Physics Based Contact Fatigue Analysis of Vehicle Powertrain Gears

Contact fatigue is a major concern on the durability of vehicle powertrain gear design. Having an effective method for gear life trend prediction will prevent over design of the powertrain gears and assure the quality of the products. The ANSI/AGMA Standard on gear contact fatigue life calculation is based on an empirical model developed from experiment data fitting. A similar approach widely used in the industry uses measured component SN curves for correspondence between loads and life cycles [1]. This method is simple. But important physical parameters such as material, lubricant, and manufacturing factors are not included in the model, therefore, the model cannot to be used for design optimization. Although some analytical models are available for the gear life prediction, they have not been accepted by the industry. On the one hand, most theoretical models are too complicated for applications. On the other hand, most product engineers are unfamiliar with these models regarding their value as well as the required effort to further develop them. This paper will apply an exist crack initiation theory to model the contact fatigue life of the powertrain gears. Such a theoretical model can be used in conjunction with the empirical model for correlations and model enhancement. In this model, the material defects, i.e., dislocations will pile up along a slip bend under cyclic loading. When the change of the Gibbs free energy reaches the maximum limit, a micro crack will be initiated to release the accumulated energy. The cycle number or the gear contact fatigue life is controlled by loading conditions, material properties, lubrication conditions, etc. A contact fatigue limit can be analytically predicted by this physics-based model.