, it
was learned that CO and N2 partial pressures do notsignificantly affect nitrogen penetration amounts andthat nitrogen penetration amounts are governed byboth NH3 partial pressures and H2 partial pressures(1% significance, degree of freedom dual adjustment
contribution ratio of 0.953).
The relationship between H2 partial pressures,
partial pressures of undecomposed NH3 and nitrogen
penetration amounts are illustrated in Fig. 11. The
spaces between the plotted results of the experiments
are approximated with a curved plane to see a visual
overview of the results of the experiments. From Fig.
11, it is apparent that the nitrogen penetration amounttends to increase as the partial pressure ofundecomposed NH3 increases and the H2 partialpressure decreases.

12
10
8
6

Amount of nitrogen penetration(10-6g/mm2 )Nitrogen penetration amount (10-6g/mm2 )To clarify the effects of carbon activities on nitrogen
penetration amounts, a series of experiments was
performed with various ac* values. In theseexperiments, the process duration was set to 9,000
seconds and the base gas flow rate was maintained at
11.5 L/min. The results are graphically plotted in Fig.
9. It is apparent that at the same partial pressure ofundecomposed NH3, the smaller ac* is, the greater thenitrogen penetration amount becomes.
In the scope of this series of experiments, when thepartial pressure of undecomposed NH3 was 0.002 atm
or greater, the surface nitrogen concentration
exhibited a saturation value at every ac* value. The
relationship between carbon activity and saturation
values of the surface nitrogen concentrations is shownin Fig. 10. When ac* falls in a range between 0.89 and
1.08, the saturation values of surface nitrogenconcentration can be linearly approximated withexpression (4).

4
2
0
0

0.001

0.002

0.003

0.004

Partial pressure of undecomposed NH3 (atm)Surface nitrogen concentration(wt%)

Fig. 9 Relationship among undecomposed NH3 andnitrogen penetration content, and carbon activity ac*

Partial pressureof undecomposed NH3
(atm)

1.1
1

Fig. 11 Relationship between partial pressures ofundecomposed NH3 and H2, and nitrogen penetration content

0.9
0.8
0.7
0.6
0.7

H2 partial pressure

0.8

0.9

1

1.1

1.2

Carbon activity (ac*)

Fig. 10 Relationship between carbon activity ac* and thesurface nitrogen concentration-48-

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

If the manner of the nitrogen penetration
mechanism is assumed to be identical to that of the
carbon penetration mechanism, then, by referring to
expression (7), the nitrogen penetration velocity canbe described by expression (8) below.

4. Nitrogen Penetration Mechanism,and Prediction of Nitrogen
Concentration Distribution
From these findings, it is apparent that regarding
nitrogen penetration during carbonitriding of JIS-SUJ2
high-carbon chromium bearing steel, the following
three factors are critical: 1) partial pressure ofundecomposed NH3 in the furnace, 2) ac* in theatmosphere (carbon activity), and 3) H2 partialpressure in the furnace. Based on these findings, thenitrogen penetration mechanism and the method forpredicting nitrogen concentration are discussed
below.

qN＝βN（NG−Ns）………………………（8）where, qN = nitrogen penetration velocity (g/mm2s);βN = nitrogen transfer coefficient (g/mm2s); NG = final
nitrogen concentration (wt%); NS = nitrogenconcentration on the material’s outermost surface(wt%).
Then, expression (8) is rewritten as expression (8’)
below by replacing NG in expression (8) with aN*-ASN.
qN＝βN（aN*・ASN−Ns） ………………（8'）

4.1 Hypothesis of the nitrogen penetration
mechanism

where, ASN is the saturation value of nitrogenconcentration (wt%).
When expression (6) is substituted in expression(8’), then the nitrogen penetration velocity is definedby expression (9).

Let us assume that the nitrogen penetration incarbonitriding of JIS-SUJ2 proceeds according to theequilibrium reaction defined in expression (5).
Expression (6) represents the equilibrium constant KX
for expression (5).

（

qN＝βN

〈N〉
＋3/2H2⇔NH3 ………………………（5）
aN*＝

PNH3
KX・PH2

3
2

PNH3
3

KX・PH2 2・ASN−Ns）………（9）In expression (9), the variables that can beinvestigated by measuring the atmosphere forcarbonitriding processes are NH3 partial pressure andH2 partial pressure. Since NS is determined based on
the actual measurements of nitrogen concentration
distribution, there are three unknown quantities in
expression (9): βN, KX, and ASN. ( NS can also be
determined through calculation involving βN, KX, ASN
and the diffusion coefficient D.)…………………………（6）where, aN* is “an unknown quantity equivalent to
the nitrogen activity” in equilibrium between
atmospheric gas and steel, and the unit of partialpressure is atm. For convenience, the equilibrium
constant KX in expression (6) is allowed to remain an
unknown quantity.
In a carbonitriding process, even if the carbon
potential, carburizing duration and processtemperature remain unchanged, the carburized depth
can vary depending on variation in the atmosphericcomponents according to Neumann et al.7) They
expressed the cause of this phenomenon with acarbon penetration velocity formula (expression (7))
that uses a carbon transfer coefficient β.

4.2 Hypothesis about nitrogen concentration
distribution in steel
Suppose that the nitrogen concentration distribution
profile is compatible with the Gaussian error function
as in the case of the carbon concentration profile, then
the nitrogen concentration distribution profile can be
expressed by the expression (10).

q＝β（CG−Cs）……………………………（7）｛ （N＝Ns・ 1−erf

where, q = carbon penetration velocity (g/mm2s); β= carbon transfer coefficient (g/mm2s); CG = final
carbon concentration (wt%); CS =carbon concentrationon the material’s outermost surface (wt%).
Being equivalent to the carbon potential, CG can be
expressed as a product of the carbon activity ac and acarbon solubility limit concentration AS. Therefore,
expression (7) can be rewritten as expression (7’)
below.

X
2√D・tt）｝………（10）where, N = nitrogen concentration at a position X(wt%); NS = nitrogen concentration on the material’soutermost surface (wt%); X = distance from thesurface of the material being processed (mm); D =diffusion coefficient (mm2/s); and tt = process duration(s). The unknown quantity in expression (10) is the
diffusion coefficient D because the diffusion coefficientis dependent on nitrogen concentration. Next, let us
assume that the diffusion coefficient is approximatedq＝β（ac・As−Cs）………………………（7’）-49-NTN TECHNICAL REVIEW No.74（2006）

concentration on a substrate. However, it is
impossible to accurately determine the nitrogenconcentration on the substrate. Therefore, the surface
nitrogen concentration is considered to be the “meannitrogen concentration of the deposited substancesand substrate.”
From the results of the experiment illustrated in Fig.
10, in the range of 0.9＜ac*＜1.1, the ac* in theatmosphere is linearly proportional to the saturation
values of surface nitrogen concentration. The
saturation value of nitrogen concentration ASN isdetermined based on this fact.

in the form of expression (11).
D＝Aexp（B・N）………………………………（11）where, A and B are unknown quantities and N isnitrogen concentration (wt%).
The value obtained by multiplying the specificgravity of the material by a value calculated byintegrating expression (10) in the depth direction is thenitrogen penetration amount in the process duration tt.
At the same time, the value obtained by integrating
expression (9) with the process duration (s) is thenitrogen penetration amount. Therefore, expression(12) can be derived from expressions (9) and (10).
tt

δ

δ

4.3.3 Calculation of KX, A and BX・ 1−erf（∫q dt＝Q＝ρ∫Ndx＝ρ∫N｛｝dx …（12）2√D・t ）The unknown quantity KX can be calculated bysubstituting the nitrogen transfer coefficient βN , thesaturation value of nitrogen concentration ac* and thenitrogen concentration NS on the outermost surface in
expression (9). They are determined from actual
measurements of the nitrogen concentration
distribution.
The diffusion coefficient was calculated on areverse analysis basis (trial and error) by altering
unknown quantities A and B in expression (11) tosolve the diffusion formula through numerical analysis.
The diffusion coefficient should coincide with theactual measurements of nitrogen concentration
distribution obtained through EPMA analysis.

s

N

0

0

0

t

where, Q = nitrogen penetration amount (g/mm2) atprocess duration tt (s); ρ= specific gravity of iron(7.85 x 10-3 g/mm3);δ= minimum depth where thenitrogen concentration is virtually zero (mm).
Accordingly, if β, KX, ASN and all the unknown
quantities for A and B are determined from the resultsof a series of experiments, then it is possible to predict
the nitrogen concentration distribution based on theassumptions described above.

4.3 Calculation of unknown quantities based on
the results of the experiments
4.3.1 Calculation of β

4.3.4 Summary of unknown quantity
calculations

Unlike the multistep carbon penetration, the reaction
formula modeling nitrogen penetration into a steel
material is believed to be simply expression (5).
Judging from the results of a series of experiments,
nitrogen penetration amounts are not influenced by
CO partial pressure and are slightly affected by N2
partial pressure. Therefore, it was assumed that thenitrogen transfer coefficient βN would remainunchanged.
When the partial pressure of undecomposed NH3,
partial pressure of H2 and ac* remain unchanged,
PNH3
3

KX・PH2 2

The unknown quantities obtained from the results of
the author’s experiments are listed in Table 4. Whenthese unknown quantities are substituted in
expressions (9) and (12), expression (13) is obtained.
By using expression (13), prediction of nitrogenconcentration distribution is possible. For example,
when a small time difference Dt is fixed, and because
the surface nitrogen concentration NS at tt = 0S is 0,

・ASN in expression (9) takes a constant

Table 4 Calculated unknown quantities
Type of unknown quantity

value. Therefore, from the actual measurements ofnitrogen concentration distribution, the nitrogen
penetration velocity qN (g/mm2s) and surface nitrogenconcentration NS (wt%) can be determined, thereby
βN can be calculated.

1.7×10-7

Nitrogen transfer coefficient:βN

Saturation value of nitrogen concentration: ASN（1.8771−1.0549・ac*）/100
Equilibrium constant: KX
Diffusion coefficient: D

4.3.2 Calculation of ASN

tt

PNH3

・
∫1.7×10 （0.006・P-7

In expression (8’), which describes nitrogen
penetration velocity, the saturation value ASN ofnitrogen concentration should be defined as the

0

∞

∫｛

（√

ρ Ns・1−erf
0

-50-

3
H2 2

2

0.006
A

6.5×10-7B

149（1.8771−1.0549・ac*）・−Ns dt＝100）

X

6.5×10-7exp（149・N ）・t）｝

dx …（13）tAtmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

The actual measurements and predicted values fornitrogen concentration distribution are graphicallyplotted in Fig. 12. NTN has learned that the predicted
nitrogen concentration distribution values match actual
measurements well with various partial pressures ofundecomposed NH3, carbon activity ac* values, H2partial pressure values and process duration values.
Thus, it is believed that the assumed values anddetermined unknown quantities related to nitrogen
penetration in this paper are valid.

expression (13) may be used in several calculations todetermine the surface nitrogen concentration andnitrogen concentration distribution.
Note, however, that prediction of nitrogenconcentration distribution with expression (13) ispossible only until the time that the surface nitrogenconcentration on the steel material being processedreaches saturation. In reality, once having reachedsaturation, surface nitrogen concentration will increase
little, if any.
Therefore, after the nitrogen concentration on theoutermost surface of the steel has reached saturation,
it is necessary to maintain the surface nitrogenconcentration at a constant level. Then, solve thediffusion formula through numerical calculations inorder to predict the nitrogen concentration distribution.

4.4 Verification by comparison of predictedvalues of nitrogen concentration distribution
and values obtained through experiments

Table 5 Experimental conditions

Test condition 1)
Actual measurements
Predicted value

0

0.2

0.4

H2

0.0016

0.354

1.03

9,000

2）

0.0020

0.622

0.97

9,000

3）

0.0016

0.356

0.94

9,000

4）

0.0010

0.332

0.88

4,500

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

(wt%)
Nitrogen concentration

Nitrogen concentration (wt%)

Predicted value

0.4

Test condition 2)
Actual measurements
Predicted value

0.2

0.4

0.6

Depth from surface (mm)Actual measurements

0.2

Process
duration(s)

Undecomposed NH300.6

Test condition 3)

0

ac*

1）

Depth from surface (mm)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

Gas component partialpressure (atm)1)

1) Converted to values under ordinary temperature and atmosphericpressure

(wt%)

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0

Test
conditions

Nitrogen concentration

Nitrogen concentration (wt%)

By comparing the actual measurements fromexperiments with predicted nitrogen concentration
distribution values, NTN has proved the validity of thenitrogen penetration mechanism and nitrogenconcentration distribution prediction method describedabove. For this verification, the results of experimentswith the test conditions summarized in Table 5 wereadopted.

0.6

Depth from surface (mm)

1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0

Test condition 4)
Actual measurements
Predicted value

0.2

0.4

Depth from surface (mm)Fig. 12 Nitrogen concentration distribution profile of experimental numbers 1)〜4)

-51-

0.6

NTN TECHNICAL REVIEW No.74（2006）

References

5. Conclusion

1) N. Tsushima: SAE Technical Paper Series (1986)
860725 (in Japanese)
2) S. Narita and K. Yokose: “Effects of the nitriding potential
caused by surface nitrogen concentration on various
types of carbonitrided case hardening steel,” Japan
Society for Heat Treatment 61st Lecture Series
Preliminary draft (2005) 5 (in Japanese)
3) C. Ohki, M. Kawakita: “Effects of carbon activity from thecarbonitriding process on high-carbon steel,” Japan
Society for Heat Treatment 62nd Lecture Series
Preliminary draft (2006) 5 (in Japanese)
4) C. Ohki, M. Kawakita: “Effects of base gas partialpressure from the carbonitriding process on high-carbon
steel,” Japan Society for Heat Treatment 62nd Lecture
Series Preliminary draft (2006) 17 (in Japanese)5) Editorial Committee of the Society of Chemical Engineers
Japan: Chemical Engineering Handbook, Maruzen (1950)
64 (in Japanese)6) R.Collin, S.Gunnarson, and D.Thulin: "Influence ofReaction Rate on Gas Carburizing of Steel in a COH2-CO2-H2O-CH4-N2 Atmosphere, "Journal of The Iron
and Steel Institute, (1972), 777
7) F. Neumann and U.Wyss: "Thermodynamische
Grundlagen zum indirekten Messen des C-Pegels, "H.T.M., vol.49, No.4, (1994) 255 (in German)

NTN has performed a systematic series ofexperiments for carbonitriding with JIS-SUJ2 highcarbon chromium bearing steel in an attempt to clarify
the nitrogen penetration mechanism associated withthis material. Also, assuming that the nitrogen
penetration mechanism is similar to the carbon
penetration mechanism, NTN has established amethod for predicting nitrogen concentration
distribution based on a series of experiments. The
findings are summarized as follows:
1) The nitrogen penetration amount increases
directly with an increases in the partial pressureof undecomposed NH3.
2) The nitrogen penetration amount increases
directly with decreases in both carbon activity
ac* and H2 partial pressure.
3) NTN has established a method for predicting
the nitrogen concentration distribution of JISSUJ2 material. This method reflectsconsiderations for the partial pressure ofundecomposed NH3, the carbon activity ac*, H2
partial pressure and the diffusion coefficientthat is dependent on nitrogen concentration.
4) NTN confirmed that, for various carbonitriding
atmospheres, the predicted steel nitrogenconcentration distribution (using NTN’s methodfor determining nitrogen concentration
distribution) closely matches actual dataobtained through EPMA analysis.

-52-

Atmospheric Control Method for JIS-SUJ2 Carbonitriding Processes

Photo of author

Chikara OHKI
Elemental Technological
R&D Center-53-

NTN TECHNICAL REVIEW No.74（2006）[ Technical Paper ]The Influence of Hydrogen on Tension-Compression andRolling Contact Fatigue Properties of Bearing Steel

Hiroshi HAMADA*
Yukio MATSUBARA**In this report, the fatigue properties of JIS-SUJ2 bearing steel
were investigated under various hydrogen pre-charging conditions.
The results demonstrated that the fatigue strength substantiallydecreased by the influence of hydrogen and that the decrease of
the fatigue strength was in direct correlation with the diffusiblehydrogen content. Additionally, double-roller rolling-sliding contact
fatigue tests were conducted to investigate the influence ofhydrogen on surface crack initiation life and microstructural change beneath contact surfaces. As aresult, surface cracking occurred much earlier as compared to the calculated life. The crack initiation
life decreased as the amount of the diffusible hydrogen increased. Unusual microstructural changes
were distinctly observed when a relatively high concentration of the diffusible hydrogen had
penetrated. In contrast, a relatively low amount of diffusible hydrogen did not always cause unusual
microstructural changes, although only marginal difference was seen in the surface crack initiation
life for varying levels of diffusible hydrogen content above 0 ppm.

investigations into the effects of hydrogen on surface
crack initiation life and microstructural change
behavior using the double-roller rolling-sliding test.

1. Introduction
In certain bearing applications with automotive
auxiliary equipment, it has been known that premature
flaking accompanied by unusual microstructuralchanges can occur under rolling surfaces, and that
one typical cause of this problem appears to behydrogen embrittlement.1) In addition, though a veryrare occurrence, premature flaking sometimes occurs
in actual applications, and the cause is difficult todetermine. Currently, many researchers believe thathydrogen-induced failures accompany microstructuralchanges. However, assuming that microstructuralchanges result only from the penetration of a large
amount of hydrogen, then there is the possibility thathydrogen can induce premature flaking withoutaccompanying microstructural change.
The effects of hydrogen on the fatigue
characteristics of bearing steel were evaloated using
an ultrasonic fatigue test. High-speed load application
during this test inhibits dissipation of hydrogen. This
paper reports the results of this test.
In addition, this paper also reports the results of the

2. Test Methods
2.1 Material
The chemical composition of JIS-SUJ2 bearing
steel, which was the material analyzed, is summarizedin Table 1. The material was subjected to a standard
quenching/tempering process under the heat
treatment conditions summarized in Table 2, and wasfinished to the specified test piece dimensions througha grinding process. The surface hardness after heat
treatment was HV747 and the residual γ content,
determined through an X-ray diffraction method, was
9.3%. These values mean that the test pieces feature
standard heat treatment quality.

2.2 Ultrasonic fatigue test method
The ultrasonic fatigue test executed was an axial
load fatigue test (stress ratio: R = –1) in which eachtest piece was brought into a resonant state by**Elemental Technological R&D Center**New Product Development Dept. New Product Development R&D Center-54-

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steel

Table 1 Chemical compositions of JIS-SUJ2 bearing steel
C

Si

Mn

0.97

0.20

0.38

P

S

0.005 0.005

Ni

Cr

Mo

O

0.08

1.35

0.03

5

stress concentration coefficient of approximately
1.03.2) The test piece is designed so that it resonates
at 20 kHz. The thread on one end is to allow it to beinstalled on the test rig.
When the test piece resonates, the stress amplitude
maximizes at the smallest diameter portion at the
middle of the narrower section. The maximum stressamplitude S can be determined by measuring thedisplacement amplitude, a, at the free end of the test
piece with a gap sensor and substituting the obtained
measurement in expression (1).3) E is Young’s
modulus, c (= (E/ρ)1/2 whereρis density) is
longitudinal wave velocity, and ω (= 2πf where f isresonance frequency) is angular frequency. l, b and β
in expression (1) can be represented by expressions(2), (3) and (4), respectively, where R (= 6 mm), H (= 2mm) and g (= 10 mm) are the maximum radius,
minimum radius and half-chord length of the test
piece, respectively.

Table 2 Heat treatment conditions
QuenchingTempering

840˚C×45min

180˚C×120minultrasonic vibration (20 kHz), causing the repeatedgeneration of stress. Further, the fatigue strength of
the test piece could be determined in a shorter timespan relative to conventional fatigue test methods.
This test method is characterized by the fact that thehydrogen-charged steel test piece is fatigued before
the hydrogen dissipates. Thus, the effect ofpenetrated hydrogen can be reliably evaluated.
A typical ultrasonic fatigue test machine isschematically illustrated in Fig. 1. The ultrasonic
fatigue test machine used for the test was Model USF2000 from Shimadzu Corporation. The entire test
system is comprised of a personal computer that
controls the test machine, an amplifier, a piezo
oscillator, an amplitude magnifying horn, and otherparts.
The shape and dimensions of the test piece for theultrasonic fatigue test are provided in Fig. 2. This test
piece is known as a circular tapered dumbbell test
piece. It consists of round columns with a uniform
cross-section from both ends to the shoulder while theside face of the narrower section between these roundcolumns is curved.
Since the test piece lacks a smooth flat section,
moderate stress concentration occurs on it with a1・coshbg・（ ω・l（ coshβg） …（1）c ）S＝a・E・β・cos
l＝cc
tan-1 （βcothβg− b tanh bg） ……（2）
ω
ω

［

］

1
b＝ g cosh-1（ HR ）………………………………（3）（ ωc ） ………………………………（4）β＝ b2−

2

The test procedure is schematically illustrated inFig. 3(a). The surface of the test piece can beroughened as a result of the hydrogen pre-charging(described later in Sec. 2.3). This roughness cancause stress concentrations that may lead to surface
initiated failures. To prevent this problem, immediately
after hydrogen pre-charging, the surface of thenarrower section of the test piece was polished withemery papers (starting with #500 and ending with
#2000) and buffed with diamond paste (1μm). The

Piezo oscillator
Amplifier

78.74

20

24.37

Personalcomputer

24.37

R1

4.5

Amplitude
magnifying horn

M6

φ4

Test piece

φ3.5

φ12

Oscilloscope

Gap gage

Fig. 2 Geometry and dimensions of ultrasonic fatigue
test specimen

Fig. 1 Schematic of ultrasonic fatigue test machine-55-

NTN TECHNICAL REVIEW No.74（2006）−

Lapping(for surface conditioning andremoval of corrosion layer)

Hydrogenpre-charging
(20h)

＋

Ultrasonic
fatigue test

10min

Pt

(a) Experimental procedures(b) Hydrogen pre-charging

Fig. 3 Experimental procedures of ultrasonic fatigue testing and schematic of hydrogen pre-charging

test sequence began by fixing the threaded portion of
the test piece to the end of the horn and then verifyingthat the test piece resonated in the frequency range of
20 kHz ± 30 Hz.
The test sequence was performed within 10 minutes
to avoid dissipation of the penetrated hydrogen, andthe time span after hydrogen pre-charging to thebeginning of the test was fixed to 10 minutes.
The conditions of the ultrasonic fatigue test aresummarized in Table 3. When a load is continuallyapplied during an ultrasonic fatigue test, heat buildup
occurs on the test piece due to the internal friction that
results from high-speed vibration. To avoid thisproblem in the test, an intermittent load application
scheme was adopted 4) in which a cycle consisting of
a 0.11-second loading period and a 1.10-second nonloading period was repeated. If a crack occurs in aductile material, its resonant frequency will decrease
greatly when the crack develops to a certain size. The
test rig was designed to stop each test run using thischange. Since the test pieces were all made of abrittle material, certain test pieces fractured completely
into two pieces. On the other hand, if a test piece did
not develop a fracture after 108 load application
repetitions, that test run was aborted.

