For New Technology NetworkR

TECHNICAL REVIEW No.84
November 2016

Green Energy Products and Machine Tool / Manufacturing Technology

http://www.ntn.co.jp

Special Issue

corporation

Special Issue: Green Energy Products andMachine Tool / Manufacturing Technology

NTN GREEN POWER PARK

corporation

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

ISSN 0915-0528 OSAKA, JAPAN

**表紙_レイアウト 1 18/06/13 13:34 ページ 2

Introduction of Green Power ParkNTN has established a "Green Power Park", an energy recycling model that supplies wind, water, and solar
powered energy to electric vehicles, vegetable factories, etc. within the Advanced Technology R&D Center site
located in Kuwana City, Mie Prefecture.
At this Park, we have installed small wind turbine generators, small hydraulic turbine generators (micro hydroturbines), wind / solar powered hybrid streetlamps, etc. developed by NTN, to conduct demonstration tests of naturalenergy related systems. Through these activities, we are proposing the use of these systems in communities close to
the daily lives of everyday citizens.
Small wind turbine power generators have achieved power generation from low wind speed through the adoptionof vertical blades, regardless of wind direction and quietness, by reducing noise through NTN's low-torque bearing
technology and special blade geometry.
Micro hydro turbines can be used where there are small scale streams, such as irrigation water ways or sewage
lines. Specially shaped propeller blades are able to efficiently capture the water stream without disruption, whichmakes it possible to install a series of micro hydro turbines in a single stream.
The electric power generated by these systems is constantly monitored and optimally controlled by the Monitoring
and Control Center for power level of generation, storage, and consumption for efficient circulation of clean naturalenergy that does not emit CO2.
We will strive for continuous improvement of this recycling model by continuing demonstration tests. We will also
make the Green Power Park publicly available to local students and customers for environmental learning andawareness of NTN’s technologies.

Overview of Green Power Park

(1)

(2)
(8)

(7)
(3)
(6)

(5)

NTN TECHNICAL REVIEW No.84
Printed and published in June 29, 2018(4)
EditorYoshinori TERASAKA

Publisher Yoichiro MOCHIDA
Published by NTN Corporation
3-17, 1-chome, kyomachibori, Nishi-ku, Osaka,J apan

(1) Small wind turbine generator 10 kW(2) Small wind turbine generator 4 kW
(3) Small wind turbine generator 2 kW
(4) Wind/solar photovoltaic hybrid streetlight 0.3 kW(5) Micro hydro turbine generator 1 kW
(6) Solar photovoltaic power generator 5 kW
(7) Monitoring and control center(8) Vegetable factory

Printed by
Nissha Printing Commmunications,Inc.

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

R

TECHNICAL REVIEWSpecial Issue;
No.84 Green Energy Products and Machine Tool
Manufacturing Technology

corporation

NTN TECHNICAL REVIEW No.84
CONTENTS
Preface

Contribution

For Green Energy Products and Machine Tool / Manufacturing Technology
Yoshinori TERASAKA

1

Outlook of Wind Energy Business2

Takao MAEDA

Perspective

Professor, Division of Mechanical Engineering, Mie University

Green Energy Business and Green Power Park

14

Natsuhiko MORI

● Special Issue for Green Energy Products
Small Wind Turbine Generator

22

Takaya ADACHI and Ryosuke KARASAWA

Micro Hydro Turbine

28

Tomoya KAWAI, Hiroki MUKAI and Tomomi GOTOU

CFD contribution to development of Small Wind Turbine Generator
Takeru ITOU, Ryosuke KARASAWA and Masato YOSHINO

34● Special Issue for Machine tool and Manufacturing Technology
Technical Trend of the Precision Bearings for Machine Tools

40

Naoki MATSUMORI and Keiichi UEDAULTAGE Serises Small Size High-Speed Precision Angular Contact Ball Bearings for Machine Tool Spindles
Mineo KOYAMA, Keisuke NASU and Takahiro KANAMOTO

Machine Tool Main Spindle Bearings with Air Cooling Spacer

46

52

Keisuke NASU, Naoya OKAMOTO and Masato YOSHINO

High-performance Sealed Cam Follower

58

Masato TSUJIHASHI and Seizo AGATA

Introduction of Linear Guides

63

Masaki KAGAMI and Keisuke KAZUNO

Plastic sliding screws

69

Naonari TANIGAWA, Norio ITOU and Tomomi TONOMURA● Technical Papers New Products
Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data
Takumi FUJITA

Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition
Takumi FUJITA, Naoya HASEGAWA, Naoya KAMURA and Toshihiko SASAKI

Speeding up of Parallel Link Angle Control Equipment

74

85

96

Kenzou NOSE, Hiroshi ISOBE and Seigo SAKATA

Propeller blade bearings for Aircraft Open Rotor engine

102

Guillaume LEFORT

Products Introduction of Composite Material for Industrial Machinery
Shinji KOMATSUBARA, Toshihiko MOURI, Takuji HARANO and Tamaki MIZUTANI

108● Award Winning Products《The Japan Society for Precision Engineering 2015 Spring Semestrial Meeting “Best Presentation Award”, “Young Engineer Award”》
114
Parallel Link High Speed Angle Control Equipment
Naoya KONAGAI《The Japan Society of Mechanical Engineers 2014 Material ond Processing Division “New Technology Development Award” 》

Materials Development about the Sintered Bearing Prepared by Unification Forming of Different Metals Powder

115

Yosuke SUGAI, Yoshinori ITOU, Toshihiko MOURI and Eiji YUASA《“Innovative Product Award” 2015 in China》

Machine Tool Main Spindle Bearings with Air Cooling Spacer

116
Wenwei WU and Michihiko KOSAKA《Equip Auto 2015 Innvation Award》

Press Connected Spline Hub Joint
Our Line of New Products

117
Sebastien GUILLAUME and Claire BIANCHIN
118

NTN TECHNICAL REVIEW No.84（2016）[ Preface ]Special Issue for Green Energy Products and Machine Tool/
Manufacturing Technology

Yoshinori TERASAKAManaging Director
With global warming being taken on as a major social issue, perspectives on the future of energy arebeing actively discussed. There is no doubt that further utilization of renewable natural energy sources
such as wind, hydro and solar power are being accelerated as alternatives to fossil fuel sources,
effective for CO2 reduction.
In particular, small-scale power generation that can be utilized in the proximity of communities andprovide stable and independent power is drawing attention to cope with recent extreme weather orlarge-scale disasters such as earthquakes.
We have started a mid-term management plan for the three year period starting in 2015 called"NTN100" for the promotion of "business development in new areas". These areas include (1) Energy
business, (2) EV business, (3) Robot associated business, and (4) Service solution business using
CMS (Condition Monitoring System)." In particular, for the area of Energy business, we have newly
established the Natural Energy Product Division to meet the social requirement of CO2 reduction. We
have developed products such as small-scale wind turbine generators with low-torque bearings andspecially-shaped blades for wind noise reduction as well as micro hydro turbines. We have also
started distribution of hybrid streetlamps. In addition, in the area of EV Business, we have developed
in-wheel motors (IWM) and conducted demonstration tests, contributing to the reduction of CO2,
together with our Natural Energy Business.
On the other hand, NTN contributes to the transformation of manufacturing with one of our baseproducts, precision rolling bearings for machine tools, commonly used in the main spindles of machine
tools. These employ NTN fundamental technologies such as materials, heat treatment and tribology.
We are glad that we could publish our Special Issue of Technical Review No. 84 for Green Energy
Products and Machine Tool/Manufacturing Technology, on this occasion of the 28th Japan
International Machine Tool Fair (JIMTOF2016) that takes place from November 17 through November
22 under the theme of "The Future Starts Here".

NTN has been the leading company in precision rolling bearings for machine tools since welaunched our ULTAGE Series of precision rolling bearings in the early 2000s, announcing new
products and always incorporating state-of-the-art technologies. Again in this issue, we are discussing
new technology trends of bearings for machine tools, as well as introducing newly developed products
such as small-size, high-speed precision angular contact ball bearings for machine tool spindles andmachine tool main spindle bearings with air cooling spacers that address high speed operation.
We will further promote technology/product development in " new business areas" to achieve salesof 1.4 trillion yen in 2025, together with new product development of rolling bearings as part of ourbase business. We will also contribute to sustainable social development through these corporate
activities, always responding to the requirements of the industry.

-1-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Contribution ]Outlook of Wind Energy BusinessTakao MAEDA
Professor, Doctor (Engineering) Department of Mechanical Engineering,
Mie University

The size of large wind turbines are increasing year by year, and somewind turbines have diameter of 160 m and more. An installation sites oflarge wind turbines were mainly onshore in the past, but are changed tomountainous regions and offshore. The technology corresponds to theinstallation sites are also developed. On the other hand, small wind turbines is useful for distributed power system,
which is introduced in advanced countries as well as developing countries of power shortage and weak electric grid.
In particular, introduction of small wind turbines is much increasing by the certification system for promotion of small
wind turbine in Japan. From the view which focuses on the difference between the large wind turbines and the small
wind turbines, this paper introduces a market trend, state of the art and the prospective of wind turbines.

However, the wind that wind turbines capture can varyin space or time. Therefore, creativity is required forextracting electric power from unstable and lowdensity wind energy. This article discusses the market
trend and differences of technologies between large
and small wind turbines, as well as from a futuretechnical perspective.

1. Introduction
The first time wind turbines for power generation
were developed was in the 1970s when aircraft
manufactures developed large wind turbines andsmall factories developed small wind turbines.
However, the high-tech wind turbines developed by
the aircraft manufacturers were not cost effective andwithdrawn from the market. On the other hand, small
factories gathered different technologies from different
areas of industry to create small wind turbines ofabout 10m in diameter through trial and error. Since
the 1980s, development has been focused ontechnologies for large wind turbines due to the policyof targeting large size turbines, and through technicalevolution the current mainstream has become
diameters of 80m and output of 2MW.
Recently, the distributed wind turbine concept has
been reconsidered and small wind turbines of several
kWs to several tens of kWs have been developed for
low power-grid areas and remote islands. In thedomestic market, activities involving small windturbines have been more widespread while somemunicipalities have introduced subsidies to small windturbines of several kWs for residential use.
Wind turbines are equipment that extract energy
from the wind and convert it to electric power.

2. Trends of the wind turbine market
2. 1 Large wind turbine market
As shown in Fig. 1 1), global installation of large
wind turbines is 432GW as of the end of 2015. Even in2015 alone, 63GW of wind turbines are newly installedwith annual growth of approx. 20%. Since the capacityof a nuclear power plant is 1GW, there are windturbines with the capacity equivalent of over 400
nuclear power plants in the world. As shown in Fig. 2 1),
China and the U.S. account for over half of the global
installed base, with China having the most. As shownin Fig. 3 1), 31GW of wind turbines were installed in2015 in China alone, which account for half of the new
global installations. There are several reasons why
wind turbine installations are active in China. First, as
opposed to large power plants such as thermal power
plants which require a long time from assessment tostart of operation, wind turbines can be installed in a-2-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 2

Outlook of Wind Energy BusinessIndia 4 %

Other
13%

New in 2015:
63 GW/yr

Brazil 4%

Annual installation capacity (MW)Installed capacity (MW)4000

Installed capacity (MW)Turkey 2%
UK 2%
France 2%
Poland 2%
Canada 2%

China
48%

Germany
9%
USA 14%

500
400

3000

300
2000
200
1000

0

100

2015

Fig. 2 Worldwide total installed capacity oflarge wind turbines 1)

2014

USA 17%2013

Germany
10%

2012

India 6%2011

Spain 5%

2010

UK 3%

2009

Cumulative total
at the end of 2015:
432 MW

Canada 3%

China
34%

2008

Italy 2%

France 2%

2007

Other
16%

Brazil 2%

2006

Fig. 1 Worldwide trend of total installed capacity oflarge wind turbines 1)

2005

Year

2004

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

2003

2002

2001

2000

1999

0

1998

100,000

2003

200,000

2002

300,000

2001

Offshore wind turbine

2000

Onshore wind turbine

400,000

1997

Installed capacity (MW)

500,000

0

Year* The total is not 100% due to a truncation error.

Fig. 3 Worldwide annual installed capacity oflarge wind turbines 1)

Fig. 4 Domestic trend of installed capacity oflarge wind turbines 2, 3)-3-Annual installation capacity (MW)installed base of offshore wind turbines is still 12GW
among the global installed base of 432GW. Therefore,
it is apparent that on-shore wind turbines are still themainstream.
On the other hand, in Japan, renewable energy
feed-in tariff programs started in July 2012 andexpansion of installation of wind turbines was
expected together with other renewable energy.
However, the requirement of an environmental
assessment equivalent for construction of large
buildings was also implemented for wind turbines from
October 2012, which prevented the introduction ofwind turbines. It takes over three years to complete
environmental assessment procedures, which slowed
down the introduction. Because of this policy, as if
applying acceleration and braking at the same time,the domestic introduction of wind turbines, as shownin Fig. 4 2, 3), is still very low with 3GW (2143 units) ofinstalled base and 0.2GW of new installation in 2015;
this only accounts for 0.5% of the total power supply inJapan. This is significantly low compared to the EU
which covers 11.4% of the demand with wind power,
which indicates a large potential for domestic wind
power introduction. Japan Wind Power Association
conducts wind potential and feasibility studies for
installation sites of wind turbines and created theroadmap shown in Fig. 5 4). This roadmap aims for
installation of 75GW of wind turbines to cover 20% of
the domestic power demand. As of February 2016, thecapacity of new wind turbines approved by the
Ministry of Economy, Trade and Industry is 2.65GW,
double the current installed base of 3GW when
installed. The objective of the present wind powerindustry is to achieve "grid parity," which is to offer
prices and reliability equivalent to that of theconventional power generation such as thermal power
plants.

few months as long as the wind conditions arefavorable. Another reason for the fast growth is that
China is promoting the wind turbine industry as anational policy for increasing employment. In Europe,
installation of "offshore wind turbines" has recently
become active; however, as shown in Fig. 1, the*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

Roadmap of wind turbine installations: vision
Actual vs. goal of wind turbine
installations (10,000 kW)

Floating wind turbine
Bottom-mounted wind turbine

7,000

Onshore wind turbine
Actual

6,000
5,000
4,000

BottomOnshore mounted

Year

Total

2010

248

245

3

OutputFloating(100
million
kWh)

0

43
230

0

2,050

1,880

2,048

1,800

2,046

1,900

2,042
2,044

3,800

2,040

7,500

2,038

2050
2,036

1,000

2,032
2,034

1,620

2,030

1,290

2,028

1,500

2,026

3,800

2,022
2,024

6,590

2,020

2040

2,018

810

2,000

2,014
2,016

380

2,010
2,012

10

580

2,008

60

2,660

2,006

1,020

3,620

2,004

1,090

2030

2,002

2020

3,000

2,000

Installed base (10,000 kW)

8,000

Fig. 5 Roadmap of domestic installed capacity of large wind turbines by Japan Wind Power Association 4)

2. 2 Small wind turbine market

growth rate has started to recover from 2015, and theannual growth rate is conservatively estimated at
20%, reaching around 1.75GW of installed capacity by2020 and annual installation capacity is estimated toreach 240 MW.

Small wind turbines are defined to have "a rotor
swept area of less than 200m2" according to theinternational standard IEC 61400-2.
Therefore, for propeller-type horizontal-shaft windturbines, those with a diameter of less than 16m and
up to an output power of 50kW are classified as small
wind turbines.
As shown in Fig. 6 5), the global installed base ofsmall wind turbines as of the end of 2014 is 945,000
units, with annual growth of 8%. As shown in Fig. 7 5),
the installed capacity is 830MW, with annual growth of
11%. The fact that the capacity growth is larger thanthe unit growth reveals that the average size isincreasing, even with the small wind turbine category.
The average size increased year on year, from
0.66kW in 2010, 0.84kW in 2012 to 0.87kW in 2014.
However, this trend varies from country to country; forexample, the average size is 0.5kW in China, 1.4kW
in the U.S., and 4.7kW in the UK. In addition, thedefinition of small wind turbines in the UK is less than
100kW, determined by an association for wind power,
and in the U.S. the definition varies from less than
100kW to less than 200 kW, depending on the state.
A significantly large installed base is found in China,
U.S., and UK. That is because, those countries, which
traditionally have large small-sized wind turbine
markets, have achieved cost reduction due to mass
production that resulted in accelerated introduction; as
opposed to countries with smaller markets having
difficulty in achieving similar cost reduction.
World Wind Energy Association created a roadmapof small wind turbines 5). The annual new installation
has been increasing by 19 to 35% until 2012, and thenthe growth rate slowed to 11% by 2013 to 2014.
However, since associations and industries
associated with small wind turbines in different
countries are taking steps to expand adoption, theSweden 0.2%
Poland 0.4%
Spain 0.8%
Argentina 1%
Japan 1%
Canada 1%
Germany 2%
UK 3%

Other 1%

USA 17%

Cumulative total
at the end of 2014:
945 k units
China
73%

* The total is not 100% due to a truncation error.

Fig. 6 Worldwide total installed units ofsmall wind turbines 5)
Argentina 1%
Spain 1%Poland 1%
Canada 2%
Ukraine 2%
Germany 3%
Italy 4%

Other 3%

UK 16%

Cumulative total
at the end of 2014:
830 MW

China
41%

US 27%* The total is not 100% due to a truncation error.

Fig. 7 Worldwide total installed capacity ofsmall wind turbines 5)-4-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 4

Outlook of Wind Energy BusinessAs shown in Fig. 8 6), among 327 small wind turbine
manufactures in the world, 74%, which represents 242
companies, manufacture horizontal axis type turbines,
18% or 60 companies manufacture vertical axis typeturbines, and 6% or 19 companies manufacture bothhorizontal and vertical axis type wind turbines. There
are no vertical axis type wind turbines for large-scale
production. Mechanically, it is more difficult to produce
vertical axis type wind turbines compared to horizontal
axis type as mentioned later. Therefore, it is noteworthy
that the vertical axis type wind turbines are produced ina certain quantity for small-scale wind turbines.
The scope of applications for small wind turbines are
as follows:

and support is significant.
In many countries, there are policies supporting
promotion of small wind turbines. Since feed-in tariffs
and purchasing of surplus electricity programs, which
are also being introduced in Japan, are enabling
revenue based on the generated power, good profit canbe achieved if reliable small wind turbines are installedat the sites with favorable wind conditions. On the other
hand, support such as favorable tax systems andsubsidies of installation costs only provide help for the
hardware of small wind turbines when installed,
regardless of whether they produce power or not.
Therefore, their contribution for sound development of
the small wind turbine industry is small.
For sound development of the small wind turbine
market, it is required to increase adoption of reliable
small wind turbines. The roles that standardization andcertification can play are significant for that. The
process of standardization is in good progress as IEC
61400-2 for design requirements for small windturbines, IEC 61400-12-1 for power performancemeasurement, and IEC 61400-11 for acoustic noise
measurement are already established and test andcertification organizations in different countries haveadopted them. Since various technologies associatedwith small wind turbines are making progress every
day, the International Energy Agency (IEA) Wind Task
27 is engaged in international joint research withinformation exchange for pre-standard research anddevelopment in different countries.
The wind condition maps addressed regarding large
wind turbines have already been created in many
countries. They have examined wind conditions at theheight of 50m or more from the ground. However, as
the small wind turbines are installed at the height of
30m from the ground, the wind condition maps for large
wind turbines are not sufficient. The forecast of windconditions is particularly difficult since there are many
obstacles that disrupt wind flow in areas closer to theground, such as buildings and trees. Therefore,
research for understanding the characteristics of local
wind behavior and prediction is indispensable in order
to increase power output and adoption rate of small
wind turbines.

• Power source for residential use• Commercial/factory power source• Power source for fishing boats and leisure boats• Hybrid power generation with solar batteries, etc.
• Power source for cattle farm, farmland, and remoteislands, etc.
• Mobile power source for excursions• Power for water pumps• Power source for water desalination and purification• Power source for remote monitoring• Material for education/research purposes• Power source for communication equipment
Promoting small wind turbines requires "costreduction," "policy", "standard and certification", and"careful examination of wind conditions." Theinstallation cost of small wind turbines is $6,230/kW in
the U.S. and $1,900/kW in China. In the UK, the cost is$6,181/kW for 1.5kW to 15kW and $4,876 for 15 kW to
100 kW 5). In spite of efforts for cost reduction by small
wind turbine manufacturers, expansion of the market isrequired for cost reduction; therefore the role of policy
Horizontal/vertical axis
wind turbines
Other 2% (6 companies)
6% (19 companies)

Vertical axis
wind turbines
18%(60 companies)

2. 3 Trend of the domestic wind turbine industry
The wind turbine industry has large supporting
industries similar to the automobile industry. A large
wind turbine consists of around 10,000 components
and its technological and economical ripple effects are
as significant as the automobile industry. However,since the related areas are so widespread into themechanical, electrical, chemical, civil engineering fields,
etc., no clear industry picture has been captured in

Horizontal axis
wind turbines
74%(242 companies)

Fig. 8 Orientation by worldwide small wind turbine
manufacturers 6)-5-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 5

NTN TECHNICAL REVIEW No.84（2016）load acting on a blade is the same during rotation
regardless of the azimuth angle (rotating direction
position); therefore, the torque acting on the blades isconstant.
On the other hand, as shown in Fig. 12, blades of avertical axis type wind turbine receive both strong and3. Wind power generation technologies

Fig. 10 Load acting on blade of vertical axis wind turbine
by rotor rotation

Tensile load

Japan. On the other hand, promotion of wind turbine
power generation related industries is being consideredfrom the viewpoint of improving global environment
problems and promoting environmental industries bygreen innovation, etc. The Japan Society of Industrial
Machinery Manufacturers is conducting research on thewind turbine related machinery industry in order tounderstand the actual industrial status, such as thecurrent activities in manufacturing of wind turbine
related machinery industries, and contribute to thepromotion of building infrastructure of a new industry 7).
This research group consists of the members of wind
power generation related businesses, related industrial
associations, and academic experts and analyzes
correlation of various businesses, studies researchstatistical methods on production, etc., analyzes
industry for production, export, etc. and studies
industrial promotion policy. Until now, Japanese wind
power related businesses have been developing
business on their own; however, this initiative is
research to precede efforts by the nation for entry into
an ever growing global market, and the neighboring
Chinese market which has become the largest in theworld. In the past few years, new introduction andrevenue of the wind turbine industry in the Japanese
domestic market have shown decreasing trend;
however, revenue started to increase from 2014 andwind turbine and component manufacturers are
expecting the global market to increase 1.5 times andthe domestic market to increase 1.6 times in 2020
compared to the market size in 2015.

Wind

Bending load

Fig. 9 Load acting on blade of horizontal axis windturbine by rotor rotation

Wind

Wind turbines are categorized into horizontal axis andvertical axis types. As shown in Fig. 9, the centrifugal
force from rotation acts on the blades as tensile force
with horizontal axis wind turbines, in contrast, thecentrifugal force acts as bending moment on the bladesin case of the vertical axis type wind turbines as shownin Fig. 10. Since a long object such as a blade is more
susceptible to bending than tensile force, vertical axis
type wind turbines are more affected by centrifugal force
on the blades. In addition, the rotation speed of theblades tends to be slower as they become larger, forexample, a few to tens of rotations per minute; therefore,
their own weight gives more impact to the overall load
than centrifugal force. In contrast, small wind turbines
tend to experience hundreds to thousands of rotations
per minute; therefore, centrifugal force affects more on
the load of the blades.
Furthermore, as shown in Fig. 11, considering
incoming wind to horizontal axis wind turbines, the wind

Torque3. 1 Mechanism and classification of wind turbines

Torque
Wind

0

90

180

270

360

Azimuth angle (blade position)(˚)

TorqueWind

Fatigue on
supporting
material

TorqueFig. 11 Torque acting on blade of horizontal axis wind turbine

Wake
flow

0

90

180

270

360

Azimuth angle (blade position)(˚)

Fig. 12 Torque acting on blade of vertical axis wind turbine-6-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 6

Outlook of Wind Energy Businessweak winds during rotation, as the downwind blade goes
through the wind that is slowed by the upwind blade(wake flow). This means that the main shaft andsupporting material experience greater fatigue as thetorque changes during rotation in the case of verticalaxis type wind turbines. Since the natural wind is notconstant, blades of horizontal axis wind turbines
experience fatigue; however, the fatigue of vertical axis
wind turbines is greater than that of horizontal axis windturbines due to the impact of wake flow.

the designer to establish specific wind conditions.
These standards are subject to review every few
years to cope with the progress of wind turbine
technologies incorporating new data and insights asthey are revised.
On the other hand, the standard for small windturbines, IEC61400-2, is also classified into similarclasses as shown in Table 3; however, since the small
wind turbines are often installed in the areas with low
wind speed, class IV is added for lower wind speed.
Considering the fact that the small wind turbines are
low and usually installed in the areas with larger
turbulence because of the buildings and trees, theturbulence category is set to 0.18, which is larger thanthose for large wind turbines.

3. 2 Design standards of wind turbines
Design standards for wind turbines are establishedby the International Electrical Commission (IEC). As
shown in Table 1, IEC61400-1 8) for on-shore large
wind turbines, IEC61400-2 9) for small wind turbines,
IEC61400-3 10) for offshore wind turbines are
established, in addition to other standards forcomponents of wind turbines.
The best known standard, IEC61400-1 for on-shore
large wind turbines, is classified into class I, II and III
depending on the wind speed (from fast to slow)
considering the different wind characteristics of theinstallation sites, and category A, B and C, for each of
the wind speed class, depending on the turbulence(from large to small), as shown in Table 2. Therefore,
there are 9 wind design specifications, i.e. 3 windspeed classes x 3 turbulence categories. In addition, a
special class S is defined for sites where theconditions exceed class I and category A, requiring

Table 3 Small wind turbine class by IEC 61400-2

Turbulence category Iref
(expected turbulence
strength at wind speed of
15m/s)

T＊

S

50

42.5 37.5

57

10

8.5

Value specified by designer

Reference wind speed Vref (m/s)(10 minute average reference
wind speed)
Average wind velocity Vave (m/s)(annual average wind speed)#

7.5 10 8.5 7.5

A+＊

0.18

A
B

0.14

C

0.16
0.12

10

6

0.18

4. 1 Wind turbines that address severe windcharacteristics
In Europe, favorable sites for large wind turbines,that is, flat with good wind conditions, are alreadysaturated with wind turbines and therefore, they are
installed in more difficult sites such as mountainous
Blade
Rotor (rotating surface)

Table 2 Large wind turbine class by IEC 61400-1 (Note: Class
T and category A+ are expected to be added to edition 4.0)@

Average wind velocity Vave (m/s)(annual average wind speed)

8.5 7.5

S$

Fig. 13 shows the components of large windturbines. The wind energy is converted into rotationalenergy by the blades and into electricity by the stepup gears and generator inside the nacelle (machineroom).

Corresponding domestic
standards (JIS)!

#

4. Technology for large wind turbines

IEC 61400-1:
JIS C1400-1(Onshore) wind turbine design requirements
IEC 61400-2:
JIS C1400-2
Small wind turbine design requirements
IEC 61400-3:
JIS C1400-3
Offshore wind turbine design requirements
Additionally, several standards are specified on noise measurement
methods, performance test methods, power quality, certification,
protection from lightening, blade load test methods, etc.

Wind turbine class@

50 42.5 37.5 30

Turbulence category Iref (nondimensional
characteristic value of turbulence strength
at wind speed of 15m/s)

Table 1 Wind turbine standards by International Electrical
Commission
International Electrical Commission (IEC)!

Value specified by designer

Wind turbine class
Reference wind speed Vref (m/s)(10 minute average reference wind speed)

Enlarged view of nacelle
Blade

Nacelle

Wind

Tower

Step-upgear

Pitch
control unit

Brake
Anemometer

Generator
Yaw control unit

Foundation

Fig. 13 Components of large horizontal axis wind turbine(＊specifies wind characteristics to be added in edition 4)-7-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 7

NTN TECHNICAL REVIEW No.84（2016）

wave data of the North Sea. However, the wave
characteristics in the Pacific-rim, including Japan, aredifferent. In addition, Japan has a rich database ofwave data accumulated over the years, so Japan is
proposing adoption of such data during the review of
the IEC standard. Inclusion of proposals from Japan in
the international standards is important for Japanesewind turbines to be adopted in the global market.
In addition, "floating wind turbines," which are
offshore wind turbines floating in the sea, are recently
drawing broad attention. "Floating wind turbines" havebeen considered as technology of the distant future;
however, the start of a demonstration test of 2MWclass floating wind turbines by Norway in 2009
instantly captured world attention. Japan is also
conducting world-leading demonstration tests offloating wind turbines, namely, 2MW floating windturbines in the Goto Islands, and 3 types of floating
wind turbines such as 2MW, 5MW, and 7MW off
Fukushima. The bottom mounted offshore windturbines were the result of cooperation between therotational machine and civil engineering of marine
structure construction. The floating wind turbines are
the result of cooperation between the wind turbinesand shipbuilding technology. Therefore, realization of
floating wind turbines is a new technological stage inthe history of wind turbines.
With this trend in the background, wind turbines are
becoming larger and larger. The largest wind turbine
in the world is installed offshore from Fukushima
Prefecture, as shown in Fig. 14, with 167m ofdiameter and 7MW of rated output and the height of
the upper tip of the blade at 189 m. Future windturbines are expected to become even larger andvarious countries are conducting research of thereference wind turbine concept of over 10MW
(diameter of 200m). In Japan, NEDO also presented
10MW reference wind turbines in the research of windturbines of over 10MW 11).
The concept that supports ultra large wind turbines
includes reduction of the number of blades from 3 to2, adoption of downwind rotor position from upwind,compact and lightweight drive trains (a set of drive
equipment including step-up gears and a generator),
etc.
The current mainstream 3-blade wind turbines haveadvantages of relatively small load variance and avisually acceptable design. On the other hand, 2-blade
wind turbines make it possible to reduce torque andattain more compact drive train by increasing rotation
speed, thus, achieving cost reduction.
Most of the current wind turbines are designed withthe upwind method in which the rotor is positioned
upstream of the tower as shown in Fig. 15; only a few

areas and offshore locations.
The challenges of installing wind turbines incomplex topography such as mountainous areas
include how to withstand fatigue produced in the windturbines due to strong wind and turbulence and how toincrease generated power in the midst of turbulent
wind. Particularly in Japan, it is important to determine
how to prevent damage due to the extreme load onwind turbines caused by typhoons. In the past, it wasunderstood that considering the impact of turbulence
due to complex topology and typhoons on windturbines was unique to Japan. However, this isbecoming a common challenge throughout the world
as there are areas of strong turbulence such as the
Pacific-rim and Mediterranean coasts, as well asapparent extreme wind cases caused by tornadoes
and cyclones. Conventional wind turbine design
requirements did not specify special wind conditions
and strong turbulence such as typhoons. IEC 61400-1
ed. 4, which is soon to be published, is expected toinclude tropical storm wind class T which withstands
strong wind from typhoons, and high turbulence
category A+ which withstands strong turbulence fromcomplex topography in mountainous areas, based on
the proposal from Japan.

4. 2 Offshore wind turbines
The advantage of offshore wind turbines is to beable to use faster and less turbulent wind than onland; however, because of the high cost offoundations, transmission lines and maintenance,large wind turbines are used to increase unit output.
Additionally," wind farms" are being created byinstalling many wind turbines in one location to pursue
economies of scale from large-scale projects.
Offshore wind turbines are affected by wind load aswell as wave load. Therefore, traditional strength
design considers the total load by simply adding wind
and wave loads together. However, this method may
create excessive safety factors; therefore, current
design methods explore optimum load evaluation bycoupled analysis of wind and wave loads. Japan also
has some offshore wind turbine installations such as a2MW turbine along the coast of Kamisu-shi, Ibaraki
Prefecture, owned by a private company, and fullscale offshore wind turbines off Choshi-shi, Chiba
Prefecture and Hibikinada, Kitakyushu-shi fromdemonstration tests by New Energy and Industrial
Technology Development Organization (NEDO).
Since evolution of the technology of offshore windturbines is significant, IEC 61400-3 for design
requirements of offshore wind turbines published in2009 is already under review. The current offshore
wind turbine design standard is specified based on-8-*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 8

Outlook of Wind Energy Businessdrive train, which accounts for a large portion of theturbine weight, is required. The current mainstream
large wind turbines of a few MW adopt high speed
drive trains, where rotor rotation of several tens of rpm
is increased by approximately 100 times through stepup gears and transmitted to the generator. However,step-up gears of a high speed-up ratio often givetrouble; therefore, direct drive trains, where a multipolar generator is adopted instead of step-up gears,
may also be used. Since the direct drive train has its
own shortfalls, such as difficulty of manufacturing andcost for rare earth permanent magnets, future windturbines over 10MW will probably adopt mid-speed
drive trains combining step-up gears with anappropriate speed up ratio and multi-polar generatorswith an appropriate number of poles.

models adopt the downwind method. It seems thedownwind method is more logical when considering
the weathercock effect; however, the downwind
method creates fatigue loading as the tower blocks
the wind and the blades go through the lower speed
wake flow. Therefore, most of the large wind turbines
adopt the upwind method where the nacelle is forced
to face upwind by the yaw control system. However,when it comes to ultra large wind turbines, the blades
may be deflected by the wind load and hit the tower;
therefore, the downwind method is considered to be
more favorable.
For cost reduction, it is essential to reduce theweight of the wind turbine; therefore, a lightweight

4. 3 Peripheral equipment of wind turbines
In addition to the technologies of large wind turbinesthemselves, various technologies are being studied forincreasing adoption rates and preventing failures.
When wind turbines become large, each component
becomes large as well, making it difficult to repair.
Especially for offshore wind turbines, it is important toprevent failure because of the limited access due tothe weather conditions and difficult repair work in thesea. The vibration and temperature of the main
components have been traditionally monitored by theSupervisory Control and Data Acquisition (SCADA)
system; however these are mainly used for stopping
operation in emergencies or abnormal conditions.
Therefore, research of the Condition Monitoring
System (CSM), for more sophisticated supervisory
control than SCADA, is underway to prevent trouble
from occurring. In addition, wind turbines use
feedback control as they adopt pitch control bymonitoring wind speed and generated power.
However, the generated power may increase if feedforward control is achieved by forecasting theincoming wind to wind turbines. Therefore, researchfor controlling operation by measuring upstream windwith riders (wind measurement equipment using laser)
installed on the nacelle is also underway. Althoughvarious riders dedicated for wind turbines havealready been developed, technology to use theupstream wind data for controlling operation of windturbines is still under development.

Fig. 14 Largest-diameter wind turbine in world(diameter: 167 m, rated power: 7 MW, height of blade
top: 189 m Courtesy : Toru Nagao)

Wind

Wind

Strong wind

Weak
wind
Upwind method

Downwind method

Fig. 15 Upwind turbine and downwind turbine-9-

*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 9

NTN TECHNICAL REVIEW No.84（2016）

Large wind turbines are equipped with pitch control
units for changing blade angles according to the windspeed. When large wind turbines stop operation, thepitch angle of the blades becomes almost 90 degrees
to completely bypass the wind to stop the rotor rotation.
When the wind starts, the blades gradually start
opening with the pitch angle changing from 90 degrees
toward 0 degrees, and the rotor starts rotating with thewind. When the wind speed is relatively low, the pitch
angle is set to almost 0 degrees, enlarging the wind
reception area of the blades to increase the generated
power. When the wind speed exceeds the rated powerof the generator, the pitch angle is adjusted to gradually
bypass the wind in order to maintain the rated output.
When the wind speed exceeds the design limit, thepitch angle is set to almost 90 degrees to completely
bypass the wind and the turbine stops operating.
On the other hand, small wind turbines are usually
not equipped with pitch control because of the high costand lack of space for incorporating the pitch control unit.
Therefore, the pitch angle is fixed to 0 degrees. With apitch angle of 0 degrees, activation of wind turbines isdifficult even when the wind starts blowing, as theblades lose momentum. Therefore, lighter blades andgenerators with small rotational resistance aredesirable. In terms of lighter blades, the mainstream
materials for large wind turbine blades are composite
materials such as fiber glass reinforced plastics forextending fatigue life. However, various materials suchas metal, composites, wood, etc., are used for small
wind turbines, not standardized materials. With respect
to generators, those with the smallest possible starting
torque are desirable to improve the starting
characteristics; however, permanent magnet typesynchronous generators are often used, which present
a cogging problem. Small wind turbine manufacturers
are manufacturing those generators internally, andthere are no general purpose generators for small windturbines addressed to market expansion. Comparing
the dimensional ratio of small wind turbines and large
wind turbines, the weight of blades and starting torqueof generators for current small wind turbines are
significantly large. Therefore, it can be assumed thatcost reduction of small wind turbines will be achieved
when lightweight and high performance components
become available through volume efficiency along withthe expansion of the market. In addition, it is difficult tostop operation of small wind turbines with the fixed pitch
angle of 0 degrees when strong wind is present.
Therefore, small wind turbines often use braking withload resistance in addition to short braking of thegenerators. In addition to these brakes, horizontal axis
wind turbines may use furling control, which is to directa wind turbine sideways, and vertical axis wind turbines

5. Technology for small wind turbines
In general, small wind turbines are not equipped withpitch control units or step-up gears since space forstoring components is limited. Also, no yaw control units
are equipped for wind direction control, only adopting
free-yaw control which passively uses wind load.

5. 1 Value of small wind turbines
Large wind turbines require sufficiently large sites for
installation. Therefore, they are often installed in remoteareas far from populated urban areas. However, sincelarge demand comes from urban areas, electric power
generated by wind turbines in remote areas is required
to be transmitted to the consumption areas bytransmission lines. Therefore, from the standpoint of
producing energy on-site at the consumption areas, it ismore desirable to install wind turbines in urban areas. In
addition, since the power grid is weak in isolated islands
and settlements, it is desirable to generate power inthose regions from an energy security standpoint, even
if installation of large wind turbines is difficult. Due tothese circumstances, it is expected that small to midsize wind turbines will be introduced in urban areas,
and isolated islands and settlements, as a "distributed
power source" with adequate capacity for the particulardemand of the region. Especially in the U.S., wherelarge areas adequate for installation of large windturbines with good wind conditions are mostly located in
the central regions of the North American continent,
and the large demand is concentrated in the coastal
areas, the cost of long transmission lines is an issue.
Therefore, the American Wind Energy Association has
been focused on the introduction of small wind turbines
for on-site power generation. Also in Japan, thepurchase price of feed-in tariffs for electricity generatedby small wind turbines is set at the highest among all
renewable energies in order to promote them.
The cost of small wind turbines per output is highcompared with large wind turbines; therefore, cost
reduction is the next challenge. In addition, labelingprograms which indicate power capacity, noise level,
tolerance (wind turbine class), etc., are being adopted
globally, so that users can select their most desiredmodel from a diverse line-up of small wind turbines.
Also in Japan, Nippon Kaiji Kyokai is offering acertification program for small wind turbines to ensure
their safety and reliability, as feed-in tariff programs
started in 2012.

5. 2 Technology challenges for small wind turbines
Since small wind turbines are not just theminiaturization of large wind turbines, they have theirown technology challenges.
-10-*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 10

Outlook of Wind Energy Business

The simplified load model allows easy calculation
assuming the running rotation speed, etc. for small windturbines; however, IEC 61400-2 only specifies thesimplified load model for small horizontal axis windturbines, not for small vertical axis wind turbines.
Globally, horizontal axis wind turbines are themainstream; however, in Japan, there are many verticalaxis wind turbines installed; therefore, creation ofdesign guidelines for small vertical wind turbines isurgently required. With these situations in the
background, a research for building the world firstsimplified load model for vertical axis wind turbines was
conducted in Japan 12, 13), and it was specified as thestandard by the Japan Small Wind Turbines
Association 14).
However, while the ultimate load safety factor in the
aeroelastic model is set to 1.35 since the precise design
is assumed in the model, the safety factor for thesimplified load model is set at 3.0, as the model allows
for a simple design. Many small wind turbines aredesigned using the simplified model with a high safety
factor, which leads to higher cost.
For horizontal axis wind turbines, the generalpurpose aeroelastic model is built, which can be used
for large and small wind turbines. However, there areno aeroelastic models for vertical axis wind turbines.
Therefore, an aeroelastic model for vertical axis windturbines is under development, aiming for cost
reduction in vertical axis wind turbines 15). In order tobuild an aeroelastic model, it is necessary to compare
and evaluate load acting on the actual vertical axis windturbine and load calculated by the model. Therefore,load acting on the vertical axis wind turbine is measured
in detail, by wind tunnel experiments, as shown in Fig.
16. Since the rotation of vertical axis wind turbines is
produced by the blades, a pressure measurement holeis created on the rotating blade surface, as shown in

may use mechanical brakes, as well.
Natural wind contains turbulence of variable duration.
Because of large rotor inertia, large wind turbines
absorb variance of wind load against high frequency
turbulence, such as instant wind speed change, only torespond with rotation against wind turbulence of a longtime scale. On the other hand, small wind turbines havesmall rotor inertia; therefore, the rotation changes evenwith instant wind speed change. The rotation rapidly
becomes high with sudden high wind speed, andconversely, the rotation rapidly becomes low or stops
with sudden low wind speed. As a result, in anenvironment where wind speed is low with significant
variation, small wind turbines are unstable, repeating
power generation and no power generation. Lighter
blades are desirable from the standpoint of starting
characteristics; however, relatively heavier blades aredesirable from the standpoint of stable output. This is atrade-off relation. In contrast, even if vertical axis windturbines have a lower output coefficient, since they
have higher rotor inertia and their operation does notdepend on the wind direction, the output is more stable.
The use of small wind turbines is categorized into anindependent power source method, separated from thegrid and grid connection methods, connected to thepower system. In the rural regions where only a small
amount of power is required, the independent powersource method is used with small wind turbines andsolar power generation.
For example, they are used for streetlights in rural
areas and as the power source for radio wave relay
stations, where the cost of installing transmission lines
is excessive. In these cases, the power generated bysmall wind turbines is stored in batteries before use. On
the other hand, if connected to the grid, power
generated by small wind turbines can be sold to powercompanies by feed-in tariff programs. However, there
are no certified power conditioners for small windturbines, which are indispensable in the connection ofsmall wind turbines and the grid, making the procedureof grid connection complicated. For increased adoptionof small wind turbines, certified power conditioners are
required, such as the case for solar-light power
generation. The grid connection requirements for small
wind turbines are currently being discussed at NEDO.

5. 3 Standardization of small wind turbines
Large wind turbines are subject to the design
requirements of IEC 61400-1 under the aeroelastic
model considering the aerodynamics of wind load andelasticity of the structure. On the other hand, small windturbines are allowed to use simplified load models, by
IEC 61400-2, which uses simplified design procedures,in addition to the aeroelastic model.

