A clam-shell grab opens and closes two linked shells using a closing line and a separate suspension. The calculator below estimates the pull and take-up of the closing tackle; the animation shows how shortening the block spacing can close the jaws.
Closing tackle and separate positioning tasks
This model uses a closing rope and a separate holding line. Its moving-block force is not the force at the shell lips or a required actuator force. Designing a different jaw drive requires the linkage geometry and loads throughout closure.
For a separate positioning mechanism, the FIRGELLI actuator force calculator covers supported mounting arrangements. Compare force, stroke and installed length in the linear actuator range, or use the replacement finder for an existing unit. These comparisons do not establish a grab or crane load rating.
Clam-shell Bucket Interactive Calculator
Set closing-line tension, supporting rope falls, overall tackle efficiency and moving-block travel. The rope path changes with the number of falls, while constant-length arms keep both shells connected.
Equation Used
- One closing tackle with equal ideal tension in its supporting legs.
- Overall tackle efficiency η is supplied by the user and applied once.
- A separate holding line supports the upper head.
- Illustrative shell radius and arm lengths scale together to give the selected block stroke.
- No calculation of jaw penetration force, payload capacity, rope strength or crane load.
The calculated force acts on the moving reeving block, not directly at the bucket lips. Jaw-edge force depends on linkage geometry and position. Efficiency is one overall tackle efficiency. The animation uses illustrative symmetric links, scaled to the entered block travel; it is not a grab capacity rating.
Closing a pair of linked shells
A closing line shortens the distance between upper and lower tackle blocks. Long support arms and the shell pivots turn that change in spacing into symmetric jaw rotation. Paying the line out reverses the motion.
The animation uses two equal long arms and equal shell radii. Their lengths remain constant as the lower joint rises. The lips meet at the closed position. A separate holding line keeps the upper head suspended.
Hiscox figure 1256 illustrates a double-chain clam-shell with linked shells. Modern rope grabs use several arrangements. The illustrated closing tackle uses the selected number of supporting falls. Its linkage proportions do not specify a particular commercial grab.
Bulk-material handling and dredging
Clam-shell buckets enclose material between two curved shells. The historical example is a dredging bucket. Modern manufacturers offer rope-operated dual-scoop grabs with separate holding and closing ropes.
The number of crane ropes is distinct from the number of supporting falls in a reeved tackle. This calculator uses the latter. Its results concern the closing block and do not establish how much material a bucket can lift.
Block pull and rope take-up
With ideal line tension T and N supporting rope legs, moving-block pull is NT. Applying an overall efficiency η gives NTη. This efficiency is applied once to the tackle, not once per sheave.
Moving the block through distance s changes the length of each supporting leg by s, so required rope take-up is Ns. The ideal work balance is T·Ns = NT·s; with the entered efficiency, output work is η times input work.
The block pull is not jaw-edge force. The linkage changes mechanical advantage with position, and weight, cutting resistance and material behavior introduce other loads.
Default closing tackle
At 1 tonne-force line tension, four supporting falls and 100% ideal efficiency, the moving-block pull is 4 tonne-force. A 2 m block stroke requires 8 m of closing-line take-up.
At 80% overall efficiency, block pull becomes 3.2 tonne-force. Rope take-up remains 8 m. Selecting eight supporting falls doubles the ideal block pull and doubles the rope take-up for the same block stroke.
The animation fits the illustrative linkage to the selected stroke. The displayed mechanism is not a measured bucket design.
What this comparison does not determine
More supporting falls increase ideal block pull but require more rope travel for a given closure stroke. Losses, reeving arrangement, structural loads and available headroom matter in a real installation.
Jaw-lip force, capture volume, penetration, fill factor and rated payload require additional geometry and load information. No capacity or service-life claim follows from this calculator.
References
- Gardner D. Hiscox, Mechanical Movements, Powers, Devices and Appliances, printed page 309, figure 1256.
- PEINER SMAG, Rope Grabs, distinction between holding and closing ropes and examples of dual-scoop and scissor grabs.
Clam-shell tackle questions
Is moving-block pull the force at the cutting lips?
No. Jaw-edge force also depends on the linkage lever arms and current position. The calculator reports tackle output only.
Does four falls mean four ropes from the crane?
No. Falls are the tensioned legs supporting the moving block. A single closing line can be reeved into several falls.
What changes when the fall count is odd?
In the illustrated reeving, odd counts use a moving-block rope anchor. Even counts use an upper-block anchor. The remaining line is reeved through the sheaves to produce the selected number of supporting legs.
Is efficiency per sheave?
No. It is the overall tackle efficiency and is applied once to the ideal force ratio.
Can these results rate a grab or crane?
No. They do not include bucket weight, payload, dynamic loading, rope strength, structure or crane capacity.
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