Volume 1 Issue 1 - Lubrication

Boundary Lubrication

The second regime, Boundary Lubrication refers to conditions where the oil film cannot prevent the asperities of lubricated surfaces, in relative motion, from coming into contact (Figure Three). As the asperities make contact, the metal literally spot-welds (adhesion) or is broken off (abrasion) causing wear of the surfaces. Friction is at maximum level and it is estimated that 70% of all wear results from boundary lubrication. Cylinder piston ring interface, camshaft lobes, tappets and connector rod bearings are some locations in the internal combustion engine where boundary lubrication exist a percentage of the time.

This graphic depicts the dynamic nature of friction and the lubrication that occurs. As components move, lubrication alternates between boundary and hydrodynamic regimes.

Elastohydrodynamic Lubrication

Elastohydrodynamic lubrication (EHL) refers to conditions where the moving surfaces have poor conformity and extreme pressures (Figure Four). Gears and rolling element bearings are the usual suspects for EHL. Oil under extreme pressure (being non-compressible) will begin to function like a solid (pseudo-solid) separating the asperities. The contact surfaces will respond elastically, actually depressing the metal. This elastic characteristic is the source of the Elastohydrodynamic lubrication terminology.

Understanding these three lubrication regimes will prepare you to distinguish how lubricants are specifically formulated to function optimally in a particular regime. For example, anti-wear additives are used for boundary lubrication while extreme pressure additives are used for elastohydrodynamic lubrication. Why the difference? Stay tuned... In succeeding articles, I will refer over and over to these basic regimes as I explain how and why lubricants are formulated for a specific task.

Elastohydrodynamic lubrication takes place in gears and rolling elements such as these ball bearings.

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