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 rims for various rim thicknesses. A finite element based computer program (FRANC, FRacture ANalysis Code) simulated gear tooth crack propagation. The analysis used principles of linear elastic fracture mechanics. Quarter-point, triangular elements were used at the crack tip to represent the stress singularity. The program had an automated crack propagation option in which cracks were grown numerically using an automated re-meshing scheme. Crack tip stress intensity factors were estimated to determine crack propagation direction. Gears with various backup ratios (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 backup ratios of 3.3 and 1.0 produced tooth fractures while a backup ratio of 0.3 produced rim fractures. For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial geometry of the crack.

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