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Technical Information on Screw-in Fittings
Screw-in fittings are form-fitting and leak-proof connectors for direct mounting in housing walls, sheet metal, or control panels. They combine a thread for fastening with a hose connection and provide mechanical guidance, medium separation, and sealing in a single component. For industrial applications, precise dimensional accuracy, material selection, and sealing concept are crucial, as these factors determine service life, leak-tightness, and chemical resistance.
Materials and Their Applications
Brass (nickel-plated) is standard for many applications: good corrosion protection, metallic dimensional stability, and economical price. For oil and air-carrying systems, as well as drinking water applications, nickel-plated brass forms are often considered a suitable choice. Stainless steel (AISI 316 / 1.4401) is used where corrosion resistance to aggressive media, saltwater, or cleaning chemicals is required. Plastic materials such as POM offer very good sliding properties, chemical resistance to many organic solvents, and are electrically insulating; PVDF (Kynar) has outstanding resistance to aggressive chemicals, strong acids, and is more temperature-resistant than many plastics. For each application, the material pairing must be checked: metallic sealing elements in plastic housings and vice versa require appropriate pre-tensioning and, if necessary, stainless steel inserts.
Shapes, Designs, and Connection Variants
Screw-in fittings exist as straight screw-in connectors, elbow connectors (90°), and with flattened or locking heads for limited installation space. The threads can be metric ISO threads (e.g., M10x1, M12x1), BSPP/G (cylindrical), or NPT (tapered). When installing in thin sheet metal, screw-in nuts with sealing rings are often used, or fittings with a collar to limit the screw-in depth. Hose connections are ribbed (for hose clamping) or conical with a union nut for clamp or press connections. For quick-assembly solutions, there are push-in or quick-release variants that are inserted without tools.
Sealing Concepts and Leakage Safety
Sealing is achieved either radially by O-rings, axially by flat gaskets, or metallically contacting in high-pressure applications. Common materials for O-rings include NBR (oil and abrasion resistance), EPDM (water and steam resistance), FKM/FPM (Viton – high temperature and chemical resistance), and PTFE inserts for very aggressive media. The selection is based on the medium, temperature range, and operating pressure. For dynamic stress or temperature fluctuations, detachable sealing concepts with replaceable O-rings are preferable to ensure ease of service. Thread flanks and collar surfaces must be flat and burr-free during assembly, otherwise even high-quality sealing technology can fail.
Operating Pressures, Temperature Ranges, and Compatibility
Operating pressure and temperature limits largely depend on the material and sealing choice. Brass fittings with NBR seals typically cover pressure ranges up to 10–16 bar and temperatures from -20 °C to +80 °C. Stainless steel variants with FKM seals are suitable for higher temperatures up to +150 °C and offer pressure resistance over 20 bar, depending on thread and wall thickness. Plastic fittings made of POM or PVDF are temperature and pressure limited: POM up to approx. +80 °C, PVDF up to +140 °C; pressure limits vary greatly with wall thickness and design. For gas applications, additional requirements for leak-tightness (e.g., helium test) and conical threads (NPT) must be observed.
Manufacturing Tolerances, Surface, and Corrosion Protection
Precise thread manufacturing (ISO 6g/6H for internal/external threads) reduces assembly effort and leakage risk. Surface treatments such as nickel plating increase corrosion protection and reduce friction, while electropolished stainless steel surfaces improve cleaning and hygiene properties. In chemically aggressive environments, attention must be paid to interference fits and galvanic corrosion pairs: stainless steel against brass can lead to corrosion under certain electrolytes; in such cases, insulation measures or uniform materials are advisable.
Connection Types and Assembly Safety
The choice of connection principle depends on the frequency of assembly and maintenance requirements. Screw-in fittings with union nuts allow for easy retightening, while clamped or press-fit variants can be more stable for vibration-rich environments. Assembly tools and torque specifications must be observed: overtightening threads leads to plastic deformation or swelling in plastic parts. For critical media, the use of torque limiters and retightening after the first operating pressure test is recommended.
Compatible Hoses and Hose Dimensioning
The hose inner diameter and the outer profile of the fitting must match; the use of clamps is mandatory for ribbed connections to absorb axial forces and pressure peaks. For soft hoses, the use of hose nozzles with an inner sleeve or clamp is recommended to avoid pinching. The length calculation and pressure loss consideration of the hose system influence dimensioning: longer hose sections or smaller inner diameters require larger cross-sections in the fitting to minimize flow losses.
Practical Application Examples
Practical Example 1: Coolant Line in a Machine Tool
In a CNC machine tool, nickel-plated brass screw-in fittings M12x1 with NBR O-rings were used to guide coolant lines to the machine base plate. The conical inner design allowed the 6/4 mm hose to be securely fastened with a stainless steel clamp. The fittings were tightened to 8 Nm and retightened after 24 hours of operation; leakage rates in the retest were 0 ml/min.
Practical Example 2: Chemical Dosing Station
For a dosing station with aggressive acids, a PVDF screw-in fitting with FFKM seal was selected. The fitting was mounted as a straight screw-in connector in the front panel, with the hose connection designed as a push-in for PTFE hoses. The PVDF construction and the FFKM O-ring achieved a durable, replaceable sealing solution that resists cleaning agents and dosing chemicals.
Practical Example 3: Pneumatic Distributor in an Assembly Cell
In an assembly cell, stainless steel screw-in fittings with BSPP thread and ribbed hose connection were used to supply compressed air to pneumatic grippers. The mounting was done in thin stainless steel sheet using a collar design with a sealing washer to dampen vibration effects. The targeted material selection allowed pressure surges over 12 bar to be safely transmitted.
- Material choice, thread type, and seal must be matched to the medium, pressure, and temperature.
Maintenance, Spare Parts, and Quality Checks
Regular visual inspection for cracks, corrosion, or deformation is mandatory. O-rings are wear parts and should be checked during scheduled downtimes and replaced if tension cracks or hardening occur. Pressure tests after assembly (e.g., 1.5 times operating pressure) and leak tests (air/water or helium test) are recommended for safety-critical applications. Document torques and used seal types in the maintenance record to ensure replacement and traceability.
Standards and Specifications
Observe relevant standards such as DIN ISO for threads, EN standards for fittings, and the applicable safety standards for pressure equipment and food contact. Material certificates (e.g., material test certificate 3.1 according to EN 10204) should be requested for critical applications.
Further technical background and application examples can be found on our technology page: https://maku-industrie.de/technik and in specific practical reports at https://maku-industrie.de/anwendungsbeispiele.
FAQ
Which seal is recommended for aggressive chemicals?
For aggressive chemicals, FFKM (perfluorinated elastomers) or PTFE components are the first choice; PVDF housings further increase resistance. Material comparisons and medium database matching are mandatory before selection.
Which thread do I choose for thin sheet metal mounting?
For thin sheet metal mounting, screw-in fittings with a collar and integrated seal or additional screw-in nuts with nut and sealing ring are recommended. This limits the screw-in depth and prevents overloading the sheet metal.
How do I ensure leak-tightness after assembly?
Assembly with the prescribed torque, use of suitable O-rings, pressure testing at 1.25–1.5× operating pressure, and retightening after initial operation ensure long-term leak-tightness. Documentation of test values in the maintenance record increases traceability.