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 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 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.
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.
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.
(in Russian, smazka), a term in engineering that has several meanings. Lubrication may refer to the control of friction between machine parts (lubricating) or to the method of introducing the lubricant into the region required. In Russian, smazka may also refer to lubricants themselves (that is, to materials which reduce friction and facilitate machining and pressure-shaping of metals) or to materials used to protect
surfaces from corrosion and to seal certain machine joints and connections.
Lubrication is the lubricating action of lubricants on surfaces in relative motion that results in a reduction of friction and surface wear. The layer of lubricant introduced between the sliding surfaces provides for a minimum friction force and a sufficiently high resistance to loads that are normal to the surfaces. Optimal lubricating action can be provided by the use of proper lubricants, the introduction of lubricants to the sliding surfaces in sufficient quantities, the proper design of regions where friction arises, and the selection of appropriate technology.
Lubricants are materials used to prevent scoring and seizure and to reduce and regulate the wear of surfaces in relative motion. Liquid cooling lubricants, used, for example, in the machining of metals, constitute a special group. Fatty oils are used in the pressure shaping of metals (forging, rolling, drawing).
The methods of introducing lubricants between sliding surfaces are governed by the properties of the lubricant and the function and the operating conditions of the regions where friction arises. Lubricating oils are usually introduced through a crankcase system or a circulating system. In the crankcase system, sliding parts are located in an airtight crankcase, and the oil is poured into the lower part of the crankcase. In circulating systems, the oil is fed through passages to the areas of friction by a pump from the crankcase or a reservoir and is returned to the crankcase or reservoir by either gravity or forced circulation. A circulating lubrication system involves filters, separators, oil sumps, oil coolers, and measuring instruments. Other methods for introducing oils include wick feed, drop feed, and lubrication by oil mist. Greases are inserted directly into the areas of friction during the manufacture of the machine or are pressed into the assembly by devices called grease fittings with grease guns or mechanical extruders. Grease cups and spring grease cups are among the other types of lubricators. Some machines, such as rolling mills and walking excavators, have centralized lubrication systems.
Protective materials are substances that prevent the corrosion of metal articles and machine parts during storage or operation. Materials used for this purpose usually consist of semisolid hydrocarbons (petrolatums). Liquid preserving oils and solid coatings are also used. To obtain a solid coating, a solution of paraffin, ceresin wax, and corrosion inhibitors in volatile petroleum products is deposited on the surface to be protected. With the evaporation of the solvent, a solid coating with a thickness of approximately 0.1 mm remains on the surface.
Sealers are materials used in sealing, for example, vacuum systems, pipe fittings, and threaded pipe connections. They are also used to facilitate assembly or disassembly of threaded and other connections. Sealers are usually greases containing up to 20 percent of such materials as graphite powder, molybdenum disulfide, and soft metals.
V. V. SINITSYN