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LUBRICANTS
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LUBRICANTS . See also:Machines consist of parts which have relative See also:motion and generally slide and rub against each other. Thus the See also:axle of a See also:cart or railway vehicle is pressed against a metallic bearing See also:surface supporting the See also:body of the vehicle, and the two opposed surfaces slide upon each other and are pressed together with See also:great force. If the metallic surfaces be clean, the See also:speed of rubbing high, and the force pressing the surfaces together considerable, then the latter will abrade each other, become hot and be rapidly destroyed. It is possible, however, to prevent the serious See also:abrasion of such opposing surfaces, and largely to reduce the frictional resistance they oppose to relative motion by the use of lubricants (See also:Lat. lubricare, lubricus, slippery). These substances are caused to insinuate themselves between the surfaces, and have the See also:property of so separating them as to prevent serious abrasion. The solid and semi-solid lubricants seem to See also:act as rollers between the surfaces, or See also:form a film between them which itself suffers abrasion or See also:friction. The liquid lubricants, however, maintain themselves as liquid films between the surfaces, upon which the bearing floats. The frictional resistance is then wholly in the fluid. Even when lubricants are used the friction, i.e. the resistance to motion offered by the opposing surfaces, is considerable. In the See also:article FRICTION will be found a statement of how friction is measured and the manner in which it is expressed. The coefficient of friction is obtained by dividing the force required to cause the surfaces to slide over each other by the load pressing them together. For clean unlubricated surfaces this coefficient may be as great as o•3, whilst for well-lubricated cylindrical See also:bearings it may be as small as o•0006. See also:Engineers have, therefore, paid particular See also:attention to the See also:design of bearings with the See also:object of reducing the friction, and thus making use of as much as possible of the See also:power See also:developed by See also:prime See also:movers. The importance of doing this will be seen when it is remembered that the See also:energy wasted is proportional to the coefficient of friction, and that the durability of the parts depends upon the extent to which they are separated by the lubricant and thus prevented from injuring each other. There is great diversity in the shapes of rubbing surfaces, the loads they have to carry vary widely, and the speed of rubbing ranges from less than one See also:foot to thousands of feet per See also:minute. There is also a large number of substances which act as lubricants, some being liquids and others soft solids. In many See also:instruments or machines where the surfaces in contact which have to slide upon each other are only lightly pressed together, and are only occasion-ally given relative motion, the lubricant is only needed to prevent abrasion. Microscopes and mathematical instruments are of this See also:kind. In such cases, the lubricant which keeps the surfaces from abrading each other is a See also:mere contamination film, either derived from the See also:air or put on when the surfaces are finished. When such lubricating films are depended upon, the friction surfaces should be as hard as possible and, if practicable, of dissimilar metals. In the See also:absence of a contamination film, most metals, if rubbed when in contact, will immediately adhere to each other. A large number of experiments have been made to ascertain the co-efficient of friction under these imperfect conditions of See also:lubrication. Within wide limits of load, the friction is proportional to the pressure normal to the surfaces and is, therefore, approximately See also:independent of the See also:area of the surfaces in contact. Although the static coefficient is often less than the kinetic at very See also:low speeds, within wide limits the latter coefficient de-creases with increaAing speed. These See also:laws apply to all bearings the velocity of rubbing of which is very small, or which are lubricated with solid or semi-solid materials. When the speed of rubbing is considerable and the contamination film is liable to be destroyed, resort is had to lubricants which possess the power of keeping the surfaces apart, and thereby reducing the friction. The See also:constant application of such substances is necessary in the See also:case of such parts of See also:machine tools as slide rests, the surfaces of which only move relatively to each other at moderate speeds, but which have to carry heavy loads. In all See also:ordinary cases, the coefficient of friction of See also:flat surfaces, such as those of slide blocks or See also:pivot bearings, is high, owing to the fact that the lubricant is not easily forced between the surfaces. In the case of cylindrical bearing surfaces, such as those of See also:journals and spindles, owing to the fact that the See also:radius of the bearing „urface is greater than that of the See also:journal or spindle, the lubricant, if a liquid, is easily See also:drawn in and entirely separates the surfaces (see LUBRICATION). Fortunately, cylindrical