Special Issue;
Products for Industrial Machinery andElemental Technologies
October 2010

ISSN 0915-0528 OSAKA, JAPANR

TECHNICAL REVIEW
No.78

Special Issue:
Products for industrial machineryand elemental technologies

corporation

NTN TECHNICAL REVIEW No.78
CONTENTS
Yoshikazu FUKUMURA

Preface
Contribution

New Machining Technologies and Tribology
Takashi NAKAMURA Vice President, Nagoya Institute of Technology

1

2● New Products and Technology for Manufacturing Equipment "NTN supports your "Mono-Zukuri" evolution."
Technical Trend of Machine Tool Bearings
Hiroshi TAKO and Yasutsugu TANAKA

Application of Ceramics to NU-Type Cylindrical Roller Bearings for Machine Tool Main Spindles
Masatsugu MORI and Takuji KOBAYASHI

Evaluation of Scratched Contact Fatigue Life with Artificial Dent of Carbonitrided SUJ2 Steel
Whose Surface Nitrogen Content is Controlled

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Chikara OHKI

Plasma Nitriding Treatment of High Alloy Steel for Bearing Components
Kazuhiro YAGITA and Chikara OHKI

Improve Productivity for Warm Ring Rolling Process of Bearing Race
Eric PELLETIER and Pierre EHINGERAir Oil Lubrication Bearings with Re-lubricating Hole on the Outer Ring for Machine Tool
Futoshi KOSUGI and Kouji NISHINO

High-speed Angular Contact Ball Bearings New 9 Series for Machine Tool
Futoshi KOSUGIBall Screw Support Angular Contact Thrust Ball Bearing Unit "BSTU Series"
Hiroki TANIMURA

Ultra Small Grinding Sludge Briquetting Machine
Akira NISHIYAMA, Michio TANAKA and Yuuki HONMA

Compact and Multifunction Controller for Parts Feeder
Kunihiko SUZUKI

NTN-SNR High Performance and Flexible Linear Modules Series AXDL
Ulrich GIMPEL and Michael WILLE● Technical Papers Technical Articles New ProductsHigh Resolution Sensor Bearing with an Index Signal
Hiroyoshi ITO, Toru TAKAHASHI, Pascal DESBIOLLES, Cyril PETERSCHMITT and Shintarou UENO

Improved Method of Rolling Bearing Fatigue Life Prediction Under Edge Loading Conditions
Haruo NAGATANILong Life Grease Added Naturally Derived Antioxidants
Yosuke TAGUCHI and Hidenobu MIKAMI

Improvement of Grease Leakage Prevention for Ball Bearings Due to Geometrical Change ofRibbon Cages

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50
54
59
63
69

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83
91
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Norihide SATO and Tomoya SAKAGUCHINTN-SNR High Performance Oil-lubricated Plummer Blocks SNOE@
Ulrich GIMPEL, Martin SOMMER and Jens ULBRICH

Development of Oil-impregnated Sintered Bearing of Low Wear and Corrosion Resistance
Kazuhiro KIMURA

Fluid Dynamic Bearing Unit for the Home Ventilation Fan
Masaharu HORI

Surgical Support System for Cerebral Aneurysm Coil Embolization
Yoshitaka NAGANO, Yukihiro NISHIO, Noriaki MATSUBARA, Shigeru MIYACHI and Hideo FUJIMOTO

Automatic Repair Technology of Fine Pattern in LCD Manufacturing Process
Hiroaki OBA

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129

● Our Line of Award Winning Products"2010 Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology (MEXT) " Science and Technology Award

The Commendation for Science and Technology by the Minister of Education,Culture, Sports, Science and Technology in Development Category

136

Chikara OHKI
"The 25th Annual Meeting of The Japanese Society for Neuroendovascular Therapy" Gold Prise for Excellence in a Dissertation Award

Development of Optical Force Sensor System for Cerebral Aneurysm Coil Embolization

137

Noriaki MATSUBARA, Shigeru MIYACHI, Yoshitaka NAGANO, Hideo FUJIMOTO et al.
"The Japanese Society of Tribologists 2009" Encourangement Award

Study of a Long-Life Thrust Needle Roller Bearing Lubricated with Low -Viscosity Lubricant

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Hiroki FUJIWARA

Our Line of New Products

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NTN TECHNICAL REVIEW No.78（2010）[ Preface ]An introduction to this special edition on products forindustrial machinery and elemental technologies

Yoshikazu FUKUMURAManaging Director

Concerns about global warming and other environmental problems have been growing rapidly. Global
responses to these concerns are increasing. In the automotive field, this means increasing fuel efficiency
and converting to electric power. The industrial machinery field has seen the development of equipmentthat uses natural energy sources, such as wind and solar power.
In these changing times, harmonious existence with the global environment is also a focus for NTN.
Using advanced technological abilities that we have so far developed, we are developing green products
based on the concepts of low torque, long life and reduced size and weight to conserve resources andenergy.
At NTN, we have published this journal to correspond with the holding of the 25th Japan International
Machine Tool Fair It features NTN’s cutting-edge technology and new products for industrial machinery,in addition to fundamental concepts and ideas, This fair, also known as JIMTOF 2010, has the theme of“Mono-Zukuri Innovation” and is being held from October 28 to November 2, 2010.
We start our journal with an introduction entitled, “New manufacturing technologies and tribology,”
contributed by Takashi Nakamura, who is an authority in the field of tribology and Vice-President of the
Nagoya Institute of Technology. This is followed by discussions about fundamental bearing technology forindustrial machinery, including machine tools, tribology, ceramics and sensor technologies, as well as ourrecent developments with new technology and products. We also describe three products we developedthat received awards this year, including the Commendation for Science and Technology by the Ministerof Education, Culture, Sports, Science and Technology and an award from the Japanese Society of
Tribologists.
At JIMTOF 2010, NTN is exhibiting with the theme, “NTN: Supporting Evolving Manufacturing(manufacturing that is good for the environment).” We are presenting new technology and products
related to industrial machinery and elemental technology. Even before now, we had brought to market the
Ultage Series, which responds to industrial machinery market needs, including high precision, highspeed, energy conservation and contribution to a better environment. This year at JIMTOF 2010, in
addition to pioneering technology for industrial machinery precision bearings, we are also making apresentation about ultra-precision inspection and measurement technology. This merges our NTN-SNR
sensor technology with unique products developed for production technology.
In the two years starting with fiscal 2009, we have sought to realize becoming a strong corporation that
does not depend on scale in our new “NTN 2010 for the Next Step” medium-term management plan. This
includes our corporate philosophy, “For New Technology Network: We shall contribute to internationalsociety through creating new technologies and developing new products.” We are also following thefundamental principle of “technological leadership.” By promoting the development of new technology andproducts, we will continue contributing to the sustainable advancement of society.

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NTN TECHNICAL REVIEW No.78（2010）[ Contribution ]New Machining Technologies and Tribology

Takashi NAKAMURA
Professor, Vice President, Nagoya Institute of Technology (NIT), D. Eng.

Some machining technologies are necessary for manufacturing real products, and thecontinuous pursuit of superior machining technologies is required to sustain and expand
manufacturing industries in Japan. New machining processes promoted by the author andothers are introduced in this paper, and the significance of these technologies and theirrelevance to tribology are explained.

1. Introduction
Following the April 6, 2010 commencement
ceremony of state-run Nagoya Institute of Technology(NIT), a seminar of Japan’s business leaders washeld. During this seminar, Yasunobu Suzuki, NTN’s
Chairman, presented a speech titled “’Dream’ and ‘Ki(spiritual energy)’—Let us have a pluralistic picture of
the world”. This speech covered topics ranging fromhis student days at NIT’s Department of Metal
Engineering to those in his career at NTN (Fig. 1). He
emphasized the values of intellectual power, physicalpower and will power, the necessity to study history
and establish ‘kata (one’s own style)’, and theimportance of viewing the world based on a wide
sense of values. The attending freshmen and theirparents ardently listened to the speech. The author
was also a member of the audience and was deeply
impressed by the speech. Beginning in 2005, NIT, inmarking the 100th anniversary of its foundation, hasbeen honoring “individuals who have been committed
to the cultivation of human resources and shown
significant achievements in the development ofeducation and research” with an NIT honorary degree.
The first person honored by NIT was Colin L. Powell,
an ex. US Secretary of State, who presented a special
lecture at NIT’s 100th anniversary. The second person
who received the honorary degree was Fujio Cho, thethen vice chairman of Toyota Motor Corp. At the 2010
seminar of Japan’s top business leaders, NIT awarded
Yasunobu Suzuki, NTN Chairman, with NIT’s 7th
honorary degree (Fig. 2). We hope he will continue
providing precious advice to encourage further
development of NIT.

Fig.1 Chairman Yasunobu Suzuki making his speech at
the seminar of business leaders at NIT

Fig.2 Commencement ceremony(left : Nagoya Institute of Technology President
Minoru Takahashi
right : NTN Chairman Yasunobu Suzuki)-2-

NTN New Machining Technologies and Tribology
social education. In reality, the precision of machines
has been improving from year to year, and this trend
is said to result from the “generating principle”.
Several machining processes are currently available
which are based on the “generating principle”. One of
the typical machining techniques is lapping of steel
balls which is described below. Note, however, thatthe term “generation machining” refers to gear
generation using a hob cutter, pinion cutter or grinding
operation for special shaped work-pieces using an
NC-controlled machine tool. For this reason, in theremainder of this paper I will call processes such aslapping a Suriawase process. Thanks to recent
advances in measuring techniques for machine tools,
it is increasingly common for operational errors andthermal deformation are measured in advance and thenecessary adjustments are simultaneously performed
during the machining operation. This practice also
may be regarded as a Suriawase process. Speakingof Suriawase, recently a number of commentators saythat Japanese auto companies manufacture cars withinternal combustion engines based on Suriawase
technology. Any manufacturer can produce EV’s
simply by purchasing the components and thenassembling them similar to the production of personal
computers—“the future of Japan seems gloomy”. It istrue that some people accept this “opinion”. This
practice is a very maternal production system, anddoes not promote the progress of technologies.
Is lapping of steel balls—a time-honored machining
process—based on the “all things belong to sphere”
principle? To answer this question, I have researched
into existing manufacturing techniques for making
balls. Fig. 3 schematically illustrates a situation wherea steel ball is held between lapping plates. Dependingon the lapping technique used, a cutout slot is formedat the bottom of circular arc slot, or a V-sectioned slot
is formed4). Now, let me recall a scene at an NTN
production site that has a US-made steel ball lapping
machine that was manufactured year ago. ThoughThis spring (2010), another enthusiastic graduate
from my laboratory joined NTN. The author has long
been supported by NTN in various aspects. In
particular, my joint researcher Fumihiro Itoigawa,
Associate Professor at NIT, was heavily supported by
NTN while preparing his doctoral thesis “Research into
precision of steel balls for precision equipment1)”.
NTN provided a guided tour through its ball production
facility and gave useful recommendations about
possible test methods. NIT is grateful for this support.
In this paper, lapping techniques for steel balls will be
described as well as new machining technology andtheir affect on tribology. Production of high precisionsteel balls is only possible with a fundamental
understanding of engineering and a great deal of
manufacturing experience and expertise. Mass
production of steel balls with a high precision level isotherwise impossible. Although it may produce ahighly accurate spherical surface, even the mostadvanced 5-axis ultra high-precision machine toolcannot manufacture such high-quality steel balls.
Therefore, in order for the new machining technology
described herein to be used properly, the technology
needs to be supported by the fundamental principles
of engineering and satisfy the latest machining
requirements.
Incidentally, before drafting this paper, the author
checked past contributions to NTN Technical Reviews
on NTN’s website. When seeing a portrait of Yoshiji
Kimura2), an ex-president of Kagawa University, theauthor recalled his cheerful high-pitched laugh and hiscomment,“Mr. Nakamura! You wish to be a contributor
to NTN’s review?” At that moment the author felt
honored to contribute to this issue of the NTN
Technical Review. Being impressed by Mr. Kimura’s
unique insight in the paper “Introduction to Tribology”3)
coauthored with the late Heihachiro Okabe (a former
leading tribology scientist in Japan), the author hopes
this paper is as insightful to the readers.

2. Techniques for lapping steel balls
During my student days at the university, I wastaught that the machining principles that governmachining accuracy are the “maternal principle” andthe “generating principle”. “Maternal principle” refers toa fact that the operating accuracy of a machine tool
(also known as the “mother machine”) is directlytransferred to the product made by the machine—a“child” can never be superior to his/her “mother”.
Nowadays it is no longer the norm for a mother alone
to take every responsibility. However, at that time I felt
this principle was interesting as education theory anddiscussed this concept in a separate paper related toFig.3 Lapping of a Steel Ball

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NTN TECHNICAL REVIEW No.78（2010）attempted to clarify the manufacturing process forbearing balls. At the same time, he proposed a
specific measure for further improving the accuracy
and efficiency of the process. His proposal was toprovide add a cutout slot at the bottom of circular arc
slot on one lapping plate as shown in Fig. 3. With thisarrangement, the three-lobed component of circularity
error would be positively inhibited. However, due tothe limitation in the measuring technique and no
manufacturing plant to try this suggestion, it has yet tobe verified that this arrangement would help produce
ultrahigh-precision steel balls. Presently, three
approaches have been proposed to the Manager of
NTN Kuwana Works. First, add an in-process
measuring operation (likely very difficult) for the slot
profiles and the steel diameter to the lapping machine.
Second, add an in-process measuring operation for
the grain size distribution of the lapping compound.
Third, adjust the lapping plate forcing load based on
the data obtained from steps 1 and 2. Essentially thecontact mode between the lapping plates and thesteel ball is elastic Hertzian contact. Since the lapping
plate forcing load causes the pure sliding position to
shift, it may be possible to control the profile of the sloton the lapping plate. Any of these approaches poses adifficult engineering challenge and requires a lot ofexperimental research. Nevertheless, these concepts
all have the potential to help create higher precisionsteel balls.

unsure that this machine is still operational today,
previously this machine was producing ultra highprecision steel balls at their maximum possible
dimensional accuracy. Thus, it is apparent that thematernal principle does not apply to the lapping
process on this machine. As many NTN employees
know, steel balls are processed on this machine in aparticular sequence. Two lapping plates are butted
together along their lateral axis with the plates having
concentrically formed circular arc-shaped slots. Eachsteel ball blank is loaded into the lapping machine
through the stationary lapping plate, and is subjected
to a lapping motion for one revolution, and thendelivered into a stocker with a large capacity. The
steel ball is allowed to sit in the stocker, and is thenloaded into the lapping machine again. The repetitionof reloading/unloading cycle has two importantfunctions. First, this technique helps the axis ofrotation of each ball change randomly. As determined
experimentally, rolling elements turning at high speeds
in angular contact ball bearings or thrust roller
bearings do not readily change their axis of rotation. In
other words, if a ball is lapped without changing its
axis of rotation, the finished ball will be egg-shaped
rather than spherical. This is the purpose of thereloading/unloading cycle on a lapping machine: toensure the area being ground is random. Second, thewear rate between the lapping plates and the grinding
rate of the steel ball are adjusted so that the optimal
shape conformity between the circular arc groove andsteel ball coincides with the time length of steel ball
final finishing. When shape-conformity is not good, theshape of final balls tends to be polygonal (ortetrahedral in many cases). When the lapping
operation for steel balls is allowed to continue withmachining parameter values unchanged, circularity
improves over time but begins to worsen after acertain point. Therefore, the lapping time must be
controlled precisely for steel balls to be finished to atarget accuracy. To this end, intuition, experience andtribological technology are important. The lapping
compound and oil used during this process affect thecircularity of the steel balls. A Manager at NTN’s
Kuwana Works once commented, “For best machining
accuracy, we even consider the weather of the day”.
In other words, highly accurate steel balls with good
circularity are manufactured by lapping under the
optimal work environment and conditions rather thanfollowing the “all things belong to sphere” principle.
After careful consideration, this idea makes sense
because we have never heard that “operating a ballbearing in clean conditions for a prolonged period
helps produce highly accurate balls”. In his doctoral
thesis, Fumihiro Itoigawa, Associate Professor at NIT,

3. Suction removal of loose material
Any metal cutting or grinding operation inevitably
creates loose material. In fact most of the energy
consumed to machine a work-piece is removing theloose material from the machine tool during theturning operation. To avoid any adverse effects on
machining accuracy from the heat generated by
cutting or grinding, loose material should be removed
from the working area as promptly as possible.
However, on certain unattended automated productionlines airborne loose material can cause problems
during its operation. Loose material can get trapped ingaps between jigs and tools, leading to a greater
dimensional mounting error or blocking the sliding
motion of the tool. In order to completely eliminate
these problems, we have created a prototype of asuction tool that removes loose material (illustrated inFig. 4) that helps improve the milling operation shownin Fig. 55). Loose material created by the machining
process hits a cover enclosing the cutting face of thetool and is then drawn into the opening in the center oftool. Use of this new suction type ball end mill will leadto a cleaner machined surface free of loose material-4-New Machining Technologies and Tribology

as shown in Fig. 5. In contrast, when a conventionalend mill is used to finish a cavity die shown in Fig. 5,loose material can bounce off the work-piece surface,interfere with the tool edge, and produce a lower
quality finished surface. To avoid this type of defect at
the final finish step in a time-consuming die machining
process, this chip suction type milling tool would be
useful. Though various engineering challenges still
remain, prototypes have been built of suction typetools for a face mill and drill and tap, and tests arebeing run on these tools. Various manufacturers,
whose specialties include cutting tools and tool
holders, machine tools and chip disposers, are likely
to support these efforts. During this academic year,
NIT has studied a “debris-free grinding” technique
where loose material from grinding is collected on thegrinding wheel side of the machine. This technique,when fully commercialized, should be good news forvarious machine component manufactures, inparticular, bearing manufacturers.

4. Laser lap welding
One technique used to weld together two
thermoplastic resin members with heat generated by alaser beam is called “laser lap welding”. Although theword “lapping” means to finish a work-piece by lap
grinding, the term “laser lap welding” is derived fromthe word “overlap”. Since in this type of process, alaser beam is directed onto a prospective weld
between two overlapped members as shown in Fig. 6,
at least one member has to be able to transmit thelaser beam. Another lap welding technique available iscalled “ultrasonic welding” where a prospective weld isheated by friction from ultrasonic vibration. Bothtechniques are unique in that only the interface at theprospective weld area is heated and the influence ofheat on other areas is limited. In an ordinary laser lap
welding process, one resin member is made of lightabsorbing material as shown in the left side of Fig. 6or the interface at the prospective weld is coated witha light-absorbing material. We have attempted todetermine optimal welding conditions by measuring
the surface roughness of the two members (which has
always been a subject of interest in the tribologicalengineering field)6). As a result, an unexpected effect
was found and a unique welding operation similar tothe one shown in Fig. 7 is now possible. Morespecifically, when a particular rough surface area on atransparent material absorbs laser beam, the surface

Laser beam

Material: tungsten carbide
Dia.: 10 mm
Number of flutes: 1
Width of suction opening: 0.9 mm

Laser beam

Molten area
Transparent
resin

Cutting edge

Colored
resin

Cover
Suctionopening
Cuttingedge
Cover
Suctionopening
Cuttingedge

Fig.6 Conventional laser lap welding vs. laser lap welding
technique for joining resin members
Loose material
SuctionFig.4 Suction type ball end mill

Fig.7 Simultaneous welding of three transparent resin(PMMA) layers

Fig.5 Finishing of cavity-type die with suction type tool-5-

NTN TECHNICAL REVIEW No.78（2010）

becomes molten and the transparent members begin
to weld together. Then the resultant weld becomestransparent and, as a result, heat generation in theweld stops and the welding process automaticallyends. This technique can be conveniently used for an
application where a plurality of resin members are
piled up as demonstrated by Fig. 7 where completionof welding begins with the layer nearest to the laser
beam emitter (although for human eyes welding of theoverlapping members appears to take place
simultaneously). This technique can be applied to a
welding process for a plurality of overlapped
thermoplastic members each having a unique melting
point. Though useful only for thermoplastictransparent resin materials, this technique is promisingfor ‘Monozukuri” as tribologists are conscious about
the usefulness of “surface roughness”.

application of wear phenomenon. When we havesucceeded in clarifying the working principle of theelectro-discharge machining process, we will reach abetter understanding of the corrosion wear
phenomenon on electric contacts, as well as the ability
to propose appropriate measures for preventing
corrosion wear of electric contacts. That is, machining
and tribology involve common considerations in direct
association with each other such as the wear ofmachining tools but also in understanding the detailsof machining and tribology. Though the author is not
sure if it is good situation—several researchers(including the author) are deeply involved in aninterdisciplinary region between these two academic
fields.
Furthermore, the suggestion that the electrodischarge machining process could be used tomachine a material not attempted before, we haveattempted to apply electro-discharge machining
technique to C-FRP, a material that is being used
more frequently in recent years. At an early stage of
research, the technique was unsuccessful—anoccurrence-avoidance cycle of short-circuit simply
recurred, while the C-FRP work-pieces, consisting ofelectrically conductive carbon fibers bonded withresin, were not properly machined by the electrodischarge process. Shin-ya Hayakawa, AssistantProfessor, and his undergraduate students attempted
to determine the optimal machining conditions,
thereby machining work-pieces like the one shown in
Fig. 8 with the wire electro-discharge machining
process8). Though the machining speed is still low, theresultant cross-sectional shape is very clear-cut andthe cut faces are free from fluffing. I believe thistechnique will be useful as a machining technique forcertain portions of work-pieces made of C-FRP, amaterial that will likely be used increasingly ontransportation equipment for lighter designs.

5. Electro-discharge machining for
C-FRP material
We have been investigating the material removal
mechanism of work-pieces subjected to an electrodischarge machining technique that is often used tomachine metal dies. It has been said that in anelectro-discharge machining process, thermal energy
from an electric discharge melts metal material andcauses vaporized machining liquid to splash themolten metal. However, there have been reports
denying this claim, based on the observation ofmachining marks and measurements of machining
efficiency. We have prepared a work environment thatsimulates the actual electro-discharge machining
process, developed an instrument that can directly
monitor the area being machined, and have recorded
pictures of the electro-discharge machining operation
using a high-speed video camera. The images
captured are amazing: immediately after the electric
discharge, a bubble made of vaporized machining
liquid expands, and then red-hot metal debris flying
from the discharge point collides with the inner wall of
the bubble and is stopped7). In other words, there
occurs a collision state when the molten metal
spontaneously splashes and the machining liquidcontains the discharge point in ultra high-pressure
atmosphere for a period of time. This is a very
important finding that will help clarify the operating
principle of the electro-discharge machining process(we are still investigating detailed mechanism for thisprocess). Textbooks for production engineeringclassify the cutting process and the grinding process
into “removal machining”, to which electro-dischargemachining belongs too. From a tribologists’ viewpoint,
removal machining may be regarded as one positiveFig.8 C-FRP material trimmed by wire-electricaldischarge machine-6-New Machining Technologies and Tribology

Oil pressure MPa

Coefficient of friction μ

The feed mechanisms of machine tools have long
been manually finished by the ‘suriawase’ techniquewith a scraper (‘suriawase’ in this context means“manual finishing by manual scraping”). Based on the‘suriawase’ principle previously described in Sec. 2, thisunique technique gives critical areas in machine tools ahigher degree of precision than possible withmachining. At the same time, delicate uneven surface
patterns formed by scraper finishing helps achieve ahigher degree of sliding quality needed for machine
tools. As higher speeds are increasingly needed formachine tools, linear rolling bearings are frequently
used since they feature lower frictional resistance inwide range of speeds. However, feed mechanisms ofmachine tools may need to withstand heavy loads andminute load variations and feature highly effective
vibration dampening performance. Fig. 9 graphicallyplots the sliding characteristics of sliding guide ways
reproduced by an experimental rig developed in ourlaboratory9).
The graph in Fig. 9 displays the measurements ofthe coefficient of friction with a wide speed range withthree lubricating oils specially developed for use on
slide ways. These characteristic curves appear to differ
from ordinary Stribeck curves. The values for thecoefficient of friction tend to be smaller in the lower
speed region where boundary lubrication occurs(usually the coefficient of friction should remain
relatively constant in this speed region). With a certain
type of lubricating oil, the coefficient drops below
0.005. The “boundary lubrication region” (i.e. boundary
film lubrication region) corresponds to a state where
the boundary oil film adsorbed by the slide way surface
governs its friction. These specially formulated sliding
lubricants form a rigid high-viscosity oil film which
mitigates solid contact, leading to the above-mentioned
characteristics. These dynamic sliding characteristics

Lub. A
Lub. B
Lub. C

0.2

0.1

0.1

0.02

0.05

0.01

0

0

Surface deformation μm

are ideal so that the table can be stopped correctly at a
specific position with an accuracy of 0.1 µm needed forhigh efficiency machine tools.
Is it possible to meet the sliding characteristics
shown in Fig. 9 simply by scraping and using a special
sliding lubricant? The answer to this question:
impossible. In our test, these characteristics were only
achieved after break-in operation for more than 20hours. I have been told that after assembly, machine
tools require break-in operation for more than two fulldays. The author believes that this practice is theultimate example of the “suriawase process”. Next, let
us consider what is occurring during the break-in
operation of the slide way surfaces. It is said that in an
ordinary break-in operation, the high points on thecontact surfaces are worn away so that flat working
faces are formed. However, the scraping process(having been performed prior to break-in operation)
already resulted in 20 bare spots per square inch(class A standard), and fairly smooth working faces.
The author thinks that during the break-in operation ofa slide way surface having undergone the scraping
process, corners of very small individual working faces
still have ultra-small peaks. Fig. 10 graphicallyillustrates a comparison between the oil pressure on
the working faces (drops are provided on both ends ofeach working face) of the slide way lubricated with a
special lubricant, with consideration of the elastic
deformation of working faces10).
This chart corresponds to a situation where theupper working faces shift to the left relative to the lower
stationary working faces. Oil pressure occurs at the
inlet side of the working faces and the elastic
deformation that occurs on each working face allows
the oil pressure to be maintained to the midpoint on theworking face. Consequently, the working faces support
loading without developing solid contact. For this test,

6. Break-in process

Oil pressure
Surface deformation
-0.05
0.0
10-3

-0.01
0

10-2

10-1

100

101

102

103

104

Length ratio

1

Fig.10 Elastic deformation and oil film pressure ofcontact surface with droop(Sliding velocity: 0.10 ms, initial oil film thickness: 1.3 mm)

Sliding speed mm/min

Fig.9 Stribeck curve of sliding lubricant-7-

NTN TECHNICAL REVIEW No.78（2010）

References

we have made calculations using oil with ordinary
lubricant viscosity. However, when a special sliding
lubricant is used and a highly viscous film of adsorbed
oil is formed on a sliding guide surface, a greater oilpressure occurs between working faces, thereby
allowing the sliding guide surface to develop a higher
load bearing capacity11).
To sum up, a slide way having undergone scraping
exhibits an ideal surface quality thanks to thetribological wear process. In other words, thisapproach helps optimize the boundary film lubricationmode, known as a labyrinth in the field of tribology,
according to the intended application by using alubricant containing an appropriate additive.

1) F. Itokawa, Research Related to the Precision of SteelBalls used in Precision Machinery—Precision
Evaluation Methods, and Analyses of Steel Ball
Lapping Processes and Ball Bearing Motion Errors(Doctoral thesis), Nagoya Institute of Technology(1993).
2) Y. Kimura, Industry-University Partnerships in R&D ofMachine Elements, NTN Technical Review No. 76(2008) 2-9.
3) Y. Kimura and H. Okabe, Introduction to Tribology,
Yokendo, Ltd. (1982).
4) F. Itokawa et al., Steel Ball Lapping using Laps with Vshape Grooves, Transactions of the Japan Society of
Mechanical Engineers, Series C, Vol. 59, No. 562(1993) 304-310.
5) T. Nakamura et al., The Present Status ofEnvironmentally Friendly Machining and its Future,
Journal of Japanese Society of Tribologists, Vol. 53,
No. 1 (2008) 21-26.
6) M. Yamakawa et al., Study on Laser Welding ofTransparent Thermoplastic Resin Plates, Transactionsof the Japan Society of Mechanical Engineers, Series
C, Vol. 74, No. 744 (2008) 149-153.
7) S. Hayakawa et al., Observation of Bubble Expansion
and Flying Debris in Parallel Flat Gap Space inElectrical Discharge Machining, International Journalof Electrical Machining, No. 14 (2009) 29-35.
8) T. Ito et al., Test Applications of Electric Discharge
Machining of Carbon Fiber-Reinforced Polymer(CFRP), 2010 Japan Society for Precision Engineering
Spring Meeting Academic Lecture Proceedings, CDROM (2010) 577-578.
9) T. Norihisa et al., Development of Friction Test
Apparatus for Slide-Guide Way of Machine Tool,
Journal of Japanese Society of Tribologists, Vol. 52,
No. 9 (2007) 45-52.
10) T. Nakamura et al., Isoviscous-EHL Mechanism of
Parallel Slide-Way with Oil Groove, Journal ofJapanese Society of Tribologists, Vol. 44, No. 4, pp.
42-48 (1999).
T. Norihisa et al., Study on Velocity-Dependent
Property of Friction in Boundary Lubrication under Low
Contacting Pressure Condition (Part 2)—
Consideration into Mechanism of Velocity-Dependent
Property of Friction Lubricated with Oil Containing
Alkyl Acid Phosphate, Journal of Japanese Society of
Tribologists, Vol. 53, No. 10 (2008) 51-58.
11) W.C. Oliver and G.M. Pharr: An improved technique fordetermining hardness and elastic modules using load anddisplacement sensing indentation experiments, J. Mater.
Res., Vol.7 No.6, 64, 1992

7. Conclusion
This paper has presented the author’s latest
knowledge regarding techniques for machining
bearing components. The topics covered include thelapping of steel balls, the suction removal of loosematerial, the laser lap welding process, the electrodischarge machining process for C-FRP, and the‘suriawase’ technique for finishing slide ways. Capableof understanding calculated precision, maternal
principle-based machining processes are suitable formass-production but do not allow for an improvementin precision and performance. ‘Suriawase’ is atechnique used to achieve a higher degree ofprecision through interactions between materials andtools and necessitates unique techniques andexperience that cannot be easily expressed in theform of mathematic formulas or tabulations. The
author strongly believes that manufacturers in Japan
should remain committed to the creation of productswith improved performance through a succession oftechniques and experience from previous generations
and continue to explore these techniques andexperiences. Tribology is an academic field thatinvolves interactions between two surfaces moving indifferent directions and has many things in common
with the ‘suriawase’ process. Being a specialist intribology, NTN CORP. has been active as a leading
bearing manufacturer around the world and has a very
dependable presence in Japan.

-8-New Machining Technologies and Tribology<Author’s profile>Takashi Nakamura
Professor, Vice President, Nagoya Institute of Technology (NIT), D. Eng.
1975 Graduates from Industrial Mechanical Engineering Dep.,
College of Engineering, NIT
1977 Completes master’s course at Mechanical Engineering
Dep., NIT
1980 Completes doctoral course for mechanical engineering at
Nagoya University
1980 Assistant at Nagoya University

-9-

1982
1983
1987
2000
2005
2010

D. Eng. (Nagoya University)
Instructor at NIT
Assistant professor at NIT
Professor at NIT
Professor at Graduate School, NIT
Vice President, NITNTN TECHNICAL REVIEW No.78（2010）[ Technical Article ]Technical Trend of Machine Tool Bearings

Hiroshi TAKO**
Yasutsugu TANAKA**

For machine tool bearings, high- speed, high rigidity and high accuracy are important.
Recently, reducing environmental impacts and costs have also become important.
This paper gives an overview of technical trends for machine tool bearings and explains
current machine tools.

capability of processing larger parts. The second
option is for lower-cost general purpose machines
accomplished through utilization of common
components, lower machining accuracy, and limitedpart size capability. NTN has designed machine tool
bearings for both the high-end and general purpose
markets.
NTN has been developing machine tool bearings
focusing on attaining both high speed performanceand higher rigidity (two conflicting requirements), as
well as higher accuracy and eco-friendliness. In thiscontext, NTN has been developing unique machine
tool bearings to support further sophistication offunctionality for 5-axis machine tools and CNC turning
centers.
As for the increased interest in general purpose
machine tools, NTN has been helping reduce both thecost and size of main spindles by developing various
technology, including grease-lubricated bearings that
do not need an additional lubricating oil supply
system.

1. Introduction
Machining efficiency and machining quality are two
criteria historically common to the machine toolindustry. Current economic times call for additional
criteria, those being reduced machining cost and adecrease on environmental impacts. To addressthese challenges, bearing manufacturers arecommitted to the development of improved materials
and the optimization of bearing internal design. As aresult, the performance of main spindles and feedsystems will improve due to higher speeds, higher
rigidity, improved lubrication, and better overall
accuracy.
This paper will present the recent engineering trendsof machine tool bearings and introduce products NTN
has developed to meet these requirements.

2. Recent engineering trends of machine
tools and machine tool bearings
Though there are signs of recovery from the 2008
global recession, worldwide demand in the machine
tool market remains sluggish. China, whose
manufacturing output was largest in the world in 2009,is rapidly improving their machine tool industry and willsoon equal the technology of Japanese and European
machine tool industry.
Due to the slow recovery and the increased impactof the Chinese market, machine tool development inJapan has been divided in two directions. The firstbeing high-end machines featuring improved
machining efficiency and accuracy as well as the

3. Engineering trend of NTN’s machine
tool bearings
Fig. 1 illustrates engineering improvements in
NTN’s precision bearings for machine tools.

3.1 Engineering trend of NTN’s angular contact
ball bearings
In the 1980s, need for high-speed main spindles
began, resulting in the development of high-speed
angular contact ball bearings. NTN’s efforts in** Industrial Business HQ. Industrial Engineering Dept.
** Industrial Business HQ.

-10-

Technical Trend of Machine Tool Bearings

Cylindrical roller bearings
Angular contact ball bearings

Main spindle bearings

dmn value
2×106

Double row cylindrical roller bearings

1×10

NN30xxHS
NN30・NN49・NNU49

dmn value
3×106
Single row cylindrical roller bearings
2×106

N10xxHSRT6・N10xxHSLT6

N10xxHSN10

dmn value
1.5×106
For bearing axial load
1×106
HTA・HTB
6
0.5×10

HTAxxxU

562xx・742xx

dmn value
3×106

For bearing radial load

MQCJ bearngs
HSC

6

2×10

1×106

HSF・HSFL

HSBxxxCAEX1
HSA
7xxx

HSE9xxU

HSE・HSLHSB

79xxU・70xxU

dmn value
2×106

For bearing radial load; w/ seal

MQGS bearngs
BNFSxxxLLB

1.5×106

BSNxxxLLB
7xxxLLB

1×106

Angular contact
ball bearings

1980

Feed system

NN30xxHSRT7NN30xxHSRT6
6

1990

1996

2000

2004

2008

Longer life
For supporting ball screw

2010 Year

BSTU

BSTxx-1E

2A-BSTxx-1B・2A-BSTxx-1BLXL (w/ seal)

BSTxx-1B

1980

1990

1996

2000

2004

2008

2010 Year* Red characters: ULTAGE Series
Fig. 1 Development history of precision bearings for machine tool

helped improve the efficiency of machining and thequality of machined surfaces. Recently, need forhigher speeds with these bearings have decreased as
the demands for die machining machines have beensluggish. Instead, there has been an increase in theneeds for cost reduction and size reduction ofmachine tools.
To address this market trend, NTN has recently
developed the “New 9-Series High-Speed Angular
Contact Ball Bearings (HSE9xxU)” and “Ball Screw
Supporting Double Row Angular Contact Ball
Bearings (BSTU)”. These two product lines will belater described in further detail in Sec. 4 “Introduction
to NTN’s Newly Developed Bearings”.

developing high speed angular contact ball bearings
began with HSA bearings that are similar to standard
types but feature smaller diameter rolling elements.
Around 1990, NTN developed “HSB bearings” thatwere improved HSA bearings featuring limited heatgeneration, as well as “HSBxxxCAEX1 bearings” thatare capable of even higher speed. In 2000, NTN
released “ULTAGE Series high-speed angularcontact ball bearings—also known as HSE
Bearings” that utilize bearing rings made from special
bearing steel featuring excellent wear resistance andanti-seizure quality. Thanks to use of special bearing
steel, HSE bearings are capable of greater mounted
preload compared with conventional bearings. Thus,
they feature both high-speed performance and greaterrigidity, helping greatly enhance functionality of themachine tool main spindles they support.
Furthermore, NTN marketed “HSF bearings” in
2002. Compared with the HSE bearings, the diameterof their rolling elements is much smaller. NTN
developed this new series of bearing products as airoil lubricated constant preload bearings for ultra-highspeed main spindles on machine tools such as die
machining machines.
Main spindle bearings allowing for higher speed

3.2 Engineering trend of NTN’s cylindricalroller bearings
Single-row cylindrical roller bearings are often used
to support machine tool equipment main spindles. As
a result of creating high-speed angular contact ballbearings for machine tools, it was now required toincrease the speed capabilities of the supporting
cylindrical roller bearings. To address this challenge,
NTN developed the “N10xxxHS bearings” thatfeature a smaller roller diameter verses that of the-11-

NTN TECHNICAL REVIEW No.78（2010）

standard NTN single row cylindrical roller bearings. In
2000, NTN developed the ULTAGE Series“N10xxHSRT6 Bearings” that incorporate a PEEK
resin cage which allow an even higher speed whenused with main spindle bearings. In 2004, a similar
modification was made to double-row cylindrical roller
bearings NN30xx often used to support turning centermain spindles. This resulted in the high-speed double
row cylindrical roller bearings “NN30xxxHSRT6
bearings” that incorporate PEEK resin cage.

developed for use on rotary tools.
Main spindles attaining much higher accuracy
employ unique bearings whose NRRO (NonRepetitive RunOut) has been decreased by limiting
variation in the diameter of rolling elements andregulating bearing ring roundness.
NTN offers angular contact ball bearings in twolevels of NRRO—0.3 mm or smaller, and 0.1 mm or
smaller. These angular contact ball bearings are usedin main spindles of high-precision machining andturning equipment.

3.3. NTN’s technology for improving bearing
accuracy

3.4 NTN’s eco-friendly bearing technology

There are JIS accuracy classes, equivalent to thosespecified in a corresponding ISO standard. Machine
tool main spindles often use bearings of JIS accuracy
class 4. Applications that need particularly highbearing accuracy employ bearing of JIS accuracy
class 2.
As stated previously, there are additional needs inthe machine tool bearing market including: bearing
users who want both high accuracy and low cost,bearings of special accuracy, or those satisfying both
JIS accuracy class 2 in terms of running accuracy andJIS accuracy class 4 in terms of dimensional
accuracy. To meet these needs, NTN added a uniqueline of products whose accuracy class is categorized
into “P42” as standard bearings to the “SealedStandard Angular Contact Ball Bearings
7xxCD/ADLLB Bearings” series products that wereAround 2000, machine tool manufacturers started
their efforts reducing environmental impacts, lowering
power consumption, and improving work
environments.
On the basis of its ULTAGE Series bearings, NTN
has established new lines of eco-friendly bearing
products illustrated in Fig. 2: the “HSL bearings” thatare based on the HSE bearings, the “HSFL bearings”
that derive from the HSF bearings, and the“N10xxHSLT6 bearings” that are essentially
improved N10xxHSRT bearings. These eco-friendly
bearings are unique in that lubricating oil is fed
through a special nozzle into the bearing, thus helping
reduce consumption of compressed air and oil 50% to
75% compared with those of conventional bearings.
At the same time, noise level of the bearing is reducedby limiting the flow of compressed air interfering with

Special nozzle

Special nozzle

HSL bearing

N10xxHSLT6 bearing

Fig. 2 Eco-friendly air-oil lubricated bearing
Minimum clearance
for lubrication

Grease chamber

Axial clearance

GreaseSE-1 greaseNon-contact seal
Spacer for filling grease

Fig. 3 BNS bearing(Enlarged view)Fig. 4 MQGS lubrication bearing-12-Technical Trend of Machine Tool Bearings

the rolling elements, thereby these air-oil lubricated,eco-friendly machine tool bearings contribute to
reduction in consumption of energy and resources aswell as improved work environments.
Use of grease-lubricated bearings can also
contribute to reduction in consumption of compressed
air, oil, and electricity. Since 2000, NTN has beendeveloping the “BNS/BNFS bearing” products shownin Fig. 3. These bearings feature grease chambers on
the outer ring rolling surfaces, a non-contact seal, andhigh-speed long-life grease “SE-1”, all of which
contribute to the improved reliability of greaselubricated machine tool bearings.
Thanks to these improvements, higher speed is also
now possible with grease-lubricated bearings. In otherwords, air-oil lubricated bearings typically used inhigher speed machine tool applications can be
replaced with grease-lubricated bearings.
NTN made another innovation in 2008—the “MQGS
Lubrication Bearings (dmn: 1.9×106)”, schematicallyillustrated in Fig. 4. This bearing design features aunique spacer that delivers a minimum amount ofbase oil to the outer ring rolling surface in order tohelp extend grease life and improve the high-speed
capability of the bearing.

series bearings due to their smaller cross-sectionalarea.
To address this drawback, NTN has developed the“HSE9xxU bearings” that feature the high-speed
performance of the HSE9 series bearings. Optimal
specifications for bearing internal design have led to
50% increase in dynamic load rating and 20%
increase in axial load rating relative to conventional
similar bearing products.
Thanks to this engineering development, it is now
possible to design compact, high-speed, high-rigidity
main spindles employing 9 series angular contact ballbearings.
Detailed information about the high-speed angularcontact bearings—9 series HSE9xxU bearings will
be provided in a separate article within this technicalreview.

4.2 Ball screw supporting bearings
In the Asian machine tool industry, ball screw
supports employ duplex angular contact ball bearings.
In the same application, European machine toolindustries use double-row angular contact ballbearings whose outer rings have mounting holes. The
advantages of this bearing type include a simpler
mounting procedure, a decreased number of parts
since the bearing can be mounted directly to thebearing housing, and no need to match bearing pairs.
To its ULTAGE series of bearing product, NTN has
added the “double row angular contact ballbearings BSTU for supporting ball screws”
schematically illustrated in Fig. 6. This bearing typeis essentially double-row angular contact ball bearing
with the outer ring having mounting holes mentioned
above, except that the BSTU bearing has a uniquely
shaped seal. Features of this bearing include highload capacity, low contact pressure and low torque, as
well as long-life grease.
Detailed information about the double-row angularcontact ball bearings BSTU bearings will be provided
in a separate article within this technical review.

4. NTN’s newly developed machine tool
bearings
4.1 New 9 series high speed angular contact
roller bearings
As shown in Fig. 5, the cross-sectional area of 9series angular contact ball bearings (often used in theEuropean market) is smaller compared with 0 series
angular contact ball bearings (commonly used inJapan). As a result, the same size spindle can beused in a more compact housing, or alternately, alarger, more rigid spindle can be used in an equivalent
sized housing.
One disadvantage is that the load capacity of the 9series bearings are smaller compared with the 0

Housing

9 series
0 series

9 series

0 series

Main spindle

Main spindle

Main spindle

Fig. 5 High-speed angular contact bearings new 9 series-13-

NTN TECHNICAL REVIEW No.78（2010）

Fig.6 BSTU bearing

BST Bearing (Current)

5. Conclusion
With its unique ULTAGE series of bearings, NTN
has been helping machine tool manufacturers satisfy
the ever demanding engineering needs for theirmachine tool products.
Machine tool manufacturers have experienced adramatic change in engineering trends for machine
tools—from higher functionality to reduction in costand size. We believe that the most challenging needsin the machine tool industry may vary as the market
demands will vary. Typical examples of engineering
challenges include eco-friendly technology, higher
functionality, and cost reduction.
In order to address the ever changing engineering
trends in bearing industry and to contribute to theengineering development of machine tools, NTN willcontinue to be committed to the development andimprovement of its bearing technology.

Photo of authors

Hiroshi TAKO

Yasutsugu TANAKA

Industrial Business HQ.
Industrial Engineering Dept.

Industrial Business HQ.

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NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Application of Ceramics to NU-Type Cylindrical Roller Bearings
for Machine Tool Main Spindles

Masatsugu MORI**
Takuji KOBAYASHI**
Ultra-high-speed operation of an air-oil lubricated NU-type cylindrical roller bearing has
been made possible by using a ceramic inner ring. A maximum speed of up to 35,000
min-1 is possible (dmn value of 3.25 million, inner ring bore of 70 mm). Devising the outerring rib structure to streamline lubricant drainage resolves the occurrence of high andbroad temperature rises around the mid-speed range, which is typical of conventional
NU-type cylindrical roller bearings, as well as rapid temperature rises at high shaft speeds. The developed bearing will allow
the practical application of NU-type cylindrical roller bearings to machine tools that require high bearing stiffness over a wide
range of operation speeds. The cage made of PEEK is guided on the air-oil nozzle outside surfaces, while rollers made ofsteel can be used even at 35,000 min-1 and control the inner ring temperature below 70˚C.

spindles, capability for much higher speed will beneeded for rear-position (that is, free-side) single row
cylindrical roller bearings.
To address this challenge, we attempted to use
ceramic inner rings (this topic will be described indetail later) to prevent occurrence of excessive
preload that will pose a direct obstacle against
achievement of higher main spindle bearing speed. In
a previous NTN Technical Review 2), we alreadyreported our experience in developing the N-typecylindrical roller bearings series having ceramic innerring (featuring double rib inner ring)—this cylindricalroller bearing type, lubricated with an air-oil lubrication
system, achieved ultrahigh-speed bearing operation
as fast as dmn (bearing pitch diameter mm×inner ringrunning speed min-1) value = 3.25×106. This speed
level is equivalent to that obtained from not-yetmounted ultrahigh-speed constant-pressure preloaded
angular contact ball bearings 3). However, the N-type
is uniquely structured in that its ceramic inner ring istightly fitted with steel spacer rings also serving as
ribs: therefore, a simpler-structured ceramic inner ringhas been needed to simplify formation and mountingof inner ring.
To address this challenge, we have developed NUtype cylindrical roller bearings (featuring double-rib
outer ring) that have ceramic rings lacking spacerrings, and achieved ultrahigh-speed bearing operation
as fast as dmn value = 3.25×106 with air-oil
lubrication. This article reports the performance of our
new engineering development.

