Effect of start-up conditions on elastohydrodynamic lubrication of metal-on-metal hip implants

Abstract

Lubrication plays an important role in the wear and friction of metal-on-metal bearings for artificial hip joints. It is generally known that both load and speed can influence the lubrication mechanism. This is particularly important in hip implants, which experience many intermittent start-ups and stops, which may adversely affect the lubrication and increase wear in metal-on-metal hip implants. The effect of start-up and gait initiation upon elastohydrodynamic lubrication (EHL) in a typical metal-on-metal hip implant has been investigated in the present study. A simple ball-on-plane model was used for a full transient EHL analysis under assumed start-up conditions represented by linear variations in both load and speed, but with different development time periods. It was shown that the duration of the start-up period had little effect on the predicted lubrication film thickness during either the start-up or the steady cyclic conditions. It was found that only two to three steps were required to establish a continuous steady cyclic variation of the predicted lubricant film thickness. This suggests that intermittent motions associated with start-up and stop may have only a small effect on the wear of metal-on-metal hip implants.