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Threaded Bulkhead Fittings: Technical Properties, Applications, and Selection Criteria

Threaded bulkhead fittings securely connect hoses to housing walls, sheet metal, or control cabinets. In this category, you will find Parker Rectus versions made of POM (polyoxymethylene) and PVDF (polyvinylidene fluoride) with G 1/4" external thread and hose nozzles for 4 mm or 6 mm inner diameter. The design of these fittings is intended to create sealed and strain-relieved feed-throughs without risking leakage or tensile stress to the interior or electronics.

Materials and Material Selection

POM offers high mechanical strength, good sliding and wear properties, and chemical resistance to many hydraulic and pneumatic media. POM threaded fittings are suitable for standard applications with moderate temperatures and medium chemical loads. PVDF is the choice for increased chemical resistance, especially against aggressive media, solvents, and high UV and heat exposure. PVDF fittings withstand higher continuous temperatures and are suitable when peroxides, acids, or aggressive organic solvents are present. For selection, consider: medium, temperature range, mechanical stress, and electrical requirements.

Designs, Connection Types, and Sealing Concepts

The offered bulkhead fittings are designed as one- or two-piece nozzles. The standard geometry combines a G 1/4" external thread for screwing into a sheet metal hole with a hose nozzle (4 mm or 6 mm). Sealing is achieved either by an integrated O-ring seat system, a flat sealing washer, or by a two-piece screw connection with a lock nut. For installation in thin sheet metal (0.5–3 mm), self-clamping variants or those combinable with a washer are offered. Common sealing materials are NBR (nitrile rubber) for general applications, EPDM for water supply and steam, and FKM (Viton) for high temperatures and aggressive oils.

Mechanical Data, Tightness, and Assembly

G 1/4" thread is industrially standardized and compatible with common nuts and screw connections. Pay attention to the correct bore execution: for a clean fit, the bulkhead hole must meet the specified opening tolerance so that the threaded fitting lies flat. During assembly, a moderate tightening torque is required to avoid damage to the plastic thread; as a guideline, 1.5–3 Nm is often applied for POM fittings, slightly higher for PVDF, depending on wall thickness and nut type. For sealing, PTFE sealing tape or liquid sealant can also be used, if approved by the manufacturer. Strain relief on the hose side can be achieved by clamps, hose clamps, or specific strain relief bodies.

Temperature and Pressure Limits

POM fittings typically cover temperature ranges from approximately -40 °C to +90 °C in continuous operation; higher temperatures are possible for short periods. PVDF fittings remain functional in ranges up to approximately +140 °C. Operating pressure depends on hose diameter, installation position, and medium; for pneumatic applications with 4 mm/6 mm hose nozzles, typical continuous pressures up to 10 bar are possible, significantly more for short periods. For safety-critical or high pressures, metallic screw connections are preferred.

Corrosion and Chemical Resistance

PVDF offers superior chemical resistance to halogenated hydrocarbons, strong acids, and bases. POM is resistant to oils, greases, hydraulic fluids, and many organic solvents, but loses resistance to strong oxidizing agents and aromatic hydrocarbons. Select material and seal according to the chemical composition of the medium, as well as temperature and concentration.

Practical Application Examples

Example 1 – Control Cabinet Perforation for Pneumatic Lines: In a control cabinet system, control air lines for valves are routed through a 1.5 mm steel sheet wall. The Parker Rectus POM threaded fitting with G 1/4" thread and 4 mm hose nozzle is screwed in from the outside, fixed on the inside with the lock nut, and sealed with an EPDM O-ring. The hose side is secured with a hose clamp to absorb tensile forces. Result: no leaks, clear strain relief, and easy disassembly for maintenance.

Example 2 – Chemical-Carrying Line Through Partition Wall: In a dosing system for aggressive acid, a PVDF bulkhead fitting with a 6 mm hose nozzle is used. The fitting is equipped with an FKM O-ring, and the bulkhead hole was manufactured with precision. Assembly is carried out with torque monitoring, additional PTFE sealing, and a stainless steel lock nut for mechanical security. Result: chemically resistant feed-through, long operation without replacement intervals.

Example 3 – Retrofit in Existing Machines: An existing conveying machine requires additional pneumatic connections in the side wall. The choice fell on POM bulkhead fittings with a larger flange diameter for better load distribution in thin sheet metal. After adjusting the bore dimensions and using washers, the solution was implemented without extensive housing modifications. Result: quick retrofitting, minimal downtime.

Selection Criteria for Ordering

Select the fitting based on medium, temperature, required hose size (4 mm or 6 mm), bulkhead thickness, desired seal (O-ring material), and mechanical load capacity. Check the installation dimensions and the G 1/4" thread size for compatibility with existing connection parts. Order PVDF versions for aggressive media or elevated temperatures. Also consider mounting accessibility and the need for lock nuts or washers.

• Key selection parameters: Material (POM/PVDF), hose Ø (4 mm/6 mm), seal (NBR/EPDM/FKM), bulkhead thickness, operating pressure, and temperature.

Compatibility, Standards, and Documentation

G 1/4" is a standardized Whitworth pipe thread size; check compatibility with existing components. For safety-critical applications, check material-related certificates and approvals for food or medical technology, if required. Technical data sheets and installation instructions can be found in the product details; further technical information and application examples are documented on our technical page: maku-industrie.de/technik as well as specific practical cases under maku-industrie.de/anwendungsbeispiele.

Quality Features and Spare Parts

Pay attention to the dimensional accuracy of the hose nozzle, clean thread execution, and tested sealing surfaces. Spare parts include lock nuts, O-rings in various elastomers, and washers. For series solutions, it is advisable to procure complete mounting kits to ensure uniform sealing and mounting properties.

Ordering Information

For Parker Rectus threaded bulkhead fittings, specify material (POM / PVDF), hose Ø (4 mm / 6 mm), required seal type, and desired quantity. Check the installation dimensions in the data sheet and, if necessary, specify bulkhead thickness and required tightening torque. If you need advice on selection or custom designs, use our technical resource at maku-industrie.de/technik or contact sales via the product page in this shop.

Maintenance and Service Life

Regular visual inspection of sealing surfaces and hoses reduces the risk of failure. O-rings should be checked for cracks, hardening, or swelling during each removal and replaced if necessary. PVDF fittings show fewer signs of aging than POM under thermal and chemical stress; nevertheless, in critical applications, proactive replacement planning is recommended.

FAQ

Which seal is suitable for water applications?

EPDM is preferred for water, steam, and many cleaning chemicals; NBR is suitable for oil and air applications. Use FKM for high temperatures and aggressive oils. Always adapt material choice to medium and temperature.

How do I measure the correct bulkhead thickness and bore?

Bulkhead thickness corresponds to the material thickness of the housing; choose a fitting with suitable flange lengths or a two-piece screw connection with a lock nut. The exact bore can be found in the technical data sheet of the fitting; a cleanly executed bore without burrs ensures tightness and a secure fit.

Can I use PTFE tape for sealing?

Yes, PTFE sealing tape can be used for the thread as an additional seal, provided the manufacturer does not prescribe specific sealants. Pay attention to clean installation and not too high tightening torque to avoid damaging plastic threads.