Recirculating pump hydraulics

Recirculating pump hydraulics describes hydraulic systems in which the pump’s delivery flow circulates at low pressure in idle and is only converted into work when a valve is actuated. This principle is widespread in compact hydraulic power packs and, in many applications, forms the basis for the reliable use of portable tools, such as concrete demolition shears or hydraulic rock and concrete splitters from Darda GmbH, in fields like concrete demolition and deconstruction, gutting works and cutting, rock breakout and tunnel construction, natural stone extraction, as well as special operations.

Key idea: in neutral, the system runs with very low hydraulic losses and heat generation; only on demand is pressure built and power converted into useful work. This makes recirculating systems particularly suitable for mobile, intermittent-duty applications with repetitive cycles and short hose runs.

Definition: What is meant by recirculating pump hydraulics?

Recirculating pump hydraulics refers to a constant-flow system with an open center (recirculating control). In the neutral state, the directional valves are interconnected so that the oil delivered by a fixed-displacement pump is returned to the tank almost without pressure. Only when a function is actuated does the recirculation path close, pressure builds up, and the flow is directed into the working circuit. The system is characterized by simple design, high robustness, and predictable switching behavior. It thus differs from constant-pressure and load-sensing systems, which follow different control strategies.

• Core characteristics: fixed pump displacement, open-center neutral, low idle losses, pressure relief protection.
• Advantages: compact design, low complexity, fast response, service-friendly setup with easily diagnosable states.
• Limitations: parallel movements are only possible to a limited extent without additional flow-sharing measures; idle flow still induces some thermal load.

Operating principle and design of recirculating pump hydraulics

The pump (typically a fixed-displacement unit, often implemented as a gear pump) supplies a continuous flow. Through an open-center directional valve, the oil returns to the tank with a low pressure differential in neutral. When actuated, the recirculation channel is partially or completely closed; the flow is directed to the actuator, and the pressure rises to the level required for the work. A pressure relief valve protects the circuit against overpressure. Check valves, throttles, or flow dividers can be added to control movements or supply multiple actuators. In tools such as concrete demolition shears or stone and concrete splitters, the valve function is sometimes integrated in the power pack and sometimes in the tool, resulting in short response times and reproducible work cycles.

Dynamic behavior: the transition from recirculation to load is governed by the valve’s metering edges and the pump’s inherent stiffness. Smooth spool actuation and suitable damping prevent pressure spikes and jerky starts, while short hose lengths further reduce compliance and dead volumes.

Components at a glance

System performance is determined by the matching of pump, directional valve, protection devices, and return path. Consistent sizing across the chain avoids bottlenecks and unnecessary heating.

Pump and drive

Fixed-displacement pumps deliver a defined volume per revolution. In mobile hydraulic power packs, such as compact power units, they are usually driven electrically or by combustion engines. Key selection criteria include flow rate, maximum system pressure, efficiency, noise, and thermal behavior in recirculating idle operation.

• Ensure favorable suction conditions (short, large-diameter suction line, low inlet losses) to avoid cavitation and noise.
• Match motor power to the product of maximum pressure and flow at duty-relevant operating points; consider duty cycle and altitude/ambient conditions.
• Where appropriate, multi-pump groups can separate auxiliary and working functions without compromising the recirculating principle.

Directional valve and control

Open-center directional valves form the heart of recirculating control. They allow low-pressure return flow in idle and connect the actuator when actuated. Depending on the tool, additional functions such as load-holding or lowering brake valves can be integrated to hold loads safely or enable precise motion.

Center configuration (true open center versus open center with float), actuation type (manual, electric, hydraulic), and metering characteristics (progressive or linear) influence controllability and heat balance. Integrated load-holding valves should be matched to the expected peak pressures and thermal expansion.

Overpressure protection and return system

A correctly sized pressure relief valve protects the system without converting unnecessary energy into heat. The return line should be designed for favorable flow to minimize losses and oil foaming. Efficient filtration in the suction and/or return lines maintains the cleanliness class, increasing the service life of pumps, valves, and cylinders.

