TECHNICAL REPORTGrease Lubrication Technology of Rolling Bearings
K. YOSHIZAKI

Friction loss of rolling bearings that support rotational motion in machinery is generally smaller than that ofsliding bearings. Moreover, rolling bearings lubricated with grease can contribute much more to reduction offriction loss in machines, machine size, and machine weight than those lubricated with oil due to smaller agitation
resistance of lubricant and simpler sealing equipment. Development and improvement of rolling bearings andgreases have been carried out to support higher performance of machinery. This paper introduces trends in greases
for rolling bearings used in main industrial fields and recent examples of research and development results.
Key Words: grease, rolling bearing, boundary lubrication, elastohydrodynamic lubrication

1. Introduction
For global environment conservation and global
warming prevention, much effort is being made regarding
the development of machinery, with the aims of achieving
smaller size, lighter weight and longer service life,
reducing friction loss and increasing efficiency. The
rolling bearing is an important elemental component
which supports the rotational motion of machinery and,in general, the friction loss of rolling bearings is smaller
than that of sliding bearings. Moreover, the lubrication
method of rolling bearings can be broadly divided into
oil lubrication and grease lubrication, however, in thecase of the latter, frictional loss caused by agitationof the lubricant is less than that of oil lubrication. In
addition, the sealing equipment can be simplified andthe machinery can be made smaller and lighter if greaselubrication is used. However, due to needs such as
making machines with even less friction loss, smaller sizeand lighter weight, more and more is being demanded
of rolling bearing performance. In order to respond tothese needs, greases for various applications are being
improved and developed. This paper introduces the trendsof greases for rolling bearings in the main industrial fields
and recent research and development trends.

2. Characteristics of Grease for Rolling Bearings
Rolling bearings are required to work across a
range of temperatures, from low to high, sometimes
at a high surface pressure where plastic deformation
will occur in the bearing material. Bearings are also
expected to support rotational motion, and sometimes
swaying motion, lightly, quietly, smoothly and for a

26

prolonged period over the duration of the product's life.
Therefore, while bearing model selection, internal design
optimization and so forth is important, so too is selecting
the appropriate grease. For rolling bearings to be able toperform well, the grease used must be heat resistant, havegood oxidation stability, work at low temperatures, havegood load carrying capacity, low torque, high speed andacoustic properties, be resistant against corrosion and soon. These characteristics are determined by combining
thickener to maintain the semi-solid state of the base oil,the main acting agent in lubricant, and various additives
to improve each characteristic.
Characteristics such as heat resistance, oxidization
stability, load carrying capacity and so on are also
necessary in grease for other applications, such as gear
grease. On the other hand, good acoustic properties are
only required in grease for rolling bearings. The contact
portion between the rolling elements of rolling bearings
and the race has a high surface pressure of several GPa,
and also performs rolling motion at a high speed. If thegrease in this contact portion contains solid particles, therolling bearing will begin to vibrate, which will create
abnormal noise.
Solids inside the grease include lumps of thickener,
which is a component of grease, and other foreign matter.
1)
As Fig. 1 shows , thickener lumps are dispersed finely inroll mill treatment when grease is manufactured, howeverroll mill treatment increases the cost of grease as it is time
consuming. Various efforts such as using a filter on thebase material and manufacturing in clean rooms are taken
to prevent foreign matter entering the grease and help
improve acoustic properties.

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Table 1 Compositions and properties of grease for alternatorbearing

Before roll mill treatment

0.1mm
After roll mill treatment

Fig. 1 Microscopic observation results of greases before andafter roll mill treatment

