Hydrodynamic lubrication of soft solids

Abstract

This investigation is concerned with the lubrication of soft solids under pure sliding. The design of the experimental rig is described in detail. Very slight modification was required to ensure the correct functioning of the rig and the results obtained are compared with those of a fairly simple theory. Particular reference is made to the functioning of human joints, and it is noted that even with severe deterioration of the articular cartilage, human joints can function satisfactorily for considerable periods of time. The results of the investigation show clearly that the manner in which human joints behave can be applied to soft bearings generally. A soft bearing reduces friction drastically at low speeds and the frictional coefficients of human joints and those obtained from the experimental rig are comparable. The soft layer, exceeding a minimum thickness, has little effect upon the performance of the bearing generally but enables hydrodynamic lubrication to persist. to very low sliding speeds. The effect of the surface roughness of the compliant sliding surfaces is evident in the form of increased friction in the boundary and in the hydrodynamic lubrication regimes. A major discrepancy between experimental results and theory has still to be satisfactorily explained but further experiments may solve this problem. One achievement of the investigation was the successful application of the "backward method" of solving the elasto-hydrodynamic problem. It is hoped that this may be useful in other applications.