1. Introduction
Bearings used to support main spindles on machine
tools need to be capable of higher speed and greaterrigidity. This is true since any main spindle that turns
together with a tool or work piece mounted onto it is
one of the critical machine tool components that
directly affects machining efficiency and accuracy ofthe machine tool, and the bearings that support themain spindle are the most critical machine elementson the machine tool 1). Other mechanicalcharacteristics any main spindle bearing needs tosatisfy include higher bearing accuracy, lower
vibration, and lower noise. Rolling bearings are mostoften used to support main spindles because theysatisfy various requirements, including costeffectiveness and maintainability of balance comparedwith hydrodynamic (static or dynamic pressure)bearings and magnetic bearings.
Typical rolling bearing types used to support
machine tool main spindles are angular contact ballbearings, cylindrical roller bearings, and tapered roller
bearings. In particular, cylindrical roller bearings are
preferred as non-locating bearings because theyboast higher load capacity and greater rigidity in theradial direction, and their inner and outer rings arecapable of moving in the axial direction relative to themain spindle. Since requirements appear to be
increasing for higher speed with the fixed position
preload bearing system (which features greaterrigidity) for rolling bearings on machine tool main
*Elemental Technology R&D Center-15-

NTN TECHNICAL REVIEW No.78（2010）

red circle) between the outer ring and oil drain
spacer/rig.
One characteristic that any machine tool main
spindle bearing needs to satisfy is avoidance ofexcessive preload. On a cylindrical roller bearing, theinner ring and outer ring freely move relative to eachother in the axial direction, so no axial preload occurs.
On the other hand, a problem can occur in the radial
direction: the inner ring expands owing to heat buildup
and greater centrifugal force resulting in particularfrom high-speed bearing operation, leading to overpreload radially; heat buildup increases between therollers and raceway surface; the resultant rapid
temperature rise can potentially lead to bearing failure.
In machine tool main spindles, jacket cooling istypically provided on the outer surface side of outer
ring, which is a stationary body, in order to prevent
heat generation on the main spindle system fromadversely affecting the entire machine tool.
Temperature on the inner ring side can readily risebecause of heat generation on the bearing and built-in
motor, as well as a structure that does not readily
release heat; consequently, a steep heat gradientoccurs across the inner ring and outer ring, andpreload on the bearing at higher speed can beexcessively large. Therefore, problem-free high-speed
operation of bearing is possible through reduction ofheat generation inside the bearing and thermal
expansion of the bearing.
Based on the above-mentioned assumption, theelemental technologies for the elements inside thebearing that allow higher speed operation will now be
described. First, the physical properties of ceramicmaterial (silicon nitride) are compared with those ofsteel for the inner ring as summarized in Table 1. Low
linear expansion coefficient of the ceramic material(30% of that of the steel material) very much helps
inhibit the thermal expansion of the inner ring. Though
the physical density of this ceramic material is as low
as 40% that of the steel material, the modulus oflongitudinal elasticity with the ceramic material is150% as great as the steel material, and, at the sametime, difference in Poisson’s ratio between these twomaterials is very small. Consequently, the centrifugal
expansion on the inner ring is limited to approximately

Use of ceramic materials in elements of rolling
bearings has long been proposed 4). In the technicalfield of machine tools, there have been an increasing
number of cases 6) where ceramic rolling elements areused in angular contact ball bearings in order to inhibit
adverse effects of gyro-moment 5) that poses problems
in particular with machine tool main spindles runningat higher speeds. However, there have been a limited
number of applications of ceramic materials tocylindrical roller bearings for machine tools. In addition
to the information already presented in NTN Technical
Review (No. 76) 2), we want to provide additional
information in this Technical Review in order todemonstrate that by utilizing benefits of ceramicmaterials, cylindrical roller bearings can offer highspeed performance comparable to that of constantpressure preload angular contact ball bearings.

2. Structure, and elemental technologies
for high-speed operation
Fig. 1 shows a cross-sectional view of the NTN’s
newly developed NU-type cylindrical roller bearing.
The structure of the NU-type cylindrical roller
bearing in Fig. 1 is characterized as follows: the innerring is made of silicon nitride (Si3N4), which is a
structural ceramic material; the cage is made of PEEK
(polyether ether ketone); the bore surface of the cagerides on the outer circumferential surface of the air-oil
nozzle spacer; the rollers and outer ring are made ofcommon bearing steel (SUJ2); the outer ring is fittedwith oil drain spacers that doubles as ribs, andlubricating oil is drained via the gaps (marked with aOuter ringAir-oil nozzle

Gap

Oil drain spacer/rib

Air-oil nozzle
spacer

PEEK cage

Ceramic inner ring

Table 1 Properties of Si3N4 and steel
Si3N4

Steel

Linear expansion
coefficient 1/K

3.2×10-6

11×10-6

Density kg/m3

3.2×103

7.8×103

Modulus of longitudinalelasticity GPa

314

211

Poisson’s ratio

0.26

0.3

Exhaust holeOil-drain channel

Fig.1 Developed NU-type cylindrical roller bearing-16-

Application of Ceramics to NU-Type Cylindrical Roller Bearings for Machine Tool Main Spindles

Outer ring

30%. More specifically, compared with the steel innerring, increase in preload is reduced with the ceramic
inner ring, and heat generation inside the bearing ismore efficiently prevented.
As previously reported in NTN Technical Review762), it is important with a cage riding system thatlubricating oil of controlled temperature is always
supplied to the cage lands, and is promptly drained
away to prevent the lubricating oil (which has become
very hot from shear heat generation) from remaining
on the guide surface. This arrangement helps inhibit
heat buildup in the bearing. As shown in Fig. 1, thelubricating oil ejected from the air-oil nozzle together
with compressed air hits the ramp of the rotating innerring, then rises along the ramp by surface tension andcentrifugal force working on it, and lubricates therollers and raceway surface. At the same time, driven
by compressed air, the lubricating oil passes the cageriding clearance from the inside of the bearing and isdrained away. In other words, fresh lubricating oil isalways supplied into the cage lands and then promptly
drained away from the bearing.
Another unique arrangement has been incorporated
into the oil-drain structure in the outer ring side. Whenthe bearing is running at a higher speed in particular,
the fresh lubricating oil supplied into the bearing tends
to remain in the vicinity of the bore of outer ring
because of the centrifugal force working on it. An
excessive amount of lubricating oil remaining in andaround the bore of outer ring will lead to increase instir resistance and, as a result, heat generation inside
the bearing. Compared with the N-type bearings, thistendency is more apparent with the NU-typecylindrical roller bearings whose outer ring include
ribs. Therefore, for trouble-free high speed bearing
operation, the oil-drain structure on the outer ring sideneeds to incorporate a special solution.
For the purpose of comparison, the N-typecylindrical roller bearing presented in NTN Technical
Review 76 2) is illustrated in Fig. 2.
The structure of this bearing is characterized in thatits ceramic inner ring is fitted with spacer rings on bothends. The ceramic inner ring is interference-fit onto
the shaft while the ring spacers are slip-fit onto theshaft. To be able to replace the steel inner ring on the
N-type cylindrical roller bearing with a ceramic innerring, it is necessary to use a ring with integrated ribs
or fitted with separate ribs. Compared with the NUtype bearing in Fig. 1, either ceramic inner ring variant
described above complicates the inner ring design -this “ceramic-based solution” poses a drawback ofsignificantly increased machining costs, as ceramicmaterial machining cost is much higher compared withsteel. To the advantage of the N-type bearings, theAir-oil nozzle

Steel rib/spacer ring

PEEK cage

Ceramic inner ringFig.2 N-type cylindrical roller bearing with a ceramic inner ringouter ring does not have a rib—lubricating oil thrown
to the outer ring side by centrifugal force therefore
does not tend to remain in the bearing. Note that thecage riding system and material of the cage in the Ntype bearings are essentially identical to those of the
NTN’s newly developed NU-type bearings shown in
Fig. 1.

3. Oil-draining capability and highspeed running performance of NUtype cylindrical roller bearings
Considering the oil-draining performance of the NUtype cylindrical roller bearings mentioned in Sec. 2, wehave developed various bearing prototypes. In thissection, we will describe the result of our investigation
into temperature-dependent characteristics of these
prototypes being run at various speeds. These
prototypes are essentially NU-type cylindrical roller
bearings, categorized into “standard oil-drain structure
variant, “oil-drain groove variant”, and “oil-drain holevariant”—their structures are schematically illustrated
in Figs. 3, 4 and 5, respectively.
In the standard oil-drain structure variant shown in
Fig. 3,, the inner ring is made of steel (SUJ2) while therollers are made of ceramic material. The outer ring rib
is independent of the outer ring, the PEEK cage is thenozzle outer surface riding type shown in Figs. 1 and
2: however, the oil-drain structure on the outer ringside is the standard type.
The oil-drain groove variant in Fig. 4 is unique inthat a separate outer ring rib is provided, wherein the
fresh lubricating oil flows through the gaps on bothends of rollers and the gap between the outer ring andrib toward the outer surface side of outer ring, and theheated lubricating oil is drained away from the bearing
through the groove formed on the outer surface ofouter ring.
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NTN TECHNICAL REVIEW No.78（2010）
Exhaust hole

Exhausthole

Oil-drain Oil-drain groovegap

Fig.4 Oil-drain groove structure NU-typecylindrical roller bearing

On the oil-drain hole variant in Fig. 5, outer ring ribs
on both sides each have six equally spaced oil-drain
holes toward their outer circumference in order to direct
the heated lubricating oil to the outside of bearing,wherein the phases of both outer ring ribs are shiftedwith each other so that the locations of the oil-drain
holes on one outer ring rib are not directly opposite tothe oil-drain holes on the other outer ring rib.
Note that the oil-drain groove variant (Fig. 4) andoil-drain hole variant (Fig. 5) both have inner rings androllers made of ceramic material, and the PEEK cageis riding on the outer circumferential surface of the airoil nozzle.
Major technical data of these test cylindrical roller
bearings and test conditions are summarized in Table
2, while the cross-sectional view of the spindle test rig
used throughout our present development work is
illustrated in Fig. 6. The test result obtained from the
NU-type cylindrical roller bearings of Figs. 3 through 5
is illustrated graphically in Fig. 7.
From the graphs in Fig. 7, it should be understoodthat apparent temperature peaks occur at around
10,000 min-1 with all designs -- the “standard oil-drain
structure variant”, “oil-drain groove variant”, and “oildrain hole variant”. This is the major reason why the
NU-type bearings have not yet been used as air-oil
lubricated cylindrical roll bearings for machine tools.
Therefore, the challenges for the present development
work were to increase the maximum allowable bearing
speed, as well as providing a bearing that canmaintain its rigidity in a wider speed range without
developing heat buildup – all in an economically viable
design. The original objective of our development
work for prototypes of the oil-drain groove variant andOil-drain hole

Exhaust holeFig.5 Oil-drain hole structure NU-typecylindrical roller bearing

the oil-drain hole variant was to improve oil-draining
performance at higher bearing speeds. Though the
maximum allowable bearing speed with the oil-drain
groove variant reached 35,000 min-1, the temperaturepeak in the medium speed range at around 10,000
min-1 still persists with either variant. Note that the test
for standard oil-drain structure variant and oil-drain-18-Standard oil-drain
structure variant

Cross-sectional plan Fig. 3Size
φ70×φ110×20
Pitch diameter
93mm
Inner ring
SUJ2 (tapered hole: 1/12 bore diameter)
Outer ring
SUJ2
Rollers
Si3N4, φ7×7, 22 pcs.
Cage
PEEK+CF30%, Nozzle outer surface riding

Oil-draingroove variant

Table 2 Test bearings (Figs. 3〜5) and conditions
associated with Fig. 7

Cross-sectional plan Fig. 4Size
φ70×φ110×20
Pitch diameter
93mm
Inner ring
Si3N4 (cylindrical bore)
Outer ring
SUJ2
Rollers
Si3N4, φ7×7, 22 pcs.
Cage
PEEK+CF30%, Nozzle outer surface riding

Oil-drain hole variant

Fig.3 Standard structure NU-typecylindrical roller bearing

Exhaust hole

Cross-sectional plan Fig. 5Size
φ70×φ110×20
Pitch diameter
93mm
Inner ring
Si3N4 (cylindrical bore)
Outer ring
SUJ2
Rollers
Si3N4, φ7×7, 22 pcs.
Cage
PEEK+CF30%, Nozzle outer surface riding

Test conditions

Oil-drain channel

Exhausthole

Initial radial clearance -3 – -4 mm
Bearing lubrication Air-oil
ISO VG32
Oil is supplied from both sides of bearing.
0.01cm3 / 10 min×2
Jacket cooling temperature Room temperature ±1˚CApplication of Ceramics to NU-Type Cylindrical Roller Bearings for Machine Tool Main Spindles

Jacket coolingof air-oil mixture was supplied to the bearing prior tooperation, while in the other scenario, no air-oil
mixture was supplied to the bearing prior to operation.
In the two cases in Fig. 8, the test bearings werequickly accelerated to 13,000 min-1 while being
lubricated with an oil-air flow rate of 0.01 cm3/10 min×
2. In the scenario in Fig. 8 (a), air-oil mixture was
supplied to the test bearing for 90 minutes prior to thetest operation, while in the scenario in Fig. 8 (b), no
air-oil mixture was supplied to the test bearing prior tostart of the test operation. When comparing the data inthe scenario (a) with that of the scenario (b), the heatrise on the inner ring with the scenario a isapproximately as much as 30˚C higher and that on theouter ring is approximately as much as 15˚C higher.
From these findings, we suspect the cause of thetemperature peak at the medium speed range is“residual heated lubricating oil” remaining in the
bearing. Therefore, an oil-drain structure boasting
positively efficient oil-draining capability in both highspeed and medium-speed ranges is needed. Also,
since such temperature peaks do not occur with the
N-type bearings, we feel we should consider oildraining behavior at around the outer ring rib, andBuilt-in motor

Test single-row
cylindrical roller bearing

Supporting DB arrangement
angular contact ball bearing

Fig.6 Section view of spindle test rig

70

80

60

Temperature ˚CInner ring temperature ˚CStandard variant (Fig. 3)
70

20000

Oil-drain groove (Fig. 4)

60
50
40

Inner ring
Bearing speed

50

15000

40
10000

30

Outer ring

20

5000

10
0

30

0

Oil-drain hole (Fig. 5)

10

20

30

40

50

Bearing speed min-1

hole variant was suspended because of sudden
temperature rise.
To be able to find a solution, we first assumed thecause for this temperature peak was poor oil-draining
performance, and attempted to verify this assumption.
Fig. 8 includes two sets of data obtained from twocases of bearing operation shown in Fig. 4—one casecorresponds with a scenario where a sufficient amount

060Time elapsed min
20

0

10000

20000

30000

(a) With pre-oil-supply

40000

Bearing speed min-1
70
60

Temperature ˚C

80Outer ring temperature ˚C

20000

70
60

Standard variant (Fig. 3)
Oil-drain groove (Fig. 4)

50

15000

50
40

Bearing speed

Inner ring

10000

30

Outer ring

20

5000

10
0

40

0

10

20

30

40

50

0
60

Time elapsed min

30(b) Without pre-oil-supply

Oil-drain hole (Fig. 5)
20
0

10000

20000

30000

Fig.8 Pre-oil-supply and temperature rise

40000

Bearing speed min-1(b) Outer ring temperature
Fig.7 Inner and outer ring temperatures vs. rotational speed-19-

Bearing speed min-1(a) Inner ring temperature

NTN TECHNICAL REVIEW No.78（2010）

believe that this approach will lead to improved oildraining capability on the outer ring side of the bearing
running at a higher speed.
Starting with these findings, we have further
continued with review and prototyping activities andhave finally reached the current cylindrical roller
bearing structure (NU-type) shown in Fig. 1. The test
conditions associated with these activities are
summarized in Table 3. Test results from the NU-type
bearing and those from the N-type bearing in Fig. 2 2)
are shown in Fig. 9. Fig. 9 (a) shows the test datafrom bearing samples with ceramic rollers, while Fig. 9(b) gives the test data from bearing samples with steel
rollers.
As is shown in Fig. 9 (a), the samples of the newly
developed NU-type as well as those of the N-type donot show a temperature peak in the medium speed
range at around 10,000 min-1, and exhibit smoothtemperature rise curves up to the targeted maximum
running speed of 35,000 min-1 (dmn value=3.25×106).
Compared with the N-type, the inner ring temperatureon the NU-type at 35,000 min-1 is 2˚C lower. Also, asapparent from the test data of the NU-type samples inthe data in Fig. 9 (b), there is no temperature peak ataround 10,000 min-1, and the temperature slowly
increases up to 35,000 min-1. The inner ringtemperature at 35,000 min-1 reads 70˚C which is 4˚Clower compared with the N-type—a very favorable
achievement.
From these findings, it has been verified that thebearing configuration illustrated in Fig. 1 (NU-type),

which comprises the ceramic inner ring (no spacerrings), the cage riding on outside surface of air-oil
nozzle, and the outer ring having oil-drain gaps, does
not exhibit the temperature peak in the medium speed
range, and can be run up to ultrahigh-speed range
without developing sudden temperature rise. With avariant of this bearing configuration that uses steel
rollers rather than ceramic, the inner ring temperature
at 35,000 min-1 is limited to 70˚C, which is the
maximum allowable temperature for commerciallyacceptable bearing operation. Compared with the Ntype in Fig. 2 2), the NU-type has the simpler-shaped
ceramic inner ring as well as steel rollers, allowing thisbearing to be offered at a commercially acceptableprice.
By the way, it is apparent that use of the ceramic
inner ring has helped achieve problem-free highspeed operation of the bearing. However, use ofceramic material in a bearing leads to additional
benefits. When a bearing having a steel inner ring is
run at a higher speed, bearing fit to the shaft can getloose owing to expansion of inner ring bore resulting
from heat and centrifugal force occurring from bearing
operation: to prevent loosening of the bearing relative80

Temperature ˚CCross-sectional plan Fig. 1
Size
φ70×φ110×20
Pitch diameter
93mm
Inner ring
Si3N4 (w/o steel spacer rings)
Outer ring
SUJ2
Rollers
Steel or Si3N4, dia. φ7×7mm, 22 pcs.
Cage
PEEK+CF30%, Nozzle outer surface riding
Fit between shaft and inner ring 5 μm, interference-fit

60
50
40

Inner ring

30
20

Outer ring
0

10000

dmn 3.25×106

20000

30000

40000

Bearing speed min-1(a) With ceramic rollers
80

Cross-sectional plan Fig. 2
Size
φ70×φ110×20
Pitch diameter
93mm
Inner ring
Si3N4 (w/ steel spacer rings)
Outer ring
SUJ2
Rollers
Steel or Si3N4, dia. φ7×7mm, 22 pcs.
Cage
PEEK+CF30%, Nozzle outer surface riding
Fit between shaft and inner ring 2 μm, interference-fit

N-type w/ ceramic
inner ring
Newly developed
NU-type70

Temperature ˚CN-type withceramic inner ring

Newly developed
NU typeTable 3 Test bearings (Figs. 1 and 2) and conditions
associated with Fig. 9

Test conditions

N-type w/ ceramic
inner ring
Newly developed
NU-type70

Initial radial clearance 0–3 mm
Bearing lubrication Air-oil
ISO VG32
Oil is supplied from both sides of bearing.
NU type 0.01cm3/10min×2 (Si3N4 rollers)
0.01cm3/6min×2 (steel rollers)
N type0.01cm3/10min×2 (Si3N4 rollers)）
0.01cm3/5min×2 (steel rollers)Jacket cooling temperature Room temperature ±1˚C

60
50
40

Inner ring30
20

dmn 3.25×106
Outer ring
0

10000

20000

30000

40000

Bearing speed min-1(b) With steel rollers
Fig.9 Inner and outer ring temperatures vs. rotational speed-20-Application of Ceramics to NU-Type Cylindrical Roller Bearings for Machine Tool Main Spindles

to the shaft, a rolling bearing of bore diameter 50 to
100 mm which is often used to support a machine tool
main spindle is interference-fitted over the main
spindle with interference allowance of 30 µm or
greater: consequently, there will be difficulty when
mounting the bearing by press-fitting. In contrast, useof a ceramic inner ring, whose expansion from heatand centrifugal force is small, will help the bearing to
be interference-fitted over the shaft with interference
allowance of 5 µm or smaller, leading to much easier
bearing mounting work.
Furthermore, the greater modulus of longitudinalelasticity of ceramic material helps enhance the rigidityof bearing. This topic is discussed in the following
section.

Displacement of bearing center μm

Steel inner ring/steel rollers
16

Steel inner ring/ceramic rollers
Ceramic inner ring/steel rollers

14

Ceramic inner ring/ceramic rollers
12
10
8
6
4
2
0

0

2

4

6

8

Radial load kN

Fig.10 Radial load vs. calculated bearing deflection

4. Improvement in bearing rigidity byuse of ceramic material

Table 4 Bearing stiffness improvement due to ceramic elements

Remember that in Section 1 in this article, we briefly
stated that any cylindrical roller bearing for machine
tool main spindle needs to be capable of not only
higher speed, but also greater rigidity. Throughcalculation, we have verified the effect of use ofceramic material in improving bearing rigidity. Here wewill describe this effect in detail.
The internal clearance of a cylindrical roller bearingof bore diameter 70 mm was selected as zero: then,
combining steel or ceramic inner ring with steel orceramic rollers, we have prepared various cylindricalbearing samples. To simulate operation of the bearing
sample on an actual machine tool, the maximum radialload applied to the bearing has been set to 7 kN.
The relation between radial load and displacementof the bearing center is graphically plotted in Fig. 10.
The result of bearing rigidity in the 3 to 7 kN region
that features good linearity within the radial load vs.
bearing center displacement, as summarized in Table
4. Improvement in rigidity with the sample using
ceramic material only for the inner ring is 7%, and thatwith the sample using a ceramic material only for therollers is 19%. Use of ceramic rollers leads to greater
improvement in bearing rigidity, since roller-to-innerring rigidity and roller-to-outer ring rigidity are
simultaneously improved. Use of ceramic material for
inner ring alone results in relatively small effect inimproving bearing rigidity: however, this arrangement
greatly contributes to improvement in high-speed
performance of the bearing.

Inner ring/rollers

Rigidity N/mIncrease %Steel/steel

6.23×108

0

Si3N4/steel

6.64×108+7

Steel/Si3N4

7.43×108+19

Si3N4/Si3N4

8.09×108+30

5. Verification of mechanical strengthof ceramic inner ring
When ceramic material is used for a bearing innerring it is necessary to prove that the inner ring has
sufficient mechanical strength against hoop stressoccurring from both heat generation and centrifugal
expansion. In the axisymmetric deformation mode, noshear stress occurs; therefore, the major stresses
involved are circumferential hoop stress, axial stress,
and radial stress, wherein on a thin-walled cylindersubjected to internal pressure and centrifugal force,the stress with the greatest impact is hoop stress.
Fig. 11 illustrates the hydraulic loading test rig wehave used to test the mechanical strength of theceramic inner ring. The test piece used is the same
inner ring used in the NU-type cylindrical roller bearing
in Fig. 1. High-pressure hydraulic oil supplied from an
outside hydraulic pump is uniformly distributed within
the bore of inner ring, and either an inner ring alone oran inner ring in a bearing assembly (complete withrollers and outer ring) can be tested. The test result is
illustrated in Fig. 12, with the hydraulic pressure
applied to the bore surface of the inner ring on the Xaxis, and the corresponding hoop stress occurring on
the bore surface of inner ring on the Y-axis. The innerring has developed fracture at a hoop stress of 500-21-

NTN TECHNICAL REVIEW No.78（2010）

MPa when tested alone, and at a hoop stress of 640
MPa when tested in the bearing assembly. These
hoop stress values are approximately three times andfour times greater than maximum commerciallyallowable hoop stress (160 MPa) for inner ring intypical cylindrical roller bearings for machine tool main
spindles. When assembled together with the rest of
the bearing, a compressive stress is applied to theinner ring in a direction which helps the compressive
stress overcomes the hoop stress; therefore the innerring in this configuration can withstand a greaterinternal pressure than the inner ring alone canwithstand.
Thus, we have proven functionality and mechanicalstrength of our newly developed NU-type cylindricalroller bearing, as illustrated in Fig. 1. Typicalphotographic views of this bearing type are given in
Fig. 13.

6. Conclusion
To enhance high-speed capability of its NU-typecylindrical roller bearing for machine tool main spindle,
NTN has introduced the following elemental
technologies:(1) Ceramic (silicon nitride) inner ring(2) Cage riding on the outer surface of the air-oil
nozzle(3) Oil drain structure with separate outer ring rib
Ceramic materials boast a low linear expansion
coefficient, low density and high modulus oflongitudinal elasticity. Thanks to these features, theceramic inner ring can resist over-preload that can
result from expansion of the inner ring while thebearing is running at a greater speed. This
improvement helps mitigate heat buildup within the
bearing, which is the biggest obstacle to problem-free

Ceramic inner ring

Ceramic inner ring

Hydraulic oil supply

Oil-drain channels(a) Bearing bodyAir-oil nozzle spacer

200

Calculated hoop stress value MPa

1000
900

Stress800

Expansion

700

150

Fracture on innerring within bearing
assembly

600
500

Fracture on inner ringalone (not in bearing
assembly)

400
300

100

50

200

Estimated maximum stress on
inner ring mounted on actualmachine tool main spindle

100
0
0

50

100

150

200

0
250

Expansion on raceway of inner ring μm

Fig.11 Hydraulic loading test rig(b) Bearing and air-oil nozzle spacers
Fig.13 Developed NU-type cylindrical roller bearing

Pressure loading on inner ring MPa

Fig.12 Inner loading pressure vs.
hoop stress and inner ring expansion-22-

Application of Ceramics to NU-Type Cylindrical Roller Bearings for Machine Tool Main Spindles

References

high speed operation of bearings.
Incidentally, stagnant lubricating oil within a bearing
can lead to higher bearing temperatures, due to shear
heat generation of lubricating oil. In addressing thisproblem, we have improved the guide surface on thecage to promote draining of oil from the slide way on
cage, and at the same time, introduced a draining
structure independent of the outer ring rib in order topromote draining of oil from an area around the outerring rib.
By adoption of the above-mentioned elemental
technologies, NTN has successfully developed an
improved variant of NU–type cylindrical roller bearingthat boasts ultrahigh-speed range, that is, dmn
value=3.25×106 (bore diameter 70 mm, bearing
speed 35,000 min-1). At the same time, we haveanalyzed the mechanical strength of the ceramic innerring, and have determined that this inner ring hasmechanical strength sufficient for commercial use of
the new NU-type bearing.
Higher functionality and improved reliability ofbearings directly contribute to better performance ofmachine tools, and pose not-yet-solved challenges forbearing manufacturers. NTN will remain committed tofurther sophistication of its bearing technologies.

1) Japan Machine Tool Builders’ Association TechnicalCommittee, Design Theory of Machine Tools(Applications)—Basic Knowledge for Mather Machine
Design (1998) 93.
2) M. Mori and T. Kobayashi, Development of HighSpeed Cylindrical Roller Bearings for Machine Tools,
NTN Technical Review No. 76 (2008) 80-87.
3) NTN, Precision Rolling Bearings Cat. No. 2260/E(2008) 160.
4) K. Rokkaku and K. Nishida, Outline of Ceramic
Antifriction Bearings, Journal of the Japan Society for
Precision Engineering, Vol. 54, No. 7 (1988) 12401244.
5) Japan Society of Mechanical Engineers, editors,
Advanced Machine Tool Technologies—Methods forHigh Speed, High Precision and Multifunctionality, 2ndedition, Kogyo Chosakai Publishing Co., Ltd. (1989) 104.
6) Japan Machine Tool Builders’ Association, Machine
Tool Design Technology Expert Committee, Design
Theory of Machine Tools (Applications)—Basic
Knowledge for Mather Machine Design, (1998) 99100.

Photo of authors

Masatsugu MORI

Takuji KOBAYASHI

Elemental Technology
R&D Center

Elemental Technology
R&D Center-23-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Evaluation of Scratched Contact Fatigue Life
with Artificial Dent of Carbonitrided SUJ2 Steel
Whose Surface Nitrogen Content is Controlled Note)

Chikara OHKI**

Scratched contact life performance is very important for rolling bearings, but there is noresearch about the relationship between nitrogen concentration and scratched contact
life. In consideration of this situation, the scratched contact life of SUJ2 (SAE52100
equivalent), which had nitrogen concentration accurately controlled, was investigated inthis research. To avoid unexpected accidents occurring due to additive contamination to
lubrication oil, artificial dents were added to the inner rings of ball bearings using a
Rockwell indenter (196N), and these were used for the scratched contact life test. The
Weibull slope of the test results from this operation condition was sharper than that oftest results from a general test method (i.e., bearing rolling contact fatigue life test in contaminated lubrication condition).
Therefore, it is thought that the test method in this research is suitable for collecting quantitative data. A significant test
result appeared that when the surface nitrogen concentration is over 0.05 mass %, the scratched contact life is superior tothat of normal SUJ2 with a reliability of 99%. Moreover, experimental results revealed that, when the surface nitrogenconcentration is 0.4 mass %, the life can be longer than that of 0.1 mass % nitrogen concentration. The sample that had
surface nitrogen concentration of 0.4 mass % demonstrated L50 life that was 4 times as long as normal SUJ2.

test”. For comparison, NTN has also performed life
test under contaminated lubrication environments4), 5).

1. Introduction
The effectiveness of carbonitriding in improving
scratched contact fatigue life1) of rolling bearing has
been already reported by Kurabe, et al., and recently
by Dommarco, et al.2), 3) Recently, it has been reported
that the life of rolling bearings in contaminatedlubrication environments (here-after referred to as“scratched contact life”) is critical for bearing life inautomotive transmissions, automotive hubs andreducers. With lubricating oil contaminated withforeign matter of higher hardness, it has been verified
that carbonitriding is effective in extending thescratched contact life of rolling bearings4), 5). Note that
there is no life data for SUJ2 material whose nitrogenconcentration has been strictly controlled6), 7).
NTN has attempted to establish data on thescratched contact life of SUJ2 material whose nitrogenconcentration was strictly controlled. Test pieces
each having artificial dents were used for this life test.
NTN used a Rockwell indenter to form artificial dents
on the raceway surface of bearing inner rings. This
test is to be called the “scratched contact fatigue life

2. Test Method
The test bearing used was a deep groove ballbearing (bearing number: 6202, 30 mm ID, 62 mm
OD, 16 mm W, nine rolling elements). To avoid theadverse effect of bidirectional crack possibly occurringon the raceway surface8), ceramic balls were used as
rolling elements (dia. 9.525 mm (3/8 inch)). The basematerial used for the inner ring and outer ring
subjected to our scratched contact fatigue life test
belongs to a same lot per JIS-SUJ2 whose chemicalcomposition is summarized in Table 1. The inner ringand outer ring have been heat-treated in a batch heat
treatment furnace (capacity 120 L). The furnace
atmosphere during the carbonitriding process forthese work pieces was analyzed by the following
techniques: CO and CO2 partial pressures were
measured by non-dispersive infrared absorption
method, H2 partial pressure by thermal
conductometry, and NH3 partial pressure by gas*Elemental Technology R&D Center

NOTE) This technical paper has been developed by adding new information to and modifying existing information in“Tetsu-to-Hagane (Iron and Steel)”, vol. 95 (2009), pp. 695-703.
-24-

Evaluation of Scratched Contact Fatigue Life with Artificial Dent of Carbonitrided SUJ2 Steel Whose Surface Nitrogen Content is Controlled

been found in both carbon distribution and nitrogen
distribution; the nitrogen distribution profile matchesclosely with the estimated nitrogen profile plotted inFig. 1. Note that carbon (or, nitrogen) concentration
values obtained from EPMA are each a sum ofconcentration of carbon (or nitrogen) dissolved in solid
Fe and concentration of carbon (or nitrogen) within
deposited matter.

chromatography. Applying the Ohki’ method, NTN hascontrolled nitrogen concentration distribution in steel
by controlling undecomposed NH3 partial pressure, H2partial pressure, carbon activity and processing
duration6), 7). Because the life of outer rings, on which
no dent is formed, is unconditionally longer than thatof inner rings, the outer ring has been subjected toordinary hardening/tempering rather thancarbonitriding. Table 2 summarizes heat treatment
conditions and grinding allowances applied. The
experimental numbers for the respective tests aredescribed by using alphanumerical notations such as
those indicating the surface nitrogen concentration ofcompleted specimens having undergone grinding
processes as listed in Table 2.
Each test piece has been heated to the temperature
listed in Table 2, and hardened by oil quenching, andthen subjected to tempering for 2 hours at 180˚C. N0
and N0C mean non-carbonitrided specimens. The
suffix “C” means a specimen intended for test undercontaminated lubrication. N005 is intended for a casewith low nitrogen concentration, while N01 and N04
are intended for specimens whose nitrogenconcentration remains at a certain level even after
grinding. After heat treatment, two test pieces havebeen taken, at random, from a group of test pieces
associated with a given experimental number. Then,for each test piece, the carbon and nitrogenconcentration distribution profiles at the midpoint ofbottom of the groove within inner ring raceway surface
have been investigated by line analysis using an
Electron Probe Micro Analyzer (EPMA). The resultant
data are graphically illustrated in Fig. 1. For any given
experimental number, no dispersion in data points has

Table 1 Chemical composition of JIS-SUJ2 used (mass %)
C

Si

Mn

P

S

Ni

Cr

Mo

Cu

0.99

0.26 0.44 0.012 0.006 0.08 1.46 0.03 0.15

O

(ppm)

9

Notation of eachspecimen

Inner ring
N0,
N0C

N005

N01

N04

Outer
ring

Partial pressure of NH3 (Pa)

―

30

213

←

―

Partial pressure of H2 (kPa)

33.4

36.5

42.5

←

33.4

Flow rate of RX gases measured 11.5
at 25˚C and 1 atm (L/min)

←

←

←

11.5

Treating temperature (˚C)

850

852

852

←

850

Duration (s)3000

8000 5300←

3000

Carbon activity ac

0.70

0.95

←

←

0.70

Machining allowance (μm)

100

110

130

50

100

Surface nitrogen concentration
after grinding (mass %)

0

0.05

0.1

0.4

0

Carbon, Nitrogen concentration (mass%)

Table 2 Heat treatment conditions and grinding allowances(N0C for the test under contaminated lubrication)

1.9

Carbon, Nitrogen concentration (mass%)

Carbon, Nitrogen concentration (mass%)

By line analysis with EPMA, NTN has determined
nitrogen concentration distribution profiles at the
middle of the bottom groove on the inner ring raceway
surface after grinding. NTN has taken measurementson two data points for each experimental number. The
results are graphically plotted in Fig. 2. The nitrogenconcentration distribution profiles generated from
actual concentration measurements match well withthose developed by subtracting the grinding
allowances summarized in Table 2 from the nitrogenconcentration distribution profiles shown in Fig.1.
Three types of nitrogen concentration profiles wereobtained from heat-treated parts. However, NTN has
prepared four types of steel specimens featuring
unique nitrogen concentration distribution profiles byvarying the grinding allowance. By applying x-ray
diffraction technique, NTN has determined values forresidual stress and the amount of retained austenite at
a depth of 50 µm from the outermost surface at the
middle of the groove surface on specimens not yet
tested. The resultant data are graphically plotted inTable. 3. The residual stress mode occurring in the

N0, N0C

1.7
1.5
1.3
1.1
0.9
0.7

Carbon0.5
0.3

Nitrogen

0.1
-0.1

0

1.9

0.1 0.2 0.3 0.4 0.5
Distance from surface mm

0.6

1.9

N005

1.7
1.5
1.3
1.1
0.9
0.7

Carbon0.5
0.3

Nitrogen

0.1
-0.1

0

0.1 0.2 0.3 0.4 0.5
Distance from surface mm

0.6N01, N04

1.7
1.5
1.3
1.1
0.9
0.7

Carbon

0.5
0.3
-0.1

1st measured profile
2nd measured profile
Estimated nitrogen profile

Nitrogen0.1
0

0.1 0.2 0.3 0.4 0.5
Distance from surface mm

0.6

Fig. 1 Carbon & Nitrogen concentration distribution
profiles before grinding-25-

NTN TECHNICAL REVIEW No.78（2010）

0.5

Nitrogen concentration (mass%)

Nitrogen concentration (mass%)

entire test is “compression” regardless of the heat
treatment conditions applied. There is no large
variation in the quantitative values of residual stress.
The amount of retained austenite is greater withspecimens of greater nitrogen concentration, and is

N0, N0C
0.4
0.3
0.2
0.1
0
-0.1

0

0.1
0.2
0.3
0.4
Distance from surface mm

greatest with specimen N04, and lowest with noncarbonitrided specimen N0.
Fig. 3 shows hardness distribution profiles withspecimens having undergone heat treatment. At the
distances from the surface, which correspond with theoutermost surface after grinding, the hardness
distribution profiles with specimens N005 and N01 aresituated somewhat higher than that of specimens N0
and N04. However, the difference is small: notgreater than 50 HV. Judging from these findings, we
believe that there are two major characteristics thatmark uniqueness of each experimental number(specimen notation), that is, nitrogen concentration
and retained austenite in steel.

0.5
N005
0.4
0.3
0.2
0.1
0
-0.1

0

0.1
0.2
0.3
0.4
Distance from surface mm

Nitrogen concentration (mass%)

Nitrogen concentration (mass%)

2.2 Life test method
0.5
N01

0.4
0.3
0.2
0.1
0
-0.1
0

0.1
0.2
0.3
0.4
Distance from surface mm

0.5

The subject tested was an inner ring of a deep
groove ball bearing. The artificial dents were formedby forcing a cone-shaped diamond Rockwell hardness
indenter (conical indenter with cone angle of 120
degrees, whose tip has a spherical face of curvature
0.2 mm) onto the midpoint of the groove with a forceof 196 N. NTN has analyzed the profile of the dentwith a 3D surface profile analyzer, and have verified
that the profile of ridge around dent on each specimen
is nearly symmetrical in both axial and circumferential
directions, with respect to the center of dent. Fig. 4
shows typical dent profiles on specimens. The numberof dents formed per inner ring specimen is 30 (equally
spaced at 12-degrees intervals).
As shown in Fig. 5, the life test rig used is capableof applying a load evenly to two test bearings. The test
bearings are lubricated with an oil bath system. The
test conditions applied are summarized in Table 4. To
be able to detect occurrence of flaking, a vibrometer
was used. While the vibration acceleration test was
set to approximately 1.5 m/s2 (frequency range: 3 Hz
to 5 kHz, effective value), the test rig was adjusted sothat the test was terminated when the vibrometer
detected the increase of vibration acceleration at 6m/s2. NTN has found that when the vibration
acceleration reaches approximately 6 m/s2, one of thedents formed on the inner ring develops significant
flaking. To eliminate the adverse effects of instrument
error between test rigs, NTN has used only one life
test rig for the life test with all the specimens.
For comparison, NTN has subjected a noncarbonitrided bearing (N0C) to a life test under a muchcommonly applied contaminated lubrication
environment4), 5) (here-after referred to as“contaminated lubrication test”). The bearing material
type life test rig, applied radial load, inner ring runningspeed and lubrication system for this test were sameas those for the scratched contact fatigue life test.

N04

0.4
0.3
0.2
0.1
0
-0.1
0

0.1
0.2
0.3
0.4
Distance from surface mm

1st measured profile
2nd measured profile

Fig. 2 Nitrogen concentration distribution profiles
after grinding
Table 3 Characteristics measured by X-ray diffraction
Notation

Residual atress (MPa)Retained austenite (%)

N0, N0C

-53

8.5

N005

-92

15.3

N01

-75

15.8

N04

-119

23.3

800
750

Hardness (HV)

700
650
600
550

N0, N0C
500

N005
450

N01, N04

400
0

0.1

0.2

0.3

0.4

0.5

0.6

Distance from surface mm

Fig. 3 Hardness distribution profiles on heat-treatedspecimens before grinding-26-

Evaluation of Scratched Contact Fatigue Life with Artificial Dent of Carbonitrided SUJ2 Steel Whose Surface Nitrogen Content is Controlled

Load movement dir.

Other test conditions were virtually same as those of
the scratched contact fatigue life test. However, thistest is unique in that no artificial dents were made on
the inner ring, while a predetermined amount of hardforeign matter (shape: spherical, amount: 0.4 g/L,
hardness: 800HV, grain size: 108 to 180 mm) was
mixed into lubricating oil.

Dent
2μm

3. Test result
3.1 Facts about flaking and dents
In the scratched contact fatigue life test, flaking has
occurred on the inner ring of all the test bearings,
while no failure has occurred on the rolling elements
and outer rings on these bearings. Major flaking has
occurred only on one of 30 dents formed on the innerring. Fig. 6 provides examples of starting point
flaking. Similar views were also found on other test
bearing specimens. Flaking started at the ridgearound the dent rather than within the dent andpropagated along the direction of shifting load. Thismode is identical to the flaking mode 9) found on theconventional test method 4), 5).
Fig. 7 shows typical views of flaked areas resulting
from the contaminated lubrication test. It will beapparent from these photos that flaking starts at theridge around dent and propagates along the direction

Dent
2μm

2mm

0μm

0μm

Dent Profile of cross section

Dent Profile of cross section

5μm

5μm
100μm

100μm
Dent
2μm

0μm

Load movement dir.

Dent
2μm

0μm

Dent Profile of cross section

Dent Profile of cross section

5μm

N04 109.9h failed

N0 36.7h failed

5μm
100μm

2mm

100μm

Fig. 4 Examples of dent's shape

Load spring
0.1mm

Dent by rockwell indentation

Support bearing
6312

Fig. 6 Examples of flaking area with artifical dent
Load movement dir.

PulleyTest bearing
6206

Test bearing
6206
Coupling

0.1mm

Dent by rockwell indentation

Oil bath

2mm

Fig. 5 Schematic drawing of rolling contact fatigue life
tester for ball bearing

N0C 19.8h failed

N0C 65.0h failed

Table 4 Test conditions of scratched contact life testing
Radial load (kN)

4.9

Radial internal clearance (μm)

20

Maximum contact stressof flat surface (GPa)3.04

Rotating speed of inner ring (min-1)

3150

LubricantTurbine oil(Dynamic Viscosity at 40˚C :58.6mm2/s)

0.1mm

Dent by contamination

0.1mm

Dent by contamination

Fig. 7 Examples of flaking area under contaminatedlubrication condition-27-

NTN TECHNICAL REVIEW No.78（2010）

Load movement dir.

of shifting load. Similar flaking took place on othertest bearing types. Fig. 8 shows views of dents
distributed on the inner ring raceway surface having
undergone the test (areas around these dents did not
develop flaking). On a certain N0C specimen, theinner ring raceway developed flaking as early as 14.7
h, and the quantity of dents formed on this raceway
surface is more than double the quantity of dents
formed on the inner ring raceway surface of another
N0C specimen in which flaking occurred at 65.0 h. It
should be understood that the tendency of dent
occurrence greatly varies in this test even when test
conditions remain unchanged.

specimens (N0C) with contaminated lubricationcompared with those from the scratched contact
fatigue life test. The Weibull slope for the former
specimen type is much gentler because dispersion in
the life values with the former specimens is great. This
seems to be because in the contaminated lubrication
life test, the quantity of foreign matters biting into the
raceway surface can vary greatly depending on thestate of distribution of hard foreign matter in the oilbath as well as the start timing for bearing operation.
As a result of the scratched contact fatigue life test,
the Weibull slope is steeper and better life data has
been obtained. We believe this is because there are
no uncertain factors such as the quantity of foreign
matters biting into the raceway surface since dents
are intentionally formed on the raceway surface andthere is little dispersion in the quantity of resultant
artificial dents (in other words, variation in vulnerable
volume where fatigue typically develops).
Fig. 10 shows Weibull distributions that represent
bearing life which is dependant on the variation insurface nitrogen concentration. Each test session has
resulted in the least variation in life data and steeper
slope. We believe this test method is useful for
predicting the life of a bearing mounted on an actualmachine. The L50 life data in Table 6 shows that thelife is 57.7 h for specimen N0 (no-carbonitrided); 117.3
h for specimen N005 (surface nitrogen concentrationof approx. 0.05mass%); 121.3 h for specimen N01(surface nitrogen concentration of approx. 0.1mass%);and 218.0 h for specimen N04 (surface nitrogenconcentration of approx. 0.4mass%). In other words,
the longest L50 life of these specimens isapproximately four times as long compared with theshortest L50 life. Greater surface nitrogen

N0C 65.0h failed

N0C 14.7h failed

1mm

1mm

Arrows mark dents formed by contaminants.

Fig. 8 Appearance of inner ring raceway surface after
contaminated lubrication test

3.2 Weibull distributions
Table 5 summarizes data obtained from the test. In
this test, NTN has determined scratched contact
fatigue life by assuming that bearing life, according toclassical theory of Lundberg-Palmgren10), follows 2parameter Weibull distribution function.
Table 6 summarizes the results of the scratch
contact life test on bearing specimens determinedfrom Weibull distributions. Fig. 9 graphically provides
the Weibull distributions for non-carbonitrided

Table 5 Results of scratched contact life test

Table 6 Test results from Weibull distributions

N0C
N0
N005
N01
N04
Cumulative Failure Cumulative Failure Cumulative Failure Cumulative Failure Cumulative Failure
life (%) time (h) life (%) time (h) life (%) time (h) life (%) time (h) life (%) time (h)
15.9

14.1

5.6

36.7

10.9

66.6

5.6

78.4

8.3

109.9

38.6

14.7

13.7

42.5

26.5

95.3

13.7

87.1

20.2

150.4

61.4

19.8

21.8

44.4

42.2

119.6

21.8

100.9

32.1

200.7

84.1

65.0

29.8

47.0

57.8

123.6

29.8

101.9

44.0

235.6

―

―

37.9

50.1

73.5

130.7

37.9

105.8

56.0

241.7

―

―

46.0

55.3

89.1

168.2

46.0

111.6

67.9

244.2

―

―

54.0

58.5

―

―

54.0

121.2

79.8

244.2

―

―

62.1

62.9

―

―

62.1

122.3

91.7

319.2

―

―

70.2

63.0

―

―

70.2

130.5

―

―

―

―

78.2

70.1

―

―

78.2

146.3

―

―

―

―

86.3

71.6

―

―

86.3

146.4

―

―

―

―

94.4

85.7

―

―

94.4

195.0

―

―

-28-

N0C

N0

N005 N01

L10 life※ (h) 8.0

38.6

67.6

N04

L50 life※ (h) 24.3

57.7

117.3 121.3 218.0

Weibull slope※ 1.70

4.68

3.42

81.7 124.9

4.77

3.38

※Calculation value from 2 parameter Weibull distribution.

Evaluation of Scratched Contact Fatigue Life with Artificial Dent of Carbonitrided SUJ2 Steel Whose Surface Nitrogen Content is Controlled

NTN has attempted to assess the life test results
obtained in the present research, by t-testing forsignificant differences with normal distributions. When
comparing two populations when normal distributionsand variances of the distributions are unknown, it will
be possible to assume a population standard deviationcommon to both populations and then test thesignificant differences of population averages basedon t-distributions14). Weibull distribution coincides withnormal distributions when its gradient is 3.2610).
The Weibull gradient in the contaminated lubrication
test is 1.7, and approximation to normal distribution
will result in greater error (skewness: 1.9, kurtosis:
3.8). The Weibull gradients observed in the scratchedcontact fatigue life fall in a range of 3.38 to 4.77, andneat to 3.26 (max. skewness: 1.1, max. kurtosis: 1.8).
The Weibull distributions appear to fairly match thestandard distributions. Table 7 summarizes data
resulting from fair approximation to standard
distributions, wherein the data for each specimen
consist of average life, standard deviation and thesum of squares. When Weibull distributions fairly
approximate (match) normal distributions, the average
life of any specimen matches a value virtually thesame as L50 life obtained from the Weibull distributions
for that specimen. For this reason, we believe that themagnitude of errors resulting from approximation tonormal distributions is small.
Table 8 summarizes the results of the t-test. Thereappears to be a noteworthy difference, when the levelof significance is 1%, between the population mean ofN0 (specimen not undergone carbonitriding) and thatof N005 or N01 (specimen having undergonecarbonitriding). Our carbonitriding techniqueapparently helps realize longer life for specimens
subjected to scratched contact fatigue life test. NTN
has learned that there is no sizeable difference, when

Cumulative failure (%)

99
80
50
20
10
5

N0C
N0
1
101

102

103

Life hFig. 9 Difference in Weibull distributions dependent on
test method used

Cumulative failure (%)

99
80
50
20

N0
N005
N01
N04

10
5

1
101

102

103

Life hFig. 10 Difference in Weibull distributions dependent onvariation in nitrogen concentration distribution

concentration leads to longer bearing life. Note, that
there is a difference in surface nitrogen concentration
between N005 and N01. The Weibull distributions
obtained from these two specimen types are virtually
identical. This topic will be discussed later in Sec. 4.