2.3 Method of hydrogen pre-charging of
ultrasonic fatigue test pieces
The hydrogen pre-charging method is schematicallyillustrated in Fig. 3(b). Cathode hydrogen precharging is a hydrogen pre-charging technique inwhich the test piece is used as the cathode and theaqueous solution of the electrolyte is electrolyzed.
During this process a portion of the hydrogen atoms
occurring on the surface of the test piece are
introduced into the test piece. This technique helps
achieve safe and easy hydrogen penetration into thespecimen. With this technique, it is possible to control
the hydrogen penetration content relatively easily bycontrolling the applied current density.
The hydrogen pre-charging conditions are
summarized in Table 4. The electrolyte solution used
was prepared by diluting sulfuric acid with thiourea,
which serves as a hydrogen rebonding inhibiting
additive. Five levels of current density in a range of
0.1–0.5 mA/cm2 were used as listed in Table 4. The
hydrogen pre-charging duration was fixed to 20 hours.
The hydrogen diffusion coefficient with the JIS-SUJ2
steel at an ordinary temperature is 3.8 x 10-11 m2/s.5) If
the hydrogen diffusion coefficient takes this value,then the relative concentration distribution of diffusiblehydrogen in the minimum diameter section (φ4) on
the ultrasonic fatigue test piece can become nearly
uniform within 20 hours.

Table 3 Conditions of ultrasonic fatigue test
20 kHzLoading frequency
Loading cycle
Stress ratio
Test atmosphere

Table 4 Hydrogen pre-charging conditions prior toultrasonic fatigue test

Intermittent
loading

Electrolyte

Loading: 0.11 sec.
Non-loading: 1.10 sec.

Pre-charging current density (mA/cm2)

R = –1 (fully bidirectional)

Pre-charging duration (h)Room temperature atmosphere-56-

H2SO4 (0.05 mol/L) +CH4N2S (1.4 g/L) (thiourea)
0.1, 0.2, 0.3, 0.4, 0.5
20The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing SteelThe test procedure is schematically illustrated inFig. 5. Either the driving roller or the following roller
was subjected to cathode hydrogen pre-charging
similar to that described previously in Sec. 2.3. The
hydrogen pre-charging conditions are summarized inFig. 6. The pre-charging duration was set to 20 hours.
In this pre-charging duration, the hydrogenconcentration distribution does not become uniform
through the interior of the test piece. However, thesurface of the test piece will have near-uniform
hydrogen concentration distribution. Prior to therolling-sliding test after hydrogen pre-charging, thesurface of the test piece was polished with emery
papers (starting with #500 and ending with #2000) andbuffed with diamond paste (1μm) to remove a thincorrosion film and avoid the effect of surface
roughness. In the rolling-sliding test, the rollers wereallowed to turn continuously for 20 hours. Taking theabove-mentioned process as one cycle, the cycle(consisting of hydrogen pre-charging and rolling of therollers) was repeated until surface cracking occurred.

2.4 Double-roller rolling-sliding test method
The concept of the double-roller rolling-sliding test isschematically illustrated in Fig. 4. To provide 2%
relative sliding at the contact point between the tworollers, the driving roller and the following roller were
each driven with an independent motor.
The test piece and test conditions are summarizedin Table 5.

Driving roller

Following roller

Fig. 4 Schematic of double-roller rolling-sliding test

2.5 Thermal desorption analysis method

Table 5 Conditions of rolling-sliding test
Driving roller

φ40mm×L12mm，Sub-curvature R60mm，Ra=0.02μm

Following roller

φ40mm×L12mm，Sub-curvature R60mm，Ra=0.02μm

Running speed(min-1)

Driving roller

3060

Following roller

3000

Max. contact bearing
pressure (GPa)

2.1
2a×2b

Contact oval form

1.375mm×0.669mm

Sliding ratio (%)

2

Lubricating oil

VG22

Lubrication system

Lubrication with felt pad

Oil film parameter (Λ)
Test atmosphere

Hydrogenpre-charging(20h)

The thermal desorption analysis was executed toinvestigate the interrelation between the hydrogenpre-charging current density and the resultant
diffusible hydrogen content. The data for the test pieceand the analysis conditions are summarized in Table
7. The hydrogen pre-charging conditions for the test
piece including the electrolyte solution and precharging duration (20 h) were identical to thosesummarized in Tables 4 and 6. Since the test piece
was a slender form, the relative hydrogenconcentration distribution inside the test piece became
sufficiently uniform after 20 hours of pre-charging.5)
Gas chromatography was used for the analysis. The
analysis was started exactly 10 minutes after
completion of hydrogen pre-charging in accordancewith the test conditions of the ultrasonic fatigue test
and double-roller rolling-sliding test.

18
Room temperature atmosphere

Lapping(for surface conditioning andremoval of corrosion layer)

Double-roller
rolling-sliding
test (20 hrs.)

Table 7 Conditions of thermal desorption
analysis of hydrogen

10min(Cycle repeated until crack occurs)

Gas chromatograph

Analyzer

Fig. 5 Experimental procedures of double-roller rollingsliding test

Carrier gas

Type

Ar

Flow rate (mL/min)20

Test piece size (mm3)Table 6 Hydrogen pre-charging conditions
for rolling-sliding test
Electrolyte
Pre-charging current density (mA/cm2)
Pre-charging duration (h)Heat increase rate (˚C/min)
Temperature range for analysis (˚C)

H2SO4 (0.05 mol/L) +CH4N2S (1.4 g/L) (thiourea)
0.3, 0.5, 1.0
20

-57-

27×1×1
3
Room temperature to 360

NTN TECHNICAL REVIEW No.74（2006）

was defined as diffusible hydrogen and the amount ofsuch hydrogen is plotted in Fig. 8.
Based on the results shown in Figs. 6 and 8, theeffects of diffusible hydrogen on fatigue strength of
JIS-SUJ2 steel were evaluated considering thefollowing two points.
First, because scattering was present in fatigue test
data, a statistical approach for handling the data wasneeded. Second, because premature flaking would
accidentally occur on JIS-SUJ2 steel accidentally inreal machines, it appeared reasonable to make theabove-mentioned evaluation based on thecharacteristics of this material at around the fatigue
limit rather than based on the high loading
characteristics of this material, the latter being theevaluation practice in conventional rolling fatigued
tests. Furthermore, the fatigue characteristics that canbe reflected in machine designs appear to be fatigue
strength-related characteristics rather than life-relatedcharacteristics. In this context, based on the JSMSSD-6-02 SN curve regression method 6) of the Societyof Materials Science, Japan, the fatigue test data wasapplied to a continuously-decreasing one-sided
logarithmic curve model. In this way, the 50% fatigue
strength curve and the 10% fatigue strength curve
were determined. The curve in Fig. 9 is the 50%
fatigue strength curve. The evaluation criterion was
10% fatigue strength after 107 load application cycles.

3. Test Results and Discussion
3.1 Ultrasonic fatigue test results
3.1.1 SN characteristics
The results of the ultrasonic fatigue test are plottedgraphically in Fig. 6. Compared to the results obtained
without hydrogen pre-charging, the fatigue strengthwith hydrogen pre-charging was much lower. In
addition, fatigue strength decreased with greater
current densities. As shown in Fig. 7, the mode offracture was a fish-eye fracture initiated at a non-metal
inclusion or other impurities within each test piece
regardless of whether or not they were subjected tohydrogen pre-charging. Thus, no macro-scaledifference in fracture mode was found.

3.1.2 Relationship between fatigue strength anddiffusible hydrogen content
The relationship between the temperature and thehydrogen release rate were obtained from the thermal
desorption analysis of hydrogen. The results areshown graphically in Fig. 8. The amount of hydrogen
released was greater with greater hydrogen precharging current densities, peaking at around 80˚C.
The hydrogen released at temperatures up to 200˚C

1400

σ＝10-A log10 N+B+D（ A, B, D：constants）………（5）1200

The relationship between the amount of diffusiblehydrogen and the 10% fatigue strength after 107 load
application cycles is illustrated in Fig. 10. Since there
is a good linear relation between these two factors, it
could be understood that the loss in fatigue strength isgoverned by the diffusible hydrogen content.

1000
0.1mA/cm2
800
0.2mA/cm2
600

0.3mA/cm2
0.4mA/cm2
0.5mA/cm2400
1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

Number of load repetitions

0.25

Fig. 6 Results of ultrasonic fatigue testing
Hydrogen release rate, ppm/min

Stress amplitude, MPa

No hydrogen pre-charging

0.5mA/cm2：6.5wt-ppm
0.20
0.4mA/cm2：5.3wt-ppm
0.15

0.3mA/cm2：4.5wt-ppm
0.2mA/cm2：3.5wt-ppm

0.10

0.1mA/cm2：2.2wt-ppm
0.05

0.00
0

100

200

300

400

Temperature ˚CFig. 7 Photograph of fracture center

Fig. 8 Results of thermal desorption analysis of hydrogen-58-

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steeltest was 36,511 hours (approx. 4 years),7),8) cracking
must not occur during the effective bearing life.
However, as shown in Fig. 11, in the result of eachtest run that underwent hydrogen pre-charging,surface cracking occurred before 80 hours had
elapsed, which is much shorter than the calculated
life. The accumulated test duration (the number ofload application cycles) before the occurrence of
cracking was shorter with greater current densities,
that is, with greater amounts of penetrated hydrogen.

Stress amplitude

50％

10％

1.282s

10% fatigue strength after
107 load application cycles

3.2.2 Relationship between microstructuralchanges and diffusible hydrogen contents
Photos of surface cracks and their circumferential
cross sections are shown in Fig. 12. Fig. 12(a) shows
the test result from a current density of 1.0 mA/cm2
and Fig. 12(b) shows the test result from a current
density of 0.3 mA/cm2. In the case of Fig. 12(a), what
is distinctive about 12(a), that is not observed in 12(b)were observed under the rolling surface. In contrast, inthe case of Fig. 12(b), no distinctive microstructuralchanges were observed. Judging from these results, itappears that distinctive microstructural changes occur
with greater amounts of penetrated hydrogen, but they
do not always occur with lower amounts of penetratedhydrogen.

107
Number of load applications

Fig. 9 Estimation of 10% fatigue strength at 107 cycles

10% fatigue life after 107 load application cycles
MPa

1100

No hydrogen pre-charging

1000
900

0.1mA/cm2

800

0.2mA/cm2

700
0.3mA/cm2

600
500

3.2.3 Correlation between the results of theultrasonic fatigue tests and the results ofthe double-roller rolling-sliding test

400
0.4mA/cm2
300

0.5mA/cm2

200
0

1

2

3

4

5

6

As previously described in the introduction, with thebearing products on the market, though a very rareoccurrence, surface-initiated flaking sometimes occurs
on the contact surface without apparent damage. The
cause of this problem could be tensile stress that isrepeatedly applied circumferentially to the rolling
contact surface. This tensile stress is greater with agreater rolling friction coefficient. It could be easily
imagined that, under such conditions, premature
flaking would tend to occur if diffusible hydrogen
penetrates the material and the fatigue strength of thematerial deteriorates. Therefore, an investigation was
conducted to determine the effect of the SN

7

Diffusible hydrogen content wt-ppm

Fig. 10 Relationship between diffusible hydrogenconcentration and 10% fatigue strength at 107 cycles

3.2 Results of roller rolling-sliding test
3.2.1 Crack initiation life
The relationship between the hydrogen precharging current densities and the number of rolling
test runs before occurrence of surface cracking in the
double-roller rolling-sliding test is illustrated in Fig. 11.
Because the calculated life under the conditions of thisCurrent density Roller subjectedto hydrogen（mA/cm2）
pre-charging

Number of test repetitions (accumulated test duration)
1 (20h)2 (40h)3 (60h)4 (80h)Driving side
0.3

Following side
Driving side

0.5
Following side
Driving side
1.0
Following side

Fig. 11 Relationship between the current density for hydrogen pre-charging and the number of repetitions to failure-59-

NTN TECHNICAL REVIEW No.74（2006）

Direction of load transfer
Direction of load transfer（a）Driving roller side, current
density of 1.0 mA/cm2（b）Driving roller side, current
density of 0.3 mA/cm2

Fig. 12 Photographs of surface cracks and their circumferential cross sections

characteristics of penetration of diffusible hydrogen on
the possibility of premature flaking. This effect was
examined by repeatedly applying circumferential
tensile stress to the rolling surface in a double-roller
test using an ultrasonic fatigue test.
The dynamics of contact between the rollers were
calculated under the loading conditions of the doubleroller test. The calculation results are illustrated in
Figs. 13(a) and 13(b). Contact pressure distribution isprovided in Fig. 13(a), while Fig. 13(b) shows
circumferential tensile stress distribution on the rollingsurface along the X-X line where the tensile stressmaximizes on the edge of the contact ellipsoid. Whenthe rolling friction coefficient is 0, the maximum tensilestress of both contact edges is 214 MPa. When therolling friction coefficient is 0.1, the maximum tensilestress of one contact edge becomes 531 MPa.
From the results of the ultrasonic fatigue test, asshown in Fig. 14, the 10% fatigue strength after 107
load application cycles is 981 MPa when there is nodiffusible hydrogen penetration. This strength issufficiently greater than the results shown in Fig.
13(b). In contrast, the 10% fatigue strength dropped to
460 MPa when diffusible hydrogen penetrated thematerial, as in the result of the ultrasonic fatigue testwith a hydrogen pre-charging current density of 0.3
mA/cm2 (4.5 wt-ppm). Consequently, if hydrogen
penetrates the material, cracks can occur at a tensilestress less than those shown in Fig. 13(b).
As summarized above, if for any reason the rolling
friction coefficient becomes greater and a large
amount of diffusible hydrogen penetrates the material
at the same time, there is a higher possibility of crack
occurrence on the rolling surface, which can lead topremature flaking. However, to verify this assumption,
it will be necessary to prove that diffusible hydrogen
penetrates the material in an amount sufficient enoughto lower the fatigue strength of the material in therolling contact mode. Detailed research efforts forquantitative verification of the effects of hydrogen on
rolling fatigue were made from the 1950’s to the1970’s. Grunberg et al. proposed a theory that theloss in rolling life of a steel material due to ingress ofwater is caused by hydrogen derived from the water.9)11) This theory was supported by Scharzberg et al.12),13)
1.0
2.5

2.0

Axial coordinate

0.5

1.5

X

0.0

X
1.0

0.5

-0.5

0.0
-1.0
-0.5

0.0

0.5［GPa］(a) Contact pressure distribution between two rollers
1400

Stress, MPa

1200
1000
800

μ＝0.1

531MPa

600
400
200

μ＝0

214MPa

0
-0.5

0.0

0.5

Circumferential coordinate mm(b) Distribution of circumferential tensile stress working on
the contact surface along the X-X line

Fig. 13 Distribution of circumferential tensile stressat contact surface along X-X line

-60-

The Influence of Hydrogen on Tension-Compression and Rolling Contact Fatigue Properties of Bearing Steel

Incidentally, the problem of loss in the effective
service life of bearings available in the market, which
results from penetrated hydrogen, has beenaddressed through various improvements inlubricants, bearing materials and surface modification
techniques. NTN bearing products improved in theseways have already been made available to themarket.

Stress amplitude, MPa

1400
No hydrogen pre-charging

1200
1000

981MPa800
600

0.3mA/cm2
460MPa400
2000
1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

Number of load applications

Fig. 14 10% fatigue strength curves

References
1) K. Tamada and H. Tanaka, Wear, 199 (1996) 245-252
2) R. E. Peterson, Stress Concentration Factors, John Wiley &Sons, New York, (1974) 51p
3) K. Salama and R. K. Lamerand, Proceedings of the 1st
International Conference on fatigue and Corrosion Fatigue
up to Ultrasonic Frequencies (1981) 109-118
4) H. Ishii, K. Yamanaka and K. Tohgo, Material Science
Research International, Special Technical Publication -1,(2001) 59-63
5) Y. Matsubara and H. Hamada, Journal of ASTM
International, 3 (2006) , Online ISSN: 1546-962X.
6) JSMS Committee on Fatigue of Materials, JSMS Committee
on Reliability Engineering: Standard Evaluation Method of
Fatigue Reliability for Metallic Materials—Standard
Regression Method of S-N Curves (JSMS-SD-6-02), Societyof Materials Science, Japan (2002) (in Japanese)
7) J. J. Coy, D. P. Townsend and E. V. Zaretsky, Analysis of
Dynamic Capacity of Low-Contact-Ratio Spur Gears using
Lundberg-Palmgren Theory, NASA TN D-8029, (1975)
8) J. J. Coy and E. V. Zaretsky, Life Analysis of Herical Gear
Sets using Lundberg-Palmgren Theory, NASA TN D-8045,(1975)
9) L. Grunberg, Proc. Phys. Soc. (London) , B66 (1953) 153161
10) L. Grunberg and D. Scott, J. Inst. Petrol., 44 (1958) 406410
11) L. Grunberg, D. T. Jamieson and D. Scott, Phil. Mag., 8(1963) 1553-1568
12) P. Schatzberg and I. M. Felsen, Wear, 12 (1968) 331-342
13) P. Schatzberg, J. Lub. Tech., 231 (1971) 231-235
14) D. E. Swets and R. C. Frank, Trans. AIME, 221 (1961)
1082-1083
15) N. Kino and K. Otani, JSAE Review, 24 (2003) 289-294

Incidentally, Swets et al. suggested that hydrogenoccurring from decomposed lubricating oil penetrates
steel materials and causes the rolling fatigue life of thematerials to become shorter.14) Additionally, Kino et al.
reported that hydrogen content in a steel material
increases after a rolling fatigue test and the increasein hydrogen content varies depending on the type of
lubricating oil used.15) Based on this knowledge aboutwater ingress and decomposed lubricating oil, further
investigation is needed to determine quantitatively
whether diffusible hydrogen penetrates the steel
material in an amount sufficient enough to lower its
fatigue strength in the rolling contact mode.

4. Conclusion
The authors have evaluated the effects of hydrogenon the fatigue characteristics of JIS-SUJ2 bearing
steel by using an ultrasonic fatigue test that is capableof high-speed loading on the material in order to inhibit
dissipation of hydrogen during the test. In addition,
through a double-roller rolling-sliding test, the authors
investigated the effects of hydrogen on the surface
crack life of the material and microstructural changes
in the material. As a result, the authors haveconfirmed the following:1) In the ultrasonic fatigue test, the fatigue strength ofthe material significantly decreased due to thepenetrated hydrogen, and there is an apparent
relation between the amount of penetratedhydrogen and the fatigue strength values after 107
load application cycles on the material.
2) In the double-roller rolling-sliding test afterhydrogen pre-charging, premature flaking occurs
due to the effect of penetrated hydrogen. A greater
penetrated hydrogen content will cause distinctive
microstructural changes in the material, while asmaller penetrated hydrogen content may notalways lead to the occurrence of distinctive
microstructural changes in the material.

Photos of authors-61-

Hiroshi HAMADA

Yukio MATSUBARA

Elemental Technological
R&D Center

New Product Development Dept.
New Product Development R&D Center

NTN TECHNICAL REVIEW No.74（2006）[ Technical Article ]Technical Trends in Bearings for Office Equipment

Chikara KATAGIRI*

Recently, the demand for digital color PPC's (Plain Paper
Copiers) and MFP's (Multi Function Printers) has been
increasing alongside personal computers, digital cameras andthe internet. During this time, NTN has been aggressively
working to develop technologies and products for this everincreasing digital media market. This report discusses therequired function of photosensitive drums and fuser rollers forthe latest PPC's and MFP's, as well as bearing technologies
for these applications.

1. Introduction

2. Structure of Color Copying
Machines and Color LBPs

Electrophotography began as a monochromatic
analog technology. Then, in 1983, Carlson (USA)proposed color xerography, and in the 1990’s, digitalelectrophotography became a technology that wasmore commonly used. As more and more people havebegun using personal computers and the Internet,
electrophotographic equipment, including digital plain
paper copiers (PPC) and laser beam printers (LBP)
have become increasingly used as imaging devices
for diverse applications worldwide.
The requirements for bearings used in digital
copying machines and printers have become moredemanding. This paper describes the most recent
requirements for these bearings and the technologies
used to satisfy these requirements.

The structure of a typical color copying machine orcolor LBP is illustrated schematically in Fig. 1. Manyrolling bearings are used in color copying machinesand color LBPs. Recently, thin deep groove ballbearings, such as those shown in Photo 1, have beenused more frequently in photosensitive sections andfusing sections of these devices. Featuring smaller
cross-sectional areas, thin deep groove ball bearings
contribute to the compact design of mechanisms incolor copying machines and color LBPs and also help
provide extra space around the bearings.

Basic process of color copying machines andcolor LBPs
1) Image exposure: The image being copied orprinted is formed as a latent image on thephotosensitive drum.
2) Developing: The toner is allowed to be attracted to
the latent image on the photosensitive drum tomake the image visible.
3) Transfer: The toner powder on the photosensitive
drum is transferred to a sheet of copy paper.
4) Fusing (fixing): The toner powder is fixed to the
copy paper by heat and pressure.

