For New Technology Network

R

Special Issue; Composite Material,
Technology and Products
for Industrial Machines
October 2014

ISSN 0915-0528 OSAKA, JAPANIntroduction of “WIND LAB.” - Test equipment
for ultra-large main shaft bearings for wind power generators

NTN has developed the world's largest large-bearing-test apparatus (“WIND LAB.”) which allows evaluation of off-shore
wind power generator main shaft bearings with an outer diameter of up to 4.2m. This will enhance our ability to develop
technology to cope with increasing demand around the world. NTN introduced the equipment at the Advanced Technology
R&D Center (Kuwana-shi, Mie-ken).
“WIND LAB.” applies loading to the bearings with a total of 6 hydraulic cylinders, in vertical and horizontal directions, tosimulate the actual conditions experienced by a wind turbine. This enables detailed technological analysis and specification
review of bearings selected for large wind power generator applications. In addition to enabling evaluation of the loadconditions in the stationary or extreme state, it can also support various types of bearings, such as self-aligning roller bearings
and tapered roller bearings, so that the customers can conduct tests tailored to their needs.
NTN develops and provides all kinds of bearings used in the components of wind power generators including main bearings,
final gears, step-up gears, and generators. Wind power generators, including those for off-shore use, are increasing in size, andmain shaft bearings require advanced technology to support rotation of blades, which can exceed 100 m in diameter.
Therefore, evaluating the actual ultra-large bearings will help accelerate development and improve bearing reliability. In
addition, we will aim to monitor the bearing conditions during the test by integrating the condition monitoring system (CMS)
for wind power generators developed by NTN (“WindDoctor ®”- page 74). Our objective is to improve the functionality ofthe bearings while continuing to further enhance the equipment diagnostic technology of CMS.
We will strive to enhance our advantage as a bearing manufacturer by using "WIND LAB." and further grow our businessfor wind power generation equipment.

Hydraulic cylinder

Bearings under test

▲ “WIND LAB.”
test equipment for ultra-large wind
power generator main shaft bearings▲ An example of an ultra-large

main shaft bearing

R

TECHNICAL REVIEW
No.82

Special Issue: Composite Material,
Technology and Products
for Industrial Machines

corporation

NTN TECHNICAL REVIEW No.82
CONTENTS
Preface
Contribution

Natsuhiko MORI

Advanced Powders Processing – Recent Powder Metallurgy Techniques and It’s Trend –Hideshi MIURA

Professor, Department of Mechanical Engineering, Kyushu University

1

2● Special Issue for Composite Material Products and Technology
Product Introduction and Complex Technology of Resin,Sintered Metal and Magnet for Growth Markets
Yoshio OKI

The Improvement in Strength of Sintered Machine Parts
Takahiro OKUNO and Naoki YASHIROHigh Performance Magnetic Core for Induction Hardening Devices
Takuji HARANO

12
21
26

The Copper-Iron-based Material Equivalent to Bronze-based Material for Sintered, Oil-impregnated Bearings 30
Tomonori YAMASHITAHigh Load Sliding Bearing Unit For Use in Copier and Printer Fusers
Kei HATTORI and Takuya ISHII

34● Special Issue for Manufacturing Technology
Machine Tool Main Spindle Bearings with Air Cooling Spacer
Yushi ONDA, Kikuo FUKADA, Yohei YAMAMOTO and Masato YOSHINONew Engineering Plastic Cage of High Speed Angular Contact Ball Bearings for Machine Tools
Wakana INOUE, Mineo KOYAMA and Soichiro ONITSUKA

Bearing Technical Calculation System for Machine Tool Spindles
Keiichi UEDA, Yushi ONDA and Shouhei HASHIZUME

Estimation Method for Friction Torque of Air-oil Lubricated Angular Contact Ball Bearings
Hiroki FUJIWARA

Real-time Monitoring of the Superfinishing Process

38
44
49
54

Kohei HIGASHI, Hajime ISA, Takakazu KITAGAWA, Takashi ONISHI and Kazuhito OHASHI

61

Lightweight Bearing for Robot

68

Sebastien BRISSON●Special Issue for CMS Supertechnology New Products
Development of Condition Monitoring System
Keisuke HASHIZUME, Akitoshi TAKEUCHI and Yuzuru TANAKA

Introduction of "ULTAGE" series : High Functionality Roller Bearings
Shogo TABATA, Kouhei TODA and Yasuhiko SHIMIZU

Technical Trend of Aircraft Bearings
Takashi NISHIKAWA, Nao HAYASHI and Akiko HAYAKAWA● Technical Papers
The Influence of the Humidity and Quantity of Hydrogen into Steel under the Abrasive Wear

74
78
83

Motohiro ITO

88

Evaluation of Shear Fatigue Properties of Medium Carbon Steels andRolling Bearing Steels up to Giga-cycle Regimes

94

Noriaki MIWA

● Our Line of Award Winning Products“2013 ‘CHO’ MONODZUKURI Innovate Components Awards” NIPPON Brand Award

Parallel Link High Speed Angle Control Equipment

98

Hiroshi ISOBE, Yukihiro NISHIO, Seigo SAKATA, Naoya KONAGAI and Hiroyuki YAMADA“The Japan Powder Metallurgy Association Awards 2013” New Materials

Development of the Oil Seal Made of Sintered Metal for Variable Valve Timing Device

99

Yosuke SUGAI and Toshihiko MOURI“The Japan Powder Metallurgy Association Awards 2013” Effort Prize

Sintered Hydrodynamic Bearing for Super Thin Fan Motor

100

Tadahiro UCHIYAMA and Shinji KOMATSUBARA“The Japanese Society of Tribologists 2013” Encouragement Award

Development of Ultrasonic Torsional Fatigue Tester for Rapid Shear Fatigue
Property Evaluation of Rolling Bearing Steels

101

Noriaki MIWA

Our Line of New Products102

NTN TECHNICAL REVIEW No.82（2014）[ Preface ]A Message for the Special Issue:
Composite Material, Technology and Products
for Industrial Machine

Natsuhiko MORI
Executive Officer

As the manufacturing industry is globalized and both the market and demand of products
are expanding throughout the world, maximizing the effective use of the minimum resources
and paying attention to the global environment has become an urgent issue. In Japan,
development and utilization of advanced technologies and materials are accelerating in orderto maintain sustainable and constructive competitiveness. Therefore, methods for utilizing
materials and some manufacturing methods are about to change significantly. Some
examples are the expanded application of CFRP and use of 3D printers.
In view of these circumstances, NTN has formed a new division, the Composite Material
Product Division in April, 2011. The purpose of this division is to consolidate and enhance theresin, sintered metal and magnetic material technologies to develop and introduce composite
products and unit module products into the market. Then, they can contribute to the fuelefficiency and electric drive of automobiles and high-performance industrial machinecomponents.
At the 27th Japan International Machine Tool Fair (JIMTOF2014), which took place from
October 30 to November 4, with the theme “’Mono-Zukuri DNA’ connects to the Future andthe World”, NTN Technical Review No. 82 was published featuring NTN’s composite
material, technology and products for industrial machines.
In this Technical Review, we had a contribution entitled, “Advanced Powders Processing –Recent Powder Metallurgy Techniques and Its Trend” from Professor Hideshi Miura.
Professor Hideshi Miura is from the Department of Mechanical Engineering, Kyushu
University, and is an authority on powder and powder metallurgy.
In the “Special Issue for Composite Material Products and Technology”, we introduce highstrength sintered metal, magnetic material and resin as well as hybrid products. In the“Special Issue for Manufacturing Technology”, we introduce technology and products thatwere exhibited at JIMTOF2014, manufactured with our proprietary technology. Finally, in“Special Issue for CMS Supertechnology New Products” we introduce basetechnology/products and monitoring technology for industrial machines.
At NTN we will celebrate our 100th anniversary in fiscal year 2017. Following our theme,“For New Technology Network: We shall contribute to international society through creating
new technologies and developing new products,” we are striving to contribute todevelopment of the industry by utilizing our proprietary technology development capabilities.

-1-

NTN TECHNICAL REVIEW No.82（2014）[ Contribution ]Advanced Powders Processing– Recent Powder Metallurgy Techniques and It’s Trend –Hideshi MIURA
Professor, Department of Mechanical Engineering, Kyushu University

Advanced powder processing techniques such as metal injection molding (MIM)
are hoped to be suitable for fabricating complex shaped components with low cost.
So far, we have applied MIM process to produce various types of industrial
materials such as ferrous and Ti alloys, especially heat treated Fe-6Ni-0.4C
showed the super high strengthened properties of 2000MPa tensile strength and
8% elongation which are superior to wrought materials. On the other hand, directlaser forming (DLF) technique is another advanced powder processing technique.
Recently, much effort has been made to apply this process to automobile,
aerospace and medical fields due to the potential of producing complex shaped Ti
alloy structures. In this paper, the above two techniques are introduced as recent
powder metallurgy techniques.

processing, as well as development of new technology
are well underway for P/M products with high density
and high performance. However, recent P/Mtechnologies are focused on machining and composite
processing technologies in the boundary areas withother material processing technologies, among others,
as shown in Table 11). For example, the powder
injection molding is a combination of P/M and plastic
molding, and powder lamination 3D printing is acombination of P/M and laser machining(CAD/CAM).Acquiring knowledge and applications ofthese new technologies should be very useful for
manufacturing/development of new materials andproducts for NTN. In this paper, I will focus on theabove two innovative and unique molding/sintering
methods, providing an overview of the technologies
and introduce you to the recent outcomes of ourresearch as an alternative to providing you with aperspective into the future.

1. Introduction
Powder metallurgy (P/M) is a metal processing
method using a phenomenon called sintering, a
process which bonds particles by heat (temperature
lower than the melting point) for extracting particularproperties from the metal and ceramics powder andplays an important role as one of the manufacturing
processes of materials and products in highindustrialized society. Liquid-phase sintering also
exists depending on the components. The biggest
advantage of P/M is to be able to shape the powder
directly into the end product (near net or net shaping)having a significant degree of freedom in material
properties, composition, thermal processing andmicrostructure.This provides properties that cannot be
achieved by the melting method. In addition, there isan economical benefit allowing volume production.
With these properties, P/M is used to produce various
components for machine structural use such as gears,
bearings and connecting rods (for automobiles) P/M isalso used to produce cutting/abrasion resistant toolingmaterials such as cemented carbide tip and molding,high melting point metal materials such as W and Mo,
porous materials such as filters and implants for living
bodies, electric contacts/power collecting materials
such as switches and materials for pantograph contact
strips, and magnetic materials such as magnetic coresand sensor rings.
Improvement of conventional processes fromproduction of powder, shaping, sintering and post-

2. Metal Injection Molding
A molding technique that allows both a high degreeof freedom for shaping and high density in a relatively
easy process has been desirable in P/M. One suchtechnique is the metal injection molding (MIM) processwith binder developed in the 1970s.
MIM process is used in many forms however thebasic steps are similar to the ones shown in Fig. 11).
The typical raw material is nearly round particles ofcarbonyl and powder produced from oxide reduction-2-

Advanced Powders Processing – Recent Powder Metallurgy Techniques and Its Trend –Table 1 Recent applications of various P/M processings
Use

Required properties

Process and materials

Anticorrosive component

True density uniform alloy

HIP, titanium

Thermal process equipment

Carburizing resistance and creep resistance

Reaction sintering, extrusion, nickel aluminide

Fine jewelry/accessories

Good appearance, complex shape

Injection molding, precious metal, titanium

Pipe, thick plate, rod, plate

True density, net shape

Spray forming, steel, super alloy

Magnetic component

Magnetic functionality, ambient resistance

Sintering, injection molding, rapidly solidified powder

Covering material

Oxidation resistance and corrosion resistanceSpray forming, intermetallic compound

Microelectronics board

Adjustment of thermal property, absorption of microwave

Injection molding, W-Cu, Mo-Cu, Al-SiC, Al-AlN

High melting-point metal

Complex shapeComplete densification method such as HIP, special alloy

Aircraft structural materialHigh relative strength

Complete densification method, rapidly solidified Al alloy

Structural material

Fatigue strength and fracture strength

Powder forging, iron alloy

Dynamic membranel filter

Small pore size, corrosion resistance

CIP, stainless steel

Abrasion-resistant structural material

Lightweight, high abrasion resistance

Hot press, boride, Al based composite material

Powderequivalent to approximately 6 wt% in case of ferrous
metal, although the percentage depends on the fillingcharacteristics of the powder. The best shaping canbe attained when the viscosity of the kneaded material
is 100 Pa・s or less however, the viscosity depends
not only on the viscosity of the binder itself but also thetemperature of the kneaded material, shear rate,
volume of the solids and type of surface wetting agentsincluded in the binder. Good uniformity is required forthe kneaded material to maintain good process control.
The viscosity, which is sensitive to the composition andvariations may also prevent good flow into a moldcavity. Usually, the viscosity of the kneaded material
can be kept within the desirable range by slightly
adding more binder.
The device used for molding is the same as thoseused for plastic injection molding. The reciprocal-typescrew driven by a motor is used for mixing/kneading
the raw material so that the kneaded material becomes
uniform, producing the required pressure to fill thekneaded material into the mold. The raw material, in
the form of pulverized particles at normal temperature,
is introduced from the charging hopper and heatedover the melting temperature of the binder as it passes
the barrel. The kneaded material of melted powder andbinder is pushed forward to instantaneously fill in themold cavity. After being sufficiently cooled down, thecompact is removed and the process is repeated. For
better molding, higher pressure or lower viscosity isrequired. Since the pressure that can be used for the
molding machine is limited and the viscosity isgoverned by the temperature, the main control factors
are temperature and pressure for the molding process.
After molding, the binder is removed from thecompact by a process called debinding. Many
debinding techniques and processes related todifferent binders have been developed which, together

BinderPreliminary
kneading

Kneading and granulation(particles)

Injection
molding
Solvent
degreasing

Heated degreasing/
preliminary sintering

SinteringFig.1 Conceptual sequence of MIM steps

and gas atomization with the average grain size of 0.5
to 15 μm to facilitate densification during sintering.
Generally, thermoplastic polymer materials are used as
the binder however; water and diverse inorganic
materials are also often used. The typical binderconsists of 70% of paraffin wax and 30% of
polypropylene with an appropriate lubrication andwetting agent to provide adhesiveness to the binder
and the powder. This type of binder completely melts
at approximately 150˚C. The amount of the binder isabout 40 vol% of the kneaded material, which is-3-

NTN TECHNICAL REVIEW No.82（2014）

Table 2 Various debinding techniqueswith the composition of the binders, account for the
majority of the patents. Table 2 2) shows various
debinding techniques.
In the industry, thermal decomposition and solvent
degreasing are mainly used, however the former still
has issues such as long debinding time (slow heating
up to 600˚C) and possible deformation of the product.
The latter has significantly overcome those issues but,many of the solvents such as acetone and ethylene arehazardous to humans and the environment and theirhandling involves additional problems. Therefore, newbinders soluble in ethanol and water are actively underdevelopment. At any rate, long debinding time isdisadvantageous for production, which considerably
limits the permissible thickness of the MIM products.
Up until a few years ago, it was considered that athickness of around 10 mm was the limit from theeconomical point of view however, with the evolution ofdebinding technology it is now possible to allow a
thickness of around 25 mm. In addition, there are otherspecial techniques such as debinding by catalyst 3) anda kind of water freezing technique called Quickset
process 4).
The next step, sintering, is processed under inert, or
reduction, atmosphere or vacuum atmosphere, similar
to the conventional P/M technique. Sintering createsstrong bonding of particles and voids are removed bydensification. Isotropic powder filling results in uniform
contraction (15–20%) as can be predicted. Therefore,
initial compacts are made a little larger so that the final
compacts fit the desirable size. After sintering, thecompacts show superior strength and uniform
composition compared to the characteristics possible
with many other manufacturing processes. Table 3
shows an example of the comparison of mechanicalproperties of various ferrous sintered materials withthis process and other manufacturing processes. The
mechanical properties of the MIM process not only
exceed the properties of the conventional P/Mmaterials but are also equivalent or superior to themolten materials in any steel types 5)-9). This indicatesthat the MIM process is effective for forming materials
with hard machining properties, as well as for
improving the material properties. The following
indicates some of the typical examples from the resultsof our research.

Process nameBinder content

Debinding condition

Heated degreasing
MACPHERSON process PE, camphor

Vacuum

WITEC process

WAX，PE

Turbulence, absorber

VI process

WAX，PE，PPHigh vacuum, evaporation(volatile)
RIVERS process

Water, methyl cellulose Anhydration within the mold

QUICKSET process

Water, PEG

Freeze dry

PE，PS，PEG

Water, methylene chloride

Solvent extraction
WITEC process

MACPHERSON process PE，PS
AMAX process

Trichlene

WAX，PE，PP
Peanut oil

Methylene chloride

Modified Polyacetal

Nitric acid, oxalic acid vapor

Chemical decomposition
BASF process

UV decomposition process WAX, acryl

UV irradiation

PE: polyethylene, PP: polypropylene, PEG: Polyethylene glycol,
PS: polystyrene, POM: polyacetal

Table 3 Mechanical properties of various alloy steels
by different material processings
Mechanicalproperties

MIM

P/M

Melting
methodHigh-speed steel
SKH10(tempered material)

Deflecting strength (MPa) 3200
Hardness (HRC)
70

2500
71

2500
67

Maraging steel
18Ni-8Co-5Mo(aged material)

Tensile strength (MPa) 1640
2〜3
Elongation (%)
47
Hardness (HRC)

1500
1〜2
35

1800
8

Martensitic
stainless steel
SUS440C (17Cr-1C)(tempered material)

Tensile strength (MPa) 1600
1〜2
Elongation (%)
53
Hardness (HRC)

SUS410
900
4
30

1950
2
57

17-4PH stainless steel Tensile strength (MPa) 1340
SUS630
11
Elongation (%)(aged material)
44
Hardness (HRC)

970
2
24

1370
14
45

Steel typepowder technique which indicates extremely highmechanical properties (equivalent to sinter forging
material) of 1800 MPa tensile strength and elongationof 3-4% which cannot be achieved by the conventional
P/M technique (tensile strength of ordinary sintered low
alloy steel by press molding is around 1000 MPa andelongation is 1-2%) 10). We conclude that this is amanifestation of superior properties of fineheterogeneous structure with locally gradient nature ofcontents unique to sintering material of mixed
poweder. This structure is the martensite phase (incase of high concentration, retained austenitic phase),
which appears according to the Ni concentration
gradient, particularly due to insufficient diffusion of
aggregated Ni powder in the Fe matrix powder, and issurrounded in mesh by the tempered martensite

2.1 Ultra-high strengthening of ferrous sinteredmaterials by MIM process
The author group, from our research, proposed a
new precise controlling method of the amount ofcarbon by atmosphere control in the MIM process.
This developed sintered low alloy steel (Fe-2%Ni-0.5%
Mo-0.4%C thermal processed material) by the mixed-4-

Advanced Powders Processing – Recent Powder Metallurgy Techniques and Its Trend –in any structure, highlighted by the arrows and aresurrounded by tempered martensite phases, seen as a
network structure. It is observed that the bigger the Ni
powder grain size and the more the Ni content, thelarger the Ni-rich phases.
Fig. 3 shows the distribution of Ni by EPMA. As it
shows the areas of high and low Ni concentration, it isclear that this is the heterogeneous structure with Nicontent gradient. It was found that when Ni powder ofsmall grain size is used, Ni is dispersed within an Fe
matrix resulting in a smaller difference in concentration.
It was also revealed that when Ni content is large, thedifference in concentration also becomes larger.
Fig. 4 shows the tensile strength obtained from thetensile test. All the test pieces show a very high valueof around 2000 MPa.

phase. As the above properties were only obtained byusing the natural phenomenon of aggregation of Ni
powder and composition of Fe-2% Ni-0.5%Mo, further
superior properties (targeting more than 2000 MPa oftensile strength, more than 5% of elongation, and morethan 600 MPa of rotational bend fatigue strength) could
be sufficiently expected by artificially changing the sizeand content of the Ni powder or sintering/thermal
processing conditions based on this steel type.
Therefore, we examined the optimum structure on the
micro-level such as the composition and percentage ofeach phase that composes the heterogeneous
structure and their dispersion state.

2.2 Method of Experiment
We used carbonyl iron powder (Fe-OM, averagegrain size: 4.4 μm, made by Fukuda Metal Foil &Powder Co., Ltd.) as the powder for this experiment.
We also used water atomization powder (AKT-Ni,
made by Mitsubishi Steel Mfg. Co., Ltd.) as the Ni to be
added, classified into 3 grain sizes. The average grain
sizes were 6, 16 and 24 μm, which we called fine,medium and coarse, respectively, and added 4, 6 and8 mass% in the mixture. As the binder, we blended 69
mass% of paraffin wax, 20 mass% of atactic
polypropylene, 10 mass% of carnauba wax and 1
mass% of stearin acid. The powder and the binder
were kneaded so that the powder ratio became 65
vol% and a tensile test plate the shape of a dumbbell
was created by injection molding using the obtained
raw material. Paraffin wax was first removed from thecompact using the solvent degreasing method in theheptane gas phase, then heated degreasing andsintering were conducted successively. Sintering was
conducted in a vacuum atmosphere of 10-1 Pa and asintering temperature of 1200-1350˚C was maintainedfor 1 hour. The sintered compact was thermal
processed in the Ar atmosphere.
After maintaining 900˚C for 30 minutes, quenching
was performed in oil and then after maintaining 200˚C
for 120 minutes, tempering was performed by air
cooling. The test piece was then tested for relative
density measured by the Archimedes method, carbon
content, structure observation, and Ni distribution by
EPMA and tensile test.

(a)

(b)

50µm

Fig.2 Representative of optical microstructures for various
Fe-Ni steel compacts sintered at 1250 ˚C for 1 hour in vacuum
atmosphere. The arrow shows the bright martensite region.
(a) As-tempered compacts of different Ni particle size,(b) As-tempered compacts of different Ni content (mass%)

(a)

(b)

2.3 Test Results and Observations
The density of the obtained sintered compacts was
95% or more of the relative density in any Ni powder
grain sizes and Ni contents. The carbon content of any
thermal processed materials was about 0.4 mass%.
Fig. 2 shows the pictures of the structure of the typicalthermal processed materials.
The specimens show that white Ni-rich phases exist

Fig.3 Representative of Ni element mappings by EPMA
analysis for as-tempered Fe-Ni steel compacts changing(a) Ni mean particle size, (b) Ni content (mass%)-5-

NTN TECHNICAL REVIEW No.82（2014）

However, hardness of the Ni-rich phase increased
nearly 20% from before the test showing an evenhigher value than the tempered martensite phase. On
the other hand, little change was observed in thehardness of the martensite phase.
We also attempted to quantify the retained austenite
before and after the tensile test by X-ray diffractometry(XRD). Fig. 7 shows the retained austenite amount
before and after the tensile test. The retained austenite
existed nearly 17% before the test and decreased to
6% after the test. From the above, it is concluded thatthe stress-induced martensitic transformation (TRIP)
occurred in the Ni-rich phase resulting in improved
hardness. It was found that this also caused ultrastrong tensile properties in the thermal processedmaterials of sintered low alloy steel withheterogeneous structure 15).

680
660
640
620
600
580
560
540
520
500

2500
2000

Hardness, HV

Tensile strength, MPa

Higher strength and elongation are seen in samples
with smaller Ni powder grain size and the test pieces
with Ni content of 6% show the maximum values. Fig. 5
shows the elongation, showing that the smaller the Ni
powder grain size, the greater the elongation and the
Ni content of 6% again shows the largest value. The
test piece with fine Ni and 6% content showed a verystrong property of 2040 MPa of tensile strength and8.1% elongation. This is the highest level among thereported sintered low alloy steels 11-14) by MIM. It is
concluded that such ultra high strength properties wereobtained because of the fine heterogeneous structure.
Fig. 6 shows the hardness of the test pieces of fine
powder with Ni content of 6% before and after thetensile test. The Ni-rich phase showed a lower value
than the tempered martensite phase before the tensile
test.

1500
1500
500

Fine

0

Medium4

Coarse6

Before8

Ni content, %Tensile test

Fig.4 Tensile strength of different Ni content andparticle size

Ni rich phase
After

Fig.6 Micro vickers hardness of each region at before
and after tensile testing

18

10
8

Retained austenite, %Elongation, %Tempered α’

6
4

Fine
Medium

2
0
4

6

8

Coarse

16
14
12
10
8
6
4
2
0

Ni content, %BeforeAfter

Fig.7 Retained austenite at before and after tensile testingFig.5 Elongation of different Ni content and particle size

-6-Advanced Powders Processing – Recent Powder Metallurgy Techniques and Its Trend –controlling laser output. In the case of Fe-Ni mixed
powder, development of improving dimensional
accuracy is also conducted by sealing the remaining
pores with shot peening however, those methods
require many processes and the strength is not
sufficient.
Therefore, a method to directly irradiate by the laser
only metallic powder for melting and binding to build
products is under research and development. Its
principle is shown in Fig. 8 17). First, a layer of powderis placed on the manufacturing table (cylinder) and thelaser irradiates the powder based on the CAD data sothat the powder melts and solidifies. Next, the following
layer is placed on the manufacturing table from thepowder supplying cylinder with a roller then the laser
irradiated the sample, fusing and solidifying thepowder. By repeating this lamination process, thedesired 3D shape can be obtained. In this case, thedimensional accuracy and surface roughness are notat the industrially satisfactory level. Due to this, metal
stereolithography composite machining technology
was developed where accurate cutting technology isadded as an additional machining process to produce
molding with short delivery time and low cost.
The following describes the recent outcome ofresearch of the author group 17-22), as they have been
focusing on biomaterial and Ti as aircraft component
material and conducting basic research of laser
forming using Ti powder in the degas/Ar gas
atmosphere as the net shape machining technology ofcomplex structure.

3. Powder Lamination 3D Printing
CAD has become a common practice fordevelopment and design of components and products.
Particularly, as component shapes are stored as 3D
CAD data, a new additive manufacturing (AM) has
been developed to shape the actual body from thedata. The most representative method isstereolithography which forms thin layers of light curingliquid resin by laser light scanning, to shape theproducts by adding layers as needed. This is applied tothe metallic and ceramic powder in a technique called
powder bed fusion (PBF; the former is called “laser
forming” hereinafter). The following methods areavailable as AM:● Material extrusion (ME) method: The most popularmethod of additive manufacturing uses continuous
extrusion of melted thermal plastic resin from a
nozzle.
● Sheet lamination (SL) method: An additive
manufacturing method that uses adhering sheet-likematerial such as paper and film.
● Directed energy deposition (DED): An additive
manufacturing method that injects droplets ofmetallic powder liquefied by plasma or laser from a
nozzle.
Recently, material development and manufacturing
techniques have evolved so that AM is not only used
simply for creating a mold to check the shape design
but also for manufacturing products of rich functionality
with a certain degree of precision. We are mainly
focusing on laser forming here.
Several techniques for supplying powder for laser
forming have been researched for laser forming using
powder 16). Initial techniques consisted of the use ofbinder content mixed with or coated on the powder
where only the binder was melted and used forbinding. Some require further sintering and others do
not. The former required degreasing and both
frequently used impregnation. With a larger amount of
resin binder, the forming speed is faster. Therefore,
with green body forming, the time is significantly
reduced. However, in instances where the resin binder
is not used, the forming time tends to be longer. In theformer case, the processes are shortened from 2
processes of degreasing/sintering and infiltration to
one process of degreasing/sintering/infiltration bychanging blending of resin (thermosetting phenol andparaffin wax) and the coating method, as well as theproperty of the base material. There are also
improvements of strength and dimensional accuracy by
HIP processing instead of infiltration, or by applying
impregnation and infiltration after liquid phase sinteringof nickel phosphor bronze and completely melting it

Fiber laser

Chamber(Ar, Vacuum)

RollerBase plate

PowderSintering table
Powder supply
table

Fig.8 Sketch of laser forming equipment-7-

NTN TECHNICAL REVIEW No.82（2014）

3.1 Ti Powder Laser Forming

The energy density is the energy input on the unit
area, P [W] is the laser output, s [mm] is the scanninginterval and v [mm/s] is the scanning speed. The
surface roughness decreases as the energy density
increases and converges at around Sa=10 μm. It isreported that the surface tension of the melted metal
decreases as the temperature increases 23), therefore,
it is considered that the wettability of the melted part
increases on the board as the temperature increases
along with the increase of energy density to form asmooth layer on the surface.
Next, we conducted a lamination experiment. We
decided that the shape of the laser formed compacts
would be the same as the test pieces for the tensile
test (14B test piece). For the forming parameters, weused 260 W and 80 mm/s, which were the conditions
to make the lowest surface roughness in the single
layer manufacturing experiment in the previous
section.
The annealing process was applied to some laser
formed compacts. Annealing consisted of maintaining
the pieces at 700˚C for 2 hours then cooling them in a
furnace. Fig. 10 shows the cross-section image of alaser formed compact. This is a high density compact
with a relative density of 99.8% with almost no voids.
Fig. 11 shows the corrosion structure. Generation of
acicular martensite by laser forming can be observed.
In addition, epitaxial growth where the said structure
grew vertically on the direction of formation was also
observed. This structure did not change between
before and after the annealing process.
Fig. 12 shows the tensile test results of the highdensity compacts. The JIS Standard for 60 types oftitanium alloy specifies that the tensile strength andelongation to e 895 MPa and 10% respectively. The

An important aspect of laser 3D forming is the highdensity and high accuracy of the product. In our
research, we have improved the relative density of thecompact up to 97.8% through the improvement of thepowder supply process. In this research, we havecreated high density compacts by improving laser
conditions, evaluating their mechanical properties andexamining the impact of the powder grain size on theside surface accuracy. This is done with the intent ofcreating compacts of even higher density and higher
accuracy.

3.2 Experiment Method
We used Ti-6Al-4V alloy powder (TILOP64 made by
Osaka Titanium technologies Co., Ltd.) by gasatomized technique as the raw material powder. The
grain size is under 45 μm and the average grain size is33.4 μm. In addition, we prepared finer powder with thegrain size under 25 μm (average grain size of 18.8 μm)
to evaluate the impact of the powder grain size on theside surface accuracy. We used ytterbium fiber laser ofthe continuous oscillation method with maximum
output of 330 W for the laser oscillator. The laser beamfrom this oscillator scans the powder with a final beam
diameter of 50 μm using the scanning system including
galvanometer mirror. The manufacturing chamber isfilled with high-purity argon (600 Pa) after being
degassed.

3.3 Test Results and ObservationsFirst, a single layer was created changing laserparameters and the surface roughness was measured
with a laser microscope (OLS-4000 from Olympus) inorder to evaluate the impact of laser parameters on thesurface roughness. Fig. 9 shows the measurement of
the surface roughness of the single layer compact overenergy density Ep [J/mm2] indicated by equation (1).

Surface roughness Sa, µm

Ep=P/(s･v) ……………………………(1)

80

mm/s

60

50
60

40

(a)

80

20
0

Fig.10 Cross sectional microstructure of laser formed
Ti compact

Scan
speed,

(b)

100
160
0

200

400

Energy density, J/mm2

600

Fig.9 Surface roughness of single layers in function of
volumetric laser energy density

Fig.11 Etched microstructures of laser formed Ti compacts(a) As – formed (b) After heat treatment-8-

Advanced Powders Processing – Recent Powder Metallurgy Techniques and Its Trend –

1217

1131

800

8.88

14.0

10.0

6.89
400

0

Tensile strength
Elongation
(b)

(a)

6.0

Elongation, %Tensile strength, MPa

1200

value. One of the causes was assumed to be theimpact of small voids still present within the material
even with high density however; the details are to be
examined in the foregoing review.
When laser scan is applied linearly on the evenly
placed metallic powder layer, the metallic powder
forms one linear compact as the metallic powder melts
and solidifies as shown in Fig. 14. At that time,
granular residues are generated around the formed
compact. Fig. 15 shows the graph indicating thediameter of the residues. It was found that the diameterof residues became smaller by increasing the laser
output and was halved when fine powder was used.
Fig. 16 shows the side surface roughness of the laser
formed compacts. Improvement of around 60% in
surface roughness was confirmed in the compact using
fine powder. A change in side surface properties wasverified by using fine powder even with the same
forming parameters however, the stable supply of
uniform powder layer is difficult with fine powder.
Therefore, further study is required for the
improvement of side surface accuracy.

2.0

Fig.12 Tensile properties of laser formed Ti compacts(a) As – formed (b) After heat treatment

tensile strength of the laser formed compacts is asufficiently higher value than the standard.
The elongation indicates a lower value, but it isimproved by the annealing process by over 10% insome samples. The fusion and solidification of thesurface of the compacts is repeated by the laser
scanning during laser forming, which generates
residual stress within the formed compacts. This
residual stress is considered to be one of the causes of
the reduced elongation. We assume that by applying
the annealing process, the residual stress wasremoved and the elongation was improved with acloser value to the JIS Standard.
Next, we also evaluated the dynamic mechanicalproperties. We introduced fatigue using the pulsating
tension test with a frequency of 30 Hz and stress ratio
of R=0.1 using an electro-hydraulic servo testingmachine. Fig. 13 shows the fatigue test results. The
vertical axis indicates fatigue strength under pulsating
tension (twice the stress amplitude). The fatigue
strength estimated from the tensile test result is morethan 600 MPa and the fatigue strength of this test was
260 MPa, a significantly lower value than the estimatedFig.14 Appearance of a laser formed line

Diameter, mm

100

Ti64_33.4µm
Ti64_18.2µm

80
60
40
20
100

150

200

Laser power, W

250

600
100

500

Roughness Rz, µm

Stress amplitude*2, MPa

Fig.15 Diameter of adhered particles

400
300
200
100
1.E+05

1.E+06

1.E+07

1.E+08Under 25µm

60
40
20

Number of cycle, N

Under 45µm

80

Right side

Left side

Fig.16 Side surface roughness of laser formed Ti
compacts using different Ti particle powders

Fig.13 S-N curve of laser formed Ti compacts
after heat treatment

-9-

300

NTN TECHNICAL REVIEW No.82（2014）

References

4. Conclusion

1) R. M. German, Powder metallurgy science, translated by Hideshi
Miura et al. Uchida Rokakuho (1996).
2) Kazuo Okamura et al. Plastic molding technology, 27 (1996) 235.
3) D Weinand et al. Plastic molding technology, 13 (1996) 19.
4) C. Quichand：Proc. of Powder Met．World Congress，EPMA，2(1996) 1101.
5) Hideshi Miura et al. Powder and powder metallurgy, 40 (1993) 393.
6) Hideshi Miura et al. Powder and powder metallurgy, 42 (1995) 353.
7) Hideshi Miura et al. Powder and powder metallurgy, 41 (1994) 1071.
8) Takeharu Baba et al. Powder and powder metallurgy, 42 (1995)
1119.
9) Hideshi Miura et al. Powder and powder metallurgy, 40 (1993) 988.
10) Hideshi Miura et al. Powder and powder metallurgy, 42 (1995) 378.
11) H. Miura, M. Matsuda, Material Transactions, 43 (2002) 343.
12) Materials Standards for PM Structural Parts, 2009 Edition, MPIF,
Princeton, NJ, (2009) 48-49.
13) Materials Standards for Metal Injection Molded Parts, 2007 Edition,
MPIF, Princeton, NJ, (2007) 16-17.
14) Metals Handbook, 10th ed., 1, ASM Int., Materials Park, OH, (1990)
430-448 and 793-800.
15) W. Harun, H. Miura et al. J. Japan Soc. Pow. & Pow. Met., 59 (2012)
677.
16) Masato Imamura，Powder and powder metallurgy，48 (2001) 415.
17) Hideshi Miura et al. Powder and powder metallurgy, 53 (2006) 740.
18) Hideshi Miura et al. Powder and powder metallurgy, 54 (2007) 707.
19) Hideshi Miura et al. Powder and powder metallurgy, 55 (2008) 738.
20) H. Miura, et al. Proceedings of the MPIF/APMI International
Conference on Powder Metallurgy & Particulate Materials (CDROM), (2009) 708.
21) H. Miura, et al. Materials Science Forum, 654-656, (2010) 2057.
22) H. Miura, et al. Proceedings of the 2012 Powder Metallurgy World
Congress & Exhibition(CD-ROM), (2012).
23) Akio Sasama, et al. : Journal of The Japan Institute of Metals，40(1976) 1030.