3. Trends in Greases for Rolling Bearingsin Main Industrial Fields
3. 1 Automotive
Recently there is a tendency to incorporate the rolling
bearings for wheels of passenger vehicles into hub
units due to advantages such as being more compact,
lightweight, maintenance-free and simpler to assemble.
In order to prevent false brinelling when vehicles aretransported and eliminate the need for maintenance, we
select JIS 1-2 penetration grade grease for hub units,urea mineral oil grease, with excellent heat resistance,
adhesiveness to the friction surface, fluidity andlubrication.
Regarding the rolling bearings used in automotive
electrical components and engine accessories, previously
there was a problem of early flaking during usage on
actual vehicles due to the microstructual change of steel
in the fixed ring. This problem was largely related to thefact that operation conditions of rolling bearings havebecome quite severe, i.e. high speed, high temperature,high load and high vibration, because devices are smaller,
lightweight and have higher performance.
This flaking is completely different to the flaking
originating from within the rolling bearing noted
previously, and is characterized by a white etching band,
therefore it is also known as "white band flaking". The
main cause of this flaking is an increase in internal stressdue to sliding, high surface pressure, shock load, etc. andit was assumed that the hydrogen created when grease in
the race breaks down and intrudes the steel facilitated theprocess of flaking. Grease to prevent white band flaking
was developed based on this hypothesis. Throughout thedevelopment of this grease, studies were conducted forthe alleviation of shock load caused by oil film as well as
reducing friction caused by additive membrane, hydrogencreation and intrusion into the steel. As a result, a grease
was developed by adding organic metal extreme-pressure
additive to urea grease with a base oil of alkyl diphenyl
ether (ADE) which has excellent oil film forming ability
as well as large kinetic viscosity and pressure-viscosity
2)
index . The properties of this developed grease andthe results of an alternator endurance test are shown in

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

Thickener

Developed
greaseDiureaDiureaPoly-a-olefin/ Alkyl diphenyl
Mineral oil
ether

Base oilBase oil kinetic viscosity2
mm /s at 40℃

56

97

Worked penetration

244

238

Dropping point, ℃

＞270

＞270

Evaporation loss, %100℃×22 h

0.15

0.19

Oxidization stability, kPa
99℃×100 h

15

10Starting

0.274

0.322

Rotation

0.042

0.048

Low temperature
torqueN・m, −30℃
99.9: Stopped
90
Cumulative failure probability, %0.1mm

Conventional
greaseConventional grease50

Developed grease
10

1
0.5
10

50

100

500 1 000

Service life, hFig. 2 Results of alternator endurance test

Table 1 and Fig. 2 respectively. By using this developed
grease, it was possible to prevent white band flaking of
the fixed ring and contribute to the long service life ofrolling bearings.

3. 2 Industrial Device Field
3. 2. 1 Wind Turbin
Figure 3 shows the typical structure of a wind turbine.
Wind turbines are configured from a main shaft, gear box,
generator and so forth, meaning many rolling bearings
are used. In many cases, oil lubrication is adopted for the
rolling bearings of the gear box, and grease for lubricationof all other parts.
The main shaft bearing is an important component
which supports the load caused by the wind working

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through the rotor and transmits rotational torque to thegear box. The grease for the main shaft bearing is required
to have low torque at low temperatures, have good load
carrying capacity, be resistant against corrosion and havea long service life. Therefore, as indicated in Table 2,lithium soap grease, lithium complex soap grease andother grease types which use poly-a-olefin and mineral3)
oil as base oils, are adopted .

equipment are used in high temperature, extremely low
speed and under high loads, making oil film formationdifficult and meaning operation takes place at theboundary lubrication regime. Conventionally, aluminumcompound soap was used as sealed grease as it hadsuperior heat resistance, however in recent years, urea
grease, which has a base oil of high viscosity, has been
4)
gaining popularity .

3. 2. 2 Iron and Steel Equipment

3. 2. 3 Railway Carriages

Rolling bearings used in iron and steel equipment
must be not only able to cope with high temperaturesand high loads, but also perform satisfactorily in severe
environments with water, dust, oxidized steel and so on.
The roll neck bearings used in roller mills are used
under high loads and shock loads. Furthermore, thelarge amount of cooling water used in the rolls and scale
intrudes the rolling bearing, leading to problems suchas softening of the grease and shortening of the rolling
bearing service life. Conventionally, lithium soap grease
with superior water resistance and load carrying capacity
or special lithium complex soap grease with high PV
performance were used, however in recent years, the useof calcium sulfonate complex grease is becoming more
popular as it has excellent extreme pressure performance,
is anti-corrosive, has good adherability and shear stability
4)when mixed with water .
The guide roll bearings used in continuous casting

