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Lubrication is the process, or technique employed to
reduce wear of one or both surfaces in close
proximity, and moving relative to each 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) 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 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,
and when such a catastrophic event does occur it
means that the lubrication has broken down.
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 removes 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 engines, 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. |
