**表紙E_レイアウト 1 13/05/17 14:48 ページ 1

TECHNICAL REVIEW No.80

http://www.ntn.co.jp

Special Issue

corporation

Special Issue;
Environment and Energy
October 2012

Environment and Energy
corporation

This catalog is printed on recycled paper for environmental protection purposes.

ISSN 0915-0528 OSAKA, JAPAN

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The newly established renewable energy management
demonstration area promotes efficient, effective energy usage

Our Advanced Technology R&D Center in Kuwana City, Mie Prefecture, is not only working towards advancing
and developing cutting edge technologies, but is also improving and integrating core NTN technologies such as
tribology, raw materials, surface modification, composite materials, and simulation technology. The slogan“Advanced Technology = Global No. 1, Global First” promotes the creation of innovative technologies that lead tostate-of-the-art solutions and global environmental conservation, such as next-generation tribo-materials, highly
functional new materials, a sustainable energy utilization system, and an advanced computing scientific simulation.
Additionally, in December of last year, a “Natural Energy Management Demonstration Area” was established to startassessing energy-related systems. In this facility, new added tracking-type solar panels and an experimental greenhouse
have been installed. These come as an addition to existing solar and wind power generation facilities to form a micro
smart grid, which includes an electric vehicle charging station. A micro hydropower generation unit was also installed
in an adjacent park to conduct power generation experiments.
We will promote effective utilization of natural energy including development of various energy-related systems
around “technologies that support rotation,” which NTN has been fostering thus far, through cultivation of plants using
natural energy.
▼Advanced Technology R&D Centerand Wind Turbines▼Renewable Energy Management Demonstration Area

NTN TECHNICAL REVIEW No.80
Printed and published in JUN 1, 2013
Demonstration under way witha linear module installed (p.19)

EditorYoshinobu YASUDA

Publisher Koji TAKAHASHI
Published by NTN Corporation
3-17, 1-chome, kyomachibori, Nishi-ku, Osaka,J apan
Printed by
NISSHA Co., Ltd▲Micro-hydropower generator installed in the nearby park
(usually covered for safety as shown in the picture. p.6)

No part of this publication may be reprinted or reproduced
without the prior permission of the copyright owner.

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R

TECHNICAL REVIEW
No.80

Special Issue:
Environment and Energy

corporation

*01-目次-巻頭言E_*01/目次/巻頭言 13/05/17 18:05 ページ 2

NTN TECHNICAL REVIEW No.80
CONTENTS

Preface

Yoshinobu YASUDA

1

Sustainable Society Powered by Renewable Energy
Contribution- An Approach from Millennium Sustainability Studies Masao TAKANO Associate Professor, Nagoya University Graduate School of Environmental Studies

2● Special Issue for Environment and Energy Engineering
Market of Wind Power Generation Industry, Technology Trends and Bearing Technology of NTN
Michio HORI, Yusuke YAMADA and Masahiko KATAOKA

Application of Condition Monitoring System for Wind Turbines

8

15

Akitoshi TAKEUCHI, Takashi HASEBA and Hiroshi IKEDA

Motorized Linear Module for Tracking System of Solar Light / Solar Heat Power Generation
Tomoyuki OWADA and Masaki KAGAMI

Development of Induction Through-hardening and Induction Tempering Methods
with Temperature Control and Microstructual Control in Bearing Steel

19

23

Takumi FUJITA and Nobuyuki SUZUKI

Rolling Bearing for Environments of Ultrahigh Temperatures

31

Miki ARIHANA and Takayuki KAWAMURA● Special Issue for Manufacturing Technology "NTN supports the "Mono-Zukuri" manufacturing through the advanced technology"
Machine Tool Main Spindle Bearings with "Air Cooling Spacer"

38

Yuushi ONDA, Mamoru MIZUTANI and Masatsugu MORI

Parallel Link High Speed Angle Control Equuipment (PHACE)

42

Hiroshi ISOBE and Yukihiro NISHIO

Development of the Lubricating Oil Supply Unit with Self-generatig Power Supply
Kaoru OMOTO and Hiroyoshi ITO

NTN Linear Modules Series

48
52

Ulrich GIMPEL, Michael WILLE, Tomoyuki OWADA and Masaki KAGAMI● Technical Papers Technical Articles New Products
Evaluation of Local Mechanical Properties of High Strength Steels by Indentation Method
Takeshi OGAWA, Noriaki SAKANAKA and Yukio Matsubara

TMR : A New Frontier for Magnetic Sensing

58
64

Christophe DURET and Shintarou UENOLong Life Technology of Grease for Journal Bearing

72

Takamasa TANAKA and Hidenobu MIKAMI

Market Trend and High-Functionality of Wheel Bearings for Off-Highway Truck

78

Kenichiro NAITO, Naota YAMAMOTO and Katsunori SONE

Multi Layer BEARPHITE

83

Yosuke SUGAI and Toshihiko MOURIIntroduction of Magnetic Material Products

87

Takuji HARANO, Shinji MIYAZAKI and Hajime KATSUURA● Our Line of Award Winning Products"The Japanese Society of Tribologists 2011" Technology Award

Advancement of Cage Stress Analysis of Rolling Bearings

92

Tomoya SAKAGUCHI, Kazuyoshi HARADA and Sadatsune KAZAMA"2011 'CHO' MONODZUKURI Innovate Components Awards" Machinery Component Award

Desktop Type Microscopic Coating Applicator

New Products Information

Motohiro UCHIYAMA

9394

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NTN TECHNICAL REVIEW No.80（2012）[ Preface ]A message for the special Environment
and Energy issue

Yoshinobu YASUDAManaging Director

It has been one and a half years since the Great East Japan Earthquake, and there is a
lot of talk regarding Japan’s energy future. Forms of renewable energy such as solar,
wind, tidal, and geothermal are currently under review worldwide, and there is no questionthat they will bring safe, clean power, and help to prevent global warming. However,renewable energy has its own share of challenges like cost and consistency; thus,
improving the efficiency and reliability of power generation systems is imperative.
Under these circumstances, NTN actively participated in offshore wind power generation
projects, smart community initiatives, and demonstrations of experimental projects inseveral regions. NTN also established the “Renewable Energy Management
Demonstration Experiment Area” within the Advanced Technology R&D Center. The
purpose of this area is to develop relevant system technologies through activities such as
cultivating plants with renewable energy.
A special “Environment and Energy” issue was published for the 26th Japan
International Machine Tool Fair (JIMTOF2012), which takes place from November 1st to
6th and focuses on “Mono-Zukuri” innovation. This issue featured NTN’s products andtechnologies that relate to reducing the environmental load. Also in this issue, we
introduce you to “Sustainable Society Powered by Renewable Energy—An Approach from
Millennium Sustainability Studies” by Associate Professor Masao Takano of Nagoya
University Graduate School of Environmental Studies, who is recognized as one of theleading experts in creating systems for the globe and society. In the following“Environment and Energy” issue, technologies and products related to renewable energy
and the environment are introduced, with a focus on wind and solar power generation.
Further, the “Manufacturing Technology” issue introduces next-generation technologies
and new products that were exhibited in JIMTOF2012, which support manufacturing
capabilities.
NTN will celebrate its centennial in 2017. We strive to contribute to the sustainable
development of the society under our corporate philosophy, “For New Technology
Network: We shall contribute to international society through creating new technologies
and developing new products.”-1-*02-寄稿文E_*02/寄稿文 13/05/17 18:06 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Contribution ]Sustainable Society Powered by Renewable Energy- An Approach from Millennium Sustainability Studies -Masao TAKANO
Nagoya University Graduate School of Environmental Studies

Future style of energy usage in Japan is at great concern after the accident of Fukushima Daiichi
nuclear plant. From the viewpoint of Millennium sustainability, the society should be powered by
not fossil fuels nor nuclear power but by renewable energy. Principal automobiles may be electricvehicles shared in communities and charged by small hydropower station in rural area.

product of the research commissioned to D. H.
Meadows, of the Massachusetts Institute ofTechnology, by the think tank “Club of Rome”, a groupof prominent scholars, at the United Nations
Conference on the Human Environment held in
Stockholm in 1972.
Dr. Meadows and his colleagues used computermodeling to simulate how factors such as world
population, food production, industrial production,
availability of resources, and pollution would change.
At this time, computer simulation was not widely
available as it is today. This made the idea of using it
to foresee the future a bold effort.
In this case, the growth means factors increase
exponentially or quadratically. For example, if the
population doubles in 50 years, it doubles in the next
50 years, which is 4 times the original population. The
world in the 20th century was, indeed, an era of thegrowth.
The program called “World3”, written by Meadowsand his colleagues, adjusted a number of parameters
to simulate the sharp increase in the population, foodproduction, and industrial production of the 20th
century, i.e., the growth. These factors were used to
predict the behavior of 100 years of the 21st century.
The picture of the 21st century shown in the result
was quite disastrous. The industrial production would
peak in around 2020 and then start a sharp decline.
This would result in a drop of agricultural production,
as it depends on industrial products such as chemicalfertilizers, pesticides, agricultural machines, andothers. The drop in agricultural production would lead

1. Introduction
The Great East Japan Earthquake on March 11,
2011, and the resulting accident at Fukushima Daiichi
Nuclear Power Plant were both shocking incidents. An
increased interest toward a sustainable society andthe concept of “energy shift” emerged. However, there
is some confusion regarding this concept. This article
will provide an introduction to the basic concept ofmillennium sustainability and discuss its specificapplications, mainly on the energy sector.

2. Basic concept of millennium
sustainability
2. 1 Learning from “Limits to Growth”
The concept of “millennium sustainability” was firstcreated in the year 2000 at the Science andTechnology Forum, which was held every year by the
Science and Technology Agency. This forum allowed
free discussion among scholars of several disciplines
for 3 days, with the intent of developing ideas for
future policies in science and technology.
One notable breakout session led by Mr. Manabu
Akaike, an expert in science and technology, and Dr.
Shin-ichi Kawakami, of Gifu University Faculty of
Education, discussed “Security, science andtechnology of the 21st century from the perspective ofEarth Science.”
A range of findings were reviewed from the “Limits
to Growth” 1), which stemmed from the Earth Systemtheory in this breakout session. “Limits to Growth” is a-2-

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Sustainable Society Powered by Renewable Energy

to the rapid decrease in food production followed by asteep reduction of population with little delay, as thefood cannot support the peak population.
The simulation results showed that the foodproduction per capita would peak in the 1980s anddecrease in the 21st century. The average life
expectancy in the world, which was drasticallyextended in the 20th century, would begin rapidly
shortening starting in the middle of the 21st century
Due to the increased mortality rate of children.
The reason for the sharp decline of industrial
production in the program is the drastic decrease ofavailable resources (underground reserve of oil, etc.,
is assumed) as a result of the rapid increase of theindustrial production in the 20th century. That would
require increased investment for extraction of moredifficult and more costly resources. The capital for theindustrial production itself would then startexperiencing shortage, triggering a downward spiral of
reduced reproduction.
The release of this paper caused a big debate in theworld involving many both in favor and against thetheory. Meadows and his co-authors publishedrevisions recalculating the assumptions in 1985 andagain in 2002, concluding that the original estimates in
the 1970s were largely correct, even with the inclusionof the subsequently accumulated data.

Earth/social system

Society

Product
Consumption

Production
Recycled resources
Waste

Waste

Renewable resources

Undergroundresources

Earth

Ecosystem
Pollution

Solar energy

Fig.1 Schematic figure of material flowin 20th century type society

Meanwhile, we adopt resources from the ecosystem
as well, such as wood and food. As these resources
are renewed, if the ecosystem is properly maintained,
the resources obtainable from the ecosystem arecalled renewable resources. The produced waste canalso be returned to the ecosystem, after it is properly
processed.
In about 1,000 years, the earth’s resources will most
likely be depleted. The graphical representation of thematerial flow will be as shown in Fig. 2. There are nolonger any underground resources, therefore, there isno pollution which makes us free from the waste
issues. We obtain resources from the ecosystem andreturn the waste to the ecosystem. In other words,
humanity will also live as a part of the ecosystem.
The society in this manner, which is alsosustainable 1,000 years ahead, is called millennium
sustainability. The basic idea of the millennium
sustainability studies is to start the first step now,
aiming for that type of society in the future.
However, that does not mean we should go back to
the Stone Age. The metal that used to beunderground before is now stored in society. We canuse it carefully through recycling. The resources in theecosystem can be used in such a manner thatenriches the ecosystem by the use of advanced
science and technology rather than destroys it. This is
the concept of millennium sustainability science andtechnology 2).

2. 2 Millennium sustainability/unsustainability
Fig. 1 is a graphical representation ofunsustainability in our current society, based on theconcept of “Limits to Growth.” Underground resources
are kept in the earth. They are removed to produce
goods. When the goods are consumed, waste is
produced. The waste is stored somewhere in the earth
as “polluted material.”
For example, oil is extracted to produce gasoline at
a refinery. We fill up our cars at gas stations and drive
them, consuming gasoline. The material changes its
form in this process and is discharged from theexhaust pipe as carbon dioxide. However, thematerial did not come from nothing and neverdisappears, following the law of conservation of mass.
At a glance, gasoline looks like it has disappeared
from the tank, but in reality, it is stored in theatmosphere as carbon dioxide.
Society depending on this material flow will sooner
or later face a dead end. The resources in the earth
will be depleted and the pollution will continue toaccumulate. The use of gasoline faces these two deadends with the depletion of oil and the global warming
issue. Nuclear power faces the same issue with thedepletion of the natural uranium and proper
radioactive waste disposal.
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NTN TECHNICAL REVIEW No.80（2012）

clear that every power company has the capacity to
produce enough power to meet the maximum demandwith the existing thermal and hydropower plants even
without the nuclear power generation. In fact, asignificant amount of thermal and hydropower
capacity has been suspended because the preferenceof power generation was given to nuclear power
plants.
As we have witnessed, although the summer of2012 was the third-hottest summer after WWII, there
was no need for planned outages as none of thenuclear power plants were running except Ooi Power
Plant of the Kansai Electric Power Company. It wasproven that the power supply is sufficient even without
nuclear power plants, except for the Kansai ElectricPower Company. Even for Kansai Electric Power
Company, it has become clear that there was no needto run the Ooi Power Plant.
In the second step, cities can be provided withcogeneration (combined supply of heat and power)
fueled by natural gas (metro gas). Small form-factor
fuel cells fueled by metro gas will gain popularity inhomes and offices. Heat produced together withelectricity is effectively used for supplying hot waterand air. The buildings will be equipped withphotovoltaic panels on the rooftop. A picture of smart
grid cities, where numerous individual power facilities
are connected by ICT, will be a reality.
Alternatively, in the rural area, every village can aim
for energy independence. It is expected that there willbe numerous villages that can become energy
independent, if they take full advantage of abundant
renewable energy resources readily available in therural areas.
And in the third step, the cities can also shift torenewable energy, aiming for the entire society to befueled by the renewable energy 100%. This is the“millennium sustainable” society independent fromsoon-to-be-depleted resources, that is, the society thatcan function even 1,000 years ahead.

Earth/social system

Society

Product
Consumption

Production
Recycled resources
Waste

Renewable resources

Ecosystem

Earth
Solar energy

Fig.2 Schematic figure of material flow
in future sustainable society

3. Three-step energy shift
3. 1 Phase-out of nuclear power
Now, let me discuss the future use of energy fromthe point of view of millennium sustainability. The
feed-in tariff program for renewable energy began inof July this year. This program ensures the purchaseof renewable power by power companies at a fixedprice when renewable power plants are constructed.
Therefore, the investment can be recovered withcertainty, making the renewable power plants
profitable. This is an epoch-making event in thehistory of the Japanese energy industry. The
construction of megawatt solar plants is in progress inmany regions, and micro-hydropower plants are alsoplanned in different regions.
Under these circumstances, the concept of “energy
shift,” which will promote nuclear phase-out by shifting
the energy to renewable power, is gaining the supportof many. However, this concept also means that the“Nuclear plants must be operated until renewableenergy can effectively replace them.” If so, nuclear
plants must be running for quite some time, as therenewable energy cannot produce the amount of
power that nuclear power plants have been producing
soon enough.
Instead, the nuclear power plants can be stopped
immediately. The energy shift should be planned inthree phases. The first phase is to immediately stop
the nuclear power plants and restart the thermal
power plants that have been halted. It will become

3. 2 Challenges of oil phase-out
Currently, about half of the ultimate oil reserve isused, approaching the peak production. It is notdepleted immediately; however, decreased production
causes price increase, as the supply cannot meet theentire demand. In fact, since 2000, the crude oil priceentered an unstable period where the price repeats
sudden jumps and drops.
The problem does not stop at the price. Since the“bottom of the tank” is now visible, the question of who
controls the remaining oil has affected two wars fought
in recent history, the Gulf War and the Iraq War.
These wars were fought by two presidents Bush and-4-

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Sustainable Society Powered by Renewable Energy

consumption of an air conditioner has reduced toabout 1/3 in 20 years. However, we are installing atleast three of such air conditioners in our homes, so
the power consumption in our homes continues toincrease even when the overall power consumption ofsociety decreases. Automobile fuel consumption has
drastically improved, however, larger cars with moreweight have lower gas mileage and most tend to buylarger cars with improved fuel consumption whenpurchasing new vehicles.
These problems cannot be solved solely by
individual efforts. To start, we must create a society
where individual air conditioners are not required foreach room and cars are not required to be driven
around town.
To achieve this, towns must be compact. The
decreased population and resulting change in urban
structure may provide the necessary condition. The
town structure needs to create a shorter distance
between work and home which does not requirecommuting by car. A different lifestyle may also be
required. In addition, forests and farmland need to beplaced around the urban area to avoid the heat island
phenomenon for more comfortable summers.
Cogeneration by biomass and area heating/cooling
can possibly be accomplished if society evolves fromnatural gas cogeneration and smart grid.

the president Hussein of Iraq. Iraq boasts the second
largest oil reserve in the world. The control of the Iraq
oil went from the Iraq government to the U.N. after the
Gulf War and then transferred from the U.N. to thehands of American and British government after the
Iraq War.
Therefore, it is imperative to phase out from thesociety dependent on oil. The energy shift is not only
for nuclear phase-out. Oil phase-out is also a criticalissue.
Natural gas is still abundant; therefore, it can beused as an interim solution. However, the bottom of
the resources may be observed in the latter half of the
21st century causing conflict over its interest.
Its procurement has already become tight. There is
tension between Japan and China over thedevelopment of a natural gas field in the East China
Sea and also the use of a large gas field developed in
Sakhalin, Russia. The original plan of laying pipeline
to Japan is stagnant, as another plan of laying pipeline
to China emerged later. There are similar tensions
over natural gas resources around the world.
Therefore, the Japanese society should aim for
100% renewable energy by 2050, phasing out all
natural gas dependency, as well. This can be Japan’s
most effective contribution to world peace. Japan’s
conversion to a society of millennium sustainableenergy utilization is not only important for protecting
the domestic lives, but also essential for creatingworld peace.

4. Toward the society of millennium
sustainable energy utilization

Current energy supply and future vision

4. 1 Renewable energy in urban areas

Biomass
Other new energy
Natural gas

In the third step of the energy shift, urban areas alsoneed to replace natural gas with renewable energy,
such as biomass, toward 2050.
However, that is not possible with the current
energy consumption. Fig. 3 shows an idea for the
future energy utilization. The amount for 2050 showsthe estimate of total available renewable energy inJapan. It is only 1/3 of the current energy supply.
In order to realize a society of 100% renewableenergy, it assumes that drastic energy conservation isnecessary. The level of conservation is about 1/3 forthe entire society, in some cases, 1/10. The Japanese
population is decreasing. It is estimated that the
population in 2050 will become about 80% of that in2010, which means the reduction of energy per capita
is not as drastic as the above. Still, significant energy
conservation is required.
Technological evolution is an important factor;
however, that is not sufficient. The energy

Photovoltaic
Hydropower
Coal

Wind
NuclearOil

20,00015,00010,0005,000

NuclearNaturalgas

Biomass

Coal
Naturalgas
Oil

Biomass
Coal

0

2008

2030

Fiscal year

2050

Fig.3 Primary energy supply;
Present situation and the future vision-5-

Photovoltaic
Wind
HydropowerPrimary energy supply PJ/Year

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NTN TECHNICAL REVIEW No.80（2012）

4. 3 Sustainable automobile utilization in the
future4. 2 Possibilities for the small-scale hydropower
Hydropower plants are a promising renewable
resource in Japan. Not large-scale hydropower plantsthat require construction of dams, but small-scale
plants of the 1 MW range or micro- hydropower of a
few to tens of kW and even smaller pico- hydropower.
Under the feed-in tariff program, hydropower of tensof kW and larger can become profitable by selling
power to power companies. Due to the staged fixedprice structure, hydropower of 1MW is particularly
advantageous. Many of these small-scale hydropower
plants were in fact constructed about 100 years ago.
Fig. 4 shows the diagram of those power plants, inwhich the water drawn upstream from the intake gate
runs down the high-pressure steel pipe and operates
the turbine. Fig. 5 is a picture of Hirose power plant in
Sakauchi, Ibigawa, Gifu Prefecture, with 3.2 MW of
output, which was constructed in 1925. It is still up todate and working properly. The construction of thistype of plant slowed down after WWII, however, withthe feed-in tariff program, it is expected to see arenewed construction boom.
Further, several manufacturers are developing
water mills in the area of micro- and pico-hydropower
plants, which are not profitable under the feed-in tariff
program. Fig. 6 is a power generation system using
axial-type turbines developed in my laboratory.
It can be set in an agricultural waterway andproduces 30 W of output. It generates power in asmall drop of water and withstands frequent dust suchas leaves in those waterways. It is expected to be
used for self-support, typically in the mountains where
no power lines are provided, or as an emergency
power source during power outages as a result of
natural causes. In the developing countries, there aremany regions without power. It is expected that theresidents in those regions can invest in and maintainthese types of power generation systems on theirown,.

How about transportation in rural areas? Rural
areas pose the biggest problem because cars are
more necessary. The mainstream cars in themillennium sustainable society in 2050 should beelectric cars, with the power source in that case beinghydropower generation. The power management tomatch the unstable photovoltaic and wind power
generation with the similarly fluctuating electricity
demand is a tough task. Instead, it stable hydropower
generation would be most suitable, and in particular,
micro-hydropower.

Fig.5 An example of small hydro power station

Head tankPressure steel pipe

Scouring sluice

Water
intake gate

Generator
TurbineRiver

Mechanism ofwaterway power plant

Drain

Fig.6 Power generation system using axial typeturbine set in a small irrigation canalFig.4 Schematic figure of small hydro power station-6-

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Sustainable Society Powered by Renewable Energy

Reference

For example, let us consider a micro-hydropower
plant of 1 MW of output. Usually, water can be taken
stably during the day and night. If the power demandduring the night is 30% of the rated output, theremaining 70%, which is 0.7 MW, can be used tocharge electric cars. This is enough to charge 200
Mitsubishi i-MiEV electric vehicles at the same time. A
charging station can be installed within a village which
can be used for car sharing.

1) D. Meadows, et al. “Limits to Growth,” Diamond, Inc.,
2005
2) Edited by Shigen-Kyokai, “Millennium sustainability –creation of coexisting/recycling civilized society,”
Japan Institute for Community Affairs, 2003
3) Masao Takano, “Big power of small hydropower,”
Quarterly magazine “Chiiki (region)” No. 9, 2012

5. Conclusion
The utilization of energy in the future should bedetermined through thorough discussions andagreements, particularly, by the younger generation.
The major issue is that there has not been such aframework thus far. It is important, from now on, to
continue discussions on how to drive the future energy
utilization to bring forward different thoughts, ideas,
and concepts. I hope this paper will serve as areference for those discussions.

<Author biography>

Masao Takano
Associate Professor, Nagoya University Graduate School of Environmental Studies
1962 Born in Yamaguchi Prefecture
1981 Enrolled in Nagoya University Faculty of Science. Received Doctorate of Science in Earth Science
1993 Nagoya University Faculty of Science supporting staff
Participated in the research project “Decoding the Earth’s Evolution” to examine the Earth history and learned theapproach of using the concept of “Coexistence of lives and Earth” to tackle the Earth history.
Realized that the era of humans is very distinctive in the 4.6 billion years of Earth history.
1996 Associate Professor, Faculty of Science
2001 Participated in the establishment of Nagoya University Graduate School of Environmental Studies, Associate Professor
Engaged in research and education toward creation of “Millennium Sustainability,” which aims for Earth and societasystem that can function 1,000 years ahead when the underground resources are depleted, working together with many
experts of different disciplines.
Also, participates in activities for realizing “Millennium Sustainable society” working together as a citizen with government
and NPOs.

-7-

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NTN TECHNICAL REVIEW No.80（2012）[ Technical Article ]Market of Wind Power Generation Industry,
Technology Trends and Bearing Technology of NTN

Michio HORI*
Yusuke YAMADA*
Masahiko KATAOKA*
Recently large-sized wind turbine generators and off-shore wind turbine
generators have been promoted globally in the field of wind turbine
generators. It is not merely growth, but also accompanying innovations
towards higher reliability, longer lives, and higher efficiency. In this paper,
the industry trends will be introduced, along with bearing technologies,
which contribute the movement of the industry.

installed wind turbines 1) for each year. The averagecapacity grew from 1.30 MW/unit in 2005 to 1.66
MW/unit in 2010. However, it is converging at 1.68
MW/unit in 2011.
The reason behind it may be that, since the majority(98.5%) of the wind turbines installed in 2011 were onshore models, they could have been restricted by not
only the cost of the wind turbine itself, but also byother conditions such as road transportation andconstruction, with reasonable maximum capacity.
Fig. 3 shows the relation between the number of
installed wind turbines last year and their capacity.
Most of the wind turbines were between 1.5 MW and
2.5 MW 1); therefore, it is considered the mainstream
for the time being.

1. Introduction
Wind power generation has evolved into clean
energy with the least impact on the environment,
without any CO2 emission.
Fig. 1 shows the trend of the total capacity of windturbines. Although the progress slowed slightly due tothe recent economic situation, the total capacity in theworld reached about 240 GW 1), which is equivalent to
60 nuclear power plants * and it is expected that thegrowth will continue in an accelerated fashion.
* Nuclear power plant: 1 million kW/station; facility availability 80%
Wind power generation facility: facility availability 20% assumed

Fig. 2 shows the average capacity per unit of newly

Total capacity GW

500
400

ActualAverage power output per station
MW

600

Estimate2011 Year

300
200
100
0
06
20

08

20

10

20

12

20

14

20

16 Year20

Fig. 1 Transition of total capacity of wind turbines

1.7
1.6
1.5
1.4
1.3
1.2

05

20

06

20

07

20

08

20

09

20

10

20

11

20

ar
Ye

Fig. 2 Average capacity per one unit of newly installedwind turbines 1)* Industrial Machinery Division New Energy Engineering Department-8-

*03_03 13/05/17 18:10 ページ 2

Number of installed turbines

Market of Wind Power Generation Industry, Technology Trends and Bearing Technology of NTN

Step-up-gears (gear box)

20000

Main bearings

Generator

15000

Rotor

10000
5000
0

5

0.7

0−

1

5−

0.7

1.5

1−

.5

−2

1.5

2.

r
5o

NacelleremoOutput capacity MW

Blade (wing)

Fig. 3 The number of installed wind turbines in 2011
and their capacities 1)

Fig. 4 Structure of wind turbine

NTN has been developing and supplying bearings
that meet the required characteristics for theapplications of main shafts, step-up gears, generators,
etc., for on-shore wind turbines, as well as thebearings for the above-mentioned offshore windturbines.
Recently, we have also been developing a
Condition Monitoring System (CMS) to address
offshore applications. We introduce you to theirtechnological trend in the following section.

2. Offshore
Because of the shortage of adequate sites for onshore installation and for power efficiency/facility
runtime improvement, wind turbine manufacturers aredeveloping larger turbines targeting offshore
applications on a full scale.
Today, more than 70% of the cases underconsideration by NTN are for offshore applications,and therefore it is expected that the average capacity
will increase again within 5 to 6 years.

4. Main bearings
3. Technology trend of bearings foroffshore wind turbines

Main bearings that support rotors are becoming
larger, however offshore wind turbines use bearings ofextremely large size that are not used in regularindustrial machines.
The mainstream specification of the bearing
configuration for the main shaft uses two self-aligningroller bearings, which have high allowance for
installation errors when they are mounted in theturbines. However, for purposes of floating bearingsand due to the light-weigh requirements for larger
offshore applications, various configurations are
emerging, as shown in Table 1.
When the main bearings are failed and need to be
replaced, the rotor and the main shaft that the main
bearings support need to be removed from the nacelleat a huge expense. The replacement cost becomeseven larger for offshore wind turbines; therefore, very
high reliability is required for the main bearings.
Evaluation of main shaft bearings using bench test
machines is made extremely difficult due to thevariations in load conditions and the necessary
evaluation time. It is therefore important to ensure
reliability using component tests and analyticaltechnologies.

Wind turbines come in many types, such as
horizontal axis and vertical axis. We will introduce you
to the representative three-blade (wing), horizontalaxis-type large wind turbines. Fig. 4 shows the nacelleportion of the induction generator, which is themainstream design today. The wind is received by theblades, which rotates the rotor and the power isconverted to electric energy by the generator.
Offshore wind turbines are not easily accessible;
therefore, ease of maintenance and reliability areemphasized and several mechanisms have beendeveloped in addition to the components in the nacelle(main shaft – step-up gear – generator).
1) Main shaft - generator (direct drive)2) Main shaft - multiple generator (friction drive)3) Main shaft - step-up gear/generator unit
4) Main shaft - hydraulic step-up gear - generator
Each type has advantages and disadvantages. The
optimum configuration may change depending on theeconomic conditions such as material cost; therefore,
every manufacturer is diligently conducting
development.
-9-*03_03 13/05/17 18:10 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

Table 1 Bearing layout of offshore wind turbine
A

B

C

LayoutStructure

Generator side

Rotor side

Generator side

Single row tapered
roller bearing

Single row tapered
roller bearing

Bearing type

Rotor side

Cylindrical roller
bearing

Double row tapered roller bearing Double row tapered roller bearing with steepwith vertex of contact angles
vertex of contact angles outside of bearing
inside of bearing

4.2 Optimization of the peripheral design ofbearings

4.1 Optimization of the internal design of bearings
In larger main bearings, rigidity that affects thedeformation resistance of the entire cage is an
important factor, in addition to the cage strength. As aresult, the impact of the rolling elements and the cage
weight on the rotational performance is evaluatedthrough analysis. Fig. 5 shows an example of the
motion analysis of a single-row tapered roller bearing.
In this analysis, operating conditions are simulated
to confirm the cage strength by calculating the loadand stress applied to the supports of the cage. With
such dynamic and static analyses, the specificationsof rolling elements and the cage can be optimized,
and a high-load capacity design in the predetermined
size can be achieved.

