Tooth Root Bending Strength of Shot-Peened Gears Made of High-Purity Steels up to the VHCF Range

The load capacity calculations for gears according to standardized methods, like AGMA 2001-D04 or ISO 6336, are intentionally conservative to ensure broad applicability in industrial practice. However, due to new applications and higher requirements, more detailed design calculations are nowadays often necessary in order to use possible strength potentials. For example, in wind power gearboxes long operating lives are necessary and in e-mobility applications, due to fewer gear stages and higher speeds at the electric motor, there are higher load cycles per tooth. Hence, higher tooth flank and root load carrying capacities up to the very high cycle fatigue (VHCF) range are desired for gears. To achieve a higher bending strength in the tooth root area of gears, one approach is to induce increased compressive residual stresses into the stressed area, e.g. by a shot-peening process. The drawback is that often there is a change in the crack mechanism. Crack initiation can now occur at non-metallic inclusions in the steel matrix. For that reason, the working hypothesis of this publication is: the higher the cleanliness the fewer the non-metallic inclusions in the material and therefore the higher the tooth root capacity of case-hardened, shot-peened gears. This working hypothesis is verified with tests on FZG back-to-back test rigs up to the very high cycle fatigue (VHCF) range. The test gear variants were manufactured from steels with different degrees of cleanliness. The gears were also examined metallographically, with a special focus on the residual stress state in the tooth root area. As a result, it could be shown that with a higher degree of cleanliness, higher tooth root load carrying capacities up to a higher number of load cycles are possible even taking the different crack mechanism into account.