Effect of Solidification Characteristics of Lubricants on Lubricating Performance Under EHL Condition

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Lubricants behaved as elastic-plastic solid at temperatures below their glass transition temperatures in shear ring tests carried out with a concentric double cylinder apparatus. In the temperature range a lubricant were sheared only within a narrow shear band in the vicinity of the inner cylinder surface. There was a critical shear stress, of which the magnitude decreased with the sliding velocity. A lubricant with high traction coefficient in two disc machine testing had a large critical shear stress.In four ball testing, the lubricant with the lowest solidification pressure made the thickest oil film. Once a thick solidifies film was formed under a condition of high sliding velocity, the thick film was retained and had an ability to prevent the rubbing surfaces from direct contact even when the sliding velocity was decreased.Under rolling/sliding contact conditions, in contrast to viscous fluid the traction coefficient decreased with an increase in the shear rate or the sliding velocity in cases where the lubricant behaved as a elastic-plastic solid. The traction coefficient of the elastic-plastic solid could be practically determined only by the value of ap, where a is the viscosity-pressure coefficient of the lubricant and p the mean contact pressure. The ap seems to be a measure of the packing state of the lubricant molecules in the contact region.