Debris effects and denting process on lubricated contacts

Due to better quality steels and advanced manufacturing process, surface initiated failure has become the dominant mode of rolling element bearings. Although, lubricants are filtered, debris contaminants are readily present in the lubricants. Debris contaminants enter the heavily loaded contact and severely damage the rolling and sliding surfaces. Surface dents caused by debris contaminants in the lubricant modify the pressure distribution and consequently alter the stress condition between the mating surfaces, which leads to significant reduction in fatigue life of the bearings. This dissertation presents the analytical and numerical studies of the debris effects and denting process on elastohydrodynamic lubricated (EHL) contacts. An EHL model with the inclusion of debris was developed to investigate the motion of debris contaminants and consequent debris effects on EHL contacts. The results indicate that the debris does not move toward the contact when the debris height is smaller than the back flow region height. Debris denting process models were developed to investigate debris denting process for heavily loaded contacts using a finite element method. A dry contact model which uses the FEM results was then developed to investigate the debris denting process. The results indicate that the debris causes significant stress in the mating surfaces and results in permanent dent with material pile-up on the mating surfaces. Based on the damage mechanics theory, spall initiation and propagation models were developed for a point contact. The dent on the mating surface enters the EHL contact which creates high pressure spikes and results in stress concentrations on the surfaces leading to surface initiated failure. The results indicate that the spall initiation in lubricated contact with surface damage is due to the accumulated plastic strain process