Lubrication Studies of Smooth Rubber Contacts
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By producing optically smooth spheres and cylinders of rubber, it has been possible to study the thickness and contour of a liquid film between surfaces by optical interferometry. Normal approach between surfaces clearly shows the entrapment of a bell of liquid and reveals the load bearing capacity of electrical double layer forces at small distances of separation. In the absence of such forces, a liquid film collapses with time leading to areas of adhesion over most of the contact region. When a normally loaded rubber sphere is slid tangentially, the entrapped bell lifts up its leading edge to form a convergent wedge of liquid capable of supporting the normal load. Such action gives the horseshoe shaped contour typical of elastohydrodynamic lubrication. Recently, the effect of relative spin between contact surfaces has been examined. An unrestrained rubber ball rolled with spin between plates in dry contact experiences tangential contact tractions resulting in a creep motion of the ball perpendicular to its direction of rolling, as anticipated by earlier work. In the presence of a lubricant, there is still creep, but in the opposite perpendicular direction. Tilt between the contact surfaces generated by elastohydrodynamic action clearly contributes to this expected result, though it does not fully explain it.
KeywordsLiquid Film Rubber Surface Bulk Viscosity Rolling Speed Elastohydrodynamic Lubrication
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