Sealing ring

Sealing rings are unobtrusive but central to the safety, efficiency, and service life of hydraulic tools and power units. In applications such as concrete crushers, rock and concrete splitters, stone splitting cylinders, and hydraulic power units from Darda GmbH, they hold pressure, separate media, and protect against dirt and moisture. Especially in concrete demolition, strip-out, rock demolition, or tunnel construction, the right sealing ring determines precise force transmission, controlled movements, and minimal downtime. In practice, well matched sealing systems reduce micro leakage, stabilize cycle times, and help protect bearings and guides from abrasive contamination.

Definition: What is meant by a sealing ring?

A sealing ring is an annular component used to seal joints, bores, and shafts against liquids or gases. In hydraulic systems it seals static connections (e.g., threaded joints, cylinder heads) or dynamic movements (e.g., piston and rod movements). Sealing rings work through positive fit, preload, and media support: the pressure exerted by the medium presses the ring into its sealing surfaces and thereby increases the sealing effect. Depending on the task, cross-sections can be round (O-ring), X-shaped (quad ring), or lip-shaped for dynamic duties. Compression set resistance and recovery behavior are decisive for long-term tightness. In many tools from Darda GmbH, sealing rings are part of a matched sealing kit, which often consists of a primary seal, wiper, back-up ring, and guiding elements for reliable interaction under alternating loads.

Construction, materials, and operating principle

Sealing rings are made of elastomeric or thermoplastic materials such as NBR (nitrile rubber), HNBR, FKM, PU (polyurethane), or PTFE-based blends. Selection and geometry are determined by pressure, temperature, motion, medium, and clearance gaps. The operating principle combines the preload in the groove with pressure energization by the medium: as system pressure rises, the sealing ring conforms more firmly to the sealing surfaces without generating unnecessarily high friction. Material specifics influence durability and friction:

• NBR: robust with standard hydraulic oils and good low-temperature flexibility within moderate ranges.
• HNBR or FKM: elevated temperature and chemical resistance where thermal cycling or aggressive media occur.
• PU: high extrusion resistance and abrasion strength for dynamic rod and piston seals.
• PTFE blends: low friction and stick-slip resistance, often combined with elastomer energizers.

Sealing rings in the hydraulic systems of Darda GmbH

Hydraulic power packs deliver pressure and flow rate; tools such as concrete crushers, rock and concrete splitters, stone splitting cylinders, combination shears, multi cutters, steel shears, and tank cutters convert this energy into controlled motion and high cutting or splitting forces. In all these assemblies, sealing rings seal the pressure chambers, prevent leakage, and protect guides and sliding surfaces against wear. Coordinated sealing, wiper, and guide concepts improve energy efficiency, increase repeatability under alternating loads, and keep hydraulic oil cleanliness within target limits.

Functions and installation locations

• Piston seal: separates the pressure and return sides in the cylinder; determines holding force at standstill.
• Rod seal (U-cup): seals the moving piston rod to the outside; reduces oil loss.
• Wiper/scraper: prevents dirt ingress into the cylinder, especially important with concrete dust and rock particles.
• Static O-rings: seal housing covers, threaded joints, valve plates, and power pack segments.
• Radial shaft seals: seal rotating drive or pump shafts in hydraulic power packs.
• Back-up rings: limit extrusion under high pressures and large clearances.

Types of sealing rings and application limits

Depending on function and load, different geometries and materials are used. Operating limits arise from pressure, temperature, surface finish, and relative speed; friction and lifetime are influenced by the achievable lubrication film and the ring compound.

O-rings

Universal, cost-efficient round cross-section seals for static and moderately dynamic tasks. Common in hydraulic power packs and valve manifolds. Require defined groove geometries and suitable back-up rings at high pressures. For dynamic use, surface finish and squeeze must be tightly controlled to avoid heat build-up.

U-cups and rod seals

Asymmetric lip profiles for dynamic movements. Widely used in concrete crushers and stone splitting cylinders. Good sealing performance under alternating pressures; require adapted surface roughness of the rod. Variants with pressure-relief features reduce the risk of pressure entrapment between rod seal and wiper.

