48 APPLIED SCIENCE 



are clamped to the pulley with bolts a, a. As the pulley revolves 

 in the direction indicated by the arrow, the tendency is for the 

 entire brake to rotate in the same direction; this is prevented by 



e 



P 



(Copyrighted by Millers Falls Co.; 



Ki<;. 33. Prony Brake as Usually Constructed. 



the weights P in the scale pan suspended from the end of lever A. 

 When the pulley runs at its normal speed, sufficient weight is 

 placed in the pan at P to balance the lever between the pins e, e, 

 which are provided to prevent the lever from revolving. The 

 power absorbed by the clamp shoes c and d is equal to the amount 

 of work which is accomplished in foot-pounds per minute by the 

 revolving shaft. 



This work in foot-pounds = NxPxLx2- 



where N is number of revolutions per minute, and L the length of 

 lever. 



The H. P. = 



2T.NPL 

 33000 



The small pulleys /, /, and the weight W are provided as a counter- 

 balance for the lever arm when the machine is at rest. The clamp 

 shoes c and d should be well lubricated. To illustrate the calcula- 

 tion, assume that an engine shaft makes 240 revolutions per minute, 

 what is the H. P. developed when a weight of 50 Ibs. is just balanced 

 at the end of a 10-ft. lever, as shown in Fig. 33? 



H. P. = 



2 - NPL 6.2832 X 240 X 50 X 10 



33000 



33000 



= 22.8 H. P. 



