TECHNICALREVIEW
Special Industrial Machinery & Automotive Products,
Issue Fundamental Technology, New Business2020-2021

ISSN 0915-0528 OSAKA, JAPANCompletely Revised “Ball and Roller Bearings” Catalog
NTN has completely revised the “Ball and Roller Bearings” catalog thatprovides comprehensive coverage of technical explanations and our productlineup for roller bearings.
Roller bearings are used in various types of machinery and devices as an important machineelement. Performance requirements are becoming increasingly sophisticated and diversified
in terms of they need to have a long operating life, be compact, be lightweight, and support
high speeds and specific environmental conditions. Particularly in recent years, there isa need for applicable technology that increases the performance of bearings as well as
peripheral technology that includes bearings. NTN is working hard to help improve the overall
performance of machinery by advancing development and improvements to respond to theseneeds.
The new ball and roller bearings catalog, which has now been completely revised, wasedited to enable customers to select the best type and size of bearing based on technicalcontent backed by the results of our development and improvements.
The main revised content is shown below.
〇 The latest content for ISO and JIS standards are included, and general commentaries

such as bearing selection, bearing periphery design, and handling have been substantially
increased.

〇 From the results of continuous improvements in material, product and production

technology, the current bearing operating life is longer than previously published.
Therefore, the basic dynamic load ratings have been revised and increased in many cases
based on actual bearing performance data.

〇 “ULTAGE” is the newest and highest performing NTN product line. Expanded ULTAGE

product offerings can be found in the revised catalog which now features bearings newly
added to the ULTAGE series in addition to the ones previously established.

We encourage you to use this revised “Ball and Roller Bearings” catalog, and hope that we
can work and grow together with our customers “to realize a NAMERAKA society”. Thank you
for your continued support.

Ball and Roller Bearings
catalog

The catalog is now easier to use with theaddition of a stepped index on the edge ofthe page and bookmark strings

TECHNICAL REVIEW
No.88

Special Issue
Industrial Machinery & Automotive Products,
Fundamental Technology, New Businesscorporation

NTN TECHNICAL REVIEW No.88
CONTENTS

Preface
Contribution

For Industrial Machinery/Automotive Products, Fundamental Technology, and New Business Areas
Masaki EGAMI

The Past and Present of AI and Prospects in Manufacture

Ken-ichi FUKUI, Associate Professor Division of Information and Quantum Sciences/SANKEN (The Institute of Scientific and Industrial Research)/Osaka University

1

2

Special Issue for Industrial Machinery Products
Rolling Bearing Development for the Future of Industrial Machinery
Etsu HARIMA

New Products and Improved Reliability of Main Bearings for Wind Turbine Generators
Michio HORI, Yusuke YAMADA

Product Development of Rolling Bearings for Railway Vehicles
Takashi NISHIKAWA, Kengo SUZUKI, Wataru ORITO, Tsukasa TOYODA

Approach to Development of Robotic Joint-Related Products

Hiromichi KOKUMAI, Hideaki TANAKA, Kosuke SUZUKI, Yuichiro KAWAKAMI

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles

Shohei HASHIZUME, Yusuke SHIBUYA, Daichi KONDO, Yohei YAMAMOTO, Hiroyuki IWANAGA

Development of a Low Contamination Generation Bearing for Servo Motors
Araki TANAKA, Naoaki TSUJI, Hideyuki MITANI, Takayuki KAWAMURA

Introduction of Composite Material Products for Industrial Machinery
Shinji KOMATSUBARA, Norikazu MUNEDA

10
15
21
27
33
38
44

Special Issue for Automotive Products
Activities and Achievements for Automotive Market Trends CASE
Koji KAMETAKA

Small and Lightweight CVJ for Rear Sub-axles
Tomoshige KOBAYASHI

Hub Bearing Module with Steering Function for Rear Wheel
Yusuke OHATA, Atsushi ITO

Creepless Ball Bearing

Hayato KAWAGUCHI, Toshiki MASUDA, Marina NAGATA, Toshiki KAWAILow Temperature Rise and Low Torque Tapered Roller Bearing
Yasuhito FUJIKAKE, Takanori ISHIKAWA

49
54
59
66
71

Special Issue for Fundamental Technology
Initiation Mechanism of Peeling in Rolling Bearings, and Its Life Estimation Method
Naoya HASEGAWA, Takumi FUJITA, Michimasa UCHIDATE, Masayoshi ABO, Hiroshi KINOSHITA

Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings
Masashi KITAI, Yoshinobu AKAMATSU, Ken-ichi FUKUI“ETFA” Bearings Strengthened by Fine Microstructure Design
Masahiro YAMADA, Naota YAMAMOTO, Chikara OHKI

77
86
99

Special Issue for New Business Areas
Application Examples and Function Improvements of the Wrist Joint Module “i-WRIST™”

Keisuke KAZUNO, Hiroshi ISOBE, Masaki KAGAMI, Jun MIDOMAE, Yuki SHIMURA, Seigo SAKATA, Yukihiro NISHIO, Naoki MARUI

Deployment and Improved Reliability of the Condition Monitoring System for Wind Turbines
Katsuyoshi SUZUKI

105
111

Award Winning Products
Nippon Brand Award of 2019 “CHO” MONODZUKURI Innovative Parts and Components Award
Hub Bearing Module with Steering Adjust Function “sHUB™”

116

New Energy Foundation Chairman Award (field of products and services) of New Energy Awards 2019
Micro Hydro Turbine

117

Hirokazu OHBA, Satoshi UTSUNOMIYA, Norio ISHIHARA, Yusuke OHATA, Atsushi ITO

Hiroki MUKAI, Fumihiko MATSUURA, Takashi ITO, Tomoya KAWAI, Yasunari KANAMURA

2019 Japanese Society of Tribologists (JAST) Encouragement Award
Mechanism for Initiation of Peeling in Rolling Contact and the Effect of Black Oxide Treatment on the Suppression of Peeling (Part 1, Part 2) 118
Naoya HASEGAWA, Takumi FUJITA, Michimasa UCHIDATE, Masayoshi ABO

Our Line of New Product
Inch Series, SAFC/SAFD Plummer Blocks

119

Tenter Clip Bearings for Film Stretching Machine120

The affiliations and titles of the authors mentioned in the text are as of March 2021.

Preface

For Industrial Machinery/Automotive Products,
Fundamental Technology, and New Business Areas

Executive Officer, CTO (Chief Technology Officer)

Masaki EGAMINTN has been issuing the NTN Technical Review, which is a technical information magazine
for the general public, every year in the fall. Over the past 15 years, we have featured
automotive products and industrial machinery products alternately every other year, asaligned with the Tokyo Motor Show and Japan International Machine Tool Fair (JIMTOF).
However, due to the global pandemic caused by COVID-19, many events which attract large
groups of people such as the Olympics, have been put off or canceled; including JIMTOF 2020
where we had planned to participate as an exhibitor. As you are aware, tremendous efforts
are being made to change the formats of these events to, for example, online exhibitions andwebinars, to communicate new information to the public and prevent an economic slowdown.
We have a keen interest to provide information regarding new products and technologies,
even under these circumstances, as much and as soon as possible to you. Therefore, we
decided to publish our Technical Review No. 88 in a new format, not focusing on specificareas as we once did, but featuring the details of both industrial machinery products, which
we exhibited in JIMTOF 2020 Online, and new automotive products, ahead of time, as a
special issue of “Industry Machinery/Automotive Products, Fundamental Technology, and New
Business Areas.”
From the title you may have an impression that it is a mixture of everything, but we will
introduce you to recent progress of rolling bearings and module products for wind turbines,
rolling stock, robots and machine tools that we focus on in the industrial machinery area,and discuss the most recent technology of data analysis through sensing and AI algorithms,
leveraging the biggest feature of rolling bearings, which is their ubiquitous nature of“being used in each and every product,” as our initiative for advancing IoT. This includes
achievements from the “NTN Next Generation Research Alliance Laboratory” established in
Osaka University in September, 2017.
In the automotive area, one important challenge is how to deal with CASE, which isperceived as a “once-in-a-century transformation.” Specifically, in this issue, we are
introducing newly developed products such as: refined rolling bearings for a smaller andlighter form factor contributing to lower fuel consumption and drive shafts, module products
which offer new functions and value, as well as the fundamental technologies of rolling
bearings which preserve adequate bearing life with new lubricant with lower viscosity that the
automotive manufacturers are developing to reduce fuel consumption.
It is always best to see the actual products because “seeing is believing,” however, we hope
this magazine will be helpful for you, under the current situation where a real exhibition is
impossible due to COVID-19. NTN has been working on “DRIVE NTN 100,” which is our midterm management plan with the basic policy of accelerating business transformation for the
next 100 years since April, 2018, when we celebrated our 100th year of foundation. We arestarting a new mid-term management plan “DRIVE NTN 100” Phase 2 in April, 2021. We
strive for further development of the industry as a whole, enhancing our R&D activities under
our corporate philosophy:“We shall contribute to international society through creating new technologies anddeveloping new products (For New Technology Network: Networking the World with New
Technology).”NTN TECHNICAL REVIEW No.88

1

Contribution

The Past and Present of AI and Prospects in Manufacture

Ken-ichi FUKUIAssociate Professor Division of Information and Quantum Sciences/SANKEN (The Institute of Scientific andIndustrial Research)/Osaka University

AI is now said to be in the third boom. In this article, the author first gives an
overview of the history of AI up to the present Deep Learning. Next, Deep Learning,which is currently undergoing rapid development, focusing on the expansion ofthat area is introduced. Then, the author introduces new issues and hot topics
arisen with the significant development of Deep Learning, such as ethical issues,
explanations and safety issues. In addition, the problems of human resources for AI
development with the expansion of application areas is discussed. Lastly, based on the above, the author
will state the prospects of AI in the manufacturing industry.

1. Introduction
AI (Artificial intelligence) has been a topic ofdiscussion in the general public and mass media sincearound 2015. In 2016, the AI software for the gameof Go “AlphaGo,” developed by Google DeepMindbeat the top human Go player, which became a bitof a sensation at the time1). That software uses atechnology called deep learning. While the excessive
hype of media coverage has calmed down, today, AI issteadily making strides into various industrial sectors
with expectation of increasing productivity.

2. History of AI: 3 booms and “winter times”1)
2.1 What is an AI?

In general, AI is described as a research andtechnology field with the objective of realizing
computer programs with the “ability to recognize
the external world, learn from the experience, thinkwith acquired knowledge and behave or have aconversation just as humans do.” However, the exact
definition of AI is controversial among researchers,
because there is no clear answer to the fundamental
question of “what is an intelligence?” In addition, “whatis an AI?” changes over time, too. For example, in thelate 1960s, when adoption of character recognitionin commercial use started, automatic recognition for
postal codes was called an AI.
Research of AI can be categorized broadly into two
directions:1) Engineering technology development to realize theintelligent processing functions in the computer
2) Research for gaining insight in human intelligent
behavior and brain functions by realizing theintelligent processing function in the computer
What the public at large talks about andexpects most is the AI of the above 1) Engineeringachievement of intelligent functions. Among different
areas of AI, deep learning has proven to achieve highaccuracy especially in recognition and identification.

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NTN TECHNICAL REVIEW No.88

2.2 History of AI(1) Dawn of AI (-1956)
When the worldʼs first computer ENIAC wasdeveloped in 1946, research for a “machine toperform human intelligent activities” started. In
this period, artificial neuron (mathematical modelon transmission of electrical signal of nervous
cells) was proposed by W. McCullouch and W. Pitts
and a program playing chess was developed by C.
Shannon and A. Turing. In 1956, the research area
on computers performing intelligent activities wasnamed “Artificial Intelligence” in the “Dartmouth
Conference” organized by J. McCarthy and others.
(2) Search/reasoning: The first boom (1957-1960s)
It was an extraordinary accomplishment, during
this period, that a computer could deal with eventhe slightest intelligent activity as it had only beenable to do calculations before. At that time, AI
was converting objects into explicit symbols, thenused logic for search and reasoning. Automatic
mathematical theorem proving was one of theresearch areas. However, in 1969, the “Frame
Problem,” the largest technical difficulty, was raised
by J. McCarthy and J. Hayes. The frame problem
describes an issue that a computer cannot deal
with all the possibilities that potentially arise in reallife since it only has limited information processing
capability (actually, current deep learning has notfundamentally resolved the “Frame Problem,” yet).
(3) Winter time ① (1970s)
It became clear that the approach of search andreasoning based on the symbols in the 60s was
only effective in toy problems (toy problems suchas a building block puzzle called Tower of Hanoi)
with clear rules and small magnitude but not withreal-life problems. One of the reasons was that the
AI programs did not have knowledge of the objects
and the second reason was that the computerswere still not powerful enough to solve large-scale
problems within a practical time frame. Therefore,
the fervor of research was gradually diminished.

2.3 Overview of AI field

Fig. 1 is an extract of the Overview of AI researchfor beginners and interdisciplinary researchers “AI
Map β 2.0”2) which was published in June 2020 by theJapanese Society for Artificial Intelligence. The websiteof the Japanese Society for Artificial Intelligenceshows 4 detailed maps from different perspectives, so
please review them if you are interested.
The first layer in Fig. 1, foundation, shows that AI
is built on the broad foundational scientific studies
such as mathematics, logic, cognitive science, brain
science and psychology. Then, AIʼs own foundation
includes logic, inference and knowledge engineering.
In the second layer, on top of the first layer, machinelearning including deep learning as the foundational
technology related more tightly with applications,language and image/voice media processing arefound. The research areas located in the upper side
within the second layer are the foundational areas
closer to applications. Then the most upper layer,
the third layer is the application area and shows
robots and real world, medical/biology, education,etc. however, the application fields of AI is, recently,
drastically expanding.
Medical and Biology
Major
Application classification
Third layer
Major classification(4) Time of knowledge: The second boom (1980s)
During this period, researchers were focusing
on handling real-life problems by incorporatingknowledge of the objects into computers. They
were called “expert systems” as they werededicated systems for individual problems. Many
commercial expert systems were developed.
In Japan, with the leadership of the Ministryof International Trade and Industry (currently,
Ministry of Economy, Trade and Industry) the
fifth-generation computer development project(ICOT) started in 1982 with different development
projects of expert systems and reasoning systems
using symbols based on them.
The principal player of the second boom wasthe expert system. However, research of artificial
neurons, which was one of the areas that opened
the field of artificial intelligence research also madegood progress with the introduction of Multi-Layer
Perceptron. Character recognition through neuralnetwork also developed in the late 1980s to the1990s. This was the period when the foundation of
the current deep learning was built.
The former approach of knowledge processing
based on the symbols represented by the expert
system is called “symbolism” and the latterapproach of computing mechanism based on themulti-layer perceptron and the numerical values ofneural network is called “connectionism.” Both ofthese contrasting approaches led their respective
times.
(5) Winter time ② (1990s-2000s)
The boom of the expert system graduallydwindled down due to its limitation of describing
knowledge base and complexity of maintaining
and updating extensive data. On the other hand,in Japan, the internet was connected for the firsttime in 1984 and the www (World-wide web) andcommunication environment drastically evolvedin the 1990s. As massive information crossed over
the web, research of data mining (technology todiscover (mining) useful knowledge from a large
amount of data) and image/information retrieval
was very heavily conducted. In addition, statisticalmachine learning based on mathematical andstatistical theories such as probability/statistics
and mathematical optimization developed.
(6) Period of data and learning: The third boom
(2013-present)
The period from 2013 to present is called thetime of “Learning” by deep learning based on
big data. Deep learning is being achieved fromthe integration of connectionism that has been
inherited from the continuing research andmathematical foundation of statistical machinelearning being developed in the 1990s and 2000s,
as well as availability of storage with the capabilityof storing and processing massive data and highspeed computing capability leveraged by a GPU(Graphic Processing Unit).

Use and sharing of
knowledge
Webintelligence

Major
Method classification
Second layer

Major
Basis classification
First layer

Environment
Human interface

Social computing
Machine learning

Logic/inference

AI and society
Industrial
application
Robots andreal world

Education
Art/entertainment
application

Agent

Metaheuristics

Language media
processing

Image voice media
processing

Knowledge engineering
Artificial intelligence basics/theory(Source: The Japanese Society for Artificial Intelligence “AI Map
β 2.0 (June, 2020)”2)
This figure is an English translation of the Japaneseversion of AI Map β 2.0.)

Fig. 1 Development from basic research to methods
and applications

NTN TECHNICAL REVIEW No.88

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The Past and Present of AI and Prospects in Manufacturing

3. Present status of AI: period of big
data and learning1)
3.1 What is machine learning?

Machine learning refers to the overall technologies
to have computers acquire learning ability. Learningis, simply put, the ability to handle various tasks
well based on past experience and data. For
example, becoming proficient at catching a ball by
predicting the trajectory after playing catch manytimes. Therefore, machine learning is a technology
for inference and prediction by studying principles
from experience and observation. Machine learning
is broadly used today, in medical diagnosis,
recommendation systems, spam filters, prediction offinancial markets, classification of DNA sequences,
pattern recognition such as voice recognition andcharacter recognition, games such as shogi (Japanesechess) and most recently, automated driving (Fig. 2).
Today, the mainstream machine learning acquired
functions to conduct tasks such as identificationand prediction from data. If the desired output is yfor the input x, then machine learning acquires they from the training data. Since
mapping function c:x
acquiring a true mapping function from the limited
amount of training data is impossible, in practice,parameters of such function are adjusted to obtain
the desired output by, for example, limiting the formof function or placing certain assumptions. Thisparameter adjustment of functions is called “learning.”
Parameter adjustment for the training data which
was used to acquire functions is relatively easy if thedegree of freedom of function is increased; however,for new unknown data, this adjustment does notwork well and is likely to fall into overfitting. One of
the challenges of machine learning is how to reduce
overfitting and increase generalization performancefor unknown data and various technologies are being
developed.

3.2 What is deep learning?

Deep learning is a general name for a neuralnetwork with many layers. Originally, the neuralnetwork was a model of transmission of electricalsignals of neural cells. However, today it is used as a
type of machine learning to learn mapping functions
corresponding to various tasks. Fig. 3 is an example ofa typical neural network of a task to identify an animal
in an image. In the learning stage, many images ofvarious animals are used and the weight between
neurons in the hidden layer is adjusted so that only
the neuron in the output layer corresponding to cats(gray dot) responds strongly when an image of a cat
is input. Conventionally, learning of a neural networkwith a deep hidden layer was difficult, however,with the recent evolution of various mathematicaloptimization techniques and development ofcomputers, learning of the network in deeper layers is
now possible.
With deeper layers, various expressions can beacquired inside the network. Before deep learning,
experts in different fields had to design input features
well, however, with deep learning, it is possible forthe network to learn features even when raw data is
directly input into the neural network. This technologyof learning features of the object problems from datais called “feature learning.” When training data can bemassively collected, this high feature learning ability
can be maximized, so deep learning currently shows
extraordinary accuracy in identifying images andvoices.
In addition, Fig. 3 depicts a simple overlay of
networks of full connectivity, however, various network
structures have been recently created by configuring
neurons with various functions and exploring different
connectivity of networks. For example, there are
Convolutional Neural Networks (CNN) which leveragespace information such as images and Recurrent
Neural Networks (RNN) which learn change intemporal sequence.

Learning Build a model
required for tasks

Cat

Input layer
Prediction

Identification

Recommendation

Hidden layer

Output layer(Source: Introduction to learning how to use AI through Python1))

Fig. 3 Typical pattern of image recognition
by deep learning(Source: Introduction to learning how to use AI through Python1))

Fig. 2 Machine learning used in various applications

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NTN TECHNICAL REVIEW No.88

3.3 Development of deep learning

Deep learning started to attract attention when it
broke the record of the conventional classification
performance with a significant margin at international
competitions of image recognition in 2012 and 2013.
Fig. 4 shows the championship records of ImageNet
Large Scale Visual Recognition Challenge (ILSVRC)3)
held from 2010 to 2017. ILSVRC is a large-scale
problem of classifying around 14 million images into
20,000 classes. Until 2011, a method combining theconventional image processing and machine learning
was winning, however, since the appearance of deeplearning in 2012, classification errors have been
drastically decreased year over year. A classification
error of a human being for classification by visual
identification is said to be around 5 %. Deep learning
has been exceeding the average human classification
since 2015.

0.25
0.2
0.15

0.28

0.26

0.16
0.12

0.1

0.07

0.05

2010 2011 2012 2013 2014 2015 2016 2017year
ZF

/G

Al

ex

N

et

0

Human error
0.036 0.03 0.023

oo VG
gl G
e
N
et
Re
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et
(E
ns
em
bl
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SE
N
et

Classification error

0.3

Deep Learning

Fig. 4 Championship Record of ImageNet Large Scale
Visual Recognition Challenge (ILSVRC)
Deep learning initially attracted attention in thefield of image recognition. Then from 2014 to 2016,its application range expanded into “action” from“recognition,” such as general game playing, AI go andautomated driving through Deep Q-Network (DQN),
which combines deep learning with Reinforcement
Learning. In 2016, when “AlphaGo” developedby Google DeepMind beat the go champion, thatnews grabbed headlines. From 2016 to 2018,
natural language processing (field of processing
languages) also made significant progress thanks to
deep learning. Conventional automated translationthat used statistical model with bilingual corpus
was replaced with deep learning, which drastically
improved translation accuracy.
In addition, the invention of the network called
Generative Adversarial Network (GAN), with which
two networks learn from each other, enabled“generation” and “imitation” and its application range
has been recently expanding to creative fields suchas art. A famous example is the successful generationof a masterpiece of the famous Dutch artist of the
17th century, Rembrandt, by learning his painting
style through “The Next Rembrandt”4), a project led byMicrosoft. In Japan, the singing voice of Hibari Misora,
who passed away 30 years ago, was recreated andfeatured on a TV program, NHKʼs Kohaku Utagassen5)in 2019 and a new manga (comic book) production ofOsamu Tezuka (also passed away over 30 years ago),“Phaedo” was published as a result of collaboration
between AI and human artists in February 2020,
through a project called “TEZUKA 2020.”6) 7)

4. Future of AI: a thought on the outlook
in the manufacturing industry fromthe front line of research
4.1 Issue of ethics

The evolution of AI, especially that of deep learning
is tremendous as we have seen in Chapter 3. Thosetechnologies are put into practice in many industrial
fields. In order to prevent unforeseen circumstances,discussions on the ethical aspect of R&D andutilization of AI are taking place throughout the world.
In Japan, the Ethics Committee was established in
the Japanese Society for Artificial Intelligence in 2014
and the “Japanese Society for Artificial Intelligence
Ethical Guidelines” was released in 2017.8) It lists
9 articles, namely, 1. Contribution to humanity, 2.
Abidance of laws and regulations, 3. Respect for theprivacy of others, 4. Fairness, 5. Security, 6. Act withintegrity, 7. Accountability and Social Responsibility,
8. Communication with society and self-development,
and 9. Abidance of ethics guidelines by AI. In addition,“AI R&D Guidelines”9) was released in July, 2017 by
the Ministry of Internal Affairs and Communications
for developers, “Social Principles of Human-Centric
AI”10) was released in March, 2019 by the Cabinet
Office for users and policy makers, and “AI Utilization
Guidelines” 11) was released in August, 2019 by the
Ministry of Internal Affairs and Communications for allparties including consumers.
Among those guidelines, technical research on
fairness has recently been picking up steam. In 2018,
AI conducted the hiring process, after learning past
resumes and the successful/unsuccessful results ofapplicants. As a result, AI rejected female applicantsfor engineering positions through the review process,as the engineering positions are mainly occupied bymale workers. This happened because the training
data was biased. Therefore, research on technologies
to fairly handle the socially sensitive attributes are
now being conducted.

4.2 Issue of explainability

Importance of the ability to explain the “basis ofdecision” is increasing as AI is being introduced invarious fields. Current mainstream deep learning
consists of several millions and tens of millions ofparameters making it impossible for human beings to
explore the basis of decision by manually analyzing
the model that AI used for learning. Explainability is
one of the challenges critical for introduction of AI, asa lack of explanation for the decision-making process
can often be a barrier of introduction.

NTN TECHNICAL REVIEW No.88

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The Past and Present of AI and Prospects in Manufacturing
The research area to add explainability to AI iscalled XAI (eXplainable AI) and it is very active in thefield of image recognition12). For example, Fig. 5 (a)shows a dog and a cat and Fig. 5 (b) highlights thearea when a “dog” is focused on in the trained deeplearning model. Accordingly, Fig. 5 (c) highlights thearea when a “cat” is focused on. In principle, gradation
is applied on the image based on the calculation of
the gradient of output response when input pixels areslightly manipulated.

(a) Original image(b) Focused area for dog (c) Focused area for cat(Source: International Journal of Computer Vision13) )Fig. 5 Visualization example of attractive areas
in deep learning with Grad-CAM

4.3 Issue of security

In 2013, it was pointed out that the identified resultof the trained deep learning model could intentionallybe altered. This leads to the risk of misinterpretationof traffic signs and “impersonation” in face recognition.
This is called an Adversarial example, Adversarial
attack and Adversarial perturbation. For example,
Fig. 6 shows that when a special perturbation (in the
middle) is added to the image input to the trained
deep learning model (left side), all the images (rightside) are identified as an “ostrich” even if they look
the same as the input images. Defense technologies
against adversarial attacks are also being researched/suggested, however, they are only symptomatic
treatment for each case, not fundamental solutions.
It is expected that fundamental defense technologies
will be developed.

Original
image

Perturbation

Original
image

Ostrich

Perturbation

Ostrich(Source: Cornell University arXiv14) )Fig. 6 Example of adversarial attack

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NTN TECHNICAL REVIEW No.88

4.4 Limitation of current AI

In 2016, an automated driving vehicle had an
accident colliding with a trailer. It was explained that
AI could not recognize the trailer as the sunlightreflected on its white body. In addition, from 2008
to 2015, a service to estimate the outbreak of
influenza from search words was provided, however, it
overestimated the actual outbreak by more than 50 %.
These problems were caused by the fundamental
issue that the current machine learning is based on
the inductive approach where laws are obtained
from past data. It works well for the “interpolated”
cases closer to the past data, but not at all for the“extrapolated” cases not present in the past data.
Research of refinement in operation is being
pursued but another direction is to incorporate
existing theories into machine learning. Collaboration
with physics which has a robust theory backgroundis gradually increasing under the names of Theoryguided Data Science and Physics-informed Machine
Learning. Our group is also looking into this area,
collaborating with experts of meteorology, toforecast the wind in the upper troposphere. We areproposing deep learning for outputting the forecast bydecomposing the components based on the equationand the residual components15) (Fig. 7).

Restriction by equations

Residual

Input feature
Output: Upper wind vector field

Fig. 7 Deep learning in consideration ofphysical models15)

4.5 Issue of AI human resource

Recently, the shortage of human resources thatcan understand and utilize AI has become a serious
issue, as AI is being introduced in various fields. Inview of this, the Japanese government established theStrategic Council for AI Technology in 2016 to study
and discuss the human resource issues with the Human
Resource Development Task Force16) 17). The task
force summarized that the development of AI humanresource with the following capabilities is critical:① Problem solving with AI technology
Ability to understand foundational technologies
of AI such as machine learning and natural language
processing and to indicate a roadmap for problem
solving from the AI standpoint② Implementation of AI technology
Ability to implement AI by programming capability
and knowledge of computer science such as algorithmand data structure③ Utilization of AI technology
Ability to capture the challenges of the respective
target fields as the AI problems
Based on the discussion at the Strategic Council
for AI Technology, education program for fostering
immediately effective players, “Learning AI throughthe Real Data”, was held at Osaka University, where
the author belongs, and the University of Tokyo
from 2017 to 2019 as a commissioned R&D projectof NEDO (New Energy and Industrial Technology
Development Organization)18). This course assumes
engineers (all fields) with a few years to 10 years ofexperience after graduation as the audience with
objectives of learning AI foundational technologies
including machine learning, deep learning andcomputer vision and putting the knowledge into
practice through exercises using real data. This NEDO
project has now completed but the lecture of Osaka
University still continues having established a nonprofit organization19).

4.6 Prospects in Manufacturing

Fig. 8 is an extract from “Study Report on Utilizationof AI Technology in Manufacturing Area”20) by the
Ministry of Economy, Trade and Industry published in
March, 2018, which summarizes manufacturing areas
where AI can be introduced by field and objective/
accomplishment. Areas of introduction span not only
the manufacturing process but also upper stream anddownstream, as well as business management.
Currently, the area that AI fits the most is theapplication of images where big data can be easilycollected. Inspection through image recognition is
the most favorable area. AI is also introduced in thearea where relatively organized data can be collected
such as fault detection and design support. On theother hand, replacement and/or succession supportof craftsmenʼs skills which involves tacit knowledge
is still a difficult area even if some advancement canbe observed. The issue here is sensing rather than thetechnical issue of AI.
The current AI is completely based on data.
Therefore, it is important to identify and collect/extract “useful data” for utilizing AI. Field knowledge is
indispensable and certain know-how is also requiredfor collecting valuable data. When the know-how ofdata collection is established and useful AI systems
are completed, the next important phase seems tobe the creation of an ecosystem where “humans
and AI collaborate” and complement each otherfor the operation in the field. Increasing individuals
who correctly understand AIʼs characteristics and“are able to successfully operate AI systems” will be
required. The methodology of AI (machine learning)system operation is called “machine learning systems
engineering.” In 2018, the Special Interest Groupon Machine Learning Systems Engineering21) was
launched within the Japan Society for Software
Science and Technology, to promote discussions andknow-how sharing.

How AI can be utilized in manufacturing
Area |
Objective/Accomplishment

Improvement of production process(Automation/improvement → energy saving/cost reduction)■ Design of production system/

Upper stream ofmanufacturing

Development of production plan

■ Optimization of traffic line

■ Automation of assembly work/

Manufacturing process

picking■ Image recognition inspection

Downstream ofmanufacturing(efficiency improvement)■ Maintenance/follow-up service

Support activities for
manufacturing■ Development of estimate■ Facility management■ Development of businessBusiness/Management

strategy■ Demand forecasting■

Inventory control■ Inspection of parts/materials(efficiency improvement)

Improvement of quality/service(Increase value to the customers)■ R&D/material design support■ Inspection of parts/materials(improvement of accuracy)

Creation of new values(What was impossible becomes possible)■ R&D/material design support(AI supporting experts)■ Analysis and implementation of■ Replacement/succession of■ Manufacturing evolution support■ Efficiency in packaging process■ Image recognition inspection■ Provision of new follow-up

energy saving■ Improvement of yield/availability■ Improvement of logistics →energy saving■ Production line management/

maintenance(Maintenance, fault prediction,energy saving)■ Marketingcraftsman's skills(Successionof skills, machining conditions,
optimization of manufacturingconditions, management and controlof reaction/fermenting process)(improvement of accuracy)■ Maintenance/follow-up service(AI supporting experts)

service(improvement of service)■ Support for succession of skills/

skills training■ Production line management(stable operation)■ Labor management/humanresource management■ Marketing■ Customer relation■ New business/new productdevelopment expenses■ Expansion of business scope(Source: Ministry of Economy, Trade and Industry “Study Report on Utilization of AI Technology in Manufacturing Area”20))

Fig. 8 AI-applicable fields in manufacturing
NTN TECHNICAL REVIEW No.88

7

The Past and Present of AI and Prospects in Manufacturing
5. Summary
The public hype of AI in the past seems to havecalmed down, however, introduction of AI is steadily
advancing. As mentioned in this paper, significant
progress of deep learning expanded the utilizationof AI, however, at the same time, new issues suchas ethics, explainability and security have emerged.
On the other hand, the current AI, which is basedon training, depends very much on advanced datacollection and preprocessing. Therefore, it has
significant potential for the Japanese manufacturingindustry to use its strength of “front line skills.” In orderfor useful AI systems to become firmly established insociety, not as a temporary “boom,” we expect to builda system that is operated effectively where AI andhumans work together, with correct understanding ofAIʼs strengths and weaknesses.

References

1) Ken-ichi Fukui (author), “Introduction to learning
how to use AI through Python,” Japan Technical
Education Development Center (2020)
2) Japanese Society for Artificial Intelligence, “AI Map
β 2.0” (June, 2020), Japanese Society for Artificial
Intelligence HP
https://www.ai-gakkai.or.jp/resource/aimap/
3) ImageNet Large Scale Visual Recognition
Challenge (ILSVRC) HP
http://www.image-net.org/challenges/LSVRC/
4) The Next Rembrandt HP
https://www.nextrembrandt.com
5) YAMAHA Corp., “Hibari Misora Vocaloid AI”
YAMAHA HP
https://www.yamaha.com/ja/about/ai/vocaloid_ai/6) KIOXIA Corporation, “TEZUKA 2020,” Kioxia HP
https://tezuka2020.kioxia.com/ja-jp/
7) Special issue “Revival of Osamu Tezuka by AI,”
Artificial Intelligence, Vol. 35, (May, 2020), 390429.
8) Ethics Committee of the Japanese Society
for Artificial Intelligence, “Japanese Society
for Artificial Intelligence Ethical Guidelines,”(February, 2017).
http://ai-elsi.org/report/ethical_guidlines9) Ministry of Internal Affairs and Communications,“AI Development Guidelines,” Promotion Council
for AI Network Society Report 2017, (July, 2017)https://www.soumu.go.jp/menu_news/
s-news/01iicp01_02000067. html
10) Cabinet Office, “Social Principles of HumanCentric AI,” Council for Social Principles of
Human-Centric AI, (March, 2019).
https://www.cas.go.jp/jp/seisaku/jinkouchinou/
pdf/aigensoku.pdf11) Ministry of Internal Affairs and Communications,“AI Utilization Guidelines,” Promotion Council
for AI Network Society Report 2019 Annex 1,(August, 2019).
https://www.soumu.go.jp/menu_news/
s-news/01iicp01_02000081.html

8

NTN TECHNICAL REVIEW No.88

12) Satoshi, Hara, “Interpretability in Machine
Learning,” The Japanese Society for Artificial
Intelligence HP, My Bookmark (May, 2018)https://www.ai-gakkai.or.jp/my-bookmark_vol33no3/
13) Ramprasaath R. Selvaraju, Michael Cogswell,
Abhishek Das, Ramakrishna Vedantam, Devi
Parikh, and Dhruv Batra, “Grad-CAM: Visual
Explanations from Deep Networks via GradientBased Localization”, International Journal ofComputer Vision, 128, (2020), 336-359.
https://link.springer.com/article/10.1007%2
Fs11263-019-01228-7
14) Christian Szegedy, Wojciech Zaremba, Ilya
Sutskever, Joan Bruna, Dumitru Erhan, Ian
Goodfellow, and Rob Fergus, “Intriguing propertiesof neural networks”, Cornell University arXiv, last
revised 19 Feb 2014.
https://arxiv. org/abs/1312.6199
15) Ken-ichi Fukui, Junya Tanaka, Tomohiko Tomita,
and Masayuki Numao, “Physics guided Neural
Network with Model Discrepancy Based on Upper
Troposphere Wind Prediction”, Proc. IEEE 18th
International Conference on Machine Learning
and Applications (ICMLA 2019) , (2020) , 414419.
16) Yasushi Yagi, “Educational program for fostering
AI human resources: Discussion at the Strategic
Council for AI Technology,” Artificial Intelligence,Vol. 33, (May, 2018), 259-264.
17) Ministry of Education, Culture, Sports, Scienceand Technology, NEDO, “Human Resource
Development Task Force Final Report,” Reference
Material 1-1, for the Fifth Strategic Council for AI
Technology (March, 2017).
https://www.nedo.go.jp/content/100862415.pdf18) NEDO, “Fostering an Industry-Ready Workforce
in the AI Field―NEDO to hold series of special
lectures at Osaka University and the University ofTokyo ―” NEDO HP News Release, (July, 2017).
https://www.nedo.go.jp/news/press/
AA5_100809.html
19) Datability Consortium “Learning AI through thereal data”
https://www.cds.or.jp/jinzai/course01/index.html
20) Ministry of Economy, Trade and Industry“Study Report on Utilization of AI Technology
in Manufacturing Area” Ministry of Economy,
Trade and Industry Commissioned Study Report,(March, 2018).
https://www.meti.go.jp/meti_lib/report/
H29FY/000119.pdf
21) Special Interest Group on Machine Learning
Systems Engineering, Japan Society for Software
Science and Technology
https://sites.google.com/view/sig-mlse/

<Author Biography>

Ken-ichi FUKUIAssociate Professor Division of Information and Quantum Sciences/SANKEN (The Institute of Scientific and Industrial
Research)/Osaka University

2003
2005-20102010
2010-2015
2015-Present
2020-Present

Completed Materials/Life Information Engineering Course (Masterʼs Program),
Graduate School of Human Informatics, Nagoya University
Specially appointed Assistant Professor, Materials Science & Technology
Research Center for Industrial Creation, Institute of Scientific and Industrial
Research, Osaka University
Received Ph.D. from Osaka University
Assistant Professor, Division of Information and Quantum Sciences, Institute of
Scientific and Industrial Research, Osaka University
Associate Professor, Division of Information and Quantum Sciences, Institute of
Scientific and Industrial Research, Osaka University
Trustee, Japanese Society for Artificial Intelligence[Specialty]

・Machine learning, data mining and its applications
・Knowledge discovery, prediction, fault detection especially from event series data
・Scope of application includes sleep, meteorology, earthquake, fuel cell, etc.

[Affiliated academic societies]

IEEE Computer Society, Japanese Society for Artificial Intelligence, Information Processing
Society of Japan, The Japanese Society for Evolutionary Computation and The Institute ofElectronics, Information and Communication Engineers[Awards]
2008

2011
2013
2013
2013
2016
2016
2017
2018

2018
2020

IEEE 8th International Conference on Computer & Information Technology (CIT),Best Paper Award
25th Annual Conference Best Presentation (oral presentation), JapaneseSociety for Artificial Intelligence
President Award, Osaka University
Encouragement Prize, Research Area, President of Osaka University
Incentive Award in 2012, Japanese Society for Artificial Intelligence
FAN Best Paper, 26th Intelligent System Symposium
The Workshops at the 14th Pacific Rim International Conference on Artificial
Intelligence (PRICAI-2016), Best Workshop Paper Award
Incentive Award in 2016, Japanese Society for Artificial Intelligence
Encouragement Prize, Research Area, Artificial Intelligence and Knowledge
Processing Research, The Institute of Electronics, Information andCommunication Engineers
Elsevier, Knowledge-Based Systems, Outstanding Reviewer
34th Annual Conference Best Presentation (international session oralpresentation), Japanese Society for Artificial Intelligence[Publication]“Understanding Machine Learning by Python and Real Data - identification/prediction/fault
detection,” Ohmsha, 2018, etc.

NTN TECHNICAL REVIEW No.88

9

Perspective

Rolling Bearing Development
for the Future of Industrial Machinery
Etsu HARIMA*NTN supplies many bearing sizes based on the needs of various
applications in the industrial machinery market. These range from ultrasmall bearings with outer diameters of a few millimeters for electronic
devices, to ultra-large bearings with outer diameters of several meters
for wind turbines and large mining machinery. In addition to existingcustomers for “core” industrial markets such as construction machinery,
agricultural machinery, and machine tools, business from customers
relating to social infrastructure is increasing. Activity with customers in
the aircraft, railway vehicle, and wind turbine markets is growing withchanges in social structure and increased awareness of environmental issues. Therefore, NTN has developed
related engineering and production systems accordingly. Due to advancements in Internet of Things (IoT)
and development of artificial intelligence (AI) technology, it is expected that the current environment willchange significantly. This report introduces advancements in NTNʼs analysis and evaluation technology, as
well as NTNʼs approach to higher bearing performance and reliability.

1. Introduction

2. Past technology development1）

In Egyptian pictures drawn thousands of years ago,people are transporting rocks by pulling them onrollers placed underneath the rocks. This is the originof rolling bearings (hereafter, “bearings”).
It is also believed that Leonardo da Vinci, an artist
in the Renaissance era, invented the basic principle
and structure of modern day bearings. Da Vinci had
drawings of bearings with almost the same structure
as the ones used today.
It has been about 100 years since NTN started
manufacturing bearings. The appearance andcomponents are almost the same as back then.
However, the performance of the components has
significantly improved thanks to the progress ofmaterials, lubricants, and processing technologies.
Bearings are now indispensable machine components
for equipment.
In this paper, the evolution of bearings in areas
such as steel manufacturing methods, manufacturing
technologies, and design technologies is reviewed.
Additionally, NTNʼs progress of analytical technologies
and evaluation technologies, as well as its initiatives
for improvement of reliability are introduced.
This paper also introduces an approach for modular
and intelligent components by integrating forwardlooking bearing technology, sensing technology, andprecise control technology, as well as new products
and services leveraged by fault detection, condition
monitoring system (CMS), and IoT.

Looking at the NTN company history, there arenumberous revolutionary years showing excellence intechnology.
In 1934, NTN developed bearings for aircrafts andits products were adopted in the engine and body of
100% domestically-produced aircraft, “Kamikaze.”This aircraft set a world record of 94 hours 17 minutes
and 56 seconds of flight between Tokyo and London.
Double-row cylindrical roller bearings and singlerow deep groove ball bearings were adopted in 1964
as journal bearings of the 0-series Shinkansen (bullet
train), which made its debut on the newly opened
Tokaido Shinkansen line. Components of the materials,
heat treatment, and inspection conditions were
rigorously examined and standardized by the thenJapan National Railways (currently, Japan Railways)
and only NTN and other designated companies wereallowed to deliver the products.
Later, the H-1 rocket, which made a successful
launch of its first test vehicle in 1986, was the firstrocket with 100% domestically-produced liquid
oxygen/liquid hydrogen engine in the second stage.
Almost all of the bearings2) used in the rocket were
produced by NTN. In 2002, NTN developed bearings
capable of ultra-high-speed rotation of 3 million
in dn*1 value in liquid hydrogen (-253 ℃ ) for the
first time in the world together with the National
Aerospace Laboratory of Japan (currently, JAXA)3).
*1 dn value: d=ID (mm) × n= shaft rotation speed (min-1)
In general, dn value exceeding 1 million is called “high-speed”
and a dn value exceeding 2 million is called “ultra-high-speed”.

* Operating Officer, Corporate General Manager, Industrial Business Headquarters

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NTN TECHNICAL REVIEW No.88

As such, NTN has been accumulating producttechnologies and manufacturing technologies sothat NTN can respond to the needs of its customers
such as “long operating life” and “high speed.” Thesefeatures are required by all industrial machines.

two developments increase lubrication reliability
by reducing heat caused by agitating resistance of
lubricant (Fig. 2).
300

2.1 Long operating life

The evolution of the steel making method has
significantly contributed to “long operating life.”
As cleanliness of modern bearings has increased,internal origin flaking due to rolling contact fatigue
caused by non-metallic inclusions is reduced underproper lubricating conditions. Operating life ofbearings made of bearing steel with standard heat
treatment (immersion quenching) are said to besemi-permanent if used under the fatigue limit load4).
Conversely, since operating conditions of the actual
bearings are severe and they are used under higher
temperature, higher speed, and higher load, theexpectation of the market is longer life with thinner
lubrication and contamination.
Past research achievements5) revealed that
improvement of steel chemical components (alloy
element) and heat treatment characteristics(toughness, hardness, etc.) can increase bearing life.
As such, NTN has provided many long-life bearings on
the market using these technologies. NTN has recently
developed the “ETFA bearing.”6) This bearing provides
twice the bearing life of the conventional long-life “ETA
bearing” and 6 times longer than the NTN standard “4Top” series with contaminated lubrication. Resistanceto foreign objects was increased thanks to special heat
treatment technology (enhanced grain refinement) of
case-hardened steel (carburized steel) (Fig. 1).

Conventional product (ETA bearing)Higher speed

200

Adoption of aircooling technology

150
100
50
0
1980

Adoption of air-oil lubrication
1990

2000

2010

2020

2030

Time

Fig. 2 Transition into higher speed machine tool bearings
Recently, demand for bearings for main shafts
that use grease lubrication has increased for easeof use and reduced environmental impact. With
grease lubrication, heat can cause degradation of
the grease, and have a significant impact on bearing
life. Therefore, reduction of heat during operation is
an important factor. NTN developed “Machine Tool
Spindle Bearing with Air Cooling Space for Grease
Lubrication”8) with its unique air-cooling technology.
This technology introduces bearing cooling throughair-cooling nozzles on outer ring spacers between
bearings to cool the inner rings via inner ring spacers(Fig. 3).
Air cooling spacer(Outer ring spacer)

Cooling air

Air cooling spacer(Inner ring spacer)

Air cooling nozzle

ETFA bearing

Fig. 1 Prior austenite grain under surface of bearing raceway

2.2 High speed

dn value×104

250

Another critical performance characteristic required
for the bearings is “high speed.” Particularly, main
shafts of machine tools, such as machining centers,
are operated at significantly higher speed to improvemachining efficiency and accuracy. NTN has beenworking on development and improvement of its
products to respond to the requirement of high-speed
operation7).
As a result, NTN has achieved ultra-high-speed
operation by adoption of ceramic balls which arelighter than steel balls and adapt well to high-speed
operation with high contact pressure, adoption ofsmaller roller elements to reduce centrifugal forceof high-speed operation, and adoption of air-oil
lubrication and air-cooling technology8). The lastFig. 3 Machine Tool Spindle Bearing with Air CoolingSpacer for Grease Lubrication

NTN TECHNICAL REVIEW No.88

11Rolling Bearing Development for the Future of Industrial Machinery

3. Evolution of rolling bearings
As mentioned earlier, the required function forbearings was to rotate smoothly under a large load fora long time. However, current bearings are not limited
to that and NTN is now putting bearings with several
more functions on the market.
New products are being developed to respond to
demand, such as wind turbines and railway vehicles
in growing markets, robots, aircrafts and machine
tools. All of these industries require state-of-the-art
technology.

