Page:EB1911 - Volume 07.djvu/77

Rh the hoist works, the time taken in raising the charged skip, discharging it, and returning it empty would be only 30 to 40 seconds. The hoist cable runs over guide pulleys placed at the top of the furnace, and the cable is often manipulated by an electrically driven winch in a cabin below. The descent of the empty skip in more modern installations is utilized to effect an even distribution of the feed from the hopper to the furnace by causing the hopper to revolve. To this end the latter is provided with an ingenious mechanism which only comes into operation as the car descends. After every charge shot into the hopper the latter is revolved a few degrees, and this has the effect of giving the delivery of the next load in another direction, so that the charges of the skip are in turn distributed over the whole area of the surface. This is deemed a most essential point in furnace-charging, and it is not one of the least recommendations of this mechanical system of furnace-charging that it can give an even feed without any hand labour whatever. A double hoist has been designed which has the advantage that if one elevator breaks down the work of the furnace is not interrupted. In this system two furnaces are connected at the top by a gantry or bridge, against which, between the furnaces, two inclined elevators are set, so that each can serve either furnace. The skips are on wheels and detachable from the elevator, and are loaded from the ore pockets at the lower terminal and drawn up on a cradle; as this reaches the top where the rails on the gantry correspond with the gauge of the skip or car, the latter is carried by its own weight down a slight incline to either furnace, discharging its contents as it passes over the conical mouth. Another advantage claimed for this system is that the rails of the cradle, when in its lowest position, correspond with the rails which lie parallel to the furnaces and run right under the store bins from which the skip is loaded. The economy to be realized from a furnace hoist will be in direct proportion to the use made of mechanical means of feed conveyance. For instance, the store bins in connexion with such elevators might be economically fed by suitable conveyors, or the material might be brought in self-unloading hoppered trucks into conveniently placed bins, ready to be drawn into the skips.

Ropeways.—A ropeway has been defined as that method of handling material which consists of drawing buckets on ropes, and by means of ropes, such buckets being filled with the material to be handled and being automatically or otherwise discharged. At what period of history ropeways were first used it is impossible to say, but the fact that pulley blocks, and even wire ropes, were known to the ancients, renders a pedigree of 2000 years at least possible. In more modern days, an old engraving shows a single ropeway in working order in 1644 in the city of Danzig. This, the work of Adam Wybe, a Dutch engineer, was a single ropeway in its simplest form, consisting of an endless rope passing over pulleys suspended on posts; to the rope were attached a number of small buckets, which evidently carried earth from a hill outside the city to the rampart inside the moat. The rope was probably of hemp. Modern ropeways worked with wire ropes date from about 1860, when a ropeway was erected in the Harz Mountains. Since then several systems have been evolved, but in the main ropeways may be divided into the single and double rope class.

The ropeway is essentially an intermittent conveyor, the material being carried in buckets or skips, and practice has proved it an economical means of handling heavy material. The prime cost of a ropeway is usually moderate, though of course it varies with the ground and other local conditions. Working expenses should be low, because under the supervision of one competent engineer unskilled labour is quite sufficient. A ropeway may be carried over ground over which rails could only be laid at enormous cost. To a certain extent ropeways are independent of weather conditions, because their working need not be interrupted even by heavy snowfalls. Their construction is very simple, and there is little gear to get out of order. Sound workmanship and good material will ensure a relatively long life. As an instance, a certain rope in a Spanish ropeway tested new to a breaking strain of 29½ tons was shown after carrying 160,000 tons (in two years’ incessant work) still to possess a breaking strain of 27½ tons. The power absorbed by a ropeway is relatively moderate, and under special conditions may be nil. The only demand it makes on the superficial area of the ground traversed is the small emplacements of the standards, which in modern ropeways are few and far between. Wayleaves, or the permission to erect standards and run the line over private land, may of course mean an item in the capital outlay. This circumstance may have checked ropeway construction in Great Britain, but it must also be borne in mind that a large portion of that country is comparatively level and well provided with railways. In building a ropeway it is essential to take as straight a line as possible, because curves generally necessitate angle stations, which mean extra capital and working cost. On the other hand, ground that would be difficult for the railway engineer, such as steep hills, deep valleys and turbulent streams, has no terror for the ropeway erector. There is a case of a ropeway of a total length of 5400 ft. with a total difference in altitude of 2000 ft.; it is claimed this ground could not be covered by a railway with less than 15 m. of line graded at 1 in 40.

Perhaps the simplest type of a single rope system is an endless running rope from which the carriers are suspended, and with which they move by frictional contact. Or the carriers may be fixed to this rope and move with it. The ropeway itself would consist of an endless rope running between two drums, one, known as the driving drum, being provided with power receiving and transmitting gear, while the drum at the opposite terminal would be fitted with tightening gear. The endless rope is carried on suitable pulleys which themselves are supported on standards or trestles spaced at intervals varying with the nature of the ground. The rope runs at an average speed of 4 m. per hour, a speed at which the bucket or skip can automatically unload itself. In the double ropeway the carrier runs on a fixed rope, which takes the place of the rails of a railway. The carrier is fitted with running heads furnished with grooved steel wheels. The load is borne by a hanger pivoted from the carrier, and is conveyed along the rail rope by an endless hauling rope at an average speed of 4 to 6 m. per hour. The hauling is operated by driving gear at one end, and controlled by tightening gear at the other end just as in the single rope system. Double ropeways have been carried in one section over 18 to 20 m., and will transport single loads of 6 cwt. to a ton or more.

Broadly speaking, the single ropeway is not so suitable for heavy loads and long distances as the double, but in this connexion the work of Ropeways Limited should be noted, which favours a single rope system. Their engineer, J. Pearce Roe, introduced multiple sheaves for supporting the rope at each standard. Thus the rope may pass over one, two or four sheaves, which are provided with balance beams that have the advantage of adjusting themselves to the angle caused by the rope passing over the sheaves, thus equalizing the pressure over a number of sheaves. A ropeway erected on this system in Japan spans 4000 yds. of very broken ground; yet only 17 trestles are used, and as each support is placed as high as possible, no one is of great height. An altitude of 1130 ft. is reached in a distance of 1200 yds. The ropeway has a daily carrying capacity of 60 tons in one direction and of 30 tons in the other. Another installation on this system, which serves an iron mine in Spain, spans 6500 yds. of very rough country, so steep that in many places the sure-footed mule cannot keep on the track. This ropeway can deal with 85 tons per hour. The greatest distance covered by this system, on one section, is 7100 yds., or about 4 m., and the carrying capacity is 45 tons per hour.

The motive power required for a ropeway will vary with the conditions. In cases of descending loads the power generated is sometimes so considerable as to render it available for driving other machinery, or it may have to be absorbed by some special brake device. In a ropeway in Japan of 1800 yds., which runs mostly at an incline of 1 in 1½, the force generated is absorbed by a hydraulic brake the revolving fan of which drives the water against fixed vanes which repel and heat it. In this way, 50 h.p. is absorbed and the speed brought under the control of a hand brake.

Aerial Cableways.—The aerial cableway is a development of the ropeway, and is a conveyor capable of hoisting and dumping at any desired point. The load is carried along a trackway consisting of a single span of suspended cable, which covers a comparatively short distance. The trackway may either run in a more or less horizontal direction, i.e. the terminals may be on the same level, or it may be inclined at such an angle that the load will descend by gravity. The trackway or rail rope rests upon saddles of iron or hard wood on the tops of terminal supports, usually known as towers. These towers may be constructed