The Mechanical Handling and Storing of Material

ELEVATORS FOR MATERIAL IN BULK 13 rush of feed, and will prevent the arching over by keeping the material constantly in motion. These devices may be used not only for feeding from large stock hoppers, but also for equalising the flow of material from smaller hoppers, which are fed intermittently by grabs, etc., or which receive at intervals the contents of trucks or railway wagons, and serve the purpose of paying the material out in a uniform stream, thereby converting an intermittent supply into a continuous one. T. he elevator should always be fed from that side on which the buckets ascend, so that the stream of material runs into the elevator buckets, meeting them on their upward journey. This will prevent the material from falling into.the elevator well, and does not necessitate the buckets dredging through an accumulation of feed. If elevators are erected at an incline, it is advisable, where possible, to feed them at a point several feet above the well into the up-going strand, as in this case very little will miss the buckets and drop into the well. Double Chains for Large Elevators.—For elevators of very large capacity two or more strands of chain are sometimes used to support the buckets, but there is a drawback to the employment of multiple strands when using ordinary malleable-iron chains, as the different strands are apt to stretch or wear unevenly and throw the buckets out of parallel with each other. It is therefore always best, when possible, to use one chain of sufficient strength, or chains with double links, that is, two links cast together with a web in the middle. These remarks refer only to ordinary malleable-iron chains, and not to steel or wrought-iron link chains, which are jointed together by steel bolts through bored holes. This latter kind of chain is always used for elevators of large capacities and for heavy materials, such as coal, and runs over polygon or sprocket wheels. Position of Elevators and their Speed of Running.—The reason why an elevator should sometimes be vertical and sometimes set in an inclined direction is this. In elevating materials of low specific gravity an elevator can be driven at a much higher speed than the elevator which is handling material of higher specific gravity, as a velocity of the material at the delivery which would not injure grain would break up coal and other heavier and friable products. Moreover, the receiving spouts and shoots would be quickly destroyed by the impact of the material. Elevators in a vertical position are therefore only suitable for specifically light material, and can be run at a circum- ferential velocity of 250 to 350 ft. per minute. Elevators for heavy material must be either wholly or partially inclined, to give a clean delivery without scattering at the much lower speed of 50 to 160 ft. per minute. At a speed of 180 ft. per minute the centrifugal force which helps fast elevators to give a clean delivery ceases altogether, so that the stuff simply falls out by gravity in slow elevators. It is for these reasons that slow elevators for specifically heavy material require so much larger buckets, chains, etc., than grain elevators of the same bulk capacity. There is a second reason why it is essential to run an elevator for heavy material up an incline, namely, the fact that it is more easily driven; for part of the load is then borne by the inclined supports instead of the whole weight hanging from the driving- gear, as in the vertical elevator. The angle at which to fix an elevator in order to get the most favourable results, without occupying too much space, is 45° to 60° to the horizon. The speed of elevators, which are fixed in a perpendicular position for dealing with grain, etc., depends upon the diameter of the pulley, i.e., the pulley on which the band which carries the buckets is running, because such an elevator, in order to deliver perfectly, must throw the grain a certain distance, which is equal to the radius of the