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Lubrication is the process, or technique employed to reduce wear of one or both surfaces in relative motion to one another by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (eg graphite, MoS₂)^[1] a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (greases) or exceptionally a gas.

In the most common case the applied load is carried by pressure generated within the fluid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces.

Lubrication can also describe the phenomenon when such reduction of wear occurs without human intervention (aquaplaning on a road).

The science of friction, lubrication and wear is called tribology.

When we talk about (adequate) lubrication smooth continuous equipment operation is assumed, with only mild wear, and without excessive stresses within the lubricated conjunctions to cause seizure at the conjunction, or break of any part of the equipment, an when such a catastrophic event does occur it means that the lubrication has broken down.

When progressively increasing the load between the contacting surfaces three distinct situations can be observed with respect to the mode of lubrication, which are called regimes of lubrication:

• Hydrodynamic lubrication (also known as fluid film lubrication); The opposing surfaces are completely separated by the fluid which carries the load by its viscous resistance.

• Elastohydrodynamic lubrication: The opposing surfaces are separated with the exception of some of their raised solid features called asperities, which deform elastically, and the energy of deformation contributes to the viscous resistance of the lubricant to support the load.

• Boundary lubrication (also called boundary film lubrication): The bodies come into closer contact at their asperities; the heat developed by the local pressures causes a condition which is called stick-slip and some asperities break off. At the elevated temperature and prssure conditions chemically reactive constituents of the lubricant react with the contact surface forming a highly resistant tenatious layer, or film on the moving solid surfaces (boundary film) which is capable of supporting the load and major wear or breakdown is avoided.

Boundary lubrication is also defined as that regime in which the load is carried by the surface asperities rather than by the lubricant^[2]

Because of the high local temperatures the function of the lubricant is not just to keep the surfaces apart, and bear the load, but also to cool the contact areas, and for this end the lubricant is constantly replaced from these areas either by the relative movement or by externally induced forces.

Lubrication is required for correct operation of mechanical systems engines, pumps, cams bearings, cutting tools etc where without lubrication the pressure between the surfacers in close proximity would generate enough heat for rapid surface damage which in a coarsened condition literally weld the surfaces together, causing seizure.

Look up lubrication in Wiktionary, the free dictionary.

• Lubricant
• Lubrication theory
• Bearing (mechanical)
• Grease (lubricant)
• Fluid management
• Oils
• Society of Tribologists and Lubrication Engineers

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