Elastohydrodynamic lubrication under non-steady conditions

Sammanfattning: The six papers presented in this thesis consider the effects on elastohydrodynamic lubrication (ehl) of non-steady conditions. The vast majority of ehl investigations, both experiemental and theoretical, utilise steady state conditions, i.e. constant load, velocity, geometry and temperature. The lubricant is, however, in conditions that are far from those of steady state. Many components operate with varying load, velocity and geometry. Some examples are gears, rolling element bearings, cams and follower mechanisms and reciprocating seals. To predict the operation of these components it is necessary to consider the transient effects. The theory of elastohydrodynamic lubrication under non-steady conditions is presented in this thesis and applied to some practical cases. Papers A and B deal with the simulation of the lubrication in the contact between an impacting ball and a lubricated surface. These investigations give understanding of lubrication under impact loading and also fundamental understanding of the dynamics of an ehl contact. Paper C is an experimental investigation of the impacting ball problem. The deformation of the surfaces in the contact region is studied for different combinations of lubricant viscosities and impact velocities. Paper D and E deal with the breakdown mechanisms of a lubricant film. It is shown that lubricant film failure occurs at high sliding velocities if the load is non-stationary. This contradicts elastohydrodynamic theory since increasing sliding velocity implies increasing entrainment of lubricant and thus thicker lubricant film and less risk for failure. It is, however, shown in Paper E that starvation can be the reason why film failure occurs even if the conjunction is lubricated by a thick layer of lubricant. Paper F is a transient simulation of a meshing involute spur gear. Film thickness, pressure, friction and subsurface stresses are computed at different positions along the line of action. Two different sets of lubricant properties are used, the first one