The life of the bearings and the contact stressbetween the rolling elements and raceways areevaluated based on the calculation of weight of eachrolling element, drawn from the structural analysis
considering the peripheral structure. This is done
because the life of the bearings is largely affected by
the peripheral structure such as the housing, and its
impact increases as the size increases.
Fig. 6 shows the housing for self-aligning roller
bearings used for inner ring rotation. When selfaligning roller bearings are used, the housing isconnected to the frame (nacelle) at the two ends.
Therefore, the load of the rolling element increases
compared with the calculation result of the rigid
element at the 8-o’clock region in Fig. 7. This isbecause the connections with the frame havesomewhat higher rigidity and resistance todeformation, the load is applied to local points.
Therefore, when used for inner ring rotation, theshape of the housing and the location of theconnection points that fit the operating conditions are
particularly important.
Fig. 8 shows an example of analytical result of
double row tapered roller bearings used in outer ringrotation. In the case of outer ring rotation, the housing
does not have connecting surfaces and since thethickness (rigidity) on the circumference is uniform, theload is evenly distributed, so the load on the rolling
elements can be minimized.
As shown, the structure around the bearings has asignificant impact on the life of the bearings.

Fig. 5 Movement analysis example ofsingle row tapered roller bearing-10-*03_03 13/05/17 18:10 ページ 4

Market of Wind Power Generation Industry, Technology Trends and Bearing Technology of NTN

shown in Fig. 9. Therefore, moment load is reducedby the positioning of the bearings inside the rotor,
resulting in the reduction of the bearing load.
As explained above, positioning of the bearings
inside the rotor and using them in outer ring rotation
contribute the most to the longevity of the bearings,
light weight of the nacelle and compact bearing size,
which is the best design.

4.4 For higher reliability
Fig. 6 Analytical model of housing for spherical roller bearing

NTN has a variety of test machines to meet therequirements of its customers. We use them toconfirm the reliability of the analytical results andcollect the basic data of lubrication for the optimum
maintenance.
Also, we are planning to commission a large test
device that can be used to evaluate bearings of 4 m ofouter diameter, which is the actual size used foroffshore wind turbines (Fig. 10). This test device isable to control not only temperature and humidity, butalso severe environmental conditions such as dust.
We will be proving high reliability by building up
verification of functionality and basic data using thesemachines.

Load distribution on rolling elementskN

Calculation result of elastic body
Calculation result of rigid bodyFig. 7 Analytical result of spherical roller bearing
Load distribution on rolling elementskN

Calculation result of elastic body
Calculation result of rigid bodyFig. 8 Analytical result of double row tapered roller bearing
Fig. 9 Coordinate system of main bearing

Therefore, NTN provides the most appropriate
bearings not only through the bearing internal design,
but also after analysis that includes the peripheral
structure.

4.3 Best layout of bearings
As mentioned above, in order to prolong the life of
the bearings, it is necessary to either make the rigidityof the housing circumference uniform or make the
distribution of rigidity as even as possible.
In general, the load on the main bearings isdetermined by the radial load on the rotor center in theFz direction, axial load in the Fx direction, and themoment load in the My direction in the coordinates

Fig. 10 Bearing testing machine for offshore wind turbine-11-*03_03 13/05/17 18:10 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

The special surface treatment raises the oil film
formation capability to prevent surface damage
caused by smearing due to metal contact. Fig. 15
shows the test results comparing standard and special
surface treated products against smearing. More than
twice the resistance to smearing was attained.

5. Bearings for step-up gears
While the size of wind turbine generators isincreasing, there are requirements for lighter and morecompact step-up gears (Fig. 11) despite increased
input torque.

Input Main
shaft side

First row

Second row

Low-speed
shaft

High-speed
shaft

Planetarygear
Calculation result of elastic body

Planetary
part

Calculation result of rigid body

OutputGeneratorside
Medium-speed
shaft

Load distribution on rolling elements kN

Fig. 12 Analytical result of planet bearing including
structure in surrounding

LargerFig. 11 Structure of general gearbox

Without misalignment

5.1 Optimization of the internal design of bearings

Contact stress

LargerContact stress

For lightweight and compact step-up gears, theradial size of the planetary portion is important.
Therefore, step-up gear manufacturers consider
thinning the housing walls and reducing planetary
gear sizes and bearings, in order to reduce the radial
size as much as possible. We determine and propose
specifications for bearings by analyzing thedeformation of and load distribution on different parts,
including the bearing peripheral structure, in addition
to the operating conditions such as the load andspindle speed (Fig. 12).
Additionally, torque and spindle speed of a step-up
gear can vary depending on the wind conditions.
Since long-life and high reliability are needed evenwhen the input condition changes, NTN applies
specifications dedicated to step-up gears.
We optimize the crowning shape that mitigatesharmful edge stress applied to the roller, which is therolling element, when high load is applied to thebearings. Fig. 13 shows the comparison of distributionof contact stress between standard crowning andspecial crowning when high load is applied. It can beseen that the edge stress is reduced by applying
special crowning to the rollers.
Also, bearings for medium/high-speed shafts
support relatively low loads. We have developed a
special surface treatment for inner/outer rings androllers for preventing surface damage by sliding, suchas smearing. (Fig. 14)

Edge stress

With misalignment

Standard crowning
Special crowning

Position along the shaft

Resistance to smearing

BetterFig. 13 Comparison of contact stresses of rolling element

Conventional
product

Fig.14 Special surface
treatment bearing-12-

Product withspecial heat
treatment

Fig.15 Examination resultof smearing

*03_03 13/05/17 18:10 ページ 6

Market of Wind Power Generation Industry, Technology Trends and Bearing Technology of NTN

5.2 Addressing changing step-up-gears

the step-up mechanism and generator, the reliability
can be further improved.

Along with larger and offshore applications, step-up
gears are also changing.
Table 2 shows different configurations of step-up
gears. Conventionally, the input rotation from the main
shaft is increased 100 times by mechanical step-up
gears, and then the power is relayed to the generator;
however, due to the requirements of higher reliability
and compact size, different types of step-up-gears arebeing proposed.

(2) Unitization of step-up gears and generator
By combining step-up gears with a generator into aunit, the size can be reduced. Compared toconventional power train, the rotation speed isrelatively slow, making the speed-up ratio 10–20 times
to simplify the structure for improved reliability.

(3) Mechanism to replace step-up gears
Speed is increased by a hydraulic pump/motor
instead of step-up gears. Since rotation speed can becontrolled by controlling the pump/motor, in addition toreliability improvement, the number of components
can be reduced.
Step-up mechanism is facing a significant period oftransition as the wind turbines are applied offshore.
The requirements for bearings significantly varydepending on each of these mechanisms. NTN isdiligently developing bearings suitable for each ofthese requirements.

(1) Multiple generator configuration
To enhance reliability, this type replaces thecomplex mechanical step-up gears with a simpler
friction-drive speed-up mechanism to drive multiple
generators. When NTN constant velocity type joints(Fig. 16) are used for torque transmission between

Table 2 Speed increasing system

Mechanical type(conventional
configuration)

MechanicalᶵᲔᘧ
step-up gears(1) Multiple-generator
configurationᦶ᧿ᘧFrictiontype
step-up
gears
ቑ㏿ᶵ

ቑ㏿ᶵ

6. Bearings for generators

GeneratorⓎ㟁ᶵ

Generally, deep groove ball bearings are used forgenerators supporting both sides of the rotor. The
shaft diameter for a 1.5-MW-class turbine is about 150mm.
NTN markets the “MEGAOHM ™” series ofbearings for generators.
This product has a special ceramic applied to theoutside cylindrical surface to the sides of the bearing
outer ring by thermal spraying (Fig. 17) and is used
not only for wind turbines but also for a wide range ofapplications such as main motors for rail cars, general
purpose motors, etc.
Also, for achieving even higher reliability foroffshore application, we have a line-up of bearings
using ceramic balls as shown in Fig. 18 instead ofsteel balls. Because of reduced temperature rise due
to ceramic balls, lubricant life can be increased.

GeneratorⓎ㟁ᶵ
GeneratorⓎ㟁ᶵ(2) Unitization of
step-up gears andgenerator

Step-up-gears
ቑ㏿ᶵ(3) Hydraulic type

Motor
䝫䞁䝥
Pump
䝰䞊䝍

＋
䠇
GeneratorⓎ㟁ᶵ

Generator

Sprayed ceramic

Fig. 16 Constant velocity joint

Fig. 17 Ceramic insulated bearing-13-*03_03 13/05/17 18:10 ページ 7

NTN TECHNICAL REVIEW No.80（2012）

Fig. 18 Ceramic balls

7. Condition monitoring system
Windmills are often installed in remote locations
away from cities, in the mountains, along the coast,
etc.. If any failure occurs in offshore windmills, inparticular, the repair cost can become huge.
To reduce this cost, the condition monitoring system(CMS) for wind turbine generators is attracting
attention.
By using CMS, conditions of wind turbine
generators can be remotely monitored for early
detection of anomalies to prevent secondary damage,which leads to improvement of availability. Thissystem will be separately discussed in detail.

8. Conclusion
Wind power is a new energy expected to growfurther and the requirements for the operating
environment, economy, and reliability are increasing.
NTN is moving forward with development andanalysis, so that we can contribute to the developmentof wind power generation, in response of thoserequirements.

Reference
1) BTM Consult：Word Market Update 2011

Photo of authors

Michio HORI

Yusuke YAMADA

Masahiko KATAOKANew Energy Engineering
Department
Industrial Machinery Division

New Energy Engineering
Department
Industrial Machinery Division

New Energy Engineering
Department
Industrial Machinery Division-14-*04_03 13/05/17 18:13 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Article ]Application of Condition Monitoring System
for Wind Turbines

Condition monitoring
sensor (vibration)

Akitoshi TAKEUCHI*
Takashi HASEBA*
Hiroshi IKEDA*

Operation information
sensor (rotation/current)

NTN has developed a Condition Monitoring System for windturbines. CMS can detect the failure of main bearings, gearboxes,
Data acquisition module
generators, and many mechanical components of wind turbines inLANtheir early stages. One feature of CMS is that users can monitor
from a remote location. This system includes a data acquisition module, data management softwareand monitoring and analysis software for a client PC and was the first in Japan to acquire GL
certification. In this paper, the structure and the diagnostic method of NTN’s CMS will be introduced.

installations will increase 2). Access to these locations
is even more difficult than on-shore installation fortransporting replacement components, which willcause longer shut down when major failures areencountered. Therefore, Germanischer Lloyd (GL)recommends the use of GL approved CMS for
offshore wind turbines.
In order to contribute to the advancement ofmaintenance, NTN has developed CMS based on its
own vibration diagnostics technology. The system was
the first in Japan to acquire GL-type approval.

1. Introduction
Japan is 13th 1) in the world, with wind power
generation capacity of 2.5 GW in 2011. It is expectedthat it will significantly increase with the import tariff
program that started in July this year. On the other
hand, as the fixed price will be reviewed annually,further efforts are required, including reduction of themaintenance cost and improvement of capacity usage.
The main components of windmills such as blades,
main bearings, step-up gears, and generators arelocated at the top of the tower. Therefore, repair and/or
replacement of the equipment requires a supersized
crane. In addition, since most of the windmills arelocated in the mountainous areas where access ischallenging, maintenance cost is high. Further, theshutdown period may be prolonged, resulting in low
availability if replacement components are not secured
before the shutdown caused by failure.
Condition monitoring systems (CMS) for wind power
generators target early detection of component
deterioration from symptoms such as vibration andtemperature, preventing proliferation of damage andenabling earlier ordering of replacement components
for increased uptime. In addition, maintenance costs
can be saved since the windmills are monitored
remotely, reducing the need for site visits.
Since adequate sites for wind power generation are
becoming scarce on-shore, it is expected that offshore

2. System configuration
NTN’s CMS consists of (1) a data acquisition
module and measurement sensors, (2) data
management software, (3) monitor/analysis software,as shown in Fig. 1. “(1) Data acquisition module”
measures vibration, etc., with the sensors installed on
the wind turbine and sends the data to the server
through the Internet for storage. “(2) Data
management software” performs a primary diagnosisof the data and issues an alarm if the data exceeds
the threshold.
In addition, when “(3) Monitor/analysis software” isinstalled on a PC connected to the Internet, alarms
and data can be monitored at a remote location fromthe wind turbine. This software also has an analysis
function, enabling detailed diagnostics.

*New Energy Engineering Department Industrial Machinery Division-15-

*04_03 13/05/17 18:13 ページ 2

NTN TECHNICAL REVIEW No.80（2012）
Blade

NacelleMeasurement sensor

Generator

Step-up gears

User or NTN

ServerMain bearings(1) Data acquisition module(2) Data management software

Data(3) Monitor/analysis software

Fig. 1 Structure of NTN’s CMS

3. Data acquisition module

4. Flow of operation and diagnosis
philosophy

Table 1 shows the specifications for “(1) Dataacquisition module” and Fig. 2 shows its externalview. The module is the world’s smallest, for easierinstallation into the nacelle, even on existing windturbines. Since it has high dustproof/waterproof
capabilities and a wider operating temperature range,
it can be used in a wide variety of environments,
including offshore applications.
Different sensors for vibration, rotation, current,
temperature, etc., can be used and changing of thenumber of channels can also be accomplished easily.

4.1 Observation mode operation
Fig. 3 shows the flow of CMS from installation tooperation. As the operating conditions of wind turbineschanges from moment to moment, the cause must be
determined if the change of vibration comes from thechange of operation conditions or damage to thebearings and gears. In order to eliminate the influenceof vibration of operating conditions, after CMS is
installed, a preliminary measurement called“observation mode operation” is conducted as apreparation of diagnosis, to determine diagnostic
operating conditions for the production stage.
With NTN’s CMS, rotational speed and generated
power are used as parameters to indicate theoperation conditions. In “observation mode operation,”
the data acquisition module is regularly activated, andthe rotational speed and generated power arerecorded.
Fig. 4 shows an example of the main shaft
rotational speed and generated power. In general,
condition 1, which has large power output around therated rotation speed, is considered diagnosis
operating condition. For diagnosis of lower speed in a

Table 1 Specification of data acquisition module
Item

Specification
Vibration (built-in acceleration sensor in the amplifier) max16ch

Measuredsignals

Temperature (thermocouple)max 6chVoltage (AC, DC, current sensor)

max 4chRotational speed (proximity sensor)

1ch

Power supply

AC90 〜 260V (50/60Hz)

Case dimension

250 × 290 × 108 mm

WeightOperatingtemperature
Protection of
enclosureElectromagnetic
compatibility

5.1 kg-20 〜 +60˚C
Degree of protection of electric machines
and appliances
Complied with EN Standards,
EN61000-6-2 and EN61000-6-4

Installation
Observation-mode
operation
Learning-mode
operation⇒ Determination of diagnosis
operation condition⇒ Determination of thresholds

Production-mode
operation
Primary diagnosis: automatic diagnosis, “Caution”
and “Alert” alarm issued
Precise diagnosis: diagnosis by experts

Fig. 2 Data acquisition module

Fig. 3 Flow of NTN’s CMS operation-16-*04_03 13/05/17 18:13 ページ 3

Application of Condition Monitoring System for Wind Turbines

Condition 1

1200
800
400
0

frequencies caused by defects of bearings and gearson the spectrum. It is also possible to compare past
waveforms with the current ones.
Rated rotational speed

Power output kW

1600

Condition 2
0

5

10

15

5. Diagnosis parameters

20

Rotational speed of main shaft min

The main components of wind turbines are theblades, main bearings, step-up gears, and generators.
NTN’s CMS addresses damage to any of thesecomponents. Table 2 shows diagnosis parameters
corresponding to the failure modes. The relation
between the failure modes and diagnosis parameters
are discussed in detail in the following:

25-1

Fig. 4 Main shaft rotational speed and electric power

period of weaker wind conditions, condition 2 withslow rotation speed may be added as a diagnosis
operation condition.

(1) Imbalance of blades
When blades are damaged, imbalance of mass willcause an increase in vibration. Imbalance appears
as a peak in rotational frequency. Imbalanceanomaly is detected taking the primary to tertiarycomponents of rotational frequency of the main
shaft as the diagnosis parameters.

4.1 Learning mode operation
After the diagnosis operation conditions are
determined, “learning mode operation” is conducted todetermine the diagnostic thresholds. In this mode,
diagnosis parameters in normal conditions arecollected. The data acquisition module is regularly
activated in the same way as “observation mode
operation,” data is collected only when the rotation
speed and generated power match the diagnosis
operation conditions.
NTN’s CMS automatically calculates the thresholds
for “Caution” and “Alert.” From the relationshipbetween the diagnosis parameters in normal
conditions and those in damaged conditions obtained
from the past failure cases and the data in normal
conditions measured in “learning mode operation,” thethresholds for the diagnosis parameters for thefollowing section 5 are automatically established. In
addition, the thresholds can be manually set, as well.

(2) Abnormal vibration of nacelle/tower
This indicates a large vibration out of norm when
they receive strong wind, etc. The vibration consistsof low frequencies; the sum of spectrum in low
frequency band among acceleration vibration isused as the diagnosis parameters.

(3) Anomaly in bearings
It is caused by separation or abnormal wear on the
raceway surface or rolling surface. The effective
value and modulation factor are used as diagnosis
parameters. The effective value is an average ofvibration and the modulation factor is an effective
value of the AC component of accelerated vibration
after envelope processing and can be used for
determination of vibration caused by a shock.

(4) Anomaly in gears
It indicates anomalies of tooth contact surface, suchas the pitching and abnormal wear of tooth flank. It
is diagnosed by tooth meshing frequency of primary
to tertiary components and the sum of spectra ofhigh-frequency band in the acceleration vibration
spectra.

4.3 Production mode operation
Under “production mode operation,” CMS regularly
conducts measurement and if a diagnosis parameter
exceeds the threshold, it issues two types of alarms,“Caution” and “Alert,” as a “primary diagnosis” in Fig.
3. The alarm can be confirmed on “(3)Monitor/analysis software.” It is also able toautomatically notify predetermined personnel via
emails.
“Precise diagnosis” in Fig. 3 is diagnosis provided
by an expert using analytical tool of “(3)Monitor/analysis software.” “(3) Monitor/analysis
software” displays diagnosis parameters, trend
analysis, and thresholds, for easy determination of thediagnosis parameter trend. It also provides filter
processing, envelope processing, and FFT processingof time waveforms, as well as a function to display

Table 2 Failure modes and diagnosis parameters
Failure mode
Imbalance of blades

Diagnosis parameters
Primary to tertiary frequency
components of the main shaft rotation

Abnormal vibration of nacelle/tower Low frequency components of nacelle-17-

Anomaly in bearings(main bearings, step-up gears,
generator)

Effective value and modulation factorof vibration in the proximity of bearings

Anomaly of gears(step-up gears)

Primary to tertiary components of toothmeshing frequency and vibration
components of high frequency bandcalculated from specification of the gears

*04_03 13/05/17 18:13 ページ 4

NTN TECHNICAL REVIEW No.80（2012）

6. Case study of diagnosis forgenerator bearings
Effective values, which are diagnosis parameters,
were obtained from the measurement of vibration inbearings with electrical pitting and normal bearings ofthe actual generator. Fig. 5 shows the relation
between main shaft rotational speed of the windturbine and vibration effective values around thebearings. The effective values are significantly larger
due to electrical pitting. The difference from thenormal bearings is more significant as the rotational
speed becomes higher.
Fig. 6 shows the result of envelope spectrum ofvibration. The outer ring defect frequency, which is afrequency that a point on the outer ring raceway
surface is in contact with the rolling element, and its
higher order components are observed. It is thenpossible to assume damage on the outer ring raceway
surface. This envelope spectrum evaluation result wasproven correct after this bearing unit with electricalpitting was disassembled and examined, as it showed
repetitive ridge marks on the outer ring raceway, as
shown in Fig. 7.

Effective value m/s2

20

Fig. 7 Outer raceway failure by electrical pitting

7. Conclusion
Functions expected of CMS for wind turbines
include not only an early detection of damages but
also the prediction of remaining life. For improving thecapacity uptime, it is important to minimize the periodof shutdown, and therefore, it is critical for the users toknow how long certain components can be used after
damage is detected. For highly precise prediction of
the remaining life, vibration measurement on theactual machines and accumulation of data fromexaminations through disassembly are necessary.
We will work together with different players such as
wind power providers, windmill facility manufacturers
and maintenance service providers to accumulatediagnostic know-how so that the system can provide
more useful information for expansion of wind power
utilization.

Bearings with electrical pitting

15
10

Reference

50Normal bearings
0

5

10

15

20

1）Global Wind Energy Council：Global Wind Report
Annual Market Update 2011
2) Takeshi Ishihara: Current situation and future
perspective of offshore wind power generation, The
Journal of the Institute of Electrical Engineers ofJapan, Vol. 131 No. 7, 20113）Germanischer Lloyd Industrial Services GmbH：
Guideline for the Certification of Condition Monitoring
Systems for Wind Turbines,2007

25

Rotational speed of main shaft min-1

Fig. 5 Main shaft rotational speed and root mean squarevalue of vibration

Amplitude m/s2

4

F0×1

F0×2

F0×3

3

F0：Frequencies of defective
outer ring

2

Photo of authors

1
0

0

200

400

600

800

1000

Frequency Hz

Fig. 6 Envelope spectrum of vibration-18-

Akitoshi TAKEUCHI

Takashi HASEBA

Hiroshi IKEDA

New Energy Engineering
Department
Industrial Machinery Division

New Energy Engineering
Department
Industrial Machinery Division

New Energy Engineering
Department
Industrial Machinery Division*05_05 13/05/17 18:15 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]Motorized Linear Module for Tracking System of Solar Light /Solar Heat Power Generation

Tomoyuki OWADA*
Masaki KAGAMI*NTN is engaged on product development in the renewable energy
sector. This article introduces motorized linear modules, which
possess the features of low cost, high performance, and easy
maintenance for solar tracking systems.

Reflective mirror

1.Introduction
Expectation for renewable energy is increasing,daily, due to growing interest in the reduction ofgreenhouse gases and the review of electrical power
supply. Particularly, solar energy utilization, such as
photovoltaic and solar thermal power generation, has
attracted attention. However, the reduction of overall
cost, including simplified associated equipment, easy
maintenance, and reduced running cost, is required,since the investment in facilities reflects the power
generation cost.
NTN has developed several motorized cylindricallinear modules with high environmental resistance,thrust power, and resolution.
In this paper, the structure and specifications ofthese modules are introduced.

Linear motion actuator

Fig. 1 Heliostat mirror

Solar light

2. Solar tracking system for solar
thermal power generation

Central Tower
Heat-collector tower

As a measure to efficiently utilize solar energy, amechanism is implemented to ensure that an
irradiated surface, such as the mirror in Fig. 1, faces
the sun at all times (heliostat). The mechanism
consists of a reflective mirror, a linear motion actuator
for tracking the solar angle of elevation, and a rotating
driver for tracking the horizontal angle.
The sunlight reflected by the heliostat is collected by
the solar collector at the central tower, as shown in
Fig. 2. The medium inside the heat collector is heated
to 500 – 1000˚C by the collection of light and used as
the heat source for steam-power generation.

Reflective mirror(heliostat)Fig. 2 Central Tower power plants* Product Engineering Department, Precision Equipment Division-19-

*05_05 13/05/17 18:15 ページ 2

NTN TECHNICAL REVIEW No.80（2012）

So far, pneumatic and hydraulic cylinders are used
for actuators to control the angle of elevation for
tracking the height of the sun, because their structuresare simple and suited for mass production. However,since they require ancillary equipment, air and oil may
leak from inside the cylinder or hose. Therefore, theuse of motors is attracting attention.

nuts for the ball screws. The trunnion bearings are incontact with 4 raceway surfaces inside the housing of
the body to obtain a smooth linear guide, as well as toreceive uneven load applied to the slide shaft tips inthe vertical and horizontal directions.

3.3 Motor, reducer
A 2-phase stepping motor and worm gear reducer
were adopted.

3. Structure and characteristics of the
motorized linear module

3.4 Installation to the bodyFig. 3 and Fig. 4 show the structure and outer viewof the motorized linear module for tracking solar light,
respectively.

Rod ends were installed at both ends of the fixedside (motor side) and floating side (slide shaft side) sothat the mirror is freely movable within a certain range.

3.1 Feed screw

3.5 Other structure

To emphasize maximum thrust, long-lasting
reliability, and position reproducibility for both forward
and backward directions (accuracy of repeatability),ball screws were adopted.

A dust control seal is provided at the inlet/outlet ofthe slide shaft to prevent dust and foreign objects fromentering. In addition, a stainless steel cover is installed
for the motor. The sensor areas and wiring are also
protected by ultraviolet-resistant and heat-resistanttubes. Finally, a weather-resistant mold agent is usedat the connection of exterior components to provide adustproof and drip-proof property.

3.2 Linear motion guideAs shown in Fig. 5, a trunnion structure wasadopted with bearings installed at the tips of 4 shafts
which extended radially from the outer surface of the

Speed reducer

Housing
Rod end

Slide shaft

Protective tube
Feed screw

Motor
A

Rod end
Fixed side

Floating side
StrokeOrigin sensor

Fig. 3 Structure of motorized linear module

Housing

Fig. 4 External appearance of motorized linear module

Bearing

Fig. 5 Part of straight guide（Magnified view A）-20-

*05_05 13/05/17 18:15 ページ 3

Motorized Linear Module for Tracking System of Solar Light /Solar Heat Power Generation

4.2.1 Accuracy of positioning

4. Application of motorized linear
module for tracking solar light

The accuracy of positioning reproducibility was 1/10
or less than the specification.
Also, the accuracy of positioning in the entire stroke
range was very high; with 0.10 mm or less as shownin Fig. 6.

4.1 Operating conditions
Table 1 shows the operating condition of the
motorized linear module. This condition assumes
operation in severe environments, such as a desert.
Reliability was emphasized for developing thisproduct.
Usually, the reflective mirror changes the direction
gradually so that it is precisely synchronized to theorbit of the sun; however, with strong wind, thereflective mirror withdraws at the maximum speed tothe horizontal position for protection from danger. The
load applied to the tip of the module is considered inthis operation.

4.2.2 Verification of thrust performance
The thrust performance required for withdrawal
operation under strong wind was evaluated with themeasurement equipment shown in Fig. 7. From thistest result, the maximum thrust was verified to beadequate.

5. Solar tracking system forphotovoltaic power generation
There are mainly three different methods for lightcollection-type photovoltaic power generation: thefresnel lens method, which utilizes concentricallyarranged annular sections of regular lenses which are
made thinner than regular lenses but have a jagged
cross section (Fig. 8); the reflective mirror method,which collects light at the central focal point byreflecting the sunlight with a concave mirror, and thenagain with a convex mirror placed above at the center(Fig. 9); and the cassegrain lens method, whichcollects light at the central point of the bottom part ofconcave mirror by reflecting the sunlight with theconcave part of the bottom of the lens, and then again

4.2 Product specification and measurement results
Table 2 shows the measurement results of the
motorized linear module product (n = 2). Thisconfirmed that the results sufficiently satisfy theoverall specifications.

Table 1 Service condition of motorized linear module
Operating condition

Item-10〜+45˚C

Ambient temperature

40〜100%

Ambient humidity
Condition for maximum thrust as neededWind speed 40m/s

Table 2 Specifications and validation resultNo.
1

Resolution

2

Maximum speed

Developed product No.1 Developed product No.2

1μm/p

1

1

2.5mm/s or more

CompliedComplied3

Accuracy of repeatability of positioning

Less than ±0.1mm以下±0.004±0.004

4

Thrust output

40m/s of wind speed equivalent

CompliedComplied5

Verification of loss of steps of stepping motor

40m/s of wind speed

No loss of steps

No loss of steps

6

Accuracy of positioning

Reference

0.096mm0.093mm7

Backlash

Reference

0.002mm

0.016mm

8Power consumption

Reference

29W

24W

Accuracy of positioning

0.08

Positioning error mm

Specification

Measured item

0.06

Motorized linear module

No.1

0.04

Loading pneumatic cylinder

No.2

0.02
0
-0.02
-0.04
-0.06
-0.08

0

100

200

300

400

Measurement point mm

500Linear scale

600

Fig. 6 Result of positioning accuracy

Load-sensing sensor

Fig. 7 Thrust force measuring device

-21-

Base

*05_05 13/05/17 18:15 ページ 4

NTN TECHNICAL REVIEW No.80（2012）

with the convex part of the top of the lens (Fig. 10). Allof these types require accurate tracking. NTN has
developed both a high-accuracy and high-rigidity typemotorised linear module so that they can be selected
depending on the size and structure of the solar
panels. Both of these specifications are currently
under evaluation with no problems in continuous
operation.

ClevisPin for body support

Slide shaft

Fig. 11 High precision spec linear module

Pin for body support
Bellows
Focal point

Fig.8 Fresnel lensFig.9 Reflective mirror

Slide shaft (inside bellows)

Fig. 12 High stiffness spec linear module

6. Conclusion
It is expected that demand for renewable energy willcontinue to increase.
We have developed motorized linear modules for
tracking systems that suit the properties of solar
thermal power generation and photovoltaic power
generation, respectively.
While they are currently under empirical evaluation,
improvement in durability and maintainability is being
addressed for product commercialization that meets
market demand.

