A complete guide to hydraulic fittings

In hydraulic systems, the integrity of each connection point often determines whether the circuit can maintain pressure, contain the working fluid, and operate without interruption. Fittings are not incidental parts. They define the flow path, provide the primary sealing interface, and must remain stable under the combined effects of pressure, vibration, and thermal cycling.

Correct selection of fittings depends on a sound grasp of how loads are transferred through the joint, the limits imposed by working pressure, the compatibility of the materials in contact, and how the sealing faces behave under repeated pressure and vibration cycles. Mistakes at this stage are a frequent cause of leakage, accelerated wear, and loss of efficiency that can propagate into costly system failures.

Because hydraulic systems operate across a wide spectrum of pressures, temperatures, and environmental exposures, engineers must evaluate each fitting choice in the context of the system’s specific requirements. This demands a structured approach that accounts for geometry, sealing strategy, and material science alongside operational constraints and industry standards. This guide covers:

What are hydraulic fittings and connectors?
Why they’re essential in hydraulic systems
Common types of hydraulic fittings
Seals and o-rings in hydraulic systems
Materials for hydraulic fittings and protection
Applications and industries for hydraulic fittings
How to select the right hydraulic fitting
Conclusion

What are hydraulic fittings and connectors?

Hydraulic fittings and connectors are mechanical interfaces that join sections of hydraulic hose, tube, or pipe to pumps, actuators, valves, and other system components. Their function is to provide a leak-tight, pressure-resistant, and serviceable connection that maintains the integrity of a closed hydraulic circuit under a wide range of operating pressures, often exceeding 350 bar (5,000 psi) in industrial systems.

Rather than being generic couplings, these components are engineered to meet strict dimensional, material, and performance standards. Their geometry governs how the joint seals (metal-to-metal, elastomeric, or a hybrid), resists loosening under vibration, and handles the cyclic loading typical of hydraulic machinery.

Hydraulic connectors typically fall into two interrelated categories:
●    Fittings – The fixed or semi-permanent interfaces used to join hoses, tubes, or pipes to other system components. Examples include flare fittings, compression fittings, ORFS (O-Ring Face Seal) fittings, and threaded adapters.

●    Quick-connect couplings (or connectors) – Designed for rapid, often tool-less, connection and disconnection without fluid loss. They are commonly used in mobile equipment, test ports, and modular systems.

The choice of fitting or connector affects flow path geometry, sealing strategy, ease of assembly, and maintainability. Because these parts sit at the junction of mechanical, hydraulic, and sometimes environmental constraints, they are specified according to factors such as pressure class, material compatibility with the working fluid, thermal stability, and compliance with standards such as ISO 8434, SAE J514, or DIN 2353.

Browse our range of hydraulic coupling protection

Why they’re essential in hydraulic systems

Hydraulic fittings are the points at which a pressurised circuit is most likely to fail. They do more than connect components: they must withstand full system pressure, maintain a leak-tight seal, and allow assembly and service without weakening the joint. A poor fitting choice or installation often becomes the first source of leakage, efficiency loss, or a safety incident. A fitting's performance depends on the geometry of its design, the mechanical strength of the joint, and its stability under vibration and repeated loading.

1. Containment of high-pressure fluid
Hydraulic circuits may operate at pressures from around 70 bar (1,000 psi) in light industrial equipment to over 420 bar (6,000 psi) in heavy-duty mobile and aerospace systems. The fittings must maintain a leak-tight seal at these pressures, even when exposed to pressure spikes, shock loading, and continuous vibration. Any breach can lead to fluid loss, performance degradation, or catastrophic failure.

2. Preservation of system efficiency
Even minor leakage or turbulence at a connection point can cause:
●    Energy loss due to pressure drop
●    Localised heating that accelerates fluid degradation
●    Reduced actuator performance

A properly selected fitting seals under full operating load and presents a smooth, correctly sized bore at the connection. By avoiding steps, gaps, or turbulence at the junction, it minimises pressure drop and keeps the system delivering its rated flow and efficiency.

