Non-steady state non-conformal contacts: friction and film thickness studies

Many machine elements work under non-steady state conditions. Velocity, load and geometry can be time-dependent, and this can influence in a significant way two important quantities of a lubricated contact: friction and film thickness. This work is devoted to non-conformal lubricated contacts under transient conditions and consists of two parts. In the first part a review is made of studies on transient conditions related to the systems themselves (as occurs in gears, valve train, piston assembly and rolling bearings), to particular operating conditions (as starting and stopping of the motion) and to variations localised in the contact (as the ones related to surface defects and roughness). The second part is specifically addressed to investigations on friction and film thickness under conditions of periodically variable velocity. Loops of film thickness and friction coefficient appear when these quantities are plotted as a function of the velocity, mainly due to squeeze effects. The friction is higher and the film thickness smaller for increasing speed than for decreasing speed. A time shift between the film thickness and the speed variations occurs. The loops amplitude increases by increasing the frequency of the speed variation.