PRINCIPLES OF MACHINES 463 



b( 'tween the two surfaces neeessary to eause one to slip over 

 the other. If it requires a foree of 2 pounds to move a 25- 

 pound weight over a surface, the coefficient of friction is 

 .08. The coefficient of friction is always greater in starting 

 a surface to slide than is required to keep it moving after it 

 is once in motion. 



MEANS OF REDUCING FRICTION 



When a load is moved, the pounds actually hauled by a 

 given tractive force can be increased in one of two ways: 

 First, by lubrication, or the interposing of a hard, smooth 

 film under the load. A sled moved over ice or well-packed 

 snow is a simple application of this principle. Secondly, 

 by the introduction of a rolling surface under the load, of 

 which the wagon is a practical application. The application 

 of these two principles for decreasing friction in the hauling 

 of loads is made use of in machines to increase their efficiency 

 by reducing the friction in the moving parts. 



Lubrication. A lubricating substance is one that may be 

 applied between sliding surfaces that will form a smooth 

 film and decrease contact between the two. The film reduces 

 the wear and heating occasioned by the motion. The sub- 

 stance suitable for lubricating material varies with the 

 physical conditions of the moving surfaces. It must be a 

 substance that will spread over and adhere to these surfaces. 

 Great pressure requires a sticky or viscous substance, and 

 heat, one that will not be burned or evaporated. The ice 

 acts as a lubricant under the sled; the water, in the moving 

 parts at the lower end of a pump rod, where the pressure is 

 light; while the interior of a gas engine requires the best 

 prepared lubricant on account of the close-fitting parts and 

 great heat. Various prepared lubricants in either solid or 

 fluid form, to meet the requirements of mechanical devices, 

 are on the market, usually under the name of oil or grease. 

 Finely ground graphite or mica in combination with grease 

 is much used for a lubricant. Heavy lubricants are used 