***Industrial Engineering Dept. Industrial Sales headquarters-62-

Technical Trends in Bearings for Office Equipment

Polygonal scannerPhotosensitive section
bearings

Photosensitive
drum

Developing
section

Fusing section
bearings
Fuser roller
F

Photosensitive section

Pressuring roller
Fusing section

Paper feeder
Paper tray

Fig. 1 Structure of color copy machine and LBP

Photo 1 Thin type deep grove ball bearing

3) Provision of metal bypasses between inner andouter rings
Recently, to meet the market price demands forcolor copying machines and color LBPs, the need has
been mounting to minimize the costs of their internalmechanisms and associated components. Given thissituation, the addition of new components and parts
should be strictly avoided, so the most promising of
the above measures seems to be the first, “prefillingwith conductive greases.”
For this reason, NTN offers bearings prefilled withoriginal greases, including types for use at room
temperature and types for use at high temperature,that have been developed in cooperation with grease
manufacturers. The performance data of thesebearings are described below.

3. Functions Required of Color
Copying Machine and Color LBP
Bearings
Photosensitive drum support bearings1) Electrical conductivity2) Resistance against chemical attack3) Stable running accuracy

Fuser roller support bearings
1) Long life under high temperatures2) Electrical conductivity
3) Prevention of frictional noise4) Resistance against chemical attack

4. Bearing Technology
Explanations of bearing technologies related to
conductivity (at ordinary and high temperatures),chemical attack resistance and frictional noise*elimination are presented below.

4.1.1 Conductive bearings for photosensitive
drum support
The typical properties of the original NTN
conductive greases for room temperature use, EP-2
Grease and EP-3 Grease, which is a variation of EP-2
with improved oil separation, are summarized in Table
1. A unique feature of these greases is the addition ofcarbon black, which is a conductive material, as athickener.

*Frictional noise is noise that occurs from interference between abearing inner ring and a shaft resulting from a loose fit.

4.1 Conductive-bearing technologies
In the past, both grounding arrangements andconductive bearings were used in the photosensitive
sections of color copying machines and color LBPs toeliminate triboelectricity. Recently, better conductivity
for bearings used for this application is needed evenmore because grounding arrangements inphotosensitive sections are being eliminated in order
to simplify entire systems and to prevent theoccurrence of electromagnetic noise. In addition, mostrecently, better conductivity for fuser roller support
bearings has also become more necessary.
The following techniques have been used to provide
electrical conductivity in rolling bearings:
1) Prefilling with conductive greases
2) Use of conductive contact rubber seals

Table 1 Typical properties of conductive greasefor photosensitive drum

Thickener
Base oil

-63-

NTN

NTN

EP-2 GreaseEP-3 Grease

Carbon black

Carbon black
Lithium soap

Synthetic hydrocarbon Synthetic hydrocarbon

Test method――

Workedpenetration

290

266

JIS K 2220

Oil separation
mass %

3.2(100˚C, 24 hrs.)

1.6(100˚C, 24 hrs.)

JIS K 2220

NTN TECHNICAL REVIEW No.74（2006）

The amount of weight loss in the bearings used forthe test described above are graphically plotted in Fig.
4. Compared with the bearing prefilled with EP-2
Grease, the bearing prefilled with EP-3 Grease
features better base oil retention power and reduced
loss of bearing weight. Thus, EP-3 was shown to beeffective in preventing outward leakage of the base oilfrom running bearings.
Encouraged by the performance of the EP-3 Grease
as described above, NTN will employ EP-3 Grease asa standard conductive grease for photosensitive drum
support bearings.

The test rig used for measuring the resistance on
conductive bearings is illustrated in Fig. 2, and thetest conditions are summarized in Table 2.

Table 2 Test condition of bearing resistance
Test bearing

6806ZZ φ30mm×φ42mm×7mm

Running speed (min-1)

100

Radial load (N)4.9

Ambient temperature (°C)

Room temperature

Control resistance (kΩ)

300

Supply voltage V
Loss in weight mg

100

Control resistor Rs
Terminal B

Terminal A

80
60
40
EP-2

20

EP-3

0
0

200

400

600

800

1000

Test duration h
Slip ring

Test bearing

Fig. 4 Results of bearing weight loss test

Drive belt
Load

4.1.2 Conductive bearings for fuser roller support
Unlike the photosensitive section, the fusing section
gets very hot (max. 250˚C). Therefore, since the fuserroller supporting conductive bearings must satisfy both
high-temperature durability and conductivity, they are
prefilled with fluorine grease. NTN high-temperature
conductive greases have been developed fromfluorine grease. The representative properties of NTN
high-temperature conductive EF-5 Grease and EF-7
Grease, which is a variant of EF-5 with a longer life,
are summarized in Table 3.
The electrical resistance of ordinary fluorine grease
prefilled bearings are about 100 kΩ. In contrast, theresistance values of the bearings prefilled with EF-5
Grease and EF-7 Grease are lower, as shown in Fig.
5. In particular, the newly developed EF-7 Grease
maintains low resistance for an extended period. As

Fig. 2 Test rig of bearing resistanceResistance value kΩThe resistance values of bearings prefilled with EP2 Grease, EP-3 Grease and a conventional grease
are plotted in Fig. 3. The bearings prefilled with EP-2
and EP-3 exhibited resistance values that are
maintained at lower levels. This means that EP-2 andEP-3 are relatively free from time-dependent
deterioration.

100
EP-2
EP-3
Conventional
grease806040Table 3 Typical properties of conductive grease for fuser roller

20
0
0

200

400

600

800

1000

Thickener

Test duration hBase oilFig. 3 Results of bearing resistance test

Workedpenetration
Oil separation
mass %

-64-

NTN

NTN

EF-5 GreaseEF-7 Grease

PTFE+graphite PTFE+graphite

Test method―

Fluorine oil

Fluorine oil

―

234

225

JIS K 2220

5.0
3.9
(200˚C, 24 hrs.) (200˚C, 24 hrs.) JIS K 2220

Technical Trends in Bearings for Office Equipment

can be seen in Fig. 6, EF-7 Grease boasts hightemperature life that is more than three times as longas that of EF-5 Grease. Therefore, NTN will employ
EF-7 Grease as a standard conductive grease forfuser roller support bearings.

4.2 Bearing technologies for chemical attack
resistance
Many resin components (mainly those made ofaromatic resins) are used in color copying machinesand color LBPs. The bearings used in these machines
are prefilled with grease as lubricant, and their outerfaces are coated with rust-preventive oil. With
particular combinations of oil and resin, cracking
and/or crazing can occur on resin components.

Table 4 Test condition of bearing resistanceOperating conditions

Test conditions

2TS3-SC05B98ZZ φ25mm×φ32mm×7mm

Test bearing
Running speed (min-1)

130

Radial load (N)220

Ambient temperature (˚C)

200

Room temperature

Control resistance (kΩ)−

300

4.2.1 Mechanism of chemical attack
Chemical attack (stress corrosion cracking) is aphenomenon where corrosion and static stress lead tocracking and embrittlement. Static stress is a stressoccurring on an object when a given static load isapplied to it. Corrosion occurs when compatibility
between the resin and the solvent is high, as thesolvent dissolves the resin or enters molecules in theresin and slackens its polymer chains and crystal
structures, leading to the occurrence of cracking
and/or crazing.

Resistance value kΩ

40
EF-5

35
30

EF-7

25
20
15

4.2.2 Chemical attack evaluation

10
5
0

0

100

200

300

400

500

600

As shown in Fig. 7, a resin test piece was bent by aparticular amount, and the sample oil was applied to aface of the test piece where tensile stress wasapplied. The test piece was then left to stand under
the test conditions summarized in Table 6.

700

Test duration hFig. 5 Results of bearing resistance test

Table 5 Test condition of high temperature grease life
Table 6 Test condition of solvent crack
Test conditions
Resin test piece (mm)2TS3-SC05B98ZZ φ25mm×φ32mm×7mm

Test bearing
Ambient temperature (˚C)

200

Running speed (min-1)

130

Radial load (N)220

Grease life
judgment criteria

1) Temperature increase on the bearing (+15°C)
2) Torque increase on the motor (1.5 times
higher than that at the beginning of the test)

150×19×3 (length x width x thickness)

Deflection ratio (%)

5Standing temperature (˚C)

70Standing time (h)72(Deflection ratio = deflection/distance between supporting points)

Resin(test piece)

EF-5

EF-7

100mm
Application of oil
0

100

200

300

400

500

600

(Distance between
supporting points)

Test duration hFig. 6 Results of high temperature grease life test

Fig. 7 Test rig of solvent crack-65-

NTN TECHNICAL REVIEW No.74（2006）

Photo 2 Example of test pieces after evaluation (occurred split and crack)

The result of evaluation for base oils used on
bearings and typically used resin materials issummarized in Table 7.

this problem, grease or a resin sleeve (heat-insulating
sleeve) was placed between the shaft and the innerring.
Through a special lubricating surface treatment on
the bore surface of the inner ring, NTN has succeeded
in decreasing the friction coefficient on the bore
surface of the inner ring by 40%. As a result, theattitude of the inner ring is not disturbed in spite of thethermal expansion and bending on the fuser roller,
and the inner ring can follow the motion of the roller,
thereby frictional noise that would occur otherwise isprevented.
The result of the durability evaluation of the special
lubricating surface treatment on the bore surface of
the inner ring is shown graphically in Fig. 8. After theelapse of 600 operating hours under the test
conditions summarized in Table 8, there was virtually
no change in the inner ring bore diameter, and it isapparent that wear did not occur on the treatedsurface.

Table 7 Results of chemical attack test
Ester oil

Fluorine oil

Mineral oil

PAO

ABS

×

◎

○

◎

PC

×

◎

○

◎

POM

◎

◎

◎

◎

PA

◎

◎

◎

◎

PBT

○

◎

◎

◎

PEEK

◎

◎

◎

◎

PPS

◎

◎

◎

◎

◎：The oil does not attack the resin much.
○：In certain cases, the oil attacks the resin.
×：The oil attacks the resin.

Color copying machines and color LBPs incorporate
many components made of ABS (acrylonitrilebutadiene-styrene) and PC (polycarbonate).
Therefore, the grease prefilled in bearings and the rustpreventive oil must be oils that do not have chemical
attack effects on resin materials.
Based on the chemical attack results of theevaluation described above, NTN is using greases
and rust preventive oils that employ PAO and fluorineoils as their base oils. Additionally, the conductive
greases previously described in Sec. 4.2 are also
types that do not cause chemical attacks.

Table 8 Test condition of wear test for special lubricant film
Test conditions

2TS3-6804ZZ φ20mm×φ32mm×7mm

Test bearing
Bearing temperature (˚C)

200

Running speed (min-1)

120

Radial load (N)294

Shaft material/surface roughness

Variation in dimension μm

4.3 Fuser roller support bearings that prevent
frictional noise
On the fuser roller support bearings, the fit between
the inner ring and the shaft is loose to simplify theassembly work. Subjected to heat and load, the fuserroller is affected by both thermal expansion in the axial
direction and bending. Due to the loose fit, the thermal
expansion in the axial direction and the bending, theattitude of the roller is disturbed, the inner ring hits the
shaft and, as a result, frictional noise occurs at afrequency of once per revolution. Previously, to control

A5052／Ra0.5

3
2
1
0
-1
-2
-3
0

100

200

300

400

500

600

700

Test duration hFig. 8 Results of wear test for special lubricant film

-66-

Technical Trends in Bearings for Office Equipment

5. Conclusion
As the functions of copying machines and printers
have become increasingly diverse and sophisticated,
the performance needs for the bearings in thesemachines has changed rapidly. NTN will further
develop its bearing technology to help satisfy
requirements for copying machines and printers andcontinue to contribute to achieving higher performanceand energy conservation.
In closing this report, the author would like toencourage the readers to consult our General Catalog
for Office Equipment Products (Catalog No. 8701/J),
which presents NTN’s products for office automationequipment, including rolling bearings, resin products,
sintered products, torque limiter units, one-way clutch
units and torque diodes.

Photo of author

Chikara KATAGIRI
Industrial Engineering Dept.
Industrial Sales headquarters-67-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Unit Products for Office Equipment

Shoji ITOMI*
Atsushi MOROOKA*
The torque limiter and the torque diode are used for high-speed
and medium-speed models of copiers and printers. NTN developed
a new torque limiter and the torque diode that can be used also for a
low-speed model.

The NTS series of products have a uniquemechanism. When the torque working between the
inner ring and the outer resin component is low, theinner ring rotates together with the outer resin
component, but when the torque becomes greater, theinner ring rotates relative to the outer resin component
while maintaining a certain torque remains between
these members.
On the old design, the constricting force from thecoil spring on the inner ring was utilized to control therotary torque in the torque generation direction. In the
inner structure of this old design, which is illustrated inFig. 1, the coil spring consists of two sections thateach has a unique diameter. The smaller diameter

1. Introduction of Ultra Compact
Torque Limiter Units
1.1 Overview
Torque limiters are incorporated in the paper feeder
sections of office automation equipment, such as
photocopying machines and printers, to provide
reliable paper stock separation. NTN has been
supplying the NTS series of torque limiter units formany years, and they have been adopted by manycustomers.
Recently, needs have continued to increase for
office automation equipment designs that use lessspace and are lighter weight. Given this, torquelimiters and other paper feeder section componentsthat are more compact are greatly needed.
Because of their sizes, the current NTS series
products could force the designers of photocopiersand printers to make extra efforts to achieve spacesaving in their designs. To address these issues, NTN
has developed the NTB14 ultra-compact torquelimiter unit.

Old design

New product

1.2 Optimization of internal structures
Each product of the current NTS series (hereafterreferred to as the “old design”) consists of an innerring, a coil spring, an outer resin component and a lid.

Photo 1 Torque limiter**Industrial Engineering Dept. Industrial Sales headquarters-68-Unit Products for Office Equipment

Larger diametersection
Torque adjustment section

Coil spring

Inner ring

Smaller diameter section
Torque generation section
Inner ring

Coil spring

Projection

Projection

Cutout on inner ring

Cutout on inner ring

Cap
Cap

Outer resin component

Outer resin component

Old design (NTS18)

Ultra-compact type (NTB14)

Fig. 1 Structure of current type (NTS18) and new type (NTB14)

section functions as a torque generation section, andthe larger diameter section serves as a torqueadjustment section. To set the torque, the lid is turned
relative to the outer resin component to twist the larger
diameter section, and the resultant constricting force
causes the smaller diameter section of the coil spring
to shrink to restrain the inner ring. On the old design,
the end hooks of the coil spring are engaged with theouter resin component and the lid. Thus, the inner ringof the old design can rotate only in the torquegeneration direction.
The newly developed ultra-compact torque limiter
unit (hereafter referred to as NTB14) does not havethe large diameter section that is the torqueadjustment section of the coil spring in the old design.
Instead, the tightening allowance between the springbore and the inner ring outer surface is responsible forcontrol of the torque generated. As a result, the outer
diameter and the length of the outer resin component
on the NTB14 design are reduced. In addition, thisnovel structure has enabled the torque limiter to becapable of rotation in both directions.

Rubber rollers
Paper stock

NTB14

Fig. 2 Example of torque limiter and rubber roller all- inone design

Benefits of use
Removal of jammed paper is easy.

1.3.3 Prevention of chemical attack
Like the old design, the new torque limiter has an
inner ring made of an oilless sintered metal, in which
the oil in the sintered metal does not pose risk ofchemical attack to the resin components.
Benefits of use• The new torque limiter does not pose problems toadjacent resin materials.
• Since the inner ring is made of an oilless sinteredmetal, there is no possibility of leaking lubricating
oil, and no relubrication is necessary.

1.3 Advantages of NTB14
1.3.1 Compact design
Old design (NTS18): 18 OD x 18 L (mm)New design (NTB14): 14 OD x 12 L (mm)Benefits of use
The torque limiter can be incorporated into therubber rollers on the paper feeder section, so thesizes of the rubber rollers can be reduced in bothdiameter and length (Fig. 2). In addition, the numberof parts associated with the rubber rollers may bedecreased and the assembly procedure for the rubberrollers can be simplified.

1.4 Durability
A typical result of a durability test is shown in Fig. 3.
The loss in torque with the new design is roughly
equivalent to that with the old design. In other words,
the durability of the new design is comparable to thatof the old design.

1.3.2 Capability of rotation in both directions
The new design is capable of rotation in both
directions.
-69-

NTN TECHNICAL REVIEW No.74（2006）

2. Introduction of the TDL16 Compact,
Lightweight Torque Diode

Test conditions
Operation cycle: 1.1 sec. on, 0.2 sec. off
Operation period: 300 hrs.
Running speed: 220 min-1

2.1 Overview
To realize full-color capability, photocopying
machines and printers are incorporating increasing
numbers of components and movable parts. However,the floor space requirements of this equipment need
to be roughly the same as with conventional
monochromatic machines. Therefore, the need formore compact components is great. At the same time,
demands are mounting for compliance with energy
conservation and environmental regulations, as well
as for satisfaction of requirements to acquire BlueAngel Mark certification. For these reasons,photocopying machine and printer manufacturers aremaking efforts to decrease power consumption for thistype of equipment.
The lock-type torque diode can hold the drives fortrays and other components in position with its
reverse-input prevention function even when theequipment is powered off. For this reason, this type of
torque diode is advantageous for saving energy
compared to arrangements in which the drive is heldin position by supplying power to the motor.
NTN has developed and continues to provide the
TDL28 lock-type torque diode for office automationequipment. However, since its components are madeof steel, it is heavy and structural limitations makedevelopment of a compact variant of this torque diodedifficult. To address this challenge, NTN has
developed the TDL16, a new torque diode thatfeatures both compact size and light weight.

40

Torque m・Nm

35
30
25
20
15

Old design

10

NTB14

5
0
0

50

100

150

200

250

300

Time hFig. 3 Results of endurance test

1.5 Standard shape and materials used andtorque setting
The standard shape of the NTB14 is illustrated inFig. 4, and the materials used are listed in Table 1.
Note NTN will supply non-standard shaped variants
upon request.
Allowable torque setting rage: 24.5–39.2 mN-m
{250–400 gf-cm}

1

2

3

4

φ14

φ6

φ9

2-2.5

2-3.2

12
2-2.5

12.4

3.9

Fig. 4 Design of NTB14

Table 1 Composition and material of NTB14
Component
Code description

Material used

Outer resin
component

Polyacetal resin (hue: black)2

Lid

Polyacetal resin (hue: white)

3

Coil spring

Spring steel

4

Inner ring−

LubricantIron-based sintered material
Chemical attack-free oil(impregnated into the inner ring)

1

Photo. 2 New type Torque Diode-70-

Unit Products for Office Equipment

2.2 Operating principle of lock-type torque diodes

Outer ring

A torque diode is essentially a reverse-input
blocking clutch on which rotation on the input side istransmitted to the output side, but rotation on the
output side is not transmitted to the input side. If therevolution on the output side is not transferred to the
input side because the output shaft is locked, then thetorque diode is defined as a lock type. If the revolution
on the output side is not transferred to the input side
because the output shaft rotates freely, then thetorque diode is defined as a free type.
The operating principle of the lock-type torque diodeis illustrated in Figs. 5 through 7.
When the input shaft is at a standstill, the rollers areforced by the spring to the wedge-shaped space
between the inner ring cam surface, which is
integrated with the output shaft, and the outer ringsliding surface. As a result, even when an attempt ismade to turn the inner ring (integrated with the output
shaft), the inner ring and outer ring are locked
together and the inner ring does not turn (Fig. 5).
The input shaft and the cage rotate together, sowhen the input shaft rotates, the cage also rotates inthe same direction. Then, the cage comes into contact
with the roller to force the roller toward the larger sideof the cam surface (toward the spring), releasing thelock (Fig. 6).
When the input shaft is further turned, the two
surfaces on the input shaft come into contact with thetwo surfaces in the hole of the inner ring and force theinner ring (integrated with the output shaft) to rotate(Fig. 7).

Rollers

Cage

Inner ring(integrated with theoutput shaft)Outer ring sliding
surface
Inner ring cam
surface

Input shaft

Fig. 5Lock is released

Rotation directionof the cage
Input shaft rotates

Fig. 6

Rotation directionof the input shaft

2.3 Structure of the TDL16 compact, lightweight
torque diodeContact point

The structure of the newly developed TDL16
compact, lightweight torque diode is illustrated in Fig. 8.

Fig. 7

Housing (resin)
Outer ring
RollerOutput shaft (resin)
φ25

Input shaft1)

SpringCage
Lid (resin)
Inner ring
34.8
Note 1) The input shaft is not built into the torque diode.

Fig. 8 Structure of TDL16-71-

Spring

NTN TECHNICAL REVIEW No.74（2006）

2.3.1 Compact design

2.3.3 Durability

The structure of the old TDL28 is characterized inthat the roller at each end of the spring is forced to thecam surface. In contrast, the TDL16 has a novel
structure in which multiple rollers at each end of thespring are forced to the cam surface. Because of thisarrangement, the outer diameter of the TDL16 issmaller than that of the TDL28 though both designs
have the same number of rollers. Thus, the TDL16
has succeeded in minimizing the loss of load bearing
capacity on the clutch.
In Table 2, the dimensions and limiting torque of the
TDL16 are compared with those of the TDL28.

Under the test conditions summarized in Table 3,
the TDL16 has successfully endured 300,000 cycles
of load applications.
Table 3 Test condition of endurance test

Table 2 Size and limiting torque of TDL16 and TDL28
TDL16

TDL28

25 OD x 35.8 L (mm)(Note that the shape of the 30 OD x 38.2 L (mm)resin housing can be altered
upon customer request.)

2.5N･m

4.9N･mNote 2) These limiting torque values have been calculated based on the maximum
surface pressures occurring on the cam surfaces. The useful operating lifeof a torque diode can vary depending on operating conditions.

70

Weight g

60
50
40
30
20
10
0

TDL16

Load torque

0.5 N-m (constant loading in one direction)

One cycle

Forward rotation for 2.5 sec.⇒standstill for 10 sec.⇒
reverse rotation for 2.5 sec.⇒standstill for 10 sec.

NTN believes that their NTB14 ultra-compact torquelimiter unit and TDL16 compact, lightweight torquediode positively contribute to space and energy
conservation in photocopying machines and printers.
NTN will further develop unit products to satisfy future
market needs.

By incorporating the resin components (output
shaft, housing, lid) shown in Fig. 8, the authors’
TDL16 compact, lightweight torque diode achieved
weight reduction of 50% relative to the TDL28 (Fig. 9).

