Tooth Flank Fracture – Investigations on the Influence of Overloads on the Fatigue Strength of Case-Hardened Gears

Tooth flank fracture (TFF) is a gear fatigue failure mode with a crack initiation in larger material depths. It has recently been observed more frequently, especially with larger gear dimensions, such as can be found e.g., in wind turbines and heavy industry applications. In the past, experimental investigations were performed by single load testing to determine the fatigue strength on the level of the endurance limit. However, for example in geared wind turbines, overloads occur due to wind gusts or emergency stops. Overloads can also occur in industrial gear units due to unforeseen impacts. This raises the question whether overloads do have an influence on the fatigue behavior of TFF. In this paper, the first test results with case-hardened gears are shown in order to investigate the influence of overloads on TFF. For the experimental investigations, TFF critical test gears with a center distance of 200 mm made of two different case-hardening steels (20MnCr5 and 18CrNiMo7-6) were considered. To investigate the influence of overloads, two stage tests were performed. The first stage represents the overload stage, which the level of the overload and the amount of load cycles was varied for. In the second stage, load levels in the range of the endurance limit were considered, which was determined beforehand. In addition to the experimental investigations, further investigations regarding microstructure, degree of cleanliness, hardness and residual stresses are presented. With these tests an initial assessment is given whether for TFF a damage line exists in S-N-curves in comparison to other gear failure modes such as tooth root breakage or pitting. Based on the results that overloads have in fact an influence on TFF, further outlook is given on possible research on a systematic investigation.