**表紙E_レイアウト 1 19/07/03 11:48 ページ 1

For New Technology NetworkR

TECHNICAL REVIEW No.86
Special Issue

corporation

Special issue on"Robotics and Sensing Products and Machine Tools"
October 2018

Robotics and Sensing Products and Machine Tools
corporation

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

ISSN 0915-0528 OSAKA, JAPAN

**表紙E_レイアウト 1 19/07/03 11:48 ページ 2

New Policy Using Natural Energy Products

NTN Corp. marked its 100th anniversary in March 2018 and started the New

Medium-term Management Plan called "DRIVE NTN100" in April 2018. As we promotevarious policies aiming for sustainable growth in the next 100 years, we conduct businessdevelopment in new areas. We do so by leveraging our collective strength in R&D,

production technology and distribution, which we have been developing over the years.
In the area of natural energy, we have started the manufacturing and distribution of small
wind turbines and micro hydro turbines utilizing our highly efficient and quiet blades.

The small wind turbine, "NTN Hybrid Street Light", which generates electricity fromthe wind and solar power, can supply electric power when and where the commercial
electric power cannot be used such as in disasters or areas where such power is not
supplied. NTN Corp. has donated its "NTN Hybrid Street Light" to five regions in Japan,

so far, for promoting renewable energy and helping those regions with disaster and crime
prevention.

We also started “Joint Research for Creating Mechanisms Regarding Disaster

Prevention Monitoring and Tourism through Use of IT” with the National University
Corporation Osaka University (hereafter, Osaka University) and its associatedorganizations as a new initiative. This joint research will integrate the "Disaster Relief
Map" developed by Osaka University with “Mimamori Robokun III” Wi-Fi▲ Experimental unit using

NTN Hybrid Street Light

communication infrastructure. This is equipped with a monitoring camera and adopts the"NTN Hybrid Street Light" as the framework and independent power supply for the experimental unit. The new initiative isworking on various experiments aimed at creating an information network for disaster prevention and disaster support,monitoring elderly citizens and children, and building information infrastructure for tourism.

The "NTN micro hydro turbine," which is a small hydroelectric generator with a unique blade shape, is able to efficiently

convert water flow energy to electric power with simple installation into irrigation channels. Different from the conventionalhydro power generators which use the difference in water levels, it does
not require expensive foundation work to create the difference in waterlevels, which results in significant cost reduction. Furthermore, it

allows serial/parallel installation of multiple units into the same water

channel for obtaining desirable output based on the number of units
installed. We have also launched distribution of “Grid Connectable

NTN Micro Hydro Turbine” this year, which allows generated electric

NTN TECHNICAL REVIEW No.86

NTN will promote proposals for locally produced and consumed

Printed and published in July 30, 2019

power to be sold to the grid.

energy by leveraging new policies using these natural energy products,▲ NTN micro hydro turbines placed in series

in the same conduitand contribute to the local communities through realization of
low-carbon society and safe/secure urban development.

Yoshinori TERASAKA
Publisher Yoichiro MOCHIDA

EditorPublished by NTN Corporation
3-17, 1-chome, kyomachibori, Nishi-ku, Osaka,J apan
Printed by
Nissha Printing Commmunications,Inc.

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

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R

TECHNICAL REVIEWSpecial Issue;
No.86 Robotics and Sensing Products
and Machine Tools

corporation

*01-目次-巻頭言_*01/目次/巻頭言 19/07/11 9:39 ページ 2

NTN TECHNICAL REVIEW No.86
CONTENTS
Preface

For Robotics and Sensing Products and Machine Tool
1

Yoshinori TERASAKA

Strategic Vision for Smart Machining Tool and Measuring Instrument
Contribution

Perspective

Yasuhiro TAKAYA

Professor, Department of Mechanical Engineering,
Graduate School of Engineering, Osaka University

Rolling Bearing and IoT – NTN’s Approach to IoT Masaki EGAMI

2

16● Special Issue for Robotics and Sensing Products
Development of "i-WRISTTM" Wrist Joint Module
Keisuke KAZUNO, Hiroshi ISOBE, Jun MIDOUMAE, Yuuki SHIMURA and Masayuki OHARA

Application of i-WRISTTM to the Robot Wrist Joint
Kenzou NOSE, Seigo SAKATA, Naoki MARUI and Naohiko SATOU

Technical Trend and Features of the Bearing for Robot
Makoto OEBISU, Hiromichi KOKUMAI, Yasuyoshi HAYASHI and Masato TSUJIHASHI

Application of Condition Monitoring System for wind turbines
Makoto MIYAZAKI and Wataru HATAKEYAMA

Development of Multi Track Magnetic Ring for High Accuracy Absolute Angle Detection
Takashi KOIKE, Yasuyuki FUKUSHIMA, Yusuke SHIBUYA and Hiroyoshi ITOUI

22

28

34

40

45

● Special Issue for Machine Tool
Development of Sensor Integrated Bearing Unit for Machine Tool Spindles
Shohei HASHIZUME, Yasuyuki FUKUSHIMA, Yusuke SHIBUYA and Yohei YAMAMOTO

Angular Contact Ball Bearings for High-Speed and Heavy-Cutting Machine Tools
Jin TAKEGAHANA, Mineo KOYAMA, Kouji JINNO and Yuya TANAKA

Machine Tool Main Spindle Bearings with Air Cooling Spacer for Grease Lubrication
Keisuke NASU, Naoya OKAMOTO, Yuya TANAKA and Tomohiko OBATAULTAGE Precision Standard Angular Contact Ball Bearing, 72U Series
Takayuki KITANO and Youhei YAMAMOTO

Plastics Sliding Sheet Used Slide Guide of Machine Tools
Takuya ISHII, Satoru FUKUZAWA and Norikazu MUNEDA

50

56

62

68

74

● Technical Papers and New Products
Lubrication Mechanism of Low Friction Seal of Ball Bearing for Transmission
Tomohiro SUGAI, Katsuaki SASAKI and Takahiro WAKUDA

Hub Bearing with Steering Function that Improves Vehicle Dynamic Performance78

Norio ISHIHARA, Hirokazu OOBA, Atsushi ITOU, Mitsunori ISHIBASHI, Makoto YAMAKADO, Yoshio KANO and TMasato ABE

84"Tapered Roller Bearings" with Early Failure Detection Sensor

91

Naota YAMAMOTO, Hiroshi UCHIMURA and Shota TOHO

Asymmetrical Spherical Roller Bearings for Wind Turbine Main Shafts
Kazumasa SEKO and Takashi YAMAMOTO

Grid Connectable NTN Micro Hydro Turbine

96

102

Takashi ITOU, kanta KIMURA and Yasunari KANAMURA● Award Winning Products“Cho” MONODZUKURI Innovative Parts and Components Award 2017, Automotive Components AwardULTAGE Tapered Roller Bearing for Automotive Application

108

Takashi KAWAI, Yasuhito FUJIKAKE, Takanori ISHIKAWA and Susumu MIYAIRIThe Society of Materials Science, Japan “The 51st symposium on X-ray material strength” Best Presentation Award

Effect of Residual Stress on Peeling of Rolling Bearings

109

Naoya KAMURA, Takumi FUJITA and Toshihiko SASAKI“The Japanese Society of Tribologists 2017 ” Outstanding Paper Award

Rolling Contact Fatigue of Thrust Ball Bearing under Low Lambda Condition

110

Takumi FUJITA, Naoya HASEGAWA, Naoya KAMURA and Toshihiko SASAKI

Our Line of New Product111

*01-目次-巻頭言_*01/目次/巻頭言 19/07/03 10:15 ページ 3

NTN TECHNICAL REVIEW No.86（2018）[ Preface ]AMessage for the special issue on"Robotics and Sensing Products and Machine Tool"

Yoshinori TERASAKAManaging Director

As the population of productive age decreases in all sectors, creating a major social problem, measuresagainst labor shortage due to the upcoming accelerated aging population and dwindling birthrate arestrongly required. The market for collaborative robots is expected to grow, as these robots are able to workin a variety of manufacturing sites such as handling, assembly and testing on behalf of the workers. On theother hand, with the labor shortage as a backdrop, the demand for additional monitoring capabilities inindustrial machinery is increasing, and approaches for further advancing unmanned or reduced manpower
operations by leveraging the widespread adoption of IoT and sensing technology are actively being
discussed.

NTN has developed a compact and high-speed appearance inspection device combining its wrist joint
module "i-WRIST" with a unique link mechanism and camera as a solution to the issue of labor shortageand quality improvement in manufacturing operations. NTN is promoting this device as one approach
toward new business development.
In addition, bearings, which are NTN's platform product and are used in rotational components of all
kinds of machinery, are also expected to have higher functionality, acting as the machine element for datacollection by including sensing capability with various embedded sensors. NTN is engaged in the joint
research and technology development with Osaka University for allowing trend control and damage
prevention during bearing operation by integrating multi-function sensors in the bearings.
At the 29th Japan International Machine Tool Fair (JIMTOF2018) held from Nov. 1 to Nov. 6 with thetheme of "CONNECT by technology for the future," we are publishing a special issue of Technical Review
No. 86 "Robotics, Sensing Products and Machine Tools." In this issue, we discuss the technical trends andfeatures of one of our platform products, industrial robot bearings, which utilize NTN's base technologies
including materials, heat treatment and tribology. The issue also introduces newly developed products
such as sensor integrated bearing units for machine tool spindles, machine tool main spindle bearings with
air cooling spacer for grease lubrication for high speed operation, and angular contact ball bearings forhigh-speed and heavy-cutting machine tools.

NTN marked its 100th anniversary in March 2018. We began a new three-year mid-term management
plan "DRIVE NTN100" in April 2018 to accelerate our business structure transformation for our next 100
years. Under our philosophy of "We shall contribute to international society through creating newtechnology and developing new products (For New Technology Network: Networking the World With New
Technology)," we will promote the research and technology development that will support the next 100
years by strengthening our unique competitive platform businesses, creating new businesses, andleveraging our core competence which targets sustainable growth of economic society.

-1-

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NTN TECHNICAL REVIEW No.86（2018）[ Contribution ]Strategic Vision for Smart Machining Tool and
Measuring Instrument

Yasuhiro TAKAYA
Professor, Department of Mechanical Engineering,
Graduate School of Engineering, Osaka University

Establishment of the innovation driven manufacturing, which enables new value
creation such as customized products and innovative products with services, is expectedin the age of non-equilibrium and strategy. The holistic measurement based on integrating
different measurement principles and multi-scale data fusion as well as smart in-process
and on-machine measurement technologies will play an important role in the predictive
control type manufacturing system. The connected smart in-process and on-machinemeasurement systems that give self-recognition ability to the manufacturing system is also the key technology inIndustry 4.0 to establish the innovation driven manufacturing.

1. Introduction

2. Innovation driven manufacturing

The paradigm shift of the socio-economic structurefrom a "society of equilibrium" to a "society of nonequilibrium" which proliferates and penetrates on a
global scale and its challenges can be interpreted as
the transition from the "age of tactics" to "age ofstrategy" as shown in Fig. 1. With the change of the
new value system and socio-economic environment,such as a sustainable society, environmental
considerations, and global standards, manufacturing
is also changing. Its status is termed the change from"linear society/age of railway" to "non-linear
society/age of great voyage."1) Attention is paid to theunpredictable destination (market) and the process ofgetting there, rather than the foreseeable andpredictable destination (product), and focus is placedon fundamental thoughts (strategy) rather than the
pursuit of higher added value through improvement
and enhancement (tactics). Along with these trends,
the challenges that cannot be solved by extending theconventional approaches are increasing. The pace ofholistic transformation in many fields from science tosociety and economy is steady and fast, even more
accelerated in recent years. This paper will identify thepotential power of strategic measurement as one of
the above trends which drives the manufacturingtransformation, namely, the 4th Industrial Revolution,and explore the future development of the predictive
production system and smart
machining/measurement.

In the efficiency driven manufacturing of the"equilibrium/age of strategy," focus was given to thestrategy to achieve addition of higher product value
through high precision and efficient production systems
and the tactics to realize it. The tactics were to rapidly
promote a highly developed production system.
Therefore, efforts were made to breakdown the
manufacturing technology into elemental technology(production tools) such as machining technology,
measurement technology, and production technology(CAD/CAE/CAM/CAT) that compose of production
systems and solve challenges of each element. That
resulted in a significant outcome. As such, the role ofmeasurement technology was weighed heavily as oneof the production tools (measurement=testing) in Fig. 2and used as a tactic for achieving the objectives given
to the "thing" (product) such as high quality, low cost,and short delivery time. When the manufacturing
process was in the "equilibrium state" for a prolonged
time, "measurement in manufacturing" as a production
tool played a satisfactory role.
Alternatively, in the innovation driven manufacturingof "non-equilibrium/age of strategy," strategy isweighed more heavily than tactics. Strategy in thiscase is new value creation, such as customized
manufacturing, creation of innovative products, and theassociated services in addition to the new approach ofhigh value addition. As the tactics to respond to thestrategy change over time, achieving predictive-2-

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Strategic Vision for Smart Machining Tool and Measuring Instrument

20th century [Change of structure and challenges]
Equilibrium
society(Paradigm shift)[Age of tactics]

Society
Economy21th century
Non-equilibrium
society[Age of strategy]

Attention ondestination

Attention onmode of transportation· Increase capacity andfrequency for transporting
more people and cargo· Condition of sea changes
time to time
· Diversification andcomplexity, impact fromvarious factors

speed for arriving[Age of railway] · Increase
early

Prediction is easy(Linear Society)[Age of great voyage]

Prediction is difficult(Nonlinear Society)· Situation is always stable· Main attention is on the destination (Product)· Situation is always changing· Main attention is on the destination (Market)
and how to get there (Process)· How to build? · What to build?
· Value as the "Purpose"· Product quality· Why such activities?
· Value as the "Means"· Production quality

Engineering· Algebraic approach· Knowledge· Solutions are derived when boundary condition is given· Combinational approach· Wisdom· Structured knowledge does not work

Measurement· One-dimensional, two-dimensional,
low-scale three-dimensional measurement· Offline measurement· Pre-process/post-process measurement· Large-scale three dimensional/
multi-dimensional measurement· On-machine/in-process measurement· Holistic measurement· Dominant equation is clear· Predictable given the initial condition· Equilibrium physics· Deterministic model· Combinational knowledge/wisdom is critical· Non-equilibrium physics/complex system science· Network modelBusiness
Production(Manufacturing)

ScienceFig. 1 Railway era to the age of discovery

the manufacturing process is transitioned to a "nonequilibrium state," the measurement, which is thesource of information (resource), is required to makeinnovative progress as a strategic tool for"measurement for 'event' creation."

Production tool: measurement = inspection
MarketProduct
Process
Production process"Thing"(high quality, low cost,[Objective] short delivery time)
Tactics
Measurement in manufacturing
Efficiency driven manufacturing

Market A

Equilibrium stateTime

Strategic tool:
measurement

Fig. 2 Equilibrium era: Measurement technology as
production tools

Product

Life cycle
Forecast"Thing"Process

Market B
Product"Thing"Information

Information

Process(diversification,[Objective] value creation)
Market C

Strategy

production systems is necessary. Therefore, the role ofthe measurement technology is significantly different
from the past, in addition to applying a predictive
production system, as shown in Fig. 3. The
measurement technology will be required to assume
the strategy itself for achieving the diversified
objectives that are heavily engaged with the entire"event" processing, such as market, product, andprocess throughout the "things" (product), and their life
cycle, tightly connected to information. Today, when

Process
Life cycle
MarketProcess
Product

Measurement in"event" creation

Product"Thing"Information

Innovation driven manufacturing

Life cycle
Forecast

Non-equilibrium stateTime

Fig. 3 The age of non-equilibrium: Measurement
technology as strategic tools-3-

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NTN TECHNICAL REVIEW No.86（2018）

respectively. For example, when Cp≧1.33, 99.1% of
workpieces fall within the allowance of 1/1.33=0.75.
Machining accuracy (error) cannot be directly"observed." Therefore, it is necessary to understandhow to "visualize" it through the "window ofmeasurement." Fig. 5 3) shows how the machining
capability index is evaluated depending on themagnitude of "uncertainty" 4) of measurement. Whenthe allowance is T, LSL and USL shows the lower andupper limits of the machining specification,
respectively. For simplicity, assume the averagemachining value (e.g. machining size) to be the center
value of T and the standard deviation that shows themachining variance to be σ. For example, if the truemachining capability index Cp (or Cpk) = 1.33, then Cp
is observed through the "uncertain window ofmeasurement", and when the machining capability isevaluated with the uncertainty U = 0.1 T (Fig. 5 (a)),
then Cp (Cpk) is evaluated as 1.24. Alternatively, when
the uncertainty U = 0.2T (Fig. 5 (b)), Cp (Cpk) = 1.04
and it is evaluated as the lower machining capability. It
also reveals that the entire uncertainty U', which
includes the uncertainty of measurement, is larger
than σ. Therefore, the magnitude of uncertainty as
the index to show that the reliability of measurement
determines the reliability against assurance ofmachining accuracy. The uncertainty of measurement
is an important index for highly efficient machining
processes based on the evaluation of machining
capability. In order to assure high precision/highaccuracy machining by the strategic measurement "tocreate value," it is necessary to use measurementswith higher reliability (lower uncertainty). As seen
above, improvement of process capability is requiredby reducing machining errors from the machining
process, that is, bias errors and variation errors fromthe absolute values, for improving the machining
accuracy. To do so, it is most effective to examine all

3. Concept of new production quality
3.1 Evaluation of machining accuracy andprocess capacity
Taniguchi predicted that the machining limit ofprecision machining technology would reach 0.1μm
in 2000, in his 1983 paper. 2) The progress of
mechanical machining technology has been significant
and it is now reaching the area of general machining
limits. The machining process is a general technology
involving many elements, such as machine tools,
workpieces, working conditions, working environment,
and products. The machining error is caused by the
influence of various factors, such as positioning
accuracy and thermal displacement of machine tools,
geometric accuracy and wear property of tools,
material and hardness of workpieces, machining
conditions, such as dry machining and processing skill
set, working environment, such as temperature andvibration, and high precision and complexity of
products. Based on the law of error propagation,accuracy of the entire machining process cannot be
improved by compensating unstable factors by stablefactors. That is, if there is even one unstable factor in
the machining process, the overall accuracy isdecreased, as it is determined by that factor.
Machining accuracy can be tackled separately
between precision and accuracy. In addition,
machining requires capability to satisfy the standard
size and allowance. Fig. 4 shows how to evaluate themachining accuracy and process capacity. If the upperand lower limits of the machining specification are LSL
and USL, respectively, and the allowance is T, thenwhen the average machining value (machining size,etc.) is the midpoint value of T and the standard
deviation is σ, the limit machining accuracy p andmachining capability index Cpk or Cp are calculated by
the evaluation equation indicated in Fig. 4,
Probabilistic density (frequency)＊Errors from the machining process include bias errors and variation errors.

Allowance T

Allowable
machining errorεThe following value is evaluated to determine if the machine tool has
machining capability to satisfy required accuracy of parts.

εAverageμBias error d【 Process capability index Cpk 】
Cpk = min

Variation errorσ

LSLStandard sizeσ

USL−μ , μ−LSL
3σ

3σ＊When average,μ, is the center value of T

Machiningaccuracy limit p：
p=d+σ

Cp =

USL−LSL
6σ

=

T

6σ

Size value X

USL（＊LSL: Lower Specification Limit, USL: Upper Specification Limit）

Fig. 4 Process capability index (Cpk, Cp) for evaluating the machining accuracy-4-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 4

Strategic Vision for Smart Machining Tool and Measuring Instrument

Machining accuracy

Measurement accuracy+=

Measurement
accuracy
MachiningaccuracyU = 0.1T

Cp = 1.33（or Cpk）

AllowanceT

LSL

Average+U

Average

Cp = 1.24（or Cpk）

Process capability
evaluation

Probabilistic density (frequency)

LSL

Process capability evaluation=

Measured quantity
U : Uncertainty

U

USL

U
T

(a) In the case that uncertainty U = 0.1T

USL
Measuredquantity

U = 0.2T

MeasuredquantityU

U’ = σ 2+U 2

Measurement
accuracy
Machiningaccuracy
Process capability
evaluation

Cp = 1.04（or Cpk）

LSL

+
U

U

Measuredquantity(b) In the case that uncertainty U = 0.2T

USL

=
U

U

MeasuredquantityT

Fig. 5 Importance of measurement uncertainty to evaluate process capability index(Reconfigured and summarized reference 3) Weckenmann et al., Precision Engineering (2000))

system are required, not only products but the process
will also drastically change, along with the aging
process of individual products in their life cycle andchange of strategy. In these cases, carefulconsideration is required for applying quality control
based on the conventional statistical concept as thesystematic error in machining errors change in theenvironment where variation of temperature and aging
of machines cause change in accuracy. In addition,
when the number of units for machining is very small
and the process tends to change, verification of
appropriateness of the statistical assumptions shownin Fig. 4 will be difficult.
Therefore, the concept of production quality is being
proposed 6), as a new production control and quality
control approach, as shown in Fig. 7, which can beapplied to small-lot or one-unit production, or
customized production solving the challenges of theconventional statistical process control approach. Thisapproach emphasizes quality/function assurance
focused on the process as the basic concept by"informatizing every process of manufacturing", notonly geometric quantity of workpieces as the index tomeasure the quality. It establishes production quality
control by defining innovative and integrated
production quality that uses enormous amount of data
(industrial big data), designing an entire product life
cycle from production logistics to maintenance basedon such definition, introducing new management /control methods for big data with advanced
technology to achieve them. The technologicalinnovation required for that includes product testing
technology, machining monitoring technology, andthe elements of the entire machining process
considering all the factors in the machining process as
subject to measurement and apply improvement to themost unstable factor.

3.2 From product (quality) to production quality
Machining accuracy is determined by the complex
superposition of machining errors that vary withvarious unpredictable factors, such as the propertiesof the machine tools and workpieces, machining
environment, and machining procedures. The
accidental errors included in the machining errors
cannot be corrected, as their cause cannot be
determined. However, they can be estimated withrepetitive measurements and, therefore, it is effective
in mass production, where the same machining isrepeated for the same components (many probabilistic
trials). Machining accuracy can be considered as thestatistically fluctuated quantity, that is, probabilisticallyvariable quantity. Therefore, the conventional quality
control approach used in mass production of theefficiency-driven manufacturing is based on themachining accuracy evaluation of the statistical model.
On the other hand, together with the increase of
demand of energy related facilities such as power
generation turbines and customized products, thedemand for aircraft parts is increasing globally, inrecent years as shown in Fig. 6 5). Domestic
production of large precision parts which require
rigorous quality control is also increasing. For small-lot
or one-unit production or customized production
where new quality control concepts and strategic
machining measurements as the base for production-5-

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NTN TECHNICAL REVIEW No.86（2018）

multi-sensor data integration technology, as shown in
Fig. 7 (a) to (h). This technology, in other words,
implies that the in-process/on-machine measurement
technology encompassing a broad range of elementsthat directly determine machining accuracy, such as
tools, machine tools, machining environment, andmachining/measurement operation, is critical. In
addition, it is expected that small-lot production andcustomized production or one-unit production will
increase in the "non-equilibrium/age of strategy,"where the adopted strategy is to generate new value
by creating innovative products. Therefore, production
quality control based on in-process/on-machinemeasurement is indispensable, which is also the key
technology for achieving predictive production system.

(100 million yen)
16,000100%

14,00012,000Accessories

75%

Engine parts

10,0008,000

50%Engine body

6,000
4,000

Boeing/Spirit (U.S.) 35%
Japan 35%
Vought (U.S.)/Alenia (Italy) 26%
Others 4%

Aircraft parts

25%

2,000
0

H21

H22

H23

H24

H25

Aircraft body0%

H26
(Forecast)
Source: "FY2014 outlook of production/export/booking of aircraft (revised)"
The Society of Japanese Aerospace Companies (August 2014)

Source: Japan Aircraft Development Corporation

H20(b) Historical trend of aircraft production volume(a) Joint development of Boeing 787

Forecast of jet passenger aircraft share in operation# of aircraft
40000

Forecast

Actual

35000
30000

A380
747

26814

25000

A340
A330

DC10,MD11
L1011

15000

13802

10234 DC8,707

10000

A300

747
787
A330
A310
A321 757

A320,MD80/MD90
727-200,737-300/400

5000

1997

2002

2007

DemandRemaining
aircraft

A319/A320
737-700/800

737-600,A318

2017

2022

120-169 seats

CS100/300 100-119 seats

328JET,ERJ130/145,ORJ200

2012

310-399 seats

787
767
230-309 seats
A350

ARJ21,SSJ100,MRJ
ERJ170/190,CRJ700/900/1000

717,727-100,737-100/200/500,TRIDENT,DC80
BAC111/F28/F70/F100,DC8

01992

400 seats or more
777

737-900 170-229 seats
A321

18590

20000

34359

2027

60-99 seats

2032

20-59 seats

Source: Japan Aircraft Development Corporation(c) Forecast of jet passenger aircraft share in operation

Fig. 6 Increasing demand for high-precision parts in aerospace industry【Concept of conventional quality control】(1) Mass production(2) Statistical process control(3) Yield management【Concept of new production quality】(1) Small batch, Customized, or even one-of-a-kind products）(2) New paradigm beyond six sigma approach(3) Zero defect manufacturing【Key elements of comprehensive production quality control】
Innovative and integrated
quality

Production logistics andmaintenance design

Management and control
methods

Advanced technologicalenablers（＊ICT: Information andCommunication
Technology）【Technological innovation required for establishing production quality】(a) Product inspection technology(b) Process monitoring technology(c) Multi-sensor data fusion technology(d) Learning techniques and cognitive computing methods(e) E-maintenance technology(f) Product traceability technology(g) Production monitoring technology(h) ICT and digital manufacturing technology(Reconfigured and summarized reference 7) Colledani et al. CIRP annals (2014))

Fig. 7 Fundamental scheme of production quality control and the required innovative technologies to establish it-6-

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Strategic Vision for Smart Machining Tool and Measuring Instrument

and relationship between machining factors acrossmachine tools, workpieces, and machining environment
and in-process/on-machine measurement. Of particularimportance as a fundamental measurement property is
accuracy and precision. For example, consideration
for questions such as if accuracy and precision forevaluating tolerance range as the base for processcapability are satisfactory and if absolutemeasurement is required due to machining bias errors
is critical. In addition, the in-process measurement
evaluation is affected by dynamic characteristics suchas transient response and frequency response.
Furthermore, properties specific to in-process/onmachine measurement are also required. These
include environment factors required for installingmeasurement devices to machine tools andhardware/software factors. For example, in the case of
on-machine 3D profile measurement using an opticalcomparator, vibration property, compactness, andswitching process are required. The advantage of inprocess/on-machine measurement is not limited to
improvement and stabilization of machining accuracy
but extended to higher efficiency ofmachining/measurement setup. As such, the scope ofmeasurement extends from workpieces to machining
state in space and time domain realizing "visualizationof machining process" by quantifying machining
factors.
The machining error, which value is constantly
maintained during the repetitive machining operation,
provides the same effect not influencing the accidental
error. Therefore, it provides a constant bias to the

4. Production quality control byholistic measurement
4.1 Current status of in-process/on-machinemeasurement
The role of the machining measurement for
production control is "informatization" of manufacturing.
In the advanced production system using diversified
and vast production information, machining
measurement plays an important role as the key
information source on the products and machining
states. The most essential purpose of machining
measurement is production control, production
automation, and test automation. In-process/onmachine measurement is the most efficient andeffective machining measurement technology engaged
directly in monitoring and controlling of machining. With
the machine tools with reproducibility, the machining
errors that vary over time during machining operation
due to factors such as deformation and displacementof machine due to change in temperature, thermal
expansion and deflection of tools, and wear may beevaluated and corrected by re-machining.
Recently, machine tools with in-process/on-machinemeasurement capability are being commercialized
and the importance of their role is increasing. These
trends are driven by diversified purposes of themeasurement. In-process/on-machine measurement
plays a central role for production/quality control aswell. Fig. 8 shows the fundamental measurement
properties required in in-process/on-machinemeasurement technology and the unique properties[Properties specific to in-process/on-machine measurement][Environmental factors]· Temperature, humidity· Vibration· Dust, etc.

[Hardware factors][Basic measurement properties]

Absolute criteria Average of data pool
(Master value)
Measurement device
Exactness[Static properties]· Exactness
Machine tool(absolute accuracy)· Precision (repeatability)· Range· Linearity
Upper limit
Lower limit· Traceability
Precision[Dynamic properties]· Transient response· Frequency response
Measured
Machining (product)
quantity
allowance· Correction of machine coordinates(geometric error)

Evaluation of motion accuracy[Machine tool] ·· Accurate
matching of workpiecereference point and machining
origin point· Measurement criteria(independent/dependent)· Non-contact/contact· Measurement speed· Compactness[Software factors]· Measurement dataprocessing (error
analysis model,
correction data
generation, stitching,etc.)· Measurement strategy(sequential three-point
method, measurement
path generation,
swing-round method,etc.)[Tool][Workpiece]· Measurement of length, diameter,
and tool edge
· Detection of relative coordinate
distance between measurement
probe and tool edge
· Evaluation of tool wear
· Evaluation of tool deflection andforming load
· Dynamic balance adjustment for
grindstone· Real time verification of machining
accuracy· Higher accuracy of machining
correction· Higher efficiency of machining
process· Evaluation of workpiece deformation[Working environment] · Real time correction of machining state changes due to changes in temperature and external vibration[Machining/measurement operation] · Pre-inspection of machining/measurement· Higher efficiency of machining/measurement setup (positioning of tools and workpieces)· Higher efficiency of measurement operation (standardization of tool path probe path software, etc.)

Fig. 8 Fundamental properties of measuring technology in machining process and roles of on-machine measurement-7-

*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 7

NTN TECHNICAL REVIEW No.86（2018）

machining accuracy. In general, evaluation ofsystematic error which cannot be found in theserepetitive measurements is difficult and often its
existence cannot be detected. However, if the factor
for systematic error is predetermined and its
magnitude is known, it can be eliminated withcorrection. Therefore, a smart machine tool which candetermine the factors of machining errors in real timewith multiple in-process/on-machine measurements is
an important concept as an elemental technology ofthe production system in conventional efficiency
driven manufacturing. The basic concept of the smart
machine tool that holistically provides machining
process monitoring, recognition, and prediction using
multiple sensors was presented around 1993 by
Moriwaki 7). Since then, active research anddevelopment has been conducted on in-process/onmachine measurement and machining control aimed
at application to practical systems, resulting inincreased development in application fields such asadvanced sensors for tool condition monitoring (TCM),
signal processing methods, decision making strategy,
etc. These trends of research activities on thechallenges of in-process/on-machine measurement
indicate that attention is paid to the solution ofindividual challenges for each element oftools/workpieces/machine tools which are the mainelements of the machining process, and thechallenges in the boundary area from interaction ofthese elements. In other words, adaptation toadvanced and complex requirements of high-speed,
multi-functional, and intelligence is sought after in
addition to higher accuracy of machining processes,
from the viewpoint of practical application of inprocess/on-machine measurement to the real system.
As the complexity of machining process increases,
interaction of tools, workpieces, and machine tools
has become more complex and the challenges of inprocess/on-machine measurement in the boundary
area have also become multifaceted and advanced.
Areas subject to in-process/on-machinemeasurement are expanding from geometric quantity tostate quantity and the challenges are not only
multifaceted, but expanding with advanced multiaxis/multi-tasking machine tools and intelligent
machining process. In addition, the requirement offurther improvement of accuracy for ultra-precision
machining is expanding into new dimensions, such as
3D free curve profile, micro complex profile, and nanosurface pattern, which are required for commercializationof advanced functional parts. Fig. 9 summarizes
positioning and scope of in-process/on-machinemeasurement technology in the on-machine systemwith tools, workpieces, and machine tools as its key
elements. It indicates the boundary areas of eachelement and relation among measurement technology
and breaks down the time-variant phenomena into
short, medium, and long period and marks their・On-machine measurement of tool wear (1976): direct/indirect in-process measurement method

Machining atmosphere(environmental change)ic
Dynamserespon・Tool state monitoring TCM (1995): Optical surface measurement sensor, accelerometer, AE sensor, etc.

[Workpiece]

・Measurement of workpiece-tool distance (1998): In-process measurement machining accuracy correction[size, shape,surfacecharacteristics,
stress,temperature, etc.]

Measurement of cylindrical surface micro profile (2006): Optical angle sensor

・Measurement of tool deflection and cutting load (2010): Camera imaging measurement
・Direct evaluation of cutting force (2011): Development of cutting force detection spindle
・On-machine measurement of machined surface
roughness (2014): Contact probe (surface roughness gauge)e sm
riv niD a・On-machine measurement of workpiece shape (2010): Laser displacement sensorg/Holdingbindin・Evaluation of motion accuracy of 5-axis machine tool (2010): Geometric error analysis by improved DBB

h
t
ec
en rs
m
m to
re facsu nea io
M eratop

・Motion accuracy measurement of 5-axis machine tool (2008): New measurement principle (two-axis scale type)・On-machine measurement of aspheric surface work
(2009): High accuracy contact probe
・On-machine measurement of tool cutting edge
shape (2009): Three-dimensional measurement by AFM
・In-process evaluation of edge motion and workpiece
shape (2009): Laser displacement sensor
・In-process evaluation of tool position (2010): Optical imaging measurement of tool length and diameter

m
p
belex
ha vibvi raor tioDynamic characteristics
of cutting and grinding[On-machine system][Tool]

Abrupt change,
phenomena thatchange in a shortperiod[size, position,
vibration,
forming load,
wear,
temperature, etc.]m[Machine tool][space error,
vibration, motion
accuracy, thermal
displacement,
wear, etc.]

n

ve is
ri n
D ha
ec
m

Wear

al n
m tio
er a
Thorm
f
de

Co

Hold
bind ing/
ing

・On-machine measurement of mirror finished surface (1992-1999): Fizeau interference, zone-plate interference,
radial shearing interference, etc.

Phenomena that Phenomena thatchange over achange over a
medium period long period

op M
er ac
de Staati hion nin formatitci
on
fac g
to
rs

・On-machine measurement for process monitoring (1983): AE sensor, accelerometer, spindle motor output, etc.

ic on
amati
yn
D formde

Evaluation of machining
dynamic characteristics
Machining statemonitoring
Geometric quantity
measurement(size, shape, dimension, etc.)

Fig. 9 Spreading roles of on-machine measuring technologies in machining process-8-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 8

Strategic Vision for Smart Machining Tool and Measuring Instrument

Therefore, a machining monitoring system 8) 9) and onmachine dimensional measurement systems are built
and installed on the machine tools based on theconcept of "multi-sensor fusion" and "multi-sensor
cooperation," respectively.
Fig. 10 (a) shows the concept of multi-sensor fusion
on-machine measurement for machining statemonitoring. By conducting state estimations combining
feature amounts from multiple sensors, characteristics
of different physical amounts can be used to improvethe detecting capability of machining monitoring. In themachining process that requires long time,measurement information (physical amount varied overtime) obtained from the respective sensors is integrated
in real time. Use of advanced signal processing todetect feature amounts that correlate with abnormality
based on the time-frequency analysis provides adaptive
threshold management in machining monitoring. Its
high adaptability to environment and abnormality
symptom detection capability reduces the risks related
to production quality, such as chattering and tool failure,
which is the most important measurement strategy forachieving a zero-defect process. In addition, in the
future it is expected to both maintain production quality
and improve process capability by integrating
estimation methods and machining monitoring
information considering multiple analysis technologies
and uncertainty, such as cutting phenomenon, cutter
path, tool/workpiece shapes, and by optimizing
machining conditions based on the process model.
Fig. 10 (b) shows the concept of multi-sensor
cooperation on-machine measurement for highprecision measurement of geometric quantity. Higher
accuracy of measurement is pursued by optimizing

positioning in the respective time scale. The relation of
in-process/on-machine measurements is indicated byring bands connecting two elements and a ring band
circling three elements. The former indicates interactionof the elements, and the latter indicates a complex
interaction area where three elements are involved. In
addition, the outer ring band that encloses the entire
diagram indicates machining atmosphere and manual
operation. The purpose of measurement is classified
into evaluation of machining dynamic characteristics,
machining state monitoring, and measurement ofgeometric quantity, such as size, shape, anddisplacement, with R&D examples shownchronologically from [a] to [ p]. For example, in the caseof [a], on-machine measurement of tool wear (1976) is
positioned as the on-machine measurement technology
with the purpose of monitoring machining state thatchanges over time in the boundary area between tools
and machine tools. Also, the in-process/on-machinemeasurement technology that corresponds to thesolution to challenges other than the interaction
between elements are only indicated with factors as thechallenges to be solved in the future.

4.2 New in-process/on-machine measurement
strategy
As the basic concept of new in-process/on-machinemeasurement by multiple sensors, an example of highfunctional measurement information is shown in Fig.10
by "multi-sensor fusion" and "multi-sensor cooperation".
In the manufacturing process of large precision parts, astrategy for proper installation and placement of inprocess/on-machine measurement system on themachine tools at the manufacturing sites is required.

Multi-sensor fusion: improved detection capability by using
characteristics of different physical quantities

Ammeter

Multi-sensor cooperation: optimized configuration ofcomponents which measure the same geometric quantities

Position
detection
probe

Opticalsensor

ThermalMotor current displacement

AE sensor

Sound
wave
Vibration

Accelerometer

Machining statemonitoring

Laser
tracker

Emissionof light[Machining abnormality,
tool wear/damage,Stressdynamic characteristics,
etc.]
Acceleration
Temperature

Straingauge
Calibration
gauge

Thermocouple(a) On-machine measurement using multi-sensor fusion

Laser
displacement
sensorHigh accuracy
measurement ofgeometric quantity[size, shape, position, attitude,
surface characteristics,
displacement, motion accuracy,etc.]

SurfaceroughnessstylusLaserinterferencecomparator(b) On-machine measurement using multi-sensor cooperation

Fig. 10 The smart on-machine measurement system using multi-sensor fusion and cooperation-9-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 9

NTN TECHNICAL REVIEW No.86（2018）

4.3 Concept of holistic measurement

and allocating respective roles for multiple components
with the purpose of measuring the same geometricquantity (e.g. size, shape, attitude, and position) basedon their measurement elements. The example of onmachine dimensional measurement systems integrated
with the large CNC lathe for machining large precisionparts consists of laser interferometer, laser tracker,
tough trigger probe, and block gauge to realize highprecision, multi-sensor cooperation for on-machinemeasurement providing absolute length calibration.
In the case of the machining process of steam
turbine rotors, the operation of moving the work frommachine tools to the measurement system was difficultand on-machine measurement was manuallyconducted using large micrometers, etc. Therefore, theproblem was that it involved many measurement
processes, resulting in increased uncertainty. To solve
this problem and achieve efficiency of the
manufacturing process and machining quality
assurance, a high precision on-machine measurement
system was developed. Fig. 11 shows an example ofon-machine measurement system for large structuralparts by "multi-sensor cooperation" targeting steam
turbine rotors (max. length of 10m or more, diameterof 1m or more, and dimensional tolerance of 0.1mm orless) of large CNC lathes. It shows configuration of thesystem which conducts absolute measurement ofwheel position and axis diameter of the steam turbine
rotor of the large CNC lathe by probing measurement
method 10). An inline length measuring system using
laser tracker is introduced as the external coordinate
system and a wireless touch trigger probe is
implemented on the tool carriage. In addition, as theparts and machine elements are both affected byshrinkage and expansion (about 0.1mm of impact per
1m if the rotor temperature changes from 20˚C to10˚C), a calibration method of absolute length isimplemented using a block gauge to correct bias
errors.

