Study of the Wall-slippage of Lubricants in Non-conformal Contact using a Novel Experimental Scheme

Project: Research

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The limited traction force with increasing in the relative sliding speed as demonstrated in a typical lubricated two disc traction test cannot be fully explained by the existing elastohydrodynamic lubrication (EHL) theory. It has probably been attributed to thermal or limiting shear stress effect. Recent numerical studies and some indirect experimental results, such as the abnormal EHL film profile which deviates from the conventional horse-shoe shaped EHL film, point out the shortfall of the no-slip boundary condition which is a commonly accepted principle in the study of fluid film lubrication. No-slip means that the liquid layer adjacent to a bounding surface has the same velocity as the solid surface. Those findings suggest that slippage may exist at the liquid/solid interface, and it may be the dominating factor in traction and lubrication. Furthermore, recent progress in surface treatment has enabled affinity change at interfaces, which really provides a new approach for controlling the operation of lubricated pairs. Slippage models have been developed. Nevertheless, no direct evidence is available to justify the postulation of wall slippage in lubricated contacts. This work is thus proposed to fill this vacuum.Recently, a novel experimental measuring scheme for the study of the wall-slippage of lubricant in high pressure non-conformal contacts has been developed in the researchers' laboratory. The technique makes use of optical interferometry to visualise the movement of lubricant entrapment in the contact of a steel ball and a glass plane. The slippage can be quantified based on the differences in the velocity of the entrapment and the entraining speed of the lubricating contact. Supported by a small internal grant, a project was carried out and the proposed technique was conceptually validated. The slippage of lubricant was proven to be visible. The promising preliminary work leads to this project to experimentally study on the slippage behaviour of pressurised lubricant in a nonconformal contact in a more systematic manner and on a larger scale.The project will aim to:further develop the test rig in our laboratory to achieve a larger measurement range and to improve its stability, and determine the appropriate parameters for the characterisation of wall-slippage in lubricated non-conformal contacts;experimentally justify the wall slippage in lubricated contact as proposed in previous studies, including those by the applicant and by others;identify the main factors dominating the interface slippage, andunderstand preliminary findings obtained by the applicant; andpropose a slippage model and experimentally determine the coefficients in the model, and develop a simulating solver to theoretically study the flow of the lubricant in lubricated pairs.Success of the research will provide a new means for studying the wall slippage of lubricants in highly pressurised contacts. An understanding of the mechanism of wall-slippage will allow more effective control of traction and lubrication for concentrated contacts. In the longer term, the outcome of the research will facilitate a control strategy for the wall slippage of pressurised lubricants, which will have potential for application in high-precision microdevices


Project number9041385
Grant typeGRF
Effective start/end date1/01/0922/08/11