Lubrication is the process or technique employed to reduce friction between, and wear of one or both, surfaces in proximity and moving relative to each other, by interposing a substance called a lubricant in between them. The lubricant can be a solid, (e.g. Molybdenum disulfide MoS2) a solid/liquid dispersion, a liquid such as oil or water, a liquid-liquid dispersion (a grease) or a gas.
With fluid lubricants the applied load is either carried by pressure generated within the liquid due to the frictional viscous resistance to motion of the lubricating fluid between the surfaces, or by the liquid being pumped under pressure between the surfaces.
Lubrication can also describe the phenomenon where reduction of friction occurs unintentionally, which can be hazardous such as hydroplaning on a road.
The science of friction, lubrication and wear is called tribology.
Adequate lubrication allows smooth continuous operation of equipment, reduces the rate of wear, and prevents excessive stresses or seizures at bearings. When lubrication breaks down, components can rub destructively against each other, causing heat, local welding, destructive damage and failure.
The Regimes of Lubrication
As the load increases on 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 object (the lubricated conjunction) and solid–solid contact is
avoided.
- Hydrostatic lubrication is when an external pressure is
applied to the lubricant in the bearing, to maintain the fluid lubricant
film where it would otherwise be squeezed out.
- Hydrodynamic lubrication is where the motion of the
contacting surfaces, and the exact design of the bearing is used to pump
lubricant around the bearing to maintain the lubricating film. This
design of bearing may wear when started, stopped or reversed, as the
lubricant film breaks down. The basis of the hydrodynamic theory of
lubrication is the Reynolds equation. The governing equations of the
hydrodynamic theory of lubrication and some analytical solutions can be
found in the reference [3].
- Elastohydrodynamic lubrication: Mostly for nonconforming surfaces
or higher load conditions, the bodies suffer elastic strains at the
contact. Such strain creates a load-bearing area, which provides an almost
parallel gap for the fluid to flow through. Much as in hydrodynamic
lubrication, the motion of the contacting bodies generates a flow induced
pressure, which acts as the bearing force over the contact area. In such
high pressure regimes, the viscosity of the fluid may rise considerably.
At full film elastohydrodynamic lubrication the generated lubricant film
completely separates the surfaces. Contact between raised solid features,
or asperities, can occur, leading to a mixed-lubrication or
boundary lubrication regime. In addition to Reynolds equation,
elastohydrodynamic theory considers the elastic deflection equation, since
in this regime elastic deformation of the surfaces contributes
significantly to the lubricant film thickness.
- Boundary lubrication (also called boundary film lubrication): The
hydrodynamic effects are negligible. 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 tenacious 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.
- Mixed lubrication: This regime is in between the full film
elastohydrodynamic and boundary lubrication regimes. The generated
lubricant film is not enough to separate the bodies completely, but
hydrodynamic effects are considerable.
Besides supporting the load the lubricant may have to perform other functions as well, for instance it may cool the contact areas and remove wear products. While 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 surfaces in close proximity would generate enough heat for rapid surface damage which in a coarsened condition may literally weld the surfaces together, causing seizure.
In some applications, such as piston engines, the film between the piston and the cylinder wall also seals the combustion chamber, preventing combustion gases from escaping into the crankcase.
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