Workpiece (protruded cylindrical part: wheel)
Reference plane

Block gauge

For the predictive production system, which is thefoundation for innovation driven manufacturing,development of smart in-process/on-machinemeasurement as the strategic tool for machining
measurement technology innovation is required torealize production quality control. As its foundation,
the measurement strategy is to realize "informatizationof every process related to manufacturing" bydeveloping high degree of "multi-sensor fusion" and"multi-sensor cooperation," and with holistic
measurement 11) based on integration of different
measurement principles and multi-scale data fusion.
For realizing holistic measurement, new theory forevaluating uncertainty, new comprehensive
measurement principle to encompass the calibration
technology, and conventional measurement principle
and broader research regarding the compatibility
among them are required.
As a specific example, a basic concept of intelligent
multi-sensor coordinate measuring machine (CMM) is
presented 12). The holistic measurement uses newmeasurement technology, such as multi-sensor CMM
with metrology X-ray CT (volumetric measurement).
With the advancement of measured data fusion
technology by integrating CAD/CAE/CAM/CAT data,
this builds the foundation for advanced production
quality control while different measurement systems
become connected with each other through theInternet. The new strategic machining measurement
which extends the concept of this measurement
fusion technology to in-process/on-machinemeasurement is the smart in-process/on-machinemeasurement that realizes environment adaptive
measurement with such properties as real-time, wide
range, and multi-point simultaneity. As shown in Fig.
12, the optical non-contact measurement technology
in a broad sense, including X-ray, is making rapid
progress and technology that can measure surface
micro-profile, 3D profile, and 3D internal structure has
already been developed. Metrology X-ray CT
technology in particular is making significant progress
and its incorporation into inline processes andapplications for assembly process inspection arebeing proposed 13). When building multi-sensor CMM
using these measurement technologies, 3D
measurement data fusion technology is required formeasurement integration of micro-profiles to complex
large 3D profiles and volumetric measurement data.
Efforts for developing measurement fusion
technology 14) is already in progress. However, sometechnical challenges still remain, specifically related
to multi-scale data fusion, such as measurement data
quality including uncertainty, standard and calibration

Laser tracker

Maximum distance
between wheel planes

Tool/reflection mirror/probe
Positioning on XZ
Carriage plane with resolutionof 0.001 mm
Z
O
X

Fig. 11 The developed on-machine measuring system
for a steam turbine rotor on CNC turning machine-10-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 10

Strategic Vision for Smart Machining Tool and Measuring Instrument

Measured data quantity● 3D internal structure measurement technology◆ 3D shape measurement technology※Surface roughness measurement technology

Large
Holistic measurement(fusion of 3D measured data)(Research stage)
Volume※Light scattering
methodSurface(Commercial stage) ◆ Light-section method

2-D
1-D
Point
Small

◆ Moire fringe method◆ Fringe pattern projection method◆ Stereo image method※Coherence scanning◆ Digital holography methodinterferometry※Stripe pattern◆ Photogrammetry
projection method

3-D

Line

●X-ray CT※Confocal microscope※Focal position detecting method※Phase shifting interferometry
Slow◆ Triangulation method◆ Optical interferometry

Measurement speed

Fast

1/timeFig. 12 Rapid progress in three dimensional measurement technologies for industrial measurement

manufacturing facilities). The key is to build predictive
production systems with self-recognition capability.
Internet of Things (IoT) which converts everything todata and connects them to the Internet through sensor
technology, smaller and faster processors, andadoption of cloud technology, industrial big data whichis collected by IoT, and cyber physical system (CPS)
which use them will play the central role as themechanism for such intelligence 15). In addition, themeasurement technology, which plays the role ofinformation resources, is required to undergotransformation as a strategy tool. Particularly, smart
in-process/on-machine measurement technology is an
important and high-quality source of production
information together with controllers and network
systems, and requires technology innovation as a
strategy tool, such as self-recognition and selfprediction embedded in the measurement systems.

methods, the basic quantity of measurement such as
one-dimensional length and displacement, and twodimensional surface micro profile with different angles
and scale.

5. Outlook and challenges of smart
machining/measurement machines
5.1 The 4th Industrial Revolution and predictive
production system
The 4th Industrial Revolution is a disruptive
transformation unlike the previous industrial
revolutions, as significant innovation is expected not
only in the manufacturing process but also across theentire product life cycle due to the rapid technologicalinnovation in information technology, as evidenced by
the promotion of Industry 4.0 in Germany andIndustrial Internet in the U.S. The ultimate objective of
the 4th Industrial Revolution is achievement of a new
predictive production system based on the total
optimization and efficiency of the manufacturing
process, development of predictive maintenance, etc.
That is, creation of new value by innovative productsthat fill the gap of "invisible value" and shift from mass
production to complete customized production (oneunit production). In the manufacturing process withcompletely different mechanisms from the past,
production quality has better adaptability than thequality control approach used in mass production.
Furthermore, the basic concept of the 4th Industrial
Revolution requires intelligence with autonomy (e.g.
self-recognition, self-maintenance, and self-prediction)
as its core property to the members of the
manufacturing system (e.g. products and individual

5.2 Smart in-process/on-machine measurement
network
The approach of intelligent machine tools by onmachine measurement using multiple sensors is aconcept for standalone machine tools. Its objective is
to increase efficiency of individual machining/measurement systems with knowledge base andincorporation into the machines. Therefore, it remains
in the scope of efficiency driven manufacturing. On theother hand, innovation driven manufacturing requires
intelligent environment adaptive in-process/onmachine measurement, i.e. open and intelligent
machine tools through integration with smart inprocess/on-machine measurement. Configuration ofmulti-sensor fusion and multi-sensor cooperation-11-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 11

NTN TECHNICAL REVIEW No.86（2018）

based on holistic measurement changes in real time
according to measured content (information on
measurement environment, measurement condition,measured amount, subject of measurement,machining condition, required accuracy, allowance,
uncertainty, etc.) to achieve advanced prediction. Its
mechanism is based on the network of smart inprocess/on-machine measurement connected in
virtual space, as shown in Fig. 13. That is, themeasurement content and measurement information
are shared among intelligent machine tools connected
to IoT, to drive predictive production systems which
control production quality. The smart in-process/onmachine measurement is required to have thecapability to sufficiently satisfy conditions of its
measurement information such as amount, speed,
diversification, and reliability. Capabilities such as selfrecognition, self-maintenance, and self-prediction are
obtained by CPS.

the activities of "visualizing" operational states acrossthe machines in the individual process, production line,
the entire production process, traceability management,collection of production process data from overseas
factories, etc. are all increasing. Therefore, it isconsidered that smart in-process/on-machinemeasurement will be increasingly important as a highquality information source. In addition, solutions using
recent digital technology are considered to be the"engineering chain" and "supply chain" indicated in Fig.
15. which also shows the mechanism to make thosechains more efficient by quickly feeding back the smart
in-process/on-machine measurement data. In contrast,
the requirements for smart in-process/on-machinemeasurement, such as improvement of reliability andquality on the measurement data, are becoming higher
and higher, as the data is more integrated, complex,
and significantly larger.
Fig. 16. indicates the transition of machining
measurement technology up to the 4th Industrial
Revolution and transformation to the strategic
machining measurement, as the foundation of
production quality control. Machining measurement
technology has been playing a role in element
technology for production systems that satisfy therequirements of automation of inspection, production
control, automation of production, etc. In that process, it
underwent a series of revolutionary improvements inmeasurement capability, such as improvements inmeasurement accuracy and speed, higher
sophistication from one dimensional to two dimensional,
etc. However, the measurement technology required forthe 4th Industrial Revolution is not an extension of
conventional technology, but a fundamental
transformation. That is, the predictive production
system that takes advantage of IoT, industrial big data,
and CPS based on real time holistic measurement

6. Summary
The most recent "2017 White Paper on Manufacturing
Industries" 16) reports the initiatives and measures thatthe manufacturing industry in Japan is taking for the 4th
Industrial Revolution. The white paper mentions that thevalue of virtual data from activities on the Internet is
predominantly dependent on the "quantity" of data
which is already dominated by the U.S. IT companies.
However, the value of the real data collected by
sensors in real life activities, such as operational data of
the factory facilities, is potentially dependent on the"quality" of data and the companies in the
manufacturing industry may be able to take a leadingrole depending on the future course of action. In fact,
the survey results regarding data collection of the
production process in Fig. 14 indicates the trend that

Which machine
tool was used?

When and where
was it measured?

VolumeVelocity
Variety
VeracityMachincond ing
ition?

RFID

Product(Virtual space)Industrial
big data

Visibility

IoT

ment
Measure t?
en
m
n
ro
vi
en

ainty?

UncertAllowance?

Value

h

whic
s of
Part ine?
hmac

Smart in-process/on-machinemeasurement
Autonomous operation by cyber
physical system (CPS) (use of
big data, AI and IoT)

＊RFID (Radio Frequency IDentifier); IC tag

Fig. 13 Connecting data of smart in-process/on-machine measurement in cyber physical space-12-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 12

Strategic Vision for Smart Machining Tool and Measuring Instrumentthat uses intelligent in-process/on-machinemeasurement is considered to achieve the ultimate
production quality control with self-recognition of theintelligent production elements.

[Collecting any data fromthe domestic factories?]
2016["Visualization" of data collection and use of data for
production process improvement such as traceability management]（n=4566）

No

33.4％

Work on process improvement by"visualizing" availability of machines
in the individual processes

Yes

Work on process improvement by"visualizing" availability of machines
in the lines and entire production
processes

66.6%

Work on process improvement by"visualizing" availability of workforce
related to lines and production process

Source: METI Survey (Dec. 2016)2015 年

7.2%【2015 survey】（n=3507）

18.1%

4.6% 12.3% 1.4%

30.0%

【2016 survey】（n=3980）

13.9%

7.5%

【2015 survey】（n=3518）

16.3%

5.0% 13.7% 1.7%

16.4%

8.9%

4.0%

Yes

Work on collection and use of data
related to production process inoverseas factories[owns overseas factories]

10.0%

Already implemented

Planned

63.5%

24.1%

16.4%

21.9%

39.1%
63.7%

38.9%

Hope to implement if possible

Source: METI Survey (Dec. 2016)

Source: METI Survey (Dec. 2015)

11.5% 7.0%

38.4%

2.9%

【2016 survey】（n=720） 10.0% 6.8%

【2015 survey】（n=626）

38.3%
63.3%

35.4%
17.8%

38.4%
63.5%

31.4%

【2016 survey】（n=3970） 9.1% 7.1%

【2015 survey】（n=3510） 10.2% 5.6%

8.9%Manage traceability of product/material
among intra-company or inter-company
factories【2015 survey】（n=3496） 14.7% 3.2% 11.8% 6.6%

40.6%

59.4％

15.5%

【2016 survey】（n=3897）（n=3751）

No【2016 survey】（n=3966）

16.7%
8.1%

No plan

27.6%
51.4%

No data collection implemented※ Note that the simple comparison to the survey of the previous year is not

appropriate as the number of survey respondents increased by approx. 20%

Fig. 14 Research results on data collection of production process

Su

pp

Order processly

Joint order process

ch

ain

(Quickly reflect customer usage data
to supply chain)

Customer

Expand and increase speed of data usage
using digital tools

Production control
Optimized production
High mix low volume production※ Improvement of productivity
and value addition by
optimization and efficiency
within company

Technology inheritance(Digitalization of expertise)
Product Design
Design support

Product planning
Planning support
R&D
R&D support

Production(Production association)

ring

ee
gin

En

n
hai

c

Predictive maintenance(intra-company)
Remote maintenance(intra-company)

Logistics/distribution
Logistics optimization
Sales forecast

Operation optimization(intra-company)

MaintenanceFollow-through service, etc.
Predictive maintenance(customer)
Remote maintenance(customer)

Operation optimization(customer)

Expand and increase speed ofdata usage using digital tools(Quickly reflect customer usage data
to planning and design)

Fundamentally new service※

Direct improvementof customer value

Creation of new added value

Productivity improvement

Fig. 15 Effective utilization of IOT in Industry 4.0-13-

*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 13

NTN TECHNICAL REVIEW No.86（2018）

Measurement data amount/measurement
accuracy/measurement speed

Revolutionary improvement of measurement capability (machining measurement):
standardization, higher accuracy, 3D/automation

Industry3.0
Industry2.0
Industry1.0

Laser interference comparator
Vernier, micrometer

Optical 3D measurement
device3D coordinate measuringmachine: CMM (1970)

2D characterization (contour form)

Block gauge (1896)

・International Meter
Convention (1875)

1D characterization (dimension)· Manual measurement· SI-unit (1960)

Industry4.0
Industrial X-ray CT
Multi-sensor CMM
Holistic measurement(fusion of 3D measured data)

3D characterization (3D form)

Volume measurement (3D volume)· Coordinate measurement,
point group measurement· Automated measurement,
computer control· High degree of freedom,
universal measurement· Offline measurement· CAD/CAM/CAE integrated
measurement· Combined measurement(multi-sensor fusion,
multi-sensor cooperation)· In-process/on-machinemeasurement

First industrial revolution

Second industrial revolution

Third industrial revolution

Fourth industrial revolution

Latter half of
18th century

Beginning of
20th century

Latter half of
20th century

Present

1900

1970

（1980s and later）

2000

Fig. 16 Revolution of strategic measurement technology in production inspired by Industry 4.0

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20058) Masahiro Uekita, Yasuhiro Takaya, Tool condition
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Engineering, Vol.24, (2000), 264-269.
4) International Organization for Standardization, “Guide
to Expression of Uncertainty of Measurement”, JCGM
100, (2008).

5) Ministry of Economy, Trade and Industry, Ministryof Health, Labor and Welfare, Ministry of
Education, Culture, Sports, Science andTechnology, 2015 White Paper on Manufacturing
Industries Overview; FY2014 Promotion measuresof core manufacturing technology (Overview),(2015)
6) Marcello Colledani, Tullio Tolio, Anath Fischer, Benoit
Iung, Gisela, Lanza, Robert Schmitt, Józef Váncza，Design and management of manufacturing systems
for production quality，CIRP Annals - Manufacturing
Technology, Vol.63/2, (2014), 773-796.
7) Byrne G, Dornfeld D, Inasaki I, Ketteler G, König W,
Teti R, Tool Condition Monitoring (TCM) — The
Status of Research and Industrial Application, Annalsof CIRP, Vol. 44/2, (1995), 541–567.

-14-*02-寄稿文E_new_*02/寄稿文 19/07/03 10:26 ページ 14

Strategic Vision for Smart Machining Tool and Measuring Instrument

13) L. De Chiffre, S. Carmignato, J.-P. Kruth, R. Schmitt,
A. Weckenmann, Industrial applications of computed
tomography, CIRP Annals - Manufacturing
Technology, Vol.63/2, (2014), 655-677.
14) A. Weckenmann, X. Jiang, K. –D. Sommer, U.
Neuschaefer-Rube, J. Seewig, L. Shaw, T. Estler，Multisensor Data Fusion in Dimensional Metrology，Annals of the CIRP, Vol.58/2, to be published, 2009.

15) Jay Lee, "Industrial big data; challenges ofmanufacturing industry toward Industry 4.0",
Nikkan Kogyo Shimbun, Ltd., 201616) Ministry of Economy, Trade and Industry, Ministryof Health, Labor and Welfare, Ministry of
Education, Culture, Sports, Science andTechnology, 2017 White Paper on Manufacturing
Industries Overview; FY2016 Promotion measuresof core manufacturing technology (Overview),(2017)<Author biography>

Yasuhiro TAKAYA
Professor, Doctor (Engineering), Department of Mechanical Engineering,
Graduate School of Engineering, Osaka University
March, 1992
April, 1992 - June, 1995
July, 1995 - June, 1997
July, 1997 - March, 2006
April, 2006 - present

Engineering Doctor, Doctor Course Completed, Precision Engineering Department,
Graduate School of Engineering, Hokkaido University
Assistant, Industrial Mechanical Engineering, School of Engineering, Osaka University
Instructor, Industrial Mechanical Engineering, School of Engineering, Osaka University
Assistant Professor, Department of Mechanical System Engineering,
Graduate School of Engineering, Osaka University
Professor, Department of Mechanical Engineering,
Graduate School of Engineering, Osaka University[Specialty]
Precision machining measurement, optical applied nano-measurement, optical applied nano/micro machining[Academic society and committee affiliations]
The Japan Society for Precision Engineering: Intelligent Nano-Measure Expert Committee Chair
The Japan Society of Mechanical Engineers: Fellow, 1st Planning Committee Chair, Manufacturing and Machine Tool Division
The Japan Society for Abrasive Technology
The International Academy for Production Engineering (CIRP): Fellow
International Measurement Confederation (IMEKO): TC14 Chair
The Japan Society for Die and Mould Technology: Director (Vice President)
Mitsutoyo Association for Science and Technology (MAST): Advisor and many others-15-*03_展望E_*03_展望new 19/07/03 10:27 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ Perspective ]Rolling Bearing and IoT – NTN’s Approach to IoT -Masaki EGAMI

With advancement of IoT, big data collected from various types of equipment
are utilized to improve service and productivity, to reduce opportunity loss byproviding appropriate maintenance and so on. Rolling bearings are integrated
and used in many familiar applications such as transportation vehicles,
electrical products, and industrial machinery. Therefore, rolling bearings are
considered as one of the most suitable data collecting elements. This report introduces NTN’s approaches to IoT
and future prospects, focusing on sensing technologies related to rolling bearings.

40 times compared with conventional products anddevelopment of a sensing system that enables
detection of absolute angles.
With regard to the detection of abnormal operationof machinery by vibration, NTN started evaluating
vibration acceleration. NTN recently moved to moresophisticated abnormality detection using artificial
intelligence (AI) such as machine learning.
We also developed "Wind Doctor TM" as theCondition Monitoring System (CMS) for wind turbines
in 2012 as the abnormality diagnostic system forequipment, which is used with domestic power
generation facilities for reduction of downtime.
In this article, we introduce this technology as our
initiative related to IoT at NTN, and describe our vision
for the future. Some of the technology and products
introduced in this article are also detailed in different
papers, so please excuse us for possible duplication.

1. Introduction
Recently, as Germany and the U.S. promote
Industry 4.0 and the Industrial Internet, respectively,
the Internet of Things (IoT) is being rapidly adopted.
Also, in Japan, Society 5.0 is being proposed andpromoted under the leadership of the government
which describes the "Super-smart society" as thesociety of the future 1-2). With IoT, data is collectedfrom any device and analyzed as big data. Initially, thefocus was placed on improvement of productivity in
manufacturing and reduction of lost opportunity by
optimizing maintenance. However, they are now
planned to be used in various fields including
improving quality of life.
Rolling bearings are used in all kinds of machines,
including transportation vehicles such as automobiles,
rolling stock, aircraft, industrial machinery such as
construction machinery, machine tools, robots,
information/office equipment such as computers andprinters, electrical appliances such as refrigerators, airconditioners and washing machines. The bearings areused as mechanical elements to support various
rotational operations.
NTN developed rolling bearing units with embedded
rotation sensors in early 1990s for industrial
machinery. NTN continued to develop new technology
for applications in industrial fields such as robotics,
with enhancement of resolution of rotation sensors to

2. NTN's rotation detection technology
and application products
2.1. Rolling bearing with rotational sensor
Optical and magnetic sensors are generally used for
detection of rotation, depending on the application. An
optical sensor has the advantage of high detection
sensibility; however, it is costlier and more susceptible
to adverse environments such as dirt. Therefore, NTN
has been dedicated to the application of magnetic*Senior Executive Officer, R&D-16-

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Rolling Bearing and IoT – NTN’s Approach to IoT –-2.2. High resolution rotational sensor

sensing systems to consider the environments whererolling bearings are used. We implemented volume
production of "bearings with rotational sensors" which
can detect both rotation speed and direction by
installing multipole (N/S poles) magnetized rings andmagnetic sensor IC facing each other in rolling
bearings 3-4) Fig. 1 shows an example of bearings withrotational sensors for industrial machinery, used for
detecting rotational speed and direction of drive motor
for forklifts. There are other products which are madeinto a unit for detecting rotational angle of hydraulicshovel booms 5).
Fig. 2 shows a hub bearing with a magnetic ring fordetecting rotation of anti-lock brake system (ABS) for
automobiles. The magnetized ring is integrated withthe bearing seal. The material is rubber with magnetic
particles. Rotation is detected by placing a magnetic
sensor IC against this magnetized ring. This system is
produced by NTN-SNR under the name "Active
Sensor Bearing (ASB ®)" and widely used globally as
the de facto standard for automotive ABS, even today.

NTN-SNR developed high resolution rotational
sensor with 40 times higher resolution than theconventional product for detecting rotation, using themagnetized ring described in 2.1 6). The sensor
structure and the measurement principle are shown in
Fig. 3. This sensor can detect small angles of lessthan the polarity gap width by dedicated IC, which
integrates multiple magnetic sensor elements andsignal processing circuits. An example of combinationof this sensor and rolling bearing is shown in Fig. 4.
This sensing system was adopted by Light Detection
and Ranging (Lidar) used for detecting obstacles
around vehicles in automotive driving support systems,
which is in the process of practical application. Thissystem detects obstacles by detecting laser lightreflected by obstacles by scanning laser light with amirror. This sensing system is integrated in the rotating
mechanism of the mirror, contributing to theimplementation of accurate angle control.
An example of detection of tire rotation in an actualHall sensor

N
0

S
π

θ

2π

Hall element
Magnetic
bypassring

Magnetic
encoder

ASIC

Side plate
Encoder(48 polar pairs)

Fig. 1 Rolling bearing with rotational sensor

NS

48 polar pairs

Conventional output(48 pulses)

Resolution 7.5˚

Developed productoutput(1920 pulses)

Resolution 0.18˚

Fig. 3 Sensing element for high resolution rotational sensor

Commercialized for rotating
mirror detection of Lidar

Fig. 2 Hub bearing with magnetic encoder

Fig. 4 Ball bearing with high resolution rotational sensor-17-*03_展望E_*03_展望new 19/07/03 10:27 ページ 3

NTN TECHNICAL REVIEW No.86（2018）vehicle by integrating this system into the ABS
sensing part of the vehicle is shown in Fig. 5 7). The
graph also shows the conventional ABS sensor signal.
It reveals that the tire rotation at the start of vehicle isdetected more in detail by the system, in the timezone indicated on the left axis. As autonomous driving
is expected to grow in the future, precise
measurement of road surface and tire rotation isrequired and therefore, application of this sensing
system is expected.

Magnetization pattern isvisualized by pasting
magnet sheet(b) Magnetization patternHigh-Resolution Sensor
ABS Sensor

0.2

Sub track (N-1 polar pairs)
Main track(N polar pairs)

Core metal(a) Outlook

0.25

Rotation speed s-1

Rubber magnetic
material

Divide this interval

0.15

Sensor 1

N

Sensor 2

N

S

N

S

e.g.
32 polar pairs
S
N

N

0.1

S
N
S
N
Double-trackmagnetic encoder

0.05

An example of detected signal(subdivide the interval between the N-S poles)

0

0

0.1

0.2

0.3

0.4

0.5

Time s

0.6

0.7

0.8

0.9

S
31 polar pairs

Sensor 1

1

32 periods
31 periods

Sensor 2

Fig. 5 Rotational signal when staring a vehicle
0°

Phase
difference

Rotational angle of shaft

360°(C) Principle for detecting absolute angles (e.g.: 32-31 polar pairs)

2.3 Absolute angle sensing by multi-trackmagnetic ring

Fig. 6 Multi track magnetic ring --- Radial type

NTN developed "multi-track magnetic rings" which
can detect the absolute angle of rotational shaft withhigh resolution and precision by combining thededicated magnetic sensor IC with the magnetic
rotational sensor technology it has developed so far 8).
The ring is made of annular core metal withunvulcanized rubber material vulcanized on the metal,
forming two magnetic tracks on the rubber magnetic
material.
Fig. 6 shows an example of radial magnetized type.
As shown in Fig. 6 (b), magnetic poles of different
number are magnetized in each of the two tracks ateven intervals with difference in phase. From thedifference of phases between the two magnetizedpatterns, the angle position is accurately calculated(Vernier principle) to detect the absolute position in a
360-degree range. This is described in more detail ina different article.
The multi-track magnetic ring can be designed in
low profile and compact size, and cables can be
included internally as it has a hollow structure. It is
expected that these magnetic rings can be used forangle or torque detection of joints in collaborative
robots which are expected to be quickly put into
practical use (Fig. 7) .

Axial type

Cabling throughthe ring
Radial typeFig. 7 Multi track magnetic ring installed in joint of robot-18-*03_展望E_*03_展望new 19/07/03 10:27 ページ 4

Rolling Bearing and IoT – NTN’s Approach to IoT –-effects on the maintenance plan.
Therefore, NTN is working on the damage detectionof bearings for large wind turbine by participating in
NEDO's project together with University of Tokyo andNational Institute of Advanced Industrial Science andTechnology (AIST), for improving abnormality
diagnosis. In this project, we have measured andcollected data on vibration installing NTN's condition
monitoring system "Wind Doctor TM" on wind turbines invarious locations in Japan, under the leadership of the
University of Tokyo. We are analyzing signals detectedby ordinary monitoring in order to find further early
symptoms using AI developed by AIST. Fig. 9 showscases of earlier detection of abnormality compared withthe conventional methods by applying AI developed by
NTN under advice of AIST.
Not only wind turbines, but many applications where
damage on bearings can cause social loss. If earlydamage or its symptoms can be detected and the time
until breakdown can be estimated, planned
maintenance can be conducted to avoid reduction of
the availability to a minimum. Currently, NTN isdeveloping AI to predict progress of damage through
NTN Next Generation Research Alliance Laboratory
established within Osaka University in September 2017.

3. Rolling bearing abnormality
detection technology

Ⅱ

Ⅰ

InitialprogressNoapparent
progress

NormalGenerator_Input_RMS_e [m/s2]

Damage (vibration)
stagesⅢ

Ⅳ

Progressinfinal stage

Warning threshold

Abnormal rate

Vibration acceleration

Operation status monitoring is required, including
surrounding machine parts in the areas where highreliability and safety are required for large equipment
such as power plants, plant facilities and rolling stock.
Sensors are integrated in the bearings or journal
boxes for sensing vibration of the rotation shaft,rotational speed, temperature, etc. Predictive safetyfunction is used for early warning of potential failure,by estimating the source of vibration from the detectedrotational speed and vibration frequency and detecting
the symptoms of increase in vibration.
The current typical abnormality detection methodsof rolling bearings are evaluation of effective values ofvibration acceleration and use of acoustic emission(AE) 9). Fig. 8 shows a transition of vibration
acceleration from normal status to breakdown ofrolling bearings 10). When the bearing is in normal
status (stage I), the vibration acceleration around thebearing is at a low level and increases when the
bearing is damaged (stage II). When the damage
reaches a certain size, the vibration acceleration
remains at the same level for a while (stage III). Whenthe damage further increases, the vibration
acceleration increases (stage IV), then it abruptly
increases to functional breakdown.

Caution threshold
Time

Fig. 8 Vibration transition of rolling bearing
with damage growth

28.0

Detection timingwith conventional
technology

24.0
20.0
16.0
12.0
8.0

Threshold

4.0
0.0

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

Time
Detection
timingwith AITime

Fig. 9 Example of early detection of failure by utilizing AI

Fig. 8 shows a simple model to set a threshold to theeffective value of vibration acceleration and issue
caution and warning signals. However, in theconventional method, the threshold should be
sufficiently larger than the initial background vibration toeliminate impact of temporary increase of vibration andnoise. There are cases where significant damage isalready given when the effective value of the signal islarge enough. In these cases, the time given until thefunctional breakdown may be very short depending on
the speed of damage progress, which causes adverse

4. Introduction of NTN's initiatives for IoT
4.1. Condition monitoring system (CMS)
for wind turbines
NTN developed "Wind Doctor TM" in 2012, as acondition monitoring system to help wind turbines
improve their availability 11) and is actively promoting itin Japan. Various measurement sensors and datacollection devices are included in the nacelle of thewind turbines (Fig. 10).

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NTN TECHNICAL REVIEW No.86（2018）

4.2 Natural energy and IoT

Measurement sensor(vibration, etc.)

NTN established the Green Energy Products
Division in 2016 to work on business development ofsmall power generation systems. It also started
distribution of NTN Hybrid Street Light which use two
types of natural energy of wind and solar light, and
NTN micro hydro turbines (micro hydro turbine).
"Green Power Park" in Advanced Technology R&D
Center in Kuwana-city of Mie Prefecture wasestablished as a part of this initiative (Fig. 13) 12). In
the Park, 3 NTN vertical axis wind turbines, one NTN
micro hydro turbine and 3 NTN Hybrid Street Light,
which were developed by NTN, are installed providing
energy generated by them to electric vehicles (EV)and vegetable factories (green power farms),
introducing energy circulating models. The IoT room
(Monitoring and Control Center) located in the site ismonitoring information on "power generation, storage
and consumption." NTN is also working on thedevelopment of energy management system pursuinghigh efficiency using IoT of this facility.
It also installed an "Independent power supply
network Mimamori Robokun III" experimental model(Fig. 14) in Osaka University for demonstration testing
within the framework of "Joint Research for Creating
Mechanisms Regarding Disaster Prevention
Monitoring and Tourism through Use of IT" with Osaka
University. This system combines "NTN Hybrid Street

Operational information(rotational speed/power output)

LAN

Data acquisition module

Fig. 10 Installation equipment of “Wind Doctor TM”
in wind turbine nacelle

Fig. 11 shows the system configuration of "WindDoctor TM" The collected data are recorded on the
dedicated server on the Internet and through signal
processing and trend analysis, system abnormalityand damage detection results are notified. As
mentioned in Section 3, we will improve the diagnosis
capability by applying abnormality detection
algorithms using AI and advance the entire system toachieve more in-depth planned maintenance (Fig. 12).

(1) Sensor within wind turbine + (2) Data acquisition module +(3) Dedicated server + (4) Analysis/diagnosis

Natural energy circulating modelGreen Power
Farm(1) Attach approx. 10 sensors to main bearing, gearbox andgenerator(2) Measure sensor signals with data acquisition module every
two hours(3) Send and store data to dedicated server through the Internet(4) Diagnose abnormality by analyzing the data

NTN vertical axis wind turbine(10 kw)

NTN vertical axis wind turbine(4 kw)

Tracking-typesolar power
generation system

Fig. 11 Components of “Wind Doctor TM” system

IoT room

NTN vertical axis wind turbine(2 kw)
NTN micro-hydropower turbine

Present state

NTN Hybrid Street Light

Electric vehicle
experience space

Fig. 13 Green power parkAI(5) Advanced preventive
maintenance (2)(3) Advanced preventive
maintenance (1)High accuracy/early
detection(1) Sale ofmodules(2) Dataacquisition (4) Deployment toother fields
Trendmanagement
Preventive
maintenanceHigher accuracy/early
detection/ labor saving
unmanned operation
through introductionof AI technology

Rolling stock,machine tools,etc.

Fig. 12 Future image of NTN’s CMS
Fig. 14 Mimamori Robokun Ⅲ experimental device-20-*03_展望E_*03_展望new 19/07/03 10:27 ページ 6

Rolling Bearing and IoT – NTN’s Approach to IoT –-Light" which uses wind and solar lightas the energy source for security
cameras, Wi-Fi communicationequipment and emergency power
supply providing a monitoring
function for local children andcommunication means in the case ofa disaster (Fig. 15). We areconducting connectivity experiments
between this experimental model and"Disaster Relief Map," verification of
power generation/storage amount,
operational experiment of Wi-Fi
communication equipment andexperiments on automatic connection
in case of communication shutdown.
We plan on using the know-how ofcommunication and signal processing
obtained in these experiments aselement technologies of bearing
abnormality diagnosis of the future.

In case ofdisaster

Area of severe damage

On-site report ofdisaster damage
by application
users

Operable during power
outage with independent
power supply

Mimamori Robokun III（Wi-Fi）(Specified low
power radio)
Bypass route

Disaster Relief Map(Osaka University)

NormalperiodDisaster Relief Map control screen(Osaka University)（LTE）

Ordinary route(Specified low
power radio)

Mesh cognitive communication(auto configuration of connectablecommunication network)

Posted information is usedby supporting personnelInternet

Connection ofcommunication
network/information（Wi-Fi）

（LTE）

（LTE）

Tourism information site
(e.g. Tourism Bureau)

Internet

Tourism application(Specified low
power radio)(Specified low
power radio)(e.g. Imai Town Tourism Bureau)

XX areamonitoring site

Local monitoring site (tentative name)(For local associations/temples/shrines)

Agreement
Agreement

Osaka University Omni Site
(Osaka University)

Fig. 15 Creation of mechanisms regarding disaster prevention, monitoring, andtourism through use of IT

NTN TECHNICAL REVIEW No. 78 (2010) 76.
7) Kentaro Nishikawa, Toru Takahashi, Christophie
Duret: Hub Bearing with an Integrated HighResokution Rotation Sensor, NTN TECHNICALREVIEW No.81 (2013) 52.
8) Takashi Koike, Yasuyuki Fukushima, Yusuke
Shibuya: High Resolution Absolute Angle Sensor
Applicable for Detecting Absolute Angle of Robot
Joints, Machine Design Vol. 62 No. 3 (2018) 55.
9) Hiroki Mano: IoT Deployment of Rolling Bearings,
Machine Design Vol. 62 No. 2 (2018) 46.
10) Toru Takahashi, Michio Hori: Technical Trends on
Bearing and Condition Monitoring System in Wind
Turbine, Tribologist Vol. 62 No. 11 (2017) 691.
11) Akitoshi Takeuchi, Takashi Haseba, Hiroshi Ikeda:
Application of Condition Monitoring System forWind Turbines, NTN TECHNICAL REVIEW No.80(2012) 15.
12) Natsuhiko Mori: Green Energy Business andGreen Power Park, NTN TECHNICAL REVIEW
No.84 (2016) 14.
13) https://www.ntn.co.jp/japan/news/press/news
201700086.html

5. Conclusion
A great number of rolling bearings are integrated inapplications, not only in manufacturing, but also invarious devices related to people's everyday life suchas clothing, food, living and transportation. They are
the optimum elements for data collection of IoT whensensors are incorporated in them. To this end,challenges regarding reduction of sensor size,
securing power supply and communication means
must be solved. NTN is committed to do its best forsolving these challenges through IoT for preserving
the natural environment, increasing people's living
standards and improving security/safety.

References
1) P. C. Evans and M. Annunziata: Industrial Internet- Pushing the Boundaries of Minds and Machines-, GE Reports Japan (2012)
2) Yoshitsugu Kimura: Measurement and life
evaluation in topology, Tribology Workshop, 29th
Conference, Preprint (2018) 1.
3) Hiroyoshi Ito, Takeshi Koike: Bearing with Integral
Revolution Sensor, NTN TECHNICAL REVIEW
No.69 (2001) 108.
4) Takeshi Koike, Tomomi Ishikawa, Hiroyoshi Ito,
Noriyoshi Mizutani: Improvement of Magnetic Flux
Leakage Durability of Integrated Sensor Bearings,
NTN TECHNICAL REVIEW No.71 (2003) 74.
5) Shoji Itomi, Hiroyoshi Ito, NTN Sensor Units forConstruction Machinery, NTN TECHNICALREVIEW No.76 (2008) 121.
6) Hiroyoshi Ito, Toru Takahashi, Pascal Desbiolles,
Cytil. Peterschmitt, Shintarou Ueno, “High
Resolution Sensor Bearing with an Index Signal”,

Photo of author

Masaki EGAMI
Senior Executive Officer,
R&D

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*04E_*15 19/07/11 9:39 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Development of "i-WRIST TM" Wrist Joint Module

Keisuke KAZUNO*
Jun MIDOUMAE*
Masayuki OHARA*

Hiroshi ISOBE*
Yuuki SHIMURA*

NTN has developed the angle control equipment applying a parallel linkmechanism which is a type of constant velocity joint. In order to expand thisequipment for appearance inspection application, we expanded the movablebending angle range from 45˚ to 90˚ and developed a wrist joint module which
can be positioned toward any posture in all hemispherical directions at highspeed. We changed the name from the conventional “Parallel Link High Speed
Angle Control Equipment” to “i-WRISTTM” and started mass production.

In this article, we introduce features, system
configuration, setup method for operation patterns, andapplications of the "i-WRIST TM" utilizing its 90° bending
angle, which NTN developed for visual inspection.

1. Introduction
Industrial robots are rapidly being deployed globally
due to the strong demand for automation and laborsaving aimed at improving productivity in
manufacturing. For example, vertical multi-joint robots
are widely used in severe manufacturing environments,
such as handling of heavy objects and welding/painting
lines. In addition, horizontal multi-joint robots andparallel link robots are also widely used in industrial
applications which require high-speed handling andtransportation of food.
On the other hand, a workforce shortage is one of theissues facing Japan, due to a decreasing birthrate andaging population 8). Particularly, there is a serious labor
shortage in work processes requiring skill sets andcompanies are employing robots in processes whichhave been traditionally handled by workers.
Since the announcement of the angle control unit
using the proprietary parallel link mechanism in 2012,
NTN has been exploring the use of robots in grease
and cleaning applications which were not well handled
by the conventional robots due to the requirement offine-tuned and quick angular movement. Recently, we
are also extending their use to visual inspection forautomotive parts and electric/electronic parts whichhave been depending on skilled workers to respond to
the new demands.

2. Configuration and features of i-WRISTTM
Fig. 1 shows an overview of i-WRIST TM.i-WRIST TM
is a wrist joint module consisting of a proprietary
parallel link mechanism and drive mechanism that
controls its orientation and synchronously controls
three motors to provide two degrees of freedom ofattitude positioning (bending angleθand revolving
angleφ) to the top end. The following shows its
features.

(1) High-speed operation
Fast operation of 0.1s/point or less* has beenachieved on fine-tuned attitude change.

(2) Wide operating angle range
Maximum bending angle of 90˚, revolving angle of360˚ x n (unlimited)(3) Smooth operation in all directions
No singular point in operation angle range withspherical link mechanism(4) Placing end effector within internal space oflink mechanism
High-speed operation is achieved by reducing the*Industrial Machinery Division-22-

*04E_*15 19/07/11 9:39 ページ 2

Development of "i-WRISTTM" Wrist Joint Module

Drivemechanism

Base-end side
1st link system

3rd link system

Parallel linkmechanism

Top-end side
2nd link system
Bend angle

Revolving angle

Fig. 1 Schematic of i-WRISTTM

moment of inertia. Cabling is placed through theinternal space to avoid twisting the cables whenrevolving operation is repeated.
(Bend angle, revolving angle)* Speed of positioning operation against points on 7 x 7matrix of 10 mm intervals

Z-Stage

3. System configuration example
i-WRIST TM can be used in a configuration thatcombines a linear motion actuator and Rz stage, as
shown in Fig. 2. Fig. 3 shows the configuration of thecontrol system. An (1) i-WRIST TM, (2) a proprietarycontroller and a (3) proprietary console (enclosed in
red line) are provided as standard offering by NTN.
Servo drivers and a linear motion actuator, etc. which
are enclosed in blue line are the parts which models
and series (specification) are specified by NTN. The
parts in black lines are selected by the customer.

Rz-Stage

Y-Stage

X-StageFig. 2 Configuration example(2) Proprietary controller(3) Proprietary console

I/O(External interface)(1) i-WRIST TM

End effector(for camera, etc.)

Wrist joint module with three reducer motors.
Servo system
driver(2) Proprietary controller
Installed software works not only for standalone iWRIST TM but also in a system configuration of iWRIST TM with linear motion actuator (XYZ axis)
and Rz Stage (Fig. 3).

Cabling

M E

M1

Motor

Controller onuser side

M E

M E

M2

M3

Motor Motor

M E

M E

M E

Rz

X

Y

M E
Z

Motor Motor Motor

Motor

Rz-StageLinearmotionactuator(X axis)(3) Proprietary console

Stand

Used to teach operation of i-WRIST TM and linear
motion actuator.

(1)

Linearmotionactuator(X-Y axis)

XYZRz-Stage

Fig. 3 Control system configuration-23-

*04E_*15 19/07/11 9:39 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

Table 2 Specification of proprietary controller

4. Key specifications

Item

4.1 i-WRISTTM
Fig. 4 shows the picture of i-WRIST TM outer view
with a bending angle of 90˚, which was most recently
developed, and Table 1 shows its key specifications.
Angular contact ball bearings are used in each rotating
pair in order to allow high-speed operation andappropriate pre-load is applied to ensure rigidity of iWRIST TM. The increase in moment of inertia due toinstallation of end effectors could be reduced byplacing them within the internal space of the parallel
link mechanism.

Specification

Size
Auxiliary unit

W65×D110×H130 mm

Rated voltage

DC24V±20%

Items to becontrolled

I/O unit (input: 32 points, output: 32 points)
i-WRISTTM, X-Stage, Y-Stage, Z-Stage,
Rz-Stage

4.3 Proprietary console
Table 3 shows the key specifications of the
proprietary console. It is a handheld device that uses
commercial hardware with console screens developed
by NTN.
The proprietary console is able to set not only the iWRIST TM configuration, but also operating conditions
and operating patterns of the linear motion actuatorand rotating mechanism (Rz axis) shown in Fig. 3(Control system configuration). In addition, its ability toregister multiple operating patterns allows for an easy
exchange of setup or tools.