Piston seals

Single- or two-piece sealing systems, often with a PTFE glide ring and elastomer energizer. High dimensional stability and low friction; suitable for finely metered splitting operations and precise cutting cycles. The interaction with guide rings is critical to prevent edge loading and uneven wear.

Wipers/scrapers

Single- or double-lip designs. Essential in concrete demolition and tunnel construction to keep dust, sludge, and chips out and thus prevent consequential damage to the rod seal and guides. Metal-cased wipers can enhance mechanical stability in abrasive environments.

Radial shaft seals

Radial seals for rotating shafts in the power pack. Material selection is based on oil, temperature, and circumferential speed. Integrated dust lips are advantageous where contamination cannot be fully excluded.

Back-up and guide rings

Technically not seals, but decisive for the function of sealing rings: they prevent edge loading, reduce tilting, and stabilize piston and rod under asymmetric loads, as can occur during gripping, cutting, and splitting. Proper clearance control with back-up rings minimizes extrusion at pressure spikes.

Selection criteria for sealing rings

Proper design takes into account the operating parameters and environmental conditions of the respective Darda GmbH application:

• Medium: mineral oil, bio-based, or water-glycol fluids affect swelling behavior and chemical resistance.
• Temperature: ambient (e.g., winter service in tunnel construction) and process heat (long-lasting cycles) define the material range.
• Pressure and pressure spikes: choose clearance and back-up rings to prevent extrusion.
• Type of motion: stroke, speed, dwell times; the goal is low friction without stick-slip.
• Contamination: dust, sand, cement fines, moisture; specify wipers and double-lip systems.
• Mechanical load: side loads and tilting due to eccentric loads in concrete crushers and multi cutters.
• Maintenance strategy: availability of sealing kits, ease of assembly, and field inspectability.
• Energy balance: target low leakage with acceptable friction to maintain efficiency and thermal stability.
• Regulatory and environmental context: consider biodegradability of fluids and spill prevention measures for the site.

Installation, assembly, and surface requirements

Proper assembly is crucial for tightness and service life.

1. Prepare edges: deburr chamfers and radii, avoid sharp edges, reduce risk of cutting damage. Lead-in chamfers and protective assembly sleeves prevent lip nicks.
2. Surface quality: for dynamic seals, low roughness with a load-bearing profile to carry the oil film and avoid abrasion. Typical rod finishes are in the range of Ra 0.1 to 0.4 micrometers with isotropic texture.
3. Cleanliness: clean components, keep particles away, cover the assembly area. Avoid fiber shedding from cleaning cloths.
4. Assembly tools: use edge-friendly aids, assembly sleeves, and suitable assembly grease or compatible oil. Do not use greases that swell elastomers.
5. Orientation: orient sealing lips toward the pressure direction; ensure twist-free installation.
6. Initial run-in: vent the system without pressure, increase pressure stepwise, and inspect sealing points.

Groove and clearance dimensions

Groove width, groove diameter, and clearance must be matched to pressure, temperature, and material. Excessive clearances increase the risk of extrusion, while grooves that are too tight can complicate preload and assembly. As a guide, static O-ring squeeze typically ranges around 15 to 30 percent depending on hardness, while dynamic applications require lower squeeze and carefully limited radial clearance.

Maintenance, diagnostics, and servicing

In heavily loaded applications such as concrete demolition and specialized deconstruction, condition-based inspection of sealing points is advisable. Early indicators include damp spots, increased oil demand, altered motion profiles, or rising temperatures on cylinders and power packs. Thermal imaging, oil cleanliness monitoring, and leakage trending over time support proactive servicing.

Typical damage patterns

• Abrasion: dull, eroded sealing surfaces caused by particles; often due to insufficient wiper performance.
• Extrusion: material being squeezed into the gap under pressure spikes; remedy by using back-up rings or smaller clearances.
• Thermal damage: hardening, cracking; usually a result of overtemperature or an unsuitable lubrication film.
• Chemical degradation: swelling, tacky surfaces; indicates media incompatibility.
• Stick-slip: jerky movement at low speeds; requires optimization of material, surfaces, and lubrication.
• Compression set: permanent deformation after load; often linked to prolonged high temperature or excessive squeeze.