bearings are by far the most See also:common and important form of bearing, and they can be so lubricated that the friction coefficient is very low. The lubricant, owing to its viscosity, is forced between the surfaces and keeps them entirely apart. This property of viscosity is one of the most important possessed by liquid lubricants. Some lubricants, such as the See also:oils used for the See also:light spindles of textile machinery, are quite thin and limpid, whilst others, suitable for See also:steam See also:engine cylinders and very heavy bearings, are, at ordinary temperatures, as thick as See also:treacle or See also:honey. Generally speaking, the greater the viscosity of the lubricant the greater the load the bearing will carry, but with thick lubricants the frictional coefficient is correspondingly high. True lubricants differ from ordinary liquids of. equal viscosity inasmuch as they possess the property of " oiliness.” This is a property which enables them to maintain an unbroken film between surfaces when the loads are heavy. It is possessed most markedly by vegetables and See also:animal oils and fats, and less markedly by See also:mineral oils. In the case of mineral lubricating oils from the same source, the See also:lower the specific gravity the greater the oiliness of the liquid, as a See also:rule. Mixtures of mineral oil with animal or See also:vegetable oil are largely used, one class of oil supplying those qualities in which the other is deficient. Thus the mineral oils, which are comparatively cheap and possess the important property of not becoming oxidized into gummy or sticky sub-stances by the See also:action of the air, which also are not liable to cause spontaneous ignition of See also:cotton See also:waste, &c., and can be manufactured of almost any desired viscosity, but which on the other See also:hand are somewhat deficient in the property of oiliness, are mixed with animal or vegetable oils which possess the latter property in marked degree, but are liable to See also:gum and become See also:acid and to cause spontaneous ignition, besides being comparatively expensive and limited in quantity. Oils which become acid attack the bearings chemically, and those which oxidize may become so thick that they fail to run on to the bearings properly. The following table shows that the permissible load on bearings varies greatly: Description of Bearing. Load in lb per sq. in. Hard See also:steel bearings on which the load is intermittent, such as the See also:crank pins of shearing machines . . . . 3000 See also:Bronze crosshead See also:neck journals . 1200 Crank pins of large slow engines . . Boo-goo Crank pins of marine engines . 400-500 See also:Main crank-See also:shaft bearings, slow marine 600 Main crank-shaft bearings, fast marine 400 Railway See also:coach journals . . 300-400 See also:Fly-See also:wheel shaft journals 150-200 Small engine crank pins . . 150-200 Small slide blocks, marine engines Too Stationary engine slide See also:block 25-125 Stationary engine slide block, usually . 3o-6o Propeller thrust bearings 50-70 Shafts in See also:cast See also:iron steps, high speed . 15 Solid Lubricants.—Solid substances, such as See also:graphite or See also:plumbago, soapstone, &c., are used as lubricants when there is some objection to liquids or soft solids, but the surfaces between which they are placed should be of very hard materials. They are frequently mixed with oils or greases, the lubricating properties of which they improve. Semi-solid Lubricants.—The contrast in lubricating properties between mineral and fatty oils exists also in the case of a pure mineral grease like See also:vaseline and an animal See also:fat such as See also:tallow, the latter possessing in a far greater degree the property of greasiness. A large number of lubricating greases are made by incorporating or emulsifying animal and vegetable fats with See also:soap and See also:water; also by thickening mineral lubricating oils with soap. Large quantities of these greases are used with very See also:good results for the lubrication of railway waggon axles, and some of them are excellent lubricants for the bearings of slow moving machinery. Care must be taken, how-ever, that they do not contain excess of water and are not adulterated with such useless substances as See also:china See also:clay; also, that they melt as a whole, and that the oil does not run down and leave the soap. This is liable to occur with badly made greases, and hot bearings are the result. Except in See also:special cases, greases should not be used for See also:quick-See also:running journals, shafts or spindles, on See also:account of the high frictional resistance which they offer to motion. In the case of fats and greases whose melting points are not much above the temperature of surrounding See also:objects it generally happens that the lubricating films are so warmed by friction that they actually melt and act as oils. These lubricants are generally forced into the bearings by a form of See also:syringe fitted with a See also:spring See also:piston, or are squeezed between the faces by means of a See also:screw-plug. Liquid Lubricants.