Friction. 511 



turc. The oils and fats suitable for liibrieanis belong to 

 both the animal ami vegetable and the mineral non-drying 

 types. Any oil or fat which becomes permanently -uinmy 

 and stiff after the exposure of service makes a very unsatis- 

 factory machine lubricant, and alt IK nigh such oils can be 

 found upon the market, selling at a lower price than the 

 best lubricants, it is seldom economy to use them because 

 they soon gum up the bearings, greatly increase the fric- 

 tion and increase the labor necessary to put the machine 

 into condition again. In special cases finely divided 

 solids, like graphite and soapstone, are used as lubricants. 



681. The Action of Lubricants. When liquids are 

 brought into contact wi^h solid surfaces to which they ad- 

 here, as in the case of water flowing through pipes, there 

 is a thin layer held to the walls of the pipe so rigidly that 

 it hardly takes part in the flow, so that instead of having 

 the friction of a liquid upon a solid, the slipping takes 

 place between layers of the liquid itself. So, too, when an 

 oil is poured into the bearings of a machine there come to 

 be two comparatively stationary layers against the two 

 metal surfaces and the sliding or friction takes place be- 

 tween layers of oil rather than between layers of metal, 

 and it is because the friction of lubricants upon themselves 

 is so much less than that between solids that they are so 

 serviceable. 



The lubricant may act in two different ways in lessoning 

 the friction: (1) by causing the chief part of the r. 

 ance to be that due to the slipping of oil over oil, which is 

 usually less than the friction between solids, and (2) by 

 the lubricant acting to fill up the smaller inequalities of 

 the two friction surfaces and in this way preventing so 

 much interlocking. 



C82. Adaptation of Lulricant to Place of Service. 



Where the speed of the sliding surfaces is relatively high, 

 nnd especially if the pressure between the hearings is imt 

 henvy, one of the thin oils will render the best service 4 , 



