Mc01 03 Recent Trends in Be

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5 Motion & Control No.1—1996

1. Introduction

Rolling bearings have an extremely long life cycle, but

their design details are under continual evolution though

their basic shape remains the same. For example, general

trends in bearings are toward increasing compactness,

lightness of weight, thinness, and reliability. These

developments are in line with the progress of automotive

technology. Other trends are toward ultra-compactness

and higher accuracy along with function sophistication andsize reduction for computers and related products.

Furthermore, new bearings are being developed as

industries related to the environment, semiconductor

manufacture, and space equipment emerge.

NSK, and specifically, its Research and Development

Center, has been engaged energetically both day and night

in research to push forward fundamental technologies that

support the advancement of bearings.

2. Recent Trends in Computer-aidedBearing Analysis

As the computational power of computers has

progressed, the design of rolling bearings relies more and

more on computers. In addition to modern technology to

calculate the load and deflection on the bearings as well as

the structural analysis of the bearing surrounding, EHL

(Elasto-hydrodynamic Lubrication) theory has provided a

clear explanation of the lubrication mechanism at the

contact section of rolling bearings. Since then, the

technology to analyze and to theoretically predict the

performance of rolling bearings has progressed

remarkably.

With the emphasis on compact and lightweight designs,a concern has emerged that the deformation of housing

and shaft may affect the bearing life substantially. This

requires elasticity analysis of the bearing-shaft-housing

system. In the case of roller bearings, in particular,

inclination of the shaft causes the contact pressure

between the rolling element and bearing ring to result in

stress concentrations near the edges of the contact area. In

this case, the contact analysis is indispensable for

determining the optimum crowning shape.

Dynamic analysis of bearing behavior during rotation is

now possible thanks to progress in computer and

theoretical analysis methods. By fully utilizing these

technologies, NSK has developed an original bearing

performance analysis software called BRAIN that stands

for "BeaRing Analysis In Nsk."

Though sophisticated in its analytical features, BRAIN

can calculate results extremely quickly (a few minutes on a

PC), and is thus frequently used as a design tool for

bearing optimization. With BRAIN, analysis can be made

for all of NSK's bearing products, including ball bearings,

cylindrical roller bearings, tapered roller bearings,

spherical roller bearings, etc. Also with this software, the

user can calculate the attitude of the rolling elements

(skew, tilt) or revolving/rotating slide of rolling elements as

well as the PV value even in the case that the inner/outerring is deformed.

Bearing performance analysis includes simulation of the

motion of bearing elements on the basis of the equilibilium

of forces and moments, acting on the inner/outer ring,

rolling elements, and cage considering frictions. By solving

the dynamic equation for each bearing element while

eliminating the assumption of “pure rolling” used in a

conventional bearing calculation, a more reliable analysis

becomes possible. In this way, this bearing performance

analysis software enables the user to predict the bearing

heat generation, rolling element attitude (skew, tilt), slide

of rolling element and bearing ring, and the PV value,

thereby promoting optimization of the bearing's internal

design dimensions to meet the customer's needs. This

Recent Trends in Research andDevelopment of Rolling Bearings at NSK Kyozaburo Furumura

Managing Director, Research and Development Center

95BT10X

Steel ball

Others

Spin slip

Gyroscopicslip

400

300

200

100

00 1 2 3 4 5 6 7 8

F r i c t i o n a l l o s s , W

Rollingresistance

(X1000)Speed, rpm

Fig. 1 Friction loss analysis of angular contact ball bearings by

BRAIN



Motion & Control No.1— 1996 6

analysis can also be used for the prediction of bearing

damage, such as seizure, smearing, abnormal wear, and

premature flaking.

As an example of a calculation with BRAIN, Fig. 1 shows

the calculated heat generation of an angular contact ball

bearing 1). This calculation shows not only the total heat

generation of the bearing, but also a breakdown of the heat

generation into its different types. It is therefore possible

to study how bearing design affects heat generation. Fig. 2

shows the ball attitude angle obtained through calculation.

The "Jones control theory" applied widely up to now does

not include the gyro moment. However, as the speed rises,

depending on whether the gyro moment is included or

ignored, leads to a substantial difference in the result. This

in turn means that the conventional analysis based on

control theory and the assumption of “pure rolling” is not

rigorous enough for accurate analysis of high-speed

bearings.

