An analytical approach for establishing case depth requirements in carburized gears

Abstract

An analytical approach for determining case depth requirements in carburized gears is presented. It is indicated that the minimum case depth requirement in carburized gears is dictated primarily by spoiling fatigue which occurs when the applied alternating shear stress exceeds the allowable fatigue strength in the vicinity of the casecore boundary. Based on these concepts, the following procedure for establishing case depth is suggested: 1) From design load and gear geometry data, compute the applied shear stress gradient at the most severe location along the path of contact. 2) Transform applied shear stress gradient into a critical shear stress gradient using suitable design factors. 3) Convert the critical shear stress gradient into a hardness gradient by applying a torsional fatigue strength = UTS = hardness relationship. The validity of this approach has been substantiated by conducting constant torque four square dynamometer tests on several transmission gears carburized to varying case depths. By analyzing present production data and by applying basic hardenability principles, the stress/strength intersection concepts have been further used to demonstrate that current case depth requirements established on the basis of commonly used empirical rules may be over-specified.