3. Structural integrity and load transfer
Fittings often do more than seal a joint; they also carry part of the mechanical load from the connected lines. A sound design resists vibration, holds torque on the threads, and keeps the joint from working loose under bending or tensile stress. When the fitting is undersized or the joint relaxes under cyclic loading, it becomes the first point to leak or crack.

4. Serviceability and modularity
Hydraulic circuits often require hoses to be replaced, lines rerouted, or components changed during maintenance. Standardised fitting designs let a mechanic apply the right torque, break and remake the joint without damaging the sealing faces, and swap parts quickly in the field.

5. Safety and environmental compliance
A failed hydraulic fitting can cause an abrupt drop in pressure, which may drive an actuator out of position, eject fluid as a high-velocity spray that can ignite or create a slip hazard, or discharge oil into the environment. To reduce these risks, many fittings now incorporate captive ring seals, dry-break couplers, or secondary containment.

We recommend you learn more in the guide our experts have written, Hydraulic hose protection to ensure safe designs.

6. Compatibility with standards and diverse fluids
Fittings bridge the gap between different tube/hose diameters, wall thicknesses, pressure classes, and material types. They also ensure reliable sealing with a wide variety of working fluids - mineral oils, synthetic esters, phosphate esters, or environmentally acceptable fluids - by combining appropriate sealing compounds and surface finishes.

Common types of hydraulic fittings

Different fitting types are optimised for specific tasks, from barbed hose connectors for simple lines to compression fittings for high-pressure applications. Understanding these categories helps engineers match each fitting to the system’s operating pressure, environment, and service requirements.

You might first find it helpful to understand hydraulic hose. See our experts’ guide, What are the components of a hydraulic hose assembly?
 

Spiral wrap protection extends the service life of hydraulic hoses by shielding them from abrasion, impact, and environmental exposure. It also organises and bundles hoses for safer, more efficient routing. Learn more about this indispensable protection in our guide, Hydraulic hose spiral wrap, explained.

Seals and o-rings in hydraulic systems

Ring seals play a central role in hydraulic assemblies by keeping the pressurised fluid inside the circuit. Their main task is to stop leakage between mating surfaces while enduring the combined effects of high pressure, temperature change, dynamic motion, and chemical exposure.

Role in system integrity

Hydraulic systems often run at pressures above 400 bar, and many components are exposed to repeated load cycles and rapid changes in flow direction. Seals and O-rings keep the fluid contained under these conditions, preventing internal bypass that reduces actuator efficiency and stopping external leakage that can create safety and environmental hazards.

Types and performance factors

The performance of a seal depends on several factors:
●    Material compatibility with the hydraulic fluid (e.g., mineral oils, phosphate esters, bio-based fluids) to prevent swelling, degradation, or embrittlement.

●    Ability to withstand working pressure and temperature. The seal must keep its shape and compression under the system’s pressure and not harden, crack, or lose elasticity as the temperature changes.

●    Seal profile and support elements. The cross-section, any lip detail, and the use of back-up rings or energisers determine how well the seal resists being forced out of its groove under high pressure.

●    Dynamic vs. static applications, as seals for moving parts (such as piston and rod seals) must accommodate friction, wear, and surface finish requirements.

Commonly used materials include NBR (nitrile rubber) for general applications, and FKM (fluoroelastomer) for higher temperatures and chemical resistance.

Reliability and maintenance

Seals are often the first parts to wear out when exposed to high pressure, heat, or contaminated fluid. Once they begin to fail, the system can lose efficiency or suffer sudden leakage. Choosing the right material, installing it correctly, and checking seals at service intervals are key to keeping equipment running. In demanding applications, back-up ring seals help stop the seal from being forced out of its groove under pressure, and wipers keep dirt out of the sealing area to slow wear.

Materials for hydraulic fittings and protection

Material selection is a critical engineering decision in hydraulic fittings and protection, directly influencing mechanical strength, pressure tolerance, corrosion resistance, and service life. Below are some common materials used.