NSK is aiming at analysis with improved reliabilitywhich allows the bearing to demonstrate its fullest

performance. Also, NSK will continue to make its best

efforts to support its customers by means of fast and

accurate software. Computer-aided bearing performance

analysis is indispensable, since NSK has to satisfy

increasingly stricter demands on bearings including speed

increase, torque reduction, life extension, and price

reduction. Also, automotive bearings and large industrial

bearings increasingly require elasticity analysis of the

bearing and its surroundings as an essential part of the

design process. High-level know-how and extensive

expertise are necessary for such analysis. In NSK, a

dedicated group of computer specialists and engineers

makes highly reliable analysis within a short period of

time by fully using appropriate FEM software and BRAIN.

3. Recent Development of TribologyTechnology

The practical achievements of EHL theory, which may

be traced to around 1960, are closely connected to the

tremendous computational power of computers. NSK ispushing forward research to improve EHL theory further.

This includes EHL analysis of high pressure on the one

hand and the so-called starved EHL problem with an

extremely limited lubricating oil amount on the other

hand.

In research relating to EHL analysis, the absolute value

of the contact pressure was around 1 GPa at maximum.

For actual rolling bearings, however, a contact pressure of

1 GPa is classified as a light load condition, since a higher

pressure of 2 GPa occurs more frequently in general use

(and even a contact pressure of 3 GPa is not uncommon in

severe cases). Since the Dowson-Higginson's formula thatis applied widely for calculation of the oil film thickness

was derived from the results of analysis under low contact

pressure conditions, there are disputes concerning its

applicability to high contact pressure conditions. Recent

computer development and calculation software have

proved their worth by producing accurate calculations for

applications under high contact pressure. Fig. 3 shows a

comparison between the calculation results of oil film

thickness made systematically up to 3 GPa by Natsumeda

and the results obtained by the Dowson-Higginson's

equation2). The results agree well with each other. In short,

this figure indicates the reliability of the Dowson-

Higginson's equation.

Fig. 4 shows the calculation of the pressure distribution.

According to this figure, the pressure distribution at high

pressure is approximately equivalent to the Hertzian

pressure distribution. A pressure spike, which is commonly

observed with EHL, does not appear. This is also a

reasonable result from the viewpoint of practical

experience.

G=2500

G=5000

0.5 1.0 1.5 2.0 3.0

2 5 10 20 50 100

Wx105

0.7

1

2

pH, GPa

: Dowson-Toyoda: Dowson-Higginson

H

c x 1 0 5 ,

H m i n x 1 0 5

: Hc: Hmin

Fig. 3 Minimum and central film thickness for varying W (U=10-11)

40

30

20

10

0

A t t i t u d e a n g l e ,

d e g r e e

0 1 2 3 4 5 6 7 8(x1000)

Speed, rpm

95BT10X

Steel ball

Si3N4 ball

Fig. 2 Attitude angle analysis of angular contact ball bearings by

BRAIN




The second topic, starved EHL, has been the subject of

systematic experimental study by Spikes, et al3). An optical

interference method was used to show the formation of the

EHL oil film. Its thickness was on the order of 10 nm. In

another study, Akagami and Aihara4) lubricated an actual

rolling bearing with an extremely small amount of oil. By

measuring the EHL oil film thickness with a displacement

method, they showed that the oil film became thin when

the oil amount decreased, as shown in Fig. 5, and that the

oil film did not often become thick even when the speedincreased. Further, they showed that such a thin oil film

might survive for a considerably long period as shown in

Fig. 6. This is considered to indicate a new lubrication

mechanism (parched EHL, etc.).

4. Technology to Enhance thePerformance of Bearings

Along with increased efficiency and labor savings as well

as size reduction and increased output, the operating

conditions of bearings have become considerably more

severe. NSK relentlessly develops assorted technologies to

enhance bearing performance to satisfy diverse customers'

needs.

4.1 Trends toward speed-increasing

technology

A bearing for an airplane gas turbine spindle must

demonstrate its high reliability under severe conditions,

including high speed, high temperature, and heavy load.

Recently, a new performance demand emerged relating to

the survival of the bearing during and after a lubricating

oil shut-off test as a means to ensure safe high-speed

operation. In other words, the bearing must not sufferdamage even though the lubricating oil supply becomes

temporarily insufficient during operation. NSK has

developed a chemical surface modification technology that

is effective at improving the above-mentioned

performance5). Using such surface modification for a three-

point contact ball bearing, the lubricating oil was shut off

at the dN value of 2.2 million to confirm its superior

seizure resistance. As shown in Fig. 7, the above dynamic

analysis of bearings has shown that the improved bearing

can have its seizure resistance enhanced through both

reduction in the friction heat generation between the ball

and bearing ring during shut-off of the lubricating oil

supply and inhibition of the local temperature rise between

the ball and groove6).