Focal point

Fig.10 Cassegrain lens

5.1 High-accuracy specification
Fig. 11 shows a motorized linear module aiming formoving small panels with high accuracy.
The body of the module is fixed to the base using
pins for supporting the body at both sides of the
housing; then, the reflective mirror is installed to theclevis at the tip of the slide shaft. The stroke is about
200 mm and a ball screw is used in the actuator forhigh accuracy, similar to the one for solar thermal
power generation. A trunnion structure is adopted forthe linear motion guide.

5.2 High-rigidity specification

Photo of authors

Fig. 12 shows the high-rigidity type solar tracking
system for accurate tracking of sunlight while strongly
supporting medium to large panels. A trunnion
structure is adopted, the same as the other types, forthe linear motion guide to cope with uneven loads andhigh rigidity. The stroke is 500 mm and an accurate
trapezoidal screw is used.
In addition, the slide shaft is equipped with bellows
for preventing dust and drips from entering.

-22-

Tomoyuki OWADA

Masaki KAGAMI

Product Engineering
Department, Precision
Equipment Division

Product Engineering
Department, Precision
Equipment Division*06_06 13/05/17 18:18 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Paper ]Development of Induction Through-hardening and Induction Tempering Methods
with Temperature Control and Microstructual Control in Bearing Steel

Takumi FUJITA*
Nobuyuki SUZUKI*
In this report, induction through-hardening and induction temperingfor bearing rings are introduced. Both technologies determine heatconditions in induction heating by utilizing temperature control andmonitoring of carbon solubility in bearing steel. This means the systemeliminates the need for qualified expertise in the heat condition designof induction through-hardening and induction tempering. The system
will promote replacements of conventional atmosphere furnaces with induction heating furnaces, andresult in a contribution to energy saving in the manufacturing process of rolling bearings andreduction of negative environmental impact.

heating could make penetration control of carbon into
the base metal with good reproducibility, which isnecessary for hardening of bearing steel. In order toclear these concerns, a design technique for theheating coil to provide uniform heating conditions anda method to control organization of the entire productare needed.
In this paper, a method for high-frequency throughhardening and high-frequency induction tempering(hereafter, IH through hardening/tempering) that canminimize the heating time by controlling theorganization of the entire steel bearing raceway ring
(JIS-SUJ2, hereafter SUJ2) is discussed.

1.Introduction
High-frequency induction heating (hereafter, IH) has
advantages in safety, environmental impact, andheating efficiency over other heating methods. It isvery safe against fire because no combustion is used,environmentally friendly because there is no heatdissipation, and efficient because no fuel storage
facility is required. In addition, since this mechanism
offers quick heating taking advantage of electricity, it
is suitable for any quick heating application. Recently,
cooking devices that take advantage of thesecharacteristics, such as cooking heaters and electric
kettles, are found in many homes.
On the other hand, IH is attracting attention inindustrial production 1), 2), due to many advantages in
production facilities, such as piece-by-piece
processing, facility space reduction, quick start/stop,and efficient processing of small-lot products. In thearea of wheel bearings, selective hardening by IH isapplied.
IH has a lot of advantages in industrial production
activities. However, there were not many uses inthrough hardening for general bearings. There are two
reasons for this. One is the perception that the
selective heating property of IH makes it difficult touniformly heat the entire product. The other is theconcern whether the high temperature and short

2. IH through-hardening/temperingsystem
2.1 Overview of the system
Preformed material of SUJ2 with 1% carbon ismade by spherical disperse precipitation of carbide
with 15% of area ratio to improve machinability. Whenthe preformed product is heated to hardening
temperature, carbon within the carbide becomes asolid solution in austenite. The hardening temperatureneeds to be adjusted so that the area ratio of carbide
becomes 6–8% for a good balance of fatigue strength,
wear resistance, and dimensional stability 3). For
through hardening of bearing rings made of SUJ2 in* Advanced Technology R&D Center-23-

*06_06 13/05/17 18:18 ページ 2

NTN TECHNICAL REVIEW No.80（2012）

an atmosphere furnace, sufficient soaking time should
be secured depending on the thickness so that theoverall carbide distribution is kept uniform. On theother hand, with IH, which is basically a selective
heating technology, uniform heating is more difficult
than with an atmosphere furnace. Therefore,technology to control the entire bearing rings material
within the set criteria is necessary. In the following, amethod of controlling the entire material based on the
result of multiple-point measurement of bearing rings
is discussed.
Fig. 1 shows a conceptual diagram of IH systemwith temperature control.
Two radiation thermometers are set at the heater tomeasure the temperature at the surface close to thecoil and far from the coil. The point closer to the coil isheated faster since more flux penetrates it. On theother hand, the point far from the coil is heated slowly
since less flux penetrates it. Now, the position of theradiation thermometer closer to the coil (1) is called
the temperature-controlling side and the position of
the radiation thermometer far from the coil (2) is called
the heating-stop-timing side. The characteristic of thistechnology is the material prediction method based on
the temperature measurement results of the material
at these positions. Together with PID control of thetemperature at the temperature-controlling side, thematerial at that position is predicted in real time, with a
PC based on the temperature measurement results. In
addition, the temperature of the heating stop timingside is also measured to predict the material at that
position, as well. If the bearing rings are cooled
immediately after the material prediction results ofboth the positions reach the control criteria of material,
hardening can be accomplished in the shortest time.
In this system, the material parameter to be predicted
at hardening is set to carbide area ratio, and theHeating coil
Radiation
Product
thermometer (1)(temperature
control side)・High frequency power
supply・Hardening water tank¡Controller
・PLC

2.2 Prediction of carbide area ratio at IH hardening
Fig. 2 shows the carbide distribution of SUJ2
hardened product made with an atmosphere furnace.
The carbide area ratio in the figure is 6.3%. With thistechnology, the carbide distribution is predicted fromthe temperature measurement results using two
thermometers. The prediction can be made by solving
the diffusion equation 4) or using empirical formula.
The method using empirical formula was adopted forthis system. We considered that the prediction
accuracy is higher with the method from experimental
results than with the diffusion equation model because
the latter needs several assumptions. The equations(1) and (2) show the empirical formula.
M = M0 exp (−(kt)n)………………………… (1)k = A exp (−E/RT)………………………… (2)M : Carbide area ratio (%)
t : Heating time
M0 : Carbide area ratio (%) of raw material
T : Temperature (K), R: Gas constantA, k, n, E: Constants obtained from experiment
A, k, n, and E in the formula are obtained byregressing the carbide area ratio of the test samples
hardened under various conditions with equations (1)and (2).

Radiation thermometer (2)(heating stop timing side)

PID temperature control + hardening¡IH unitmaterial parameter to be predicted at tempering is set
to hardness after tempering.
Furthermore, the configuration of PC and controller
in Fig. 1 can also be used in the configuration with
programmable PLC, fast-response AD and DA
conversion unit, controlling unit with PID control, and atouch panel.

Temperature
measurement results¡Personal computer・Temperature control (PID)
・Material prediction such asorganization
・Determination to stop heating

10μm
Fig. 2 SEM image of carbide distribution of SUJ2
quenched by atmosphere furnaces

Fig. 1 Schematic of induction heating systemwith temperature control-24-

*06_06 13/05/17 18:18 ページ 3

Development of Induction Through-hardening and Induction Tempering Methods with Temperature Control and Microstructual Control in Bearing SteelThe carbide area ratio is obtained by analyzing theelectron microscopic image of the organization of test
samples. Fig. 3 shows the results from the calculationof the relation between heating conditions and carbide
area ratio, using the regression formula obtained fromcarbide area ratios of 50 samples of different types.
The carbide area ratio significantly reduces in a shorttime when the heating temperature is higher.

In addition, the calculation of the carbide area ratio isstarted from the austenite temperature when the
carbide starts penetrating.
Fig. 5 shows an example of calculation of carbide
area ratio from the temperature measurement results
at the temperature-controlling side and the heatingstop-timing side. PID is controlled so that thetemperature-controlling side remains constant at
880˚C. The carbide area ratio decreases after thetransformation point is passed along with the rise of
temperature. If we set the allowable range of carbide
area ratio to 6–10%, the shortest heating time, withwhich each temperature measurement position
reaches the allowable range, is 120 seconds. Fig. 6
shows the carbide distribution of the heating-stoptiming side of the bearing rings hardened under theconditions of Fig. 5. The carbide area ratio of Fig. 6 is
9.5% and matched, in general, with the calculation
result of Fig. 5.

Carbide area ratio %

18
16
14
12

800˚C

10
8
6

1000˚C

850˚C

950˚C

900˚C

4
2
0
0

500

1000

1500

2000

2500

3000

Time sec

Fig. 3 Carbide area ratio as a function of holding time
before quenching of SUJ2
1100

Temperature ˚C

Equations (1) and (2) predict the change of carbide
area ratio at constant temperature over time; however,since the temperature of bearing rings while being
heated is not constant, some ingenuity is required for
calculating the carbide area ratio from the temperature
measurement results. Fig. 4 shows the schematic oftemperature measurement results. When the graph isenlarged, you can see that the temperature changes
in steps. The carbide area ratio Mi can be calculatedusing equations (3) and (4) based on temperature Ti
at each step.
n
Mi = M0 exp(−(ki (ti＋Δt)) ) ……………… (3)Heating stop timing side (carbide)

1000
900
800

20

Temperature control side (carbide)
Upper limit
10%

15
10

700

Lower limit 6%

600
500
0

25

Temperature control side (temperature)

Heating stop timing side (temperature)
20

40

60

80

100

5

Carbide area ratio %

1200

0
120

Time sec

Fig. 5 Specific example of carbide area ratio calculated
by equations 3 and 4

ti = n ln (M0 /Mi-1) / Anexp(nE /RTi) ……… (4)ti indicates the time necessary for heating to reachthe temperature of the next step, converted from thechange of the carbide area ratio of the current step. Δt
is the sampling period for temperature measurement.

Mi

Temperature■■■■■■■■■■■■■■■■■■■■■■■■
Ti■■■■■■■■■■■■■■■■■■■■■■■■M2
M1 T2■■■■■■■■■■■■■■■■■■■■■■■■
T1■■■■■■■■■■■■■■■■■■■■■■■■

10μm■■■■■■■■■■■■■■■■■■■■■■■■
Δt
Fig. 6 SEM image of carbide distribution of SUJ2 after
quenching. The austenitization condition and thepredicted carbide area ratio are shown in Fig. 5

Time■■■■■■■■■■■■■■■■■■■■■■■■

Fig. 4 Schematic of time-temperature chart■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■■-25-

*06_06 13/05/17 18:18 ページ 4

NTN TECHNICAL REVIEW No.80（2012）

2.3 Prediction of hardness with IH tempering

Table 1 Heat condition for inner and outer rings of 6206
ball bearing

In tempering, the hardness after tempering isestimated in real time from the temperature
measurement results 5). Equations (5) and (6) show
the empirical formula for obtaining hardness after
tempering.
X=X0−(X0−Xf )(1−exp(−(k' t)n' ) ………… (5)k' = A' exp(−E' / RT ) ………………………… (6)No Location

Hardening

Tempering

1 Outer ring

900˚C IH heating
Carbide area ratio 6–8%
Room temperaturesoluble water cooling

180℃×2h
Thermostatic chamber
heating

2

Inner ring↑

830 〜 860℃
3 Outer ring Atmosphere furnace heating
90˚C semi-hot oil cooling

X : Hardness after tempering (HRC)t : Heating time
X0 : Hardness after hardening (HRC)
Xf : Hardness of raw material (HRC)
T : Temperature (K), R: Gas constantA', k', n', E': Constants obtained from experiment4

Inner ring

230˚C IH uniform heating
Hardness HRC62↑↑

900˚C IH heating
5 Outer ring Carbide area ratio 6–8%
Room temperaturesoluble water cooling
6

Inner ring

7 Outer ring
8

Fig. 7 shows the relation of heating time andhardness after tempering for each tempering
temperature. This relation is obtained, in the same
way as Fig. 3, by regression analysis of the results ofhardness measurement of test samples obtained
under systematically changed tempering conditions. In
addition, the real time calculation for hardness after
tempering for IH is the same as that of casehardening.

Hardness after tempering HRC↑

Inner ring

230˚C IH uniform heating
Hardness HRC62↑↑

Standard product(hardening/tempering with atmosphere furnace)

68(a) Outer ring Heating coil

66
64

180˚C

62

230˚C

60

280˚C

58
56
0

330˚C
30

60

Heating time

90

120

150

(b) Inner ringmin

Heating coilFig. 8 Measuring position of Vickers hardness

Fig. 7 Relationship between hardeness and tempering time

Table 2 Vickers hardness for inner and outer rings of6206 ball bearings

2.4 Material examination results
Using this technology, IH through hardening andtempering was applied to 6206 ball bearing inner/outerrings made of SUJ2. Table 1 shows the list of test
samples. Vickers hardness of 8 types of test samples
in Table 1 was measured at 9 points shown in Fig. 8.
Table 2 shows the average hardness and standard
deviation at all the measurement points. Standard
hardness of bearings made of SUJ2 was, in mostcases, HRC 60-63 (HV696-772). The variance ofhardness in products was within 2 points in HRC

No Location
1 Outer ring
2

Inner ring

3 Outer ring
4 Inner ring
5 Outer ring
6

Inner ring

7 Outer ring
8 Inner ring

-26-

Hardening Tempering
IH

FurnaceheatingFurnaceheating

IH

IH

IH

FurnaceheatingFurnaceheating

Hardness HV
Average763

Standard deviation
7.5

734

24.8

721

6.1

710
702

6.5
5.3

710

11.9

738

5.5

743

8.3

*06_06 13/05/17 18:18 ページ 5

Development of Induction Through-hardening and Induction Tempering Methods with Temperature Control and Microstructual Control in Bearing Steel

Cumulative damage probability %(approx. 50 points in HV). The quality of 6206 ballbearing inner/outer rings processed with thistechnology was good, well within these ranges.

3. Characteristics of IH throughhardening/tempering product3.1 Rolling element fatigue test
Rolling fatigue tests were conducted using the 6206
ball bearing life tester shown in Fig. 9 under theconditions in Table 3. Fig. 10 and 11 show the test
results under clean oil lubrication and lubrication withcontamination. Under rolling fatigue tests with clean oil
lubrication, all three subjects completed testing
without any damage for 5984 h. Assuming the Weibull
slope is the same as the standard products, 10% lifeof IH through hardening/tempering is calculated to2115 h 6), 7), with 90% of reliability, and is equivalent to
the life level of 1624 h for standard bearings.
Also, the life of IH through-hardening/tempering
products under contaminated lubrication showed nodifference from the standard products, after thesignificance test 6), 7).

99.0
90.0
70.0
50.0
10.0
5.0
1.0
0.5

IH through-hardening/temperingStandard

0.1
100

1000

10000

Time hCumulative damage probability %Fig. 10 Comparison of RCF lives for 6206 ball bearing
treated by induction heating and conventional heating(clean lubrication)

Loading
spring

99.0
90.0
70.0
50.0
10.0
5.0
1.0
0.5
0.1
1

IH through-hardening/temperingStandard

10

Time h100

1000

Fig. 11 Comparison of RCF lives for 6206 ball bearing
treated by induction heating and conventional heating(contaminated lubrication)

Test bearings
Driving pulley

3.2 Static crush test and ring crack fatigue test
Table 4 shows the static crush test result of 6812
ball bearing outer rings. IH throughhardening/tempering products showed the same
strength against static crush as the standard products.
Fig. 12 shows the results of a crack fatigue test
using rings of 37.8 mm outer diameter × 29.8 mm
inner diameter × 15 mm width. The continuous drop
single logarithm curve model, standard from theSociety of Materials Science, Japan, 8) was applied tothe data obtained from the fatigue test for analysis.
The 10% fatigue strength of IH throughhardening/tempering product after 107 times was
about 50 MPa lower than the standard product.
However, since there was no significant differencewith a 5% risk factor, both can be considered thesame in strength.

Fig. 9 Rolling contact fatigue (RCF) test rig for 6206 ball bearing

Table 3 Conditions of RCF life testing for 6206 ball bearing
Item

Clean oil lubrication Lubrication with foreign objects

Bearing

6206C3

Maximum contact surface pressure
Maximum shear stress depth
Rotational speed
Lubrication

Foreign objects
Size of foreign objects
Hardness of foreign objects
Estimated life

3.2GPaZ 45˚ =0.16mm
Turbine oil with noadditives
VG56 circulation
lubrication

3000min -1

NTNstandard foreign objects
0.4g/L
Turbine oil with noadditives
VG56 circulationlubrication
KHA30
Gas-atomized powder

100〜181μｍ
HV700〜800

126h (no foreign objects)-27-*06_06 13/05/17 18:18 ページ 6

NTN TECHNICAL REVIEW No.80（2012）
Test piece on the follower side

Table 4 Compressive strengths of SUJ2 rings treated byinduction heating and conventional heating
Heat treatment

Type6812 Ball bearing IH through-hardening/temperingouter ring
Φ78×Φ60×10
Standard deviation

Test piece on the driver side

Strength (kN)
Average Standard deviation
1.50

0.13

1.48

0.02

Lubrication felt
Follower side

Driver side

Load

Fig. 13 Ring to ring type test rig
1400

IH through-hardening/tempering

1300

Table 5 Condition of peering test

Standard

TesterStress MPa

1200

Test piece on the driver side

1100

900
800

φ40×t12，Axial straight

Radial load

1245N

Maximum contact surface pressure
Contact ellipse

10% fatigue strength after 107 times 853.7MPa
10% fatigue strength after 107 times 808.9MPa700
1.E+04

1.E+05

1.E+06

Loading cycle

Atmosphere temperature
Lubricating oil

1.E+07

Oiling method

Loading cycle

3.3 Peeling test and smearing test

2.3GPa

1.48×0.71 mm
4.8×105

Room temperature

Turbine oil with no additives VG46
Felt oiling

14
12
10
8
6
4
2
0

Under thin lubrication conditions, the rolling
elements and raceway rings are in direct contact
causing surface damage with small peeling andcracking. Also, in applications with slide bearings,
surface damage caused by smearing can beobserved. These surface damages are evaluatedusing a two-cylinder tester shown in Fig. 13.
Table 5 shows the peeling test conditions. The
peeling test is conducted by driving the driver-side
shaft with a motor, allowing free rotation of thefollower side. The resistance against peeling wasevaluated with the average of peeling area ratios atthree positions on the rolling contact surface of thedriving side. Fig. 14 shows the peeling test results.
The peeling area ratio of the IH throughhardening/tempering products was equivalent with thestandard products.
Table 6 shows the smearing test conditions. For thesmearing test, the driver side and follower side are
driven with different motors, in order to give sliding at
the contact surface. The test samples are fixed on
both shafts under a predetermined load. Both test
samples are driven for 3 min at 200 min-1 for runningin, then maintaining the rotation speed of the follower
side at 200 min-1. The rotation speed of the driver side2000 min-1

16

Peeling area ratio %Fig. 12 SN Curves of SUJ2 rings treated by induction
heating and conventional heating. The stress ratio is 0.1.

φ40×t12，Axial curvature R60

Test piece on the follower side
Rotational speed

1000

Two-roller testing machine

IH through-hardening/temperingStandard

Fig. 14 Results of peeling test for SUJ2 rings treated byinduction heating and conventional heating

Table 6 Condition of smearing test
TesterDriver side/follower side test pieceTwo-roller testing machine

φ40×t12，Axial curvature R60

Radial load

980N

Contact ellipse

1.37×0.66 mm

Maximum contact surface pressure 2.1GPa
Lubricating oil
Oiling method

Time to smearing

sec

Rotational
speed

Driver side
Follower side

Turbine oil with no additives VG46
Felt oiling

Step up from 200 min-1 in increments of 100 min-1
Constant at 200 min-1

500
400
300
200
100
0

IH through-hardening/temperingStandard

Fig. 15 Results of smearing test for SUJ2 rings treatedby induction heating and conventional heating-28-

*06_06 13/05/17 18:18 ページ 7

Development of Induction Through-hardening and Induction Tempering Methods with Temperature Control and Microstructual Control in Bearing Steelis increased from 200 min-1 in 100-min-1 steps every
30 seconds. The smearing is detected by vibration,and resistance against smearing was evaluated by the
duration of smearing. Fig. 15 shows the smearing test
results. The resistance against smearing of IH
through-hardening/tempering products was equivalent
with the standard product.

Table 7 Merits of induction heating on manufacturing

Features・Inline integration with other facilities
・Heat history tracing of each product・Efficient treatment of small-lot products
・Short shutdown/star-up time・High freedom of layout

sootSafety/environment ・Low
・Flammable gas facilities are not required

3.4 Dimensional stability

Ratio of dimensional change
due to aging ×10-5

High-dimensional stability is required for the
raceway rings for rolling bearings. Fig. 16 shows thedimensional change due to aging when they were kept
at 230˚C for two hours. IH throughhardening/tempering products had smaller
dimensional change than standard products.

30
25

size of facility are required for application to small
products.
For larger products, the difference in productivity
between IH and atmosphere heating will be reduced;
however, it should be noted that different challenges
will surface, such as reduced advantages due tolarger facilities, decarburization and generation ofscale due to increased uniform heating time in theatmosphere, and more difficult uniform heating.

Condition：230˚C×2h

20

4.2 Effect of IH on reduction of environmental load

15

Table 8 shows the estimates from the energysaving and CO2-reduction viewpoints on the effect of
IH on reduction of environmental load. Its power
consumption can be reduced to 74% of power
consumption of electric furnace per kg of productweight (compared to our existing product). Also, CO2
emission can be reduced to 56% of the emission ofheaters that use city gas. In the calculation, CO2
emission per consumed electric power unit was
assumed to be 0.341 kg/kWh, which is electricity-CO2
emission equivalent in the Chubu region in FY2010.

10
50

IH through-hardening/temperingStandard

Fig. 16 Dimensional change of SUJ2 rings treated byinduction heating and conventional heating

4. Consideration
4.1 Advantage of IH in production

Table 8 Reduction effect of using induction heating on
negative environmental impact

As shown in Table 7, IH has many advantages overan atmosphere furnace. However, of special note is
the fact that the stop/start-up time of the facility isalmost zero and that no flammable gas is required.
These points open the possibility of elimination of 24hour operation and elimination of use of flammablegas, which are today instituted in the conventional
heat treatment plants of rolling bearings.
The advantages listed in Table 7 assume theproduction volume, facility cost, and facility space are
equivalent to atmosphere furnace. IH shows highproductivity of quick heating in applications such as
surface hardening, providing equal or moreadvantages over atmosphere furnace heating.
However, for through hardening, since relatively long
heating is required, more units are required for
production of the same volume. Therefore, reductionof cycle time, reduction of facility cost and compact

Heating method

AtmosphereElectricity for heating kWh/kg
City gas Nm3/kg

RX gas Nm3/kg

CO2 emission g/kg

-29-

IH

ElectricalCity gas

0.934

1.261

0

0

0

Nitrogen gas Nitrogen gas RX gas
0

318

0

0.185

430

566

0.464

*06_06 13/05/17 18:18 ページ 8

NTN TECHNICAL REVIEW No.80（2012）

Reference

5. Conclusion

1) Takao Yamazaki: Review of High Frequency
Induction Heat Treating Technology during Past 50
Years, Netsu Shori 50，6 (2010)580-588.
2) Kazuhiro Kawasaki, Yoshitaka Misaka, Fumiaki
Ikuta: Towards the High Performance Induction
Heat-Treatment Accompanied with W-Eco(Ecological & Economical) Effect, Netsu Shori, 50，
4 (2010) 368-376.
3) Saburo Shiko, Kazuo Okamoto, Shozo Watanabe:
Effect of Metallographical Factors on the Rolling
Fatigue Life of Ball Bearing Steel, Tetsu- to –Hagane, 54，13 (1968)1355-1366.
4) Takumi Fujita, Nobuyuki Suzuki: High Frequency
Overall Heating Hardening Method for Bearing
Race under Temperature Control, 61st Meeting of
the Japan Society of Heat Treatment, presentationmaterial, (2005) 41-42.
5) Takumi Fujita, Nobuyuki Suzuki: High Frequency
Overall Heating Hardening Method for Bearing
Race made of SUJ2 under Temperature Control,
62nd Meeting of the Japan Society of Heat
Treatment, presentation material, (2006) 25-26.
6) Takumi Fujita: Rolling Contact Fatigue Life Test
Design and Result Interpretation Methods
Maintaining Compatibility of Efficiency andReliability, NTN Technical Review, 76 (2008) 31-38.

In this paper, a method for achieving IH throughhardening/tempering in the minimum processing time,
while controlling the quality of the entire bearing rings
within the control criteria, was discussed. This
technology is a proprietary system of NTN, which evenallows engineers inexperienced with IH to optimallydesign the processing conditions. We believe thissystem will accelerate the use of IH for heat treatmentof rolling bearings, resulting in improved environmentof production sites and energy saving.

7) T. Fujita: Rolling contact fatigue life test design andresult interpretation methods maintaining compatibilityof efficiency and reliability, J. ASTM international, 7, 6
(2010)Paper ID JAI102492.

8) The Society of Materials Science, Japan,
Committee on Fatigue of Materials, Committee on
Reliability Engineering, Metal Materials Fatigue
Reliability Evaluation Standard JSMSSD-6-02 –SN
Curve Regression Method, The Society of Materials
Science, Japan (2002).

Photo of authors (titles are at the time of development)

Takumi FUJITA

Nobuyuki SUZUKI

Advanced Technology
R&D Center

Advanced Technology
R&D Center-30-

*07_07_2 13/05/17 18:21 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]Rolling Bearing for Environments of Ultrahigh Temperatures

Miki ARIHANA*
Takayuki KAWAMURA**
In recent years the demand of bearings which are used at 400 degree
C for tenter clips, glassmaking facility for solar panels etc,, has been
increasing. Usually, plain bearings are used for such ultra-hightemperature application in atmosphere.
NTN developed the “ULTAGE Deep Groove Ball Bearings for
Ultrahigh Temperature Environment” instead of plane bearings, which
has the advantage of low torque and efficient operation because of rolling bearing. On the other hand in
vacuum environment, NTN adopts “Ultra-clean WB Bearing” by using the application is under 300 degree C.
NTN is now developing new products to achieve 400 degree C for higher performance.
This report introduces the design, feature and performance of “ULTAGE Deep Groove Ball Bearings for
Ultrahigh Temperature Environment” in atmosphere environment, “Ultra-clean WB Bearing” in vacuum
environment and current further study of it.

1. Introduction
Recently, demand for heat resistance is increasing
for bearings used in tenter clips and glassmaking
facilities, etc. and today, their use at around 400˚C isparticularly increasing.
Traditionally, bearings used in 150 to 250˚C oftemperature have heat resistance by adoption of ashield plate made of steel and a seal made of fluoro
rubber, or injection of heat resistant fluorinatedgrease, etc. Plain bearings are mainly used for hightemperature environments at around 400˚C; however,requirement for rolling bearings are increasing forlower torque.
In addition, demand for bearings for use in vacuum
and ultrahigh temperatures for manufacturing

equipment of solar panels is increasing as the global
market for solar cells expands. Operating temperatureof products for this use can reach up to 400˚C;therefore we have worked to establish forward-looking
and superb bearing specifications that withstandvacuum and 400˚C environments.
In this paper, we introduce you to “ULTAGE Deep
Groove Ball Bearings for Ultrahigh Temperature
Environment” of Ultrahigh Temperature Graphite(UTG) type for atmosphere environment and “Ultraclean WB Bearing” for ultrahigh temperature vacuum
environment, which are already introduced in a series
and their current improvement development status.

* ULTAGE is a name created from the combination of “ultimate,” signifying refinement, and “stage,”signifying NTN’s intention that this series of products be employed in diverse applications and is thegeneral name for NTN’s new generation of bearings that are noted for their industry-leading performance.

**Industrial Machinery Engineering Dept., Industrial Machinery Division**Advanced Technology R&D Center-31-

*07_07_2 13/05/17 18:21 ページ 2

NTN TECHNICAL REVIEW No.80（2012）

2. ULTAGE Deep Groove Ball Bearings for
Ultrahigh Temperature Atmosphere
Environment

ensuring smooth rotation.
However, because of the graphite solid lubricant,bearings must not be dropped or exposed to a shock
load.

2.1 Features(1) Long life under atmosphere/ultrahighenvironment

2.3 Solid lubrication mechanism
A solid lubricant with the longest life was selected
after the essential testing of different types and blendsof solid lubricants under the conditions listed in Table
1. In this test, the life was determined when the total
bearing torque reached 147 mN·m. The test results
are shown in Fig. 2. The adopted lubricant was
confirmed to have more than 4 times longer life
compared with WS2 (tungsten disulfide). Fig. 3 shows
the supply mechanism of solid lubricant.
As the bearings rotate, the transfer film transferredfrom solid lubricant to the balls, and then from theballs to the raceway surface of the inner/outer rings
keeps adequate lubrication.1)

Approximately double the durability compared tofluorinated grease for high temperature
specification, due to the adoption of solid lubricantof special composition blend and internal structure
for supplying adequate amount of solid lubricant(2) Lower torqueApproximately a quarter of the starting torque andhalf of the dynamic torque compared to fluorinatedgrease by adoption of ceramic balls and consistent
supply of solid lubricant(3) Improved handling with sealed structure
Reduced leakage of solid lubricant from insidebearings by side ring and non-contact shield plate
installed inside the bearings(cannot be used in a clean environment)

Table 1 Evaluation test condition of lubricant

2.2 Structure
Fig. 1 shows the structure of ULTAGE Deep
Groove Ball Bearings for Ultrahigh Temperature
Atmosphere Environment. These bearings use agraphite solid lubricant with a special composition
blend and optimized lubricant amount to achieve longlife. Also, its structure prevents balls from being offset
causing inner/outer rings to be dislocated even when
the solid lubricant wears out, by installation of a Ushaped metal cover. In addition, the optimum shape ofsolid lubricant, and shield plate and lubricant siderings for guiding rotation of the solid lubricant make itideal for the solid lubricant to remain in contact for

Test bearings

608（φ8×φ22×7）

Load

196N

Rotational speed

1000min-1

Temperature

400˚C

Operating time h

60
50
40
30
20
10
0

WS2

MoS2

WB

Developed
lubricantFig. 2 Evaluation results of solid lubricantsMetal cover
Solid lubricant

Solid lubricant

Transfer
film

Metal coverOuter ring

Ball

Ball

Side ring

Inner ring
Solid lubricant particles

Fig. 3 The supply mechanism of solid lubricant

Fig. 1 ULTAGE Deep groove ball bearings for theatmosphere and ultrahigh temperature environment-32-

*07_07_2 13/05/17 18:21 ページ 3

Rolling Bearing for Environments of Ultrahigh Temperatures

2.4 Various performance tests
2.4.1 High temperature durability evaluation test

2.4.2 Running torque test
The running torque of ULTAGE Deep Groove Ball
Bearings for Ultrahigh Temperature Atmosphere
Environment was evaluated under the conditions
shown in Table 3. As a result, as shown in the torquechart of Fig. 6, the starting torque was found to beabout one-fourth and the dynamic torque was about
half, respectively, compared to the conventional hightemperature fluorinated grease type.