Material	Density (g/cm³)	Tensile/Yield Strength (MPa)	Continuous Service Temp. (°C)	Indicative Pressure Tolerance† (bar)	Abrasion Loss (DIN 53516, mm³) – the lower, the better
PVC (rigid)	~1.38	45–60 (tensile)	–15 to +60	Up to ~20 (low-pressure sleeves/guards)	250–400
PES/PO (Polyethersulfone / Polyolefin blends)	1.3–1.4	70–85 (tensile)	–50 to +180	~40–60 (moderate-pressure connectors)	~150–250†
POM (Acetal, incl. Delrin®)	1.39 – 1.42	65 – 75 (tensile)	–40 to +100	~60 – 100 (moderate-pressure fittings)	80 – 120
Aluminium	2.70	240–310 (yield)	–200 to +200	Up to ~350 (lightweight fittings)	Negligible‡
HDPE	0.94–0.96	25–31 (tensile)	–50 to +80	Up to ~15–20 (low-pressure guards/caps)	150–250
Nylon 6	1.12–1.14	65–85 (tensile)	–40 to +120	20–40	~70–120 (moderate-pressure fittings/clamps)
PP	0.90–0.91	30–40 (tensile)	–20 to +100	Up to ~20–30 (low-pressure)	180–300
FKM (Fluoroelastomer)	~1.8	7–12 (tensile)	–20 to +200	Sealing only (not load-bearing)	150–250
EPDM (Ethylene-Propylene Rubber)	~0.9	7–12 (tensile)	–40 to +150	Sealing only (not load-bearing)	150–250
Rubber	~1.0	~10–20 (tensile)	–30 to +100	Sealing only (not load-bearing)	80–120
Silicone (VMQ)	~1.1	7–10 (tensile)	–60 to +200	Sealing only (not load-bearing)	400–600
Stainless Steel (304)	7.90–8.00	520–650 (yield)	–200 to +400	>350 (high-pressure fittings)	Negligible‡
Woven nylon	~1.1 (fibre)	Fibre-based	–40 to +120	Not structural (used as protective cover)	Typically <100§

Notes

† Indicative Pressure Tolerance: Pressure tolerance values are approximate, material-based guidelines only. Actual working pressure depends heavily on the component’s geometry, wall thickness, manufacturing method, and safety factors. Always refer to the fitting manufacturer’s rated working pressure for design and selection.
‡ Negligible Abrasion Loss: Metals such as aluminium and stainless steel do not show measurable loss under the DIN 53516 polymer abrasion test. Their real-world wear is governed by contact surface finish, lubrication, load, and operating environment, rather than by the bulk material itself.
§ Woven Nylon Performance: Woven nylon sleeves act as sacrificial, flexible protective layers. Their reported abrasion loss is an indicative comparative value, as actual performance depends on weave density, yarn thickness, and test configuration.

Chemical resistance

Selecting materials for hydraulic fittings requires an understanding of their performance in contact with hydraulic fluids, fuels, water, and other service chemicals. The table below provides typical reference ranges for chemical resistance, derived from industry standards (ASTM D471, ISO/TR 10358, ASTM G31) and representative supplier datasheets for common grades of each material.

These figures are intended for preliminary engineering screening only. They are not results from a single, controlled laboratory test and should not be used as certified design data. 

