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Strategies for Optimizing Greases
to Mitigate Fretting Wear
Juan Bosch Giner and G. L. Doll
Mechanical Engineering Department,
The University of Akron
September 2021
Outline
• Project Definition and Motivation
• Experimental Methods
• Lube Tests
• Tribotests
• Results
• Rotational Fretting
• Translational Fretting
• Summary of Conclusions
2
Project Definition and Motivation
3
• Greases are ubiquitously utilized in sealed rolling element bearings.
• When the bearings experience continuous rotational motion, adequate
lubricant films are usually generated between the rolling elements and the
raceways.
• However, during periods when rotational motion is interrupted, intimate
contact between the asperities on the rolling elements and the raceways
can occur.
• If the bearing is subjected to vibrations or low amplitude oscillations during
these non-rotational periods, fretting-type wear of the raceways can occur.
Fretting wear on the
inner raceway of a
deep-groove ball
bearing (left) and
tapered roller
bearing (right)
Project Definition and Motivation
4
• Previous research performed in our laboratories has revealed that the
success of these approaches greatly depends on the type of fretting
motion experienced by the bearing or other mechanical component [*].
• Our research has found that if fretting results from linear oscillations,
solid additives such as boric acid or molybdenum disulfide may be more
effective than bleed rate in mitigating fretting-type wear.
• On the other hand, bleed rate may be more effective than solid additives
if the fretting wear resulted from rotational oscillations.
* Saatchi, A., 2019. The Effect of Grease Composition on Fretting Wear (Doctoral
dissertation, University of Akron).
Experimental Methods
5
• Materials
▪ Greases: Lithium complex 100, 220, and 460 cSt.
▪ Oils: 1 and 5 wt.% Synthetic base oil 100, 220,
and 460 cSt.
▪ Solid Additives: 1 wt.% CuO, ZrO2, and ZrO
nanoparticles.
Experimental Methods
6
• Experiments
• Grease Characterizations:
• Bleed Rate - ASTM D6185
• Consistency - ASTM D1403
• Rheology
• Fretting Tests:
• Translational: HFRR - ASTM D7594
• Rotational: Modified Fafnir
• Wear Analysis:
• Wear Volume: Zygo
• Mass Loss
Experimental Methods: Grease characterization
7
Test Parameters: ASTM D1403
50 g of worked grease
25 ℃One half size equipment
Cone penetration
Test Parameters: ASTM D6184
100 ℃ oven for 30 hours
Bleed rate
Experimental Methods: Rheology
8
Oscillatory stress sweep parameters:
10-6
10-5
10-4
10-3
10-5
10-4
10-3
10-2
10-1
Modulu
s (
MP
a)
Oscillation Stress (MPa)
Storage modulus
Loss modulus
-20
-10
0
10
20
Tan(delta)
Tan(d
elta)
Parameter Value
Pre-shear 3.259 Pa for 1 min
Frequency (Hz) 1
Stress sweep (Pa) 1 - 1000
Grease (mL) 2
Temperature (℃) 40 and 80
Experimental Methods: HFRR
9
Test Parameters:
0 10000 20000 30000 40000
0.2
0.4
Friction c
oeff
icie
nt
Cycles
Parameter Value
Stroke length (µm) 40
Frequency (Hz) 25
Load (N) 10
Grease (mL) 2
Number of cycles 45,000
Experimental Methods: Modified Fafnir
10
Test Parameters:Parameter Value
Load (kN) 18
Frequency (Hz) 8
Rotation (degrees) 3
Grease (mL) 2
Number of cycles 720,000
Experimental Methods: Zygo
11
Grease Characterization
12
Grease Addition
Penetration
distance ½ size
testing
NLGI grade
100 cSt
As-received 141.8 ± 1.8 2
1% CuO 135.3 ± 0.8 2
1% ZrO2 138.4 ± 1.7 2
1% ZrO 134.3 ± 1.5 2
1% Base oil 138 ± 1.3 2
5% Base oil 141.9 ± 0.9 2
220 cSt
As-received 143.4 ± 0.5 2
1% CuO 143 ± 0.7 2
1% ZrO2 137.5 ± 1.3 2
1% ZrO 141.8 ± 1.1 2
1% Base oil 139.3 ± 1.4 2
5% Base oil 143.2 ± 1.3 2
460 cSt
As-received 140.8 ± 2.1 2
1% CuO 138.5 ± 1.8 2
1% ZrO2 142.5 ± 1.6 2
1% ZrO 141.7 ± 0.8 2
1% Base oil 142 ± 1.2 2
5% Base oil 143.3 ± 0.5 2
Consistency results
Grease Characterization
13
460
cSt B
ase
460
+ 1%
CuO
460
+ Zr
O2
460
+ 1%
ZrO
460
+ 1%
BO
460
+ 5%
BO
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Weig
ht
loss (
g)
Grease
220
cSt B
ase
220
+ 1%
CuO
220
+ Zr
O2
220
+ 1%
ZrO
220
+ 1%
BO
220
+ 5%
BO
