Preparations used to decrease the friction between opposed solid faces, which auses, as is supposed by some, from true cohesion. The solid varieties include natural and artificial graphite, talc, mica, and other substances, such as flowers of sulphur” and whitelead, which are used for curing hot bearings. The natural graphite is usually of the socalled flake variety, and varies in size of particles from in. and less than in., whilst the artificially produced kind is amorphous, and ground even finer tbai; the natural product. The latter is sold under the trade names of “Aquadag” anil “Hydrosol” when in admixture with water, and as “Oildag,” “Oleosol,” and “Kollag” when in admixture with oil. Generally speaking, solid lubricants are applied dry in cases where for special reasons it is inadvisable or not possible to use liquid or semisolid lubricants, although they are usually employed in admixture with oil or as ingredients in greases. “Aquadag” used as a cylinder lubricant has been found advantageous where solid friction occurs, as in wormgear, although equally good results have been obtained by the use of natural flake graphite; and it has been concluded that the lubricating value of graphite depends upon its chemical purity. The lubricating value of oil depends upon somethin? not yet properly understood; it is not viscosity. What, however, is required in a liquid lubricant is that it shall penetrate into the narrow spaces between journal and bearing, thus “wetting” or spreading over the surfaces which are in motion together. The function of a lubricant is to keep metal surfaces separate with a minimum expenditure of energy, Vegetable and animal oils possess greater value than mineral oils obtained from crude petroleum and coal tar, which are also used as lubricants in common with the so called rosin oil, obtained by the destructive distillation of resin. They are generally graded in accordance with their specific gravities and viscosities. The socalled “germ process” of lubrication employs one or more fatty acids with mineral oil as the instrument of lubrication, i or 2 per cent, being incorporated according to chemical circumstances. This is said to reduce the coefficient of friction from 00084 to 00052that is, some 25 per cent, on a frictiontesting machine. Oils exhibit a rapid increase in viscosity with pressure, this increase being much greater for the mineral than for animal and vegetable oils.
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