The breaking strain of a one-inch wire rope is set down in the foregoing table at 16 tons, and its weight at 1.55 pounds per foot; the breaking strain of a two-inch wire rope is 56 tons, and its weight 7.05 pounds per foot: consequently, the relative strength is in favor of the small rope as to its weight, and indirectly, as to use, it is much more in favor of the small rope, since the bending backwards and forwards over a drum, even of large diameter, is more injurious to a large rope than a small one. It would require 4.75 feet of one-inch rope to weigh as much as one foot of two-inch rope: consequently, the breaking strain of four and three-quarter one-inch ropes would be 76 tons against 56 tons as the breaking strain of a two-inch rope; or four one-inch ropes would bear a breaking strain of 64 tons,-8 tons more than a two-inch rope,—and, if worked together as a flat rope, would last double as long as the larger rope.
Flat ropes, therefore, are stronger and more durable than round ones of the same weight, and are to be preferred for colliery purposes. The great difficulty seems to be in their wear together. If not carefully put together, one may bear much more strain than the other; but this difficulty can be obviated either by the manufacturer or the operator. Great care should be taken in putting on the sockets: if done at the mines, the rope should be stretched powerfully and carefully, in order to equalize the strain. If this is done, and the rope well put together, it will outlast two round ropes of the same strength.
Another objection may be raised in the lapping of the rope around the drum; but the true principle is to lap the rope upon itself on a narrow drum or wheel, just wide enough to receive the breadth of the flat rope. The abrasion in this case is not greater than it would be in a round rope, which abrades powerfully against its own side as it winds on the drum. The winding of a flat rope is directly on itself, and the abrasion is, therefore, less than that of a round rope, which winds against itself with a powerful rubbing process, on account of the indirectness of the pull. In this respect the advan tage is with the flat ropes and narrow drums. But there is also another advantage in starting loads from deep mines, with small drums increasing as they draw near the top. This is a great help to the machinery, as the load, though of equal weight, exerts a much greater strain on the machinery and rope at 600 feet distance than at 300 feet, independent of the increased weight of the rope at the greater depth. Six hundred feet of two-inch rope will weigh nearly two tons. To start this with a load of eight tons gives ten tons at the greatest depth and nine tons at half the depth. But the effect on the machinery is much greater, since it requires as much power to start 8 tons at 600 feet as 10 tons at 300 feet distance. With the common 12-feet drum required for a two inch round rope, the leverage of the pinion is just the same through all the drag,—as much at starting as on stopping. But with the flat rope and narrow drum, starting at
6 feet diameter and ending with 8 or 10, the leverage or power of the engine is much greater at the commencement than at the end, on the same principle that it is easier for a man to work a windlass with a six-inch drum than one with a twelve-inch drum. But in the case of the flat-rope colliery drum, the diameter increases as the rope winds on, and as the weight of the load decreases.
We have no doubt that a good steel wire rope of three-quarter inch diameter, in a band of four ropes, would be much more powerful than a round rope of two inches, and that it would wear out at least four round ropes; the advantage given to the machinery would save several thousand dollars in the first cost of the power, and be the source of constant saving in the generation of steam.
Powerful machinery, with large drums and large round ropes, is very effective and available at our large colliery establishments. Steam is not so much of an object, since coal is plentiful and cheap; but simplicity, permanence, and reliability are important considerations, and should have the precedence over all other questions in mining economy. We do not, therefore, advance the above as a dogmatic rule, but simply suggest the availability of the mode proposed as equally permanent and reliable with the best machinery now in use at the anthracite collieries, and as possessing greater lifting power with less steam and mechanical strength and more economy. We do not propose to recommend any mere theoretical project, but such improvements as tend to simplify and economize.
Of course, hemp ropes are now obsolete in mining economy as a general thing, since they bear no comparison to wire in cost, weight, or effect; and chains are still more objectionable for deep mines and heavy weights, however they may be made; while bands of steel or sheet iron are not much better, and are objectionable in all mining operations: therefore wire ropes are in all respects superior.
A chain capable of drawing 10 tons from a depth of 600 feet would weigh over 14,000 pounds, or 10,000 pounds more than the weight of a wire rope to accomplish the same purpose; while a steel rope to draw an equal load from the depth of 1000 feet would weigh 4000 pounds less than an iron wire rope for the same purpose.
We make the load of 8 tons as the maximum in our deep slopes. It would be unne cessarily heavy in a deep shaft,—say 1000 feet. At such a depth a wire rope of two inches diameter would weigh 7050 pounds, or over three tons. If we add to this 7 tons as the weight of coal, car, and cage, it would still make a load of ten tons to start with, which is as much as a two-inch rope ought to be subjected to at that depth, as much as the heaviest machinery yet built in this region for mining purposes is capable of doing, and as much as it is desirable to lift at one time.