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Analysis on Failure of Hub Bearing Greases Based on Rheology
Wu Baojie, Mi hongying, Liu qinglian, Buoxiaochuan, Chen deyou,
Tianjin Branch, Lubricant Company, Sinopec Corp Tianjin 300480, China.
Presented at NLGI’s 79th Annual Meeting, June, 2012, Palm Beach, Florida, USA
becomes larger with higher base oil chemical polar
ity, because of the van der Waals forces. Cann (ref
10) studied the effect of van der Waals and capillarity
forces on grease base oils under steady state condi
tions. Thermal energy and force were reported to make
the base oil leak from the thickener, and caused the
grease structure to start to disintegrate. Hurley (ref 11)
found that the grease modulus does not recover com
pletely after shearing under higher temperature and
shear stress conditions. Temperature was seen to have
a bigger effect than shear. MartIn-Alfonso (ref 12) had
obtained a higher modulus, more adhesiveness and
lower elasticity grease when using high viscosity base
oil and polymer in laboratory testing. The aim of this
paper is to try to determine the reasons for the failure of
hub bearing grease by testing different grease rheologi
cal properties under controlled temperature and shear
rate on a Physica MCR3O1 rheometer.
Introduction
Rheology is the science of deformation and flow of
matter. Lubricating greases demonstrate a variety of
rheological characteristics such as visco-elasticity, plas
ticity and Thixotropy. These parameters are influenced
by the grease formula and manufacturing process.
Typically, rheological characteristics are determined by
three differing measurement modes: steady shearing
tests; transients tests and dynamic oscillation tests.
The last type of test causes the least amount of dam
age to the grease thicker structure. In rheological
terms, the output parameters from a rheometer are the
storage modulus G’ and the loss modulus G”. These
parameters can be related to the lubricating grease
internal network structure. G’ represents grease elastic
behavior and describes the deformation energy from
the shear stress (-r) which is stored temporarily. G” is a
measure of the loss of deformation energy. Shear strain
(y) is the key parameter which represents fluidity.
Under a moderate shearing rate, the grease rheology
mathematical model can be described as follows:
t =
tO
+
~(du/dz)n
Experiment
Materials
For this experiment, two different types of grease
thickener and four different base oils were used in five
grease samples, along with two different types of poly
mer. The base oils chosen included paraffinic mineral
oils, PAO and a blend of both mineral oil and PAO.
The characteristics of the five grease samples are
listed in Table 1.
equation 1
The apparent viscosity (11) is affected by the base
oil viscosity and the concentration of the thickener.
In bearings, the grease’s main functions are sealing
and lubrication. There is an intermediate zone between
these two grease functions at which the shear rate is
zero. High temperatures and high shear rates can make
the grease run out of the bearing. Thus yield stress and
apparent viscosity are the key parameters for governing
this behavior. Läuger (ref 8) observed that both grease
storage modulus G’ and the loss modulus G” and
the yield stress increase when the temperature falls.
Thomas (ref 9) reported that the storage modulus G’
Measuring system
All experiments were performed using a Physica
MCR 301 rheometer. This device is equipped with an
electronically commutated synchronous motor and
an air-bearing drive. The technical data is as follows:
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4ff
Table 1
The properties of five greases
Items
ASTM
Method
jinzhi-1
jinzhi-2
Sample
jinzhi-3
jinzhi-4
jinzhi-5
Li. Comp.
lithium
Li. Comp.
lithium
Li. Comp.
