Prediction of contact fatigue for the rough surface elastohydrodynamic lubrication line contact problem under rolling and sliding conditions

• Main Author: Qiao, Hua
• Published: Cardiff University 2005
• Subjects: 621.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583691

The study of elastohydrodynamic lubrication (EHL) with rough surfaces results in a transient problem due to time varying geometry. In conditions where the A ratio (the ratio of smooth surface film thickness to composite roughness) is low, significant pressure deviations from the corresponding smooth surface results occur. The relative motion of asperity features on the two surfaces leads to cycling of these pressure deviations as surface features move through the nominal contact area. Micropitting, which is a form of surface distress seen in gears, may well be viewed as rolling contact fatigue at the roughness asperity scale as a result of this load cycling. In order to understand the failure mechanism associated with rough surface EHL, a full theoretical model of lubrication of gear contacts under low A conditions is presented in this thesis. The model takes account of the real gear operating conditions in terms of loads, speeds, surface roughness and lubricant properties to predict pressures, film thickness, temperatures, and friction between the teeth. Subsequent contact stress analysis was performed to determine the stress history developed in the contacting solids. Conventional pitting is usually associated with failure beneath the surface of a rolling contact and in the past has been linked with the occurrence of the maximum shear stress that occurs in the classical Hertzian solution for smooth surfaces. In this thesis, a numerical procedure for predicting contact fatigue damage in an EHL line contact between two rough surfaces is developed to take the features of variable amplitude multiaxial fatigue into account due to the roughness effect. The way in which the calculated fatigue damage varies with changes in factors such as slide roll ratio, speed, viscosity, rough surface profiles, and A ratio adopted for the analysis is demonstrated in a series of numerical examples using roughness profiles taken from gears in micropitting experiments.