Some insights into micro-EHL pressures

An analytical model is developed in this paper which relates the major comp onent of micro-EHL pressure responses to lubricant properties, roughness ge ometry, contact load, velocity, and slide-to-roll ratio. Analyses are then conducted showing the effects of system parameters on this micro-EHL pressu re. For a Newtonian lubricant with an exponential pressure-viscosity law, t his pressure would be large unless the contact practically operates right a t pure rolling. The magnitude of the pressure rippling is largely independe nt of the slide-to-roll ratio, and smaller wavelength components of the sur face roughness generate larger micro-EHL pressures. With less dramatic pres sure-viscosity enhancement such as the two-slope model, the micro-EHL press ure is generally smaller and sensitive to the slide-to-roll ratio larger wi th higher sliding in the contact. Furthermore, this pressure-viscosity mode l yields a micro-EHL pressure that becomes vanishingly small corresponding to sufficiently small wavelength components of the roughness. For a shear-t hinning non-Newtonian lubricant, such as the Eyring model, with an exponent ial pressure-viscosity law, substantially less micro-EHL pressure rippling is generally developed than its Newtonian counterpart. While the pressure r ippling is insensitive of the slide-to-roll ratio like its Newtonian counte rpart, it vanishes corresponding to sufficiently small wavelength component s of the roughness. The analyses revealed that a key factor resulting in a smaller micro-EHL pressure with the two-slope model or the Eyring model is the lower viscosity or shear-thinned effective viscosity in the loaded regi on of the contact. Since EHL traction is proportional to this viscosity, co ntacts lubricated with oils exhibiting higher traction behavior would devel op larger micro-EHL pressures and thus would be more vulnerable to fatigue failure.