Enhanced Calculation Method for Tooth Flank Fracture Risk with Consideration of Tensile Residual Stresses in Larger Material Depths

Residual stresses have a significant influence on the load carrying capacity of gears. High near surface compressive residual stresses can lead to an increase in pitting and tooth root bending load carrying capacity up to 50%. Tooth flank fracture (TFF) is a gear fatigue failure mode, which is in contrast to pitting or tooth root breakage not initiated at the surface but in larger material depths beneath the active flank. Therefore, the residual stresses in larger material depths are decisive for TFF. In these larger material depths, the residual stress conditions are almost unknown up to now. It is assumed, that tensile residual stresses are present in these larger depths, which can have a negative influence on the TFF load carrying capacity. So far there are no validated methods known to estimate or predict these tensile residual stresses. As a result, these residual stresses have not yet been considered in the calculation methods for the risk of TFF. This paper presents a calculation method to assess the residual stresses in gears, based on comprehensive numerical simulations of the case carburizing process including typical material characteristics and heat treatment process parameters. The calculated residual stress profiles also consider the existent tensile residual stresses in larger material depths. Based on these predicted residual stress profiles, calculation methods for TFF, including the standard calculation of ISO/TS 6336-4, were extended to consider not only compressive but also tensile residual stresses, which was not possible so far. Based on recalculations of experiments in test rigs and failed gears of different industrial applications, the extended calculation method is validated and maximum material exposure limits for this extended calculation method are presented.