thickener
concentration, %
11.5
10.5
14.8
10.2
12.2
base oil and type
mineral
mineral
PAC
mineral
PAO/mineral
tackifier (PIB/OCP)
content, %
0.5 (PIB)
0,5 (PIB)
5 (COP)
0
0
thickener type
base oil viscosity
(100/40°C), mm2/s
D445
18/220
9.7/1 00
10/110
9.7/100
10/77
penetratiøn, worked
60, 0.1mm
D21 7
255
240
243
241
281
NLGI grade
D217
2.5
3
3
3
2
dropping point, °C
D2265
290
197
300
195
300
leakage (104°C, 6h), g
Dl 263
0
0
0.1
0.1
0.1
Oil separation
(100°C, 24h), %
D1472
0.8
0.9
0
0
1.2
torque 0.01 pNm to 200 mNm (resolution: 0.001 pNm)
angle resolution: 0.012 prad
axial load: 0.01 -50 N (resolution : 0.002 N)
frequency: 1 x 1 0~ to 100 Hz;
temperature range: -40 to +200°C.
Rheology measurement
The tests were carried out at 80°, 120°, and 150°. This
is because as the grease approaches the dropping
point it becomes fluid and it is important that the sam
ples were still non-Newtonian materials. The rheology
measurement modes are below:
(1) Controlled-stress rheology measurement under
oscillatory strain amplitude sweep. Under angular fre
quency of 10 s~, the strain amplitude was increased
steadily from 0 to 100%. The storage modulus G’ and
the loss modulus G” were measured.
(2) Controlled-strain rheology measurement under
steady stress sweep. The grease’s apparent viscosity
and stress during shear rate ramping were measured.
Based on experimental data, Herschel-Bulkley empiri
cal correlations were obtained via regression.
For our experiments the grease samples were
applied on a cone/plate measuring system with a gap
height of 1 mm. A Schematic of the cone-on-plate
rheometer is in Figure 1.
plate
4
I.
Figure 1
—
5chematic of the cone-on-plate rheometer
~
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VOLUME 77, NUMBER 3
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Field test
Test protocol: 20 Steyr trucks (model for
Don gfeng EQ1 11 8GA) were chosen as
the test vehicles on the position locality
where the altitude is about 3000 meters.
The ambient temperature during the test is
15 to 35°. This test is used for predicting
the performance of greases in automotive
wheel bearings. No leakage was allowed
after travelling distances of 3000 km or
6000 km or more.
Figure 2— CRC standard color plate
Test Methodology: Take the hub wheel apart and
check the grease and oil film thickness and distribution,
the quantity of grease in the cage and grease loss near
the bearing seals. CRC standard color plates were used
to evaluate the color of the roller bearings, If the rollers
after driving had seen a different temperature, then the
CRC standard color plate stuck in the bearing inner ring
would reflect (see Figure 2) the increased temperature.
After the visual inspection, the wheel bearing parts were
disassembled and samples taken for analysis. The tests
and analyses carried out were based on the sample
quantity obtained. Typical tests such as the dropping
point and micro-cone penetration were carried out.
Changes in the soap fibers, before and after use, were
examined by a scanning electron microscope (SEM).
G’
Strain (%)
(b) 120°C
0.01
0.1
1
Strain (%)
Strain (%)
(a) 80°C
(c) 150°C
Figure 3—Storage modulus G and the loss modulus C” plotted against strain at 80, 120 and 150°C
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NLGI SPOKESMAN, JULY/AUGUST 2013
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£
Results and Discussion
Amplitude Sweeps
will strengthen its van der Waals forces and increase
hydrogen bonding, but high temperatures will make the
force weaker. So if grease thickener is simple lithium,
the elastic modulus will decrease with the environment
temperature increase, but we determined that with
complex lithium grease, the modulus is greater at 1200
than that of at 80°, as was seen in jinzhi-1 and jinzhi-2
grease samples. Sanchez (ref 15) also reported this
type of findings in his research.
