AGMA 18FTM11 PDF

Power density is a key factor in gear design. Increasing the power density enables engineers to use smaller gears for their applications which lead to smaller and lighter gear boxes. The benefit for example for the automotive industry is less moving load in the vehicles and therefor a reduction of fuel consumption and subsequently a reduction of CO2 emission. The limiting factor for the increase in power density of gears is the material strength in regard to the critical failure mode.

The most common way to design gears is using industry standards in which the material strength can be obtained either from fatigue limit tables, which are based on test rig results, or from the calculation of local material data (e.g. based on hardness, residual stress, and oxidation) by means of empirical formulae. Due to the limited empirical data, a lot of averaging and approximations are used to make the available standards applicable to a wide range of applications. To cover for the uncertainties due to this fact, the gears will be designed on the safe side and a near maximum power density will not be achieved.

To design the gear closer to the power density limit, a high level of information concerning the load carrying capacity for the specific failure mode is necessary. In this paper, the three major gear failure modes pitting, tooth root breakage and flank fracture will be discussed. Those failure modes have in common that they start with a local exceeding of the material strength in regard to their specific load situation. This paper will show how local FEA-based calculation approaches can be used to design gears closer to their power density limits for pitting, tooth root breakage and flank fracture. The calculation results will be validated in running tests on different test rigs. The testing results will then be compared to the results of the local FEA-based calculation approaches as well as to the common industry standard ISO 6336.