A Novel Technique for Slip Length Measurement in Elastohydrodynamic Lubricated Contact

Energy saving and CO2emission reduction are currently major global concerns. As tribologists, we can contribute to advances in these areas by developing a better understanding of lubrication fundamentals to enhance the efficiency of engines and machines. Two years ago, we initiated a study on boundary slip at the liquid/solid interface in elastohydrodynamic lubrication (EHL), which is a largely unexplored area. Gaining a proper understanding of EHL boundary slip is not only fundamental in tribology in general, but also has a significant impact on better lubrication for typical engine components such as bearings, gears, etc. We have recently successfully developed a technique for measuring slip length in EHL, a key parameter to quantify slip. This is the first method ever proposed and the only method to date that facilitates the quantification of boundary slip in EHL.However, under current testing arrangements, tests can be run only at extremely low speeds and at shear rates of less than 1 s-1. The view that slip length is dependent on shear rate is controversial and can be resolved only on the basis of data obtained over a wide range of shear rates. Obtaining such data will allow for a comprehensive EHL slip model to be established.What do we intend doing? Experimentally, we will increasethe sliding speed by three orders of magnitude from the present available, i.e. up to shear rates of 103s-1, using a high-speed camera;the accuracy of slip-length measurements by imposing reciprocating motions; andthe range of specimen oils of different viscosities through slight modifications to the setup. Theoretically, the movement of oil in EHL which is governed by both boundary slip and controversial high-pressure fluid rheology will be numerically simulated. Since the slip effect will now have been obtained, the high-pressure rheology can be inferred by comparing the numerical and experimental data.The project deliverables includea novel technique for quantifying slip length in EHL;the effects of pressure and speed on slip length;a comprehensive slip model for EHL; andan appropriate high-pressure rheological model.We presented the method for the first time at the recent Tribology World Congress (Kyoto, September 2009). Our presentation was well-received and we were encouraged to produce more original data for a variety of oils under a wider range of operating conditions.There are a number of unresolved issues on the nature of slip length. This novel technique can certainly help move the field forward.