Previously, axle bearings for bullet train carriages used
oil bath lubrication, but the shift to grease lubrication
was made in 1997 when the 500 series bullet trainscommenced operation. Because sealing equipment canbe simplified, axle bearings have been made smaller andlightweight, and can now support high-speed operation.
There are also expectations that the axle bearings can5)
be made maintenance-free . For the packed grease, inorder to increase the length of time between inspections,
lithium soap grease was adopted. Lithium soap grease ismade from hydro-treated mineral oil, which has excellent
oxidization stability compared to common mineral oil
and a high viscosity index. Grease-packed bearings
using the same model grease is adopted on the carriagesof the Kyushu (800 series) bullet train that commenced
operation in March of 2004.
Since the introduction of the induction motor and shift
to brushless, the inspection period of the traction motor,
which is the power source of a railway carriage, hasdepended on the rolling bearing using grease lubricationand grease service life. In bullet trains, induction motors
were adopted in carriages from the 300 series and, sothat the rolling bearings could support high speed/hightemperature operation, grease made from a combinationof lithium complex soap, which has excellent heat
resistance, and mineral oil was adopted. This grease is
now used in the later model bullet train carriages andmany traction motors. To further increase the length oftime between inspections, grease made from the samelithium compound soap with a synthetic base oil isbecoming commonplace.

Main shaft bearing, housing
NacelleBladeGenerator
Gear box
Main shaft
Rotor

Tower

Fig. 3 Typical structure of wind turbine

Table 2 Compositions and properties of grease for wind turbine main spindle bearing

28

Grease A

Grease B

Grease C

Grease D

Grease E

Thickener

Lithium soap

Lithium/
calcium soap

Lithium
complex soap

Calcium sulfonate
complex

Lithium
complex soap

Base oil

Mineral oil

Mineral oil

Mineral oil/
PAO

Mineral oil/
PAO

PAO

Base oil kinetic
viscosity
mm2/s at 40℃

200

400

130

80

460

Penetration grade

Grade 1

Grade 2

Grade 1

Grade 2

Grade 1/2

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Additive-free
ZnSul

SbDTC
SbDTC+ZnDTC

ZnDTC
SbDTC+ZnSul

5

15

0.22
Friction coefficient, lWhen used under hydrodynamic lubrication conditions,sufficient lubrication can be maintained with an oil film
formed from the base oil, however, the conditions of use
are becoming increasingly severe, (e.g. high temperature,high load), and in order to improve lubrication whenused under mixed or boundary lubrication conditions, anoiliness-improving agent and extreme-pressure additive
are added. There are sulfur and phosphorous extremepressure additives, however it has been discoveredthat organometallic extreme-pressure additives best
represented by organomolybdenum compound, areeffective in preventing white band flaking. To verify this,an evaluation of the friction and wear characteristics oforganometallic extreme-pressure additives and an analysisof the formation and structure of surface film generated
by tribochemical reaction were performed.
Regarding the friction and wear characteristics of ADEbased urea grease combined with different organometallic
extreme-pressure additives, it was discovered that in somecases, the effect of reducing friction was good, however
the friction coefficient was sometimes great, or, wear
prevention was good, however, friction reduction was
small. Characteristics differed depending on the variety
6)of extreme-pressure additive used . Furthermore, a study
was carried out to see the improvement in the friction
and wear characteristics when two types of additives
were used with the extreme-pressure additive. Figure 4
shows the change over time of the friction coefficient,and a reduction in friction and wear was recognizedwhen zinc additive was used, proving that the friction andwear characteristics improved when additives were used
7)
together rather than independently .
These friction and wear characteristics are determinedby the surface film generated by tribochemical reaction,
and as such, it is important to find out the chemical
structure and thickness of this surface film. Therefore, thesurface film was analyzed using surface analysis devices
such as Auger Electron Spectroscopy (AES).
For example, Fig. 5 shows the AES analysis resultsof a friction surface after a friction and wear test whenantimony dithiocarbamate (SbDTC) and a zinc additive
were used together. The surface film structure is thought
to be formed from layers of antimony oxide and sulfide on
the top and a layer of zinc sulfide underneath, suggestingthat antimony oxide and sulfide were effective for friction
reduction while zinc sulfide was effective for preventing
7)
wear .
Furthermore, a comparison was made of the surface
films formed in the case of rolling contact and sliding

