66 MECHANICS 



pull P is exerted on a chain, the fixed pulleys are rotated. 

 The chain winds up on one side of the large pulley and 

 unwinds, at a slower rate, from the small pulley. As a result. 



FIG. 5 



FIG. 6 



FIG. 7 



the load W is lifted slightly at each turn of the fixed pulleys. 

 The weight W that can be lifted by a force P is found by the 

 formula 



R-r 



the letters R and r designating the radii of the large and small 

 fixed pulleys, respectively. 



Wheel and Axle. The device known as the wheel and axle 

 consists of two cylinders of different sizes connected rigidly, 

 so that they turn together. The rope carrying the load to 

 be raised is fastened to the smaller cylinder and the rope on 

 which the pulling force is exerted is wound on the larger cylin- 

 der, as shown in Fig. 7. The pull F causes the larger cylinder 

 to turn, rotating the smaller cylinder at the same rate. The 

 rope attached to the weight W is thus caused to wind on the 

 smaller cylinder, and the load is raised. If R and r represent, 

 respectively, the radii of the large and small cylinders, the 

 force F required to lift a weight W, and the weight W that may 

 be lifted by a force F, are given by the formulas 



F = and W = 

 R r 



BELT PULLEYS 



Solid and Split Pulleys. Besides being used with ropes or 

 chains for the hoisting of loads, pulleys are extensively employed 

 with belts for transmitting power. Belt pulleys may be divided 



