A calculation model for rating the gear load capacity based on local stresses and local properties of the gear material

The paper presents a material-physically based model for calculating the load capacity of gears taking into account the different stress conditions in the tooth root and on the tooth flank and considering decisive material properties due to different heat treatment processes. The basis of the model is a local comparison of the occurring stresses and the available strength values in the gear volume. The complex stress condition on the tooth flank is illustrated. On the flank occurs a three dimensional stress condition with a rotating principal stress system. Furthermore the influences of residual stresses and from the contact of technical rough surfaces on this stress condition are discussed. Especially the described complex stress condition on the tooth flank requires the definition of an adequate criterion of failure. For this purpose a variant of the Schubspannungsintensitätshypothese (SIH) is proposed. The applicability of the proposed model is verified with numerous results of experimental investigations. Some of the calculations are presented here. Altogether there is a good agreement between the test results and the calculated results especially for the amount of the fatigue limit, the kind of damage (e.g. tooth root breakage, pitting and tooth breakage in the region of the flank) and the position of the beginning of the damage in the material at and below the surface. The calculation model is successfully used at the Gear Research Centre (FZG, TU Munich) in several research projects and currently will be refined more in detail for the damage type of tooth breakage in the region of the gear flank.