—Generally speaking, all bearings which it is necessary should run with as little friction as possible must be sup-plied with liquid lubricants. These may be of animal, vegetable or mineral origin. The mineral oils are mixtures of See also:hydrocarbons of variable viscosity, flashing-point, See also:density and oiliness. They are obtained by See also:distillation from See also:American, See also:Russian and other petroleums. The fixed oils obtained from animal and vegetable substances are not volatile without decomposition, and are found ready made in the tissues of animals and See also:plants. Animal oils are obtained from the adipose See also:tissue by See also:simple See also:heat or by boiling with water. They are usually either colourless or yellow. The oils of plants occur usually in the seeds or See also:fruit, and are obtained either by expression or by means of solvents such as See also:ether or See also:petroleum. They are of various shades of yellow and See also:green, the green See also:colour being due to the presence of See also:chlorophyll. The-fundamental difference between fixed oils and mineral oils exists in their behaviour towards See also:oxygen. Mineral oils at ordinary temperatures are indifferent to oxygen, but all fixed oils combine with it and thicken or gum more or less, generating heat at the same See also:time. Such oils are, therefore, dangerous if dropped upon See also:silk, cotton or woollen waste or other combustible fibrous materials, which are thus rendered liable to spontaneous ignition. Liquid lubricants are used for all high speed bearings. In some cases the rubbing surfaces See also:work in a See also:bath of the lubricant, which can then reach all the rubbing parts with certainty. Small engines for motor cars or road waggons are often lubricated in this way. In the case of individual bearings, such as those of railway vehicles, a See also:pad of cotton, worsted and See also:horse See also:hair is kept saturated with the lubricant and pressed against the under See also:side of the journal. The journal is thus kept constantly wetted with oil, and the film is forced beneath the See also:brass as the axle rotates. In many cases, oil-ways and grooves are cut in the bearings, and the lubricant is allowed to run by gravity into them and thus finds its way between the opposing surfaces. To secure a steady feed various contrivances are adopted, the most common being a See also:wick of cotton or worsted used as a See also:siphon. In cases where it is important that little if any See also:wear should take See also:place, the lubricant is forced by means of a See also:pump between the friction surfaces and a constant film of oil is thereby maintained between them. For the spindles of small machines such as clocks, watches and other delicate mechanisms, which are only lubricated at See also:long intervals place for theoretical discrepancies. However, in the attempts at generalization which followed the See also:reformation of See also:science, opportunity was afforded for such discrepancies in the mere enunciation of the circumstances in which the so-called laws of friction of motion are supposed to apply. The circumstances in which the great amount of empirical See also:research was conducted as to the resistance between the clean, See also:plane, smooth surfaces of rigid bodies moving over each other under pressure, invariably include the presence of air at atmospheric pressure around, and to some extent between, the surfaces; but this fact had received no See also:notice in the enunciation of these laws, and this constitutes a theoretical departure from the conditions under which the experience had been obtained. Also, the theoretical See also:division of the See also:law of frictional resistance into two laws—one dealing with the limit of See also:rest, and the other asserting that the friction of motion, which is invariably less in similar circumstances than that of rest, is independent of the velocity of sliding—involves the theoretical See also:assumption that there is no asymptotic law of diminution of the resistance, since, starting from rest, the See also:rate of sliding increases. The theoretical substitution of ideal rigid bodies with geometrically See also:regular surfaces, sliding in contact under pressure at the common regular surface, for the aerated surfaces in the actual circumstances, and the theoretical substitution of the See also:absolute See also:independence of the resistance of the rate of sliding for the limited independence in the actual circumstances, prove the See also:general See also:acceptance of the conceptions-(1) that See also:matter can slide over matter under pressure at a geometric-ally regular surface; (2) that, however much the resistance to sliding under any particular pressure (the co-efficient of friction) may depend on the See also:physical properties of the materials, the sliding under pressure takes place at the geometrically regular surface of contact of the rigid bodies; and (3) as the consequence of (1) and (2), that whatever the effect of a lubricant, such as oil, might have, it could be a physical surface effect. Thus not only did these general theoretical conceptions, resulting from the theoretical laws of friction, fail to indicate that the lubricant may diminish the resistance by the mere See also:mechanical separation of the surfaces, but they precluded the See also:idea that such might be the case. The result was that all subsequent attempts to reduce the empirical facts, where a lubricant was used, to such general laws as might reveal the