Fig. 16 Wind tunnel experiment of vertical axis wind turbine(Mie University Large Wind Tunnel)-11-*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 11

NTN TECHNICAL REVIEW No.84（2016）0.15

circumferential speed to wind speed). In addition, as
shown in Fig. 20, behavior such as stalling can belearned by measuring flow around the blades of thevertical axis wind turbines during operation, improving
accuracy of the aeroelastic model. After analyzing theperformance of vertical axis wind turbines and thedetails of load from these wind tunnel experiments,
final evaluation of the aeroelastic model will be
performed to consider the impact of unsteady wind on
performance and load of the vertical axis windturbines. We believe that, through these studies, theaeroelastic model for vertical axis wind turbines will be
developed and low-cost and highly reliable verticalaxis wind turbines can be designed.
1.2

Pressure holeposition

0.10
0.05

Nondimensionalx coordinate

Blade thickness ratio

Fig. 17, and the static pressure on the blade surface ismeasured by the pressure sensor installed on therotating system. An example of the measured pressure
distribution is shown in Fig. 18. It can be seen that thepressure distribution varies depending on the azimuth
angle of the blades. By integrating the area enclosed bythis pressure distribution and calculating the rotational
direction component, the rotational force that acts on
the blades can be calculated as shown in Fig. 19. This
figure reveals that the rotational force of running
vertical axis wind turbines can be changed by theazimuth angle of the blades, as well as by thecircumferential speed ratio λ (ratio of rotor

1.1

0.00

6m/s

1.0

-0.05
-0.10
-0.15
0.0

0.4

0.2

0.6

0.8

Position of blade chord direction

0.9
-0.4

1.0

Fig. 17 Pressure taps on blade of vertical axis wind turbine

-0.3

-0.2

-0.1

0.0

0.1

0.2

Nondimensional y coordinate

0.3

0.4

Fig. 20 Flow around rotating blade of vertical axis windturbineNondimensional pressure coefficientunder uniform flow pressure-25
0 [deg]-20

6. Summary

95 [deg]-15

Large wind turbines are becoming ultra large as
installation sites move from onshore to offshore,reaching a diameter of 200m and output of over
10MW.
The generated power is proportional to the squareof the rotor diameter and the cost of the wind turbine
increases proportional to the cube of the rotor
diameter; therefore, the cost per generated power
becomes higher as the size increases. Therefore,
progress in NTN's practical research related to theCondition Monitoring System (CMS) aimed for
improved availability and prevention of failure is highly
expected. In addition, some innovative technology
development is also expected, for example, anexperimental study of airborne wind turbines isunderway, where only a propeller is floating in the air,suspended by wires, eliminating the tower which
accounts for a large proportion of the wind turbine
cost.
On the other hand, small wind turbines, which are
deployed in different sites and applications from large
wind turbines, also face the challenge of cost
reduction for increased introduction. Therefore, theindustry is addressing adoption of common
components for small wind turbines and a certification
program of equipment for grid connection. In addition,

185 [deg]
270 [deg]

-10
-5
0
5
10
0.0

0.2

0.4

0.6

0.8

1.0

Position of blade chord direction

Nondimensional tangential force

Fig. 18 Pressure distribution on blade of vertical axis
wind turbine in operation

3.5
λ=1.383.0

λ=2.192.5

λ=2.58

2.0
1.5
1.0
0.5
0.0
-0.5
-1.0

0

90

180

Azimuth angle (˚)

270

360

Fig. 19 Tangential force on blade of vertical axis windturbine-12-*03-寄稿文E_*02/寄稿文 18/06/13 13:45 ページ 12

Outlook of Wind Energy Business8) International Electrical Commission, Wind Turbines – Part
1: Design requirements, Edition 3.0 Amendment 1,(2010).
9) International Electrical Commission, Wind Turbines –Part 2: Design requirements for small wind turbines,
Edition 3.0, (2013).
10) International Electrical Commission, Wind Turbines – Part
3: Design requirements for offshore wind turbines,
Edition 1.0, (2009).
11) New Energy and Industrial Technology Development
Organization, Achievement Report on Natural Energy(including Wind Energy) Technology Research andDevelopment/Wind Power Generation Advanced
Practical Research and Development/Research of Wind
Turbine over 10 MW (component technology) FY2013 toFY2014, New Energy and Industrial Technology
Development Organization (NEDO), (2015) pp.3_1-3_29.
12) Takao Maeda, and five others, Wind tunnel experimenton flow and fluid force of straight-bladed vertical axis
wind turbine, turbo machine, 42 (2), (2014) pp.100-106．
13) Maeda, T., Kamada, Y., Murata, J., et al., Measurementsof flow field and pressure distribution of straight-bladedvertical axis wind turbine, Proceedings of European Wind
Energy Association Conference 2013, Web site, (2013)
8p.
14) Japan Small Wind Turbines Association, JSWTA 0001
Standard on performance and safety of small windturbines, ed. 2, (2013) pp.9-17.
15) Junsuke Murata, Takao Maeda, Yasunari Kamada,
Tatsuhiko Ogasawara, Kento Shimizu, Development andValidation of Analysis Method for Straight-Bladed VAWT,
Proceedings of the 37th Wind Energy Application
Symposium, (2015) pp.361-364.

it is expected that a global standard for aeroelastic
analysis codes will be established for small windturbines also, like their large counterparts, whichenables design of low-cost small wind turbines byreducing excessive safety factors while ensuring
reliability. This should contribute to the wide adoptionof small wind turbines.

References
1) Global Wind Energy Council, Global Wind Report Annual
Market Update 2015, (2015) pp.8-19.
2) Situation of wind power generation in Japan, New Energy
and Industrial Technology Development Organization(NEDO) home page
http://www.nedo.go.jp/library/fuuryoku/state/1-01.html(accessed on 6/25/2016)3) Cumulative installed base of wind turbines as of the endof 2015, Japan Wind Power Association (JWPA) home
page http://log.jwpa.jp/content/0000289449.html(accessed on 6/25/2016)4) Japan Wind Power Association, JWPA Wind Vision
Report - Aiming for truly reliable power supply - , Japan
Wind Power Association (2016) p.3.
5) Pitteloud, J.D. and Gsanger, S., Small Wind World Report
2016, Summary, World Wind Energy Association (2016).
6) Gsanger, S. and Pitteloud, J., Small Wind World Report
2014, Summary, World Wind Energy Association, (2014).
7) Research report on wind power generation related
machinery industry, March, 2016, Research Committee
on Wind Power Generation Related Machinery Industry,
The Japan Society of Industrial Machinery Manufacturers(2016) pp.3-47.

<Author biography>

Takao Maeda
Professor, Doctor (Engineering) Department of Mechanical Engineering, Mie University
1991
1991
1992
1995-1996
1997
2005
2006
2006-20072009

Doctoral course, Graduate School of Engineering, Nagoya University, Mechanical Engineering and MechanicalEngineering Second Specialization, Completed
Supporting staff, School of Engineering, Nagoya University
Instructor, Faculty of Engineering, Mie University
Fellowship Researcher, TU Delft Wind Energy Institute
Assistant Professor, Faculty of Engineering, Mie University
Professor, Faculty of Engineering, Mie University
Professor, Graduate School of Engineering, Mie University
Head of Mie University Satellite Venture Business Laboratory
to present Head of Energy and Environmental Engineering Research Center, Mie University[Specialty]
Fluid engineering, fluid machinery, wind turbine engineering[Academic society and committee affiliations]Japan Wind Energy Association: Director (2005-), Program Committee Chairperson (2006-), Vice President (2014-)
Turbomachinery Society of Japan: Director (2011-2012)
The Japan Society of Industrial Machinery Manufacturers: Research Committee on Wind Power Generation Related Machinery
Industry, Chairperson (2010-)
The Japan Electrical Manufacturers' Association: Wind Power Generation System Standardization Committee, Vice Chair (2013-)
New Energy Foundation: New Energy Industrial Conference, Wind Power Committee, Vice Chair (2014-)-13-*04_展望E_*03_展望new 18/06/13 13:46 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Perspective ]Green Energy Business and Green Power Park

Natsuhiko MORI*

Vertical axis wind turbine
Vegetable plant 1

Monitoring andcontrol center

Vegetable plant 2

From the point of view of energy supplying and environmental
protection, the renewable energy is the ultimate one to realize asustainable society.
"NTN100", NTN’s medium-term management plan whichstarted in April 2015, has been promoting the businessdevelopment of a new area, "Energy Business" using the naturalenergy of wind and water.
This article introduces the environment surrounding Small Wind Turbines and Micro Hydro Turbines which
NTN is developing, and "Green Power Park" established in April, 2016, proposing the energy recycling society
with low carbon emission using the familiar power generation technology.
Solar photovoltaic
power
generator
太陽光発電装置

Vertical axis wind turbine

Micro hydro turbine

EV demonstration
space

Vertical axis wind turbine

Hybrid street light

Solar thermal 0.4%
Geothermal
5.4%
Waste
7.1%

1. Introduction
Since the Industrial Revolution which started in the
18th century in England, humans have been using
underground resources such as carbon, oil, andnuclear fuel, converting them into energy for driving
civilization and increasing consumption ofunderground resources.
As a result of the massive consumption of theseunderground resources, concerns have been raised
about the ongoing stable supply of energy andenvironmental destruction in recent years.
Energy such as solar light, solar heat, hydraulicpower, wind power, biomass and geothermal power iscalled "natural energy" and can be renewed in a
relatively short time, even when used. Therefore, it iseffective for supplementing fossil fuels such as oil andcarbon, which are currently the main energy sources
in Japan.
In addition, natural energy is an excellent energy
source because it emits little CO2, which can cause
global warming when it is used for power generationand producing heat.
Wind power is the largest natural energy source
used for electric power generation, which accounts for
2.7% of total global power generation and almost 1/2
of all natural energy (48%) (Fig. 1).

Solar photovoltaic
10.6%

Tidal 0.1%

Global naturalenergy power
generation

Wind
48.4%

Biomass
28.0%

Fig. 1 Recyclable energy power generation in the world 1)

2. Working on energy business
NTN is moving forward with business developmentin the following new areas in the mid-term businessplan, "NTN100", which started in April, 2015 for theperiod until March, 2018.
(1) Energy business (use of natural energy)(2) EV business (vehicle automation, safety
revolution)(3) Robot associated business (working and livingwith people)(4) Service solution business (use of big data)*Senior Executive Officer and Corporate General Manager, New Product and Business Strategic Planning Headquarters-14-*04_展望E_*03_展望new 18/06/13 13:46 ページ 2

Green Energy Business and Green Power Park

This article will discuss the environment aroundsmall wind turbines and micro hydro turbines that NTN
is working on.
NTN signed an agreement for exclusive
implementation of patents and designs on innovativeand highly efficient blades for wind and hydro turbinesin late 2014, obtaining the rights for exclusive use of
those patents and designs. NTN combined knowledge
accumulated in low-torque/lubrication technologies
and this efficient blade technology together toadvance development of products in a special project
until March 2016, and established the Natural Energy
Product Division on April 1, 2016 for development,
launch, and broad distribution of commercial products.
This business corresponds to the above mentioned(1) Energy business, including small wind turbines andmicro turbines aiming for 30 billion yen of sales by
2025.

SWT Installed Capacity World Market Forecast 2009-2020

370

400

433

2020

2019

2018

2017

2016

2015

2014

2013

2012

2010

2011

Size Limit EUR/kWh Country/
Region1-10kW

0.185

Japan

Fig. 2 Wind power installed capacity in the world 1)-15-

Cumulative Installed SWT Capacity (MW)Size Limit EUR/kWh
＜20kW

0.418

≧20kW

0.167

＜10kW

0.095

10-350kW

0.092

0.074

Nova Scotia

＜50kW

0.332

＜30kW

0.220

Portugal＜3.68kW

0.432

Off-grid

0.190

Slovenia

＜50kW

0.095

＜50kW

0.250

Switzerland

＜10kW

0.247

＞50kW

0.090

UK

＜100kW

0.207

Off-grid

0.100

USA

Israel

2020

2019

0

＜10kW

Greece

637

2018

2017

500

Ontario
Cyprus

497

2016

2015

2014

2013

2012

2011

2010

2009

2008

2007

2006

2005

2004

100

Chinese Taipei

100

2003

1,000

Canada

200

2002

1,500

400

Country/
Region

300

0

2,000

Table 1 Small wind feed-in tariff picking worldwide 3)

712
565

600

The purchase price of electric power in the U.S. is,although slightly different dependent on the regions, inthe range of 16.2 to 17.2 yen/kWh and in the UK, 27.3
yen/kWh (conversion rate: 132 yen/EUR). In Japan,
the purchase price from sizes less than 20kW is very
high, at 55 yen/kWh, based on the feed-in tariff
program (Table 1).

792

500

2001

Wind power installed capacity

600

2,500

3. 2 Purchase price of electric power from small
wind turbines in key countries

Italy

CAGR
(2001-2015):21.3%(2015-2020):12.8%

3,000

Fig. 3 SWT installed capacity world market 2)

Total introduction of new wind turbines in 2015 was
433GW (Fig. 2) with only 0.1% of them being small
wind turbines, generally defined as less than 100kWof output, equivalent to around 300MW per year.
Breakdown by country indicates that China
accounts for the largest share, followed by the USA
and UK, the top three countries accounting for the
major portion of the total share. In Japan, small windturbines are defined as less than 20kW by the Electric
Utility Industry Law and Nippon Kaiji Kyokai
Certification program with only 4,200kW of installedbase by 2014.
The small wind turbine market includes over 100
models in many countries, which indicates that the
global market for small wind turbines is still in the early
development stage with an expectation for annual
growth of around 20% moving forward (Fig. 3).

700

Cumulative Installed SWT Capacity(MW)80003. 1 Installed base and forecast of small windturbines

800

3,500

Annual Forecast Installed SWT Capacity(MW)

1,000

2009

Annual Installed SWT Capacity (MW)Annual Installed SWT Capacity(MW)3. Environment around small windturbines and micro hydro turbines

(GW)
900

4,000

1,200

Lithuania

1-20kW

0.285

Indiana5-100kW

0.130

20-200kW

0.262

Hawaii

＜20kW

0.123

0.2-1.0MW

0.146

20-100kW

0.105

＜15kW

0.250

＜15kW

0.180

Vermont

*04_展望E_*03_展望new 18/06/13 13:46 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

3. 3 Small wind turbine market in Japaneffects during blackout caused by an earthquake ortsunami for activating CCTV cameras, supplying
power for light at evacuation centers and cell phones,
etc. Solar panels and small wind turbines are
effective; however, the power output period may not
be stable.
A hybrid method combining solar panels, small windturbines and batteries is advantageous; however, due
to cost, a combination of solar panels and small windturbines may be more desirable.

The small wind turbine market in Japan is estimatedto be around 2.2 billion yen in 2020, accounting foraround 17% of the global market in monetary base
(14% in unit base). The market is expected to grow 4
to 5% per year (Table 2).

Table 2 Japan & worldwide market of small wind turbine 4)
ActualEstimate

unit: million yen, units

Forecast

2011

2012

2013

2014

2015

2016

2017 2020

Japanese Amount 1,300
market Units 1,700

1,500

1,700

1,800

1,800

1,800

1,900

2,200

2,000

2,300

2,500

2,600

2,600

2,740

3,200

3. 5 Small hydraulic turbine market in Japanand micro hydro turbines

Overseas Amount 10,400 10,900 11,300 11,500 11,700 11,800 12,000 12,800
markets Units 18,400 19,400 20,100 20,700 21,200 21,500 46,600 23,300

The installed base of small hydraulic turbines inJapan up to 2012 and the market size between 2013
and 2020 are shown in Table 3. Small hydraulicpower is defined for capacity less than 1,000kW(1MW) which is subject to the RPS law (note).

3. 4 Application of small wind turbines for thedisaster prevention field(Note) "Act on Special Measures Concerning New Energy
Use by Operators of Electric Utilities" published in June
2002, which is to promote use of new energy bymandating use of electric power generated by new energy
over a certain volume to utility companies.

Reflecting on the damage from recent disasters,
municipalities are looking to multiple measures ofprotection from both structure and non-structure
standpoints, drastically changing the assumption ofmagnitude of potential disasters.
In Kochi Prefecture, near the anticipated seismic
center of the Great Nankai Trough Earthquake,evacuation towers are being built as the risk of an
ultra quick tsunami is high after an earthquake hits theregion, with tower construction plans at 117 sites
within the prefecture (Fig. 4).
Also, a plan to install CCTV (closed circuit
television) cameras for collecting information on
disasters is well underway. In these disasterprevention areas, it is critical to ensure the availabilityof a stable and independent power supply with little

Table 3 Market of micro hydro turbine in Japan 4)
ActualEstimate2011 2012 2013
Japanese Amount 1,370 1,170
market Capacity (kW) 5,220 4,450

Unit: million yen, capacity: kW

Forecast

2014 2015 2016 2017 2020

2,370

2,290

1,900

1,700 1,700

1,600

9,050

9,140

7,740

6,850 6,800

6,600

Due to the feed-in tariff program, in particular themarket of 200 to 1,000kW with a purchase price of 29
yen/kWh extended until 2014, and the proportion of
the share of less than 200kW for industrial facilities
with a purchase price of 34 yen/kWh is expected toincrease after 2015.
There are over 10,000 sites which are adequate forsmall hydraulic power generation in Japan, anddemand in Niigata and Gifu is high for power
generation using river water. For use of agricultural
water ways, Fukushima, Aichi, and Tochigi arepromising. The reconstruction demand continues in
the Tohoku region. Support from the Ministry ofAgriculture, Forestry and Fisheries, and local
governments can also be expected.
Asaka Canal in Fukushima Prefecture, originatingfrom Lake Inawashiro, consists of many waterways
that run into Abukuma River with a total length ofaround 50km and are used for drinking water andfarmland irrigation. There are many spots adequate
for small hydraulic power generation in this canal withstable flow rate; therefore, NTN is conducting
demonstration projects of small hydraulic power

Shimoda-Mito evacuation tower against tsunami in Shimannto City,
Kochi Prefecture 5)
Second tower w/9m in height (18.6m above sea level) next to the existing
tower w/6m in heightMax. capacity: 320, Equipped with life jackets at the highest platform;
Cost: 19.53 million yen

Fig. 4 Evacuation tower in Kochi-16-*04_展望E_*03_展望new 18/06/13 13:46 ページ 4

Green Energy Business and Green Power Park

Overview of three major canals and potential
capacity of small hydraulic power generation

generation. By promoting small hydraulic power
generation business, ripple effect for new businessusing the canal can be expected.
Small hydraulic power generation provides highly
efficient energy conversion and stable power
generation for 24 hours due to a stable water flow
(Table 4). Conventional hydraulic power generation
involves water entitlement and complex legal
procedures, in addition to the high installation cost ofhead type hydraulic power plants. Smaller hydraulicpower plants, in particular, have not been actively
promoted. However, micro hydro turbines, which canbe easily installed using stream water, are attracting
attention from this successful demonstration project(Fig. 5, 6). For details, see [Product Introduction]"Micro Hydro Turbine."

Agricultural water ways in Japan extend 400,000km.
The length of three major canals, Asaka Canal(Fukushima prefecture), Nasu Canal (Tochigi
prefecture) and Lake Biwa (Shiga and Kyoto
prefectures) are 500 km, 357 km and 45 km,
respectively. An overview of these three major canals
and calculated potential power generation capacity frommicro hydro turbines are shown in Table 5.
Assuming installation of 100kW/km (100 units of 1kW
installed every 10m) in 20% of the main watercourse,
the respective capacity from Asaka Canal is 1,040kW,
330kW in Nasu Canal, and 220kW in Lake Biwa
Canal.

Advantage

Much higher energy conversion efficiency compared withother power generation methods
Higher energy density and stable power generation
Small footprint compared with solar photovoltaic power
generation
Abundant endowments not yet developed

Disadvantage

Table 4 Advantages & disadvantages of micro hydro turbine

Complex legal procedures such as River Act
Limited installation sites
Entitlements of stake holders with water use
Low awareness

Table 5 3 major canals and endowments of
micro hydro turbine
RegionTotal extension
Main watercourse
Branch aqueduct
Endowments applicableto NTN micro hydroturbines (estimate)

Asaka Canal Nasu Canal Lake Biwa Canal
Fukushima
Tochigi
Shiga and Kyoto
Prefecture
Prefecture
Prefectures
45km
Approx. 500km 357km
52km
16.3km11.1km
96km
96km
8.4km
1,040kW

330kW

220kW

3. 6 Feed-in tariff program for renewable energy
in Japan
The feed-in tariff program (FIT) started on July 1,
2012 mandating utility companies to purchase electric
power generated from natural energy sources (solar,
wind, water, geothermal and biomass) at a pricedetermined by the government for a certain duration(Fig. 7).
The actual certification of small wind turbines
amounted to 4,500kW by the end of 2015; however,
the actual approved grid connection was limited to364kW, or 8% of certification. The progress of theactual grid connection is slow, partly due to unstable
output of wind power influenced by weather
conditions, which in turn affects the stable supply of
the grid (Fig. 8, 9).
The ultimate energy for achieving sustainable
society is natural energy. Therefore, widespread useof natural energy is desired, by technological pursuitof stable supply and resolution of cost challenges.

Fig. 5 Micro hydro turbine
Downstream
Canal

Serial installation is possible
in one stream
Upstream

Fig. 6 Instllation of micro hydro turbine-17-*04_展望E_*03_展望new 18/06/13 13:46 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

Wind power

20kW or more

Less than 20kW Offshore wind power（※）

Capacity (kW)
5,000

Purchase price

22 yen + tax

55 yen + tax

36 yen + tax

Purchase duration

20 years

20 years

20 years

350

Cumulative certified capacity
Number of cumulative certifications

4,500
4,000
3,500

※Access by boat, etc. is required both for installation and operation/maintenance.

Number ofcertifications

300
250

3,000

1,000kW or more, 200kW or more,less than 30,000kW less than 1,000kW

Hydropower

24 yen + tax

Purchase price

29 yen + tax

20 years

Purchase duration（※）

14 yen + tax

Purchase price
Purchase duration

21 yen + tax

20 years

20 years

1,500
1,000
500

20 years

Small to mid-hydroturbines using "1,000kW or more, 2,00kW or more,existing aqueduct less than 30,000kW less than 1,000kW

150

2,000

Less than
200kW
34 yen + tax

20 years

200

170

2,500

0

8

2

6

100

2,294

41

16

9
6

50

314

39

1 2 3 4 5 6 7 8 9 10 1112 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 112013

Less than
200kW

2014

0

2015

Year and month

25 yen + tax

Fig. 8 Approved small wind turbine by FIT 7)

20 years※Upgrade of electric facility and penstock using existing installed aqueducts.

Solar
photovoltaic

Less than 10kW
Purchase of excessive power
Dual generation/purchase of excessive power
Support of output Support of output Support of output Support of output
control not required control required※ control not required control required※

Purchase price
Purchase duration

31yen

33 yen25 yen

10 years

Number ofcertifications

Capacity (kW)Installed capacity
Installed turbines

300

27 yen
10 years

39

18

Tohoku, Hokuriku, Chugoku, Shikoku, Kyushu and Okinawa Power Companies.

100

Solar photovoltaic

10kW or more

Purchase price

24 yen + tax

Purchase duration

20 years

0

9

9

9

11

12

13

4

5

5

6

6

7

2

3

3

3

6

6

6

27 27 27

40 43 47 47

4

5

6

7

8

9

10

11 12

2

1

3

4

22

5

108
6

30

26

364
285

13

5

2014

20

137 160

7

8

40
35

31

200

※Device to support output control is required in the area for demand/supply control of Hokkaido,

45

25
20
15

209

10
5

9

10

11

0

2015

Year and month

Fig. 7 IFIT purchase price in FY2016 (per 1kWh) 6)

Fig. 9 SMT System interconnection result with
Electric Power Co. 7)

6,300m2 (Fig. 10). There are vegetable plants andcharging stations for electric vehicles, powered byelectricity generated from natural energy, for
We have established Green Power Park, equipped
demonstration of recirculation of natural energy.
with wind, water, and solar power generation devices
Also installed are 6 small wind turbines (vertical axis
within the Advanced Technology R&D Center sitewind
turbines), consisting of 2kW, 4kW, and 10kW
located in Kuwana City, Mie Prefecture, in an area of
each, and 3 hybrid street lights. In addition,
NTN's two in-wheel motor vehicles, Q'mo, arecharged with power generated by naturalenergy.
Furthermore, an IoT room is provided foroverseeing the power generation condition
Vertical axis wind turbine
and battery charging condition of the entire
park.
Monitoring andVegetable plant 1
Vegetable plant 2
control center
The park was selected as one of the sites
Solar photovoltaic
for"2016 Junior Summit in Mie" from April 22
power
generator
太陽光発電装置
to 28, 2016 in Kuwana City, Mie Prefecture,and introduced as a model for low carbon andVertical axis wind turbine
energy recirculating society without disrupting
Micro hydro turbine
EV demonstration
space
Vertical axis wind turbine
nature.

4. Introduction of Green Power Park

Hybrid street light

Fig. 10 Green Power Park-18-

*04_展望E_*03_展望new 18/06/13 13:46 ページ 6

Green Energy Business and Green Power Park

4. 1 Small wind power generation unit

6m

Small wind power generation units are defined ashaving output of less than 20kW, which do not
require major construction work and installable innarrow spaces in public parks and car parking areas.
Their roles are diverse, as they can be used tosupply power on isolated islands where no grid
power is available, as an emergency power supply
during disaster, or as clean energy for regularresidential homes.

(1) Characteristics of vertical axis wind turbines
NTN's vertical axis wind turbines produce little windnoise due to their proprietary thick blade geometry. In
addition, they can efficiently generate power withnatural wind from any direction with significantly varied
wind conditions (Fig. 11).

Fig. 12 Hybrid street light

4. 2 Micro-hydro power turbine

17m

Head-type hydraulic power plants, in general,
require major construction for retaining water.
In contrast, micro hydro turbines are small hydraulicpower generation systems capable of generating
power using small water flows, such as water
channels and streams, and are easy to install (Fig.
13).

Fig. 11 10kW Wind Turbine(2) Hybrid street lights
Hybrid street lights are a 0.3kW clean energy power
generation system using two types of natural energy :wind and solar light (Fig. 12).
They generate power with both wind turbines andsolar panels during the daytime and charge the built-in
battery. They also generate power with wind turbines
to light the LED lamps. The capability of generating
power with both wind and solar light makes it aneffective power supply for emergencies.
This can also benefit risk management devices
such as cell phone charging capability andsurveillance cameras.

Fig. 13 Micro hydro turbine
and tank for display-19-*04_展望E_*03_展望new 18/06/13 13:46 ページ 7

NTN TECHNICAL REVIEW No.84（2016）

4. 3 Tracking-type solar power generation

4. 5 Q'mo

Based on the "tracking-type solar power generation"
made by another manufacturer, the NTN-developed
cylinder type motor driven linear module is installed(Fig.14). Adoption of ball screws for a feed screw
mechanism achieved high repeat positioning accuracy
and durability for use over a long period of time. In
addition, application of grooves on the internal
raceway of the housing, used as a linear motion
guide, and adoption of rolling bearings as the guideroller made it possible to realize smooth linear motion
and high rigidity.

Q'mo is an electric vehicle that allows "pivot turning"and "lateral movement." This is a new-generation
ultra-small electric mobility vehicle, which allows
movement, such as the pivot turn not found inconventional vehicles with built-in in-wheel motors on
all four wheels (Fig. 16).
"Q'mo@" with an improved operability "multi-driving
system", obtained a light vehicle license plate andbecame the world‘s first electric mobility vehicle
permitted to drive on public roads.

Fig. 14 Tracking-type solar power generation

Fig. 16 Q'moQ'mo

4. 4 Vegetable plant

4. 6 Monitoring and control center

Vegetable plants use natural energy within GreenPower Park, supplying power for hydroponic
cultivation of vegetables such as lettuces andtomatoes (Fig. 15).
We are aiming to increase yield and establish amodel for sustainable recirculating of vegetable plants
by efficient and systematic cultivation with no impact
from natural environmental change and reduced useof power.
First vegetable plant: 300m2
Second vegetable plant: 1,000m2

The monitoring and control center allows efficient
distribution of power from various operational
conditions by monitoring and controlling power
generation, storage and consumption conditions inreal time on all the equipment in the Green PowerPark (Fig. 17).
In addition, a display that shows real time indicationof power generation and storage conditions of theentire Green Power Park are installed in the lobby of
the Advanced Technology R&D Center (Fig. 18).

Fig. 15 Vegetable plantFig. 17 Monitoring and control center-20-*04_展望E_*03_展望new 18/06/13 13:46 ページ 8

Green Energy Business and Green Power Park

Fig. 18 Green Power Park monitoring display

References

5. Conclusion

1) Sustainable Japan [Energy] Global installed base of windturbines and business environment-2015 Overviewhttp://sustainablejapan.jp/2016/05/06/wind-powermarket/11154
2) Small Wind World Report 2014<http://uwea.com.ua/uploads/docs/SWWR_2014.pdf>
p.12.
3) Small Wind World Report 2014<http://uwea.com.ua/uploads/docs/SWWR_
2014.pdf> p.10.
4) Fuji Keizai "2013 New market of electric energy system."5) Referred from Shimanto City Website, Publication
Shimanto April, 2013 issue,
http://www.city.shimanto.lg.jp/gyosei/pdf/h25/kouhou04.
pdf p.1.
6) Agency for Natural Resources and Energy Website
http://www.enecho.meti.go.jp/category/saving_and_new/saiene/kaitori/kakaku.html
7) Source for actual small wind turbine FIT certifications,and actual grid connection installed base:
http://www.fit.go.jp/statistics/public_sp.htm

Renewable natural energy, such as wind, water,and solar power, is critical for achieving a sustainable
society, and more advanced utilization is required.
NTN is making contributions for achieving a lowcarbon and energy recirculating society in the world
through small wind turbine and micro hydro turbine.

Photo of author

Natsuhiko MORI
Senior Executive Officer andCorporate General Manager,
New Product and Business Strategic
Planning Headquarters-21-

*05E_03 18/06/13 13:46 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Small Wind Turbine Generator

Takaya ADACHI*
Ryosuke KARASAWA*

Small wind turbine as one of renewable energies has received
remarkable attention and rapidly become popular due to lowinitial-cost and space-saving for construction compared to large
wind turbine. NTN is taking on development of the small windturbine using our technology.
This paper describes the technology and features regarding
NTN’s small wind turbine products.

The small wind turbine is defined by JIS C 1400 asa wind turbine with a swept area (area of turbine
blades receiving wind) of 200m2 or less and output ofless than 20kW, as defined by the Electricity Business
Law 1, 2).
Large wind turbines face challenges such as highinitial costs per station and restriction of installationsites due to high noise and environmental issues.
In contrast, small wind turbines are less restricted
by installation costs and construction duration, andcan be easily installed in public places such as parks
and schools due to their small size. In addition, they
can be easily deployed as an emergency power
supply during natural disasters.

1. Introduction
Utilization of natural energy has been an urgent
matter in the world from the standpoint of global
warming and depletion of fossil fuels. Japan, with few
natural resources, depends on other countries for the
majority of energy resources, including oil. Therefore,
it is imperative to improve the energy supply structure.
In addition, utilization of diverse energy sources,
including natural energy, is being sought out morethan ever since the experience of the Great East
Japan Earthquake 1).
Hence, a feed-in tariff program was started in July,2012 as a policy for promoting natural energy inJapan, with particular emphasis on small windturbines with the highest purchase price of 55
yen/kWh (in the case of small wind turbines of lessthan 20kW).
NTN is developing highly efficient and very quiet
small wind turbines for commercialization byleveraging their proprietary blades. This article
introduces their characteristics and progress of theirdevelopment.

3. Types of small wind turbine generators
There are many different types of small windturbines; however, in general, they can be broadlyclassified into horizontal axis wind turbines (HAWT)and vertical axis wind turbines (VAWT), from theposition of the rotational axis against the winddirection. They can also be classified into lift type ordrag type, depending on the principle of operation.
Fig. 1 shows the typical wind turbine types.
Each type has its own characteristics, which arebriefly described in the following paragraphs 1-3).
Horizontal axis wind turbines have a rotational axis
parallel to the wind direction and include propeller
type, sail-wing type, Netherlands type and multi-wing
type. Horizontal axis wind turbines need to make theturbine rotational blades face the wind direction.

2. About small wind turbine generators
Wind turbine generation is a power generating
method which produces electric power by transmitting
the rotating motion produced by the wind power to thegenerator. Wind that exists everywhere in nature is atypical clean energy that does not emit any CO2 orwaste.
*Engineering Dept. Natural Energy Products Division-22-

*05E_03 18/06/13 13:46 ページ 2

Small Wind Turbine Generator

with their larger torque, they are frequently used for
power operation such as pumping water and milling.

Type of wind turbines
Propeller
Horizontal
axis

Lift
type

Sail-wingMulti-wing

Lift
type
Verticalaxis

4. NTN's proprietary wing design

Netherlands
Propeller type

NTN's proprietary wing design can be used in bothvertical axis and horizontal axis wind turbines. Fig. 3
shows their respective designs.
Characteristics common to both horizontal andvertical axis wind turbines are as follows:

Darrieus
Giromill
Straight-wing

Darrieus type

Giromill type

Savonius
Drag
type

Paddle1) Little wind noise even under strong windcondition
2) High wing efficiency

Cross-flow
S-type rotor

Savonius type Cross-flow typeFig. 1 Wind turbine category 4)

The control method for tracking the wind direction
so that turbine rotational blades face the wind is called"yaw control", and includes forced yaw control, whichuses a control device to force the yaw control andfree-yaw control, which uses air flow for autonomous
control. Wind turbines with rotational blades locatedon the upstream side of the tower are called upwind
type, and those with rotational blades located on thedownstream side of the tower are called downwind
type (Fig. 2).
Vertical axis wind turbines have their rotational axis
perpendicular to the wind direction (often perpendicular
to the ground), and include Darrieus type, straightwing type, Savonius type and cross-flow type. Verticalaxis wind turbines can rotate by receiving wind fromany direction; therefore they do not require yaw
control.
Lift type and drag type are categorized by theprinciple of operation. Lift type mainly uses lift force
and includes propeller type, Darrieus type andstraight-wing type. Lift type wind turbines can produce
high circumferential speed ratio of a few times to 10
times that of the wind speed, which make themsuitable for power generation.
On the other hand, drag type wind turbines mainly
use drag force and include Savonius type and crossflow type. Drag type wind turbines cannot produce
higher rotational speed than the wind speed; however,Wind flow

Vertical axis

Fig. 3 Design of NTN blade

Common horizontal axis three-blade propeller typewind turbines are characterized by having the bestrotational speed for efficient power generation on thehigh side, as well as thinner wings which produce highwind noise. This does not cause any problems if
installed in remote areas with no residential areas
nearby; however, this high noise creates problems if
installed in populated areas such as towns and parks.
NTN wind turbines with proprietary wings have the bestrotational speed for efficient power generation on thelower side, compared to those of other manufacturers,
and the unique thick wing design reduces wind noise
even under strong wind conditions. In addition, theunique tip design of the winglet reduces the wingtip
vortex and resulting loss, eliminating most of the windnoise (Fig. 4).
The high efficiency of the wing is attributed to two
characteristics of the wing: the first is the winglet on
the tip of the wing and the second is the unique wingcross-sectional design. The winglet is a small wing
generally seen on the main wing of aircrafts, also
called a wingtip. The wingtip prevents vortex, which
may be produced at the tip from the lower surface of
the wing to the upper face, resulting in improved fuelefficiency (Fig. 5). NTN's winglet adopts a uniqueangle and tip shape design on top of the winglet

Wind flow

Upwind type

Horizontal axis

Downwind typeFig. 2 Type of horizontal axis wind turbine-23-

*05E_03 18/06/13 13:46 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

Winglets

Fig. 4 Change of the flow by winglets

VortexLow
pressureHighpressureLow
pressure

ReducedvortexFig. 6 Result of fluid analysis

Highpressure

Fig. 5 Effect of Winglet
Fig. 7 Picture of Wind Tunnel

design adopted by aircrafts, to reduce wingtip vortex
and energy loss due to vortex leaving the wing, which
has improved the wing efficiency. The result of fluid
analysis is depicted in Fig. 6.
In addition, the cross sectional shape of the wingwith a unique thickness produces rotational force froma complete stop even with no pitch angle, and creates
great lift force at the optimum rotational speed.
Furthermore, no pitch angle means reduced
resistance for rotation, maintaining rotational speed
even with weaker wind. When the wind starts blowing
again, the wings can capture the wind with the
optimum rotation speed - increasing efficiency invaried wind conditions.
NTN conducted wind tunnel experiments in order toobtain basic data of wind turbine wings. The windtunnel experiments were conducted at Mie University,
which possesses one of the largest wind tunnel
facilities in Japan. The wind tunnel facility is acirculation flow type (Fig. 7) where the turbine wasplaced in the open measurement area. The
experimental wind turbine is shown in Fig. 8.
Collection of accurate data from the wind tunnel
experiment enabled significant improvement insimulation accuracy. The results were used for design,
which helped reduce the development time. We willcontinue developing highly efficient and safe windturbines.

Fig. 8 Wind Turbine for laboratory test

5. Hybrid street lights
This is a hybrid power generation system combining
wind power generators and solar power generators.
Their appearance is shown in Fig. 9, specifications
are shown in Table 1, and the structure is shown in
Fig. 10.
NTN hybrid street lights are an independent power
supply system and can be used as street lights where
no power cables are available, or as an emergency
power supply in the case of disasters, such as-24-

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Small Wind Turbine Generator

NTN hybrid street lights can be installed in many
places such as parks, schools, city halls, hospitals,
disaster evacuation centers, bus stops and parking
areas-serving as a symbol of the town. They can also
be used as advertising towers by installing electronic
signs, or as surveillance towers by installing
surveillance cameras. The appearance can becustomized by user request. Fig 11. shows several
examples of customization including the installation ofa bench.
Circular LED

Fig. 9 Hybrid street light
Table 1 Specification of hybrid street light
0.2kW

Rated output

0.8m

Rotor diameterWind power
generator

Solar electric
generation
LED illumination

Wing length

1.2mWind turbine type

Vertical axis

Brake
Survival wind speed

Electric
60 m/s(max. instantaneous wind speed)

Rated output

90W

Power consumption(nominal)
Illuminance

10W

Customizable with change of LED light andaddition of bench.

Fig. 11 Customization example

900lm

6. 10kW vertical axis wind turbine※Product appearance and specification are subject to change without notice.

The rate for purchasing power from small windturbines of less than 20kW in Japan in FY2016 is 55
yen/kWh, which is the highest in the FIT program, forthe duration of 20 years.
Stable and efficient power generation is required for
selling electric power at the rate of 55 yen/kWh during
this period.
In order to deploy small wind turbines, certificationmust be obtained from the Minister of Economy, Tradeand Industry. Nippon Kaiji Kyokai, which was
authorized as the certification body for wind power
generation products in 2013, issues type approval"ClassNK", which is required for application of FIT.
NTN's 10kW vertical axis wind turbines are currently
in development with specifications to meet theClassNK type approval.
The structure of 10kW vertical axis wind turbines isshown in Fig. 12, and the specifications are shown in
Table 2.
This 10kW vertical wind turbine also produces little
wind noise, like the hybrid street lights, even understrong wind conditions, and generates power
efficiently even in the area of frequently changing winddirection due to the vertical axis structure.
In addition, it adopts NTN's expertise in bearings
and rotary machines acquired over many years for the
optimum design of low-torque and long-life drive line.

1.2m

0.8m

Wind turbine
Solar panel

6m

Circular LED simulates
sunlight filtered throughfoliage.

LED light
Battery andcontroller

Fig. 10 Structure of Hybrid street lightcharging stations for mobile telephones and a powersource for telecommunication equipment.
The battery of this product can turn on the LED
lamp for approximately 5 days, when fully charged,
assuming use of 12 hours/day.
They can operate electric brakes under strong windconditions for slower rotation, bypassing wind power,
as a safety measure. Being highly safe and very quiet,-25-

*05E_03 18/06/13 13:46 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

Highly efficient and quiet small vertical axis windturbines can be installed in places where conventional
wind turbines could not be installed.
In March, 2016, we built an experimental windturbine in NTN's Advanced Technology R&D Center(Fig. 13). We are collecting data from thisexperimental turbine for further improvement.

7. Safety of 10kW vertical axis windturbines
The specifications for small wind turbines forClassNK were developed based on IEC international
standards, specifying safety, reliability, performanceand noise property of wind turbines.
Particularly, strict requirements are in place forsafety issues, as failure due to over-rotation duringstorms may result in a severe accident5).
NTN adopts dual brakes, namely, mechanical andelectrical brakes, to be used under strong windconditions.
In addition, the safety of wind turbines is assured byadopting a design of applying mechanical brakes evenwhen power is lost due to submergence of the control
panel in assumed natural disasters such as lightning,
earthquakes and tsunami.
Wings are also required to be highly safe andreliable.
The safety of the wing strength is ensured bystrength tests using the actual wings, in addition to
analytical strength verification (Fig. 14).
ClassNK process verifies these safety requirementson the strength and system safety, as well as design
documents including quality of manufacturing. In
addition, it is required to conduct a type approval test
to ensure stable operation for the specified period※1by building the wind turbine for approval at the testsite. NTN plans to conduct this type approval test in2017 for ClassNK type approval.

17m

7m

6m

Fig. 12 Structure of 10kW vertical axis wind turbine

Table 2 Specification of 10kW vertical axis wind turbine
Rated output

10kW

Rated wind speed

12m/s

Rotor diameter

6m

Wing length

7m

Turbine type

Vertical axis

Brake

Mechanical, electric (dual brake)

Survival wind speed

60m/s (max. instantaneous wind speed)※1: Requirements of type approval test
(1) Secure operation during the test period(2) Operation for more than 6 months(3) Power generation for over 2,500 hours regardlessof wind speed(4) Power generation of over 250 hours at the windspeed of 10.2m/s or more
(5) Continuous test for over 25 hours under the windcondition of 15m/s or more

Fig. 14 Result of FEM analysis

Fig. 13 10kW vertical axis wind turbine-26-

*05E_03 18/06/13 13:46 ページ 6

Small Wind Turbine Generator

8 . Wing performance

9. Conclusion

We have conducted performance test of NTN's wingat the large wind tunnel test facility at Mie University.
The test was conducted by using two types of
specimens with the same swept area, namely, NTN'svertical axis blades with proprietary wing geometry
and winglets as shown in Fig. 15, and conventional
wings without winglets as shown in Fig. 16 measuring
the torque of each rotational speed under certain windspeeds for performance verification.
Fig. 17 shows the results under a wind speed of
6m/s. Compared with the conventional specimen,
NTN's wing was proven to be approximately 1.5 times
higher in performance.

In this article, we have discussed the characteristics
of recent natural energy, small wind power generation
systems, and NTN's proprietary technology anddevelopment.
It is expected that the products will be used invarious places under different environments due totheir low cost, small footprint, and ease of deployment.
In the market, addressing the needs for a low
carbon society, development of small wind power
generators is well underway. NTN will also contributeto this trend by incorporating its own unique
technologies to improve its products every day, withclear differentiation from the competition. With thisapproach, we will continue proposing new productsthat contribute to the revenue of the Natural Energy
Division and address market demand.

