The condition in which surfaces are completely separated by a continuous film of lubricating fluid is commonly referred to as Hydrodynamic or Full Fluid Film Lubrication. Although hydrodynamic lubrication is the ideal lubrication scenario, in many situations it cannot be maintained. Hydrodynamic lubrication is limited by the lubricant’s viscosity, the rotation speed or RPM and by component loading. An increase in speed or viscosity increases oil film thickness. An increase in load decreases oil film thickness.
Boundary Lubrication is a condition in which the lubricant film becomes too thin to provide total surface separation. This may be due to excessive loading, low speeds or a change in the fluid’s characteristics. In such a case, contact between surface asperities (or peaks and valleys) occurs.
Friction reduction and wear protection is then provided via chemical compounds rather than through properties of the lubricating fluid. The third type lubrication situation is known as Elastohyrodynamic Lubrication (EHD or EHL). This situation occurs as pressure or load increases to a level where the viscosity of the lubricant provides higher shear strength than the metal surface that it supports. As a result, the metal surfaces deform elastically in preference to the highly pressurized lubricant. This increases the contact area and decreases the effectiveness of the lubricant. To minimize friction an effective lubricant should be able to handle the pressures and speeds of the surfaces it will separate.