Limiting torque2)

250min-1

3. Conclusion

2.3.2 Lightweight design

Shape

Running speed

TDL28

Fig. 9 Weight of TDL16 and TDL28

Photos of authors

Shoji ITOMI

Atsushi MOROOKA

Industrial Engineering Dept.
Industrial Sales headquarters

Industrial Engineering Dept.
Industrial Sales headquarters-72-

Unit Products for Office Equipment-73-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Honing Sludge and Electro Furnace Dust Briquetter

Shouzo GOTOH*
Kanji NAKAMURA*
Being an ISO14001-certified company, NTN is responsible forhelping establish recycling technology to contribute to our recyclingoriented society. At first NTN started recycling grinding sludge with agrinding sludge briquetting machine, achieving Zero-emission withcost reduction. In 2002 NTN has established UNITOP, anenvironmental venture business based on briquetting technology.
Horning sludge and electro furnace dust were picked up as the next
targets. After testing, practical machines for horning sludge andelectro furnace dust were developed. These machines will contributeto environmental load reduction.

well as cost reduction by eliminating industrial waste
disposal expenses. To further promote our waste
reduction efforts, in 2002, we founded UNI TOP, anenvironmental venture company that boasts a uniquegrinding sludge briquetting technique.
We have also succeeded in meeting the needs ofour customers for technical assistance in their solidliquid separation and solidifying efforts. We are now
committed to the commercialization of a briquetter forhoning sludge and electric-arc-furnace dust.

1. Introduction
In 1998, the NTN Iwata Works became the firstmember of the NTN Group to acquire ISO 14001
certification. At that point, among NTN’s waste,grinding sludge was the type that accounted for thelargest environmental impact. In 1998, all of NTN
produced approximately 8,000 tons of grinding sludge
that was disposed of by industrial waste disposal
agents. As a part of our efforts to acquire ISO 14001
certification, we began a commitment to recycling
technologies with the aim of both recycling grinding
sludge and reducing industrial waste disposal costs.
Through the use of a solid-liquid separation techniquefor grinding sludge. We successfully established therecycling system that is illustrated schematically in
Fig. 1. In 2000, we developed a grinding sludge
recycling machine capable of commercial operation
and began a full-scale grinding sludge recycling
operation. Three years later, we succeeded inrecycling nearly all the grinding sludge produced by
the entire NTN Group. Consequently, we achieved“zero emissions” of waste, mitigating environmental
impacts, which is the main objective of ISO 14001, as

Plant

Higher-level
cycle

Separationand collection

Steel mill

Grinding sludge

Reuse
Classification

Solid-liquid separation
immediately aftersludge generation

Recycling
Coolant

RecycleReduceReusedVolumereduction

Grinding sludge briquetter

Qualityassurance

Briquette

Prevention of
spontaneous
combustion

Fig. 1 3R recycling for grinding sludge*Equipment Development Dept. Production Engineering R&D Center-74-

Honing Sludge and Electro Furnace Dust Briquetterhoning sludge briquetting system. As a result, theauthors verified that honing sludge could undergo
solid-liquid separation and solidification through theuse of this system.
Encouraged by this finding, the authors fabricatedthe prototype honing sludge briquetting system shownin Fig. 5. In the fall of 2005, this system wascommissioned at a customer automaker’s site andallowed to run continuously on a trial basis. The
automaker was satisfied with the basic functions of
this system. Fig. 6 illustrates the prototype machine
schematically, and Fig. 7 shows the honing sludge
used in the authors’ test. Fig. 8 is a view of a briquetteobtained from the authors’ test, and Fig. 9 illustrates aview of the filtered and recovered coolant. For
reference, a photographic view of the coolant inprocess is shown in Fig. 10. From a comparison ofFig. 9 and Fig. 10, the readers can see that thecoolant is cleaned by filtration through solid-liquid
separation and solidification using paper filters. The
authors’ newly developed honing sludge briquetter
has the following advantages:
1) Reliable solid-liquid separation andsolidification: Labor time saving throughstable operation

2. Honing Sludge Briquetters
In 2004, NTN received a request from an automakerthat wished for us to develop a briquetter that could
solidify the sludge that results from the honing work on
engine cylinder bores. We first attempted to meet thisrequest with our grinding sludge briquetting technique.
Regrettably, the sludge started to leak through themechanical gaps between the die and the pressurizing
shaft when pressurizing was begun. We investigatedthe cause of difficulty in solid-liquid separation andsolidification, learning that the unique shape of theswarf (shavings) from the honing process is thecause. A microscopic photograph of honing swarf isshown in Fig. 2, while a microscopic photograph ofgrinding swarf is provided in Fig. 3. Throughcomparison of the two photos, it is apparent that
grinding shavings are curled and can easily become
entangled with each other, while honing shavings arerolled and cannot easily become entangled. To realize
solid-liquid separation and solidification of honing
sludge, the authors first tested a method, as shown in
Fig. 4, in which the mechanical gaps are blocked withpieces of filter paper that allow only the coolant todrain away while retaining the honing swarf in the

Paper filters

100μmFig. 4 Test method of horning sludge briquetting

Fig. 2 Magnified horning swarf
Machine dimensions
Width :1,265mm
Depth : 596mm
Height :1,560mm

100μm

Fig. 3 Magnified grinding swarf

Fig. 5 Test machine for horning sludge briquetting-75-

NTN TECHNICAL REVIEW No.74（2006）

AC servo motor(for pressurizing)
Vacuumchuck

Sludge hopper

Pressurizing rod
AC servo motor(for positioning the die)Briquette
Die
Discharge chutePressurizing station

Filter feedstationSludge feedstationFig. 6 Mechanism of horning sludge briquetter

2) Separation into briquettes and clean coolant:
Filtration is achieved in conjunction with solidliquid separation
3) Driven completely by electric power: Energy
saving, quiet running, optimal pressure control
4) Compact design: The briquetter can beinstalled onsite, so honing sludge can beprocessed immediately after generation
Fig. 7 Horning sludge

3. Electric Arc Furnace Dust Briquetter
In cooperation with Daiwa Steel Corporation, NTN
has developed an electric arc furnace dust briquetter.
By further advancing the technology that has beendeveloped through its grinding sludge briquetters.This
high-performance system is based on Daiwa’s uniquepelletizing technology and NTN’s unique briquetting
technology. In this system, electric arc furnace (EAF)
dust and carbon are premixed and pelletized. The
resultant pellets are loaded into the EAF dust
briquetter without blending with a binder, which isoften used in conventional EAF dust briquetters.
Incorporating a novel pelletizer, our EAF dust
briquetter can produce briquettes of stable strength tohelp improve melting efficiency in the electric arc
furnace. As a result, our EAF dust briquetter will leadto reductions in the amount of industrial wastes andelectric power consumption for the furnace. Insummary, our EAF dust briquetter can be defined as
an eco-friendly system that will contribute to rapid
recovery of investment.

Fig. 8 Horning swarf briquette

Fig. 9 Recovered horning
oil after paper filtration

Fig. 10 Horning oil inhorning machine

3.1 EAF dust recycling
The concept for EAF dust recycling is shown in Fig.
11. The dust occurring in the electric arc furnace iscaptured with a filter. It is mixed with water and-76-

Honing Sludge and Electro Furnace Dust Briquetter

Scrap

Electric arc furnace

Briquette（φ150×120H）
Product

Slag

Exhaust

Dust

FilterDust

CarbonWater

Pelletizer
Pellets

Iron
content
Commissioning tooutside disposal agents

Dezincification
systemZinc bullion

NTN EAF dust briquetter
Fig. 11 Recycling of EAF (electric arc furnace) dust

carbon, which functions as a reducing agent and fuel,and the mixture is formed into pellets. The resultant
pellets are not suitable for direct loading into anelectric arc furnace due to their low mechanicalstrength and difficulty in handling. Therefore, thepellets are formed into briquettes with the authors’
newly developed briquetter, and then the briquette isloaded into the electric arc furnace. Since EAF dust isrepeatedly recycled, the concentration of zinc, which
has a relatively low boiling point, in the EAF dust will
increase and pose a problem in operating the electricarc furnace. Therefore, when the zinc concentration
reaches a specific upper limit, the iron content isseparated from the zinc content. The iron content isreloaded into the furnace while the zinc content isrecycled as a valuable material.
Thanks to this recycling scheme, the disposal of
EAF dust by landfill, no longer necessary. The numberof zinc removal cycles can be decreased, thereby
dramatically reducing the costs associated withindustrial waste disposal.

Upper punch

Die
Pellets

Lower punch

Fig. 12 Moulding method

2) Effects of various factors on the strength ofbriquettes
The authors investigated the effects of factors
associated with materials and formation conditions on
the mechanical strength of the briquettes obtained.
The results of these investigations are shown in Figs.
13 through 16. The mechanical strength of thebriquettes was translated into indexes by dropping
each briquette from 1 meter above to a 5-cm thicklayer of raw material powder and then counting thenumber of fractured pieces resulting. The greater theindex is, the greater the mechanical strength of thebriquette. By appropriately selecting the water andcarbon content in the pellets and the formation

3.2 Overview of the authors’ briquetter
1) Formation principle
The principle of this briquette formation method isschematically illustrated in Fig. 12. The pellets areloaded into the die, and then the upper and lower
punches apply force to the pellets in the die to convert
the pellets into a briquette.
-77-

NTN TECHNICAL REVIEW No.74（2006）

Moldability index
Lower⇔ Higherpressure for the briquette, the necessary formation
strength can be obtained for the briquette. A lower
formation speed is advantageous in attaining a higher
briquette formation strength. However, the authors
have selected an appropriate formation speed inconsideration of the throughput of the briquetter.

⇔

Lower

3) Briquetter configuration
HigherAfter the pellets are loaded into the inlet hopper, acycle consisting of metering, water spraying, loading
into the die, formation and discharge is executed
automatically. Through setup for optimal briquetting
conditions and rationalized machine design, stablebriquette strength and higher productivity are attained.
The major technical data for the authors’ EAF dust
briquetter are summarized in Table 1.

Water content wt%

Moldability index
Lower⇔ HigherFig. 13 Water content and briquette strength

Table.1 Main specification of dust moulder
Model
Lower

⇔

HDM-150

Pellets consisting of EAF dust, blended withBase materials carbon and water

HigherCarbon content wt%

Moldability index
Lower ⇔ Higher

Fig. 14 Carbon content and briquette strength

Briquette size

Dia. 150 mm x 120 mm H, approx. 6.5 kg

Cycle time

17 secMachine size

3600 W x 5550 D x 4300 H (mm);weight: approx. 1500 tons

Throughput

800 tons/month (24 hours/day operation for 30 days)

Power supply

200 V, 3-phase, 50 kVA capacity

3.3 Advantages of the authors’ technology

Lower

⇔

1) A pelletizing technology to produce pellets byadding a measured amount of carbon and water tothe EAF dust, useful in obtaining briquettes ofstable mechanical strength (patent pending).
2) This technology provides reliable briquetting of thepellets (patent pending).
3) The briquetter can be directly connected to thepelletizer to establish a fully automated briquetting
system.
4) Non-processed EAF dust is not loaded into thefurnace. This helps improve the working
environment.

Higher

Molding pressure MPa

Moldability index
Lower ⇔ Higher

Fig. 15 Moulding pressure and briquette strength

Small

⇔

3.4 Benefits from adoption of the authors’ briquetter

Large1) Prevention of EAF dust from being emitted asindustrial waste and reuse of iron resources⇒ Helps achieve zero emissions2) Elimination of binder expenses, as well as the fees
for commissioning briquetting work to an outsideagent⇒ First in the industry! Realization of briquetting
without using a binder

Molding speed mm/sec.

Fig. 16 Moulding speed and briquette strength-78-

Honing Sludge and Electro Furnace Dust Briquetter3) Decrease electricity expenses throughimprovement of melting efficiency with electric arc
furnaces (in comparison to conventional EAF dust
recycling)⇒ Realization by mixing in carbon and use ofheavier briquettes
4) By concentrating zinc, dramatic reduction in theamount of dezincification processing⇒ Improved melting efficiency leads to simpler
dezincification process, which helps realize
significant cost reductions5) Significant cost-reduction for transportation tooutside agents and improved logistics⇒ Significantly reduced burden of transportation tooutside agents through improvements in
dezincification process, etc.
6) Improved working environment, and simpler
handling of briquettes⇒ No handling of powder is needed, greatly
improving the working environment

4. Conclusion
Through further developments in these technologies
as, described below, the authors strongly believe that
NTN’s unique honing sludge briquetting technology
and EAF dust briquetting technology will contribute to
the mitigation of environmental impacts.

Honing sludge briquetting technology
This technology will be applicable to various sludge
types that cannot be solidified or briquetted with agrinding sludge briquetter. NTN is currently attempting
to apply this unique technology to solid-liquid
separation for ultrafinish-derived sludge so as tofurther mitigate environmental impacts from NTN’s
production activities.

EAF dust briquetting technology
The authors began the development work for thistechnology because it appeared to be advantageousin converting pellets into molded briquettes. Currently,
we expect that our novel technology for directly
converting powdered materials into briquettes will becommercialized and eventually adopted in various
powder-solidification applications to realize therecycling of resources that were previously difficult torecover.

Photos of authors

Shouzo GOTOH

Kanji NAKAMURA

Equipment Development Dept.
Production Engineering R&D Center

Equipment Development Dept.
Production Engineering R&D Center-79-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Ultra-Clean Bearing for Clean Environments

Ryouichi NAKAJIMA*
Masaaki HONDA*
NTN manufactures Ultra-Clean bearings for clean
environments used for a semiconductor and LCD panel
fabrication equipments, vacuum evaporation systems, etc.
NTN developed Ultra-Clean NC bearings and Ultra-Clean
WB bearings to supply the demand for a long-life under hightemperature and heavy load conditions.

Conventional oil-impregnated resin cages are madeof sintered resin materials, and, consequently, theirporosity, the ratio of pores to retain lubricating oil, was
20% at maximum. In contrast, NTN’s newly developed

1. Overview
Extremely low dust generation is needed for the
bearings used in the systems and machines that
manufacture semiconductor devices and liquid crystal
display panels, as well as for those used in thevacuum evaporation systems for these products.
Recently, demand has been mounting for longer
maintenance intervals for these devices and systems.
Given these demands, the bearings used in thesesituations must be capable of durable operation in
high-temperature and high-load environments. To
address these engineering challenges, NTN has
developed a unique lubrication system, and with this,they have successfully developed unique Ultra-Clean
NC bearings that feature long life in vacuums underhigh bearing pressure and unique Ultra-Clean WB
bearings that boast long life in high-temperatureenvironments. This report introduces these uniquebearings.

Fig. 1 Ultra-Clean NC bearing
NC：Network Cell

2. Introduction to NTN Ultra-Clean NC
Bearings (Fig. 1)
For lubrication, the authors adopted a porous resin
cage that was developed independently by NTN. This
cage is impregnated with a low-vapor-pressure oil(Fig. 2).

Pore
×400

Fig. 2 Porous resin cage

*Industrial Engineering Dept. Industrial Sales headquarters-80-

Ultra-Clean Bearing for Clean Environments

Table 2 Test conditions

oil-impregnated porous resin cage features 35% orhigher porosity thanks to a special manufacturing
technique, allowing this cage to retain more lubricatingoil. Compared to conventional ultra-clean bearings,
NTN’s new ultra-clean bearings are capable of longer
life while maintaining low dust generation in high load
applications.

Deep groove ball bearing (#608) φ8×φ22×7 mm

Test bearing

2.1 Evaluation of various characteristics
2.1.1 Evaluation of dust generation characteristics

Degree of vacuum

10-5PaRunning speed

500 min-1

Temperature

Ordinary temperatureThrust load

196N

Rotating torque mN・m

50

Under the test conditions summarized in Table 1,
the test bearing was allowed to run for 150 hours, andthe amount of resultant dust generated during theduration of the test was measured. As summarized inFig. 3, the dust generated with the Ultra-Clean NC
bearing was equivalent to the amount generated bybearing types usually used for clean applications (alow-dust-generating prefilled-grease type for vacuumuse and a special PTFE-coated type).

40
30
20
10
0

Ultra-Clean
NC bearing

2.1.2 Rotating torqueLow-dust-generating
prefilled-grease typefor vacuum-use

SpecialPTFE-coated
type

Fig. 4 Results of rotating torque

NTN evaluated the rotating torque for the UltraClean NC bearing under the test conditions
summarized in Table 2. As illustrated in Fig. 4, theUltra-Clean NC bearing features a lower torquecompared to the conventional bearing (special PTFEcoated type).

2.1.3 Endurance evaluation
Each bearing type was evaluated at each of thethree thrust load levels specified in Table 3. As can beseen from the data in Fig. 5, the Ultra-Clean NC
bearing boasts a longer life in all three loading

Table 1 Test conditions of dust generation

Degree of vacuum

10-5PaRunning speed

50 min-1Size of particles being counted

9.8N
Ordinary temperature

Degree of vacuum

10-5PaRunning speed

500 min-1

Temperature

Ordinary temperatureThrust load

196N，686N，981N

End of life criterion

150 hrs.

The point when a sudden increase in torque occurred

Ultra-Clean NC bearing

1000

Low-dust-generating prefilled-grease type for vacuum-use
Special PTFE-coated type
10000

100

Cessation of test run

Cessation of test run

Cessation of test run

196N

686N

981N

1000

Life hNumber of dust particles generated

Duration

Deep groove ball bearing (#608) φ8×φ22×7 mm

Test bearing

0.2μm or greater

Thrust load
Temperature

Table 3 Test conditions of endurance test

Deep groove ball bearing (#608)
φ8×φ22×7 mm

Test bearing

10

100

10
1

Ultra-Clean
NC bearing

Low-dust-generating
prefilled-grease typefor vacuum-use

SpecialPTFE-coatedtype

1

Fig. 3 Results of dust generation test

Fig. 5 Results of endurance test-81-

NTN TECHNICAL REVIEW No.74（2006）

conditions when compared to the conventional
bearings. In particular, under a high-loading condition,
the life of the authors’ Ultra-Clean NC bearing is morethan 10 times as long compared to the conventional
bearings subjected to the same demanding
conditions.

results in Fig. 8, the rotating torque with the UltraClean WB bearing is much lower than that with theconventional bearing (special PTFE-coated type).

Table 4 Test conditions of rotating torque
Deep groove ball bearing (#608) φ8×φ22×7 mm

Test bearing

3. Introduction to NTN Ultra-Clean WB
Bearings (Fig. 6)
Aware of the lubricating power of tungsten disulfide(hereafter, “WS2”), NTN adopted one rolling element
made of this material (Fig. 7) in the Ultra-Clean WB
bearing. When the WS2 rolling element rolls in the
bearing, WS2 is transferred onto the rolling surface
and functions as a solid lubricant. Additionally, sinceWS2 can withstand a maximum temperature of 400˚C,
which is much higher heat resistance compared toconventional special PTFE coating, the Ultra-Clean
WB bearing is an ideal choice for vacuum and hightemperature environments such as those on vacuum
evaporation systems and sputtering systems.

Degree of vacuum

10-5PaRunning speed

500 min-1

Temperature

Ordinary temperatureThrust load

196N

Rotating torque mN・m

25

20

15

5

0

Ultra-Clean
WB bearing

SpecialPTFE-coated
type

Fig. 8 Results of rotating torqueWS2 rolling element

3.1.2 Endurance
1) High-temperature (300˚C) endurance test
The applicable test conditions are summarized inTable 5, while the test results are illustrated in Fig. 9.
The life of the Ultra-Clean WB bearing is more than
five times as long as that of the conventional bearing(special PTFE-coated type).

Fig. 6 Ultra-Clean WB bearing
WB：WS2 Ball
Lubrication with tungsten disulfide ball

2) High-load endurance test
The test conditions used are summarized in Table
6, while the test results obtained are illustrated in Fig.
10. The life of the Ultra-Clean WB bearing is morethan nine times as long as that of the conventional
bearing (special PTFE-coated type).

4. Conclusion
Fig. 7 WS2 ball

Products incorporating flat panel displays, such as
personal computers and flat-panel TV sets, havebecome used more commonly, and demands are
growing rapidly for bearings capable of use in theclean-environment manufacturing systems for theseproducts. NTN believes that our Ultra-Clean NC
bearings and Ultra-Clean WB bearings, which areenhanced variants of conventional Ultra-Clean

3.1 Evaluation of various performance
considerations
3.1.1 Rotating torque
Under the test conditions summarized in Table 4,
the rotating torque of the Ultra-Clean WB bearing in a
vacuum was evaluated. As can be seen from the-82-

Ultra-Clean Bearing for Clean Environments

Table 5 Test conditions

Table 6 Test conditions
Deep groove ball bearing (#608) φ8×φ22×7 mm

Test bearing

Deep groove ball bearing (#608) φ8×φ22×7 mm

Test bearing
Degree of vacuum

10-5PaDegree of vacuum

10-5PaRunning speed

1,550 min-1

Running speed

500 min-1

Temperature

300˚C

Temperature

Ordinary temperature

9.8N

Thrust load

Thrust load
End of life criterion

256 N

End of life criterion

The point when a sudden increase in torque occurred

The point when a sudden increase in torque occurred

100000

10000

Cessation of test run
10000

Cessation of test run

Endurance hEndurance h

1000

100

10

1

1000
100
10
1

Ultra-Clean
WB bearing

SpecialPTFE-coated
type

Ultra-Clean
WB bearing

Fig. 9 Results of high temperature endurance test

SpecialPTFE-coated
type

Fig. 10 Results of heavy load endurance test

Table 7 Performance of Ultra-Clean bearing series
Ultra-Clean
bearing

Ultra-Clean
bearing

Ultra-Clean
NC bearing

Ultra-Clean
WB bearing

SpecialPTFE-coating

Low dust-generating
vacuum grease

Porous resin cage+
low vapor pressure oil

Tungsten disulfide(WS2)

Temperature range tolerance (˚C)

〜+260

〜+200

〜+150

〜+400

Degree of vacuum tolerance (Pa)-6

Lubrication system

10 〜atm-5

10 〜atm-5

10 〜atm

10-5〜atm

Load bearing capacity

△

◎

◎

◎

Low torque feature

○

△

◎

◎

Dust generation

◎

◎

◎

△

Corrosion resistance

△

△

△

△

◎：Optimal○：Good△：Acceptablebearings, will satisfy the characteristics required ofthese manufacturing systems: low dust generation,
tolerance of high vacuum conditions, and higher levelsof endurance and heat resistance. NTN has already
added new products to its Ultra-Clean bearing Series(see Table 7). We will continue to develop uniqueproducts that satisfy market needs, and we hope tocontribute to improvements in various vacuum
systems by helping extend their maintenance intervals
and reduce their energy consumption.

