TECHNICAL PAPER

Low Torque and Low Noise Technology for Ball Bearings
Under Grease Lubrication
K. MIYAKE T. TSUDA

K. YOSHIZAKI

A study was conducted to verify the effect of grease on the low torque property and low noise property which areparticularly highly sought bearing performances. We focused on the viscoelasticity of grease as the dominating
factor of bearing rotational torque. As a result, we clarified that the higher the viscoelasticity, the lower the torque,and that grease families with short carbon chains were superior in terms of low torque characteristic. Moreover,bearing noise increases when grease thickener becomes trapped between the raceway and rolling element causing
the oil film thickness to vary, therefore low noise was better achieved if thickeners with low Young’s modulus andsmall particle size were used as this minimizes oil film variance.
Key Words: grease, bearing torque, grease noise, young modulus, particle size

1. Introduction
In recent years, demands relating to energy-saving,such as minimizing the use of fossil fuels and reducing
power consumption, have been increasing. Particularly
with regard to the industrial motor field, which is said
to account for 55% of domestic annual electricity
consumption, there is a strong requirement to improveenergy efficiency. In reality, a Top Runner Program wasformulated in 2011 targeting the three-phase motors usedin plant pumps, fans and so on, and higher-efficiency
motors have become mandatory. As such, strong demands
have also arisen relating to torque loss caused by bearings
used in these motors and there is much pressure from theindustry to develop grease-lubricated ball bearings with
low torque performance.
Many motors are already used for automotive
applications, however it is predicted that, due to thepromotion of electrification, the quietness of automobiles
will continue to improve in the future. For this reason,
there is a need to further improve the quietness of greaselubricated ball bearings.
The impact of grease on the low torque performance ofball bearings has been studied to date and many research
reports have been published. In regards to bearing torque,
Watabe and company have conducted an evaluation
using lithium soap grease and shown that a correlation
exists between the yield stress of grease and the bearing
1)
rotational torque . Moreover, Akutsu and company focus
on the thixotropy of grease and have concluded that thetendency for grease viscosity to decrease due to shearing

26

depends on the three-dimensional network structure of2)
thickener . However, hardly no reports have been madeof case studies quantifying the factors affecting bearing
rotational torque. As such, this research focused on
viscous transition stress as a new factor affecting bearing
rotational torque and, as a new concept deriving fromthickener-originating loss, introduced stirring loss energy
to investigate the correlation between the compositionand properties of grease, bearing torque and stirring loss
3)
energy .
Regarding bearing noise, Endo and company discuss
the smoothness of the thickener film from analysis resultsof EHL (elasto-hydrodynamic lubrication) thickener film
thickness and have concluded that grease which forms a4)smooth thickener film has high quietness . Furthermore,
Igarashi and company have studied the effect of baseoil viscosity of grease and thickener fiber diameter onbearing noise and demonstrated that the greater thebase oil viscosity and the smaller the fiber diameter, the5)more bearing noise is reduced . Komiya investigated therelationship between the base oil viscosity and penetrationof grease and the size of solid components and clarified
that the lower the base oil viscosity and the greater the
penetration, the louder bearing noise caused by thickener
6)
becomes . In response to these theories, we believe thatthe mechanical property of thickener affects bearing
noise, and devised a technique to measure the Young’s
modulus of thickener bulk using a nanoindentation.
Furthermore, using samples with differing Young’s
modulus and particle diameter, which are affecting
bearing noise, we measured the change in the EHL film

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−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

Table 1 Compositions and properties of test grease
Sample sign

T1

Thickener Composition (Diurea)
Carbon chain length

GreaseT3

T4

MDI-Octylamine MDI-Decylamine MDI-Dodecylamine MDI-Octadecylamine
8

10

12

18

SP value, (J/m )21 88721 47821 06820 250

Amount, mass%

15

15

15

18

246

258

3 1/2

Base oil

T2

Composition

PAO

SP value, (J/m3)1/2

17 18240℃
Kinetic
2
viscosity, mm /s 100℃

30.5

Density

15℃

0.827

Penetration

60W

5.82
251

248

MDI：Methylenediphenyl 4.4'-Diisocyanate
Table 2 Compositions and properties of test grease
Sample sign
Thickener

