Seal

Seals are unobtrusive yet central components of high-pressure hydraulics in demolition and extraction technology. They keep hydraulic oil where pressure work is performed, protect moving parts from dirt and moisture, and ensure reproducible force transmission. In applications such as concrete demolition shears and hydraulic rock and concrete splitters, the quality of the sealing determines function, service life, and safety-from hydraulic power units, cylinders, and valve blocks to hoses and fittings. Optimized sealing concepts reduce leakage, stabilize performance over the entire duty cycle, and contribute to environmental protection through minimized oil loss.

Definition: What is meant by a seal?

A seal is a component or system that prevents the unwanted escape or ingress of fluids, gases, or particles. In hydraulics this includes dynamic seals (for example on rods and pistons) and static seals (for example between housing parts). The goal is a controlled sealing effect under defined movements, pressures, and temperatures-with the lowest possible friction and minimal wear. Performance is assessed by leakage behavior, friction level, and wear rate across typical load collectives.

Design and function of seals in hydraulics

Hydraulic sealing systems often consist of matched elements: primary and secondary seals for the pressure side, wipers against dirt ingress, guide rings to absorb transverse loads, and back-up rings to prevent gap extrusion. In the cylinders of rock splitters, concrete demolition shears, or combination shears these components work together to convert pressure into linear or shearing force. Proper sequencing of elements and avoidance of trapped intermediate volumes prevent pressure build-up between seals.

Seal types at a glance

• O-rings and X-rings for static and moderately dynamic sealing
• Rod seals (lip, U-, and compact rings) for reciprocating motions
• Piston seals (single- and double-acting) for pressure separation in the cylinder
• Wipers (single/double) against dust, mud, and splash water
• Guide rings for guidance and to prevent metal-to-metal contact
• Flat gaskets and molded seals on housings, covers, and valve blocks
• Rotary seals at oscillating or rotating interfaces
• Chevron or V-packings for high-pressure, heavy-duty applications
• Bonded seals for screw connections and hydraulic fittings

Materials and resistance

Common materials are NBR/HNBR for mineral hydraulic oils, FKM for elevated temperatures and chemically demanding media, PTFE for low friction and high sliding speeds, and PU for robust, wear-resistant lip seals. Fabric-reinforced composite materials are used for guide and back-up rings. For water-glycol or phosphate-ester media, EPDM or special FKM grades may be required; for low-temperature flexibility VMQ can be considered. Compatibility with biodegradable ester-based fluids and resistance to hydrolysis, abrasion, and ozone must be evaluated. Selection depends on medium, temperature, pressure, speed, and environmental conditions on the construction site.

Seals in products of Darda GmbH

In the hydraulic systems of Darda GmbH, seals assume different functions along the force chain depending on the product family-from the power pack through lines to the tool head.

• Hydraulic splitters: Cylinder seals must safely handle high-pressure spikes and alternating loads with shock-like stresses; wipers protect against abrasive particles from rock and concrete.
• Concrete demolition shears: Rod and piston seals ensure sensitive, precise jaw movement; low friction supports controlled closing and opening during deconstruction.
• Hydraulic power packs: Static seals on pumps, valve blocks, and fittings prevent leakage; media- and temperature-resistant materials are crucial.
• Combination shears, multi cutters, steel shears, Tank Cutter: Dynamic seals on actuators and slewing points operate under varying load spectra; good wipers minimize dirt ingress.
• Splitter cylinders: Seal systems reliably separate the pressure and return sides and keep the splitting force stable, even with varying split strokes.
• Hoses and fittings: Gasketed connectors rely on intact sealing washers and correct tightening torques to avoid weeping and micro-leaks.

Influence of application areas on sealing

The operating environment decisively shapes the design and material selection of the seals.

• Concrete demolition and special deconstruction: Cement dust and rebar abrasion demand effective wipers and robust guides; cyclic pressure changes foster gap extrusion, which is limited by back-up rings.
• Strip-out and cutting: Frequent load changes and precise movements require low-friction seals with good stick-slip behavior for finely metered force transmission.
• Rock breakout and tunnel construction: Moisture, slurries, and temperature fluctuations promote corrosion and media changes; water-resistant materials and double wipers are advantageous.
• Natural stone extraction: High abrasiveness due to rock flour requires wear-resistant wipers and adapted surface roughness on rods and guides.
• Special operations: Extreme temperatures, special media, or long standstill periods require adapted materials and preservation measures.
• Cold-start at low ambient temperatures: Elastomer hardening increases breakaway forces; suitable compounds and start-up procedures mitigate effects.

Design: groove geometry, tolerances, and surfaces

Seal performance depends on correctly dimensioned grooves, suitable installation squeeze, and controlled clearance gaps. Rod and cylinder bore surfaces must exhibit seal-matched roughness and bearing ratios to limit wear, micro-leakage, and stick-slip. Back-up rings reduce gap extrusion at high pressures. In addition, controlling ovality and concentricity, rounding edges with adequate radii, and documenting tolerance stack-ups improves reproducibility and service life.

