The Mechanical Handling and Storing of Material

DISCHARGING BY MEANS OF SKIPS AND GRABS 4°9 Fig. 579. Pneumatic Grab of the Beard Dredging Co. the drum d during the process of opening. As soon as the apparatus has thus been discharged the main chain begins to lower, when the levers ll, being released from the obstruction, are pulled back into their original position by the springs ss, while the pawls dd hold the drum d in its position, and thus keep the jaws open as the grab plunges into the material to be elevated. If the rope or chain be now further lowered, the part b sinks by its own weight into the opening in the top of the framework, and in so doing the pawl c is pushed to one side and held in that position by another small catch. The hoist chain is now pulled up by the crane, b is now no longer held in position, and as it is pulled out of the framework, pulls with it the chain m so as to unwind it from the drum i>. The revolutions of the drum, and with it of the spindle a, wind up the two small chains kk, and pull up the levers tt, thus closing the jaws. As soon as the grab is thus closed, the chain is lowered once more for a foot or so; B re-enters its former position, and is now again held in place by the pawl c, which has in the meantime been released. As b re-enters the frame, the chain m is released and lies loosely on top of the load as the drum is now fixed. The grab is now ready to be lifted and swung into position for delivery. Pneumatic Grab.—A grab operated by compressed air is built by the W. H. Beard Dredging Co., of New York. The jaws are opened and closed by means of air pressure. The contention of the inventor is this, that the grab when embedded in the material to be raised should be closed without pulling a chain or rope, as this might have a tendency to loosen the hold of the jaws upon the material, with the effect of diminishing the load to be raised. From the chain of the grab (Fig. 579) is suspended a frame a., which carries the air cylinder b and the crosshead guide which supports levers cc. Although this grab has only one chain its manipulation is somewhat com- plicated. There are two air-pipes to be raised and lowered with the grab, one to connect the upper and one to connect the lower end of the cylinder with the compressed air vessel. This is effected in the following manner: A drum is fixed to the jib-head of the crane, and to its hollow spindle the two air-pipes are attached by means of stuffing boxes on either end. The two flexible air-pipes leading to the grab are rolled upon the drum, one end of each being connected with the main pipe through the stuffing boxes. As the grab descends, the pipes unroll from the drum and cause the latter to revolve, thus winding a rope (on the end of which is a balance weight) upon the drum, so that as soon as the grab ascends again the weight on the rope turns the drum in an opposite direction and winds the two pipes up again, so that they are always taut no matter in what position the grab may be. To manipulate the grab a three-way valve is used, two of the exits communicating with the upper and lower ends of the cylinder, and the other with the compressed air vessel. Hydraulic Grab.—This grab (Fig. 580) is similar to the last mentioned, but is operated by hydraulic power. The arrangement of the jaws and their guides is similar to the Priestman principle. The casting upon which the jaws are hinged is connected to the cylinder by