Metal injection molding (MIM) and powder lamination
3D printing by laser are introduced as new powder
forming methods from our research and the former
MIM is already recognized as an industrially usefulforming method. However, creating large products isstill difficult because of deformation due to their own
weight during the debinding process and longprocessing time. If these problems are resolved, thescope of application is expected to be greatly
expanded.
On the other hand, metallic powder lamination 3D
printing has just begun with the ability of forming
complex structure on the outside, as well as the inside.
However, as the powder is melted and laminated,particularly roughness of the side surface and theentire dimensional accuracy need to be improved.
There are also issues associated with volume
production but, this is unique from the manufacturing
viewpoint and attracting a lot of attention. Based on
this, further development is very much expected.

<Author biography>
Hideshi Miura
Professor, Doctor of Engineering, Department of Mechanical Engineering, Kyushu University
1997
1985

Steel Metallurgy Course (Master) completed, Department of Mechanical Engineering, Kyushu University, AssistantDoctor of Engineering received from Kyushu University, Assistant Professor, Department of Materials Development Engineering,
Kumamoto University
1992-1993 Visiting Researcher, Rensseaer Polytechnic Institute, USA
1995
Professor, Faculty of Engineering, Kumamoto University
2004
Professor, Mechanical Engineering, Graduate School of Engineering, Kyushu University
2011present President, Education and Research Center of Manufacturing, Kyushu University[Specialty]
Material processing, powder metallurgy, metallic injection molding, laser powder lamination 3D printing[Academic society and committee affiliations]Japan Society of Powder and Powder Metallurgy: Board Director (1996-2010), Vice President (2003-2010), President (2012-2014), Adviser (2014-)
The Japan Institute of Metals and Materials: Subcommittee V, Manager (1995, 1996), Vice Chairman (1997, 1998), Chairman (1999, 2000)
The Iron and Steel Institute of Japan: Division Steering Committee Member (Powder/Particle Forum Chair) (1998-2012),
Division Vice Chair (2012-2014), Division Chair/Board Director (2014-2016)
American Powder Metallurgy Institute: Powder Injection Molding International Symposium Organization Committee Member (1995, 1996, 1998-)
P/M Sci. & Tech. Briefs Editorial/Review Member (1999-2005), International Conference Program
Committee Member (2004-)
The Japan Society of Mechanical Engineering: Materials and Processing Division, 81st Period Steering Committee Member (2nd TechnicalCommittee Chair) (2003-2004), Division Vice Chairman (2005-2006), Division Chair (2006-2007),
Fellow (2010-)
The Japan Society for Technology of Plasticity: Board Director/Publishing Business Committee Chair (2007-2009), Kyushu District Head (2010-2012)
Conference Steering Committee Chair, Publication Editorial/Review Committee Member, Award Selection Committee Member, Representative,
Council Member, Program Committee Member of Powder Metallurgy International Conferences, Organizing Committee Member, Chair of the above
academic societies

-10-

-11-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Article ]Product Introduction and Complex Technology of Resin,
Sintered Metal and Magnet for Growth Markets

Yoshio OKI*NTN Composite Material Products Division has developed themechanical parts and bearings, and the unit module products by a wide
range of materials and technology such as resin material, sintered metal,
magnetic material and fluid dynamic pressure technology.
In order to meet the market to be diversified, NTN is working to develop the future markets while
creating new material properties by promoting fusion of each technology and Composite Materials which
are applying tribology, bearing material, high-precision processing technology and magnetic materials.
This paper reports the Developed Products of Composite Products for the further growth markets.

2. Product line and production system ofComposite Material Products Division

1. Introduction
NTN’s Composite Material Products Divisionconsolidates and integrates fluid dynamic pressuretechnology and technologies of NTN’s relatedcompanies, which include NTN Engineering Plastics
Corp., NTN Powder Metal Corporation and Nippon
Kagaku Yakin Co., Ltd., to develop unit module
products to be used in the automotive and industrial
industries. Currently, NTN is working specifically on thefollowing activities, focusing on the growing markets
such as electric vehicles and energy:

NTN’s composite material products include mainly
resin, sintered metal, and magnetic materials. These
products are produced by the NTN’s four relatedcompanies shown in Fig. 1. Key products developed by
the NTN companies are introduced in Section 3 and thesubsequent sections. Various technologies/products
are also covered by different articles.

Composite Material Products Division

BEAREE®

1) Development and proposal of not only components
but unique unit products by combining resin andsintered metal
2) Development of light and compact sliding bearings
as an alternative to rolling element bearings3) Sintering of machine parts which require highstrength4) Product development for industrial and automotive
industries applying magnetic material technologies
5) Proposal of “only one” technology including
amorphous magnetic material into growing markets

NTN Engineering Plastics Corp.

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

BEARPHITE®

NTN Powder Metal Corporation
Sintered oil-impregnated bearings,
sintered machine parts

NIKAMET®
Nippon Kagaku Yakin Co., Ltd.
Magnetic material products, resin molding products,
sintered machine parts

Hydrodynamic BEARPHITE

NTN Manufacturing (Thailand) Co.,Ltd

Hydrodynamic bearing units

Fig. 1 Products and production system of composite
material products* Composite Material Products Division-12-

Product Introduction and Complex Technology of Resin, Sintered Metal and Magnet for Growth Markets

inferior crack resistance against impact. The cost is alsoa challenge. Since BEAREE AS5704 bearings aremainly made of PPS resin, they have superior
properties in self-lubricity, chemical resistance, and ahigh degree of freedom in design using injection
molding. It is easy to provide lubrication grooves on theinternal surface of the bearing bores, end faces, andretainers (D-cut, projection, etc.) on the outer surface ofthe bearings which can be molded into a single piece
without the machining process.

3. Resin material products
Resin components have a high degree of freedom fordesign and are used in diverse fields as alternatives tometal components since they are inexpensive and canbe made lightweight, compact, and corrosion resistant.
NTN develops and designs resin materials, specificallyworking toward higher product functionality byleveraging the resin properties and improving bearing
load properties by creating composite material withmetal. The following is the recent resin material:

Water(1) Resin sliding bearings1) for electric water pump
Use of electric and hybrid vehicles is increasing
because of to the reduction in CO2 emissions.
Conventional vehicles driven by internal combustion
engines are equipped with centrifugal pumps forcirculation of radiator water for engine cooling. These
are driven by the engine rotation which is transmitted tothe pump shafts through the engine belts. Contrary tothis, electric vehicles do not have engines and hybrid
vehicles stop engines with a start/stop function.
Therefore, electric pumps are required for inverters orengine cooling systems. These electric water pumps
are mainly high-efficient and compact centrifugal pumps
with magnetic drives.
Fig. 2 shows the structure of the magnetic-drive
centrifugal pump. This structure allows fluid, such as
antifreeze liquid, to flow by rotating impellers integrated
with magnetic casing using the magnetic force of themagnet installed on the motor shaft as it rotates. As
shown in Fig.3, BEAREE AS5704 made ofpolyphenylene sulfide (PPS) resin with special filler
blended in is used as the bearings in the center of theimpeller. These bearings have sliding contacts
between the internal bore of the bearing and thestationary shaft and the bearing side face and the thrustpad. These sliding contacts support the radial and axial
loads respectively.

Coil

ImpellerThrust pad

MagnetThrust pad

Water
Stationary shaft
Bearing

Fig. 2 Structure of magnet drive centrifugal pump

Fig. 3 Plastic sliding bearings for electric water pump
(BEAREE AS5704)

Table 1 Comparison of features various bearings[Features](1) Friction in the water is 1/5 or less compared with

Types ofbearings

sliding bearings made of universal PPS resin(2) Reduced friction damage to mating material,
such as stainless steel(3) Larger degree of freedom for design due toinjection molding(4) Compatible with antifreeze, acid, and alkaline
fluid

BEAREE
AS5704
Injection
molding

CarbonPhenol resin
PTFE(graphite blended) (graphite blended)

Machining

Injection/
compression
molding

Machining

◎

○

△

×

◎

◎

△

◎

○

×

△

○

Crack by impact

○

×

×

○

Deformation by
impactDegree of freedom
for design

○

○

○

×

◎

×

○

×

Machining
methodWear resistance(under water)
Chemicalresistance
Water absorption
dimensional stability

Table 1 shows a comparison of performance among
typical sliding bearings used in fluid. Bearings made ofcarbon excel in wear resistance in fluid; however, theyshow a lower degree of freedom in shape because they
are machined from formed materials and possess

Price

-13-

◎

×

○

×

◎ Excellent○ Good△ Fair× PoorNTN TECHNICAL REVIEW No.82（2014）

Fig. 4 shows the specific wear rate of BEAREE
AS5704 and various PPS resin materials with glass
fiber, carbon fiber, and PTFE resin blended in.
AS5704 is superior in wear resistance whencompared to other PPS materials because it has
special filler materials added to the PPS resin.

results in a longer life when compared to generalpurpose resin sliding screws.
NTN has recently developed resin sliding screws forhigh load, which have been limited to lower load
applications, and proposing them for a cleaner
production environment and increased load capacity.
NTN has created a composite nut applying PPS resin
sliding material onto the surface of threads of a brass
nut as indicated in Fig. 6, by injection molding, andincreased the heat transfer and load capacity. The
brass has a special surface treatment to improveadhesion with the resin. This composite structure
improved not only the bearing load capacity but also
durability.
The resin sliding screws for high load have twice theaxial load capacity and 9-times higher nut static
breaking load when compared to the conventionalsliding screws. In addition, the life under high loadcondition is 3 times longer than the conventional resin
sliding screws.

Specific wear rate

500
400
300
200

×10-8mm3 100
N・m
0

BEAREE
AS5704

PPS resin+glass fiber

PPS resin
PPS resin++carbon fiber PTFE resin

Fig. 4 Wear factor of PPS bearings[Features]Comparison with conventional products (NTN
resin sliding screw)(2) Resin sliding screws for high load 2)(1) Permissible axial load: 2 times(2) Nut static breaking load: 9 times(3) Friction: 1/3

Feed screws, which convert rotational motion of the
motor to linear motion, are used for carrying units ofmedical and food equipment. Feed screws include ballscrews and resin sliding screws.
Table 2 shows the comparison of their performance.
Ball screws are superior in load capacity and screw
efficiency, although they are an expensive option. In
addition, as they are lubricated with grease, they are
not suitable for use where dispersion of grease anddeterioration due to high temperatures are a concern.
On the other hand, resin sliding screws have lower load
capacity but they do not require lubrication allowing foruse in a broader range of applications, including
vacuum or high-temperature ambient. They are alsoless expensive and quieter during operation.
NTN’s sliding screws made of precision resin consistof PPS resin nuts and stainless steel screw shafts asshown in Fig. 5. Due to the PPS resin having improved
wear/friction property by blending solid lubricant, it also

Resin nut

Screw shaft

Fig. 5 Resin sliding screws
Composite nut
BronzePPS resin

Table 2 Comparison of features various sliding screws
Item

Ball screw

Lubrication

Required(grease)Resin sliding
screw
Not required

Resin sliding screwfor high load
Not required(can also be usedwith lubrication)(can also be usedwith lubrication)

Load resistance

◎

△

○

Screw efficiency

◎

△〜○

○

Noise

△

○〜◎

◎

Heat resistance

△

△〜○

○

◎ Excellent○ GoodScrew shaft

Cross section structure

Fig. 6 Resin composite sliding screws for high load

△ Fair-14-

Product Introduction and Complex Technology of Resin, Sintered Metal and Magnet for Growth Markets(3) Hybrid PEEK bearings 2)

The hybrid PEEK bearings have the following featurescompared to conventional PTFE resin wrapped bushings:

The majority of power consumption of an air
conditioner is occupied by the compressor. Therefore,
higher efficiency is required to reduce energy
consumption of an air conditioner. Fig. 7 shows the
structure of a compressor and the installation positionsof the bearings. The bearings are required to have
improved friction/wear properties and resistance against
heat (the refrigerant liquefied by over-compression canmake the refrigerant oil leak from friction).
Therefore, NTN is developing hybrid PEEK bearings
with improved friction/wear properties and resistanceagainst being burnt using composite material ofsintered alloy and PEEK resin.
Fig. 8 shows the structure of hybrid PEEK bearings.
Different from polytetrafluoroethylene resin wrappedbushings made by the impregnation/sintering process
on the sintered intermediate layer of a steel plate, thesebearings are made with a layer of 0.5 mm thick PEEK
resin applied on the internal surface of the sintered alloybushing by injection molding. PEEK resin is suitable forinjection molding and excels in heat, wear, chemical,
and fatigue resistance. In addition, it also considerably
improved heat transfer properties in the composite
structure with metal compared to the bearings with only
PEEK resin material.

[Features]Compared to conventional products (PTFE resin
wrapped bushing)(1) Friction coefficient: 2/3, wear: 1/3(2) Heat resistance: more than 5 times

4. Sintered metal products
NTN Powder Metal Corporation works on highfunctionality of conventional oil-impregnated sinteredbearings and develops high-strength machine parts
mainly for automotive applications. In the automotive
applications, sintered products are increasingly used asa part of the component cost reduction, which accounts
for approximately 60% of the sintering market or
approximately 9.2 kg per vehicle. The following is theintroduction of high-strength sintered materials andvarious types of sintered bearing products developed
for this growing market:(1) High-strength sintered materials 4)
Sintered machine parts for automotive applications
include engines, drive components, and in-vehicle oilpumps and starter motors. Fig. 9 shows an example ofsintered gears. In addition, Fig. 10 shows the resultsfrom a comparison test of ring compressive fatigue
strength of high-strength sintered materials. The fatigue
strength of the developed products is about 2.3 times
that of the conventional products. The sintered machineparts require not only high-strength but also support forhigh precision and the ability for complex shaping.
Additionally, we are optimizing material composition,
manufacturing method, and thermal processing to
expand applications starting from replacement of cold
forging materials.

Fixed scroll
Revolving scroll
Thrust bearing
Bearing

Motor[Features](1) True density ratio: achievement of 96%(2) Fatigue strength: 2.3 times compared to theconventional products(3) Young’s modulus: achievement of 180 GPa

Main shaft

Fig. 7 Structure of air conditioner compressors

0.5mm

Sintered alloy bush

Example : Thinner gear
Left : Development

PEEK resin

Fig. 8 Structure of hybrid PEEK sliding bearing

Fig. 9 High strength sintered gear

-15-

NTN TECHNICAL REVIEW No.82（2014）[Features](1) Simultaneous forming of two types of powdermaterials(2) Different properties between the inner layer
and outer layer

True density ratio %

8.0
7.8
7.6
7.4

g/cm3 7.2

600

340
150

96

500
100

91

7.0
6.8

g/cm3

Density
True density ratio%

Fatigue limit MPa

Conventional
product

7.1

91
150

Developed
product

Bearing
steel

96
340

100
500

7.5

400
200
0

Fatigue limit

Density

Fatigue strength MPa

Fig 12 shows comparison of the friction coefficients
between this newly developed product andconventional product (bearings that use the inner layermaterial of the newly developed product only).
This developed product uses the high-wear resistantmaterial in the inner layer only but has the equivalentfriction property as the conventional product, whichuses the same material for the entire product.

MPa

7.8

Fig. 10 Ring compressive fatigue strength(2) Multi-layer sintered oil-impregnated bearings 5)
With increased infrastructure development in theworld including emerging markets, demand forconstruction machines, such as hydraulic shovels, isalso increasing. For joints of these hydraulic shovels,use of sintered oil-impregnated bearings is increasing
for longer oiling intervals. Higher strength is also
required due to severe operating conditions.
Conventionally, oil content is secured in low density to
prolong oiling intervals while strength and hardness are
improved by thermal processing, then dimensional
accuracy is achieved by machining the bearings.
The multi-layer sintered oil-impregnated bearings
shown in Fig. 11 have two types of powders withdifferent properties molded simultaneously into one
piece so that the inner layer has sliding and wear
resistant properties while the outer layer has a highstrength property. This process can eliminate the needfor thermal processing such as carburized hardening
and the machining process to achieve a significant
reduction of the manufacturing workload.

Friction coefficient

0.30

Developed bearings

0.25

Conventional bearings

0.20
0.15
0.10
0.05
0

0

1

2

3

4

5

Time hFig. 12 Friction coefficient measurements(3) Corrosion resistant sintered bearings 4)
Bearings for valves in fuel pumps and exhaust gas
recirculation (EGR) systems require high corrosion
resistance as they are in contact with fuel, etc. The
typical material for this is cupronickel-based materials.
However, this material contains a rare metal, nickel,
with the requirement for high corrosion resistantbearings without rare metal increasing. NTN has
focused on aluminum bronze which exhibits good
corrosion resistance resulting in corrosion resistantbearings with better corrosion resistance and sliding
property than cupronickel-based sintered bearings.
Fig. 13 shows an example of bearings for a fuelpump.

[Features](1) Corrosion resistance equivalent or superior tocupronickel-based material(2) Improved corrosion resistance without usingrare metal(3) Significant improvement of wear resistanceover cupronickel based material

Outer layer: high-strength structure material

Inner layer: bearing part; high wear resistance

Table 3 shows the test results of corrosion resistanceagainst organic acid and sulfur. The test piece wasFig. 11 Multi-layer BEARPHITE for jointsof hydraulic excavators-16-

Product Introduction and Complex Technology of Resin, Sintered Metal and Magnet for Growth Markets

acceleration/deceleration, which requires step-up(amplification) of the dedicated battery output voltage.
Therefore, step-up reactors are used for the system
circuit components, which should support large
current/high frequency as well as have a compact sizefor reducing electric cost. The current hybrid vehicles
use Fe-Si (ferrite-silicon alloy powder) with thecompression manufacturing method. In addition, forlarger current to be applied with higher EV output, Fe(pure iron powder) products with high saturation
magnetic flux density manufactured using thecompression method are being considered. However,these materials have large iron loss with large heatgeneration and the step-up reactor tends to becomelarger in order to avoid this.
Fig. 14 shows the performance comparison ofdifferent core material for step-up reactors. The Fe-Si
and Fe products made by the above mentioned
compression method exhibit large iron loss. In
comparison, the amorphous products by Nippon
Kagaku Yakin Co., Ltd. have smaller iron loss and canbe manufactured by injection and compression molding.
The newly developed “hybrid amorphous core”, inparticular, is a combination of injection amorphous coreand compression amorphous core. Fig. 15 shows the
structure of hybrid amorphous core. Compression

Fuel pumpto engine

Fuel tankFig. 13 Sintered bearing for fuel pump
Table 3 Result of anti-corrosion examination
Weight change rate %Developed
Cupronickel
material
based material(1) Corrosion resistance test
against organic acid(2) Corrosion resistance test
against sulfur

0.14

0.15

0.10

0.40

immersed in test fluid at the set temperature and time
and the variation was calculated from the weight
difference before and after test.

<Test condition>(1) Corrosion resistance test against organic acid
・Test fluid: Organic acid concentration 2%
・Temperature: 50˚C・Test time: 100 hours

Compressed
Fe material
Compressed
Fe-Si material (µ100 〜 300)
µ :Relative permeability(µ60 〜 100)
Injected amorphous
40
(µ10)
50

(2) Corrosion resistance test against sulfur
Iron loss

・Test fluid: Gasoline with 300 ppm of sulfur
・Temperature: 80˚C・Test time: 300 hours
The comparison between the newly developed materialand cupronickel based material revealed favorable
results, with an equivalent friction coefficient and thespecific wear rate approximately 1/10 of the cupronickel
based material.

kW

30
20

m3 10
0
0.0

Injected/compressed
Hybrid amorphous(µ20 〜 40)<Reference> ferrite(µ700 〜 10,000)

Compressed amorphous
(µ60)
0.5

1.0

1.5

2.0

2.5

Saturated magnetic flux density T

5. Magnetic material products

Fig. 14 Comparison of features various core material

Nippon Kagaku Yakin Co., Ltd. develops andpromotes magnetic material products aiming at theconverting mechanical components into functional
components. 6)
In this article, we are introducing products for growing
the market, particularly by applying its unique
technologies.

Compression amorphous core

Injection amorphous core

(1) Reactors for automobiles 7)
The motor drive of the recent hybrid vehicles use
dedicated batteries. These vehicles have achieved
drive performance equivalent to fuel-driven engines inFig. 15 Structure of hybrid core-17-

NTN TECHNICAL REVIEW No.82（2014）(2) Dual track magnetic encoder

amorphous with small iron loss is used for the area ofelectric-magnetic conversion for lower heat generationand injection amorphous with superior shaping flexibility
is used for the magnetic conduit for retransmitting theconverted magnet to the coil. Fig. 16 shows ananalytical example of heat generation of the hybrid
core, which indicates appropriate placement of therespective cores. In addition, Table 4 shows acomparison of hybrid core performance against
injection and compression amorphous cores,
respectively. The hybrid core exhibited favorable results
in any property enabling manufacturing of pod-shaped
core, which is difficult with compression amorphous
core.

Industrial machine robotics require high positioning
accuracy and use high-resolution absolute angle
sensors which requires a highly accurate encoder.
NTN is introducing our newly developed dual trackmagnetic encoder. Fig. 17 shows the structure of this
newly developed product. Hard magnetic material isinjected on the core metal of compressed ferrous soft
magnetic material. This diagram shows an axial typedual track encoder with inner and outer ring patterns ofa different number of accurately magnetized poles. The
following are the features of the developed product:[Features](1) Supports opposite angle detection(2) Angle accuracy: ±0.2˚(3) Supports hollow structure[Features](1) Can be used in applications where large
current or high frequency is required(2) Compact and low heat generation bycomposite structure(3) Higher degree of freedom for reactor shapedesign by injection molding

Compressed core

In addition, Fig. 18 shows an absolute angle
measurement method using this developed dual trackmagnetic encoder. Two sensors facing inside andoutside magnetized rings, respectively, detect thesignals. The absolute angle can be accurately
measured by calculating the relative displacement of
the patterns from the magnetic sensor signals, using a
principle called caliper or Vernier principle.

Injected core

125℃
Hard magnetic material:
dual track magnetization

Coil
Ferrous soft
magnetic mandrel

Fig. 16 Temperature distribution analysis of hybrid coreFig. 17 Structure of dual-track magnetic encoder
Table 4 Comparison of features various reactor coreSingle core coil(conventional product)
Compression core Injection core
For large
current
For high
frequency(low loss)

Compact△○(large press required)(DC bias: good)

○

△

○

×

(Permeability: high)(Permeability: low)○Heat
suppressed(Heat transfer: good)

Flexibleshaping(CNC press required) (Injection molding: allowed)△

×○

Hybrid core coil(newly developed
product)

New sensor IC (off the shelf product)
Sensor 1

Hybrid core

Example of detected signals(NS poles can be precisely divided)
8 periods

Sensor 2○

7 periods
0°○○
Dual track magnetic encoder○

Rotation of shaft

90°

Calculate angle position from thedisplacement (phase difference) of twomagnetic patterns (caliper principle,
Vernier principle)

Consists of two rings, inner and outer;
Different number of poles for each ring○

Fig. 18 Absolute angle measurement system-18-

Product Introduction and Complex Technology of Resin, Sintered Metal and Magnet for Growth Markets(3) Choke coil for MRI 6)

This is used in this developed product and thegenerally-used ferrite/iron-silicon (Fe-6.5Si) soft
magnetic material.
In addition, Fig. 23 shows the frequency response of
the inductance when weak current is applied. Fromthese properties, the following features can be pointed
out and this choke coil is suitable for this application
which requires large current and stable high-frequency
response characteristics:

The MRI shown in Fig. 19 is a medical device used
to provide images of sections of the human body usinglarge magnetic energy. The choke coil integrated in thisdevice is an electronic component to generate large
current for creating a gradient magnetic field during
testing.
Fig. 20 shows the components used for the developed
choke coils. The coil, covered by an insulating case, isincorporated in the injection amorphous core to make thechoke coil shown in Fig. 21.
Fig. 22 shows the inductance deviation rate (DC bias
characteristic), which serves as a measure of themagnetic field generation capability when appliedcurrent is swept for amorphous soft magnetic material.

Rate of change of inductance %[Features](i) Inductance deviation rate is small even whenhigh current is applied compared to theconventional ferrite and Fe-6.5Si.
(ii) Change rate of inductance in the highfrequency range is smaller than ferrite.

Fig. 19 Medical magnetic resonance imaging

120

Amorphous
Fe-6.5Si
Ferrite

100
80
60
40
20
0

0

100

200

300

Current A

Fig. 22 DC bias characteristicsa）Coilb）CaseRate of change of inductance %120c）Injection amorphousFig. 20 Parts of choke coil

100
80
60
40

Amorphous
Fe-6.5Si
Ferrite

20
0

1

10

100

Frequency kHz

Fig. 23 Frequency characteristics

Fig. 21 Choke coil for MRI power supply

-19-

1000

NTN TECHNICAL REVIEW No.82（2014）

6. Conclusion
In this article, new products with composite materials
for growing markets are mainly introduced.
With increased fuel efficiency and use of electric
drive components in automobiles, being lightweight is a
focal point in the development of components andcomposite material products.
In addition, NTN is also proposing new products tocontribute to the development of our customers in the
broad field of industrial machines, including medical
devices, food equipment, robotics, etc.
NTN is poised to actively develop products byintegrating a wide range of materials and technologies
in response to the trend of the consistently growing
markets and contributing to these markets.

References
1) Norio Itoh, Takuya Ishii: Plastic Bearings of Electric
Pump for the Next Generation Cell, NTN TECHINICALREVIEW, No. 77 (2009) 51-55.
2) Takuya Ishii, Yoshihide Himeno: Functional
Improvement of Resin Sliding Bearing Through the
Combination with Metal, Journal of the Japan Society ofPolymer Processing, 25 (2), 73, 2013
3) Takuya Ishii, Ken Yasuda: Hybrid PEEK Sliding
Bearing, NTN TECHNICAL REVIEW, No. 79 (2011)125-129.
4) Tomonori Yamashita, Tomokazu Sonozaki: The
Introduction of Sintered New Products for Automobile,
NTN TECHNICAL REVIEW, No. 81 (2013) 74-77.
5) Yosuke Sugai, Toshihiko Mouri: Multi Layer
BEARPHITE, NTN TECHNICAL REVIEW, No. 80(2012) 83-86.
6) Takuji, Harano, Shinji Miyazaki, Hajime Katsuura:
Introduction of Magnetic Material Products, NTN
TECHNICAL REVIEW No. 80 (2012) 87-91
7) Takayuki Oda, Shinji Miyazaki, Eiichiro Shimazu: The
Reactor Core for HEV Boost Converter, NTN
TECHNICAL REVIEW No. 81 (2013) 46-51

Photo of author

Yoshio OKI
Composite Material
Products Division-20-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Article ]The Improvement in Strength of Sintered Machine Parts

Takahiro OKUNO*
Naoki YASHIRO*

Sintered machine parts are used in many industrial fields. Especially,
these are widely used for automotive parts. It is essential for theincreasing use to improve mechanical properties of sintered parts.
However, the pores in the sintered body are dominant factor for thestrength of sintered parts. Therefore, various kinds of trial to decreasepores in the sintered body, or to increase sintered density were reported.
This article introduces the method of improvement of density and strength, and the mechanicalproperties of high-density sintered machine parts.

1. Introduction

2. Sintered alloy steel

Sintered machine parts have been adopted in diverse
fields as they can be made into complex shapes andare suitable for inexpensive, large volume production.
The production volume of sintered machine parts inJapan amounted to approx. 90,000 tons in 2013 andmore than 90% of that volume was for transportation
machines.
Recently, in particular, sintered machine parts havebeen actively used in the automotive industry, with 9.0
kg used per vehicle; 50% of these are for the engine. 1)
In order to expand the scope of sintered machinepart applications, improvement in their mechanicalproperties is vital. However, since sintered compact isporous, ordinary sintered machine parts are not asstrong as molten steel parts. Therefore, reduction ofpores in the sintered compact, namely, by methods toincrease density, is being attempted to improvemechanical properties.
In this paper, we will present methods to make thesintered machine parts higher density/higher strength
through materials or compacting and machining
processes and improvement in mechanical properties.

Sintered alloy steel is used in many sintered machineparts 2).
Powders for sintered alloy steel have a particle sizeof approx. 300 μm or less and include alloy elements
such as copper (Cu), nickel (Ni), chromium (Cr) andmolybdenum (Mo), in addition to iron (Fe). Other
elements include those specified in JIS Z 2550, which
complies with the ISO Standards that specify theirchemical contents. The specification for sintered alloysteel is different from molten steel. For example, Cu,
which is treated as an impurity in molten steel, is
actively used in sintered alloy steel for improving
sintering properties at low temperature and enhancing
sliding properties. Also, carbon (C) is added as graphite
powder to be solidified into iron during sintering. This is
to avoid significant deterioration of formability if graphite
is solidified into iron in the powder form as the powder
becomes hardened. Material manufacturers aredeveloping proprietary powders outside thespecification so that the optimum materials can beselected depending on the intended use.
The base metal powder is either mixed powder withall the additive elements, completely alloyed powder
with additive elements pre-alloyed and homogenized, ordiffusion alloyed powder which uses pre-alloyedadditive elements with no negative effect of compaction
and the remaining additive elements diffusion bondedby thermal diffusion.

* Advanced Technology R&D Center-21-

NTN TECHNICAL REVIEW No.82（2014）

2.1 Fe-Cu-C baseTable 1 shows the classification of metal powder oftypical sintered alloy steel; Table 2 shows the featuresof completely alloyed powder and diffusion alloyed
powder; Fig. 1 shows SEM micrographs. The following
are typical sintered alloy steels used for machine parts.

Fe-Cu-C based material is widely used as sinteredmaterial for machine parts. Cu has a melting point of1085˚C and melts under the general sintering
conditions of sintered alloy steel.
Therefore, Cu exists in the liquid phase between ironparticles and is alloyed as sintering progresses. This
results in high-strength. In addition, Cu is known forvolume expansion when sintered with Fe powder 3). It
can be used to control size change since the volume
generally shrinks with sintering. Mixed powder isfrequently used mixing pure iron powder with purecopper powder or graphite, etc.

Table 1 The classification of metal powder of typicalsintered alloy steel
Powder type
Mixed powder

Sintered alloy steel
Fe-Cu-C baseCompletely alloyed
powder

Fe-Ni-Mo-C base
Fe-Cr-Mo-C baseDiffusion alloyed
powder

Fe-Ni-Mo-C base
Fe-Cu-Ni-Mo-C base

2.2 Fe-Ni-Mo-C base
Fe-Ni-Mo-C based material is used to obtain highstrength sintered compact by various tempering andannealing processes after sintering. Wear resistancecan also be obtained by applying a surface hardening
process in addition to high-strength. Improvement ofsteel toughness can also be expected by Ni. Ni is
frequently used with completely alloyed powder or
diffusion alloyed powder.

Table 2 Feature of completely alloyed powder anddiffusion-alloyed powder
Completely alloyed
powderDiffusion alloyed
powderHomogeneity of powder

High

Low

Compaction property after
forming process

Low

High

2.3 Fe-Cu-Ni-Mo-C base
Fe-Cu-Ni-Mo-C based material is made by diffusion
bonding of fine powders of Cu, Ni, Mo, etc. to pure ironpowders. This is done to combine the features of theabove two previous materials. It is currently widely usedas high-strength sintering material since it has both highcompaction and excellent mechanical properties.

2.4 Fe-Cr-Mo-C baseCr, as an element for alloy, is generally used asmachine structural parts in molten steel 4). On the other
hand, when used as sintering material, it is difficult to
reduce oxygen content as it has high affinity withoxygen. This means that high compaction and highdensity are difficult to obtain. However, with recent
technological progress by material manufacturers,
powders with low oxygen content are developed withimproved compaction properties. Fe-Cr-Mo-C basedmaterial is used in completely alloyed powder due tothe above mentioned affinity with oxygen.

(a) Completely alloyed powder(b) Diffusion alloyed powder

Fig. 1 SEM micrographs of completely alloyed powderand diffusion-alloyed powder-22-

The Improvement in Strength of Sintered Machine Parts

3.4 Infiltration

3. Methods for densification/strengthening

Infiltration is a method to infiltrate melted material of alow-melting point into the pores of a sintered compact.
Material is communicated from the surface using
capillary phenomenon to fill in the pores 10). Specifically,
the sintered compact and the green compact of lowmelting point material are placed in contact with eachother and heated to a temperature more than themelting point of the green compact and less than themelting point of the sintered compact for infiltration.
Since it is possible to close the pores in the sinteredcompact, it is used in the applications where high airtightness is required such as compressor components
or hydraulic pressure resistant components.

Densification methods can be performed duringmolding, sintering and after sintering. Table 3 shows
those densification methods in each process.

Table 2 The method of densification at the production process
Process
During molding
During sintering
After sintering

MethodWarm forming
Warm forming with mold lubrication, etc.
2 forming 2 sintering
Infiltration, etc.
Sinter forging, etc.

3.5 Sinter forging
3.1 Warm forming

Sinter forging is a method to obtain product byforming and sintering metallic powder, and inserting theheated sintered compact into a mold for closed die
forging 10). A regular forging method, such as hot
forging of sintered compact, with a shape close to theformed product, is also adopted. Since it is possible toobtain a value close to the true density, it is used inconnecting rods of vehicle engines, etc.

Warm forming is a method to obtain high-density
green compact by heating the powder and mold to 80300˚C during molding of metal powder. This reduces
the yield strength of metal powder and increases
compaction property 5).
Since this method is lower in cost compared with the
2-forming 2-sintering method and sinter forging method
described later, it is used for sprockets of vehicle
engines, etc. 6)

4. Development of high density sinteredmachine parts

3.2 Warm forming with mold lubrication

NTN is working on development of high density
sintered machine parts with potential for various
applications. The following is an example of mechanicalproperties of the developed materials.

In general, metallic soap, such as Zn and Ca, or aresin based lubricant is added/mixed to reduce friction
between the metal powder and mold when metal
powder is molded; however, since the lubricant
disappears during sintering, it inhibits densification.
Warm forming with mold lubrication is a method to
reduce lubricant by pre-applying the lubricant to themold, in addition to the above warm forming method, toobtain high density green compact 7). Since it ispossible to obtain higher density sintered compact than
the warm forming method, it is used in reactors of
hybrid vehicles, etc. 8)

4.1 Method for making test piece
We made a 23.2 mm OD×16.4 mm ID×7 mm H
ring-shaped test piece by warm forming of 80˚C fromthe Fe-Ni-Mo-C based material. This was made ofdiffusion alloyed powder of Fe and diffusion bonded Ni
and Mo with a predetermined amount of graphite mixed
with lubricant, using floating die method. Next, thegreen compact was sintered in a mixed atmosphere ofN2 and H2, then carburized tempering/annealing wasapplied. In addition, we prepared Fe-Cu-Ni-Mo-C basedmaterial as conventional high-density sintered compact
as a control.

3.3 2-forming 2-sintering
Also called 2P2S (2Press-2Sinter). This is a method
to obtain high density sintered compacts by first forming
powder under a predetermined pressure and applying
preliminary sintering with a relatively low temperature of
1000˚C or below. Then the compact is formed under apredetermined pressure again and sintered with ahigher temperature than the preliminary sintering. Thisis used in synchronizer hubs of vehicle transmission,etc. where strength is required 9).