The high temperature durability of ULTAGE Deep
Groove Ball Bearings for Ultrahigh Temperature
Atmosphere Environment was evaluated as shown in
Table 2 and Fig. 4. As a result, they were proven to
double the life of conventional fluorinated grease typeat its operating limit of 350˚C.
Also, even in the atmosphere and a temperature of
400˚C, where the fluorinated grease type cannot be
used, they were proven to have an equivalent
durability as the fluorinated grease type at 350˚C.

Table 3 Measurement conditions of rotating torque6200（φ10×φ30×9）

Test bearings
Load

98N

Rotational speed

530min-1

6200（φ10×φ30×9）

Rotation type

Inner ring operation

Temperature

Room temperature

Table 2 Test condition of high temperature durability
Test bearings
Load

49N

Rotational speed

530min-1

Rotation type

Outer ring rotation

Temperature

350, 400˚C

Test bearings

Driving motor

Running torque mN・m

100

HeaterInsulation

60
40
20
0
-20

Coupling

Existing standard grease type (for up to 250˚C)
Developed product (for up to 400˚C)

80

0

500

1000

1500

Operating time sec

Fig. 6 Results of rotating torqueFig. 4 High temperature endurance testing machine

3. Introduction to rolling bearings for use
in vacuum and ultrahigh temperature

1600(Ultra-clean WB Bearings and their improved version)Operating time h

1400

Since oil and grease cannot be used for lubricating
rolling bearings for use in high temperature andvacuum environments, a solid lubricant is used. A
method of forming solid lubricant film directly on the
raceway through spattering, etc. has been used forsupplying solid lubricant to rolling elements. Thismethod is widely used because it works from the
beginning of the bearing operation; however, thebearings start wearing quickly when the film on the
raceway surface wears out, which ultimate results inseizure. Therefore, long life cannot be expected.
NTN uses a sintered tungsten disulfide ball(hereafter WS2 ball) replacing a steel ball forcontinuous supply of solid lubricant and created “Ultraclean WB Bearings” (Fig. 7). 2)

1200
1000
800
600
400
200
0

Developed High temperature Developed
fluorinated grease productproduct
350˚C
350˚C
400˚C

Fig. 5 Test results of high temperature durability-33-*07_07_2 13/05/17 18:21 ページ 4

NTN TECHNICAL REVIEW No.80（2012）

3.2 Various performance tests

In this section, these Ultra-clean WB Bearings andthe current status of development for improved
durability under vacuum and high temperature (400˚C)conditions are introduced.

In this section, results from various performance
tests for Ultra-clean WB Bearings with sintered balls of
the best mix of WS2 are provided.

3.2.1 Running torqueThe running torque for Ultra-clean WB Bearings
was tested in a vacuum environment under theconditions in Table 5. The result indicated that itprovides lower torque than the conventional special
PTFE film type, as shown in Fig. 9.

WS2 sintered ball

Table 5 Measurement conditions of rotating torqueFig. 7 Ultra-clean WB bearing

Test bearings

608（φ8×φ22×7）

Vacuum level

10-5PaRotational speed

500min-1

Temperature

Room temperatureThrust load

196N

3.1 Durability under high temperature for solid
lubricant sintered balls

30

Running torquemN・mDifferent types of solid lubricants and blends werereviewed for bearings with solid lubricant sintered balls
as shown in Table 4. The result showed that the WS2balls surpassed MoS2 balls in durability against hightemperature and 95% mix of WS2 proved to provide
the longest life.3)

Table 4 Test conditions of endurance test for solid lubricants
Test bearings

608（φ8×φ22×7）

Vacuum level

10-5PaRotational speed

1550min-1

Temperature

200˚C

Thrust load
Determination of life

When abrupt increase of torque is observed

20

10

0

Ultra-clean
WB Bearings

Special PTFE
film type bearings

Fig. 9 Results of rotating torque

83.4N

3.2.2 Durability under high temperature (300˚C)
Durability under high temperature of Ultra-clean WB
Bearings was evaluated under the test conditions inTable 6. The result showed that the high temperature
durability of Ultra-clean WB Bearings was 5 times
higher than the conventional special PTFE film type,as shown in Fig. 10.

Stopped1000

Operating time h

100
10
1

Table 6 High temperature endurance test conditions

0.1
0.01

WS2
60%

WS2
80%

WS2
90%

WS2
95%

MoS2 MoS2
60% 95%

Fig. 8 Test results of endurance test for solid lubricants-34-

Test bearings

608（φ8×φ22×7）

Vacuum level

10-5PaRotational speed

1550min-1

Temperature

300˚C

Thrust load
Determination of life

9.8N
When abrupt increase of torque is observed

*07_07_2 13/05/17 18:21 ページ 5

Rolling Bearing for Environments of Ultrahigh Temperatures

3.3 Development of Ultra-clean WB for improvement

Operating time h

10000

It is expected that the operating temperature ofbearings for vacuum/high temperature conditions foruse in solar panel manufacturing equipment will reachup to 400˚C. Since the Ultra-clean WB Bearings
developed and marketed by NTN (hereafter,
conventional product) can only be used in continuous
operation up to 300˚C under vacuum condition, NTN isdeveloping bearings that can be used under 400˚C.

Stopped1000

100

3.3.1 Specification of bearings
10

Ultra-clean
WB Bearings

Endurance was evaluated in the following two
specifications:
Specification (1): Replaced a conventional stainlesssteel ball with a ceramic ball (oneWS2 sintered ball)Specification (2): Added another WS2 sintered ball of
the Specification (1) so two balls areused (see Fig. 12)

Special PTFE
film type bearings

Fig. 10 Results of high temperature endurance tests

3.2.3 Heavy load endurance
Heavy load endurance of Ultra-clean WB Bearings
was evaluated under the test conditions in Table 7.
The result showed that the heavy load endurance of
Ultra-clean WB Bearings was 9 times higher than theconventional special PTFE film type, as shown in Fig.
11.

Table 7 Heavy load endurance test conditions
Test bearings

608（φ8×φ22×7）

Vacuum level

10-5PaRotational speed

500minTemperature

Room temperatureThrust load
Determination of life

When abrupt increase of torque is observed

Operating time h

10000

Ceramic ball

-1

WS2 sintered ballFig. 12 Arrangement of ceramic balls and WS2 balls ofdevelopment bearing ②

256N

3.3.2 Endurance under ultrahigh temperature(400˚C)
StoppedThe newly developed product and conventional
product were evaluated under the test conditions
shown in Table 8. The result showed that thedeveloped product was more than double inSpecification (1) and more than 60 times inSpecification (2) in endurance compared to theconventional product, as shown in Fig. 13.

1000

100

Table 8 Ultra-high temperature endurance test conditions
10

Ultra-clean
WB Bearings

Special PTFE
film type bearings

Fig. 11 Results of heavy load endurance tests-35-

Test bearings

608（φ8×φ22×7）

Vacuum level

10-5PaRotational speed

1000min-1

Temperature

400˚C

Thrust load
Determination of life

When abrupt increase of torque is observed

196N

*07_07_2 13/05/17 18:21 ページ 6

NTN TECHNICAL REVIEW No.80（2012）

4. Summary

Operating time h

10000

In this paper, we have introduced ULTAGE Deep
Groove Ball Bearings for Ultrahigh Temperature
Atmosphere Environment, which can be used in
atmosphere and ultrahigh temperature of 400ºC. We
will proceed to develop bearings for further
applications such as tenter clips and glassmaking
facilities.
We have also introduced Ultra-clean WB Bearingsand bearings for use in vacuum and ultrahightemperature of 400˚C, currently under development, tobe used in solar panel manufacturing equipment, themarket for which is globally expanding.
NTN strives to contribute to extending maintenanceintervals, improving reliability, and saving energy ofvarious systems for use in ultrahigh temperature bydeveloping new products that match the requirementsof the market.

1000

100

10

Developed
bearings①

Developed Ultra-clean
bearings WB Bearings(conventional②
product)

Fig. 13 Results of ultra-high temperature endurance test
at 400˚C

Reference
1) Japanese Society of Tribologists, Solid Lubricant
Technical Committee, New Edition Solid Lubrication
Handbook, 1st edition, Yokendo, P56 (2010)
2) Ryouichi Nakajima, Masaaki Honda: Ultra-Clean
Bearings for Clean Environment, NTN TECHNICALREVIEW No. 74 (2006) 76–81
3) Hideyuki Tsutsui, Masakazu : Endurance ofbearings with sintered tungsten disulfide balls in
vacuum, Japanese Society of Tribologists,
Tribology Conference 2007-5 Proceedings (2007)
173–174

Photo of authors

Miki ARIHANA

Takayuki KAWAMURA

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division

Advanced Technology
R&D Center-36-

*08_*09 13/05/17 18:22 ページ 1-37-

*08_*09 13/05/17 18:22 ページ 2

NTN TECHNICAL REVIEW No.80（2012）[ Technical Article ]Machine Tool Main Spindle Bearings
with “Air Cooling Spacer”

Yuushi ONDA*
Mamoru MIZUTANI*
Masatsugu MORI**High speed and high rigidity performances are always required for high precision bearings for machine
tool main spindles. However, since both high speed and high rigidity increase operating temperature, it
has been very difficult to achieve high speed and high rigidity performance at the same time. In this paper
new “air cooling spacer” technology, which will be able to decrease the operating temperature of bearings
by applying cooling air onto the outside diameter of inner ring spacer located, is introduced.

(core-cooling main spindle) and a device to adjust
running bearing pressurization; however, thesemethods require complex main spindle designs. 2)
To cope with this issue, NTN is developing air
cooling spacer technology , which will be able todecrease the operating temperature of bearings byapplying cooling air onto the outside diameter of innerring spacer . This technology can be used to achieve
high speed or high rigidity of machine tools, or both,without complicating the main spindle design. In thispaper, the performance of this air cooling spacer isintroduced.

1. Introduction
Machine tools are used for machining parts ofautomobiles, aircrafts, and construction equipment.
Therefore, machine tools are called “mothermachines,” and are under constant pressure toincrease machining efficiency and improve quality ofmachined products.
In order to improve machining efficiency, increase ofrotational speed of the main spindles is effective.
However, when the bearings run in high speed, thecentrifugal force on the rolling elements will increase.
Therefore, lightweight ceramics are used for rolling
elements to reduce this centrifugal force. 1)
Also, for quality improvement of machined products,
bearings are required to improve rotational precisionand increase shaft rigidity. For improving rotational
precision, it is effective to increase the number ofrolling elements and improve dimensional precision for
inner/outer rings. On the other hand, for higher rigidityof bearings, it is effective to increase the size of rolling
elements and raise the pressurized amount.
These improvements tend to increase temperaturein the bearings; therefore, it is difficult to achieve highspeed and high rigidity at the same time. However, forrecent machining centers and field-assisted
machining, both high speed and high rigidity aredesired for space saving and process integration.
Technology for fulfilling high speed and high rigidity
has been realized by supplying coolant in the spindle

2. Structure of bearings with air
cooling spacer
Fig. 1 shows the structure of bearings with air
cooling spacer. The outer ring spacer installed in thecenter of a back-to-back (DB) arrangement is provided
with environment friendly nozzles, which are air
cooling nozzles, independent from the air oil supplying
nozzle for lubricating bearings. The air cooling nozzles
are offset from the spindle center at three locations on
the circumference of the ring, pointing toward the
direction of rotation. Room temperature air injectedfrom the air cooling nozzles passes between the innerring spacer and outer ring spacer, and through thebearings swirling toward the direction of the rotation tocool off the bearings.

**Machining Tool/Aerospace Engineering Dept., Industrial Machinery Division**Advanced Technology R&D Center-38-

*08_*09 13/05/17 18:22 ページ 3

Machine Tool Main Spindle Bearings with “Air Cooling Spacer”

up to 17,000 min-1 without air cooling, then while thespeed was maintained at 17,000 min-1, pressure ofcooling air was gradually increased to examine its
relation with the temperature of the bearings. The
result is shown in Fig. 4. The higher the cooling air
pressure, the more the temperature dropped. The
slope of the temperature drop depended on the offset
amount.
The condition for the least temperature drop waswhen the offset amount was 0, which is to inject air at
the right angle to the inner ring spacer. The more theoffset amount, the lower the temperature becomes.
Also, to confirm the relation between the offset
amount and the temperature drop, the air pressure
was maintained at 400 kPa. From Fig. 4, the optimum
offset amount was assumed to be between 25 and34.5 mm; therefore, offset amount of 33 mm was also
evaluated. The result is shown in Fig. 5. The
maximum temperature drop, which is the best air
cooling effect, was found to be at the offset amount ofapprox. 30 mm, which is at about 80% of the radius of
the inner ring spacer outer diameter.
In addition, if the direction of air injection is set
backward to the rotation the temperature drop of theinner ring was about 3˚C even though it was cooled at
400kPa; therefore, it is important to match thedirection of cooling air injection with the direction of

Air cooling spacer(outer ring spacer)

Cooling air nozzle
Test bearings

Cooling air

Environment friendly
nozzle for air oil lubrication

Motor side

Inner ring spacer

A

OffsetAir cooling spacer(outer ring spacer)
Inner ring spacer

Cooling air nozzle

Direction of inner ring rotation

Test bearings andair cooling spacer

View A

Fig. 1 Structure of the bearing with air cooling spacer

Driving motor

Supporting bearings

3. Evaluation of bearings with air
cooling spacer
3.1 Comparison of temperature
3.1.1 Evaluation of air cooling spacer setting
Fig. 2 shows an overview of the test machine used
for the evaluation and Table 1 shows the test
conditions. The bearings under test are arranged
back-to-back (DB) and driven by a motor. The outerring of the bearings and outer tube of the motor arecooled by circulating oil during the test.
The effect of the air cooling spacer was confirmedby the temperature of inner/outer rings.
First, the impact of the injection points of the cooling
air nozzles and amount of cooling air on thetemperature of bearings was evaluated. Fig. 3 shows
the injection points of the nozzles in four different
positions from (1) to (4), with different offset amount(hereafter, offset amount) from the center of thespindle. In the evaluation test, the spindle was sped

Fig. 2 Structure of testing equipment

Table 1 Test conditions
Test bearings

7! 0×!1 10×20
5S-2LA-HSE014 equivalent

Pressurization method Fixed position pressurization (ON after incorporation)
Method: Air oil
Supply oil amount: 0.03mL/5min
Lubrication
Lubricating oil: ISO VG32
Air flow: 40NL/min
Outer cylinder cooling Yes, tuned to room temperature (21±1˚C)-39-*08_*09 13/05/17 18:22 ページ 4

NTN TECHNICAL REVIEW No.80（2012）34.5

the rotation and to inject air to the surface of therotational spacer, in order to increase the cooling
effect.

25
13

②

③

④

①

3.1.2 Combined test with the air oil lubrication
nozzleNext, air cooling effect was examined between
standard nozzles widely used for air oil lubrication
shown in Fig.6 and environment friendly nozzles. The
result showed that even with the standard nozzles, aircooling effect could be confirmed, in the same way as
the environment friendly nozzles, as shown in Fig. 7.
In addition, the rotational speed for the inner ringtemperature to reach 60˚C was 20,500 min-1 whencooling air was injected, compared to 16,000 min-1

④
③
②
①

①
②
③

④

Fig. 3 Off set amount of cooling nozzles from the center

70

0

Amount of cooling air NL/min

97

157

217

65

338

34.5mm
25mm
13mm
0

60

Temperature ˚C

278Standard air oil
lubrication nozzle

Cooling air nozzle

55
50

Inner
ring

45
40
35
30

Outer
ring
0

100

200

300

400

500

Pressure of cooling air kPa

Fig. 4 Air pressure (amount of cooling air) vs.
temperature by each offset amountFig. 6 Assembly with standard nozzles

16

Temperature decrease ˚C

Temperature of inner ring ˚C

70

14

Inner ring

12
10
8
6
4

Outer ring

2
0

50

10

20

30

40

With aircooling(400kPa)

40
30
20
10
0

0

Without air cooling

Environment friendly nozzle (Fig.1)
Standard nozzle (Fig.6)
Environment friendly nozzle
Standard nozzle

60

0

5000

10000

15000

Rotational speed min

Cooling nozzle offset amount mm

Fig. 5 Temperature decrease by cooling nozzle offset amount

20000
-1

Fig. 7 Test result of inner ring temperature

-40-

25000

*08_*09 13/05/17 18:22 ページ 5

Machine Tool Main Spindle Bearings with “Air Cooling Spacer”

without cooling air, meaning that an increase inoperating speed of 28% was achieved with cooling air
injection.

4. Summary
We have introduced bearings with air coolingspacer for high speed and high rigidity for bearings ofmachining tool main spindles. Air cooling spacer canbe applied not only to angular contact ball bearings
evaluated in this paper, but also cylindrical roller
bearings and, with some more improvement, bearings
with grease lubrication, contributing to the reduction oftemperature for the entire main spindle.
NTN will continue evaluating this air cooling spacer
to concurrently achieve high speed and high rigidity ofmachine tool main spindles.

3.2 Comparison of generated noise
Noise level of standard air oil lubrication nozzle andenvironment friendly nozzle are compared with orwithout cooling air from the cooling air nozzle. The
noise was measured with a broadband precisionsound-level meter placed at 45˚ × 1 m high. Fig. 8
shows the rotational speed and results frommeasurement of A-weighting sound level.
With no air cooling, the standard nozzle generated
82–86 dBA as opposed to the lower 76–79 dBA of theenvironment friendly nozzle. With the standard nozzle,
the injected high-speed air directly hits the rolling
elements, while the environment friendly nozzle injects
air to the inner ring; therefore, it was evident that theenvironment friendly nozzle produces less noise fromthe past experiments. 3)
On the other hand, with air cooling, the standard
nozzle generated 81–86 dBA while the environment
friendly nozzle produced 87–97 dBA. Due to massive
cooling air, the environment friendly nozzle produced
more noise; however, no difference was recognizedwith or without cooling air with the standard nozzle,
resulting in lower noise level than the environment
friendly nozzle.
It is conceivable that the reason for the lower noiselevel with the standard nozzle is that the opening
between the air oil nozzle’s inner cylindrical surface
and inner ring spacer outer cylindrical surface issmaller than in the case of the environment friendly
nozzle, and that the cooling air is diffused at a position
more distant from the rolling elements due to thedifference of the position of the nozzle tips.
We will study and evaluate further for reducing thenoise.

Reference
1) NTN Catalog: Precision Rolling Bearings, CAT. No.
2260-IV/J 11.06.03
2) Kenji Fujii, Masatsugu Mori: Pressurization Switch
Unit, NTN TECHNICAL REVIEW No. 60 (1992)
3) Kenji Fujii, Masatsugu Mori, Yoshimi Ota: Noise
Reduction for Machine Tool Main Spindle Air Oil
Lubrication Bearings, The Japan Society for
Precision Engineering, Autumn Meeting 2000
Proceedings (2000) 449.

Noise level dBA

100
95
90
85

(400kPa)8075
70

Environment friendly nozzle (Fig.1)
Standard nozzle (Fig.6)
Environment friendly nozzle
Standard nozzle

Without air cooling

65
60
55
50

Authors

With aircooling

0

Background noise
5000

10000

15000

Rotational speed min

20000

25000

-1

Fig. 8 Noise level with inner ring spacers with different type

-41-

Yuushi ONDA

Mamoru MIZUTANIMasatsugu MORI

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Advanced Technology
R&D Center*09_*09 13/05/17 18:23 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Article ]Parallel Link High Speed Angle Control Equipment(PHACE)
Hiroshi ISOBE*
Yukihiro NISHIO*NTN has developed a Parallel link High speed Angle Control
Equipment (PHACE) that can control the angle with two-degrees-offreedom quickly and over a wide range (max bend angle : 90 deg.).
Adapting the unique parallel link mechanism enables both compact
size and wide operating range. In addition, by developing the drive
mechanism and control methods, positioning accuracy is improved by more than ten times compared toconventional equipment, and higher speed control is realized.. Furthermore, we have developed ateaching console that facilitates equipment implementation on the production line.

This parallel link mechanism, as shown in Fig. 1,
places three rows of link systems (1st link system, 2ndlink system, 3rd link system) each consisting of baseside arm member, central link member, and tip-side
arm member, in parallel, between the base-side link
hub and the tip-side link hub. The joints of these linkmembers are arranged in turning pairs, which use
rolling bearings such as deep groove ball bearings
and angular ball bearings to reduce rotational
resistance. In addition, backlash in the joints iseliminated by applying pressure to the bearings. Also,
as shown in Fig. 2, the rotational axes of central linkmember and arm member, and the rotational axes oflink hub and arm members are arranged so that theymeet at one point, to enable a configuration thatcombines two spherical link mechanisms (mechanism
to allow each link to operate around the sphere) as
shown in Fig. 3. As a result, high speed and broad
operation range are achieved because the constantdistance between the center of the base-side link hub
and the tip-side link hub is maintained, and the baseside and the tip side operate symmetrically against theintersecting plane of two spherical links. Also, cables
and tubes can be inserted through an ample space inthe center and those cables, etc. are not twisted evenwhen the turn is repeated on one direction.

1. Preface
NTN has been developing angle control equipmentthat can control tip angle by actuators at high speed
and with two degrees of freedom, throughimprovements to the constant velocity joint linkmechanism.1)
By increasing the number of axial drives of the
Parallel Link High-Speed Angle Control Equipment(hereafter referred to as PHACE) from 2 axes to 3
axess, positioning accuracy is improved to ±0.05 mm
or less, and backlash is eliminated from the driving
mechanism. In addition, we have developed a highly
maneuverable teaching console anticipating its
introduction to production lines.
This paper will introduce the features of PHACE,
explain the measurement method for accuracy of
position reproducibility, and provide an example of
implementation on grease application equipment.

2. Parallel link mechanism
In general, a parallel link mechanism is amechanism to connect base and end effector withmultiple links in parallel, with different types of
implementation such as the Steward-Gough type, therotational type, and the linear fixed type.2) The parallel
link mechanism adopted in PHACE is different fromthose widely used, and includes a unique structure
described as follows.
*New Product Development R&D Center-42-*09_*09 13/05/17 18:23 ページ 2

Parallel Link High Speed Angle Control Equipment (PHACE)

2nd link system
Tip-side link hub

3. Structure and features of thedeveloped product3rd link system

Fig. 4 shows the developed PHACE. PHACE
consists of a fixed driving mechanism and movableparallel link high-speed angle displacement
mechanism (hereafter, high-speed angle displacement
mechanism) and is installed with the tip side of thehigh-speed angle displacement mechanism
downward. Also, teaching operation is conductedusing the dedicated teaching console shown in Fig. 5.
This product was designed for applying grease andtherefore, the bend angle θ (see Fig. 3) is limited to45˚or less; however, it has capability of up to 90˚.
Table 1 shows the main specifications of PHACE.

Tip-side arm material
Central link material
1st link systemBase-side arm material
Base-side link hub

Fig. 1 Unique parallel link mechanism

Crossing
Central link material

Base-side link hub
Bearing

Axis ofrotation

Drive mechanism

Axis of rotation

Parallel linkhigh speed angle
displacement
mechanism

Tip-side link hub
Tip-side arm material

Base-side arm material

Fig. 4 PHACE

Fig. 2 Connecting Section between each part

Angle of traverseφ

Spherical link
(tip-side)
Central link material

Bend angleθ
Tip-side link hub
Center of
spherical link

Spherical link(base-side)
Base-side link hub

Fig. 5 The dedicated teaching consoleIntersecting
plane

Table 1 PHACE specification

Cable

Item
WeightFig. 3 Spherical link mechanism

10kg

Maximum payload(inertia moment)

Max. 1.0kg（6300kg・mm2）

Operating range

Bend angleθ：±45deg
Angle of traverseφ：360deg(unlimited)

Accuracy of positioning
reproducibility
Motor output-43-

Specification±0.05mm or less50W×3

*09_*09 13/05/17 18:23 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

3.1 Parallel link high-speed angle displacement
mechanism

Motor

Fig. 6 shows the high-speed angle displacement
mechanism and Table 2 shows its main specifications.
Angular ball bearings are set for each turning pair andpressure is applied to eliminate backlash in the joint.

Motor

Drive powertransmission
mechanism
Motor

Base-side link hub(a) Flat structure of the drive mechanism

Base-side arm material
Central link material

Coupling

Motor

Spur gear

Tip-side arm material
Tip-side link hub

Fig. 6 Parallel link angle displacement mechanism

Table 2 Specification for the parallel link angle
displacement mechanism
Item

Specification

Size (outer diameter×height)

100×90mm

Weight560g

Material

Aluminum + steel

High speed angle
displacement mechanism

Base-side arm material(b) Driving force transmission mechanism
Fig. 7 Drive mechanism

3.2 Driving mechanism
Motor rotational angle

Fig. 7 shows the driving mechanism. As shown in
Fig. 7 (a), each of the three link systems is equippedwith a motor and the size of the driving mechanism isabout 450 mm including the motor. Also, the motors
are placed horizontally to achieve a low profile
structure. The drive power transmission mechanism,as shown in Fig. 7 (b), transmits power from the motor
to the base-side arm member through a spur gear
attached to the base-side member.

Arm rotational angle β1 to β3
Transformation
Inversetransformation

HeightTip-side
link hub

Bend angleθ
Angle of traverseφ

YWork plane

3.3 Control method
X

PHACE controls the attitude using relational
expression between the arm rotational angles β1 to
β3 of the base-side arm member and the attitude ofthe link hub at the tip side (bend angle θ, angle oftraverse φ). As shown in Fig. 8, the inverse
transformation of this relational expression can givethe arm rotational angles β1 to β3 from the commandof the tip-side link hub attitude (bend angle θ, angleof traverse φ) and control the arm rotational angles β1
to β3 with three motors. As a result, the positioning
speed can be increased even with small motors. In
addition, we have improved the positioning accuracy
by adopting a control method to cancel out backlashof the drive mechanism by allowing small mutualinterference of three base-side arm members.

Fig. 8 Control method

3.4 Dedicated teaching console
Here is an overview of the dedicated teaching
console developed for introduction to production lines.
Table 3 shows the key specifications. This console
enables command input for the tip-side link hub
attitude by inputting the bend angle θ and angle oftraverse φ, and in addition, allows teaching by
inputting position with the XY coordinates of the actual
work plane, as shown in Fig. 8. Furthermore, it
includes a teaching function that can memorize theposition adjusted by fine tuning operation of the-44-*09_*09 13/05/17 18:23 ページ 4

Parallel Link High Speed Angle Control Equipment (PHACE)

4.1 Measurement method

control buttons on the console, shown in Fig. 9, whichbrings improved efficiency when the work order ischanged on the shop floor and prevents operational
errors. Other operations such as setting conditions foreach motor gain and registered pattern, and XY stage
teaching as described below (see 5. Examples ofapplication) are also available.

As shown in Fig. 11, for an arbitrary command
pose, positioning from 16 directions was conducted for10 cycles each, and accuracy of positioning
reproducibility of the tip of the tip-side link hub was
calculated.
As shown in Fig. 12, a virtual plane is defined as aplane perpendicular to the center axis of the tip-side
link hub, at the distance D from the rotational center of
the tip-side link hub (equivalent to the position of the
tip of the tip-side link hub). The attained pose (Px, Py)
is defined as the position where the center axis of the
tip-side link hub and the virtual plane intersect. As
shown in Fig. 13, accuracy of positioning
reproducibility is defined as the average of theattained poses and two times the maximum distance
between each attained pose (Px, Py) and the average(“±maximum distance”). The attained pose (Px, Py) is
calculated by the following equation (1).

Table 3 Console specification
Item

Specification

Size (W×L×H)

224×174×87.1mm

Weight1kg or less

(Px ,Py)=(D×tan(αy−αy), (D×tan(αx−αx)) ……(1)where, αx，αy：inclination angles around axis X, Y

αx，αy：average ofαx,αyof each command pose
The output signal of this inclination angle sensor
varies by very small vibration. Therefore, the output
signals were passed through low-pass filter and to an

Fig. 9 Operation screen of the teaching console

Command pose（θ，φ）

4. Measurement of accuracy of position
reproducibility by inclination angle
sensorAs shown in Fig. 10, a 2-axis inclination angle
sensor is installed at the tip of PHACE to measureaccuracy of position reproducibility of the tip position.
The measurement method and condition was basedon JIS B 8432 “Manipulating Industrial Robots –Performance Criteria And Related Test Methods.” 3)

Fig. 11 Positioning direction

D
Center of rotation

PHACE

Inclination
angle sensor

Tip-side
link hubA

Mounting fixture

Virtual plane

Fig. 10 Device for the measurement of the positioning
accuracy

Fig. 12 Measuring method-45-

*09_*09 13/05/17 18:23 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

4.4 Consideration

analog meter as differential inputs then the moving
average was calculated (fluctuation of measured
values: ±0.01 mm or less).