N1

N4
MDI

Amount, mass%

15
PAO

Penetration

N5

Aliphatic Alicyclic Aromatic Aliphatic Alicyclic Aliphatic

Composition
kinetic viscosity, mm2/s

GreaseN3

Isocyanate
Amine

Base oil

N2

40℃

30.5

100℃

5.82

60W

Average particle size of thickener, μm

256

329

356

267

302

2.8

2.4

1.3

0.3

13

thickness in order to investigate the mechanism of bearing
noise exacerbation.

2. Test Method
2. 1 Test grease
2. 1. 1 Low torque performance
Table 1 shows the compositions and properties of thevarious greases used to study low torque performance.
By using the same base oil and isocyanate (MDI), andchanging only the amine composition, we prepared fourtypes of urea grease with differing molecular structures(T1, T2, T3, T4). For the base oil, we used a synthetichydrocarbon oil (Poly-Alpha-Olefin: PAO) which is low
in impurities and widely used as a base oil of grease.
Unlike lubricant oil, grease contains the solid componentof thickener, and we believe that this solid component
is a causing factor of stirring resistance and affects thefluidity of grease. Here, in order to quantitatively evaluate
the stirring resistance caused by thickener, we used an
aliphatic diurea, made from reacting and synthesizing
aliphatic amines of different alkyl chain length and 4.4'diphenyl methane diisocyanate (Methylenediphenyl
4.4'-Diisocyanate: MDI) in base oil as the thickener. Also,in order to eliminate the effect of grease penetration,

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

which is believed to affect stirring resistance, we
controlled thickener amount and miniaturization
processing conditions to standardize the worked
penetration (JIS K2220) at 250±10.

2. 1. 2 Quietness
Table 2 shows the compositions and properties ofthe various greases used to study quietness. All of thesegreases used the same base oil and isocyanate. PAO wasused as the base oil and MDI was used as the isocyanate.
PAO6, which is classified as a low-viscous grease baseoil for rolling bearings, was used as the base oil, due tothe high demand to reduce torque loss of rolling bearingsand based on the anticipation that the affect of thickener
on the noise property would be prominent. The urea
compound, which is a thickener, was made from reacting
and synthesizing isocyanate and amine in base oil. For
this investigation, we prepared the samples of N1, N2
and N3, all of which had different amine compositions.
Amines used for each of the samples were of the straightchain alkyl group for sample N1, the monocyclic alkyl
group for sample N2 and the aromatic alkyl group for
sample N3. Moreover, sample N4 was prepared by
combining the two types of amines used in N1 and N2
and sample N5 used the same amine as N1 but had a27

−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−different miniaturization processing condition and larger
average thickener particle diameter.

2. 2 Test conditions
2. 2. 1 Bearing rotational torqueFigure 1 is a schematic view of the apparatus used tomeasure bearing rotational torque and Table 3 provides
the test conditions. For the test bearing, we used a deepgroove ball bearing (62022RU) with a non-contact seal
and a crown type resin cage filled with 0.72 grams ofgrease. Rotational torque was found by measuring thetangential force applied to housing assembled with atest bearing in a load cell and multiplying this by halfof the housing’s outer diameter. Furthermore, using abearing with 0.11 grams of base oil applied to the balls,
we measured rotational torque in the case of only asmall amount of base oil lubrication, and established thetorque causing stirring resistance deriving from thickener
as the difference between rotational torque caused bygrease lubrication and rotational torque caused by a small
amount of base oil lubrication, then calculated the time
integral of this torque causing stirring resistance derivingfrom thickener then defined the obtained loss energy as
stirring loss energy. We did this as we believed it would
make it possible to consider not only the starting torqueand rotational torque during stable operation, but also all
variations in rotational torque during the test period.

