Figure 33 shows the prony brake as generally constructed. The clamp shoes c and d 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 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.

Fig. 32.   Principle of Prony Brake.

(Copyrighted by Millers Falls Co.)

Fig. 32. - Principle of Prony Brake.

Fig. 33.   Prony Brake as Usually Constructed

(Copyrighted by Millers Falls Co.)

Fig. 33. - Prony Brake as Usually Constructed.

This work in foot-pounds =NxPxLx2π where N is number of revolutions per minute, and L the length of lever.

The H. P. =-----------2π NPL / 33000

The small pulleys f, f, and the weight W are provided as a counterbalance for the lever arm when the machine is at rest. The clamp shoes c and d should be well lubricated. To illustrate the calculation, assume that an engine shaft makes 240 revolutions per minute, what is the H. P. developed when a weight of 50 lbs. is just balanced at the end of a 10-ft. lever, as shown in Fig. 33?

H. P. = 2πNPL / 33000 = 6.2832 X 240 X 50 X 10 / 33000 = 22.8 H. P.