AGMA 13FTM09

Tooth root load-carrying capacity is one of the determiningfactors in gear design. In addition to the strength of the materialitself, the existing state of stress significantly influences toothroot load-carrying capacity.

Based on extensive experimental investigations of gears, thebeginning of the fatigue strength range is generally set3×10⁶ load cycles, which common calculation methods,like ISO 6336, also take into account. According to this, standardtest methods for tooth root bending endurance strength usuallyassume a load cycle limit of 3-6×10⁶. However, currentas well as completed studies on tooth root loadcarrying capacityshow tooth root fractures with relatively high numbers of loadcycles in a range of general fatigue strength and above. Analysisof these fracture surfaces shows that these late breakages areoften initiated by small inclusions or microstructural defects inthe material. These tooth fractures that initiate with cracks underthe surface have a negative effect on the tooth root load-carryingcapacity in the range of high cycle fatigue.

Therefore, experimental investigations regarding high cyclefatigue have been carried out in a pulsator test rig on gears ofvarious sizes, materials and residual stress conditions. As aresult, depending on the existing residual stress condition, thereare different levels of tooth root load carrying capacity,different failure behaviors in high cycle fatigue and differenttypes of damage. Especially for test variants with high residualstresses, the size of the gear and the cleanness of the materialhave an impact on the tooth root load-carrying capacity and thedamage pattern.

This paper discusses the different fracture modes by means ofexamples. Furthermore, it presents the influence of residualstresses, size and material cleanness on the tooth rootload-carrying capacity and on the type of tooth root fractures withcrack initiation on and under the surface. These influences will beadditionally confirmed by examples of experimental testresults.

Edition:

13

Published:

09/01/2013

Number of Pages:

16

File Size:

1 file , 1.3 MB