EFFECT OF RIM THICKNESS ON GEAR CRACK-PROPAGATION PATH

Analytical and experimental studies were performed to investigate the effect of rim thickness on gear tooth crack propagation. The goal was to determine whether cracks grew through gear teeth or through gear ri ms for various rim thicknesses. A finite element based computer progra m (FRANC, FRacture ANalysis Code) simulated gear tooth crack propagati on. The analysis used principles of linear elastic fracture mechanics. Quarter-point, triangular elements were used at the crack tip to repr esent the stress singularity. The program had an automated crack propa gation option in which cracks were grown numerically using an automate d re-meshing scheme. Crack tip stress intensity factors were estimated to determine crack propagation direction. Gears with various backup r atios (rim thickness divided by tooth height) were tested to validate crack path predictions. Gear bending fatigue tests were performed in a spur gear fatigue rig. From both predictions and tests, gears with ba ckup ratios of 3.3 and 1.0 produced tooth fractures while a backup rat io of 0.3 produced rim fractures. For a backup ratio of 0.5, the exper iments produced rim fractures and the predictions produced both rim an d tooth fractures, depending on the initial geometry of the crack.