In this paper, the point at which G’ is equal to G” is
called the flow point. The strains at the flow point (y) is
the key parameter which can bind the static region and
the flowing region. If y increases, more time is needed
to make the grease flow. By our rheological experi
ment for five grease samples, we found that the higher
grease thickener concentration and higher base oil
viscosity polarity can make the value of y higher. The
G* represents the grease deformation energy which is
stored and lost under the shear stress, and it has every
relevance to grease yield stress. High grease thickener
concentration, higher base oil viscosity and greater
polarity, the presence of polymer will increase the elas
tic energy. Under different environmental temperatures,
the effects of these important factors on the rheology
of the grease will alter. In the five grease samples, we
can see the different factors affecting the properties in
differing ways:
the grease thickener concentration order from high to
low is:
jinzhi-3 > jinzhi-1 > jinzhi-2 > jinzhi-4 > jinzhi-5;
the grease base oil viscosity order from big to small is:
jinzhi-1 > jinzhi-3 > jinzhi-2>
jinzhi-4 > jinzhi-5;
the base oil polarity order from
strong to weak is:
jinzhi-1 > jinzhi-2 > jinzhi-4>
jinzhi-5 > jinzhi-3.
When the temperature increases
the effect of base oil and polymer
for elastic modulus will be reduced,
but the grease thickener and oil
polarity have a bigger influence. An
increase in temperature will make
the thickener structure swell up with
base oil and make the attraction
force between thickener structure
weakened because of increased
kinetic energy and greater Brownian
motion. The elastic modulus of the
grease was then determined. By
analysis of experiment data, we
found that the sample of jinzhi-5 has
a higher modulus than the sample
of jinzhi-4. This appears to be
because jinzhi-5 is a complex lithium
grease. The diacid in the thickener
Table 2
Data of G’ and G” from the five greases and strains at the flow point
Yield point
Sample
T,
Flow_point
00
G’ (G”)
Pa
G’, Pa
G”, Pa
80
115000
18500
10
4880
10
120
125000
18300
4.9
8100
4.9
150
35600
8550
4.22
2470
4.22
80
81600
12200
3.1
8200
3.1
120
81000
15100
1.2
8220
1.2
150
21500
3350
0.7
3900
0.7
80
35900
4850
2.4
7200
2.4
120
39200
3580
4.1
3600
4.1
150
15800
1670
2.2
1900
2.2
80
25600
2670
2.95
5050
2.95
120
23700
4200
3.25
3500
3.25
150
15200
2850
1.15
3700
1.15
80
51400
3410
1.35
9340
1.35
120
38900
2790
1.23
4600
1.23
150
11700
2140
1.1
2350
1.1
~‘
%
~“
~
__________
jinzhi-1
jinzhi-2
jinzhi-3
jinzhi-4
jinzhi-5
~
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VOLUME 77, NUMBER 3
NLGI
y value of the sample jinzhi-2 is the smallest at high
temperature because there was a special polymer
tackifier in the formulation. When the temperature is
increased, this polymer tackifier molecule will spread
out and decrease the binding force of thickener struc
ture, and weaken its power of thickening. So this effect
will make the y value smaller and change the visco
elasticity of the grease, and allow it to flow easier.
400
*
T
jinzhi2
*
300
*
C”
0~
jinzh~3
jinzhi4
jinzhi5
>‘
CO
0
C)
C’)
200
>
*
–
100
Steady Stress Sweeps
~,
____________________________
I
The steady stress sweep test show us how the appar
ent viscosity of the five grease samples change when
both shear rate and temperature increase. The data
is shown in Table 3. Figure 4 show us that all grease
samples apparent viscosity are dropping gradually until
they reach a stable value during the increasing shearing
rate. This behavior is caused by the grease’s thickener
molecular attraction forces, which are influenced by the
arrangement of the thickener particles, and become
lower when the units are ordered under continuous
shearing force. The stable temperature and shear rate
make the thickener structure behave in a steady state
manner, so the grease’s apparent viscosity stabilizes at
a certain value. At the same time, we could see that it
take less time to reach a stable viscosity value at high
temperature, the reason may be that the grease thick
ener particles are easily destroyed in a high tempera
ture environment.
