The connection thread is a central detail in hydraulics that determines sealing integrity, operational safety, and performance. Especially in robust applications such as concrete demolition, building gutting, rock breakout, or natural stone extraction, it reliably connects the power unit, hydraulic hose line, and tool. For tools from Darda GmbH – such as concrete demolition shears or Darda rock and concrete splitters – the correct selection and assembly of the connection thread are crucial to transfer high-pressure energy with minimal losses and to avoid leaks.
Definition: What is meant by connection thread
A connection thread is the standardized screw profile on hydraulic components through which lines, couplings, or adapters are connected with a positive fit and – depending on the design – self-sealing or connected with an additional sealing element. There are internal and external threads, parallel and tapered versions, as well as systems with sealing edges, O-rings, or flat seals. In the hydraulic tool chains of Darda GmbH – for example, between the hydraulic power pack, hose, and concrete demolition shear or rock splitting cylinder – the connection thread provides a dimensionally accurate, pressure-resistant, and releasable connection.
Technical features and thread types in hydraulics
Various thread standards are used in high-pressure hydraulics. They differ in flank angle, taper, sealing principle, and designation. It is important that systems cannot be mixed arbitrarily. Commonly used are:
- Parallel pipe threads (BSPP, “G” per ISO 228): cylindrical, sealing with a flat seal, O-ring/carrier, or bonded seal; common marking, e.g., G 1/4.
- Tapered pipe threads (BSPT or “R”, NPT/NPTF): conical, sealing via thread flanks; depending on the standard, require a suitable sealant; examples: R 1/4, 1/4–18 NPTF.
- Metric threads with O-ring (ISO 6149): cylindrical, face sealing with an integrated O-ring groove; e.g., M18×1.5 (O-ring seat).
- Cutting ring/tube fittings (DIN 2353, 24° cone): metallic sealing via cone and cutting ring; in light, heavy, and extra-heavy series.
- 37° cone (JIC/UNF, ISO 8434-2): metallic sealing via a 37° sealing cone; typical fine threads, e.g., 9/16–18 UNF.
- ORFS (O-Ring Face Seal): face sealing via an O-ring at the end face; very good leakage security under vibration.
Also important in practice are the flank angle (e.g., 55° for BSP, 60° for NPT/UNF), the pitch (threads per inch or millimeters), the nominal diameter, and the sealing location (in the thread, at the cone, or at the end face). These characteristics determine pressure resistance, sealing behavior, and achievable flow rate on concrete demolition shears, rock and concrete splitters, combination shears (HCS8), multi cutters, steel shears, tank cutters, and the hydraulic power packs of Darda GmbH.
Relevance in use with concrete demolition shears and rock and concrete splitters
In concrete demolition and special deconstruction, in building gutting, in rock breakout and tunnel construction, as well as in natural stone extraction, tools are exposed to high load cycles, vibration, dust, and moisture. Connection threads must reliably seal under these conditions, tolerate repeated coupling, and provide sufficient cross-section for the required flow rate. With concrete demolition shears, the proper thread interface between power unit, hose, and shear determines whether power peaks are transmitted cleanly. With rock and concrete splitters, the connection also needs to be insensitive to particles and impact – a robust, standard-compliant threaded connection reduces micro-leaks and downtime.
Sealing principles: tapered, face sealing, and metal-to-metal
Tapered self-sealing
Tapered threads (e.g., NPT/NPTF or BSPT/R) seal via the thread flanks. Assembly is usually performed with a suitable sealant. Advantages are compact design and high holding force; the downside is limited reusability with frequent disassembly.
Face-sealing systems
Parallel threads (G, ISO 228) seal with an additional sealing element such as a bonded seal. ORFS and ISO 6149 versions use an O-ring at the end face or thread seat surface. They are particularly leakage-proof under vibration, which has proven itself in harsh demolition scenarios.
Metal-to-metal cone
24° (DIN 2353) or 37° cones (JIC) seal via precisely machined conical surfaces. Advantage: high pressure resistance and good alignability; critical is a clean, undamaged sealing edge.
Selection criteria for connection threads
- Pressure and temperature range: The thread system must safely cover the maximum operating conditions.
