1. Tribological behaviour and wear prediction of molybdenum disulfide grease lubricated rolling bearings under variable loads and speeds via experimental and statistical approach
Gabi Nehme Saeed Ghalambor
Department of Mechanical Engineering Department of Mechanical Engineering,
University of Balamand University of Texas at Arlington
El-koura, Lebanon Arlington, Texas
Introduction
Program set up adjustments using Plint machine software (TE92) for various testing procedures such as loads and speeds
In this research. wear and bearings grease deterioration with and without MoS2 are shown and statistical equations to predict wear with respect to speed, MoS2 concentration, and loads are calculated. Tests are carried out using a 4 ball method wherein spectrum loads different schedules are applied for different testing procedures; scheduled load # 1: 393 N-786N-393N-786N and schedule load # 2: 786N-393N-786N-393N, the grease is heated to 75°C and the test is conducted at a speed of 1200 rpm or 600 rpm for revolutions and the wear scar is measured thereafter. Auger electron spectroscopy (AES), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) were used in order to highlight wear and friction for different lubricant blends and to understand the influence of MoS2 and variable loads on grease.
Comment
Step No.
Next step
Loop count
Step time
Data mode
Motor enable
Load Newton
Specimen Temperature
Clutch Enable
Counter Reset
Speed rpm
Drive enable
Test
Control unit
Apply Load 1 2
CStp+
Disabled
785 20
Disengaged
Reset/on
Enabled
Current time
Wait for 75 C 3 OR 75
Elapsed time
Per test
Reset PID 4
Engaged
Current step
Run Test 5
900
10 sec
Enable
1200
Total time
Per step
Decrease rpm 6
392 7
Residual time 8
Test status 9
Test file 10
Data file 11
Data points
In step 12
Total data 13 14
start
Stop motor 15
stop
Wear scar responses at different interactions for schedule load # 1 and # 2 using Design of Experiment (DOE) analysis. MoS2 concentrations vary between 0 and 3% for rotational speed between 600 and 1200 rpm
Desirability response at different interactions for schedule load # 1 and # 2 using Design of Experiment (DOE) analysis. MoS2 concentrations vary between 0 and 3% for rotational speed between 600 and 1200 rpm.
ANOVA (Analysis of Variance) Equations for Wear Data Using DOE optimization.
Equations for both scheduled loads imply that a reduction of MoS2 to 2 % is significant and does not affect the wear especially when using schedule load#2
Wear Scar Width Equations for ramping up and ramping down Scheduled Loads
Scheduled Load #1 (ramping up) =
E-004 *Rotational speeds *% Concentration
E-005* Rotational speeds* % Concentration
Scheduled Load #2 (ramping down) =
E-005*Rotational speeds *% Concentration
- 2.00E-004* Rotational speeds*% Concentration
Coefficient of friction as a function of the number of revolutions were measured and plotted directly into an excel file. Post-test analysis were examined using a JEOL JSM 845 scanning electron microscopy (SEM) at the end of each test providing that the test balls are cleaned with hexane-acetone mixture to remove the debris and grease from the surface. The SEM images for the wear surface were obtained for all the tests of milled MoS2 grease at 50X which correspond to the lower magnification images for easy comparison of WSD’s.
Specific areas on the surfaces for 2 wt. % MoS2 concentrations for schedule 1 and 2 loadings were evaluated using Auger electron spectroscopy (AES) and energy dispersive spectroscopy (EDS) along with scanning electron microscopy to determine the elements present on the surface and to compare the effects of load variations at lower MoS2 concentrations.
Objectives
Screen the AW and EP properties of lubricating MoS2 greases using Four Ball Tribometer where the ball arrangement results in wear scars over the three balls and a circular wear mark over the top ball.
Examine the grease formulations under spectrum loading conditions for two different speeds.
Evaluate Wear surfaces using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).
Determine the wear performance of greases under spectrum loading conditions to simulate actual applications rather than testing it under the ASTM standards wherein the test parameters are held constant throughout the test.
Methods
Results
Friction coefficient and Scanning electron images (SEM) of the wear track for repeated tests of schedule load # 1 and #2 tests at 600 and 1200 rpm and reduced MoS2 additives ( 2 %) in lithium based grease.
-Three grease blends were prepared in batches of approximately 200 grams using Kitchen Aid blender (4.25 liters capacity with power rating of 250 watts)
-Grease had a kinematic viscosity of 220 cSt at 40oC).
- Hexane was used as dispersant to avoid particles agglomeration. 3 wt. %, 2 wt. % and 0 wt. % batches of ball milled MoS2 were mixed with lithium base grease using a blender for 1 hour and the sequence of adding MoS2 to the grease and mixing it is important for the final preparation.
-Tests were carried out using a Plint Four-Ball wear tester (Model number TE92).
-A program was written for each specific condition to adjust for the speed and load according to the desired schedule where continuous steps (CStp+) were used.
-Tests for the different factors considered in the analysis and the measured wear scar averages are presented in the Table then analyzed using design of experiment to evaluate the ANOVA equations using different factors.
Friction coefficient vs. No. of revolutions and wear scar behavior of Lithium based two different scheduled loads and 600 rpm with 3% MoS2 and without MoS2
Molybdenum and sulphur EDS maps of the wear scars for schedule load #2 & 2% MoS2 additives concentrations in lithium base grease
Factor A: Rotational speed: rpm
Factor B: Concentration MoS2 0-3%
Factor C: Schedule load # 1 vs. Schedule load # 2
Response: Average wear scar width (mm)
600
0.0
Scheduled Load # 1
1.57
3.0
0.97
1.56
0.96
1200
1.52
0.94
1.53
0.92
Scheduled Load # 2
0.93
1.54
1.22
0.88
1.21
Graphs of concentration (a.u) versus sputtering time (sec.) of oxygen, Molybdenum, and sulfur for Grease with 2% MoS2; (a) represents schedule load # 2 and 1200 rpm speed; (b) represents schedule load # 1 and 1200 rpm speed.
Friction coefficient vs. No. of revolutions and wear scar behavior of Lithium based two different scheduled loads and 1200 rpm with 3% MoS2 and without MoS2
Conclusions
Results, validation through trials and optimization revealed that spectrum loadings are appropriates for reduced MoS2 grease in high pressure applications.
Under schedule load # 2 where the tests are ramped down, the coefficient of friction and WSD were lower when compared to constant loading or ramping up conditions as reflected in the SEM images.
Higher rotational speeds played very important roles in the analysis resulting in reduced wear in several tests and enhanced the formation of antiwear film as reflected in the ANOVA equations
Friction coefficient vs. No. of revolutions and wear scar behavior of Lithium based two different steady state loads and 2 speeds of 600 and 1200 rpm with 3% MoS2.