Influence of Real Lubricant Density-Pressure Behavior on the Dynamic Response of Elastohydrodynamic Lubricated Conjunctions

The current work investigates the influence of real lubricant density-pressure behavior on the dynamic response of elastohydrodynamic lubricated conjunctions. Such a response is often based on a non-realistic universal equation of state, despite longstanding evidence of its lack of support by measurements. A finite element framework is employed to investigate the damping and stiffness characteristics of line contact EHD lubricating films, subject to a harmonic loading. Both the equivalent stiffness and damping coefficients of lubricating films are found to increase with the base applied external load, and its amplitude of oscillation. They decrease however with increasing mean entrainment speed and load oscillation frequency. That is, they both increase as lubricant films get thinner. By comparison with the real density-pressure response of a highly compressible silicon oil, the universal equation of state is shown to underestimate the lubricant film's stiffness and damping characteristics. The relative deviations in equivalent damping and stiffness coefficients can reach up to about 12% and 25%, respectively. Therefore, realistic lubricant characteristics should always be considered. In particular, the use of the universal equation of state should not be taken for granted, as is customary in the EHL literature. Lubricant density-pressure response is not of a secondary nature, when it comes to predicting the dynamic performance characteristics of EHL conjunctions.