For New Technology NetworkR

TECHNICAL REVIEW No.83

http://www.ntn.co.jp

Special Issue

corporation

Special Issue for
Automotive Module Products
October 2015

Automotive Module Products
corporation

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

ISSN 0915-0528 OSAKA, JAPANMechanical clutch unit (MCU)
for next-generation steering

Ball screw drive module
for electrohydraulic brakes

Clutcht h forf seatt lifterlift

NTN TECHNICAL REVIEW No.83
Printed and published in March 31, 2018
EditorSensor embedded
third-generation hub bearings

Press connect spline hub joint(PCS-H/J)

Various bearings

Yoshinori TERASAKA

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

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

R

TECHNICAL REVIEWSpecial Issue;

No.83 Automotive Module Products

corporation

NTN TECHNICAL REVIEW No.83
CONTENTS
Preface

Yoshinori TERASAKA

1Contribution

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile
into Autonomous Service Robots

2

Hisashi TAKAHASHI

Perspective

Professor, Graduate School of Science and Technology, Shizuoka Institute of Science and Technology

Electrification of the Automobile and Module Products of NTN
Makoto OKASAKA

16● Special Issue for Automotive Module Products
Two Motor On-board Drive System
Ryou YUKISHIMA, Hidenori KARASAWA, Aiko MYOUKI, Katsunori SATOU, Yusuke MAKINO, Takeshi KANDA and Tomohiro SUGAI

Rear-wheel Independent Steering System
Yasuhiro MATSUNAGA, Tatsuji INOUE, Hironori TOKUNAGA and Yukihiro NISHIOMCU (Mechanical Clutch Unit) for Next Generation Steering
Naotsugu KITAYAMA and Shintaro ISHIKAWA

Clutch for Seat Lifter
Masayoshi YAMADA and Yasumasa HIBI

Next-generation Drive-shaft “ADS Module”
Masafumi OOSUGI and Tatsuro SUGIYAMATechnology Trends and Products for Accessory Drive Belt Systems
Ayumi AKIYAMA and Hiroo MORIMOTO

Auto Tensioner with Variable Damper Mechanism for ISG-equipped Engines
Hiroo MORIMOTO and Yuta MOCHIZUKI

Products Introduction of Composite Material for Automotive
Kayo SAKAI, Eiichirou SHIMAZU, Takahiro GOTOU, Hajime ASADA and Takuya ISHII

20

26

32

36

40

43

48

54

● Technical Papers New Products
Modeling of Retained Austenite Transformation in Bearing Steels
Christine SIDOROFF, Pierre DIERICKX and Pr. Michel PEREZ

Evaluation of Rolling Contact Fatigue by X-ray Diffraction Ring
Naoya KAMURA, Takumi FUJITA and Toshihiko SASAKI

Influence of Induction Heating Conditions on Bearing Steel Characteristics
Hiroshi YUKI and Miyu SATO

Grease Lubricated High-speed Deep Groove Ball Bearing for EV and HEV Motor
Masahiko SATODA and Goro NAKAO

Deep Groove Ball Bearing for Alternator Corresponding to 200˚C
Tomoyoshi IZUTSU and Goro NAKAO

3D Microgeometry Measuring Machine
Hiroaki OBA

High-speed Microscopic Coating Applicator
Akihiro YAMANAKA

60

67

75

81

86

91

98

● Award Winning Products“CHO” MONODZUKURI Innovative Parts and Components Award, "2014 Automotive Components Award"

Mechanical Clutch Unit for Direct Adaptive Steering

104

Naotsugu KITAYAMA and Shintaro ISHIKAWA
AVEC'14 "Best Paper Award"

Roll Control Using Tire Longitudinal Forces Integrated with Tire Force Distribution of
Full Drive-by-Wire Electric Vehicle

105

Yuta SUZUKI, Tomohiro SUGAI, Junichi HIRATA and Nobuyuki SUZUKI

Our Line of New Products

106

NTN TECHNICAL REVIEW No.83（2015）[ Preface ]Message for the Automotive Module Products Special Issue

Yoshinori TERASAKAManaging Director

Due to growth of vehicles in emerging markets, the total number of vehicles produced globallyhas reached well over 80 million units, which has created an imminent challenge of reducing fuelconsumption for mitigating global warming. In this regard, auto manufacturers are responding tothis challenge by introducing eco-friendly vehicles with various modified drive systems such as
vehicles with improved traditional internal-combustion engines, hybrid vehicles and electricvehicles.
For addressing these social requirements, NTN is not only exploring further improvement inminiaturization/lightweight technology, low friction technology and long-life technology that wehave been successfully developing, but also accelerating development of various highly functional
module products by combining electronic control technology to these technologies.
On this occasion of the 44th Tokyo Motor Show from Oct. 28 to Nov. 8, with the theme of “YourHeart Will Race”, this special issue was created featuring state-of-the-art module
products/technology for vehicles. In this issue, we have asked Professor Hisashi Takahashi of
Shizuoka Institute of Science and Technology to explain the trend of motor control technology,
which is indispensable for module products.
In addition, we are introducing “automotive module products” in this special issue, including a
two-motor on-board driving system characterized by both high motion performance and safetyfunctionality, MCU for next-generation steering systems and a next-generation lightweight drive
shaft, “ADS Module”, that uses new bonding technology. Further, we present basic research
papers that affect the material evaluation methods and material properties, as well as bearing
products for automotive use such as the grease lubricated high-speed deep groove ball bearing
and bearing for alternators that can withstand 200˚C.
At NTN, we will celebrate our 100th anniversary in fiscal year 2017. Starting fiscal year 2015
through fiscal year 2018, we will drive our mid-term management plan "NTN100" building
foundation for ongoing growth in the next 100 years, following our theme, “For New Technology
Network: We shall contribute to international society through creating new technologies anddeveloping new products,” contributing for sustainable development of the industry through our
eco-friendly product development.

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NTN TECHNICAL REVIEW No.83（2015）[ Contribution ]Highly Efficient and Responsive Motor Drive Technologies
Enable Robotics to Evolve Automobile into Autonomous Service Robots

Hisashi TAKAHASHI
Professor, Graduate School of Science and Technology,
Shizuoka Institute of Science and Technology

Electric motor-driven automobiles(EV) are getting a great deal of attention overconventional internal combustion engine-powered automobiles since EV is seen as
a potential solution to the environmental pollution and energy crisis possible in the
not too distant future. Electric motors, the power source of EV, are responsive incontrolling the torque and it makes precise control of regenerative brake and vehicle
body dynamics possible. Making use of the advantage of electric motor, byincorporating the vehicle navigation system, imaging, and radar technologies, which
are now affordable to wide range of automobiles, automobiles are being transformed
from a means just to transport to a machine of robotics for even safer, more efficient, and more comfortable mobility. To
make this happen, smaller-sized and highly efficient motors and inverters, and others like batteries with greater capacityand extended longevity are in great demand. This paper presents newest or most recent technologies essential fordeveloping those building blocks in the automobile evolution including motor control technologies, application technologies
of inverter and converter, sensorless control technologies, how to deal with the problem caused from the current leakage,and others which are existing and waiting extended study.

On the other hand, since EVs use generators to
produce electric power, the driving force can beobtained without the need of a mechanicaltransmission, depending on the design of the motors
and controllers. In addition, with in-wheel motors where
the motors are installed inside the hubs, ( i.e. a motoris placed in each wheel) power can be produced bymultiple distributed motors, which significantly
increases the freedom of vehicle layout design.
Furthermore, EVs have higher torqueresponsiveness than ICEs and are capable ofproducing any desired torque in positive or negativerotation ranges by controlling the motors. The
technology to regenerate, store and reuse the motion
and potential energy of vehicles is impractical toincorporate with conventional ICEs. By introducing adrive system utilizing multiple motors, the driving power
can be more precisely controlled in order to enable
vehicle attitude control and stable driving.
By combining new control systems, such as a highresponse control system for motor torque, a steer-bywire system and various sensors, it is possible tocreate vehicles with higher performance andfunctionality. By adopting car navigation systems,
cameras, radars, etc., the robotization of automobiles

1. Introduction
Solutions for problems associated with global
environment and energy consumption have become amajor focus. Expectations for a shift from vehicles
using internal combustion engines (ICEs) , which
consume a lot of energy and emit CO2, to eco-friendly
electric vehicles, is increasing. For electric vehicles togain momentum, the development and broad utilizationof compact and long-life batteries, batteries with fastcharging capabilities and a large number of charging
stations are expected; however, the shift to electricvehicles without internal combustion engines that
consume fossil fuel will continue to take place.
EVs can incorporate new functions and features
which have been impractical for conventional vehicles
with internal combustion engines, thus a new vehiclecommunity is about to be created. In many cases,
vehicles which utilize fossil fuel have their internal
combustion engines placed in the front or back of thevehicles, and the energy produced by ICEs istransmitted to the wheels through the transmission anddrive train. This arrangement prevented freedom ofdesign in the placement of the different components.

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Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots

In order to achieve compact, lightweight and highly
efficient ICEs, the turbo charger, which is a device that
supplies a compressed mixture of fuel and air orcompressed air into the cylinders, is coming into the
spotlight. This device uses exhaust gas from theinternal combustion engine to turn a turbine to drive
compressors in order to create compression. However,at lower rotation speeds, sufficient charging is notavailable, resulting in a delayed response called turbo
lag.
Currently, charging systems called electricallyassisted turbo chargers or super chargers, which havefast responses even at lower rotation speeds, arebeing developed by combining recent electronic control
technology and high performance motors 1)-5). The
electric super charger system is composed of acompressor and motor to easily control the charging
pressure and capacity without depending on theexhaust pressure from the internal combustion engine,which contributes to a faster response.
These devices often use permanent magnet
synchronous motors (PMSM) driven at 100,000 –
200,000 rotations/min. To achieve these high speed
rotations, the motor drive frequency can reach several
kHz. Therefore, reduction of motor winding electric
time constants, improvement of power density andsmall form factors, and creating inverter systems thatenable precise power control are required. Mechanicalcomponents, such as bearings for high speed rotationthat can withstand vibration and high temperatures
must also be developed.
The motors used for electrically assisted turbo
chargers and super chargers are required to be driven
at high efficiency and controlled with precise
identification of the magnetic pole positions of the rotor.
Resolvers are often used for detecting the magnetic
pole positions, which makes the motor system size, as
well as the detection circuits and detection wiring,increase, resulting in higher costs. Consequently,
systems to accurately estimate the magnetic polepositions without using rotor position sensors areneeded.
Fig. 1 shows the transient analysis result of a 1.4
liter gas engine to reach the target torque. As shown in
the figure, the rise time can be shortened by 35%
compared with conventional turbo chargers using
exhaust gas.

that enables safer transportation is about to begin.
For EVs to gain popularity, the development ofcompact and high-efficiency motors, the achievementof higher efficiency in drive circuits such as inverters
and converters and the development of low-costbatteries with high capacity, long life and low loss,
capable of high speed input/output of large currents, isrequired. Permanent magnet synchronous motors(PMSM) that use rare earth magnets such as NdFeB
magnets are often used as the motors for driving
power.
However, for manufacturing cost reduction and toensure a stable supply, the use of induction motors(IM) and switched reluctance motors (SRM) that do not
use rare earth magnets is expected to increase.
For EVs, the battery voltage can be 200V or more,
the drive voltage of the main drive motors can be 500Vor more and the output of the motors can be 100kW or
more. For drive circuits, low-loss power elements thatcan be used under high voltage and high currents are
required.
In particular , low-loss power devices allow thecooling structure of inverters to be changed from watercooling to air cooling, which contributes to a compact
size and low cost, as well as the efficient use ofelectricity. Heat radiation design and measures against
heat are essential. Also, drive methods, control circuits
and implementation technologies for simplifying heatradiation systems are required. To achieve thesemethods, higher performance of power devices andefficiency in controlling methods are required, as wellas the technology development of control systems formore efficient energy regeneration and storage ofregenerated energy. It is also important to develop
measures against noise emitted from the inverters andconverters to protect the electronic data.
In this paper, the current state and futuretechnologies such as recent motor control technology,
properties of various motors, application technologies
for inverters and converters, sensor-less control
methods and issues associated with leakage currents
due to motorization are described.

2. Recent control technologies
2.1 Higher efficiency of ICEs
Whether powered by ICEs or electric motors,
completely new vehicles are required to be highly
energy efficient and environmentally friendly with low
emissions.
ICEs for automobiles are now being equipped with
control systems using advanced controls and motors
for the purpose of making the ICEs highly efficient andenvironmentally friendly.
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NTN TECHNICAL REVIEW No.83（2015）

1.2
1.0

motoring and regeneration modes are used in amanner similar to moving forward. In the motoring
mode, the inverter transmits the electrical energy fromthe battery to the motor and in the regenerative mode,the motor operates in generative mode, transmitting
kinetic energy from the motor to the battery. Therefore,
the converter, which controls the motor drive voltage,
needs to convert electric power in both directions.
Fig. 3 shows the rotation speed - torquecharacteristics required for the main driving motor. In
the case of automobiles, the motor is used in the whole
region surrounded by the line. Therefore, the motors
for the main engine are required to have the following
properties.

Comparison of transient response(engine rotation: 2000 rpm)
△35％

Target torque

Torque

0.8
0.6
0.4

Turbo charger
Electrically assisted turbo charger
2-stage turbo(electric super charger + turbo charger)

0.2
0.2
-1

0

1

2

Time s34(1) Compact and lightweight
The weight of the motor increases the rolling friction
between the tires and the road surface and climbing
torque on slopes; therefore, light weight is an importantfactor. In addition, a structure to quickly radiate jouleheat produced inside the motor due to copper loss andiron loss is indispensable.

Fig. 1 Example of transient response ofinternal combustion engine 3)
Source: Yukio Yamashita, Seiichi Ibaraki, et al., Development of Electric Supercharger.
Contributing to the Downsizing of Automobile Engine, Mitsubishi Heavy Industries
Technical Review, Vol. 47 No.4, pp.12-17 (2010) Fig. 3.

2.2 Properties required for motors of automobiles
The motors used in automobiles are required tooperate from below -40 to +85˚C indoors and frombelow -40 to +120˚C in engine rooms (in some cases,+150˚C or more). In addition, motors positioned inplaces other than the cabin must have a structure thatavoids any problems caused by the adhesion of ironsand contained in dust and sand. Furthermore, highspeed response is also required along with the
improvement of the control performance.
For auxiliary motors, low-cost brushed DC motors(DCM) with simple control circuits are often used.
In applications where compactness, long-life and low
noise are required or high efficiency is required,permanent magnet synchronous motors (PMSM) are
used.
Motors used as the main engine of electric vehicles
are required to have various rotation speed/torqueproperties depending on the driving conditions such as
low speed/high torque application where start/stop
operations are frequently repeated, driving on flat
terrain or climbing slopes and driving on highways.
Motors for main engines operate in four quadrants, as
shown in Fig. 2. When moving forward, the direction ofrotation (forward direction) and torque are both positive
and the motor is used in the motoring mode where theelectrical energy is converted to mechanical energy.
When decelerated, the direction of rotation is positive,
however, torque is negative, resulting in the generativemode where the mechanical energy is converted into
electrical energy. Moving in reverse, the direction ofrotation and torque are simply reversed, where both

CW rotation speed
Forwardregenerating

Forwardmotoring

Negative
torquePositive
torqueReversemotoring

Reverseregenerating

CCW rotation speed
Fig. 2 Motor condition quadrants

TorqueClimbing/
accelerating
Normal driving
Driving inurban area

High speed
driving
Rotation speed

Fig. 3 Demanded characteristics of main driving motor-4-

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots(2) Large power density

prevent the windings from ejecting out due tocentrifugal force during high speed rotation will be
required.
These types of motors are expected to exhibit highefficiency when driven at constant high speed for along distance; however, efficiency drops with repetitive
starts and stops over short distances.

There is an increase in the maximum output powerof the motors. It is feasible by increasing the maximum
torque and rotation speed. However, to increase therotation speed, there are still many challenges such asincreasing the mechanical strength of the rotor,
development of bearings capable of high speed
rotation and the reduction of the electric time constantof the inverters and motors that can support motor
drive frequencies of several kHz.

2.3.2 Permanent magnet synchronous motor(PMSM)(3) High efficiency

There are two types: the surface magnet (SPMSM)
type, where the magnets are mounted on the surfaceof the rotor, and the interior magnet (IPMSM) type,where the magnet is embedded inside the rotor. For
the main drive application, IPMSM, which can use
magnet torque and reluctance torque, is widely used.
There are two types of windings, distributed orconcentrated and the distributed winding is used forthe effective utilization of field weakening. It is widelyused as the main drive motor for EVs and hybrid
vehicles since it has high efficiency, even in theapplication of repetitive starts and stops over shortdistances.
Vector control can be used for field weakening toachieve high speed rotation; however, efficiency is
reduced.
Also, since the coercive force of rare earth magnets
such as NdFeB magnets decreases at hightemperatures, resulting in demagnetization andreduction in the magnetic force, care should be taken
in limiting the operating temperatures.
IPMSM has the following properties, compared withSPMSM.
(a) Since the permanent magnets are embedded
inside the rotators, cracking and scattering ofmagnets due to high speed rotation is unlikely.
(b) SPMSM requires the permanent magnet to beprocessed in an arch whereas IPMSM can use
plate-shaped magnets, resulting in manufacturing
cost reduction.
(c) As both torques from the magnet and reluctancecan be used, high torque is achieved.
(d) Taking advantage of rotor saliency, the position
sensor-less control system can be easily
configured.
(e) Leakage flux may be generated at the end of themagnet.
(f) As the q-axis inductance is large, q-axis armature
reaction is large, likely to be affected by theincrease of terminal voltage and magnetic
saturation.

Since the EVs are powered by batteries, the travel
distance from the stored energy is an issue. Losses
with motors are broadly classified into machine loss,
iron loss and copper loss. Machine loss comes fromloss from winding and bearing loss of the rotor. Iron
loss is a loss in the magnetic circuit and can be
reduced by selection of magnetic steel sheets andconfiguration of magnetic circuits. Copper loss is thejoule loss from winding resistance and currents, andcan be reduced by reducing the winding resistance.
Since the copper loss is proportional to the square of
the current, loss reduction is more effective when thedrive voltage is increased and the current is reduced toobtain the same power output. In this case, theconfiguration of drive circuits and measures against
leak currents will become critical technical challenges,
as will be mentioned later.

2.3 Types and characteristics of main drive motors
The main drive motor for EVs is used with the direct
current from the battery. Motors with the least
consumable parts are used here. They are mainly
induction motors (IM), permanent magnet synchronous
motor (PMSM), synchronous reluctance motor (SyRM)
and switched reluctance motors (SRM). In Japan,
permanent magnet synchronous motors (PMSM)
driven by 3-phase power are generally used.
These motors are expected to be compact,
lightweight and capable of producing high power output
in order to attain longer travel distance, with low
vibration and low noise. A wide range of rotation
speed must be supported from standstill to high speed
driving, and high efficiency is desired for effective useof the battery power.

2.3.1 Induction motor (IM)Induction motors are relatively inexpensive androbust, and highly efficient control by vector control isalso available. In addition, since permanent magnets
are not used, they are less susceptible to the operating
environment. However, since the rotor includes
windings, the cooling systems to radiate joule heat are
difficult to build. In addition, mechanical structures to-5-

NTN TECHNICAL REVIEW No.83（2015）

2.3.3 IPMSM capable of highly efficient control

T= p×Ψa×iq = p×Ψa×Ia

When the d-axis current of IPMSM is set as id, theq-axis current as iq, effective value of phase current as
Ie, and the current phase asβ, then the following
relations can be found:provided, la = iq .
With SPMSM, since the d-axis current is 0, the suppliedcurrent for the same torque will become the minimum,
resulting in high efficiency. On the other hand, with
IPMSM, since the reluctance torque Tr cannot be used,the control system must be built with special consideration.
With IPMSM, current phaseβthat maximizes the motor
generated torque T exists. That is, it is possible togenerate torque most efficiently for the current la. The
current phase la to obtain the maximum torque can be
expressed in the following equation:

Ia = iq2＋id 2 = 3 Ie
id = −Ia sin(β)
iq = Ia cos(β)

The torque from the permanent magnets of IPMSM,

Tm can be obtained from the following equation when
the number of pole pairs is set as p:
Tm = p ×Ψa×iq = p×Ψa×Ia cos(β)

On the other hand, torque from reluctance Tr can beobtained by the following equation when the numberof pole pairs is p:

β=sin-1p
Tr = p (Ld −Lq)×id ×iq = −(Ld −Lq)×Ia2×sin(2β)−Ψa + Ψa2 +8(Lq −Ld)2×Ia24(Lq −Ld)×Ia

The d-axis current id and the q-axis current iq in thisequation are as follows:2

where, Ψe is the effective value of the interlinkage fluxof one phase of the permanent magnets: Ψa= 3 Ψe .
The overall torque of IPMSM, T is the sum of thetorque from the permanent magnets Tm and torque fromreluctance Tr :id = −Ia sin(β)

iq = −Ia cos(β)
Highly efficient motor control can be achieved byobtaining the current phaseβ for the motor current laand controlling the d-axis current id and the q-axis
current iq of IPMSM.

T = Tm +Tr= p ×Ψa×iq +p(Ld −Lq)×id ×iqp= p×Ψa×Ia cos(β)+ − (Ld −Lq)×Ia2×sin(2β)
22.3.4 Synchronous reluctance motor (SyRM)
and switched reluctance motor (SRM)

Since the torque from the permanent magnets Tm canbe expressed as

SyRM and SRM both use reluctance torque, notpermanent magnets. With SyRM, the rotor is driven by
the rotating magnetic field created by the windings,
similar to inductance motors and permanent magnet
synchronous motors, while the method of SRM drives
the rotor by providing currents to the windings applying
switched voltages. It is necessary to drive these motors
by detecting or estimating the rotor position in order togain an efficient reluctance torque.
The position sensor-less driving method of SRM will
be described later.
Since these motors do not use permanent magnets
and the rotors are made of iron core, their application
is expected to increase. In addition, the traditional
issues of vibration and noise associated with SRM arebeing addressed in recent research with developmentof structures with lower vibration and noise, for broader
implementation.

Tm = p×Ψa×Ia cos(β)
the maximum torque from the permanent magnets is
obtained when the current phaseβ= 0 degrees. On theother hand, torque from reluctance Tr can be obtainedby the following equation when the number of polepairs is p:p
Tr = −×(Ld −Lq)×Ia2×sin(2β)
2Therefore, the current phaseβ for the reluctance torque

Tr becomes the maximum at either 45 or -135 degrees.

Hence, the current phaseβ that creates the overall
torque of IPMSM, is in the range of 0 - 45 degrees. Also,
since the torque from the permanent magnets Tm is
proportional to the current and the torque from thereluctance, Tr is proportional to the square of the current,
and the current phaseβincreases as current la increases.
IPMSM is characterized by the controllability of the
maximum torque by controlling the current phaseβ.
When the d-axis current id is controlled to be always
set to 0, the motor generated torque T is proportional to
the q-axis current iq as shown in the following equation:

2.4 In-wheel motor
In-wheel motors are the built-in motors within thewheels, with the direct drive method which drives thetires directly by motors, and the reducer method, which
incorporates gear speed reducers within the motors.
-6-

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots(a) Non-insulated converter

RectifiercircuitConverter
circuit

Battery

Converter
circuit

Battery

The DC-DC converters with electric coupling
between the input-side power supply and output-side
power supply are called non-insulated type. In thistype, in general, the ground is commonly used asshown in Fig. 4 (a). On the other hand, the insulated
type provides no DC coupling between the input-side
power supply and output-side power supply using atransformer in between. The voltage obtained from thetransformer is converted to DC voltage through therectifier for output. The selection of type depends onthe circuit type used; however, the insulated type ismore effective for suppressing diffusion of noise.

They use multi-pole permanent magnet synchronous
motors (PMSM), and outer-rotor type and pancake
type are used to obtain large torque with the directdrive method and inner rotor type is also used with the
reducer method.
Even if the reducer method requires an increased
number of reducer components, since it can easily gainhigh torque, compact and lightweight motors can beachieved. The mechanical design using in-wheel
motors is significantly different from the on-board
method using differential gears and transmission,increasing the degree of freedom in the vehicle design.
With the in-wheel motor method, each wheel can be
independently and precisely controlled, significantly
enhancing driving stability and performance 6)-10).
However, the increased unsprung weight below thesuspension may adversely affect the comfort level ofpassengers due to lower suspension responsiveness.
Therefore, vehicle systems design must be madeprecisely.
In addition, the cooling system to radiate the heatfrom copper loss and iron loss of motor windings
applied to each wheel is a critical technology.

(b) Insulated converter

Fig. 4 Non-insulated and insulated converters

2.5.1 Non-insulated type bidirectional converter
In general, it is configured with circuits with acommon ground and is capable of transmitting powerin both directions. Fig. 5 shows a non-insulatedbidirectional converter circuit using MOSFET(surrounded by dotted line) and an inverter circuit. This
circuit is able to create the voltage supplied to theinverter, by stepping up the DC supply voltage from thebattery.
In this case, MOSFET1 remains OFF and the duty
ratio of the ON/OFF switch for MOSFET2 is controlled
so that the terminal voltage Vc of the capacitor C, is set
at the specified voltage.
On the other hand, when the regenerative energy is
generated, the current (charge) is supplied to thecapacitor C through the inverter, to increase the

2.5 Converter and inverter
The power source of the main drive motor is DC
power from the battery. A converter is a device thattransforms DC voltage to any desired DC voltage, andan inverter is a device that creates AC voltage from DC
voltage. The converter is required to supply electric
power from the battery to the inverter and motor in the
motoring mode and convert kinetic energy to electricenergy and store it in the battery in the regenerating
mode. Therefore, the converter is required to have thecapability to flow the current in both directions. Those
converters are called bidirectional step-up/down DCDC converters and used in the power supply circuits of
EVs.

Non-insulated bidirectional
converter circuit

Inverter circuit

Fig. 5 Non-insulated bi-directional converter and inverter circuits designed with MOSFETs-7-

NTN TECHNICAL REVIEW No.83（2015）

terminal voltage of the capacitor Vc. The control circuit
controls the capacitor voltage Vc, and when the voltage
exceeds the specified voltage, it adjusts the duty ratioof the ON/OFF switch of MOSFET1 to allow the energy
stored in capacitor C to be appropriately stored in thebattery.

is PWM controlled so that the regenerative current(current from V4) or current supplied to the battery(charging current) becomes the specified value.

2.5.3 Inverter
Inverters are the circuits to generate AC voltage
from DC voltage. Three-phase permanent magnet
synchronous motors are usually used for vehicles,
because of the gains in efficiency. The basic circuit
configuration of the three-phase inverters consists of 6
power devices, as Fig. 7 shows, and each powerdevice is controlled by switching. The wiring depictedwith bold lines in the figure carries large currents.
Wiring is implemented using copper bars, etc. toreduce the impact of inductance.
Inverters control power devices with PWM to supply
sinusoidal current to the motor windings by switching.
The higher the switching frequency (PWM frequency),
the more the reduction of motor magnetic noise andcurrent ripples can be achieved. As the switching lossof power devices is proportional to PWM frequency,
the vehicle systems use a frequency of 5-10 kHz.
Currently, IGBT based on silicon (Si) is widely usedas power devices for vehicle drive systems; however,PWM frequency, circuit configuration, radiation
method, etc. are expected to change significantly,
when devices using silicon carbide (SiC) and gallium
nitride (GaN) emerge.
For the main drive motor of vehicles, permanent
magnet synchronous motors of 50-100 kW are used
and the voltage of approx. 500V is used for2.5.2 Insulated type bidirectional converter
The insulated type bidirectional converter has its
input/output terminals decoupled from the DC
standpoint but is able to control bidirectionaltransmission of power. Fig. 6 shows an example of an
insulated type bidirectional converter circuit capable ofstepping up/down the voltage. The following is themechanism of power transmission.

(1) Transmitting power from V1 to V4(i) In the case of V1 > V4
S1, S2, S7, S8, S9, S10 and S12 are all turned OFF.
S3 and S6, as well as S4 and S5 are alternately
switched ON/OFF at the duty ratio of 50%. The voltage
V2 and V3 will become close to voltage V1. S11 is
PWM controlled so that the output voltage V4 becomes
the specified voltage.

(ii) In the case of V1 < V4
S1, S7, S8, S9, S10 and S12 are all turned OFF. S3and S6, as well as S4 and S5 are alternately switched
ON/OFF at the duty ratio of 50%. S11 remains ON. S2
is PWM controlled so that voltage V4 becomes thedesired output voltage. Voltage V2 and V3 will become
higher than voltage V1.

(2) Transmitting power from V4 to V1(i) In the case of V4 > V1
S2, S3, S4, S5, S6, S11 and S12 are all turned OFF.
S7 and S10, as well as S8 and S9 are alternately
switched ON/OFF at the duty ratio of 50%. S1 is PWM
controlled so that the regenerative current (current
from V4) or current supplied to the battery (charging
current) becomes the specified value.

Current sensor(ii) In the case of V4 < V1
S2, S3, S4, S5, S6 and S11 are all turned OFF. S7 andS10, as well as S8 and S9 are alternately switched
ON/OFF at the duty ratio of 50%. S1 remains ON. S12Inverter control circuit

Fig. 7 Three-phase inverter circuit

Fig. 6 Insulated bi-directional step up & down converter circuit-8-

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots

compactness and high efficiency, supplying large
currents. The motor voltage is expected to increase
further along with the development of power devices.
Therefore, ingenuity in design is needed to maintainthe insulation of the wiring and providing thicker wiring(bus) to avoid the impact from inductance, as well as
reducing the inductance. In general, copper bars are
frequently used to configure the wiring. Also,
capacitors of approx. several 100 μF with good highfrequency characteristics (devices with small ESR and
ESL, such as ceramic capacitors) need to be inserted
between power supplies into each arm of the inverterin order to reduce the noise due to switching.
Since the power source voltage supplied to inverters
has large fluctuations, a control system with fast
response is required to obtain the specified motor
torque even with the voltage fluctuation. Also, a small
form factor is required for installation in the vehicles,with special consideration on the configuration of thecooling system.
For selecting power devices, it is required toconsider sufficient capabilities for supply voltage andmotor drive current. The devices are required to havesmall switching losses at high PWM frequencies, aswell. In addition, inverter control circuits for driving thedevices in high speed are also required.

(a) Distributed winding(b) Concentrated winding

Fig. 8 PMSM windings

conductive vehicle body and capacitance is present
between the windings and the motor enclosure.
The inverter controls power by PWM control(switching control) to supply adequate current to
PMSM based on the torque command. The powerdevice in the drive circuit provides high frequency
switching. The stray impedance between the power
supply and the load connected with the inverter andvehicle body is the main cause of electromagnetic
noise and leakage current.
Fig. 9 shows the stray impedance in the motor drive
circuit. In the figure, Cf 1 to Cf 5 indicate examples ofstray capacitance, Lf 1 and Lf 2 indicate examples ofstray inductance. Stray capacitance produces leakage
current and diffuses noise when the voltage changes
quickly. On the other hand, stray inductance produceshigh voltage when the current changes quickly, which
may destroy the device. Transient voltage and leakage
current due to stray inductance and capacitanceproduce noise.
Since motors have stray capacitance between thewindings and the motor enclosure, leakage currents
are produced inside the motor enclosure when PWM
voltage is applied to the motor windings as it acts on
the stray capacitance, etc. that exists in electronic
circuits such as batteries and inverters.

2.6 Leakage current of vehicles
Permanent magnet synchronous motors (PMSM) of
50-100 kW are used as the main drive motor andpower is supplied to the motor with bidirectional step
up/down DC/DC converters and inverters. Recently,
the supply voltage to inverters has been stepped up
from the battery voltage of approx. 200 V to approx.
500 V, to reduce the drive current.
As shown in Fig. 8, the windings of PMSM arewound on the iron core, with insulation in between. Fig(a) shows distributed windings and (b) indicatesconcentrated windings. The motor is installed on the

Stray inductance

Stray capacitanceFig. 9 Stray impedance of inverter circuit and motor-9-

NTN TECHNICAL REVIEW No.83（2015）

3. Switched reluctance motor (SRM) toachieve high speed rotation
3.1 Characteristics and rotor position
estimation methods
The switched reluctance motors (SRM) have asimple structure not requiring permanent magnets,
therefore, they are suitable for driving at high speed orunder high temperatures11)-12). In order to operate
SRM at variable speeds, it is necessary to obtain therotor position information similar to permanent magnet
synchronous motors. However, most rotor position
detection devices that are used in general industry
fields, such as encoders and Hall effect sensors, arenot suited for severe environments such as vehicle
use. Several methods are proposed such as estimating
the angles from driving voltage/current, taking
advantage of the rotor rotation angle dependency
properties of SRM winding inductance, and obtaining
the angles from the change in inductance bysuperimposing the high frequency voltage on thedriving voltage, etc.13)-16).
There are other methods of estimating the rotor
position such as using observer or analytical models toestimate the rotor positions from multiple parameters
not in linear relation with the rotor position, using somekind of artificial intelligence, or applying signals otherthan the driving voltage on the motor windings, etc. 17)-19).
In this section, two methods of estimating the rotor
positions not affected by driving voltage and current
are proposed, when the windings are driven by PWM.

Fig. 10 Wire connections of SRM

3.2.1 Rotor position detection method
Fig. 11 shows a rotor position detection circuit that is
not affected by PWM voltage and winding current,
driven by a circuit with a common mode network(CMN) 20). Fig. (a) is a high frequency injection circuit,and (b) is a detection circuit for high frequency signal
injected into a different phase. When the phase, where
the high frequency is injected, is aligned with the teethof the stator, the high frequency signal levels in theother two phases match. By injecting high frequency
waves into only one phase, the rotor position isdetected based on the high frequency detection signals
obtained from the other two phases, and when thealignment is detected, the phase where the highfrequency is injected is switched and the above
operation is repeated. This is how the rotor position isdetected. Fig. 12 shows equivalent circuits shown in
Fig. 11 for the high frequency signal and common
mode signal detection circuits. In the case of highfrequency signals, the circuit impedance is 0 and theamplitude of the detection signal can become large. On
the other hand, in the case of differential mode signals,
the circuit impedance is high and the output signal
becomes small.

