Grease Degradation in R0F Bearing Tests

This paper is the second in a series that examines grease lubrication mechanisms and failure in rolling element bearings. The aim of the work was to understand the grease condition changes during use and the relationship to lubrication performance and failure. R0F bearing tests were carried out with two lithium hydroxystearate greases and the effects of the temperature, the speed, and the additive package on lubrication life was studied. Post-test, one pair of bearings (fail and non-fail) was dismantled and grease distribution and condition assessed. IR spectroscopy was then used to determine the lubricant composition and the oxidation level of the grease remaining on the shields, the inner raceway, and the cage pockets. The additive package increased the grease life by 100-700% depending on the test condition. Most of the grease remaining in the bearing was found on the shields, with only trace amounts in the cage pockets or close to the rolling track. The IR analysis showed that the composition of the shield sample was similar to the fresh grease although the base oil oxidation was evident and this increased with the running time. The cage pocket and inner raceway films contained a number of chemical species; these included the base oil and the thickener and their oxidation products. The study concludes that after an initial running-in period the “active” lubricant is heavily degraded grease, which contains oxidized species from the base oil and the thickener. Different failure mechanisms are identified depending on the test condition. High-speed tests that fail relatively quickly are due to poor boundary lubrication performance or cage failure rather than the lubricant reaching its “oxidation” limit. Long-term tests at slower speeds suffer considerable base oil oxidation. Under these conditions, failure is due to a reduction in the amount and/or mobility of the raceway lubricant.