Axial load
Spring
Housing

2. 2. 2 Viscous transition stress
A viscoelasticity measurement apparatus was used
to measure the viscous transition stress of the grease.
Figure 2 is a schematic view of the measured portion.
Grease was wedged between two parallel surfaces and ameasurement was made under the conditions shown in
Table 4. We measured the storage modulus G' and loss
modulus G" when strain control was applied to graduallyincreasing sine wave strain. Grease was vibrated witha strain of 0.01 to 1 000% and frequency of 1 Hz. At
this time, storage modulus G' is an elastic component
holding the stress accumulated inside the grease and loss
modulus G" is a viscous component dissipated by theapplied energy, which acts as a heat source. When theapplied strain is small, G"/G' is less than 1, which is a
stress response close to an elastic body, and as the strain
gradually increases, G" increases. As G"/G' gradually
increases G"/G' becomes equal to 1, and loss modulus
G" is equal to storage modulus G', the stress is known as“yield stress.” However, strictly speaking, yield stress isthe size of the stress when a force is applied to an object
which exceeds the elastic limit, therefore the strain on theobject increases rapidly and it cannot return to normal. As
such, yield stress is not appropriate as an expression ofgrease fluidity. Therefore, we have defined the stress whencomplex modulus G*=G"/G'=1 as the viscous transition
stress of grease rather than yield stress and tested this as
the fluidity of grease. The greater this viscous transition
stress of grease is, the greater the force when grease
begins to flow while the bearing rotates, and it becomes
more difficult for the grease inside the bearing that has
been discharged once through the raceway to flow backinto the raceway, which indicates excellent channelingperformance.

Load cell
Strainf=1 HzTest bearing

SpindleShaft

Fig. 1 Schematic view of torque test apparatuss

Table 3 Rotational torque test conditions
Test bearing

6202

Axial load, N

44

Grease inclusion quantity, %35

Oil inclusion quantity, g

0.11

Pmax, GPa

0.93
−1

28

Rotation speed, min

1 800

Temperature, ℃

25±2

Test time, s

1 800

Parallel plate
Greaseu25
1mm

Stabilizer panel

Fig. 2 Schematic view of transition stress of viscoelasticity
measurement method

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−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

Table 5 Particle size test conditions

Table 4 Rheological property test conditions
Plate
Clearance, mm
Frequency, Hz
StrainTemperature，℃

Test method

u25 Parallel plate11−5

Laser diﬀractometry

Mesurement range, μm

0.01〜3 500

Carrier ﬂuid

TolueneNumber of measurement0

7×10 〜6×10
25±2

50 000−1

Agitation speed, min

3 000

Scattering intensity, %15±5

2. 2. 3 Thickener relative surface area
The relative surface area of the thickener particles wasderived from the measurement result of thickener particle
size distribution. A laser diffraction type particle size
distribution measuring apparatus was used to measure
the thickener particle size distribution. Table 5 shows
the measurement conditions. To measure the thickener
particle size distribution, we diluted 1 gram of grease
for the low torque study in small amount of toluene,
injected to the circulating toluene in the measuringapparatus, then irradiated it with a laser 50 000 times
to measure the scattering pattern. As Fig. 3 shows, lightscatters at a small angle in relation to the laser beam in
the case of large particles, but scatters at a large angle inthe case of small particles. Here, the thickener particles
are deemed spherical and, using the Mie theory for lightscattering, the diameter for a sphere that creates the same
scattering pattern as the thickener particles was derived
as the valid diameter of the particles. This valid diameter
was established as particle diameter to obtain particle
size distribution. From the results of the particle size
distribution measurement obtained here, Formula ⑴ wasused to derive the surface area of thickener particles withparticle diameter d . Here, “A ” is the surface area of the2
thickener particle [μm ], “d ” is the particle diameter [μm],3“V a” is the overall volume of all the particles [μm ], and“R ” is the volume fraction [%] occupied by the particles
with the particle diameter d .
2
4p（d / 2 ）× Va × R
3
4p（d / 2 ）
/3

A＝

⑴

From the sum total of the surface area obtained for allparticle diameter d , the surface area of the thickener for
the unit of volume was found, then by multiplying the
thickener amount TC [%], the relative surface area S of
the thickener in the defined grease was found by applying
Formula ⑵.