Photos of authors-83-

Ryouichi NAKAJIMA

Masaaki HONDA

Industrial Engineering Dept.
Industrial Sales headquarters

Industrial Engineering Dept.
Industrial Sales headquarters

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]Development of New RCT Bearing
for Axleboxes and Insulated Bearing with Shields

Ryutaro OKA*
In recent years railroad carriages have been increasingly
considered to be eco-friendly, energy efficient, and are viewed
as a good means of transportation. Demand for railroad
bearing business grows greatly in China and Europe.
High reliability is sought after in a bearing for railroad
applications because of public visibility. There is also a strong
demand from the railroad companies for longer time between
maintenance periods.
Therefore the problems facing railroad bearings is how toeffectively maintain reliability and extend the time between
maintenance periods. NTN has developed an Insulated Bearing with shields and the new RCT
Bearing, which could solve these problems and satisfy the Chinese and European markets.
This report discusses laboratory evaluation test results concerning the performance of Insulated
Bearings with shields and the new RCT Bearing.

the manufacturers of bearings for rolling stock now
have to address the challenge of realizing longer
maintenance intervals while maintaining high levels of
bearing reliability.
Recently, we have developed unique bearing
products to address these challenges and to fulfill theneeds of the very promising Chinese and European
markets. They are our new RCT bearings foraxleboxes, which is a line of high performance
axlebox bearings and our sealed insulated bearings
for traction motors. This report describes the results ofevaluation tests for our new RCT bearing foraxleboxes and our sealed insulated bearings.

1. Introduction
Recently, the usefulness of rolling stock has been
appreciated even more because trains are
increasingly regarded as an eco-friendly means oftransportation that excels in energy efficiency. In
particular, there have been serious efforts for theconstruction of intercity railway infrastructure and theintroduction of high-speed railway systems in China.
Furthermore, in the European market, which accounts
for 50% of the world’s railway equipment demand,higher train speeds are needed to accommodate therecent expansion of the EU. Thus, in these markets,
the demands for rolling stock bearings have been
increasing dramatically.
The typical bearings for rolling stock include traction
motor bearings for power transmission systems, drive
system bearings used in the gearboxes (drives) thattransmit the motor output to the axles, and axle
bearings that support the weights of the train cars.
Each of these bearings is a critical component whose
performance directly affects the operation of railroads,
and, therefore, they must all be highly reliable. At thesame time, the end users who operate the trains aredemanding longer maintenance intervals. Therefore,

2. New RCT Bearings
In the European rolling stock axlebox bearing market,
metric-series bearings have been used commonly, andsuch bearings are also often adopted in the Chineserolling stock market. We have recently developed a newline of RCT bearings for axleboxes that satisfy the EN
standards, which are commonly adopted in theEuropean and Chinese rolling stock markets.
*RCT bearings: sealed double-row tapered roller
bearings used on rolling stock axles.

*Industrial Engineering Dept.

-84-

Development of New RCT Bearing for Axleboxes and Insulated Bearing with Shields

Test conditions

2.1 Development of the New RCT Bearing
We incorporated the three new technologies below
in a conventional RCT bearing to develop our new
RCT bearing, and we subjected this new product line
to evaluation tests according to the relevant EN
standards. The features of the new technologies andthe test results are provided below:
1) Adoption of a resin cage2) Adoption of an anti-fretting plate3) Adoption of a light contact seal

Table 1 Condition of endurance test
Run pattern at 2125 min-1

Bearing speed (min-1)
Radial load (kN)

48.0±9.8

Axial load (kN)

0±14.7

Lubrication system

Grease lubrication

Run duration

Equivalent to 700,000 km of travel

Test results

2.2 Resin cage（wt%）

The resin cage is made of a special resin materialfeaturing impact resistance and is expected to provide
longer lubrication life and boast better mechanicalstrength.

Verification of the benefits of adopting a resin cage
1) Inhibition of the occurrence of wear dust(iron dust)
The occurrence of iron dust from a steel cage ingrease was compared with the occurrence from aresin cage. Consequently, we have verified that use ofa resin cage can effectively inhibit the occurrence ofion dust on running bearings.
Test bearing
Double row tapered roller bearing (120 ID x 220 OD
x 155/160mm)

Standard value of the Railway Technical ResearchInstitute of Japan (iron quantity 1.0%)
Iron quantity (wt%)
1.2
1.03
1.0
0.8
0.6
0.4
0.2
0.02

0

Resin cage

Sheet steel cage

Fig. 2 Quantity of iron in grease after endurance test

Resin cage

Light contact seal

Longer grease life andlighter weight

Lower bearing temperature

Soft, thin sheet(brass sheet with rubber lip)
Fretting control

Fig. 1 New RCT Bearing-85-

NTN TECHNICAL REVIEW No.74（2006）

2) Cage with improved mechanical strength
A fall impact test was performed and determinedthat the resin cage excels in mechanical strengthcompared to the current NTN steel plate cage type,which has been performing well and has been
accepted favorably in the bearing market.
Test conditions

2.3 Verification of the effectiveness of antifretting measures
The state of the backing rings after an endurance
test was investigated. As a result, we learned that ouranti-fretting measure is effective.
Test conditions
Table 3 Test condition of prevention of fretting test

Table 2 Condition of fall impact test

Bearing speed (min-1) 2125 (equivalent to train operation at 330 km/h)

Vibration acceleration (m/s2)

9800 (fall height: 80 mm)

Radial load (kN)

54.9

Fall cycle frequency (cpm)30

Axial load (kN)

6.5

Run duration (h)7273

Test results

Test bearing

Vibration-isolating
pneumatic spring

Fig. 3 Fall impact test machine

Backingring

Anti-fretting
backing ringStandardbacking ring

Fig. 5 Form of backing ring width after examination

Test results

2.4 Verification of the effectiveness of a light
contact seal

Test was suspended after 360,000 cycles

Number of fall cycles

400000Through rotation tests, the run temperature on thelight contact seal was compared with that of astandard seal (with spring). As a result, it was
discovered that compared with the standard seal, thelip temperature of the light contact seal is about 20°C
lower. (The temperature measurement point was
about 2 mm directly beneath the lip on the seal sliding
section.)

300000200000

Crack occurrence after 93,000 cycles

1000000

Steel cage

Resin cage

Fig. 4 Test result of fall impact test

Standard seal

Light contact seal

Fig. 6 Shape of oil seal-86-Development of New RCT Bearing for Axleboxes and Insulated Bearing with Shields

Test results

Test conditions
Table 4 Test condition of light contact seal
Radial load (kN)

68.3

Axial load (kN)

13.6

Table 7 Test result of EN standard
Bearing Bearing
1
2

Bearing speed (min-1)

2216 (equivalent to train operation at 330 km/h)

Run pattern

Repeated forward/backward running cycles(according to EN standard)

Peak value
Running-in
operation

Test results
Table 5 Test result of light contact seal
Outer ring Outer ring
A row
B row

Seal
A row

Seal
B row

AxleboxStandard Max
seal Ave

56.6

56.5

73.9

78.4

50.5

50.3

49.9

65.2

69.6

45.2

Light Max
contact
seal Ave

44.2

46.4

45.9

52.8

41.5

41.3

42.7

43.0

49.6

38.7

EN
standard

Inner ring

97.9

98.8

―

Outer ring

80.1

78.5

―
5°C or lower
for 2 hours

Value atstable state

Inner ring

45.7

47.9

Outer ring

37.3

36.8

Value atstable state(average value)

Inner ring

43.9

Outer ring

34.1

90 or lower

Endurance
Difference in outer ring temperature
test
between Bearing 1 and Bearing 2

15 or lower

20 or lower

Difference in outer ring temperature
between adjacent cycles

10 or lower

20 or lower

2.6 Summary

2.5 Result of EN standard running test

By adopting a resin cage, a light contact seal andan anti-fretting measure, NTN has realized a journalbearing that satisfies the EN standard and is capableof extending maintenance intervals while ensuring ahigh degree of reliability.

The samples were subjected to a running test that
conforms to the relevant EN standard. As a result, it
has been verified that the new RCT bearing satisfies
the EN standard.

3. Sealed Insulated Traction Motor
Bearing

Note: data converted to equivalent value in a 20˚C environment

In order to realize higher train speeds as well as topromote increased use of LRVs, electric motors thatare more compact are greatly needed to realize lighter
trains. At the same time, railway companies aredemanding extended maintenance intervals todecrease maintenance costs.
Currently, traction motor bearings are relubricatedafter every 600,000 to 900,000 km of operation. The
maintenance intervals of traction motors are usually
governed by the lubrication lives of the bearings, andcan vary depending on whether or not the traction
motor is a brushless design and whether or not the
lubricant is an insulating type. Therefore, until now,
long-life greases have been developed in an attempt
to extend lubrication life. Since bearing life is extended
only when the bearing is effectively lubricated for along period, the importance of the structure around thebearing, including the grease pockets, has been
attracting attention.
NTN has recently developed a sealed insulated
bearing that allows compact design and does notneed maintenance work for up to 1,200,000 operatingkm.

Fig. 7 Test machine for Journal bearing

Test conditions
Table 6 Test condition of EN standard
Endurance
test

Running-in operation
Bearing speed (rpm)

554

1108

1662

2216

2216

Train speed (km/h)

83

165

248

330

330

13.6

13.6

68.3

Radial load (kN)
Axial load (kN)

3.4

Cooling air speed (m/s)

Approx. 10 (0 when no rotation is in progress)

Run duration (h)6.8

24 for each step

10.2

2910

Axial loading: One loading cycle consists of a 5-second loading period and a
5-second no-loading period (bi-directional)

3.1 Features of sealed insulated bearings
The traction motors for rolling stock, such as thoseon Shinkansen high-speed trains, use insulated-87-

NTN TECHNICAL REVIEW No.74（2006）

bearings with outer rings that have insulation layers on
their outer circumferential surfaces and side faces.
The outer ring of the insulated bearing has an annularseal to provide grease pockets in order to extend the
lubrication life of the bearing.
To ensure insulation performance, the annular seal
is made of a resin, and its inner profile has a special
form to prevent grease leakage even underdemanding conditions of high temperature and severe
vibration. Compared with the conventional design, thesize of the new design is much more compact.

Test bearing

Vibrator

Fig. 9 Test machine with vibration
Grease pockets
Insulating
layer

Insulating
layer

Test results
Neither NU214 nor 6311 developed grease leakage
or loose seals.

3) Operation performance and endurance testi) Rapid acceleration test
The temperature characteristics of the test bearings
in rapid acceleration were determined. Consequently,
it has been verified that each test bearing was capableof problem-free operation without developing rapid
temperature rise or grease leakage.

Annular seals

New design

Conventional design

Fig. 8 Structure of sealed insulated bearing

3.2 Performance evaluation test
1) Insulation performance

Test bearings
NU214 and 6311

The effect that the seal groove formed on the outerring has on the insulation resistance performance wasinvestigated. As a result, it has been verified that theinsulation resistance with the new insulated bearing is1000 MΩ or higher (relative to an applied voltage of
500 V), which is equivalent to that of current ceramic
insulated bearings.

Test conditions
Table 9 Test condition of high speed
Bearing speed(run pattern)(min-1×min)
Radial load (kN)2) Rotation vibration test
The new insulated bearing was subjected to arotation vibration test. As a result, it has been verified
that under operation with vibration, the new bearing
does not develop grease leakage or a loose seal.

Test bearing

1800×60→6180×30→
8600×2 (The acceleration for all the run patterns
was 242min-1/s.)
2.6

Test bearing

Test bearings
NU214 and 6311
Test conditions
Table 8 Condition of rotation test with vibration
Bearing speed (min-1)

6500

Radial load (kN)

1.5

Ambient temperature (˚C)

25±5

Axlebox vibration acceleration (m/s2)
Vibration frequency (Hz)300
(Sweep between the upper and lower
frequencies in a 50-sec. period)

Fig. 10 Test machine for traction motor bearing-88-Development of New RCT Bearing for Axleboxes and Insulated Bearing with Shields

Test results
NU214

6311

Temperature increase on outer ring80

Bearing speed

60

×100mm-1

90

Bearing speed

×100mm-1

100

70

90

60
50
40

20
10
0
0

0.5

1

1.5

2

Bearing speed

70

40
30

Temperature increase on outer ring80

50
Temperature
increase ˚C

Temperature
increase ˚C

Bearing speed

100

30
20
10
0
0

0.5

Time h1

1.5

2Time hFig. 11 Result of high speed testii) Endurance test
An endurance test simulating 2,400,000 km of travel
was executed. The new insulated bearing did not
develop grease leakages or experience rapid
temperature increases. Deterioration of the bearing
and grease after the endurance test was minor, andthe bearing still remained fully operative.

4. Conclusion
NTN’s newly developed RCT bearings foraxleboxes and sealed insulated bearings have been
described and the results of tests with them havebeen presented.
The need for safer and more reliable rolling stock
will continue to grow. At the same time, railway
operators worldwide will further demand decreased
maintenance costs for their rolling stock.
NTN’s newly developed RCT bearings foraxleboxes and sealed insulated bearings describedabove constitute useful means for extending themaintenance intervals and improving the reliability of
the journal bearings that represent the most criticalrolling stock components.

Table 10 Result of Grease analysis after endurance test
SampleX-ray fluorescence
spectrometry (wt%)
Fe

Cu

NU214

0.03

6311
Fresh grease

Spreadingconsistency

Oil separation
ratio
(%)

0.02

200

6.2

0.05

−

220

3.2

0.00

0.00

250

−

150〜350

30 or lower

Temperature ˚CRailway Technical Research 0.5 or lower 0.3 or lower
Institute control value

Temperature onouter ring of 214

100
90
80
70
60

Temperature onouter ring of 6311

Room
temperature

References
1) Suzuki M. et al.: Degradation of Lubricants Used inElectric Cars

50
40
30
20
10
0
0

1000

2000

3000

4000

5000

6000

Photo of author

Duration hFig. 12 Result of endurance test

3.3 Summary
NTN has successfully developed a novel traction
motor bearing that boasts insulation resistance andhigh-speed running performance comparable toconventional ceramic insulated bearings and iscapable of longer maintenance intervals due to its
enclosed construction.

Ryutaro OKA
Industrial Sales
headquarters-89-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]High Load Capacity Cylindrical Roller Bearings

Takuya OZU*NTN has developed high load capacity cylindrical roller
bearings appropriate for gearbox of wind turbines and otherindustrial machinery.
By using "Keystones" instead of the conventional cage,
the new bearings have increased the load capacity without
reducing speed performance.
As a result, the new bearings have 1.5 times or more therating life of current caged bearings.

1. Introduction

2. Structure and Features

The gearbox on wind turbines is located high off theground and is therefore difficult to access for service.
Therefore, the bearings in the gearbox must provide
long life with a high degree of reliability. Even though
the size of wind energy farms has been growing inrecent years, demand for smaller, lighter nacelles
have been on the rise in order to mitigate loads on
towers. Given this, compact bearings for gearbox
applications that boast greater load carrying capacity
are greatly needed.
In response to these needs, NTN has developed aunique high load capacity cylindrical roller bearing
product (Fig.1) that incorporates keystones in place ofa cage. As a result, our new cylindrical roller bearing
product boasts a greater load bearing capacity without
loss in running performance. The rated life of the new
product is more than 1.5 times longer than that ofconventional caged cylindrical roller bearings.

In the new NTN design, resin-made keystones are
situated between the rollers to prevent roller-to-roller
contact. Since these keystones are independent ofeach other, tensile stress otherwise resulting fromroller-to-roller contact and separation does not affectthem. As a result, the size and number of the rollers
can be increased because the keystones can bethinner (Fig. 2).
In Table 1, the performance of the new design iscompared with that of a conventional design.

¡Longer life
The size and the number of the rollers are optimized
to enhance the load carrying capacity of the bearing.
As a result, a rated life that is more than 1.5 times aslong as that of a conventional caged bearing design.

¡Higher speed
Roller-to-roller contact occurring on full complement
roller bearings is avoided by use of a unique keystone
design. Through optimization of the shape and roller
guiding scheme with the keystones, limiting speed andsmearing resistance equivalent to a caged design
have been achieved.

*Industrial Engineering Dept. Industrial Sales headquarters-90-

High Load Capacity Cylindrical Roller Bearings

The size and number of rollers
cannot be increased because
the mechanical strength of thecage needs to be maintained.

Keystone (resin)New
designCage

Fullcomplement
roller

A roller comes into contact with an adjacent roller that is rotating in
the opposite direction, leading to greater frictional resistance.

Fig. 1 High Load Capacity Cylindrical Roller Bearings

Fig. 2 Comparison of structure with conventional CRB¡Handling ease

3. Evaluation Test (1)Even when the inner ring is removed, the rollers on
the keystone design do not disassemble. Thanks tothis feature, the inner ring and outer ring subassembly can be handled independently, leading toeasy bearing installation on machines.

The following tests were performed by assumingthat each test bearing was used on the output shaft ofa gearbox for a wind turbine.

3.1 Test bearings
In general, the non-locating bearing on the output
shaft is often an NU or NJ cylindrical roller bearing
type, in the case the test bearings used were
NJ2324E. The new design tested is a prototype
identical to a conventional design, but that has anincreased number of rollers compared to theconventional design.
In Table 2, the technical data for the conventional
design (with a cage) is compared to that of the new
design.

Table 1 Comparison of performance with conventional CRB
Conventional
designwith cage

Fullcomplement
rollerType

New design

Bearing size (mm)φ150×φ320×108( equivalent to NJ2330E)

Dynamic rating life (kN)

1330

1160

1410

Static rating life (kN)

1740

1600

1870

Rating life ratio

1.6

1

1.9

Allowable running speed(oil) min-1

2000

2000

1000

Table 2 Test bearings
Conventional design(machined cage)Type

New design(rolling element separators)

Appearance

φ120×φ260×86

Bearing size (mm)←φ38×62

Roller size (mm)←13

Number of rollers

15

-91-

NTN TECHNICAL REVIEW No.74（2006）

As can be seen from the test results plotted in Fig.
4 , the temperature rise on the outer ring was limited
to a stable level of approximately 15˚C. (Additionallysmearing did not occur during the test.)

3.2 Temperature increase comparison test
The running performance of the new design wascompared with that of the conventional design interms of temperature increase.
As shown in Fig. 3, the temperature increase
pattern with the keystone design closely matches thatof the conventional design, and the new bearing wascapable of trouble-free operation up to 3500 min-1(dn/dmn = 420,000/670,000).

3.4 Oil-resistance test with separator pieces
To determine oil resistance resin test pieces madeof the same material as that used for the keystones
were immersed in the lubricating oils that are used inwind turbine gearboxes. The tensile stress of the test
pieces was taken and plotted in Fig. 5.
As can be seen from the data, the test pieces
immersed in each of the two lubricating oils did notexhibit significant losses in mechanical strength.

3.3 Acceleration/deceleration test
To evaluate resistance to the smearing that canoccur in non-load operation, the keystone bearing was
subjected to an acceleration/deceleration test under aset of low-load conditions.

Temperature rise on outer ring ˚C100

Test conditions
・Loading condition：
Radial load Fr= 98 kN・Rotation speed：
Up to 3500 min-1
・Lubricating oil：
Turbine VG56
・Lubricating system：
Circulating lubrication90

New design

80

Conventional design

70
60
50
40
30
20
10
0

0

1000

2000

3000

4000

-1

Rotation speed min

Fig. 3 Temperature rise

Temperature rise on outer ring ˚C

80

Test conditions
・Loading condition：Radial load
Load of housing weight(210 kg) only・Rotation speed：0⇔1800 min-1
・Lubricating oil：
Turbine VG56
・Lubricating system：
Circulating lubrication
・Run duration： 24 h

Run pattern

min-1
60

1800

5s
7s

12s

40
4s

0
20

0
0

5

10

15

Run duration hFig. 4 Accelerated test result

-92-

20

25

High Load Capacity Cylindrical Roller Bearings

Tensile strength MPa

80

Test conditions
・Test piece：JIS 7113
No. 1 dumbell test piece・Oil used for immersion：
Synthetic oil equivalent to VG320
from Company A
Synthetic oil equivalent to VG320
from Company B
・Immersion temperature：100˚C・Immersion duration：2000 hrs.

60

40

20

A
B

0
0

500

1000

1500

2000

Immersion duration hFig. 5 Oil resistance of separator

4. Evaluation Test (2)Planetary gear

Additional tests were performed with an N-typecylindrical roller bearing (a type that has a doubleribbed inner ring). This type of bearing is often used inplanetary gear applications.
On certain planetary gears, the bore surface of thegear may be used as a raceway surface for the outerring. With this type of arrangement, an N-typecylindrical roller bearing that lacks an outer ring isoften used (Fig. 6). Bearings for planetary gearing areusually used with their outer rings allowed to rotate.
However, for the these tests, the inner ring wasallowed to rotate.

4.1 Test bearings
In Table 3, the technical data for the conventional
design (full complement roller bearing) is comparedwith that for the new keystone design.

Fig. 6 Example of using N type

Table 3 Test bearings
Conventional design(full complement roller bearing)Type

New design(complete with rolling element separator assembly)

Appearance

φ101.6×φ190.2×76.6

Bearing size mm←

φ29.5×60

Roller size mm←

17

Number of rollers

16

-93-

NTN TECHNICAL REVIEW No.74（2006）

conventional design. There was an approximately
10°C maximum difference in temperature increase
between the new and conventional designs. In
addition, after undergoing the test. The conventional
design exhibited smearing, something not apparent in
the keystone design (Fig. 8).

4.2 Temperature rise comparison test

Test conditions
・Loading condition：
Radial load Fr= 69 kN・Rotation speed：
Up to 2000 min-1
・Lubricating oil：
Turbine VG56
・Lubricating system：
Oil bath

Temperature rise on outer ring ˚CTo evaluate the running performance of the
keystone design, the temperature increasecharacteristics were compared to those of theconventional design.
As shown in Fig. 7, the temperature increase of the
keystone design was limited compared to the70

New design
Conventional
design60

Aborted when abnormal
vibration occurred.

