The Mechanical Handling and Storing of Material

402 THE MECHANICAL HANDLING OF MATERIAL crane. On the spindle a thread is cut to which a nut is fitted, the latter being of an easy fit and so weighted as to prevent it from revolving with the spindle. Instead of that, it passes along the thread till a point is reached which forms an obstruction to its further progress, when it has to revolve with the spindle, which movement is used to throw the apparatus out of gear. The Jaeger Two-Chain Grab.—The grab illustrated in Fig. 569 is built by J. Jaeger, of Duisburg, and is designed for an approximate capacity of 2| cub. yds.; it will therefore handle about 2 tons of coal at a time. The framework which carries the jaws is of a triangular shape, built of angle iron with gusset plates, and stiffened with flat iron bars. The upper cross-bar consists of two channel irons, and there is a second and movable cross-bar of similar construction, which is guided in its up-and-down motion on two iron bars of square section marked a a. This movable cross-bar is connected by two pairs of toggle levers to the two jaws of the grab which are fixed to the point from which the radius of the jaw body is described. The jaws are hinged on the Hone principle. The chain which carries the load is divided into two before entering the grab Fig. 569. Jaeger’s Two-Chain Grab. and the action is, therefore, identical on either side, each chain being led over three pulleys which are mounted between the two channel irons forming the fixed and movable cross-bars. The illustration shows the chain and the pulleys in the position in which the two cross-bars are close together, and in which the grab is therefore closed. The opening of the grab is effected by a steel rope which is attached to it slightly out of the centre; thus the grab hangs a little to one side during the emptying process. If the one rope is now slackened, and the hauling rope held in position, the movable cross-bar descends, which movement is accelerated by a cast-iron weight. The downward movement of this cross- bar forces the jaws asunder as shown. The jaws of this grab open extra wide, the toggle levers forcing them further apart than they would move by their own weight. The same firm are also building a grab for handling larger lumps, in which the movement is transmitted by means of wheels instead of chains and blocks'(see Figs. 570 and 571). In this case the jaws are keyed to the spindles which support them, and are manipulated by the wheels, which are also keyed to the spindles. The chain supporting the load acts on two drums which are mounted on the same spindle, and placed close together, and these drums revolve in opposite directions, coinciding with the movement