Lubricants may be divided into three general kinds or classes - fluid, plastic, and solid. To the first-named class belong the various oils; to the second, the greases; and to the third, such substances as graphite, talc, soapstone, or mica.

Where the speed of a machine is high and the pressure great, oils are, in nearly all cases, the most satisfactory lubricants to use. They cling to the contact surfaces and thus form an elastic coating to the metals and keep them apart. Oils also absorb the frictional heat and carry it away. Other advantages of oils are: (1) they can be obtained in almost any desired grade or density, from the thin oils to the heavy, dense oils; (2) they do not become rancid or gummy; and (3) they contain no free acids.