Chains for hoisting weights are made from a good grade of wrought iron, which has a tensile strength of from 40,000 to 48,000 lbs. per square inch. Chains used for raising weights should never be made from steel, as it is not so strong under shock as wrought iron, and does not weld so readily. Because of the possibility of the weld not being as strong as the balance of the link, the strength of the chain is not reckoned as twice the strength of the bar from which it is made. When buying chains in the open market it is advisable to base the computation of strength on the lowest tensile strength of iron used for the purpose, i.e., 40,000 lbs. to the square inch.
The strength of a chain link is 1.63 times the strength of the bar from which it is made. The strength referred to is the breaking, or tensile, strength. It is never safe to strain to anywhere near the breaking point, because every time a piece of metal is strained to a point beyond its elastic limit it is permanently stretched and weakened. For this reason, it is never considered advisable to strain a chain to more than one-half the amount shown by the method given for computing the tensile strength. In other words, the proof test of a chain should be about 50% of the ultimate resistance of the weakest link.
If, for example, the tensile strength of a chain made from 1/2 inwrought iron is 40,000 lbs. per square inch, the safe working strength may be calculated as follows:
Area = Diameter squared X .7854 = .5 X .5 X .7854 = .19635 .19635 X 40,000 = 7854
7854 X 1.63 = 12,802 lbs. = ultimate breaking strength 12,802 X .50 = 6401 lbs. = proof test, or safe working strength.