Another system of ropeway, which, however, is not so generally suitable, consists in having only one endless rope. The bearing rope, in this case, is an endless rope driven by an engine; the buckets are suspended by hooks to the rope, which is carried on rollers at the posts. There is, however, a liability of the buckets Scale 11⁄2 Feet to 1 Inch FIG. 480.-Ropeway: rollers and buckets. to slide along the rope when approaching the posts, owing to the steepness of the curve. This is especially the case where the gradient of the ropeway itself is steep. CHAPTER XVII. TRANSMISSION OF POWER: RODS, ROPES, AIR, WATER, POWER is transmitted from steam-boilers on the surface into the mine in the following ways: Steam. This is taken down the shaft in pipes to work engines near the bottom, and is also conveyed along the roads to considerable distances, in some cases as far as 1200 yards. In order to prevent excessive condensation, the pipes must be thickly covered with non-conducting composition. Expansion joints must be introduced, and in the levels and inclines the pipes must be carried on rollers or suspended between the expansion joints; otherwise there will be continual breakages of joints. This is probably the cheapest mode of transmitting power into the mine, but there are several objections. A leakage of steam is annoying; should the steam-pipes burst, the steam might be dangerous, and the pipes must be so placed that in such a contingency the steam will not pass through any working place. The heat from the steam is often inconvenient, and in some cases would be apt to cause spontaneous combustion. The percentage of loss from condensation depends on the amount of power transmitted, because the actual loss is proportional to the circumference of the pipe from which the heat is radiated. But the friction of a gas in a pipe varies inversely as the fourth or fifth1 power of the diameter, so that whilst a 2-inch pipe might be sufficient for 10 H.P., a 4-inch pipe would be sufficient for 50 or 60 H.P. In the latter case the radiating surface of the iron pipe is double; the radiating surface of the covering composition is, however, only one and a half times, whilst the power transmitted is from five to six times as great. Therefore the loss taken in percentages will be about one-third for the larger power of what it was for the smaller power. If, in the first instance, 60 per cent. of the steam was condensed, in the second instance only 20 per cent. would be condensed. It is generally taken as the fifth power. Professor Merivale has given a good deal of attention to this question, and the reader is referred to his interesting papers in the "Transactions of the North of England Institute of Mining Engineers," vol. 35, part 3, and vol. 36, part 1. Mr. Merivale describes an installation where the steam from a cylindrical boiler, 29 feet long and 5 feet in diameter, is taken down a shaft to drive three engines underground. The first engine, 12-inch cylinder, 2-feet stroke, is 342 yards from the boilers, and the steam passes through 5-inch pipes. The second engine is 950 yards further, supplied by 24-inch pipes; the engine is 8-inch diameter, 1-foot stroke. The third engine, 120 yards further, 14-inch steam-pipes, is 6-inch diameter, with a 10-inch stroke. The total distance is 1414 yards from the boilers. The pipes, steam-traps, and engines are covered with Wormald's composition, and in the shaft, which is rather wet, the pipes are covered with felt and lead. The engines in the pit work from 5 hours to 11 hours a day. The boilers consume 427 lbs. of rough small coal per hour, and evaporate 273 gallons of water at 56° at a pressure of 35 lbs. Of this 273 gallons, 57, or about 21 per cent., are collected at the steam-traps in the pit, showing that the total loss by priming and condensation is only 21 per cent. of the steam-power. It is, however, probable that the loss by condensation is rather greater than that indicated by the water condensed in the steam-traps, as some of the condensed water may very likely pass through the steam-engines. However that may be, there is no doubt that this installation of Professor Merivale's demonstrates that steam may be carried with great economy to a distance of nearly a mile, and if, instead of a small horse-power and intermittent working, there had been a large horse-power and continuous working, the loss by condensation would have been very much less for the reasons given above. Compressed Air.-Compressed air is the most usual and most generally approved method of transmitting power to mines, because it can be taken into any part of the mine without danger or inconvenience, and applied to any kind of work, whether pumping, hauling, winding, ventilating, coal-cutting, or rockdrilling (see Fig. 481). The air-compressor is simply an air-pump (see Fig. 482). The air is sucked in at one valve, and driven out from another valve into a receiver. I, I, are the inlet valves, and 2, 2, are the delivery valves. In case cooling water is admitted into the cylinder, the delivery valves must be at the bottom. An arrangement of engine and air-compressor erected by the writer is shown in Fig. 482a. In this case the steamcylinders had a diameter of 24 inches, and the air-cylinders 25 inches, and the stroke 4 feet. They were coupled engines. The air-pressure can be raised to a higher pressure than the steam if desired. In this case the boiler pressure was between 40 and 50 lbs., and the air was compressed to about the same pressure. In order to conduce to the economy of steam, a cut-off valve, FIG. 481.-Transmission of power. 1, steam-engine; 2, compressor; 3, air-receiver; 4, hauling-engines; 5, fan; 6, pump; 7, coal-cutter. worked by cam-gearing, was fixed on the steam-pipe. In order to maintain the air in the receiver at a uniform pressure without the constant attendance of an engine-man, the throttle-valve was controlled by a governor; the 3 5 ૐ I, governor consisted of a piston working in a small cylinder on the air-receiver, which lifted a weighted lever and was connected with the throttle-valve. If the air-pressure fell, the throttle-valve was opened wider; if the air-pressure rose, the throttle-valve was closed, and in this manner the air-pressure was maintained nearly uniform. In case, however, an air-pipe should burst (a most unlikely contingency) and the air-pressure rapidly fall, the governor lever would automatically disconnect itself from the throttle-valve, which would be instantly closed by the action of a weighted lever. Where inlet valves, similar to those in the sketch, are used, there must be a guard of perforated plate inside the valves, so as to obviate the possibility of a broken valve falling into the cylinder. FIG. 482.-Transmission: air-cylinder. The compression of air produces a great deal of heat, just as spray, the finer the better, as it then mixes better with the air, and the cooling is more complete with a less quantity of water. A great quantity of water in the cylinder, as all mechanical engineers know, is a danger to the safe working at a high speed, and a great 21.0----- 17 PIPE LINE SPRING LINE INLET VALVE FOR AIR CYLINDER. LEATHER RING VALVE SEATING RECEIVER Scale 1⁄2 Size. the expansion produces cold. In order to keep the cylinders cool, it is usual to place them in a tank of water, through which a stream is maintained. Sometimes a jet of water is admitted into the cylinder. This jet should be in the form of a very fine |