The Mechanical Handling and Storing of Material

THE AUTOMATIC WEIGHING OF MATERIAL CHAPTER XLIII THE AUTOMATIC WEIGHING OF MATERIAL When handling material automatically, it is often desirable to keep a record of its weight; also of the total tonnage dealt with, so as to check deliveries of coal, grain, etc. It is also essential to keep a record of the contents of vessels filled mechanically, as well as of silos, bunkers, and other receptacles. Automatic weighing machines which are used as a means to this end may be classified under three types. In one the material is weighed at the point of receiving from or delivering to the elevators or conveyors which handle the material, in which case it actually passes through a weighing machine. A second type of weigher is attached to certain kinds of conveyors, and records the weight of the passing load without bringing the material itself in contact with the hopper of a weighing machine. There is also a third type which is used for weighing material intermittently in larger quantities. There are advantages and disadvantages in ea"ch method. In the first type, termed Hopper Weighing- Machines, the record of the weight is more accurate. This is, of course, an important matter if material of a valuable nature is being handled, but these machines have their limitations. To begin with, the material to be weighed must be of a fairly uniform size, that is to say, big pieces must not be mixed up with small particles. Then again, with these machines considerable headroom is necessary, which is not always available, as a stationary hopper must be provided into which the elevator or conveyor can deposit the material before feeding it into the weighing machine, and there must also be a second hopper beneath the weighing machine to receive the material leaving it. In addition to these two hoppers, the height •of the weighing machine itself must be taken into consideration. This difficulty may be overcome by the use of an elevator, but there is also the drawback incidental to the breakage of friable material, and the consequent production of dust. In the case of ■coal the last objection would probably be the most urgent. The second type, termed Continuous Weighing Machines, will record with sufficient accuracy coal, ore, cement, etc., as it is passed along by a conveyor of the belt, tray, plate, or gravity bucket type. The driving mechanism derives its motion from the conveyor itself, so that the record is not deranged by any variation in the speed of the conveyor. These machines require no extra headroom, produce no dust, and are not calculated to triturate the most friable material; but on the other hand, the record obtained is not so reliable as that of the hopper weighing machines. The action of the first and second types is more or less continuous, as a constant stream passes through or over both kinds of weighing machines. Even with a hopper type the delivery can be made to a certain extent continuous.