TRANSMISSION OF POWER 



289 



As shop machines are usually arranged so that they may be 



disconnected from the power while the shafts are moving, 



it is very important in 



starting or stopping them 



to avoid sudden jars or 



changes, as such sudden 



movements are dangerous 



to the machine. To avoid 



this danger a device, called 



the fast and loose pulley 



(Fig. 145), is used. This 



consists of two pulleys 



placed on the shaft, the 



one being firmly fixed, and 



the other loose so that it 



may easily turn while the 



shaft remains at rest, or 



vice versa. The belt is made to pass over either pulley by 



means of a 

 forked guide; 

 if on the fast 

 pulley, the 

 machine 

 moves; if on 

 the loose pul- 

 ley, the ma- 

 chine remains' 

 at rest. 



FIG. 144. Split Iron Pulley. Pul- 

 leys are split to permit their being 

 applied quickly to any shaft al- 

 ready in place. 



FIG. 145. Countershaft with Fast and Loose Pulley 

 on Right. A shifting rod throws the belting from 

 one pulley to the other. 



332. Speed 

 of Pulley. 



The size of the pulley governs the speed of the machinery and 

 this speed is determined by the relative movements of the 



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