scope:

Tooth root load-carrying capacity is one of the determining factors in gear design. In addition to the strength of the material itself, the existing state of stress significantly influences tooth root load-carrying capacity.

Based on extensive experimental investigations of gears, the beginning of the fatigue strength range is generally set 3×10⁶ load cycles, which common calculation methods, like ISO 6336, also take into account. According to this, standard test methods for tooth root bending endurance strength usually assume a load cycle limit of 3-6×10⁶. However, current as well as completed studies on tooth root loadcarrying capacity show tooth root fractures with relatively high numbers of load cycles in a range of general fatigue strength and above. Analysis of these fracture surfaces shows that these late breakages are often initiated by small inclusions or microstructural defects in the material. These tooth fractures that initiate with cracks under the surface have a negative effect on the tooth root load-carrying capacity in the range of high cycle fatigue.

Therefore, experimental investigations regarding high cycle fatigue have been carried out in a pulsator test rig on gears of various sizes, materials and residual stress conditions. As a result, depending on the existing residual stress condition, there are different levels of tooth root load carrying capacity, different failure behaviors in high cycle fatigue and different types of damage. Especially for test variants with high residual stresses, the size of the gear and the cleanness of the material have an impact on the tooth root load-carrying capacity and the damage pattern.

This paper discusses the different fracture modes by means of examples. Furthermore, it presents the influence of residual stresses, size and material cleanness on the tooth root load-carrying capacity and on the type of tooth root fractures with crack initiation on and under the surface. These influences will be additionally confirmed by examples of experimental test results.