

THE SCREW 



301 



The wedge. If an inclined plane is pushed underneath or 

 within an object, it serves as a wedge. Usually a wedge con- 

 sists of two inclined planes (Fig. 167). 

 A chisel, an ax, and the prongs of a 

 fork are illustrations of wedges. Per- 

 haps the most universal form of a 

 wedge is our common pin. Can you 

 explain how this is a wedge ? 



The screw. Another valuable and 

 indispensable form of the inclined 

 plane is the screw. This 

 consists of a metal rod 

 around which passes a 



ridge. Figure 168 shows clearly that a screw is 

 simply a rod around which (in effect) an inclined 

 plane has been wrapped. 



The ridge encircling the screw is called the thread, 

 anc ^ tne distance between two successive threads is 

 a simple called the pitch. It is easy to see that the closer 

 the threads and the smaller the pitch, the greater 

 is the advantage of the screw, and the less is the force needed 

 in overcoming resistance. A corkscrew is a familiar illustration 

 of the screw. 



167 -By means of a wedge. 

 the stump is split. 



A screw ^s 



FIG. 169. Useful applications of the screw. 



