| The effects of branching and carbon-to-oxygen (C:O) ratio on the rheology and elastohydrodynamic lubrication (EHL) of two branched and two linear perfluoropolyalkylether (PFPAE) fluids were studied. The branching and the higher C:O ratio increased the pressure-viscosity coefficients, film thickness and traction of these fluids. The properties of the branched fluids were more sensitive to temperature. The results show that for similar-viscosity fluids, the linear PFPAE with higher C:O ratio is more desirable for wide temperature use.;The effects of surface features such as asperities (bumps) and dents on elastohydrodynamic lubrication of elliptical contacts were studied under pure sliding. A symmetric surface feature was used on the stationary surface such that the center of the feature was at the center of the conjunction. A modified Reynolds equation was derived by using the circular non-Newtonian fluid model, and was solved numerically by using a finite element system approach. Streamlines were used to clarify the flow patterns near the surface feature. The effect of asperity size on the pressure, film thickness, surface shear stresses, and flow patterns inside the EHL contact were studied. The asperity width ranged from 10% to 61% of the longitudinal semi-width of the Hertzian contact. With respect to the central film thickness for smooth surfaces, the asperity height ranged from 20% to 66% of the film thickness. The pressure, surface shear stresses and other related parameters increased sharply near the surface feature. With increasing asperity width the central film thickness asymptotically approached the value for the smooth surfaces; and with decreasing asperity width the central film thickness approached zero. For the wider asperities, the pressure spike was spread over a larger area, resulting in higher elastic deformations of the asperities. Therefore, the narrower asperities are more likely to cause a transition from full-film elastohydrodynamic lubrication to partial elastohydrodynamic lubrication. |