Pneumatic brake

References

Torque meter

1) New Energy and Industrial Technology Development
Organization (NEDO): NEDO Renewable Energy
Technology White Paper, 2nd Edition (2013). Chapter 1,
4-11, Chapter 3, 4-92) Japan Small Wind Turbines Association: Small Wind
Turbine Guide Introductions 2nd Edition (2012), 6-29.
3) Izumi Ushiyama: Wind Energy, Ohmsha, (2005), 49-53.
4) Referred from New Energy and Industrial Technology
Development Organization (NEDO) website:
http//www.nedo.go.jp/content/100544818.pdf
5) Japan Small Wind Turbines Association, JSWTA 0001
Standard on performance and safety of small windturbines, ed. 2, (2013) 1-6.

Generator

Fig. 15 NTN's vertical axis wind turbine

Fig. 16 General vertical axis wind turbine (non winglet)

Efficiency ratio (%)

250
200

NTN's wing
Conventional wing

Photo of authors

150
100
50
0
0.0

0.5

1.0

1.5

2.0

Circumferential speed ratio ※2

2.5

Fig. 17 Result of performance※2 Circumferential speed ratio is the ratio of rotational speed(circumferential speed of wing) and wind speed.

-27-

Takaya ADACHI

Ryosuke KARASAWA

Engineering Dept.
Natural Energy Products Division

Engineering Dept.
Natural Energy Products Division

*06E_03 18/06/13 13:47 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Micro Hydro Turbine

Tomoya KAWAI*
Hiroki MUKAI*
Tomomi GOTOU*
Demand of renewable energy such as hydroelectric power generation isincreasing rapidly from rise of global warming and the interest to theenergy mix. The one for which hydroelectric power generation uses thegap of the water is general. NTN has developed micro hydro turbine forsmall water flow (NTN micro hydro turbine) which is used in agricultural
canal and industrial canal of existence.This paper introduces the features
and the structure of the products of NTN.

natural energy at a predetermined price in order topromote the use of natural energy and make it easierfor businesses to recover investments from naturalenergy development 2).
Fig. 2 shows the purchase price of hydraulic powerin the FIT program.
The rate of 34 yen/kWh for hydraulic power lessthan 200kW is more favorable than the other hydraulicpower generation classes. Hydraulic power generation
is more limited by location than solar power or wind
power; however, it provides stable power generation
with small variation. Therefore, the market is expected
to grow moving forward.
This article discusses the flow-through small
hydraulic power generator (micro hydro turbine) that
NTN is developing with highly efficient blades thatpossess unique geometry.

1. Introduction
Energy issues are a global challenge that must be
dealt with in order to achieve a sustainable society.
Some of the key initiatives include reducing
greenhouse gas emissions, increasing renewal energy
usage to phase out fossil fuel dependency, anddiversifying sources of natural energy.
The estimated energy mix for FY2030 includes
more than double the amount of natural energy
compared to the mix before the Great Earthquake of
East Japan, indicating an expectation of future growth 1).
Fig. 1 shows the estimated energy mix for FY2030.
In Japan, a feed-in tariff (FIT) program was started
in July, 2012 as a policy to expand the introduction ofnatural energy. FIT is a program that mandates utility
companies to purchase electric power generated by
Energy mix/amount (100 million kWh)FY2030
Oil

315

3%

Coal

2,810

26%

LNG

2,845

27%

Nuclear2,317 〜 2,168

22 〜 20%

Energy Saving

2,366 〜 2,515

22 〜 24%

10,650

100%

Total

Solar
Wind
Geothermal
HydraulicBiomass

Oilapprox.
3%

FY2030
749

7.0%

182

1.7%

102 〜 113

1.0 〜 1.1%

939 〜 981

8.8 〜 9.2%

394 〜 490

3.7 〜 4.6%

FY2030

Beforeearthquake av.
of 10 years

LNG
approx. 27%

LNG
approx. 27%

Coalapprox. 26%

Coalapprox. 24%

Renewable
approx. 11%

Nuclearapprox.
22 - 20%

Nuclearapprox. 27%≥1,000kW,<30,000kW≥200kW,<1,000kW<200kW

Purchase price

24 yen + tax

29 yen + tax

34 yen + tax≥1,000kW,<30,000kW≥200kW,<1,000kW<200kW

14 yen + tax

21 yen + tax

25 yen + tax

Purchase duration

Oil approx. 12%
Renewable
approx.
22 - 24%

Hydraulic

Small/medium
hydraulic using
existing
waterway（※）

Purchase price

Purchase duration

20 years

20 years

20 years

20 years

20 years

20 years※Using existing waterways and upgrading electric facilities and high pressure steel pipes.

※Values are approximate

Fig. 1 The power supply construction in 2030 1)

Fig. 2 Price of the hydroelectric power generation 2)*Engineering Dept. Natural Energy Products Division-28-

*06E_03 18/06/13 13:47 ページ 2

Micro Hydro Turbine

2. About micro hydro power generation

4. Types of hydro turbines

Hydraulic power is produced by turning hydraulicturbines with water and transmitting the motion energy
to a generator. Hydraulic power is a type of naturalenergy, like solar or wind power, that is free ofgreenhouse gas emissions.
The "Guidebook for Implementation of Micro
Hydropower Generation (NEDO 2003)" categorizes
those with an output of 100kW or less as microhydropower generators. In addition, relatively small
power generators that produce 1000kW or less may becollectively called small hydraulic power generators 3).
In this article, micro hydropower generators will becalled micro hydro turbines. Table 1 shows thecategorization of hydraulic power generators.

The hydro turbines used for hydraulic power
generators are classified by their shape and structure
and are broadly categorized into impulse water
turbines and reaction water turbines 5). Fig. 3 shows
the types of hydro turbines.
Impulse water turbines use water injected from thenozzle which is flushed onto the bucket of the impeller
to rotate the turbine. For example, the Pelton turbine
shown in Fig. 4 corresponds to this category.
Reaction turbines use the speed energy andpressure energy of the water to rotate the impeller.
For example, the Francis turbine shown in Fig. 5
corresponds to this category.

Table 1 Division of hydroelectric generator 4)
Division
Large hydraulicMedium hydraulic
Small hydraulicMini hydraulicMicro hydraulicPower output kW

Type ofhydroturbine≥100,000
10,000〜100,000
1,000〜10,000
100〜1,000≤100

PeltonImpulseturbines

Cross-flow
Turgo impulse
Francis

Reaction
turbines

Propeller

KaplanDiagonal flow (Deriaz)
TubularStraight flow

Fig. 3 Form of hydroelectric generator 5)

Medium hydraulic power generation faces challenges
such as high installation costs and restricted installationsites.
In contrast, micro hydro turbines can be easily
installed in existing water channels, such as
agricultural water ways, which can reduce the overallcost. In addition, they can be used for charging
batteries and as an independent power supply in thecase of emergencies, such as natural disasters.

BucketNozzleNeedleFig. 4 Pelton wheel 5)

3. Flow-through hydropower generation
Existing mainstream hydraulic power generators arehead type. Generated power is determined by the flowrate and effective head, as shown in equations (1) and(2).

Generator

CasingGenerated power = Theoretical hydropower ×η (1)Theoretical hydropower: P =g × Q × H(2)Impeller(runner)

Where η is efficiency, g is gravity, Q is water use, His effective head.
On the other hand, the generated power of microhydro turbines developed by NTN is determined by theswept area of the micro hydro turbine and flow rate, as
shown in equation (3).
Generated power: P =1/2×ρ× A × v 3×η

Guide vane

Fig. 5 Francis turbine 5)(3)

Where,ρ is water density, A is swept area, v is flowrate andηis efficiency.
-29-

*06E_03 18/06/13 13:47 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

5. 1. 1 Thick cross sectional structure

5. Structure and characteristics of
micro hydro power turbines

The cross sectional structure is thickened toincrease lift force in the rotational direction. This
allows for a reduction in the pitch angle. By reducing
the pitch angle, the resistance on the rotational
direction is reduced to increase lift force and power
generation efficiency.

NTN is developing flow-through small hydraulicturbines (micro hydro turbines) with proprietary wing
shapes which can be installed in existing agriculturaland industrial waterways. Installation costs can besignificantly reduced since there is no need to changeor add waterways. The key configuration is shown in
Fig. 6 and Table 2.

5. 1. 2 Winglet structure
As shown in Fig. 8, a wingtip vortex is observed
when the tip of the wing is flat. The wingtip vortex
disrupts the flow behind the wing and reduces lift
force. For example, with aircrafts, the design called
winglet, which bends the tip of the wing as shown in
the right side of the Fig. 8, prevents reduced fuelefficiency by the wingtip vortex. NTN turbine wings
have winglets with a proprietary shape and tip angle
that reduce energy loss by reducing the wingtip
vortex.

Table 2 Example of structure
Component
Turbine wingGear box
Generator
BeamSpecifications
φ900mmGear ratio 2.5
Permanent magnet synchronous
generator Rating 1.5kW
Adjusted based on waterway

5. 1 Hydro turbine wings
Hydro turbine wings are shown in Fig. 7 . Wings
with NTN's proprietary geometry improve upon theefficiency of conventional generator wings through thefollowing distinctions:.
∙ Thick cross sectional structure∙ Winglet structure∙ Wide wing tip

Winglets

Fig. 8 Change of the flow by Winglets

5. 1. 3 Wide wing tip
The wing is gradually widened toward the tip toincrease the force applied to the wingtip. Thisefficiently produces running torque.

Fig. 7 Blade of hydro turbine

BeamTurbine wing

Generator
Battery or sale to gridGear box

Fig. 6 System configuration-30-

*06E_03 18/06/13 13:47 ページ 4

Micro Hydro Turbine

5. 2 Low torque/low environmental load bearings

5. 5 Serial installation of multiple units

Low torque bearings that NTN has been developing
for many years are used for rotational components toachieve low friction loss. In addition, NTN grease forfood processing machines is used as a lubricant forbearings and gears in order to minimize the impact on
the surrounding environment, even when accidentallyleaked into waterways.

Micro hydro turbines can efficiently convert energyof running water into electric power with NTN's
proprietary wing geometry. There is no interference
when multiple units are installed in a single waterway
for increasing power output. Fig. 11 shows a diagramof serial installation.
Downstream

5. 3 Ease of installation
Waterway

As shown in Fig. 9, micro hydro turbines can beinstalled on waterways only by placing beams whichspan the width of the waterways. As shown in Fig. 10,
a turbine can be installed with one mobile crane andthree workers within one hour. In addition, there is noneed for upstream flow to be stopped or other major
installation work.

Upstream

Fig. 11 Instllation of micro hydro turbine
Beam
Wall of channel

6. Measures against incoming foreign
objects
6. 1 Debris screen
Since micro hydro turbines are installed in the open
waterways, countermeasures against incoming debris
are required. The type and amount of debris varydepending on the environment of the waterways;
however, debris screens such as the one shown in
Fig. 12 are typically used. Debris screens prevent
foreign objects larger than the designed size;
however, foreign objects need to be removed from thescreen. Even if automatic removal systems areavailable, they are expensive and not appropriate forsmall hydraulic power generation systems. Therefore,
regular maintenance is required. We are working on asolution to simplify debris screen maintenance work.

Fig. 9 Installation image to a canalFig. 10 Installation operation

5.4 Use of existing waterways
Since micro hydro turbines generate power by using
the flow of running water, no head is needed;
therefore, no major construction, such dams or
headraces, is required. This results in lower initialcosts and easier installation.

Fig. 12 Dust removal screen-31-*06E_03 18/06/13 13:47 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

6. 2 Debris removal by controlling generator rotation

7. Demonstration test conducted at
Asaka Channel

Foreign objects caught in the rotating wings arepressed into the wings with water pressure and are
not likely to come out. Therefore, rotation can be
slowed down to reduce the water pressure so that thewater flow washes the foreign objects away. Fig. 13
shows the results of a test to wash away foreign
objects by controlling generator rotation. The time
period it took for the generator to reduce its power
output to 60% was increased by more than threetimes (3 days to 10 days). We are continuing
verification testing for implementation in commercial
products.

100With generator rotation control
Without generator rotation control

80
Output ratio %A demonstration test was conducted at New Asaka
Channel in Sukagawa-shi, Fukushima Prefecture from
June to August, 2016. The test overview and crosssection of the Channel under the demonstration test
are shown in Fig. 15 and Fig. 16, respectively.
Asaka Channel is one of the three largest channels
in Japan and runs from Lake Inawashiro through the
Ou Mountains. We obtained significant cooperationfrom the Land Improvement District, relevant
authorities and organizations for this demonstration
test.
Generation performance, impact on the waterway,
maintainability, etc. were verified during thisdemonstration test. The test conditions are shown in
Table 3. The measured data of flow rate andgenerated power, as well as the calculated result,

60
40
20

Downstream

0
1st
day

2nd
day

3rd
day

4th
day

5th
day

6th
day

7th
day

8th
day

9th
day

10th
day

Fig. 13 The reduction preventive effect of the amount ofelectric power generation by the generator control

6. 3 Turbine axis cover
Upstream

Foreign objects entangled around the axis are hard
to remove, resulting in reduced efficiency. An axis
cover is used as a countermeasure against
entanglement of foreign objects on the axis which may
affect rotation of the turbine wings, as shown in Fig. 14.

Fig. 15 Verification test at Asaka canal

2,100mm1,200mm

Turbine wingFig. 16 Cross section of Asaka canal
Axis cover

Table 3 Electric power generation test condition
Item
Turbine wing
Generator
Generator control
Measured flow rate

Gear box

Fig. 14 Cover of blade axis

-32-

Condition
φ900mmRating 1.5kW
Electronic load
1.6 m/s

*06E_03 18/06/13 13:47 ページ 6

Micro Hydro Turbine

References

which assumes the generated power is proportional tothe cube of the flow rate, is shown in Fig. 17. The test
result is the output of the generator and does notreflect controller efficiency. We will use the obtained
data in development for commercialization. In
addition, we will continue our test in order to verify
durability.

1) Ministry of Economy, Trade and Industry: Long-term
Energy Supply and Demand Outlook and related materials2) Ministry of Economy, Trade and Industry home page:"Nattoku!" Renewable energy
http://www.enecho.meti.go.jp/category/saving_and_new/s
aiene/kaitori/kakaku.html
3) Ministry of Economy, Trade and Industry, Agency for
Natural Resources and Energy, Pacific Consultants Co.,
Ltd.: Overview of Guideline for Introduction of
Small/Medium Hydraulic Power Generation Plan,
February, 2014
4) Agency for Natural Resources and Energy home page:
Small Hydraulic Power Generation Information Site
https://www.env.go.jp/earth/ondanka/shg/page01.html
5) Ministry of Economy, Trade and Industry, Agency for
Natural Resources and Energy home page: Small
hydraulic power generation information site
http://www.enecho.meti.go.jp/category/electricityand_gas/
electric/hydroelectric/mechanism/waterwheel/

1,200

Test result
Calculated value(from this test result)Power output W

1,000
800
600
400
200
0

0

0.5

1

1.5

2

2.5

Flow rate m/s

Fig. 17 Relations of flow velocity andelectric power generation

8. Conclusion
In this article, we have discussed types andcharacteristics of hydraulic power generation, thestructure of NTN's proprietary high-efficient wingtechnology and debris removal methods of microhydro turbines. Safe and highly reliable micro hydroturbines are necessary as they use agricultural andindustrial waterways, which are very much involved in
the lives of the local communities. We will continue ourbench tests and demonstration tests to develop highly
efficient and reliable micro hydro turbines andintroduce them into the market.

Photo of authors

Tomoya KAWAI

Hiroki MUKAI

Tomomi GOTOU

Engineering Dept.
Natural Energy Products
Division

Engineering Dept.
Natural Energy Products
Division

Engineering Dept.
Natural Energy Products
Division-33-

*07E_03 18/06/13 13:47 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Technical Paper ]CFD Contribution to Development of
Small Wind Turbine Generator

Takeru ITOU*
Ryosuke KARASAWA*
Masato YOSHINO**
Currently, renewable energy products are attracting in the world. NTN isdeveloping small wind turbine products which are able to easily construct by
low-initial-cost. This paper describes a Computational Fluid Dynamics (CFD)of blade which is one of evaluation methods for small wind turbine.

and application of CFD required high performancecomputers, such as super-computers. Recently, withthe evolution of computers, regular computers haveacquired the same performance as super-computersof the past by combining image processing software 2).
This makes the CFD analysis readily available toanyone who owns a computer and has the skill to run
such analysis. .
CFD analysis can be conducted relatively easily byusing commercial CFD analysis software.
The use of CFD analysis brings not only a reductionof development costs, but also acquisition of detailed
information about the flow field (e.g. air flow), which isone of the greatest benefit.
Revealing the details of the flow field is extremely
helpful for developing new products in terms ofdesign/development of products that use flow, and for
improvement in efficiency, etc. However, the solution
acquired from the CFD analysis is not an exact
solution but an approximate solution, and may lead toan unrealistic solution if the wrong conditions are
given. In order to ensure reliability of the solution fromthe CFD analysis, it is necessary to verify thealignment of solutions between test data from windtunnel tests, etc and the CFD analysis.
In this article, we will report the results of the CFD
analysis for the blades of the wind turbine generator
developed by NTN and the results of the tests
conducted with the wind tunnel at Mie University.
Then we will compare the two outcomes.

1. Introduction
With the advancement of computers, calculation
using Computational Fluid Dynamics (CFD) has
become more accessible. Traditional evaluation withwind tunnels and field tests requires a number of
specimens for tests to be created according toevaluation conditions which may significantly increase
cost. In addition, large specimens require large windtunnels, meaning that availability and usable
equipment may be limited. Furthermore, when windtunnels are not used outdoor tests pose various
challenges. For example the topology in Japan does
not offer abundant wind or wind with a stable direction.
Development of CFD has potential for solving thesechallenges.
CFD is a numerical analysis method for calculating
fluid behavior using computers, which has beendeveloped along with the improvement of computer
performance. In general, it involves dividing the space
where fluid exists into grids and then apply NavierStrokes equations, which are the dominant equations
for fluid motion. The equations are applied todiscretized grid space, solving differential equations
numerically as a boundary value problem or initialvalue problem. CFD started in the 1960's and thanks
to very active research for applications to real
problems, significant progress was achieved in the
1980's 1). However, up until the 1980's only
laboratories of national research institutions and large
enterprises could use this method because research**Engineering Dept. Natural Energy Products Division**Advanced Technology R&D Center-34-

*07E_03 18/06/13 13:47 ページ 2

CFD Contribution to Development of Small Wind Turbine Generator

2. CFD analysis
2. 1 Analysis model
We applied CFD analysis to the blades of thehorizontal axis wind turbine (HAWT) and the verticalaxis wind turbine (VAWT). We call the forward
direction of the blade rotation “the leading edge” andthe backward direction “the trailing edge”.
NTN's blade is a lift type blade with quiet operationeven in high-speed rotation thanks to the proprietary
thick blade cross sectional geometry. The thickgeometry allows smooth air flow along the bladesurface producing lift force toward the forward
direction of rotation, creating torque even with small
pitch angle (angle of blade relative to the rotationalaxis). In other words, air resistance during windlessconditions is small. In addition, the unique tip designof the blade reduces the blade tip vortex and theresulting loss, achieving high quietness. The blade
under analysis is shown in Fig. 1, the cross sectionof the horizontal axis blade is shown in Fig. 2, andthe cross section of the vertical axis blade is shownin Fig. 3, respectively.

B

B

Direction of
motionCross section B-B
Direction of
motionFig. 3 Wing and cross-section of VAWT

2. 2 Analysis conditions
The analysis conditions of the horizontal axis andvertical axis turbines are shown in Table 1.
In general, the appropriate wind speed for wind
power generation is said to be 6m/s or more.
Therefore, the CFD analysis was conducted at a windspeed of 6m/s. We used the commercially available
fluid analysis software STAR-COM+ for this analysis.
The method used was the finite volume
method/implicit unsteady analysis. Raynolds-averagemodel (RANS) was used for the turbulent flow model.
The boundary layer grids around the blade were set to
20 layers, and the growth rate between grids was set
to 1.5 to observe air flow turbulence. In addition, inorder to increase computation speed, the horizontal
axis wind turbine was divided into five increments
along the rotational direction, and the vertical axis
wind turbine was divided into two in the verticaldirection.

WingletDirectionof rotation
Directionof rotation

Table 1 Analysis condition
Fig. 1 Analysis model (left:HAWT, right:VAWT)Wind speed m/s Rotational speed min-1
10
20
30
40
50
60
70
80
Horizontal axis
6
90
blade
100
110
120
130
140
150
200
250
50
Vertical axis
6
100
blade
150
Type

Wind direction

A

A

Cross section A-A
Direction of rotation
Direction of rotation

Fig. 2 Wing and cross-section of HAWT

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*07E_03 18/06/13 13:47 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

3. 3 Wind tunnel test specimen

3. Wind tunnel test

The outline drawings of the horizontal axis andvertical axis wind turbines used in this test are shownin Fig. 5 and Fig. 6, respectively. Torque meters were
set to each specimen to calculate the power of theblades from the running torque and from the rotational
speed. The specimens were equipped with pneumatic
brakes for safety reasons.

3. 1 Wind tunnel facility
The wind tunnel facility at Mie University was used.
The appearance and overview of the facility is shownin Fig. 4.

Torque meter
Generator

Pneumatic brake
31m

Cornervane

Sealed test
sectionWire meshscreenHoneycomb flow
straightenerBlower

9.5m

Fig. 5 HAWT test modelOpen test
sectionWire meshscreenWind direction

Honeycomb flow
straightener

Air inlet

4.5m

Exhaust port forsmoke experiment

Pneumatic brake

Fig. 4 Wind tunnel

Torque meter

Generator

3. 2 Wind tunnel test conditions
Conditions of the wind tunnel are shown in Table 2.

Fig. 6 VAWT test model

Table 2 Wind tunnel test condition
Type

Wind speed m/s

Horizontal
axis blade

6

Verticalaxis blade

6

Rotational speed min-1
20
40
60
80
100
120
140
160
180
200
220
240
260
280
298
18
36
54
72
90
108
116

4. Results
4. 1 Horizontal axis wind turbine
Results of the CFD analysis and the wind tunnel
test for the horizontal axis wind turbine are shown in
Fig. 7. From Fig. 7, it was verified that the results of
the CFD analysis and the wind tunnel test were mostlyaligned and that the analysis was adequate.
The kinetic energy of the wind that goes through theswept area (area where the blades of the turbine
receive wind) is hereby called blade power.
The increase of the blade power to 50 - 100 min-1
range of rotational speed was then examined with the
CFD analysis. The pressure fields around the blade at
the rotational speed of 50 min-1 (Point A) and 70 min-1(Point B) are shown in Fig. 8, and the flow field isshown in Fig. 9, respectively. Fig. 9 is a diagram
showing the speed vector around the cross section of
the blade at the point where negative pressure is-36-

*07E_03 18/06/13 13:47 ページ 4

CFD Contribution to Development of Small Wind Turbine Generator

Blade power (W)

80
60

B

40

A

20
0

blade, a strong negative pressure was generated on
the leading edge, resulting in increased torque. From
the above, in the case of horizontal axis turbines
where the blade rotational speed and circumferential
speed ratio change at every point from the center ofrotation to the tip of the blade, it is critical to make theshape of the blade match both the wind speed andcircumferential speed throughout the wind turbine inorder to generate strong negative pressure. The
analysis model used for this CFD analysis ofhorizontal axis blades captured separation andadhesion of air flow, indicating that this analysis model
was adequate.

Wind tunnel test result
Analysis result

100

0

50

100

150

200

250

300

Rotational speed min-1

Fig. 7 Result of HAWT wind tunnel test and CFD

Approx.
50%

50 min-1 (Point A)

Negativepressure

4. 2 Vertical axis wind turbine
The results of the CFD analysis and wind tunnel test
for the vertical axis wind turbine are shown in Fig. 10.
From Fig. 10, it was verified that the results of the
CFD analysis and wind tunnel test were mostlyaligned around 50 min-1. However, the results of the
CFD analysis and the wind tunnel tests differ around
100 min-1 and over. This error is believed to be aconsequence of the fact that the resistance of the arm
which connects the main spindle and blade was notconsidered.
In general, air resistance is expressed as follows:

70 min-1 (Point B)

Fig. 8 Blade pressure fieldWind
direction

Separation range

Air resistance = Ct×1/2×ρ×A×V

2

Ct = drag coefficient ρ= air density
A = project area
V = velocity
Direction of motion

50 min-1 (Point A)

and proportional to the square of the velocity.
Therefore, resistance at the arm becomes greater as
the circumferential speed increases. For confirmation,
we conducted the CFD analysis of the blades with arm
and main spindle at 100 min-1. The model with arm isshown in Fig. 11, and the CFD analysis result isshown in Fig. 12, respectively.
As shown in Fig. 12 errors from the test data
decreased from around 30% to 7% by considering

70 min-1 (Point B)

Fig. 9 Blade flow field

generated (approximately along 50% of the blade
length) with a rotational speed of 50 min-1 and 70 min-1.
From Fig. 8, it is revealed that the negative
pressure is generated at the middle point of the bladewith a rotational speed of 70 min-1. On the other hand,no negative pressure is observed at the middle of theblade with 50 min-1. This indicates that at 70 min-1, theblade power was increased by the presence of strong
negative pressure. In addition, from Fig. 9, it wasobserved that air flow does not exist along the lower
side of the blade with 50 min-1, but exists with 70 min-1.
This suggests that the air flow changed as theincoming wind angle changed due to increased blade
circumferential speed and the air flow was accelerated
on the blade’s surface. As the air flow on the bladesurface accelerated without being separated from theWind tunnel test result
Analysis results

Blade power (W)

150
120
90
60
30
0 0

0

50

100

150

Rotational speed min-1

Fig. 10 Result 1 of VAWT wind tunnel test and CFD-37-

*07E_03 18/06/13 13:47 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

At point A, where the rotational moment of theblades reaches the maximum, the pressure fieldaround the blades creates a positive moment that
contributes to power generation (Fig. 14).
Wind flowing in from the outside diameter of theblade (1) is accelerated by the forward blade shapeproducing strong negative pressure on the forward
blade (1). This means that the force rotating theturbine is created at the blade (1).
At point B, where the rotational moment of theblades is minimum, the pressure field around theblade will create a negative moment to reduce therotation (Fig. 15).
Strong positive pressure can be confirmed at theforward portion of the blade (1). The positive pressure
is generated because the wind collides with the blade
and stays in the area, which pushes the turbine in the
opposite direction. In addition, the blade (2) is notworking on either side since the variation of pressure
distribution around the blade is small. In other words,
with this wind direction and blade position, there is aforce pushing the turbine rotation backward.
In practice, the turbine does not stop due to theinertia of rotation.
As summarized above, we could verify theperformance of the blade with NTN's proprietary
shape by using the CFD analysis.

arm resistance. This means that arm resistancecannot be ignored when conducting the analysis. It is
generally understood that it is hard to conduct
accurate calculations with the CFD analysis for verticalaxis wind turbines because the air flow on the bladesurface changes in complex patterns. However, forthis CFD analysis, the results matched well with thewind tunnel test results at low speed, even though
only the blade was considered. Also, errors could be
sufficiently reduced at higher speed, when the arm
was also considered.
Next, the pressure field around the blades wasverified by the CFD analysis. Since the vertical axis
wind turbine we consider in this article is a 2-blade
system, one rotation produces a 2-cycle fluctuation in
the process. In the case of the rotational speed of theblade being 100 min-1, the fluctuation will be as shownin Fig. 13.

B(a) Without arm(b) With arm

Fig. 11 Analysis model with arm and shaft

Wind tunnel test result
Analysis results (blade only)
Analysis results (blade+principal axis+arm)

150

Blade power (W)

②
①

120
90

Wind
direction

60

Rotational directionof blade

30
0

0

50

100

150

Rotational speed min-1

Rotational moment (Nm)Fig. 12 Result 2 of VAWT wind tunnel test and CFD

15

A

Wind
direction

10

Rotational direction① of blade

②

5
0
-5

Negativepressure

B
0

0.1

0.2

0.3

0.4

0.5

0.6

Time sFig. 14 Blade pressure field(above:surface, below cross-section)

Fig. 13 Blade rotational moment-38-

*07E_03 18/06/13 13:47 ページ 6

CFD Contribution to Development of Small Wind Turbine Generator①

5. Conclusion②

Wind
direction

We have evaluated NTN's small wind power
turbines with wind tunnel tests as well as the CFD
analysis. We obtained the following results from thetest data and analysis results:
・The results from the wind tunnel tests and the CFD
analysis matched. By using the method adopted inthis article, performance of wind turbines can be
estimated by using the CFD analysis.
・The output mechanism could be reviewed bypressure distribution/flow field obtained from the
CFD analysis.
・By revealing the output mechanism, improvement
points of the blade shape such as the crosssectional shape and size were clarified.

Rotational
directionof blade

Positivepressure①
Wind
direction

Rotational directionof blade

Use of the CFD analysis greatly contributes to the
reduction of development cost and new productdevelopment. We will continue working on therefinement of blade analysis technology using the
CFD analysis to develop natural energy products withhigher efficiency for business growth.

②

Fig. 15 Blade pressure field(above:surface, below:cross-section)

References
1) Yoshiaki Nakamura, "Historical Review and Prospect on
CFD", Proceedings of the 28th Symposium on
Computational Fluid Dynamics, Presentation No. S01-1,
JSFM, (2014) P1.
2) Hisaaki Daiguji, "History of the Japan Society ofComputational Fluid Dynamics", the Japan Society ofComputational Fluid Dynamics Journal, Vol. 10, No. 3,(2002) P233.

Photo of authors

Takeru ITOU

Ryosuke KARASAWA

Masato YOSHINOEngineering Dept.
Natural Energy Products Division

Engineering Dept.
Natural Energy Products Division

Advanced Technology
R&D Center-39-

*08E_*09 18/06/13 13:48 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Technical Article ]Technical Trend of the Precision Bearings
for Machine Tools
Naoki MATSUMORI*
Keiichi UEDA*NTN has been developing, manufacturing and selling the precision
bearings for many years to support the progress of the machine tools which
are called “Mother Machine” and “Mother of Industry”, too. And the bearings
have been also globally supporting the manufacturing of the automobile
industry and non-automobile industries like medical, energy, transportation
and aerospace. In this article, technical trend of the precision bearings formachine tools are introduced below with trend histories and NTN activities.

1. Introduction

2. Transition of technology trends inbearings for machine tools

Recently, machine tools have further improved theirperformance in order to address diverse requirementsfrom various industries, including automobiles,
aircrafts and I.T. In addition to being equipped with thebasic performance of the "Mother Machine" such as"high speed, high rigidity and high precision," newmodels are also being developed one after anotherwith new capabilities and concepts. Such capabilities
and concepts include (1) integrated capabilities tocope with consolidated manufacturing processes, (2)5-axis manipulation to achieve complex and highly
precise processes such as curved surfaces, (3) small
form factors for energy saving and leaving a small
footprint, (4) multi-functionality for improved operabilityand (5) condition monitoring mechanisms for highreliability 1).
To support the above efforts, precision bearings formachine tools are also making progress with newtechnologies and new products. For example, newtechnology has been developed to achieve both highspeed and high rigidity, which at a high level, areusually in a trade-off relationship, as well as
technology to further improve lubricant reliability forgrease lubricated high speed main shafts.
In this article, we will discuss technology trends inprecision bearings for machine tools, including pasttransitions and our previous work.

Recently, machine tools have been bipolarized intoa high speed/high precision machine group, including
mainly 5-axis machine tools and micro precision
machine tools, and a low-priced machine group for
customers in emerging countries.
In this way, technology trends in machine tools(next page) are diversified; as such, precision
bearings used for machine tools also require new
characteristics in addition to high speed, high rigidity
and high precision.
Fig. 1 shows the requirements for precision
bearings for machine tools. They can be broadly
divided into three phases.
First, until around 2000, bearing technology for
improving the basic performance of machine tools,
i.e., high speed, high rigidity and high precision andthe resulting products were required. With thisbackground, the following products were developed:
ceramic bearings which are lighter than steel bearings
and therefore more appropriate for high-speed andhigh contact pressure; small size rolling elements with
reduced centrifugal force at high-speed operation; andair-oil lubrication with increased lubrication reliability
enabling ultra-high speed operation.
Subsequently, during 2001 to 2014, the requirement
for high speed, high rigidity and high precisioncontinued, but long-life and environmental
considerations were added as important criteria. Here,*Industrial Machinery Engineering Dept., Industrial Business HQ.

-40-

*08E_*09 18/06/13 13:48 ページ 2

Technical Trend of the Precision Bearings for Machine Tools[Phase 3]2015 -

Small form factor
Technical
support service[Phase 2]
2001 - 2014

Long life

Long life
Environment
friendly[Phase 1]
2000
High speed /high rigidity

High speed /high rigidity

High precisionHigh precision

Environment friendly
High speed /high rigidity
High precision

Fig. 1 Needs for the precision bearings for machine tools

retain as much grease as possible near the raceway 2).
In addition, environmental consideration means
energy saving and eco-friendliness. Specifically, thismeans increased capability of grease lubrication
bearings with a low environmental burden in highspeed areas, low consumption of lubricant oil andcompressed air for air-oil lubrication, suppressed oil
mist, and low noise generation to reduce the physicalburden on the machine tool workers. To address theserequirements, eco-friendly air oil lubricated bearings
using proprietary nozzles to supply air oil to the outer
diameter surface of inner rings, which is subsequently
supplied in the bearings with centrifugal force weredeveloped. Angular contact ball bearings with relubricating holes on the outer rings were also
developed 2).
At the present (2015 and after), small machine tools
are being actively developed resulting in the technicalrequirements for small size bearings becoming more
rigorous. For example, some small machine tools use
compressed air supplied to the main spindle for air
cooling and air seals for the small space requirementof the main spindle, instead of ordinary housing
cooling (outer cylinder cooling).
With grease lubricated bearings, grease may leakoutside the bearings due to compressed air; therefore,
measures must be taken.
In addition, as the products, machining methods
and users of the machine tools have become
diversified, the considerations and proposals fordesigning main spindles have increased; therespective requirements are increasing, together withdesign frequency of the main spindles. To support
design efforts for the main spindles and reduce thedevelopment lead time of machine tools, a "technical[Machine Tools; Recent Technical Trends]➢ Increase of processing difficult-to-cut materials
such as titanium and ultra-hard materials,
resulting in further high rigidity requirements formain spindles➢ Demand for high efficiency/high precision
processing by multi-spindle/multi-function
machines➢ Small machine tools for processing small objects(Objective: smaller footprint for machine
installation, more installed machines per square
meter, energy saving, cost reduction, etc.)➢ Pursuit of automation, high intelligence,
improved operability by high functionality of NC
interface and software➢ Support of IoT (Internet of Things)➢ Prevention of coolant and chips/powder fromentering into main spindles➢ Prevention of burnout of bearings and othermachine components from poor lubricant➢ Energy saving, reduction of environmental
burden➢ Cost reduction by consolidating main spindles➢ Insight of the status of the actual machine
operation (CMS※ Technology)※ CMS: Abbreviation of Condition Monitoring Systemlong-life means the improvement of durability forgrease lubricated bearings, with reduced costs andreduced load on the environment compared with air-oil
lubrication. For example, high speed/high precision
bearings dedicated to grease lubrication weredeveloped with technologies such as grease for highspeed and long-life operation, double sealed bearings
to increase retention of grease, and grease pockets to-41-

*08E_*09 18/06/13 13:48 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

calculation system for bearings of machine tool main
spindle" is being developed. The new tool will allow
customers to automatically calculate the characteristic
values of bearings and main spindle in a short amountof time 3).
To address the above requirements, NTN has beenlong engaged in research and development,
manufacturing and distribution of precision bearings
for machine tools, resulting in the market introductionof the "ULTAGE Series" High Speed Angular Contact
Ball Bearing "HSE Type", which was developed in
2000.
ULTAGE is a name created from the combination of"ultimate," and "stage,” to represent NTN's pursuit for
the highest level of precision in machine tool bearings.
Our concept is to achieve ultra-high speed, highprecision and high reliability while coordinating
harmony with our environment.
The history of precision bearings for machine tools,
ULTAGE Series, is shown in Fig. 2. Since thedevelopment of High Speed Angular Contact Ball
Bearing "HSE Type" and Angular Contact Ball Bearing
with High Speed Seal "BNSxxxLLB Type," many new
products have been introduced in the market. In Fig.
2, the concept of each product is broken down bycolor. Similarly to Fig. 1, blue in Fig. 2 represents"high speed, high rigidity and high precision," green

represents "long life and environmental friendly", andyellow represents "small form factor and technicalsupport service", indicating that the ULTAGE Series
addresses various needs for precision bearings formachine tools.
In the next section, we will discuss the technicaltrends in "high speed, high rigidity and high precision,""long life and environmental friendly", and "small form
factor and technical support service", included in the
ULTAGE Series.

3. Technical trends of "high speed,high rigidity and high precision"
In order to increase the rigidity of bearings,
increasing the pre-load during assembly andincreasing the load carrying capacity of bearings are
effective. However, these adjustments lead to a rise intemperature during operation resulting in deteriorationof machining accuracy due to thermal expansion.
Therefore, it is critical to suppress heating during
operation in order to increase the rigidity of bearings
for faster operation 4). Specific methods to reduceheating during operation include adoption of theaforementioned ceramic rolling elements and small
size rolling elements, as well as optimization of the
bearing contact angle and improved raceway

Small diameter high speed angular contact ball bearings
Bearing Technical Calculation System for Machine Tool Spindles
Machine tool main spindle bearings with air cooling spacer
High-speed angular contact ball bearings HSE-T9
Air-oil lubricated high-speed angular ball bearings with re-lubricating hole on the outer ring HSEW
Ball screw support angular contact thrust ball bearings BSTU
High-speed angular contact ball bearings HSE9xxU
Standard angular contact ball bearings 79xxU / 70xxU
Angular contact ball bearings for axial loads HTAxxxU
High speed double-row cylindrical roller bearings NN30HSRT6Standard sealed angular contact ball bearings 7xxxLLBSuper high-speed angular contact ball bearings HSF，Eco-friendly air oil lubricated super high speed angular contact ball bearings HSFL
Ball screw support angular contact thrust ball bearings 2A-BST (open type), 2A-BST LXL (sealed type)High speed single-row cylindrical roller bearings N10HSRT6，Eco-friendly air oil lubricated super high speed single-row cylindrical roller bearings N10HSLT6High speed angular contact ball bearings HSE，Eco-friendly air oil lubricated super high speed angular contact ball bearings HSL
High speed sealed angular contact ball bearings BNSxxxLLB

2000

2004

2008

2012

Fig. 2 Development history of NTN precision bearings for machine tools ULTAGE series

-42-

2016

*08E_*09 18/06/13 13:48 ページ 4

Technical Trend of the Precision Bearings for Machine Tools

geometry of the inner/outer rings.
Special bearing steel with superior wear resistanceand anti-galling properties is also used for raceways toincrease durability against contact pressure duringhigh-speed/high load operation and under high preload conditions 5).
Various improvements are also made for cages,
such as changing the material from conventional hightension brass to PEEK (polyetheretherketone) resin inorder to reduce the weigh to 1/4 or less 6); adoption of
the most appropriate shape for the supply/drain of oilin air oil lubrication; and retention of grease in greaselubrication 7). Precision bearings for machine tools
incorporating the above measures are shown in Fig. 3.
The most recent trend is the increased demand for
5-axis machine tools with multiple machining
capabilities and complex machine tools, which require
bearings for the main spindles to offer both high speed
and high rigidity at a high level. To that end, it isimportant to keep the temperature of the bearings low
during operation. Therefore, we are proposing"machine tool main spindle bearings with air coolingspacers" and applying air-cooling technology to thebearings (Fig. 4) 4, 8).
Machine tool main spindle bearings with air coolingspacers are equipped with air cooling nozzles in
addition to oil supply nozzles for air-oil lubrication, on
the outer ring spacers. These are incorporated
between angular contact ball bearings in back-to-backarrangements (DB arrangement). Air cooling nozzles
are at offset positions from the axial center,; thecompressed air at room temperature injected fromthese nozzles (hereinafter cooled air) goes through thespace between the inner and outer spacers, as well as
inside the bearings, swirling in the rotational directionof the axis. The cooling air removes heat from thesurface of the inner ring spacer to cool it down.
The air nozzles of the cooling air are offset so thatthe cooling air stays around the surface of the innerring spacer longer. This extends the heat exchange
time between the cooling air and the inner ring spacer
surface, and increases the cooling effect of the innerring spacer.
When the inner ring spacer is cooled down, theadjacent bearing inner ring is also cooled. As a result,the difference in temperature between the bearing
inner ring and outer ring, and the contact stress on the
raceway surface can be reduced to achieve both highspeed and high rigidity at a high level.
To meet the precision requirements for bearings
used in machine tools, ultra-high precision bearings
were developed which reduce Non-Repetitive Run-Out(NRRO) of the bearings. This is done by controllingthe size variation of rolling elements per bearing and

(1)

(2)

(3)

(4)

Fig. 3 NTN precision bearings for machine tools
ULTAGE series(1) Standard angular contact ball bearings 79xxU/70xxU(2) High speed angular contact ball bearings HSE(3) Super high speed double-row cylindrical roller bearings NN30HSRT6(4) Super high speed single-row cylindrical roller bearings N10HSRT6

Outer ring spacer

Air cooling nozzle

Inner ring spacer

Fig. 4 Machine tool main spindle bearings
with air cooling spacer

controlling the angle components of raceway
roundness. NTN supports bearings with NRRO of
0.3μm or less, which are adopted in the ultra-highprecision machining center and the main spindles oflathes 5).

4. Technical trends in long-life andenvironment support
As described in Section 2, long-life means improved
durability for grease lubricated bearings, with reduced
costs and reduced environmental impact comparedwith air-oil lubrication. "High-speed sealed angularcontact ball bearings BNSxxxLLB" (Fig. 5) are based-43-

*08E_*09 18/06/13 13:48 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

on the high-speed/high rigidity bearings described in
the previous section, but with the additional of double
seals for higher retention of grease and grease
pockets for retaining grease closer to the rolling
elements in order to achieve durability of over 20000
hours in ultra-high speed operation of dmn value 1)
1,400,000.
For environmental requirements, eco-friendly air oil
lubricated bearings (Fig. 5) were developed enabling
the reduction of lubricant oil and compressed air
consumption, as well as the reduction of oil mist andlow noise.
Furthermore, in recent years, angular contact ballbearings with outer ring supply holes are being seen in
the market, which are equipped with re-lubricating holeson the outer ring instead of the conventional method of
supplying air oil from the outer ring spacer (Fig. 6) 9).
This reduces the spacer width because a nozzle hole is
not necessary, and contributes to the compact design
and increased rigidity of the main spindles by placing
bearings on the tip side of the machine tool. "Bearings
with re-lubricating holes on the outer ring (HSEW)" canreduce the consumption of lubrication oil andcompressed air by using air oil lubrication method fromthe outer ring. In addition, wind noise and oil mist canbe reduced by an optimized oil supply hole size.