4.2 Mechanical properties
Fig. 2 shows sintered density and radial crushingstrength. The sintering density of the developedmaterial is 7.63 g/cm3 which is approx. 97% of true
density of 7.8 g/cm3. The radial crushing strength is2180 MPa which is an improvement of 30% or morefrom the 1630 MPa of the control. Fig. 3 shows the
cross section micrograph of the sintered compact. It
-23-

2500

8.0

2000

7.5

1500

7.0

1000

6.5

500

6.0

Developed
material

Control

Repetitive load

8.5

Radial crushing strength MPa

Sintered density g/cm3

NTN TECHNICAL REVIEW No.82（2014）

Fig. 4 Schematic the method of measuring ring
compressive fatigue strength0

Fig. 2 Sintered density and radial crushing strengthσσmax
Stress amplitude

σmin
Average stressN

Fig. 5 The stress wave form at fatigue test

600(b) Control
Density 7.01g/cm3

Stress amplitude MPa(a) Developed material
Density 7.63g/cm3

Fig. 3 Optical micrographs of sintered partcan be observed that the number and size of theinternal pores are both reduced.
We evaluated the fatigue properties of the highdensity sintered compact with the ring compression
fatigue test. The ring compression fatigue test evaluatesfatigue strength by repetitively applying compressive
load on the ring outer surface which produces tensilestress on the inner surface of the ring in the direction of
the load. Fig. 4 shows an overview of the measuringmethod. The test condition was set with a stress ratio
R=0.1 and load frequency 50 Hz. The fatigue strength
was defined as the stress amplitude when the repetitive
load of 107 times did not break the test piece. Here, thestress ratio is a ratio of the minimum stress over the
maximum stress and the stress amplitude is theamplitude of the variable stress waveform shown in
Fig. 5. Fig. 6 shows the stress-endurance diagram of
the fatigue test. The fatigue strength of the developedmaterial was 405 MPa, which was 2.25 times thecontrol (180 MPa). The fatigue strength of thedeveloped product is equivalent to molten steel,
SCM440 thermal refined steel.

Developed material
Control

500
400
300
200
R=0.1
100
1.0E+04
1.0E+05

1.0E+06

1.0E+07

1.0E+08

Number of repetitive loads

Fig. 6 Stress-endurance diagram

4.3 Improvement of mechanical properties
by cold working
We attempted to improve mechanical properties ofthe sintered compact made in Section 4.1 by applying
cold working to reduce pores. The cold working wasapplied after sintering and before thermal processing.
Fig. 7 shows an overview of cold working. The sinteredcompact is placed between the outer restraint jig andthe compression jig. The sintered compact was rolled
by 0.25 mm, pressing the sintered compact by rolling thecompression jig; then carburized tempering/annealing
was applied.

-24-

The Improvement in Strength of Sintered Machine Parts

References

Fig. 8 shows the cross section micrograph. By thiscold working, the compact was densified in the area 0.5mm from the surface, which is twice the rolling amountwith porosity of less than 0.5%. The fatigue strength
after cold working was 450 MPa, which is more than a
10% improvement from 405 MPa before cold working.

1) Kazunori Arai, Toru Shimizu: SOKEIZAI Industrial
Yearbook 2013, 5. Powder Metallurgy, Sokeizai, 55, 5(2014) 52-58.
2) Edited by: Japan Society of Powder and Powder
Metallurgy: A Handbook of Powder and Powder
Metallurgy, Uchida Rokakuho (2010) 236.
3) Hidenori Kuroki, Shinya Fujisawa: The Mechanism ofDimensional Changes of Iron-Copper Powder Compacts
during Sintering, Powder and Powder Metallurgy, 38, 5(1991) 571-576.
4) Edited by The Japan Society for Heat Treatment:
Guidebook for Heat Treatment, Taiga Publishing (2008)
13.
5) Hironori Suzuki, Yoshikazu Seki, Kazuhisa Fujisawa:
Properties of "KHD-Segless" Warm Compaction Steel
Powder, R&D Kobe Steel Engineering Reports, 52, 3(2002) 70-73.
6) Akira Fujiki, Yukihiro Maekawa, Yoshimi Sugaya,
Takashi Shibano: Development of Warm Compacted
P/M Sprocket for Automotive Engines, Journal of theJapan Society of Powder and Powder Metallurgy, 51, 7(2004) 507-514.
7) Mikio Kondoh: Development of High Accuracy, HighDensity Powder Compaction Technology, Journal of theJapan Society of Powder and Powder Metallurgy, 54, 7(2007) 506-512.
8) Takeshi Hattori, Shin Tajima, Mikio Kondoh, Masaki
Sugiyama, Toshiya Yamaguchi, Hidefumi Kishimoto,Tomo Okochi, Takanobu Saito, Satoshi Takemoto,
Yuichiro Fujita: Development of High-Density and Lowloss Powder Magnetic Core for Reactor in Hybrid
Vehicles, (1) Si volume of Fe_Si based compact coreand impact on molding pressure, Proceedings of Japan
Society of Powder and Powder Metallurgy, Autumn
2009 Conference, Nagoya 2009, (2009) 99.
9) Edited by: Japan Society of Powder and Powder
Metallurgy: A Handbook of Powder and Powder
Metallurgy, Uchida Rokakuho (2010) 245.
10) Edited by: Japan Society of Powder and Powder
Metallurgy: A Handbook of Powder and Powder
Metallurgy, Uchida Rokakuho (2010) 154.

Load

Outer restrained jig

Compression jig
Test piece

Fig. 7 Schematic cold working(a) Before cold working(b) After cold working

Fig. 8 Optical micrographs of sintered part5. Conclusion
In this paper, we presented
densification/strengthening methods of sinteredmachine parts and mechanical properties of highdensity sintered machine parts. We have obtained highdensity sintered compacts by applying optimum
methods in each process and we have obtained fatigue
strength equivalent to molten steel SCM440 thermal
refined steel. In addition, it is possible to further improvestrength by applying cold working, etc.
We are poised to expand application of thisdeveloped material to the machine parts that require
high strength, as well as to continue developing
materials with even higher performance and expanding
their applications as well.

Photo of authors-25-

Tadahiro OKUNO

Naoki YASHIRO

Advanced Technology
R&D Center

Advanced Technology
R&D Center

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]High Performance Magnetic Corefor Induction Hardening Devices
Takuji HARANO*

Nihon Kagaku Yakin Co., Ltd. of the NTN group develops various
magnetic materials satisfying demand properties. In this report, it is
attached to the heating coil part of the induction hardening device andintroduces about a high-performance magnetic core in a magnetism
characteristic and a mechanical characteristic to be used for control of
the quenching depth.

1. Introduction

2. Magnetic core for induction
hardening heating coils

Machine structural parts, such as bearings andgears, are strengthened by the thermal process for
improving reliability. The drive parts for automobiles are
required to have strength in certain areas of complex
shapes; therefore, the best thermal process is applied
to the required area with induction hardening.
Magnetic material called "core" is attached to theheating coil of induction hardening equipment("magnetic core"). In this paper, we are presenting ahigh performance magnetic core that exhibits superior
magnetic characteristics as well as mechanical strength
developed for induction hardening.

One of the induction hardening parts for automobiles
is a drive shaft. Fig. 1 shows some examples of its
application.
A magnetic core installed on the back of the heating
coil of induction hardening equipment can accelerate
induction heating by concentrating magnetic flux on thework.
On the other hand, if it is installed on the front of theheating coil, it can block the magnetic flux to prevent
heating where hardening is not needed. Therefore, it isan essential component for heating coil of induction
hardening equipment.
When the target work has a complex shape and thedepth of hardening area needs to be adjusted, induction
heating can be completed by adjusting the shape, size,

Drive shaft
Inner rolling surfaceof outer ring

Stem

Shaft
Induction hardened partFig. 1 Example of induction hardened parts (Drive shaft)* Engineering Dept., Nihon Kagaku Yakin Co, Ltd.

-26-

High Performance Magnetic Core for Induction Hardening Devices

3.3 Features and performance of developed
magnetic cores
3.3.1 Structure and features

number, direction and position of the installed magnetic
cores. This controls the hardening depth of the work.
Nihon Kagaku Yakin Co., Ltd. developed highperformance magnetic core by combining iron powder
and thermosetting resin as shown in Fig. 2.

Fig. 3 shows the structure of the developed highperformance magnetic cores.
We used iron powder for powder metallurgy as themagnetic powder, added a small amount ofthermosetting resin with high adhesive effect, andapplied insulating layer on the iron powder surface by
granulation process. This is a composite material which
reduces the damage to the insulation layer by applying
compaction molding at low pressure and thermalhardening.
This process brought a magnetic material that isstrong yet has low contacts among magnetic powders.
It has low iron loss, even in the high frequency range,
so that the material can be used in a broad frequency
range.
The features of the developed high performancemagnetic cores are as follows:・Saturation magnetic flux density: 1300 mT・Relative permeability: 54
・Iron loss [10KHz/200mT]: 1480 kW/m3
・Radial crushing strength: 150 MPa

Fig. 2 High performance magnetic core

3. Development of high performancemagnetic core
3.1 Background of the development
Commonly used ferrite based magnetic core has
relatively small iron loss even in the high frequency
range. However, a larger size may be required tomaintain hardening performance due to its low
saturation magnetic flux density. Magnetic steel sheet
cannot be used, even if it has high magnetic flux
density, because the iron loss becomes larger in thehigher frequency range.
Commercially available cores or base material of
powder compact based cores manufactured by powder
metallurgy are frequently used as magnetic cores
installed on heating coils of induction hardening
equipment. They are expensive, exhibit low material
strength, and are challenging to machine and handle.
Therefore, our task was to develop magnetic coresthat have better characteristics than the conventional
material, can be produced with a simple manufacturing
process, and are suitable for volume production.

Pore

Iron powder

Resin

Fig. 3 Structure of the high performance magnetic core
(Image)3.2 Development concept
Since the magnetic cores for heating coils of
induction hardening equipment are used in the highfrequency range (several kHz to 100 kHz), the following
characteristics are required:(1) High saturation magnetic flux density(2) High relative permeability(3) Good frequency response (small variation of
inductance against variation of frequencies)(4) Low iron loss(5) Superior mechanical strength-27-

NTN TECHNICAL REVIEW No.82（2014）

3.3.2 Performance(1) Magnetic characteristics(ii) Variation of inductanceFig. 5 shows the rate of inductance variation, which
serves as a measure of the magnetic field generation
capability in the high frequency range, among thefrequency characteristics of the magnetic material.
The rate of inductance variation of the developedmaterial is equivalent to the conventional material in all
ranges, including the high frequency range, with bothmaterials showing small inductance variation up to
about 100 kHz.
As a result, it was revealed that this characteristic isstable up to the high frequency range.

(i) Saturation magnetic flux density,
relative permeability
Table 1 shows a comparison of the saturation
magnetic flux density and relative permeability of thedeveloped and conventional products. Fig. 4 shows thegraph indicating the relation between the magnetic flux
density B and magnetic field strength H (B-H curve).
The developed product improved approximately 10%
in saturation magnetic flux density and approximately
30% in relative permeability over the conventional
product. In addition, the rise of the magnetic flux density
B is faster over the magnetic field strength H indicating
its high efficiency. When the developed magnetic core
is installed on the back of the heating coil of theinduction hardening equipment introduced at the
beginning of this paper, it can accelerate induction
heating by concentrating magnetic flux on the work.

Rate of inductance variation %120

Table 1 Saturation magnetic flux density and relative
permeability
Material

Saturation magnetic
flux densitymT

Relative
permeability

Developed material

1300

54

Conventional material

1200

40

100
80
60
40

Developed material

20
0Conventional material
1

10

100

1000

Frequency kHz

Fig. 5 Frequency characteristics(iii) Iron loss
Fig. 6 shows iron loss of the developed andconventional materials in the high frequency range (10kHz) equivalent to the induction hardening condition.
The iron loss of the developed product at 200 mT
was approximately 15% smaller than the conventional
material indicating less energy loss; a favorable result.

1400

1000
800
600
400

Developed material
Conventional material

200
0

0

5000

10000

15000

20000

25000

Frequency : 10kHz

3000

30000

Magnetic field strength H A/mIron loss kW/m3

Flux density B mT

1200

Fig. 4 B-H curve

Developed material
Conventional material

2500
2000
1500
1000
500
0

0

50

100

150

Flux density mT

Fig. 6 Iron loss

-28-

200

250

High Performance Magnetic Core for Induction Hardening Devices(2) Mechanical strength

4. Conclusion

The developed magnetic core is a strong material,
where iron powders are adhered with thermosetting
resin and hardened, as mentioned above. The
mechanical strengths of the developed andconventional materials were compared by obtaining
radial crushing strength (Fig. 7), applying compressive
stress on the annular test piece, and using the following
equation:K=

F (D−e)L・e2

In this paper, we presented the high performancemagnetic core with superior magnetic and mechanicalcharacteristics to be installed on the heating coil of
inductance hardening equipment.
The developed magnetic core showed superior
magnetic characteristics compared to the conventional
product and significantly improved mechanical strength.
By applying this high performance magnetic core toinductance hardening equipment, we believe we canachieve not only stable quality and improved reliabilityof inductance hardened components but also reduced
size of these materials through enhanced strength andhardness of the structural components. This willcontribute to manufacturing lighter vehicles.

K ：Radial crushing strength (MPa)F ：Maximum load at break (N)L ：Length of hollow cylinder (mm)D ：Outer diameter of hollow cylinder (mm)e ：Wall thickness of hollow cylinder (mm)The result is shown in Table 2. The radial crushingstrength of the developed material is approximately 5
times stronger than the conventional material.

References
Load1) Takayuki Oda, Shinji Miyazaki, Eiichiro Shimazu: The
Reactor Core for HEV Boost Converter, NTN
TECHNICAL REVIEW No. 81 (2013) 46-51
2) Takuji, Harano, Shinji Miyazaki, Hajime Katsuura:
Introduction of Magnetic Material Products, NTN
TECHNICAL REVIEW No. 80 (2012) 87-91
3) Shin Tomogami: Technical Trends in Constant Velocity
Universal Joints and the Development of Related
Products, NTN TECHNICAL REVIEW, No. 75 (2007)
10-15.
4) Japan Electronics and Information Technology
Industries Association: Soft Magnetic Metal Materials

Fig. 7 Method for radial crushing strength measuremen

Table 2 Mechanical strength
Material

Densityg/cm3

Radial crushing strength
MPa

Developed material

6.1

150

Conventional material

6.4

30

Photo of author

Takuji HARANOEngineering Dept.,
Nihon Kagaku Yakin Co, Ltd.

-29-

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]The Copper-Iron-based Material Equivalent to Bronze-based Material
for Sintered, Oil-impregnated Bearings
Tomonori YAMASHITA*

The materials of the sintered oil-impregnated bearing are classified intobronze-based, copper iron-based and iron-based in general. Bronze basedmaterials are superior in sliding an acoustic feature, the problem is thecopper powder is high cost compared with iron powder, so it was necessary
to reduce its quantity. By adopting special copper powder, we developed
new materials having same or surpassing sliding and acoustic features as in
the existing bronze-based materials, even with the small amount of copper
inside. This report introduces the sintered oil-impregnated bearing usingdeveloped materials.

between sliding properties, durability, and cost, from thebase copper-iron-based material.

1. Introduction
Sintered, oil-impregnated bearings are widely used in
automobiles and industrial machines and their materials
are broadly classified into bronze-based, iron-basedand copper-iron-based, which combines the former twomaterials. When superior sliding performance isrequired in a bearing application, such as electric parts(power windows, fan motors, etc.), copying machines,or laser printers, bronze-based material is suitable.
However, because of the recent increase in the cost of
copper, NTN has developed a new material as analternative to bronze for sintered bearings. In this paper,
we present the properties of this copper-iron-basedmaterial equivalent to bronze sintered material for
sintered, oil-impregnated bearings.

3. Features of Special Copper Powder
Table 1 shows the composition of the developedmaterial and Fig. 1 shows the cross section of abearing using a special copper powder based on thisdeveloped material composition. The upper side of the
figure is the bearing bore surface, where a thin layer ofcopper can be observed. The special copper powder
tends to aggregate on the surface of the powder
compact; therefore, a green compact can be producedin the regular molding process without needing any
special processes, such as plating. On the other hand,
many general copper and iron powders added in theblending process are contained inside the compact.
Fig. 2 shows the bearing bore surface. Copper is
exposed on approx. 60% or more of the bore surface
which contributes to the excellent sliding properties.

2. Material of Sintered, Oil-impregnatedBearings
Among sintered, oil-impregnated bearings, bronzebased material is superior in its sliding properties, ironbased material is superior in its durability, and copperiron-based material has both properties.
On the other hand, since the price of iron is lessexpensive than copper, the sliding properties, durability,
and cost are in a trade-off situation. NTN determinedthe correct composition of copper and iron based on theapplicable use and conditions considering the balance*Engineering Dept., NTN Powder Metal Corporation-30-

The Copper-Iron-based Material Equivalent to Bronze-based Material for Sintered, Oil-impregnated Bearings

Table 1 Chemical components of the developed product

The friction coefficient of the developed material was
the lowest at the beginning of operation and it was thesame as the bronze-based material at the time before
stabilization. In addition, the friction coefficient 10
minutes later was at the same level as the bronzebased material and even lower than the copper-ironbased material containing an equivalent amount ofcopper.
From these results, it was found that the developed
product has excellent initial fitness and that its friction
properties are equivalent or superior to the bronzebased material.

Chemical components wt%
Developed
material

Cu

Sn

C

Fe

15〜22

0.5〜2.5

0.5〜2.5

Remainder

Copper thin layer

CopperUpper layer of the
bearing bore surface<Test condition>
・Peripheral speed: 75 m/min
・Contact pressure: 1.2 MPa
・Bearing size: φ6×φ12×6J
・Shaft material: SUS420J2
・Test temperature: Room temperature
・Test time: 10 min.

Iron

Fig. 1 Section of the developed material

Iron
Friction coefficient

0.14

Graphite

Copper

0.13
0.12
0.11
0.10
0.09
0.08
0.07

Fig. 2 Macrophotograph of bearing bore

0

1Bronze-based

2

3

4

5

6

Time minCopper-iron-based(20% Cu)

7

8

9

10

Developed
material

Fig. 3 Result of initial fitness evaluation

4. Performance of the Developed Bearings
We considered initial fitness, limiting PV value, wear
resistance, and low temperature properties whenevaluating the performance of the developed products.

4.2 Limiting PV Value
PV value, which is an index to determine theoperation limit of the bearings, is indicated by the
product of the contact pressure applied to the bearings
P and peripheral speed V.
The larger the value, the tougher the operation
condition.
Fig. 4 shows the measurement results of the limiting
PV value of different bearings. This test is conducted bymaintaining a constant rotational speed and applying aload on the bearings. After a predetermined time, if thefriction coefficient becomes stable, the weight of theload is increased. This is repeated. If the friction
coefficient does not stabilize after the predetermined
time, the PV value at that time is considered as thelimiting PV value.

4.1 Initial Fitness
The sintered, oil-impregnated bearings have a highfriction coefficient at the beginning of operation which
gradually decreases. This phenomenon is called initial
fitness.
Since motors are required to have stable operation
characteristics, a low friction coefficient from the
beginning of operation and early stabilization isrequired. Therefore, we evaluated the initial fitness ofthe developed product.
In addition to the developed product, we also
evaluated bronze-based material and copper-ironbased material containing 20% copper, equivalent to
the developed product. The result is shown in Fig. 3.
-31-

NTN TECHNICAL REVIEW No.82（2014）
10

Bore surface wear µm

The friction coefficient of the developed material
behaved the same way as the bronze-based material
and was lower than the copper-iron-based material
containing an equivalent amount of copper. In addition,
the limiting PV value of the developed material was
significantly higher than the bronze-based product andequivalent with the copper-iron-based material.
From this result, it can be determined that thedeveloped material has an equivalent friction coefficientas the bronze-based material in the low PV value range
and maintains a low friction coefficient in the high PV
value range, as well. Therefore, it can be used in the
broad PV value range.

Friction coefficient

0.10
0.08
0.06
0.04
0.02
100 150 200 250 300 350 400 450 500 550 600

Bronze-based

MPa・m/minCopper-iron-based(20% Cu)

4
3
2
1

Bronzebased

Copper-ironbased(20% Cu)

Developed
material

When used in an automotive application, a noise may
be produced at the beginning of operation in low
ambient temperatures - (abnormal noise at a lowtemperature). The cause of this noise is considered tobe the metal-on-metal contact between the shaft andthe bearings.
With sintered, oil-impregnated bearings, therotational shaft is supported by the oil film formed
between the shaft and the bearings which may contract
in low temperatures, causing less lubrication andresulting in metal-on-metal contact.
Fig. 6 shows the measurement result of oil film
forming rates in low temperature, as an evaluation of oil
film forming behavior. As a comparison, themeasurement result in room temperature is also shown.
The oil film forming rate of 100% indicates non-contact
and 0% indicates full contact.
In Fig. 6 (1), it was found that the oil film forming rateof the bronze-based product was poor, indicating metalon-metal contact even 30 minutes later. On the other
hand, the developed product showed a good oil film
forming rate right after the beginning of the test,
transitioning to a non-contact state in an early stage. As
a result, we consider that the developed product has
superior oil film forming properties to bronze-based
product in low temperatures, contributing to low noiseand vibration in a low-temperature environment. In
addition, as shown in Fig. 6 (2), the developed productindicates better oil film forming properties than thebronze-based product suggesting superior operational
life and wear resistance.

0.12

PV value5

4.4 Low Temperature Properties

0.14

50

6

Fig. 5 Result of wear resistant evaluation

0.16

0

8
7

0

<Test condition>
・Shaft material: SUS420J2・Bearing size: φ6×φ12×6
・Test temperature: Room temperature09Developed
material

Fig. 4 Result of limiting PV value measurement

4.3 Wear ResistanceFor the evaluation of wear resistance, the wear was
estimated from the difference of the bearing bore
diameter before and after the operation test. Fig. 5
shows the evaluation results.
It was found that the wear amount of the developed
product is less than that of the bronze-based materialand copper-iron-based material, presenting excellent
wear resistance.
<Test condition>
・Peripheral speed: 38 m/min
・Contact pressure: 4.0 MPa
・Bearing size: φ6×φ12×6
・Shaft material: SUS420J2
・Test temperature: Room temperature
・Test time: 8 hours<Test condition>
・Shaft material: S45C
・Contact pressure: 0.51 MPa
・Peripheral speed: 93.4 m/min
・Shaft material: SUS420J2
・Test time: 30min.
・Test temperature: room temperature, -40°C-32-

The Copper-Iron-based Material Equivalent to Bronze-based Material for Sintered, Oil-impregnated Bearings

Oil film forming rate %100

5. Expansion of Applications of theDeveloped Product80

The features of the developed product as observed in
the evaluation results of the previous section are as
follows:(1) Equivalent friction properties as the bronzebased product(2) Equivalent limiting PV value properties as thecopper-iron-based product(3) Superior wear resistance(4) High oil film forming properties in low
temperatures
Because of these features, the application of thedeveloped material can be expanded into diverse areas
as an alternative to bronze-based material. In particular,
its application in electric motors, such as the starter
motor, is expected due to its good low-temperature
characteristics.
In industrial machines, it can also be applied to thesmall diameter motors where large loads are applied,such as vibration motors and stepping motors.

60
40
20
0

0

5

10

15

Test time min

20

25

30

Developed material

Bronze-based

Fig. 6 (1) Result of oil film forming evaluation (-40°C)

Oil film forming rate %

100

%

80
60
40
20
0

0

5

10

15

Test time min

Bronze-based

20

25

6. Conclusion

30

In this paper, we presented the newly developed
copper-iron-based material equivalent to bronze
sintered material for sintered, oil-impregnated bearings.
We are poised to expand our sales by leveraging theunique features of these products and continue ourdevelopment aiming at further enhancement.

Developed material

Fig. 6 (2) Result of oil film forming evaluation(Room temperature)

Photo of author

Tomonori YAMASHITAEngineering Dept.,NTN Powder Metal Corporation-33-

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]High Load Sliding Bearing Unit For Use
in Copier and Printer Fusers
Kei HATTORI*
Takuya ISHII**
As one of the cost reduction for business machine typified in copiers and printers,
rolling bearings is required replaceable from sliding bearings. Because of using
bearing conditions at fuser roller of medium and high-speed models are very hard, it
is difficult to apply the sliding bearing. Therefore, NTN developed “High Load Sliding
Bearing Unit”, that combined the metal inner ring and PPS resin outer ring, has
twice load capacity,lower friction and wear properties in comparison withconventional resin sliding bearings. This article introduces the characteristic andperformance of “High Load Sliding Bearing Unit”.

into the paper. The heat from the heating roller istransferred to the fuser roller through the belt. This
makes the fuser roller hotter than the pressure roller.
The bearings support both the fuser roller and the

1. Introduction
With the increased demand for lower cost copiersand printers manufacturers are examining replacing
rolling bearings in the fuser roller with sliding bearings.
However the operating conditions in the fuser roller isdemanding and the use of sliding bearings is notalways possible. The use of either rolling bearings orresin sliding bearings 1) is determined depending on
the specification of the model. Since resin sliding
bearings have inferior load carrying capacity whencompared with rolling bearings, the use of resin sliding
bearings is limited to relatively light-load models.
In this paper, we will present a “High Load Sliding
Bearing unit” with improved load carrying capacity. This
bearing combines an outer ring made of polyphenylensulfide (PPS) resin and an inner ring made of metal.

Pressure Fuser roller
rollerHeating roller

Optical unit

Print paper
Development,
exposure unit

Transfer belt

Fuser assembly

Fig. 1 Laser beam printer2. Bearing specifications and issues forthe fuser roller

Fuser roller
Fuser roller bearing

2.1 Bearing specifications for the fuser roller
Fig. 1 shows the design of a laser beam printer while
Fig. 2 shows the design of a fuser assembly. The
image captured in the optical unit is replicated onto thetransfer belt with toner in the development andexposure unit. The replicated image is then moved fromthe transfer belt and fixed onto paper at the fuser roller.
During this process, the paper passes between thefuser roller and the pressure roller, is heated toapproximately 200°C and has the toner/image pressed

Toner
Separating claw

Shaft

Pressure roller

Pressure

Fig. 2 Fuser assembly**Composite Material Engineering Dept., Composite Material Products Division**Engineering Dept. NTN Engineer Plastic Corp.

-34-

Paper

Pressure roller bearing

High Load Sliding Bearing Unit For Use in Copier and Printer Fusers

pressure roller are supported by. and In operation thepressure roller is pressed onto the fuser roller, usuallyby springs that transmit the force through the bearings.
Therefore, the bearings of the fuser roller and pressureroller are always pressed in one direction and are used
under shaft rotation.

properties. The material for the inner ring is common
bearing steel while a fluoride based grease is used.
With this design and materials, we have improved
friction/wear properties of the bearing, made it usable
under the high load conditions, and given the bearing aself-aligning property, as well.
Table 3 provides a comparison between resin sliding
bearings, a high load sliding bearing unit and rolling
bearings. The high load sliding bearing unit, which has
a hybrid structure, has the following features comparedwith the resin sliding bearings:

2.2 Issues for bearings in the fuser roller
Resin sliding bearings are widely used in low-speed
machines with maximum printing speeds of 30 sheets
per minute, since the load on the pressure roller isrelatively low. However as the printing speed increases
both the temperature and pressure of the fuser roller
increases in order to quickly print the toner. It becomesdifficult for sliding bearings to survive under these
operating conditions necessitating the move to, rolling
bearings that possess higher load capacities.
There are other issues that a designer needs toconsider when using resin sliding bearing in theapplication such as different friction/wear characteristics
depending on the material of the bearing, the shaft fit,
shaft surface roughness and uneven wear due todeflection and/or misalignment of the shaft.

GreaseFlange(any shape allowed)(applied between innerand outer rings)

Slit

Rotation stop

(any shape allowed)

Inner ring

Outer ring

Fig. 3 Structure of high load sliding bearing unit

3. Features of the high load sliding
bearing unit

Aligning

Fig. 3 shows the design of the high load sliding
bearing unit while Fig. 4 shows a partial cross section.
The high load sliding bearing unit is composed of two
components, a PPS resin based outer ring and a metal
based inner ring. The two components contact between
the inner convex spherical surface of the outer ring andthe outer concave spherical surface of the inner ring.
Operating clearance is provided between the outer ringand the inner ring to accomodate thermal expansion
while lubrication is supplied by grease applied to thesliding surface. By making the outer concave sphericalsurface of the inner ring larger than the inner convex
spherical surface of the outer ring the contact area (andfriction) is reduced. The clearance in the bearing allows
for the formation of a grease sump resulting in stable
friction condition.
Table 1 shows the materials of the outer ring, theinner ring and the grease. The electrical characteristics
required for bearings mounted in copiers and printers
vary depending on the design. To accommodate thesedifferent designs there are two types of high load sliding
bearing units each with different materials for the outerring and for the grease, namely, non-conductive andconductive. The respective outer rings use the
BEAREE AS materials shown in Table 2. These are
PPS resins with superior high temperature
propertiesand special additives to improve friction/wear

Outer ring

Inner ring

Fig. 4 Cross section of high load sliding bearing unit
Table 1 Constituent material
Component

Non-conductive type

Conductive type

Outer ring

BEAREE AS5056(non-conductive)

BEAREE AS5965(conductive)

Inner ring

Bearing steel

Bearing steel

GreaseNon-conductive
fluoride based grease

Conductive fluoride
based grease

Table 2 Basic properties of outer ring resin material
Item

Unit

BEAREE AS5056 BEAREE AS5965

Volume resistivity

Ω・cm

×1014or more

×103

Specific gravity

−

1.58

1.62

Tensile strength

MPa

58

43

Coefficient of thermal expansion ×10-5/˚C

8.7

7.3

Thermal conductivity W/(m・K)

0.32

0.38

Note: The above properties (room temperature) are the typical values.

-35-

NTN TECHNICAL REVIEW No.82（2014）
Coupling(Features)(1) A superior friction/wear property in a hightemperature environment(2) Twice the load carrying capacity(3) High degree of freedom for designing shafts(shaft material and surface roughness does not
impact bearing performance)(4) Consistent sliding property with aligning design

Motor

Housing
Bearingunder test

Mating shaft
Thermocouple
HeaterLoad

Table 3 Comparison of features various bearings
Item
Structure

Fig. 5 Radial test machine

Resin sliding High load sliding Rolling bearing
bearingbearing unit
Resin based outer ring Metal basedouter ringSingle resin Metal based inner ring Metal based inner ring
Steel ball

Table 4 Test condition
Item

Description

Load carrying capacity

△

○

◎

Shaft rotational speed

16.5 m/min (210min-1)

Friction property

△

○

◎

Contact pressure

Wear property
Degree of freedom
for shaft design
Aligning property

△

○

◎

Mating shaft

Aluminum alloy

Temperature

180˚C (surface of mating shaft)

○

○

×

○

△

Thermal insulation

◎

◎

△

Cost

◎

○
○ Good20h

0.2△△ Fair× PoorWear amount mm◎ Excellent

Time

4. Performance of high load sliding
bearing unit
4.1 Friction/wear property

0.1

0.1
0

Fig. 5 shows the test machine used for measuring
the friction coefficient and wear amount. The bearing
under test was installed in the housing, to which load
was applied upward from underneath the housing. Thenthe shaft was rotated to measure the friction coefficientand resulting wear. The bearing under test had an innerdiameter of 25 mm and width of 7 mm. The inside of the
shaft was heated and the surface temperature wascontrolled using a thermocouple.
Table 4 shows the test conditions. The friction
coefficient and amount of wear for both the high load
sliding bearing unit (developed product) and the resin
sliding bearing (conventional product) were measured.
Both the developed product and the conventional
product are of the non-conductive type. The
conventional product is a sliding bearing made of PPS
resin with polytetrafluoroethylene resin blended in.
Fig. 6 shows the amount of wear and dynamic
friction coefficient forthe bearings. We defined theamount of wear in the developed product by measuring
the increase in the operating clearance between the
inner and outer ring from the test. The amount of wear
and the dynamic friction coefficient of the developed
product were 1/3 and 1/2, respectively, when comparedto the conventional product.

00.2

Conventional
product

Dynamic friction coefficient×0.95 MPa (166N)

Developed
product

Fig. 6 Friction and wear property

4.2 Limiting PV property
One important operating condition of sliding bearings
is known as the PV value. This is defined as a productof the contact pressure at bearing sliding surface P and
the sliding velocity V. The limiting PV values were
evaluated under the test condition shown in Table 5.
We defined allowable contact pressure as the
maximum contact pressure where the increase of theoperating clearance between the inner and outer rings
from the test was 0.1 mm or less, through a number oftests with different loads at the predetermined rotation
speeds. Here, the contact pressure is a value of load
divided by the product of the inner diameter and widthof the bearing.
Fig. 7 shows the P-V curve, which shows the relation
between the rotation speed of the shaft and allowablecontact pressure of the developed product andcompares this with the PV curve for conventional
product. This test revealed that the developed productis superior in load carrying capacity as the allowable-36-

High Load Sliding Bearing Unit For Use in Copier and Printer Fusers

contact pressure was twice that of the conventional
product.
Fig. 8 shows the time-dependent change of dynamic
friction coefficient of the developed product with the shaft
rotation speed of 10 m/min and contact pressure of 1.66
MPa.This revealed that the dynamic friction coefficient
was low and consistent at approximately 0.06.
We also evaluated bearings of the conductive type.
The conductive type bearings showed equivalentfriction/wear results and limiting PV values as the
conductive type.

dimensions similar to those of equivalent rolling bearings.
However it should be noted that it is also possible toprovide thinner bearing units with smaller outer
diameters. This is achieved by reducing the thickness ofboth the outer and inner rings. In addition, as the outerring is manufactured by injection molding, the size andshape of the flange and rotation stop can be arbitrarilychanged and the number of peripheral components,
such as the retaining ring, may be reduced.
However, since the resin outer ring has a slit, arotation stop needs to be provided on the outer ring .

Table 5 Test condition

Table 6 Dimensions of high load sliding bearing unit

Item

Description

Shaft rotational
speed

5m/min (64min-1)
10m/min (128min-1)
16.5m/min (210min-1)

Mating shaft
Temperature

Main dimensions mm
FlangeInner diameter Outer diameter outer diameter

Width

Allowablealigning angle
mm/mm

20

32

34.8

7

1/100

Aluminum alloy

25

37

39.8

7

1/100

200˚C (surface of mating shaft)

30

42

44.8

7

1/100

35

47

49.8

7

1/100

Time

100h

Notice : Allowable misalignment is a reference level to vary according to condition of use.

Allowable contact pressure MPa

4Developed product6. Conclusion

Conventional product3

We have developed a high load sliding bearing unit
with significant improvement on the load carrying
capacity and friction/wear properties in a hightemperature environment. Additionally the new bearing
allows for a high degree of freedom in designing theshaft and alignment accuracy. With this development,
we are able to provide an alternative to rolling bearings
for the fuser rollers of copiers and printers.
In addition, the high load sliding bearing unit can beapplied to various other applications by changing
materials of the components. NTN is looking at
expanding the use into food processing equipment andmedical equipment.

2
1
0

0

5

10

Shaft rotational speed

15

m/min

20

Fig. 7 Limiting P-V curve

Dynamic friction coefficient

0.40

Shaft rotational speed：10 m/min
Contact pressure：1.66 MPa
Temperature : 200 ˚C

0.35
0.30
0.25

References

0.20

1) Yoshio OKI, Takumi HAYASHI, Takuya ISHII: Characteristics
and Applications of Super Engineering Plastics, Journal ofJapanese Society of Tribologists 49 (7), (2004) 31.

0.15
0.10
0.05
0

Photo of authors
0

20

40

60

Time h80

100

Fig. 8 Coefficient of dynamic friction

5. Dimension of bearings
Table 6 shows an example of dimensions for a highload sliding bearing unit mainly used in the fuser rollers
for copiers and printers. This table shows bearings with-37-

Kei HATTORI

Takuya ISHII

Composite Material Engineering Dept.,
Composite Material Products Division

Engineering Dept.
NTN Engineer Plastic Corp.

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]Machine Tool Main Spindle Bearings with Air Cooling Spacer

Yushi ONDA*
Kikuo FUKADA*
Yohei YAMAMOTO**
Masato YOSHINO***NTN developed “Machine Tool Main Shaft Bearing with Air CoolingSpacer” with the air cooling technology for machine tool main spindle
bearings, which realizes high-speed and high-rigidity at the same time withhigher level than ever. In this paper, bearing cooling effect through thetechnology and technical activities for practical use are introduced below
with tests and analysis results.

now developed a “cooling spacer 3),” by applying coolingtechnology for bearings as a new approach. In thispaper, we present the structure and mechanism of“machine tool main spindle bearings with air coolingspacer” which is an improved version of thisdevelopment to the practical level.

