Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/96

7(5 COAL [MIXING. pull of the chains attached to the pulleys c c, which turn the broad toothed portions outwards, and away from the guides. The connection with the rope is made by the slide bar C and spring catch h having a projecting trigger, which, if the cage is lifted too high, strikes against the cross-bar of the framing k, and detaches the cage, which is then left hanging by the catches to the guides in the pit. The use of safety catches is more common in the collieries of France, Belgium, and Germany than in Eng land, where they are not generally popular, owing to their uncertainty in action, as they are often fouud to fail when most wanted. The constant drag of the catches on the guides when the rope slacks is also objectionable, but this has been overcome to a great extent in a very in genious contrivance invented by Mr Calow, where the catches are not brought into action unless the cage is actually falling clear of the rope, with a certain acquired momentum of its own. The only real safeguards against accidents in winding are to be found in constant vigilance, in maintaining the ropes in working efficiency, and in the use of proper signals and brake power in the engine house. The speed attained by the load in the shaft in the best- appointed English collieries is very considerable, and may be paralleled with that of a fast railway train. At Shire- oaks Colliery, Nottinghamshire, the cage with a load of 34 cwts. of coal in five tubs, and weighing in all 60 cwts., or with the rope at the bottom 92^ cwts., is raised from a depth of 51 G yards in 45 seconds, corresponding to an average of 35 feet per second, or 24 miles per hour, the maximum speed when the load is mid- way being 50 feet per second, or nearly 35 miles an hour. The ropes used are round, of steel wire, weighing 13 Ibs. to the yard, winding on to a spiral drum, increasing from 17 to 20 ftet in diameter. Thera are two engines with vertical cylinders, 32 inches diameter and 6 feet stroke, developing a useful effect of about 320 horse-power. The guide pul leys are 12 feet in diameter. The above may be taken as a good example of the mo dern class of winding engines, such as are required to draw from 600 to 1200 tons in the shift of 10 hours. When the pits are of small depth it is better to increase the weight of the load than to draw at a very high speed, as the loss of time in filling and unloading or striking the cages is the same for a short as for a long journey, so that it becomes advantageous to diminish the number of journeys for a given quantity of coal drawn. The great amount of dead weight required to be raised in the ordinary system of winding (e.g., in the instance given above, the total weight moved is nearly four times that of the nett load drawn, that of the ropes being nearly 1 1 times as much as the latter), has led to the proposal of various plans to obtain a more mechanically economical method, but none of these have at present been brought into successful use. One of the latest is that of M. Blanchet, who proposes to draw a number of tubs linked together into a long vertical train in a closed tube about 5 1 feet in diameter, by exhausting the air above them in the manner adopted in the pneumatic tubes used for the transmission of parcels. An experimental apparatus of this class has been recently constructed at Creusot, in France, designed to lift a cage with 9 tubs, attached to a piston, weighing in all about 12| tons. Striking When the cage arrives at the surface, or rather the plat- ail( j " form forming tho working top above the mouth of the pit, scrcening.it is received upon the keeps, a pair of hinged gratings which are kept in an inclined position over the pit-top by counterbalance weights, so that they are pushed aside to allow the cage to pass upwards, but fall back and receive it when the engine is reversed. The tubs are then removed or struck by the landers, who pull them forward on to the platform, which is covered with cast-iron plates; at the same time empty ones are pushed in from the opposite side. The cage is then lifted by the engine clear of the keeps, which are opened by a lever worked by hand, and the empty tubs start on the return trip. When the cage has several decks, it is necessary to repeat this operation for each, unless there is a special provision made for load- ipg and discharging the tubs at different levels. An arrangement of this kind for shifting the load from a large cage at one operation has recently been introduced by Mr Fowler at Hucknall, in Leicestershire, where the trains are received into a framework with a number of platforms cor responding to those of the cage, carried on the head of a plunger movable by hydraulic pressure in a vertical cylinder. The empty tubs are carried by a corresponding arrangement on the opposite side. By this means the time of stoppage is reduced to a minimum, 8 seconds for a three-decked cage as against 28 seconds, as the operations of lowering the tubs to the level of the pit-top, discharging, and replacing them are performed during the time that the following load is being drawn up the pit. The tub when brought to the surface, after passing over a weigh-bridge, where it is weighed and tallied by a weigher specially appointed for the purpose by the men and the owner jointly, is run into a tipping cage, and the contents are dis charged into an inclined screen with bars about 1 inch to 1 -| inches apart. The large coal remaining passes through a spout into a railway waggon placed below, the discharge being regulated by a valve at the lower end. The small coal passing through is either sold as such, or may be lifted by an elevator to a second series of screens, either fixed or rotating, with half-inch apertures. These make a further separation of larger pieces, which are sold as "nuts," while the small, or slack, passing through is sent to the coke ovens, if the quality of the coal is suitable. Asa rule, non- caking coals are not very closely screened, as the small is of comparatively little valu?, and therefore must have a proportion of larger sizes mixed with it to form saleable slack. Figs. 22-24, representing the surface arrangements Illustr adopted at a pair of pits in the Wigan district, may be tions f taken as fairly representative of the fittings of a large ^ ra ^ ( modern colliery, where a considerable output of coal has to men t 8 . be screened and loaded in an ordinary working day of less than twelve hours. The details, of course, will vary, ac cording to the nature of the outlet or vend, which may be by retailing into carts sent by purchasers, or by canals or railways, or by a combination of all three. In the example selected, the coal is loaded directly from the screens into full-sized trucks, each carrying from 6 to 8 tons, on a main line of railway. Of the two pits, one is an upcast, and is surmounted by a chimney at the surface, the drawing being confined to the downcast, which is 310 yards deep and 10| feet in diameter. GOO tons of coal are drawn from this depth in 10 hours by a pair of direct-acting engines, with vertical cylinders working a spiral drum, in creasing from 13i feet to 1 7| feet in diameter. The pit-head frame is of wood, with guide pulleys 7 feet in diameter, a much, smaller size than is now usually adopted ; the iron wire drawing-ropes are round, weighing 5 ft) to the yard. Double-decked cages of a light construction in wrought iron are used, carrying four tubs at a time. The landing platform is raised upon pillars 20 feet above the surface of the ground, and covered with iron plates. As soon as the cage arrives at the surface, the tubs are run into tumbling cages, which discharge their contents on to fixed screens, witL bars of 1 to 1^ inch aperture. The large coal passes by a shoot directly into the railway waggon, while the first screenings fall into a channel below, which is traversed by

a series of scrapers attached to an endless chain, and are