A failure mode called“flank breakage” is increasingly observed in different applications of cylindrical and bevel
gears. These breakages typically start from the active flank approximately in the middle of the active tooth
height and propagate to the tooth root of the unloaded flank side. Crack initiation can be localized below the
surface in the region between case and core of surface hardened gears. This failure mode can neither be
explained by the known mechanism of tooth root breakage nor by the mechanism of pitting. Even bevel gears
in truck and bus applications are at the risk to suffer from subsurface fatigue, if the optimum utilization of the
material should be achieved. In this case a balance between the flank breakage and pitting risk has to be
found. The purpose of this paper is to describe a new material, physically based calculation method to
evaluate the risk of flank breakage versus the risk of pitting. The verification of this new method by
experimental tests is exemplarily shown.