The Mechanical Handling and Storing of Material

BAND CONVEYORS 89 It is a surprising fact that the power consumed by a band conveyor running empty may be as much as 35 per cent, to 75 per cent, of that consumed by the loaded conveyor, the percentage depending principally upon the load, length, and speed of the conveyor. It is obvious that the power consumed by the terminals does not increase pro rata with the length of the conveyor, and, therefore, the power thus consumed by the terminals of short conveyors may be the principal factor, but it becomes more and more insignificant as the conveyor is lengthened, whilst the power consumed in turning the idlers increases in direct proportion to the length and load of the conveyor, so that with short conveyors running idle the percentage of power consumed is smaller than with long conveyors. The power required for band conveyors for grain is less than with any other grain conveyor. An 18-in. band travelling at 500 ft. per minute would convey 50 tons of grain per hour, and if 100 ft. long would consume 4’5 H.P. Speed and Capacity.-—The speed and capacity of belt conveyors depends entirely on the nature of the material to be handled and the width and shape of the belt. Materials like grain and seeds can be conveyed at the highest speed, as such materials will not be damaged by being delivered at a great velocity. The limit of speed is, therefore, only set by the resistance of the air to the passage of the material. If the grain or seeds are heavy, a speed of 500 to 700 ft. per minute is admissible, whilst if it contains much dust or chaff, or if the grain itself is light, like oats, the speed should not exceed 400 to 600 ft. per minute. Wheat and barley are generally conveyed at a speed of 500 to 550 ft., whilst maize, beans, and other heavy seeds may be conveyed at the maximum speed of 600 ft. per minute. The speed of belts for coal and minerals is altogether different. Here the speed is, in many cases, limited by the friability of the material and the consequent dust creation and deterioration of some materials through the impact at the delivery point. This refers particularly to coal and coke. There is an exception to this in cases in which the material does not deteriorate by breakage at the impact, but where the band has to run slower on account of fine material being mixed with the coarse, so that the same con- sideration comes into force as in the case of grain conveyors. The best speed for large pieces is 150 to 250 ft. per minute, and the speed increases in proportion for material of a smaller size to a maximum speed of 700 to 750 ft. per minute. The shape of the belt has also a great influence on the capacity, and flat or very shallow trough belts cannot carry as much as deeper troughed belts. The following table gives the safe load for a belt, in cubic feet per minute, by multiplying the area of a uniform load with the speed in feet at which the belt travels:— Table showing Section of Load in Square Feet which can be Carried with Safety in a Constant Stream on Belt Conveyors if the Material is Small and Uniform Width of Belt in Inches. Area or Cross Section of Load in Square Feet. Width of Belt in Inches. Area or Cross Section of Load in Square Feet. For Flat Belts. For Troughed Belts. For Flat Belts. For Troughed Belts. 10 0 015 0'05 30 0'203 0-47 12 0 026 0-06 32 0'253 0-53 14 0-042 0-08 34 0-29 0-60 16 0-055 o-ll 36 033 0-66 18 0-07 0'15 38 0-367 0-75 ‘20 0-085 0-18 40 0-41 0-83 22 0-114 0-23 42 0’455 0-91 24 0139 0-28 44 0-506 1-00 26 0'164 0'33 46 0’557 1-08 28 0-19 0-40 48 0-608 ri2