3.2 Rotor position estimation method!
The proposed sensor-less SRM driving method
provides leader lines at the connection points of the
opposing serially connected windings as a driving
terminal and high frequency signal terminal, and isused for the injection and detection of signals. Fig. 10shows a part of the connection diagram when this
proposal is applied to a 4-pole 6-slot SRM. It shows the
cross section of the rotor having the salient poles in thecenter. 6 windings from L1 to L6 are placed as thestators. It shows the availability of leader point T in theconnection of windings L1 and L4. The same wiring
applies to the remaining two pairs of windings.
The symmetric properties, when the SRM isregarded as a transformer from this connection, can beapplied to reduce the interference of high frequency
signals and driving voltage. In addition, the accuracy ofthe detection can be increased by reducing theinterference of the driving voltage to the detection
circuit, by inserting a differential mode choketransformer into the circuit where high frequency
signals are injected/detected.
-10-

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots

Motorwindings

Motorwindings

Winding1

Winding1

High frequency
wave injection

Winding2(a) High frequency signal injection
circuit for detection

Detection
signalWinding2(b) Detection circuit for high frequency signal
injected to different phases

Reception Reception
signalsignalReception Reception
signalsignalFig. 11 Rotor position detecting circuit with CMN

Detection
signalDetection
signalOnly received high frequency
signal is outputted

Fluctuating signals such as motor drive
voltage do not appear(a) Equivalent circuit of high frequency
signal detection(b) Equivalent circuit for differential
mode signal

Fig. 12 Equivalent circuits for the high-frequency signal and differential mode signal detection circuits

3.2.2 Example of implementation
Fig. 13 shows an example of implementing the
proposed sensor-less rotor position detection method.
In the figure, OSC is an oscillator, CTL is a control
circuit and DET is the detection circuit. The detection
circuit detects the amplitude of waveforms using thegeneral envelope detection or product detection
circuits, etc. The control circuit CTL selects a CMN toinject signals into by controlling the switch group,
detects the timing for commutation by examining theamplitude detected from the other phases and notifies
the power circuit. Fig. 14 shows the measurement
results of self-inductance to the rotor angle.

Fig. 13 Proposed sensor-less rotor position detecting method

Self inductance [µH]

125
100
75
50
25
-40

-30

-20

-10
0
10
20
Roller angle θ [deg]

30

40

50

Fig. 14 Relation of self inductance to rotor angle-11-

NTN TECHNICAL REVIEW No.83（2015）

3.2.3 Experiment results

more of rotating speed by adjusting the timing ofcommutation (advancing) in the experiment.
In addition, this method is applicable for the rotor
position detection of the motors where the inductancechanges depending on the rotor position such as
IPMSM.
This method provides a relatively easy detection of
the rotor position; however, magnetic saturation at thestators can affect the mutual inductance which
prevents the correct detection of position. Therefore,
further study is required.

Fig. 15 shows the waveforms from the experiment at
10,000 min-1 of rotating speed with no load. They are,starting from above, the output waveform of the Hall
effect sensor installed for monitoring the rotor position,
the A-phase current and overlapping B-phasedetection waveform νDB and the C-phase detection
waveform νDC. The Hall effect sensor generates 4period pulses per rotation. In the experiment, advance
control is applied to compensate for the delay ofcurrent phases.
Fig. 16 shows waveforms of the experiment at
100,000 min-1 with no load. In the experiment, therotating speed was controlled by monitoring the Hall
effect sensor waveform and adjusting the DC rail
voltage to obtain the specified rotating speed.
However, the control of the rotating speed can be
automatically adjusted.
We have confirmed that the sensor-less rotor
position control can drive the motor at 200,000 min-1 or

3.3 Rotor position estimation method@
The previous section proved that, with SRM, therotor position can be detected when the inductance ofwindings is measured. In production systems, motor
windings are connected to power circuits such as
inverters and driven by PWM. The inductance must be
measured while the voltage is applied and currents arecarried in the windings.

Hall sensor
10V/div
Current A phase
20A/div

νDB and νDC
0.5V/div

500 µ s/div

Fig. 15 Waveform at 10,000min-1

Hall sensor
10V/div
Current A phase
20A/div

νDB and νDC
0.5V/div

500 µ s/div

Fig. 16 Waveform at 100,000min-1-12-

Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots

L4 in parallel connection.
This method is characterized by its ability to detect
the rotor’s salient pole position regardless of the
motor’s driving conditions, since the impedance of the
motor windings can be observed without any impact
from the current supplied from the power circuit by theuse of toroidal core.

In this section, we propose a new method of
measuring the self-inductance of 4-pole 6-slot SRMs
that have parallel connections of phase windings,
which is not affected by the driving voltage/current,
even with magnetic saturation. 21)-22) Fig. 17 shows
the connection and winding inductance measurement
circuit for 4-pole 6-slot SRMs. The windings in parallel
connection are excited by currents supplied by driving
circuits based on the rotor position, and the rotor
generates torque.

3.3.3 Rotor position detection method
As shown in the previous section, we have proventhat the inductance of each phase of SRM in parallel
connection can be measured while the motor is inoperation. By using this method, when the inductanceof three-phase windings is measured and if theinductance levels of the phases not excited match,then the alignment to maximize the self inductance ofthe excited phases is occurring. Highly efficient drives
can be achieved by appropriately controlling theswitching timing of the current rising and falling points
by detecting the rotor positions from these conditions
and using the estimated rotor position information. Thismethod requires more leader lines; however it ischaracterized by being able to detect rotor positions
without any impact from magnetic saturation.

3.3.1 Equivalent circuit
Fig. 18 shows an equivalent circuit configuration of
one winding. In this circuit, two wires that supply
current to the two windings in parallel connection areplaced through the toroidal core alternately to cancelout the magnetic flux generated by the currents. It is
possible to measure the inductance of parallel connectionof the two windings L1 and L2, by measuring theimpedance of the winding (between Terminals a and b)
placed around the toroidal core.

3.3.2 Measurement principle and characteristics
In the circuit of Fig. 17, the terminals A and B areconnected to the power circuit that drives the motorand supplied with an agitation current. The current
supplied to terminal A runs through winding L1 andreaches terminal B through the winding on the toroidal
core. Similarly, the current runs through winding L4.
The magnetic fluxes inside the toroidal core are
mutually cancelled because the directions of thewinding current are different; therefore, windings L7
and L8 cannot be seen from the power circuit. If weconsider the output impedance of the power circuit as
0, the impedance observed from winding L9 placed on
the toroidal core is the impedance of windings L1 and3.3.4 Summary of this method
We have proposed a method of sensor-lessdetection of SRM rotor positions and confirmed theoperation of this method through simulation, as well aswith a test circuit using this method where high speed
rotation can be achieved with sensor-less position
detection. These methods are characterized by thefollowing:With estimation method!, no electronic components
are required for the motor if an intermediate tap wiring
for CMN is added for the number of phases between

Toroidal core

Toroidal coreFig. 18 Impedance measuring principle

Impedance measurement

Fig. 17 Impedance measuring circuit-13-

NTN TECHNICAL REVIEW No.83（2015）

in-wheel motors, by installing reducers. By reducing
the unsprung weight, the responsiveness is increased
and the comfort is improved. For compactness, highspeed rotating motors and reducers that can withstandhigh speed rotation are required. In addition, as power
assisted turbo chargers and superchargers, used forhigher efficiency of internal combustion engines,
generate pressure by the high speed rotation of
turbines, bearings are also required to withstand thehigh speed rotation. Also, the magnetization of
bearings by leakage flux of motor windings andmagnets cause the generation of currents during
rotation, resulting in electric corrosion and lower
product life; therefore, bearings with low magnetization
properties are required.
Motors will be used, more than ever, for service
robots and vehicles, with increased demands, forwhich higher efficiency, low vibration/noise and longlife will be required. Bearing technology to support
these rotational bodies are expected to be a criticalelement and further development at NTN is highly
expected.

the motor and power circuit and CMN is installed in theelectronic circuit. Therefore, the rotor position can be
estimated under high temperatures while the motor isin operation. In addition, since it is not affected by thewinding current, no complex calculation or filter design
is required.
Estimation method@ requires more leader lines
connected to the motor, however, it is a highly practicalmethod since the rotor position can be estimatedwithout any impact from magnetic saturation.
These rotor position detection methods should besuitable for applications where installing magnetic poleposition sensors is difficult such as in vehicles.

4. Challenges
Current state and future technologies such as recent
motor control technologies, the properties of various
motors to be used in vehicles, application technology
for inverters and converters, sensor-less control
methods and issues associated with leakage currents
due to motorization have been described.
Vehicles, not only electric vehicles but also vehicles
with internal combustion engines, are making progressin higher efficiency using motors and electronic control
technology. Furthermore, in recent years, driverassistance features such as automatic parking
systems, pedestrian detection alarms and collision
prevention functions are being installed. Safetyfunctions are also being installed due to the progressthat has been made. In the future, safer and morefriendly automobiles for environment and people will be
developed, including autonomous driving, and it isexpected that automobiles will be heading in the
direction of robotization with high intelligence in both
function and performance.
For these automobiles, motors and electronic control
are essential systems and critical technology. In
addition, since the vehicles are for humans to operate
and ride, driving and riding comfort must beemphasized. Informing the driver of the external forces
applied to the vehicle and changes in the environmentis also important, and various sensors are used forthis. Software to comprehensively evaluate these typesof information is also necessary.
In order to increase the responsiveness to vehicle
motion, it is necessary to reduce the weight of vehicles
and increase the control performance.
Reduction of weight can be accomplished with alighter body, as well as the smaller and lighter main
drive motors and heat radiation systems including
inverters, batteries, etc.
Smaller and lighter main drive motors are importantfactors that can be achieved comprehensively, even for

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Evaluation of the High Temperature Turbine Wheel for
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Industries Technical Review, Vol. 43, No. 3 (2006)
3) Yukio Yamashita, Seiichi Ibaraki, et al.,Development of Electric Supercharger to Facilitate theDownsizing of Automobile Engines, Mitsubishi Heavy
Industries Technical Review, Vol. 47 No.4, pp.12-17(2010)
4) Toshihiko Noguchi，et al.，220,000-r/min，2-kW
Permanent Magnet Motor Drive for Turbocharger，International Power Electronics Conference，IPECNiigata (2005)
5) Toshihiko Noguchi, Yosuke Takata, Yukio Yamashita,
Yoshimi Komatsu, Seiichi Ibaraki, 220000-r/min, 2-kW
PM Motor Drive for Turbocharger, IEEJ Transactions on
Industry Applications Vol. 125, No. 9, pp. 854-861 (2005)
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The Society of Instrument and Control Engineers, 50 (3),184-188 (2011)9) Satoshi Murata, Development of In-Wheel Motor Drive
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Highly Efficient and Responsive Motor Drive Technologies Enable Robotics to Evolve Automobile into Autonomous Service Robots

10) Suzuki, Kano, Abe, et al.: ”Roll Control Using Tire
Longitudinal Forces Integrated with Tire ForceDistribution of Full Drive-by-Wire Electric Vehicle”,
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Ltd. (2012)13) Mohammad S. Islam, Iqbal Husain, Robert J．
Veillette and Celal Batur: "Design and Performance
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Ehsani: "Inductance Model-Based Senseless Control of
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pp.1568-1573 (2004)
15) Estanislao Echenique, Juan Dixon, Roberto Crdenas and
Ruben Pe a: “Sensorless Control for a Switched
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and Neural Networks”, IEEE Transactions on Industrial
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Control Method for SRM Based on Variation of Phase
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Ichinokura: "A New Sensorless Drive Method of Switched
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Yamamoto, A method of driving SRM for improved
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<Author biography>

Hisashi Takahashi
Senior Member, Institute of Electrical Engineers of Japan, Doctor (Engineering),
Professor, Graduate School of Science and Technology, Shizuoka Institute of Science and Technology[Specialty]
Electronic Circuit Engineering, Control Engineering, Motor Applications Engineering(Motor Control, Robot Control, Medical Engineering, Research on Parametric Estimation)
1975

Graduated from Faculty of Electrical Engineering, Polytechnic University, Assistant, then Instructor of
Faculty of Electrical Engineering of Polytechnic University, Associate Professor, Electrical Information
Specialty, Graduate School of Engineering
1997-1999
Dispatched to Tianjin University of Technology and Education in Tianjin, China as professional in control
technology (Long-term Expert) in a National Project.
Since 2000
Member, Program Committee, IEEJ
2001
Special lecture on control technology at the Electrical Electronic Engineering, Nanyang Technological
University in Singapore (Short-term Expert)
2003-2007
Member, Technical Committee of Rotational Machines, IEEJ
2003-2005
Chairperson, Investigating R&D Committee for Advanced Technology of Small Motors, IEEJ
2005-2007
Chairperson, Investigating R&D Committee of Performance Improvement and Evaluation Technology by
Applications for Small Motors, IEEJ
Since October 2011 Professor, Faculty of Science and Technology and Graduate School of Science and Technology,
Shizuoka Institute of Science and Technology
Since 2012
Chairperson, Power Electronics Business Development Program, Hamamatsu Agency for Innovation
Since April 2013
Director of the Yaramaika Education Site, Director of the Yaramaika Innovation Center for Engineering Designand Manufacturing
Shizuoka Institute of Science and Technology
Has held many positions such as Academic Society Committee Member, University External Evaluation
Committee Member, Subsidy Review Committee Member, etc.

-15-

NTN TECHNICAL REVIEW No.83（2015）[ Perspective ]Electrification of the Automobile andModule Products of NTN

Makoto OKASAKA

About 250 years ago, the very first “auto-mobile” started to run in theworld, and now, full-scale developments of “autonomous” car are started.
On the other hand, the world is facing a serious problem of global warming
caused by greenhouse gases mainly CO2, and each region and country
began to move regulation of severe CO2 emissions, what we call, strict fueleconomy regulation to the automobile.
Automobile manufacturers are developing downsized engines as next-generation vehicle and electrification
devices for low fuel consumption. This paper introduces modular products by NTN contributing to upgrade ofvehicle electrification in various places.

majority will be shifted to the next-generation vehicles
with electric motor technology. EVs/FCVs with no CO2emission at all are not yet widely spread, with lessthan half of the total number of vehicles.
In other words, even in 2050, more than half of all
the vehicles sold will be using fossil fuel, including
HEVs, that emit CO2.
During this transition, components of automobiles
are being developed with electrification in mind, withcomplete transition to 48V-based technology for
improved fuel efficiency. Automobile manufacturers
are addressing electrified vehicles for “running","turning” and “stopping” with the upcoming transition
to self-driving in mind and developing system/module
products for that will be important items for future
improvement of fuel efficiency.
In this paper, some examples of NTN module
products that contribute to the improvement offunctions in this transition to electrified vehicles will be
introduced.

1. Introduction

Passenger LDV sales (millions)

In 1769, a “self-propelled” vehicle powered bysteam engine, “Cugnot’s Wagon”, ran as the worldfirst automobile in France under King Louis XV’s
regimen.
Approximately 250 years since the development of“self-driven” vehicles started, the world is facing aserious problem called global warming. A significant
contributor for this problem is said to be greenhousegasses, mainly consisting of carbon dioxide (CO2).
Countries around the world are working on regulating
this CO2 emission.
CO2 regulations are particularly strict in Europe(EU), specifying a corporate average CO2 emissions
as 2021 as 95 g/km per company.
This requires more than 38% of fuel efficiency
improvement when compared with 2008 efficiency
standards. The automobile manufacturers areworking on development and promotion of nextgeneration automobiles. These include hybrid
vehicles (HEV/P-HEV) with less CO2 emission andbetter fuel efficiency, electric vehicles (EV) and fuelcell vehicles (FCV) with no CO2 emission, and a shift
from the conventional fossil fuel vehicles such as
gasoline and diesel engine vehicles.
However, according to the outlook of newpassenger vehicle sales in 2050 by the International
Energy Agency (IEA) 1), as shown in Fig. 1, thevehicles only driven by fossil fuels such as gasoline
and diesel will decrease to around 10%. The vast200
H2 Fuel cell
180
Electricity
Plug-inhybrid160
Hybrid140
CNG/LPG
120
Conventional diesel
Conventional gasoline
100
80
60
40
20
0
2005
2010 2015
2000

Sourse:IEA 2010.

2020

2025

2030

2035

2040

2045

2050

Fig. 1 Annual light-duty vehicle sales by technology type* Executive Officer, Automotive Business HQ-16-

Electrification of the Automobile and Module Products of NTN● For “running”,

2. Module/system products addressing
electrification of vehicles… In-wheel motor system… In-wheel motor system for electric commuters… 2-motor onboard drive system… Automatic transmission module/system

NTN has been developing in-wheel motors to
replace from internal combustion engines that
consume fossil fuel, for reduction of emission of thegreenhouse gas CO2 2).
In addition, in order to help improve fuel economy ofthe existing vehicles with internal combustion engines,
NTN has been developing and launching electricmodular products of operational/control components,
since 2004 as shown in Fig. 2, supporting “by wire”
technology. These components are based on
products such as ball screws, considering that
improvement of mechanical efficiency by refinedelectric control of operational/control components isimportant for improvement of fuel efficiency.
Furthermore, electric module products and theirsystems are indispensable for next-generation electricvehicles (EV) and fuel cell vehicles (FCV), which do
not use fossil fuel at all. NTN is developing electricmodule products and system products including theircontrol shown below and in Fig. 3, for improvement ofeach of the “running”, “turning” and “stopping”
functions of vehicles, using demonstration tests withthe actual vehicles.

● For “turning”,… Mechanical clutch unit for next-generation steering… Steer by wire control system… Rear wheel independent steering system● For “stopping”,… Electric braking system with parking mechanism… Electric hydraulic braking system

3. Addressing next-generation vehicles
with electrified modules
The worlds-first automobile, “Cugnot’s Wagon”,
collided with a mural the following year, causing theworlds-first vehicle accident.
Approximately 250 years since then, thedevelopment of "self-driving" vehicles has greatly
accelerated. This was triggered by the development
and market introduction of “collision prevention”
vehicles that include radars and/or stereo cameras
and automatic braking systems.
Since self-driving vehicles control the “running”,“turning” and “stopping” operations, which wereconventionally conducted by humans, development ofelectrified modules/systems is strongly required.

Steer by wire

¡MCU for next-generation steering

Brake by wire
Shift by wire

Shift by wire

¡Ball screw for automaticmanual transmission

Throttle by wire

¡Ball screw unit forcontinuous variable valve

2004

2007

¡Actuator for electric
hydraulic braking(A) Electrified modules/systems for “running”¡Actuator for parking
shift

2008

The next generation vehicles will “run” with adriving system equipped with multiple motors. NTN isdeveloping in-wheel motor systems and 2-motor
onboard drive systems (Fig. 4) as a running system toimprove traveling stability of vehicles against many
disturbances. These disturbances include braking byunlimited variable control and speed reduction torquefor the left/right wheels with independent motors. Thisdevelopment will also improve performance of“turning” by controlling vehicle attitude with torquevectoring from the added torque to the left and rightwheels.

2012

Fig. 2 Electrification with by-wire technology

Module/system products for next-generation vehicles
In-wheel motor systemfor electric commuters
Electric braking systemwith parking capabilities

In-wheel motor
systemSteer by wire
steering system

2-motor onboard
drive system
Annecy, France

Ise-City, Mie Prefecture

Public road demonstration driving projectsof vehicles with the systems installed

Fig. 4 Torque-vectoring mechanism with 2 motor
on board driving system

Fig. 3 Modular & system products with electrification
for next-gen. vehicles-17-

NTN TECHNICAL REVIEW No.83（2015）

Vehicles driven by internal combustion engines, thatstill emit CO2, and will still account for more than halfof the vehicles in 2050 are equipped withtransmissions. Transmissions use hydraulic pumps tooperate and cause low fuel efficiency. Therefore, thehydraulic mechanisms for operation systems, such as
gear shifts, are being replaced by electric shifts.
Large trucks and light passenger cars continue to
use gear-based manual transmissions that arecharacterized by low-cost and high transmission
efficiency. However, the use of automatictransmission systems with electrified modules usingball screws in the transmission mechanism is
expanding. This change reduces workload of thedrivers, particularly for aged drivers.
Continuously variable transmissions (CVT) operatewith improved fuel efficiency when compared to a
traditional transmission. However the vehicle fuelefficiency of a CVT has been further improved. Fuelloss due to hydraulic pumps that operate pulleys
holding the metallic belt is not negligible. Therefore,
the CVT pulley driving system using hollow ball screw
units is an effective improvement of fuel efficiency forthe next-generation CVTs (Fig. 5 and 6).
Some examples include, downsized turbo engineswith broad torque band and diesel engines with large
absolute torque. Both are widely used due to low fuelconsumption, and the efficient transmission of CVT’s
pinpoint input power is reduced. It is important toprovide coordinated control of both pulleys in the input
and output sides with an electrified ball screw unit toachieve more than 95% transmission efficiency 4).

(B) Electrified modules/systems for “turning”
The worlds-first steer-by-wire vehicle using an NTN
mechanical clutch unit (MCU), shown in Fig. 7,
became commercially available in 2014, opening a
new chapter of self-driving.
This MCU makes it possible to establish type II
steer-by-wire which does not connect steering wheel
and tire during normal operation but immediately
connects in case of failure, an indispensable module
product for self-drive vehicles (Fig. 8).
The next-generation vehicles will evolve from“collision prevention vehicles” to “steering away
vehicles”. Type III steer-by-wire is when the vehicle
autonomously steers to avoid obstacles.
In other words, the next-generation vehicles will not
be equipped with column assist electrified steering,
but rack and pinion assist. A downstream assist
method will be the main steering method.
NTN has developed a rear wheel independent
steering system including toe angle control byproactively steering individual rear wheels to improvevehicle stability during braking and turning (Fig. 9). A
new electrified system where the tire geometry incontact with the road is individually controlled will be
developed to optimize turning.

Motor

Primary pulley

Hollow ball screw

Fig. 7 MCU for steer-by-wire

Belt
Reducer gear

Fig. 5 CVT pulley driving module
Hydraulic
Input

OutputSensorElectric

Differential
angle control mechanism

Input

OutputECUElectric actuator

Reaction
force
actuator
ClutchSensor #1,#2

SensorECU

Reaction
force
actuator

Power steering
unit

Steering actuatorsteering unit

Steering actuatorsteering unit

Type!:
Direct connection

Type@:
Mechanical link control

Type#:
Electrical link control

Fig. 8 Steer-by-wire mechanisms

Fig. 6 Electric CVT pulley driving system-18-

ECU#1,#2

Electrification of the Automobile and Module Products of NTN

4. Conclusion
In next-generation vehicles with self-driving in mind,
electric modules/systems which operate without steps
will be used by developing electrification ofoperational/control system components.
NTN is poised to develop module/system products
for each of “running”, “turning" and "stopping" basedon NTN technology. Since the next-generation vehicle
performance will depend on these components, thisdevelopment is critical.

Fig. 9 Rear-wheel independent steering system(C) Electrified modules/systems for “stopping”
The maximum regeneration of energy during
braking was made possible by continuous variable
control of braking hydraulic pressure with electric
hydraulic braking system using NTN’s ball screws in2014 (Fig. 10).
For “stopping” of next-generation vehicles, electricbrakes shown in Fig. 11 will be adopted for all wheels.
Until then, a hybrid system, namely, electric hydraulicbraking system, driven by ball screws will be adopted
for the front wheels, and electric braking system will
be adopted for rear wheels from fail-safe design
standpoint.

Aluminum case

Ball screw

1) “Technology Roadmap Electric and plug-in hybrid electricvehicles”, IEA, p14 (2011)2) NTN TECHNICAL REVIEW No 79, (2011) 22-28
3) Nikkei Business Publications, Inc. “HEV/EV Element
Component Technology 2012-2013”4) Nikkei Business Publications, Inc. “Automobile
Component Industry: Path to Growth”
5) Nikkei Business Publications, Inc. “Next-Generation
Vehicles” 2015

Bolt
Idler gear
Output gear
SleeveBraking forceIN

DC motor
Rotational torque

References

Braking force
required by driver
Braking force by hydraulic force

Braking force by motor(regenerative brake)

OUT
Thrust force generation(pushing hydraulic cylinder)

Supporting
bearings

Start of braking

Stop

Fig. 10 Ball screw module to electric braking system

Photo of author

Fig. 11 Electric mechanical braking system

Makoto OKASAKA
Executive Officer,
Automotive Business HQ-19-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Two Motor On-board Drive System

Ryou YUKISHIMA*
Hidenori KARASAWA*
Aiko MYOUKI*
Katsunori SATOU*

Yusuke MAKINO**
Takeshi KANDA**
Tomohiro SUGAI***

Two motor type electric vehicle has improved driving performances because the left
and right wheels are driven by independent motors in accordance with driving conditionsof the vehicle. NTN has focused on the two motor on-board type and developed “Twomotor on-board drive system" that consists of motor drive units with an inverter.
This paper introduces overviews of this system and effects of independent left/rightwheel drive.

driving units consisting of a reducer, motor, andinverter.
In this paper, we will introduce an overview of thissystem and the effect of independent driving
mechanisms for the left and right wheels.

1. Introduction
Recently, electric vehicles have gained attention asa measure against environmental problems such as
global warming and air pollution, as well as energy
supply problems with many initiatives taken by various
factors for broader adoption 1).
The driving mechanism of electric vehicles can bebroadly classified into the onboard method where themotor is placed on the vehicle body and the driving
force is transmitted to tires through drive shaft 2), andthe in-wheel motor method where the motor is placed
within the wheels to drive tires directly 3), 4), 5). The
onboard method can be implemented onto thevehicles with suspension structure adopted by theconventional vehicles without any change.
The onboard method can be further classified into
the 1-motor method and the 2-motor method. With 1motor electric vehicles, the driving torque from thesingle motor is distributed to left and right wheels
evenly by differential gears. On the other hand, with 2motor electric vehicles, the left and right wheels are
driven by their own motors depending on the driving
conditions; therefore, it is possible to improve theturning performance and driving stability during driving
on low friction roads.
NTN has focused on the 2-motor onboard method
for these characteristics and developed a “2-motor
onboard driving system”. This system combines motor

2. System configuration
2.1 Motor driving unit
Fig. 1 and 2 show the appearance of the developed
2-motor onboard drive system and structure of the
motor drive unit. Table 1 shows the specification of
the motor drive unit and Fig. 3 shows an image of theunit installed on the vehicle.
This drive unit consists of two motors and a reducer
which transmits power from each motor by reducing
rotational speed. The drive unit has a symmetricstructure so that drive shafts of the same length canbe applied to the left and right wheels. The coupling of
this drive unit and drive shafts is placed in the centerof the unit. This structure allows the drive shafts to belonger to place the reducer closer to the wheels. Thiscan reduce the impact of position of the drive unit andreduce the variation of operational angle of the drive
shafts due to vertical movement of tires. This results in
an improved degree of freedom in designing andinstallation of the unit on the vehicles.

***Drive System Engineering, EV Module Business HQ***Control System Engineering, EV Module Business HQ***Chassis System Engineering, EV Module Business HQ-20-

Two Motor On-board Drive System

InverterFig. 3 Image of the vehicle equipped with the system
Motor drive unit

2. 2 Motor

Drive shaft

The specification of the motor is shown in Table 2.
Improvement of reluctance torque output wasachieved by adopting distributed windings.

Hub bearings

Fig. 1 Appearance of two motor on-board drive system

Table 2 Motor specifications
Motor

Reducer

Motor
StatorRadial gap type permanent magnet
synchronous motoMotor type
Winding method

Rotor

Maximum torqueMaximum output
Maximumrotational speed

Distributed winding
180Nm
50kW

8000min-1

2. 3 Inverter
The specification of the inverter is shown in Table 3.
This inverter houses all the circuits required for controlof two motors in one enclosure. A portion of the circuit
is common for both motors. With this configuration,
the volume was reduced by 30% compared to cases
installing individual inverters.

Fig. 2 Section of motor drive unit

Table 3 Inverter specifications

Table 1 Motor drive unit specifications
Maximum output

Maximum torque

1100Nm※

1300min-1

Maximumrotational speed(Equivalent to 150 km/h assuming tire diameter of 0.6 m)

Reducer type

Parallel axis gear method

Reduction ratio
Cooling method

Power supply voltage
for driving
Power supply voltage
for control
Maximum output

50kW※

Driving method

1/6.16
Motor part: water cooling
Reducer part: air cooling

Cooling method※Value per wheel-21-

DC 250〜400V
DC 12V
50kW × 2

Sine-wave PWM
Water cooling

NTN TECHNICAL REVIEW No.83（2015）

3. Performance

4. Vehicle motion control

The efficiency of 1-motor and 2-motor drive units
are shown in Fig. 4 and 5. The efficiency of the entire
system including the inverter is shown in Fig. 6. The
motor achieved 90% or more efficiency in a wide
range of operational conditions and the maximum
efficiency of the motor drive unit and entire system
was 93% and 91%, respectively.

4.1 Effect of independent drive for left and rightwheelsDirect Yaw-moment Control (DYC), which controls
vehicle motion by giving direct yaw-moment onto thevehicle body, is introduced more frequently, with the
purpose of improving turning performance and driving
stability. In general, yaw moment in DYC is generated
by either differential gears with a driving torquedistribution mechanism or friction brakes to givedifferent driving/braking force to the left and rightwheels 6), 7).
The driving force control by electric motors has
advantages for higher precision and responsivenessin generating torque compared with internal
combustion engines. In addition, it can control not only
driving torque but also braking torque. Furthermore,since this system drives the left and right wheels by
their dedicated motors, it can control the vehicle yaw
moment in any way, as required, with high precision.
Fig. 7 shows the concept of DYC by independent
left and right drives. Typically, there is a delay inresponse of vehicle yaw-moment from steering. If yaw
moment can be generated in the right timing against
steering, the turning performance will increase.
By using the advantages of motor driving force
control, the slip ratio can be controlled to effectively
reduce slippage of tires when driving on a low friction
road. In the verification test with the actual vehicles
described in this paper, we applied Yaw-rate feedback
Traction Control System (Y-TCS). Y-TCS reduces
disturbance of the vehicle attitude by optimallycontrolling the slip ratio of the left and right wheels in acoordinated manner, in addition to the above slip ratio
control.

Efficiency %
100

200

95

Motor torque N·m

160

90

Maximum
efficiency: 95%

120

85
80
75

80

70
65

40
0

60
55
0

2000

4000

6000

8000

50

10000

Motor rotational speed min-1

Fig. 4 Motor efficiency
Efficiency %
100

1200

Output axis torque N·m

95
1000

90

800

85

Maximum
efficiency: 93%

80

600

75
70

400

65
60

200

55
0

0

400

800

1200

50

1600

Output axis rotational speed min-1

Fig. 5 Motor drive unit efficiency
Yaw moment

Efficiency %
100

1200

Output axis torque N·m

95
1000

Driving force

90

800

85

Maximum
efficiency: 91%

600

80

Motor drive unit

75

Fig. 7 Concept of DYC by the independent driving ofeach wheel

70

400

65
60

200

55
0

0

400

800

1200

Braking force

1600

50

Output axis rotational speed min-1

Fig. 6 Motor drive system efficiency-22-

Two Motor On-board Drive System

4.3 Verification with actual vehicles
4. 3. 1 DYC

4.2 Vehicle with the system equipped
Fig. 8 and 9 show the appearance of the test
vehicle equipped with this system and the view of thesystem installation from underneath, respectively.
The specification of the vehicle with the system isshown in Table 4. The vehicle is based on acommercially available vehicle with this systeminstalled on the rear side of the vehicle body.
Underbody structure such as suspension, hub
bearings and knuckles are not changed from the basevehicle.

Fig. 10 shows the test results from driving with sinewave steering angle input and Fig. 11 shows the
picture of this driving test. Without DYC, the delay of
the yaw-rate response was 50 ms against the changeof steering angle. With DYC, the response wasapprox. 30 ms, which is a reduction of 60%. DYC
showed an improvement in vehicle responsiveness.

Vehicle speed 50km

90

20

50ms

0

0

-90
-180

40

-20
0

0.5

1.0

1.5

Yaw rate (deg/s)

Steering angle (deg)

180

-40
2.0

Time sSteering angle

Yaw rate(a) Without DYC

Motor drive unit

Front of vehicle

Suspension lower arm

Vehicle speed 50km

20

90

30ms

0

0
-20

-90
-180

40

0

0.5

1.0

Time sSteering angle

1.5

Yaw rate (deg/s)

Steering angle (deg)180

Fig. 8 Appearance of the vehicle equipped with the system

-40
2.0

Yaw rate(b) With DYC

Fig. 10 Results of driving test with sine-wave steering
angle input
Drive shaft

Fig. 9 State of the system mounted onboard

Table 4 Specifications of vehicle equipped with the system
Vehicle weight

1100kgCapacity2

Drive shaft

Maximum output
Maximum torque (wheel)
BatteryType

Capacity

Rear 2 wheels

50 kW x 2 wheels

1100 Nm x 2 wheels
Li-ion23.3kWh

Fig. 11 Driving test with sine-wave steering angle input-23-

NTN TECHNICAL REVIEW No.83（2015）

4.3.2 Control of slip ratio

Left rear wheel speed min-1

Fig. 12 shows the change of speed of the driving
wheels at the quick start of vehicles from a complete
stop on the road with a low friction factor μ between
the road surface and tire. The left and right wheels
showed almost the same behavior; therefore, Fig. 12
only shows the behavior of the left rear wheel. By
application of the slip ratio control, the wheel slips
immediately after starting, and then tire rotation is
sufficiently controlled following the target value.
Furthermore, in order to verify the effect of adding
Y-TCS on the slip ratio control, we conducted thequick start test from complete stop on the split low
friction road with different μ for the left and rightwheels.
The result and the picture of the test are shown in
Fig. 13 and 14, respectively. In this test, the steering
angle was held at 0 degree (straight) and theacceleration pedal was stepped in to the maximum.
As shown in Fig. 13, with Y-TCS, the deviation fromthe target route is reduced to less than 1/2 comparedto without Y-TCS. This shows that the coordinatedcontrol of the left and right driving force according to
disturbance of the vehicle attitude improves thesteering stability when driving on a low friction road.

600

Low friction
road surface

Fig. 14 Split friction road driving test

5. Conclusion
In this paper, we have introduced the structure andperformance of the developed 2-motor onboard
driving system. Independent drive on the left and rightwheels by this system can improve both turning
performance and driving stability of vehicles.
We will continue with the evaluation of the vehicle inFig. 15 for further improvement of performance andreliability and address further enhancements.

Target wheel speed
With slip ratio control
Without slip ratio control

500
400

Reference
1) Ministry of the Environment: Next Generation Vehicle
Guidebook 2014, (2014)
2) Sano, et al.: Cutting-edge EV component technologies
used in the “i-MiEV”, Mitsubishi Motors Technical Review,No. 22, (2010) 23-28.
3) Murata: Development of In-wheel-motor drive unit,
Proceedings of Society of Automotive Engineers ofJapan, No. 28-10 (2010) 5-9.
4) Okamoto, et al.: Development of High Efficiency In-wheel
Motor System for using 20 inch Wheel Mounted Motor,
Toyo Denki Review, No. 117, (2008) 12-17.
5) Itoh, et al.: In-Wheel Motor System, NTN TECHNICALREVIEW, No. 79, (2011) 22-28.
6) Kunii, et al.: Development of SH-AWD (Super HandlingAll Wheel Drive) System, Honda R&D Technical Review,Vol.16, No.2, (2004) 9-16.
7) Ushiroda, et al.: Development of Super AYC, Mitsubishi
Motors Technical Review, No. 15, (2003) 75-78.

