Friction of hybrid and steel ball bearings in mixed lubrication

Hybrid ball bearings with rolling elements made of silicon nitride show important friction, temperature, and power loss benefits compared to full steel bearings under mixed lubrication. This is of particular interest for electrical vehicle (EV) range extension in the automotive industry, where hybrid bearings are used in the electric motor with integrated gearbox to avoid electrical erosion damage. Same benefits can be obtained for industrial electric motors with high frequency variable speed drives where the hybrid bearing acts as an insulator against electrical currents. In this study bearing temperatures and system torque are measured in a modified FZG gearbox for both hybrid and steel ball bearings. The gear set was designed with a small helix angle to create an axial preload to the sets of 6306 deep groove ball bearings with C3 clearance. To reach mixed lubrication conditions, a small amount of low viscous oil was used operating at high temperatures up to a maximum of 100°C. Results in this study show that for mixed lubrication the hybrid bearings run 2-4°C cooler compared to steel ones, while the temperature gradient is similar. Due to lower friction in the hybrid bearings, the FZG test box efficiency is on average improved with 3%.