3.3 t-test for significant differences
Experimentally developed Weibull distributions canbe used to compare bearing life4, 5, 11). However, if thenumber of test bearing specimens is low, the resultant
Weibull distributions are less reliable, making thiscomparison practice not always relevant. If this is thecase, it would be necessary to make comparisonsbased on significant differences by applying statistical
data processing techniques. However, there havebeen few researches about the t-test for significant
differences with Weibull distributions. In researching ttest technique for this significant difference in Weibull
distributions, Johnson has adopted a statistical
approach, and Fujita has applied a technique to use
random numbers developed by a computer12, 13).
However, these approaches are not yet regarded as
fully established techniques. To address this issue,

Table 7 Approximations to normal distribution
N0

N005

N01

N04

Average life (h)

57.3

117.3

120.6

218.2

Standard deviation

14.2

34.3

31.5

64.4

Sum of squares

2211.95869.810903.729040.0

Table 8 Resulta of t -test
Objects

Standard deviationof populationt0

t(φ,0.1)t(φ,0.01)

N0 and N005

22.475

5.339

1.746

2.921**

N0 and N01

24.416

6.350

1.725

2.845**

N005 and N01

32.378

0.204

1.746

2.921

N01 and N04

47.107

4.539

1.734

2.878****Significant difference is present when level of significance is 1%.

-29-

NTN TECHNICAL REVIEW No.78（2010）

the level of significance is 10%, between the
population mean of N005 and that of N01. There is aconsiderable difference between the population meanof N04 (whose surface nitrogen concentration is ashigh as approximately 0.4mss%) and that of N01,
when the level of significance is 1%. From thefindings obtained in our scratched contact fatigue life
test, we can judge that our carbonitriding techniquepositively help extend life of SUJ2 material used as
bearing components and this effect will be enhancedwith higher nitrogen concentration in carbonitrided
case layer.

obtuse and their curvature radii are greater comparedwith the shapes on similar areas on the noncarbonitrided specimens (N0). Retained austenite
seems to trigger generation of work-induced
martensite, which leads to a greater work hardening
coefficient. The carbonitrided specimens feature
greater amounts of retained austenite. There isresearch that investigates the relation between
physical properties of steel materials and the shapesof dents (indentations) formed on specimens made ofthese steels17). According to this research, the apexesof ridges around dents on steel are duller with greater
work hardening coefficients. Thus, the results
obtained from our scratched contact fatigue life test
appear to be relevant. However, in comparison withpre-test data in Fig. 11, the post-test data in Fig. 12
indicate that the heights around the apexes of ridges
around dents decreased to those as low as nearly half
the original heights. The shapes near the apexes of
the ridge around the dents maintain their shape
regardless of the material and elapsed test time.
According to previous researches, the shapes of theridge around the dents in rolling contact mode varies
greatly in the earlier stages of fatigue life (until

4. Discussion
From the findings obtained in our scratched contact
fatigue life test, NTN has determined that our
carbonitriding technique applied to SUJ2 (typicalbearing steel material) positively helps extend the lifeof bearing components made from SUJ2 material.
This life-extending effect is more apparent when thenitrogen concentration is higher in the carbonitrided
case of a bearing component made of SUJ2. The
next section describes the reasons for why ourcarbonitriding technique helps extend the service lifeof bearing components made from SUJ2.
A report about scratched contact life of bearing steel
material, states that a smaller gradient of the inside
face of dent (dent depth divided by dent diameter)
means a longer scratched contact fatigue life of the
bearing component in question15). However, since thevariation of the shapes of dents is small, as previously
shown in Fig. 4, the gentle slope inside the scratchderived dents does not account for the longer life ofcarbonitrided bearing components in the present
scratched contact life test.
Other reports show that greater stress concentration
will occur as the fatigue on the ridge around dents
progresses. The scratched contact fatigue life of thematerial in question will be shorter if the value
obtained by dividing the curvature radius on the ridgearound the dent is divided by the dent radius issmaller11) and/or if the angle at apex of ridge issmaller16) (acute angle). The shapes of ridges(downstream in the shift direction of load) arounddents before and after scratched contact fatigue life
test are illustrated in Figs. 11 and 12. Note that the
inner raceway surface on the dent-free specimen was
taken at the Y-axis origin, and the intersection point of
the ramp of ridge around dent and the Y-axis is taken
at the X-axis origin.
Fig. 11 shows that before the test, the shapes ofapex of the ridge around the dents on the
carbonitrided specimens (N005, N01, N04) are more

2.0
1.8

N0
N005
N01
N04

dent height (μm)

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0

25

50

75

100

125

150

Distance from the standard point μm

Fig. 11 Shapes around dents before rolling contact fatigue

2.0
1.8

N0
N005
N01
N04

dent height (μm)

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0

0

25

50

75

100

125

150

Distance from the standard point μm

Fig. 12 Shapes around dents after rolling contact fatigue-30-

Evaluation of Scratched Contact Fatigue Life with Artificial Dent of Carbonitrided SUJ2 Steel Whose Surface Nitrogen Content is Controlled

approximately 1/8 flaking life, and does not change
greatly afterwards18). It is estimated that while fatigue
progresses, stress concentration remains virtuallyunchanged regardless of the steel type. Thus, wethink that the true cause of longer life by carbonitriding
is not caused by the shapes of ridges around dents,
but from the enrichment in retained austenite.
We believe that since our carbonitriding technique6), 7)
does not lead to decarburizing, the sum of carbon andnitrogen concentration in carbonitrided case increases
by the concentration of interstitial nitrogen portion.
For this reason, we currently think that solid-solution
strengthening19) caused by carbonitriding may
contribute to longer fatigue life of steel specimens.
However, Weibull distribution obtained from specimen
N005 whose surface nitrogen concentration stands atapproximately 0.05mas% is nearly identical to that ofspecimen N01 whose surface nitrogen concentration
is as high as approximately 0.1mss%. Therfore,
surface nitrogen concentration alone cannot explain
longer rolling fatigue life. To address this issue, wehave attempted to determine, using a market-available
thermodynamics phase equilibration state calculation
software package, the interrelation between nitrogenconcentrations in steel and the concentration of
nitrogen dissolved (here-after referred to nitrogenconcentration in solid) in austenite (here-after referredto “γ-Fe”). SUJ2 contains chemical compounds C,
Si, Mn and Cr. To simplify calculation, Si and Mn
have been excluded from the calculation, and theequilibrated state for 1mass% C-1.5mass% Cr-Fe
system including addition of nitrogen has been
analyzed. The test temperature has been set to
850˚C, which is same as the carbonitriding
temperature. On a commercial production basis, steel
work pieces are heated, and then hardened.
Therefore, the concentration of nitrogen dissolved inmartensite should be determined, but this calculationis very difficult. At the same time, because thehardening mode in the present research is oil
quenching, we thought that nitrogen concentration inmartensite did not greatly differ from that in γ-Fe at
850˚C. Furthermore, though nitrogen is present, otherthan as CrN, in the form of element that substitutes
carbon in cementite ((Cr,Fe)3(C,N)), the amount of thisnitrogen portion is very small, and we have ignored
this portion in the calculation. Table 9 summarizes
the calculation results of nitrogen concentration in γFe at 850˚C. When total nitrogen concentration is
0.05mass%N, two phases (γ-Fe + cementite) arepresent in γ-Fe at 850˚C. When the total Ni
concentration falls in a range of 0.1 to 0.3mass%N,
three phases (γ-Fe + cementite + CrN) exist. Withthe total Ni concentration range of 0.4 to 0.6mass%,

Table 9 Calculation results of nitrogen concentration in γ-Fe
at 850˚C (1mass%C-N-1.5mass%Cr-Fe)
Total nitrogenconcentration(mass%)

Mass of CrN
per steel 1g (g)Nitrogenconcentrationin γ-Fe※ (mass%)

Phase
at 850˚C

0.05

0

0.050

γ-Fe※+cementite

0.1

0.001

0.077

0.2

0.005

0.092

0.3

0.009

0.115

0.4

0.012

0.154

0.5

0.014

0.211

0.6

0,015

0.286

γ-Fe※
+cementite+CrN

γ-Fe※+CrN※ Austenite-Fe

only two phases (γ-Fe + CrN) remain. When the total
nitrogen concentration stands at 0.05mass%N, CrN
phase does not occur, thereby the resultant nitrogenconcentration in γ-Fe remains equivalent to the total
nitrogen concentration. In contrast, when total
nitrogen concentration is 0.1mass%N, CrN phaseoccurs, and the nitrogen concentration in γ-Fe
increases to approximately 0.077mas%. Also, whentotal nitrogen concentration reaches 0.4mass%N, CrN
phase occurs too. Owing to higher total nitrogenconcentration, nitrogen concentration in γ-Fe
increases to approximately 0.154mass% (this is twice
as high as with a case where the total Ni concentration
stands at 0.1mass%N). The reason why the mean lifeof N005 does not differ from that of N01 seems thatthe total Ni concentrations on these steels are virtually
the same. From these findings, NTN believes thatscratched contact fatigue life of carbonitrided bearing
steel will be longer with higher total dissolved nitrogenconcentrations.

5. Conclusion
We have attempted to obtain life data and better
repeatability of the fatigue life of deep groove ballbearings made of SUJ2 steel whose surface nitrogenconcentration is accurately controlled. For thispurpose, we intentionally formed dents on the
raceway surface of the inner ring and then subjected
the inner rings to scratched contact fatigue life tests.
The results obtained from the test are summarized
below:
1) The Weibull distributions of any specimens
obtained from our test feature smaller scatteringand steep Weibull slopes as compared with theconventional life test under contaminatedlubrication with hard foreign matters. NTN believes
that this is because of the elimination of the-31-

NTN TECHNICAL REVIEW No.78（2010）

uncertain factor of biting of foreign matters into the
raceway surface. Our test method is suitable forobtaining life data of good repeatability, and will
make a useful test method for predicting bearing lifeon actual machinery.
2) The L50 lives, determined from the Weibull
distributions, are 57.7 hours with specimens notundergoing carbonitriding; 117.3 hours, 121.3 hours
and 218.0 hours with specimens whose surface
nitrogen concentrations are 0.05mass%, 0.1mass%and 0.4mass%, respectively. The longest life isapproximately four times as long as the shortest
life. In other words, higher surface nitrogenconcentration means longer life.
3) The Weibull distributions were approximated tonormal distributions, whose significant differenceswere tested with the t-test method. Previously,
Weibull distributions obtained from conventional
contaminated lubrication life test could not beapproximated to normal distributions because ofgentle Weibull slopes. In contrast, our test method
has revealed that the mean life of carbonitrided
specimens is, when the level of significance is 1%,
longer than that of non-carbonitrided specimens.
The mean life of specimens featuring higher
surface nitrogen concentration, such as 0.4mass%,
is, when level of significance is 1%, longer thanthose of specimens with surface nitrogenconcentration 0.05mass% or 0.1mass%.
4) As a result of the calculation with thermodynamic
phase equilibration diagram, the life of carbonitrided
steel material appears to be higher with higher
concentration of nitrogen dissolved in solid Fe.

References
1) N. Mitamura, J. Jpn. Soc. Tribol., Vol. 53 (2008), 641.
2) H. Kurabe, T. Araki, Tetsu-to-Hagané, Vol. 53 (1967),
1305.
3) R. C. Dommarco, K. J. Kozaczek, P. C. Bstias, G. T.
Hahn, C. A. Rubin, Wear, 257 (2004), 1081.
4) K. Maeda, Kougyou Kanetsu, Vol. 38 (2001), 2.
5) C. Ohki, SAE Technical Paper Series (2004), 2004-010634.
6) C. Ohki, Tetsu-to-Hagané, Vol. 93 (2007), 220.
7) C. Ohki, Tetsu-to-Hagané, Vol. 94 (2008), 42.
8) Y. Fujii , K. Maeda, Wear, Vol. 252 (2002), 787.
9) T. Sada, T. Mikami, J. Jpn. Soc. Tribol., Vol. 49 (2004),
948.
10) S. Shimizu, Introduction to Reliability Design for
Mechanical Engineering, Suurikougakusha-Sha Co.,
Ltd., Tokyo (2006) 34.
11) Y. Murakami, Machine Design, Vol. 39.
12) L. G. Johnson, The Statistical Treatment of Fatigue
Experiments, Elsevier Publishing Company, St. Louis,(1964), 3.
13) T. Fujita, Proceedings of Tribology Conference,
Japanese Society of Tribologists, Aichi (2008), 439.
14) S. Moriguchi, Statistical Methods, Newly Revised
Edition, Japan Standards Association, Tokyo (1989)
121.
15) A. Lubrecht, C. H. Venner, S. Lane, B. Jacobson, E.
Ioannides, Proceedings of the Japan International
Tribology Conference, Aichi (1990), 185.
16) C. Ooki, Proceedings of Tribology Conference,
Japanese Society of Tribologists, Tokyo (2000) 169.
17) S. Maki, J. Mater. Test. Res. Assoc. Jpn., Vol. 39(1994) 127.
18) H. Kakishima, E. Kimoto , A. Korenaga, T. Yoshioka,
Proceedings of Tribology Conference, JapaneseSociety of Tribologists, Tokyo (1997) 443.
19) M. Kato, Introduction to the Theory of Dislocations,Shokabo Publishing Co., Ltd., Tokyo (1999) 179.

Photo of author

Chikara OHKI
Elemental Technology
R&D Center-32-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Plasma Nitriding Treatment of High Alloy Steelfor Bearing Components

Kazuhiro YAGITA**
Chikara OHKI**
Bearings for special purposes, including aerospace engines, should have a very highreliability in consideration of safety. In addition, they need to have extended maintenanceintervals in consideration of economics. In order to achieve these, it is necessary to preventthe unexpected failure of bearings caused by surface damage from exposure tocontaminated lubricant. A countermeasure to contaminated lubricant is to provide a
hardened layer to bearing races to reduce the effect of contamination. One method of
surface modification is low-temperature nitriding as a kind of surface hardening. In
particular, plasma-nitriding (ion-nitriding) is a treatment that does not use ammonia and has
few environmental impacts, while being relatively suitable for high alloy steel. This report
describes examples of plasma-nitriding applications.

contaminated lubrication and extended bearing
replacement intervals are needed.
To make bearings more resistant against failurecaused by foreign contamination in lubricating oil,various techniques are available, including improved
heat treatment techniques1)–3). One example of is asurface hardening process for forming a hardened
layer on bearing raceway surfaces. Plasma nitriding,also referred to as ion nitriding, is a variation of a lowtemperature nitriding process.that, thanks to the
sputtering effect4), can realize nitriding relatively easily
on high alloy steels for which gas nitriding would be
difficult. NTN has been using M505) or M50NiL6)(Table 1) for bearings used under high-temperatureand high-speed conditions, and the plasma nitriding
process is suitable for these high alloy steels.
Depending on process conditions, low-temperaturenitriding process can cause a chemical compound
layer (also known as white layer) to occur on thesurface of steel or cause precipitates (Fig. 1) to occur
in grain boundary. These phenomena can adversely
affect the rolling fatigue life of the bearing. Any
nitriding technique applied to bearing components
must not develop a chemical compound layer orintergranular precipitates. At the same time, a certain
degree of nitrided case depth needs to be achieved
such that the surface of bearing components
withstands the adverse effects caused by

1. Introduction
Bearings used under high-temperature and highspeed conditions are required to satisfy a high degreeof reliability; therefore, these bearings use materials
with cleanliness that is much better than ordinary
bearing steel. These highly clean steel materials areresistant against internally propogated flaking that
often begins at non-metallic inclusions within steel,resulting in longer life.
Unfortunately, bearings are not always lubricatedwith clean oil, and operating conditions for bearings
with contaminated lubrication may be very severe. In
such severe operating conditions, contamination will
dent bearing races before internally propagatedflaking occurs, and the resulting indentations canprovide propagation points for flaking. In thisdemanding situation, bearing life is shorter than underinternally propagated flaking mode and can varydepending on a variety of factors including quantity ofcontamination, and the size and hardness of foreigncontaminants.
When a bearing fails due to foreign contamination,
the failure usually occurs suddenly and unexpectedly.
This failure mode is particularly critical for safety inaerospace engine bearings since engine trouble
during flight can require an emergency landing. Also,for economic reasons, longer bearing life under*Elemental Technology R&D Center-33-

NTN TECHNICAL REVIEW No.78（2010）

contaminants in lubricating oil. This technical paper
describes our experience in applying a plasma
nitriding technique to M50 and M50NiL which satisfies
the above-mentioned requirements.

2. Plasma heat treatment
This section contains an overview of plasma heat
treatment. Various plasma heat treatment machines
are currently available. In the basic arrangement
common to these machines, a vacuum pumpevacuates the interior of the machine, and a DC
voltage is applied to a work-piece using the workpiece as a cathode and the inside wall of the machineas an anode. The work-piece is subjected to nitriding
and/or carburizing through application of DC voltage.
When a DC voltage is applied to electrodes under
reduced atmospheric pressure, glow discharge occurs
across the electrodes (Fig. 2). During glow discharge,
electrons are released from the cathode toward theanode. These electrons collide with gas molecules
introduced into the machine, converting the gas
molecules into ions. The resulting ions are accelerated
toward the cathode(that is, the work-piece),
penetrating into the workpiece surface when theycollide with the surface7).
One outstanding advantage of plasma heat
treatment is the lower environmental impact it posescompared with heat treatment with a controlled
atmosphere furnace. For example, nitriding with acontrolled atmosphere furnace often uses ammonia
gas. In contrast, using a plasma nitriding technique, asteel material can be nitrided with nitrogen andhydrogen. Also, with plasma nitriding, the intended
work-piece alone is heated without heating the entire
interior of the furnace—a practice necessary for acontrolled atmosphere furnace.
Fig.3 shows schematics of typical plasma heat
treatment equipment configurations. With cold-wall or
hot-wall configurations where a bias voltage is applied
to a work-piece, a clean surface is exposed on thework-piece through sputtering where ions collide withthe work-piece. Consequently, the surface of a

Table 1 Chemical compositions of M50 and M50NiL
C

Si

Mn

Ni

Cr

Mo

V

M50

0.80

0.2

0.3

0.1

4.0

4.3

1.0

M50NiL

0.13

0.2

0.2

3.4

4.1

4.3

1.2

0.05mmFig. 1 Typical intergranular precipitates (etched in nital)

Fig. 2 Appearance of glow discharge

ScreenHeater• Built-in heater• Uniform temperature
distribution

Advantage• Heating is achieved
with glow dischargealone.

(c) Active screen(b) Hot wall
Advantage

Advantage(a) Cold wall

• Bias voltage is applied to anarea called “screen”.
• Work-piece is heated by heatradiation from the screen.

Fig. 3 Schematic diagram of plasma heat treating equipment-34-

Plasma Nitriding Treatment of High Alloy Steel for Bearing Components

1200

3.0
C concentration
N concentration
Hardness

2.5

1000

2.0

800

1.5

600

1.0

400

0.5

200
0
0.5

0
0

0.1

0.2

0.3

HV hardness

C&N concentrations wt%

material is nitrided in a short time even if the material
type needs a longer time for nitriding with a gas
nitriding or vacuum nitriding process. However,conventional plasma heat treatment techniques havedisadvantages such as poor surface smoothness
resulting from sputtering. To address this problem, aunique configuration known as the “active screen”
method has been developed, wherein a bias voltage is
not applied to the work-piece.
In the test performed for this paper, we have used acold wall type plasma heat treatment equipment, as
well as a radiation thermometer to help control thetemperature of the plasma heat treatment equipment.

0.4

Depth from surface mm(a) M50

3. Typical applications of plasma
nitriding
3.1 Intergranular precipitates
As mentioned previously, formation of a chemicalcompound layer on the work piece surface, andintergranular precipitates are challenges associatedwith low temperature nitriding. These problems canprovide a propagation points for flaking or fracture,and can very often adversely affect the rolling fatigue
life of a bearing. This section provides information
about the occurrence of intergranular precipitates.
Prior to nitriding, specimens were subjected toconventional heat treatment such as hardening andtempering, and then M50 specimens (immersion
quenched) and M50NiL specimens (carburized) were
subjected to Ar sputtering, and were allowed to standat 480˚C for 30 hours; during this course, a gas
mixture (N2:H2:CH4=79:80:1, volume ratio) was
introduced into the furnace in order to nitride thespecimens. The CH4 was added to the gas mixture toprevent minor decreases in nitrogen concentration
around the surface of specimens during lowtemperature nitriding8), 9).
We have analyzed the carbon and nitrogenconcentration distribution profiles on the nitrided
specimens with Electron Probe Micro Analyzer(EPMA), and measured cross-sectional hardness
profiles on each specimen using a Vickers hardness
tester (Fig. 4). Nitrogen has penetrated to a depth ofapproximately 0.1 mm with M50 specimens, andapproximately 0.2 mm with M50NiL specimens. As
shown in the photos of microstructures in Fig. 5,
etching with nital caused white precipitates to become
visible along the grain boundary. Intergranularprecipitates occurred to a depth of approximately 0.05
mm with M50 specimens, and approximately 0.2 mm
with M50NiL specimens. The analysis has revealed
that these intergranular precipitates consist of ironnitrides, in particular, Fe3N.

C concentration
N concentration
Hardness

2.5

1200
1000

2.0

800

1.5

600

1.0

400

0.5

200

0
0

0.1

0.2

0.3

0.4

HV hardness

C&N concentrations wt%

3.0

0
0.5

Depth from surface mm(b) M50NiL
Fig. 4 Depth profiles of carbon, nitrogen content andhardness of plasma nitrided specimen

0.05mm(a) M500.05mm(b) M50NiL
Fig. 5 Microstructures of plasma nitrided specimen (etched in nital)-35-

NTN TECHNICAL REVIEW No.78（2010）

3.2 Inhibition formation of intergranular precipitates

quickly penetrate the steel material, nitrogen atoms
will be very slowly diffused in the steel material, andnitrogen atoms of higher concentration will remainaround the surface of the steel material, thereby
causing intergranular precipitates to occur in thisregion. Thus, it will be difficult to achieve a sufficiently
deep nitrided case while inhibiting occurrence ofintergranular precipitation by plasma nitriding alone.

We have subjected to M50 and M50NiL specimens
to plasma nitriding test, and have learned that whileintergranular precipitates tend to occur in steel
materials with higher nitrogen concentration,intergranular precipitates may or may not occur at agiven nitrogen concentration.
From the findings in Sec. 3.1, we suspected thatcarbon concentration governs occurrence ofintergranular precipitates, and reinterpreted the test
results by considering not only nitrogen concentration
but also carbon concentration. Consequently, it has
become apparent that, as illustrated in Fig. 6,
intergranular precipitation occurs only when the sumof carbon concentration and nitrogen concentration inbase steel materials is higher than approximately
1.7wt%. Note that the higher concentration data
resulting from precipitates contained in the areas foranalysis have been excluded from analysis. Fromthese findings, we have learned that for better plasma
nitriding results, we need to remain aware not only of
nitrogen concentration alone but also of the sum ofcarbon concentration and nitrogen concentration.
We have also learned that when the plasma
nitriding duration is set longer in order to achieve anecessary nitrided case depth, nitrogen concentration
around the surface layer will become higher andintergranular precipitates occur before much nitrogenis diffused into the steel specimens (this state is
known as “diffusion controlled” mode).
To summarize, intergranular precipitation will notoccur when nitrogen is allowed to penetrate only ashallow depth below the surface for a short durationby plasma nitriding; However, a problem will occur
when plasma nitriding is applied to steel specimens
for a longer duration in order to obtain a sufficiently
deep nitrided case. That is, though nitrogen atoms will

3.3 Discussion of dispersion treatment conditions
When plasma nitriding process alone is applied to
M50 and M50NiL specimens, it is difficult to achieve a
nitrided case depth sufficient for bearing components
without causing intergranular precipitates to occur.
Therefore, we have attempted to achieve a sufficientnitrided case depth without triggering intergranularprecipitation by performing a diffusing process after
plasma nitriding.
Generally, completing a diffusion process at ahigher temperature and for a longer duration iseffective to increase the diffusion depth of any
element diffused into steel material. We have triedvarious diffusion process conditions, and learned that
if the diffusion process is performed for a prolonged
time after plasma nitriding, the core hardness of thebase steel material will decrease even though the
diffusion process temperature is lower than thetempering temperature. When the core hardness ofthe base steel material deteriorates even though asufficient nitrided case depth is achieved, the material
cannot be used for bearing components. Therefore,
we have to establish diffusion process conditions that
do not lead to a decrease in the core hardness ofbase steel material while maintaining a satisfactorily
large nitrided case depth.
Using M50 and M50LiN specimens, we haveattempted to determine the relationship between thetemperature and duration of the diffusion process anddeterioration in core hardness of the base steel
material (Fig. 7) as well as determine the nitrided casedepth.The triangular region illustrated in Fig. 8
defines relevant diffusion process conditions. The subsection below describes the result of combining
plasma nitriding processes and relevant diffusion
processes.

C+N concentration wt%

2.5

2.0

1.5

1.0

0.5

0

No intergranularprecipitation

Occurrence of intergranularprecipitation

Fig. 6 Relationship between sum of carbon and nitrogencontent and occurrence of intergranular precipitates-36-

Lower temperature

3.0
C concentration
N concentration
Hardness

2.5
2.0

1200
1000
800

1.5

600

1.0

400

0.5

200

0

0

0.1

0.2

0.3

HV hardness

C&N concentrations wt%→Higher hardness

HV hardness

Plasma Nitriding Treatment of High Alloy Steel for Bearing Components

0
0.5

0.4

Depth from surface mm(a) M50→Longer duration
Process time

Diffusion temperature

Fig. 7 Relationships among treatment temperature, timeand matrix hardness

C concentration
N concentration
Hardness

2.5
2.0

1200
1000
800

1.5

600

1.0

400

0.5

200

0

0

0.1

0.2

0.3

0.4

0
0.5

Depth from surface mm

Conditions suitablefor diffusion process
Insufficientnitrided case
depth

3.0

HV hardness

C&N concentrations wt%

Higher temperature(b) M50NiL
Fig. 9 Carbon and nitrogen concentration and hardness
profiles of plasma nitrided specimen, after diffusion
treatment

Deterioration incore hardness

Diffusion time

Fig. 8 Schematic diagram of diffusion treatment
condition

3.4 Example of treatment involving diffusion
process
Prior to plasma nitriding, specimens were subjected
to conventional heat treatment. M50 specimens(immersion quenched) and M50NiL specimens(carburized) were subjected to Ar sputtering in plasma
heat treatment equipment, and were then allowed tostand at 430˚C for 10 hours. During this process, gas
mixture (N2:H2=1:1, volume ratio) was introduced into
the equipment to plasma-nitride the specimens. The
nitrided specimens were allowed to cool to room
temperature in the furnace, and then removed fromthe equipment and loaded into a constant temperature
diffusion chamber where they were kept at 430˚C for
160 hours.
Fig. 9 shows carbon and nitrogen concentration
profiles and cross-sectional hardness profiles of
specimens having undergone the diffusion process.
Fig. 10 provides photos of microstructures of plasma
nitrided specimens having undergone natal etching.
Nitrogen has penetrated to a depth of approximately
0.15mm in M50 specimens, and approximately 0.2

0.05mm(a) M500.05mm(b) M50NiL
Fig. 10 Microstructures of plasma nitrided specimen, after
diffusion treatment (etched in nital)-37-

NTN TECHNICAL REVIEW No.78（2010）mm in M50NiL specimens. The sum of carbon andnitrogen concentration on nearly the entire nitrided
region is less than 1.7wt% with the exception oflocations with high concentration due to detection ofprecepitates.
We have compared the test results in Sec. 3.4 withthose in Sec. 3.1 (Fig. 4), find that increase inhardness is limited in the nitrided area due to the
reduced amount of nitrogen atoms; notwithstanding,the surface hardness of this area exceeds 900 Hv andthe microstructure in this area does not exhibit
intergranular precipitation. We believe that our
process, consisting of the plasma nitriding anddiffusion techniques described in this paper is suitable
for treating bearing components subjected to rolling
fatigue load.

may be performed as necessary in a vacuum furnace
or atmosphere furnace.
Finish grinding is performed after diffusion process
in order to correct changes in surface roughness
resulting from plasmas nitriding or possible changes(though very small) in shape or dimensions of thework-pieces.
As a result of these process steps, a sufficiently
deep nitrided case is formed on bearing component
while occurrence of intergranular precipitates isinhibited and the hardness of steel material ismaintained at a same level that is obtained by heattreating M50 or M50NiL with conventional heat
treatment process.

3.5 Procedure for applying plasma nitriding
technique to bearing components

To determine the degree of surface damage
resistance provided by plasma nitrided steel material,
we have adopted a double roller rolling fatigue test.
Fig. 12 illustrates the test rig used, and Table 2
summarizes the test conditions applied. The driving
cylinder is driven by an electric motor, while the driven
cylinder is driven by the driving cylinder. The driving
cylinder, featuring a rough-textured outer surface, ismade of SUJ2. This cylinder was allowed to cause
peeling damage on the smooth-textured outer surface
of driven cylinder made of M50 or M50NiL.We haveassessed the severity of the peeling damage andevaluated the effectiveness of plasma nitriding
through comparison peeling damage severity between

4. Verification of effect of plasma nitriding

Fig. 11 shows a process flow that appears to be
relevant for applying plasma nitriding to bearing
components made of M50 or M50NiL based on thefindings mentioned previously. Among the steps in
Fig. 11, those framed in red are processes that areadded to apply plasma nitriding. The steps necessary
for this procedure are described below:• Treat the work-pieces in a conventional heat
treatment process, and then rough-grind the workpieces. Be sure to keep grinding allowance for finish
grinding.
• Using plasma heat treatment equipment, subject thework-pieces to Ar sputtering and plasma nitriding.
• To achieve a necessary case nitrided depth, performa diffusion process under relevant conditions.
• Finish-grind the work-pieces, then treat the workpieces in a manner identical to those that are notsubjected to plasma nitriding.
Note that the diffusion process need not be
executed in plasma heat treatment equipment, andHeat treatment

Driven cylinder
No secondary
curvature

Driving cylinder
Secondarycurvature R60

Fig. 12 Schematic diagram of double roller test

Conventional heat treatment
process(hardening and tempering, etc.)

Table 2 Conditions of double roller rolling test
Rough grinding
Driving cylinder
Ar sputtering in Ar-H2 atmosphere
Plasma nitriding Nitriding in N2-H2 atmosphere
Diffusion

Dia. 40×L12, secondary curvature R60, Ra-0.5 μm,
standard heat-treated product made of SUJ2

Driven cylinder

Dia. 40×L12, no secondary curvature, Ra = 0.02 μm

Running speed

2000min-1

Max. contact pressure

2.3GPaNumber of loading cycles 4.8×105 cycles (4 hrs.)
Finish grinding

Fig. 11 Process flow for producing plasma nitrided components-38-

Ambient temperature

Ordinary temperature

Lubricating oil

Additive-free turbine oil VG46

Lubrication system

Lubricating felt pad

Plasma Nitriding Treatment of High Alloy Steel for Bearing Components

non-plasma nitrided inner ring specimens made of
M50NiL (carburized) that have been heat-treated withconventional process only, and have formed Rockwell
indentations in a manner identical to that mentioned
above. Using the test rig in Fig. 14 and applying thetest conditions in Table 3, we have tested the rolling
fatigue life of 6202 bearing samples having
incorporated these inner rings.
The test results are graphically plotted in Fig. 15.
From this chart, it is apparent that the plasma nitrided
specimens feature longer life and are much moreresistant against damage caused by contaminatedlubrication.

driven cylinder specimens having undergone
conventional heat treatment only and driven cylinder
specimens that were plasma nitrided and thensubjected to diffusion process under the process
conditions mentioned in Sec. 3.4 before being finishground.
Fig. 13 shows some examples peeling damage
appearance on driven cylinders made of M50. With
both M50 and M50Nil materials, peeling damage has
occurred on the outer surface of non-plasma nitrided
driven cylinder. In contrast, no peeling damage has
occurred on the outer surface of driven cylinders thatwere plasma nitrided and then subjected to a diffusion
process. To summarize, plasma nitriding apparently
improves surface damage resistance of the steel
bearing components.
Next, we manufactured 6202 bearing specimens
with M50NiL material (this material is used for bearing
ring on aero-engine) subjected to plasma nitriding,and diffusion processes prior to finish grinding.We
then evaluated the life of these speciments undercontaminated lubrication. For the bearing life test, wehave formed, with a load of 20 kgf, 30 equally spaced
Rockwell indentations at the bottom of the raceway
groove in the inner ring raceway surface of each test
specimen in order to simulate the indentations thatare caused by foreign contamination in lubricating oil.
For comparison purposes, we have also prepared

Loading coil spring
Loading ball bearing 6312

Test bearing 6206

Test bearing
6206

Coupling
Drive pulleyFig. 14 Schematic of rolling contact fatigue life test rig forball bearings
Table 3 Test conditions of rolling contact fatigue life test
Bearing

6206C3

Radial load

6.86kNMax. contact pressure (inner ring) 3.44GPa
Max. contact pressure (outer ring) 3.56GPa
0.2mm

Running speed

3000min-1

Lubricating oil

Circulating lubrication with VG56

Cumulative failure probability %(a) M50
99
80
50
20

(b) M50 (plasma nitriding + diffusion)

Nitrided bearing5

L10 22.9h
e
3.11
1

0.2mm

Conventional bearing

10

101

L10 179.1h
e
1.76
102

Life h103

104

Fig. 15 Test results of rolling contact fatigue life test

Fig. 13 View of peeling damage-39-

NTN TECHNICAL REVIEW No.78（2010）

References

5. Conclusion

1) H. Kurabe and T. Araki, Rolling Fatigue Characteristics
of Carburized or Carbonitrided 1% Cr Steel at
Elevated Temperatures, Vol. 53, No. 11 (1967) 1305.
2) K. Maeda, H. Nakashima, N. Tsushima, Proc. of theInt. Tribology Conf. Yokohama, JAST (1995) 1387.
3) C. Ooki, K. Maeda and H. Nakashima, Improving
Rolling Contact Fatigue Life of Bearing Steels ThroughGrain Refinement, NTN Technical Review No. 71(2003) 2-7.
4) K. Nishiguchi, Y. Takahashi, M. Tanaka, Pre-Prints of
the National Meeting of JWS, 51 (1992) 334.
5) For example, Aerospace Material Specifications (AMS)6491.
6) For example, Aerospace Material Specifications (AMS)6278
7) Example when processing nitride, J. Walkowicz, Surf.
Coat. Technol. (2003) 174-175, 1211
8) M. Lovonyak, Proc. of the 17th IFHTSE Congress(2008) 1 (2009) 61.
9) Kaoru Ikenaga, Proc. of the 17th IFHTSE Congress(2008) 2 (2009) 685.

We have attempted to determine optimal conditions
for plasma nitriding M50 and M50NiL materials used
for bearing components subjected to high-temperatureand high-speed conditions, and have attained thefollowing findings:
1) When the sum of carbon concentration andnitrogen concentration in a steel material exceeds
1.7wt%, intergranular precipitates can more readily
occur, which adversely affect the quality of bearing
components in question.
2) Nitriding alone cannot achieve a sufficiently deep
nitrided case while inhibiting the occurrence ofintergranular precipitates;a diffusion process needs
to be added.
3) Even when the diffusion temperature is below thetempering temperature, diffusion process withprolonged duration will cause the hardness of thebase steel material to decrease; Therefore, thework-pieces need to be subjected to diffusion
process under relevant process conditions that help
realize a sufficiently deep nitrided case while notcausing a decrease in hardness of the base steel
material.
4) Peeling damage resistance of steel material isimproved through plasma nitriding and diffusion
processes.
5) The hardness of the outermost surface and surface
layer is improved on M50 and M50Nil specimens
having undergone appropriate plasma nitriding anddiffusion processes. Compared with M50 and
M50NiL specimens not undergone plasma nitriding,these plasma nitrided specimens boast improved
damage resistance and rolling surface withindentations exhibit longer life.

Photo of authors

Kazuhiro YAGITA

Chikara OHKI

Elemental Technology
R&D Center

Elemental Technology
R&D Center-40-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Article ]Improve Productivity for Warm Ring Rolling Processof Bearing Race

NTN-SNR

Eric PELLETIER*
Pierre EHINGER*

NTN-SNR improved the accuracy of numerical simulations for the warm ring rolling
process in order to achieve productivity improvement and scrap reduction. Since 1960,
NTN-SNR has applied the warm ring rolling process, and has been able to realize theimprovement of forming tool life and the reduction of cycle time for manufacturing andmachine adjustment through the improvement of numerical simulation technology.

bore surface of the blank to form the blank into its
intended cross-sectional shape and enlarge the blankto its target diameter.
As schematically illustrated in Fig. 2, the outercircumferential profile of a work piece blank isregulated with the outer dies (1) and (2), ensuring thatthe profile of the roll-forming tool is accuratelytransferred to the work piece blank. In summary, NTNSNR’s unique roll-forging technique produces highprecision ring-shaped bearing components through arolling process that forms the bore and outercircumferential surfaces without the need for any
turning operation.
Fig. 3 offers a diagram showing the spiral trajectoryof the roll-forming tool that is a unique feature of the
NTN-SNR process. By this spiral motion, a work pieceblank is formed to its intended shape and dimensions,
simultaneously reducing its wall thickness andenlarging its diameter.

1. Preface
Machine component manufacturers across theglobe have long faced the challenge of improving
productivity in order to reduce production costs. To
this end, NTN-SNR has been applying a warm ringrolling process to the production of its bearing
raceways, eliminating the need for a turning process.
In order to further improve the effectiveness of thewarm ring rolling process, NTN-SNR has been
optimizing a numerical simulation technique that aidsin the warm ring rolling process. In this article, wereport our efforts toward improving our warm rollforging technique, taking the outer ring of a doublerow angular contact ball bearing as a typical workpiece.

2. Structure and features of warm rollforging system
Fig. 1 schematically illustrates difference between aconventional roll-forging technique and NTN-SNR’s
unique warm roll-forging technique.
In the conventional roll-forging technique, therotating roll-forming tool is forced to the bore surfaceof the work piece blank to form the blank into its
intended cross-sectional shape and enlarge the blankto its target diameter while reducing the blank wall
thickness.
NTN-SNR’s warm roll-forging system is very uniquein that a work piece blank is placed in two outercircumferential profile regulating dies, whereupon the
spirally-moving roll-forming tool is forced against the

Rotary rolling rollOuter circumferential
profile regulating die(outer die)Roll-forming tool

Work pieceblankOrdinaryroll-forging process

NTN-SNR’s unique warm
roll-forming process

Fig. 1 Comparison of rolling process*NTN-SNR ROULEMENTS Production Engineering R&D-41-

NTN TECHNICAL REVIEW No.78（2010）

Main frame
Outer circumferential
profile regulating die (1)(stationary outer die)Outer circumferential
profile regulating die (2)(movable outer die)Roll-forming toolRoll-forming tool
rotation drive

Outer circumferential
profile regulating die (1)(stationary outer die)Roll-forming toolspiral motion drive

Outer circumferential
profile regulating die (2)(movable outer die)Roll-forming toolpush-in drive
Rack mechanism
actuator

Fig. 4 View of warm-rolling machine
Fig. 2 Fixed die and roll forming tool

the outer die (2), a work piece blank is loaded into
the space between the outer rollers and the rollforming tool; once the work piece has been formed
it is removed from the warm-rolling system, again
by the movable outer die (2).
The main frame of the machine supports the rollforming tool. Its inclined angle allows the formed
products to be automatically delivered by gravity. In
other words, its simple construction removes the needfor a product delivery mechanism.
Instead of the previous hydraulic drive, the newmodel of the machine is actuated by an NC system.
Consequently, operation of all the mechanisms within
the machine is now centrally controlled to inhibit
variation in motion that can occur from variations inmechanical components. Furthermore, by fully utilizing
position information from the NC controller, simulation
accuracy has been improved, resulting in the
reduction of rolling cycle time.

Z
10.00

5.00

Y

0.00

-5.00
-10.00-15.00-10.00

-5.00

0.00

5.00

Fig. 3 Spiral trajectory for roll forming tool

3.1 Numerical simulation
To reduce machining cycle time, extend tool life,and decrease setup time for its warm-rolling machine,
NTN-SNR has been developing a numericalsimulation tool that helps optimize operation of its
warm roll-forging process and roll-forming tool.

3. Feature of the NTN-SNR warmrolling machine, and NTN-SNR’s
commitments to improvement inproductivity

3.1.1 Temperature analysis

A view of NTN-SNR’s unique warm-rolling machineis shown in Fig. 4. This machine operates as
described below:
1) The roll-forming tool drive causes the roll-forming
tool to turn.
2) The roll-forming tool spiral motion drive, rackmechanism actuator, and roll-forming tool push-in
drive are linked with each other through gearing tocause the roll-forming tool to follow a spiraltrajectory.
3) As indicated in Fig. 2, the outer die (1) is stationary,
while the outer die (2) is movable. By operation of

An example of the results of temperature analysis
on various areas of the warm-rolling machine during
the warm roll-forging process is given below.
Fig. 5 illustrates the temperature distribution on awork piece blank during warm roll-forging; Fig. 6
shows the temperature distribution on the outer dies;and Fig. 7 provides the temperature distribution on theroll-forming tool. Fig. 8 provides a comparison
between the values resulting from numericalsimulation and empirically measured values byplotting the time elapsed and temperatures occurring-42-

Improve Productivity for Warm Ring Rolling Process of Bearing Raceon the roll-forming tool during warm roll-forging
operation.
From this diagram, it can be seen that the result of
numerical simulation closely matches the measured
values for temperatures of the roll-forming tool, both atits outer surface and within the tool (20 mm beneaththe surface). In other words, the numerical simulation
tool developed by NTN-SNR exhibits a very highdegree of accuracy.

3.1.2 Analysis for stress and deformation
Fig. 9 offers an example of the results of analysis
on stresses occurring on the roll-forming tool during
warm roll-forging, while Fig. 10 illustrates the resultsof deformation analysis in the same situation.
Fig. 11 illustrates the relation between run time of
the above-mentioned machine and load applied to theroll-forming tool in the machine during warm rollforging. Differences in various data sets between the
pre-operation analytical values and the actual values
obtained from the machine during operation were fed
back to the simulation system. Then, reanalysis wasFig. 5 Temperature of blank during rollingFig. 6 Temperature of outer die during rollingFig. 7 Temperature of roll-forming tool during rolling

expérience 3mm

expérience 20mm

simulation 3mm

Fig. 9 Stress distribution on roll-forming tool during
rollingsimulation 20mm

450

Temperature ˚C

400

Analytical surface temperature
values of roll-forming tool

Measured surface temperature
values of roll-forming tool

350
300
250

Analytical values and measured values insideroll-forming tool (20 mm beneath the surface)

200
150

Time elapsed

Fig. 8 Relation between time and temperature during rollingFig. 10 Deformation distribution on roll-forming tool
during rolling-43-

NTN TECHNICAL REVIEW No.78（2010）

225
200

Through improvement of its numerical simulation
technique for warm ring rolling process, NTN-SNR has
been endeavoring to optimize process conditions,while always remaining careful not to deteriorate theeffective life of the roll-forming tool.
By our efforts, we have achieved approximately
20% reduction in cycle time, compared with theprevious technique, as shown in Fig. 12.
NTN-SNR will remain committed to developing newtechnologies targeting improved productivity, reduced
scrap production, and simplified processes. Further
improvement in its forging technology is to be capableof forming more complicated, medium-sized ringshaped bearing components. In doing so, NTN-SNR
will remain fully conscious of the conservation of the
global environment.

40

Location of tool
being pushed in
Déplacement Crémaillère

175

Efforts essais

150

Effort molette simulation
effort molette simulation

125

35
Actual value 30

25
20

100
15

75

Analytical value(post-operation)

50
25

4. Conclusion

Analytical value(pre-operation)

Load tonf

Displacement in tool push-in location mm

performed using the newly-fed data: It was found that,in terms of the peak load acting on the roll-forming
tool, the analytical values (post-operation) closely
match the measured values. This means that oursimulation method fairly accurately represents thetiming and magnitude of the peak load applied to theroll-forming tool during roll-forming operation.

10
5
0

0
Time elapsed

Displacement in tool push-in location mm

Fig. 11 Load applied to roll-forming tool during rolling

250
Previous technique
200
150
New technique
100

50
0
Time elapsed

Fig. 12 Cycle time

Photo of authors

Eric PELLETIER

Pierre EHINGER

NTN-SNR ROULEMENTS
Production Engineering R&D

NTN-SNR ROULEMENTS
Production Engineering R&D-44-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Air Oil Lubrication Bearings with Re-lubricating Holeon the Outer Ring for Machine Tool

Futoshi KOSUGI**
Kouji NISHINO**

The best design specification was established for angular contact ball bearings with outer ring
re-lubricating holes for machine tool main spindles. In this report, we introduce the features
and the test data for these air oil lubrication bearings with outer ring re-lubricating holes formachine tools.

1. Introduction

2. Bearing design details – machine
tool bearing design with outer ring
re-lubricating holes

Air-oil lubrication systems are often used tolubricate machine tool main spindle bearings, wherein
the lubricating oil is traditionally fed into the interior ofeach bearing through a ring spacer having relubricating holes. There are also some present-day
European machine tools featuring machine tool
bearings where oil penetrates more directly into thebearing interior through outer ring re-lubricating holes.
Lubricant oil flow in this manner will lead to various
benefits including improved lubrication efficiency andeliminating the need of a separate ring spacer
component. This article describes NTN’s uniqueversion of “air-oil lubricated machine tool main spindle
bearings with outer ring re-lubricating holes” including
the basic design concept and performance test
results.

Re-lubricating holeO-ringFig. 1 illustrates the comparison between NTN’s
new design concept and the traditional bearing/spacer
system used now. The traditional system requires oil
to flow through the separate spacer.
The NTN design also includes an O-ring on eachside of the outer ring to prevent oil leakage out thesides, while oil is fed into the bearing via thecircumferential oil groove and holes.

3. Basic advantage for the bearing tohave an outer ring with re-lubricatingholes
We checked heat and noise generation between
NTN’s new design idea with the traditional bearing /spacer system.
Fig. 2 schematically illustrates the test rig (mainspindle type test rig) for this test. Table 1 summarizes
the test conditions.

Circumferential oil groove

Bearing having outer ring with re-lubricating holes

Re-lubricating hole

Ring spacer with re-lubricating holes

Ring spacer with re-lubricating holes (standard bearing)

Fig. 1 Design of bearing*Industrial Business HQ. Industrial Engineering Dept.

