M MACHINE I >K \N\ING AND M 



VIII. PULLEYS. 



Velocity I ring. Let two pulleys A in 



be connected by a belt, and let tli- ters be D, and D a ; 



and let their >j.erds. in revolutions per minute, be N, a 

 respectively. If there is no slipping, the speeds of the rims of 

 the pulleys will !>< tin- same as that of the belt, and will there 

 fore be equal. Now the speed of the iini<>f A is evidently 

 = D, xS-UHixN. : while the speed ,,1 the rim of I 1 , 

 I), x:vi416xN 2 . HanoeD,x 3-1416 xN,D,x 3-1416 xN* 



and then-fore *' = !' 



.Nj D, 



Putteys for Flat Bands. In cross section the rim of a 

 pulley for carrying a flat band is generally curved as shown 

 in tigs. 41 and 42, but very often the cross section is straight. 

 The curved cross section of the rim tends to keep the Land 

 from coming off as long as the pulley is rotating. Sometimes 

 the rim of the pulley is provided with flanges which keep the 

 band from falling off. 



Pulleys are generally made entirely of cast iron, but a great 

 many pulleys are now made in which the centre or na 

 is of cast iron, the arms being of wrought iron cast into the 

 nave, while the rim is of wrought sheet iron. 



The arms of pulleys when made of wrought iron are 

 in \ariably straight, but when made of cast iron they are very 

 often curved. In ti::. H, which shows an arrangement of two 

 cast-iron pulleys, the arms are straight ; while in fig. 42, whirh 

 another cast-iron pulley, the arms are curved. Through 

 unequal cooling, and therefore unequal contraction of a cast- 

 iron pulley in the mould, the arms are generally in a state of 

 11 or compression ; and if the arms are straight they are 

 very unyielding, so that the n >ult of this initial stress is often 

 the l>reaking of an arm, or of the rim where it joins an arm. 

 With the curved arm, however, its shape permits it to yield. 

 and thus caus*- a diminution of the stress due to unequal con- 

 traction. 



