The Mechanical Handling and Storing of Material

UNLOADING AND LOADING APPLIANCES CHAPTER XXX DISCHARGING BY MEANS OF SKIPS AND GRABS Boats and barges as well as railway trucks usually deliver material at the point where the mechanical appliances for its further conveyance are located, and here the transfer has to take place. Hand labour at this point is out of the question if the plant is to be economical, or if labour-saving appliances can do the work wholly or even partially. Perhaps the most frequently used appliances at this stage are skips and grabs. I o operate any kind of bucket for unloading purposes a crane of some description is necessary, such cranes often being fixtures on the quay side, and when large vessels have to be discharged they are used in pairs or in even greater numbers. Not infrequently they are mounted on rails so as to be readily moved to any point on the quay at which the boat to be unloaded may most conveniently be moored. It is not, however, so much the object of this chapter to give a technical description of cranes, as to deal more in detail with such skips and grabs as are manipulated by cranes and transporters. The oldest form of skip consists of a large bucket suspended slightly below its centre of gravity by an arched bail with a catch, which keeps the bucket in an upright position. Such buckets have a capacity of from 1 to 3 cub. yds., and are lowered into the boat and there filled by hand. When full, they are raised by the crane and swung to the unloading position, when the catch being withdrawn, the buckets tilt and discharge their contents, This mode of unloading is used for grain as well- as for minerals, but has to a great extent been superseded by the self-filling grab, as the latter dispenses with hand labour in filling. Grabs are frequently used for excavating purposes, perhaps as much so as for handling grain and minerals. Although the earliest grab known only elates from 1703 there is every reason for believing that appliances of this type were in use at an even earlier period. In 1703 the French Academy of Science approved of the design of a machine for excavating purposes, a description of which was published by them. This appliance was the invention of a M. Gouffe, and had the outward appearance of the modern grab. It was provided with serrated edges that cut into the ground, and was operated by two ropes and a bar which gave the cutting edges a downward pressure, the two ropes being manipulated by two windlasses. There are other designs too numerous to mention which have from time to time sprung up, only to disappear again. 1 he grab of to-day is perhaps one of the most familiar of modern labour-saving appliances. Self-Dumping Buckets.—Before passing to a more minute description of grabs, one or two types of mechanical self-dumping buckets may be mentioned, one of which 392