-45-

NTN TECHNICAL REVIEW No.78（2010）

14000

40

Temperature ˚CTemperature on outer ring
Bearing running speed

12000

35 Air flow rate on bore dia.

10000

1.2 mm relubricating
hole: 40 NL/min

8000
6000

30

4000
25

0

1

2

3

4

5

6

7

Bearing running speed min-1[Bearing specimen having ring spacer with relubricating hole]2000

Run time h[Bearing specimen having outer ring with relubricating hole]40

Table 1 Test conditions

Materials

Bearing ring: special bearing steel
Rolling elements: ceramic material

Contact angle

25˚

Type of preload

Fixed position preload(preload on mounted bearing: 98 N)

Oil feed rate

0.03 mL/10minLubrication system

Air-oilLubricating oil

VG32

Number of relubricating holes

One/bearing

Jacket coolingYes

Temperature ˚C

φ100×φ150×24

Bearing running speed

12000

Air flow rate on bore dia.
0.8 mm relubricating
hole: 25 NL/min

35

10000
8000
6000

30

4000
25

2000
0

1

2

3

4

5

6

7

Run time hFig. 3 Bearing temperature

Temperature on jacket ˚C

Test bearing

14000

Temperature on outer ring

Bearing running speed min-1

Fig. 2 Test spindle

Fig. 3 provides outer ring temperature data for
speeds up to up to 13,000 min-1 using fixed position
preload bearing samples. Notice that the temperature
increase profile using the traditional standard bearing /ring spacer system (oil hole bore dia. 1.2 mm, air flowrate 40 NL/min) was virtually same as the bearing
sample using outer ring re-lubrication holes (bore dia.
0.8 mm, air flow rate 25 NL/min).
The data shown in Fig. 3 shows the temperaturerise results while running at 13,000 min-1, but varying
the oil and air flow rate. Fig. 4 gives temperature rise
result after fixing the air flow rate at 15 NL/min, but the
lubrication injection intervals at 0.03 mL/shot werevaried. When the re-lubrication interval was set at 1
shot per minute for the bearing with outer ring relubricating holes, over-lubrication occurred and thebearing temperature actually rose significantly (heatbuildup). When re-lubrication intervals were reduced
to once every 25 minutes, the traditional bearing / ring
spacer system exhibited only minor heat rise. We
believe that, even though the total oil flow rate into
bearing is decreased, the standard bearing / ring
spacer system becomes under-lubricated as it isreadily affected by an “air-curtain” effect created by60
Outer ring w/lube hole (φ0.8)
Ring spacer w/lube hole (φ1.2)

Fixed air-oil flow rate 15 NL/min

50
Trend in temperature
increase

Temperature
increase

40

30

0

5

10

15

20

25

30

Relubrication interval min (0.03 mL/1 shot)

Temperature on jacket ˚CFig. 4 Amount of oil and bearing temperature

60
Outer ring w/lube hole (φ0.8)
Fixed oil feed rate 0.03 mL/10 min
Ring spacer w/lube hole (φ1.2)

50
Somewhat unstable temperature

40

30

0

5

10

15

20

25

Air flow rate NL/min

Fig. 5 Volume of air flow and bearing temperature

-46-

30

Air Oil Lubrication Bearings with Re-lubricating Hole on the Outer Ring for Machine Tool

re-lubricating hole (bore dia. 0.8 mm or 1.2 mm).
However, when the air flow rate is reduced to 15
NL/min, the bearing with the non-traditional bearing
using an outer ring re-lubricating hole running faster
actually generated a lower noise level. These noiselevel differences seem to result of an air condition
created when the air is injected through each relubricating hole configuration.
Fig. 7 shows measurement results and therelationships between the re-lubricating hole bore
size, air pressure, and air flow rate. When the airpressure is constant, air flow rate is dependent on the
re-lubricating hole diameter. Naturally, the smaller thehole, the lower the air flow rate with constant airpressure. Similarly, when the re-lubrication hole size
is constant, a lower air flow rate results with a reduced
air pressure.
Fig. 8 illustrates the relationship between the relubricating hole diameter and air flow velocity (air jet
speed) for various air flow rates (calculated values are
based on assumption of no loss).
When the re-lubricating hole diameter is constant,the air flow velocity becomes lower as the air flow rate
becomes lower. When the air flow rate is constant,the air flow velocity becomes lower as the nozzle
diameter becomes larger.

the high speed movement of rolling elements aroundthe bearing center.
Fig. 5 is a graphic that illustrates test results when
the oil flow rate was fixed at 0.03 mL/10 min and the
air flow rate was varied. When the air flow rate is
reduced, temperature fluctuation occurs with thebearing / ring spacer system. This phenomenon
seems to be the result of an “air-curtain” effect thatkeeps lubricating oil flow from remaining smooth, andthe bearing becomes under-lubricated.
From these results, we can see that the oil and air
flow rates for a non-traditional bearing that uses anouter ring with re-lubricating holes (bore dia. 0.8 mm)
can be reduced, and this bearing type can operatewith an air flow rate of 10 - 20 NL/min and oil feed rate
ranging from 0.03 mL/5 min to 0.03 mL/25 min.
Fig. 6 is a graph showing the noise levels measuredof bearing test samples. Bearings with both the nontraditional outer ring with re-lubricating hole system(bore diameter either 0.8 mm or 1.2 mm) and the
traditional bearing / ring spacer system (spacer relubricating holes of bore dia. 1.2 mm) were compared.
The traditional bearing with a ring spacer / relubricating hole (bore diameter 1.2 mm) exposed to an
air flow rate of 40 NL/min had a noise level generally
less than the non-traditional bearing with an outer ring

1400

100

Air flow velocity m/s

Noise level dBA

1200
95
90
85
Outer ring w/lube holes 1-φ0.8-25 NL/min
Outer ring w/lube holes 1-φ0.8-15 NL/min
Outer ring w/lube holes 1-φ1.2-25 NL/min
Ring spacer w/lube holes1-φ1.2-40NL/min

80
75

0

2000

4000

6000

8000

10000

12000

○ mark: four conditions in Fig. 6

40NL/min
25NL/min
15NL/min12.5NL/min

1000
800
600
400
200
0 0.8

14000

Bearing running speed min-1

0.9

1

1.1

1.2

1.3

1.4

1.5

Bore dia. mm

Fig. 6 Noise level

Fig. 8 Air flow velocity

60

Air flow rate NL/min

Lube hole dia.
50

φ1.5
φ1.2

40

φ1.0
φ0.8

1000
2000
3000
〔dB〕
100
3,107Hz
Number of passes
90with rolling elements
80 relative to outer ring

4000

5000〔Hz〕

70

30

60
20

50
40

10
0
0.1

30

13,000min-1
0.2

0.3

0.4

0.5

Air pressure MPa

Fig. 7 Air flow volume

Fig. 9 Frequency of bearing noise-47-

NTN TECHNICAL REVIEW No.78（2010）

In the case of non-traditional bearings with outerring re-lubricating holes, high-pressure air is injected
through the re-lubricating holes and directly reaches
the rolling elements. Also, the distance that the
injected air travels is much shorter as compared tousing the traditional bearing / spacer system. The
reason why the noise level is greater with the nontraditional, outer ring-lubrication system to be that the
injected high-pressure air reaches the rolling elementswith less velocity loss causing the rolling elements todevelop a whistling sound (Fig. 9). Information shownin Fig. 6 shows that the noise level of the nontraditional bearing with outer ring re-lubricating holes(bore dia. 0.8 mm) and an air flow rate of 15 NL/min islower when compared to the same bearing
experiencing an air flow rate of 25 NL/min. The noiselevel is also lower when the re-lubricating hole bore
diameter is 1.2 mm (25 NL/min). It is believed that thisis due to the difference in air flow velocity. Air flow
velocity in the traditional ring spacer system is limited
to only 600 m/s. It seems that the “air curtain” effecthelps reduce the air flow velocity before air reaches
actually the rolling elements.

flow velocity, but maintaining air pressure at
approximately 0.3 MPa and also supply a air flow rateof at least 20 NL/min: as illustrated in Fig. 10. The
non-traditional outer ring design has two equally
spaced re-lubricating holes measuring 1.5 mm
diameter as well as a circumferential groove with across-sectional area equivalent to a 0.8 mm diameter
re-lubricating hole.
Usually, when an outer ring is fed with an airpressure of 0.3 MPa and has two bore 1.5 mm
diameter re-lubricating holes, the air flow rate per holeis about 40 NL/min, and the air flow velocity is around
400 m/s. Therefore, we decided to adopt a bearing
specification that helps limit the supply air flow rate toapproximately 20 NL/min relative to an air pressure of
0.3 MPa (see Fig. 11). We also used a smaller outerring circumferential groove than usual to help control
the air flow rate. Furthermore, using of a two-holedesign helps cut the air flow rate in half and limit the
air flow velocity to 400 m/s or lower.
Fig. 12 compares phase differences of relubricating holes on the housing and outer ring with air
flow rates. The air flow rate reduction created by thecircumferential groove is particularly apparent when

4. Improved bearing having outer ringwith re-lubricating holes
Cross-sectional area

Sec. 3 provides information showing that the noiselevel (whistling noise) of a bearing with the nontraditional outer ring re-lubricating system is less than
than the traditional bearing / ring spacer system evenwith improved oil flow efficiency. It seems that a
reduction in air flow rate and air flow velocity will
reduce noise level.

≒（π/4）×0.82

φ1.5
Flow rate of supply air A

Method of reducing air flow rate:
Lower the air supply pressure and use a smaller
re-lubricating hole bore diameter

A/2

Method to decrease air flow velocity:

A/2

Flow rate ofdischarge air

Reduced air flow rate and use a larger relubricating hole diameters
However, because a reduced air flow rate will affectlubrication oil capacity in the supply line tube to thenozzle1), the flow rate needs to be at least 20 NL/min.
In a real-life commercial operation it would probably
be appropriate to set air pressure between 0.3 to 0.5
MPa: the re-lubricating hole bore diameter should be
1.2 to 1.5 mm to reduce the air flow velocity while
maintaining an air flow supply rate of 20 NL/min. In
our investigation, we adjusted the air flow velocity to
400 m/s or lower. We then investigated the nozzle
parameters to satisfying all of the above-mentioned
requirements.
As a result of our investigation, we have developed
a special bearing specification capable of reducing air

Fig. 10 Improved design60
2-Φ1.5 (Flow rate of supply air)2-Φ1.5 (Flow rate of discharge air per lub hole)

Air flow rate NL/min

50
40
30
20
10
0

0.1

0.2

0.3

Air pressure MPa

Fig. 11 Air flow volume

-48-

0.4

0.5

Air Oil Lubrication Bearings with Re-lubricating Hole on the Outer Ring for Machine Tool

Air flow rate per lube hole NL/min

the hole diameter is 1.5 mm. When the two re-lube
holes are more than 90 degrees apart, this apparently
does not significantly affect the overall air flow rate. A
maximum flow rate reduction per outer ring relubrication hole is achieved by using “axisymmetricallysituating” (equally spaced) holes each with a 1.5 mm
diameter and situating the housing relube holes
exactly at the midpoint between the holes on the
bearing outer ring.
Fig. 13 graphically illustrates the result of assessing
our improved bearing design. Noise level of our new,
non-traditional design outer ring hole system with thehole geometry and distribution mentioned above islower compared to using a non-traditional bearing withouter ring oil holes a 0.8 mm diameter or a traditional
bearing / spacer system.

5. Conclusion
We have tested bearing samples designed with a
non-traditional, outer ring re-lubricating hole systemand evaluated the results to adjust the details to an
optimal bearing design. Heat generation of our
improved, non-traditional bearing design concept isequivalent to the traditional bearing / ring spacer
system where oil is injection nozzle while its noise
remains the same. Consequently, redesigning toincorporate a re-lubrication nozzle to the new design
onto the ring spacer is needed, but allows the bearing
system to be smaller by eliminating the ring spacer.
This helps to create an overall more compact machine
tool main spindle design that has increased tool
rigidity when re-locating the bearing more towards theouter end of tool than when using a traditional bearing/ spacer system. In addition, tests results prove that
our improved, non-traditional bearing design using
outer ring re-lubricating holes is reliably lubricated,
and we learned that the air flow rate and oil
consumption can be reduced compared to that of the
traditional bearing system that uses a separate spacer/ re-lubricating hole. At the same time, we haveclarified that changing a re-lubricating hole bore
diameter size affects bearing noise generation. We
plan to continue our efforts to improve bearing
lubricating systems and conditions to allow evengreater machine tool main spindles speeds. It is
encouraging to see how our lower-noise, air-oil
lubricated bearing helps improve the functionality ofmachine tool main spindles.

80
70
60
50
φ0.8-0.5MPa
φ1.5-0.5MPa
φ1.5-0.35MPa

40
30
20
10
0

0˚

(One-hole)45˚(One-hole)90˚(One-hole)135˚(One-hole)180˚(One-hole)90˚(One-hole)

Phase difference between lube holeson housing and those on outer ring

Fig. 12 Phase and air flow volume

References

1001) Y. Akamatsu and M. Mori, Minimizing Lubricant Supply
in an Air-Oil Lubrication System, NTN Technical
Review No. 72 (2004) 12-19.

Outer ring w/lube hole 2-φ1.5-25 NL/min

Noise level dBA

95

Outer ring w/lube hole 1-φ0.8-25 NL/min
Ring spacer w/lube hole (standard) 1-φ1.2-40 NL/min

90

85
80

Photo of authors
75

0

2000

4000

6000

8000

10000

12000

14000

Bearing running speed min-1

Fig. 13 Noise level-49-

Futoshi KOSUGIKouji NISHINO

Industrial Business HQ.
Industrial Engineering Dept.

Industrial Business HQ.
Industrial Engineering Dept.

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]High-speed Angular Contact Ball Bearings
New 9 Series for Machine Tool

Futoshi KOSUGI*

Many 9 series angular contact ball bearings are used for the main spindles of machine tools in
Europe. However, demand for them has risen also in Japan recently. We improved the current
design and developed new 9 series high-performance angular contact ball bearings.
We introduce the features and the performance test data of these new 9 series high-speed
angular contact ball bearings in this report.

1. Introduction

2. Advantages of 9 Series bearings

In order to shorten lead time by addition of processintegration capability, an increasing number of machine
tool models have recently been adopting unique main
spindles whose angle can be varied during machining
operation, and examples of such main spindles include
milling main spindles on 5-axis machine tools andcombined machine tools.
Main spindles on these machines swing to change
their angle, and a sufficiently large space is needed onthese machine tools to accommodate the motion of
main spindles. Therefore, any main spindle in thesemachines needs to be compact in size, and the main
spindle bearing also needs to be compact in size. For
this reason, 9 series bearings are used because of
their low cross-sectional heights and smaller width.
This article hereunder describes the features andperformance of the NTN’s newly developed “highspeed angular contact ball bearing 9 series” products.

Fig. 1 provides information that helps clarify theadvantages of 9 series bearings in comparison with 0
series bearings. Because of lower cross-sectionalheight, 9 series bearings have the following
advantages over 0 series bearings:• When main spindle diameter (bearing bore
diameter) is same with both series (Fig. 1a) for agiven main spindle diameter, a bearing product ofsmaller outside diameter may be used. Thus, a morecompact gearbox design is possible, which leads tosize reduction for main spindle.
• When bearing outside diameter is same with both
series (Fig. 1b)
In an application where there is limitation on the
bearing outside diameter dimension, use of a 9series bearing can lead to larger main spindle
diameter. This feature will allow a main spindle
design that provides greater shaft rigidity.

0 series

9 series

0 seriesa) Comparison for same shaft diameter (bearing bore diameter)

Fig. 1 Feature of 9 series angular contact ball bearing*Industrial Business HQ. Industrial Engineering Dept.

-50-

9 seriesb) Comparison for same bearing outside diameter

High-speed Angular Contact Ball Bearings New 9 Series for Machine Tool

On the disadvantage side, having balls smaller thanthose of 0 series, 9 series bearings are inferior to 0
series bearings in terms of their radial load capacityand maximum allowable axial load. To eliminate thisproblem, NTN has developed 9 series bearing variants
that boast greater load capacity.

20% (Fig. 3). Consequently, the new 9 series bearing
products boast longer life and increased allowanceagainst axial load that occurs during tool change
process on machine tools; thus, these bearings will be
used in a larger scope of applications.
Fig. 4 provides axial rigidity graph and radial rigidity
graph with high-speed angular contact ball bearings(new design and old design), internal bore diameter
100 mm, with constant pressure preload (GN preload).

3. Improved functions on New 9 Series
products
3.1 Load rating and allowable axial load

3.2 Result of operation test

Our newly developed high-speed angular contact
ball bearing products—new 9 series—feature improved
internal design that include rolling elements ofincreased diameter as shown in Fig. 2, thereby theload rating has been improved by a maximum of 50%
and the allowable axial load has been increased by

The result of operation test with the high-speed
angular contact ball bearings (new design and old
design) is described below.
Fig. 5 schematically illustrates the construction of
the test rig (main spindle type test rig) used for the test.
Fig. 6 summarizes a result of temperature rise teston the new and old bearing design variants withceramic balls, air-oil lubricated, and with fixed position
preload.
The pattern of heat rise on the outer ring of our new
design, up to dmn = 1.45×106 (12,000 min-1) is
identical to that of the old design. However, at higher
speed range, the temperature with the new design issomewhat higher (1 to 2˚C). A possible cause is alarger ball size, which leads to greater PV value(contact surface pressure x slip velocity).

New design

Old design

Fig. 2 Design of new productBasic dynamic load rating

140

Basic static load rating

Allowable axial load

120

kN

100

New design

Old design

80
60
40
20
0

φ50

φ100

φ150

φ50

φ100

φ150

φ50

φ100

φ150

Fig. 3 Basic load ratings and allowable axial load (steel ball, contact angle 20˚)

6

Radial displacement μm

Axial displacement μm

15
New design
Old design
10

5

0

0

500

1000

1500

2000

2500

4
3
2
1
0

3000

Axial load N

New design
Old design

5

0

500

1000

1500

2000

Radial load N

Fig. 4 Rigidity graph (bearing bore dia. 100 mm, constant pressure preload)

-51-

2500

3000

NTN TECHNICAL REVIEW No.78（2010）

Fig. 7 graphically plots the variation in bearing
preload in this test: the diagram shows that the new
design helps reduce possible increase in preload byapproximately 15% compared with the old design. To
sum up, the temperature on outer ring on our new
design is somewhat higher compared with the olddesign: nevertheless, our new design boasts improved
anti-seizure property because temperature difference
across its inner ring and outer ring is smaller,
minimizing the increase in preload.
Fig. 8 illustrates test result with constant pressure
preload type bearing specimens.
The temperature on the outer ring of our new design
is higher by 1˚C compared with the old design:
nevertheless, our new design is capable of stable
operation up to ultrahigh-speed region of dmn = 2.40×
106 (20,000 min-1).

5000
New design
Old design

Preload N

4000

Approx. 15%
reduction
in increasein preload

3000
2000
1000
0

0

5000

15000

10000

Bearing running speed min-1

Fig. 7 Correlation between speed and preload<Test conditions> 5S-2LA-HSE920UDBP4 (New design)
5S-2LA-HSE920DBP4 (Old design)
Air-oil lubricated
Air supply rate: 40 NL/min
Oil supply rate: 0.03 mL/5 min
Fixed position preload: 1800 N
w/ jacket coolingTemperature rise on outer ring ˚C

Test bearing

Drive
system20
18

New design
Old design

16
14
12
10
8
6
4
2
0

0

5000

10000

15000

20000

Bearing running speed min-1

Fig. 5 Test spindle

Fig. 8 Test results (Constant pressure preload, Air-oil)<Test conditions>Temperature rise on outer ring ˚CTemperature rise on outer ring ˚C<Test conditions> 5S-2LA-HSE920UDBP4 (New design)
5S-2LA-HSE920DBP4 (Old design)
Air-oil lubricated
Air supply rate: 40 NL/min
Oil supply rate: 0.03 mL/5 min
Preload on mounted bearing: 49 N
w/ jacket cooling20
18

New design
Old design

16
14
12
10
8
6
4
2
0

0

5000

10000

15000

20000

20
18
16
14
12
10
8
6
4
2
0

2LA-HSE920UDBP4 (New design)
2LA-HSE920DBP4 (Old design)
Grease lubrication
Prefilled grease: SE-1
Fixed position preload: 49 N
w/ jacket cooling

New design
Old design

0

5000

10000

15000

Bearing running speed min-1

Bearing running speed min-1

Fig. 6 Test results (Fixed position preload, Air-oil)Fig. 9 Test results (Fixed position preload, Grease)-52-

High-speed Angular Contact Ball Bearings New 9 Series for Machine Tool

Fig. 9 summarizes test result with grease-lubricatedspecimens having steel balls, with fixed position
preload setting. Even under these conditions, heat riseon the outer ring of the new bearing design remains
low up to dmn = 1.44×106 (12,000 min-1).

4. Conclusion
Compared with old products, our newly developed
high-speed angular contact ball bearing 9 series
products boast greater radial load capacity andallowable axial load, and longer life and increased load
capacity during tool change process on machine tools.
In terms of functions, the bearing temperature on our
new design is 1 to 2˚C higher compared with olddesign. Nevertheless, the temperature gradient acrossthe inner ring and outer ring on our new design is low
and increase in preload in high-speed bearing
operation is also low. Thus, our new design excels in
anti-seizure property.
NTN will add its “high-speed angular contact ballbearing new 9 series” products, which boast compact
size and enhanced functionality, to its ULTAGE Seriesline of products, and will actively market these novel
products.

Photo of author

Futoshi KOSUGIIndustrial Business HQ
Industrial Engineering Dept.

-53-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Ball Screw Support Angular Contact Thrust Ball Bearing Unit
"BSTU Series"

Hiroki TANIMURA*

NTN had developed the BST series of single row angular ball bearings forball screw support in the ULTAGE series. Now, NTN has developed the BSTU
series of double row angular ball bearings for the ULTAGE series.
The BSTU series products have high basic load ratings (highest levels in theworld as bearings for ball screw support), low torque, and high dust-proofing.

1. Introduction

2. Structure and advantages

NTN has been marketing ball screw support
bearings—the ULTAGE Series “ball screw support
single row angular contact thrust ball bearing unit
(BST series)” products.
Recently, NTN has developed and commercialized
a novel high load rating double row angular contact
ball bearing unit and has added it to the ULTAGE
series line of products.
Our single row BST series products are typicallyused in Asian markets: while our newly developed
BSTU series of products, standard ball screw support
angular contact thrust ball bearing products, areintended for the European market.
Our newly developed BSTU series high capacity
double row angular contact ball bearings unit products
boast a higher load bearing capacity—highest amongball screw support bearings in the world. At the sametime, adoption of a novel seal helps realize lower
running torque and positive dust-proofing
performance.
NTN has named this new series “BSTU (Ballscrew Support angular contact Thrust ball bearing
Unit) series”.

Fig. 1 shows the structure of a BSTU series
product.
The advantages of the BSTU series bearings are
hereunder described.

BST series
Single row (DB duplex)

BSTU series

Fig. 1 Design of BST and BSTU

2.1 Realization of higher load capacity
Through its unique design featuring the maximum
possible number of larger steel balls, NTN has
realized greater load capacity, as shown in Figs. 2
and 3. These load capacity values are highest among
the world’s ball screw support bearings.

*Industrial Business HQ. Industrial Engineering Dept.

-54-

Basic dynamic load rating kN

Ball Screw Support Angular Contact Thrust Ball Bearing Unit "BSTU Series"

2.3 Longer life

（BSTU3080：φ30×φ80×28）

40

Any ball screw support bearing is subjected tovibration during rapid traverse (the cutting tool
approaches the work piece) and low speed rotation inmachining, as well as during cutting operation.
Therefore, the bearing has to satisfy very demanding
requirements for its lubrication. To prevent possible
disruption of oil film caused by vibration and shock, wehave adopted urea-based grease as standard
because of its excellent fretting resistance quality andgrease life.
The performance of NTN’s standard urea-based
grease “L588 ” will be described later in Sec. 3.

30

20

10

NTN’s
new design

Competitor
A’s design

Basic static load rating kN

Fig. 2 Basic dynamic load rating (BSTU3080)

2.4 Improved mountability

70

As shown in Figs. 5 and 6, the outer ring isprovided with mounting bolt holes so that the bearing
is easily mounted to the housing.

60

New seal
design

50

40

NTN’s
new design

Competitor
A’s design

Fig. 3 Static load rating (BSTU3080)

2.2 Low torque and reliable dust-proofing
performance
By adopting a novel seal lip shape shown in Fig. 4(light contact seal), we have achieved both reliable
dust-proofing performance and lower running torqueof the bearing.
A unique light-contact type main lip helps realize
reliable dust-proofing performance and lower runningtorque of the bearing: the side lip positively prevents
the grease in the bearing from flowing out.
This pre-greased bearing is an easy-to-handle, ecofriendly design—the user does not need to rinse anddegrease the bearing and prefill grease into thebearing when assembling the bearing at the user’s
site.

Bolt mounting holeFig. 5 Bearing design

BSTU seriesSide lip

Seal(Benchmark) BST series

Fig. 6 Bearing assemblyLight-contact typemain lip
Inner ring

Fig. 4 Seal design-55-

NTN TECHNICAL REVIEW No.78（2010）

2.5 Adoption as new standard series[Example of bearing designation]

The BSTU line of products feature bore
diameter of 20 to 100 mm, and outside
diameter of 68 to 200 mm.
The bearing designation system of thisnew series is described to the right.

P4U
BSTU 30 80 LLX / GN P42U / L588
Seal code
OutsidediameterGrease code
(standard prefilled grease)
Precision code

Bore
diameter
Series name

Preload code

3. Assessment of performance
3.2 Torque characteristics

The results of performance test on the BSTU series
bearings are described below.

Fig. 8 provides information about running torqueoccurring on the test bearings running in one direction
only. Regardless of running speed, the BSTU test
bearings boast stably low running torque.

3.1 Temperature characteristics
The test bearings were run, simulating typical run
patterns for machine tool bearings. Consequently, as
shown in Fig. 7, the NTN’s BSTU series bearings
boast excellent temperature characteristics (limited
temperature rise) thanks to adoption of the “new lightcontact type seal”. We believe that owing to positively
limited temperature rise, the BSTU series bearings
least contribute to thermal elongation of the ballscrew, and is capable of high-speed operation onmachine tools.

<Test conditions for torque characteristics test>
Bearing number: BSTU3080LLX/GNP4U/L588
Preload: 2,700 N
Lubrication system: Grease lubrication(Amount of prefill: 1.6 gr, 25% the bearing space)

0.35
0.30

Torque N・m<Test conditions for temperature rise test>
Bearing number: BSTU3080LLX/GNP4U/L588
Running speed: 1,000–5,000 min-1
Preload: 2,700 N
Lubrication system: Grease lubrication(Amount of prefill: 1.6 gr, 25% the bearing space)

0.25
0.20
0.15
0.10
NTN’s new design
Competitor A’s design

0.05
0.00
0

0.2S

3S

500

1000

1500

2000

2500

Running speed min-1

0.2S 2S

Fig. 8 Test results of rotational torque
Running
speed

Max. running speed

Standstill
0min-1Time elapsed

Temperature rise on inner ring ˚CRun pattern for temperature rise test
30
25
20
15
10
NTN’s new design
Competitor A’s design

5
0
1000

2000

3000

4000

5000

Running speed min-1

Fig. 7 Test results of temperature rise

-56-

3000

Ball Screw Support Angular Contact Thrust Ball Bearing Unit "BSTU Series"

3.3 Limiting speed
Limiting speed min-1

Because of limited temperature rise and stably low
torque, the BSTU bearings are capable of operation at
higher speed range (Fig. 9).
Bearing number: BSTU3080LLX/GNP4U/L588
Lubrication system: Grease lubrication(Amount of prefill: 1.6 gr, 25% the bearing space)

5000
4000
3000
2000
1000
0

NTN’s
new design

Competitor
A’s design

3.4 Grease characteristics
The BSTU bearings adopt urea-based grease“L588” that boasts excellent fretting resistance quality
and grease life. Fig. 11 shows the results of fretting
corrosion test. In this test, a stationary ball is forced toa plate as shown in Fig. 10, and the plate ishorizontally reciprocated to develop reciprocating
friction, thereby the characteristics of the grease are
evaluated based on the depth of resultant wear.
Compared with lithium grease, the NTN’s standard
urea-based grease “L588” boasts excellent fretting
corrosion resistance quality, and is suitable for ballscrew support bearings that are subjected to inchingmotion and oscillating motion.

Fig. 9 Limiting speed

Load

Ball
(material: SUJ2)

Plate(material: SUJ2)

Reciprocating friction

Fig. 10 Test method of fretting corrosion<Test conditions for fretting corrosion test>
Load: 98 N
Max. contact pressure: 2,560 MPa
Number of load applications: 8.6 (x105 cycles)
Test duration: 8 h
Sliding cycles: 30 Hz, amplitude: 0.47 mm
Ambient conditions: Room temperature, atmosphere

1.2

Wear ratio

1.0
0.8
0.6
0.4
0.2

3.5 Dust-proofing characteristics

0.0

L588

On machine tools, chips and coolant can reach anarea around the ball screw. Therefore, the ball screw
support bearing needs to be resistant against ingressof dust.
Fig. 12 exhibits the result of dust-proofing test on
our newly developed ball screw support bearing.
Thanks to excellent dust-proofing feature of the novel
light-contact type seal, there is no ingress of foreign
matters into the bearing.

Li grease

Fig. 11 Test results of fretting corrosion<Test conditions for dust-proofing test>
Bearing number: BSTU3080LLX/GNP4U/L588
Running speed: 2,200 min-1
Lubrication system: Lubrication with prefilled grease
Dust: Grain size 5 to 75 µm(Ingredients: SiO2, Fe2O3, Al3O3, etc.)
Hue of dust: Brown
Test duration: 1 hr.
Before test

Fig. 12 Results of dust test

-57-

After test

NTN TECHNICAL REVIEW No.78（2010）

4. Conclusion
NTN’s newly commercialized ball screw support
bearing “BSTU series” products boast higher load
capacity and lower torque, which mark the world’s
best performance levels, as well as improved dustproofing quality, longer life and easier mounting
procedure. To be able to help realize the eco-friendly
society, bearing manufactures have to realize
engineering innovations to satisfy various
requirements on various areas in machine tools. In
this context, our BSTU products provide solutions toneeds for longer bearing life and lower energy
consumption.
To be able to cope with various market needs, NTN
will remain committed to development efforts for nextgeneration bearings through continued improvements
and challenges for higher functionality.

Photo of author

Hiroki TANIMURA
Industrial Business HQ
Industrial Engineering Dept.

-58-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Ultra Small Grinding Sludge Briquetting Machine

Akira NISHIYAMA*
Michio TANAKA*
Yuuki HONMA*

We had already established an economical recycling technology that separates thesteel and grinding lubricant in grinding sludge and had developed a grinding sludge
briquetting machine.
This time, we developed the PRG-III model ultra-small grinding sludge briquetting
machine for small plants that produce low volumes of grinding sludge.

established a recycling technology (Fig. 1) for reusing
recovered metal sludge and coolant as useful
resources.
Consequently, NTN has achieved not only greatcost reduction in treating its grinding sludge but also
“zero emission” (no grinding sludge is disposed
outside NTN’s site).
In marketing this technology, NTN has been
supplying two types of grinding sludge briquetting
machines to cope with varying volume of grinding
sludge occurring in the users of NTN briquetters:
these types are model PRG-I whose compression

1. Introduction
Since acquisition of ISO 14001 certification in 1998,
NTN has been making efforts in mitigating its
environmental impacts in order to help realize therecycling-oriented society. In business year 2000,
NTN developed an economically justifiable techniqueto separate grinding sludge, occurring from its bearing
manufacturing process, into metal and coolant. Also amachine tailored for this purpose (that is, grinding
sludge briquetting machine) would need to be
designed and created. Thus NTN has successfully

Production plant
Steel mill
Grinding sludge

Reuse
Reuse

ReduceRecycleVolumereduction

Grinding sludge
briquetting machine

Recycling

Briquettes

Fig. 1 Grinding sludge recycle system*Production Engineering R&D Center-59-

NTN TECHNICAL REVIEW No.78（2010）

sludge briquetting machine model PRG-III:

power is rated at 1,000 kN (processing capacity: 30
t/month) and model PRG-IIL whose compression
power is rated at 520 kN (processing capacity: 20
t/month). From these marketing efforts, NTN haslearned that certain large-scale manufacturers would
prefer to dispose of the grinding sludge generated by
the smaller machining lines on-site.
To address this trend, NTN has developed, basedon its so-far accumulated grinding sludge briquetting
technique, a novel super-compact grinding sludge
briquetting machine for small-scaled production
sites—this new development is NTN’s model PRG-III
that requires a smaller floor space.

(1) Automatic setup of compression conditions
Knowledge based setup, and optimization forcompression conditions(2) Compact
Floor space: 40% as small compared with PRG-I
Machine weight: 40% as low, compared with PRG-I
Capacity for treating grinding sludge: 5 tons permonth(3) User-friendly
Improved operability and maintainability(4) Eco-friendly
Reduction in electricity consumption

2. Concept of Model PRG-III

3. Specification and advantages

Fig. 2 illustrates percentage proportions of numberof manufacturers in terms of grinding sludge
emissions particularly in the fields of automotive
components, bearings, and steel products.
This diagram shows that monthly grinding sludge
emission is not greater than 5 tons with 46%
manufacturers. In other words, grinding sludge
emissions are relatively low with many manufacturers,
and these companies want a grinding sludge
disposing equipment that needs a smaller floor space.
In addition, we have learned that because physicalproperties (in particular, water content) of grinding
sludge are unique to each manufacturer and can vary
every day, the grinding sludge briquetting machine
has to be capable of adjusting compression conditions
according to the physical properties of the intended
grinding sludge.
From these findings, we have established thefollowing concept for our novel supercompact grinding

An appearance of our newly developed
supercompact grinding sludge briquetting machinemodel PRG-III is shown in Fig. 3, and a result ofcomparison of its specification with those of theconventional models PRG-I and PRG-II L issummarized in Table 1.
Fig. 4 below shows appearances of briquettes
obtained from the models PRG-I and PRG-III.
The major advantages of the machine are :40 t or less
4%

Inlet

Height 1,200

Outlet

50 t or less5%
Width 1,300

Depth 600

30 t or less7%

Fig. 3 View of PRG-#

20 t or less
18%

PRG-!

5 t or less
46%

PRG-#
φ70
φ40

10 t or less20%

Fig. 2 Breakdown of manufacturers in percentages
by monthly grinding sludge emission

Fig. 4 Briquets of PRG-! & PRG-#-60-

Ultra Small Grinding Sludge Briquetting Machine

Table 1 Grinding sludge briquetting machine type and main specification
Major characteristics

PRG-!

PRG-@L(New) PRG-#

Capacity(water-soluble sludge)

max. 80kg/h(30 t/month)max. 50kg/h(20 t/month)max. 13kg/h(5 t/month)

Dehydrating performance in %

90wt%

90wt%

90wt%

Compression force

max. 1000kN

max. 520kN

max. 300kN

Briquette size

φ70

φ60

φ40

Approx. dimensions W×H×D

2510×2200×850

2220×1950×620

1300×1200×600(Benchmark) Floor space needed

2.1m2

1.4m2

0.8m2 (40%, compared with PRG-I)

Machine weight

2500kg1800kg

800kg(1) Adoption of automatic compression
conditions setup system (knowledge based
setup, and optimization for compression
conditions)[3] A database of briquetting conditions other thancompression conditions has been established andthe machine is equipped with this database:consequently, the operator can change over theprocessing conditions for handling a new grinding
sludge type, by simply manipulating one pushbutton switch.
[4] This new machine type incorporates a uniquemonitor system that is capable of data
management, including updating of production
records, maintenance records, and fault
occurrence log. The unit also contains an early
warning system, alerting the user of necessary
maintenance needed to the machine. The machineis also capable of real-time monitoring of its status
at a remote site so that a fault on the machine ispromptly corrected.

For optimal briquetting of various grinding sludge
types, each having a unique water content,
compression conditions (compressing force, andcompressing velocity), need to be set up according to
the water content of sludge. To address thischallenge, we have developed a unique solution—that
detects compressing force and compressing velocity
during briquetting cycle. The water content is thenestimated based on the resultant measurements,
thereby optimal compression conditions (compressing
force and compressing velocity) are automaticallyadjusted. Consequently, we have achieved stablebriquette formation.

(4) Improved stability in forming briquettes,
improved material loading and delivering(2) Space saving (compact design)
The machine adopts a rack that integrally consistsof a main base, a hydraulic oil tank, a compact
compressing mechanism and a compact feeder. Thus,
the machine needs a smaller floor space measuring
1,300 mm in width and 600 mm in depth (40% as
small, compared with conventional PRG-I).
Also, the machine boasts light weight assummarized in Table 1 (40% as low, compared withconventional PRG-I).

Adoption of the previously-mentioned automatic
compression conditions setup system helps optimize
the briquetting conditions.
To ensure more reliable sludge loading andbriquettes discharging, we have incorporated thefollowing features into the machine:[1] To prevent stagnant sludge flow at the sludge
loading port and feeding section (that is, bridging ofsludge), a reversing cycle is provided for thestirring paddle unit in the sludge loading port aswell as the sludge feed screw. At the same time,the motors for the stirring paddle unit in the sludge
loading port and the sludge feed screw n can bearbitrarily changed so that the machine is capableof handling sludge types of various physicalproperties.
[2] To prevent failure in delivering briquettes, we haveintroduced a newly designed discharge section(Fig. 5) that obliquely delivers grinding sludgederived briquettes in the direction same as thatalong the delivery chute. Also, we have adopted a(3) User-friendly (improved operability andmaintainability)
Aiming at realization of user-friendly machine, wehave achieved improved operability andmaintainability. The typical improvements achieved
are :[1] The total height is limited to 1,200 mm to mitigate
difficulties in feeding grinding sludge into themachine.
[2] A touch-panel type operator’s control panel has
been adopted to improve operator-friendliness ofthe machine.
-61-

NTN TECHNICAL REVIEW No.78（2010）

uniquely structured slide that positively prevents
ingress of sludge and coolant so as to improve
durability of the slide.

4. Conclusion
So far, NTN has remained committed to developmentof recycling techniques typically intended for disposalof grinding sludge emitted from centralized coolant
filtration equipment in large-scale manufacturing sites.
NTN has recently developed the “super-compact
grinding sludge briquetter model PRG-III” thatincorporates a unique control system capable of
optimizing compression conditions (compressing
pressure and compressing velocity) in accordancewith the water content of an intended grinding sludge
type. We expect that this equipment will be
conveniently used in disposing small lots of varying
physical properties released from small-scale coolant
filtration equipment, etc.
One additional application of our super-compact
briquetter is solidification of various materials such as
paper dust, cutting chips, dehydrated sludge, andtumbler sludge. To be able to solidify a diversity ofmaterials, NTN will further continue R&D works for theequipment to help achieve mitigation of environmental
impacts from manufacturing activities so as to realize
the recycling-oriented society.

(5) Energy saving
The machine adopts an inverter-driven hydraulicpump as well as a novel pump control system thatallows the hydraulic pump on the machine to run only
when needle for actuating the hydraulic cylinder: as aresult, power consumption has been reduced by 14%
compared with conventional hydraulic pump control
systems that require the hydraulic pump to run
continuously.

Discharge cylinder

Direction fordelivering briquettes
Briquette forming section

Discharge section

Flow direction
in delivery chute

Delivery chuteFig.5 Discharge section in PRG-#

Photo of authors

Akira NISHIYAMAMichio TANAKA

Yuuki HONMAI

Production Engineering
R&D Center

Production Engineering
R&D Center

Production Engineering
R&D Center-62-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Compact and Multifunction Controller for Parts Feeder

Kunihiko SUZUKI*

NTN parts feeders that automatically line up and supply parts are acceptedby manufacturing in various fields, and are contributing to the improvementof automation and productivity in manufacturing processes. NTN developed
a new series of controllers for parts feeders. Features of these controllers
are that they are lightweight, compact and multifunction. Moreover, a special
vibration sensor has been newly developed. This article introduces thefeatures of these new small controllers for parts feeders.

Fig. 1 provides comparison with the old controller
model in appearance. The new model is shown in theright—its outside dimensions are much smaller though
its control capacity is equivalent to that of the oldmodel.
In Table 1, the technical data of our new controllers
are compared with those of the old controller.
To accommodate an increased number of functions,the quantity of terminals including I/O signal terminals
has doubled: therefore, a new compact terminal block
has been designed, and at the same time, the entire
control panel on the new controllers can be fully
opened to provide a sufficiently large space for wiring
work (see Fig. 2).
Furthermore, in addition to development of the newcontrollers, we have also developed a novel vibration
sensor. Though compact in size, the new controllers
are more immune to noise interference thanks to theirimproved circuitry: they allow for extension of leadwires using an ordinary terminal block or cable, etc.;
thereby wiring work on the controller at the site ismuch easier.

1. Introduction
Vibratory parts feeders are commonly used in
production lines of various industrial fields, which
include car manufacturing, electric appliancemanufacturing, food processing and semiconductor
device manufacturing. Controllers that control
vibration of these vibratory parts feeders have to becompact and light weight while being capable ofsupporting ever diversifying applications.
NTN has commercialized a new series of parts
feeder controllers aiming at size reduction, lighter
weight and multi-function capability. This reportprovides information about the advantages of these
new controller products.

2. Overview of NTN’s new Parts Feeder
controllers
In 1996, NTN marketed a digital controller for parts
feeders—the first in the bearing manufacturingindustry in Japan. Its higher control accuracy andimproved operation, not obtained from conventional
analog controllers, was highly appreciated. This
controller become the standard digital controller forparts feeder in the manufacturing industry in Japan.
The controller models described in this document are
NTN’s third generation parts feeder controllers, andboast both compact light-weight design and multifunction capability through incorporation of additionaluser-friendly functions thanks to unique digital control
technology.

3. Advantages of the new controllers(1) Compact size, and light weight
In terms of the physical volume, the size of the newmodels—K-ECH45/K-ECJ45—are 60% of the size ofthe old model (see Table 1 and Fig. 3).
The challenge to size reduction of the controller is
how to reduce the surface area needed for heatradiation and interfacing arrangement.

*Precision Equipment Division Product Engineering Department-63-

NTN TECHNICAL REVIEW No.78（2010）

Fig. 1 Externals view (right side : new model)

Fig. 2 Wiring enclosure (right side : new model)

Table 1 Comparison table of new and old model
New model(low capacity)

Model number

New model(high capacity)

K-ECF25 K-ECG25 K-ECH45

Characteristics
External dimensions W×H×D[mm]82×140×142

100/200V

100/200V

Control capacity

Features of new model

K-ECJ45

60×140×142

Supply voltage

Old model
K-ECA49

K-ECA46

80×180×180
100V

200V

4.5A

4A

30〜500Hz

30〜500Hz

30〜250Hz

Multi-speed function

Can drive SMD feeder.

Panel + 3 stages

Panel + 3 stages

No

Internally includes three speed memories.

Analog voltage command

Yes

Yes

No

Capable of speed change from outside device.

Valve control function

Yes

Yes

No

Can be connected to three valves.

No work-piece warning function

Yes

Yes

No

Capable of switching over between sensor 1
and sensor 2.

Fault signal output

Yes

Yes

No

Fault signal is triggered when a fault occurs.

Alarm signal output

Yes

Yes

No

Alarm signal is triggered when alarm situation occurs.

F/V curve
Constant voltage function

Yes

Yes

Yes

18 sets

18 sets

2 sets

Yes

Yes

Yes

Greater selectable range for load protection curves.

Constant amplitude function

No

Built-inNo

Built-inOptional; externally provided

Resonance point tracking function

No

Built-inNo

Built-in

No

2-φ15

142

5

30

15

K-ECH45, K-ECJ45
9
1

5
30

160

170
(35)

140

160

15

2-φ15

60

142

3

5
60

(9)

82

60

Fig. 3 New controller outline dimension

-64-

2

15

140

5

K-ECF25, K-ECG25(35)

Over-load protection function

170

Functions

2A

Allowable frequency range

Compact size as small as approx. 60% theold controller of a same control capacity
New model is capable of inputs of both 100 V
and 200 V.

(9)Compact and Multifunction Controller for Parts Feeder• Control for solenoid valve

To be able to reduce heat radiation area, it is mostimportant to mitigate heat generation from the built-in
parts. The heat generation on the controller has been
reduced by adoption of an appropriate FET and
improvement in the drive circuit; thereby the surface
area needed for heat radiation has been reduced by
20%.
Furthermore, the location for mounting the radiation
fins on the new controllers is a side face instead of therear face on the old controller; thereby a greaterradiation area is available and the size of the radiation
fins can be reduced by half. Note that the rated
current greatly differs between the medium sized
general-purpose vibrator and large-sized generalpurpose vibrator. Previously, one controller model has
been used regardless of necessary control capacity.
In contrast, two differently sized controller types are
now available—the larger size controller features
greater control capacity. The smaller capacity
controller (K-ECF25/K-ECG25) that is responsible forcontrol of vibrators up to a medium size, boasts a
reduced size—46% of the old model.
On the new controllers, the surface area is large
enough for interfacing work, including wiring work isprovided since the entire control panel can be fully
opened (see Fig. 2). Now, the wiring space is as large
as the front face of the controller: at the same time,
the signal circuit employs a compact plug-in-type
terminal block. Consequently, the new controllers
boast an increased number of I/O points despite theirsmaller space requirement.
Furthermore, adoption of smaller radiation fins has
led to a light-weight enclosure design that does not
require greater mechanical strength: consequently,
thinner wall thickness can be adopted, leading to 30%
weight reduction with the entire controller.

When the parts feeder is a two-track (two-lane) typeas shown in Fig. 4, a conventional parts feeder
controller needs to control the solenoid valve for a
relevant track (lane) in accordance with the current
status detected by a work-piece presence sensor, byusing a compact programmable logic controller(hereinafter referred to as “PLC”). This system posesa cost problem since an additional PLC, DC power
supply, casing, etc. will be needed in order to control
two air-supply solenoid valves. NTN’s new series ofparts feeder controllers do not need additional control
components in this control scenario because the newcontroller models each include a PLC controlling onechip microcomputer that contains control functions
needed for various parts feeder types. What theoperator must do on our new parts feeder controller is
to select an intended function and then set up timer
duration. The user of the parts feeder controller does
not need to a programming tool and software specificto an intended parts feeder. NTN’s parts feeder
controllers are very easy to use.

Work-piecepresence sensor 1
Work-piecepresence sensor 2
Compressed
air 1
Compressed
air 2Fig. 4 Two tracks bowl feeder(2) Multi-function capability• Overload protection• Switch-over between carrier frequencies

The new controllers are capable of setting the rated
current of the vibrator so that the vibrator connected tothe controller is reliably protected against a possible
overload. The level of current flowing to the electromagnet in the vibrator can greatly vary depending on
the dimension of gap between the magnet and thearmature. Previously, incorrect setup of the controller
could cause overcurrent to flow the electro-magnet.
The NTN’s new controllers automatically regulate their
output voltage in order to protect the electro-magnet.
This feature has been available with the NTN’s old
controller model: however, the new controllers feature
improved detection accuracy and the lower limit of
their allowable setting rage is 0.2 A instead of 0.5 A on
the old model.

Our new parts feeder controller models adopt
sinusoidal wave PWM* technique to drive parts
feeders. To improve control accuracy for thistechnique, a higher carrier frequency for modulation ispreferable: however, higher carrier frequency can
trigger higher harmonic noise which can adversely
affect measuring instrument involved. The controller
models K-ECH45/K-ECJ45 permit the operator to
select one of three carrier frequencies. This ability tochange over carrier frequencies helps mitigate
adverse effect of higher harmonic noise.
* PWM: Acronym for Pulse Width Modulation: one
technique to generate AC waveform.