1. Introduction
For machine tool main spindles, high rigidity and highprecision are required for machining difficult-to-cut
materials and parts with complex shapes. The
characteristics of the machine tool main spindles varydepending on the machining content and the target
workpiece because, for example, the machining ofmolding for machine parts and components for medical
equipment requires high-speed and high-precisionrotation. Therefore, the demand for equipment withmultiple machining capabilities such as 5-axis
processing machines and field-assisted machining
equipment is increasing. The main spindles andbearings are required to have high-speed performanceand high rigidity.
To increase rigidity of the bearings, it is effective toraise preload during assembly and to increase thepayload capacity of the bearings. However, thosemeasures may cause high heat during operation,
preventing high speed operation and degradingmachining accuracy due to thermal expansion of themain spindles. Therefore, it is important to reduce heat
during operation in order to achieve high-speed
operation with increased rigidity of the bearings.
NTN has been working on higher speed, higher
rigidity and lower noise for the bearings of the main
spindles by improving air oil lubrication 1), 2). We have

2. Structure
Fig. 1 shows the structure of the bearings with air
cooling spacer. NTN’s proprietary environment friendly
air oil nozzle 4),, which enables reduction of air volume,
oil quantity and noise, is applied to the outer ring spacer
installed between back-to-back angular contact ballbearings (DB arrangement), in addition, air cooling
nozzles are installed independent from the air oil
nozzle.
The air cooling nozzles are arranged offset from thecenter of the shaft at three locations on thecircumference toward the direction of rotation. The
room temperature air (hereafter, “cooling air”) injectedfrom the air-cooling nozzles passes between the innerspacer and outer spacer and through the bearings
swirling toward the direction of the rotation to cool the
bearings.

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

Machine Tool Main Spindle Bearings with Air Cooling Spacer

Cooling air nozzle

Air cooling spacer(outer ring spacer)

Bearings with
air cooling spacer

Cooling air nozzle
OffsetAir cooling spacer(outer ring spacer)

Cooling air (inlet)

Inner ring spacer

Environment friendly air oil
lubrication nozzle 4)
Inner ring spacer

Direction of rotation(inner ring)
Cooling air (outlet)

Arrow A

Cooling air (outlet)
A

Fig. 1 Structure of the bearing with air cooling spacer

result is shown in Fig. 2.
If the air cooling nozzles are not offset from thespindle center, the cooling air diffuses toward the axial
direction after colliding against the outer surface of theinner ring spacer, and then the air is exhausted out of
the spacer. Therefore, the bearing cooling effect fromthe cooling air is small.
On the other hand, when the nozzles are offset, thecooling air flows along the circumference of the outersurface of the inner ring spacer after its collision to thesurface in high speed, remaining on the surface for alonger time.
Therefore, the bearing cooling effect from the cooling
air is large.

3. Cooling mechanism
The bearings with air cooling spacer take the heatgenerated from the bearings from the surface of theinner ring spacer with the cooling air. When the cooling
air stays longer in the spacer, the heat exchanging time
between the cooling air and the surface of the inner ring
spacer increases, which improves the cooling effect of
the bearings. In addition, faster air flow increases theheat taken out of the surface of the inner ring spacer
per hour, increasing cooling efficiency of the inner ring
spacer and the bearing inner ring.
We have modeled the cooling air flow inside thebearings with air cooling spacer by fluid analysis. The

Direction of rotation(inner ring)

Cooling air diffuses
after colliding with
the inner ring spacer
and leaves the surfaceof the inner ring spacer
outer surface.

Cooling air nozzle

Flow

OffsetInner ring spacer
outer surface
Direction of rotation(inner ring)

Cooling air nozzle

Cooling air flows along
the inner ring spacer
outer surface in highspeed.

Cooling air nozzle

Cooling air nozzle
Slow

Outer ring spacer
inner surface

Cooling air nozzle

Cooling air nozzle

Fast

Slow

When cooling air nozzles are not offset

Flow
When cooling air nozzles are offset

Fig. 2 Cooling air flow in bearing spacer (Fluid analysis results)

-39-

Fast

NTN TECHNICAL REVIEW No.82（2014）

In case of more practical cooling air flow of 100
NL/min, the temperature difference between the innerand outer rings can be reduced by approx. 4˚C and the
maximum contact pressure of the inner ring runway
surface can be reduced by approx. 6% at 23,000 min-1(dmn value of 2,070,000), therefore, the maximum
rotation speed of the bearings can be increased byapprox. 5%.
The bearing preload during assembly of the main
spindle of 0 N can be increased up to approximately
150 N.
As a result, by adopting this technology we canachieve both high-speed and high-rigidity.
On the other hand, in Fig. 5, we verified that thenoise becomes loud in the medium/low speed range,
when the cooling air flow is 300 NL/min. We discuss
this reduction of noise in the following section.

4. Cooling effectWe have conducted a high speed operation test of
the bearings to verify the cooling effect of the bearings
with air cooling spacer. Fig. 3 shows the structure of thetester, Table 1 shows the test conditions, and Fig. 4
and Fig. 5 show the rise of temperature and noise levelof the bearings, respectively. The offset level of thenozzles is determined to be 80% of the outer radius of
the inner ring spacer based on past test results 1).
In Fig. 4, the temperature rise of the inner ring issmall when cooling air is supplied, compared with no
cooling air. From this result, we verified that thedifference of the temperature between the inner andouter rings at the rotation speed of 23,000 min-1 (dmnvalue of 2,070,000) decreases approx. 14˚C when thecooling air flow of 300 NL/min is supplied.
By this reduction of temperature difference between
the inner and outer rings, the maximum contactpressure of the inner ring runway surface from inside
the bearings during operation can be reduced byapprox. 35% and the maximum rotational speed can beincreased by approx. 20%. In addition, the bearing
preload during assembly of the main spindle of 0 N canbe increased up to approx. 1,300 N.

Temperature difference between
inner ring and outer ring ˚C

30

Bearings with
air cooling spacer

Cooling air (inlet)

No cooling air

25

Cooling air: 100 NL/min

20

Cooling air: 300 NL/min

Cooling air: 200 NL/min

15
10
5
0
-5

Driving
motor

0

5000

10000

15000

20000

Speed of rotation min

25000

-1

Fig. 4 Cooling air amount and temperature difference
between inner ring and outer ring
Cooling air (outlet)

Cooling air (outlet)

100
No cooling air
Cooling air: 100 NL/min

Fig. 3 Structure of test machine
Noise level dBA

95

Table 1 Test conditions
Bearings under test
Pressurization method
Speed of rotation
Lubrication method
Oil level
LubricantLubrication air flow

φ70×φ110×20
Equivalent to 5S-2LA-HSL014
Fixed location pressurization(pressurization after assembly ON)

Cooling air: 300 NL/min
9085

0〜23000min-1
Air oil lubrication
0.03ml/10min
ISO VG32

80

30Nl/min

0

5000

10000

15000

Speed of rotation min

Outer cylinder cooling Yes, synchronized with room temperature (21±1˚C)
Spindle position

Cooling air: 200 NL/min

Horizontal

20000
-1

Fig. 5 Cooling air amount and noise level

-40-

25000

Machine Tool Main Spindle Bearings with Air Cooling Spacer

into this bearing part, we provided a space between the
inner ring spacer and outer ring spacer which extends
to the circumference. Fig. 7 shows the fluid analysis
result of this design. The cooling air injected to thisspace flows to the circumferential direction without
diffusing immediately after being injected, then flows
into the bearing space slowing down the speed. In thiscase, the collision between the cooling air flown in andthe air curtain is reduced compared with the case of
Fig. 6; therefore, the noise becomes low.
In order to verify the above space effect, we
conducted an operational test. This result is shown in
Fig. 8. We verified reduction of noise by provision of aspace between the inner and outer ring spacers.

5 Initiatives of noise reduction
In order to investigate the cause of the louder noisewith increased cooling air flow, we modeled the cooling
air flow by fluid analysis.
The analysis result when the air cooling spacer widthof 22 mm is shown in Fig. 6. A part of the cooling air
diffuses to the axial direction also, immediately after
being injected from the air cooling nozzles flowing into
the bearings maintaining the fast flow speed. It isconsidered that this cooling air and the air curtain
produced around the rotating rolling element collide
strongly with each other, which caused a lot of noise.
In order to reduce the speed of the cooling air flowing

Cooling air nozzle

22

Air coolingspacerAir coolingspacerDiffuses horizontally, as
well, immediately after
being injected from thecooling air nozzle

Location where
air curtain is
produced in the
bearings
Inner ringtapered surface

Slow

Space

Cooling air injected into thisspace does not diffuse in
the axial direction but in thecircumferential direction
immediately after being injected

Location where
air curtain is
produced in the
bearings

Loud noise produced
as the air flow maintaining fast speed into
bearings collides with
air curtain

Inner ringtapered surface
Inner ring spacer
outer surface

Inner ring spacer
outer surface
Inner ringtapered surface

22

Cooling air nozzle

Inner ringtapered surface

Location where air
curtain is producedin the bearings

Location where air curtain is produced in the bearings

Fast

Fig. 6 Cooling air flow near inner ring spacer surface(Fluid analysis results)

Slow

Lower noise due to low
speed when air flow
collides with air curtain
Location where air
curtain is producedin the bearings

Location where air curtain is produced in the bearings

Fast

Fig. 7 Cooling air flow near inner ring spacer surface
for the case of special space(Fluid analysis results)-41-

NTN TECHNICAL REVIEW No.82（2014）

66

100
No cooling air

Noise level dBA

No space, cooling air: 300 NL/min
With space, cooling air: 300 NL/min

95

Cooling air nozzle

90

Air cooling spacer

85

80

Fig. 9 Bearing with air cooling spacer of 66mm width

0

5000

10000

15000

20000

25000

30

Temperature difference between
inner ring and outer ring ˚CSpeed of rotation min-1

Fig. 8 Special space of air cooling spacer and noise level

6. Relation between the space widthand noise
In order to verify the relation between the spacer
width and noise, we conducted a test with the cooling
air spacer width of 66 mm, as shown in Fig. 9, which is
triple the size of the 22 mm spacer width described in
the previous section. Fig. 10 shows the rise of
temperature of the bearings and Fig. 11 shows theresulting noise level.
With Fig. 10, we verified that the temperature
difference between the inner and outer rings is reduced
by approx. 7.5°C and the maximum contact pressure of
the inner raceway surface is reduced by approx. 20% at
the cooling air flow of 300 NL/min and with 23,000 min-1(dmn value of 2,070,000). As a result, the maximum
rotation speed of the bearings can be increased byapprox. 15% and the bearing preload during assemblyof the main spindle of 0 N can be increased up to
approx. 500 N.
Also, we verified that the temperature difference
between the inner and outer rings is reduced by approx.
2.5°C and the maximum contact pressure of the inner
raceway surface is reduced by approx. 5% at thecooling air flow of 100 NL/min and with 23,000 min-1(dmn value of 2,070,000). As a result, the maximum
rotation speed of the bearings can be increased byapprox. 5% and the bearing preload during assembly ofthe main spindle of 0 N can be increased up to approx.
90 N. From the above, we verified the bearing cooling
effect also with the cooling air spacer width of 66 mm.
On the other hand, no increase of noise level wasobserved with the cooling air spacer width of 66 mm,with the cooling air flow of 300 NL/min and no space
between the inner and outer ring spacers, from Fig. 11.

No cooling air

25

Cooling air: 100 NL/min
Cooling air: 200 NL/min

20

Cooling air: 300 NL/min

15
10
5
0
-5

0

5000

10000

15000

Speed of rotation min-1

20000

25000

Fig. 10 Temperature difference between inner ring andouter ring of bearings with air cooling spacer 66mm width100
No cooling air

Cooling air: 100 NL/min
Cooling air: 200 NL/min

Noise level dBA

95

Cooling air: 300 NL/min

90

85

80

0

5000

10000

15000

Speed of rotation min-1

20000

25000

Fig. 11 Noise level of bearings with air cooling spacer
66mm width-42-

Machine Tool Main Spindle Bearings with Air Cooling Spacer

In order to understand the reason, we analyzed thecooling air flow with the cooling air spacer width of 66
mm. The result is shown in Fig. 12. With the cooling air
spacer width of 66 mm and no space between the innerand outer ring spacers, a part of the cooling air diffuses
to the axial direction immediately after being infectedfrom the cooling air nozzles. However, since thedistance to the bearing part is longer and the air flow
speed was reduced by the air viscosity while it passed
the space between the inner and outer ring spacers, it
is considered that the resulting noise due to the collision
with the air curtain was reduced.

7. Conclusion
NTN achieved a high level of speed and rigidity byusing the air cooling technology for the bearings ofmachining tool main spindles. We strive to continue
contributing to "Monozukuri” as we believe that the“Machine Tool Main Spindle Bearings with Air CoolingSpacer” will contribute to higher speed, higher rigidity
and higher reliability of the machine tools.

References
1) Kenji Fujii, Masatsugu Mori, Yoshimi Ota: Noise
Reduction for Machine Tool Main Spindle Air Oil
Lubrication Bearings, The Japan Society for Precision
Engineering, Autumn Meeting 2000 Proceedings (2000)
449.
2) Yoshinobu Akamatsu and Masatsugu Mori: Minimizing
Lubricant Supply in an Air-Oil Lubrication System, NTN
TECHNICAL REVIEW, No 72 (2004).
3) Yushi Onda, Mamoru Mizutani and Masatsugu Mori:
Machine Tool Main Spindle Bearings with “Air CoolingSpacer”, NTN TECHNICAL REVIEW, No 80 (2012).
4) NTN Catalog: Precision Rolling Bearings, CAT. No.
2260-V/J 12.09.01.

A part of cooling air diffuses in the axial direction
immediately after being injected from the cooling
air nozzles
Location where
air curtain is
produced in the
bearings

Because of the long distanceto the bearings, cooling air
speed is reduced before it
flows in the bearings due toair viscosity as it passes
through the space

Inner ringtapered surface
Inner ring spacer
outer surface
Inner ringtapered surface

Slow

Location where air
curtain is producedin the bearings

Cooling air flow speed around theinner ring spacer surface

Fast

Fig. 12 Cooling air flow in bearing 66mm spacer(Fluid analysis results)

Photo of authors

Yushi ONDA

Kikuo FUKADA

Yohei YAMAMOTO

Masato YOSHINO

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division

Advanced Technology
R&D Center-43-

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]New Engineering Plastic Cage of High Speed
Angular Contact Ball Bearings for Machine Tools
Wakana INOUE*
Mineo KOYAMA*
Soichiro ONITSUKA**
To support the high-speed, high-rigidity and high-reliability requirements ofmachine tools, NTN developed new resin cage of high speed angular contact
ball bearings for machine tools. The new cage has good characteristics inhigh-temperature range and optimized pocket design. With the modifications,the cage can be applied higher speed range than conventional one. In thisdocument, we introduce the design points and test data.

properties, as well as a special surface modification
technique for internal design of inner and outer rings toachieve higher speed, rigidity and reliability. By thisadoption, higher speed operation was made possible
while maintaining preload after assembly of the main
spindles allowing broad use in the main spindles ofhigh-speed lathes and machining centers.
It is possible to allow use of steel balls or ceramicballs as rolling elements and to select contact angles of
inner and outer rings (15°, 20° and 25°) depending on
the operational conditions of the main spindles.

1. Introduction
The speed of spindle rotation for machine tools has
been increasing for improving quality of the machined
surface and machining efficiency as represented by themachining centers for mold manufacturing. In order to
respond to these requirements, NTN has introduced
high speed angular ball bearings, ULTAGE Series *1
HSE type, applying special materials and special
surface treatment onto the inner and outer rings of the
bearings, as well as optimum design for the internal
components.
The HSE bearings adopt machined cages made ofphenol resin as standard; however, for mid/low speed
main spindles of dmn value (an index of the speed ofrotation) of 1,000,000 or lower, cages made ofpolyamide resin are widely used considering the cost.
We have designed cages made of newly developedpolyamide resin applicable for a higher speed range
where machined cages made of phenol resin havebeen traditionally used.
In this paper, we present features and evaluation
results of the developed cages.

Cage· High speed main spindle: Machined cage with phenol
resin· Mid/low speed main spindle: Cage with polyamide resin(different shape)
Raceway rings(inner ring, outer ring)· Special material+ surface treatment
Contact angle(15°, 20° and 25°)
Rolling element· High speed main spindle: Ceramic ball· Mid/low speed main spindle: Steel ball

2. High speed angular contact ballbearings - HSE typeThe high speed angular contact ball bearings - HSE
type (shown in Fig.1) employs a special materialfeaturing improved wear resistance and anti-seizure

Fig. 1 High speed angular contact ball bearings HSE type**Machining Tool/Aerospace Engineering Dept., Industrial Machinery Division**Industrial Machinery Engineering Dept., Industrial Machinery Division※1 ULTAGE is a name created from the combination of "ultimate" and "stage” signifying NTN’s intention to pursue the ultimate performance of precision bearings.

-44-

New Engineering Plastic Cage of High Speed Angular Contact Ball Bearings for Machine Tools

High speed operation of main spindle

3. Features of cages with new resin
3.1 Issues with cages made of the conventiona
polyamide resin

Centrifugal force
applied to the inner ringand rolling element

Rolling heat generation
between rolling element
and raceway

Fig. 2 shows the conditions of the cage pocket after
high speed operation of bearings with the cage made of
conventional polyamide resin at over 1,000,000 of dmn
value. The contact area of the cage pocket and therolling element locally melts and wears depending on
operating conditions. This damages progresses as
tough operation continues, and may cause abnormalrise of temperature or seizure of the bearings.
The damage process of the cage pocket is shown in
Fig. 3. High speed sliding contact is observed between
the cage pocket and rolling element in high speed
operation of main spindles of machine tools. Fig. 4
shows the relation between the speed of the main
spindle rotation and the peripheral speed of the rolling
element. In case of main spindles of machining centerwith the main spindle diameter of 70 mm and speed ofrotation 15,000 min-1, the peripheral speed of therotating element can reach approximately 35 m/s, andover 40 m/s with higher speed spindles.
In addition, the preload of the bearings increases withspeed, due to the temperature difference between the
inner and outer rings, which expands because of theheat generated between the rolling element and
raceway and the centrifugal force applied to the innerring and the rolling element. This leads to the increaseof the load on the rolling element, which causes tighter
contact between the cage pocket and the rolling
element.
As a result, the heat generated in the cage pocket
can locally increase, causing potentially poor
lubrication, melting, and wear.

Large rise of temperature
for the bearings(large difference oftemperature between the
inner and outer rings)
Increase of preload of bearing(increase of load on the rolling element)
Strong contact between
the cage pocket androlling element

Slide contact between
the cage pocket androlling element

Heat generation at the cage pocket contact area
Poor lubrication at the cage pocket contact area
(further heat generation)
Wear/melt of cage pocket

Rolling element surface velocity m/s

Fig. 3 Cage damage process in high speed operation

Initial wear/melt

HSE014 (φ70×φ110×20)

50

Expansion of applications
by making the cage forhigher speed operation

40
30
20
10
0

Main spindle for high speed
machining center (φ70×15000min-1)

Operation
range of theconventional
cage

0

0

5000

10000

15000

Bearing rotating speed min-1
0.5

1.0

dmn value

1.5

×106

20000

2.0

Fig. 4 Relationship between bearing rotating speed androlling element surface velocity

New

After high speed operation

Fig. 2 Conventional cage pocket after high speed
operation-45-

NTN TECHNICAL REVIEW No.82（2014）

3.2 Adoption of polyamide resin material witha high melting point

rolling element due to the decrease of dynamic modulus
of elasticity and the resulting heating can be reduced.
To evaluate the developed cage material, we made acage with the same shape as the conventional cages
for comparison.
Fig. 6 shows the test spindle, Fig. 7 shows the test
result with air oil lubrication, and Fig. 8 shows the test
result with grease lubrication.
Even if no difference in rise of temperature wasobserved between two cages, the resin of thedeveloped cages with both air oil and grease lubrication
demonstrated durability for applications of higher speed
than the conventional polyamide resin.

Dynamic modulus of elasticity

Pa

As mentioned in the previous section, it is necessary
to apply the materials with a high glass transition pointand high melting point, in particular, to prevent local
wear and melt of the cage pocket, as shown in Table 1,
for resin cages to allow higher speed of rotation.
Table 2 shows the characteristics of the developed
cage material. We adopted a high melting point semiaromatic polyamide as the material for the developed
cage. It has a higher melting point and glass transition
point compared with the conventional polyamide used
for cages, with smaller strength deterioration anddimensional change due to high temperature.
Fig. 5 shows the relationship between the dynamic
modulus of elasticity*2 to temperature.
The dynamic modulus of elasticity of polyamide forthe conventional cages starts decreasing from approx.
50°C, around the glass transition temperature; however,polyamide with a high melting point for the developed
cages has a higher glass transition temperature,
maintaining a gradual decrease of dynamic modulus ofelasticity up to around 120°C. Therefore, when the cage
pocket is heated by the high speed slide contact withthe rolling element, increase of the contact area with the

×1010
1.2

Developed cage
material

1.0
0.8
0.6

Conventional
cage material

0.4
0.2
0

-50

0

50

100

150

200

250

Temperature ˚CFig. 5 Relationship between the dynamic modulus ofelasticity to temperature

Table 1 The necessary characteristic of cage material
for high speed operation
Required properties

Purpose· Low water absorption and1 dimensional change· Low linear expansion coefficient

Small dimensional dispersion,
allows stable operation2· High tensile strength and flexuralstrength

Bearings
under test

Supporting
bearing

Little deformation in highspeed operation· High glass transition temperature
Little wear/melt in high speed· High melting point
slide contact
4 · Superior sliding property
3

5 · Good injection moldability

Stable quality

6 · Low cost

Broader applicability without
cost restriction

Motor side

Table 2 Characteristic of cage material
Item

Conventional cage
material

Resin type

Polyamide

Developed cage
material
High melting point
polyamide

260

300

58

125

160

175

Melting temperature Tm

˚C

Glass transition temperature Tg ˚CTensile strength

Water absorption ratio

MPa
％

2〜3

Fig. 6 Test spindle

0.2※2 Dynamic modulus of elasticity is obtained from the response of the sinusoidal amplitude given to the test piece.

-46-

New Engineering Plastic Cage of High Speed Angular Contact Ball Bearings for Machine Tools

3.3 Shape optimization of cages【Test condition】
Bearingunder test

5S-2LA-HSE014DB (contact angle of 20˚)
φ70×φ110×20 back-to-back assembly

PreloadFixed position preload
Preload during assembly: 300 N

Air oillubrication

Supply amount: 0.03mℓ/8 min
Air flow: 40 Nℓ/min

Rise in temperature of outer ring ˚CExternal cylindercooling
35

The shape of the pocket of the developed cage isshown in Fig. 9. We have provided space for lubrication
close to the contact area between the rolling element
and cage pocket for the developed cage. As a result, incase of air oil lubrication, smooth supply/drain of
lubricating oil was made possible at the contact point of
the rolling element and the cage pocket. In case ofgrease lubrication, it also contributed to the retention ofgrease in the proximity of the contact point increasing
lubrication reliability in high speed operation.
In addition, the guiding surfaces, which are thecontact area of the developed cage with the rolling
element in rotational and axial directions, are made flat(ref. (1) and (2) of Fig. 9) to reduce generation of heatby minimizing the contact area with the rolling elementcompared with the circular hole contact area of theconventional cages with polyamide resin.

Yes

Rise inDeveloped cage material temperature

30

Conventional cage material

25
20
15
10
5
0

0

0

10000

20000

30000

Speed of rotation min-1
1

0.5

2

1.5

2.5

dmn value ×106

Circular hole contact areawith rolling element

3

Plain (1) guide torotational direction
Space

Rotational
direction

Fig. 7 Test result under air-oil lubrication with ceramic ball

【Test condition】
Bearingunder test

2LA-HSE014DB (contact angle of 20˚)
φ70×φ110×20 back-to-back assembly

PreloadFixed position preload
Preload during assembly: 300 N

GreaselubricationNTN SE-1

Rise in temperature of outer ring ˚CExternal cylindercooling
30

Conventional cage

Yes

Conventional cage material

Rise intemperature

Rise intemperature

4. Performance of the developed cage
We evaluated the HSE bearings using ceramic ballswith air oil lubrication and grease lubrication, and using
steel balls with grease lubrication in high speed
operation with the test spindle shown in Fig. 6.
The results are shown in Fig. 10, Fig. 11 and Fig. 12.
We verified that the developed cage experienced
reduced the rise of temperature enabling higher speed
operation.

20
15
10
5
0

0

0

5000

Developed cage

Rolling
element

Fig. 9 Cage pocket shape

Developed cage material

25

Plain (2) guide to
Axial direction
axial direction

10000

15000

Speed of rotation min-1

0.5

1

dmn value ×106

20000

1.5

Fig. 8 Test result under grease lubrication with steel ball-47-

NTN TECHNICAL REVIEW No.82（2014）【Test condition】
Bearing
2LA-HSE014DB (contact angle of 20˚)
under test
φ70×φ110×20 back-to-back assembly【Test condition】
5S-2LA-HSE014DB (contact angle of 20˚)
φ70×φ110×20 back-to-back assembly

PreloadAir oillubrication

Rise in temperature of outer ring ˚CExternal cylindercooling
35

Fixed position preload
Preload during assembly: 300 N
Supply amount: 0.03mℓ/8 min
Air flow: 40 Nℓ/min

PreloadGreaselubrication
External cylindercooling

Yes

Rise in temperature of outer ring ˚CBearingunder test

Rise inDeveloped cage material temperature
Conventional cage material

30
25
20
15
10
5
0

0

0

10000

20000

1

0.5

2

1.5

3

2.5

×106

dmn value

Fig. 10 Test result under air-oil lubrication with ceramic ballRise in temperature of outer ring ˚CGreaselubrication
External cylindercooling
30

Yes

20

Rise intemperature
Rise intemperature

15
10
5
0

0

5000

0

0.5

10000

15000

Speed of rotation min-1
1

dmn value

×106

1.5

20000

Conventional cage material

Rise intemperature

25
20
15
10
5
0

5000

0

0.5

10000

15000

20000

Speed of rotation min-1
1

dmn value ×106

1.5

2

We have improved the high speed performance of
cages for high speed angular contact ball bearings formachine tools over the conventional cages made ofpolyamide resin by adopting high melting point
polyamide resin and a new cage shape.
We have enabled cages made of newly developedpolyamide resin applicable for a higher speed range
where machined cages made of phenol resin havebeen traditionally used. We will introduce this cageusing ceramic balls with air oil lubrication for HSE
bearings to be used with dmn value of 1,600,000.
We strive to work on further improvement of
performance of the bearings for main spindles
contributing to higher speed, rigidity and reliability ofmachine tools.

NTN SE-1

Conventional cage material

25

Rise intemperature

5. Conclusion

Fixed position preload
Preload during assembly: 300 N

Developed cage material

Yes

Fig. 12 Test result under grease lubrication with steel ball

【Test condition】
Bearing
5S-2LA-HSE014DB (contact angle of 20˚)
under test
φ70×φ110×20 back-to-back assembly
PreloadNTN SE-1

Developed cage material

30

0

30000

Speed of rotation min-1

35

Fixed position preload
Preload during assembly: 300 N

25000

Photo of authors

2

Fig. 11 Test result under grease lubrication with ceramic ball-48-

Wakana INOUE

Mineo KOYAMA

Soichiro ONITSUKA

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division

NTN TECHNICAL REVIEW No.82（2014）[ Technical Article ]Bearing Technical Calculation System
for Machine Tool Spindles

Customer

Keiichi UEDA*
Yushi ONDA*
Shouhei HASHIZUME*

Data sentby email

Data received
by email

24 hour/7 day
supportIn recent years, machine tool and spindle manufactures have been developing various products which canmeet the new customer demands. To support the activities, NTN developed the bearing technical calculation
system for machine tool spindles. Machine tool and spindle manufactures can calculate various
characteristics like spindle rigidities and fatigue life of spindle bearings through the system in short time.
Features, mechanism and functions of the system are introduced below.

Therefore, NTN developed the Bearing TechnicalCalculation System for Machine Tool Main Spindles that
customers can use to automatically calculatecharacteristics of the bearings and main spindles in ashort time. This new tool will help our customers with
their designs and contribute to reductions in lead time
for development of machine tools.
In this paper, we present the features, configuration,and functions of this system.

1. Introduction
The main spindles of machine tools are an essential
element in determining the machining accuracy andproductivity of machine tools. Their critical properties,
such as rigidity, are determined by the type,arrangement, placement, preload, etc., of the bearings.
Therefore, when the manufacturers of machine tools
and manufacturers specialized in spindles (hereafter,"customers") design main spindles, they need to obtainspecifications of the bearings and properties of the main
spindles by calculation and verify if they satisfy thetarget objectives.
Since the these calculations are generally performedby the bearing manufacturers, they are required to
calculate the properties of multiple main spindle design
candidates quickly and in detail based on theinformation from the customers’ operating conditions,etc. In addition, materials, machining processes, andmachine tools users have become increasingly
diversified in the recent years with additional
requirements such as complex functionalities,
downsizing, and environment friendliness in addition to
the conventional requirements of high speed, highrigidity and high precision. In order to design main
spindles that meet these requirements, more reviews
and propositions are required with increasingly frequent
design opportunities.

2. Overview
The technical calculation system we developed cancalculate characteristics of 10 main spindle bearing
layouts of different types, arrangements and preload
methods. The overview is shown in Fig. 1.
The customer enters data required for calculationsuch as the bearing type and operating conditions into
the dedicated Excel ®※1 file and sends it to NTN’s
dedicated server by email. The server automatically
performs the calculation based on the input informationand immediately responds back to the customer withthe calculation results such as the rigidity of the main
spindle and bearing life. Traditionally, NTN required atleast 1 day ※2 from receipt of the calculation requestfrom the customer until the report with the calculation
results was sent back to the customer. However, withthis system, the response is sent back to the customer*Machining Tool/Aerospace Engineering Dept., Industrial Machinery Division※1 Excel ® is a trademark or registered mark of Microsoft Corporation in the United States of America or other countries.
※2 Required time for technical calculation of bearings for machine tool main spindles, in general, and may vary depending on the content
of bearing calculation.

-49-

NTN TECHNICAL REVIEW No.82（2014）

3.2 Configuration

within approx. 10 minutes of the customer’semailrequest.
In this system, the server handles all processes fromreceipt of the email requests to automatically sending
the email responses. Therefore, the time required fortransferring information between the customers and
NTN can be significantly reduced. Additionally,customers are free to submit as many calculations as
they desire, making it possible to narrow down main
spindle design plans very quickly. Furthermore, as theserver is able to work on a 24 hour, 7 days a weekbasis, customers can use the system without no time
restrictions.
Though the design process leading to the finalization
can be shortened by this system, contributing tosignificant development lead time reductions for main
spindles and machine tools, the final specifications ofbearings must still be determined by discussions
between the customer and NTN.

This system is shown in Fig. 1 and consists of thefollowing elements ① – ③ . Any customer with Excel ®
(Excel 97-2003 or later) and an email system can use
this system. No special settings, such as installation of
dedicated programs, is required for customers: one of
the benefits of this system.
① Dedicated server (installed at NTN)
② Dedicated program installed in ①
③ Dedicated Excel ® file for exchanging input/output
information between the customer and ①

3.3 Flow
The system flow is as follows: The sequence ① – ⑤
corresponds to ① – ⑤ in Fig. 1. After step ⑤ , thespecification of the bearing is finalized in a discussion
with the customer and NTN.

3. System features, configuration, and flow

① The customer enters required information into the dedicatedExcel ® file.

3.1 Features
Industry-first technical calculation system with
automatic processing② The customer attaches the file of ① to an email and sends
it to the NTN dedicated server.

(1) Fast technical calculation of bearings for main
spindles③ The NTN dedicated server automatically calculates the data.

10 minutes after sending a request email(traditionally, at least 1 day)④ Approx. 10 minutes after ②, an email is sent from the NTN
dedicated server to the customer. The Excel ® file attachedto the email contains the calculation result (specificationsof the bearings and properties of the main spindle
determined by them).

(2) Technical calculation of many types of main
spindle layouts
10 types of main spindle layouts can be processed(3) Automatic calculation within the dedicatedserver⑤ The customer confirms the calculation results and evaluates
if they satisfy the objective. If they do not satisfy theobjective, the customer can repeat ① to ④ as many times
as the customer wants.

24×7 operations(4) Simple communication of input/output data
Adoption of a practical Excel ® file

Customer①Create entry sheet¡Main spindle layout¡Bearing
model number

Send calculation results:
approx. 10 minutes② Sendinput sheet

Bearing TechnicalCalculation System
for Machine Tool Spindles③Automatic calculation
support⑤Design main spindle
based on output sheet¡Bearing property¡Main spindle property④ Receive
output sheet

Fig. 1 System flow

-50-

¡Receive email¡Automatic calculation
Automatic¡Send calculation
result to customer calculation

24 hour/ 7 day support

Bearing Technical Calculation System for Machine Tool Spindles

Table 1 Input and output items

4. System input/output[Items regarding main spindle]· Main spindle layout
Select from 10 layouts· Operating conditions
Speed of rotation, external load, etc.

4.1 Input/Output items
Table 1 shows input/output items for this system.
As described in the previous section, the customer
enters information in the predefined Excel ® file and thesystem calculates and responds with the calculation
results (output) to the customer.
In the input, the items and operating conditions
regarding the main spindles are entered. 10 types oflayouts can be selected for main spindles, as shown in
Fig. 2, so various design candidates for main spindles
can be created.
Preload, spring constant, and rolling contact fatigue
life of bearings, as well as rigidity during assembly(radial and axial direction) to the main spindles are
obtained as output. The customer verifies the result andevaluates if it meets their objectives.
Regarding the main spindle layouts, NTN is
expanding the number of types moving forward.

Input item

Output item

[Items regarding bearings]· Bearing model number, bearing position· Preload, space· Engagement· Amount of adjustment of inner/outer ring
spacer, etc.
[Bearing properties]· Preload, spring constant, rolling fatigue life
[Main spindle rigidity]· Rigidity during assembly (radial, axial)

Bearings on the front side
Bearings on theback side

Layout 6

Layout 1
Constant preload spring

Layout 2

Layout 7

Layout 3

Layout 8

Layout 4

Layout 9

Layout 5

Layout 10

Fig. 2 Spindle layout-51-

NTN TECHNICAL REVIEW No.82（2014）

4.2 Input/Output screen examples

to the output screen of the file in Fig. 4, andautomatically sends the file to the customer.
The customer can receive the calculation results
approx. 10 minutes after sending email to the server.

The customer selects layouts that need to be
examined from the main spindle layout group shown in
Fig. 2 and enters the required information into theshaded cells of the dedicated Excel ® file shown in Fig. 3.
The file has a function to check the entered items;
therefore, the customer can easily check if any enteredinformation is missing or wrong.
After entry of the required information, the customer
sends the file to NTN's dedicated server by email.
The server which receives this email automaticallycalculates based on the input information and enters
the calculation results, such as the main spindle rigidity,

5. Calculation examples
As a part of the output examples shown in Fig. 4, thecalculation results of main spindle axial rigidity whenbearings, arrangements and preload methods in Fig. 5
are examined against the main spindle layout 1 of Fig. 2
are shown in Fig. 6.
The time needed to narrow down design candidates

Fig. 3 Example of input sheet (Spindle layout 1, extract)

Fig. 4 Example of output sheet (Spindle layout 1, extract)-52-

Bearing Technical Calculation System for Machine Tool Spindles

Arrangement

Front side
Rear side
High speed angular ball
Cylinder rolling
contact bearings
bearings
5S-2LA-HSE020
N1018HSK
Two-row back-to-back (DB)―

Preload method

Fixed position (GL preload)

Bearing

Front side―

Axial displacement at origin µm

Axial displacement at origin µm

Fig. 7 Technical study example of spindle layout 4

14
12
10
8
6
4
2
1

1.5

2

Axial force kN
Axial rigidity of spindle

2.5―

Rear side

Front side

16

0.5

Fixed position (GL preload)

Rear side

18

0Preload method

Bearing

Fig. 5 Technical study example of spindle layout 1

0

Arrangement

Front side
Rear side
High speed angular ball
Cylinder rolling
contact bearings
bearings
5S-2LA-HSE020
N1018HSK
Four-row back-to-back (DTBT)―

3

Fig. 6 Example of calculation result(Axial rigidity of spindle layout 1)

18
16
14
12
10
8
6
4
2
0

0

0.5

1

1.5

2

Axial force kN
Axial rigidity of spindle

2.5

3

Fig. 8 Example of calculation result(Axial rigidity of spindle layout 4)

can be reduced, given that the calculation of propertiesof multiple main spindle design candidates can be
performed quickly. and comparedFor example, the rigidity can be quickly comparedwhen double-row back-to-back (DB) bearings areapplied to the main spindle of shaft diameter 100 mm
on the front side shown in Fig. 5 and four-row back-toback (DTBT) bearings in Fig. 7 are applied (mainspindle layout 4 ). In this case, it is observed that the
four-row back-to-back bearings shown in Fig. 8 havehigher rigidity than the two-row back-to-back bearings
shown in Fig. 6.