NSK is also proceeding with development of ceramic

Amount of oil

mm

0.8

0.6

0.4

0.2

0

0 2000 4000 6000 8000 10000 12000

3

20

10

5

1

0.5

0.1

0.0250.01

F i l m t h i c k n e s s , µ m

Rotational speed, rpm

VG32 VG68 VG100Experiment

Theory

Fa = 19.6Nn = 3600 rpm

Initial

amount: 10 mm

3

0.5

0.4

0.3

0.2

0.1

00 500 1000 1500 2000

Running time, hrs

F i l m t h i c k n e s s , µ m

1

0.5

0

1

0.5

0

W=3.07x10-4

W=1.08x10-3

(pH=3.0GPa)

(pH=1.6GPa)

p / p H

p / p H

Fig. 4 Pressure distribution and film shape for varying

Load (U = 10-11, G = 5 000)

Fig. 5 Oil-film thickness measurement of ball bearing with trace oil

lubrication

Fig. 6 Film thickness during long term test




(Silicone Nitride) bearings for airplane gas turbines, since

they can satisfy the demand for light weight and high

speed. A hybrid bearing that is equivalent to an M50

material bearing (except for its balls which were made

from Silicone Nitride) was tested at the dN value of 2.55

million. The test results confirmed its superior survival

performance after the lubricating oil supply shut-off.

Experimental results are shown in Fig. 8.

The next generation of gas turbines requires bearings

that can run reliably at temperature and speed conditions

far exceeding the current limits. NSK participates in the

AMG project which is sponsored by a consortium that is

conducting long-term research and development of

fundamental technologies necessary for ultra-high

performance gas turbines for the 21st century. NSK’s

responsibility is to create a new generation of high-speed

and high-temperature bearings.

4.2 Improvement of axial load capacity

Higher axial load capacity cylindrical roller bearings arerequired as substitute bearings to achieve compact yet low-

cost machines, thereby eliminating the original

requirement for thrust bearings in certain applications.

NSK is undertaking basic research on the lubrication

characteristics between the collar that supports the axial

load and the roller end surface in order to further enhance

the thrust load capacity of the above cylindrical roller

bearing. According to model tests and to analysis by

partial EHL, the seizure of the cylindrical roller bearing

collar depends on the contact condition between the roller

end surface and collar. Local seizure is found to grow until

seizure of the whole collar7)

. Currently, a model test isunder way to check for change in the roughness due to

run-in and to investigate the progress of seizure.

4.3 Trends toward lower torque

Nowadays, tapered roller bearings used in automobiles

must have dynamic frictional loss reduced, or torque

reduced, in order to save energy. It is essential for the

reduction of the torque of the tapered roller bearing that a

stable oil film be formed over the contact area between the

roller and rib. A few years ago NSK achieved operation at

low torque on the basis of EHL theory (Fig. 9) 8).

To reduce the dynamic friction further in a low-torque

bearing, decrease in the viscosity of the lubricating oil used

and reduction of the lubricating oil amount proved to be

effective measures. However, these measures cause an

increased possibility of seizure. Consequently, NSK is

proceeding with the research and development of a tapered

roller bearing with low torque and superior seizure

resistance.

Chemicallymodified

Non-modified

5 10 15 20 25 30 350

120

100

80

60

40

20

Time after oil shut-off, s

F r i c t i o n l o s s a t

i n n e r r a c e w a y ,

W

Low-torque bearing

R u n n i n g t o r q u e ,

k g f c m

10

5

1000 2000 30000

Rotational speed, rpm

Conventional bearing

Fig. 8 Friction loss due to slip between inner ring raceway and

rolling element

Fig. 9 Development of low-torque tapered roller bearings based on

analysis

Bearing Rev. speed(rpm)

Axial load(kN)

NormalSurface

14650

14650

16.7

16.7

16.7

SurfaceTreated

17000Si3N4

Ball

Seizure

Si3N4 Ball

Normal surface

Oil

re-supply

(30.6 s)

Surface treated

10 20 300

70

60

50

40

30

20

10

I n n e r r i n g t e m p e r a t u r e r i s e ,

° C

Time after oil shut-off, s

Fig. 7 Bearing temperature rise after oil shut-off




5. Vibration and Noise ReductionTechnology of Bearings

Recently, “quietness” has been highlighted as one of the

sales points for electric home appliances (air conditioners,

washing machines, vacuum cleaners, etc.) andautomobiles. This means that the bearings used in these

machines must meet increasingly stricter demands calling

for lower vibration and less noise. Research and

development projects are under way to satisfy such

demands.