W 331mm

D 286mm
H 202mmTable 3 Specification of proprietary console(a) Attitude with bend angle 0˚

Item
Outer dimension
Display device
Display size
WeightRated voltage
Registrable
operating patterns

Specification
W224×L174×H107 mm
Color

W115.2×L86.4 mm
0.96 kg or less
DC24V±20%50 patterns (100 points max. can beregistered for each operating pattern)(b)Attitude with bend angle 90˚Fig. 4 External view of i-WRISTTM

5. Setting of operating patterns
Typically, special knowledge is required to operate
robots; however, the proprietary controller simply
performs the points registered as sequential operating
patterns. Therefore, operating patterns can be easily
set without the need for special knowledge.

Table 1 Specification of i-WRISTTM
ItemSpecification

Size
WeightW331×D286×H202 mm

Degree of freedom
Operating angle range

5.4 kg2
Bend angle: 90˚

Revolving angle: 360˚×n (unlimited)

5.1 Teaching

Repeated positioning accuracy ±0.05˚ or less

Teaching methods vary depending on thecoordinate system being used. They are broadly
categorized into "base coordinate system" which isbased on the entire system and "work coordinate
system" which is based on the work.

Portable mass
1.0 kg（0.025 kgm2）(permissible moment of inertia)
Motor output

50 W

4.2 Proprietary controller
Table 2 shows the key specifications of the
proprietary controller. It uses commercial hardware
with proprietary software developed by NTN. It does
not require any special knowledge and allows for
intuitive operation to perform complex calculations
such as coordinate conversion.

5.1.1 Base coordinate system
Fig. 5 (a) shows the conceptual diagram of the basecoordinate system. In this method, the job buttons on
the proprietary console screen shown in Fig. 6 areused to move the X-Stage, Y-Stage, Z-Stage, iWRIST TM, Rz-Stage to the desired positions and-24-

*04E_*15 19/07/11 9:39 ページ 4

Development of "i-WRISTTM" Wrist Joint Module

angles to register those positions and attitudes (Sx,Sy, Sz,θ,φand Rz). This does not require anunderstanding of the coordinate system to set theoperating patterns.

base coordinate system and work coordinate system
are preset on the proprietary console. Therefore, theteaching process can be performed efficiently.
The position coordinates of work points and work
direction can be edited while the coordinates on the
3D model of the work (work coordinates) are
confirmed on the PC screen by the developed point
data editing software as shown in Fig. 7. The edited
software can be written on the proprietary console in abatch process. The operating pattern data of the
proprietary console can also be stored in the PC as
backup data.

5.1.2 Work coordinate system
Fig. 5 (b) shows the conceptual diagram of the work
coordinate system. In this method, the actual working
position (XYZ coordinates) and working direction (θ'andφ') on the work to be operated are specified.
The proprietary controller automatically calculates
the attitude of i-WRIST TM and linear motion actuator if
the distance from the top end of i-WRIST TM to thework position of the end effector and the positions of

Input area for position coordinates and operating direction

3D model(Bend angleθ,
Revolving angleφ)Z-Stage（Sz）

OperatingscreenZ
Y

Base coordinate system

Y-Stage（Sy）

Input area for operating condition
X

Fig. 7 Point data edit software

5.2. Other functions

Rz-Stage
X-Stage(rotation angle Rz) （Sx）

As shown in Table 4, not only commands for eachactuator position and angle can be input, but thefollowing conditions can also be set.

(a) Base coordinate system(1) Operation mode
Direction of approach（θ, φ）（Wx, Wy, Wz）
Work

PTP (point to point) operation, which registers thepoint for operation to stop, and CP (continuouspath), which allows continuous and synchronous
movement of i-WRIST TM and other components(linear motion actuator, etc.), can be set.

Z-StageZ
YWork coordinates

Y-StageX(2) JOB code

Rz-Stage

The JOB code is the number (0-63) to be output tothe user side controller. When JOB codes are set
for each position, the user side controller candetermine the operation at each point. The user
can define, for example, "take a picture withcamera when JOB code is ’1’, and do nothing
when it is ’2’."

X-Stage(b)Work coordinate system

Fig. 5 Conceptual diagram of the coordinate system

Table 4 Other function
Position/angle
No.

Sz

θ

φ
45

2

200 150 100

45

150

4

150 150 100

5

-25-

Sy

90

3

Fig. 6 Operation screen of proprietary console

Sx

(1)
(2)
(3)
i-WRISTTM Rz-Stage Operation JOB Speedmode code ratio

30

1

Jog
buttonStage
coordinates
50

10100 14080

50 100

90

90

Rz

0

0

0

1

90

0

0

1

0

0

0

1

90

90

180

−90

45

45

180

0
0

0
0

1
1

*04E_*15 19/07/11 9:39 ページ 5

NTN TECHNICAL REVIEW No.86（2018）(3) Speed ratio

Table 5 Measurement results of repeated positioning accuracy

This function reduces the speed of movement to theset point. For example, this can be used when
vibration needs to be controlled on only a few points
among many to conduct secure operation. The
desired stable operation can be achieved without
significant increase of overall tact time.

Point
P1
P2
P3
P4
P5
P6
P7
P8
P9

6. Evaluation test

Repeated positioning accuracy [˚]±0.008±0.007±0.006±0.010±0.005±0.009±0.003±0.009±0.006In this section, we describe the basic performance
test of i-WRIST TM and its results.
The measurement method and condition werebased on JIS B 8432 (Manipulating Industrial Robots –Performance Criteria And Related Test Methods).

6.1 Repeated positioning accuracy
Fig. 8 shows the measuring device of repeatedpositioning accuracy.
A high-precision inclination sensor is installed on the
top end of i-WRIST TM which measures inclination angles
around two axes to calculate unidirectional repeatedpositioning accuracy from its sensor output signal.
As shown in Fig. 9, nine typical attitudes were
defined as the measurement points of i-WRIST TM and
30 cycles of measurement were conducted. The
unidirectional repeated positioning accuracy wasdefined as the variation ±3σaround the average ofthese measurement results. An example of the resultis shown in Table 5. It was revealed that the repeatedpositioning accuracy was within ±0.05˚, as set in thespecification.

6.2 Reliability test
A continuous operation of i-WRIST TM was conducted
for a target run time of 2 years (5000 hours) assuming
its use in the production line of an automotive factory.
As a result, the repeated positioning accuracy was still
within ±0.05˚ after the continuous operation without
any sign of abnormality in appearance. The
continuous operation was conducted without any
maintenance work, such as greasing.

7. Deployment for visual inspection
application
Fig. 10 shows some of the applications where wehave deployed i-WRIST TM so far. Among them, we
have focused our development on the visual inspection
as the demand for automated operation is rapidly
increasing.
Most of the current visual inspections are conductedby workers, holding the work by hand and moving
them around in various directions. Fig. 11 shows anexample of prototype visual inspection system
configuration, which NTN created to automate thisprocess.
In this configuration, XYA and Rz (rotational axis)
stages are used to move the work and at the sametime, the attitude of camera and light mounted on thei-WRIST TM are changed. Inspection directly from theside is possible because the maximum bending angle
is expanded to 90˚. Also, the addition of Rz-Stage,
which rotates the work, shortened the stroke of XYStages to make the entire device more compact.
Table 6 shows the main specifications of thisprototype. Although it may vary depending on the
shape and size of the work and processing speed of
the image processing system, the device is able toinspect 60 points of the work with φ100 × 100 mm(height) in about 8 seconds.

Stand

Thermometer/hygrometer

Inclination sensor

Fig. 8 Measuring device of repeated positioning accuracy

Fig. 9 Measuring posture of repeated positioning accuracy-26-

*04E_*15 19/07/11 9:40 ページ 6

Development of "i-WRISTTM" Wrist Joint Module

Wrist joints of robot (assembly)

Welding

Cleaning

Grease application

8. Summary
Since the development of the angle control unit using
the proprietary parallel link mechanism in 2012, NTN
has been expanding its use in grease and cleaning
applications, while refining the product technology.
Now, we have developed i-WRIST TM for visualinspection to further take advantage of its capabilities.
As we move forward, since the labor population
decreases, the demand for automating processes suchas visual inspection, which have been dependent on
people, is expected to grow rapidly. We are poised tocontinue developing robotic products to contribute toautomation and manpower saving, and to further
improve productivity and quality in manufacturing for the
betterment of the society.

Visual inspectionWork
Linear motion actuator

Fig. 10 Application examples of i-WRISTTM

Camera + light

References
1) Keisuke Sone, Hiroshi Isobe, Koji Yamada, Wide Angle
Active Link Equipment, NTN TECHINICAL REVIEW No. 71(2003) 70-73.
2) Hiroshi Isobe, Yukihiro Nishio, Parallel Link High Speed
Angle Control Equipment (PHACE), NTN TECHNICALREVIEW, No. 80, (2012) 42-47.
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) 809810.
4) Hiroshi Isobe, Yukihiro Nishio, Seigo Sakata, Naoya Konagai
and Hiroyuki Yamada, 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) 605-606.
6) Kenzou Nose, Hiroshi Isobe, Seigo Sakata, Naoki Marui,
Naoya Konagai: Parallel Link High Speed Angle Control
Equipment - Enhancement of Performance by Improvement,
Proceedings of the Japan Society for Precision Engineering
Spring Meeting in 2016, (2016) 483-484.
7) Kenzou Nose, Hiroshi Isobe, Seigo Sakata, Parallel Link
High Speed Angle Control Equipment, NTN TECHNICALREVIEW, No. 84, (2016) 96-101.
8) Japan Robot Association, Need to Know This! Basic
Knowledge for Utilizing Robots

Work

Rz-Stage

Y-Stage
Z-Stage

X-StageFig. 11 Configuration example of appearance inspection
equipment

Table 6 Specification of prototype model
Item

Specification

Size

W800×D850×H2000 mm

Size of the work to beinspected

φApprox. φ100 × 100 mm (H)Tact time (for ref. only)

Approx. 8 sec/60 points

Photo of authors

Keisuke KAZUNOHiroshi ISOBE

Jun MIDOUMAE

Yuuki SHIMURA

Masayuki OHARA

Industrial Machinery Division

Industrial Machinery Division

Industrial Machinery Division

Industrial Machinery Division

Industrial Machinery Division-27-

*05E_*15 19/07/03 15:06 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ Technical Paper ]Application of i-WRIST TM to the Robot Wrist Joint

Kenzou NOSE*
Naoki MARUI*

Seigo SAKATA*
Naohiko SATOU*

While the working population is decreasing, in the industrial field, robots come into
use for operation of hand work by human, and to work in cooperation with human
there. In such case, the robots are required to perform delicate hand work in a safemanner and at a sufficient speed.
This report introduces the application of i-WRISTTM (Parallel Link High Speed Angle
Control Equipment) that has proven in a grease dispensing and external inspection toa wrist actuator of robot arm.

the base) and top-end part (which is used to install theend effector), making each link system joint a turning
pair. The structure of this unit has pre-loaded angularcontact ball bearings for the turning pair to reduce
friction and to eliminate the gap in the joints.
The attitude of the top-end part is defined by thebend angleθ, which is the inclination angle between
the central axis of the base-end part and central axisof the top-end part, and the revolving angleφaroundthe central axis of the top-end part looked at from thecentral axis of the base-end part.
The attitude of the top-end part (θandφ) isdetermined by fixing the attitude of two base-end sidearms in each link system. i-WRIST TM uses
synchronous control of three motors to improve
repeated positioning accuracy and rigidity.

1. Introduction
Industrial robots have been making a significant
contribution to factory automation in manufacturing byincreasing productivity and production volume. In
manufacturing facilities, production line efficiency has
been pursued though the deployment of robots,
special-purpose machinery and workers. However,
due to a shortage in the labor population, the demand
for automating refined manual operations, which has
been traditionally difficult with conventional robots, isincreasing.
NTN adopted its proprietary parallel link mechanism
to develop a compact "i-WRIST TM" (previously known as
Parallel Link High Speed Angle Control Equipment 1)-5))which has a wider operating range with two degrees of
freedom for fast and highly accurate angular
positioning, and is actively pursuing the developmentof broader applications in a configuration with a linear
motion actuator. In this article, we describe theapplication of i-WRIST TM to actuator for robot wrist
joints.

Bend angleθ

Central axis of
top-end part

Revolving angleφ
φ=0°
3rd link system
Top-end side arm

2. Overview of parallel link mechanism

2nd link system

Top-end part2.1 Basic configuration
Fig. 1 shows a conceptual diagram of the parallel
link mechanism (hereafter, link mechanism) of iWRIST TM. The link mechanism takes three rows of linksystems, which consist of a base-end side arm, central
link and top-end side arm, (the 1st - 3rd in Fig. 1) placedparallel between the base-end part (which serves as

Central link

1st link systemBase-side arm

Base-end part
Central axis of base-end partFig. 1 Conceptual diagram of i-WRISTTM mechanism*New Product Development R&D Center-28-

*05E_*15 19/07/03 15:06 ページ 2

Application of i-WRIST TM to the Robot Wrist Joint

2.2. Features

them to be easily handled. In addition, cables will
not be twisted even with continuous rotation in one
direction.

The following are the main features of the linkmechanism when it is applied to actuators for robot
joints.

2.3 Comparison with other types of robots(1) Wide operating range

Table 1 shows a comparison of the robot combining
i-WRIST TM and linear motion actuator with generalvertical multi-joint robots and parallel link robots.
Fig. 2 shows the wrist joint (3-axis of top end) of ageneral vertical multi-joint robot.
The vertical multi-joint robot (Table 1B) has
advantages such as larger operating range andheavier transferable load; however, reducing the tacttime for small movement is difficult because of thelarge movement of multiple joints that results fromeven a small attitude change. Since the wrist joint of
the vertical multi-joint robot consists of three rotational
joints(revolving axis, bend axis and rotational axis),
and the revolving axis needs to be moved a greater

It has a very wide operating range with a maximum
bend angle θ of 90° and revolving angle φ of
360°×n (unlimited), which allows for a wide range
of approaching options to the subject from different
angles.

(2) High-speed/highly accurate operation
Fast and highly accurate movement, particularly
for small attitude changes compared to multi-joint
robots.

(3) Lightweight/compact operable parts
Positioning accuracy is improved in high speed
operation by reducing weight and moment ofinertia of operable parts through shifting the driving
parts to the base-end side. This also makes thelink mechanism more compact, which is desirable
for avoiding interference of the apparatus evenwhen the operable range is broad.

Revolving axis

Bend axis(4) No singularity in operating range
Teaching can be done relatively easily because
there is no need to care for singularity.

(5) Easy handling of cables and tubes
Cables and tubes for hands and grippers to beinstalled at the top-end part can be run through theinternal space of the link mechanism, which allows

Rotating axis

Fig. 2 Wrist joint of ordinary vertical articulated robot

Table. 1 Comparison of other type robot
StructureA+
Linear motion actuator

B

Vertical multi-joint robot

C

Parallel link robot

Features
Strongarea

Non-contact work,
assembling of small objects· High speed positioning with small changeof attitude
Advantage · No singularity in operating range· Cables can be run through link internal
space (Cables not twisted even whenrevolving operation is repeated)

Challenges· Certain footprint is required even when
not in use· Lighter transferable loadWelding, transfer· Wide operating range· Heavier transferable load
· Smaller footprint when not in use(robot arm can be folded)

Transfer (pick and place)· Faster parallel movement· Slower positioning speed with small change · Larger equipment size relative to operatingof attitude (depending on attitude)range· Singularity in the range (special knowledge · Certain footprint is required even when notand experience needed for teaching)in use-29-

*05E_*15 19/07/03 15:06 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

distance even for a small attitude change of the top
end, smooth and quick motion is not its strong point.
Therefore, it is not suitable for applications requiring
fast and detailed assembling operation.
In addition, due to singularities in the operating
range, operators with special knowledge andexperience are required for teaching. Furthermore,robots with powerful and high-speed operation, otherthan collaborative robots, require fencing around theentire operational range for safety, which results in alarger required footprint.
General parallel link robots (Table 1C) are usedmainly for pick and place applications due to their fastand parallel operation. The size of the entire
equipment including driving mechanism and required
footprint are large compared to the operating range.
Therefore, these robots do not usually fit in theexisting space replacing workers for assembly work.
The robot proposed by NTN which combines the iWRIST TM and a linear motion actuator (Table 1A)
provides fast positioning speed and small overall
movement for a small change of attitude. The iWRIST TM is characterized by smooth and quick
movement in any direction as its compact andlightweight parallel link mechanism at the top end has
two degrees of freedom. Compared to the wrist joint ofa vertical multi-joint robot which requires large
movement of the revolving axis closer to the base forpositioning, the i-WRIST TM is able to perform fast
operation in all directions, which allows a shortening oftact time compared with the wrist joint of a verticalmulti-joint robot. The vertical multi-joint robot's
operating axes are serially configured; therefore, when
the joint closer to the base (revolving axis of the wrist
joint) needs to be moved a greater distance, themoment of inertia will be large preventing fast
movement. On the other hand, i-WRIST TM canachieve high-speed and smooth movement because
the moment of inertia is reduced by the compact
parallel link mechanism. In addition, as the required
space for installation is relatively small, it is considered
to be a good fit for automating the process previously
operated by workers.

3. Deployment for robot wrist joints
3.1 Actuator for wrist joints
We have made a prototype actuator (hereafter,
actuator) for the robot wrist joint 6). Table 2 shows themain specifications of this prototype.
The actuator shown in Fig. 3 is made by reducing
the size of i-WRIST TM in the radial direction for lighter
weight and adding one degree of freedom for rotating
the overall i-WRIST TM . A concentric reducing
mechanism is used for the rotating mechanism for
rotating the overall i-WRIST TM , so it is called the geardrive type (hereafter, GD type).
In addition, the maximum bend angle of the linkmechanism is 90˚. Together with the revolving angle
φof the i-WRIST TM and rotating angleθz of therotating mechanism, the rotation of the rotating
mechanism can be converted to rotation around therotating angle of the top-end part, equiangularly, inany bend angle.
Fig. 4 shows the structure of the revolving pairs
serving as connections between the base end part andbase end arm. The base end arm is connected to the
output axis of the reducer unit installed on the baseRotating axis of top-end partTop-end part
Link mechanism
Transmission mechanism
Base-end part
Rotating angleθz
Drive mechanism
Rotating axis ofrotating mechanism

Fig. 3 Wrist joint actuator featurung i-WRISTTM(with gear drive)

Table. 2 Specification of wrist joint actuator with GD
Item

Size
(outer diameter×height)
WeightDegree of freedom

φ102×115 mm (link)
3 kg
3Operating angle range

Bend angle: 90˚
Revolving angle: 360˚×n (unlimited)
Rotating angle: 360˚ (-175˚ to 175˚)

Repeated positioning
accuracy±0.05˚ or lessTransferable load
(weight)

0.5 kgMotor output-30-

Specification

φ145×278 mm (overall)

30 W×3 (link)
50 W×1 (rotating mechanism)

*05E_*15 19/07/03 15:06 ページ 4

Application of i-WRIST TM to the Robot Wrist Joint

Base-side arm
Rotating drive mechanismon the input side
Input axis

Motor for link mechanism
attitude control

Rotating drive
mechanismon the output side
Output axis

Transmission
mechanism

Bearing

Belt for rotating mechanism

Reducer Unit

Fig. 4 Revolute pairs serving as connections between
proximal end side link hubs and proximal end link member

Motor for rotation control

Fig. 5 Wrist joint actuator with belt drive

end part through the output side rotation drive. The
rotation of the attitude control motor transmitted to the
input axis of the reducer unit through the transmittermechanism, such as gears and belts, is reduced by the
reducer unit and transmitted to the base end arm
through the rotation drive of the output side. The input
rotation drive fixed on the base end arm is rotatably
connected to the input axis of the reducer unit throughthe bearings. As such, the actuator a compact
configuration while improving the supportive rigidity ofthe base end arm by incorporating the reducer unit
within the base end arm structure which supports bothends of the rotational axis of the base end arm. In
addition, the impact of backlash within the transmission
mechanism can be reduced by placing thetransmission mechanism on the input side of the
reducer unit.
The GD type rotating mechanism has an advantage
in placing the reducing mechanism of high reduction incompact configuration. However, since the motor forcontrolling angular rotation is positioned so that therotating axis of the rotating mechanism is concentricwith the central axis of the base end part, the drive
mechanism does not have internal space. Therefore,
the benefit of running the cables and tubes through theinternal space of the link mechanism is not attainable.
On the other hand, the BD type, which uses a belt
drive, places the motor of the rotating mechanism
through the belt offset from the center of the rotating
mechanism and therefore, can provide a large internal
space. Fig. 5 shows the structure of the BD type. The
BD type uses the belt for the link mechanism, as well,which allows the motor for attitude control to bepositioned offset in parallel to the input axis of the
reducer unit so that the height of the link mechanism isreduced. In addition, the ability of running cables forthe motor for attitude control of the link mechanism
through the internal space of the drive mechanism
makes cabling work very easy and broadens theoperating range of the rotation angle.

3.2 Linear motion dual-arm robot
Fig. 6 shows a linear motion dual-arm robot, whichis configured with two sets of the combination of thisactuator and a linear motion actuator with orthogonalaxes (XYZ stage). In order to automate the existing
manual work, the actuators are inclined at 45˚ toward
center relative to vertical axis (Z axis) to facilitate
collaborative work of the two arms. This linear motion
dual-arm robot controls the attitude and position of theend effector with this actuator and the XYZ stage,
respectively. It is able to perform various works byoperating XYZ movement maintaining the attitude(asynchronous operation), and through synchronous
movement of this actuator and the XYZ stage(synchronous operation).
The asynchronous operation allows for theapproach attitude to be determined by controlling theattitude of the grabbed work with this actuator, thentransfer it horizontally with the XYZ stage, which is

XYZ-Stage

Actuator for wrist joints

Fig. 6 Liner motion type dual arm robot featuring
i-WRISTTM-31-

*05E_*15 19/07/03 15:06 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

easily predictable by the operator when teaching.
Therefore, it is suitable for assembly work. Fig. 7
shows operation of insertion/removal of shaft. The
operation consists of insertion and removal of the
shafts through holes in different angles, which is
performed by determining the attitude of shafts toward
the holes by this actuator and inserting/removing theshafts in linear motion with the XYZ stage.
The synchronous operation facilitates refined
motion of the top end of the end effector. Fig. 8 shows
placing of seals on a curved surface and Fig. 9 shows
attaching/removal of O-rings, as some examples of
the applications. In these examples, 3D tracing motion
on a curved or plane surface, which has been difficult
to automate, is achieved by the coordinated trajectory
control with this actuator and the XYZ stage, in a
combination of the parallel link mechanism and linear
motion actuator.
As shown in these examples, NTN is currently
pursuing application of i-WRIST TM to robot wrist joints
for use in assembly tasks.

O-ringFig. 9 Operation for installing and removing O-ring

4. Conclusion
We have made a an actuator prototype for a robot
wrist joint based on the proprietary parallel link of two
degrees of freedom with addition of one rotational
degree of freedom, which achieves shape and motion
close to the human wrist joint. In addition, we havebuilt a prototype of a linear motion dual-arm robot thatcombines this actuator and a 3-axis linear actuatorand are pursuing the development of robots which canachieve refined manual work by taking advantage ofthe smooth and fast operation unique to the iWRIST TM, verifying differences from vertical multi-joint
robots and parallel link robots.
As we move forward, we will explore further
downsizing this actuator and the entire system andpursue collaborative work with human workers byadding features such as a collision detection function.
We are poised to contribute to the evolution of
automation/robotics in manufacturing, to enablereplacement of manual work, such as assembly ofsmall parts, which has been difficult with conventional
robots.

Hand(air chuck)Work(shaft)Work (hole)Fig. 7 Operation for inserting and removing shaft

Curvedsurface
Seal

Fig. 8 Operation for put a seal on a curved surface-32-

*05E_*15 19/07/03 15:06 ページ 6

Application of i-WRIST TM to the Robot Wrist Joint

References
1) Hiroshi Isobe, Yukihiro Nishio, Parallel Link High
Speed Angle Control Equipment (PHACE), NTN
TECHNICAL REVIEW, No. 80, (1988) 42-47.
2) 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), 80981.
3) Hiroshi Isobe, Yukihiro Nishio, Seigo Sakata,
Naoya Konagai and Hiroyuki Yamada, 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) 10871088.
4) 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) 605-606.
5) Kenzou Nose, Hiroshi Isobe, Seigo Sakata, Naoki
Marui, Naoya Konagai: Parallel Link High Speed
Angle Control Equipment - Enhancement of
Performance by Improvement, Proceedings of theJapan Society for Precision Engineering Spring
Meeting in 2016, (2016) 483-484.
6) Kenzou Nose, Hiroshi Isobe, Seigo Sakata, Naoki
Marui, Yuuki Shimura: Parallel Link High Speed
Angle Control Equipment - Application to Wrist
Joint Actuator for Robots, Proceedings of theJapan Society for Precision Engineering Spring
Meeting in 2017, (2017) 761-762.

Photo of authors

Kenzou NOSE

Seigo SAKATA

Naoki MARUI

Naohiko Satou

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center-33-

*06E_*15 19/07/03 15:06 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ Technical Article ]Technical Trend and Features of the Bearing for Robot

Makoto OEBISU*
Yasuyoshi HAYASHI*

Hiromichi KOKUMAI*
Masato TSUJIHASHI*

Recently, the robotics market has become very active, and demand is
expanding each and every day.
Various types of mechanical equipment, such as reduction gears, areincorporated into robotics, and many bearings are used. NTN manufactures awide variety of bearings for robotics applications, and in this section, thefeatures of bearings used for robotics will be explained based on the technicaltrends in the industry.

activities including welfare, healthcare and nursing
care. Recently, it is expected that these robots will be
rapidly introduced and used in a broader area toincrease productivity in the service industry, secure
workforce for healthcare/nursing care services, etc.
Robotics is an area that will be adopted globally, as
the machines perform tasks in place of human
workers, where future growth is highly expected. NTN
produces many bearings for robots. In this article, we
describe the technical trend of bearings for industrial
robots, in particular, their application area andfeatures.

1. Introduction
Robots can be broadly classified into industrial
robots, field robots and service robots (Fig. 1).
Industrial robots are used in many manufacturing
factories including automobiles and electricalequipment with demand in relatively simple andrepetitive tasks; however, the demand is shifting to amore advanced and automated production process.
Japan has a competitive edge in industrial robots andis expected to advance development leveraging its
strength. Field robots are engaged in outdoor activities
such as on farmland, in the sea and at disaster sites;
therefore, robots which perform well at such sites are
expected to be developed. Service robots are used in
an environment where interaction with people isrequired, such as at home, at the workplace, and in

Industrial robot

2. Market needs of industrial robot
Industrial robots are required to have not only basic
features such as to grab objects at a predetermined
position and accurately work on them, but also (1)superior cost performance, (2) accuracy andperformance suitable for the required work, easyteaching and correction (accuracy and operability), (3)small and light bodies with small footprints (compact
form factor), and (4) contribution for high availability of
production line (reliability and maintainability), in order
to be broadly deployed in various industrial fields.
In the industry, the demand for automation by
adoption of robots is recently increasing in order todeal with the labor shortage in the production lines
and to realize stable product quality. The demand for
robots in assembly and transfer processes isincreasing, for example, from the capital investment inelectronics industry due to growing demand of smart(Example)
Assembly robot
Transportation robot

Field robot(Example)
Construction robot
Agricultural robot
Disaster prevention robot
Underwater robot

Service robot(Example)
Cleaning robot
Living assistant robot
Welfare robot

Fig. 1 Classification of robot*Application Engineering Dept., Industrial Machinery Division-34-

*06E_*15 19/07/03 15:06 ページ 2

Technical Trend and Features of the Bearing for Robotphones, automotive related industry and IoT (Internetof Things). In addition, the demand for collaborative
robots which work with human workers, is also a
growing trend and expands the market size forindustrial robots. Based on these trends, therequirements for industrial robots such as (1) increaseof productivity by reducing tact time, (2) refined
workability with improved positioning accuracy andrepeatability during work transfers by increasing
rigidity of robots, (3) securing work space by reducing
the size and footprint of robots and (4) improved
maintainability by increasing maintenance intervals,
are increasing as the market needs. Table 1 shows
the market trend of the recent performancerequirements for industrial robots.
Industrial robots are equipped with reducers andgas balancers where bearings are used in moving
parts. Based on the aforementioned trends, bearings
are required to be smaller in size and higher in rigidity
with high-load capacity and long operating life.

B axis: wrist bend

T axis:wrist rotation

U axis: upper arm

Balancer
L axis: lower arm
S axis: revolution

Fig. 2 6 axis articulated robot

reducer for large robots and (2) strain wave gearing
reducer for small robots as examples of applications
where bearings are used.

Table. 1 Market trend of industrial robot
Required performance
Cost performance(robot productivity)

Recent market needs
・Reduction of tact time(1) Eccentric differential reducer
An example of eccentric differential reducers is
Nabtesco Corporation's precision reduction gears RV.
Fig. 3 shows the structure of the precision reduction
gears RV. This reducer consists of the 1st stage
reducer made of a spur gear and the 2nd stage
eccentric oscillating reducer with a pin gear
mechanism. When the crankshaft rotates, the RV gear
moves eccentrically. It rotates in the opposite directionof the input rotation by the difference of number ofgears with the internal gear, which is taken out as theoutput. This mechanism is lightweight and has highrigidity with high tolerance to overload due to the highnumber of simultaneously engaged gears which also
provides low backlash and low vibration with smooth
and accurate torque transmission.
This reducer is equipped with needle roller bearingsin the eccentric portion of the crankshaft and tapered
roller bearings in the journal portion. Each bearing isrequired to have high rigidity and high load carrying
capacity. For the main bearings, thin section type
angular contact ball bearings are used in pairs which
are required to have high rigidity (moment rigidity) inorder to ensure positioning accuracy, which is an
important property of a robot.

High accuracy and operability ・High positioning accuracy(precise work)
・Increased repeatability
Space saving(securing work space)
High reliability andmaintainability

R axis: wrist revolution

・Compactness
・Longer maintenance intervals

3. Structure of industrial robots andareas bearings are used
3.1. Structure of industrial robots
The typical industrial robot is the vertical multi-joint
robot which moves like a human from the shoulder tothe wrist, among many types (Fig. 2).
When a robot is operated with heavy objects, eachjoint drive requires large torque; therefore, reducers
are used to amplify output torque of servomotors. A
large robot, in particular, is equipped with balancers
which produce balance power against the gravity load
to complement servomotor power and contribute toenergy saving.

3.2. Reducers for industrial robot and areas
bearings are used
As robots of different sizes are emerging in recent
years, large and small reducers are used accordingly.
In this article, we describe (1) eccentric differential-35-

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NTN TECHNICAL REVIEW No.86（2018）

Shaft

Internal gearThin section typeelastic toothed gear

Main bearing(thin section type angular contact ball bearing)Thin section typeelastic ball bearing
Needle roller bearing

Oval axis

Spur gear

Input gear
RV gear
Pin

Fig. 4 Structure of strain wave gearing

Crank shaft

Internal gear

Tapered roller bearings

Fig. 3 Structure of precision reduction gears RV 1)

Oval axis

0°(2) Strain wave gearing reducer

180°

Thin section
type elastic
toothed gear

The strain wave gearing reducer is mainly used as
the small reducer mechanism (outer diameter of 100
mm or less). Fig. 4 shows the schematic diagram ofits structure. It consists of a thin section type elastic
toothed gear made of a thin, elastic metal ring withgear cutting, it incorporates an oval shaft where thin,elastic ball bearings are inserted and the elastic
toothed gear is in contact with the thick, rigid internalgear at the major axis. This provides a uniquereducing mechanism cleverly utilizing the elastic
deformation. The operating principle is shown in Fig.
5. If the internal gear is fixed while the oval shaft
rotates clockwise once, the thin section type elastic
gear rotates counterclockwise by the difference of thenumber of gears between the thin section type elastic
gear and the internal gear (e.g. thin section typeelastic gear has two teeth less) which is extracted as
the output. Fig. 5 shows the status where the oval
shaft rotated half circle and the thin section typeelastic gear moved counterclockwise by one tooth.
This reducer has concentric and simple structure withcompact form factor and provides a significant
reduction ratio of 1/30 to 1/320. Since both teeth are
simultaneously engaged like wedges, providing great
contact ratio without backlash, the reducer ischaracterized by averaged gear errors, high angle
transmission accuracy, and large torque capacity.
Stress amplitude of tension and compression isrepeatedly applied to the outer ring’s outer diametersurface of the thin section type elastic ball bearing,
while internally engaging reaction force due to torquetransmission is applied in addition to preload due todeformation of the oval part. It is important to provide

Thin section type elastic ball bearing(elastic deformation in oval shape)

Fig. 5 Operating principle

both fatigue strength against cracking of outer ring androlling contact fatigue life when designing thin section
type ball bearings. Furthermore, consideration isrequired for the angled shape, surface roughness,
residual stress of the surface, etc. in addition to
selecting the optimum thickness and thermal
treatment specification so that the raceway cantolerate high stress amplitude.

3.3. Balancer for industrial robots and areas
bearings are used
Fig. 6 shows an example of the balancer structure
for robots. This device is applied to complement thepower of the servo motor when the robot arm isextended and contracted for energy saving. Recently,
the balancer has moved from coil type to gas type forcompactness and high load capacity. In addition,
longer maintenance interval is required for highavailability.
Self-aligning roller bearings may be used for the rod
cylinder support area of this balancer, considering
ease of incorporation. For this purpose, it is necessary
to select bearings considering abnormal wear andfretting damage, in addition to large load carrying
capacity, as it is difficult to form an oil film because oflarge load and small oscillating angle.

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*06E_*15 19/07/03 15:06 ページ 4

Technical Trend and Features of the Bearing for Robot

carrying capacity. Balancers for robots are required tohave high reliability in addition to compactness, while
self-aligning roller bearings for rod cylinder support are
required to have long life. NTN is conducting R&D forvarious bearings following the recent technology trend
to meet the market demand.

Coil type

Table. 2 Required performance of bearing for industrial robot
Areas where bearings are applied
Main shaft

Crank shaft・Compact (space saving)・High rigidity
・High-load capacity
・Long life

Reducer for
robot

Gas type

Balancer for robots(rod cylinder support)

Compact

Required performance・Compact (thin section type)・High rigidity
・Long life

・High-load capacity
・Long life

4.2 Compact angular contact ball bearings
NTN designs, manufactures and promotes compact
thin section type angular contact ball bearings for main
shafts of precision reduction gears.
Fig. 7 shows a comparison of cross section with the
79-Series standard type (the outer diameter of the
bearings is uniformly designed). The thin section type
angular contact ball bearings achieved approx. 30%
reduction on the radial direction , approx. 25%
reduction on the axial direction in size and approx.
55% reduction of weight.
For this reduction, thinning of the raceway is
indispensable; however, the resulting deformation of
the raceway (distortion and curve) becomes achallenge. NTN achieved volume production of thinsection type angular contact ball bearings by
establishing machining technology including
optimization of thermal processing conditions forcontrolling deformation (Fig. 8).

Fig. 6 Structure of balancer

4. Technical trends and characteristicsof the bearings for industrial robots
4.1. Technical trends of the bearings forindustrial robots
As mentioned above, the main use of the bearings
for industrial robots is for reducers and balancers.
These bearings are required to have "compactness,""high rigidity," "high load carrying capacity" and "longlife." Particularly, compactness of bearings isimportant; however, that means downsizing of rolling
elements due to reduction of cross section, whichtranslates into reduction of rigidity, load carrying
capacity and operating life. Namely, the compactness
and high rigidity, high load carrying capacity and long
operating life are in a trade-off relation, so it isimportant for bearings to achieve compactness without
reduction of rigidity, load carrying capacity, andoperating life. Table 2 shows the requirements forbearings for industrial robots.
For reducers of robots, while high rigidity, highoutput torque and high efficiency are required,compactness tends to be required, as well. Angularcontact ball bearings for main shafts are required tomaintain rigidity and long life, while thin section type isalso required. The needle roller bearings forcrankshafts are required to pursue space saving within
their type, as well as high rigidity and high load

79-SeriesThin section type
angular contact ball bearing
for main shaft

Fig. 7 Cross-view comparison of Ultra thin angularcontact ball bearing-37-

*06E_*15 19/07/03 15:06 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

Table 3 shows a comparison between HWT (J)Type and standard type (PK Type). The major featureof the HWT (J) Type is that it enabled increasing
number of rollers compared with the PK Type, by
eliminating the columns on the inner diameter side of
the cage, to achieve high rigidity, high-load capacity
and long operating life (Table 4).
Furthermore, the roller surface is carbonitrided,
strengthening tolerance against damage to the surface
due to foreign objects penetrated into lubricating oil oroil film shortage on the raceway, contributing to
improvement in reliability and bearing operating life.
Fig. 8 Ultra thin angular contact ball bearing
Table. 3 High load capacity needle roller bearing

With regard to rigidity (moment rigidity) andoperating life (rolling fatigue life) which are in a tradeoff relation with reduction, optimal design technology
has been established by increasing the distance to theaction point of bearings in order to increase themoment rigidity and reducing the contact stress insidebearings to avoid reduction of operating life. Fig. 9shows the relation between the moment rigidity ratio
and operating life ratio against the preload ratio. It
reveals that the rolling fatigue life of the thin section
type angular contact ball bearings is equivalent to the
79-Series standard type and the moment rigidity is
increased by approx. 40% even if they are smaller insize compared to the 79-Series standard type.

Standard typeHigh-load capacity type

PK Type(Machined cage)

HWT Type
HWTJ Type(Machined cage) (Pressed cage)

Limited number of rollers
due to columns on the
inner diameter side

More rollers can be placed as
there are no columns on the
inner diameter side

Table. 4 Comparison of performance for PK type 2)Thin section type angular contact
ball bearing_moment rigidity

79-Series_moment rigidity

Thin section type angular contact
ball bearing_calculated operating life

79-Series_calculated
operating life

8

8

Moment rigidity+ approx. 40%

6

7
6

5

5

4

4

3

3

2

2

1

1

0

0

0

1

2

3

4

5

6

Preload ratio

7

8

9

10

Comparison＋20-30%＋20-35%＋15-25%×1.5-2＋15-25%

Life ratio

7

Moment rigidity ratio

Item
Number of rollers
Basic static load rating
Basic dynamic load rating
Rolling fatigue life
Rigidity

4.4 Long operating life for self-aligning roller
bearing
NTN deploys "ULTAGE" Series "EA Type" as theself-aligning roller bearings for rod cylinder support ofrobot balancers in the market.
The "EA Type" has improved the load carrying
capacity (basic dynamic load capacity) by max. 65%(Fig. 10), achieving top level performance (Fig. 11).
In addition, the "ULTAGE" Series features sealed
type (WA Type) with seals on both sides (Fig. 12) in
its line-up, as well, which allows enclosing special
grease with superior wear resistance and fretting
resistance, contributing to improvement ofmaintainability and bearing life.

Fig. 9 Moment stiffness and lifetime ratio
at loading preload

4.3 High rigidity and high-load capacity ofneedle roller bearings
NTN is deploying high-rigidity and high-load
capacity needle roller bearings (HWT (J) Type) in themarket for cranks of precision reducers.
-38-*06E_*15 19/07/03 15:06 ページ 6

Technical Trend and Features of the Bearing for Robot

5. Summary

Conventional
product

NTN produces many bearings for robots. In thisarticle, we described the technical trends and featuresof bearings for industrial robots.
It is expected that the robot demand will grow alongwith rapid development of the service fields, androbots are expected to work closely with humans.
NTN is poised to continue contributing to thedevelopment of robots by product development andimprovement of bearings for robots.

EA Type

Basic dynamic load rating+ 69% (max) compared
to conventional typeFig. 10 ULTAGE Spherical roller bearing (Type EA) 3)

Basic dynamic load rating kN

120

References
1) Naptesco Corporation, Nabtesco Catalogue,
Precision Reducer RV CAT 180420, (2018) Cover
2) Katsufumi Abe, New High-Capacity HWTJ Type
Pressed Cage and Needle Roller Assemblies,
NTN TECHNICAL REVIEW, No.71, (2003) 52-55.
3) Yukihisa Tsumori, New Spherical Roller Bearings“ULTAGE” Series Type EA and EM, NTN
TECHNICAL REVIEW, No.77, (2009) 78-82.
4) NTN Corp. Sealed Self-Aligning Roller Bearing(ULTAGE WA Type), NTN TECHNICAL REVIEW,
No.79, (2011) 138.

