INTRODUCTION: Friction in contacts of machine elements is considered as a basic indicator which to a large extent predetermines energy demands and efficiency of the entire machine. Most of the lubricated contacts are designed to work in elastohydrodynamic lubrication (EHL) regime since this regime ensures low friction. A lubricant passing through EHL contact is exposed to severe conditions involving rapid changes in pressure and temperature which have a major influence on its viscosity. In addition, a lubricant film thickness can fluctuate considerably if the contact operates under non-steady state conditions/vibrations where vibrational motions are generally smaller than the size of contact.1 In the light of these circumstances, EHL friction results mainly from shearing of high viscous lubricant in the central area of contact.
There have been only a few attempts at analysis of EHL friction according to rheology of high-pressurized lubricants and considering non-steady state operating conditions of EHL contacts. Using free sliding oscillating approach, it was highlighted that the piezoviscous behavior together with the changes in film thickness should be considered when dynamic frictional response is investigated.2,3 It was demonstrated as well the total friction corresponds to frictional responses in sub-contact areas considering their film thicknesses.4 It should be noted that the Cox-Merz rule is there hardly applicable.5
It is clear that the EHL frictional response under micro-oscillations remains a challenging tribological problem. Therefore, the aim of this study is to experimentally investigate the dynamic frictional response of lubricating film in a smooth point contact under lateral micro-oscillations (sub-contact vibrations) and describe its frictional responses in main and lateral direction of the contact.
METHODS: Sliding EHL contact is formed between a rotating ball (25.4 in diameter; 100Cr6 bearing steel) and a fixed window (glass or sapphire; coated with layer of chromium). Besides rotation, the ball performs lateral harmonic motions (vibrations) perpendicular to the direction of the ball rotation. Frictional forces were measured simultaneously in both the directions under various operating conditions (contact pressures, contact areas, frequencies and stroke lengths of lateral oscillatory motion). Optical interferometry method6 was employed to determine film thickness distribution with the contact. The knowledge about the film thickness together with the sliding speed of contact surfaces are necessary to determine a shear strain rate. Similarly, a mean shear stress can be calculated according to frictional responses and contact pressure. This approach enables to link the lubricant frictional response with its rheology and operating conditions.