Thermal effects on surface failure in gears

All current standards and design relationships for surface durability of gears are based on contact stress considerations. Thermal effects are considered only for scoring failures when the destruction of the lubricating film occurs as a result of temperature rise. Temperature increase on the surface reduces the film thickness and causes the solid to solid contact. The objectives of this dissertation are to consider the mechanism of thermal stress and thermal cycling during tooth contacts for dry and lubricated conditions and its effect on the allowable mechanical and thermal strength for different machining processes and geometrical conditions. The main goal is to develop a dimensionless relationship to take into consideration the combined effect of the thermal and mechanical stress in predicting the surface failure of gears for different design, manufacturing and operating parameters. The result of the study shows that the gears with low number of pinion teeth and high gear ratios are more influenced by the thermal effects. Also the allowable mechanical stress is reduced with decreasing center distance. The surface roughness is found to have a significant effect on the temperature rise on asperities but not on the bulk surface. Fine ground gears are generally less affected by the thermal stress since the heat input is due to viscous sheer. High carbon steel gears are significantly more sensitive to the thermal stress than low carbon steel because of the significant change of their ultimate strength with temperature and their high susceptibility to thermal fatigue and thermal shock.