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CONVEYORS
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CONVEYORS . " Conveyor " (for derivation see See also:CONVEYANCE) is a See also:term generally applied to See also:mechanical devices designed for the purpose of moving material in a See also:horizontal or slightly in. dined direction; in this See also:article, however, are included a variety of appliances for moving materials in horizontal, See also:vertical and combined horizontal and vertical directions. The material so handled may be conveyed in a practically uninterrupted stream, as in the See also:case of See also:worms, bands and pushplate conveyors, or See also:elevators carrying See also:grain or See also:coal, &c.; or it may be conveyed from one point to another, intermittently, that is to say in a See also:succession of See also:separate loads, as happens with single bucket elevators, See also:furnace hoists, rope and See also:chain haulage, and also in the case of ropeways and aerial cableways. Some of these devices are of See also:great antiquity, others are of quite See also:modern origin. The principles of their construction are See also:simple and easy of understanding, but by See also:variations in the details of their construction the engineer has adapted these few appliances to the most varied See also:work. At one end of the See also:scale they may be used for such See also:light duties as conveying the goods See also:purchased by a customer to the packers and bringing them back made up into a See also:parcel or for taking his See also:money to the See also:cashier and returning the See also:change. At the other they are adopted for handling large quantities of heavy material at a minimum See also:expenditure of human labour. Coal, for instance, a more or less friable substance, the value of which is seriously diminished by fracture, may be mechanically handled with a minimum See also:risk of breakage. The difficult problem of handling the contents of See also:gas retorts and See also:coke ovens, and of simultaneously quenching and conveying the glowing material, has been solved. Perhaps an even more astonishing piece of work is the manipulation of the See also:iron from the blast furnace; for instance, liquid See also:metal is See also:drawn from a furnace into pouring pots which in their turn See also:discharge it to and distribute it over a See also:pig-iron casting See also:machine, which is practically a conveyor for liquid metal, consisting of a strand of moving moulds from which the solidified pigs, after cooling in See also:water, are automatically removed after reaching the loading terminal over the railway trucks. Certain types of conveyors may be made to combine efficiently, with their See also:primary work of transport, complex sorting, sifting, drying and weighing operations. See also:Worm Conveyors.—The worm conveyor, also known as the Archimedean See also:screw, is doubtless the most See also:ancient See also:form of conveyor. It consists of a continuous or broken blade screw set on a spindle. This spindle is made to revolve in a suitable trough, and as it revolves any material put in is propelled by the screw from one end of the trough to the other. Such conveyors have been used in See also:flour-See also:mills for centuries. The writer has seen in an See also:East Anglian See also: The spiral or See also:anti See also:friction conveyor (fig. 4) was introduced about 1887. In this case a narrow spiral, which passes See also:con-centrically round the spindle, with a space between both, is fixed to it at set intervals by small blades, each of which is itself fixed by its shank and a nut to the spindle. The spiral may be made of almost any See also:section, from a round See also:bar about z in. in See also:diameter to L or T section, but is preferably a See also:flat bar. Worms are fitted into wooden or iron troughs leaving a clearance of to 4 in. The spindle must be supported at suitable intervals by bearings, preferably of the See also:bush type. A continuous worm, being more rigid than a paddle worm, needs fewer supports. The lid of the worm trough should be loose, not screwed on, because in case of an See also:accumulation of feed through a choke in a delivery spout the paddles of a paddle worm would be broken, or a continuous worm stripped, unless the material could throw off the lid and relieve the worm. The ratios of the See also:pitch of the worm to the diameter must be regulated by the nature of the material to be conveyed, and will vary from one-third to a pitch equal to, or even exceeding, the diameter. The greater the pitch the larger the capacity, but also the greater the See also:driving See also:power required, at the same See also:speed, For handling materials of greater specific gravity, such as See also:cement, &c., it is advisable to use a smaller pitch than for substances of See also:lower specific gravity, such as grain. The capacity of a continuous worm exceeds that of either a paddle or spiral conveyor of the same diameter, pitch and speed. As regards the relative efficiency of paddle and spiral conveyors a series of careful tests made by the writer indicated that, run at a slow speed the paddle worm, but at a high speed the spiral worm, has the greater efficiency. There is of course a speed at which the efficiency of both types is about equal, and that is at 150 revolutions per See also:minute for conveyors 4 to 6 in. in diameter. The power necessary to drive worm conveyors under normal conditions is very considerable; a continuous worm of 18 to 20 in. diameter See also:running at 6o revolutions per minute will convey 5o tons of grain per See also:hour over a distance of a See also:hundred feet at an expenditure of 182 to 19 H.P. A material like cement would require rather more power because of the greater friction of the cement against the blades and the trough. Delivery from a worm conveyor can be effected at any desired point, all that is necessary being to cut an outlet, which should preferably he as wide as the diameter of the worm, because the worm delivers only on its leading See also:side, and is practically empty on the other side, so that a smaller outlet might only give exit to a portion of the feed, unless it was on the leading side. A See also:special form of worm conveyor is the tubular (fig. 5), which consists of an iron See also:tube with a continuous spiral fitted to its inner periphery, or of iron or wooden tubes of square sections fitted with fixed baffle plates inside. In working it revolves bodily on suitable rollers. This type is more costly than the See also:ordinary worm conveyors, and also requires more power. Its efficiency is, moreover, easily impaired if run at too high a speed, because the centrifugal