3.1 Bearings for large wind turbines andCAE*2 analysis technology

Large wind turbines have rapidly evolved as anapplication area of bearings over the last 20 years. In
addition to the requirement of a design life of over 20
years, highly reliable large-scale bearings are in highdemand for offshore wind turbines. These bearing
have outer diameters of over 2 m.
The magnitude and direction of load on bearings
can vary depending on the wind conditions, andthere are up to several millions of directionalload combinations. Also, because of the size ofthe bearings, tests using the actual products are
extremely difficult. Therefore, CAE analysis technology
for deformation of bearings and their mating shafts
and housings is indispensable (Fig. 4).
*2 Abbreviation of Computer Aided Engineering

Front row

(Close to the blade)

Rear row(Far from the blade)

Front row Rear row

α1 α2Wind load
Axial load

Center rib
α2＞α1
L2＞L1

Fig. 5 Left: Loading condition, Right: Asymmetrical design

3.2 Bearings for machine tools and IoT

Bearings for main spindles of machine tools are
required to offer ultra-high-speed operation withultra-high accuracy. However, the new trends ofintegration and combination of machining processes
require a single tool to handle deep cutting withmedium to low speed rotation with high load
resistance and fine cutting with high speed rotation.
NTN developed the “Angular Contact Ball Bearingfor High-Speed and Heavy-Cutting Machine Tools”
in 2018 by optimizing the internal design of theconventional high-speed angular contact ball bearing“HSE type” achieving 30 % higher load carrying
capacity and permissible axial load while maintaining
high speed performance10).
In addition, there is high demand for condition
monitoring and IoT functions for reliability. NTN has
newly added load detection and wireless functions tothe unit “Sensor Integrated Bearing Unit”11) which had
been announced previously with integrated sensors in
the outer ring spacer adjacent to bearing for realizing
advanced condition monitoring (Fig. 6).
<Integrated in outer ring spacer>・Sensor (load, vibration, temperature)・Electromagnetic generator
・Wireless module
Wireless

Double-row tapered
roller bearing

Cylindricalroller bearing

Fig. 4 Analysis model of wind turbine main shaft system
NTN also develops unique products contributing
to long operating life. For main bearings, self-aligningroller bearings have been used which allow installation
errors and may lead to early failure due to wear on the
raceway.
Noting that a larger load is applied to the rear row(farther row from the blade) of bearings than thefront row (closer row to the blade), NTN unveiled the“asymmetric self-aligning roller bearing”9) adopting
NTNʼs unique design of different roller lengths andcontact angles on rows in 2017 (Fig. 5).

12

NTN TECHNICAL REVIEW No.88

Fig. 6 Sensor Integrated Bearing Unit for Machine
Tool Spindles

3.3 Rolling bearings for railway vehicles andinsulation technology

Bearings are used in the journal, driving mechanism,and main electric motor locations of railway vehicles.
Bearing performance and quality directly affect thesafety of railway vehicles. Therefore, particularlyhigh reliability is required among industrial machine
bearings.
NTN is also working on technology development to
respond to recent demand of high-speed operationand extension of maintenance intervals. Stray current
corrosion can occur in the journal and main traction
motor bearings. Usually, measures are taken on
vehicles and its carriages, however, insulating bearings
themselves can be a very effective measure. NTN has
developed the “MEGAOHM™ Series”12) with ceramicspray or resin coating on the bearing outer diameter.
This development has been was received very well(Fig. 7).
Ceramic spray
insulation coating

3.4 Condition monitoring service of bearings

NTN is currently developing a condition monitoring
service (hereafter, “CMS”*3) for predictive maintenanceby IoT and NTN sensing technologies in order to
respond to the demand of energy saving, long andstable operation, and reduction of life cycle cost.
*3 Abbreviation of Condition Monitoring System

In the wind turbine area, NTN has already developed
the condition monitoring system “Wind DoctorTM.” Thisdevelopment has been implemented in wind turbine
systems with very favorable feedback (Fig. 8).
For CMS to contribute to reduction of maintenancecost and improvement of equipment availability inthe wind power generation business, highly accurate
prediction and fault detection technology are required
so that repair work can be scheduled at the right timewith the remaining life of the components known.
For improved accuracy, data analysis technology anddiagnosis algorithms must be developed, which integratedifferent kinds of information, such as operation andmanagement of facilities, not only measurement datafrom sensors.
From these perspectives, NTN is promoting
development of the CMS technology, effective forsupporting maintenance activities, and intends tomake specific proposals in the areas of railway vehicles,machine tools, and machinery equipment.

Deep Groove
Ball Bearing

Cylindrical roller bearing

Fig. 7 Ceramic insulated MEGAOHMTM series① Data collection system② Data management software③ Monitoring/analysis software

Sensors for measurement(vibration, etc.)

Dedicated server
Information of operation(rotational speed, generated power)
Data collection system

User/NTN

Fig. 8 System configuration of Wind Doctor™

NTN TECHNICAL REVIEW No.88

13

Rolling Bearing Development for the Future of Industrial Machinery

4. Future Development
Changes in the market environment are about tooccur.
・Carbon neutral
・DX(Digital transformation), IoT(Internet of Things)
・Reduction of productive population ratio
Challenges for rolling bearings are also changing
in the midst of these changes. “Carbon neutral”
raises expectations of renewable energy, especiallywind turbines. The wind turbine facility will become
increasingly larger and include offshore installations.
Companies who can design and produce bearings that
support rotors and blades of these turbines are few
and far between, however, NTN can.
As mentioned previously, NTN is increasing thereliability of its bearings and contributing to the
reduction of maintenance cost and increase of
equipment availability in the wind turbine businessthrough utilization of CMS.
NTN will also work on making bearings intelligent
in the backdrop of “DX (digital transformation) and
IoT (Internet of Things).” Moving forward, leveraging
NTNʼs sensing and analysis technologies, as well as
AI (Artificial Intelligence), NTN will offer services
to support efforts by users on fault prediction andpredictive maintenance of bearings.
Adoption of automation and robots will accelerate
due to the “reduction of productive population ratio.”In the area of industrial robots, high rigidity bearings
are required for precise positioning. NTN is focusedon development of bearings with higher rigidity within
limited space. NTN is also marketing the “i-WRIST™”
Wrist Joint Module, which leverages NTNʼs basetechnology of bearings13).
Bearings are also becoming a commodity due tothe fierce competition of low-cost products. NTN istackling the challenge of cost reduction as an importanttask by optimizing procurement of materials andcomponents.

5. Summary
The rolling bearing will continue to be a criticalmachine element. NTN must strive to lead the world intechnology development on the eternal objectives of“long operating life,” “ultra-high-speed,” “high rigidity”
and “high reliability.” NTN is focusing on bearings thatleverage IoT, condition monitoring and data collectionof rotating machines, peripheral machines that utilize
data, and development of applications, so that after
10 or 20 years, everyone will be amazed how rolling
bearings have changed.
The NTN industrial machinery business remains
indispensable, supporting development of world
industry based on bearing technology and leveraging
digital technology.

References

1) The history of NTN: 100 years (2019)2) Masataka Nosaka, Satoshi Takada, Makoto Yoshida,
Research and development of cryogenic tribology ofturbopumps for rocket engines, Aeronautical and Space
Sciences Japan, Vol. 58, Issue 681, (2010) 303-313.
3) NTN press release “Receives the JapaneseSociety of Tribologists, Technology Award” NTN
website, https://www.ntn.co.jp/japan/news/press/
news20030530_3.html
4) NTN Ball and roller bearingsʼ catalog, CAT.
No.2203/J, A-19-21.
5) Kikuo Maeda, Hirokazu Nakashima, Hiroshi
Kashimura, Development of Long Life TAB and
ETA Bearings and Their Automotive Applications,
NTN TECHNICAL REVIEW, No.65, (1996) 17-22.
6) Masahiro Yamada, Naota Yamamoto, Chikara
Ohki, “ETFA” Bearings Strengthened by Fine
Microstructure Design, NTN TECHNICAL REVIEW,
No.88, (2020-2021) 99-104.
7) Keiichi Ueda, Technical Trend of the Precision
Bearings for Machine Tools, Tool Engineer, Vol. 60
No. 16, (2019) 41-43.
8) Kazuki Sonoda, Tomohiko Obata, High Speed
Machine Tool Main Spindle Bearings with Air
Cooling Spacer for Grease Lubrication, JapaneseSociety of Tribologists, Proceedings for Tribology
Conference 2020 Fall (2020) B-11 95-96.
9) Kazumasa Seko, Takashi Yamamoto, Asymmetrical
Spherical Roller Bearings for Wind Turbine Main Shafts,
NTN TECHNICAL REVIEW, No.86, (2018) 96-101.
10) Jin Takegahana, Mineo Koyama, Kouji Jinno, Yuya
Tanaka, Angular Contact Ball Bearings for HighSpeed and Heavy-Cutting Machine Tools, NTN
TECHNICAL REVIEW, No.86, (2018) 56-61.
11) Shohei Hashizume, Yasuyuki Fukushima, Yusuke
Shibuya, Yohei Yamamoto, Development of Sensor
Integrated Bearing Unit for Machine Tool Spindles,
NTN TECHNICAL REVIEW, No.86, (2018) 50-55.
12) Hideji Ito, Insulated bearing “MEGAOHM” series,
NTN TECHNICAL REVIEW, No.71, (2003) 48-51.
13) Keisuke Kazuno, Hiroshi Isobe, Masaki Kagami, Jun
Midomae, Yuki Shimura, Seigo Sakata, Yukihiro
Nishio, Naoki Marui, Application Examples andFunction Improvements of the Wrist Joint Module“i-WRIST™”, NTN TECHNICAL REVIEW, No.88,(2020-2021) 105-110.
Photo of authors

Etsu HARIMA
Operating Officer,
Corporate General Manager,
Industrial Business Headquarters

14

NTN TECHNICAL REVIEW No.88

Technical Article

New Products and Improved Reliability of Main Bearings
for Wind Turbine Generators
Michio HORI*
Yusuke YAMADA*Wind turbine generators, which are becoming an increasingly moremainstream source of renewable energy, are becoming larger. Furthermore,bearings used in these wind turbines are required to be reliable. This section
introduces NTNʼs latest products and design approach for spherical roller
bearings used in many onshore turbines and single-row tapered roller
bearings that are used in offshore turbines more and more.

1. Introduction

2. Composition of main bearing

Adoption of renewable energy is being promoted
worldwide to address global warming, as exemplified
by the Paris Agreement, an international accord
for reducing greenhouse gas emissions. Last year,
renewable energy, excluding hydropower, exceeded
nuclear power in global share of power generated wind power accounted for about 5 % of the total1).
To broaden adoption of renewable energy, it will
be important to reduce the LCOE (Levelized Cost of
Energy) which is calculated by dividing costs of power
generation, including construction, operation, andmaintenance costs, by the projected amount of power
generated during the operating period. Turbines have
improved utilization as a result of increased towerheights, lengthening of blades, and improved blade
shape. Off-shore wind power has been increasingly
adopted in recent years. Since construction costs
are higher than on-shore wind power, there is a needto develop ultra-large turbines, often exceeding 10
MW, extend service life, and improve maintenanceefficiency.
In wind turbines, bearings are used at the support
section for the rotor shaft (main shaft), gearbox, andgenerator as shown in Fig. 1. Additionally bearings areused to allow pitch control of each blade, yaw controlof the machine head, and within the pitch and yaw
reducers which drives them. This article describes
efforts by NTN to improve reliability and achieve
optimization of the main bearing used at the main
shaft support section.

The main bearing is the most important part for
supporting the wind load received by the blades
and transmitting driving power to the generator.
The bearing type used varies depending on thecomposition of the drivetrain. As shown in Fig. 2,
there are two basic categories: the gearbox type, andthe ʻdirect driveʼ type with no gearbox.
Type ① in Fig. 2 has traditionally been the mostcommon choice for the gearbox type installed
on-shore. In this type, the main shaft is typicallysupported by a single main bearing within apillowblock housing and a bearing inside the gearbox.
Spherical roller bearings tolerant of mounting error
are used. Type ② , in which support is provided using
two spherical roller bearings, is also used in many onshore turbines. However, with type ② , sliding mustbe allowed at the outer ring outer diameter surface
on the free side to cope with thermal expansion andcontraction of the main shaft. To ensure reliability
over the long term, there are also structures which
use inward-facing (direct mount) double row tapered
roller bearings in the fixed position, and cylindricalroller bearings capable of accommodating thermal
expansion and contraction by sliding in the axial
direction in the floating position.
With the direct drive type, on the other hand,reliability is better due to the reduced number ofparts. A directly-linked generator enables power
generation at a low rotational speed by making
permanent magnets multipolar, however, thisincreases the size of the generator rotor. Therefore,
the main shaft must have a large diameter and loading
is supported by a compact double row tapered roller
bearing with a large contact angle (Type ⑤ ).
In addition to the direct drive type, a multi-polesynchronous generator of the “gearbox + medium
speed type” balances reliability and cost and is also
used in off-shore turbines. Due to the increased size of

Rotor

Blade

Main
Gearboxbearing
Generator

NacelleFig. 1 Internal Wind Turbine construction* Application Engineering Dept., Industrial Business Headquarters
NTN TECHNICAL REVIEW No.88

15

New Products and Improved Reliability of Main Bearings for Wind Turbine Generators
wind turbines, there is increased use of back-to-backsingle row tapered roller bearings instead of double
row tapered roller bearings with a large contact angle
which have a large impact on productivity and cost
(type ② , ③ ). With this bearing type, it is possible tooptimize service life and system rigidity through theuse of preload. Bearing setting must be done by thewind turbine manufacturer, but at NTN, we support
users through analysis to determine the setting range.
Gearbox typeType ①

Bearing Gearbox

shaft due to the wind load. If we assume an upwind
wind turbine, i.e., the mainstream type with the rotor
receiving the wind located on the upwind side, then alarger load acts on the rear bearing row away from theblade than on the front bearing row near to the blade(Fig. 3).
Front row(near to blade)

Rear row(away from blade)

Front row Rear row

Direct Drive typeType ④α1 α2Wind load

Main bearing + bearing in gearbox
Bearing type: SRB

Type ②Main bearing ×2

Bearing type: TRB×2 or DRTRB+CRB

Type ⑤Main bearing ×1

Main bearing ×2

Bearing type: MBRG

Bearing type: SRB×2 or TRB×2 or DRTRB+CRB

Type ③Main bearing ×2 (in rotor)

α2＞α1
L2＞L1

Axial load

Generator

Type ⑥Main bearing ×2 (in rotor)

Fig. 3 Loading condition

Centerrib

Fig. 4 Asymmetrical design

At NTN, we noticed these characteristics of theapplied load, and in 2017 we issued a press release on
an asymmetrical spherical roller bearing. This bearing
has a unique design featuring different roller lengths
and contact angles among the different rows2) (Fig. 4).
These changes increase calculated life by about 2.5
times, and enable reduction of inner diameter by about
10 % and weight by about 30 % in a bearing with life
comparable to the conventional product. If this bearing
is used in a wind turbine, bearing downsizing canbe achieved, which can contribute to reducing size,
weight, and cost of the entire wind turbine (Fig. 5).

Bearing type: TRB×2 or DRTRB+CRB Bearing type: TRB×2 or DRTRB+CRB

SRB: Spherical roller bearing TRB: Tapered roller bearing
DRTRB: Double row tapered roller bearing（face-to-face）
MBRG (moment bearing): Double row tapered roller bearing（back-to-back）
CRB: Cylindrical roller bearing

Fig. 2 Relationship between drive train and bearing typeThe following will introduce NTNʼs efforts witha focus on spherical roller bearings, which are themainstream choice for on-shore turbines, and single
row tapered roller bearings, which are increasingly
used in off-shore turbines.

3. Extending life of spherical roller bearings
The aforementioned spherical roller bearings areadvantageous due to their tolerance for mounting
error, and the ability to use independent housingsfor each bearing. However, there are cases wherepremature damage occurs due to operational wear of
the raceway surface. NTN is helping to extend the lifeof these types of bearings and minimize turbine sizeby using unique technology which accounts for theapplication conditions particular to main bearings.

3.1 Asymmetrical design of different rows

The main bearing is subjected to a radial load acting
vertically on the shaft due to the weight of the rotor
and blade, and axial loading acting horizontally on the16

NTN TECHNICAL REVIEW No.88

Reduction of about
10 % in inner diameter
Reduction of
about 30 % in weight
Conventional productDeveloped product240/600B
240/530B

Fig. 5 Example of design for downsizing

3.2 DLC coating(1) Structure
The rollers of spherical roller bearings are barrel
shaped. Within the spherical roller bearing, differential
slippage*1 occurs, where the bearing rotates withslippage at the contact points between the roller
contact surface and raceway surface. Also, rotation
repeatedly stops and starts due to wind conditions,and the bearing is lubricated with grease. If poor
lubrication (insufficient oil film) is experienced during
operation, wear may occur at the raceway surface
due to metal contact, starting from points with a highPV value (i.e., the product of the contact stress (P)and sliding velocity (V)) and progressing into a twopeaked form. As a result, stress is then concentrated
at the lines of pure rolling contact where no wear
occurs, and this may cause flaking and cracking of the
raceway surface (Fig. 6)．
*1 Differential slippage: Slippage attributable to differences in speed in therotation direction between the roller and raceway

Wear occurring from thepoints with high PV value

Stress concentration on the two peak-shapedlines of pure rolling contact (protrusions)

Flaking due to stressconcentrationLines of pure rolling
contact(2) Verification of effectiveness
Verification testing was carried out to confirm thesuperiority of the DLC coating. Testing was done with
two types of bearings: a small model bearing and afull scale test bearing.
① Evaluation using a small model bearing(ϕ 120 ×ϕ 180 × 60)

Flaking mechanism

Fig. 6 Damage mechanism
The rotating inner ring experiences loading about theentire circumference of the raceway surface, but theouter ring is fixed in the housing, and thus the load zone
is concentrated in a specific range, and damage occurs
due to repetitive loading.
With the aforementioned asymmetrical design, thePV value can be reduced by about 30 %, so the design iseffective to some degree in reducing wear. On the other
hand, it is difficult to avoid metal contact in a state withinsufficient oil film, and other approaches were needed
to further improve reliability. Thus we developed abearing with a DLC (diamond-like carbon) coating
applied to the roller contact surface (Fig. 7). The DLC
coating used by NTN employs a three-layer structure:① a metal sub-layer to increase adhesion to the basematerial, ② an intermediate layer to act as a hardness
gradient between the metal sub-layer and the top layer,and ③ a high hardness top layer of DLC coating. The
coating has outstanding adhesion and wear-resistancegiven the differential slippage particular to sphericalroller bearings and lubrication conditions where oil film
formation is inadequate (Fig. 8).

A combined radial and axial load was applied,similar to an actual turbine, and wear status of theouter ring load region was compared every 50 hours,
under accelerated conditions where the outer ring
raceway surface of a standard product flakes at 300
hours (Fig. 9).

}

Flaking
partFig. 9 Outer ring raceway after 300 hours
Fig. 10 shows the test results. The wear progression
speed of the asymmetrical product was about 2/3
that of the standard product. On the other hand,about 5 μm of wear occurred initially with the DLC
coating product, but after that wear did not progress
and the results were extremely good.

Test bearing

Changes in wear depth of outerring load region

DLC coating

Fig. 7 DLC coating spherical roller bearing
Pmax= 2.5 GPa
10 % slippage, poor
lubrication conditions

Standard productWear depthmAsymmetrical productWear depthm

DLC coating productWear depthmAppearance of coated surface
0 times

1×105 times

1×106 times

1×107 times

DLC
No DLC coating delamination

Fig. 8 DLC coating delamination test

Fig. 10 Test results with model size bearings

NTN TECHNICAL REVIEW No.88

17

② Evaluation using actual-size bearing(ϕ 600 ×ϕ 870 × 272)

To accelerate testing, as with the small model
bearing, evaluation was done under conditions where
the standard product flakes at 720 hours.
Fig. 11 shows the condition of the outer ring
raceway surface load region after testing. Flaking isevident on the standard product, but not on the DLC
coating product, where the results were extremely
good with only 2 m of wear.
Standard product

DLC coating product

Countermeasures for raceway surface wear

New Products and Improved Reliability of Main Bearings for Wind Turbine Generators② DLC coating④ DLC coating +asymmetrical design① Standard design③ Asymmetrical design

SmallerFig. 12 Lineup of spherical roller bearings forwind turbine main shaft
Flaking*2 Skew: Roller inclination over its normal axis of rotation in roller bearings

4. Size reduction of tapered roller bearings
Front

Rear

Front

Rear

2 μm

Here we introduce the characteristics and design
approaches for size reduction of the back-to-backsingle row tapered roller bearings (Fig. 13) that are
being increasingly used in off-shore turbines.

Fig. 11 Test results with full scale bearings

3.3 Proposed specifications of sphericalroller bearings

In light of the above, NTN proposes the following
for spherical roller bearings used as the main bearingsin wind turbines (Fig. 12).
① Standard design: This type employs asymmetricalrollers, and has a center rib on the inner ring.
Structurally, rollers are supported at three points: theinner/outer ring raceway surfaces, and a center rib on
the inner ring. This prevents skew*2 of the rollers, andsuppresses slippage between the raceway and rollers.
② DLC coating: This type is based on the standard
design, and is used when wear damage needs to be
suppressed.
③ Asymmetrical design: This is used when there is aneed to extend life (through flaking countermeasures)and reduce turbine size (bearing size reduction).
④ DLC coating + asymmetrical design: This typelowers initial cost by reducing overall size andweight of wind turbines, and can potentially improvereliability and reduce generation costs through morestable operation.

18

NTN TECHNICAL REVIEW No.88

Fig. 13 Single row tapered roller bearing

4.1. Design parameters

Wind loads act on the blades and the weight of
the rotor itself act on the main bearing as momentloads (Fig. 14), thus maintaining a large distance
between the load centers of the two main bearings in
the system is key for reducing the resultant bearing
load. By using single row tapered roller bearings ina back-to-back arrangement, the distance between
load acting points can be kept large, which enables
reduction of bearing size and weight.

1.40

Ratio

1.20

Equivalent
load

1.00
0.80

Rated load

0.60
0.40
1.00

1.20

1.30

1.40

Contact angle ratio

Moment loadDistance between
load acting points

Fig. 16 Example 2 Effect on the axial load side row
due to the change in contact angle of nonaxial load side row

Fig. 14 Relationship between applied load andbearing span
In determining the contact angle and distance
between load centers, optimal values must be
considered in light of use conditions. Characteristics
based on differences in contact angles are presented
next.

(2) Relationship of axial setting and life
When the axial setting is 0, as indicated in Fig. 17,
the larger the contact angle, the longer the bearing life.
However, the rate of decrease in life increases as theaxial gap decreases (i.e., when preload is increased),
and there is a reversal at more than 1,050 m
preload. This is because axial rigidity and resultant
applied axial load increases with a larger contact angle.

(1) Effects on life
In Example 1, considered in Fig. 15, we confirmed
the effects when the contact angle was varied only forthe axially loaded row. As the contact angle increased,
both the equivalent load and bearing capacity
decreased, but the capacity decreased less, so life
increased.
In Example 2, considered in Fig. 16, we verified theeffects of the axially loaded row when the contact
angle was varied only for the non-axially loaded row.
In this case, the results showed that life decreased
regardless of the fact that the distance between load
centers increased due to the increased contact angle.
This is because the induced axial load increased when
the contact angle of the non-axially loaded row wasincreased. This shows that increasing the distance
between load centers does not necessarily lead tolonger life directly, and that the induced axial load
must also be taken into account. That is, increasing
the distance between load centers by maximizing thecontact angle of the axially loaded row is related tolonger life of both bearings.
1.60
1.40
1.20

Equivalent load

1.00

Rated load

0.80
0.60
1.00 1.10 1.20 1.30 1.40 1.50 1.60

LifeLife ratio

4.2 Verifying effects due to differences incontact angle

Ratio

Life
1.10

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-2,000Smallcontact angle
Large
contact angle-1,500-1,000-500

Axial gap μm

0Fig. 17 Effect of axial clearance and bearing life fordiffering contact angles(3) Effects of temperature variation on life
With back-to-back single row tapered roller
bearings, radial expansion and contraction of the
bearing, as well as axial elongation and contractionof the shaft and housing, both affect the axial setting.
However, the effect of axial elongation/contraction of
the shaft and housing is small, and the effect of axial
expansion/contraction of the bearing is dominant.
Variation in the axial setting was calculated by taking-1,050 m where life reverses as a basis, as shownin Fig. 17, and assuming the inner ring operating+5 ℃ hotter than the outer ring (Fig. 18). If thecontact angle is small, then the rate of decrease inlife becomes smaller, as shown in (2), but the amountof change in the axial setting increases, so caution isnecessary.

Contact angle ratio

Fig. 15 Example 1 Relationship between contact
angle and bearing life for axial load side row

NTN TECHNICAL REVIEW No.88

19

Life ratio

New Products and Improved Reliability of Main Bearings for Wind Turbine Generators

2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-2,000

Small contact angle
Large contact angle

Smallcontact angle
Large
contact angle-1,500-1,000-500

Axial Setting μm

0Fig. 18 Effect to bearing life for temperature change(4) Relationship of aspect ratio and capacity tobearing cross section
As shown in Example 1 considered in Fig. 15, lifechanges with variation in the contact angle, but the
aspect ratio*3 of the bearing cross section may havean effect. As shown in Fig. 19, with an aspect ratio of
1.1, that is a cross section that is larger in the radial
direction, it is evident that the rate of decrease inbearing capacity when the contact angle is increased
from α 1 to α 2 is smaller than the aspect ratio 0.9.
This is because it is easier to achieve increased roller
diameter and roller length within the smaller aspectratio even when the contact angle is increased. On
the other hand, if the contact angle is reduced, thendecreasing the aspect ratio also makes it easier toachieve increased bearing capacity.
*3 Aspect ratio: Bearing cross section height / Bearing group width

applied axial load increases under preload to theextent that axial rigidity increases. Another point forcaution is that, when the contact angle is reduced,
increased variation in axial setting may result in
response to temperature changes.
Design with single row tapered roller bearings
must be based on a good understanding of the above
characteristics, but in actual practice, selectionof the contact angle also varies depending on thepermissible bearing size. Therefore, at NTN, we will
utilize analysis tools and know-how we have acquired
through our extensive experience to collaborate withwind turbine manufacturers from an early stage, toprovide support to establish drivetrain designs.

5. Summary
This article has introduced the efforts of NTN to
improve reliability and achieve optimization of main
bearings used in wind turbines.
As the size of bearings continue to increase, wewill respond by proposing new technologies andnew products to help optimize designs and improvereliability, including manufacturing methods, andthereby help broaden adoption of renewable energy
in collaboration with wind turbine designers.

References

1) P:Statistical Review of World Energy 2020.
2) Kazumasa Seko, Takashi Yamamoto, “Asymmetrical
Spherical Roller Bearings” for Wind Turbine Main
Shafts, NTN TECHNICAL REVIEW. No.86, (2018)
96-101.

Larger radial cross section (aspect ratio 1.1)

Cr16 %
reduction
Contact angle: α1Contact angle: α2Wider axial cross section (aspect ratio 0.9)

Contact angle: α1

Cr19 %
reduction

Contact angle: α2

Photo of authors

Fig. 19 Relationship between aspect ratio of crosssection and load rating

4.3 Summary of effect verification

By increasing the contact angle, it is possible
to reduce the radial reaction load, but there isalso an increase in the induced axial load, and it isnecessary to check effects on life and contact stress.
Furthermore, main bearings are initially set into
preload, but care must be given to the fact that the20

NTN TECHNICAL REVIEW No.88

Michio HORI

Yusuke YAMADA

Application Engineering
Dept., Industrial Business
Headquarters

Application Engineering
Dept., Industrial Business
Headquarters

Technical Article

Product Development of Rolling Bearings
for Railway Vehicles
Takashi NISHIKAWA*
Wataru ORITO*

Kengo SUZUKI*
Tsukasa TOYODA**

In order to ensure the safety of railway transportation, it is required todevelop bearings for railway vehicles with respect for high reliability of
products. Providing bearings to public transportation services such as railway
transportation service and contributing to society is significant as corporate
activity. NTN has been working on development of bearings for railway
vehicles for many years. This article introduces our product development activity of bearings for railway
vehicles.

1. Introduction
Bearings used in the axles, drive units, and traction
motors of railway machinery (Fig. 1) are crucial tothe overall safety of the vehicle. They are therefore
required to meet particularly high quality andreliability standards for an industrial application.
These bearings must have the strength andperformance to withstand not only large static loads,
but also the elevated dynamic loads associatedwith operation of the vehicle. The bearings mustalso withstand continued exposure to extreme
temperature environments, humidity, and dust.
Traction motor bearings

Drive unit bearing(pinion side)

Axle
bearing

Axle

Axle

Drive unit bearing(gear side)

Fig. 1 Railway Bearing Application
To meet these reliability requirements, we at NTN
have worked for many years to develop high quality
bearings for axle, drive unit and traction motor
applications. NTN is also a leading supplier of railway
bearings both inside and outside the country of Japan.
In 2010, NTN acquired the International Railway
Industry Standard (IRIS)Note 1) certification, further
establishing NTN as a leader among Japanesecompanies in ensuring quality management
systems that conform to international standards for
manufacturers of railway vehicles bearings.

This article introduces efforts to develop bearings
for railway vehicles.
Note 1) The specifics of these provisions were inherited by theInternational Standard Technical Specification ISO/
TS22163 (Railway Quality Management System (RQMS))
in 2017.

2. Technology trends in bearings forrailway vehicles and development efforts
2.1 More compact and highly functional axle
bearings

Axle bearings for railway applications need tosupport both the static weight of the vehicle and the
dynamic loading created during operation. Double
row tapered roller bearings and double row cylindricalroller bearings with outer diameter dimensions of
ϕ 210– ϕ 250 mm are commonly used for thisapplication and double row tapered roller bearings
with grease lubrication are more common today.
To meet the need for higher speeds and longer
maintenance cycles, NTN has developed andimproved special-purpose bearings that employ oil
seals to reduce heat generation during operation.
Reinforced resin cages and measures to suppress
fretting between the inner ring and backing ringNote 2)
are also used.
Note 2) Fretting occurs at the contact surface due to repetitive
minute relative displacement of the inner ring and rearcap during axle bearing use. Our patented technology(patent no.: 4060232) suppresses fretting and prevents
ingress of wear debris into the bearing by mounting a
metal plate with gasket.

* Application Engineering Dept., Industrial Business Headquarters** Product Design Dept., Industrial Business Headquarters
NTN TECHNICAL REVIEW No.88

21

Product Development of Rolling Bearings for Railway Vehicles
In addition to development aiming at improving
basic performance, NTN has also developed products
to meet the need for compactness and increased
functionality in recent years. Examples of thesedevelopments include the short-type sealed axle
bearing unit and the ceramic coated insulated axle
bearing.

Conventional oil seal

Conventional
product

2.1.1 Development of short-type sealed axle
bearing unit

(1) Background
Shortening the axial dimension of the axle bearing
unit makes the structure around the axle box morecompact and improves rigidity, making for an effective
solution at preventing fretting wear. However,there have been limitations to this approach due toconstraints with the oil seal size.
Occasionally this issue is resolved by using a shieldplate instead of an oil seal. However, the simpler
design of the shield plate often introduces issues withsealing performance.
NTN developed the short-type sealed axle
bearing unit to simultaneously achieve goodsealing performance while also minimizing the axial
dimension of the axle bearing unit for increased
fretting resistance. (Table 1, Fig. 2).
(2) Features and specifications- More compact, lighter weight- Fretting resistance- Improved reliability, reduced maintenance costs

15 % reduction

Oil seal with shorter axial
direction dimension

Developed
product

Fig. 2 Short-type sealed axle bearing unit
(3) Evaluation test(3-1) Bearing rotating test
A durability test was carried out equivalent to running
1.2 million km at 420 km/h. There was no abnormal risein temperature during testing, and no problems were
evident in the bearing or grease condition after testing(Table 2, Fig. 3).
Table 2 Test condition

Table 1 Specification of short-typesealed axle bearing unit

Item

Conditions

Radial load

91.4 kN (fixed)

Axial load

16.7 kN (5 s load, 25 s no load)

Specifications

Rotational speed

Max. 2,685 min-1 (equivalent to 420 km/h)
Forward/reverse rotation (4 h cycle)

Bearing type

Sealed double row tapered roller
bearing

Wind cooling speed

10 m/sTest time

3,429 h (equivalent to 1.2 million km)

Bearing main
dimensions

φ 130 ×φ 240 × 160/160 mm

Basic dynamic load
rating1,040 kN

Basic static load rating

1,870 kN

Axial dimension of
bearing unit

Compared with our conventional
product–15 %Bearing unit weight

Compared with our conventional
product–10 %Temperature ℃ Vibrational acceleration G

Item

120

Outer ring row A
temperature

Outer ring row B
temperature

Atmospheric temperature

Radial vibrational acceleration

Axle box
temperature

Temporary rise in temperature
due to enfolding of grease

100
80
60
40
20
0

0

500

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

Test time hFig. 3 Bearing rotating test result

22

NTN TECHNICAL REVIEW No.88

(3-2) Test to check sealing performance
A rotational vibration test confirmed that grease
sealing performance of the short-type sealed axle unit
bearing is superior to the shield plate specifications,and comparable to the conventional product (Table 3,
Fig. 4).
Table 3 Test condition
Item

Conditions

Rotational speed

Max. 2,685 min-1 (equivalent to 420 km/h)

Vibration frequency

50 Hz

Acceleration

50 – 200 m/s
Increased in 50 m/s2 increments every 24 h2

Table 4 Specification of ceramic-insulated axle bearing
Item

Specifications

Bearing type

Sealed double row tapered roller bearing

Insulation specifications

Ceramic coating on outer ring

Bearing dimensions

ϕ 135 ×ϕ 240 × 140/130 mm

Basic dynamic load rating

770 kN

Basic static load rating

1,270 kN

Insulation resistance

10 M Ω min. (DC500 V)

Ceramic coated insulated outer ring
Thermal sprayed ceramic coating
To counter electrical corrosion:
Prevents passage of current
Oil seal

Developed product:
No grease leakage

Conventional product:
No grease leakage
Vertical vibration
Rotation and vibration test
Grease leakage
Shield plate:
Grease leakage occurs

Fig. 5 Ceramic-insulated axle bearing(3) Evaluation test
A strength evaluation was carried out of the ceramic
coating under load conditions equivalent to actual use
conditions. No cracking, chipping or other damage
to the coating was evident after testing, and there
were confirmed to be no problems with insulation
resistance (Table 5, Fig. 6).
Table 5 Test condition

Fig. 4 Sealing test result

2.1.2 Development of ceramic coatedinsulated axle bearing(1) Background
Insulation is often employed around the axle
bearings in railway vehicles to prevent electricalcorrosion, which can cause a multitude of performanceissues depending on the specific operating condition.
However, the most effective measure for countering
electrical corrosion is to insulate the bearing itself.
To optimize electrical corrosion prevention in railway
applications, NTN has worked to develop a ceramiccoated insulated axle bearing. (Table 4, Fig. 5)(2) Features and specifications- Ceramic coating on outer ring
- Prevention of electrical corrosion- Improved reliability and reduced maintenance cost

Item

Conditions

Static strength test

Radial load: Max. 110 kN

Fatigue strength test

Radial load: 28.6 – 54.3 kN
Number of loading cycles: 107

Load

Housing
BearingOuter surface of ceramiccoated insulated outer ring
(Photo taken with ring placedin the vertical axis direction)

Axle

Test method

Result of post-test
coating penetrant inspection

Fig. 6 Strength test result of ceramic-insulation
coating

NTN TECHNICAL REVIEW No.88

23

Product Development of Rolling Bearings for Railway Vehicles
2.2 Improved reliability of resin-insulated
bearing for traction motor

In the traction motor, NU type cylindrical roller
bearings are used on the drive side, and deep grooveball bearings are used on the counter-drive side.
These bearings support the rotor.
The drive and counter-drive side bearings are both
typically grease lubricated with an outer diameter of
ϕ 120 – ϕ 170 mm.
To prevent electrical corrosion within these
induction motors, NTN has developed a ceramicinsulated bearing in which the ceramic is plasma
sprayed on the outer ring. NTN has also developed aresin-insulated bearing made with injection molded
reinforced resin (Table 6, Fig. 7). The former is mainly
used in Shinkansen (bullet train) vehicles, while thelatter is mainly used for conventional railroad lines.
To better manage changes in the environmental
conditions of traction motor bearings which haveoccurred in recent years, NTN has also been working
to develop resin-insulated bearings with improved
heat dissipation.

2.2.1 Development of resin-insulated
bearings(1) BackgroundTo reduce lifecycle cost, railway manufacturers
are increasingly looking for methods to lengthen
the maintenance cycle of traction motors. The
maintenance cycle of a traction motor is determined
mainly by the lubrication life of the bearing. To
extend the lubrication life, it is essential to limit heatgeneration within the bearing during operation.
Railway manufacturers are also increasingly adopting
fully-closed traction motors1), which typically
expose the traction motor bearings to more severe
temperatures than the standard open motor type.
In extending the maintenance cycle and developing
applications for fully-closed traction motors,
improving the heat dissipation of resin coatings is vitalto achieve higher reliability.
(2) Features and specifications- Improvement of heat dissipation of injection
molded resin coating for outer rings- Improved reliability and reduced maintenance costs
Table 6 Specification of resin-insulated bearing
Item
Bearing type
Insulation
specifications
Bearing
dimensions

Specifications
Cylindrical roller
bearing

Deep groove ballbearing

Injection molded resin coating of outer ring
NU214

ϕ 70 ×ϕ 125 mm

6311

ϕ 55 ×ϕ 120 mm

Basic dynamic
load rating

83.5 kN

71.5 kN

Basic static load
rating

95 kN

45 kN

Insulation resistance24

100 M Ω min. (DC500 V)

NTN TECHNICAL REVIEW No.88

Injectionmoldedresin coating

Cylindrical roller bearing

Deep groove ball bearing

Fig. 7 Resin-insulated bearing(3) Evaluation test(3-1) Outer ring temperature
Evaluation was carried out by heating the racewayof the resin-insulated outer ring with a heater tosimulate elevated temperatures during bearing use,
and then measuring the outer ring base material outersurface temperature (Table 7).
The test results showed that changes in the materialof the resin coating resulted in a drop of about 5 ℃ in
the outer ring base material outer surface temperaturewhen compared to the conventional product (Fig. 8).
Table 7 Test condition
Item

Conditions

Prototype

Resin-insulated outer ring (NU214)

Heating method

Heating of outer ring raceway using
special-purpose jig with cartridge heater

Heating temperature

120 ℃ (special-purpose jig part)(3-2) Change in bolt fastening force
The outer ring of the resin-insulated bearing iscompressed in the axial direction due to the bolt
fastening force of the traction motor's bearing box
and end cap. Therefore, a drop in bolt fastening force
occurs due to deformation in the film thickness of theresin coating on the outer ring.
We incorporated the developed resin-insulated
outer ring and conventional product into a test jig,
fastened the bolt of the test jig with the specified
fastening force, and applied compressive force inthe thickness direction to the resin coating widthsurface. The raceway was heated to 120 ℃ × 100 h,
and cooled to room temperature, and the drop in bolt
fastening force was evaluated (Table 8).
The test results showed that the developed productwas more effective at reducing change in bolt fastening
force by roughly 20 % compared to the conventional
product (Fig. 9).
Table 8 Test condition
Item

Conditions

Prototype

Resin-insulated outer ring (NU214)

Bolt fastening

4-M10, axial force 18 kN/bolt

Heating method

Heating of outer ring raceway using
special-purpose jig with cartridge heater

Heating temperature

120 ℃ (special-purpose jig part)

Test time

100 h

60
60
50
50

Decrease
Decrease inin fastening
fastening force
force (average)(average) kN
kN

Outer
Outer ring
ring base
base materialmaterial outer
outer surface
surfacemax.
℃max. temperaturetemperature ℃

65
65
55
55

Pinion-side bearing

10
10

70
70

60.6
60.6
55.7
55.7

45
45
40
40
35
35
30
30
Developed Conventional
Conventional
Developed
product
product
product
product

Fig. 8 Temperature ofouter ring

88
66

7.9
7.9
6.1
6.1

44
22
00
Developed Conventional
Conventional
Developed
product
product
product
product

Fig. 9 Change in bolt
fastening force

2.3 Improved seizure resistance performanceof drive unit bearings

It is typical for tapered roller bearings to bearranged face-to-face in a parallel Cardan drive unit
(Fig. 10).
The pinion-side bearing supports the pinion axle
weight, part of the coupler weight, and the gear
engagement load. The gear bearing supports part of
the gearbox weight and the gear engagement load.
In terms of main bearing dimensions, the outer
diameters of the pinion bearing and gear bearing are,respectively, ϕ 150 – ϕ 180 mm and ϕ 280 – ϕ 330
mm, and lubricating oil is supplied to the bearing by
gear rotation.
In recent years, drive units using cylindrical roller
bearings have also been developed2)-4), but the major
bearing type is still tapered roller bearings. With thistype, an axial load acts on the tapered roller bearing,and seizure resistance performance of the pinion-side
bearing is especially important.
The next section introduces efforts to improve
the seizure resistance performance of the pinionside tapered roller bearing in low temperatureenvironments.

2.3.1 Improved seizure resistance performanceof tapered roller bearings for drive units
in low temperature environments(1) Background
While some research has been done on the effectsof the bearing axial clearance and surrounding
temperature have on the rise in bearing temperaturein connection with seizure of the pinion-side bearing
in low temperature environments5), there havebeen no examples of observing the status inside adrive unit and measuring bearing load under low
temperature conditions. Thus, we improved the
product by observing the state inside the drive unit,
and measuring the bearing load.

(2) Internal observation of drive unit
The upper cap of the drive unit was switched to one
made of acrylic resin, and we took a video in orderto observe the situation inside the drive unit at low
temperature up to seizure of the pinion-side bearing(Table 9).

Gear side bearing

Fig. 10 Bearing arrangement in driving-unit
Table 9 Test condition
Item

Conditions

Test bearing

Drive unit pinion-side tapered roller
bearing

Atmospherictemperature-15 ℃

Maximum rotational
speed

5,772 min-1

Lubricating oil

Lubricating oil for drive unit

Axial gap

60 % of design value(to reproduce seizure)[Results of observing situation inside the drive unit ina low-temperature environment (Fig. 11)]
① At startup, high-viscosity lubricating oil brought upby the gear is supplied to the pinion engagement
section, and some of that is supplied to the pinionside bearing.
② After that, viscosity of the lubricating oil decreases
with rising temperature, and the lubricating oil
deposited on the walls inside the drive unit flows
along the inner walls due to the flow of air insidethe unit.
③ Immediately before the pinion-side bearing seizes,
the inside of the drive unit gradually clouds up due
to lubricating oil and the pinion momentarily gives
off sparks.
LightingVideocameraTop cap made ofacrylic resin
Gear

Pinion axle
Gear Lubricating oil Pinion Lubricating oil① At startup② During operation

Spark③ When seizure occurs

Fig. 11 Internal state of driving-unit in low-temp.
environment
NTN TECHNICAL REVIEW No.88

25

Product Development of Rolling Bearings for Railway Vehicles

When seizure occurs:

100

240

・Increased axial load
・Rise in outer ring temperature

80

Rotational speed

60

140
90

40
Axial load

20
0
-20

40

Outer ring temperature

0

2

-10
4

6

8

10

12

3. Summary
This article has introduced efforts to develop
bearings for railway vehicles at NTN. Development
of bearings for railway vehicles often proceeds inclose collaboration with development of vehicles andequipment. The examples presented in this article
are some of NTNʼs development activities for railway
vehicle bearings.
Contributing to society by developing and providing
bearings for public transportation services such as
railway transportation is significant as a corporate
activity. Going forward, we will continue active efforts
to develop technology and products.

References

190
Axial load kN

Temperature ℃, Rotational speed ×100 min-1(3) Results of measuring axial load
The axial load of the pinion-side bearing wasmeasured in order to understand the process up to
seizure of the pinion-side bearing. The measurement
results (Fig. 12) revealed a mechanism where, after
starting from a low-temperature condition, theaxial clearance disappeared before long due to thedifference in thermal expansion of the bearing innerring, pinion axle, and gearbox. After a few minutesof operation, the axial load increased, resulting inseizure. It was also confirmed that an axial load as
high as 150–200 kN acts on the pinion-side gear
when seizure occurs.

-60

Time minFig. 12 Bearing temperature and axial load(4) Effectiveness of countermeasure design
To improve seizure resistance performance ofthe tapered roller bearing for drive units in lowtemperature environments, it is effective to improve
seizure resistance strength of the roller large end face
and the inner ring rib surface. Suppressing the rise intemperature is also important to limit the reductionin the axial clearance. An improvement effect wasevident in comparative testing of the conventional
product and the improved product by incorporatingthese countermeasures (Fig. 13).

1) Toyo Denki Seizo 100-Year History Editorial
Committee, A One-Hundred Year History of Toyo
Denki Seizo, Toyo Denki Seizo K.K. (2018) 234-235.
2) Emiko Cho, Ken Iwanami, Fumihiko Suzuki, Ryohei
Shimamune, Development of Double Helical Gear
Driving Device, JR EAST Technical Review, No.53(2015) 29-32.
3) Hiroshi Ono, Satoshi Hayashi, Technology Trends inGearing for High-Speed Railways, THE TRIBOLOGY,
2017.5, (2017) 40-42.
4) Takafumi Nagatomo, Railway Vehicle Bearings andRelated Technology Trends, Tribologist, Vol. 63, No.
2 (2018) 100-105.
5) Ken Takahashi, Takafumi Nagatomo, Behavior of
Bearing Clearance in Starting Gear Units of Railway
Vehicles, Japan Society of Mechanical Engineers,
2019 Conference Proceedings, S11320.

Photo of authors

Outer ring temperature ℃,
Rotational speed ×100 min-1

100
Seizure occurs withconventional product8060

Rotational speed

40

Outer ring temperature ofconventional productTakashi NISHIKAWA

Application Engineering Dept., Application Engineering Dept.,
Industrial BusinessIndustrial Business
Headquarters
Headquarters

Outer ring temperature of
improved product20

Kengo SUZUKI

0
-20

0

2

4

6

8

10

12

14

16

18

20

Time minFig. 13 Start-up test result in low-temp. environment

26

NTN TECHNICAL REVIEW No.88

Wataru ORITO

Tsukasa TOYODA

Application Engineering Dept.,
Industrial Business
Headquarters

Product Design Dept.,
Industrial Business
Headquarters

Technical Article

Approach to Development of Robotic Joint-Related
Products
Hiromichi KOKUMAI*
Kosuke SUZUKI***

Hideaki TANAKA**
Yuichiro KAWAKAMI***In recent years, the robot industry has been expanding due tothe market demand for improved productivity and labor savings.
In particular, those represented by smaller size robots, typicallycollaborative robots, are expected to grow rapidly in the future. For
various devices incorporated into robotʼs joints, such as reduction
gears and rotary encoders, many bearings are used. This paper
outlines NTNʼs latest developments for these applications.