0.2
0.18
0.16
0.14
0.12

0

10

20

Test time, min.

Fig. 4 Friction coefficient of greases combination SbDTC
with Zn additives

2 000
S
1 500
Intensity

4. 1 Tribological Characteristics
4. 1. 1 Organometallic Extreme-Pressure Additives

contact8). Figure 6 shows the analysis results of a surface
film formed in rolling contact. The surface film formedby rolling contact is made mainly from antimony and leadoxides, which differed from the surface film formed bysliding contact, made mainly from sulfide. It is believed
this is due to differing friction conditions and whether or
not there is contact with oxygen.

Fe

1 000

Zn

500
0

Sb, O
0

50

100

150

200

250

Analysis depth, nm

Fig. 5 AES analysis results of sliding friction surface(SbDTC + ZnSul)

1 200
Fe

1 000
Intensity

4. Recent Research and Development Trends

800
Sb

600

O

400

Zn

200
0

S

C
0

25

50

75

100

125

150

Analysis depth, nm

Fig. 6 AES analysis results of rolling friction surface(SbDTC + ZnSul)

29

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4. 2 Rheological Characteristic

Due to the importance of protecting the global
environment and improving human safety, a shift fromlead and chloride compounds to alternative additives
has already taken place in the extreme-pressure additive
field. However, new stipulations and restrictions on theeffects of additives towards humans and the environment
may be made in the future, therefore the research anddevelopment of an even safer additive is important.
Bismuth is part of Group 15, the same family as
arsenic, however compounds of bismuth are used inpharmaceuticals, cosmetics, etc. and it has relatively low
toxicity compared with other heavy elements. Bismuth isalso used in lead-free soldering alloy. Furthermore, there
is research that claims organic bismuth compound could
9)
serve as an alternative to lead extreme-pressure additives
and as such the characteristics of bismuth compoundas an extreme-pressure additive were researched. As
Fig. 7 shows, this research confirmed that bismuth
dithiocarbamate (BiDTC) has excellent friction and wear
10)
characteristics .
Furthermore, an evaluation of tribologicalcharacteristics under sliding friction conditions and ananalysis of the surface film formed under sliding friction
conditions and rolling friction conditions were performed
for a BiDTC-added grease which uses ADE as its base
11)
oil . As Fig. 8 shows, under sliding friction conditions,
the BiDTC-added grease has equivalent friction and wear
characteristics to the SbDTC-added grease, and, as Fig. 9shows, it was discovered that the surface film formed wasmade from bismuth and sulfide. The surface film formedby rolling friction was, as shown in Fig. 10, made frombismuth, oxygen and sulfur, showing that the surface
films formed by sliding friction and rolling friction
differed. This is thought to depend on the same factors as
SbDTC, i.e. differing friction conditions and whether or
not contact is made with oxygen.

