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? 



22 



(Take T: = .) 



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 

 Prony Brake. 



of 



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.? 



3.1416 X 126.04 X 200 X 5 



- = 32,997.272 or approximately 33,000 



12 



ft.-lbs. 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 