1. Introduction① Increasing productivity by reducing tact time (higher

In recent years, robot replacements for human
workers have become popular worldwide as a way toresolve labor shortages and stabilize product quality.
There has been particularly active capital investmentin the automotive and electronic components markets
due to increased demand relating to IoT (Internetof Things). The robot market is also expanding inJapan due to further growth in demand, due to labor
shortages brought on by the decreasing birthrate andaging population, and labor-saving driven by surging
personnel costs in developing countries.
Typical examples of robots include: industrial
robots installed on manufacturing lines for welding,painting, transfer, and other tasks; field robots thatwork outdoors at locations such as farms, at sea,
and disaster sites; and service robots that interact
with people in medical and nursing care settings,
and the home and workplace. There is expected
to be particularly rapid dissemination of compact
robots such as collaborative robots that work withhuman workers, and service robots to cope withdiversification of services.
NTN has developed numerous bearing and sensor
related products as mechanical components for use
in robotic joints1)-5). Here we will introduce our efforts
to develop products for compact robots that will see
market growth going forward.

② Improved positioning accuracy and repeatability

2. Robot market needs
In addition to the basic function of robots̶
grasping objects at determined positions and working
accurately̶recently, market needs have been growing
in the following areas:

speed)

during work transfer (higher precision)③ Simplification of teaching (improved operability)④ Securing work space by reducing the footprint ofrobots (greater compactness)⑤ Improved line operation rate by lengthening

maintenance intervals (improved reliability andmaintainability)

3. Robot structure and joint mechanisms
Vertical articulating robots which perform the same
movements as the human arm, from the shoulder tothe wrist, are one typical type of robot (Fig. 1). These
robots need to transfer heavy objects at high speed
to a determined position with high accuracy. Thisrequires not only driving force and rigidity of eachjoint, but also accurate positioning control. Therefore,high-rigidity precision reduction gears with high torquedensity are used at joint locations, and positioningis performed through feedback control using angle
detection by rotary encoders (Fig. 2).
In compact robots, it is essential to reduce thesize and weight of these joint mechanisms, andstrain wave gearing and lightweight, compact rotary
encoders are used as compact precision reducers(outer diameter ϕ 100 mm or less).
NTN is working to develop products such as
bearings for use in rotational supports of strain wave
gearing and multi-track magnetic rings enabling
detection of absolute angle and rotation speed.

*
Application Engineering Dept., Industrial Business Headquarters** Robotics Sensing Engineering Dept., Industrial Business Headquarters*** Product Design Dept., Industrial Business Headquarters
NTN TECHNICAL REVIEW No.88

27

XXXXXXXXXXXXApproach to Development of Robotic Joint-Related Products

Axis 5: Wrist bend

Axis 4: Wrist
revolution

Axis 3:Upper arm

Axis 6: Wrist rotation

Axis 1:
Revolution

Axis 2: Lower arm

Fig. 1 6 axes articulating robot

external gear. The operating principle is shown in
Fig. 4. If the internal gear is fixed while the oval shaft
rotates clockwise once, the external gear rotatescounterclockwise by the difference in the numberof teeth between the external gear and the internalgear (e.g. the external gear has two less teeth) whichis extracted as the output. Here, the diagram at rightin Fig. 4 shows the situation where the oval shaft has
rotated through half a turn and the external gear has
moved counterclockwise by one tooth. This reducer
has a concentric and simple structure with compact
form factor and provides a significant reduction ratioof 1/30 to 1/320. Since both teeth are simultaneously
engaged like wedges, providing a high contact
ratio without backlash, the reducer is characterizedby averaged gear errors, high angle transmission
accuracy, and high torque capacity. Also, a crossroller bearing is used for the main bearing supporting
output rotation of the external gear, and since thisbearing is compact and has high moment stiffness,it also ensures the positioning accuracy, which is themost important characteristic for a robot.
Internal gear

Magnetic sensor

Thin section typeelastic ball bearing

Input side

Motor

Cross roller bearing

Multi-track magnetic ring

Fig. 2 Structure of robotic joint part

Output side

Precision reduction gear

Oval shaft

Thin section typeelastic toothed gear
(external gear)(2nd axis: Lower arm)

Fig. 3 Structure of strain wave gearing (Unit type)

4. Development of bearings for strain wave gearing
4.1 Points where bearings are used in strain
wave gearing

Fig. 3 shows a sample schematic diagram of thestructure of strain wave gearing (unit type) that canamplify output torque of a servomotor (reduce speed)
and apply a moment load. A thin section elastic ballbearing (which plays the role of the wave generating
mechanism) and cross roller bearing (which supports
the moment load on the output side) are used in thisreducer.
Now the operating principle of this reduction
gear will be explained. It consists of a thin section
elastic toothed gear (external gear) made of a thin,elastic metal ring with gear teeth. It incorporates athin, elastic ball bearing into which an oval shaft isinserted. The elastic toothed gear is in contact withthe thick, rigid internal gear and the major axis of
the oval. This provides a unique reducing mechanism
cleverly utilizing the elastic deformation of the28

NTN TECHNICAL REVIEW No.88

Internalgear

Oval shaft

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

Fig. 4 Operating principle of strain wave gearing

The highly-robust, specialized design of the thinsection elastic ball bearing and cross roller bearing
is essential to maintain stable performance andoperation of the reducer and achieve high reliability.

4.2 Thin section elastic ball bearing

The engaging reaction force from the gear is
supported in the oval deformed state so that the thinsection elastic ball bearing used in strain wave gearing
provides torque transmission as a wave generating
mechanism. At this time, the inner ring engages tightly
with the oval shaft, and rotates while being deformed
into an oval, and the deformed state of the outer ring
repeatedly changes via the rolling element. In addition
to the preload due to this oval deformation, reaction
from the gear during torque transmission also actson the inside of the bearing. In other words, a thinsection elastic ball bearing needs to have the following
characteristics to accurately transmit high torque overthe long term.
① Longer life through high load capacity

design (improved rolling fatigue life)

element is unloaded and contracts in diameter, and thus
compressive stress is produced at the outer ring outer
diameter. Furthermore, during operation the deformed
inner ring rotates, and the stress distribution of theouter ring changes moment to moment in the rotation
direction. The major and minor axes are alternatelyinterchanged, and thus it is evident that repeated tensileand compressive stress are applied to the outer ringouter diameter. NTN has achieved ① Longer life throughhigh load capacity design (improved rolling fatigue life),② Flexibility in response to elastic deformation of theinner/outer raceway, and ③ Improved fatigue strength
against cracking of the outer ring, by developing an
understanding of the correct stress situation throughthis type of stress analysis, and thereby selecting the
optimal thickness for suppressing stress amplitude, andoptimizing the corner shape design to prevent stressconcentration.

② Flexibility in response to elastic deformation of theinner/outer raceway

Max. tensile stress③ Improved fatigue strength against cracking of theouter ring

To satisfy these conditions, the basic design uses alarger rolling element, thinner inner and outer rings,
and an increased rolling element filling rate. Table 1
shows a comparison of cross-section form with the
68-series standard product, a typical thin section
bearing series. The thin section elastic ball bearing has
thinner inner and outer rings than the 68-series. The
rolling element ratio with respect to the cross section(cross section ratio) is roughly 1.2 times larger, andthe load capacity is about 1.4 times larger. This design
allows realization of both elastic deformation andhigher load capacity of the inner and outer raceway.
Table 1 Features of thin section elastic ball bearing

Major axis side

High

Minor
axis side

Max. compressive stress

Low

Fig. 5 Outer race stress analysis of thin section elasticball bearing

Thinner

Schematic diagram
Thin section elasticball bearing

68-series

Cross section ratio

1.2

1Load capacity ratio

1.4

1* Cross section ratio, load capacity ratio: Ratio taking 68-series to be 1

Here, we strive for ① “Longer life through high load
capacity design (improved rolling fatigue life)” and toensure tensile and compressive stress amplitude areapplied due to repeated elastic deformation of theouter ring via the rolling element during operation. We
must take into account not only rolling fatigue life, butalso fatigue strength against cracking of the outer ring.
Fig. 5 shows an example of outer ring stress analysis.
Analysis was done using a 1/4 cut model of the outer
ring, while taking into account elliptical deformation andexternal load. On the major axis side, the outer ring ispushed to the diameter expansion side via the rolling
element, and thus tensile stress is produced at the outerring outer diameter. On the minor axis side, the rolling

Also, in a thin section elastic ball bearing, the innerand outer rings are extremely thin, and thus one issue
for manufacturing is how to suppress deformation
during machining and heat treatment. However, we havecommercialized a thin section type elastic ball bearing
through thin machining technology previously cultivatedby NTN and optimization of heat treatment conditions(Fig. 6).

Fig. 6 Cut model of thin section elastic ball bearing
NTN TECHNICAL REVIEW No.88

29

XXXXXXXXXXXXApproach to Development of Robotic Joint-Related Products

4.3 Cross roller bearing

Main bearings used in strain wave gearing must
directly support the moment load acting on a robotic
joint, and, even when holding a large moment load,
they must have high moment stiffness and support
rotation with long-term stability while suppressing
axis inclination and bearing internal stress (contact
stress). Therefore, the bearing must have ① greatercompactness, ② higher moment stiffness, and ③
longer life. To achieve these functions, a cross roller
bearing is used which has no cage, and a larger
number of rollers arranged with alternating contact
angles (Fig. 7).

Moment stiffness

Life

Suitable preload range LongHigh

Rollers

Life

Moment stiffness

Outer ringshows the relationship of moment stiffness and life
to preload. With increasing preload, there is also an
increase in moment stiffness. Life, on the other hand,tends to increase in the low preload region due tosuppression of preload loss in response to an externalload. In the high preload region, life tends to decrease
due to an increase in contact stress. NTN has achieved:① greater compactness while simultaneously realizing② higher moment stiffness and ③ longer service life bysetting the optimal preload range.

Low

Short
Low

Fig. 8 Moment stiffness and lifetime at preload forcross roller bearing

Inner ring

Fig. 7 Structure of cross roller bearing
Table 2 shows a comparison when tapered roller
bearings and deep groove ball bearings are used, as
typical bearing types other than a cross roller bearing,for the main bearing of strain wave gearing. Tapered
roller bearings have outstanding moment stiffness
and load capacity, but use of two rows is a reqirement
for supporting a moment load in both directions. This
results in issues in terms of compactness. Deep
groove ball bearings are compact, but there are issues
in terms of moment stiffness and load capacity. Thus,
the cross roller bearing is the most functional for thisapplication.
Table 2 Comparison of bearing types for strain wave
gearing main shafts

Type① Compact② High moment

stiffness③ High load capacity(long life)

Cross roller
bearing

Tapered roller
bearing

Deep grooveball bearing

〇

×

〇

〇

〇

×

〇

〇

×

To achieve ② “high moment stiffness” with a crossroller bearing, the design must increase internal
preload, and thus the bearing internal gap is set to anegative gap (preload). On the other hand, increasing
preload involves an increase in contact stress, and thus
reduction in service life must be considered. Fig. 8

30

NTN TECHNICAL REVIEW No.88

PreloadHigh

5. Development of multi-track magnetic
ring for rotary encoders
5.1 Application of multi-track magnetic rings
to robotic joints

To achieve accurate positioning control of robots, it isnecessary to detect the rotation angle, rotation direction,and rotation speed on the output side of the motor
which drives the joint with high accuracy. That output isused for feedback control.
Rotary encoders are broadly divided into two
types: optical and magnetic. The optical type has highposition detection accuracy and resolution, but has
low environmental resistance because it is affectedby aspects of the ambient environment, such astemperature variation, dust, and oil. The magnetic type,on the other hand, is highly resistant to environmental
aspects, and can be used in ordinary work environments.
NTN has developed a multi-track magnetic ring foruse with absolute angle detection type rotary encoders3)(Fig. 9).

Fig. 9 Multi Track Magnetic Ring

This multi-track magnetic ring uses thin, press-formed
core metal, and is hollow with a large diameter. Therefore,
it is ideal for robots whose wiring passes through hollow
shafts such as joints, and helps to save space. Fig. 10shows an example using multi-track magnetic rings and
iC-MU6) series sensor ICs at robotic joints.
Radial TypeMulti Track Magnetic Ring (MTR)

iC-MU series

the absolute angle by internally calculating. Using
parameter setting inside the iC-MU, it is also possible
to generate highly-divided output up to a maximum of
20 bits (resolution: approx. 0.00034 ). Fig. 12 shows
the results of measuring absolute angle error underideal conditions, where the relative positional deviation
between the multi-track magnetic ring and iC-MU
is minimal. An angle error of ± 0.025°is achieved,
although this is only a reference measuring result.
The multi-track magnetic ring introduced in thisarticle is the OFF-AXIS type, in which the inner diameterpart is fixed to the axis outer diameter part. NTN has
also commercialized an ON-AXIS type angle sensor unit,
in which a magnet is fastened to the axis end4). Also, as
rotation detection technology and application products,
NTN has commercialized bearings with rotational
sensors5) (Fig. 13), hub bearings with sensors, andother products.

Substrate
Multi Track Magnetic Ring (MTA)

Axial Type

Fig. 11 Image of Combination of iC-MU andMulti Track Magnetic Ring

Fig. 10 Application example of Multi Track Magnetic
Ring in robotic joint(Conceptual illustration, provided by: iC-Haus GmbH)

5.2 Multi-track magnetic rings

The multi-track magnetic rings developed at NTN
consist of different numbers of magnetic poles arranged
in two tracks, and we have realized high-accuracy
magnetization by accumulating magnetizing technology
for the magnetic rings used in bearings with rotational
sensors.
The iC-MU (Fig. 11) used in a set with a multitrack magnetic ring provides a Hall element and signal
processing circuit in a single package. It reads the phasedifference of the magnetic poles in the two tracks withdifferent numbers of magnetic poles, and can detect

Fig. 13 Integrated Rotation Sensor Bearing

Angle error deg.

0.15
0.1
0.05
0
-0.05
-0.1
-0.15
0

30

60

90

120

150

180

210

240

270

300

330

360

Rotation angle (position) deg.

Fig. 12 Angular accuracy measurement result when combining the Multi Track Magnetic Ring anddedicated iC-MU
NTN TECHNICAL REVIEW No.88

31

XXXXXXXXXXXXApproach to Development of Robotic Joint-Related Products

5.3 Developing a multi-track magnetic ring
seriesAfter starting mass production of multi-trackmagnetic rings, first of the radial type with 64/63
polar pairs (inner diameter 44 mm, outer diameter
51.5 mm, width 8.2 mm, weight 10.7 g), an axial
type was added with 64/63 polar pairs. NTN has also
developed a compact type with 32/31 polar pairs(MTR32 and MTA32) (Fig. 14) with an eye toward
non-robot applications, such as electric tools andcompact motors to support self-driving automobiles.
To meet market needs, NTN is developing a seriesof multi-track magnetic rings with the model names
shown in Table 3.
Going forward, we will consider marketing an ultracompact type with 16/15 polar pairs and a large typewith 128/127 polar pairs to meet user needs.
MTR64
MTR32

MTA64
MTA32

Fig. 14 Comparison of 64/63 pole pairs and small size
32/31 pole pairs

6. Summary
NTN is developing products applicable to robotic
joints, and this article has introduced those efforts.
Going forward, needs for compact robots are
expected to diversify due to expansion of the service
field and diversification of collaborative work withpeople, and product development will be needed tocover these new needs.
NTN will keep a close eye on industry trends, andcontinually move forward with product development
suited to market needs.

References

1) Makoto Oebisu, Hiromichi Kokumai, Yasuyoshi
Hayashi, Masato Tsujihashi, Technical Trend andFeatures of the Bearing for Robot, NTN TECHNICALREVIEW, No.86, (2018) 34- 39.
2) Hiromichi Kokumai, Technology Trends in Robot
Bearings, THE TRIBOLOGY, No.11, (2019.11) 1214.
3) Takashi Koike, Yasuyuki Fukushima, Yusuke
Shibuya, Hiroyoshi Itou, Development of Multi
Track Magnetic Ring for High Accuracy Absolute
Angle Detection, NTN TECHNICAL REVIEW, No.86,(2018) 45-49.
4) Shoji Itomi, Hiroyoshi Itou, NTN Sensor Units forConstruction Machine, NTN TECHNICAL REVIEW,
No. 76, (2008) 118-125.
5) Hiroyoshi Itou, Takashi Koike, Bearings with
Rotational Sensors, NTN TECHNICAL REVIEW, No.
69, (2001) 108-116.
6) iC-Haus GmbH, iC-MU off-axis nonius encoderwith integrated hall sensors.

Table 3 NTN Multi Track Magnetic Ring corresponding to sensor IC (iC-MU series) of iC-Haus
Pole Pair Number(Main Track / Sub Track)
Multi Track Magnetic Ring

Pole Pitch of Main Track (mm)
1.28

1.50

2.00

32/31

MTR32
MTA32

MTR32-1
MTA32-1

MTR32-2
MTA32-2

64/63

MTR64
MTA64

MTR64-1
MTA64-1

MTR64-2
MTA64-2

iC-MU

iC-MU150

iC-MU200

Magnetic Sensor made by iC-Haus

Photo of authors

Hiromichi KOKUMAI

Hideaki TANAKA

Kosuke SUZUKI

Yuichiro KAWAKAMI

Application Engineering
Dept., Industrial Business
Headquarters

Robotics Sensing
Engineering Dept., Industrial
Business Headquarters

Product Design Dept.,
Industrial Business
Headquarters

Product Design Dept.,
Industrial Business
Headquarters

32

NTN TECHNICAL REVIEW No.88

New Products

Development of Sensor Integrated Bearing Unit for
Machine Tool Spindles
Shohei HASHIZUME*
Yusuke SHIBUYA**
Daichi KONDO**
Yohei YAMAMOTO***
Hiroyuki IWANAGA****
Machine tools require not only fundamental features like high speed, highrigidity and super precision capabilities, but also condition monitoring and“Connected Industries” related technology. NTN developed the “Sensor
Integrated Bearing Unit for Machine Tool Spindles” in 2018 and secured many
positive responses from the manufacturing industry. NTN has recently added
load detection function and wireless system to the unit, based on the additional requirements. This reportintroduces the features, structure, and performance of the unit.

1. Introduction
Machine tools support monozukuri in various
types of industry, including automotive, aircraft,medical equipment and IT, and the market demand
for them is becoming increasingly sophisticatedand diversified1)2). Particularly in recent years, thediminishing working population has created a need to
improve production efficiency. There is an increased
need to detect abnormalities at an early stage toprevent damage to the spindle and spindle bearing,which are key components within machine tools,
and to prevent such factors as downtime and spindle
replacement associated with this damage. Generally,a sensor is installed on the outer diameter surface ofthe spindle to detect any abnormalities as a method
to measure the temperature and vibration. However,there is a problem with this method in that the sensor
is easy to install but the measuring position is far fromthe bearing raceway surface, which makes it difficult
to detect any abnormalities at an early stage such assudden temperature rises in the bearing.
To solve this problem NTN has integrated sensors
in the spacer, a bearing unit component, to performsensing around the bearing raceway surface. This
has allowed us to develop a “Sensor Integrated
Bearing Unit” for Machine Tool Spindles3) that enables
advanced condition monitoring. This product was
displayed as a reference exhibit at the 29th Japan
International Machine Tool Fair (JIMTOF2018) heldin 2018. As a result, we received a great deal ofpraise from the market for this product and also
received many requests to add a load inspection
function and wireless technology, and so proceededwith developing these items. The developed product(hereafter, this bearing unit) can be applied to lathes
and machining centers as shown in Fig. 1, and thisbearing unitʼs features, structure, and performance
test results are introduced below.
Lathe spindle

Machining center spindle

Bearing unit mounted part

Bearing unit
mounted partFig. 1 Machine tools and spindles(Left : Lathe, Right : Machining center)*
Application Engineering Dept., Industrial Business Headquarters**
New Product Development R&D Center*** Product Design Dept., Industrial Business Headquarters**** Robotics Sensing Engineering Dept., Industrial Business Headquarters
NTN TECHNICAL REVIEW No.88

33

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles

2. Structure and Function of the Sensor
Integrated Bearing Unit
This bearing unit monitors the condition and detects
abnormalities at the spindle on machines tools, and thespindle bearing. To achieve this, the bearing unit has 3
types of sensors for load, temperature and vibration built
into the outer ring spacer, which is arranged between
two rows of back-to-back angular contact ball bearings.
Furthermore, the unit has a built-in generator as anindependent power source and a wireless module toprovide wireless technology. Fig. 2 shows this bearing
unitʼs structure, Fig. 3 shows an example of application
on a machine tool spindle, and Table 1 shows thefunction and purpose.
In addition, when this bearing unit was announced
in 2018, it was integrated with 3 types of sensors fortemperature, heat flow and vibration. Among thesesensors, the heat flow sensor has been replaced with aload sensor.

Load sensor

Temperature sensor, vibration sensor

Wireless module

Generator (rotor and stator)

Load Path

Fig. 2 The sensor integrated bearing unit
Outer ring spacer

Housing outer diameter surface of spindle

Spindle bearing

Inner ring
spacerFig. 3 Application example of the sensor integrated
bearing unit to the machine tool spindle

2.1 Sensors
2.1.1 Load Sensor

The load sensor detects loads applied on the
bearing, such as bearing pre-load and external loads
applied to the spindle.
This bearing unit has a newly developed, compact

34

NTN TECHNICAL REVIEW No.88

and high-sensitivity load sensor built into the load
path that can detect sudden increases in pre-load that
occurs prior to bearing seizure. This contributes tothe prevention of such seizure. Furthermore, detecting
pre-load for bearings after spindle installation makes it
easier to manage the pre-load for such bearings and is
expected to reduce the number of man-hours required
for assembly in comparison with the conventional
method of calculations based on spindle rigidity andthe method of calculations based on the spindleʼsnatural vibration frequency. It is also possible to detect
the external load applied to the spindle and this canbe used to monitor the machining conditions in termsof changes in the cutting load to contribute towards
improving the machining quality and productivity.
Additionally, there is the potential to also use this todetect collisions between the tool and the workpiece.

2.1.2 Temperature Sensor

The temperature sensor detects heat generated
from the bearing as a result of spindle rotation and thecutting load. Generally, when measuring the bearing
temperature during operation, the temperature ismeasured at the housing outer diameter surface of thespindle where the sensor is easy to install, and then the
bearing temperature is estimated based on this value.
However, fluid cooling channels are provided for the
bearing and motor between the bearing and housing
outer diameter surface, which means the temperature islower than when the bearing temperature is measured
directly. Furthermore, the housingʼs heat capacity islarge and the sensor requires some time to detect
temperature changes. Due to this fact, it was difficult toaccurately know the temperature of the bearing.
This bearing unit measures the temperature of theouter ring spacer in close proximity to the bearing.
When compared to measuring at the housing outer
diameter surface, it provides a more accurate
measurement of the bearing temperature and canincrease the reliability of condition monitoring.

2.1.3 Vibration Sensor

A vibration sensor is used to detect roughness,
peeling, and seizure on the bearing raceway surface
due to insufficient lubrication that commonly occurs inmachine tool spindle bearings, and to detect abnormalindentations due to collisions between the spindle and theworkpiece. Generally, when measuring bearing vibration
during operation, the vibration is often measured at the
housing outer diameter surface of the spindle because
this makes it easier to install the sensor; the same as forthe temperature sensor. However, any vibration originatingfrom the bearing is dampened when measured throughthe housing.
This bearing unit has a vibration sensor built into theouter ring spacer in close proximity to the bearing. Since
the vibration is measured at a position close to the bearing
raceway surface it is possible to measure the initial stageof an abnormality and even the smallest level of vibration
with good sensitivity. Furthermore, there is the potential
to also use this to detect collisions between the tool andthe workpiece; the same as for the load sensor.

Table 1 Functions & purpose examples of the sensor integrated bearing unit
Function

Purpose・Early detection for signs of bearing seizure
Detects pre-load for spindle bearing(While spindle is rotating and after spindle installation) ・Reduces man-hours for spindle assembly・Detects the machining load applied to the① Load sensor

spindle, and monitors machining(Contributes to improving the machining
Detects external loads applied to the spindle
quality and productivity)
・Detects collisions between the tool and theworkpiece(Used to reduce spindle damage
and investigate the cause of any damage)
Detects temperature changes in the spindle
and spindle bearing② Temperature sensor

・Monitors the condition of the bearing

raceway surface

・Monitors the condition of the bearing③ Vibration sensor④ Independent powersource⑤ Wireless module

raceway surface

Detects vibration changes in the spindle andspindle bearing

・Detects collisions between the tool and theSupplies power required for the sensors andwireless module・No need for an externally connected cable orworkpiece(Used to reduce spindle damage
and investigate the cause of any damage)wiring space・Reduces man-hours to assemble the spindle
Provides wireless communication to a location
・No need to change the spindle structure

outside the spindle for detected data

2.2 Wireless Technology

This bearing unit has a built-in independent powersource and wireless module. This means that there isno need for an externally connected cable and wiring
space used to transmit data and to supply power,
achieving the same level in handling properties asconventional bearings with spacers that do not havesensor integration. The following sections describe theindependent power source and wireless module.

2.2.1 Independent Power Source

The bearing unit uses a compact and high output
electromagnetic generator that generates power using
the rotation of the spindle. A power-generating rotor
is arranged at the inner ring spacer while a powergenerating stator is arranged opposing it at the outerring spacer. This setup supplies the power required forthe sensors and wireless module by generating powerfrom the relative rotation of the inner and outer ring.

3.Evaluation Test
This section introduces the measurement results takenusing wireless technology with the load sensor that was
newly added to this bearing unit. Evaluation test results
are shown for “Detects pre-load for spindle bearing(While spindle is rotating)” and “Detects external loads
applied to the spindle” in section ① of Table 1.
The test machine used to simulate the machine tool
spindle is shown in Fig. 4. This bearing unit was set up on
this test machine and the evaluation test was performed.
A high speed angular contact ball bearing with ceramicballs (HSE type) was used for the test bearing. A two-row
back-to-back arrangement was chosen for the bearing
to simulate a machine tool spindle. This bearing unit
wirelessly transmits data detected by the sensors using
the wireless module to a receiver set up at a location
external to the test machine.

2.2.2 Wireless Module

A wireless module is built into the outer ring
spacer to enable data detected by the sensors
to be transmitted wirelessly to a location outside
the spindle. The wireless module is compact andconsumes only a small amount of power to ensure it
can operate using power from the above mentioned
independent power source with consideration forcommunication standards, the frequency used, signal
strength, and safety, and to maintain sufficient spacerequired for its antenna even when integrated in thespacer.

Drive
motor

External load
(Axial direction)

Fig. 4 Structure of test spindle
NTN TECHNICAL REVIEW No.88

35

Development of Sensor Integrated Bearing Unit for Machine Tool Spindles
3.1 Bearing Pre-load Detection Test

A performance test was conducted to detect the
pre-load of the bearing while the spindle is rotating.
Table 2 shows the test conditions. Fig. 5 shows themeasurement results from the load sensor using
wireless technology that was newly added to thisbearing unit. For comparison, the graph also shows
the bearing pre-load (theoretical value) obtained fromthe spindleʼs rotational speed and the bearingʼs innerand outer ring temperature which were measured
separately form this bearing unit. From the test results
we confirmed that it is possible to estimate the preload of the bearing while the spindle is rotating using
the load sensor on this bearing unit.
Table 2 Test condition of the bearing preload detection

sensor on this bearing unit can estimate the externalload in the axial direction while the spindle is rotating.
Table 3 Test conditions of the external load detection
Test bearing

ϕ 70 ×ϕ 110 × 20
Part equivalent to 5S-2LA-HSE014(high speed angular contact ball bearing withceramic balls)

Pre-load
methodFixed position pre-load (pre-load of 750 N
after spindle installation)

Rotational
speed

5,000 min-1

Lubrication
methodAir oil lubrication

Lubricating
amount0.03 mL/10 min
ISO VG32

Lubricating air
flow rate

30 NL/min

Pre-load
methodFixed position pre-load (pre-load of 750 N
after spindle installation)

Fluid coolingchannelYes, room temperature tuning

Rotationalspeed

0 - 14,000 min-1

Axis position

Horizontal axis

Lubrication
methodExternal load

(Axial direction) 0 - 3 kN

Air oil lubrication

Lubricating
amount0.03 mL/10 min

Lubricating oil

ISO VG32

Lubricating air
flow rate

30 NL/min

Fluid coolingchannelYes, room temperature tuning

Axis position

Horizontal axis

External load

NonePre-load (estimated value) kN

4Load sensor

3

Load sensor

2

1

0

0

1

2

3

Fig. 6 Test result of the external load detection

Pre-load (theoretical value)

4.Collaboration with Industrial IoT Platform

2

1

0

5,000

10,00015,000

Rotational speed min-1

Fig. 5 Test result of the bearing preload detection

3.2 External Load Detection Test

An external load test was conducted to detect theexternal load applied to the spindle. Table 3 shows thetest conditions. During this test the spindle was rotated
at 5,000 min-1 and an external load was applied in theaxial direction. Fig. 6 shows the measurement results.
From the test results we confirmed that the load

36

4External load kN

30

External load (estimated value) kN

Lubricating oil

Test bearing

ϕ 70 ×ϕ 110 × 20
Part equivalent to 5S-2LA-HSE014(high speed angular contact ball bearing withceramic balls)

NTN TECHNICAL REVIEW No.88

In order to use the data detected by this bearing
unit, this bearing unit must be connected to amachine tool and an industrial IoT platform(Fig. 7). When used for machine control that requires
high responsiveness, the bearing unit should beconnected to the machine tool (data transmission
target ① ). It is often necessary to develop a dedicatedconnection program or similar such program in thiscase. Meanwhile, when monitoring the conditionand conducting predictive maintenance that uses
measurement data, it is more common to connect
to a management system in the form of an industrial
IoT platform (data transmission target ② ). Since theconnection program is provided, the connection canbe relatively easily established in this case.
For example, connecting this bearing unit to anindustrial IoT platform such as Edgecross*1 enables

information for the load, temperature and vibration
to be imported. Utilizing the various measurement
data imported into the industrial IoT platform not only
enables condition monitoring of the bearing, but it
also enables monitoring of the machining conditions.
The data can also be stored on a server, which alsoenables the use of big data.

Data transmission target ①
Machine toolSensor integrated
bearing unit

Industrial
loT platform

Data transmission target ②
loT platform

Machine tool

Etc
NTN App statusmonitoring and diagnostic

Fig. 7 Communication examples of the sensor
integrated bearing unit
This bearing unit uses an independent power source
to supply power and send data wirelessly, which can
reduce the man-hours required to install sensors andalso simplify the move to an IoT environment.
NTN has developed Wind DoctorTM, a Condition
Monitoring System for wind turbines that fully utilizes
bearing analysis and our diagnostic expertise, andprovides a monitoring service for power generationcompanies4).
Additionally, we are also currently developing adiagnostic application for industrial IoT platforms thatincorporates the knowledge and expertise we gained
from Wind DoctorTM and plan to propose solutions
employing this bearing unit.

5.Summary
Further enhancement of condition monitoring
functions is required for machine tools against suchfactors as the backdrop of a diminishing working
population.
NTN has developed the “Sensor Integrated Bearing
Unit” for Machine Tool Spindles and announced thisproduct at 2018 JIMTOF2018 in response to thisneed. While promoting proposals to customers, wehave received many requests to add a load detection
function and wireless technology so have developed a
new unit to achieve this purpose.
We will continue to make further improvements inperformance and in the practical use of this productinto the future. We are also aiming to improve
technology for predictive maintenance using AI
technology to continue contributing towards more
efficient maintenance and operation of equipment,
including machine tools.

References

1) Naoki Matsumori, Keiichi Ueda, Technical Trendof the Precision Bearings for Machine Tools, NTN
TECHNICAL REVIEW, No.84, (2016) 40-45.
2) Keiichi Ueda, Technical Trend of the Precision
Bearings for Machine Tools, Bearing & Motion Tech,
No.002, (2016) 33-35.
3) Shohei Hashizume, Yasuyuki Fukushima, Yusuke
Shibuya, Yohei Yamamoto, Development of Sensor
Integrated Bearing Unit for Machine Tool Spindles,
NTN TECHNICAL REVIEW, No.86, (2018) 50-55.
4) Makoto Miyazaki, Wataru Hatakeyama, Applicationof Condition Monitoring System for wind turbines,
NTN TECHNICAL REVIEW, No.86, (2018) 40-44.

*1 : A Japanese-developed open software platform in thefield of edge computing to enable FA and IT collaboration.
(From Edgecross Consortium official website https://www.
edgecross.org/en/)

Photo of authors

Shohei HASHIZUMEYusuke SHIBUYA

Daichi KONDO

Application Engineering
Dept.,
Industrial Business
Headquarters

New ProductDevelopment R&D Center

New ProductDevelopment R&D Center

Yohei YAMAMOTO Hiroyuki IWANAGA
Product Design Dept.,
Industrial Business
Headquarters

Robotics Sensing
Engineering Dept.,
Industrial Business
Headquarters

NTN TECHNICAL REVIEW No.88

37

New Products

Development of a Low Contamination Generation Bearing
for Servo Motors
Araki TANAKA*
Hideyuki MITANI**

Naoaki TSUJI**
Takayuki KAWAMURA***NTN has developed low contamination generation bearings for servo motors
and has experienced increased demand for these bearings in recent years. This
bearing type generates 90 % less contamination and requires 50 % less torquethan conventional bearings; this allows the servo motor to be downsized andincreases the motorʼs power output. This report introduces the features andperformance of the low contamination generation bearing.

Table 1 Requirements of Servo Motor Bearings

1.Introduction
With increasing demand for improved productivity
and reduced labor costs against the backdrop ofa diminishing working population, performance
improvement is required for FA devices and motors(Fig. 1), the power sources of devices such asmachine tools and industrial robots1).
NTN has recently developed a low contamination*1
generation bearing to decrease the size and increase
the power output of servo motors for industrial robots.
In addition to technical trends for servo motor
bearings (Fig. 2), this developed productʼs featuresand evaluation test results are introduced below.

Fig. 1 Motor

Fig. 2 Servo motor bearings

2.Development BackgroundIn recent years, demand for servo motors for use inmachine tools and industrial robots has increased. A
servo motor features a rotation detector and brake, or
control device, that enable it to control rotation with ahigh level of precision. Generally, a sealed and greased
deep groove ball bearing with cost advantages is used
in a servo motor to support high speed rotation. Table 1
shows the characteristics required for servo motors andtheir supporting bearings.

Application

Required Servo Motor Required Bearing
Characteristics
Characteristics

Machinetool

High speed
rotation

Industrial
robot

Other Bearing
Characteristics

High speed rotation Plastic ribbon cage
Long grease life

ME-1 Grease

Compact

Low contamination
generation

Low contamination
generating grease

High power output

Low torqueContact seal

Servo motors used for machine tools rotate athigh speeds and are often used to drive a spindle.
Therefore, the bearing must have good high speed
rotational performance and a long grease life. NTN
has historically used “ME-1”3) high temperature, longlife grease and its proprietary “plastic ribbon cage”(Fig. 3). This cage has the advantage of extending
the grease life and prevents centrifugal expansionthat inhibits high speed rotation. NTN also offers
its commercialized4) “ULTAGE*2 Deep Groove Ball
Bearings for High-Speed Servo Motors” (Fig. 4) thatcombine long operating life with high speed rotation
at a d mn value*3 of 1 million.

Fig. 3 Plastic ribbon
cage2）

Fig. 4 ULTAGE deep groove
bearings for highspeed servo motors4）*1 Throughout this report, contamination refers to lubrication emitted by the bearing.
*2 ULTAGE is the name for NTNʼs new generation of bearings that is noted for its industry-leading performance. The name is created by combining“ultimate”, signifying refinement, and “stage”, signifying NTNʼs intention for this product to be used for a variety of applications.
*3 d mn is a value that represents the bearingʼs rotational performance.
d mn=d m(bearing rolling element pitch diameter, mm) × n(rotational speed, min-1)*
Application Engineering Dept., Industrial Business Headquarters** Product Design Dept., Industrial Business Headquarters*** Advanced Technology R&D Center

38

NTN TECHNICAL REVIEW No.88

Servo motors used in industrial robots are often
used to drive joints and, like those used in machine
tools, are required to be compact. These bearings mustalso generate a high power output to increase the axis
speed and acceleration/deceleration rates of the joints
they support. A control device is typically mounted
near the bearings of industrial robots. However,bearing lubrication can splash onto and stick to thesecontrol devices which causes a drop in detection
accuracy and braking performance. To prevent this,sealed device could be used to protect the control
device, but this increases the size of servo motor andresults in a less compact system. A low contamination
generation bearing is required to prevent lubricationfrom exiting the bearing and sticking to the control
device when a servo motor does not equip sealed
device between its bearing and control device.
Furthermore, low torque generation is required toachieve high power output from the servo motor. NTN
has therefore commercialized a low contamination
generation bearing1) that utilizes a seal arrangementthat prevents pressure from increasing inside thebearing, the main cause of contamination generation,and requires low torque. This bearing also uses
“EP-1” grease, a low contamination-generating grease.
Each year the requirements for low contamination
generation and low torque become more stringent,and further performance improvement is required.

3.Product Structure and Features
The “Low Contamination Generation Bearing forServo Motors” (Fig. 5) is a deep groove ball bearing
with a newly developed low contamination-generating
grease and a newly developed contact seal on bothsides of the bearing. In comparison with conventional
NTN products, this bearing both reduces the amountof contamination generated by the bearing by
approximately 90 % and the rotational torque by
approximately 50 %.

Outer ring

Newly developed
low contaminationgenerating grease

Steel ball
Cage

Newly developed
contact seal

3.1 Low Contamination Generating Grease

When bearings rotate, compressive and shearing
forces are applied to the grease within the bearing.
This causes the base oil to separate from thethickener. Furthermore, fine particles are released.
As explained in section 2, lubrication can exit
the bearing and splash onto the servo motor
control device. After collecting and analyzing thiscontamination, NTN found it to be a component of
the base oil. However, the grease thickening agent
was not detected. It is therefore necessary to preventthe base oil in the grease from separating andevaporating (Fig. 6). More specifically, increasing thesurface tension of the base oil5) and decreasing thegrease consistency is considered to be effective forlow contamination generation. The following pages
show the test results that confirm this effect.
Furthermore, since contamination generated by
the bearing is caused by heat generated by bearing
rotation and internal bearing pressure increase, it isalso necessary to devise a means to stop the internalbearing pressure from rising.

Bearing rotation

Heat generated frombearing rotation andpressure increase

Compressive and shearing
forces applied to grease

Separation and evaporationof grease base oil

Dust generationfrom bearings

Fig. 6 Dust generation from bearings

Inner ring

Fig. 5 Low Contamination Generation Bearing forServo Motors

NTN TECHNICAL REVIEW No.88

39

Development of a Low Contamination Generation Bearing for Servo Motors

1) Relationship Between Base Oil Surface
Tension and Contamination Generation5)

16 types of bearing lubrication with different baseoil surface tensions were applied onto test bearing partnumber 608 with an average thickness of 1 m.
The amount of contamination generated was theninvestigated while rotating the test bearing inner rings at
a speed of 1,000 min-1 with an axial load (Fa) of 10 N. A
light scattering particle counter with a total volume of
9 L was used to count particles of contamination withsizes of 0.1 m or larger for 30 minutes. The trial wasthen continued for 8 total hours. Fig. 7 shows the results.
A distinct trend was observed: the higher thebase oil surface tension, the smaller the amount ofcontamination generation at both the initial stage ofrotation and after 8 hours had elapsed. This means that
selecting a base oil with a high surface tension can beeffective for low contamination generation.

Table 2 Characteristic of the developed grease

1,000,000

No. of particles generated/9 L

Another distinct trend was observed: the lower theconsistency of the grease, the smaller the amountof contamination generation for both of the tested
thickening agents. This means that optimizing the typeand amount of thickening agent to achieve low grease
consistency can be effective for low contamination
generation.
Selecting a base oil with a high surface tension andoptimizing the type and amount of thickening agent(Table 2) reduces the contamination generated byapproximately 60 % compared to “EP-1”, the current
low contamination generation grease (Table 3，Fig. 9)．
In addition, this developed grease satisfies therequirements for high temperature durability andwear resistance for servo motor applications. An
appropriate additive was selected that has noadverse effect on the base oilʼs surface tension andconsistency.

Developed low
contamination
generation grease

Current grease
EP-1

Thickening agent

Urea

Urea

Base oil

Synthetic oil

PAO

Base oil viscosity, mm /s (40 ℃ )

130

47

Mix consistency, 60 W (25 ℃ )

220

220

Initial stage of rotation
After 8 hours

100,000

10,000

1,000

100

2

0

20

40

60

Surface tension mN/mTable 3 Condition for contamination generation test

Fig. 7 Relationship between base oil surface tension
and contamination generation

Test
bearing

6900

Axis
orientation

Vertical2) Relationship Between Consistency andContamination Generation

Load
condition

Fa = 30 N

Test
temperature

Constant 120 ℃

Rotation
speed

Constant
6,000 min-1

Test time

200 hours

1,000,000

Thickening Agent A
Thickening Agent B

100,000

10,000

Amount of contamination generated isreduced by approximately 60 %with the developed grease

100

50

0

EP- 1

Developed grease

Fig. 9 Contamination generation test of thedeveloped grease*

1,000

100

150

Comparison for amount ofgenerated contamination

No. of Contamination Particles Generated/9 L

After adjusting the type and amount of thickening
agent, 16 test grease types, 8 with thickening agent
A and 8 with thickening agent B and all with different
trial consistencies, were applied on test bearing partnumber 608 with a total weight of 0.1 g. The same test
method used in section 1) above was used to calculate
the amount of contamination generated after 8 hours
of testing had elapsed. Fig. 8 shows the results.

0

100

200

300

400

Consistency

Fig. 8 Relationship between grease consistency andcontamination generation
40 NTN TECHNICAL REVIEW No.88* The contamination generation test used a test machine thatsimulates a servo motor. A glass plate that simulated a rotation
detector was used to measure the mass of oil that stuck to theglass plate.

As described in section 3.1, in order to reduce
contamination generated by the bearing, pressure
increases inside the bearing should be prevented. To
help prevent this internal bearing pressure, a contact
seal can be installed on the control device side and a
non-contact seal can be installed on the opposite side.
This allows air intake/exhaust through the bearing andprevents pressure differential from occurring between
the interior and exterior of the bearing.
In recent years, control devices are installed on bothsides of the bearing, necessitating low contamination
generation on both sides of the bearing. Furthermore,
low bearing torque is required to increase servo motor
power output.
The newly developed NTN contact seal adopts
the following two points compared to a conventional
contact seal (Fig. 10).
① A slit is added to the seal outer diameter to allow

air intake/exhaust through the bearing.

② The shape of the seal lip is changed.

To confirm the effect of this seal slit ① , a seal was
installed on one side of a bearing and oil was injected
into the opposite side to check whether oil seeped outof the slit. This indeed resulted in oil seeping throughthe slit and confirmed air intake/exhaust air throughthe bearing is possible (Fig. 11).
Conventional seal

Developed sealof the seal groove.
If the seal lip of a conventional seal that makes
contact with the inside of the seal groove deforms
towards the outside of the bearing, the seal/inner ringinterference, tension force of the seal lip, and rotational
torque will increase. The developed NTN seal prevents
this seal lip tension force from increasing wheninterference increases due to the shape of the seal lip(Fig. 12).
Table 4 and Fig. 13 show the bearing rotational torquewith usage of the developed seal. The developed seal
reduces the rotational torque by approximately 60 %compared to the conventional seal.
1,000
Lip tension force mN

3.2 Low Contamination Generation, Low
Torque Seal

Conventional seal

800

Developed seal

600
400
200
0
Small

②

Seal lip

Contact
surface(inside)Contact
surface(outside)

Table 4 Conditions for Torque Test
Test bearing

6900 (ϕ 10 ×ϕ 22 × 6)
Each seal installed only on one side

Load condition

Fa = 39 N

Rotation speed

Constant 3,600 min-1

Axis position

Vertical axis

Lubrication

Small quantity of VG32 oil applied

Enlarged view of slit (before injecting oil)Oilseeping
Slit position

Enlarged view of slit (after injecting oil)Fig. 11 Performance test of the seal slit
Furthermore, for conventional seals, the seal lip
makes contact with the inside of the inner ring seal
groove. In contrast, the lip of the developed seal
makes contact with the outside of the seal groove. This
ensures interference between the seal lip and inner ring
seal groove and prevents contamination from exiting
the bearing, even when the pressure inside the bearing
increases and the seal lip deforms towards the outside

Rotational torque comparison

150

Fig. 10 Characteristics of the Developed Seal

Large

Fig. 12 Relation between seal lip interference andtension forth (FEM analysis)

①

Slit

Seal lip interference

Developed seal reduces
rotational torqueby approximately 60 %

100

50

0

Conventional seal

Developed seal

Fig. 13 Torque test of the developed seal

NTN TECHNICAL REVIEW No.88

41

Development of a Low Contamination Generation Bearing for Servo Motors

4.Evaluation Test
4.1 Test Conditions

Contamination generation and rotational torquetests were conducted to evaluate the performance of
this NTN developed product. Tables 5 and 6 show thespecifications of the evaluated bearings and the test
conditions for each evaluation.

These factors enable further performance
improvements for servo motors and can contributetowards productivity improvements for industrial
robots.

Table 5 Specifications of Test Bearings
Conventional productDeveloped product

Bearing

6900（ϕ 10 ×ϕ 22 × 6）

Seal

One side: Contact seal Both sides: Contact seal
One side: Non-contact seal
(Developed seal)

GreaseEP-1

Amountof grease
injected

Developed grease
Approximately 0.1 g

Same specifications as Table 5

Load condition

Fa = 39 N

Rotation speed

Constant 3,600 min-1

Axis position

Vertical axis

4.2 Test Results

Figures 14 and 15 show the results of thecontamination generation test while Fig. 16 shows theresults of the rotational torque test. The test results
reinforced the conclusions drawn earlier in this report.
This developed bearing both reduces the amount ofcontamination by approximately 90 % and reduces
the rotational torque by approximately 50 % whencompared with NTN conventional product.
The contamination generation test used a test
machine that simulates a servo motor, the sameas shown in Fig. 9 in section 3.1. A glass plate
was installed next to the bearing to measurecontamination accumulation. Fig. 14 shows a photoof the exterior view of the glass plate before and after
the test. The cloudy section of the glass plate after
the test is the result of oily substances generated by
the bearing. The greater the amount of contamination
generation, the thicker this cloudy section becomes.
As shown in Fig. 15, the developed product has alower amount of contamination generation than theconventional product.
Application of this developed bearing in a servo
motor means that a sealed device is no longer
necessary to prevent contamination generated fromthe bearing from sticking to the control device. It alsoenables servo motors to be more compact and resultsin higher power output by reducing the rotational
torque of the bearing.

