Figure 12-28.-Hook block.
A clamshell consists of hoist drum lagging,
clamshell bucket, tag line, and wire ropes to operate
holding and closing lines. On some crane models, the
hoist drum lagging (hoist drum diameter) can be
changed to meet the speed or pull requirements for
clamshell operations. Once a crane is rigged with a
clamshell, the crane is referred to by the name of the
When changing attachments from a hook block to a
clamshell, check the operators manual for the correct
length of wire rope reeving; for example, some crane
models require 300 to 400 feet of wire rope for hook
block operations and only 100 to 200 feet of wire rope
for clamshell operations. Too much wire rope on the
hoist drum during clamshell operations will cause the
wraps of wire rope to loosen on the hoist drum and cross
wind, resulting in crushed wires and kink spots in the
wire rope. This is very expensive, because the wire rope
is usually no longer useful for hook block operations.
Changing the length of rope requires unreeving the
hook block wire rope and reeving the correct length of
wire rope for the clamshell. This may be a
time-consuming effort, but saves you from having to
replace 300 to 400 feet of wire rope when the crane is
rigged for hook block operations.
The clamshell bucket (fig. 12-29) is two scoops
hinged together in the center with counterweights
Figure 12-29.-Clamshell bucket.