Lubrication

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.

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lubrication

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

The use of lubricants to reduce friction and wear. Whenever two bodies in contact are made to slide relative to one another, a resistance to the motion is experienced. This resistance, called friction, is present in all machinery. Approximately 30% of the power of an automobile engine is consumed by friction. Friction and wear can be significantly reduced, and thus relative motion of machine parts made possible, by interposing a lubricant at the interface of the contacting surfaces; the machine elements designed to accomplish this are called bearings. Bearings can be lubricated by solids such as graphite or, more commonly, by liquids and gases. See Antifriction bearing, Friction, Lubricant, Wear

Conventionally, lubrication has been divided into (1) fluid-film lubrication (hydrostatic, hydrodynamic, and elastohydrodynamic), where the sliding surfaces are separated by a relatively thick, continuous film of lubricant; and (2) boundary lubrication, where contact surface separation is but a few molecular layers and asperity contact is unavoidable.

Hydrostatic bearings

Hydrostatic films are created when a high-pressure lubricant is injected between opposing (parallel) surfaces (pad and runner), thereby separating them and preventing their coming into direct contact. Hydrostatic bearings require external pressurization. The film is 5–50 micrometers thick, depending on application. Though hydrostatic lubrication does not rely on relative motion of the surfaces, relative motion is permitted and can even be discontinuous. Figure 1 is a schematic of a hydrostatic bearing pad. To handle asymmetric loads, hydrostatic systems generally employ several evenly spaced pads. Hydrostatic bearings find application where relative positioning is of extreme importance. They are also applied where a low coefficient of friction at vanishing relative velocity is required.

Hydrodynamic bearings

Hydrodynamic bearings are self-acting. To create and maintain a load-carrying hydrodynamic film, it is necessary only that the bearing surfaces move relative to one another and ample lubricant is available. The surfaces must be inclined to form a clearance space in the shape of a wedge, which converges in the direction of relative motion. The lubricant film is then created as the lubricant is dragged into the clearance by the relative motion. This viscous action results in a pressure build-up within the film (Fig. 2). The fact that hydrodynamic bearings are self-generating and do not rely on auxiliary equipment makes these bearings very reliable. Hydrodynamic journal bearings and thrust bearings are designed to support radial and axial loads, respectively, on a rotating shaft.

Rolling contact bearings

Journal and thrust bearings are conformal bearings; that is, the opposing bearing surfaces conform in shape. Ball and roller bearings, also known as rolling contact bearings, are counterformal. Counterformal bearings always operate in the hydrodynamic mode, but because the contact area in these bearings is small the pressure attains high values, in the range of 1–3 gigapascals (10,000–30,000 atm). In consequence, the surfaces deform elastically and the lubricant viscosity increases by several orders of magnitude.

Lubricants

Today, mineral oils manufactured from petroleum are the most common liquid lubricants. The manufacturer of petroleum lubricants can choose from a wide variety of crude oils, and the choice is of great importance because the lubricating oil fraction of crude oils varies widely.