Plastic seals are central building blocks of modern hydraulic systems. In tools and attachments for concrete demolition, special deconstruction, rock demolition and tunnel construction, they maintain pressure, minimize friction and protect against ingress of dirt. Especially in the cylinders of concrete demolition shears as well as in rock and concrete splitters from Darda GmbH, seals made of elastomers and thermoplastics act as precise barriers between hydraulic fluid, environment and moving components. Careful selection and design increase service life, operational safety and availability—without losing sight of efficiency and environmental compatibility.
Definition: What is a plastic seal
A plastic seal is a sealing element made of polymer materials—typically elastomers (e.g., NBR, HNBR, FKM, PU elastomers) or thermoplastics (e.g., PTFE, POM)—that prevents the passage of liquids, gases or particles. In hydraulics, a distinction is made between static seals (such as O-rings in housing joints) and dynamic seals (e.g., rod and piston seals in cylinders). A plastic seal operates as a system comprising material, profile, groove geometry, counterface and lubricant film. In concrete demolition shears and rock and concrete splitters from Darda GmbH, seals must withstand high pressures, shock loads, abrasive particles and changing temperatures without hardening, swelling or extruding.
Material types and properties
The choice of sealing material depends on medium, temperature, pressure, speed, clearances and environmental conditions. Elastomers provide elastic recovery and good tolerance compensation, while thermoplastics deliver low friction and high extrusion resistance. Material combinations are common, such as PTFE lip seals with an elastomer preloading element or backup rings made of POM/PEEK to bridge gaps.
Elastomers for hydraulics
- NBR (acrylonitrile butadiene rubber): proven in mineral oils; good sealing performance, well-balanced price-performance ratio; limited temperature and ozone resistance.
- HNBR: increased temperature and media resistance compared to NBR; suitable for higher continuous loads and fluctuating pressures.
- FKM (fluoroelastomer): very good resistance to high temperatures and many oils; low gas permeation; comparatively high cost, lower cold flexibility.
- PU elastomers (polyurethanes): high abrasion resistance, good extrusion resistance; suitable for dynamic rod and piston seals; note hydrolysis resistance depending on formulation.
- EPDM: good in aqueous media; generally not suitable in mineral oil hydraulics—always check media compatibility.
Thermoplastics and composite solutions
- PTFE: extremely low friction, wide temperature range, very good chemical resistance; tends to cold flow, therefore often combined with preloading elements.
- POM and PEEK (as backup rings): increase extrusion resistance, stabilize gap areas at high pressures and temperatures.
- TPE/PA compounds: modern compromise between dimensional stability and elasticity for specific profiles.
Friction, sealing gap and service life
Friction determines response behavior, energy demand and thermal balance. A balanced sealing gap reduces leakage without excessive wear. Material, profile and groove geometry must be matched to avoid stick-slip, compression set and abrasion.
Seal types in hydraulic cylinders of concrete demolition shears and rock and concrete splitters
- Rod seals (U-cups, lip seals): seal to the outside; crucial against leakage and ingress of dirt.
- Piston seals: separate the pressure chambers; high dimensional stability and extrusion resistance required.
- Wipers: keep dust, mud and debris away from the sealing system—essential in concrete demolition and tunnel construction.
- O-rings/X-rings: static sealing of covers, flanges, threaded connections; also as preloading elements in PTFE profiles.
- Rotary seals/shaft seals: on pump or motor shafts in hydraulic power packs.
- Backup rings: secure seals against extrusion in large gaps or pressure spikes.
- Guide rings (not seals, but system-relevant): guide piston/rod and protect seals from transverse forces.
Factors influencing sealing performance
- Pressure and pressure spikes: impact-like load changes in special deconstruction require extrusion-resistant profiles and suitable backup rings.
- Temperature: cold starts, summer heat, self-heating due to friction—consider material flexibility and compression set.
- Medium: mineral oils, water-containing fluids or rapidly biodegradable hydraulic oils—check media compatibility.
- Speed/stroke: influences lubricant film and heat; too little movement can lead to starved lubrication.
- Surfaces: appropriate roughness and hardness of counterfaces prevent abrasion and lip wear.