S ＝ R A（ d ）× TCd⑵

The reason why the thickener particles were deemed
spherical was because it was verified from results of adifferential interference microscope observation that the
thickener in the grease contained many agglomerated
particles with a shape close to a sphere.

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

Scattering light
Scatter pattern
Laser
Particle size : small
Thickener particle
Scattering light
Scatter pattern
Laser
Particle size : large
Thickener particle

Fig. 3 Schematic image of particle size measurement

2. 2. 4 Bearing noise value
The bearing noise was tested based on the bearing
vibrational acceleration. Table 6 shows the measurement
method for bearing vibrational acceleration rate (bearing
noise value). 0.72 grams of test grease for the quietness
study was injected into a bearing (62022RU) and sealed
in by attaching a non-contact rubber seal. Next, we set
the bearing in the spindle of the measuring apparatus,
applied an axial load of 20 N then used a piezoelectric
acceleration sensor to measure the vibrational acceleration
rate in the radial direction of the bearing outer ring, whichis greatest in the initial stage of rotation, when the bearing−1
was rotated at an inner ring speed of 1 800 min .

Table 6 Bearing noise test conditions
Items

Conditions

Bearing
I.D×O.D×W, mm

62022RU
15×35×11, mm

Grease quantity, g

0.63

Axial load, N

20

Rotation speed, min−1

1 800

Measurement time, s129

−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

2. 2. 5 Thickener Young’s modulus
1) Measurement sample adjustment method
We prepared the samples used for the measurementof the thickener Young’s modulus in accordance withthe conditions shown in Table 7 and the method shownin Fig. 4. After isolating the thickener from the grease,
we applied the thickener to the bottom of the die, theninjected phenolic resin on top of this and finally used heatcompression to form a sample specimen with embedded
resin. We believed that by using these adjustment
conditions it would be possible to keep the effect oftemperature on the thickener to a minimum and, due tothe pressure also being small, keep the effect of sample
fabrication to a minimum also.
Table 7 Sample preparation method
Item
Resin

Condition
Phenolic

Heating temperature, ℃

130Pressure, MPa

1.5

Compress time, s

900

2. 2. 6 Film thickness
In this test, optical interferometry was used to measure
the thickness of the grease EHL film in a point-contact
state comprised of a glass disc and steel ball. Figure 5
is a schematic image of the film thickness measuringapparatus and Table 9 shows the condition used whenmeasuring film thickness. The measurement itself
involved using a glass disc with chrome vapor-depositedon it and a 38.1mm-diameter steel ball for bearings,
pressing the ball up against the glass disc and measuring
the thickness of the grease oil film under pure rolling
conditions from the order of the interference fringes.
For the measurement, we performed a test under a low
speed condition (5mm/s), in which the film thickness
becomes thinner, and observed the inflow of thickener tothe contact interface. Also, with consideration to bearing
rotational speed, we conducted a test under a high speed
condition (100mm/s) and measured film thickness fromthe observation result of interference fringes.
Table 9 Measurement conditions of oil film thickness
Item

Condition

Load, N
Heat compression

Thickener

Thickener

90

Grease coating thickness, mm1

Temperature, ℃

25

Rotation speed, mm/s

5 100

Measurement time, min1Fig. 4 Schematic image of sample preparation method
PC

Camera2) Thickener Young’s modulus
Table 8 shows the measurement conditions for the
thickener Young’s modulus. Nanoindentation was used as
the measuring apparatus. After applying an indentationload, we found the Young’s modulus from the inclinationof the unloading curve when the indentation load wasremoved. We set the indentation load in a preliminary
measurement so that the indentation depth would be onetenth or less of the thickener’s thickness.

