Toi:s|oN TKSTS. 



< >\\ iug to the fact that shafts of circular section are l.y 

 far the most commonly used this form is almost always 

 m ployed for torsion specimens. The specimens should be 

 turned, and provided with enlarged ends for holding pur- 

 poses. 



109. Torsional Testing Machines. --The conditions 

 which must obtain in a test under a torsions! stress, and 

 the functions to be performed by the machine, are as 

 follow : Imagine a truly circular bar of the metal to be 

 tested. Then, in order that the test may be properly 

 carried out : 



(a) One end of the bar must be rigid!}* held, and, at 

 the same time, a twisting moment in the form of a couple 

 applied at the other end, this couple acting in a plane at 

 right angles to the axis of the bar. 



(6) Ihe usual manner of imposing the moment is by 

 having a weight acting at the end of a single arm rigidly 

 attached to one end of the bar, bending being prevented 

 by having the enlarged end rotating in a long bearing. 



(c) When the twisting moment is applied in the manner 

 described, any strain which takes place causes the arm at 

 which the load is applied to sink from its horizontal posi- 

 tion, and, in order that this horizontal position may be at 

 all times maintained, it is necessary to have some means of 

 taking up the twisting strain. 



(d) The shaft must be so held as to be free to vary its 

 length, that is, one end must be free to move with respect 

 to the other in the direction of its length. 



(e) There must be no possibility of a distortion taking 

 place in any way except in a rotary direction round the 

 axis of the bar. 



It is difficult to fully carry out all these conditions, and 

 most existing machines have some defects. The general 

 plan of most torsional testing machines is much the same, 

 and is somewhat as follows : 



(a) The twisting moment is usually applied by means 

 of a horizontal beam, provided with a jockey-weight, much 

 in the same way as in an ordinary tension machine ; in 

 fact, some few machines are used for both torsion and 

 tension tests. The bar to be tested is attached to the 

 beam in such a way that its axis coincides with the axis 

 of the knife-edge of the beam. The beam is provided 

 with a hole into which the end of the test bar fits, not too 

 tightly, and is held in place by keys or other grips. 