force asserts itself and counteracts the propulsion, which in this case is effected by gravity. Some experiments made in 1868 by See also:George Fosbery Lyster, engineer of the See also:Liverpool docks, gave convincing results (see Proc. Inst. Mech. Eng., See also:August 1869). The tubular worm conveyor is suitable where a granular material has to be moved over a comparatively See also:short distance, say from one See also:building to another on the same level, and where no See also:bridge is available for the See also:installation of any other kind of conveyor. Conveyors of this type have, however, come into use for conveying hard and cutting substances over consider-able lengths. Ordinary worm conveyors are practically debarred from use for such substances on See also:account of the short See also:life of the intermediate bearings, which are not necessary with externally supported tubular worms. To sum up, worm conveyors are of the simplest construction and of small See also:prime cost. The terminals again are much less expensive than those of most other kinds of conveyors. When the distance to be traversed by the material is short, the worm conveyor has this See also:advantage, that it is cheaper than other kinds of conveyors. If it be desired not only to convey but also to mix two or more materials, such as cement and See also:sand in a dry See also:state, or poultry See also:food, this appliance is thoroughly well adapted for the work. On the other See also:hand, there is .a grinding See also:action exercised on any material conveyed, and when hard or cutting sub-stances are handled the See also:wear and See also:tear on the conveyor blades, trough and bearings is very great, and the power absorbed by a worm conveyor is a sensible See also:item. See also:Band Conveyors.—The inventor of band conveyors for the handling of grain and minerals was G. F. Lyster, who, as already mentioned, in 1868 carried out exhaustive experiments at the Liverpool docks, where he established the band conveyor as a grain-handler. For See also:granaries the band conveyor is an ideal appliance. Its capacity is great, and it can be run at relatively high speeds with a moderate expenditure of power. The band conveyor of to-See also:day is an endless See also:belt of See also:canvas or more often See also:india-See also:rubber with insertion, and when fitted with the usual receiving and delivery appliances can be used to handle grain from or into granaries and also to feed bins or sections of a See also:ware-See also:house. The endless bands run over terminal pulleys, and are also supported on their way by a series of See also:guide rollers, which are in greater number on the loaded than on the empty strand. The band is usually run quite flat, except that at the point or points where the grain is fed on it is slightly hollowed for a few feet, by means of two curving rolls which are set obliquely so asto make it trough-shaped. The supporting or guide rollers are 4 in. to 6 in. in diameter, and are sometimes made of wood, but more often consist of See also:steel tubes to which spindles with conical end gudgeons are secured. The gudgeons generally run in suitable bush-bearings, which should be well lubricated. Band conveyors should be driven on the delivery and not the receiving terminal, as the tight side of the band is the flattest. The guide rollers, for ordinary grain conveyors, are fitted to the upper or working side of the. band at intervals of about 6 ft., and at distances of 12 ft. on the lower or return strand. In cases where both strands of the band are used for carrying grain, the lower strand must be supported by as many rollers as the upper. Under such conditions, terminal pulleys must be of larger diameter than usual, the See also:object being to throw the two strands farther apart, so as to give sufficient space between the two strands to spout the feed in and out again at the other end. The two strands can be run any distance apart by the use of two additional pulleys for the terminals. This arrangement would be in See also:place where it was desired, as it might be, to run one strand of the band along the See also:top See also:floor of the granary to distribute, while the other strand travelled along the ground-floor or See also:basement to withdraw, the grain. Band conveyors are kept tight, when the band is not very See also:long, by a tightening See also:gear, similar to that used on elevators, and consisting of two screws which push or better pull the two pedestals of one terminal See also:pulley farther away from the other terminal. If the band is of such length that an See also:adjustment of 4 to 5 ft. on the tightening gear is not sufficient, it is advisable to use In place of screws a tightening pulley, over which the belt passes, but which is itself held in tension by weights. The choice of the exact tightening gear will depend on various considerations, the length of the belt, the type of throw-off See also:carriage used, and the quality of the belt all being factors to be considered. The throw-off carriage (fig. 6), which serves to withdraw material from the band at any desired point, is a simple but ingenious appliance consisting essentially of guide pulleys which by raising one See also:part of the band and lowering the other have the effect of causing the grain to quit the See also:surface of the band at the point where it is deflected upwards. The grain is thus See also:cast clear of the band, and into the See also:air, being caught as it falls in a hopper and spouted in any desired direction. Throw-off carriages differ in certain details, but the principle is the same. For feeding a band conveyor it is important to give the material a horizontal velocity, approaching that of the band. The grain should therefore be fed through a spout rather less in breadth than See also:half of the width of the band, and set at an incline of 42i° to the horizontal. Band conveyors run at a speed of 40o to 60o ft. per minute, according to the nature of the material; oats, for instance, would be liable to be blown off the band at a speed in excess of 500, which would be suitable for See also:wheat. Nuts, See also:maize and the heavier seeds could be carried at 600. The power See also:consumption by a grain-laden band compares favourably with any other form of conveyor. An 18-in. band Too ft. in length running 500 ft. per minute would carry 50 tons per hour at an expenditure of only 4.5 H.P. While the band conveyor is an ideal conveyor in warehouses and mills, it is also capable of rendering See also:good service in handling such heavy materials as coal and minerals. Of course for such purposes the band and its fittings must be of much more substantial construction. 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