A hydraulic power pack for a diaphragm wall grab should be sized from the specified jaw closing force, cylinder effective area, operating pressure and required closing speed, with the BAUER DHG-V cylinder-force ratings of 1,200 kN or 1,800 kN used as verification points [S5].
The calculation must distinguish force from power: 210 bar acting on a 1,000 cm² piston produces 2,100 kN, while a 250 L/min flow at that pressure requires 87.5 kW before system losses, cooling duty and duty-cycle allowances are added [S4].
Set the Force Basis from Grab Geometry and OEM Data
The governing design input is the force required to close the grab jaws against the excavation resistance, not the grab mass or soil capacity, while the BAUER DHG-V specifies cylinder forces of 1,200 kN or 1,800 kN for trench widths of 600–1,500 mm and trench lengths of 2,400–3,800 mm [S5].
Published equipment limits show why jaw force cannot be selected from trench width alone. The KHD B360 is designed for diaphragm widths of 600–1,500 mm, lengths of 2,200–4,000 mm and depths up to 80 m [S1]. A broader dimension envelope does not establish a higher clamping force, so the cylinder and grab mechanism must provide the selected force separately.
For procurement, record at least four values before choosing the pack: required closing force in kN, maximum system working pressure in bar, commanded cylinder speed in L/min, and expected duty cycle. If the supplied documentation gives only a 1,200 or 1,800 kN cylinder rating, treat that as an equipment rating rather than proof that the same force reaches both jaws at every bucket width [S5].
Convert Jaw Force into Required Cylinder Pressure
Hydraulic pressure is calculated from the load and the actuator area using Pressure (bar) = Force (N) / Area (cm²) [S4]. The jaw load must first be assigned to the relevant cylinder or cylinder group, and the effective annulus area used where a double-acting rod-side calculation applies.
At 210 bar, a 400 cm² effective piston area develops 840 kN, a 600 cm² area develops 1,260 kN and a 1,000 cm² area develops 2,100 kN, using the relationship Pressure = Force / Area [S4].
A practical sizing table makes the selection reviewable:
These force values follow the cited pressure-area relationship [S4]. A pack pressure above the calculated requirement adds little value if the cylinder, links or grab frame are the limiting components, and matching line pressure does not by itself prove adequate jaw closure.
Size Flow and Motor for the Closing Cycle
Hydraulic pressure sets force, but flow sets movement speed. Required flow is obtained from actuator displacement and target cycle time: a cylinder requiring 1 litre of oil in 10 seconds needs 6 L/min [S4]. The same flow governs several functions only if the circuit can meter or sequence them; simultaneous peak demand may require separate circuits.
Motor power is calculated as Motor Power (kW) = Flow Rate (L/min) × Pressure (bar) / 600 [S4]. At 210 bar, 100 L/min requires 35.0 kW, 150 L/min requires 52.5 kW, 200 L/min requires 70.0 kW and 250 L/min requires 87.5 kW before accounting for transmission efficiency, backpressure and the engine reserve.
The comparison should separate two operating objectives. A 200 L/min circuit at 210 bar calculates to 70.0 kW, while a 100 L/min circuit at the same pressure calculates to 35.0 kW and requires twice the time to move the same cylinder volume [S4].
Do not size the engine from nameplate pressure and rated flow without checking the pump's actual operating point. The pump, relief valve, motor cooling and hydraulic oil cooler must sustain the selected combination without continuous relief-valve operation.
Compare Pack Configurations Against Four Decisions
Selection should compare four criteria: closing force, cycle speed, installed power and thermal reserve.
BAUER's vertically installed cylinder and push-rod arrangement confirms that force transmission includes the grab linkage [S5]. The pack should therefore be selected to work with the complete mechanism, not treated as an isolated cylinder circuit. A balanced design sizes pressure from the calculated load, flow from the required cycle and engine power from the operating flow and pressure [S4].
Account for Reservoir, Cooling and Duty Cycle
A correctly sized reservoir supports fluid availability, air separation and heat dissipation.
For a diaphragm wall grab, reservoir volume should be checked against the hydraulic circuit's requirements before the power pack is selected [S4].
Clamping at high pressure for long periods generates heat that cannot be removed by reservoir volume alone. Check the hydraulic oil type, cooler capacity, ambient temperature, allowable tank temperature and duty-cycle profile. If the grab spends 210 bar of continuous holding time, include standby cooling and filtration requirements; no research figure supports a universal cooler tonnage or oil-volume rule for this grab class.
Controls also affect sizing. A proportional or directional valve can reduce throttling losses and control jaw velocity, while a solenoid valve may cause pressure loss and abrupt motion. Steering plates in 0, 4, 8 and 12 configurations and an optional TD 4 turning device are available on the DHG-V, so hydraulic demand can change with the selected machine configuration [S5].
Match the Pack to the Carrier and Work Envelope
The power pack must fit the carrier's power, weight and hydraulic interfaces, not only the cylinder requirement. The Zoomlion underground diaphragm wall hydraulic grab lists a 660 kN maximum hoisting force, 2 × 330 kN maximum line-pull force and bucket sizes of 0.8, 1.0, 1.2 and 1.5 m [S7].
Those hoisting and line-pull figures are separate from jaw closing force. A system can have adequate line pull while its clamping circuit cannot close the jaws under the required soil resistance. Keep the hydraulic functions separate in the duty schedule: hoisting, jaw extension, jaw closure and steering may compete for pump capacity.
Two published diaphragm wall grab configurations illustrate the design range. The KHD B360 reaches 80 m depth with 600–1,500 mm diaphragm widths and 2,200–4,000 mm lengths [S1]. The DHG-V covers 2,400–3,800 mm trench lengths, 600–1,500 mm widths, 1,200 or 1,800 kN cylinder forces and 17–37 t equipment weight [S5]. Extension sets can also change the working envelope, including a DHG V 600 × 2,800 mm configuration [S3].
Use the encyclopedia entry for diaphragm wall grabs to separate the attachment from its carrier, then verify every final dimension and force against the exact OEM configuration. Extension width or trench length should not silently change the pressure, pump or engine selection.
Commission the Force and Heat Test
A calculated power pack still requires a commissioning test that separates force from function. Test the pack at the specified relief pressure of 210 bar where the reference grab is rated for that pressure [S2], verify the actual cylinder force against the 1,200 kN or 1,800 kN DHG-V rating selected for the machine [S5], and record pressure loss across the valve, hose and cylinder connections.
During the test, verify at least three operating points: no-load cylinder travel, low-resistance closing and maximum-resistance closing. At each point, record pressure in bar, flow in L/min, elapsed closing time in seconds and oil temperature in degrees Celsius. Compare calculated motor power with measured shaft input, because the formula provides a hydraulic estimate before losses [S4].
Acceptance should confirm that the pack reaches the specified force without excessive pressure drop, maintains the specified closing time without prolonged relief operation and keeps oil temperature inside the fluid and component limits. A 210 bar hydraulic pressure alone is not a passing result: the grab must close, the drive train must remain within its limits and the cooling system must control heat over the intended duty cycle.
The next procurement decision is to obtain the exact grab cylinder data, jaw load schedule and carrier engine rating, then calculate the pack at a stated pressure, flow, force and duty cycle. If the calculation lands near a 250 L/min and 210 bar duty point, the preliminary hydraulic requirement is 87.5 kW before system losses, which makes engine and cooling verification the immediate trackable checks [S4].