300
200
100
0
0

1

2

3

4

5

6

7

Time sFig. 12 Results of low friction road test

Lateral travel distance m

4
3

With Y-TCS

Low friction
road surfaceµ=approx. 0.2

2

Without Y-TCS

1

Target route0-1

Ultra low friction
road surfaceµ=approx. 0.05

-2
-3
-4
-10

0

10

20

30

Ultra low friction
road surface

40

Forward travel distance m

Fig. 13 Results of split friction road test-24-

Two Motor On-board Drive System

Fig. 15 Appearance of converted vehicle

Photo of authors

Ryou YUKISHIMA

Hidenori KARASAWA

Aiko MYOUKI

Katsunori SATOU

Drive System Engineering,
EV Module Business HQ

Drive System Engineering,
EV Module Business HQ

Drive System Engineering,
EV Module Business HQ

Drive System Engineering,
EV Module Business HQ

Yusuke MAKINO

Takeshi KANDA

Tomohiro SUGAI

Control System Engineering,
EV Module Business HQ

Control System Engineering,
EV Module Business HQ

Chassis System Engineering,
EV Module Business HQ-25-

NTN TECHNICAL REVIEW No.83（2015）[ Technical Article ]Rear-wheel Independent Steering System

Yasuhiro MATSUNAGA*
Tatsuji INOUE*
Hironori TOKUNAGA*
Yukihiro NISHIO*

Rear-wheel steering in automobiles is a function to assist vehicle stability at midium to high speed driving as wellas improve cornering at low speed driving and is already installed in some luxury-class models. NTN announced the“Rear-wheel Independent Steering System” that utilizes steer-by-wire technology for electronically-controlled steeringin 2013, and has made further enhancements for even better response and a more compact, lighter weight design.
This report introduces the “Rear-wheel Independent Steering System” that is capable of toe angle control with a
one body combined-type steering system.

(2) Anti-phase

1. Introduction

Mainly used in the low speed range. The rear
wheels are steered in the reverse direction of the front
wheels when turning to make the turning radius
smaller.

Four wheel steering (4WS) systems have long been
known for improving driving stability by steering therear wheels.
4WS systems were introduced into the market in the
1980s and later rear wheel steering by mechanicalmeans was introduced. Unfortunately, drivers may
have felt uncomfortable by the lag in the transmissionof power. However, recent developments in electronic
components and control technologies have made it
possible to steer the wheels by motor drive. Becauseof the improvement in responsiveness, it is now
possible to eliminate this uncomfortable feeling;
consequently, this technology is being adopted,
especially in luxury vehicles for enhanced driving
stability.
Four wheel steering is a steering system to steer
the rear wheels along with the front wheels dependingon the driving patterns, and is classified into themodes shown in Fig. 1. The characteristics of eachmode are as follows:(3) Toe-in
The front side of the rear wheels is steered inside(when the distance between the front sides of bothtires is compared to the distance between the rearsides of both tires, the rear distance is wider). It
improves the vehicle’s straight line stability.

(4) Toe-out
The front side of the rear wheels is steered outside(when the distance between the front sides of bothtires is compared to the distance between the rearsides of both tires, the front distance is wider). Not
usually used.
NTN developed steer-by-wire steering system forfront wheels in 2011 1) and now applies thistechnology for developing rear wheel independent
steering system capable of controlling each of theabove mentioned modes.
In this paper, we will report the system configuration
and results of the vehicle test.

(1) In-phase
Mainly used in the mid/high speed range. The rear
wheels are steered in the same direction as the front
wheels when turning, to improve the stability andmaneuverability for changing lanes and reduce cornering.

*New Product Development R&D Center-26-

Rear-wheel Independent Steering System(2) Ant-phase(3) Toe-in(4) Toe-out

Front

Front

Front

Front

Steering state(1) In-phaseFig. 1 Operation mode of 4WS

The linear motion mechanism uses hollow DC
brushless motors for compactness, and the motor
torque is transmitted to the trapezoidal screw throughthe planetary gear reducer to drive the rotationrestrained steering shaft.
The detailed internal structure is shown in Fig. 4
and 5. A large thrust external force (lateral withstandload) generated by stationary steering etc. is received
by the needle thrust bearings located in the central
part. The rolling motion in the radial direction issuppressed by the sliding bearings installed at the tipsof the steering shaft and the trapezoidal screws, and

2. System overview
2.1 StructureThis system, which steers the rear wheels
according to the angle of the steered front wheels,
makes it possible to achieve in-phase and anti-phasesteering control, as well as independent toe angles forleft and right rear wheels by adopting an independentand symmetrical linear motion mechanism for the left
and right wheels, as shown in Fig. 2. In addition, as
shown in Fig. 3, since it can be installed on the
chassis, there is no impact on the driving/riding
comfort level due to the change of unsprung weight.

Sliding bearing
Steering
shaft

Planetary gear reducer

Trapezoidal
screw

Trapezoidal
screw

Steering
shaft

Planetary gear

Trapezoidal
screw

Needle thrust bearing

Needle radial bearing

Fig. 4 Internal structure (Central part of actuator)
Hollow motor

Housing

Hollow motor
Magnet holder (magnet)

Fig. 2 Actuator structure

Slidingbearing

Hall IC

Resolver
Steering shaft

Fig. 3 Actuator layout

Lock

Fig. 5 Internal structure (Peripheral part of steering shaft)-27-

NTN TECHNICAL REVIEW No.83（2015）

2.3 Control system

the needle radial bearings supporting the outer
diameter of the material that works as the carrier forthe trapezoidal screw and the planetary gear. There isno displacement of the trapezoidal screw due to thethrust external force (lateral withstand load) since theefficiency for the reverse input to the trapezoidal
screw is low. In addition, resin is used as the material
for the planetary gear for quietness.
In addition, a resolver is used for controlling the
motor. Furthermore, the flux changed due to the
movement of magnets installed on the steering shaft
is detected by the Hall effect IC to determine theabsolute position of the steering shaft.

ECU

ActuatorPowersupplyAbsolute position
sensor

2.2 Specifications of the actuator
Upper
ECU

Table 1 shows the actuator of this system. The
lateral withstand load is 3.5 kN or more on one sideand is capable of addressing the upper medium to
executive class of vehicles of around 4.9 m in total
length.
Fig. 6 shows the measurement results of therelation between the lateral withstand load and shaft
speed of the steering shaft. It can achieve a shaft
speed of 30 mm/s or more with 2.5 kN of lateralwithstand load.

CPU

Motor rotational
angle sensor

Motor drive
circuitBrushless DC motor(for steering left wheel)

Mutual monitoring
ECU(Omitted as the structure is thesame as for steering left wheel)

Absolute position
sensorMotor rotational
angle sensor
Brushless DC motor(for steering right wheel)

For steering right wheel

Battery

For steering left wheel

The control system consists of two ECUs which
control the respective motor, as shown in Fig. 7. Two
ECUs receive the target values of the steering angles
according to the vehicle status from the upper ECU
and determine the positions of each motor by PWM
control.
These two ECUs have the functions of mutual
monitoring and partial redundancy to increase safety.

Fig. 7 Block diagram of control system

Table 1 Specification of actuator
Item

Specification

Linear-motion method

3. Rear wheel steering control method

Trapezoidal screw

Shaft motion level

In order to confirm the effect of this system with theactual vehicle, “Vehicle speed response type front
wheel steering angle proportional control” 2), which is
the typical control law when rear wheel steering is
added to a vehicle, was adopted. In this paper, we will
describe this effect from the basic motion properties ofthe vehicle against the steering operation.
The degree of freedom for the vehicular motion
generated from steering is lateral, yawing and rolling,
however, we can ignore rolling as the verticalmovement is a secondary motion and if we set thefollowing assumptions, we can consider the motion of
a vehicle running at a constant speed in the horizontal
X－Y plane shown in Fig. 8.
・Vehicular traveling velocity is constant・Tire properties of the left and right tires are thesame±7 mm (max. motion level ±10 mm)

Motor

Brushless DC12V

Rotation sensor

Resolver (4X)

Reducer

Shaft absolute position detection

Planetary gear reducer mechanism
Hall IC

Lateral withstand load (one side)

3.5 kN or more

Maximum steering angle※
Maximum feeder speed
Mass±2.5 degrees

30 mm/s (at lateral withstand load of 2.5 kN)
9 kg

Case size

φ81×375mm※Reference only as the values vary depending on the vehicle structure
45

Shaft speed mm/s

40
35
30
25
20
15
10
5
0
0.0

1.0

2.0

3.0

Lateral withstand load kN

4.0

5.0

Fig. 6 Lateral load-rear steering speed characteristics-28-

Rear-wheel Independent Steering System

When the lateral slip angle is small, the cornering
forces acting on the tire Yf and Yr are proportional to thelateral slip angles of the front and rear tires βf and βr.
By taking the counterclockwise angle positive, thecornering force, when the lateral slip angle is positive,
can be written as follows, since the cornering force actson the negative y direction in Fig.8:

Yf = _Kf βf ……………………………………（7）Y = _K β ……………………………………（8）rwhere,
Kf ：Front wheel cornering power
Kr ：Rear wheel cornering power

Fig. 8 Motion of vehicle

In the above equations, when a small steering angle
is given to the front and rear wheels, the lateral slip
angle of the front and rear tires βf and βr can beapproximated as follows:

As mentioned above, the basic vehicular motion
equation in a horizontal plane ignoring rolling can be
expressed as follows:mVI( dβdt +r ) =Y +Y +Y +Y ……………（1）
f1

f2

r1

r2

where,
β ：Lateral slip angle of the vehicular center ofgravity
V ：Vehicular traveling velocity
m ：Inertial mass of the vehicle
Yxx：Cornering force given to tires
r ：Yaw angle velocity
l ：Yawing inertial moment of the vehicle
lf ：Distance between the vehicular center ofgravity and the front shaft
lr ：Distance between the vehicular center ofgravity and the rear shaftβr =β _

lr r _ δ
r ……………………………（10）
V

(

lf _
r δf ……………………（11）
V

(

lf _
r δf ……………………（12）V

Yf = _Kf β +Yr = _Kr β _))By assigning equations (11) and (12) to equations (5)and (6), the following motion equations can be obtained:

mV

{

}

dβ
2
(l K _l K ) r+2(Kf +Kr) β + mV +V f f r r
dt

= 2Kf δf + 2Kr δr …………………………（13）2(lf Kf _lr Kr) β + I2

22(lf Kf +lr Kr)
dr
r
+
V
dt

= 2lf Kf δf _ 2lr Kr δr ………………………（14）The above equations are the basic motion equations
describing the vehicular motion for the front and rear
wheels in the horizontal plane.
“Vehicle speed response type front wheel steering
angle proportional control” adopted with the vehicular
tests, the front steering angle and rear steering angle
can be written as follows:

2Yf = Yf 1+Yf 2 …………………………………（3）2Yr = Yr1+Yr2 …………………………………（4）By considering this force to be the force on the y
direction in Fig.8, equation (1) and (2) can be written
as follows:Ilf _r δf ………………………………（9）V

Assigning equations (9) and (10) to equations (7) and(8), the cornering force can be expressed as follows:

If we assume that there is no difference between theproperties of the left and right tires themselves, thenthere is no difference in the cornering forces felt by bothtires. Therefore, if we set the cornering force of the front
and rear tires as Yf and Yr, respectively, we get thefollowing equations:( dβdt +r ) = 2Y +2Y ……………………（5）fβf =β +

where,
βf ：Front wheel steering angle
βr ：Rear wheel steering angle

dr= lf (Yf 1+Yf 2) _ lr (Yr1+Yr2) ……………（2）
dt

mV

r r

r

δf =

dr
= 2lf Yf _ 2lrYr …………………………（6）dtδ………………………………………（15）nδr =k δf =
-29-

k δ
n ………………………………（16）NTN TECHNICAL REVIEW No.83（2015）

where,
δ ：Steering angle
n ：Gear ratio of front wheel steering systemk ：Steering angle ratio

Table 2 Specifications of test vehicle
Item

f

f

f

2

Steering angle ratio

k

1.0
0.5
50

100

150

-0.5
-1.0
-1.5

±2.5 degrees

10 degrees/s (axial force: 2.5 kN)

The test was conducted in the double lane change
course with a dry surface, as shown in Fig. 10, withthe vehicle entering the course at the speed of 70
km/h and maintaining the speed as constant as
possible, and the yaw rate and lateral acceleration
against the steering angle were recorded. Fig. 11
shows the results.
When the rear wheel steering function is OFF (Fig.
11 (a) and (c)), there will be a delay in the trackabilityof the yaw rate and the lateral acceleration against thesteering angle requiring the driver's unnecessary
steering manipulation. On the other hand, when therear steering function is ON (Fig. 11 (b) and (d)), therear wheels turn approx. 1.5 degrees in-phase withthe front wheels at 70 km/h. As a result, it is revealed
that the linearity of lateral acceleration and yaw ratewith respect to the steering angle are improved. Thismeans that the driver is operating the vehicle as
desired, with improved trackability of the yaw rate andlateral acceleration against the steering handling ,proving the effect of the rear wheel steering for driving
stability.
With actual vehicles, there is a difference between
these quantitative data and the feeling that the drivers
sense when steering. By applying various tunings at
the vehicle level, we are ready to develop thistechnology by leveraging the advantages of 4WS that
contribute to driving stability.

2

f

0

Approx. 1700 kg

4.2 Response evaluation

If we set the proportional constant of the rear wheel
steering angle against the front wheel steering angle as
set in equation (17), we can achieve zero lateral slip
angle of the vehicular center of gravity in steady-state
circular turn, which, theoretically, aligns the vehicular
direction and its traveling direction. The steering angle
ratio k is determined by the vehicle specifications. Withthe vehicle we used in this test, it provides anti-phaseat lower speed and in-phase at higher speed with theswitch occurring at around 50 km/h of speed, as shownin Fig.9.

0.0

Rear wheel drive

Rear-wheel max. steering speedrr

Drive method

Rear-wheel max. steering angle

ml
V )
(
K
2ll
k0 = _……………………（17）
ml
l (1+
V )
2ll K
r

4940×1940×1500（L×Ｗ×Ｈ mm）

WeightBy assigning the equations (15) and (16) into theequations (13) and (14), calculating k so that the lateralslip angle β of the vehicle center of gravity becomes 0,
and by rearranging the equation, we obtain:
lr 1_

Specification

Vehicle size

Vehicle speed km/h

Fig. 9 Steering angle ratio

4. Vehicle test
4.1 Test vehicle and driving conditions

3m

We installed this system in the test vehicle with thespecifications shown in Table 2 and conducted driving
tests on the test course. We used “Vehicle speed
response type front wheel steering angle proportional
control” described in the previous section to determine
the rear wheel steering level according to the front
wheel steering angle and the vehicle speed, andevaluated the vehicle response under actual driving.

1m
2.3m

3m
13.5m

11m

12.5m

Fig. 10 Test course-30-Rear-wheel Independent Steering System

30
20

20

10

Yaw rate deg/s

Yaw rate deg/s

30

1st time
2nd time

0
-10
-20
-100

-50
0
50
Steering angle deg

100

0
-10

-30
-150

150

(a) Steering angle-yaw rate
(rear wheel steering function OFF)

10
8

1st time
2nd time

6

Lateral acceleration m2/s

Lateral acceleration m2/s

10

-20

-30
-150

10
8

1st time
2nd time

4
2
0
-2
-4
-6

-8
-10
-150

-100

-50
0
50
Steering angle deg

100

150

(c) Steering angle-lateral acceleration(rear wheel steering function OFF)

-100

-50
0
50
Steering angle deg

100

150

(b) Steering angle-yaw rate
(rear wheel steering function ON)
1st time
2nd time

6
4
2
0
-2
-4
-6

-8
-10
-150

-100

-50
0
50
Steering angle deg

100

150

(d) Steering angle-lateral acceleration(rear wheel steering function ON)

Fig. 11 Linearity control test result at 70km/h

of autonomous vehicles, market expansion of thesemodule products to contribute to the performanceenhancement of safety, stability and convenience is
expected to continue. To be prepared for further
development of the electrified modules including
vehicle control technologies, we strive to develop new
module/system products that contribute to theelectrification, improved fuel consumption andenhanced driving performance of vehicles.

5. Conclusion
In this paper, we introduced the "rear wheel
independent steering system". With actual vehicle
tests, we have confirmed the effect of the in-phasemode, however, since this system is able toindependently and proactively control the toe anglesof the left and right wheels regardless of thesuspension structure, it is possible to apply various
steering controls according to the driving conditions.
By using these features, it is expected that toe-in forimproved driving stability and anti-phase mode for
automatic parking, etc. are expected to be applied.
Along with the electrification of vehicles, theadoption of by-wire technology and the development

References
1) Katsutoshi MOGI, Tomohiro SUGAI, Ryo SAKURAI andNobuyuki SUZUKI, Development of New Steer-by-wire
System, NTN TECHNICAL REVIEW No79, (2011) 42-50.
2) Masato ABE: Automotive Vehicle Dynamics (2nd Edition),
Tokyo Denki University Press (2012) 5-226.

Photo of authors

Yasuhiro MATSUNAGA

Tatsuji INOUE

Hironori TOKUNAGA

Yukihiro NISHIONew Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center-31-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]MCU (Mechanical Clutch Unit)
for Next Generation Steering

Naotsugu KITAYAMA*
Shintaro ISHIKAWA*

The next generation steering system which controls steering with electric
signal starts to be adopted for the purpose of automated cruise or
alleviating driver’s fatigue. On the other hand, the system must be obtained
something mechanical back up function in case of emergency condition such as electric failure.
By using the NTN’s own MCU (Mechanical Clutch Unit) technology, NTN has developed “smaller” backlush type
new MCU for the next generation steering system. And it has very useful feature which makes a quickly locking
when the electric power is shut off, it is our unique technology.

failure occurs.
Type 3 is similar to type 2 without the mechanicalbackup mechanism. Instead, an electrical backup
system, like those in aircrafts, is installed to introduce
redundancy. Current regulation requires a mechanicalbackup mechanism for steering when electrical failure
occurs, which makes it difficult to introduce Type 3
into the market.
Therefore, the mainstream SBW technology forvehicles is expected to be the type 2 for the time
being, with a reliable mechanical backup mechanism.
NTN has developed a mechanical backup
mechanism: a compact and high torque capacity
mechanical clutch unit (MCU) with reduced backlash
for next generation steering. The MCU contains a
newly adopted ball cam mechanism that converts
linear motion to rotational motion and has the optimum
layout of rollers in the roller clutch, based on our
proprietary technology.

1. Introduction
Recent improvements in function and reliability ofelectronic control capabilities have enabledconventional mechanical control systems to be
replaced by electrical control systems, as is the casewith brake and acceleration by wire. One technology
leading this trend is steer by wire (SBW), which is thenext generation steering system aimed atautonomousdriving and reduction of driver fatigue.
SBW can be broadly classified into three types, as
shown in Fig. 1 1).
Type 1 is a system that changes steering angles by
operating angle control mechanisms consisting of aconventional steering system, motor and reducer.
Type 2 transmits the steering angle of the steering
wheel by an electrical signal to the steering actuator tocontrol the vehicle steering angle, with a mechanicalbackup mechanism to enable steering when electrical

Operational angle
control mechanism

Steering wheel

Steering reaction actuator
Mechanical backup
mechanism

Power steering

Steered wheel（a）Type1

Steering actuator（b）Type2

Fig. 1 Types of next generation steering*Automotive Business HQ-32-Steering actuator（c）Type3

MCU (Mechanical Clutch Unit) for Next Generation Steering

Table 1 Specifications of MCU for next generation
steering system

2. Application area of MCU for next
generation steering(introduction of direct adaptive steering※)

φ85×190mm

Response time

0.1 s or less

Rated torqueDirect adaptive steering (DAS) is the world’s firstnext generation steering system that Nissan Motor
introduced to the market. NTN’s MCU is installed as
the mechanical backup mechanism of this DAS (Fig. 2).
DAS releases the roller clutch coupling by supplying
current to the electromagnetic clutch inside the MCU,
and connects the steering wheel and the steering
rack, that had been mechanically connected, withelectrical signaling. When electrical failure occurs andcurrent to the electromagnetic clutch is lost, theembedded roller clutch is immediately engaged tomechanically connect the steering wheel to thesteering rack, enabling traditional vehicle steering.
Steering wheel

Size

80N・m

3.1 Adoption of ball cam mechanism
The typical electromagnetic clutch transmits powerfrom the input shaft to the output shaft when current is
supplied. No power is transmitted when current is not
supplied. This is called positive operation. However,as the backup mechanism of SBW, the roller clutch
must be engaged to transmit power when electric
failure occurs (reverse operation).
We adopted a ball cam in the developed MCU,
which converts the linear motion of theelectromagnetic clutch to rotational motion. It is placed
between the electromagnetic clutch and roller clutch toadjust the circumferential position of two cages. In thisconfiguration, the roller clutch is released whencurrent is supplied to the electromagnetic clutch, andpower is not transmitted from the input shaft to the
output shaft. When current is not supplied to theelectromagnetic clutch, the roller clutch is engaged totransmit power from the input shaft to output shaft,achieving reverse operation (Fig. 3).

MCU

Steering shaft

Steering rack

Electromagnetic clutch

Direct adaptive steering

Photo: Courtesy of Nissan Motor Corp.

Ball camFig. 2 Layout of MCU for DAS

Roller clutch

3. NTN proprietary technologysupporting
MCU for next generation steering
The MCU for next generation steering requires thefollowing three points as a backup mechanism:(1) Secure engagement and release of roller clutch(2) Reduction of backlash while the roller clutch isengaged(3) Low friction
We have created a compact MCU with high torquecapacity and reduced backlash by:(1) adopting a new ball cam mechanism(2) placing a pair of rollers in the roller clutch area
The design of our MCU contains the roller clutch
and electromagnetic clutch made into a single unit,
which is our proprietary technology. Table 1 shows
the specification of the MCU for the next generation
steering system.

Fig. 3 Structure of MCU

3.2 Improvement of roller placement
Conventional MCUs are single roller clutch systems
with one roller placed on the cam surface. However, in
the MCU for next generation steering, we introduced atwin roller clutch system where two rollers are placedin a pair to reduce backlash when the rotational
direction is switched (Fig. 4).

※Registered trademark of Nissan Motor Corp.

-33-

NTN TECHNICAL REVIEW No.83（2015）
αα<Engaging roller clutch (mechanical coupling state)>ββ(5) When current is not supplied to theelectromagnetic clutch, the rollers are maintainedat the engaging position by the pressure of thespring located between the rollers.
(6) When power is given from the input shaft to the
output shaft, the rollers are pushed into the “wedge”
made between the input shaft cam surface and theoutput shaft, making mechanical connection.

α＞β(a) Conventional MCU(b) MCU for next generation
steering system

Fig. 4 Reducing backrush

4. Evaluation test
3.3 Operation principles

4.1 Torsional endurance test

The following indicates the operation principles forreleasing and engaging the roller clutch (Fig. 5).

Fig. 6 shows the torsional endurance test result ofapplying 1,000,000 cycles of load while measuring therepetitive rated torque in the CW/CCW direction at the
input shaft, with the roller clutch engaged. The rated
torque of roller clutches is determined by the repetitive
torque load, and is 80 N∙m for the MCU for next
generation steering. There were no issues, such asincrease of backlash, breakage or wear before andafter the test.

<Releasing roller clutch (SBW state)>(1) When current is supplied to the electromagnetic
clutch, cage 1 is attracted to the electromagnetic
clutch (linear motion in axial direction).
(2) The linear motion of cage 1 is converted torotational motion by the ball cam mechanism andcage 1 and 2 rotate in opposite directions(conversion from linear motion to rotational motion).
(3) Due to the counter rotation of cage 1 and 2, thedistance between the rollers is reduced.
(4) Due to the gap between the output shaft and therollers, power from the input shaft (steering wheel)
is not transmitted to the output shaft, resulting in
SBW state.

4.2 Measurement of rotational torqueFig. 7 shows the result of the rotating torque testwhen the input shaft is rotating in one direction while
the roller clutch is released. The increase in rotational
torque remained within 10% of the rotational torque ofthe embedded bearing, accomplishing low friction.

Roller clutch released (SBW state)

Roller clutch engaged (mechanical coupling state)⑤②
Cage 2

Cage 2①⑤

SpringSpringRollerRollerOutput shaft

Output shaft
Cage 1Cage 1

A

B

View from arrow A (roller clutch released)
Cage 2

Cage 1④ Gap

RollerView from arrow B (roller clutch that can be engaged)
Output shaft

RollerRollerRollerInput shaft③

Input shaft cam surface

Fig. 5 Roller clutch mechanism (Pattern diagram)

-34-

⑤

⑥ WedgeMCU (Mechanical Clutch Unit) for Next Generation Steering100

CW:80N・m

80
60

Torque N·m

Torque N·m

40
20
0
-20
-40

Rating 80 N·m

-60
-80

CCW:80N・m-100

Torsional angle deg.

Time sec

Fig. 6 Result of torsional endurance test

Fig. 8 Result of Static torsional strength test (example)

5. Conclusion
Difference from the rotating
torque of embedded bearing:
within 10%
Torque N·m

In this paper, we have introduced NTN’s MCU for
next generation steering, with reduced backlash fromthe adoption of a ball cam, and placing a pair of rollers
in the roller clutch area. These developments are
NTN's proprietary technology. We will launch it in themarket as a product to add backup function to next
generation steering systems, contributing to improved
reliability.

Rotating torqueof embedded
bearing

Rotating torque of MCU
for next generation
steering system

Reference
1) Automotive Electronics Research Group, TER
Sougougiken, Corp., Current Status and Future ofAutomotive By-Wire Technology, 2014 Edition,
Sougougiken, Corp, (2014) 68-97.

Fig. 7 Result of rotating torque test

4.3 Static torsional strength test
Fig. 8 shows the result of static torsional strength
test with torque applied in one direction while the roller
clutch is engaged. When the roller clutch is engaged,
it possesses the rated torque of 80 N∙m or more.

Photo of authors

Naotsugu KITAYAMA

Shintaro ISHIKAWA

Automotive Business HQ

Automotive Business HQ-35-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Clutch for Seat Lifter

Masayoshi YAMADA*
Yasumasa HIBI*NTN has been developing torque diode for seat lifter that adjusts the height of aseat cushion with manual action. Market deployment has been begun since 2002.
For safety improvement of seat, high brake capacity and reliability are required fortorque diode.
In this paper, we will introduce the structure and our product line-up.

Lever forreclining

1. Introduction
Requirements for vehicle safety have increased
more than ever in recent years. Countries around theworld are conducting assessments for improved
vehicle safety and the assessment criteria forpassenger protection performance are made stricter
every year.
Seats for vehicles (hereinafter called seats) are oneof the components which significantly affect thepassenger protection performance. NTN has beendeveloping clutches for seat lifters which are capableof manually adjusting the seat height to the desired
position*1, and then held, contributing to the safety
and comfort of seats.
This product uses NTN’s unique mechanism, the“Torque Diode,” and we introduce its function,mechanism and line-up in this paper.
*1 Jointly developed with Shiroki Corporation

Lever for seat lifter(seat height adjustment)

Fig. 1 Installation condition of torque diodeFig. 2 shows the layout of the torque diode and thesector gear installed on the seat.
When a load is applied to the seat, the torque istransmitted to the pinion gear (output shaft) of thetorque diode via the sector gear (link mechanism) andthe seat height is held by locking the torque diode.
On the other hand, when the lever is operated fromthe input side, the pinion gear rotates and the entire
seat moves up/down and forward/backward by the linkmechanism of the sector gear, and so the seat height
can be adjusted.

2. Function of the torque diodeA torque diode is a bidirectional engaging clutch
based on a roller clutch mechanism, that can be used
to transmit the rotating force from the input to the
output side, whilelocking the rotating force from beingtransmitted from the output to the input side. By using
this function, the up/down seat height can becontinually adjusted, and its position held, byoperating the lever installed on the side of the seat .
Fig. 1 shows the typical installation of the torquediode on the seat.

*Automotive Business HQ-36-

Clutch for Seat Lifter

Torque diodeSeat surface

Load bypassenger
weightSector gear

Rotating directionof pinion gear

Movement of seat

Rotating directionof sector gear

Fig. 2 Layout of torque diode

Structure/operation principle(1) The mechanism to hold the seat height

3. The mechanism and operating
principle of the torque diodeFig. 4 shows the torque transmission path when theseat height is held. The load from the seat istransmitted from the pinion gear to the outer ring on
the brake side fixed to the stationary seat frame via
the roller of the brake-side clutch; consequently, theoutput shaft becomes locked in order to maintain theseat height. Fig. 5 shows the pattern diagram of thecross section of the clutch on the brake side, for thisexample. A pair of rollers and a spring are inserted
into the wedge-shape space formed by the output
shaft and the inner diameter of the outer ring, andbecause the rollers are pushed into the narrow area of
the wedge-shape space by the spring, the assembly islocked.

The torque diode for the seat lifter is required tohave both(1) An ability to hold the seat height, and(2) An ability to move the seat up and down bycontrolling the lever.
As shown in Fig. 3, the torque diode consists of two
clutches, a clutch on the brake side with the above
mechanism (1) and a clutch on the lever side with thefunction of transmitting the up/down operation of thelever to the clutch on the brake side. With these two
clutches, the output components of the lever-side
clutch (inner ring) and the input components of thebrake-side clutch are made into one assembly.

Lever-side clutch
Transmits torque to the brake-side
clutch according to the lever operation
Input material on
the lever side(outer ring)

Brake-side clutch
Holds the seat height

Output material on
the lever side (inner ring)and input material on Y
the brake side

Outer ring on the brake side
(fixed to the frame)

Z
Pinion gear

Roller on thelever side

Output shaft

Y

Z

Cross section of clutch on the lever side (Y-Y)

Roller on thebrake side

Cross section of clutch on the brake side (Z-Z)

Fig. 3 Structure of torque diode for seat lifter-37-

NTN TECHNICAL REVIEW No.83（2015）(2) The mechanism to move the seat up anddown by controlling the lever

X

When the seat is moved up/down by the lever
operation, the rollers on the brake-side clutch need tobe released from the wedge-shaped space and thetorque from the lever-side clutch needs to betransmitted to the pinion gear and the sector gear. The
torque transmission path when operating the lever isshown in Fig. 6. By using this lever, the torque istransmitted to the output shaft of the brake-side clutch
via the lever-side outer ring, rollers and inner ring.
At the start of the rotation of the inner ring, the sideof the pillar section of the inner ring pushes the roller
onto the brake-side clutch, moving it to the wider areaof the wedge-shape space (unlocking the output shaft;
Fig. 7 (a)).
Rotating the inner ring further, the rotating force of
the inner ring is transmitted to the output shaft by thecontact of the elongated hole of the inner ring and the
protruded portion of the output shaft (Fig. 7 (b)).
When the lever stops moving, the rollers arepushed to the wedge-shaped space by thecompressive force of the spring placed between therollers, and the locking state is restored, where therotation of the output shaft is locked in both
directions, (Fig. 5).

Output shaft
pinion gear part

X

Load on theseat side
Outer ringon the brake side

Fig. 4 Torque flow at neutral position
Outer ring

Roller (lock-side)

SpringInner ring
Output shaft

Fig. 5 Neutral position of brake-side clutch(cross section viewed from X-X)

W

Outer ring on
the lever side

Inner ring pillar
section side

RollerInner
ring

Torque is transmitted by the contact ofelongated hole portion of the inner ringand the protruded portion of the output shaft

ContactW

Outputshaft

Lever operationin the rotating
direction of theinner ring

Protruded
portion of
the output
shaft
Gap

Inner ringelongated hole(a) Unlocked output shaft(gap in the protruded portionof the output shaft)

Fig. 6 Torque flow at switching position

Contact

Rotation of
output shaft(b) Rotating output shaft(contact with the protruded
portion of the output shaft)

Fig. 7 Switching position of brake-side clutch(cross section of viewed from W-W)

-38-

Clutch for Seat Lifter

Table 1 Product line-up of torque diode for seat lifter
Specification

Characteristics(1) 80N・m(2) φ50×32mm(3) 330g

・Thin outer ring is adopted as the outerring of the brake-side clutch
・Lightweight and compact design

2nd generation(1) 130N・m(2) φ60×25mm(3) 380g

3rd generation

・Thinner and higher torque realized bythinner outer ring of the brake-side
clutch(1) 170N・m(2) φ61×25mm(3) 390g

・Improved safety against collision byhigher torque, yet maintaining similar
installation size as the 2nd generation

4th generation

1st generation

Appearance(1) 230N・m(2) φ61×27mm(3) 420g

・Improved safety against collision byeven higher torque than the 3rd
generation by longer rollers

・Improved seat holding tolerance
against rear-ended accidents

・Improvement in comfort level alongwith larger seat cushion(1) Holding strength (2) Clutch OD x Installation width (3) Weight

4. Product line-up and features

5. Conclusion

Since starting mass production in 2002, we havebeen responding to the market needs of increasing
seat height holding capability and tighter spaceconstraints . We have a line-up of products, up to the
4th generation, which have been adopted by many
auto manufacturers and vehicle models (Table 1 andFig. 8).

In this paper, we have introduced the function andmechanism of the torque diode, invented by NTN, aswell as the product line-up. The torque diode is an
important product contributing to the improved safety
and comfort of seats. We will continue toimprove/develop products that meet market needs.

Reference
1) Masahiro KAWAI, NTN TECHNICAL REVIEW No. 79(2011) Torque Diode for Seat Lifters

Holding torque N·m

250

4th generation

200

Photo of authors

3rd generation

150
100

Thinner form factor

Improvement ofholding strength

2nd generation

1st generation

50
0
40

35

30

25

Installation width mm

20

15

Fig. 8 Performance comparison-39-

Masayoshi YAMADA

Yasumasa HIBI

Automotive Business HQ

Automotive Business HQ

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Next-generation Driveshaft “ADS Module”

Masafumi OOSUGI*
Tatsuro SUGIYAMA*

Recently, low-fuel consumption and weight reduction are important problems worldwide in the automotive
industry. And, development speed-up and development man-hour reduction according to standardization of a carcomponent are also important problems. Responding to these problems, NTN proposes to worldwide automobile
manufacturers the ADS module (Advanced Drive Shaft module) which modularized the constant velocity joint (CVJ)and hub-bearing (H/B) for drivetrain with a new concept and high manufacturing technology.

1. Introduction

2. Structure and features

The power from the engine is transmitted to the tires
through the transmission, differential gear (hereinafter,
differential) and drive shaft. Because the differential
gear shaft, which is the shaft to input power, and the
hub bearings (H/B), which support the tires, rotate atdifferent angles, the drive shaft uses a constantvelocity joint (CVJ). This ensures that torque istransmitted smoothly and with constant velocity.
The requirement specifications for a differential, H/B
and torque vary depending on the vehicle.
Conventional drive shafts require combining CVJs andshafts individually designed according to the required
specifications. Each vehicle requires repeating trial
productions and evaluations, which results in longer
development lead time.
NTN possesses technologies accumulated fromresearch and development activities with thebackground of one of the world’s top class sales
volume, and state-of-the-art manufacturing
technologies such as the "press-connect method"
joint, which will be described later, and electron beam
welding, 1). By using these technologies, we areproposing the next generation drive shaft called“Advanced Drive Shaft (ADS) module”. ADS
combines various components that are different
depending on the vehicle types, such as shafts withdifferent lengths and shapes and CVJ components
commonly designed according to the required torques.

