Lubrication refers to the action of rendering something smooth or slippery.
Lubrication occurs when opposing surfaces are completely separated by a lubricant film. The applied load is carried by pressure generated within the fluid, and frictional resistance to motion arises entirley from the shearing of the viscous fluid.
Hydrodynamic lubrication is a system of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film having sufficient pressure to separate the surfaces. Under hydrodynamic lubrication, which is also referred to as “full film” lubrication, moving engine surfaces are actually separated by an oil film, eliminating all metal contact.
Elastohydrodynamic lubrication is the lubrication that occurs between rolling bodies.
Boundary lubrication is the case where a complete fluid film does not develop between potentially rubbing surfaces; the film thickness may be reduced to permit momentary dry contact between wear surface high points or asperities, a condition characteristic of boundary lubrication. Boundary lubrication occurs whenever any of the essential factors that influence the formation of a full fluid film are missing. The most common examples of boundary lubrication include bearings, which normally operate with fluid film lubrication but experience boundary lubricating conditions during routine starting and stopping of equipment, gear teeth contacts and reciprocating equipment.