(1)

(3)

Fig. 6 New bearings for long-life and environmental friendly
Left : High speed angular contact ball bearings with re-lubricating
hole on the outer ring HSEW
Right : "Grease Lubricating Unit with Self-generating Power Supply"
for main spindles

A "grease lubricating unit with self-power generator" isalso under development for main spindles to further
expand the allowable range of operation at high speeds,
and reduce the environmental burden (Fig. 6) 10). This
unit will be incorporated in the outer ring spacer
adjacent to the bearings and be equipped with apower generator, pump and lubricating oil. It will havea self-powered generator which leverages thetemperature difference that arises between the
bearing inner ring and outer ring during bearing
operation to enable automatic re-lubrication without anexternal supply of electric power or lubricating oil.
Currently, efforts for commercialization are underway.

5. Technical trends in small form factors
and technical support services(2)

Demand for small spindles is growing due to theincrease of the machining needs for small products
such as smart phones and precision/micro processing
for medical device components, as well as the highly
active development of low-cost/small footprint machine
tools for emerging countries.
Some small spindles use compressed air supplied
into the spindles for air cooling and air seals, for small
space requirement of spindles, instead of ordinary
housing cooling (outer cylinder cooling). With greaselubricated bearings, grease may leak outside thebearings due to the compressed air; therefore,
measures must be taken. Sealed bearings are effective
for preventing grease leakage; however, it will beharder for compressed air to flow within the spindles,
reducing the cooling effect and blocking the capability
to prevent foreign objects from entering. Therefore,
NTN developed the "ULTAGE Series Small Size Highspeed Angular Contact Ball Bearings" (Fig. 7) 11).
These bearings adopt newly designed seals toprevent grease leakage and increase lubrication life,
while ensuring the easy flow of compressed air. They
also achieve reduced vibration by optimizing theinternal design of the bearings.
Recently, machine tool manufacturers, and(4)Fig. 5 NTN precision bearings for machine tools
ULTAGE series(1) Standard sealed angular contact ball bearings 7xxxLLB(2) High speed sealed angular contact ball bearings BNSxxxLLB(3) Eco-friendly air oil lubricatedhigh speed angular contact ball bearings HSL(4) Eco-friendly air oil lubricatedsuper high speed single-row cylindrical roller bearings N10HSLT6-44-

*08E_*09 18/06/13 13:48 ページ 6

Technical Trend of the Precision Bearings for Machine Tools

meetings by the customers and NTN; however, thissystem can shorten the design process before then,contributing to a reduced lead time for development of
the main spindles and machine tools.
Compressed air flow
through spindle

6. SummaryFig. 7 ULTAGE small size high-speed precisionangular contact ball bearings

As the materials, shapes, processing methods andprocessing conditions of the products are diversified,
the requirements for performance of machine tools
and precision bearings used by those tools are
expected to be even more rigorous.
NTN will continue to work to further improve theperformance and quality of its products, as well as
enhance the technical support and proposals,
including enhanced speed and capabilities of technicalcalculation, in order to respond to customer needs
sincerely and precisely.

customers specializing in spindles, are developing awide array of products addressing user needs. To
support these development efforts, we havedeveloped the "Bearing Technical Calculation System
for Machine Tool Spindles". This tool allows customers
to automatically calculate the properties of precisionrolling bearings used for main spindles of machine
tools such as the main spindle rigidity and bearing life,
very quickly (Fig. 8) 3).
This system can calculate the properties of 10 typesof main spindle layouts with varied bearing types,
arrays and pre-load methods. First, customers will
send the data required for the calculation, such as
bearing type and usage conditions, in the dedicatedExcel® file to the dedicated server by email 2). Upon
receiving the data, the server automatically conducts
calculations based on the input data, andautomatically sends the calculation results such as thespindle rigidity and bearing life to the customer,
instantaneously. Conventionally, it takes at least one
day to respond to customers with the report of thecalculation results from the time we received therequests 3). However, this system produces thecalculation results in approximately 10 minutes fromthe time the customer sends the email.
In addition, since the customers can conduct
calculations as many times as required, they canshorten the time needed to choose between spindle
design options.
Furthermore, since the server is available at all
times, customers can use the system without any time
restrictions.
The final specifications of bearings require review

References
1) Shouhei Hashizume: New Technologies on Precision Bearings for
Machine Tools, Machine Design, Vol. 60, No. 1 (Jan issue, 2016) 28.
2) NTN Catalog: Precision Rolling Bearings, CAT. No. 2260-IV/J,
14.10.02 82.
3) Keiichi Ueda, Yushi Onda and Shouhei Hashizume: Bearing
Technical Calculation System for Machine Tool Spindles, NTN
TECHNICAL REVIEW, No. 82, (2014) 49.
4) Yushi Onda, Kikuo Fukada, Yohei Yamamoto and Masato Yoshino:
Machine Tool Main Spindle Bearings with Air Cooling Spacer, NTN
TECHNICAL REVIEW, No. 82, (2014) 38.
5) Hiroshi Tako and Yasutsugu Tanaka: Technical Trend of Machine
Tool Bearings, NTN TECHNICAL REVIEW, No. 78, (2010) 8.
6) Naota Yamamoto and Mamoru Mizutani: High Speed and Long Life
Double-Row Cylindrical Roller Bearings, NTN TECHNICALREVIEW, No. 72, (2004) 42.
7) Keiichi Ueda: ULTAGE Standard Angular Contact Ball Bearings,
79U/70U type, NTN TECHNICAL REVIEW, No. 72 (2004) 30.
8) Keisuke Nasu, Naoya Okamoto and Masato Yoshino:
Machine Tool Main Spindle Bearings with Air Cooling Spacer, NTN
TECHNICAL REVIEW, No. 84, (2016) 52.
9) Futoshi Kosugi and Kouji Nishino: Air Oil Lubrication Bearings with
Re-lubricating Hole on the Outer Ring for Machine Tools, NTN
TECHNICAL REVIEW, No. 78 (2010) 41.
10) Kaoru Omoto and Hiroyoshi Ito: Development of the Lubricating Oil
Supply Unit with Self-generating Power Supply, NTN TECHNICALREVIEW, No. 80, (2012) 48.
11) Mineo Koyama, Keisuke Nasu and Takahiro Kanamoto: ULTAGE
Series Small Size High-Speed Precision Angular Contact Bearings
for Machine Tool Spindles, NTN TECHNICAL REVIEW, No. 84,(2016) 46.

Photo of authors
Customer

Data transmission
through email

Data reception
through email

24 x 7 support

Fig. 8 Bearing technical calculation system
for machine tool spindles-45-

Naoki MATSUMORI

Keiichi UEDA

Industrial Machinery
Engineering Dept.,
Industrial Business HQ.

Industrial Machinery
Engineering Dept.,
Industrial Business HQ.

*09E_*09 18/06/13 13:49 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]ULTAGE Serises Small Size High-Speed Precision
Angular Contact Ball Bearings for Machine Tool Spindles

Mineo KOYAMA*
Keisuke NASU*
Takahiro KANAMOTO**

For high-speed, long-life and low vibration demand of small size machine
tools and small size spindle, NTN developed small size high-speed angular※1
contact ball bearings for machine tool spindles and added it to the ULTAGE
family. The bearings can hold more grease than current bearings through the
new designed seal and modified grease filling, even if compressed air goes
through the bearing. Moreover the bearings perform lower vibration through the process modification of raceways. In
this document, we introduce the design points, test datas and analysis results .

machining accuracy of the spindle, improving the qualityof the machined surfaces, and reducing vibration.

1. Introduction
Recently, machining needs for small products suchas smart phones and precision/micro processing needs
for medical device components are increasing. Also, thedevelopment of low-cost machine tools with a small
footprint for emerging countries is highly active.
Therefore, the demand for small spindles is increasing.
Many small spindles adopt a structure to use
compressed air for internal cooling and to prevent
foreign objects from entering. However, when greaselubrication is used, compressed air can cause thegrease to leak from the bearings, resulting in a shorter
lubrication life.
Sealed bearings are effective for preventing grease
purge, however, it is harder for the compressed air toflow within the bearings, reducing the cooling effect on
the spindle and limiting the capability to prevent foreign
objects from entering.
The newly developed "small size high speed angularcontact ball bearing" adopts a new seal design toreduce grease purge while securing easy flow ofcompressed air inside bearings. There are measures inplace to better retain the grease by adjusting the seal
position and pressure. In addition, the processing of the
bearing raceway was optimized, increasing product2. Small spindle structure
Fig. 1 shows a general schematic of a small size
spindle with a built-in motor. Medium to large highspeed spindles with built-in motors usually adopt theouter cylinder cooling method, where heat is removed
from the motor and bearings by circulating coolant tostabilize the spindle temperature. This cooling method
requires ancillary facilities and running costs, as well asa larger spindle outer diameter. Also for medium tolarge high speed spindles, a Labyrinth, or air seal
structure, is often used to protect the tip of the tool byblocking cutting fluid and chips from the machining
location.
In contrast, in the case of small size spindles withlimited space and low cost requirements, compressed
air is supplied from the rear side of the spindle to cool
down the motor and bearings. The compressed air,which is exhausted from the tip of the tool, also works
as an air seal. For this cooling method to work, thebearings used in small size spindles require easy flowof compressed air through the spindle and grease
retention for high lubrication reliability.

** Industrial Machinery Engineering Dept., Industrial Machinery Division** Product Design Department, Industrial Machinery Division※1 To the left of the photo is one side of the developed bearing (front side/black seal) and to the right of the photo shows the other side of the product(back side/orange seal).
※2 ULTAGE is a name created from the combination of "ultimate," and "stage,” signifying performance in any application expressing NTN’s idea of
exploring the ultimate performance of precision bearings.

-46-

*09E_*09 18/06/13 13:49 ページ 2

ULTAGE Serises Small Size High-Speed Precision Angular Contact Ball Bearings for Machine Tool Spindles[Spindle rear side][Spindle front side (Tool head)]Compressed air is exhausted through the spindle front side,bocking cutting fluid and chips from entering during operation.

Compressed
air supply
Motor

Motor and bearings are cooled down with the compressed air.

Fig. 1 Example of small size spindle

bearing width allows grease lubrication from bothsides of the bearing, for an increased total fill amount.
Also, a grease pocket is created close to the outerring raceway to increase grease retention. In addition,
the processing method of the bearing raceway was
optimized, increasing product machining accuracy ofthe spindle, improving quality of the machined
surfaces, and reducing vibration.
In the next section, we will discuss the results of theanalysis conducted before adopting this specification
and an example of the test results.

3. Benefits of the developed product3. 1 Achieving both compressed air flow andgrease retention
As described in the previous section, compressed
air is supplied inside the spindle and runs through theinside of the bearings. The developed product ensuresthis compressed air can flow while increasing grease
retention. Fig. 2 shows the challenges faced by theconventional product and the benefits of thedeveloped product.
The open type conventional product 1) has a wide
opening for easy compressed air flow, but the greaseis likely to escape. The sealed type conventional
product reduces grease purge risk, but thecompressed air does not flow easily through the small
gap in the seal lip.
On the other hand, the developed product adopts a
new seal design to reduce grease leakage whileallowing easy flow of compressed air. The increased

Conventional
Product
Open TypeGrease3. 2 Flow of compressed air through thedeveloped seal
Fig. 3 shows a fluid analysis simulation of thecompressed air flow through the developed productwhen actually incorporated into the spindle. The flow
is shown when compressed air is supplied from one
end of the double-row back-to-back arrangement, andexhausted out the other end.

Conventional
Product
Sealed Type

Compressed
air

Small gap
at the seal lip

Compressed
airEasy air flow, but harder
to retain grease[Seal]
With new seal design, compressed
air flow is improved and grease isretained to increase lubrication life.

Easier to retain grease, butharder for compressed air to flow
Developed
Product[Outer ring]
Equipped with grease pocket toincrease grease retention.
[Grease]Grease amount is increased at both sidesof the bearing to be less affected by the
direction of compressed air flow.
Internal structureor easy compressed
air flowCompressed
air[Cage]
Inner diameter is expanded
to secure easy flow ofcompressed air.

[Inner/outer rings]
Enlarged width to provide increased grease
space. Optimized process for raceway
surface quality to achieve low vibration.

Fig. 2 Small size high-speed angular contact ball bearings（Comparison of conventional design and development design）-47-

*09E_*09 18/06/13 13:49 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

3. 3 Bearing grease behavior due tocompressed air

The developed product allows an almost straightstream-line from the front side of the first bearing
where the compressed air is supplied to the back side,then to the front side of the second bearing where thecompressed air is exhausted. The adoption of a newly
designed seal with an increased gap between the seal
inner diameter on the back side and inner ring allows
this stream-line flow.
The flow rate of the compressed air is high in thespace between the seal inner diameter and the innerring, while it is low in the space between the seal innerdiameter and the outer ring.
From this, it is expected that this design will prolong
lubrication life by retaining abundant grease in thespace between the seal inner diameter and the outer
ring, which is less affected by the compressed air.

Cage

Front side seal

Ball

In the previous section, we described the
expectation of improved grease retention in the space
between the seal inner diameter and outer ring withadoption of the new seal design. In order to confirm,
the behavior of the grease was analyzed using fluid
analysis. The result is shown in Fig. 4. Condition 1
shows the status of the grease after operation, when agrease amount of approximately 30% of the bearing
space is injected mainly on the back side of each of
the bearings. Abundant grease is retained in thespace between the seal inner diameter and outer ring,which is less affected by the compressed air.
However, the grease amount retained in the bearingson the exhaust side of the compressed air is lower[Analysis condition]·Bearing inner diameter: φ10·Spindle rotational speed: 60,000 min-1·Compressed air supply: 0.3 MPa (30 NL/min)

Back side seal

Outer ringCompressed
air supply

Exhaust ofcompressed air
Inner ring
Low

Flow rate of compressed air

HighFig. 3 Fluid analysis result of compressed air flow through the bearings
Grease amount approx. 30% of bearing space is injected,
Condition 1 mainly on the back side
Front side seal

Cage

Ball

Back side seal

[Analysis condition]·Bearing inner diameter: φ10·Spindle rotational speed: 60,000 min-1·Compressed air supply: 0.3 MPa (30 NL/min)·Grease SE-1 2 )·Analysis started from the assumed grease position after
pre-conditioning operation·
shading indicates grease position after transition.

Compressed
air supply

Exhaust of compressed airGrease is gradually released
Grease of the exhaust side bearing is reduced

Condition 2

Increased grease amount approx. 50% of bearing space is injected, both on the back side and front side-> As applied with the developed productCompressed
air supply

Exhaust of compressed air

Transition of grease due to compressed air flow is suppressed by filling the space on the exhaust side with grease

Fig. 4 Fluid analysis result of grease movement figure by compressed air through the bearings-48-

*09E_*09 18/06/13 13:49 ページ 4

ULTAGE Serises Small Size High-Speed Precision Angular Contact Ball Bearings for Machine Tool Spindles

3. 4 Evaluation test resultscompared with the bearings on the supply side.
Condition 2 uses a grease amount of approximately
50% of the bearing space, injected both on the backside and front side of each bearing. In this case, thegrease is sufficiently retained in the bearings on theexhaust side also, which means that a longer
lubrication life can be expected.
In the case of a back-to-back arrangement,
compressed air flows from the front side of a bearing
to the back side of another set of bearing. Therefore, itis necessary to increase grease retention regardlessof the direction of compressed air flow, which makes
condition 2 more favorable, as an increased amount ofgrease is retained, both on the back side and front
side of each bearing.

In order to verify the actual effect of the fluid
analysis results described in the previous section, atest was conducted by operating a spindle with thedeveloped product built in. The spindle was operated
at a speed of 60,000min-1 for 500 hours, continuously,
and the grease amount and grease condition inside
the bearings was confirmed after the test. Results areshown in Fig. 5.
In this test, compressed air of 0.3MPa (30NL/min)
was used, which is the highest level recommended forthis size of bearings.
The developed product uses a grease amount ofapproximately 50% of the bearing space, injected bothon the back side and front side of each bearing. The[Test condition]

Bearing inner diameter: φ10
Ceramic ballGrease: SE-1 2 ) Amount approx. 50%, 30% of the bearing space
Spindle Rotational Speed: 60,000 min-1 continuous
Compressed
Fixed position Pre-load: 8N (test bearings),
air(support bearings on the rear)
Spindle position: Horizontal
Compressed air supply to main spindle: 0.3 MPa (30 NL/min)

Supplyside

Back side

Front side

Bearing

Developed Product

Seal

Yes

Grease fill method

Bearing space
ratio(Volume ratio)%

reduction

40
30
20
10

Position of bearing vscompressed air direction
Residual grease condition of the exhaust Side bearings after test

Seal effectResidual

Grease amountafter test

Supplyside

Front side

Comparison 1
Comparison 2(for verification of seal effect) (assuming conventional product)
No
No

Injected on both back side and front side
Approx. 50% of the bearing space

50

Exhaustside

Injected mainly on the back side
Approx. 30% of the bearing space
Effect on
conventional Product4 timesthe residual amount30%
15%
Exhaustside

Supplyside

Exhaustside

8%
Supplyside

Exhaustside

Back side
Abundant residualgreaseFront side

Observation

Sufficient grease observedin inner diameter of outer ringand cage

Although sufficient grease is
Reduced grease both on theobserved, less grease on the back side and front sideback side than the developed
product, the back side is in theflow of the compressed air

Fig. 5 Grease amount and situation after 500 hour driving under compressed air supply through the spindle-49-

*09E_*09 18/06/13 13:49 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

decreased to 8% of the bearing space. No
abnormalities were found in the test bearings, whichsuggests continued operation is still possible;
however, in order to retain stable lubrication for a longtime under the condition of compressed air flow, it isconsidered important to supply an increased amountof grease on both sides, as shown in the developed
product.

grease amount after 500 hours was decreased to 30%of the bearing space; however, that amount is still
sufficient for continued operation and grease retention
around the rolling elements, outer ring inner diametersurface, and cage surface was verified to be sufficient.
Next, a test without a seal was conducted to verify
the effect of the seal (Fig. 5 Comparison 1). In thecase of Comparison 1, the grease in the bearings on
the exhaust side had decreased to 15% of the bearing
space. This amount is also sufficient for continued
operation; however, the grease amount at the back of
the exhaust side bearing is lower than in thedeveloped product.
Subsequently, another test was conducted with agrease amount of approx. 30% of the bearing space,
for comparison with the conventional product. Results
are shown in Fig. 5 Comparison 2.
Grease in the bearing on the exhaust side had

4. Durability under compressed air flow
Currently, NTN is conducting a test on a spindle
built with the developed product, supplying 0.3MPa(30NL/min) of compressed air, for over 6,000 hours ofcontinuous operation at the speed of 60,000 min-1(Fig. 6). Additional test samples to gather data on
durability are to be added.

Compressed air exhaust
Test bearings

Compressed air
supplyOperation time, hDeveloped
Product

0

1,000

2,000

3,000

4,000

5,000

Over 6,000h of continuous pperation[Test condition]
Bearing inner diameter: φ10, Ceramic Ball
Grease amount approx. 50% of the bearing space, Grease: SE-1 2)
Spindle rotational speed: 60,000 min-1 continuous operation, back face in combination,
Fixed position: 8N (built-in time), (support bearings on the rear)
Spindle position: horizontal, compressed air supply to main spindle: 0.3 MPa (30 NL/min)Fig. 6 Durability test in high-speed operation under compressed air supply through the spindle

-50-

6,000

*09E_*09 18/06/13 13:49 ページ 6

ULTAGE Serises Small Size High-Speed Precision Angular Contact Ball Bearings for Machine Tool Spindles

5. Reduction of bearing vibration

6. Summary

The application range of small size spindles is
expanding from conventional drilling and otherauxiliary uses such as deburring to sub-spindles ofmachine tools and machining of small products.
Among others, applications for precision microprocessing are promising, which require low vibrationin order to increase the quality of the machined
surface.
The developed product has achieved approximately
50% of vibration reduction by optimizing the bearing
internal design 3) and the processing quality of the
raceway surface on the inner and outer rings (Fig. 7).
By reducing the vibration in bearings, the vibration
property of the spindle is improved, which should
contribute to the expansion of applications for small
size spindles.

The developed ULTAGE Series Small Size HighSpeed Angular Contact Ball Bearings achieve both
improved compressed air flow and increased retentionof the required grease amount. These improvements
are made without degrading the spindle cooling andair seal effects, allowing the continued use ofcompressed air flow inside the bearings to cool thespindle, while contributing to a long lubrication life
under high speed operation.
NTN will continue to work on performance
improvements of bearings for the main spindles, inorder to further improve reliability of machine tools.

1) Hiroshi Takiuchi and Futoshi Kosugi: "ULTAGE" Series
Precision Bearings for Machine Tools, NTN TECHNICALREVIEW, No. 72, (2004) 26.
2) Takayuki Kawamura: Research on the Lubrication
Mechanism of Grease for High Speed Bearings, NTN
TECHNICAL REVIEW, No. 76, (2008) 39.
3) Tomoya Sakaguchi and Yoshinobu Akamatsu: Simulationfor Ball Bearing Vibration, NTN TECHNICAL REVIEW,
No. 69, (2001) 69.

50% reduction of vibrationcompared to conventional productVibration ratio

1.0

References

0.5

Conventional
Product

Bearing inner diameter: φ10
Grease: SE-1 2)
Spindle rotational speed:
60,000 min-1

Developed
Product

Fig. 7 Effect of vibration reduction

Photo of authors

Mineo KOYAMA

Keisuke NASU

Takahiro KANAMOTO

Industrial Machinery
Engineering Dept.,
Industrial Business HQ

Industrial Machinery
Engineering Dept.,
Industrial Business HQ

Product Design Department,
Industrial Business HQ-51-

*10E_*09 18/06/13 13:53 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Technical Article ]Machine Tool Main Spindle Bearings
with Air Cooling Spacer
Keisuke NASU*
Naoya OKAMOTO**
Masato YOSHINO***NTN developed “Machine Tool Main Spindle Bearing with Air Cooling Spacer” which can perform high-speed and highrigidity at the same time with higher level than ever, through the original air cooling technology. We carried out theperformance tests of the cooling technology in various practical applications of machine tool spindles. In this paper, theresults of the above mentioned tests and fluid analysis regarding the cooling technology are introduced as follows.

1. Introduction

2. Structure and cooling mechanism

The main spindles of machine tools are required tohave both high rigidity and high precision whenmachining difficult-to machine materials orcomponents with complex shapes. Additionally, highspeeds and highly precise rotation are required tomachine molds for machine components and medical
field components. Although the requirements for themain spindles of machine tools vary depending on themachining method and the products to be machined,
there has recently been increased demand for 5-axis
machine tools with multiple machining capabilities, aswell as complex machine tools which require spindles
and bearings that offer both high speed and highrigidity at a high level 1). To that end, it is important tokeep the temperature of the bearings low during
operation, and NTN has developed "machine tool
main spindle bearings with air cooling spacers", andapplied its proprietary air-cooling technology to thebearings 2).
In this development, we demonstrated the cooling of
the bearings as well as noise reductions by improving
the shape of the outer ring spacer. This time, in orderto demonstrate its practicality, we implemented thistechnology with bearings and operating conditions
which are commonly found in machine tool main
spindles and verified the cooling effects.

The structure of bearings with air cooling spacers isshown in Fig. 1 and the fluid analysis results areshown in Fig. 2. NTN's proprietary eco-friendly air-oil
lubrication nozzle 3, 4) is applied to outer ring spacers
between the angular contact ball bearings in a backto-back arrangement (DB arrangement) for reductions
in air-oil and noise. Additionally, a separate air cooling
nozzle is included.
The air cooling nozzles are at offset positions about
the center axis. The compressed room temperature air
injected from these nozzles (hereinafter cooling air)goes through the space between the inner and outerspacers as well as the inside of the bearings, revolving
in the rotational direction of the inner ring. The cooling
air removes heat from the surface of the inner ring
spacer to cool it down.
The air nozzles of the cooling air are offset so thatthe cooling air remains near the surface of the innerring spacer longer, allowing more time for the cooling
air to remove the heat from the inner ring spacer
surface, thereby increasing the cooling effects of theinner ring spacer.
When the inner ring spacer is cooled down, theadjacent bearing inner ring is also cooled. As a result,the difference in temperature between the bearing
inner ring and outer ring (hereinafter, inner/outer ring*** Industrial Machinery Engineering Dept., Industrial Business HQ*** Product Design Dept. Industrial Business HQ*** Advanced Technology R&D Center-52-

*10E_*09 18/06/13 13:53 ページ 2

Machine Tool Main Spindle Bearing with Air Cooling Spacer

Air cooling nozzle

Air cooling spacer(outer ring spacer)

Bearings with
air cooling spacer

Air cooling nozzle
OffsetAir cooling spacer(outer ring spacer)

Cooling air (inlet)

Inner ring
spacer

Eco-friendly air-oil
lubrication nozzle

Inner ring
spacerInner ringrotational direction
Cooling air (outlet)

View A

Cooling air (outlet)
A

Fig. 1 Structure of the bearing with air cooling spacer

Outer ring spacer
inner diameter surface

Air cooling nozzle

Air cooling nozzle

OffsetInner ring spacer
outer diameter surface

Air cooling nozzle

Inner ringrotational direction

Inner ringrotational direction
Air cooling nozzle

Air cooling nozzle

Air cooling nozzle
Slow

Flow rate

Fast

Slow

Flow rate

Fast

When air cooling nozzle is not offset

When air cooling nozzle is offset

Cooling air collides with the perpendicular inner ring
spacer immediately after it is injected from the air
cooling nozzle and spreads out. Therefore, the time
it stays on the surface of the inner ring spacer is shortwithout a sufficient cooling effect.

Cooling air swirls around on the inner ring spacer inthe direction of inner ring rotational direction, because
the cooling nozzle is offset from the center of thespindle, and stays longer on the surface. The cooling
air removes more heat from the inner ring spacer
surface during that time increasing the cooling effectof inner ring spacer and inner ring.

Fig. 2 Cooling air flow between inner and outer spacers (Fluid analysis results)

temperature difference) is reduced, and the contact
stress on the raceway surface decreases to allow for
both high speed and high rigidity at a high level.

The nozzle offset was set to 80% of the radius of
the inner ring spacer outer diameter 2).

3. Cooling effects

Table 1 Test conditions
Bearing arrangement

In order to verify the cooling effects with bearingsand operating conditions which are commonly found inmachine tool main spindles, high speed operation
testing was conducted under the four conditions
shown in Table 1.
-53-

Main spindle drive method

DB(Back-to-back)[Test 1]
Motor direct[Test 2]
Built-in motor

DTBT(Tandem back-to-back)[Test 3]Motor direct[Test 4]
Built-in motor

*10E_*09 18/06/13 13:53 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

3. 1 Cooling effects of back-to-back arrangement(DB arrangement)

condition where heat is not easily released because
the bearings (which are heat sources) are close toeach other.
In the case of a direct motor drive, inner/outer ringtemperature difference at 17,000 min-1 (dmn value
1,530,000) was confirmed to decrease by approx. 2˚Cwith cooling air of 100NL/min, approx. 5.5˚C with
200NL/min, and approx. 7.5˚C with 300 NL/mincompared with no cooling air conditions. Due to these
decreases in inner/outer ring temperature difference,
the maximum contact stress on the bearing raceway
surface at 17,000 min-1 is reduced by approx. 10% at
100NL/min, approx. 15% at 200NL/min, and approx.
20% at 300NL/min.
As a result, the stress at 17,000 min-1 with no
cooling air supply and the stress at 21,000 min-1 with asupply of 300NL/min cooling air are equivalent, whichallows for a 25% increase in speed.
In the case of the built-in motor drive, theinner/outer ring temperature difference at 17,000 min-1(dmn value 1,530,000) was confirmed to decrease byapprox. 5˚C with cooling air of 100NL/min, approx. 8˚Cwith 200NL/min, and approx. 11˚C with 300 NL/min

Cooling effects of the bearings with air coolingspacers under DB arrangement was confirmed withthe high speed operation testing. The test conditions
are shown in Table 2, the structure of the tester isshown in Fig. 3, and the test results are shown in Fig.
4. This test was conducted with a DB spacer width of
22mm, almost the same width as the bearing, aTable 2 Test conditions for DB arrangement
φ70 x φ110 x 20
5S-2LA-HSL014 equivalent(Eco-friendly air-oil lubricated angularcontact ball bearings with ceramic balls)
Fixed position pre-loading(Pre-loading ON after built-in)

Test bearings
Pre-load method

0〜23,000min-1

Rotational speed

Air-oil lubrication

Lubrication method

0.03mL/10min

Amount of lubricant

ISO VG32

LubricantLubrication air flow rate
Cylinder cooling

30NL/minYes, Sync w/room temperature
Horizontal spindle

Spindle position

Bearings with
air cooling spacer

Cooling air (inlet)

Bearings with
air cooling spacer

Cooling air (inlet)

Built-in motor

Drive
motor

Cooling air (outlet)

Cooling air (outlet)

Supporbearings

Cooling air (outlet)

Cooling air (outlet)

Built-in motor drive main spindle

Motor direct drive main spindle

Fig. 3 Test spindles for DB arrangment30

No cooling air

25

Cooling air of 100NL/min

Inner/outer ring averagetemperature difference ˚C

Inner/outer ring averagetemperature difference ˚C

30

Cooling air of 200NL/min

20

Cooling air of 300NL/min

15
10
5
0
-5

0

5,000

10,00015,000

Rotational speed

20,000

min-1

Results with motor direct drive main spindle

Cooling air of 100NL/min
Cooling air of 200NL/min

20

Cooling air of 300NL/min

15
10
5
0
-5

25,000No cooling air

25

0

5,000

10,00015,000

Rotational speed

min-1

20,00025,000Results with built-in motor drive main spindle

Fig. 4 Relation between cooling air amount and temperature difference between inner ring and outer ring (In case of DB arrangment)-54-

*10E_*09 18/06/13 13:53 ページ 4

Machine Tool Main Spindle Bearing with Air Cooling Spacer

confirmed with a high speed operation test. The
configuration of the tester is shown in Fig. 5 and test
conditions are shown in Table 3. This test was also
conducted with a DB spacer width of 22mm, similar to
the test with DB arrangement in the previous chapter.
The results for the direct motor drive are shown in
Fig. 6. The inner/outer ring temperature difference at
17,000 min-1 (dmn value 1,530,000) was confirmed todecrease by approx. 6˚C with 300NL/min comparedwith no cooling air conditions. Due to this decrease of
inner/outer ring temperature difference, the maximum
contact stress on the raceway surface at 17,000 min-1
is reduced by approx. 15%, allowing for a 17%
increase in speed.
Subsequently, a test with a built-in motor drive wasconducted. With this drive method, the bearing
temperature is bound to be affected by the heat fromthe motor as the built-in motor is close to the bearings.
To verify this effect, a high-speed test was first
conducted without cooling air. The inner/outer ringtemperature difference at 17,000 min-1 (dmn value
1,530,000) is shown in Fig. 7. In this test, larger
inner/outer ring temperature differences werecompared with no cooling air conditions. Due to these
decreases in inner/outer ring temperature difference,
the maximum contact stress on the bearing raceway
surface at 17,000 min-1 is reduced by approx. 15% at
100NL/min, approx. 20% at 200NL/min, and approx.
25% at 300NL/min. As a result, the contact pressure
at 17,000 min-1 with no cooling air supply and thecontact pressure at 22,000min-1 with a supply of
300NL/min cooling air are equivalent, which allows fora 30% increase in speed.
In either main spindle drive method, the cooling
effects of bearings with air cooling spacers wereverified.
In addition to the test with a spacer width of 22mm
as described earlier, a test with a spacer width of
66mm was also conducted, verifying the cooling
effects 2).

3 .2 Cooling effects of tandem back-to-backarrangement (DTBT arrangement)
Following the evaluation test with DB arrangementin the previous chapter, cooling effects of the bearings
with air cooling spacers under DTBT arrangement was

Cooling air (inlet)

Bearings with
air cooling spacer

Bearings with
air cooling spacer

Built-in motor

Cooling air (inlet)

Supporbearings

Drive
motor

Cooling air (outlet)

Cooling air (outlet)

Cooling air (outlet)

Motor direct drive main spindle

Cooling air (outlet)

Built-in motor drive main spindle

Fig. 5 Test machines for DTBT arrangment

Table 3 Test conditions for DTBT arrangement

Pre-load method
Rotational speed
Lubrication method
Amount of lubricant
LubricantLubrication air flow rate
Cylinder cooling
Spindle position

Inner/outer ring average temperature
difference on the rows B and C ˚CTest bearings

30

φ70 x φ110 x 20
5S-2LA-HSL014 equivalent(Eco-friendly air-oil lubricated angularcontact ball bearings with ceramic balls)
Fixed position pre-loading(Pre-loading ON after built-in)
0〜23,000min-1
Air-oil lubrication
0.03mL/10min
ISO VG32
30NL/minYes, Sync w/room temperature (21 ± 1°C)
Horizontal spindle

No cooling air

25

Cooling air of 100NL/min
Cooling air of 200NL/min

20

Cooling air of 300NL/min

15
10
5
0
-5

0

5,000

10,00015,000

Rotational speed

20,000min-1

25,000Fig. 6 Relation between cooling air amount andtemperature difference between inner ring and outer ring
(Coupling direct motor system, in case of DTBT arrangment)-55-*10E_*09 18/06/13 13:53 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

1,530,000) decreased by approx. 7˚C at rows C and
D. When cooling air of 150NL/min was supplied
between both DB and DT for a total of 300NL/min(Fig. 8 (3)), the inner/outer ring temperature difference
at 17,000 min-1 (dmn value 1,530,000) was verified todecrease by approx. 5.5˚C at row C and approx. 5˚Cat row D.
From the above, this test has verified that rows C
and D, which are mostly affected by the heat producedby the built-in motor, can be intensively cooled down
by concentrating the supply of cooling air between DT,
closer to the built-in motor.

Inner/outer ring temperature difference ˚Cobserved on bearings closer to the motor. Therefore,
methods to cool down row C and row D, whichshowed larger inner/outer ring temperature differences
and higher maximum contact stress on the raceway
surface, were considered.
The relationships between the supply point ofcooling air, supply amount, and inner/outer ringtemperature difference are shown in Fig. 8. Whencooling air was supplied at 300NL/min between DB(Fig. 8. (1)), the inner/outer ring temperature
difference at 17,000min-1 (dmn value 1,530,000)
decreased by approx. 7˚C at row C and approx. 3˚C at
row D as compared to the case where no cooling air
was supplied. When cooling air of 300NL/min was
supplied between DT (Fig. 8 (2)), the inner/outer ringtemperature difference at 17,000min-1 (dmn value

20

4. Fluid analysis result
In the DTBT arrangement of the built-in motor drive
main spindles described in the previous chapter, whencooling air is supplied between DT closer to the built-in
motor, the cooling air is supplied to the space on thefront side of row C and to the space on the back sideof row D. We simulated how the air oil and cooling air
would flow through those spaces with fluid analysis.
Results are shown in Fig. 9.
The cooling air injected from the air cooling nozzle
revolves along the surface of the inner ring spacer inthe direction of spindle rotation, then flows to row D,
which has a larger exhaust space, and finally exhausts
in the spindle’s direction.
Additionally, Fig. 9 also shows the streamline of air
oil injected from the air-oil lubrication nozzle with a redline. We have verified that the streamline of air oilreached the bearing raceway surface on all four rows,
including rows C and D, which are closer to the air
flow nozzle.

Row A Row B Row C Row D
No cooling air at 17,000 min-1
Motor

M

15

10

5

0

Built-in motor drive
main spindle(Reference)
Motor direct drive main spindle

Fig. 7 Temperature difference between inner ring and outerring without cooling air (Coupling direct motor system and builtin motor drive system, in case of DTBT arrangment)

Inner/outer ringtemperature difference ˚C

20

Row A

Row B

Row C

Cooling air of
300NL/min

No cooling air

M15

No cooling air at 17,000 min-1

Row D
Cooling air of
300NL/minM

Cooling air of
150NL/min 150NL/min

M

M

10

5

0

No cooling air

Cooling air of 300NL/min
between DB

Cooling air of 300NL/min
between DT

Cooling air of 150NL/min eachbetween DB and between DT

(1)

(2)

(3)

Fig. 8 Relation between cooling air point and each bearings temperature difference between inner ring and outer ring
(In case of DTBT arrangment)-56-

*10E_*09 18/06/13 13:53 ページ 6

Machine Tool Main Spindle Bearing with Air Cooling Spacer

Air cooling
nozzle5. Summary

Bearing No Bearing No Bearing
Bearing
CoolingA
B
C
D
coolingcooling
air ofair

air

Air-oil lubrication nozzle
for row C

Slow

NTN has developed "machine tool main spindle
bearings with air cooling spacers" with air coolingtechnology. In order to demonstrate its practicality, we
verified the cooling effects by applying this technology
on bearings and operating conditions which arecommonly found in machine tool main spindles, andconfirmed achievement of high speed and high rigidity
at a high level.
This development can contribute to theadvancement of machine tools in any application withregard to higher speed, higher rigidity, and higher
reliability. We will continue working on enhancements
and contributing to performance improvements ofmachine tools.

300NL/minStream line of air-oil injectedrom air-oil lubrication nozzle
for row C

Cooling air flow rate around theinner ring and inner ring spacer surface

Fast

References
1) Shouhei Hashizume: New Technologies on Precision
Bearings for Machine Tools, Machine Design, Vol. 60, No.
1 (Jan issue, 2016) 28.
2) Yushi Onda, Kikuo Fukada, Yohei Yamamoto and Masato
Yoshino: Machine Tool Main Spindle Bearings with Air
Cooling Spacers, NTN TECHNICAL REVIEW, No. 82,(2014) 38.
3) Kenji Fujii, Masatsugu Mori and 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.
4) Yoshinobu Akamatsu and Masatsugu Mori: Minimizing
Lubrication Supply in an Air-Oil Lubrication System, NTN
TECHNICAL REVIEW, No. 72, (2004) 12.

Fig. 9 Cooling air flow on inner ring and inner spacer surface(Fluid analysis results)

Photo of authors

Keisuke NASU

Naoya OKAMOTO

Masato YOSHINO

Industrial Machinery
Engineering Dept.,
Industrial Business HQ

Product Design Dept.
Industrial Business HQAdvanced Technology
R&D Center-57-

*11E_*09 18/06/13 13:54 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]High-performance Sealed Cam Follower

Masato TSUJIHASHI*
Seizo AGATA*

Cam follower is used for guide rail, index machine and so on. In thesemechanisms, cam follower is often mounted outside exposing position. Therefore
cam follower might be exposed to fluid such as cutting oil, and improvement ofseal performance to prevent grease leakage is needed.
This article introduces the feature and the performance of reinforced seal type
cam follower.

1. Introduction

2. Structure of typical cam followers

Cam followers are stud type track roller assemblies
comprised of rollers and thick-walled outer rings. The
outer ring is designed to roll directly on a track
surface. Cam followers are mainly used as eccentricrollers and guide rollers.
Cam followers can be categorized into two groups:
cage type and full-complement roller type. The caged
type is best suited for high-speed operation because
the rollers are guided by the cage. In contrast, the fullcomplement roller type has a larger load rating thanthe caged type and is more suited for high load
(typically low-speed) applications.
Depending on where they are used, the grease
inside the bearings may leak, causing an early failure
due to insufficient lubrication. NTN's cam followers
include a sealed type as the standard offering.
However, the seals do not completely work to prevent
grease leakage as they are provided mainly forblocking dust from entering the bearing.
In this article, we will describe the high performancesealed cam follower developed to cope with thegrease leakage issue.

The structure of typical cam followers is shown in
Fig. 1.
These cam followers consist of a stud, an outer ring,two side plates and several rollers. Since the outerring rolls directly on the track of the customer’s
application, the outer ring is designed with a thickcross-section so that it can withstand impact loading.
In addition, full-complement cam followers comprisedof a cage and full-complement rollers are available inthe same envelope dimensions. Furthermore, rubber
seals are available as an option for these cam follower
bearings. However, keep in mind that the rubber sealof the standard series is for sealing out dust from thebearing and not for excluding liquids from entering thebearing.

Outer ring
RollerSide plate

Stud
Seal

Fig. 1 Structure of cam follower* Industrial Machinery Engineering Dept., Industrial Business HQ-58-

*11E_*09 18/06/13 13:54 ページ 2

High-performance Sealed Cam Follower

3. Usage in machine tool applications
under ambient conditions
Cam followers are frequently used in carry guideand cam drive areas of the indexing cam mechanism
in machine tool applications 1).
Fig. 2 shows a common machine tool application
where cam follower bearings are used (a turretmechanism) 2).

Rust on worn surface
Before test

After test

Wear depth: 0.5 mm

0.5Circled in red:
cam follower locations

Profile of worn
surface shown
below(indicated by
red arrow inphoto)

0

-0.5
0

2

4

6

8

10 mm

(a) StudFig. 2 Example of cam follower for machine tool

Since the cam drive assembly is located in themachine interior, it is directly exposed to cutting oil.
When cam followers are used in this environment,
grease leakage prevention is very important.
Furthermore, for environmental reasons, recently
cutting oil is more commonly using a water base 3).
Water-based cutting oil is more likely to mix withgrease if it enters the cam follower compared toconventional cutting oils. Therefore, now it is evenmore important to stop cutting oil from penetrating
cam followers to prevent lubrication failure which leads
to premature bearing failure.

Before test

Profile of worn
surface shown
below(indicated by
red arrow inphoto)

After test

Wear depth: 0.5 mm
0.05
0

-0.05
0

2

4

6

8

10 mm

(b) Raceway of outer ring

4. Wear condition from cutting oil
penetration
An example of wear when cutting oil penetrated amachine tool cam follower bearing is shown in Fig. 3
The worn bearings were equipped with standard
rubber seals and its design was equivalent to thebearing shown in Fig. 1. When the failed bearings
were inspected, it was revealed that the grease wascompletely gone. It was determined that the internal
grease leaked when cutting oil entered the cam
follower. Once the grease was used up, the oil film
inside the cam follower was no longer maintained,
allowing the components to have metal-to-metal
contact with each other and resulting in severe wear
on both the stud and the outer ring race. Rust wasalso observed on the bearing components. NTN has

Before test

Profile of worn
surface shown
below(indicated by
red arrow inphoto)

After test

Minimal wear

0.05

0
-0.05
0

2

4

6

8

10 mm

(c) Roller
Fig. 3 Example of worn cam follower with greaseleakage-59-*11E_*09 18/06/13 13:54 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

developed a high-performance sealed cam follower toaddress such reduced life issues caused by greaseleakage.

