The Mechanical Handling and Storing of Material

398 THE MECHANICAL HANDLING OF MATERIAL rule the grab is dropped down on the material, which is bound to break under its weight, but there is no occasion to do this with any grab, as it makes little or no difference to the amount raised. I hey will fill just as readily when laid gently down on the material, as the action is in this case a gathering one, and the grab through this action buries itself in the heap. With grabs of this kind used in this way there need be no more breakage than in filling skips by hand. The Priestman Grab- —Ihis grab was one of the first to be placed on the niaiket in this country. It was originally operated by two chains, one for closing and hoisting, and the other for opening. There are now, however, many different designs of grabs in use working with two, three, or four chains or ropes, as well as others operated by a single chain only. An ordinary design of “Priestman” Grab is illustrated in Figs. 560 and 561. a is Figs. 560 and 561. Priestman Two-Chain Grab. the hoisting and closing chain, which is fixed at one end to the drum B, and at the other to the ciane hoisting barrel. Ihe opening chain (or frequently wire rope) is attached to the head of the grab at c. I he two jaws d d are hinged together by the strong central shaft e. The method of operation is as follows : The opening rope is held on the brake, and the hoisting chain is paid out ■ the shaft e thereupon falls, and the jaws open, as shown in dotted lines (Fig. 560). Ihe action of opening unwinds the side chains f f from the two ends of drum b, and this winds a short length of hoisting chain on to the centre portion of the drum. The grab is then lowered open on to the material to be lifted, and the opening rope being released, the hoisting chain is wound up, thus causing the side chains J f to haul down the top shaft e, thereby closing and filling the grab. On further winding up the chain a, the grab is lifted with its load to the place of discharge, and is emptied in the manner already described. It will thus be seen that if the full weight is carried on chain a, the grab is closed, and if from point c, it is open. I o increase the digging power, a multiple arrangement of sheaves is frequently fitted in place of the drum B, and different designs of jaws are provided for handling various materials.