The ADS module is a combination of the tire sideand the differential side or “EBW drive shaft”, which
will be discussed later. The tire side is made byconnecting a CVJ and a H/B that is standardized with
common architecture based on torque requirementwith the “press connect method”. The differential side
is made by connecting a CVJ similarly standardized
with common architecture based on torquerequirement and individually designed stem part withelectron beam welding.

2.1 Common specifications
The specifications of the internal components suchas inner rings, cages, balls, and boots whichconstitute CVJ are already standardized with thecommon architecture.
With the ADS module, NTN is designing CVJs on
the tire side and the cup of the CVJ on the differential
side with the common architecture. The H/B, the stemon the differential-side CVJ, and the shaft to beengaged between the tire-side CVJ and the differentialside CVJ are to follow the individual specifications
according to the vehicle model (Fig. 1 and 2).

2.2 Features of PCS-H/J
For connection, the splines are pre-formed on the
inner diameter of H/B with smaller tooth width than theouter ring stem splines (pre-spline) and gaps areprovided between the tips of the stem teeth and thebottom part of the grooves of H/B, enabling a bolt joint*Drive Shaft Engineering Dept. Automotive Business HQ-40-

Next-generation Driveshaft “ADS Module”

PCS-H/J

EBW drive shaft

Press connect joint

Specification varied
by vehicle

Commonspecification

Joint with electron
beam welding

Specification varied
by vehicle

Fig. 1 Structure of ADS module

Fig. 2 Structure of ADS module (photo)and reducing the load required for the joint process.
This joining method is called the press connect
method (PCS). By this design, the assembly processof vehicle production line of the automotive
manufacturers is not required to be changed for
assembling the PCS-H/J (Fig. 3).
This joining method is characterized by significant
reduction of weight and elimination of backlash of the
spline joint. With this method, there is no gap in the
joint as opposed to the conventional spline joint;
therefore, the input torque can be uniformly borne by
the entire circumference. We could also reduce thelength of the CVJ stem by approx. 65%. Furthermore,
the weight was reduced to 0.94 kg/unit (approx. 12%
compared with the conventional unit in case of
application to the C segment) by thinning the insidewall of the hub ring and the use of a bolt instead of a
nut.

-41-

Hub bearing

Press connect joint

Constant velocity joint

Fastening bolt

Fig. 3 Structure of PCS-H/JNTN TECHNICAL REVIEW No.83（2015）

2.3 Features of EBW drive shaft

3. Summary

Electron beam welding (EBW) is developed forconnecting the common cup (outer ring) and the stem
(shaft) (Fig. 4).
In the case of long stem type, the joint of EBW drive
shaft can be made closer to the cup when comparedwith conventional friction welding. Therefore, EBW isa more suitable joining method for common cups.
In the case of short stem type, the cup has been
conventionally integrated. However, by separating thestem, which shape can vary depending on thevehicles, we could achieve commonalty of cups.
The welding quality is better than the other welding
methods because it is done in a vacuum, with lesswelding flaws and sufficient strength to meet therequirements.
Furthermore, since there is no flash on the joints,
the turning process can be eliminated after the joining
process.

The ADS modules can reduce the development
lead time of drive shafts and contribute to the vehicle
ride quality and fuel economy by making the module
products smaller and lighter.
We will proactively propose the ADS modules to theauto manufacturing industry.

Fig. 5 Application

Reference
1) Takayuki NORIMATSU, Tsutomu NAGATA, Press
Connect Spline Hub Joint, NTN TECHNICAL REVIEW
No. 81, (2013) 58-63.

Cross section of joint
電子ビーム溶接接合部断面
by electron beam welding

Fig. 4 Structure of EBW driveshaft

Photo of authors

Masafumi OOSUGITatsuro SUGIYAMA

Drive Shaft Engineering
Department,
Automotive Business HQ

Drive Shaft Engineering
Department,
Automotive Business HQ-42-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Technology Trends and Products forAccessory Drive Belt Systems

Ayumi AKIYAMA*
Hiroo MORIMOTO**

As a superior car in the mileage, strong and mild HEVs are increasing and theaccessory drive belt system is diversifying. On the other hand, requests for theaccessory drive belt system are changing rapidly under the influence of ISG(Integrated Starter Generator) and engine downsizing.
This paper introduces the technology trends and products of NTN for theaccessory drive belt system corresponding to the change of the engine.

1. Introduction[Idler pulley]

Various expectations have been given to
automobiles throughout their history of 100+ years.
They must be capable of transporting people andcargo quickly and securely to distant places, as wellas providing pleasure and relaxation in our daily lives.
Automobiles have been responding to these
expectations by adding more and more complex
equipment with greater functionality. In addition tothese continually growing expectations, the next
generation of vehicles is further required to deal withprotecting the environment and saving resources.
NTN has been developing and marketing light andcompact auto tensioners and pulleys for accessory
drive belt systems to respond to these requirements 1).
Fig. 1 shows an engine overview and examples of
NTN products for engine accessory applications. The
focus of this paper is to introduce the structure andfeatures of NTN’s auto tensioners and pulleys.

[Auto tensioner]

Alternator[pulley, bearing]

Turbo charger[bearing]
Compressor for A/C[pulley, bearing, seal]

Water pump[module, pulley, bearing]

Fig. 1 The view of automotive engine with NTN products

transitioned from a multiple belt-setting, whereindividual belts drive each accessory pulley from thecrank pulley, to a single belt-setting (serpentine
method) which reduces the total length of the engine.
This transition caused a large variation of accessory
belt tension due to thermal expansion/compression of
the entire engine and degradation of belt over time.An
auto tensioner is adopted to absorb this variation oftension and extend the belt life.

2. Current status of accessory drive
belt systems
2.1 Configuration of accessory drive belt system
Vehicle engines drive each accessory system suchas water pump, A/C compressor and alternator bytransmitting the rotation of the crankshaft throughpulleys and belts. The belt setting method has**Application Engineering Dept. Automotive Business HQ**Automotive Product Design Dept. Automotive Business HQ-43-

NTN TECHNICAL REVIEW No.83（2015）

2.2 Fuel saving technology for the accessory
drive belt systemused today is deeply pushed in by this increased belttension, creating slippage between the pulley and belt.
This can not only cause a squeaking sound andreduced belt life, but also may result in failure torestart the engine. If the initial tension is set higher inorder to prevent slippage during the high transmission
torque needed to restart the engine, friction loss
during steady driving becomes high, resulting inadverse fuel economy effects.
In general, loads on accessory drive belt systems
are expected to continually increase. For example,
with the recent trend of downsizing engines andadding turbochargers, the variation of crankshaft
rotational speed will be larger due to the reduced
number of engine cylinders. Furthermore, with the
adoption of a 48V battery in progress in Europe, it isexpected that loads on the alternator (generator) will
increase along with larger battery capacity. Therefore,
accessory belts and pulleys, will be required to haveincreased durability and higher load capacity. It is theauto tensioner that reduces these loads on accessory
drive belt system, and unprecedented new functions
will be required to balance these loads.

Today, adoption of an idling stop system isincreasing as a fuel saving technology. Traditionally,
the starter motor was used to restart the engine after
idling stop. However, to reduce the time required torestart the engine, and to reduce the noise andvibration, the belt drive ISG (Integrated Starter
Generator) method is being adopted, using thealternator as the starter motor (Fig. 2). The ISG
method is characterized by compatibility with theconventional accessory system layout, superb quiet
operation due to the friction-belt transmission
mechanism, and short engine restarting time.
The ISG method does present a challenge for the
auto tensioner, in that the area of high tension and low
tension of the belt switches when changing between
restarting the engine and steady driving (Fig. 3).
Typically, the auto tensioner is set at a position where
the belt is in low tension during steady driving. Whenthe engine restarts with the ISG, the area of hightension and low tension switches, and the tension at
the belt position where auto tensioner is set isinstantly increased. The conventional auto tensioner

3. NTN’s auto tensioners and pulleys
for accessory drive belt systems
The following is an overview of NTN's auto
tensioner as well as steel and resin pulley options foruse in engine accessory drive belt systems.

ISG

Integrated Starter
Generator

1) Auto tensioners
ISG belt

The accessory drive belt system transmits driving
power from the crank pulley to each accessory
component pulley through friction of the drive belt. The
auto tensioner for the accessory system belt functions
to guarantee a minimum tension, as well as to reduce
any excessive tension in the system.
Auto tensioners for accessory system belts can bebroadly classified into mechanical auto tensioners andhydraulic auto tensioners, depending on the damper
method used to absorb the variations in belt tension.
The mechanical system provides damper effectthrough friction resistance with material such as resin,along with compression/torsion springs. Therefore, thedamper properties are significantly changed byvariation of the friction coefficient. In addition, themaximum load capacity is limited by the property of
the adopted spring. It is not suitable to use this type of
auto tensioner for engines with an ISG system,
because they cannot withstand the instant high loads.
On the other hand, the hydraulic auto tensioner
provides damper effect by the resistance of oil passing
through the internal oil channel. Therefore, it ispossible for the hydraulic tensioner to cope better with

Auto tensioner

Water pump be

lt

Crank pulleyFig. 2 Engine equipped with ISG 2)

ISG
starting

Steadydriving
Auto tensioner

ISG pulley

ISG pulley(drive pulley)

Pushed in

Crank pulley(drive pulley)

AccessorypulleyFig. 3 Modes of belt span at the time of steady driving
and engine starting-44-

Technology Trends and Products for Accessory Drive Belt Systems

for lower fuel use intensifies, NTN has developed thevariable damper type auto tensioner. This new auto
tensioner automatically changes the damper
properties such that the belt tension is maintained low
during normal driving and high belt tension is
generated when starting the engine to prevent
slippage. This product is introduced in “AutoTensioner with the Variable Damper Mechanism for
ISG-equipped Engine” in this issue of the Technical
Review.

the sudden change of belt tension and high load
compared with the mechanical system.
NTN has developed and launched highly reliable
hydraulic auto tensioners with advanced functions.
Our compact and high load short type auto tensioners
have been adopted for use on the aforementioned
engines equipped with ISG systems. Fig. 4 shows the
structure of NTN’s short type auto tensioner for highload. This product can sufficiently address the sudden
increase of belt tension when restarting an engine withan ISG.
Since the hydraulic auto tensioner follows variationof belt tension very closely, when used on an engine
with a large variation of tension, the tensioner reaction
force may increase more than necessary resulting inexcessive belt tension. In order to prevent thisexcessive belt tension, NTN has developed a newauto tensioner with a relief mechanism, improving on
the short type auto tensioner. Fig. 5 shows the
structure of the hydraulic auto tensioner with relief
mechanism.
Currently, short type auto tensioners for high loads
are applied to engines with ISG systems. However,they require a higher setting for belt tension comparedwith ordinary (non-ISG) engines, which results inslightly higher belt friction losses. As the competition

2) Steel pulleys
Steel pulleys for accessory systems combine steel
pulley material and dedicated bearings in one unit.
These can be broadly classified into pulleys made bymachining the pulley material and pulleys
manufactured by press forming. The machined pulleys
are characterized by high load carrying capacity andreliability; however, they may become costly in volume
production due to the machining process.
On the other hand, pressed pulleys can be made at
low cost and are suitable for volume production;
however they have a lower degree of freedom in theirdesign compared with machined pulleys. Pressedpulleys are limited in producible section geometries,
and as a result, it has been difficult to use them forhigh load applications. Accordingly, NTN has
developed the “Low cross-section high strengthpressed pulley” which has a strength equivalent tomachined pulleys, yet is producible with the highly
productive press forming process. Fig. 6 shows thecomparison of cross-sections between theconventional pressed pulley and the low cross-section
high strength pressed pulley.
As shown in Fig. 6, the new design has
proportionally less cross section area against the totalheight compared with the conventional product. Dueto this smaller cross-section, the size of the bearing

Rod
Leak gap
Valve sleeve
Pressure chamber
Check valve

Fig. 4 Short type auto tensioner for high-loadRelief valve

Conventionalpressed pulley

Low section high strengthpressed pulleyFig. 6 Cross-section of conventional pulley anddeveloped product with press forming

Fig. 5 Auto tensioner with relief valve-45-

NTN TECHNICAL REVIEW No.83（2015）

properties of the resin to form complex shapes and be
insert molded with other components.

can be made larger without changing the outer
diameter of the pulley. In addition, by making thepulley material thicker, the load carrying capacity ofthe pulley and bearing can be increased with thesame installation size.
On the other hand, recently,Additionally, due to therecent improvement of accessory belt bending
durability, smaller pulleys are being adopted. In thiscase, it is also possible to maintain the bearing size
and make the pressed pulley outer diameter smaller
through use of this new low cross section highstrength design.

Resin based

3) Resin pulleys

Resin pulley
for high load

Steel pulley

Resin pulleys can be used for a significant reductionof weight compared with steel pulleys. However,
because of their lower load capacity, in addition tochallenges with wear and environment tolerance, they
were not typically selected for use in real world
applications.
However, since the requirement for improvement offuel economy has increased, adoption of resin pulleys
is now expanding by both European and North
American auto manufacturers. The lightweight and low
inertia properties of resin pulleys lead to improved fueleconomy. The material mainly used for resin pulleys isthermoplastic polyamide resin. For improvement of
mechanical properties, PA66+ organic reinforcement
material is now being adopted instead of PA6+
inorganic reinforcement material.
NTN is introducing a “high load resin pulley” withincreased load tolerance. A phenol resin has been
adopted, which is thermosetting resin instead of theconventional thermoplastic resin. With this material,
these new resin pulleys are able to withstand an
equivalent load to a steel pulley, which was notpossible with the conventional resin pulleys. Phenol
resin is generally considered as hard and brittle, butwith recent progress in technologies for various
additives, materials with improved toughness havebeen developed. In addition, the manufacturing
process has recently evolved, enabling injection
molding with this new material that is equivalent to theprocess used with thermoplastic resin. Also, the costand productivity of compression molding, which used
to be the mainstream process, has been improved.
Fig. 7 shows the appearance of a steel pulley and aresin pulley. Through adoption of the thermoplastic
resin, these pulleys can be used in a relatively hightemperature environment, achieve 1/3 of the weightand equivalent load capacity as the steel pulley of thesame installation size.
In addition, it is expected that this material will be
used to make various types of pulleys, leveraging the

Fig. 7 Steel pulley and resin pulley

4. Future trends of accessory drive belt
systems

Grams CO2 per Kilometer normalized to NEDC

Fig. 8 shows the fuel consumption regulations in
key regions. The EU has the strictest target of
reducing the CO2 emission to 95 g/km by 2021. To
achieve this target, the EU focused on mild hybrid
vehicles using a 48V power supply.
The EU auto manufacturers are aiming at further
fuel efficiency improvement by not only by restarting
the engine after an idling stop but also by
regeneration, assist, and driving with an electric motor
depending on the conditions, lengthening the time theengine is not in use.
Downsizing of engines, along with the adoption ofturbochargers and idling stop mechanisms are being
increasingly used in many vehicle models as aneffective means for improving fuel economy. On the

US-LDV

260

Canada-LDV

240

EU

220

Japan
China

200
180

S.Korea

Mexico 2016:173
Canada 2016:170

160

Australia
MexicoS.Korea 2015:153

140
120

China 2020:117
Japan 2020:105

100

US 2025:107

EU 2020:95

80
60
40
2000

2005

2010

2015

2020

2025

(1) China’ s target reflects gasoline vehicles only. The target may be lower after new energy vehicles are concidered.
(2) US, Canada, and Mexico light-duty vehicles include light-commercial vehicles.

Fig. 8 Mileage regulation of key regions(CO2 emissions, g/km) 3)-46-

Technology Trends and Products for Accessory Drive Belt Systems

other hand, it is expected that the load on the belts,
pulleys (bearings) and auto tensioners of accessory
drive belt system will increase because of the increasein variation of crank rotation and accessory belttension as the number of engine cylinders is reduced.
With the continuing technology development forlower fuel consumption, a comprehensive optimizationof the entire accessory belt drive system will become
critical. This is especially the case when applying
associated technologies such as hybrid systems,
downsized turbocharged engines, and a 48V power
supply.

5. Conclusion
In this paper, we have discussed the technologicaltrends of accessory drive belt systems and introduced
the NTN products to be used with those systems;
auto tensioners and various pulleys.
Since the regulations for fuel consumption are
becoming stricter in key regions, the demand for lower
fuel consumption will further increase. In order to
respond to these requirements, NTN will address thediversified market needs, based on the desirable formof the entire accessory drive belt system throughcontinued development of our auto tensioners andpulleys.

Reference
1) Seiji SATO, Technology Trends in Auto Tensioners, NTN
TECHNICAL REVIEW, 79 (2011) 83-89
2) For example, Auto Prove Web home page3) ICCT (International Council on Clean Transportation),
2013

Photo of authors

Ayumi AKIYAMA

Hiroo MORIMOTO

Application Engineering Dept.
Automotive Business HQ

Automotive Product Design Dept.
Automotive Business HQ-47-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Auto Tensioner with Variable Damper Mechanism
for ISG-equipped Engines

Hiroo MORIMOTO*
Yuta MOCHIZUKI**

It is necessary to set the belt tension rather higher than usual to prevent the belt fromslipping at the time of the engine restarting when we applied a conventional auto tensioner tothe accessary drive belt of the engine equipping with ISG and it causes harm for millage
improvement.
This paper introduces the new developed variable damper auto tensioner which contributes
to the mileage improvement of the ISG-equipped car.

1. Introduction
between steady driving and engine restarting, auto
tensioners used with ISG systems are required tohave new functions.
In order to respond to these requirements, NTN has
developed a new “variable damper auto tensioner for
ISG-equipped engines” for the accessory drive belts ofvehicles with this feature.

Currently, all motor vehicles are being required toaddress a number of environment protection andresource saving concerns. An increased number ofvehicles are being equipped with an idling stopmechanism as one of the means for improving fueleconomy of the conventional internal combustion
engine vehicle (Fig. 1). This trend is expected tocontinue, and the method of restarting the engine isshifting to the integrated starter generator (ISG). Thisis a belt driven method where the crankshaft is driven
by the accessory belt from the alternator. This engine
restarting method is characterized by quick engine
restarting time and quiet operation, compared withusing the traditional starter motor.
For the accessory drive belt system of mostvehicles (Fig. 2), an auto tensioner is used to maintainan adequate belt tension. This prolongs the life of thebelt and accessory systems and also prevents
vibration and noise due to belt slippage 1). To
effectively reduce vibration of belt, the auto tensioner
is usually placed at the lower tensioned area of thebelt when the engine is operating, namely, after the
crank pulley (driving side). However, when using the
ISG method for idle stop, the auto tensioner must be
set at the area where the belt is in high tension whenthe engine restarts, namely, before the ISG pulley.
Since the areas of high tension and low tension switch

Production Volume

120

mil.

100
80

Idling stop vehicle

60

Fuel Cell
Electric

40

Hybrid-Mild
Hybrid-Full
ICE:Stop/Start

20
0
Apr-Mar.2010

ICE

Apr-Mar.2013

Apr-Mar.2016

Apr-Mar.2019

Fiscal Year

Fig. 1 Trend of automotive propulsion system 2)**Automotive Product Design Dept. Automotive Business HQ**Functional Experiment Dept. Automotive Business HQ-48-

Auto Tensioner with Variable Damper Mechanism for ISG-equipped Engines

Auto tensioner

Duringsteady driving

ISG

Auto tensioner

ISG pulley

ISG
restarting

ISG pulley(drive pulley)

Crank pulleyHigh tension at
ISG restarting

Crank pulley(drive pulley)

Accessory systempulley(a) During steady driving

Accessory drive belt(b) ISG restarting

Fig. 3 Figure of FEAD layout

Fig. 2 Accessory belt drive system

2. Characteristics of engines adopting
the belt driven ISG method

3. Variable damper auto tensioner
for ISG equipped engines

Fig. 3 shows a diagram of a typical accessory
system layout of a vehicle engine. Fig. 3 (a) shows
the system status under steady driving conditions.
During this state, the crank pulley is the driving pulley.
Since the crank rotation speed changes due to the
combustion process of each cylinder, the belt tension
fluctuates at the frequency of engine rotation x (1/2) x
number of cylinders. By minimizing the reaction force
generated with this fluctuation of the belt tension(hereinafter, tensioner reaction force), we can reduce
the average belt tension, increasing the life of the belt
and accessory systems, as well as, improving fueleconomy by reduction of friction losses. In otherwords, the smaller the tensioner reaction force to thebelt, the more desirable.
On the other hand, Fig. 3 (b) shows the systemstatus when the engine is restarting (hereinafter, ISG
restarting). The ISG pulley acts as the driving pulley toturn the stopped crank pulley through the belt. At the
moment of ISG restart, the belt tension between the
ISG pulley and the crank pulley suddenly increases.
As a result, the auto tensioner is deeply pushed in,creating lower tension. Therefore, the tensioner
reaction force must typically be set at a higher set
value to avoid belt slippage.
It seems then, that auto tensioners in engines
equipped with an ISG are required to provide
conflicting functions; low tensioner reaction force
during steady driving and sufficient tensioner reaction
force to avoid slip during ISG restarting.

3.1 Structure
Fig. 4 and 5 show the cross-sections of theconventional product and the new variable damper
auto tensioner for ISG equipped engines (hereinafter,
developed product), respectively. The main feature of
the developed product is the ability to switch thetensioner reaction force according to driving
conditions with a simple mechanism and without useof electrical control, etc.
The conventional product creates a pressurechamber and leak gap with the valve sleeve and rod.
Along with the extension/compression of the auto
tensioner, the rod is pushed in/out against the valve
sleeve.
For the conventional product, the hydraulictensioner reaction force is generated by elastic
deformation of oil in the pressure chamber and flow
resistance as oil passes this leak gap.
The developed product has a pressure chamber
and two leak gaps created by the valve sleeve,plunger and rod. We can describe the radial gap
between the outer diameter of the rod and the innerdiameter of the plunger as the first leak gap. The
radial gap between the outer diameter of the plunger
and the inner diameter of the valve sleeve is thesecond leak gap. In addition, a second check valve isformed by the rod, plunger, retaining ring and switch
spring. The second check valve is open when thepressure in the pressure chamber is low, since theplunger is pushed toward the pressure chamber by
the switch spring. The tensioner reaction force
generated is adjusted by the opening/closing and thesize of two leak gaps.

-49-

NTN TECHNICAL REVIEW No.83（2015）

valve sleeve. As the plunger is pushed by the switch
spring, it is fixed to the tip of the rod contacting theretaining ring, moving with the rod. The pressure of
the pressure chamber increases as the oil in thepressure chamber is compressed, and the oil leaks
from the pressure chamber via the first leak gap
between the outer diameter of the rod and the innerdiameter of plunger, and through the second checkvalve. The second check valve remains open as thepressure in the pressure chamber is still lower than
the force of the switch spring pushing the plunger.

Rod
Leak gap
Valve sleeve

Pressure chamber

Fig. 4 Structure of conventional product

Plunger
RodFirst leakgap

Rod
Switch spring
Second check valve

Tensioner reaction force

Check valve

Extended ← Tensioner length → ShrunkPressurechamberFirst leak gap

Fig. 6 Region where auto tensioner shrinks and reaction
force increases

Plunger
Second leak gap
Valve sleeve(2) Region where the auto tensioner shrinks,
yet the reaction force remains mostly
unchanged (Fig. 7)

Retaining ring
SpringPressure chamberFirst check valve

Fig. 5 Structure of development product

3.2 Operation principle
Fig. 6-8 illustrate the relationship between thechange in length of the auto tensioner and theresulting tensioner reaction force when the developed
product is extended/shrunk at an amplitude equivalent
to that of ISG restarting conditions. The graphs shownin the right side of each diagram are called Lissajous
patterns. The Lissajous patterns of the developed
product can be divided into three regions according tothe operational conditions. The following is thedescription of the internal operation of the auto
tensioner for each region:

Secondcheck
valve(1) Region where the auto tensioner shrinks andthe reaction force increases (Fig. 6)

Tensioner reaction force

In this region, the force of the switch spring pushing
the plunger is balanced with the pressure in thepressure chamber. Therefore, only the rod moves, into
the valve sleeve as the auto tensioner shrinks, and thechange in the pressure chamber volume is smaller.
As the pressure continues to increase, the switch
spring gradually shrinks, maintaining the balanced
condition. When the compression of the switch spring
reaches the stroke of the second check valve, thesecond check valves closes.

Second check valve
ClosedExtended ← Tensioner length → Shrunk

In this region, the auto tensioner starts shrinking
from its most extended condition. As the auto
tensioner shrinks, the rod and plunger move into theFig. 7 Region where auto tensioner shrinks but reaction
force remains unchanged-50-

Auto Tensioner with Variable Damper Mechanism for ISG-equipped Engines(3) Region where the auto tensioner shrinks andthe reaction force increases again (Fig. 8)

resulting in adverse fuel economy effects due toincreased friction.

In this region, the pressure in the pressure chamber
exceeds the force of the switch spring pushing theplunger. Therefore, the second check valve closes
and the plunger moves into the valve sleeve with therod. At the same time, the pressure of the pressurechamber is increasing and the oil leaks from thesecond leak gap formed by the outer diameter of theplunger and the inner diameter of the valve sleeve.

Fig. 10 shows the Lissajous pattern considering theamplitude that occurs during ISG restarting. At ISG
restart, since the belt tension instantly increases, alarge tensioner reaction force is required.
The conventional product with high reaction force
specifications can ensure the belt tension required for
ISG restarting, not allowing any belt slippage.
However, the conventional product with low reaction
force spec. cannot ensure the belt tension required for
ISG restart, as the tensioner reaction force isinsufficient, likely leading to slippage between the belt
and pulleys.

Tensioner reaction force

Secondleak gap(2) Belt tension adjustment with theconventional product during ISG restartTensioner reaction force

Extended ← Tensioner length → Shrunk

Conventional product(low reaction force spec.)
Conventional product(high reaction force spec.)

Fig. 8 Region where auto tensioner shrinks and reaction
force increases again

4. Achieving the ideal damper property

Excessive belttensionLow tension

4.1 Comparison of properties between theconventional and developed products

Amplitude: small

ExtendedWith the conventional product, the conflicting
properties, that is, the reduction of belt tension during
steady driving and addition of relatively large belttension at ISG restarting, cannot be simultaneously
achieved.

Tensioner length

Shrunk

Fig. 9 Lissajous pattern during steady driving(conventional product)(1) Belt tension adjustment by the conventional
product during the steady driving
Tensioner reaction force

Fig. 9 shows the Lissajous pattern considering
steady driving conditions. During steady driving
conditions, since the fluctuation of the belt tension issmall, the extension range of the auto tensioner isalso small. Therefore, the tensioner reaction force
generated by the extension/shrinkage is small with theconventional product designed for low belt tension(low reaction force spec.). On the other hand, with theconventional product designed for high belt tension
required for ISG restarting (high reaction force spec.),
the tensioner reaction force becomes large even if theamplitude of the auto tensioner is small. Therefore,
the belt tension becomes unnecessarily large,

Conventional product(low reaction force spec.)
Conventional product(high reaction force spec.)
Requiredhigh belttensionInsufficient
belt tension

ExtendedTensioner length

Amplitude:
large

Shrunk

Fig. 10 Lissajous pattern at ISG restarting(conventional product)-51-

NTN TECHNICAL REVIEW No.83（2015）(3) Belt tension adjustment with the developed
product(a) Developed productRotational speed

ISG pulley starts

ISG pulley
Crank pulleyEngine starts

Developed product(large amplitude)
Developed product(small amplitude)
ISG restarting
high reaction force

Time(b) Conventional product(high reaction force spec.)
ISG pulley
Crank pulley

Steady driving
low reaction force
ExtendedTensioner length

Rotational speed

Tensioner reaction force

Fig. 11 shows the Lissajous pattern of thedeveloped product. The tensioner reaction force canbe maintained low during steady driving, similar to theconventional product with low reaction force spec.
However, during ISG restarting, a large tensioner
reaction force can be instantly generated similar to theconventional product with high reaction force spec.

Shrunk

Fig. 11 Lissajous pattern of developed product

ISG pulley starts

Engine starts

4.2 Verification of the effect with the actual
vehicleNTN has installed the developed product on an
actual vehicle and measured; (1) pulley rotation speed
when restarting the engine with the ISG, and (2)tensioner reaction force during steady driving. As acomparison, the conventional product (high reaction
force spec.) was also measured.

Time

Fig. 12 Measurement results of pulley rotational speed
at ISG restarting(1) Restarting the engine with the ISG(2) Auto tensioner reaction force during steady
driving

Fig. 12 (a) and (b) show the measurement resultsof the developed product and the conventional product(high reaction force spec.), respectively. We havedefined the engine restarting point as the moment
when the crank pulley rotation speed starts stabilizing,
and we compared the time from the moment the ISG
pulley rotation started until the engine restarting point.
From the test, the time until the engine restarting
point was not later than the conventional product withhigh reaction force spec. and the correct ISG
restarting was verified. During the measurement, no
belt slippage occurred.

Fig. 13 shows an example of the auto tensioner
reaction force measurement results at a certain
rotation speed of the engine. Graph (a) shows theresults of the developed product when installed on an
actual vehicle and (b) shows the results of theconventional product (high reaction force spec.). The
developed product reduced the maximum value of thetensioner reaction force by around 50% comparedwith the conventional product. From this, we can
expect an improvement of fuel economy throughreduction of tension on the accessory belt.

-52-

Tensioner reaction force N

Auto Tensioner with Variable Damper Mechanism for ISG-equipped Engines

5. Conclusion(a) Developed productTensioner reaction force N

NTN has developed a variable damper auto
tensioner for use on ISG equipped engines that can
automatically adjust the accessory belt tension
according to the driving conditions. NTN has verified
the developed product’s effectiveness by testing it on
an ISG equipped engine. When the developed productis installed, the starting time of the crank pulley isabout the same as the conventional product with highreaction force spec. At the same time, the tension of
the accessory belt can be reduced up to 50% during
steady driving, with which an improvement of fueleconomy can be expected.
As the requirement for improving fuel economy
further increases, NTN aims to contribute to global
environmental protection by marketing this developed
product worldwide.

A

Time sA(b) Conventional product(high reaction force spec.)

Time sReference
1) Seiji SATO, Technology Trends in Auto Tensioners, NTN
TECHNICAL REVIEW, No. 79 (2011) 83-89
2) HIS Data (2014)

Fig. 13 Measurement results of tensioner reaction force
during steady driving

Photo of authors

Hiroo MORIMOTO

Yuta MOCHIZUKI

Automotive Product Design Dept.
Automotive Business HQ

Automotive Product Design Dept.
Automotive Business HQ-53-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Products Introduction of Composite Material
for Automotive

Kayo SAKAI*
Takahiro GOTOU**
Takuya ISHII***

Eiichirou SHIMAZU*
Hajime ASADA**NTN group produces sliding bearings, sliding parts and magnetic components,
which are manufactured from the composite materials. They are blended powder
metal or resin with additives to impart various functions according to our own
formula. This article introduces various products made of such composite
material used in automotive applications for high mileage and light-weighting.

components, machine parts such as gears and cams,
magnetic components used for sensors, and coilcomponents for electrical noise removal and voltage
regulation. In this paper, we introduce composite
material products contributing to low fuel consumption,compact size and electrification of vehicles.

1. Introduction
NTN is developing/manufacturing composite
material products. The products are made by blending
various powder additives with resins and metallic
powders according to our proprietary blending formula.
These products have required characteristics desiredby the companies shown in Fig. 1. The composite
material products can be made for many applications
by combining material powders and blending them. In
addition, the powder material can be molded into
complex shapes, allowing efficient manufacturing.
Because of these properties, NTN is proposing
composite material products to a broad range ofapplications and fields and manufacturers. Examples
are sliding bearings made of resin and sintered metal,
including fluid hydrodynamic bearings, sliding

2. Low torque seal ring for transmission
Automatic and continuously variable transmissions
in vehicles use hydraulic mechanisms to change thevelocity of the vehicle. Seal rings are used in thehydraulic circuits of transmissions, installed between
the rotating shaft and housing. These are thecomponents that seal oil and maintain pressure within
the hydraulic circuit. Then seal rings are pressed toboth the housing and shaft by hydraulic pressure andslide against the rotating shaft. Around 4 to 10 resin oil
seal rings (hereinafter, seal rings) of 15 – 60 mm outer
diameter are used per transmission for sealing oil inthe sliding area.
Reduction of the torque of the seal ring means
improvement of the transmission efficiency, andreduction of oil leakage means a more efficient andcompact hydraulic pump. Both contribute positively tothe fuel economy of vehicles. In order to maintain low
torque and low oil leakage for a long time, it isnecessary to minimize wear of the seal rings and therotational shafts, which are the mating components on
which the seal rings slide.

BEAREE TM

Composite Material
Product Division

NTN Engineering Plastics Corp.

Resin sliding bearings, resin mechanical components

BEARPHITE TM, NIKAMETTM

NTN Advanced Materials Corp.

Oil impregnated sintered bearings,
sintered machine parts, magnetic material products

Hydrodynamic BEARPHITE

NTN Manufacturing (Thailand) Co.,Ltd

Fluid hydrodynamic bearings

Fig. 1 Composite material products of NTN group***Composite Material Product Division, Composite Material Product Division***Engineering Department, NTN Advanced Materials Corp.
***Engineering Department, NTN Engineering Plastics Corp.

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Products Introduction of Composite Material for Automotive

Housing

Side wall of
shaft groove

Seal ring

Seal ringSide wall of
shaft groove

Housing

Oil pressure

Trapezoidal seal ring

Lubricating oil

Rectangular seal ring

Lubricating oil

NTN has developed “low torque seal rings” whichhave reduced torque by 60% compared withconventional products. The seal rings have superb
properties such as low wear and low oil leakage due
to newly developed V-shaped lubrication grooves on
the side wall of the PEEK (Polyetheretherketone) resin
seal rings 1), 2) as shown in Fig. 2. These parts are inmass production..

Rotational shaft

Rotational shaft
Miter

Fig. 3 Operating of seal ring

V-shaped
lubrication groove

reduced torque by newly forming the V-shaped
grooves on the rectangular seal rings. The low torqueseal rings exhibit a balanced performance, providing
the following features, compared to the conventional
rectangular seal rings.

Fig. 2 Groove shape of low torque seal ring

2.1 Features[Features]

Typical seal rings have rectangular or trapezoidal
cross-sections. Their respective operations areillustrated in Fig. 3. A comparison of the properties ofthese seal rings with the respective cross section
geometries and the developed low torque seal ring areshown in Table 1.
As the rectangular seal rings contact the rotational
shaft along the entire side wall, they exhibit low oilleakage and large torque. Conversely, the trapezoidal
seal rings have line contact with the shaft resulting in
low torque and high oil leakage. Also, they tend towear faster due to the line contact, resulting inunstable properties.
The low torque seal rings were developed tomaintain a superb low oil leakage property, and(1) 60% lower torque(2) 10 times better wear resistance(3) Equivalent low oil leakage property
The material used is our proprietary material,
BEAREE PK5301, which is the PEEK resin blended
with special filler, with a long proven track record inthis application.