50
40
30
20
10
0
0

500

1000

1500

Rotation speed min-1

Fig. 7 Temperature rise

Fig. 8 Bearing condition after test

-94-

2000

2500

High Load Capacity Cylindrical Roller Bearings

5. Conclusion
A unique high-load capacity cylindrical roller bearing
design incorporating keystones has been described,
and the results from evaluation tests with this new
design have been discussed. The author believes that
this new bearing design, which features theadvantages not only of bearings with cages but also
those of full complement roller bearings, will contributesignificantly to longer bearing life and improved
reliability for transmission systems.
The new high-load capacity cylindrical roller bearing
design can be applied to systems other than
gearboxes, so NTN will market this design as a highload capacity bearing product line.

Photo of author

Takuya OZUIndustrial Engineering Dept.
Industrial Sales headquarters-95-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]The Rust Guard Bearingsof Highly Corrosion-resistant Bearings

Takashi YAGI*The environment of bearings for steel mill machinery has
recently become more severe. For example, the time intervals
between machine maintenance has increased and thecomposition of the water used in the rolling process varies. Lastly,
there is a growing demand for long-life bearings for the steel millindustry. To combat these tough conditions, NTN has developed
the Rust Guard bearings of highly corrosion-resistant bearings.
This publication describes the Rust Guard bearings in detail.

1. Introduction
The steelmaking industry worldwide has recently
been undergoing a strong restructuring drive to ensure
the availability of iron resources, including iron ores,
and to prevent the oversupply of steel products in the
future. To accomplish this, the production lines inpresent-day steel mills are being used to their full
capacity, while extending maintenance intervals forsteel mill machinery in order to improve productivity.
The durability of the bearings used in various
applications in steel mills, such as the production andtransportation machinery for raw materials and steel
products, directly affects the productivity of suchmachinery. If a bearing failure occurs in an
unexpectedly short time (hereafter referred to as“premature failure”), a long time is necessary for
restoration and significant monetary loss results. NTN
has developed the RustGuard™ bearing line of highly
corrosion-resistant roll neck bearing products forrolling lines where damaged bearings can greatly
affect the operability of production lines. The goal indeveloping the RustGuard™ line is to prevent thepremature failure of bearings.

Photo 1 View of a Rolling Mill in the Rolling Process

Fig. 1 Cross-sectional View of a Roll-neck Bearing
on a Rolling Millsurface, and flaking that starts from the surface, inwhich flaking begins at damage on the outermost
raceway surface. Many cases of premature bearing
failures result from flaking that starts on the surface.
Therefore, flaking that starts on bearing surfaces mustbe actively inhibited to ensure reliable operation of therolling lines in steel mills.

2. Background for Developing
RustGuard™
Flaking, which is a typical bearing failure, can be
categorized into flaking that starts internally, in which
fissures start inside immediately below the raceway*Industrial Engineering Dept. Industrial Sales headquarters-96-

The Rust Guard Bearings of Highly Corrosion-resistant Bearings

The initial efforts to realize longer life for roll neckbearings have included an attempt to achieve a higher
bearing capacity by increasing the number of rollers
through adoption of a pin-type cage and efforts toprevent failure that starts internally by adopting
cleaner bearing materials. However, despite these
efforts, premature failures still occur frequently
because flaking that starts on the surface isunavoidable. A starting point for flaking could be arusty spot or dent resulting from the ingress of wateror foreign matter into the bearings, and the production
sites in steel mills are usually contaminated with muchwater (rolling water and cooling water) and foreign
matter (scale and the like). To address this problem,the lubricant and chock seal were improved, andsealed bearings have also been incorporated into
components such as the work rolls in cold rollingmachines. Despite these attempts, ingress of waterand foreign matter has been unavoidable.
NTN has already marketed the EA Bearings, which
are specially heat-treated products boasting limited
loss in life even in contaminated conditions. This
product line has attained a good reputation, and, when
installed on a hot rolling machine, this bearing typehas achieved L10 life three times as long comparedwith conventional bearings.
Incidentally, the problem known as “downtimeinduced rust” (Photo 2) should be noted. This resultsfrom the ingress of water into the bearing and occurs
when the bearing is at a standstill while the rolls arebeing polished or inspected. To address this type ofrusting, NTN developed a special phosphatingprocess (hereafter referred to as the “previous
specification”) and has adopted this technique in
conjunction with EA Bearings on actual steel millmachinery, verifying its effectiveness.
However, despite our efforts, the need to further
improve the corrosion resistance of bearings has been
mounting because much longer maintenance intervals
are needed at the workplaces of steel manufacturers.

3. Features of RustGuard
The author’s newly developed RustGuard coating isa unique type of coating with improved corrosion
resistance that was developed through further
improvement of the previous special phosphatebearing coating (previous specification).
A phosphate coating is a type of a chemical
conversion coating, in which chemical reactions withthe compositions of the steel base material form
crystals and an accumulation of these crystals creates
the phosphate coating. Generally, phosphate coating
is used to provide the initial sliding fit for sliding
sections or to prepare a base for the application ofpaint. The purpose of phosphating on the bore surfaceof the inner ring of ordinary roll neck bearings is toprovide initial sliding fit for sliding sections.
Capable of retaining lubricating oil between its
crystals due to the capillary effect, the phosphatecoating also provides corrosion resistance. However,
because the base material is somewhat attacked
during the initial coating formation process, theroughness of the base material surface (boundary withthe coating) will deteriorate and can lead to loss inrolling life even though the oil retention capability ofthe base material will be improved. Therefore, theordinary phosphate coating cannot be applied to the
raceway surface of a bearing.
To address this problem, NTN has long been
applying a special phosphate coating that does notlead to loss in rolling life by employing a special
miniaturization technology. Photo 3 shows rollers of
the previous specification that underwent operation fora duration equivalent to five times as long as thecalculated life L10 and new non-treated rollers. Bothsamples were partially submerged for 24 hours in tap
water. From the state of rusting at the water surface ofeach sample, it can be seen that the rollers of theprevious specification maintained corrosion resistanceafter use.

Flaking resulting fromdowntime-induced rust

Downtimeinduced rust

Photo 2 Examples of Downtime-Induced Rust

-97-

NTN TECHNICAL REVIEW No.74（2006）

Very limited rusting at the water surface

Significant rusting at the water surface

Test conditions
Bearing: 22318
Load: Fr= 0.3 Cr
Running speed: 2500 min-1
Test duration: 1670 hours(Five times L10 time)
Non-treated roller(new)Previous specification roller(after rolling operation of 1670 hours)

Photo 3 The rollers were soaked in water for 24hours

Incidentally, the previous specification was, inprinciple based on compatibility with bearing steel,
and was not an optimal specification for the casehardening steel used for roll neck bearings. Therefore,
we improved the previous specification, anddetermined the optimal crystal size throughconsideration of compatibility with components of casehardening steel. By using a special miniaturization
technology, we successfully developed the RustGuard
coating that boasts much improved corrosion
resistance.

protective coating is worn down when the bearing isrunning, and rust occurs on the rollers when the
bearing is at a standstill. The author’s corrosion
resistance test is intended to simulate use with actual
steel mill machinery, so the test pieces were allowed
to stand in a rust-promoting environment after
operation, and then their rolling life was evaluated.
The author has a wide range of experience withinvestigations into bearings that have been used invarious actual steel mill machines. Based on thisexperience, the author set up a rust-promoting
environment for simulation so that the severity of ruston the previous specification test pieces was
4. Corrosion Resistance Test
equivalent to that on the samples from actual steel millmachines operated in rather severe operating
On actual steel mill machinery, each bearing goesconditions (Fig. 2).
through a repeated cycle of running and stopping. The
As a result of the corrosion resistance test, theauthor’s new design exhibited a
Based on the findings of the investigation into the bearings used on the actual steel mill machinery,smaller variation in life and boasted aadjustment was made so that the severity of rust on the previous specification rollers was the same
longer life with an L10 that is 3.5 times
level as with the rollers on the actual steel mill machinery.
greater than the previous specification.
1 Rolling: 180 min
2 Standing in rust-causing
3 Rolling: evaluation
Incidentally, in a clean environment,
4155 MPa (line contact)environment
4155 MPa (line contact)
Number of loading cycles:
Number of loading cycles:
Immersion in 0.1% aqueous
RustGuard samples have exhibited
20400 cpm
NaCl solution (40˚C) for 300 min.
20400 cpm
lifespans virtually equivalent to thoseN308 (special)
Upper rollof the previous specification samples.
Lubrication

6306

NU306

Test piece
Dia. 12 x 2L
(mm)

Driving rollSupport roll
CylinderDia. 20 x 20L(mm)

Surface pressure:4155 MPa (line contact)
Loading frequency:
20400 cpm
Treatment same for both dia. 12
and dia. 20 (mm)n= 6Material: Case hardening steel

Cumulative failure probability (%)

Lower roll
99
80

RustGuard™
L10 = 7.4 x 107 cycles
L50 = 8.8 x 107 cycles

50
20
10

Previous specification
L10 = 2.1 x 107 cycles
L50 = 3.2 x 107 cycles

5

1
106

107

108

109

Life (number of loading cycles)

Fig. 2 Bearing failure mode simulation-98-

5. Anti-Seizure Property
Confirmation Test
Other than the downtime-induced
rusting, the typical causes ofpremature bearing failure on actual
steel mill machinery include lubrication
failure and seizure caused by a failed
lubrication system (for example, aclogged nozzle on an oil-mist
lubrication system).
Boasting good oil retention andcorrosion resistance quality, theRustGuard coating does not readily
coagulate.
Fig. 3 summarizes the results of a

The Rust Guard Bearings of Highly Corrosion-resistant Bearings

Operating conditions
Bearing: 4T-30309D
Load: Fr= 8 kN
Running speed: 1000min-1
Lubrication: Application of trace amount of VG320

140

Run time min

120
100

Duration after
temperature increase
Duration before
temperature increase

Seizure did not occur

80
60

Seizure40
20
0

Post-test photo: non-treated rollers

Post-test photo: RustGuard rollers

Untreated

RustGuard

Fig. 3 Seizure resistance confirmation test

treated bearing products, the average life of
RustGuard bearing products used in this type of
application is sometimes more than four times as long.

comparison test that simulated an underlubricatedstate. After the start of operation, the temperatures of
the non-treated rollers and RustGuard rollers firststabilized and then increased. The non-treated rollers
seized after 5 minutes of temperature increase. In
contrast, the RustGuard rollers did not seize evenafter 60 minutes of temperature increase, showing
their good anti-seizure qualities.

7. Conclusion
The operating conditions for the roll neck bearings
on rolling machines in steel mill machinery are very
demanding, and can lead to bearing failures that startfrom the surface. We have developed andcommercialized the RustGuard product that actively
resists the occurrence of “downtime-induced rust,”
which is a typical cause of bearing failures that startfrom the surface. Furthermore, RustGuard also
features good anti-seizure properties. In summary, theRustGuard line of products is highly suitable to theneeds of steelmaking sites, including, for example, thestable operation of bearings in actual machinery.
NTN will remain committed to the development of
products that satisfy customer worksite needs.

6. Examples of Applications in Actual
Machinery
The major objectives of adopting the RustGuard
coating, though varying somewhat between different
steel manufacturer sites, include the extension ofmaintenance and replacement intervals, thesimplification of maintenance works and theachievement of more reliable steel-making operations.
Both in Japan and in other countries, RustGuard
bearings are used on work rolls and backup rolls incold rolling machines, in work rolls in hot rollingmachines and in guide rolls in continuous castingmachines.
The life (L10) of the RustGuard bearings in the workrolls of actual cold rolling machines has been twice aslong as that of the previous specification bearings. In
addition, compared to the non-treated bearings fromother bearing manufacturers, the bearing replacement
intervals with the previous specification bearings havebeen increased to 1.3 times as long, while those with
the RustGuard bearings have been extended to 1.7
times as long.
Incidentally, as with steel manufacturing works,
papermaking plants using previous specification
products (phosphate coating for bearing steel) havebeen experiencing problems of downtime-induced
rust, particularly with the suction rolls used in thepaper making process. Compared with the non-Photo of author

Takashi YAGI
Industrial Engineering Dept.
Industrial Sales headquarters-99-

NTN TECHNICAL REVIEW No.74（2006）[ New Product ]“SSB” of Sliding Aseismic Isolution Unit Multi-ply Rubber

Norio ITOH*
Takuya ISHII*

SSB with multi-ply rubber is newly developed seismic isolation
unit which has higher vertical rigidity and wider applicability than
SSB with a single ply rubber, by arranging the number of rubber
ply and thickness. New SSB is a low friction sliding seismic isolator
with a friction coefficient of 0.024 that is equivalent to the current
SSB. The expected applications are from low to high buildingssuch as condominiums, offices, factories, hospitals and schools,
which are not only newly built but also retrofitting existing ones.
This article introduces the characteristic of multi-ply rubber SSB(Ministry of Land, Infrastructure and Transport authorization number MVBR-0304) and basic
properties comparing with the current single layer rubber SSB which achieved business result.

Infrastructure and Transport certification number
MVBR-0304).

1. Introduction
Most of the damage resulting from an earthquake issecondary damage caused by collapsing buildings
and falling furniture. Since the Great Hanshin
Earthquake in 1995, many seismically isolated
apartment buildings and detached houses have been
built. The probability of another large earthquake inthe next 30 years, including powerful earthquakes
near the Tokai or Tonankai regions, in particular, isapproximately 60%. Given this, more seismicallyisolated apartment buildings and detached houses are
expected to be built. The seismically isolating
members used for these buildings can be categorized
into multi-ply rubber isolators, sliding/rolling bearingsand dampers. The sliding aseismic, multi-ply rubber
Super Sliding Bearing (SSB) developed by NTN is aunique sliding bearing product that has overcome thetechnical challenges of conventional sliding bearings
to provide stable performance for an extended periodof time at a lower friction coefficient. This paper,
through comparison with a conventional single-ply
rubber SSB product already on the market, describes
the features and basic characteristics of NTN’s new
multi-ply rubber SSB (Japanese Ministry of Land,

2. What are Seismically Isolated
Structures?
A seismically isolated structure can be defined as abuilding that is made resistant to earthquakes by
isolating it from the vibration of the ground rather thanincreasing the strength and rigidity of the building sothat it can withstand severe seismic vibrations. As
summarized in Fig. 11), compared with earthquake
resistant structures and vibration control structures,
Earthquake
resistant structure

Vibration control
structure

Seismicallyisolated structure

The building withstands
the seismic vibration
with its strength

An energy-absorbing
device absorbs seismic
vibration in order todampen the vibration
affecting the building

The building is supported
on a seismically isolating
device to isolate it fromseismic vibration

Fig.1 Concept of earthquake resistant structure,vibration control structure, and seismic isolated structure*Engineering Dept. NTN Engineering Plastics Corp.

-100-

“SSB” of Sliding Aseismic Isolution Unit Multi-ply Rubber

the severity of the seismic vibration is dampened byseismic isolation to prevent damage to the structuresand maintain the functionality and habitability of thebuildings.

schematically illustrated in Fig. 3, and the materials in
its major components are summarized in Table 1. The
SSB is comprised of a multi-ply rubber integrated
structure with laminated rubber and steel members, aswell as sliding members arranged in series. The SSB
has the following features.

3. Reasons Why Low-Friction Sliding
Bearings Are Needed for Buildings

Features of the multi-ply rubber SSB
1) Compared with a conventional single-ply rubber
SSB, the authors’ new design features a greaterrubber component shape factor (= rubber
diameter/4 x thickness of one ply of rubber). As aresult, the new design boasts greater verticalrigidity and smaller rubber deformation.
2) Compared with a conventional single-ply rubber
SSB, the NTN’s new design features a long-term
carrying load that is approximately twice as great.
In addition, the combination of the thickness ofeach rubber ply and the total number of rubber plies
can be altered arbitrarily. This feature allows
greater freedom in designing aseismic structures.
3) NTN’s new design involves a unique combinationthat is comprised of a sliding member made ofPTFE and a corresponding sliding plate made of

The time required for a building to vibrate once iscalled its natural period. For many seismic vibration
modes, it is known that the severity of the vibration isgreatest for periods of 1 second or less and that theseseismic vibration periods are approximately the sameas the natural periods of structures that are notseismically isolated. Thus, resonance vibration occurs
in these structures and consequently they can beseverely damaged. As schematically illustrated in Fig.
21),, SSBs can provide buildings with longer natural
vibration periods because of their low friction
coefficients. More specifically, compared toconventional sliding bearings made from a
combination of PTFE sliding material and a noncoated stainless steel plate, our SSBs succeeded in
reducing the vibrating force (seismic force) affecting
the building to approximately 1/3 of its original value.
As a result, since the sizes and numbers of pillars andbeams can be decreased, building designers cancreate larger dwelling spaces.

FlangeIntermediate steel plate
Rubber
Backing steel plate

FlangeIntermediate steel plate

Elastic
bearing

Sliding member

Rubber
Backing steel plate

4. Composition and Features of theMulti-Ply Rubber SSB

LengthSliding member

Sliding plate + coating

The composition of a multi-ply rubber SSB is

Width
Sliding plate mounting plate
Displacement and acceleration
are greater on the upper floors.

Displacement and acceleration
are virtually equal on all the floors.

Earthquake resistant structure(non-aseismic structure)
Natural period: 0.6 sec.

Seismically isolated structure
Conventional sliding bearing(μ: approx. 0.10)
Natural period: 2-4 sec.

Elastic bearing

Displacement and acceleration are reduced further.

Sliding plate + coating

Sliding plate mounting plate

Fig. 3 Outlined structure of SSB
Table 1 Material of major parts of SSB
Member

NTN part designation

Composition

SSB（μ=0.024）
Natural period: 4-6 sec.

Multi-ply rubber member

BEAREE ER4080

Natural rubber

Fig. 2 Comparison of vibration strength of the building
with seismic isolated structure

Sliding member

BEAREE FL3045

PTFE w/ filler

Coating member

BEAREE SP7001

Thermo-setting resin w/ filler-101-

NTN TECHNICAL REVIEW No.74（2006）

stainless steel (equivalent to JIS SUS-304). As aresult, the authors’ multi-ply rubber SSB has
achieved a friction coefficient of 0.024 against asurface pressure of 15 N/mm2. Consequently, theaseismic period of building structures with theauthors’ multi-ply SSBs can be made much longer(4 sec. or greater) and their response accelerations
can be much lower.
4) The rotation generated on the sliding bearings by,for example, the bending of beams in response toan earthquake or deformation caused byconstruction error or the creeping of beams is
mitigated by the multi-ply rubber. Furthermore,
NTN’s multi-ply rubber SSB is a seismicallyisolating member that smoothly changes mode fromelastic deformation of the multi-ply rubber to slidingin the event of an earthquake.
5) When a minor earthquake occurs, sliding does notoccur but elastic deformation of the multi-ply rubber
occurs. In a medium or larger earthquake, sliding of
the multi-ply rubber also occurs to dampen theseismic force.

SSBs are similar to those of multi-ply rubber isolators,
the design of seismically isolated building structures
can also be achieved more easily.

6. Dynamic Friction Coefficient
6.1 Comparison of a conventional sliding bearing
and the NTN’s multi-ply rubber SSB
The dynamic friction coefficient of a conventionalsliding bearing that combines a PTFE sliding member
with a non-coated stainless steel plate was comparedwith that of the multi-ply rubber SSB. The results
obtained are plotted in Figs. 5 and 6. Even though the
dynamic friction coefficients depend on surfacepressure and velocity, the dynamic friction coefficientsof NTN’s multi-ply rubber SSB are less than 1/3 to 1/5of those of the conventional sliding bearing.
As for the dynamic friction coefficients for theauthors’ multi-ply rubber SSBs, the authors haveobtained the surface pressure dependency formula

Test
conditions

5. Vertical Load Characteristics
The relationships between vertical displacement
and vertical load (600-mm diameter rubber) for themulti-ply rubber SSB and a conventional single-ply
rubber SSB are graphically plotted in Fig. 4. Since theshape factor for the multi-ply rubber SSB isapproximately three times greater than with the singleply rubber SSB, the vertical rigidity is approximately
three times as great and the rubber displacement isapproximately 1/3 with the multi-ply rubber SSB whencompared with the single-ply rubber SSB.
When sliding bearings are used on a seismicallyisolated building structure, they are adopted inconjunction with multi-ply rubber isolators that restore
the building to its original state. Since the verticalrigidity and rubber displacement of the multi-ply rubber

Dynamic friction coefficient

0.15
Conventional sliding bearing
0.12

Multi-ply rubber SSB

0.09

0.06

0.03
0
0

10

Test
conditions

Multi-ply rubber
Dynamic friction coefficient

Vertical load kN

2000
1000

1.5

50

60

70

Sliding member diameter: 400 mm
Excitation waveform: Triangular wave
Surface pressure: 15 N/mm2
Amplitude: ±150 mm
Number of excitations: 10
Conventional sliding bearing

3000

1.0

40

0.15

4000

0.5

30

Fig. 5 Relation of the friction coefficient vs. pressure

Single-ply rubber

0
0.0

20

Surface pressure N/mm2

6000
5000

Sliding member diameter: 400 mm
Excitation waveform: Triangular wave
Velocity: 150 mm/sec
Amplitude: ±150 mm
Number of excitations: 10

2.0

2.5

Multi-ply rubber SSB

0.12

0.09

0.06

0.03

0
0

Vertical displacement mm

100

200

300

400

500

Velocity mm/s

Fig. 4 Relation between vertical displacement andvertical load of a multi-ply rubber and a single ply rubber

Fig. 6 Relation of the friction coefficient vs. velocity-102-

“SSB” of Sliding Aseismic Isolution Unit Multi-ply Rubberand velocity dependency formula below based on thedata from their experiments, and these are being
considered in the structural design of seismicallyisolated building structures.
Surface pressure dependency formula (for velocity
15 mm/s)
μ= 0.0618σ-0.351
where, σ: surface pressure (N/mm2)
Velocity dependency formula (for surface pressure
15 N/mm2)
μ= 0.0173 (V/10)0.132 where, V: velocity (mm/s)

8. Conclusion
The NTN R&D Center, which is shown in Photo 1,
is a typical example of a seismically isolated building
structure supported by the authors’ SSB, and Photo 2
is a view of an installed SSB. Capable of withstanding
an earthquake as severe as the Hanshin Earthquake
level, the NTN R&D Center is expected to function as
a disaster relief center for the NTN Iwata Works. NTN
remains committed to improving the reliability of ourmulti-ply rubber SSBs. We have been providing ourcustomers with a full line of products that are capableof withstanding conditions in a variety of seismic load
isolating applications. The NTN believes that theirproducts can contribute to saving human lives andprotecting property during seismic disasters.

