Gear crack propagation path studies-guidelines for ultra-safe design

Design guidelines are established to prevent catastrophic rim fracture failure modes when considering gear tooth bending fatigue. Analysis is performed using the finite element method with principles of linear elastic fracture mechanics. Crack propagation paths are predicted for a variety of gear tooth and rim configurations. The effects of rim and web thicknesses, initial crack locations, and gear tooth geometry factors such as diametral pitch, number of teeth, pitch radius, and tooth pressure angle are considered. Design maps of tooth/rim fracture modes including effects of gear geometry, applied load, crack size and material properties are developed. The occurrence of rim fractures significantly increases as the backup ratio (rim thickness divided by tooth height) decreases. The occurrence of rim fractures also increases as the initial crack location is moved down the root of the tooth. Increased rim and web compliance increases the occurrence of rim fractures. For gears with constant pitch radii, coarser-pitch teeth increases the occurrence of tooth fractures over rim fractures. Also, 25 pressure angle teeth has an increased occurrence of tooth fractures over rim fractures when compared to 20 pressure angle teeth. When comparing gears with the same number of teeth or for gears with constant diametral pitch, varying size has little or no effect on crack propagation paths.