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

Electromechanically loaded contacts, which have relative motion between the contacting parts, experience severe damage compared to mechanically loaded contacts. The electromechanical environment occurs when different types of current flow through the bearings of traction motors due to the usage of electronic speed control devices. The current passage through the contact depends on the voltage potential developed across the contact. Grease is commonly used as a lubricant, and degradation and evaporation of lubricant due to the joule heating effect are concerns in electromechanical contacts. This study reports the current–voltage–friction characteristics of lithium mineral oil grease 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 increased applied voltage. Two critical voltages are identified: one where the friction is observed to rise and the other 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 for operating bearings safely from the perspective of grease lubricant using simplified ball-on-disk experiments.