Elastohydrodynamic film formation at the start-up of the motion

Abstract

This paper describes an experimental study of elastohydrodynamic (EHD) lubricating film formation during the start-up of motion of a point contact from rest. EHD film thickness was measured using ultra-thin optical interferometry. It was found that film thickness behaviour depends strongly upon acceleration. When motion starts, most lubricants form a front that travels across the contact with its initial thickness unchanged. At some point, depending upon the acceleration, a second front often forms, so that the lubricant film has a characteristic stepped profile. This behaviour occurs in both pure sliding and pure rolling conditions. The development of film profile over time can be used to chart the speed of motion of the fluid film in the contact. In pure rolling conditions, the first film front travels through the conjunction at a speed lower than the entrainment speed in the first half of the contact and higher in the second half. In pure sliding it travels at a velocity higher than the entrainment speed across the whole width of the contact. The leading edge moves as a core of lubricant with a velocity constant through 60 per cent of its thickness. At very high accelerations, the central film thickness shows damped oscillations about the steady state final entrainment speed value. These oscillations are similar to those found theoretically by other workers for an accelerated slider bearing using a Navier-Stokes analysis. It is believed that some of the non-classical EHD behaviour observed on start-up may result from the finite rate of momentum transfer across the fluid film.