110
100
90
80
70

NTN

EA TypeCompanyA

Company
B

CompanyC

Fig. 11 Comparison of load capacity

Seal

Seal

従来品

EAタイプ

Fig. 12 ULTAGE Spherical roller bearing 4)

Photo of authors

Makoto OEBISU

Hiromichi KOKUMAI

Yasuyoshi HAYASHIMasato TSUJIHASHI

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Application Engineering Dept.

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*07E_*15 19/07/03 15:07 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Application of Condition Monitoring System
for wind turbines
Makoto MIYAZAKI*
Wataru HATAKEYAMA*NTN has developed Condition Monitoring System using sensing technology
and vibration diagnosis technology, and provides information services dedicatedto the maintenance of wind turbines. This article introduces some of thediagnostic methods used in the CMS and results obtained in validation tests,
and gives a perspective of CMS for wind turbines.

By monitoring measurement data from sensors
installed in the equipment in operation, it will be easier
to detect symptoms of failures. Therefore, a condition
monitoring system (CMS) which aims for online
condition monitoring and diagnosis from themeasurement data sent from the sensors is required.
By using CMS, the condition monitoring can be
conducted remotely to save maintenance costs. In
addition, time for taking precautionary measures canbe secured by detecting failed parts early, resulting in
improved availability. Furthermore, by detecting early
warning signs, scheduled maintenance can be
conducted at an optimum time, for efficient equipment
operation without significant loss of power.
NTN developed CMS taking advantage of theaccumulated sensing and vibration diagnosis
technology and marketing it under the name of Wind
Doctor TM (Fig. 1) 2), 3).
In this article, we describe the data analysis method
using Wind Doctor TM and some examples of its
application.

1. Introduction
1.1. Background
According to the report from Global Wind Energy
Council (GWEC2017) 1), the installed capacity of windturbines in 2017 was 529.1 GW, an increase of 10.6%
from the previous year and it is expected that the trend
will continue in the coming years. There are fewer
places onshore appropriate for wind turbines;
therefore, development of higher efficiency windturbines suitable for areas of low wind speed andoffshore wind turbines are accelerated. In addition,
selling price of electric power continues to decreaseas the deployment of the wind turbine businessexpands, which dictates further reduction ofmaintenance cost and increase of availability of windturbine facilities. Therefore, advanced maintenancewith constant monitoring of facility conditions andoperation to prevent prolonged shutdown due tofailures are required.

1.2 Overview of CMS
The conventional, periodic measurement ofvibration and abnormal noise, and analysis of grease
and lubricating oil have been conducted as condition
monitoring of wind turbines. However, from safetyconcerns, it is difficult to check for vibration andabnormal noise during operation, as it is not possible
to work on the equipment during operation. Therefore,
inspection is mainly conducted during outage, which
may delay the detection of symptoms of failure. Whenunscheduled repair becomes necessary, repair parts
may not be readily available for outages causing
prolonged shutdown resulting in lost revenue.

Fig. 1 Data acquisition module 2) 3)*Robotics Sensing Engineering, Industrial Machinery Division-40-

*07E_*15 19/07/03 15:07 ページ 2

Application of Condition Monitoring System for wind turbines

which deal with nature, the operating environment
such as wind speed and direction constantly change.
Wind Doctor TM records the operating conditions
together with sensor signals to reduce the impact ofdifferent operating conditions, extracting measured
data under similar conditions and using them fordiagnosis of the equipment status 4).
When an abnormal condition is determined bydiagnosis, frequency analysis of vibration data isconducted to estimate the cause of the abnormal
condition. However, when the rotating speed aroundthe axis drastically changes due to change of windspeed and direction, the accuracy of frequency
analysis will decrease and cause of the abnormal
condition may not be easily estimated. The analysis
tool of the Wind Doctor TM condition monitoring
software includes a function to correct the variation of
the rotational speed around the axis in order toprevent deterioration of the analysis accuracy. Fig. 3
shows the frequency spectrum before and after thecorrection. The frequency spectrum after thecorrection (b) has much clearer peaks compared tothat of the frequency spectrum before the correction(a), enabling more accurate analysis.

2. Configuration of CMS and analysis
method
2.1. System configuration
Fig. 2 shows the system configuration of Wind
Doctor TM 4). The data collection system periodicallymeasures the equipment using the sensors installedon the measured equipment and saves the measured
data to the server on the Internet. Various sensors
such as for vibration, displacement and temperature,
can be used as the measurement sensor. The data
management software manages the measured data inthe database on the server and issues an alert when
an anomaly is detected. The condition monitoring
software is a graphical user interface (GUI) containing
a communication function with the server. It not only
provides ability to visually inspect the monitoredequipment but also analysis of the measured data
through filtering, envelope processing, and frequency
analysis, etc.

2.2 Monitoring/analysis method
In order to monitor changes and trends inequipment, it is desirable to measure the data while
the equipment is running. However, for wind turbines(1) Data acquisition module(2) Data management software(3) Condition Monitoring Software

Measurement
sensor(vibration, etc.)

Operational
information(rotational speed/
power output)

Dedicated server
Data acquisition
moduleUser/NTN

Fig. 2 System configuration of Wind Doctor TM 4)

150000150000Power

200000Power

200000100000100000

50000
0

50000
0

1000

2000

3000
4000
5000
Frequency Hz

6000

7000

0
0

8000

(a) Frequency spectrum before correction

1000

2000

3000
4000
5000
Frequency Hz

6000

7000

(b) Frequency spectrum after correction

Fig. 3 Frequency spectrum．(a) Raw data, (b) Corrected data

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8000

*07E_*15 19/07/03 15:07 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

analysis of the envelope curve of the filtered vibration
waveform, and used for analyzing vibration
components due to damage. In the envelope spectrumat the point A, we could observe the peak of the rolling
element damage frequency (2 fb), which is thefrequency of the contact of a point of the rolling
element with the inner and outer ring, and thesideband at both sides of the peak, which is themodulated component of the rolling element rotation
frequency (2 fb ± fc). From this property, it wasestimated that there was a damage to the rolling
element of the bearing and the generator vibration
increased due to that damage.
In the envelope spectrum at the point B (Fig. 6),
which is the point where the vibration further increased
from the point A, we could observe the peaks of innerring damage frequency (fi) and outer ring damage
frequency (fo) in addition to the rolling element
damage frequency (2 fb). Therefore, it was estimatedthat the damage progressed to the raceways of theinner and outer ring in addition to the rolling element.
At point B, the facility verified the equipment status
and recognized that abnormal noise occurred. The
facility replaced the bearing and investigated theremoved bearing by disassembling it. Damage due tostray current corrosion was observed on the racewaysof the inner and outer rings of the bearing (Fig. 7) 5).

3. Examples of recent applications
NTN has been participating in Research andDevelopment of High Level Application of Wind
Turbine Technology (Research and Development of
Smart Maintenance Technology) conducted by NEDO(New Energy and Industrial Technology Development
Organization) together with the University of Tokyo,
etc. conducting demonstration tests on how to use
CMS in the maintenance of wind turbines throughout
Japan by installing Wind Doctor TM, in order to increaseavailability of wind turbines in Japan. The followingshows some examples of the measured data.

3.1 Case study of damage on the generator bearing

Warning threshold

50

Replacement ofbearing

40

B
30

A

m/s2

Vibration acceleration RMS value

Fig. 4 shows the data acquired from the vibration
sensor installed on the generator drive shaft. The
horizontal axis of the graph indicates dates and thevertical axis indicates vibration RMS values, whichshow the level of vibration. The graph was made by
extracting measured data while the wind turbine wasoperating at 85% or more of the rated rotational
speed.
The vibration RMS value started to increase amonth before the measurement point A and exceeded
the caution threshold at the point A. Fig. 5 shows theenvelope spectrum of the measured data at point A.
The envelope spectrum is the result of frequency

Caution threshold

20

10
0
2/1

3/3

4/2

5/2

6/1

7/1

7/31

8/30

9/29

10/29

11/28

12/28

1/27

Fig. 4 Trend graph of RMS value at generator drive end
5.0

Frequency Hz

Frequency Hz

Fig. 5 Envelope spectrum at A

Fig. 6 Envelope spectrum at B

-42-

500

450

400

350

300

250

0

500

450

400

350

300

250

200

150

100

0.0
50

0.0

1.0

200

1.0

2.0

150

2.0

3.0

100

3.0

4.0

50

Vibration acceleration m/s2

4.0

0

Vibration acceleration m/s2

5.0

*07E_*15 19/07/03 15:07 ページ 4

Application of Condition Monitoring System for wind turbines

the peak of the inner ring damage frequency (fi) of
medium speed shaft bearing and sideband of themodulated components of the medium speed shaft
rotation frequency (fi ± fr, 2fi ± fr) on both sides of thepeak were observed; therefore, it was estimated theexistence of damage at the inner ring of the bearing
immediately after installation of the CMS. However,since the vibration RMS value was at relatively low
level, they decided to continue observing it,
determining tentative caution threshold based on thedata in the first month.
After the installation of the CMS, the vibration RMS
value maintained similar level; however, the vibration
RMS value started to climb up at the point D, 6
months after the installation, and exceeded thecaution threshold at point E. The envelope spectrumat point E (Fig. 10) showed the inner ring defectfrequency (fi) and the sideband of its modulated
components (fi ± fr) more pronounced than point C;
therefore, it was estimated that the damage from theinner ring of the bearing had progressed.
They reported the progress of the damage on the
inner ring of the bearing at point E to the facility. Then,the facility replaced the bearing. The raceway of theremoved bearing inner ring showed multiple axial
cracks, as well as flaking, possibly originating from thecracks (Fig. 11) 5).

Fig. 7 Outer raceway failure of electrical pitting 5)

In this case, use of the CMS could detect thesymptom of the failure 2 months before the facility
recognized the anomaly at the site. Therefore, when
the facility received the report, it could prepare for the
replacement of the bearing with ample time before thewinter season started when the strong wind was
expected, and avoided outage during the mostdesirable time of power generation.

3.2 Case study of damage on the gearbox bearing

0.6
0.5

E

Replacement ofbearingD

C Caution threshold

0.4
0.3
0.2

5/31

6/30

7/30

8/29

9/28

10/28 11/27 12/27

1/26

2/25

3/26

4/25

5/25

6/24

7/24

8/23

9/22

10/22 11/21 12/21

300

0
5/1

250

0.1

200

m/s2

Vibration acceleration RMS value

Fig. 8 shows the trend graph of RMS values
obtained from the vibration sensor installed on the
gearbox planetary. The graph was made by extracting
measured data while the wind turbine was operating at
85% or more of the rated rotational speed.
In the envelope spectrum of the point C (Fig. 9),
which is immediately after the installation of the CMS,

Fig. 8 Trend graph of RMS value at gearbox planetary
5.0

Frequency Hz

Frequency Hz

Fig. 9 Envelope spectrum at C

Fig. 10 Envelope spectrum at E

-43-

500

450

400

0

350

1.0
0.0

500

450

400

350

300

250

200

150

100

0.0

50

1.0

2.0

150

2.0

3.0

100

3.0

4.0

50

Vibration acceleration m/s2

4.0

0

Vibration acceleration m/s2

5.0

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NTN TECHNICAL REVIEW No.86（2018）

It is important to continue data collection andanalysis on the actual wind turbines and improve theanomaly detection technology of slow-rotating parts inorder to achieve accurate prediction of the remaininglife.

5. Conclusion
In this article, we described the methods for data
analysis using the CMS and introduced someexamples of application of Wind Doctor TM, which wehave experienced in the R&D activities of NEDO's
Smart Maintenance Technology project.
We will promote advancement of the CMS
technology so that it can contribute to the reduction ofmaintenance cost and improvement of the equipment
availability for wind turbine business.

Fig. 11 Inner raceway failure 5)

In this case, the symptom of damage wasdiscovered by the initial diagnosis after installation andaccurately detected the progress of the damage bycontinuous monitoring. The facility received theanalysis result from the CMS and could plan adequate
preparation and replacement of the bearing at the besttime.

References
1) GLOBAL WIND ENERGY COUNCIL，GLOBAL
WIND Report 2017，(2018)．
2) Takeuchi, Haseba, Ikeda: Application of Condition
Monitoring System for Wind Turbines, NTN
TECHNICAL REVIEW No.80 (2012) 15-18.
3) Hashizume, Takkeuchi, Tanaka, Development of
Condition Monitoring System, NTN TECHNICALREVIEW No. 82, (2014) 74-77.
4) Wataru Hatakeyama, The Recent Applications of
the Condition Monitoring Systems for Wind
Turbines, THE TRIBOLOGY No. 356, (2017) 3841.
5) NEDO, FY2017 Final Report "Research andDevelopment of Natural Energy Technologies
Based on Wind Power, Etc. / Research andDevelopment of Advanced Applied Technologies /Research and Development of Smart MaintenanceTechnologies (Analysis) (Risk Analysis, etc.)(2008)

4. Challenges ahead
In order to minimize the outage due to repair workand reduce the loss of power and maintenance cost,
the prediction technology to appropriately determine
the limit of the component operating life and plan therequired repair work at the right time will be critical.
However, the challenge for the wind turbines is thedifficulty to accurately predict the remaining life since
the process to the failure is quite different for eachcase due to varied operating environments such as
wind conditions. In addition, for low-speed rotation
parts such as the main bearing, the progress ofdamage is so slow that a long observation time isrequired in order to collect sufficient data in theprocess to failure.

Photo of authors

Makoto MIYAZAKI

Wataru HATAKEYAMA

Industrial Machinery Division
Robotics Sensing Engineering

Industrial Machinery Division
Robotics Sensing Engineering-44-

*08E_*15 19/07/03 15:08 ページ 1

NTN TECHNICAL REVIEW No. 86（2018）[ New Product ]Development of Multi Track Magnetic Ringfor High Accuracy Absolute Angle Detection
Takashi KOIKE*
Yusuke SHIBUYA*

Yasuyuki FUKUSHIMA*
Hiroyoshi ITOU**

As the introduction of industrial robots progresses, more accurate work is
required. Therefore, it is necessary to detect the angle of each joint of the robot
arm with high accuracy. NTN has developed a "Multi-track magnetic ring" thatcan realize absolute angle detection with high accuracy at low cost by applying
our precise magnetization technology.

methods. For detecting methods, there are opticalmethods and magnetic methods. The former detects
the light from a light emitting device transmitted
through the slit plate or reflected by the reflection plateusing a photoreceiver. The latter detects the change ofangle by the change of direction and magnitude of themagnetic field. For output methods, there are theincremental method and absolute method. The former
repetitively outputs a signal corresponding to thechange of angle so that the relative angle can bedetected. The latter outputs the current angle
information so the absolute angle can be determined.
For robots, angle sensors using the absolute method
are more suitable since it requires no initialization. For
the multijoint robot, which has made remarkableprogress in recent years, angle sensors with highresolution and accuracy are required. The servo motor
widely used for this application integrates an opticalangle sensor (rotary encoder) and a motor withsuperior properties for these requirements.

1. Introduction
The recent trend of IoT (Internet of Things), which is
to connect "things" to the Internet, is expanding torobotic applications. In developing countries such as
China, adoption of industrial robots is increasing due
to the rise in labor costs, global price competition, andrequirement for product quality. In Japan, labor
shortages due to a decreasing birthrate and aging
population has become a serious problem. This issue
is raising expectations for not only industrial robots in
manufacturing, but also for service robots in medical,
nursing/welfare, logistics, and security fields 1).
Japan has provided many industrial robots and theircomponents to the world such as reducers, servo
motors, and various sensors. Recent development ofindustrial robots enabling more complex andadvanced operation with artificial intelligence (AI)requires advanced components with high functionality.
For precision work, positioning accuracy of the tip of
the robot arm requires highly accurate detection of
joint angles.
NTN has developed "Multi-track Magnetic Ring" forhighly accurate angle detection, applying its precise
magnetization technology 2).

Detection
methodOpticalOutputmethodIncremental
Absolute

Angle detection
device2. Angle detection methods

Magnetic

In general, angle detection devices (hereafter, angle
sensors) are broadly categorized as shown in Fig. 1,
depending on the detecting methods and output

Incremental
Absolute

Fig. 1 Classification of angle sensor**New Product Development R&D Center**Robotics Sensing Engineering, Industrial Machinery Division-45-

*08E_*15 19/07/03 15:08 ページ 2

NTN TECHNICAL REVIEW No. 86（2018）

direction within the cast iron or die cast aluminum
housing. The sensor technology developed in detectionof rotational speed of hub bearings for automotive use 78) is used in broad applications such as rotational speed
control of servo motors, angle detection of steering, etc.
However, the technology is yet to be applied in robot
joints, which require angle detection of high accuracy.
NTN also newly developed the "multi-track magnetic
ring" which can detect the absolute angle of rotational
shaft with high resolution and accuracy by combining
the dedicated magnetic sensor IC (refer to Section 5.2)
with the magnetic rotational sensor technology it hasdeveloped.

3. Magnetic angle sensor
The magnetic angle sensor of the absolute method
is configured as shown in Fig. 2, with a pair of N- andS- pole permanent magnets attached to the rotational
axis and a magnetic sensor placed against the pair on
the fixed side on the coaxial direction. For themagnetic sensor, hall element and MR (Magneto
resistance) element are used.
In general, this configuration of using permanent
magnet and magnetic sensor is inferior to the opticalangle sensor (rotary encoder) in resolution andaccuracy. However, it is less expensive and lesssusceptible to environment such as oil, dirt, and dust.
Its application to robots will greatly expand if resolution
and accuracy are improved.

5. About multi-track magnetic ring
5.1 Overview
Fig. 3 (a) shows the appearance of the developed"multi-track magnetic ring."
The multi-track magnetic ring consists of a core
metal made of press formed steel sheet and rubber
magnetic material. The rubber material is kneaded
magnetic material, vulcanized and attached on theouter periphery of the core metal. Two magnetic tracks
are formed on the outer diameter periphery of therubber magnetic material, and as shown in Fig. 3 (b)and (c). The main track is magnetized with 64 N/Spole pairs, and the sub track is magnetized with 63pole pairs.
In addition to the multiple magnetic ring of "radial
magnetized type," shown in Fig. 3, multi-trackmagnetic ring of "axial magnetized type" was also
developed as shown in Fig. 4. Table 1 shows the key
specification.

Magnetic sensor

Magnet

Shaft

Fig. 2 Angle detection of magnetic absolute method

4. Development of rotation/angle sensors
NTN implemented volume production of "bearings
with rotational sensors" for industrial machinery which
can detect both speed and direction of rotation with theincremental method. This method uses a multipole (N/Spoles) magnetized ring and magnetic sensor IC to deep
groove ball bearings 3-4).
NTN also developed "bearings with high resolution
rotational sensors" which have higher resolution than"bearings with rotational sensors" by installing a
multipole (N/S poles) magnetized ring and magnetic
sensor IC with multiple hall elements to deep grooveball bearings. These bearings can provide a maximumof 40 times of incremental output by multiplying the N/Spole pairs by 40 inside the magnetic sensor.
Furthermore, NTN also developed "bearings with highresolution rotational sensor of origin signal output type"which can also output origin signals by using a speciallypatterned N/S multipole magnetized ring 5).
NTN developed and commercialized "angle sensor
units" of the absolute method for construction
machinery 6), which incorporate a permanent magnet
and magnet sensor IC facing each other in axial

Sub track (63 polar pairs)
Main track (64 polar pairs)

Rubber magnetic material

Magnetization pattern visualizedby pasting magnet sheet(b) Magnetization pattern

Core
metal
(a) Appearance(c) Image of magnetization patternFig. 3 Multi Track Magnetic Ring-radial type-46-

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Development of Multi Track Magnetic Ring for High Accuracy Absolute Angle Detection

Core metal

Rubber magnetic material(a) Appearance

64 magnetized polar pairs from detector 1 and 63
magnetized polar pairs from detector 2. It can also
detect the absolute angle by using the fact that theirphase difference changes from 0˚ to 360˚ with one
rotation. Therefore, it can detect which polar pair
position it is in the magnetic track of 64 polar pairs. In
addition, this magnetic sensor can provide
multiplication of 12 bits by accurately reading themagnetic intensity between the magnetic poles. It is
possible to divide the angle information of one polar
pair into 212. Considering 64 polar pairs (26), it can
output angle information of 18 bits (resolution of
approx. 0.0014˚) 6). Depending on the parametersetting of the magnetic sensor IC, it is possible tooutput a maximum of 20 bits (resolution of approx.
0.00034˚).
In order to achieve such high resolution, NTN isdeveloping proprietary magnetizing technology andcontrolling the width of each magnetic pole of themulti-track magnetic ring very accurately.

(b) Image of magnetization patternFig. 4 Multi Track Magnetic Ring-axial type

Table 1 Main specification for Multi Track Magnetic Ring
Item

Part number

Magnetized typeOD (mm)ID (mm)Width (mm)Weight (g)

MTR64

MTA64

Radial

Axial

φ51.5

φ56

φ44

φ41

8.2

4.0

10.7
9.8
Number of magnetized
64/63 polar pairs
polar pairs(Main track: 128 poles, sub track: 126 poles)
MagnetRubber magnet

Core metal

SPCC (rolled steel plate) thickness 0.6 mm

Operating temperature
range (°C)

−40 to＋110

5.3 Features
1) High resolution/high accuracy
As mentioned above, it can output angle informationof maximum of 20 bits (resolution of approx.
0.00034˚).
Fig. 6 indicates an example of measurement ofangle error from the angle sensor combining the multitrack magnetic ring and the magnetic IC sensor
described in Section 5.2. This example has an idealalignment with the least possible installation errors,
achieving the angle error of ±0.025˚.

5.2 Principle of absolute angle detection andresolution
Fig. 5 shows the principle of absolute angle
detection by multi-track magnetic ring. The magnetic
sensor IC placed against the multi-track magnetic ring
integrates two detectors facing each magnetic track(detector 1 and detector 2) and absolute angle
calculation unit. The iC-Haus GmbH's IC (iC-MU) can
also be used 9).
The multi-track magnetic ring shown in Table 1
obtains output signal with the cycle corresponding toMagnetic sensor

2) Hollow structure = Lightweight and compactness
Since its structure is hollow and its diameter islarge, it is possible to run cables inside when it isapplied to the joints of robots. This will contribute to
the compactness and lightweight for robots.

Multi-track magnetic ring

Detector 1

S

N

S

N

S

N

Detector 2

S

N

S

N

S

N

S

N
S

S
N

N
S

S

N

64 polar pairsNS

63 polar pairs

Detector 1
Detector 2
Detector 1 phase
Detector 2 phase
Phase difference0°

Rotating angle

Fig. 5 Absolute angle detection principle

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*08E_*15 19/07/03 15:08 ページ 4

NTN TECHNICAL REVIEW No. 86（2018）

Fig. 7 shows an image of the application of multitrack magnetic ring and magnetic sensor IC to thejoints of robots.
A multi-track magnetic ring is fixed on the output
stage of the reducer and a printed circuit board
implemented with the magnetic sensor IC is installedin the facing position to the magnetic track.
Since the multi-track magnetic ring is fixed on therotational shaft by press fitting, the installation requires
little space contributing small form factor of the joints.
The adoption of multi-track magnetic ring is moreadvantageous than the conventional rotary encoder interms of installation space. Its compactness willcontribute to the increased degree of freedom fordesigning robots with many rotational axes.

6. Application for torque sensors
The developed multi-track magnetic ring assumes
application for torque sensors in addition to theaforementioned absolute angle detection for robot
joints.
High resolution angle detection of 18 to 20 bits canbe achieved by using the multi-track magnetic ring.
Therefore, torque can be detected by placing multitrack magnetic rings and magnetic IC sensors on bothends of the torsion bar and calculating the torsion
amount of the torsion bar from the difference of output
angles of each magnetic sensor IC.
Fig. 8 shows an example of the multi-trackmagnetic ring installed on the actual robot. The multitrack magnetic ring of 64/63 magnetized polar pairs is
installed in each joint of the 7-axis robot arm(TOROBO ARM) developed by Tokyo Robotics, Inc.
and used for detection of absolute angle and torque.
This design was adopted because it achieves a highdegree of freedom thanks to the hollow large diameterof the multi-track magnetic ring and accuracy of angle
detection.

3) Environmental resistance
Environmental resistance of magnetic property ofthe multi-track magnetic ring is superior to the opticalmethod. It protects against vibration, high temperature,
dirt and dust, oil mist, etc. enabling its application in anenvironment where optical encoders cannot be used.

0.15

Angle error ˚

0.1
0.05
0
-0.05
-0.1
-0.15

0

60

120

180

240

300

360

Rotating angle (position) ˚

Fig. 6 Angular accuracy measurement result when combining the Multi Track Magnetic Ring and dedicated magnetic sensor IC
Multi-track magnetic ring
Motor

Speed reducer
Printed circuit board(magnetic sensor IC)

Fig. 7 Application example of the Multi Track Magnetic Ring to robot joint-48-

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Development of Multi Track Magnetic Ring for High Accuracy Absolute Angle Detection

References
1) New Energy and Industrial Technology
Development Organization (NEDO): NEDO RobotWhite Paper (2014).
2) Koike, Fukushima, Shibuya: High Resolution
Absolute Angle Sensor Applicable for Detecting
Absolute Angle of Robot Joints, Machine Design
Vol. 62 No. 3 (2018) 55.
3) Ito, Koike: Bearing with Integral Revolution Sensor,
NTN TECHNICAL REVIEW No.69 (2001) 108-116.
4) Koike, Ishikawa, Ito, Mizutani: Improvement ofMagnetic Flux Leakage Durability of Integrated
Sensor Bearings, NTN TECHNICAL REVIEW
No.71 (2003) 74-79.
5) Ito, Takahashi, Pascal Desbiolles, Cyril
Peterschmitt, Shintarou Ueno, High Resolution
Sensor Bearing with an Index Signal, NTN
TECHNICAL REVIEW No. 78 (2010) 70-76.
6) Itomi, Ito, NTN Sensor Units for Construction
Machinery, NTN TECHNICAL REVIEW No.76(2008) 118-125.
7) Pascal DESBIOLLES, Achim FRIZ, Developmentof High Resolution Sensor Element MPS40S andDual Track Magnetic Encoder for Rotational Speedand Position Measurement, NTN TECHNICALREVIEW No. 75, (2007) 36-41.
8) Nishikawa, Takahashi, Christophie Duret: Hub
Bearing with an Integrated High-Resokution
Rotation Sensor, NTN TECHNICAL REVIEW
No.81 (2013) 52-57.
9) iC-Haus GmbH，iC-MU off-axis nonius encoderwith integrated hall sensors

Fig. 8 Application example of the Multi Track Magnetic Ring
(Source: Tokyo Robotics, Inc.)

7. Future development
There are two types of multi-track magnetic rings
developed this time: 64/63 polar pair radial magnetized
type and axial magnetized type. From now on,
development will continue on the smaller diameter typewith fewer polar pairs of 32/31 magnetized polar pairs.
In order to respond to the market demand for
improved design degree of freedom and various robotic
applications, we will continue developing products withhigher resolution and accuracy that have a lighter andthinner form factor.

Photo of authors

Takashi KOIKE

Yasuyuki FUKUSHIMA

Yusuke SHIBUYA

Hiroyoshi ITOU

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D CenterIndustrial Machinery Division
Robotics Sensing Engineering-49-

*09E_*09 19/07/03 15:08 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Development of Sensor Integrated Bearing Unit
for Machine Tool Spindles

Shohei HASHIZUME*
Yusuke SHIBUYA**

Yasuyuki FUKUSHIMA**
Yohei YAMAMOTO***

Recently, machine tools are required not only high speed, high rigidity and superprecision, but also condition monitoring function and the "Connected Industries" related
technology strongly for higher reliability. To realize the above mentioned machine tools,
NTN has developed the "Sensor Integrated Bearing Unit for Machine Tool Spindles" todetect the various conditions like temperature, heat flux and vibration from the bearings.
The unit has the various sensors inside the outer spacer beside the bearings. This
report introduces the features, mechanism and performance of the unit.

Main spindleof lathe1. Introduction

Main spindle ofmachining center
Implemented
bearing unit

Machine tools support manufacturing in different
industries including automotive, aircraft, medicine andIT, and their requirements are increasingly diversified.
In recent years, various machine tools with new
functionality and concepts such as multifunction
numerical control and built-in condition monitoring
capability have been developed 1), 2).
Recent machine tools, in particular, are required tocontribute to unmanned or labor saving operation andimproved productivity by using advanced condition
monitoring and control technologies as well as IoT, inview of labor shortages due to a decreasing birthrate
and aging population. Therefore, early detection of
anomalies in main spindles and their supporting
bearings in machine tools is highly desirable in orderto prevent unexpected damage and the resulting
production downtime required for main spindle
replacement.
We have newly developed this "sensor integrated
bearing unit for machine tool spindles" as a functional
product to solve the above mentioned challenges. Thisunit, applicable to lathes and machining centersshown in Fig. 1, integrates various sensors into theouter ring spacer to enable condition monitoring of
nearby bearing raceway surfaces. The following
sections introduce the features, configuration andevaluation results of this bearing unit.

Implemented
bearing unit

Fig. 1 Machine tools and spindles(Left : Lathe, Right : Machining center)

2. Damage modes of bearing units formachine tool spindles and featuresof the developed product
Bearing units for machine tool spindles are usedwith lighter loads compared with other bearings forindustrial machines. Therefore, they rarely experienceflaking 3) due to material fatigue, but the main damage***Application Engineering Dept., Industrial Machinery Division***New Product Development R&D Center***Product Design Department, Industrial Machinery Division-50-

*09E_*09 19/07/03 15:08 ページ 2

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles

3. Structure of the sensor integrated
bearing unit
Fig. 3 shows the structure of this bearing unit.
Three types of sensors are integrated in the outer ring
spacer that is inserted between two back-to-back (DB)angular contact ball bearings. The following sections
describe those sensors.

(a) Surface roughness, peeling and burnout due to poor
lubrication
The photo shows burn-out of angular contact ball bearings(inner ring raceway surface)Burn-out due to shortage of air oil supply

Outer ring spacer

Sensor board(temperature sensor, heat flow sensor,
vibration sensor)

Fig. 3 The sensor integrated bearing unit(b) Indentation due to collision of the main spindle andworkpiece[Temperature sensor]

The photo shows ball pitch indentation produced on the inner
raceway surface of angular contact ball bearing (roundness
measurement result; indicated by arrows)

This sensor detects heat produced within the
bearings due to rotation and the spindle’s cutting load.
Sensors for measuring bearing temperature during
operation are usually installed on the housing’s outersurface due to ease of installation; however, since a
cooling path exists between the bearings and the
housing’s outer surface, the measured temperature
becomes lower than the bearing temperature. In
addition, measuring temperature at the housingmeans that it takes time to detect a rise in temperaturewhen the bearing is suddenly heated, due to the large
heat capacity of housing. Therefore, it is difficult toquickly determine a sudden heat-up of the bearings.
This bearing unit can measure the temperature of the
bearings more accurately compared withmeasurement on the outer surface of the housing, as
it measures the temperature at the outer ring spacer
adjacent to the bearings.

Fig. 2 Damage example of machine tool spindle bearing

modes are "(a) surface roughness, peeling, andburnout due to poor lubrication" and "(b) indentation
due to spindle collision with the workpiece" (Fig. 2).
The developed bearing unit incorporates various
sensors in the outer ring spacer adjacent to thebearings. This enables early detection of anomalies
compared to conventional measurement on the outersurface of the main spindle. In addition, a heat flow
sensor is newly incorporated to improve
responsiveness. Damage mode "(a) surface
roughness, peeling and burnout" can be detected bytemperature sensors, heat flow sensors and vibration
sensors, and damage mode "(b) indentation" can bedetected by vibration sensors (Table 1).

[Heat flow sensor]
As the spindle rotates, there will be a difference intemperature between the inner and outer rings(hereafter, inner/outer ring temperature difference) of
the bearing unit for spindles. The main reason for thisis the difference of heat dissipation between the innerand outer rings. The inner ring with less dissipationshows higher temperature than the outer ring. The
inner/outer ring temperature difference becomeslarger as the bearing rotates at higher speeds,
increasing preload within the bearings and contact
surface pressure on the rolling surfaces. In addition,

Table 1 Major damage mode of machine tool spindle
bearings and measurement items
Temperature Heat flow Vibration(a) Detection of surface roughness,
peeling and burnout due topoor lubrication(b) Detection of indentation due tocollision of the main spindle
and workpiece

○

○

○
○

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NTN TECHNICAL REVIEW No.86（2018）spindles. In addition to the integrated sensors within
the developed bearing unit, sensors were also
installed at the conventional measurement positions
on the outer surface of the housing for comparison
purposes. For correlation purposes, the temperatureof the bearing outer ring outer diameter surface wasalso measured (Fig. 4).

the inner/outer ring temperature difference also
increases when operating with damage and burnout
caused by poor lubrication within the bearings. The
heat flow sensor adopted in this development catches
minor changes better than an ordinary temperature
sensor and measures the heat flux going from theinner ring with higher temperature to the outer ring,and is used for anomaly detection due to theinner/outer ring temperature difference.

(4) (5)[Vibration sensor]
Used for anomaly detection of both damage mode
"Table 1 (a) surface roughness, peeling and burnout
damage" and damage mode "Table 1 (b) indentation."
In general, for measuring vibration of bearings inoperation, a vibrometer is installed on the outersurface of the housing, for ease of installation, similar
to the aforementioned temperature sensor. However,since vibration of the bearing in operation is measured
through the housing, the measured vibration levels are
reduced from those at the bearing. Therefore,
detection is not possible until an anomaly progresses
to a sufficiently high level of vibration. The developed
bearing unit, on the other hand, integrates thevibration sensor into the outer ring spacer adjacent tobearings, which makes it possible for the sensor todetect small vibrations at the initial stage of anomaly
with good sensitivity.

(6)

Motor
(7)
(1) (2) (3)[Sensor integrated bearing unit](1) Temperature sensor(2) Heat flow sensor(3) Vibration sensor[General measurement method](4) Temperature on the outer surface of housing (thermocouple)(5) Vibration on the outer surface of housing(6) Temperature on the bearing outer ring outer diameter surface(thermocouple)(7) Temperature on the bearing inner ring inner diameter surface(thermocouple)

4. Performance evaluation test

Fig. 4 Test spindles, measurement items and points

4.1 Test conditions, tester configuration
Fig. 4 shows configuration of the tester, modeled
after a machine tool main spindle. Condition detection
performance was evaluated by integrating thedeveloped bearing unit. Test conditions are shown in
Table 2. The test unit, comprised of an ultra-highspeed angular contact ball bearing with ceramic balls(HSE Type), was operated under the test conditions.
Bearings were arranged in a double-row back-to-backconfiguration (DB) as is common for machine tool

4.2 Test results
Outputs of the various sensors in the bearing unit
are shown in Fig. 5 for the performance test describedin Table 2 and Fig. 4. It was verified that all the
sensors performed well from the low speed range tothe ultra-high speed range (dmn value: 1.44 million).
Measurements from the developed bearing unit andthe conventional measurement method described in
Section 4.1 were also compared. Fig. 6 and Fig. 7
show the temperature and vibration measurement
results. It was verified that readings from thetemperature sensor integrated into the developed
bearing unit outer ring spacer were closer to thetemperature at the bearing outer ring outer diametersurface than the readings from the outer surface of themain spindle housing. It was also verified that the unit
provided higher readings for vibration than
measurements from the outer surface of the housing.
In addition, comparisons were also made for an
acceleration/deceleration test where the rotational
speed was changed at certain cycles. Fig. 8 andTable 2 Conditions of functional tests
Test bearing

φ70×φ110×20
Equivalent to 5S-2LA-HSE014(Ultra high speed angular contact ball bearing with ceramic balls)

Preload method

Fixed position preload (preload of 750N after incorporation)

Lubrication method

Air oil lubrication

Lubricating oil

ISO VG32

Rotational speed

Supply oil amount

Lubricating air flow

Outer cylinder cooling
Attitude of axis

0〜16,000min-1
0.03mL/10min
30NL/minYes, controlled to room temperature
Horizontal axis

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*09E_*09 19/07/03 15:08 ページ 430(1) Temperature sensor

20
1.5(2) Heat flow sensor

1.0
0.5
0.0
-0.5

Vibrationm/s240(3) Vibration sensor

0-40

Rotational speed
min-1

24000
16000
8000
0

0

1

2

3

4

5

6

7

8

9

10

Operating time hFig. 5 Fundamental test result※1

45
40

40
35

Temperature
˚C

Temperature
C˚

35
30

30

25

25

20
15
10

0

5000

10000

15000

Fig. 6 Temperature measurement result※1(measurement point comparison)

15

8000
0

10

20

30

40

50

Operating time min

20

60

70

80

0

(3) Vibration sensor(6) Outer surface of housing
Rotational speed

15
10

5

Vibrationm/s2

Vibration
m/s2

10

16000

Fig. 8 Temperature measurement result in the frequent
cycle test※1 (measurement point comparison)(3) Vibration sensor(5) Outer surface of housing
Rotational speed

15

20

10

20000

Rotational speed min-1

20(1) Temperature sensor(4) Outer surface of housing(6) Bearing outer ring outer diameter surface
Rotational speed

45(1) Temperature sensor(4) Outer surface of housing(6) Bearing outer ringouter diameter surface

0

Rotational speed
min-1

-5

5
0
-5

-10

16000

-10

16000

-15

8000

-15

8000

-20

0

2

4

6

Operating time h

8

10

0

Rotational speed
min-1

Heat flux
kw/m2

Fig. 9 show the temperature and vibration measurement
results. It was verified, in the acceleration/deceleration
test also, that readings from the temperature sensor
integrated into developed bearing unit outer ring
spacer were closer to temperature readings at thebearing outer ring outer diameter surface than thereadings from the outer surface of the main spindle
housing. The vibration sensor also gave higher
readings than the measurements taken on the outersurface of the housing.
Fig. 10 shows the heat flow sensor outputs during
the acceleration/deceleration test. The heat flow
sensor has good responsiveness versus
acceleration/deceleration of the rotational speed andthe resulting change of inner/outer ring temperature
difference, and improves the accuracy of anomaly
detection as well. The inner/outer ring temperature
difference is defined as the difference in temperature
between the bearing inner ring bore surface and theouter ring outer diameter surface.

40

-20

Fig. 7 Vibration measurement result※1(measurement point comparison)

0

10

20

30

40

50

Operating time min

60

70

80

0

Rotational speed
min-1

Temperature
C˚

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles

Fig. 9 Vibration measurement result in the frequent cycle
test※1 (measurement point comparison)※1 Refer to Fig.4 for measurement positions for each item.

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NTN TECHNICAL REVIEW No.86（2018）

1.00

30

0.50

45

0.00

25

-0.50

20
15

1.05

10

0.55

5

0.05

0

-0.45

Fixed position preload (preload of 750N after incorporation)

Attitude of axis

Horizontal axis

Constant at 18,000min-1
A few drops of lubricating oil were injected
Lubrication method when the main spindle was assembled(no lubrication during the operation)
Outer cylinder cooling
Yes, controlled to room temperature

55

24000

Temperature
C˚

8000
0

10

20

30

Operating time min40

50

Fig. 10 Measurement result in the frequent cycle test ※1(heat flux, temperature and temperature deference between IR and OR)
Inner/outer ringtemperature difference
C˚

Bearing burn-out was simulated in order to test
symptom detection for this damage mode. Table 3
shows the test conditions. The tester shown in Fig. 4
was used, in a manner similar to the basic
performance evaluation test and theacceleration/deceleration test. In this test, only a verysmall amount of lubricating oil was injected into thebearing when the main spindle was assembled inorder to cause the test bearing to burn out easily. In
addition, a limiter was set to automatically stop thetester when the motor becomes overloaded due toburn-out of the test bearing.
Fig. 11 shows the relationship between heat flux
and temperature, as well as inner/outer ringtemperature difference and vibration. The point in time
when motor overload was detected is shown by theblue dotted line in Fig. 11. The test results revealed
that the heat flux shows an increase of output earlier
than temperature, inner/outer ring temperature
difference and vibration; therefore, it is considered that
this measurement is more effective for early detectionof burn-out symptoms.
In Fig. 11, the reason why the vibration value
increased even after the tester automatically stopped
after detection of overload is because the motor andthe main spindle continued rotating by inertia underthe burn-out condition.

