TYPES OF CONVEYORS One of the oldest conveyor devices is the worm or screw con veyor. It consists of a stationary trough of wood or steel plates, in which a helix rotates and pushes the material fed into it from end to end. Its capacity is relatively small, it is apt to in jure friable materials, and its driving power is high; on the other hand, where mixing of the material is essential, as in the handling of poultry food, flour, etc., the worm is quite good; it has the further advantage that material can be fed on from any number of points and can likewise be withdrawn at any alternative point or points, through openings in the base of the trough, which can be closed by sliding gates when not required. There are a number of types of the worm conveyor, one of which is shown in fig. 2.
The pushplate, scraper or drag conveyor is similar in prin ciple to the foregoing, but the material is pushed along by an end less running chain or chains, to which are attached dragging or pushing plates. This device may be used for handling larger quan tities than the worm conveyor, but it shares its advantages as well as disadvantages, so far as power consumption and possible injury to the material is concerned. It is fed similarly to the worm, in and out at any number of points, when the idle return run of the chain passes over the top of the working run. In fig. 3 this is illustrated diagrammatically.
The U-link conveyor is an obvious modification of this type, in which the chain itself is so formed that it will drag the material along without any other attachment. The links are in the form of the letter U.
The De Brouwer or push-bar conveyor is widely employed in gas works for handling incandescent coke. This always runs in a trough filled with water. The trough is made with a renewable cast-iron base and the two chains, one on either side, which sup port the pushing bars, are protected in recesses, and thus do not come in contact with the coke.
All types of these conveyors, including those yet to be de scribed, are fitted with power-driven sprockets at their delivery ends, and at their other ends with similar terminal sprockets ar ranged with tension take-ups for keeping the chains taut.
the band or belt conveyor, illustrated diagrammatically in fig. 4. This device consists essentially of two terminal drums over which an endless band travels, supported on its carrying as well as on its return run by idler rollers, pitched closely on the loaded run and two or three times the distance apart on the return run. Such idlers are ordinary small diameter rollers of steel in some instances, as, for example, f or handling goods in packing cases; or more frequently a combination of three or more shorter rollers which give the band a trough-like shape when bulk material such as grain or coal is handled in large quantities. The idlers for the con veying strand may be as close as 2 to 3 ft. apart, or even up to a distance of 6 feet.
The belt or band itself can be made of various materials, that being chosen which will best suit the goods handled. Compound cotton-duck-and-rubber belting is most frequently employed. The pulley side has a rubber coating of about in., while on the edges and working side are thicker layers of rubber, finally the whole is vulcanized. For light work, cotton belting of the Gandy type may suffice, and the idlers are sometimes replaced by a board of hard wood. Balata belting is also used, as well as woven wire off" or tripper gear is necessary. Such a throw-off device is shown diagrammatically in fig. 5. The full lines denote the machine in action, the band passing over idlers Al and B' ; as shown in dotted lines, idlers A and B are out of action when the band with its load passes by.
The Shuttle Conveyor is an important application of the band conveyor. Its use can best be visualized by an example. For instance, in a very long boiler house, where it is essential that the coal be delivered in a central position, an elevator or a skip hoist is installed to lift the coal to a level above the bunkers. Instead of providing two band conveyors, one on the right and the other on the left, for distributing purposes, only one length is installed, reaching from the elevator or skip hoist to the bunker most remote from the central position. This is mounted on rails and is made reversible so that it can be run either to the right or to the left.