NTN TECHNICAL REVIEW No.88

After test (developed product)

Fig. 14 Photo of Glass Plate After Testing
150

Comparison for amount ofcontamination generated

Test bearing

After test (conventional product)

Amount of contamination generated
is reduced by approximately 90 %with the developed product

100

50

0

Conventional productDeveloped productFig. 15 Contamination Generation Test Results
150

Rotational torque comparison

Table 6 Condition for torque test

42

Before test

Rotational torque is reduced
by approximately 50 % withthe developed product
100

50

0

Conventional productDeveloped productFig. 16 Torque test of the developed bearing5.Summary
With increasing demand for improved productivity
and reduced labor costs against the backdrop ofa diminishing working population, performance
improvement is required for FA devices and motors,
the power sources of devices such as machine tools
and industrial robots.
NTN has developed the low contamination
generation bearing for servo motors that significantly
reduces rotational torque and generated bearing
contamination to help make servo motors used inindustrial robots more compact and provide higher
power output.
NTN aims to further improve bearing performanceand quality in the future and provide outstanding
support for customer requests by strengthening its
technical support and product proposals.

References

1) Hikaru Ishida, Technical Trend of Servo Motor
Bearings, THE TRIBOLOGY, No.388, (2019) 4244．
2) Chikara Katagiri, Kenichiro Naito, Next Generation
Deep Groove Ball Bearing for High-Speed
Servomotor, NTN TECHNICAL REVIEW, No.72,(2004) 46-51.
3) Hidenobu Mikami, Development of Long Life
Grease for High Speed Application “ME-1” Greasefor Motor Bearings, NTN TECHNICAL REVIEW,
No.72, (2004) 20-25.
4) NTN ULTAGE Deep Groove Ball Bearings for Highspeed Servo Motors, CAT.No.3103, 11. 05. 03,(2011).
5) Takayuki Kawamura, Masami Minami,
Masakazu Hirata, Evaluation of Dust Generation
Characteristics of Lubricating Oil due to Rolling
Bearings, Japanese Society of Tribologists,
Proceedings for Tribology Conference, (2000) 8586.

Photo of authors

Araki TANAKA

Naoaki TSUJI

Hideyuki MITANI

Takayuki KAWAMURA

Application Engineering
Dept., Industrial Business
Headquarters

Product Design Dept.,
Industrial Business
Headquarters

Product Design Dept.,
Industrial Business
Headquarters

Advanced Technology
R&D Center

NTN TECHNICAL REVIEW No.88

43

New Products

Introduction of Composite Material Products for Industrial
Machinery
Shinji KOMATSUBARA*
Norikazu MUNEDA**
Composite Material Products Division has developed
and produced bearings, mechanical parts, and unit module
products made of multi materials such as plastic, sinteredmetal, and magnetic material using tribology as a basic
technology. In this article, we introduce composite material products used in various
advanced fields of industrial machinery.

1.Introduction
New markets are emerging and growing for
products equipped with advanced technology
according market trends in the field of industrial
machinery. Machine parts with tribologicalcharacteristics that are better than existing
technologies are required for new products in thisadvanced field. Creating lightweight or low-friction
parts to save energy and control CO2 emissions are
an example of this. The Composite Material Product
Division has been using and developing materials
such as plastics, sintered metal and magnetic material
for products that meet these higher performancerequirements, and has been proposing and supplying
these products to our customers. This paper
introduces examples of applications for composite
material products in advanced fields.

2. Hydrodynamic BEARPHITE Bearing
for Thin Cooling Fan Motors
There has been a rapid growth in demand formobile devices such as laptops for use in teleworking
and online courses. A superior quietness is requiredfor cooling fans (Fig. 1) equipped in these types ofdevices that covers an extensive range of low to highrotational speed. NTNʼs Hydrodynamic BEARPHITE(Fig. 2) has the reputation of achieving this quietness
and reliability, and has been widely adopted in thincooling fans.
Hydrodynamic BEARPHITE provides non-contact
support for the shaft and bearing through oil-filmpressure generated within the bearing clearance dueto the hydrodynamic effect. Therefore, it is a bearing

an excellent quietness. Generally, oil-film pressure
increases proportional to the rotational speed forhydrodynamic BEARPHITE bearings. Like when the
bearing is rotating at high speeds, it is important tomaintain the oil-film pressure required to provide
non-contact support even at low speeds when the oilfilm pressure drops. By improving the Hydrodynamic
BEARPHITE material, we have maintained a quietness
at low rotational speed, a requirement of cooling fans.

Hydrodynamic BEARPHITE

Fig. 1 Fan for cooling

1 mm

Fig. 2 Hydrodynamic BEARPHITE* Hydrodynamic Bearing Engineering Dept., Composite Material Product Division** Plastics Engineering Dept., Composite Material Product Division

44

NTN TECHNICAL REVIEW No.88

2.1 Features＜ Test conditions ＞

There are 2 types of Hydrodynamic BEARPHITE:
the copper and iron based material EZ06 which is astandard material, and EZ17 with increased oil-filmpressure. Table 1 shows the material composition.
Table 1 Material composition

Shaft material SUS420J2
Bearing dimensions: Inner diameter 2 mm, outer
diameter 4 mm, width 3 mm
Surface pressure 0.5 MPa, room temperature, test
time of 2 min
Lubricating oil kinematic viscosity (40 ℃ ) 12 mm2/s

EZ17

Cu

Remaining amount

Remaining amount

Sn

1-3

1-3

C

0.5 - 2.5−

Fe

38 - 42

38 - 42

Hydrodynamic BEARPHITE is a type of oilimpregnated sintered bearing manufactured using
a powder metallurgy method. So it has pores on
the bearing surface and interior. While the pores
have an important function of maintaining andsupplying lubricating oil, they can also be the causeof decreasing of generated oil-film pressure. In orderto maintain a quietness at a low rotational speedof 1,000 min-1 required for thin cooling fans used in
mobile devices, it is important to control the pores tomaintain a high oil-film pressure.
In case of EZ17, the pores are micronized to
improve the oil-film pressure by optimizing theparticle distribution and shape of the material
powder. Fig. 3 shows a comparison of the bearing
inner diameter surface. Pores on EZ17 are clearlymicronized than EZ06.

2,000

non-contact min-1

Chemicalcomposition %EZ06

Rotational speed that can support

Material

Required level

1,000

0

EZ06

EZ17

Fig. 4 Test result of contact of shaft and BEARPHITE(Shaft outer diameter 2 mm)

3.Plastic Bearings for Electric Water Pump1)
Heat pump water heaters using natural coolant,
residential fuel cell cogeneration systems, and waterheated floors are some of the technology used forresidential equipment to support ZEH (Net Zero
Energy House). Electric water pumps are used
for their circulation and cooling systems. NTNʼsunderwater plastic (BEAREE AS5704) sliding bearing
is used in these electric water pumps.

3.1 Structure of Electric Water Pump

Fig. 5 shows the structure of a magnetic drive water
pump as a typical electric water pump.
Water OUT
0.1 mm

0.1 mmPore (black area)

Fig. 3 Enlarged photo of inner surface(Inner diameter 2 mm)

2.2 Performance

We compared the rotational speed at which EZ06
and EZ17 can provide non-contact support for theshaft by determining whether contact occurs between
the shaft and Hydrodynamic BEARPHITE using anelectrical resistance method. Fig. 4 shows the results.
EZ17 can reduce the rotational speed at which
non-contact support can be provided for the shaft by
30 % compared to EZ06. This shows that it is possible
to provide non-contact support even at the low
rotational speed of 1,000 min-1 required for cooling
fans.

Drive magnet
Pump
chamberThrust pad

Motor

Water IN

Motor shaft

Shaft (fixed)

Rotor(rotational)

Impeller Sliding bearing

Driven magnet

Fig. 5 Structure of magnetic drive electric water pump

NTN TECHNICAL REVIEW No.88

45

Introduction of Composite Material Products for Industrial Machinery＜ Test conditions ＞

Ring on disc type testing machine
Mating material SUS304
Surface pressure 0.4 MPa, speed 25 m/min
Underwater, room temperature, test time 50 hours
700

0.6

600

0.4

500

0.2

400

0

300
200
100
0

BEAREE
AS5704

PPS＋
GF

PPS＋
CF

○Dynamic friction coefficient

Low-friction properties and high wear resistancein a liquid are required for bearings to achieve powersaving and long-life electric water pumps.
Furthermore, chemical resistance and dimensional
stability in water or acidic and alkaline liquids arealso required. For magnetic drive water pumps,
the bearing is integrated with the impeller using acomposite mold, therefore the bearing outer diametersurface must be provided a retainer and rotation lock(D-shaped cut, protrusion, etc.) from the impeller.
Therefore, the flexibility of the shape design is also
important.
Due to the above requirements, electric water
pumps use plastic bearing made of polyphenylene
sulfide (PPS), polyether ether ketone (PEEK) andpolytetrafluoroethylene (PTFE) or carbon bearing.
Among plastic bearing, PPS bearing is less wear
and longer operating life than PTFE bearing, and arecheaper than PEEK bearing. PPS bearing also has
the benefit of a high flexibility of the shape design
since it can be injection molded. Consequently, NTNʼs“BEAREE AS5704 Bearing” for underwater (Fig. 6)
which has excellent friction and wear properties, andchemical resistance by combining fillers such as solid
lubricants and stiffeners with PPS, is used on electricwater pumps.

PPS have high rigidity and excellent water, chemicaland heat resistance, although it is comparatively low
cost. PPS lacks the mechanical properties and friction
and wear properties by itself, so it is combined withfillers. The friction coefficient and wear resistancechange depending on the conditions of use like theenvironment (the atmosphere, oil, or water), theload, sliding speed, and mating material. Bearings forelectric water pumps slide on a soft stainless steel
mating material in fluid such as water.
Fig. 7 shows the results from a friction and wear
test performed underwater on 4 types of bearings.
These bearings are the BEAREE AS5704 bearing, and
PPS combined with glass fiber (GF), carbon fiber (CF)and polytetrafluoroethylene (PTFE).
The CF combined bearing has a low specific wear
amount but has high dynamic friction coefficient. The
PTFE combined bearing has 30 to 40 % lower dynamic
friction coefficient than both the GF and CF combined
bearings but has less wear resistance. The BEAREE
AS5704 bearing has a lower friction coefficient than
the PTFE combined bearing and its specific wear
amount is under 1/5 of the CF combined bearing.
The BEAREE AS5704 bearing is combined withfillers selected appropriately for using in electricwater pumps, so the wear amount of the bearing andmating material is small, and it has excellent friction
and wear properties underwater in comparison with
PPS bearings combined with other fillers.

x 10-8 mm3/(N·m)

3.2 R equired Performance and Bearing
Material

3.3 Comparing BEAREE AS5704 Bearing
with Other Bearings□Specific wear amountA rotor integrated with a plastic impeller, driven
magnet and sliding bearing is housed inside the pumpchamber and supported by the shaft using a sliding
bearing. The drive magnet installed on the motor
shaft rotates the rotor by magnetic force to transfer
water sucked by the impeller into the pump chamber.
Since radial load and axial load occur when the rotor
rotates, the bearing surface of inner diameter andshaft, and the bearing end surface and thrust padslide underwater respectively. Generally, the bearing isinserted into a mold along with the driven magnet andthe plastic impeller is integrated using an injection
molded composite mold.

PPS＋
PTFE

Fig. 7 Results of friction and wear test of PPS bearings

10 mm

Fig. 6 BEAREE AS5704 bearings for using in water46

NTN TECHNICAL REVIEW No.88

Carbon bearing also used to use in electric water
pumps. As shown in Fig. 8, the BEAREE AS5704
bearing has the same or greater friction and wear
properties as carbon bearing, therefore adoption of
BEAREE AS5704 bearing is increasing by replacingfrom carbon bearing.
Table 2 shows the comparison in performance of
the BEAREE AS5704 bearing and carbon bearing.

Since carbon bearing is machined product made frommolding material, so it is expensive and has a low
flexibility of the shape design. Furthermore, it is fragileand easy to chip due to impact, so it is necessary to
be taken care with handling during transportation andfixing them to rotors.
Meanwhile, BEAREE AS5704 bearing is an injection
molded product, so the retainer, rotation lock andlubrication grooves on bearing inner diameter surface
and end surface can be easily designed. Furthermore,
it is cheaper compared to carbon bearing and easy tohandle since it is difficult to chip.
＜ Test conditions ＞

30

0.15

25

0.10

20

0.05

15

0.00

10
5
0

BEAREE
AS5704○Dynamic friction coefficientx 10-8 mm3/(N·m)□Specific wear amount

Ring on disc type testing machine
Mating material SUS304
Surface pressure 1 MPa, speed 125 m/min
Underwater, room temperature, test time 10 hours

CarbonFig. 8 Results of friction and wear test of BEAREE
AS5704 bearing and carbon bearing
Table 2 Comparison between BEAREE AS5704
bearing and carbon bearing
Item

BEAREE AS5704
bearing

Carbon bearings

Processing method

Injection molded

Machined

Friction coefficient(underwater)◎◎

Wear resistance(underwater)◎○

Water and chemical
resistance◎◎

Chip due to impact○△

Flexibility of shapedesign

◎

△

Cost

◎

△

◎：Excellent 〇：Good △：Acceptable

4. Plastic Sliding Screws for Medical
Devices Such as Eye Testers, Dialysis
Machines and Anesthesia Apparatus2)
Ball screw and sliding screw are available for feedscrew that convert the rotational movement of
the motor into linear motion, and these screws areused depending on their application and conditionsof use. Ball screw have a high allowable load andexcellent screw efficiency, but is not suitable foruse in environments where grease cannot be used.
Meanwhile, plastic sliding screw has a low allowableload but can be used without lubrication, produce
low-noise, and is light and compact.
Due to these features, NTN plastic sliding screws
are also adopted in inspection devices for COVID-19.
In terms of other medical devices, these screws havebeen installed for eye testers, dialysis machines
and anesthesia apparatus. In other fields, there arealso adopted for such as substrate positioning on
semiconductor manufacturing equipment.

4.1 NTN Plastic Sliding Screw Structure andRequired Performance

Plastic sliding screw is assembled plastic nut andstainless steel screw shaft. During shaft rotation, theplastic nut and screw shaft slide, so low friction andwear properties are required for the plastic nut. As
shown in Fig. 9, the plastic nut material of the “NTN
Plastic Sliding Screw” is “BEAREE AS5000”, which has
combined with PPS and a special solid lubricant. NTN
has a lineup of 16 standard types with a screw shaft
outer diameter of 4 to 12 mm and leads of 1 to 36 mm.
Generally, polyoxymethylene (POM), polyamide,
and PPS are used for the plastic nut material on theplastic sliding screw and are combined with a solid
lubricant to provide low friction properties. The
durability of plastic sliding screw depends on thewear resistance of the plastic nut which is improved
through the use of a filler. However, combining fibrousfiller increases the friction coefficient and generates
sliding noise.

BEAREE AS5000
plastic nut
Stainless steelscrew shaft

Fig. 9 NTN Plastic sliding screws

4.2 Comparison between NTN Plastic Sliding
Screws and Other Sliding Screws

A special fibrous PTFE is combined to BEAREE
AS5000 to produce a plastic nut that has low friction
and wear properties. Fig. 10 shows wear propertiesNTN TECHNICAL REVIEW No.88

47

Introduction of Composite Material Products for Industrial Machinery＜ Test results ＞

Ring on disc type testing machine
Mating material S45C
Surface pressure 0.4 MPa, speed 25 m/min
Room temperature, no lubrication, test time 50
hours＜ Screw test conditions ＞

Screw specification: Shaft diameter 8 mm, lead 24mm, 6 threads
Load 100 N, rotational speed 500 min-1, room
temperature, no lubrication
0.6

Amount of increasein axial clearance mm

without lubrication based on an element test for
3 types of sliding screws: BEAREE AS5000, PPS
combined with a general granular PTFE, and PPS
combined with granular PTFE and CF. The specificwear amount of BEAREE AS5000 combined withfibrous PTFE is 1/5 lower compared with that of PPS
combined with granular PTFE, and half compared withthat of PPS combined with granular PTFE and CF as
stiffener.

0.5

POM plastic sliding screw

0.4

Upper limit for allowance level

0.3
0.2

NTN plastic sliding screw

0.1
0

0

Specific wear amountx 10-8 mm3/(N·m)

100

150

200

600

Fig. 11 Result of wear test of plastic sliding screws

400

5. Conclusion

200
0

BEAREE
AS5000

PPS＋
PPS＋
granularPTFE granularPTFE＋CF

Fig. 10 Result of wear test of PPS
POM plastic sliding screw which is assembled anoil-impregnated POM plastic nut and a stainless steelscrew is widely used in the market. Fig. 11 shows
the results of a wear test for this POM plastic sliding
screw and the NTN plastic sliding screw. Generally,a sliding screw can be used as long as the amountof increase in axial clearance due to operation is 0.3
mm or less. In contrast, the POM plastic sliding screw
which exceeds 0.3 mm for the amount of increase inaxial clearance at a 60 km of sliding distance, the NTN
plastic sliding screw is 0.15 mm even at 200 km, so it
has a long operating life.

Photo of authors

Shinji KOMATSUBARA Norikazu MUNEDA
Hydrodynamic Bearing
Engineering Dept.,
Composite Material
Product Division

48

50

Nut travel distance km

800

Plastics Engineering Dept.,
Composite Material
Product Division

NTN TECHNICAL REVIEW No.88

This paper introduced examples of adopting
bearing and element products made of plastics andsintered metal in various fields for the purpose of lownoise, energy-saving, compact design, lightweight andlong operating life.
In future, societies will emerge driven by newtechnologies such as robotics, hydrogenation and IoT.
The Composite Material Product Divisioncontributes developing the industrial machinery
field by developing products to further improvefunctionality and making low friction and element
products for growing markets.

References

1) Takuya Ishii, Ken Yasuda, Plastic Sliding Bearings forElectric Water Pumps in Hybrid Vehicles and Electric
Vehicles, Latest Technology for Plastics Used in
Automotive and Aircraft, (2016)167-176.
2) Naonari Tanigawa, Norio Itou, Tomomi Tonomura,
Plastic sliding screws, NTN TECHNICAL REVIEW,
No.84, (2016) 69-73.

Future Development

Activities and Achievements for Automotive Market
Trends CASE
Koji KAMETAKA*In this article, we explain our considerations and actions of research & technological development forfuel economy regulations, autonomous driving and car sharing, in the change of business conditions
represented by “CASE”. In other words, we introduce our development of technologies and products whichmeet the needs of the market.
Today, automobile industry is facing a great change. We are to develop in advance of the market trends,
improving efficiency and accuracy with high speed. We also mention our CAE simulation technologies that
support the research & technological development based on our core competence.

in urban areas. The current utilization rate per hourfor passenger cars is said to be around 5 %. If thisutilization rate increases due to car sharing, there will
be a demand to improve the durability of all parts,
including NTN parts, or provide vehicles with various
types of sensor functionality such as failure detection.

1. Introduction
Europe implements the strictest fuel efficiency
regulations, and it will be necessary to further improvefuel efficiency by approximately 15 % in 2025
versus 2021 regulations. Automobile manufacturers
will continue to improve fuel efficiency into the
future by decreasing vehicle weight, improving
aerodynamics, engine downsizing, and further shiftingfrom mild hybrids to strong hybrids and fully electricvehicles. (Fig. 1)
Furthermore, as shown in Table 1, we know thatthe level of autonomous driving can be divided into
4 to 5 stages. Current mainstream autonomous
driving systems are considered to be very effective in
reducing traffic congestion at level 1, reducing roadaccidents due to human error at level 2, and reducing
the environmental impact at level 3 due to the factthat they reduce the burden on the driver. However,
it is necessary to closely monitor industrial trends forlevel 4 and 5 until transportation system management
has been established a little further in the future.
Recently, car sharing has been increasing mainly
2017

Environmental
changesCar
evolution

2018

2019

2021

Aging society
Spread of Advanced
Driver-Assistance Systems (ADAS)

48 V mild HV

PHV expansion

2023

2024

2025

Carbon-freesociety

Next-gentransportation system
Autonomous driving (Level 3)

Sharing
Connected
EV

Autonomous
driving

Autonomous driving (Level 4)

Various companiesʼ EV launch Activation
Sensing technology

17.8 km/L (130 g/km)
EURO6e2022

Car sharing

Eliminate
traffic accidents

Electrification

Stricter fuelefficiency standards

2020

24.6 km/L (95 g/km)

27.3 km/L (81 g/km)

EURO7Fig. 1 Market trend of automobile* Executive Officer, Deputy Corporate General Manager of Automotive Business Headquarters, Corporate General
Manager of Electric Module Products Division
NTN TECHNICAL REVIEW No.88

49

Activities and Achievements for Automotive Market Trends CASE
Table 1 Issues and measures for each level ofautonomous driving
Level

Challenges

Measure1

Reduce traffic
congestion

Can reduce traffic congestion by
managing the transportation system

2

Reduce traffic
accidents

Reduce accidents due to humanerror by controlling the vehicle witha autonomous driving system

3

Reduceenvironmental
impactReduce unnecessary acceleration or
deceleration and reduce environmental
impact by reducing traffic congestion

4

Movement assist
for the elderly
and disabled

Assisted movement for people
who are unable to drive using
autonomous driving vehicles

5

Labor-saving

Resolve driver shortages using
unmanned driven vehicles

2. Market Needs and Responding to These Needs
with NTNʼs Research and Development
There is a strong demand from the market to
produce lighter and more compact bearings that have alower friction and such from the perspective of reducing
fuel consumption and saving energy (Table 2).
NTN has been making progress in developing highperformance products and deploying them to themarket more than our competitors based on our
accumulated technology and knowledge. We have alsoestablished the NTN Next Generation Research Alliance
Laboratories at Osaka University (in 2017) to conduct
research into next-generation technology that cannot
be achieve through corporate research alone, and havecontinued to incorporate findings from this research
into NTN technology.
For example, we took up the challenge of producing
an axle bearing that has approximately 60 % lower
friction than conventional products and achieveda product to meet this goal. Grease-filled bearings
have a seal to prevent the grease from leaking andprevent muddy water from penetrating from outside
the bearing. It was necessary to establish an optimal
design for the sealing characteristics and friction
characteristics, which was the root of the challenge
concerning friction. We were also successful in
reducing the friction of transmission ball bearingsby 80 %. In the past, a contact-type seal was used
to prevent hard foreign objects, such as gear debris
generated inside the transmission, from entering thebearing. However, after coming up with a bold new
design for the seal we achieved an 80 % reduction in
friction.
The electrification of auxiliary equipment driven
by the internal combustion engine is also making
progress due to the electrification of drivetrains.
Therefore, low friction, lightweight and compact
bearings are required for the purpose of achieving
further energy reduction. It is necessary to also
respond to speed increases that accompany motors
becoming faster. NTN has explored solutions for theseissues as it has continued to do so in the past. Our
core competencies lie in technologies that include
multi-material technology, heat treatment technology,

50

NTN TECHNICAL REVIEW No.88

high-precision machining technology, measurement
technology, surface processing technology, andsimulation technology. NTN has a dedicated teamof researchers and engineers who excel in thesecore competencies in our Research and Engineering
Division. We believe the time has come to contributetowards the automotive industry by using these corecompetencies and various human resources to further
expand the playing field to meet market needs suchas improved fuel efficiency.
We have also received many requests for highperformance module products, beginning withbearings, utilizing these core competencies. All ofwhich have the goal of reducing traffic accidents,
providing freedom of movement in a safe andsecure manner, bringing convenience to users, andestablishing environmental measures. NTN has beendeveloping electric module products by integratingsuch things as motors and controllers throughcollaboration with other companies and in-housedevelopment. Some of these products have alreadyentered the preliminary stage of mass production.
More specifically, products such as electric oil pumps,
electric water pumps, and electric parking locks
for which demand is increasing. This also includes
products such as Hub Bearing with Motor Generator
Function (eHUB) and Hub Bearing with Steering Assist
Function (sHUBTM), which provide multi-functionality
to the axle bearing, a bearing that has taken the top
global share.
Table 2 Market trend and needs
Market trend

Market needs

1・Electric module products
・Lower friction
Environmental
・Higher speed
technology(Low fuel and power ・More compact and lightweight・Greater efficiency
consumption)
・Greater reliability

2

Car sharing

・Greater reliability
・Durability

3

Comfort・Low-noise profile

4

Safety and・Sensors, control technology (linearity)
autonomous driving

3. Developed Products
3.1 Electric Module and Actuator
Development and Application Examples

We have developed an electric module and actuator
constructed from a motor, sensor and mechanicalbody that utilizes such as motor design technology
and bearing/ball screw product technology, which
are some of our core competencies. More specifically,
the BIII-series (Fig. 2) combines a hollow high power
output brushless DC motor and a ball screw, witha linear motor mechanism arranged in the hollow
section of the motor. The SP-series (Fig. 3) is alsoavailable for use in applications that require a thinand high torque rotational actuator by incorporating a
reducer in the hollow section of the BIII-series.

The following are application examples proposedto the market, beginning with the electric module andactuator.
(1) Electric hydraulic brake (BIII-series)
Applying the coaxial type BIII-series with built-inball screw to the master cylinder shaft can createa more compact electric hydraulic brake system.
(2) Electric variable valve timing control (SP-series)
Incorporating the SP-series into the camshaft
can achieve a more compact and highly efficientelectric variable valve timing mechanism.
(3) Electric oil pump (SP-series)
Incorporating a thin and highly efficient SP motorand integrating it with a controller can create amore compact electric oil pump.
・Compact design due to coaxial arrangement
・Optimized size, torque and output using a

polymerized structure for the magnet and core

・Shares the magnet and coil, which are the main

components of a motor

・Built-in non-contact linear position sensor

Fig. 2 BⅢ series

products, and has improved vehicle fuel efficiency
by approximately 0.53 % (Fig. 5). This producthas already been proposed to several automobile
manufacturers and is undergoing prototype evaluation
and being ordered by customers.
(1) Low friction due to the seal
Approximately 50 % of rotational torque froma hub bearing is friction due to sliding with theseal. The seals ability to resist muddy water is
an important characteristic and the following
two points are challenges in maintaining thischaracteristic and lowering the friction. The
friction was reduced for both the outer seal andinner seal making full use of each element test andsimulation technology.
① Labyrinth structure, lip shape and material② Optimal design for lip contact surface, lip application
dedicated grease(2) Low friction due to grease inside the bearing
The following 3 points are challenges present forresistance and friction that accompany greaseagitation and such. Low friction was achieved by
optimizing the thickening agent and additive andlowering the viscosity and friction of the base oil.
① Rolling viscous resistance between the rolling
element and raceway surface② Friction due to micro slips at the rolling contact
zone of the rolling element and raceway③ Resistance due to grease shearing between therolling element and cage pocket surface
Outer seal
Inner seal

Hub bolt

・Brushless DC motor series
・Hollow structure based on the motor unit fromthe BIII-series

・Can be applied as a thin and high torque rotational

actuator by incorporating a reducer in the hollow section

・Built-in rotation angle sensor

Inner ring
Rolling element
Hub ring

Outer ring

Fig. 3 SP series

3.2

Axle Bearing Low Friction and Modular
Products
3.2.1 60 % Reduction in Friction“Low Friction Hub Bearing III” (Fig. 4)“Low Friction Hub Bearing III” is constructed withseals, grease and an internal bearing design in pursuitof lowering the friction, reduces the rotational torqueby about 60 % when compared to conventional

Rotational torque ratio

Fig. 4 Low Friction Hub Bearing Ⅲ

1

Seal sliding
resistanceHub bearing (overall)Reduction of approx. 60 %rotational torque

0.5

0Bearing rolling
resistance-70 %Conventional
products

Developed
product-50 %Fig. 5 Performance of Low Friction Hub Bearing Ⅲ

NTN TECHNICAL REVIEW No.88

51

Activities and Achievements for Automotive Market Trends CASE
3.2.2 Hub Bearing with Steering Assist
Function “sHUB TM” (Fig. 6)

Since the steering device arranged at the front wheels ofa vehicle is mechanically connected to the tires on both sides(left and right) of the vehicle, the steering angle for the left andright tire is uniquely determined against the steering angle ofthe steering wheel. When making a large turn at low speed, theturning radius of the path through which each of the left and rightwheel passes through is different. Therefore, it is necessary thatAckermann geometry (a setup in which the steering angle forthe inside wheel is large and the outside wheel is small) is used
for the left and right wheel to ensure the vehicle turns smoothly
in theory. Furthermore, the turning center of a vehicle changes
depending on the vehicle speed in a medium to fast speed range
so the ideal left and right wheel angle will also change.
NTN developed the “Hub Bearing with Steering Assist
Function” (hereafter, sHUBTM), which grants the hub bearing witha function that can control steering geometry depending on theconditions.
sHUBTM achieves reliable and comfortable driving by controllingthe angle of the left and right wheel separately according to thevehicle driving conditions. This product is a module system that
improves straight running stability and driving reliability when
cornering and also contributes towards avoiding dangers during
an emergency such as when a vehicle wheel slips while drivingon a low coefficient of friction road. It can also save energy whendriving by reducing cornering drag when turning the vehicle
normally. It is a module that achieves vehicle maneuverability anda comfortable driving experience with the aim of applying it tosuch things as avoidance operation in future autonomous driving
technology. The following are its features.
(1) assembled on both left and right wheels to separately set theleft and right steering angle(2) Ideal angle control that matches driving conditions at eachwheel separately
・Energy-saving driving with reduced cornering drag・Stabilizes the vehicle attitude even during emergencies
such as to avoid dangers(3) Can be assembled regardless of the type of suspension
systemCan be assembled without significant modification to theexisting vehicleʼs driven wheels (front and rear)(4) Provides a compact and lightweight solution using an optimal
internal bearing design
Furthermore, sHUBTM for the rear wheels described in thispaper was designed for the purpose of further improving vehicle
maneuverability and achieving safe, comfortable, and energysaving driving by assembling it on the vehicleʼs rear driven wheels
to control them. We have also received many inquiries about its
application on front-wheel-drive vehicles.
KnuckleActuator

NTN TECHNICAL REVIEW No.88

Within this revolutionary period typified by CASE (Connected,
Autonomous, Shared & Services, Electric) and against the backdropof such things as the increase in vehicle travel distance due tovehicle electrification and car sharing, the move to higher efficiency
power transmission devices is underway. Furthermore, due toreduction in the amount of lubricating oil in devices and the switch
to low-viscosity oil, there has been a greater need for bearings towork under harsh lubrication conditions and for bearings with lower
torque.
To fill this need, we developed the “Low Temperature Rise and
Low Torque Tapered Roller Bearing” for automobile transmissions
and differentials. By optimizing the newly designed plastic cage andthe internal bearing design, we achieved the worldʼs highest level in
low temperature rise (capability for protecting against seizure) andlow torque performance. The following are this productʼs features.
(1) Low temperature rise:
10 times improvement in comparison with NTN standard type(worldʼs highest level)
・Reduces temperature rise even under harsh lubrication
conditions due to decreasing amount of lubricating oil andreducing lubricating oil viscosity
・The concave shape of the plastic cage enables oil to besupplied to the roller end face when there is insufficientlubrication to reduce temperature rise・Reduces temperature rise with a design that improves the
lubricity of sliding contacts between the roller end face andinner ring cone back face rib surface(2) Low torque:
66 % reduced torque compared to NTN standard type (worldʼshighest level)
・Plastic cage reduces excessive lubricant inflow into thebearing, reduces rotational torque due to lubricant stirring
resistance・Smaller bearing size with longer operating life due to
optimized internal bearing design and roller bearing design
・Reduces rotational torque due to reduction in rolling contact
length between rolling elements (rollers) and raceway (innerand outer rings)
Since we were able to create a compact bearing size with thisproduct, it contributes to making devices more compact andlightweight, and further increasing the amount of space inside thevehicle as well as improving driving comfort, in addition to providing
greater efficiency for power transmission devices, and making
vehicles more fuel and power efficient.

Steering axis

Hub bearing

Fig. 6 Structure of sHUB™

52

3.3 Support for Next-Generation Mobility with Worldʼs
Highest Level in Low Temperature Rise and Low Torque“Low Temperature Rise and Low Torque Tapered Roller
Bearing” (Fig. 7)

Fig. 7 Low Temperature Rise and Low Torque Tapered Roller Bearing4. CAE Simulation Technology
In order to quickly launch competitive products on themarket that meet market needs, it is essential to make researchand technology development work more sophisticated andefficient. We have introduced CAE simulation technology asa means of this achievement. This technology allows to verify
the performance of products including peripheral components
at the design stage. This makes it possible to optimize design
variables before creating a prototype, and reduce the numberof prototype builds and tests. These factors can achieve
improved work efficiency, reduce development costs, andshorten development lead times.
To ensure that designers can easily use CAE simulation
technology required to develop products, we have beendeveloping our own CAE system and program, and part of it isintroduced here.
(1) FEM analysis automated system (Fig. 8)
FEM analysis is used when designing axle bearings toevaluate the stress and rigidity. In the past, this work was
performed by dedicated CAE personnel but after developinga system that automates model creation, calculation, andresults processing required for the analysis, it is now possiblefor designers to perform this analysis by themselves. This
has enabled us to reduce the time needed to hold meetings
between designers and dedicated CAE personnel and thetime waiting for analysis to begin, and also speed up designchanges influenced by the calculation results.
(2) Engineering calculation program for transmission
bearings (Fig. 9)
Progress is being made for multi-stage and increasingly
complex transmissions to respond to environmental
requirements. To design bearings used in such products,
we have developed an engineering calculation programthat enables designers to easily create a model of theentire transmission and simulate it. This has enabled us
to perform batch calculations for the bearing operating
life and the load acting on each bearingʼs rolling element
inside the transmission, a task that was traditionallyperformed using a combination of several programs.
Further using a dynamic analysis system1) for the bearing
alone enables us to calculate stress generated at thecage as well2) 3). Using these programs has enabled us toefficiently perform advanced optimal design for bearings
that match the characteristics of transmissions.

Fig. 9 Simulation sample of transmission bearing

5. Conclusion
This paper describes NTNʼs views and how thecompany will proceed concerning research andtechnology development with regards to fuel efficiency
regulations, autonomous driving technology and car
sharing amid environmental changes typified by CASE.
We believe that the information contained here has
provided you with an understanding of our productdevelopment and market expansion that is superior
to our competitors based on NTNʼs accumulated
technology and knowledge.
All developed products introduced here are theresults of improving and incorporating elemental
technology. We will continue to provide global
proposals as specifications that can apply theseproducts to next-generation mobility.
We sincerely hope that by providing products that
resolve the various issues related to automobiles into
the future will enable us to contribute to achieving a
NAMERAKA smart mobility society.

References

1) Mariko Sekiya, Integrated Bearing Dynamic Analysis
System (IBDAS), NTN TECHNICAL REVIEW,No.79,(2011)119-123.
2) Tomoya Sakaguchi, Kazuyoshi Harada, Sadatsune
Kazama, Advancement of Cage Stress Analysis of
Rolling Bearings, Tribology Congress Spring 2012
Tokyo, (2012) 233.
3) Tomoya Sakaguchi, Kazuyoshi Harada, Sadatsune
Kazama, Advancement of Cage Stress Analysis of
Rolling Bearings, NTN TECHNICAL REVIEW, No.80,(2012) 92.
Photo of authors

Koji KAMETAKA

Fig. 8 FEM analysis sample of axle bearing

Executive Officer,
Deputy Corporate General
Manager of Automotive
Business Headquarters,
Corporate General Manager ofElectric Module Products Division

NTN TECHNICAL REVIEW No.88

53

New Products

Small and Lightweight CVJ for Rear Sub-axles

Tomoshige KOBAYASHI*

FF-based 4WD vehicles generally have a smaller rear torque distribution than the front, so small load
capacity is applied to the constant velocity joints for the sub-axle.
There is a market demand for small-sized CVJs for rear sub-axles, similar to the “R series”; “R series
is a compact and lightweight CVJ for rear of main axle, which is already in mass production and pursues
miniaturization and weight reduction.
Introducing the features and performance of the developed “small and lightweight CVJ for rear subaxles”.

1. Introduction1) 2)
Passenger cars are categorized by front-wheel drive(FF), rear-wheel drive (FR) or 4-wheel drive (4WD)
driving methods, with 4WD vehicles further classified
as being based on an FF layout or FR layout.
Constant velocity joints (CVJs) that are used as
driveshafts for driving the front wheels of FF vehicles
and 4WD vehicles have the role of transferring powerfrom the engine or motor smoothly to the tires. These
CVJs need to move with the tires steering and thesuspension changing, and as such, consist of fixed
type CVJs with a larger work angle used on the tireside, and sliding type CVJs that can slide in the axial
direction used on the differential side
While rear driveshafts of FR vehicles and 4WD
vehicles can actually use fixed axles (rigid axle withthe left and right wheels connected in a line along
the same axle), vehicles designed for ride comfort
and driving stability require CVJs because they use
suspension systems that allow the left and right axles
to move up and down independently (independent
suspension systems) (Fig. 1)．Yet they differ from the
CVJs used for front wheels because the rear tires are
not used for steering so a large operating angle is notneeded.
NTN has developed a wide variety of CVJ types
including fixed types and sliding types that provide
different functions to suit specific applications.
Furthermore, each type of CVJ is also available by load
capacity in a range of sizes, from small (such as #75
for compact cars) to large (such as #113 for large
vehicles).
Until recently NTN had used front wheel CVJs as
rear wheel driveshafts, however as large operating
angles are not required for rear wheel CVJs, NTN
has developed the small and lightweight R Series3)
with a minimal operating angle. This product has
been released in sizes with high load capacities andmarketed toward European automotive manufacturers
for use as main axles in FR vehicles.
More recently, rear wheel CVJs have also beendeveloped as small and lightweight CVJs in sizes withlow load capacities for sub axle applications.

Differential

Application

Fig. 1 Application of CVJ for rear

2. Developed Product Structure and Features
SUVs have been dominating the automotive market
in recent years, and there are many models that areavailable with 4WD powertrains. C segment or smaller
compact SUVs that are particularly popular in Japanand Europe also come in 4WD configurations basedon an FF platform in order to maximize cabin space.
In general FF-based 4WD vehicles have a smaller
torque distribution to the rear compared to the front,
and in many cases small CVJs are used for the rear.
In light of this, NTN adapted its existing lightweight“R Series” rear driveshaft to develop the small size
“Small and Lightweight CVJ for Rear Sub-axles.”A comparison of the developed “Small andLightweight CVJ for Rear of Sub-axle” and the* CVJ Development Dept., Automotive Business Headquarters

54

NTN TECHNICAL REVIEW No.88

conventional product is shown in Fig. 2.

reduction) diameter achieved on the sliding type CVJ
in particular.
Note that as the vehicle mounting structure differs
for each customer, the mass calculations shown in
Table 1 do not include the stem section (fitting shaft
for vehicle mounting) in the shape shown in Fig. 2.

The developed product mass and outer diameter
reduction rate from the conventional product is shownin Table 1. Compared to the conventional product, thefixed type CVJ is approximately 6.2 % lighter and thesliding type CVJ approximately 29 % lighter in weight.
This corresponds to a 429 g lighter weight per
driveshaft for the #75 size, making each vehicle 858 glighter when both left and right driveshafts are taken
into account.
The outer diameter has also been reduced
significantly, with an 8.5 mm smaller (12.3 %Developed
product

Conventional
product

Stem

Fixed type CVJ

Boot

Sliding type CVJ

Stem

Fig. 2 Development product VS Conventional product (#75 size)
Table 1 Development product mass and outer diameter reduction rate (#75 size)
Developed Product

Mass Reduction Rate

Diameter Reduction Rate

Fixed type CVJ

6.2 ％

1.9 ％

Sliding type CVJ

29.0 ％

12.3 ％NTN TECHNICAL REVIEW No.88

55

Small and Lightweight CVJ for Rear Sub-axles

3. Small and Lightweight Fixed Type CVJ
for Rear Sub-axles
Fixed type CVJs for front wheels have a maximum
operating angle of 47 to 50°as they need to cater tovertical movement of the suspension and turning of
the wheels with steering.
In contrast, CVJs for rear wheels only need to caterto the vertical movement of the suspension, and assuch an operating angle of 20°or less suffices on mostvehicles when actual usage conditions are considered.
As some vehicles may need an operating angle of 20°
or more for mounting or handling requirements, themaximum operating angle of fixed type CVJs is 30°.
In the conventional product, the internal parts(inner ring, cage, ball) of the existing 47°design wereadopted together with the 30°design, where theouter ring is shorter in the axial direction to achieve alighter weight. To make the developed product evenlighter in weight, each part was shortened in the axial
direction, as well as a shorter design incorporated in
the radial direction.

diameter balls could also be used to successfully makethe outer ring outer diameter and cage smaller, which
helped achieve a small size and light weight (Fig. 3).
Ball

Outer ring

Developed
product

Cage
Inner ring

Conventional
product

Fig. 3 Comparison of outer dia. and axial length

3.2 Functional Evaluation

The functional requirements are as outlined below.

① Strength: equivalent to conventional product (atmaximum angle)

A comparison of the lighter weight #75 size
developed product and the conventional product isshown in Table 2.
Table 2 Comparison of development product andconventional product
Item

Developed Product

Conventional Product

Maximum
operating angle [°]30

30

Outer diameter [mm]ϕ 67.6

ϕ 68.9No. of balls

6

6

Mass [g]834Compared toconventional product- 6.2 ％

889

② Durability: equivalent to conventional product

Examples of strength test results for the developed
product are shown in Fig. 4, and durability test resultsin Fig. 5.
The developed product meets the development
targets set based on past test data for the conventional
product, and was verified as having equivalent
strength and durability as the conventional product.

n=3
Breakage torque3.1 Features

Development target
SUB-ASSY

Fig. 4 Static torsion strength test result of developed)
product ( =20°
No.

The conventional product was based on the CVJ
for front wheels, with the maximum operating angle
limited to 30°and only the outer ring shortened in theaxial direction. However, there was excess thickness
in the material of each part designed with a maximum
operating angle of 47°.
The developed product was designed for rear
wheels so the maximum operating angle was madesmaller, which results in a smaller maximum input load
and allows each part to have thinner material. Smaller

56

NTN TECHNICAL REVIEW No.88

Developed
product
durability
test

Damage Chart/Inspection Time (h) n=4
80

160

250

330

1
2
3
4
：No defects

Development target

Fig. 5 Durability test result of developed product ( =6°)

4.1 Features

A comparison of the lighter weight #75 size
developed product and the conventional product isshown in Table 3.
Table. 3 Comparison of development product andconventional product
Item

Developed Product

Conventional Product

Maximum
operating angle [°]20

23

Outer diameter [mm]ϕ 60.5

ϕ 69

No. of balls

8

6

Mass* [g]910Compared toconventional product- 29 ％

1,284

Conventional
product

Fig. 6 Comparison of ball groove length (red line)
As the balls have a smaller diameter than theconventional product, the width of the cage window,
and thus the width of the cage, could be reduced.
Limiting the maximum operating angle and using asmaller ball PCD also decreases the range of motionof balls within the cage window, thus the length of thecage could be made shorter. Limiting the operating
angle also allows for a smaller taper angle to be
used for the outer diameter, which makes it easier toachieve the thickness for the cage and a smaller sizein the radial direction (Fig. 7).
Cage widthCage width
Taper
angle

Taper
angle
Window length

While there are many variations of sliding type CVJs
available, FF vehicles and the front wheels of 4WD
vehicles use either tripod or ball type CVJs. Tripod
type CVJs have a low sliding resistance to limit engine
vibrations being transmitted to the tires, steering andother parts of the vehicle body via the CVJs, while ball
type CVJs are able to reduce rattle in the drivetrain.
For a long time, NTN had been using ball typesliding double offset joints (DOJ) as small CVJs forthe rear wheels, but had been using the CVJs outlined
above for front wheels.
The developed product was also applied to the
DOJ to develop a new small and lightweight sliding
type CVJ for rear wheels by limiting the maximum
operating angle (23°→ 20°) for the rear wheels only.

Developed
product

Window length

4. Small and Light Sliding Type CVJ for
Rear Wheels4)

Window width
Window width

SUB-ASSY

Developed product* Slide amount: calculated at 45 mm

The conventional DOJ was designed with 6 balls,
however the developed product was designed with
8 balls with the goal of reducing size and weightconsiderably.
Using 8 smaller diameter balls for transmitting
torque helps to better disperse the load acting oneach ball to achieve a smaller size and lighter weight.
Limiting the maximum operating angle and using asmaller ball PCD decreases the range of movementof balls in the axial direction and reduces the groove
length of the inner and outer rings. The result is thatthe inner and outer rings, and the axial length of thecage, can be reduced to achieve a more compact size
as well as a lighter weight (Fig. 6).

Conventional productFig. 7 Comparison of development product cage andconventional product cage
As the width of the cage and inner ring can beshortened, the length of the outer ring cup can be
reduced while ensuring the same slide amount (L1< L2 in Fig. 8), thereby making it shorter in the axial
direction than the conventional DOJ.
Smaller diameter internal parts also mean the outerring can be a smaller outer diameter.

NTN TECHNICAL REVIEW No.88

57

Small and Lightweight CVJ for Rear Sub-axles
Ball

5. Boot and Amount of Grease

Outer ringCage
Developed
product

Limiting the maximum operating angle also
allowed for a small and lightweight boot to be used.
Furthermore, the amount of injected grease could
also be reduced due to the smaller size of the CVJ andboot.
Small and lightweight boot designs for fixed type
CVJs and sliding type CVJs are being developed withthe aim of reducing the weight of driveshafts evenmore.

Inner ring

Conventional
product

Fig. 8 Comparison of outer dia. and axial length

4.2 Functional Evaluation

The functional requirements are as outlined below.

① Strength: equivalent to conventional product (atmaximum angle)② Durability: equivalent to conventional product

Examples of strength test results for the developed
product are shown in Fig. 9, and durability test resultsin Fig. 10.
The developed product meets the development
targets set based on past test data for the conventional
product, and was verified as having the equivalent
strength and durability as the conventional product.

6. Conclusion
This article outlined the features and performanceof the “Small and Lightweight CVJ for Rear Sub-axles”
developed for use with the sub-axle rear wheels ofFF-based 4WD vehicles.
With the growing demand for small SUVs, there has
been an increase in models designed using FF-based
4WD vehicles. The developed product can also be
used for rear-wheel drive vehicles that need small size
CVJs. With more environmentally friendly designs anessential requirement of recent vehicle development,
this significantly lighter weight developed product isanticipated to meet a range of market requirements.