Condition
1 kg

Sensor + mounting fixture

Commandrotational speed

8min-1

Equivalent to synthesizedrotational speed for base-side
arm materials

Direction of
positioning

16 (9)

Value in parentheses is for
θ= 45˚See command pose in Table 5

Attained pose

αyAttained pose (Px，Py)

Y

Average of attained posesX

αx

Maximum distanceX

Fig. 13 Repeat positioning accuracy

4.2 Measurement condition
Table 4 shows the measurement conditions. The
maximum values of this equipment were used for load
and rotational speed.
Table 4 Measurement condition

9

Measurement points
Number ofmeasurements

160 (90)

RemarkValue in parentheses is for
θ= 45˚

PHACE

4.3 Measurement results

1024 pixel

Item
Weight of load

The validity of the measurement of accuracy of
positioning reproducibility of this equipment using an
inclination angle sensor was considered. The
unidirectional accuracy of reproducibility, 3) separately
measured, was about twice the accuracy ofmultidirectional accuracy of positioning reproducibility
shown in Table 5. Therefore, we determined that theaccuracy of positioning reproducibility of this equipment
is represented by multidirectional accuracy of
positioning reproducibility.
We also verified the results with the accuracy
measurement by image processing shown in Fig. 14.
Measurement by image processing can be highly
accurate by taking an enlarged image of a 4-quadrant
circle marker and calculating to the resolution less than
the image pixel size (fluctuation of measured values:±0.005 mm or less). When comparing the accuracies of
positioning reproducibility by these measurement
methods, we verified that they were equivalent.
This measurement method using an inclination angle
sensor does not require changing settings ofmeasurement equipment for each command pose andthe measurement can be made automatic by indicating
the recording timing of the sensor output signals withtrigger signals, etc., resulting in a shorter testing period.
Therefore, we decided to select the measurement
method of using an inclination angle sensor formeasuring accuracy of positioning reproducibility of thisequipment, based on the above consideration.

Quarter circle marker
Measuring Point

Table 5 shows the measurement results ofaccuracy of positioning reproducibility. The accuracyof positioning reproducibility is max. ±0.025 mm.

(a) Measuring Point

1024 pixel(b) Image data

Fig. 14 The measuring method using image processing
Table 5 Measurement result
Command pose
θ [deg]φ [deg]Accuracy of positioning
reproducibility (mm)00±0.01225

0±0.01725

90±0.02325

180±0.01925

270±0.0184545±0.01445

135±0.01845

225±0.02545315±0.021

5. An example of implementation(Grease Applicator)
An example of grease application equipment
configuration using PHACE is shown in Fig. 15 andFig. 16. With this configuration, a dispenser (constantliquid delivery equipment) is attached at the tip of the
tip-side link hub of PHACE, which is installed
downward against the stand, so the dispenser canapply grease on the work-piece placed on the XY-46-

*09_*09 13/05/17 18:23 ページ 6

Parallel Link High Speed Angle Control Equipment (PHACE)

stage. This dispenser can discharge grease at a
distance by controlling high pressure air with anelectromagnetic valve. Three motors used by PHACE
and two motors used by the XY stage are collectively
regulated by one controller. With this configuration,
one dedicated teaching console can perform theteaching operation and parameter setting for both the
PHACE and the XY stage.
The advantages of the grease applicationequipment in this configuration are as follows:(a) One sample of dispensing(b) Another sample

Fig. 17 Sample of dispensing(1) Reduction of tact time
About 10 points of grease application per second
can be made resulting in reduction of tact time.

6. Summary(2) Compact footprint
As the movable components of PHACE are small,the equipment can be made compact.

This paper has introduced the structure of Parallel
Link High-Speed Angle Control Equipment (PHACE)
with two degrees of freedom, described themeasurement method of positioning accuracy, andprovided an application example.
NTN is planning to expand the market of this device
in addition to grease application equipment, for such
purposes as adhesive applicators camera or laser
inspection devices, camera platforms, and so on by
expanding functions and reducing cost.

(3) Enables grease application for components
with complex shape
Grease can be applied from a slanted orientationby controlling the dispenser attitude by PHACE;
therefore, grease can be applied to objects that are
not directly seen from the top as in Fig. 17 (a) andthe vertical side of column-like objects as in Fig. 17(b).

Reference
1) Keisuke Sone, Hiroshi Isobe, Koji Yamada: High
Angle Active Link Equipment, NTN TECHNICALREVIEW No. 71 (2003) 70–73
2) The Robotics Society of Japan, Robotics Hand
Book, New Edition, Corona Publishing, 297, 2005
3) Japanese Standards Association, JIS Handbook
14, 2011 edition, 1585–1662, 2011

PHACE

Stand
Dispenser

XY stage

Fig. 15 System configuration example of grease dispensing

Motor driver
Motor
High speed angle
displacement
mechanism

Dedicated teaching
consoleUpper controller

Stand

Photo of authors

DispenserWork
platform

XY stage

Work

Motor

Controller

Motor driverFig. 16 System configuration-47-

Hiroshi ISOBE

Yukihiro NISHIONew Product Development
R&D Center

New Product Development
R&D Center*10_*09 13/05/17 18:25 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Article ]Development of the Lubricating Oil Supply Unit
with Self-generating Power Supply
Kaoru OMOTO*
Hiroyoshi ITO*
For the purpose of life time extension of the grease lubricated bearing formachine tool spindle, NTN developed a lubricating oil supply unit with the selfgenerating power supply.
This lubricating oil supply unit, which includes a self-generating power
supply, pump, and lubricating oil, is built into the outer ring spacer. Testing on
angular contact ball bearings confirmed that it is possible to supply adequate
lubrication by using a standalone lubrication supply unit.

1. Introduction
converter), power supply/controller, pump, andlubrication oil tank are built-in inside of the unit to draw
lubrication oil out with the pump and supply oil to thebearing raceway through the nozzle using thegenerated power.
The nozzle is installed near the bearing outer ring
raceway, as shown in Fig. 2 (a). Fig. 3 shows thenozzle hole and (a) and (b) show the status before
and after discharge of lubricating oil.

For bearings with grease lubrication, maintaining an
oil film for a long time with minimum grease is a criticaltask. NTN has been developing a “New greaselubrication system, MQGS (Minimum Quantity base oilof Grease Supply lubrication),” 1) which supplies asmall quantity of grease base oil while maintaining oil
films.
Based on the minimum oil supply technology wehave cultivated, we have added an electronic
application technology to develop a lubrication supply
unit with a self-generating power supply which
supplies grease lubrication to the bearings.
This unit lubricates itself without any external power
supply or external control. In this paper, we will
discuss this lubricating oil supply unit with self-power
generation (hereafter referred to as the “lubricating oil
supply unit”).

2. Configuration
2.1 Overall structure
Fig. 1 shows the external view of the lubricating oil
supply unit with self-generating power supply. The
lubricating oil supply unit is installed on the inside of
the outer ring fixed spacer and can be separated fromthe bearings, as shown in Fig. 2 (a). Also, as shownin Fig. 2 (b), power generator (thermoelectric

Fig. 1 Lubricating oil supply unit with self-power
generator*Industrial Machinery Engineering Dept., Industrial Machinery Division-48-

*10_*09 13/05/17 18:25 ページ 2

Development of the Lubricating Oil Supply Unit with Self-generating Power Supply

Outer ring fixed
spacerAngular contact ballbearings

Lubrication oil supply unit
Inner ring rotational
spacerNozzleMount
Enlarged

Grease reservoir(a) Section view of lubricating oil supply unit and bearing

Power supply/controller

Power generator

Thermoelectric converter(Peltier module)

Outer ring fixed
spacerElectric storage
deviceInner ring rotational
spacerPump

Lubrication oil tankNozzle(b) Inside of lubricating oil supply unit
Fig. 2 Schematic structure of lubricating oil supply unit with self-power generator

Lubrication oil(colored red)Nozzle hole(a) before discharge(b)after discharge
Fig. 3 Discharge of lubricating oil-49-

*10_*09 13/05/17 18:25 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

2.2 Power generator and power generation method

3. Features

The electrical control block of this unit is shown in
Fig. 4. A power generation method to obtain sufficientenergy for powering the control devices and charging
the electric storage device is necessary.
In general, heat, vibration, magnetism(electromagnetic induction) are used for power
generation. This lubricating oil supply unit adopts amethod of converting differences in the temperature
between the inner and outer rings (thermal energy)
generated by the rotation of bearings into electricalenergy by using a thermoelectric converter.
The thermoelectric converter used in this unit is thePeltier module, which generates electricity when thereis temperature difference between side A and side B
in Fig. 5. Since the Peltier module is small, it can beinstalled between the inner ring spacer and the outerring spacer of the bearings.

(1) No external power or lubrication is necessary
since the self-generating power supply andlubrication reservoir are built-in.
(2) Same configuration of bearings and spacers as
the conventional unit.
(3) Lubricant supply is minimized by utilizing
microcomputer control to disperse theoptimum amount of lubricant at the appropriate
intervals.

4. Performance evaluation
4.1 Power generation characteristics
By installing the power generator in the spindle
shown in Fig. 6, the relationship between charging
voltage and charging time from power generatedusing the difference in temperature between the
inner/outer rings of the bearings was verified. The
bearing specification of this test is shown in Table 1
and the power generation test conditions are shown in
Table 2. A double layer capacitor was used as theelectric storage device and the time needed to reachan arbitrary charging voltage target was evaluated.

2.3 Controller
The amount and interval of lubricating oil supplied tothe bearings are controlled by the program of thecontroller.

2.4 Electric storage device
The electric storage device stores power generated
by the Peltier module and ensure the pump operates
consistently by maintaining a constant voltage.

PowergenerationPowergenerator

ThermoenergyTest bearings

Electric storage
devicePower supply

Pump

DriverFig. 6 Spindle for test
Controller(microcomputer)

Table 1 Specifications of test bearing

Fig. 4 Block diagram of electrical control

Ceramic plate

Side A

Test bearings

φ100×φ150×24

Contact angle

20˚

Rolling element material

Si3N4

Cage material

Laminated phenolic resin

Sealed bearing greaseMP-1（9g contained）

Table 2 Conditions of power generator
Side B
Electrode

Attitude of axis
Horizontal axis
Pressure whenincorporated into bearings 100 N (pressure at fixed position)

N-type material
P-type material

Fig. 5 Structure of peltier module-50-

Rotational speed
Outer cylinder coolingtemperature

10,000min-1，15,000min-1

Electric storage device

Electrical double layer capacitor (rated 0.1 F)

Room temperature ±1˚C

*10_*09 13/05/17 18:25 ページ 4

Development of the Lubricating Oil Supply Unit with Self-generating Power Supply

4.3 Durability evaluation

Fig. 7 shows the power generation characteristics.
It can be seen that the charging time is reduced when
the temperature difference between the inner andouter rings is larger.

Bearings for machine tool main spindles with greaselubrication are required to have a durability of 20,000
hours or more. Therefore, we are currently continuinga durability test based on conditions listed in Tables 4
and 5, setting the optimum amount and interval of
lubricating oil supply.

Charging voltage V

Charging voltage target

Table 4 Specifications of test bearing and lubricating oil supply unit

Difference in temperature 5˚C
Difference in temperature 7˚C

0

1

2

3

4

5

Charging time hFig. 7 Characteristic of power generation

Test bearings

φ100×φ150×24

Contact angle

20˚

Rolling element material

Si3N4

Cage material

Laminated phenolic resin

Sealed bearing grease

MP-1 (9 g contained)

Supply lubrication oil
Amount and interval of
supply oil

MP-1 base oil
Automatically adjusted depending on the
bearing conditions.

Table 5 Conditions of endurance test
Attitude of axis

Horizontal axis

Pressure when incorporated into bearings

100 N (pressure at fixed position)

4.2 Dispenser operation

Rotational speed

15,000min-1

In order to verify the diffusion of the lubricating oil,the oil was colored red and the dispenser was
installed in the spindle shown in Fig. 6. The test
conditions are shown in Table 3.
Fig. 8 (a) shows the grease condition before
dispensing lubricating oil and (b) shows the grease
condition after lubricating oil is dispensed. It wasverified that the grease became red overall after
lubricating oil was dispensed. The lubricating oil
supply unit was verified to operate correctly as the oil
was uniformly diffused in the grease reservoir close to
the bearing raceway indicated in Fig. 2 (a).

Outer cylinder cooling temperature

Room temperature ±1˚C

5. Summary
In this paper, we have discussed the lubricating oil
supply unit with self-generating power supply that
operates using the difference in temperature between
the inner and outer rings in the bearings. Traditionally,
with grease lubrication, it was difficult to maintain along and optimum lubrication environment; however,
we have solved this issue with the developed
lubricating oil supply unit. We will extend thistechnology to general industrial machines for energy
saving and reduction of environmental load.

Table 3 Conditions of lubricating oil supply
Attitude of axis

Reference

Horizontal axis

1) Sun-Woo Lee, Tadaaki Maeda: Minimum Quantity
base oil of Grease Supply Lubrication Angular
Contact Ball Bearings, NTN TECHNICAL REVIEW
No. 76 (2008) 88–93

Pressure when incorporated into bearings 100 N (pressure at fixed position)
15,000min-1
Rotational speed
Outer cylinder cooling temperature Room temperature ±1˚C
Sealed bearing grease

MP-1 (9 g contained)

Supply lubrication oil

MP-1 base oil

Operating time

Approx. 100 hours(a) Before lubricating oil discharge(b) After lubricating oil discharge

Photo of authors

Fig. 8 State of grease before and after discharge-51-

Kaoru OMOTO

Hiroyoshi ITO

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division*11_*09 13/05/17 18:26 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]NTN Linear Modules Series

Ulrich GIMPEL*
Michael WILLE*
Tomoyuki OWADA**
Masaki KAGAMI**NTN linear modules are establishing credibility from customers as
supplying best suited products. In this document, the feature andapplications of NTN linear module series are introduced.

1. Introduction

2. NTN linear module products

Automated facilities are being implemented in morediverse sectors, such as automobile, machine/parts,
semiconductors, flat panels, food, and medicine, forincreased productivity, increased efficiency, and cost
reduction. This paper introduces the features andapplications of the NTN linear module for use incarrying mechanisms, positioning devices, and lifts.

Table 1 shows an overview of NTN linear modules
and Table 2 shows their dimensions. Four series areavailable, which are (1) small type, (2) parallel type,(3) large type, and (4) table type. They offer a fast
operation of up to 10 m/s, driven by a timing belt, anda long stroke of up to 10 m with the large-type AXS
series.

Table 1 Outline of NTN linear modules(1) Small type:
AXC Series
Features

Driving
methodLinearmotionguide

Number of rows of incorporated
linear motion guideLinear guideRoller guide

Feed screw drive
Timing belt drive
Omega drive

Rack and pinion drive

Maximum speed m/s
Maximum length m

Maximum static permissible
moment N・mSmall, compact
Single row
Applicable

Applicable

Applicable

Applicable

Applicable(2) Parallel type:
AXDL Series
Small,
high load capacity
Multi-row

Applicable

Applicable

Applicable
Applicable―(3) Large type:AXS Series
Supports transport ofheavy object
Multi-row

Applicable(4) Table type:
AXLT Series
Supports highly
accurate positioning
Multi-row

Applicable

―

―

―

Only ball screw drive

Applicable
Applicable

Applicable

―
―

―

―

10.0

10.0

6.35

10.00

3.50

5.900
Frame size

3.550
Frame size

21.000
Frame size

10.000
Frame size

8.00

120×120mm

240×100mm**NTN-SNR ROULEMENTS Engineering Bielefeld**Product Engineering Department, Precision Equipment Division

-52-

10.0

400×300mm―
2.5

455×70mm

*11_*09 13/05/17 18:26 ページ 2

NTN Linear Modules Series

Table 2 Lineup of NTN linear modules
Type(1) Small type

Frame size
Series/model
A×B mm40 40 × 40
AXC

60

80

60 × 60

B

80 × 80A

120 120 ×120

240 240 ×100A

200 200 ×100A

460 400 ×300

225 225 × 40
325 325 × 50

2.2 Parallel type: AXDL Series

8.00
6.10
6.35

10.00

The outer view of AXDL110S is shown in Fig. 2.
This series is equipped with two linear motion guides
on both sides of the driver to provide larger load
capacity and rigidity. Particularly, it provides increased
pitch and yaw rigidity, making it possible to be used
for a long time under severe conditions, such as
uneven loading.
It also provides the same dust control andmaintainability as the AXC Series. A linear motion
guide can be selected from two either linear guide orroller guide and a driving method can be selected fromfeed-screw drive or timing-belt drive. This series is
considered the by the users as most well roundedamong the size, load capacity, rigidity, and accuracy.

3.50

155 155 × 30
AXLT

10.00

8.00

10.00

AXS 230 200 ×160 B
280 280 ×170

(4) Table type

6.00

8.00

3.00

120 120 ×120
(3) Large type

6.00

6.10

110 110 × 50

(2) Parallel type AXDL 160 160 × 66 B

selected from three types, feed-screw drive, timing
belt drive, or omega drive. The details are discussed
in Section 4.
The standard equipment now includes a dust
control seal to prevent foreign objects from entering
the frame, as well as a mechanism for facilitating easy
greasing and exchange of the timing belt, which allcontribute to the improved maintainability.
This series can be used not only as a uniaxial
application, but also as a part of multi-axial systems
combined with other products of the same AXC series
or different series.

3.50

B

455 455 × 70

A

3.20
3.20

They have a large static permissible moment that issuitable for carrying large or heavy materials andmaking compact equipment. They also provide
diverse motor couplings, which allow a combination ofdifferent motor types.
NTN linear modules can be combined with those ofdifferent series or frame sizes, linear motion guides, or
driving methods in order to address required operation
conditions such as applications, loads, or installation
positions.

110mm
65mm

2.1 Small type: AXC Series
The outer view of AXC80Z is shown in Fig. 1. It isequipped with a single-row linear motion guide and adriver in the slim and light aluminum frame. Its small
form-factor provides the same load capacity as thelarger competitive products due to the adoption of ahighly rigid linear motion guide, which helps the users
to realize smaller equipment.
A linear motion guide can be selected from two
types, which are the ball-circulating-type linear guideor roller guide, which consists of a shaft and bearings,
depending on the application. A driving method can be

Fig. 2 AXDL Series AXDL110S

2.3 Large type: AXS Series
Fig. 3 shows an example of combined use with
AXS280Z as a horizontal axis. High rigidity and highload capacity are achieved by the large aluminum
frame and high-performance linear guide. It is mainly
used for horizontal transportation of heavy loads over
100 kg, as an upper axis of a gantry structure called abeam, and for lifting heavy loads with vertical axes.
This series includes an ultra-large size type that
other competitors do not manufacture. The frame sizeof AXS460 is 400 mm wide × 300 mm high. A drive
method can be selected from rack-and-pinion drive,
timing-belt drive, or omega drive, with 10 m of
maximum stroke, 10 m/s of maximum speed, and

79mm
100mm

Fig. 1 AXC Series AXC80Z-53-

*11_*09 13/05/17 18:26 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

required to operate consistently for a long time. This
product achieved low heat, low noise, long life, andlong maintenance-free operation by adopting a ballretainer with grease pockets, as shown in Fig. 6.

1000 kg of maximum transport weight supported. (See
5.2 An example of application to a machine tool)

330mm

Cage

250mm

Fig. 3 Application of AXS280Z

Ball

2.4 Table type: AXLT Series
AXLT155, shown in Fig. 4, uses aluminum extruded
material of positioning table type for the frame. The
linear module products from other manufacturers aremostly limited to transportation; however, this series
can also be used as an accurate positioning device,as it combines precision ball screws and multi-row
high-performance linear guides.
Although the bellows are equipped to prevent
foreign objects from entering the frame, maintenancecan be conducted without removing them since thelubrication hole is provided on the side of the stage.

Fig. 5 Linear guide

Grease pockets

Fig. 6 Retainer with grease pocket
155mm
60mm

3.2 Roller guideFig. 7 shows the roller guide as installed into the
AXC Series in transparent view. A roller guide canalso be selected with the AXDL Series.
As shown in Fig. 8, it has a simple structure whererolling bearings with grooved outer rings traverse
between two rod-shaped guiding shafts in thealuminum frame, yet it has high load capacity with amaximum operating speed is 10 m/s.

Grease lubrication
hole

Fig. 4 AXLT Series AXLT155

3. Linear motion guide
The linear motion guide to be incorporated into NTN
linear modules can be selected from linear guide androller guide, depending on the operating conditions
such as load, speed, accuracy, and the required cost.

3.1 Linear guideFig. 5 shows the internal structure of a ballcirculating-type linear guide. The linear module isFig. 7 Roller guide-54-*11_*09 13/05/17 18:26 ページ 4

NTN Linear Modules Series

drive AXC Series. It uses a highly durable timing beltthat does not stretch even after a lengthy operation.
Its responsiveness is improved by adjusting the belttension to the optimum value when it is assembled
into the module.

Roller bearings withgrooved outer ring

Guiding shaft

4.3 Omega driveThis driving method, which can be applied to AXC
and AXS Series, uses a driving pulley and a motor(omitted from the figure) on the stage (carriage), withboth ends of the timing belt fixed on the frame, as
shown in Fig. 10. It is called “omega drive” because
the shape of the driving pulley wrapped by the timing
belt looks like the Greek letter Ω (omega). It is used toprovide independent motions of multiple omega
driving stages installed on the module of a single axis,
or to move the frame by fixing the stage, for example.

Fig. 8 Guiding system of roller guide

4. Driving method
There are 4 types of driving methods that can be
combined with the module, i.e., (1) feed screw drive
by a ball screw or trapezoidal screw, (2) timing-belt
drive, (3) rack-and-pinion drive, and (4) omega driveusing a timing belt.

Deflection pulley

4.1 Feed screw driveThis method can be applied to AXC, AXDL, and
AXLT Series (not AXS Series), selecting either a ballscrew or trapezoidal screw depending on the load,
speed, and application.
The standard specification of the ball screw is JIS
accuracy class Ct7 with lead typical motion error of 52µm/300 mm or less. If even higher accuracy is required,a precision grinding ball screw can also be selected.
A trapezoidal screw is mainly used in a simplified
positioning device where cost is emphasized. The
lead typical motion error is 100 µm/300 mm (or 200µm/300 mm depending on the size), which is inferior
to the ball screw; however, the accuracy of positioning
reproducibility after it is incorporated into the module
is ±0.100 mm or less and the supported maximum
speed is 115 mm/s. Therefore, it has a high-cost
performance.

Driving pulley

Carriage

Timing belt

Fig. 10 OMEGA drive

4.4 Rack-and-pinion drive
The rack-and-pinion drive, which is only applicableto AXS Series, is used for lifting load in vertical motion
or as a beam in the gantry structure. It is able totransport a maximum of 1000 kg of load due to its highpower transmission capability despite the simple andlight structure.

4.2 Timing-belt driveThis can be selected for AXC, AXDL, and AXS
Series (not AXLT) and is a driving method appropriate
for high-speed transport. Fig. 9 shows the timing-beltTiming belt

Pulleyshaft

Fig. 9 Timing belt drive

Fig. 11 Rack-and-pinion drive-55-*11_*09 13/05/17 18:26 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

construction material (large wall panels) as well as
small applications such as industrial products and infood and medical sectors, due to their many
variations.

5. Applications of NTN linear modules
5.1 Combined approach
Fig. 12 shows a gantry structure combining several
NTN linear modules.
Small-type AXC Series are used for the two bottom
axes to reduce the size of the overall equipment. In
order to drive both axes at the same time, theconnecting shaft is used to transmit the power of the
motor rotation in this mechanism. The parallel-type
AXDL Series is used as the beam axis to support theoverhang moment load of the Z-axis driving moduleand the large type AXS Series is used for Z-axis toenhance the lifting performance.
As shown in this example, it is possible to configurea system taking advantage of features of each
module.

AXS Series

NTN Linear Module AXS Series(length 6000; stroke 3000; width 280 mm)

Connecting shaft

Fig. 13 Example of use for loader

AXC Series

AXDL Series

Fig. 12 Combination reference example

5.2 An example of application to a machine toolFig. 13 shows an example of using a timing belt
drive AXS Series for the work automatic attach/detachloader. In order for a robot exceeding 100 kg in
weight, which is suspended from the beam, toattach/detach the work quickly and accurately after its
position is determined, it is necessary to keep thedisplacement of the tip of the hand caused by the
robot operation and movement of the load within therequired accuracy. This was achieved by the AXS
Series with high rigidity.
Fig. 14 Example of use for service vehicle

5.3 Special applications
Fig. 14 shows its application on the lateral slide
mechanism for the operator cockpit of a roadpavement service vehicle.
The linear modules have been used in very large
applications such as automatic sprayer equipment ofpesticide and water in greenhouses and planting
facility and gantry-type cranes for transporting-56-

*11_*09 13/05/17 18:26 ページ 6

NTN Linear Modules Series

6. Summary
NTN linear modules are excellent for rigidity andreliability and have an extremely proven track record,
especially in Europe.
We will continue to contribute to the facility
automation by proposing solutions with high cost
performance to the new user requirements for theirfacilities and new development challenges using NTN
linear modules.

Reference
Ulrich Gimpel, Michael Wille, NTN-SNR High
Performance and Flexible Linear Modules Series
AXDL, NTN TECHNICAL REVIEW No.78 (2010) 6469

Photo of authors

Ulrich GIMPEL

Michael WILLE

Tomoyuki OWADA

Masaki KAGAMI

NTN-SNR ROULEMENTS
Engineering Bielefeld

NTN-SNR ROULEMENTS
Engineering Bielefeld

Product Engineering
Department,
Precision Equipment Division

Product Engineering
Department,
Precision Equipment Division-57-

*12_*15 13/05/17 18:27 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Paper ]Evaluation of Local Mechanical Properties of High Strength Steels
by Indentation Method
Takeshi OGAWA*
Noriaki SAKANAKA**
Yukio MATSUBARA**High strength steels are used for important components such as rolling
bearings, for which surface treatments such as induction and carburized
hardenings are applied. Strength designs for these components requirematerial properties at the local areas, since the material properties aredifferent at the treated locations. The objective of the present study is toevaluate the local mechanical properties of high strength steels by the dual indenter method. Nondimensional П function is developed for 118 degree trigonal pyramid indenter using finite element
method. Dual-indenter method is conducted by indenters with the apex angles of 115 and 118
degrees for JIS-SUJ2 and JIS-SUJ3. The results reveal that good agreements are achieved between
stress-strain curves of tensile testing and those of the dual-indenter method. The local mechanicalproperties are evaluated by the dual-indenter method for induction-hardened and carburizedcomponents. There are some differences in stress strain curves at the locations of the components
by the influence of the heat treatments.

1. Introduction

2 Material under test and test piece

For important components such as rolling bearings,
hardening is applied for either all the components or
only local areas where high strength is needed bysurface treatments such as induction and carburized
hardenings. Since the local areas where surface
treatment is applied have different material properties,
local evaluation for those local areas is required whenmechanical properties are evaluated. A dual indenter
method 1) has been proposed for evaluating local
mechanical properties as an indentation method. Oneof the authors (Ogawa) has been applying the dual
indenter method to diverse materials for themechanical evaluation 2). However, it was revealed
that the prediction on high-strength-bearing steel did
not always match the tensile testing results.
Therefore, in this study, a method of estimating
mechanical properties of high-strength materials wasstudied using the indentation method and finite
element method (FEM).

The chemical components of materials under test,
high-carbon chrome- bearing steel JIS-SUJ2 and JISSUJ3, are shown in Table 1. The standard overall
hardening was applied to the materials under test, andthen, they were tempered at 180–350˚C and used as
test pieces. Induction-hardened and carburizedmaterials were also used as test pieces. The surfaceof test pieces was treated with emery paper polishing,buffing, and lapping with diamond paste.

Table 1 Chemical compositions (mass %)
C

Si

Mn

Ni

Cr

Cu

SUJ2 1.02

0.27

0.43

0.014 0.007

P

S

0.05

1.48

0.09

SUJ3 0.99

0.47

1.06

0.014 0.006

0.07

1.06

0.14

**Professor, Department of Mechanical Engineering, College of Science and Engineering, Aoyama Gakuin University**Advanced Technology R&D Center-58-

*12_*15 13/05/17 18:27 ページ 2

Evaluation of Local Mechanical Properties of High Strength Steels by Indentation Method

4.2 Finite Element Method (FEM) analysis

3. Testing machine and conditions

A general-purpose non-linear structural analysis
program, MSC.MARC Mentat 2005, was used foranalyzing newly generated П functions in thismechanical property evaluation using dual-indenter
method. In the previous indentation tests with trigonal
pyramid indenters, it has been verified that the threedimensional analysis result of trigonal pyramidindenters is equivalent to the analysis result from theaxisymmetric model of conical indenters with the same
depth-projected ratio 3). Axisymmetric model with 2549
elements and 2658 nodes simulating conical indenterof apex angleθ= 77.37˚ was used for the indenter of
φ= 118˚ in the FEM analysis. Fig. 1 shows theanalysis model. For generating new П functions, totalof 72 combinations were analyzed with a range ofmaterial parameters, namely, Young’s modulus E = 50
to 300 GPa, yield stressσy = 0.1 to 5.0 GPa, andwork-hardening exponent n = 0.1 to 0.5. The Poisson’s
ratio was set toν= 0.3 for all combinations.
The relationship of E*/σr and C/σr obtained fromthis analysis is shown in Fig. 2. The analysis result is
indicated by one curve for each value of workhardening exponent n in the analysis conditions. For

Six indentation tests were conducted for eachcondition, using Dynamic Ultra Micro Hardness Tester
DUH-W201 of Shimadzu Corporation. Tests were all
conducted at room temperature (approximately 20˚C)with the loading/unloading speed of 10.1 mN/s and the
maximum test force of 1961 mN. In addition, trigonal
pyramid indenters with apex anglesφof 100˚, 115˚,
and 118˚ were used for the testing.