2.2 PerformanceFig. 4 shows the relation between oil pressure andtorque. The low torque seal rings have approximately
60 to 70% lower torque than the rectangular sealrings. Dependency on the hydraulic pressure is also
relatively low.
Providing V-shaped lubrication grooves on the side

Table 1 Comparison of features of various seal rings
Item
Cross section

Rectangular seal ring Trapezoidal seal ring

Low torque seal ring

RectangularTrapezoidal

RectangularNo

No

V-shape

Full contact

Line contact

Full contact otherthan groove

Diagram ofcross section
Lubrication grooveon side wall
Contact with sidewall of shaft groove
Torque△◎〜△○Oil leakage

◎

△

◎

Wear resistance

○

△

◎

Performance balance

○

△

◎

◎：Excellent○：Good-55-△：FairTorque (for 2 seal rings) N·m

NTN TECHNICAL REVIEW No.83（2015）

1.5

evaporate. This leads to contamination in thesurroundings, and therefore lubricant and greasecannot be used in this application. New sliding
materials that can be used in such dry environment
where lubricant cannot be used are needed. NTN isdeveloping self-lubricating, sintered sliding material
with improved lubrication by adding a large amount ofgraphite, which is a solid lubricant.

φ50 seal ring, 4000min-1, ATF (110℃)
Rectangular seal ring

Low torque seal ring
1.0

0.5

0.0
0.0

3.1 Features
0.5

1.0

1.5

Oil pressure MPa

2.0

2.5

Since the bulk density of graphite powder is low, if alarge amount is added, the fluidity of the material
powder decreases, limiting the shape and size thatcan be molded. Also, even if the shape can bemolded, the processing cycle could become longer,
incurring relatively higher cost. The developed selflubricating sliding material ensures high degree of
freedom of shapes and productivity even with theincreased level of graphite by increasing fluidity ofgraphite powder with a special process. Thisdeveloped material exhibits a superior low friction
property and wear resistance, even in the environment
where lubricating oil cannot be used.

Fig. 4 Torque versus oil pressure

wall of the rectangular seal rings makes the contact
area with the shaft groove side wall smaller than therectangular seal rings. The hydrodynamic effect, whereoil pressure increases at the end of the V-shaped
grooves, reduces the contact pressure and improves
lubrication on the sliding area, achieving low-torqueproperties. The higher the hydraulic pressure, the more
oil enters onto the sliding surface from the lubrication
groove; therefore, the torque dependency on hydraulicpressure is considered to be smaller.
In addition, since the lubrication condition on thesliding surface is improved by the V-shaped
lubrication grooves, the wear resistance of the low
torque seal rings is about 10 times better than therectangular seal rings.

[Features](1) Superior low friction and wear property in the dry
environment(2) High degree of freedom for the shape is ensured
by the special graphite
The chemical composition of the developed productis shown in Table 2. This product has more than twice
the amount of graphite compared with the ordinary
bronze based material.

3. Self-lubricating sliding components
In order to increase visibility of night driving, theadoption of Adaptive Front-lighting Systems (AFS),
which directs the headlights axis to the traveling
direction, is becoming more popular 3). Fig. 5 shows
an example of the sliding components for AFS.
With the AFS, sliding materials (bearings) are
required for the supporting components of theheadlamps. The area inside the headlamp cover
becomes very hot due to the heat from the lamp andliquid lubricant or grease on the sliding materials may

Table 2 Chemical components of developed productMaterial

Chemical composition %Sn (tin) C (graphite)

Cu (copper)

Ordinary bronze base material

Remained

7〜11

1〜2

Developed productRemained

7〜11

4〜7

3.2 Performance
The test conditions for friction and wear propertiesof the self-lubricating sliding material are shown in
Table 3, and the test results are shown in Table 4.
The friction coefficient of the developed product isabout half of the ordinary bronze based material
without oil content, and the wear resistance isincreased up to about 40 times of the ordinary bronze
based material.

Fig. 5 Self-lubricating slide material (for AFS)

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Products Introduction of Composite Material for Automotive

4.1 Features and performance

Table 3 Test conditions for friction and wear property(1) Friction test(2) Wear test

Load N

98

15

Rotational speed min-1
Test piece size (mm)(ID x OD x width)

1000

450

Mating material
Test time min
Lubrication condition

φ6×φ12×6←

SUJ2

SUS4209060

No oil content (dry)←Fig. 7 shows the choke coils made with amorphousalloy powder developed by NTN. NTN’s AL60
compression molding core 5) and AS10 injection
molding core 5) can be used in various shapes not only
for toroidal type but also pot type and EER type.
Fig. 8 shows the relation between frequency andpermeability of various soft magnetic materials used in
choke coils. Permeability is an index that indicates theconversion capability between electrical power andmagnetic power. If permeability decreases, noise
reduction effect decreases. Therefore, it is desirable
that permeability remain the same with increasing
frequency. Comparing the frequency at whichpermeability reduces 10% for each material, an
ordinary ferrite core is around 6 kHz and a Fe-Si
compression molded core is around 2 MHz. NTN’s
AL60 compression molded core is around 5 MHz, and
NTN’s AS10 injection molded core is around 10 MHz,
exhibiting a stable noise reduction effect in a highfrequency bandwidth more than twice that of theconventional material.
The magnetic energy which can be stored in the coreis determined by the saturation magnetic flux density,
which is dependent on the material of the core and thecore volume. Therefore, for designing the core, material

Table 4 Specific wear rate and friction coefficient(1) Friction coefficient

0.2〜0.3

Ordinary bronze
based material
0.45(2) Specific wear rate
m3 / (N・m)×10-12

0.08

3.56

Developed product

4. Choke coil for high frequency application
In recently produced electrified vehicles, many
power supply systems, such as DC/DC converters
and inverters, are installed. Choke coils are used toreduce the noise content, added mainly to the direct
current, for higher efficiency of these power supply
systems. Choke coils are also a measure against
radio frequency interference to the peripheral
equipment. The vehicle systems tend to be driven byhigher frequencies for the purpose of achieving higher
efficiency and more compact power supplies;
therefore, choke coils are required to maintain stableproperties for high frequency bandwidth with low loss.
Fig. 6 shows a toroidal choke coil as an example ofchoke coils. Choke coils are composed of soft
magnetic material, called the core, and conductor wire
wound around the core. Choke coils store electricalenergy as magnetic energy and prevent current flow ofincreasing frequencies as stored energy increases 4).
Choke coils use these properties to filter highfrequency noise. NTN develops choke coils of various
shapes.

AL60 compression
molding core

AS10 injection
molding core

Change ratio of permeability %Fig. 7 Various choke coils

Conductor wireCore

Fig. 6 Toroidal choke coil

120
100
80
60
40
20
0
0.001

AS10

AL60

Fe-Si
based

Ferrite

0.01

0.1

1

Frequency MHz

10

100

Fig. 8 Frequency response of magnetic permeability-57-

NTN TECHNICAL REVIEW No.83（2015）

and shape must be determined so that the operating
condition does not exceed the energy that can be
stored. The condition that exceeds the energy that canbe stored in the core is called magnetic saturation, andwhen magnetic saturation occurs the coils do not
function as choke coils. Fig. 9 shows the relationshipbetween magnetizing force and magnetic flux density(B-H curve). When saturation magnetic flux density ishigh, storage capacity of magnetic energy is high, andthe core can be made to have a small volume. Fe-Si
based compression molded core, which has a large
saturation magnetic flux density, is often used for
applications with large current.
Fig. 10 shows energy losses in different cores
during electrical energy to magnetic energy conversion.
Fe-Si based compression molded cores have larger
core losses than the ferrite core and the efficiency isinferior to the ferrite core. NTN’s AL60 compression
molded core and AS10 injection molded core havesmall core losses. Particularly the AL60 compression
molded core has an excellent low core loss property
equivalent to the ferrite core.
Based on these points, AL60 compression moldingcore and AS10 injection molding core have excellent

Magnetic flux density mT

400

frequency response, large saturation magnetic flux
density and low core losses; therefore, they can makecompact and low-loss high frequency choke coils.

4.2 Composite core
NTN’s AL60 compression molded core has a large
saturation magnetic flux density and particularly small
core losses. On the other hand, the AS10 injection
molded core has highly stable permeability in highfrequencies and a high degree of freedom for design
because it is injection molded. By taking advantage ofthese properties, cores using either of the two types ofmaterials, or composite cores combining bothmaterials, are designed. The cores are designed
depending on parameters like current value and size,
as shown in Fig. 11. By this approach, more efficientand compact power supplies can be expected.

AS10 injection
molding core
AL60 compression
molding core

Coil

Magnetic saturation

350
300

Fig. 11 Composite core

250
200
150
100

AS10

50
0

Fe-Si
based

0

1000

2000

3000

4000

5000

Magnetic field strength A/m

5. Conclusion

AL60
Ferrite
6000

7000

100

120

In this paper, we have introduced recent composite
material products contributing particularly to higher
fuel economy, compact size and electrification ofvehicles.
Various new technologies are being rapidly
developed and proposed in the automobile industry, in
addition to electrification, compactivity and high fueleconomy. We are aiming for continuous productdevelopment and propositions responding to the newrequirements in the automotive industry, by advancing
and integratingcomposite material technologies.

Fig. 9 B-H curve

6000

AS10

Iron loss kW/m

5000

AL60

4000

Fe-Si
based

3000

Ferrite

2000
1000
0

0

20

40

60

80

Frequency kHz

Fig. 10 Core loss characteristics (50 mT)-58-

Products Introduction of Composite Material for Automotive

Reference
1) Yutaka HAYASHI, Jiro MATSUMOTO, Noboru
UMEMOTO: Introduction of Engineering Plastic Seals,
NTN TECHNICAL REVIEW No. 65 (1996) 54.
2) Yoshio OKI, Takumi HAYASHI, Takuya ISHII:
Characteristics and Applications of Super Engineering
Plastics, Journal of Japanese Society of Tribologists 49(7), (2004) 31
3) Automotive Technology Handbook Editing Committee
Planned/Edited: Automotive Technology Handbook 2.
4) Jiro TOGAWA, Coil / Transformer Design of Switching
Power Supply
5) Takayuki ODA, Shinji MIYAZAKI, Eiichiro SHIMAZU: The
Reactor Core for HEV Boost Converter, NTN TECHNICALREVIEW No. 81 (2013) 46-51.

Photo of authors

Kayo SAKAI

Eiichirou SHIMAZU

Takahiro GOTOU

Hajime ASADA

Takuya ISHII

Composite Material
Engineering Dept.
Composite Material Product Division

Composite Material
Engineering Dept.
Composite Material Product Division

Engineering Department
NTN Advanced Materials Corp.

Engineering Department
NTN Advanced Materials Corp.

Engineering Department
NTN Engineering Plastics Corp.

-59-

NTN TECHNICAL REVIEW No.83（2015）[ Technical Papers ]Modeling of Retained Austenite Transformation in Bearing Steels

Christine SIDOROFF*
Pierre DIERICKX*
Pr. Michel PEREZ**

Bearing steels are heat treated to obtain martensitic microstructures providing high hardness necessary forgood rolling contact fatigue performance. If specific actions are not taken, austenite is generally retained after
heat treatment in the final component with a more or less important content. Depending on the requirements ofeach application, retained austenite can be desired because of beneficial effects such as improvement of rolling
contact fatigue performance, mostly in contaminated lubricating conditions, or can be avoided if dimensional
stability is needed for example in cases of bearings operating for long durations at high temperatures. This paper
discusses the modeling of retained austenite transformation and its dimensional consequences in SUJ2, taking
into account all the phase evolutions occurring during tempering and in service.

1. Introduction
Steels such as SUJ2 are easily hardened to therequired level, by austenitizing at temperatures above
AC1, quenching and tempering. The metallurgicalstructures obtained after hardening are not stable andwill evolve in service towards more stable states,
resulting in size changes or dimensional instability.
The main non-stable phase often associated tothese evolutions is retained austenite, remaining in
high-carbon alloyed steels’ microstructure after
quench.
These phenomena have been extensively describedin existing publications, but the purpose of this paper
is to explain the complex metallurgical mechanisms
behind the dimensional instability, and to present work
realized in NTN-SNR to estimate the relative
contributions of the different individual phenomena inorder to build a model to predict dimensionalvariations occurring during service.

To attain long service lives, bearing ring materials are
selected to provide a minimum hardness of 58 HRC for
rolling contact fatigue (RCF) resistance (in conjunction
with the appropriate inclusion cleanliness), and a goodwear resistance by choosing the appropriate steel
grade, steelmaking process, and heat treatment.
If no specific requirements such as structural fatigue
or impact resistance are expressed, through-hardened
steels are used, such as SUJ2 steel in the case ofstandard temperature conditions (hereafter mentioned
as standard bearing steels).
Another important feature that bearing steels mustretain in service through the life of the mechanicalcomponent is the initial internal clearance of the
bearing, and its interference with the surroundingparts. Therefore, the dimensional change must be
limited at the service temperature for standard bearing
steels by selecting the appropriate heat treatment.

**NTN-SNR Roulements, Technology and Innovation Department**Institut National des Sciences Appliquées de Lyon, MATEIS Laboratory-60-Modeling of Retained Austenite Transformation in Bearing Steels

When carbon is rejected from martensite andcarbides precipitate, two opposite effects take place:--- Contraction of the martensite because of lattice size
reduction (tetragonal loss and volume reduction oflattice cell);--- Slight expansion because of carbide formation, nothigh enough to compensate for the contraction
previously mentioned.
These two effects result in a continuous contraction,
as long as carbon precipitates intoε-carbides, andFe3C carbides.

2. Metallurgical phenomena and theirdimensional consequences
2. 1 Metallurgical phenomena
After austenitizing and quenching, themicrostructure of SUJ2 is mainly martensitic, andcomposed of the following constituents:• Martensite, with a high carbon concentration above
the equilibrium carbon content in ferrite;• Retained austenite, not transformed at room
temperature after quench because of the alloying
elements and carbon concentration,• Undissolved carbides, not fully dissolved during
austenitizing.
These two first constituents, martensite andretained austenite, are metastable and evolve inservice towards a more stable state.
Martensite tempering is often described using thefollowing stages associated with temperature ranges
in which they are likely to take place 1, 2). Thesephenomena can happen at lower temperatures,provided time is left for these transformations to occur.

Transformation of austenite
Austenite has a face-centered cubic (fcc) structure,
which is the densest existing structure. When it
transforms into ferrite and cementite, the volume
increases.
Fourth stage
Carbide coarsening and spheroidizing, recovery andrecrystallization result in a macroscopic contraction,
rarely referred to in technical publications.
For as-quenched martensitic microstructures, during
thermal aging, the dimensional evolution shows asuccession of contraction (carbon rejection andεcarbide precipitation), expansion (austenite
decomposition) partially compensated by cementite
precipitation, contraction (end of cementite
precipitation), and again contraction with a slightchange in slope (structure evolutions during the fourthstage). This succession of events is represented inFig. 1 for an anisothermal aging of SUJ2 steel
obtained by differential dilatometry with a heating
speed of 5˚C/min. The same succession can beobserved for isothermal aging, if tested times are longenough.

Stage 0 (T<100˚C)
Carbon atoms segregate to dislocations (it canrepresent up to 0.2 wt.%C).
Stage 1 (100<T<200˚C) α’ → α’’ +εCarbon leaves the oversaturated martensite(α’→α’’), and creates metastable ε-carbides(Fe2,4C).
Stage 2 (200<T<300˚C) γR → α + Fe3C
Retained austenite (γR), also a metastable phase,
decomposes into a mix of ferrite (α) and cementite(Fe3C).
Stage 3 (250<T<350˚C) α’’ +ε→ α + Fe3C
Carbon leaves the impoverished martensite (α’’→α)
to form cementite carbides (Fe3C) to the detrimentofε-carbides.

Retained austenite (expansion)
Stage 2γR→α + Fe3C

0.0000Dimensional variations

Stage 4 (T>350˚C)
Carbides coarsen and spheroidize. The structure is
subjected to recovery, and then recrystallization.
To summarize, two different phenomena occur
during the first stages of tempering: precipitation ofcarbides from martensite (stage 0 + stage 1 + stage 3),
and transformation of retained austenite (stage 2).

-0.0005-0.0010-0.0015-0.0020

Martensite (contraction)-0.0025

Stage 0Stage 1α’→ α” +ε-0.0030

2. 2 Dimensional consequences

0

These phase evolutions and transformations
produce macroscopic dimensional variations 3, 4).

100

200

→

Stage 3
α+Fe3C

300

400

Temperature（℃）

Stage 4500

Fig. 1 Anisothermal aging of as quenched SUJ2

Precipitation from martensite
Martensite has a tetragonal lattice, and its
parameters are dependent on carbon concentration 5).

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600

NTN TECHNICAL REVIEW No.83（2015）

3. 2 Use of TEP to the study of phase evolutions
during aging of SUJ2

3. Analysis and modeling of thedimensional evolutions duringisothermal aging of SUJ2

Thermoelectric power measurements have beenused to follow the kinetics of carbon precipitation frommartensite. The principle of TEP is to apply to thestudied steel specimen a temperature gradient
between its two extremities by contact with two purecopper blocks, and to measure the resulting voltage
induced by the Seebeck effect between the twojunctions. TEP is sensitive to the crystal defects
influencing electron diffusion and phonon propagation,such as atoms in solid solution, and is therefore aninteresting tool to characterize and quantify
precipitation of carbides from oversaturated martensite
during thermal aging.
Samples of rectangular shape according to Fig. 2
treated according to Table 1 were aged at different
temperatures from 110˚C to 505˚C to durations up to
300h. Aging was interrupted at logarithmic increasingtime steps to obtain the TEP evolution for eachsample at room temperature, expressed as the TEP
variation ΔS between the initial state (as quenched)
and aged at considered time (Fig. 3).
ΔS increases in two consecutive sigmoidal steps,
identified in Fig. 3 as A and B, associated to theprecipitation of carbon from oversaturated martensite,
into metastableε-carbides, and Fe3C (to the detrimentofε-carbides), respectively. These stages occur
sooner when temperature increases, based on thetime-temperature equivalence presented later. At hightemperature and times, ΔS continues to increaselinearly, in step C, and is assumed to be caused by
the phenomena occurring during the fourth stage oftempering: recovery of the dislocation structure,

An original approach to the study of dimensional
stability of SUJ2 has been conducted by NTN-SNR 6-8)
using Thermoelectric Power (TEP) measurements.
The aim of the study was to predict the macroscopic
dimensional variations as a function of a thermal
aging, based on calculations of each microstructuralcontribution. This work comprises two stages:--- Quantification of volume fraction and carbon content
of each microstructural constituents through TEP
measurements as a function of aging time andtemperature,--- Use of these data as input for calculations ofdimensional variations of each contribution, and an
average calculation to obtain the macroscopic
dimensional variation.

3. 1 Thermal treatments studied
The study was conducted on samples heat treatedaccording to the cycles of Table 1. A standard asquenched martensitic heat treatment (referred to as H)
was studied as well as a similar state followed by asubzero treatment (referred to as HF) to destabilize apart of the retained austenite (F as ‘freeze’ was added
to the name of the sample).
X-Ray Diffraction (XRD) was used to determine theinitial retained austenite contents, on cylinder-shaped
samples (according to Fig. 2).

Table 1 Thermal treatments performed on SUJ2 6)Martensitic thermal treatment
H

HF

850˚C / 15min

850˚C / 15min

5

Oil

Oil

4.5

60˚C / 5min

60˚C / 5min

4-80˚C / 1h

3.5

4.7

3

Quench /isothermal holding
Water rinsing
Subzero treatment----

Initial retained
austenite content(vol.%)

10.3

Φ20

ΔS μv/K

Austenitizing

4

1

Φ6

X-ray diffraction

0.5

Dimensional measurement0
1.E+00

60
TEP

2
1.5

50
5

2.5

1.E+01

1.E+02

1.E+03

1.E+04

Time (min)Fig. 3 TEP evolution of the martensitic heat treatment as
a function of time at different aging temperatures, with(HF) and without (H) subzero treatment – H110 standsfor H treatment 6-8)

1

Fig. 2 Samples used for X-Ray diffraction,
TEP and Dimensional measurements-62-

Modeling of Retained Austenite Transformation in Bearing Steels

retained austenite decomposes into a mix of ferriteand cementite.
--- Stage C: Recovery of dislocation structure andcoarsening of martensite laths occur.
Based on these assumptions and scenarios, ananalytical approach was built, using Johnson-MehlAvrami-Kolmogorov 7) (JMAK) empirical formulae forthe kinetics of the precipitation of ε-carbides andFe3C, and for the recovery of dislocations and lathcoarsening.
The macroscopic TEP is considered to be thevolume average of the two individual contributions to
TEP of the martensitic phase (the variation of carbon
in solid solution in the martensite, and the recovery),and of the retained austenite. The TEP master curvesof H and HF heat treatments as a function ofequivalent time at 110˚C were then used to adjust a
number of parameters while others were measured orgathered from publications 7). The interpolatedparameters were all confronted to values available in
the literature and found to be in agreement with them7), and the interpolated TEP curves fit quite well withthe experimental data (Fig. 4).

recrystallization, and the coarsening of carbides.
Subzero treated HF samples, with a lower initial
retained austenite content compared to H samples,
present during stage B a slightly lower ΔS . Thisdifference is assumed to be linked to thedecomposition of retained austenite that seems tooccur simultaneously to the precipitation of Fe3C
carbides.
As increasing temperature seems to shift the curves
towards shorter times, the results were treated with atime-temperature equivalence according to an
Arrhenius law 7), to transfer measurements of alltimes and temperatures to equivalent times at 110˚C,
the lower aging temperature investigated, using
activation energies of 120 kJ/mol for temperatures
between 110 and 240˚C, and 190 KJ/mol for higher
temperatures (close respectively to the values forcarbon precipitation during tempering of martensite,
and for recovery in α-iron).
These master curves (given for H samples in Fig. 4)
were then used to estimate the evolution of volume
fraction and carbon content of the different
constituents of the microstructure, through theirindividual contribution to TEP evolution, according to
the following scenario:--- Stage A:ε-carbides are precipitated from the carbon
rejected from the oversaturated martensite.
--- Stage B: Fe3C carbides precipitate using the carbon
segregated on dislocations, the carbon remaining insolid solution in martensite, and released from thedecomposition of ε-carbides that dissolve because
they are less stable than Fe3C. Simultaneously,

3. 3 Estimation of macroscopic dimensionalvariations
The macroscopic dimensional variations can becalculated from the individual contributions of eachphase, as far as the microstructure of SUJ2 can beunderstood as a composite material. This can beachieved either through the use of simple models,
where the minimal and maximal value of dimensionalvariations were estimated by averaging the stresses(uniform strain – Voigt) or the strains (uniform stress –Reuss) in the volume, either with more complex
approaches such as self-consistent model or Ponte
Castaneda-Willis (also called homogenization
techniques), that take into account the morphology
and relative layout of the phases, and their mechanicalinteractions.
For a given time and temperature, it was thenpossible to estimate the macroscopic dimensionalvariation limits (here using Voigt and Reuss models forease of understanding). Carbon concentrations in the
different phases and the fraction of decomposed
retained austenite are the main input parameters of
this model (for detailed expressions, see 7-8) ). The
experimental measurements, realized on long
cylinder-shaped samples (according to Fig. 2), are ingood agreement with the estimations (Fig. 5).

5
4.5
4

ΔS μv/K

3.5
3
2.5
2
1.5
1
0.5
0
1.E+00

1.E+03

1.E+06

1.E+091.E+12

1.E+15Equivalent time at 110˚C (min)Fig. 4 Experimental (dots) and modeled (curves) TEP
evolution for martensitic heat treatment H: macroscopic
TEP evolution (ΔSH) and individual contributions to TEP
evolution (ΔSε, ΔSθ, ΔSγR, ΔSR) as a function ofequivalent time at 110˚C 6-8) – H110 stands for H
treatment aged at 110˚C, εforε-carbides, θforcementite, γR for retained austenite, R for Recovery-63-

NTN TECHNICAL REVIEW No.83（2015）

compensated by the expansion associated to the
carbides formation (εand Fe3C), so that the overall
contraction of the evolution of the martensitic phase is
not so high. The expansion induced by retained
austenite decomposition is of the same order ofmagnitude to that of Fe3C precipitation (superimposed
curves in Fig 6.b), but is enough to lead in total to a
macroscopic expansion.

ΔL/L

0.001

0

-0.001

ΔL/L

0.001

0

4. Industrial practice-0.001

4. 1 Selection of tempering

ΔL/L

0.001

In industrial practice, hardened bearing steels arenever used as quenched, but tempered, in order toreduce the inherent brittleness of quenched highcarbon steels.
Tempering after quenching is the beginning of theaging process described in§2 and§3 andcharacterized in§3, and will be more or less advanced
depending on the tempering conditions. Depending on
the extent of the tempering i.e. temperature andduration, stage A and possibly stage B described in§3
occur during tempering.
In service, the material will undergo an additional
thermal aging, similar to tempering, during which thephase transformations initiated during temperingcontinue.
Depending on tempering conditions, the component
will present an expansion in service if only stage A
occurred during tempering, or a contraction if stages A
and B (total decomposition of retained austenite)occurred. It is to be reminded that dimensional
evolutions can occur even at low temperatures giventhat enough time has passed.

0-0.0011

10100

Time（min）

1000

10000

Fig. 5 Experimental (dots) and modeled (curves for Voigt
and Reuss estimations) dimensional variations ofmartensitic H heat treatment at experimental
temperatures 6,7)

0.002

0.002

0.001

0.001
Dimensional evolution ΔL/L %Dimensional evolution ΔL/L %As example of use, the estimation of dimensionalvariations for equivalent time at 110˚C are represented
in Fig. 6 for H and HF martensitic heat treatments,
with two different initial retained austenite contents 7-8).
It is possible to compare the influence of retained
austenite content for similar austenitizing temperatureand quench.
It is interesting to compare the relative order ofmagnitude of the different phenomena, displayed inFig. 6. The contraction of the martensite lattice is large
and is the only contraction contribution; it is largely

0-0.001-0.002-0.003-0.004-0.005
1.E+000-0.001-0.002-0.003-0.004

1.E+04
1.E+081.E+12Equivalent time at 110˚C (min)-0.005
1.E+001.E+16(a)1.E+04
1.E+081.E+12Equivalent time at 110˚C (min)1.E+16(b)Fig. 6 Estimated macroscopic dimensional variations (Voigt, Reuss) and individual contributions of phases to macroscopic
dimensional variations (ε, θ, α’, γR) for equivalent times at 110˚C - (a) H martensitic treatment with 10.7 vol.% retained
austenite, (b) HF martensitic heat treatment with 4.7 vol.% retained austenite 7-8) -εstands for ε-carbides, θ forcementite, γR for retained austenite, R for Recovery-64-Modeling of Retained Austenite Transformation in Bearing Steels

conclusions and the ones also obtained by otherauthors are summarized in the following:• An increase of austenitizing temperature and timeincreases the amount of retained austenite after
quench;• A quicker cooling rate produces lower retained
austenite contents;• Interrupted quenching (martempering or
marquenching), commonly used for SUJ2 to reduceheat treatment distortions and quench cracking
likelihood, increases the amount of retained
austenite;• Higher tempering temperature and times will lead tolower retained austenite contents, even if
temperatures lower than 200˚C will not lead tosignificant lowering of retained austenite content in
SUJ2.

Finally, in the case of SUJ2, depending on theapplication, different temperings are used, resulting indifferent properties and advantages/shortcomings:--- ‘Low temperature’ or standard tempering (only stage
A occurs during tempering, 1 to 2h at 170-180˚C):
the hardness obtained is high for good RCF
performance and no dimensional stability is
guaranteed.
--- ‘Higher temperature’ tempering or stabilizing
temperings (stages A and B occur during tempering,for example 2h at 230˚C): the hardness obtained islower than that of standard temperings, but a higher
dimensional stability is obtained, as no expansion
due to retained austenite decomposition will occur,
but contraction at long operating times, or hightemperatures, or combinations thereof. Thesetemperings are used for bearings subjected tohigher temperatures.
The actual temperature/times of NTN-SNR existing
standard tempering (D treatment), and stabilizing
tempering (S) were used to calculate the equivalent
times of aging at 110˚C for a SUJ2 steel, so that thetime corresponding to the beginning of service for D
and S temperings are reported in Fig. 6.

4. 3 Reducing retained austenite content byheat treatment
Different methods can be used to reduce retained
austenite content:¡Subzero treatment consists in an additional
subzero quenching after the standard quenching toroom temperature, but small amounts of retained
austenite are always retained. It generally consists
in an isothermal holding between -100 and -70˚C.
¡Water rinsing consists in submitting thecomponents to a soaking in a water tank at 1520˚C, after oil quench usually realized between 40
and 60˚C. It has been shown that this step can
reduce a portion of the retained austenite, andresultant dimensional variations.
¡Tempering (stabilizing heat treatment) is to date themost effective way to suppress retained austenite.
For SUJ2 and other low alloyed steels, temperingmust be performed at temperatures higher than
200˚C for a significant reduction of retained
austenite content, and at least 230˚C for efficient
removal of retained austenite 9).

4. 2 Influence of heat treatment conditions on
retained austenite
In the previous paragraph, it is shown that it ispossible to obtain different hardness/dimensional
stability compromises depending on heat treatment
parameters.
It is also important to add that heat treatment
solutions are in practice also selected for otherconsiderations, for example to minimize distortions, toavoid quench cracking or depending on furnacetechnology available, and that many criteria areaccounted for to design the heat treatment.
Many studies have been carried out on theinfluence of heat treatment conditions of classicalbearing steel on the amount of retained austenite 9-12),and its thermal stability, through its practicalconsequences on dimensional stability.
Pearson 9) has illustrated for a variability of industrial
heat treatment conditions and facilities, the dispersionof size change that can be found for standard treated
SUJ2: for an aging of 3000h at 150˚C, the size
expansion can vary from 7x10-4 to 11x10-4, illustrating
the influence of the different heat treatment conditions
used depending on the bearing manufacturer andfacilities.
Franklin 12) performed an extensive study in the
1970s on the effect of all these parameters on
retained austenite content (without measuring its
resultant dimensional size change). His main
-65-

NTN TECHNICAL REVIEW No.83（2015）4) Bhadeshia, H.K.D.H. “Steels for Bearings”, Progress inMaterials Science, 2012, Vol. 57, pp. 268–435.
5) Cheng, L., Brakman, C.M., Korevaar, B.M., Mittemeijer,
E. “The tempering of iron-carbon martensite; dilatometricand calorimetric analysis”, Metall. Trans. A, 1988, Vol.
19A, pp. 2415-2426.
6) Sidoroff, C. “Analyse Microstructurale et Modélisation
des Evolutions Dimensionnelles de l’Acier 100Cr6 :Structures Martensitique et Bainitique”. PhD Thesis
INSA Lyon, 2002.
7) Perez, M., Sidoroff, C., Vincent A., Esnouf C.
“Microstructural Evolution of Martensitic 100Cr6 Bearing
Steel During Tempering: From Thermoelectric Power
Measurements to the Prediction of Dimensional
Changes”, Acta Materialia, 2009, Vol. 57, pp. 3170–
3181.
8) Sidoroff, C., Perez, M., Dierickx, P., Girodin, D.
“Advantages and Shortcomings of Retained Austenite in
Bearing Steels : a Review”, Bearing Steel Technology,
ASTM STP 1580, J. M. Beswick, Ed. American Society
for Testing and Materials International, WestConshohocken, PA, 2015, pp. 312-348.
9) Pearson, P.K. “Size Change of Through Hardened
Bearing Steels at Application Temperatures”, SAE
Transactions A, 1997, Vol.106, N°.2, pp. 170-175
10) Duval, P., Murry, G., Constant, A. “Stabilité
Dimensionnelle dans le Temps. Contribution à l'Etude
des Facteurs Métallurgiques Affectant la Stabilité
Dimensionnelle des Aciers”, Mécanique-Electricité,
1968, Novembre, pp. 28-37.
11) Hengerer, F., Nierlich, W., Volkmuth, J., Nutzel, H.
“Dimensional Stability of High Carbon Bearing Steels”,
Ball Bearing Journal, 1988, Vol. 231, pp. 26–31.
12) Franklin, J., Hill, P., Allen, C. “The Effect of Heat
Treatment on Retained Austenite in a 1% Carbon /Chromium Steel”, Heat Treatment of Metals, 1979, Vol.
6, pp. 46-50.

5. Conclusions
The issue of dimensional stability cannot be
simplified to the content of retained austenite present
in the bearing ring, and complex microstructural
evolution mechanisms modelled in this paper are
responsible for the observed macroscopic dimensionalvariations.
The model presented here takes into account one
initial heat treatment. Other NTN-SNR studies on theevolution of this model enable the evaluation ofdimensional variations for other heat treatment
conditions.
Nevertheless, the only efficient way to improve
dimensional stability in service when needed (ifservice time and temperatures are high enough tocreate dimensional changes modifying the internal
clearance or loss of fit with mating components) is toselect the tempering with an appropriate temperaturethat will bring the microstructure closer to equilibrium
and remove the remaining retained austenite.

References
1) Speich, G.R., Taylor, K.A. “Tempering of Ferrous
Martensites. Martensite : a Tribute to Morris Cohen”,
Edited by Olson G.B. and Owen W.S. United States ofAmerica, ASM International, 1992. pp. 243-275.
2) Porter, D.A., Easterling, K.E. “Phase Transformations in
Metals and Alloys”, London : Chapman and Hall, 1992,
514p.
3) Roberts, C.S., Averbach, B.L., Cohen M. Trans. ASM,
1953, Vol. 45, pp. 576.

Photo of authors

Christine SIDOROFF

Pierre DIERICKX

Pr. Michel PEREZNTN-SNR Roulements
Technology and Innovation
Department

NTN-SNR Roulements
Technology and Innovation
Department

Institut National des Sciences
Appliquées de Lyon
MATEIS Laboratory-66-

NTN TECHNICAL REVIEW No.83（2015）[ Technical Paper ]Evaluation of Rolling Contact Fatigue
by X-ray Diffraction Ring
Naoya KAMURA*
Takumi FUJITA*
Toshihiko SASAKI**In this report, a novel approach for evaluating the progression of rolling
contact fatigue (RCF) is introduced. The progression of RCF is evaluatedbased on tri-axial stress state and the degree of martensite grain orientation
which are characterized by two parameters: σeq and S/S0, respectively.
From the behavior of the σeq and S/S0 during RCF, it is found that the
progression of RCF can be divided into three stages as follows. First, σeq approaches the yield stress of SUJ2
in the early loading stage; about 103 cycles. The S/S0 does not change in this stage. After drastically increasing
the σeq up to 103 cycles, the S/S0 increases. This shows that the martensite grain orientation on the RCF
surface begins to be formed after the 103 cycles. Finally, σeq gradually decreased while developing peeling on
the raceway although the orientation of texture continues to progress.
This is a new finding obtained by applying the Debye ring analyzer to the evaluation of RCF. The evaluation of
RCF by Debye ring is a promising method, not only to investigate mechanism of RCF, but also to allow forquantitative estimation of the progression of RCF.

method, tri-axial stress measurement is also possible.
Furthermore, information on the orientation of crystal
grains can be obtained from the intensity distributionof the Debye rings (obtained on the detector). By
using this information obtained by unconventional
methods, it should be possible to accurately estimate
residual life from the complex mechanism of rolling
contact fatigue.
In this paper, we will study the relation between the
crystal orientation of martensite (obtained from the
Debye rings) and tri-axial residual stress (obtained by
Sasaki-Hirose method) in the progression of rollingcontact fatigue of two cylindrical specimens.

