The Composition of Reaction Layers on Rolling Bearings Lubricated with Gear Oils and Its Correlation with Rolling Bearing Performance

Various commercially available gear oils with similar viscosity were subjected to bearing life tests under identical mixed lubrication conditions. The test results revealed that bearing performance varied significantly depending on the chemistry of the gear oil used. Surface analysis with secondary neutral mass spectrometry (SNMS) techniques showed that the best-performing gear oil with respect to bearing life formed a reaction layer that was mainly composed of oxygen with a low concentration of sulfur and phosphorus and the worst-performing gear oil formed a reaction layer that was mainly composed of sulfur and phosphorus with significantly less oxygen. In this article, a semi-empirical equation is proposed to correlate the composition of the reaction layer (i.e., the oxygen, phosphorus, and sulfur concentration profile) with bearing performance. Plausible mechanisms of the observed correlation are discussed.