290 



THE INCLINED PLANE, WEDGE, AND SCREW. 



longitudinally ; and that the screw, though incapable of revolving, is capable 

 of moving longitudinally. In that case, by turning the nut A B upon the screw 

 by the lever, the screw will be urged in the direction C D or D C, according 

 to the way in which the nut is turned. 



The apparatus may, on the contrary, be so arranged, that the nut, though 

 incapable of revolving, is capable of moving longitudinally ; and the screw, 

 though capable of revolving, is incapable of moving longitudinally. In this 

 case, by turning the screw in the one direction, or in the other, the nut A B 

 will be urged in the direction C D or D C. 



All these various arrangements may be observed in different applications to 

 the machine. 



A screw may be cut upon a cylinder by placing the cylinder in a turning- 

 lathe, and giving it a rotatory motion upon its axis. The cutting point is then 

 presented to the cylinder, and moved in the direction of its length, at such a 

 rate as to be carried through the distance between the intended thread, while 

 the cylinder revolves once. The relative motions of the cutting point and the 

 cylinder being preserved, with perfect uniformity, the thread will be cut from 

 one end to the other. The shape of the threads may be either square, as in 

 fig. 8, or triangular, as in fig. 10. 



Pig. 10. 



The screw is generally used in cases where severe pressure is to be excited 

 through small spaces; it is therefore the agent in most presses. In fig. 11, 



Pig. n. 



the nut is fixed, and by turning the lever, which passes through the head of 

 the screw, a pressure is excited upon any substance placed upon the plate im- 

 mediately under the end of the screw. In fig. 12, the screw is incapable of 

 revolving, but is capable of advancing in the direction of its length. On the 

 other hand, the nut is capaye of revolving, but does not advance in the direc- 

 tion of the screw. When the nut is turned by means of the screw inserted in 

 it, the screw advances in the direction of its length, and urges the board which 

 is attached to it upward, so as to press any substance placed between it and 

 the fixed board above. 



In cases wliere liquids or juices are to be expressed from solid bodies, the 

 screw is the agent generally employed. It is also used in coining, where the 