References

1) Shin Tomogami, Technical Trends in Constant Velocity
Universal Joints and the Development of Related
Products, NTN TECHNICAL REVIEW, No. 75, (2007)
10-15.
2) Shinichi Takabe, History of Constant Velocity Joints,
NTN TECHNICAL REVIEW, No. 85, (2017) 40-45.
3) Tomoshige Kobayashi, Light Weight Drive Shaft for FR
vehicle “R series,” NTN TECHNICAL REVIEW, No. 85,(2017) 78-83.
4) Hiroshi Tone, Kenji Terada, Masamichi Nakamura,
Summary of E Series, High Efficiency Constant Velocity
Joints, NTN TECHNICAL REVIEW, No. 70, (2002) 1823.

Breakage torquen=3

Development target

Fig. 9 Static torsion strength test result of developed)
product ( =20°
No.
Developed
product
durability
test

Damage Chart/Inspection Time (h) n=4
80

160

250

330

1
2

Photo of authors

3
4：No defects：Continuous operation possible

Development target

Fig. 10 Durability test result of developed product)( = 6°

Tomoshige KOBAYASHICVJ Development Dept.,
Automotive Business
Headquarters

58

NTN TECHNICAL REVIEW No.88

New Products

Hub Bearing Module with Steering Function for Rear Wheel

Yusuke OHATA*
Atsushi ITO*
We have developed a hub bearing module, ʻRa-sHUBʼ with steeringfunction incorporating the steering angle adjustment mechanism in the hub
bearing. This product can be attached to rear driven wheels, by controlling
the right and left wheel with separate actuators, we can control the angle of
the right and left wheel independently. Therefore, the dynamic performanceof the vehicle is significantly improved.

1. Introduction

2. Background

There are Systems available that improve turning
capabilities at low driving speeds and enhance vehicle
stability at medium to high driving speeds by making
corrections to the steering angle of the wheels to suitthe vehicleʼs driving conditions.
NTN developed the “Hub Bearing Module with
Steering Adjust Function2)-4)(sHUBTM)” that turns thefront wheels by incorporating a steering function into
the “Hub Bearing1)” of which NTN has the leading
global market share.
NTN has verified in a number of actual driving tests
that independently correcting the steering angle ofthe left and right front wheels can improve the driving
performance of vehicles in a range of situations, fromordinary driving to harsh driving conditions, as well asfrom very low driving speeds up to high speeds.
The shape and configuration of each part has now
been modified so that this system can be assembled
to the rear wheels. This report covers the basic
performance test and actual driving test that wereconducted.
Conventional rear wheel steering systems havebeen limited to suspension structures such as multilink systems, and large operating angles were difficult
to achieve due to the structure of the system. The
Hub Bearing Module with Steering Function for Rear
Wheel (hereafter, Ra-sHUB) incorporates a steering
shaft in the hub bearing for an integrated steering
angle adjustment mechanism, with its compact
design meaning it can be assembled in the same way
as conventional hub bearings.
The result can be applied as a rear wheel steeringsystem capable of large operating angles that caneven be assembled to rigid axle systems such as
torsion beams, regardless of the suspension structure.

Rear wheel steering systems first became available
in mass-produced vehicles in the 1980s, however
many commented on the unnatural way the vehicle
behaved with the driverʼs steering operation. The
advances that have been developed to control
technology prove this unnatural feeling can be
minimized, and such systems have increasingly beenused in luxury cars and sports cars in recent years.
Rear wheel steering systems have the following
advantages.
● Low-speed cornering
Turning the rear wheels in the opposite direction(reverse phase) to the front wheels reduces
the turning radius, allowing tighter turns to bemade in the same way as a vehicle with a shorter
wheelbase.
● High-speed cornering
Turning the rear wheels in the same direction(same phase) as the front wheels and controllingthe centrifugal force to reduce the yaw moment
achieves the same level of stability as vehicles witha longer wheelbase.
● Better fuel efficiency
Vehicles generally have rear wheels set to toe-in
to increase stability during braking, however thisincreases running resistance and in turn reduces
fuel efficiency. Reducing running resistance with thetires parallel to the driving direction during ordinary
driving, and changing to toe-in when braking canhelp achieve both safety and low-fuel consumption
driving.

* New Business Search and Development Dept., New Product and Business Strategic Planning Headquarters
NTN TECHNICAL REVIEW No.88

59

Hub Bearing Module with Steering Function for Rear Wheel

3. Concept
The aim of control with Ra-sHUB assembled to therear wheels is to increase the dynamic performanceof the vehicle, as well as achieving safe, comfortableand energy-efficient driving. The system is also
anticipated to be used with future automated driving
technologies. The main concepts are as outlined
below.
① Assembled on the left and right rear wheels
for independent control of steering angles.
This allows each wheel to be controlled at the
optimum angle to suit driving conditions
・Energy-efficient driving with minimal corneringdrag・Excellent response and stable vehicle stanceeven during emergency situations like hazard
avoidance② Can be assembled to existing vehicles without
requiring any modifications, regardless of thetype of suspension system③ Small and lightweight due to optimized internal
design④ Assembled within wheels to maintain vehicle
design flexibility
Fig. 1 shows Ra-sHUB assembled to a vehicleʼs rear
wheels. The Ra-sHUB controller separately calculates
and controls the target angle of the left and rightRa-sHUB based on vehicle information such as speed
and angle of the steering wheel.
Front of vehicle

Control range ofsteering angle
Hub bearing

Conventional
steering device

Actuator

Vehiclespeed

Steering
wheel angle
Steering axis

Controller

Base partFig. 2 Ra-sHUB attached to rigid axle
The basic layout of the rear wheel with Ra-sHUB
assembled is shown in Fig. 3. When all four wheels of
the vehicle are in equal contact with the ground, thesteering axis is slightly behind the point of contact
between the tire and ground. This results in groundcontact point located forward of the steering axis,
so when the reverse input load from the groundcontact point acts on the outside tire of the turnwhen cornering, the large load applied to the outside
tire forces it toward a natural toe-in direction, which
increases the cornering limit performance for morestable driving.

Steering axis

Ground contact
point
Front of vehicle

Fig. 3 Layout of rear wheel with Ra-sHUB

4. Configuration and Specification
4.1 Components

Ra-sHUB

Fig. 1 Vehicle image with Ra-sHUB(Attached to rear wheels)
Fig. 2 shows the layout with Ra-sHUB assembled
to an ordinary rigid axle (torsion beam). Turning
the rear wheels when a rigid axle is used generally
requires movement of the entire rigid axle, however, arear wheel steering system can be achieved easily byattaching Ra-sHUB to the hub bearing mounting faceof the torsion beam.

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NTN TECHNICAL REVIEW No.88

Fig. 4 shows the components of Ra-sHUB.
Ra-sHUB is comprised of three core components: anactuator, base part, and hub bearing. The role of eachcomponent is as follows.
・Actuator: fixed to the base part, and consistsof a motor, reducer and trapezoidal
screw for steering the hub bearing.
・Base part: connects to the suspension systemof the vehicle.
・Hub bearing : supports the base against rotation
around the rotation axis and aroundthe steering axis of the wheel.

Table 1 Specifications of the Ra-sHUB prototype

Base part

Steering axis

Actuator

Item

ValuesMaximum steering torque

350 NmPower supply voltage

24 V

Maximum steering angle± 3.5 deg

Hub bearing

Maximum steering angular speed

16 deg/s

Arm

5. Basic Performance Test
The basic characteristics of the developed productrequired for assembling it to vehicles were verified.

5.1 Frequency Response

Hub bearing
Base part

Hub bearing

Fig. 4 Components of Ra-sHUB
The motor of the actuator is controlled at the besttire angle by the controller based on the input suchas the vehicle speed and steering wheel angle. The
rotation from the motor is converted to linear motion
by the trapezoidal screw via the reducer, and thescrew drives the end of the arm via the joint to steer
the hub bearing around the steering axis.
The reverse input load from the tires is also blocked
by the self-lock mechanism of the trapezoidal screw,which helps to reduce motor power consumption.

0.5

4.2 Specifications

Fig. 5 shows the appearance of the developed
product designed for the rear wheels of a frontwheel drive C segment vehicle (with torsion beamrear suspension structure). The specifications of thedeveloped product are shown in Table 1.

Amplitude gain dB

Joint
Actuator

A frequency response characteristics test was
conducted with the wheels of a test vehicle (in astationary state) assembled with Ra-sHUB placed ona turntable. The response of the actual steering angles
was verified by applying a steering angle command in
the form of a 0.5 degree amplitude sinusoidal wave to
Ra-sHUB and changing the frequency.
The test results are shown in Fig. 6. At frequenciesof 4 Hz or less, the amplitude gain (shown as the logof the ratio between the amplitude value of the actual
steering angle and the command value, Fig. 6 (a)) andthe absolute value of the phase difference (Fig. 6 (b))
were small, and deemed to be at a level that can beused to control the stance of the vehicle without any
issues.

0
-0.5
-1
-1.5
-2

Actuator0

1

2

Steeringaxis

Hub bearing

Fig. 5 Prototype of the Ra-sHUB (Left rear wheel use)

4

5

6

7

8

9

6

7

8

9

Frequency Hz(a) GainPhase difference deg

Base

3

10
0
-10
-20
-30
-40
-50
-60
-70

0

1

2

3

4

5

Frequency Hz(b) Phase difference
Fig. 6 Frequency response characteristic

NTN TECHNICAL REVIEW No.88

61

Hub Bearing Module with Steering Function for Rear Wheel
5.2 Small Steering Angle Durability Test

4

Steering angle deg

Many repeated minor adjustments around thecentral steering position are expected during
actual driving, so the durability was verified under
the conditions shown in Table 2. Fig. 7 shows thecomparison of response characteristics before andafter the durability test. There was no change in
response characteristics with respect to commands
before and after the test, and there was no differencein the response waveform. This verified that there
was no decrease in performance even after repeatedminor steering adjustments.

3

Command value

2

Actual value (-40 ℃)

1Actual value (-20 ℃)

0Actual value (0 ℃)-1Actual value (20 ℃)-2

Actual value (40 ℃)
Actual value (60 ℃)

-3
-4
-0.2

0

0.2

0.4

0.6

Time sTable 2 Durability test condition
Fig. 8 Waveform of steering speed

Steering Angle± 0.5 deg

Steering frequency

5 Hz (sinusoidal wave)

Load from road surface

3.8 kN (equivalent to vehicle weightacting on each wheel)

No. of repeated steering
cycles107 cycles

Command value

Actual value (after test)

Actual value (before test)

0.6

25

0.4
0.2
0
-0.2
-0.4
-0.6

0

0.1

0.2

Time s0.3

0.4

Fig. 7 Comparison of response characteristic

5.3 Temperature Characteristics

The usage temperature of the Ra-sHUB was set atthe same temperature as hub bearings in ordinary
vehicles at -40 to +120 ℃ . The change in steering
speed within environment temperature ranges waschecked. The test involved immersing the controller
and sample into a constant temperature bath andkept at a set temperature for one hour before testing
operation. Fig. 8 shows the measured waveforms ofsteering speed at each environmental temperature.

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NTN TECHNICAL REVIEW No.88

Steering speed deg/s

Steering angle deg

1
0.8

The steering speed was determined from theactual steering angle inclination of the Ra-sHUB whenapplied with stepped steering commands (-3.5 ⇒+3.5 deg (Fig. 8 dotted line)).
Fig. 9 shows the relationship between the steering
speed and environmental temperature. While adecrease in steering speed that is thought to becaused by an increase in internal resistance at low
temperature ranges was observed, the design meetsspecifications and has minimal impact on vehicle
control.

20
15
10
5
0

-40

-20

0

20

Temperature ℃

40

60

Fig. 9 Relations of temperature and steering speed

5.4 Rigidity and Strength Tests

The rigidity and strength when mounted to thevehicle was verified for factoring in external forcesfrom the road surface when driving, by individually
applying loads in the vertical direction (z direction)and the longitudinal direction (x direction), andmoments around the x axis. The XYZ directions areshown in Fig. 10.
Fig. 11 shows the relationship between the loadfrom each direction and displacement. There wereno major differences in rigidity in the vehicleʼsvertical and longitudinal directions. Fig. 12 shows therelationship between the moment load in the camber(x axis) direction and the flange inclination. No
deformation or damage was observed after removing
the load under all conditions.

the same line.
Data at points A to E in Fig. 14 to 16 correspond toeach of the positions A to E in Fig. 13.

Hub bearing flange

Z

Z

Y

X
Fig. 13 Course layout

Fig. 10 Axis direction of the product1

Displacement mm

0.8
0.6
0.4
0.2
-30

-20

0
-10
0
-0.2

10

20

30

-0.4
-0.6

Vertical direction-0.8

Longitudinal direction

Fig. 14 shows the vehicle speed and steering
wheel angle during the test, and Fig. 15 shows thelongitudinal and lateral acceleration. When driving at
the same speed (84 km/h) at position B and following
the same line at the tight radius position C, steering
control with Ra-sHUB made cornering possible
without exceeding the grip limit (0.4 G) of the tires oncompacted snow roads, and there was also a 7 % rateof decrease in speed. Compared to driving without
Ra-sHUB steering, the vehicle could drive through thewhole course approximately 2 seconds faster.

-1

Vehicle speed km/h

Load kN

Fig. 11 Wheel rigidity

0.8
0.6
0.4
0.2
-3

-2

-1

0
-0.2

0

1

2

B

A

-0.4
-0.6
-0.8
-1

Moment load kNm

Fig. 12 Moment rigidity

6. Verification of Effective Ra-sHUB
Rear-wheel Steering
To verify the improvement in vehicle cornering
performance with Ra-sHUB steering, Ra-sHUB wasassembled to the rear wheels (torsion beam) ofa C segment test vehicle and actual driving tests
conducted.
The rear wheel steering angle was changed with
Ra-sHUB to suit driving conditions to control thevehicleʼs slip angle.
Fig. 13 shows the layout of the course on acompacted snow road used for driving tests. The
test compared differences in vehicle behavior with orwithout Ra-sHUB steering operation while followingDE

C 7 % reduction
Without Ra-sHUB
With Ra-sHUB

0

10

20

Time s3040(a) Vehicle speed

3

Steering wheel angle deg

Hub flange inclination deg

1

100
90
80
70
60
50
40
30
20
10
0

60

B

40
20

D

A

0
-20
Without Ra-sHUB

-40
-60

0

10

E

C

With Ra-sHUB

20Time s3040(b) Steering wheel angle

Fig. 14 Test results (Speed, Steering Angle)

NTN TECHNICAL REVIEW No.88

63

Hub Bearing Module with Steering Function for Rear Wheel

20 % lower lateral G
at position C

0.4

Compacted snow roadtire grip
limit (0.4 G)

0.3

A

0.2

D

0.1
0

B

-0.2
-0.3

Without Ra-sHUB
With Ra-sHUB

-0.8

-0.6

C

-0.1

-0.4

E

-0.4
-0.5
-0.2
0

0.2

0.4

0.6

Lateral acceleration G

0.8

Fig. 15 Test results (Longitudinal acc, Lateral acc)
As shown in Fig. 16, there is a closer linear
relationship between the yaw rate and steering wheelwith Ra-sHUB steering compared to without Ra-sHUB
steering, and the improvement in vehicle operation isclear.
20

Yaw rate deg/s

C

10

-60

-40

0

-20

Without Ra-sHUB
With Ra-sHUB

D
0

20

-10

40

Target steering command
value and actual angle deg

Rear left wheel

0.4
0.2

-0.6
-0.8

20

30

40

Command value
Actual angle

Time sFig. 17 Ra-sHUB command value and actual angle
The driverʼs workload was also estimated using
the steering entropy method5), which calculates thedriverʼs workload from how smooth the steering angle
changes.

64

20
10

NTN TECHNICAL REVIEW No.88

-2

-1.5

-1

-0.5

0

0.5

1

1.5

Table 3 Steering entropy (90 percentile)

0.8

-0.4

30

Fig. 18 Prediction error

B

-20

10

With Ra-sHUB

60

The command value and actual angle match
during this Ra-sHUB operation as shown in Fig. 17,
and operation was observed to be in line with thecommand value without any delay.

-0.2 0Without Ra-sHUB

40

Error deg

Fig. 16 Test results (Steering angle, Yaw rate)

0

50

0

Steering wheel angle deg

0.6

The difference (prediction error) between thetarget steering angle and actual steering angle ateach measurement position is calculated, with thesteering entropy corresponding to the 90th percentileof absolute values of the prediction error distribution.
This means that if 90 % of the data lies within therange of prediction error ±α margin, the steering
entropy is α. A smaller value here indicates thatsteering is close as possible to the ideal smoothness.
Fig. 18 shows the prediction error distribution
calculated from the results of driving tests, andTable 3 shows the values for steering entropy. This
table shows that the steering entropy value is lower
with Ra-sHUB steering compared to without Ra-sHUB
steering, indicating that the vehicle is easier to driveby the driver and that smoother operation is possible.

Distribution ratio %Longitudinal acceleration G

0.5

Without Ra-sHUB

With Ra-sHUB

0.73 deg

0.56 deg2

7. Conclusion
This article outlines the configuration, specifications
and basic performance of the “Hub Bearing Modulewith Steering Function for Rear Wheel.”
Ra-sHUB is a system that allows rear wheel steeringwith large operating angles that can be assembled torigid axle systems such as torsion beams, regardlessof the suspension structure. The article outlines how
Ra-sHUB is effective for improving vehicle dynamic
performance and operation in driving tests conducted
on compacted snow roads.
Practical applications for Ra-sHUB will be
developed in the future, including compatibility withvarious suspension systems and further improvements
in functionality.

References

1) NTN Corp. Hub Bearings Catalog, NTN homepage,https://www.ntn.co.jp/japan/products/catalog/
ja/4601/index.html
2) Norio Ishihara, Mitsunori Ishibashi, Hirokazu Ooba,
Atsushi Ito, Makoto Yamakado, Yoshio Kano,
Masato Abe, Hub Bearing with Integrated Active
Front Steering Function that Improves Vehicle
Dynamic Performance, Proceedings of Society ofAutomotive Engineers of Japan, (2018) 20185263.
3) Norio Ishihara, Mitsunori Ishibashi, Hirokazu Ooba,
Atsushi Ito, Makoto Yamakado, Yoshio Kano,
Masato Abe, Hub Bearing with Integrated Active
Front Steering Function that Improves Vehicle
Dynamic Performance [2nd Report], Proceedingsof Society of Automotive Engineers of Japan,(2019) 20191269.
4) Satoshi Utsunomiya, Norio Ishihara, Yusuke Ohata,
Atsushi Ito, Hub Bearing Module with Steering
Adjust Function (sHUBTM), NTN TECHNICALREVIEW, No. 87, (2019) 18-23.
5) Takayuki Kondoh, Tomohiro Yamamura, Nobuyuki
Kuge, Perez Miguel, Takashi Sunda, Developmentof a Real-time Steering Entropy Method for
Quantifying Driverʼs Workload, Transactions of theSociety of Automotive Engineers of Japan, Volume
46 Issue 1, (2015) 167-172.

Photo of authors

Yusuke OHATA

Atsushi ITO

New Business Search andDevelopment Dept.,
New Product andBusiness Strategic
Planning Headquarters

New Business Search andDevelopment Dept.,
New Product andBusiness Strategic
Planning Headquarters

NTN TECHNICAL REVIEW No.88

65

New Products

Creepless Ball Bearing

Hayato KAWAGUCHI*
Marina NAGATA**

Toshiki MASUDA*
Toshiki KAWAI***As housings and bearing rings are made thinner to accommodate
downsizing or reduced weight transmissions for improved fuel consumption
in automobiles, outer ring creep is more likely to occur.
NTN has developed a “Creepless ball bearing” capable of preventing creep
during single direction load conditions. This article introduces the featuresand performance capabilities of the Creepless ball bearing.

1. Introduction

2. Features

Deep groove ball bearings are frequently used asgear support bearings in automotive transmissions.
These bearings are often configured with a loose fit
between the outer ring and housing, which could
result in the outer ring experiencing creep dependingon factors such as bearing specifications, loose fit
conditions or load conditions. Creep of the outerring causes wear on the fitting surfaces of the outerring and housing, and increases misalignment or
inclination of the shaft which results in problems likeabnormal noises or vibrations in the apparatus1).
The growing demand for greater fuel efficiency inrecent years has led to motors being developed withmore gears or more compact and lightweight designs.
To achieve this, housings and bearing raceways tend
to be designed with thinner raceways; however, thiscan result in outer ring creep occurring more often.
As such, there has been increasing demand for creepresistant bearings in recent years.
The creepless ball bearing developed here isable to prevent creep when a load is applied from a
specific direction as in the case of progressive wave
type creep2), where the outer ring slips in the same
direction as the rotation of the inner ring.
This article provides an outline of a newly developed
product that is easy to assemble as it features thesame components as standard bearing types.

The features of the developed Creepless Ball
Bearing (hereafter, developed product) are as follows.
Superior creep resistance* : prevents creep
No creep wear
(under NTN test
conditions)
Ease of assembly: equivalent to standard
type
Durability: equivalent to standard
type (under NTN test
conditions)* Under conditions with load applied in a specific direction

3. Structure and Performance
3.1 Types of Bearing Creep2)

There are two main types of bearing creep
depending on the direction of rotation andphenomena. Table 1 outlines the various types ofouter ring creep. Outer ring creep in the opposite
direction of rotation occurs when there is clearance
between the outer ring and housing, or elastic
deformation and slippage of the outer ring. Outerring creep in the same direction as rotation occurs
when there is outer ring distortion due to the rolling
element load being applied in progressive waves. NTN
calls this “progressive wave type creep” (which occurs
when load is applied to the bearing in one specificdirection).

*
Automotive Engineering Dept., Automotive Business Headquarters** CAE R&D Center*** Automotive Testing Dept., Automotive Business Headquarters

66

NTN TECHNICAL REVIEW No.88

Table 1 Types of outer ring creep2）
Type
Creep
(opposite
direction ofrotation)

Illustration① Loose fit for outer ring and housing
Clearance
Clearance
Clearance

Rolling
Rolling
element
element
Rolling
Raceway
Raceway
element
Raceway
Housing
Housing

Direction ofof loadload
Direction
Direction
Direction ofofrotational load
load
rotational
Direction of load
Rotational
Rotational
Direction of
direction
ofloadbearing
direction
bearingrotationalof
Creep
Creep direction
direction ofof
Rotational
raceway
raceway
direction of bearing
Creep direction of・Occurs with rotating
load
Housing
raceway
・Caused by clearance between the outer ring

Progressive wave type creep only occurs when
the load on the bearing is in one direction. By
using the following formula, progressive waves canbe prevented. However, extremely thick sections
are required for standard bearings, which makes
preventing progressive wave type creep difficult in
reality (Fig. 2).
Raceway thickness t / pitch interval between rolling
elements w > 0.6
Rolling
element

outer diameter and housing inner diameter② Interference fit for outer ring and housing

Raceway(example of tea canister with rubber band around it)
RubberRubberband
band
Rubber
band

Tea
Tea
canistercanisterTeacanister
Table
Table
Table

Direction
Direction ofof loadload
Traveling direction
direction
Travelingofcanisterof canisterof load
Direction
Rotational
direction
Rotationaldirection
direction
Travelingof canistercanisterofof canister
CreepdirectionofCreep directionofRotational
direction
rubber band
band
rubberof canister
Creep direction ofrubber band・Occurs with rotating load
・Caused by elastic deformation and slippageof the raceway

Creep
(same
direction as
rotation)→ Progressive
wave typecreep

Rolling
Rolling element
element
Raceway
Raceway
Rolling element
Direction ofof loadload
Direction

Housing
Housing

Direction
Direction ofofbearing rotation
rotationbearing
Directionof loadCreepdirection
Creep direction
Directionofofof raceway
raceway
bearingrotation

Housing

Creep directionof raceway

Raceway・Occurs when load is in one direction
・Caused by outer ring distortion

3.2 Mechanism of Progressive Wave Type
Creep2)

Details of the mechanism are shown below (Fig. 1).
(1) Rolling element load acts on the outer ring(2) Outer ring material immediately below isdisplaced(3) Outer ring surface becomes wavy up to several
microns depending on the magnitutde of therolling element load
(4) When inner ring rotates, the rolling elements
also move(5) Surface waves become progressive waves andcause outer ring creep

Raceway

Fig. 2 Thickness of bearing ring and rolling elements

3.3 Overview of Developed Product3.3.1 Aim of Design

The gear support sections of transmissions often
suffer from progressive wave type creep due to thebearing fit and load conditions. By focusing on themechanism of progressive wave type creep, NTN
developed a product to stop creep by blocking
progressive waves in the outer ring.

3.3.2 Appearance

The components are the same as the standard type
bearing, but the developed product features an arcshaped undercut across the full width of part of theouter diameter of the outer ring (Fig. 3, 4).
The size of the new product is interchangeable withstandard bearings, and does not impact assembly into
the housing.
Undercut

Fig. 3 Appearance of developed product
UndercutOuter ring outer
diameter

Load ofTraveling direction of rolling element
rolling element
Raceway elongation
Rotation direction of
Elliptic motion of surface
rolling element
Raceway contraction
Creep direction of raceway
Rolling
element
Rolling element
Raceway

Housing
Housing

Fig. 4 Structure of developed productFig. 1 Creep by strain

NTN TECHNICAL REVIEW No.88

67

XXXXXXXXXXXX
Creepless Ball Bearing

3.3.3 Mechanism to Prevent Creep

An image of the mechanism that prevents creep isdescribed below and shown in Fig. 5.
(1) Bearing load acts on outer ring after passing
through the inner ring and rolling elements(2) Outer ring outer diameter surface deforms in
the radial direction due to the load from therolling elements(3) Undercut used to avoid contact with the housing(4) Progressive waves caused by distortion on outerring outer diameter surface are blocked, which
prevents creep
Inner ring Bearing load

1.20

Creep speed mm/sec

Standard type bearings transmit progressive waves
in the load region if the outer diameter surface ofthe outer ring deforms and makes contact with the
housing, causing creep in the same direction. The
developed product however, avoids contact with the
housing in the region of the undercut, which blocks
progressive waves and stops creep from occurring.

Standard type

1.00

AC bearing
Coated type0.80

(conventional product)

Developed product

0.60
0.40
0.20
0.00

No creep
0.1

0

0.2

P/C

0.3

0.4

0.5

Fig. 6 Creep speed test results
The order of creep speed at high loads was as follows.
High ← Creep speed → Low
Conventional product ＞ AC bearing ＞
Coated type ＞ Developed productRolling element
Outer ringContact due to distortionin outer diameter surface(a) Standard type

No contact due to distortionin outer diameter surface

Undercut(b) Developed productFig. 5 Mechanism image for creep stop
Outer ring creep is prevented when the undercut ispositioned in the load zone; even when the undercut
is positioned outside of the load zone, creep isprevented when the outer ring undercut enters theload region as a result of creeping. Accordingly, there
is no need to factor in phase when installing thedeveloped product into the housing.

While the developed product generated around
10-times the tensile stress compared to the standard
type, this was not at a level that could cause failureand had several times the safety factor for allowabletensile stress of bearing steel (SUJ2).
Table 3 Strength analysis condition

3.3.4 Creep Speed

Tests were conducted to verify the speed atwhich the developed product would creep. The test
conditions are shown in Table 2 and test results
shown in Fig. 6.
Four types of samples were tested alongside thedeveloped product, including standard type bearings,
NTNʼs conventional creep prevention AC bearings3),
and bearings with coated outer ring outer diameters.
Creep speed in the standard type bearings andAC bearings increased as the load was increased.
However, there was no creep in the developed producteven with high loads verifying that progressive
waves in the outer ring were blocked and creep wasprevented by the new design.

Bearing number

6208

Load Fr (P/C)

4 standards from 0.1 to 0.4

Inner ring rotational speed min

6,000

Lubricant

CVT fluid

Bearing outer ring temperature ℃

50-1

NTN TECHNICAL REVIEW No.88

Standard Type

Developed Product

Bearing
number6208

UndercutYes(immediately below
load region)

No

Load Fr
(P/C)

0.4 *
(top of ball)Bearing fits

Clearance fit(outer ring outer diameter / housing innerdiameter)
Rigid body(housing innerdiameter)

Analysis model
image

Table 2 Test condition

68

Tests were conducted using FEM analysis to verify
the stress on the outer ring outer diameter surface ofthe developed product.
The analysis conditions are shown in Table 3 andanalysis results shown in Fig. 7.

Outer
ring
Inner
ring

Inner and outer rings areelastic body
Rolling elements are
rigid body

Rigid body
(shaft outer
diameter)
Rolling
element<No undercut>

Enlarged view

0.5

Housing

3.3.5 Impact on Outer Ring Undercut
Strength<With undercut>

* The rolling element load distribution of the developed productdiffers from the standard type due to the undercut.
As indicated in these analysis conditions, the maximum rolling
element load of the developed product is lower than thestandard type when the undercut is immediately below theload region. To consider the worst case scenario, the samerolling element load as the standard type has been used for the
analysis.

When the undercut is at a 45°position with respect
to the load direction, the rolling fatigue life decreases(around 5 %) compared to the standard type;however when the undercut is immediately below theload, it actually increases (around 6 %). This is due tothe change in the maximum rolling element load ascompared to the standard type. As such, the impact
on the rolling fatigue life due to the undercut is small.

3.4 Evaluation Result of Developed Product3.4.1 Outer Ring Crack Test (static)

49

429

(a) Standard type(b) Developed productFig. 7 Strength analysis result

A static crack test was conducted to verify thestrength of the outer ring outer diameter surface
undercut included in the developed product.
The test jig is shown in Fig. 8. The undercut of thedeveloped product was simulated with a load using a
special jig, with one rolling element positioned in theload region to concentrate the load at the undercut.
A precision universal testing machine was used as thetester to apply a static load from the inner ring, withthe cracking load deemed to be the point when theload decreased suddenly.

3.3.6 Impact on Operating Life

Load applied from inner ring

The load distribution on the rolling elements in thedeveloped product differs from standard bearings dueto the position of the undercut in relation to the load
direction. As such, the impact on the rolling fatigue life
was calculated theoretically.
To understand how the undercut affects thedeveloped product, the balance of forces wascalculated by factoring in both the elastic deformationof the outer ring and housing as well as the amount ofelastic contact between the balls and raceway due toHertzian contact.
Table 4 shows a list of the analysis results.
Table 4 Calculation result
Developed ProductStandard Type

Bearing number

Immediately
below
undercut

Load applied fromone rolling element

Simulated undercut

Fig. 8 Outer ring crack tester
The test results are shown in Fig. 9.
The cracking load was determined to have a safety
factor of two times or more against the basic staticload rating C0, and as such was deemed acceptablewith respect to static destruction.

6208
No

Load Fr (P/C)

Yes(45°position)

Cracking

50

Yes(immediately
below)

45
40

2.5x

35

0.4

Load KN

Undercut

Undercut 45°
position

Change ofundercut angle

30
25
20
Basic static load rating C0

15

Illustration

10
5
Undercut position Undercut position

Rolling
fatigue life L10(compared tostandard type)

1

0.95

1.06

0

0

0.2

0.4

0.6

0.8

1

Displacement mm

1.2

1.4

Fig. 9 Crack test results

NTN TECHNICAL REVIEW No.88

69

XXXXXXXXXXXX
Creepless Ball Bearing

3.4.2 Outer Ring Crack Test (dynamic)

A dynamic crack test was conducted to verify thestrength of the outer ring outer diameter surface undercut
included in the developed product.
To evaluate the fatigue strength, a load exceeding thebasic dynamic load rating was applied for more than 107
cycles. The test conditions and results are shown in Table 5.
Table 5 Test condition and result

Test
conditions

Bearing number

6208

Load Fr (P/C)

1.2

Inner ring rotational
speed min-1

2,500

Lubricant

CVT fluid

Bearing outer ringtemperature ℃

Natural temperature rise

Operating time

Suspended at double the 1 × 107
cycles passed by the rolling element(equivalent to 3-times or more therolling fatigue life)

Test result

No failure in standard typeor developed product

Under these conditions, the developed product wasdeemed acceptable with respect to dynamic destruction.

3.4.3 Durability Test

The rolling fatigue life was verified with a durability test.
The test conditions are shown in Table 6. The undercut was
positioned at 45°from the direction of the applied load where
the rolling element load is the highest. The operating time was
set to be suspended at 10-times the calculated operating life.
From the results shown in Table 6, the developed productwas deemed acceptable with respect to rolling fatigue life
under these conditions.

4. Conclusion
This article provided an outline of the Creepless Ball
Bearing. The developed product features an undercut
in part of the outer diameter surface of the outerring to successfully prevent outer ring creep caused
immediately below the load when the bearing rotatesin a specific direction. Creep is traditionally restrictedby using external parts or applying a coating; however
the developed product completely prevents all creep
by including a simple modification to the outer ring
without any changes to the internal components orenvelope dimensions of the standard type bearing
and without affecting assembly to the housing.
The Creepless Ball Bearing is anticipated tocontribute to making automotive transmissions
smaller, more lightweight and more fuel efficient, andwill be actively released in various markets. Efforts will
also be made to promote product development withthe aim of further increasing performance.

References

1) Takayuki Teramoto, Yutaka Sato, Prediction Methodof Outer Ring Creep Phenomenon of Ball Bearing
Under Bearing Load, Transactions of the Society ofAutomotive Engineers of Japan, Volume 46 Issue 2
March, (2015) 355.
2) Tsuyoshi Niwa, A Creep Mechanism of Rolling
Bearings, NTN TECHNICAL REVIEW, No. 81, (2013)100-103.
3) NTN “Ball and Roller Bearings Catalog” CAT. No.
2203/J, (2020) B-15.

* This durability test used a high-rigidity housing to ensure that
there was no creep.

Photo of authors

Table 6 Test condition and result

Test
conditions

Bearing number

6208

Load Fr (P/C)

0.6

Inner ringrotational speed
min-1

3,000

Lubricant

CVT fluid

Bearing outer ringtemperature ℃

Natural temperature rise

Operating time h

L10h × 10
Rotating
direction

Hayato KAWAGUCHI

Toshiki MASUDA

Automotive Engineering
Dept., Automotive
Business Headquarters

Automotive Engineering
Dept., Automotive
Business Headquarters

Fr

Undercut position
Undercut

45° position

Test configured at position withhighest rolling element load
Test result

70

No failure in standard type ordeveloped product (suspended at
10-times calculated operating life)

NTN TECHNICAL REVIEW No.88

Marina NAGATA

Toshiki KAWAICAE R&D Center

Automotive Testing Dept.,
Automotive Business
Headquarters

New Products

Low Temperature Rise and Low Torque Tapered Roller
Bearing
Yasuhito FUJIKAKE*
Takanori ISHIKAWA*
Power transmission devices such as automobile transmissions anddifferentials are becoming smaller due to environmental regulations,and the use of low-viscosity oil and reduced oil quantity are advancing
for high efficiency and power savings. The tapered roller bearings usedin such power transmission devices are required to withstand severe
conditions and to have low friction. In order to meet these demands,
NTN has developed a tapered roller bearing with a new shape resin cageand incorporated design technology for low torque improvement and low temperature rise, thus attaining
the worldʼs highest level for both low torque performance and high seizure resistance. This paper introduces
the structure and performance of the developed product “Low Temperature Rise and Low Torque Tapered
Roller Bearing”.

1. Introduction
The automotive industry is facing major changes1)
such as the transition to Connected, Autonomous,
Shared, Electric (CASE) and smart mobility.
As companies boost their social responsibility
toward the environment with measures suchas Corporate Average Fuel Economy (CAFE)
regulations2), increasingly stringent CO2 regulations
means companies are focusing on electrification as a
viable solution.
With the growing need for safety and comfort whiledriving, efforts are not only focusing on automateddriving and collision safety, but also on achieving agreater freedom of layout design and more cabin
space with the use of compact power transmission
units.
The background to this shift in mobility is that
automotive power transmission devices like thetransmission and differential use an internal lubricant
with lower viscosity and reduced quantity becausesuch devices are becoming more compact with lower
torque performance.
Tapered roller bearings that have a high load
capacity are used in power transmission devices.
However, these bearings have a higher rotational
torque compared to ball bearings and as such sufferfrom sudden temperature increases under severe
lubrication conditions. To achieve a longer operating
life and lower temperature rises, NTN released the“ULTAGE Tapered Roller Bearing for Automotive
Application”3) in 2017.
Tapered roller bearings that support the powertransmission devices mounted in vehicles required
even lower torque and greater protection against

seizure, so efforts into enhancing these technologies
were continued with the development of a newresin cage shape to meet such demands. This was
combined with lower torque and lower temperaturerise design technologies to develop the “Low
Temperature Rise and Low Torque Tapered Roller
Bearing” that achieves the worldʼs highest level in low
torque performance and protection against seizure.
This article outlines the structure and features of thisbearing.

2. Structure
An overview of the structure of the developed Low
Temperature Rise and Low Torque Tapered Roller
Bearing is shown in Fig. 1. The features ① to ④ shownin Fig. 1 correspond to the following explanations of
the structures in the developed product.
● New resin cage shape① The inner diameter on the small side of the cage

has been reduced for a smaller clearance with theinner ring small side rib outer diameter surface.
This suppresses the amount of lubricant inflow
into the bearing, which reduces stirring resistancedue to the lubricant to achieve a lower torque.
② An open concave shape has been applied to theinner diameter side of the cage pocket largeside edge. The corners within the concave shaperetain lubricant inside the concave shape due tosurface tension, and feed lubricant to the roller
end face after operation starts, thus limiting any
sudden temperature increases.

* Automotive Engineering Dept., Automotive Business Headquarters
NTN TECHNICAL REVIEW No.88

71

Low Temperature Rise and Low Torque Tapered Roller Bearing● Optimized design of rollers④ A more compact roller design and smaller roller

pitch circle diameter design have been used.
The former decreases the rolling contact line
between the rollers and the inner and outer rings.
Reducing the rolling contact line decreases therolling viscosity resistance, which achieves a lower
torque. The latter reduces the circumferential
speed of the rolling contact between the rollers
and the inner and outer rings, which limits rolling
resistance to achieve a lower torque.

④ Smaller roller pitch

circle diameter

applied to pocket
large side edge

RollerInner ring
① Smaller clearance

between cage innerdiameter and inner ringsmall side rib

Resin cage

The internal design for a longer operating life usedin NTNʼs proprietary “ULTAGE Tapered Roller Bearing
for Automotive Application” technology, as well as an
optimal design5) of factors affecting the bearing load
rating (such as outer ring angle, and angle and lengthof rollers), have been used to limit the decrease inbearing rated life while allowing for smaller size, andas a result achieving lower torque.
Table 1 shows an example of tests conducted on
the small and lightweight design with load conditions
expected to be acting on transmission support
bearings of medium size passenger vehicles. The
developed product test example has a 25 % reduction
in bearing width dimension and 44 % lighter weightcompared to conventional tapered roller bearings.
This results in a shorter rolling contact length as well
as a smaller roller pitch circle diameter, which helps toachieve low torque.
In contrast, the smaller size results in a reductionin the bearing dynamic load rating and operating life,
so carbonitriding treatment was used to maintain theoperating life.
Table 1 Study example of downsizing and weight reduction
by applying the development specification③ Optimal design

between roller end
face and inner ringlarge side rib① Conventional Product② Developed Product

Tapered roller bearing 32007X4)

Tapered roller bearing18

3. Features
The features of the Low Temperature Rise and
Low Torque Tapered Roller Bearing (hereafter, thedeveloped product) compared to conventional
tapered roller bearings (conventional product)4) are asshown below.
(1) Worldʼs highest level in low torque performance: reduces rotational torque by 66 % compared
to conventional product(2) Worldʼs highest level in protection against seizure: up to 10-times protection against seizurecompared to conventional product3.1 Reduced Rotational Torque3.1.1 Reduced Lubricant Stirring Resistance

The raceway of tapered roller bearings is inclinedwith respect to the center of rotation, which creates aphenomenon called pumping action where lubricant
flows from the small side to the large side of thedeveloped product when it is rotating. To prevent
more lubricant than is necessary flowing into thebearing due to pumping action, the developed
NTN TECHNICAL REVIEW No.88

13.5

Scale
drawingofbearing
crosssection

φ62

Fig. 1 Structure of “Low Temperature Rise and Low
Torque Tapered Roller Bearing”

72

3.1.2 Bearing Internal Optimal Design

φ35Roller pitch circle diameter

Outer
ring

② Concave shapeproduct uses the cage shape in Fig. 1 ① , which limits
torque loss caused by lubricant stirring resistance andthus achieves a lower torque.

φ58.5

inner ring large side rib③ An optimal design has been used to improve
lubricity of the sliding contact between the roller
end face and inner ring large side rib.

φ34

● Optimal design between the roller end face andSize

ϕ 35 ×ϕ 62 × 18

ϕ 34 ×ϕ 58.5 × 13.5
(width reduced by 25 %)Mass (kg)0.223

0.125(44 % lighter weight)

Dynamic
load
rating (N)46,00028,500Material
Carburized case-hardened
heat
steel
treatment

Carbonitrided bearing
steel

Operatinglife

Meets standards

Meets standards

Contact
stressMeets standards(with edge stress)

Meets standards(without edge stress)

Gravity

Bottom(a) Analysis model image ofbearing front

Roller(b) Direction of gravity andbearing top position crosssectional image

Fig. 2 Definition of oil flow analysis

The analysis results are shown in Fig. 3. Lubricant
was observed being retained in the concave shape ofthe cage pocket large side edge. This analysis helped
determine the optimum shape within the concave
section for retaining an amount of lubricant that canbe supplied to the roller end face.

(a) Cage concave shape(same view as Fig. 3 (b))

5

4

4

3

3

2

2

1

1

0

0

Comparison of protection
against seizure(lubricant
flows down)

Oil poor
conditions

Development
specifications

Direction of gravity

Oil richconditions

Conventional standard
specifications

Top

5

Development
specifications<Lubricant flow analysis conditions>・Model conditions (Fig. 2): the analyzed cage pocket was
positioned at the bearing top (12 oʼclock) to simulate
supporting a horizontal shaft(selected the position with the worst lubricant retention)
・Lubricant: ATF (120 ℃ )・Analysis conditions: unsteady flow analysis ofmulti-phase flow (VOF)
・Analysis step 1: rotate bearing in lubricant
until lubricant flow within
bearing is steady・Analysis step 2: stop rotation of bearing. Stop
supplying lubricant
・Analysis step 3: analysis ends when outflow(movement) of lubricant
within bearing stops

The inner ring large side rib and roller end face oftapered roller bearings make sliding contact with eachother, leading to concerns over sudden temperature
increases due to the lack of oil film formation when
there is insufficient lubricant or low-viscosity lubricant
is used. As such, the lubricity (oil film formation) needs
to be improved on the sliding contact components toensure greater protection against seizure.
The inner ring large side rib uses a straight shape tomake this sliding contact more steady, and the roller
end face uses an optimal rounded design to improve
lubricity (oil film formation).
Fig. 4 shows the results of seizure protection testsof the development specifications with a roundedroller end face used to check the effectiveness of
this design. The test conditions were set to abundantlubricant conditions (oil rich) and extremely low
lubricant (oil poor). Better lubricity (oil film formation)
was achieved under both conditions for thedevelopment specifications with a rounded roller end
face, thus indicating better protection against seizurecompared to the conventional standard specifications.

Conventional standard
specifications

The effectiveness of the lubricant retention functionthat the concave shape applied to the cage pocket largeside edge achieves was verified using flow analysis of the
lubricant. The analysis conditions were as shown below.

3.2.2 Optimized Shape of Tapered Rollers

Comparison of protection
against seizure

3.2 Greater Protection Against Seizure
3.2.1 Concave Shape Applied to Pocket
Large Side Edge

Fig. 4 Seizure resistance evaluation results for development
specification roller end face radius(Comparison result when the seizure resistance ofthe conventional standard specification is set to 1)

4. Bearing Performance
4.1 Rotational Torque Test

A rotational torque test was conducted on the
bearing itself to check the effectiveness of reduced
torque of the developed product.
The test results are shown in Fig. 5. As outlinedin the previous section, the developed product was
found to have a significant 66 % reduction in torquecompared to the conventional product due to theeffects of lower rolling friction and lower stirring
resistance of the lubricant.

(b) Lubricant flow analysis
results(red parts: lubricant)(bearing shown as semi-transparent)

Fig. 3 Oil flow analysis results (Enlarged view of large diameterend face recessed shape of cage pocket)
NTN TECHNICAL REVIEW No.88

73

Low Temperature Rise and Low Torque Tapered Roller Bearing

with different dimensions for outer diameter andassembled width)
・Axial load
: 3,000 N
・Rotational speed: 5,000 min-1
・Lubricant conditions : oil bath lubrication, ATF (50 ℃ )Time for outer ring temperature
to reach 100 °Cs<Test conditions>・Tested bearing size: Conventional product ø35 × ø62 ×
18 (Table 1 ① bearing)
Developed product ø34 × ø62 × 16
(same internal structure as Table 1 ② bearing,

800

710 seconds

700
600
500
400
300
200

72 seconds

100
0Conventional productDeveloped productRotational torque N·cm

60

Fig. 6 Seizure resistance evaluation test results

50
40

-66 %

30
20
10
0

Conventional productDeveloped productFig. 5 Bearing rotational torque test results

4.2 Protection Against Seizure Evaluation Test

An evaluation test under the following conditions
was conducted to check the effects of protection
against seizure of the developed product. Test
conditions were set with a low amount of lubricant
based on the assumption of using a low-viscosity
lubricant in power transmission devices or simulating
the quick acceleration of electric vehicles. The bearing
was applied with a set miniscule amount of lubricant,and then operated at room temperature in a nonlubricated state to test the time taken until thetemperature of the bearing outer ring reached 100 ℃.
The test results are shown in Fig. 6. The
conventional product standard steel plate cage has no
lubricant retention function, and the rib between the
inner ring and rollers is the standard design, and thus
the bearing reached 100 ℃ in 72 seconds. In contrast,
the developed product features the new resin cage
shape outlined in the previous section, and in this test
the initial amount of adhered lubricant increased 1.3
times compared to the conventional product. With
the optimized roller end face and inner ring large rib,the test duration was approximately 10 times longer
at 710 seconds, which is an immense improvement toprotection against seizure.
<Test conditions>・Tested bearing size: Conventional product ø35 ×
ø62 × 18 (Table 1 ① bearing)
Developed product ø34 × ø62 × 16
(same internal structure as Table 1 ② bearing,with different dimensions for outer diameter andassembled width)
・Lubricant: ATF (25 ℃)
・Rib contact stress : approx. 200 MPa
・Rib sliding speed : approx. 2.5 m/s

74

NTN TECHNICAL REVIEW No.88

4.3 Longer Operating Life Under Lubrication
Conditions that Contain Contaminants

An operating life test was conducted on the developed
product under lubrication conditions containing hardcontaminants. The operating life of power transmission
devices may be affected if hard contaminants are mixed
in with lubricant. While there are various conditions ofhard contaminants contained in the lubricant of powertransmission devices, this test assessed the operating
life under the following conditions.
The test results are shown in Fig. 7. Under thecontaminant conditions of this test, the developed
product was deemed to have an ample effective
operating life with respect to the rated operating
life. Furthermore, no deformation or other wear wasobserved in the new plastic cage shape after the test.

