LUBRICANTS, substances having a low degree of cohesion applied to solid surfaces rubbing against each other; the object being to reduce the friction at the points of contact. The theory as to lubricants is that they interpose an almost frictionless film between the sur faces to which they are applied. The require ments of a good lubricant, therefore, are a minimum of internal cohesion and sufficient body to resist being squeezed out of place by the pressure to which they may be subjected — as, for example, in the bearings of a heavy shaft. Other important qualities demanded are freedom from all corrosive ingredients and also from any component which has a tend ency to absorb oxygen and become gummy. Lubricants for use in the cylinders of gasoline engines must possess the further qualifications of having a high flash point and a relatively very high fire point.
The lubricants generally in use belong to the group of oils and greases. Outside of these hut one substance ranks with them in im portance, and that is graphite. Although nearly all of the so-called non-drying oils and most of the greases have been used at one time or another as lubricants, it is now recog nized that animal oils and fats are liable to de compose with use, developing acids which are corrosive, and thus destructive to the bearings in which they are used; and oils of vegetable origin have a tendency to become gummy within a brief period. For all fine machinery, therefore, the preferred lubricants are the min eral oils derived from crude petroleum. For
many purposes, however, sperm oil, lard oil, whale oil, neatsfoot oil and tallow oil, among the animal oils, and olive oil, rape oil and cas tor oil among the vegetable oils are in consid erable use, both alone and in mixtures with one another and with the various mineral oils. Solid tallow is also used in mixtures with vari ous oils to form lubricating greases. Castor oil has proved of special value in lubricating the high-speed engines of the aeroplane and the racing automobile. Graphite, preferably of the type, is used on the finest of ma chinery, usually in mixture with oil or grease.
The fine lubricating oils are made from crude petroleum by first distilling off the naph thas and the illuminating oil, and then distilling the residue with superheated steam in a vac uum. The lower temperatures give the lighter spindle oils, and the higher give cylinder oils of greater viscosity up to a buttery paste, called cup grease. These lubricants are graded to ex tremes of nicety for the pressures under which they must work and the temperatures they must endure, and the manufacturers offer them in almost endless variety. Consult Archbutt, L, and Deeley, R. M., 'Lubrication and Lubri cants) (London 1912) ; Battle, J. R., 'Lubricat ing Engineer's Handbook) (Philadelphia 1916) ; Hurst, G. H., 'Lubricating Oils, Fats and Greases) (London 1911).