Due to improved material qualities, new surface finishing methods and increased heat-treatment process reliability, flank surface damages, such as pitting or micropitting, can be prevented more and more in a reliable manner. At the same time this may result in an increase of unexpected flank damages with crack initiation below the surface of the loaded gear flank, for example tooth flank fracture.

As stated in previous publications, tooth flank fracture is characterized by a crack initiation below the active flank surface due to shear stresses caused by the Hertzian contact and crack propagation in direction of both the active flank surface and the core area. The main mechanisms leading to tooth flank fracture have been investigated in different research projects and have already been internationally published. Damages caused by tooth flank fracture usually result in a total breakdown of the gear unit.

By now, an ISO technical specification for calculation of tooth flank fracture load capacity of case hardened spur and helical gears is in preparation. This technical specification is based on a simplified calculation method for assessing the risk of tooth flank fracture for case-carburized gears which has been derived from a sophisticated material-physically calculation approach. Parameter studies have proved that the results provided by the simplified approach are in very good accordance to the results of the underlying material-physically based model.

In previous publications on the topic of tooth flank fracture by FZG, failure descriptions were given and calculation methods to assess the risk of tooth flank fracture damages as the above mentioned were already described. Furthermore, this paper aims at giving guidance on how to use the proposed technical specification correct and efficient for the design of industrial gearboxes. Therefore, the basic principles of the ISO/DTS calculation approach for assessing the risk of tooth flank fracture as well as the main influence parameters and common application fields are presented in a first step. Different approaches for estimating hardness and residual stress depth profiles are compared to measured data and consequently recommendations are given on how to choose the right method for estimating hardness and residual stress depth profiles if there are no measured data available.

Based on a detailed 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 parameters and material characteristics can be quantified regarding their susceptibility for tooth flank fracture damages. Furthermore, the practical applicability of the available draft technical specification is shown and critical design parameters are identified. Finally, benefits of a computer aided calculation of the risk of tooth flank fracture damages according to the new draft technical specification are shown.