EHD phenomena in grease lubricated contacts

This thesis examines the rheological behaviour and film formation of greases and oils under elastohydrodynamic (EHD) conditions. It approaches the lubrication of non-conforming contacts from tribological as well as from dielectric point of view. The experimental work was carried out on a point contact formed between a ball and the flat of a disc, which was either transparent, for optical interferometry study or steel in order to evaluate the conditions in real-life machine components. In the second case electrical capacitance method has been employed to study film formation. The experimental equipment has also been adapted to study the starvation behaviour of grease lubricants in point EHD contacts under vibrations. Dielectric properties of lubricants have been studied in static conditions and correlated to their performance in EHD conditions. The dielectric constant of greases indicates in general higher value than corresponding base oils and the dielectric relaxation time of greases is proved to be shorter. It has been found that in EHD conditions higher-polar greases show higher shear stress, while the dielectric constant of the lubricating film decreases with increasing contact pressure. The effect of three parameters upon grease EHD films was evaluated: operating speed, load and vibration of the contact. The results showed that greases possess an intrinsic time to starvation related to the operating speed but independent of the exerted load. The results also showed that a high speed was not necessarily associated with the high likelihood of occurrence of starvation. The vibration tests revealed that the lateral motion perpendicular to the rotating direction helped maintain the inlet region flooded and, under oscillations, starvation of the contact seems to never occur. This would suggest that machine elements such as rolling element bearings could operate under the fully flooded conditions as vibrations are almost inevitable in any machinery. Finally, the cavitation phenomenon was also investigated and found that the rheological model of greases could be predicted from the observation of the cavity pattern.