Light
Microscope
Glass discBall

Chromium coating

Load

Fig. 5 Schematic image of oil film thickness measuring tester

Table 8 Measurement conditions of Young's modulus
Item

Condition

Number of measuring location

4

Load, μN

500

Load time, s5

Wait time, s

330

3. Results and Observations
3. 1 Bearing rotational torque and stirring loss
energyFigure 6 shows the change over time of rotational
torque by using greases or base oil which were injected
into bearings. We confirmed that the rotational torquewith grease lubrication was higher than the rotational
torque with a small amount of base oil lubrication.
Moreover, because a difference in rotational torque wasrecognized between the various greases for studying low

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

30
20
T1

10
0

Base oil
0

600

1 200

1 800

30
20

T2

10
0

Base oil
0

600

1 200

1 800

Time, s(a) Running torque of T1(b) Running torque of T2

30
T3

20
10

Base oil
040Time, s40

0Rotational torque, mN・m

40

600

1 200

1 800

Rotational torque, mN・m

Rotational torque, mN・m

Rotational torque, mN・m−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

40
30
T4

20
10
0

Base oil
0

600

1 200

1 800

Time, sTime, s(c) Running torque of T3(d) Running torque of T4

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

Under grease lunbrication
Stirring loss energy

Under oil lubrication

0

1 800
Time, sFig. 7 Schematic of stirring resistance30
Stirring loss energy, J

torque performance, it was apparent that the terminal
alkyl chain length of thickener urea does have an affecton rotational torque. As shown in Fig. 7, in order to test
overall rotational torque variation throughout the durationof the test (e.g. initial stability of rotational torque, etc.),
we found the time integral of torque caused by stirring
resistance and calculated the energy lost during bearing
rotation due to stirring resistance deriving from thethickener. The colored portion of the figure indicatesthe stirring loss energy, and the smaller this stirring
loss energy is, the more difficult it becomes for greasethat has been discharged from the raceway once, to reenter, therefore indicating channeling type. It is also
indicated that grease which maintains a low absolutevalue for rotational torque from the initial phase has
superior rotational torque stability. Figure 8 shows thestirring loss energy of the greases for studying low torqueperformance. In the same way as for rotational torque, T4
shows the highest stirring loss energy while T1 shows thelowest. These results clarify that the terminal alkyl chain
length of thickener has an effect on stirring resistanceand, as the alkyl chain length increases, both stirring loss
energy and rotational torque increase. This is believed tobe because the fluidity of the grease overall is changed
due to the terminal alkyl chain length of thickener.

Running torque, mN・mFig. 6 Rotational torque of sample grease

20

10

0

T1

T2

T3

T4

Fig. 8 Stirring loss energy of sample grease31

−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

3. 2 Stirring loss energy and viscous transition
stressface. Meanwhile, in the case of grease with low viscoustransition stress, there is a large volume of residualgrease, therefore stirring resistance is high. This means
that, by designing grease with a thickener urea of a shortterminal alkyl chain length and high viscous transition
stress, it is possible to improve channeling performance.
We believe that this result is consistent with the theory ofgrease stirring resistance becoming smaller when the ballmoves.

Figure 9 shows the measurement results of viscoustransition stress in greases for studying low torqueperformance. A trend of viscous transition stressincreasing as the terminal alkyl chain length of thickener
urea grows shorter (short chain length) is recognized,
therefore it is clear there is a difference in fluidity.
Figure 10 shows the relationship between the obtained
viscous transition stress and the stirring loss energy
deriving from thickener, however in the greases forstudying low torque performance, a trend was recognized
whereby the higher the viscous transition stress becomes,
the more the stirring loss energy decreases. Figure 11
shows the results of an observation of the grease adhesion
status within the bearing. For T4, which has a longterminal alkyl chain length (long chain length), a large
amount of grease is adhered to the ball surface, but incomparison, only a small amount of grease is adhered
to the ball surface but large on seal for T1, which has
a short chain length. It is believed that for T1, which
has high viscous transition stress, rotational torque was
reduced as the movement of the ball did not inhibit thegrease and a stable channeling status was maintained,
therefore stirring resistance was suppressed. From theseresults, we ascertained that, regarding the flow of grease
inside a bearing, the grease moves due to the centrifugal
force generated in the initial phase of rotation then by
the movement of the ball and cage until finally it adheres
to a location, such as inside the seal, where it does notcontribute to stirring resistance. Afterwards, in the case ofgrease with high viscous transition stress, only the small
amount of grease which is not removed from the adheredportion and remains in close proximity to the ball andraceway, as well as the base oil separated from the greaseadhered to the seal and cage, are supplied to the raceway

