Finite Element Analysis of the Stress Distribution in Silicon Nitride Bearing Balls

Silicon nitride balls in hybrid bearing applications are operated under heavy loads and/or high speeds. To better understand the role of the surface crack size in the failure mechanism of these balls, finite element analysis (FEA) was used to analyze the stress magnitude and direction at the crack tip on the surface of the silicon nitride ball under simulated loading and motion. The objectives of this study are to obtain the stress magnitude in a silicon nitride ball as a function of load and to obtain the stress magnitude at the crack tip of the surface flaw as a function of flaw size. ANSYS 5.3 was used for the 2D model. Loads between 1000 to 12000 lbs are applied to the ball to determine the stress field in the ballbearing system. The comparison between load, compressive stress and contact length for the FEA and Hertzian solutions are very close. It is found that tensile stress develops at the crack tip when the ball is subjected to a vertical loading force and a friction force between the ball and bearing raceway. The maximum tensile stress occurs at the crack tip. The stress distribution at the crack tip tends to open the crack so as to extend it towards the free surface, resulting in chipping. The FEA results explain the surface damage observed in silicon nitride balls which failed in bearing applications.