<Test conditions>・Tested bearing
Developed product : carbonitrided bearing steelSize: ø30 × ø65 × 22
Dynamic load rating : 47,500 N
・Load conditions: 42 % of dynamic load rating・Rotational speed: 2,000 min-1
・Lubricant: ATF (approx. 70 ℃)
・Contaminant average : ① 20 µm, ② 90 µm
particle diameter(tested with a mix of ①
and ② contaminants)
・Total amount of contaminants : 0.1 g/L・Rating life (L10h): 149 h300
250

L10h=255.3 h

200

L10h
150
h

Rated life

100
50
0

Developed productFig. 7 Life test results under contamination
lubrication condition

4.4 Comparison with Tandem Type Double
Row Angular Contact Ball Bearing

In recent years during the bearing selection stage,
there have been cases where ball bearings are
selected to achieve lower torque instead of tapered
roller bearings. With this in mind, a functional
comparison was made between the developed
product and the Tandem Type Double Row Angular
Contact Ball Bearing (Fig. 8) that is used to support
differential pinion shafts.

The sizes of the ① Ball bearing and the ②
Developed product in Table 2 were selected tosimulate operating life under load and rotational
speed conditions expected when used as a reardifferential pinion shaft support bearing for large
size passenger vehicles. As a reference comparison,
the ③ Conventional tapered roller bearing (33108U)
is shown, as it has the same inner diameter as the② Developed product and a similar outer diameterand dynamic load rating. The results of this bearing
comparison revealed that the ② Developed productwas 60 % lighter weight than the ① Ball bearing.
For the bearings shown in Table 2, rotational torque
was calculated at conditions based on the rotational
speed expected with large size passenger vehicles.
The calculation results are shown in Fig. 9.
The ③ Conventional product is shown with thehighest rotational torque, and while the ② Developed
product had a higher torque than the ① Ball bearing
up to a vehicle speed of 5 km/h, it was observed witha lower torque than the ① Ball bearing at speeds over
5 km/h.

Fig. 8 Structure example of tandem type double row
angular contact ball bearing① Ball Bearing

Tandem type
double row
angular contact
ball bearing③ Conventional

Tapered roller
bearing

Taperedroller bearing
33108U4)

Product

Product50

100

Vehicle speed km/h

Fig. 9 Bearing rotational torque calculation results

φ75

φ40

φ77.1

φ95

φ40

26

21

ϕ 40 ×ϕ 95
× 40

ϕ 40 ×ϕ 77.1
× 21

ϕ 40 ×ϕ 75
× 26

Mass
(kg)

1.06

0.423 (60 %lighter weightcompared to ①
Ball bearing)

0.498 (53 %lighter weightcompared to ①
Ball bearing)

Dynamic
load
rating (N)Large side:
50,000
Small side:
42,50083,50088,000Size

05 km/h

40

φ40

Scale
drawingofbearing
crosssection② Developed

Rotational torque

Table 2 Study example of “Low Temperature Rise and Low
Torque Tapered Roller Bearing” with the same
calculation life as tandem type double row angularcontact ball bearing

Tapered roller bearing 33108U4) (③ Conventional product)
Tandem type double row angular contact ball bearing (① Ball bearing)
Tapered roller bearing (② Developed Product)

NTN TECHNICAL REVIEW No.88

75

Low Temperature Rise and Low Torque Tapered Roller Bearing
Comparisons of the calculation results of bearing
rigidity with applied load in both the radial and axial
directions are shown in Table 3. Although the ②
Developed product has a smaller size than the ① Ballbearing, both radial rigidity and axial rigidity wereverified as equivalent or better.
Table 3 Bearing rigidity calculation results① Ball Bearing② Developed ProductTandem type double row
angular contact ballbearing

Tapered roller bearing

40

Radialrigidity

21

φ40

φ77.1

φ95

⇒

2.6-times
higher rigidity

Equivalent
Axial
rigidity⇒

Photo of authors

Yasuhito FUJIKAKE

Takanori ISHIKAWA

Automotive Engineering
Dept., Automotive
Business Headquarters

Automotive Engineering
Dept., Automotive
Business Headquarters

76

The automotive industry is said to be facing a
once-in-a-100-year period of change, and while
usage conditions of bearings will become even more
stringent than before, they will be expected to have aneven higher level of reliability. The “Low Temperature
Rise and Low Torque Tapered Roller Bearing” featured
in this article is a new product that is sure to help
vehicles achieve a higher mechanical efficiency andbetter electrical efficiency, as well as contribute toincreased uptake of electric vehicles. NTN will focus
on technical innovation of the new technologies
attained through this development and continue
contributing to a smooth “smart mobility society” withthe release of this new product.

References

φ40

Scale
drawingofbearing
crosssection

5. Conclusion

NTN TECHNICAL REVIEW No.88

1) Ministry of Economy, Trade and Industry, Ministryof Land, Infrastructure, Transport and Tourism,“Smart Mobility Challenge” project
https://www.mobilitychallenge.go.jp/
2) Ministry of Land, Infrastructure, Transport andTourism, “Report on New Fuel Efficiency Standards
for Passenger Vehicles”
https://www.mlit.go.jp/report/press/
jidosha10_hh_000217.html
3) Yasuhito Fujikake, Takanori Ishikawa, Susumu
Miyairi, ULTAGE Tapered Roller Bearing for
Automotive Application, NTN TECHNICAL REVIEW,
No. 85, (2017) 51-55.
4) NTN “Ball and Roller Bearings Catalog” CAT. No.
2203/J
5) Daisuke Imada, Tsuyoshi Niwa, Takashi Ueno,
Tomohisa Uozumi, Development of a System
for Rolling Bearing Design Optimization, NTN
TECHNICAL REVIEW, No. 79, (2011) 98-102.

Technical Paper

Initiation Mechanism of Peeling in Rolling Bearings,
and Its Life Estimation Method
Naoya HASEGAWA*
Michimasa UCHIDATE**
Hiroshi KINOSHITA***

Takumi FUJITA*
Masayoshi ABO***

Peeling, which consists of spalls and cracks of about 10 m in size, is one of the common failure modesof rolling bearings. Peeling is known to occur under poor lubrication conditions which result in directcontact of surface roughness asperities. However, the initiation mechanism of peeling is not yet completely
understood. In this study, we investigated the peeling formation mechanism by observation of rolling
contact surfaces, surface topography measurements, residual stress measurements, and contact analysis.
These results clarified that initial peeling cracks arose from notches which formed due to plastic contact of
surface roughness asperities.
We also developed a peeling life estimation method based on the above formation mechanism. The
method can estimate peeling life under pure rolling and boundary lubrication conditions.

1. Introduction
In order to reduce friction, the recent trend has
been towards the use of low viscosity oil lubrication.
The chance that rolling bearings (hereafter, “bearings”)
are used in thin lubricating conditions is therefore
increasing. Thus, clarifying the damage mechanismof bearings under such conditions has become an
important engineering task.
Peeling is one type of failure experienced bybearings operating under thin lubricating conditions
and is indicated by dense area of spalls and cracks
approximately 10 μm in size1). Peeling is likely tooccur when the surface roughness is high and theoil film parameter Λ (ratio between the minimum
oil film thickness obtained from the EHL Theory andthe square root of the square sum of the root-meansquare roughness of two surfaces) is low2). Therefore,
repeated stress applied on the direct contact
area (hereafter, “real contact area”) of the surface
roughness is the cause of peeling. Although, thepeeling mechanism is roughly understood, the details
remain unclear3) 4). The authors studied the formation
mechanism of peeling in detail by reproducing peelingwith a two-cylinder type testing machine, observing
the rolling contact surface, and conducting various
analyses5). In addition, the mechanism of crack
formation during peeling was quantitatively examinedby estimating the repeated stress applied to the realcontact area using the surface topography and themeasurement of residual stress6). Chapter 2 of thispaper explains the peeling formation mechanism
obtained from this study.
In order to examine the reliability of bearings
under thin lubricating conditions, a tool to estimate

peeling life is also required. The authors developed alife estimation method based on the above peelingmechanism7). Chapter 3 of this paper describes thismethod for estimating peeling life.

2. Peeling formation mechanism
2.1 Two-cylinder test

The two-cylinder type tester shown in Fig. 1
was used to reproduce peeling. A driving cylinder
attached to a motor is put in contact with a driven
cylinder, both of which are rotated under a pure rolling
condition without slippage. Lubrication was provided
through an oil saturated felt pad in contact with thetest pieces. The test pieces were both 40 mm OD and
12 mm in length, with a 60 mm crown radius placed
axially along the outer diameter surface of the driving
cylinder. The material of the test pieces is JIS-SUJ2
with ordinary quenching and tempering. A groundfinish was applied to the outer diameter of the driving
cylinder while superfinishing was applied to the driven
cylinder. Two types of driving cylinders were prepared,
one with black oxide treatment (Test No. 2) andthe other without such treatment (Test No. 1). The
purpose is to examine how the reduction of surface
roughness (hereafter, “running-in”)8) progresses due toblack oxide treatment affects the formation of peeling.
The condition of the black oxide treatment was set
based on DIN 509389). Table 1 shows the surface
roughness, hardness and thickness of the black oxide
layer and Table 2 shows the test conditions. The
test was interrupted after each loading cycle, withanalysis to be described later taking place, then thetest continued until the total number of loading cycles
reached 5.0 x 105.

*
Advanced Technology R&D Center** Faculty of Science and Engineering, Iwate University*** School of Engineering, University of Hyogo
NTN TECHNICAL REVIEW No.88

77

Initiation Mechanism of Peeling in Rolling Bearings, and Its Life Estimation Method

Driving cylinder

2.3 Estimation of repeated stress of rolling
contact surface

Load

Lubrication felt
Driven cylinder

Motor

Fig. 1 Two-cylinder type rolling contact fatigue tester

2.2 Observation of rolling contact surface,measurement of shape and analysis ofsurface roughness

Under the conditions shown in Table 1, peelingis likely to form on the driven cylinder with lower
surface roughness. Therefore, peeling formation was
examined by observing the rolling contact surface ofthe driven cylinder with an optical microscope andscanning electron microscope (hereafter, “SEM”). For
observation of the crack section of peeling, a focusedion beam (FIB) system was used to prepare the test
pieces.
Topography of the rolling contact surface of bothcylinders was measured by confocal laser scanningmicroscopy and used in the estimation of repetitive
stress on the rolling contact surface, as describedlater. In addition, the relationship between the changeof surface topography on the driven cylinder andformation of peeling was investigated. Furthermore,
to study the relationship between the degree of
running-in and the formation of peeling, two types of
3-dimensional roughness parameters on the surface
topography of the driving cylinder (standard deviation
σ* of height of protrusions and arithmetic mean
curvature Spc) were analyzed. Based on the theoryof rough surface contact of Greenwood, etc.10), theseverity of contact at the real contact area decreases
as the product of σ* and Spc (σ*・Spc) is reduced. In
section 2.5.2, the relationship between σ*・Spc and theprogress of peeling will be discussed.

Triaxial stress components of contact stress that act
on the real contact area were estimated during thetest by elastic analysis using the boundary element
method11). In this analysis, the oil film parameter
during the test was low ( Λ ≈ 0.10), so we assumed
there was no load supported by the oil film. The subject
area of the analysis was set to the area surrounding
the center of the contact ellipse where the apparent
contact pressure is relatively large. The details of theanalysis are discussed in reference 6).
Furthermore, under the condition that the oilfilm parameter is low, plastic deformation occurs in
the contact region and residual stress is producedon the rolling contact surface. In this study, triaxial
stress components of the residual stress on thedriven cylinder were measured with the X-ray stressmeasuring method12) and the area detector method.
The triaxial stress components that actually acted
on the rolling contact surface can be estimated withthe following equation (1) by adding the resultsfrom the above contact analysis and X-ray stressmeasurement.
（1）

Where, the subscripts ʻconʼ and ʻresʼ indicate contact
stress and residual stress, respectively. As discussed
later, plastic deformation of the rolling contact surface
is involved in the formation of the initial cracks inpeeling. Therefore, in the study of the formation
mechanism of peeling, the discussion uses von Mises
stress (hereafter, “Mises stress”) which indicates theyield condition of the material. Mises stress σvm can beobtained by the following equation (2) by using thetriaxial stress components obtained in equation (1).
（2）

Table 1 Surface roughness, hardness and thickness of the black oxide layer for test cylinders
Surface roughness (Ra) μm

78

Test No.

Drivingcylinder

1

0.75

2

0.70

NTN TECHNICAL REVIEW No.88

Drivencylinder0.02

Hardness
DrivingcylinderThickness of black oxide layer μm
Drivencylinder61.5 HRC

Drivingcylinder2.0

Drivencylinder-Rolling
Rolling direction
direction

Table 2 Test conditions of the RCF testing
Items

Conditions

Lubricantpoly- α -olefin, VG5

Rotational speed min-1

2,000

Load kN

2.25

Maximum contact pressure
GPa

2.3

Oil film parameter Λ

0.09 - 0.11 (at 40 ℃ )Total number ofloading cycles

5.0 × 105

Crack-like mark
Crack-like mark100 µm
100 µm

100 µm
100 µm(a) No. 1, 0.1 × 105 rev.

(b) No. 1, 2.5 × 105 rev.

100 µm
100 µm

100 µm
100 µm(c) No. 1, 5.0 × 105 rev.

(d) No. 2, 5.0 × 105 rev.

2.4 Results of experiment and analysis
2.4.1 Observation of rolling contact surfaceof the driven cylinder

Fig. 2 shows optical micrographs of the rolling
contact surface on the driven cylinder. Test No. 1
shows crack-like damage at 0.1 × 105 loading cycles.
At 2.5 × 105 loading cycles, the damage increased,
then at the 5.0 × 105 cycles, several small spalls
approximately 10 μm in size are also observed. On
the other hand, no damage formed on No. 2 evenafter 5.0 × 105 loading cycles.
Fig. 3 shows the topography of the rolling contact
surface on the driven cylinder. Fig. 3 (a) shows the pretest surface topography created by superfinishing. In
Fig. 3 (b) Test No. 1 at the 1.0 × 105 loading cycles,
the wavy pattern indicates a larger amplitude andperiod than before the test, as well as multiple small
protrusions. These protrusions matched the locationof the damage seen in Fig. 2 (b). On the other hand,Fig. 3 (c) Test No. 2 at 1.0 × 105 loading cycles showsa wavy pattern, however, its amplitude is small and nosmall protrusions are observed.

(a) No. 1, Before testingFig. 2 Optical micrographs of rolling contact surfaces
for driven cylinders(b) No. 1, 1.0 × 105 rev.

(c) No. 2, 1.0 × 105 rev.

Fig. 3 Surface topographies of driven cylinders measured by laser microscopy

NTN TECHNICAL REVIEW No.88

79

Initiation Mechanism of Peeling in Rolling Bearings, and Its Life Estimation Method

(a)

(b)

(c)

Fig. 4 SEM images of the crack-like marks on the driven cylinder of test No. 1 at 0.1 × 105 rev.

Fig. 4 shows the damage seen on the Test No. 1
driven cylinder at 0.1 × 105 loading cycles observed
by SEM. Fig. 4 (a) - (c) show higher magnification
views from different observation points. In Fig. 4 (a),protruded areas (B and D in the figure) exist adjacent
to grooves on both sides (A and C in the figure). Thismeans the aforementioned wavy pattern was formedby plastic deformation. In Fig. 4 (b), the protruded
area of the wave is deformed as if pressed in thevertical direction, from which a notch (E in the figure)
was formed. A notch (F in the figure) was also formed
in Fig. 4 (c), however, it was folded down along theslope of the adjacent groove.
Fig. 5 shows the damage on the Test No. 1 driven
cylinder at 5.0 × 105 loading cycles observed by SEM
from the axial direction. A crack which propagatedfrom the notch (area enclosed by dotted line in the
figure) can be seen.

the residual stress of the driven cylinder at 0.5 × 105
loading cycles, at which point there was almost no
difference in residual stress between Tests No. 1 andNo. 2.
1,300

Test No. 1

1,100

No. 2
1,300

σvm,res

MPa

1,100

900

900

700

700
500

0

0.05 0.1 0.15 0.2 0.25

500
0

1

2

3

4

5

Number of cycles (×105)

Fig. 6 Relationship between residual stresses atrolling contact surfaces of driven cylinders
and loading cycles
Table 3 Tri-axial stress components of residual stresses
for driven cylinders at 0.5 × 105 rev.
Fig. 5 Cross-section SEM image of the crack-likemark on the driven cylinder of test No. 1 at
5.0 × 105 rev.

2.4.2 Measurement result of residual stressFig. 6 shows the relationship between the residual
stress on the rolling contact surface of the driven
cylinder and number of loading cycles. The value ofthe vertical axis σvm,res is the Mises stress calculated
from the triaxial stress components. The inserted
graph in the figure is an enlarged view during the earlyloading cycles. σvm,res of Tests No. 1 and No. 2 stabilized
at 1,050 - 1,100 MPa after 0.1 × 105 loading cyles.
The rate of increase of σvm,res was slightly slower for
No. 2. Table 3 shows the triaxial stress components of80

NTN TECHNICAL REVIEW No.88

Test No.

Residual stress GPa
σx

σy

σz

τxy

τyz

τzx

1

-1.53

-1.34

-0.37

-0.02

-0.01

0.00

2

-1.52

-1.43

-0.41

-0.01

0.02

0.00

2.4.3 Surface roughness parameters ofdriving cylinder

Fig. 7 shows the relationship between the surface
roughness of the driving cylinder and the loading
cycles. In each test, σ*・Spc decreased immediately
after the start of testing, however, Test No. 2
decreased more than Test No. 1. This means that theseverity of contact due to running-in is much lower in
Test No. 2 when compared to Test No. 1.
0.14
0.12
0.1

σ*•Spc0.08
0.06

Test No. 1

0.04
0.02

No. 2

0

0

1

2

3

4

5

Number of cycles (×105)

the depth direction, at 0.5 × 105 loading cycles. In the
figure, Mises stress that considered the residual stressand Mises stress that did not consider residual
stressare both plotted. The stress values
on the vertical axis indicate the mean values of Mises
stresses at points where the stresses are the largest
among the neighboring stress values (within a square
area of 6.25 μm centered on the own points) on thex-y plane of depth z. The area enclosed by a dottedline in the figure shows the yield stress of the bearing
in Tests No. 1 and No. 2
steel (1.8 - 2.0 GPa13)).
exceeded the yield stress in all the areas up to 5 mof depth.
in Test No. 2 was smaller than Test
No. 1 at all depths. In addition, the maximum value of
occurred at about 0.5 m for both tests. The
stress value for Test No. 2 at this depth was about 40
was% smaller than for Test No.1. In all tests,
about 10 % smaller than.

Fig. 7 Relationship between the surface roughness ofdriving cylinders and loading cycles(σ*：Standard deviation of peak height，
Spc：Arithmetic mean peak curvature)

2.4.4 Estimation result of repeated stressapplied to rolling contact surface

Fig. 8 shows the pressure distribution of the realcontact area on the rolling contact surface obtained bycontact analysis. On the rolling contact surface of Tests
No. 1 and No. 2 before testing, contact pressure of morethan 10 GPa was observed. Areas of more than 10 GPa
were found on Test No. 1 at 0.5 × 105 loading cycles,
however most areas on the rolling contact surface wereless than 10 GPa for Test No. 2.
Fig. 9 shows the distribution of Mises stress acting
on the rolling contact surface of the driven cylinder, inyµm(Circumferential
direction)

y
µm

y
µm

y
µm

x
µm
(Axial direction)

x
µm

x
µm

x
µm

(a)
No. 1, Before testing(b)
No. 2, Before testing(c)No. 1, 0.5 × 105 rev.

(d)
No. 2, 0.5 × 105 rev.

Fig. 8 Contact pressure distributions at real contact area on rolling contact surfaces

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Initiation Mechanism of Peeling in Rolling Bearings, and Its Life Estimation Method

6○： σvm,con(z)●： ^σvm(z)5Test No. 1

σvm(z)^

or4

σvm,con(z)GPaformation will be larger when plastic deformation on
the rolling contact surface is high. In fact, the plastic
deformation of the driven cylinder in Test No. 1,
where peeling formed, was more significant than on
the driven cylinder of Test No. 2 where peeling did not
form.

3

No. 2

2.5.2 Relationship between repeated stressof rolling contact surface and peelingformation

Yield stress of JIS-SUJ2

It is assumed that a stress ofover the yieldstress was applied to the driven cylinders in Tests No.
1 and No. 2 and plastic deformation continued evenafter “running-in” and stabilization of residual stress(after 0.5 × 105 loading cycles) . It is also understoodthat the plastic deformation of the driven cylinder was
more significant on Test No. 1 than Test No. 2 because
up to 5 m of depth was larger . From theabove, it seems that peeling life (formation of initial
cracks and degree of progress) should be shorter as
the repeated stress on the rolling contact surface islarger and the degree of plastic deformation is greater.
Both cylinders in Tests No. 1 and No. 2,
was approximately 10 % smaller thanatthe same depth. This indicates that the Mises stressacting on the rolling contact surface was reduced dueto the impact of the residual stress, so the residual
stress must affect the peeling life. However, as
shown in Table 3, there was no difference between
the residual stresses in Tests No. 1 and No. 2, so theimpact of residual stress was not apparent.
In test No. 2 where black oxide treatment wasapplied to the driving cylinder, the Mises stress acting
upon the rolling contact surface at 0.5 × 105 loading
cycles was smaller than No. 1. This is due to the lower
σ*・Spc on the driving cylinder in Test No. 2, indicating
better “running-in”, was the main reason that peeling
did not occur on the driven cylinder in Test No. 2.

2
1
0.0

1.0

2.0

3.0

Depth μm

4.0

5.0

Fig. 9 Depth dependence of von Mises stress on therolling contact surface of the driven cylinder at
0.5 × 105 rev.

2.5. Discussion
2.5.1 Crack formation mechanism in peeling

From observation with the optical microscopein Fig. 2, small peeling spalls formed on the driven
cylinder in Test No. 1 which seem to be a result of
propagation of the damage that initiated from small
cracks at 2.5 × 105 loading cycles (hereafter, “initial
cracks”). Also, it is assumed that the initial cracks
were formed from micro-scaled plastic deformation
originating on the driven cylinder during rotation,
based on measurement of the surface shape in Fig. 3and observation by SEM in Fig. 4. Based on theseresults, Fig. 10 shows a schematic diagram of theformation mechanism of peeling cracks. Initially,asperity on the rough surface of the driving cylinder
contacts the rolling contact surface of the driven
cylinder, which creates a wrinkle-like pattern ofplastic deformation on the driven cylinder (Steps
1-2). Subsequent asperities, contact the plastically
deformed surface of the driven cylinder, flattening
the protrusions. Additional rolling creates notch
folding which leads to stress concentrations andinitial cracking (Steps 3-4). A similar phenomenon is
also observed in a study of micro-pitting of gears by
Mallipeddi, etc14). Based on the above mechanism,it can be assumed that the risk of peeling crack

Fig. 10 Formation mechanism of a crack in peeling on the driven cylinder

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3. Development of peeling life estimation
3.1 Overview of life estimation method

We have developed a peeling life estimation
method based on the findings of the mechanism
described in Chapter 27). Fig. 11 shows an overview
of the life estimation method. First, conduct rolling
contact fatigue tests under various operating
conditions to obtain a peeling life and a historyof repeated stresses acting on the rolling contact
surface. (hereafter, “stress history”) for each test (Step1). The stress history changes depending on runningin and change of residual stress. Therefore, estimationmust be done based on the contact analysis using
the actual measured surface topography and residual
stress measurement. Then, obtain peeling S-N curves(Stress – Number of cycles to failure) by regression
analysis by applying Minerʼs rule to the data obtainedin Step 1 (Step 2). Finally, estimate the peeling life
under any conditions using this S-N curve. At thistime, conduct a preliminary test simulating operating
conditions of the subject of estimation, obtain stresshistory under those conditions (Step 3) and estimate
the life using the S-N curve from the obtained stresshistory and Minerʼs rule (Step 4). S-N curves must be
prepared for each steel type and thermal treatmentof the rolling elements. The preliminary tests can be
conducted up to the loading cycles where running-in
and residual stress become stable. In our experience,
running-in and residual stress stabilize by 104 loading
cycles, under the condition that the oil film parameter
is lower than 1.5.
Currently, it is difficult to accurately simulaterunning-in and change in residual stress of rolling
elements used in various conditions. In this estimation
method, the impact of running-in and change ofresidual stress can be considered based on preliminary
test that simulates the actual operating conditions.
Impact of the materials can also be considered if S-N
curves for each steel type and thermal treatment are
prepared.

Stress-life data of experiment 2
Stress-life data of experiment 3

Peeling

Stress-life data of experiment 4[Step 2 Create S-N curve]

Regression
analysis

StressStress-life data of experiment 1

Stress[Step 1 Collect data for peeling life and stress history]

Number of cycles

Peeling life

[Step 3 Preliminary test under the conditionsof the subject for life estimation]

StressObtain stress history data
up to the limited loading cycles
Assume constant stressafter thisNumber of cycles[Step 4
Life calculation]

Map with S-N curve(Apply Miner s rule)

Estimated peeling life

Fig. 11 Flow of the estimation method of peeling life

3.2 Various assumptions for this life
estimation method

It must be noted that the following are assumed inthis life estimation method.
1) Repeated stress that produces peeling (dominantstress) is Mises stress.
2) The relationship between peeling life and repeatedstress follows Minerʼs rule.
3) Loss of fatigue layer of the surface layer due towear is negligible.
4) The relationship between peeling life Lth andrepeated stresscan be expressed by the
S-N curve of the double logarithmic model in thefollowing equation (3).
（3）

Where, A, B and Sf are constants specific to the materials.
Using Mises stress as the dominant stress of peelingin 1) should be appropriate considering that formationof peeling is affected by the magnitude of plastic
deformation of the rolling contact surface. Use of
Minerʼs rule and the S-N curve of the double logarithmic
model in 2) and 4) is generally adopted in otherresearch involving rolling fatigue life estimation15) 16) andthis method also followed that practice. The assumptionof 3) impact of wear, should be considered appropriate
under the conditions that the depth of wear on therolling contact surface is relatively small. It should be
noted that the scope of application of this life estimation
method is currently limited to pure rolling and boundary
lubrication conditions.

3.3 Validation of life estimation accuracy

The accuracy of this life estimation method was
examined by conducting two - cylinder type tests similar
to Fig. 1 under various test conditions. Table 4 shows
the test conditions and Fig. 12 shows the relationshipbetween the estimated peeling life (Lest) and the actual
life (Lact). S-N curves were generated with the test datafrom Test No. 1 - 7 and the estimated life of each test
was obtained from the stress history of up to 104 cycles.
In the figure, the median, minimum and maximum
values of the life ratio (Lact/Lest) are indicated. Thesestatistics values within the range of this test condition
was 0.89, 0.49 and 1.82, respectively. This accuracy is
the same or higher than other life estimation methods17)(90 % reliability interval of Lact/Lest under thin lubrication
condition is 0.98-4.3), therefore, this life estimation
method should be applicable as one of the peelinglife estimation methods. The tests of No. 8 and No. 9
were additionally conducted after generating the S-N
curves. The life ratios of these tests were 0.61 and 1.07,
respectively, thus, the validated life estimate of thesame accuracy as those in Test No. 1 - 7.
Moving forward, we plan to develop methods
applicable to conditions with slippage and mixed
lubrication, in order to expand the scope andapplication of this life estimation method.
NTN TECHNICAL REVIEW No.88

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Initiation Mechanism of Peeling in Rolling Bearings, and Its Life Estimation Method
Table 4 Test conditions of the RCF testing for validation of the life estimation accuracy
Surface roughness (Ra) µm

Test No.

Driving cylinder

Driven cylinder

Rotational speed
min-1

Maximum contact
Pressure GPa

Oil film
parameter Λ
0.11

1

0.75

2

0.40

3

0.35

4

0.30

5

0.75

6

0.75

2,000

7

0.40

1,000

8

0.50

9

0.75

2,000

0.21
2.3

0.24
0.28

0.02

500

2,000

0.06
1.6

0.12
0.17

2.3

0.17
0.11

Estimated life (Lest) × 105 cycles

4. Summary
10
4

Test No. 1
7

1
8

3
2

9

Lact / Lest (All test)

6

0.1

Median : 0.89
Min : 0.49
Max : 1.82

5

0.01
0.01

0.1

1

10

Actual life (Lact) × 105 cycles

Fig. 12 Relationship between estimated peeling lives
and actual peeling lives

84

NTN TECHNICAL REVIEW No.88

In this paper, we examined the formation
mechanism of peeling based on experiments andanalysis. We also introduced a life estimation method
based on the peeling formation mechanism.
1) Initial peeling cracks are produced from thenotches which are formed by plastic deformation of
protrusions on the rolling contact surface.
2) The developed life estimation method can estimate
peeling life under pure rolling and boundary
lubrication conditions with the same or better
accuracy than other life estimation methods. Moving
forward, we also plan to develop life estimation
methods applicable to conditions with slippage andmixed lubrication.
This paper is a compilation of 2 papers publishedin “The Tribologist,” the Journal of the JapaneseSociety of Tribologists, Vol. 63, Issue 8 (2018) 551
and Issue 9, (2018) 618, “Mechanism for Initiationof Peeling in Rolling Contact and the Effect of Black
Oxide Treatment on the Suppression of Peeling (Part
1 and 2)” and another paper published in its English
Journal, “Tribology Online” Vol. 14, Issue 3 (2019)131, “Estimation Method of Micropitting Life from S-NCurve Established by Residual Stress Measurements
and Numerical Contact Analysis.” We appreciate
generosity of the Japanese Society of Tribologists,
who permitted this publication.

References

1) Noriyuki Tsushima, Hirokazu Nakashima, Hiroshi
Kashimura, Typical Failures of Bearings, NTN
TECHNICAL REVIEW, No. 57 (1990) 59.
2) Y. Akamatsu, et. al., SAE Paper, 891909, (1989).
3) K. Maeda, N. Tsushima & H. Muro, The Inclinationof Cracking in the Peeling Failure of a Ball Bearing
Steel and its Relationship to the Inclination of the
Principal Residual Stress, Wear, 65, (1980) 175.
4) A. Oila, B. A. Shaw, C. J. Aylott & S. J. Bull,
Martensite Decay in Micropitted Gears, Proc.
IMechE. Part J. J. Eng. Trib., 219, (2005) 77.
5) Naoya Hasegawa, Takumi Fujita, Michimasa
Uchidate, Masayoshi Abo, Mechanism for
Initiation of Peeling in Rolling Contact andthe Effect of Black Oxide Treatment on the
Suppression of Peeling (Part 1), The Tribologist,
63, 8, (2018) 551.
6) Naoya Hasegawa, Takumi Fujita, Michimasa
Uchidate, Masayoshi Abo, Mechanism for
Initiation of Peeling in Rolling Contact andthe Effect of Black Oxide Treatment on the
Suppression of Peeling (Part 2), The Tribologist,
63, 9, (2018) 618.
7) N. Hasegawa. T. Fujita, M. Uchidate, M. Abo & H.
Kinoshita, Estimation Method of Micropitting Life
from S-N Curve Established by Residual StressMeasurements and Numerical Contact Analysis,
Tribology Online, 14, 3, (2019) 131.
8) V. Brizmer, K. Sradler, M. V. Drogen, B. Han, C.
Matta & E. Piras, The Tribological Performance ofBlack Oxide Coating in Rolling/Sliding Contacts,
STLE Tribol. Trans., 60, 3, (2017) 557.
9) DIN 50938, (2003).
10) J. A. Greenwood & J. B. P. Williamson, Contact ofNominally Flat Surfaces, Proc. Roy. Soc. London,
A295, (1966) 300.
11) M. Uchidate, Comparison of Contact Conditions
Obtained by Direct Simulation with Statistical
Analysis for Normally Distributed Isotropic
Surfaces, Surface Topography, Metrology andProperties, 6, 3, (2018) 034003.

12) Toshihiko Sasaki, Shun-ichi Takahashi,
Katsunari Sasaki, Yuichi Kobayashi, A Studyon Improvements in Multiaxial Stress Analysis
with Area Detector Type Diffraction Method,
Transactions Vol. A of The Japan Society of
Mechanical Engineers, 75, 750 (2009) 219.
13) E. Yhland, Static Load-Carrying Capacity, Ball
Bearing Journal, SKF, 211, (1982) 1.
14) D. Mallipeddi, M. Norell, M. Sosa & L. Nyborg,
Influence of running-in on surface characteristics
of efficiency tested ground gears, Tribology Int.,
115, (2017) 45.
15) L. Houpert & F. Chevalier, Rolling Bearing Stress
Based Life-Part I, Calculation Model, Journal of
Tribology, 134, 2, (2012) 021103, 1.
16) S. Shimizu, Fatigue Limit Concept and Life
Prediction Model for Rolling Contact Machine
Elements, Tribology Transactions, 45, 1, (2002)
39.
17) J.Gnagy, L. Houpert & F. Chevalier, Rolling Bearing
Stress Based Life-Part II, Experimental Calibration
and Validation, Journal of Tribology, 134, 2, (2012)
021104, 1.

Photo of authors

Naoya HASEGAWA

Takumi FUJITA

Michimasa UCHIDATE

Masayoshi ABO

Hiroshi KINOSHITA

Advanced Technology
R&D Center

Advanced Technology
R&D Center

Faculty of Science andEngineering,
Iwate University

School of Engineering,
University of Hyogo

School of Engineering,
University of Hyogo

NTN TECHNICAL REVIEW No.88

85

Technical Paper

Development of a Machine Learning Algorithm to Improve
Defect Detection Accuracy for Rolling Bearings
Masashi KITAI*
Ken-ichi FUKUI***

Yoshinobu AKAMATSU**

Detection of rolling bearing defects is important for machine maintenance. Previous studies by theauthors attempted to detect artificial defects of various sizes on the outer ring raceway surface of a rolling
bearing, and found that key features for successful detection depend on the defect size. This study shows
an improvement in accuracy of the defect detection process by using a size-based feature selection
approach coupled with a two-step outlier detection method.

1. Introduction
Rolling bearings are indispensable elements formany rotating machines and their use spans across
many industries such as automotive, aircraft, andindustrial plants. When rolling bearings suffer damage,it may not only affect the accuracy and operating
efficiency of rotating machines but may also haveserious consequences for the machines themselves if
the damage propagates and expands. Therefore, it isimportant to accurately detect any damage in rolling
bearings.
A frequently used diagnostic approach for rolling
bearings is analysis on vibration acceleration or
vibration data of Acoustic Emission (AE). The reason
for this is because measurement of the data is easy
and can be conducted while the target machines arein operation. Methods of detecting damage on rolling
bearings using vibration data include, for example,
condition monitoring by analyzing changes in trendsof different statistical data such as effective value andpeakedness calculated from vibration acceleration1),
condition monitoring by tracking changes ofcharacteristic frequency peaks after FFT process2)
and detecting formation of initial cracks using AE3).
However, these methods require an understanding ofvibration characteristics of rolling bearings and signal
processing for analysis. Therefore, a simpler diagnostic
approach is desirable.
On the other hand, an approach using machinelearning has recently been attracting attention as amethod to evaluate damage conditions of rotatingmachines as it does not require an understandingof analysis and signal processing. We can see someexamples in faulty symptom detection of hydraulicpower plants5) using the One Class Support Vector
Machine (OC SVM)4), faulty vibration detection ofdriving machines7) using Nearest Neighbor Data

Description6) and fault detection of wind turbines9)
using the tandem connection approach of the Deep
Neural Network/Gaussian Mixture Model8).
We also attempted fault detection by OC SVM,
creating artificial faults of different sizes on outer
raceways of rolling bearings, and found that small
defects could not be detected depending on therotational speed of the main shaft10). After examining
various feature quantities (considering domain,frequency filter, and measurement direction ofvibration acceleration), we verified that variations in
defect size change feature quantities that are usefulfor defect detection11). Therefore, it is necessary to
conduct feature selection against different defectsizes to improve the accuracy of defect detection.
In this study, we propose a new defect detection
method combining feature selection against
various artificial defect sizes and the two-stepoutlier detection method. As a preliminary test, weselected Local Outlier Factor (LOF)12) as a method
to achieve the highest defect detection accuracy,
after comparing the defect detection accuracy ofvarious outlier detection methods. Additionally, as aresult of investigation into classification accuracy ofnormal bearings and bearings with small defects byRandom Forest (RF)13), it was determined that RF wasappropriate to capture features of vibration due todefects as it could achieve high classification accuracy.
Therefore, RF was adopted for feature selection
against bearings with artificial defects of different
sizes and normal bearings, after which the effect offeature selection by LOF on defect detection accuracy
was examined.
Finally, the proposed method considering
preliminary test results and the original method
were compared to evaluate the defect detection
performance of the proposed method. By conducting
feature selection according to defect sizes and two-*
Product and Business Strategic Planning Dept., New Product and Business Strategic Planning Headquarters** New Product and Business Strategic Planning Headquarters*** The Institute of Scientific and Industrial Research, Osaka University

86

NTN TECHNICAL REVIEW No.88

step outlier detection, it was shown that the defectdetection accuracy was improved compared to theexisting research results10) 11).
In the following section, relevant research regarding
fault detection of rolling bearings using machinelearning will be reviewed. In Section 3, the outlier
detection method and feature selection method thatare used in this research are discussed. In Section
4, the evaluation test and results of the proposed
method are presented along with the final conclusion.

2. Relevant research
Recently, diagnostic approaches using machinelearning and a combination of machine learning andvibration analysis have been attracting attention
as diagnostic approaches for rolling bearings androtating machines that involve rolling bearings.
For example, Li et al. succeeded in determining
a fault location for rolling bearings with higher
accuracy than the existing methods, targeting rolling
bearings with defects set on surfaces of the innerand outer ring raceways and rolling elements18), byfeature extraction with Local Mean Decomposition14)
and Multiscale Permutation Entropy15), feature
selection with Laplacian Score (LS)16), and finally byclassification with Improved SVM-BT, which is theirown unique improvement of the Support Vector
Machine based of Binary Tree (SVM-BT)17). Also,
Bugharbee et al. improved detection accuracy offaults on surfaces of inner and outer ring raceways
and rolling elements of bearings under different
rotational speeds by analysis of main components
using Singular Spectrum Analysis19) and the creationof a fault detection threshold based on Mahalanobis'
Distance, using vibration data of normal bearings20).
Shao et al. classified defects using Deep Belief
Network22) which optimizes the number of neurons ineach hidden layer by Particle Swarm Optimization21)
targeting rolling bearings with defects of different
sizes on the outer and inner ring raceways and rolling
elements. Additionally, they showed its advantage
of classification accuracy over other classification
methods such as SVM and Bayesʼ estimation23).
However, Bugharbee et al. target bearings with
only one type of defect in each part of the outerand inner raceway and rolling element surfaces anddo not state the impact of the size of defects on
detection accuracy20). Li and Shao classify defects
by supervised learning as the method of defectdetection and do not discuss defect detection
accuracy under unsupervised learning18) 23).

3. Proposed method
3.1 Overview

The proposed method takes the input data v,
which is the feature vector calculated from measured
vibration acceleration data of rolling bearings, andcalculates the anomaly score a by feature selection
using the classification method and two-step outlier
detection method. Fig. 1 shows a flow diagram ofdefect detection using the proposed method. The
defects to be detected are divided into 3 stages basedon their status. The importance of each feature is
calculated using the classification method for eachdefect status, after which the feature selection is
performed (Feature Selection in Fig. 1).

Fig. 1 Flow of defect detection
The calculation of importance by the classification
method is only done during training. During testing,feature selection is directly performed using theimportance calculated during training. After feature
selection, each outlier detection method is trained andMtested individually for each input data v′ ( ), M ∈ {1 2(M)3} and the respective anomaly score a is calculated(Outlier Detection (1st) 1 to Outlier Detection (1st) 3
M
in Fig. 1). The input data after feature selection v′ ( )are targeting different defect statuses, respectively,
and therefore defect statuses other than theirtarget may not be detectable. Therefore, multiple
MM
anomaly scores a( ) obtained from input data v′ ( ) arehandled as one vector (anomaly vector a in Fig. 1)
for corresponding measured data. A final anomaly
score a is calculated by outlier detection against theanomaly vector a (Outlier Detection (2nd) in Fig. 1).
In this research, after the anomaly score is calculated
based on the proposed method on all the input
data obtained from the measured data of the target
to be tested. The ratio of input data for which theanomaly score exceeds the defect detection thresholddetermined in advance is evaluated as an anomaly
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Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings
ratio. The presence of any defect in the target data isdetermined based on the magnitude of the anomaly
ratio.

1.0
0.8

3.2 Input/output data

The input data v are the vectorized statistics of themeasured vibration acceleration data divided into
segments at a fixed time interval considering domain(time, frequency, and quefrency), band pass filter(BPF), and the direction of sensor measurement. The
output data is the anomaly score a obtained by theoutlier detection method of the second step.

3.3 Evaluation metrics

Anomaly Ratio (AR) and Area Under Curve Score(AUC Score)24) are used as metrics to evaluate defectdetection accuracy using the proposed method. The
calculation method of AR and AUC Score is shown in
the following:

3.3.1 Anomaly ratio

The anomaly ratio AR is calculated using thefollowing equation based on anomaly score a which is
calculated by the proposed method against the input
data v that can be obtained from consecutive multiple
segments in the measurement data. athreshold is a defectdetection threshold to be predetermined and set as
the average of the anomaly score against the normal
data + 5 × standard deviation of the anomaly score
against the normal data. In addition, NDS indicates thenumber of segments included in the measurement
data of the evaluation target.

ROC Curve

TPR

0.6
0.4

AUC Score

0.2
0.0

0.0

0.2

0.4

0.6

0.8

1.0

FPR

Fig. 2 ROC Curve and AUC Score
By setting FPR at a certain defect detection
threshold on the horizontal axis and TPR on thevertical axis and plotting the relation of FPR and TPR
in the graph when the defect detection threshold
is changed, the Receiver Operating Characteristic
Curve (ROC Curve) is obtained. An example of ROC
Curve is shown in Fig. 2. AUC Score is defined as thearea below the ROC Curve. If the normal data andanomaly data can be completely separated, the AUC
Score becomes 1.0. On the other hand, in random
identification, the AUC score will be 0.5. This means
that, if the AUC Score is close to 1.0, then the createdmachine learning model has a high identification
performance.

3.4 Machine learning method

For the outlier detection method, OC SVM, LOF,
and Isolation Forest (IF)25) were used and for thefeature selection method, RF and LS were used. The
following is a discussion of each of these methods.
For implementation of the machine language method,
Python 2.7 and Scikit-learn 0.19.0 were used.

3.4.1 One Class Support Vector Machine

3.3.2 Area Under Curve Score

As evaluation metrics of identification performance,
when positive is correctly identified as positive, it iscalled True Positive (TP); when negative is correctly
identified as negative, it is called True Negative (TN);when negative is incorrectly identified as positive,
it is called False Positive (FP); and when positive isincorrectly identified as negative, it is called False
Negative (FN). The False Positive Rate (FPR) and
True Positive Rate (TPR) are given in the following
equations, respectively:

OC SVM is a method to determine the identification
boundary that classifies normal data and anomaly
data by mapping normal data to a certain feature
space and obtaining the radius and center of thehypersphere where normal data is preferably includedin the feature space. When unknown data is obtained
and mapped to the feature space, if it is mapped
outside the radius of the hypersphere, the target data
is regarded as an anomaly. By selecting the kernel
function used for mapping into feature space, nonlinear problems can be handled. In this research,
Gaussian kernel was used for the kernel function.

3.4.2 Local Outlier Factor

LOF is an outlier detection method based on thedensity of the feature space. Outliers are based on
an assumption that they distribute in a range withlow density in the feature space. For each point on
the feature space, the marginal density is calculated.
When the marginal density of the target point andneighboring points are almost the same, the target
point is regarded as normal. On the other hand, when88

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the marginal density of the target point is lower thanthe marginal density of the neighboring points, thetarget point is regarded as an anomaly.

3.4.3 Isolation Forest

IF is a method to calculate the anomaly score basedon the average number of splits until certain data isisolated from the other data by random selection offeature quantities and split points. Since normal data
have similar characteristics to other normal data,
the average number of splits for each normal data
to become isolated will be large. On the other hand,since anomaly data have different characteristics
from normal data, the average number of splits foreach normal data to become isolated will be small.
Leveraging these characteristics, target data isregarded as an anomaly when the average number ofsplits required to isolate from certain data is smaller
than other data.

The test was conducted with a normal bearing
without artificial defects (ND in Table 1) and bearings
with artificial defects of different sizes discussed
above, attached to the test equipment. The main shaftof the test equipment was operated at the rotational
speeds of 1,000, 1,500 and 2,000 min-1 and vibration
acceleration in the radial, horizontal, and axial
directions was measured and used for evaluation.
Evaluation was made individually for each rotational
speed of the main shaft. One measurement data ofDvibration acceleration of a specific direction X ( ) = [x1x2 … xi … xN],D ∈ {Axial, Radial, Horizontal} involved 33
times of measurement for each artificial defect sizewith sampling frequency of 50 kHz and sampling timeof 20 sec. Where, index i indicates an order of time
sequence and xi indicates an instantaneous value ofvibration acceleration amplitude of index i. In addition,
considering the impact of bearing recombination
on vibration, bearings were recombined after 3
measurements.

3.4.4 Random Forest

Acceleration Vibration Sensor

RF is a classification method based on ensemble
learning using decision trees. Multiple training sets
are generated from the input data by sampling withreplacement, and then each training set is classified
based on a decision tree. Importance of the feature
quantity is also calculated based on the information
obtained from the training stage of RF. In thisresearch, the importance calculated by RF was used in
effective feature selection according to defect status.

Test Bearing

3.4.5 Laplacian Score

LS is a method to create a neighborhood graph,taking input data as nodes, and perform feature
selection based on the graph Laplacian for eachneighboring node in the graph. In this research, valuesof Laplacian score calculated for each feature quantity
were used in effective feature selection according to
defect status.