3.0

40

2.0

35

1.0
0.0
4.0

Thermocouple ((7)-(6))(2) Heat flow sensor

40

3.0

30

2.0

20

1.0

10
600

0.0
4.0

(3) Vibration sensor(4) Outer surface of housing(2) Heat flow sensor

300

Vibration
m/s2

4.0

45

30
50

4.3 Bearing burn-out simulation test

5.0(1) Temperature sensor(4) Outer surface of housing(2) Heat flow sensor

50

16000
0

(Ultra-high speed angular contact ball bearing with ceramic balls)

Pressurization method
Rotational speed

1.55

Thermocouple ((7)-(6))(2) Heat flow sensor

φ70×φ110×20
Equivalent to 5S-2LA-HSE014

Test bearing

3.0

0

2.0

-300

1.0

-600
24000

0.0

Rotational speed
min-1

Heat flux kw/m2

1.50

Heat flux kw/m2

35

Heat flux kw/m2

40

Table 3 Conditions of the bearing seizure test

2.00

Heat flux kw/m2(6) Bearing outer ring outer diameter surface(2) Heat flow sensor

Heat flux kw/m2

Rotational speed
min-1

Inner/outer ringtemperature differenceC˚

Temperature
C˚

45

18000
12000
6000
0

0

10

20

30

Operating time s

40

50

Fig. 11 Measurement result in the seizure test※1(heat flux, temperature and temperature deference
between IR and OR, vibration)

the workpiece using a vibration sensor, a simulated
indentation was made on the bearing raceway
surface, and a test was conducted. Table 4 shows thetest conditions. The tester in Fig. 4 was used in amanner similar to the basic performance evaluation
test and the acceleration/deceleration test. Fig. 12shows the simulated indentation made on the bearing
raceway surface.
Calculated values of bearing forced vibration
frequencies during operation at 1,800 min-1 are shown

4.4 Bearing indentation test
In order to test anomaly detection of indentations on
the raceway surface produced by spindle collision to-54-

*09E_*09 19/07/03 15:08 ページ 6

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles

in Table 5. Fig. 13 shows the vibration output
frequency analysis results for the developed bearing
unit and the conventional measurement method
described in Section 4.1 4). The test result revealed
that both the vibration sensor of the developed unit
and the measurements taken at the main spindle
housing outer surface detected the frequency causedby the damage on the outer ring when the indentation
depth is 60 μm.
On the other hand, since indentations produced byspindle collision with the workpiece vary from several
μm to several tens of μm in field applications, we willcontinue performing tests with varied depths and sizesof indentation.

4(3) Vibration sensor

3(m/s 2Rms) 2

342Hz

2

684Hz

1026Hz1

1368Hz

1710Hz00.03(5) Outer surface of housing(m/sRms) 22

1710Hz0.02
1026Hz

0.01
0

342Hz
0

500

684Hz

1368Hz1000

Frequency Hz

1500

2000

Fig. 13 Vibration frequency analysis of dent bearing※1

5. Summary
Table 4 Conditions of the running test of dent bearing
Test bearing

Due to diversified market needs and changes in social
structure, machine tools are required to evolve withfurther enhancement of condition monitoring functionality
and support of IoT. In order to respond to these needs,
we have newly developed this "sensor integrated
bearing unit for machine tool spindles."With this development, we have achieved highly
sensitive condition detection compared with theconventional measurement on the main spindle outersurface. In addition, the responsiveness of condition
monitoring was improved by adopting heat flow sensors.
We will continue to refine this development for further
improvement of performance and commercialization. In
addition, we will work on the detection of otherconditions such as load, as well as enhancement ofcondition monitoring functionality.

φ70×φ110×20
Equivalent to 5S-2LA-HSE014(Ultra-high speed angular contact ball bearing with ceramic balls)

Pressurization method Fixed position preload (preload of 750N after incorporation)
Rotational speed

1,800min-1

Simulatedindentation

One location on the outer ring raceway
Depth of 60 μm (1.5mm×3.8mm)

Simulated indentation

3D image of indented bearing

Enlarged photo of indented bearing

References

Fig. 12 Dent on the bearing raceways

1) Naoki Matsumori, Keiichi Ueda: Technical Trend of thePrecision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No. 84 (2016) 40.
2) Keiichi Ueda: Technical Trend of the Precision Bearings
for Machine Tools, Bearing & Motion-Tech, September
2016 issue No. 002, 33.
3) NTN Catalog: Rolling Bearings, Catalog, A-17.
4) Keisuke Hashizume, Akitoshi Takkeuchi, Yuzuru
Tanaka: Development of Condition Monitoring System,NTN TECHNICAL REVIEW No. 82, (2014) 74.

Table 5 Calculation result of bearing frequency in 1800min-1
Number of rolling elements
passed against the outer ring(Hz)

1st order 2nd order 3rd order 4th order 5th order
341

682

1023

1364

1705

Photo of authors

Shouhei HASHIZUMEIndustrial Machinery Division
Application Engineering Dept.

Yasuyuki FUKUSHIMANew Product Development
R&D Center

Yusuke SHIBUYA

Yohei YAMAMOTONew Product Development
R&D CenterIndustrial Machinery Division
Product Design Dept.

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*10E_*09 19/07/03 15:09 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Angular Contact Ball Bearings for High-Speedand Heavy-Cutting Machine Tools
Jin TAKEGAHANA* Mineo KOYAMA*
Kouji JINNO**
Yuya TANAKA**NTN has developed new angular contact ball bearings, well suitable formachine tool spindles especially taking advanced multi-tasking which is thelatest trend of the manufacturing field. The bearings perform 30% higher basic
load rating and higher allowable axial load through internal design
optimization. Thus the bearings support the well balanced machine tools
show high-speed and heavy-cutting capability. In this document, we introduce
the design points and test data.

main spindle and workpieces are recently increasing.
The impact load applied to the bearings of the main
spindle due to collision may produce indentation on
the raceways of the bearing inner/outer rings
depending on the magnitude of the load, which
impedes smooth and high precision rotation of themain spindle. Therefore, bearings of the main spindle
are also required to have high load resistance toprevent/mitigate occurrence of indentation. To
respond to these trends and requirements, NTN has
developed bearings for main the spindle to support
high-speed rotation and heavy cutting based on thehigh-speed angular contact ball bearing "HSE Type."
In this article, we describe the features and theevaluation test results of the developed product.

1. Introduction
Recently, requirements for higher machine tool
performance are increasing in order to address thediverse needs of different industries.
The recent trends of these requirements include (1)"increase of machining and cutting of difficult-to
machine material and hard material like titanium, andrequirement of a more rigid main spindle to deal withthem," (2) "integration and multi-functionality ofmachining process for higher efficiency," (3)"requirement of five-axis tool for machining complex
shapes with high precision" and (4) "requirement forsmaller machine tools to adapt to the size of
workpiece." To cope with these trends, "one chucking
for the entire process," which is to process theworkpiece from heavy cutting in mid to low speed
rotation of the main spindle to refined cutting in highspeed rotation by one machine tool, is demanded. As
a result, the bearing for the main spindle of machine
tool needs to provide a high level of both high-speed
capability and high rigidity, which are in a trade-off
relationship (Fig. 1) 1).
Also, efforts for reducing non-cutting takt time are in
progress to "further increase productivity," which is an
important factor for machine tools; consequently, thespeed of the main spindle and table feed rate of recent
machine tools is increasing. In addition, recent
machine tools are required to process more complex
shapes. With these changes, unexpected collision
events between the tools attached to the tip of the(2) Integration of process
and multi functionality ofmachine tools(3) 5-axis machine tool

(1) High rigidity requirement for
main spindles to deal with machining
and cutting of difficult-to machinematerial and very hard material(4) Small form
factor of machinetool

One chuck for entire processing
One unit to process from mid/low speed rotation
range (heavy-cutting) to high-speed rotation
range (refined cutting)

Bearing to fulfill high-speed capability
and high rigidity in an elevated levelis required

Fig. 1 Technical trend of machine tools
and spindle bearings 1)**Application Engineering Dept., Industrial Machinery Division**Product Design Department, Industrial Machinery Division-56-

*10E_*09 19/07/03 15:09 ページ 2

Angular Contact Ball Bearings for High-Speed and Heavy-Cutting Machine Tools

smooth and highly precise rotation. This axial load
limit is called permissible axial load (stationary) andNTN defines it as the load leading to any of thefollowing 2) :· The end of the contact ellipse produced between therolling element and the raceway overtakes theshoulder either in the inner ring or outer ring (Fig. 4).
· The contact surface pressure of the raceway reaches
3,650 MPa※2 on either the inner or outer ring.

2. Background of development
NTN's high-speed angular contact ball bearing HSE
Type adapts for broad usage from high-speed to ultra
high-speed range. Consequently, many units havebeen adopted for main spindles of machine tools for along time. However, machine tools are expected to
evolve with even higher speed of main spindle rotation
for further productivity gain. Under thesecircumstances, adoption of the HSE Type is expected
to expand.
As another approach of productivity increase,machine tools actively incorporating integration ofmachining processes, multi functionality, and heavycutting capability are recently increasing. In thesecases, dmn value※1 of the main spindle rotational
speed is approximately 1.2 million and the bearings forthe main spindles are required to have both highspeed capability and rigidity, which are in a trade-off
relationship. Therefore, as shown in Fig. 2, thechallenge of the bearings for main spindles is to fulfill
both high-speed capability and high rigidity at anelevated level.

Impact load
Unexpected collision
between tools
and workpieces

High

Both high-speed and heavy-cutting are
required for main spindles and bearingsof main spindles for increasing productivityof machine tools

Main spindle rigidity
Low

In order to prevent/mitigate indentation, permissible
axial load (stationary) needs to be increased andtolerance against collision must be enhanced.

Fig. 3 Image of the unexpected collision between tool
and work piece, and the resulted dent on the bearing
raceways caused by the collision

Heavy
cutting

Lathe

0Multi-function
machine toolmachining center

Range supported bycurrent machine tools Grinder
50

100

High-speed
cutting

Mold machine tool

150

200

250

Main spindle high-speed capability (dmn value)×104

Indentation producedon raceway

High

Fa

2a

H

H : Height of bearing
shouldera : Semi-major axis ofcontact ellipse
Fa : Axial load

Fig. 2 New needs for main spindles and spindle bearingsof machine tools
Fig. 4 Overflow of the contact ellipse from the raceway

In addition, impact load (large axial load) may be
applied to the main spindle and bearings of main
spindles of machine tools due to unclamping load
while exchanging tools during off-operation or
unexpected collision of tool attached to the tip of themain spindle and the workpiece (Fig. 3).
When the impact load exceeds the tolerance,
indentation may occur on the raceway, impeding※1 dmn value expresses the rotational performance of bearings. dm (diameter of rolling element (mm)) × n (rotational speed (min-1))※2 Contact surface pressure 3,650MPa is a value to produce permanent deformation of 0.00002 to 0.00005 times the rolling element
diameter.

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*10E_*09 19/07/03 15:09 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

This load varies depending on the magnitude of the"lag-lead" of rolling elements and the magnitude of theload applied to the rolling elements (hereafter, rolling
element load). Since the developed product uses
larger rolling elements than the conventional product,as mentioned above, the rolling element load is likely
to increase. In addition, for heavy-cutting, a relatively
larger machining load is applied. Therefore, a large
load may be applied to the cage, as well. Therefore,
an enhanced cage with larger cross section than theconventional product was adopted to increase
reliability of the cage strength (Fig. 7).

3. Features of developed product3.1 [Support of heavy-cutting (1)]increase of load capacity
The developed product optimized the internal
design such as larger rolling element diameter, inorder to support heavy-cutting capability while still
maintaining the high-speed capability of theaforementioned HSE Type (hereafter, conventional
product) (Fig. 5). The developed product improved theload capacity, increasing the rated load by approx.
30% (Fig. 6).

Developed
product

Conventional product(HSE Type)

Fig. 5 Conventional design and developed design

80

Rated load kN

70
60
50
40

Basic dynamic load rating
Conventional productDeveloped productImprovement ofapprox. 30%

Conventional
product

Basic static load rating

Developed product(enhanced type)Fig. 7 Cross section of cage
(Comparison of conventional design and developed design)

Improvement ofapprox. 30%30

3.3 Increase of permissible axial load (stationary)20

The permissible axial load (stationary) of thedeveloped product is increased by approx. 30%
compared to the conventional product by optimizing
the internal design (Fig. 8). This will prevent/mitigate
indentation on the bearing raceway surface when
impact load is applied.

100Over twice
larger crosssection area

φ70

φ100

φ120

φ70

φ100

φ120

Bearing bore diameter mm

Fig. 6 Basic load rating of conventional design anddeveloped design (ceramic balls, contact angle 20˚)

3.2 [Support of heavy-cutting (2)]improvement of cage strength
Permissible axial load(stationary) kN

90

The rolling elements of rolling bearings rotate with socalled "lag-lead" which is a very small difference ofrelative speed due to geometrical accuracy of the
bearing, installation accuracy onto shaft and housing,external load, etc. The rolling elements with relatively
faster rotational speed tend to shift forward in the cage
pocket (traveling direction). The rolling elements withrelatively slower rotational speed tend to shift backwardin the cage pocket (opposite side of the traveling
direction). Consequently, the rolling elements makecontact with the cage pocket to apply load on the cage.

80

Conventional productDeveloped productImprovement of70

約 approx.
30％向上
30%

60
50
40
30
20
10
0

φ70

φ100

φ120

Bearing bore diameter mm

Fig. 8 Static allowable axial load(ceramic balls, contact angle 20˚)-58-

*10E_*09 19/07/03 15:09 ページ 4

Angular Contact Ball Bearings for High-Speed and Heavy-Cutting Machine Tools

4.2 Test results

4. Evaluation test of developed product

Test results are shown in Fig. 10 and 11. The
developed product shows rise of temperature similar
to the conventional product while increasing the load
capacity. It was verified that the operation exceeds
dmn value of 1.2 million, shown in Fig. 2, up to dmn
value of 1.62 million (18,000 min-1) (Fig. 10). It wasalso verified that the bearings other than row C also
showed a stable rise in temperature (Fig. 11).

4.1 Test condition
We have conducted high-speed operation tests ofthe developed product. The configuration of the testeris shown in Fig. 9 and the test conditions are shown in
Table 1.
This test used a bearing of inner ring inner diameter
70 mm, which is widely used for main spindles of themachining center. Assuming main spindles where highrigidity is required such as heavy cutting, a four-row
back-to-back arrangement (DTBT) fixed position
preload was used, setting 1,400 N of preload afterincorporation of bearing into the main spindle, which
exceeds medium preload of the conventional product.
In addition, the enhanced type of Fig. 7 was adopted
for the cage of the developed product and withphenolic resin material.

Rise of outer ring temperature of
row C bearing ˚C20(ddmn value 0.9 - 1.5 million)

Developed product

16
14
12

The developed productincreases load capacity
while maintaining the rise of
temperature similar to theconventional product.

10
8
6
4
2
0

Test bearing:
4-row back to backarrangement (DTBT)

Outer cylindercooling IN

High-speed/heavycutting range

Conventional product

18

0

5,000

0

0.5

10,00015,00020,000

Rotational speed min-1
1.0

dmn value x 106

1.5

1.8

Fig. 10 High-speed running test result (C-row bearing)
Row RowA
B

Row Row
C
D

Motor
20

Rise of temperature of outer ring
˚C

Outer cylindercooling OUT

15

Table 1 Test conditions

Cage
specification

φ70×φ110×20×4 rows (DTBT)
Equivalent to 5S-2LA-HSE014(Ceramic ball, contact angle 20°)
Conventional productPhenolic resin cage

Fixed position preload

Rotational speed

0〜18,000min-1

Preload after incorporation
Lubrication method

1,400N0.03mL/8min

Lubricating oil

ISO VG32

Attitude of axis

Horizontal axis

Amount of air

Outer cylinder cooling0

Developed
product

4.3 Permissible axial load (in operation)
Based on the temperature data obtained in the
aforementioned test, permissible axial load (inoperation) was calculated for each bearing in therange of dmn value of approximately 1.2 million. The
result indicated that when dmn value is 1.17 million(13,000 min-1), the developed product was Fa=9.8 kN
compared to the conventional product of Fa=6.8 kN,
verifying that the external load applicable to the main
spindle is increased by approx. 30%. In addition,
similar result was obtained for dmn value of 1.35 million(15,000 min-1) (Fig. 12).

Air oil lubrication

Supply oil amount

Conventional
product

5

Fig. 11 Outer ring temperature rise in spindle rotation
speed 15,000 min-1

Developed product

Preload method

Row Row Row Row
A
B C
D

10

Fig. 9 High-speed running test spindle

Test bearing

Row Row Row Row
A
B CD

35NL/minYes-59-

*10E_*09 19/07/03 15:09 ページ 5

Permissible axial load (in operation) kN

NTN TECHNICAL REVIEW No.86（2018）14

Conventional product12

same as Table 1 except that the cage specification isdifferent.
The test result revealed that the developed productwith high-melting point PA cage also showed a rise intemperature similar to the conventional product. It wasverified that the operation is possible, in addition to
dmn value of 1.2 million, shown in Fig. 2, up to dmn
value of 1.62 million (18,000 min-1) (Fig. 13). It wasalso verified that the bearings other than the row C
also showed a stable rise in temperature (Fig. 14). In
this test, conventional product with ordinary polyamide
resin (hereafter, PA) cage was also evaluated. The
result showed an unstable rise in temperature at dmn

Developed productImprovement of approx. 30%

10
8
6
4
2
0

15,000min-1
13,000min-1(dmn value 1.17 million) (dmn value 1.35 million)

Fig. 12 Dynamic allowable axial load(ceramic balls, contact angle 20˚)

Table 3 Test conditions

5. Application of injection molding
plastic cage

φ70×φ110×20×4 rows (DTBT)
Equivalent to 5S-2LA-HSE014(Ceramic ball, contact angle 20˚)

Test bearing
Cage
specification

5.1 Material of plastic cage
In the previous section, we verified that thedeveloped product shows equivalent rise intemperature as the conventional product by
optimization of the internal design. In this test,
phenolic resin (hereafter, PF) was used for the cage.
Since the phenolic resin is thermosetting, it has theadvantage of higher tolerance against a rise intemperature due to high-speed rotation of the bearing.
On the other hand, consideration is required for
productivity and cost since it is fabricated bymachining. In this section, adoption of injection
molded cage, which has an advantage in productivity
and cost, is also considered.
For high-speed operation of injection molded cage,research using high-melting point polyamide resin(hereafter, high melting point PA) shown in Table 2
has been conducted 3) .

Rise of outer ring temperature of
row C bearing ˚C

20

Conventional productDeveloped productPA cage

High-melting point
PA cage

PF cage

Conventional product(PF cage)
Conventional product(PA cage)

18
16
14

High-speed/heavycutting range(dmn value 0.9 - 1.5 million)

Developed product(high-melting point
PA cage)

12
10
8
6
4

Unstabletemperature at
15,000min-1

2
0

0

5,000

0

0.5

10,00015,00020,000

Rotational speed min-1
1.0

dmn value x 106

1.5

1.8

Fig. 13 High-speed running test result (C-row bearing)
Table 2 Characteristics of resin cage material

Category

Glass transition
temperature
Melting point

Production method

High-melting
point polyamide
Polyamide(high-melting point(PA)
PA)
Thermosetting
Thermoplastic

20

Phenol(PF)

Cutting

158˚C

58˚C

158˚C

260˚C

Rise of temperature of outer ring
˚C

Material

15

Row Row Row Row
A B
C D

Row Row Row Row
A B
C D

Row Row Row Row
A B
C D

10

Injection molding

5.2. Evaluation test
Enhanced cage was fabricated with high-melting
point PA in Fig. 7, and high-speed operation test of
the developed product was conducted under theconditions of Table 3. The test conditions were theUnstabletemperature
although low

50Conventional
product(PF)

Conventional
Developed
product
product(PF)(high-melting point PA)

Fig. 14 Outer ring temperature rise in spindle rotation
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Angular Contact Ball Bearings for High-Speed and Heavy-Cutting Machine Tools

value of 1.35 million (15,000 min-1) and could notsupport the high-speed/heavy-cutting range (dmn value
0.9 - 1.5 million). We will proceed with volume
production of the developed product, considering
adoption of the high-melting point PA cage, as well.

6. Summary
We have developed "Angular Contact Ball Bearingsfor High-Speed and Heavy-Cutting Machine Tools" inorder to meet new trends such as integration ofmachining processes and multi-functionality ofmachine tools. The developed product optimizes theinternal design based on the conventional high-speed
angular contact ball bearing "HSE Type," byincreasing load capacity and permissible axial load byapprox. 30% while maintaining high-speed capability.
In addition, adoption of high-melting point polyamide
resin (high-melting point PA) with advantageous
productivity and cost was also studied for thedeveloped product. We will also conduct performance
evaluation with grease lubrication.
We are poised to continue working on the
improvement of bearing performance in order to support
higher functionality of machine tools.

References
1) Naoki Matsumori, Keiichi Ueda: Technical Trend of
the Precision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No. 84, (2016) 40.
2) NTN Catalog: Precision Rolling Bearings, CAT. No.
2260-VII/J. 16.11.03 21.
3) Atsushi Tokuda, Takumi Hayashi: Impact of Heat
Resistance of Plastic Cage for High-Speed
Operation of Angular Contact Ball Bearing,
Japanese Society of Tribologists, Tribology
Conference 2017 Fall Takamatsu

Photo of authors

Jin TAKEGAHANA

Mineo KOYAMA

Kouji JINNO

Yuya TANAKA

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Product Design Dept.

Industrial Machinery Division
Product Design Dept.

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*11E_*09 19/07/03 15:09 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Machine Tool Main Spindle Bearings with Air Cooling Spacer
for Grease Lubrication
Keisuke NASU*
Yuya TANAKA**

Naoya OKAMOTO**
Tomohiko OBATA***

Recently, machine tools are required multitasking for various
processes and works. Though high speed and high rigidity ofspindle bearings are trade-off relationship each others, they aredemanded to achieve higher level performancecoinstantaneously. And the application of grease lubricatedspindle bearings has been increasing in the last years because of the strongly trend of the simplification of devices
and systems and environmental concerning. NTN has been developing the application of the air cooling spacer to thegrease-lubricated spindle bearings to meet the above mentioned two requirements.

Machine tool main spindle bearings with air coolingspacers are the proprietary technology that NTN
developed and is used in products with air oil
lubrication.
In this article, functional evaluation of the cooling
effect, in particular, when air cooling spacers areapplied to grease lubrication and the arrangement forcommercialization are described.

1. Introduction
The required properties of main spindles varydepending on the machining needed. High rigidity andaccuracy spindle are required when working difficult-to
machine material or partial processing of complex
shapes. On the other hand, high-speed and highly
precise rotation is required for machining molds formachine components and components used in the
medical field. Therefore, demand for 5-axis machine
tools with multiple machining capabilities and complex
machine tools, which require bearings for main
spindles, to offer both high speed and high rigidity are
increasing 2).
Demand for bearings in the main spindles withgrease lubrication is increasing for simplification offacilities and reduction of environmental burden. One
concern of grease lubricated bearing is that heat
accelerates degradation of grease thus significantly
impact the lubrication life 3). Therefore, it is critical toreduce heat during the operation of the bearings.
There have been several measures taken to meet thischallenge. For example, grease with superior highspeed properties, and heat resistance andoptimization of internal design 4) In this development
we have attempted to apply air cooling spacers to tool
main spindle bearing 1) to grease lubrication as a new
approach.

2. Structure and cooling mechanism of air
oil lubrication air cooling spacer
In the following, we will describe the basic structure
and cooling mechanism of the "machine tool main
spindle bearings with air cooling spacers," by using
actual application example..
Air nozzles are created in the outer ring spacer
which is inserted between two back to back angularcontact ball bearings (Fig. 1). These nozzles are atoffset positions from the axial center and the cooling
air in normal temperature injected from these nozzles
goes through the space between the inner and outerspacers, as well as inside the bearings, swirling in therotational direction of the inner ring. The cooling air
removes heat from the surface of the inner ring spacer
to cool it down.

***Application Engineering Dept., Industrial Machinery Division***Product Design Department, Industrial Machinery Division***Advanced Technical Research Center-62-

*11E_*09 19/07/03 15:09 ページ 2

Machine Tool Main Spindle Bearings with Air Cooling Spacer for Grease Lubrication

As shown in the fluid analysis result of Fig. 2, whencooling air is injected vertically to the inner ring spacer(left figure), the cooling air immediately spreads out inthe axial direction, which minimize the cooling effect.
On the other hand, when the air nozzles of the cooling
air are offset (right figure), the cooling air stays aroundthe surface of the inner ring spacer longer to extend
the time for the cooling air to remove heat from theinner ring spacer surface, which increases the cooling
effect of the inner ring spacer.
As inner ring spacer cooled down, the adjacentbearing inner ring also cooled down. Usually, when

Cooling air nozzle

Bearing with
Air Cooling Spacer

Air cooling spacer(outer ring spacer)

bearings rotate, the inner ring is more heated than theouter ring. Therefore, the preload increases by thedifference in thermal expansion. In this coolingtechnology, the difference of temperature between the
bearing inner ring and outer ring can be reduced bycooling the inner ring. As a result, preload and contact
surface pressure on the raceway can be reduced toachieve both high speed and high rigidity at a highlevel. Table 1 and Fig. 3 show an example of test
results of the machine tool main spindle bearing with
air cooling spacer 1).

Cooling air nozzle

OffsetCooling air (inlet)

Eco-friendly nozzle
for air oil lubrication

Air cooling spacer(outer ring spacer)
Inner ring
spacerInner ring
spacerDirection of
inner ring rotation
Cooling air (outlet)

Cooling air (outlet)

View A

A

Fig. 1 Machine tool main spindle bearings with air cooling spacer (for air oil lubrication) 1)

OffsetInner ring spacer
outer diameter surface

Cooling air nozzle

Direction of
inner ring rotation

Direction of
inner ring rotation
Cooling air nozzle

Cooling air nozzle

Cooling air nozzle
Slow

Outer ring spacer
inner diameter surface

Cooling air nozzle

Cooling air nozzle

Flow rate

Fast

Slow

Flow rate

Fast

When cooling nozzle is not offset

When cooling nozzle is offset

Cooling air collides to inner ring spacer vertically
immediately after injected from air cooling nozzle andspreads to axial direction. Therefore, the cooling air only
stays on the surface of the inner ring spacer for a shorttime, with insufficient cooling effect.

Having the air cooling nozzle in an offset position from the centerof the shaft, the cooling air swirls in the inner ring radial direction
and stays on the surface of the inner ring space for a long time.
Therefore, the cooling air can remover more heat from thesurface of the inner ring spacer, increasing the cooling effect of
the inner ring spacer and inner ring.

Fig. 2 Cooling air flow between inner and outer spacers (Fluid analysis results) 1)-63-

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NTN TECHNICAL REVIEW No.86（2018）

From the test results in Fig. 3, we verified thecooling effect of the "air-oil lubrication bearings with air
cooling spacers" by cooling air from low-speed to highspeed rotation, as well as the fact that the cooling
effect is greater as the cooling air supply is larger.

Table 1 Test conditions for high speed running test of themachine tool main spindle bearings with air cooling spacer(air oil lubrication)
Test bearing

φ70×φ110×20
Equivalent to 5S-2LA-HSL014(Eco-friendly air-oil lubrication angularcontact ball bearing with ceramic balls)

Preload method

Fixed position preload (preload of 0N after incorporation)

Lubrication method

Air oil lubrication

Lubricating oil

ISO VG32

Rotational speed

3. Structure of grease lubrication air
cooling spacer

0 - 23,000min-1
0.03mL/10min

Supply oil amountFig. 4 shows the structure of the "machine tool main
spindle bearing with air cooling spacer" when it isapplied to grease lubrication. The cooling air nozzles
are formed in the outer ring spacer. It is then inserted
between the back to back angular contact ballbearings with grease lubrication seals.

30NL/minLubricating air flow

Outer cylinder cooling Yes, thermal tuning to room temperature (21±1˚C)

Inner/outer ring temperature difference ˚CAttitude of axis

30

Horizontal axis

4. Cooling effect of grease lubrication
air cooling spacer

No cooling air

25

Cooling air 100NL/min

20

Cooling air 300NL/min

Cooling air 200NL/min

A high-speed operation test was conducted to verify
the cooling effect of the "machine tool main spindle
bearing with air cooling spacer" when it is applied togrease lubrication. The test condition is shown in
Table 2 and the structure of the tester is shown in Fig.
5.
In this test, the upper limit of the cooling air supply
was set to 150 NL/min. considering the commercializedversion. The offset amount of the nozzles was set to80% of the radius of the inner ring spacer outer
diameter, based on previous test results 5).
Fig. 6 shows the test results of the high-speed
operation test. It was verified that the inner/outer ring

15
10
5
0
-5

0

5,000

10,00015,00020,00025,000

Rotational speed min-1

Fig. 3 Result of high speed running test of the machine tool
main spindle bearings with air cooling spacer (air oil lubrication)

Cooling air nozzle

Air cooling spacer(outer ring spacer)

Bearing with
Air Cooling Spacer

Cooling air nozzle

OffsetCooling air (inlet)

Air cooling spacer(outer ring spacer)
Inner ring
spacerInner ring
spacerDirection of
inner ring rotation
Cooling air (outlet)

Cooling air (outlet)

View A

A

Fig. 4 Machine tool main spindle bearings with air cooling spacer (for grease lubrication)-64-

*11E_*09 19/07/03 15:09 ページ 4

Machine Tool Main Spindle Bearings with Air Cooling Spacer for Grease Lubrication

Inner/outer ring temperature difference ˚Ctemperature difference at 20,000 min-1 (dmn value 1.8
million) decreases approx. 2.5˚C with 150 NL/min ofcooling air supply compared to no supply. Because ofthis decrease of inner/outer ring temperature
difference, the maximum contact stress on the
raceway surface at 20,000 min-1 can be reduced byapprox. 10%. These results bring about the following
effect. (1) The temperature at 18,000 min-1 with no
cooling air supply and the temperature at 20,000 min-1
with supply of 150 NL/min cooling air are equivalent,
which enables higher speed by approx. 10%. (2)Preload of the bearings after incorporation into themain spindle can be increased, which enables
increase of main spindle axial rigidity by approx. 10%(Fig. 7). In addition, improvement of lubrication
durability can be expected. This point will be further
verified.

Preload method

Fixed position preload (preload of 40N after incorporation)

Lubrication method

Grease lubrication

Rotational speed
Grease

10
8
6
4
2
0

0

50

0〜20,000min-1
SE-1

Outer cylinder cooling Yes, thermal tuning to room temperature (21±1˚C)
Attitude of axis

Cooling air 150NL/min

5,000

10,00015,00020,000Fig. 6 Result of high speed running test of the machine tool
main spindle bearings with air cooling spacer(grease lubrication)

Axial displacement µm

Test bearing

No cooling air

12

Rotational speed min-1

Table 2 Test conditions for high speed running test of themachine tool main spindle bearings with air cooling spacer(grease lubrication)
φ70×φ110×20
Equivalent to 5S-2LA-BNS014LLB(High speed angular contact ball bearing
with grease lubrication seals and ceramic balls)

14

Increase of axial rigidity by approx. 10%
when Fa=1.0 kN

40
30
20
10

No cooling air

Horizontal axis00.0

Cooling air 150NL/min
0.2

0.4

0.6

0.8

1.0

Axial load kN
Cooling air (inlet)

Fig. 7 Relation between cooling air amount andaxial rigidity (grease lubricated air cooling spacer)

Bearing with
Air Cooling Spacer

5. Challenges for commercialization ofgrease lubrication air cooling spacer

Motor

Cooling air (outlet)

Spacerexhaust port

From the results of the previous Section, it wasverified that the machine tool main spindle bearing
with air cooling spacer has cooling effect for greaselubrication, as well. On the other hand, by observing
the bearing after the high-speed operation test, grease
was found attached in the seal gap of the bearing withcooling air supply, as shown in Fig. 8 . A small amountof grease was observed leaking from the width surfaceof the bearing.
In order to increase lubrication durability of the
bearings, it is desirable to hold as much grease as
possible inside the bearings. Therefore, it is necessary
to reduce the leakage of grease from the width surface

Cooling air (outlet)

Fig. 5 Structure of test spindle(grease lubricated air cooling spacer)-65-

*11E_*09 19/07/03 15:09 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

Outer
ring

Outer
ring

Inner
ring

Inner
ring

No supply of cooling air

Bearing after test with
150 NL/min of cooling air supply

spacer was divided into two along with the introductionof the slope for ease of assembly.
The improved spacer with these changes in
specification is shown in Fig. 10 (a). The improved
spacer is expected to provide better cooling effect as
the amount and time of adhesion of air to the innerring are increased from the standard spacer. As
shown in the result of the analysis of Fig. 10 (b), asmooth flow in the tangential direction of the inner ring
was verified with reduction of flow into the bearings.
Table 4 shows the percentage of cooling air flown
to the bearings and exhaust port, and the ratio of heattransfer between the standard spacer and theimproved spacer. With the standard spacer, 50% of
the supplied air flows through the bearings; however, it
is reduced to approximately 10% with the improved
spacer. The heat transfer also shows improvement of
50%. Improvement of cooling effect can be expectedwith grease lubrication, while preventing greaseleakage by using the improved spacer.

(Observation of bearing width surface)
Outer
ring

Outer
ring

Inner
ring

Inner
ring

No supply of cooling air

Bearing after test with
150 NL/min of cooling air supply

Leaked grease(Observation of bearing width surface without seal)

Fig. 8 Grease status inside the bearing
after the high speed running test

Table 3 Fluid analysis conditionsof the bearings to a minimum. A possible cause of thegrease leakage was the flow of cooling air into thebearings. Thus, a change of the spacer shape wasstudied to let the air exhaust from the exhaust port
created on the air cooling spacer, so that the air does
not flow into the bearings.
The cooling air flow was simulated under the fluid
analysis condition shown in Table 3.
Fig. 9 (a) shows the enlarged view of the standard
spacer shape evaluated with Fig. 5. Fig. 9 (b) shows
the result of the analysis. When cooling air is supplied
to the nozzles of the standard spacer, the cooling air
collides with the inner ring spacer and then spreads
out to the axial direction with a fast flow rate while
maintaining a fast flow rate in the radial direction. A
part of the injected cooling air reaches the exhaust
port placed on the outer ring spacer; however, a partflows into the bearings. It was estimated that thegrease leaked, affected by the air flow into thebearings.
The cooling air flow in the axial direction can becontrolled by increasing the gap between the nozzle
and the inner ring spacer. By securing sufficient spaceat the opening of the nozzle, air can flow adhering to
the inner ring, maintaining the flow rate toward thetangential direction, which is the direction of injection.
In addition, by creating a slope with large inclination
on each side of the inner spacer, the air flow into thebearings can be reduced by guiding the air to theexhaust port, not in the axial direction. The inner ring

Inner ring rotational speed

19,100min-1

Air pressure

0.2MPa(Equivalent to 150 NL/min of air supply)

Temperature

Inner ring 50˚C; other 27˚C

Air cooling spacer(outer ring spacer)

Spread in theaxial direction

Inner ring spacer

Fig. 9 (a) Standard designed Fig. 9 (b) Cooling air flow ofspacer shapeStandard designed spacer(Fluid analysis results)
Air cooling spacer(outer ring spacer)

Divide theInner ring
spacer intotwo

Flow to the tangential
direction on the
inner ring

Increased
gap

Slope

Fig. 10 (a) Development
designed spacer-66-

Fig. 10 (b) Cooling air flow ofdevelopment designed spacer(Fluid analysis results)

*11E_*09 19/07/03 15:09 ページ 6

Machine Tool Main Spindle Bearings with Air Cooling Spacer for Grease Lubrication

References

varies significantly and the cooling air flow canchange depending on the drive methods and exhaust
methods. Therefore, we will develop prototypes andcontinue verifying the superiority of the improved
spacer.

1) Keisuke Nasu, Naoya Okamoto and Masato
Yoshino: Machine Tool Main Spindle Bearings with
Air Cooling Spacers, NTN TECHNICAL REVIEW,
No. 84, (2016) 52.
2) Naoki Matsumori, Keiichi Ueda: Technical Trend of
the Precision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No. 84, (2016) 40.
3) Takayuki Kawamura: Research on the Lubrication
Mechanism of Grease for High Speed Bearings,
NTN TECHNICAL REVIEW, No. 76, (2008) 39,
Machine Design, Vol. 60, No. 1 (Jan issue, 2016)28.
4) NTN Catalog: Precision Rolling Bearings, CAT. No.
2260-VII/J. 16.11.03.82.
5) Yushi Onda, Kikuo Fukada, Yohei Yamamoto andMasato Yoshino: Machine Tool Main Spindle
Bearings with Air Cooling Spacers, NTN
TECHNICAL REVIEW, No. 82, (2014) 38.

Table 4 Fluid analysis results ofstandard spacer and development spacer

Flow rate of thecooling air

Exhaust port
Inside bearing

Heat transfer ratio

Standard
spacerImproved
spacer

41%
59%

87%
13%

1

1.5

6. Summary
Demand of bearings for main spindles using greaselubrication is expected to grow for higher functionality
and reduced environmental burden of machine tools.
At NTN, we have attempted to apply the "machine tool
main spindle bearing with air cooling spacer," whichuses NTN's proprietary air cooling technology togrease lubrication as a new approach and verified its
effect. We will continue to work on improving theperformance and refinement of the product, such as
ensuring grease retention, toward commercialization.

Photo of authors

Keisuke NASU

Naoya OKAMOTO

Yuya TANAKA

Tomohiko OBATA

Industrial Machinery Division
Application Engineering Dept.

Industrial Machinery Division
Product Design Dept.

Industrial Machinery Division
Product Design Dept.

Advanced TechnicalResearch Center-67-

*12E_*09 19/07/03 15:10 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]ULTAGE Precision Standard Angular Contact Ball Bearing,
72U Series*Takayuki KITANO**Youhei YAMAMOTO*

NTN has developed the new ULTAGE precision standard angular contact
ball bearing, the 72U series for productivity improvement and energy reducing
consumption of industrial machinery with a focus on machine tool. The newly
developed bearing has adopted the new phenol resin cage. We introduce thedesign points and results of the above mentioned tests of the “NTN’s ULTAGE
precision standard angular contact ball bearing 72U series” that have created a new epoch in the areas of load
resistance, high-speed and lubrication performance in this paper.

this successful outcome and the market trend, we
have newly developed the 72U Series, which is thehigh-function angular contact ball bearing in thedimension series "72."

1. Introduction
Recently, further increases in productivity andreductions in energy consumption are demanded inindustrial fields including machine tools, in view ofsocial challenges such as the reduction of the laborpool due to decreasing birthrates and aging
population, and environmental issues. To respond tothese demands, higher output and higher efficiency
are required.
Bearings are one of the key components ofindustrial machines, often referred to as "essential andindispensable in the industry." Many bearings areused in the rotational parts of machinery affecting heatdissipation, efficiency and output; therefore,
improvement in performance of the bearings is the key
to achieving improvement in performance ofmachinery.
Angular contact ball bearings have higher load
carrying capacity and rigidity than deep grove ballbearings, and are used in many machines in broad
industry fields.
NTN has increased performance of angular contact
ball bearings of the "70" and "79" dimension series,
which are broadly used as bearings for machine tool
main spindles, and is already marketing them as
70U/79U Series in the line-up of high functional
products under the name of "ULTAGE"※1 2). Based on

2. Features of developed product
The precision standard type angular contact ballbearings are superior in load carrying capacity andrigidity compared with the angular contact ballbearings of special high-speed specification, which
use rolling elements of smaller diameter 3); however,their application was limited to lower speed.
In this development, the goal was set to expand theapplication range and further increase the load
carrying capacity while ensuring both rigidity and highspeed capability, which are in a trade-off relationship,by reviewing the internal design of the 72 Series -which intrinsically has a larger load carrying capacity
and higher rigidity than the 70U/79U Series.

2.1 Load resistance
The newly developed product, 72U Series, achieved
approximately 1.1 times greater basic dynamic load
rating, approximately 1.2 times greater basic staticload rating and approximately 1.3 times longer basic
rating life compared to the conventional product (Fig.
1 and Fig. 2).

