Analysis of Crack Severity on Power Density Increment of Gears

Helical gears are commonly used in industry as they have advantages of higher power density, quieter operation etc., compared to spur gears. Conventional gear design is based on various design criteria, including durability and bending strength load rating. In recent times, fracture failure modes are gaining importance in addition to conventional failure modes. Stresses due to operating fatigue loads and internal residual stresses can cause fatigue fracture failure on the surface, sub-surface of gear flanks or at tooth root of gears. During gear design, various parameters are optimized, and one such parameter is the helix angle that is optimized for power density and gear noise. However, effect of possible defects (voids and inclusions) in the gear tooth is not usually considered in these calculations. This paper is a study on severity of defects in a gear blank relative to power density increment. Three different gear geometries (spur and helical gears with two different helix angles) each with similar defects are considered. Finite Element Analysis (FEA) is used to analyze Tooth Interior Fracture (TIF), and study variation of Stress Intensity Factor (SIF) with crack size and helix angle. It is seen that power density increment of a common gear blank through helix angle increment poses a higher risk of crack severity, as the same gear blank is exposed to higher operating loads.