20 



TORSION. 



Wherefore 



G 

 IT 



. . . (VII.) 



2 h+ 



For metals the value of in is as nearly as possible 4, 

 so that 



_G 2 

 E T 



n. Torsion. Torsion may be defined as rotational 

 shearing. 



If the shaft represented in the figure has its end, 

 marked A, held and rotated by the couple P x 2a, and 

 has its other end held, and rotated in the opposite direc- 

 tion by an equal couple P x 2a, the plane of one end will 



FIG. 8. 



revolve, or tend to revolve, with respect to the other end. 

 If the material is elastic, a line drawn on the surface of the 

 shaft parallel to the axis previous to the application of the 

 stresses will be deflected so as to form an angle with 

 its original position, as indicated by the second line. The 

 short portion of the shaft enclosed between the two 

 parallel planes marked C and D is shown on an enlarged 

 scale on the following Fig. 9. 