4. Indentation test
4.1 Dual indenter method
In this study, a trigonal pyramid indenter with apex
angle of 118˚, which has relatively small plastic
deformation, is used in addition toφ= 100˚ and 115˚,
as the dual-indenter method is applied. 3) The
following analysis was conducted using a conicalindenter with apex angle θ, where the ratio of
projected area and h of the indenter is the same. In
general, indentation force F and indentation depth h
are in the relationship of F = Ch2 and can be
expressed as follows after a dimension analysis:
F = F ( E*, n, σr, θ, h) …………………… (1)θ=77.37˚

Where E* is the complex Young’s modulus, n is thework-hardening exponent including influence of elastic
deformation of indenter. σr is called the typical stressthat characterizes the plastic deformation region.
When the П theory is applied to the equation (1), thefollowing is obtained 3).
F

σr h2

=

C

σr

（σ

=Π

E*

r

）

，n，
θ ……………… (2)The typical stress-strain relationship for eachindenter can be obtained by finding this П function foreach of the indenters of apex anglesφand testing two
types of indenters, and using the П function and theexperiment result, that is, the slope C, which is F
against h2 in the load process. n is determined fromthe relationship of these two points. Also, from thegenerally known stress-strain relationship, the yieldstress σy and the strength coefficient K can beobtained from the following equation: whereεr is the
typical strain.

（ σ ε）

σr =σy 1+

E

y

r

240
200

Π118 = C118 /σr 118

σr = KεnrnFig. 1 Model for FEM analysis………… (3)160

Fitting curve

120
80

φ
θ
φ=118˚
θ=77.37˚εr118 =0.02

40
0

0

n=0.1
n=0.3
n=0.5

500

E*/σr118

Fig. 2 Dimensionless function of 118˚ indenter

-59-

1000

*12_*15 13/05/17 18:27 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

the determination of εr, the fitting to the curve istested inεr = 0.001 increments to obtainεr = 0.020
for the indenter ofφ= 118˚. As a result, therelationship of E*/σr and C/σr is expressed by one
curve independent from the work-hardening exponentn and the non-dimensional function П118 is obtainedby the following equation:
C118

σr 118Mechanical properties were evaluated by the dualindenter method using the newly generated Пfunction for the 118˚ indenter. Fig. 3 shows theevaluation results of SUJ2 and SUJ3, tempered at
180˚C, using indenters ofφ= 115˚ and 118˚, andφ=
100˚ and 115˚. It can be seen that the evaluation tests
using the 115˚ and 118˚ indenters provide similar
results as the tensile test results. On the other hand,the dual-indenter method using the 100˚ and 115˚（σ ）= 43.25ln

3000

True stress σ, MPa

5.1 Evaluation of mechanical properties ofbearing steel

E*

r 118

- 55.90 …… (4)

3000

(a)
True stress σ, MPa

Π118 =

5. Test result and observation

2000

1000

0

100˚ indenter
115˚ indenter
118˚ indenter
Tensile test
118 & 115˚
115 & 100˚

SUJ2 180˚C

0

2

4

6

True strain ε, %

2000

1000

0

8

(b)

100˚ indenter
115˚ indenter
118˚ indenter
Tensile test
118 & 115˚
115 & 100˚

SUJ3 180˚C

0

2

4

6

True strain ε, %8Fig. 3 Stress-strain curves obtained by tensile testing and dual-indenter method for SUJ2 (a) and SUJ3 (b)3000

Yield stress σy, MPa

Yield stress σy, MPa

3000

2000

SUJ2

1000

0

118 & 115˚
115 & 100˚
Tensile test

(a)
200

2000

0

300

SUJ3

1000

118 & 115˚
115 & 100˚
Tensile test

(b)
200

Tempering temperature C˚

300

Tempering temperature C˚Work-hardening exponent n

Work-hardening exponent n

Fig. 4 Results of yield stresses for SUJ2 (a) and SUJ3 (b)SUJ2118 & 115˚
115 & 100˚
Tensile test

0.5

0

(a)
200

300

SUJ3

118 & 115˚
115 & 100˚
Tensile test

0.5

0

(b)
200

Tempering temperature C˚

300

Tempering temperature C˚

Fig. 5 Results of work hardening exponents for SUJ2 (a) and SUJ3 (b)-60-*12_*15 13/05/17 18:27 ページ 4

Evaluation of Local Mechanical Properties of High Strength Steels by Indentation Method

not well simulated with the FEM analysis that generates
П functions, it is assumed that the significant pile-up
prevented valid results to be generated in the dualindenter method using indenter of 100˚.

indenters gave significantly different results from thetensile tests. Fig. 4 and Fig. 5 show the yield stressσyand work-hardening exponent n obtained from thedual-indenter method. Since the indenters of 115˚ and118˚ provided similar values as the tensile test results,
evaluating correct tempering temperature dependencyof the mechanical properties, the validity of the dualindenter method using those 115˚ and 118˚ indenters
is verified.
In order to review the cause of the significant
difference between the test results using the indentersof 100˚ and 115˚ and the tensile tests, impressions
were observed. Fig. 6 shows the photo of impressionsof eachφand AFM observation result. It can beobserved that the smaller the apex angleφ, the larger
the pile-up around the impressions. As the pile-up is

5.2 Evaluation of mechanical properties of
surface treated components
The local mechanical properties were evaluated fortest pieces cut out from the induction-hardenedcarbon steel S53C and carburized chrome steel
SCr420 components. In order to evaluate local
mechanical properties on these test pieces, hardness
distribution was verified to identify the range of effectof the respective heat treatments. The distribution ofVickers hardness HV is shown in Fig. 7 and Fig. 8.
The measurement was carried out with Vickers micro-

Depth h, µm

2

(a)

0

SUJ2 180˚C

φ=100˚
A-A’
B-B’

-2

C-C’
0

Depth h, µm

220

Position µm

40

(b)

0

SUJ2 180˚C

φ=115˚
A-A’
B-B’

-2

C-C’
0

20

Position µm

402Depth h, µm(c)

0SUJ2 180˚C

φ=118˚
A-A’
B-B’

-2

C-C’
0

20

Position µm

40Fig. 6 Shapes of impressions and height profiles measured by AFM formed by 100°(a), 115°(b) and 118°(c) indenters-61-

*12_*15 13/05/17 18:27 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

hardness tester MVK-G3, made by Akashi. The test
force is 2940 mN (300 gf) and measurement was
taken toward the direction of the arrow in the picture at
the same intervals. HV of induction-hardened
component in Fig. 7 shows distribution of 304 to 746.
In contrast, HV of carburized hardened component in
Fig. 8 shows distribution of 383 to 744. As such, thedifference of HV by heat treatment methods can be
confirmed.
Defining the region of 700 or higher of HV as “highhardness” and 450 and lower as “low hardness”, thelocal mechanical properties were evaluated using thedual-indenter method. The 115˚ and 118˚ indenterswere used for indentation test. The results are shownin Fig. 9 and 10. In the high-hardness region, the test
result shows large yield stress and it is possible toconfirm the relationship between the hardness and theyield stress. When the induction-hardened
components and carburized-hardened components
are compared in the high-hardness region, thehardness of the induction-hardened components ishigher and the yield stress is also larger. In the lowhardness region, however, different from the highhardness region, the carburized-hardened

Vickers hardness

800
600

400
200

0

0

2

4

6

Distance from edge, mm

Fig. 7 Shape and hardness distribution chart of induction-hardened specimen

Vickers hardness

800
600

400
200

0

0

2

4

Distance from edge, mm

6Fig. 8 Shape and hardness distribution chart of carburized specimen

3000

Hard

2000

115˚
118˚

1000

0

True stress σ, MPa

True stress σ, MPa

3000

Soft
0

2

4

True strain ε, %

6

2000

Soft
1000
115˚
118˚
0

8

Fig. 9 Stress-strain curves of induction hardened specimen

Hard

0

2

4

True strain ε, %68Fig. 10 Stress-strain curves of carburized specimen-62-*12_*15 13/05/17 18:27 ページ 6

Evaluation of Local Mechanical Properties of High Strength Steels by Indentation Method

Referencecomponents are superior in both hardness and yieldstress.
Induction hardening has a hardening effect only on
the surface but the internal organization remains thesame. On the other hand, carburized hardening
hardens the surface and also the internal organization
to some extent. The test result clearly shows thesecharacteristics of the respective heat processes,
indicating the clear difference of the impact of the heatprocesses in σ-ε curve of the local areas.

1) Nagahisa Ogasawara, Wakako Makiguchi,
Norimasa Chiba, “Plastic Properties Determination
Method for Power-Law Hardening Material Using
Plural Triangular Pyramid Indenters”, Transactionsof the Japan Society of Mechanical Engineers. A,
Vol. 70, No. 698 (2004), pp. 1529-1534.
2) Akio Yonezu, Takeshi Ogawa and Mikio Takemoto,
Key Engineering Materials，Vols．353-358，pp.22232226 (2007)．

3) Takashi Ono, Akio Yonezu, Takeshi Ogawa, Jun
Akimitsu, “Evaluation of Mechanical Properties andStrength Characteristics of Micro Region Using
Indentation Method”, Journal of Material Testing
Research Association of Japan, Vol. 49, No. 3(2004), pp. 149-156.

5. Summary
The dual-indenter method using trigonal pyramidindenters of apex angles of 115˚ and 118˚ applied tothe test pieces of high-strength steel with different
heat processes revealed equivalent stress-strain
properties as the test results of tensile tests.
With the evaluation of mechanical properties of
surface-hardened components, the difference of
induction-hardened and carburized-hardened heatprocesses was verified from the stress-strain
properties of the local areas.
Also, pile-up was confirmed around the impressionsof the 100˚ indenter. When pile-up is formed, therelationship between the indentation force and depth
is affected and the dual-indenter method cannot beapplied.
When stress-strain properties obtained from thedual-indenter method are used for the analysis of theactual machines, it is possible to conduct moredetailed elastic/plastic analysis than conventional
methods.

Photo of authors

Takeshi OGAWA

Noriaki SAKANAKA

Yukio MATSUBARA

Professor,
Department of Mechanical Engineering,
College of Science and Engineering,
Aoyama Gakuin University

Advanced Technology
R&D Center

Advanced Technology
R&D Center-63-*13_13 13/05/17 18:27 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ Technical Paper ]TMR: A New Frontier for Magnetic Sensing

Christophe DURET*
Shintarou UENO*

We present here a new linear magnetic sensing technology, based on
Tunnel Magneto Resistance (TMR). The TMR principle derives from
Spintronics, a new branch of Physics which received the 2007 Nobel
Prize in Physics. Contrary to “traditional” electronics, a Magnetic Tunnel
Junction (MTJ) uses not only the electric charge of the electron, but also
its quantum spin value.
During the CAMEL project, we have designed MTJs for sensor applications, i.e. for having,compared to other existing technologies, greater performances, extended working ranges and design
capabilities, the robustness needed for harsh mounting and working conditions and of course a costcompatible with a market introduction. The fascinating properties of the TMR sensing elements wehave developed open the door to many new possibilities of magnetic sensing.

1. NTN-SNR magnetic sensing
technology
1.1 Bearings with active sensor ASB®
NTN-SNR has developed the active sensor
bearings, ASB®, the technology for detecting wheel
speed used for Antilock Brake System (ABS) andElectronic Stability Program (ESP) for vehicles, as
shown in Fig. 1. The ASB®,, which helps in reducing
the weight and space of vehicles and improves therobustness and zero velocity detection in the severe
conditions, is a defacto global standard for detecting
wheel velocity in the automobile industry. The
following shows the configuration of an ASB®,:

Fig. 1 Example of ASB integration in a roller bearing unit

• Target magnet
Multi-pole ring magnet integrated with the bearing
seal. It generates an alternating magnetic field withfrequencies proportional to the rotational speed of thewheel.

1.2 Development and applications of highresolution rotation sensor MPS40S
After the development of the ASB®, continuing withthe research of a low-cost sensor for accurately
detecting angles of rotation and for controlling rudder
angle of the steering wheel and electric motor, wedeveloped the special-purpose IC MPS40S 1) with abuilt-in Hall effect device and signal processing circuit.
As shown in Fig. 2. MPS40S is applicable for a wide

• Detection device
This uses the magnetoresistance effect or Hall
effect. It measures the alternating magnetic field thatthe target magnet generates with no contact andcalculates the wheel rotation speed to be used by the
ABS and ESP.

*NTN-SNR ROULEMENTS Research & Innovation Mechatronics-64-*13_13 13/05/17 18:27 ページ 2

TMR: A New Frontier for Magnetic Sensing

range of pole widths, has the capability of outputting
high-resolution pulse signals and origin position
signals, and satisfies the quality standard AEC-Q100
as the electronic device for automobile use.
MPS40S is also targeted at industry machines suchas the rotational control of servo motors. Fig. 3 shows
the high-resolution sensor bearing with origin signal
output 3), which integrates MPS40S and bearings for
improving ease of assembly with target equipment.
As an application of MPS40S, Fig. 4 shows a highprecision magnetic angle sensor 4), which detects theabsolute angle using the Vernier principle. This sensor
is applicable to tubular shafts, where the existing
sensors shaft end placements was not applicable. By
integrating the multi-pole ring magnet and the bearing
rotating ring and placing the detection device with abuilt-in signal processing circuit to the fixed ring, theaxial space adjustment is not required whenassembled. The absolute angle can be accurately
measured even in high-speed rotation. This is done bysetting the dedicated multi-pole ring magnet, which
has two rows of concentric magnetic patterns with adifferent number of magnetic poles inside and outside,
and calculating the relative displacement of thepatterns from the magnetic sensor signals.

2. Characteristics of various magnetic
sensors
In order to apply magnetic sensing technology to awide range of applications, it is necessary to consider
basic performance of magnetic detection sensors(such as output accuracy, frequency bandwidth, andpower consumption), as well as operation range
against air gap and temperature, size of the sensor,
installation allowance, robustness, and cost. Sensors
in severe operating environments such as automobile
and industrial machine applications typically use theHall device, anisotropic magnetoresistance (AMR)
device, and giant magnetoresistance (GMR) device
for magnetic field detection 5). The following showstheir respective characteristics:• Hall device
It is of very good linearity and is inexpensive. On theother hand, it has limited sensitivity and large power
consumption and offset drift.

• AMR element
It has better sensitivity than the Hall device and thehysteresis is small. Linearity, offset-free operation,and high magnetic field detection can be achievedby a barber pole sensor structure, flip driving
method, and arranging a stabilizing magnet.
However, in a high-temperature environment, slightdeterioration of sensitivity and linearity is observed.

Fig. 2 Appearance of MPS40S

Outer ring

Connector

Magnetic encoder

Substrate
Sensor IC
Sensor case

Inner
ring

Type C spring

Fig. 3 High resolution sensor bearing

Fig. 4 High precision magnetic angle sensor-65-*13_13 13/05/17 18:27 ページ 3

NTN TECHNICAL REVIEW No.80（2012）• GMR element

resistance, and when it is parallel, it indicates low
resistance. That is, the resistance changes dependingon the relative angle of magnetization direction of twolayers. This phenomenon is called the Tunnel
Magneto Resistance (TMR) effect *2. In order todetect an external magnetic field using this effect, thedevice is configured keeping the magnetized directionof one ferromagnetic material layer (called fixed or pin
layer) fixed and allowing the magnetized direction of
the other ferromagnetic material layer (called freelayer) to change depending on the external magnetic
field.
The TMR effect under low temperature wasdiscovered in 1975 6); however, it did not attract
attention, at that time, as the application was difficult.
However, in 1995, when a change of almost 20% ofmagnetic resistance was achieved (the maximum
value at that time) at room temperature in the low
magnetic field, the TMR effect was immediately placed
under spotlight 7) 8).
A recent research reports that using crystalline MgO(100) for the insulation layer as an alternative toamorphous Al2O3 yields very high magnetoresistancechange of about 600% 9) in room temperature 10) 11).
While the TMR effect has been recently used in thehard disk read head, application development formagnetoresistance memory (MRAM 12)) for read/writeof quantum information of 0 and 1 and for magnetic
sensor has just begun 13) 14). Also, in order to apply
the MTJ element to a magnetic sensor, the resistanceneeds to be changed depending on the external
magnetic field strength and direction, as opposed to
the MRAM and hard disk head.

It has the highest sensitivity and widest permissible
range of input magnetic field strength. However, theperformance, such as linearity, significantly
deteriorates as soon as the temperature or input
magnetic field strength exceeds the operating
range.

3. Tunnel Magneto Resistance (TMR)
effectThe basic theory of the MTJ element *1 is based on
spintronics that uses the quantum spin condition in
addition to the electric property of the electron basedon the existing electronics theory. When Pr. A. Fert
and Pr. P. Grünberg received the Nobel Prize inPhysics in 2007, Spintronics attracted immediate
attention.
As shown in Fig. 5, the MTJ element generally has
the structure of an insulation layer of aluminum oxide(Al2O3) or magnesium oxide (MgO) in the middle oftwo stacked ferromagnetic layers, similar to a GMR
element. With the MTJ element, the current flows
vertically through these layers while the electrons in aferromagnetic layer pass through the insulator byquantum-mechanical effect and are injected to theother ferromagnetic layer by the tunneling effect. In
this case, the electric resistance to the current
changes significantly depends on the relative angulardifference of the magnetized direction of two
ferromagnetic layers. When the magnetized directionof two layers is antiparallel, it indicates highFerromagnetic layer (fixed layer)

4. CAMEL Project
Insulation
layer

NTN-SNR has been focusing on the research anddevelopment of the TMR effect since 2000 15) 16) 17),and has filed several patents such as laminatedstructure of the MTJ element and sensor structure. It
also accelerated technology development andapplication from 2005 to 2010 in the CAMEL (CApteur
Magnétique à effet tunnEL) project. Sponsored by the
French National Research Agency (ANR), the project
was conducted as an academic-industry partnershipof NTN-SNR (end-user project leader), Sensitec(making of MTJ element), and two public research
institutions.
The project established the MTJ element aimed atsensor applications and linear magnetic sensing
technology. It evaluated how they would adapt to the
production and high-volume production using theexisting production line of AMR/GMR elements owned
by Sensitec 18). An example of the linear magnetic

Ferromagnetic layer (free layer)

Fig. 5 Representation of a MTJ in an antiparallel
magnetization configuration (left) and in a parallel
magnetization configuration (right). The black arrow
represents the current going through the junction＊1 MTJ element: Magnetic Tunnel Junction element.

An element made of a stack of layers with an insulation
layer of a few nm of thickness called a barrier layer ortunnel barrier in the middle, and two ferromagnetic
layers made of ferromagnetic metal on both sides.

＊2 TMR Effect: Tunnel Magnetoresistance Effect.

A phenomenon that resistance changes by MTJ
element.

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*13_13 13/05/17 18:28 ページ 4

TMR: A New Frontier for Magnetic Sensing

sensing using the TMR effect is shown in Fig. 6. As
shown in the graph to the right, it was determined thatthe measured result mostly matched with the linear
approximation and the hysteresis was sufficiently
small.

In addition, this project received the Yves Rocard
prize in 2010 from the French Physics Society for its
achievement in advancing the basic technology topractical utilization.

Fig. 6 Example of transfer curve for a MTJ with the following structure

AMR Sensor
Chip

5. State-of-the-art magnetic sensing
using the TMR effect
The following discusses important properties of thesensor using the TMR effect (hereafter, “TMR
Sensor”), which was developed applying thistechnology. The details of the structure and test
results are omitted in this paper. For more information,refer to the references 17) 18) 19).

Button Cell

TMR Sensor
Chip

5.1 Basic performance
1) Low power consumption
The power consumption of the TMR Sensor is
1/100th of the AMR sensors and 1/1000th of the Hallsensors 19), since the internal resistance of the MTJ
element is high. Even when the peripheral signal
processing circuits are considered, the power
consumption is still sufficiently low which enables long
operation over the system product life with small
batteries or energy harvesting devices. Therefore, it isexpected to be used in self-supported wirelesssensors. As an example, a comparison image of thenumber of button cells required for six-monthcontinuous operation is shown in Fig. 7.

Fig. 7 Power and Cost comparison of an AMR and a
TMR sensor after 6 months of continuous use-67-*13_13 13/05/17 18:28 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

2) High detection accuracy

The sensitivity of the TMR sensor is 10 times that of
the AMR sensor and 3 times that of the GMR sensor.
Its internal noise is so small that it can also be used
for magnetic field measurement of geomagnetism inµT scale. Therefore, as shown in Fig 8, detection withmaximum of 5 mm of air gap is possible, whichsignificantly expands the freedom of design. For
example, a wall of non-magnetic material such as
stainless steel, aluminum, or plastic can be installed
between the sensor and the ring magnet for improving
dust-, oil-, or heat-resistant properties.

The sensor detection accuracy is determined by the
combination of linearity, hysteresis property,
sensitivity, and S/N ratio. The newly developed TMR
sensor has the following characteristics compared tothe sensors using the existing AMR/GMR/Halldevices.
The linearity of a Hall sensor for detecting highmagnetic field and the hysteresis property of an
AMR/GMR sensor for detecting low magnetic field areboth at a very high level. However, they are notalways good across all the properties because they
exhibit both advantages and disadvantages. On theother hand, the TMR sensor exhibits a high level of
performance across all the properties compared toother sensors, and thus, it can be used for detecting awide range of magnetic fields. Also, it is possible toverify the position sensing accuracy by the evaluation
kit discussed in the next Chapter.
When used for detection of absolute angles, theAMR sensor can only detect a range of 180˚ ofexternal magnetic field, while the TMR sensor can
detect a range of 360˚ (one full rotation). The TMR
sensor can be applied to angle detection placed on
the shaft end by using its superior properties of widerange of input magnetic field strength, good linearity,and low hysteresis.
For magnetic detection sensitivity, although theperformance of SQUID *3 is superior, it is proven thatthe 1/f noise and thermal noise of MTJ element can be
reduced to the lnT/Hz1/2 level. This implies realizationof high space resolution and low cost for medical andnon-destructive test applications 20).

80

Magnetic Field kA/m

70
60
50
40
30
20
10
0

0

1

2

3

Air Gap mm

45Fig. 8 Magnetic field generated by an elastoferrite polering as a function of the air gap

5.3 Freedom of design
1) Small size
Different from the AMR element and GMR element,
the MTJ element uses electric current that flows
vertically through the layers that compose the device.
Therefore, it is possible to significantly reduce thearea of the sensor by improving the manufacturing
process such as the etching machining performanceand side-etching performance. MRAM, where therecording density is the most critical parameter, canbe configured with 1 µm2 or less of area per MTJ
element. On the other hand, for sensor application, it
is necessary to ensure the operation by securing theminimum dimension in the bonding pad. That is also
possible with about 30% less space than the AMR
sensor.

5.2 Operating range
1) Wide operating temperature range
In high-temperature life test, it is confirmed that
there is no atom diffusion within the MTJ element
even in high temperature of over 150˚C. Also, different
from other sensors, the TMR sensor can be designed
to increase sensitivity as the temperature rises tocompensate the reduction of magnetic field in hightemperature. Therefore, it can detect a magnetic fieldeven in the high temperature range with the same air
gap as in room temperature.

2) Wide air gap range *4
Air gap setting for the AMR sensor for detecting
wheel speed in ABS, etc., is usually set at 2 mm oreven at 1 mm or less when higher resolution or
accuracy is required.

2) Larger installation tolerance
Magnetic sensors for speed and position detection
are susceptible to inaccuracy of sensor and magnet
installation. When they are not properly installed,
signals may not be detected or the detection accuracy
may be deteriorated.

＊3 SQUID: Superconducting Quantum Interference Device＊4 Air gap: Distance between sensor and magnetic target-68-*13_13 13/05/17 18:28 ページ 6

TMR: A New Frontier for Magnetic Sensing¡Sensor chips can be made smaller so that more
chips can be manufactured from one wafer.
¡Inexpensive ferrite magnets can be deployed
instead of expensive rare-earth magnets since avery low magnetic field can be detected.
¡Low power consumption drive is possible due tosimplified signal processing circuit, as theresistance of the device itself is high and amplifying
circuitry is not required.

The TMR sensor operates from a low magnetic fieldof -0.5 to 0.5 kA/m to a high magnetic field of -15 to 15
kA/m, as shown in Fig.6. It is possible to apply themost appropriate design for the application and theinstallation tolerance can be larger.

5.4 Robustness
1) Electrostatic discharge (ESD) capability
The MTJ element can be regarded as a capacitor
with a very thin insulator of nanometer scalesusceptible to high insulation breakdown risk.
However, the risk is reduced as follows:¡To increase dielectric strength voltage by improving
the quality of the insulation layer¡To distribute ESD voltage per MTJ element byserially connecting multiple MTJ elements¡To reduce the risk of ESD by controlling the processof connecting the TMR sensor and signal
processing circuit for quick handling, as the ESD ismostly found in the sensor packaging process.
After introducing the above measures, the testing
based on the machine model after installation onto theprinted wiring board revealed improvement ininsulation breakdown risk.

3) Cost reduction of the entire system
In applications such as electric motors and ABS,
consideration for assembly and installation of theentire system configuration including peripheral
components is required, not only the sensing
functionality.
When the air gap and installation tolerance isbroader, such as the case of the TMR sensor, therequirement for assembly/installation can be reduced
and overall reduction of manufacturing cost isachieved from a simplified assembly process.

6. TMR sensor evaluation kit
NTN-SNR and Sensitec created three types of
evaluation kits for detection of rotation and position
and are providing them to potential users for thebroader use of the TMR sensor technology (Fig. 9 and
10). The configuration of the evaluation kits is shownas follows:2) Tolerance against high magnetic field
As shown in Fig.6, the resistance of the TMR has
reversed property against the external magnetic field;
therefore, the TMR sensor does not break down evenwhen it is exposed to a strong magnetic field produced
by a rare-earth magnet.

5.5 Cost
1) Facility investment cost
As previously noted, the MTJ element is made by
the same manufacturing facility as the AMR/GMR
elements that Sensitec owns. In other words, the TMR
sensors can be manufactured by using similar process
and materials as the manufacturing for the GMR
elements, and any special facility investment for thevolume production is not required.

2) Cost reduction effect by TMR sensor
Fig. 9 TMR Evaluation PCB

The following shows the cost reduction effects:

Fig. 10 The three TMR Evaluation Kits-69-

*13_13 13/05/17 18:28 ページ 7

NTN TECHNICAL REVIEW No.80（2012）

Kit (1)current sensors, speed sensors and position sensors,
image processing equipment, and the medical field.
NTN group including NTN-SNR will actively pursue
further improvement and new market development of
the TMR sensors.

Low power consumption drive demo set• Sample board with TMR sensor for 360˚ angle
detection to be placed at shaft end• SPI communication by low-power consumption
micro-computer• Solar cell
• Magnet to be placed at shaft end

Reference001) P. Desbiolles, A. Friz: Development of High
Resolution Sensor Element MPS40S and Dual Track
Magnetic Encoder for Rotational Speed and Position
Measurement, NTN TECHNICAL REVIEW, 75(2007) 36-41.
02) http://www.ntn-snr.com/industry/fr/en-en/file.cfm/
MPS40S_datasheet_rev0.pdf?contentID=4826
03) H. Ito, T. Takahashi, P. Desbiolles, C. Peterschmitt,
S. Ueno: High resolution sensor bearing with an index
signal, NTN TECHNICAL REVIEW, 78 (2010)04) http://www.ntn.co.jp/english/news/news_files/new_
products/news201000026.html
05) C.P.O. Treutler: Magnetic sensors for automotive
applications, Sensors and Actuators A91, 2-6 (2001)06) M. Jullière: Tunneling between ferromagnetic films,
Physics Letters, vol 54A, p225-226, 1975.
07) T. Miyazaki, N. Tezuka: Giant magnetic tunneling
effect in Fe/Al2O3/Fe junction, Journal of Magnetismand Magnetic Materials 139, L231-L234 (1995)08) J.S. Moodera, L.R. Kinder, T.M. Wong and R.
Meservey: Large Magnetoresistance at Room
Temperature in Ferromagnetic Thin Film Tunnel
Junctions, Physical Review Letters, vol. 74, p. 32733276, 1995.
09) S. Ikeda, J. Hayakawa, Y. Ashizawa, Y. M. Lee, K.
Miura, H. Hasegawa, M. Tsunoda, F. Matsukura, H.
Ohno: Tunnel magnetoresistance of 604% at 300K
by suppression of Ta diffusion in CoFeB/MgO/CoFeB
pseudo-spin-valves annealed at high temperature,
Applied Physics Letters, 93, 082508 (2008)
10) S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki,
K. Ando: Giant room-temperature magnetoresistancein single-crystal Fe/MgO/Fe magnetic tunnel
junctions, Nature Materials, 3, p868-871, (2004)
11) S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice,B. Hughes, M. Samant, S-H Yang: Giant tunnellingmagnetoresistance at room temperature with MgO(100) tunnel barriers, Nature Materials,3, p862-867(2004)
12) B. N. Engel and al.: A4-Mb Toggle MRAM Based ona Novel Bit and Switching Method, IEEE
Transactions on Magnetics, VOL. 41, NO. 1, p132136, JANUARY 2005

Kit (2)

Active sensing *5 demo set• Sample board with TMR sensor• High-resolution pulse output (phase A/B)
• Analog output• Multi-pole magnet (linear and ring magnet)

Kit (3)Passive sensing *6 demo set• Sample board with TMR sensor and bias
magnet• High-resolution pulse output (phase A/B)
• Analog output• Linear and ring gear

7. Summary
The TMR sensor has the following significantly
superior properties and high potential as a nextgeneration magnetic sensor for automobile andindustrial machine applications:¡Basic performance (low power consumption/highoutput accuracy/high sensitivity)¡Operating range (high temperature/wide air
gap/360˚ angle detection/detection of extremely
low magnetic field)¡Freedom of design (smaller sensor size/larger
installation tolerance)¡Robustness (ESD capability/high magnetic field
tolerance)¡Low cost
Taking advantage the TMR sensor and its use inmany broader applications is expected, such as
electronic compass, switches, non-destructive testing,＊5 Active sensing: A detection method with multi-pole magnet as the detection target＊6 Passive sensing: A method, with a magnet placed behind the sensor, to detect change of magnetoresistance as a magnetic

body passes through.

