The behaviour of low-amplitude surface roughness under line contacts: Non-Newtonian fluids

Abstract As a rough surface passes under an elastohydrodynamically lubricated (EHL) contact the surface profile is modified, with long-wavelength components being reduced in amplitude. The behaviour of the shorter wavelengths is complex. Where both surfaces have the same velocity-pure rolling-the roughness passes through the contact unaltered. Where there is relative slip and the fluid is assumed to be piezoviscous and Newtonian, analysis predicts that the roughness will be flattened. It is replaced by a surface profile with a different wavelength travelling through the contact at the entrainment velocity. The amplitude of this complementary wave may be smaller or larger than the roughness it replaces. This behaviour has not been found experimentally and does not occur in soft contacts where piezoviscous effects are absent. This paper extends a previous analysis for Newtonian fluids to include shear rate effects. It shows that while these effects are relatively unimportant for pure rolling they have a major influence where there is relative sliding of the surfaces. The formation of the complementary wave is largely eliminated and, at short wavelengths, instead of being modified the original roughness passes unchanged through the contact.