6.2 Durability
The durability of the multi-ply rubber SSB isindicated in Fig. 7 in terms of the trend with the friction
coefficients of the authors’ multi-ply rubber SSB. The
grouping of the friction coefficient values is fairly
stable. Therefore, a total sliding distance of 180 m fora seismically isolating device appears to be sufficient.

Test
conditions

Sliding member diameter: 400 mm
Surface pressure: 15 N/mm2
Excitation waveform: Triangular wave
Velocity: 150 mm/sec

Dynamic friction coefficient

0.15

0.12

Photo 1 Appearance of NTN R&D center

0.09

Building structure
0.06

SSB

0.03
0.00

0

30

60

90

120

150

180

Sliding distance m

Fig. 7 Durability of the friction coefficientFoundation

Photo 2 Installed SSB

7. Multi-Ply Rubber SSB Variations

References
1) Multi-ply Rubber for Seismic Isolation Editorial
Committee: Handbook of Multi-ply Rubber for
Seismic Isolation, Rikoh Tosho. (In Japanese)

Each building structure needs to cope with uniquecolumn loading considerations. Thus, NTN offers ninestandard multi-ply rubber SSB types with bearing
diameters ranging from 300 to 1300 mm.

Photos of authors
Table 2 Specification of multi-ply rubber SSB standard series
Bearing diameter (mm)Long-term bearing load (kN)
Friction coefficient
Variation in friction coefficients (%)
Horizontal rigidity (kN/m)
Variation in horizontal rigidity (%)
Vertical rigidity (MN/m)

300〜1300
1,060〜19,910
0.024(15N/mm2 standard surface pressure)±30
4,712〜22,122±20

Norio ITOH

Takuya ISHII

4,607〜30,222

Engineering Dept.
NTN Engineering Plastics Corp.

Engineering Dept.
NTN Engineering Plastics Corp.

-103-

NTN TECHNICAL REVIEW No. 74（2006）

Introduction of an Award-Winning Product2005 Monozukuri Buhin Taisho Grand Prix Machine Component Award

Monodrive Two-Way Feeder

Toshio OGUSU*The Monodrive Two-Way Feeder is a parts feeder
where parts can be moved in two directions (the line
supply side and the other side) by one driving unit.
This reduces the driving unit and controller set fromtwo pieces to only one piece. Therefore, the feeder
becomes smaller and lighter, with reductions in power
consumption, cost, and maintenance.

Shimbun, Ltd., a leading Japanese industrial paper.
This award was given to outstanding machinecomponents among those developed or brought tomarket in the 2005 business year. In this report, theauthor would like to introduce this unique product tothe readers.

1. Introduction
NTN’s Monodrive Two-Way Feeder is a uniquecompact vibratory parts feeder optimized for feeding
many part types in smaller lots, and boasting greatly
decreased floor space requirements compared withconventional bowl-type parts feeders.
As many bulk parts and components are now
manufactured in plants outside Japan, automatic
assembly lines in Japan are increasingly required tobe capable of producing more product types in smaller
lot sizes, thereby demands are growing for lessexpensive, more compact parts feeding devices. So
far, the conventional parts feeder systems used forthis purpose have generally consisted of a cylindricalbowl feeder type that aligns the parts by vibration anda linear feeder type that feeds the parts to theassemblers. In this arrangement, the floor spacerequired for the bowl feeder section is often great.
Therefore, believing that a unique driving unit capableof the entire process of parts alignment and feeding
would dramatically raise user satisfaction, the author
began his commitment to the development andcommercial release of the Monodrive Two-Way
Feeder™ product.
The design concept and novel operating principle ofthis product have been highly evaluated, and it evenreceived the 2005 Monozukuri Buhin Taisho Machine
Component Award sponsored by Nikkan Kogyo

2. Overview of the Monodrive Two-Way
FeederPhoto 1 shows the appearance of the author’s
monodrive two-way feeder.
In Table 1, the features of the author’s monodrive
two-way feeder design are compared with those of
conventional compact vibratory parts feeder systems.
Various compact parts feeder designs that do not
employ bowls have been available (Table 1).
However, these products have drawbacks, including
the need for two driving units and the difficulty ofadjustment of the entire system.
Requiring multiple assemblies, conventional parts
feeder systems that use two linear feeders do notsatisfy the needs for less expensive, compact parts
feeding systems. Therefore, the author tried various
ideas for parts feeding systems in which one linear
feeder unit could feed the parts. As a result of theseattempts, the author has incorporated a vibrationdirection switching leaf spring unit (Photo 2) into a
conventional linear feeder. Thus, the author has*Product Engineering Department Precision Equipment Division-104-

Monodrive Two-Way Feeder

Flow direction of workpieces

Alignment chute(aligns workpieces and supplies themto the next process)Driving unit (1 linear feeder)
Return trough(stores workpieces and supplies them to thealignment chute)

Controller: 1 unit

Photo 1 Monodrive Two-Way Feeder

Table 1 Space-saving parts feeding method (vibration type)

Conventional
designs

Type

SystemReturn feeder 1 Linear feeder x 2

Advantages

Disadvantages

Both feeders are controlled independently of the others

2 sets of drive units and controllers arenecessary

Return feeder 2

1 lower driving unit
+2 chutes

1 drive unit and 1 controller

Difficulty of adjustment (for resonance point with 2
vibration systems) System-specific control is impossible

Drum feeder

Rotary hopper+ linear feeder

Storage of workpieces

Difficulty in supplying workpieces
2 sets of drive units and controllers are necessary

Monodrive
two-way feeder

Linear feeder+ return chute

1 drive unit and 1 controller
Easy adjustment and maintenanceEasy storage and input of workpieces

System-specific control is impossible

Vibration-direction switching leaf spring(forward-backward)inWorkpieces

return troughWorkpieces in chuteLinear-feeder leaf spring (forward-backward)

Photo 2 Composition of leaf spring (Monodrive Two-Way Feeder)-105-

NTN TECHNICAL REVIEW No. 74（2006）

succeeded in generating vibration of not only thealignment and feeding section but also of the return
section with just one driving unit and one controller.
Furthermore, the vibration direction of the alignment
and feeding section is opposite the vibration directionof the return section.
The author has adopted a vibration-direction
switching leaf spring with rigidity in the leaf thickness
direction that is lower than the leaf spring rigidity of thelinear feeder. Consequently, the vibration in the leaf
spring thickness is dampened while only thelongitudinal vibration is utilized so that the vibrating
angle with the linear feeder (alignment feeder) isconverted to the opposite direction, which isnecessary for the driving motion on the return side.

is a lighter parts feeding system that contributes toreduced energy consumption. Compared with the
combination of a conventional bowl feeder and alinear feeder, both the weight and energy
consumption of the author’s system are less than
1/3 (data by NTN).
4) Simple construction makes it an optimal choice
for multiple product type and small-lot
production
The author’s feeder arrangement is very simple. It
essentially consists of an ordinary linear parts
feeder with the addition of a vibration direction
switching leaf spring unit, and it boasts excellent
maintainability. The author’s system is capable ofhandling a variety of workpiece types just by
mounting the relevant chutes.
5) Possibility of CAD design for the tooling(alignment section)
Three-dimensional tooling efforts such as thoseneeded for conventional bowl feeder systems areusually not necessary with the author’s system.
Thus, the tooling on the author’s feeder system canbe readily designed using a CAD system.

2.1 Advantages
The author’s monodrive two-way feeder boasts thefollowing advantages.
1) One linear feeder propels transfer chutes in two
directions—the alignment/supply side and thereturn side (patent pending)
The completely new operation principle combines
the resonance phenomenon, which has been usedcommonly, with forced vibration movement.
2) Reduction of installation space
The greatest advantage of the author’s parts feeder
is the reduction of installation space. Comparedwith the combination of a conventional bowl feeder
and a linear feeder, the floor space requirement for
the author’s design is approximately 1/2 or less(Fig. 1).
3) Energy conservation and lighter weight
The author’s linear feeder system is capable ofparts storage, alignment and supply, and therefore

2.2 Technical data
The main technical specifications of the monodrive
two-way feeder are given below.
¡Models: Three models—MD10, MD20 and MD30
¡Size
MD10: 366L x 120W x 189H (mm)MD20: 458L x 166W x 229H (mm)MD30: 663L x 205W x 342H (mm)*The size of the alignment chute is not included.
¡Mass of feeder
MD10: 9 kg (The chute is not included.)Bowl-type(conventional design)

Monodrive
two-way feeder

Fig. 1 Comparison of space-saving-106-

Monodrive two-way feeder

Bowl-type (conventional design)

Monodrive Two-Way Feeder

MD20: 17.5 kg (The chute is not included.)
MD30: 49 kg (The chute is not included.)¡Return trough inclination angle:
5˚ as standard (variable in a range of 4˚ to 7.5˚)¡Length of the chute mounted (additional mass)
MD10: 450-600 mm (1.1 kg)
MD20: 550-800 mm (2.0 kg)
MD30: 850-1100 mm (7.0 kg)¡Applicable controller: K-ECA46 (or K-ECA49)¡Suitable workpiece types:
Small/medium-sized parts (for machines,
electronics and plastics)
Bolts, pins, lightweight parts
Moderately oily parts and various other workpiecetypes

the author’s monodrive two-way feeder fulfills userneeds for reduced floor space requirements, lighter
weight, economy in energy consumption and resource
conservation.
Each component of the author’s feeder, including
the tooling, is very simple. Consequently, unlike
conventional “handmade” parts feeders, the author’sparts feeder boasts unprecedented quality andstability. As a result, the author’s novel parts feeder
has been highly evaluated for new applications and in
new industrial fields.
So far, we have brought three medium and large
linear feeder product models to market. In the future,
we would like to expand the range of this uniqueproduct by adding, small-sized models and specialpurpose feeders to best meet different user
requirements.

3. Conclusion
The author’s monodrive two-way feeder describedabove features a very simple construction comprisedof one driving unit and one controller, and is still
capable of the functions of storage, alignment andsupply necessary for any parts feeder. In summary,References
1) T. Ogusu: Automation Advancement, Vol. 35, No. 1,(2006). (In Japanese)

Photo of author

Toshio OGUSU
Product Engineering Department
Precision Equipment Division-107-

NTN TECHNICAL REVIEW No. 74（2006）

Introduction of an Award-Winning Product2005 Technology Prize of the Japanese Society of Tribologists"Hybrid Bearphite" a Sintered Metal Sliding Bearing
Inserted with Plastic
Masaki EGAMI*

This report introduces the new "Hybrid Bearphite" sinteredmetal sliding bearing with a plastic sliding material on its innersurface. Because the plastic layer is only 0.25 mm thick,temperature changes give minimal dimension changes. Thisassures a higher precision over a wide temperature range (1060˚C). Furthermore, combining the plastic with a special filler
gives an extremely low friction coefficient of 0.05 (1/2 thefriction coefficient of conventional plastic sliding bearings),
resulting in low rotational torque.
This new sliding bearing "Hybrid Bearphite" received theJapanese Society of Tribologists technology prize.

The Hybrid Bearphite line of products has achieved
the above-mentioned accuracy and lower runningtorque at a decreased cost. The Japanese Society of
Tribologists has evaluated this achievement andawarded their 2005 technology prize to NTN’s Hybrid
Bearphite.

1. Introduction
Our Hybrid Bearphite is a unique high-precision lowfriction sliding bearing that consists of a sintered metalbacking, and a bore surface of thinly formed resin
sliding material. 1) The features of the Hybrid Bearphite
are described below.

2. Development background1) Excellent running accuracy that is comparable
to sintered oil-impregnated bearings
This feature is accomplished with a thin insert
molding of resin on the bore surface of a sinteredmetal sliding bearing.
2) Lower friction (friction coefficient μ= 0.05)
A high-performance sliding resin is used as a
coating on the inner bore surface of the bearing.
This unique resin features a friction coefficient halfthat of conventional resin materials.
2) Our Hybrid Bearphite sintered metal sliding
bearing can be used in conjunction with soft
shaft materials such as aluminum.
Hybrid Bearphite products can readily be used toreplace rolling bearings due to their low prices. In
addition, they offer an ideal choice for a compact
sliding bearing design.

Rotary mechanisms on photocopying machines andprinters (Fig. 1) employ a variety of bearing types. In
this field of application, rolling bearings excel overusual sliding bearings in terms of running accuracy,
rotational torque and durability, but they are more
expensive. Furthermore, sintered oil-impregnatedbearings are less expensive compared to rolling
bearings and excel over resin bearings in terms of
running accuracy and rotational torque. On the other
hand, they accelerate the wear of the soft shafts(aluminum shafts, Ni-plated shafts, etc.) typically usedin printers and photocopying machines because they
are lubricated in boundary lubrication mode.
Incidentally, resin bearings are the least expensive,
and allow soft metal materials, including aluminum, tobe used as a shaft material. However, the materials*Elemental Technological R&D Center-108-

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

Fusing section

Photosensitive
drum bearing

Developing section
bearing

Paper stock travel

Transfer section
bearing
Transfer belt

Transport roller

Toner cartridge

Photosensitive drum
Imaging

Imaging

Imaging

Imaging

Fig. 1 Schematic of copying machines and printers

for resin bearings, used in this type of application, canhave disadvantages. During the molding process, theirshrinkage rate and coefficient of linear thermal
expansion are large and, as a result, their dimensional
accuracy and stability are poor.
Mechanisms, such as the photosensitive drum
whose running accuracy greatly affects print quality,
incorporate rolling bearings because rolling bearings
boast better functionality even though they are
relatively expensive. Concequently, there has been agrowing need for a bearing suitable for this applicationthat are both less expensive and more accurate.

Sintered metal alloy

Resin sliding material

Fig. 2 Concept of HYBRID BEARPHITE

3. Development of Hybrid Bearphitebacking. At the same time, through improvement of
the insert molding process, the thickness of the resin
layer is regulated to 0.25 mm so that the resin layer
features good dimensional accuracy and thedimensional stability of the resin layer is high relative
to temperature variation. If the end faces are eachprovided with a resin layer, then the sliding bearing
can bear not only a radial load but also an axial load.

3.1 Concept for the newly developed bearing
To satisfy the needs described in Section 2 , wedeveloped the Hybrid Bearphite, based on a conceptthat consists of the following parts. 1) An insert
molding technique is used to form a thin resin layer on
the inner bore surface of a metal backing to achieve
better dimensional accuracy. 2) The metal backing is
made of sintered metal alloy to ensure effective
bonding to the resin layer due to its anchoring effect.
3) The resin layer is made of a low friction sliding
material so that the rotational torque with the bearing
is low. The newly developed Hybrid Bearphite bearing
is schematically illustrated in Fig. 2.
A resin layer formed inside the bore surface of ametal member will usually develop peeling on themetal-resin interface due to shrinkage occurring frommolding operation with the resin.
With our Hybrid Bearphite sliding bearing, however,
the resin material is allowed to fill the pores in thesintered metal so that the resultant anchoring effecthelps prevent the resin layer from peeling off the metal

3.2 Development of resin sliding material
Since the main purpose of the Hybrid Bearphite
sliding bearing is the replacement of rolling bearings,
the friction on its sliding surface needs to be low.
However, the friction coefficient of any conventional
resin sliding material is 0.1 or higher,2) so such amaterial would not serve for this sliding bearing. To
address this problem, we attempted to develop a newresin sliding material with a friction coefficient half thatof conventional resin sliding materials.
The friction-wear characteristics of a resin areusually improved by blending in oil or a solid lubricant
such as polytetrafluoroethylene (PTFE) or graphite.
-109-

NTN TECHNICAL REVIEW No. 74（2006）

Use of oil is effective to decrease the friction
coefficient. Until now, NTN had been attempting tolower the friction coefficient for sliding bearings byusing an oil dispersion technique with a polymer alloy,
for example.3)
However, conventional techniques had limitations
because the addition of an excessive amount of oil
can lead to deterioration in the mechanical strengthand formability of a sliding material. Therefore, wedeveloped a technique that adopted porous silica asan oil retaining material (oil-impregnated silica).
Thereby, we attempted to increase the content of oil
included in the resin while maintaining goodmechanical characteristics and formability of theresultant sliding resin material.4)
Compositions and frictional characteristics ofmaterials prepared by blending different proportions ofoil-impregnated silica into ultrahigh-molecular-weight
polyethylene (UHMWPE) are summarized in Table 1
and Fig. 3. Compared with the material blended withoil alone 2, the material blended with oil-impregnatedsilica 3 exhibited a lower dynamic friction coefficientthough its oil content was equal to that of material 2.
With an increase in the amount of oil-impregnatedsilica (material 4), the dynamic friction coefficient
further decreased, reaching a level lower than a third
the level obtained with a PTFE-based sliding material.
Having undergone the test, the sliding surface ofmaterial 4 was inspected, and the oil-impregnatedsilica exposed on the sliding surface was found as aresult (Fig. 4). In addition, the static oil seepage frommaterial 4 at room temperature and 15 MPa conditions
was measured. As shown in Fig. 5, the oil seepage
increased as the time elapsed and reached 6% in 100
hours. Assuming that only the oil near the surface layer
seeped out this level of leakage is too great.
From these findings, we estimated the low friction
mechanism of this material as shown in Fig. 6. Inother words, we believe the following theory about themechanism. Since the oil-impregnated silica serves as
the load bearing point, solid-to-solid contact isinhibited, making it easier for an oil film to form.

Oil-impregnatedsilica10μm

Fig. 4 Surface of the material 4 after sliding test

Seepage wt%

8.00

Table. 1 Composition of test specimens for sliding tests
Material

Composition (vol%)1

UHMWPE (100)2

UHMWPE (85)／Silicone oil (15)3

UHMWPE (82)／Oil-impregnated silica＊(18)4Pressure: 15 MPa
Room temperature

6.00
4.00
2.00
0.00
0

20

40

60

UHMWPE (70)／Oil-impregnated silica＊(30)Dynamic friction coefficient

0.30
0.25
0.20

100

Fig. 5 Relationship between loading time and oozed oilamount from the material 4＊1:5 porous silica to silicone oil ratio

Two test runs

80

Time hTest rig: oscillating ring-on-disk
Specific pressure: 15MPa
Velocity: 0.1m/s
Counterpart material: SUS304 (Ra 0.1μm)
Characteristic to be measured:dynamic friction coefficient 5 minutes later

Resin material

Transfer of oil

Oil-impregnatedporous silica

Oil film

0.15
Silicone oil added
0.10

Porous silica added

Counterpart material

Increased amount added

0.05

Operating mechanism of oil-impregnated silica (estimated)

0.00
UHMWPE100%

UHMWPE15%
silicone oil

UHMWPEUHMWPE
3% porous silica 5% porous silica
15% silicone oil 25% silicone oil

PTFE

Load-bearing point – decrease in true contact area
Oil supply source – decrease in shear strength

Fig. 3 Friction properties of the materials filled with oilimpregnated-silica

Low μFig. 6 Estimated mechanism of low friction of thedeveloped material-110-

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

Furthermore, the oil-impregnated silica particles linkwith each other and function as passages for the oil.
Through these passages, the oil inside the material iscontinuously supplied to the sliding surface, thereby
giving the material a stable low friction coefficient.
The resin sliding material 4 was modified so that its
melted viscosity was suitable for injection molding andwas adopted for the resin layer on the Hybrid
Bearphite.

resultant anchoring effect, the resin layer does notcome off the metal backing. Therefore, we attempted
to optimize the porosity ratio of the surface of thesintered metal as well as the injection molding
conditions.
Fig. 8 shows an example of optimization of thesurface porosity ratio with a sintered metal backing.
Fig. 8(a) illustrates a standard sintered metal for an
oil-impregnated sliding bearing. Fig. 8(b) provides anexample of optimization of the surface porosity ratio
through adjustment for grain size of the starting metal
powder and molding pressures. Both Fig. 8(a) and
Fig. 8(b) show the surfaces of the sintered metal prior
to insert-molding with a resin material (black areas
denote pores) as well as a view of the surfaces of thesintered metal after the resin layer formed by insertmolding had been stripped off by force. The surface
shown in Fig. 8(a) where the resin layer was stripped
off does not show traces of the previously depositedresin, suggesting that the resin did not fill the pores.
In contrast, with the example in Fig. 8(b), the resin
layer itself was split, and this suggests that thebonding of the resin layer to the sintered metal wasvery strong. As can be understood from this fact, thebonding strength of the sintered metal backing andresin layer is very high with our Hybrid Bearphite.

3.3 Insert injection molding to sintered metalbacking
The factors that help ensure high accuracy with theHybrid Bearphite sliding bearing are summarized inFig. 7. As already mentioned, if a resin layer is formedon the bore surface of a conventional metal member,peeling will unavoidably occur on the metal-resin
interface due to molding-induced shrinking with theresin material. For our Hybrid Bearphite sliding
bearing design, a resin material is allowed to fill thepores on the sintered metal so that, due to the

Sintered metal backing

Porosity ratio
Strength and rigidity

Resin sliding material

Linear expansion coefficient

4. Performance of Hybrid Bearphite

Melting viscosity
Injection-molding conditions

Fig. 9 shows the appearance of our Hybrid
Bearphite, which is essentially a sintered metal sliding
bearing with a resin insert. The example in this photo
has a resin layer on its side faces in order to be able
to bear not only a radial load but also an axial load.
The thickness of the resin layers on the bore surface
and end faces is 0.25 mm. The performance of ourHybrid Bearphite is described below.