Extreme-pressure additives are most effective mainlyin the regimes between boundary lubrication and mixed
lubrication. If used together with the improved lubrication
state from the boundary to the mixed lubrication regime,
and from the mixed to the hydrodynamic lubrication
regime, extreme-pressure additives contribute toimprovement of performance by reducing the friction andwear of rolling bearings, extending service life and soon. In grease lubrication, the lubrication state is affectedsignificantly by the rheological characteristic of the grease

Friction coefficient, l

0.3

lmax
lmin

0.2

Wear depth

0.1

0

①

②

①

Additive-free

②

BiDTC

①

②

SbDTC

①

②

3.5
3
2.5
2
1.5
1
0.5
0
−0.5

Wear depth, µm

4. 1. 2 Bismuthic Extreme-Pressure Additive

ZnDTP(1 and 2 show the no. of tests)

Fig. 8 Friction and wear characteristics of BiDTC-added
greaseElemental analysis area

Many

0.5mm

Fig. 9 Analysis results of friction surface after sliding friction
test
Bi-added oil (2 500 ppm)
Bi-added oil (5 000 ppm)

1mm

Elemental
analysis area

0.2

0.1

0

Many

Friction coefficient, l

0.3

Bi-added oil (7 500 ppm)
Bi-added oil (15 000 ppm)
Base oil

0

500

1 000

1 500

Slide time, sec

Fig. 7 Friction characteristics of ADE oil containing BiDTC
Fig. 10 Analysis results of friction surface after rolling friction
test

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itself as well as conditions of use such as temperature,load and rotational speed. Thus, it is important tounderstand the rheological characteristic of grease.
Furthermore, the need to lubricate with only a small
amount of packed grease is growing from the perspectiveof reducing friction loss caused by the agitation resistanceof grease and the need to reduce grease consumption.
However, one of the concerns of using a small amount ofpacked grease is that the amount needed for lubrication
will not be supplied to the contact portions. It isnecessary to understand the rheological characteristicof grease in this lubrication-deprived state and make itappropriate in order to increase the amount of grease bycontributing effectively to lubrication, hence improving
the performance of rolling bearings.
Conventionally, the discussion on fluidity of grease
mainly focused on worked penetration therefore firsta study of the correlation between the rheologicalcharacteristic of grease in this lubrication-deprived state
12)
and worked penetration was carried out . As Fig. 11
shows, the anti-seizure characteristic under sliding contact
in a deprived-lubrication state is effected by penetration.
It was discovered that, it takes a long time before thegrease seizes when penetration is high, and a correlation
exists between grease fluidity to the friction surface andthe shear viscosity and yield stress of the grease reserveat the entrance to the contact portion. Furthermore, therespective impact of thickener types and base oil kinetic
viscosity on the rheological characteristic was studied
and it was discovered that, even if the penetration was thesame, the grease fluidity would differ depending on the13, 14).
thickener type

4. 3 Elastohydrodynamic Lubrication Film
Forming Ability
The theoretical analysis and measurement of theelastohydrodynamic lubrication (EHL) film of grease
have been carried out in comparison with the EHL film
forming ability of base oil. To date, studies have been
done on the impact of thickener type, amount, fiber
15)
structure, base oil type, grease penetration and so on .
Figure 12 shows the results of an evaluation of grease
400

16)film forming ability using optical interferometry . In
this evaluation, no extra grease was supplied after grease
was applied before measurement. In the case of common
grease, the grease film breaks early on, as is the casefor Grease A, but by adjusting thickener type, volume,
worked penetration and so on, the grease film can be
maintained for an extended period of time, as is the casefor Grease B.
The following section introduces 2 research cases ofgrease film forming ability of urea grease in the lowspeed regime.
In the first case, the results of measuring grease film
thickness up to the extreme low-speed regime showthat the grease film thickness in the low-speed regime
is a different order of magnitude to the film thickness
17)of base oil . Even in the low-speed regime, horseshoeshaped interference marks, typical for EHL film, wereobserved, and the increase in film thickness was assumed
to be caused by hydrodynamic lubrication. Meanwhile,in the second case, while the grease film thickness did
increase in the low-speed area, observation results of the18)
sealed-in grease film and FT-IR analysis results of the19)
disk rolling contact surface led to the assumption thatthe grease film became thicker due to the adherence andaccumulation of thickener on the rolling contact surface
during rotation.
The above two cases both show that urea grease has
superior grease film forming ability in low-speed regimes,
however, the respective mechanisms differ. In the firstcase, it is assumed the grease film forming ability is
due to hydrodynamic lubrication, while in the latter it
is assumed to be a result of boundary film formation.
This is thought to be due to the difference in evaluation
conditions. Moreover, these evaluations were carried out
using optical interferometry and verification using actual
rolling bearings is still necessary.