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Weig
ht
loss (
g)
Grease
100
cSt B
ase
100
+ 1%
CuO
100
+ Zr
O2
100
+ 1%
ZrO
100
+ 1%
BO
100
+ 5%
BO
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Weig
ht
loss (
g)
460 Grease 220 Grease 100 Grease
Bleed rate results
Results: Rheology
14
Crossover Modulus
220
cSt B
ase
220
+ 1%
CuO
2
220
+ Zr
O2
220
+ 1%
ZrO
220
+ 1%
BO
220
+ 5%
BO
0.002
0.004
0.006
0.008
0.010
0.012C
rossover
modulu
s (
MP
a)
Grease
Flow point 40C
Flow point 80C
100
Base
100
+ CuO
100
+ Zr
O2
100
+ Zr
O
100
+ 1%
BO
100
+ 5%
BO
0.001
0.002
0.003
0.004
0.005
0.006
Cro
ssover
modulu
s (
MP
a)
Grease
Flow point 40C
Flow point 80C
460
cSt B
ase
460
+ Zr
O2
460
+ 1%
CuO
2
460
+ 1%
ZrO
460
+ 1%
BO
460
+ 5%
BO
0.002
0.004
0.006
0.008
0.010
Cro
ssover
modulu
s (
MP
a)
Grease
Flow point 40C
Flow point 80C
460 Grease 220 Grease 100 Grease
Results: Rheology
15
460 Grease 220 Grease 100 Grease
Crossover Stress
460
cSt B
ase
460
+ Zr
O2
460
+ 1%
CuO
2
460
+ 1%
ZrO
460
+ 1%
BO
460
+ 5%
BO
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
Cro
ssover
poin
t (M
Pa)
Grease
Crossover point 40C (40C)
Crossover point 80C (80C)
220
cSt B
ase
220
+ 1%
CuO
2
220
+ Zr
O2
220
+ 1%
ZrO
220
+ 1%
BO
220
+ 5%
BO
0.00010
0.00012
0.00014
0.00016
0.00018
0.00020
0.00022
0.00024
0.00026
0.00028C
rossover
poin
t (M
Pa)
Grease
Crossover point 40C (40C)
Crossover point 80C (80C)
100
Base
100
+ CuO
100
+ Zr
O2
100
+ Zr
O
100
+ 1%
BO
100
+ 5%
BO
0.00016
0.00018
0.00020
0.00022
0.00024
0.00026
0.00028
0.00030
0.00032
0.00034
0.00036
0.00038
Cro
ssover
poin
t (M
Pa)
Grease
Crossover point 40C
Crossover point 80C
Translational Fretting Results
16
220
cSt B
ase
220
cSt C
uO
220
cSt Z
rO2
220
cSt Z
rO
220
cSt 1
% B
O
220
cSt 5
% B
O
16000
20000
24000
28000
32000
36000
Volu
me d
ow
n (m
3)
Grease
Volume down
100
cSt B
ase
100
cSt C
uO
100
cSt Z
rO2
100
cSt Z
rO
100
cSt 1
% B
O
100
cSt 5
% B
O
16000
20000
24000
28000
32000
36000
Volu
me d
ow
n (m
3)
Grease
Volume down
460 Grease 220 Grease 100 Grease
HFRR data after Zygo analysis
Volume down plots
460
cSt B
ase
460
cSt C
uO
460
cSt Z
rO2
460
cSt Z
rO
460
cSt 1
% B
O
460
cSt 5
% B
O
16000
20000
24000
28000
32000
36000
40000
Volu
me d
ow
n (m
3)
Grease
Volume down
Translational Fretting Results
17
460 Grease 220 Grease 100 Grease
HFRR data after Zygo analysis
Pit depth plots
220
cSt B
ase
220
cSt C
uO
220
cSt Z
rO2
220
cSt Z
rO
220
cSt 1
% B
O
220
cSt 5
% B
O
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0M
ax d
epth
(m
)
Grease
Max Scar depth
100
cSt B
ase
100
cSt C
uO
100
cSt Z
rO2
100
cSt Z
rO
100
cSt 1
% B
O
100
cSt 5
% B
O
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Max d
epth
(m
)Grease
Max Scar depth
460
cSt B
ase
460
cSt C
uO
460
cSt Z
rO2
460
cSt Z
rO
460
cSt 1
% B
O
460
cSt 5
% B
O
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Max d
epth
(m
)
Grease
Max Scar depth
460
cSt B
ase
460
+ 1%
CuO
460
+ Zr
O2
460
+ 1%
ZrO
460
+ 1%
BO
460
+ 5%
BO
0
5
10
15
20
25
Weig
ht
loss (
mg)
Grease
220
cSt B
ase
220
+ 1%
CuO
220
+ Zr
O2
220
+ 1%
ZrO
220
+ 1%
BO
220
+ 5%
BO
0
1
2
3
4W
eig
ht
loss (
mg)
Grease
Rotational Fretting Results
18
100
cSt B
ase
100
+ 1%
CuO
100
+ Zr
O2
100+
ZrO
100
+ 1%
BO
100
+ 5%
BO
0
2
4
6
Weig
ht
loss (
mg)
460 Grease 220 Grease 100 Grease
Modified Fafnir results
Summary and Conclusions
19
• All greases displayed the same consistency NLGI grade 2.
• Bleed rate was mitigated by ZrO.
• CuO and ZrO2 also mitigate fretting wear from tangential motion.
• ZrO promoted more fretting wear from tangential motion.
• Deeper pit depths were promoted by oil bleed (460 and 220) and powders, being
the worst ZrO.
• ZrO and ZrO2 promotes more fretting wear on rotational motion.
• CuO mitigates fretting wear on rotational motion.
• Low oil bleed (1 wt.%) do not significantly affect fretting from rotational motion,
sometimes promoting it. However, higher oil bleeds (5 wt.%) showed enhanced
protection.
Acknowledgements
20
TESL Students and Staff
NLGI Project Mentor: Dr. Kuldeep Mistry (The Timken Company)
NLGI Research Grant
