A CENTRAL FILM THICKNESS FORMULA FOR ELASTOHYDRODYNAMIC LUBRICATION OF CYLINDERS WITH SOFT INCOMPRESSIBLE COATINGS AND A NON-NEWTONIAN PIECEWISE POWER-LAW LUBRICANT IN STEADY ROLLING MOTION

The Reynolds equation for lubrication of rigid cylinders with soft inc ompressible coatings and a non-Newtonian piecewise power-law lubricant in symmetric steady rolling motion is presented. A central film thick ness formula is given for a low ratio of the coating thickness to the contact half-width. The maximum dry pressure gradient is used for the maximum fluid pressure gradient present in this formula. The former is determined by using an asymptotic value for the dry contact half widt h and a thin-layer approximation for the vertical surface displacement of the elastic coating. It is found, when applied to the human ankle joint, that the effect of increased viscosity of the normal synovial f luid for low shear rates does not influence the fluid film thickness f or entraining velocities encountered in walking, It follows that the s ynovial fluid in the ankle joint behaves as Newtonian during walking. (C) 1997 Elsevier Science S.A.