366 APPLIED MECHANICS 
Ss 
Fra. 557. Fic. 558. Fic. 589. Fia. 560. 
on to the pulley is a little nearer the base than the part in front of it; 
but once in contact with the pulley, it remains in contact without slip- 
ping until it leaves on the other side. The result is, therefore, that the 
belt ultimately reaches the highest part, and to prevent it falling off a 
similar pulley is placed, as shown by the dotted lines. From this the 
form shown in Fig. 558 is obtained. 
The straight rim is used when it is necessary to move the belt from 
one part of the rim to another, as in the case where a pulley drives a 
pair of fast and loose pulleys. 
It must be borne in mind, however, that the convex section of rim 
only helps to keep the belt on the pulley when the belt and pulley rotate 
together. If the belt should slip through the resistance being too great, 
it will fall off the pulley more readily if the rim is convex than if it is 
straight. 
For hemp or cotton ropes the pulley rim has V shaped grooves, as 
shown in Fig. 559, the angle of the V being generally about 45°. - The 
rope does not rest on the bottom of the groove, but on its sides only, 
so that it is wedged in, causing the resistance to slipping to be much 
greater. 
For wire ropes the design is altered so that the rope rests on the 
bottom of the groove, as shown in Fig. 560. The resistance to slipping 
is increased, and the wear of the rope reduced by lining the bottom of 
the groove with some material softer than metal, such as leather, wood, or 
tarred oakum. 
311. Fast and Loose Pulleys.—The motion of a shaft driven from 
another by belt gearing may be stopped or reversed SE 
without affecting the motion of the driving shaft by 
using a combination of fast and loose pulleys. A “fast” 
pulley is one which is fixed to the shaft, while a ‘‘ loose” 
pulley is one which can turn freely on the shaft. Fig. 561 
shows a pair of such pulleys, F being the fast pulley 
secured to the shaft by a key K, and L is a loose pulley. 
When the belt is on F the shaft A revolves, and when 
the belt is shifted to L the motion of A is stopped. Fie. 561. 
The belt is shifted from one pulley to the other by pressing on one edge 