1. Introduction
It has been well documented (within many reports)that fatigue associated with microstructural changes,
hardness changes, and the formation of residual
stress can be observed under the raceway of rolling
bearings), 1), 2).
This is an attempt to estimate the residual life ofrolling bearings, to determine the risk of damage torolling bearings. Previously, three parameters(residual stress, half width and retained austenite
assuming plane stress condition) obtained by X-ray
stress measurement device were used in the
research. However, since the fatigue of rolling
bearings varies depending on the operation
conditions, it is difficult to accurately estimate theremaining life.
Recently, an analytical device (which can detect
Debye rings) was developed and is currently being
studied for use in material evaluation 3). Specifically,
Debye rings can be used for measuring stress with asingle incident angle (cos α method 4)). With the cos αmethod, the stress can be obtained quickly comparedwith the conventional methods. In addition, if SasakiHirose method 5), which is an extension of cos α

2. Theory of Tri-Axial StressMeasurement by X-ray
2.1 Stress Measurement Method with Debye rings
When an X-ray is incident to an object of crystalline
structure, a diffraction phenomenon occurs within the
crystal that satisfies Bragg conditions, as indicated by
the following equation (1).
2d ∙ sinθ= nλ……………………………………… (1)***Advanced Technology R&D Center***Faculty of Human Sciences, Kanazawa University-67-

NTN TECHNICAL REVIEW No.83（2015）

which is an extension of the cos α method. The
following is a description of the Sasaki-Hirose method.

Where,
d : Lattice spacingθ : Bragg angle
n : Integer
λ : Wavelength of X-ray

2.2 Sasaki-Hirose Method
The strain εα to the central angleαof Debye ring canbe expressed as the equation (2) using the radius of
Debye ring Rα.
εα= 1 2θ0 _π+ tan-1 Rα cotθ0 ……………(2)2CLWhere,
θ0：Bragg angle without strain
Rα：Radius of Debye ring at the positionα
CL：Distance of X-ray from the radiation point to
the detector
The relation betweenεαand stress can be expressedby the following equation in the coordinate system inFig. 3.
εα=σx 1 [n12 _υ(n22 + n32 ) ]E+σy 1 [n22 _υ(n32 + n12 ) ]Eσ+ z 1 [n32 _υ(n12 + n22 ) ]E
2(1+υ) n n +τyz 2(1+υ) n n
+τxy
1 2
2 3
E
E
+τxz 2(1+υ) n3 n1 …………………………(3)E
Where,
E：Young’ s modulus of X-ray
υ：Poisson’ s ratio
In addition, n1 to n3 are direction cosine ofεαand canbe expressed by the following equations:

In general, metallic materials (for industrial use) areoften poly-crystalline possessing a random crystalline
orientation. When tensile stress is applied to thesetypes of materials, the lattice spacing of the crystal
grains vary (as shown in Fig. 1)as well as thediffraction angle of each crystal grain changes.
Therefore, X-ray stress measurement is accomplishedby calculating stress using the difference between thediffraction angles (due to the orientation of crystals 6).).
If the specimen has a crystal grain suitable for X-ray
stress measurement (the diameter of crystal grain is
30 μm or less in the state without orientation or
aggregate structure 7)), diffraction X-ray is generated
within a conical shape centered around incident X-ray(shown in Fig. 2). The diffraction X-ray is measured bya two-dimensional “Debye Rings” detector (as crosssections of a cone).
For the X-ray stress measurement theory using
whole Debye rings, the cos α method (proposed by
Taira, etc.) and a method using a imaging plate (IP) asa detector (proposed by Yoshioka, etc. 8), and Sasaki,
etc. 9)). The cosα method is an analysis method thatassumes a plane stress condition; the Sasaki-Hirose
method is a tri-axial stress measurement method,

( )]

[

n1 = cosηsinψ0 cosφ0_ sinηcosψ cosφ cosα0

_ sinηsinφ sinα0…………………………(4)0n2= cosηsinψ0 sinφ0_ sinηcosψ sinφ cosα00+ sinηcosφ0 sinα …………………………(5)n3= cosηcosψ0 + sinηsinψ0 cosα ……………(6)Fig. 1 Changes of lattice spacing by applying tensile stressFig. 3 Relationship between Debye ring on Imaging
Plate (IP) and strainεαFig. 2 Schematic drawing of incident X-ray and Debye ring-68-

Evaluation of Rolling Contact Fatigue by X-ray Diffraction Ringσy _σz can be calculated when the relation of equation(15) at φ0 = π/2 rad (90˚ ) is used.
σz is obtained from the following equation.

Where,
η ：Supplementary angle of Bragg angle(η= π/2_θ)ψ0：X-ray incident angle
φ0：Angle made by incident X-ray and X-axis
Then, to obtain each stress component from theequation (3), strains at the central anglesπ＋α, π−αand −α are expressed asεπ＋α,επ−α andε-α,respectively, with the following parameters newly defined.
1a1(φ0)≡ 2 [(εα _επ＋α) ＋ (ε-α _επ−α)] ……(7)1a2(φ0)≡ 2 [(εα _επ＋α) _ (ε-α _επ−α)] ……(8)From equations (3) to (8), a1 and a2 atφ0＝0 are
expressed as follows:E(εα_ X ) ……………………………(17)σz = _
1 2υ
X can be expressed by the following equation.
2 (1+υ)X=(τxy n1 n2 +τyz n2 n3 +τxz n3 n1)E+ 1 (σx _σz ) [n12 _ν(n22 + n32 )]E+ 1 (σy _σz ) [n22 _ν(n32 + n12 )] ………(18)E
Since each stress component and direction cosine are
already known in equation (18), the value of σz can beobtained. In addition, equation (17) includes a term ofεα,and since σz can be obtained for each central angle of
Debye ring, σz is determined as their average.
The above is the tri-axial stress analysis method by the
Sasaki-Hirose method and 6 stress components can beobtained by three X-ray irradiations indicated by Fig. 5.

1+υ
σx _σz ) sin 2ψ0 + 2τxz cos2ψ0]
E [(× sin 2ηcosα ……………………………(9)a1(0)= _a2(0)=

2 (1+υ)[τxz sin ψ0 +τyz cosψ0]E× sin 2ηsinα …………………………(10)When ψ0=0 in equations (9) and (10), namely, when
X-ray is incident perpendicularly onto the specimen(perpendicular incident), the following equations can beobtained with the shear stressesτxz andτyz:
τxz= _
τyz=

3. Experiment Conditions
3.1 Two-Cylinder Test Conditions
Fig. 4 shows the schematic overview of the twocylinder testing machine and Table 1 shows the test
conditions. The test was conducted under the purerolling conditions. No. 1 to No. 3 are conducted under
the conditions of using a specimen of small surface
roughness for the driving side (mirror surface drive),
No. 4 is conducted under the condition of using a
specimen of large surface roughness for the drivingside (rough surface drive). In the two-cylinder test, it is
known that the damage progression of the cylinder inthe mirror side is faster with rough surface drive rather
than mirror surface drive 10), 11), and we haveinvestigated its effect on the evaluation of fatigue by
X-ray.
The two-cylinder test specimens are made of JIS
SUJ2 with standard heat treatment. The shape iscylindrical of φ40×12mm and the radii of curvature in
the axial direction are indicated in Table 1. No. 1 is aboundary lubricating condition with the maximum
contact stress (Pmax) of 2.77 GPa and the oil film
parameter (Λ) of 0.3. No. 2 has mirror surface finish
for both specimens (driving/following sides) to comply
with the elastohydrodynamic lubrication condition(Λ>3). No. 3 uses mirror surface finish for the drivingside specimen and ground surface for the following
side specimen, similar to No. 1, however, Pmax is 2.20
GPa, which is a lower surface pressure than No. 1.
Lubrication oil was supplied by contacting a felt pad

E1 ∂a1(0)…………………(11)2 (1+υ) sin 2η∂cosα∂a2(0)
E
1…………………(12)2 (1+υ) sin 2η ∂sinα

In addition, equations (9) and (10) indicate that a1
and a2 are linear to cosα and sinα, respectively withthe slope to be expressed in the following equations:∂a1(0)
1+υ
σx _σz ) sin 2ψ0 + 2τxz cos2ψ0]
=_
E [(
∂cosα
× sin 2η …………………………(13)()∂a2(0)( ∂sin α ) = 2 (1E+υ) [τ sin ψ +τ cosψ ]
xy

0

yz

0

× sin 2η …………………………(14)In equations (13) and (14), since τxz andτyz are
already obtained by equations (11) and (12), whenψ0≠0, σx _σz andτxy can be obtained by the following
equations:∂a1(0)
E
1
1
σx _σz= _1+υ sin2η sin 2ψ0 ∂cosα()_ 2τxz cot 2ψ …………………………(15)
0

τxy =

E
1
1
2 (1+υ) sin 2η cosψ0∂a2(0)( ∂sinα )_τyz cot ψ ……………………………(16)0-69-NTN TECHNICAL REVIEW No.83（2015）

Table 1 Two-cylinder type test conditions

Table 2 X-ray measurement condition
Cr-Kαα-Fe (211)

Characteristic X-ray

No.

Rq µm
Load Pmax Curvature in the axial direction
kN GPaDrivenDriving Driven
DrivingΛ13.92 2.77

0.3

Tube condition

0.3

Incident angle ψ0 rad (deg)Exposure time s

2
3
4

↑
↑
1.96 2.20
3.92 2.77

∞
60

60
∞

0.022
0.025
0.030
0.985

0.987
0.010
0.945
0.029

Diffraction (hkl)

30kV, 1mA

φ2

X-ray irradiated area

10
0.3

0, 0.524 (0, 30)
72

Fig. 4 Schematic drawing of two-cylinder type test

Fig. 5 Relationship between coordinating system inmeasuring object and location of IP

containing additive-free turbine oil (ISO VG32) to thespecimens. The rotational speed of the driving side
specimen was set at 500 min-1.

is depicted in Fig. 6. In this paper, as we set theincident angle of X-ray as 0 rad (0°) and 0.524 rad(30°), the average penetration depths of X-ray are 5.4
μm and 4.6 μm, respectively. Therefore, the X-ray
measurement results that we describe later indicate
information on the organization and stress variation up
to approx. 5 μm from the surface of the specimens.

3.2 X-ray Measurement Conditions

Average penetration depth µm

We used μ-X360 (made by Pulstec Industrial Co.,
Ltd.) as the X-ray diffraction ring analyzer. Thisequipment can analyze the Debye rings (as thediffraction X-ray is detected in two dimensions).
Table 2 shows the X-ray measurement conditions.
We used the coordinate system (shown in Fig. 5) forthe measurement, and the Sasaki-Hirose method
described in Chapter 2 for the stress analysis. The
irradiation range of the X-ray is φ2, which is smaller
than the major axis of the contact ellipse (2.96 mm)
calculated from the test conditions, therefore, only therolling contact area can be evaluated.
The depth that the X-ray can penetrate in thematerial (X-ray penetration depth) is given by thefollowing equation 12).

6
5
4
3
2

0

10

20

30

ψ0 deg40

50

60

Fig. 6 Relationship between X-ray incident angle andaverage penetration depth

1 cos2ηcos2ψ0+ cosψ0 sin2ηsinψ0 cosα(19)
Tα=μ .
(1 + cos2η) cosψ0+ sin2ηsinψ0 cosα
Where,
Tα: Penetration depth of diffraction ray at the position α
μ: Ray absorption coefficient of iron against Cr-Kαray 13)(889.76 cm-1)
By using equation (19), the relationship between theincident angle ψ0 and the average penetration depth-70-Evaluation of Rolling Contact Fatigue by X-ray Diffraction Ring

3.3 Evaluation Indices

4. Results and Observations

As the evaluation indices for rolling contact fatigue,
parameter S/S0 , which is defined for the purpose ofquantifying change of Debye rings caused by theequivalent stress (Mises stress) σeq and the crystal
orientation, is used. S is the standard deviation ofdiffraction intensity to the central angle of Debye ringand S0 is the value of S before the test. With the
Debye ring with heterogeneous intensity distribution
as shown in Fig. 7 (a), the peak intensity against α isshown in Fig 7 (b) and its standard deviation isindicated by S in the right diagram. On the other hand,
σeq is expressed by equation (20).
σeq =

｛
1
2

4.1 Effect of the Oil Film Parameter
Fig. 8 shows the relation between σeq and thenumber of cycles for driving specimens of No. 1 andNo. 2. σeq of No. 2, which is the elastohydrodynamic
lubrication condition, exhibits almost no change evenwhen the number of cycles becomes large; however,σeq of No. 1 exhibits the maximum value at the
beginning of the rolling contact, increasing to 1134
MPa, which is close to the yield stress of SUJ2. Then,σeq gradually decreased along with peeling. Fig. 9shows the condition of the raceway at the end of thetest. This peeling effect was already observed fromthe early stage (driving cycle: 12 x 104).
Fig. 10 shows the Debye rings obtained from thespecimens before and after the test. With No. 1,
heterogeneous Debye rings were observed after the
test. Fig. 11 shows the relation between the driving
cycles and S/S0 of No. 1 and No. 2. S/S0 of No. 2 did
not change until the end of the test, however, S/S0 of
No. 1 increased as the driving cycle increased. This
indicates that the rolling contact fatigue (under theboundary lubrication condition) is a phenomenon withorientation of martensite crystal grains.

(σx _σy)2 + (σy _σz)2 + (σz _σx)2…(20)+6 (τ2xy+τ2yz+τ2xz)｝

α（a）Example of Debye ring

0

60

120

180

240

α deg

300

No.1
No.2

1200

σeq MPa

Intensity a.u.

Intensity a.u.

1400

S

360

1000
800
600
400

（b）Intensities at diffraction peak for central angle
and its standard deviation
Fig. 7 Definition of parameter S

0

10

20

30

Number of cycle ×10440

50

Fig. 8 Relationship between von Mises stress σeq andnumber of cycles for driving specimens of No.1 and No.2 tests

Fig. 9 Peeling for driving specimens of No.1 test after
RCF of 48×104 cycles-71-

NTN TECHNICAL REVIEW No.83（2015）

1400（b）No.1（a）
Before RCF

σeq MPa

1200（C）No.2
After RCF

1000
800

No.1
No.3

600

Fig. 10 Debye rings for before and after RCF tests ofNo.1 and No.2 tests at 48×104 cycles

400

0

10

20

30

Number of cycle ×1044050（a）σeq

10

10

No.1
No.2

8

S/S0

S/S0

8
6
4

4

No.1
No.3

2

2
0

6

0
0

10

20

30

Number of cycle ×104

40

50

0

10

20

30

Number of cycle ×1044050（b）S/S0

Fig. 11 Relationship between S/S0 and number of cycles
for driving specimens of No.1 and No.2 tests

Fig. 12 σeq and S/S0 depending on number of cycles
for driving specimen of No.1 and No.3 tests

4.2 Impact of Load

4.3 Change of X-ray Measured Values within
the Low-Driving Cycle Range

Fig. 12 shows the change of σeq and S/S0 against
driving cycles of No. 1 and No. 3. The behavior of σeq
and S/S0 was almost the same between No. 1 and 3,
with peeling observed almost at the same time.
The reason why there was no difference information of σeq , behavior of crystal orientation andtiming of peeling (even with different loads) is because
there was no difference in severity of asperity contact
between the two. Rdq 14), which indicates severity ofasperity contact, exceeds 0.175 rad (10°) before thetest for following side cylinder in No. 1 and 3. Thisvalue is much higher than Rdq of the regular bearings(0.017～0.070 rad). Contact pressure generated at theasperity contact area of on the surface of large Rdq isestimated to exceed the limit that the SUJ2 materials
can support in any load conditions. Therefore,
increase of load does not affect the local contactpressure but only to the increase of real contact area.
This is also verified by the X-ray analysis. This is thereason why the load did not affect the formation of σeq
and behavior of crystal orientation.

The fact that σeq and the shape of the Debye ringsin the boundary lubrication condition quickly change inthe driving cycle range up to 105 has been shown
above. In the following, we describe the analysis
results of driving cycle range up to 105.
Fig. 13 shows change of σeq and S/S0 when rolling
contact test was conducted up to 5 x 104 under thesame conditions as No. 1. σeq increased from 533
MPa of before the test to 1084 MPa at 103 driving
cycles and stayed mostly constant in the range of1080 to 1200 MPa up to 5 x 104. On the other hand,
S/S0 showed little change up to 103 and thenincreased after 103 revealing the fact that the rolling
contact fatigue progresses with generation of residual
stress then crystal orientation.

-72-

1400

1400

1200

1200

σeq MPa

σeq MPa

Evaluation of Rolling Contact Fatigue by X-ray Diffraction Ring

1000
800

800

No.1

600

600
400

1000

400
100

101

102

103

104

No.4
100

101

104

105

104

105

（a）σeq（a）σeq
10

25

8

20

6

15

S/S0

S/S0

103

Number of cycle

Number of cycle

4

No.1
No.4

10
5

2
0

102

0
10

0

10

1

10

2

10

3

Number of cycle

10

4

100

101

102

103

Number of cycle（b）S/S0（b）S/S0

Fig. 13 σeq and S/S0 depending on number of cycles
up to 5×104 for No.1 test

Fig. 14 σeq and S/S0 depending on number of cycles
for the driving specimen of No.1 and No.4 tests

4.4 Impact of Rough Surface Drive
Fig. 14 shows the change of σeq and S/S0 for mirror
surface drive cycle of No. 1 and rough surface drive
cycle of No. 4. σeq and S/S0 exhibited similar behavior
in No. 1 and 4 up to the cycles 5 x 104, then No. 4
showed larger increase of S/S0 thereafter. In addition,
peeling was observed from around 5 x 104 both for No.
1 and 4; however, the progress was faster for No 4
showing significant difference at the cycles of 5 x 105 in
the number and magnitude of damages on the raceway
as shown in Fig. 15. That is, rough surface drive
promotes significant crystal orientation after peelingstarts and the progress of peeling was also faster.
These results show that the progressions of rolling
contact fatigue for No. 1 and 4 are different after
cracking occurs. Kaneta et al.11) indicate that the
behavior of crack opening can vary between thedriving side and following side when cracks andlubricating oil are present on the raceway and this isconsidered to be the reason why the progressions of
peeling in No. 1 and 4 are different. We will study themechanism of generation and growth of cracks, aswell as the mechanism of the formation of crystal
orientation, in more detail.

（a）No.1（b）No.4
Fig. 15 Peeling on raceway at 5×105 cycles-73-

NTN TECHNICAL REVIEW No.83（2015）

3) For example, Makoto Tominaga, Yoshiaki Akiniwa, Influenceof Measurement Area on the X-ray Stress Measurement
Using 2D Detector, The Society of Materials Science, Japan,
Proceedings of the 47th Symposium on X-ray Studies on
Mechanical Behavior of Materials, (2013) 1-6.
4) Shuji Taira, Keisuke Tanaka and Toshiharu Yamasaki, A
Method of X-Ray Microbeam Measurement of Local Stressand Its Application to Fatigue Crack Growth Problems,
Journal of the Society of Materials Science, Japan, 27, 294,
(1978) 251-256
5) Toshihiko Sasaki, Shunichi Takahashi, Katsunari Sasaki andYuuichi Kobayashi, A Study on Improvements in Multiaxial
Stress Analysis with Area Detector Type Diffraction Method,
Transactions of the Japan Society of Mechanical Engineers.
A, Vol. 75, No. 750 (2009), 219-227.
6) Keisuke Tanaka, Kenji Suzuki and Yoshiaki Akiniwa,
Evaluation of residual stress by X-ray diffraction –Foundation
and application, 1st edition, Yokendo Ltd. (2002) 99-105.
7) X-ray Stress Measurement Standard, The Society of Materials
Science, Japan, (1982), 79-84.
8) Yasuo Yoshioka, Takeshi Shinkai and Shinichi Ohya,
Application of imaging plate to micro-beam X-ray diffraction,
Proceedings of the 26th Symposium on X-ray Studies on
Mechanical Behavior of Materials, The Society of Materials
Science, Japan, (1989) 122-127.
9) Toshihiko Sasaki and Yukio Hirose, Stress measurement fromone diffraction ring using imaging plate, Proceedings of the
30th Symposium on X-ray Studies on Mechanical Behavior ofMaterials, The Society of Materials Science, Japan, (1994)
22-27.
10）S. Way，Pitting Due to Rolling Contact，Journal of Applied
Mechanics, 2, 2, (1935) A49-A58．
11) Motohiro Kaneta, Yukitaka Murakami and Hirohiko
Yatsuzuka, Fracture mechanics considerations of Way’s
hypothesis on crack growth in lubricated rolling/sliding
contact, Lubrication, 30, 10, (1985) 739-744.
12) Toshihiko Sasaki and Yukio Hirose, A method of neutronstress measurement using the cos α method, Transactions of
the Japan Society of Mechanical Engineers. A, Vol. 71, 704,
(2005) 670-676.
13) Rigaku Corporation, X-ray diffraction handbook, Edition 6,(1998) appendix 4.
14）E. Ioannides，G. Bergling & A. Gabelli, An Analytical
Formulation for the Life of Rolling Bearings, Acta Polytechnica
Scandinavica, Mechanical Engineering Series,137,(1999)．
15) Naoya Kamura, Takumi Fujita, Toshihiko Sasaki and Yoichi
Maruyama, Tribology Conference Spring 2015, Tokyo (2015)
Himeji Proceedings, Evaluation of Rolling Contact Fatigue by
X-ray Diffraction Ring, Conference Number F12 (2015).
16) Naoya Kamura, Takumi Fujita, Yoichi Maruyama andToshihiko Sasaki, Evaluation of Rolling Contact Fatigue by Xray Diffraction Ring, Proceedings of the 49th Symposium on
X-ray Studies on Mechanical Behavior of Materials, (2015)
58-61.

5. Conclusion
We have evaluated the progression of the rolling
contact fatigue (under boundary lubrication and fluid
lubrication conditions) by the equivalent stress (Mises
stress) σeq obtained by X-ray diffraction ring analyzer
and S/S0 , which indicates the heterogeneity of the
Debye rings. The following is the summary of theresults:1) σeq of the rolling contact surface increases closer
to the elastic limit of SUJ2 in early stage of around103.
2) After a rapid increase up to the cycles around 103,
σeq remained almost constant until small peelingstarted.
3) In this experiment, the crystal orientation started
after the end of the significant increase of σeq ,around the point of 103.
4) As the peeling grows, σeq decreases; however, thecrystal orientation continues to progress.
5) σeq and crystal orientation behavior did notchange, even if the specimens of driving andfollowing sides are switched up to the point peelingstarted; however, crystal orientation was moresignificant with the rough driving after peelingstarted.
This paper has been edited based on the original
manuscripts 15) and 16). We thank Japanese Society of
Tribologists and the Society of Materials Science,
Japan, who gave us the permission for publishing it.

Reference
1) For example, Hiroshi Muro, Noriyuki Tsushima and Masatoshi
Tokuda, Changes of Residual Stress due to Rolling Contact,
Journal of the Society of Materials Science, Japan, 18, 190,
(1969) 615-619.
2) Noriyasu Oguma, Prediction of Residual Fatigue Life of
Bearings, Part 1: Application of X-ray Diffraction Method,
KOYO Engineering Journal, 161, (2002) 26-31.

Photo of authors

Naoya KAMURA

Takumi FUJITA

Toshihiko SASAKI

Advanced Technology
R &D Center

Advanced Technology
R &D Center

Faculty of Human Sciences
Kanazawa University-74-

NTN TECHNICAL REVIEW No.83（2015）[ Technical Paper ]Influence of Induction Heating Conditions
on Bearing Steel Characteristics
Hiroshi YUKI*
Miyu SATO*
In order to evaluate the characteristics of SUJ2 bearing rings hardened by induction
heating (IH) in comparison to those heat treated in a furnace, we researched suitable
IH conditions that achieve the same properties as furnace heating. Dimensionalchange ratio, static load capacity, fracture strength, and shear fatigue strength were
evaluated with specimens that were induction hardened with varying temperaturesand carbide area ratios. Suitable conditions were determined. Basic rating lives of ballbearings made with the suitable IH conditions were found to be 4 times longer thantheir calculated fatigue life.

lost and the pinning effect cannot be obtained, whichleads to reduced strength due to grain coarsening.
Conversely, shortage in solid-solution results inunsatisfactory hardening. Therefore, in this study, we
have targeted the carbide area ratios of 4%, 8% and12%, and the heating temperatures of 900˚C, 950˚C
and 1000˚C, which are higher than the furnace
heating, in order to investigate the relationship ofsolid-solution carbon concentration and various
properties. For tempering, we have set 3 levels,
namely, ordinary tempering condition of atmospherefurnace, 180˚C x 2h and 240˚C x 43 s targeting 62
HRC, and 310˚C x 37s targeting 60 HRC, assuming
high-temperature and short-time tempering of IH. As
such, we considered 27 levels of 3x3x3 for evaluation.
The processing conditions are marked with theheating temperature, carbide area ratio and tempering
condition connected by hyphens in this order, such as“900-4-180”. Table 1 indicates the relationshipbetween the heating temperature and heating time for1. Introduction
Changing the heat source of the overall heathardening of high carbon chromium bearing steel type
2 (hereinafter, SUJ2) from furnace to induction heating(hereinafter, IH) makes it possible to save energy,
labor and lowering the environmental load.
On the other hand, IH has disadvantages in
productivity compared with the furnace heating andtherefore, high temperature heating is often adopted toreduce the processing time per work. When hightemperature heating is adopted, hardening starts fromthe austenite single phase region of the SUJ2
material, which may bring properties different from theproducts hardened from two-phase regions ofaustenite phase and cementite phase in furnace
heating.
Therefore, in this research, we have explored the
optimum induction heating process condition andcompared the resulting product with the furnace
heated products.

Table 1 IH test conditions

2. Test method

Temperature
˚C

Quenching
Target carbide area ratio

4%8%316.0s
57.5s
65.0s
14.5s
10.3s
4.0s
TemperingTarget hardness HRC Temperature ˚C
2h
62
180
62
240
43.2s
60
310
36.8s

2.1 IH thermal process conditions

900
950
1000

Although knowing solid-solution carbon
concentration in the base material is important since it
affects various properties, it is difficult to directly
measure the concentration.
Therefore, we have controlled the carbide area ratio
instead of the solid-solution carbon concentration.
When carbon solid-solution is excessive, carbide is*Advanced Technology R&D Center

-75-

12%
11.0s
3.0s
0.7s

NTN TECHNICAL REVIEW No.83（2015）

2.3 Basic property of each IH thermal
processing condition

each carbide area ratio and the relationship between
the heating temperature and heating time for eachtargeted hardness after tempering.

We have used the ring-shaped test specimens of
φ60×φ54×15 for investigating the material properties
of IH heated-quenched products. Fig. 1 shows the
carbide distribution of the IH products. The photos A,
B and C in the figure are the IH products processed
under the conditions of 950˚Cx65.0 s, 1000˚C x 4.0 s
and 900˚C x 11.0 s, respectively. It can be observedthat the microstructures are different by the heating
conditions. Table 3 indicates hardness, retained
austenite after tempering and prior γ grain size.

2.2 IH thermal process methods
The test specimens are SUJ2 materials from thesame lot.
Table 2 shows the chemical composition of the test
specimens.
Table 2 Chemical composition of used SUJ2 (mass %)
C

Si

Mn

0.98

0.27

0.42

Ni

Cr

Mo

Cu

0.011 0.007 0.06

P

S

1.47

0.03

0.08

3. Determination of the optimum condition
3.1 Investigation of dimensional variation ratio
3.1.1 Test method

We have used a high frequency induction heater forheating the test specimens.
With the feedback control using the temperature of
the central area of the outer diameter of the test
specimens, the temperature of the test specimens
were maintained constant. After heating is completed,
the test specimens were cooled for quenching in thecold oil of 70˚C until the specimens become around
100˚C.

A : 950˚C×65.0s

We have used the ring-shaped test specimens of
φ60×φ54×15.
Thermally treated rings were maintained at 230˚C
for 2 hours and the dimensional change ratios before
and after heating were determined. The number oftests was set to n=3 for each level. The specimens
with the dimensional change ratio of less than 0.050%
were regarded as passed.

B : 1000˚C×4.0s

C : 900˚C×11.0s

Fig. 1 Cementite distributions
Table 3 Hardness, amount of retained austenite and prior γ grain size after tempering 1)

Heating
Temperingtemperature temperature Hardness
HRC
˚C
˚C

900

950

1000

180

62.8

310

60.2

240
180
240
310
180
240
310

62.5

Prior γRetained
γ amount crystal grain Hardness
HRC
Grain number
%

14.6
14.5
7.3

62.5

17.0

60.2

9.8

62.8

17.9

62.0

17.2

60.2

11.1

62.3

Target carbide area ratio

4%

16.9

11.0

9.4

10.0

62.1
62.4
60.9

8%

Prior γRetained
γ amount crystal grain Hardness
HRC
Grain number
%

10.5
11.2
5.7

62.3

13.0

60.0

7.8

62.3

13.1

62.1

17.3

60.1

9.6

62.2

-76-

16.5

11.0

10.8

10.2

61.0
60.2
57.9

12%

Prior γRetained
γ amount crystal grain
Grain number
%

7.9
8.6

61.3

10.2

59.1

6.0

61.4

10.7

62.4

14.6

60.2

7.8

62.2

11.0

5.4

14.8

11.0

11.0

Influence of Induction Heating Conditions on Bearing Steel Characteristics

3.1.2 Evaluation results

depths of the remaining impressions were measured
using a three dimensional surface structure analyzing
microscope. The number of tests were set to n=3 foreach level.
The following is how we organized the test results. It
is known that there is a linear relation between
logarithm of the maximum contact surface pressure
Pmax and logarithm of δ/DW, which is depth ofimpression δ divided by the diameter of the ceramicball DW 2). In addition, the static rated load is known tobe the load where the sum of total permanent
deformation of the rolling element and that of the
raceway becomes 1/10000 of the diameter of rolling
element 3). The permanent deformation generated bythis test is only on the test specimen equivalent to the
raceway, however, since the ratio of permanent
deformations on the raceway and rolling element isalmost the same 4), we have defined Pmax when 2δ/DW
becomes 1/10000 as the static load-carrying capacity.
Pmax , where 2δ/DW = 1/10000 was calculated fromlinear approximation, was obtained by the least-squares
method from the plotting of log (Pmax )−log (δ/DW).

Table 4 shows the dimensional change ratios. The
dimensional change ratio tends to become large when
the carbide area ratio is 4%. Under the tempering
conditions of heating temperature of 1000˚C targeting
62 HRC (180˚C and 240˚C), all the specimens with any
carbide area ratio failed. Fig. 2 shows the relationshipbetween retained austenite amount and dimensionalchange ratio. There was a strong correlation between
the retained austenite amount and dimensional change
ratio for the same hardness.
Table 4 Dimensional change ratio (×10-5)
Tempering
Heating
temperature temperature
˚C
˚C

900

950

240
310
180
240
310
180
240
310

4%

8%

12%

71
65

36

19

45

38
25

24

78

47

75

47

60

38

82

77

51

62

54

48

40
35
39
26
59
55

3.2.2 Test results

41

Fig. 3 shows the static load-carrying capacities. The
static load-carrying capacities of the products in the
red areas of the figure are lower than those of thefurnace heated products with the risk factor of 5%.
That means that the static load-carrying capacities
are less when heated with 1000˚C.

90
80
70
60
50

310˚C tempering(60 HRC)

40

5.5

30

180˚C tempering+
240˚C tempering(62 HRC)

20
10
0
0.0

5.0

10.0

15.0

Pmax GPa

Dimensional change ratio x 10-5

1000

180

Target carbide area ratio

20.0

Retained austenite amount %Fig. 2 Relationship between amount of retained γ
and dimensional change ratio

5.0

4.5

Furnace heatedproduct
900-4-180
900-4-240
900-4-310
900-8-180
900-8-240
900-8-310
900-12-180
900-12-240
900-12-310
950-4-180
950-4-240
950-4-310
950-8-180
950-8-240
950-8-310
950-12-180
950-12-240
950-12-310
1000-4-180
1000-4-240
1000-4-310
1000-8-180
1000-8-240
1000-8-310
1000-12-180
1000-12-240
1000-12-310

4.0

3.2 Investigation of static load-carrying capacity
3.2.1 Test method

Fig. 3 Static load-carrying capacities

We have used flat plates of 6×15×3 which were cut
out from the rings of φ60×φ52×15 thermally treatedunder various conditions as the test specimens.
The most broad surface was processed with mirror
finish and applied loads of 421.3, 599.9 and 822.9 N
which are equivalent to the maximum contact surfacepressure Pmax = 4.0, 4.5 and 5.0 GPa with a 3/8 inch
bearing made of silicon nitride ceramics on thesurface. The load velocity was 3 N/s and the loadholding time was 120 s. After removing the load, the

3.3 Investigation of static fracture strength
3.3.1 Test method
We have used the ring-shaped test specimens of
φ60×φ54×15. We have defined the static fracture
strength as the stress converted from the load when
the rings were broken by applying load of 1 mm/min
using a commercially available tensile/compression
testing machine. The number of tests were set to n=5
for each level.
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NTN TECHNICAL REVIEW No.83（2015）

3.3.2 Test results

3.4.2 Test results

Fig. 4 shows the static fracture strength. The
specimens from the heating temperature of 1000˚C
were excluded from the test as they failed the test forthe dimensional change ratio and the static loadcarrying capacity was low, as described in 3.1 and 3.2.
The static fracture strengths of the products in the redareas of the figure are lower than those of the furnace
heated products with the risk factor of 1%. It wasdetermined that the strengths of the products with the
carbide area ratio of 4% were lower under thetempering conditions targeting 62 HRC in the heating
temperature of 900˚C and all the tempering conditionsin the heating temperature of 950˚C.

