PRINCIPLES OF MACHINES 467 



multiply the diameter of the drive pulley in inches by the 

 number of revolutions per minute and divide the product 

 by the diameter of the driven pulley in inches. The quotient 

 will be the number of revolutions of the driven pulley per 

 minute (commonly designated as r. p. m.). 



Belting. A common form of transmission of power is by 

 means of a belt running from the drive pulley to a pulley on 

 the device to be driven. The transmission of power between 

 the pulleys and belt is by friction. When the load becomes 

 too heavy, one or both pulleys slip within the belt and the 

 driven machine stalls. To increase the power of the belt, 

 it is necessary to increase the friction by widening the belt 

 and the face of the pulley or by increasing the diameter of 

 both drive and driven pulleys. Where only a small amount 

 of power is transmitted, a shaft may be driven at an angle 

 to the drive shaft by means of an idler, "a pulley without 

 a load," placed in line with both pulleys, which will turn the 

 belt in the required direction. With a light load a belt 

 may be crossed between the pulleys which turns the driven 

 pulley in the opposite direction to that of the drive pulley. 

 Heavy loads require that the belt run straight from pulley 

 to pulley and in the direction of the drive pulley. For a 

 belt to operate properly, it should run as slack as is possible 

 without slipping, be tight on the under side, the pulley centers 

 not too close together, and the pulleys crowned (slightly 

 curved on the surface) and exactly in line. "Crowned" 

 pulley is distinguished from "straight-faced." Where a 

 straight-faced pulley is required (as, for example, opposite 

 a pair of "tight and loose ") a "crowned " pulley will not work. 

 a) Leather belting : Leather is the standard belting mate- 

 rial where it is dry. It should run with the smooth 

 side next the pulley, and, to prevent cracking, should be 

 occasionally treated with neats-foot oil or a belt dressing. 

 Leather belts for pulleys larger than 10 inches in diameter 

 may be had in two-ply or more which is a multiple thickness 

 of leather. 



