Development of Grease
focusing on Irn~r@vec~i /~nergy Efficiency
Masamichi Yamamoto
Junichi Imal
Kyodo Yushi Co., LTD
1. Introduction
In modern times, where energy and resource
saving strategies are becoming more important for
environmental conservation, the control of air pollutants
such as C02 has been one of the most urgent global
issues. Among other C02 emission sources, plant
facilities and automobiles are considered to be the
two major contributors. In plant facilities, production
machinery and equipment as well as motor driven fans
and pumps consume large amounts of electricity and
power plants consequently emit significant amounts of
C02 to cover the electricity demand in the facilities. As
for automobiles, there has been a downward trend in
the amount of C02 emissions since the 1990s thanks to
the growth of the eco-car. Still, looking at C02 emission
sources by sectors, transportation sector is highly
influential against the industrial, residential and energy
sectors.
Accordingly, electric and automobile sectors are
working on energy- and resource-saving strategies
including a reduction of power consumption by
improving the torque characteristics of an electric motor.
To be more specific, component and material designs
and lubricants are being reviewed for potential torquereducing effects, and grease in its unique “semi-fluid”
state has attracted considerable attention being
extensively studied to find out application-specific
formulations.
This study describes a design concept and study case
to develop a rolling element bearing grease with torque
reducing effects.
2. Torque generatingfactor during rolling element
bearing operation
The torque during rolling element bearing operation
occurs accompanied by friction resistance which is
classified roughly into three types: grease’s churning
resistance, rolling-viscous resistance and seal
resistancel).2). Greases are involved especially in
the two torque-generating factors: resistance acting
on the rolling contact surfaces of rolling elements in
elastohydrodynamic lubrication (EHL) and lubricant’s
churning resistance observed at any other site (Fig. 1).
Considering that these two factors are, basically,
dependent on grease base oil viscosity, a low viscosity
base oil was conventionally used to decrease bearing
torque. However, a low viscosity oil had limited
effectiveness as it could only form a thin oil film and
shortens the rolling fatigue life of the bearing.
Thus, a new low-torque technique without using a low
viscosity base oil, or with using a low viscosity base oil
not adversely affecting the rolling fatigue life has been of
great interest to bearing manufacturers.
– 18 VOLUME 78, NUMBER 4
NLGI
3. Design conceptfor low torque
3.1 Low torque by preventing metal-metal contact
at low speed
Recently, greases in rolling contact at very low
speeds were found to form a much thicker EHL film
compared to oil lubrication. Endo et al. conducted
an optical interferometric study (Fig.2) on EHL film
formed by some typical greases. In their interferometric
measurements(Fig.3), a base oil behaved according to
the EHL theory in both low- and high-speed ranges.
Whereas, greases approached asymptotically to the
base oil at high speed but formed a thick EHL film at
low speed. In particular, urea-thickened greases formed
a thicker EHL film than lithium-thickened greases,
and a horse shoe-shaped interference fringe typically
observed in the EHL contact, suggested that the EHL
effect was dominating at a low speed, being responsible
for the increase in the film thickness 3)4). Their study
implied that urea and some greases can form a robust
EHL film even at very low speeds in the partial EHL
regime and are applicable to a wide range of lubrication
regimes. Subsequently, Doe et al. reported their study
on an EHL film thickening effect at low speed, based on
measurements of isolation voltage and frictional torque
between inner and outer rings of deeply grooved ball
bearings subjected to axial load (Fig.4). The results of
isolation voltage measurements (Fig.5) showed that a
base oil maintained stable electrical continuity below
100 rpm, the current flowed less easily at 200 rpm and
was almost insulated at 600 rpm. By contrast, grease
was not only insulated at high speed but also conducted
almost no electricity at 1rpm. These results confirmed
the EHL film thickening effect of grease in real rolling
element bearings operating at low speeds (Fig.6) and
agreed well with the findings of Endo’s study. In their
frictional torque measurements (Fig.7), base oils formed
an EHL film at high speed and a drop in resistance of the
film with decreasing speed, decreased torque. At a lower
speed, metal contacts occurred and increased torque. By
contrast, greases had a low torque even at low speed, at
which base oils had a high torque.
