Transition from Elastohydrodynamic to Partial Lubrication

Abstract

The recognition and understanding of elastohydrodynamic lubrication (EHL) represents one of the major developments in the field of tribology in the last half of the twentieth century. The revelation of a previously unsuspected lubrication film is clearly an event of some importance in tribology. In this case it not only explained the remarkable physical action responsible for the effective lubrication of many nonconformal machine elements such as gears, rolling-element bearings, cams, and continuously variable traction drives, but also brought order to the understanding of the complete spectrum of lubrication regimes, ranging from boundary to hydrodynamic.

The transition from EHL to partial lubrication has been the focus of the last twenty years of the twentieth century. That is, if (for example) in various machine element applications the pressures are too high or the running speeds are too low, the lubricant film will be penetrated. Some contact will take place between the asperities, and partial lubrication (sometimes referred to as “mixed lubrication”) will occur. The behavior of the conjunction in a partial lubrication regime is governed by a combination of boundary and fluid film effects. Interaction takes place between one or more molecular layers of boundary lubricating films. A partial fluid film lubrication action develops in the bulk of the space between the solids.

It is important to recognize that the transition from elastohydrodynamic to partial lubrication does not take place instantaneously as the severity of loading is increased, but rather a decreasing proportion of the load is carried by pressures within the fluid that fills the space between the asperities of the solids. Recent results showing the transition from elastohydrodynamic to partial lubrication while considering an individual asperity are discussed.