130 



; SHACKLES. 



and so prevent the gripping being all on one side. The 

 same device is used in the Adamson machines. This is 

 shown in Fig. 56. Here M, M, are the coned bushes, and 

 N, N, the wedges. Messrs. Riehle attain the same 

 object, which is really the preservation of the pull along 

 the axis of the bar, by making the faces of the wedges 

 convex in section, as shown on Fig. 57. For every 

 machine a number of sets of double pairs of wedges 

 must be kept, in order to accommodate test specimens 

 of different thicknesses. Wedges of the shape shown 

 in Fig. 55 are used for flat test pieces, generally those 

 cut from plates. For round bars, wedges of the section 



YJ_. . _J 



FIG. 60.* 



shown on Fig. 58. (A), or sometimes of the section (B). Square 

 bars may also be accommodated in the wedges marked A. 



On Fig. 59 are shown photo views of the three types of 

 wedges described. 



In the Werder machine, shackles of the form shown on 

 Fig. 60 are employed. By having the recess carried right 

 through the shackle, plates of great width may be held. 



A holding device, now hardly ever used, which was at 

 one time the only one employed, was obtained by drilling 

 two large holes in the ends of the bar, these ends being 

 enlarged so as to allow of sufficient metal at the sides of 

 the holes to secure ample strength for the ends : into these 

 holes were inserted pins, which were themselves attached 

 to the machine ; in this way the whole force of the pull 



Unwin's "Testing," p. 177. 



