Simulation and Observation of Transient Effects in Elastohydrodynamic Lubrication

Many engineering components in engines and machines are of a counter conformal geometry (e.g. valve trains, rolling element bearings and gears) and impose not only high shear rates and temperatures but also extremely high pressures on the lubricant. The effect of pressure is to elastically deform the sliding/rolling contact geometry. Of crucial importance to the engineering and lubricant designer is the magnitude of the lubricant film thickness generated under these severe conditions. Steady state isothermal elastohydrodynamic (EHD) lubrication is now relatively well understood by the use of engineering correlations, first propounded by Dowson et. al., for both line and point contacts, which are used as design tools. However, with ever increasing demands to improve the efficiency of machines, these correlations do not satisfy all the design needs, especially under reversal conditions, where wear is a major problem. In this paper, detailed computer simulations of the oil film shape in EHD line and point contacts under transient operation are reported. The line contacts derive from kinematic models of a cam and follower valve train and an involute spur gear system, and the point contact corresponds to a laboratory rheometer. The simulation of transient effects induced by reversal of lubricant entrainment velocity is discussed and compared with experimental results obtained using a thin film optical interferometry technique and high speed video recording. This provides convincing experimental validation of the models. In both the simulated and experimental studies the region of minimum oil film thickness is seen to move rapidly from one side of the Hertzian loaded region to the other during lubricant entrainment velocity reversal, proceeding via an intermediate geometry that entraps high viscosity oil and prevents the film thickness ever falling to zero. The simulation of transient effects induced by shock loading is also discussed. This appears to involve a similar oil entrapment effect with surprising implications for the operation of gears and cam and follower systems and the understanding of boundary friction and wear.