Effect of Perfluoropolyether Fluids on Life of Thrust Ball Bearings

Abstract

Perfluoropolyether (PFPE) fluids are used satisfactorily on magnetic recording media, in the aerospace industry, and in satellite instruments. These oils have been used as hydraulic fluids, high-temperature liquid lubricants in turbine engines, and base oils for high-temperature greases. Despite considerable research on PFPE, the rheological characteristics at high pressure and their effects on the life of ball bearings have not been investigated completely. First, high-pressure density measurements of three kinds of commercial PFPE fluids (fluid K, fluid D, and fluid F) were made from temperatures 293 K to 333 K and pressures up to 1.2 GPa for estimation of the free volume. The density-pressure-temperature relation and viscosity-pressure-temperature relation of PFPE fluids were proposed based on the free volume and the phase diagram. Next, the effect of PFPE fluids on ball bearing fatigue life was determined by systematic tests using thrust ball bearings. The general tendency of PFPE fluids shows that bearing life increases with the film parameter. The elastohydrodynamic lubrication (EHL) film thicknesses were studied and compared with experimental measurements. The results show that for fluid F, which contains an acetal group (-OCF 2 O-), the viscosity was decreased by mechanical shear at the high shear rate in the EHL inlet and the EHL oil film was not easily formed. The beneficial effect of high film parameter is therefore reduced for fluid F by permanent viscosity loss; thus, contacts occur in ball bearings and shorten the bearing life.