- Flow demand: Nominal size and free cross-section influence the working speed of concrete demolition shears or splitting cylinders.
- Sealing principle: Vibration tolerance and frequency of disassembly often argue for face-sealing systems.
- Material and media compatibility: Select sealing materials (O-rings) to match the hydraulic oil.
- Standardization: Uniform thread standards across the fleet simplify service and spare parts stocking.
- Regional availability: Metric/BSP in Europe, UN/NPT in other markets – relevant for international projects.
- Installation space and protection: Provide thread protection caps, kink and impact protection, especially on construction sites.
Assembly, tightening torque, and sealants
Preparation
- Clean and inspect threads and sealing faces, and keep them closed with protective caps until assembly.
- Check O-rings/bonded seals for damage and use only matching, undamaged sealing elements.
Screwing together
- Observe the component manufacturer’s tightening torques and installation guidelines; avoid over-tightening.
- Use only suitable sealants on tapered threads; for ORFS/ISO 6149 do not apply additional pastes on the sealing face.
- Align bends/adapters in the installation position, then tighten to the final torque.
Verification
- Perform a pressure test under operating conditions; check for weeping, dripping, or settling effects.
- Plan a follow-up inspection after the first load cycles.
Compatibility and adapters in practice
Adapters can bridge standards (e.g., from G to ORFS). Each additional interface, however, increases complexity, leakage risk, and potential pressure losses. For mobile operations – such as special deconstruction with concrete demolition shears – few, clearly defined interfaces are advantageous. Mixing metric and inch fine threads without a secure seat (cone/face seal) should be avoided. Marking on the power unit and on hydraulic hose lines should be unambiguous to prevent mix-ups.
Sizing and marking systems
Threads are designated by nominal diameter, pitch, and standard. Examples for orientation: G 1/4 (BSPP), R 3/8 (BSPT), M18×1.5 (ISO 6149), 9/16–18 UNF (37° cone), 1/4–18 NPTF. To identify an unknown connection thread, it is advisable to:
- Determine outer or inner diameter with calipers.
- Determine pitch with a thread gauge (mm or TPI).
- Check the sealing principle: tapered, face sealing with O-ring/bonded seal, or cone.
- Compare flank angle (55°/60°) and the shape of the sealing face.
- Cross-check with standard tables and document permanently.
Typical failure patterns and test notes
- Weeping/dripping after pressurization: O-ring pinched, wrong diameter, or contaminated sealing face.
- Cracks/overstretching at the thread root: excess torque or wrong adapter; remove component from service.
- Damaged cones: Misalignment when starting the thread; inspect sealing face, replace component if necessary.
- Wrong sealant on tapered threads: chemical incompatibility or particles in the system; use a suitable product.
- Reduced flow: Nominal size too small or tight cascades of adapters; optimize cross-section.
Safety, environment, and documentation
Work on hydraulic connection threads is always carried out depressurized and clean. Escaping oil must be collected in an environmentally proper manner; leak-tightness tests should be performed regularly. Maintenance requirements follow the manufacturer’s specifications and the generally accepted rules of technology. Clear, sustainable documentation of the thread standards installed in power units, hoses, and tools of Darda GmbH facilitates service and reduces downtime – especially in special operations with rotating teams.
Practical reference: connection thread on the concrete demolition shear and hydraulic power pack
In use, the hydraulic power pack (hydraulic power units from Darda) is coupled via a quick coupling with a preceding connection thread to the hose that leads to the concrete demolition shear. The thread interfaces on the power unit, coupling, and tool must be of the same standard and compatible in sealing concept. Clearly assign supply and return, insert sealing elements, observe tightening torque, then test under load. This ensures that the force required for concrete demolition can be transmitted safely and reproducibly.
Maintenance and spare parts management
Wear parts such as O-rings or bonded seals are replaced preventively. Threads receive dust protection caps, especially on the construction site. For rock and concrete splitters as well as concrete demolition shears, a standardized spare parts inventory for the thread standards in use is recommended. Clean storage, clear labeling, and regular visual inspection keep connections reliably tight.