Highly loaded gears are usually case hardened to fulfill the high demands on the load carrying capacity. Several factors, such as material, heat treatment, or macro and micro geometry, can influence the load carrying capacity. Furthermore, the residual stress condition has a significant influence on the load carrying capacity. The residual stress state results from the heat treatment and can further be modified by manufacturing processes after the heat treatment, for example grinding or shot peening.

A variety of investigations was performed in several research projects, concerning the influence of residual stresses on the load carrying capacity limits of gears. The investigations were focused on the tooth root bending strength as well as the flank load carrying capacity. The gears were analyzed in unpeened, mechanical cleaned, and shot peened condition. The investigations included different materials, e.g. 16MnCr5 or 18CrNiMo7-6, and gear sizes.

Compressive residual stresses generated by shot peening, for example, result in an increased tooth root bending strength. The tooth root bending strength of shot peened gears can be increased by more than 50% compared to gears in the unpeened condition. Due to mechanical laws, the increase of the load carrying capacity is limited. In the case of shot peened gears which are highly loaded, other failure mechanisms may arise like subsurface-initiated cracks. Shot peening can also significantly increase the flank load carrying capacity. Due to a shot peening process, the surface of the gears is influenced. As a consequence, other failure mechanisms can occur, such as micropitting. Furthermore, shot peening and the resulting compressive residual stresses can also be used to repair grinding burn or to avoid facing edge tooth flank fractures.

All in all, the investigations show that shot peening can significantly increase the load carrying capacity of case hardened gears. Furthermore, correlations between the residual stress state and the load carrying capacity limits were determined. This paper will give an overview of the main results of different investigations and discuss influences of the residual stress condition on different failure modes of case hardened gears.