The Influence of Base Oil Properties on the Friction Behaviour of Lithium Greases in Rolling/Sliding Concentrated Contacts

Abstract

This study investigates the influence of base oil type and viscosity on the frictional behaviour of lithium-thickened bearing greases. A series of model lithium greases were manufactured by systematically varying viscosity and type of base oil, so that the influence of a single base oil property could be studied in isolation. In addition, selected greases were blended with oleic acid, with the purpose of evaluating its effectiveness in further reducing grease friction. Friction coefficient and film thickness were measured in laboratory ball-on-disc tribometers over a range of speeds and temperatures. For a specific oil type, the influence of base oil viscosity on friction was found to be closely related to its effect on film thickness: greases formulated with PAO oils covering a wide range of viscosities gave very similar friction at the same nominal film thickness. For a given base oil viscosity, base oil type was found to have a strong influence on grease friction under all test conditions. PAO-based greases generally produced lower friction than mineral- and ester-based greases. Addition of oleic acid to the test greases did not significantly affect friction within the range of test conditions employed in this study. The results provide new insight into the frictional behaviour of greases, which may be used to help inform new low-friction grease formulations for rolling bearing applications.