SampleTransition stress ofviscoelasticity, Pa

1 500

1 000

500

0

T1

T3

T4

Fig. 9 Measurement result of viscous transition stressStirring loss energy, J

30
T4
20

T3

T2

10
T1
0
500

1 000

1 500

Transition stress of viscoelasticity, Pa

Fig. 10 Correlation between transition stress ofviscoelasticity and stirring loss energy

T1
Side of tabs

T2

T4
Back side

Side of tabs

Back side

Observation of theballs and retainer

Grease amount0.559 g0.570 g0.079 g0.045 gObservation of theseals

Grease amountFig. 11 Observation result of test bearing

32

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−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

5

0
0.1

T3

10

1 000

Particle size, µm

Volume ratio, %10Volume ratio, %Particle size distribution

T2

5

0
0.1

1 000

T4

10

Volume ratio, %T1

Volume ratio, %Sample500.1Particle size, µm

10

1 000

Particle size, µm

5

0
0.1

1 000

Particle size, µm

Average grain size, μm

2.23

3.682

3.93

4.624

Relative surface area

8 953

8 154

7 632

7 472

Fig. 12 Particle size distribution and relative surface area of thickener

Table 10 Measurement result of bearing noise

Transition stress ofviscoelasticity, Pa

1 500
T1

Sample sign
1 000

T2

T4
500
6 500

7 500

9 500

Relative surface area of thickener

Fig. 13 Correlation between relative surface area ofthickener and viscoelasticity

N2

N3

N4

N5

Aliphatic
Aliphatic Alicyclic Aromatic
Alicyclic

Amine

T3
8 500

N1

Aliphatic

Average particle
size of thickener,
μm

2.8

2.4

1.3

0.3

13

Bearing noise
value, G

0.029

0.157

0.243

0.021

0.186

Table 11 Measurement result of Young’s modulus

3. 3 Factors affecting viscous transition stress
As it was elucidated that viscous transition stress has
a large affect on the reduction of rotational torque, we
studied the factors affecting viscous transition stress. As
one of the factors affecting viscous transition stress whichindicates grease fluidity, we focused on the interaction
between the thickener and base oil contained in grease.
While there are said to be a number of physical property
values that indicate this, we believed the relative surface
area of the thickener particles, which expresses theinterfacial area between the thickener and base oil, tohave a particularly large effect. Figure 12 shows theparticle size distribution and calculated thickener relative
surface area for each of the greases. It was elucidated that,in contrast to the short-chain T1, whereby surface area
was large due to increased number of small thickeners
with small particle diameter, long-chain T4 had a small
relative surface area due to increased number of thickener
with large particle diameter. Here, Fig. 13 shows theresults of studying the correlation between the obtained
thickener relative surface area and viscous transition
stress, however, a tendency for the viscous transition
stress to increase as the thickener’s relative surface area
became larger, was clarified. In the case of thickener withlarge relative surface area and small particle diameter,
the number of particles increase, therefore the distance
between the thickener particles is shorter and there is

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

Sample sign
Youngʼsmodulus,
MPa

N1

N2

N3

N4

N5

n＝1

685

1 568 3 382

783

706

n＝2

729

1 678 2 621

821

627

n＝3

734

1 526 2 592

689

687

n＝4

685

1 663 2 273

755

592

Average708

1 609 2 717

762

653

greater interaction between particles. Moreover, it isbelieved that the fluidity of the grease is inhibited due tothe small particles dispersing in the oil and taking on a
pseudo-slurry state.