The seals used in the standard cam follower series(hereinafter, conventional seals) are molded
exclusively with rubber, which is not very resistant toexternal pressure and is insufficient to prevent fluid
penetration. Conventional seals are primarily designed
for excluding dust and are sufficient for that purpose.
On the other hand, the seals NTN just developed(hereinafter, developed seals) are designed with acore metal insertion together within the rubber molding
to ensure rigidity of the seal to prevent deformationfrom external pressure. In addition, double lips wereincorporated into the seal structure and the contact
surface was optimized to improve both its antipenetration performance and minimize its operating
torque.

5. High performance sealed cam follower
5. 1 Features(1) Superior sealing performance
Anti-fluid penetration performance: 80% reduction in
fluid penetration into bearing seal compared toconventional seal

(2) Dimensionally compatible
The outer diameter of the new design isdimensionally compatible with the standard series
since only the seal and internal bearing geometry waschanged. Also, common grease is compatible with the
new seal design.

5. 3 Roller follower applications
Roller followers are bearings used in similar
applications as cam followers. The structure ofordinary roller followers is shown in Fig. 6. Roller
followers are cam followers with the stud substituted
with an inner ring and are used when the spindle isprovided by the customer. Since the internal structureis the same as the cam followers, the developed
product described in this article can also be applied toroller follower bearings.

5. 2 Bearing specifications
Structure changes were minimized to ensurecompatibility with the standard series. Specifically, themodifications focused on seal geometry and the seal
contact with the outer ring, stud and side plate. A
comparison of the sealing structure of the current andnew cam follower designs parts is shown in Fig. 4.
The geometry of the bearing areas not shown inthese diagrams is the same as Fig. 1. A picture of a
cam follower with the new seal design is shown in
Fig. 5.

Seal

Outer ringSide plate

RollerInner ring(a) Conventional
seal design(b) New
seal design

Fig. 4 The shape of the seal

Fig. 6 Structure of roller follower

Fig. 5 Cam follower with development seal-60-

*11E_*09 18/06/13 13:54 ページ 4

High-performance Sealed Cam Follower

6. Performance
6. 1 Comparison of anti-fluid penetration performanceFig. 7 and Table 1 show the test rig and test
conditions of the anti-fluid penetration performancecomparison test.
The evaluation method for test performance is tocompare the water content of the internal residualgrease after testing is completed. The results of thistest are shown in Fig. 8 and post-test pictures ofbearings are shown in Fig. 9.
Fig. 8 displays a significant improvement in the antifluid penetration performance. The water content inside
the bearings with newly developed seals was reduced
by 80% compared to bearings with conventional seals.

(a) Conventional product

Drive spindle
Test bearings

Load cell

Load

Load spring(b) Developed productFig. 9 Inside of bearing after the test

LM GuideFig. 7 Test rig

6. 2 Starting torque

Table 1 Test condition

BearingsizeStud diameter

Outer diameter
Total width
GreaseRotational speed
Load
Temperature
Test run time

Water injection
procedure

Conventional
sealed productφ12Newsealed productBecause of the structural change to improve antifluid penetration performance (increased sealinterference), the new cam follower design has anincreased starting torque. To determine the relative
impact of this higher torque, a starting torqueverification test was conducted. The test conditions
are shown in Table 2 and the corresponding test
results are shown in Fig. 10. The developed cam
follower product showed an increase of 30% instarting torque compared to the conventional design.

←
←
φ32
45
48.5
Shell Alvania grease S3Color: light brown
500 min-1 (inner ring rotation)
392Ｎ
Room temperature

30 minutes
Spray 20ml of water with a syringe every
15 seconds, aiming at the contact area
between the seal lip and side plate

Table 2 Test condition
12

Water content %10
8

Conventional seal
New seal
No seal with grease

Stud diameter
BearingOuter diametersize
Total width

6

Grease4Temperature
Load

2
0

Fig. 8 Water content after the test

-61-

Conventional
sealed productNew
sealed product

φ12
φ32

←
←

45

48.5

Shell Alvania grease S3
Room temperature
No load*11E_*09 18/06/13 13:54 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

14
12
Starting torque mN·m

7. Summary

Conventional sealed productNew sealed productBy enhancing the seals of cam followers from thecurrent design to the new design, we could
significantly improve anti-fluid penetration performance.
The new improved seals are able to replace existingseals without increasing the size of the bearing outer
diameter to easily improve bearing anti-fluid
penetration performance. Therefore, we will actively
promote the use of these bearings in customer
applications.

10
8
6
4
2
0

Fig. 10 Test result of starting torque

References
1) Yuichi Danjo, Applications of Cam Followers and Roller
Followers, NTN TECHNICAL REVIEW No. 55 (1989), 6172
2) Satoshi Nakamura, Miniature Cam Followers for Index
Table, NTN TECHNICAL REVIEW No.60 (1992), 67-74
3) Shigehiro Iwata, Eco-Friendly Water Soluble Cutting Oil Development Trends - Journal of the Japan Society for
Precision Engineering, No.68, 7, 2002, 919-922

6. 3 Verification of outer ring slippage
With the verification test mentioned in section 6.2,
an increase in starting torque was confirmed in the
newly developed product. This increased starting
torque can hinder outer ring rotation and cause
slippage with the track surface. Therefore, to verify ifslippage occurs between the outer ring and track
surface during operation, a test was conducted withthe actual load. When the bearing load is 4% or less ofCor, the rolling elements inside the bearing may be
impeded from performing a smooth rolling motion.
Therefore, 0.04Cor was determined to be the practicalminimum load for this bearing and was used as thetest load. The rig used for this test is the same asshown in Fig. 7. No slippage was observed during
testing under the conditions shown in Table 3. This
test result confirmed smooth bearing rotation during
operation under the practical load which determined
there is no outer ring slippage problem with the new
bearing design.

Table 3 Test condition

BearingsizeStud diameter
Outer diameter
Total width

GreaseLoad

Conventional
sealed productNew
sealed product

φ12
φ32

←
←

45

48.5

Shell Alvania grease S3
810Ｎ (0.04C0r)

Test run time

200, 500, 1000min-1
30 seconds at each rotational speed

Verification method

Visual inspection

Rotational speed
Temperature

Photo of authors

Room temperature-62-

Masato TSUJIHASHI

Seizo AGATA

Industrial Machinery
Engineering Dept.,
Industrial Business HQ

Industrial Machinery
Engineering Dept.,
Industrial Business HQ

*12E_*09 18/06/13 13:54 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Introduction of Linear Guides

Masaki KAGAMI*
Keisuke KAZUNO*
Man has moved heavy loads since ancient times using rotation and linear
movement or a combination of both. Linear guides based on these movements
have realized the linear motion with high capacity, compact, long stroke andhigh accuracy.
Linear guides have been used for various applications, for example; machine
tools, assembling machines, automation machines, and have many past
results. This article introduces basic structure and features and applications oflinear guides.

runner block and are sent back to the track surface. In
this way, the balls continue rolling in an unlimited
circular motion, enabling guidance with low friction 3).

1. Introduction
Automation in manufacturing is expanding in order
to increase efficiency and save manpower at
production sites. The guidance mechanisms ofautomated equipment are required to be lightweightand have a high load carrying capacity, high rigidity,
small footprint, high precision, low friction andenvironmental resistance.
Currently, NTN has significant business for linear
guides led by the European market. Recently, we
started production and sales of second generation
linear guides, an improvement on conventional (first
generation) linear guides. Second generation linear
guides have achieved an increased load carrying
capacity, increased life, and lower noise comparedwith conventional products by optimizing the geometry
and surface finish of various components.
In this article, we will describe the basic
configuration, features and applications of our linear
guides 1, 2).

Runner block
Rolling elements (balls)
End plateRail

End seal

Fig. 1 Basic structure of linear guides

2. 2 Error compensation capability
Components and support structures which are used
to install linear guides have deviations in flatness andparallelism. In addition, errors can occur in the
installation of linear guides, as well. These errors canbe compensated for/averaged by using multiple
rails/runner blocks (Fig. 2). In the case of thecommonly used two-rail/two-runner block
configuration, the running accuracy of the movablestage increases 80% compared to the mounting
surface flatness (Table 1).

2. Linear guide basics
2. 1 Basic configuration
As shown in Fig. 1, the linear guide assembly
mainly consists of a rail, runner block, rolling elements(balls), end plate, and end seal. The balls roll between
the rail and runner block track surface and are picked
up by the end plate at the end of the runner block.
Then, they travel through the circular path of the* Robot Engineering Department, Industrial Business HQ-63-

*12E_*09 18/06/13 13:54 ページ 2

NTN TECHNICAL REVIEW No.84（2016）20µm

3. 2 Linear guides with retainer
45

In general, linear guides use a type with balls
almost completely filling the circulation path (full-balltype). In addition, we offer linear guides with a retainer
to hold and separate the rolling elements (Fig. 4).

µm

Mounting
surface A

100µm

Mounting surface
flatness of machine bed(cutting finish-machining)

90µm

Mounting sidesurface C
Mounting sidesurface D

Mounting
surface B[Features of linear guide with retainer]∙ Superior high speed operation∙ Low heat generation∙ Low noise∙ Smooth motion∙ Improved lubrication performance∙ Uniform distribution of load
∙ Long life

Measuring jig

Movable stage

Linear scale

Linear guideRunning accuracy of
installed linear guide

The retainer prevents metal contact between
adjacent the balls (Fig. 5). Compared with the full-ball
type with no retainer, the relative slip speed at the ballcontact surface is 1/2 and the contact pressure is also
lower, resulting in reduced heat generation and wear.
In addition, the retainer has a pocket structure forretaining lubricant, through which lubricant is delivered

Fig. 2 Error compensation
Table 1 Mounting surface flatness and running accuracy
Straightness μm

Mounting
surface
Base
Side

A
B
C
D

Mounting surface
flatness
100
90
20
45

Running
accuracy
17
4

3. Features of linear guides
A wide series of NTN linear guide products forvarious industries and uses are available for users to
select the best specifications for their particular needs.

3. 1 Standard linear guide and miniature linear guide
The linear guide series can be categorized into
standard linear guides and miniature linear guides(Fig. 3). Standard linear guides provide a lineup withrail widths of 15 - 55 mm, and miniature linear guides
provide a lineup with rail widths of 7 - 15 mm. In the
future, we will expand the range of the lineup tosmaller and larger rail widths.

Standard linear guideFig. 4 Linear guides with retainer

Full-ball type(conventional product)

Retainer type

Miniature linear guideFig. 3 Lineup of Linear guides

P = Contact pressure

F = Force between balls

A = Contact area

Fig. 5 Schematic view of the contact surfaces-64-*12E_*09 18/06/13 13:54 ページ 3

Introduction of Linear Guides

3. 3 Interchangeability of linear guides

to the balls to reduce metal wear. This extends thelinear guide life and maintenance interval, as well as
reducing noise during operation (Fig. 6).
The retainer also keeps the distance between theballs constant to achieve stable circulation of the balls(Fig. 7). This reduces variation of running resistanceof the runner blocks, achieving smooth and quiet
operation (Fig. 8) 1, 2).

Various models with different accuracy classes andpre-loads are available depending on operation
conditions and applications, taking into considerationinterchangeability.
Rails and runner blocks of the Standard Linear
Guide Series are interchangeable for precisionclasses of medium through high–precision, andpreload classes of zero through medium. This
improves operability and maintainability. Table 2
shows the combination of accuracy class and preloads and whether they are interchangeable.

80

Noise dB70

LGBXH25FN
Full-ball type60

Table 2 Interchangeability of linear guides

LGBCH25FN
Retainer type

50

Accuracy class

Regular High Precision Super Ultra superSeries class class class precision precision
class
class
(M)
(H)
(P)
(S)
(U)

40
0

25

50

75

100

Speed m/min

125Pre-load

30

Fig. 6 Noise level of linear guides

se

Noi

Full-ball type
C1

Micro gap
(Z0)
Micro pre-load (Z1)
Medium pre-load (Z2) LGB
Heavy pre-load (Z3)
Special pre-load (ZX)
Micro gap
(Z0)
Micro pre-load (Z1) LGM
Medium pre-load (Z2)

○
○
○
△
△
△
△
△

○
○
○
△
△
△
△
△

○
○
○
△
△
△
△
△

LGB: Standard linear guide, LGM: Miniature linear guide○：operable, interchangeable△：operable, not interchangeable
̶：not operable

C2

△
△
△
△
△
̶
̶
̶

△
△
△
△
△
̶
̶
̶

Retainer typeFig. 7 Comparison of the ball distance

Drive resistance N

80

In order to fully take advantage of the linear guideperformance, appropriate lubrication is required. Whenlubrication is insufficient, not only does the resistanceof various components increase, but the life is alsosignificantly shortened.

LGBXH25FNZ1, Speed 0.6 m/s

70
60

Fluctuation range 36%

50

Full-ball
type(conventional
product)

40
30
20

4. 1 Lubricant

10

NTN offers a wide range of lubricants so that users
can select the best lubricant depending on the specificenvironment and condition of operation. The following
is an example of different lubricant types.
∙ Lubricating oil: superior cooling effect∙ Grease: superior attenuation and maintainability
Low viscosity base oil grease is appliedfor high speed operation

0

0

25

80

Drive resistance N

4. Lubrication

50

75

100 125 150 175 200 225 250

Stroke mm

LGBCH25FNZ1, Speed 0.6 m/s

70
60

Fluctuation range 6%

50

Retainer
type

40
30

4. 2 Lubrication method

20

The linear guide is lubricated through grease zerks
or lubrication fittings installed in runner blocks. By
using a grease gun, automatic lubrication dispenser,
or centralized lubrication system, the lubricant can beeasily supplied (Fig. 9).

10
0

0

25

50

75

100 125 150 175 200 225 250

Stroke mm

Fig. 8 Comparing driving resistance-65-

*12E_*09 18/06/13 13:54 ページ 4

NTN TECHNICAL REVIEW No.84（2016）

5. Seal options
A combination of multiple seals can be chosen toachieve the most appropriate sealing depending on
operating conditions. An example of seal options isshown in Fig. 12.
Grease gun

Centralized
lubrication
systemAutomatic
lubrication
dispense

(6)

(5)

(4)

(1)

(2) (3)Fig. 9 Tools for lubrication

4. 3 Lubrication from the top of runner blocks
Lubrication is also possible from the lubrication
adapter on top of the end plate by attaching a
dedicated adapter to the runner block (Fig. 10).
(2)
(3)
(1)

(1) End seal
(2) Inner seal
(3) Side seal(4) Multi-layer seal
(5) Double seal(6) Metal scraper(1) O-ring _1(2) O-ring _2(3) Adapter

Fig. 12 Sealing options

6. Application to linear modules
Linear modules incorporate linear guides for highload capacity and long stroke applications (Fig. 13,
Fig. 14) 4).
Fig. 10 Lubrication adapter

4. 4 Lubrication system LU1The lubrication system LU1 is capable of lubricating
the track surface of the rail when installed on the end
plate of the runner block (Fig. 11). Since it provides
continuous lubrication, maintenance free operation
can be achieved for an extended period.
For regular use, no oil supply is required for LU1.
However, if oil shortage is a concern, such as for use
in severe environments, it is also possible to re-supply
oil from the top of the cover. In addition, even when
LU1 is installed, grease zerks and lubrication fittings
can also be directly installed 1, 2).

(1)

(2)

Runner block
Rail

Fig. 13 Linear module with linear guides

(4)

(3)

(1) Lubricant supply felt
(2) Oil absorber(3) Cover(4) Frame

Fig. 11 Structure of LU1

Fig. 14 High load capacity linear module for lifting axis

-66-

*12E_*09 18/06/13 13:54 ページ 5

Introduction of Linear Guides

Table 3 Main components

For long stroke applications exceeding 10 meters inlength, shipping to the installation site may become
problematic. In such cases, the linear module can bedivided into pieces of several meters for transportationand then connected at the installation site. By using
linear guides which are pre-cut at a predetermined
length, the linear module can be easily
divided/connected, making it possible to shorten thetime for installation work at the site.
In addition, all linear guides used in linear modules
come with retainers. Adoption of linear guides withretainers allows quiet, long-life and long maintenancefree operation.
Linear guides are currently used in various
automation facilities in automobile and industrial plants
by leveraging these features.

[Rail]

Appearance

Feature·Integrated magnetic scale(with protective cover)·Rail length: up to 2,500mm·Wireless power supply
capability (transmitter)[Sensor unit]·Built-in detection sensor head
·Positioning signal output(wireless)·Wireless power supply
capability (receiver)[Receiver unit]·Positioning signal reception(wireless)·Signal output to controller·Power supply to rail

7. Wireless linear scale system
Direction of motion

Rail

Linear scales are often used for devices that requireprecise positioning, such as semiconductor test
equipment and machine tools. Currently, we aredeveloping a "wireless linear scale system" which
combines linear guides and sensor units (Fig. 15) 5).

Magnetic scale

Runner block
Sensor unit
Controller
Power supply
Receiver unit

Power supply (wired)Power supply (wireless)Positioning signal (wired)
Positioning signal (wireless)Fig. 16 System model

runner block, adjustment work is not required. Labor
can be significantly reduced compared with ordinary
linear scale installation work.
As the sensor unit is capable of wireless signal
output and wireless power supply –signal/power
cables are not required (Fig. 17).

Fig. 15 Wireless measuring linear system

7. 1 System configuration
Key components of the wireless linear scale areshown in Table 3 and the system model is shown in
Fig. 16.

[Features]∙ Compact∙ Easy assembly∙ No cables are required for the sensor unit
∙ Cover to prevent foreign objects from entering

7. 2 System features
The wireless linear scale system consists of a railwith a magnetic scale installed on the side and asensor unit connected to the runner block. As opposedto the conventional linear scale system which requires
a significant space for installation, this system hasachieved compact size by integrating the scale andsensor within the linear guide.
Since the gap between the detection sensor headand magnetic scale is adequately controlled by theContamination of the magnetic scale is prevented
by the protective cover. In addition, sensor units andrunner blocks can be equipped with the seal option
shown in Fig. 12. With these measures, even if foreign
objects are attached to the rail, they can be prevented
from entering the sensor unit.
-67-

*12E_*09 18/06/13 13:54 ページ 6

NTN TECHNICAL REVIEW No.84（2016）

Runner block

8. Summary

Communication module
Power supply module

In this article, we discussed the basic structure,features and applications of linear guides. We will
strive for their introduction into global markets forautomation and labor saving at production sites.

Protective cover

Magnetic scale

References

Protective cover

1) NTN Linear Guide Catalogue, CAT No. 6018/J
2) NTN-SNR, Linear Guide Catalogue, (2015)
3) Mitsumasa Wada, Linear Guide for High Load, THE
TRIBOLOGY No.342, (2016) 16-18
4) NTN, Linear Module Catalogue, CAT. No. 6107-II/J

Detection sensor headFig. 17 Inside structure of wireless sensing unit

Photo of authors

Masaki KAGAMI

Keisuke KAZUNO

Robot Engineering
Department,
Industrial Business HQ

Robot Engineering
Department,
Industrial Business HQ-68-

*13E_*09 18/06/13 13:55 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Plastic Sliding Screws

Naonari TANIGAWA*
Norio ITOU**
Tomomi TONOMURA*
Plastic parts have high design freedom and high corrosion resistance,
can carry out weight saving and compactification and are low in cost.
Therefore, plastic parts substitute metal parts in many industrial fields.
Plastic sliding screws (plastic nut and metallic screw shaft) are one of them,and these are widely used for food machinery and medical machinery parts.
This article introduces features and performance of plastic sliding screws.

have a lower permissible load capacity but are well
suited for broad ambient conditions, such as invacuums and high temperatures. In addition, they are
quieter and inexpensive. Furthermore, they can
reduce weight and size. However, corrosion
resistance and heat resistance vary depending on thematerials for screw shafts and nuts; therefore,
appropriate selection based on their application isrequired. In this article, we will discuss NTN's plastic
sliding screws (hereinafter, sliding screws) and NTN's
high-load plastic sliding screws (hereinafter, high-loadsliding screws) with increased permissible load
capacity 1, 2).

1. Introduction
Feed screws, which convert the rotating motion of
motors to linear motion, are used in the load carrying
mechanism of industrial machines, food machines,
medical devices, etc. Feed screws are categorized
into ball screws and sliding screws. Sliding screws
with copper-based nuts are used with grease. On theother hand, screws with plastic-based nuts can beused without lubrication.
Table 1 shows the comparison of the performanceof NTN's plastic sliding screws with othermanufacturers' sliding screws and ball screws. Ballscrews are characterized by large permissible loadand screw efficiency; however, they are expensive.
Since they are lubricated with grease, these screws
are not suitable for use in environments where grease
performance deteriorates, such as under water or inhigh temperatures. In contrast, plastic sliding screws

2. Sliding screws
Sliding screws consist of plastic nuts and stainlesssteel screw shafts, as shown in Fig. 1. Table 2 shows
the materials and the specifications.

Table 1 Comparison of features various sliding screws
Item

Sliding screw

NTN plastic
oil-impregnated POM Ball screw
sliding screw by another manufacturer
Not required
Operablewith lubricant

Not required
Operablewith lubricant

RequiredGrease△×

Screw accuracy
Screw efficiency

○

△〜○

◎

○

△〜○

◎

Noise
Heat resistance

◎

○

△

◎

△

△

Lubrication
Corrosion resistance◎Screw shaft

Plastic nut

Fig. 1 Plastic sliding screw** Engineering Dept. NTN Engineering Plastics Corporation** Planning Dept. NTN Engineering Plastics Corporation-69-

*13E_*09 18/06/13 13:55 ページ 2

NTN TECHNICAL REVIEW No.84（2016）

The plastic nut (BEAREE AS5000) is based onpolyphenylenesulfide (PPS) and blended with special
fluorocarbon resin and solid lubricants to improve
friction/wear resistance properties. As the impact
resistance is also improved, they are more durable
than regular plastic sliding screws.
Fig. 2 shows the comparison of properties of
BEAREE AS5000, which is the material for our plastic
nuts, and commercially available PPS-based sliding
material with fluorocarbon resin. BEAREE AS5000 isparticularly superior in wear resistance and friction
coefficient, in addition, improvement is made to impact
strength, which is a weak point of PPS. Fig. 3 shows
the cross section SEM image. The improved propertyof BEAREE AS5000 against commercially available

PPS with fluorocarbon resin was made possible by thethree dimensional presence of the special
fluorocarbon resin in the molding.
A comparison test of our sliding screws against
other manufacturers' sliding screws was conductedunder the conditions listed in Table 3 up to a travel
distance of 200 km. In general, an increase of axial
clearance of up to 0.3 mm due to operation isconsidered to be permissible. As shown in Fig. 4, theincrease of the axial clearance of the sliding screw ofanother manufacturer exceeded 0.3 mm with a travel
distance of around 70 km. In contrast, the increase of
the axial clearance of our sliding screw was 0.2 mm or
less, even after a travel distance of 200 km, whichindicated it was still usable for continuing operation.

Table 2 Specification of our plastic sliding screws

Table 3 Test condition of wearing test

Material
BEAREE AS5000

Screw shaft

SUS304 (thread rolling)

Cumulative lead error

Ct10（JIS B 1192）±0.21／300ｍｍ

Superior

4

10Elongation

Elastic coefficient

Large

High

Strength

Commercially available
fluorocarbon resin (granular)
600µm

BEAREE AS5000

0.5

Sliding screw fromanother manufacturer

0.4
0.3

NTN's plastic sliding screw

0.2
0.1
0

0

50

100

150

200

Fig. 4 Relationship between axial clearance increase
and nut motion distanceFig. 2 Features of BEAREE AS5000

100µm

0.6

Travel distance km

High

Special PTFE
(fibrous form)

None (dry friction)[Typical technical data]

Friction coefficient

2

100Ｎ
500min-1

Rotational speed
Lubrication

Low

3

6

Number of threads
Axial load

Commercially available
PPS sliding material withfluorocarbon resin"

5Impactstrength

24ｍｍ

BEAREE AS5000

Wear resistance
High

8ｍｍ

Nominal lead

Increase of axial clearance mm

Accuracy class

Screw nominal diameter

UsablePart
Plastic nut

Commercially available PPS
with fluorocarbon resin

Fig. 3 Cross section SEM images-70-

*13E_*09 18/06/13 13:55 ページ 3

Plastic Sliding Screws

Table 5 shows comparison of load carrying capacity
and Fig. 7 shows variation of shaft surface
temperature during operation. The plastic nut material
used in both nuts is BEAREE AS5000 with improved
friction/wear resistance. However the high-load sliding
screw uses thin-walled composite molding which
provides better heat dissipation; therefore, thetemperature rise at the screw surface is approx. 1/4 ofthe sliding screw.
In addition, the permissible axial load capacity of thehigh-load sliding screw is double and the static fractureload is nine times higher than the sliding screw, due tothe composite molding effect.

3. Examples of applications
The sliding screws are adopted by medical devices
and food machines since they can be used with no oil
or grease lubricant. Table 4 shows a record of actualuse. Among others, Fig. 5 shows an example ofapplication in food fryers and noodle boilers.
Table 4 Applications
Field
MedicalName of device

Adopted part

Vision tester

Focusing XY axis

Analysis equipment

Specimen carrying drive shaft

Pump actuator shaft
Dialyzer
Elevator drive shaft
Anesthesia apparatus
Cup carrying drive shaft
Vending machine
Foodprocessing
Fryer/noodle boiler elevator
Processing machinemachinePacking elevator
Wrapping machine
Semiconductor
manufacturing equipment Substrate positioning driver
Other
Electronic microscope Specimen positioning XY axis

Composite nut
BEAREE AS5000
Aluminum basematerial

Screw shaft

Food fryer elevatorsliding screw

Composite nut

Fig. 6 Plastic sliding screw for heavy condition
Converts rotating motion
from motor to linear motion

Table 5 Comparison of load limit
Sliding screw

Permissible axial load

High-load
sliding screw

600N

1,500N

Static fracture load of nut

2.7kN

24kN

Item

Fig. 5 Application for food fryer

Surface temperature ofscrew shaft
˚C

400

4. High-load sliding screws
Since the sliding screws use plastic nuts, there wasa risk of flange failure or thread wear when the load
significantly exceeded the permissible load capacity.
However, lubrication-free and low-noise operation is
also required for high-load applications, which havebeen addressed by sliding screws for low-load
applications. In order to address this requirement,
NTN has developed high-load sliding screws withmetal composite material as shown in Fig. 6.
BEAREE AS5000, which was discussed in Section
2, is used for injection molded threads on top of thealuminum-based nut, as a composite nut, to improve
permissible load capacity and heat dissipation.
The aluminum is processed with special etching to
improve adhesiveness with BEAREE AS5000.

Sliding screw
High-load
sliding screw

300
200
100
0

0

5

10

15

Nut traveling distance m[Screw test condition]

Screw specification: shaft diameter: 12mm, lead: 2mm, thread: 1
Load: 600N, Rotational speed: 200 min-1, room temperature,no lubrication"

Fig. 7 Surface temperature of screw shaft

-71-

20

*13E_*09 18/06/13 13:55 ページ 4

NTN TECHNICAL REVIEW No.84（2016）

nuts was also observed. The result is shown in Fig. 10- 12. In all of these exposure tests, reduction ofadhesiveness was 5% or less. In addition, no
abnormality was found, such as rust, in theappearance of the nuts after exposure tests.

Next, Fig. 8 shows the increase of axial clearance
after operation.
The high-load sliding screw has less wear, approx.
1/3 of the sliding screw. Since the rise in surface
temperature is small, the elastic coefficient of the
BEAREE material is maintained and reduction offrictional shear is small. Therefore, there is little change
in screw efficiency with good friction properties.
Adhesiveness was evaluated during theenvironmental resistance test under the exposure
conditions in Table 6. Adhesiveness was tested bymeasuring the maximum load when only BEAREE
AS5000 was shorn (Fig. 9). In addition tomeasurement of adhesiveness, the appearance ofAdhesion strength MPa

10Measured data
Average

6
4
2

0

100

200

0.15

400

500

Fig. 10 Adhesion strength variation in high temperature
exposure test

0.10
0.05
0.00

300

Time h10High-load
sliding screw

Adhesion strength MPa

Increase of axial clearance
mm

8

0

0.20

Exposure to hightemperature (150˚C)

Plastic
sliding screw[Screw test condition]

Screw specification: shaft diameter: 12mm, lead: 2mm, thread: 1
Load: 400N, Rotational speed: 100 min-1, room temperature,no lubrication, nut travel distance: 0.7km

Fig. 8 Axial clearance increase

Exposure to constanttemperature/humidity(80°C, 95%RH)

8

4
2

0

100

Table 6 Test condition of environment resistanceCondition
150˚C×500hrs

Exposure to constant temperature/humidity

80˚C, 95％RH×500hrs

Immersion in hot water

90˚C×500hrs

200

300

Time h400

500

Fig. 11 Adhesion strength variation in temperatureand humidity controlled test

10

Adhesion strength MPa

Item

Average

60

Exposure to high temperature

Measured data

Shear direction

Test jig
Aluminum basematerial

Average

6420BEAREE AS5000

Measured data

Immersion in hot water

8

0

100

200

300

Time h400

500

Fig. 12 Adhesion strength variation in hot water test
Fig. 9 Schematic of adhesion strength test-72-

*13E_*09 18/06/13 13:55 ページ 5

Plastic Sliding Screws

Permissible axial load N

Fig. 13 shows the relation between the permissible
axial load and rotation speed of sliding screws andhigh-load sliding screws. A test of 20 sets was
conducted, defining 200 times of back and forthmotion under a predetermined rotational speed andload as one set (the screws were cooled down to room
temperature by stopping the motion between each set
with a total of 2,000 times of back and forth motion).
The permissible axial load was determined as the
maximum load before the increase of axial clearance
reached 0.1 mm without nut fracture. The high-loadsliding screw had a permissible axial load twice ashigh as the sliding screw.

1,800

5. Conclusion
In this article, we have discussed NTN's plastic
sliding screws which convert the rotating motion of
motors into linear motion in lubricant (oil or grease)
free operation, and NTN's high-load plastic sliding
screws with high permissible load capacity.
Market needs for lightweight and small sliding
screws will grow further in many fields, including
medical devices and food processing machines. NTN
will continue addressing those requirements byoffering products to contribute to customer productdevelopment.

References
High-load sliding screw

1,500

1) Takuya Ishii and Yoshihide Himeno, Functional
Improvement of Resin Sliding Bearing Through e
Combination with Metal, Journal of the Japan Society ofPolymer Processing, 25 (2013), 2, 73
2) Yoshio Oki, Product Introduction and Complex
Technology of Resin, Sintered Metal and Magnet for
Growth Markets, NTN TECHNICAL REVIEW No. 82(2014), 12

Sliding screw

1,200
900
600
300
0

0

100

200

300

400

Rotational speed min-1[Screw specification]screw shaft: 12mm, lead: 2mm, 1 thread[Test condition]
Room temperature, no lubrication (dry), stroke ±1,180mm,
A test of 20 sets was conducted defining 200 times of backand forth motion under the predetermined rotational speed
and load as one set (screws were cooled down to room
temperature by stopping the motion between each set with a
total of 2,000 times of back and forth motion)
The permissible axial load was determined as the maximum
load before the increase of axial clearance reached 0.1mm
without nut fracture.

Fig. 13 Speed and axial load limit

Photo of authors

Naonari TANIGAWA

Norio ITOU

Tomomi TONOMURAEngineering Dept.
NTN Engineering Plastics
Corporation

Planning Dept.
NTN Engineering Plastics
Corporation

Engineering Dept.
NTN Engineering Plastics
Corporation-73-

*14E_*15 18/06/13 13:56 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Technical Paper ]Strategy of Rolling Contact Fatigue Life Testing
and Interpretation of Life Data
Takumi FUJITA*

0.0020

Probability g(x)0.00180.0016e=0.5

0.00140.00120.00100.0008

e=1.0

e=2.0

0.0006e=3.0

Rolling contact fatigue (RCF) life testing is essential to evaluate
rolling bearing performance. Generally, the probability density
0.0000
distribution of RCF lives follow a Weibull distribution, and its wide
0
500
1000
1500
2000
2500
3000
Time hvariation in life data is common. Thus, the test design and resultinterpretation without statistics are highly likely to lead toincorrect conclusions for rolling bearing performance. This report introduces statistical procedures of the test
design and result interpretation in two types of RCF testings which are called "fixed time test" and "accelerated
test". In addition, several calculations applied in the procedures are explained.
0.00040.0002that there are no performance issues if no failures areexperienced for a predetermined time or loading
cycles, and accelerated tests are defined as tests tocompare performance from the life estimated by the
Weibull plots. The details of calculations are shown in
the appendices.

1. Introduction
The life data of rolling bearings (hereinafter,"bearings") is known to follow the Weibull distribution 1, 2).
Fig. 1 shows the results of calculation of the ratio of
90% life over 10% life from the cumulative distributionfunction of the Weibull distribution for each shapeparameter value (hereinafter, "Weibull slope"). Since
the Weibull slope indicating variation of bearing life
data is usually around 1.1 (10/9, 9/8) 1-3), it is not
exceptional if the life test of bearings contains datawith a difference of around 16.5 times within 10 life
data. As it is clear from this fact that the life data of therolling contact fatigue life test (hereinafter, "life test")
has significant variation, the planning andinterpretation of the life test needs to be based on
statistical evidence to avoid the risk of deriving awrong conclusion.
Therefore, the planning and data interpretation oflife tests are of high interest for engineers andresearchers who are engaged in the actual life tests.
This paper discusses how to proceed with truncatedtests and accelerated tests in the form of case studies.
Here, truncated tests are defined as tests to decide

1000

L90 / L10

100

10

1

0

1

2

3

Weibull slope

45Fig. 1 Relationship between L90/L10 and Weibull slope* Advanced Technology R&D Center-74-

*14E_*15 18/06/13 13:56 ページ 2

Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data

Appendix 2. If it is probable to meet the target life andthere is room for the extended test period, the test cancontinue with the remaining undamaged test bearings.
The equation in Appendix 2 can also be used to set
the truncated life of the remaining bearings.

2. Truncated test
2. 1 Truncated test procedure
Fig. 2 shows the truncated test procedure. In thetruncated test, test condition and target life (10% life or
50% life) are determined first. Then, the number oftest samples to be provided and the truncated life to
be set for guaranteeing the target life are examined.
The relationship between the number of test samples
to meet the target life and the truncated life can beobtained by using the equation shown in Appendix 1.
There are other factors to be considered, such as the
restriction of test period and number of testers, fordetermining the actual number of test samples andtruncated life.
After the test starts, if the test samples survive until
the end of the predetermined truncated life, the test
bearings are considered to have passed, but if failures
are encountered, it is necessary to consider whether
the test should continue. From the viewpoint ofefficiently reducing the test time, it is recommended to
set the number of test samples to 6 or more, ordesirably, 9 or more 4). For deciding if the test should
continue, probability of meeting the target life bycontinuing the test with the remaining undamaged test
bearings, as well as if there is enough room forextending the test period are examined. The
probability can be obtained by using the equation in

2. 2 Truncated test case study No. 1
Case study 2-2-1:
A request was made to conduct a test over 4
months (approx. 3,000h) for guaranteeing that
10% life of test bearings is equivalent to or better
than 10% life of the volume production bearings(the Weibull slope is assumed to be e=1.85,
L10=1,600h) using three test bearings. Five testers
are available for 5 months (approx. 3,600h).
The following is an explanation of how to proceedwith the truncated test under the conditions of casestudy 2-2-1. First, use the equation (A1.6) of Appendix
1 5) to calculate the truncated life T for guaranteeingthat 10% life is 1,600h or more when the number oftest samples is three, as follows:

T = (1600−0) .

(ln (1−0.01.90)
3ln (1−0.01.10))

1/1.85+ 0 =4680

Determine test condition and target life
Set number of test samples and truncation life

Longer than truncation life

PassedStart test

Failure before truncation life

Probability of meetingthe target life is examined

Failure before truncation life

Probable to meet the target life

Cannot meet the target life

FailedRe-determine the truncation life
No problem withthe test period

Start test
Longer than truncation life

PassedFig. 2 Procedure in fixed time test-75-

*14E_*15 18/06/13 13:56 ページ 3

NTN TECHNICAL REVIEW No.84（2016）spread sheet programs (solver function in the case of
Microsoft Excel®).
In this calculation, the truncated life with theremaining three test bearings is 4,319h. Negotiations
with the relevant parties is required to determine if thisextended test period is allowed.

Where, the location parameter (hereinafter,"minimum life") is assumed to be 0; however, from theresult of enormous number of past experiments 6-9),
the relational expression between 10% life andlocation parameter 10) can also be used for theassumption of the minimum life value. In addition, theWeibull slope which indicates variation of tests should
also be assumed; however, the actual value from past
results can be entered.
In this calculation, the truncated life to fulfill thetarget life of 1,600h with a confidence level of 90%
was calculated to be 4,680h, when the test variation
was assumed to be the same as the past result(Weibull slope of 1.85) and the number of test
samples was three. Since 4,680h exceeds therequired test period of 3,000h, measures to shorten
the truncated life must be taken. If the number of test
bearings can be increased, the truncated life can be
reduced since there are additional testers available. If
the test samples are increased to five, the truncatedlife can be reduced to 3,551h. In the following, we
assume that the test period can be extended to 5
months (3,600h) and the following case study 2-2-2 isexamined.
The reason why we assumed a confidence level of
90% in the above calculation is that, since general
textbooks of reliability engineering 11-13) do not present
χ, F, t distribution table below a confidence level of
90%, we believed that the confidence level of 90% orabove was common practice. In this paper, theconfidence level is set to 90% hereafter.

2. 3 Truncated test case study No. 2
Case study 2-3-1:
The test started with the conditions of casestudy 2-2-1 and a sample failed at 800h, which isshorter than the truncated life of 3,551h.

The following discusses how to examine thepossibility of guaranteeing the target life by continuing
the test under case study 2-3-1. In addition, a method
for setting the truncated life of the remaining test
bearings required for guaranteeing the target life isalso discussed in case the test is to continue.
Table 1 is the calculation result of probability thatcannot guarantee the target life from the life data of
the first broken test bearing using the equation(A.2.14) in Appendix 2. From the table, it is revealed
that, if a test sample fails at 800h, the lot cannot meet
the target life with an 80 to 90% probability.
If, even under this situation, a decision is made tocontinue the test, the truncated life of the remaining
test bearings can be calculated as follows (5,030h)
using the equation (A.2.14) in Appendix 2 therefore,
the test will continue for that duration, if it can beaccommodated.

Case study 2-2-2:
The test started with the plan of five test
samples and a truncated life of 3,551h. When thetest time reached 2,000h, two testers failed. The
test continued with the remaining three testers as
there were no other alternatives.

・Calculation ofαα= 5400[(ln (1−0.01×90)) ()

2000−0 1.85
T −0 1.85
2+3 R
1600−01600−0

1600

0

1.85

・Calculation of truncated life of the remaining test
bearings

In the following, the truncated life for guaranteeing
the target life with the remaining three testers is
calculated under the conditions of case study 2-2-2.
Using the equation in Appendix 1, the truncated life
TR was obtained as shown in the following:

1−0.01×10 = exp

−
(
) ｝
α
｛

0.01.10 = 1−exp −1.85

Ti +1 = 0 + 5400 .

ln

[ −B
1

-1

]

1
= 5030(0.01. 90,2,5−2+ 1)

The above re-calculation of truncated life does notconsider the life data of the failure at 800h. If that data
should also be considered for the calculation oftruncated life, Nelson's calculation can be used 14).

]

TR = 4319
TR can be solved by using a non-linear equation
with a convergence calculation function of ordinary-76-

*14E_*15 18/06/13 13:56 ページ 4

Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data

Table 1 Confidence levels to determine if the required
L10 (1,600h) cannot be achieved. These are calculated
from life data for the bearing which is firstly failed before
designated suspension time without failure.
Life of first failed bearing(h)Confidence level
(%)

1,579

60

1,011

80

test period, etc., the number of test samples isdetermined by considering all of the above. When thelife test is started and life data at multiple levels are
obtained, the respective lives can be estimated by the
Weibull plot. The decision for significant difference
between the estimated lives of two levels (significancetest) can be made by the calculation result from themethod in Appendix 3, or in a simplified manner, byusing approximation equations (A4.1) to (A4.3) shownin Appendix 4. In addition, it is recommended that theinterpretation of the significance test be made basedon the rule shown in Appendix 5.

70

1,301

90

675

95

459

99

193

3. 2 Accelerated test case study

3. Accelerated test

Case study 3-2-1:
The following data was obtained from the life
test with seven test bearings made of material A
and seven test bearings made of material B(Table 2).

3. 1 Accelerated test procedure
Fig. 3 shows the accelerated test procedure. Also inaccelerated tests, the test conditions and number oftest samples must be determined first. In order todetermine the number of test samples, it is necessary
to identify a sufficient level of test samples and clarify
the criteria used. For example, if a specific criteria forreliability is clarified, such as, "I want to set the
number of test samples to secure reliability level thatcan determine a significant difference with a 90%
confidence level, when there is a difference of about
double in the estimated 50% life", the number of test
samples can be calculated by assuming test variation(Weibull slope) 5).
The relation between the number of test samples
and the life ratio to determine the significant differencein estimated life at an arbitrary percent point(hereinafter, "significant life ratio") can be obtained byusing the random number simulation in Appendix 3.
In the actual situation, since there are restrictions in
the number of available test samples, usable testers,

Table 2 Life data for material A and B
Life data (h)Material A

Material B

24

45

20
31

15
80

42

100

65

180

52
79

140
220

The following describes the significance test
procedure and interpretation of the significance test
results of the life data. Fig. 4 shows the Weibull plotsof material A and material B. The Weibull slope ofmaterial A is larger than material B, indicating that thevariation of life data is small. In addition, there are no
significant differences between the lives of material A
and B for 10% life; however, the life of material A isshorter than material B for 50% life. The significancetest by random number simulation in Appendix 3 ismade for the life data of these two levels. In thiscalculation, the Weibull slope of 2.07 and 1.15 wereassumed, which are the estimated values of material
A and B, respectively, and a confidence level of 90%
was used. Significant life ratios at the percent points
10 and 50 are 5.39 times and 2.12 times, respectively.
The ratio of 10% life and 50% life of material A andmaterial B are 1.07 and 2.21, respectively, which aresmaller than the significant life ratio of 5.39 for 10%
life and larger than the significant life ratio of 2.12 for
50% life. Therefore, the decision of significant
difference is "not significantly different" for 10% life

Determine test condition andnumber of test samples
Start life test

Estimate life by Weibull plots of two levels of dataSignificance test of two levels of life estimation
Fig. 3 Procedure in accelerated test-77-

*14E_*15 18/06/13 13:56 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

The significant life ratio obtained from theseapproximation expressions are 4.48 times for 10% lifeand 1.94 times for 50% life, which is interpreted as"not significantly different" for 10% life and"significantly different" for 50% life, which is the sameas the result obtained from the calculation in
Appendix 3.