-70-*13_13 13/05/17 18:28 ページ 8

TMR: A New Frontier for Magnetic Sensing13) www.nve.com/Downloads/AAT001.pdf14) http://cache.freescale.com/files/sensors/doc/data_
sheet/MAG3110.pdf?pspll=115) G. Malinowski, M. Hehn, S. Robert, O. Lenoble, and
A. Schuhl: Magnetic origin of enhanced top
exchange biasing in Py/IrMn/Py multilayers , Phys.
Rev. B 68, 184404, 2003
16) G. Malinowski, M. Hehn, F. Montaigne, E. Jouguelet,
and A. Schuhl: Intrinsic thermally compensated fieldsensor based on single magnetic tunnel junctions,Applied Physics Letters, 84, 1204, 200417) G. Malinowski, M. Hehn, F. Montaigne, A. Schuhl, C.
Duret, R. Nantua, G. Chaumontet: Angular magnetic
field sensor for automotive applications based onmagnetic tunnel junctions using a current loop layout
configuration, Sensors and Actuators A, 144,
p263–266 (2008)18) B. Negulescu, M. Hehn, A. Gerken, J. Paul, C. Duret:
Magnetic tunnel junction linear field sensor forindustrial applications, Proc. of 10th MR Symposium,
Wetzlar, 200919) J. Paul, R. Lehndorff and C. Duret:Ressourceneffizienz am Beispiel magnetoresistiver
Sensoren, Proc. of MikroSystemTechnik Kongress,
201120) R. Guerrero, M. Pannetier-Lecoeur, C. Fermon, S.
Cardoso, R. Ferreira and P.P. Freitas: Low frequency
noise in arrays of magnetic tunnel junctions
connected in series and parallel, Journal of Applied
Physics, 105, 113922 (2009)

Photo of authors

Christophe DURET

Shintaro UENO

NTN-SNR
Research & Innovation
Mechatronics

NTN-SNR
Research & Innovation
Mechatronics-71-

*14_*15 13/05/17 18:29 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[Technical Article]

Long Life Technology of Grease for Journal Bearing

Takamasa TANAKA*
Hidenobu MIKAMI**
Journal bearings are very important components of railway vehicles as they
directly affect the vehicle’s running stability. Sealed type double row tapered
roller bearings with grease packed are traditionally used as journal bearings.
The grease packed in the bearing has a tendency to move out from contact
surface of raceways. Good oil providing capability of a grease to the racewaysof a bearing is an important factor for the grease life.
NTN Engineering tested grease packed double row tapered roller bearings to investigate the relationshipbetween oil separation of the grease and metal to metal contact of between bearing rollers and raceways. As aresult, it was observed that bearings would be kept in good lubrication conditions by using the greases which
had proper oil separation performance. This paper reports the effect of lubrication condition of rolling bearing
due to oil separation of utilized the grease.

This paper investigates the effect of different
greases’ oil separation properties on journal bearing
life of railroad vehicles with the intent of optimizing
grease and bearing life.

1. Introduction
There are typically three types of bearings used withrailroad vehicles. These include bearings for the main
motors used for driving the vehicles, bearings for thedrive equipment used for transmission of driving
power, and journal bearings to support the wheel
axles. All of these bearings require a high reliability
since they are directly related to the running of thevehicles. With the requirements for longer life comerequirements for longer intervals between
maintenance, making extending bearing life andmaintaining reliability a difficult task 1).
Sealed-type double-row tapered roller bearings withgrease lubrication are mainly used as journal bearings
for use with railroad vehicles as longer grease life isrequired for longer maintenance intervals.
Improvements to such components as bearing seals
and cages have helped to reduce the deterioration
rate of grease in journal bearings. Since sealed-type
double-row tapered roller bearings have line contact
between the roller and the raceway, they require
increased lubrication in comparison to ball bearings,
which have point contact. Therefore, longer
lubrication life through improvement of the grease
itself is critical.

2. Journal bearings
Under normal operation, journal bearings are
installed in the journal box to support the wheel axle.
While in operation, radial load from the vehicle weight,
axial load from the vehicle movement, and vibration
are applied to the journal bearings. While various
bearing types were investigated, adopted, and used in
the past, sealed-type rotating end cap tapered roller
bearings (RCT), shown in Fig. 1, are the mostprevalent design today.
Sealed-type rotating end cap tapered roller bearings
consist of an inner ring, outer ring, rollers, cage, oilseal, slinger, back cover, etc. 2) For a lubricant, agrease with a lithium soap as the thickener andmineral oil as a base oil is generally use. Grease isinjected between the rollers and the cage but as thebearings rotate, grease is often pushed outward
toward the (5) oil seal. Since sealed-type rotating end
cap tapered roller bearings have a larger space
between (3) the rollers and (5) the oil seal compared**Construction Machinery and Railway Engineering Department, Industrial Machinery Division**Advanced Technology R&D Center-72-

*14_*15 13/05/17 18:29 ページ 2

Long Life Technology of Grease for Journal Bearing

(6)

(5)

(3)

(1)

(2)

(7)

4. Oil separation property of grease
The oil separation property of grease varies
depending on the type and amount of base oil andthickener, as well as the manufacturing method of thegrease. For this test a total of 44 types of grease withvarious types and amounts of base oil and thickener
were prepared to investigate the relationship between
the grease composition and oil separation properties.
For measuring oil separation properties, the following
centrifugal oil separation testing method was usedassuming that grease attached to the bearing cagewould be separated from oil by centrifugal force.

(4)(1) Inner ring (2) Outer ring (3) Roller (4) Cage
(5) Oil seal
(6) Slinger(7) Back cover

4.1 Grease samples under test

Fig. 1 RCT bearing for axleboxes

Table 1 shows the composition of grease samples
prepared for this test. For thickener, three types of urea
compound and one type of lithium soap were selected
and used with different blending amounts. For base oil,mineral oil and synthesized hydrocarbon oil were used.

to other types of rolling bearings, it is necessary to pay
attention to fluidity when selecting sealing grease. The
rate of deterioration of grease in journal bearings is
one important element for determining themaintenance intervals of the vehicles. The bearings
are disassembled, inspected, and greased during theperiodical maintenance.
So far, various measures have been taken to extend
the maintenance intervals. Cages that in the past weremade of steel were replaced with cages made of resin
to reduce the metal abrasion powder, which can cause
grease deterioration. In addition to this, low-heat
generating seals were adopted to control degradationof grease due to temperature rise 3). Although thesemeasures gradually extended the maintenanceintervals, the requirement for further extension is still
very high 4), requiring longer life of the grease itself.

Table 1 Composition of test grease
Group

M1

Thickener

Aliphatic Alicyclic Aromatic Li Aliphatic Alicyclic Aromatic Li
urea
urea
urea soap urea
urea
urea soap

Amount of thickener
wt%
Base oil
Viscosity of base oil
mm2/s (40˚C)

7〜13
6

M2

7〜10
4

M3

18〜30
6

M4

9〜13
6

S1

S2

S3

S4

11〜24 17〜28 22〜41 16〜34
6
6
6
4

combinations combinations combinations combinations combinations combinations combinations combinations

Mineral oil

Synthesized hydrocarbon oil

100

46

4.2 Centrifugal oil separation test
The oil separation property of the grease samples
was investigated using the centrifugal separator. Afterrotating the centrifugal tube with 1.0 ± 0.1 g of greasesample under 3.9 × 104 G for 1 h, the weight of theseparated oil within the centrifugal tube wasmeasured, as shown in Fig.2. The centrifugal oil
separation ratio was calculated as the ratio of theweight of separated oil against original grease using
the following equation (1).

3. Grease lubrication
The grease properties to be improved for long life
vary depending on the use location and conditions.
Within the rolling bearings, grease itself penetrates
in the rolling surface providing lubrication at the
beginning of the operation; however, after some timein operation the base oil becomes the main vehicle forlubrication. Toward the end of lifecycle, seizure may
be caused by shortage or deterioration of oil 5).
For cylindrical roller bearings, studies have alreadybeen conducted showing the relationship between
lubrication life of bearings and oil separation property.
It concluded that the optimum oil separation property
would provide a longer bearing life 6).
In this paper, we investigated how the oil separation
property of grease would affect lubrication of rotating
end cap tapered roller bearings.

Amount of separated oil in the test (g)Centrifugal oil separation ratio (%)= ——————————————————— ×100…… (1)Original amount of grease (g)Fig. 3 and 4 show the measurement results as thecentrifugal oil separation ratio. The centrifugal oil
separation ratio decreases when the thickener isincreased in any grease sample. When compared withthe same amount of thickener, the centrifugal oil
separation ratio varies depending on the type ofthickener or base oil.
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NTN TECHNICAL REVIEW No.80（2012）

With mineral oil as base oil, as shown in Fig. 3,
when aromatic urea (M3) is used as thickener,
centrifugal oil separation ratio tended to be higher;
however, no significant difference was observed
among thickeners other than aromatic urea. When
synthesized hydrocarbon oil was used as base oil, as
shown in Fig. 4, the centrifugal oil separation ratio
tended to be higher than when compared the cases
where mineral oil was used as base oil. Whenaromatic urea was used as thickener, the centrifugal
oil separation ratio tended to be higher too.

As a result, it is determined that the type andblending ratio of thickener as well as the type of baseoil affect the centrifugal oil separation ratio.

5. Grease endurance test using rolling
bearings
5.1 Test method
Five types of grease with different centrifugal oil
separation ratio, M1-12% (expressed in group-amountof thickener), S2-17%, S3-22%, S4-16%, and S4-31%
were sealed between the cage and inner ring of roller
bearings. For this endurance test, 20g of grease wasused in each bearing. Fig. 5 shows the overview of
the test equipment and Table 2 shows the test

Table 2 Endurance test condition
Separated oil

Number of bearings Two (back to back)Radial load
10.78kN(per 2 bearings)

Centrifugal
tube

40

M1
M2
M3

30
25

M4

20
15
10
5
0

0

10

20

30

Amount of thickener wt%

40

50

Fig. 3 Oil separation of test grease (M1 to M4)

1.5V

V

3000min -1

Temperature

Room temperature

Test duration

2000–10000 h

40

S1

35

S2
S3

30
25

S4

20
15
10
5
0

0

10

20

30

Amount of thickener wt%

40

Fig. 4 Oil separation of test grease (S1 to S4)

Metal contact ratio measurement circuit
100kΩ

Rotational speed

Centrifugal oil separation ratio %Centrifugal oil separation ratio %Fig. 2 Schematic diagram of centrifugal oil separation test

35

Tapered roller bearing（φ70×φ110×31）

Bearing

GreaseTest bearings

Bearing rotation pulley10kΩ

Supporting bearings

Supporting bearings

Fig. 5 Endurance test equipment

-74-

50

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Long Life Technology of Grease for Journal Bearing

5.2 Status change of the grease after test

conditions. For bearings under test, two units ofrotating end cap tapered roller bearings (φ70 × φ110× 31) were used. The bearings were rotated at a
speed of 3000 min-1 with an applied radial load of
10.78 kN. During the test, the metal contact ratio
between the inner and outer rings of the rolling
bearings was measured using the electric resistancemethod. The test equipment was stopped after apredetermined duration and the oil reduction ratio of
the grease measured. The calculation method of
metal contact ratio and the oil reduction is shown inequations (2) and (3), respectively.

Fig. 6 shows the behavior of S4-16% grease during
the endurance test. At the beginning of the operation,
100% of grease is inside the bearings. Some grease
moves to the sealed area as the bearings start tooperate (shown in Fig. 7). It was observed that thegrease mostly remained after 2000 h of operation.
Fig. 8 shows the oil reduction ratio of the grease
remaining inside the bearings and the grease attachedto the sealed area. Since the oil reduction ratio of thegrease remaining inside the bearings continues toincrease after 2000 h, it is understood that the
lubrication of the raceways is maintained using only
the oil separated from the grease that remained inside
the bearings.
Fig. 9 shows the relationship between the oil
reduction ratio of the grease inside the bearings andthe centrifugal oil separation ratio. There is anexponential correlation between the two. The oil
separation of the grease inside the bearings in thistest is assumed to be caused by the centrifugal force.

Voltage when insulated (V) – Voltage under operation (V)Metal contact ratio (%)= ————————————————————————— ×100… (2)Voltage when insulated (V)Amount of thickener in the grease after operation (%)
Oil reduction ratio (%)= (1- ——————————————————————— ×100…… (3)Initial amount of thickener in grease (%)

100
Present inside bearing

S4-16%

15

Attached in sealed area

Oil reduction ratio %Weight of remaining grease g

20

10

5

0

0

2000

4000

6000

8000

40
20

0

2000

Test duration hFig. 6 Change of grease amount by test duration

4000

6000

Test duration h

8000

10000

Fig. 8 Relationship between test duration and oil reduction

Inside bearing

50

Oil reduction ratio %Sealed area

Attached in sealed area

60

0

10000

Present inside bearing

S4-16%

80

Fig. 7 Transfer state of grease in operation

40

After 2000 h of operation

30
20
10
0

0

10

20

30

40

50

Centrifugal oil separation ratio %Fig. 9 Reduction rate of bearing inside grease after test-75-

*14_*15 13/05/17 18:29 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

100

100

80

80

60

60

40

40

20

20

0

0

2000

4000

6000

8000

0
10000

Test duration hFig. 12 Bearing temperature and metal contact ratio(S2-17%)

80

80

80

60

60

60

60

40

40

40

40

20

20

20

20

2000

4000

6000

8000

Metal contact ratio %

0

0
10000

Test duration h

80

60

60

40

40

20

20

0

0

2000

4000

6000

8000

Bearing temperature ˚C

80

4000

6000

Test duration h

100

100

Metal contact ratio %M1-12%

2000

8000

0
10000

Fig. 13 Bearing temperature and metal contact ratio(S3-22%)

Fig. 10 Bearing temperature and metal contact ratio(S4-16%)

100

0

100

0
10000

Fig. 11 Bearing temperature and metal contact ratio(M1-12%)

100

80

80

60

60

40

40

20

20

0

Test duration h

S4-31%

0

2000

4000

6000

Test duration h

8000

Metal contact ratio %0S3-22%

Metal contact ratio %80

S4-16%

Bearing temperature ˚C

100

0Bearing temperature ˚C

S2-17%

100

100

Bearing temperature ˚CBearing temperature ˚CFig. 10 to 14 show the temperature of the outer ringof the bearings and metal contact ratio between the
inner/outer rings measured for the bearings with thefive types of grease. S4-16% grease shown in Fig. 10
became unstable by both metal contact ratio andbearing temperature specifications after 3000 h oftest. M1-12% grease in Fig. 11 and S4-31% grease in
Fig. 14 had high metal contact ratio and bearing
temperature from the beginning; therefore, the tests
were terminated in 2000 h and 5000 h, respectively. In
contrast, S2-17% grease in Fig. 12 and S3-22%
grease in Fig. 13 were stable over 7500 h ofoperation.

Metal contact ratio %5.3 Oil reduction ratio of the grease after test

0
10000

Fig. 14 Bearing temperature and metal contact ratio(S4-31%)-76-

*14_*15 13/05/17 18:29 ページ 6

Long Life Technology of Grease for Journal Bearing

Table 3 summarizes the relationship between thecentrifugal oil separation ratio and lubrication statusmeasured as the metal contact ratio during thebearing endurance test. It was observed that thegreases with a high or extremely low centrifugal oil
separation ratio caused unstable bearing lubrication
when used in tapered roller bearings under these
operating conditions.
Grease with a high centrifugal oil separation ratiodelivers a large amount of lubrication oil to the
raceway early in terms of the life of the grease. It is
assumed that this lubrication oil is depleted after some
operation causing ill lubrication for longer term
operating. Grease with an extremely low centrifugal oil
separation ratio does not sufficiently deliver lubrication
oil to the raceway; therefore, the lubrication is notgood from the beginning of operation.
On the other hand, it is assumed that grease withthe appropriate centrifugal oil separation ratio
continues to deliver lubrication oil to the raceway
throughout the operation, without depletion,
maintaining a good lubrication state for a long time.

3) Greases with centrifugal oil separation ratio of
20–35% show good lubrication for long periodstime.
4) For improving the grease life of tapered roller
bearings used in wheel axles, controlling the oil
separation property of the utilized grease is one of
the important factors.
We have studied the grease composition that
contributes to extending life of journal bearings forrailroad vehicles considering that the centrifugal oil
separation property is one of the important factors forextending grease life for journal bearings. Based on
the findings obtained in this study, we would like toconduct tests under closer operating conditions to theactual vehicles to verify the grease composition of
appropriate oil separation properties in an effort tocontribute to longer life and reliability improvement forjournal bearings of railroad vehicles.

Reference
1) Tsuyoshi Ichiki: Technology Standard andMaintenance Technology; New Vehicle
Maintenance System for JR East Japan Railway,
Rolling Stock and Technology No. 79 (2002) 6-112) Kosoku Sharyoyou Rinjiku Kennkyu Iinnkai:
Tetsudo Rinjiku, Maruzen, 175-179
3) Ryutaro Oka: Development of New RCT Bearing for
Axleboxes and Insulated Bearing with Shields, NTN
TECHNICAL REVIEW No. 74 (2006) 80-85.
4) Japan Railway Engineers’ Association: the Railway
systems of Japan Coming after 20 Years, 213-221.
5) Japanese Society of Tribologists, Grease TechnicalCommittee, “Lubrication Grease: Basics andApplications”, Yokendo, 82-83.
6) Shigeki Komatsuzaki: 48th Tribology Advanced
Course, Lubricant and Tribology, (1993).

Table 3 Centrifugal separation ratio and metal contact
ratio of grease
Grease code

Centrifugal oil
separation ratio wt%

Metal contact ratio during
bearing operation

S4-16

38.0

△

S3-22

34.0

○

S2-17

22.0

◎

S4-31

4.2

×

M1-12

3.6

×
◎

○

△

×

BetterWorse
Judgment (metal contact ratio)

6. Conclusion
The impact that the oil separation property ofgrease on lubrication of bearings was investigated by
testing and comparing the durability of tapered roller
bearings with grease types of different oil separation
properties. The study was completed with the purposeof pursuing grease with long life for journal bearings ofrailroad vehicles. The following findings wereobtained:
1) Grease with a centrifugal oil separation ratio close
to 40% shows good lubrication at the beginning;
however, the lubrication performance degrades as
the operation continues for a longer time.
2) Grease with an extremely low centrifugal oil
separation ratio of 5% or less has low oil delivery
capability; therefore, the state of lubrication is notgood from the beginning.

Photo of authors-77-

Takamasa TANAKA

Hidenobu MIKAMI

Railway Engineering
Department,
Industrial Machinery Division

Advanced Technology
R&D Center*15_*15 13/05/17 18:30 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]Market Trend and High-Functionality of Wheel Bearings
for Off-Highway Truck
Kenichiro NAITO*
Naota YAMAMOTO**
Katsunori SONE**

Off-Highway Trucks are generally used in very severe mining
conditions operating 24 hour duty cycles in natural environment
with dust and contamination. In order to satisfy such market needs,
NTN developed high functionality of wheel bearing which is one of the major parts of the truck. In thispaper, NTN new product features and specifications are introduced.

1. Introduction

2. Filter seal integrated large-sized bearing

Off-Highway Trucks in mines are generally used
under very severe conditions of continuous operationfor 24 hours per day., in dusty natural environment ofsand and particles. In addition, “productivity/highefficiency,” is valuated by the transported loads perhour; “durability/reliability” by supporting thisproductivity for a long period of time and“safety/comfort” for the workers is also required.
In order to address these market needs, NTN has
developed new products shown in Fig. 1 with highfunctionality for wheel bearings, which are the criticalcomponents of these trucks and is actively proposing
them to the market. In this paper, we will discuss (1)Filter seal integrated large-sized bearings, (2) IC tag
integrated bearing, and (3) Integrated rotation sensor
large-sized bearing, which were developed by NTN.

Wheel bearings for off-highway trucks require
special measures against severe operating
environment and improved reliability in addition to longlife and maintenance-free operation. However,damage due to denting on the bearing raceways
caused by contamination from dust and particles into
lubricant and metal abrasion powder from the gears
around bearings is very common in only a short timeof operation.
Filter seal integrated large-sized bearing developed
by NTN (Fig. 2) does not allow foreign objects in the
lubricant to enter into bearings, thus resulting in longer
life; therefore, it is expected that the intervals for
replacing the bearings will be extended. The following
are the features and test results of these bearings.

Market needs

Required properties
for bearings

Corresponding bearing
technologies

Durability/reliability

Long life/high reliability

Measures for preventing
foreign objects(1) Filter seal integrated large-sized
bearing

Productivity/high efficiency

Efficient managementof bearings

Recording of management
information in bearings(2) IC tag integrated bearing

Safety/comfort

High functionality/integration

Integration withrotation sensor(3) Integrated rotation sensor
large-sized bearing

NTN products

Fig. 1 Market needs for wheel bearings and NTN new products**Construction Machinery and Railway Engineering Department, Industrial Machinery Division**New Energy Engineering Department, Industrial Machinery Division-78-

*15_*15 13/05/17 18:30 ページ 2

Market Trend and High-Functionality of Wheel Bearings for Off-Highway Truck

Filter seal
(fixed to inner ring)

Lip

Ring (material: resin)
Meshed area(material: resin)

Outer ring

Outer ring

Inner ring
Cage

Inner ring

Meshed areaa) Outer view of bearingb) Sectional view of bearing and filter seal

Fig. 2 Filter seal integrated large size bearing (Tapered roller bearing)

2.1 Features(1) Extending bearing replacing cycles
Denting on the bearing raceways can beprevented by the filter seal equipped with meshthat prevents foreign objects in the oil fromentering into the bearings without blocking the flowof lubricant oil.

Test bearing(2) Compact design
Existing bearings can be replaced as the filter sealis attached without changing key dimensions(inner diameter/outer diameter/width) of the
bearings.

Motor(3) Operable under vibration
They can be used with vibration acceleration of 10
G.

Fig. 3 NTN outer ring rotational test machine
for large size bearings

2.2 Rotational test under the condition offoreign objects in the lubricant<Test condition>
Test machine: NTN outer ring rotational test
machine for large-sized bearings (Fig. 3)
Bearings under test: Tapered roller inch bearings,
back-to-back combination
Fig. 4 Metal particle

Rotational speed: 50 min-1
Lubricant oil: Diesel engine oil ISO VG100
Foreign objects (Fig. 4): Size: max. 0.7 mm
Hardness: HRC 56 to 60
Amount: 2000 mg/L

<Test results>
The control effect of denting by the filter seal was
confirmed by observing dents on the inner ring
raceway after the test, as shown in Fig. 5. As a result,no dents were observed on the inner ring raceway
when the filter seal was attached. Thus, it was
confirmed that the foreign objects were blocked.

30mm

30mm

30mm

30mm

a) Without filter seal
(using conventional product)b) With filter seal
(using this product)

Fig. 5 Inner ring raceway after rotation testwith metal particle-79-*15_*15 13/05/17 18:30 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

2.3 Summary(3) Management items can be freely set

Filter seal integrated large-sized bearing blocks
foreign objects in the lubricant oil entering into thebearing, and therefore, it is expected that the actual
life of the bearing will be extended.

Management items can be set according to theusers’ needs.

3.2 Specifications
Tables 1 and 2 show the specifications of the IC tag
and the reader/writer and Fig. 7 and 8 show theirappearance.

3. IC tag integrated bearings
Off-highway trucks are periodically inspected andrepaired. For critical components such as bearings,
maintenance records such as inspection date andoperating hours are managed and kept. As bearings
are used for a long time it is challenging to simplify the
management and to , secure inspection records
against loss. Thus, management by IC tags (RFID:
Radio Frequency Identification) was seen with highexpectation. However, the existing IC tags do notallow reading or writing while embedded in metal anduse of non-magnetic material for enabling read/write
operation makes the tag size too large; therefore, ICtags have not been practical.
NTN has developed IC tag integrated bearings sothat the quality information and operation history canbe directly recorded on the bearings (Fig. 6). It is
possible to read/write information on IC tag embedded
directly in the bearings and to set management
information for each user. In addition, bearing quality
information including inspection records can be
confirmed. The features, specifications, and theexamples of usage are discussed in the following:

Table 1 Specification of IC Tag
Specification

Item
Memory capacity

112 bytes

Carrier frequency

13.56 MHz

Compliance standard

ISO 15693

Transmission distanceWithin 2 mm

Maximum operating temperature

120˚C

Dimension

φ4.5 ×4 mm

Table 2 Specification of reader and writer (PDA※1 type)
Item

Specification

Carrier frequency

13.56 MHz

Compliance standard

ISO 15693

Operating temperature range-10〜50˚C

Dimension

79 ×164×25 mm

Weight320 g"1: PDA: Personal Digital Assistant

4mm

φ4.5

Fig. 7 IC Tag (Embed in bearing side face)Fig. 6 IC Tag integrated bearing①

3.1 Features(1) Quality information can be read directly fromthe actual bearings
Quality information of the bearings at factory
shipment such as accuracy of dimension for innerdiameter, outer diameter and width, space
information, and serial number can be read out.

②

① PDA
② Pen-shaped antenna(2) Operating history of the bearings can be
written and read out by the users
The inspection information can be directly
written/read from the bearings.

Fig. 8 Reader and Writer (PDA Type)-80-*15_*15 13/05/17 18:30 ページ 4

Market Trend and High-Functionality of Wheel Bearings for Off-Highway Truck

3.3 Examples of usage

4. Integrated rotation sensor largesized bearing(1) Read the bearing data from the factory shipment
and compare it with the user data at periodical
inspections.
(2) Periodically write the bearing management
information and manage the change of therecorded data.
(3) Store the bearing history data as electronic data on
a PC and manage all the information related to thebearings of the entire machine.

As the off-highway trucks are driven on unpaved
roads in the mines, slippage of wheels may cause
accidents. As a countermeasure to avoid slippage
trucks are equipped with , anti-lock brakes andtraction control systems. These systems detect therotation of the wheels to control them. The wheel
rotation detection sensors are usually installed
separately from the wheel bearings. In order toaccurately detect the rotation speed, position
adjustment is required for associated rotation sensor
components.
NTN has developed an integrated rotation sensor
large-sized bearing with a pulse detection sensor on
the stationary inner ring,and a pulsar ring on therotating outer ring, (Fig. 11).
By integrating the rotation sensor and bearings,
sensor positioning adjustment is not required and thenumber of components is reduced. In addition, a
disconnection prevention mechanism is adopted forsensor wiring so that even when slippage occurs
between the axle and bearing inner ring(, called
creep), rotational speed or direction can be detectedwithout causing damage or errors to the sensor. The
following are the features and specifications of an
integrated rotation sensor large-sized bearing:■ Examples of management items
1. Bearing quality information
Fig. 9 shows an example of data that NTN records
at the factory shipment.
2. User management information
Fig. 10 shows an example of data that users canwrite, in addition, for the maintenance.

3.4 Summary
IC tag integrated bearings allow necessary
information to be read/written directly from thebearings; therefore, the management of information
can be simplified even in operating conditions wherelong-time reliability is required.

4.1 Features(1) Integration of rotational sensor and bearings
Reduced installation work, reduced number ofcomponents, and reduced size of peripherals.

(2) Mechanism of preventing disconnection of
wiring even if creep (slippage) occurs
between the axle and inner ring is adopted
Rotation speed can be detected even when thecreep occurs at the inner ring.

(3) Operable in oil
No errors for rotational speed detection when
immersed in the engine oil (oil temperature 120˚C× 2000 h) *2*2: May be affected depending on the oil or additive types.

Fig. 9 Example of input bearing quality information(4) Usable under vibration
No errors in rotational speed detection undervibration acceleration of 10 G.

4.2 Structure
Fig. 11 shows an example of the structure oftapered roller bearings used in rotational outer ring.

Fig. 10 Example of input user management information-81-*15_*15 13/05/17 18:30 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

Pulsar ring

Rotation sensor

Outer ringA

WiringView A

Inner ring

Pulsar ring
Fixed to rotational ring(outer ring)
Rotation sensor
Fixed to stationary ring(inner ring)a) Outer view of bearingb) Sectional view of bearing and rotation sensor
Fig. 11 Integrated rotation sensor large size bearing

4.3 Specification of a rotational sensor and an
example of output

4.4 Summary
Integrated rotation sensor large-sized bearing does
not require position adjustment associated withassembly of sensor components, as the bearing andthe sensor are integrated. This brings a reduced
number of components and promises cost reduction.

Table 3 shows the specification of a rotational
sensor and Fig. 12 shows an example of output
waveform.

5. Conclusion

Table 3 Specification of rotation sensor
Item

Specification

Sensor type

Back magnet type Hall IC

Resolution

192 pulses/rotation *3

Output phases

Single phase *4

Supply voltage

4〜24V (DC)

Output waveform

Open collector (square wave)

Response frequency

12 kHzSink current

25 mA or lessMaximum operating temperature

120˚C

In this paper, three new products that NTN
developed are discussed as the high-functionality
version of wheel bearings for off-highway trucks usedin mines.
The demand for mineral resources continues toexpand against a backdrop of population growth andurbanization in the emerging countries, and the mining
development is expected to continue in the future.
NTN will continue developing products that respondto the requirements of productivity, reliability, andsafety for mining machines such as off-highway
trucks.

