Elastohydrodynamic film formation at the start-up of the motion

This paper describes an experimental study of elastohydrodynamic (EHD) lubr icating film formation during the start-up of motion of a point contact fro m rest. EHD film thickness was measured using ultra-thin optical interferom etry. It was found that film thickness behaviour depends strongly upon acce leration. When motion starts, most lubricants form a front that travels acr oss the contact with its initial thickness unchanged. At some point, depend ing upon the acceleration, a second front often forms, so that the lubrican t film has a characteristic stepped profile. This behaviour occurs in both pure sliding and pure rolling conditions. The development of film profile o ver 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 t he conjunction at a speed lower than the entrainment speed in the first hal f 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 t he contact. The leading edge moves as a core of lubricant with a velocity c onstant through 60 per cent of its thickness. At very high accelerations, the central film thickness shows damped oscilla tions about the steady state final entrainment speed value. These oscillati ons are similar to those found theoretically by other workers for an accele rated slider bearing using a Navier-Stokes analysis. It is believed that so me of the nonclassical EHD behaviour observed on start-up may result from t he finite rate of momentum transfer across the fluid film.