THE .ELEMENTS OF MACHINERY. 



109 



How is power ^'*^' ^°'^®'" ^^ commonly applied to the screw by means of 



appUed to the a lever, either attached to the nut, or to the head of the screw, 

 Screw ? ^ ggpjj Jq Yig. 92. By varying the length of this, the power 



may be indefinitely increased at the point of resistance. The screw, there- 

 fore, acts with the combined power of the lever and the inclined plane. 



Thus, in Fig. 92, /d is the lever, c the nut, 

 a d the screw, and e the block upon which the 

 substance to be pressed is placed. As in all the 

 other simple machines, the advantage in this ia 

 estimated by the relative distances passed over 

 by the power and the weight. If the distance 

 of the spiral threads of the screw is 1 inch, and 

 the handle of the screw, that is the lever, is 2 

 feet in length, then the extremity of the lever 

 will describe a circle of over 12 feet in turning 

 once round, but the screw will only advance 1 

 inch. The ratio between the power and the 

 weight will be, therefore, as 1 inch to 1 2 feet, or 

 as 1 to 144. Consequently, if a man is capable 

 of exerting a force of 60 pounds at the end of the lever, the screw will ad- 

 vance with a force of 8,640 pounds. If the distance of the threads had been 

 i an inch, the power exerted by the screw would have been doubled. In 

 this illustration friction has not been taken into account ; this will diminish 

 the total effect nearly one fourth. 



How is the ad- 244. The advantage gained by the screw is 

 by'ttfe^ scl°ew ^^ proportion as the circumference of the circle, 

 estimated? described by the power (that is by the handle 

 of the lever) exceeds the distance between the threads of 

 the screw. 



Hence the enormous mechanical force exerted by the screw is rendered 



evident. There is no limit to the smallncss of the distance between the 



threads except the strength it is necessary to give them ; and there is no limit 



to the magnitude of the circumference to be described by the power, except 



the necessary facOity for moving it. FiG. 93. 



■nn,»i. „ f 245. The screw is 



what are fa- 

 miliar applica- generally used where 



Sct!w?^ ^^^ great pressure is to be 

 exerted through small 

 spaces ; hence its application in presses 

 of ail kinds; for extracting the juices 

 of seeds and fruits, in compressing cot- 

 ton, hay, etc., as also for coining and 

 punching. For the two latter opera- 

 tions it is caused to act with enor- 



