Parameter study on the calculated risk of tooth flank fracture of case hardened gears

Due to better material qualities, new surface finishing methods and better heat-treatment process reliability, flank surface damages, such as pitting or micropitting, can be prevented in a reliable manner. This results in an increase of unexpected flank damages with crack initiation below the surface of the loaded gear flank, for example tooth flank fracture. Tooth flank fracture is characterized by a crack initiation below the active surface due to shear stresses caused by the Hertzian flank contact and crack propagation in direction of both the active flank surface and the core area. Damages caused by tooth flank fracture usually result in a total breakdown of the gear unit. The main mechanisms leading to tooth flank fracture have been investigated in different research projects. By now, an ISO technical specification for calculation of tooth flank fracture load capacity for case hardened spur and helical gears is in preparation. Based on the available draft technical specification, a parameter study on the load capacity calculation of the damage mechanism tooth flank fracture has been performed in order to identify characteristic influence factors. Furthermore, the tooth flank fracture load capacity was compared to the pitting load carrying capacity for different example gearings. Based on the parameter study, the influence of surface hardness, hardness gradient and core hardness on the damage mechanism tooth flank fracture is characterized. With these findings, different heat treatment processes and material characteristics can be quantified regarding their susceptibility for tooth flank fracture damages. Besides the properties of the heat influenced near-surface zone, different residual stress profiles and geometrical parameters (radii of curvature, module …) have been analyzed, too. Based on the performed parameter study, design limits for practical application have been derived and are presented in the present paper. These derived design limits allow a fast estimation of the exposure concerning tooth flank fracture for a given gear unit design.