Effects of Load System Dynamics on the Film Thickness in EHL Contacts During Start Up

By means of numerical simulations the effects of the loading system on the contact dynamics of an EHL contact during start up have been studied. The work was initiated by experimental results obtained for the start up situation on a ball on disk apparatus in which strong film thickness oscillations were observed. In this paper it is shown how much, under operational conditions as they appear in a ball on disc rig, inertia, and stiffness related to the configuration of the loading system can influence the film thickness and mutual approach. For this purpose a model was used in which the usual force balance equation has been replaced by an equation of motion for the loading system in which an inertia and spring effect appear. It was shown that for large accelerations, the loading system does induce oscillations in the film thickness, and predictive formulas for the oscillation frequency derived from a dry contact analysis are shown to be accurate. The nature of the oscillations is explained in relation to the general aspects of time dependent solutions to EHL problems. However, the predicted oscillation amplitudes are small compared to what is seen in the experiments and although some of the phenomena shown in the solutions presented here also appear in the experimental results the experimentally observed oscillatory behavior appears to be of a different nature. As the model used here has given good predictions of the dynamic behavior of single contacts in ball on disc experiments in earlier studies it is concluded that it is unlikely that the oscillatory behavior observed in the experiments is only due to the dynamics of the loading system and other effects must play a role.