6. Conclusion
NTN developed the Bearing Technical Calculation
System for Machine Tool Spindles which can
automatically calculate properties of bearings and main
spindles in a short time, for precision rolling bearings to
be used for the main spindles of machine tools.This
system will support diverse product development ofmachine tool manufacturers and spindle manufacturers.
With this system, NTN is poised to provide fastertechnical service and obtain higher customer
satisfaction, as well as to contribute to reduced leadtime for machine tool development.
NTN will explore easy-to-use, useful, and user friendly
systems by the further improvement of this system.

Photo of authors

Keiichi UEDA

Yushi ONDA

Shouhei HASHIZUME

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division-53-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Paper ]Estimation Method for Friction Torqueof Air-oil Lubricated Angular Contact Ball Bearings
Hiroki FUJIWARA*
Angular contact ball bearings for high-speed spindles are lubricated by air-oil
lubrication. A main factor of bearing friction torque is rolling viscous resistance.
On the air-oil lubricated bearings, oil starvation leads to decrease in the rollingviscous resistance. In this article, the expression of deduction factor of rollingviscous resistance under the driving condition is proposed. It is derived bycomparing between rolling viscous resistances estimated from measured friction
torques of angular contact ball bearings under the air-oil lubrication and calculatedrolling viscous resistances under the full lubrication. The friction torque of the bearing is computed withconsideration for differential slip, spin, elastic hysteresis loss, cage guiding friction, and the starved rollingviscous resistance. The starved rolling viscous resistance is obtained for product of the deduction factor andthe rolling viscous resistance by the NTN original regression formula under the full lubrication.

The friction torque of rolling bearings using oil-bathlubrication can be calculated by considering differential
slip, spin, rolling viscous resistance, shear resistance ofthe oil film between the rolling element and cage
pocket, etc. 1).
However, this calculation method is applicable only tothe case where lubricating oil is abundant and cannot
be applied to air-oil lubrication as its friction torque is
1/10 to 1/5 that of bearings with sufficient lubrication.
There are no reports that examine the estimation offriction torque for air-oil lubrication. Although there areattempts to theoretically estimate the starvation statusof the ball pushing through the lubricating oil on the
raceway, given certain assumptions of lubricating oil
quantity 2), 3), it is difficult to accurately find the
lubricating oil quantity on the raceway in air-oil
lubrication. Therefore their application to practical use ischallenging.
In this paper, we propose an estimation method for
friction torque of air-oil lubricated angular contact ballbearings in their unique starvation status. The friction
torque of these bearings consists of differential slip,
spin, rolling viscous resistance, elastic hysteresis loss
and friction between the cage guiding surface and therolling element.
We considered that the lubricating oil film of thesebearings produced a level of no mixed boundary
lubrication, assumed that the friction torques due todifferential slip and spin are not affected by the

1. Introduction
High speed spindles are required to contain heat inorder to reduce thermal expansion of the main spindle.
Air-oil lubrication has been adopted to reduce torque ofthe bearings, therefore reducing heat generation. Air-oil
lubrication enables accurate control of lubricating oil
quantity by adjusting the plunger pump to supply theminimum required lubricating oil directly to the raceway
surface by suspending the oil in air. Through thisoperation, stirring resistance of the lubricating oil can bealmost eliminated and the lubricating oil quantity
between the rolling element and raceway is reduced tothe bare minimum.
Lubricating oil above a specific amount at the inlet ofthe elastohydro dynamic lubrication (hereafter, EHL)
contact between the rolling element and raceway
presents sufficient lubrication where the thickness of the
lubricating oil film does not increase anymore. However,from the viewpoint of damage control, sufficientlubrication is not necessarily required. Only oil film large
enough to separate the two surfaces from contact isrequired. Since the rolling viscous resistance, which is
the main element of friction torque, is affected by the
lubricating oil quantity at the EHL contact inlet, therolling viscous resistance can be reduced by actively
creating insufficient oil quantity, called "starvation", tothe level of having no oil film between the rolling
element and raceway breaks.
*Advanced Technology R&D Center-54-

Estimation Method for Friction Torque of Air-oil Lubricated Angular Contact Ball Bearings

T0 ：Standard value on the ball pass cycle
U ：Dowson's dimensionless parameter on velocity
W ：Dowson’s dimensionless parameter on the
point contact load
α0 ：Viscosity-pressure factor of lubricating oil
α ：Contact angle
β ：Inclination of the ball’s axis of rotation
δ ：Distance from the pure rolling point to thecontact point
μt ：Coefficient of traction
μt max ：Maximum coefficient of traction
ν ：Kinematic viscosity of lubricating oil
ν0 ：Standard value on the kinematic viscosity of
lubricating oilτ ：Shear stress due to Couette flow of contact area
between the ball and raceway ring
φr ：Reduction factor of rolling viscous resistance
φre ：Reduction factor of rolling viscous resistance(experimental value)ω ：Angular velocity
ω0 ：Standard value on angular velocity
ωb ：Rotational angle of the ballstarvation, and applied an experimentally obtained
reduction factor to the rolling viscous resistance. This
reduction factor is proposed as an equation
experimentally identified comparing the experimental
value of the friction torque with the calculated value of
sufficient lubrication.

2. Symbols
a ：Long axial radius of contact ellipse
a0 ：Standard value on the long axial radius of contact
ellipseC ：Reduction factor constant of rolling viscous resistance(includes standard values of the parameters)
C’ ：Reduction factor constant of rolling viscous resistancedp ：Pitch circle diameter of a ball
F! ：Traction generated inside differential slip
F@ ：Traction generated outside differential slip
FHD ：Dimensionless rolling viscous resistanceFr ：Rolling viscous resistanceFs ：Traction
G ：Dowson's dimensionless material parameterk ：Ellipticity of contact ellipse
l! ：Length of the contact point and pure rolling pointin the radius direction inside the differential slip
l@ ：Length of the contact point and pure rolling pointin the radius direction outside the differential slip
M ：Moment around rotational axis on the ball
Mr ：Friction torque of the bearing due to friction between
the ball and raceway in the rolling direction
m ：Moment due to F! and F@
Pb ：Component force of the oil film reaction force on
the ball in the rolling direction
Pe ：Component force of the oil film reaction force in therolling direction in the equivalent system
P1 ：Component force of the oil film reaction force appliedon object 1 (ball) in the rolling direction
P2 ：Component force of the oil film reaction force appliedon object 2 (raceway) in the rolling direction
r1 ：Radius of object 1 (ball)r2 ：Radius of object 2 (raceway)
rb ：Radius of the ballrb' ：Distance from the center of the ball to the purerolling point
re ：Equivalent radius
S! ：Inside area of differential slip of inner ring contact
S@ ：Outside area of differential slip of inner ring contact
s ：Slip ratio
sm ：Slip ratio to give the maximum coefficient of tractionsT ：Index on the ball pass cyclesν ：Index on the kinematic viscosity of lubricating oil
sα ：Index on the long axial radius of contact ellipsesω ：Index on angular velocity
T ：Pass cycle of the ballSubscriptsio：Inner ring
：Outer ring

3. Static analysis of angular contact ballbearings with only preload
High speed spindle angular contact ball bearings are
typically arranged with 2-rows or 4-rows on the front
side and used with fixed position or constant pressure
preload. Although minor radial load may be applied
during operation depending on the use of spindles, we
ignored the effect of the radial load and assumed pureaxial load with only preload applied to discuss the
motion of a ball. Regarding the static analysis of angularcontact ball bearings, Fujii has provided a detailed
review 4) and Fujii has determined rotational axis of theball based on Jones' control ring theory 5). With Jones’
theory, the ball only rolls on the raceway of the ringwhich has larger friction and only spin occurs on theother raceway. However, in actuality, the rotational axisof the ball should be determined by the way that theforce and moment are balanced and the consumed
energy is minimized. In the following sections, we will
discuss hydrostatic movement of the ball considering
additional force due to slippage.

3.1 The force and moment produced due toslippage between the ball and raceway
Fig. 1 shows the schematic diagram of an axial crosssection of one ball and the inner/outer ring indicating theforce due to slippage within the contact ellipse.
-55-

NTN TECHNICAL REVIEW No.82（2014）

When a tangent force is applied to the ball as shownin Fig. 1, the force can be obtained with the following
equation:∫∫Fs,o =∫
τo dS −∫
τo dS

Fs,i = τi dS@− τi dS!
@

The moment of the ball around the pure rolling pointdue to slippage can be calculated with the following
equation:∫∫…………………… (3.2)
mo =∫τo l dS ＋∫
τo l dSmi = τi l!dS!＋ τi l@dS@…………………… (3.1)

!

!

@

@

The moment of the ball around its rotational axis due
to slippage can be expressed as follows:mo∫…………… (3.3)
Mo =∫(r’b＋δ)τo dS = rb Fs,o−mo
Mi = (r’b＋δ)τi dS = rb Fs,i−mi

F!,o

F@,o
ωb

From Fig. 2, the peripheral force equilibrium on theball is:

F@,i

F!,i

!

Fs,i−Fr,i＋Pb,i−Fs,o＋Fr,o−Pb,o = 0 ………… (3.4)

miand as Zhou-Hoeprich 6) states, the rolling viscous
resistance has the following relation:

Fig. 1 Direction of forces and moments from slip

Pe re = Pl rl = P2 r2 = 2Fr re …………………… (3.5)
Therefore, it can be rewritten as follows:

re,i
re,o
Fs,i−Fr,i＋2 r Fr,i−Fs,o＋Fr,o−2 r Fr,o= 0 … (3.6)

The moment of the ball around the pure rolling pointdue to slippage can be calculated with the following
equation:bb

The equilibrium of the moment around the rotational
axis of the ball can be expressed,∫∫…………………… (3.2)
mo =∫τo l dS ＋∫
τo l dSmi = τi l!dS!＋ τi l@dS@

rb cos (αi−β) (−Fr,i＋Fs,i )＋rb cos (αo−β) (−Fr,o＋Fs,o )@−mi cos (αi−β) −mo cos (αo−β) = 0 ……… (3.7)

The moment of the ball around its rotational axis due
to slippage can be expressed as follows:

and from equation (3.3), it can be rewritten as follows:∫…………… (3.3)
Mo =∫(r’b＋δ)τo dS = rb Fs,o−mo＋Mi cos (αi−β) ＋Mo cos (αo−β) = 0 ……… (3.8)

!

!

@

rb cos (αi−β) (−Fr,i )＋rb cos (αo−β) (−Fr,o )Mi = (r’b＋δ)τi dS = rb Fs,i−mi

F!,o

F@,o

mo

Fr,o
Pb,o
Fr,i

Pb,i

ωb

F@,i
F!,i

mi

ωi

β

αi
Fig. 2 Forces and moments on a ball

-56-

αo

Estimation Method for Friction Torque of Air-oil Lubricated Angular Contact Ball Bearings

The moment applied to the outer ring of Fig. 3 canbe expressed by Equation (3.9):

4. Starvation effect to rolling viscous
resistance( d2 ＋ r cosα ) (F ＋F )−m cosαdd
F ＋M cosα=(＋ r cosα ) F ＋
2
2

Mr =

p

p

b

o

r,o

b

o

r,o

s,o
p

o

s,o

4.1 Theoretical regression of rolling viscous resistanceoo

The main undertaking of EHL is to study oil film
thickness. Hamrock-Dowson 8), Chittenden et al. 9) andothers are proposing many theoretical regression
equations. On the other hand, only a few reports exist
on rolling viscous resistance, and with regard to line
contact, no equations based on EHL theory are foundexcept for Zhou-Hoeprich's equation 6). Also, withregard to point contact, few reports exist on equations
to easily obtain rolling viscous resistance. The equation
proposed by Houpert 10) lacks material parameter and noconsideration was given to a viscosity-pressure factor. It
is natural to think that the rolling viscous resistance isaffected by the viscosity-pressure and therefore we do
not adopt Houpert’s equation in this paper.
Fujiwara is proposing the theoretical regression
equation regarding point-contact rolling viscous
resistance by defining dimensionless rolling viscous
resistance with Equation (4.1), and summarizing it as
Equation (4.2) using a point-contact dimensionless
number of Dowson, etc. 11).

o…………………… (3.9)
This is the friction torque of the bearings generated
by the contact between the ball and raceway.

Fr,o
Fs,o = F@,o−F!,omoFig. 3 Forces and moment on the outer race used to
calculate outer ring friction torque

FHD =

Frαo
re2

………………………………… (4.1)

FHD = 44.6U 0.694G 0.961W 0.401｛1−0.962exp (−0.818k)｝………………………………… (4.2)
We used the equation for rolling viscous resistanceunder sufficient lubrication to obtain the rolling viscous
resistance under air oil lubrication by multiplying a
reduction factor to account for the lubricant starvation.

The rotational speed, orbital speed and the inclinationof the rotational axis of the ball are determined so thatthe force and moment of the ball are balanced and thefriction torque is minimized.
By determining the behavior of the ball in a certain
state, the slip distribution and rolling viscous resistancecan be calculated. The tangent force is the contactpressure at the observation point multiplied by thecoefficient of traction. The coefficient of traction can begiven as the function of slip ratio.
We adopted Lee-Hamrock’s circular model 7) shownin Equation (3.10).

μt =s / sm1＋ (s / sm ) 2

4.2 Reduction factor of rolling viscous resistance
When a ball passes on the raceway, lubricating oil on
the raceway is pushed away by the ball but moves backto the center of the raceway before the next ballpasses, thus the lubricating oil quantity at the EHL oil
film inlet is recovered. It is considered that the recovery
quantity of lubricating oil is affected mainly by the ballpassage cycle T, kinematic viscosityν, and long axial
radius of contact ellipse a . The longer the passage
cycle of ball T, the larger the recovery amount. The
smaller the kinematic viscosityν, the greater therecovery amount because lubricating oil can move
more easily. The smaller the contact ellipse long axial
radius of the ball, the greater the recovery amountbecause the distance the lubricating oil has to move isshorter. It is also possible that centrifugal force on the
lubricating oil may also affect the recovery amount. Witha large centrifugal force, lubricating oil moves outward
on the raceway and therefore the recovery amount isreduced on the contact area on the inner ring side.

=μt max ……………… (3.10)

In this model, the maximum coefficient of traction
μmax and the slip ratio sm must be assumed so we haveset μt max=0.05 and sm=0.003.
The rolling viscous resistance is calculated
considering the starvation effect which is discussed indetail in the next section. The numerical calculationconsiders the forces due to this slip and rolling viscous
resistance for a convergent calculation.

-57-

NTN TECHNICAL REVIEW No.82（2014）

In addition to rolling viscous resistance, other factors
to impact the friction of rolling bearings are differential
slip, spin and elastic hysteresis loss. The behavior of
the ball is determined by the balance between traction
due to slip produced by various frictions and theresistance against those frictions. Therefore, resistancecannot be separated into these various factors.
However, we consider that the sum of the friction
torques given by each factor is the friction torque of the
bearings. In this study, in order to identify the reduction
factor of rolling viscous resistance, we compared thefriction torque obtained from the experiment removing
the calculated friction torques of differential slip, spin,
elastic hysteresis loss and friction within the cage
guiding surface, and the friction torque due to rollingviscous resistance obtained by calculation assuming
sufficient lubrication. We defined the value of the rollingviscous resistance obtained from the experimental data
divided by the calculated value of sufficient lubrication
as the experimental reduction factor of rolling viscous
resistanceφre . We determine C, sT, sν, sa, sω ofEquation (4.4) so that the square of the difference
betweenφr andφre becomes the smallest. We canthen obtain mathematically precise values if we conductmultiple linear regression. However, for the purpose ofthis report, due to the accuracy of the experiment andassumptions given, we do not have to define accurate
values. Instead integers or similar values such as0, ±1/2, ±1, ±2, …can be selected for sT, sν, sa and sω.
For C, it is sufficient to round to two significant figureson the average of ratios betweenφr andφre when C=1
is assumed.
Skipping the details of the data, we identifiedφr under
the above considerations using experimental data offriction torque by varying the bearing size, ball material,
speed of rotation, preload and lubricating oil viscosity,
we could obtain the following equation:

However, the recovery amount is increased on theouter ring side because the lubricating oil is forced tomove toward the center of the raceway. Since it is notpossible to separate the torque produced at the innerring and outer ring by the experiment, the impact of the
reduction of torque at the inner ring side offset by theincrease of torque at the outer ring side is consideredusing angular velocity ω. Although lubricating oil
quantity affects starvation, in the case air oil lubrication,
the impact was determined to be relatively small as aresult of the experiment shown in Fig. 4, so we ignore
will it in this discussion.

Friction torque N・m

0.4

Bearing: inner diameter 100 mm, ceramic ball specification
Preload: 2360 N
Lubricating oil: ISO VG32

0.3

0.2
20000 min-1

0.1

015000 min-1
10000 min-1
0

0.5

1

1.5

Lubricating oil quantity mL/h

2

Fig. 4 Effect of oil quantity on friction torqueThe reduction factor of the rolling viscous resistanceis a function of the amount of lubricating oil recovered
and therefore can be considered to be as a function of
the ball passage cycle T, kinematic viscosityν andlong axial radius of contact ellipse a . The reduction
factor for the rolling viscous resistance can have avalue between 0 and 1 and it is adequate to consider
that each parameter has a standard value when
determining the starvation factor 1. Therefore, we
define the reduction factor of the rolling viscous
resistance in the following equation:
Sa

Sν

S

φr = 3.5 × 10-11

This matches with the physical perception that thelonger ball passage cycle T makes largerφr because it
allows longer time for recovery of lubricating oil, smaller
kinematic viscosity ν makes largerφr because it allows
lubricating oil to move quickly for recovery, and smaller
contact ellipse long axial radius a makes largerφr
because the distance lubricating oil needs to move forrecovery is shorter. In addition, ω which we introduced
to consider the impact of centrifugal force wasnegligible. Since the coefficient 3.5 x 10-11 includes T0,
ν0 and a0, it has a dimension of [m4/s1.5].

Sω

T Tν a ω
φr = C’…………………… (4.3)
T0 ν0 a0 ω0Where, when T＞T0, T = T0
whenν＞ν0, ν=ν0
when a＞ a0, a = a0
when ω＞ω0 ω = ω0However, since it is difficult to obtain the standard
values of T0，
ν0，a0 andω0, by experiments, Equation (4.3)
is converted to Equation (4.4) to identify C, ST, Sν, Sa, Sω.

φr =

C’
S

S

Sν

Sa

Sν

Sa

Sω

T0 T ν0 a0 ω0SSω=C T T ν a ω

Sν

Sa

T 0.5
………………………… (4.5)
νa 2

Sω

T Tν a ω………………………… (4.4)-58-

Estimation Method for Friction Torque of Air-oil Lubricated Angular Contact Ball Bearings

friction torques, and added them to the final friction
torque. We referred to Kakuta’s analysis 12) for the
calculation of elastic hysteresis loss and calculated thefriction within the cage guiding surface as the boundary
friction caused by the centrifugal force. The effect of theelastic hysteresis loss and the friction within the cage
guiding surface is small. The friction torque produced
between the ball and raceway ring accounts for 60 90% of the overall friction torque.
Fig. 5 shows the experimental and calculated valuesof friction torque for comparison. Fig. 5 (a) shows theresults of angular contact ball bearings with an innerdiameter of 70 mm, using ceramic balls, a standard

5. Comparison of the calculated value
and experimental value of friction
torque considering starvation
We calculated the rolling viscous resistance using the
reduction factor obtained in section 4.2 and obtained
the friction torque of the rolling bearings considering
impact of slip, etc. We compared this calculated value
with the value from the experiment. Although the friction
force of elastic hysteresis loss and friction within thecage guiding surface should also be included in thesimultaneous equation, their impact is relatively small.
So we obtained them individually, converted to the

0.3
0.2
0.1
5000

10000
15000
20000
Rotational Speed min-1

0.2
0.1
5000

10000
15000
20000
Rotational Speed min-1

25000

Bearing: inner diameter 70mm
Ceramic ball specification
Preload: 1470N Lubricating oil: ISO VG68

Bearing: inner diameter 70mm
Steel ball specification
Preload: 1470N Lubricating oil: ISO VG32

0.5

Experimental
Calculated

0.2
0.1
5000

10000
15000
20000
Rotational Speed min-1

Experimental
Calculated

0.4
0.3
0.2
0.1
0

25000

0

5000

10000
15000
20000
Rotational Speed min-1

25000(c) Comparative example 3(d) Comparative example 4

Bearing: inner diameter 100mm
Ceramic ball specification
Preload: 2360N Lubricating oil: ISO VG32

Bearing: inner diameter 70mm Ceramic ball specification
Rotational speed: 1800min-1
Lubricating oil: ISO VG32

0.5

0.4
0.3
0.2
Experimental
Calculated

0.1
0
0

0.3

(b) Comparative example 2

0.3

0.5

Experimental
Calculated(a) Comparative example 1

0.4

0
0

Bearing: inner diameter 70mm
Ceramic ball specification
Preload: 1470N Lubricating oil: ISO VG10

0.4

0
0

25000

Friction torque N・m

0.5
Friction torque N・mFriction torque N・m

0.4

0
0

Friction torque N・m

0.5

Experimental
Calculated

Friction torque N・mFriction torque N・m

0.5

Bearing: inner diameter 70mm
Ceramic ball specification
Preload: 1470N Lubricating oil: ISO VG32

5000

10000
15000
20000
Rotational Speed min-1

0.3
0.2
0.1
0
0

25000

(e) Comparative example 5

Experimental
Calculated

0.4

1000

2000
Preload N(f) Comparative example 6

Fig. 5 Comparative example of experimental results to calculated values of friction torque

-59-

3000

NTN TECHNICAL REVIEW No.82（2014）

Referencespreload applied, and lubricated with ISOVG32
lubricating oil. The calculated values are a little smaller
than the experimental values however they are roughly
consistent. Fig. 5 (b) and (c) are the results withdifferent kinematic viscosity of lubricating oil. Fig. 5 (d)shows the results with steel balls and Fig. 5 (e) is theresult with a different bearing size. Fig. 5 (f) shows thecomparison with a modified preload. In all cases, thecalculated values duplicated the experimental values
with accuracy sufficient for practical application.

1) Hiroki Fujiwara, Kenji Fujii: Rolling Bearing Torque in Oil
Bath Lubrication, Japan Society for Precision
Engineering Spring Conference Academic Lecture
Proceedings (2002), 219.
2) Takashi Nogi: An Analysis of Starved EHL Point
Contacts with Reflow, Tribologist, 59 (2014) 239-250.
3) Kenichi Shibasaki, Masato Taniguchi, Marie Oshima:
Development of Numerical Method for Coupled
Simulation of Starved EHL and Macro Flow,
Proceedings of Japan Tribology Conference Tokyo(2010-5), 245-246
4) Kenji Fujii: Research on High-Speed Rolling Bearings,
Meiji University, Doctoral Dissertation (2001).
5) A. B. Jones: A General Theory for ElasticallyConstrained Ball and Radial Roller Bearings under
Arbitrary Load and Speed Conditions，Trans. ASME J.
Basic Eng., 82 (1960) 309-320．
6) R. S. Zhou and M. R. Hoeprich: Torque of Tapered
Roller Bearings, Trans. ASME, J．Tribol., 113 (1991)
590-597．
7) R. T. Lee and B. J. Hamrock: A Circular Non-Newtonian
Fluid Model: Part I – Used in Elastohydrodynamic
Lubrication, Trans．
ASME, J. Tribol., 112 (1990) 486-496．
8) B. J. Hamrock and D. Dowson: Isothermal
Elastohydrodynamic Lubrication of Point Contacts, Part
2 - Ellipticity Parameter Results, Trans. ASME, J. Lub.
Tech., 98 (1976) 375-383．
9) R. J. Chittenden, D. Dowson, J. F. Dunn and C. M.
Taylor: A Theoretical Analysis of the Isothermal
Elastohydrodynamic Lubrication of Concentrated
Contacts II. General Case, with Lubricant Entrainment
along Either Principal Axis of the Hertzian Contact
Ellipse or at Some Intermediate Angle, Proc. R. Soc．
Lond., A 397 (1985) 271-294．
10) L. Houpert: Piezoviscous-Rigid Rolling and Sliding
Traction Forces, Application: The Rolling Element-Cage
Pocket Contact, Trans. ASME, J. Tribol., 109 (1987)
363-371．
11) Hiroki Fujiwara: Rolling Viscous Resistance of Point
Contact EHL, Proceedings of Japan Tribology
Conference Tokyo (2009-5), 119-120.
12) Kazuo Kakuta: Friction Moment of Radial Ball Bearings
under Thrust Load, Transactions of the Japan Society of
Mechanical Engineers 27-178, 3 (1961) 945-956.

6. Conclusion
We proposed an estimation method for friction torqueof air-oil lubricated angular contact ball bearings. Since
air-oil lubrication does not achieve a sufficientlubrication condition, we used experimental values forfriction torque to identify the reduction factor of therolling viscous resistance. This was done by comparing
the rolling viscous resistance estimated from theexperimental values of friction torque and the calculated
values of rolling viscous resistance of sufficientlubrication. The friction torque of air-oil lubrication canbe calculated by solving the equilibrium of forces andmoments considering the rolling viscous resistancemultiplied by the reduction factors, differential slip,
traction produced by the spin, and adding the elastic
hysteresis loss and friction within the cage.
This technology can be used not only for improving
accuracy of the selection of high-speed bearings andpreloads, but also for contributing to further
development and improvement of high-speed bearing
and spindle design.

Photo of author

Hiroki FUJIWARA
Advanced Technology
R&D Center-60-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Paper ]Real-time Monitoring of the Superfinishing Process

Kohei HIGASHI*
Hajime ISA*
Takakazu KITAGAWA*
Takashi ONISHI**
Kazuhito OHASHI**
The advantage of superfinishing is that, the fine surface of workpiece can beobtained by the transition of machining states from cutting action to finishing
one, while the condition of superfinishing stone surface changes selfsharpening to wear during one processing cycle. In conventional superfinishingprocess, to obtain the fine surface, the superfinishing stone, machining
condition and timer control have been determined by empirical laws. In this study, we have monitored themachining force during machining process, and have judged the roughness of workpiece by calculating
the machining force ratio.

In view of these issues, the authors developed atechnology to monitor the superfinishing process in realtime 1), 2). In this study the in-process monitoring
technology was controlled to determine the finish
accuracy over a preset threshold, focusing on themachining force ratio, which is the ratio of the thrust andprincipal forces applied to the grinding stone. In
addition, the improvement over a conventional
machining cycle based on fixed time was examined andis included in this report.

1. Introduction
Rolling bearings (hereafter, bearings) are said to be
the “central core” of the machine industry and to havesupported the growth of Japan's manufacturing industry(such as automobile and machine tools) with highperformance and consistent quality. Bearings arealways required to provide high quality, low cost andshort lead time as machine components for highvolume production. This is the reason for thedesignation “central core” and production technologies
to achieve these requirements are also required.
A high level of performance is required for thesebearings from three basic elements: low torque, low
noise and long life. Superfinishing machining
technology (hereafter, “superfinishing"), which ispresented in this paper, is a very important machining
process in bearing manufacturing, has a significant
impact on these three elements of bearing
performance.
Conventional superfinishing has typically involved
selecting grinding stones and machine conditions basedon experience and determining machining time basedon desired accuracy. In addition, the machining
requirements are affected by the accuracy of the priorprocess and the condition of the grinding stones.

2. Characteristics of superfinishing andmachining models
2.1 Characteristics of superfinishing
Superfinishing is a machining process in which astick-shaped, fine-grained stone with a relatively low
hardness is applied to a work piece under a certain
pressure with oscillating motion, rotational motion andfeed motion between the stone and the work piece anda large amount of processing solution is supplied 3).
Fig. 1 shows the transition cycles of the stone during
superfinishing. An important characteristic ofsuperfinishing involves the initial contact between thework piece and the stone and subsequent relative
motion which creates new cutting edges. This action is**Production Engineering R&D Center**Graduate School of Natural Science and Technology, Okayama University, Division of Industrial Innovation Sciences-61-

NTN TECHNICAL REVIEW No.82（2014）

2.2 Superfinishing model

called self-dressing of the grinding stone.
Fig. 1 (a): The initial stage of the cycle is called thegrinding state, where the stone removes the top roughsurface of the work piece while the stone also selfdresses, smoothing the work piece.
Fig. 1 (b): The final stage of the cycle is called thepolishing state, where self-dressing of the stone
concludes and the stone becomes loaded and there isa rapid improvement in the roughness of the workpiece.
Fig. 1 (c): A new work piece is introduced whichcauses rapid self-dressing of the grinding stone due tothe roughness of the new work piece. This overall cycle
repeats. Although required in normal grinding, thegrinding stone is not dressing in the superfinishingprocess.

Fig. 2 shows a general model of the superfinishingprocess. Pressure is applied vertically to the grindingstone which oscillates perpendicularly to the rotation of
the cylindrical work piece. When focusing on themachining action of the grinding stone, it moves in asinusoidal motion on the surface of the work piece. The
maximum velocity vector on the below sinusoidal curveis called "grinding directional angleθ" and is expressedby Equation (1).θis an important indicator ofsuperfinishing processing conditions which is commonlyused.
a ： Grind stone oscillation amplitude (mm)f ： Grind stone oscillation frequency (c/min)
D： Work piece diameter (mm)
N： Work piece rotational speed (min-1)

For machining accuracy, superfinishing produces agood non-directional surface roughness due to thesmaller amplitude motion and lower machining force
than ordinary cutting or grinding. In addition, since
superfinishing generates less heat during machining, it
minimizes the affected work layer. As a result thisprocess ensures the machined raceway surface is
acceptable for bearings with high wear resistance.

Oscillation

θ=tan-1 (af/DN ) ………………………………（1）In theory, a stronger grinding action results from alargerθand a stronger polishing action results from asmallerθ.
However, in reality, since there are countless grainson the grinding stone, a smooth and quality finish canbe obtained.

PressureφPressure

Grind stone
Work piece

πa ff(a) Grinding state

Loadeda

Stone

Work piece

Polishing surface

θ
Vs
Vmax

(b) Polishing stateFig. 2 Superfinishing model

Pre-processedsurface

Loadedπa fcosφ(c) Self-dressing

Fig. 1 Mechanism of superfinishing cycle

-62-

Vw

Real-time Monitoring of the Superfinishing Process

2.3 A superfinishing example

Coolant
nozzleSizing sensor

The raceway, in contact with the bearing rolling
elements (balls, rollers, etc.), is required to have goodsurface roughness and quality surface properties,
particularly with no surface protrusions, to minimize
noise.
As an example, Fig. 3 shows a comparison of theinner ring raceway surfaces of a deep-groove ballbearing after grinding and after superfinishing (partnumber 6203 with envelope dimensions of: ID = 17 mm,
OD = 40 mm & width = 12 mm). An after grinding
surface roughness of Ra 0.3 was improved to Ra 0.03
after superfinishing with good surface properties closeand no significant protrusions.

Three-component
force sensor

Work piece

Oscillation
devicez

Grind stoneLaser
displacement
sensor

Stone holderyx

Air cylinder

Fig. 4 Experimental setup
Table 1 Sensing device and evaluation points
Sensing device

Displacement

Evaluation item
Stock removal(2) Laser displacement
sensorDimension change
Advancement ofgrind stone table(3) Three-component
force sensor

Grinding resistanceagainst grind wheel(1) Sizing sensor

Wear of stone
Thrust force
Principal force
Force on the oscillating direction

Ra 0.03

Ra 0.3

4. Investigation of superfinishingmachining characteristics
After grinding process

4.1 Test conditions
After superfinishing process

To understand the machining characteristics ofsuperfinishing and to estimate the formation process ofthe finished surface, an experiment was conductedunder the test conditions shown in Table 2.
The test used a grinding stone wheel (WA1200) witha thickness of 5 mm and the work piece was made frombearing steel pre-processed to a cylindrical shape witha surface roughness of Ra 0.3 – 0.4. In addition, waterinsoluble grinding fluid was used as the coolant. The
grinding directional angleθselected was 3.2˚ andbased on experience is positioned between grinding
and polishing processing, allowing for continuoustransitions from the grinding state to the polishing state.
The machining time was set at 20 seconds max. andallowed an arbitrary 2 – 20 seconds to find themachining state during the process.

Fig. 3 Improvement of surface roughness

3. Experimental setup
Fig. 4 shows the experimental apparatus used forthis study. In order to model the setup shown in Fig. 2,
a cylindrical stepped round bar was selected as thework piece. The work piece is supported on both sides
and rotated by the lathe shaft. The grinding stone isheld by a fixture and pushed horizontally against thework piece by an air cylinder in a direction
perpendicular to the work piece. The grinding stone,
holding fixture and air cylinder are attached to a table
which oscillates axially with respect to the work piece.
Table 1 lists the sensing devices used in this study tomonitor the machining process and the items that were
evaluated. They include a sizing sensor to measure thework piece diameter, a laser displacement sensor tomeasure the movement of the grinding stone table as
an indication of the stone wear, and a three-component
force sensor, installed between the stone and the air
cylinder, to measure the machining force applied to thestone. In this study, these sensors were used todetermine the finish accuracy by monitoring themachining force applied to the grinding stone.

Table 2 Experiment condition in one step
Grinding stone
Work piece
Grinding stone oscillating amplitude
Cooling flow
Grinding stone oscillating frequency
Shaft rotational speed
Grinding direction angle
Pressure on grinding stone
Machining time

-63-

Spec：WA1200 20A
Size：t5×W30 mm
SUJ2 φ20（HV770）0.75mm2.0L/min (Water-insoluble)
1870 c/min
1230 min‐1
3.2˚
0．54 MPa
2〜20s

NTN TECHNICAL REVIEW No.82（2014）

4.2 Test results

Work piece

Fig. 5 shows a diagram of the sensing devices in themachining area including forces and distances. Fig. 6
shows the monitoring data over 20 seconds ofmachining under the test conditions defined in Table 2
with the elapsed time on the horizontal axis. The top
figure shows a plot of the advancement of the grind
stone stand d and stock removal R. The bottom figure
shows the plot of the thrust force P and principal force Q
applied to the grinding stone.
The movement of dimension d barely changes after 3
seconds from the beginning of machining. Therefore, itis theorized that the grinding stone became loaded andthe machining state transitioned to polishing. Looking at
the change in the machining force, the thrust force P isconstant at 80 N as the pressure to the stone isconstant; however, we could verify that the principal
force Q gradually decreases after 3 seconds.
Focusing on this change, the machining force ratio
P/Q (hereafter “force ratio”) was added to the plot. It
was observed that after a brief decrease, the force ratio
gradually increased over time. This trend is theorized tobe related to the roughness of the finishing surface.

Grinding stone

R/2

Laser
displacement
sensorP

In-process
gauge

d

Q

Fig. 5 Diagram of sensing devices in machining area

4.3 Formation process of a finished surface
In order to understand the roughness formation
process on a finished surface, the relationship between
force ratio and roughness was examined. Experiments
were conducted under 7 different conditions withmachining times of 2 – 20 seconds under the test
conditions shown in Table 2. As a result, therelationship between the machining force ratio (after
machining is completed) and the roughness of thefinished surface was determined. The results from eachexperiment are shown in Fig. 7. As the machining timeincreased, the machining force ratio increased while theroughness of the finished surface reduced. By
calculating the correlation coefficient between the force
ratio and the surface roughness at 7 points, a very highcorrelation of -0.97 was obtained. Therefore, it is highly
probable that roughness of the finished surface can be
determined by monitoring the machining force ratio.

