140 Sir C. Fox on the size of Pins connecting Flat Links [Mar. 30, 



a close approximation, that, as about the same force is required for shearing 

 as for breaking wrought iron by extension, it would be necessary to give the 

 pin a cross section equal to the sectional area of the smallest portion of the 

 link only. The fact of the possibility of links being torn and destroyed by 

 the pin being too small to present the necessary bearing surface, although 

 quite large enough to resist the calculated shearing force brought to bear 

 upon it by the links, seems hitherto not to have attracted notice ; but as 

 the strength of a chain depends upon the proper extent of surface being 

 offered by the pins of the links to pull against, such a mode as the one de- 

 scribed has been proved by experiment to be altogether fallacious. For by 

 this mode of estimating, the size of a pin passing through links 10 inches 

 wide and of uniform thickness (that is, not having the head thicker than the 

 body of the link) would be something less than 3| inches in diameter, 

 whereas (as will presently be shown), in order to get the whole benefit from 

 such a link, the pin must be somewhat more in diameter than 6 inches, and 

 for the following reasons. 



In wrought iron the initial forces necessary to extend, or diminish by 

 compression, the length of a bar are practically the same ; and hence it 

 arises that unless the surface of the pin on which the semicylindrical sur- 

 face of the hole in the link bears is as great as the smallest cross section of 

 the link itself, the head will be torn by the pin ; and since to provide this 

 necessary surface it is essential to have a pin of much larger size, the 

 question of its ability to resist the operation of shearing never arises, and 

 the whole subject resolves itself into one of bearing surface. 



If the pin be too small, the first result on the application of a heavy pull 

 on the .chain will be to alter the position of the hole through which it 

 passes, and also to change it from a circular into a pear-shaped form (vide 

 fig. 2), in which operation the portions (A A, figs. 2 & 3) of the metal in 

 the bearing upon the pin become thickened in the effort to increase its 

 bearing surface to the extent required. But while this is going on, the 

 metal around the other portions (B B, figs. 2 & 3) of the hole will be 

 thinned by being stretched, until at last, unable to bear the undue strains 

 thus brought upon it, its thin edge begins to tear, and will, by the continu- 

 ance of the same strain, undoubtedly go on to do so until the head of the 

 link be broken (or, rather, torn) through, no matter how large the head 

 may be ; for it has been proved by experiment that by increasing the size 

 of the head, without adding to its thickness (which, from the additional 

 room it would occupy in the width of the bridge, is quite inadmissible), no 

 additional strength is obtained. 



Acting upon the principle above described, most engineers have made 

 the pins of their chains far too small, whereby much money has been 

 wasted in making the links of a size, and consequently of a strength, of 

 which it was, through the smallness of the pins, impossible to obtain the 

 full benefit. Indeed to such an extent has this been carried, that in one of 

 the most noted suspension bridges hitherto constructed, a very large sum 



