Gear Rim Failure Prediction Based on Fracture Mechanics

Abstract

Gear design changes impact on gear crack propagation trajectory is investigated through numerical study. General purpose linear elastic fracture mechanics software, FRANC2D and FRANC3D, are used to simulate 2D and 3D gear crack propagation. FRANC can model non-planner, arbitrary shape crack surface for crack tip stress distributions, stress intensity factors, and crack propagation analyses. Maximum tensile stress and NASGRO4 fatigue crack growth models are employed to predict crack propagation direction and life. Three-dimensional idler gear crack propagation simulation shows the predicted crack trajectory is close to the field observation. Various 2D models are simulated to investigate the crack trajectory impact factors and design strategies to prevent gear rim failure. As shown in previous studies, the initial crack position and orientation play pivot role to control gear failure mode - tooth or rim. For a fixed crack position, this study shows the ratio between bend stress and centrifugal stress dominates gear fracture mode. The less centrifugal stress, the crack more likely to break tooth, while lower bend stress more likely lead to break rim. To prevent rim failure through increasing the rim thickness results in a significant weight penalty. The larger the gear, more the weight penalty. Based on the simulation results, the recommended design strategy is to evaluate gear rim failure risk during the gear train layout phase. It is difficult to be improved at the individual gear design phase.