*Product Design Department, Industrial Machinery Division※1 ULTAGE is a name created from the combination of "ultimate," and "stage,” signifying performance in any applications expressing
NTN’s idea of exploring the ultimate performance of precision bearings 1).

-68-*12E_*09 19/07/03 15:10 ページ 2

ULTAGE Precision Standard Angular Contact Ball Bearing, 72U Series· The contact surface pressure of the raceway reaches
3,650 MPa※2 on either the inner or outer ring.
※2 Contact surface pressure 3,650MPa is a value to produce
permanent deformation of 0.00002 to 0.00005 times therolling element diameter.

Fa

s

ime

1.1 t

40
30
20

0

43kN

47kN

Conventional
product

Developed
product

s

ime

1.1 t

150
100
50
0

【Developed product】7210UC
Part number 【Conventional product】7210C
φ50mm ×φ90mm ×20mm
of bearing
Contact angle 15˚steel ball specification

149kN

165kN

Conventional
product

Developed
product【Developed product】7220UC
Part number 【Conventional product】7220C
φ100mm ×φ180mm ×34mm
of bearing
Contact angle 15˚steel ball specification

Fig. 1 (a) Basic dynamic load rating (7210C)

Fig. 1 (b) Basic dynamic load rating (7220C)

50

200

40

Basic Staic load rating kN

Basic Staic load rating kN

H

200

50

10

2a

H : Height of bearing
shouldera : Semi-major axis ofcontact ellipse
Fa : Axial load

Fig. 3 Overflow of the contact ellipse from the raceway

Basic dynamic load rating kN

Basic dynamic load rating kN

For machine tool main spindles and bearings,
impact load (large axial load) may be applied due tounclamping load while exchanging tools during offoperation or unexpected collision of tool attached tothe tip of the main spindle and the workpiece.
When the impact load exceeds the design limit,
indentation may occur on the raceway, impedingsmooth and highly precise rotation. This axial load
limit is called permissible axial load (stationary) andNTN defines it as the load leading to any of thefollowing 4).
· The end of the contact ellipse produced between therolling element and the raceway reaches theshoulder of either the inner ring or outer ring
raceway. (Fig. 3).

es

im
1.2 t

30
20
10
0

31kN

Conventional
product

37kN

150
100
50
0

Developed
product

s

ime

1.2 t

127kN

147kN

Conventional
product

Developed
product【Developed product】7210UC
Part number 【Conventional product】7210C
of bearing
φ50mm ×φ90mm ×20mm
Contact angle 15˚steel ball specification【Developed product】】7220UC
Part number 【Conventional product】7220C
φ100mm ×φ180mm ×34mm
of bearing
Contact angle 15˚steel ball specification

Fig. 2 (a) Basic static load rating (7210C)

Fig. 2 (b) Basic static load rating (7220C)-69-

*12E_*09 19/07/03 15:10 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

temperature under high speed operation, an optimum
internal design of rolling elements and raceway
shapes plus consideration of supply and drainage of
lubricating oil are required.
When grease lubrication is used, consideration for
grease deterioration due to heat (namely, grease life)
is important; therefore, an internal bearing design for
grease retention is required.
Even if the conventional product uses the cage
made of polyamide resin, the newly developed productadopts the cage made of phenolic resin which has
superior properties for high-speed operation with both
air-oil lubrication and grease lubrication, and adopts a
new shape designed for improved lubricating
capability. The key points are described in thefollowing section.

The newly developed 72U Series achieved
approximately 2.5 times larger permissible axial load,
significantly exceeding the conventional product, byreviewing the internal design such as the size of theheight of shoulder (Fig. 4 and 5).

es

40

7

tim

30

2.

Permissible axial load kN

50

20
10
0

15kN

41kN

Conventional
product

Developed
product

2.2.1 New-shape phenolic resin cage
The groove on the inner diameter surface of the
new-shape phenolic resin cage (hereafter, new-shapecage) was deepened compared with the standard
design to achieve the following two objectives (Fig. 6):(1) Stabilize cage behavior by improving the cage sothat it is actively guided to the outer ring, arranging
the contact position of the rolling elements andcage pockets.
(2) Extend the grease life by improving the grease
retention, expanding the space for holding thegrease.

【Developed product】7210UC
Part number 【Conventional product】7210C
φ50mm ×φ90mm ×2mm
of bearing
Contact angle 15˚steel ball specification

Fig. 4 Allowable axial load (7210C)

tim
es

150

4

100

Phenolic resin cage
Polyamide resin cage
[Conventional product] 【Standard shape】 【New shape】

2.

Permissible axial load kN

200

50

0

67kN

Conventional
product

162kN

Developed
product【Developed product】7220UC
Part number 【Conventional product】7220C
φ100mm ×φ180mm ×34mm
of bearing
Contact angle 15˚steel ball specification

Fig. 5 Allowable axial load (7220C)

Fig. 6 (a) Appearance of resin cage

2.2 High-speed capability (lubricating capability)

Phenolic resin cage
Polyamide resin cage
[Conventional product] 【Standard shape】 【New shape】

For air-oil lubrication, not only the supply of
lubricating oil into the bearing but also the drainage of
lubricating oil out of the bearing is important. Withoutsmooth supply and drainage, lubricating oil may
remain inside the bearing without good heat transfer,resulting in a possible rise in temperature andpotential seizure 5), 6). To maintain a stable<Objectives of inner diameter groove>· Stabilize cage behavior· Improve grease retention performanceFig. 6 (b) Bore diameter shape of resin cage-70-

*12E_*09 19/07/03 15:10 ページ 4

ULTAGE Precision Standard Angular Contact Ball Bearing, 72U Series

2.2.2 Modification of inner ring shape

Fig. 7 and 8 show the results of high-speed
operation test of the cages with standard shape andnew shape. It was verified that the temperature of the
bearing outer ring of the new-shape cage, newly
adopted this time (○ in the graph) was equivalent toor below the standard shape cage (□ in the graph),both for air-oil lubrication and grease lubrication.
Rise of temperature of outer ring
˚C

30

Since the inner ring outer diameter (front side),which is the loaded side of the newly developed 72U
Series, affects the permissible axial load, dimensions
such as the shoulder height were reviewed. On theother hand, the inner ring outer diameter (back side),which is the anti-loaded side, was designed with lower
height than the conventional product to make it easier
to supply lubricating oil and grease injection between
the cage and the inner ring raceway surface by
combining it with the aforementioned new-shape cage.
Particularly in the case of air-oil lubrication, this allows
supply of lubricating oil from the slanted direction(nozzle target), which was not possible with theconventional product, reducing the restriction of sizefor installing a spacer with nozzle between bearings,
improving the degree of freedom for designing theperipheral structure of the bearings (Fig. 9).

New-shape phenolic resin cage

25

Standard-shape phenolic resin cage

20
15
10
5
0

0

5,000

10,00015,00020,00025,00030,000

0

35

70

105

140

175

210

Rotational speed min-1[Test condition]

dmn value x 104

φ50mm ×φ90mm × 20mm double-row
Contact angle 15˚, steel ball specification

Preload after incorporation

1.275 N (Constant pressure preload method)

Lubrication method

Air oil lubrication
40NL/min0.03 mL/5min /1 shot
ISO VG32

Amount of air
Amount of oil supply
Oil type

Outer cylinder cooling

Conventional
product

7210UCDB

Part numberof bearing

Inner ringouter diameter(front side)

Yes

Fig. 7 High-speed test of standard and new cage(air-oil lubrication)

Rise of temperature of outer ring
˚C

30

Developed
product

New-shape phenolic resin cage

25

Standard-shape phenolic resin cage

20

Inner ringouter diameter(front side)

15
10
5
0

Inner ringouter diameter(back side)00[Test condition]
Part numberof bearing

5,000
10,000
Rotational speed min-1
35

70

105

Fig. 9 Section of ULTAGE precision standard
angular contact ball bearing 72U series

7210UCDB

φ50mm ×φ90mm × 20mm double-row
Contact angle 15˚, steel ball specification

Preload after incorporation

980 N (Constant pressure preload method)

Lubrication method
Grease brandGrease lubrication

Outer cylinder cooling· Inner ring outer diameter (back side) is designed
lower than the conventional product· Space for supply of lubricating oil and grease injection
is significantly expanded by combining it with the
new-shape cage

15,000dmn value x 104

Inner ringouter diameter(back side)

SE-1

Yes

Fig. 8 High-speed test of standard and new cage
(grease lubrication)-71-*12E_*09 19/07/03 15:10 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

2.3 Rigidity

3. Operation test results

In order to achieve both rigidity and high-speed
capability of the newly developed 72U Series, which
are in a trade-off relationship, we checked the impactof the internal bearing design of rolling elements andraceway shapes on rigidity, and when found to besignificant, restricted the change to the minimum sothat the rigidity can be maintained.
The comparison of the rigidity between the newly
developed product and the conventional product isshown in Fig. 10 and 11. Although the rigidity of thedeveloped product is slightly lower than theconventional product, it is almost at the same level.

The operation test results under a constantpressure preload of the newly developed 72U Series
and the conventional product are shown in Fig. 12 and13. It was verified that the temperature of the bearing
outer ring of the developed product (○ in the graph)
was equivalent to or below the conventional product(△ in the graph), both for air-oil lubrication and greaselubrication.
This developed product achieved improvement ofload carrying capacity and permissible axial load. On
the other hand, rigidity of this developed product ismaintained at the same level as the conventional
product, and by combining with the aforementioned
new-shape cage, high-speed capability is also
maintained at the stable temperature similar to or
below that of the conventional product, achieving
approximately 1.6 times larger dmn value※3 of 1.8
million for air-oil lubrication and approximately 1.1
times larger dmn value of 1 million (both with contact
angle of 15˚ and use of steel balls).

Difference of displacement at Fa = 3 kN: approx. 0.8 μm
Axial displacement µm

40Developed product30

Conventional product

20
10
0

0

1

Axial load kN

2※3 dmn value expresses the rotational performance of bearings.
dm (diameter of rolling element (mm) x n (rotational speed(min-1)

3【Developed product】7210UCDBPart number 【Conventional product】7210CDB
of bearing
φ50mm ×φ90mm × 20mm double-row

30

Rise of temperature of outer ring
˚CContact angle 15˚, steel ball specification

Preload afterincorporation GN preload (fixed position preload method)

20
15

Fig. 10 Axial rigidity (7210CDB)

10Difference of displacement at Fa = 5 kN: approx. 0.4 μm
Axial displacement µm

40

5
0

0

5,000

10,00015,00020,000
Rotational speed min-1

25,00030,000

0

35

70

175

210

Developed product30

Conventional product20[Test condition]

100Developed product
Conventional product

25

Part numberof bearing
0

1

2

3

4

5

【Developed product】7220UCDBPart number 【Conventional product】77220CDB
of bearing
φ100mm ×φ180mm ×34mm double-row
Contact angle 15˚, steel ball specification

Preload afterincorporation GN preload (fixed position preload method)

Fig. 11 Axial rigidity (7220CDB)

105

140

dmn value x 104【Developed product】7210UCDB【Conventional product】7210CDB
φ50mm ×φ90mm × 20mm double-row
Contact angle 15˚, steel ball specification

Preload after incorporation

1,275 N (Constant pressure preload method)

Lubrication method
Amount of air
Amount of oil supply
Oil type

Air oil lubrication
40NL/min0.03 mL/5min /1 shot
ISO VG32

Outer cylinder coolingYes

Fig. 12 Running test (air-oil lubrication)-72-

*12E_*09 19/07/03 15:10 ページ 6

ULTAGE Precision Standard Angular Contact Ball Bearing, 72U Series

5. Summary

Rise of temperature of outer ring
˚C

30

Developed product
Conventional product25
20

The newly developed ULTAGE Precision Standard
Angular Contact Ball Bearing 72U Series achieved
improved load resistance and equivalent or betterhigh-speed capability compared to the conventional
products, while maintaining rigidity.
The developed product is one of the precision
bearings that satisfies the market requirement formachine tools and other industrial machines withongoing requirements for higher functionality.
We are poised to work on the improvement of theexisting Series including the Standard Series anddevelopment of new Series for our precision angularcontact ball bearings.

15
10
5
0

0

5,000
10,000
Rotational speed min-1

15,000

0

35

105

70

dmn value x 104[Test condition]【Developed product】7210UC【Conventional product】7210C

Part numberof bearing

φ50mm ×φ90mm × 20mm double-row
Contact angle 15˚, steel ball specification

Preload after incorporation

980 N (Constant pressure preload method)

Lubrication method
Grease brandGrease lubrication

Outer cylinder cooling

Yes

References
1) Hiroshi Takiuchi and Futoshi Kosugi, "ULTAGE"
Series Precision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No. 72, (2004) 26 page.
2) Keiichi Ueda, ULTAGE Standard Angular Contact
Ball Bearings, 79U/70U type, NTN TECHNICALREVIEW, No. 72 (2004) 30 page.
3) Hiroshi Tako and Yasutsugu Tanaka: Technical
Trend of Machine Tool Bearings, NTN
TECHNICAL REVIEW, No. 78, (2010) 8 page.
4) NTN Catalog: Precision Rolling Bearings, CAT. No.
2260-VII/J. 16.11.03, (2016) 21 page.
5) Keiichi Ueda, Technical Trend of the Bearings for
Machine Tools, Eco-friendly Technologies, THE
TRIBOLOGY No. 188, (2003, 4) 19 page.
6) Keiichi Ueda, Technical Trend of the Precision
Bearings for Machine Tools, NTN TECHNICALREVIEW, No. 84, (2016) 40 page.

SE-1

Fig. 13 Running test (grease lubrication)

4. Series configuration
The configuration of the newly developed 72U
Series is shown in Table 1. The inner diameter of the
bearings is configured from 10 to 130 mm with threecontact angles (15˚, 25˚ and 30˚). We also added
ceramic balls in addition to steel balls for rolling
elements.

Table. 1 ULTAGE precision standard angular contact
ball bearing 72U series
Steel ball
specification
Ceramic ball
specification

Contact angle 15˚

72XXUC

Contact angle 25˚

72XXUAD

Contact angle 30˚

72XXU（A）

Contact angle 15˚

5S-72XXUC

Contact angle 25˚

5S-72XXUAD

Contact angle 30˚

5S-72XXU（A）

Photo of authors

Supported bearing size: inner diameter from 10 to 130 mm(common to both specifications)-73-

Takayuki KITANO

Yohei YAMAMOTO

Industrial Machinery Division
Product Design Dept.

Industrial Machinery Division
Product Design Dept.

*13E_*09 19/07/03 15:10 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Plastics Sliding Sheet Used Slide Guide of Machine Tools

Takuya ISHII*

Satoru FUKUZAWA*

Norikazu MUNEDA*

Machine tools are required to operate at high speed, high precision. The
models of using plastics sliding sheet in guide are increasing. When cutting
resistance is high, the models are superior in processing precision by low
vibration. Low friction properties and wear resistance are required to plastics
sliding sheet. In this report, characteristic of sliding sheet formed from PTFE
and special filler is introduced.

load. Therefore, it is not suitable for processing
objects with high cutting resistance.
The sliding guide type has a large area of contact
due to plane contact leading to a high rigidity;
therefore, it is suitable for processing objects with highcutting resistance. By using a plastic sliding sheet on
the guideway, a high vibration damping property canbe achieved and stick slip can be largely prevented
compared to sliding between two metal materials. In
addition, seizure can be avoided even under poor
lubrication conditions.
Fig. 1 shows an overview diagram of the machine tool
sliding guide type. Recently, as the cases of processing

1. Introduction
Machine tools process workpieces using relative
motion between the tools and workpieces; therefore,
performance of the guideway which moves either the
tool or workpiece has a significant impact on processing
accuracy. There are two types of guideways, rolling typeand sliding type, which are selected depending on theuse. The rolling type, such as a linear guide, moves
tools or workpieces with rollers. The sliding type either
uses lubricating oil on the guideway for sliding between
two metal surfaces or uses a plastic sliding sheet
adhered to one of the metal surfaces for sliding between
the other metal surface. For the sliding sheet material,
polytetra fluoroethylene (PTFE) is used for its low friction
and high anti-galling properties. The properties of theplastic sliding sheet play a critical role since the sliding
surface is in mixed lubrication condition.
In this article, we describe the sliding sheet formachine tool guideways made of BEAREE FL3307,
which is PTFE blended with special filler material, for
improved friction wear properties under the mixed
lubrication condition.

Table 1 Characteristic of guide type in machine tools
Item
Rigidity

Vibration damping properties
Friction resistance
Stick slip

Rolling guidetype(Linear guide)

Sliding guide type
Plastic sliding Metal sliding○◎○○◎：Excellent

○
○
△
△

△
△
◎
◎

〇：Good△：FairMoving part to fix tools
or workpieces

2. Characteristics of various guide types 1)
Table 1 shows the features of different guide types.
The rolling guide type has low friction resistance andis capable of moving tools or workpieces at highspeed without causing stick slip; therefore, it provides
high processing accuracy. However, since the area ofcontact is small due to line contact of the rollers,
rigidity is low and vibration may occur under heavy

Guideway

Fig. 1 Slide guide type of machining tools*Precision Plastic Engineering Dept. Composite Material Product Division-74-

*13E_*09 19/07/03 15:10 ページ 2

Plastics Sliding Sheet Used Slide Guide of Machine Tools

difficult-to-cut materials, such as carbon fiber reinforced
plastics, are increasing, machine tool models with sliding
guideways using plastic sliding sheets are becoming
more popular due to their superior processing accuracy
with low vibration even when the cutting resistance ishigh.

4. Features and manufacturing processof PTFE plastic sliding sheet
PTFE has low friction coefficient, high non-adhesive
properties, and a high heat tolerance up to a continuous
operating temperature is 260˚C. It also has excellent
chemical and weathering resistance enabling its use on
the guideway, which is in contact with lubricating oil andcoolant for a long time. PTFE alone, however, has poor
friction and compression creep resistance and a large
linear expansion coefficient; therefore, fillers such as
glass fiber, carbon fiber, bronze powder, graphite, etc.
are blended into the material. The friction/wear
properties change depending on the fillers and blend
ratio. It is important to choose the right filler and blend
ratio for the specific use.
Fig. 3 shows the manufacturing process of sliding
sheets for machine tool guideways. As PTFE has highmelt viscosity preventing injection molding, a plastic
sheet of 0.8 to 1.5 mm thickness is manufactured byskiving the material from a compression molding.
Skiving is a manufacturing process to obtain sheetsfrom the mold material by slicing it with a blade whilerotating the mold material.
Since PTFE has high non-adhesive properties andcannot be adhered as is, the surface is chemicallyetched by treatment liquid so that it can be adhered.
NTN has prepared an adhesive dedicated for adhering
the plastic sheet on the guideways made of cast iron,etc. which has resistance against lubricating oil andcoolant. The tensile shear strength of cast iron and
BEAREE FL3307 plastic sliding sheet using thisadhesive is 6 MPa and has a sufficient safety factor
against frictional shear of 0.06 MPa calculated from theused contact pressure and friction coefficient (whencontact pressure is 1 MPa and friction coefficient is0.06).

3. Performance required for plastic
sliding sheet
In the sliding guideway type, oil grooves are createdon the guideway to facilitate the formation of alubricating film. In addition, scraping is also applied on
the surface to create a uniform contact condition withmicro dimples of several tens of μm 2). Fig. 2 shows
the surface conditions before and after scraping on a
BEAREE FL3307 plastic sliding sheet. Scraping isdone by hand by skilled technicians. The plastic
sliding sheet has an advantage over metal material foreasy scraping.
The micro dimples on the guideway created byscraping increases lubricating oil retention andstabilizes friction resistance to a low level. When theplastic sliding sheet wears out or is deformed by theload, the dimples on the sliding surface are reduced
which increases the friction resistance; therefore, theplastic sliding sheet requires good wear andcompression creep resistance.
For machine tools, high speed transfer of tools or
workpieces is required in order to reduce machining
cycle time. Therefore, it is desirable for plastic sliding
sheets to have low starting static wear coefficient anddynamic friction coefficient, as well as low contactpressure and speed dependency 3).

Mixture of raw materials(PTFE and filler)
Compression molding(compression at room temperature)

Firing (380 to 400˚C)

10mm

Before scraping

After scraping

Skiving

Fig. 2 Hand scraper surface of plastics sliding sheet

Compression molding

Chemical etching process
Cutting
Shipping inspection/packaging

Skiving
Width 300mm, length 30m

Fig. 3 Manufacturing process of plastics sliding sheet-75-

*13E_*09 19/07/03 15:10 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

5.2 Contact surface/speed dependency ofdynamic friction coefficient

5. Performance of NTN plastic sliding sheet

The dynamic friction coefficient was measured bychanging the contact pressure and speed under the test
condition shown in Table 3. Fig. 5 shows the relation
between the contact pressure and the dynamic friction
coefficient and Fig. 6 shows the relation between the
speed and the dynamic friction coefficient.
The dynamic friction coefficient tends to decreaseas the contact pressure increases. In the contactpressure dependency test, the dynamic friction
coefficient of NTN's product was slightly lower than thecomparison products, and the dependency was small
when the contact pressure was higher than 0.3 MPa.
In the speed dependency test, the dynamic friction
coefficient of NTN's product was equivalent to or lessthan the comparison products with small speed
dependency.
Fig. 7 and 8 are the photographs of the plastic sheetof NTN's product and Comparison Product A, taken byscanning electron microscope. Fillers are exposed on
the sliding surface of the plastic sheet.

A comparison of the performance of 3 types ofplastic sliding sheets (BEAREE FL3307 (hereafter,
NTN product), glass fiber (GF) blended PTFE
(hereafter, Comparison Product A) and carbon fiber(CF) and bronze powder blended PTFE (hereafter,
Comparison Product B)) was performed. Table 2
shows the comparison of performance.
NTN product has higher tensile strength andelongation with superior friction wear properties andcompression creep properties. Details of the test
method and results of the friction wear andcompression creep properties are described below.

Table 2 Comparison of features plastics sliding sheets
NTN product

Sheet material

BEAREE
FL3307

GF blended
PTFE

20MPa

15MPa

○

△

Friction properties properties

PhysicalComparison
product A

Item

Tensile strength
Elongation
Static friction property

Contact pressure
dependency of dynamic
friction coefficient
Speed dependency ofdynamic friction
coefficientCompression creep properties

220%

200%

Comparison
product B
CF and bronze
powder blended
PTFE

13MPa
180%

△

○

△

△

○

○

△

○

△

△

〇：Good△：FairTable 3 Test condition of dynamic friction

Contact pressure
Speed
StrokeLubricating oil
Temperature

5.1 Friction wear test method

Dynamic friction coefficient

Speed dependency
test

0.1 to 1.0 MPa
0.5MPa30m/min10〜50m/min±100mmLubricating oil for machine tool
Room temperature

0.10

Comparison product A

0.08

Comparison product B
NTN product

0.06
0.04
0.02
0.00
0.0

0.2

0.4

0.6

Contact pressure MPa

0.8

1.0

Fig. 5 Coefficient of dynamic friction versus surface pressure
0.10

Dynamic friction coefficient

Load

The reciprocating motion tester shown in Fig. 4,
which simulates the guideway of machine tools, wasused for the friction wear test. A vertical load wasapplied to a metal block test piece, which has theplastic sliding sheet adhered to it, and the metalmating material was set in reciprocating motion. The
plastic sliding sheet was not scraped but oil grooves
were applied after adhesion. The metal mating
material is meehanite cast iron with surface roughnessof Ra 0.25 μm which is induction hardened andpolished on the sliding direction. Lubricating oil formachine tools was applied on the sliding surface.
The friction coefficient was calculated by dividing
the friction force, which was measured by the load cellattached to the mating material, by the vertical load.

Metal mating material

Contact pressure
dependency test

Item

Block test piece（30×30×10mm）
Plastic sliding sheet(thickness 1mm)

Fig. 4 Diagram of reciprocating motion test

Comparison product A

0.08

Comparison product B
NTN product

0.06
0.04
0.02
0.00

0

10

20

30

40

Speed m/min

50

60

Fig. 6 Coefficient of dynamic friction versus velocity-76-*13E_*09 19/07/03 15:10 ページ 4

Plastics Sliding Sheet Used Slide Guide of Machine Tools

Special filler material

Specific wear rate
mm3/(N・m)

X10-8 1086
420Electron microscope image

10m/min

30m/minFig. 9 Specific wear of NTN Plastics sliding sheet

Fig. 7 Surface of NTN plastics sliding sheet
Glass fiber

Table 4 Wear test condition
Item
Contact pressure
Speed
StrokeLubricating oil
TemperatureTime

Description
0.5MPa10m/min, 30m/min±100mmLubricating oil for machine tool
Room temperature
100h

The specific wear rate of NTN's product is very low
at any speed and is superior in wear resistance.

Fig. 8 Surface of comparative-A plastics sliding sheet

6. Summary

In Comparison Product A, the direction of fiber filler
material is random, with some only exposing the endsof the fibers. Fiber fillers are not likely to fall out whena frictional shear force is applied due to their shape;
however, the friction coefficient does increase due tothe resistance of the fiber end’s edges. This is also
true with Comparison Product B, which is blended withcarbon fibers. NTN's product has fillers scattered on
the sliding surface; however, these fillers are not fiberlike and not anisotropic; therefore, they are unlikely toincrease friction coefficient.
The sliding surface can easily form an oil film due tothe oil grooves and micro dimples (from scraping);
however, it is in mixed lubrication condition as theplastic sliding sheet is partially in contact with themating metal material. When this contact becomesplane contact, lubricating oil is ejected, resulting in ahigher friction coefficient. It is assumed that NTN's
product creates point contact at the exposed fillers;
therefore, lubricating oil remains on the contact
surface, resulting in a good lubrication condition andlow friction coefficient.

In this article, we introduced the BEAREE FL3307
Plastic Sliding Sheet, used in the sliding guideways ofmachine tools. This material mix has low friction, lowwear properties, and a small compression creep
deformation rate, which contributes to higher speed
and higher machining accuracy of machine tools.
We will continually improve our material blending
technologies for lower friction/wear and develop
products that meet the demands of higher functionality
and energy efficiency in various fields.

References
1) Japanese Society of Tribologists: Tribology Handbook, 1stissue, Yokendo, (2001) 343.
2) Haruhisa Sakamoto: Triboloty Technology and Trends in
Machine Tools, THE TRIBOLOGY No. 331, (2015) 18-20.
3) Fumio Obata: Research and Development of low sliding
friction coefficient/High Chatter Resistance Sliding Linear
Guide, Innovation Japan 2009---University Trade Fair,
Conference No. PM-08.

Photo of authors

5.3 Wear properties
Fig. 9 shows the specific wear rate of NTN's productwhen tested for wear under the test conditions shownin Table 4. Specific wear rate is the volume of wear
per sliding distance per load, which is calculated fromthe measurements before the test and after the test.
The smaller the specific wear rate, the better the wear
resistance.
-77-

Takuya ISHII

Satoru FUKUZAWA

Norikazu MUNEDA

Composite Material Product Division
Precision Plastics Engineering Dept

Composite Material Product Division
Precision Plastics Engineering Dept

Composite Material Product Division
Precision Plastics Engineering Dept

*15E_*09 19/07/03 15:11 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ Technical Paper ]Lubrication Mechanism of Low Friction Seal of Ball Bearing
for Transmission
Tomohiro SUGAI*
Katsuaki SASAKI**
Takahiro WAKUDA**
A low friction seal for use in ball bearings developed for automotive transmissions
has arc-shaped (half-cylindrical shaped) micro bumps on the sliding surface of theseal lip. This generates a lubrication oil film between the seal lip and inner ring by awedge effect in the bump area, thereby achieving low friction.
This article reports the mechanism of fluid film formation and friction resistanceby soft EHL (ElastoHydrodynamic Lubrication) analysis.

than the magnitude of roughness), due to micro EHL(elastohydrodynamic lubrication) from the roughsurface, shear resistance is large even if the seals arein a fluid lubrication condition. This new seal, on theother hand, produces a pressure gradient in the
lubricating oil that flows along the circumference of theinner ring with micro bumps that are larger than thesurface roughness of the seal and inner ring, toactively create oil film of lubricating oil on the sliding
surface between the seal lip and bearing inner ring.
This facilitates fluid lubrication, reduces the rotational
torque, and improves the peripheral speed limit.
In this article, we review the investigation results of
the oil film formation mechanism and friction
resistance using soft EHL analysis, which provides
coupled analysis of the elastic deformation of theentire seal, including bumps, and the fluid pressure of
lubricating oil.

1. Introduction
Contact type seals may be used in bearings forautomotive transmissions to reduce the impact onbearing life caused by hard foreign objects penetrating
the bearings, such as wear debris from the gears in
the transmission. However, since these seals come incontact with the bearing inner ring, frictional torqueduring rotation may increase. Furthermore, inapplications where high-speed rotation is required,such as recent EVs and HEVs, the use of contact typeseals is difficult due to the restricting seal peripheral
speed limits. Although further torque reduction is arequirement for bearings used in automotivetransmissions, since torque reduction and sealing
performance are conflicting properties, it has beendifficult to achieve significant torque reduction while
ensuring a high level of seal performance.
To solve this challenge, NTN has developed a low
friction ball bearing seal for transmission applications
that uses arc-shaped (half-cylindrical shaped) micro
bumps in equal distance on the surface of the seal lipthat slides against the bearing inner ring 1). This seal
achieves lower torque while satisfying the required
bearing life, since the height of the bumps is smaller
than the size of the foreign objects that affect bearing
life, based on the relationship between the foreign
object size in the lubricating oil and bearing operating
life. Ordinary seals are considered to operate in amixed lubrication or fluid lubrication condition 2), 3);
however, since the formed oil film is very thin (less

2. Soft EHL analysis
2.1 Analysis model and program
Fig. 1 shows the developed seal. The figure on theleft shows the cross section of the bearing with theseal and the figure on the right shows the cut out viewof the seal along the circumference. Arc-shaped (halfcylindrical shaped) micro bumps in equal distance on
the seal sliding surface contact the bearing inner ring.
The bumps are curved in the axial direction as well,providing point contact with the inner ring sliding
surface when they are installed in the bearing as**Advanced Technology R&D Center**Automotive Bearing Engineering Dept., Automotive Business Headquarters-78-

*15E_*09 19/07/03 15:11 ページ 2

Lubrication Mechanism of Low Friction Seal of Ball Bearing for Transmission

is used in structure analysis for calculating shapes
since the elastic deformation is small. However, whenrubber seals are analyzed, semi-infinite elastic body
approximation does not work as the elastic
deformation is large; therefore, non-linear structure
analysis is required 4). Changes of the lubricating oil
properties due to pressure are not considered, since
the increase of pressure in the oil is small because theseal deforms more easily due to the increased
pressure. Seal deformation, including that of thebumps, is calculated using general purpose non-linear
analysis software with the finite element method, andthe oil film pressure is calculated using the solution of
the following Reynolds equation that describes
viscous fluid motion on the lubricating surface 5).

Arc-shaped (half-cylindrical shaped)
micro bumps

Fig. 1 Low friction seal

shown in Fig. 2. As shown in Fig. 3, at the bumps, thedistance from the sliding surface gradually decreases
in the circumferential direction, namely, direction of theflow, which, while rotating, produces fluid pressure in
the lubricating oil on the sliding surface in a gradient,forming an oil film between the seal sliding surface
and the inner ring. When enough oil film is formed on
the sliding surface, it creates a fluid lubrication
condition and frictional torque is expected to decrease
compared with the conventional seal without thesebumps. The thickness of the oil film between the seal
lip and bearing inner ring, and seal frictional torquedue to fluid viscosity resistance are calculated by thesoft EHL analysis coupling the oil film pressure andseal deformation.
In general, when oil film analysis of rolling bearings
is performed, semi-infinite elastic body approximation

∂
∂x

（

u=−

Fig. 2 Deformed shape of the seal lip mounted in bearing
Pressure distribution

Seal

Oil
flow
Direction of rotation

）

z
1 ∂p
z（h−z）+ 1−
2η ∂ xh（）u

0

The first term of the right side is the Poiseuille flow
induced by the difference of pressure and the second
term of the right side is the Couette flow induced by
the motion of the wall surface. For the cavitation
condition, Swift-Steiber's condition 6) was used. In
addition, a periodic boundary condition was used as
this is an axisymmetric problem.
By repeating structure analysis and fluid analysis, aconvergent solution was obtained. Since the oil film
pressure significantly changes with a small change ofoil film thickness, and the seal deformation issignificantly affected by a small change of pressure,convergence properties may be degraded by strong
non-linearity in the coupled analysis. By appropriately
controlling the relaxation amount according to theconvergence condition in an iterative computation
when the Reynolds equation is solved with therelaxation method 7), we have enabled calculation withgood convergence properties, as shown in Fig. 4. The
calculation was performed in the following two steps:
First step: The condition of the bearing with innerring, outer ring and seal is calculated with structure
analysis only. This is equivalent to the condition of
the seal while not rotating.
Second step: Using the calculation result of the first

Point contact(b) When mounted
in bearing(a) Before deformation） （Where, x: coordinate in the flow (circumferential)
direction, y: coordinate in the direction perpendicular
to flow (rotational axis), ρ: density, h: oil film
thickness, η: lubricating oil viscosity, p: pressure, u 0:velocity on the sliding surface. In addition, flow rate in
the x direction u is a function of the coordinate in the
direction of the film thickness z and can be expressedby the following equation.

Inner ring
Micro bumpsρh3 ∂p∂ ρh）∂ ρh3 ∂p（+
= u0
∂ y 12η ∂ y∂x12η ∂x

Inner ring

Fig. 3 Wedge action at the micro bump-79-

*15E_*09 19/07/03 15:11 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

step as the initial condition, coupled analysis withthe fluid is performed. This is equivalent to theconditions when the seal is rotating. The initial oil
film thickness is set by exploring the inner ring
position where the load capacity and tension force
due to oil film pressure are balanced, with the seal
shape as calculated in the first step (not rotating).
In the first half of this step, inner ring position isreturned to the predetermined position.
In the calculation of torque, a fluid lubrication
condition was assumed with the seal lip sliding surface
and the inner ring well separated by the oil film andonly torque due to viscosity resistance of the
lubricating oil considered.

along with the flow at the minimum film thickness
position. Approximately 0.22 MPa of maximum
pressure was observed. The minimum oil film
thickness was about 1μm. The composite roughness

Table. 1 Properties and conditions
Seal material

Acrylic rubber

Bump radius (mm)1.5

Number of bumps

180

Lubricating oil dynamic viscosity (40˚C) (mm2/s)
Rotational speed (min-1)

1500

Bearing inner ring OD (mm)64Bump height (μm)

40

26

MPa

Bu
m

p

1.E+2

Residual

1.E+1

0.20

1.E+0

0.15

Direction of rotationof inner ring(not shown)

1.E-1
1.E-2
1.E-3

0

50

100

150

200

250

300

0.10
0.05

350

Torque N・m

Number of iterations in the convergent calculation

0.00

0.005

MPa

0.004

0.20

0.003

0.15

0.002

0.10

0.001
0.000

0

50

100

150

200

250

300

0.05

350

Number of iterations in the convergent calculation

0.00

Fig. 4 Convergence of calculation

Fig. 5 Calculation result of oil film pressure distribution

2.2 Conditions and results of calculation
The properties and operating conditions of thedeveloped seal under review are shown in Table 1.
The rubber rigidity and lubricating oil viscosity were
defined as a function of temperature considering
thermal expansion of the rubber. In this article, impact
from surface roughness, stress relaxation of rubber,pressure to lip seal by oil flow within the transmission,
shaft run-out, etc. are not considered. Fig. 5 and 6
show an example of the calculation results. Fig. 5
shows oil film pressure distribution against the seal lip
sliding surface, while the diagram above shows the
picture of the seal lip sliding surface viewed from theinner ring side. Oil film pressure is shown at the bump.
Fig. 6 shows oil film thickness and pressure distribution

25

15

Pressure p

Oil film
thicknessh

0.20

Pressure p
MPa

Oil film thickness h
μm

20

0.25

0.15

10

0.10

5

0.05

0

-300

-200

-100

0

100

200

300

0.00

Coordinate in the flow direction X μm

Fig. 6 Oil film thickness and pressure distribution-80-*15E_*09 19/07/03 15:11 ページ 4

Lubrication Mechanism of Low Friction Seal of Ball Bearing for Transmission

Minimum oil film thickness μmof sliding surface is about Rq 0.22 μm. Sufficient oilfilm is formed on the surface compared to theroughness, creating a fluid lubrication condition. Fig. 7
shows the relationship between temperature andminimum film thickness. The oil film thickness is
reduced as the temperature rises; however, theminimum film thickness is about 1μm even at 150˚C,
which is sufficient compared to the roughness. Fig. 8
shows the relationship between temperature and seal
torque. The indicated torque comes from two seals
used in one bearing. In addition, factors making up thetotal torque are categorized into the following threegroups and color coded: torque caused by the
Poiseuille flow at the bumps, torque caused by the
Couette flow at the bumps, and torque caused by the
Couette flow at the non-bump area, which is thesliding surface between the bumps. Since the increasein pressure at the non-bump area is small, torquecaused by Poiseuille flow at the non-bump area is also
small and not shown. The torque decreases as thetemperature rises, to approximately 0.007 Nm at
120˚C. The main factor of the torque is the Couette
flow at the bump area. Fig. 9 shows the relation
between the number of bumps and torque. When
bumps are fewer, torque due to the Couette flow at the
non-bump area increases. The reason for this is thatthe area of the sliding surface at the non-bump area
increases and, in addition, the gap of the sliding
surface of non-bump area decreases. When thesliding surface gap decreases, the shear resistanceand seal torque increase. When there are fewerbumps, the distance between the bumps increases,
and the sliding surface of the non-bump area
undergoes elastic deformation due to the tension
force, moving it closer to the inner ring sliding surface
and reducing the gap between the sliding surfaces.
Fig. 10 shows a schematic diagram of the non-bumparea. Fig. 11 shows the relationship between thenumber of bumps and the gaps between bumps under
non-operating conditions. In the figure, the gap in thearea between bumps is shown as a ratio against the

5
4
3
2
1
0

20

40

60

80

100

120

140

Temperature ˚CFig. 7 Relationship between temperatureand minimum film thickness
0.04

Seal torque N・mPoiseuille flow (at bumps)
0.03

Couette flow (at bumps)
Couette flow (at non-bump area)

0.02

Seal

p area

Bump
0.01
0.00

Seal lip sliding
surface
20

40

60

80

100

Temperature ˚C

120

140

Gap in the area
between bumps

Gap between sliding
surfaces of non-bump area

Gap in the area between bumps
%

100

0.020

Poiseuille flow (at bumps)
Couette flow (at bumps)

0.015

Couette flow (at non-bump area)
0.010
0.005
0.000

Inner ring

Fig. 10 Schematic drawing of elastic deformation ofnon-bump area

Fig. 8 Relationship between temperature and seal torqueSeal torque N・m

Bump area Non-bum

80

100

120

Number of bumps

180

240

80
60
40
20
0

50

100

150

200

250

Number of bumps

300

350

Fig. 11 Relationship between number of bumps and gaps in
the middle between bumps under non-operating condition

Fig. 9 Relationship between number of bumps
and seal torque-81-

*15E_*09 19/07/03 15:11 ページ 5

NTN TECHNICAL REVIEW No.86（2018）of light reflected on the bump surface, after it istransmitted through the oil film. The pressure of therubber bump to the disc is controlled by the Z-stage. In
this experiment, the pressure and speed are set asparameters.
The measurement results and calculation resultsfrom the soft EHL analysis are compared in Fig. 13.
Both results show the same trend and approximate
magnitude, verifying an oil film thickness of around 1
μm. However, the measured value is larger than theanalysis results in the range where the oil film isthinner. The reason for this difference may be theimpact from the measuring range and vibration of thesapphire disc surface. The analysis shows theminimum film thickness; however, the measurement
result is the average of a 30 x 30 μm area; therefore,
the measurement value of the oil film thickness may
have been larger. Also, there was approximately 0.35
μm of disc surface vibration due to the rotation, which
may have resulted in a larger measurement value ofoil film thickness.

height of the bumps. In this study, when the number ofbumps is around 100 or less, the gap in the area
between bumps becomes 0, which means that thesliding surface of the non-bump area comes in contact
with the inner ring. However, during operation, an oilfilm is formed by the pressure gradient created when
the non-bump area is close to the inner ring, resulting
in no direct contact between the surfaces.
When the number of bumps increases, torque dueto the Couette flow at the bump area increases. The
more bumps, the lower the tension force per bump;therefore, the minimum film thickness of the bumparea increases and the torque increases, because thetotal area of sliding surface of the bumps increases.
Under the conditions of this study, the lowest torqueresult occurred when the number of bumps wasaround 100.