These results demonstrated that frictional torque of
the bearing can be considerably decreased even under
severe low speed and high load conditions, where metal
contacts leading to high torque are likely to occur in oil
lubrication, by the technique of selecting an appropriate
thickener that will impart an EHL film thickening effect
to a low viscosity oil-based grease5).
3.2 Low torque by reducing churning resistance
Figure 8 shows torque-generating factors for deeply
grooved ball bearings operating at 8000rpm. The
grease-related resistance makes up more than half of all
torque-generating factors, specifically by grease churning
resistance and rolling viscous resistance accounting for
45°o and l6°o, respectively.
Churning resistance of grease in bearings can be
reduced by the following two approaches: decreasing
viscous resistance, i.e., apparent viscosity of grease with
respect to shear rate and optimizing the behavior of
grease in bearings. The former can be done, when the
same base oil is used, by reducing thickener content
(Fig.9). That means, a grease needs to be softened or
formulated with a thickener having an exceptional
thickening performance. In the torque measurements for
two greases of the same base oil viscosity and penetration
but a different thickener (Fig. 10), grease A having a
low thickener, content had a low torque indicating
grease churning resistance is significantly affected by
the amount of thickener. As for the latter, it should
be noted that a grease is usually packed to fill about
20-30% of bearing void and makes a channel which has
a large impact on torque level. The behavior of grease
in bearings can be divided into two distinct periods
as shown in Fig. 11: churning period, at which grease
is present in all shear area while being churned and
channeling period at which a certain portion of grease
stays in a fixed position to be involved in lubrication
and the other portion is eliminated making a channel.
The impact of churning resistance is more substantial at
the churning period than at the channeling period6). A
thickener with short fibers is effective in inducing early
channeling while long and tangled thickener fibers can
stimulate churning (Fig. 12). In the torque measurements
for two greases of the same base oil viscosity but a
different thickener (Fig.13), thickener I had a stable
torque within a short time due to early initiation of
19channeling.
NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
These results confirmed that frictional torque can be
decreased by selecting an appropriate thickener to optimize
the behavior of grease in bearings instead of using a low
viscosity base oil.
3.3 Low torque by reducing rolling viscous resistance
In case of bearings with a contact angle like angular
contact ball bearings and tapered roller bearings, toque
is greatly affected by traction in grease-lubricated
EHL contacts. There have been attempts to reduce
traction focusing on “hardware” aspects, for instance
by improving bearing internal factors. On the other
hand, grease is considered to have much potential for
further traction reduction. Under the same operating
conditions, contribution of traction differs significantly
depending on the molecular structure and formulation
of lubricating oil (Fig. 14). With lubricating oils of similar
formulation and/or structure in the same series, viscosity
at normal pressures has minimal impact because traction
is associated most closely with a viscosity-pressure
coefficient, a7). Therefore, naphthenic mineral oils having
“bulky” branched molecular structure with a higher a have
a higher traction coefficient than paraffinic mineral oils
(Fig.15) 7). Oils with a lower a are effective in reducing
traction, meaning that oils with a low traction coefficient
such as poly-alpha-olefin can effectively decrease torque.
The results of traction measurements showed that grease
I formulated with flat structured poly-alpha-olefin had a
lower traction coefficient than a mineral oil-based grease
(Fig.16).
4. Study case Rolling element bearing grease with
torque-reducing effect
–
Table. 1 summarizes formulation and general properties
of a grease developed for use in automobile bearings. The
development objective was an improved torque-reducing
effect over the whole speed range. Based on the abovementioned findings, the grease was thickened with urea
to prevent an increase in frictional torque at low speed
by forming a thick EHL film to avoid metal contact. To
ensure low torque at high speed, an urea thickener with
exceptional thickening performance was used so as to
decrease the solid content and apparent viscosity of the
grease. A synthetic hydrocarbon oil was used as a base oil.
