MECHANICS 



67 



into two classes, namely, solid pulleys and split pulleys. A 

 solid pulley is one in which the arms, hub, and rim are cast 

 in one solid piece, as shown in Fig. 1. A split pulley is one that 

 is cast in halves that are afterwards bolted together, as shown 

 in Fig. 2. The latter style of pulley is more readily placed on 

 or removed from a shaft than is the solid pulley. Pulleys are 

 generally cast in halves or parts when they are more than 

 6 ft. in diameter. This is done on account of the shrinkage 

 strains in large pulley castings, which renders the pulleys liable 

 to crack as a result of unequal cooling of the metal. 



FIG. 1 



FIG. 2 



Wooden Pulleys. Although most belt pulleys are made of 

 cast iron, wrought iron, and steel, wooden pulleys have come 

 into extensive use. These are built of segments of wood 

 securely glued together, maple being the wood ordinarily 

 used. It is possible to procure wooden split pulleys that are 

 fitted with removable bushings, thus allowing the same pulley 

 to be adapted readily to shafting of different diameters. 

 Wooden pulleys are somewhat lighter than cast-iron pulleys 

 for the same service. 



Driving and Driven Pulleys. The pulley that imparts 

 motion to the belt is called the driving pulley, or the driver, 

 and the one that receives motion from the belt is called the 

 driven pulley, or simply the' driven. When two pulleys are 

 connected by a belt, the speeds at which the pulleys run are 

 inversely proportional to their diameters. Thus, if two 

 pulleys have diameters of 12 in. and 24 in., the speed of the 

 smaller is to the speed of the larger as 24 to 12, or as 2 to 1 



