Rebounding lubricant layers

Most research in elastohydrodynamic lubrication has concentrated on the immediate contact area between the two elements. In the case of ball or cylindrical roller bearings, the finitely separated rolling elements may have, between them, a free‐surface lubricant film on each of their races. This film, formed by the wake emanating from one pair of elements, becomes the inlet boundary condition for the following pair. Its possible variation in shape with time is thus important when estimating the lubricant film thickness. With this practical situation in mind, the variation with time of the wake formed behind a ball sliding in a thin viscous lubricant layer on a plate has been studied. Using thin‐film theory and an initial wake cross‐section similar to those observed experimentally, the Navier Stokes equations are solved to yield solutions which describe the subsequent alteration in the wake shape with time. It was found that the track centre of the wake remains at, or slightly below, its chosen initial height for some time before commencing rebound towards the undisturbed layer condition. This behaviour is confirmed by some simple experiments carried out on a ball‐and‐plate machine which also indicate that the track centre height, measured just downstream of the ball, is initially less than the minimum film thickness under the ball and remains at or near this condition for some time before rebound commences. The experiments have also shown that time taken for the track centre height to regain the minimum height under the ball is found to depend on the lubricant properties, the undisturbed layer thickness and the depth of immersion of the ball.