13.860 14.868 14.364 14.364 1.3.860 13.356 13.356 12.852 12.348 13.860 14.868 14.364 13.356 12.852 13.860 13.356 12.852 16.380 15.876 15.372 14.868 14.364 12.348 13.860 14.868 14.364 13.860 13.356 16.884 16.380 15.876 15.372 14.868 14.364 13.860 17.388 16.884 16.380 17.892 17.388 16.884 16.380 14.868 14.364 15.876 15.372 14.868 18.396 17.892 17.388 16.884 16.380 15.876 15.372 16.884 16.380 15.876 94 19.908 19.404 17.388 16.884 18.900 18.396 17.892 17.388 20.412 19.908 19.404 18.900 18.396 17.892 17.388 20.916 20.412 19.908 19.404 18.900 18.396 17.892 20.916 19.908 19.404 18.900 18.396 16.380 16.884 Link-Belt Co. Rope CRANE CHAIN AND HOOKS Material for Crane Chains. The best material for crane and hoisting chain} is a good grade of wrought iron, in which the percentage of phosphorus, sulphur, silicon and other impurities is comparatively low. The tensile strength of the best grades of wrought iron does not exceed 46,000 pounds per square inch, whereas mild steel with about 0.15 per cent carbon has a tensile strength nearly double this amount. The ductility and toughness of wrought iron, however, is greater than that of ordinary commercial steel, and for this reason it is preferable for chains subjected to heavy intermittent strains, because wrought iron will always give warning by bending or stretching, before breaking. Another important reason for using wrought iron in preference to steel is that a perfect weld can be effected more easily. Strength of Chains. - When calculating the strength of chains, it should be observed that the strength of a link subjected to tensile stresses is not equal to twice the strength of an iron bar of the same diameter as the link stock, but is a certain amount less, owing to the bending action caused by the manner in which the load is applied to the link. The strength is also reduced somewhat by the weld. The following empirical formula is commonly used for calculating the breaking load, in pounds, of wrought-iron crane chains: in which W = breaking load in pounds, and D = diameter of bar (in inches) from which links are made. The working load for chains should not exceed one-third the value of W, and, in many cases, it should be less. When a chain is wound around a casting and severe bending stresses are introduced, a greater factor of safety should be used. 1/4 3/4 1/4 I 1231600 1/2 134 32 5 9 13695 I 2 312 * These figures apply only to a rope in fairly good condition. Care of Hoisting and Crane Chains. All chains used for hoisting heavy loads are subject to deterioration, both apparent and invisible. The links wear, and there is also an alteration in the nature or fiber of the material, owing to the Cable Diam. strains, shocks, etc., producing crystallization. Chain wear can be reduced considerably by occasional lubrication. The life of a chain can also be prolonged by frequent annealing, as this restores the fibrous quality of the material to some extent, although there may be a slight decrease in the tensile strength. To anneal a chain, heat it to cherry-red and allow it to cool slowly. This should be done every six months, and oftener if the chain is subjected to unusually severe service. Chains should be examined periodically for twists, as a twisted chain will wear rapidly. Any links which have worn excessively should be replaced with new ones, so that every link will do its full share of work during the life of the chain, without exceeding the limit of safety. Chains for hoisting purposes should be made with short links, so that they will wrap closely around the sheaves or drums without bending. The diameter of the winding drums should be not less than 25 or 30 times the diameter of the iron used for the links. Studded Chains. - Tests have demonstrated that the ultimate breaking strength of a chain with studded links is less than that of an unstudded chain. This is probably due to the fact that the open links of an unstudded chain collapse until the sides are approximately parallel, so that the stresses are lower than in the studded links, the sides of which are prevented from collapsing by the studs. The principal function of the stud is to prevent the chain from kinking and catching, so that it will run free from chain lockers, etc. The stud also prevents the chain from becoming rigid under heavy strains. |