The nominal capacity of a clamshell grab, expressed in cubic meters per shell, rarely matches the actual operational capacity on site. This difference is not an arbitrary safety margin, but the result of three interacting variables: the physical nature of the material, the moisture present at the moment of grabbing, and the geometric constraints of the extraction point. Ignoring this gap leads to oversizing equipment or, worse, underestimating an installation's actual cycle time.
Fill Factor: The Variable That Corrects the Spec Sheet
The fill factor expresses the percentage of the grab's nominal volume that is actually occupied by material on each pass. Under favorable conditions, with fine and homogeneous granulometry materials, this factor sits between 90% and 100% of nominal capacity. However, when the material has coarse granulometry, irregular shape, or low-slip surfaces, actual fill can drop to 70-80%, even with shells correctly sized for the product's density.
This drop is neither linear nor predictable from material specs alone: it depends on how the product behaves at the moment the shells close, the penetration angle of the lips, and whether prior compaction exists at the grabbing point. This is why calculating operational capacity cannot rely solely on the material's bulk density; it requires incorporating a correction coefficient specific to each application, adjusted with field data whenever possible.
Wet Material: When Fill Factor Isn't the Only Adjustment
Moisture introduces a second variable that acts independently of the granulometry-based fill factor. Wet material tends to compact within the shell, which may seem beneficial at first glance since it reduces voids, but it also causes adherence to the internal walls and hinders drainage during closure. The net result depends on the material type: in sludge or slurry with high liquid content, usable volume decreases because part of the load is lost through drainage between the moment of grabbing and discharge; in pasty materials or those with intermediate cohesion, however, moisture can improve retention and partially offset unfavorable granulometry.
This behavior is common in wastewater treatment plant screening pits, in foundries with wet sand, or in process sludge storage, and requires shells with a profile and drainage system adapted accordingly: the drainage holes, their size, and their distribution directly influence how much usable volume is actually retained until the discharge point.
Pit Geometry: The Physical Limit That Governs the Cycle
Beyond the material itself, operational capacity is limited by the available space for maneuvering. The pit's depth, its width in plan, and the grab's access angle determine how far the tool can open before touching the walls or the bottom, which in practice sets a maximum working opening below the equipment's nominal opening.
This restriction has a direct effect on cycle time: a grab that cannot open fully needs more passes to extract the same volume, which increases the number of maneuvers per hour and, consequently, wear on cylinders and joints. In narrow or limited-access pits, this geometric factor can be more decisive than the fill factor itself in determining the equipment's actual performance.
From Spec Sheet to Real Operational Capacity
The correct calculation combines all three variables: nominal capacity, corrected by the material's actual fill factor (affected by granulometry and moisture), and checked against the maximum opening allowed by the working point's geometry. Only this way do you arrive at an operational capacity figure that reflects expected plant performance, not just the shell's theoretical volume.
Stemm, manufacturer of electrohydraulic clamshell grabs and orange peel grabs
At Stemm we design clamshell grabs and electrohydraulic orange peel grabs adapted to each combination of material, moisture, and working space. If you need to size the right equipment for your application, check out our online configurator and get an estimated quote.