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NTN TECHNICAL REVIEW No.78（2010）(3) Novel vibration sensor
We have also developed a novel vibration sensor.
To detect vibration, piezoelectric elements are often
used. In the aspects of electrical characteristics,
piezoelectric elements feature much greater output
impedance and this fact leads to more demanding
requirements from the cables connected to thepiezoelectric elements. Our newly developed sensor
not only includes a built-in band-path filter that allows
only signals of the necessary band to pass but also
features lower output impedance to alleviate
limitations on the cable to be connected to thevibration sensor. Consequently, when wanting toextend the cable for the vibration sensor, the user ofvibration sensor can extend the line by using an
ordinary cabtyre cable and terminal block without
adopting a shielded cable. Use of a shielded cable
was necessary for conventional vibration detection
systems with piezoelectric elements. Wiring work ismuch simplified.
Fig. 5 shows an appearance of our novel vibration
sensor. Fig. 6 gives a view of cable extension with alength of ordinary cabtyre cable; this type of
connection was difficult with the old controller model.
Fig. 6 Typical arrangement of cable extension for
new vibration sensor

Fig. 5 Vibration sensor

Sensor cable

Terminal block

Fig. 6 Example of cable expansion
for new type vibration sensor

that the signals from the vibration sensors are kept atconstant level. Typical examples of control
characteristics of our new controller obtained from thisconstant amplitude function and resonance point
tracking function described below are shown in Fig. 7.
Our new controllers are capable of automatic
regulation of their output voltage so that they restore
the amplitude on the parts feeder to the original leveleven when a load on parts feeder has increased (for(4) Constant amplitude operation modeOn our two new controller models, K-ECG25 and KECJ45, the magnitude of amplitude of vibration isadjusted with an amplitude dial to a target value. The
controller automatically regulates its output voltage so

K-ECG25 Percent variationin amplitude relative to load mass
Constant amplitude mode

5%

0.9

0%

0.8

-5%

0.7

Load current A

Variation in amplitude %Constant voltage mode

K-ECG25 Variation
in load current relative to load mass

-10%
-15%
-20%
-25%

0.6
0.5
0.4
0.3

-30%

0.2

-35%

0.1

-40%
0

Resonance point tracking mode

0.0
2

4

6

8

10

0

Load mass kg

2

4

6

8

10

Load mass kg

Suppose a load (work-pieces in the bowl) on the parts feeder is gradually increased. In the conventional constant voltage
mode (no amplitude feedback is available), the amplitude will decrease; in contrast, in the constant amplitude run mode
(amplitude feedback is available), change in amplitude is minimized because load current is increased as necessary. At thesame time, it should be understood that in the resonance point tracking run mode (feedback of amplitude and resonancepoint is available), change in amplitude is mitigated and a load current necessary to achieve a given amplitude is low
compared with the constant amplitude mode.

Fig. 7 Constant amplitude characteristic (typical example)-66-

Compact and Multifunction Controller for Parts Feeder

mode automatically starts and a corresponding modeindicator lamp will light up. The operator of our new
controllers can easily execute the necessary setup
and checkup procedures.
Since resonance point auto tracking function alone
does not provide stable vibration frequency for thevibratory parts feeder, our new controllers inresonance point auto tracking mode unconditionallyenable the constant resonance mode.
The resonance point tracking function is useful inparticular in realizing high-speed feeding of workpieces: however, this function may be less effective interms of inching-speed work-piece feeding
performance or reliability of the parts feeder
depending on the settings on the vibrator and/or thetype of work-pieces handled by the parts feeder.
With the new controllers, the “LIMIT” indicator lamp
will light up when the parts feeder is running while
being controlled by the constant amplitude function
and if the control limit is reached. There may be cases
where necessary amplitude is no longer achieved due
to reasons including excessive fatigue on the leaf
spring in the vibrator. The “LIMIT” indicator lamp willlight up so that the user can maintain the parts feeder
in question.

example when the work-pieces have been loaded into
the parts feeder). The parts feeder can keep supplying
work-pieces at constant vibration frequency.
Previously, the constant amplitude function on our old
controller model could be enabled only after manualadjustment with GAIN potentiometer. The newcontroller models are capable of automatic GAIN
adjustment. New controller models help the user
always select an optimal GAIN without relying on thejudgement of an operator of the controller.

(5) Resonance point tracking run mode
The resonance point of a given parts feeder canvary depending on the factors including the quantity ofwork-pieces in its bowl. Our two new vibratory parts
feeder controller models K-ECG25 and K-ECJ45 arecapable of automatically tracking the resonance pointof the parts feeder being controlled and accordingly
control the vibration frequency of the parts feeder. A
vibratory parts feeder utilizes resonance phenomenon
to amplify vibration on it to line up and feed workpieces to an assembling machine. For this reason, theparts feeder will operate at optimal efficiency when thevibration frequency matches the resonance point. This
arrangement may not be advantageous in thatvibration-feed operation at resonance frequency isvulnerable to outside disturbance and as a result
stable vibration is not readily achieved. To address
this problem, our old controller model was designed torun at a frequency where its vibration is stable,
somewhat higher than the resonance point, eventhough the load current is higher than that inresonance point-matched mode. The new controller
models are capable of resonance point tracking runmode where the controller detects occurrence ofresonant state based on signals from vibration
sensors, thereby the controller automatically regulates
the vibration frequency so that the parts feeder isalways locked to resonant state. Vibration on thefeeder remains stable even when the vibration
frequency coincides with the resonance point, thereby
load current needed for the test parts feeder controlled
by the new controller to generate a given amplitude isapproximately 20% lower compared with the oldcontroller, (percentage decrease in necessary current
can vary depending on the model, settings andoperating conditions, etc. of the vibrator).
To enable resonance point tracking function, acertain adjustment is necessary, however, the newcontroller models automatically achieve thisadjustment thanks to their auto-calibration function.
The operator does not need to waste time for thisprocedure. On the NTN’s new parts feeder controllers,
once calibration work is complete, an intended run(6) Other functions
The new controller models are capable of handing
various functions and signals, and examples of which
include multi-speed capability, control of setup voltage(amplitude) with analog voltage, control dial lockup(inhibition of manipulation), no work-piece detection
alarm, fault signal and alarm signal. A user of parts
feeder can achieve a diversity of unique control
functions by using one unit of the NTN’s parts feeder
controller instead of developing a purpose-specificcontrol circuit. Consequently, the user of our parts
feeder controllers will enjoy benefits such as reduction
in costs for production equipment and stock ofmaintenance parts.

(7) Costs
NTN has attempted to minimize cost increase over
the old model though introducing more functions;
thereby the users of NTN’s new parts feeder
controllers can use a wider scope of control function ata price approximately comparable to that of the oldmodel.

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NTN TECHNICAL REVIEW No.78（2010）

4. Conclusion
This paper has provided information about theadvantages of NTN’s four multi-function, compact andlight-weight controller models for parts feeders. NTN
will include as an option to the lineup of parts feeder
controllers a special step-up transformer unit thatallows a 200 V rated parts feeder to be operated witha 100 V power supply.
Because of a diversity of possible applications,parts feeders need to be capable of handing a wide
variety of work-piece types. NTN’s new parts feeder
controllers satisfy most of these requirements, andcope with higher levels of users’ needs. NTN willremain committed to development of novel parts
feeder controller products that are accepted by moreusers as they help realize the best combination
between characteristics of parts feeder proper, optimal
tooling and appropriate control scheme.

Photo of author

Kunihiko SUZUKI
Precision Equipment Division
Product Engineering Department-68-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]NTN-SNR High Performance andFlexible Linear Modules Series AXDL

Ulrich GIMPEL*
Michael WILLE*
The AXDL linear modules series has been developed as a positioning
module with high rigidity to meet high requirements for accuracy, zero
maintenance and dynamics. A concept-oriented CAD design has resulted in an
extremely flexible program to respond to various specifications, such as sizeand functions, from customers and innovative synchronous-belt clamping andcomplete sealing that ensure long service lives and high operating safety.
AXDL modules can be combined with the complete AXC/AXLT series ranges
by using a standardized system of connecting and fastening elements.

1) Selection of a drive unit allowed

1. Foreword(1) Toothed-belt drive unit
Fig. 1 shows the appearance of an AXDL linear
module equipped with a toothed-belt drive unit. The
stage is fastened to the toothed belt, and is moved by
the toothed belt motor-driven from the drive head. The
toothed-belt drive unit is capable of being advanced at
a high speed of 10 m/s as a result of combining it withthe track roller parallel guide described later.
The AXDL series toothed belt drive module ischaracterized by the method of fixing the toothed beltthat facilitates the replacement of belts. In common
toothed belt drive units, toothed belts are often
arranged at locations at which it is difficult to handle
the inside of the unit, requiring many man-hours forbelt replacement. For AXDL series products, a new
fixing method has been developed, as shown in Figs.
2 and 3, in which the part on the cross-section of thestage through which the toothed belt is passed isformed as a space with a wedge-shaped crosssection, into which a wedge member is squeezed as if
the toothed belt were to be inserted there. This fixing
method allows the toothed belt to be fixed securely
and replaced without removing component parts.

Linear modules have been incorporated into various
manufacturing machines for automobiles, photovoltaic
generation systems, lumber processing, aircraft,packaging, and medical treatment systems since the1980s. They are used as basic component parts for
portal Cartesian coordinate robots including tip units
and pick-and-place units of robots and are an
indispensable technology for automation and transfer
mechanisms. Depending on their applications, they
are required to have high positioning accuracy orendurance under load. They are used in a variety ofworking environments ranging from outdoor locations
to clean rooms. This article introduces AXDL linear
modules developed by NTN-SNR to respond todiversifying market needs.

2. AXDL Series
2.1 Features of the AXDL series linear module
In an AXDL series linear module, a driving unit, aguide mechanism and a moving stage are joined to analuminum frame with a unique structure. The mostoutstanding feature is its high general versatility thatallows the optimum driving unit and guide mechanism
to be combined to the frame freely.
The features of the AXDL series linear module areas follows:*SNR Wälzlager GmbH Engineering Bielefeld-69-

NTN TECHNICAL REVIEW No.78（2010）Stage

distance between the supports and must not be
exceeded.
NTN-SNR developed a wire-driven ball screw
floating support mechanism shown in Fig. 5, making it
possible to handle a high-load condition through an
improvement of the shape of the support unit. Thismechanism is structured to allow a pair of support
units with a coupling element to move axially as theball screw nut moves. Fig. 6 shows an example ofaction of a floating support equipped with a pair ofsupport units. Applying support units makes it possible
to reduce the support-to-support distance incomparison with a structure having a fixed support
bearing only. Increasing the number of support units
has made it possible to increase the allowablerotational speed as shown in Fig. 7.

Drive unit
(Toothed belt drive unit)

Fig. 1 AXDL240Z linear module with toothed-belt drive

Wedge member
Protective screw

Fig. 2 Fixed unit of toothed belt
Protective screw

Wedge member

Stage
Guide
mechanism

Fig. 4 AXDL240S: linear module with ball screw drive

Frame

Fig. 3 Fixed method of toothed belt

(2) Ball screw drive unit
Fig. 4 shows the AXDL ball screw drive module.
In the AXDL module equipped with a ball screw
drive unit, the type of feed screw and the accuracy
class of ball screws can be selected depending on theperformance required.
In the standard specification, rolled ball screws withthe accuracy class of T7* are used, while ball screwsof up to accuracy class T3** can be selected for highaccuracy positioning applications. In addition,
trapezoidal screws can be selected for adjustment
action application not requiring positioning accuracy.
When a ball screw drive unit is used for high-speedtransfer applications, it is necessary to be careful not
to exceed the allowable rotational speed of the ballscrew. The allowable rotational speed is determinedby the shaft diameter of the ball screw and the

Floating support mechanism

Fig. 5 Floating support system
Shaft end fixing and support bearing
Support unit

Ball screw shaft

Ball screw nut(a) Before moving(b) After moving*Class C7 in the JIS notation** Class C3 in the JIS notation

Fig. 6 Floating support （case of 2 support units）-70-

Allowable rotational speed min-1

NTN-SNR High Performance and Flexible Linear Modules Series AXDL

3) The latitude in selecting a drive motor is large.

NoneFirst pairSecond pair
Third pair
Fourth pair

4,000
3,000

Motor flanges, couplings, and reduction gears
allowing the combination with a motor with the best
drive characteristics.

2,000
1,000
0

250

2.2 Improvement of the design of the AXDL
series products
750

1,250 2,250 2,750 3,250 3,750 4,250 4,750 5,250 5,750

The AXDL series products are born out of theincorporation of a variety of improved technologies
into the existing AXC series. The points ofimprovement are shown below:

Ball screw length mm

Fig. 7 Limiting rotation speeds1) Optimization of the frame shape
Since the cross-sectional shape of the frame affects
the rigidity of the unit, it is necessary to increase themoment of inertia of area in two directions shown in
Fig. 10 , Ix and Iy.
The frame is composed of extruded aluminum
members. In manufacturing extruded parts, aluminumbars (billets) are extruded through a trimming die toform extruded parts with a desired cross-section. By
induction-heating billets to about 500˚C in advance,
adequate material liquidity associated with forming is
obtained. The forming conditions are optimized so thatstable drift velocity may be obtained even when the
thickness varies locally in the same cross-section. The
straightness of the AXDL 240Z product is optimized
within the physical limits of the extrusion process bymeans of the finite element analysis so that it may beless than 0.5 mm/m (in conformity with DIN EN 120202).
The cross-sectional structure of the frame manufactured by the extrusion shown in Fig. 11 exerts asignificant effect on the rigidity of the linear moduleand that of the unit as a whole. Rigidity shows theamount of deformation of the unit due to load.This is
an important element determining the function anddurability of the unit.

2) Selection of a guide mechanism(1) Linear guideLinear guide using a ball with a holder is applied.
This guide mechanism has a high load capacity,
having a higher rigidity, a higher accuracy, and alonger life than the track roller described later. Fig. 8
shows a cross-sectional view combined with a linear
guide perspective view.
(2) Track roller guide unit
Generally, the rated operating speed of a linear
module using a linear guide is up to 5 m/s. With the
AXDL series, however, combining the track roller
guide mechanism with the toothed belt drive as shownin Fig. 9 allows the system to reach a speed of 10
m/s.

Linear guide(a) Linear guide perspectiveStage(b) Cross-section of themounted parts

PR
Fig. 8 Linear guide systems with ball chains.
MC
Track roller guide unit

PT
MB

Stage

MA
(a) Example of mountingof a track roller guide

IX(b) Cross-sectionof mounting

PL

Fig. 9 Track roller guide units

Fig. 10 Coordinate of linear module

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IY

NTN TECHNICAL REVIEW No.78（2010）

2) Optimization of rigidity of component
members

Rigidity was measured on AXDL 240 double-row
linear guide products (with a height of 25 mm),double-row track roller guide products (with a track
diameter of 47 mm) and conventional AXC 120 singlerow linear guide products according to the coordinate
system shown in Fig. 10 . The rigidity is denoted with
the inclination angle produced by the moment load,
converted into the displacement at 1 m ahead. The
experimental results are shown from the moment load
MA (Fig. 12), MB Fig. 13), and MC (Fig. 14).

Fig. 15 shows an example of the calculation of theeffect on the device rigidity of different module
component parts by means of finite element analysis,
which was carried out in addition to the optimization of
the frame cross-sectional shape in order to increase
the rigidity of the unit as a whole. An analysis carriedout on an imagined condition of combined component
parts (the track roller guide and the stage in the caserepresented by Fig. 15 ) allows mutual effects ofmembers on one another to be evaluated and theeffect on the entire unit to be estimated.

3) Improvement of the method of joining thelinear guide unit with the ball screw drive unit
When the linear guide unit is joined with the ballscrew drive unit, it is important to prevent stress thatwill affect the durability of the entire unit fromoccurring. As shown in Fig. 16 , NTN-SNR has
improved the method of joining the nut holder of theball screw with the linear guide unit. The structure issuch that the joint with the linear guide unit is formed

Fig. 11 Cross section structure of flame

Displacement μm

0.2

Displacement μm

0.4

AXDL240 Linear guide
AXDL240 Track roller
AXC120

0.1

0

0

20

40

60

80

0.3

0.2

0.1

0

100

Moment load Nm

AXDL240 Linear guide
AXDL240 Track roller
AXC120

0

20

40

60

80

Moment load Nm

Fig. 12 Comparison of the rigidity in MA direction

Fig. 14 Comparison of the rigidity in MC direction

Displacement μm

0.3
AXDL240 Linear guide
AXDL240 Track roller
AXC120
0.2

0.1

0

0

20

40

60

80

100

Fig. 15 FEM analysis result for AXDL

Moment load Nm

Fig. 13 Comparison of the rigidity in MB direction

-72-

100

NTN-SNR High Performance and Flexible Linear Modules Series AXDLvia the beam that projects only in the direction of theradius from the ball screw nut holder. This reduces thegeneration of stress due to the error in relative
positions between the ball screw and the linear guideunit. This improvement also eliminates the adjustment
during the assembly process and thereby preventing
the occurrence of defects in assembling.

4) Improvement of dust-tightness by means of
optimization of the seal structure

Nut holder

The dust-tightness of the unit is important from theviewpoint of improving the durability and reducing
maintenance work. As shown in Fig. 17, a seal strip
and a side strip, both made of polyamide (PA), areprovided to improve the dust-tightness.

Beam5) Increased ease of assembling to the unit
Fig. 18 shows the structure of the drive head of the
toothed belt drive unit. For AXDL series products, theease of assembling is improved by assembling the
motor drive shaft into the drive head. This makes it
possible to secure the compactness of the unit evenwith a large diameter pulley incorporated into theassembly, providing the following advantages:⇒ Restraining the installation height of the system⇒ Securing a large effective stroke length ascompared with the total length of the system

Fig. 16 Connecting between ball screw and guide system

3. Example of application of the AXDL
series product

Side seal

Fig. 19 shows an example of application of the
AXDL series product to a two-shaft screw driving robot
for automobile production systems.
Using the AXDL series product with a parallelarranged two-row guide mechanism in this application
has made it possible to accommodate a large angularmoment (MC). In a quest to prevent vibration
associated with the movement of the shaft, the rigidityof each module has been increased.

Side seal

Y axis: Servo-motor-driven AXDL 160 toothed belt
module equipped with a planetary reductiongear
Z axis: AXDL 110 ball screw drive module equippedwith a motor-mounting flange and a coupling toconnect with a servo motor
The Z-axis is equipped with an electric driver to screw
the valve cover to the cylinder head of an
automobile engine.

Seal strip

Fig. 17 Sealing design

Drive head

Drive shaft (pulley)

Fig. 18 Structure of motor drive head-73-

NTN TECHNICAL REVIEW No.78（2010）

AXDL 110 (Z axis)

AXDL 160 (Y axis)

4. Conclusion
The high-rigidity AXDL series linear module has
been developed to meet strict requirements for
accuracy, freedom from maintenance, and motion
responsiveness. The unification of design concepts
has made it possible to combine a drive unit and aguide mechanism, both suitable to a particularapplication, to flexibly meet diversified customer
needs varying in terms of the size and function. Added
to this have been technologies of ball screw support
schemes, methods of fixing toothed belts, and perfectsealing of internal component parts to achieve a longservice life and high stability of operation.
Furthermore, the connection and fixing brackets for
the AXDL series products are standardized to allow
the AXDL series products to be used in combination
with all conventional AXC and AXLT series products.

Fig. 19 Example of application :Screw-driving unit for automotive parts

Photo of authors

Ulrich GIMPEL

Michael WILLE

NTN Wälzlager GmbHEngineering Bielefeld

NTN Wälzlager GmbHEngineering Bielefeld

-74-

-75-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Article ]High Resolution Sensor Bearing with an Index Signal

NTN Hiroyoshi ITO***
Toru TAKAHASHI***NTN-SNR Pascal DESBIOLLES***
Cyril PETERSCHMITT***
Shintarou UENO***
This bearing with a rotation sensor is used for the purpose of speed control androtational direction detection of servomotors used in, for example, battery forklifts.
The sensor outputs two phase A and B signals. NTN and NTN-SNR have now newly
developed a high-resolution rotational sensor bearing with an index signal output.

Outer ring

1. Foreword

Connector

Substrate
Sensor IC

Magnetic
encoder

NTN has added to its lineup “rotation sensorequipped bearings” that combine a shaft and arotation sensor into a unit as products that supports
electrical control of industrial machines such as
rotation control of servo motors 1), 2). Using a sensor IC(MPS40S) 3) made by NTN-SNR ROULEMENTS, amember of the NTN group, NTN has developed, jointlywith NTN-SNR, a “bearing equipped with a home
position signal output type high-resolution rotation
sensor” that allows the home position signal (phase Z)
to be outputted. Thanks to the Hall elements installedinside, the NTN-SNR’s sensor IC is capable ofincreasing the rotation detection capability by up to 40
times and of outputting the home position signal. In
addition, this newly developed product has adopted aconnector-based connection system to improve theease of mounting it to devices, with the power andsignal output wires separate from the main body.

Sensor case

Inner
ring

Type C spring

Fig. 1 Structure

6mm

Substrate Sensor IC

7mm

2. Construction

Fig. 2 Sensor IC soldered with circuit board

Fig. 1 shows the construction of the newly
developed product.
(1) Inner ring side: a press-fitted magnetic encoder(2) Outer ring side: A type C spring fastens the sensor
case, and the substrate fastened to the sensor
case is equipped with a sensor IC and a connector.
Fig. 2 shows how the sensor IC is mounted. A
QFN28 package is used to achieve compact
implementation.

3. Features
The features of the newly developed product and of
the sensor IC used in the product are described
below.

***Industrial Business HQ. Industrial Engineering Dept. **New Product Development R&D Center Mechatronics Research Dept.
***NTN-SNR ROULEMENTS New Product and Mechatronics R&D-76-

High Resolution Sensor Bearing with an Index Signal

3.1 Features of the newly developed productthe magnetization pitch, for example) occurs, thefunction provides notification that an error
occurred.
(5) The width of the magnetic pole, number ofmultiplication, and form of signal output can be
specified by programming;(6) With the phase difference between phase A andphase B signals being accurate, an increase in the
resolution can be achieved by means ofquadruplication processing.

The newly developed product has the following
features:(1) Capable of detecting rotation without contact andfree from the possibility of performance
deterioration due to wear;(2) A rotational angle resolution of 0.14˚ is achievedon the 6206 size bearing because the capability ofoutputting a rotation signal is up to 40 times that of
the current product;(3) Capable of detecting the rotational speed anddirection by means of the signal output in phases A
and B;(4) Capable of detecting the home position by meansof phase Z signal output (For example, this isapplied to the detection of a motor shaft, theposition of the steering, and the like.); and(5) Ease of mounting the bearing as a result of the
adoption of a connector-based connection system.

Table 2 Specification of sensor IC
Required magnetic flux density±5 mT or morePower supply voltage

5V±10％

Current consumption

40 mA maxCompatible width of magnetic pole
1.15–3 mm(that can be specified)
Maximum frequency of
5kHz
input magnetic field
Number of multiplication
1x, 2x, 4x, 8x, 16x, 32x, 5x, 10x, 20x, 40x(that can be specified)

Table 1 shows the comparison of specifications
between the newly developed size 6206 product andthe current one.

Signal output

Phase A, Phase B, Phase Z

Output form (that can be specified)

5 V TTL level or open drain

Package

TSSOP20 or QFN28

Acquired certification

AEC-Q100※※Certification standard for the stress test for automotive integrated circuits

Table 1 Specifications of 6206 size
Newly developed productCurrent product

Number of output pulses
from phases A and B

2,560 pulses/rotation

64 pulses/rotation

Resolution

0.14 degrees/pulse

5.625 degrees/pulse

Home position signal

AllowedーHow to connect signals

Connector-based
connection

Wiring-based
connection

4. Characteristics of the output waveform
Fig. 3 shows the comparison of the rotation signal
output waveform between the current product and the
newly developed one in terms of the magnetic pole ofthe magnetic encoder. The current product outputs asignal of a single pulse per pair of N-S magnetic poles(64 pulses per rotation), while the newly developed
product outputs a signal of 40 pulses (2560 pulses per
rotation). The rotation signal shown in Fig. 3 denotes
phase A signal only, for both the current and the newly
developed product. In reality, rotation signals from twophases are outputted; that is, phase A signal andphase B signal having a 90-degree phase difference
from phase A (a phase difference equal to a quarter
pulse). The rotation signal from the newly developed
product is multiplied 40 times inside the sensor IC
compared to the rotation signal from the current
product. Fig. 4 shows how to generate the home
position signal by using a magnetic encoder. The
magnetization pattern of the magnetic encoder iscomposed of double row tracks layed out in the axial
direction. Of the two tracks, the track that ismagnetized in equal pitches is called the HR track andthe one magnetized in uneven pitches is called the RP
track. They have the following roles:

3.2 Features of the sensor IC
Several Hall elements are installed in a line in thesensor IC (MPS40S). Signals from these Hall
elements are combined with a specially designed
multiplication circuit; this allows the magnetic patternin the magnetic encoder to be divided up to 40 times
internally to increase resolution. The major features ofthis IC are shown below and the specifications inTable 2.
(1) A single IC package is capable of outputting
signals from phases A, B, and Z, with theassembling of a sensor made easy;(2) The adoption of an ultra-small IC package allows
assembling to be performed in a small space;(3) A decrease in the magnetism in the encoder canbe detected;(4) Equipped with a self-diagnosing function thatmonitors the state of the sensor IC and encoder
and if an abnormality (an extreme abnormality of-77-

NTN TECHNICAL REVIEW No.78（2010）

HR track: For outputting signals from phases A andmagnetized so that the magnetic phase differs fromother positions. The home position signal is outputtedwhen the magnetic force of the RP track exceeds thethreshold value at the position at which the polarity of
the HR track changes (the zero crossing position).

B
RP track: For outputting signals from the home
position (phase Z)
The portion in the RP track near the position atwhich the home position signal is detected isMagnetic pole of themagnetization encoder
Rotation signal of thecurrent productRotation signal of the
newly developed product0.14 degrees (2,560 pulses/rotation)
5.625 degrees (64 pulses/rotation)

Fig. 3 Comparison of output form of current product and newly developed product

Magnetic encoder

The magnetic phase changes.
Sensor IC
RP track
HR track

HR track
Magnetic force

Position at which the home position
signal is detected

Zero crossing position
Phase A output
Phase B output
Phase Z output

Fig. 4 Generation of output wave form of index signal-78-

Portion exceeding thethreshold value
RP track
ThresholdvalueHigh Resolution Sensor Bearing with an Index Signal(1) Adjoining pitch error (%)｜(Tn−Tn＋1｜) /Tn×100: Maximum in a single rotation
Specified range: 10% or less
(2) Duty ratio (%)
Tp/Tn×100:: Maximum in a single rotation
Specified range: 35 to 65%(3) Phase difference (degrees)
TAB/Tn×360: Maximum in a single rotation
Specified range: 45 to 135 degrees

5. Important specifications of different
parts
Tables 3 to 5 show important standard
specifications of the newly developed product.
(1) The specification for the bearing is selectedconsidering the use of the bearing in supporting
the motor;(2) Adoption of a magnetic encoder magnetized indouble tracks; and(3) Two of the core wires of the five-core connector
are assigned to each of the ground connections
and one to each of phases A, B, and Z signal
outputs.

Phase AoutputPhase B
outputTp

Table 3 Specification of bearing
Bearing size

6206 (φ30×φ62×16)

Radial internal clearance

C3 (0.013–0.028 mm)

Sealed-in grease

Motor-use long life grease

Seal

Contact type rubber seal

Cage

Ribbon steel sheet cage

Allowable rotational speed

4687min-1 ※Working temperature range-40 to +120˚CTn

6.1 High temperature shelf test (at 120˚C)
Output accuracies of sensors that underwent a hightemperature atmosphere condition of 120˚C for 0, 300,
600, and 1,000 hours were measured at room
temperature. Fig. 6 shows the test results. The initial
sensor output accuracy was maintained after each of
the different shelf test hours.

Table 4 Specification of magnetic encoder
NBR

Magnetic material

FerriteTn＋1

Fig. 5 Definition of sensor output accuracy※Limit imposed by the sensor response frequency (5 kHz)

Rubber material

TAB

Number of magnetized poles 64 N poles and as many S poles

6.2 Low temperature shelf test (at -40˚C)

Direction of magnetization Radial direction (Double tracks)
Surface flux density

Output accuracies of sensors that underwent a lowtemperature atmosphere condition of -40˚C for 0, 300,
600, and 1,000 hours were measured at room
temperature. Fig. 7 shows the test results. The initial
sensor output accuracy was maintained after each of
the different shelf test hours.

17 mT or more (at a gap of 0.5 mm)Table 5 Specification of connector
Number of terminals

5

Applicable wire size

AWG26

6.3 Thermal shock test(-40˚C×60 minutes⇔Room temperature×10
minutes⇔120˚C×60 minutes)
The thermal shock test was conducted between 40˚C and 120˚C, and output accuracies of sensors
that underwent 0, 100, 600, and 800 cycles were
measured at room temperature. Fig. 8 shows the test
results. The initial sensor output accuracy wasmaintained after each of the different numbers ofcycles.

6. Evaluation tests
The evaluation tests described in Subsections 6.1 to
6.4 were conducted to measure the accuracy of
sensor output. The accuracy of sensor output isevaluated by means of the adjoining pitch error, duty
ratio, and phase difference defined below. (See Fig.
5.) The specified range to be used as an evaluation
standard was determined by taking into consideration
error factors arising from the measuring system (suchas accuracy in fabricating an encoder) and installation
errors. If an error exceeds this range, an abnormality
is suspected.
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NTN TECHNICAL REVIEW No.78（2010）

Adjoining pitch error

Phase A
Phase B

15

Relative error %Relative error %Adjoining pitch error

Specified range: 10% or less

12
9
6
3
0

Beforetest

After
300 hours

After
600 hours

15

Specified range: 10% or less

12
9
6
3
0

After
1,000 hours

Beforetest

After
300 hours

After
600 hours

Time

Phase A
Phase B
65

Specified range: 35 to 65%

Specified range: 35 to 65%

60

55

Ratio %

Ratio %

60

50
45
40

55
50
45
40

Beforetest

After
300 hours

After
600 hours

35

After
1,000 hours

Before
test

Time

After
300 hours

After
600 hours

After
1,000 hours

Time

Phase difference

Phase difference
135

135

Specified range: 45 to 135 degrees

120

Angle in degrees

Angle in degrees

Phase A
Phase B

Duty ratio

65

105
90
75
60
45

After
1,000 hours

Time

Duty ratio

35

Phase A
Phase B

Beforetest

After
300 hours

After
600 hours

105
90
75
60
45

After
1,000 hours

Specified range: 45 to 135 degrees

120

Beforetest

After
300 hours

After
600 hours

After
1,000 hours

Time

Time

Fig. 6 Result of high temperature environment test

Fig. 7 Result of low temperature environment test-80-

High Resolution Sensor Bearing with an Index Signal

6.4 Vibration test

Phase A
Phase B

Adjoining pitch error

Sensor output accuracies were measured at room
temperature before and after undergoing vibration
under the conditions shown in Table 6 and Fig. 9. A
cycle of frequencies varying continuously from 5 Hz to
200 Hz in 15 min was repeated. Fig. 10 shows how
the vibration test was performed and Fig. 11 shows
the test results. The initial sensor output accuracy wasmaintained after 24-hour (96 cycles) vibration.

Relative error %15

Specified range: 10% or less

12
9
6
3
0

Beforetest

After
100 cycles

After
600 cycles

After
800 cycles

CyclesTable 6 Condition of vibration test
Phase A
Phase B

Duty ratio

5

38.62

200

Acceleration (G)

0.5

30

30

10

10

0.37

Amplitude (mm)65

Specified range: 35 to 65%

60

Ratio %

Frequency (Hz)Vibration condition(one cycle)

55
50
45

The frequency is varied graduallyfrom 5 to 200 and then down to 5 Hz
in 15 minutes.

Direction of vibration

Axial direction

Test duration

24 hours (96 cycles)

4035
Beforetest

After
100 cycles

After
600 cycles

After
800 cycles

30

Cycles
G 0.5
Go and returnin 15 minutes
Phase difference
0Angle in degrees

135

Specified range: 45 to 135 degrees

120

5

38.62
Hz

200

Fig. 9 Relation between vibratory acceleration andfrequency

105
90
75
60
45

Vibrator body
Beforetest

After
100 cycles

After
600 cycles

Test sample

After
800 cycles

CyclesFig. 8 Result of heat shock test

Fig. 10 Condition of vibration test-81-

NTN TECHNICAL REVIEW No.78（2010）

6.5 Results of evaluation
Phase A
Phase B

Adjoining pitch error

These results show that:(1) The initial accuracy of the sensor output is within
the specified range, allowing one to judge the initial
performance to be without problems. The sensor
output accuracies remained within the specified
range after the sensors had undergone the hightemperature, low temperature, thermal shock, andvibration tests, allowing one to ascertain productdurability;(2) The difference observed in adjoining pitch errors
between phase A output and phase B output isestimated to be due to the effect of the magnetic
field of the RP track of each phase.

Relative error %15

Specified range: 10% or less

12
9
6
3
0

Before vibration

After vibration

Phase A
Phase B

Duty ratio
65

Specified range: 35 to 65%

Ratio %

60

7. Afterward

55

In this paper, the authors have introduced a newly
developed technology for a high resolution sensor
bearing with an index signal (built to the specificationof phase A and B output pulses of 2,560 pulses/
rotation). They will make efforts to advancecommercialization development of rotation sensor
bearings using this new technology.

50
45
40
35

Before vibration

After vibration

Phase difference

They will also continue pushing ahead aggressively
to broaden the market reach of high resolution rotation
sensor bearings built to the specification of 1,280 pulses
per rotation. This product has already completed thecommercialization development stage.

135

Specified range: 45 to 135 degrees
Angle in degrees

120
105
90
75
60
45

References
Before vibration

1) H. Ito and K. Koike, Bearing with Integral Revolution
Sensor, NTN Technical Review No. 69.
2) T. Koike, T. Ishikawa, H. Ito and N. Mizutani,
Improvement of Leakage Magnetic Flux Resistance ofIntegrated Sensor Bearings, NTN Technical Review
No. 71 (2003) 74-79.
3) P. Desbiolles and A. Friz, Development of High
Resolution Sensor Element MPS40S and Dual Track
Magnetic Encoder for Rotational Speed and Position
Measurement, NTN Technical Review No. 75 (2007)
36-41.

After vibration

Fig. 11 Result of vibration test

Photo of authors

Hiriyoshi ITOToru TAKAHASHI

Pascal DESBIOLLES

Cyril PETERSCHMITTShintarou UENO

Industrial Business HQ
Industrial Engineering Dept.

Industrial Business HQ
Industrial Engineering Dept.

NTN-SNR ROULEMENTS
New product andMechatronics R&D

NTN-SNR ROULEMENTS
New product andMechatronics R&D

NTN-SNR ROULEMENTS
New product andMechatronics R&D-82-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Improved Method of Rolling Bearing Fatigue Life Prediction
Under Edge Loading Conditions

Haruo NAGATANI*

Various types of life prediction methods have been proposed that calculate
total lifespans by synthesizing the lives of subdivided stressed volumes. Thesemethods seem to be devised to deal with any stress distribution. However fromthis study, it has been found that the life prediction accuracy of these methods
decrease when stress concentration occurs. In response to this, an improved
method is proposed. This method calculates the total life by synthesizing the
lives at the locations where stress concentrations occur.

edge load problem occurring in the necking section for
grinding on a rolling surface and to (b) the one
occurring in the joint between the cylinder section andthe crowning section in cut crowning as shown in Fig.
1. Other methods 10) 11) of determining a crowning
profile have been suggested; however, those methods
are intended to determine crowning profiles to reduce
contact pressure and stress, not intended to calculate
the bearing life.
Recently, as is seen in the theory of Ionnides andHarris (referred to as IH in the following) 12), various
methods have been suggested that determine the lifeof the entire bearing by dividing the surface loaded bystress into finite pieces, calculating the life of each,and then combining them. These methods are
considered to be applicable to any stress distribution
condition and should be applicable to the life
projection for cases where edge load occurs;
however, this has not been ascertained adequately.

1. Foreword

R

More than half a century has passed since
Lundberg and Palmgren (referred to as LP in thefollowing) established a theory on the life of rolling
bearings 1) 2), and problems with the theory such as its
lack of capability of considering shear force (tangential
force) on the surface caused by slip or the initial orresidual stress due to fit have been pointed out, withcorrection to these problems having been reported inmany ways. 3)-6) However, these problems occur only
when the theory is used under special conditions, andin most cases, the subjects above do not pose
problems. For this reason, designing and examination
based on the LP theory is still in practice now from theviewpoint of respecting past track records also.
On the other hand, the design of crowning isimportant in designing roller bearings. When an
imbalance in sharing of load exists between the edge
section and the middle section or when misalignment
exists, that region will be damaged in a short timeshould a greater load is exerted on either of both. To
avoid a problem of this kind, it is necessary to provide
a suitable crowning.
The method of Ito and Sugiura 7) is well-known as amethod of determining a crowning profile. On thebasis of results of various life tests conducted with theamount of crowning of full-crowning rollers being
varied, they used the method of Harris 8) and Moyer et
al. 9), proposing an optimum crowning design method.
However, this method cannot be applied to (a) the(a) Undercut part for grindings(b) Connecting part of cut crowning

Fig. 1 Examples that Ito and Sugiura's method
cannot be applied*Automotive Business HQ. Automotive Engineering Dept.

-83-

NTN TECHNICAL REVIEW No.78（2010）
Table 1 Rolling fatigue test results 7)

In actual designing, machining restrictions and costproblems often prevent one from defining an optimum
crowning shape, forcing one to determine a crowning
shape within the performance limit of the current
processing machine. It is necessary to determine
whether the life of the bearing designed in this way
can meet the requirement; in doing so, if one canobtain a tool to predict the bearing life accurately,such tools are extremely useful. This paper discusses
a method of predicting the life under the effects edge
loading that can handle such problem.

Test No.

V2

Load (kN)

V3

V4

V5

13.72

V6

V7

V8

17.35 11.76

9.8
1200

Crowning
radius (mm)

300

480

890

1200

∞

480

890

L10×104

560

1240

930

900

400

410

2200 4010

L50×104

2160 4640 2360 1640

745

1330 5100 7250

Weibull slope

1.4

1.4

2.0

3.0

1.6

2.2

3.2

Edge failure/
0/14
test frequency

0/15

7/15 12/15 13/15 3/15

6/15

8/15

3.2

Roller

2. Rolling fatigue test conducted by
Ito and SugiuraoTable 1 shows the result of the rolling fatigue test
conducted by Ito and Sugiura 7). They conducted therolling fatigue test in which they changed the crowningradius of a full-crowning cylindrical roller with φ12×
12 and the load.
Fig. 2 shows an example of the contour of theamplitude of subsurface shearing stress that occurred
this test. The figure shows the half of the contour inthe axial direction for the reason of the symmetry. Fig.
3 shows the coordinate system used in the calculationof the shearing stress amplitude. The horizontal axis
in Fig. 2 denotes the direction of the axis of the roller
and the vertical axis the direction of the depth, with thecontour lines of stress amplitude plotted in the figure.
Photo (a) shows a state of contact without edge load,
exhibiting a stress concentration in the middle of theaxial direction (at x = 6). Photo (b) shows aconcentration of stress both at the edge (x = 0.5) andin the middle. Photo (c) shows the result of calculationof stress on a roller without a crowning, exhibiting aremarkable concentration of stress at the edge. Photo(b) shows a weaker concentration of stress in the
middle than that in Photo (a), exhibiting a relatively
uniform stress distribution.

x

z

Race

Fig. 3 Contour plots of orthogonal shear stress amplitude

3. Prediction based on the current life
model

z mm

The current life prediction model is applied to theresults of the experiment described above to ascertain
the prediction accuracy of the model. This paper
discusses the four life models shown below. In thefollowing analysis, indexes e, c, and h are coefficients,
being 9/8, 31/3, and 7/3 for a roller bearing,
respectively.
Model-A
This is a model based on the concept of the LP
theory, but it does not use the method of stresscalculation in the LP theory; it calculates the life by

z mm

z mm

V1

x mm

x mm

x mm

（a）V1

（b）V4

（c）V5

Fig. 2 Contour plots of orthogonal shear stress amplitude-84-

Improved Method of Rolling Bearing Fatigue Life Prediction Under Edge Loading Conditions

Model-Dmeans of equation (1) on the basis of the maximum
value among the entire shearing stress distribution
determined by numerical calculation and the depth ofthat maximum value. S denotes the survival
probability,τ0 the maximum amplitude of shearing
stress, z0 the depth ofτ0, V the stress volume, a thecontact width,rthe perimeter length of the rollingsurface, and N the number of times of loading.

This model is worked out to allow one to consider
stress distribution on the basis of the LP theory 13).
(See Fig. 5.) The portion below the surface is notdivided in the direction of depth but in the direction ofaxis only, with disk-shaped slices formed after
division. The life of a slice is calculated using the
maximum amplitude of stress to which a slice issubjectedτ0j and its depth z0j , and combining the lifeof each slice. In this way, the significance of the
progress of cracking in the LP theory as it is can bebrought into each slice. a j denotes the width of eachslice.

1
τ0c
τ0c
e
e
∝ h ・N・V = h−1 ・N ・ar…………（1）
S
z0
z0

log

Model-B, and -C
These are methods based on the IH theory, inwhich the subsurface portion is divided (see Fig. 4.),and the life in the domain VR in which the roller
undergoes stress greater than the fatigue limit iscalculated to determine the life as a whole. The
calculation equation is given by equation (2).τudenotes the stress value of each slice, tu the fatigue
limit, and ΔVi the volume of each slice. With model B,
it is assumed that the fatigue limitτu is 350 MPa, andwith model C, it is assumed that the fatigue limit does
not exist. The stress volume VR of model C is assumed
to be the same as the stress volume at a fatigue limitof 350 MPa. Although the IH theory does not explicitly
show the stress that provides the reference for life
calculation, the shearing stress amplitude is used inthis paper as in the LP theory. z' is the depth weighted
with stress, taking the same value common to all
slices.

LaminaRoller
o

Depth

Race
Roller lengthzFig. 5 Laminas in Model-DlogRz (τ−τ ) dV Σz (τ−τ )ΔV
∫
z'=
=
……（3）
∫ (τ−τ ) dV Σ(τ−τ )ΔV
VR

u

i

i

u

i

VR

u

i

u

Σ

Σ
eff

Fig. 6 shows the results of the life calculationaccording to the four models described above. Fig. 6
shows the ratios of life with reference to V1. The scale
graduations are provided up to 20; the scale isexceeded depending on the model.

Σ

VR

τ0jc
τ0jc
1
e
∝
・N e ……（4）
j N =
h ΔV・
h−1 ajrS r z0jr z0jeff(τi−τu)c1e∝・ΔV・………………（2）
i N
S
z' h
V

log

x

i

VR

20
16
14

x
Life ratiooRollerTest result (L10)Model-A
Model-B
Model-C
Model-D

18

Dividedvolume

Depth

Race

12
10
8
6
4
2

Roller length0zFig. 4 Subdivide volumes in Model-B and -C

V1

V2

V3

V4

V5

V6

V7

V8

Fig. 6 Comparison of life predictions and test results-85-

NTN TECHNICAL REVIEW No.78（2010）

In the results of life tests shown in Fig. 6, model A
capture overall trends in samples V2 and V5, while theagreement deteriorates in samples V3, V4, V7, andV8. Model B shows a similar trend, with theagreement becoming poorer in samples V3, V4, V5,
V7, and V8. By contrast, model C in which the fatigue
limit is not specified shows a better agreement thanmodel B. When the fatigue limit is taken into
consideration as in model B, the value of the stresscterm of equation (2), (τi −τu) , decreases as a resultof subtracting the fatigue limit component τu from theactual stress; accordingly, it is considered that theagreement with the experimental values becomespoor. Model D shows a trend similar to that of model C
for which the fatigue limit is not considered.
Table 2 brings together the evaluations of theresults of calculation based on the four models
described above.
Of the results of calculation using the four life
models above, those obtained from model A, model C,
and model D show a relatively satisfactory agreementwith experimental values, but the agreement in allcases cannot be said to be enough. Models C and D
are ones that combine the life values of divided slices;in a case in which the edge load is strict as in sample
V5, however, the calculated life becomes larger thanthe life obtained through experiments. On the other
hand, model A, which does not combine life values ofdivided slices, captures the results of experiments on
sample V5 best. This suggests that the method model
A follows is effective in making it easier to reflect theeffect of the edge on the life. The reason for a poorer
agreement with experiments in models C and D withsevere edge load is considered in the following.

4. Discussion
The distribution in the axial direction of the life ofeach slice in model D is investigated on sample V5.
Life values represented by LIFE Parameter shown inequation (5) are shown in Fig. 7. As shown in thecomparison with equation (4), this value is perfectly
proportional to the life. In equation (5), the stress isexpressed in MPa and the length in mm.
c

V4

V5

V6

V7

V8

Model-A

○

×

×

△

△

△

×

Model-B

○

×

×

×

○

×

×

Model-C

△

△

△

×

○

○

○

Model-D

○

△

△

×

○

○

△

1.E-26

LIFE Parameter

V3

1

In Fig. 7, intervals between calculation points (slice
widths) are shortened, as shown in Fig. 3, to express
the abrupt change in stress at the edge.
In Fig. 7, discontinuity appears in the section near
the edge in which the slice width is small (x =0.49–0.65 mm) and stress is concentrated; however,the life is not shortened so much as in the middle
section. The reason for this is as follows: the slice
width is reduced to one tenth of that in the middle inorder to express stress concentration; as a result thevalue in equation (5) is lowered, and taking thereciprocal causes the life to be lengthened. In thisway, the effect of the volume on the life is very great,with a slice with a smaller volume having a longer life.
The life of the edge load section must become shortbasically; however, it has already become equal tothat of the middle section at the stage shown in Fig. 7.
However, combining them to determine the life as awhole further weakens the effect of the edge load. In
other words, with the volume in the middle far greaterthan that of the edge, the effect of a decrease in thelife in the edge is thinned out in the process of
combining life values.

Table 2 Validation of the 4 models
V2h−1

LIFE Parameter=τ0je ・z0je ・aje ………………（5）○：Errors from experimental values up to about 30%△：Errors from experimental values from 30% to twice
×：Errors from experimental values above twice

1.E-27

1.E-28012

3x-coordinates

4mm

Mesh interval is 0.1 mm
Mesh interval is 0.01 mm

Fig. 7 Life distribution of V5

-86-

5

6

Improved Method of Rolling Bearing Fatigue Life Prediction Under Edge Loading Conditions

On the basis of the consideration described above,
an improved model was worked out. Following theconcept of the LP theory, the improved model gives
consideration only to the stress amplitudeτ0peak and its
depth z0peak at several parts in which stress isconcentrated, expressing the life of one of those partsin the following equation:ln1

τ0 peakc∝

Speak

In the full-crowning in which the experiment was
conducted, the parts in which stress is concentrated
are two in the edge and one in the middle, withequation (6) applied to these three parts to calculate
the life of different parts. Since the life is determinedby the stress in the middle only when edge load isabsent, this model is quite identical to the LP theory.
The value of the life as a whole L is determined by
combining the life values of these parts in which stressis concentrated. The life value combining the life valueof each of the three parts in which stress isconcentrated under the full-crowning condition is given
by equation (7). The three sum values for i areassociated with two locations in the edge and with one
in the middle.
3

Σ(1/L ) …………………………………（7）(1/L)e=

i

600
400
200
0

1

2

3

4

5

6

φv in equation (6) is a coefficient to correct the life
value depending on how uniform the stress is, being
called the “stress uniformity coefficient.” The threeparts in which stress is concentrated are identified on
the assumption that the peak of stress amplitude isdistinct; depending on conditions, however, thedegree of stress concentration in the middle becomes
lower, making a peak not distinct. To clarify thiscondition, Fig. 8 shows the axial distribution of the
maximum values of shearing stress observed on
samples V1, V4, and V5. Fig. 8 is equivalent to a

1200
1000

800
600
400
200
0

e

i=1

1000

800

・φv・N peake・ar …………（6）z0 peakh−1

1200

1000

0

5. Improved model (Multi-point LP method)

Max shear stress amplitude
MPa

1200

Max shear stress amplitudeMPa

Max shear stress amplitude
MPa

To express the detail mentioned above numerically,
combining LIFE Parameter for the range x = 0.5–0.64
mm (in the edge) yields 2.95E-29, while doing thesame operation for the range x = 0.8–11.2 mm yields
1.08E-29 (in the middle). This means that the life of
the middle section is reduced to one third of that in theedge section in terms of calculation.
As seen in Table 1, however, sample V5 shows that
13 of 15 actual breakages have occurred in the edge
section; therefore, the life of the edge section should
be shorter than that in the middle section. As shown
above, the method shown above does not agree withthe real phenomenon at all. This is considered to be
an adverse effect of combining life values of slices.
This mechanism is common to methods of using
stress distribution to combine life values. It isconsidered that a similar mechanism works when thecalculated life is longer than the experimental life ofsample V5 in model C based on the IH theory.
By the way, when a notched flat sheet undergoes
the fatigue test, rupture starts at the notch section onwhich stress concentrates; for this reason, only thestress exerted on that section is considered and not
combined with the life value calculated from the stressexerted on other parts. Assuming that most of cracks
occur at the location at which the maximum amplitudeof shearing stress occurs, the LP theory makes
calculation using the stress and its depth at thatlocation only. In other words, the LP theory practices
the very calculating method just mentioned above. It isconsidered that the stress volume V in the LP theory
is intended for the consideration of the effect of thelength of the race, not for the consideration of stress
distribution.
As described above, the method of model A thatuses the value at the part in which stress isconcentrated is effective in making it easier to reflect
the effect of the edge on the life. However, model A
needs to be improved because of the disagreement of
the result of calculation with the result of experiments
in other T.P.s such as samples V3, V4, and V8.