：Material A

99.0
e=2.07, L10 =17.3, L-50=43.0
90.0 L63.2 =51.3

F (t) %

50.0
：Material B
e=1.15, L10 =18.5,
L-50= 95.2, L63.2 =131

10.0
5.0
1.0
0.5
0.1

4. Conclusion
1

10

Time

h

100

1,000

In this paper, the procedures of the truncated test
and accelerated test in rolling contact fatigue life tests
were discussed. In the rolling contact fatigue life test, it
is important to plan the test and process the life data
based on the statistical evidence because variation oflife data is significant. However, it is not easy for
beginners who are not familiar with the data that
follows the Weibull distribution. In this paper, planningof the rolling contact fatigue life test and interpretationof the results are described with the specific casestudies so that even beginners can conduct the tests. I
hope that this paper serves as a good reference forengineers and researchers who are engaged in rolling
contact fatigue life tests.

Fig. 4 Weibull plots for material A and B

and "significantly different" for 50% life. However, thisresult is not contradictory. This result can be said,"comparing the lives of material A and material B,
there is no difference in frequency of short life data
rarely observed at 10%; however, the average life ofmaterial A is shorter than material B." Since thissignificance test result is as shown in Fig A5.1 (b) of
Appendix 5, the decision "the life of material A isshorter than material B" is considered to bereasonable.
When the approximate expressions (A4.1) and(A4.2) in Appendix 4 are used for the significancetest, the Weibull slope of 1.61, which is an average of
two levels of Weibull slope, should be assumed.
The following shows the significant life ratio
calculated by using the equation in Appendix 4.
・Significant life ratio for 10% life

L10R = 1+[

10.007474｛ ln9.106 (1.051−0.01.90)7 ｝3.151

1/1.61]= 4.48・Significant life ratio for 50% life

L50R = 1+[

10.009349｛ ln36.74 (1.051−0.01.90)7 ｝3.108

1/1.61]= 1.94-78-

*14E_*15 18/06/13 13:56 ページ 6

Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data

Reference10) ISO/TS16281, (2006).
11) For example, Jiro Yamauchi: Statistical Tables,
Japanese Standards Association, (1986) 241.
12) For example, Shigeichi Moriguchi: Quality Control
Course: New Edition Statistical Method Rev,
Suurikogakusha, (1999), 262.
13) For example, Shigeichi Moriguchi: New Edition JUSE
Numerical Tables, JUSE Press Ltd., (2000), 5.
14) W. Nelson: Weibull analysis of reliability data with few or
no failure, Journal of Quality Technology, 17 (1985) 140.
15) W. Nelson: Analysis of Life Data, JUSE Press Ltd.,(1988) 272.
16) R. B. Abernethy, The New Weibull Handbook 5th edition,(2006) chapter 6-15.
17) R. B. Abernethy, The New Weibull Handbook 5th edition,(2006) chapter 6-19.
18) Takumi Fujita and Toshihiko Sasaki: Significance Test of
Rolling Contact Fatigue Life Estimated by Weibull Plots,
Tribologist, 59, 10 (2014) 667-673.
19) L. Johnson: The Statistical Treatment of Fatigue
Experiments, Elsevier (1964) 37.
20) M. Matsumoto & T. Nishimura: Mersenne twister: A623dimensionally equidistributed uniform pseudorandom
number generator, ACM Transaction on modeling andcomputer simulations special issue on uniform random
number generation, 8 (1998) 3.
21) For example, Tetsuaki Yotsuji: Probability Distribution
Random Number Generating Method for Calculator
Simulation, Pleiades Publishing House, (2010) 53.

1) G. Lundberg, & A. Palmgren: Dynamic capacity of rolling
bearing, Acta Polytechnica 7, Mechanical Engineering
Series, 1, 3 (1947).
2) G. Lundberg, & A. Palmgren: Dynamic capacity of roller
bearing, Acta Polytechnica 96, Mechanical Engineering
Series, 2, 4 (1952).
3) Shigeo Shimizu: Introduction to Reliability Design for
Machinery, Surikogakusha, Tokyo, (2006) 36.
4) Takumi Fujita: Design method for life test to achieve bothspeed and reliability and statistical test method for test
results (2nd Report) - Consideration for points toremember in analysis of life data that follow the Weibull
distribution - ,Rolling Elements Technical Committee of
the Japan Society for Precision Engineering, Proceedingsof Academic Frontier Symposium of Meiji University,(2010).
5) T. Fujita: Rolling Contact Fatigue Life Test Design and
Result Interpretation Methods Maintaining Compatibility of
Efficiency and Reliability, J. ASTM international, ASTM
STP 1524, 7 (2010) 179-198, Paper ID JAI102492.
6) T. Tallian: Weibull distribution of rolling contact fatigue lifeand deviations therefrom, ASLE Trans., 5, 1(1962) 183.
7) B. Snare: How reliable are bearings, The ball bearing
journal, 162(1970) 3.
8) Masao Kuroda: Load rating and life of rolling bearings,
Lubrication, 17 (1972) 569.
9) Junzo Okamoto, Takeo Yoshioka, Takashi Fujiwara,
Kiyoshi Fujita, Tokio Kitahara and Shizuo Koizumi:
Experimental study regarding the minimum life of rolling
bearings, Lubrication, 22 (1977) 307.

Photo of author

Takumi FUJITA
Advanced Technology
R&D Center-79-*14E_*15 18/06/13 13:56 ページ 7

NTN TECHNICAL REVIEW No.84（2016）

Arranging equation (A1.4) with relational expression(A1.5), equation (A1.6) can be obtained.

Appendix 1
Equation to calculate the relation between
truncated life and number of test samples
when there are no failures
If, from the population of the cumulative failure
probability F(x) , N test bearings are extracted, theprobability Pk that the life of k bearings among themfall below the time T (or loading cycles) follow binomial
distribution of (A1.1)15).
Pk =
=

N!
k! (N−k) !
N!
k! (N−k) !

F k (T ) (1−F (T ) )R (T )N−k

T1 = (Ln−γ) .
Where,
n : n of n% life

N−k

Where,F (T )：cumulative failure probability of the Weibull distribution
R (T )：cumulative probability of survival, F (T )=1−R (T )Therefore, the probability that the number of test
samples with life that falls below time T becomes 0
among N samples (none in N samples) can be
expressed with k =0 , by equation (A1.3) using
cumulative distribution function F(x) of the Weibull
distribution of equation (A1.2).

N

Π
i =1

[ ｛ （ α ）｝]｛ （ α ）｝x−γ e

= exp −x−γ

N!
0! (N−0) !

N−0

T−γ

e

(1−R (T ) ) 0 =R (T )N………………… (A1.3)

u

e

e

｛ （ α ）｝ ｛ （ α ）｝｛ （ α ）｝t1−γ

R (T1) = exp −×exp −

tN−γ et2−γ×……… (A1.7)｛ （ α ）｝ ｛ （ α ）｝1−0.01C =exp −t1−γ e×exp −t2−γ e×e｛ （ α ）｝

…×exp −

tN−γ……… (A1.8)

Arranging equation (A1.8) with relational expression(A1.5), equation (A1.9) is obtained 16).
1−0.01n =exp

｛ （ α ）｝
T1−γ e＋γ … (A1.6)

situation that the above mentioned data is obtained can
only occur with a probability of 0.1 when N test samples
are extracted from the population ofα=αu, which is rare,
the assumption ofα=αu is discarded with a probabilityof 0.9. That is, this event should be interpreted as theprobability thatαof the population becomes overαu is90%. From the above, the relation between the numberof test samples N thatα≧αuwith confidence level of
C % and the truncated life of the undamaged bearings
can be expressed by the equation (A1.8).

When α=αu, if the probability of equation (A1.3) is0.1, among N extracted bearings from the population
α=αu, the event that bearings with a life less thantime T is zero has a probability of 0.1, which is rare;
therefore, the assumption of α=αu discarded with aprobability of 0.9 (confidence level of 90%)
(null hypothesis). That is, this event should be interpreted
as the probability thatαof the population becomesoverαu is 90%. From the above, the relation between
number of test samples N that α = αu with confidence
level C % and a truncation life T1 of the undamaged
bearings can be expressed with equation (A1.4).
1−0.01C =exp −N

N ln (1−0.01n )α=αu, if the probability of equation (A1.7) is 0.1, the｛ （ α ）｝

= exp −N

1/e

Applying the same concept mentioned earlier, when…………………… (A1.2)

R (T )……… (A1.5)uln (1−0.01C )…×exp −eWhere,
e：Weibull slope,α：scale parameter,
γ：location parameter
P0 =

e

Ln−γ

Equation (A1.6) is an expression showing the relation
between the number of test samples N that canguarantee the target life of Ln with a confidence level of
C%, and truncated life T1 of the undamaged bearings.
Next, the situation that N bearings stay undamaged
until the end of their life data t1, t2……tn is examined.
From the population with a cumulative probability ofsurvival R(x), N test bearings are extracted. Since thelife data t1, t2……tn are simultaneously obtained, thejoint probability can be expressed as follows:(1−R (T ) ) k … (A1.1)

R(x) =1−F(x) =1− 1−exp −｛（ α ）｝（）

F(x) =0.01n =1−exp −………… (A1.4)-80-[ Σ（ ）]ln (1−0.01C )N

t1−γ ei =1Ln−γ……… (A1.9)

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Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data

From equations (A2.4) and (A2.5), the following
relation can be obtained.

Appendix 2
Equation used in the case study in Section 2.3
If, from the population of the cumulative failure
probability F(x) , N test bearings are extracted, theprobability Pk that the life of k bearings among themfall below the time T (or loading cycles) follow binomial
distribution of (A2.1).
N!

Pk =

F k (T ) (1−F (T ) )k! (N−k) !
N!

=

R (T )

k! (N−k) !

N−k

｛

u

k

k =0

1=

k

k =0

e

……………… (A2.9)

When equation (A2.8) is rewritten using the relation＋of equation (A2.9), equation (A2.10) is obtained.
N

1
Pk =
β(i, N−i ＋1)
k= iΣ… (A2.3)

F (Ti＋1)∫t

i−1(1−t )N−idt0……… (A2.10)

This leads to the cumulative distribution functionof the beta distribution function expressed byequation (A2.11).
1
Pk =β(i, N−i ＋1)
k= i

N−k

F (Ti＋1)∫t

i−1(1−t )N−idt

0=β(F (Ti＋1), i, N−i ＋1)

F k (Ti＋1) (1−F (Ti＋1)) … (A2.4)

k

k= i

∫

Γ(x＋y)
1=
Γ(x) Γ(y )β(x, y )ΣΣP ＋ΣP……… (A2.7)

On the other hand, the relation of (A2.9) exists
between the gamma function and beta function β(x, y)(different from beta distribution function).

N

N

dt

……… (A2.8)……… (A2.11)

From equations (A2.6) and (A2.7), equation (A2.12)
is obtained.

On the other hand, since the probability that all
events occur is 1, equation (A2.5) is obtained.
i−1

N−i

F (Ti＋1)

ΣkΣ k! (N−k) !

(1−t )

Γ(N＋1)
t i−1 (1−t ) N−i dt
Pk =
(i)
(N−i
Γ
Γ
＋1)
k= i
0

i−1

N!

i−1

0

N

k =0

i−1

and can be

A recursive equation can be obtained if partial
integration is applied to the right side of the equation(A2.7), to derive equation (A2.6). Next, when Γ
function is used to rewrite the factorial part of theright side of equation (A2.7), equation (A2.8) is
obtained.

ΣP

=

15)

F (Ti＋1)

k= iSimilarly, the relation between the truncated life
Ti+2 of i failed bearing that α≧αu with a confidence
level of C % and the number of test samples N canbe expressed by equation (A2.4).
1−0.01C =N!

ΣP = (i−1) ! (N−i ) !∫t｛ （ α ）｝（ α ）｝ ｛ （ α ）｝u

(A2.6)

i＋1

expressed by equation (A2.7).

N

Using the same concept as Appendix 1, the relation
between truncated life T2 of one failed bearing thatα≧αu with a confidence level of C % and the numberof test samples N can be expressed by equation (A2.3).

T2−γ e

N−k

i＋1

k

k= i

P0 ＋P1 =R (T2) N ＋NR (T2) N−1 (1−R (T2)) … (A2.2)

×exp −kN

On the other hand, when the second shortest life
data of N test samples extracted from the cumulative
failure probability F(x) exceeds the time T2 is when
there are 0 and 1 test bearing which lives fall shorter
than the time T2 among N test bearings. Therefore,
the probability that the second shortest life data
among N test bearings exceeds the time t 2 can beobtained by equation (A2.2) using the law of the sumof probability 17).

T2−γ eN!

ΣP is called order statistics

R (T )：cumulative probability of survival, F (T )=1−R (T )Nexp−(N−1)

k

k= i

Where,F (T )：cumulative failure probability of the Weibull distributionuNΣP k! (N−k) ! F (T ) (1−F (T ))

=

N−k

T2−γ

k

k= i

(1−R (T ) ) k … (A2.1)

1−0.01C = P0＋P1 =exp −N

ΣP

0.01C =β(F (Ti＋1), i, N−i ＋1) = 0.01C ………… (A2.12)……………………… (A2.5)-81-

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NTN TECHNICAL REVIEW No.84（2016）

When equation (A2.12) is modified using the inverse
function of the cumulative distribution function of thebeta distribution function and the inverse function of
cumulative failure probability F(x) , we obtain equations(A2.13) and (A2.14).

Appendix 3
Significance test using random simulation18)(calculation method for significant life ratio)
This appendix discusses how to conduct significancetest of life at an arbitrary percent point using Weibull
random number. Random simulation can identify
variation of life ratio from two levels of data obtained
from two sets of data extracted from the population of
the same life. If the life ratio obtained from the actual
test is greater than the variation of the life ratio
obtained from the random simulation, it means thatthe lives of two levels contain a bigger difference thanthe variation, in other words, a significant difference.
The variation of life ratio obtained from this random
simulation is the significant life ratio. The followingshows the calculation method of significant life ratiousing Weibull random number.

B -1(0.01C, i, N−i ＋1) = F (Ti＋1) ………… (A2.13)
Ti＋1 = F -1 (B -1 (0.01C, i, N−i ＋1) )e= γ＋α. ln

1[ 1−B (0.01C, i, N−i ＋1) ] (A2.14)-1Where,
B -1 (0.01C, i, N−i ＋1)：inverse function of cumulative
distribution function of the beta
distribution function,
C：confidence level %,N：number of test samples
i：truncation life of undamaged condition is
calculated by i=1, truncation life of failure of 1…ksamples is calculated by i = 2…k+1(1) With Weibull, plot two levels of data obtained fromthe life test of test samples N1 and N2 and find therespective Weibull slopes e1 and e2. For life
estimation with the Weibull plots, use the calculation
method in Johnson's literature 19).
(2) Determine the percent point for calculating variationof life ratio.
(3) Assume the lives of two levels Ln,1 and Ln,2 at thepercent point determined in (2) n are the same
(value of 1), obtain the scale parameter of Weibull
distributionα1 andα2 .
(4) Generate Weibull random numbers that follow the
Weibull distribution of two levels obtained from (1)to (3) above, for the number of test samples, N1
and N2 , respectively. Weibull random numbers
can be obtained by obtaining uniform random
numbers generated by the Mersenne Twister
method 20) and applying the inverse function
method 21) to them.
(5) Plot the two sets of Weibull random numbers
generated in (4), estimate the lives at percent
point n determined in (2) and obtain the life ratio.
(6) Repeat the operation of (4) and (5) for more than
10,000 times to obtain the range of life ratio witha confidence level of C %.

In addition,α of equation (A2.14) can be obtainedby substituting the target life Ln into equation (A1.5),
if the target life is available. This equation is forcalculating the truncated life of the remaining test
bearings. However, it can be applied for the decision]of not being able to meet the target life with aprobability of 90% if the first test bearing fails before
the truncated life, because when i is substituted by 1
andC is substituted by 10, it indicates that the target
life can only be met with a probability of 10% during
the truncated life of all samples.
As shown above, truncated life can be calculatedusing the inverse function of the cumulative distributionfunction of the beta distribution function. The cumulative
distribution function of the beta distribution function
can be obtained, in principle, from binomial probability
and if the conditions of the binomial probability(random extraction of test bearings, sufficient numberin the population) are established, it gives the exact
result in the truncated life calculation. In addition,
regarding the inverse function of the cumulative
distribution function of the beta distribution function,
the calculation of equation (A2.14) can be easily
performed since ordinary spreadsheet programs
provide the functions (in case of Microsoft Excel®,betainv function).

The range of life ratio obtained from the above
procedure indicates degree of variation with aprobability of C % when life ratios are compared byextracting N1 and N2 test bearings from two levels of
populations with the Weibull slopes of e1 and e2, andthe lives at the percent point n are the same. Therefore,
if the life ratio of two levels is greater than the variance,
the lives of the two levels have significant differencewith a confidence level of C %.
-82-

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Strategy of Rolling Contact Fatigue Life Testing and Interpretation of Life Data

Appendix 4
Approximation equation for significant life
ratio18)

Appendix 5Interpretation of significance test result
The following describes the interpretation ofsignificance test results under each situation of assumed
2 levels of Weibull plots.

Significant life ratio varies depending on the numberof test samples for each level N1 and N2, Weibull slope
e1 and e2, percent point for comparing life difference n,and confidence level C. Appendix 3 indicated how
to calculate the significant life ratio that changes bythese six parameters using Weibull random numbers.
This method, however, has a disadvantage of having
to create calculation programs.
In the calculation using Weibull random numbers,
Weibull slopes indicating the variation of test andnumber of test samples are assumed to be different
between two levels; however, in the actual tests, theWeibull slopes are often similar if the test conditions
and the steel type are the same, and number of test
samples tends to be the same unless there is any
special reason. Also, the percent points to compare
the life difference are almost always 10, 50 and 63.2%.
The percent point of 63.2% is the percent point that
corresponds to the scale parameter of the Weibull
distribution. Therefore, the approximation equation
which can calculate the significant life ratio of twolevels for the same number of test samples andWeibull slopes at the percent points of 10, 50 and
63.2 with a confidence level of C% can cover a vast
majority of the situations. The following shows
approximation equations of significant life ratio
obtained by a regression analysis of significant life
ratio from random simulation.

[

L10R = 1+

10.007474｛ ln9.106 (1.051−0.01C )N ｝3.151

]

1/e

]

1/e

]

1/e

Cumulative failure probability

(a)

[

1
0.009349｛ ln36.74 (1.051−0.01C )N ｝3.151(A4-1)[

L63.2R= 1+

3.059
0.01068｛ ln41.48 (1.051−0.01C )N ｝

L50

L10

(b)

(A4-2)

・Significant life ratio for 63.2% life1B(b) Significance test results show that the varianceof life A is small (Weibull slope of A is large) andno significant differences exist for 10% life but
A's life is shorter for 50% life. We can interpretthat there are no differences in a short life thatrarely occurs with a probability of 10%, but there isa difference for the average life. Even if A's life isequivalent to B for 10% life, it is shorter for 50%l ife,
which is the average life; therefore, the interpretation"A has a shorter life than B" is considered appropriate.

・Significant life ratio for 50% life

L50R = 1+A

Life

Cumulative failure probability

・Significant life ratio for 10% life(a) Significance test results show that the variationsof life of both levels are the same (the same
Weibull slope) and A has a shorter life for both
10% life and 50% life. In this case, the interpretationis "A's life is shorter than B."(A4-3)

A

B

L50

L10

Life

Where,
N：number of test samples, e：Weibull slope,
C：confidence level
Applicable range：N≦50, 0.5≦e≦10, 80≦C≦99(c) Significance test results show that the varianceof life of A is large (A's Weibull slope is small)and A's life is short for 10% life and no significant
difference for 50%.
Even if A's life is equivalent for 50% life, which isthe average life, under the situation that it is shortfor 10% life, it indicates that A's life has a larger
variation and unstable quality; therefore, the-83-*14E_*15 18/06/13 13:56 ページ 11

NTN TECHNICAL REVIEW No.84（2016）(f) Significance test results show that the variance oflife A is large (the Weibull slope of A is small) andno significant differences exist for 10% life but A'slife is longer for 50% life. A's life is mostly long;
however, it may be almost the same for a short lifethat may occur rarely with a probability of 10%. In
this case, the interpretation "Although A's life islonger than B, its variance is large" is considered tobe adequate.

interpretation of "Although A's life is equivalent to
B, its variation is large" is considered to beadequate.

L50
A
B

L10

Cumulative failure probability

Cumulative failure probability

(c)

Life

A

L50

L10

(g)

L10

Life

(e) Significance test results show that the variance oflife of A is small (A's Weibull slope is large) and A'slife is long for 10% life and no significant differences
for 50%. To indicate that A's life is equivalent for
50% life, which is the average life, with small
variation, the interpretation of "Although A's life isequivalent to B, its variation is small" is considered
to be adequate.
(e)
Cumulative failure probability

L50(g) Significance test results show that the variances oflife for both are equivalent (the same Weibull slope)and A's life is long for either 10% or 50% life. In thiscase, the interpretation is "A's life is longer than B."

(d)
B

A

B

Life

Cumulative failure probability

Cumulative failure probability(d) Significance test results show that the variances oflife for both are equivalent (the same Weibull slope)and there are no significant differences for either
10% or 50% life. In this case, the interpretation is"A's life is equivalent to B."

(f)

B

L50

L10

Life

A

L50

B

L10

Life

-84-

A

*14E+_*15 18/06/13 13:56 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Technical Paper ]Rolling Contact Fatigue Life of Thrust Ball Bearing
under Low Lambda Condition

Takumi FUJITA*
Naoya HASEGAWA*
Naoya KAMURA*
Toshihiko SASAKI**

Rolling contact fatigue (RCF) testings of thrust ball bearing underboundary lubrication condition (low lambda condition) areconducted while changing test conditions of load, rotational speed
and combination of surface roughnesses for bearing component
parts. The RCF test results and measurements of surface
roughnesses of before and after the tests show that the raceway surface with larger roughness promotes thesurface initiated failure of the other component. In addition, they demonstrate that the behavior of change insurface roughness during rolling contact depends on the test conditions and influences RCF life accordingly.
These results suggest that RCF life of rolling bearing used under low lambda conditions should be estimatedconsidering not only analytical relationship between repeated stress in subsurface and surface roughness
during RCF, but also experimental database of the running-in behavior depending on RCF conditions.

Λ conditions does not only depend on Λ, but also on
various parameters of surface roughness andoperating conditions.
Currently, the most recent and generally well known
life prediction methods for low Λ condition are ISO(International Organization for Standards) 281 29)
based on research outcome by Ioannides et al. 28) andlife formula by JIS (Japan Industrial standards)
B1518 30). This life formula includes a coefficient
called κ, which is used to correct predicted life
depending on lubrication conditions; therefore, using
this coefficient makes it possible to predict life underlow Λ conditions. However, this coefficient does nottake into account severe conditions of projection
contact of surface roughness (root mean square slope
Rdq of 4˚ or more). Therefore, life prediction underthose conditions cannot be performed with the life
formulae of ISO and JIS. In addition, as Akamatsu etal. point out, life under the low Λ conditions is affectedby operating conditions and various parameters of
surface roughness; however, the life formulae by ISO
and JIS cannot make a life prediction by taking these
conditions into account. ISO and JIS may posit that life
formulae considering large surface roughness, special
operating conditions or special surface characteristics
are not generally required. However, considering that

1. Introduction
In an effort to reduce friction in automobiles andindustrial machines, development of low viscosity
lubricants is underway. Along with this trend, the useof rolling bearings in low lubrication condition(hereinafter, low Λ condition, where Λ is a film
parameter which is the ratio of oil film thickness andsum of the square root mean square roughness of two
objects) will increase; therefore, the importance ofprediction technique and life improvement technologyof rolling contact fatigue life (hereinafter, "life") underlow Λ conditions is expected to increase.
The study on life under low Λ condition became
active in the latter half of the 1960s, along with thedevelopment of theory of elastohydrodynamic
lubrication (EHL theory), both domestically andinternationally 1-19) . The relation between film
parameter Λ and life ratio based on the research of
Tallian 1) and Skurka 2) is called the ASME (American
Society of Mechanical Engineers) recommended
diagram 3) and is a very well known study in the field.
Since the latter half of the 1980s, Akamatsu et al.
have been studying the relation between various
parameters on surface roughness and life 20-24), rolling
speed and life 25-27), and pointed out that life under low** Advanced Technology R&D Center** Professor, Faculty of Human Sciences, Kanazawa University-85-

*14E+_*15 18/06/13 13:56 ページ 2

NTN TECHNICAL REVIEW No.84（2016）

obtained from the Hamrock-Dawson's film thickness
calculation formula 32) was 0.1 or less in all conditions.
For lubricant oil, JX Nippon Oil & Energy turbine oilwith no additives was used for all tests. For Test Nos.1- 9, steel rolling elements of root mean squareroughness (hereinafter, "Rq") of 0.22 μm were used.
Among them, with Test Nos. 1 - 6, the effect of surface
roughness of the inner/outer rings and load can bechecked, with Test No. 1 and No. 7, the effect of therolling speed can be checked, with Test No. 7 and No.
8, the effect of oil temperature can be checked, andwith Test No. 3 and No. 9, the effect of load period canbe checked. Test Nos. 10 - 12 intentionally created anaggregation of micro peeling (hereinafter, "peeling")
on the inner/outer rings of the bearings by making thesurface of the steel rolling elements rougher. Here,peeling is defined as the aggregation of several 10 100 μm of length, and flaking is defined as anindividual delamination larger than that size. The effectof load on peeling can be checked with Tests No. 10 12.
Table 1 shows the calculated life L10h , calculatedwith a method used for general life calculation of ballbearings 33), and the values of correlation factor for
operating conditions read out from the ASME diagram(hereinafter, “a3 ”).
Since the ASME diagram does not present a3
values when Λ is 0.6 or less, the value of 0.164 under
the column a3 is the minimum value read out from the
ASME diagram.

limit design technology is required at the actual
production sites of rolling bearings due to a restrictionin processing methods or for pursuing productivity
improvement or reduction of production costs, that theoperating conditions of rolling bearings are widelydifferent by users and that bearings with special
surface characteristics are actually used 31),
establishment of universal life prediction methods
which can take those conditions into account aredesirable.
For establishment of life prediction methods underlow Λ conditions, various life tests under low Λ
conditions and observation of the findings from a
tribology viewpoint are indispensable; however, there
are few reports of systematic life tests under low Λ
conditions. Particularly, there are no reports regarding
life tests under low Λ conditions for thrust ballbearings. In this paper, life test results under low Λ
conditions of thrust ball bearings are reported withconsideration of causes that determine life and life
prediction under low Λ conditions based on the test
results.

2. Test method
Table 1 shows the life test conditions. The test was
conducted with thrust ball bearings of part number
51105 by changing surface roughness of theinner/outer rings and steel rolling elements, oil type, oiltemperature, number of rolling elements, test load androlling speed. The inner/outer rings and steel rolling
elements of the thrust bearings are made of highcarbon chromium bearing steel type 2 (hereinafter,"SUJ2") with standard hardening and tempering. The
hardness of the inner/outer rings and steel rolling
elements of thrust bearings were HRC62 and HRC64,
respectively. Oil bath lubrication was used and Λ

Table 1 Test conditions
Test
No.

1
2
3
4
5
6
7
8
9
10
11
12

Surface roughnessμm
Inner andouter ring

Rolling
element

0.25
0.25
0.25
0.65
0.65
0.65
0.25
0.25
0.25
0.25
0.25
0.25

0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.22
0.54
0.54
0.54

The number
Contact Rotaion
Loadof rolling
stress speedkN-1element
GPa
min

3
3
3
3
3
3
3
3
6
3
3
3

5.88
4.26
2.70
5.88
4.26
2.70
5.88
5.88
5.39
5.88
2.70
0.98

3.9
3.5
3.0
3.9
3.5
3.0
3.9
3.9
3.0
3.9
3.0
2.1

-86-

1500
1500
1500
1500
1500
1500
500
500
1500
1500
1500
1500

Viscosity
grade,
Temperature

Oil Film
parameterΛ

VG32, 110˚C
VG32, 110˚C
VG32, 110˚C
VG100,110˚C
VG100, 110˚C
VG100, 110˚C
VG150, 110˚C
VG32, 70˚C
VG32, 110˚C
VG150, 110˚C
VG150, 110˚C
VG150, 110˚C

0.071
0.073
0.076
0.058
0.059
0.061
0.069
0.072
0.076
0.081
0.086
0.092

Calculated Correlation
factor
life
a3
L10h

14
37
145
14
37
145
42
42
73
14
145
3031

0.164
0.164
0.164
0.164
0.164
0.164
0.164
0.164
0.164
0.164
0.164
0.164

*14E+_*15 18/06/13 13:56 ページ 3

Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition

occurred at the inner/outer rings; however, in Test
Nos. 5 and 6, the steel rolling elements were partiallydamaged. The damage mode in Test Nos. 1 - 9 wasflaking and in Test Nos. 10 – 12, peeling. In Test No.
10, both peeling and flaking were observed. In
addition, in Test Nos. 10 - 12, the peeling location wascharacteristic and was found more on the innerdiameter side of the rolling contact trajectory.
Fig. 3 shows the relation between the estimated lifeand calculated life. The number near each plot in the
figure indicates the Test No.
The calculated life of the horizontal axis takes a3
into account.
The following is an itemized summary of thefindings from Test Nos. 1 to 12.
1) In Test Nos. 1 - 9, the ratios of the estimated life
and the calculated life were in the range between
0.6 and 2.6, and no enormous difference existed
between the estimated life and calculated life under
any conditions.
2) In Test Nos.10 - 12, the ratios between theestimated life and the calculated life (L10 /a3 L10h )were in the range from 0.004 to 0.4, and thedifference between the estimated life and calculated
life was more significant under small loadconditions.
3) The surface roughness of the inner/outer rings
before the test was different for Test Nos. 1 - 3 andTest Nos. 4 – 6, but the ratios of the estimated lifeand calculated life were 0.7 – 0.94 and 0.6 - 1.1,
respectively, and no significant life differences wereobserved between the two conditions.
4) In Test Nos. 1 - 3 and Nos. 4 - 6, the effect of load
was observed respectively, and the life wasinversely proportional to the cubes of the load.
5) The effect of rolling speed was observed in Test
No. 1 and No. 7, and the ratio of the estimated lifeand calculated life of Test No. 7 with slow rolling

3. Test results
3. 1 Life test results
Fig. 1 shows an example of the Weibull plots (Test
No.1) and Table 2 shows the estimated lives obtained
from the Weibull plots of Test Nos.1 - 12, respectively.
For life estimation with the Weibull plots, the L. G.
Johnson method 34) was used for calculating medianrank and analysis was made by adjusting the Weibull
slope to 1.1. The reason why the Weibull slope wasadjusted is to make a comparison with the calculated
life (calculated using Rolling Bearing Catalog 37) ,)which is calculated with a Weibull slope of 1.1 35, 36)(10/9 for point contact and 9/8 for line contact, to be
exact). In Test Nos. 10 - 12 where the damage mode
was peeling, since each peeled piece is small,vibration that occurs at the initial stage of peeling was
small, making it difficult to stop the test at the initialstage of peeling. Therefore, the life data in Table 2
shows the stage where peeling had proceeded until it
was detected by vibration.
Fig. 2 shows typical delamination modes in Test
Nos. 1 to 9 and Nos. 10 to 12. Most of the damage

F (t), %

99.0
90.0
70.0
50.0
10.0
5.0

1.0
0.5
0.1
0.1

Be-ta-binomial 90 % confidence Bounds

1

10

100

Time, hFig. 1 Weibull plots of failure time for test No.1

Table 2 Results of RCF testing
Test No.

The number of test specimen
Flaking of Inneror Outer ring
Peeling of Inneror Outer ring
The number of
failure parts Failure of Rolling
element

Suspended
Estimated L10 (h)Estimated L50 (h)Calculated L10h
a3L10hL10㸭(a3×L10h)

1
10

2
10

3
13

4
10

5
14

6
11

7
10

8
11

9
11

10
10

11
8

12
8

10

9

8

9

7

7

10

11

9

10

0

0

0

0

2

1

3

1

0

0

1

10

8

8

0

1

0

0

4

2

0

0

0

0

0

0

0

0

3

0

0

1

0

0

1

0

0

0

5.4 22.5 2.5
29.8 124.9 14.1
37
146
14
6.0 23.9 2.3
0.89 0.94 1.10

6.6
36.3
37
6.0
1.09

13.5
74.7
146
23.9
0.56

10.2
56.4
42
6.9
1.47

17.8
98.1
42
6.9
2.57

10.6
58.7
73
11.9
0.89

1.0
5.4
14
2.3
0.43

1.6
8.8
14
2.3
0.69

-87-

1.8
2.0
10.2 11.0
146 3031
23.9 497.1
0.077 0.004

*14E+_*15 18/06/13 13:56 ページ 4

NTN TECHNICAL REVIEW No.84（2016）

3. 2 Surface roughness of inner/outer rings andsteel rolling elements before and after test

speed was about 2.1 times greater than Test No. 1.
6) In Test Nos. 7 and No. 8, the effect of oil
temperature can be seen and the ratio of theestimated life and calculated life of Test No. 8 withlow oil temperature was about 1.7 times greaterthan Test No. 7.
7) In Test Nos. 3 and No. 9, the effect of loading cycle
can be seen and the ratio of the estimated life andcalculated life of Test No. 9 with a short loading
cycle was not much different from Test No. 3.

Bearings with low Λ conditions are considered tobe affected by surface roughness of the inner/outerrings and steel rolling elements of the bearings. The
following shows the results of the surface roughness
investigation of the inner/outer rings and steel rolling
elements before and after the tests.
Table 3 shows the average (average ofmeasurement data of test samples shown in the table)of Rq and root mean square slope (hereinafter, Rdq) of
the inner/outer rings and steel rolling elements before
and after the test. Where, Rq and Rdq show the degreeof surface roughness of raceway and slope of
projections of surface roughness, respectively, andare the parameters used as indices to indicate severityof projection contact of surface roughness in the paper
by Ioannides et al. 28). These measurement results are
the values obtained by using a stylus type surface
roughness measurement instrument/profilometer, in a
direction perpendicular to the rolling contact trajectoryfor inner/outer rings and an arbitrary direction for steel
rolling elements of bearings. The measurement
positions are selected from undamaged areas. In
addition, JIS 38) was followed for conditions ofmeasurement.
From Table 3, Rq and Rdq before and after the testfor each area where the surface roughness conditionsof the inner/outer rings were different before the test
(Test Nos. 1 - 3 and Test Nos. 4 - 6) were extracted
and compared. The results are shown in Fig. 4. Afterthe tests, Rq and Rdq of the inner/outer rings were
found to be smaller than before the tests in allconditions, indicating that the surface of theinner/outer rings was "run-in" after testing (reduction of
Rq and Rdq during testing is called "run-in"). In Test
Nos. 1 - 3 and Test Nos. 4 - 6 where surface
roughness levels of the inner/outer rings were quite
different, Rq and Rdq of the inner/outer rings exhibited
differences after the tests, and Rq and Rdq of Test
Nos. 4 - 6 were greater than Test Nos. 1 - 3 before thetest and after delamination started. After the tests, Rq
and Rdq of the inner/outer rings of Nos. 1 - 3 were0.07 - 0.15 μm and 0.54 - 1.60˚, respectively and Rq
and Rdq of the inner/outer rings of Nos. 4 - 6 were0.26 - 0.43 μm and 1.03 - 3.48˚, respectively. On theother hand, the steel rolling elements did not run-in
well as opposed to the inner/outer rings in any
condition, and Rq after testing indicated little reduction
in all conditions. In addition, Rdq did not show
significant reduction as observed with the inner/outerrings.
Rq and Rdq after Test Nos. 1 - 12 extracted from
Table 3 are shown in Fig. 5. The reason why Fig. 5(a)(a) Flaking for Test No.1

(b) Peeling for Test No. 12
Fig. 2 Typical failure for test No.1 and test No.12

Estimated life from experiments (L10h), h

1,000

100

7
5

10

4

2

1

1

0.1
0.1

8

3
9

6
12

11

10

1

10

100

1000

Calculated life (a3×L10h), hFig. 3 Correlation diagram between calculated life
(a3 ×L10h ) and estimated life from experimental results-88-*14E+_*15 18/06/13 13:56 ページ 5

Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition

Table 3 Average of surface roughness before and after testing
Test
No.

1

2

3

4

5

6

Parts

Before testing
Rq
Rdq
N
μm
°

After testing
Rq
Rdq
N
μm
°

I
O
E
I
O
E
I
O
E
I
O
E
I
O
E
I
O
E

10
10
5
10
10
5
10
10
5
10
10
5
10
10
5
10
10
5

7
7
21
7
7
21
10
10
30
10
10
10
14
14
14
11
11
11

0.24
0.26
0.22
0.24
0.26
0.22
0.24
0.26
0.22
0.65
0.64
0.22
0.65
0.64
0.22
0.65
0.64
0.22

7.24
7.68
3.78
7.24
7.68
3.78
7.24
7.68
3.78
12.48
12.63
3.78
12.48
12.63
3.78
12.48
12.63
3.78

0.07
0.09
0.22
0.07
0.14
0.20
0.11
0.15
0.19
0.28
0.26
0.26
0.41
0.34
0.20
0.38
0.43
0.22

0.58
0.54
2.16
0.86
0.96
2.89
1.60
1.56
2.10
1.08
1.03
3.04
1.85
1.82
2.23
2.52
3.49
2.11

Test
No.

7

8

9

10

11

12

Parts

Before testing
Rq
Rdq
N
μm
°

After testing
Rq
Rdq
N
μm
°

I
O
E
I
O
E
I
O
E
I
O
E
I
O
E
I
O
E

10
10
5
10
10
5
10
10
5
10
10
6
10
10
6
10
10
6

10
10
30
11
11
32
11
11
66

0.24
0.26
0.22
0.24
0.26
0.22
0.24
0.26
0.22
0.24
0.26
0.54
0.24
0.26
0.54
0.24
0.26
0.54

7.24
7.68
3.78
7.24
7.68
3.78
7.24
7.68
3.78
7.24
7.68
9.52
7.24
7.68
9.52
7.24
7.68
9.52

0.17
0.13
0.17
0.11
0.13
0.21
0.14
0.18
0.23

0.63
0.88
1.86
0.67
0.62
1.23
1.28
1.44
2.94

Peeling
10

0.34

3.79

Peeling
7

0.36

5.30

Peeling
7

0.39

5.85

O : Outer， I : Inner， E : Element， N : Number

1
0.9
0.8

Rq μm

0.7

- (d) do not show the results of the inner/outer rings
from Test Nos.10 - 12 is because peeling wasobserved throughout the raceways of the inner/outerrings. Observing the inner/outer rings after the tests,
Rq and Rdq tended to decrease more when contactpressure was higher. No significant difference,
depending on the test conditions, was observed for Rqof steel rolling elements after the Test Nos. 1 - 9;
however, the difference was smaller for Rdq in Test
Nos. 7 and 8 where the rolling speed was lower. In
Test Nos. 1 - 9 and Test Nos. 10 – 12, where surface
roughness levels of steel rolling elements were quite
different, Rq and Rdq of steel rolling elements exhibited
difference after the tests, and Rq and Rdq of Test No,.
10 - 12 were greater than Test Nos. 1 - 9 before thetest and after delamination started. For the steel rolling
elements for Test Nos. 1 - 9, Rq showed little difference
before and after the tests; however, Rdq showeddecrease from 3.78˚ at the initial stage of testing to 1.23
- 3.04˚, although there were some variances between
different Test Nos. For the steel rolling elements in
Test Nos. 10 - 12, Rq decreased from 0.54 μm of theinitial stage to 0.34 - 0.39 μm, and Rdq decreased
from 9.52˚ of the initial stage to 3.79 - 5.85˚. The
surface of steel rolling elements did not "run-in" wellcompared to inner/outer rings in any Test Nos.

Before testing
After testing for Pmax=3.9GPa
After testing for Pmax=3.5GPa
After testing for Pmax=3.0GPa

0.6
0.5
0.4
0.3
0.2
0.1
0

Inner
ring

Outer Rolling Innerring element ring

Test No.1〜3

18
16
14

Rdq °

12

(a) RqOuter Rollingring element

Test No.4〜6

Before testing
After testing for Pmax=3.9GPa
After testing for Pmax=3.5GPa
After testing for Pmax=3.0GPa

10
8
6
4
2
0

Inner Outer Rolling Inner Outer Rollingringring element ring
ring element
Test No.1〜3
Test No.4〜6

3. 3 Relation between test conditions anddamage mode(b) Rdq

In Test Nos. 1 - 12, most of the inner/outer rings ofthe test samples were damaged (Table 2). In Test
Nos. 5 and 6, where Rq and Rdq of the inner/outerFig. 4 Average of Rq and Rdq before and after testing fortest No. 1 to 6-89-

*14E+_*15 18/06/13 13:56 ページ 6

NTN TECHNICAL REVIEW No.84（2016）

0.8
0.6

Rq= 0.24 µm
before testing of No.1〜3 and No.7〜9

0.5

1.0

0.4
0.3

Rq= 7.24˚
before testing of No.1〜3 and No.7〜9

0.8
0.6
0.4

0.2

0.2

0.1
0.0

Rdq= 12.48˚ before testing of No.4 to 6

1.2

Rdq ˚

Rq μm

1.4

Rq= 0.65 µm before testing of No.4 to 6

0.7

1

2

3

4

5

Test No.

6

7

8

0.0

9

1

2

3

(a) Rq of inner ring
0.8

1.4

0.5

1.0

0.4
0.3

9

Rq= 7.68˚
before testing of No.1〜3 and No.7〜9

0.8
0.6

1

2

3

4

5

Test No.

6

7

8

0.0

9

1

2

3

(c) Rq of outer ring
0.8
0.6

1.2

Rdq ˚

Rq= 0.22 µm
before testing of No.1 to 9

0.4
0.3

5

Test No.

6

7

8

9

1.4

Rdq= 9.52˚ before testing of No.10 to 12

1.0

0.5

4

(d) Rdq of outer ring

Rq= 0.54 µm before testing of No.10 to 12

0.7

Rq μm

8

0.2

0.1

Rq= 3.78˚
before testing of No.1to 9

0.8
0.6
0.4

0.2

0.2

0.1
0.0

7

0.4

0.2
0.0

6

Rdq= 12.63˚ before testing of No.4 to 6

1.2

Rq= 0.26 µm
before testing of No.1〜3 and No.7〜9

Rdq ˚

Rq μm

0.6

5

Test No.

(b) Rdq of inner ring

Rq= 0.64 µm before testing of No.4 to 6

0.7

4

1

2

3

4

5

6

7

Test No.

8

9

10 11

12

1

(e) Rq of rolling element

2

3

4

5

6

7

Test No.