Service life and influencing factors

Service life results from material selection, surface condition, cleanliness of the medium, and the load spectrum. High side loads, frequent load changes, finely dispersed concrete dust, and pressure spikes from fast valve switching impose additional stress on seals. Clean hydraulic fluid, appropriate filtration fineness, and consistent use of wipers increase uptime in daily operation. Stable temperatures, correct rod finish, and verified alignment of guides further reduce wear and extend overhaul intervals.

Standards, dimensions, and tolerances

Many sealing rings follow common dimensional series and tolerance systems. Widely used standards and tolerance series exist for O-rings, as well as for radial shaft seals and guide rings. These specifications support interchangeability, but must be reconciled case by case with the specific groove geometry and the material-specific squeeze. Examples include established dimensional frameworks for O-rings and radial shaft seals; for piston and rod seals, housing dimensions are often standardized. Information on standards serves as guidance and does not replace application-specific design.

Safety and environmental aspects

Hydraulic systems are under pressure. Before working on sealing points, depressurize systems and secure against re-energizing. Accumulators and suspended loads can retain residual energy and must be made safe. Escaping oil must be captured and disposed of in an environmentally responsible manner; the handling of operating supplies follows the respective safety data and applicable regulations. Spill kits, suitable personal protective equipment, and clean working practices reduce environmental and occupational risks. The statements herein are general in nature and not tailored to individual cases.

Storage and handling of sealing rings

Store seals cool, dry, dark, and protected from ozone sources. Avoid contact with plasticizers, solvents, or sharp edges. Open packaging shortly before installation. Do not overstretch or kink elastomer seals; precondition PTFE rings before installation and place them stress-free. Use first-in first-out practices and keep labels with batch data for traceability.

Application areas: specifics and best practices

Concrete demolition and specialized deconstruction

High dust exposure, sharp particles, and impact-like load changes require robust wipers, abrasion-resistant rod seals, and stable guides. In concrete crushers, a combination of double-lip wiper and pressure-energized rod seal has proven effective. Back-up rings limit extrusion during pressure spikes. Additional metal-cased wipers and contamination barriers can further stabilize performance in abrasive, wet sludge.

Strip-out and cutting

Varying media exposure (moisture, corrosion particles), frequent load changes, and partly long partial-load phases argue for media-resistant materials and low-friction piston seals. In multi cutters and steel shears, stable guide rings secure the seal under lateral loading. Where fine control is required, low-stiction sealing stacks improve cut quality and repeatability.

Rock demolition and tunnel construction

Moisture, mud, and cold stress wipers and elastomers. Materials with an extended temperature window and good hydrolysis resistance are advantageous. In rock and concrete splitters and stone splitting cylinders, hefty load changes favor a robust seal-and-guide configuration. Protective bellows or additional scraper stages can be beneficial in extreme grit conditions.

Natural stone extraction

High cycle counts and varying splitting forces demand wear resistance and low friction. Low leakage keeps the force path stable and improves the reproducibility of splitting results. Monitoring oil cleanliness and seal wear indicators pays off in longer service intervals.

Practical design tips

• Material selection: NBR for standard hydraulic oils; HNBR or FKM for higher temperatures; PU for dynamic, abrasion-intensive tasks; PTFE blends for low friction.
• System design: size clearances for the maximum pressure; include back-up rings when manufacturing tolerances are large.
• Surfaces: piston rods with a load-bearing microstructure; surfaces that are too smooth can disrupt the lubrication film.
• Filtration: clean fluid reduces abrasion and extends the service life of seals and guides.
• Test strategy: after assembly, increase pressure stepwise and visually inspect the sealing zones.
• Avoid pressure traps: where necessary, use pressure-relieved rod seals or vented wiper concepts to prevent interstage pressure build-up.
• Lubrication management: pre-lubricate sealing contact areas with compatible system fluid to reduce start-up wear.

Role of the sealing ring for performance

An appropriate sealing ring determines cylinder holding force, the repeatability of cutting and splitting movements, energy efficiency through low friction losses, and operational safety through minimized leakage. In Darda GmbH tools such as concrete crushers and rock and concrete splitters, it is therefore a central functional element between the hydraulic power pack and the tool mechanics. Robust sealing concepts translate directly into availability, predictable maintenance intervals, and total cost of ownership.