LUBRICATE encyclopedia topics

The regimes of lubrication

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:

Fluid film lubrication is the lubrication regime in which through viscous forces the load is fully supported by the lubricant within the space or gap between the parts in motion relative to one another (the lubricated conjunction) and solid-solid contact is avoided.

Hydrostatic lubrication is a special case of fluid film lubrication in which an external pressure is applied to keep the lubricant in the conjunction, enabling it to support the external load.
Hydrodynamic lubrication is also a special case of fluid film lubrication which occurs when the lubricant is able to support the load without external pressure, through hydrodynamic forces alone, which deform the shape of the interposing lubricant film into a wedge shape and drags the lubricant into the film, so that the externally applied load can be supported.

Elastohydrodynamic lubrication: The opposing surfaces are separated but there occurs some interaction between the raised solid features called asperities, and there is an elastic deformation on the contacting surface enlarging the load bearing area whereby the viscous resistance of the lubricant becomes capable of supporting 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 pressure 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.

Beside supporting the load the lubricant may have to perform other functions as well, for instance it may have to cool the contact areas, or to remove wear products, and for carrying out these functions the lubricant is constantly replaced from the contact areas either by the relative movement (hydrodynamics) or by externally induced forces.

Lubrication is required for correct operation of mechanical systems pistons, pumps, cams, bearings, turbines, 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.