Service life of cylindrical and bevel gears under variable load and stresses

Transmissions are usually loaded by variable external loads under real operating conditions. The decisive load for a gearbox is in most cases the applied torque. Commonly used allowable stress numbers Hlim/Flim (ISO) or HP/FP (AGMA) for calculating the load carrying capacity of cylindrical, bevel and hypoid gears are usually derived from single stage tests carried out on pulsators or back-To-back test rigs. Variable loads can be considered in the calculation of the load carrying capacity by using application factors, overload factors or more complex standards like ISO 6336-6, which was recently revised. In case of variable loads, the calculation of the load carrying capacity of gears is quite different to bearings. According to ISO 6336-6, a safety factor is determined for gears and according to ISO 281, a service life is determined for bearings, respectively. Whereas all of these calculation methods only consider a global safety or lifetime, continuously progressing failures like micropitting or wear can-especially on bevel and hypoid gears-also lead to locally varying stresses even if only a constant external load is applied. This paper is intended to give a brief overview of currently applied methods to consider variable loads in the design process of cylindrical as well as bevel and hypoid gears. Therefore, the scope of application of these methods is shown and critically analyzed for the damage mechanisms pitting, tooth root breakage and tooth flank fracture. Especially the changes made in the revised version of ISO 6336-6 are shown in detail. Furthermore, the influence of locally changing stresses on the pitting load carrying capacity is explained on bevel and hypoid gears. A method to assess such influence is shown for constant external loads.