8

9

10 11

12

(f) Rdq of rolling element

Fig. 5 Average of Rq and Rdq after testingfor test No.1 to 12

tests; however, little difference was observed in life
(Table 2 and Table 3).
In Test Nos. 7 and 8, where the rolling speed waslow, Rdq of the steel rolling elements was smaller thanin other conditions and life tended to be longer thanthe calculated life (Table 2 and Table 3).
In Test Nos. 1 - 9, where Rq and Rdq of the steel
rolling elements at the initial stage of tests wererelatively small, no enormous difference was observed
between the estimated life and calculated life (Fig. 3)
and the life was inversely proportional to the cubes ofthe load. On the other hand, in Test Nos. 10 - 12,
where Rq and Rdq of the steel rolling elements at theinitial stage of the tests were relatively large, life wasalmost constant regardless of the load and thedifference from the calculated life was more significant
for smaller loads (Fig. 3).

rings were large and load was relatively low, damage
was observed in the steel rolling elements of sometest samples (Table 2).
In Test Nos. 1 - 9, where Rq and Rdq for steel rolling
elements were relatively small, the damage mode wasflaking, and in Test Nos. 10 - 12, where Rq and Rdq
were large, the damage mode was peeling (Table 2
and Fig. 2). In Test No. 10, both flaking and peelingwere observed (Table 2).
In addition, peeling was more frequently observedon the inner diameter side of the rolling contact
trajectory (Fig. 2).

3. 4 Relation between surface roughness and life
In Test Nos. 1 - 3 and Test Nos. 4 - 6, where Rq and
Rdq of the inner/outer rings were quite different before
the tests, Rq and Rdq showed differences after the-90-

*14E+_*15 18/06/13 13:56 ページ 7

Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition

Rdq, is more appropriate as an index to indicate
aggressiveness to the counterpart; however, in view thatin the case of tests with a small rolling speed, there waslife difference while no differences were observed in Rq
but there were differences in Rdq, and that there is aview that the increase of contact stress in the truecontact point can be obtained by Rdq in Appendix A ofthe paper by Ioannides et al. 28), Rdq may be more
appropriate as an index to indicate aggressiveness on
the raceways of the counterpart. Fig. 6 shows thecontact condition of two surfaces with the same Rq anddifferent Rdq in schematic form. Considering that the topof the projections of the large Rdq surface is likely to fall
into the small Rdq surface, the area around the
projections may be close to the status under hydrostaticpressure stress, which may be advantageous for fatigue.
At any rate, it is considered that the life under low Λ
conditions, such as the tests we have conducted, isdetermined by the aggressiveness of the trajectorysurface of the counterpart to the area of delamination.
Previously, we discussed that the lower rolling speed
contributed to the run-in of the rolling contact surface ofthe steel rolling elements. It can be also interpreted thatwhen the rolling speed is low, life is relatively longer, so
the increase of the loading cycle may have contributed
to the reduction of surface roughness of the steel rolling
elements. Therefore, we have reviewed the relation
between the rolling speed with constant load cycle andRq and Rdq of steel rolling elements after the tests. The
test conditions are shown in Table 4 and test results in
Fig. 7, respectively. As it is evident from the figure, Rq
and Rdq are likely to decrease as the rolling speed
becomes lower. Taking advantage of this property, a test
was conducted to clarify the phenomenon of extended
life when the trajectory surface is run-in with low speed
rotation. The test conditions are shown in Table 5 andtest results in Fig. 8, respectively. Before conducting thetest under the regular condition of 1,500 min-1, the livesof the bearings were compared with run-in operation
and without run-in operation shown in Table 5 .
All three test samples with run-in operation showed
a life of 4,000h or more, and all of four test samples
without run-in operation showed a life of 40h or less.
This result is an example that run-in operation withslow rotation can significantly affect life.

4. Discussion
In this Chapter, observations are made on the test
results from the previous Chapter and considerations
required for life prediction under low Λ condition andongoing challenges are discussed.

4. 1 Effect of Rq, Rdq and run-in on progress ofrolling contact fatigue
In our testing, we observed that the inner/outer ringsof most of the test samples experienced damage. The
reasons are assumed as follows:
Since the steel rolling elements are less "run-in"compared to the inner/outer rings, the surface
roughness of the steel rolling elements became
relatively larger than the inner/outer rings, so that therolling contact surface of the rolling elements "attacked"
the raceways of the inner/outer rings (hereinafter, theproperty that the parts with larger Rq and Rdq promote
progress of fatigue of the trajectory surface of thecounterpart is called "aggressiveness.") With thrust ballbearings, the inner/outer rings receive fatigue on thesame raceway. In contrast, the steel rolling elementschange the rolling contact surface due to friction force
produced by spinning within the bearings, resulting in
the progress of fatigue throughout the entire sphericalsurface. In addition, since the steel rolling elements are
harder than the inner/outer rings, they have an
advantage over the inner/outer rings from a standpointof strength, which is another reason why the inner/outerrings receive damage before the steel rolling elements,
in addition to higher aggressiveness of the rolling
contact surface of the rolling elements. However, under
the test conditions where Rq and Rdq were large during
testing (Test Nos. 5 and 6), there were cases where thesteel rolling elements were damaged before theinner/outer rings. Therefore, the aggressiveness of therolling contact surface of the counterpart of the damaged
parts is assumed to be the largest factor for determining
life. In addition, under conditions where there weredifferences in surface roughness of the inner/outer rings
before the tests (Test Nos. 1 - 3 and Nos. 4 - 6), no
significant differences in life were observed. That can bealso attributed to the fact that there were no differencesin aggressiveness of the rolling contact surface of therolling elements since there was little difference in Rq
and Rdq of the rolling elements before and after the tests
in Test Nos. 1 - 6. Furthermore, the reason why the life
under low rolling speed conditions was longer than thecalculated life compared with other conditions assumes
that the aggressiveness of the rolling elements on the
inner/outer rings was reduced due to better "run-in"
under the low rolling speed conditions. It is not clear
which surface roughness parameter, between Rq and

Surface 1

Surface 2

Rdq1 > Rdq2

Fig. 6 Schematic of contact condition for raceway
surfaces with same Rq and different Rdq (Rdq 1>Rdq 2)-91-

*14E+_*15 18/06/13 13:56 ページ 8

NTN TECHNICAL REVIEW No.84（2016）
Table 4 Test condition to investigate Rq and Rdq after
rolling contact under different rotation speed

Table 5 Test condition to investigate effect of running
operation on RCF life

Type number of rolling bearing

51105

Type number of rolling bearing

51105

Surface Roughness of inner andouter ring before rolling contact

Rq = 0.25μm, Rdq = 7.45°

Surface Roughness of inner andouter ring before rolling contact

Rq = 0.25μm, Rdq = 7.45°

Surface Roughness of rolling
element before rolling contact

Rq = 0.54μm, Rdq = 9.52°

Surface Roughness of rolling
element before rolling contact

Rq = 0.54μm, Rdq = 9.52°

The number of rolling element
Load and maximum contact stressLubricating oil
Temperature of Lubricating oil
Rotation speed
Oil film parameter
The number of rotation

3
2.7 kN, P max = 3.0 GPa
Turbine oil 䚷VG32
110˚C
250, 500, 1,500 min-1
0.008 〜 0.048
261,000

0.35

4.5

0.30

4.0

2.5
2.0

0.15
0.10

Rq

0.05

Rdq
0

500

1,000

1,500

The number of rotationin running-in operation

261,000 (8.7hours)

Surface Roughness of inner andouter ring after running-in operation

Rq = 0.23μm, Rdq = 1.15°

Surface Roughness of rolling
element after running-in operation

Rq = 0.03μm, Rdq = 0.95°

Rdq °

Rq μm

3.0

0.20

3
2.7 kN, P max = 3.0 GPa
Turbine oil
䚷VG32
110˚C
Running-in operation: 250min-1
RCF testing: 1,500min-1

Rotation speed

3.5

0.25

0.00

The number of rolling element
Load and maximum contact stressLubricating oil
Temperature of Lubricating oil

1.5

Without
running-in
operation

1.0
0.5
0.0
2,000

Rotation speed min-1

Withrunning-in
operation

Fig. 7 Rq and Rdq after rolling contact under different
rotation speed
The number of rotation is 261,000

1Akamatsu and Miller et al. report that when the rolling
speed is in the range of 0.05 - 16.4msec of passing timeof contact ellipse (the smaller value indicates faster rolling
speed; hereinafter, "contact time"), life is proportional tothe power of 0.4 of the contact time 26). In the test we
conducted, the contact time under slow rolling speed andfast rolling speed was in the range of 0.25 - 0.96msec,
indicating a difference of about four times. The
improvement of life due to this difference can be
estimated to be around 1.7 times, which mostly matcheswith the life tests conducted (Test Nos. 7 and 8). The
report by Akamatsu and Miller et al. does not mention thereason why life is extended with a slow rolling speed. We
assume that life is extended because the raceway
becomes more run-in with slow rolling speeds. Now, thereason why slow rolling speeds result in more run-in isstill unclear, which is a question to be addressed in the
future.

10100Life h1,000

10,000Fig. 8 Results of RCF testing with and without running-in
operation

formula. In the discussion, the probability of survival of
the rolling bearings S can be expressed by equation (1).

（ ）（
（

ln

1
S

= ln

+ ln

1

S hertzian-scale_stress1
S subsurface_stress

）（
）
+ ln

1
S surface_stress）（1）

Equation (1) indicates that the probability of survivalof rolling bearings is determined by the probability ofsurvival for each damage mode generated by thefollowing three types of stress.
(1) Hertzian-scale_stress of the first term: Stressgenerated under apparent contact (Hertziancontact) surface and stress that generates ordinary
subsurface initiated flaking under good lubrication
conditions(2) Surface_stress of the second term: Stress togenerate damage from the surface by frictional
force induced by contact of projections on rough4. 2 Effect of Rq, Rdq and tangential force ondamage mode
Appendix A of the paper by Ioannides et al., which was
the basis of ISO281:2007 published in 2007 28),
discusses the concept of correction coefficient aISO of life-92-

*14E+_*15 18/06/13 13:56 ページ 9

Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition

the life of peeling is not inversely proportional to the
cubes of the load, but mostly constant to the load,correction is not possible by multiplying the conventional
life formula by the factor obtained from the ASME
diagram. Peeling seems to occur with projections of large
Rdq creating high contact stress on the raceway of thecounterpart. Considering that, when peeling is about tooccur, the contact stress on the projection contact area isreaching the final limit which the bearing steel can
support as it goes through work hardening and residual
stress generation (hereinafter, "shake down limit" 39) ,these test results can be rather easily described. As
described in the paper by Ioannides et al. 28) , if Rdq
indicates the contact stress of the projection contact area,
when its value exceeds a certain value, the contact
stress of the projection contact area will have reached
the shake down limit regardless of the load condition, andthe change of load only affects the contact area.
Therefore, since the contact stress of that projection
contact area is the same in any condition, peeling occurs
mostly on the same loading cycle. This seems to be thereason why the peeling life becomes insensitive to theload. Considering this mechanism, peeling should stop
occurring if the maximum Rdq of each area becomes lessthan a certain value (in our tests, 4˚ or less) before acertain loading cycle (in our testing, 105 - 106). Thismeans that if peeling occurs in the bearings or not can beexperimentally confirmed by examining if Λ during thetest becomes 3 or more, Rdq of each area decreases to
4˚ or less before loading cycle of 105 - 106, as a
guideline.
The reason why peeling was observed more on the
inner diameter side of the rolling trajectory seems to bethat the inner diameter side of the inner/outer rings ofthrust bearings is the driven side in two cylinder tests,
and the outer diameter side is the driving side. Where,
the driven side and driving side are also called the low
speed side and high speed side, the indicating raceway
where the direction of action of the tangential force is thesame as the direction of rolling and the raceway where
the tangential force is in the opposite direction,
respectively. The fact that life decreases on the low
speed side is also indicated by Way's study 40), and there
is a discussion that the reason for decrease of life is dueto the difference of stress states and difference of crack
propagation behaviors 41,42). In any event, it seems thatthe reason why the damage due to peeling was moresignificant on the inner diameter side of the rolling
trajectory is because the peripheral speed of the innerdiameter side of the inner/outer rings was slower than the
peripheral speed of the steel rolling elements and the
direction of action of tangential force was aligned with therolling direction on the inner diameter side of theinner/outer rings.

surfaces with a relatively moderate slope (Rdq≦4˚)under sparse lubrication conditions(3) Subsurface_stress of the third term: Stress togenerate damage from the top surface by locallylarge alternating shear stress that works on thearea immediately below the high contact pressure
from the contact of projections on rough surfaces
with a relatively large slope (Rdq>4˚) under sparse
lubrication conditions
In the following, we examine the test results assumingthat the stress that determines the life of rolling bearings
can be classified as above:
Under the condition in the initial stage, where Rq and
Rdq for steel rolling elements were relatively small (Test
Nos. 1 - 9), the damage mode was flaking, and under thecondition, where Rq and Rdq were large (Test Nos. 10 12), the damage mode was peeling. Considering that in
Test Nos.1 - 9, Λ was 0.11 or less at the end of the test
(after run-in), and Rdq was less than 4˚ in all areas after
the test, even if it was more than 4˚ for inner/outer rings
before the test, the stress to determine the life isassumed to be (2) Surface_stress. The damage mode bythis (2) Surface_stress was flaking in most test samples.
On the other hand, in Test Nos. 10 - 12, considering thatΛ was smaller than the cases of Test Nos. 1 - 9, andRdq of the steel rolling elements was considered to bealmost always 4˚ or more during the test, the stress that
determines the life is assumed to be (3) Subsurface_stress. There is room for discussion if the stress which
generated peeling in Test Nos. 10 - 12 was alternating
the shear stress working on the immediately below
projection contact area of surface roughness; however, it
can be assumed that when Rdq is large, damage modechanges with the severity of contact of projections ofsurface roughness, creating peeling before flaking
generated by (2) Surface_stress. In one test (Test No.
10) both flaking and peeling were observed. That seems
to be the result of racing by two damage modes, namely,
flaking due to (2) Surface_stress and peeling created bycontact of raceways with a large Rdq. In Test Nos. 1 - 9,life of flaking by (2) Surface_stress is inversely
proportional to the cubes of load; however, in Test
Nos.10 - 12, life of peeling is mostly constant to the load.
Therefore, in Test No. 10 where the load was large, there
were no differences in the life of both damage modes,
and flaking and peeling appeared at the same time
(Table 2).
The reason why the life of flaking by (2) Surface_stressis inversely proportional to the cubes of the load is
unclear. However, this relation indicates that life can bepredicted only by multiplying the conventional life formulaby the life adjusting factor obtained from the ASME
diagram for flaking by (2) Surface-stress. In contrast, as-93-

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NTN TECHNICAL REVIEW No.84（2016）

4. 3 Life prediction under low Λ condition andfurther challenges

5. Conclusion
In this paper, life test results under low Λ conditions
for thrust ball bearings were discussed. Also, factors
determining life under low Λ conditions and life
prediction methods were examined.
1) Under the test conditions in this report, theinner/outer rings of the bearings experienced
damage in most cases. This is because surface
roughness of the steel rolling elements did notdecrease as much as the inner/outer rings during
the tests and the rolling contact surface of therolling elements attacked the raceways of theinner/outer rings.
2) No differences in life were observed in the tests
with different inner/outer surface roughness before
the tests. That is because there were no differencesin surface roughness of the steel rolling elements.
3) Under the conditions of slow rolling speed, the life
tended to be longer than the calculated life. That is
because decrease of surface roughness of steel
rolling elements during the tests was moresignificant under slow rolling speeds.
4) Two damage modes, namely, flaking and peelingwere observed under the test conditions in thisreport. Which damage mode occurs is determinedby how much surface roughness decreases during
the tests.
5) In the case where the damage mode was peeling,
life was insensitive to the loads. That is because thesurface pressure of the true contact area had
exceeded the shake down limit under the conditionof severe contact of surface roughness.
6) More peeling was observed on the inner diameterside of the rolling contact trajectory. That is
because the inner diameter side of the inner/outerrings is in line with the direction of the action oftangential force and the rolling direction.
7) For establishment of a more universal life prediction
method under low Λ conditions, estimation
technology of stress status of the true contact area
considering constantly changing Λ by run-in,
surface roughness, residual stress and hoop stress,
and a database of run-in behavior depending on
operating conditions are required.

From the above life test results and subsequent
observation, the following conclusions were obtained forlife prediction under low Λ conditions.
1) Life under low Λ conditions is affected by surface
roughness of the counterpart of the damaged part.
2) Surface roughness of each part of the rolling bearings
changes during operation and those changes areaffected by operating conditions such as load, rotation
speed, etc.
3) Under low Λ conditions, two damage modes, namely,
flaking and peeking are observed and the types ofstress that cause the damage are different.
4) Flaking occurs due to surface stress by friction force
induced by the contact of projections of surface
roughness, and the life is affected by Rdq.
5) Peeling occurs by contact of raceway and rolling
contact surface with Rdq of more than a certain value,
and the life is affected by the relation between the
direction of tangential force and rolling direction.
From the above, for life prediction under low Λ
conditions, technology to estimate stress conditions on
the top surface is required considering Λ that
constantly changes by run-in, surface roughness andfrictional force. In addition, it is considered that a
database of experiments with behavior of changes insurface roughness depending on operating conditions
is required.
If the status of stress that repeatedly acts on the
raceway by Λ and surface roughness that changes
during operation can be found, fatigue can becalculated in a time series using SN diagram,
Goodman diagram, Miner's law, etc. enabling more
universal life prediction. Another consideration to bemade is the residual stress distribution distributed
below the rolling contact surface and hoop stress byfitting. In this report, although residual stress and hoop
stress were not discussed, these stresses affectequivalent stress of the raceway and stress ratio. For
establishing a more universal life prediction method
under low Λ conditions, estimation technology of thestress status of the true contact area considering
constantly changing Λ by run-in, surface roughness,
residual stress and hoop stress, and a database of
run-in behavior depending on operational conditions
are required.

This paper is a transcript of the paper "Rolling
Contact Fatigue of Thrust Ball Bearing under Low
Lambda Condition" published in "The Journal ofJapanese Society of Tribologists" Vol. 60, No. 11(2015) 741 of the Japanese Society of Tribologists
with minor revisions. We thank the Japanese Societyof Tribologists, who gave us permission for publishingit here.
-94-

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Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda Condition

References
21) Y. Akamatsu, N. Tsusima, T. Goto, K. Hibi & K.Ito:
Improvement of roller bearing fatigue life by surface
modification, SAE technical paper series 910958 (1991).
22) Y. Akamatsu, N. Tsusima, T. Goto & K. Hibi: Influence of
surface roughness skewness on rolling contact fatigue life,
Trib. Trans., 35, 4 (1992) 745.
23) Yoshinobu Akamatsu: Improvement of Roller Bearing Fatigue
Life by Surface Roughness Modification, Lubrication, 37, 7(1992) 533.
24) Fuyuki Ito and Yoshinobu Akamatsu: Effect of Roughness ofCounterpart on Rolling Contact Fatigue Life, Proceedings ofTribology Conference, Spring 1995, (1995) 535.
25) J. R. Miller: Roller bearing life estimation at low speed andhigh stress, Proceeding of international tribology conferencein Yokohama, (1995) 1381.
26) J. R. Miller & Y. Akamatsu: Effect of low speed on roller
bearing fatigue life, Trib. Trans., 40, 1 (1997) 129.
27) Yoshinobu Akamatsu: Effects of Low Speed on Rolling
Bearing Fatigue Life, NTN Technical Review, 67 (1998) 59.
28) E. Ioannides, G. Bergling & A. Gabelli: An Analyticalformulation for the life of rolling bearings, Acta Polytechnica,
Mechanical Engineering Series, 137(1999).
29) ISO281:2007: Rolling bearings -Dynamic load ratings andrating life, (2007).
30) JIS B 1518: Rolling Bearings - Dynamic Load Ratings andRating Life (2013).
31) NTN Catalog: HL Bearings, CAT. No 3020/J.
32) For example, Masayoshi Muraki: Tribology, Science ofFriction and Lubrication Technology, Nikkan Kogyo Shimbun,
Ltd., (2008) 216.
33) For example, Junzo Okamoto: Calculation of Ball Bearings,(1991) 65.
34) L. Johnson: The Statistical Treatment of Fatigue
Experiments, Elsevier (1964) 37.
35) G. Lundberg & A. Palmgren: Dynamic Capacity of Rolling
Bearing, Acta Polytechnica 7, Mechanical Engineering
Series, 1, 3 (1947).
36) G. Lundberg & A. Palmgren: Dynamic Capacity of Roller
Bearing, Acta Polytechnica 96, Mechanical Engineering
Series, 2, 4 (1952).
37) NTN Catalog: Rolling Bearings, Overview Catalog, CAT. No.
2202-X/J.
38) JIS B 0601: Geometrical Product Specifications (GPS)Surface texture: Profile method-Terms, definitions andsurface texture parameters, (2001).
39) T. A. Harris: Rolling bearing analysis 4th ed., Wiley & Sons
Inc., (2001) 853.
40) S. Way: Pitting due to rolling contact, J. Appl. Mech., 2, 2
(1935) A49.
41) Norimune Soda: Bearings, Iwanami-Shinsho, (1964) 176.
42) Motohiro Kaneda, Yukitaka Murakami and Hirohiko
Yatsuzuka: A Fracture Mechanical Study on Contact Fatigue
Crack Propagation, Lubrication, 30, 10 (1984) 739

1) T. E. Tallian: On competing failure modes in rolling contact,
ASLE Trans., 10, 4 (1967) 418.
2) J. C. Skurka: Elastohydrodynamic lubrication of roller bearings,
J. Lubr. Technol. Trans. ASME, 92, 2 (1970) 281.
3) Rolling-Element committee lubrication division of ASME,
Catalog Card Number 170-179492 (1971).
4) C. H. Danner: Fatigue life of tapered roller bearings underminimal lubrication films, ASLE Trans., 13, 4 (1970) 241.
5) Y. P. Chiu, T. E. Tallian & J.I. McCool: An engineeringmodel of spalling fatigue in rolling Contact, I.: The
subsurface model, Wear, 17 (1971) 433.
6) T. E. Tallian & J. I. McCool: An engineering model ofspalling fatigue failure in rolling Contact, II.: The surface
model, Wear, 17 (1971) 447.
7) T. E. Tallian: An engineering model of spalling fatigue failurein rolling contact, III.: Engineering discussion and illustrative
examples, Wear, 17 (1971) 463.
8) R. A. Hobbs: Fatigue lives of ball bearings lubricated withoils and fire-resistant fluids, EHL symp. IME C1 (1972) 1.
9) C. A. Moyer: Relating lubrication parameters and rolling
contact fatigue life in mixed EHL regime, EHL symp. IME C1(1972) 95.
10) G. H. G. Vaessen & A. W. J. de Gee: Rolling contact fatigue
of maraging steels of different production history: Influenceof film thickness/roughness ratio, EHL symp. IME C7 (1972)
40.
11) S. Anderson & T. Lund: Ball bearing endurance testing
considering elastohydrodynamic lubricatoin, EHL symp. IME
C36 (1972) 138.
12) I. M. Felsen, R. W. McQuard & J. A. Marzani: Effect of sea
water on the fatigue life and failure distribution of floodlubricated angular contact bearing, ASLE Trans., 15, 1(1972) 8.
13) J. Y. Liu, T. E. Tallian & J. I. McCool: Dependence ofbearing fatigue life on film thickness to surface roughness
ratio, ASLE Trans., 18, 2 (1975) 144.
14) D. F. Li, J. J. Kauzlarich & W. E. Jamison: Surface
roughness effects on fatigue in partial EHD Lubrication, J.
Lubr. Technol. Trans. ASME, 98, 4 (1976) 530.
15) N. G. Popinceanu, M.D. Gafitanu, S.S. Cretu, E.N.
Diaconescu & L. T. Hostiuc: Rolling contact fatigue life andEHL theory, Wear, 45 1 (1977) 17.
16) Masatoshi Tokuda, Mitsutoshi Nagabuchi and Shigeo Ito:
Peeling Damage on Rolling Contact Surface of Bearings,
Bearing Engineer, 45 (1977) 8.
17) Kyozaburo Furumura, Shinichi Shirota and Kiyoshi
Hirakawa: The Subsurface-initiated and Surface-initiatedRolling Fatigue, NSK Bearing Journal 636 (1977) 1.
18) Hirotoshi Takada, Susumu Suzuki and Etsuo Maeda: Effectof lubrication on fatigue life of rolling bearings, Lubrication,
26, 9 (1981) 645.
19) Hirotoshi Takada: Life and surface roughness of rolling
bearings, Lubrication, 27, 2 (1982) 105.
20) Y. Akamatsu: Peeling damage due to rolling contact fatigue,
SAE technical paper series 891909 (1989).

Photo of authors

Takumi FUJITA

Naoya HASEGAWA

Naoya KAMURA

Toshihiko SASAKI

Advanced Technology
R&D Center

Advanced Technology
R&D Center

Advanced Technology
R&D Center

Professor,
Faculty of Human Sciences,
Kanazawa University-95-

*15E_*15 18/06/13 13:57 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Speeding up of Parallel Link Angle Control Equipment

Kenzou NOSE*
Hiroshi ISOBE*
Seigo SAKATA*NTN has developed an improved Parallel Link Angle Control Equipmentthat can control the angle with two-degree-of-freedom fast and widely.
Enhancing the drive mechanism, link part, and control method of the uniqueparallel link mechanism, this equipment can be improved repeat positioning
accuracy and positioning speed more than our conventional equipment.
Furthermore, we had developed the grease dispensing system andappearance inspection system there are adapted this equipment.

side link hub serves as the base of the mechanism
and the top-end side link hub is used to install the end
effector. The joints of each member of the linksystems are turning pairs.
The base-end side arm of a 3-row link system isconnected to its central link on one end and the baseend side link hub on the other end. The center lines of
the rotation axes of the base-end side arm, central link
and base-end side link hub all intersect at one point.
Having the same structure at the top-end side, thisparallel link mechanism has a structure that iscomposed of two "spherical links". Each member
moves on the two spheres S and S', with therespective intersections at their centers as shown in
Fig. 2. The attitude of the parallel link mechanism isdefined by angles: bend angle θ and the angle oftraverse φ. The bend angle θ is the inclination angle
between the central axis of the base-end side link hub
and central axis of the top-end side link hub. The
angle of traverse φ is the angle of rotation of thecentral axis of the top-end side link hub when viewed
from the central axis of the base-end side link hub.
Because of this structure, the base-end side and the
top-end side always move symmetrically against thebisecting plane made by the intersection of two
spheres S and S', which compose two sphericallinks 2).
This parallel link mechanism is characterized by awide operation range and high-speed/high-precision

1. Introduction
The parallel link mechanism is drawing attention for
its high-speed handling of industrial robots. Thismechanism connects base and end effectors using
multiple parallel links, and has various types ofconfigurations, such as the Steward-Gough type,rotational type and linear fixed type 1).
The parallel link angle control unit that the authors
have developed adopts a special parallel link that isdifferent from the previously mentioned types. It issmaller and has a wider angle of movement, whichallows it to achieve fast and high precision two-degreeof freedom angular positioning 2).
This article discusses the configuration,
performance, control technology and applications of
the improved unit, which offers higher speed than theprevious unit.

2. Basic configuration of the parallel
link mechanism
Conceptual diagrams of the parallel link mechanismand spherical link of this system are shown in Fig. 1
and Fig. 2. This mechanism takes three rows of linksystems, each consisting of a base-end side arm,
central link and top-end side arm (the 1st - 3rd in Fig.
1), and places them parallel between the base-end
side link hub and top-end side link hub. The base-end* New Product Development R&D Center-96-

*15E_*15 18/06/13 13:57 ページ 2

Speeding up of Parallel Link Angle Control Equipment

End effector

motion. It is also characterized by the ability to makethe moving part light and compact by separating the
movable part and driving part, and by possessing no
singular points within the operation range due to theadoption of the spherical link structure. In addition, it isable to run cables and tubes through the central space
without being twisted, even with continuous rotation in
one direction.

Spur gear

Parallel linkmechanism

Drivingmechanism
0°
Angle oftraverseφ
Bend angleθ

BaseFig. 3 The structure of Parallel Link Angle Control
Equipment（conventional）

3rd link system

1st link system
Top-end side link hub

Parallel linkmechanism2nd link system

Fig. 1 Unique parallel link mechanism

Drivingmechanism

Angle of traverseφBase-end side
spherical link (S')
Top-end side arm
Central link

Base-endside arm
Base-end side
link hubEnd effector(a) Entire view

Bend angleθ
Top-end side
link hub

Base

Base

Motor

Center of
spherical link
Bisecting
plane
Cable
Base-end side
spherical link (S)Fig. 2 Conceptual diagram of spherical link
φ500mm

3. Parallel link angle control unit(b) Drive mechanism

The conventional unit is shown in Fig. 3, theenhanced version is shown in Fig. 4, and conceptual
diagrams are shown in Fig. 5 and 6. In both versions,the movable range of the bend angleθis within 45˚;
however, it can be extended to a max. of 90˚ by using
an optional design.
Since the internal space of the conventional unit issmall and the end effector is installed at the top-end,
there is a long distance from the base to the top-endof the end effector and a large moment of inertia,
which makes it difficult to achieve faster operation. In

Fig. 4 The structure of Parallel Link Angle Control
Equipment（improvement）-97-

*15E_*15 18/06/13 13:57 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

precision. Furthermore, the increased internal space
allows for easier arrangement of cables.

addition, the cabling arrangement of the end effector iscomplex. Furthermore, it has safety and maintenanceissues because the spur gear of the driving
mechanism is exposed.
The following discusses the improved model thatsolves the above issues by adopting an integrated
structure of a reducer and motor for the driving
mechanism, and enhancing the link structure.

φTop-end side
link hub

3. 2 Driving mechanism
The attitude of the top-end side link hub expressedby the bend angleθand angle of traverseφcan be
determined when two of the three angles of rotation on
the base-end side arm are identified. However, as
shown in Fig. 4 (b), each arm of this unit is driven by
its respective motor to cancel out the backlash of thedriving mechanism by restricting the 3-row linksystems in order to improve positioning accuracy.
The conventional unit uses a spur gear to transmit
the driving force of the motor to each base-end sidearm 2). The driving mechanism of the improved unit
(Fig. 4) eliminates the exposed spur gear of theconventional unit, and replaces it with a motor with the
reducer directly installed on each base-end side arm.
As a result, the outer diameter of the entire system islarger, but the height is reduced and safety andreliability are improved.

φ=0°θ
Center of spherical linkon base-end side

Top-end sidearm
Central link

Center of
spherical linkon top-end side

Base-end side
link hub(base)

Base-end side arm

3. 3 Teaching console
A teaching console (hereinafter, "console") has
been developed for introduction to production lines.
Fig. 7 shows a conceptual diagram of position
registration by XYZ coordinates, Fig. 8 (a) shows the
JOG operation screen and Fig. 8 (b) shows theconsole screen for inputting the coordinates. These
operation screens are used for the registration andfine tuning of the position using a cross key, and for
directly entering the XYZ coordinates. In addition,
registration of multiple movement patterns and settingof operational conditions can be made. These
capabilities are effective for setting and resetting
movement, and improving efficiency at work sites. In
addition, editing software has been developed, which
can be used on personal computers to easily adjust
movement patterns. Furthermore, this editing software
enables data to be read from the console in order toadjust and back up of the movement patterns.

Fig. 5 Conceptual diagram of parallel link mechanism(improvement)

δ2
β2
φ

δ3

β1

β3
α

φ=0°
δ1（δ=0°）Fig. 6 The arrangement of the base end link mechanism

3. 1 Parallel link mechanism
This unit uses pre-loaded angular contact ballbearings for the turning pair of the parallel linkmechanism to reduce friction and eliminate the gap
between the joints. The improved model adopts an
integrated base-end side link hub and base, andplaces the turning pair on the outer diameter sidewhich creates more internal space in the parallel linkmechanism than the conventional unit so that the end
effector and cabling for the end effector can be placed
inside. These modifications reduce the moment ofinertia when the end effector is installed, whichenables faster operation while maintaining motion

Z

Y

X
Fig. 7 Move imagine by installation alignment-98-

*15E_*15 18/06/13 13:57 ページ 4

Speeding up of Parallel Link Angle Control Equipment

4. 2 Operating accuracy and vibration reducing
control algorithm
An algorithm to improve operating accuracy was
introduced by the Preshaping Command Input method 3).
The operating accuracy was determined by evaluating
grease application accuracy when it was applied in thegreasing operation, as described later.
The grease application was evaluated by a visualinspection after applying grease on evaluation sheets
under a predetermined condition Evaluation sheets,
shown in Fig. 10, were used to test how grease wasapplied to several points on the sheets by changing
acceleration and deceleration. The grease application
diameter is approx.φ3.5mm and the application point
diameter isφ1mm. The evaluation was scored bygiving plus 1 point if the application point wascompletely covered by grease, 0 points if the point
was marginally covered, minus 1 point if partiallycovered, and minus 2 points if not covered at all, as
shown in Table 1.

(a) JOG operation screen(b) Coordinate input screen
Fig. 8 Operation screen of teaching console

4. Control method
4. 1 Method for instructing angles to motors
Angles are instructed to motors by obtaining theangles of rotation for each base-end side arm βn (β1,β2, β3) from the instructed angles (θandφ). The
three motors are synchronously controlled to start andend operation at the same time by obtaining thedifference between the current angles at each baseend side arm β’n (β’1, β’2, β’3) and the target angles
of rotation from the instructed angles βn (β1, β2, β3).
The instructed speed is given as the synthesis ofrotational speed of the three base-end side arms V
and the instructed speed for each motor Vn is
calculated by equation (1). In addition, the motors are
driven by "ramp-up/down" control in the shape of a
trapezoid as shown in Fig. 9.
Vn=

42
41
40

③

⑤

0

①
④

⑥

O→①→O→②→O→③…→ 41 →O→ 42

Fig. 10 Grease dispensing sequence

Table 1 Evaluation method of grease dispensing
accuracy
Application
accuracyV（β n−βn）（β1−β1）2＋（β2−β2）2＋（β3−β3）2

◎

○

▲

×

0

-1

-2

Grease(n= 1, 2, 3) ……………（1）Instructed
speed

②

Application
position

Speed

Application point

Vn

Evaluation
score
Time
Accelerationtime

Decelerationtime
Operation time

Fig. 9 Drive pattern of motor

-99-

1

*15E_*15 18/06/13 13:57 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

Evaluation results are shown in Fig. 11. The natural
frequency of the unit used in the test was 53
ms/period. The vibration of the tip after positioning
could be reduced by setting the acceleration anddeceleration time to one period of the unit’s natural
frequency. This reduction in tip vibration enables highapplication accuracy.
In a similar test, application accuracy deteriorated
after a short traveling distance. It is assumed thatwhen the traveling distance is short, the actual
acceleration/deceleration time is shorter than the
period of the natural frequency, which prevents the
suppression of vibration when the tip stops. Therefore,
when the traveling distance is small, the control
algorithm is changed to automatically calculate theinstructed speed to make the acceleration anddeceleration times equivalent to one period of thenatural frequency. This is accomplished by eliminating
the constant speed time in the trapezoid previously
mentioned, making it a triangle shape. As a result, thevibration of the tip after positioning was reduced.

5. Application examples
5. 1 Grease applicator
The specifications and performance comparison of
the conventional unit and improved unit when they
were applied to grease applicators are shown in Table
2. An example configuration of the grease applicatorusing the improved unit is shown in Fig. 12 (a).
In this configuration, the work is moved by the XY
stage and the attitude of the dispenser is controlled bythis unit. This configuration uses a non-contact typedispenser which injects grease by compressed air.
The improved unit encloses the end effector in theinternal space of the parallel link mechanism.
Therefore, the distance between the nozzle position of
the dispenser installed on the top-end side link hub
and the base is shorter. As a result, the positioning
accuracy of the nozzle during repetitive operation
could be improved (Table 2). Also, since the momentof inertia for the positioning system is smaller, thenatural frequency becomes larger, allowing higher
application speed.
The grease applicator achieved a high speed
application of 10 points/sec on the matrix sheet (※1).
It also enables an automated greasing process forworks with complex shapes by application fromslanted directions. Fig. 12 (b) shows an example ofapplication on the side of the gear which is notpossible by application from the top 4).

45
40

Evaluation

35
30
25
20
15
10

5.2 Visual inspection system

5

The configuration of the visual inspection systemusing the improved unit is shown in Fig. 13. In thisconfiguration, the work is moved by the XYZ stage atthe bottom, and the attitude of the camera is controlled
by the unit. Inspection of the sides and edges of thework, as well as detection of missing bolts in recessed
areas, can be automated in a compact configuration
without using multiple cameras.

0
0.03

0.04

0.05

0.06

0.07

0.08

0.09

Acceleration (deceleration) time s

Fig. 11 Grease dispensing accuracy

Table 2 Performance comparison
Without dispenser
Size mm(Outer diameter x height)
With dispenser
Mass (kg)Max. carrying capacity kg

Conventional unit
Improved unit
φ400×107.5
φ500×190
φ400×262.5
φ500×23010
1.0Movable range ˚Bend angle θ: 0-45
Angle of traverse φ: 0-360 (unlimited)

Positioning accuracy
for repetitive operation

Angle ˚Nozzle position mm±0.065
±0.22

±0.15

Natural frequency (Hz)(When dispenser is installed)

25.6

27

Application speed s/point ※1

0.095

0.089

※1 Application time per point when applying grease on 49 points of matrix sheet (7 x 7 point, 10mm of interval)(grease amount: 5mg/point)-100-

*15E_*15 18/06/13 13:57 ページ 6

Speeding up of Parallel Link Angle Control Equipment

Parallel link type angle control unit

6. Summary
We have developed a parallel link angle control unit
using a special parallel link mechanism with tworotational degrees of freedom and the end effector
placed inside the internal space. By reviewing thedriving mechanism and controlling methods, we have
improved the life, safety, positioning accuracy andoperation speed from conventional units. We areworking on deploying units in production facilities to be
used for grease application and visual inspection. In
the future, we will work on expanding deployment into
many other fields such as medical/welfare and living
environments, which will further contribute to theevolution of automation and robotization.

Dispenser

Work

XY stage(a) System configuration of grease dispensing

Gear

References
1) The Robotics Society of Japan, Robotics Hand Book, New
Edition, Corona Publishing, 2005, 297
2) Hiroshi Isobe, Yukihiro Nishio: Parallel Link High Speed
Angle Control Equipment (PHACE), NTN TECHNICALREVIEW, No. 80, (2012), 42-47.
3）Neil C. Singer, Warren P. Seering, Preshaping Command
Inputs to Reduce System Vibration, J. Dyn. Syst. Meas.
Control，Vol.112，No. 1，（1990）76-82
4) Hiroshi Isobe, Yukihiro Nishio, Keisuke Sone, Hiroyuki
Yamada, Yoshio Fujikawa: Parallel Link High Speed
Angle Control Equipment, Proceedings of the Japan
Society for Precision Engineering Spring Meeting, 2013,(2013), 809-810.

Grease(b) Grease dispensing sample
Fig. 12 System configuration example of grease
dispensing

Parallel link typeangle control unit
CameraXYZ stage

Enlarged view of work

Fig. 13 System configuration example of visual inspection

Photo of authors

Kenzou NOSE

Hiroshi ISOBE

Seigo SAKATA

New Product Development
R&D Cente

New Product Development
R&D Cente

New Product Development
R&D Cente-101-

*16E_*15 18/06/13 13:58 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Propeller Blade Bearings for Aircraft Open Rotor Engine

Guillaume LEFORT*The Propeller Blade Bearings for Open Rotor Engine SAGE2
were developed by NTN-SNR in the frame of the Clean Sky
aerospace programme. The goal of this project is to design,
manufacture and test the propeller blade bearings for the upstream
and downstream propellers of SAGE 2 Open Rotor.
The propeller blades of the two rotors are attached to engine
cases and are orientated at various angles by the two independent
Pitch Control Mechanisms.
The propeller blade bearings ensure the transfer of aerodynamic and centrifugal loads to the respective engine
cases while allowing the pitch change of the blades.
Based on its know-how, its scientific knowledge and Research studies, NTN-SNR developed a new bearingarchitecture, completed a general mitigation test plan, successfully passed the validation tests and delivered thebearings for the Open Rotor Sage2 demonstrator.

1. Introduction

2. Context of the project

The SAGE2 counter-rotating open rotor is anaircraft engine concept developed in the frame of theEuropean Union Programme “Clean Sky”. It is themost attractive aircraft engine concept to reduce thefuel consumption and the gas emission (CO2, NOx, …).
In fact, this concept allows improving the propulsive
efficiency and the thermal efficiency while growing up
the by-pass ratio (higher the by-pass ratio is, lower thefuel consumption is with the same weight).
However, before introducing this concept in
production, several technical issues had to be solved.
A lot of innovative works had to be studied for the
integration of the open rotor to the aircraft and for the
propulsive part (gearbox, propeller blade design,propeller blade integration: pitch change mechanism,
propellers blade bearings).
NTN-SNR was selected by the EU to design,develop and manufacture the propeller blade bearing
prototypes for this demonstrator, which are one of thecritical parts for Open Rotor Engine.
In this paper, we briefly discuss about the context ofthe project and present the propeller blade bearings
developed by NTN-SNR for this engine.

2. 1 Clean Sky programme
Clean Sky is the most ambitious aeronauticalresearch programme ever launched in Europe. Its
mission is to develop breakthrough technologies tosignificantly increase the environmental performancesof airplanes and air transport, resulting in less noisy
and more fuel efficient aircraft, hence bringing a key
contribution in achieving the Single European Sky
environmental objectives. 6 technologies are studiedin Clean Sky (see Fig. 1).

Fig. 1 Technologies studied by Clean Sky programme*NTN-SNR Roulements, R&D Aerospace Design Office-102-

*16E_*15 18/06/13 13:58 ページ 2

Propeller Blade Bearings for Aircraft Open Rotor Engine

2.4 Customer specification and technicalchallenges

In the frame of this R&D programme, NTN-SNR
answers to a call for proposal about propeller blade
bearing activity in the frame of Sustainable And Green
Engine technologies (SAGE).
The NTN-SNR proposal has been classified as the
top ranked one, against others proposals and NTNSNR has been officially selected on the project on
October 19th 2012.

Regarding the propeller blade bearings, NTN-SNR
has a very good experience in the concepts forcommercial turboprop aircraft already in production.
However, due to the SAGE2 technical specificationand the location of the propellers, none of the current
blade bearing designs can be used on the open rotor
concept. The Fig. 4 and Fig. 5 show the differences
between current blade bearing specification and
SAGE2 open rotor blade bearing specification.
The 4 main bearings specific challenges for thisinnovative application were:- Criteria 1: High level of loads (especially centrifugal
force) with +15% compared to the current application- Criteria 2: Large temperature range and high level ofmaximum temperature (-55°C to 180°C): twice
higher than current application- Criteria 3: Smaller allocated room to design thesolution than existing one- Criteria 4: The sealing device has to be inserted in
the new propeller blade bearing. The current design
doesn’t include the sealing device inside the bearing.