0
2
I
6
4
8
10
1~
8
10
12
8
10
5hear Rate (s~’)
(a) 80°C
(C’
a
‘~‘
200
0
2
4
6
5hear Rate (s~’)
(b) 120°C
300
Field Test
In general, grease apparent viscosity decreases
when increasing the temperature, the extent of the
decrease is affected by the grease base oil viscosity,
thickener type and if a polymer is present and the char
acteristics of the polymer. Below 12000, the base oil
viscosity and polymer play a role on the apparent
viscosity, whereas above 12000 the thickener type
is the main factor.
Table 3 illustrates that the jinzhi-2 sample has
higher viscosity than jinzhi-3 because the polymer PIB
has better dissolving power than the polymer OCP.
When grease base oil viscosity are the same, lithium
Cr)
(‘5
a
>~
C’)
0
C)
0
2
4
6
5hear Rate (s-’)
(c) 150°C
Figure 4
—
The apparent viscosity with low shear rate at 80°C, 120°c and 150°C
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NLGI SPOKESMAN, JULY/AUGUST 2013
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complex greases have higher apparent viscosity than
simple lithium grease, for example, the complex grease
jinzhi-5 and lithium grease jinzhi-4 under 120°C. It was
not expected that the apparent viscosities of jinzhi-4
and jinzhi-5 at 1500 are larger than that at 120°. It is
thought that the reason for this is that more base oil is
separated by high temperature energy and this oil, after
a sufficient period of time, is oxidized into a gel-like
material on the rheometer cone plate. The fact that the
two grease rheology index n (from equation 1) become
very small at 150° is further evidence.
The testing grease would begin to run off when the
average temperature of the wheel hub reached —80 to
90°C, from which we could calculate the bearing tem
perature is about 120°C. From visual inspection, it is
obvious that both grease jinzhi-3 and jinzhi-1 have thick
films on the bearing rolling surfaces, and the greases
are evenly distributed in the cage; jinzhi-2 appeared to
be thinner, but had a good adhesion to the bearing roll
ers. From the analysis data (see Table 2 and Table 3):
the distribution of the grease in the bearings is related
to the strain amplitude, the grease volume in the cage
is related to elastic modulus, and the adhesion prop
erty to the roller is related to the apparent viscosity.
The result shows that the hub bearing grease, which
has such characteristics as elastic modulus value of
exceeding 8 x 1 0~ Pa, strain amplitude of approximate
4%, apparent viscosity of more than 20 Pa.s at 120°C,
should have better performance in this road test. 3000
6000km running journey had less damage on the
grease structure in the bearings, so the results of drop
ping point, cone penetration and soap fiber of grease
in SEM after the test have no significant changes. From
this it was thought that the main factor which made
the wheel bearing grease fail is the temperature. We
could not obtain enough sample from the bearings
assembled with grease jinzhi-4, and jinzhi-5 because
of their high operation temperature and loss of grease,
so it was not possible to gather the SEM and related
test data.