0

1

2

3

4

5

6

800
600
400
200
0

0

1

2

3

4

Axial location mm

Axial location mm

Axial location mm(a) V1(b) V4(c) V5

Fig. 8 Distributions of max orthogonal shear stress amplitudes

-87-

5

6

NTN TECHNICAL REVIEW No.78（2010）

Fig. 9 shows diagrammatically the axial distributionsof stress amplitude shown in Fig. 8 into four types.
Sample V1 provides a Hertz contact without edge
load, corresponding to a quadratic stress distribution
shown in Fig. 9 (a). As described above, the φv value
in this case is unity. As edge load is intensified, thestress distribution changes to those shown in Figs. 9(b) and (c) and finally to that shown in (d); in Fig. 9(d), stress concentration is not regarded as existing in
the middle, and for this reason, the stress in the
middle is neglected. However, since the edge exhibitsa remarkable decrease in the life than the middle,
combining life values including that in the middle
yields about the same result. Sample V4 shows acharacteristic relatively close to that shown in Fig. 9(c). Sample V5 shows a characteristic similar to thatshown in Fig. 9 (d). It may be better to express thedegree of stress concentration by means of thecurvature in the middle of the stress distribution shownin Fig. 8. However, this needing complex calculation,
the degree of stress concentration is expressed by thearea occupied by the stress in Fig. 8. This area isgiven by equation (8).

diagram that plots the values obtained by searching
the maximum values of shearing stress amplitude in
the direction of depth on the contour diagram shown in
Fig. 2. Fig. 8 shows how the maximum stressamplitude that determines the life of each slice inmodel D is distributed in the axial direction.
Fig. 8 shows that sample V4 exhibits a considerably
lower stress concentration in the middle than sample
V1 and the stress distribution has been madeconsiderably uniform. It is estimated that such state
appears when the crowning profile comes close to the
optimum shape. This corresponds to a case in which
the radius of the notch is very large in the fatigue teston the notched flat sheet described earlier. The
maximum stress values are about the same at any
position in the axial direction, and it becomes
impossible to predict where a crack starts dependingon the unevenness in strength of the material. It isconsidered, in such a case, that a mechanism similar
to the dimensional effect may cause the life to be
shortened, with some kind of correction being
necessitated. However, what this means is simply an
increase in the odds of the occurrence of cracks dueto the unevenness of materials, not affecting the life
substantially. Determining this value needs to conductdetailed experiments; however, φv is calculated undera certain supposition here. For this reason, thefollowing idea of calculation is intended merely toshow guidance in considering the decrease in life in
the middle.
Stress uniformity coefficient φv
As seen from equation (6), aφv value greater unity
shortens the life. In the following, the approach todetermine the value of this coefficient is described. Ina stress distribution such as that observed in a normal
Hertz contact, it is considered that a sufficient stressconcentration occurs and a stress uniformity
coefficient needs not considering; therefore, it isassumed thatφv = 1 holds. Since the degree of stressconcentration on the edge is more intense than in the
middle,φv is always equal to unity. What matters isthe stress in the middle, and as this becomesincreasingly uniform, the value of φv increases.

ΣS= τyzj maxΔaj ………………………………（8）The extent for which area S is calculated isdetermined with the edge domain excluded in order toavoid being affected by the stress peak in the edge.
Although the roller chamfer of T.P. is 0.5 mm, the part
extending 1 mm from the roller end face was excludedin the present discussion.
In general, area S is between area Sa shown in Fig.
9 (a) and area Sc shown in Fig. 9 (c), with areas Sa
and Sc being expressed by equations (9) and (10),respectively. φv is set at unity in Fig. 9 (a) and at 2 inFig. 9 (c), and φv values between unity and 2 aredetermined depending on area S. Therefore,φv can be
expressed as shown in equation (11):

g

g

g

g

Sa

τmax

Axial location(a) Hertziancontact

StresshStresshStresshStressh

Sb

Sc

Axial location

Axial location(b) Slightly edge(c) Uniform stressloading
Fig. 9 Type of max. shear stress distribution at center part

-88-

Sd
Axial location(d) Severe edge
loadingImproved Method of Rolling Bearing Fatigue Life Prediction Under Edge Loading Conditions

the result from sample V2 than to that from sample
V3, while the result from sample V4 shows a peakwith models A, B, C. and D. On the other hand, thepeak life value appears between samples V2 and V3,
capturing the life value peak in the experimental
results successfully. Table 4 shows the result of
evaluation of the improved method in comparison withthe result obtained using model D.
A shortcoming with the improved method is theestimation of the calculated life value at a lower value
compared with the experimental value under severeedge load condition as in sample V5. This is due tothe fact that despite a change in the stress value dueto the plastic deformation caused by the high contactpressure exerted by edge load, the calculation ofstress values on the basis of the theory of elasticity
has yielded an estimation of a higher stress value thanan actual one. For this reason, the estimation is on thesafe side from the design viewpoint, and this will pose
no practical problem.

6. Results of calculation using theimproved modelFig. 10 shows the result of calculation using theimproved model. The illustration shows the result of
calculation not using the stress uniformity coefficient(φv = 1) and that of calculation using model D. Table 3
shows the stress uniformity factors used in calculation.
It is clearly seen that the improved method exhibits
a far better agreement with experimental results than
the results obtained on four models (Fig. 6). In
particular, the improved model excels other fourmodels in that it can be used to determine theoptimum crowning radius. In other words, the optimum
crowning radius under a load of 13.72 kN can be
determined using the results of the life test on
samples V1 to V5. Ito and Sugiura consider that the
optimum crowning radius is 830 mm, slightly closer to

7. Afterword
Methods of predicting the life under the effect ofedge load occurring in the roller bearing werediscussed.
It was clarified that the commonly-used life
calculation method of dividing subsurface volume into
slices and calculating and combining the life values ofindividual divided slices does not yield a correct life
prediction due to a great effect exerted by the stressvolume.
As an improved method, the “multi-point LP
method” was proposed in which, on the basis of an
idea that each of several parts where stress isconcentrated forms the starting point leading tobearing damage, the life value at each starting point todamage is calculated by the LP theory and the values
thus calculated are combined. This method introduces
the “stress uniformity factor” to take into consideration
the phenomenon of fatigue-based life shortening
similar to the dimension effect occurring when the
stress distribution becomes uniform. To determine thestress uniformity factor, the method of determining it
using the area formed by the axial distribution of the
maximum values of shearing stress was employedtemporarily. To determine this value, it will benecessary to accumulate experimental data as in theprocess in which the dimension factor was determinedin the field of strength of materials in the past.

12

Test result (L10)
Improved method
Improved method (φv=1)
Model-D

10

Life ratio

8
6
4
2
0

V1

V2

V3

V4

V5

V6

V7

V8

Fig. 10 Life ratio of improved model
Table 3 Stress uniformity factors for center part
V4

V5

V6

Stress uniformity
1.000 1.128 1.582 1.731
factor

V1

V2

V3

V7

V8

―

1.302 1.498 1.586

Table 4 Validation of the improved method
V2

V3

V4

V5

V6

V7

V8

○

○

○

△

○

○

○

Improved method (φv=1) ○

△

○

△

△

△

○

Model-D

△

△

×

○

○

△

Improved method○

This paper has been prepared with additions and modifications on thebasis of a paper titled “Improved Method of Roller Bearing Fatigue
Life Prediction Under Edge Loading Conditions” (Tribology
Transactions, Vol. 53, issue 5, pp. 695–702).

○：Errors from experimental values up to about 30%△：Errors from experimental values from 30% to three times
×：Errors from experimental values above three times-89-

NTN TECHNICAL REVIEW No.78（2010）

References
1) Lundberg, G., and Palmgren, A., Dynamic Capacity of
Rolling Bearing, Act a Polytechnica, MechnicalEngineering Series, Vol. 1, No. 3 (1947).
2) Lundberg, G., and Palmgren, A., Dynamic Capacity of
Rolling Bearings, Act a Polytechnica, MechnicalEngineering Series, Vol. 2, No. 4 (1952).
3) Harris, T., A., and Ragen, M., A., Spitzer, R., F., The
Effect of Hoop and Material Residual Stresses on the
Fatigue Life of High Speed, Rolling Bearings,
Tribology Transactions, Vol. 35, No. 1 (1992) 194-198.
4) Yu, W., K., and Harris, T., A., A New Stress-Based
Fatigue Life Model for Ball Bearings, Tribology
Transactions, Vol. 44, No. 1 (2001) 11-18.
5) Kudish, I., A New Statistical Model of Contact Fatigue,
Tribology Transactions, Vol. 43, No. 4 (2000) 711-721.
6) Kotzalas, M., A Theoretical Study of Residual Stress
Effects on Fatigue Life Prediction, Tribology
Transactions, Vol. 44, No. 4 (2001) 609- 614.
7) S. Ito and I. Sugiura, Investigation into the optimal
amount of crowning in linear contact turning fatigue
tests, NTN Bearing Engineer, No. 48 (1982) pp. 18-26.
8) Harris, T. A., The Effect of Misalignment on the
Fatigue Life of Cylindrical Roller Bearings Having
Crowned Rolling Members, Transaction of the ASME,
Journal of Lubrication Technology, Vol. 91, Series F,
No. 2 (1969) 294-300.

9) Moyer, C. A., and Neifert, H. R., A First Order Solution
for the Stress Concentration Present at the End of
Roller Contract, Transaction of the ASLE, Vol.6 (1963)
324-336.
10) H. Fujiwara and T. Kawase, Logarithmic Profile ofRollers in Roller Bearings and Optimization of their
Profiles, Transactions of the Japan Society of
Mechanical Engineers C, Vol. 72, No. 721 (2006) 338345.
11) M. Harada, S. Kamamoto, Y. Fujiwara, Roller Bearing
Crowning Form Designed in Consideration of InternalStress, Conference on Tribology 2000-5 proceedings(2000), C6, pp. 171-172.
12) Ioannides, E., and Harris, T. A., A New Fatigue Life
Model for Rolling Bearings, Transaction of the ASME,
Journal of Tribology, Vol. 107, July (1985) 367-378.
13) H. Hasegawa, Fatigue Life Estimation for Roller
Bearings Under Edge Load Occurrence, Transactionsof the Japan Society of Mechanical Engineers C, Vol.
74, No. 742 (2008) 1609-1616.

hoto of author

Haruo NAGATANI
Automotive Business HQ.
Automotive Engineering Dept.

-90-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Long Life Grease Added Naturally Derived Antioxidants

Yosuke TAGUCHI*
Hidenobu MIKAMI*
It is generally known that preventing the oxidation degradation of oil isnecessary to develop long-life grease. The evaporation of oil has been
controlled by having used the best antioxidant up to now. In attempting to
make long-life grease, an additive with a function that is superior toconventional antioxidants is needed. Therefore, we adopted naturally-derived
antioxidants of plant origin as additives and aimed to develop long-life grease
while maintaining low noise and low torque.

1. Introduction

2. Mechanism of oxidative deterioration

Rolling bearings are used in rotary components on
various machines to help improve efficiency of themachines and reliability of the rotary components.
Generally, any rolling bearing requires a lubricant, andthe most commonly used lubricant type for thispurpose is lubricating grease.
When used for an extended time at a higher
operating temperature, a lubricating grease will be
deteriorated by oxidation: therefore, an antioxidant willneed be added into the grease to help inhibit
deterioration in its lubricating performance. Various
antioxidants have been commercially used for thispurpose. In particular, phenolic antioxidants boast
greater benefits when added into grease, since they
themselves make stable radicals that help terminate
chain reactions while they donate hydrogen atoms.
Most phenolic antioxidants used in lubricating greases
are phenolic derivatives having a tertiary butyl group 1, 2).
Recently, bearings have been increasingly used inapplications where high temperature environments arepresent. Therefore, lubricating greases used for thesebearings need to be more durable in high temperatureenvironments, and very effective antioxidants have tobe added to these greases.
We have recently researched naturally derived
antioxidants, which have not been used as
antioxidants for industrial lubricating oils, and havestudied their oxidation inhibition capability as additives
for lubricants. Also, we have determined influences ofthese antioxidants as additives in lubricating greases
onto various lubricant characteristics including life of
the lubricating grease.

Generally, researchers believe a lubricating oil will
undergo oxidative deterioration in the following
sequence 3)–5).
<Step 1> Owing to factors including high temperature,a lubricating oil will be decomposed into
alkyl radicals.
<Step 2> An alkyl radical will react with one oxygen
molecule; thereby a peroxide and alkyl
radical will be generated.
<Step 1> The peroxide generated in Step 2 will reactwith two normal lubricating oil molecules,
producing alkyl radical.
<Step 1> RH→R・<Step 2> R・+O2→ROO・
ROO・+RH→ROOH+R・<Step 3> ROOH+ 2RH→
ROH+H2O+2R・
RH: hydrocarbon (lubricating oil)
R・: alkyl radical
ROO・: peroxy radical
ROOH: peroxide
To be able to withstand use for an extended period
at a higher temperature, lubricating oils usuallycontain antioxidant additives. Antioxidants used forthis purpose can be categorized into two types—(1)chain stoppers (primary antioxidants), and (2)peroxide decomposers (secondary antioxidants).
These types differ in oxidation inhibition mechanism.
The known examples of the chain stopper include
phenolic stoppers and amine stoppers, and those of*Elemental Technology R&D Center-91-

NTN TECHNICAL REVIEW No.78（2010）

the peroxide decomposer include phosphites,
phosphates, thio ethers, and thio esters.
Being chain stoppers, phenolic antioxidants each
donate a hydrogen atom derive from a hydroxyl group
to a peroxy radical that occurs during the growthphase in oxidative deterioration mechanism of alubricating grease (Formula 1) to turn the radical into aharmless radical. The resultant phenoxy radical isthen resonance-stabilized (Formula 2), and thenreacts with another peroxy radical to make a nonradical compound 6) (Formula 3), thereby oxidative
deterioration of the target lubricating oil is inhibited.
OH

extracted from onion: we have added thesesubstances into greases in an attempt to extend
grease life. Structural formulas of curcumin andquercetin are shown in Table 1, and appearances ofthese substances are shown in Fig. 1. These
substances are generally used as health foods or foodantiseptics.

Table 1 Naturally derived antioxidants
Description

Structural formulaO

Curcumin(turmeric)

O・

HO

OH
H3CO

OCH3

+ROOH (Formula 1)

+ROO・

O

OH
OH
Quercetin(onion)

O

O・

HO

O

OH
OH O

・

(Formula 2)

H
H

・

O

O

OOR

・
H +ROO・H

or(Formula 3)

O

O

10mm
Curcumin

+ROO・
H

O

・

Quercetin

Fig. 1 Appearance of naturally derived antioxidants
H

OOR

3. Natural antioxides

4. Weight decrease of greases athigher temperature“Natural antioxide” is the generic name for plant orvegetable derived constituents whose molecules eachhave a plurality of phenolic hydroxyl groups (hydroxyl
groups linked to aromatic rings such as benzene ring,and naphthalene ring). Natural antioxidants arecontained in a diversity of plants or vegetables. Well
known plant- or vegetable-derived antioxidants include
curcumin, flavonol, isoflavone, tannin, catechin,quercetin, and anthocyanin. These antioxidants havebeen proven to efficiently function as antioxidants
when taken into human body 7).
In the present study, we, in expectation of oxidation
inhibition effects of natural antioxidants for bearing
grease, have focused our study efforts onto curcuminthat is extracted from turmeric and quercetin that is

To be able to achieve longer life for lubricating
greases, it is necessary to inhibit oxidative
deterioration of base oil (lubricating oil). Examples ofweight decrease with lubricating oils having been
allowed to stand at a higher temperature aregraphically plotted in Fig. 2. When a given lubricating
oil is allowed to stand at a higher temperature, its
weight decreases as time elapses. Once a period toinitiation of oxidation of lubricating oil (induction
period) ends, its weight begins to drop suddenly. It
appears that when oxidized, lubricating oil isdecomposed, and the decomposition products readily
evaporate, leading to weight decrease of the
lubricating oil2). Consequently, the induction period of-92-

Long Life Grease Added Naturally Derived Antioxidants

Weight decrease rate wt%

lubricating oil lacking antioxidant is shorter while thesimilar period of lubricating oil having antioxidant islonger. Note that the addition of less effective
antioxidant will result in shorter induction period withthe lubricating oil.
We have added natural antioxidants andconventional antioxidants into lubricating oils, anddetermined the magnitude of oxidative deterioration
inhibition effects of these antioxidants. The
conventional antioxidants used were phenolicantioxidant having a tertiary butyl group (AO-1) andamine antioxidant (AO-2) (Table 2). The lubricating oil
used was ester oil (33 mm2/s, at 40˚C). 1wt% ofantioxidant was added to the ester oil and thespecimen was then allowed to stand at 150˚C, thereby
the weight decrease rate of the specimen wasdetermined 8). The results are graphically plotted inFig. 3.

The lubricating oil specimens lacking antioxidant
begins oxidative deterioration immediately after startof the test, and the weight decrease rate becomes
greater as time elapses. In contrast, the lubricating oil
having antioxidant shows inhibition of weight decrease
for a certain period. The specimen containing either
phenolic antioxidant (AO-1) or amine antioxidant (AO2) exhibits apparent weight decrease of lubricating oilat around 500 hours after start of the test. In contrast,
the specimen having natural antioxidants such as
curcumin or quercetin shows little weight decrease of
lubricating oil, even after 2,000 hours has elapsed. A
longer period before weight decrease of lubricating oil
becomes apparent means more positive inhibition ofoxidative deterioration of the lubricating oil. Thus, we
believe that these natural antioxidants excel inoxidation inhibiting performance and both curcuminand quercetin are positively effective antioxidants forester oil.

60
Lubricating oil: ester
50

W/o antioxidant

5. Application of natural antioxidants
to lubricating grease

Temperature: 150˚C

40
Induction period
30

W/ antioxidant

Li-soap/ester grease is typically used as motor
bearing grease, and boasts excellent reduction inrunning torque and noise, possibly occurring on motor
bearings.
We have added natural antioxidant to Li-soap/ester
grease, and evaluated the improvement to hightemperature life of grease and positive influence in
reducing noise and torque on the bearing lubricatedby the grease.

W/ antioxidant

20
10
0

0

500

1000

1500

2000

Time hFig. 2 Typical weight decrease of lubricating oil
Table 2 Conventional antioxidants
Description

Structural formula

Phenolic
antioxidant(AO-1)HO

Weight decrease rate wt%

Amine
antioxidant(AO-2)

Samples of a particular NTN rolling bearing type(#6204) were each prefilled with grease containing
curcumin or quercetin or conventional antioxidant, andthese samples were subjected to a high-temperature
grease life test. The compositions of the test greases
are summarized in Table 3.
The four different greases summarized in Table 3.
were subjected to high-temperature grease life test
(per ASTM D3336). The test rig used is schematicallyillustrated in Fig. 4, and the results are summarized inFig. 5. The amount of lubricating grease used for
prefilling is equivalent to 38% (1.8 g) relative to thebearing internal volume. The number of repetitions foreach test sequence (number of test runs) was n=3.
The grease life of each grease type was calculated bydetermining the mean for three test runs. The Test
Greases C and D, each having natural antioxidant,
boasts longer life—approximately 10 times as long asthat of the Test Grease D that lacks antioxidant andapproximately three times as long as that of the Test

OH

H
N

C8H17

5.1 High-temperature grease life

C8H17

60
No addition
AO-1
AO-2
CurcuminQuercetin

50
40
30
20
10
0
0

500

1000

1500

2000

Time hFig. 3 The weight decrease rate of lubricant-93-

NTN TECHNICAL REVIEW No.78（2010）

5.2 Difference in effect with varied amount ofaddition

Grease B that contains amine antioxidant (AO-2).
When considering the result of this test, together withthe result of weight decrease test in the previous
section (see Fig. 3), we think that an antioxidant
additive that more positively helps inhibit weightdecrease of lubricating oil also contributes to longer
life for the associated lubricating grease.

We have attempted to determine an optimal amountof addition of natural antioxidant. For this purpose, wechose curcumin as the natural additive tested andadded in Li-soap/ester grease. Curcumin was addedin six levels of amount, in a range of 0 to 10wt%
concentration relative to the amount of grease. The
resultant grease specimens were each prefilled into arolling bearing, and thereby their high-temperature
lives have been determined. The test results areshown in Fig. 6. The grease specimen containing
2wt% of curcumin boasts longest grease life, while
addition of 5wt% or greater results in shorter grease
life. To sum up, there appears an optimal amount ofaddition about curcumin. Note, that, a much greater
amount of curcumin added resulted in reduced effectin extending grease life. The most probable cause forthis problem is: when the amount of curcumin added
exceeds 2wt%, a certain portion of addition remains
non-solved in the grease, and the insoluble matterpromotes vibration of the bearing, causing lubricity of
the grease to deteriorate.
Based on these findings, we believe that curcuminand quercetin are efficient antioxidants for lubricating
greases and positively extend grease life.

Table 3 Test grease
A

B

ThickenerCD

Li-soap

Base oil

Ester

Base oil viscosity

mm2/s 40˚C

26

Additive―

AO-2

Amount added wt%

―

2

2

2

Worked penetration 60W 25˚C

250

245

245

245

Curcumin Quercetin

HeaterTest bearing
Coil spring
Supporting
bearing

PulleyThermocouple
To motor

Bearing: 6204ZZ
Temperature: 150˚C

Running speed: 10,000 min-1
Load: Fa=Fr=67N

500

Li-soap/ester oil grease

Fig. 4 High temperature bearing endurance test machine
Grease life hBearing: 6204ZZ
Temperature: 150˚C

400Running speed: 10,000 min-1
Load: Fa=Fr=67N

1400

300
200
100

Grease life h

1200
0
1000

0

1

2

3

4

5

6

7

8

9

10

Additive concentration wt%

800

Fig. 6 Varying influence of amount of curcumin added(0.7 g prefill)

600
400
200

5.3 Noise reduction characteristics

0
A

B

C

D

The noise level needs to be as low as possible withbearings, in particular for motor bearings. Being solid
at an ordinary temperature, a natural antioxidant
needs to be dissolved in grease in order to help
maintain low noise levels on bearings. Fig. 7 shows
photomicrographs of curcumin added in amount of
2wt% to commercially available Li-soap/ester grease.
Fig. 7(a) shows a state where curcumin remains

Test grease

Fig. 5 Result of high-temperature bearing durability test
(1.8 g prefill)-94-

Long Life Grease Added Naturally Derived Antioxidants

up to 10,000 min-1 in several steps, and the test
bearing was allowed to run for 1 hour at each speed
setting, thereby the mean running torque at eachrunning speed was measured.
We have learned that the grease consisting ofcommercially available Li-soap/ester oil grease and2wt% curcumin additive provides bearing runningtorque equivalent to that obtained from a lubricating
grease not containing curcumin, at any bearing
running speed. We may judge that addition of
curcumin does not lead to increased bearing runningtorque.

simply mixed with the grease and curcumin particles
are apparently present. In contrast, Fig. 7(b) illustrates
a state where curcumin has been fully dissolved in thegrease thanks to an improved grease manufacturing
process.
The noise levels of greases either containing or notcontaining dissolved curcumin were measured with anAnderon meter. The resultant measurements are
summarized in Fig. 8. The noise level of the grease
specimen containing dissolved curcumin wasapproximately equivalent to that of the specimen notcontaining curcumin. Because curcumin can be
dissolved in Li-soap/ester oil grease, the grease canhelp the bearing run without developing higher runningnoise.

0.1mm(a) Insoluble state

Test bearing

0.1mm

Load cell(b) Fully dissolved stateFig. 7 Photomicrograph of curcumin addition grease
Fig. 9 Vertical torque examination machine
Bearing: 608LLB
Amount of grease prefill: 0.118 g
Running speed: 1,800 min-1 Load: Fr=1.5N
2

Running torque mNm

Anderon value

Bearing: 6204LLB
Load: Fa=19.6N

10W/o curcumin added

35
30
25
20
15
W/ curcumin added
W/o curcumin added

10
5
0
0

W/ curcumin added

Amount of grease prefill: 1.8 g

2000

4000

6000

8000

10000

Running speed min-1

Fig. 8 Noise levels of various greases
Fig. 10 Rotational torque of various greases

5.4 Running torqueThe running torque needs to be as low as possible
with bearings, in particular for motor bearings in orderto minimize power consumption of the electric motors.
We have determined the effect of addition of curcuminonto bearing running toque. Fig. 9 schematicallyillustrates the test rig used, and Fig. 10 graphicallyplots the result of the running torque measurement.
The measuring operation was performed while
increasing the bearing running speed from 1,800 min-1

-95-

12000

NTN TECHNICAL REVIEW No.78（2010）

benefit—the percentage of grease prefill in bearing
internal space volume can be reduced from 38% to25% (reduction ratio: 30%) without causing decreasein grease life (Fig. 11).
Also, the bearing running torque with the curcumincontaining grease is 35% smaller, compared with therunning torque with bearings prefilled with standard
grease not containing curcumin (Fig. 12).
It is also possible that reduction in amount of grease
prefill helps prevent grease leakage andcontamination around the bearings.

6. Decreased bearing running torque by
reduction in amount of grease prefill
High-temperature durability of any rolling bearing
can be governed by the amount of grease prefill in the
bearing. Life of a given rolling bearing at higher
temperatures is longer with a greater amount ofprefilled grease, while the running torque of a rolling
bearing is lower when the amount of prefilled greaseis smaller. Thus, smaller amount of prefilled grease
will help realize lower running torque of the bearing, ata cost of shorter bearing life.
Addition of a natural antioxidant in grease will
positively extend life of the grease. In other words, agrease containing natural antioxidant will help reduce
the amount of grease prefilled in a bearing without
decreasing grease life in the bearing when comparedwith a conventional grease.
For the purpose of this section, we have added anatural antioxidant in varying amounts into Lisoap/ester oil grease to prepare specimens, and by
testing the specimens attempted to determine therelation between the amount of lubricating grease
prefill and grease life, and the amount of grease prefill
and bearing running torque. Fig. 11 graphically plots
the relation between the amount of grease prefill andgrease life, while Fig. 12 shows the relation between
the amount of grease prefill and bearing runningtorque. Compared with grease not containing
curcumin, the useful life of grease containing 25wt%
of curcumin is about three times as long. Thus, use of
curcumin-containing grease leads to a unique

7. Conclusion
From the currently available test results, we haveverified that use of lubricating oil or Li-soap/ester oil
grease containing natural antioxidant used to lubricate
a bearing will help realize not only low running noiseand low running torque on the bearing but also longer
effective life for either lubricating oil or grease.
A natural antioxidant may be added to a lubricating
grease in order to inhibit oxidation of the lubricant in
the grease and help extend high temperature life of
the lubricating grease. This type of grease, even if
prefilled in a smaller amount, can maintain useful lifeof the bearing. At the same time, smaller amount ofprefill means lower stirring resistance of the grease;
thereby the running torque of the bearing will bedecreased.
It appears highly possible that natural antioxidants
can be applied to those lubricants having
compositions other than the Li-soap/ester oil

Percentage of running torque %(when running torque on bearing prefilled with standard
non-curcumin containing grease is taken as 100)

Bearing: 6204ZZ
Running speed: 10,000 min-1
Temperature: 150˚C Load: Fa=Fr=67N
1400
W/ curcumin added
1200

W/o curcumin added

Grease life h

1000
800

Possible reduction
in grease prefill

600
400

200
0
10

20

30

40

Amount of grease prefill(percentage relative to bearing internal space volume) %Bearing: 6204LLB
Load: Fa=19.6N

Amount of grease prefill: 1.8 g
Running speed: 10,000 min-1

100

80

60

30% reductionin amount ofgrease prefill

40

20

0
0

20

40

60

80

100

Percentage of grease prefill %(when the amount of non-curcumin containing
grease prefill is taken as 100)

Fig. 12 Influence of amount of grease on rotating torqueFig. 11 Relation between amount of grease andlongevity-96-Long Life Grease Added Naturally Derived Antioxidants

combination described above. We expect theirapplicability in a diversity of lubricants. We believe that
longer grease life together with decreased amount ofgrease prefill in bearings will positively contribute to
reduction in environmental impacts through decreased
energy consumption, lower cost, and prevention ofcontamination of the environment around the bearing
resulting from possible grease leakage.

References
1) H. Mikami, Book of Synopses ITC Kobe (2005) 145.
2) H. Mikami, Development of Long Life Grease for High
Speed Application—ME-1 Grease for Motor Bearings,
NTN Technical Review No. 72 (2004) 20-25.
3) Y. Ohkatsu, T. Aoshima and K. Yamaguchi, Molecular
Design of New Phenolic Antioxidants, Chemical
Society of Japan, No.11 (1998) 711-721.
4) J.L. Reyes-Gavilan, Paul Odorisio, NLGI Spokesman,64, 11 (2001) 22.
5) J. Igarashi, Nisseki Technical Review, 32, 3 (1990) 5
6) T. Watanabe, Functions and Applications of
Antioxidants, Junkatsu Keizai, pp. 6-3 (2003).
7) H. Nishino , Development of Free Radical Theory andFood Products for Ageing Prevention, CMC TechnicalLibrary, pp. 14-21 (1999).
8) Y. Taguchi, H. Mikami, Effectiveness of Natural
Antioxidants on the Oxidation and Deterioration of
Lubrication Grease, Proceedings of Tribology
Conference, pp. 283-284 (2010).

hoto of author

Yosuke TAGUCHI

Hidenobu MIKAMI

Elemental Technology
R&D Center

Elemental Technology
R&D Center-97-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Improvement of Grease Leakage Prevention for Ball Bearings
Due to Geometrical Change of Ribbon Cages

Norihide SATO*
Tomoya SAKAGUCHI*
Grease leakage from sealed grease bearings shortens bearing life andpollutes nearby parts. Therefore, the seal shape was changed and theincluded grease amount was decreased. However, these changes reduce
performance, such as causing higher torque, and shorten bearing life.
Generally, grease leaks from between the inner seal groove and the seal,
which rotate relative to each other. In this report, in order to improve theprevention of grease leakage from sealed deep groove ball bearings, newgeometrical cage designs were developed. These cages were confirmed
experimentally to have excellent performance in grease leakage prevention.

2. Mechanism of grease purge and thestate of adhesion of grease to theribbon steel cage

1. Introduction
To avoid grease purge from a greased and sealed
bearing a number of measures can be taken. The
pressure the seal lip exerts on the inner ring can beincreased1), the shape of the seal lip can be changed
to help keep the grease in2), or the amount of greasein the bearing can be decreased. Unfortunately thesechanges can increase the shaft torque, the cost of the
bearing, or shorten the life respectively.
Grease purge occurs between the seal and theinner ring seal groove that rotate against each other. It
has been found that grease purge occurs when thegrease can adhere to a surface close to an opening.
For this reason, it is postulated that if the adhesion ofgrease to certain bearing components can besuppressed, grease purge can be reduced. On thebasis of this concept, a cage with a new shape wasdeveloped in an attempt to improve the resistance togrease purge.

A ball bearing with a full contact-seal and filled witha lithium/ester based grease (6203 LLU, ribbon steelsheet cage containing 870 mg of grease) was run at a
speed of 3,600 min-1 under a radial loading. The test
was conducted under both inner ring and outer ringrotation. Following the test, the post test bearing mass
was measured and the results from inner ring rotation
were compared to the results for outer ring rotation. It
was found that the bearings run under outer ringrotation saw five times the mass decrease compared
to inner ring rotation. All mass decreased were foundto e the result of grease purge.
Fig. 1 shows the results of running a bearing at
3600 min-1 for five seconds in either inner ring or outerring rotation with no seal. Grease does not adhere to
the inner ring seal groove with the inner ring rotating,while a large amount of grease is seen adhering therewith the outer ring rotating. This allows one toestimate that the adherence of grease to the inner ring
seal groove is one of the prerequisite for the large
amount of grease purge under outer ring rotation.
It is theorized that grease purge in a bearing withfull contact seal occurs by the mechanism shown in
Fig. 2:*Elemental Technology R&D Center-98-Improvement of Grease Leakage Prevention for Ball Bearings Due to Geometrical Change of Ribbon Cages(1) Outer ring rotation causes the grease to move tothe inner ring seal groove(2) The heat from bearing operation raised thetemperature of the air inside the bearing resulting
in an increase in pressure(3) The increased pressure pushes the seal lip
outward opening a path for grease purge to occur

3. Behavior of grease adhering to theinner ring seal groove
In order to observe how grease moves under outerring rotation a test was conducted where a bearing
with a minute amount of grease was rotated for fiveseconds.
A 6203 type bearing was filled with 60 mg ofgrease. The grease was located between the outerring raceway and the cage. After the bearing was
rotated it was found that the grease had migrated to
the leading edge, with respect to rotation, of the cage
pocket (Fig. 3). It was theorized that grease adheringto the rolling element was scratched off by the innerdiameter of the cage after which the greases collected
at the leading edge of the cage pocket. Fig. 4 shows
the grease state when 320 mg of grease is placed in
the bearing. The amount of grease on the leadingedge of the cage pocket increases. Additionallygrease is found to adhere to the middle of the cage
pocket and to the outer diameter of the inner ring.

The above progression suggested that if the amountof grease adhering to the inner ring seal groove canbe reduced grease purge in a sealed bearing can be
reduced. To achieve this, the shape of the cage was
examined.

No grease on inner ring seal groove
Rotationa) Inner ring rotation

Outer ring rotation
Wiped grease

Grease on inner ring seal grooveb) Outer ring rotation
Fig. 1 Location of grease in a ball bearing with a ribbon
cage after operation

Wiping locationof grease

Fig. 3 Grease dispersal in a ball bearing with 60 mg ofgrease after outer ring rotation

Pressure rise of inside air

RotationPushedout

Grease123Outer ring rotation

Fig. 2 Grease purge mechanism from a sealed ballbearing

Grease on pocket
center area

Grease on outersurface of innerring

Fig. 4 Grease dispersal in a ball bearing with 320 mg ofgrease after outer ring rotation-99-

NTN TECHNICAL REVIEW No.78（2010）

These test results suggests the grease follows the aspecific route to the inner ring seal groove, assuming
outer ring rotation and a ribbon steel cage.

to rotate more grease is deposited into the innerring seal groove. Under inner ring rotation thegrease at GO in Fig. 5 b) is scattered back into thebearing by centrifugal force, helping to prevent
grease purge. However, under slower inner ringrotational speeds the centrifugal force is reduced
leading to the same behavior as that observed inouter ring rotation.

I) Grease adhering to the rotating rolling element is
scraped off by the inner diameter of the cage
pocket and attaches to location Gw in Fig. 5 a).
II) An increase in the accumulated amount of greaseat Gw on the edge of the pocket causes grease toalso attach to the outer diameter of the inner ring at
GO in Fig. 5 b).
III) An increase in grease at GO causes grease toattach to the inner diameter surface of the cage at
Gt. This only occurs at the middle part of the cage
pocketIV) As the amount of grease at Gt increases, part ofthat grease is pushed out into the inner ring seal
groove at Gg in Fig. 5 d). As the bearing continues

4. Development of a cage that reduces
grease buildup to the inner ring seal
groove4.1 Working out an improved cage
The discussion in Section 3 suggests three
concepts would be effective in suppressing theadherence of grease to the inner ring seal groove:(1) Reducing the ability of the inner diameter of thecage pocket to scratch off grease off the outer
diameter of the inner ring (position GO), thereby
reducing grease accumulating on the pocket edge;(2) Preventing grease from moving from the leadingedge of the cage pocket and attaching to the outer
diameter part of the inner ring; and(3) Preventing the movement of grease to the innerring seal groove from the edge of the cage pocket.

Gwa) Wiping stepOn the basis of these concepts, three prototype
cages were made to confirm the effect of cage designof grease on the behavior of grease. All of thesecages were fabricated from common ribbon steel
sheet cages with minor alterations (referred to asreference cages in the following).
The cage shown in Fig. 6 has a larger chamfer to awide area in the middle part of the cage pocket (dottedzone). This cage aims at suppressing the scratchingaction of the edge on the inner diameter side of thecage pocket (concept (1)) and at suppressing the
movement of grease to the inner ring seal groove(concept (3)). This cage was called the ‘widely
recessed cage’.
The cage shown in Fig. 7 is an example ofrealization of concept (1), in which only the area of thecage that scratches the grease is chamfered. This
cage is called the ‘diagonally recessed cage’. Its effectis considered smaller than that of the widely recessed
cage described above.
Fig. 8 shows a cage with an cut-out where the cage
pocket would contact the outer diameter of the innerring. This aims to avoid the effects of concepts (2) and(3) by increasing the gap between the edge of thecage pocket and the outer diameter of the inner ring.
This cage is called a narrow pocket width cage.

Gob) Grease transfer step to outer surface of inner ringGtc) Grease transfer step to pocket center area

Ggd) Grease shift step to inner seal groove
Fig. 5 Grease movement from ball surface to inner ring
seal groove-100-Improvement of Grease Leakage Prevention for Ball Bearings Due to Geometrical Change of Ribbon Cages

4.2 Effect of preventing grease from adheringto the inner ring seal groove

diagonally recessed cage, with no attachment ofgrease to the inner ring seal groove observed. While
this was a satisfactory result this was a realization of
only concept (1) for grease purge reduction.
Fig. 11 shows the result obtained from the narrow
pocket width cage. While the amount of grease
scratched off by the edge of the pocket (Gw in Fig. 5)
is large, the adherence of grease to the inner ring seal
groove was not observed.
In short all three cage designs worked at
suppressing the adherence of grease to the inner ring
seal groove.

Figs. 9 to 11 show results of the outer ring rotation
test for each of the new cage designs. Compare thiswith the results for the standard cage shown in Fig. 1(the amount of sealed-in grease: 870 mg).
Fig. 9 shows the result obtained with the widely
recessed cage. No attachment of grease to the innerring seal groove observed. Additionally grease was
not found in the outer diameter part of the inner ring
and at the center of the outer surface of the pocket,
with the anticipated effect obtained.
Fig. 10 shows the result obtained with theFig. 6 Design for the widely recessed cage with large
chamferFig. 9 Grease deposition in bearing using the widely
recessed cage (Fig. 6)

Fig. 7 Design for the diagonally recessed cage with two
small chamfers

Fig. 10 Grease deposition in bearing using diagonally
recessed cage (Fig. 7)

Shape of the base cage

Large amount of the wiped grease

Fig. 8 Design for the narrow pocket cage

Fig. 11 Grease deposition in bearing using narrow
pocket cage (Fig. 8)-101-

NTN TECHNICAL REVIEW No.78（2010）

narrow pocket width cage.
Figs. 12 to 14 show the effects of the cage changes(the standard cage for comparison, diagonally
recessed cage, and narrow pocket width cage).
These three cage types were built into bearings andgrease leakage tests were performed under outer ringrotation conditions (Table 1) in which grease leakage
would normally easily occur. The test criterion was
based on a visual check for purged grease to theoutside of the bearing.
Table 2 shows the results of the grease leakage
test. Six base cages out of 15 showed greaseleakage. However, the improved cages showedsatisfactory results of one diagonally recessed cageand no narrow pocket width cage showing leakage.
Figs. 15 to 16 show the amount of grease attachedto the inside of the seal in a bearing with the standard
cage and the narrow pocket cage respectively. A large

5. Effect of the improved cages on theresistance to grease leakage
The cages used for testing in Section 4 werestandard cages that had been modified by additional
cutting steps. For additional testing cages made using
the standard die stamping, as in volume production,
were examined.
It was found that a widely recessed cage madeusing the die stamping process caused the outersurface of the cage to swell outward into where thebearing seal is located. This would necessitate achange in the design of the bearing inside. For thisreason, the evaluation of production cages was limited
to two types, the diagonally recessed cage and theTable 1 Test conditions
Bearing

6203LLU (Sealed ball bearing)

Outer ring rotation speed，
3600
min-1
Operation time，min

15
870 mg, Lithium soap-Ester,
Penetration 255

GreaseLeakage detection

Visual inspection

Fig. 12 Standard cage with ball
Table 2 Test results
Recessed points

Fig. 13 Diagonally recessed cage with ballCage shape

Leakages / Total

Base cage (Fig.12)

6 / 15Press cage of diagonally recessed pocket(Fig.13)1 / 15Press cage of narrow pocket width (Fig.14)

0 / 15

Grease on seal lip area

Fig. 15 Adhered grease on the inside of seal with thestandard cage (Fig. 12)

Narrowed areas

No grease on seal lip area

Fig. 16 Adhered grease on the inside of seal with thenarrow pocket width cage Fig. 14)

Fig. 14 Narrow pocket cage with ball-102-

Improvement of Grease Leakage Prevention for Ball Bearings Due to Geometrical Change of Ribbon Cages

face side of the rivet presser. When a bearing is run
under moment load, cracks often occur on theboundaries between the plane with rivet holes and thepocket (referred to as a pocket end in the following).
Also the analysis illustrated in Fig. 17 shows that the
maximum principal stress occurs at the same positionin the modified cage.
Figs. 18 and 19 show the distributions of principal
stress in the two improved cages. The upper andlower limit values of the stress contour are similar
between the three cage designs. In the diagonally
recessed cage shown in Fig. 18, the maximum
principal stress occurs at the pocket end as with thestandard production cage, while in the narrow pocket
width cage shown in Fig. 19, the maximum principal
stress occurs in the middle of the pocket.
Fig. 20 shows the ratios of the principal stressvalues at the pocket end and at the middle of thepocket of the different cage designs to the maximum
principal stress value of the standard production cage.
A standard cage and a diagonally recessed cage, bothwith the same width in the radial direction, exhibit thesame stress value. By contrast, a narrow pocket width
cage is narrow in width in the radial direction in the
pocket and for this reason has low rigidity. As a result,the stress in the middle of the pocket inside increases
relatively, being 6% greater than the maximum value
with the base cage.
Due to the higher stresses in narrow pocket width
cage care should be taken to ensure that the strengthof the cage is sufficient under operating conditions.

amount of grease is observed in the seal lip of the
bearing with the standard cage, but no grease isobserved in the seal lip of the bearing with the narrow
pocket cage. It was concluded that the improvement in
the shape of the cage has made it possible tosuppress the flow of grease to the inner ring seal
groove, thereby reducing grease purge from thebearing.
This test demonstrated that a cage capable ofreducing the adherence of grease to the inner ring
seal groove also reduced the amount of grease purge.

6. Examination of the strength of
improved cages
The new cages differ in shape from that of thestandard cage. It was anticipated that this will change
how much stress occurs in the cage and where thestress is concentrated. The cage strength was
examined by means of a static structural analysis
based on the finite element method.
For this analysis the following assumptions weremade; the rolling element moves relatively to thedirection of revolution and gives contact load andfrictional force to the pocket plane.
Fig. 17 shows for the results for the standard
production cage. To speed the analysis only a sectionof cage was examined. This section consisted of anarea between rivet holes. Circumferential
displacement lock was given to the cross-section ofdivision, and axial displacement lock to the matingHigh stressHigh stress

Low stress

Low stressMaximum stress point

Maximum stress pointa) Pocket-back sidea) Pocket-back side

Acting area of contact load

Acting area of contact loadb) Pocket sideb) Pocket side
Fig. 17 Principal stress distribution in standard
production cage (Fig. 12)

Fig. 18 Principal stress distribution in diagonally
recessed pocket cage (Fig. 13)-103-

NTN TECHNICAL REVIEW No.78（2010）

High stress

Low stress

7. Conclusion
With the aim of reducing grease purge from agreased and sealed ball bearing, the behavior ofgrease adhering to the inner ring seal groove wasstudied. As a result an improved cage capable ofreducing the leakage was designed and evaluated.
The conclusions of this evaluation are as follows:
1) It was theorized that an increase in the internal
temperature of a sealed bearing causes a pressure
on grease located on the inner ring seal groove
resulting in grease purge. It was also theorized thatthe adherence of grease to the inner ring seal
groove is caused by (1) scratching off of greaseadhering to the rolling element at the edge on the
inner diameter side of the cage pocket, (2)migration of grease from there to the outer diameterpart of the inner ring, and (3) pushing out of grease
to the inner ring seal groove.
2) Changing the shape of the cage made it possible tosuppress the adherence of grease to the inner ring
seal groove. Any of the following changes will resultin lowered grease purge: (1) it reduces the action ofscratching off grease adhering to the rolling
element by the edge on the inner diameter side of
the cage pocket, thereby reducing the amount ofaccumulated grease, and (2) it increases the gap
between the cage and the outer diameter of theinner ring.
3) Building a cage capable of suppressing theadherence of grease to the inner ring seal groove
into the bearing improves the resistance to grease
purge substantially.
4) Regarding the strength of the cage, the maximum
stress of the narrow pocket width cage is 6%
greater than that of a standard production cage. For
this reason, it is necessary, before applying theimproved cage, to conduct a cage strength
evaluation under the operating condition to ensurethat the cage has sufficient strength.

a) Pocket-back side
Maximum stress point

Acting area of contact loadb) Pocket side
Fig. 19 Principal stress distribution in narrow pocket
width cage (Fig. 14)

1.5

Pocket end

Stress ratio

Pocket center
1.0

0.5

0.0

1

3

3

1 Base cage (Fig. 12)
2 Diagonally recessed pocket cage (Fig. 13)
3 Narrow pocket width cage (Fig. 14)

Fig. 20 Stress ratios of cages at pocket end and centerusing structure strength analysis

Previously grease purge had been addressed bymeans of improvements in the seal or an adjustmentof the amount of sealed-in grease; however, thepresent study has made it clear that an improvementin the cage shape enables grease purge to be
mitigated substantially.
Furthermore, since using the improved cage
described above allows for higher grease fills,
compared to a standard cage, improvements in life
can be expected.

-104-

Improvement of Grease Leakage Prevention for Ball Bearings Due to Geometrical Change of Ribbon Cages

References
1) NTN, Ball and Roller Bearings Cat. No. 2202/E (1997).
2) S. Nozaki, M. Okasaka, Y. Kubota and S. Akabe,
Trends in Automotive Instrument and Auxiliary Bearing
Technologies, NTN Technical Review No. 65 (1996)
65-72.
3) N. Sato & Sakaguchi, The Influence of the Retainer
Form on Grease Adhesion to the Inner Ring Seal
Groove, Proceedings of Tribology Conference Tokyo,
2008-5 (2008) 135.
4) N. Sato, T. Sakaguchi and M. Kawamura,
Development of a Corrugated Iron Retainer for Ball
Bearings that Prevents Grease Leakage, Proceedingsof Tribology Conference Nagoya, 2008-9 (2008) 323.
5) T. Sakaguchi and Y. Akamatsu, Simulation for Ball
Bearing Vibration, Proc. Int. Trib. Cont., Nagasaki
2000, 3 (2000) 1795.