Friction, tightness, and energy efficiency

There is a trade-off between tightness and friction. Excessive interference increases sealing but also raises friction and temperature. Friction-optimized materials (for example PTFE compounds) and tuned spring or lip geometries help balance efficiency and tightness. Micro-textured sealing lips, pressure-relief notches, and optimized lubrication regimes reduce breakaway forces and energy consumption in continuous operation.

Managing pressure spikes

Rapid pressure rises can damage sealing lips and cause oil film collapse. Low-surge valving, adequate support of the seal, and avoidance of sharp edges in transitions improve safety. Accumulators, damping orifices, and flow-optimized lines lower local peaks and protect sealing systems during shock events.

Assembly and installation practice

Careful assembly is crucial for seal service life-especially on cylinders of concrete demolition shears and hydraulic splitters.

1. Break and deburr edges; provide lead-in chamfers.
2. Ensure a clean, particle-free environment; clean components.
3. Use compatible assembly and lubricating agents; avoid media mixing.
4. Use installation tools; avoid twisting and damage.
5. Check installation position; verify correct seating of back-up and guide rings.
6. Vent the system gently; monitor the first pressure cycles.
7. Orient sealing lips toward the pressure side; confirm correct preload.
8. Precondition materials as specified, for example gently warm elastomers or size PTFE rings before assembly.
9. Record batch codes and torque values to ensure traceability of critical joints.

Maintenance, inspection, and replacement

Coordinated maintenance routines prevent consequential damage and unplanned downtime.

• Visual inspection: Detect oil film, dirt deposits, damaged wipers.
• Functional check: Smooth, jerk-free movements, constant holding forces, no drift.
• Observe pressure and temperature behavior; note unusual heating.
• Replace seals in good time based on operating hours, cycles, or condition indicators.
• Store spare parts cleanly and protect them from UV/ozone.
• Monitor filter differential pressures and cleanliness targets; adjust service intervals accordingly.

Hydraulic fluid, filtration, and cleanliness

Compatibility between seal material and hydraulic fluid is fundamental. Particles and water content act as wear accelerators; effective filtration and regular oil maintenance are therefore essential.

• Select media with an eye on temperature window and chemical resistance.
• Design fine filtration to suit the application and flow rate.
• Perform regular oil condition analyses to detect aging and ingress early.
• Establish clean assembly and filling processes.
• Define cleanliness classes and moisture limits, for example by particle coding and saturation index, to sustain seal performance.

Typical damage patterns and causes

• Extrusion cracks: Excessive clearances or missing back-up rings at high pressures.
• Lip abrasion: Mismatched roughness, dirt ingress, incorrect lubrication regime.
• Thermal damage: Excessive friction, inadequate heat dissipation.
• Chemical swelling or embrittlement: Media incompatibility, additive effects.
• Ozone and UV cracking: Improper storage, weathering effects.
• Twisted O-rings: Incorrect assembly, unsuitable groove geometry.
• Compression set: Permanent deformation due to heat and sustained compression over time.
• Hydrolysis in PU: Water contamination at elevated temperature weakens material structure.

Diagnostic notes

Seal damage patterns allow conclusions about the root cause. A systematic analysis of installation situation, operating parameters, and oil condition supports the selection of longer-lasting sealing concepts. Photographic documentation and microscopy of fracture or wear zones facilitate reliable attribution of failure mechanisms.

Storage and handling of seals

Unused seals should be stored cool, dry, protected from light, and without stress. Avoid contact with plasticizers, solvents, or ozone-laden air. Original markings ensure traceability and material identification. Implement first-in-first-out handling and keep items in sealed, labeled packaging until use.

Safety and environmental aspects

Hydraulic leaks impair occupational safety and the environment. Targeted sealing, regular inspections, and immediate remediation of leaks reduce risks. Follow the guidance in the operating manual and maintenance documentation; in case of doubt, qualified specialists should be consulted. Use drip trays and spill kits where appropriate and comply with applicable environmental regulations.

Practical relevance: concrete demolition and rock breakout

During deconstruction of reinforced concrete, a concrete demolition shear must grip repeatedly and close in a controlled manner. Low-friction rod seals and intact guide rings stabilize the motion, while wipers keep cement dust away. In natural stone extraction, fine abrasive particles act on splitter cylinders; double wipers, suitable materials, and clean assembly increase service life. After impact-intensive operations, wiper and rod surface condition should be checked to prevent accelerated lip wear.

Quality requirements and standard references

Recognized standards and guideline sets exist for O-rings, groove geometries, and surfaces. In practice, a combination of suitable tolerances, appropriate materials, and documented testing processes proves effective. For Darda GmbH the following applies: Only the sum of seal design, manufacturing quality, and care in use ensures the lasting performance of hydraulic splitters, concrete demolition shears, and other hydraulic tools. Reference frameworks such as ISO specifications for O-rings and housings (for example ISO 3601 and ISO 7425) or rotary sealing principles support interoperable design and verification without prescribing specific products.