-,
Table 3
Rheological equations at different temperatures
Rheological equation, -t
Sample
T=80°C
jinzhi-1
=
+
jinzhi-2
t =
142.17
+
jinzhi-3
t
129.89
jinzhi-4
t=
jinzhi-5
~
Sample
191.2
driving
distance,
km
jinzhi-1
6000
jinzhi-2
6000
jinzhi-3
6000
jinzhi-4 <3000
jinzhi-5 <3000
— ~
=
=
= -t0+
i1(du/dz)°
T=120°C
164.47y0.35~54
223.13
+ 27.287066357
t
=
174
+
6.45367095460
=
114.88
+
47.189Y°~~~~
+ 210.997032772
=
127.28
+
16.051 7076046
t =
129.99+ 112.127044243
=
116.08+
13.3667072523
t=35.5+46.861Y02~23
117.42
=
76.178
+
31 6.927028987
t =
T=150°C
120.1 970.38278
t
t =
+ 15.55670.7016
=
71.273
+
1 5.9657079743
71.13
+
3.3392Y10~
9.7783
+ 25.2397036204
Table 4
The data of the greases and the bearings after field test
maximum average
1/4 penetration
remain
temperature
dropping point,
(after/before test) oil film
(after/before
test)
°C
on
cage
of hub cup, °C
0.1mm
86.6
88.3
77.5
97
150
281/290
195/1 97
272.5/300
193.5/1 95
—-/300
62/61.6
56/57.6
62/58.4
69/57.9
—/68.5
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VOLUME 77, NUMBER 3
thick
thin
medium
a little
none
95%
80%
80%
30—50%
run off
run-off distribution
none
few
none
many
all
good
medium
good
bad
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(b) jinzhi-1 after test
(d) jinzhi-2 after test
(c) jinzhi-2 before test
(e) jinzhi-3 before test
Figure 5 — SEM micrographs of fiber structure (~nzhi-1 jinzhi-2 jinzhi-3) before and after the field test
Discussion
By analyzing the rheology data and field testing
results, we can see that the high temperature caused
by wheel hub brakes is the key reason for the failure
of hub bearing greases. The high temperature energy
make the main grease rheology parameters remarkably
changed, for example, elastic model, grease apparent
viscosity and strain amplitude at flow point. Many com
ponents of grease are very sensitive to the environmen
tal temperature. In this case, base oil viscosity, polymer
form and thickener polarity, which play a role on rheol
ogy parameters, are significant effected. Based on our
experience, we can conjecture how grease thickeners
are formed: first, the orientation force and hydrogen
bonding were driving forces, which made the grease
thickener molecules become grease thickener crystal
on one-dimension. Second, those thickener crystals
would be grown up the entire thickener fiber by the dis
persion force and orientation force. Lastly, the grease
thickener network was formed by the dispersion force.
The gel, which is composed of thickener and base oil in
grease together, is called grease thickener micelle.
The base oil viscosity index and aniline point have a
lesser effect, but the dispersion force and van de Waals
force have a greater effect, so the grease yield force
and elasticity modulus became more significant. The
grease thickener which was composed of polar organic
salts, had a higher absorbability of base oil when there
were more thickener micelles in the grease samples.
The reason may be that a high percentage of thickener
makes the orientation force, dispersion force and van
de Waals force stronger (ref 16). The lithium complex
grease has a greater elastic modulus because of the
diacid in the formula. The polymer in the grease disturbs
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NLGI SPOKESMAN, JULY/AUGUST 2013
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7
the formation of thickener crystal because it has less
polarity and a higher molecular weight; the result is a
grease with good flow properties.
In summary, during the running of hub bearings at
1200, higher elastic modulus make the grease have
stronger energy to keep its structure, and make it more
difficult for it to be able to leak from bearings. Higher
strain amplitude could lengthen the time which it takes
the grease sample to get from the solid to liquid state.
Higher grease apparent viscosity could make them
have a better hold capability on bearing surface at
various shearing rates.
Reference
1. S K Yeong, P F Luckham, Th F Tadros. Steady flow
and viscoe-Iastic properties of lubricating grease
containing various thickener concentrations [J].
Journal of Colloid and Interface Science, 2004,
274 (1): 285-293.
2. I Couronnéa, P Vergnea, L Ponsonnetb, N Truong
Dinhc, D Girodind. Influence of grease composi
tion on its structure and its rheological behavior [J].
Tribology and Interface Engineering Series, 2000,
38: 425-432.
3. L Salomonsson, G Stang, B Zhmud. Oil/Thickener
Interactions and Rheology of Lubricating Greases [J].
Tribology Transactions, 2007, 50 (3): 302-309.
Conclusion
1) By the analysis of rheology data of new and field
test grease samples, we can see that the high tem
perature from the wheel hub brake is the key reason
for the failure of hub bearing greases, the high tem
perature energy make the main grease rheology
parameters change significantly.
