The Short-Time Compressibility of Elastohydrodynamic Lubricants

Elastohydrodynamic (EHD) lubricants are subjected to very large pressures (several GPa) for very short times (10−4 seconds) in typical EHD contacts. However, measurements of EHD lubricant compressibilities to date have primarily been made for quasistatic deformations and only for relatively low pressures. This paper presents some experimental results on the variation of the density of the lubricant 5P4E over a very wide range of pressures and over two distinct timescales, from 10−6 seconds to 10−4 seconds. The very short time (10−6 seconds) data are obtained from plate-impact experiments, and the data near 10−4 seconds are obtained with a new experimental technique using the compression Kolsky bar. It is observed that the commonly used Dowson-Higginson relationship represents too stiff a response at the high pressures for these short times, at least for this synthetic lubricant. A full finite deformation analysis of the plate impact problem is used to obtain the material response function for the large compressibilities observed. On the basis of these results, we suggest a new form of the pressure-density relationship for an elastohydrodynamic lubricant that appears to hold over the entire range of pressures and for time durations on the order of those actually occurring in elastohydrodynamic lubrication.