PRINCIPLES OF MACHINES 469 



form of belt. Their lengths change with moisture condi- 

 tions, and they can be used only where it is not necessary 

 to maintain a fixed distance between pulleys. They are 

 suitable for the belting of a farm motor to a portable machine 

 or where a belt tightener can be used. Belts should be of 

 sufficient width and pulleys large enough to transmit the 

 power without the application of resin or other substances 

 to the face of the pulley. These materials injure leather 

 belts and destroy rubber. The power transmitted by a 

 single-ply leather belt can be calculated as follows: 



h n - VW 

 n -P- 1000 



where V = velocity of belt in feet per minute and W = width 

 of belt in inches. 



Where it is not practicable to use endless belts, a variety 

 of metallic fasteners may be found in the market for connect- 

 ing the ends, some of which are very desirable for high-speed 

 belts or where idlers are used. The common method of 

 fastening belts used on the farm is by lacing the ends together 

 with a belt lace which is a narrow strip of rawhide cut from 

 lace or whang leather. Before the lacing is done the ends 

 of the belt must be cut square with the edge, holes punched, 

 and the leather inserted as shown in Figure 358, page 

 470. There should be no knots or crossing of the lace on the 

 side next the pulley. 



d) Chain-and-sprocket or link belts: Sprocket wheels 

 attached to the shaft in the same way as a pulley, and con- 

 nected by endless chains made up of rectangular-shaped 

 links, are used a great deal on agricultural implements. 

 The links are made of malleable iron or steel, and one can 

 readily be taken out or put in. They are a positive drive 

 and do not permit slipping. In the case of an overload the 

 power must stall or the machine will break. On account of 

 the links not fitting the sprockets snugly there is a given 

 amount of lost motion. These belts are noisy and cannot 

 be operated at high speeds. 



