FARM MACHINERY 



the holes should be made with a small punch, the thick- 

 ness of the belt from the edge and twice the thickness 

 apart. The lacing should not be crossed on the pulley 

 side of the belt. 



48. Pulleys. Pulleys are made of wood, cast iron, and 

 steel. They are also constructed solid or in one piece 

 and divided into halves. It is best to have a large cast 

 pulley divided, as the large solid pulley is often weakened 

 by contraction in cooling after being cast. For most 

 purposes the iron pulley is the most satisfactory, as it is 

 neat and durable. Belts do not cling to 

 iron pulleys well, and hence they are often 

 covered with leather to increase their driv- 

 ing power. Often the driving power is in- 

 creased one-fourth in this way. 



Puller's are crowned or have an oval face 

 in order to keep the belt in the center. The 

 tendency of the belt is to run to the highest 

 point, as shown in Fig. 17. The pulley that 

 imparts motion to the belt is called the 

 driver and the one that receives its motion 

 from the belt the driven. 



49. Rules for calculating speed of pul- 

 leys. Case I. The diameters of the driver 

 and driven and the revolutions per minute 

 of the driver being given, to find the number of revolu- 

 tions per minute of the driven. Rule : Multiply the 

 diameter of the driver by its r.p.m. and divide the product 

 by the diameter of the driven; the quotient will be the 

 r.p.m. of the driven. 



Case II. The diameter and the revolutions per minute 

 of the driver being given, to find the diameter of the 

 driven that shall make any given number of revolutions 



E 



FIG. 17 SHOW- 

 ING THE 

 EFFECT OF 

 CROWN ON 

 PULLEYS 