See also:separate functions of the complex circumstances on which lubrication depends, completely failed. Thus until 1883 the science of lubrication had not advanced beyond the empirical See also:stage. This See also:period of stagnation was terminated by an accidental phenomenon observed by See also:Beauchamp See also:Tower, while engaged on his research on the friction of the journals of railway carriages. His observation led him to a See also:line of experiments which proved that in these experiments the general See also:function of the lubricant was the mechanical separation of the See also:metal surfaces by a layer of fluid of finite thickness, thus upsetting the preconceived ideas as expressed in the laws of the friction of motion. On the publication of Tower's reports (Prot. Inst. M.E., See also:November 1883), it was recognized by several physicists (B.A. See also:Report, 1884, pp. 14, 625) that the See also:evidence they contained afforded a basis for further study of the actions involved, indicating as it did the circumstances—namely, the properties of viscosity and cohesion possessed by fluids—account of which had not been taken in previous conclusions. It also became apparent that continuous or steady lubrication, such as that of Tower's experiments, is only secured when the solid surfaces separated by the lubricant are so shaped that the thickness at the ingoing side is greater than that at the outgoing side. When the general equations of viscous fluids had been shown as the result of the labours of C. L. M. H. Navier,1 A. L. See also:Cauchy,' S. D. See also:Poisson,' A. J. C. See also:Barre de St Venant,4 and in 1845 of See also:Sir G. See also:Gabriel See also:Stokes,5 to involve no other assumption than that the stresses, other than the pressure equal in all directions, 1 mein. de l'Acad. @826), 6, p. 389. Mem. See also:des say. ttrang. I. 4o. 3 Mem. de l'Acad. (1831), 10, p. 345. ' B.A. Report (1846) 5 See also:Cambridge Phil. Trans. (1845 and 1857). and are often exposed to extremes of temperature, the lubricant must be a fluid oil as See also:free as possible from tendency to gum or thicken by oxidation or to corrode metal, and must often have a low freezing-point. It must also possess a maximum of " oiliness." The lubricants mostly used for such purposes are obtained from See also:porpoise or See also:dolphin See also:jaw oils, See also:bean oil, See also:hazel See also:nut oil, neatsfoot oil, sperm oil or See also:olive oil. These oils are exposed for some time to temperatures as low as the mechanism is required to work at, and the portion which remains fluid is separated and used. Free acid should be entirely eliminated by chemical refining. A little good mineral oil may with See also:advantage be mixed with the fatty oil. For all ordinary machinery, ranging from the light See also:ring spindles of textile See also:mills to the heavy shafts of large engines, mineral oils are almost universally employed, either alone or mixed with fatty oils, the general rule being to use pure mineral oils for bath, forced or circulating pump lubrication, and mixed oils for drop, siphon and other less perfect methods of lubrication. Pure mineral oils of relatively low viscosity are used for high speeds and low pressures, mixed oils of greater viscosity for low speeds and high pressures. In selecting oils for low speeds and great pressures, viscosity must be the first See also:consideration, and next to that " oiliness." If an oil of sufficiently high viscosity be used, a mineral oil may give a result as good or better than a pure fixed oil; a mixed oil may give a better result than either. If a mineral oil of sufficient viscosity be not available, then a fixed oil or fat may be expected to give the best result. In special cases, such as in the lubrication of textile machines, where the oil is liable to be splashed upon the fabric, the See also:primary consideration is to use an oil which can be washed out without leaving a stain. Pure fixed oils, or mixtures composed largely of fixed oils, are used for such purposes.
In other special cases, such as marine engines working in hot places, mixtures are used of mineral oil with See also:rape or other vegetable oil artificially thickened by blowing air through the heated oil, and known as " blown " oil or " soluble See also:castor oil."
In the lubrication of the cylinders and valves of steam, See also:gas and oil engines, the lubricant must possess as much viscosity as possible at the working temperature, must not evaporate appreciably and must not decompose and liberate fatty acids which would corrode the metal and choke the steam passages with metallic soaps; for gas and oil engines the lubricant must be as free as possible from tendency to decompose and See also:deposit See also:carbon when heated. For this See also:reason steam cylinders and valves should be lubricated with pure mineral oils of the highest viscosity, mixed with no more fixed oil than is necessary to ensure efficient lubrication. Gas and oil engines also should be lubricated with pure mineral oils wherever possible.
For further See also:information on the theory and practice of lubrication and on the testing of lubricants, see Friction and Lost Work in Machinery and See also: M. Additional information and CommentsThere are no comments yet for this article.
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