The crane is a versatile piece of equipment that can be equipped with various attachments to perform a number of different operations. These attachments include a hook block, a clamshell, and a dragline.
A crane that is rigged with a hook block is the primary unit for lifting an objector load, transferring it to a new place by swinging or traveling and then placing the load. Figure 12-28 shows an eight-part line rigged hook block.
The number of parts of a line rigged on the hook block is important for figuring the capacity of the crane. Most crane load charts show the rated capacity of the crane for different parts of the line; for example, a crane that is capable of being rigged with a eight-part line is rigged with a six-part line. The eight-part line gives the crane a greater lifting capacity; therefore, you must check the load chart for the six-part line capacity to avoid overloading the crane.
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 attachment.
When changing attachments from a hook block to a clamshell, check the operator's 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.
bolted around the hinge. The two hoist drum wire ropes on the crane are rigged as the holding and closing lines for controlling of the bucket. An example of a clamshell rigging configuration is shown in figure 12-30.
The tag line winder (fig. 12-31) controls the tension on the tag line that helps prevent the clamshell from twisting during operations. Like the clamshell bucket, the tag line winder will exchange with most makes or models of cranes in the same-size range.