Choosing a clamshell grab based on its nominal volume is one of the most common mistakes in loading equipment specification. The volume listed in the datasheet is only the starting point; the actual operational capacity depends on three interacting variables that, if not taken into account from the outset, result in oversized equipment, below-expected performance, or in the worst case, overloads that compromise operational safety.
Nominal volume is not usable volume
Every clamshell grab has a geometric volume defined by its physical dimensions, and this is the figure that appears in catalogues. However, the volume actually used in each cycle depends on the material: its flowability, its granulometry, and how the grab responds when closing over it. Fine, free-flowing materials allow fill factors close to 100%, while coarse or angular materials can reduce it considerably.
Bulk density: from cubic metres to tonnes
Once the actual load volume per cycle has been estimated, the next step is to convert it into tonnes. This is where the material's bulk density comes into play, expressing how much one cubic metre weighs under real working conditions. This value varies enormously depending on the material: shredded biomass can weigh less than 300 kg/m³, while compacted gravel exceeds 1,800 kg/m³ and dense scrap metal can reach 2,500 kg/m³.
Multiplying the usable volume by the bulk density gives the actual load in tonnes per cycle, which is the figure that must be checked against the capacity of the carrying equipment under the expected working conditions. If that load exceeds the equipment's limit, the grab volume must be reduced. Systematically operating under overload conditions degrades the hydraulic and mechanical components and drastically reduces the equipment's service life.
Granulometry and particle size
The granulometry of the material conditions both the grab's design and its operational performance. It has direct consequences on grab closure, material losses during hoisting, and wear on the lips and teeth.
For fine, uniform granulometry materials, a smooth-lip grab with good sealing is sufficient. For coarse materials or those with large, irregularly shaped particles, penetration teeth are required to ensure proper anchoring before closure. For highly abrasive materials such as silica, crushed basalt, or recycled aggregates, the hardness of the steel used in the lips and jaws becomes a critical design factor.
Overlooking granulometry in the specification typically results in material spillage during transport, incomplete closures, and a maintenance frequency far higher than anticipated.
Work cycles and hourly output
The operational capacity of a clamshell grab is not measured in tonnes per cycle alone, but in tonnes per hour. And that depends on the number of cycles the equipment can complete in that time.
A complete cycle includes the grab's descent, closure over the material, hoisting, travel to the discharge point, opening, and return. Duration varies according to the type of crane, working height, transport distance, and manoeuvring speed. In intensive port operations it can range between 60 and 90 seconds; in operations involving greater heights or distances it can exceed two minutes.
Multiplying the load per cycle by the number of cycles per hour gives the actual hourly output. It is this figure, not the grab's nominal volume, that should be used to validate whether the equipment meets the operational requirements.
Design your clamshell grab with Stemm
At Stemm, we calculate the right clamshell grab for each application taking all these factors into account: the material's bulk density, its granulometry, the expected work cycle, and the limits of the carrying equipment. We do not manufacture standard grabs for non-standard applications.
If you know the material you need to handle and the equipment you have available, our online configurator allows you to set up and request a quote for your clamshell grab in just a few minutes, with technical specifications tailored to your actual operation.
