The state-of-the-art engineering uses gears under high loads and demanding ambient conditions. Because of the high sliding- and rolling-strain in the gear tooth contact the tooth surface changes continuously beginning with the first tooth contact. With an increasing time of operation, appearances of fatigue, like micropitting, can be noticed. Micropitting is the macroscopic image of material fatigue which is characterized by a high density of micro cracks in the tooth surface. These fine cracks expand at the beginning only a few micrometres from the tooth surface into the subsurface material layer. In some cases, and under continuing load impacts these micro cracks join to larger crack networks, which after some time may lead to extensive material break-outs on the tooth surface, the so-called pittings. This crack growth from the surface is induced by local stress intensities in the crack area. Another reason for the occurrence of pittings are inclusions with strongly different stiffnesses within the microstructure of the gear's material. They also cause local stress intensities which initiate fatigue. In contrast to the pre-described surface driven crack growth, these kinds of cracks are initiated in a subsurface material layer where also a high shear stress is operating on the material. In case of further crack growth under continuing load, these cracks can reach the tooth surface and are in this case also a driver of pitting occurrence. This contribution analyses and summarizes the fatigue mechanisms on gear tooth flanks and introduces a model and simulation method for these mechanisms. This algorithm considers the exact tooth geometry, the local load distribution, the local speed ratios and the tribological conditions. These variable and interdepend parameters are calculated in every simulation step and are used to determine the local pressure, local sliding speeds and further parameters of a discrete point on the tooth flank. By using these discrete load conditions algorithms are executed which calculate a discrete wear value and wear gradient for every point of the tooth flank. This local wear values can be used to calculate the profile form deviation. Another simulation result is a crack density value which is an indicator and measure for micropitting. These results are summarized over all simulation steps and can be readout at any point of time. These simulation results have already been validated with trails on various gear test rigs. A new aspect of this simulation is the simulation of pitting failures. Every simulation step is extended with calculations for crack initiations at the surface as well as below the surface. Depending on the local load conditions a length for short and long cracks on the surface is summarized over the simulation steps. If the surface crack length reaches a certain value with a correlating orientation at a discrete point of the tooth flank a pitting is initiated. Based on the fatigue algorithms which are used for bearing fatigue life calculation since decades the sub-surface crack initiation is determined.
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