4. I. Couronne, G. Blettner, P Vergne. Rheological
Behavior of Greases: Part I—Effects of Composition
and Structure [J]. Tribology Transactions, 2000,
43(3): 61 9-626.
5. Draft DIN 51 81 0-2, Testing of lubricants-Testing
rheological properties of lubricating greases; Part 2:
Determination of the flow point using oscillatory
rheometer with a parallel-plate measuring system,
July 2009, Beuth Verlag, GmbH, Berlin.
2) The hub bearing grease, which has such char
acteristics as elastic modulus value of exceeding
8 x 10~ Pa, strain amplitude of approximate 4%,
apparent viscosity of more than 20 Pa.s at 120°C,
would have perfect high temperature performance
and longer service life in a wheel bearing.
6. Yoo J, Kim K. Numerical Analaysis of Grease
Thermal Elastohydrodynamice Lubrication Problems
Using the Herschel-Bulkley Model [J]. Tribology
International, 1997, 30 (6): 401-408.
3) By choosing lithium complex as the grease thickener,
it enhances the elastic module, increasing base oil
viscosity to enlarge grease strain amplitude, add
ing a special polymer in the grease formula to boost
grease apparent viscosity, we can get better perform
ing grease at high temperatures in hub bearings.
7. Huang Y P, Wang Y J, Song C L, et al. Analysis on
Seal Space and Determination of Optimum Filling
Grease Amount for Seal Ball Bearings [J]. Bearing,
2001,9:1-4
—30—
VOLUME 77, NUMBER 3
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13. J Lauger and P Heyer. Temperature-Dependent
Rheology and Tribology of Lubrication Greases
Investigated with New Flexible Platform for
Tribological Measurements on A Rheometer [C].
Advanced Tribology, 2010, Part 3, I., 61-63.
8. J Lauger, P Heyer. Temperature-Dependent
Rheology and Triobology of Lubricant Greases [J].
NLGI Spokesman, 2009, 72 (10): 9-19.
9. Thomas L, Nael Z, Bernhard K. Influence of Base
Oil Polarity and Thickener Type on Visco-Elastic
Properties Investigations with Strain Sweep
Rheometry at +25 °C and +80
[J]. ELGI, 2010.
14. Whittingstall P Controlled stress rheometry as a
tool to measure structure and yield at various tem
peratures [J]. NLGI Spokesman, 1997, 61(12):
12-23.
00
10. P M Cann, H A Spikes. Fourier-transform infrared
study of the behavior of grease in lubricated con
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15. M C Sanchez, J M Franco, C Valencia, C
Gallegos, F Urquiola and R Urchegui. Atomic
Force Microscopy and Thermo-Rheological
Characterisation of Lubricating Greases [J].
Tribology Letter, 2011, 41(2): 463-470.
ii. S Hurley, P M Cann. Examination of grease
structure by SEM and AFM techniques [Jj. NLGI
Spokesman, 2001, 65 (5):i7-26.
12. J E MartIn-Alfonso, G Moreno, C Valencia, M C
Sanchez, J M Franco and C Gallegos. Influence of
soap/polymer concentration ratio on the rheologi
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Engineering Chemistry, 2009, 15 (5): 687-693.
16. P M Lugt. A Review on Grease Lubrication in
Rolling Bearings [J]. Tribology Transactions, 2009,
52 (4): 470-480.
ABOUT THE AUTHORS
Mi Hongying Sinopec Corporation
Mi received a Doctorate degree in Oil
Application from the China University of
Petroleum. She is an engineer for Beijing
POL Research Institute.
Baojie Wu Sinopec Corporation Mr.
Wu earned his Master of Science degree
in mechanical design from ZheJiang
University, China. He is currently the
Director of the Technology Center of
Tianjin Branch Lubricant Company,
Sinopec, responsible for research and
development of greases.
—
Qinglian Liu
—
Sinopec Corporation
—
—
—
—
No photo or biographical information at press.
—31
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NLGI SPOKESMAN, JULY/AUGUST 2013