Injection pressure
Die temperature
Die structure (gate)

DesignResin layer thickness

Fig. 7 Factors to achieve high precision

Surface of sintered metal
before insert molding

Surface with resin layer
stripped off(a) Standard sintered metal
Resin(b) Optimized sintered metal

Fig. 9 Appearance of HYBRID BEARPHITE

100μｍ

Fig. 8 Inner surface of sintered back metal-111-

NTN TECHNICAL REVIEW No. 74（2006）

4.1 Friction-wear characteristics

4.2 Dimensional accuracy

The Hybrid Bearphite, which is provided with theinsert-molded resin sliding material in its bore surface
was combined with an Ni-plated shaft and was run
under a radial load of 29.6 N and a running speed of
240 min-1. The resultant friction coefficient isgraphically plotted in Fig. 10. For comparison, thecharacteristics of a sintered oil-impregnated bearing(mineral oil-impregnated copper sintered bearing) arealso plotted in the same diagram.
The friction coefficient of the sintered metal oilimpregnated bearing was high during the earliest
stage of operation, but eventually decreased as
running-in proceeded, and finally became stable at
0.06. In contrast, the friction coefficient with the Hybrid
Bearphite always remained stable at about 0.06 evenat the earliest stage of operation. The wear with thesintered metal oil-impregnated bearing after 500 hoursof operation reached 10μm while the Hybrid
Bearphite was as small as 6μm. This means that thefriction-wear quality of the Hybrid Bearphite sliding
bearing is better than that of the sintered metal oilimpregnated bearing.
In a test with aluminum shafts, the sintered metal
oil-impregnated bearing caused the aluminum shaft tobe worn rapidly. In contrast, our Hybrid Bearphite
exhibited performance equivalent to that shown in Fig.
10.

The dimensional accuracy data and features of ourHybrid Bearphite are summarized in Table 2. The
Hybrid Bearphite boasts dimensional accuracy
comparable to that of a sintered oil-impregnatedbearing. Furthermore, since the resin sliding layer on
the Hybrid Bearphite expands and shrinks relative tothe reference surface, it features extremely small
dimensional variation due to temperature variations(Fig. 11). Note that greater dimensional variation witha bearing leads to larger shaft clearance, resulting in
greater variation in running accuracy.
For the Hybrid Bearphite with a bore diameter of 7.5mm, the resin layer thickness was adjusted to 0.25mm and the linear expansion coefficient of the resin
layer was assumed to be 14 x 10-5/˚C. Thus, thetemperature-dependent variation in the bore diameterof the Hybrid Bearphite was calculated (reference
temperature: 25˚C) and was compared (Fig. 12), withthat obtained from an ordinary resin sliding bearing
comprised solely of resin material. This diagram also
includes the actual measurement data at 60°C. The
steepness of the straight line representing theTable. 2 Dimensional properties and characteristics of
sliding bearings

Dynamic friction coefficient

0.25
Hybrid Bearphite

Radial load: 29.6 N
Running speed: 240 min-1
Counterpart shaft: Ni-plated

0.20

Sintered
oil-impregnatedbearing

0.15

0.05

Characteristic

HybridBearphite

Resin sliding
bearing

Sintered metal
oil-impregnated bearing

Bore diameter

±5μm

±15μm

±5μm

Outside diameter

±15μm

±15μm

±15μm

Bore roundness

10μm max.

50μm max.

10μm max.

Concentricity

15μm max.

30μm max.

15μm max.

Cylindricity

10μm max.

40μm max.

10μm max.

Dimensional variation

○

×

○

Sliding relative
to aluminum

○

○

×

Sliding relative to
SUM + Ni-plating

○

○

△

0.00
0

100

200

300

400

500

Duration h

Lowtemperature

Fig. 10 Friction property of HYBRID BEARPHITE

Reduction in sintered bore
Shrinking of resin layerHightemperature

Increase in sintered bore
Expansion of resin layer

Interface is firmly bonded
Reference surface for expansion and shrinking
Bearing with smaller change in bore diameter relative to variation in temperature

Fig. 11 Dimensional change of HYBRID BEARPHITE
against temperature change-112-

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

was very small. This means that the bonding is very
firm between the sintered metal and the resin materialof our Hybrid Bearphite.

Variation in bore diameter μm

50Bearing bore diameter: 7.5 mm
Resin linear expansion coefficient: 14×10-5/˚C
40

Ordinary
resin bearing
Hybrid Bearphite

30

4.4 Chemical attack resistanceResin materials have been used increasingly on
bearing housings in products such as officeautomation equipment. Because of the requirements
for satisfying dimensional accuracy and impact
strength, the resins often used on housings areamorphous resins that are not sufficiently oil-resistant.
Examples of such resins include polycarbonate (PC)and acryronitrile-butadiene-stryrene copolymer (ABS).
If an operating resin housing comes into contact withan inappropriate oil, it can develop cracks. This
problem is known as solvent cracking, and it occurs
when the compatibility of a resin with an oil is high.
This allows the oil to penetrate and disentangle thepolymer molecules that constitute the resin, causing
the resin to develop cracks and/or crazing.
Polycarbonate test pieces measuring 152 mm inlength, 12.5 mm in width and 6.35 mm in thickness
were prepared. Each oil was applied to the area of thetest piece where the maximum tensile stress occurs,
and a bending load was exerted for a specific duration(Fig. 13). Thus, the chemical attack quality of each oil
was evaluated.
The support distance adopted was 100 mm and thetest temperature was 75˚C. As can be seen in Table
4, silicone oil, which is included in the resin layer of

20

Actual measurementwith Hybrid Bearphite

10

0
25

35

45

55

65

Temperature ˚CFig. 12 Dimensional stability of HYBRID BEARPHITE

dimensional variation is governed by the design of theresin layer thickness. In other words, if the thicknessof a resin layer is determined through consideration of
the dimensional variation of the shaft, and theclearance between the shaft and the bearing can be
maintained at a constant dimension for a wider
temperature range. This is a revolutionary new feature
for resin bearings. Consequently, our Hybrid Bearphite
can realize higher running accuracy at a wider
temperature range, an achievement not previously
attained with conventional resin bearing technologies.

4.3 Bonding strength of the resin layer
The resin layer must be firmly bonded to thesintered metal in order to maintain a high level ofdimensional accuracy in a sintered metal sliding
bearing. The results of a heat shock test aresummarized in Table 3 to provide data about thebonding strength of the resin layer. In this test, theHybrid Bearphite sliding bearing was subjected to 100
heating-cooling cycles, with each cycle consisting of 1hour at -20˚C followed by 1 hour +60˚C. Through this,the dimensions before and after the heat shock test
and the bonding strengths between the sinteredmetals and the resin layers were inspected.
The dimensional variation occurring on our Hybrid
Bearphite sliding bearing after undergoing the test

Displacement
PC

Application of oil
Support distance：100ｍｍ

Fig. 13 Test rig for chemical attack

Table. 4 Chemical attack of oils for polycarbonate
Condition Displacement
Oil type

(mm)

1.5

3.0

Time (h)

3

3

3.5

3.5

3

196

○○○○○

No oilPolyethylene glycol ×××××
Polyol ester oil

Table. 3 Inner diameter change of HYBRID BEARPHITE
after 100 cycles of heat shock test
TP
1

Before testmm

After testmm

Variationμm

State ofbonding

7.501

7.500

1

No peeling

Ether oil (ADE oil)

×××××
○○×××

Mineral oil○○○○○ ○○×××

PAO＊○○○○○ ×××××

Silicone oil＊＊○○○○○ ○○○○○ ○○○○○

2

7.510

7.508

2

No peeling＊：polyαolefin oil＊＊：Oil blended in Hybrid Bearphite

3

7.508

7.504

3

No peeling

Symbols in the table: ○: no crack occurrence, ×: crack occurrence-113-

NTN TECHNICAL REVIEW No.74（2006）

the Hybrid Bearphite, does not damage thepolycarbonate by chemical attack.

5. Conclusion
Our Hybrid Bearphite, a sintered metal sliding
bearing with a resin insert, boasts higher accuracy
and lower torque at a decreased cost. This sliding
bearing will be used in a diverse variety of industrial
fields other than office automation equipment, and its
applications include the replacement of rolling
bearings as well as provision of improved functionality
for sliding bearings. We will remain committed to
improving the physical properties, including the heat
resistance, of Hybrid Bearphite in order to contributeactively to the development of compacter and lighter
designs that allow lower energy consumption andreduced costs for a variety of equipment.

4.5 Electrically conductive Hybrid Bearphite EC
Electrophotographic printers and copying machines
utilize static electricity to form an image and transfer
the toner to paper stocks. For this reason, if the
bearings on the photosensitive drum aretriboelectrically charged, the static electricity and toneron the photosensitive drum (Fig. 1) will be adversely
affected and the latent image on the photosensitive
drum will be disturbed. The static charge on certain
bearings other than those on the photosensitive drum
needs to be eliminated. This is achieved with agrounding mechanism, but provision of the grounding
mechanism causes the number of associated parts toincrease. Therefore, such bearings are sometimes
required to be electrically conductive.
Any ordinary resin material is electricallynonconductive and does not satisfy this requirement.
Therefore, to address this challenge, we developed
the electrically conductive Hybrid Bearphite EC byadding nanometer-sized carbon particles into the resin
sliding material . The volume resistivity of this resin is
3 x 103Ωcm, and this performance value is sufficientfor elimination of static charge from the bearing. Its
friction characteristics for sliding action with a Niplated shaft are summarized in Fig. 14. Addition ofcarbon particles caused the friction coefficient toincrease to approximately 0.1. However, this level ishalf or less compared with that of other electricallyconductive resin sliding materials available on themarket. Incidentally, the physical properties of theHybrid Bearphite EC, including chemical attack
resistance, are equivalent to those of the electricallyinsulating Hybrid Bearphite.

0.40

Bearing dimensions: ID 8 mm, width 5 mm
Radial load: 29.6 N
Rotation speed: 120 min-1
Counterpart shaft: Ni-plated

Friction coefficient

0.35
0.30

References
1) Egami, Shimizu, Ohira: Tribology Meeting Draft
Proceedings 2006-5 Tokyo (2006) p. 313. (InJapanese)
2) Egami, Hirose: Development of Plastic Sliding
Materials for OA Equipment, NTN Technical Review,
No.67 (1998) p.98. (In Japanese)3) Shimazu, Egami, Shimokusuzono: Tribology Meeting
Draft Proceedings 1998-10 Nagoya (1998) p. 379. (InJapanese)
4) Tsutsui, Egami, Ishii: Tribology Meeting Draft
Proceedings 2003-5 Tokyo (2003) p. 77. (InJapanese)

Conductive Hybrid
Bearphite EC
Hybrid Bearphite(non-conductive)
Resin sliding materialon the market

0.25
0.20
0.15
0.10
0.05
0.00
0

5

10

15

20

Test duration hFig. 14 Friction property of electrically conductive“HYBRID BEARPHITE EC”-114-

"Hybrid Bearphite" a Sintered Metal Sliding Bearing Inserted with Plastic

Photo of author

Masaki EGAMI
Elemental Technological
R&D Center-115-

NTN TECHNICAL REVIEW No.74（2006）[ New Products Information ]DOJ Compact High-Angle Constant Velocity Joint
Lightweight and compact with a maximum operating angle of 30˚DOJ

Features

Structure¡External dimensions: one size more compact(approximately 4%) compared to the current
30˚ DOJ¡Weight: approximately 6% lighter¡Strength and durability: improved actual
strength and durability through optimizationof track contact angle/contact ratio, and cageoffsetOne size more compact

Applications
Previous product¡SUVs and other vehicles that require greater
operating angles
Typical application-116-

New productNew Products Information

PTJ High-Angle Low-Vibration Constant Velocity Joint
The PTJ low-vibration constant velocity joint boasts the world’s highest level of NVH performance※1 andachieves a maximum operating angle of 30˚ for the first time in the world※2※1

Performance associated with vehicle noise and vibration.

※2

In the low-vibration tripod CVJ category

Features¡Enlarged operating angle range
Operation possible up to 30°
Operation at
maximum angle

low-vibration range
The roller cassette ismaintained parallel to theouter ring axis to ensure
low vibration.

Improved product:
approximately 17˚Improved product:
approximately 30˚Previous product:
approximately 15˚Previous product:
approximately26˚Structure

60
Average of current PTJ
30° PTJ95

50
40
30
20
10
0

0

2

4

6

8

Trunnion

Lower
cassette¡Crossover SUVs
and other vehicle
types that require
greater operating
angle and lower
vibration
Example ofapplication

-117-

10

12

Angle (deg)Applications
Outer ring

Outside diameter:same as the current PTJ

Comparison of induced thrust
between 30° PTN and current PTJ
Induced thrust 3rd component（N）rms¡Extended operating angle
range that ensures low vibration
From 15° to 17°
Operation in

Typical application

14

16

18

20

NTN TECHNICAL REVIEW No.74（2006）

Super-Wide Operating Angle (54˚) Fixed CVJ (TUJ)
θmax. = 54°)
The world’s first fixed CVJ capable of a super-wide operating angle (θResults of a case study that assumes a 4˚ increase in the operating angle with a fixed joint on an ordinary
medium-sized FF vehicle

Approximately 70 cm (13%) decrease in the minimum turning radius of the vehicle(when the wheelbase is the same as a conventional CVJ)

Approximately 40 cm (15%) increase in the wheelbase(when the minimum turning radius is the same as a conventional CVJ)

Example of use

Features¡Maximum operating angle (θmax): 54˚(current UJ: 50˚)¡Outer ring outside dia.: Same as with UJ¡Mechanical strength, durability: Equivalent tothose with UJ

Sliding constant velocity joint

Applications¡Large-sized and medium-sized FF and 4WD
vehicles boasting smaller minimum turning
radiusFixed constant velocity joint

Front axle

-118-

New Products Information

Ultra-Low Torque Tapered Roller Bearing
Significantly lower running torque(1/2 that of previous tapered roller bearing in the same class),
without loss in bearing life and rigidity

Performance

Features

Torque measurement result

27.25

1.2

Running torque (Nm)¡Realization of lower torque:
27.25
1/2 that of previous bearing¡Compact bearing size: 10%or greater reduction of
φ100
outside bearing diameter(same life and rigidity
compared to previous
Previousbearing)30309D model

φ90

φ45
New product

1
0.8
0.6
0.4
Previous productNew product0.2
0Engineering challenges and solutions

0

500

1000

1500

2000

Bearing speed (r/min)
Challenge 1: Realization of lower torqueMeans for realization: Implementation of
optimization in design

Running torque is 1/2 as low as the previous product at apractical speed range of 500–2000 r/min.

Adoption of a new cage form with reduced oil stirring
resistance

When lubricated with clean oil, the new product boasts
a bearing life 2.8 times as long as thatof the 4T product.

Axial displacement (μm)

Challenge 2: Maintain bearing life comparable Oil flow
to that of the previous bearing
Means for realization: Adoption of FA heat
treatment
Image of oil flow

Challenge 3: Maintain rigidity
comparable to that of theprevious bearing
Means for realization: Adoption ofa high load capacity design
The cage is situated near the outerring and a maximum possible numberof rollers are incorporated.

Bearing rigidity calculation result
40
35
30
25
20
15
10
5
0

Previous productNew product

0

5

10

15

20

25

30

Axial load (kN)Previous product

The new product has rigidity nearly equivalent to that of theprevious productNew product

Shape of cage pocket-119-

NTN TECHNICAL REVIEW No.74（2006）

High Load Capacity Tapered Hub Bearing for SUVs
Realization of 20% longer bearing life and
7% higher rigidity in the same amount of space
New productNarrowerPrevious product

Wider

Structure
Previous cageHigh load capacity cage

Enlarged view of rollers and cage

Enlarged view of rollers and cage

Enlarged view of rollers and cage

Enlarged view of rollers and cage(equivalent)

Cage
RollerClearance
between rollers

The number of rollers is nearly
equal to that of rollers used on
full complement bearings.

Features

A smaller distance between the cage and theouter ring helps realize reduced roller-to-roller
clearance without loss of the mechanicalstrength of the cage.

Features¡20% increase in bearing life (calculated life)¡7% increase in bearing rigidity¡Wheel bearings for SUVs

-120-

Clearance
between rollers(smaller)New Products Information

Hub Bearing with Built-In Low-Torque Hermetic Seal
Twice the sealing performance and a 15% reduction in running torque in thesame seal space through optimization of structure, form and rubber material

Features

Structure¡Sealing performance: twice the resistance tomuddy water¡ Lower torque: 15% decrease in runningtorque (30% decrease with seal alone)

Muddy water
resistance cycles

Muddy water test result

Twice
as great
Enlarged view of seal
Previous productNew productDual side lip(greater sealing
performance)

Previous productNew product

Torque test result

Side lip

Running torque

15% decreasein running torque

Radial lip(contact)

Seal
BearingPrevious productRadial lip (non-contact)
Adoption of low torque rubbermaterial (lower torque)

Applications¡Wheel bearings for passenger cars

New product-121-

NTN TECHNICAL REVIEW No.74（2006）

Hub Bearing with Built-In Active Sensor
No need for adjustment of sensor air gap or machining of knuckle

Features¡No need for adjustment of sensor air gap¡No need for machining work or assembly tosecure the sensor to the knuckle¡Protection against flying rocks and preventionof jamming by foreign objects

Means for realization
Conventionalarrangement

New product¡Sensor is built into the hub bearing¡Sensing face is surrounded by ametal core

Sensor is secured
to the knuckle.

KnuckleMetal core
SensorConstant
velocity joint
Magnetic encoder ring-122-

Air gapNew Products Information

Low-Emission Capable Laminated Conrod Module• Shell-type needle roller bearing that can operate under minimal lubrication conditions to help
comply with emission control• Realization of compact size with NTN’s unique laminated conrod• Proposal of total VA/VE with modular design involving conrod that incorporates the shell-type
needle roller bearings described above

Modular design

Thin sheets are pressed
into conrod shapes

Laminating by caulking

Press-fitting

Features

Shell-type needle roller bearings
capable of operation with minimal
lubrication

Applications¡Through optimized design of the cage usedon shell-type needle roller bearings and thehigher precision of the outer ring, this conrod
boasts excellent anti-seizure performanceeven under a severely reduced lubrication
condition that is four times as demanding as
conventional lubrication conditions (75%
reduction in the amount of mixed oil).
¡This conrod product features a uniquelaminated construction prepared by pressing
thin sheets. As a result, it is very compact,with a minimum width that is 17% (6% interms of outside dimensions) more compact
than conventional conrod products formed byfine blanking techniques.
¡NTN offers a modular conrod product that is
capable of operation with minimal lubrication.
This is prepared by press-fitting shell-type
needle roller bearings into its uniquelaminated conrod component. Being highly
functional and compact, this product also
serves for VA/VE applications.

¡2-cycle engines for agricultural machinery(pruning machines, spraying machines,
chainsaws, etc.)

Small-end
face-side bearing

Large-end
face-side bearing

ConrodCross-sectional view of a 2-cycle engine
for agricultural machinery-123-

NTN TECHNICAL REVIEW No.74（2006）

High-Speed, High-Quality Color Filter Repair System
Capable of coping with large-sized substrates,
the system features multi-functionality and advanced functions

Features

Advantages of the NTN’s unique applicator mechanism

1) Realization of quick repair

1 High-speed mechanism
specialized for application(for X1, X2, Y, and Z axes)¡A dedicated high-speed drive mechanism is provided for repair fluid
application¡A combination of a dedicated applicator needle and a repair fluid
container is provided for each repair fluid→No need to rinse the applicator needle; simpler color switching¡The tip of the applicator needle is refilled by repair fluid during
application→No need for separate repair fluid filling

The time needed for repair isreduced by the high-speed, highprecision drive motion of ultracompact linear motors.

q
Z axis
X1 axisOpticalobservation▼
70% reduction in repair timecompared to previous repair systems

Color switching without rinsing
and reduced repair time are
realized.

2) Realization of high-quality repair¡Each applicator needle has a specially treated tip. The
amount of the repair fluid applied is adjusted precisely
based on the time of contact between the color filter
substrate and the applicator needle

3 Unified support structure
for applicator needles andrepair fluid containers→Prevention of overflow of repair fluid to defect-free areas

4 Filler-applicator needle

Y axisResist (repair fluid)
containerw

Filling of applicator needles withrepair fluid is not necessary.

▼
Improvement in repair quality

The amount of liquid applied isadjusted in a range from a few
picoliters (one trillionth of liter) totens of picoliters. Repair quality isimproved because any defect canbe filled with a very small amountof the repair fluid.

Example of repair

Repair fluid
container
Applicator nozzler
Conventional
systemNew system

Coating is removed by lasers

X2 axis2 Provision of four applicator systemneedles
Applicator nozzle

Repair fluid is applied-124-

New Products Information

Advanced Functionality Controller for Parts Feeder
Power consumption by parts feeders can be reduced
by 30% compared with previous controllers

Features

Means for realization

1) Optimal vibration frequency automatic adjustment
function

1) The drive signal (voltage, current) to the vibrator
electromagnet is accurately measured [drive
signal measuring device]2) Based on the measured values, (i) deviation fromthe optimal vibration frequency and (ii) the amountof transportation (vibration amplitude) are
estimated [CPU]3) The electromagnet is energized based on thevibration frequency and amplitude ordered by the
CPU [inverter]→Automatic adjustment of optimal frequency andconstant amplitude functions are realized without
use of an external vibration sensor¡Automatic adjustment to the most efficient frequency→Approximately 30% energy conservation

2) Constant amplitude function allows parts to betransported in an optimal state.
¡Amplitude regulation: within 3% (under conditions that result in
amplitude variation of 10% or greater with previous controllers)

3) No need for a vibration sensor¡Work to mount a vibration sensor and lay wires is not necessary→The new controller can be easily installed to replace an older
controller¡No problems such as broken vibration sensor leads→Reduction of malfunctions

Amplitude regulation

Result of amplitude variation test relative
to variation in the mass of the load
10%
0%
-10%
-20%
-30%
-40%
-50%
-60%
-70%
-80%

Controller block diagram

Setting 100V
Control ON
Setting 80VControl ON
Setting 100V
Control OFF
Setting 80VControl OFF

New
system

Improvementq Drive signal measuring device
Vibration
Vibrator
electromagnet

w CPU

Previous
system

0

10

20

30

Parts feeder

Controller

40

Load mass (kg)e Inverter circuit-125-

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Printed in Japan

NTN TECHNICAL REVIEW No.74
Printed and published in November 1, 2006
EditorKenji Okada

Publisher Hitoshi Takai
Published by NTN Corporation
3-17, 1-chome, kyomachibori, Nishi-ku, Osaka,J apan
Printed by
NISSHA Co., LtdNo part of this publication may be reprinted or reproduced
without the prior permission of the copyright owner.