Application Note | Lubricant Stability
Mechanical Stability of Lithium Complex Grease Using Tribometer
Detecting Grease Breakdown Under Mechanical Working from the Friction Signal
Research & Experimental Testing
Andrea Herrmann
Visual Design & Editorial
Andrew Shore
Introduction
Greases made from lithium complex soap are known for their improved properties compared to other alkali metal soaps. These include better resistance to water, temperature and mechanical working. Although lithium soap greases may be more expensive, the advantages offset the increased cost with improved performance. Understanding the enhanced performance of the grease is critical to its use; specifically, we investigate its mechanical stability.
Importance of Tribology Inspection for Quality Control
The tribometer plots friction as it relates to speed, load, time, revolutions or distance. With this capability, the improved mechanical stability of lithium soap greases can be studied through the forces of friction and wear. The purpose of tribology research is ultimately the minimization and elimination of losses resulting from wear and friction of rubbing surfaces. Tribology research is vital to greater production efficiency, application performance, controlled replacement of breakdowns and, most importantly, the cost savings that allow industrial growth.
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Measurement Objective
The NANOVEA Pin-on-Disk Tribometer is used in rotative mode (linear could also have been used). A steel ball tip is used against a steel sample coated with lithium complex grease. The load is 5 N with a constant speed of 150 rpm. Two separately formulated lithium complex greases are tested for comparative evaluation.
NANOVEA T2000 High Load
Pneumatic Tribometer
Test Conditions
A flat or sphere-shaped indenter is loaded onto the test sample with a precisely known force, and the frictional force between the ball and the rotating disc is measured from very small deflections of the arm using a strain gage sensor. The test parameters and environmental conditions are summarized in Table 1.
| Test parameter | Value |
|---|---|
| Load | 5 N |
| Duration of test | 10 min |
| Speed | 150 rpm |
| Radius | 4 mm |
| Revolutions | 1487 |
| Ball diameter | 6 mm |
| Ball material | Steel |
| Substrate material | Steel |
| Lubricant | Samples provided |
| Atmosphere | Air |
| Temperature | 24 °C (room) |
| Humidity | 40% |
Table 1: Test parameters and environmental conditions.
Results
The main numerical results are summarized in Table 2, and the coefficient of friction over the test is shown for each sample, with the points of (A) initial failure and (B) complete failure marked.
| Sample | Initial failure (revs) | Complete failure (revs) | Max COF | Min COF | Average COF |
|---|---|---|---|---|---|
| Sample 1 | 200 | 310 | 0.113 | 0.106 | 0.109 |
| Sample 2 | 360 | 460 | 0.136 | 0.128 | 0.132 |
Table 2: Summary of results.
Sample 1
Sample 2
Conclusion
Using the NANOVEA Tribometer, the mechanical stability of the two lithium complex greases was comparatively evaluated. The graphs clearly show the change in consistency after mechanical working. Sample 1 begins to lose stability at 200 revs, with complete loss of stability at 310 revs, whereas Sample 2 begins to lose stability at 360 revs, with complete loss of stability at 460 revs. This is a user friendly, simple yet extremely precise test method for comparatively evaluating the mechanical stability of grease formulas.
The sensitivity of the sensor allows small changes in friction to be detected, and the full speed range from 0.01 rpm (and lower) to 2000 rpm allows the full range of lubrication behavior to be studied. The NANOVEA Tribometer can be equipped with a temperature module to study these properties up to 900 °C. A liquid cup can also be used to test lubricants by full submersion, or an optional lubrication module adds drop-by-drop or spray modes.
Frequently Asked Questions About Grease Stability Testing
Why are lithium complex greases used?
Compared with other alkali metal soaps they offer better resistance to water, temperature and mechanical working, which offsets their higher cost.
How was mechanical stability tested?
A 6 mm steel ball ran in rotative pin-on-disk mode on a greased steel disc at 5 N and 150 rpm on a 4 mm radius for 10 minutes, with friction recorded throughout.
How is grease breakdown detected?
The friction coefficient changes as the grease’s consistency changes under mechanical working, marking an initial failure and then a complete failure.
Which grease was more stable?
Sample 2: it began losing stability at 360 revolutions and failed completely at 460, against 200 and 310 revolutions for Sample 1.
What were the friction levels?
Average COF was 0.109 for Sample 1 and 0.132 for Sample 2.
Can lubricants be tested under other conditions?
Yes. Speeds from 0.01 rpm to 2000 rpm, a temperature module up to 900 °C, a liquid cup for full immersion, or drop-by-drop and spray lubrication.