In apparent viscosity measurements using a rheometer
(Fig.17), the developed grease had approx. 3000 less
apparent viscosity compared to a conventional grease
(Fig. 18) indicating that apparent viscosity is highly
dependent on solid content in grease. In traction
coefficient measurements (Fig.19), the developed poly-~
olefin based grease had a lower traction coefficient than
mineral oil-based grease A, being in good agreement with
the results with base oil alone as mentioned earlier.
In torque measurements (Fig.20), the developed grease
with a low apparent viscosity and a low traction coefficient
was proved to have a torque-reducing effect showing about
a 500o reduction in running torque in relation to churning
resistance and rolling viscous resistance.
5. Summary
This report introduced a study case where a rolling
element bearing grease having improved torque-reducing
effect was developed without relying on the conventional
low viscosity base oil technique. We successfully achieved
low torque and long service life at the same time, instead
of using a low viscosity base oil which often shortens
bearing life, by selecting the best combination of base oil
and thickener and identifying the most effective thickener
(solid) content to ensure a sufficient EHL film formation
and to optimize the grease behavior in bearing.
The developed bearing grease with improved torquereducing effect can be beneficial to the resource- and
energy-saving strategies in various industries.
References
1) Tedric A.Harris: Rolling Bearing Analysis Second
Edition (1984)
2) Rolling Bearing Engineering, edited by Editorial
Committee of Rolling Bearing Engineering, Yokendo
(1978)
3) Endo et al. : Tribology Conference Proceedings
5(2008) 181.
4) Don et al. : Tribologist, 57,8(2011)62.
5) Don et al. : Tribologist, 56,1(2011)24.
6) Hoshino : Junkatsu: 25 (1980) 547
7) Masayoshi Muraki : Junkatsu,33,11,(1988) 811
20 VOLUME 78, NUMBER 4
I
-~
~
‘4%
ft
Churning resistance of grease
Rolling-viscous resistance
Fig.1 Grease and torque-generatingfactors
a……
Pattei n
observation
….a…………ØiI
CCDcamera~
Spectronietei~
~
S..—
Optical III~III,.,flhII
~I
microscope
.
a
S
:
Steel ball
..
•
H
~H
G ass disk
Thickness
measurement
__________
Load
Fig.2 Ultra thin EHLfilm thickness measurement system
– 21 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
1000
E
C
U,
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LJ-OHSI
U,
•
•
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A.
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U
.~
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4.
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S
u-si
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LL
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h.
(a) O.004m/s
(b) tLOlmIs
(c)022m1s
(d)O.6lmls
Base oil
C
U,
C.)
0001
001
01
Entrainment speed/ms-i
Fig.3. EHLfilm thickness measurement 3),4)
–
Spline
shaft
~
•
______________
r4 —
Aa
________
A
Fa
(II
Servo
motor
Load
celi~j
Test bearings 6204
4.
Torque
Electrical
potential
Fig.4 Bearing torque tester
–
– 22 VOLUME 78, NUMBER 4
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Fig.5 Electrical potential
–
1000
E
C
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GREASE
100
0
1.)
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10
9-F~
y’.
C
a)
0
—
1
1
4.
9-BASE OIL
—
9—’
100
10
Entrainment speed I rn/s
1000
Fig.6 EHL oilfilm thickness vs speed rolling element bearing
–
–
– 23 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
0.2
BASE OIL Visc. A < B < C
GREASE
A
By thickener amount
I
Cu
High
Shear rate. s~1
Low
Fig.9 Reduction in churning resistance
–
Spline shaft
(1
0.3
Motor
Test bearing
i;
Load
ccli
•
-Apparatus• Bearing lubrication performance tester
(as illustrated above)
-Bearing 6204 contact seal (Deep groove bail bearing
-Rotational speed 800rpm
-Load. Fa=500N 6204
.
Penetration : 280
Base oil:7Omm2is @40 °C
Grease B
(thickener amount =21%)
02
101
~
Gre~se1A~
~(thickener.amount1i1P6)
0
10
0
.
