152 APPLIED SCIENCE 



the movement or pressure of the earth. Slates and marbles 

 are examples of this class. 



167. Object of Lubrication. Lubrication is the applica- 

 tion or introduction of some substance that will cling to or 

 flow between two surfaces and thus prevent friction. Bear- 

 ings and joints of engines and machinery are lubricated to 

 keep the various metal surfaces from coming in direct contact, 

 and thus to prevent excessive friction and consequent heat- 

 ing. (See Fig. 34, page 49.) Perfect lubrication is secured 

 when the surfaces are separated by means of the thinnest 

 possible film that is sufficient to prevent heating. A thick 

 film is harmful because it tends to produce fluid friction. 



168. Kinds of Lubricants Oils. 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. 



169. Greases. Greases are suitable for use on slow- 

 moving machinery where the pressure is not great. Even 

 where the speed is comparatively high, but the pressure is 

 light, a grease will often give excellent results, if the proper 



