The Mechanical Handling and Storing of Material

394 THE MECHANICAL HANDLING OF MATERIAL supported by the triggers j. chain. The triggers turning Fig. 553. Self-Discharging Skip of Taylor & Hubbard. As in lifting, the weight of the bucket is again taken by the on the fulcrums j1 are pulled outwards by their springs, so that on again lowering, the sleeve h drops, and its shoulder H2 engages the arms k, releasing the triggers j, and allow- ing them to return to their normal position ready for the next operation. An intermediate form between the skips just de- scribed and the grab is illustrated in Fig. 554,1 which shows the jaws in an open and in a closed position. This appliance is used for a variety of purposes—for unloading minerals and grain for instance—in which case the recep- tacle is filled by hand, and then swung by means of the crane into position so as to deliver to the spot at which the discharge is required, when the contents can be deposited by means of the two outer chains. It will be readily seen that the discharge of coal from this appliance is more gentle than from the skip previously described. In the earliest grabs, owing to their slow speed of working, chains were used for lifting and closing the jaws ; the higher speed at which grabs are now worked makes chains unsuitable on account of the greater wear, their noisiness, and their ability to break without giving any warning. Generally speaking, ordinary chains are rarely used, but mostly those of special design are employed. Grabs may be roughly divided into two sections— those working with one rope and those with two or more. The inducement for the employment of one-rope grabs is that existing cranes can be used to manipulate them, while for two-rope grabs it is necessary to install a special winding gear on the crane in order to accommodate and Fig. 554. Self-Discharging Skip with Semicircular Jaws. separately actuate the two ropes. With a one-rope grab the single rope when tightened will first close the grab and then immediately lift it. The cycle of movements in the one-rope grab is practically unalterable. It is lowered open upon the material, closed and 1 Magavne of the Thames Ironworks, June 1900.