• Set the relief valve with a defined margin above required working pressure but below component ratings; verify stability across temperature.
• Dimension the return filter and its bypass so that back pressure remains low; avoid continuous operation with an open bypass.
• Provide air separation in the tank (flow calming, sufficient dwell time) to reduce foaming and aeration.

Hydraulic design for concrete demolition shears and stone and concrete splitters

Flow rate and working speed

The working speed of a tool depends directly on the flow rate. In concrete demolition shears, it influences opening and closing of the jaws as well as cycle times. In stone and concrete splitters, it determines the speed of the spreading cylinder up to the point where splitting pressure is reached. Oversizing can cause unnecessary heating, while undersizing extends cycles and reduces productivity.

As a rule of thumb, actuator speed scales with v ≈ Q/A (flow over effective area). For fast approach and controlled work stroke, staged metering or separate circuits can be advantageous.

Pressure level and force requirement

The required force results from pressure times effective piston area. Cutting and splitting processes require high pressures to sever or fracture brittle materials. Typical designs operate in the range of several hundred bar. Adequate safety margins are important so the system can withstand load spikes when a concrete demolition shear bites in or when the splitting wedge is set.

Account for area ratios of differential cylinders, potential pressure intensification in blocked volumes, and the strength of hoses, fittings, and couplings. Elastic compliance in lines can temporarily store energy and should be considered in safety assessments.

Hoses, couplings, and pressure losses

Line cross-sections, couplings, and hose lengths have a noticeable influence on pressure drop and heating. Short, large-diameter lines with flow-optimized couplings reduce losses. Clean couplings and dust-protected parking positions contribute to system cleanliness and prevent malfunctions in valves and seals.

• Target oil velocities: approximately 2 to 3 m/s in return lines and 3 to 5 m/s in pressure lines for compact mobile circuits.
• Minimize sharp bends and unnecessary adapters; each transition adds local losses and potential leak points.
• Use abrasion protection and proper clamp spacing to prevent hose chafing and vibration-induced failures.

Applications and typical workflows

In concrete demolition and special demolition, recirculating pump hydraulics enables fast, finely metered opening and closing of concrete demolition shears. In gutting works and cutting, it supports repetitive cycles with stable repeatability. In rock breakout and tunnel construction, stone and concrete splitters benefit from the direct pressure buildup when transitioning from low-pressure recirculation to the working circuit. In natural stone extraction, the robust recirculating control allows long shifts with consistent performance. In special operations – for example with limited energy supply – the simple design of recirculating pump hydraulics is an advantage for integration into compact hydraulic power packs from Darda GmbH.

• Typical cycle for shears: approach in recirculation with low resistance, controlled closing under load, brief pressure hold if required, then release to open center.
• Typical cycle for splitters: fast insertion and spreading up to contact, build pressure to splitting threshold, fracture propagation, depressurize and retract.

Thermal management, oil, and filtration

Viscosity and temperature window

The oil viscosity must remain within the recommended range across the entire operating envelope. Oil that is too thin degrades lubrication and increases leakage losses; oil that is too viscous reduces volumetric efficiency and prolongs start-up phases.

• Aim for operating oil temperatures that keep viscosity in the mid-range of the fluid’s specification; typical targets are around 40 to 60 °C depending on the oil grade.
• Select oil with a suitable viscosity index for the expected ambient spread; review compatibility with seal materials.

Cooling strategies in recirculating operation

Although little pressure heat is generated in idle, flow-induced heat is still present. Heat is dissipated via tank surface, lines, or integrated coolers. Operation adapted to ambient conditions – such as breaks, shutting down at standstill, or variable-speed drives – limits oil temperature.

Consider thermostatically controlled cooler circuits, sufficient airflow around the power pack, and contamination monitoring on cooler fins. Where noise is critical, low-speed fans with appropriate shrouding can balance cooling performance and acoustic emissions.

Filtration and cleanliness classes

Fine filtration in the return line and suitable suction protection prevent particle ingress. Especially with frequent tool coupling, as is common with concrete demolition shears and stone and concrete splitters, a consistent cleanliness strategy pays off.

