Lubricants

Lubricants. Machines consist of parts which have relative motion and generally slide and rub against each other. Thus the axle of a cart or railway vehicle is pressed against a metallic bearing surface supporting the body of the vehicle, and the two opposed surfaces slide upon each other and are pressed together with great force. If the metallic surfaces be clean, the 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 abrasion of such opposing surfaces, and largely to reduce the frictional resistance they oppose to relative motion by the use of lubricants (Lat. lubricare, lubricus, slippery). These substances are caused to insinuate themselves between the surfaces, and have the property of so separating them as to prevent serious abrasion. The solid and semi-solid lubricants seem to act as rollers between the surfaces, or form a film between them which itself suffers abrasion or 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 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 0.3, whilst for well-lubricated cylindrical bearings it may be as small as 0.0006. Engineers have, therefore, paid particular attention to the design of bearings with the object of reducing the friction, and thus making use of as much as possible of the power developed by prime movers. The importance of doing this will be seen when it is remembered that the 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 foot to thousands of feet per minute. There is also a large number of substances which act as lubricants, some being liquids and others soft solids. In many instruments or machines where the surfaces in contact which have to slide upon each other are only lightly pressed together, and are only occasionally given relative motion, the lubricant is only needed to prevent abrasion. Microscopes and mathematical instruments are of this kind. In such cases, the lubricant which keeps the surfaces from abrading each other is a mere contamination film, either derived from the 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 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 coefficient of friction under these imperfect conditions of lubrication. Within wide limits of load, the friction is proportional to the pressure normal to the surfaces and is, therefore, approximately independent of the area of the surfaces in contact. Although the static coefficient is often less than the kinetic at very low speeds, within wide limits the latter coefficient decreases with increasing speed. These 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 constant application of such substances is necessary in the case of such parts of 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 ordinary cases, the coefficient of friction of flat surfaces, such as those of slide blocks or 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 journals and spindles, owing to the fact that the radius of the bearing surface is greater than that of the journal or spindle, the lubricant, if a liquid, is easily drawn in and entirely separates the surfaces (see Lubrication). Fortunately, cylindrical bearings are by far the most 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 oils used for the light spindles of textile machinery, are quite thin and limpid, whilst others, suitable for steam engine cylinders and very heavy bearings, are, at ordinary temperatures, as thick as treacle or 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 animal oils and fats, and less markedly by mineral oils. In the case of mineral lubricating oils from the same source, the lower the specific gravity the greater the oiliness of the liquid, as a rule. Mixtures of mineral oil with animal or 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 substances by the action of the air, which also are not liable to cause spontaneous ignition of cotton waste, &c., and can be manufactured of almost any desired viscosity, but which on the other 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 gum and become 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 ℔  
per sq. in.  
---|---  
Hard steel bearings on which the load is intermittent, |    
such as the crank pins of shearing machines | 3000  
Bronze crosshead neck journals | 1200  
Crank pins of large slow engines | 800-900  
Crank pins of marine engines | 400-500  
Main crank-shaft bearings, slow marine | 600  
Main crank-shaft bearings, fast marine | 400  
Railway coach journals | 300-400  
Fly-wheel shaft journals | 150-200  
Small engine crank pins | 150-200  
Small slide blocks, marine engines | 100  
Stationary engine slide block | 25-125  
Stationary engine slide block, usually | 30-60  
Propeller thrust bearings | 50-70  
Shafts in cast iron steps, high speed | 15  
  
Solid Lubricants.—Solid substances, such as graphite or 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 vaseline and an animal fat such as 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 soap and water; also by thickening mineral lubricating oils with soap. Large quantities of these greases are used with very 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, however, that they do not contain excess of water and are not adulterated with such useless substances as china 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 special cases, greases should not be used for quick-running journals, shafts or spindles, on 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 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 syringe fitted with a spring piston, or are squeezed between the faces by means of a screw-plug.

Liquid Lubricants.—Generally speaking, all bearings which it is necessary should run with as little friction as possible must be supplied with liquid lubricants. These may be of animal, vegetable or mineral origin. The mineral oils are mixtures of hydrocarbons of variable viscosity, flashing-point, density and oiliness. They are obtained by distillation from American, 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 plants. Animal oils are obtained from the adipose tissue by simple heat or by boiling with water. They are usually either colourless or yellow. The oils of plants occur usually in the seeds or fruit, and are obtained either by expression or by means of solvents such as ether or petroleum. They are of various shades of yellow and green, the green colour being due to the presence of chlorophyll. The fundamental difference between fixed oils and mineral oils exists in their behaviour towards 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 time. Such oils are, therefore, dangerous if dropped upon 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 work in a 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 pad of cotton, worsted and horse hair is kept saturated with the lubricant and pressed against the under side of the journal. The journal is thus kept constantly wetted with oil, and the film is forced beneath the 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 wick of cotton or worsted used as a siphon. In cases where it is important that little if any wear should take place, the lubricant is forced by means of a 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 long intervals and are often exposed to extremes of temperature, the lubricant must be a fluid oil as 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 porpoise or dolphin jaw oils, bean oil, hazel nut oil, neatsfoot oil, sperm oil or 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 advantage be mixed with the fatty oil.

For all ordinary machinery, ranging from the light ring spindles of textile 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 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 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 rape or other vegetable oil artificially thickened by blowing air through the heated oil, and known as “blown” oil or “soluble castor oil.”

In the lubrication of the cylinders and valves of steam, 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 deposit carbon when heated. For this 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 information on the theory and practice of lubrication and on the testing of lubricants, see Friction and Lost Work in Machinery and Mill Work, by R. H. Thurston (1903); and Lubrication and Lubricants, by L. Archbutt and R. M. Deeley (1906).

(R. M. D.)