The term injection pressure describes the local pressure acting at the contact surface between tool and workpiece when hydraulic forces are transferred onto a limited area. In the practice of demolition, deconstruction, and rock excavation, injection pressure largely determines whether concrete breaks in a controlled manner, steel is separated, or natural stone splits along existing planes of weakness. For applications by Darda GmbH—such as with concrete demolition shears or rock and concrete splitters—the safe determination, adjustment, and monitoring of this pressure is crucial for efficiency, precision, and component preservation.
Definition: What is meant by injection pressure
Injection pressure is the contact pressure p between an acting force F and the effective contact area A (p = F/A). It occurs where jaws, wedges, blades, or shears transfer load into the workpiece. Injection pressure is not identical to the system or hydraulic pressure in the power unit; it is the result of force transmission (cylinder area, lever kinematics, wedge geometry) and the real, often dynamically changing contact area. In concrete, sufficient injection pressure leads to microcracks, crack initiation, and controlled separation. In steel it determines whether plastic flow begins and a shear or separating cut succeeds. In rock, injection pressure induces radial tension around the borehole that is used for splitting.
Physical fundamentals and distinction from system pressure
Hydraulic power units generate system pressure, which is translated into forces via cylinders. The resulting force acts through tool geometries (jaw levers, wedge faces, cutting edges). Decisive is the local contact pressure on the workpiece: it depends on the real contact (roughness, positive fit, edge radius) and load distribution during deformation. The smaller the effective contact area, the higher the injection pressure—up to the point where material limits, safety requirements, or tool wear impose constraints.
Injection pressure in application: concrete demolition shears and stone and concrete splitters
Across the Darda GmbH portfolio, different tools rely on injection pressure as their operating principle. Two central examples are concrete demolition shears as well as stone and concrete splitters; in addition there are combi shears, multi cutters, steel shears, tank cutters, and special stone splitting cylinders.
Concrete demolition shears
Concrete demolition shears transfer hydraulic force through jaws into reinforced concrete. Serrated contact surfaces concentrate injection pressure to crack the concrete cover and expose the reinforcement. Balancing sufficiently high contact pressure with the protection of adjacent components is essential, for example in concrete demolition and special deconstruction or during strip-out and cutting.
Stone and concrete splitters
Splitters use wedges and counter-wedges to apply injection pressure in boreholes to build radial tensile stresses. Critical factors include wedge angle, lubrication, borehole diameter and depth, as well as the homogeneity of the material. In rock excavation and tunnel construction and in natural stone extraction, precise injection pressure enables controlled, low-vibration rock removal.
Other tools
Combi shears, multi cutters, steel shears, and tank cutters apply injection pressure at cutting edges or pressing faces to shear or separate cross-sections. Stone splitting cylinders focus pressure in linear contact zones, for example in natural stone sizing or special operations.
Influencing variables on injection pressure
- Tool kinematics: cylinder area, lever ratio, and wedge geometry determine force transmission.
- Contact area: tooth geometry of the jaws, edge radii, material pairing, and surface condition govern the real pressing area.
- Material properties: compressive strength of concrete, tensile strength, fracture toughness; yield strength and toughness of steel; anisotropy and jointing in natural stone.
- Hydraulic conditions: system pressure, oil temperature, viscosity, pressure loss in lines, response time of the power unit.
- Borehole quality (splitters): diameter, roundness, spalling, axis deviation, depth, moisture.
- Environmental factors: temperature, moisture, chemical influences (e.g., chloride-contaminated environment) that change material behavior and friction.
- Wear: wear of teeth, wedges, and cutting edges enlarges the contact area and lowers injection pressure.
From hydraulic pressure to contact pressure: estimation and calculation
The chain is: hydraulic pressure → cylinder force → force at the tool tip → injection pressure at the contact area. Approximation: p = F/A. For concrete demolition shears, a jaw force in the range of several hundred kN can act on a few square centimeters, resulting in contact pressures that exceed local concrete tensile strength and trigger cracking. In splitters, injection pressure at the wedge rises as contact area decreases and friction is reduced; the resulting radial tension around the borehole exceeds the tensile strength of concrete or natural stone and leads to splitting.
Practical note on dimensioning
For concrete: the local injection pressure must be sufficient to create the combination of tensile and shear stresses needed for crack initiation, without unnecessarily introducing high loads into adjacent components. For steel, contact pressure must reach the yield point to initiate a shear fracture. Gradual pressure increase and monitoring of crack sounds, split progression, and tool travel are proven practices.