The low-vibration characteristic is of vital importance for

AV and OA equipment, such as VCR, HDD (hard disk

drive), etc. In particular, the vibration of a bearing in an

HDD can adversely affect the memory, so such vibration

must be reduced by all means. As years go by, HDD

spindle motor bearings are becoming smaller and have

better accuracy. The accuracy of a bearing considered asan independent unit is 0.1 µm or less in terms of NRRO

(Non Repetitive Runout). The capacity of HDD will be

enlarged further in the future to reach 10 GB to 20 GB. It

is presumed that NRRO required for the bearing will be

0.02 µm or less in the year 2000, as shown in Fig. 109).

Therefore, the NSK R&D Center is actively researching

how to improve the performance of HDD spindle motor

bearings.

Noise is a problem related to human senses, and thus,

the noise needs to be reduced especially in household

appliances that are used daily such as air conditioners,

VCRs, etc. When vibration is considered, the bearing noise

contains a frequency component in the higher frequency

range and is distributed over a wide bandwidth. This

makes it difficult to adopt a countermeasure specifically

against noise, but it is possible to take countermeasures

against vibration.

Factors responsible for bearing noise include the shape

error (waviness, roughness of bearing components) and the

grease. To solve such problems, low-noise greases with

optimum base oil viscosity, consistency, and soap fiber

structure have been developed specifically for this purpose.These greases are designed not only to achieve satisfactory

acoustic performance during the initial period, but also to

offer low-noise performance over a long period of use.

6. Lubricating Grease

Lubricating grease for rolling bearings plays an

important role in assuring a long bearing service life.

Approximately 80% of bearings are lubricated with grease

because it helps to simplify the surrounding parts of the

bearing. To meet increasing demands for maintenance-freeoperation, about 70% of the above-mentioned bearings are

sealed or shielded ball bearings. The grease used affects

the performances of the bearing, including long life

(flaking, seizure, sound), low friction torque, low

temperature rise, low noise, low vibration, and high rust-

preventive capacity. For example, a sealed ball bearing,

without any subsequent grease supply, eventually suffers

damage (seizure, etc.) due to degradation of the grease,

which results in a shorter service life. It goes without

saying that the grease performance is one of the key

factors controlling the performance of bearings10).

6.1 Present grease development in NSK Standard greases for rolling bearings till the middle of

the 1970s were lithium soap/mineral oil, natrium

soap/mineral oil, and lithium soap/silicone oil greases. The

demands of customers, which were growing increasingly

stricter concerning enhanced performance, life, and

reliability, seemed not to be fully satisfied with the

improvements brought by the above greases. Against that

background, NSK decided to develop its own grease in-

house, instead of relying on greases developed by outside

suppliers.

NSK development has focused on ball bearings for

electric machinery and for information fields as well as forautomotive electric devices and engine auxiliary

equipment fields, in which the grease contributes

considerably to the bearing performance. To meet the

needs for heat resistance and low-temperature

performance under application conditions, a synthetic oil-

based grease is frequently used. Over the years, NSK has

formulated many dedicated greases and now offers about

30 different greases. NSK is ready to recommend an

appropriate grease for your application.

In electric machinery and information processing fields,

several types of lithium soap/ester oil grease have been

developed to meet the demands for low noise, low torque,

and satisfactory acoustic life performance. Moreover, in

electric devices and engine auxiliary equipment fields,

more than ten types of urea/synthetic oil (ester, polyalpha-

25.4

2.0

1.0

( µ m )

( µ m )

20

10

01984 1990 1994 2000

12.7

6.4

4.2

2.0

1.24

0.380.3

0.190.13 0.06

Track pitch

Motor NRRO

Bearing

year

Fig. 10 Improvements in track pitch and motor NRRO of hard disk

drives




olefin, ether) greases have been developed to satisfy the

demands for superior high-temperature resistance, high

speed rotation, and rust prevention.

