Current–Voltage–Friction Characteristics of Grease in Electromechanically Loaded Sliding Contacts

Abstract

Electromechanically loaded contacts which have relative motion between the contacting parts experience severe damage compared to mechanical-loaded contacts. Such electromechanical environment occurs in the bearings of traction motors through which different types of currents flow, due to the usage of electronic speed control devices. The passage of current through the contact depends on the voltage potential developed across the contact. Grease is frequently used as a lubricant, and degradation and evaporation of lubricant due to the joule heating effect is a concern in the electromechanical contacts. In this study, the current-voltage-friction characteristics of lithium mineral oil grease are reported using a ball-on-disk configuration under combined electrical and mechanical loading. The characteristics indicated a transition of the lubricated contact from a non-conducting state to a conducting state with an increase in applied voltage. Two critical voltages are identified: one where the friction is observed to rise and the other is where the current flow rapidly increases leading to accelerated damage to the lubricant by inducing a significantly high temperature. The study helps in identifying permissible voltage levels to operate bearings safely from the perspective of grease lubricant using simplified ball-on-disk simulated experiments.