Stock removal

0.2

5Advancement of grinding wheel
0
6

0
100
80

5
Thrust force P

60
40

3

P/Q

20
10

4

2

Principal force Q
0

5

10

Time sStock removal µm

10Machining force ratio P/Q

Machining resistance N

Advancement ofgrinding wheel mm

0.4

1
20

15

Correlation coefficient：-0.97

0.4

5
4

0.3
Machining force ratio
0.2

3

0.1

2

0

Roughness of finished surface
0

5

10

Time s15

Machining force ratio

Roughness of finished surface µm

Fig. 6 Machining characteristics during one stage processing

1
20

Fig. 7 Relationship between machining force ratio andsurface roughness-64-Real-time Monitoring of the Superfinishing Process

the end of the machining cycle (20 seconds total), theforce ratio was slightly below 4 and the roughness ofthe finishing surface (Ra) was 0.052.

5. Machining characteristics underforced state transition
5.1 Test conditions

Work piece
Stone oscillating amplitude
Coolant flow
Machining condition
Stone oscillating frequency
Main shaft rotational speed
Grinding direction angle
Stone pressure
Machining time

Advancement of grinding wheel

5

Stock removal

0100

Thrust force P

80

0
6
5

60

4

P/Q

40

Principal forceQ

20

3
2

Grinding
5

1µm

0

1mm

10

Time sPolishing15

Stock removal µm

10

0.2

10

Table 3 Test conditions in two steps
Stone

0.4

Machining force ratio P/Q

Machining force N

Advancement ofgrinding wheel mm

When using the superfinishing process during
normal production, it is necessary to remove the workpiece’s preprocess surface (stock removal) andachieve final finish accuracy. To meet theserequirements, it is common to program machining
conditions for both grinding and polishing and havethe cycle changed sequentially by the timer. That way
stock removal is ensured and the stone is forced to beloaded for polishing.
Therefore, the conditions were set for the test
machine grinding and polishing processes in two stages
and the machining characteristics were investigated.
Table 3 shows the experimental conditions. The
grinding stone, coolant and work piece were equivalent
to those shown in Table 2 and the wheel swing speed
was set to 670 min-1 for both grinding and polishing.
The grinding directional angle was changed by reducing
the stone oscillation frequency from 1,870 c/min (cycles
per minute) for grinding to 500 c/min for polishing toconstitute the forced transition condition from grinding topolishing. The machining time for both grinding andpolishing each lasted 10 seconds.

1
20

Ra 0.052
Rz 0.649

Fig. 8 Machining characteristics during two stage processing

Spec：WA1200 20A
Size：t5×W30 mm
SUJ2 φ20mm（HV770）

6. Determination of finishing accuracy
via threshold

0.75mm2．0L/min (Water-insoluble)

Grinding (1st step) Polishing (2nd step)1870 c/min
670 min-1

6.1 Method for determining threshold

500 c/min

A high correlation between the machining force ratio
P/Q (force ratio) and the finish accuracy (roughness)
was verified by the results shown in Fig. 6 and 7.
Therefore, to estimate the finish accuracy (roughness)in real-time during the machining process, the force
ratio was monitored during the machining process and it
was compared to a threshold. Fig. 9 shows a diagramof the signal processing system. The output signals
from the three component force sensor are transferredto the processing unit after the A/D conversion. The
processing unit has an output terminal which sends thesignal to the air cylinder which in turn applies pressure
to the stone.
Fig. 10 shows the flowchart used for roughness
determination. The two-step machining conditions thatwere used are listed in Table 3. At the beginning of themachining process, the part is ground for 10 seconds
for pre-process stock removal. Next, after switching topolishing conditions, the force ratio is continuously

670 min-1

6.0˚

1.6˚

0.54 MPa0.54 MPa

10s

10s

5.2 Test results
Fig. 8 shows the monitoring data from the machining
test conditions listed in Table 3. For the first 10
seconds, the large machining directional angle is set to
the grinding state. During this time, the grinding wheeland stock removal are both progressing at a constant
speed and the machining force of the grinding stone
and the machining force ratio (P/Q or force ratio) areconstant. After 10 seconds, when the grinding
directional angle is reduced to transition into thepolishing state, both stone advancement and stock
removal cease and the force ratio starts to increase. At-65-

NTN TECHNICAL REVIEW No.82（2014）
Three-component
force sensor

calculated and the result is compared to apredetermined threshold at regular intervals. The scanperiod was set to a 0.1 second interval. When the force
ratio becomes larger than the predetermined threshold,the finishing process is deemed complete and a signal
is sent to the air cylinder to retract the grinding wheel
away from the work piece.

A/D
conversion

Thrust force P

CPU
processing

Principal force Q

Air cylinder

Fig. 9 Signal processing system

6.2 Threshold determination results
The plots in Fig. 11 show the measurement resultsof P, Q and P/Q when the thresholds are set to 3.4 and
3.7, respectively, as well as the surface roughness
before and after the machining process. When thethreshold was set to 3.4, P/Q exceeded 3.4 after 5
seconds of the polishing cycle. Roughness of thefinished surface (Ra) was 0.064 but small irregularities
such as grinding marks were found along theroughness curve. When the threshold was set to 3.7,
P/Qexceeded 3.7 after 17 seconds of the polishingcycle. However, this time the roughness of the finishedsurface was 0.037 and no grinding marks wereobserved along the roughness curve. Based on theseresults, it was verified that setting a higher threshold for
P/Q results in a longer polishing time for improved
roughness of the finished surface and no grinding
marks.

Start of machining
Rotation of work piece/
oscillation of stone
Advancement of grinding wheel
1st step (grinding) condition
Timer 10 seconds
2nd step (polishing) condition
0.1 sec scanning

P/Q：Threshold
Forced setback of stone
Stop work piece/
stone operation

6.3 Relationship between the force ratio (P/Q )and surface roughness

Machining ends

In order to obtain a straightforward relationshipbetween the force ratio (P/Q) threshold and roughness,roughness and polishing time were plotted against P/Q

Fig. 10 Judgement signal flow chart

40

P/Q

10

Principal force Q
0

10

Time s

3
2
1
30

5

Threshold

60
40

4
3

P/Q

20

Principal force Q
0

10
Beforemachining

1mm

Ra 0.340
Rz 2.492

Ra 0.064
Rz 0.814

6

Thrust forceP

80

10After thresholdreached

Beforemachining

Ra 0.364
Rz 2.717

20

4

100

Time s1µm

205Machining force N

Threshold60

6Machining force ratio P/Q

Thrust forceP

80

1µm

Machining force N

100

End of machining with force ratio P/Q=3.7

20

2
1
30

After thresholdreached1mm

Ra 0.037
Rz 0.459

Fig. 11 Comparison of surface roughness after processing using threshold force ratio-66-

Machining force ratio P/Q

End of machining with force ratio P/Q=3.4

Real-time Monitoring of the Superfinishing Process

0.20

20

0.15

15

0.10

10

Roughness offinished surface

5

0.05
0

7. Conclusions
Technology was created to monitor the superfinishingprocess via an in-process gauge and the following
results were obtained:(1) A method was proposed to automatically determine
the finish accuracy by in-process monitoring ofmachining force ratio, thus monitoring the machining
state in the superfinishing process.
(2) Under polishing conditions, the machining force ratio
P/Q , which is calculated from the thrust force P and
the principal force Q applied to the stone, was foundto increase as the machined surface roughness
became lower.
(3) The machining cycle time can be reduced by ending
the machining process as soon as the machining
force ratio P/Q exceeds a preset threshold. This iscompared to the conventional machining method ofsetting machining conditions of all parts for the sameamount of time.

References
1) Takahiro Iguchi, Taskashi Onishi, Kazuhito Ohashi,
Kohei Higashi, Shinya Tsukamoto: Development of
Monitoring Technology for Superfinishing Process, The
Japan Society for Abrasive Technology, Proceedings ofAbrasive Technology Conference (2011) 297.
2) Shingo Yamashita, Takashi Onishi, Kazuhito Ohashi,
Kohei Higashi, Takahiro Iguchi, Ryosuke Yamamoto,
Shinya Tsukamoto: In-Process Monitoring of Machining
State in Superfinishing – Determination of Machining
State by Machining force ratio -, The Japan Society for
Abrasive Technology, Proceedings of Abrasive
Technology Conference (2012) 45.
3) Toshikatsu Nakajima, Norihiko Narutaki: Kikai Kakogaku(1984) 195.
4) T. Onishi, K. Ohashi, K. Higashi, S. Yamashita, T. Iguchi
and S. Tsukamoto, In-Process Monitoring of Machining
State in Superfinishing, Proceeding of the 7thInternational Conference on Leading Edge
Manufacturing in 21st Century (LEM21), (2013) 89-92．

Polishing time s

Roughness of finished surface µm

thresholds set at 0.1 intervals (shown in Fig. 12 ). As
the threshold is increased, the polishing time lengthens
and roughness improves.
However, when the threshold exceeds 3.7, there isno significant improvement in roughness even though
the machining time continues to increase.
To compare the superiority of this targeted machining
process with a conventional method using fixedmachining time, additional testing was performed. Inorder to obtain a roughness (Ra) of 0.052 (indicated by
the red dotted line in Fig. 12 ), which is equivalent to
the finishing surface roughness of the conventional
method (as shown in Fig. 2 ), a threshold of 3.6 was set
for this test. The time required for the polishing step was
6 seconds, which is a 40% reduction from the 10
seconds used by the conventional machining method.
From the above results, it is possible to control theroughness of the finishing surface by monitoring themachining force ratio and governing the process using
the force threshold. As a result the productivity can be
improved by reducing the polishing time compared to aconventional cycle which machines parts for a fixedamount of time.

Polishing time
3.0
3.4
3.5
3.6
3.7
Grinding
Threshold against P/Q
state

3.8

0

Fig. 12 Relationship between finished surface roughness
and polishing time at different threshold force ratios

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

Kohei HIGASHI

Hajime ISA

Takakazu KITAGAWA

Takashi ONISHI

Kazuhito OHASHI

Production Engineering
R&D Center

Production Engineering
R&D Center

Production Engineering
R&D Center

Graduate School of Natural
Science and Technology,
Okayama University, Division ofIndustrial Innovation Sciences

Graduate School of Natural
Science and Technology,
Okayama University, Division ofIndustrial Innovation Sciences-67-

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]Lightweight Bearing for Robot

Sebastien BRISSON*

Robotic arms do have many qualities, but intelligence is not their best point.
It is necessary and important to be programmed move by move. They work in
a XYZ coordinates world associated to θX θY θZ for rotations. Moreover,programming and maintenance are matter for specialists. Their size andpotential tremendous force imply large secured areas and heavy procedures
when an incident occurs.
Some trends can be found such as Cobotics that have new technologies of
motion control and can coexist with surrounding humans.
Based on its know-how, experience and Market understanding, NTN-SNR prototyped a bearing conceptthat aims to accompany the Robotics Megatrends. Making the bearing lighter, less expensive by tuning its
stiffness according to real needs of coming technologies.

process, when parts are conveyed by robots, the robots
have to grab the parts in the same way and transportthem to the predefined locations following the same
trajectory every time. Therefore, the positioning of therobot and objects must always be accurate. This is true
with welding and painting processes as well, so
accurate positioning and stable operation is again
required.
Fig. 1 shows a typical perpendicular articulated 6axis robot for industrial use. The programming for
operation is achieved by expressing and modeling theaccurate motion of the robots in X, Y, Z and θX, θY, θZ
axes, as shown in Fig. 2, for each process to beaccomplished.
Therefore, good performance of today’s industrial
robot arms is determined by the following items:• Accuracy of positioning• Accuracy of positioning reproducibility• Rigidity
With the performance of these items, motions are
combined to establish automatic operation of anindustrial robot. In addition, sufficient rigidity to support
the payload capacity of the robot is required.
Fig. 3 shows the performance required for the current
industrial robots. Today bearings for industrial robots
are heavy due to the necessary control of axis, highprecision, high rotational speed (acceleration) and highrigidity required.

1. Introduction
Many characteristics are required for industrial
robotic arms; however, the following two points are
particularly important:• Robot arm design must conform to the demand forseveral sizes and use in a broad range of applications
while also considering the cost of the robot arm.
• Higher functionality of robot arms and improved
operability
To satisfy these requirements, particularly from afunctionality standpoint, robot arms use technology todetermine various conditions in addition to high rigidity,
broad operational range and easy operation. Therefore,
bearings supporting the robot arms are required to be
designed to conform to these technology trends.
In this paper, we briefly discuss industrial robots andpresent the lightweight bearings for robots developed
by NTN-SNR.

2. Structure and capability of robots
The basic functionalities of today’s industrial robots
have evolved first from positioning, then to grabbing
objects with "hands" to more detailed tasks such asassembling, welding, painting, etc. which previously
was the work of humans. For example, in the assembly*NTN-SNR ROULEMENT Industry Innovative Developments-68-Lightweight Bearing for Robot

Fig. 4 shows an image of the industrial robot line-up.
In general, the maximum payload capacity is approx.
1,500 kg, however, the majority of applications are 20
kg or less. High rigidity is required not only for
maintaining accuracy of positioning, as mentioned
earlier, but also for supporting the weight of themanipulator.

3. Cobotics and new technology
3.1 Cobotics
Cobotics derived from "cobot", an abbreviation ofcollaborative robot, means a collaborative robot that
operates in an environment surrounded by humans.
Fig. 5 shows examples of operation. Cobots detect theco-working humans and other robots when they
approach the cobots and work on their own without
making contact with others. Cobots also work forautomated or assisted driving of vehicles. For example,
they can sense how humans work and assist them with
their work or change their behavior accordingly.
The payload of cobots is usually 20 kg or less andthese robots are becoming lighter and smaller.
However, these approaches are being adopted by large
units, as well.

Manipulator

Controller

Fig. 1 6 axis robot (manipurator and controller)

Example of autonomous cobot

Fig. 2 Modelling and programing for motion

Example of cobot to follow man’s movements
while multiplying its strength

Fig. 5 Payload of industrial robots

Fig. 3 Specification of usual robots

Fig. 4 Payload of industrial robots-69-

NTN TECHNICAL REVIEW No.82（2014）

The cobots are mainly deployed for tasks with lessadded value, time consuming tasks and repetitive tasksof critical process. Cobots are also aimed at reducing
the load of workers and preventing injuries such asarthritis.

4. Bearings for robots developed by
NTN-SNRIn Europe, NTN-SNR is a leading bearing
manufacturer of reducers for robots and maintains ahigh share not only for industrial robots but also forwork/motion-assist robots.
Fig. 7 shows an example of the specifications for the
manipulator and bearings of common industrial robots.
The rigidity required for the bottom most turning
bearings and the wrist rotating bearings at the tip of thearm are significantly different even for the same robot.
Therefore, the bearings to be used in the robot must be
individually selected depending on the task, payload,
position and operation of the robot.
Due to the above reason, crossed roller bearings
shown in Fig. 8 or four-point contact ball bearings areused for turning, rotating and oscillating robot joints andthe bottom most bearings need to support the weight ofthe manipulator.
NTN-SNR focused on developing a more lightweightbearing while maintaining the previously mentionedfunctional requirements.

3.2 New technologies
Among the technologies on robots, the following aredirectly related to control:• Image processing technology• Force sensing technology• Laser measurement technology
Fig. 6 shows some examples of movement using theabove technologies. These technologies enable robots
to understand the shape of the work, support it, andaccurately transport it after computational processing
for various movements.
They also assist operation with other robots andadjust distance with the workers in the proximity.

Movement of vision and image treatment

Force sensing

End of arm Bearing
Down to 0.3kg
Down to 20 Nm/Arcmin

Base of arm Bearing
Up to 11.5kg
Up to 1800 Nm/Arcmin

Laser measurement

Fig. 7 Manipulator and specification example of bearing

Picking

Deburring

Sorting

Fig. 6 Example of movement by new technologies

Fig. 8 NTN-SNR crossed roller bearing-70-Lightweight Bearing for Robotside of the resin parts, so that no displacement on theouter ring raceway of both sides is created in the radial
direction.
There are several ways to connect both sides of theresin parts, such as use of rivets as shown in Fig. 12 ①,
or to insert steel-made sleeves and fixed by bolts asshown in ②.

5. Lightweight bearings for robots
The design concept of NTN-SNR lightweightbearings for robots is a composite material of resin andsteel. Conventionally, inner and outer rings are speciallyshaped and made of steel because they are attached tothe peripheral components with bolts, etc., however, thedeveloped product uses steel only for bearing raceway
assembly and rolling elements and adopts resin for the
specially shaped part to be attached to the peripheral
components.
The application of resin must function without
affecting the bearing peripheral components.
Therefore, we also considered the operating
environment of industrial robots in the selection of resinmaterial and adopted nylon resin of high-melting pointwith excellent strength.
Also, the developed bearings can replace theconventional bearings without redesigning of robot
joints and reducers.
In the following, we present the types of lightweightbearings for robots.

Outer ring
(resin made, separate
pieces on both sides)

Sintered component
Outer ring(steel made; separate
pieces on both sides)
Rolling element
Inner ring (steel part)
Inner ring (resin part)Fig. 10 Lightweight bearing for robot (separate type)
Offset δ

5.1 Non-separable type
With the non-separable type, resin is injection moldedon the inner and outer rings of the deep groove ballbearings, as shown in Fig. 9. The rigidity is increased
by reducing the internal clearance of the bearings.

Outer ring (resin part)Fig. 11 Assemble example of lightweight bearing(separate type)

Outer ring (steel part)② Steel-made sleeve

Rolling element① RivetInner ring (steel part)
Inner ring (resin part)Fig. 9 Lightweight bearing for robot (non-separate type)

5.2 Separable typeThe separable type is the crossed roller bearings
shown in Fig. 10, to obtain higher rigidity than the nonseparable type which adopted deep groove ballbearings. In the assembly of the bearings, only the
inner ring is molded in a single piece with resin and theresin parts of the outer rings are assembled from bothsides of the steel parts after the rollers are inserted.
Also, as shown in Fig. 11, the widths of the separable
resin parts on each side are different. The reason is thatthe outer rings made of steel are engaged only to one

Fig. 12 Fastening example of lightweight bearing

5.3 Rigidity
The developed bearings have reduced rigidity
compared with the conventional bearings because thespecially shaped part of the inner/outer rings is replacedwith resin. Table 1 and Fig. 13 show an example of thecomparison of rigidity of the separable type (crossed-71-

NTN TECHNICAL REVIEW No.82（2014）

roller bearings) by FEM analysis.
In case of cobotics discussed in Section 3, which arecontrolled by image processing and force sensors, thehigh speed and high rigidity requirements of current
robots are not required; therefore, this reduction of
rigidity of the developed product should be in thepermissible range.

6. Conclusion
We have discussed the lightweight bearings for
robots developed by NTN-SNR. We have also
summarized the recent trend and new technologies of
the robot industry and the resulting requirements to thebearings in Fig. 14.
As shown in Fig. 3, the control of axis for the robot
motion has been emphasized until now. However, it isbelieved that the requirements on being lightweight andlower in cost will be more emphasized more in the
future instead of high speed and high rigidity, due tothe increasing control with image processing andsupport of workers by cobotics.
We developed the bearings for robots discussed inthis paper to cope with this trend. We will continue ourdevelopment with a close eye on the trends of the robot
industry and are poised to contribute to thedevelopment of the European and global industry byproviding this product for various industrial robots.

Table 1 FEM Analysis to compare the stiffness between
developed bearing and current bearing

Bearing main sizes ｍｍ

φ40×φ110×26
40

Analysis ① Axial load kN
conditions ② Moment load Nm① Axial rigidity mm(axial movement)
Analysis
results ② Moment rigidity deg(bearing inclination angle)

Example of analysis ①(Axial rigidity)

Developed
bearings

Current
bearings

1000
0.020

0.150

0.04

0.25

Example of analysis ②(Moment rigidity)

Developed
bearings

Fig. 13 FEM analysis example

Fig. 14 Technology trend for robot and requirement ofbearing

5.4 Weight
Table 2 shows the comparison of weight between theconventional bearings made of steel and the developed
product of the equivalent size. The developed bearings
reduced the weight by 63% for the separable type and
73% for the non-separable type compared to theconventional bearings, resulting in a significant weight
reduction.
Photo of author

Table 2 Weight comparison between developed bearing
and current bearing
Bearing type

Current bearings(separatecrossed roller)

Developed
bearings(separatecrossed roller)

Developed
bearings(non-separate,
deep groove ball)

1.00
̶

φ40×φ110×29
0.37
63

0.27
73

Appearance
Bearing size mm
Weight kg

Reduction factor %Sebastien BRISSON
NTN-SNR ROULEMENTS
Industry Innovative Developments-72-

Condition Monitoring System (CMS) for Wind Power Generators

Wind Doctor ®

Data acquisition
moduleR

Data management/monitor/
analysis software

NTN’s Condition Monitoring System (CMS) for Wind
Power Generators consists of a data acquisition module,measurement sensors, and data management/monitor/
analysis software and enables monitoring of bearing
condition, etc. from a remote location. It helps early
detection of errors to prevent expansion of damages andensures availability of power generators.
CMS: abbreviation of Condition Monitoring System

NTN’s system products supporting stable operationof offshore wind power generation-73-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Article ]Development of Condition Monitoring System

Keisuke HASHIZUME*
Akitoshi TAKEUCHI*
Yuzuru TANAKA*
NTN has developed Condition Monitoring System using bearing
diagnosis technology. This article introduces CMS’s products, andexplains necessity of CMS in the market and expansion in the future.

at wind turbine sites in Japan, and have started
acquiring large amounts of data. In this paper, we
discuss the reason why CMS is drawing attention,technical details of analysis through CMS, and its futuretechnology trends.

1. Introduction
Aiming at the prevention of global warming, energy
conservation and the reduction of power generation
cost, demonstration and full-scale production of power
generation devices using renewable energy are wellunderway. In Japan, the feed-in tariff program forrenewable energy (photovoltaic, wind, hydro,
geothermal and biomass power) started in July 2012
and their deployment is expected to accelerate. Amongthese, the cost of wind power generation is low among
renewable energies, and many sites are actuallydeployed as key power generation facilities in Europe,
which is a leading region for wind power generation. It
is expected that wind power generation will further
expand as larger wind power generation facilities aredeveloped for off-shore deployment, and large-scale
introduction in the Asian region is expected as well.
NTN introduced the Condition Monitoring System(CMS) for wind power generation as a product for the
renewable energy market that supports the nextgeneration power industry, taking advantage ofvibration analysis technology and sensing technology.
Through various improvements since its introduction,
we now market this system as Wind Doctor ® as shownin Fig. 1. We are increasing the number of installations

2. Condition Monitoring System (CMS)
Condition Monitoring System is a system tocontinuously or intermittently acquire data remotelyfrom several sensors, such as vibration sensors,
displacement sensors and temperature sensors which
are attached to the target machinery, for early detectionof component anomalies based on the acquired data.
By early detection of anomalies, damage progression
can be limited, which results in reduced maintenancecosts, and by determining the damaged area,
appropriate preparation and treatment can be
performed.

3. Market demands of CMS
Now, we will discuss the market demands of CMS,
taking the power generation business as an example.
In order to reduce power generation costs, thereduction of initial facility investment, the introduction ofhighly efficient generation technologies and theimprovement of equipment availability are necessary.
That is, after switch-over, maintaining stable operationis critical, and downtime must be reduced to aminimum. In general, stable operation of power
generation is considered to be a basic requirement, andunscheduled downtime has a significant impact on
consumers.
As a countermeasure to avoid such downtime,
periodical inspection of equipment and preventive

Fig. 1 Data acquisition module, Wind Doctor ®
*CMS Engineering Dept., Industrial Machinery Division-74-

Development of Condition Monitoring System

dedicated server through the Internet. Based on thisacquired data, various analyses and precise diagnoses
are done using dedicated software applications
installed on the user PC. Fig. 3 shows the base screenof this condition monitoring. When an anomaly isdetected, an alarm is displayed on the screen, andemergency emails are sent to the designated email
addresses.
This software also displays the trend view of RMS
values as shown in Fig. 4 and vibration wave patternsof acquired data as shown in Fig. 5. In addition, it also
has capabilities of various calculations and analyses
such as the envelope signal processing as shown in
Fig. 6 based on vibration wave patterns as shown in
Fig. 5 and FFT spectrum analysis and display as
shown in Fig. 7 It also allows arbitrary setting of bandpass filters to maximize the signal-to-noise ratio (S/Nratio), which is appropriate for displaying defectfrequencies of the bearings and step-up gears
diagnosed as damaged.

maintenance are necessary, but it is also important togather information beforehand in order to identify
anomalies and their locations based on early andaccurate detection of changes in equipment condition In
fact, with the gas turbine generation and steam power
generation, various sensing technologies, such as
vibration, temperature, rotation speed, pressure,displacement and acoustic response, are used forcontinuous monitoring and for associated maintenance
operations.
On the other hand, in the field of industrial machines,
some businesses, including those with long productionlines such as steel, papermaking and chemical plants,
are conducting preventive maintenance with immediate
effect by introducing their own CMS with the objectivesof maintaining supply capabilities and ensuring stable
quality.

4. Examples of equipment and applications
NTN is moving forward with the development andmarket introduction of CMS related technology
including special sensors. In this section, we describe
some of the typical examples of NTN's commercial
products.

4.1 CMS for wind power generation
The following is the data acquisition module, Wind
Doctor ® targeted for use in wind power generation
applications as shown in Fig. 1 in the previous section.
This module works with a 100V-240VAC power
supply and is equipped with 17 channels of signal
inputs. It also complies with IP65 level of JapaneseIndustrial Standard for electrical machinery andapparatus protection code JIS C 0929 for its dust/water
proof performance.
In order to achieve highly stable operation, it is also
equipped with data protection and cooling capabilities
during power outages as a hedge against risk. ThisWind Doctor ® module is operated as part of a system
as shown in Fig. 2. The output data in multiple formats
from various sensors are encrypted and uploaded to a

Fig. 3 Home screen of monitoring software

Fig. 4 Trend view of RMS value

Condition monitoring
softwareVarioussensors

ServerAnomaly
DectionCaution/warning
Precisediagnostics
ReportingWind Doctor ®

Fig. 2 Layout of CMS for wind turbines-75-

NTN TECHNICAL REVIEW No.82（2014）

In addition, the raw data can be downloaded in CSV
format so that users can freely process the data.
These operations can be run repeatedly without
limitation, so they can be used as an effective data
analysis tool.

vibration data is automatically sent to the server throughthe Internet using a dedicated application as shown in
Fig. 9. The server determines normal/abnormal status
of inner rings, outer rings and rolling elements of the
bearings through calculation and analysis and sends
the results back to the smartphone.
Fig. 10 shows an example of the operation screen
and result display screen on the smartphone. Fig. 11
shows FFT analysis results, Fig. 12 shows the overallvalues and Fig. 13 shows a trend view of the RMS
values. These data are stored on the dedicated server
not only on a temporary basis, but also past data can

Fig. 5 Wave pattern of acquired data

Fig. 6 Envelope signal processing of acquired data
Fig. 8 Handy type failure detection device
Smartphone
BluetoothServerFig. 7 FFT Analysis Results
Vibration pick-up
A/D converter

4.2 Handy type failure detection device
Fig. 8 shows the handy type failure detection device
developed for condition monitoring and maintenance ofgeneral industrial machines such as general-purpose
motors and fans.
This system consists of a vibration pick-up andpocket-sized A/D converter that are provided by NTN
and link to a user-provided smartphone. This productwas realized by applying bearing diagnostics
technology which has been developed over the years
with the goals of performing diagnostics “easily”,"consistently”, “on the spot” and “in a short time”.
Conventional all-in-one type diagnostics systems
calculate and analyze the measured vibration data
within the system, and require large displays for easy
identification of detailed data, limits how far their size
and weight can be reduced. In contrast, by using
smartphones, displayed data can be easily recognizedby zooming in to the most detailed view.
In addition, maintenance personnel of manufacturing
equipment and production lines need to carry thesystem together with tools and paperwork during theirmaintenance and inspection activities.
Therefore, NTN’s handy type failure detection device
is made very compact and portable for themeasurement of vibration. In this system, the acquired

Data display

Fig. 9 Layout of handy type failure detection device
Operation button

Judgment Results

Fig. 10 Operation screen and judgment screen

Fig. 11 FFT Analysis Results-76-Development of Condition Monitoring System

be downloaded; therefore, the data can be used for
fixed-point monitoring of facilities such as displaying thetrend view of past measurement results.

prediction based on much broader information will beenabled by integrating the data obtained from SCADA
with data from CMS which is useful for early detectionof anomalies.
The ultimate function required for CMS is not only
early discovery of damage, but also prediction ofremaining life. It is important to know how to minimize
the period of downtime due to anomalies; andspecification of operating limitations is frequently
requested in order to secure preparation time.
However, because the progression from anomaly
detection to failure can vary significantly depending on
the operating conditions, disturbances andenvironment, predicting remaining life is fraught withchallenges. In the future it may become necessary toemploy new analysis methods and to introduce precise
diagnostics technologies incorporating not only vibrationand displacement data but also acoustic and visual
sensing data.

Fig. 12 View of overall values

7. Conclusion
We have discussed why CMS is necessary and also
NTN’s products and activities. We are poised tocontribute to further improve availability and reduce
maintenance burden of not only wind power generation
but also industrial machines in general, by continuallyadvancing high functionality and high performance dataacquisition devices and providing highly accurate anduseful technology information services.

Fig. 13 Trend view of RMS values

5. Participation in the NEDO (New
Energy and Industrial Technology
Development Organization) project
NTN is participating in the “NEDO Advanced
Practical Research and Development of Wind Power
Generation” (R&D of Smart Maintenance Technologies)
project with other organizations such as the Universityof Tokyo, to perform experimental evaluations of CMS.
This research and development aims to increase
average availability of domestic wind power generation
facilities from the current level below 20% to over 23%
by developing highly-efficient maintenance techniquesand conducting demonstration tests on actual windturbines. NTN will conduct demonstration tests for thisR&D project by installing Wind Doctor ® modules to thewind power generation facilities across Japan thereby
promoting the further advancement of CMS equipment
diagnostic technology.

References
1) Akitoshi Takeuchi, Takashi Haseba, Hiroshi Ikeda:
Application of Condition Monitoring System for Wind
Turbines, NTN TECHNICAL REVIEW, No. 80 (2012)15-18.

Photo of authors

6. Future development
Power plants in general including wind power arebuilding remote monitoring/controlling systems(SCADA: Supervisory Control and Data Acquisition) toacquire operational data of their facilities and entire
sites. We believe that advanced control such as
automatic trend analysis and degradation trend-77-

Keisuke HASHIZUME

Akitoshi TAKEUCHI

Yuzuru TANAKACMS Engineering Dept.,
Industrial Machinery Division

CMS Engineering Dept.,
Industrial Machinery Division

CMS Engineering Dept.,
Industrial Machinery Division

NTN TECHNICAL REVIEW No.82（2014）[ New Product ]Introduction of "ULTAGE" series :High Functionality Roller Bearings
Shogo TABATA*
Kouhei TODA**
Yasuhiko SHIMIZU***

When it comes to the operational demands put on equipment (suchas iron and steel machinery, or wind power generation) and industrial
machinery (such as construction and mining machinery, or gearboxes),
customers are looking to maximize output and efficiency under more
severe usage conditions, like higher vibration and impact loads. In response to these increasedemands, NTN developed the ULTAGE® Series of cylindrical ,tapered and thrust spherical roller
bearings, which deliver the highest load capacity and allowable speed by optimizing the internal
design and crowning profile. NTN will release these bearings as the ULTAGE Series which reducethe internal friction resulting in less energy needed to operate.

1. Introduction

2. ULTAGE Cylindrical Roller Bearings

Technology development that emphasizes global
environmental protection is well underway for every
industrial machine.
Bearings used in rotational components of industrial
machines are required to have an improved “long life”,“high load carrying capacity”, “high speed”, and “ease ofuse”. To meet this demand NTN has developed theworld's-highest level of new-generation bearings known
as the “ULTAGE ®※” series. This series started with
precision rolling bearings for main spindles of machine
tools and was followed by standardization of new
spherical roller bearings, roll neck sealed type 4-row
tapered roller bearings for rolling mills, etc. Since thennew cylindrical roller bearings, tapered roller bearings
and thrust spherical roller bearings have been added to
the ULTAGE series. These bearings achieve long life,
high load carrying capacity and high speed and are
discussed in the following:

The ULTAGE cylindrical roller bearing is optimized
for rolling contact so that the contact pressure
distribution between the rolling elements and rolling
bearing rings becomes uniform. The features of thisbearing are as follows:※

2.1 Features(1) Load carrying capacity (compared with theconventional cylindrical bearings)• Basic dynamic load rating: 1.2 x• Basic rating life: 1.8 x(2) Limiting speed (compared with theconventional cylindrical bearings)• Max. 1.2 times under oil lubrication condition(3) Resin cage material is applied as standard• Applicable for oil and grease lubrication(4) Allowable inclination (misalignment)
ULTAGE: 1/500 (0.002 rad.)Conventional cylindrical bearing : 1/2000 (0.0005 rad.)ULTAGE ® is a name created from the combination of “ultimate,” signifying refinement, and “stage,” signifying NTN’s
intention that this series of products be employed in diverse applications. This is the general name for NTN’s new
generation of bearings that are noted for their industry-leading performance.

***Industrial Machinery Engineering Dept., Industrial Machinery Division***New Energy Engineering Department, Industrial Machinery Division***Construction Machinery and Railway Engineering Department, Industrial Machinery Division-78-

Introduction of "ULTAGE" series : High Functionality Roller Bearings

2.2 Structure

Stress concentration point of
conventional bearing crown design

Fig. 1 shows an example of the structure of NU typecylindrical roller bearings.
High

Adoption of larger diameter rollers and optimized
crowning shape enabled use under tough conditions
such as larger inclination and heavy load, increasingload carrying performance.

Reduction ofedge stressContact pressure1) Optimized internal design

Low2) Resin cage material is standard

Roller length

Glass fiber enhanced polyamide resin is used as thematerial. Resin cages are lightweight andcharacterized by low torque and low noise.

Cage¡Roller guiding method is adopted¡Resin material cageULTAGE
Conventional Bearing Design

Fig. 2 Contact stress distribution of rollers

2.4 High Speed Operation( increase of allowable rotational speed)

Outer ring

The ULTAGE cylindrical roller bearings reduces thecontact pressure between the rollers and rolling bearing
rings and improved the contact conditions between therollers and rib section. This improves lubrication
performance inside the bearings and reduces heatgeneration allowing a 20% improvement of rotational
speed compared with the conventional bearings. Fig. 3
shows the test result of oil lubrication temperature rise.

※Steel-base, brass-base and machined
cages are also possible depending on
the operating conditions<<Test Condition>>

Inner ring

Bearing part number: NU307E (φ35×φ80×21)
Radial load: 3.5 kN (5% Cor)
Bearing internal clearance: CN clearance (0.025 - 0.050)
Lubrication: Turbine oil VG32 circulation lubrication

Roller¡Optimum crowning shape¡Large diameter rollers are adopted

Fig. 1 Design of ULTAGE cylindrical roller bearing
Temperature rise ˚C80

2.3 High Load Carrying Capacity
Fig. 2 shows the stress distribution under thefollowing calculation conditions.
The ULTAGE cylindrical rolling bearing has
significantly reduced edge stress compared with theconventional bearings by optimizing the roller crowning
shape. In addition, smoothly blending the straight area
and crowning area of the outer surface of the roller byapplying a new processing method, we have reduced
concentration of stress at the starting point of thecrowning area seen in the conventional bearing crowndesigns . This reduces the contact stress even undertough conditions, resulting in improved load carrying
performance and bearing life.

ULTAGE
Conventional Bearing Design

60
40
20
0

4000

6000

8000

10000

12000

14000

Rotational speed min-1

Fig. 3 Temperature rise test results

2.5 Standardized Series
The ULTAGE design will be the standard forcylindrical roller bearings with an outer diameter of 180
mm or less.

