Types of Conveyors

With regard to the speed at which band conveyors may run, this may be said, broadly, to be 600 ft. per minute when handling maize, etc., while when handling lump coal it should not be more than 15o to 25o ft. per minute.

Articulated Band Conveyors.

A number of conveyors are in use on the same principle as the band conveyor, but where end less chains replace the band and to which are attached a variety of carrying elements. These may be likened to, and in fact are, articulated band conveyors. These all run at relatively slow speeds. The type most frequently used is the slat or apron conveyor. It has the usual two sprocket terminals. Every link or every other link of the chains is provided with slats of iron, or of wood, furnished at their ends with small supporting rollers. (See fig. 6.) and even Swedish charcoal steel bands, which are quite as flexible as any of the foregoing.

The only disadvantage of the band conveyor is that while it can be fed at any number of points by making adequate provision, the withdrawal of the material en route is somewhat complicated. It is true that, with an oblique plough, material can be scraped off at intermediate points if the band speed is low, but even then it is a makeshift and shortens the life of any but a steel band, for which such ploughs are therefore ordinary standard practice. For all textile and textile-rubber bands what is known as a "throw For handling small coal, etc., conveyors with steel slats, bent up at the sides, are employed. They are called continuous trough conveyors and form articulated troughs. A modification is intro duced for the purpose of effecting intermittent delivery. It is known as the Tipping-Tray Conveyor and is illustrated in fig. 7.

Reciprocating Conveyors.

(See fig. 8.) These arse essen tially steel troughs which are set into frequent reciprocating motion, so that with the forward stroke the material travels with the trough, while the trough returns to its initial position without the material, which travels forward in an almost continuous stream.

Coal Face Conveyors.—All the foregoing types are employed in collieries, but their construction is somewhat different on ac count of the confined head-room in the coal seams. They are generally from i oo to i8o yd. long to reach from gate to gate, and are so built that they can be readily taken down as the coal face recedes and be re-erected elsewhere.

When inclines have to be negotiated in favour of the load, what is known as a retarding conveyor is employed, similar in principle to that last described, but with wire ropes instead of chains to which cast-iron discs are clamped at regular intervals.

Such devices need no driving power, but rather a brake, if the incline is steep enough.

All types of chain conveyors so far discussed employ endless chains running on vertical sprockets, so that the two strands are disposed one above the other. In another type, known as general purpose con veyors, the chains run over horizontal sprockets, so that the two strands are side by side, or by the addition of guide idlers they may run over a more complex path on an essentially horizontal plane.

Gravity Roller Conveyors.

Gravity roller conveyors con sist of benches, the upper surface of which is provided with light rollers of two to three inches in diameter, placed close together and running in ball bearings disposed at a downward gradient of 2% to 5%, so that any object with a flat base, such as a packing case, can run down by gravity.

A great variety of types of continuous handling machinery are employed in modern factories. Sometimes when conveying mate rial from one point to another it is exceedingly difficult to deter mine which of the many devices obtainable is best suited for the specific purpose in hand. The accompanying diagram, fig. 9, will be helpful in such a case. On the left hand side it shows a hopper A, containing a material which will run down an incline of and which will therefore convey itself by gravity to any one of the conveying devices represented by radial lines in the diagram. The circles to the left of the datum line represent the receiving terminals of all types of conveyors or elevators. The devices shown by circles in a variety of positions may be what are usually known as bucket elevators, bucket conveyors, etc., according to the incline at which they work. At an angle of 9o° to the datum line, and above, such a device would be a bucket elevator, and would remain so at any angle to about 67.5°, when it would merge, stage by stage, into a bucket, tray, push-plate, slat, apron or band conveyor, the latter on the datum line.