- Contamination: cement dust, rock particles, metal chips; robust wipers and good filtration are key factors.
- Clearances/tolerances: properly designed grooves and guides minimize leakage and extrusion.
- Assembly quality: damage during installation is a common cause of leakage.
Design, groove geometry and surfaces
Groove geometry determines preload, stability and leakage behavior. Rounded edges, defined lead-in chamfers and suitable groove widths prevent edge tearing and overcompression. Counterfaces should be uniformly and finely machined so that a load-bearing lubricant film can form. In cylinders of concrete demolition shears and rock and concrete splitters, a robust design of the rod seal with a high-performance wiper is worthwhile to keep particles out.
Gap control and extrusion safety
Large gaps between moving components increase the risk of extrusion. Backup rings made of POM/PEEK or hard PU compounds reduce this risk. In pressure spikes, stepped seal profiles help to dissipate pressure gradually.
Installation, maintenance and replacement
- Clean components, remove burrs and sharp edges; deburr and chamfer installation edges.
- Work only with medium-compatible assembly oil or grease; evenly wet the sealing lips.
- Insert seals without stress, do not twist; use auxiliary tools with smooth surfaces.
- Observe installation orientation (pressure side to the correct side); orient wipers correctly.
- After assembly, perform functional and leak testing with a slow pressure ramp.
Preventive measures in concrete demolition
- Regularly inspect wipers and rod seals; replace early in case of severe abrasion.
- Keep hydraulic oil clean: filtration and oil conditioning reduce wear particles.
- For continuous dust exposure, consider double-acting wipers and dirt-resistant materials.
Hydraulic power packs: static plastic seals
In hydraulic power packs from Darda GmbH, O-rings, flat gaskets and shaft seals seal housings, fittings and shafts. Important aspects are compression set (permanent deformation), cold flexibility and media resistance. At low temperatures, elastomers shrink, which can promote leakage; appropriate material selection and defined preload compensate for this.
Environmental conditions in tunnel construction and special deconstruction
Wet environments, alkaline concrete slurries and sharp-edged particles place heavy stress on seals. High-performance wipers, hydrolysis-resistant PU seals and low-wear PTFE profiles increase service life. In rock demolition and natural stone extraction, shocks and transverse forces act—guide rings protect the sealing lips from tilting.
Failure patterns and root cause analysis
- Abrasion of the sealing lip: rough counterfaces, particles; remedy: surface optimization, better wipers, filtration.
- Extrusion/nibbling: excessive gaps, high pressure spikes; remedy: backup rings, harder materials, reduce gap.
- Compression set: permanent deformation; remedy: suitable elastomer material, temperature management.
- Chemical attack/swelling: incompatible medium; remedy: media check, alternative materials.
- Thermal cracks/overheating: friction too high; remedy: low-friction profiles (e.g., PTFE composites), ensure lubrication.
- Installation damage: cut sealing lips; remedy: installation aids, chamfer edges, correct tools.
Sustainability, safety and legal aspects
Tight hydraulics reduce oil losses, protect soil and lower operating costs. In sensitive areas, it may be sensible to use media-compatible seals with rapidly biodegradable hydraulic fluids. Safety-relevant sealing points should be checked regularly. Legal requirements on emissions and water protection depend on location; compliance is the operator’s responsibility, and technical information is always general and non-binding.
Testing and quality assurance in operation
- Pressure and leak tests after assembly and at intervals.
- Visual inspection for scoring, hardening, crack formation on sealing lips and wipers.
- Oil analyses for particles and water content; document filter condition.
- Verify dimensional accuracy of groove and counterface during overhaul.
Relation to other products and application areas
In addition to concrete demolition shears as well as rock and concrete splitters, combination shears, multi cutters, steel shears, tank cutters, rock splitting cylinders and hydraulic power packs from Darda GmbH also benefit from durable plastic seals. The requirements are similar: high pressure fluctuations, abrasive media, varying temperatures and the need to balance leakage, friction and wear. A material- and application-oriented seal selection supports process reliability in gutting and cutting, concrete demolition and special deconstruction, rock demolition and tunnel construction as well as in special operations.