The effect of couple stress fluids on elastohydrodynamic lubrication (EHL) behaviour in smooth as well as rough line contacts is investigated numerically at low speed-high load and high speed-low load conditions. A non-dimensional couple stress parameter represents the molecular length of the additives. The EHL characteristics computed for couple stress fluids are found to have strong dependence on couple stress parameter. Significant increase in minimum film thickness due to couple stress fluid is observed. Also, the surface roughness effects on EHL behaviour are influenced by couple stress fluid to an extent depending upon the speed-load conditions.
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