Time (mm)
Fig. 10- Frictional torque measurement Grease A, B
—
– 25 NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
20
30
Churning
Cage Grease
.
v~1~
*
Channeflng
Spa e
I s./
Bali
Fig.11 Churningperiod and channelingperiod 7)
–
Thickener I
—
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Fig.12 Thickenerfiber
–
Spline shaft
01
008
Thickener II
Motor~$~y4
c.I)
[1
—
Eloctilcal
.
~!
potential
,r°4
002
-Bearing : 6204(pair, parallel)
-Load: Fr200Nieach
-Rotational speed: 5000rpm
Thic1n~ r I
0
~
0
6(X)
12(X)
time(s)
Fig.13 Frictional torque measurement Thickener land II
–
–
–
26
–
VOLUME 78, NUMBER 4
Tesi oil
Kiiieinat~c visc.
1
0.10 2
3
0.09 4
(nm~_s@40eC)
I ,Conuiwrcial ~ynthctic traction oil
31.1
2 .Contrn~rcia1 synthetic t~ction oil
37,2
iyrnhetic traction oil
96.1
oil
45.6
3.Cornniercial
4.Commercial symbelictraction
5. Alkylbenzenc
w
Zi 0.07
E
01)6
U
3S.8
6.Coninicrcial syi~tlietjc traction
oil
0.05
9.72
0.03
6
10.3
0.02
31.8
0.01
8
9
7.”~apbtheiiic mvneraL
oil
26.9
8.Naphthenic myncral
oil
9,Paraffinic m~iera1
~ 0.08
oil
10. Commercial synthetic engme oil
61.51
1l.Polya olefm
32 0
I—
10
20 40 60 80 100120140
(I
00 temp. °C
Fig. 14 Traction coefficient 6)
–
[Low traction coefficient]
~High traction coefficient]
V
Flat molecular structure without
asperily
Projections get lodged in molecular
surface asperity
Fig.15 Traction coefficient 26)
–
Grease K ~MlneraIoil)
00?
Disk
AAA A A
~ 0,05
••
10,04
Grea5e
A
•,.•
.
Steel bal
Greas~ j
U
001
0
Apparatus; EHL oil film thickness measurement system
10
Sliding ratio, %
-Max. contact pressure: 0.5GPa Temp.: 250C
Rotational speed :0.5m(s (constant steel bell speed)
SI ding ratio 0~20 Steel ball ~19.O5mm
—
–
—
Fig.16 Traction measurement
–
(PA~)
.
0c~
–
•.•••
20
Properties
Developed grease
Conventional Grease
Thickener
Thickener %
Base oil Type
DiureaA
II
Mineral oil
DiureaB
22
Mineral oil
+
Kinematic
40°C
viscosity
100°C
mm 2/~
Worked penetration
Synthetic_hydrocarbon
69.0
9. 19
74.7
9.30
280
300
Table. 1 Generalformulation and properties
Motor
-Rheometer
Plate: Cone type
Plate radius R;12.5mm
Plate distance:OO5mm
Cone angle:2°
-Temp. :25°C
Grease
–
Plate
–
–
–
Rheometer
Fig. 17- Rheometer
20
Mean of shear rate series 101-103
Cu
a
5
0
Developed grease
Conventional Grease
Fig.18 Apparent viscosity developed grease and conventional grease
–
–
– 28 VOLUME 78, NUMBER 4
0.07
Conventional Grease
—.
•
o
o uu~
A
,~
AAAA
AAA
21% reduction
A
~O.O2
A
0.01
Developed Grease
0.00
0
5
10
15
20
Sliding ratio, %
Fig.19 Traction coefficient developed grease and conventional grease
–
–
0.4
~ 0.3
Grease
z
Developed Grease
I
0
~0.1
Sealing
resistance
0
0
10
Time (mm)
20
Fig.20 Torque developed grease and conventional grease
–
–
–
29
–
NLGI SPOKESMAN, SEPTEMBER/OCTOBER 2014
30