![Steam engine lubrication](https://upload.wikimedia.org/wikipedia/commons/7/71/Steam_engine_lubrication.jpg)
Lubrication is the process or technique of using a
lubricant to reduce
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
and
wear and tear
Wear and tear is damage that naturally and inevitably occurs as a result of normal wear or aging. It is used in a legal context for such areas as warranty contracts from manufacturers, which usually stipulate that damage from ''wear and tear'' w ...
in a contact between two surfaces. The study of lubrication is a discipline in the field of
tribology
Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication and wear. Tribology is highly interdisciplinary, drawing on many academic f ...
.
Lubrication mechanisms such as fluid-lubricated systems are designed so that the applied load is partially or completely carried by
hydrodynamic
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including '' aerodynamics'' (the study of air and other gases in motion) a ...
or
hydrostatic
Fluid statics or hydrostatics is the branch of fluid mechanics that studies the condition of the equilibrium of a floating body and submerged body "fluids at hydrostatic equilibrium and the pressure in a fluid, or exerted by a fluid, on an imme ...
pressure, which reduces solid body interactions (and consequently friction and wear). Depending on the degree of surface separation, different lubrication regimes can be distinguished.
Adequate lubrication allows smooth, continuous operation of
machine element
Machine element or hardware refers to an elementary component of a machine. These elements consist of three basic types:
# '' structural components'' such as frame members, bearings, axles, splines, fasteners, seals, and lubricants,
# '' mech ...
s, 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.
Lubrication mechanisms
Fluid-lubricated systems
As the load increases on the contacting surfaces, distinct situations can be observed with respect to the mode of lubrication, which are called lubrication regimes:
*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.
**In hydrostatic lubrication, external pressure is applied to the lubricant in the
bearing to maintain the fluid lubricant film where it would otherwise be squeezed out.
**In hydrodynamic lubrication, the motion of the contacting surfaces, as well as the design of the bearing, 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 Reynolds Equation is a partial differential equation governing the pressure distribution of thin viscous fluid films in lubrication theory. It should not be confused with Osborne Reynolds' other namesakes, Reynolds number and Reynolds-averaged N ...
. The governing equations of the hydrodynamic theory of lubrication and some analytical solutions can be found in the reference.
*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. Due to the strong coupling between lubricant hydrodynamic action and the elastic deformation in contacting solids, this regime of lubrication is an example of
Fluid-structure interaction. The classical elastohydrodynamic theory considers
Reynolds equation The Reynolds Equation is a partial differential equation governing the pressure distribution of thin viscous fluid films in lubrication theory. It should not be confused with Osborne Reynolds' other namesakes, Reynolds number and Reynolds-averaged N ...
and the elastic deflection equation to solve for the pressure and deformation in this lubrication regime. Contact between raised solid features, or ''asperities'', can also occur, leading to a mixed-lubrication or boundary lubrication regime.
* Boundary lubrication is defined as that regime in which the load is carried by the surface asperities (high points) rather than by the lubricant. This is the effect that makes
Ultra-high-molecular-weight polyethylene
Ultra-high-molecular-weight polyethylene (UHMWPE, UHMW) is a subset of the thermoplastic polyethylene. Also known as high-modulus polyethylene, (HMPE), it has extremely long chains, with a molecular mass usually between 3.5 and 7.5 million amu. ...
"self-lubricating".
*Boundary film lubrication: The hydrodynamic effects are negligible. The bodies come into closer contact at their asperities (high points); 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.
*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 such as
pistons,
pumps,
cam
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...
s,
bearings,
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating ...
s,
gear
A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic ...
s,
roller chains
Roller chain or bush roller chain is the type of chain drive most commonly used for transmission of mechanical power on many kinds of domestic, industrial and agricultural machinery, including conveyors, wire- and tube-drawing machines, pri ...
,
cutting tool
In the context of machining, a cutting tool or cutter is typically a hardened metal tool that is used to cut, shape, and remove material from a workpiece by means of machining tools as well as abrasive tools by way of shear deformation. The major ...
s 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
An epileptic seizure, informally known as a seizure, is a period of symptoms due to abnormally excessive or synchronous neuronal activity in the brain. Outward effects vary from uncontrolled shaking movements involving much of the body with l ...
.
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.
If an engine required pressurised lubrication to, say,
plain bearing
A plain bearing, or more commonly sliding contact bearing and slide bearing (in railroading sometimes called a solid bearing, journal bearing, or friction bearing), is the simplest type of bearing, comprising just a bearing surface and no roll ...
s, there would be an
oil pump and an
oil filter
An oil filter is a filter designed to remove contaminants from engine oil, transmission oil, lubricating oil, or hydraulic oil. Their chief use is in internal-combustion engines for motor vehicles (both on- and off-road ), powered aircraft, r ...
. On early engines (such as a
Sabb marine diesel), where pressurised feed was not required
splash lubrication
Splash lubrication is a rudimentary form of lubrication found in early engines. Such engines could be external combustion engines (such as stationary steam engines), or internal combustion engines (such as petrol, diesel or paraffin engines). ...
would suffice.
See also
*
References
External links
Machinery Lubrication magazineInternational Council for Machinery Lubrication
{{Authority control
Tribology
Lubricants