<<Calculation Conditions>>
Bearing part number: NU304E (φ20×φ52×15)
Radial load: 7.9 kN
Inclination: 1/500 (0.002 rad)-79-

NTN TECHNICAL REVIEW No.82（2014）<<Calculation Conditions>>

3. ULTAGE Large Tapered Roller Bearings

Bearing part number: 30316U (φ80×φ170×42.5)
Radial load: 102 kN
Inclination: 1/600 (0.0017 rad)The ULTAGE large tapered roller bearings have thefollowing features by optimizing the internal design ofconventional bearings:

3.1 Features(1) Load carrying capacity (compared withconventional tapered roller bearings)

High

Contact pressure

ULTAGE
Conventional Bearing Design• Basic dynamic load rating: 1.3 x• Basic rating life: 3 x(2) Limiting speed (compared with conventional
tapered roller bearings )Low• Max. 1.1 times under oil lubrication condition

Small
diameter(3) Reduction of running torque(4) Long-term dimensional variation

Large
diameterRoller length

Fig. 5 Contact stress distribution of rollers• 1/10 of bearing steel, 1/4 of case hardening steel(5) Allowable inclination (misalignment)
ULTAGE: 1/600 (0.0017 rad)Conventional tapered roller bearings: 1/2000 (0.0005 rad)3.4 High Speed Operation( increase of allowable rotational speed)

3.2 Structure

The ULTAGE large tapered roller bearings have
improved the allowable rotational speed by 1.1 times
compared with the conventional units by reducing therunning torque and temperature rise by optimizing thesliding contact area of the rollers and inner rings asshown in Fig. 6.

Fig. 4 shows the structure of this bearing.

Outer ring
¡Bearing steel with special
thermal treatment is adopted

Cage

Roller1

Inner ring
¡Bearing steel with special
thermal treatment is adopted¡Optimum rib design

1 Optimum contact point
for the roller
2 Enhanced precision of rib
surface finish

Roller¡Optimum crowning shape

2

RibFig. 6 Contact point between inner ring large end rib
face and roller large end faceFig. 4 Design of ULTAGE taper roller bearing

3.3 Higher Load Carrying Capacity(1) Temperature Rise Test

The ULTAGE large tapered roller bearings have
improved load carrying capacity compared with theconventional bearings by optimizing the rollercrowning. This optimized crowning reduces excessive
contact stress and provides unform contact stress
distribution. This reduction of contact stress results inimproved bearing life even under tough conditions..
Fig. 5 shows the roller contact stress distribution
under the following calculation conditions.

Fig. 7 shows the temperature rise test results of the
ULTAGE large tapered roller bearing compared with an
NTN’s conventional bearing and anothermanufacturer’s bearing .

<<Test Condition>>
Bearing part number : 32940XUE1 (φ200×φ280×51)
Lubrication: Turbine oil VG56 circulation lubrication
Bearing internal clearance: C2 (0.100 – 0.260)-80-

Temperature rise ˚C

70

Dimensional change ratio

Introduction of "ULTAGE" series : High Functionality Roller Bearings

ULTAGE
Conventional Bearing Design
Another manufacturer's
Bearing Design

60
50
40
30
20
10
0

0

500

1000

1500

Case
hardening steel
ULTAGE
0

2000

Rotational speed min-1

Conventional(Bearing steel)

500

1000

1500

2000

2500

(Holding temperature:120˚C)

Elapsed time hFig. 7 Temperature rise test results

Fig. 9 Dimensional change test results(2) Running Torque Test

4. ULTAGE Thrust Spherical Roller Bearings

Fig. 8 shows the running torque test results of the
ULTAGE large tapered roller bearing compared with an
NTN’s conventional bearing and anothermanufacturer’s unit.

The ULTAGE thrust spherical roller bearings havethe following features by optimizing the internal designof conventional bearings:<<Test Condition>>

4.1 Features(1) Load carrying capacity (compared with theconventional units)

Bearing under test: 30316U (φ80×φ160×42.5)
Axial load: 6.9 kN
Lubrication: Turbine oil VG32
Temperature: Room temperature• Basic dynamic load rating: 1.7 x (max.)• Basic rating life: 6 x (max.)(2) Limiting speed (compared with the conventional
units)Torque N・cm

1200

• Allowable rotational speed: 1.2 x (max.)
ULTAGE
Conventional Bearing Design
Another manufacturer's Bearing Design

1000
800

(3) Window type punched steel cage is adopted• Applicable for oil and grease lubrication(4) Heat resistant up to 200°C

600• Adoption of special heat treatment

400

4.2 Structure2000Fig. 10 shows the structure of this bearing.
0

100

200

300

400

Rotational speed

500

600

700

min-1

Cage
¡Window type punchedsteel cage

Fig. 8 Running torque test results

Shaft rolling bearing washer¡Optimized groove curvature¡Optimized rib curvature

3.5 Reduction of Long-Term Dimensional Variation
The operating long-term dimensional variation was
reduced to 1/10 or less compared with the conventional
bearings using the bearing steel (1/4 or less comparedwith case hardening steel) by adopting special thermal
treated (heat resistant dimensional stability treatment)bearing steel. This result is shown in Fig. 9.

Housing rolling bearing washer¡Optimized groove curvature

3.6 Standardized Series

Roller¡Large diameter rollers

are adopted¡Maximum number of

The ULTAGE design will be the standard for metricseries bearings with an outer diameter of 270 mm or
more.

rollers

Fig. 10 Design of ULTAGE thrust spherical roller bearing-81-

NTN TECHNICAL REVIEW No.82（2014）

1) Optimized internal design50

Temperature rise ˚C

The diameter and length of the rollers are maximized
and the curvature of the groove and rib is optimized.

2) Adoption of the window type punched steel cage
By adopting a punched steel cage, lubricant caneasily flow to the inside of the bearings enabling use ofeither oil or grease as lubricant.

ULTAGE
Conventional Bearing Design

40
30
20
10
500

4.3 High Load Carrying Capacity

1000

1500

Rotational speed

The ULTAGE thrust spherical roller bearings
achieved the world's-highest level of load carrying
capacity by maximizing the diameter and length of therollers of conventional bearing .

2000

2500

min-1

Fig. 11 Temperature rise test results, vertical shaft

Temperature rise ˚C50

4.4 High speed operation( increase of allowable rotational speed)
The ULTAGE thrust spherical roller bearings adopts
a window type punched steel cage as opposed to the
supported-ring-guided machined steel cage of theconventional bearing . In addition, optimization of the rib
curvature of the shaft rolling bearing washer reduces
the rise of temperature which improved allowablerotational speed by up to 1.2 times compared with theconventional bearing .
Fig. 11 and 12 show the test result of temperaturerise of vertical and horizontal shaft tests. In both tests,
the rise of temperature was lower than the conventional
bearing .

40

ULTAGE
Conventional Bearing Design

30
20
10
500

1000

1500

2000

Rotational speed min-1

2500

Fig. 12 Temperature rise test results, horizontal shaft

5. Conclusion
Three new types of roller bearings are now added to
the NTN next-generation bearings, the ULTAGE Series.
As technology innovation of industrial machines for
global environmental protection is currently underway,
we believe that bearings featuring high load carrying
capacity and high speed will meet the market
requirements of industrial machines by being
lightweight, compact and having a long-life.
We are poised to continue with the development of
next-generation bearings with further improvement andhigher functionality that respond to various market
demands.

(1) Vertical Shaft Test<<Test Condition>>
Bearing part number: 29418E
Axial load: 98 kN (5% Coa)
Lubrication: Turbine oil VG32 circulation lubrication(2) Horizontal shaft test<<Test Condition>>
Bearings under test: 29418E
Axial load: 98 kN (5% Coa)
Lubrication: Turbine oil VG32 circulation lubrication

4.5 Standardized Series
The ULTAGE design will be the standard for the
29400Series and bearings with an outer diameter of
130 - 320 mm.

Photo of authors-82-

Shogo TABATA

Kouhei TODA

Yasuhiko SHIMIZU

Industrial Machinery
Engineering Dept.,
Industrial Machinery Division

New Energy
Engineering Dept.,
Industrial Machinery Division

Construction Machinery and
Railway Engineering Dept.,
Industrial Machinery Division

NTN TECHNICAL REVIEW No.82（2014）[ Technical Article ]Technical Trend of Aircraft Bearings

Takashi NISHIKAWA*
Nao HAYASHI*
Akiko HAYAKAWA**
There are many different types of bearing for aircraft applications.
NTN supplies bearings to jet engine manufacturers all over the world.
Recently due to new customer requirements for reducing weight ,environmental impact and higher performance, bearing systems are
becoming very complicated that require higher reliability and quality
controls.
In this paper, NTN’s design theory to jet engine main shaft bearings along with the industry trends
are documented.

processes occur in clean room environments, andcustomer-authorized quality assurance representatives
make final product release decisions.
In addition, NTN’s development includes hybrid
bearings with rolling elements made of ceramic andraceway rings made of bearing steel, for which demandis expected to grow, nitrided bearings in which raceway
rings and rolling elements are nitrided, as well as its
own test rig which evaluates dry-run performance todetermine if the bearings can be used without seizure,even if the supply of lubricant is interrupted for a certain
period.
NTN has technology development sites, in addition to
production sites, in Japan and France. Centered onthese sites, NTN has been directly dealing with the key
jet engine manufacturers, helicopter manufacturers andaerospace businesses not only in Japan and France
but around the world, providing a number of high quality
products for many years. Recently, NTN’s bearings are
adopted by General Electric’s GENX engine which isused in Boeing 787, Rolls Royce’s Trent XWB engine
expected to be used by Airbus A350, Pratt & Whitney’s
GTF engine expected to be used by Mitsubishi MRJ,
etc., which are raising NTN’s presence in the aerospaceindustry.
NTN’s bearings are used in various parts of aircraft in
addition to main shafts of jet engines, such asaccessory gear boxes (AGB), flight control systems(FCS), landing gears and auxiliary power units (APU).

1. Introduction
Demand and sales of commercial aircraft are growing
steadily, together with rapid growth of emerging
markets, as well as the global expansion of low cost
carriers (LCC). Aircraft employ many bearings, andNTN also provides bearings to the key jet engine
manufacturers. In this paper, we discuss NTN's
activities on the bearings for aircrafts, and bearings formain shafts of jet engines which are used underparticularly tough conditions.

2. NTN’s activities on bearings for aircraft
Development and manufacturing of aerospace
products requires considerably high technology. NTN
has established bearing plants dedicated to aerospace
applications at NTN Kuwana Works (Mie Prefecture)
and NTN-SNR Argonay Works (France) to guarantee
high precision and high reliability with utmost quality
control. To support this, these plants utilize customerapproved special processes and certified inspectors
accredited by the international certification program
NADCAP (National Aerospace and Defense
Contractors Accreditation Program). Special process
operations for the aerospace industry and certification
programs of various customers may include heat
treatment, surface plating & treatments, non-destructive
testing, etc. In addition, final assembly and inspection**Machining Tool/Aerospace Engineering Dept., Industrial Machinery Division**Kuwana Works, Industrial Machinery Division-83-

NTN TECHNICAL REVIEW No.82（2014）

Inner diameter of bearings mm

3. Introduction of bearings for main
shaft of jet engines
3.1 Types of jet engines
There are four types of aircraft turbine engines:
turbojet, turbofan, turboprop and turboshaft, and theformer two are usually called jet engines. The mostwidely used engines are two-shaft turbofan engines
featuring high efficiency and low noise; however, threeshaft engines which enable optimization of compressor
design are also used. There are also GTF engines
equipped with a planetary gear mechanism for drivingfans with reduced speed.
Fig. 1 shows the basic structure of a turbofan engine.
In this configuration, the air absorbed from the front sideof the engine is compressed by the compressor, which
is sent to the combustion chamber where fuel is burned
and the exhaust drives the turbine. There are two
driving systems for the turbine, compressor drive andfan drive, and most of the thrust of a turbofan engine is
obtained from the exhaust of the fan.
Fan

dn 2.4×106

150

100

50

0
0

dn 2.0×106
dn 1.6×106

10,00020,00030,00040,00050,00060,00070,00080,000-1

Fig. 2 dn value of bearings(2) High temperature
The bearings for jet engine main shafts are exposed
to temperature of approx. 200°C during operation. In
addition, the turbine remains in the so-called heat soakback state where heat is stored with no escape evenafter the engine stops, which raises the temperature of
the bearings to over 300°C. Therefore, low dimensionalvariation and high fatigue life under high temperatureare required.

Fan exhaust jet
Turbineexhaust jet

Compressor

dn 2.8×106

Bearings for jet engine
Ball bearings for gas turbine engine
Cylindrical roller bearings for gas turbine engine
Ball bearings for industrial machines
Cylindrical roller bearings for industrial machines
Tapered roller bearings for rail car shafts

Rotational speed min

Additional turbine

Air inlet

200(3) Reliability and traceability
Since aircraft safety is an absolute requirement, thebearings are also required to have high reliability. We
maintain high reliability by conducting full inspections inevery step of bearing manufacturing process, such as
procurement of material, forging, turning, heat
treatment, machining and surface treatment, to verify
manufacturing conformity including various certification
contents mentioned in the previous section.
In the event of any possible trouble such as damaged
bearings, the ability to trace all manufacturing data of
every process is also required.

Combustion
chamber Turbine

Fig. 1 Outline structure of Turbo fan engine

3.2 Characteristics required for bearings of jet
engine main shaft
Key characteristics required for the bearings of jet
engine main shaft are:• Suitable for high-speed rotation• Operable in high temperature• Highly reliable and traceable

3.3 Main features of bearings of jet engine main shafts(1) Bearing types(1) High speed

Ordinary jet engines have two shafts, one for thecompressor/turbine on the high speed/high pressure
side, and the other one for the fan/compressor/turbineon the low speed/low pressure side. The bearings havea role to support them.
The bearing types for jet engine main shafts aremainly three-point contact ball bearings and cylindricalroller bearings as shown in Fig. 3. The three-point
contact ball bearings support load in the axial and radial
directions.
They can support both directions of axial loads. The
inner rings of these bearings can be split to incorporate

An index to evaluate high speed property of bearings
includes dn value (bearing inner ring inner diameter(mm) x inner ring rotational speed (min-1)). Fig. 2 showsdn value of the bearings of the typical use in generalindustrial machines. The bearings of jet engine main
shafts are used under the condition that exceeds dn
2.0×106, therefore, they are required to have superior
fatigue life and anti-galling properties. In addition,
significant hoop stress is placed on the bearing innerring due to centrifugal force; therefore, the fracture
strength to withstand the force is also required.
-84-

Technical Trend of Aircraft Bearings

as many balls as possible to ensure long life under alarge load. The cylindrical roller bearings are used as
floating-side bearings in order to absorb longitudinal
expansion/contraction of the engine’s main shaft while
supporting radial load.
Fig. 4 shows the bearings for main shafts used on
Boeing’s passenger aircraft B767.

Hardness HRC

70
65
60
55

M50

50
45

SUJ2

40
35

30
100

Outer
ring
Ball

250

300

Temperature ˚C

350

400

450

350

Inner
ring

300

(b) Cylindrical roller
bearing

Hoop Stress MPa(a) Three-point contact
ball bearing

200

Fig. 5 Hardness of bearing steel for high temperature

RollerCage

150

Fig. 3 Bearing types for jet engines

250
200
150
100
50
0

(a) Three-point contact ball bearing

0.0

1.0

2.0

3.0

dn x106

4.0

Fig. 6 Hoop stress on bearing inner ring(b) Cylindrical roller bearing

Fig. 4 Examples of main shaft bearings of jet engine
M50NiL(2) Bearing material
The material of bearings for jet engine main shafts
requires long life, dimensional stability, high strengthand high wear resistance, particularly under hightemperature. For raceway rings and rolling elements,
high speed steel such as molybdenum-based AISI M50
or tungsten-based T1 is used. As shown in Fig. 5, M50
has hardness of around 58HRC at 300°C and highdimensional stability.
When bearings are used under high speed rotation,
centrifugal force is applied to the inner rings and thetensile stress on the circumferential direction of theinner rings (hoop stress) will increase. In case ofthrough-hardening steel such as M50, once exfoliation
starts at the inner rings, cracking may progress very
fast resulting in fracture of the inner rings; therefore,
heat-resistant carburized steel AISI M50NiL may be
used. Fig. 7 shows the comparison of fracture
toughness.
Cages are frequently used with SAE4340 steel
hardened to around 30HRC.
Usually, the bearings of aerospace applications must
only use the material certified by the aircraft
manufacturers or engine manufacturers.
NTN also procures the material from supplierscertified by the engine manufacturers.

M50
0

10

20

30

40

50

Fracture Toughness MPa・m1/2

60

Fig. 7 Comparison of fracture toughness(3) Bearing design
In order to absorb vibration produced at the main
shaft in high speed rotation, the outer ring is unitizedwith the vibration absorbing components with springs
which are supported by the oil damper, as shown in
Fig. 8. Currently, the vibration absorbing components
are most likely unitized with the outer ring in order toreduce the weight and number of components.
Fig. 9 shows an example of the commerciallyavailable unitized bearings.
The cylindrical roller bearings have low load carrying
requirements because they only support the main shaft
weight as the radial load and the force that the innerring drives to the rollers is also relatively small. Sliding
between the rollers and the raceway may cause earlywear. Therefore, in some cases, the outer diameter is-85-

NTN TECHNICAL REVIEW No.82（2014）

most widely used as the lubricant for jet engines. In
addition, lubricant oil with superior oxidation stability
under high temperature and sludge resistance is also
developed and standardized as MILPRF-23699 HTS.
Along with this, the conventional oil MIL-PRF-23699
was standardized as MIL-PRF-23699 STD.
The lubricant must be delivered to the rolling contact
surface of the bearings and the contact area between
the cage guiding surface and land. Therefore, forced
lubrication is required, such as oil jet lubrication where
the high speed lubricant is injected from the nozzle(shown in Fig. 11).
Along with the high speed rotation of the main shaft,a “wall” of air, or so-called “air curtain”, is formed due tothe rotation of the rolling elements and the cage, which
prevents sufficient delivery of lubricant oil injected fromthe nozzles. In that case, under-raceway lubrication isused to deliver oil to the required area with thecentrifugal force. Fig. 12 shows an example of underraceway lubrication.
The oil jet injected from the nozzle is captured by theoil scoop and guided to inside the main shaft. Then, theoil is supplied into the bearing with the centrifugal force.
Since the under-raceway lubrication delivers the oil
directly into the inner ring of the bearings, the cooling
effect on the bearings is higher than with oil jet
lubrication.

made into an oval shape so that preload is given to therollers when they are assembled into the housing.

Outer ring ＋ Vibration absorbing component
SpringOil ring

Oil dumper

Unitized
configuration

Conventional
configuration

Fig. 8 Unitized bearings

Fig. 9 Unitized model of a bearing
Outer surface: Ellipse
Raceway surface:True circle

Lubricant
inlet

Outer
ring

Lubricantinlet(a) Discrete state
Outer surface:True circle

Land orshoulderOil jetRaceway surface:
Ellipse

Ball

PreloadDividedinner rings

Cage

PreloadHollow shaft(b) Assembled stateFig. 11 Oil jet lubrication

Fig. 10 Non-true circle cylindrical roller bearing

Lubricantoutlet(4) Lubrication

Oil scoop

Mineral oil based SAE1010 was used as lubricant ofearly jet engines, however, its fast oxidative
degradation due to the use under high temperature
affected the bearing life.
The first solution to this challenge was thedevelopment of synthesized ester base MIL-L-7808.
Then, MIL-L-23699 was developed with improved load
carrying performance and currently, MIL-PRF-23699 isOil jetOil cooling and hole for lubricant

Fig. 12 Under-raceway lubrication-86-Technical Trend of Aircraft Bearings(5) Surface treatment

4. Conclusion

On the cage guide surface, sliding friction between
the cage and the land cannot be prevented; therefore,
good friction properties and wear resistance are
required. In order to obtain good friction properties,
secure delivery of lubricant oil to the cage guide surface
is required and the above mentioned under-raceway
lubrication is used. In addition, the cage surface is silver
plated to prevent contact between the similar metals.
The main standard for this silver plating is AMS2414.
The land surface on the rolling bearing rings shownin Fig. 13 may be TiN coated, which is a type of hardceramic, in order to improve wear resistance.

Bearings are used in various locations of aircraft,including jet engines. NTN has been providing a
number of high quality bearings for many years, andguaranteeing high precision and high reliability throughcomprehensive quality control.
Bearings for jet engines are expected to face
increasingly severe operating conditions considering
the ongoing development of high performance jetengines. NTN will continue to advance research anddevelopment of new technologies recognizing theimportance of bearings in jet engines. At the same time,
we are poised to contribute to the aerospace industry
by providing our high technology and quality tocustomers around the world through our participation in
new development projects with jet engine andhelicopter manufacturers, among others.

References
1) Masuji Matsuoka: Koku Kagaku Koza Vol. 11 Jet
Engine (Structure)2) NTN TECHNICAL REVIEW, No.67 (1998) 7．
3) NTN Bearing Engineer, Vol. 21, Issue 1 (1972) 61.
4) NTN Naoki Matsumori, Bearings for Jet Engine Main
Shafts to Protect Safety of Aircrafts, Tool Engineer, Vol.
54 No 13, Taiga Publishing Co., Ltd. (2013) 61.

Fig. 13 TiN coating

3.4 Technology development for bearings of jet
engine main shafts
Recent jet engines are designed to operate at higher
temperature and pressure for lower fuel consumption
and lighter weight, and the structure of the bearings forjet engine main shafts is becoming more complicated.
Therefore, reliability of bearings must be improved. This
important development task is being pursued on
several fronts:

1) Surface modification
Common causes of bearing degradation are exfoliation
and adhesion, which originate from surface defects suchas indentations produced by hard foreign objects.
Raceway hardening processes via surface modification
are being developed to reduce such degradation andachieve high reliability.
Photo of authors

2) Rolling element material
We are considering use of ceramics (Si3N4) which islight with the specific gravity of approx. 40% of M50 andexhibits good friction and wear properties for the rolling
elements in order to reduce the contact pressure between
the rolling elements and the raceway rings due to highspeed rotation. However, since high reliability is requiredfor its application in bearings for jet engine main shafts,
we are working on establishment of a high precision nondestructive testing method for detecting defects.

-87-

Takashi NISHIKAWA

Nao HAYASHI

Akiko HAYAKAWA

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Machining Tool/Aerospace
Engineering Dept.,
Industrial Machinery Division

Kuwana Works,
Industrial Machinery Division

NTN TECHNICAL REVIEW No.82（2014）[ Technical Paper ]The Influence of the Humidity and Quantity of Hydrogen
into Steel under the Abrasive Wear
Motohiro ITO*
An early bearing damage is occurred and accompanied by unique
white microstructures in the raceway subsurfaces. The main cause isthought to be the result of hydrogen embrittlement caused by generatedhydrogen from decomposition of lubricant and containing water. It isconsidered that hydrogen embrittlement is occurred by penetrated
diffusible hydrogen. There is a method to detect diffusible hydrogen insteel. This method measures the specimen that is worn under abrasive wear by thermal desorption
analysis. In this report, we tested abrasive wear in space that is controlled humidity and investigatedthat relationship between humidity and penetrated diffusible hydrogen into steel via abrasive wear.

1. Introduction

2. Wear test

Early flaking together with distinctive white structuralchange observed in roller bearings is considered to bemainly caused by hydrogen 1). The mechanism of thisphenomenon is assumed that the hydrogen producedby hydrolysis of the lubricant or water inside thebearings penetrates the steel causing the steel tobecome brittle and separate 2)- 4). In addition, thehydrogen which causes the embrittlement is considered
to be diffusible hydrogen.
In order to detect the diffusible hydrogen, two
methods can be used. One method is to havehydrogen electrochemically penetrate into steel andcapture the permeated hydrogen as the ionizedcurrent 5). The second method is to have hydrogen
penetrate into steel under a corrosive environment andapply thermal desorption analysis (TDA) 6).
Diffusible hydrogen derived from water or lubricant
hydrolysis can be detected by thermal desorption
analysis after abrasive wear of steel by emery paper
while water or oil is dropped on the steel 7). However,diffusible hydrogen is also detected in the dry abrasion
test where no water or oil is applied. Therefore vapor in
the atmosphere may also contribute 7).
In this paper, we have investigated the amount ofhydrogen that penetrated into steel and the impact oftemperature on the amount through abrasive wear
under the environment with controlled temperature andhumidity.

Fig. 1 shows an overview of the test equipment andTable 1 shows the wear test conditions. In this test,
emery paper #220 was set on the rotary surface table
and three SUJ2-material disks were set on thespecimen holder. The chamber is designed to provide
air circulation with which the temperature and humidity
are controlled by an air conditioner. The load on thespecimen is provided by a weight with a target of 85.4
kPa of contact pressure. The wear test was conducted
applying 5 ml of test oil on the emery paper before thetest and then dropping 1 ml/min of test oil during thetest. Both the rotary surface table and the specimen
holder rotate with the rotational speed of the rotarysurface table, at the center of the specimen holder,
being 0.59 m/s and the rotational speed of thespecimen holder, at the center of the specimen, being
0.03 m/s.
Fig. 2 shows the shape of the specimens and Table
2 shows the processing conditions of the specimens.
The specimen is SUJ2 processed with ordinary
quenching at 850˚C and tempering at 180˚C x 2 h.
Table 3 shows various wear test conditions using
mineral oil, water-added oil and fluorine-based oil.
For mineral oil, additive-free turbine oil VG100 wasused.
The temperature in the test chamber was set to 20˚C
and 30˚C and several levels of relative humidity values
were used ranging of 25 to 95%RH. The wear test

*Advanced Technology R&D Center-88-The Influence of the Humidity and Quantity of Hydrogen into Steel under the Abrasive Wear

Specimen holder

duration was 40 minutes.
In order to verify the impact of water mixed in oil, awear test was also conducted using water-added oil.
For water-added oil, additive-free turbine oil VG100 withpure water was used. The amount of added water was
2, 5, 10 and 15 wt%, with a set temperature of 30˚C and
two relative humidity conditions, 25%RH (low humidity)
and 95%RH (high humidity).
Diffusible hydrogen in steel after the wear test is
considered to be derived not only from water but also
from the decomposition of the mineral oil. In order toverify the impact of hydrogen from hydrolysis, a wear
test was conducted with fluorine based oil which does
not contain hydrogen in the chemical structural formula.
For fluorine-based oil, additive-free perfluoro-polyether
oil with kinematic viscosity at 40˚C adjusted to 100
mm2/s was used. A set temperature of 30˚C andrelative humidity of 25, 60 and 95%RH were used in thetest chamber. The wear test duration was set at 20, 30
and 40 minutes.

Intake and exhaust of
temperature and humidity
controlled air

WeightGears

Test oilSpecimen

Emery paper

ChamberFig. 1 Schematic of test rig

Table 1 Wear test common condition
Number of specimens

Three

No. of emery paper#220

Table rotation speed at thecenter of specimen holder

0.59 m/s

Holder rotation speed at thecenter of specimen

0.03 m/s

Contact pressure

3. Thermal desorption analysis

85.4 kPaDrop rate

After the wear tests, 3 specimens were cleaned andadded to the analyzer tube for thermal desorption
analysis. The rate of temperature rise was set to180˚C/h and the final test temperature was set to540˚C. A thermal conductivity detector (TCD) was usedas the hydrogen detector. Fig. 3 shows an example of
TDA profiles. The diffusible hydrogen was defined as
the total amount of hydrogen emitted up to 200˚C.

1 ml/min

5

t1

6

R

Surface area：104mm2

Fig. 2 Specimen shape (unit: mm)
0.03

Material

Emitted hydrogen ratio

SUJ2

Thermalprocessing Quenching temperature: 850˚C, tempering: 180˚C×2 h

Table 3 Each wear test conditions
Item

Test oilAmount ofadded waterSet temperature
Set relative
humidity
Abrasivewear time

Mineral oil

Water added oil

Fluorine-based oil

Additive-free turbine oil VG100
Perfluoro-polyether oil
2, 5, 10, 15 wt%
0 wt%
0 wt%
(Pure water)
20, 30 ˚C
30 ˚C
25〜95 %RH

25, 95 %RH

40 minNo testAfter test (30℃ 25%RH)

wt-ppmmin

Table 2 Disposal condition of specimen

0.02
Diffusiblehydrogen
0.01

0
0

100

200

300

Temperature ℃

400

Fig. 3 An example of TDA profiles

25, 60, 95 %RH
20, 30, 40 min

-89-

500

NTN TECHNICAL REVIEW No.82（2014）

0.20

Amount of diffusible hydrogen wt-ppm

4. Mineral oil
Fig. 4 shows the TDA profile of the specimen before
and after the abrasive wear tests. No diffusiblehydrogen emission was detected in the “no test” case.
The emitted diffusible hydrogen increased as therelative humidity increased at both 20˚C and 30˚C.
Fig. 5 shows the relation between the relative
humidity and amount of diffusible hydrogen. At both set
temperatures of 20˚C and 30˚C, a high correlation wasobserved.
Fig. 6 shows the relation between the absolute
humidity and amount of diffusible hydrogen.
The absolute humidity was obtained based on themeasured temperature and relative humidity.
There was a strong correlation between the absolute
humidity and amount of diffusible hydrogen with little
impact of temperature.
The results show that the amount of diffusible hydrogenin steel is dependent on the amount of vapor in the unit
volume of air.

Emitted hydrogen ratio

wt-ppmmin

0.012

Set temperature 20℃
94%RH
72%RH
44%RH
No test

0.010
0.008

100

200

0

100

200

Temperature ℃

Amount of diffusible hydrogen wt-ppm

Amount of diffusible hydrogen wt-ppm

0.20
Set temperature 30℃
Set temperature 20℃
0.15

0.10

0.05

0

20

40

60

Relative humidity %RH

80

0

0

5

10

15

20

25

30

g/m3

Fig. 7 shows the relation between the amount ofadded water and amount of diffusible hydrogen. Both in
the low humidity and high humidity cases, as theamount of water increased, the amount of diffusiblehydrogen increased. The amount of diffusible hydrogenin the low humidity test, excluding the 15 wt% water
added oil, was less than the amount of diffusiblehydrogen in the high humidity test, excluding the waterfree oil.
When water added oil was used, the amount ofdiffusible hydrogen increased compared to the waterfree oil in both high and low humidity. This indicates thatthe water added to the grease as well as the small
amount of water vapor in the air both contributed to as
diffusible hydrogen sources to the new steel surface
produced during abrasion.

Fig. 4 TDA profiles of specimens before and afterabrasive wear testing

0

0.05

5. Water added oil

0.002
0

0.10

Fig. 6 Relationship between absolute humidity andamount of diffusible hydrogen

0.004

0

0.15

Absolute humidity

Set temperature 30℃
93%RH
63%RH
40%RH
26%RH
No test0.006

Set temperature 30℃
Set temperature 20℃

100

0.25

0.20
Set temperature 30℃

0.15

0.10

0.05

High humidity (set relative humidity: 95%RH)
Low humidity (set relative humidity: 25%RH)

0

0

5

10

Amount of added water

wt%

15

Fig. 7 Relationship between amount of containing water andamount of diffusible hydrogen in cases of water containing oils

Fig. 5 Relationship between relative humidity andamount of diffusible hydrogen-90The Influence of the Humidity and Quantity of Hydrogen into Steel under the Abrasive Wear

5.1 Relationship between humidity and watercontained in oil

However, the amount of diffusible hydrogen in the
low humidity test, excluding the 15 wt% water added oil(0.153wt-ppm), was less than the amount of diffusiblehydrogen in the high humidity test, excluding water-freeoil (0.176wt-ppm). This shows that the impact on
diffusible hydrogen into the steel after abrasion waslarger for water vapor in the air than that of water added
directly to the test oil.
Table 4 shows the amount of water involved in theabrasive wear test and the amount of diffusiblehydrogen. Since the volume of the test chamber is0.0104 m3, the amount of vapor contained in the test
chamber air is 0.080g in low humidity and 0.271g inhigh humidity. When 15 wt% of water was added to thetest oil, with a drop rate of 1 g/min and an abrasion test
duration of 40 minutes, the amount of water in the
dropped oil was 6g. The total amount of water when 15
wt% water added oil was used in low humidity was
6.080g (6 + 0.080g).
On the other hand, when water-free oil was used inhigh humidity, the amount of water was 0.271 g. Thisshows that while the amount of water involved in theabrasive wear test was larger in the case of 15 wt%
water added oil in low humidity versus that of water-free
oil in high humidity, the amount of diffusible hydrogenwas smaller. While the reason why the amount ofdiffusible hydrogen varies depending on the state ofwater is unclear, the test result showed humidity had alarger impact on the amount of diffusible hydrogen insteel after the abrasion test.

It was shown that the amount of diffusible hydrogenin steel after the abrasive wear test depends on theamount of vapor in the air. It was also shown that the
impact of humidity is larger than the impact of added
water to the lubricant. Therefore, we investigated thewater content of the oil when exposed to different
humidity conditions in order to verify the impact ofhumidity on water content of the oil.
Table 5 shows the test conditions for measuring thewater content. The same additive-free turbine oil VG100
was used. A set temperature of 30˚C and relative
humidity of 30, 60 and 90%RH were used for the test
chamber. The container with test oil was exposed in thetest chamber for 30 minutes, then the water content
was measured.
Fig. 8 shows the measurement results of the watercontent in the oil. As the absolute humidity increased,
the water content in the oil also increased.
This means that in high humidity the amount of watercontained in oil increased.
It is considered that as humidity increases, watercontained in the oil increases and becomes the sourceof hydrogen. Therefore, diffusible hydrogen in steel
depends on the amount of water vapor in the air.

Table 5 Water content of oil test condition
Test oil

Table 4 Amount of water and amount of diffusiblehydrogenin wear test

Volume of chamber space
Absolute humidity

Amount of water in the chamber

Low humidity(25%RH)+added water(15 wt%)High humidity(95%RH)+no added water(0 wt%)

7.6 g/m3

25.9 g/m3

約 0.0104 m3

0.080 gTest timeAmount of added water
Amount of water in oil

Total amount of water
Diffusible hydrogen

0.271 g

1 g/min

Drop rate
15 wt%
6g

6.080 g0.153 wt-ppm

40 min30˚C30, 60, 90 %RH
30 min0.005

Amount of water content wt%

Item

Additive-free turbine oil VG100

Set temperature
Set relative humidity
Exposure time

0 wt%
0g

0.271 g0.176 wt-ppm

0.004

0.003

0.002

0.001

0

0

5

10

15

Absolute humidity

20

g/m3

Fig. 8 Water content of oil

-91-

25

30

NTN TECHNICAL REVIEW No.82（2014）

but also from decomposition of mineral oil. Therefore,
we conducted an abrasive wear test using fluorinebased oil to obtain correlation between the increase of
humidity and increase of diffusible hydrogen. Since
fluorine-based oil does not contain hydrogen in thechemical structural formula, diffusible hydrogendetected in the abrasive wear test using fluorine-based
oil derives only from water.
In addition, since the abrasive wear tests indicate alinear increase of diffusible hydrogen within the test
range, it is assumed that diffusible hydrogen is not
saturated in the steel but is in a transient state.
Therefore, it is possible that the impact of humidity will
be more significant when abrasion takes longer timethan these tests.
The impact of humidity on the amount of hydrogen
penetrated in steel has not been reviewed until now.
However, this result indicates that the impact ofhumidity is a significant factor.