3. 4 Factors affecting bearing noise value
Table 10 shows the results of measuring the bearing
noise value of the greases for studying quietness withdifferent amine types and miniaturization processing
conditions. The table shows both the bearing noise
value after one second’s worth of rotation as well as theaverage particle diameter of the thickener measured fromthe diameter of the particle size distribution, taken witha laser diffraction method. Comparing the samples of
N1, N2 and N3, it was observed that the bearing noise
value varied greatly depending on the amine type of the
thickener, with noise value being smallest for the aliphatic
diurea, second smallest for the alicyclic type and finally,33

−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

Sample sign

be minimized and the smaller the bearing noise value
becomes. If thickener Young’s modulus is low, we inferthat the variation amount of the thickener when it becametrapped was greater, therefore it was possible to minimize
the vertical vibration of the rolling element and reducebearing noise value.
Meanwhile, when we investigated the relationshipbetween thickener Young’s modulus and bearing noise
value using the samples of N4 and N5, which havediffering thickener average particle diameters, thecorrelation seen in Fig. 14 was not obtained. Previous
research has reported that there is correlation between
the thickener particle diameter and bearing noise value,
whereby the smaller the particle diameter, the more4)
the noise value reduces , and we verified that ourinvestigation results demonstrated the same tendency.
This is because both the thickener Young’s modulus andaverage particle diameter are the factors affecting bearing
noise value.

0.3
Aromatic
Bearing noise value, Glargest for the aromatic type. Moreover, we confirmed
that, compared to N1 and N2, the bearing noise value forsample N4, a combination of the aliphatic and alicyclic
types, was lower. Meanwhile, for sample N5, which used
the same aliphatic amine as N1 but had a bigger average
particle diameter, it was confirmed that the bearing noise
value significantly increased compared to N1.
Table 11 shows the measurement results for Young’s
modulus. When a comparison was made between thesamples of N1, N2 and N3, with differing thickener
types, it was observed that Young’s modulus was thelowest in N1, which used aliphatic amine, second lowest
in the alicyclic type of N2 and highest in the aromatic
type of N3. Furthermore, the Young’s modulus for both
sample N4 and N5 had values close to that of sample
N1. The resin-embedded test specimen on which thismeasurement was performed was prepared by applying
1.5 MPa of compression stress, however this is extremely
small compared to the absolute value obtained for theYoung’s modulus of the thickener on this occasion, andwe deemed there to be no affect.
Based on the results obtained on this occasion, we
considered the factors of grease that affect bearing noise
value. Of the various greases used to study quietness,the samples of N1, N2 and N3, which use thickeners
of differing amine types but have an average particle
diameter of around the same size (within the range of 1.3- 2.8 μm) were used to evaluate the effect of the thickener
Young’s modulus on bearing noise value. The results areshown in Fig. 14. There is a strong correlation between
bearing noise value and thickener Young’s modulus andthe lower the Young’s modulus, the smaller the bearing
noise value. We believe that, when the contact face ofthe rolling element and raceway is flowed into not onlyby the base oil of grease but also thickener, the smaller
the vertical vibration amount of the rolling element
when thickener is trapped the more oil film variation can

N3
0.2

Alicyclic
N2

0.1

Aliphatic
R2 = 0.9836

0

N1
0

1 000

2 000

3 000

Young's modulus of thickener, MPa

Fig. 14 Effect of Young’s modulus of thickener on bearing
noise

N1

N2

N3

N4

N5

Max.

690

790

800

630

850

Min.

550

550

550

520

600

Variation range

140

240

250

110

250

5mm/s
Speed
100mm/s

Oil film thickness, nm

Fig. 15 Measurement result of oil film thickness

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−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

