lubrication

lubrication

[loo-bri-keyt]
lubrication, introduction of a substance between the contact surfaces of moving parts to reduce friction and to dissipate heat. A lubricant may be oil, grease, graphite, or any substance—gas, liquid, semisolid, or solid—that permits free action of mechanical devices and prevents damage by abrasion and "seizing" of metal or other components through unequal expansion caused by heat. In machining processes lubricants also function as coolants to forestall heat-caused deformities.

Types of Lubricants

Lubricants can be classified by their origin—animal (e.g., sperm oil, goose grease), vegetable (e.g., soybean oil, linseed oil), or mineral (e.g., petroleum, molybdenum sulfide). From ancient times until the late 19th cent. lubricants were obtained from vegetable oils or animal fats and oils. Today most are derived from mineral oils, such as petroleum and shale oil, which can be distilled and condensed without decomposition. Synthetic lubricants, such as silicones, are of great value in applications involving extreme temperatures. In certain types of high-speed machinery films of gas under pressure have been successfully used as lubricants.

Types of Lubrication

Differing widely in viscosity, specific gravity, vapor pressure, boiling point, and other properties, lubricants also offer a wide range of selection for the increasingly varied needs of modern industry. But whatever their derivation or properties, the purpose of lubricants is to replace dry friction with either thin-film or fluid-film friction, depending on the load, speed, or intermittent action of the moving parts. Thin-film lubrication, in which there is some contact between the moving parts, usually is specified where heavy loads are a factor. In fluid, or thick-film, lubrication a pressure film is formed between moving surfaces and keeps them completely apart. This type of lubrication cannot easily be maintained in high-speed machinery and therefore is used where reciprocating or oscillating conditions are moderate.

Application of Lubricants

Efficient operation of machinery largely depends not only on the lubricant selected but also on its method of application. Lubricants formerly were applied by hand, but modern machinery requires exact methods that can be precisely controlled. For most machinery, different methods of lubrication and types of lubricants must be employed for different parts. In an automobile, for example, the chassis is lubricated with grease, the manual transmission and rear-axle housings are filled with heavy oil, the automatic transmission is lubricated with a special-grade light oil, wheel bearings are packed with a grease that has a thickener composed of long fibers, and the crankcase oil that lubricates engine parts is a lightweight, free-flowing oil.

Application of Liquid Lubricants

Mechanical devices to supply lubricants are called lubricators. A simple form of lubricator is a container mounted over a bearing or other part and provided with a hole or an adjustable valve through which the lubricant is gravity-fed at the desired rate of flow. Wick-feed oilers are placed under moving parts, and by pressing against them they feed oil by capillary action. Horizontal bearings are frequently oiled by a rotating ring or chain that carries oil from a reservoir in the bearing housing and distributes it along the bearing through grooves or channels. Bath oiling is useful where an oil-tight reservoir can be provided in which the bearing journal may be submerged; the pool of oil helps to carry away heat from contact surfaces. Splash-oiling devices are used where gears, bearings, or other parts contained in housings have moving parts that dip into the lubricant and splash it on the bearings or into distribution channels. Centralized oiling systems usually consist of a reservoir, pump, and tubes through which oil is circulated, while heaters or coolers may be introduced to change the viscosity of the lubricant for various parts of the system. Many oiling operations are automatically synchronized to start and stop with the machinery.

Application of Semisolid and Solid Lubricants

Grease lubricants are semisolid and have several important advantages: They resist being squeezed out, they are useful under heavy load conditions and in inaccessible parts where the supply of lubricant cannot easily be renewed, and they tend to form a crust that prevents the entry of dirt or grit between contact surfaces. Grease is a mixture of a lubricant and a thickener; often it is made from a mineral oil and a soap. It may be applied in various ways: by packing enclosed parts with it, by pressing it onto moving parts from an adjacent well, by forcing it through grease cups by a spring device, and by pumping it through pressure guns. Solid lubricants are especially useful at high and low temperatures, in high vacuums, and in other applications where oil is not suitable; common solid lubricants are graphite and molybdenum disulfide.

Introduction of any of various substances between sliding surfaces to reduce wear and friction. Lubricants may secondarily control corrosion, regulate temperature, electrically insulate, remove contaminants, or damp shock. Prehistoric peoples used mud and reeds to lubricate sledges, timbers, or rocks. Animal fat lubricated the axles of the first wagons and continued in wide use until crude oil became the chief source of lubricants. Crude oil has been the basis of products designed for the specific lubricating needs of automobiles, aircraft, locomotives, turbojets, and all other power machinery. There are three basic varieties of lubrication: fluid-film (in which a fluid film completely separates sliding surfaces), boundary (in which the friction between surfaces is determined by the properties of the surfaces and properties of the lubricant other than viscosity), and solid (used when liquid lubricants lack adequate resistance to load or temperature extremes). The principal lubricants are liquid, oily materials (petroleum-based or synthetic, and including greases); solids (such as graphite, molybdenum disulfide, soft metals, waxes, and plastics); and gases.

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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 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.

References

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