Measurement, monitoring, and calibration
Injection pressure is rarely measured directly. In practice, system pressure and tool travel are recorded and, via known transmissions, used to infer contact pressure. A reliable measurement chain from the hydraulic power pack to the tool tip is important.
- Define measuring points: pressure gauge or sensor at the hydraulic power pack, optionally additional acquisition near the cylinder to reduce the influence of line losses.
- Calibration: regular verification of sensors according to manufacturer specifications; comparative measurements with reference gauges.
- Documentation: record pressure profile, temperature, tool condition, and material description for traceability.
- Indicators: tool stroke, closing speed, acoustic signatures (crack formation) as qualitative markers of contact pressure.
Injection pressure by field of application
Concrete demolition and special deconstruction
Concrete demolition shears use concentrated pressure to break cover concrete and expose reinforcement. The goal is controlled removal with minimal secondary damage. Stone and concrete splitters enable selective deconstruction of massive components where vibration and noise control are required.
Strip-out and cutting
In strip-out, combi shears and multi cutters are used, where injection pressure at the cutting edges is decisive. Precise settings reduce deformation of adjacent structures and facilitate clean separation by type.
Rock excavation and tunnel construction
Splitters and stone splitting cylinders generate high injection pressure via wedges in boreholes. In homogeneous rock this enables controlled break-out without blasting, which can be advantageous especially in tunnel construction.
Natural stone extraction
Targeted application of injection pressure along natural joints allows gentle splitting of large blocks. Suitable borehole spacing and wedge geometry reduce scrap and improve block quality.
Special application
For tank cutters and steel shears, injection pressure, cutting-edge geometry, and material condition (e.g., coatings, residual stresses) must be carefully matched to achieve controlled separating cuts with minimal spark generation.
Best practices for setting injection pressure
- Material analysis: concrete grade, reinforcement ratio, moisture; for natural stone, jointing and grain bonding; for steel, strength class and wall thickness.
- Tool inspection: condition of jaw teeth, wedges, cutting edges; lubrication on splitting wedges; correct seating of pins and mounts.
- Hydraulic check: system pressure per specification, oil temperature, filter condition, check hose lengths and diameters for pressure loss.
- Contact optimization: clean, properly aligned seating of the jaws; correct drilling pattern for splitters (diameter, depth, axis accuracy).
- Incremental load increase: raise pressure in steps, observe crack progression, hold load, increase again until separation initiates.
- Follow-up and relief: reposition the tool during splitting; with shears, cyclically open/close to propagate the crack and manage component stresses.
Common failure patterns, causes, and remedies
- Insufficient separation despite high system pressure: effective contact area too large (worn teeth/wedges)—regrind/replace tools, improve seating.
- Lateral spalling in concrete: injection pressure too concentrated near edges—correct jaw position, increase pressure more slowly, increase distance to the edge.
- Wedge jams in the borehole: friction too high or borehole out of tolerance—clean, lubricate, ensure correct diameter and roundness.
- Excessive heating and slow stroke: viscosity/temperature issue or pressure loss—check oil temperature, inspect filters/hoses, adapt power unit performance.
- Delayed crack progression: unsuitable drilling pattern or reinforcement blocking—adjust borehole spacing, account for reinforcement layout.
Material- and geometry-driven aspects
Concrete compressive strength influences how high the injection pressure must be to initiate cracking. Tensile strength and fracture toughness govern crack growth. For steel, yield strength and impact toughness are relevant; sharp cutting edges concentrate injection pressure and reduce shear forces. In natural stone, anisotropy leads to direction-dependent requirements for contact pressure; joints promote splitting at lower pressure.
Resource efficiency, emissions, and component preservation
Correctly set injection pressure reduces energy demand, limits tool wear, and lowers noise, dust, and vibrations. Especially in urban deconstruction and on sensitive structures, targeted contact pressure from concrete demolition shears and wedge-driven pressure from stone and concrete splitters can reduce secondary damage.
Safety and general legal notes
Safe work with hydraulically generated injection pressure requires appropriate personal protective equipment, proper setup of the equipment, and compliance with applicable regulations. Load limits of tools and components must not be exceeded. Inspection and maintenance intervals must be observed; documenting pressure settings and work processes supports verification. Legal requirements may vary regionally; adherence to local regulations and recognized rules of engineering practice is generally required.