as in the case of worms, bands and pushplate conveyors, or
elevators carrying grain or coal, &c.; or it may be conveyed
from one point to another, intermittently, that is to say in a
succession of separate loads, as happens with single bucket
elevators, furnace hoists, rope and chain haulage, and also in
the case of ropeways and aerial cableways. Some of these
devices are of great antiquity, others are of quite modern origin.
The principles of their construction are simple and easy of
understanding, but by variations in the details of their construction
the engineer has adapted these few appliances to the most
varied work. At one end of the scale they may be used for
such light duties as conveying the goods purchased by a customer
to the packers and bringing them back made up into a parcel
or for taking his money to the cashier and returning the change.
At the other they are adopted for handling large quantities of
heavy material at a minimum 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 risk of breakage. The difficult problem
of handling the contents of gas retorts and 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 iron from the blast furnace;
for instance, liquid metal is drawn from a furnace into pouring
pots which in their turn discharge it to and distribute it over a
pig-iron casting machine, which is practically a conveyor for
liquid metal, consisting of a strand of moving moulds from which
the solidified pigs, after cooling in 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 primary work of transport, complex
sorting, sifting, drying and weighing operations.
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| Fig. 1.—Early Flour Mill Conveyor.[1] |
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| Fig. 2.—Paddle Worm Conveyor. |
Worm Conveyors.—The worm conveyor, also known as the Archimedean screw, is doubtless the most ancient 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 flour-mills for centuries. The writer has seen in an East Anglian mill which was over 250 years old disused screw conveyors, probably as old as the mill, consisting of spindles of octagonal shape, made of not too hard wood, around which a broken blade screw was formed by the insertion at regular intervals of small blades of hard wood (fig. 1). Modern worm conveyors usually consist of a spindle formed of a length of wrought iron piping, to which is fitted either a broken or continuous worm. In the former case (fig. 2) the worm is composed of a series of blades or paddles arranged like a spiral round the spindle; each blade is fixed, by means of its shank, in a transverse hole in the spindle, and the shank is held in position by being tapped and fitted with a nut. In this way is formed, out of separate blades, a practically complete screw, technically known as a “paddle worm.” The lengths or sections of the worm run to about 8 ft., the various lengths being coupled by turned gudgeons, which also serve as journals for the bearings. In the so-called continuous worm conveyors the screw is formed of a continuous sheet-iron spiral (fig. 3). Sometimes a narrow groove is cut in spiral form on the spindle, and in this groove the sheet-iron spiral is secured.
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| Fig. 3.—Continuous Worm Conveyor. |
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| Fig. 4.—Spiral or Anti-Friction Conveyor. |
The spiral or anti-friction conveyor (fig. 4) was introduced about 1887. In this case a narrow spiral, which passes concentrically 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 section, from a round bar about ½ in. in diameter to L or T section, but is preferably a flat bar. Worms are fitted into wooden or iron troughs leaving a clearance of 18 to 14 in. The spindle must be supported at suitable intervals by bearings, preferably of the 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 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 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 driving power required, at the same speed. For handling materials of greater specific gravity, such as cement, &c., it is advisable to use a smaller pitch than for substances of 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 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 running at 60 revolutions per minute will convey 50 tons of grain per hour over a distance of a hundred feet at an expenditure of 18½ 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 be as wide as the diameter of the worm, because the worm delivers only on its leading 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.
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| Fig. 5.—Tubular Worm Conveyor. |
A special form of worm conveyor is the tubular (fig. 5), which consists of an iron tube with a continuous spiral fitted to its inner
- ↑ The illustrations in this article are taken, by kind permission, from the Proceedings of the Institution of Civil Engineers.