 

Material	Mineral-Oil Hydraulic Fluids (% Volume Swell, 70 h @ 100 °C)¹	Phosphate-Ester Fluids (% Swell)¹	Water / Aqueous (% Swell)¹	Tensile Retention after Exposure (%)¹	Notes
PVC (rigid)	< 2 %	10 – 15 %	< 2 %	85 – 90 %	Rigid; good with mineral oils but softens in ester fluids
PES/PO (Polyethersulfone / Polyolefin blends)	< 1 %	< 2 %	< 1 %	> 95 %	High-performance engineering thermoplastic
POM / Delrin® (Acetal, POM-H)	< 1 %	3 – 5 %	< 1 %	> 90 %	Excellent vs fuels, oils; limited vs strong acids/bases
Aluminium	N/A	N/A	N/A	N/A	Excellent in neutral fluids; may corrode in acidic/chloride media
HDPE	< 1 %	< 1 %	< 1 %	> 95 %	Chemically inert to most fluids
Nylon 6	1 – 3 %	5 – 8 %	2 – 3 %	85 – 90 %	May absorb water but retains mechanical integrity
PP	< 1 %	< 2 %	< 1 %	> 95 %	Very good vs. acids / bases
FKM (Fluoro-elastomer)	< 2 %	< 5 %	< 2 %	80–90 %	Excellent vs petroleum oils and most synthetic fluids; fair in hot water/steam
EPDM (Ethylene- Propylene Rubber)	> 10 % (swells)	< 2 %	< 2 %	70–85 %	Good in water, steam & phosphate-ester fluids; poor in petroleum oils and fuels
Rubber	< 5 %	30 – 50 %	5 – 8 %	70 – 80 %	Excellent vs. petroleum oils; poor vs. phosphate esters
Silicone (VMQ)	20 – 40 %	50 – 80 %	< 1 %	40 – 60 %	Excellent temperature / weathering resistance but swells in hydrocarbons
Stainless Steel (304)	N/A	N/A	N/A	N/A	More economical ferritic grade; lower acid / chloride resistance
Woven nylon	1 – 3 %	5 – 10 %	2 – 3 %	80 – 90 %	Protective sacrificial barrier, not chemical-containment

Notes & reference
1.    Polymer swell and tensile-retention data: typical published ranges from ASTM D471 (rubbers) and ISO/TR 10358 (plastics) at stated temperature and immersion times; also cross-checked with manufacturer datasheets (BASF Ultramid PA6, LyondellBasell PP, HDPE, Solvay PES, major NBR and VMQ compound suppliers).

2.    Corrosion rates for metals: typical laboratory immersion results from ASTM G31 in aerated 3 % NaCl at ambient temperature; practical field performance varies with pH, temperature, surface finish and protective coatings.

Important: These values are indicative ranges for preliminary material screening. Actual performance depends on the exact formulation, test conditions, and service environment. Always verify with the material supplier for critical applications.

Applications and industries for hydraulic fittings

Hydraulic fittings are fundamental to any system that uses pressurised fluid to transmit power or control motion. 

Industries	Typical Applications
Fluid-Power Hose Manufacturing	Assembly of high-pressure hydraulic hoses for OEM and aftermarket use; integration with hose guards, clamps, and wear protectors; pressure and impulse testing of hose assemblies
Construction & Agriculture	Hydraulic lines for excavators, loaders, cranes; steering, braking, and lift systems on tractors and harvesters; hose routing and abrasion protection in equipment exposed to soil, rock, and debris
Machinery & Equipment	Hydraulic circuits in presses, stamping machines, and injection moulding machines; clamping and actuation in CNC and fabrication equipment; connections for automated production lines
Industrial Trucks, Tractors, Trailers & Stackers	Hydraulic power for lift trucks, pallet stackers, and mobile platforms; power steering and braking systems; hose protection against chafing and impact in tight routing paths
Automotive Manufacturing & Service	Hydraulic lines in assembly robots, stamping presses, and test rigs; brake and clutch line service; hose and fitting protection in under-chassis and engine-bay environments
Chemical-Processing Plants	Hydraulic and fluid-handling lines for pumps, mixers, and reactor controls; corrosion-resistant fittings and protective sleeves for hoses exposed to aggressive chemicals and cleaning agents
Machine Shops & Welding / Heavy Manufacturing	Hydraulic power units for presses and clamping devices; coolant and fluid-handling circuits near heat and welding spatter; hose protection in high-temperature zones
Oil & Gas Field Services	High-pressure hydraulic power units for drilling rigs and well-head control; hydraulic lines on mobile fracturing and pumping units; abrasion, flame, and chemical protection for hoses in onshore and offshore conditions
Mining, Quarrying, Drilling & Tunnelling	Hydraulic hoses for drills, bolters, and tunnel-boring machines; protection against sharp rock edges, vibration, and abrasive dust; fire-resistant and antistatic protection for underground safety compliance
Forestry & Agricultural Machinery	Hydraulic lines for harvesting heads, loaders, and tree-handling equipment; abrasion and UV protection for hoses exposed to rough terrain and weather
Marine, Sub-Sea & Offshore Oil & Gas	Hydraulic systems for winches, cranes, and sub-sea tooling; corrosion-resistant fittings and hose protection in salt-water and humid environments; fire-resistant and antistatic hose protection for offshore platforms

