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The behaviour of greases in elastohydrodynamic contacts

The ability of grease to generate elastohydrodynamic (EHD) films has been studied and interpreted in terms of the response of grease structure to contact conditions. For most liquid lubricants, EHD film thickness depends upon the rheological properties of the fluid in the contact inlet. This mechanism is reasonably well understood for fluids that are Newtonian in behaviour. However, greases have complex bulk structures, the scale of which is larger than the dimensions of a contact inlet. The EHD performance of grease must therefore depend upon the response of grease structure to the high-shear inlet conditions. In this study, EHD film thickness has been measured for a range of greases using optical interferometry. In parallel work, grease structural changes have been monitored directly in the inlet of EHD contacts using reflection-absorption infrared spectroscopy. By using these two approaches in combination, changes in composition and loss of bulk grease structure have been observed and correlated with the ability of greases to form films under EHD conditions.