What are popular hydraulic fitting types?

We all know about using the STAMP method (Size, Temperature, Application, Media, Pressure) when choosing the right hydraulic hose fitting. The goal is always to have a leak-free connection that can handle the specific application. But what are the most popular fittings, and why are they so common?

The most common hydraulic fitting styles are O-ring, Mated Angle, and Tapered Thread. Each of these styles comes in different variations for different applications.

O-ring Fittings

There are three types of O-ring fitting designs: O-ring Boss, O-ring Face Seal, and O-ring Flange. Each design utilizes an O-ring to create a secure seal.

- O-ring Boss (ORB) Fittings: The O-ring in ORB fittings is situated between the threads and wrench flats, encircling the outer diameter of the male threaded end of the fitting. It seats against the female port to provide a very reliable leak-tight seal. ORB fittings can be either adjustable or non-adjustable.

- Adjustable O-ring Fittings: Typically elbows or tees, these fittings are installed in a fixed direction and use a backup washer to force the O-ring into the cavity as the lock nut is tightened. They are considered adjustable because they can be aligned toward a mating member before being fully tightened.

- Non-adjustable O-ring Fittings: These include plugs and straight connectors that fit directly into a port without requiring alignment. ORB fittings are highly preferred in many applications because they provide optimal sealing in medium to high-pressure systems, as recommended by the National Fire Protection Association (NFPA).

- O-ring Face Seal (ORFS) Fittings: In ORFS fittings, the O-ring is situated in the face of the fitting to create a seal between two connected metal surfaces. When the connection is tightened, the O-ring compresses against the flat face of the mating fitting, tube, or other mating female port. While Nitrile 90 Durometer O-rings are most frequently used in ORFS fittings, other compounds, such as fluorocarbon, are used in very high-temperature environments depending on the application.

With design engineers increasing system pressures, which can result in added vibration, O-ring face seals are becoming even more popular due to their resistance to high-vibration applications.

Mated Angle Fittings

The two most popular mated angle (flare) hydraulic fittings are JIC 37-degree and SAE 45-degree. JIC 37-degree flare fittings are often used for high pressure applications in off-road and agricultural equipment, such as hydraulic loaders, excavators and tractors.

SAE 45-Degree Flare Fittings: These fittings are more frequently seen in low to medium-pressure applications, such as military and aerospace equipment. The flares and mating seats of these fittings can only mate with a corresponding angle and are not interchangeable with other angles.

Flare fittings have tapered ends that fit into a flared piece of tubing. They are seated in place with a threaded nut, providing a leak-tight and pressure-resistant seal. The tubing is flared using a flaring tool, such as a mandrel or rolling cone, though pneumatic flaring tools are also used for quick and accurate assembly.

Angled hydraulic flare fittings have either parallel or straight threads, but the threads don’t form the seal. Instead, the seal is formed when the two mating flared angle seats (male and female) are joined together.

JIC Fittings: Also known as flare fittings, SAE 37-degree tube fittings, and SAE J514 fittings, JIC fittings are the most popular type of hydraulic fittings in America and are generally interchangeable with AN fittings. They are frequently used in hydraulic hose assemblies as the hose end adapter for the commonly used female JIC swivel fittings. JIC fittings are rated for higher pressures in correlation with the specific tubing size and often incorporate an elastomeric O-ring seal for a leak-proof connection. They are commonly manufactured from nickel alloys, brass, carbon steel, and stainless steel. The most common JIC 37-degree flare fittings are made of carbon steel, with operating temperatures ranging from -65°F (-53°C) to +400°F (204°C).

SAE 45-Degree Fittings: These fittings are machined with a 45-degree flare seating surface and are frequently used in automotive, refrigeration, and air conditioning applications, as well as in military and oil and gas industries for fuel equipment, plumbing, gasses, and vacuum lines. They are not compatible with JIC connections due to the different flare angle. Typically made of brass, they are designed for use with low-pressure brass, copper, steel, or aluminum tubing. SAE 45-degree flare fittings are easy to reassemble many times without affecting the seal, provided they are not over-tightened. Overtightening can lead to distortion, cracking, or cross-threading due to brass’s soft material. They are ideal for low and high-temperature applications, able to withstand intermittent temperature ranges of -65°F (-54°C) to 250°F (121°C). However, at wide temperature ranges, the maximum allowable pressure drops significantly.

NPT, BSPT, and Metric Tapered Thread Fittings

Tapered thread fittings achieve a metal-to-metal pressure-tight seal when assembled. Unlike O-ring and mated angle hydraulic fittings, tapered threads require the use of a thread sealant (such as Teflon tape) to create a pressure-tight seal. They are used to effectively seal pipes for fluid and gas transfer.

Tapered threads make effective seals because the male and female threads compress and wedge together, providing a strong, leak-resistant connection. NPT (National Pipe Thread) is the most common tapered pipe thread in the United States and Canada, though it is not compatible with British BSPT (British Standard Pipe Taper) or Metric pipe threads. Metric threads are the most common style in Europe and have a precise inner and outer cylindrical diameter measured in millimeters.

The thread angle, or included angle, is 60 degrees for NPT and Metric threads, while BSPT threads have a 55-degree angle. Their thread shapes are also different, with NPT threads having flattened peaks and valleys, and BSPT threads being rounded.

The distinctive feature of metric threads is the V-shaped thread form, with both male and female threads being parallel. Metric threads come in coarse and fine pitches. Coarse threads are more popular and have a default pitch size, whereas fine threads have a smaller pitch size. To identify the difference, coarse threads are distinguished only by diameter, while fine threads are recognized by both diameter and pitch size.