2. 2 Open Rotor Architectures
The Open Rotor architecture is an aircraft engine
composed of a gas generator and 2 propellers. Thegas generator provides the mechanical energy torotate the 2 propellers, as shown on the Fig. 2.
This concept combines the qualities of theturbopropulsor (propulsive efficiency, thermal
efficiency) and the benefits of the turbojet engine(higher flight speed, lightness,…).

Load daN

Fig. 2 Open Rotor architecture

2. 3 Schedule
The 34 months duration project has cost €1.5
million with 50% of EU contribution.
The schedule is described on the Fig. 3.
Rotational speed of propeller min-1

Fig. 4 Loads comparison between current applications
and SAGE2 Open rotor specification

Fig. 3 Project schedule (civil year)

Fig. 5 Propeller blade Bearing allocated room
differences between current applications and SAGE2
Open rotor specification-103-

*16E_*15 18/06/13 13:58 ページ 3

NTN TECHNICAL REVIEW No.84（2016）B) develop a new complex tribological system in orderto minimise the friction torque and avoid any
raceway wear (criteria 1 and 2)C) develop an innovative sealing system answering tocriteria 4.
Regarding all the risks existing on this project a
mitigation plan was established and completed.

3. Technical solution
3. 1 Objectives
The goal of this project was to design, manufacture
and test the propeller blade bearings for 2 propellersof SAGE 2 Open Rotor (See Fig. 6).
The propeller blades of the two rotors are linked toengine cases (bearing housing) and are orientated atvarious angles. The propeller blade bearings ensure
the transfer of aerodynamic and centrifugal loads to
the respective engine cases while allowing the pitchchange of the blades.

A) Bearing architecture
Based on criteria 1 and 3, several architectures ofbearings have been defined and calculated using
specific new adapted Finite Element methodology.
This new methodology has been developed during
the project because the constraints of integration,
especially the complex thermal gradient and thestiffness of parts. Eventually, only two architectures
meet the specifications and based on thesecalculations (See Fig. 7), the best bearing architecture
was selected. The selected architecture is a double
row angular contact ball bearing.

Fig. 7 Stress distribution of finite element calculation
Fig. 6 Location of Propeller Blade bearings on the engineB) Tribological system selection
These propeller blade bearings are one of thecritical Open Rotor Architecture parts. Without
efficient, reliable and safety propeller blade bearings,
no Open Rotor Engine could be introduced in mass
production on the commercial civil aircraft.
The main functions of these bearings are:• Allow oscillatory rotation between the engine caseand the propeller blade shaft• Transmit the centrifugal and aerodynamic loads fromthe propeller blade to the engine case
• Resist to the environment (thermal, fluids,..)• Ensure the stiffness between the propeller blade root
and the engine case
• Resist to a load caused by the loss of propeller blade

It is important to highlight that the contact lubrication
is a key parameter for the success of this project
because both the life duration of the bearing and thebearing starting torque depend on the contact
lubrication. Due to the extreme specification, thelubrication cannot be done by one type of lubricant.
Therefore, NTN-SNR has developed a complex
tribological system to allow for good bearing
lubrication (See Fig. 8). This selection was divided
into three steps:

3. 2 Description of the technical studies
To meet the specification requirements, 3 innovative
works were conducted:
A) design a new bearing architecture addressing
criteria 1, 2, 3 and partially 4

Fig. 8 Complex tribological system developed-104-

*16E_*15 18/06/13 13:58 ページ 4

Propeller Blade Bearings for Aircraft Open Rotor EngineB-1) Exploration of existing solutions

temperature tests were performed to validate the good
behaviour of the solutions at 180°C, where thereference is not valid. These tests performed theequivalent in service time on the three best solutions.

The aim of this task was the exploration of allexisting lubricant solutions. This task was based on
NTN-SNR experiences, literature and supplierexperiences. It is important to highlight that the major
criterion for the lubricant is the capacity to sustain very
high contact pressure (up to 4 GPa). It is also achallenge for the surface treatment.
12 potential greases and several surface treatments
were studied.
Only the four best greases and the best surface
treatment were further studied in the following step.

Conclusion: Based on all these tests, NTN-SNR
defined and developed a customised complex
tribological system which has the same
performance as the propeller blade root “inservice” solution in an environment respecting therequirements criteria.

C) Sealing deviceB-2) Tribological solution definition

Intensive work has also been done to design
complex sealing devices to avoid the leakage of
lubricant outside of the bearing and the penetration ofpollution inside the bearing. In fact the bearing sealing
system is composed of two seals: the upper seal andthe lower seal as shown on the Fig. 10. The maximal
equivalent pressure applied on the upper seal by thecentrifugation of the grease is about 16 bars withsafety factor. In addition, at low temperature, thegrease has to have a low viscosity to ensure a good
lubrication of bearing.

The target of this step was to select three bestsolutions regarding the global tribological behaviour.
NTN-SNR explored the different types of lubricants(greases and surface treatment defined previously)
and the combination of the different types. The impactof the bearing ring materials and their heat treatment
was also evaluated. The ring and ball materials are
established by the bearing sizing. Several solutions
were studied for three other components.
The three best tribological systems were selected
regarding laboratory test results.

B-3) Validation of the selected solutions
The aim of this task was to test three best selected
solutions in representative conditions, studying the“False Brinelling Effect” on three test benches: at room
temperature, at high temperature and at low
temperature. The first two test rigs already existed at
NTN-SNR and the last one had to be built (See Fig. 9).

Fig. 10 Sealing system

These 3 solutions were compared with the solutionof reference used on the current application for thetest at room and low temperature. However, the highThe detailed study of sealing, including functional
analysis, risk analysis and product Failure Mode andEffect Analysis dedicated to the sealing, was donewith the supplier. Due to the specificity of therequirement and to optimise the integration aspect(torque, weight, cost …), the architecture of the lower
and upper seals used on the bearing assembly is not
the same. The Finite Element calculations showedlarger displacements of the seal seatings than existing
applications. Therefore, the seal design has to be
reinforced to take into account these displacements.
Due to the high level of risk, a sealing validation
plan was performed. It consists of six tests:• Material compatibility tests:
These are laboratory tests to validate the good
behaviour of the seal material with the materials(fluids or chemical components) that could be incontact with the seal during its life.
Fig. 9 False Brinelling test bench-105-*16E_*15 18/06/13 13:58 ページ 5

NTN TECHNICAL REVIEW No.84（2016）• “Raffer test”: ( shown on Fig. 11)
This is a fatigue pressure test to study the
behaviour of the seal regarding complex cycles andendurance.

Oil under pressure
Oil under pressure
Seal
Housing
Ring

Shaft

Fig. 11 “Raffer test”
Fig. 12 Starting test measurement test bench• Adhesive test:

TorqueThis test aims to validate the good adherence
between the elastomer part of the seal and theframe.
It is important to highlight that an adhesive issue
could result also in a non-optimised seal design
which puts a high level of strain on the link between
the elastomer and the frame.

• Dismounting test:
This test measures the load necessary todisassemble the seal of the bearing in order todemonstrate that there is no displacement of theseal and deflector during the engine test.

Load value daN

Fig. 13 Example of torque measurement results obtained• Starting torque measurement on seal:
This test aims to measure the starting torque ofthe seal alone. The torque value measured by thistest is used to compare the starting torque of thethree solutions.

3. 3 Results
After the completion of the three innovative works, thebearings were manufactured. After that, the prototype
bearings were validated:• For criteria 1, 2 (partially) and 3, the good behaviour ofthe bearing under load was validated with tests on ballbearing on a compression/torsion test machine (seeFig. 14). The results are shown on the Fig. 15. The twobearings successfully passed the limit and ultimate
load tests. And the torque under load is two times
lower than the starting torque requirements included in
the course of the project. The two bearings will allow:• Good behaviour of the engine, allowing oscillatory
rotation between the engine case and the propeller
blade, transmitting centrifugal and aerodynamic loads
from the propeller blade to the engine case, ensuring
the stiffness between the propeller blade root and theengine case
• Safe working of the engine, avoiding the loss ofpropeller blade.
• The criteria 4, is only validated by the “Raffer” tests
performed on the seals.

• Starting torque measurement on standard
representative bearing in terms of diameter,ball size and number of balls:
This test mitigates the risk of having a starting
torque higher than the specification. In fact, if the
bearing torque is too high, the actuator which
controls and changes the blade pitch angle cannot
modify the angular position of the blade. Thus theengine does not work. A specific test methodology
was developed for this project. The Fig. 12 shows
the test bench.
The results are shown in the Fig. 13.
The best seal design regarding performance and life
duration was assembled on the bearing prototypes.

-106-

*16E_*15 18/06/13 13:58 ページ 6

Propeller Blade Bearings for Aircraft Open Rotor Engine

4. Conclusion
In conclusion, during this project, NTN-SNR has
developed:- a new compact self-lubricated architecture of bearing- a new complex tribological system to allow a good
lubrication of bearing under extreme conditions(centrifugation loads and high temperatures)- a new sealing device compliant with high pressure
fatigue safety margin.

LVDT

Brushless motor

Force/torquesensorHydraulic cylinder

All these innovations will be definitely validated on
SAGE2 Open Rotor demonstrator during a ground test
campaign. The Open rotor demonstrator will be tested
in the South of France at the end of CY2016.
The bearing solution developed in the frame of thisproject by NTN-SNR will now become one of thetechnical standard for all the blade root application(unducted or ducted propeller) which works in highspeed conditions, high temperature environment, in asmall allocated room where sealing and self-lubricant
are requested. (See Fig. 4)

Angular position ˚

Axial load kN

Fig. 14 Compression/torsion test machine

0

These bearings contribute to the positiveenvironmental impact of the Open Rotor engine:- CO2 emission reduction:-15% to - 17%- Noise reduction:-6 to -9 dBTime sFig. 15 Results of the validation tests

The impact of the innovative technologies
developed for these bearings could be applied on
several aerospace applications (propeller blade
bearings, fan blade bearings, main rotor blade
bearings, swash plates, ...). Moreover the tribologicalsystem could be applied on other bearings for industry
market or automotive market.

The project developed and thoroughly tested thebearing prototypes up to TRL5. As a result, the bearings
for the two SAGE 2 demonstrator propellers with thesubstantiation documents have been delivered to thecustomer.
The bearing and its 13 different components were
manufactured following all the aerospace quality
standards.

4 patents were applied based on the results ofthese studies.
Out of 482 projects, NTN-SNR was ranked within
the 10 best projects.

Photo of author

Guillaume LEFORTNTN-SNR Roulements
R&D Aerospace
Design Office-107-

*17E_*15 18/06/13 13:58 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ New Product ]Products Introduction of Composite Material
for Industrial Machinery
Shinji KOMATSUBARA*
Toshihiko MOURI**
Takuji HARANO**
Tamaki MIZUTANI***NTN Composite Material Product Division has developed the mechanicalparts and bearings, and the unit module products by a wide range ofmaterials and technology such as resin material, magnetic material and fluid
dynamic pressure technology. This paper introduces our products that meetvarious required characteristics of industrial machinery such as medical instrument, food machinery, hydraulicequipment and information device etc.

sintered metal products, hydrodynamic bearings for thintype fan motors used for information devices, and resin
sliding components, specifically resin-based sliding
bearing units and resin rolling bearings with excellent
handling properties. for

1. Introduction
NTN's composite material business is working on thedevelopment of products leveraging technologies related
to resin material, sintered metal, magnetic material andfluid dynamic, and combining these technologies withunit module products.Our composite material products
meet the various demands of the industrial machineindustry and automobile industry as products that:
achieve both high functionality and productivity by
combining a blend of additives to give several functions
to the materials, and have unique designs that are only
possible to make by using a metallic mold. These
products are produced by the associated companies
indicated in Fig. 1. In this article, we will introduce you toa choke coil used in MRI medical devices, as an
example of magnetic material products. We will also
introduce high strength machine parts with mechanicalproperties equivalent to ingot materials as an example of

2. Choke coil for high current MRIThe MRI (magnetic resonance imaging) device,
shown in Fig. 2, is required to take clear pictures in ashort time. Therefore, it uses high currents to produce
strong magnetic fields and high frequency drives torealize high-speed image processing. For power supply
circuits, choke coils for high currents are used to remove
noise generated in circuits. Choke coils are required tomaintain low-loss and stable properties up to highcurrent/high frequency bandwidths.
NTN developed magnetic materials using low-loss
amorphous alloy powder, which can be used for
injection molding, and applied it to choke coils for MRI 1).

Composite Material
Product Division

BEAREE®

NTN Engineering Plastics Corporation

Resin sliding bearings, resin injection molding products,
resin machining, coating parts

BEARPHITE®, NIKAMET®
NTN Advanced Materials Corporation
Sintered oil-impregnated bearings, sintered machine parts,
magnetic material products, resin injection molded products"

Hydrodynamic BEARPHITE

NTN Manufacturing (Thailand) Co.,Ltd

Fluid hydrodynamic bearing parts

Fig. 1 Composite material products of NTN Group

Fig. 2 Medical Magnetic Resonance Imaging***Fluid Hydrodynamic Bearing Dept. Composite Material Product Division***Engineering Dept. NTN Advanced Materials Corporation***Engineering Dept. NTN Engineering Plastics Corporation-108-

*17E_*15 18/06/13 13:58 ページ 2

Products Introduction of Composite Material for Industrial Machinery

2. 1 Features

120

Inductance change rate %Fig. 3 shows NTN's choke coils for MRI power
supply. The magnetic, core covering exterior isgenerally made by compression molding ofamorphous alloy powder. However, amorphous alloy
powder has poor moldability and is limited in size andshape. It is also fragile, so it is not easy to handle.
NTN has succeeded in expanding the range of shapes
for magnetic cores by making the material injection
moldable.
Presently, 125A - 300A types are in volume
production.

100
80
60
40

AS10

Fe-6.5Si
Ferrite

20
0

1

10

100

1,000

Frequency kHz

Fig. 5 Frequency characteristics

Since AS10 is stable with small inductance change
in a high current/high frequency, it is a material
suitable for operating conditions of choke coils for highcurrents.
Moving forward, we will expand the use of thismaterial for various applications in industrial machines
and automobile industries which require high currents
and stable high frequency responses.

Magnetic core
Type 300A

Type 260A

Fig. 3 Choke coil for MRI power supply

3. High-density/high strength sinteredmachine parts

2. 2 Performance
The material used for magnetic cores is thecomposite material "AS10", made of iron-based
amorphous powder that does not have a crystalline
structure, and resin material.
AS10 was compared to the conventional ferrite/ironsilicon (Fe-6.5Si) magnetic material with an
inductance deviation rate (DC bias characteristic),
which serves as a measure of the magnetic field
generation capability when a high current is applied.
The result is shown in Fig. 4.
In addition, the inductance change rate (frequency
response) when frequency is changed to a highfrequency bandwidth is shown in Fig. 5.

Inductance change rate %120

One of the manufacturing methods of machine parts
is powder metallurgy. Powder metallurgy is superior inproductivity and material yield compared withmachining from ingot steel, and reduces the cost ofmachining by applying a near net shape process.
However, sintered machine parts usually have poorer
mechanical properties than ingot steel due to pores in
the part that are inherent to the process.
NTN developed sintered machine parts with higher
density and strength than the conventional productsby optimizing the material, mold and heat treatment 2, 3).
This expanded the range of application of sinteredparts in areas where ingot steel was used for it’smechanical properties. In this article, a sinteredmachine part with high density and high strength isintroduced, taking oil pumps for industrial machines as
an example.
Oil pumps for industrial machines are widely used forconstruction machines, machine tools and printing
machines which use hydraulic power as the powersource. Fig. 6 shows the structure of a vane pump,which is a type of oil pump. Its components havecomplex shapes, and therefore, powder metallurgy issuitable for production of these components.
However, along with the recent increase of compact
and lightweight oil pumps, the components are

AS10

Fe-6.5Si
Ferrite

100
80
60
40
20
0

0

100

200

300

Current A

Fig. 4 DC bias characteristics-109-

*17E_*15 18/06/13 13:58 ページ 3

NTN TECHNICAL REVIEW No.84（2016）

material for sintered machine parts. Density increases
as the molding pressure increases for both materials;
however, the conventional product plateaus at 7.2 g /cm3,
while the developed product reached 7.6 g/cm3.

becoming smaller and thinner. Therefore, inapplications under high pressure, with an increased riskof fluctuation of hydraulic pressure/flow rate that leads
to component fracture, conventional sintered machineparts cannot be used. NTN is proposing the use of thedeveloped high density/high strength sintered machineparts for rotors and cam rings shown in Fig. 6.

3. 1. 3 Heat treatment
Heat treatment is applied for increasing themechanical properties. Carburizing, quenching,tempering, and carbonitriding, etc. can be used as theheat treatment process.

Rotor

3. 2 Mechanical properties
The mechanical properties of the developed
product, which uses the material described in theprevious section and molded with higher pressure(0.8GPa) and treated with carburizing, quenching andtempering, were compared with the mechanicalproperties of a conventional material for sinteredmachine parts and also the properties of ingot steel(high carbon chromium bearing steel). Fig. 8 shows
the comparison of ring compression fatigue strength.
In addition, Fig. 9 shows the comparison of themeasurement result of Young’s modulus.
While the density of the conventional product was
7.1 g /cm3, the density of the developed product was
7.5 g /cm3, and the true density ratio against steel ofthe developed product was 96%. As a result, thefatigue strength of the developed product was 340
MPa, approximately 2.3 times that of the conventional
product.
The Young’s modulus of the developed product was
180 - 200 GPa, equivalent to the ingot steel, SUJ2.

Vane
Cam ring

Fig. 6 Vane pump

3. 1 Methods for manufacturing high-density/
high strength sintered materials
Increasing density and reducing pores are the mosteffective measures for increasing strength of sinteredmachine parts. The following describes methods for
manufacturing high-density/high strength sinteredmaterials

3. 1. 1 Material

Fatigue strengthMPa

We have developed a material with high compression
properties for high density when used for powder
molding. Graphite with treatment for anti-segregation
was mixed with a partially alloyed powder of Ni and Mo
diffusionally joined to Fe. Additionally, lubricant wasadded at a certain proportion for ensuring release fromthe mold and preventing seizure.

3. 1. 2 Molding pressure and density
Fig. 7 shows the relationship between the molding
pressure for powder molding and the resulting density
for the developed product as well as a conventional
8.0

400
300
200
100
0

g/cm 3

Fatigue strength

MPa%

Conventional
product

Developed
product

Ingot steel
SUJ2

91

96

100

7.1

150

7.5

340

7.8

500

Fig. 8 Ring compressive fatigue strength
Developed product

220
Young's modulus GPa

Density g/cm3

500

Density

True density ratio

Conventional product
7.5

7.0

6.5
0.2

600

0.4

0.6

0.8

1

Molding pressure GPa

1.2

200
180
160

Fig. 7 Relations of compacting pressure and the density

Ingot steel
SUJ2

Conventional
product

140
120
7.0

1.4

Developed
product

7.2

7.4

Density g/cm3

7.6

7.8

Fig. 9 Relations of density and the Young's modulus-110-*17E_*15 18/06/13 13:58 ページ 4

Products Introduction of Composite Material for Industrial Machinery

Presently, we are proceeding with ongoingdevelopment for higher density/strength of this material
for even broader deployment of this technology.

NTN has succeeded in both retaining productivity
and improving load carrying capacity by developing a
new material, optimizing the size and shape of theparticles. Fig. 11 shows the comparison of the innerdiameter surface of the developed and conventional
bearings. It reveals that the pores of the developed
product are refined compared with the conventional
product.

4. Thin type hydrodynamic BEARPHITE
for fan motor applications
Quietness is highly required for fan motors forcooling mobile terminals such laptops, and NTN's fluidhydrodynamic bearings "Hydrodynamic BEARPHITE"
are used to achieve this requirement. Hydrodynamic
BEARPHITE offers high levels of quietness by noncontact support of shafts and bearings through an oilfilm. In recent trends of thin enclosures, installed
cooling fan motors are also becoming thin. As thelength of the supporting bearings also becomes short,the load carrying capacity of the bearings decreases.
They produce noise when the shaft and bearings
come into contact due to a lack of load carrying
capacity. Therefore, high load carrying capacity isrequired for thin-type hydrodynamic BEARPHITE.
NTN has developed thin-type hydrodynamic
BEARPHITE to solve the above issue by applying a
new material. The product is shown in Fig. 10.

Developed
product

Pores (black area)
Conventional
product

Fig. 11 State of inner surface

4. 2 Formation of oil film

Fig. 10 Appearance of Hydrodynamic BEARPHITE

4. 1 New material
Hydrodynamic BEARPHITE is a type of sintered,oil-impregnated bearing manufactured by a powder
metallurgy process and therefore contains pores in theouter and inner surfaces of the bearings. While thesepores have an important function of retaining anddelivering lubricating oil, the pressure generated by
the hydrodynamic effect escapes through these pores
into the bearings, and it is necessary to control thepores in order to ensure high load carrying capacity.
In general, pores on the outer and inner surfaces ofthe bearings can be refined by using powder of minuteparticles to suppress the generated pressure. On theother hand, this is likely to produce more burrs since
the minute particles of the powder penetrate the small
gaps of the mold and deteriorate fluidity andmoldability.

Rotational speed min-1

5,000
4,000
3,000
2,000
1,000
0

Oil film formation ratio %Conventional
product

100
80
60
40
20
0

Shaken Still
Rotational speed min-1

5,000
4,000
3,000
2,000
1,000
0

Oil film formation ratio %Developed
product

The formation of an oil film was compared between
the developed product and conventional product usedin actual fan motors. Results are shown in Fig. 12.
Assuming the actual operating conditions, the fan
motor experienced cycles of being repeatedly shaken
and then left undisturbed while it was in operation, andmeasurements were taken as to whether the shaft andhydrodynamic BEARPHITE were in contact using theelectric resistance method. Based on the detectedvoltage, the contact status was set ast 0% of the oilfilm formation ratio and non-contact status as 100% of

Shaken

Still

Shaken Still Shaken

100
80
60
40
20

2sec

0

Fig. 12 Test result of contact of shaft and BEARPHITE-111-

*17E_*15 18/06/13 13:58 ページ 5

NTN TECHNICAL REVIEW No.84（2016）

5. 1. 1 Features

the oil film formation ratio. Even in severe conditions
when the conventional product indicated contact
status, the oil film formation was good in thedeveloped product, indicating that load carrying
capacity had improved.
From these results, it was determined that thedeveloped product is able to maintain a high level ofquietness for fan motor applications, even in more
severe operating conditions.
Hydrodynamic BEARPHITE using newly developedmaterial will be applied not only for thin fan motors formobile devices, but also for larger cooling fan motors
such as for general industrial machines andautomobiles. In addition, we would like to actively
expand application of this product to other areas, notonly fan motors. And we will continue our development
for further improvement of functionality andproductivity.

(1) Light yellow with a clean appearance(2) Excellent friction/wear resistance, chemical resistanceand water resistance properties(3) Extremely low friction coefficient at start-up and ultra
low speed operation(4) Good compatibility with stainless steel(5) Usable in high temperatures (260˚C)

5. 1. 2 Specific wear rate
Since food machines are exposed to high pressurecleaning methods, performance in dry and wet
conditions is emphasized. Therefore, the specific wear
rate of FL3642 and other materials were compared indry conditions and in water. The test conditions areshown in Table 1 and results in Fig. 14.
Compared with PTFE material containing glass
fiber, which is a sliding material, the specific wear rateof FL3642 is higher in dry conditions, but 1/60 thewear rate value of the PTFE material in water. Also,
compared with the sliding material for water use,
FL3700, it has equivalent performance in both dry andwet conditions 4).

5. Resin products for food machines
Food machines used in food production lines andfacilities are under strict control in order to provide thesafety and security of food. From selection of materials
that comply with the "Standard for Instruments,
Containers and Packages Made of Synthetic Resin"based on the Food Sanitation Act, to the requirement ofhigh temperature/high pressure cleaning for every
production process, the operating conditions are strict for
sanitation control. Therefore, stainless steel and resinmaterials are used for food machine components.
NTN distributes resin products for food machines
leveraging the material and design benefits of resin.

Table 1 Test conditions for wear test
Tester
Surface pressure
Peripheral speed
Mating material
LubricationTimeｘ10-8mm3/(N・m)Specific wear rate ofresin material

100,000

5. 1 Sliding material BEAREE FL3642 for food machines
Sliding materials for food machines are required tohave good friction/wear resistance, heat resistance,
chemical resistance and water resistance properties, aswell as a clean appearance. For example, carbon isundesirable because of its dark, black color.
Fig. 13 shows BEAREE FL3642, broadly used as thesliding material for food machines.

10,000Dry

Thrust tester
0.98MPa
32m/min
SUS304
Dry, water
50h

Water

1,000
100
10
1

FL3642For food machines(light yellow)

FL3700Under water(black)With glass fiber
PTFE material

Fig. 14 Comparison of comparative abrasion quantityof FL3642 with other NTN resin materials

5. 2 Resin sliding bearing unit for special environments
The bearing units shown in Fig. 15 are used forchain conveyor bearings for cleaning machines anddisinfection systems. The fluorocarbon resin sliding
bearings are enclosed between the stainless steel
inner ring and outer ring, and incorporated in the
housing. They are grease-less and maintenance-freebearing units, which are easy to install in themachines. Table 2 shows the constituting materials.
The design and manufacture of this product is tailoredto the specifications of our customers.

Fig. 13 Products of BEAREE FL3642-112-

*17E_*15 18/06/13 13:58 ページ 6

Products Introduction of Composite Material for Industrial Machinery

Resin bearings

Housing

Table 3 Constituting material
Component parts
Cage
Ball

Set screw

Material
BEAREE FE5002
Ceramic

Inner ring
Outer ring

Fig. 15 Sliding bearing unit for special environment
Table 2 Constituting material
Component parts
Inner ring
Outer ring
Set screw
Resin bearings
Housing

Material
SUS304
SUS304
SUS304
BEAREE FL3642/FL3700
SCS13 (Stainless cast steel)Fig. 16 Plastic thrust rolling bearing

6. Conclusion

5. 3 Resin based thrust rolling bearings

This article introduced various composite material
products made of resin, sintered, and magnetic
materials, with new designs and technology formeeting diverse requirements of industrial machines.
NTN will create modules and units based on theseproducts. We are poised to propose value-added
products that meet the market trend of being easy tohandle for customers, and contribute to thedevelopment of industrial machines.

These bearings are light and stainless thrust rolling
bearings that combine NTN's proprietary resin cages
and ceramic balls. The constituting materials areshown in Table 3 and an image of the product isshown in Fig. 16. The material of the cage isfluorocarbon resin blended with special additives. The
cage has excellent heat resistance and a low
coefficient of friction to prevent adhesion of food.
In addition, excellent handling properties wereachieved by creating a unique shape for the cage
pocket so that the balls are retained when installing
bearings to the equipment, yet easy to remove fordisassembling and cleaning. Currently, these bearings
are used by food machine manufacturers and used infood contact areas.

References
1) Takuji Harano, Shinji Miyazaki and Hajime Katsuura:
Introduction of Magnetic Material Products, NTN
TECHNICAL REVIEW No. 80 (2012) 87-91.
2) Tomonori Yamashita and Tomokazu Sonozaki:
Introduction of Sintered New Products for Automobile,
NTN TECHNICAL REVIEW No. 81 (2013) 74-76.
3) Takahiro Okuno and Naoki Yashiro: Improvement inStrength of Sintered Machine Parts, NTN TECHNICALREVIEW No. 82 (2014) 21-25.
4) Yoshio Oki and Takuya Ishii: Sliding property of PTFE
composite material under water, THE TRIBOLOGY No.
148 (1999) 48-49.

5. 3. 1 Features(1) Lightweight and rust-free material(2) Usable in high temperatures (200˚C)(3) Excellent handling properties(4) Easy to detect foreign objects after cleaning due tolight color
Photo of authors

Shinji KOMATSUBARA

Toshihiko MOURI

Takuji HARANO

Tamaki MIZUTANI

Fluid Hydrodynamic Bearing Dept.
Composite Material Product
Division

Engineering Dept.
NTN Advanced Materials
Corporation

Engineering Dept.
NTN Advanced Materials
Corporation

Engineering Dept.
NTN Engineering Plastics
Corporation-113-*18_21受賞案件E_*18,19_受賞案件 18/06/13 13:59 ページ 1

NTN TECHNICAL REVIEW No. 84（2016）

Award Winning Products
The Japan Society for Precision Engineering, Spring Meeting, 2015, Best Presentation Award
2015 The Japan Society for Precision Engineering, Young Engineer Award

Parallel Link High Speed Angle Control Equipment
Naoya KONAGAI

Parallel link high speed
angle control equipment

1. Overview
We have developed Parallel Link High Speed Angle Control
Equipment (hereinafter, "equipment") by applying the special
parallel link mechanism of two rotational degrees of freedom,and deployed this equipment to grease dispensing, automatic
welding, and appearance inspection. This equipment, whichallows high-speed/high precision positioning and granularmotion in a wide operating range that accommodates a 90°
working angle and 360° traverse angle, was recognized at
The Japan Society for Precision Engineering Spring Meeting,2015, and received Best Presentation Award and Young
Engineer Award.

Base side
Tip of torch

Tip sideWelding torch
XY stage
Wire supply
equipment

Fig. 2 System configuration of automatic welding machinemechanism facing upward to reduce the moment of inertia,
and the welding torch is positioned so that it penetrates theinternal space of the link mechanism in order to achieve fastand refined operation. By controlling the position of thewelding torch in this configuration, it can weld around theentire circumference of columns with the welding torch in an
inclined position.

2. About Parallel Link High Speed Angle Control
Equipment
The equipment achieved smooth operation with tworotational degrees of freedom and high-speed/high precision
operation by controlling all three links of the 4-joint, 3-linkparallel link mechanism, canceling backlash with a balance ofdriving forces from the three motors, and reducing settling
time by controlling acceleration/deceleration1 to 4) Fig. 1 shows
the standard offering of this equipment. Speed and positioning
repeatability are improved by enclosing the end effector in theinternal space of the link and reducing the moment of inertia.
Due to its high speed and superior maneuverability, theequipment has been adopted in grease applications where
short takt time and high reliability are required, such as inproduction lines of automotive components. Furthermore, it isalso deployed in automatic welders, leveraging its capability ofdrawing fine trajectory 5).

References
1) Hiroshi Isobe, Yukihiro Nishio: Parallel Link High Speed Angle
Control Equipment (PHACE), NTN TECHNICAL REVIEW, No. 80,(2012), 42-47.
2) Hiroshi Isobe: Parallel Link High Speed Angle Control Equipment –Basic Configuration and Application for Grease Dispensing,
Machine and Tool, February Issue, (2013) 83-87.
3) Hiroshi Isobe, Yukihiro Nishio, Keisuke Sone, Hiroyuki Yamada,
Yoshio Fujikawa: Parallel Link High Speed Angle Control
Equipment, Proceedings of the Japan Society for Precision
Engineering Spring Meeting in 2013, (2013), 809-810.
4) Hiroshi Isobe, Yukihiro Nishio, Seigo Sakata, Naoya Konagai,
Hiroyuki Yamada and Yoshio Fujikawa: Parallel Link High Speed
Angle Control Equipment - Implementation on Grease Application
Equipment -, Proceedings of the Japan Society for Precision
Engineering Spring Meeting in 2014, (2014), 1087-1088.
5) Naoya Konagai, Hiroshi Isobe, Seigo Sakata, Kenzou Nose,
Hiroyuki Yamada and Yoshio Fujikawa: Parallel Link High Speed
Angle Control Equipment, Proceedings of the Japan Society for
Precision Engineering Spring Meeting in 2015, (2015), 605.
6) Keisuke Sone, Hiroshi Isobe and Koji Yamada, Actuator Unit with
Two Degrees of Freedom for Bending Angle of 9 Degrees - High
Angle Active Link - , Machine Design, Vol 47, No. 11, (2003) 60.

3. Deployment to automatic welders
Fig. 2 shows the configuration of an automatic welder using
this equipment with a welding torch. The parallel linkmechanism for automatic welders is a mirror symmetric type 6)
and each component is redesigned from the standard
products to secure space for installing the welding torch. Inorder to install heavy and long welding torches, the equipment
is fixed on the XY stage with the tip of the parallel link

Photo of author
End effector
Parallel linkmechanism

Drivemechanism

Naoya KONAGAI
New Product Development
R&D Center

Fig. 1 Parallel link high speed angle control equipment-114-

*18_21受賞案件E_*18,19_受賞案件 18/06/13 13:59 ページ 2

NTN TECHNICAL REVIEW No. 84（2016）

Award Winning Products2014 "The Japan Society of Mechanical Engineers, Materials & Processing Division,
Division General Award (New Technology Development)

Materials Development about the Sintered Bearing Preparedby Unification Forming of Different Metals Powder
Toshihiko MOURI Yoshinori ITOU

Yosuke SUGAI

Eiji YUASA

Outer
layer

1. Introduction"Material Development on Sintered Bearings Prepared by
Unification Forming of Different Metal Powder," presented in
the 2014 Annual Meeting sponsored by The Japan Society of
Mechanical Engineers, received the division general award in
the Materials & Mechanics/Materials & Processing Division.
This article describes the development of materials for"multi-layer oil-impregnated sintered bearings" for joints ofhydraulic shovels (Fig. 1), etc. These multi-layer oilimpregnated sintered bearings have been developed by NTN,
and achieve low friction and wear resistance, and highstrength while being low cost 1).

Inner
layer

General ferrous
structural materialHigh strength ferrous
structural material

Fig. 2 Structure of the Multi Layer BEARPHITE®

The outer layer uses strong and less costly ferrous material
blended with a metal with a low melting point to improvestrength.

Bearings in joints

3. Features
Multi-layer oil-impregnated sintered bearings exhibit highstrength and superior wear resistance with a two-layered
structure using two different materials.
1) Radial crushing strength: 500 MPa or more without
heat treatment
2) Possible to correct distortion after heat treatment witha sizing operation; no machining is required
Fig. 1 Application example

4. Summary
This product achieves both low friction and wear resistanceand high strength by the integrated forming of different
materials. We will continue contributing to the market throughdevelopment of new products.

2. Structure
The structure of multi-layer oil-impregnated sinteredbearings is shown in Fig. 2. Multi-layer oil-impregnatedsintered bearings are comprised of two layers, with different
materials in the inner and outer layers. They are characterized
by integrated forming of this two-layer structure via powder
molding, instead of press fitting or adhesion. The inner layer ismade of hard ferrous material, with copper added to achieveboth low friction and wear resistance.

References
11) Yosuke Sugai and Toshihiko Mouri: Multi-Layer BEARPHITE, NTN
TECHNICAL REVIEW 80, (2012) 83-86.

Photo of authors

Toshihiko MOURI

Yoshinori ITOU

Yousuke SUGAI

Eiji YUASAEngineering Dept.
NTN Advanced Materials
Corporation

Engineering Dept.
NTN Advanced Materials
Corporation

Sales Technology Dept.
Composite Material
Product Division

Emeritus Professor,
Tokyo City University-115-

*18_21受賞案件E_*18,19_受賞案件 18/06/13 13:59 ページ 3

NTN TECHNICAL REVIEW No. 84（2016）

Award Winning Products
2015 Annual Innovation Product Award

Machine Tool Main Spindle Bearings
with Air Cooling Spacer
Wenwei WUMichihiko KOSAKA
direction of the spindle at positions offset about the axial
center. The compressed room-temperature air injected fromthe nozzles goes through the space between the inner andouter spacers as well as inside the bearings. It then revolves
in the rotational direction of the spindle and cools down the
bearings.
This product has been introduced in separate articles indetail 1, 2).

1. Introduction
NTN's "Machine Tool Main Spindle Bearings with Air
Cooling Spacer" (hereinafter, "Bearings with Air CoolingSpacer") was recommended in the technical magazine "MM
Modern Manufacturing", which has the largest circulation in
China (25,000 copies/issue, 52 issues/year), as a candidate
for "Annual Innovation Product Award" for 2015 and received
the award based on votes from the internet and expert
judges.
"Bearings with Air Cooling Spacer" are products using newtechnology which can allow for both high speed and highrigidity at a high level for the main spindles of machine tools byapplying air cooling technology to the bearings.

3. Summary
NTN has achieved both high speed and high rigidity at ahigh level by using air cooling technology to the bearings usedin the main spindles of machine tools. "Bearings with Air
Cooling Spacer" developed with this new technology are
expected to allow for higher speed, higher rigidity, andimproved reliability, as well as contribute to overall
manufacturing.

2. Structure of bearings with air cooling spacer
Structure of "Bearings with Air Cooling Spacer" is shown in
Fig. 1. NTN's proprietary eco-friendly air-oil lubrication nozzle
is applied to the outer ring spacers between the angularcontact ball bearings in a back-to-back arrangement (DBarrangement) for reductions in air-oil and noise. Additionally, aseparate air cooling nozzle is included.
Three air cooling nozzles are placed along thecircumference of the bearing oriented in the rotational
Air cooling nozzle

Air cooling spacer(outer spacer)

Bearings with air
cooling spacer

References
1) Onda, Fukada, Yamamoto and Yoshino: Machine Tool Main
Spindle Bearings with Air Cooling Spacer, NTN TECHNICALREVIEW, No. 82, (2014) 38-43
2) Nasu, Okamoto, Yamamoto and Yoshino: Machine Tool Main
Spindle Bearings with Air Cooling Spacer, NTN TECHNICALREVIEW, No. 84, (2016) 52-57

Air cooling nozzle
OffsetAir cooling spacer(outer spacer)

Cooling air (inlet)

Inner ring spacer

Eco-friendly nozzle
for air-oil lubrication
Inner ring spacer

Inner ringrotational direction
Cooling air (outlet)

Cooling air (outlet)

Arrow view A

A

Fig. 1 Structure of the bearing with air cooling spacer

Photo of authors

Wenwei WUMichihiko KOSAKA

Industrial Machinery
Technology Department,
NTN (China) Investment Corporation

Industrial Machinery
Technology Department,
NTN (China) Investment Corporation-116-

*18_21受賞案件E_*18,19_受賞案件 18/06/13 13:59 ページ 4

NTN TECHNICAL REVIEW No. 84（2016）

Award Winning Products
EQUIP AUTO 2015 INNOVATION AWARD

Press Connected Spline Hub Joint
Sebastien GUILLAUME

Claire BIANCHIN

1. Introduction
Equip’Auto is the largest Automotive Aftermarket Show inFrance. It takes place every two year in Paris.
The promotion of innovations useful for aftermarket is one ofthe priorities of Equip’Auto, with the “Grand prix internationaux
de l’innovation”. This challenge awards gold, silver & bronze
trophy, in 5 categories.
NTN-SNR attended the show, with a stand to exhibit the range
of NTN-SNR automotive
products dedicated toaftermarket.
NTN-SNR applied for the“Grand prix internationaux de l’innovation” with the presentationof a light weight wheel hub, inthe category of ‘OEM & newtechnologies’.

4. NTN-SNR award
With an attendance of more than 95,000 visitors, and thepresence of 1,400 exhibitors (who 60 % of international), EQUIP
automobile 2015, who is kept from 13 till 17 October, confirmed
its position of international, essential meeting in France.
The “Grand prix internationaux de l’innovation” exists since1985. The jury consists of more than 60 journalists, from morethan 20 countries.
The NTN-SNR presentation to the jury explained theadvantage of this innovation: CO2 emission reduction, mileage
increase, noise reduction, and also the possibility of easy
maintenance by the short length of the CVJ spline.
The jury awarded NTN-SNR for this innovation, with the silver
trophy.

2. Structure of Press Connected Spline (PCS) hub
joint with aluminum centering
The innovation presented by NTN-SNR is a Press Connected
Spline Hub joint, developed by NTN, with an aluminum wheel &break disc centering developed by NTN-SNR.

5. Summary
Press connected Spline Hub Joint & aluminum centering is asolution to answer automotive market demand. This product also
demonstrates the strength of NTN group, with, on one hand, thepossibility to propose improvements on
both the wheel hub & CVJ, and on theother hand, the common input of both
Japan & France R&D center.
NTN-SNR will continue to promote
Press connected Spline Hub Joint &aluminum centering to European car
makers.

Compare to a standard solution, the mass of the wheel hub &CVJ assembly is reduced by 21.2%, with 18.2% by PCS & 3%
by aluminum centering.
This solution has been applied on a Renault demonstration
vehicle called ‘Eolab’ that achieve a mileage of 2L / 100km.

3. Detail explanation of aluminum centering
The target of aluminum centering development is to savemass, but also to simplify the manufacturing of the hub. The hub
is easier to forge, and the small splines of PCS are possible tomanufacture with a standard process (the tool can passthrough).
The geometry of the aluminum centering has been optimizing
by FEM with two criteria:
1) Stay below 170 MPa of stress in thealuminum under the axial load of the nut
due to tightening, to be able to use low cost
aluminum for mass production.
2) Keep the displacement in the centering
area below 5 microns.

Photo of authors-117-

Sebastien Guillaume

Claire Bianchin

NTN-SNR ROULEMENTS
Automotive business unit
R&D – Wheel bearing

NTN-SNR ROULEMENTS
Automotive aftermarket
communication

*21_26製品紹介E_*20_26製品紹介 18/06/13 14:00 ページ 1

NTN TECHNICAL REVIEW No.84（2016）[ Our Line of New Products ]Lightweight Rear Drive Shaft
30% lighter than the conventional product・Significant weight reduction achievedby new joints (REBJ and REDJ) with dedicated design for compact rear drive shafts!

Lightweight fixed-type
CVJ (REBJ)

Thin-wall type hollow shaft

Wheel side
Compact boots dedicated for rear side

Lightweight slide-type
CVJ (REDJ)

Differential
gear side

Features

Structure

Lightweight/compact

Comparison with conventional products (EBJ+EDJ)(1) 2.2 kg lighter (30% less than conventional products)
per drive shaft(2) 3-5% reduction of CVJ outer ring outer diameter∙ Fixed type/slide type CVJ with dedicated design for therear side∙ Thin-walled type hollow shaft∙ Compact boots dedicated for the rear side

Applications¡Rear drive shaft for rear wheel drive and four
wheel drive vehicles-118-*21_26製品紹介E_*20_26製品紹介 18/06/13 14:00 ページ 2

NTN TECHNICAL REVIEW No.84（2016）[ Our Line of New Products ]Auto Tensioner with Variable Damper Mechanism
for ISG* Equipped Engines
Achieve both belt slip prevention when starting the engine and low fuel consumption,
which was difficult with conventional products, with the variable damper mechanism

Features

Structure

Achievement of both improved fueleconomy and high stability

Adoption of dual leak system(1) Mechanism to automatically switch the oil flow pathdepending on the operating condition of the engine(2) Check valve structure that opens/closes with thebalance of spring force and hydraulic power(1) Maintain low belt tension for regular driving to improvefuel economy(2) Prevent belt slip by instantaneously generating highbelt tension when the ISG functions as a starter(3) Equivalent size and durability as the conventional
product

Applications

Dual leak system¡Tension adjustment for automobile engine
accessory drive belts
Auto tensioner

ISG
Accessory
drive belt

* Abbreviation of Integrated Starter Generator. Motor with integrated generator and starter.

-119-