Since the middle of 1980, the poly V belt has been used

to drive the electric devices and auxiliary equipment. This

in turn caused frequent flaking with structural change inan alternator. Though the frequency could be reduced

through improvement of the bearing material and design,

this problem is not yet fully eliminated. However, the

urea/ether oil-type MA7 grease has been developed and

proved effective in eliminating the occurrence of any

flaking for the last five years11). Fig. 11 shows the effects of

MA7 grease.

Currently, more greases are being developed. Greases

thus developed include a grease for roller bearings, a low-

splashing grease, a fretting resistant grease, and a fluoric

grease for high temperature use.

6.2 Prediction and equation for grease lifeNSK has supplied grease life equations in bearing

catalogs. These equations were formulated through

accumulation of extensive data from grease life tests in the

laboratory. For the mixed use of lithium soap and urea

greases in the bearing, NSK has studied the degradation

level, establishing the degradation limit value for

estimating the remaining service life12). Subsequently,

experiments were made to classify general greases into

five types according to their composition. Based on this

classification, a life equation has been prepared to enable

calculation of the average life of grease when the bearing

number and bearing conditions are known. Fig. 12 shows asummary of experimental results which provided a basis

for the grease life equation13). The ability to estimate the

grease life more precisely contributes to better machine

design and maintenance scheduling.

7. Long Life and High Reliability

Given satisfactory lubricating conditions with sufficient

oil film formation and successful prevention of mixing-in of

foreign particles, then the fatigue strength of materials

and its variance are key factors governing the durability

and reliability of a bearing. Based on joint research with

steel makers conducted over a long period of time, NSK

has succeeded in co-developing the world's cleanest

bearing steel. As shown in Fig. 13, this extra purified steel,

or EP steel, offers an over five-fold longer life and provides

higher reliability than existing highly clean steels.

In actual practice, however, lubricating conditions are

often less than satisfactory. In response, NSK's research

99

90

50

10

5

1

10 5 102 5 103 5 104

Grease MA7

L10 cal.

Grease E

P e r c e n t f a i l e d ,

%

Life, hrs

Fig. 11 Rolling contact fatigue life of actual engine test under up-

down cyclic revolution (E grease vs. MA7 grease)

10000

1000

100 100 120 140 160 180

Rotational speed: 3 000 rpm (top)

10 000 rpm (bottom)

Load ratio (P/Cr): 0.014

Temperature, °C

G r e a s e l i f e ,

h r s

Fig. 12 Effect of temperature on grease life

99

90

50

10

5

12 5 102 2 5 103 2 5 10410

Life, hrs

SAE52100 (N = 30)

EP steel

SAE52100 (N=53)

Existing steel

L10

cal. P e r c e n t f a i l e d ,

%

Fig. 13 Results of fatigue life test for two high-cleanliness steel

bearings (6206 deep groove ball bearing)




has focused on long-life and high-reliability under tough

lubricating conditions where mixing-in of foreign particles

is expected. HTF and STF bearings of NSK have been

developed to provide long life under contaminated

conditions with foreign particles. With contaminated

lubrication, the life of the bearing is determined by thestress concentration at the edge of the dent. The aim is,

therefore, to reduce such stress concentration. A study of

material properties that alleviate stress concentration at

the edge of the dent showed that a hard material can

reduce such concentration provided the retained austenite

content is kept high. Retained austenite, however, is

traditionally associated with a lower hardness material.

Therefore, raising the hardness while simultaneously

maintaining the retained austenite appears to be a

conflicting demand.

To meet this contradictory requirement, a new steel was

developed. It is an SAC steel that is combined with a newlydeveloped carbonitriding treatment.

Material A, which is hard yet contains a large amount of

retained austenite, was developed by dispersing extremely

fine carbon nitrides uniformly. This newly invented

technology is covered by patents issued in the USA, UK,

and Japan. As shown in Fig. 14, HTF and STF bearings

are expected to attain a life that is three to five times as

long as existing composition bearings under contaminated

conditions14).

The HTF bearing is made of an economical Mn-Cr

material and is applied mainly for automobile applications.

The STF bearing uses a Cr-Mo material that satisfies

extremely strict performance requirements. This material

is employed in increasing quantities in the steel, machine,

and automobile industries.

The values obtained from calculations with the currently

available equation suffer a drawback. Namely, they are

shorter than the actual life under clean lubricating

conditions but longer than the actual life under conditions

where mixing-in of foreign materials is expected.

