LUBRICANTS. The lubricants are of three classes; solid, semi-solid and liquid. Various forms of graphites or plumbago and soapstone constitute the first class. The second class is com posed mainly of the animal fats and of greases that are made of a combination of fats and oils thickened either with a calcium soap or a sodium soap, or with a combination of the two. The third class consists of a great variety of oils that are derived from the animal, vegetable and mineral kingdoms. Animals as well as vegetables secrete a fatty matter which serves as a reserve supply of food for the growing organisms. The fat is generally present in the form of small globules suspended in the cell liquids.
In animals, the fat is in layers and the oil from this fat is rendered by heating or by boiling with water. The most abundant supply of vegetable oil is found in the seeds or the fruits. The oil is rendered by pressing out ("expression") or by solvents such as ether or petroleum naphtha. (See OILS, FATS AND WAXES.) The lubricants are obtained from the minerals through the distillation of brown coal or peat, of shale and of bituminous coal; although lubricants have not been produced to any great extent from these sources. The greatest amounts are obtained from crude petroleum found in many parts of the world and these are the lubricants that are being used to-day on the greatest proportion of the world's machinery. The petroleum lubricants are recovered by distillation of the crude in steam stills or vacuum stills. The lubricants all have a characteristic greasy touch and an individual taste and smell, all subject to some variation through methods of manufacture. The manner in which the lubricant is made also has certain influences upon its greasi ness, its cohesion or power of holding together so that it will drag between wedge-like surfaces, and its ability to cling to metal.
All fluid lubricants have certain physical characteristics that can be determined in the laboratory, such as specific gravity, which is the weight of the oil as compared to the weight of a similar amount of distilled water. The flash and the fire points denote the temperature at which vapours are emitted that form a lean mixture with the air and can be ignited to produce a flash.
At a higher temperature more vapour is emitted and the mixture burns steadily. The cloud test represents the temperature at which wax begins to form in the paraffin oils. The pour test is determined by the temperature at which the oil ceases to flow. The "fluidity" of the oil at various temperatures is the measure of its viscosity. (See Viscosinr.) The carbon residue (Conrad son) test indicates the percentage of residue left after the oil has been completely burned. Saponification and emulsification values, acidity, colour and many other tests are also made.
The foregoing tests have been standardized by the American Society for Testing Materials (see A.S.T.M. Transactions) and have been adopted by the U.S. Government, by all refiners and by the industries. (See the U.S. Government Master Specifica tions for Lubricants and Liquid Fuels.) In the practical work of lubrication, the viscosity, the carbon residue and the tests that indicate the ability of a lubricant to withstand physical change are the most important.
Viscosity.—The fixed lubricants, that is, the animal or vege table oils, have a viscosity curve that is a characteristic of the oil and indicates its source. The mineral oils can be made of any desired body or viscosity at any temperature by methods of distillation or by blending the heavy-bodied with the light bodied lubricants. Viscosity is a necessary feature of an oil's ability to support loads; it is also a measure of its internal fric tion and an indication of its film thickness. The amount of frictional resistance to the motion of machinery is entirely de pendent upon the viscosity of the lubricant and its suitability for the conditions under which it has to work. The heat generated in a bearing, if it all could be collected and measured, would be found to be the precise measure of the power lost in overcoming the solid friction of the dry surfaces moving on each other or of the fluid friction of the lubricant between such surfaces. This heat, as it develops, flows out through the metal and is radiated to the air, its rate of flow being dependent upon the construction of the bearing and the kind and location of the machine. (See