Fig. 6 shows the shear fatigue strength. The
specimens from heating temperature of 1000˚C were
excluded similar to 3.3.
The tempered products with 310˚C targeting 60 HRC
tend to have lower fatigue strength; however, the other
IH heated products had almost the same strength. For
detailed comparison, Fig. 7 shows the stress amplitude
where the load cycle is 3 x 109. The load cycle of 3 x
109 causes exfoliation in the bearing life test under the
maximum contact surface pressure of Pmax =2.5GPa.
Assuming that the standard deviation of fatigue strength
is constant regardless of stress, detection of significant
difference with the significance level of 5% revealed
that the fatigue strengths after 3 x 109 cycles of all the
IH heated products were equivalent or better comparedwith the furnace heated products.

3000
1000

2500

Stress amplitude MPa

2000

950-12-310

Fig. 4 Static fracture strength

900-4-180

900

900-8-180

850

900-8-240

800

900-12-180

750
700

950-4-310

650

950-8-240600

950-8-310

550
1.0E+07

3.4 Evaluation of shear fatigue strength
For rolling bearings, it is known that the shear
fatigue is dominant 5). Therefore, we have evaluatedthe shear fatigue strength using an ultrasonic torsional
fatigue tester 6).

1.0E+08

1.0E+09

1.0E+10

Load cycle

950-12-180
950-12-240

700
600

950-12-240

950-12-180

950-8-310

950-8-240

950-4-310

400

900-12-180500
900-8-240

Fig. 5 shows the appearance of the test specimen.
The area of evaluation was mirror treated to eliminate
impact from surface roughness. The intermittent load
method was applied alternating loads and pauses withresonance frequency of 20 kHz and stress ratio of R=1 (perfectly reversed loading). When there was no
damage up to 1010 times, the test was terminated.

800

900-8-180

3.4.1 Test method

900

Furnace heatedproduct

Stress amplitude MPa

Fig. 6 Shear fatigue properties

900-4-180

950-12-240

950-8-310

950-12-180

950-8-240

950-8-180

950-4-310

950-4-240

950-4-180

900-12-310

900-12-240

900-8-310

900-12-180

900-8-240

900-8-180

900-4-310

900-4-240

1500
1000

Furnace heatedproduct

950

Furnace heatedproduct
900-4-180

Tensile stress MPa

3500

Fig. 7 3 × 109 fatigue strength
36.6
26.5
6.8

3.5 Summary of the test for determining suitableconditions

Fig. 5 Schematic drawing of test specimen

Table 5 is a list of the dimensional change ratio andresults from strength evaluation.
The products with properties equivalent to or better
than the furnace heated products are marked with “○”
and those with lower strengths are marked with “×”.
Those not tested are marked with “—”.

6.8

13

φ4

φ3.5

φ9

(10)

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Influence of Induction Heating Conditions on Bearing Steel Characteristics

Table 5 Experimental results
Heating
Temperingtemperature temperature
˚C
˚C

180
900

240
310
180

950

240
310
180

1000

240
310

Aging dimensionalchange ratio

4%
×

8%
○

12%
○

Shear fatigue
strength

Static fracture
Static load-carrying
strength
capacity
Target carbide area ratio

4%
○

8%
○

12%
○

4%
×

8%
○

12%
○

4%

8%

12%

○

○

○

×

○

○

○

○

○

×

○

○

̶

○

̶

○

○

○

×

○

×

○

○

○

̶

̶

̶

×

○

○

○

○

○

×

○

○

̶

̶

○

×

○

○

○

○

○

×

×

○

̶

○

○

×

○

○

×

○

×

×

○

○

○

○

̶

×

×

×

×

○

○

̶

̶

̶

̶

̶

̶

×

×

×

×

×

○

̶

̶

̶

̶

̶

̶

×

○

○

×

×

×

̶

̶

̶

̶

̶

̶

3.6 Observation

surface after hardening. The data for the carbide area
ratio of 8% is also listed for comparison. Under thecondition of the carbide area ratio of 8%, 62 HRC was
secured regardless of the heating temperature or
thickness. On the other hand, under the condition of
the carbide area ratio of 12%, the hardness was about
61 HRC and with the heating temperature of 900˚Cand thickness of 4.0 mm, the hardness was 60 HRC,
since the solid-solution carbon content is lower. Thisshould be caused by partially incomplete quenchingwith slower cooling speed due to thickness. Since the900˚C-12% product has large insoluble carbide
content and less amount of elements which improvequenching property such as carbon and chromium inthe base material, in addition to unstable austenite
due to low heating temperature 7), hardness is reduced
even by slight reduction of cooling speed. Therefore,
the 900˚C-12% condition is determined to be
unpractical.
Accordingly, Table 7 shows the suitable conditions
where all properties are equivalent or better than the
furnace heated products.

From the results of this test, we considered thepractical range of IH thermal processing conditions
that can obtain equivalent performance as the furnace
heated products.
First, we take the 310˚C tempering. The 310˚Ctempering is the tempering condition targeting reduced
dimensional change ratio by reducing retained
austenite volume, with sacrifice of hardness. Becauseof this, while the dimensional change ratio of 180˚Ctempering and 240˚C tempering resulted in “×” for1000˚C -8% and 1000˚C -12%, that of 310˚Ctempering resulted in “○”. However, because of lower
hardness, while the static load-carrying capacity forthe 180˚C tempering and 240˚C tempering resulted in“○”, that of 310˚C tempering resulted in “×”, and theoverall evaluation also resulted in “×”.
Although the static load-carrying capacities of
900˚C-8% and 950˚C-8% are “○” even with the 310˚Ctempering, since the 180˚C tempering and 240˚Ctempering conditions also resulted in “○”, it is notnecessary to select 310˚C tempering with lower
hardness. From the above, we have determined thatthe 310˚C tempering condition is practicallyunnecessary.
Next, we take the shear fatigue strength of “950-8180”, which test was not conducted. The results from900˚C-8% and 950˚C-12% show that the strength of
180˚C tempered products are equivalent to or higher
than the 240˚C tempered products. From this fact, it ispossible to predict that 180˚C≧240˚C, and since “9508-240 is marked with “○”, “950-8-180” should also be“○”. Therefore, we have determined that the overall
evaluation of “950-8-180” is also “○”.
Since the condition of carbide area ratio of 12% has
reduced hardening property, there is a concern ininsufficient hardness for thick works. Table 6 shows
the thickness of the test rings and the actual
measurement of hardness immediately below the

Table 6 Hardness of each sample thickness
Heating
temperature
˚C
900
950

Carbide area
ratio
˚C

Thickness
4.0mm

Thickness
3.4mm

12
8

60.1

12

62.1

61.0

61.2

61.3

8

62.1

62.1
62.3

Table 7 Suitable IH conditions for JIS-SUJ2
Heating
temperature
˚C
900
950

-79-

Carbide area
ratio
˚C

Tempering conditions

12

180˚C×120minor
240˚C×43.2s

88NTN TECHNICAL REVIEW No.83（2015）

4. Bearing life test under the suitable
IH thermal treating conditions

5. Conclusion
We have explored IH thermal processing conditions
that can obtain the equivalent performance as thefurnace heated products for the purpose of changing
the heat source of the overall heating of SUJ2
materials for quenching from furnace heating to
induction heating and obtained the following insights:
1) The thermal treatment conditions that can secure
equivalent performance as the furnace heatedproducts under the test conditions described above
are as shown in Table below:

We have created raceways for deep groove ballbearings under the suitable conditions mentioned in
the previous Chapter and conducted bearing life test.

4.1 Test method
The bearing life test was conducted with clean oil
lubrication. Fig. 8 shows the overview of the tester
and Table 8 shows the test conditions. The test
specimens were from furnace heated products and IH
thermal processed products under suitable conditions.
The bearing model type of 6206 and the steel balls of
SUJ2 were used.

Uniform
temperature
˚C
900
950

Table 8 Test conditions of the ball bearing RCF life test

Tempering condition
180˚C×120minor
240˚C×43.2s2) We have conducted the bearing life test using the
raceways made under the above suitable conditions.
The result showed that the life of the createdbearings was more than 4 times the basic rated life.
3) When the heating temperature is raised up to 1000˚C
under the conditions described in this paper, thedimensional change ratio is degraded. If tempered inhigh temperature to improve the dimensional change
ratio, hardness is reduced and static load-carrying
capacity is lowered.

6206C36.86kN
Inner ring/steel ball = 3.11GPa
Contact stress Pmax
Outer ring/steel ball = 3.21GPa
3000min-1
Rotational speed of inner ring
Turbine oil VG56 circulating oil supply
Lubrication
126h
Basic rated life
1000h
Termination
Bearing
Radial load

Load spring

Support bearing 6312

Reference

Test bearing
6206
Coupling

Pully

Carbide area
ratio
%
8
12
8

Test bearing
6206

1) JIS Handbook 2007 Ferrous Materials and Metallurgy 1G0551
p. 665 Appendix C Table 1.
2) Eschmann, Hasbargen, Weigand: Ball and Roller Bearings―
Their Theory, Design and Application―, London, (1958)．
3) JIS B 1519:2009 “Rolling bearings – Static load rating”.
4) Tedric A. Harris: Rolling Bearing Analysis Fourth Edition, John
Wiley & Sons, Inc., (2001) 824．
5) Junzo Okamoto: Dynamic Load Capacity of the Rolling
Bearings, Roller Bearings – Lundberg-Palmgren Theoretical
Discussion -, Seibunsha, (1990) 1.
6) Sakanaka, Matsubara, Shimamura and Ishii: NTN
TECHNICAL REVIEW, 79 (2011) 104.
7) Zenji Nishiyama: Martensitic transformation, Maruzen, (1971)
219.
8) Takumi Fujita: Rolling Contact Fatigue Life Test Design and
Result Interpretation Methods Maintaining Compatibility of
Efficiency and Reliability, Journal of ASTM International, Vol. 7,
No. 6, (2010).

Oil bath

Fig. 8 Schematic drawing of rolling contact fatigue life
tester for ball bearings

4.2 Test results
Table 9 shows the test results. The entire eighttested furnace heated products and the entire eighttested “900-8-240” products, as well as the entire
seven tested “950-8-240” products exceeded 1000
hours and the test was terminated. The bearing life
made from IH thermally treated products can secure
product life of more than 4 times of the basic rated lifeof 126 hours, equivalent to the furnace heatedproducts (930 hours when n is 7) 8).

Photo of authors

Table 9 Life Test Results
Conditionn「900-8-240」

8

Furnace heated product「950-8-240」

Note: Basic rated life: 126 h

Time

8

>1015 〜 1303 h all terminated7>1075 〜 1159 h all terminated>1207 〜 1901 h all terminated-80-

Hiroshi YUKI

Miyu SATO

Advanced Technology
R&D Center

Advanced Technology
R&D Center

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Grease Lubricated High-speed Deep Groove Ball Bearing
for EV and HEV Motor
Masahiko SATODA*
Goro NAKAO*

EV and HEV are tending to increase the output by increasing the rotational
speed to compensate for the output torque reduction by miniaturization of the
motor. Therefore, there is an increasing high-speed rotation request also fordeep groove ball bearings are used for the motor support.
This paper introduces NTN grease lubricated high-speed deep groove ballbearing for EV and HEV motor, and explains its features and performances.

The motor drive section of EV/HEV consists of a
motor and reducer unit. One of the development trendsof EVs and HEVs is smaller drive unit, which requires
higher speed of rotation to maintain the output torque.
Bearings that support motor rotation are generally
used of grease lubrication, and higher rotation speed
increases the temperature of the bearings by centrifugal
force of the cage and diffusion of grease. In this paper,
we introduce a grease lubricated high-speed deep
groove ball bearings for motors of higher speed rotation
for EVs and HEVs, which was made possible byreviewing the materials/shapes of cages and blending
properties of grease of the conventional bearings.

rolling contact surface ⇒ Rise of temperature andincrease of load on the cage ⇒ Seizure and damage
on cage.
Fig. 2 shows the resin cage and grease condition
immediately before the damage occurred, when theconventional grease lubricating deep groove ballbearings were placed in high speed rotation.
A trace of hard contact of the rolling elements canbe observed inside the cage pocket surface of theresin cage. In addition, it is assumed that the cage isdeformed by the centrifugal force leaving no space
between the cage pocket surface and rolling elements.
Furthermore, degraded grease is attached inside
the cage pocket surface indicating that lubrication waspoor.

2. Grease lubricated high-speed deep
groove ball bearings for EVs and HEVs

High speed rotation of main shaft

1. Introduction

2.1 Challenges for the bearings with higher rotation
speed for EVs and HEVs and their measures
In order to achieve higher rotation speed it isnecessary to maintain lubricant on the rolling contact
surface between the raceway and rolling elements(rolling contact surface) and between the contact
surface of the cage and rolling elements (pocket
surface). In case of grease lubrication particularly
grease diffuses with centrifugal force, which isdifferent from oil lubrication. This causes a shortage ofgrease on the rolling contact surface and pocket
surface resulting in poor lubrication.
Fig. 1 shows the damage process during in life ofbearings in high-speed rotation. Poor lubrication on

Centrifugal force
applied to grease

Centrifugal force
applied to cage

Centrifugal force
applied to rolling
elements

Diffusion ofgreaseInterference withrolling elements
due to deformation

Increase ofload on cage

Shortage of
lubrication on
rolling surface

Heating andwear of cage

Accelerated heating inside bearing
Seizure of rolling surface, damage/seizure of cage
Fig. 1 Bearing damage process in high-speed operation*Automotive Product Design Dept. Automotive Business HQ-81-

NTN TECHNICAL REVIEW No.83（2015）

Fig. 3 shows the condition of the grease just before
the damage occurred.
Large amount of grease was attached to the side of
the seal including grease with no discoloration.
From the above observations, we presumed that thequick damage in high speed rotation was caused by
locally insufficient lubrication due to the shortage ofnecessary lubricant supply. The grease was scraped
off from the rolling elements because the pocket
space was reduced by the deformation of the cagefrom the centrifugal force, therefore we developed acage to cope with.
We also developed grease that is retained in theneighborhood of the rolling element surface so that thebase oil is sufficiently supplied in high speed rotation.

- Shape and material of the cage to reduce
deformation in high speed- Strength of the cage against damage- Improvement of lubricating ability of the grease inhigh speed rotation
Outer ring

Seal

Steel ball

Cage
Inner ring

Fig. 4 Grease lubricated high-speed deep groove ballbearing for EV/HEV motor
Trace of contact withrolling elements

2.3 Developed cage(1) Shape and material of cage
We made the following three improvements for
reduction of deformation in high speed rotation
conditions:- Adoption of high-strength material⇒Improvement of rigidity of the cage
- Thicker bottom of pocket to reduce deformation⇒Improvement of rigidity of the cage
- Reduction of wall thickness between the cage
pockets⇒Reduction of centrifugal force
Fig. 5 shows the difference between theconventional product and developed product. In
addition, the difference between the materials of thecurrent product and the developed product is shown in
Table 1.

Trace of degradedgreasePocket surface

Fig. 2 Cage condition immediately before damage occurred
Seal (inside bearing)Thick bottom of pocket

Fig. 3 Grease situation on damage verge

Provision of greasepassage

2.2 Structure and features of the developed
bearings

Reduction of walls
between pockets

By solving the issues described in the previous
section, the developed bearings achieved more than
twice the high speed performance over theconventional bearings.

Conventional Developed
product
product

Fig. 5 Shape comparison of current and developed cage¡High speed performance
Table 1 Material comparison of current and developed cage

Developed bearings dmn value*1 1.08×106(Conventional bearings dmn value: 0.40x106)

Item
Conventional material Developed material
Melting point
260˚C＞300˚CSpecific weight
1.32
1.28
Tensile strength
170MPa274MPa260MPa410MPaBending strength
7.6GPa22.3GPa
Bending elastic modulus (23˚C)
3.5GPa
14.1GPa
Bending elastic modulus (120˚C)*1 Product of the diameter of bearing rolling element pitch circle
diameter and rotational speed

In order to achieve the above high speed performance,
we focused on the following for development of the new
bearings:-82-

Grease Lubricated High-speed Deep Groove Ball Bearing for EV and HEV Motor(2) Strength of cage

maximum stress of the developed product is 1/2 orless than the conventional product, resulting in an
improvement of endurance as the resin cage.

We verified static fracture strength of the cage using
the strength tester shown in Fig. 6. Fig. 7 shows thecomparison of the cage static fracture strength of theconventional product and the developed product. As
shown, the cage fracture strength was improved byapproximately 1.5 times.
Fig. 8 and 9 indicate the analysis results of the cage
deformation and cage stress of the conventional
products and developed products at the dmn value of
1.08x106 and 120˚C. In addition, Fig. 10 and 11 show
the relationship of rotation speed and the maximum
cage deformation, and the relationship of rotation
speed and the maximum cage stress. With theconventional products, the phenomenon as explainedin Fig. 2 may occur at the dmn value of around0.60x106 as the cage and the rolling elementsinterfere. The deformation of the developed products
is about 1/10 of the conventional product, leaving
enough space between the cage and rolling elementseven in the high speed rotation range. In addition, theMaximum stress

Developed
product

Conventional
product

Fig. 9 Analysis result of cage stress

Conventional product

Maximum radial deformation
Small
Large

Direction of load

Developed product

Pocket space in cage

0

Cage under test

250

500

dmn value

750

×103

1000

1250

Fig. 10 Relationship between speed and cage deformation

Conventional product

Generated maximum stress
Small
Large

Fixed

Fracture strength

StrongFig. 6 Cage strength tester

Developed product

Weak

0

Conventional product

Maximum stress

250

500

750

1000

1250

dmn value ×103

Developed productFig. 11 Relationship between speed and cage stressFig. 7 Static fracture strength of cage

Maximum
deformation

Maximum
deformation

Conventional
product

Developed
product(3) Improvement of lubricating property in highspeed rotation
The resin cages can reduce heat generation by self
lubrication and their wear particles and do not promoteoxidative degradation of grease, different from themetallic cages. Furthermore, by providing grooves foroil passage, as shown in Fig. 5, the grease scraping
effect is reduced and favorable lubricating property is
maintained even in high speed rotation.

Fig. 8 Analysis result of cage deformation-83-

NTN TECHNICAL REVIEW No.83（2015）

Cumulative damage probability %2.4 Developed grease(1) Specification
Table 2 shows the properties of the greases. The
developed grease is changed to synthetic oil so thatthe base oil can better penetrate onto the rollingsurface even in high speed rotation of bearings, evenwith relatively low base oil viscosity.
For thickener diurea was adopted and exhibits
superior high temperature life. The consistency was
set low (hard) to reduce diffusion of grease by
centrifugal force to retain the grease close to therolling element surface.

5
5 7 1
×101

2

3

5 7 1
×102

2

3

Life h

5 7 1
×103

The endurance test was conducted with three typesof samples, labelled A,B and C, including theconventional bearings and developed products. Table
3 shows the test conditions and Fig. 14 shows thehigh-speed endurance test results. With theconventional bearings (sample A), lubrication on therolling surface was not sufficient in high speed rotation
as most grease was not used causing poor lubricationand seizure/damage (Fig. 3). Sample B, where
grooves were provided as oil passage to the sample
A, exhibited a longer life with improved oil passage.
Even though the sample B exhibited sufficiently
improved life, we adopted new grease with reduced
diffusion while retaining supply capability of lubricating
oil in high speed rotation to create the developed
bearing (sample C). The developed bearing achieved
a significant improvement of life, effectively using thegrease in bearing.

Diurea220(2) Performance
Grease life test was conducted with the test
machine and test conditions shown in Fig. 12. The
evaluation results of high temperature grease life areshown in Fig. 13. The developed grease exhibitedaround 1.2 times longer life than the conventional
grease.
Bearingunder test10

2.5 Endurance performance of the developed
bearings(1) High speed endurance test

Table 2 Grease properties

Diurea280

20

Fig. 13 Grease life evaluation results

Conventional grease Developed grease
Item
Base oil
Ester oil
Synthetic hydrocarbon oil
Viscosity of base oil
33mm2/s
48mm2/s(40˚C)

Thickener
Consistency

Conventional productDeveloped product

95
90
80
70
60
50
40
30

Bearingunder test
Load
2×Fr

Table 3 Test conditions and sample specification
Rotational speed
Load
Temperature
HeaterSample A
Sample B

Bearing under evaluation
Rotation speed

Sample C

6204LLB（Steel plate cage）
10,000min-1

Load2,000N

Temperature

150˚C

dmn value: 1.08×106

0.05C（C：Basic dynamic load rating）
120˚C
Conventional bearing

Conventional bearing+developed cage

Developed bearing (B+developed grease)
Target life

Sample C

Fig. 12 Grease life evaluation tester / test conditions
Sample B
Sample A
Life hFig. 14 High-speed endurance test results-84-

Grease Lubricated High-speed Deep Groove Ball Bearing for EV and HEV Motor(2) Temperature rise test

Table 5 Test conditions

Fig. 15 shows the results of the temperature rise
test under the conditions listed in Table 4. With theconventional bearings, abnormal temperature rise wasshown in high speed rotation, and contact of the cageand rolling elements was observed. However, with the
newly developed bearings, no abnormal temperaturerise was observed, maintaining a favorable condition.

Rotational speed
Load

Temperature

dmn value: 0.135〜1.08×106
Room temperature

Temperature

2 hours for each rotational speed

Holding time

Temperature rise(outer ring temperature) ˚C0Time (cycle)

80

Fig. 16 Rapid acceleration/deceleration test

Conventional productDeveloped product7060

Abnormaltemperature rise;terminated50

3. Conclusion

40

The developed bearing introduced in this article has
the following characteristics.
High speed rotation: Can be used under the condition
with the dmn value of 1.08x106
Equivalent to the rotation speed of 20,000 min-1,
in case of bearings with inner diameter of 40 mm
・Limit rotation speed: Twice or more than theconventional bearings

30
50
10
0

120˚C

dmn value
×106

0.05C（C：Basic dynamic load rating）

Load

0.1C（C：Basic dynamic load rating）

0.70

Table 4 Test conditions
Rotational speed

dmn value 0⇔0.70×106(Rapid acceleration/deceleration test: Fig. 16 )

0

250

500

dmn value

750

×103

1000

1250

[

Fig. 15 Temperature rise test results

The developed grease lubricating high speed deep
groove ball bearings for EVs and HEVs are promisingfor high speed performance required for EVs andHEVs; therefore, we will actively promote it in themarket. In addition, we will continue developing
products to help EVs and HEVs evolve further withhigher performance.

(3) Rapid acceleration/deceleration test
We conducted rapid acceleration/deceleration testof the developed product under the test conditions
listed in Table 5. The conventional bearing failed inless than 100 hours; however, the developed bearing
continued to function more than 1000 hours with noissues and no abnormalities.

Photo of authors

Masahiko SATODA

Goro NAKAO

Automotive Product Design Dept.
Automotive Business HQ

Automotive Product Design Dept.
Automotive Business HQ

]

-85-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]Deep Groove Ball Bearing
for Alternator Corresponding to 200˚C
Tomoyoshi IZUTSU*
Goro NAKAO*
According to electrification of automotive auxiliary equipment progresses,
electric power consumption of vehicle will increase. As enhanced power
generation is required and usage environment tends to become hightemperature, alternator bearing is also required to support high temperature.
This article introduces the feature and the performance of the 200˚C capablebearing for automotive alternator.

1. Introduction

2. Bearings for alternators with
operating temperature of 200˚C

Requirements on vehicle components are broad
including improvement of reliability, lighter weight,
compactness and electrification demands. These
requirements continue to become more severe.
In addition, today as significant progress has been
made in electrification of vehicles for improved
comfort, safety and fuel economy, further increases in
the generating capability of alternators is required.
Among other specifications, the bearings foralternators are required to have high temperature
resistance, brittle flaking resistance, anti-corrosive
properties, and low temperature properties.
Along with the increase of power generation
capability, bearings are also required to havespecifications with resistance to high temperature.
Therefore, NTN has developed ball bearings foralternators which can be continuously used withintemperature envelopes of 180˚C 1).
Furthermore, anticipating that the operating
temperature of alternators will increase up to 200˚C,
NTN has developed bearings to support such use. In
this paper, “Deep Groove Ball Bearing for Alternator
Corresponding to 200°C” is introduced, which is twice
more resistant than the conventional products, withimprovement of bearing material, cage, seal andgrease performance.

2.1 Features(1) Support to 200˚C
Bearing operating temperature range: -40 to 200˚C(2) Long life
Grease life, brittle flaking life: More than two times
compared with the conventional product(3) High corrosion resistanceGrease anti-corrosive performance: More than twotimes compared with the conventional products
Metal anti-corrosive performance: More than 5times compared with the conventional products

2.2 Structure
Fig. 1 shows the structure of the bearing foralternators operating up to 200˚C.
Outer ring
Seal

Cage
Steel ballInner ring

残留オーステナイト量%

Fig. 1 Deep groove ball bearing for alternator to 200˚C*Automotive Product Design Dept. Automotive Business HQ-86-

Deep Groove Ball Bearing for Alternator Corresponding to 200˚C

2.3 Bearing specifications(1) Inner ring/outer ring(3) Seal
To prevent leakage of grease and foreign objects
from entering, a contact type rubber seal with theuniquely designed seal lip shown in Fig. 4 wasadopted. Fluorine-contained rubber, which has goodcompatibility with the developed grease as describedlater, was adopted as the rubber material whichallowed its use for temperatures up to 200˚C.

High carbon chromium bearing steel (SUJ2) is
adopted for the inner ring and outer ring that compose
the raceways, with improved dimensional stability inhigh temperature by special heat resistance treatment, as well as improved brittle flaking properties. Fig. 2
shows the comparison results of brittle flaking life.

Heat tolerant material ofdeveloped bearings

Cumulative damage probability %Heat tolerant material of
conventional bearings
95
90
80
70
60
50
40
30
20

Seal
Inner ring

10
5
1

×101

2

3

5

7

Life h1

2

3

Fig. 4 Seal shape

×102

Fig. 2 Brittle flaking life comparison(4) Grease
NTN has been developing/adopting urea based
grease with improved performance in all aspects suchas high temperature resistance, brittle flaking
resistance, corrosion resistance, low temperature
property, etc. These properties are required withbearings for auxiliary equipment 2). However, when
support for 200˚C is considered, fluorine contained
grease which has superior high temperature
resistance was adopted, since urea based grease hassignificant precipitation of amine, which attacks rubber
seals. However, conventional fluorine contained
grease has several issues such as increase of torqueduring cold starts of alternators, increase of selfheating and metal anti-corrosive properties. Therefore,
we developed a new fluorine contained grease
improving these issues.
The basic properties of this grease are shown in
Table 1.

(2) Cage
Cages of bearings for alternators are required tofunction in a broad temperature range, in addition tohigh strength and tolerance to withstand the constantload from the balls caused by rapid
acceleration/deceleration and installation error against
the shaft. Metallic cages may drive degradation by
metallic wear particles or if oil film between balls andcage becomes very low.Therefore, PPS resin was
adopted as the cage material, featuring high strengthand heat tolerance as longer grease life can be
expected.
Fig. 3 shows the analysis results of the stress on
the cage, generated when the bearing rotates at
20,000 min-1. Since the maximum stress at the
weakest part is 57 MPa and the tensile strength of thismaterial when degradation of 200˚C x 1,000 h is
assumed is 125.4 MPa, the safety factor is more than
two.

Table 1 Grease properties

Thickener
Base oil
Dynamic viscosity 40˚Cof base oil
100˚C
mm²/s
Worked penetration (60W)
Drop point (˚C)

Developed fluorine Conventional fluorine
contained grease
contained grease
PTFE
PFAE

PTFE
PFAE

17.5

18.0

Test method: JIS K 2283, JIS K 2220

Fig. 3 Results of cage stress analysis

-87-

130
280
−

160

265〜295
232 or more

NTN TECHNICAL REVIEW No.83（2015）

2.4 Performance(1) High temperature resistance

Bearingunder test

6202（φ15×φ35×11）

Temperature

200˚C

Rotational speed min-1

Fig. 6 shows the result of evaluation for grease life
under the temperature of 200˚C with the measurement
equipment and test conditions listed in Fig. 5. The
developed grease has a proprietary additive blended
in to prevent metal contact and reduce wear providing
more than two times the seizure resistance comparedwith the conventional grease.

Bearing size0

1 cycleOperating cycle

Coil spring

Fig. 7 High temperature rapid acceleration/deceleration
test conditions

Support bearing
PulleyHeater1200

Bearing size

Rotational speed

Life h10006204（φ20×φ47×14）

Loademperature

800
600
400

10,000min-1

200

Fr =Fa = 67N

0

200˚C

Fig. 5 Grease life tester/test condition

Developed
greaseConventional
greaseFig. 8 High temperature rapid acceleration/deceleration
test results

7000
6000

Life h5000(3) Brittle flaking property

4000

Fig. 9 shows the evaluation results of brittle flaking
property under the NTN conditions. The developed
grease has longer life than the target life by more than
twice.

3000
2000
1000
0

Developed
greaseConventional
greaseBearing size
Rotational speed

Fig. 6 Grease life test results

Temperature

6203（φ17×φ40×14）
Rapid acceleration/
deceleration conditions
Room temperature

700(2) Rapid acceleration/deceleration property

600

The actual operating conditions of alternators
include rapid acceleration/deceleration with large
loads. Therefore, endurance needs to be evaluatedunder the actual operating conditions.
Fig. 7 indicates the high temperature and rapid
acceleration/deceleration test conditions which matchwith the actual operating conditions. The results areshown in Fig. 8.
As the results of the rapid acceleration/deceleration
test show, the developed grease exhibited longer life
than the conventional grease by more than two times,
even under the actual operating conditions.

Life h

500
400
300
200
100
0

Developed
greaseConventional
greaseFig. 9 Brittle flaking life test results-88-Deep Groove Ball Bearing for Alternator Corresponding to 200˚C(4) Corrosion resistance(6) Anti-corrosive property to metal

Fig. 10 shows the corrosion resistance test
conditions for the developed grease and the results.
As the result of the corrosion resistance test, thedeveloped grease exhibited significant improvementon the corrosive resistance.

In general, fluorine bonds with iron (Fe) when it
comes in contact with metallic active surface causing
corrosion and increasing wear. To address thisproblem, the developed grease has a unique additive
which adheres on the metal surface to reduce reactionof fluorine and iron, then consequently metallic wear.
Fig. 12 shows measurement results of wear amount(iron content) inside bearings under the hightemperature resistance evaluation and Fig. 13 shows
the appearance of the bearings after the test. With thedeveloped grease, the iron content was 1/5 of theconventional grease, showing the effect of the additive
to reduce metallic corrosion from fluorine.

Test sample
Outer ring of tapered roller bearing (Model #: 30204)
Pre-processing
Immersed in 1% sodium chloride water solution
before test
Test atmosphere Left in moist atmosphere of 40ºC for 48 hours
Measurement of rust ratio

35
30
25
20

Bearing size

15

Rotational speed
Load

10
5
0

Developed
greaseConventional
greaseFig. 10 Rust test results(5) Low temperature property
The operating environment of alternators can
include low temperature operation down to -40˚C.
Grease inside bearings may be hardened at thesetemperatures which may prevent the alternator fromrunning smoothly. The low temperature torque test
conditions and its results are shown in Fig. 11. The
developed grease has improved low temperature
torque to 1/6 or less compared with the conventional
grease.

Bearing size

6204（φ20×φ47×14）

Starting torqueMaximum torque at start

Rotational torque

Average torque of 15 second

Rotational torque mN·m

Temperature

1400
1200
1000

10

6204（φ20×φ47×14）
10000min-1

Fr =Fa = 67N

220˚C (accelerating condition)

Temperature
Iron content X-ray intensity kcps

Rust development ratio %Evaluation method

Developed grease

8

Conventional grease

6
4
2
0

0

200

400

Time h600

800

Fig. 12 Iron weight measurement results

Bearingexternal view

Bearing
internal viewSealinternal view

-40˚C

(Developed grease) after 640 h

Starting torque
Rotational torque

800
600
400

(Conventional grease ) after 640 h

2000Fig. 13 Internal conditions of bearings after test
Developed
greaseConventional
greaseFig. 11 Rotation torque measurement results-89-

NTN TECHNICAL REVIEW No.83（2015）(7) Temperature rise of self heat generation

3. Conclusion

Since the alternator rotates at high speed, it isnecessary to verify that it does not become
unexpectedly hot due to self heat generation inside
the bearings. Fig. 14 shows the test equipment andconditions and Fig. 15 shows the measurement
results of the temperature rise.
The developed grease has stable result even at
20,000 min-1.
Bearing size

Rotational speed

The features of the bearings for alternators
operating in 200˚C introduced in this paper are as
follows:- By improving the bearing material, cage, seal andgrease, the bearing can now be used in a wide
temperature range from -40˚C to +200˚C.
Since this developed product increases the hightemperature resistance and prolongs the life by twice,application of these bearings in high temperatureenvironment due to further increase of power
generation capability of alternator can be expected.
We believe they contribute to further electrification ofvehicle components.

6203（φ17×φ40×12）
5,000〜20,000min-1
300N

Load
Temperature
Measurement position

Room temperature
Outer ring (when stabilized)
2×300N

Reference
1) Ikuo FUJINIWA, Makoto MURAMATSU and Tadahisa
TANAKA, Accessory Technology Trends and ProductDevelopments, NTN TECHNICAL REVIEW No. 75 (2007),
110-1152) Takayuki KAWAMURA, Hidenobu MIKAMI, Developmentof NA103A Long-life Grease for Automotive Components,
NTN TECHNICAL REVIEW No. 75 (2007), 116-123

Rise of temperature ˚CFig. 14 Grease life tester and test condition
25
20

Developed grease
Conventional grease

15
10
5
0

5,000

10,00015,00020,000

Rotational speed min-1

Fig. 15 Measurement results of temperature rise

Photo of authors

Tomoyoshi IZUTSU

Goro NAKAO

Automotive Product Design Dept.
Automotive Business HQ

Automotive Product Design Dept.
Automotive Business HQ-90-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]3D Microgeometry Measuring Machine

Hiroaki OBA*NTN has developed a 3D microgeometry measuring machine that canmeasure a height length of 20 micrometer only for 0.6 seconds and is thefastest in class.
By the development of unique image analysis software, in addition toadapting a scanning white light interferometry and a high-speed camera,
speeding up has been realized.
The image analysis software has various functions that are 3D viewer,
profile viewer, automatic measurement function of height and volume,
analysis function of surface roughness, etc.
When this equipment is mounted on NTN desktop fine pasting system,
the shape of the ink immediately after pasted can be checked.

1. Introduction

2. Overview of 3D Microgeometry
Measuring Machine

In the microprocessing field, it is important tomanage the surface properties and dimensions offinished work after etching, precision machining,
application of chemical solution, etc. In order tomaintain the required product quality, visual inspection
is conducted during processing and after completionof work. Microprocessing has been applied particularly
within the electronic component field where
advancement in micro wiring patterns is significant
and the medical device field where superior reliability
is required. Consequently, improvement of productivity(by reducing inspection time) and even higher quality
are now required, however most commerciallyavailable 3D measurement systems require more thanseveral seconds for measurement, which is notpractical for verification along the production line.
NTN has developed the fastest 3D microgeometry
measuring machine (hereinafter, "this measurement
system”) in its class by adopting a white lightinterferometer and high-speed camera and developing
unique image analysis software, allowing a minimum
measurement time of 0.6 seconds for a measurement
range of 20 μm. This paper introduces the features ofthis system.

