TRANSMISSION OF POWER 287 



the shafts should be placed farther apart, say 20 to 25 ft., 



and a sag of 3 to 4 in. will be permitted. If possible, shafts 



should be arranged so that the sag of the belt will increase 



the arc of contact with the pulley. 



If they are not so arranged, the 



sag will lower the efficiency of the 



belt, 



329. Rope Drives. Some- 

 times instead of a pulley, a wheel 

 with grooves on its circumference 

 (Fig. 142) is used for rope trans- 

 mission. The use of rope for the 



transmission of power is more com- ^ IG - 142. Pulley use d for 



. - , T . Rope Drawing, 



mon in Europe than in the United 



States. The advantages claimed for this method of power 

 transmission are: 



(a) A larger amount of power is transmitted. 



(b) A rope can be run in any direction or to any distance. 



(c) Smooth and quiet running is obtained. 



(d) Electrical disturbances are absent. 



(e) Economy is obtained in first cost and in maintenance. 

 (/) There is an absence of slip. 



For successful work the pulleys must be large in diameter 

 and must have a smooth surface where the rope bears upon 

 them. The speed and the load on the rope must be only such 

 as experience has shown to be economical. When these con- 

 ditions are fulfilled a rope drive is one of the most satisfactory 

 methods of transmitting power. 



330. Measurement of Coiled Belting. When belting is 

 purchased it is not necessary to uncoil it to determine its 



