The Mechanical Handling and Storing of Material

ELEVATORS FOR MATERIAL IN BULK 27 hold one charge of the material Fig. 23. Driving Gear for Worm Elevator. by the constant impact of the grain, not to mention heavier and harder substances. A sheet of india-rubber about | in. thick secured to the point of impact will last longer than a cast-iron plate of the same thickness. The shoots themselves should be at an incline of say 5 to 10 per cent, greater than the angle of repose of the materials handled.1 Automatic Feeding Devices for Elevators.—If a system of feeding or unloading is employed which is more or less intermittent, and if the material so unloaded is to be handled further by elevators or conveyors so that the further progress is to be continuous, the feed must be changed from an intermittent to a continuous one. The importance of an even feed in all cases is obvious, as too rapid a heaping up of material may lead to chokes, besides causing an unfair strain which might lead to breakages and an undue consumption of power. If the material be delivered from railway or contractors’ trucks, ropeway skips, or grabs, the load should be deposited in a hopper (with an adjustable outlet or gate) sufficiently large to to be conveyed. The feed of the succeeding ele- vator or conveyor can be equalised in such a way that the hopper pays out its contents at the rate at which the elevator or conveyor can take it, while the truck or grab brings the next load. In some cases and with some materials it is necessary to provide the exit of the hopper with a mechanical feeding device in order to get uni- form delivery. This is the case where the material is not like grain, seedSj or small coal of uniform size, but where large pieces of coal and ore are mixed with the small. One effective device is an inclined table which slides backwards and forwards under the opening of the hopper and allows a certain quantity to leave the hopper at each stroke, which quantity can be adjusted by opening or closing a vertical slide or gate in the side of the hopper. A fairly uniform delivery can also be obtained by having a short spout from the hopper to the conveyor, the incline of which can be varied to give a slower or faster feed. A good plan frequently adopted in gasworks is to fit under the hopper a pair of break rollers which reduce the size of the coal so that it can be more easily handled by the succeeding elevators and conveyors; but even in such cases the feeding devices here mentioned should be used, as the work of a breaker will always improve in proportion to the uniformity of its feed. Fig. 24 shows a mechanical feeding device for elevators in which a short Zimmer conveyor is used, into which the coal falls through a receiving hopper above. In this case the hopper is fed intermittently by narrow gauge trucks. By the use of a slide or gate the quantity of material it is desired to convey can be regulated. This feeding device can of course be used equally well for feeding other kinds of conveying appliances. It is always well, where possible, to drive the feeding device from the machine which is to be fed, so that when the elevator or conveyor is at a standstill the feed may also auto- matically cease. This is of great importance, for if arranged otherwise the conveying machine at its feed end would on starting be so full of material, that in attempting to 1 For inclines of shoots for delivery of coat see page 195.