2.1 Principle of Measuring Height1) Configuration of White Light Interferometer
Fig. 1 shows the white light interferometer adopted
in this system for detecting focusing position. The
white light interferometer is configured by combining a
two-beam interference objective lens with themicroscope lens barrel.

High-speed camera

Microscopelens barrel
Correcting lensImaging lensFilterHalf mirror

LED light
Piezo stage

ObjectTwo-beaminterference
objective lensFig. 1 White light interferometer*New Product Development R&D Center-91-

NTN TECHNICAL REVIEW No.83（2015）

2.2 Fast Measurement Processing

In general, the Michelson type two-beam interference
objective lens is used for low magnification and theMirau type is used for high magnification. In this system,
the Michelson type is used for x2.5 and x5 and theMirau type is used for x10, x20 and x50 magnifications.
Fig. 1 shows an example of configuration of the Mirau
type two-beam interference objective lens.

In order to measure the distance D between levels
as shown in Fig. 3, the two-beam interference
objective lens must be moved for a distance greaterthan D.
The speed of the move is restricted by image
capture speed of the camera to be used. In thissystem, we adopted a high-speed camera which cancapture more than 1,000 images per second. An
algorithm for fast operation was created by allowing
the image analysis software to calculate heightmeasurement information from an image during
capture of the following image. From the above, we
achieved a minimum measurement time of 0.6
seconds for a height measurement range of 20 μm.

2) Measuring Principle- Generating Principle of Interference Fringes
Fig. 2 shows a diagram of the Mirau type two-beaminterference objective lens. This objective lens has areflective mirror on the central part of the objective lenssurface and a semi-transparent mirror in the middle offocal plane of the objective lens. The semi-transparent
mirror induces interference between the reflected lightsfrom the surface of the object and from the reflective
mirror. The intensity of the interfered light is maximized
when the focus on the objective surface is obtained asindicated by the red lines in Fig. 3.
Images are captured by moving the two-beaminterference objective lens in the direction of the opticalaxis in order to obtain the coordinates where theinterference light maximizes its intensity for each pixel,
from which the focusing position, namely, the height
information of the object surface, is calculated.
In order to obtain the coordinates on the optical axis
from the intensity of interference light, it is necessary tocapture images by moving the lens at a constantinterval in the direction of the optical axis. Since theaccuracy of this interval affects the measurement
accuracy, a piezo stage was implemented for moving
the two-beam interference objective lens.

2.3 Configuration of the Measurement System
A photo of the measurement system is shown in
Fig. 4.
Fig. 5 shows the configuration of this systemequipped with automatic XY stage and automatic Z
stage. The stroke of each stage is selectabledepending on the size of the object to be measured. In
addition, the system can be configured differently from
Fig. 5, such as integrating only the optical head, thepiezo stage, two-beam interference objective lens, andcamera. For example, if integrated into the desktop
micro application system, it is possible to verify theshape of an ink material immediately after applicationof the material.
Table 1 shows the system’s main specifications.
The resolution of the camera, and therefore the visual
field size, is 640 pixels horizontally and 480 pixels
vertically. The standard for height measurement range
is 100 μm, depending on the stroke of the piezo
stage combined in the system.

Objective lensReflective
mirrorSemi transparent
mirrorReflective light

Object

LED light head

Monitor

High-speed camera

Fig. 2 Mirau interferometry objective lens

Microscopelens barrel

D

Change of interfering
light intensity

Keyboard

Piezo stageTwo-beaminterference
objective lensFig. 4 Appearance of 3D microgeometry
measuring machine

Fig. 3 Intensity variation of interfering light-92-

3D Microgeometry Measuring Machine

LED light head

LED light
controller

High-speed camera
Automatic Z stage

MonitorMicroscope lens barrel

Computer

Piezo stage
controller

Mouse

Two-beam interference
objective lensAutomatic XY stage

Keyboard

Piezo stage

Automatic XYZ stage controller

Fig. 5 Configuration example of 3D microgeometry measuring machine
Table 1 Main specifications of 3D microgeometry measuring machine
Item
High-speed camera
Visual field size
Lighting device
Height measurement range/resolution
Computer
Analysis software
Power supply

Description

Monochrome CCD camera, resolution horizontal 640 pixels x vertical 480 pixels
Measuring frame rate: Over 1,000 images/second
x2.5
1,280×960μm

x5
x10
x20
x50
640×480μm 320×240μm
160×120μm
64×48μm
White light LED coaxial episcopic illumination
100μm／5nm
CPU: High performance unit, Memory: 4 GB
License key required
Single phase 100V±10％ 50／60Hz 15A（1.5k VA）

range of 30 μm was performed in around 0.9
seconds. The actual measured height of the wiring
was 20 μm.
Fig. 6 (c) shows the 3D image of a metallic object
made of stainless steel. A height measurement rangeof 500 μm was performed in around 15 seconds. The
distance between the upper and bottom surfaces was
380 μm. In addition, the 2,000 μm stroke piezo stage
was used for this measurement.

2.4 Example of Measurement
Fig. 6 (a) shows the 3D image of ink applied at aconstant interval on a glass plate using the desktop
micro application system. A height measurement
range of 20 μm was performed in around 0.6
seconds. The actual measured height of the ink was 2μm.
Fig. 6 (b) indicates the 3D display of a printed
wiring board after etching. A height measurement

230μm
380μm

2μm

（a）Pasted ink on glass substrate

20μm（b）Printed wiring board

Fig. 6 Measurement result examples

-93-

40˚

（c）Machined part

NTN TECHNICAL REVIEW No.83（2015）

2.5 Verification of Reproducibility

3. Image Analysis Software

Fig. 7 (a) indicates the measurement results of aheight master (Height master made by Mitsutoyo) of
10 μm. Fig. 7 (b) shows the results of repeatedmeasurements. An average result of 10.374 μm andstandard deviation of 7 nm (reproducibility: 0.1%) wereobtained for the repeated measurement. The
reproducibility was calculated from 10 repeatedmeasurements, assuming the measurement results
follow the normal distribution with a reliability of 95%.

The operation screen of the system is shown in Fig.
8. The operation screen consists of the various
controls necessary for measurement and analysis, aresult display area to show the secondary feature
values such as cross sections, an image display area
to show the real-time display of the images capturedby the camera and the stage operation area.
By integrating the operation buttons and result
display area, no new operation screens need to beopened during the measurement process allowing
simple operation with two clicks of a mouse.
Image display area

Result display area

10μm

Measured value μm(a) Height master: level difference of 10 micrometers

10.50
10.48
10.46
10.44
10.42
10.40
10.38
10.36
10.34
10.32
10.30

Operating button

Stage controller

Fig. 8 Operation screen

3.1 Display of the Measurement Results
Three display functions for measurement results areavailable: color display, 3D display and cross section
display.
1

2

3

4

5

6

7

Measurement cycles

89¡Color Display Function

10

Fig. 9 shows the color display result of a cylindricalobject.
The color display function displays the contour-line
map in color, with blue indicating the lowest area andred the highest area.

(b) Reproducibility of height measurement

Fig. 7 Height master and reproducibility

Color display
A

A

Fig. 9 Color display-94-

3D Microgeometry Measuring Machine¡3D Display Function

and their difference, height and distance of thespecified two points, or the cumulative heights
between those two points are displayed.

This function displays a 3D image of the measured
object. It is possible to zoom in/out, shift and rotatewith simple mouse operation. Fig. 6 and 7 weredisplayed by using this function.

3.2 Data Correction Function¡Cross Section Display Function

Three data correction functions are available:interpolation, smoothing and surface correction. These
functions are used to clean up the measurement
results by interpolating missing results and eliminating
noise due to measurement errors for cleaner display
or to eliminate the inclination or curvature of surface
as the preprocessing of the analysis function
described later.

The height measurement results along withhorizontal and vertical lines specified by the mouse on
the color display, or a line determined by any two
specified points, are displayed. The horizontal axis
indicates the height and the vertical axis indicates the
pixel position. Fig. 10 shows the cross-section displayof A-A’ from Fig. 9 . At the bottom of the cross-section
display, the maximum measurement result, minimum¡Interpolation
This system may not be able to obtain theinterference light required by the operating principle
when the height changes quickly changes. The
measurement result shown in Fig. 11 (a) has missingheight data in the black area indicated by the arrows.
The interpolation function estimates the height of
those pixels from the height of the proximate pixels.
Fig. 11 (b) shows the 3D display after correction byinterpolation.

Cross-section A-A' of Fig. 9¡Smoothing
This function eliminates spike-shaped data mixed in
the measurement results by smoothing. When thereflective light is low, the maximum intensity of theinterference light may be erroneously measured. Inthat case, the smoothing function averages the data toeliminate the erroneously measured results.

Fig. 10 Cross-section display（a）3D indication after height measurement（c）3D indication after smoothing（b）3D indication after interpolation（d）3D indication after quadric surface correction

Fig. 11 Application examples of data processing functions-95-

NTN TECHNICAL REVIEW No.83（2015）¡Surface Correction

displayed in the title field on the top left of themeasurement window. In the example of Fig. 13, a
spherical object with a height of 3.444 μm relative tothe base surface, diameter of 174 μm and volume of
0.041 nl was measured.

Two types of surface correction functions areavailable: plane correction and quadric surface
correction. The plane correction converts a sloped
surface into a plane and the quadric surface correction
converts a quadric surface into a plane. Fig. 11 (a)indicates the 3D display of the cylindrical object of 6
mm of diameter. The 3D images after interpolation,smoothing and quadric surface correction are shownin Fig. 11 (b), (c) and (d), respectively. It can be seenthat Fig. 11 (d) eliminated curvature in Fig. 11 (c).

3.3 Analysis function
Three analysis functions are available: automaticlevel measurement, automatic volume measurement
and surface roughness analysis.
（a）3D indication¡Automatic Level Measurement Function
This function automatically calculates the maximum
height (level difference) of the tested area from thebase surface. Fig. 12 shows the measurement resultsof a 10 μm height master. The area surrounded by
the red frame in Fig. 12 (b) is the base surface andthe area surrounded by the blue frame is the tested
area which can be arbitrarily set. The detected level
difference is displayed in the title field on the top left inthe measurement window. In the example of Fig. 12
the calculated level difference was 10.266 μm.

（a）3D indication（b）Calculation result

Fig. 13 Example of volume measurement¡Surface Roughness Analysis Function
Two functions are available: line roughness arearoughness. In the line roughness calculation,roughness information is calculated along a horizontal,
vertical or angled line segment. In the area roughness
calculation function, roughness information is
calculated from the measured values within thespecified range. Fig. 14 is an example of line
roughness measurement result, calculated along the
red line segment.

（b）Calculation result

Fig. 12 Example of measurement of level difference¡Automatic Volume Measurement Function
This function is used to calculate the diameter,height and volume of a sphere automatically. Fig. 13
shows the measurement of volume of spherical ink
applied by the desktop micro application system. The
area surrounded by the red frame in Fig. 13 (b) is thebase surface and the yellow frame indicates thebounding rectangle of the spherical ink, which is
automatically detected by image processing. The
height of the detected ink, diameter and volume are

Fig. 14 Display example of line roughness-96-

3D Microgeometry Measuring Machine

4. Conclusion
This paper introduced the features of the 3D microgeometry measurement machine capable of thefastest operation in its class.
The micro machining technologies is used inresearch and development in various fields such as
semiconductors, electronics and medicine, and theirapplication range is expanding. The requirement for
quickly measuring, displaying and verifying microshaped objects is expected to grow, as high reliability
is required in the production line for maintaining
product quality.
With this system we will continue to improve
performance and enhance functions for the productsthat contribute to the functions and quality of the microprocessed products in research and development, as
well as in the production process where high reliability
is required.

Reference
1）Kieran Gerald Larkin, “Efficient nonlinearalgorithm forenvelope detection in white lightinterferometry”, J. Opt.
Soc. Am. A, Vol.13（1996）, No.4, 832-843
2) Toyohiko Yatagai, “Recent Progress in Optical
Interferometric Measurement", Journal of the Japan
Society of Precision Engineering Vol. 51 (1985), 33-40
3) Hiroshi Kubota, “Coherency of Light and its Application”,
SEISAN KENKYU Vol. 15 (1963), No. 10, 410-414
4）Akiko Harasaki, Joanna Schmit and C. Wyant, “Improved
vertical-scanning interferometry”, APPLIED OPTICS,
Vol.39（2000）, No.13,2107-2115

Photo of author

Hiroaki OBA
New Product Development
R&D Center-97-

NTN TECHNICAL REVIEW No.83（2015）[ New Product ]High-speed Microscopic Coating Applicator

Akihiro YAMANAKA*
In 1990, NTN started manufacture and sell the system to repair defects that haveoccurred in the production process of flat panel display, there are many adoption
proven.Core technology of this repair system are high-precision positioning
technology with precision stages, fine processing technology with laser beam andfine pasting technology with pasting needle. In particular, the fine pasting
technology is a unique pasting method of NTN, and we can paste many kinds ofliquids in a stable with this technology, there are many adoption proven in the
repair application for the color filter of a liquid crystal display.
This paper explains newly developed high-speed microscopic coating applicator
for the purpose of application deployment of fine pasting technology.

1. Introduction

2. Microscopic application technology
using application needle method

NTN has been developing and providing patterncorrection devices since 1990, which repair flat panel
display (FPD) defects produced in the manufacturing
process, for use by many FPD manufacturers. These
devices make a significant contribution towards
reducing the environmental load and FPD
manufacturing cost by reducing the number of failed
boards which otherwise would have been discarded.
The core technologies of these devices are highly
precise positioning technology with precision stages,
fine film micro-machining technology with laser, andmicro application technology using application needles.
Particularly, the micro application technology is the NTN
proprietary technology which received the mechanicalcomponent award of the “Cho” MONODZUKURI
Innovative Components Awards from the Nikkan Kogyo
Shimbun Ltd.
This paper introduces the high-speed microscopic
coating applicator, which is newly developed for
deployment in new fields, leveraging the microscopic
application technology principle, and superb feature ofadaptation of various types of liquid.

2.1 Shape of the application needle andapplication principle
The application needle has a flat surface on theconical tip, as shown in Fig. 1. When the applicationmaterial (ink) is supplied to the application needle, thematerial moves up along the application needle bysurface tension, as shown in Fig. 2, leaving a fixedamount on the flat surface of the tip. Stablemicroscopic application is achieved by transferring thisapplication material onto the flat surface of theapplication surface, as shown in Fig. 3.

InkApplication needle

Fig. 1 Shape ofneedle tip

Before ink is
supplied

After ink is
suppliedtime

Fig. 2 Supply of application material to application needle*New Product Development R&D Center-98-

High-speed Microscopic Coating Applicator

Application needle

Ink
Zoomed view ofneedle tip

Glass substrate(1) Application needle
in contact with base plate

Zoomed view ofneedle tip(2) Application needle
starts ascending(3) Application needle completes
ascending (application completed)

Fig. 3 Pasting process

2.2 Configuration of application needle unit

occurs, unlike the conventional method which uses a
nozzle. Therefore, one of the devices greatest
features is that it can reliably apply materials with abroad range of viscosity. The comparison with theconventional application methods is given in Table 1
and an example of application by the applicationneedle method is shown in Fig. 6.
In this example the application material was a silver
paste with a high viscosity and even after 500 uses
there was only a slight change in application
diameter, proving consistent application.

The application needle unit is configured with theapplication needle and application material container,
which are supported by the application needle
support, as shown in Fig. 4. The application needle
and application material container are fixed to theapplication needle support by a magnet to allow easy
replacement.
The application material container has holes at the
top and bottom so that the application needle can beinserted through. The needle is inserted from the top
hole. The application needle is constantly immersed in
the application material injected to the applicationmaterial container, maintaining the application material
adhered to the tip of the application needle.

ApplicationneedleDispenser nozzle
Applicationmaterial
container

Inkjet head

Nozzle hole
Base plate

Application needle
support(1) Dispenser method

Applicationmaterial
container

Nozzle hole
Base plate(2) Inkjet method

Fig. 5 Conventional application methods
Table 1 Comparison with the conventional application method
Conventional application methods
Dispenser
InkjetmethodmethodDiameter >15μm Diameter >100μm Diameter >20μm> several pR>several μR
5pR to 2000pR
ApplicationneedlemethodApplication material
containe

Fig. 4 Support construction of application needle andapplication material container

Application size
Application amount

Viscosity of
application material

1〜1×105mPa・s 50〜3×105mPa・s

Clogging<Application>

When the material is to be applied, the tip of theapplication needle is pushed out from the bottom holeof the application material container, and the applicationneedle tip makes contact with the surface to apply theapplication material, as shown in Fig. 3. The applicationneedle is fixed to the sliding mechanism which canmove up and down, and the contact pressure is kept
constant.

Microscopic applicationof high-viscosity material

1〜40mPa・s

No

Yes

←

○

△

×

Application diameter: approx. 75μm,
Viscosity of application material: approx. 8 x 104mPa·s

2.3 Features of application needle method
Fig. 5 shows two examples of conventional
application methods. Since the application needle
method supplies the application material on the outer
perimeter of the application needle, no clogging

1-100 cycles

401-500 cycles※No change from 1-100 cycles

Fig. 6 Example of application-99-

NTN TECHNICAL REVIEW No.83（2015）

The sliding mechanism was integrated into theapplication needle holder to reduce the size andweight of the application needle support. Due to thesefeatures, the impact load at the instance that theapplication needle makes contact with the application
surface was reduced to approximately 0.06 N and thecontact load during application to approximately 0.03 N.
Table 2 shows the comparison between the newly
developed high-speed application mechanism and theconventional application mechanism.

3. High-speed microscopic coating applicator
As the microscopic application technology isextended to new applications such as volume
production of electronic devices and circuit pattern
drawing, the precision and consistency of theapplication method gained a great reputation.
However, the challenge was the reduction of tact.
Therefore, we have developed an application
mechanism to achieve application time of 0.1 second
as opposed to 2.4 seconds required by theconventional FPD pattern correction devices, which is
24 times faster.

Table 2 Comparison of conventional pasting unit
Application speed
Application method
Conventional
application mechanism 2.4 sec/application

3.1 High-speed application mechanism

High speed
0.1 sec/application
application mechanism

We have implemented a combination of a motorand cam for driving the application needle up anddown as the high-speed application mechanism. The
rotation of the motor is converted to linear motion
using a cam to move the application needle up and
down.
Fig. 7 shows the high-speed application
mechanism. We have optimized the motor control andcam geometry to reduce the strong impact from highspeed vertical motion at the instance the applicationneedle makes contact with the application surface,and reviewed the structure of the application needle
holder to reduce the weight of the application needle
support. Fig. 8 shows the application needle holder.

Ratio

x24

Contact load
Approx. 0.07N
Approx. 0.03N
Less than half

3.2 High-speed microscopic coating applicator
Fig. 9 shows an example of the high-speed
microscopic coating applicator (hereinafter, applicator)
with the high-speed application mechanism. The
system shown here is equipped with 2 application
mechanisms which make it possible to install multiple
application mechanisms in a system. By installing
multiple units, the system can be used for applicationof different sizes and different materials.

Observation optics

Motor
Cam

High-speed
application
mechanism(2 units installed)

Applicationneedle holder
Applicationmaterial
container

XY stage

Fig. 7 High-speed application mechanism
Fig. 9 High-speed microscopic coating applicator
exampleApplicationneedleFig. 8 Application needle holder-100-High-speed Microscopic Coating Applicator

1) User interface(4) Matrix application mode

The operation of this system is controlled by input
from the mouse in the operation screen shown on thedisplay of the computer. Fig. 10 shows an example ofthe operation screen.
This screen displays an image of the application
surface in the observation image display area. The
position for material application can be chosen byclicking the mouse on this image. The mouse can also
be used to move the XYZ stage by clicking in the XYZstage operation instruction area shown in the bottom
right of Fig. 10.

By specifying the pitch and number of dots for XY
directions, a matrix is applied.

(5) Rectangle application mode
By specifying the top left and bottom right corners of
the rectangle with the mouse, a rectangular area isapplied.

(6) Area application mode
Similar to the rectangle application mode, byspecifying the top left and bottom right corners, adiamond shaped area is applied.
In addition to the above, the following automaticbulk application operations are optionally available.

(7) Automatic bulk application to the registeredcoordinates

Observation image display area

As shown in the observation image display area ofFig. 10, the positions indicated with the preregistered coordinates are automatically applied inbulk.

(8) Automatic bulk application with CAD data
Automatic bulk application is available by the image
data of the BMP format and CAD data of the DXF
format.

3) Other options
Our 3D micro-geometry measurement machine,which is introduced separately, can be optionallyinstalled, which allows verification of the application
shape and evaluation of the application area
immediately after the application. Fig. 12 shows anexample of the measurement of application shape.

XYZ stage operation instruction area

Fig. 10 Operation screen of high-speed microscopic
coating applicator2) Application mode
There are 6 types of application modes provided as
standard modes in Fig. 11.

Fig. 11 Application modes
Fig. 12 Measurement of application example(1) Point application mode
Application occurs at the cross mark position of theobservation image display area (center of image).

4) Specification and configuration example ofhigh-speed microscopic coating applicator(2) Free application mode

An example of specification of the basic
configuration is shown in Table 3 and the outline viewis shown in Fig. 13
This compact system can be operated with 100VAC
and requires an area of 300 square millimeters tooperate. Trial applicator can be rented out forevaluation.

Application occurs at the position specified by themouse click in the observation image display area.

(3) Line application mode
By specifying the starting point and end point of aline by the click of the mouse in the observation
image display area, a line is applied at thepredetermined pitch.
-101-

NTN TECHNICAL REVIEW No.83（2015）

Table 3 Specification of high-speed microscopic coating applicator as an example
Item
Application unit

Description

Application speed: 0.1 s/application or more
Accuracy of application positioning: ±15μm or less (reproducibility)

Number of installable needles: 1
Application needle Diameter of application needle tip: ：φ10μm or larger (determined by prearrangement)

Optical unit

Microscope lens barrel: Clear vision single eye microscope lens barrel
Objective lens: Ultra long operating distance objective lens: 1 (x10)Visual field on image: 0.48x0.64mm
Lighting: 100W halogen lamp
Cold light with fiber (length of fiber: 1m)
XY stage

Stage unit

Control unit

Z stage: Stroke: (determined by prearrangement)
Resolution: 0.1μm/pulse
Maximum speed : 200mm/s: Effective stroke : 35mm
Resolution: 0.2μm/pulse
Maximum speed : 10mm/s

Computer
Main unit : FA/PC
Monitor : 17 inch LCD color display (1 unit)
Language : OS/Application Japanese(Windows 7 Professional)

CCD camera
Episcopic illumination

450

Z stage

Objective lens
XY stage
Application unit

300

300

Fig. 13 Outline view of high-speed microscopic coating applicator5) Examples of application

recognized in various applications and it is adopted involume production process.
As mentioned above, application materials of abroad range of viscosity can be applied precisely,
quickly, and reliably by using the high-speed
microscopic coating applicator. We believe the use ofthis technology will be extended to many other fields.

Currently, specific evaluations and studies are being
conducted in various fields including correction ofelectrodes of high-density printed wiring board (PWB),
mounting of electronic components, application of
catalytic material on sensors, application of agents inbio-related area, application of conductive paste in theprinted electronics field, etc. Fig. 14 shows examplesof the application of low-viscosity material andconductive paste on a printed wiring board. As shownin Fig. 14 (3), the electrode pattern can be directly
drawn by applying the conductive paste. The highspeed and stable operation of this proposal is already-102-

High-speed Microscopic Coating Applicator

Viscosity: Approx. 70 mPa·s,
application diameter: approx. 50μm

4. Conclusion
This article introduced the high-speed microscopic
coating applicator.
NTN is conducting research aimed at thedevelopment of highly reliable application of a broad
range of materials, in addition to the high speed
application we introduced, with the microscopic
application method using our unique applicationneedles.
The demand for fast and stable microscopic
application is growing in new fields such as electronic
micro-components and bio-related fields. We willcontinue to develop systems to contribute to research,development, and production in these fields.

(1) Application of low-viscosity application material(color ink) example
Viscosity: Approx. 8x104 mPa·s

DisconnectedareaElectrode
Width: 50μm

Before correction

Applicationarea

After correction(2) Correction of electrode in printed wiring board example

Electrode

Reference

Application area
Application
width: 20μm

- Yamanaka, Monthly magazine “Display”, Dec. 2004 issue.
- Yamanaka, “Complete Works of color filter material
development and manufacturing processs”, Technical
Information Institute Co., Ltd., July 2006 issue- Yamanaka, Monthly magazine “Display”, Dec. 2006 issue.
- Saruta, NTN TECHNICAL REVIEW No. 72, (2004).
- Yamanaka and Matsushima, NTN TECHNICAL REVIEW
No. 72, (2004).
- Yamanaka and Matsushima, NTN TECHNICAL REVIEW
No. 76, (2008).
- Suzuki, Monthly Tribology, No. 294 (2012) 34-36.
- Uchiyama, “Precision thin film coating technology using wet
process", Technical Information Institute Co., Ltd., Aug
2014 issue(3) Application on PWB example

Fig. 14 Application example

Photo of author

Akihiro YAMANAKANew Product Development
R&D Center-103-

NTN TECHNICAL REVIEW No. 83（2015）

Award Winning Products2014 “CHO” MONODZUKURI Innovative Components Awards Automotive Components Award

Mechanical Clutch Unit for Direct Adaptive Steering

Naotsugu KITAYAMA

Shintaro ISHIKAWA

Cage Clearance

1. Introduction

Roller Spring Cam surface

Outer ring

When electrical
power is supplied

NTN’s “Mechanical Clutch Unit for Direct Adaptive
Steering*” (hereinafter, "MCU for DAS") received theAutomotive Component Award at the "2014 “CHO”
MONODZUKURI Innovative Components Awards” sponsoredby the Monodzukuri Promotion Council and the Nikkan Kogyo
Shimbun, Ltd. (Assisted by Ministry of Economy, Trade andIndustry, The Japan Chamber of Commerce and Industry).
“MCU for DAS” is a product adopted as the mechanicalback-up mechanism for the world’s first DAS system for mass
produced vehicles that Nissan Motor Co., Ltd. introduced tothe market. It uses a compact configuration combining anelectromagnetic clutch and roller clutch, and switches torquetransmission on and off depending on the presence ofelectrical power.

Roller clutch released
When electrical
power is shut
Roller clutch engaged

Fig. 1 Roller clutch mechanism

Steering side

Ball cam unit
Roller clutch unit

* Registered trademark of Nissan Motor Co., Ltd.
Tire side

Electromagnetic
clutch unit

Fig. 2 Component

3. Conclusion
This product realized the reliability that customers requireby adopting a linear/rotation conversion mechanism (ballcam) into NTN’s proprietary roller and electromagnetic clutch.
We will continue to develop new products that support nextgeneration vehicle
technologies.

Photo: Courtesy of Nissan Motor Co., Ltd.

2. Structure of mechanical clutch unit
“MCU for DAS” is a product that NTN developed as a newmechanical element to switch the torque transmission
between the input and output shafts.
When DAS is functioning, the roller clutch is released byelectrical power supplied to the electromagnetic clutch, andthe steering operation is transmitted to the tires throughelectrical signals. On the other hand, when the electrical
power to the electromagnetic clutch is not supplied, the roller
clutch is instantly engaged to mechanically transmit thesteering operation to the tires, achieving a fail safe capability.
In addition to the electromagnetic clutch and roller clutch, apair of rollers are arranged, as well as the newly adopted ball
cam mechanism (converting the axial linear motion to rotating
motion) to reduce the backlash when mechanically connected,
realizing the clutch characteristic of high torque capacity.
This product is introduced separately in detail.

Commendation of Cho-monodzukuri award

Photo of authors-104-

Naotsugu KITAYAMA

Shintaro ISHIKAWA

Automotive Business HQ

Automotive Business HQ

NTN TECHNICAL REVIEW No. 83（2015）

Award Winning Products
AVEC’14 Best Paper Award

Roll Control Using Tire Longitudinal Forces Integrated withTire Force Distribution of Full Drive-by-Wire Electric Vehicle
Yuta SUZUKI

Tomohiro SUGAI

Junichi HIRATA

Nobuyuki SUZUKI

1. Introduction

3. Test results from an actual vehicle

The paper of the above title, which presented the results fromjoint research with the Kanagawa Institute of Technology,
received the “AVEC’14 Best Paper Award” at the “International
Symposium on Advanced Vehicle Control” which took place atTokyo University of Agriculture and Technology in September,2014. This symposium is an international conference regarding
vehicle motion performance and control, sponsored by theSociety of Automotive Engineers of Japan. Since its first
conference held in 1992 in Yokohama, it has been held in
various venues throughout the world such as Germany, theU.S. and Taiwan every two other year, and has become one of
the world’s leading places for automotive engineers andresearchers to present state-of-the-art technologies. In thisarticle, we briefly introduce this award winning research.

We conducted a lane change experiment to the left lane,assuming an emergency avoidance situation. Fig. 2 shows theresults. The tire load ratio on the rear turning inner wheel issignificantly increased by the longitudinal force when only roll
control is applied (green line) compared to no control (blue line).
However, when roll control, integrated with the tire force
distribution control (red line) is applied, the tire load ratio is
uniform between the left and right wheels without a significant
increase on the rear wheels. From this experiment, using an
actual vehicle, we could verify that the roll control withlongitudinal forces requires an integration with vehicle
stabilization control, such as tire force distribution control.
11Without control
With roll control only
With integration

0.9
0.8
0.7

0.9
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0.7
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0.6

2. Control method

0.5

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0.3

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0.4

By installing in-wheel motors for each wheel, it is possible toindependently control the driving force for each wheel very
quickly and accurately. In addition, the controllability will
increase with full drive-by-wire vehicles, which are able toindependently steer four wheels. Fig. 1 shows an experimental
vehicle equipped with the above system.
When longitudinal forces are applied to the tires from the inwheel motors, the vehicle receives vertical force through thesuspension mechanism. Past research has demonstrated thatby applying longitudinal forces in opposite directions, between
the front and rear wheels or between the left and right wheels,
vehicle roll can be controlled by leveraging this effect.
However, due to the behavior of the tires, the active use of
longitudinal forces reduces the lateral forces. Consequently, theapplication of this roll control during a turn, where high lateral
gravitational forces are put on the vehicle, increases the tireload ratio on the turning inner wheels, reducing the lateralforces, which may make the vehicle behavior unstable.
Therefore, leveraging the properties of the full drive-by-wirevehicle, the increase of the tire load ratio can be reduced duringroll control, by applying the roll control while distributing thelongitudinal forces and lateral forces to all four wheels so thatthe load on each wheel becomes uniform.

0.2

0.2

0.1

0.1
0

0

0.5

1

1.5

2

2.5

0

1

1

0.9

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0

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0

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1

1.5

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2.5

0

Fig. 2 Work-load of each tire

4. Conclusion
We will continue exploring vehicle motion control leveraging
motor response and high accuracy of in-wheel motors to makethe EV system more effective.

Photo of author (presenter)Yuta SUZUKI
Chassis System Engineering,
EV Module Business HQ

Fig. 1 Experimental Full Drive-by-Wire EV-105-

NTN TECHNICAL REVIEW No.83（2015）[ Our Line of New Products ]Self-Aligning Roller Bearing (ULTAGE EMA type)
Optimized internal design: maximized size and number of rollers
The world’s highest level of high load-carrying capacity is achieved!

Features

Structure(1) World’s highest level of load-carrying
capacity

Outer ring・Special heat treatment
・Optimized curvature
・Equipped with oil hole and groove- Basic dynamic load rating: Improved by up to 20%*

(2) Superior vibration resistance andimpact load tolerance
Cage- Cage strength: Improved by up to 55%*

・High strength machined cage・One piece outer ring guided(3) Low heat generation- The rise of temperature in the bearings is reduced
by up to 10% under vibration/impact load test
conditions*(4) Heat resistant up to 200°C- Dimensional stability and long life are achieved in ahigh temperature environment by adopting special
thermal treatment*Compared with our UA type product

Inner ring

Applications

・Special heat treatment
・Optmized curvature¡Crushing machines, grinders
disintegrators, vibrating screening
machines, reducers, etc.

Roller・Adopted large diameter rollers
・Maximized number of roller-106-

NTN TECHNICAL REVIEW No.83（2015）[ Our Line of New Products ]Wireless Measurement Unit
for Condition Monitoring System (CMS)World’s first! The condition monitoring capability is improved by using
wireless communication for condition monitoring of wind power generators!

Features

Structure(1) Wireless communication creates aconnection between the data collection
device and sensors¡Example of synchronous type wind turbine,
bearing outer ring and rotor rotation- Compact design allowing for ease of installation on
rotating components - Measurements are possible
even in areas where cabling installation isprevented- Simultaneous measurement of two signals(2 channel input)

Blade
Generator (rotor) bearing/outer ring
Rotating side

Example:Vibration sensor(installed on the main
rotor (rotating section))

Condition
monitoring software

Bearing
inner ring
Rotating side(2) Available for off-shore and severe
conditions- Dust/water proof performance complies with
JIS C 0929 IP65

Wireless measurement unit

Accesspoint

Wind Doctor ®
data collection
device¡Installed on the Main rotor (rotating section)¡Power is supplied through slip ring

Applications¡Wind power generators,
various machines,
test/inspection devices, etc.

Size and weight of unit
Size : 130 mm W x 180 mm D x 102 mm H
Weight : 550 g(Water protection cover is available)-107-

ServerNTN TECHNICAL REVIEW No.83（2015）[ Our Line of New Products ]Resin-made Thrust Rolling Bearings
for Food Processing Equipment
Resin-made rolling bearings with superior performanceand easy handling

Features

Structure

Adoption of heat tolerant resin and ceramic balls

Raceway(1) Lightweight and corrosion resistant design(2) Ensures heat tolerant performance up to 200˚C・Optimum material is proposedsuch as PEEK based resinmaterial for superior heat
resistance, corrosion resistanceand rolling contact fatigue
resistance

Maintenance conscious material blending anddesign(3) Superior handling performance is achieved due toeasy cleaning(4) White color makes it easy to detect foreign objects
after cleaning

Rolling element (ball)Applications
Resin made cage

¡Food processing equipment (stirrers and pumps),
medical devices, underwater equipment,
clean-room environments

・Rust-free ceramic material

・White color makes it easy to detect foreign objects
after cleaning
・Pocket design allowing easy disassembly/reassemblyof balls-108-

Aerospace

Three-point contact ball bearings
for jet engines

High speed railroad

Bearings for axle shafts of
Hokuriku Shinkansen cars

NTN's advanced
technologies andservices actively
used invarious fields

Automotive
Press connect spline
hub joint

Sensor-integrated
3rd generation hub bearings

Machine tools
Bearings for ball screw support

Angular contact ball bearings
with lubrication holes

Aftermarket
Technical service car

Various maintenance tools-109-