A model for gear life with surface and subsurface survival: Tribological effects

The paper describes a model for gear fatigue life based on the well-established methods used in dynamic load capacity of machine components. The model applies the Weibull's weakest link of material strength and the Lundberg-Palmgren theory for bearing dynamic load ratings. Significant, of the present model, is the introduction of a well-defined separation between the risk of surface initiated failures and the traditional subsurface fatigue of the gear contact. This opens new possibilities for the use of specialized endurance models to predict the surface life of the gear contact. A further advantage of the proposed model is the use of a statistical definition of the fatigue life of gears. Current design practices for gears are condition based rules. Essentially it is required to satisfy a fatigue limit stress which is further reduced using service penalty factors. Contrary to this approach, the proposed model makes use of a L10 life associated to a 90% reliability for the gear population. Comparison between the experimentally obtained gear endurances and L10 predicted life, using the present theory, indicates the ability of the model to account for the gear endurance and the contribution of the surface to the performance. This new approach is felt to be of advantage for future progress in gear design.