WIRE ROPES, CHAINS, AND CORDAGE.
Wire ropes are now generally used in the place of chains and hemp ropes. A greater strength with less weight is obtained by the former than either of the latter. A tarred hemp rope of the best quality, weighing 2 pounds to the fathom (6 feet), and 3 inches in circumference, will not sustain more than 3 tons without great danger of breaking.
Chains from best American charcoal iron will bear a greater breaking strain by one fourth than those made of English iron ; but it would not be safe to work them at a higher proof than one-fourth of the breaking proof.
There is a singular circumstance connected with iron not generally understood, or yet scientifically explained. Iron which is cold rolled, or drawn from an annealed or soft condition, will bear a greater direct weight, as bolts or wire ropes, by one-half, than when heated ; that is,, a wire which will bear 1000 pounds after being drawn through the wire machinery, or, in other words, new wire, will bear only 500 pounds after being heated red-hot : therefore a wire rope containing the same weight of iron will bear double the strain which chains will endure without breaking, and may be safely worked at threefold the load that can be risked on chains of the same weight.
It will be noticed from the foregoing tables that the breaking strain, or the weight required to break a certain length of rope or chain, is not in proportion to weight. A t-inch chain manufactured from round iron contains more than three times the amount of iron, and is nearly four times the weight, of a one-inch wire rope, and yet the breaking strain is about the same ; while a hemp rope to bear the same strain must be 7 inches in circumference and double the weight of the wire rope. Steel wire ropes will bear four times the burden of an equal weight of hemp rope, and eight times the burden of an equal weight of chain ; while the working strain that each will bear is in the same proportion, but in favor of the iron wire over both hemp ropes, and chains, and still more in favor of steel wire over the iron wire. But if iron or steel wire ropes are heated before use, their strength is reduced one-half. In putting on the sockets at the ends of the ropes, that part is weakened if heated for the purpose, and if short lengths are experimented on they always part at the socket ; but in practical working the load is never over one-fourth of the breaking strain, or one-half the resist ance at the socket, and, consequently, it is still double the required strength : more over, the wear and tear, and, in fact, the greatest strain, are towards the upper end of the rope, or that part which winds on the drum, and this part always gives way first.
The following table, furnished by Fisher Hazard, Esq., the Mauch Chunk Pennsyl vania manufacturer of wire ropes, gives the relative practical working dimensions and strength of hemp ropes, wire ropes, and chains. It will be noticed that the working strain is put at less than one-fourth of the breaking strain; while the breaking strain is considerably less than is obtained from experiment. This, however, is a safe prac tical test, and may be relied on ; for while a piece of chain or rope 12 feet long may bear 30 tons, the same rope or chain 300 feet long might not bear more than 20 tons. The foregoing tables were given to show the relative general strength of ropes and chains: the following one, as a guide to their practical working strength.
It is scarcely possible to manufacture wire ropes with any thing but good charcoal iron, and impossible to make them from ordinary common iron; and the same result holds good to a greater extent with steel wire than with iron wire, since the iron must be good in the first place to produce steel, and the steel must be uniformly good to produce fine wire.
But this is not the case with hemp ropes: almost any kind of material can be made up into ropes of this description in such a manner as to make it impossible to detect the quality and strength of the rope without actual experiment. The same thing may be said of chains: almost any kind of merchantable iron can be forged into chains, and it is impossible to detect the quality without experiment or practice. A Finch chain, or one manufactured from Finch iron, may stand a breaking strain of 24 tons if made of good iron; but if from poor iron it may snap, without warning, at 5 or 10 tons. In the first case, the chain will stretch considerably before breaking; but in the latter it breaks suddenly without giving to the strain. A good link will yield until it becomes parallel or straight before it breaks; but a poor link will snap off without warning. Chains are also apt to give way at the point of welding, if badly made, even with good iron ; and when we have both poor smiths and poor iron to contend with or guard against, the danger is great : therefore, in regard to safety as well as economy, wire ropes are far superior to chains, for mining purposes particularly.