*3: Resolution when bearing outer diameter is φ420
*4: Two-phase output when detection specification is on the rotational direction

12.0

Voltage V

10.0

Photo of authors

8.0
6.0
4.0
2.0
0

0

12.5

25

37.5

50

62.5

Kenichiro NAITO

Time msec

Naota YAMAMOTO

Railway Engineering
Railway Engineering
Department,
Department,
Industrial Machinery Division Industrial Machinery Division

Fig. 12 Example of output style-82-

Katsunori SONE
New Energy Engineering
Department,
Industrial Machinery Division*16_*15 13/05/17 18:31 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]Multi Layer BEARPHITE

Yosuke SUGAI*
Toshihiko MOURI*
Bearing for the joints of hydraulic excavator, are used under heavy loadcondition cases such as vibration and shock. Therefore it is necessary toensure wear-resistance and endurance for the bearing. Generally bearings
which are made from iron metal bush or are oil-impregnated and sinteredare used, which have high cost.
NTN developed the “Multi-layer oil-impregnated sintered bearing (Multi Layer BEARPHITE)” by
using a production method which consists of using different materials on the inner and outer side tomaintain a balance between performance and cost.

bearings secure surface strength by carburized
hardening. Therefore, all the bearings become hard,which prevents from applying sizing work that is usedby sintered bearings for correcting the dimension.
Therefore, the precise dimension is secured by cutting
after the heat treatment.

1. Introduction
Oil-impregnated sintered bearings are usually used
for light load application such as for small motors;
however, for special purposes they can be used asheavy load slide bearings for the joints of hydraulicexcavator as shown in Fig. 1. These bearings use ahighly hardened and durable material, which isstrengthened by carburized-hardening heat treatment
process, etc., and then cut for precise dimension.
They are more expensive than the bearings for lightload applications, which are made by press forming,
sintering, and sizing treatment.
In order to solve this problem for the two types ofapplications, NTN has developed the “two-color
molding” method, which forms a single piece unit
consisting of different materials for the inner layer andouter layer of the unit, and marketed as the “multilayer oil-impregnated sintered bearings (Multi Layer
BEARPHITE)” using a highly hardened and durable
material for the inner layer of the bearings and regularhigh-strength structural material for the outer layer.

2. Conventional bearings
The conventional bearing material mixes 15% to
20% of Cu to prevent galling with the axle as well as
Mo and Ni to obtain strength through heat treatment.
As shown in the left column of Fig. 2, conventional

Fig. 1 Oil-impregnated sintered bearing for hydraulicexcavator*Engineering Department, NTN Powder Metal Corp.

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NTN TECHNICAL REVIEW No.80（2012）[Conventional bearings]

Mixture[Developed bearings]
Two-color molding method
Inner-layermaterial
mixtureOuter-layermaterial
mixturePress forming

Two-color molding

Sintering

Sintering

Fig. 3 View of bearing

Carburized hardening/tempering
SizingCuttingOil-impregnated sintering

Oil-impregnated sintering

Fig. 2 Manufacturing process

Enlarged sideview

3. Developed bearings

Outer layer

High-strength structural
material

3.1 Structure and features
Fig. 3 shows the appearance of “multi-layer oilimpregnated sintered bearings," Fig. 4 shows theenlarged view of the boundary between the inner layer
and outer layer, and Table 1 shows the chemicalcomposition. For inner-layer materials that compose
the inner diameter surface, regular material is mixed
with Cu, Mo, and Ni for strength, wear resistance anddurability. In addition, for the outer layer, material
equivalent to JIS-SMF4030 is used, which generally isused for machine structural components with highstrength and is workable for sizing.
The right column in Fig. 2 shows the manufacturing
process. Since high-strength structural material was
adopted for the outer layer, no carburized hardening
was required and the correction of dimension throughsizing work was adopted.
The following displays the features:

Inner layer

Superior material forwear resistanceFig. 4 View of multi-layer area

Table 1 Chemical composition
Composition

Inner layer(inner cylindricalsurface)[Features]
Outer layer(outer cylindricalsurface)¡Low hardness material was used for the outer layer
enabling correction of dimension by sizing work
¡Same material as the conventional bearings wasused for the inner layer securing high sliding
performance-84-

Mixed ratio wt%

Cu

15〜20

Fe

RemainderC

0.3〜0.8

Ni

1.5〜3.5Mo

0.5〜1.5

Cu

2〜5

Fe

RemainderC

0.2〜0.8

*16_*15 13/05/17 18:31 ページ 3

Multi Layer BEARPHITE

3.2 Bonding status of inner/outer layer boundary

developed bearings are shown in Table 4. Both innercylindrical surface and outer cylindrical surface ofbearings were measured for hardness.
Radial crushing strength was equivalent to that of
the conventional bearings, and hardness of innercylindrical surface was equivalent to that of theconventional bearings. On the other hand, hardness ofthe outer cylindrical surface was lower and isappropriate for dimension correction by sizing work.

Fig. 5 and 6 show the concentration distribution of
Fe and Ni at the inner/outer-layer boundary of the
bearings made by the two-color molding method.
Fe is uniformly distributed in both inner and outerlayers without segregation. Ni, which is only mixed in
the inner layer, gradually changes the concentrationfrom the boundary toward inside the outer layer,
indicating that Ni is spread to the outer layer as aresult of the bonding at the boundary.

3.3 Accuracy of dimension

Table 2 Diameter difference of inner diameter

In order to verify the correction effect of dimensionof the developed bearings by sizing work, thedimension of the sintered product before and after
sizing was measured. The results are shown in
Tables 2 and 3.
The diameter variation after sizing is smaller than
before sizing for both the inner and outer diametersindicating that the dimension accuracy has improved.
On the other hand, no dimension correction was
possible with the conventional bearings.

Diameter variation mm
Test item

Before sizing

After sizing

Developed bearings

0.025

0.015

Conventional bearings

0.200

Sizing not available

Table 3 Diameter difference of outer diameter
Diameter variation mm
Test item[Measurement condition]
Dimension of bearings: inner diameter 35 × outer
diameter 45 × length 35
Measurement values: inner/outer diameter variation(max. – min. value of diameter)

Before sizing

Developed bearings

0.060

0.030

Conventional bearings

0.200

Sizing not available

Table 4 Strength and hardness measurements
Radial crushing strength
MPa

Hardness
HRA

Developed bearings

540

Inner layer 54
Outer layer 46

Conventional bearings

550

54

Measured item
Test item

3.4 Strength/hardness
Radial crushing strength and hardness of theOuter layer

After sizing

Inner layer

Outer layer

Fe

Inner layer

NiFig. 5 Fe concentration distribution of surfactant

Fig. 6 Ni concentration distribution of surfactant-85-

*16_*15 13/05/17 18:31 ページ 4

NTN TECHNICAL REVIEW No.80（2012）[Measurement condition]

4. Summary

Dimension of bearings: inner diameter 35 × outer
diameter 45 × length 35
Measurement of radial crushing strength: ring
compression rate 10 mm/min.
Measurement of hardness: inner/outer cylindricalsurface of bearings HRA“Multi-layer oil-impregnated sintered bearings” using“two-color molding” method, which forms different
materials into one piece, is discussed.
Since it exhibits different properties in one bearing
unit, this forming technology is expected to contributeto high functionality and low cost of not only joints ofhydraulic excavators but also many bearings andmachine components in different sectors of industrial
machines and automobiles.

3.5 Friction property
The friction coefficient of the developed bearings
was equivalent to that of the conventional bearings(Fig. 7).

[Measurement condition]
Pressure on friction surface: 100 MPa
Atmosphere temperature: 70˚C
Slide velocity: 0.014 m/s
Dimension of bearings: inner diameter 6 × outer
diameter 12 × length 4
Mating cylinder material: S45C induction hardenedmaterial

Friction coefficient

0.30

Developed bearings
Conventional bearings

0.25
0.20
0.15
0.10
0.05
0.00

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Time hFig. 7 Friction coefficient measurements

Photo of authors

Yosuke SUGAI

Toshihiko MOURI

Engineering Department,
NTN Powder Metal Corp.

Engineering Department,
NTN Powder Metal Corp.

-86-*17_*15 13/05/17 18:31 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Product ]Introduction of Magnetic Material Products

Takuji HARANO*
Shinji MIYAZAKI*
Hajime KATSUURA*
Nippon Kagaku Yakin Co., Ltd., as one of the NTN group companies,
develops various types of magnetic material products that meet customer
demand for characteristics.
The major product is the Amorphous core which has characteristics of high saturated magnetic
flux-density, low core loss and superior frequency characteristics.
This article introduces the features and applications of these products.

Hard magnetic material has high coercive force andis used as a permanent magnet. Magnetic material formagnetic recording is classified for this category.

1. Introduction
In recent years, requirements for reactors installedin electric vehicles (EVs) and hybrid electric vehicles(HEVs) include high current and frequency, as well as
high reliability in severe conditions, to which theconventional ferrite core technology may not be able
to respond.
Nippon Kagaku Yakin Co., Ltd., one of the NTNGroup companies, develops various types of magnetic
material products. In this paper, magnetic material
products that present high saturated flux density withlow core loss and good frequency response are
discussed together with their application examples.

3. Soft magnetic materials(1) High property injection core1) Material property
Amorphous injection material (AS10) is a complex
material made of iron-based amorphous particles that
do not have crystalline structure and resin material.
Table 1 shows the material property of AS10.

Table 1 Material properties of AS10

2. Classification of magnetic material
products of Nippon Kagaku Yakin
Magnetic material products are mainly divided into
soft magnetic material and hard magnetic material.
Soft magnetic material is characterized by low
coercive force and high permeability and used invarious magnetic material products using amorphousmagnetic material with low core loss and goodfrequency response. Particularly, an injection coremade by injection molding of this material has a highdegree of freedom for the shape and superior DC bias
characteristic.

Material

Densityg/cm3

Radial crushing strength
MPa

AS10

4.65

80

2) Features
Fig. 1 shows the inductance deviation rate (DC bias
characteristic), which serves as a measure of themagnetic field generation capability when appliedcurrent is swept for AS10 and the conventional
ferrite/iron-silicon (Fe-6.5Si) magnetic material.
In addition, Fig. 2 shows the frequency response of
the inductance when weak current is applied. Fromthese properties, the following features are derived:*Engineering Department, Nippon Kagaku Yakin Co., Ltd.

-87-

*17_*15 13/05/17 18:31 ページ 2

NTN TECHNICAL REVIEW No.80（2012）

3) Application examples● Choke coil¡Inductance deviation rate is small even when highcurrent is applied compared to the conventional
magnetic material ferrite and Fe-6.5Si.
¡Inductance deviation rate in high frequency range issmall compared to ferrite with stable properties.
From the above, AS10 is suitable for applications
requiring high current and stable high-frequency
response characteristics.

Inductance variation rate %120

Since high current can be applied and highfrequency response is stable, it is applied to chokecoils. Specifically, choke coils are applied to power
supply for medical MRI and industrial machines (Fig. 3
to 6). In this application, reduction of volume andweight of about 1/8 was achieved compared to theconventional ferrite products.

AS10
Fe-6.5Si
Ferrite

100
80
60
40
20
0

0

100

200

300

Current value A

Fig. 1 DC bias characteristics

Fig. 3

Medical magnetic resonance imaging

Inductance variation rate %

120
100
80
Coil

Case

60

Fig. 4 Parts of choke coil

40

AS10
Fe-6.5Si
Ferrite20
0

Amorphous
injection core

1

10

100

1000

Frequency kHz

Fig. 2 Frequency characteristics

Fig. 5 Choke coil for MRI power supply-88-

*17_*15 13/05/17 18:31 ページ 3

Introduction of Magnetic Material Products

increased mutual contact of metal particles since it
contains more iron-based amorphous particles.
¡Inductance deviation rate of frequency responsewith AL60 in high frequency range is smaller
compared to ferrite, providing stable properties.

120

Inductance variation rate %Choke coil

Amorphous core (a component of choke coil)Fig. 6 Power supply circuit of industrial equipment

AL60
Ferrite

100
80
60
40
20
0
0

5

10

15

20

Current value A(2) Amorphous-based dust core1) Material property

Fig. 7 DC bias characteristics

Amorphous-based dust core material (AL60) is acomplex material made of iron-based amorphous
particles that do not have crystalline structure with 1–2
vol% of additives.
Table 2 shows the material property of AL60 and
Table 3 shows relative permeability of AL60 and
AS10.
Since AL60 contains only a little additive including
resin, i.e., 1/40 to 1/20 of AS10, injection molding is
not possible. In addition, AL60 needs special moldand high forming pressure since iron-based
amorphous particles are hard.

Inductance variation rate %

120
100
80
60
40

AL60
Ferrite

20
0

10

0

100

1000

Frequency kHz

Fig. 8 Frequency characteristics

2) Features
Fig. 7 shows the DC bias characteristic and Fig 8shows frequency response of AL60 and ferrite. Fromthese properties and Tables 2 and 3, the following
features are derived:¡Relative permeability of amorphous-based dust corematerial (AL60) is 6 times that of amorphous
injection material (AS10) (Table 3).
¡On the other hand, inductance variation rate of
AL60 is higher than AS10. This is caused by

3) Application examples● Reactive core material for voltage booster of HEV
A reactor is included in the voltage boosting circuit
for increasing motor torque by increasing the battery
voltage. In this case, AL60, which has high relative
permeability and low core loss, is applied as thereactive core material shown in Fig. 9.

Table 2 Material properties of AL60
Material

Densityg/cm3

Radial crushing strength
MPa

AL60

5.5

40

Table 3 Relative permeability of AL60 and AS10
Material

AL60

AS10

Relative permeability

60

10After forming

After assembly

Fig. 9 Reactive coil core for HEV-89-

*17_*15 13/05/17 18:31 ページ 4

NTN TECHNICAL REVIEW No.80（2012）(2) Injection bonded magnet
1) Material property

4. Hard magnetic materials

Table 5 shows the material properties of isotropic
injection bonded magnet (AS4M), which uses Nd-FeB-based magnetic particles.

Fig. 10 shows a map of hard magnetic materials.
Nippon Kagaku Yakin has a line-up of bonded
magnetic products from compression molding to
injection molding of hard magnetic materials.

Table 5 Material properties of AS4M

(1) Compression bonded magnet
1) Material property
Compression bonded magnet is a complex material,
which is made by compression molding, mixing
magnetic particles such as ferrite magnet and resinmaterials. Table 4 shows the material properties ofisotropic compression bonded magnet (ME12M),
which uses either granulated or Nd-Fe-B-basedmagnetic particles.

Densityg/cm3

Radial crushing strength
MPa

ME12M

6.4

110

5

10

AS4M

4.6

80

3) Application examples
Fig. 11 shows application examples of compression
bonded magnet and injection bonded magnet, which
are hard magnetic materials, to an automobile.
¡Compression bonded magnet
Rudder angle sensor, exhaust sensor¡Injection bonded magnet
Sensor for key-less entry
Position, angle, and rotational speed of the target
object are detected using change of magnetic flux
density and switching of magnetic poles.

¡Maximum energy product: 8 to 12 MGOe
¡Good squareness that allows a wide range ofmagnetic circuit designs

0

Radial crushing strengthMPa¡Maximum energy product: 4 to 6 MGOe
¡Injection bonded magnet surface is covered by resinskin layer providing good anti-corrosion property¡No anti-corrosion paint is required¡High design degree of freedom even forcomplicated shapes due to injection molding2) Features

1600

Densityg/cm32) Features

Table 4 Material properties of ME12M
Material

Material

Coercive force H kOe
15

20

25

30
14

Nd-based sintering(anisotropic)
Sm-Co-based
sintering

250

0 0

10

Nd-based bond(anisotropic)
Nd-basedcompression bond(isotropic)

750

500

12

8

Nd-based
injection bond(isotropic)

Ferritesintering

6

Hard magnetic material
Target area

4

Ferrite
bond

2

500

1000

1500

Coercive force H kA/m

Fig. 10 Map of hard magnetic materials

-90-

2000

0
2500

Residual flux density Br kG

1000

AlnicoResidual flux density Br mT

1250

*17_*15 13/05/17 18:31 ページ 5

Introduction of Magnetic Material Products

Injection bond magnet
Key-less entry

Compression bond magnet

Flux density mT

Rudder angle sensor

D

A

B
0

N-pole side

Rudderangle
sensorS-pole side

Key-less
entry

40
20
0
-20
-40
-60
-80
-100
-120

Key-less entry magnet flux
density distribution

60

120

Switch ON/OFF
based on changeof flux density

C
180

240

Angle ˚The magnet rotates
proportionally to thesteering angle

Fig. 11 Application for automobile

5. Summary
We believe that the magnetic material products
discussed in this paper will be applied to the electronic
circuit components of electric vehicles, and contributeto downsizing of industrial machines and reduction ofenergy loss. We are poised to continue ourdevelopment of magnetic material products that
respond to the market needs as new energy
proliferates.

Photo of authors

Takuji HARANO

Shinji MIYAZAKI

Hajime KATSUURA

Engineering Department,
Nippon Kagaku Yakin Co., Ltd.

Engineering Department,
Nippon Kagaku Yakin Co., Ltd.

Engineering Department,
Nippon Kagaku Yakin Co., Ltd.

300

-91-

360

Rotational angle isdetermined by the
number of switchoversof the magnetic poles*18,19_受賞案件_*18,19_受賞案件 13/05/17 18:33 ページ 1

NTN TECHNICAL REVIEW No. 78（2012）

Presentation of award-winning articles
Awarded: “The Japanese Society of Tribologists 2011” Technology Award

Advancement of Cage Stress Analysis
of Rolling Bearings
Kazuyoshi HARADA,

Dimensionless Maximum-Principal stressTomoya SAKAGUCHI,

1. Overview
Common methods in which to increase the capacity of abearing are to increase the size and/or quantity of rolling
elements. As such, cage design becomes more difficult
due to the reduction of remaining volume within the
bearing. In many cases an appropriate design can bemade, but in certain applications excessive cage stress
may be seen. Therefore, a cage must be analyzed toensure proper strength. We have been developing thetechnology to obtain cage stress by analyzing it within its
application 1) to 4). This technology received the “2011Japanese Society of Tribologists Technology Award,” which
is given to excellent new technologies in tribology. This
article will provide an overview of this analytical method.

Sadatsune KAZAMA

2.5
2.0
1.5
1.0
0.5
0.0

0

2000

4000

6000

8000

Carrier rotation speed min

-1

10000

Fig. 2 Cage stress vs. carrier rotation speed1)(Z) and is used in a planetary reduction gear. The
maximum principle stress of the cage has been normalized
to the fatigue limit of the cage material and based on therotational speed of the carrier. In the cases where rotational
speed is 6000 rpm or more and Z is 11, the stresscalculated exceeded the fatigue limit. Likewise,
experiments show that under these conditions, cages
frequently failed. The cause of failure was assumed to be
fatigue breakdown. The results indicate that this analysis iseffective.
This journal bearing model is being displayed in the firstfloor lobby of our Industrial Technical Center in Kuwana-shi,Mie.

2. Characteristics of the analytical system
This technology is built as a CAE system 3) for analyzing
common bearing types in complex operating conditions
such as planetary motion and crank rotation (Fig. 1). Thissystem enables calculation of cage stress during operation,by coupled analysis of the motion of the rolling elements
and cage as well as the elastic deformation of the cage.
Rolling element to raceway contact analysis is performedby precise modeling of the contact geometry, effects of a
dynamic oil film, and including the elastic deformation of thecage using the mode synthesis method 5), a method which
is superior in calculation speed compared to the finite
element method. Accuracy of the stress calculation has
also been improved by the way in which the cage mode
synthesis method is applied. The analysis is typically used
for three-dimensional models, however two-dimensional
analysis is also possible; another advantage in the speed of
calculation 3).

References
1) T. Sakaguchi, et al.: Dynamic Analysis of Cage Stress in Needle
Roller Bearings Under Planetary Motions, IJTC2007, 44189.
2) T. Sakaguchi, et al.: Dynamic Analysis of Cage Stress in Tapered
Roller Bearings Using Component-Mode- Synthesis Method,
ASME J. Tribol., 131(2009)011102.
3) Sekiya: Integrated Bearing Dynamic Analysis System (IBDAS),NTN TECHNICAL REVIEW, 79 (2011) 119.
4) Sakaguchi, et al.: Advancement of Cage Stress Analysis of Rolling
Bearings, Tribology Conference, Spring 2012, Tokyo (2012) 233.

3. An examples of analysis
Fig. 2 shows cage stress results of a two-dimensional,
needle roller bearing analysis which uses 10 or 11 rollers

Photo of authors (Affiliations are at the time of presentation at a conference)

Fig. 1 Analyzed rolling bearings-92-

Tomoya SAKAGUCHI

Kazuyoshi HARADA

Sadatsune KAZAMA

Advanced Technology
R&D Center

Advanced Technology
R&D Center

Needle Roller Bearing
Engineering Department,
Automotive Business HQ

*18,19_受賞案件_*18,19_受賞案件 13/05/17 18:33 ページ 2

NTN TECHNICAL REVIEW No. 78（2012）

Presentation of award-winning articles
2011 “CHO” MONODZUKURI Innovative Parts and Components Awards” Machinery Component Award

Desktop Type Microscopic Coating Applicator
Motohiro UCHIYAMA

1. Introduction
The “Desktop-Type Microscopic Coating Applicator” isa specifically designed tool to repair LCD panel color
filters. Its ability to apply a microscopic coating of a few
pico liters of liquid was recognized for its compact size,ease of operation, and high coating performance. For
these features, it was given the 2011 “CHO”
MONODZUKURI Innovative Parts and Components
Awards” Machinery Component Award.
This device has also received many inquiries including
some from advanced research institutions. These
organizations studied its application to micro-coating in
the area of:¡material coating process for special sensors¡application of adhesive on small optical components¡application of material on electronic devices, and¡dividing specimens into small quantities in bio-relatedareas
All these processes conventionally depended on
manual procedures or processes.

Fig. 1 Desktop type microscopic coating applicator

2. Background
NTN is the leading producer of mechanical devices
that repair missing or mixed color defects in LCD panel
color filters. Due to the recent trend towards larger LCD
panels, the size of current repair devices used in the
production lines has increased to over 4 meters. The
smaller desktop size of this product allows use in small
preproduction applications or research and development
work, in addition to LCD panel manufacturing.

References
1) Akihiro Yamanaka, Desktop Type Microscopic Coating Applicator,
its Technology and Features, Monthly JETI, Vol. 58, No. 11, 101102, 2010.
2) Katsuyoshi Suzuki, Development and Application of Desktop Type
Microscopic Coating Applicator, Monthly Tribology, No. 294, 3436, 2012

3. Features
For applying a predetermined amount of liquid, thedispenser method and ink jet method are well known.
However, with these methods, liquid is delivered througha very thin tube or hole. With this technique, clogging
has always been a problem. This device adopts a uniqueapplication method using an applicator needle with asmall amount of liquid on the tip. Light contact, transfers
the liquid to the target object. Therefore, it can handle awide variety of liquids of different viscosity without the
problem of clogging of nozzles, etc.
Also, for ease of operation, setup uses a mouse toaccurately position the application to specified points.
We hope this product helps advance research inelectronics and medical/bio sciences, and contributes tothe further development of new technologies and newmaterials.

Photo of author

Motohiro UCHIYAMA
Product Engineering Department,
Precision Equipment Division-93-*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 1

NTN TECHNICAL REVIEW No.80（2012）[ New Products Information ]Electric Braking Systemwith Parking Capability
A lightweight and compact design with enhanced convenience and safety
is achieved by integrating an electric braking systemwith parking brake functionality!

Features

Structure[1] Enhanced convenience and safety

CaliperBrake pad

• Parking operation is simplified by electronic controlof parking brake(Auto-parking feature, release reminder feature)

Brake disk

Motor[2] Lightweight, compact• Parking brake unit (solenoid) is placed in the space
between linear motion mechanism and motor
Linear motion
mechanism(planetary roller screw
mechanism)

Applications

Intermediate gear
Parking mechanism

Detailed view of parking mechanism¡Electric vehicle, hybrid vehicle(electric parking brake)

Solenoid
Motor

Latch pinIntermediate gear

Parking ON/OFF switchedby inserting/removing latch
pin.
Latch hole-94-

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 2

New Products Information

Press Cut Spline Hub Joint “PCS-H/J”
Achievement of significantly lighter weight and higher performanceis obtained by the unique joint mechanism of hub bearing
and constant velocity joint!
Constant velocity joint

Hub bearing

Press cut joint

Bolt for tightening

Features

Structure[1] Lightweight

Hub bearing• 12% lighter than the current third-generation hub
bearing + constant velocity joint

Press cut joint[2] High performance• By our unique press cut joint technology with small
spline inside hub ring “Zero” clearance between
hub bearing and constant velocity joint• Reduction of stick slip noise
Bolt for tightening

Constant velocity joint

Applications¡Axles for passenger cars

Spline boundary-95-

No gapGap

Press cut spline (section)

Ordinary spline fit (section)

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 3

NTN TECHNICAL REVIEW No.80（2012）

Next-generation high-efficiency fixed
constant velocity joint “CFJ”
CFJ : Cross groove Fixed Joint

Contributing to vehicle fuel economy with the world’s top-level transmission
efficiency is the adoption of spherical cross groove technology!

Features

Structure[1] World’s top-level transmission efficiency

Spherical cross groove
Arc-shaped inner/outer ring balls are alternately inclined
toward rolling groove shaft• Torque loss rate: 50% reduction compared to theconventional product, EBJ

Inclination of theadjacent tracks ismirror symmetrically set[2] Low heat dissipation• Temperature rise: 50% reduction compared to theconventional product, EBJ

F1[3] Maximum operating angle
• 47˚

F2
Outer ring

Applications

Cage

Inner forces F1 and F2 are generated
in adjacent tracks in reverse directions(with conventional products, they were
all facing the same direction)¡Drive shaft and propeller shaft for SUV,ECO car, etc.

Ball

Inner ring

Inner forces are offset resulting
in reduced torque loss

Torque loss rate %2.0

Drive shaft

CFJ
1.5
1.0
0.5
0.0

Propeller shaft

Conventional product, EBJ

50% reduction compared toconventional product EBJ,
in high operating angles (8–12˚)

0

3

6

9

12

15

18

Joint angle ˚

-96-

21

24

27

30

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 4

New Products Information

High speed/low torque deep groove ballbearing for EV and HEV
High-speed rotation and over 50% in reduction of running torqueare achieved by adopting new type of resin for cages!

Features

Structure[1] High speed rotation①③• dn value: 1.2 million
In the case that the bearing inner diameter is 40 mm,
rotational speed: 30000 min-1[2] Low torque①②• Over 50% reduction when compared to our standard
deep groove ball bearing with oil lubrication.

Applications① Two new resin cages of the same type are adopted② Controls in-flow of oil inside bearings③ Optimized internal bearing specification¡Motor and reducer for EV and HEV
Speed reducer
Running torque mN・m

200

Motor

150

100

50
0
0

Developed product (applied to oil lubrication)-97-

Steel cage
Resin cage
Developed cage

1000

2000

3000

4000

Rotational speed min-1

5000

6000

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 5

NTN TECHNICAL REVIEW No.80（2012）

Deep groove ball bearing
for high thrust loadSignificant improvement of thrust load capability for deep groove ball bearing!
Tapered roller bearing can be replaced by this new technology!

Features

Structure[1] Thrust load capacity• Three times the conventional standard deep
groove ball bearing

①

②

②

①

[2] Bearing running torque• 60% reduction compared to tapered roller bearing[3] Reduction of assembly work time
• No pre-load adjustment required for incorporating
bearings

Applications① Thrust load side profile is added based on the standard
deep groove ball bearing② Adopted new resin cage combining small diameterand large diameter¡Transmission for vehicles

Running torque mN・m

Replacement of tapered roller bearing with developed product(deep groove ball bearing)

Rear differential gear

0.25
0.20

-98-

Reductionof 60%

0.15
0.10
0.05
0.00

CVT

Comparison of running torque(bearings for differential gear side: same size)

Developed
product

Tapered roller bearing30208

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 6

New Products Information

Removal Device of Microscopic Foreign Object
New Repair technology contributes to the increased productivityof FPD board manufacturing process!

XY stage for switching
objective lenses

Objective lensSmall XYZ stage(for driving needle)

Dedicated needle

Features

Structure[1] Removal of foreign objects using a special
needle with slotted screwdriver-shaped tip

Removal of foreign objects by dedicated needle
Foreign objects are removed by being pushed by the needle tip(needle is installed on the stage with mobility to XYZ directions)⇒Allows the removal of micron-scale foreign objects
that was difficult using conventional removal
methods such as laser cutting, grinding, etc.

Dedicatedneedle[2] Specify foreign objects repair areas using
a mouse on the monitor⇒Allows the removal of foreign objects while
minimizing damage to surrounding components

Dedicatedneedle[3] Reliable collection of removed foreign objects orresidue from foreign objects adhered to the needle
using suction or adhesive rollers⇒Cleaning process can be simplified or eliminatedafter repairs

Top view

Foreign
objects

Top view

SurfacelayerSide view

Side view

Before removal

After removal[Application example of “Removal Device ofMicroscopic Foreign Object” ]Applications¡Removal of foreign objects from flat
panel display (FPD) substrate¡Removal of foreign objects from photomask substrate for exposure▼

LCD repair systems

2200 mm (W) × 3200 mm (D) × 2500 mm (H)-99-

*20_26製品紹介_*20_26製品紹介 13/05/17 18:33 ページ 7

NTN TECHNICAL REVIEW No.80（2012）

Elliptic Vibratory Parts Feeder
High-speed transfer and low noise are achievedby horizontal elliptical vibration!
Bowl typeLinear motion type
Work flow
Work flow

Structure

Features

Conventional product[1] Improved feeding capacity by highspeed transfer

Vibration direction• Bowl type: 12 m/min. (150% compared toconventional products)• Linear motion type: 10 m/min. (160% compared toconventional products)

Work flow

Horizontal
direction
Vertical direction[2] Achieve low noise of alignment feeding•Improved quietness with smooth motion by ellipticalvibration(Noise reduced 10% compared to conventional
products)

Work is thrown up by diagonal vibration generated by the
combination of horizontal magnetic attractive force and reaction
force of a slantedly installed leaf spring (simple to-and-fro motion)[3] Pitching problem inherent in linear
motion type part feeders is resolved

Developed product (elliptical vibration)
Vertical direction

Work flow

Applications

Horizontal direction¡Alignment feeding of low-profile/lightweight components for automobiles,
electronics, and medical equipment

Elliptical vibration is generated by two independently installed
electromagnets in horizontal and vertical directions. Transfer
suitable to work property can be achieved by flexible control ofdriving force in two directions.

-100-