Figure 15 shows the results of an observation ofinterference fringes under the speed conditions of 5mm/s
and 100mm/s as well as the results of a film thickness
measurement obtained from the order of the interference
fringes under the condition of 100mm/s. Under the low
speed condition of 5mm/s when film thickness becomesthinner, we were able to clearly observe variation in
the oil film caused by inflow of thickener particles.
Even under the high speed condition of 100mm/s, werecognized oil film variation believed to be caused by the
thickener. This observation was of samples with largelydiffering Young’s modulus and average particle diameter
however, for all of the samples, thickener particles flowed
into the contact face and we judged it was not possible
to perform control so that the thickener did not becometrapped in the contact face.
We found film thickness from the film thickness at
the center of where the glass disc and steel ball makecontact. Moreover, we performed observation in five
fields of vision with consideration to the point that film
thickness may vary due to thickener inflow and found thefilm thickness variation width by subtracting the smallest
film thickness from the largest film thickness. The resultsof this measurement showed that the film thickness
variation width was smallest for N4, the sample using a
combination of the aliphatic and alicyclic amines and next
smallest for N1, which used an aliphatic amine alone. We
also confirmed a wide film thickness variation range forN2 and N3, which used an alicyclic and aromatic amine
in isolation respectively, as well as for N5, which had alarge thickener particle diameter.
We believe that the reason why the thickener Young’s
modulus and average particle diameter are the factors
which affect bearing noise value is because thickener
flow in between the rolling element and raceway becomestrapped and causes oil film variation, which in turncauses the bearing to vibrate. Here, we investigated therelationship between the film thickness variation width,
which was measured from the observation results using
an optical interference method, and bearing noise value.
Figure 16 shows the results.
We recognized a strong correlation between the film
thickness variation width caused by grease thickener andbearing noise value. For the N4 sample, which had low
Young’s modulus and a small average particle diameter,
we infer that it was possible to lower bearing noise value
due to the oil film variation damping effect being great
when thickener became trapped.

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)

0.3
Vibrational acceleration, G

3. 5 Effect of film thickness variation on bearing
noise value

N3

R2 = 0.9677
0.2

N2
N5

0.1
N4

N1

0
100

150

200

250

300

Amount of oil film variation, nm

Fig. 16 Relationship between amount of oil film thickness
and bearing noise

4. Conclusion
We quantified the effect of the compositions andproperties of grease on rolling bearing torque. As grease
properties, we introduced the concepts of viscoustransition stress as a new indicator to substitute yieldstress and, in regards to bearing torque, stirring resistanceenergy, which is the time integration of torque causedby thickener. As a result of studying the correlation
between terminal alkyl chain length, stirring loss energy
and viscous transition stress for aliphatic diurea grease,
we elucidated that the shorter the chain length, the lower
the stirring loss energy and higher the viscous transition
stress. We inferred that, the shorter the chain length ofaliphatic diurea grease is, the greater viscous transition
stress increases due to a higher collision frequency
between thickeners as a result of small particle diameterand a large number of particles.
In regards to the reduction of bearing noise value,
we conducted an investigation focused on the aspectsof thickener elastic modulus and oil film variation. The
results showed that bearing noise value is favorably low
for diurea grease with a small elastic modulus made fromaliphatic amine, as well as for grease combining alicyclic
and aliphatic amines for which both particle diameter andoil film variation are small.

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−Low Torque and Low Noise Technology for Ball Bearings Under Grease Lubrication−

References
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2)

3)

4)

5)

6)

E.Watanabe, Y. Toda: Low Torque Technology by Grease
and Application to Motor Bearings, JAST Tribology
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T. Akutsu, M. Masuko, A. Suzuki: Structural Change of
Lubricating Grease Considered from Viscous Reduction
by Shear, Tribologist, Vol. 52, No. 6 (2007) 445-452. (inJapanese)
M. Nitta, T. Tsuda, H. Arai, K. Sakamoto, K. Sakai:
Effect of Transition Point of Viscoelasticity of Diurea
Grease and Molecular Structure of Thickener on Running
Torque of Ball Bearing, Tribologist, Vol. 61, No. 10(2016) 699-708. (in Japanese)
K. Matsubara, D. DONG, T. Endo: Low Noise Greases
for Bearings, NLGI SPOKESMAN, Vol. 72, No. 6 (2008)
25.
T. Igarashi, N. Kamiya: Effect of Lubricants on Vibrationand Sound of Ball Bearings, Journal of Japan Society ofLubrication Engineers, 17, 3 (1972) 154. (in Japanese)
H. Komiya: Effect of Contamination in Lubricant on
Noise of Ball Bearing, NLGI SPOKESMAN, Vol. 56,
No. 5 (1992) 173.

K. MIYAKE**

36

T. TSUDA*

K. YOSHIZAKI*

Material R&D Dept., Research & Development
Headquarters

JTEKT ENGINEERING JOURNAL English Edition No. 1015E (2018)