EFFECT OF VELOCITY OF BELT 23 



be transmitted and it should be determined first. The arc 

 on the smaller pulley is the one that should be considered, as 

 the horsepower transmitted is limited by it. If the smaller 

 pulley is the driver, it limits the amount of power that can 

 be delivered to the belt, while if it is the driven, the arc 

 limits the amount of power that the belt can deliver to the 

 pulley. The curves, Fig. 2, show the influence of arc of con- 

 tact upon horsepower transmitted. The tables on pages 

 96 to 1 01 indicate the arc of contact of the belt on the 

 smaller of a pair of pulleys for the usual range of pulley sizes 

 and center-line distances. The arc that the belt subtends on 

 the larger pulley can be found by subtracting the figure 

 given in the table from 360 degrees. 



Effect of Velocity of Belt. Equally or more important 

 than arc of contact is the velocity of the belt. With a given 

 velocity, up to the point at which centrifugal tension begins 

 to manifest itself, the horsepower transmitted varies directly 

 with the velocity. As explained in Chapter I, the effect of 

 centrifugal tension reaches serious proportions in the neigh- 

 borhood of 4000 ft. per minute. This is clearly shown by the 

 curves, Fig. 3, which illustrate the relation between velocity 

 and horsepower transmitted. The tables on pages 102 to 

 107 give the -belt velocity obtained by a varying number of 

 revolutions per minute of pulleys of different diameters 

 within the usual range of shop sizes. 



Use of Horsepower Tables. Having determined the arc 

 of contact of the belt on the smaller pulley and the velocity, 

 the horsepower that the belt will transmit per square inch 

 of cross-sectional-area can be read directly from the proper 

 section of the horsepower table on pages 91 to 94. In the 

 third and ninth columns of the table will be found the tensions 

 per square inch of cross-section under which the belt should 

 be put on the pulleys ; the fourth and tenth columns show the 

 tension below which it should not be allowed to fall in service, 

 and the point at which the belt should be retightened to the 



