Elastohydrodynamic lubrication analysis of metal-on-metal hip implants under start-up and stopping conditions

The importance of start-up and stopping was considered in the elastohydrodynamic lubrication analysis of a metal-on-metal hip implant. The main variables investigated included changes of both the entraining velocity and the load during start-up or stopping in different time periods. It was shown that a powerful squeeze-film action was capable of maintaining the lubricating film, which was generated due to entraining motion prior to stopping, for a long time after deceleration was initiated. The effects of the time period of deceleration between 0.1 s and 0.2 s and the load decrease from 2500 N to 2000 N on the predicted lubricating film thickness after the commencement of deceleration were found to be very small. Under start-up conditions, it was found that the film profiles changed from those under the initial steady-state to those under the final equilibrium conditions, in a relatively short period of time of about 1 s. The effects of the time period, load increase from 2000 N to 2500 N and starting film thickness of 1 mm/s or 5 mm/s on the predicted film thickness at 1 s after the start-up were found to be small. It is concluded that metal-on-metal hip implants can also enjoy substantial fluid film lubrication even under these adverse start-up and stopping conditions.