The Mechanical Handling and Storing of Material

DISCHARGING BY MEANS OF SKIPS AND GRABS 4°7 being required. The operation of rigging the chain in the grab is exactly similar to reeving a double and single chain block, and can be effected by any unskilled labourer. As soon as the grab has been lowered on to the coal (the crosshead c being at the bottom of the guides, and the sliding block a at the top of the guides) the sliding block will, by its own weight, and the overhaul of the chain, run down the guides until the crosshead pin enters the hole in the bottom of the sliding block. This also lifts the balance weights and the machine becomes locked. The grab is now ready to dig, and on starting the crane to “heave up,” the sliding block and crossheads are pulled up between the guides at a leverage of 4 to 1, until the grab is completely closed. The load is then lifted to the Figs. 575 and 576. Hone Grab with Four Jaws (sometimes called an “ Orange-Peel ” Grab). required height, and the top portion of the grab must also be so far through the ring G that the long end of the releasing lever has been depressed by the ring, and having passed through it, has swung back again against the top pin. On lowering the grab, the releasing lever E comes in contact with the top edge of releasing ring G, thus lifting the stop plate and turning the locking pin until the two notches come opposite each other, when the crosshead, being released, falls from the sliding block, and the jaws and closing rods drop with it. The piston rod gently descends at the same time, thus preventing shock to the machine, or the scattering of the material. On lowering clear of the ring, the releasing lever and locking pin reassume their positions by means of their balance weights, and the grab is once more ready to be lowered into the barge. Figs. 575 and 576 show the same grab made with four jaws instead of two. This form is used chiefly for excavations under water and for extra stiff clay.