Choosing a clamshell grab or an electrohydraulic orange peel grab does not end with selecting the right model for the material's density and granulometry. Behind every loading operation there is a hydraulic circuit that must be correctly sized to deliver the force and speed that operation demands.
While the operational capacity of a piece of equipment depends on what is being handled, the actual performance on site depends on how the hydraulic circuit driving it has been calculated: its working pressure, the power unit's flow rate, and the cylinder diameter. These three variables are what ultimately determine the cycle time that can be achieved.
What a hydraulic circuit is and why its sizing is critical
A hydraulic circuit is the set of hydraulic components (power unit, pumps, valves, cylinders, hoses, and reservoir) that converts mechanical energy into fluid energy to generate movement and force in the grab or orange peel grab. The hydraulic schematic of each piece of equipment defines how these elements are connected and how flow is distributed among them, but the schematic alone does not guarantee performance: the pressure and flow rate values used in the sizing determine whether the equipment will open and close its jaws at the speed required for the intended work cycle.
An oversized circuit means unnecessary cost in components and energy consumption. An undersized one results in longer cycle times, oil overheating, and premature wear of seals and cylinders. Correct sizing is therefore a technical balance between force, speed, and the service life of the hydraulic system itself.
Working pressure: the force each jaw requires
Working pressure determines the closing force a clamshell grab can exert, or the gripping force of an orange peel grab. This force must overcome the resistance the material presents to penetration and compaction, which varies according to its bulk density and cohesion. Dense or compacted materials require higher pressures to achieve complete filling without the jaws opening prematurely under overload.
However, increasing working pressure is not a universal solution: every component in the circuit (seals, hoses, fittings) has a nominal pressure limit, and operating close to that limit on a sustained basis accelerates material fatigue and multiplies the risk of leaks. Sizing therefore always starts from calculating the minimum force required for the most demanding material the equipment will handle, and only then is the system's working pressure defined with an appropriate safety margin.
Power unit flow rate: cycle speed
If pressure determines force, the flow rate delivered by the hydraulic power unit determines speed. Hydraulic flow systems regulate how many litres per minute reach the cylinders, and that figure, combined with the internal volume of each cylinder, directly sets how long it takes for the jaw to open or close fully.
This is where many equipment selection projects make the same mistake: sizing the flow rate for a single working condition, without considering that the actual cycle time of an installation also depends on external factors, such as the crane or excavator positioning time between each movement. A higher flow rate reduces jaw opening and closing time, but if the real bottleneck is arm travel, that additional flow rate does not translate into more cycles per hour, only into higher energy consumption and unnecessary pump wear.
Cylinder diameter: the balance point between force and speed
Cylinder diameter is the variable that connects pressure and flow rate. The larger the diameter, the greater the surface area on which pressure acts and therefore the greater the closing force at the same pressure. But a larger-diameter cylinder also requires more flow rate to move at the same speed, because its internal volume is greater.
This is why two pieces of equipment with the same hydraulic power unit and the same working pressure can have different cycle times: their cylinder diameters are calculated to prioritise force (dense, compacted materials) or speed (lightweight materials, high handling volumes per hour). Stemm's design starts from this relationship to adjust each grab or orange peel grab to the actual working profile of the installation, rather than applying a one-size-fits-all sizing criterion to any material.
How these three parameters interact
The desired cycle time is, in practice, the result of solving an equation with three unknowns: the force the material demands (pressure and cylinder diameter) and the speed the installation's productivity demands (flow rate and cylinder diameter). Increasing one of these parameters without adjusting the other two throws the entire system out of balance. This is why hydraulic circuit sizing cannot be done in isolation: it must always start from the material to be handled, the target work cycle, and the real constraints of the installation in question, whether crane, excavator, or manoeuvring space, before setting pressure, flow rate, and diameter.
Real-time monitoring: Stemm's IoT Module
Sound theoretical sizing is only confirmed with real operational data, and this is where Stemm's IoT Module comes in. This telemetry system records working pressure, effective flow rate, and actual cycle times in real time on equipment in the field, making it possible to compare the hydraulic circuit's actual behaviour against the design parameters.
This information has a dual value. On one hand, it confirms whether the initial sizing remains adequate when working conditions change, for example if the installation begins handling a denser material than originally anticipated. On the other, it provides a historical database that facilitates hydraulic circuit redesign in future equipment upgrades, avoiding the need to recalculate from scratch and allowing pressure, flow rate, and cylinder diameter to be adjusted using real operational data rather than estimates.
Stemm, manufacturer of electrohydraulic orange peel grabs and clamshell grabs
The performance of a clamshell grab or hydraulic orange peel grab depends not only on the material it handles, but on how the hydraulic system driving it has been sized. Working pressure, power unit flow rate, and cylinder diameter are three interdependent variables that define the actual cycle time of an installation, and calculating them correctly is as decisive for productivity as the choice of equipment itself. If you want to calculate the most suitable hydraulic circuit configuration for your operation, you can use Stemm's online configurator and obtain a proposal tailored to your work cycle.
