COAL MINING MACHINERY. Dur ing the past 50 years numerous machines have been introduced for mining coal. In the United States coal mining by machinery has proved most successful and divides the field with drill ing and blasting. 56 per cent of the coal product of the United States is now machine mined. In Europe, where coal mining by machines had its origin, the development has been slower. In Great Britain less than 10 per cent of the coal output is mined by machines. In the United States the practice has been to adapt the machines to the methods of mining in use, while abroad the attempt is often made to modify the mining methods to suit a particular type of machine. To obtain the best results methods and machines must be so planned and so operated as to secure the maximum of efficiency. The installation of mining machines, with the necessary plant for the production and transmission of power, re quires large capital expenditure, with operating, maintenance and depreciation charges, and therefore machines should be kept steadily at work. A loss of 25 per cent of machine time means that one-fourth of the invested cFital is idle and earning no profit to meet the fixed charges. It is particularly important to increase the length of the working faces, and to reduce the distance between the different places to be worked by the same machine, in order to in crease the time that the machine is working without interruption, and to reduce the idle time when the machine is moved from one place tc another. The machines should not be permitted to stand idle when ready for work, which means that the operations of timbering, track shifting, breaking down and loading of the coal, and especially the movement of the loaded and empty coal cars, must be so organized and systematized as to keep the mining machines at work. Incidentally such organization of the mine work reduces the cost of these auxiliary operations and increases the output.
The advantages which have led to the large use of coal mining machines are in part direct saving of cost, and in part indirect increase of profits. The direct savings are often small, the
indirect gains are sometimes large. The direct saving of labor cost is largely offset by increased wages paid to the machine miners and to their helpers. Machine mined coal contains less slack or fine coal than that mined powder, and the coal sells at a higher average price. Machine mining lessens the amount of powder used, a comparatively saving, but lessens the chance of fire-damp and coal dust explosions, avoiding destruction of life and property, a very great gain. Machine mining increases the out put both directly and indirectly, and thus in creases the earnings on the invested capital, and repays that capital in a shorter term of years.
Coal-mining machines are operated by com pressed air or by electricity. Electric power is much more efficient on account of the smaller power losses in generation and transmission. Compressed air is safer and should be used in mines in which there is danger of fire-damp explosions. By the use of explosion proof motors, storage battery locomotives and care fully insulated transmission lines, the employ ment of electricity is made more safe and by effective ventilation of the mine workings the chance of fire-damp explosions is reduced. The tendency to-day is toward increased use of electric power.
Coal mining machines are of five types : (1) Rotary boring machines; (2) rotary bar cutters; (3) rotary disc machines; (4) endless chain machines; (5) puncher machines. They are also divided according to their special applica tion or their method of operation into: (1) Heading machines; (2) breast machines; (3) short wall machines; (4) long wall machines; (5) centre cut machines; (6) shearing ma chines; (7) loading machines; (8) combined cutting and loading machines; (9) conveyors.
Rotary boring machines are chiefly used for boring blast holes in coal or in the softer rocks. They have been used for boring air-ways of small diameter and for boring entries.