New standardized calculation method of the tooth flank fracture load capacity of bevel and hypoid gears

Bevel and hypoid gears are widespread in automotive, industrial, marine and aeronautical applications for transmitting power between crossed axles. Future trends show that the demands on bevel and hypoid gears for higher power transmission and lower weight are continuously increasing. A major aspect in the design process is therefore the load carrying capacity regarding different failure modes. Beside typical fatigue failures like pitting and tooth root breakage, which are the results of cracks initiated at or just below the surface, there are also failures caused by cracks starting in greater material depth in the area of the active flank that can be observed on bevel and hypoid gears. These cracks typically propagate to the tooth root area of the unloaded flank and to the surface of the active flank. The failure mode known as tooth flank fracture occurs particularly frequently on large spiral bevel and hypoid gears because this gear type shows larger equivalent radii of curvature compared to spur and helical gears. As a result of the larger equivalent radius of curvature the maximum shear stress occurs in a larger material depth, where the material of a case hardened gear shows a decreased strength. Important parameters influencing the tooth flank fracture load capacity are geometry, operating conditions, material and heat treatment of the gear set. Tooth flank fracture usually leads to the total breakdown of the gearbox and generally occurs suddenly and unexpected since the crack initiation and propagation takes place below the tooth surface and therefore cannot be identified within visual inspections. This paper will give an overview of the subsurface failure mode known as tooth flank fracture on bevel and hypoid gears. Further a newly developed standardized calculation method for determining the tooth flank fracture load capacity based on the geometry of virtual cylindrical gear according to the standard ISO 10300 (2014) will be explained in detail.