3. Verification test
3.1 Oil film thickness
The oil film thickness at the seal bump area wasmeasured using the infrared microspectroscopic
method. The infrared spectroscopic method is amethod of structural analysis and quantification ofsubstances that uses infrared rays applied to the test
pieces and analyzes the spectrum obtained fromtransmitted or reflected light. When infrared rays aretransmitted through substances, light in a certain
frequency range is absorbed by the substance and theintensity of light through the substance is weakened.
The intensity of light through the substance and thelight path length are correlated. Fig. 12 shows theschematic diagram of the measurement device. Since
acrylic rubber absorbs infrared rays, 0.3 μm of gold isvacuum deposited on the surface of the acrylic rubber
so that the light is reflected on the rubber surface. The
test piece, which simulates a seal bump, is pushed
into the rotational sapphire disc and infrared rays areirradiated to the disc from the opposite side. Oil film
thickness can be estimated by measuring the intensity

Oil film thickness μm

2.0

Analysis（11mN）

1.5

Experiment（22mN）
Analysis（22mN）

1.0
0.5
0.0

0.0

0.1

0.2

0.3

0.4

Sliding speed m/s

0.5

0.6

0.7

Fig. 13 Comparison of experimental results with analysis
ones of film thickness

3.2 Bearing torque
The bearing torque was measured by installing thedeveloped seal on a bearing designed for use in atransmission. The specification of the seal and the test
conditions were the same as shown in Table 1. The
lubricating method was an oil bath with the uppersurface of the lowest rolling element at the oil level. In
addition, 754 N of radial load was applied to thebearing. This measurement result and measurement
results of bearings using the conventional contact typeseal and non-contact type seal without bumps areshown in Fig. 14.
When compared to the bearing using theconventional contact type seals (without bumps), thetorque of the new seal with bumps is reduced by 80%
or more. Conventional contact type seals are
considered to operate in mixed lubrication or fluid

Micro infrared spectrometer
Sapphire discOil film
thickness
Sapphire discExperiment（11mN）

Rubber bump
Precision Z-Stage
Load detector

Fig. 12 Schematic drawing oil film thickness
measuring device-82-*15E_*09 19/07/03 15:11 ページ 6

Lubrication Mechanism of Low Friction Seal of Ball Bearing for Transmission

lubrication conditions, including solid contact;
however, since the formed oil film is very thin due tomicro EHL (elastohydrodynamic lubrication) from therough surface, shear resistance is large even if theseal is in a fluid lubrication condition. On the other
hand, the developed seal forms sufficient oil film at thebumps creating a fluid lubrication condition, which isassumed to be the one of the contributing factors forthe reduction of friction torque. In addition, as shownin Fig. 9, when the number of bumps is not too small,since the torque at the non-bump area due to the
Couette flow is small compared to the torque due tothe Couette flow at the bumps, the distance of sliding
surfaces in the developed seal is increased, whichshould also contribute to the reduction of friction
torque.
The increase of torque compared to a bearing withthe conventional non-contact type seal is around 0.01
Nm. Since the seal torque of non-contact type seal isestimated to be very small, the seal torque of thedeveloped seal is estimated as around 0.01 Nm,
which is equivalent to the calculation results shown in
Fig. 8 and 9.

Bearing torque N・m

0.4

4. Conclusion
We have developed an oil film analysis program forinvestigating the mechanism of oil film formation andfriction resistance of a newly developed seal. This seal
generates oil formation between the sliding surfaces ofthe seal lip and bearing inner ring due to the arcshaped (half-cylindrical shaped) micro bumps set inequal distances on the seal lip toward the bearing
inner ring sliding surface. Within the range ofconditions evaluated in this article, the developed seal
facilitated oil film formation due to the fluid lubrication
effect of the bumps and produced lower torque thanconventional contact type seals.
Although not mentioned in this article, interference,bump height, and variation of rubber properties also
impact bearing torque. With the developed program, it
is possible to predict torque of the newly developed
seal and optimize the specification of the bumps.

References
1) Sasaki, Wakuda, Sugai, Ultra-low Friction Sealed
Ball Bearing for Transmission, NTN TECHNICALREVIEW, No.85,(2017) 62-66.
2) Mizuta, Research on clarification of lubrication
mechanism of oil seal for rotational body andreduction of friction, Doctoral Dissertation (2013)
3) Kaneta, Sealing Theory, Tribologist, Vol. 43, No. 2,(1998), 125-130
4) Kaneta, Analysis of Soft EHL, Tribologist, Vol. 38,
No. 10, (1993), 890-894.
5) Yamamoto, Kaneta, Tribology, Rikogakusha (2007)
6) Nakahara, Fluid Film Behavior, Journal of Japan
Society of Lubrication Engineers, Vol. 26, No. 3(1981), 146-152
7) C.H.VENNER, A.A.LUBRECHT, MULTILEVELMETHODS IN LUBRICATION, ELSEVIER, (2000)

Seal with bumps
Conventional contact type seal

0.3

Conventional non-contact type seal

0.2
0.1
0.0

20

40

60

80

100

Bearing temperature ˚C

120

140

Fig. 14 Experimental results of bearing torquePhoto of authors

Tomohiro SUGAI

Katsuaki SASAKI

Takahiro WAKUDA

Advanced Technology
R&D Center

Automotive Business Headquarters
Automotive Bearing Engineering Dept.

Automotive Business Headquarters
Automotive Bearing Engineering Dept.

-83-

*16E_03 19/07/03 15:11 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ Technical Paper ]Hub Bearing with Steering Function that Improves
Vehicle Dynamic Performance
Norio ISHIHARA*
Mitsunori ISHIBASHI**
Masato ABE**

Hirokazu OOBA*
Makoto YAMAKADO**

Atsushi ITOU*
Yoshio KANO**NTN has developed a hub bearing with a next generation steering function capableof improving the motion performance of the vehicle by incorporating the steering angle
adjustment mechanism in the hub bearing.
In this developed product, it is possible to independently correct and steer the left
and right wheels according to the driving situation of the driver, but in this report wereport on the result of the implementation in the mode to synchronize the left and right.

sHUB(Right wheel)sHUB(Left wheel)

1. Introduction
We are developing "Hub Bearing with SteeringAssist Function (hereafter, sHUB)" which integrates
hub bearings and steering angle adjustment
mechanism. As shown in Fig. 1, it can be installed in
the conventional front wheel steering and suspension
system to independently correct the steering of eachwheel with a command signal from the controller
based on the vehicle information.
sHUB is a system intended to improve vehicle linear
stability and fuel efficiency, as well as to improvevehicle cornering stability and risk avoidance inemergencies such as slippage. The system also has
potential for applications in future autonomousvehicles for risk avoidance behavior.
We have studied a simulation of the vehicle withsHUB and verified that this system would improvevehicle responsiveness against yaw rate and lateral
acceleration.
The specification for sHUB was determined basedon this analysis result and a prototype unit was
created. The prototype (for right wheel) is shown in
Fig. 2.
We have conducted an engine bench test of the
prototype and verified that the basic performancesatisfies the target values. In addition, we haveverified effect of this control law with an actual vehicle
test.
This article reports these test results.

Existing
steering system

Steering wheel

Controller

Fig. 1 Image of Vehicle with sHUBs

Steering-assist shaft

KnuckleActuator

Hub bearing

Fig. 2 Prototype of sHUB**New Product Development R&D Center**Department of Vehicle System Engineering, Kanagawa Institute of Technology-84-

*16E_03 19/07/03 15:11 ページ 2

Hub Bearing with Steering Function that Improves Vehicle Dynamic Performance1+ Tβsδ(s)δh (s)2ζss2 δh1+ ωn + ωn2（2）

1+ T r sδ(s)δh (s)2ζss2 δh1+ ωn + ωn2

（3）

lr
I
s2
s+
δ
V
2lK
r
ÿ (s) =Gδ (0)(s)δh (s)2ζss2 δh1+ ωn + ωn2（4）β(s) =Gδβ(0)2. Vehicle motion control law
2.1 Vehicle motion control with front wheel steering
An improved version of the Matlab/Simulink ®
Vehicle Motion Simulation Model 1) developed at the
Yamakado/Kano Laboratory of Kanagawa Institute ofTechnology was applied to the vehicle model with
sHUBs installed in both front wheels. With this model,
the potential for improvement of motion performance
was verified. In addition, this simulation model wasused to study the best control parameter settings.
This simulation model can reproduce change ofvehicle attitude since it takes into consideration theanti-dive and anti-squat effect with the suspension linkmechanism against longitudinal and lateral force tothe tires 2), 3), 4).

r (s) =Gδr (0)ÿEquation (1) is the control law to determine vehicle
tire angle to improve responsiveness of vehicle yaw
rate and lateral acceleration. The sHUB corrective
steering angle is the difference between this angle
and the tire steering angle with the driver's steering
action. By substituting this control law to equations (2),(3) and (4), and adjusting the parametersα1,α2 andα3, responsiveness against lateral slip angle β, yaw
rate r and lateral acceleration ÿ can be improved.
Parametersα1,α2 andα3 are the values corresponding
to damping ratio ζ, natural frequency ωn and lateral
acceleration ÿ , respectively. This simulation can
reproduce the basic vehicle motion well, such that thevehicle motion performance (responsiveness to yaw
rate and lateral acceleration) against steering
degrades as the driver's steering speed or lateral
acceleration increases.
The actual simulation was conducted with a twowheel model (wheels on both sides are controlled withthe same angle) and the impact of the parameters α1,
α2 andα3 on the steering responsiveness wasinvestigated. The parameters were studied based on
the results 5).
Fig. 3 shows the results of the analysis when a lanechange (approximately 4.0 m of width) was made withsteering action of 0.5 Hz sinusoidal wave.
The values determined by the above study wereused for the control parametersα1,α2 andα3 ofsHUB. From the top, (a) angle of steering wheel, (b)front wheel steering angle, (c) operating angle speedof sHUB, (d) vehicle yaw rate and (e) vehicle lateral
acceleration are shown. In these graphs, the red line
indicates the case when corrective steering was madeby sHUB and the blue line indicates the case without
correction. In both cases, the same angle of steering
wheel was used.
From the analysis result of the front wheel steering
angle, it was revealed that sHUB increased the tire
steering angle at the beginning of the corrective
steering and controlled the corrective steering angle tobe zero at the beginning of returning the steering
wheel. The sHUB steering angle corresponds to the

2.2 Control for improving steering responsiveness
The following shows the control law to improve theresponsiveness of vehicle yaw rate and lateral
acceleration against the driver's steering action. The
symbols are defined as follows:n: Steering gear ratioV: Vehicle speed
I: Vehicle yaw inertia momentl: Wheel baselr
: Distance between the vehicle center ofgravity and rear wheel shaft
Kr: Tire cornering power per rear wheelβ(s) : Laplace transformation of lateral slip angle
βof vehicle center of gravityr (s) : Laplace transformation of yaw rate rδ(s) : Laplace transformation of front wheel tireangle δδh (s) : Laplace transformation of steering angle δhÿ (s) : Laplace transformation of lateral
acceleration ÿ ̈ωn : Natural frequency of vehicle response
against steering actionζ: Damping ratio of vehicle response against
steering actionGδβ (0)，Gδr (0)，Gδÿ (0): Vehicle response
parameters against steering action
δ
(s)
δh
2ζ
1
1+ ωn s + ωn2 s21= n2ζ(1+α1)
1
1+
s+ 2
s2
ωn (1+α2)2
ωn (1+α2)

(

)

×

l
I s2
l
1+ α3 r + (1−α3) r s +
Vβ=0
V
2lKr
lr
l
1+
s+
s2
2lKr
V

1+

（1）

-85-

*16E_03 19/07/03 15:11 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

difference between the red line of the front wheel
steering angle (vehicle with sHUB) and the blue line
(ordinary vehicle) shown in Fig. 3 (b).
With this control, it was verified that responsiveness
improved, as faster rising of yaw rate and lateral
acceleration and smaller phase lag of yaw rate andlateral acceleration were observed against the driver's
steering action.
In addition, we have studied the target of sHUB
required for improving responsiveness to vehicle yaw
rate and lateral acceleration based on the result ofanalysis conducted with varying speed and lateral
acceleration within the range of ordinary operation.
Table 1 shows the study results. Improvement ofvehicle responsiveness can be expected whensatisfying the performance of maximum steering angleof ±3 deg and maximum angular speed of 10 deg/s.

3. Structure of sHUB
3.1. Components
Fig. 4 shows the components of sHUB. sHUB
consists of hub bearing, knuckle and actuator. The
roles of the respective components are described asfollows:· Hub bearing : rotary support of tire; rotary supportof steering-assist shaft against knuckle· Knuckle: connection with vehicle suspension· Actuator: fixed to knuckle and steers hub
bearing

KnuckleSteering angle
deg

100

Hub bearing

Steering-assist
shaft

Actuator50
0-50

Front wheel steering angle
deg

-100

0

0.5

1

0

0.5

1

10

1.5
2
2.5
3
(a) Steering angle

3.5

4

4.5

5

1.5

3.5

4

4.5

5

Motor

Speed
reducer50-5Vehicle lateral acceleration
G

Vehicle yaw rate
deg/sOperating angle speed of sHUB
deg/s

-10

10

2

2.5

3

(b) Front wheel steering angle

Trapezoidal
screw

Actuator

50

KnuckleHub bearing

Fig. 4 Components of sHUB

-5
-10

0

0.5

1

0

0.5

1

10

1.5
2
2.5
3
3.5
(C) Operating angle speed of sHUB

4

4.5

5

4

4.5

5

The motor of the actuator is controlled by thecontroller with vehicle speed and steering wheel angle
as the input conditions. The rotary motion of the motor
is transmitted to the trapezoidal screw nut through areducer and converted to linear motion. The linear
motion part of the trapezoidal screw drives the end of
the arm installed on the hub bearing, which steers thetire around the steering shaft.
The reverse input from the tire is blocked by the
self-lock mechanism of this trapezoidal screw.
Therefore, power consumption of the motor can besaved. (It’s unclear what is trying to be communicatedhere, please expand as necessary.)

20
0
-20
-40

1.5

2

2.5

3

3.5

(d) Vehicle yaw rate
1
0.5
0

-0.5
-1

Without
correction
0

0.5

1

1.5

2

2.5
3
3.5
Time s(e) Vehicle lateral acceleration

4

With sHUB
correction
4.5

5

Fig. 3 Simulation Results

Table. 1 Target Spec of sHUB
Item

ValuesMaximum steering angle
Maximum steering angular speed±3 deg10 deg⁄s-86-

*16E_03 19/07/03 15:11 ページ 4

Hub Bearing with Steering Function that Improves Vehicle Dynamic Performance

3.2 sHUB specificationline) was given to the controller to drive the sHUB tosteer and the response speed was obtained. The
response speed was defined as the maximum rising
speed of the sHUB steering angle (Fig. 7 : solid line).
Fig. 8 shows the steering maximum angular speed ofsHUB against the load torque.
When the steering torque increases, the maximum
angular speed decreases; however, we could verify
the angular speed to satisfy the objective. (Pleaseclarify this last sentence.)

The specifications of the prototype that satisfies thetargets of Table 1 and installable without changing thesteering geometry of the test vehicle are shown in
Table 2. The weight was set to 13.8 kg, which adds 5
kg or less to the standard components (knuckle andhub bearing) of the original vehicle.
Table. 2 Specifications of sHUB Prototype
Item

ValuesMaximum steering torqueSupply voltage

350 NmMaximum steering angle

Maximum steering angular speed
Weight24 V±3.5 deg

4

16 deg/s

313.8 kg

Steering angle deg

2

4. Bench test result
4.1 Property test equipment
Fig. 5 shows the configuration of the test
equipment. The prototype knuckle was fixed to thetest equipment and load was applied to a point at theflange edge 120 mm away from the steering shaft by
the air cylinder to simulate the reaction force from tire.
The actual load on the flange was measured by aload cell while sHUB was in operation.

1
-3

-4

-2

-1

0
-1

0

1

2

3

4

-2
-3
-4

Motor command value deg

Fig. 6 Relationship between Motor Command Valueand Steering Angle

4
3

Steering angle deg

Load cell

120mm

Air cylinder

Fig. 5 Test equipment of sHUB

2

Command
value

1
0
-1
-2

Actualmeasurement-3-4

4.2 Relationship between the motor command
value and steering angle

Time sFig. 7 Input Value and Steering Angle

Steering maximum angular speed
deg/sFig. 6 shows the relation between the motor
command value and steering angle. The steering
angle is the angle of the hub flange surface actually
measured.
It was verified that this prototype operated linearlywith respect to the command value.
Since the tire is fixed to the hub flange surface withbolts, the angle of the hub flange surface and theangle of tire are the same.

4.3 Response speed
While the steering torque is applied (load applied at
120 mm from the shaft by the air cylinder) to the hub
bearing, the command value shown in Fig. 7 (broken

25
20
15
10
5
0
0

100

200

300

400

500

Steering torque N・mFig. 8 Maximum Angular Speed on Steering Torque-87-

*16E_03 19/07/03 15:11 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

5.3 Steering transient response test

5. Actual vehicle test results

In order to verify the improvement of the vehicle
responsiveness with the sHUB controlled by thiscontrol law, a "pulse steering input" test was
conducted according to "Transient Response Test
Procedures for Passenger Cars (JASO Z110:2003)." 6)Due to limitations in the test site space, the test
conditions were set at a vehicle speed of 80 km/h andpulse width of 0.5 s.Fig. 11 shows the operatingprocedure of the steering wheel. The triangular wave
is the input through the driver's steering action.
The analysis results of frequency response of yaw
rate (a) and lateral acceleration (b) against thesteering wheel angle are shown in Fig. 12, from thedata obtained in this test. The red line indicates theresult with corrective steering by sHUB and the blueline is the result with no correction. The tests wereconducted three times each and similar results wereobtained.
By using this control, the vehicle performance wasincreased as the peak value increased and the phaselag improved, both for yaw rate and lateral
acceleration, in the steering wheel angle frequency
range (0.5 to 2.0Hz) used in ordinary driving.
Therefore, the driver feels better vehicle
responsiveness against the steering action and betteroverall maneuverability.

5.1 Parameter setting

The values of the parametersα1,α2 andα3 were
determined by the sensitivity evaluation results fromthe single lane change test and the slalom runningtest of the test vehicle with sHUB.

5.2 Response test
The system was installed on the test vehicle toverify the mechanical responsiveness of sHUB against
the command value.
The test vehicle traveled on the single lane change
course shown in Fig. 9 at a speed of 80 km/h. The
road surface was asphalt (dry).
Fig. 10 shows the command value to sHUB for single
lane change with the black line and the actual corrected
steering angle with the red line. Fig. 10 (a) shows thedata of the entire test and (b) shows the enlarged viewof the area indicated by the broken line in (a) As shownin (b), there was a delay of around 20 ms max.;
however, it was insignificant in the driving test.

Starting point

3.5m

Steering angle

2.5m
15m

15m

Fig. 9 Course Layout for Single Lane Change

1Fig. 11 Input Steering Condition
0

0.5

1

1.5

2

-1
-1.5

Time s(a) Full test data
1

Controller
command
value
20ms

0.5
0.25
0

-0.25 1.3
-0.5
-0.75
-1

1.35

1.4

90

1.45

Phase degsHUB angle deg

0.75

0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

1.5

Actualmeasurement

0.1

0.05

0.1

1

10

0
90

60

60

30

30

0
-30
-60
-90

0.1

1

Frequency Hz(a) Yaw rate

Time s0.2

0.15

Phase deg0Lateral acceleration
m/s2/deg

0.5

-0.5

Time

Pulse width

Yaw rate 1/s

sHUB angle deg

1.5

10

0.1

Fig. 10 Mechanical Response Motion for Single Lane
Change

10

1

10

0
-30
-60
-90

0.1

Frequency Hz(b) Lateral acceleration
Without
correction(b) Enlarged view of portion

1

With sHUB
correction

Fig. 12 Transient Response Characteristic-88-

*16E_03 19/07/03 15:11 ページ 6

Hub Bearing with Steering Function that Improves Vehicle Dynamic Performance

5.4 Single lane change driving test

The delay of peak for the vehicle yaw rate andlateral acceleration against the driver's steering action
was also small with improved responsiveness.
Fig. 14 shows the driving trajectory of this test. The
corrective steering by sHUB prevented over steering
when the driver returned the steering wheel whiledriving.

A single lane change test was conducted toevaluate the vehicle driving limit performance(behavior, responsiveness, convergence performance,etc.). The course layout was the same as Fig. 9.
The vehicle entered the starting point at a speed of
80 km/h to run with no acceleration after passing thestarting point. Fig. 13 shows the result of this test. The
red line indicates the case when the correction wasmade for the tire angle by sHUB and the blue line
indicates the case without correction. There was nodifference in the vehicle speed depending on thecorrection.
When corrective steering was made by sHUB, thesteering action of the driver was smaller overall, andthe driver's corrective steering amount after lanechange was smaller as well. Therefore, it must havebeen easier for the driver to maneuver the vehicle,
with intuitive operation.

Distance in Y directionm

5

Vehicle speed
km/h

0

0

10

20

30

40

50

Without
correction

60

70

With sHUB
correction

Distance in X direction m

Fig. 14 Vehicular Swept Path

5.5 Slalom driving test
A slalom driving test was conducted to verify thedriving stability.
Fig. 15shows the course layout of this test. In thistest, the vehicle entered the starting point at a speedof 50 km/h.
Fig. 16 and 17 show the change of yaw rate andlateral acceleration against the steering angle. The redline indicates the case when the corrective steering
was made by sHUB and the blue line indicates thecase without correction.
Controlling the sHUB allows smaller steering wheel
action and improved linearity of yaw rate and lateral
acceleration, with which the driver can drive thevehicle with ease.

50
25
0

0.5

0

0.5

0

0.5

1

1.5

2

2.5

3

1

1.5

2

2.5

3

1

1.5

2

2.5

3

(a) Vehicle speed

50
0
-50

(b) Steering angle

10
5
0
-5
-10

50

(c) Wheel steering angle

Starting point

25
0

15m

-25

Lateral acceleration
G

-50

0

0.5

1

1.5

2

(d) Yaw rate

2.5

2

60

1

40

0
-1
-2

0

0.5

1

1.5
Time s22.53(e) Lateral acceleration
Without
correction

15m

15m

15m

15m

Fig. 15 Course Layout for Slalom

3

Yaw rate deg/sec

Steering angle
degWheel steering angle
deg

1

-3

100

Yaw rate
deg

2

-2

75

-100

3

-0

100

0

4

20
0
-20

-60
-200

With sHUB
correction

Without
correction

-40
-150

-100

-50

0

50

With sHUB
correction
100

150

200

Steering angle deg

Fig. 13 Test Result

Fig. 16 Relationship between Steering Angle and Yaw Rate-89-

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NTN TECHNICAL REVIEW No.86（2018）

Additional note:

Lateral acceleration G

1.5This article is a transcript of the translation of the original
paper "Hub Bearing with Integrated Active Front Steering
Function that Improves Vehicle Dynamic Performance" into
Japanese from the Proceedings for the Spring Meeting ofSociety of Automotive Engineers of Japan, 2018 No. 58-18
20185263.

1
0.5
0
-0.5
Without
correction

-1
-1.5

-200

-150

-100

-50

0

50

Steering angle deg

With sHUB
correction
100

150

References

2001) Takuma Sekiya, "Hub Bearing with Integrated
Active Front Steering Function that Improves
Vehicle Dynamic Performance," Kanagawa
Institute of Technology, Graduation Thesis
16YKN02, (2016)
2) Katsuyoshi Miyamoto, "A study on fundamental
vehicle specifications and handlingcharacteristics," Kanagawa Institute of Technology,
Graduation Thesis 12ABE05, (2012)
3) Takuma Sekiya, Rio Tanaka, Makoto Yamakado,
Yoshio Kano, Masato Abe, "A Study on Vehicle
Handling Dynamics using a Simple Description
Type of Three Dimensional Vehicle Model",Proceedings for the Fall Meeting in 2017,20176253, (2017), p1365-1369.
4) Katsuyoshi Miyamoto, Hiroyuki Matsumoto, Isao
Kuwayama, Yoshio Kano, Masato Abe, "Effect ofVehicle Fundamental Specifications on Steering
Characteristics Relying on Tire Cornering
Characteristics (Second Report)" Proceedings of
the Society of Automotive Engineers of Japan
Annual Congress, 2014, 2014753, No.122-14,(2017), p1-4.
5) Masato Abe, "Automotive Vehicle Dynamics," (2nd
Edition), Tokyo Denki University Press (2012)
6) JASO Z110, 2003 "Automotive Standard: Transient
Response Test Procedures for Passenger Cars"

Fig. 17 Relationship between Steering Angle and GY

6. Conclusion
We have developed a hub module with steeringassist function (sHUB) which is installed in both sidesof the front wheels to improve driver maneuverability.
With appropriate corrective steering of both wheels
according to traveling conditions, improvement of thevehicle motion performance was achieved.
With vehicle motion simulation, the responsivenessof steering was improved and the control law forrealizing the ideal vehicle behavior was established.
From the above simulation result, the target
specification for the sHUB was determined, a benchtest using a prototype was conducted andperformance to satisfy the target specification wasverified.
Improvement of vehicle motion performance withthe above control law was verified by installing thesHUB in the test vehicle.
On an ongoing basis, we will continue studying thecontrol law to independently control each sHUB on theright/left wheel and verify its effect with an actual
vehicle test.

Photo of authors

Norio ISHIHARA

Hirokazu OOBA

Atsushi ITOU

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center

Mitsunori ISHIBASHI Makoto YAMAKADO
Kanagawa Institute ofTechnology
Department of
Vehicle System Engineering-90-

Kanagawa Institute ofTechnology
Department of
Vehicle System Engineering

Yoshio KANO

Masato ABE

Kanagawa Institute ofTechnology
Department of
Vehicle System Engineering

Kanagawa Institute ofTechnology
Department of
Vehicle System Engineering

*17E_03 19/07/03 15:12 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]"Tapered Roller Bearings"
with Early Failure Detection Sensor
Naota YAMAMOTO*

Hiroshi UCHIMURA**

Shota TOHO**

In this paper, we introduce "Tapered Roller Bearings" with early failure detection
sensor, which can detect flaking that may occur on bearings during operation.
Continuous use of bearings with flaking failure leads to a risk of damage toperipheral components such as the shaft and housing due to abnormal bearing
rotation. Furthermore, such flaking might also cause secondary damage such as
damage to gear parts when flaking debris from bearings is dispersed andbecomes stuck in the gears. Therefore, when flaking occurs on bearings, it is best
to stop operation promptly. However, until now, it was difficult to detect flaking on bearings because it wasundetectable due to the vibrations or noise of the entire machine. The developed product utilizes a magnetic
sensor for the tapered roller bearing, with its own split type filter seal structure. Therefore the developed productcan detect failure and prevent flaking debris from flowing out of the bearings.

1. Introduction

2. Structure and features

With regard to bearings used in construction
machinery applications with oil lubrication, early
detection of damage (flaking) is one strategy for
reducing the life cycle cost (LCC) in addition to theusual approach of increasing life and reliability of the
bearing itself. Flaking is a phenomenon wherein therolling surfaces of a bearing exfoliate in scales,
propagating to the entire rolling surface (Fig. 1) with
continued operation and inducing noise and vibrationthat prevents smooth rotation. In addition, if flaking
fragments circulate within the machine, othermechanical components, such as gears, may bedamaged. Therefore, it is critical to detect early signs offlaking and its progress in order to reduce LCC.
In this article, we describe a “tapered roller bearing
with early failure detection function” (hereafter,
developed product) as a solution to this challenge.

The developed product consists of a tapered roller
bearing, which is widely used under heavy loadconditions, including those in construction machinery,
and a "seal ring with filter" (hereafter, seal ring) withintegrated magnetic metal sensor (Fig. 2). The seal
ring prevents flaking fragments from migrating outside
the bearing while maintaining lubricating oil for
circulation. In addition, the integrated magnetic metal
sensor can provide early detection of flaking.

Connecting part
Magnetic metal sensor

FilterSeal ring with filter

Configuration ofcomponents

Fig. 1 Inner ring of tapered roller bearing
which flaking had spread

Assembled
product

Fig. 2 "Tapered Roller Bearings"
with early failure detection sensor**Application Engineering Dept., Industrial Business Headquarters**Product Design Department, Industrial Business Headquarters-91-

*17E_03 19/07/03 15:12 ページ 2

NTN TECHNICAL REVIEW No.86（2018）

2.1 Magnetic metal sensor

2.2 Seal ring with filter

The developed product uses a magnetic metal
sensor to detect flaking. Fig. 3 shows a circuit
diagram for the sensor. Two permanent magnets areplaced on the circuit board and when a flaking
fragment passes through the space between them,the output voltage drops.

In applications with circulating lubrication, bearingsand other mechanical components such as gears areoften lubricated together. In these cases, when flaking
occurs, the flaking fragments are dispersed within themachine together with the lubricating oil and may
damage other mechanical components. Therefore, aseal ring with the following specifications was made toprevent the spread of flaking fragments from thebearing.
· Make the gap between the seal ring and inner ringsmaller (Fig. 5※1).
· Provide multiple magnets, as well as pockets to holdthe captured flaking fragments in the seal ring.
· Provide multiple filters to maintain ease of oil
circulation (Fig. 6).
Fig. 7 shows how a flaking fragment is captured by
the seal ring.

(A)

OutputGND
Permanent
magnetPower supply
5V

Fixed resistanceΩ

Fig. 3 Sensor circuit diagram

In this method, the distance (A) between the two
permanent magnets in Fig. 3 affects the sensitivity forthe detection of flaking fragments. The greater the
distance is, the lower the detection sensitivity
becomes. On the other hand, when the distance (A) is
too small, the system detects wear debris smaller than
the flaking fragments; therefore the detection accuracyof flaking fragments degrades (Fig. 4). The developed
product optimized the distance (A) to increase thedetection accuracy of flaking fragments.

Flow of lubricating oil(pump action)
Magnet Sensor

FilterSeal ring with filter※1

Inner ringsmallerdiameterside

Metal particles (size of wear debris is assumed: approx. 10 μm)
Metal particles (size of early flaking pieces is assumed: approx. 100 μm)

Not detected Detected

Sensor sensitivity

Wear debris pocket

FilterDistance between
magnets

Gap

Fig. 5 Cross sectional view of developed productOptimum distancesetting of (A)

Close

Inner ringlargerdiameterside

Distant

A

Fig. 4 Magnet distance and sensor sensitivity

MagnetA’

A - A' cross section
MagnetFig. 6 Filter and built-in magnet

Flaking piece

Fig. 7 Magnet with flaking pieces-92-

*17E_03 19/07/03 15:12 ページ 3"Tapered Roller Bearings" with Early Failure Detection Sensor

usually used in environments with high external
vibration; therefore, the threshold of vibration is often
set at 2 to 3 times higher than the level at stable
operation, so that those external vibrations are notdetected as a failure. In these settings, a rise such asthat in Fig. 11 is not identified as a failure, and themachine will continue to operate after flaking occurs,
which may induce further propagation of flaking andcause damage to other machine elements.
Therefore, the developed product is verified to be
more effective for early detection of flaking than theordinary method of detecting abnormalities in thevibration signature.

The seal ring is a split type design consisting of two
semi-circular components, as shown in Fig. 2, forease of assembly into the target machine. Two
different styles, type-A and type-B, with differing
connection methods (Fig. 8) are used. When thetapered roller bearing and seal ring are inserted into
the housing together, type-A is used, where aconnecting part is inserted in the groove of the innerdiameter of the housing 1). When the seal ring is fixed
to the shoulder or end part of the housing, type-B isused. In either case, the seal ring is fixed to the
housing, and even when creeping (phenomenon
wherein the outer ring rotates during operation) occurs
the sensor cables do not become disconnected.

Radial load
Vibration sensor
placement
Developed productAxial load

Motor

Connecting parttype A

Connecting parttype B

Fig. 8 Types of connected parts
Fig. 9 Test equipment

3. Performance test results

Table. 1 Test Condition

3.1 Detection performance of flaking fragments
The evaluation test shown in Fig. 9 and Table 1
was conducted to verify the detection performance offlaking fragments. In order to promote flaking, an
indentation was applied to the rolling contact surface
of a rolling element before the test (Fig. 10).
Fig. 11 shows the test results. Since a voltage dropof the magnetic metal sensor output was observed
102 hours after the beginning of the test, the test
equipment was shut down and the bearing wasinspected. Flaking on the rolling contact surface of arolling element was verified (Fig. 12).
In this test, we also conducted measurement using
an accelerometer on the outer surface of the housing,which is an ordinary failure detection method, andmade a comparison with the measurement using themagnetic metal sensor. As a result, the accelerometeron the outer surface of the housing also detected thechange of vibration 102 hours after the beginning of
the test, in like manner to the magnetic metal sensor.
However, in construction machinery, which is the main
application of the developed product, bearings are

Bearing

Tapered roller bearings

Rotational speed

2,000min-1

Inner diameter x outer diameter x width

Lubricating oil

φ65×φ130×37 mm
Turbine oil (VG46)

Lubrication method

Circulating lubrication

Axial load

100kN
2,070MPa

Radial load

Maximum contact surface pressure

12.5kN

Indentation
10mm

Fig. 10 Test bearing before the test-93-

*17E_03 19/07/03 15:12 ページ 440
35

Vibration value m/s2

16

Threshold of vibration value
Outer surface of housing

30

Developed product(integrated sensor)

25

Anomaly could not be determinedby the measurement on the outersurface of the housing

12

8

20
15

Anomaly was detected by the developed product

4

10
5
0
95

96

97

98

99

100

101

102

103

104

0
105

Sensor voltage output of developed product V

NTN TECHNICAL REVIEW No.86（2018）

Fig. 14 Test bearing after the continuous test

Operating time hFig. 11 Test result

Flaking

4. Development of additional functionality
In addition to the flaking detection introduced in thisarticle, other types of condition monitoring, such as
bearing temperature and vibration, are also being
performed. In the case of the bearing temperature andvibration, measurements are often conducted at theouter surface of the housing due to the restriction of
sensor placement; therefore, sufficient sensitivity or
reliability may not be available because of the
distance from the bearings.
In addition, measurement of shaft rotational speed
is also performed for controlling machine output.
However, early failure may occur due to the severe
external environment when sensors are attached tothe machine exterior.
Therefore, the addition of temperature androtational speed sensors to this development productwas studied (Fig. 15).

10mm

Fig. 12 Test bearing after the test

3.2 Capture performance of flaking fragments
In construction machinery, the developed productmay continue to operate even after flaking is detected.
Therefore, seal rings are required to capture the flaking
fragments to prevent damage to other machineelements. After the test in Section 3.1, we resumed theevaluation test shown in Table 1 by re-installing the test
bearing into the test equipment shown in Fig. 9.
The result was that the motor indicated overload 13
hours after the beginning of the test, and the test
equipment was turned off so that the bearing could be
inspected. It was verified that the flaking had
propagated on the rolling contact surface of the rolling
elements (Fig. 13). We also observed a significant
number of flaking fragments in the seal ring, verifying
the effect of the developed product for preventing theflaking fragments from spreading outside of the
bearing (Fig. 14).

Magnetic metal sensor

Connecting part
Temperature
sensorSeal ring with filter

Rotational speed sensor

Fig. 15 Prototype image

4.1 Results of the temperature sensor test
A temperature sensor was installed in the seal ringand temperature of the lubricating oil circulating within
the bearing was measured. For comparison purposes,
the temperature was also measured at the outersurface of the outer ring of the bearing.

10mm

Fig. 13 Test bearing after the continuous test-94-

*17E_03 19/07/03 15:12 ページ 5"Tapered Roller Bearings" with Early Failure Detection Sensor

Fig. 16 shows the temperature measurement result
during the flaking fragment capture performance testof Section 3.2, until the test equipment was turned off.
This test determined that the temperature sensor
installed in the seal ring indicated about 5˚C ofdifference at 2 hours after the beginning of the testcompared with the measurement at the outer surfaceof the outer ring; however, the difference shrank toless than 2˚C after 8 hours, which was almost
equivalent.

3,500

Rotational speed min-1

3,000
2,500
2,000
1,500
1,000
500
0

Temperature ˚C

120

Rotational speed of main spindle of the test equipment
Measurement of rotational speed of cage by the developed product
Main spindle rotational speed(converted; cage rotational speed x coefficient)

Measurement on the outer surface of outer ring
Integrated sensor in the seal ring with filter

0

50

100

Operating time sec

150

200

Fig. 17 Rotational speed instrumentation result

100

5. Conclusion
This article introduced the structure and features ofthe developed product, “tapered roller bearing withearly failure detection function."
This developed product is expected to detect theearly stages of bearing damage (flaking), as well as
prevent secondary damage to peripheral components
due to the spread of flaking fragments, thereby
reducing life cycle costs. In addition, it can also
incorporate various sensors for multi-sensing and IoTapplications.
We are committed to continue the development of
products with high added value.

80

60

0

2

4

6

8

Operating time h

10

12

14

Fig. 16 Temperature rise test result

4.2 Results from the rotational speed sensor test
A rotational speed sensor capable of operating inlubricating oil 2) was installed inside of the seal ring tomeasure rotational speed of the cage. The rotational
speed of the cage and that of the main spindle werecorrelated, and it was verified that the converted valueof the measured cage rotational speed matched thespindle rotational speed of the test equipment (Fig.
17).
In the future this sensor is expected to be
implemented in the developed product as a rotational
speed sensor for use in lubricating oil.

References
1) Naota Yamamoto, Hiroshi Uchimura, Shota Toho:
Tapered Roller Bearing with an early failure
detection function, Hydraulics & Pneumatics (2017)19-23
2) Kenichiro Naito, Naota Yamamoto, Katsunori
Sone: Market Trend and High-Functionality ofWheel Bearings for Off-Highway Truck, NTN
TECHNICAL REVIEW, No.80 (2012) 78-82.

Photo of authors

Naota YAMAMOTO

Hiroshi UCHIMURA

Shota TOHO

Industrial Business Headquarters
Application Engineering Dept.

Industrial Business Headquarters
Product Design Dept.

Industrial Business Headquarters
Product Design Dept.

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*18E_03 19/07/03 15:12 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Asymmetrical Spherical Roller Bearings
for Wind Turbine Main Shafts*Kazumasa SEKO**Takashi YAMAMOTO**Wind turbine main shaft bearings are subjected to axial loading caused by windloads. The use of spherical roller bearings for such applications results in one row
being subjected to larger loads as compared to the opposite row. Additionally,
spherical roller bearings have a characteristic of rolling and sliding due to inherent
internal geometry. These conditions, combined with insufficient lubrication at theroller/raceway contact, lead to wear at the outer raceway surface. As a result
damage might occur particularly in the outer ring raceway surface.
NTN has developed “Asymmetrical Spherical Roller Bearings” for wind turbine
main shafts that have an asymmetrical design utilizing different length rollers and a different contact angle between
internal left and right roller rows in order to address these issues.

required for bearings for larger and off-shore windturbines since the replacement cost is significant once
failures are found.