• Align cleanliness targets with component sensitivity (for example, servo-grade valves versus robust gear pumps) and reference common cleanliness codes.
• Use dirt caps and clean work practices at quick couplings; avoid opening the circuit in dusty environments whenever possible.

Control variants and energy efficiency

Even with the recirculating principle, efficiency potential can be leveraged: pressure-compensated flow control valves for uniform speeds, location-appropriate motor power, variable-speed drives to reduce idle load, or priority valves for defined function sequences. The goal is to build pressure only when work is actually performed and otherwise run in recirculation with favorable flow.

• Unloading logic: where practical, bypass additional throttling elements in neutral to keep idle losses minimal.
• Standby concepts: reduce motor speed or switch off at longer pauses to contain temperature and noise while preserving readiness.
• Function sequencing: simple logic elements (sequence or priority valves) can stabilize repeatability without complex control systems.

Occupational safety and normative guidance

Hydraulics operate at high pressure. Lines, couplings, and valves must be inspected regularly; damaged components must be replaced immediately. When working with concrete demolition shears and stone and concrete splitters, crushing and splinter hazards must be considered. Personal protective equipment, safe placement of the hydraulic power pack, depressurizing before coupling, and compliance with applicable state-of-the-art rules are fundamental measures. Notes on inspection intervals, documentation, and commissioning should generally be observed without assessing the individual case.

• Secure hoses against whip, shield potential burst zones, and maintain safe distances during pressure tests.
• Implement lockout procedures for maintenance, including verified depressurization of both pressure and return lines.
• Provide clear labeling of pressure, return, and any auxiliary connections to prevent misconnections.

Maintenance, servicing, and troubleshooting

Inspection priorities

Oil level and condition, filter loading, tightness, hose condition, coupling cleanliness, pump noises, and temperature development are key checkpoints. Before events such as prolonged downtimes or transports, a renewed visual and functional inspection is recommended.

1. Perform brief functional checks after filter or oil changes to verify correct flow direction and relief settings.
2. Document pressure and temperature at representative operating points to establish trend baselines.

Typical symptoms and possible causes

• Slow movement: insufficient flow, clogged filters, throttle points, oil too viscous.
• Excessive heating: high recirculation losses, tank too small, contaminated cooler surfaces, valve slightly open continuously.
• Pressure drop under load: leaking lines, internal leakage at cylinders/valves, pressure relief set too low.
• Pulsations and noise: air ingress, pump cavitation, unfavorable suction conditions.
• Jerky movements: contaminated valves, missing pressure compensation, fluctuating viscosity due to temperature.
• Tool drift or unintended motion: leakage in load-holding valves, spool not centered, external load effects.
• Foaming oil: air entrainment through suction leaks, over-agitated return, insufficient tank dwell time.

Comparison with constant-pressure and load-sensing systems

Compared to constant-pressure systems, recirculating pump hydraulics avoids high pressure-holding losses in idle. In comparison with load-sensing systems, the design is simpler and particularly service-friendly in compact power packs. For applications with single functions, repeatable cycles, and short line runs – as with concrete demolition shears or stone and concrete splitters – it is a proven choice. For complex multiple functions or parallel movements, pressure- or consumer-oriented control can offer advantages.

In addition, recirculating systems typically have lower part counts, straightforward diagnostics, and predictable acoustic behavior at idle, whereas consumer-oriented controls can provide superior multi-function coordination and partial-load efficiency in complex machinery.

Planning and integration into mobile hydraulic power packs

For integration into hydraulic power packs from Darda GmbH, it is advisable to clearly assign flow rate and pressure to the respective tools used. If several tools are operated sequentially, changeover or selector valves with clearly defined neutral positions are useful. For simultaneous functions, sufficient flow reserves and suitable priority or flow-dividing elements should be planned. Transport, setup, and hose routing should be chosen so that the recirculation line remains short and low-pressure, and the power pack receives sufficient fresh air for cooling.

• Standardize quick couplings and label circuits consistently; verify pressure rating and flow capacity of all connectors.
• Keep the tank above the pump inlet where possible to support inlet conditions; avoid unnecessary elevation differences in suction lines.
• During commissioning, confirm relief settings, verify open-center neutral flow, and record baseline temperatures under representative duty.