 

How to select the right hydraulic fitting

Selecting the right hydraulic fitting is critical for both safe operation and consistent system performance. The STAMP framework is a practical checklist that helps engineers weigh the main factors involved in selection.

●    Size: The fitting’s diameter and thread specification must match the hose, tube, or port to which it connects. Common thread standards include NPT, BSP, and ISO metric, and choosing the correct one prevents leaks and misalignment.

●    Temperature: Take into account both the operating temperature of the hydraulic fluid and the ambient conditions where the system operates. Certain materials can become brittle in low temperatures or lose mechanical strength at high temperatures.

●    Application: Identify what the fitting has to do in the circuit. A line exposed to heavy vibration may need a fitting that will not shake loose, while a line that is disconnected often for testing or service is better served by a quick-connect type.

●    Assembly: Look at how the fitting will be fixed to the hose or tube. Crimped fittings give a permanent, high-pressure joint, whereas reusable fittings can be removed and refitted in the field when a hose has to be replaced.

●    Material: Select a fitting material that is compatible with the hydraulic fluid and capable of withstanding the system’s pressure, temperature, and environmental exposure.

●    Pressure: Verify that the fitting’s rated working pressure comfortably exceeds the maximum pressure of the hydraulic circuit. Underrated fittings can deform or fail under peak load.

●    Fluid Compatibility: Check that the hydraulic fluid, including any additives, will not react with or weaken the fitting material. If the match is poor, the fitting can corrode, swell, or break down ahead of its expected service life.

Conclusion

Correctly matching the fitting type, material, and pressure rating to the application improves safety, reduces downtime, and extends the life of both hoses and hydraulic equipment.

Proper selection should always be supported by accurate technical data, compliance with relevant standards and specifications (such as ISO, SAE, DIN), and an understanding of the operating environment. High-quality fittings, when correctly matched to the application, reduce downtime, improve safety, and extend equipment longevity.

Expert resources

For further technical guidance and up-to-date specification data, consult:
●    International standards:
○    ISO 8434 (Metallic tube connections for fluid power)
○    SAE J514 and J1453 (Hydraulic tube fittings and O-ring face seal fittings)
○    DIN 2353 (Compression fittings for fluid power)

●    Testing standards:
○    ASTM D471 (Rubber property changes in liquids)
○    ISO/TR 10358 (Chemical resistance of plastics
○    ASTM G31 and ASTM B117 (Corrosion testing of metals)

●    Industry references:
○    NACE MR0175 / ISO 15156 for materials in corrosive oil and gas environments
○    Manufacturer datasheets for grade-specific mechanical, thermal, and chemical performance

●    Safety and compliance:
○    Check your local regulations for fire safety, environmental protection, and pressure-equipment directives

Note: Always confirm design-critical properties, such as pressure rating, fluid compatibility, and temperature limits, using manufacturer-supplied test data for the specific fitting grade and configuration being installed.

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Free CADs are available for most solutions, which you can download. If you’re not quite sure which hydraulic connector or protection solution will work best for your application, our experts are always happy to advise you. Whatever your requirements, you can depend on fast dispatch. 

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Questions?

Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application at 0345 528 0474.