THE SCREW. 



361 



THE SCREW. 



[From Dr. Dion. Lardner's Lectures, now in course of publication, by Greeley & McElrath.] 



In the applisation of the screw, the weight or 

 re.sistance is uot, as in the inclined plane and 

 wedqe, placed upon the surface of the plane or 

 Fi?. 8. thread. The power is usually 

 transmitted by cau.sing the screw 

 to move in a concave cylinder, on 

 the interior surface of which a spi- 

 ral cavity is cut, coiTesponding ex- 

 actly to the thread of the screw, and 

 in which the thread will move by 

 turning round the screw continual- 

 ly in the same direction. This hol- 

 low cylinder is usually called the 

 nut or concave acrew. The screw 

 surrounded by its spiral thread is 

 represented in fig. 8 ; and a section 

 of the same playing in the nut is represented 

 in fig. 9. Fig. 9. 



There are several ways in which the effect of 

 the power raaj' be conveyed to the resistance 

 by this apparatus. 



First, let us suppose that the ntit A B is fixed. 

 If the scre\^' be continually turned on its axis, 

 by a lever E F inserted in one end of it, it will 

 be moved in the direction C D, advancing every 

 '• 'volution through a space equal to the distance 



;\veen two contiguous threads. By turning 



!■ lever in an opposite direction, the screw 

 \\\\\ be moved in the direction D C. 



If the scr(!\v be fixf d, so as to be incapable 

 either of moving longitudinally or revolving on 

 its axis, the nut A B may be turned upon the 

 screw by a lever, and will move on the screw 

 toward C or toward D. according to the direc- 

 tion in which the lever is turned. 



In the fomier case, we have supposed the nut 

 to be absolutely immovable ; and, in the latter 

 case, the screw to be absolutely immovable. It 

 may happen, however, that the nut. though ca- 

 pable of revolvinL', is incapable of moving lon- 

 gitudinally ; and that the screw, though incapa- 

 ble of revolving, is capable of moving longitudi- 

 nally. In that case, by turning the nut A B up- 

 on the screw by the lever, the screw will be 

 tirgcd in the direction C D or D C. according to 

 tlie way in which the nut is turned. 



The ajiparatus may, on the contrarj', be so ar- 

 ranged that the nut. though incapable of revolv- 

 ing, is capable of moving longitudinally ; and 

 the screw, though capable of revolvin?. is inca- 

 pable 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 di- 

 rection C D or D C. 



All these various arrangements may be ob- 

 served in different applications to the machine. 



A screw may be cut upon a cylinder by pla- 

 cing 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 be- 

 tween the intended thread, while the cylinder 

 revolves once. The relative motions of the cut- 

 ting 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 tri- 

 angular, as in fig. 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 press- 

 es. In fig. 11, tlie nut is fixed, and by turning 



Fig. 10. Fig. 11. 



the lever, which passes through the head of the 

 screw, a pressure Fig. 12. 



is excited upon 

 any substance 

 placed upon the 

 plate immediate- 

 ly under the end 

 of the screw. — 

 In fig. 12, the 

 screw is inca- 

 pable of revolv- 

 ing, but is capa- 

 ble of advancing 

 in the direction 

 of its length. On 

 the other hand, 

 the nut is capable 

 of revolving, but 

 does not advance 

 in the direction 

 of the screw. — 



When the nut is turned by means of the screvv 

 inserted in it, the screw advances in the direc- 

 tion of its length, and urges the board which is 

 attached to it upward, so as to press any sulistance 

 placed between it and the fixed board above. 



In cases where liquids or juices are to be ex- 

 pres.sed from solid bodies, tlie screw is the agent 

 generally employed. It is also used in coining, 

 where the impression of a die is to be made up- 

 on apiece of metal, and in the same way in pi-o- 

 ducing the impression of a seal upon wax or 

 otlier substance adapted to receive it. When 

 soft and light materials, such tis cotton, are to 