1,000

Installed base (actual)
Installed base (predicted)

Installed base GW

800
600
400

238

283

319

433

488

592

650

774

841

20
07
20
08
20
09
20
10
20
11
20
12
20
13
20
14
20
15
20
16
20
17
20
18
20
19
20
20
20
21
20
22

5

159

198

370

540

712

06

20

3

04

1

02

20
0

20
0

0

121
74 94
48 59
24 31 39
20

200

20

Wind power generation has evolved globally into aclean energy with little impact on environment and noCO2 emissions.
According to an announcement from Global Wind
Energy Council (GWEC), the installed capacity of windturbines at the end of 2017 was 540 GW, an increaseof over 5 times from the previous 10 years. In addition,according to GWEC's market prediction, the increase
will continue at the pace of approximately 9% to 10%
a year (Fig. 1).
Previously, adoption of wind turbines has beenpromoted by national policy such as Feed-in Tariff(FIT). However, Europe and the U.S. are promoting
wind turbines to become a profitable energy source
without subsidies so that it can compete against
thermal or hydroelectric power generation, by
gradually reducing subsidies.
Wind turbine manufacturers are now engaged in fulldevelopment of off-shore turbines for improving power
generation efficiency and equipment availability, as
well as a countermeasure against a reduction in thenumber of sites adequate for on-shore wind turbines.
The construction costs of off-shore turbines are
significantly higher compared to on-shore models; asa result, the power generation capacity per facility islarger, in some cases up to 12 MW.
On the other hand, much higher reliability is

20
0

1. Introduction

ActualForecast

Fig. 1 Global cumulative installed wind capacity 1)

2. Structure of wind turbines
2.1 Location where bearings are used
Wind turbines come in different types such as
horizontal axis and vertical axis. We will introduce you
to the representative type for large commercial
use:the three-blade (wing) horizontal axis type.
Fig. 2 shows the nacelle portion of the induction
generator, which is the mainstream design for on-**Application Engineering Dept., Industrial Business Headquarters**Product Design Department, Industrial Business Headquarters-96-

*18E_03 19/07/03 15:12 ページ 2

Asymmetrical Spherical Roller Bearings for Wind Turbine Main Shafts

100%

Rotor

Yaw Drive
Blade

Generator

Frequency of use (estimate)

GearboxMainbearings

NacelleBack to back single row
tapered roller bearings

Double row tapered roller bearings
with large contact angle

Cylindrical roller bearings +double row tapered roller bearings

Spherical roller bearings

0%

1

2

3

4

5

6

7

8

Output capacity NW

Tower

Fig. 2 Internal structure of wind turbine

Fig. 3 Relationship between generation capacity
and type of main shaft bearings 2)

shore turbines with around 2 MW of output. Receiving
wind energy, the blades enable the rotor to rotate; therotational speed is increased by the gearbox, which isconverted to electric energy by the induction
generator.
Bearings are used to support the rotor shaft, within
the gearbox and generator, as well as to allow pitch
control of each blade, yaw control of the tower top andwithin the reducer which drives them. Approximately
20 to 30 bearings are used per wind turbine.

3. Cases of damage and countermeasures
3.1 Technical challenges that the market faces
As mentioned before, high reliability is required forbearings of wind turbines; however, there are cases
where main bearings fail before the theoretical
calculated operating life. Demand for long operating
life is high, especially for spherical roller bearings,
which are the current mainstream model widely used
for main bearings of wind turbines with around 2 MWof generating capacity.
For main bearings of wind turbines, in addition toradial load applied vertically from the weight of therotor and blade, unidirectional axial load is appliedhorizontally from the wind; therefore, larger load isapplied to the rear row (away from the blades)
compared with the front row (near the blades) in
upwind model※1 turbines, which is the current
mainstream model (Fig. 4).

2.2. Type of main bearings
Main bearings that support rotor shafts havebecome larger and larger over the years. Off-shore
wind turbines use bearings of extremely large size,
over 2m of outer diameter, which are not used inregular industrial machines. In addition, different typesof bearings are used depending on the power
capacity. In wind turbines of around 2 MW ofgeneration capacity, spherical roller bearings are often
used as they have high load capacity and superior
allowable capability toward mounting errors.
On the other hand, for models with over 2 MW ofcapacity, bearing types used in them are varied suchas back-to-back single row tapered roller bearings,
double row tapered roller bearings with large contact
angle, combination of cylindrical roller bearings anddouble-row tapered roller bearings, etc., depending on
the structure and power generation methods ofdifferent wind turbine manufacturers. In the case ofoff-shore models with over 5 MW of capacity, tapered
roller bearings are more frequently used because of
their advantage of contact angle and moment load
carrying capacity. Fig. 3 shows the relationshipbetween generating capacity and type of main
bearings.

Front row(near to blade)

Rear row(away from blade)Wind load path
Axial load

Fig. 4 Loading condition of SRB for main shaft bearings※1 Upwind type: wind turbine with the rotor to receive the windpositioned in the upwind side.

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*18E_03 19/07/03 15:12 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

In addition, due to the metal contact between the
raceway surface and the rollers from rolling andsliding, typical of spherical roller bearings※2 (Fig. 5),
and poor lubrication (insufficient oil film), stepped wear
may propagate on the surface of the raceway from thepoints with a high PV value※3. Due to this phenomenon,
stress concentration on the rolling-only points, where
no wear occurs, may cause flaking and cracking,especially on the rear row outer ring where high load
is applied (Fig. 6, Fig. 7 (a) - (c)). While uniform load
is applied about the entire raceway of the rotating
inner ring, load is concentrated within a certain loadzone of the fixed outer ring; therefore, when the load
is repeatedly applied, damage may occur.

Wear occurring from the points
with high PV value

Lines of purerolling contact(a) Area of wear occurrence
Stress concentration on the Linesof pure rolling contact

Same speed difference
appears on the outerring as the inner ringSliding
velocity

Rotational speed of
raceway at each point
Rotational speed ofroller at each point(b) Area of stress concentration occurrence
A：Roller rotational speed < raceway rotational speed
B：Roller rotational speed > raceway rotational speed
C：Roller rotational speed = raceway rotational speed

Flaking due to stress concentration

Fig. 5 Image of rolling and sliding in SRB

Flaking at lines of pure rolling contact(c) Area of flaking occurrence
Fig. 7 Mechanism of flaking on non-slip line
Fig. 6 Flaking on non-slip line

※2 Rolling and sliding: sliding due to the difference of rotational speed between the roller and raceway.
※3 PV value: product of contact pressure (P) and rolling and sliding velocity (V ).

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Asymmetrical Spherical Roller Bearings for Wind Turbine Main Shafts

3.2 Countermeasure by asymmetric design

product of 240/530B (φ530×φ780×width 250) are thesame. By adopting this developed product when windturbines are newly designed, bearings can be
downsized, contributing to compact and lightweightwind turbines overall (Fig. 11).
The developed product has a center rib on the innerring so that the roller position is supported at threepoints, namely, the inner ring raceway, outer ring
raceway and the inner ring center rib (Fig. 8). This
prevents skew※4 of rollers to reduce sliding between
the raceway and the rollers.

NTN has reviewed the design of rollers for use
under the conditions typical to wind turbines toimprove operating life and wear resistance anddeveloped "Asymmetrical Spherical Roller Bearings(hereafter, developed product) as a measure tocounteract the aforementioned damage. Specifically,
the developed product adopts a smaller contact angle
for the front row and larger contact angle for the rearrow, as well as longer rollers for the rear row andshorter rollers for the front row to efficiently carry
uniaxial wind loading at the rear row and to actively
accept radial loading at the front row. With thischange, the load can be appropriately shared by therollers of the front and rear rows (Fig. 8) 3), 4).

Front row

Conventional
product
240/600B

Rear row

L1

Developed
product
240/600B

L2

Approx. 2.5 times
the conventional product
0

0.5

1.0

1.5

2.0

2.5

3.0

Life ratio

Fig. 9 Comparison result of calculation life of
conventional SRB and asymmetrical SRBα1α2Center rib
Conventional productα2 ＞α1Developed product

L 2 ＞ L1

PV value

Reduction of
approx. 30%

Fig. 8 Cross section of Asymmetrical SRB

Center rib sideThis developed product can be designed within thesame dimensions as the conventional product;
therefore, it can replace existing conventional productin the operating wind turbines to enable longer
operating life and prevent early failure, contributing to
the reduction of maintenance cost. The developed
product has around 2.5 times the calculated operating
life of the conventional product and achieves around
30% reduction of PV value, which is an indicator for
wear, under the typical environment conditions of thewind turbine main bearings (Fig. 9 and 10).
Alternately, the design allows approximately 10%
reduction in bore diameter for a bearing withequivalent life as the conventional product, resulting inapproximately 30% less weight. For example, theoperating life of the conventional product of 240/600B(φ600×φ870×width 272) and that of the developed

Small rib side
Roller length
in the axial direction

Fig. 10 Comparison result of PV value on rear side row
of conventional SRB and asymmetrical SRB

Conventional
product
240/600B

Reduction of 11%
in bore diameter
Reduction of 29%
in weight

Developed
product
240/530B

Fig. 11 Example of design for down sizing※4 Skew: roller inclination over its normal axis of rotation inroller bearings.

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NTN TECHNICAL REVIEW No.86（2018）

Fig. 13 shows the test results and the calculation
results of the rolling element load distribution of eachbearing. By comparing the test results, it was revealed
that the rise of temperature of the rear row of thedeveloped product was 2 to 3˚C lower than theconventional product, and the difference intemperature between the front row and rear row was
also smaller. From this result, we can conclude thatthe developed product efficiently distributes the load toeach row compared with the conventional productunder the average load conditions of wind turbines, as
shown in the calculation results of the rolling element
load distribution.

4. Evaluation of the bearings of theactual size
Fig. 12 and Table 1 shows the test equipment andtest conditions, respectively. The load was assumed
to be a combination of radial load and axial load
averaged from bearing loading of actual wind turbines.
The test was conducted by measuring the operating
temperature under three discrete rotational speeds
which occur within actual wind turbines.

Radial load
Temperature
measurement point
for the front row

Temperature
measurement point
for the rear row

Table. 1 Test condition of actual size bearing test
Conventional
product

Test bearing

Test bearing front side

Test bearing

Bearing size (mm)Bearing design

Axial load

Test duration (h)Load (kN)

Fig. 12 Testing machine for actual size bearings test

Temperature rise
50min -1

Temperature rise ˚CConventional
product

Front row

10

Front row
Rear row

Rear row

5
16

N

0

15

0

115

20,000

20

0

Axial

40,00010min -1

25

392（0.06Cr）

Radial

60,00030min -130

32 at each rotational speed

Test bearing front side45
35

Asymmetric

10, 30, 50 (step-up)

Load distribution on rolling elements

50
40

ID 600 x OD 870 x W 272

Standard

Rotational speed (min-1)

Test bearing rear side

Developed
product

32

48

Time h64

80

96

Test bearing rear side
Test bearing front side50

60,000Developed
product

Temperature rise ˚C

4540

40,00035

20,000

30
25

0

20
15

Front row

10

Front row
Rear row

Rear row

5
0

N

0

16

32

48

Time h64

80

96

Test bearing rear side

Fig. 13 Test result and calculation result of load duration distribution with actual size bearings-100-*18E_03 19/07/03 15:12 ページ 6

Asymmetrical Spherical Roller Bearings for Wind Turbine Main Shafts

References

5. Summary

1) GWEC, GLOBAL WIND STATISTIC 2017,(February, 2018), pp 3.
2) Michio Hori, Natsumi Itou, Makoto Shizunai, Toru
Takahashi, Technical Trends of Bearings for Wind
Turbines, Japan Wind Energy Association, Special
Issue for the 40th Anniversary, Vol. No. 123(November, 2017), pp 454.
3) Kazumasa Seko, NTN New Products, Introductionof "Asymmetric Spherical Roller Bearings" for Wind
Turbine Main Shafts, Clean Energy, Vol. 307, (Feb,2018), pp 67.
4) Kazumasa Seko, Development of "Asymmetric
Spherical Roller Bearings" for Wind Turbine Main
Shafts, THE TRIBOLOGY, No. 368, (Apr, 2018),
pp 16.

With spherical roller bearings used for main
bearings of wind turbines, flaking and cracking may
occur due to wear on the outer ring raceway of therear row. This damage may be a result of rolling andsliding movement typical of these types of the
bearings and poor lubrication, as well as unidirectional
axial wind load applied to the rear row.
NTN developed "Asymmetrical Spherical Roller
Bearings" for wind turbine main shafts which havedifferent lengths and contact angles for the rollers ofdifferent rows. The following are features of thedeveloped product.
· Asymmetric design with different lengths andcontact angles for rollers of different rows.
· Improved calculated life of approx. 2.5 times(compared with the conventional product)*· 30% reduced PV value (compared with theconventional product) for improved wear
resistance*· Design that allows bearings with equivalent life
as the conventional product with approx. 10%
less bore diameter and approx. 30% less weight* Calculated under average fatigue load applied to the windturbine main bearings assumed by NTN.

Photo of authors

Kazumasa SEKO

Takashi YAMAMOTO

Industrial Business Headquarters
Application Engineering Dept.

Industrial Business Headquarters
Product Design Dept.

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*19E_*09 19/07/03 15:13 ページ 1

NTN TECHNICAL REVIEW No.86（2018）[ New Product ]Grid Connectable NTN Micro Hydro Turbine

Takashi ITOU*

kanta KIMURA*

Yasunari KANAMURA*

Demand of renewable energy such as hydroelectric power generation isincreasing rapidly from rise of global warming and the interest to the energy mix.
NTN already commercialized the NTN Micro hydro Turbine which is used inexisting agricultural canal and industrial canal and makes electric power
available in the place where the no power system exists. NTN develops Grid
Connectable NTN Micro hydro Turbine. This paper introduces the features andthe structure of the developed products .

Securing the emergency power supply in case ofnatural disasters is also a great challenge. Althoughstandby generators are being installed for emergency
power sources, storage of fuel is another challenge.
Renewable energy does not require storage of fueland can be used as an emergency power source.
NTN has productized a Green Power Station, which
stores power from photovoltaic power generation andwind power generation into batteries and is used forlighting at night or as an emergency power source.
Hydraulic power generation is a stable powersource with little variance compared to photovoltaic orwind power generation. NTN has developed andproductized the "NTN Micro Hydro Turbine" whichuses the energy of running water in waterways 2). NTN
Micro Hydro Turbine is an independent power source
which assumes private consumption by storing
generated power in batteries for supply in the areas
without good power infrastructure or as an emergency
power source in case of disasters.
On the other hand, feed-in tariffs (FIT) started inJuly 2012 to promote renewable energy in Japan. FIT
is a program to require power companies to purchasepower generated by renewable energy at a fixed price.
Introduction of this program facilitates recovery ofcapital investment; therefore, installed bases ofphotovoltaic power generation is significantly growing.

1. Introduction
The energy issue is a global challenge that we haveto deal with to achieve a sustainable society. Some of
the key initiatives include reduction of greenhouse gas
emissions related to recent climate change, energy
saving for phasing out fossil fuel dependency, anddiversifying energy sources by promoting use ofrenewable energy. Fig. 1 shows the actual power mix
before the Great East Japan Earthquake and theprediction for FY2030. Renewable energy, inparticular, is assumed to more than double from theactual ratio before the Great East Japan Earthquake,
indicating high expectations 1).

100％
90％
80％
70％

Energy saving11%

Nuclear27%

Renewable
Around 22-24%
NuclearAround 20-22%

60％
50％
40％
30％
20％

Total thermal
62%

Ave. of 10yrs
before earthquake

Biomass
3.7 〜 9.1％
Wind around 1.7%
Photovoltaic
Around 7%

Total thermal
Around 56%

10％0％Geothermal around 1.0%

Hydropower
Around 8.8-9.1%

FY2030Fig. 1 The power supply construction in FY2030*Engineering Dept. Green Energy Products Division-102-

*19E_*09 19/07/03 15:13 ページ 2

Grid Connectable NTN Micro Hydro Turbine

NTN's own market research revealed that there wasstrong demand for hydraulic power generation of thegrid connectable type, so that the generated power
can be sold, taking advantage of FIT. In addition,
since the purchase price for hydraulic power
generation of less than 200 kW is more profitable than
200 kW or more, as shown in Table 1, adoption of
hydraulic power generation of less than 200 kW will belikely to grow 3).
In this article, we describe the structure andfeatures of "Grid Connectable NTN Micro Hydro
Turbine" that NTN has developed which allows sale ofgenerated power.

3. Structure of Grid Connectable
NTN Micro Hydro Turbine
Fig. 3. shows the outer view of NTN micro hydroturbine, Table 2 shows the specification and Fig. 4
shows the configuration of Grid Connectable NTN
Micro Hydro Turbine.
The AC power generated by NTN micro hydroturbine is converted to DC power by the controller andinput to the power conditioner.
The power conditioner is a device to convert the DC
power to AC power and transmit it to the grid. The
controller and the power conditioner are collectively
called the grid connectable system, which enables
power generated by the NTN micro hydro turbine to beefficiently transmitted to the grid and sold to the utility
company.

Table. 1 FIT price of hydro turbine
1,000 kW or more 200kW or more
andandless than 5,000kW less than 1,000kW

Less than
200kW

FY2017FY201824 yen + tax

FY2019FY2020Purchase period

29 yen + tax

34 yen + tax

20 years

2. Grid connection
Grid connection means that power generation
facilities such as photovoltaic power generation andhydraulic power generation are connected to powertransmission lines or power distribution lines of utility
companies, as shown in Fig. 2. By connecting power
generation facilities to the grid, the generated power
can be sold to the utility companies.

Fig. 3 NTN Micro Hydro Turbine

Table. 2 Specification of NTN Micro Hydro Turbine
NTN
micro hydroturbineGridconnectablesystemGrid

Utilitycompany

Recommended
Rated
flow rate waterway exceeding
(mm)
m/s

Hydro turbine
blade diametermm

Output
kW

600

0.4

2

Width : 700
Depth : 700

900

1.0

2

Width : 1,000
Depth : 1,000

1,300

2.0

2

Width : 1,400
Depth : 1,400

Fig. 2 Image of grid connection
NTN
micro hydroturbineGrid connectable system

Controller

GridPower conditioner

Fig. 4 System configuration of Grid Connectable
NTN Micro Hydro Turbine-103-

*19E_*09 19/07/03 15:13 ページ 3

NTN TECHNICAL REVIEW No.86（2018）

The following is the control process of MPPT (Fig. 5):(1) Reduce the rotational speed of the turbine blades
from N1 to N2 to increase the power output from
P1 to P2.
(2) Reduce the rotational speed of the turbine blades
from N2 to N3 to increase the power output from
P2 to P3.
(3) When the rotational speed of the turbine blades
decreases from N3 to N4, power output changes
from P3 to P4.
(4) As the power output decreases to P4, therotational speed is increased back to N3.
(5) When the rotational speed of the turbine blades
increases from N3 to N2, power output changes
from P3 to P2.
(6) As the power output decreases to P2, therotational speed is decreased back to N3.
By repeating this process, generated power ismaximized.

4. Functions of the components ofthe grid connectable system(1) Controller
The controller consists of rectifying circuitry which
rectifies the power generated by the generator andcontrol circuitry which controls the Grid Connectable
NTN Micro Hydro Turbine. The controller has thefollowing functions:
1) Converts (rectifies) the AC power generated by thegenerator to DC power and inputs it to the power
conditioner.
2) Monitors the generated power and rotational speedof the generator.
3) Controls the rotational speed of the hydro turbine
blades in order to push out foreign objects
entangled in the blades 2).

(2) Power conditioner
The power conditioner is a device to convert the DC
power converted by the controller to AC power, which
is transmittable to the grid, and transmit it to the grid.
In the grid connectable system of "Grid Connectable
NTN Micro Hydro Turbine," the maximum power point
tracking (MPPT) is adopted for the control ofmaximizing power generation.

Generated power

Maximum point of generated power
(6)
(4)
(5)

5. Power generation control of Grid
Connectable NTN Micro Hydro Turbine
MPPT is a control to automatically maximize power
generation.
By applying MPPT to power generation control forhydraulic power generation, the load to the generator
is changed to control the rotational speed of theturbine blades, while measuring generated power, forthe speed to obtain the maximum power generation.
The Grid Connectable NTN Micro Hydro Turbine hasmodels for 0.4 kW, 1 kW and 2 kW of power
generation and the standard implementation of MPPT
for systems with less than 2 kW power generation is
the first in the industry. MPPT implementation in the
Grid Connectable NTN Micro Hydro Turbine has thefollowing benefits:

P3
P2
P4

(3)

(2)
(1)

P1

N4 N3 N2 N1
Turbine blade rotational speed

Fig. 5 Control Method of MPPT

However, when MPPT was applied to hydraulicpower generation, there were cases where themaximum power generation could not be attained by
the following:
When the flow rate and water level of the waterwaychanged abruptly in a short time and deviated from the
maximum power point of MPPT, the control could not
follow the change and the load to the generator
continued to increase causing a shortage of turbine
blade torque driven by the water flow and reduction of
turbine blade rotational speed. This phenomenon
occurs when, in Fig. 6 "Rotation speed of blades vs.
torque characteristic diagram," the operating condition,
which is usually at point (1), moves to the stall region
such as point (2), causing reduction of rotational
speed and torque of turbine blades, resulting in lower
power generation.

· Eliminates the need for measuring the width, waterlevel, and flow rate of the waterway to which thesystem is to be installed and adjusting the controller
to the result, achieving a significant reduction in
installation time and cost.
· Enables maximization of power generation, evenwhen the water level or flow rate changes in thewaterway.

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*19E_*09 19/07/03 15:13 ページ 4

Grid Connectable NTN Micro Hydro Turbine

The Grid Connectable NTN Micro Hydro Turbine
compares and controls the turbine blade rotational
speed and generated power at all times so thatmaximum power is attainable and the operating
conditions never move into the stall region.
The problem has been resolved by incorporating
control to detect the deviation of the operating point anddrive the MPPT to return to the maximum power point,even when the conditions move into the stall region.

(1)means it has tracked the maximum power generation
well.

Flow rate
High

Turbine blade torque

Stall region

Fig. 7 Field test
(2)

Table. 3 Test condition
Item

Condition

Hydro turbine blade diameter

Flow rate
Low

φ900

Water depth
Flow rate in the demonstration test

Turbine blade rotational speed

Fig. 6 Rotation speed of propeller vs torquecharacteristic diagram

670 mm
2.0 m/s(a) When electronic load is connectedNTN
micro hydroturbineController

Electronic load

6. Demonstration test in the actual waterway
The demonstration test of the developed gridconnectable system was conducted to evaluate
performance. The configuration of the demonstration
test is shown in Fig. 7 and the test condition is shownin Table 3. The demonstration test verified the tracking
performance of the maximum power generation by
MPPT of power conditioner. The waterway used for thedemonstration test did not have enough water and a
portion of the turbine blades was out of water.
First, the maximum power generation of thewaterway under the demonstration test was verified byconnecting the electronic load to the controller output,
as shown in Fig. 8 (a), changing the generated
voltage (proportional to the rotational speed of theturbine blades) and measuring the generated power.
The results are shown in Fig. 9. The maximum
generated power was 749 W.
Next, the power conditioner was connected, as
shown in Fig. 8 (b) for the measurement. In thisdemonstration test, simulated power supply load wasused to simulate the condition of grid connection.
Fig. 10 shows the results of the generated power.
The generated power was 738 W on average, which(b) When power conditioner is connectedNTN
micro hydroturbineController

Simulated powerPowersupply loadconditioner

Fig. 8 Test lig layout of field test

Generated power W

1000
800
600

749W

400
200
0

0

100

200

300

Generated power voltage V

Fig. 9 Result of field test

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400

*19E_*09 19/07/03 15:13 ページ 5

NTN TECHNICAL REVIEW No.86（2018）

Operating lever

Generated power W

1000
800

Support beam

600

738 W (ave)400

NTN bearing units(both sides)

200
0

0

200

400

Time sec

600

800
Rotating axis

Fig. 10 Result of field test

Fig. 13 Lever type

7. Mechanism for easy operation "Easy Lifter"

Coupling

When waterways are swollen by heavy rain fromtyphoons, etc, driftwood and debris may damage thehydro turbines. This should be avoided. If turbine
blades can be lifted above water level, they can be
more safely operated. NTN micro hydro turbine can beequipped with a mechanism, “Easy Lifter,” totemporarily lift the turbine blades from the water inemergency.
In addition, this mechanism can also be used tosafely and easily lift the blades without requiring heavymachinery such as cranes for periodic maintenance,
allowing time and cost reduction for maintenance andinspection tasks. Fig. 11 shows the condition of
normal operation and Fig. 12 shows how the blades
are lifted up.
There are two ways for operating "Easy Lifter." Fig.
13 shows the lever type, where the turbine connectedto the rotational shaft is manually operated by a leverpivoting at the NTN bearing unit, and Fig. 14 shows
the worm gear type where electric motors can bemounted.

Fig. 11 Normal operation

Support beam

NTN bearing units(both sides)

Worm gear

Rotating axis

Fig. 14 Worm gear type

8. Conclusion
In this article, we have described the functions andfeatures of "Grid Connectable NTN Micro Hydro
Turbine" which allows the sale of electric power
generated by NTN micro turbine, configuration andperformance of the controller and power conditioner of
NTN's highly efficient and proprietary grid connectablesystem, and "Easy Lifter", which allows temporary
lifting of the turbine blades from the water.
The market for hydraulic power generation using
agricultural waterways, etc. is expected to grow withhigh demand for hydraulic power generation systemwith shorter recovery of capital investment and ease ofoperation. We will continue to work toward higher
efficiency and respond to the needs for broader
adoption of micro hydro turbines.
NTN is also working on fulfilling related products
using natural energy, including photovoltaic and wind
power generation for diversification of energy sources
and achievement of a sustainable society.
For example, the demand and application area are
expanding for NTN Hybrid Street Light 4), which wasFig. 12 After lifting up blade unit-106-

*19E_*09 19/07/03 15:13 ページ 6

Grid Connectable NTN Micro Hydro Turbine

References

launched in 2016. NTN Hybrid Street Light has been
adopted, as an independent power source andstructure for the application “Disaster Relief Map,” thelargest disaster rescue/prevention map in Japandeveloped by Osaka University and “Mimamori
Robokun III,” Wi-Fi communication infrastructure
equipped with a monitoring camera developed by the“National Activity Support Network for Local
Municipalities,” a non-profit organization 5). "Mimamori
Robokun III" is equipped with a monitoring camera
and Wi-Fi equipment as a standard, as well as a LED
light, and requires stable power supply even in anenvironment not favorable for wind or daylight
conditions. Therefore, a higher power capacity versionof the hybrid street light was newly introduced at theGreen Power Station.
We will continue proposing locally produced andconsumed energy through development andmarketing of products that use natural energy in orderto achieve a low-carbon society. In addition, we willcontribute to the safety and security of local
communities by demand stimulation at the local
governments and local community associations.

1) Ministry of Economy, Trade and Industry: Longterm Energy Supply and Demand Outlook andrelated materials2) Tomoya Kawai, Hiroki Mukai, Tomomi Gotou,
Micro Hydro Turbine, NTN TECHNICAL REVIEW,
No. 84, (2016) 29.
3) Ministry of Economy, Trade and Industry home
page: "Nattoku! Renewable Energy"4) NTN news on new products, "NTN Launches
Extremely Silent Hybrid Street Lights,"
https://www.ntnglobal.com/en/news/new_products/
news201600059.html
5) NTN news on new products, NTN Participates in“Joint Research for Creating Mechanisms
Regarding Disaster Prevention Monitoring andTourism through Use of IT,”https://.ntnglobal.com/en/news/press/news
201700089.html

Photo of authors

Takashi ITOU

Kanta KIMURA

Yasunari KANAMURA

Green Energy Products Division
Engineering Dept.

Green Energy Products Division
Engineering Dept.

Green Energy Products Division
Engineering Dept.

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NTN TECHNICAL REVIEW No. 86（2018）

Award Winning Products2017 “CHO” MONODZUKURI Innovative Parts and Components Awards, Automotive Components AwardULTAGE※1 Tapered Roller Bearing
for Automotive Application
Takashi KAWAI

Yasuhito FUJIKAKE

Takanori ISHIKAWA(3) World-class high speed rotational performance:
Permissible rotational speed – improved by approx. 10%(4) Allowable inclination angle (magnitude of misalignment):
Allowable inclination angle - Up to 4 times

1. Introduction"ULTAGE* Tapered Roller Bearing for Automotive
Application" 1) realizing the world's highest standard of highload capacity and high-speed rotational performance received
the Automotive Components Award of the 2017 “CHO”
MONODZUKURI Innovative Parts and Components Award
sponsored by MONODZUKURI Nippon Conference andNikkan Kogyo Shimbun, Ltd.
In order to obtain the longest possible bearing life based ontheory, a "new optimizing design technology for internal designof bearings" was build and "proprietary roller processing
technology" was established as original technology.

Fig. 2 shows an example of calculation of contact stress
distribution at axial cross section of raceway. With ULTAGE
Tapered Roller Bearing for Automotive Application, adoptionof special crowning leads to improvement of bearing operating
life by minimizing the overall contact stress distribution while
suppressing excessive pressure at the end of the contact area
(edge load).
Large

Contact stress is
minimized by
optimized roller
shapeContact stress

Increase of temperature is reduced by optimized shape

Susumu MIYAIRI

Outer ring

Conventional tapered roller bearings2)
ULTAGE Tapered Roller Bearings
for Automotive Application

Small
Cage

Inner ring

RollerRoller raceway axial direction widthFig. 1 Structure of ULTAGE Tapered Roller Bearing

Fig. 2 Contact stress distribution on raceway

for Automotive Application

2. Structure

4. SummaryFig. 1 shows the overall structure of ULTAGE Tapered
Roller Bearing for Automotive Application.

While the operating conditions of tapered roller bearings forautomotive application are becoming increasingly severe,requirements for smaller size and longer operating life are
increasing. We want to introduce our ULTAGE Tapered Roller
Bearings for Automotive Application into a broad market tomeet those requirements and contribute to vehicle fueleconomy.

3. Features
The following shows the characteristics of ULTAGE
Tapered Roller Bearing for Automotive Application incomparison with conventional tapered roller bearings 2):(1) The world’s highest load capacity:
Basic dynamic load rating - 1.3 times(2) Long operating life (compared with the basic rating life):
Standard type (bearing steel; standard quenching) 2.5 times or more
High-functionality type (FA process) - 3.8 times or more

References
1) Yasuhito Fujikake, Takanori Ishikawa, Susumu Miyairi: ULTAGE
Tapered Roller Bearing for Automotive Application, NTN
TECHNICAL REVIEW, No. 85, (2017) 51-55.
2) NTN, Roller Bearings, Overview Catalog, CAT. No. 2202 - VII/J.

Photo of authors※1 ULTAGE TM is a name created from the
combination of "ultimate," signifying
refinement, and "stage,” signifying
NTN’s intention that this series of
products be employed in diverse
applications and is the general namefor NTN’s new generation of bearings
that are noted for their industryleading performance.

Takashi KAWAI

Yasuhito FUJIKAKE

Takanori ISHIKAWA

Susumu MIYAIRI

Automotive Business
Headquarters
Automotive Bearing
Engineering Dept.

Automotive Business
Headquarters
Automotive Bearing
Engineering Dept.

Automotive Business
Headquarters
Automotive Bearing
Engineering Dept.

Automotive Business
Headquarters
Automotive Bearing
Engineering Dept.

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*20_22受賞案件E_*18,19_受賞案件 19/07/03 15:14 ページ 2

NTN TECHNICAL REVIEW No. 86（2018）

Award Winning ProductsSociety of Materials Science, Japan,"51st Symposium on X-ray Studies on Mechanical Behavior of Materials, the Best Presentation Award"

Effect of Residual Stress on Peeling of Rolling Bearings
Naoya KAMURA

Takumi FUJITA

Toshihiko SASAKI

1. Introduction
This study revealed that rolling contact fatigue is aphenomenon that involves not only the change of residual
stress, half width and retained austenite, but also crystal
orientation. This was demonstrated by investigating the
progression of rolling contact fatigue that results in peelingdamage using X-ray residual stress measurement with a twodimensional detector 1), 2).
The presentation of this study at the " 51st Symposium on
X-ray Studies on Mechanical Behavior of Materials",sponsored by The Society of Materials Science, Japan
received the "Best Presentation Award" which is presented to
selected young researchers. In this report, an overview of thispresentation is presented.

(a) Before testing(b) 106 cycles

Fig. 1 Diffraction rings before and after testing
18

Carbonitrided specimen
Standard thermal treated specimen

16
14

S/S0

12

2. Overview

108
6

The rolling contact fatigue test was conducted under leanlubrication conditions, using a two-roller testing machine. The
surfaces of the specimens were measured with an X-ray
diffraction ring analysis system using a two-dimensional
detector. The residual stress measurement, retained austenite
measurement and crystal orientation was investigated using
triaxial stress analysis theory. For the specimens, standard
heat treated SUJ2 and carbonitrided bearing steels were used,
and the change of X-ray analysis values due to rolling contact
fatigue of these specimens was investigated. Fig. 1 shows anexample of X-ray diffraction ring obtained from the test.
Fig. 2 shows the change of the parameter S/S0, whichindicates progression of crystal orientation. The test revealed
that the carbonitrided specimen delays the progression of
crystal orientation compared with the standard heat treatedspecimen. It is assumed that the retained austenite and solid
solution element (mainly nitrogen) are affecting this result.

4
2
0

1

10

102

103

Loading cycle

104

105

106

Fig. 2 Relationships between progress of crystal orientation

S/S0 and the number of loading cycles
S0：Standard deviation of intensity distribution of diffraction ring
before testing.
S：Standard deviation of intensity distribution of diffraction ring
while testing.

3. Future development
The estimation of rolling contact fatigue using X-ray
analysis is an important technology to facilitate prediction of
failure and quick evaluation of rolling contact fatigue of rolling
bearings. We will continue to advance this study on an
ongoing basis.

Photo of authors

References
1) Naoya Kamura, Takumi Fujita and Toshihiko Sasaki: Effect ofresidual stress on peeling of rolling bearings (2nd report),
Proceedings of the 51st Symposium on X-Ray Studies on
Mechanical Behaviour of Materials, (2017) 109-112.
2) Naoya Kamura, Takumi Fujita and Toshihiko Sasaki: Evaluation of
Rolling Contact Fatigue by X-ray Diffraction Ring, NTN TECHNICALREVIEW, No. 83, (2015) 67-74.

-109-

Naoya KAMURA

Takumi FUJITA

Toshihiko SASAKI

Advanced Technology
Headquarters

Advanced Technology
Headquarters

Faculty of Human Sciences,
Kanazawa University

*20_22受賞案件E_*18,19_受賞案件 19/07/03 15:14 ページ 3

NTN TECHNICAL REVIEW No. 86（2018）

Award Winning Products“Japanese Society of Tribologists 2017 Best Paper Award"

Rolling Contact Fatigue of Thrust Ball Bearing
under Low Lambda Condition
Takumi FUJITA

Naoya HASEGAWA

Naoya KAMURA

Toshihiko SASAKI1000

1. Introduction
The subject paper 1) submitted to the "Tribologist", thejournal of the Japanese Society of Tribologists, was awarded
the best paper for FY2017. An overview of this paper is asfollows:

Actual life h100

2. Overview

8

4

0.1
0.1

2

1

1

Various factors that can affect the rolling contact fatigue life
under lean lubrication conditions, or low lambda conditions,were examined. These testing conditions were based on the
results of rolling contact fatigue life tests of thrust ball bearings
under low lambda conditions. The difference between thecalculated operating life and actual life was investigated bysystematically changing the surface roughness, load androlling speed of the rolling elements and raceway (Fig.1). The
difference between the calculated operating life and actual life
was also reviewed based on the difference in surface
roughness, or fitting, data before and after the test and theexamination results of peeling damage 2).
As a result, two types of surface initiated exfoliation werediscovered. In this paper, exfoliation of smaller particles close
to the size of the width of the rolling trajectory is called flaking,and exfoliation of larger particles is called peeling. The
generation of these two types of exfoliation is determined by
the initial surface roughness of the rolling elements andraceway and fitting condition. It was also found that the fitting
behavior is dependent upon the operating conditions, and theload on the rolling elements determines the exfoliation type.
The exfoliation type of photos 1 through 9 in Fig.1 is flaking,and that of photos 10 through 12 is peeling.
It is NTN’s claim that the exfoliation that changes
depending on the operating conditions should also be
considered in the estimation of rolling contact fatigue life ofrolling element bearings under low lambda conditions. This
assertion, based on the findings detailed in this report, is notreflected in the conventional ISO or JIS fatigue life estimation
formulas. Based on the results of this study, the following
should be the next subject of ongoing research for rolling
contact fatigue life estimation of rolling element bearings
under low lambda conditions:

7
5

10

12

11

10

1

9

3
6

10

100

Calculated life (a3×L10h）

1000

Fig. 1 Correlation diagram between calculated life (a3×L10h)

and estimated life from experimental results.
(1) Estimation of fitting based on lubrication conditions androlling element material(2) Estimation of surface roughness and load distribution of
lubricating oil while considering the effect of fitting(3) Estimation of internal stress under true contact pointconsidering the result of (2) and residual stress(4) Establishing the relationship between stress (3) andoperating life (S-N diagram)

3. Future development
Research of a rolling bearing life estimation method under
low lambda conditions is an important research subject thatcan determine the reliability of rolling bearings. The continued
use of low viscosity lubricating oil used to meet the industry
requirement of higher efficiency of machinery necessitates thisresearch. NTN will continue to study the subjects discussed inthis paper in order to contribute to the improvement of thereliability of our rolling bearings.
References
1) Takumi Fujita, Naoya Hasegawa, Naoya Kamura, Toshihiko Sasaki:
Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda
Condition, Tribologist, 60, 11 (2015) 741-751.
2) Takumi Fujita, Naoya Hasegawa, Naoya Kamura, Toshihiko Sasaki:
Rolling Contact Fatigue Life of Thrust Ball Bearing under Low Lambda
Condition, NTN TECHNICAL REVIEW, No. 84, (2016) 85-95.

Photo of authors

Takumi FUJITA

Naoya HASEGAWA

Naoya KAMURA

Toshihiko SASAKI

Advanced Technology
Headquarters

Advanced Technology
Headquarters

Advanced Technology
Headquarters

Faculty of Human Sciences,
Kanazawa University-110-*20_22受賞案件E_*18,19_受賞案件 19/07/03 15:14 ページ 4

NTN TECHNICAL REVIEW No.86（2018）[ Our Line of New Products ]Short Type Sealed Journal Bearing Unit
for Rolling StockFeaturing secure sealing and short axial length of journal bearing units

Cage

Inner ring

RollerOuter ring
Newly developed oil seal
placed within the bearing width

Structure

Features(1) Approximately 15% shorter axial length and 30%
reduced journal deflection, contributes toimproved journal bearing durability

Placing the newly developed oil seal within the journalbearing width allows secure sealing and short axial lengthof journal bearing units.

(2) Rated load capacity, high-speed rotational
performance and grease sealing performanceequivalent to conventional products
Newly
Developed
product

Applications¡Journal bearing for rolling stock

Reduction of approx. 15%(Product application)
Journal bearing unit
in rolling stock

Conventional
product-111-

*20_22受賞案件E_*18,19_受賞案件 19/07/03 15:14 ページ 5

NTN TECHNICAL REVIEW No.86（2018）[ Our Line of New Products ]Compact Plunging Type Constant Velocity Joint
for Propeller Shafts (HEDJ-P)
Plunging type constant velocity joint for propeller shafts provides
compact and lightweight design

Outer ring

Ball

Shaft

Cage

Inner ring

Features

Structure¡Weight: 17% reduction

Compact and lightweight design achieved by focused
development on the most critical functions of constantvelocity joint for propeller shafts.

(compared with conventional products)¡Outer ring outer diameter: 6% reduction(compared with conventional products)¡Strength and durability:
equivalent to the conventional products

Applications¡Constant velocity joint for Propeller Shafts

Product application in propeller shaft-112-

*20_22受賞案件E_*18,19_受賞案件 19/07/03 15:14 ページ 6

NTN TECHNICAL REVIEW No.86（2018）[ Our Line of New Products ]Ultra High Angle DOJ (DOJ-W)
for Multipurpose Off-road Vehicles※
Plunging constant velocity joint with a maximum operating
angle of 39 degrees
Features
Outer ring

Ball

Cage

Shaft

39 degree
angle

Inner ring

Features

Maximum operating angle andouter ring outer diameter¡Maximum operating angle: 39 degrees(conventional product is 30.5 degrees)¡Strength and durability:

120
Outer ring outer diameter mm

equivalent to the conventional products¡Outer ring outer diameter:
minimum increase in outer diameter dimensionof less than 8% over compact design

Applications¡Drive shafts for multipurpose off-road vehicles

110

Developed product
Existing product

100
90
80
70
20

25

30

35

40

Maximum operating angle (degree)

Product application in
off-highway vehicle※Generic name for 4-wheel buggy vehicles with round steering wheels which are gaining popularity mainly in the U.S.A. for a wide range ofapplications from business to leisure. It is also referred to as SSV (Side by Side Vehicle), UTV (Utility Task Vehicle) or ROV (Recreational Off
highway Vehicle).

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