AGMA 18FTM15 PDF

Planetary gear stages are commonly used in many different fields of application, for example wind turbine or automotive gearboxes. Due to the meshing with two counterparts, the planet gear’s teeth are loaded in two directions. This condition is called reverse loading. In reverse loading, the flank is not stressed more than in any other gearbox application. The root however is subjected to reverse bending which significantly reduces tooth root load-carrying capacity. A reduction factor of Y_M = 0.7 as a percentile of the nominal load-carrying capacity with strictly unidirectional load is usually associated with reverse bending in the tooth root. The reduction of load capacity is a function of stress ratio as well as fatigue strength and mean stress sensitivity of the gear material. Local calculation approaches are capable of predicting reverse bending load capacity in greater detail than ever. Free tooth root fillets that deviate from conventional trochoidal or circular root contours can significantly increase tooth root load-carrying capacity and can partly compensate for the reverse bending effect.

The authors have developed a method which is capable of optimizing the tooth root geometry based on load conditions that apply for a planet gear in planetary gearbox applications. The approach considers local material characteristics such as hardness, fatigue strength and mean stress sensitivity as well as residual stresses and different stress ratios that result from the mesh with the sun and ring gear. The approach offers a detailed tooth contact analysis based on the Finite Element Method which is capable of considering tooth flank modifications as well as cross-influences between teeth, gear body and loaded flank areas when calculating the load distribution and tooth root stresses.

The authors highlight the potentials of increased tooth root load-carrying capacity at the planet gear according to a local calculation method. Furthermore, the possibility of changes in the gear design in regard to excitation or bearing forces due to the optimized root fillet is described. This leads to better running behavior in terms of power density and noise behavior of the whole gear stage. Ultimately, it is shown that an optimization of the planet gear leads to an overall improved design of the whole gear stage not only in terms of tooth root load-carrying capacity but of all critical design criteria.