GRAB, OR GRAPPLE, DREDGERS.
IOI
In excavating, the bucket is closed by the cliain, which continues taut
during lifting. When the chain is slackened the roller falls and allows the
grippers to engage. Then, on hoisting, the grab is pulled open.
In the double chain system of the Priestman type (figs. 64 and 65) the
outer corners of the bucket are connected, by hinged arms, to a horizontal
bar, or cross piece, which is capable of vertical movement in the central
groove of the frame. One chain from the jib-head is attached to this bar,
and any tension in it causes the bucket to open; the other chain, from the
jib-head, is wound round a drum on the pivot, the unwinding of which,
with the assistance of two subsidiary chains connected to the horizontal
bar previously mentioned, pulis the latter down and causes the bucket to
close.
lns/2 3 eO [2 3Feer
Rigs. 62 and 63. —Section and Elevation of Peters’ Grab.
The single chain system has the advantage of being athxable to an
ordinary crane, while the double chain system involves the provision of a
special crane, but it has the following important points in its favour :—
1. It has fewer working parts, and those of less delicate adjustment.
2. The opening and closing of the bucket can be effected at any point
in the lift, whereas, with the one exception of the Peters machine, a single
chain grab has always to be lifted to the height of the suspending piece
before discharge can be made. If the latter should close upon some
immovable object below water, it could not be opened again without
lowering the suspending piece, or without the aid of a diver. A false
or empty lift has to be completed as well as a full one.