Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/95

Rh WINDING.] COAL hollow, are or were in general use in the numerous shallow pits working the thick coal in the neighbourhood of Dudley, &c. The best modern engines for drawing in collieries are usually direct-acting, with either horizontal or vertical cylinders. In the north of England a single engine with a heavy fly-wheel is often used, but the more general arrangement is to have two engines coupled to the opposite ends of the winding drum-shaft. In almost all cases steam is used at high pressure without condensation. The drum, when round ropes are used, is a plain broad cylinder, with flanged rims, and cased with soft wood packing, upon which the rope is coiled ; the breadth is made sufficient to take the whole length of the rope at two laps. One drum is usually fixed to the shaft, while the other is loose, with a screw link or other means of coup ling, in order to be able to adjust the two ropes to exactly the same length, so that one cage may be at the surface when the other is at the bottom, without having to pay out or take up any slack rope by the engine. For flat ropes, the drum or bobbin consists of a solid disc, of the width of the rope fixed upon the shaft, with numerous parallel pairs of arms or horns, arranged radially on both sides, the space between being just sufficient to allow the rope to enter and coil regularly upon the preced ing lap. This method has the advantage of equalizing the wark of the engine throughout the journey, for when the load is greatest, with the full cage at the bottom and the whole length of rope out, the duty required in the first revolution of the engine is measured by the length of the smallest circumference ; while the assistance derived from gravitating action of the descending cage in the same period is equal to the weight of the falling mass through a height corresponding to the length of the largest lap, and so on, the speed being increased as the weight diminishes, and vice versa. The same thing can be effected in a more perfect manner by the use of spiral or scroll drums, in which the rope is made to coil in a spiral groove upon the surface of the drum, which is formed by the frusta of two obtuse cones {laced with their smaller diameters outwards. This plan, though mechanically a very good one, has certain defects, especially in the possibility of danger resulting from the rope slipping sideways, if the grooves in the bed are not perfectly true. The great size and weight of such drums are also disadvantages, as giving rather unmanageable dimensions in a very deep pit. The use of a counterbalance chain for the winding engines is common in the collieries of the Midland dis tricts of England. In this method a third drum is used to receive a heavy flat link chain, shorter than the main drawing-ropes, the end of which hangs down a special or balance pit. At starting, when the full load is to be lifted, the balance chain uncoils, and continues to do so until the desired equilibrium between the working loads is attained, when it is coiled up again in the reverse direc tion, to be again given out on the return trip. The surface arrangements of a modern colliery are often of considerable extent and complexity, the most important feature being the pit-frame carrying the guide-pulleys or rope-rolls which lead the drawing-ropes from the verti cal line of the pit to the engine-drum. This consists essentially of an upright framework, carefully braced together, and strutted by diagonal beams against the wall of the engine-house, or other solid abutment. It is gene rally necessary to have a clear head-room, 10 or 20 feet or more, for the working arrangements at the surface above the level of the ground, especially in flat countries ; the pit- frames are made of considerable height, from 50 to 70 feet being not uncommon ; and when, as is generally the case, they are made of wood, they afford opportunities for the exercise of skilful carpentry. Of late years, however, wrought iron pit-frames have been adopted to some extent, which allows of a comparatively simpler construction being used, the main elements of the frame consisting of hollow latticed pillars and beams, similar to the construc tion now generally adopted for the pillars of railway signals, but of course of a more solid construction. They have one great advantage over wooden frames, in not being liable to destruction by fire, an accident which has occasionally happened with the latter. The guide-pulleys for iron or steel wire-ropes are made of very large dimen sions, to avoid strain upon the wires by sudden change of direction when moving at a high speed. The usual construction is a deep channeled rim or tire of cast-iron, from 7 to 20 feet in diameter, supported by numerous thin wrought iron arms, inclining inwards from a central cast iron boss, a form combining rigidity with comparative lightness. They are in fact very similar to the driving wheels of the large modern bicycles, supposing a channeled rim to be substituted for the india- rubber tire. To prevent accidents from the Safety breakage of the rope on the shaft, catches. or from overwinding when the engine is not stopped at the right moment, whereby the cage may be C Fig. 21. FIGS. 19-21. White and Grant s Safety Catch. dragged up to the head pulleys (both which kinds of acci dent are unhappily not uncommon), various forms of safety catch and disengaging hooks have been proposed. These consist of variously-constructed toothed levers, cams, or eccentrics, mounted upon transverse axes, attached to the top of the cage, whose function is to take hold of the guides, and support the cage in the event of its becoming detached from the rope. They are generally applied by means of springs acting against the pull of the rope. Figs. 19-21 represent a form of safety catch, introduced some years since by Messrs White and Grant of Glasgow. The catches BB consist of partially toothed eccentrics, which when released are forced inwards against the wooden guide a by the coiled springs d d, as shown in fig. 21. When the rope is drawing, the catches are lifted by the