46 



APPLIED SCIENCE 



It really makes no difference in the result whether the 

 inclined plane is wound in a spiral or circular path, or left 

 straight; the wedging action will be there 

 just the same. This means that all screw 

 threads, nuts, bolts, etc., are circular or spiral 

 wedges. The ease with which a screw turns 

 and ascends depends on the slowness of the 

 ascent, that is, on the number of turns, or threads, in a given 

 distance. 



f >j THRfAO 



FIG. 30. Helix. 



46. Jack Screw. The ordinary jack screw is a good 

 example of the wedge principle. It is a screw in combination 

 with a lever. 



Figure 31 shows a common jack screw. The thread is the inclined 

 plane or wedge and the circumference of the screw or thread cor- 

 responds to the base of the plane. The 

 force P on the handle is the force acting 

 parallel to the base and the weight W is 

 the weight lifted. The same rule which 

 is used for the wedge will now apply. If 

 the length of the handle, the pitch of the 

 thread (the distance between two succes- 

 sive threads), and the force applied to the 

 handle are known, the weight which can 

 be lifted, neglecting friction, can easily be calculated. 



Referring to Fig. 31, for every turn of the handle the weight is 

 raised an amount equal to the pitch : 



P x C = W X p 



where P, W, and p are as shown in the sketch and C is the circular 

 distance the end of the handle moves through in making one turn, 

 or 



C = 2 x R 



* IG< ^' 



