Ceramic Materials in Hybrid Ball Bearings

Abstract

A computational study was conducted to make comparisons regarding heat generation, frictional torque, and ball wear between conventional all steel bearings using M50 steel balls and hybrid bearings using silicon nitride (Si3N4) ceramic balls. Both were modeled using M50 steel races and the study was accomplished for radially loaded bearings only. Although most realistic applications also include axial loads on angular contact ball bearings, this study helps to recognize and quantify the performance increases of hybrid bearings due to slip and tractive rolling losses. Performance benefits for the hybrid bearings due to reduced centrifugal loads and the higher elastic modulus of silicon nitride balls are primarily demonstrated. Lubrication is assumed and reflected in the wear and friction coefficients, although hydrodynamic losses were nor accounted for. This configuration could occur with air/oil or grease lubrication or with dry lubricating films where environmental conditions might preclude the use of wet lubrication. Including hydrodynamic losses could be a future extension of this work. The predicted losses are calculated based mainly on the micro-slippage mechanism Poritsky (1) and elastic deformation mechanisms Johnson (2) in Hertzian contact regions. As part of the study, software was developed to predict micro-slippage, tractive forces, and traction coefficients between bearing races and balls. Energy losses (heat generation and torque) and wear rate are predicted for varying race curvature ratios and angular velocities. Predictions are for bearings with an outer race of 152.4 mm and ten 25.4 mm diameter rolling elements. Example calculations predict Si3N4, ceramic ball bearings reduce frictional losses over M50 steel ball bearings by approximately 20 percent and wear rates are reduced by about 50 percent. Presented at the 53rd Annual Meeting in Detroit, Michigan May 17–21, 1998