Photo of authors

Norihide SATO

Tomoya SAKAGUCHI

Elemental Technology
R&D Center

Elemental Technology
R&D Center-105-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]NTN-SNR High Performance@
Oil-lubricated Plummer Blocks SNOE@
Ulrich GIMPEL*
Martin SOMMER*
Jens ULBRICH*
Oil-lubricated plummer blocks are used for large plant industrial machinery.
NTN-SNR has developed the new SNOE@series of plummer blocks.
Compared to the previous SNOE plummer blocks, these were redesigned withgreater housing rigidity and a labyrinth sealing system, and they have
improved performance and reliability.

1. Introduction

2. Oil-lubricated Plummer Block SNOE II

Oil-lubricated plummer blocks are used extensively
in plant-related facilities using industrial machinery.
For example, plummer blocks are used in air blowers
for ventilation or drying installed in steel andpetrochemical plants and thermal power stations or incrushers in mining facilities as shown in Fig. 1.
Plummer blocks used in these facilities must meet therequirements of higher speed and shortened
maintenance work to improve operational efficiency.
They must meet the environmental requirement ofpreventing oil leakage while in use outdoors. In
response to these requirements, we have modified
existing oil-lubricated plummer blocks (SNOE series)
and developed the SNOE@series products which aresuperior in terms of functionality and reliability while
being designed to use common parts.

2.1. Construction
Fig. 2 shows the construction of the newly
developed product. This product consists of a two-parthousing using a series 222 or 223 self-aligning roller
bearing (cylindrical hole). To lubricate the systemefficiently, this product is mounted with the splash ringhanging from the slinger on the axis side; the ring

Housing
Self-aligning roller bearing
Air bleed valve

Splash ring(Upper part: hanging)Labyrinth-winged coverSlinger
Felt seal grooved coverFelt
Oil surface
Oil panSlingerPlummer block

Splash ring
(Lowermost part: in the oil)

Felt seal
groovedcover

Felt- or grease-filled

Fig. 1 Application example of oil-lubricated plummer
block (large size air blower)

Fig. 2 Oil-lubricated plummer block SNOE@ (fixed side)*SNR Wälzlager GmbH Engineering Bielefeld

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NTN-SNR High Performance Oil-lubricated Plummer Blocks SNOE@

2.2.2 Sealing performancefeeds lubricant from the oil pan to the bearing during
operation.
The product adopts the optimum labyrinth
construction, using a felt seal compatible with highspeed rotation. The housing must be supported
securely to allow the bearing inside to rotate stably
under severe environmental and load conditions.
Installing the product on a common machine, twoplummer blocks made up of the fixed side and the freeside bearing are used. In particular, in response to thelarge axial displacement of a large-sized facility inoperation, the housing of the plummer block used on
the free side is designed to accommodate a large
displacement of the bearing.

This product is provided with a non-contact labyrinth
seal on both sides of the bearing. By optimizing thedesign of the labyrinth ring (labyrinth spacer) on therotating side and the labyrinth ring (cover withlabyrinth) on the fixed side, the lubricant splashedonto the bearing is collected reliably to the oil pan
inside the housing without leaking out of the plummer
block.
The felt seal grooved cover shown in Fig. 2, added
to the labyrinth seal, enables one to add felt
depending on the surrounding environmental
conditions such as the shaft rotational speed,
temperature, humidity, and presence of dust.
Furthermore, with this cover provided with a hole tofeed grease to the felt seal groove, the grease seal
scheme (grease-filled) may be adopted.

2.2 Functions
2.2.1 Lubrication
The self-aligning roller bearing inside the housing islubricated with the oil stored in the lower part of the
housing. In addition to ordinary oil-bath lubrication,
lubricant is fed into the bearing by the splash ring,placed off-center with the axis, in the groove provided
on the labyrinth spacer as shown in Fig. 3. In otherwords, the splash ring rotating with the shaft draws up
lubricant from the ring immersed in the oil pan in thelower part of the housing, splashing the lubricant tothe upper part of the bearing.

2.3 Features of plummer block SNOE@
2.3.1 Housing material
As shown in Table 1, gray cast iron (EN-GJL-250(DlN GG25; JIS FC250)) characterized by large
attenuation was used as SNOE housing material in
the past.
In response to the recent increase in load andspeed, however, nodular graphite cast iron (EN-GJS600-3 (DlN GGG60; JIS FCD600) that excels in shock
resistance and vibration characteristics is adopted.

Table 1 Specification of ductile materials

EN standard

Existing
material

Material used in the
newly developed product

GJL-250

GJS-600-3

DIN

GG25

GGG60

JIS

FC250

FCD600Tensile strength

N/mm2

250

Young’s modulus N/mm2 103,000
Hardness (HB 30)

Oil pan (oil bath for the ring)―180 - 250600
174,000
190 - 2702.3.2 Optimum design of the housing bodySplash ring

In order to minimize the effect on the strength ofplummer blocks and on the bearing and bearing fitting
face, the rigidity was increased without changing theoutside dimensions and the tying method for the twopart housing, as shown in Fig. 4. In addition, thehousing was designed, while considering the heatdissipation of the bearing while rotating, to improve
lubrication and the bearing life.
The features of optimized housing design are
shown below.

Fig. 3 Location oil pick up ring-107-

NTN TECHNICAL REVIEW No.78（2010）(1) Two-part housing clamping section(2) Bearing fitting face(3) Housing(4) Fastening bolt area

Upper partof the housing

Lower partof the housing

Existing product: SNOE

Newly developed product: SNOE@

Fig.4 Design of housings(Features)the fastening bolts shown in Fig. 9 for improved
rigidity. The upper and the lower housing are held by
the tapered securing pins to provide accurate
positioning in maintenance and inspection.

• Adoption of the two-pillar construction system inthe bearing support in the lower part of the housing• Reinforcement between clamping bolts over thepart from the lower part to the upper part of the
housing• Reinforcement of areas surrounding mounting bolts• Change in the distance between the clamping bolts(both ends)• Improvement of heat dissipation from the clamping
face

0.033

(1) Pillar construction

0.031
0.030

Displacement

Displacement mm

0.032

0.029
0.028
0.027
0.026
0.025

In changing over from the conventional product tothe one with two pillars, the construction was designed
to maintain the volume of the oil pan. The finite element
analysis method (FEM) shown in Fig. 5 was used todetermine the pillar installation angle at which thedeformation of the bearing fitting face is minimized.
As shown in Fig. 6, it was confirmed that the
deformation of the bearing fitting face is smaller at thepillar angle of 25 degrees. The analysis of deformationin the upper and the lower part of the housing under thiscondition was conducted as shown in Figs. 7 and 8.

0.024

16

20

24

28

32

36

Angle ˚Fig. 6 Relationship between rib angle and deformation offitting area

(2) Construction of the area around theclamping of the two-part housing
The two-part housing of the newly developed
product is provided with a reinforcement rib between
Fig. 7 Deformation of bearing fitting area (upper part)

Hollow part
Pillar partFig. 5 FEM-analysis modelFig. 8 Deformation of bearing fitting area (bottom part)

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NTN-SNR High Performance Oil-lubricated Plummer Blocks SNOE@

Reinforcement rib(Upper part)
Taperedsecuring pin

Fig. 10 Position of fastened bolts

Housing
Reinforcement rib(Base part)Fig. 9 Fastened area of sprit type housing

Fastening face(contact area)(3) Position of clamping bolts
The clamping bolts fastening the upper and thelower housing are placed closer to each other on eachside, as shown in Fig. 10, to prevent them fromseparating from each other and to suppress the shiftof the bearing fitting surfaces (sections).

Fastening bolt tying area

Fig. 11 Contact area of housing(4) Housing fastening face
As shown in Fig. 11, the ground contact face of the
housing is stabilized against load by not providing theboring in the lower part of the bearing fitting part, withthe heat from the bearing being radiated sufficiently.
The area surrounding the spot at which fastening
bolts are connected is kept solid to allow the tension
due to bolt clamping to be absorbed; this contributes
to suppressing the effect on the deformation of the
housing as a whole.
Discharge(5) Other examples of fastening

Cooling pipe

Installing sensors capturing vibration, temperature,and other physical quantities on this housing makes it
possible to control the rotational state of plummer
blocks. For this reason, holes for placing a sensor areprovided in different parts of the housing. The oil levelof the lubricant can be checked constantly with an oil
gauge.
When a high temperature is expected during
operation, installing a cooling pipe in the oil pan space
as shown in Fig. 12 allows heat to be removed fromthe oil and transferred to the heat exchanger installed
outdoors.
On the other hand, when the bearing is used in acold region, preheating the lubricant is necessary
before starting machines; for this purpose,consideration has been given to installing a heaterand a thermostat as shown in Fig. 13.

SupplyFig. 12 Housing with integrated cooling pipe

Splash ring
Oil panThermostat

HeaterFig. 13 Preheating device of lubricant oil-109-

NTN TECHNICAL REVIEW No.78（2010）

2.3.3 Seal construction

labyrinth winged cover to the oil pan (3).
When lubricant further penetrates into the labyrinth(8), it is returned from the labyrinth (8) due to theconical form of the slinger outside diameter part of thefelt seal grooved cover (6), being collected via thepath from the opening (7) of the lower part of thelabyrinth winged cover to the oil pan (3).
Besides the action of the labyrinth seal function,
providing a felt seal in the felt groove enables lubricant
outflow to be prevented more efficiently.

The labyrinth seal is used as a common seal
construction of plummer blocks. With the existing
SNOE series product, the free side and the fixed side
labyrinth alike are designed so that each cover (boththrough and non-through (for use at the shaft end))
may meet individual customer specifications. To
address this situation, a unified design specification
was used for labyrinths while accounting for different
processing methods to reduce the number of partsand simplify specifications.
In SNOE@series products, as shown in Fig. 14, theslinger and felt seal grooved cover for the fixed sideand those for the free side, to be used in combination
with the labyrinth winged cover for the fixed side
bearing and the one for the free side bearing, areconstructed in the same design.
This measure eliminated the necessity of
substantial design changes to address special
applications (reinforcement of sealing performancesuch as grease seal and resistance to desert
environments).

Labyrinthwinged coverSlingerLabyrinthseal part(Compatible withspecial cases)
Felt seal
grooved coverSplash ring

：Flow of the lubricant

To the lower part via theperimeter of the circle

Labyrinth cover
Oiling groove(To the housing
inside)Splash ring

Oil drain opening(Lower part: notched area)

Shaft end coverFig. 15 Labyrinth sealing design of fixed side
Throughhole cover(2) Labyrinth seal function of the free side bearing
As shown in Fig. 16, the construction of thelabyrinth on the free side is basically identical to thaton the fixed side. However, when the axial
displacement (distance) of the bearing is the greatest
with reference to the labyrinth winged cover, thelabyrinth passage is secured, though the slinger
position becomes shallow. On the other hand, theslinger position is determined so as to avoid theinterference of theslinger with the labyrinth
cover should the slinger
position become theleast remote; thisassures the function of
the labyrinth sealadequately.

Oil drain groove(To the oil pan)Fig. 14 Labyrinth sealing design of SNOE@(fixed side)(1) Labyrinth seal function of the fixed side bearing
Fig. 15 shows the construction of the labyrinth.
Lubricant is splashed, inside the housing, from the oilpan (3) by the splash ring to lubricate the upper part ofthe bearing. At the same time, lubricant attaches tothe inclined surface of the labyrinth winged cover (1)or the outside diameter face (4) of the slinger (2) andis collected into the oil pan (3).
When splashed lubricant does not fall onto thelower part of the housing but flows into the labyrinth
inside (the area around the inside diameter of thelabyrinth winged cover (5)), lubricant attaches to thefelt grooved cover (6) and is then collected via theroute from the opening (7) of the lower part of theFig. 16 Labyrinth sealing
design of floating side

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NTN-SNR High Performance Oil-lubricated Plummer Blocks SNOE@

2.4 Evaluation(2) Test results

Since the sealing performance is referred to as animportant function of an oil-lubricated plummer blockin an operating facility, the oil leakage evaluation test
was conducted.

As shown in Fig. 18 no oil leakage was observed in
all of the tests, and abnormalities in the bearing andinterference in the labyrinth were not observed, withthese satisfactory results proving high sealing
performance.

(1) Test conditions
Table 2 shows typical test conditions for evaluating
the sealing performance of a labyrinth seal. Thisevaluation was not applied to the felt seal and grease,but the gap between the shaft and the felt seal
grooved cover (the exit from the labyrinth) wasmonitored continuously as shown in Fig. 17.

Upper part

Table 2 Test condition of lubricant leak (example)
Basic scheme of aplummer block

SNOE@218

Type of bearing used(self-aligning roller bearing)22218（φ90×φ160×40）

Test atmosphere temperature

20˚C (RT)

Lubricant viscosity

40 mm2/s

Bearing rotational speed

3,000 min-1 (max)

Shaft inclination (misalignment)

0˚

Lower partFig. 18 Gap area after lubricant leak test (no leakage)

0.25˚

Test duration

Four hours for each test item

Criteria

Visual inspection;measurement of the oil level(oil gauge)

3. Conclusion
This paper introduces NTN-SNR oil-lubricatedplummer blocks.
In addition to addressing an increase in efficiency
arising from the improvement of performance ofindustrial plant facilities and from the reduction inmaintenance work, the authors will give constantconsideration to the environment under severe
working conditions such as the outdoor environment
and push ahead with a further improvement in
performance.

Felt seal grooved cover
Housing

Monitored section(Labyrinth)
Shaft

Fig. 17 Test plummer block (monitoring oil leak)

Photo of authors

Ulrich GIMPEL

Martin SOMMER

Jens ULBRICH

NTN Wälzlager GmbHEngineering Bielefeld

NTN Wälzlager GmbHEngineering Bielefeld

NTN Wälzlager GmbHEngineering Bielefeld-111-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Development of Oil-impregnated Sintered Bearingof Low Wear and Corrosion Resistance
Kazuhiro KIMURA*
Recently, oil-impregnated sintered bearings, which are slipping bearings,
are used for a variety of applications because their steady sliding performancecan be demonstrated over a long period of time without adding oil.
In this text, we added manganese sulphide (MnS) as a solid lubricant
because MnS has sliding characteristics equal to oil-impregnated sinteredbearings made of steel, and we introduce an oil-impregnated sintered bearing
made of stainless steel that has excellent resistance to corrosion.

sulfide (MnS). Table 1 shows how the solid lubricants
react with the base metal under heated conditions.
Solid lubricants are required not to react below thesintering temperature (1,200˚C) of the base metal.
However, C, MoS2, and Sb produce compounds under
this condition which tend to exhibit increased
hardness; for this reason, C, MoS2, and Sb cannot be
used as solid lubricants for sintered bearings.
Bi, BN, and MnS do not produce reactants below
1,200˚C, the sintering temperature of stainless steel,and the wear and friction tests showed that MnS
exhibits the best sliding characteristic; for this reason,
manganese sulfide (MnS) was selected as the solid
lubricant. Fig. 1 shows an enlarged picture ofmanganese sulfide (MnS) powder.

1. Introduction
Generally speaking, sintered oil-retaining bearings
made of bronze or iron as a base material often use
graphite (C) or molybdenum disulfide (MoS2) as solid
lubricant 1). However, bronze- and iron-basedmaterials were found inferior in corrosion resistance in
special environments (in a gasoline atmosphere, forexample) causing deterioration of sliding performance.
To solve this problem, we have developed, jointlywith the Nagoya Municipal Industrial ResearchInstitute, stainless steel sintered oil-retaining bearings,
made of austenite-based stainless steel which areexcellent in corrosion resistance. They have a sliding
characteristic equivalent to that of iron-based sinteredoil-retaining bearings as a result of the selection andquality of an adequate solid lubricant.
This paper presents the materials, basic
characteristics, and performance of the newly
developed product.

Table 1 Reaction by temperature with stainless steel

2. Materials of the newly developed
product

Temperature

Reaction

1100

Deposition of (Cr, Fe)7C3

1200

Fusion

Molybdenum disulfide (MoS2)

800

Production of compounds
after decomposition

Antimony (Sb)700

Production of compounds

1200

No reaction

Solid lubricant
Graphite (C)For bearings using stainless steel as the base metal
to achieve an excellent sliding characteristic, theselection of a solid lubricant plays an important role.
The solid lubricants selected to evaluate the basemetal and their sliding characteristic were bismuth(Bi), boron nitride (BN), graphite (C), molybdenum
disulfide (MoS2), antimony (Sb), and manganese

Bismuth (Bi)Boron nitride (BN)Manganese sulfide (MnS)*NTN Powder Metal Corp. Engineering Dept.

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Development of Oil-impregnated Sintered Bearing of Low Wear and Corrosion Resistance• Temperature: 50±5˚C• Humidity: 95±5%RH• No protective layer of lubrication
Fig. 2 shows the appearance of the bearing after
the test.
Rust was observed on the surface of the iron-based
sintered bearings 24 hours after the start of the test,
while it was not observed on the surface of the newly
developed product 200 hours after the start of the test.

Newly developed productIron-based sintered bearing

Fig. 1 Manganese sulfide (MnS)

3. Basic characteristics of the newly
developed productFig. 2 Result of humidity cabinet test

3.1 Features of the newly developed product<Features>
3.3 Evaluation of corrosion resistance ina special environment(1) The newly developed product excels in seizure
resistance and wear resistance as a result of using
manganese sulfide (MnS) as solid lubricant;(2) It excels in corrosion resistance because
manganese sulfide (MnS) does not hinder theformation of passive state membrane by chromiumoxide in stainless steel; and(3) Its sliding characteristics excel even in special
environments (gasoline atmosphere).

Excellent corrosion resistance is required of fuelpump bearings as they are used in gasoline or biofuel, which have been found to be highly corrosive
when used around past products.
For this reason, bronze-based bearings, widelyused as sintered oil-retaining bearings, and the newly
developed product, both without a protect oil coating,were immersed in a sulfur-containing solution forcomparison.

3.2 Corrosion resistance
Table 2 shows the chemical composition of the
newly developed product. Its base metal is austenitebased stainless steel, a material excellent in corrosion
resistance.
A humidity cabinet test was conducted to ascertain
the corrosion resistance of the newly developed
product and iron-based sintered bearings.

(Test method)• Solution: Liquid mixture of regular gasoline,
ethanol, and sulfur• Solution temperature: 60˚C
• Test duration: 100 h• Bearing size: Inside diameter: 6 mm; outside
diameter: 12 mm; width of 6 mm

(Test method)
The newly developed product and iron-based
sintered bearings were left alone in a thermostaticbath for 200 hours under the following conditions toobserve how rust occurred:Fig. 3 shows the appearance after the test and Fig.
4 the change in dimensions due to corrosion.
After the test, discoloration was observed on thebronze-based sintered bearings, and the inside
diameter, outside diameter, and width all showedchanges of at least 0.1 mm, evidence of corrosion. By
contrast, the newly developed product showed nochange in appearance, with change in dimension
being one-tenth or less than that found in bronzebased bearings.

Table 2 Chemical composition
Chemical composition (%)
Fe

Cr

Ni

MnS

OthersResidue

15–20

10–15

1–3

1 or less-113-

NTN TECHNICAL REVIEW No.78（2010）

Bronze-based sintered bearing

After the test

Before the test

Newly developed product

Change in dimensions mm

Fig. 3 Result of corrosion test

discoloration. Fig. 5 shows the surface state and Fig.
6 the result of analysis.
The manganese sulfide (MnS) surface is blackenedby oxidation, while the stainless steel surface near the
manganese sulfide (MnS) area does not exhibitchanges. The results of the analysis before and after
the test show that oxygen (O), which was notobserved in the sample before the test, was detectedin the sample after the test. Also, a slight amount ofoxygen (O) was observed on the surface ofmanganese sulfide (MnS). That being said, thestainless steel surface exhibited no change, whichshows that manganese sulfide (MnS) did not affect thecorrosion resistance of the base metal.

0.20
0.15
0.10
0.05
0.00
-0.05
-0.10

Newly developed productBronze-based bearing

-0.15
-0.20

Inside diameter(φ6)

Outside diameter
(φ12)

Width
(6)

Fig. 4 Dimensional change of after test

3.4 Evaluation of the corrosion resistance ofsolid lubricant, manganese sulfide (MnS)
The humidity cabinet test was conducted to see
how sulfur-containing manganese sulfide (MnS)
affects the base metal, stainless steel, when the newly
developed product undergoes corrosion positively.

Before the test

(Test method)• Temperature: 60˚C
• Humidity: 90%Rh• Test duration: Observation was conducted atintervals of 168 hours, and the test was to be ended
when a change was observed on the surface.

After the test

Stainless steel
Manganese sulfide (MnS)

Manganese sulfide (MnS)

Fig. 5 Surface before and after corrosion test

After 1,176 hours of testing, the surface of the newly
developed product exhibited discoloration. The test
was ended in order to perform SEM/EDX-EPMA
analysis on the areas of the part showing the most-114-

Development of Oil-impregnated Sintered Bearing of Low Wear and Corrosion Resistance

SEM/EDX (Before the test)
SEM image

SEM/EDX (After the test)O (oxygen)

S (sulfur)

Fe (Iron)

SEM/EPMA (After the test)

Fig. 6 Analysis result

3.5 Sliding performance of the stainless steel
sintered bearing(Test method)• Peripheral speed (V): 37.7 m/min• Contact pressure (P): 0.2–1.5 MPa• Lubricant: Mineral oil• Atmosphere: Room temperature

To compare the coefficient of friction between the
newly developed product and the iron-based sinteredbearing, the testing machine shown in Fig. 7 wasused to measure the coefficient of friction. Table 3
shows the physical property values of the newly
developed product and iron-based sintered bearing.

1

2 3 4 5 67

8

* With the peripheral speed kept constant, the PVvalue was varied by varying the contact pressure.
Fig. 8 shows the relationship between the PV value
and the coefficient of friction. Both materials show
their coefficients of friction at about 0.1, which is about
the same as that of an iron-based sintered bearing.
Fig. 9 shows the initial adaptive characteristic at aperipheral speed of 37.7 m/min and a contactpressure of 1.0 MPa and the change in the coefficientof friction over a 500-hour period of time.
Like iron-based sintered bearings, the newly
developed product exhibits a high coefficient of friction
immediately after the start of the test, but the value ofthe coefficient of friction begins to decrease graduallyand shows about the same value as that of iron-based
sintered bearings at the end.

1 Load cell
2 Housing
3 Test bearing
4 Test shaft
5 Pressure ring
6 Roller bearing
7 Load lever
8 Weight

Fig. 7 Test rig
Table 3 Basic characteristics
Pressure
Surface Oil content
Densityring strength hardness
Vol.%
g/cm3
MPa
HRF
Newly developed
product
Iron-based sinteredbearing

6.5–7.0 150 or more 70 or more 8 or more
5.9–6.3 200 or more 40 or more 18 or more

※Physical property values are typical ones.

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NTN TECHNICAL REVIEW No.78（2010）

4. Points to notice in using stainlesssteel sintered bearings

Coefficient of friction μ0.30

Iron-based sintered bearing

0.25

In sintered bearings, generally speaking, lubricant
retained in the bearing by means of impregnation
seeps to the bearing surface to form oil film with theeffect of preventing the contact between the shaft andbearing surfaces. For this reason, the presence of
lubricant is indispensable to maintaining a good sliding
characteristic; the same characteristic is true of the
newly developed product2). Accordingly, the sliding
performance deteriorates remarkably without
lubrication or under high temperature conditions inwhich the lubricant cannot perform its role sufficiently.
A solid lubricant (MnS) works to prevent abnormal
friction, however, since there is a danger that the
lubricant will lead to abnormal heating of the sliding
part and abnormal wearing of the shaft and bearings,
it is necessary to select a lubricant compatible with thetemperature of the atmosphere.

Newly developed product
0.20
0.15
0.10
0.05
0.00
0

10

20

30

40

50

60

PV value MPa・m/min

Fig. 8 Comparison of frictional properties

Coefficient of friction μ

0.40
0.35

Iron-based sintered bearing

0.30

Newly developed product0.25

5. Conclusion

0.20
0.15

A new sintered oil-retaining bearing with excellent
corrosion resistance and low friction has beendeveloped by means of adopting stainless steel as thebase metal and manganese sulfide (MnS) as the solid
lubricant. The newly developed product has a sliding
characteristic equivalent to that of a conventional ironbased sintered bearing, with its corrosion resistance
improved substantially. The result of this development
work will contribute to further lengthening of theservice life of sintered bearings under special
environments.

0.10
0.05
0.00
0

100

200

300

400

500

Test duration hFig. 9 Measurement result of frictional properties

References
1) Teruhisa Watanabe , Powder Metallurgy New Edition,
Gijutsu Shoin, 65, 1995.
2) Teruhisa Watanabe, Powder Metallurgy New Edition,
Gijutsu Shoin, 63, 1995.

Photo of author

Kazuhiro KIMURANTN Powder Metal Corp.
Engineering Dept.

-116-

NTN TECHNICAL REVIEW No.78（2010）[ New Product ]Fluid Dynamic Bearing Unit for the Home Ventilation Fan

Masaharu HORI*
As a rule, the installation of the ventilation equipment has come to be required forall buildings that have rooms where people live since the revision of the BuildingStandard Law. Therefore, the demand for home ventilation fans has increased forordinary families and the demand for high reliability and low noise for home
ventilation fans has also increased.
NTN has developed a fluid dynamic bearing unit with excellent reliability andquietness that is easily used to replace rolling bearings. This fluid dynamic
bearing unit can be applied not only for use in home ventilation fans, but also inother applications where rolling bearings are used and quietness is demanded.
This report introduces this fluid dynamic bearing unit for home ventilation fans.

In addition, in response to the strong request of aventilator manufacturer having production lines usingroller bearings, we decided to develop a new productthat can be handled in a similar way as current roller
bearings.
Fig. 1 shows how the rotor and the fluid dynamic
bearing are assembled and Fig. 2 the bearing as aunit. With the aim of reducing costs, we adopted moldproduced products in all component parts of a fluid
dynamic bearing. Press-machined products wereadopted in both of two housings and sintered oilretaining bearings were used for the rotating bodies.
Two housings are fastened by means of bonding as
shown in Fig. 2; forming the cross-section of each of
the two housings in an L shape enlarged theoverlapping of the bonded parts of the housings,
resulting in increased joint strength.
Figs. 3 and 4 show the shapes of fluid dynamic
grooves.
The thrust grooves were provided on the inside end
surface of both housings. The radial grooves were
provided on the outside diameter surface of therotating body.
The oil membrane formed by these fluid dynamic
grooves supports the rotating body in perfect noncontact state in both radial and thrust directions.

1. Introduction
With the aim of reducing the room concentration ofchemical substances causing sick house syndrome,
the revised Building Standard Law took effect on July
1, 2003 that regulates building materials andventilation facilities used in buildings.
This has made it obligatory, as a rule, to installmechanical ventilation facilities in all buildings
because of emissions from furniture even if building
materials do not diffuse formaldehyde.
Obligatory installation of ventilators in living rooms,
bedrooms, and other rooms as a result of the law
revision has increased the need for freedom fromnoise in particular, bringing about requests of
replacing roller bearings with sliding bearings. In
response to these requests, we have developed afluid dynamic bearing that has the same dimension asthat of a roller bearing and that can replace it easily;
the new bearing is presented below.

2. Construction and features of the
new bearing
Currently, roller bearing 695 (a deep-grooved ballbearing with dimensions of φ5×φ13×4 mm) is usedmainly, and the major dimensions of the newly
developed product was adapted to those of bearing
695 as shown in Fig. 2.

*Fluid Dynamic Bearing Unit Division Engineering Dept.

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NTN TECHNICAL REVIEW No.78（2010）

To fabricate groove processing molds, the molddesign technology was used that had been cultivatedin the production of fluid dynamic Bearphite, currently
mass-produced for use in support bearings for harddisk drives (referred to as HDDs in the following). This
allowed high-accuracy fluid dynamic grooves to be
produced. Fig. 5 shows the picture of the appearanceof the thrust groove section.

Fluid dynamic bearing unit

Rotor

Fig. 1 Schematic layout of rotor and fluid dynamic
bearing unit
4

Housing(Inside)Housing(Outside)

φ5

φ13

Fluid dynamic
bearing section

Fig. 5 Photo of thrust groove
Rotating body

2.2 Method of processing radial grooves
Unlike fluid dynamic Bearphite currently in mass
production, radial grooves must be provided on theoutside diameter surface of a rotating body; therefore,a groove processing method based on strong sizing,
which is a conventional means of processing on the
Bearphite inside diameter surface, cannot be adopted.
To solve this problem, the porosity of sintered oilretaining bearings was used to adopt the component
rolling scheme; this allowed satisfactory bearing
surfaces to be obtained. Fig. 6 shows the picture of
the appearance.

Adhesive application
position
Bonded partFig. 2 Cross-section shape of fluid dynamic bearing unit

Fluid
dynamic
grooveSide face of the housing inside

Fig. 3 Thrust groove

Provided on the outside
face of the rotating bodyFig. 6 Photo of radial groove

2.3 Assembling method
Fig. 4 Radial groove

A method of assembling proven in mass-producing
HDD-use fluid dynamic bearings was applied.
Three members were brought into intimate contact
with one another first, and then one housing was
moved by the distance equal to the axial gap, andadhesive was applied to the housing to bond andfasten other two housings, and after this, the bearing
was impregnated with lubricant.

2.1 Method of processing thrust grooves
The housing was a stamped product, andincorporating the fluid dynamic groove processing into
the stamping process prevented an increase in the
total number of processes.

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Fluid Dynamic Bearing Unit for the Home Ventilation Fan

2.4 Load capacity

extends over 22 years when converted into theworking condition of a ventilator (60˚C), satisfying theservice life requirement of 15 years shown in Table 2.
The durability is being tested on an actual device
described in subsection 3.5.

The rotating body weight of the actual device
requires a radial load capacity in the horizontal
condition of 1 N or more.
To meet this requirement, we calculated the load
capacity using the program for analyzing thecharacteristics of a porous oil-retaining bearing unit
developed by us to determine the bearing
specification.
Table 1 shows the conditions for analysis and Fig.
7 the results of analysis. As shown in Fig. 7, it was
ascertained that a radial gap of 20 mm allows a load
capacity of 1.44 N to be secured.

3.2 Oil membrane formation test
To make sure that oil membrane is formed during
operation, the NTN fluid dynamic bearing test unit
shown in Fig. 8 was used to measure the oilmembrane formation rate.

1) Evaluation conditions• Test machine: NTN-made fluid dynamic bearing unit
test machine• Measuring item: Oil membrane formation rate; torque• Rotational speed: 2,000 min-1• Lubricant: Lubricant for fluid dynamic bearing unit
(Used in mass production of HDD fluid dynamic
bearings)• The oil membrane formation rate is measured bymeans of the electric conduction between the main
shaft and the housing.

Table 1 Analysis condition
2000 min-1

Rotational speed

25˚C

Temperature of atmosphere

6

Load capacity N

5
4
3

Axial load application jig

Housing

Main shaft

Test bearing

2
1
0

10

0

20

30

40

gap result
μmFig. 7 Radial
Analysis
RotationFig. 7 Analysis result

3. Evaluation of performance

MagnetRadial load application jig

3.1 Working conditions of a ventilator bearing

Fig. 8 Schematic view of test rig

Table 2 shows common functional requirements forventilator bearings.
Since a fluid dynamic bearing rotates on oil film
without coming into contact, the service life of abearing depends on the evaporation of lubricant. The
lubricant used this time is identical to the one used in
mass-produced HDD fluid dynamic bearings, with anextremely low evaporation rate of less than 1 wt% at
80˚C after 50,000 hours in service.
It is ascertained experimentally that the evaporation
speed roughly doubles due to a temperature rise of
10˚C. Accordingly, the service life of the bearing

2) Results of evaluation

15 years

Ambient temperature-10–60˚C

Ambient humidity

0–100%

Rotational speed

2000 min-1

4.0

3000

2000

1000

0

Torque mN・m

4000

3.0

2.0

1.0

0

100

Oil membrane formation rate %Service life

5.0

min-1

Table 2 Functional requirements

5000

Rotational speed

Fig. 9 shows that the oil membrane formation rate
reaches 100% at an early stage of rotation with oilmembrane formed completely and that the bearing
rotates without contact.

Oil membrane formation rate
8060

Rotational speed
40

20Torque2min

0Fig. 9 Evaluation result of fluid dynamic bearing unit

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NTN TECHNICAL REVIEW No.78（2010）

3.3 Temperature characteristic test

1) Measuring conditions

The variation of rotational speed resulting fromtemperature variation was investigated.

• Test machine: Motor installed on the actual device(not equipped with a fan)
• Rotational speed: About 3,000 min-1(Applied voltage: AC 100 V)• Temperature of atmosphere: 25˚C
• Motor position: Shaft horizontal• Lubricant: Lubricant for fluid dynamic bearing unit
• Distance to the microphone: 100 mm• Measuring environment: Anechoic room (Groundnoise: 13.5 dBA)

1) Evaluation conditions• Test machine: Motor installed on the actual device• Applied voltage: AC 100 V• Measuring item: Rotational speed• Temperature of atmosphere: -10–60˚C• Lubricant: Lubricant for fluid dynamic bearing unit

2) Results of evaluation
In a fluid dynamic bearing, as shown in Fig. 10, theincrease in lubricant viscosity in the low temperature
range causes the rotational speed to lower; however,the customer requirement of 2,000 min-1 wasachieved.

2) Results of evaluation
As shown in Fig. 12, the fluid dynamic bearing unit
produces less noise than the current roller bearing.
The audible frequency range is commonly said tobe from 20 Hz to 20 kHz, and the frequencies that areuncomfortable to human ears are said to be near 3
kHz. The fluid dynamic bearing exhibited lower values
in the entire range of interest. A difference in noise
value of 14.5 dBA shows that the fluid dynamic
bearing produces about one fifth of the noise thecurrent bearing does. Since the fluid dynamic bearing
unit is driven without contact, the sound produced by
the bearing is extremely small.

Motor rotational speed min-1

5000
4000
3000
2000
1000

0
-20

0

20

40

60

80

Noise value dBA

Temperature of atmosphere ˚CFig. 10 Evaluation result as electric motor

3.4 Sound measurement on the actual device
Noise produced by the newly developed productwas compared with that produced by roller bearings.
Fig. 11 shows the method of measuring noise. The
method of measuring noise from a ventilator is
specified by JIS (Noise test: JIS C9603); themeasurement is conducted using the noise meter
specified by JIS C1502 (normal noise meter).
For the reason of comparing noise between theroller bearing and the fluid dynamic bearing unit, themeasurement was conducted with the fan removed
and hence without the effect of wind noise. In addition,
since noise values were small, the measurement was
conducted with the distance to the microphone
shortened to 100 mm, one tenth of the specified value.
Ventilator

1m

50
40
30

Roller bearing
Noise value overall:
38.4 dBA

20
10
0
-10
Fluid dynamic
bearing unit
Noise value overall:
23.9 dBA

10

Frequency

15

20

kHz

Fig. 12 Result of noise measurement

3.5 Endurance test on the actual device
An endurance test was conducted using the motor
on the actual device. This test was conducted to makean evaluation at 80˚C in contrast to 60˚C, the upper
limit of the required temperature of the atmosphere.
Accordingly, this test is one with a four-timeacceleration compared with the experimental resultwith a double evaporation rate for an increase intemperature of 10˚C, described in subsection 3.1.

Microphone

1) Endurance test conditions• Test machine: Motor installed on the actual device(not equipped with a fan)
• Rotational speed: About 3,000 min-1(Applied voltage: AC 100 V)• Temperature of atmosphere: 80˚C

1m

Microphone
1m

Microphone

Fig. 11 Method of noise measurement-120-

Fluid Dynamic Bearing Unit for the Home Ventilation Fan• Motor position: Shaft horizontal• Lubricant: Lubricant for fluid dynamic bearing unit

oil layer and that in the water layer.

2) Results of evaluation• Mixing ratio: Test oil: 75 g; water: 25 g
• Test temperature: 93˚C
• Catalyst material: Copper• Stirring speed: 6 min-1• Test duration: 48 hours

1) Test conditions

As shown in Figs. 13 and 14, both the rotational
speed and the vibration value do not exhibit significant
changes after 1,000 hours in operation, showing asatisfactory result. The test will continue for two years
to check functional aspects and to determine theevaporation of the lubricant on the basis of thedecrease in the weight.

As shown in Table 3, the lubricant for the fluid
dynamic bearing unit exhibits a smaller acid value
increment and a smaller water layer acid value thanthose the base oil for ester-based grease for roller
bearings does, being resistant to hydrolysis andexcelling in humidity resistance in terms of thecharacteristics of the lubricant itself.

Roller bearing
Fluid dynamic
bearing unit

4000

Table 3 Result of hydrolytic reactivity

3000

Item to be studied
2000

Before test

Rotational speed

min-1

5000

2) Test results

1000

Dynamic viscosity 40˚C

［mm2/s］
Acid value［mgKOH/g］
Dynamic viscosity 40˚C

［mm2/s］

0
200

400

600

800

1000

1200

After test

0

Operation time hFig. 13 Result of rotational stability

Acid value［mgKOH/g］
Acid value increment［mgKOH/g］
Water layer acid value［mgKOH/g］

2.5

Lubricant for fluid
dynamic bearing unit

25.44

12.3

0.71

0.05

25.51

12.24

1.06

0.09

0.35

0.04

6.23

2.72

Roller bearing
Fluid dynamic
bearing unit

2.0

4. Conclusionm/s2
Vibration value

Base oil for esterbased grease

We have developed a fluid dynamic bearing unit for
home-use ventilators that capitalizes on the freedom
from noise and high reliability, features of a fluid
dynamic bearing, and that has the same dimensions as
those of the current roller bearings and can be used in
the same manner as those current roller bearings.
The newly developed product can be extended toother applications needing freedom from noise and highreliability than ventilators; we will extend it into various
series to apply it to other product models andapplications.

1.5

1.0

0.5

0.0
0

200

400

600

800

1000

1200

Operation time hFig. 14 Result of vibration stability (Radial direction)

3.6 Hydrolysis test on lubricant
Photo of author

Unlike HDDs, ventilators are expected to be used inhigh-humidity environments. In consideration of thesecircumstances, a comparative test on hydrolysis was
conducted between the base oil of ester-based grease
for roller bearings, used in ventilators, and lubricant for
fluid dynamic bearing units.
The test method was based on ASTM D 2619: thetest oil and water were mixed, and the mixture wasstirred at a high temperature and in the presence of
catalyst to observe the change in the acid value in the

Masaharu HORI
Fluid Dynamic Bearing Unit Division
Engineering Dept.

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NTN TECHNICAL REVIEW No.78（2010）[ Technical Paper ]Surgical Support System
for Cerebral Aneurysm Coil Embolization
Yoshitaka NAGANO*
Yukihiro NISHIO*
Noriaki MATSUBARA**
Shigeru MIYACHI**
Hideo FUJIMOTO***
Endovascular coil embolization is becoming a standard treatment option forpeople with cerebral aneurysms. The embolization method is the filling of ananeurysm with embolization coils by the combined manipulation of a
microcatheter and a coil delivery wire. The manipulation requires the skilledcollaboration of two surgeons. Thus, we have developed a new surgical support system that can be operated byonly one surgeon. Basic verification for this system was conducted using a silicone dummy aneurysm. The
results confirmed that the delivery of the wire at a constant velocity is an effective method for decreasing coilinsertion force, and that only one surgeon can perform the same coil embolization procedure. In this paper, we
report on the system features and the results of verification.

as little force as possible is required. The doctor
moves the tip of the microcatheter to a site into which
the coils can be easily inserted while carefully
monitoring the insertion force of the wire. Positioningof the tip of the microcatheter is influenced by pushingand pulling of the coils, and therefore fine adjustmentof the force depending on the movement of the coils is
required to operate the microcatheter. On the other
hand, a guiding catheter is used to guide a
microcatheter, which has a soft tip, near an arterial
aneurysm.

1. Introduction
In the medical field, treatments with minimized sizeof surgery scars to reduce the physical demands ofpatients are widespread. A typical example of suchtreatments is a therapy using a catheter. This
procedure involves inserting a thin tube called acatheter into a duct of the human body and treating anaffected part from inside the duct. Recently, thecatheter therapy is also used in the field of treating
thin and tortuous cerebral blood vessels, and has
been applied to treatments for preventing the ruptureof cerebral aneurysm, which is a cause ofsubarachnoid hemorrhage. Coil embolization shown in
Fig. 1 is a procedure that involves blocking the bloodflow within the aneurysm by filling the cerebral
aneurysm with platinum coils at a predetermined
density to purposely form a blood clot and prevent therupture of the aneurysm 1), 2).
A microcatheter is inserted from the femoral arteryin the groin to the cerebral aneurysm, and then adelivery wire with detachable Note 1) coils connected
at its tip is inserted and maneuvered through themicrocatheter from outside the body. Since theaneurysm has thin blood vessel walls and is easily
ruptured, a maneuver which can insert the coils with

Patient

Y connector

Cerebral aneurysm
Platinum coil
Micro catheter

GuidingcatheterMicrocatheter

Deliverywire

Cerebral blood vessel (parent artery)Fig. 1 Endovascular coil embolization of cerebral
aneurysm
Note 1) Electric systems which detach the coils from the wireby energizing the delivery wire are widely used.

*** New Product Development R&D Center Mechatronics Research Dept.
*** Department of Neurosurgery Nagoya university, Graduate School of Medicine.
*** Department of Engineering, Nagoya Institute of Technology Graduate School-122-

Surgical Support System for Cerebral Aneurysm Coil Embolization

The procedures of coil embolization are divided into
two types: one in which two doctors operate themicrocatheter and the wire, respectively, with both oftheir hands; and the other in which one doctor
operates the microcatheter by the left hand and thedelivery wire with the right hand 3), 4). In the latter case,a highly skilled maneuver for simultaneously operating
both the catheter and the wire with the left and righthands, respectively, is required. It is therefore often
the case that two doctors are involved in thetreatment. Mutual understanding between the doctors
about “the sense of the wire insertion force felt by thehand” and “pushing and pulling operation of thecatheter” is necessary.
In the past, NTN has developed a sensing systemfor cerebral aneurysm therapy which measures anddisplays the manual insertion force of the delivery wire
working in collaboration with Nagoya Institute ofTechnology and Nagoya University 5), 6). As shown in
Fig. 2, the sensing system consists of an optical force
sensor integrated into a medical device called a Y
connector for passing the delivery wire through thecatheter. Converting the manual insertion force of thewire into sound pitch and presenting the force audibly
provides the circumstance that allows all the medical
staff in the operating room to share the sameunderstanding of the provided insertion force 7) to 9).
By extending the function of this sensing system,
we developed a cerebral aneurysm coil embolization
support device which allows a single doctor tosimultaneously and coordinately operate the catheter
and the wire as in the two-doctor procedure 10). In thisreport, we will discuss the coordinated manipulation of
the catheter and the coil, the construction of thedeveloped support device, and the verification results
confirming the advantages of this device.

2. Coordinated manipulation ofcatheter and coils
If the contraction of the volume of a coil lump, called
coil compaction 2), occurs after coil embolization, bloodflows back into the aneurysm and the effect inpreventing a rupture is lost. In order to prevent coilcompaction, the coils need to be inserted into theaneurysm uniformly and densely 11). The doctor inserts
the coils into a site which is estimated not to bedensely filled with the coils so that an excessive force
is not exerted on the aneurysm.

2.1 Painting phenomenon
Coil embolization is performed by visually observing
the state of the aneurysm and the coils with afluoroscope. To estimate the sites which are notdensely filled with the coils, a head-swinging
phenomenon 9) called “painting” Note 2) generated at the
tip of the microcatheter shown in Fig. 3 (a) is used.
This phenomenon is generated by the interaction
between the coils inserted and the inner walls of theaneurysm, in the following manner:(1) Parts of the coils are restrained by the friction
between the inner walls of the aneurysm and thecoils which have been already inserted.
(2) When the coils are inserted further in this state,
they start to undergo pressure deformation with therestrained parts as the fulcrums, and the tip of the
microcatheter starts to swing (painting).
(3) The coils then buckle, and the tip of the
microcatheter is released from the restraint by thecoils and returns to its original position in thepainting movement.
In a space which is densely packed with the coils,
the tip of the microcatheter is restrained by the coils,
and painting does not occur. In this state, an
excessive force is required to insert the coils, andforced insertion may lead to the rupture of theaneurysm.

Saline inlet
Catheterconnecting part

Delivery wire inlet(hemostatic valve)

2.2 Maneuver by two doctors
Two doctors who operate the microcatheter and thedelivery wire, respectively, communicate throughconversations while checking the fluoroscopic images,
and as shown in Fig. 3 (b), insert the coils by thefollowing procedure:(1) During the coil insertion, when no painting isobserved and resistance is felt during insertion, thedoctor who operates the wire temporarily stops theinsertion of the delivery wire.

Insertion force
sensor

Unit: mmFig. 2 Sensing system for cerebral aneurysm treatment

Note 2) Named so since the movement of the catheter
resembles brush strokes.

-123-

NTN TECHNICAL REVIEW No.78（2010）(2) The doctor who operates the catheter retracts thetip of the microcatheter to such a degree that it is
not completely removed from the aneurysm, andthen inserts the catheter back into the aneurysm.
(3) When the tip of the catheter moves to a site which
is not densely packed with the coils, the doctor
who operates the wire can insert the wire (coils)
with a small insertion force.

3. Developed support device
3.1 Overall construction

Arterial aneurysm
Micro- Parent artery
Coils
catheter

Painting
Friction

Pressure(a) Behavior of coil and microcatheter tip

Retract microcatheter

Advance microcatheter

Advancedelivery wire

Site densely
packed with
coils

Arterial aneurysm

Coils

Fig. 4 shows the overall construction of this device.
In this construction, as in a conventional system, thedoctor operates the microcatheter with both hands
and operates the delivery wire by a foot switch. The
wire is delivered at a constant speed by the wiredelivery device. The insertion force of the wire is
detected by the cerebral aneurysm sensing system,
converted into sound pitch information, and istransmitted to the doctor. The delivery speed isadjusted by a switch. This construction allows
performance of coil embolization without a doctor who
operates the wire.
Fig. 5 shows the appearance of the sensing systemand the wire delivery device. To allow the doctor tocheck the state of the patient at all times, thismechanism is small in size for use on the surgery
table. It also has the function to allow the operator toopen the top cover and operate the wire directly by thehand. This function characterizes the system in that it
can be used not only to move the delivery wire before
insertion of the coils and recover the delivery wire after
the coils are detached, but can be also switched to aconventional treatment mode during the operation 12).

Doctor who
Doctor who
operates the catheter operates the wire
Speaker(audible presentation of insertion force)(b) Collaborative manipulation of microcatheter and delivery wire

Patient

Fig. 3 Coil insertion by two surgeon

Microcatheter

Insertion force of wire
Switch for changing
insertion speed
Sensing system
Wire delivery
device2.3 Operation of microcatheter(Two-hand operation)

The doctor who operates the microcatheter
conducts fine positioning operation of the
microcatheter by predicting the degree of bending of
the microcatheter in the blood vessel, so that the tip ofthe microcatheter is not removed from the aneurysm
during insertion of the coils. Even in the case where
the tip is not moved, the pushing and pulling
operations of the microcatheter are necessary. For
example, the microcatheter is kept pushed so that the
microcatheter is not pushed back during insertion of
the coils.
Since the insertion and removal of the microcatheter
involve opening and closing of the hemostatic valve(see Fig. 2) of the Y connector, it can be smoothly
performed if the hemostatic valve is operated by theleft hand while retaining the Y connector, and themicrocatheter is operated by the right hand.

