PULLEYS, INCLINED PLANES, AND WEDGES 47 

 Problem on the Jack Screw 



A jack screw has a single thread, seven turns to the inch, and 

 a handle 18 in. long. If a force of 50 Ibs. is applied to the end 

 of the handle what weight can be lifted, neglecting friction? 



(Take - - ^-) 



47. Measurement of Machine Power. It is often very 

 desirable to determine the power 

 necessary to operate a machine. 

 This may be done by means of 

 instruments called dynamometers. 

 The prony brake is one of the most 

 simple and familiar examples of the 

 dynamometer. 



(Copyrighted by Millers 

 Falls Co.) 



FIG. 32. Principle of 

 Prony Brake. 



Figure 32 represents one type of prony 

 brake in which a fixed band of leather, 

 or rope, is in contact with a portion of 

 the circumference of a pulley or drum 

 A . The band has one end attached as shown at B, while the weight 

 C is hung at the other end. 



The formula to find the foot-pounds per minute is : 



3.1416 X diam. of pulley X rev. per min. X weight 

 12 



As an example, if the pulley of a band brake is 126.04 in. in diameter 

 and makes 200 revolutions per minute, while a weight of 5 Ibs. 

 hung at end of the band just affects the speed, what is the H. P.? 



126.04 X 200 X 5 



3.1416 



12 



= 32,997.272 or approximately 33,000 

 ft. -Ibs. per minute = 1 H. P. 



48. Another Form of Prony Brake. Figure 33 shows the 

 prony brake as generally constructed. The clamp shoes c and d 