Therefore, NSK proposes a new life equation derived from

theoretical analysis of life-test data from research labs and

practical life-data gathered from customers15):

Lnas = a1 • a2 • a3 • a4 • a5 • (C/P)p

a4 : Environmental coefficient indicating the state of mixing-in of foreign materials

a5 : Fatigue limit factor based on consideration of

the fatigue limit of the bearing material

Introduction of “a4” has enabled prediction of the

bearing life used in the lubricating oil contaminated by

foreign materials. Concerning the rolling fatigue, it has

been said that there is no fatigue limit. As the cleanness of

the bearing steel has been enhanced considerably, data

has emerged indicating the possibility of the existence of

the fatigue limit. In this equation, “a5” has been introduced

for the term C/P.This equation offers the advantage of being familiar,

since it resembles the ISO calculation equation. Therefore,

the relative relationships can be clearly and easily

understood since the past experience of technicians and

engineers serves as a foundation.

8. Conclusions

NSK persistently pursues research and development in

areas that are the foundation of bearing technology. Based

on its long history and extensive achievements in making

rolling bearings, NSK constantly invents and innovates

products and technology. NSK contributes to the progress

of industries throughout the world by offering its bearing

technology expertise, since bearings are considered to be

“essential elements of machines.” Bearings are, like the air

we breathe, something you take for granted, so you rarely

think about it. Bearings are, nevertheless, vital for the

smooth and proper functioning of machines.

References1) Aramaki, H. et al, “The Performance of Ball Bearings with

Silicone Nitride Ceramic Balls in High Speed Spindles forMachine Tools,” ASME, JoT, Vol. 110 (1988) 693-698.

2) Natsumeda, NSK Technical Journal No. 654 (1992) 19, [In

Japanese].

3) Johnson, G., Wayte, R. & Spikes, H., “The Measurement and

Study of Very Thin Lubricant Films in Concentrated

Contacts,” Trib. Trans., Vol. 34 (1991) 187-194.

4) Akagami, K. and Aihara, S., “Experimental Study of Starved

EHL in a Small Ball Bearing Lubricated with Minute Amount

of Oil,” Proc. ITC Yokohama 95 (JAST), [to be published]

5) Hachiya, K. et al, Preprint of JAST Tribology Conference

Tokyo (1992) 331-334 [In Japanese].

6) Ijuin, S. et al, “Improving the Survivability of High Speed Ball

Bearings under Oil Shut-off Conditions by Chemical

Modification of the Surface,” Tribology Trans., Vol. 38, No. 2

(1995) 403-409.

99

90

50

10

5

1 10 102 103

Life, hrs

P e r c e n t f a i l e d ,

%

HTF

STF

Conventionalbearing

2 5 2 5 2 5

Fig. 14 Fatigue life of bearings made of different materials under

contaminated lubrication (deep groove ball bearings)




7) Aramaki, H. et al, NSK Technical Journal, No. 659 (1995) 14

[In Japanese].

8) Aihara, S., “A New Running Torque Formula for Tapered

Roller Bearings under Axial Load,” Trans. ASME, JoT, Vol.

109 (1987) 471-477.

9) Ichiyama, Y., J. JSPE, Vol. 61, No. 9 (1995) 1239-42 [In

Japanese].

10) Naka, M., Koizumi, H., Ishihara, S., Touma, K., “High-speed

Ball Bearings with Grease Lubrication,” Proc. Japan Int. Trib.

Conf., Nagoya (1990) 749-755.

11) Murakami, Y., Naka, M., Iwamoto, A., “Long Life Bearings for

Automotive Alternator Applications,” SAE Paper 950944

(1995).

12) Ito, H., Tomaru, M., Suzuki, T., “Physical and Chemical

Aspects of Grease Deterioration in Sealed Ball Bearings,”

Lubrication Engineering, Vol. 44, No. 10 (1988) 872-879.

13) Ito, H., Koizumi, H., Naka, M., “Grease Life Equations for Ball

Bearings,” Proc. ITC Yokohama 95 (JAST) [to be published].14) Furumura, K., Murakami, Y., Abe, T., “The Development of

Bearing Steels for Long Life Rolling Bearings under Clean

Lubrication and Contaminated Lubrication,” Creative Use of

Bearing Steels, ASTM (1994) 199-209.

15) Takata, H., Furumura, K., Murakami, Y., “Development of a

New Method for Estimating the Fatigue Life of Rolling

Bearings,” Proc. ASME Symposium, Orlando (1995) 11-18.

Kyozaburo Furumura

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Mc01 03 Recent Trends in Be