CHAPTER V 

 PULLEYS, INCLINED PLANES, AND WEDGES 



37. Simple Form of Pulley. The pulley is a machine 

 which in its simplest form consists of a grooved wheel, made 

 of wood, brass, or iron, with a rope or chain passing over it, 

 fixed in a framework, and free to revolve. 

 As the type of pulley shown in Fig. 17 

 turns on an axle fixed in one place it is 

 called a fixed pulley. 



W 



FIG. 17. A Fixed 

 Pulley. 



Such a device makes it easier for a man stand- 

 ing on the floor to raise a weight by pulling 

 on the end of the cord at P than if he 

 pulled the weight straight up by the cord with- 

 out any pulley, or carried the weight up a flight 

 of stairs. 



A pulley may be considered as a rotating lever 

 which is used simply to change the direction of 

 a force. The belt or rope does the work, not 

 the wheel. There is no leverage in a single fixed 

 pulley, and if the weight is 50 Ibs., it takes a pull 

 of 50 Ibs. at P (ignoring the slight friction of the wheel axle) to 

 raise it. In Fig. 17 the lever arms in the pulley are equal to 

 the radius and the fulcrum is at the center; that is, in a pulley 

 16 in. in diameter one arm would be 8 in. on one side and the other 

 8 in. on the other side of the fulcrum. 



38. Block and Tackle. The advantage of the single pulley 

 may be increased by combining several pulleys, as is done in 

 the case of the appliance called the block and tackle. 



35 



