An Experimental Approach to Predict the Behaviour of EHD Contacts under Harmonic Loading

Abstract

Rolling element bearings, gears and other machine components operate in the lubrication regime known as elastohydrodynamic (EHD), where lubricant film thickness is governed by the hydrodynamic action of the converging surfaces, the elastic deformation of the non-conforming surfaces and the increase of lubricant’s viscosity with pressure. Classic EHD theory indicates that under the assumption of fully flooded conditions in the contact inlet zone the central film thickness increases with the product of entrainment speed and dynamic viscosity and is very little dependent of load variations. EHD lubricating films most often work under transient conditions of load, geometry or speed. These make the behaviour of the lubricant film different from steady-state conditions. Previous studies showed that the squeeze effect generates film perturbations due to entrapment of lubricant inside the contact. This phenomenon becomes significant when both entrainment and squeeze are presented. In practical applications, this type of film perturbation will generate pressure fluctuations, which in turn may influences the fatigue life of the contact. In the present paper the results of a systematic experimental study for predicting the occurrence of film thickness perturbation phenomenon under variable load condition are shown. In the case of a mean load larger than the amplitude of the load variation, it was found that the film perturbation depends of the frequency of the load cycle, the entrainment speed of the lubricant and the overall lubricant film thickness; it also depends of the ratios between the mean load and the amplitude. Film perturbations occur in contacts for which the ratio between half the load cycle period and the average time of transit of the lubricant through the contact in the loading phase of the vibration is less than 2. In the case of mean load smaller than the amplitude, film thickness perturbation has been observed in every single test regardless of entrainment speed, frequency and lubricant viscosity.