5. Conclusion
It is predicted that machinery will continue to be madesmaller, lighter, able to operate at higher speeds and soon, making the environment in which rolling bearings areused even more severe, while at the same time increasing
the demand for rolling bearings with low torque and

Seizure time, s

350
300

Grease A(Starved lubrication)

250
200
150
240

Grease B(Fully lubrication)
260

280

300

Penetration (60 W)

Fig. 11 Relationship between penetration and seizure time

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)

time

Fig. 12 Observation results of grease film

31−Grease Lubrication Technology of Rolling Bearings−

a long service life. Furthermore, efforts to reduce
environmental load are also an important issue. In orderto support these trends, rolling bearings must evolve. In
the same way, grease, an "element" of rolling bearings,
must also evolve. Grease must become high performingand highly functionable by using additives technology
and rheology/oil film forming ability analysis. It isbelieved this will contribute to the advancement of rolling
bearings.

17) T. Endo, D. Dong and Y. Kimura: Proceedings of JAST
Tribology Conference, Tokyo (2008-5) 181.
18) Y. Ikeda, M. Mitsuoka, H. Iwamatsu and H. Komiya:
Proceedings of JAST Tribology Conference, Tokyo(2011-5) 377.
19) H. Komiya, Y. Ikeda, M. Mitsuoka and H. Iwamatsu:
Proceedings of JAST Tribology Conference, Tokyo(2011-5) 379.

References
1)

JTEKT CORPORATION: Mechanism and structure ofrolling bearings, SHUWA SYSTEM CO., LTD. (2011)112.
2) H. Komiya: Sekkeikougaku (journal of Design
Engineering), 35, 6 (2000) 194.
3) K. Yoshizaki, K. Matsuo, K. Yamakawa, H. Matsuyama,
T. Saito: International Tribology Conference Hiroshima
2011 Extended Abstract, B2-11 (2011).
4) K. Nagano: Jyunkatsu Keizai (Journal of Economic
Maintenance Tribology), 551 (2011) 37.
5) T. Oyama: Koyo Engineering Journal, 154 (1998) 29.
6) R. Nakata and H. Komiya: Proceedings of JAST
Tribology Conference, Osaka (1997-11) 255.
7) N. Kato, R. Nakata and H. Komiya: Proceedings of JAST
Tribology Conference, Tokyo (1998-5) 408.
8) H. Komiya, R. Nakata and K. Yoshizaki: Proceedings of
JAST Tribology Conference, Sendai (2002-10) 7.
9) D. K. Tuli, R. Sarin, A. K. Gupta, A. H. Kumar, M. M.
Rai, A. K. Bhatnagar: Lubrication Engineering, 51, 4(1995) 298.
10) M. Suzuki, H. Komiya, S. Okamura and X. Matano:
Proceedings of JPI Annual Meeting, Morioka (2005) 341.
11) K. Yoshizaki and Y. Shirai: Proceedings of JAST
Tribology Conference, Tokyo (2009-5) 95.
12) R. Nakata, K. Kimura and H. Matsuyama: Proceedingsof JAST Tribology Conference, Tokyo (2010-5) 277.
13) R. Nakata, K. Kimura and H. Matsuyama: Proceedingsof JAST Tribology Conference, Fukui (2010-9) 287.
14) R. Nakata, K. Kimura, H. Matsuyama and S. Saito:
Proceedings of JAST Tribology Conference, Tokyo(2011-5) 375.
15) Technical Committees on Grease, JAST: Fundamentals
and Applications of Lubricating Grease, yokendo co., ltd.
(2007).
16) JTEKT CORPORATION: Mechanism and structure ofrolling bearings, SHUWA SYSTEM CO., LTD. (2011)
101.

K. YOSHIZAKI**

32

Material Engineering R&D Dept., Research &Developmemt Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1010E (2013)