6. Fluorine-based oil
Fig. 9 shows the TDA profile of the specimen before
and after the wear tests. After the abrasive wear test,
diffusible hydrogen was detected from every specimen
and the amount of diffusible hydrogen increased as therelative humidity increased (in their respective abrasion
duration). As the abrasion time increased, diffusiblehydrogen increased.
Fig. 10 shows the relationship between the relative
humidity and amount of diffusible hydrogen withfluorine-based oil. There is a high correlation observed
for all abrasion times.
Fig. 11 shows the relationship between the abrasion
time and amount of diffusible hydrogen.
As the abrasion time increased, diffusible hydrogen
increased for a given relative humidity.
In the abrasive wear test using mineral oil, diffusiblehydrogen was detected from steel and a highcorrelation was observed between the absolute
humidity and diffusible hydrogen. From this, it was
shown that the amount of diffusible hydrogen in steel
after an abrasive wear test depends on the amount ofvapor contained in the unit volume of air. However, incase of mineral oil, the source of diffusible hydrogen
could derive not only from the vapor contained in the air

0.010
0.008

Abrasive wear time 40min
Abrasive wear time 30min

Abrasive wear time 20min
0.10

0.05

0

0

20

0.004

0

100
200
Set temperature 20℃

0

100
200
Set temperature 20℃

Abrasive wear time 40min
93 %RH
63 %RH
24 %RHNo test

0.010
0.008
0.006
0.004
0.002
0

40

60

Relative humidity

%RH

80

100

Fig. 10 Relationship between relative humidity and amountof diffusible hydrogen in cases of fluorine based oil

0.002

0.012
wt-ppmmin

Abrasive wear time 30min
88 %RH
63 %RH
22 %RHNo test0.006

0Emitted hydrogen ratio

Amount of diffusible hydrogen wt-ppm

Abrasive wear time 20min
91 %RH
63 %RH
26 %RHNo testAmount of diffusible hydrogen wt-ppm

Emitted hydrogen ratio

wt-ppm
min

0.012

0.15

0

100

Set temperature 20℃

0.15

0.10

0.05

0

200

High humidity(set relative humidity: 95%RH)
Middle humidity(set relative humidity: 60%RH)
Low humidity(set relative humidity: 25%RH)

0

10

20

30

40

50

Abrasive wear time min

Fig. 9 TDA profiles of specimens before and after abrasive
wear testing in cases of fluorine based oil for wear durationof 20, 30, 40 min

Fig. 11 Relationship between wear duration and amountof diffusible hydrogen in cases of fluorine based oil-92-

The Influence of the Humidity and Quantity of Hydrogen into Steel under the Abrasive Wear

References

7. Conclusion

1) K. Tamada, and H. Tanaka, "Occurrence of brittle
flaking on bearings used for automotive electricalinstruments and auxiliary devices" Wear, 199 (1996),
pp．245-252．
2) M. Kohara, T. Kawamura and M. Egami，"Study on
Mechanism of Hydrogen Generation from Lubricants"
Tribology Transactions, 49 (2006), pp. 53-60．
3) L. Grunberg，"The Formation of Hydrogen Peroxide on
Fresh Metal Surfaces" Proc．Phys. Soc (London). B66(1953), pp. 153-161．
4) P. Schatzberg and I. M. Felsen, "EFEEECTS OF
WATER AND OXYGEN ROLLING CONTACT
LUBRICATION" Wear, 12 (1968), pp. 331-342．
5) Shigeru Asano, Yoshio Fujishima, Namio Ohtani, “TheDiffusivity of Hydrogen in Iron at Room Temperature”,
Journal of The Japan Institute of Metals，Vol. 37, Issue
3 (1973), pp. 301-306.
6) M. Nagumo, M. Nakamura, and K. Takai, "Hydrogen
Thermal Desorption Relevant to Delayed-Fracture
Susceptibility of High-Strength Steels"
METALLURGICAL ANDMATERIALS TRANSACTIONS
A, 32A (2001),pp. 339-347．
7) Hiroyoshi Tanaka, Hiraku Tanimoto, Joichi Sugimura,"Observation of Hydrogen Permeation into Fresh
Bearing Steel Surface by Thermal Desorption
Spectroscopy”, Proceedings of Japan Tribology
Conference Tokyo (2010-5), pp. 203-204.

We have investigated the impact of humidity on theamount of hydrogen penetrated in steel by conducting
an abrasive wear test in a chamber under controlledtemperature and humidity and applying thermal
desorption analysis. As a result, the following wererevealed:
1) When the abrasive wear test was conducted using
mineral oil, the amount of diffusible hydrogen
increased as the relative humidity increased for agiven temperature. When absolute humidity wasused instead of relative humidity in the arrangementof data, a strong correlation was observed with theamount of diffusible hydrogen.
2) When the abrasive wear test was conducted withpure water added to mineral oil, the amount ofdiffusible hydrogen increased both in low and highhumidity.
Within the range of ambient temperature in this test,
the amount of diffusible hydrogen in the low humidity
test, excluding the 15 wt% water added to oil, wasless than the amount of diffusible hydrogen in thehigh humidity test, excluding no water added to oil.
3) Even when fluorine-based oil was used, which does
not contain hydrogen atoms in the chemical
structural formula, diffusible hydrogen was detectedfrom the steel. The amount of diffusible hydrogen
increased when the abrasive wear duration wasincreased.

Photo of author

Motohiro ITOAdvanced Technology
R&D Center-93-

NTN TECHNICAL REVIEW No.82（2014）[ Technical Paper ]Evaluation of Shear Fatigue Properties in Medium Carbon Steels
and Rolling Bearing Steels up to Giga-cycle Test Regimes
Noriaki MIWA*

We have developed an ultrasonic torsional fatigue tester which enables for the
rapid evaluation of shear fatigue properties of high strength steels. This reportpresents the test results of shear fatigue properties of induction hardened
medium carbon steels, and rolling bearing steels, up to giga-cycle regimes as
evaluated by the developed tester.

1. Introduction

2. Impact of boron (B) on shear fatigue
properties of induction hardened medium
carbon steels up to 1x109 cycles

Repetitive torsional load is applied to the shafts ofconstant velocity universal joints, which are used in
front-wheel drive and four-wheel drive vehicles andindustrial machines. Therefore, it is important toinvestigate shear fatigue properties up to giga-cycle test
regimes in order to guarantee long-term reliability of thematerials. As one of the stresses governing generationof cracks before internal-originated cracking in rolling
bearings , NTN can report completely reversed
alternative shear stress acted repeatedly on thesurface 1). Therefore, it is important to evaluate shear
fatigue properties in order to determine life of rolling
bearings.
The shear fatigue life can be obtained by a torsional
fatigue test. However, since the load frequency of theconventional torsional fatigue tester is low, evaluation
up to 1x109 cycles has been practically impossible.
Therefore, we developed an ultrasonic torsional fatigue
tester (USTF tester) aimed at rapid evaluation of shear
fatigue properties of high strength steel. Since thecommercially available torsion vibration converters have
low power, we optimized the shapes of torsion
amplitude magnifying horns and specimens andsucceeded to provide shear stress amplitude of around
950 MPa max 2). In this paper, we discuss the following
shear fatigue property evaluation results using thedeveloped ultrasonic torsional fatigue tester:(1) Impact of B on shear fatigue properties of induction
hardened medium carbon steels up to 1x109 cycles(2) Relationship between shear fatigue life and rolling
fatigue life of bearing steels obtained by an
ultrasonic torsional fatigue test.

2.1 Background
Induction hardened medium carbon steel S40C isused for the shafts of ordinary constant velocity joints.
When the shaft diameter is relatively small, the material
with increased manganese (base steel) is used to
improve the quenching property and when the shaft
diameter is large, boron (B) is added to the base steel
or its equivalent (boron-added steel) to further increase
the quenching property. NTN has evaluated shear
fatigue properties of shaft materials up to 1x109 cycles
with an ultrasonic torsional fatigue tester andinvestigated the impact of added boron on the fatigue
properties.

2.2 Test method
Boron-added steels, where 19 ppm of boron and 420
ppm of Ti are added to the base steel , were used as
test materials. Fig. 1 shows the induction hardening
conditions and Fig. 2 shows the diagram of thesample. The gray area of Fig. 2 is heated to adjust thehardness of the hatched area including the smallest
diameter area to 700 HV. Then, the specimen istempered at 170˚C x 1h and finished by grinding. To
eliminate the impact of surface roughness, emery
abrasion (#500 and #2000) and diamond wrapping(grain size of 1 μm) were applied for a mirror finish
before the ultrasonic torsional fatigue test. Resonancefrequency of 20000 Hz and stress ratio R = -1
(completely reversed) were used as the test conditions.
When a shear fatigue crack is generated andpropagated in the sample, resonance frequency*Advanced Technology R&D Center-94-Evaluation of Shear Fatigue Properties in Medium Carbon Steels and Rolling Bearing Steels up to Giga-cycle Test Regimes

decreases.
It was determined that the sample was damaged
when resonance frequency exceeded the preset range,
and the test was terminated. Since continuous loads
would cause the specimen to heat up in the USTF test,
an intermittent load method was used, alternating loads
and pauses 3). In the case of shear stress amplitudes ofmore than 700 MPa, 0.11 sec of load and 1.1 sec ofpause were used until damage was detected fromheat generation.
In the case of less than 700 MPa, 0.11 sec of loadand 1.1 sec of pause were used 1x107 times, then theconditions were changed to 1 sec of load and 0.2 secof pause. When no damage was detected up to 1x109
cycles , the test was terminated. The real loadfrequency of 0.11 sec of load and 1.1 sec of pause is1803 Hz and that of 1 sec of load and 0.2 sec of pause
is 16529 Hz 2).

850

Shear stress amplitude MPa

800

B-added steel (dashed line)

750
700
650
600
550
500
450
105

Base steel (solid line)
Base steel (Shear-mode crack in axial direction)
Base steel (Shear-mode crack in circum. direction)
B-added steel (Shear-mode crack in axial direction)
B-added steel(Shear-mode crack in circum. direction)
106

107

108

Nnmber of cycles

109

1010

Fig. 3 Shear fatigue properties of Base and B-added
steels evaluated by USTF tester

Temperature ˚C

1050
25kHz

5

water-soluble
quenching(a) Base steel(b) B-added steel

Fig. 4 SEM images of fracture surface at initial shear
fatigue cracking

10

Time sec.

intergranular enhancement elements 4); its effect could
not be confirmed since the fracture in theseexperimental conditions were transgranular.

Fig. 1 Induction heat treatment condition

3. Relationship between shear fatigue life androlling fatigue life of bearing steels obtained
by ultrasonic torsional fatigue testing.

Fig. 2 Specimen shape and hardened region

3.1 Background
2.3 Results and observation

Shear fatigue properties of carbon steel for machineparts S53C and heat-resistant carburized steel M50NiL(AMS6278) were evaluated. In addition, we investigated
the relationship between the shear fatigue properties
and rolling contact fatigue (RCF) test results of these
two steels and SUJ2 bearing steel 2) for which previous
test results exsist.

Fig. 3 shows the S-N curves of the base steel andthe Boron-added steel. S-N curves were obtained byapplying the fatigue test results to the continuous drop
single logarithm curve model of the Standard
Regression Method of S-N Curves from the Society ofMaterials Science, Japan. Fatigue strength of the basesteel and Boron-added steel after 1x108cycles with
10% fracture probability was 636 MPa and 643 MPa,
respectively, almost equal.
Shear type initial cracks started at the smallest
diameter , and propagated and transitioned to tensile
type. Shear type initial cracks are either axial or radial
and the latter are plotted with a horizontal line in Fig. 3.
Fig. 4 shows the SEM images of the fracture surface atinitial shear fatigue cracking. Both materials exhibited
transgranular fracture. Although B is reported to be

3.2 Test method
Table 1 shows the chemical contents of S53C and
M50NiL, used for the USTF test samples Fig. 5 shows
the cross section hardness distribution of the USTF
samples at minimum diameter. To eliminate the impact
of surface roughness, emery abrasion (#500 and#2000) and diamond wrapping (grain size of 1 μm)
were applied for a mirror finish before the ultrasonictorsional fatigue test.
-95-

NTN TECHNICAL REVIEW No.82（2014）

The USTF test conditions are the same as those in
Section 2.2. When there was no damage up to 1x1010
cycles, the test was terminated.
Fig. 6 shows the schematic diagram of the RCF test
equipment and Table 3 shows the test conditions. The
samples are driven by the driving roller to cause rolling
fatigue with the 3/4 inch steel balls in contact under highcontact stress conditions. When damage occurs in thesample , vibration of the tester increases, which wasused as the termination signal.

The diameter and length of the RCF samples were
12 mm and 22 mm, respectively.
Table 2 shows the number of lots, total number ofsamples , as well as the average, maximum andminimum hardness measurements of samples
evaluated in the tests.

Table 1 Chemical compositions of S53C and M50NiL
used for USTF specimens (mass %)
Cr

Mo

V

0.54 0.17 0.86 0.011 0.008 0.02 0.14

-

-

C
S53C

Si

Mn

P

S

Ni

3.3 Results

M50NiL 0.12 0.20 0.25 0.005 0.002 3.40 4.08 4.15 1.18

Fig. 7 shows the shear fatigue properties of three
types of steels, namely, the two steel types tested inthis paper and SUJ2 2) . S53C was the lowest both infatigue strength at finite life and fatigue limit (442 MPa)among the three steel types evaluated so far. M50NiL
was the highest both in fatigue strength at finite life andfatigue limit (680 MPa) among the three steel types.
The slopes of shear fatigue properties of the three steel
types were not significantly different within these fatigue
strength cycles.
Table 4 shows the average, maximum and minimum
values of 50% life of these three steel types in the RCF
tests previously conducted. The overall performance of
50% life was S53C<SUJ2<M50NiL.

Vickers hardness HV

900
800
700
600
500

S53C
M50NiL

400
0.0

0.5

1.0

1.5

2.0

Depth from surface mm

Fig. 5 Hardness distributions of S53C and M50NiL at
minimum circumference of USTF specimens
1200

Hardness (HRC)

Numberof lots

Total numberof specimens

Ave.

Min.〜Max.

9

124

62.4

62.0〜62.8

SUJ2
S53C

M50NiL2
3

Shear stress amplitude MPa

Table 2 Number of lots, total number and hardness of RCF
specimens

34
63

61.0
63.8

60.5〜61.5
62.4〜64.5

Driving roller

SUJ22)S53C
M50NiL1000

τw0=680MPa (M50NiL)

800
600
400

τw0=577MPa (SUJ2)2)
τw0=442MPa (S53C)

200
0
105

Specimen
3/4” ball

106

107

108

109

Number of cycles

1010

1011

Fig. 7 Shear fatigue properties of three kinds of bearing
steels evaluated by USTF tester

Table 4 RCF test results
Guide roller

Fig. 6 Schematic of RCF tester
SUJ2

Table 3 RCF test conditions
Ball
Max. contact pressure
Loading rate
Lubricant

S53C

M50NiL3/4” (19.05mm)×2 made of SUJ2
5.88 GPa

46240 c/min

Turbine oil VG68

-96-

RCF life L50 (cycles)

Numberof lots

Average

Minimum〜Maximum

9

1.5×108

8.5×107〜2.1×108

2
3

3.5×107
1.0×109

3.3×107〜3.7×107

7.3×108〜1.2×109

Evaluation of Shear Fatigue Properties in Medium Carbon Steels and Rolling Bearing Steels up to Giga-cycle Test Regimes

3.4 Observation(2) S53C was the lowest both in fatigue strength at finite
life and fatigue limit (442 MPa) of shear fatigue
properties among the three steel types evaluated.
On the other hand, M50NiL was the highest both infatigue strength at finite life and fatigue limit (680MPa).
(3) The slopes of shear fatigue properties of the threesteel types were not significantly different withinthese fatigue strength cycles.
(4) Linear correlation was found between shear fatigue
life within finite life (fracture probability 50%) atstress amplitude 700 MPa and rolling fatigue life L50
from previous rolling fatigue testing.

Fig. 8 shows shear fatigue life within finite life
(fracture probability 50%) at a stress amplitude of 700
MPa of three steel types and rolling fatigue life L50. A
linear relationship was observed between the data sets.
Therefore, it was implied that the USTF test may
become an alternative to the RCF test. That is, it could
be an effective tool for determining relative superiority of
RCF life.

Shear fatigue life at stressamplitude of 700MPa cycles

109

108

This paper has been edited based on the original
manuscripts 5) and 6). We thank the Iron and SteelInstitute of Japan and Japanese Society of Tribologists,
who gave us the permission to publish it.

107

SUJ2

S53C

References

M50NiL
106
107

108

109

1) G. Lundberg and A. Palmgren: DYNAMIC CAPACITY OF
ROLLING BEARING, Acta Polytechnica, MechanicalEngineering Series, 1-3, (1947) 5-58.
2) Noriaki Sakanaka, Yukio Matsubara, Yoshinobu
Shimamura, Hitoshi Ishii: Rapid Evaluation of Shear
Fatigue Properties of Rolling Bearing Steels for Lifespans
Up to Gigacycle Range, NTN TECHNICALREVIEW, No.
79, (2011) 104-110.
3) H. Ishii, K. Yamanaka and K. Tohgo: GIGA-CYCLEFATIGUE STRENGTH IN SOME HIGH STRENGTHSTEELS BY ULTRASONIC FATIGUE TESTING, Material
Science Research International, Special TechnicalPublication-1, (2001) 59-63.
4) M. Yamaguchi: FIRST-PRINCIPLES STUDY ON THE
GRAIN BOUNDARY EMBRITTLEMENT OF METALS BY
SOLUTE SEGREGATION: PART1.
IRON (FE) -SOLUTE (B, C, P, AND S) SYSTEMS,
Metallurgical and Materials Transaction A, 42, 2, (2011)319-329.
5) Sakanaka, Matsubara: Impact of Boron on Shear Fatigue
Properties of Induction Hardened Medium Carbon Steels
for Lifespans Up to Gigacycle Range, CAMP-ISIJ, 26,(2013)316.
6) Miwa: Relationship Between Shear Fatigue Life of Bearing
Steel obtained from torsional fatigue test and Rolling
Fatigue Life, Tribology Conference 2013 Autumn
Proceedings (2013-10) E6.

1010

RCF life L50 cycles

Fig. 8 Relationship between shear fatigue life and RCF life

4. Conclusion
Since torsional loads are applied repeatedly to theshafts of constant velocity joints, it is important toevaluate their shear fatigue properties. We evaluatedshear fatigue properties of the materials with increased
manganese on S40C (base steel) and boron (B) addedon the base steel or its equivalent material up to 1x106 cycle regimes with an ultrasonic torsional fatigue tester.
On the other hand, it is important to evaluate shear
fatigue properties in order to determine rolling fatigue
life, since alternate shear stress is considered to be oneof the stresses governing crack generation and its
initial propagation causing internal-originated damageof the rolling bearings. Therefore, we evaluated shear
fatigue properties of carbon steel for machine parts
S53C and heat-resistant carburized steel M50NiL(AMS6278). In addition, we investigated the relationshipbetween shear fatigue properties and rolling fatigue life
test results of three steel types including two steel types
in this paper and bearing steel SUJ2 which had beenpreviously evaluated.

Photo of author(1) Fatigue properties of base steel with increased
manganese to S40C and boron-added steel(fracture probability 10%) after 108 cycles weresimilar with 636 MPa and 643 MPa, respectively,
exhibiting no difference in shear fatigue properties.

Noriaki MIWA
Advanced Technology
R&D Center-97-

NTN TECHNICAL REVIEW No. 82（2014）

Award Winning Product2013 “CHO” MONODZUKURI Innovative Components Awards Nippon Brand Award

Parallel Link High Speed Angle Control Equipment
Hiroshi ISOBE,

Yukihiro NISHIO,

Seigo SAKATA,

Naoya KONAGAI,

Hiroyuki YAMADA

Parallel link high speed
angle control equipment

1. Introduction
NTN developed Parallel Link High Speed Angle Control
Equipment (hereafter, equipment) applying the special parallel
link mechanism of rotational 2-degrees-of-freedom, and applied
this equipment to grease dispensing. This equipment wasrecognized for its ability to achieve high speed and highly
accurate positioning in a broad operating range, with anoperating angle of 90˚ and angle of traverse of 360˚, andreceived the 2013 “CHO” MONODZUKURI Innovative
Components Awards Nippon Brand Award.

Stand
Dispenser
Work

X

XY stageY

Fig. 2 System configuration example of grease dispensing

2. Backgroundcomplicated shapes by enabling application at varying angles.
The equipment is capable of setting commanded angles byspecifying XY coordinates on the operating stage, to facilitateteaching input tasks upon introduction into a new productionline.
We hope that the equipment will be used in various fields
such as medical, service, entertainment, etc. in addition to
production facilities, and will contribute to further advancementof automation and robotization where Japan is competitive.

The parallel link mechanism adopted in this equipment was
originally developed as a constant velocity joint for automobiles,
achieving high operating angle with a maximum of 90°. Aiming
for applications other than automobiles, we continued
developing this parallel link mechanism as angle control
equipment by combining actuators with it, and with aninnovative driving mechanism and control methods, we could
achieve smooth operation of rotational 2-degrees-of-freedom
plus high speed/high accuracy operations 1)-5). Recognized for
its high speed and superior operability, it is used in the grease
application mechanism in the production lines of automobile
parts, where reduced tact time and high reliability are required.

References
1) Keisuke Sone, Hiroshi Isobe, Koji Yamada: Wide Angle Active Link Equipment,
NTN TECHNICAL REVIEW No. 71, (2003) 70-73.
2) Hiroshi Isobe, Yukihiro Nishio: Parallel Link High Speed Angle Control
Equipment (PHACE), NTN TECHNICAL REVIEW, No. 80, (2012) 42-47.
3) Hiroshi Isobe: Parallel Link High Speed Angle Control Equipment – Basic
Configuration and Application for Grease Dispensing, Machine and Tool,
February Issue, (2013) 83-87.
4) Hiroshi Isobe, Yukihiro Nishio, Keisuke Sone, Hiroyuki Yamada, Yoshio
Fujikawa: Parallel Link High Speed Angle Control Equipment, Proceedings of
the Japan Society for Precision Engineering Spring Meeting in 2013, (2013)809-810.
5) Hiroshi Isobe, Yukihiro Nishio, Seigo Sakata, Naoya Koganei, Hiroyuki
Yamada, Yoshio Fujikawa: Parallel Link High Speed Angle Control
Equipment—Implementation on Grease Application—, Proceedings of theJapan Society for Precision Engineering Spring Meeting in 2014, (2014) 10871088.

3. Implementation on grease applicationequipment
Fig. 1 shows the equipment developed for grease
application, and Fig. 2 shows an example of grease applicationequipment configuration where a dispenser for contact-lessapplication is attached to the tip of this equipment. In thisconfiguration, the equipment moves the work on the XY stage
and controls the behavior of the dispenser. It achieves highspeed application of around 10 points per second andautomates the grease application process on workpieces with

Drivingmechanism

Photo of authors (presenters)

Parallel linkmechanism

Tip

Fig. 1 Parallel link high speed angle control equipment-98-

Hiroshi ISOBE

Yukihiro NISHIO

Seigo SAKATA

New Product Development
R&D Center

New Product Development
R&D Center

New Product Development
R&D Center

NTN TECHNICAL REVIEW No. 82（2014）

Award Winning Product2013 Japan Powder Metallurgy Association, New Product Award in New Materials Division

Development of the Oil Seal Made of Sintered Metal
for Variable Valve Timing Device
Yosuke SUGAI

2.3 Improved machinability and accuracy by optimized sintering
conditions
In addition to the enhanced strength by adding low-melting point
metals, we have added a sizing process to improve dimensional
accuracy. This is possible by the optimization of the sintering
conditions. Fig. 3 shows the dimensional accuracy. By ensuring thedeveloped material made of almost only ferrite phase, we can achieve
excellent sizing properties and required dimensional accuracy.

1. Introduction

Transverse rupture
strength N

Variable valve timing devices for automobiles are used in various
types of vehicles for fuel efficiency, processing of exhaust gas, etc.
and demand is expected to increase particularly in the emerging
markets.
Hydraulic variable valve timing devices are equipped with multiple
oil seals, as shown in Fig. 1, to divide hydraulic chambers.
Conventional oil seals are made of resin; however, we have developed
oil seals made of sintered alloy and introduced them into the market.
The characteristics required for oil seals are sealing capability andstrength. If dimensional accuracy is not sufficient, oil may leak from thegaps with the surrounding components and affect the operating
responsiveness of variable valve timing devices. Insufficient strength
may cause deformation and damage to the devices.
The developed oil seals made of sintered metals received “The 35th
Japan Powder Metallurgy Association, New Product Award in New
Materials Division” which is awarded to superior new technologies inpowder metallurgy. In this article, we present an overview of thisdevelopment.

Oil seal

Toshihiko MOURI

400
300
200
100

Developed material
sintered compact

Segregation-freeprocessed powder

Resin

Fig. 2 Comparison of transverse rupture strength
4.0

Hydraulic chamber

Cp 2.00.0

Camshaft

0

Total length Cp

Straightness Cp

FG64

Cam pulley

Segregation-free Segregation-freeprocessed powder processed powder+
sizingDeveloped
material+
sizingFig. 3 Dimensional precision (Cp value)

Fig. 1 Point-of-use of the oil seal

2. Features of the developed product3. Achieved results from the development

2.1 Good dimensional stability due to segregation-free processing
Ordinary ferrous sintered material, FG64, has unstable dimensional
accuracy. This is due to segregation of graphite contained in thematerial which prevents proper flow of powder and leads to unstablecompact weight during the molding process, resulting in reduced
dimensional accuracy.
Therefore, we used ferrous powder and segregation-freeprocessing to graphite to improve the powder flow and to stabilize thecompact weight. This resulted in good dimensional stability.

By using segregation-free graphite powder, the addition of tin andoptimization of sintering conditions, we can achieve good dimensional
accuracy and sealing abilities , B We will continue our development ofoil seals made of sintered metal to improve the function of variable
valve timing equipment.

Photo of authors
2.2 Strength enhancement by mixing low-melting point metal
In order to ensure seal strength with sintered materials, a low
melting point metal, tin (Sn), was selected and mixed in the optimum
proportion. Since tin has good wettability with copper (Cu), when it issintered below the melting point of Cu, it is diffused to facilitate bondingof ferrous powders, which improves strength of the sintered product .
Fig. 2 shows the comparison of transverse rupture strength as anindex of the strength of seals.
Transverse rupture strength evaluates bend strength of the material
by applying bending loads at three points on the test sample without
cutouts. As a result, the developed material almost doubled in strengthcompared with the material made of resin.

-99-

Yosuke SUGAI

Toshihiko MOURI

Engineering Dept.
NTN Powder Metal Corporation

Engineering Dept.
NTN Powder Metal Corporation

NTN TECHNICAL REVIEW No. 82（2014）

Award Winning Product2013 Japan Powder Metallurgy Association, Effort Prize

Sintered Hydrodynamic Bearing for Super Thin Fan Motor
Tadahiro UCHIYAMA

Shinji KOMATSUBARA

2mm

Peak area

1. Introduction
As mobile devices rapidly expand, the cooling fan motors used inthese devices are becoming increasingly thin. The use of thesedevices is becoming much more diverse and the operating
environment is also becoming much more challenging. Though theoperating environment is becoming more challenging, therequirements for the bearings are to provide lower noise and morereliable operation than ever.
By leveraging high precision and high rigidity of the conventionalsintered hydrodynamic bearings, we have developed sinteredhydrodynamic bearings for super thin cooling fan motors (Fig. 1).
In this article, we present an overview of this product development,
which received “2013 Japan Powder Metallurgy Association, Effort
Prize.”

Conventional
product

1.8mm
Groove area
4mm

Fig. 2 Example of sintered hydrodynamic bearing
Shaft
4mm
Housing

Thrust washer

Sintered hydrodynamic
bearings

Fig. 3 Example of a super thin fan motor application
2.2 Optimization of the hydrodynamic groove machining conditions
In order to improve hydrodynamic bearing performance, we worked
on higher precision molding for forming hydrodynamic grooves as away to improve groove precision. We have optimized thehydrodynamic groove machining conditions for molding and also
improved the molding precision. We also optimized hydrodynamic
groove machining conditions for the bearings such as timing andmachining margin during the dynamic groove machining process to
obtain precise and stable groove depth and shape and enabled highprecision machining of hydrodynamic grooves.

Developed
product

Fig. 1 Appearance of sintered hydrodynamic bearing

2.3 New development of lubricant oil
Due to the more severe operating conditions and property
requirement, we also worked on improving the lubricant. We developed
a lubricant oil which is appropriate for the bearings of super thin fan
motors considering reduced rigidity in high temperature and increased
bearing loss in low temperature, etc.

2. Description of development
Sintered hydrodynamic bearings have a structure shown in Fig. 2.
Sintered bearings have herringbone-like hydrodynamic grooves
formed on the inner surface of the sleeve-shaped bearing rings where
fluid lubrication film in the bearing clearance produces pressure(hydrodynamic pressure) by the rotation of the shaft to support therotary shaft. In addition, as the material is the sintered alloy, lubricant iscontained within the bearings resulting in a good anti-seizure property.
Fig. 3 shows an example of a super thin fan motor. The height ofthe motor is 4 mm and the length of the shaft must be 2 mm or less.
When the length of the bearings is short, rigidity and supportive force
are reduced; therefore, how to maintain bearing rigidity is the main
challenge. We have focused on the specification of hydrodynamic
grooves and their machining conditions. By optimizing the thehydrodynamic grooves, we have succeeded in development of sinteredhydrodynamic bearings for super thin fan motors with a shaft length of
2 mm or less.

3. Conclusion
We have developed sintered hydrodynamic bearings for super thin
fan motors by optimizing the design of the dynamic groove
specification, optimized dynamic groove machining conditions and useof a newly developed lubricant oil. Its superior quietness and reliability
were recognized and the bearings were widely adopted by fan motor
manufacturers in Japan and abroad. These bearings are highly
recognized by end users such as the PC manufacturers and expansionof further adoption can be expected.

Photo of authors

2.1 Optimization of the hydrodynamic groove specification
As mentioned earlier, with shorter bearing length comes decreased
bearing rigidity which may cause noise or short product life. Therefore,
we created our own analytical program for theoretical analysis basedon the rotor weight and motor structure, etc. which affect the bearing
performance. We then optimized the design parameters such as
hill/groove ratio of dynamic bearings, angle of groove and width ofgroove for super thin fan motors. We also conducted a verification test
using motors to establish hydrodynamic bearing specification backed
by theory and evaluation of the actual units in the early stage andsignificantly reduced the development and design time.

-100-

Tadahiro UCHIYAMA

Shinji KOMATSUBARAEngineering Dept.
NTN Powder Metal Corporation

Fluid Dynamic Bearing Unit Dept.
Composite Material Product Division

NTN TECHNICAL REVIEW No. 82（2014）

Award Winning Product2013 Japanese Society of Tribologists Effort Prize

Development of Ultrasonic Torsional Fatigue Tester for
Rapid Shear Fatigue Property Evaluation of Rolling Bearing Steels
Noriaki MIWA

It is not possible to directly measure shear stress applied tothe smallest diameter area of the specimen. Therefore, first we
obtained the relationship between the maximum shear stressand torsional angle by FEM analysis, then the relationshipbetween the amplifier output and torsional angle throughexperimenting, and finally, identified the relationship between
the amplifier output and the maximum shear stress, to control
the load stress.

1. Introduction
As one of the stresses of rolling bearings governing crack
generation before internal-originated flaking and its initialprogress, we can point out almost completely reversed
alternating shear stress acting repeatedly on the surface. The
shear fatigue life can be obtained by torsional fatigue test.
However, since the load frequency of the conventional torsional
fatigue tester is low, evaluation up to the giga-cycle waspractically impossible. Therefore, we developed an ultrasonictorsional fatigue tester (vibration frequency of 20000 Hz,
completely reversed) aiming at rapid evaluation of the shear
fatigue property of high strength steel for rolling bearings 1). This
achievement was published in the journal of Japanese Societyof Tribologists, “Tribologist”, and received the 2013 JapaneseSociety of Tribologists Effort Prize.

3. Evaluation of bearing steel and estimationof fatigue limit contact pressure
Fig. 2 shows shear fatigue properties of SUJ2 up to gigacycle regimes evaluated with the developed tester. We havedevised an estimation method of fatigue limit contact pressure
from the shear fatigue limit which is drawn from the obtained
shear fatigue properties.
The fatigue limit contact pressure of SUJ2 estimated from theshear fatigue limit was almost identical with the ISO Standard.

2. Production of ultrasonic torsional fatigue
testerFig. 1 shows the configuration of the tester. The main
components are a torsional vibration converter, amplifier andtorsion amplitude magnifying horn. Since the rated output of theconverter was small, the shape of the specimen and theamplitude magnifying horn needed to be adjusted. The
specimen is dumbbell shaped and shear stress becomeslargest on the surface of the smallest diameter area whentorsional load is applied. Shear stress on the specimen isincreased as the size of the specimen increases; however,resonance became unstable and we could not continue the test.
Therefore, we limited the size of the specimen so that stable
resonance is provided at up to 90% of the amplifier's output andincreased the magnifying rate by improving the shape of thetorsional magnifying horn so that high shear stress could bestably given.

Shear stress amplitude MPa

1200
1000

S-N diagram

800
600

τw0＝577 MPa

400
200
0
105

106

107

108

109

Number of cycles

1010

1011

Fig. 2 Shear fatigue property of SUJ2 evaluated bydeveloped tester

Converter of
torsional vibration

References
1) Miwa, Matsubara, Shimamura, Ishii, Tribologist, 58, (2013) 658-666.

Photo of author

Specimen
Horn to magnify
torsional angle

Noriaki MIWA
Advanced Technology
R&D Center

Fig. 1 Schematic of ultrasonic torsional fatigue tester-101-NTN TECHNICAL REVIEW No.82（2014）[ Our Line of New Products ]Super Low Friction Hub Bearing
Significant reduction of hub bearing rotational friction during vehicle operation,
Results in approximately 0.3% improved fuel economy!

Features

Structure

33% reduction in bearing rotational friction(compared with the conventional units)

Minute dimples
in lip surface(1) Newly developed low friction grease with improved
ingredient composition and low viscosity madeexclusively for bearings(2) Minute dimples implemented to the seal lip sliding
surface

Outer seal

Inner seal

Applications¡Axles for passenger cars

Low friction grease
exclusively for bearings-102-

Our Line of New Products

EBW Driveshaft
EBW : Electron Beam Welding

Contributes to shorter development lead time
for automobile manufacturers with an innovative new production method!
Driveshaft on the differential gear side
Outer ring cup section(standard design)

Stem section(individual design for each model)

Tire side

Differential gear side

Welded section: Electron beam welding

Driveshaft (entire unit)

Features

Structure

Contributes to shorter development leadtime for new models and higher reliability

Characteristics of electron beam welding(compared with friction welding)(1) Common outer ring cup section which had beendifferent in vehicles in the past (only the stem issubject to individual designs)(2) Secure and high precision welding in a short time withelectron beam welding

Electron beam
welding

Friction
welding

Welding time◎○

Strength of welded section◎◎

Concentric accuracy of welded section○△

Secondary processing after welding

Not required

Required

Item◎Excellent

Applications¡Driveshafts for passenger cars

-103-

○Good

△Fair

NTN TECHNICAL REVIEW No.82（2014）

Low Section High Strength Press Pulley
By optimally shaping the design and improving press molding,the industry's highest load resistance is achieved!

Features

Structure(1) Steel sheets 1.5 times thicker than conventional press
pulleys can be applied⇒Improves the pulley load resistance by two times or more

Pressed steel sheet(2) Low pulley section of approximately 70% ofconventional products⇒Improves bearing operating life and load capacity(3) High durability under high temperature conditions andexcellent wear resistance against foreign matterssuitable for application in vehicles for emerging markets

Applications¡Automobile engine accessory belt drive system
Air-conditioner compressor

・ Optimal design achieved by utilizing analysis
technology

Low section highstrength press pulley・ Improved press technology results in a reduced
section and thicker sheets
・ Revised thermal processing conditions for astronger mold, preventing mold damage andseizure.

Alternator
Crank pulley

Accessory belt

-104-

Our Line of New Products

Long-life Needle Roller Bearing Unit
More than 10 times the operating life compared to conventional productswith contaminated lubrication!

Features

Structure

Long-life roller bearing unit achieving more than
10 times the operating life with contaminatedlubrication compared to conventional products

Roller

Shaft

Cage

(1) Special (heat treatment) for rollers and shafts forlonger operating life with contaminated lubrication(2) Improved roller crowning shape to limit edge stressdue to bearing installation error(3) Optimal design for the cage pocket shape to reduce
introduction of contaminants(4) Added thrust washer discharge groove to improve
contaminant discharge (oil discharge quantity: 6 times
the conventional method)

Planetary gear(partially cut away)

Thrust washer
Gear and bearing cut away
(planetary reducer)

Applications¡Planetary reducers for construction andagricultural machinery
Swing drive(Reduction gear box)

Final drive(Reduction gear box)
Hydraulic excavator-105-