4. Evaluation Test
4.1 Test equipment

Fig. 3 shows an outline diagram of the test
equipment used for evaluation of defect detection
accuracy and Table 1 shows shapes and sizes ofartificial defects created on the test bearings. Rolling
bearings (angular contact ball bearing, model No.
7216) were used for the test, with cylindrical holes ofdifferent sizes (2b to 8b in Table 1) and a rectangulargroove sufficiently larger than the cylindrical holes (RG
in Table 1) created on the outer ring raceway surface.
The size of the cylindrical holes was set as 2 to 8
times the minor axis radius (b) of the elliptic elastic
contact area of ball and outer ring raceway surface.
For example, 2b means the diameter of the cylindricalhole is twice the size of b. These artificial defects
assume flaking produced as a result of rolling fatigue,
with the cylindrical holes simulating the initial stageof flaking and the rectangular groove simulating the
progressed state of flaking.

Fig. 3 Test equipment
Table 1 Shape and size of artificial defectSymbolShape of DefectSize mm

ND

None

-

2b

Hole

φ0.32

4b

Hole

φ0.64

6b

Hole

φ1.02

8b

Hole

φ1.36

RG

Rectanguler
GrooveWidth 2, Height 10,
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Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings

4.2 Calculation of input data vj

The measurement data of vibration acceleration

X (D) was divided into segments at an interval of 5

rotations of the main shaft, and one segment data yj ( )= [x( j − 1)n+1 x( j − 1)n+2 … x( j − 1)n+k … x( j − 1)n+n] was taken foreach segment. Where, n is the number of data points
included in the segment data, j = 1,2,3,…,」 N/n｣ is thesegment number and index k indicates the order oftime series. Therefore, the total number of segment
Ddata yj ( ) which are obtained from a measurement(D)data X is as shown in Table 2.
D

Table 2 Number of segment data yi in eachmeasurement data X
Rotation Speed

Number of Segment Data

1,000 min-1

66-1

1,500 min100

2,000 min-1

133

Table 3 Kind of band-pass filter
FilterFrequency Range(Hz)

Raw

None

Low1

20-200Low2

20-1,000Mid1

200-2,000

Mid2

1,000-5,000

High12,000-20,000

High25,000-20,000

The amplitude data after BPF processing on eachDsegment data yj ( ) in each frequency range shown in
D,TIME).
Table 3 was defined as time domain data yj (In addition, the amplitude data in frequency
domain obtained by envelope processing26) and FFT
D,TIME)was definedprocessing on time domain data yj ((D,SPEC)and the amplitude
as frequency domain data yj
data obtained by another FFT processing on frequency
D,SPEC)was defined as quefrency domain
domain data yj (D,R(D,CEPS). For feature quantities, for yj ( ), R ∈ {TIME,
data yj
SPEC, CEPS}, statistics such as modulation value(MOF) was used in addition to effective value (OA),
maximum value (MAX), crest factor (CF), Kurtosis(KS) and skewness (SKN), which are frequently used,in general, in the diagnosis of rolling bearings. The
following shows the calculation method of effective
value, maximum value, crest factor, Kurtosis andskewness:

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NTN TECHNICAL REVIEW No.88

Where, D is the direction of measurement, R is
D,Rdomain, j is the segment number, yj,k ( ) is the element
(D,R)
(D,R)
(D,R)
is mean of yj, σj
is standardof index k, yj
D,Rdeviation of yj ( ). Modulation value was defined asD,R
the effective value of yj ( ) after envelope processing.
Also, input data vj was set as a vectorized feature
quantities from different measurement directions
and domains in segment data yj. Table 4 summarizes
the number of parameters included in domain, BPF,
statistics and direction of sensor measurement tobe considered for calculation of feature quantities.
Each input data vj is composed of total of 378 feature
quantities considering domain (3), BPF (7), statistics(6), direction of sensor measurement (3).
Table 4 Number of parameters included in each input data vj

Number of
Parameters

Regions

BPFs

Statistics

3

7

6

Sensors Features3

4.3 Selection of feature quantities by
variation coefficient

378

Among various feature quantities included in
input data vj, some quantities exhibit larger variation
due to the recombination of bearings rather thanvariation due to the different sizes of artificial defects.
Therefore, variation coefficient (standard deviation
divided by mean) was calculated for each feature
quantity calculated from segment data yj of normal
bearings, which was used for training, and quantities
which variation coefficient exceeded 0.3 were
excluded from the target of training and testing, as
preprocessing. Approximately 20-40 % of feature
quantities, among all the feature quantities calculated
in Section 4.2 are excluded by this processing, by theselection of measurement data X used for training.
These feature quantities, selected by variation
coefficient, were used for training and testing,calculating Z Score for each feature quantity based on
the input data vj of artificial defect size ND, which wasused for training described later.

4.4 Selection of training data and test data

Considering the impact of recombination of
bearings on defect detection accuracy, a combinationof measurement data X was randomly changed forlearning and evaluation of the proposed method.
Fig. 4 shows the details of the combination of
measurement data X, when feature selection isconducted on artificial defect sizes ND and 2b,which is indicated by the red dotted lines in Fig. 1.
During training, from 33 measurement data X foreach artificial defect size, 24 measurement data X
were selected for artificial defect sizes ND and 2b,
respectively, then the importance of feature quantity
was calculated and features were selected by theclassification method based on each of the input data
vj. The input data after feature selection obtainedby this process was set as v′ j 2b. The superscript 2b
indicates that the input data after feature selection
v′ j uses a set of feature quantities configured for theartificial defect size 2b. Then, the input data v′ j 2bof artificial defect size ND was used for the training
outlier detection method, and hyperparameters wereselected, so that the specificity is the highest by 10fold cross validation, and anomaly score aj 2b was
calculated.
During the test, anomaly score aj 2b was calculatedusing the outlier detection method against the input
data v′ j 2b obtained from 9 measurement data X foreach artificial defect size.
Similarly, feature selection was conducted for the
combination of artificial defect sizes of ND and 4b, as
well as ND and 6b and anomaly scores aj 4b andaj 6b were calculated from the input data after featureS
selection v′ j 4b and v′ j 6b. In addition, anomaly score aj ( )(S)against input data after feature selection v′ j , S ∈ {2b,4b, 6b}, which can be obtained by this processing,was set as one anomaly score vector aj = (aj 2b, aj 4b,
aj 6b) for each corresponding segment. The anomaly
S
score vector aj against input data v′ j ( ) for training/
testing was used for training/testing of the secondstage outlier detection method. In addition, we madesure that the measurement data X to be used fortraining were not duplicated with those for testing.

Fig. 4 Selection of measurement data X for train and test

4.5 Testing defect detection accuracy

Defect detection accuracy was tested by calculating
the anomaly ratio of each defect size using equation(1) from the anomaly score obtained by the outlier
detection method against the measurement data X ofthe test target. When rotational speed of 1,500 min-1
is set as a target, there are 9 measurement data X
for test targets of each defect size and 100 segment
data yj obtained from each measurement data, fora total of 900 segment data yj for the test target ofeach defect size. The anomaly ratio for each artificial
defect size was calculated by using the anomaly score
calculated by the outlier detection method against
S
input data vj, v′ j ( ) or anomaly score vector aj fromeach segment data yj. Measurement data X used fortraining was randomly changed 50 times and defectdetection accuracy was tested by mean and variationof anomaly ratios on differences among the trainingdata.

4.6 Preliminary test
4.6.1 Comparison of outlier detection methods

Fig. 5 shows the relation between artificial defectsize and anomaly ratio when feature selection is
not made, at rotational speeds of 1,000, 1,500 and
2,000 min-1, for each outlier detection method. In
addition, Fig. 6 shows the result of calculation of
AUC Score from different outlier detection methods
for artificial defect sizes of ND and 4b, and Table 5shows mean, maximum value, minimum value, andstandard deviation of hyperparameters obtainedby cross validation of different training data. In thetable, ν indicates the lower limit of support vector
ratio in OC SVM, γ indicates hyperparameter ofGaussian kernel, n-neighbors indicates neighborhood
number in calculation of marginal density of LOF,
and n-estimators indicates number of estimators inensemble learning of IF.

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Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings

OC SVM

LOF

2b

6b

Table 5 Variation of hyper parameters for differenceof train data

IF

Anomaly Ratio

1.0
0.8
0.6
0.4
0.2
0.0
ND

4b

8b

RG

Defect Size(a) Rotation speed: 1,000 min-1

OC SVM

LOF

2b

6b

IF

Anomaly Ratio

1.0
0.8
0.6
0.4
0.2
0.0
ND

4b

8b

RG

Defect Size(b) Rotation speed: 1,500 min-1

OC SVM

LOF

2b

6b

4.6.2 Calculation of feature importance byRandom Forest

IF

Anomaly Ratio

1.0
0.8
0.6
0.4
0.2
0.0
ND

4b

8b

RG

Defect Size(c) Rotation speed: 2,000 min-1

Fig. 5 Comparison of outlier detection methods
OC SVMLOF

IF

AUC Score

1.0
0.9
0.8
0.7
0.6
0.5
1,000

1,500

2,000

Rotation speed min-1

Fig. 6 AUC Score for each method

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NTN TECHNICAL REVIEW No.88

Fig. 5 reveals that the average anomaly ratio ofartificial defect sizes 2b - 6b is the highest in LOF.
Fig. 6 shows that LOF gives the highest defect detection
accuracy for AUC Score also. However, in any method,
the average anomaly ratio of artificial defect size 2b
is 0.3 or lower, which is lower than the artificial defectsize 4b or larger. From Table 5, it was estimated thatthe variation of anomaly ratio was produced alongwith variation of hyperparameter values depending
on different training data. Therefore, feature selection
with RF was performed to improve defect detection
accuracy.

Classification accuracy of artificial defect size 2b,which was difficult to detect by the outlier detection
method of the previous section, was tested using
RF. Initially, in order to confirm the influence ofhyperparameter of RF, the number of decision trees of
RF at the rotational speed of 1,500 min-1 was changed
to 100, 1,000 and 10,000, and the maximum depthof decision trees to 10, 100 and 1,000. However,there was no difference in significance level in theinfluence of artificial defect sizes of ND and 2b onclassification accuracy. Therefore, we set the numberof decision trees to 1,000 and the maximum depthof decision trees to 100 and conducted evaluation ofclassification accuracy and feature selection for eachrotational speed. Fig. 7 shows classification accuracyof artificial defect sizes ND and 2b, at rotational
speeds of 1,000, 1,500 and 2,000 min-1. Table 6
shows an example of extracting 10 feature quantities,
which are selected based on difference in training
data, after feature selection by RF, against artificial
defect sizes of ND and 4b;ND and 6b; and ND and 2bat rotational speed of 1,500 min-1. Feature quantities
of high importance common to classification of
artificial defect sizes ND and 2b and classification of
artificial defect sizes ND and 4b are indicated in boldfont. Feature quantities of high importance common
to classification of artificial defect sizes ND and 4b
and classification of artificial defect sizes ND and 6b
are indicated in bold font with an underline. Feature
quantities are expressed by statistics - domain - BPF(direction of measurement) in the table.
Fig. 7 reveals that artificial defect size 2b can beOriginal
Feature(4b)Anomaly Ratio

classified with very high accuracy by RF regardlessof rotational speed. In addition, Table 6 reveals that
2 feature quantities are common in classification of
artificial defect size ND and 2b and classification of
ND and 4b, and only 1 feature quantity is common toclassification of artificial defect sizes ND and 4b andclassification of ND and 6b, while no feature quantity
is common to classification of artificial defect sizes ND
and 2b and classifications of ND and 6b. Therefore,
all the defect sizes cannot be tested with the samefeature quantity.

Feature(2b)Feature(6b)

1.0
0.8
0.6
0.4
0.2
0.0
ND

2b

4b

6b

8b

RG

Defect Size(a) Rotation speed: 1,000 min-1

1.0

Original
Feature(4b)

0.6
0.4
0.2
0.0
1,000

1,500

2,000

Rotation speed min-1

Anomaly Ratio

Accuracy

0.8

Fig. 7 Classification accuracy by Random Forest

1.0
0.8
0.6
0.4
0.2
0.0
ND

2b

4b

8b

RG

(b) Rotation speed: 1,500 min-1

Anomaly Ratio

Original
Feature(4b)Feature vector composed of the selected 10 feature
quantities of high importance with respect to eachS
artificial defect size was set as input data v′ j ( ), anddefect detection accuracy by LOF was tested. Fig. 8
shows the relation between artificial defect size andS
anomaly ratio against input data v′ j ( ) after feature
selection. Anomaly ratios calculated from input datav′ j 2b, v′ j 4b and v′ j 6b which selected feature quantities
of high importance to the classification of artificial
defect sizes ND and 2b, ND and 4b, and ND and 6b
are shown in the figure as Feature(2b), Feature(4b)and Feature(6b), respectively. In the same figure,anomaly ratios directly calculated from input data v′ jbefore feature selection using LOF for each defectsize are shown as the original method (Original). The
results from input data vj before feature selection
are the same as Fig. 5. In addition, Table 7 shows
the mean, maximum values, minimum values, andstandard deviation of neighborhood numbers in themarginal density calculation of LOF, obtained by crossS
validation for input data v′ j ( ) after feature selection.

6b

Defect Size

Table 6 Features with high importance to defect size
(Rotation speed 1,500 min-1)

4.6.3 Defect detection accuracy by inputS
data v′ j ( ) after feature selection

Feature(2b)Feature(6b)Feature(2b)Feature(6b)

1.0
0.8
0.6
0.4
0.2
0.0
ND

2b

4b

6b

8b

RG

Defect Size(c) Rotation speed: 2,000 min-1

Fig. 8 Anomaly ratio for each input data v′ j ( )SFig. 8 reveals that, when feature quantities of highimportance are selected for artificial defect size 2b,
anomaly ratio of artificial defect size 2b increases
compared to the case when feature quantities of highimportance for artificial defect sizes 4b and 6b are
selected.
On the other hand, when the above feature
quantities are selected, anomaly ratios for artificial
defect sizes 4b and 6b are lower than anomaly
ratios for artificial defect size 2b, and the detection
accuracy decreases for those other than the targeted
artificial defect size. In addition, Table 7 shows thatvariation of the neighborhood number does not
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Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings

Rotation

VectorSpeed

Original

Feature(2b)Feature(4b)Feature(6b)Standard

AverageMax

Min

1,000 min-1

13.3

24

2

8.0

1,500 min-1

9.8

25

2

8.6

2,000 min-1

8.4

29

2

8.3

1,000 min-1

16.0

28

3

7.5

1,500 min-1

14.0

28

5

6.4

2,000 min-1

24.4

29

16

4.0

1,000 min-1

23.1

28

14

4.9

1,500 min-1

14.4

28

2

8.9

2,000 min-1

18.1

27

2

7.2

Deviation

1,000 min-1

15.2

28

2

7.8

1,500 min-1

17.2

27

5

7.0

2,000 min-1

24.4

29

15

4.6

4.6.4 Outlier detection accuracy by 2-step
LOF and impact of number of feature
quantities used for feature selection

In order to resolve the problem of reduction of
anomaly ratio other than artificial defect size targeted
for feature selection, the proposed method uses
S
anomaly score for input data v′ j ( ) after feature selection
as the input data on the second stage of LOF as one
vector (anomaly score vector aj) for each segment
corresponding to the original measurement data. Thenthe proposed method compares the defect detection
accuracy again. Therefore, defect detection accuracy ofthe proposed method varies depending on the numberof feature quantities in feature selection of input datav′ j (S). Fig. 9 shows the relation between the number ofSfeature quantities in feature selection of input data v′ j ( )and the mean and variation of anomaly ratios of artificial
defect sizes ND, 2b, 4b and 6b in the proposed method
at the rotational speeds of 1,000, 1,500 and 2,000 min-1.
In addition, Table 8 shows the relation between thenumber of feature quantities in feature selection and themean, maximum values, minimum values, and standard
deviation of the neighborhood number.
Fig. 9 shows that the number of feature quantities
to improve defect detection accuracy varies dependingon the rotational speed. In any rotational speed, theanomaly ratio of artificial defect sizes 2b and 4b
decreases when the number of feature quantities is 5
or less. In addition, the anomaly ratio of artificial defectsize 2b increases at the rotational speed of 1,500 min-1
when the number of feature quantities is 30 or more,
however, the anomaly ratio of artificial defect sizes 2band 6b decreases at the rotational speed of 2,000 min-1
and the average anomaly ratio of artificial defect size ND
increases up to 0.2. Table 8 shows no difference in theneighborhood number for different numbers of feature
quantities.

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NTN TECHNICAL REVIEW No.88

ND
Anomaly Ratio

Feature

2b

4b

6b

1.0
0.8
0.6
0.4
0.2
0.0
1

2

5

10

30

50

100

150

Number of Selected Features(a) Rotation speed: 1,000 min-1

ND
Anomaly Ratio

Table 7 Variation of the neighborhood numbers ofLocal Outlier Factor for the difference of
train data

In the practice of defect detection methods,
misjudgment of anomaly for normal bearings should
be avoided. Also, the larger the difference of anomaly
ratios between normal bearings and bearings withsmall defects, the easier the defect detection will
be. Therefore, selection criterion for number offeature quantities adequate for defect detection wasdetermined as the anomaly ratio of artificial defect size
ND to be 0.2 or lower and the margin between artificial
defect sizes ND and 2b to be large. Furthermore, whenthe margin between artificial defect sizes of ND and 2b
is about the same, features with less numbers of feature
quantities should be selected. Based on the above
criterion, the number of feature quantities adequate for
defect detection was determined to be 30 for rotational
speed of 1,000 min-1, 150 for rotational speed of
1,500 min-1 and 10 for rotational speed of 2,000 min-1.

2b

4b

6b

1.0
0.8
0.6
0.4
0.2
0.0
1

2

5

10

30

50

100

150

Number of Selected Features(b) Rotation speed: 1,500 min-1

ND
Anomaly Ratiochange depending on the different training data.
Therefore, defect detection accuracy for each inputS
data v′ j ( ) after feature selection must be evaluatedcomprehensively.

2b

4b

6b

1.0
0.8
0.6
0.4
0.2
0.0
1

2

5

10

30

50

100

150

Number of Selected Features(c) Rotation speed: 2,000 min-1

Fig. 9 Influence of number of features on feature selection

Table 8 Comparison of the neighborhood numbers ofLocal Outlier Factor for the number offeatures
Number of
Features

1

2

5

10

30

50

100

150

RotationStandard

Average

Max

Min

-1

1,000 min

11.2

24

3

7.4

1,500 min-1

14.9

24

6

7.0

2,000 min-1

16.9

29

8

7.8

1,000 min-1

21.5

29

4

8.3

1,500 min-1

21.3

27

11

5.8

2,000 min

-1

16.2

24

8

7.0

1,000 min-1

16.0

25

5

6.5

1,500 min-1

17.8

29

2

7.9

2,000 min-1

17.7

29

9

6.7

1,000 min-1

16.3

29

6

8.5

1,500 min

-1

18.0

25

3

6.6

2,000 min-1

22.0

29

16

4.2

1,000 min-1

16.9

28

3

10.5

1,500 min

-1

18.1

28

6

6.7

2,000 min-1

21.8

29

13

5.9

1,000 min

-1

20.7

29

8

7.4

1,500 min-1

21.5

29

2

9.4

2,000 min-1

16.6

28

2

8.5

-1

1,000 min

15.6

23

8

5.5

1,500 min-1

16.1

29

2

9.4

-1

2,000 min

21.3

29

2

8.1

1,000 min-1

16.6

29

5

7.2

1,500 min-1

15.6

26

9

5.1

2,000 min

17.7

29

8

7.1

Speed

-1

Deviation

4.7 Defect detection accuracy of the
proposed method

Fig. 10 shows the relation between artificial defectsize and anomaly ratio by the proposed method(Proposed) which uses the appropriate number offeature quantities for defect detection selected by theprevious section at rotational speeds of 1,000, 1,500
and 2,000 min-1. As a comparison, anomaly ratios
directly calculated from input data vj before feature
selection using LOF for each artificial defect size areshown as the original method (Original). Results of
the original method are the same as Fig. 5. Also,
the anomaly ratio for each artificial defect size using
Laplacian Score16) as the feature selection method
is depicted in the figure as the comparison method(Comparison). Fig. 11 shows the comparison of AUC
Score among the proposed method, original method
and comparison method against artificial defect sizes
ND and 2b. In the anomaly ratio of Fig. 10 and AUC
Score of Fig. 11, * is marked on the difference ofsignificance level of 5 % between the original method
and the proposed method, and the comparison
method and the proposed method, and ** is marked
on the difference of significance level of 1 %. Table 9shows the mean, maximum value, minimum value,and standard deviation of the neighborhood numbersof the proposed method and original method.
Fig. 10 indicates that the anomaly ratios of artificial
defect size 6b or more by the proposed method arealmost 1 regardless of rotational speed and eventhose for artificial defect size 2b significantly increased
compared to the original method and comparison
method. Also, for artificial defect size 4b, the anomaly
ratio by the proposed method significantly increased
at rotational speeds of 1,000 and 1,500 min-1 over
the original method, and regardless of rotational
speed over the comparison method. From Fig. 11,
AUC Score can be improved by using the proposed
method at any rotational speed over the original andcomparison methods. At rotational speeds of
1,000 min-1 and 1,500 min-1, although the anomaly
ratio of artificial defect size ND shows significant
difference between the original method and the
proposed method, AUC Score also significantly
improved with the proposed method. Therefore,
use of the proposed method improved the defectdetection accuracy.
Table 9 reveals no difference in variation ofneighborhood number of LOF between the original
and the proposed methods.

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Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings

Original

Proposed

1.0
0.8

**

** **

AUC Score

Anomaly Ratio** **Original

Comparison

*

0.6
0.4

1.0

0.0
ND

2b

Anomaly Ratio

Original

4b

6b

8b

RG

**

0.7
0.5

** **

Proposed
**

1,500

1,000

2,000

Rotation speed min-1

Comparison*

1.0* : Significance level 5 %** : Significance level 1 %Fig. 11 Comparison of AUC Score between original,
comparison and proposed method
Table 9 Comparison of the neighborhood numbers ofLocal Outlier Factor between original andproposed method

*

0.6
0.4
0.2
0.0
ND

2b

4b

6b

8b

RG

MethodMin

1,000 min-1

13.3

24

2

8.0

1,500 min-1

9.8

25

2

8.6

2,000 min-1

8.4

29

2

8.3

1,000 min-1
Outlier
Detection
1,500 min-1(2nd)(Proposed) 2,000 min-1

16.3

29

6

8.5

18.0

25

3

6.6

22.0

29

16

4.2

Original

Proposed**Comparison
*

** **

1.0
0.8
*

0.4

Standard

Max(b) Rotation speed: 1,500 min-1 (150 Features)

Original

Rotation

Average

Defect Size

Anomaly Ratio

**

0.8

(a) Rotation speed: 1,000 min-1 (30 Features)

Speed

Deviation

0.2
0.0
ND

2b

4b

6b

8b

RG

Defect Size(c) Rotation speed: 2,000 min (10 Features)-1* : Significance level 5 %** : Significance level 1 %Fig. 10 Comparison of anomaly ratios between
original, comparison and proposed method

96**

Comparison*

0.9

Defect Size

0.6

*

0.6

0.2

0.8

**

Proposed

NTN TECHNICAL REVIEW No.88

5. Future challenges
The test equipment used for this evaluation showedlarge variation due to recombination of bearings. It isconsidered that recombination has some impact on
the test environment. For validation of the proposed
method, evaluation with completely different test
equipment is necessary. Currently, data collection
using new test equipment is underway, with a plan tovalidate the proposed method moving forward.
In the proposed method, feature selection withsupervised learning is conducted for calculating
anomaly score in the first stage of LOF and vibration
data of bearings with defects are used as supervised
data. However, in practice, it is almost impossible
to obtain vibration data of bearings with defects. Asa solution to this problem, for example, a method
to improve defect detection accuracy by extracting
features which do not depend on rotational speed or
equipment can be considered, using vibration analysis
methods such as Persistent Homology27) and Dynamic
Mode Decomposition28). Or using data obtained froma theoretical analysis model instead of measurement
data from the actual equipment as the supervised
data can be considered, by using the theoreticalanalysis model29) such as Dynamic Analysis in featureselection by Random Forest. Or, using Transfer
Learning, data from test equipment or a theoreticalanalysis model may be used for diagnosis of otherequipment as supervised data. We will study these
possibilities for improvement.

6. Summary
Various anomaly detection methods were comparedfor defect detection accuracy against vibration
acceleration data of bearings with artificial defects.
Insights obtained from this study are as follows:• When 3 types of outlier detection methods, One
Class Support Vector Machine, Local Outlier Factor
and Isolation Forest, were compared for defectdetection accuracy, Local Outlier Factor provided
the highest defect detection accuracy. However, inall methods, defect detection accuracy of artificial
defect size 2b was lower than artificial defect size 4b
or larger.
• Evaluation of classification accuracy of artificial
defect size by Random Forest revealed that feature
quantities of high importance for classification of
bearings with and without defects are different
depending on the size of artificial defects. In
addition, feature selection by Random Forest for
micro artificial defect size improved the defectdetection accuracy of the targets.
• The proposed defect detection method successfully
provided a significant improvement of defectdetection accuracy for small artificial defect size
without reducing detection accuracy of larger defectsize, compared to the cases of no feature selection
by Random Forest.

Acknowledgment

This paper was produced by editing the original
paper (Kitai, Akamatsu and Fukui, Defect Detection
Method for Rolling Bearing Including Micro Defectby Feature Selection and Two Step Outlier Detection
Method, Transactions on Mathematical Modeling and
its Applications, The Information Processing Society ofJapan Vol. 12, (2019) pp 32-42). We appreciate thegenerosity of the Information Processing Society ofJapan.

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of a High Dimensional Distribution, NeuralComputation, Vol. 13, (2001) 1443–1471.
5) Takashi Onoda, Norihiko Ito, Hideaki Koreeda,
Data Indicating Malfunction Sign of HydroelectricPower Plants, The transactions of the Institute ofElectrical Engineers of Japan. D, Vol. 131, No. 4,(2011) 448–457.
6) Tax, D.M.J., One Class Classification, Ph.D thesis,
Delft University of Technology (2001).
7) Minoru Kondo, Tatsuro Takashige, Shinʼichi
Manabe, Hiroshi Kanno, Abnormality Detection
in a Contaminated Diesel Engine by a Vibration
Monitoring Method, RTRI Report, Vol. 30, No. 4,(2016) 47–52.
8) Hermansky, H., Ellis, D.P. and Sharma, S., Tandem
Connectionist Feature Extraction for Conventional
HMM Systems, IEEE International Conferenceon Acoustics, Speech, and Signal Processing.
Proceedings, Vol. 3, (2000) 1635–1638.
9) Takanori Hasegawa, Jun Ogata, Masahiro
Murakawa, Tetsuji Ogawa, Advancement of
anomaly detection technology for wind turbine
system based on representation learning fornormal and faulty classification, 39th Symposium
on Wind Energy Application (2017)10) Masashi Kitai, Hideyuki Tsutsui, Anomaly
Diagnosis of Angular Ball Bearings using One
Class Support Vector Machine, Proceedings for
Tribology Conference 2017 Fall (2017) C41.
11) Koma Kato, Ryoji Tani, Hideyuki Tsutsui,
Relationship between defect size and various
vibration features in angular contact ball bearings,
Proceedings for Tribology Conference 2017 Spring(2017) F33.
12) Breunig, M.M., Kriegel, H.P., Ng, R.T., and Sander,J., LOF, Identifying Density-Based Local Outliers,
Management of Data, Vol. 29, (2000) 93–101.
13) Breiman, L., Random Forests, Machine Learning,
Vol. 45, (2001) 5–32.
14) Smith, J.S., The Local Mean Decomposition and
Its Application to EEG Perception Data, J.R.Soc.
Interface, Vol. 2, (2005) 443-445.
15) Aziz, W. and Arif, M., Multiscale Permutation
Entropy of Physiological Time Series, 9thInternational Multitopic Conference, (2005) 1-6.
NTN TECHNICAL REVIEW No.88

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XXXXXXXXXXXX
Development of a Machine Learning Algorithm to Improve Defect Detection Accuracy for Rolling Bearings16) He, X., Cai, D. and Niyogi P., Laplacian Score for
Feature Selection, Adv.Neural Inform. Process.
Syst., (2005) 16.
17) Cheong, S., Sang, H.O. and Lee, S.Y., Support
Vector Machines with Binary Tree Architecture forMulticlass Classification, Neural Inform. Process.
-Lett. Rev., Vol. 2, (2004) 47-51.
18) Li, Y., Xu, M., Wei,Y., and Huang, W., A New
Rolling Bearing Fault Diagnosis Method Base on
Multiscale Permutation Entropy and Improved
Support Vector Machine Based Binary Tree,
Measurement, Vol. 77, (2016) 80-94.
19) Kilundu, B., Chiementin, X. and Dehombreux, P.,
Singular Spectrum Analysis for Bearing DefectDetection, Journal of Vibration and Acoustics, Vol.
133, No. 5, (2011) 051007.
20) Bugharbee, H.A. and Trendafilova, I., A New
Methodology for Fault Detection in Rolling
Element Bearings using Singular Spectrum
Analysis, The International Journal of Condition
Monitoring, Vol. 7, No. 2, (2018) 26-35.
21) Zhu H., Wang Y., and Wang K., Particle Swarm
Optimization (PSO) for the Constrained Portfolio
Optimization Problem, Expert System, Vol. 38,(2006) 10161-10169.
22) Hinton G. E. and Osindero S., A Fast Learning
Algorithm for Deep Belief Nets, Neurocomputing.
Vol. 18, (2006) 1527-1554.
23) Shao, H., Jiang, H., Zhang, X., and Niu, M., Rolling
Bearing Fault Diagnosis Using an Optimization
Deep Belief Network, Measurement Science andTechnology, Vol. 26, (2015) 1-17.
24) Tsuyoshi Ide, Masashi Sugiyama, Anomaly
Detection and Change Detection, Kodanshsa,(2015) 11–12.
25) Liu, F.T., Ting, K.M. and Zhou, Z.H., Isolation
Forest, Eighth IEEE International Conference on
Data Mining, (2008) 413–422.
26) Toshio Toyota, Procedure of anomaly diagnosisof rotating machines, Japan Institute of Plant
Maintenance, (1991) 94–96.

Photo of authors

Masashi KITAI

Yoshinobu AKAMATSU

Ken-ichi FUKUIProduct and Business Strategic
Planning Dept.,
New Product and Business
Strategic Planning Headquarters

New Product andBusiness Strategic
Planning Headquarters

SANKEN (The Institute of
Scientific and Industrial
Research), Osaka University

98

NTN TECHNICAL REVIEW No.88

27) Umeda Y., Time Series Classification via
Topological Data Analysis, Transactions of theJapanese Society for Artificial Intelligence, Vol. 32,
No. 3, (2017) 1-12.
28) Kota Dohi, Naoya Takeishi, Takehisa Yairi, Koichi
Hori, Anomaly detection of acoustic data using
dynamic mode decomposition, 32nd National
Conference of the Japanese Society for Artificial
Intelligence, 1P2-02, (2018).
29) Gupta, P.K., Advanced Dynamics of Rolling
Elements, Springer-Verlag, (1984).

New Products“ETFA” Bearings Strengthened by Fine Microstructure Design

Masahiro YAMADA*
Chikara OHKI*

Naota YAMAMOTO**

NTN designed a special heat treatment to improve bearing properties by
resulting in a fine microstructure of steel. The resulting fine microstructure
consists of homogeneous martensite blocks and precipitations of a refinedsize. Testing shows specimens with fine microstructure had high impact
strength and wear resistance. Fine microstructure test bearings also showedsuperior rolling contact fatigue lives under both clean and contaminatedlubrication conditions. NTN calls these bearings “ETFA”. In this paper, we explain the details of ETFA bearings.

1. Introduction
With a shift toward lower fuel consumption of
automobiles and increasing compactness of industrial
machinery, there is a trend toward increasingly severe
bearing operating conditions. Typically these include
higher contact surface pressure and lower viscosityof lubricating oils. These increasingly harsh use
environments may promote problems such as flaking
that originates from an indentation and shorten
bearing life. Therefore, there is a need for higher
bearing strength.
To achieve higher bearing strength, it is crucial tostrengthen the steel material. There are a numberof methods for strengthening steel and one of these
involves refinement of crystal grains1). This can be
regarded as a strengthening method that hinders
dislocation motion due to the increase in grainboundaries and changes the plastic properties ofsteel. There has previously been discussion of theeffects of the size of prior austenite grains (referredto hereafter as “prior γ grains”) on strengthening by
the refinement of crystal grains in quenched steel.
However, there are few examples of comparative
examination of the size of martensite grains actuallyformed due to quenching2). In recent years, it has beenthought that the size of martensite blocks (referredto hereafter as “blocks”) that are the effective crystal
grains is the dominant factor in determining thestrength of steel. Therefore, to strengthen steel
through crystal grain refinement, it is necessary to
produce a more refined martensite structure rather
than refining the prior γ grains3)-6).
The authors have already developed long-life, highstrength bearings made of JIS SUJ2 in which blocks
were refined through special heat treatment7). Thesethrough hardened bearings had blocks strengthened
through refinement, and were enhanced withimproved anti-wear performance through nitriding7)-9).
Recently, we have used the aforementioned heat
treatment technology to develop new bearings madeof low-carbon steel with refined blocks for longer lifeand higher strength. NTN calls these newly developed
bearings “ETFA,” and we are currently moving towardpractical application.
This article describes the details of these ETFA
bearings.

2. Fine microstructure of ETFA
2.1 Material and state of precipitation
formation

The material used is low-carbon steel containing
Cr and Mo. The fine microstructure of low-carbon
steel is controlled by applying carbonitriding
treatment and grain refinement treatment. Table 1
shows a comparison of ETFA characteristics with thecharacteristics of conventional carbonitriding treatedbearings (referred to hereafter as the “conventional
product”)10). Compared to the conventional product,
the prior γ grains, blocks, and precipitations of ETFA
are finer than the conventional product, and thenumber density of precipitations is high.
Fig. 1 shows a typical example of the depth
distributions of carbon and nitrogen concentrations
in ETFA measured with an Electron Probe Micro
Analyzer (EPMA). Multiple sharp rises in concentration
attributable to precipitations were confirmed in thesurface layer. It is evident that carbides, nitrides, andcarbonitrides are present at high density and withhigh refinement.
Fig. 2 shows a scanning electron microscope (SEM)
image of prior γ grain boundaries and precipitationsin the raceway surface layer. The crystal grains of
ETFA are fine and precipitations are homogeneously
dispersed.

* Advanced Technology R&D Center** Application Engineering Dept., Industrial Business Headquarters
NTN TECHNICAL REVIEW No.88

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“ETFA” Bearings Strengthened by Fine Microstructure Design
Table 1 Feature of the developed bearings
Conventional product

ETFA

Heat treatment

Carbonitriding
quenching

Carbonitriding
quenching
Grain refinement
treatment

Carbon concentration

Standard

Standard

Nitrogen concentration

Standard

Standard

Residual γ amountStandard

Standard

Size of precipitations

Standard

Fine

Number of precipitations

Standard

Numerous

Size of prior γ grainStandard

Fine

Size of martensite blocks

Standard

Fine

Hardness

Standard

Standard

2.2 Condition of block formation

Fig. 3 shows images of prior γ grains and blocks
in the raceway surface layer. The block images areinverse pole figure crystal orientation maps measured
using the electron backscatter diffraction (EBSD)
method. The prior γ grains and blocks of ETFA are
fine compared to the conventional product. Fig. 4
shows the {011} pole figure for blocks. The block
crystal orientation density is lower overall than thatof conventional products and crystalline orientation islow.
Fig. 5 shows the average grain size of these blocks
and their average aspect ratio. These values were
calculated for blocks corresponding to a specific area
ratio. That is, blocks within the field of observation
are added together in order of their area, and this is
the average value for those grains when their total
area corresponds to 30, 50, or 70 % of the block total
area. The difference in block diameter compared tothe conventional product increases as the area ratio
decreases, and a similar trend is event for the aspectratio as well.

Martensite block

Prior austenite grain

Conventional product

50 μm

1.5

Concentration mass%

Item

1.0

Carbon

0.5

0.0
0.00

Nitrogen

0.50

1.00

1.50

2.00

Depth from surface mm

Fig. 1 Typical depth profiles of C and N concentrationsof the developed bearings
Conventional product

ETFA

Fig. 2 SEM-micrographs of prior austenite grainboundary and precipitations near race
surfaces of the bearings
As indicated above, the formation of precipitations
and blocks is refined and homogeneous on the
raceway surfaces of ETFA bearings.

ETFA

50 μm

111

5 μm

5 μm
001

101

Fig. 3 Observation images of prior austenite grain and martensite block near race surfaces of the bearings

100

NTN TECHNICAL REVIEW No.88

Conventional product

ETFA

Martensite block average size μm

Fig. 4 {011} pole figure of martensite block near race surfaces of the bearings

5.0

Conventional product

4.0

Support bearing
Test bearing

ETFA

3.0
2.0
1.0
0.0

30

50

70

Area ratio %Martensite block average aspect ratio

SpringTest bearing
Pulley5.0

Conventional product

4.0

ETFA

3.0
2.0
1.0
0.0

30

50

70

Area ratio %Fig. 5 Martensite block size and aspect ratio near race
surfaces of the bearings

3. Rolling fatigue life of ETFA bearings
3.1 Rolling fatigue life test conditions

Rolling fatigue life testing was carried out under bothclean and contaminated lubrication conditions. Fig. 6
shows a schematic drawing of the test rig, and Table 2
shows the test conditions7). The test bearing used in
the life test was a tapered roller bearing. The life testwith clean lubrication was carried out with N=5 forbearing A (inner diameter 30 mm × outer diameter
62 mm × width 17.25 mm). To improve the reliabilityof evaluation results in the life test with contaminatedlubrication, testing was carried out with N=6–9 forbearings A and B in two sizes (inner diameter 85 mm
× outer diameter 165 mm × width 56–58 mm). To
increase contact surface pressure of the raceway intesting of bearing B, the number of rolling elements
was reduced from the standard 17 to 5.

Fig. 6 Schematic drawing of the life test rig
Table 2 Conditions of the life test
Item

Bearing A

Bearing B

Basic dynamic
radial load rating

48.5 kN

Conventional
product: 150 kN
ETFA: 168 kN(Number of rolling
elements: 5)

Radial load17.64 kN

53.9 kN

Axial load

1.47 kN

29.4 kNMax. contact
surface pressure

2.5 GPa

Conventional
product: 2.9 GPa
ETFA: 2.8 GPa

Inner ring rotational
speed

2,000 min-1

500 min-1

Lubricating oil
viscosity

ISO VG 56

ISO VG 100

Contaminant typeHigh speed toolsteel dustHigh speed toolsteel dust

Contaminant size

100 - 180 μm

100 - 180 μm

Contaminant
hardness

700 - 800 HV

700 - 800 HV

Amount ofcontaminant inlubricating oil

1.0 g/L

0.4 g/L

3.2 Rolling fatigue life under clean lubrication
conditions

After operation for 10,000 h or more in all cases,
the ETFA bearings were stopped in undamaged
condition. Therefore, life under clean lubrication
conditions was clearly longer than the basic rating life
under these test conditions (L10h = 243 h).
NTN TECHNICAL REVIEW No.88

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“ETFA” Bearings Strengthened by Fine Microstructure Design

3.3 Rolling fatigue life under contaminatedlubrication conditions

Fig. 7 shows results of the test under contaminatedlubrication conditions. Rolling fatigue life of the ETFA
bearing under contaminated lubrication conditions
is roughly 2 times or more greater than that of theconventional product.
Flaking due to rolling fatigue can be broadly divided
into two types depending on differences in the pointof damage origin: surface originated and subsurface
originated11)-13). In both cases, the magnitude ofstress concentration due to rolling has an effect onlife. Therefore, it is conjectured that ETFA has someeffectiveness in easing stress concentration during rolling.
In flaking under contaminated lubrication conditions,the damage originates at the margin of an indentation
formed by a hard foreign body. This sort of indentationoriginated flaking is thought to occur due to applicationof repeated normal stress at the surface normal to the
raceway tangent direction14). The fatigue strength of lowcarbon steel after carburizing improves with greaterrefinement of prior γ grain15). Therefore, rolling fatigue
life of the developed product under contaminatedlubrication conditions is thought to improve due to blockrefinement.

Cumulative damage probability %99
Conventional
product

80
50

ETFA
20
10
5
Bearing A
1
101

102

103

Life ha) Bearing A

4. ETFA impact strength
4.1 Impact test conditions

A Charpy impact test was carried out at room temperatureand low temperature (-20 ℃). The test specimen was the
U-notch type with dimensions of 55 mm × 10 mm × 10
mm. The test method conformed to JIS Z 2242.

4.2 Impact strength

Table 3 shows the results of the Charpy impact test.
Impact strength of ETFA was roughly 1.8 times that ofconventional material.
Table 3 Charpy impact values of the specimens atroom temperature and – 20 ℃

[J/cm2]

Test material
Test temperature
Room temperature
Low temperature (-20 ℃）

Conventional
material
5.3
5.0

ETFA
9.8
9.5

4.3 Reason for improving toughness

Fig. 8 shows observation images of the fracture surface
near the notch bottom of the impact test specimen,
and Cr mapping images of the fields of observation. A
SEM was used for observation, and energy-dispersive
X-ray spectroscopy (EDX) was used for Cr mapping.
Enlargements of the observed fields a) and d) are givenin b) and e). Mapping images were obtained for observed
fields b) and e). In addition, the region of ductile crack
formation prior to fracture is indicated with a dotted linein the images for observed fields a) and d)16).
The main fracture morphology of the conventional
material was grain boundary fracture at prior γ grainboundaries. The width of the ductile crack formation
region was about 5 μm. Also, dispersion of Cr-based
precipitations was confirmed at the prior γ grainboundary surface, and the size of the precipitations wasroughly 500 nm or less.
Conventional material

ETFA

Cumulative damage probability %99
80

Conventional
product

50
20
10

ETFA

5
Bearing B
1
101

102

Life h103b) Bearing B
Fig. 7 Results of the life tests under contaminatedlubrication conditions
Fig. 8 SEM-micrographs and EDX-mappings of the fracturesurfaces near the U-notch after impact test

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NTN TECHNICAL REVIEW No.88

On the other hand, the main fracture morphology ofETFA was transgranular fracture. Width of the ductile
crack formation region was about 10 m. Numerous
dimples were confirmed at the fracture surface, andtheir size was roughly 50 – 500 nm. Dispersion of Crbased precipitations was also confirmed at the fracturesurface. Their size was roughly 100 nm or less.
For the above reasons, it is thought that grainboundary segregation of precipitations containing Cr is
suppressed in ETFA, and prior γ grain boundaries arestrengthened17). Also, the region where ductile cracking
forms expands, and thus it is likely that fracture
stress has been increased due to transgranular plastic
deformation18)-20). Improved toughness likely occurred
due to refinement of blocks and precipitation structure.

5.2 Wear resistance

Table 5 shows the specific wear rate obtained fromthe wear test. The specific wear rate of ETFA wascompared with that of the conventional material, andit was about 1/2 or less. Based on the order of thespecific wear rate, it is conjectured that the main wear
mode is adhesive wear 22).


Table 5 Specific wear rate of the specimens[ × 10-10 mm3/(N･m)]

Test material
Opposing material

Conventional
material

ETFA

Conventional material

701

120

ETFA

357

166

5.3 Reason for improved wear resistance

5.ETFA wear resistance
5.1 Wear test conditions

A Savin type wear test was conducted at room
temperature21). Fig. 9 shows a schematic drawing of
the test rig, and Table 4 gives the test conditions. The
test specimen was sheet-shaped with dimensions
15 mm × 6 mm × 3 mm. The rotating circularplate serving as the opposing material has an outer
diameter of 40 mm, and a subcurvature of R 60.
Load

Test specimen
Opposing
material
Felt padFig. 9 Schematic drawing of Savin type wear test rig

Fig. 10 shows optical microscope images, SEM
images, and oxygen mapping images obtained
through EDX, of the test specimen wear traces.
The wear traces of the conventional material form
a number of streaks due to wear scratching in thesliding direction (vertical direction). ETFA, on the other
hand, exhibits wear scratches with a fine line width
over a wide range. Oxidation was evident in the region
where these wear scratches occurred.
Compared with conventional material, ETFA has
high toughness, and hard, fine precipitations are
dispersed homogeneously over the surface, and thoseprecipitations and high toughness are thought to raise
the shear resistance of the direct contact surface. As
a result, it is likely that severe wear was suppressed,and high surface pressure was maintained at thecontact surface, so that oxidation at the wear surface
was promoted 23)24).
Conventional material

ETFA

Table 4 Condition of the wear test
Item

Setting conditions

Load

50 N

Initial max. contact surfacepressure

0.49 GPa

Sliding speed

0.05 m/s

Sliding time

60 minLubricating oil viscosity grade

ISO VG 2

Lubrication type

Felt pad lubrication

Opposing material diameter

40 mm

Opposing material
sub-curvature

R 60

Opposing material surface
roughness

Ra 0.01 m

Fig. 10 Optical micrographs, SEM-micrographs andoxygen mappings of the wear traces for the
specimens

NTN TECHNICAL REVIEW No.88

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“ETFA” Bearings Strengthened by Fine Microstructure Design

6. Conclusion
This article has described in detail “ETFA” roller
bearings strengthened through fine microstructure
design. The rolling fatigue life and strengthcharacteristics of these bearings greatly surpass
conventional bearings, and thus ETFA can copewith the increasing severity of roller bearing use
environments.
We at NTN will continue improving our heat
treatment and material technologies, and contributing
to the development of roller bearings.