Guidingcatheter

Delivery wire

Support device
Foot switch(operation of delivery wire)

Fig. 4 Structure of the surgical support system

Top cover(when open)

Deliverywire

Sensing system

Microcatheter
Wire
deliverydeviceUnit: mmFig. 5 Photo of the sensing system and the delivery wire
driving unit-124-Surgical Support System for Cerebral Aneurysm Coil Embolization

3.2 Wire delivery device

aneurysm was formed in the shape of a terminal witha diameter of 4 mm, using a transparent silicone to
impart appropriate plasticity and enable observation of
the coils inside the aneurysm. The number of the coils
inserted was two so that the packing fraction was 20%
or higher 13), which does not cause coil compaction.

Fig. 6 shows the construction of the developed wiredelivery device. A direct-current motor is connected toa driving roller via reduction gears, and drives thedelivery wire in the longitudinal direction. To prevent
slipping and horizontal shift of a super-thin delivery
wire (diameter: about 0.3 mm), a guide rail is provided
at the center of the driving roller. A follower roller is
attached to the top cover via a thrust spring, and thedelivery wire is held between the driving roller and thefollower roller by fixing the top cover onto the bodypart. The maximum insertion speed of the delivery
wire is 2.0 mm/s based on measurement results of theinsertion speed of manual wire delivery by a doctor.

4.1 Speed and insertion force
Table 1 shows the results of coil insertion wheninserting the delivery wire at a constant speed. The
insertion was conducted with the microcatheter fixed.
The coil used was a two-dimensional type whichdevelops into a screw-like shape when it comes out of
the catheter tip. The first coil has a development
diameter Note 3) of 4 mm, the same as the diameter ofthe simulated aneurysm, and the second coil has asmaller diameter of 3 mm. The overall length of thefirst coil was 80 mm, while two different lengths of 60
mm (packing fraction: 21%) and 80 mm (24%) wereselected for the second coil for comparison of packing
fraction.
The coils were inserted at different speeds varied by
0.5 mm/s. As a result, insertion of the first coil, due toa packing fraction as low as 12%, was successful at
the maximum speed evaluated, i.e., 2.0 mm/s. On theother hand, the second coil was not inserted into theaneurysm and pushed out to the parent artery side
when driven at a high speed from 1.0 mm/s to 1.5mm/s or higher. The higher the packing fraction, thelower the upper limit of the speed which allows
successful insertion.
Fig. 8 (a) shows the insertion force trend of thedelivery wire when the first coil was used. Insertion
force is clearly less for the insertion at a constant
speed of 1.0 mm/s shown in red than for theintermittent insertion by the manual operation of thedoctor shown in blue, the former being about half thelatter. The maximum force of the former is also 1/2 orTop cover (opening and closing portion)
Delivery wire
Thrust spring

Follower roller

Reduction
Direct-current
gears
motor
Driving roller

Guide rail

Fig. 6 Structure of the delivery wire driving unit

4. Evaluation of continuous insertion
Fig. 7 shows an experimental evaluation system. A
simulated aneurysm was placed in water, and water
was injected into a guiding catheter, a microcatheter,a Y connector, and a sensing system. A delivery wire
inserted into the catheter was then advanced, and thestate of the coil embolus was evaluated by observing
it with a microscope. The overall length of the catheter
is 1.5 m, and while overall length of the delivery wire is
1.75 m (excluding the coil portions). The simulated

Y connector
Microscope
GuidingcatheterTable 1 The delivery wire speeds for completed filling coil insertion
Number of
Coil specification Volume packing Wire speed forcoils inserted Shape-Diameter-Length fraction of coils completed fillingcoil insertion[Coil No.][mm][%]
[mm/s]Y connector(sensing system)

Microcatheter

Silicone simulated aneurysm(diameter: 4 mm)

Support device

2D-4-80120.5〜2.0

2D-3-60

21

0.5〜1.02

2D-3-80

24

0.5

Water

Note 3) The coils return (develop) to their original twodimensional (spiral) or three-dimensional (basket-like)shapes once they are out of the catheter tip. The coildiameter at this time is referred to as the development
diameter, and the development diameter of the coils inserted
is selected depending on the diameter of the aneurysm.

Water

Coils

1
2

Deliverywire

Fig. 7 Evaluation system for the surgical support system-125-

NTN TECHNICAL REVIEW No.78（2010）

and the coil is restrained by the wall of the aneurysm.
In this rigid state, the insertion force increases incorrespondence with the insertion of the coils, and thepainting phenomenon occurs at the tip of the catheter.
As shown in Fig. 8 (a), sawtooth wave patterns as inFig. 9 (a) are frequently found with the intermittent
insertion by a doctor. This is presumably because
static friction is exerted when the insertion is paused,
and the coil is restrained by the wall of the aneurysm.
In contrast, in case of 1.0 mm/s, the development
state of the coils often changes as shown in Fig. 9 (d),indicating that the coil is sliding and moving on thewall of the aneurysm. In case of wire insertion at ahigh speed, it is thought that the frictional force is little
so that the coil cannot be restrained by the wall of theaneurysm. In Fig. 9 (b), although a sawtooth wave
increase of the insertion force, which possibly results

less compared with the latter. A decrease in theinsertion force directly leads to a decrease in the riskof a rupture of the aneurysm during insertion of thecoils. Therefore, continuous insertion at a constant
speed is considered useful.

4.2 Insertion force and developed stateFig. 8 (b) shows the developed states of the coils
after being inserted by the manual operation of a
doctor (intermittent insertion) and by the wire delivery
device (continuous insertion). The development by theintermittent insertion of the doctor resembles the stateof the insertion at a low speed of 0.5 mm/s, in which
the coil is twisted. In contrast, when the speed is1.0
mm/s or higher, in which the insertion force became
less, the coil was not twisted, and developed into a
desirable spiral (cylinder) shape. Such developments
can hardly be achieved by human operation. In order
to examine the relationship between the insertion
force and the developed state in more detail,photographs of the developed states of the coils weretaken at one second intervals as shown in Fig. 9. The
coil used was a three-dimensional coil which develops
into a basket shape (development diameter: 4 mm,
overall length: 40 mm). Photographing was started
when about half of the overall length of the coil wasinserted.
In case of 0.5 mm/s, as shown in Fig. 9 (a),sawtooth wave patterns are frequently found in theinsertion force trend over time. It can be confirmed
from Fig. 9 (c) that the developed states of the coils
have not changed. This is presumably because static
friction force is exerted due to low speed insertion,

Insertion force N

0.5

0.2
0.1
0

20

40

60

80

Time sec(a) Low speed continuous insertion (0.5 mm/s)

Insertion force N

0.5

Intermittent insertion by doctor

Insertion force N

Initiation of coil photographing

0.3

0.0

0.5
0.4

0.4

Continuous insertion by delivery
device (insertion speed: 1.0 mm/s)

0.4

0.2
0.1
0.0

0.3

Initiation of coil photographing

0.3

0

20

40

60

80

Time sec(b) High speed continuous insertion (1.0 mm/s)

0.2
0.1
0.0

20

0

40

60

80

Time sec

Time photo/sec(a) Comparison of coil insertion forces

Intermittent
insertion
by doctor

0.5mm/s

1.0mm/s(c) Photos of the deployed 3D coil by low speed coil insertion(0.5mm/s)

2.0mm/sContinuous insertion

Time photo/sec(b) The photos of the deployed 2D coil(d) Photos of the deployed 3D coil by high speed coil insertion(1.0mm/s)

Fig. 8 Intermittent and continuous insertion

Fig. 9 Comparison of insertion speed using 3D coil-126-

Surgical Support System for Cerebral Aneurysm Coil Embolization

5.2 Evaluation of operability

from the exertion of static friction, is found in part, theinsertion force is almost constant at slightly lower than
0.1 N.

We had four doctors (very skilled to 3 to 4 years ofexperience) subjectively evaluate the operability ofthis system. The results are shown below.

5. Evaluation of coordinatedmanipulation by hand and foot(1) Since the insertion force of the coils is audibly
presented by a change in sound pitch, the delivery
wire can be operated while always checking thestate of the coil insertion by fluoroscopic images.
The coils are inserted at a constant speed, and it istherefore easy to predict the development
movement in the aneurysm.
(2) Compared to wire operation, fine insertion andretraction operations of the catheter based on theobservation of painting are comfortable since it is a
two-hand operation as in a conventional system.

5.1 Results of coordinated manipulation
Since the second coil cannot be inserted simply byfeeding the wire at a constant speed, “manual
operation of the catheter” and “foot operation of thedelivery wire” by the doctor were added. The results ofsuch insertions are shown in Table 2.
When the overall length of the coil was 60 mm,
insertion of the coil was successful at all insertion
speeds used in the evaluation (0.5 mm/s to 2.0 mm/s)
by additionally employing the manual operation of thecatheter. On the other hand, when the overall lengthof the coil was 80 mm, the gaps between coils weredifficult to find even when the doctor moved the tip ofthe catheter as appropriate since the packing fraction
was high, and the coils could not be inserted simply by
operating the catheter in some cases.
When there is an indication that the coils arepushed out from the entrance of the arterial aneurysm,
stopping of the delivery wire by the foot switch andpulling back operation were additionally performed,
and gaps between the coils were carefully searched tocomplete the insertion.
As mentioned above, it was confirmed that in astate when the packing fraction of the coils is high, asingle doctor could manipulate the catheter and thedelivery wire coordinately and simultaneously using
this system to insert the coils.

6. Conclusion
In cerebral aneurysm coil embolization, which has
been conventionally performed by two doctors
collaboratively, a new support device which can be
performed only by a single doctor was developed.
This device is small in size for use on the surgery
table, and can switch to a normal operation mode
performed by the hand of the doctor at any time. The
results of the verification conducted using a silicone
simulated aneurysm are shown below.
(1) It was confirmed that the continuous insertion at aconstant speed requires 1/2 or less the maximum
force for inserting the wire than the intermittent
insertion by a doctor.
(2) Since the insertion force of the coils is audibly
presented, the doctor can operate the catheter byhand and the delivery wire by foot simultaneously
while checking the state of coil development byfluoroscopic images as in a conventional system.
(3) Since the delivery wire is inserted at a constant
speed, it is easy to predict the coil development in
the aneurysm.

Table 2 Delivery wire insertion speeds with collaborative
manipulation
Coil specification
ManualShape-Diameter-Length operation of[mm]
microcatheter

2D-3-60

2D-3-80Foot
operation ofdelivery wire

Wire speed forcompleted fillingcoil insertion[mm/s]

No

Advance only

0.5〜1.0

Yes

Advance only

0.5〜2.0

No

Advance only

0.5

Yes

Advance only

0.5

Yes

Advance andRetraction

0.5〜2.0

In the future, the operability of the system in cases
where the shapes of the aneurysms are difficult forcoil insertion and the advantages of this system will beverified in further detail in simulated surgery using
blood vessel models and animals.

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NTN TECHNICAL REVIEW No.78（2010）

References
1) S. Miyachi, Neuroendovascular Treatments, Gendai
Igaku, 52-1 (2004) 73-84.
2) A. Hyodo, the Ordinary Method of Cauterization Inside
Cerebral Aneurysms and GDC Treatment Techniques,in A. Hyodo and S. Nemoto, eds., Endovascular
Surgery for Cerebral Aneurysms using GDCs, Igaku
Shoin, Ltd. (1999) 30-34.
3) S. Miyachi, Setup and Angiography Practitioners andAssistants, J. Yoshida and S. Miyachi, eds.,
Neuroendovascular Treatment Do's & Don'ts 2nd
Edition, Igaku Shoin, Ltd. (2006) 16-20.
4) Y. Ito, Fundamental Techniques for Intraaneurysmal
Embolization of Cerebral Aneurysms, in H. Kikuchi,ed., Collection of Recent Cases of Endovascular
Treatment of Cerebral Aneurysms, Pioneering Medical
Techniques Laboratory , (2005) 49.
5) Y. Nagano, A. Sano, M. Sakaguchi and H. Fujimoto,Development of a Super Fine and Long Linear Sensor
for Physiological Applications—Application for
Cerebral Aneurysm Coil Embolization, Transactions of
the Society of Instrument and Control Engineers, Vol.
44, No. 3 (2008) 278-284.
6) Y. Nagano and H. Fujimoto, Development of a Sensing
System for Cerebral Aneurysm Treatment, NTN
Technical Review No. 76 (2008) 63-67.
7) Y. Nagano, A. Sano, M. Sakaguchi, H. Fujimoto, S.
Miyachi, T. Ohshima, N. Matsubara, O. Hososhima
and J. Yoshida, Development of a Sensing System for
Cerebral Aneurysm Coil Embolization: Evaluation ofInsertion Force Using Simulated Aneurysms Using
Arterial Models and In Vivo Imitations, Journal ofJapan Society of Computer Aided Surgery, Vol. 10,
No. 2 (2008) 147-153.

8) N. Matsubara, S. Miyachi, Y. Nagano, T. Ohshima etal., A novel pressure sensor with an optical system forcoil embolization of intracranial aneurysms, Journal of
Neurosurgery, 111-1 (2009) 41-47.
9) N. Matsubara, S. Miyachi, T. Ohshima, O. Hososhima,
T. Izumi, A. Tsurumi, T. Nishikori, J. Yoshida, Y.
Nagano, A. Sano, M. Sakaguchi and H. Fujimoto,Development of an optical force sensor system for
cerebral aneurysm coil embolization, Journal of
Neuroendovascular Therapy, Vol. 2, No. 2 (25thAnnual Meeting of the Japanese Society for
Neuroendovascular Therapy).
10) Y. Nagano, A. Sano, N. Matsubara, S. Miyachi, Y.
Nishio, O. Hososhima, T. Ohshima, T. Wakabayashi
and H. Fujimoto, Development of a Single Surgeon
Surgery System for Cerebral Aneurysm CoilEmbolization, Journal of Japan Society of Computer
Aided Surgery, Vol. 12, No. 2 (2010) 101-107.
11) N. Sakai, Endovascular Treatment of Unruptured
Cerebral Aneurysms, in H. Kikuchi, ed., Pioneering
Medicine Series 9: Endovascular Treatment for
Neurological Surgeons—Practice and Prospects,
Pioneering Medical Techniques Laboratory , 2001,
173-180.
12) http://www.jscas.org/guideline.htm
13) Y. Kawanabe, A. Sadato, W. Taki and N. Hashimoto,Endo vascular Occlusion of Intracranial Aneurysms
with Guglielmi Detachable Coils, Correlation Between
Coil Packing Density and Coil Compaction, Acta
Neurochir (Wien), 143-5 (2001) 451-455.

Photo of authors

Yoshitaka NAGANO

Yukihiko NISHIO

Noriaki MATSUBARA

Shigeru MIYACHI

Hideo FUJIMOTO

New Product Development
R&D Center
Mechatronics Research Dept.

New Product Development
R&D Center
Mechatronics Research Dept.

Department of Neurosurgery
Nagoya university,
Graduate School of Medicine.

Department of Neurosurgery
Nagoya university,
Graduate School of Medicine.

Department of Engineering,
Nagoya Institute of Technology
Graduate School-128-

NTN TECHNICAL REVIEW No.78（2010）[ Technical Article ]Automatic Repair Technology of Fine Pattern in LCD
Manufacturing Process

Hiroaki OBA*NTN has been working on the development of repair technologies to boost theyield of color filters (CF) and thin film transistors (TFT), which are components ofliquid crystal displays (LCD). Recently, customer demand for greater productivity
in the repair process has increased, and we have developed automatic repair
technologies in response.
This paper presents an overview of the automatic repair function used in an
LCD repair system. As automatic repair functions, automatic defect detection,
repair position calculation, ink color selection and detection of abnormal repairs,for example, are realized by applying image-processing technologies. These
functions improve the efficiency of repair processes and meet customer demands
for high productivity.

automation of defects detection, calculation of repair
positions, selection of ink color for the repair, andrepair abnormalities detection.
In this paper, after a brief description of the liquid
crystal display repair device, the automatic repairfunction NTN developed will be presented.

1. Introduction
As the demand for LCD television sets increase, theupsizing of substrates as characterized by the
adoption by panel manufacturers of the 10th
generation glass substrate with a side over threemeters is in progress, while the need for defect repair
devices contributing to the improvement of production
yield is increasing. NTN has developed and is selling
repair devices for color filters that apply ink to defects
with a repair needle, while other repair devices cut
circuits on TFT substrates with YAG laser to remove
defects. They are implemented in a vast number ofliquid crystal manufacturing processes.
Of these repair devices, the shortening of tact time
and the improvement of repair quality are required. In
addition, requirements for efficiency improvement in
manufacturing processes and labor saving haveincreased recently to reduce panel cost. In responseto these requirements, NTN has engaged in thedevelopment of automatic repair functions that will
provide effective labor saving tools.
In the past, the operator who observed the defectstate displayed on the monitor chose the machining
repair conditions and specified the repair position.
NTN developed an image processing algorithm of ourown suitable to defects to be repaired, and by using
the outcome of this development, NTN realized the

2. Liquid crystal display repair device
Fig. 1 shows the appearance of the liquid crystal
display repair device. This device consists of a chuckmechanism that fastens the substrate, a repair headthat is equipped with a repair function, and an XYZ
stage that moves the repair head to the defectposition.

Fig. 1 Appearance of LCD repair system* Precision Equipment Division Product Engineering Department-129-

NTN TECHNICAL REVIEW No.78（2010）

into the device, the substrate is chucked and aligned,
and the repair head moves to the defect position. The
information on the defect position is acquired from theinspection device while the substrate is being carriedin position.
After the repair head has been moved to the defectposition, the substrate surface is focused on for thedefect to be centered. After this, the defect is detected
to identify the kind of defect (black defects, white
defects, and so forth), and depending on the kind of
defect, the repair measure (laser irradiation or ink
application) is chosen and implemented.
Furthermore, whether repair abnormalities remain
after the repair is checked. When an abnormality isdetected such as a remaining defect in a laserirradiated area or a white defect in an ink-appliedarea, the buzzer is sounded with the automatic repair
work interrupted and the operation is changed over tomanual operation by the operator.
When a substrate is judged to be repaired correctly,
the repair head is moved to the next defect to repeat
identical automatic repair operations, carrying out
repair operations on all defects on the substrate.
Processes needing repair operations in liquid crystal
manufacturing processes are either so-called
upstream ones such as TFT substrates and color filter
substrates or downstream processes in which liquid
crystals sealed into cells or modules are processed.
Depending on the process introduced, the defectkind and the repair methods vary. In the following, thefunction of automatically repairing color filter
substrates and TFT substrates is described.

The repair head is equipped with an optical system
to observe defects on substrates, a laser with which
patterns are cut and defects are removed, and an ink
applying mechanism that applies ink to spots wheredefects were removed.
Special hardware is not necessary to implement the
automatic repair function. Images fit for image
processing are captured with a CCD camera bycontrolling the reflecting light source and thetransmission light source, taken into the computer ofthe repair device and processed arithmetically, andthe repair device is controlled in accordance with theresults of the image processing; these processes
make it possible to allow the automatic repair function
to work.
Fig. 2 shows the general flow of automatic repair
processes. When a substrate to be repaired is broughtStarting repairing
Carrying substrate in
Chucking and aligning the substrate
Moving the repair headto the defect position
Centering
Detecting a defect and determining
the repairing position
Repairing operations(laser irradiation and ink application)

3. Function of automatically repairingcolor filter substrates

Check of repairing abnormalities

For this function, a dedicated image processing
algorithm was developed that allows (1) the defectdetection, (2) the repair position calculation, (3) the
correction ink color determination, and (4) the
detection of abnormal repair to be processed
automatically.
Defects occurring in the color filter are white defects
and black defects shown in Fig. 3 (a). A white defectis a result of a missing part of the membrane formedon the substrate and the original RGB colors are
missing. A black defect is one characterized by the
mixture of colors as a result of the black matrixes
membranes swelling out or RGB colors due todefective shapes and other causes. In repairing these
defects, the defect is removed by means of laser inthe first place regardless of the defect kind, and after
this, the part that the defect has been removed isrepaired by applying ink on it. In addition, theDetecting anabnormality

No

Yes
Calling the operator
Manual operation by the operator
Yes

Next defectNo
Carrying substrate out
End of repair

Fig. 2 Flow chart of repair-130-Automatic Repair Technology of Fine Pattern in LCD Manufacturing Process

3.2 Method of determining repair positions

hardening treatment of ink by means of an UV lightsource or the drying treatment by means of an IR lightsource is conducted as an after-treatment.
The above automated treatment not only allows aseries of operations to be carried out as a batch
treatment automatically in sequence, but also allows
one to expect that the finish and quality of operations
are equivalent to those of operations carried out by anoperator. In other words, this means that it is required
to automatically perform the defects recognition andthe indication of repair positions, and the repair ink
color selection that have been done visually by theoperator, and that it is also required to detect
abnormalities occurring during the course of automatic
repairing to assure that repair has been done properly.

As described above, in repairing the color filter, adefect is irradiated with laser to remove it first, andthen the area that a defect is removed is covered withink for repairing. For this purpose, it is necessary to
determine the laser irradiation position and the ink
application position automatically. This function uses
the information on the position and size of the defectobtained through the processing of defect detection todetermine these positions.
Fig. 4 shows an example of determining the repair
position. When the defect is smaller than the circle onwhich ink is applied as shown in Fig. 4 (a), the repair
position is determined so that the ink application
center aligns with the defect center. When the defectis larger as shown in Fig. 4 (b), repairing work isdivided into several operations to be conducted. The
number of repairing operations is determined by thedefect size and the size of the circle on which ink isapplied.
Actual automatic repair requires a treatment that isequivalent to the repair quality achieved by anoperator and is more efficient. For this reason, thealgorithm contains ordered steps to minimize damage
to the normal region near a defect and the number oftimes of repair.

3.1 Detection of defects
Defects are detected by comparing an image
containing a defect with a normal image condition
registered in advance and extracting parts that do notmatch the latter image.
With Fig. 3 (a) showing an image containing a
defect and Fig. 3 (b) one showing the normal
condition, they are compared with each other withregard to density; the result of the comparison isshown in Fig. 3 (c). In Fig. 3 (c), areas in which thedensity agrees between two images are shown ingray, and of areas in which the agreement of density
is not seen, areas darker than the normal image isshown in black and areas lighter than the normal
image are in white. This function recognizes areas inblack as black defects (candidates) and areas in white
as white defects (candidates), and area with thedifference between black and white larger than thethreshold value are detected as defects in the end.

Circle to which
ink is applied
Laser irradiation
scope(a) When the defect is smaller than the diameterof the circle to which ink is appliedWhite
defect(b) When the defect is larger than the diameterof the circle to which ink is appliedBlack
defectFig. 4 Calculation example of repair positions(a) Image of a defect(b) Image of normal
condition(c) Result of density comparison
Fig. 3 Example of defects on color filter panel and defectdetection-131-

NTN TECHNICAL REVIEW No.78（2010）

3.3 Determination of the repair ink color

repeated laser irradiation for removal as shown in Fig.
6 (b). However, when laser is used for a long time, adecrease in power may prevent removing defects;
therefore, if a defect cannot be removed after apredetermined number of removal operations, it isjudged as abnormal.

In repairing a defect, ink with the normal color area
must be applied to the defect; this requires
determining the ink color to be applied.
In this treatment, the color information on the same
region in the normal image as that in the extracted
defect is obtained by means of image processing, andthe color information obtained is compared with thatwhich is associated with each ink beforehand tochoose an ink having the closest color information.
Using the example of the defect image and thenormal image, how to determine the repair ink color is
explained below. Suppose, for example, that pieces ofcolor information cR, cG, cB, and cBlk are assigned tofour kinds of repair ink R, G, B, and Black(abbreviated as Blk in the following). If, in Fig. 5, thepiece of color information on position (b) in the normal
image, which corresponds to position (a) in the defectimage, is judged as the closest to the color information
piece cG, ink G that has the color information piececG is chosen as the repair ink.

（a）

（b）

Image of a defect

Image of normal condition

3.4.2 Inspection after ink application
After the application of ink, the presence of whitepatches that are not filled with ink, as shown in Fig. 7(a), and that of areas on which a large quantity of ink
is dropped, as shown in Fig. (b), are detected. Whenthe viscosity of ink changes or when the needle
cleaning is not enough, such phenomena occur rarely.
As in the defects removal by means of laser
irradiation, the possibility is high that a desired result
is not obtained after the repetition of operations, andsuch phenomena are judged as abnormal.

Irradiation domain(a) Remaining defects after laser irradiation

Defect of interest

Fig. 5 Ink color selection for repair

First time

Second time(b) Case in which laser irradiation is repeatedFig. 6 Check after laser cutting

3.4 Detection of repair abnormalities
In repair operations carried out by an operator
manually, he or she inspects images visually from time
to time in the course of operation to check to see if
repair operations are carried out properly. For
instance, he or she proceeds with his or her
operations visually checking to see if defects remain
not removed after laser irradiation and how a defecthas been filled with ink after ink application.
This function carries out inspection based on image
processing after each automatic laser irradiation andink application to ensure that repair treatment has
been performed properly.

Before application

After application(a) Presence of white patches after application
Target size of application

3.4.1 Inspection after laser irradiation

Before application

After laser irradiation, the irradiated domain is
inspected to make sure that defects are not remaining.
Fig. 6 (a) shows an example of remaining defects.
Defects such as foreign objects with an elevation may
not be removed with single laser irradiation, needing

Dropped ink

After application(b) Dropped ink
Fig. 7 Check after ink pasting-132-

Automatic Repair Technology of Fine Pattern in LCD Manufacturing Processby the inspection device on the previous process;(2) The defect position displayed on the monitor is
detected by a method suitable for the defect inquestion such as the method described in
Subsection 3.1 and the comparison of repeatedadjoining patterns, and is moved to the center ofthe screen again (Fig. 8).
(3) The scope of laser irradiation is changed to thespecified size or the one equivalent to the defectand the defect is irradiated by laser. The machining
conditions such as laser power and oscillation
frequency are registered in the device in advance.
(4) The result of repair is checked by the method
equivalent to that described in Subsection 3.4.1 orcomparing images before irradiation with ones
after irradiation; if no defects are left, the workpieceis judged as normal. If part of defects is foundremaining, they are irradiated by laser again. If
defects remain after the repetition of the specified
times of laser irradiation, the workpiece is judged
as abnormal.

4. Function of automatically repairing
TFT substrates
Major repair operations on TFT substrates include
removing defects by means of laser and cutting part ofelectrical circuits formed on the substrate. Repair
operations include finding breaks in continuous
electrical wires on a substrate and also applying metal
paste on them with an application needle to restore
continuity. Repair operations using laser are divided
roughly into the two kinds that follow:(1) Carrying out repair operations predetermined asneeded by defects, such as removing a defect andcutting part of a pattern; and(2) The operator confirms a defect visually anddetermines the detail of the repair operation.
Operations falling under category (1) can be
automated by combining them with image processing.
The repair head is moved to the defect position, andthe accurate defect position is detected by image
processing, and then the defect is moved to the centerof the laser irradiation scope. After this, the result of
the repair operation by laser irradiation is checkedusing image processing.
Operations falling under category (2) areconsiderably complex. Even if the defect position canbe identified by using image processing, repair is notalways carried out at that position. For this reason, theoperator makes an assessment of the defect statevisually and selects the repair method. To address anincrease in the time needed for repair operations, afunction is needed for the device to carry out
automatic repair operations on a defect separatelyfrom the operator while he or she is defining therepair details operations for another defect.
In repair operations using an application needle torestore breaks in a group of continuous electricalwires, on the other hand, the inspection device on theprevious process is not capable of identifying whereon continuous wiring a break has occurred. To
address this problem, the repair device is required tobe equipped with a function to identify where a breakhas occurred.
For this function, too, the algorithm has beendeveloped with the aim of minimizing operations
carried out by the operator so that this function may
contribute to reducing repair operation time and labor.
In addition to flat panel displays, this inspection
algorithm can be applied to uses aiming at trimming.

Defect(a) After the shift of the defect position
Scope of laser
irradiation(b) The defect is moved to the center of the monitor
Fig. 8 Centering of defect

4.2 Automatic cutting of a specified position
The steps of automatic repair operations by which a
specified position on a pattern is irradiated with laser
to form a redundant circuit and similar purposes areshown below:(1) The head is moved to the defect position detectedby the inspection device on the previous process;(2) The region to be cut by means of laser irradiation
is detected from among the patterns displayed on
the monitor by image processing such as patternmatching, and moved to the center of the screen

4.1 Automatic removal of defects by means of laser
The following are automatic repair steps to remove
defects such as foreign objects adhering to a pattern.
(1) The head is moved to the defect position detected-133-

NTN TECHNICAL REVIEW No.78（2010）of an automatic repair function.
Fig. 11 shows how this automatic repair function
follows a defect. Tracking starts from screen A. The
repair device acquires the number of the line marked
with a black circle in screen A. The coordinates of theblack circle are calculated from the pieces of design
information such as the line number and line-to-line
pitches. The design information is registered in thedevice in advance.
With the head starting from A and moving fromstage to stage in order of A, B, and C, image data iscaptured in sequence, and the defect is detected after
the binarization processing and comparison with theadjoining patterns. When a defect is detected, thestage is stopped there and repair operations suitable
to the defect are conducted automatically. Fig. 11
shows a straight-line-like pattern; however, tracking
can also be performed on a pattern that twists
midway.
When a defect is a break in wiring, metal paste isapplied to the broken part with an application needle
to recover continuity. When a defect is a short circuit,
it is irradiated with laser to be removed for repair.

as shown in Fig.9.
(3) The scope of laser irradiation is changed to thespecified size and the workpiece is irradiated withlaser, the machining conditions such as laser
power and oscillation frequency are registered in
the device in advance.
(4) The result of repair is checked by the method
equivalent to that described in Subsection 3.4.1 orcomparing images before irradiation with ones
after irradiation; if no defects are left, the workpieceis judged as normal. If part of defects is foundremaining, they are irradiated by laser again. If
defects remain after the repetition of the specified
times of laser irradiation, the workpiece is judged
as abnormal.
Region to be cut(a) After the shift of the defect position
WiringScope of lase
irradiation
Wire break

Short circuit(b) The defect is moved to the center of the monitor
Fig. 10 Defect example of line patternFig. 9 Centering of cutting target

4.3 Automatic defect tracking and repair ofcontinuous wiring

A

Automatic repair operations are applicable also to abreak and a short-circuited part of continuous wiring
shown in Fig. 10.
These defects are detected by electric inspection,
but the inspection device in the previous process
gives the information on the number of the wire inwhich a defect exists, not the information on theabnormal point coordinates.
For this reason, in manual repair operations by theoperator, the repair head was moved to the start of thewiring, from which the operator moved the repair headalong the wiring with a tool such as a joystick while
following the head visually to identify a default.
However, this method took a great many hours to
identify a defect, which necessitated the development

B

C

Fig. 11 Defect search-134-Automatic Repair Technology of Fine Pattern in LCD Manufacturing Process

5. Conclusion
This paper presented the automatic function of theliquid crystal display repair device.
The flat panel display market will continue widening,with the screen becoming larger and finer. This will, it
is anticipated, require repair devices to be capable ofrepairing more defects efficiently and with goodquality, while requirements for shorter tact times andlabor saving becoming still stronger.
Aiming at the development of repair devices
capable of responding to new requirements frommanufacturing processes and contributing to theimprovement of the yield of flat panel displays, we willcontinue making an effort to enhance functions of therepair device.

References¡M. Saruta, Sagasaka , High Performance Repair
Equipment and its Underlying Technologies, NTN
Technical Review No. 64.
¡S. Shimizu and H. Ohba, Technology for the Detection
and Repair of Micro-Defects Occurring in Transparent
Thin-Film on LCD Panels, NTN Technical Review No.
68 (2000) 81-85.
¡M. Saruta, Multi-Repair System for Color Filters, NTN
Technical Review No. 72 (2004) 52-55.
¡A. Yamanaka and A. Matsushima, Development of a
Linear Changer with a Built-in Pasting Unit for a Flat
Panel Defect Repair System, NTN Technical Review
No. 76 (2008) 138-143.

Photo of author

Hiroaki OBA
Precision Equipment Division
Product Engineering
Department-135-

NTN TECHNICAL REVIEW No. 78（2010）

Presentation of award-winning articles
Awarded: Fiscal 2010 Minister of Education, Culture, Sports, Science and Technology Prize in the Science and Technology Fields

The Commendation for Science and Technology by the Minister of
Education, Culture, Sports, Science and Technology in Development Category
Chikara OHKI
reduce the dimensional change rate over time below thatof conventional long-life products (carbonitridingprocessed bearing steel of the second class).
Fig. 1 shows crystal grains of an FA-treated bearing
and those of a standard bearing, while Fig. 2 shows theresult of the life test under the condition of lubrication withforeign bodies mixed.

1. Introduction
A Fiscal 2010 Minister of Education, Culture, Sports,
Science and Technology Prize was awarded to us for theDevelopment of High-performance Bearings Reinforced bymeans of Grain Refinement (Product name: FA-processedbearing1)). FA-processed bearings are made of steel
material consisting of crystal grains whose size is reducedby means of special heat treatment. The size of the crystal
grains is now less than half the size of conventional crystal
grains used in bearing steel. These bearings have been in
full-scale mass production for five years, currently usedmainly in valve gears of automotive engines.

Prior-ausutenite grains

FA

Conv.

2. BackgroundIn response to the current trend in machine facilities ofweight reduction, downsizing, cost reduction, andresources-saving construction, technology that will allow
roller bearings, made with general-purpose steel, toachieve a long service life under severe working
conditions has been desired. This technology uses highcarbon chromium bearing steel (a general-purpose
bearing steel) undergoing a specific heat treatment
technology; however the performance is equivalent tohigh-performance steel containing rare earth metals. We
developed a heat treatment method by which crystal
grains are made finer while they are kept in granulated
state. The result obtained by this method of heat
treatment was used to develop high-strength, long-life
bearing materials based on the enhancement of solid
solubility through the use of nitrogen, enhancement
through the refinement of crystal grains, andenhancement through the precipitation of carbides andnitrides.

0.05mm

0.05mmFig. 1 Grain sizes of FA treated and conventional
99

Failled percent

80
50

10
5

1
100

101

Life h102

103

Fig. 2 RCF life under contaminated lubrication condition

References
1) C. Ohki, Improving Rolling Contact Fatigue Life of
Bearing Steels through Grain Refinement, SAE
Technical Paper Series (2004) 2004-01-0634.

3. Performance
The development of FA treatment has enabled us topositively increase the bearing life when the bearing is
operating with contaminated lubrication. the failure life
under the condition of lubrication with foreign bodies
mixed in the bearing The life of bearings which undergo
the FA treatment and operate with contaminatedlubrication go beyond that of conventional standard
products (bearing steel of the second class subjected tostandard heat treatment). It also enabled us to raise thestatic rated load, which is theresistance to plastic
deformation . The FA treatment has also enabled us to

Photo of author

Chikara OHKI
Elemental Technology R&D Center-136-

NTN TECHNICAL REVIEW No. 78（2010）

Presentation of award-winning articles
The 25th Annual Meeting of The Japanese Society for Neuroendovascular Therapy JNET Gold Award-winning paper

Development of Optical Force Sensor System
for Cerebral Aneurysm Coil Embolization*
Noriaki MATSUBARA**, Shigeru MIYACHI**, Yoshitaka NAGANO***, Hideo FUJIMOTO**** et al.

1. Introduction

3. Results

Recently, cerebral endovascular treatments using
catheters have been rapidly developing. Such treatments
typically include coil embolization for preventing a
cerebral aneurysm rupture, which is a cause ofsubarachnoid hemorrhage. In embolization, inserting
coils with an excessive force may cause a rupture of theaneurysm,resulting in severe complications, Removal of
the tip of the catheter from the aneurysm may result ininsufficient embolus formation. We will report thedevelopment of a device which measures the insertion
force of the coils using an optical system, present a
device overview and the results of its basic experiments.

As shown in Fig. 2, the maximum insertion force when
the coils were inserted into the simulated aneurysm did
not exceed 0.3 [N], and a bimodal distribution of the force
was observed, indicating that an accurate measurementof the insertion force of the coils was possible.
0.5
Coil A First trial Insertion successful
Coil A Second trial Insertion successful
Coil B First trail insertion unsuccessful
Coil B Second trail insertion unsuccessful

Insertion force N

0.4
0.3
0.2
0.1

2. Procedure
0.0

The coils are connected to the tip of the delivery wire,
and then inserted into the aneurysm by operating thewire. A Y connector and an optical sensor areincorporated into the catheter (Fig. 1) which improves theability of the device to insert the delivery wire. The wirepassing through the catheter is “bent” by the insertion
force. A LED light1) shines on the wire which then createsa shadow; the amount of bending in the shadow is used
to determine the insertion force. The coils were inserted
into silicone simulating the insertion into an aneurysm,
and the insertion force was measured by this device.

Catheterconnecting part

LED

0

20

40

60
Time sec

80

100

120

Fig. 2 Measurement results of insertion force

4. Conclusion
A device for measuring the insertion force of coils by
an optical sensor was developed. Wider clinicalapplication of this device will allow safer and morereliable performance of embolization.

References

Delivery wire inlet

1) Y. Nagano, A. Sano, M. Sakaguchi and H. Fujimoto,Development of a Super Fine and Long Linear Sensor
for Physiological Applications, Vol. 44, No. 3 (2008)278-284.
Optical sensor

Photo of authors (Representative)

Fig. 1 Force sensor****Abstract from JNET (Neuroendovascular Therapy), 2-2 (2008), 113-118****Department of Neurosurgery Nagoya university, Graduate School of Medicine.
****New Product Development R&D Center Mechatronics Research Dept.
****Department of Engineering, Nagoya Institute of Technology Graduate School-137-

Noriaki MATSUBARA

Yoshitaka NAGANO

Department of Neurosurgery
Nagoya university,
Graduate School of Medicine.

New Product Development
R&D Center
Mechatronics Research Dept.

NTN TECHNICAL REVIEW No. 78（2010）

Presentation of award-winning articles
Awarded: 2009 Japanese Society of Tribologists Prize of Promotion

Study of a Long-Life Thrust Needle Roller Bearing
Lubricated with Low -Viscosity Lubricant 1)
Hiroki FUJIWARA
double rows to decrease the slip velocity. The motion of aroller turns into pure rolling on its pitch circle; therefore,
arranging rollers in a double-row configuration causes theslip velocity on the edge section to be reduced. Using
these technologies, we obtained a service life of about
eight times that of a conventional single-row straightbearing as shown in Fig. 2.

1. Introduction
Thrust needle roller bearings are used in compressorsof automotive air conditioners; the mixture fluid ofrefrigerant and refrigerating machine oil is used as
lubrication. Recently the viscosity of this mixed fluid has
been lowered to increase compressor efficiencies,
unfortunately this causes the lubrication performance todramatically decline. To address this situation, theextension of service life under the use of low viscosity oil
is required. In this study, experiments and theoreticalconsiderations were conducted on the mechanism bywhich thrust needle roller bearings lubricated with lowviscosity oil are damaged. The results of the study wereused to extend the life of thrust needle roller bearings.
The results of the study were presented at the World
Tribology Congress 2009, and for the reason of an
outstanding contribution to the improvement of reliability
and energy efficiency of rotary machines, the 2009
Japanese Society of Tribologists Prize of Promotion wasgiven to this study.

Spin
Rolling viscous resistance
Elastic hysteresis loss

Frictional torque N・m

0.4
0.3
0.2
0.1
0

Membrane thickness Membrane thickness
ratio 9 (VG32)
ratio 1 (VG2)Fig. 1 Analysis of frictional torque factor
10

2. Elucidation of the damaging mechanism

L10 life ratio

8

A standard thrust needle roller bearing operating under
low lubricant viscosity causes flaking to occur in the drive
sprocket raceway surface in a short time. Elements of thedrive sprocket such as the heat treatment method, shapeaccuracy, and materials were not found to effect theflaking. Investigation of the flaked raceway surface
revealed that flaking originating in the surface had
occurred on the edge on the inside diameter side of theroller trace. Based on this observation, we estimated thatheating and accompanying plastic flow had caused thedamage. This theory was confirmed by a surface
analysis. In order to identify the heating quantitatively; we
introduced the theory of mixed lubrication to theoreticallyanalyze friction torque due to spin, rolling viscous
resistance, and elastic hysteresis loss. This revealed thatthe dominant factor governing friction torque under the llubrication using low viscosity oil is slip due to spin asshown in Fig. 1.

6
4
2
0

Single row

Double row

Straight roller

Single row

Double row

Crowning roller

Fig. 2 Results of life test

References
1) H. Fujiwara and K. Tamada, Study of a long-life thrustneedle roller bearing lubricated with low viscous
lubricant, Proceedings of WTC 2009 (Kyoto, Japan),
F-232, p.350.

Photo of author

3. Extension of bearing life
Heating depends on frictional force and sliding velocity.
In order to decrease frictional force; crowning wasprovided to decrease contact pressure on the roller edge
part in which slip is large. In addition, rollers were put in

Hiroki FUJIWARA
Elemental Technology R&D Center-138-

Large bearing for front axle
OD: 590 mm

Bearing for planetary reducer
OD: 220 mm

Large bearing for rear axle
OD: 810 mm

NTN bearings support the power of construction equipment-139-

NTN TECHNICAL REVIEW No.78（2010）[ New Products Information ]Intelligent In-wheel Unit
Axle Unit*that NTN proposes for next-generation electric vehicles(*consists of an in-wheel motor, an electric brake and a multi-axis load sensor)

Features

Applications(1) Compact, lightweight in-wheel motor thatcombines a high-speed, high-efficiency,
high-output motor and the world’s fastest
cycloid decelerator with large load capacity;(2) Increased fuel efficiency and driving safety.

¡Electric vehicles, Hybrid vehicles, fuel cellvehicles

In-wheel motor

Construction

Multi-axis load sensor

Electromechanical brakeSensor information
Steering angle
Accelerator opening

In-wheel motor
Electromechanicalbrake
Load sensor

Motor control
Brake control

Wheel speed (motor speed)

Vehiclecontroller

Driving power (motor current)
Braking power
Ground load

Steering angle control
Suspension control

Cornering force-140-New Products Information

Low Friction Hub Bearing
Improves vehicle fuel efficiency by reducing
straight-line driving friction!

Features

Construction(1) Use of a combined-groove curvature for the
raceway
Reduction of rolling friction while
maintaining the same bearing durability as
conventional type(2) A reduced initial preload range
Reduction of rolling friction(3) Use of a low friction seal

Flexion point
Curvature: A

Curvature: B
Contract position
during turning
Contract position
during straight-line
drivingαβFriction

Total friction reduced by 40%

Depending on contact position with steel ball,a different curvature radius is applied.
• During turning: Curvature radius A(Same as conventional type)↓
Same durability as conventional type
• During straight driving: Curvature radius B↓
Torque reduction

Seal

▲15%
▲40%

Rolling

Curvature radius B > Curvature radius A▲25%
Conventional

Applications

New¡Wheel hub bearings for passenger cars
-141-

NTN TECHNICAL REVIEW No.78（2010）

Multi Axis Load Sensor Integrated Hub Bearing
Improves advanced-vehicle safety control
with the world’s most precise load measuring technology!

Features

Construction(1) Able to detect ground-contact loads forthe four wheels separately.
(2) Loads detectable at any time from thevehicle being stopped to traveling at ahigh speed.
(3) Three-axis (Fx, Fy, Fz) loads detectablewith high accuracy (±5% FS).
⇒Precise vehicle control through very
accurate assessment of road conditions(4) Loads measured close to the tires (hubbearings).
⇒Road condition changes detected in a
response time of 0.05 sec.
(conventionally, 0.2 sec.), which
represents a difference of 4.2 m
distance traveled at 100 km/hour.

SensorFx：Driving force, controlling force
Fy：Cornering force
Fz：Vertical force

Fz

Fy

Fx

Applications
Wheel hub bearings for passenger cars
¡Vehicle stability control-142-

New Products Information

Light and High Drive Shaft
for Rear Wheel Drive Cars
Reduced weight and torque increase
the fuel efficiency and ride comfort of luxury cars!

Disc specifications

Stem specifications

Features (compared to conventional products)

Construction (comparison with conventional products)(1) Lightweight: weight reduced by 16%.

¡Balls with reduced diameter and a smaller
and lighter inner ring, outer ring and cage;¡Lighter shaft with a thin-walled, hollow body;¡Optimally angled ball slots helps reduce
frictional resistance occurring on balls inaxially shifting motion, reducing heatgeneration.

(2) Highly efficient: torque loss reduced by
40%.

Applications¡Automotive drive shaft
For use in the rear wheels of RWD and 4WD
automobiles

Cross grooves,tilted in opposite
directions

Outer ring
Balls

Cage
Inner ring

Shaft

New

Driveshaft dedicatedto rear drive vehicles
Conventional

Stem specifications-143-

Disc specifications

NTN TECHNICAL REVIEW No.78（2010）

Low Torque Thrust Needle Roller Bearing
Greater fuel economy, 50% less rotational torquethan conventional products!

Features (comparison with conventional products)

Construction(1) 50% reduction in bearing rotational
torque‘Fully crowned rollers’, arc-shaped in axial direction
at outside cylindrical surface, are used.

(2) Limited bearing temperature rise
Outside-diameterside

Applications

Inside-diameterside¡Transmission for gasoline-fuelled vehicles
and hybrids

Arc-shaped at roller end
Rollers contact the cage at
the center of roller end surface
where circumferential speed ofroller is slow.

Smoothed outside diameter pocket surface of the cage

Thrust needle roller bearing-144-

New Products Information

World’s Smallest Bore Hydrodynamic Bearing
Achieving the world’s smallest bore in a hydrodynamic bearing,
thanks to a special grooving technique !

0.5mm

Plastic(Integral molding)Hydrodynamic
BearingFeatures

Construction(1) World’s smallest bore hydrodynamic
bearing:

φ0.6

0.6 mm shaft diameter using precision
repeatable performance of electrocasting(2) Integration with plastic parts ispossible:
Integral molding is made possible by plastic
injection molding(3) Quieter sound level and high reliability:
Rotational shaft is supported by hydrodynamic
effectMolded plastic
motor housing
Fan

Applications□16

Hydrodynamic
Bearing
φ0.6¡Fan motor for mobile equipment

Shaft

4

-145-

NTN TECHNICAL REVIEW No.78（2010）

Desktop Type Microscopic Coating Applicator
Ideal for the R&D or production of
prototype/small-volume liquid crystal

Features

Construction(1) Repair ink coating function
1) Able to apply liquids such as a drug on
the order of picoliters.
2) Unique coating method using a needle
eliminates nozzle clogging.

Coating needle

Coating liquid container(2) Compact desktop size for easy
operation
1) Footprint occupied by the applicator isabout a 30 cm square.
2) Powered by 100 V AC.
3) Easily operated with a mouse.

Coating liquid<Construction of the coating unit>
Standing by

Coating-liquid
supplyApplications

Applicator actions

In action<Flow of coating actions>R&D or small-volume production of:¡Biological products and new drugs,¡Prototype electronic circuits (ICs), or¡Liquid materials such as conductive paste,¡ Assembly of micro-machines and othermicroscopic components.

-146-

New Products Information

High Performance Large Type Gantry XY Table
Large type precision positioning drive unit attains
light weight, high rigidity, and low cost
Helps shorten setup time and reduce drive motor output !

Features

Construction¡Light weight:
60% compared with conventional type
(Upper-axis beam)

306

384mm

Function and performance responding to
production equipment and inspection
equipment for large type glass substrates
etc. are realized.
0m

15
0m

m

m

¡High rigidity:
120% compared with conventional type
(Upper-axis beam)Appearance of the gantry-type upper-axis steel tube beam

¡Table traveling speed:
120% compared with conventional type(Max. speed: 2,000 mm/s)

Applications¡Making and inspecting flat panels;¡Making solar panels.

-147-