References

1) R. A. Grange, ASM Trans Quart., 59, (1966) 26.
2) G. Krauss, Deformation and Fracture in Martensitic
Carbon Steels Tempered at Low Temperatures,
Metallurgical and Materials Transactions A, (2001)
861.
3) Tadashi Maki, Imao Tamura, Morphology andSubstructure of Lath Martensite in Steels, Tetsuto-Hagane (Iron and Steel), 67, (1981) 852.
4) Yuuji Kimura, Kaneaki Tsuzaki, Trends inTechnology for Refinement of Martensite
Structure, The Special Steel, 52, (2003) 12.
5) S. Morito, H. Yoshida, T. Maki and X. Huang, Effectof Block Size on the Strength of Lath Martensite
in Low Carbon Steels, Materials Science andEngineering A, 438-440, (2006) 237.
6) Akinobu Shibata, Martensite in Iron-based Alloy̶the Formation Mechanism of Microstructure
and the Origin of Mechanical Property ̶, Materia
Japan, Bulletin of the Japan Institute of Metals, 50,(2011) 254.
7) Chikara Ohki, Improving Rolling Contact Fatigue
Life of Bearing Steels Through Grain Refinement,
SAE Tech. Paper Series, (2004) 01-0634.
8) Nobuyuki Mouri, Improvement of Carburized Steel
Wear Resistance by Heat Treatment, Tribology
Conference Proceedings, 2008, Fall, Nagoya.
9) Nobuyuki Mouri, Kazuhiko Taguchi, Improvementof Carburized Steel Wear Resistance by Heat
Treatment, NTN TECHNICAL REVIEW, No. 76,(2008) 17.
10) Kikuo Maeda, Hirokazu Nakashima, Hiroshi
Kashimura, Development of Long Life TAB and
ETA Bearings and Their Automotive Applications,
NTN TECHNICAL REVIEW, No. 65, (1996) 17.
Photo of authors

Masahiro YAMADA Naota YAMAMOTO
Advanced Technology
R&D Center

104

Application Engineering
Dept., Industrial Business
Headquarters

NTN TECHNICAL REVIEW No.88

Chikara OHKI
Advanced Technology
R&D Center

11) W. E. Littmann and R. L. Winder, Propagation ofContact Fatigue from Surface and Subsurface
Origins, Transactions of ASME, D, 88, (1966) 624.
12) T. E. Tallian and J. I. McCool, An Engineering Modelof Spalling Fatigue Failure in Rolling Contact,
Wear, 17, (1971) 447.
13) Y. P. Chiu, T. E. Tallian and J. I. McCool, An
Engineering Model of Spalling Fatigue Failure in
Rolling Contact, Wear, 17, (1971) 433.
14) Toru Ueda, Takashi Sakaguchi, Naoya Seno,
Shigeru Okita, Nobuaki Mitamura, Flaking
Morphology in a Contaminated Lubrication
Environment ̶ Effects of Tangential Force on
Indentation-Originated Flaking ̶, NSK Technical
Journal, 685, (2012) 58.
15) C. A. Apple and G. Krauss, Microcracking and
Fatigue in a Carburized Steel, MetallurgicalTransactions, 4, (1973) 1195.
16) Toshiro Kobayashi, Evaluation of Impact Fracture
Characteristics of Metal Materials at Low
Temperatures using the Instrumented Charpy Test
Method, Bulletin of the Japan Institute of Metals,(1973) 546.
17) Riichi Murakami, Yunhe Kim, Kazuhiro Kusukawa,Fundamentals of Material Strength and Fracture,
Nishinihonhouki Shuppan, (2005) 62.
18) Seiichi Karashima, Metal and Alloy Strength,
Japan Institute of Metals and Materials, (1972)
134.
19) Toshiro Kobayashi, Koichi Takai, Evaluation of
Mechanical Fracture Behavior of Metal Materials
at Low Temperature, Fuji Electric Journal, 46,(1973) 235.
20) T. Hanamura, F. Yin and K. Nagai, Ductile-Brittle
Transition Temperature of Ultrafine Ferrite/
Cementite Microstructure in a Low Carbon
Steel Controlled by Effective Grain Size, ISIJInternational, 44, (2004) 610.
21) Makio Mizuno, Measurement of Abrasion Loss,
Journal of Japan Society of Lubrication Engineers,25, (1980) 801.
22) Mechanical Engineers Handbook B1, Japan
Society of Mechanical Engineers, (1985) 61.
23) Tadashi Sasada, Wear, Yokendo, (2008) 48.
24) Yuji Yamamoto, Motohiro Kaneta, Tribology 2nd
Edition, Ohmsha, (2010) 195.

Technical Article

Application Examples and Function Improvements of theWrist Joint Module “i-WRIST™”
Keisuke KAZUNO*
Masaki KAGAMI*
Yuki SHIMURA*
Yukihiro NISHIO**

Hiroshi ISOBE*
Jun MIDOMAE*
Seigo SAKATA**
Naoki MARUI**NTN developed an angle control device that uses a parallel link mechanism
which is a type of constant velocity joint1)-7) and started mass production
under the product name “ i-WRIST™” in August 20188).
Since then, we received many inquiries for improved automation, laborsavings, improved appearance, improved inspection process and acquired
more advanced needs for product specifications. We have just developed theupgraded product to meet these needs and have significantly improved its
functions. We would like to introduce the upgraded product.

1. Introduction

2. i-WRISTTM Overview

The use of industrial robots is expanding at atremendous rate as a means of boosting productivity
due to increasing demand for automation and laborsaving methods. The shortage of labor is becoming aserious issue in Japan due to its shrinking population,and installation of industrial robots is gaining traction
as a means of overcoming this issue.
In 2012, NTN announced its angle control
equipment based on its proprietary parallel linkmechanism. It was marketed for greasing andcleaning applications that require fast and precisechanges in position that industrial robots havedifficulty with. After changing the name to “i-WRISTTM”
Wrist Joint Module from 2018 (product registration in2019), NTN began mass-production with the aim oftargeting visual inspection applications of automotive
and electronic components that had relied on manuallabor in the past.
This article focuses on visual inspection applications
developed by NTN, as well as the features,functionality and examples of compatible systems forthe further enhanced i-WRISTTM “IWS Series.”

Fig. 1 shows a conceptual diagram of i-WRISTTM,
which identifies the parallel link mechanism anddrive mechanism that control its positioning. The
three motors mounted on the drive mechanism areunder synchronous control to achieve two-degree of
freedom angular positioning (bending angle, swing
angle).
Controlling two of the three rows of link systems(1st to 3rd link systems) of i-WRISTTM can be used for
unique angular positioning, however motors mounted
to the link systems of all three rows eliminate
backlash in the drive mechanism and achieves greater
positioning accuracy.
Baseend side

Drive mechanism

1st link system

3rd link system

Topend side

Parallel linkmechanism2nd link system
Bendingangle

Swing angle

Fig. 1 Schematic of i-WRIST™

* Industrial Business Headquarters** New Business Search and Development Dept., New Product and Business Strategic Planning Headquarters
NTN TECHNICAL REVIEW No.88

105

Application Examples and Function Improvements of the Wrist Joint Module“i-WRIST™”② Dedicated controller③ Dedicated console

I/O，Ethernet

Controller(External interface)XXXXX XEnd effector
MECHATROLINK-Ⅲ*

Servo driver

Cabling
Servo motor

M1

M2

M3

Rz

X

Y

Stand

Z

Components supplied by NTN①, ②, ③①

Rotation
actuator

unit

Linearactuator

Linearactuator

Components purchased by customer(Specified by NTN)
Components purchased by customer(selected by customer)* MECHATROLINK-III is a registered trademark of MECHATROLINK Members Association

Fig. 2 Configuration example of automation equipment

Fig. 2 shows a basic configuration example ofautomation equipment that includes i-WRISTTM.
i-WRISTTM is incorporated into the equipment witha combination of linear actuators and rotation
actuators. NTN supplies ① i-WRISTTM, ② Dedicatedcontroller and ③ Dedicated console as a set, while theoverall equipment is designed and made by the user
or a system integrator.

pattern was set as ① → ② → ③ → ④ → ① → ④ → ③ →② → ① . The results show that i-WRISTTM completed
the pattern in 1.4 seconds compared to the pan
and tilt mechanism at 4.0 seconds, verifying that
i-WRISTTM moved 2.8-times faster.

①

2.1 i-WRISTTM Features

Production sites are implementing more methods
that use automation and labor-saving technologies
based on robots. Yet as robots are not perfect andsuffer from a range of issues, userʼs automation andlabor-saving efforts often do not go as planned, which
required more time and cost to develop dedicateddevices.
i-WRISTTM provides three main features that canresolve these issues faced by production sites.
1) High-speed operation
Conventional wrist joint modules with pan andtilt mechanisms move slowly when approaching
workpieces from many directions for visual inspectionsof complex shapes, which meant there was a limit in
the reduction of takt time. i-WRISTTM controls minutechanges in position at high speed, making it possible
to achieve further reductions in takt time.
A comparison of i-WRISTTM with pan and tilt
mechanisms is shown by the movement required forapproaching the four points in Fig. 3. The operating

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a) i-WRISTTM(comparison video)②③

b) Pan and tilt mechanism

Fig. 3 Operation pattern for comparison with view
from the end effector side2) Ease-of-use
The majority of industrial robot manufacturers
generally use their own proprietary robot language,
which required specialist knowledge for the
programming when changing over or teaching therobots.
In contrast, changing configuration parametersor teaching i-WRISTTM can be performed on the
dedicated console screen, eliminating the need for any
specialist programming language expertise. Operating
patterns are created using the dedicated console
screen shown in Fig. 4 to enter each coordinatedirectly for the i-WRISTTM or associated linear
actuators. Another method involves moving i-WRISTTM
or linear actuators to their desired positions using JOG
operations and then registering the coordinates. NTN
also provides software (Fig. 5) for creating operating
patterns efficiently using a computer. These i-WRISTTM
operations can be acquired by simply taking a halfday course．

Height approx.
2m

Fig. 4 Operation screen of dedicated console

Input area for position coordinates and operating direction

Width approx.
0.8 m

Depth approx. 0.85 m

Fig. 6 Compact equipment (image)
Conventional robot

3D modelOperatingscreenInput area for operating conditions

Fig. 5 Point data edit software3) Space efficiency
The problems with using robots to achieve
automation is the installation space or the majorchanges required to existing production lines.
i-WRISTTM is designed to be used with a
combination of linear actuators and rotation
actuators. The configuration of the system thatrotates workpieces requires around half the strokeof the linear actuator (XY stage), which makes theoverall system size more compact. To operate on aworkpiece around ø100 mm and a height of 100 mm
in size, NTN managed to design a system size with awidth of 800 mm and a depth of 850 mm, meaning
automation equipment can be installed in the space
where manual labor is performed (Fig. 6).
Fig. 7 also shows an example image comparing the
installation size when building automation equipmentusing a conventional robot (multijoint robot) ori-WRISTTM. This example illustrates how using
i-WRISTTM means a system can be built using aroundhalf the space.
As such, i-WRISTTM can help resolve problems suchas insufficient space at a production site.

Approx. 1.5 m

Approx. 0.8 m

Fig. 7 Installation size comparison example ofautomation equipment (image)

3. Example Application of Visual Inspection
SystemSince beginning mass-production of i-WRISTTM
in 2018, NTN has mainly marketed it for visualinspection applications. In particular, it has beenpraised for its capability of approaching workpieces
with complex shapes from many directions at a highspeed. Specific examples of target workpieces include
aluminum diecast components, forged parts, pressedmoldings and resin moldings.

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Application Examples and Function Improvements of the Wrist Joint Module“i-WRIST™”

3.1 Example of Off-line System Configuration

Fig. 8 shows an example configuration of a visualinspection system designed as an off-line system. In
this configuration, i-WRISTTM is installed on a stand ina downward attitude, and a camera and lighting unit
are mounted to the end of i-WRISTTM. The workpieceis moved and rotated with linear actuators (XYZ
stage) and a rotation actuator.
This configuration allows workpieces to beapproached from multiple directions with angle
changes of i-WRISTTM and workpiece movement/
rotation, which means the linear actuator (XY stage)
stroke can be shortened for a more compact overallsystem size.
Linear actuator (Z stage)i-WRISTTM

Linear actuator (Y stage)

4. i-WRISTTM "IWS Series"
While the product had been marketed toward visualinspection applications until now, the i-WRISTTM “IWS
Series (hereafter, IWS Series)” has been developed toaddress new needs.
The main functions of the IWS Series are as below.

4.1 i-WRISTTM Installation Direction

Fig. 10 shows the typical installation directions anddefinitions of coordinates for the bending angle andswing angle.
When mass-production began in 2018, theinstallation direction was limited to that shown in
Fig. 10 a).
The IWS Series has extended functionality forcalculating the i-WRISTTM angle and coordinates ofassociated actuators that are used, to also enableinstallation in the upward (Fig. 10 b)) and vertical(Fig. 10 c)) directions.

Workpiece
Rotation actuator
Bending
angle

Swing
angle

Linear actuator (X stage)

Fig. 8 Configuration example of off-line systema) Downward

3.2 Example of In-line System Configuration

Fig. 9 shows an example configuration of a visualinspection system designed as an in-line system. In
this configuration the i-WRISTTM with a camera andlighting unit is mounted in a downward attitude to thelinear actuator (XYZ stage). This allows it to approach
workpieces from many directions while they are
moving along an existing conveyor belt.
Linear actuator (X stage)
Linear actuator (Z stage)

Swing
angle

Bendingangle

Linear actuator (Y stage)b) Upward

i-WRISTTM
Workpiece

Swing
angle
Bendingangle

Conveyor belt

Fig. 9 Configuration example of in-line systemc) VerticalFig. 10 Installation direction

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4.2 Compatible System Configurations

The IWS Series significantly expands on the various
possible system configurations of i-WRISTTM and theassociated linear actuators and rotation actuator thatcan be used.
Devices that can be mounted to i-WRISTTM had
been limited to the end effector (camera and lighting
unit for visual inspection applications), however with
the IWS Series, workpieces can now be mounted.
This gives users a much broader range of system
configuration options to select from.
There are two i-WRISTTM coordinate systems: a“base coordinate system” based on the orthogonal
coordinates of the entire system, which defines theangle of the i-WRISTTM and rotation actuator as well as
the position coordinates of the linear actuators; anda “work coordinate system” based on the orthogonal
coordinates of the workpiece that defines the position
and working direction of the workpiece. Similar tothe previous designs, the system has been developed
using the software included in the dedicatedcontroller for calculating the coordinates of the entire
system. This allows coordinates to be calculated forthe system without the need for users or system
integrators to develop and test complex coordinate
calculations.
Fig. 11 shows typical examples of system
configurations compatible with the IWS Series.
Users can select a configuration to suit their desired
application.
Configuration examples (1) and (2) are system
configurations that are compatible with the previous
system, and correspond to the configurations shownin Fig. 8 and Fig. 9 above.
Configuration examples (3), (4) and (5) areextended system configurations that are now
available with the IWS Series. Configuration example(3) rotates i-WRISTTM with the rotation actuator sothat the direction of the end effector can be adjusted
to suit the position of i-WRISTTM. Example (4) is aconfiguration for handling workpieces and is suited
to visual inspections of small, lightweight workpieces
like resin molded parts. Configuration example (5)has i-WRISTTM installed on a rotation actuator at an
inclined angle, where the rotation actuator is actuallymounted on a linear actuator to enable positioning
in the XYZ stage directions. This configuration means
workpieces can be viewed from a lower angled
approach, and in visual inspection applications, meansinspections can be performed around almost theentire workpiece.

Linear actuator(Z stage)Linear actuator(X stage)Linear actuator(Y stage)(end effector)

Linear actuator(Z stage)

Rotation actuator(workpiece)
Linear actuator(Y stage)(end effector)

Linear actuator(X stage)(workpiece)

Example configuration (1)Example configuration (2)Rotation actuator
Linear actuator(Z stage)Linear actuator(Z stage)(end effector)(end effector)(workpiece)(workpiece)
Linear actuator(Y stage)Linear actuator(Y stage)Linear actuator(X stage)

Example configuration (3)Linear actuator(Y stage)Linear actuator(X stage)

Example configuration (4)Linear actuator(X stage)Linear actuator(Z stage)
Rotation actuator(workpiece)(end effector)

Example configuration (5)Fig. 11 Typical examples of system configurations

4.3 Compliance with Safety Standards

IWS Series has been designed with specifications
that meet ʻISO10218-1 (JIS B 8433-1)ʼ standards forthe safety of industrial robots.
The first area of compliance with the IWS Series
is the addition of a motor with a brake for the servo
motor that acts as the i-WRISTTM drive mechanism.
In the event of a power outage or an unexpectedalarm being triggered, the brake stops the motor fromrotating so there is no unexpected movement.
Operating modes have also been defined clearly as“Auto mode,” “Manual low-speed mode” and “Manualhigh-speed mode” to comply with safety standards
for industrial robots. An example of this is when users
switch from auto mode to manual mode to performteaching̶operation will be forced to manual lowspeed mode. The dedicated controller software has
been updated to ensure that the movement speed ofthe linear actuator is 250 mm/s or less as specified in
the safety standards.
As i-WRISTTM is a module product (embedded
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Application Examples and Function Improvements of the Wrist Joint Module“i-WRIST™”
system) that forms part of automation equipment,
users or system integrators are expected to install
appropriate safety and protective devices stipulatedin ISO10218.

4.4 External Interface

The IWS Series helps to boost convenience andversatility with greater support for external interfaces.
The external interface of the previous i-WRISTTM
only included a parallel input/output (I/O) for
connecting with the userʼs controller. Operating
patterns created in advance via teaching using thededicated console can be specified with I/O signals
from the userʼs controller, which provides compatibility
with sequential control that issues commands tobegin automated operation. The I/O provides fastercommunication speeds than serial communications
such as “Ethernet” and other industry networking
protocols and enables control using relatively simple
signals, a feature that has always been highly regarded
by users.
In addition to the existing I/O, the IWS Seriesalso includes “Ethernet” to significantly increase
the amount of possible data communications suchas connecting to user controllers from multiple FA
equipment manufacturers. This means point data(coordinates) of operating patterns can be loaded
and written to the dedicated controller from userʼscontrollers to increase the flexibility of operating
commands. For instance, after a workpiece is loaded
onto visual inspection equipment and scanned withimage processing, the coordinates and angles forapproaching the workpiece can be changed before
automatically starting inspection operations. Anotherpractical example is completing visual inspections
with a specific sequence of operating patterns, andthen performing additional inspections at a desired
coordinate or angle.

References

1) Keisuke Sone, Hiroshi Isobe, Koji Yamada, WideAngle Active Link Equipment, NTN TECHNICALREVIEW, No. 71, (2003) 70-73.
2) Hiroshi Isobe, Yukihiro Nishio, Parallel Link High
Speed Angle Control Equipment (PHACE), NTN
TECHNICAL REVIEW, No. 80, (2012) 42-47.
3) Hiroshi Isobe, Yukihiro Nishio, Keisuke Sone,
Hiroyuki Yamada, Yoshio Fujikawa, Parallel Link
High Speed Angle Control Equipment, Proceedingsof the Japan Society for Precision Engineering
Spring Meeting in 2013, (2013) 809- 810.
4) Hiroshi Isobe, Yukihiro Nishio, Seigo Sakata, Naoya
Konagai, Hiroyuki Yamada, Yoshio Fujikawa, Parallel
Link High Speed Angle Control Equipment Implementation on Grease Application Equipment -,Proceedings of the Japan Society for Precision
Engineering Spring Meeting in 2014, (2014) 10871088.
5) Naoya Konagai, Hiroshi Isobe, Seigo Sakata,Kenzou Nose, Hiroyuki Yamada, Yoshio Fujikawa,
Parallel Link High Speed Angle Control Equipment,
Proceedings of the Japan Society for Precision
Engineering Spring Meeting in 2015, (2015) 605606.
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 theJapan Society for Precision Engineering Spring
Meeting in 2016, (2016) 483-484.
7) Kenzou Nose, Hiroshi Isobe, Seigo Sakata, Speeding
Up of Parallel Link Angle Control Equipment, NTN
TECHNICAL REVIEW, No. 84, (2016) 96-101.
8) Keisuke Kazuno, Hiroshi Isobe, Jun Midoumae,
Yuuki Shimura, Masayuki Ohara, Development of“i-WRISTTM” Wrist Joint Module, NTN TECHNICALREVIEW, No. 86, (2018) 22-27.

5. Summary
Since the development of the angle control
equipment in 2012 based on the proprietary parallel
link mechanism, NTN has been marketing it for
greasing and cleaning applications. To take fulladvantage of its capabilities, i-WRISTTM was developedand started being mass-produced in 2018 for visualinspection applications. Feedback received after
mass-production has now been included as standard
specifications and developed as the IWS Series.
With further declines in the working populationin the future, demand for automation is expected to
grow even more for applications like visual inspections
that had previously relied on manual work. In
addition to visual inspection applications, NTN is also
focusing more on development of robotic productsthat can help enhance automation and laborsaving technologies at worksites as well as increase
productivity and quality as a way of contributing tosociety.

Photo of authors (Representative)

Keisuke KAZUNO
Industrial Business
Headquarters

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Technical Article

Deployment and Improved Reliability of the Condition
Monitoring System for Wind Turbines
Katsuyoshi SUZUKI*NTN is providing information services using condition monitoring
system for wind turbines. This article introduces market trends of the
CMS and gives some future prospects of the CMS service for windturbines.

1. Introduction
Unparalleled natural disasters and abnormal
weather are frequently occurring all over the world.
Urgent action is needed to reduce CO2 emissions andother greenhouse gases, that are an underlying causeof these phenomena, and to achieve a low-carbon
society. Therefore, renewable energy is garnering
a great deal of attention; as solar and wind power
generation is increasing worldwide.
Wind power in Japan has been trailing behind
Europe and the U.S., but is currently expanding due tonational policies such as a Feed-in-Tariff (FIT) system.
Due to Japanʼs location, it faces a harsher climatic
environment than Europe, caused by factors suchas sudden squalls, turbulence, typhoons, and winterlightning, making problems with wind turbines not
unusual. Accelerated steps are being taken to help
achieve stable operation, by accurately assessing theequipmentʼs condition, and conducting appropriate
and efficient maintenance to reduce downtime, which
includes emergency shutdowns.

2. NTNʼs vision
In wind power sector, NTN supplies specialpurpose bearings, these bearings play a vital role inensuring wind turbines achieve higher performanceand a larger size. We also help improve the efficiencyof wind turbine maintenance by offering a windturbine Condition Monitoring System (CMS) and CMS
services employing that system.
CMS services is aimed at monitoring anddiagnosing the condition of wind turbines based ondata such as vibration and rotational speed, gatheredfrom the wind turbines. In addition to the highquality manufacturing (“monodzukuri ”) that NTN has
cultivated over many years, we are focusing efforts aspart of our “service and solution business” strategy ofproviding intangible products such as information andproposals beneficial to our users.
NTNʼs CMS has a track record of adoption in over

200 wind turbines, with the number of installations
increasing year after year. We are highly regarded bypower producers as a top supplier in Japan, in partdue to our CMS services.

3. CMS for wind turbines
3.1 History of CMS for wind turbines

Full-fledged adoption of CMS for wind turbines
began in Europe around 2000 with the goal of
detecting drivetrain problems, caused by sporadic
malfunctions. With the dissemination of on-shore
wind throughout Europe and the growth of offshore wind primarily in Northern Europe, the CMS
is increasingly being used as a standard tool for
condition based maintenance (CBM).
On the other hand, Japanʼs domestic wind industry
has focused on conventional time based maintenance(TBM). Until recently, maintenance procedures suchas replacing parts, diagnosing equipment, checking
for abnormal noise, and analyzing grease andlubricating oil, have only been carried out periodically.
However, there were also cases of overtreatment orover-servicing, where parts that had not reached theirservice life were repaired to prevent malfunction,and cases where entire units were replaced due
to risk management concerns. Additionally, whenidentifying problem points, we often relied on theheuristics of skilled maintenance workers. Although
the dissemination and growth of the CMS in Europe
was well known in Japan, one factor which delayed
adoption in Japan were the features of the CMS ̶ i.e.,
data analysis capability, accuracy of information, andcost effectiveness ̶ which had not been fully verified.
Recently, the importance of CMS adoption and data
analysis have been increasingly recognized for windturbines in Japan, due to the increase in active use of
IoT and big data in various fields.

* Industrial Business Headquarters
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Deployment and Improved Reliability of the Condition Monitoring System for Wind Turbines

3.2 Needs of wind power producers

Wind power producers aim to increase theiravailability factor, and lower running costs by reducing
downtime from inspections, repair, and emergency
shutdown. With that goal in mind, power producers
use the following criteria for maintenance.
1) Beginning in early autumn and continuing into
early spring, when wind conditions are stable, thetop priority is power generation, and unplanned
maintenance is avoided.
2) Large-scale maintenance is concentrated in thesummer.
3) To minimize shutdown time, investigating
failures and resolving problems such as abnormal
noise or vibration, analysis and repairs must be
done quickly and efficiently.
4) Faulty parts and equipment must be accurately
identified and necessary parts obtained in aplanned manner.

The system configuration is shown in Fig. 1. Data
gathered from wind turbines is recorded and stored
in a cloud server. Through various types of signal
processing and analysis inside the server, the system
detects problems with the components of the drive
train, and automatically notifies a preset point ofcontact (automatic first decision).
If signs of damage or changes are identified
from additional diagnosis by our engineers, that
information is communicated through a report withrecommendations to the service subscriber.
Automatic collection/storage of data

Automatic 1st decision

Monitoring,
analysis

Equipment tocollect data

3.3 Advantages of CMS for wind turbines

Wind turbines are built based on meticulous
surveys of wind conditions, stringent environmental
assessments, and consent from the local area.
Construction sites are often difficult to access, as they
typically include remote areas, such as mountaintops,
or offshore locations far from population. Additionally,
the nacelle, which houses the wind turbine drivetrain,
is located 60–80 m above ground, preventing
servicemen from climbing the tower in cases wherestrong winds or lightning is present. Furthermore,when identifying issues, it is best to directly checkfor vibration or abnormal noise during operation.
However, due to safety, work cannot be performed
during the power generation operation. Therefore,
the usual approach is to carry out inspection whilein an operation shutdown condition or during lowspeed rotation, where there is a risk of missing signsof malfunction.
In response to the concerns above, adoption of
the CMS enables remote monitoring of the drivetrainof wind turbines even during the power generation
operation. The CMS also enables identification ofproblem points and early detection of malfunction
by identifying slight changes in conditions from
data. These condition changes are difficult to pickup, despite the skilled maintenance workers. The
CMS has made it possible to determine whether
maintenance is needed, and gauge the urgency, sothat replacement parts can be obtained and replacedat periodic inspection or other planned shutdowns.
This efficiency helps reduce shutdown periods andlost profits.

3.4 Configuration of Wind DoctorTM system

In 2012, NTN marketed Wind DoctorTM, a CMS forwind turbines. Since then, we have offered monitoring
results and condition information, derived from thegathered data, to power producers, wind turbine
manufacturers, and showcased the effectiveness of
the CMS through discussion of wind turbine events.

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Appropriate
maintenance

User

Report of information/results

Fig. 1 System configuration of Wind Doctor™
When monitoring changes or trends in mechanicalequipment, it is best to conduct measurements while
the equipment is running at constant speed. However,wind turbines interact with nature, wind speed
and direction is constantly changing, leading to an
unavoidable decline in monitoring accuracy. In orderto minimize the effects of the operating conditions,
the Wind DoctorTM records those conditions together
with sensor signals. We then focus on similar
conditions within the data set, extract applicable data,
and use that for diagnosis.

3.5 The uniqueness of Wind DoctorTM

In addition to providing standard CMS functions,
we strive to differentiate Wind DoctorTM from otherproducts. Examples of this are described in 3.5.1 to3.5.3.

3.5.1 Event recording function

Normally, the CMS performs measurements witha fixed sampling cycle. When monitoring trends ofa wind turbine with no problems, sampling a few
times a day is sufficient. On the other hand, it is best
to shorten the measurement intervals in order toevaluate changes and record sudden events caused byworsening conditions on wind turbines experiencing
issues. Setting the interval to zero and acquiring
data continuously is ideal; however, this increases
the operating load on the CMS unit, and causes anenormous increase in the amount of recording doneby the server, increasing difficulty.
Thus, the Wind DoctorTM is equipped with anadditional event recorder function. If an external signal
trigger or a preset internal threshold is exceeded,
then the function automatically records measurement

data for a certain time before and after, taking thatmoment as a starting point. This enables data to be
captured when a sudden disturbance or malfunction
occurred during the interval, and use of the data inanalysis. A specific application example is described in
section 4.

15 min-1; therefore, the issue of vibrational energy
due to damage is small, and the S/N ratio isextremely low. To improve analysis accuracy withWind DoctorTM, weʼve incorporated the technique ofidentifying disturbance noise using the spectrogram
shown in Fig. 3.

3.5.2 CMS dedicated communication unit

Frequency Hz

12,000Fig. 2 Communication Unit

10,500
9,000
7,500
6,000
4,500
3,000
1,500
0

5

10

15

20

25

30

35

40

Time Sec

Fig. 3 Spectrogram
To conduct high-accuracy vibration analysis of
the rotor, ideally data is measured while rotating at
a constant speed, but as mentioned above, a windturbine interacts with natural unpredictable wind, andthus fluctuations in rotational speed inevitably arise,as indicated in Fig. 4. Analysis accuracy declines dueto these fluctuations, and in some cases it becomeschallenging to detect abnormalities.

Fluctuation in rotational speed %There are two methods of sending the CMS data:using the wind turbine LAN, or installing an additional
CMS dedicated line with a separate channel.
The former can use the existing communication
environment of the wind turbine; however, a robust
network security must be provided to prevent
unauthorized access and viruses. In contrast, whena CMS dedicated line is installed, it is completely
isolated from the wind turbine LAN. In the eventof unauthorized access to the wind turbine or the
CMS, the effects will not spill over to the other side.
The CMS dedicated communication unit in Fig. 2
is a data communication tool employing a mobile
phone line and router, and self-checking functions are
better than the commercial communication modules
called IoT gateways. More specifically, the unit isequipped with a function for automatic recovery
if malfunctions occur such as a communication
error or communication freeze due to a drop inelectromagnetic wave strength.
The unit is also linked with a data acquisition
module, and setting can be done to force a reboot of
the data acquisition module when issues arise in themeasurement operation.

20
10
0
-10
-20

0

5

10

15

20

25

30

35

40

Time Sec

Fig. 4 Rotation fluctuation

3.5.3 Improved analysis accuracy

A wind turbine is composed of multiple drive
units and parts, and operates under conditions
exposed to the natural environment. Various typesof noise inevitably occur inside the nacelle, andsuperposition of this noise on the sensor signal isunavoidable. Specifically, there is a mixture of noisefrom various directions around the main shaft, which
is fastened solidly to the nacelle frame. Examples
include transmitted vibration from the hub, operation
vibration from the motor for yaw drive, squeaking
due to wind gusts and stress, operation vibrationof hydraulic pumps, and so forth. Additionally, therotational speed of the main shaft is low, at about

As a countermeasure, Wind DoctorTM has a function
to correct fluctuations in rotational speed. Fig. 5
shows the raw frequency spectrum before correction,
and Fig. 6 shows the frequency spectrum after
correction. The corrected graph clarifies the frequency
peaks and shows the effectiveness of the correction.

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0.30
0.25
0.20
0.15
0.10
0.05
0

5

10

15

20

25

30

35

40

45

50

Frequency Hz

Fig. 5 Raw frequency spectrum

Vibration acceleration m/s2

Vibration acceleration m/s2

Deployment and Improved Reliability of the Condition Monitoring System for Wind Turbines

0.30
0.25
0.20
0.15
0.10
0.05
0

5

10

15

20

25

30

35

40

45

50

Frequency Hz

Fig. 6 Corrected frequency spectrum

4. Event recording function applications
This section describes applications of the event
recording function in 3.5.1.
Presently, the New Energy and Industrial Technology
Development Organization (NEDO) is engaged in a
research and development project on technology formore sophisticated management of wind turbines. The
goal of this project is to raise the availability factor ofwind turbines to 97 % or higher, with the support of
the University of Tokyo, National Institute of Advanced
Science and Technology (AIST), and Chubu University
to establish the necessary technology.
Chubu University is working to achieve moresophisticated wind turbine management by organicallylinking lightning detection technology with windturbine issues, and developing an understanding oflightning strike characteristics. They are specificallyusing a lightning detection system exemplified by
the Rogowskii coil in Fig. 7 to analyze the effects
of lightning strikes. This involves comprehensive
comparative examination of: ① Lightning strike datasuch as current wave crest values and electric charge,② Lightning damage data such as malfunction points
and conditions, and ③ SCADA and CMS data.
Winter lightning, a peculiar phenomenon in Japan,
has become the catalyst for these efforts. The coastsof Hokuriku and Tohoku regions of Japan are suitablesites for wind turbines due to their favorable windconditions. However, extremely high energy lightning,
strikes the wind turbines frequently during the winterin these regions. These frequent strikes lead toconstant accidents, where the blades of wind turbines
become damaged. To prevent secondary damage
in the surrounding area from fragmented pieces
falling from lightning strikes, it becomes necessary
to shut down operations, and promptly carry out aninspection, either visual or using other techniques,
before resuming operations. Decline in the availability
factor due to lightning issues and the associatedresponse is 4-5 %. Therefore, there is a demand for
operation improvements that help raise the availability
factor, such as enabling remote decision-making todetermine whether to operate or shutdown based ondata analysis.

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Rogowskii coilFig. 7 Rogowskii coil
NTN is not a direct participant in this NEDO project,
but we are assisting by using the event recording
function of Wind DoctorTM to acquire and accumulate
data during normal conditions and lightning strikes,
and making that information available to the project.
The configuration of the system is shown in Fig. 8.
A lightning detection system at the base of the tower
detects the high current that flows during a lightning
strike, and sends out a lightning strike signal. This istransmitted to a signal repeater in the nacelle, via
optical fibers installed inside the tower, and input asa trigger signal to the data acquisition module. The
event recorder function inside the Wind DoctorTM
activates, and records vibration data for a specified
time before and after the lightning strike.
We have installed this system in multiple windturbines in the Hokuriku region, and are continuing togather data.

6. Conclusion

Signal repeater
Lightning strikecurrent

Data acquisition
moduleOptical fiberLightning strikecurrent

Lightning detector(Rogowskii coil)Fig. 8 CMS with Lightning detection system

5. Issues for the future
As previously noted, it is difficult to detect problems
due to the low signal level at parts which rotate at
low speed such as the main shaft and gearbox input.
Maintenance costs are high for these components,
and there is a pressing need to accurately assess theirstatus. Therefore, as a provider of the CMS services,
we need to establish highly accurate problem
detection technology. Another important issue isdetecting indications of parts missing or falling out,
which may lead to sudden accidents. Users have highexpectations from more sophisticated CMS.
Additionally, further evolution of the entire system
is needed, including improvements to the malfunction
detection performance, implementing machinelearning and other types of AI, suited for cutting-edge
technical innovation, and linking with prediction ofdamage magnitude and progression.

This article has covered market trends, efforts by
NTN, and future prospects relating to CMS for windturbines.
Broader use of renewable energy is the key toprotecting the environment and achieving a lowcarbon society. Therefore, we need to discoverequipment issues, detect malfunctions early, andcarry out proper maintenance. We are convincedthat the CMS and its associated services will play anincreasingly important role in this sector.
Going forward, we will continue advancing by
integrating tangible and intangible assets through IoT,
and contribute to society through a variety of services.

References

1) Wataru Hatakeyama, The Recent Applicationsof the Condition Monitoring Systems for Wind
Turbines, THE TRIBOLOGY, No. 356, (2017) 38- 41.
2) Makoto Miyazaki, Wataru Hatakeyama, Applicationof Condition Monitoring System (CMS) for Wind
Turbines, NTN TECHNICAL REVIEW, No.86, (2018)40-44.
3) NEDO Fiscal 2019 Result Report Meeting,“Research and Development on Technology
for Wind Power Generation, etc. / Researchand Development for Sophisticated Practical
Application of Wind Power / Research andDevelopment on More Sophisticated Technology
for Wind Turbine Management”

Photo of authors

Katsuyoshi SUZUKI
Industrial Business
Headquarters

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Award Winning Products
Nippon Brand Award of 2019 “CHO” MONODZUKURI Innovative Parts and Components Award

Hub Bearing Module with
Steering Adjust Function “sHUB™”
Hirokazu OHBA* Satoshi UTSUNOMIYA* Norio ISHIHARA* Yusuke OHATA* Atsushi ITO*

Vehicle1. Introduction
In order to tackle environmental challenges andcreate a society free of traffic accidents, new vehicles
are being developed incorporating self-driving andelectric propulsion to help improve automobile safety
and reduce energy consumption.
sHUBTM assists in achieving safety, comfort, andenergy-efficient driving in internal combustion engine
vehicles, electric vehicles, and self-driving vehicles, byproperly applying correct control to the tire angles tosuit any situation, with varying speeds.
In recognition of the details stated above, sHUBTM
received the Nippon Brand Award of the 2019 “CHO”
MONODZUKURI Innovative Parts and Components
Award.

This hub bearing components include a steering
shaft and an actuator incorporating a linear motion
mechanism, fastened to a knuckle, and tires aresteered by the actuatorʼs power, with the steering axisof the hub bearing as the center.
Steering axis

Actuator

KnuckleExisting steeringsystemActuator
Steering
operation
Vehicle speed

Steering
axis

Controller

Fig. 2 Vehicle with sHUB™ installed(on front wheels)

4. Summary

2. Structure

KnuckleRange of angle for adjustments±3.5°

Hub bearing

Fig. 1 Structure of the sHUB™

3. Features
The following characteristics of the vehicle shown in
Fig. 2 have been improved by providing control usingsHUBTM for the vehicleʼs front wheels:
1) Improved vehicle stability when driving straight2) Cornering performance3) Safe control of tire angle before tires slip while
maneuvering
4) Reduction of running resistance and improved fuelconsumption

We are currently moving toward commercializationand testing is taking place for this product in test
vehicles. As indicated in another article in thisreview we are also working on creating sHUBTM forrear wheels, and we expect to increase automobile
safety through the development of new automotive
products.

References

1) Norio Ishihara, Hirokazu Ohba, Atsushi Ito,
Mitsunori Ishibashi, Makoto Yamakado, Yoshio
Kano, Masato Abe, Hub Bearing with Steering
Function that Improves Vehicle Dynamic
Performance, NTN TECHNICAL REVIEW, No. 86,(2018) 84-90.
2) Satoshi Utsunomiya, Norio Ishihara, Yusuke Ohata,
Atsushi Ito, Hub Bearing Module with Steering
Adjust Function (sHUBTM), NTN TECHNICALREVIEW, No. 87, (2019) 18-23.
Photo of authors (Representative)

Hirokazu OHBA** New Business Search and Development Dept., New Product and Business Strategic Planning Headquarters

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NTN TECHNICAL REVIEW No.88

Award Winning Products
New Energy Foundation Chairman Award (field of products and services) of New Energy Awards 2019

Micro Hydro Turbine
Hiroki MUKAI* Fumihiko MATSUURA* Takashi ITO* Tomoya KAWAI** Yasunari KANAMURA*

1. Introduction

3. Product Specifications of Typical Model

Micro Hydro Turbine (Fig. 1) received the New
Energy Foundation Chairman Award at the 2019 New
Energy Awards hosted by the New Energy Foundation.
The New Energy Awards commends products thatare outstanding in promoting adoption and raisingawareness of new energy. Micro Hydro Turbine
generates power by simply placing it in an existingwater channel with a flowing current. It is very friendly
to the global environment and is contributing to SDGs
on a global scale. It is highly regarded because it helps
to reduce maintenance costs of irrigation facilities
by supplying and selling power at locations without
electrical infrastructure.

The model with a blade diameter of 90 cm achieves
a rated output of 1 kW when the flow rate is 2 m/s(Table 1).
Table 1 Product Specifications of Typical Models
TurbinePropeller hydro turbine for flowing water

Generator

Permanent magnet synchronous generator

Blade diameter

60 cm, 90 cm, 130 cm

Rated power
output1 kW (90 cm model at flow rate of 2 m/s)

Recommended
water channel

At least 100 cm width and depth

Size/weight

H190 cm × W230 cm* × D170 cm, 170 kg*Varies depending on the width of the water channel.

4. SummaryFig. 1 Micro Hydro Turbine

2. Product Design
Micro Hydro Turbine is shown in Fig. 2. The turbine
consists of high-efficiency blades connected to a
generator that is mounted on a frame for installation
on water channels. There are two types of controllers
for power generation; one for charging batteries
as an independent power source and the other forconnecting to the grid so that generated power canbe sold. This versatility allows the product to respondto broad market requirements for power generation.

Generator

Battery or gridconnection

Ordinary hydro turbines require major construction
work to create different water levels. The issue withmajor construction is its cost and damage to theenvironment. Micro Hydro Turbine can be installedby simply placing the unit on an existing water
channel with the beams fitted to the width of thewater channel, with little to no construction costs andenvironmental impact.

References

1) Tomoya Kawai et al., Micro Hydro Turbine, NTN
TECHNICAL REVIEW No. 84, (2016) 28-33.
2) Takashi Ito et al., Grid Connectable NTN Micro
Hydro Turbine, NTN TECHNICAL REVIEW No. 86,(2018) 102-107.

Photo of authors (Representative)

Strut
Support frames(both sides)

Controller
High-efficiency blade

Fig. 2 Configuration of Micro Hydro Turbine

Hiroki MUKAI** Engineering Department, Green Energy Products Division** Business Development Dept., Green Energy Products Division
NTN TECHNICAL REVIEW No.88

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Award Winning Products
2019 Japanese Society of Tribologists (JAST) Encouragement Award

Mechanism for Initiation of Peeling in Rolling Contact andthe Effect of Black Oxide Treatment on the Suppression of
Peeling (Part 1, Part 2)
Naoya HASEGAWA

Takumi FUJITA

1. Introduction
Papers1)2) submitted to the Tribologist , the Journalof the Japanese Society of Tribologists, received thesocietyʼs Encouragement Award for FY2019. The
following provides an overview of these papers.

Michimasa UCHIDATE

Masayoshi ABO

smaller with black oxide treatment and only the protrusionsof the black oxide layer wear during rolling (Fig. 1).

2. Overview
This research examined the initiation mechanism of
peeling, a type of roller bearing failure which occurs underpoor lubrication conditions, and the effect of black oxide
treatment on the rolling contact surface to suppress peeling.
In the first paper1), a peeling simulation test (RCF test) was
carried out with a two - cylinder type tester. The mechanismof initial crack initiation of peeling was investigated based on
observation of the rolling contact surface and various analysis
results. In the second paper2), the repeated stress acting at
the true contact point was estimated by examining the rolling
contact surface and measurement of residual stress. The
relationship with progression of peeling was also investigated.
As a result of the above efforts, the following findings wereobtained.
① The initial crack of peeling originated at a notch formed
due to plastic contact by roughness asperity of the rolling
contact surface.
② At the rolling contact surface, maximum shear stress acts ina direction inclined by about 45°from the radial direction.
It is likely that the initial crack of peeling progressed due tothe action of this maximum shear stress.
③ When black oxide treatment was applied to the rolling
contact surface, it promoted a drop in surface roughness(running-in), and the repeated stress acting at the truecontact point was lessened. As a result, there was a
reduction in the formation of notches due to plastic
deformation, and it became more difficult for peeling tooccur.
④ In the black oxide product, running-in is promotedby phenomenon where the surface roughness become

Fig. 1 Mechanism of running-in on rolling contact
surface with the black oxide layer 1)

3. Future development
The findings obtained in this research will contribute toguidelines for the development of life enhancements forroller bearings. Going forward, we hope to make use of
these findings to offer users bearings with higher reliability.

References

1) Naoya Hasegawa, Takumi Fujita, Michimasa Uchidate,
Masayoshi Abo, Mechanism for Initiation of Peeling in
Rolling Contact and the Effect of Black Oxide Treatment on
the Suppression of Peeling (Part 1) ̶ Discussion of Crack
Initiation Based on Experimental Results, Tribologist, 63, 8,(2018) 551-562.
2) Naoya Hasegawa, Takumi Fujita, Michimasa Uchidate,
Masayoshi Abo, Mechanism for Initiation of Peeling in
Rolling Contact and the Effect of Black Oxide Treatment on
the Suppression of Peeling (Part 2) ̶ Relationship between
Progression of Peeling and Stress Under the Rolling Contact
Surface, Tribologist, 63, 9, (2018) 618-628.

Photo of authors

Naoya HASEGAWA

Takumi FUJITA

Michimasa UCHIDATE

Masayoshi ABO

Advanced Technology
R&D Center

Advanced Technology
R&D Center

Faculty of Science andEngineering,
Iwate University

School of Engineering,
University of Hyogo

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NTN TECHNICAL REVIEW No.88

Our Line of New Product

Inch Series, SAFC/SAFD Plummer Blocks

High-strength plummer blocks with a new design, resistant to shock and vibration

X bar shape on bottom for reinforcement

Features① High strength② Dust and moisture resistant③ Improved ease of handling④ Multi-purpose

X bar shape on bottom to help reinforce base, greater thicknessof top and side parts for reinforcement
Prevents ingress of water, mud, and debris by using labyrinth
seals
Use of alignment pins in the base for easier mounting andremoval of outer housing halves
Enables mounting of accessory parts (taconite seal, closing
cover, etc.) to suit various environmental needs

Specifications
Material
Size
Shaft
diameterGray cast iron

(SAFC series)

Ductile cast iron (SAFD series) 15 % increase in static fracture strength (compared to SAFC)
SAFC/SAFD509 – SAFC/SAFD544
ϕ1-7/16 inch (ϕ36 mm) – ϕ7-15/16 inch (ϕ202 mm)

Application
Steel facilities, mine facilities, transportation port facilities, etc.

NTN TECHNICAL REVIEW No.88

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Our Line of New Product

Tenter Clip Bearings for Film Stretching Machine

Low torque and high durability, while dramatically improving grease leak resistance
Newly-developed cage withlarger inner diameter

Outer ring
Ball
Inner ringNon-contact seal

Enclosure oflong-life grease* Representative illustration showing medium
temperature specifications

Structure① High reliability

By using a newly-developed cage with larger innerdiameter and long-life grease, together with a noncontact seal bearing, this product achieves both low
torque and grease leakage resistance, and improved
seizure resistanceGrease leakage reduced by 70 %(compared to conventional
product)② High durability Seizure resistance improved by
40 % (compared to conventional
product)③ Low torque
Bearing rotational torque 1/4 thatof a contact seal bearing

Developed
product

Applications
Guide rollers for tenter clips of film stretching machines
Lineup includes:
1) Medium-temperature specifications for bearing
temperatures up to 230 ℃*
2) High-temperature specifications for bearing
temperatures up to 300 ℃

Clearance between
cage and inner ring

Features

Movement of grease to the outer diameter sideof the inner ring is suppressed and flow to theoutside is prevented by optimizing clearance
between the cage and inner ring.

Stretching and transferof plastic film

Conventional
product
Tenter clip movement

Grease scraped from the ball surface by thecage may accumulate on the inner diametersurface of the cage, move to the outer diametersurface of the inner ring, continue on to the seal
groove, and flow to the outside.

Tenter clip

Clips film

Rail

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NTN TECHNICAL REVIEW No.88

Bearing