How do hydraulic fitting threads work?

Threads for fittings in hydraulics function through the creation of a mechanical connection between two parts, which are a male thread and a female one that seals the thread's angle by itself or on an O-ring that is elastomeric, or via the cone or flare angle that mates, dependent on the thread type employed (NPT, BSPP, BSPT, JIC, ORFS, or the metric DIN/ISO). Selecting the incorrect thread type or mixing different threads is one of the most frequent reasons for leaks from hydraulics. Knowing the process by which each thread family makes its seal is crucial to anyone who is assembling or troubleshooting the hydraulic circuit.

Hydraulic systems operate at high pressures, typically between 1,500 and 6,000 psi, and even higher in certain industrial and mobile applications. With these pressures, the most mismatched fittings can leak fluid, reduce effectiveness, or even fail completely. Threads are the place where the failure usually occurs, and that's why engineers require more than a basic understanding about "screw it in until it's tight." They must be aware of the mechanisms behind the ways that each thread type creates and maintains the seal.

The two main kinds of threads for hydraulics.

Before comparing standards for individual use, it is helpful to divide the hydraulic threads into two functional types.

Threads that seal

The threads for sealing are designed to ensure that the threads form the pressure seal. Female and male threads are tapered. And when they are tightened, the sides of the threads wiggle against one another with an increase in interference. National Pipe Taper (NPT) and British Standard Pipe Taper (BSPT) both fall in this category. Because the seal depends on metal-to-metal (or metal-to-thread-sealant) contact along a tapered path, over-tightening or under-tightening directly affects sealing performance—there's a narrow window where the joint is tight enough to seal but not so tight that it cracks the fitting or galls the threads.

The threads are non-sealing (mechanical join) threads.

Threads that are not sealing are straight, non-tapered, and the primary function is to hold the two parts of the fitting mechanically. A seal actually is made elsewhere—typically an O-ring, a metal-to-metal cone/flare, or even a bonding washer. British Standard Pipe Parallel (BSPP), JIC 37deg flare, ORFS (O-Ring Face Seal), and SAE Straight-Thread O-Ring boss fittings are all based on this method. Since the thread doesn't do the sealing job, they tend to be more tolerant as well as more durable and less susceptible to leaks following repeated disassembling and reassembling.

What is the process by which every major standard for threads is able to create its own seal?

The NPT (National Pipe Taper)

NPT threads taper approximately 1.79 degrees per inch (3/4 inch for each foot of width). When the male and female tapers connect, the crest on one thread is deformed slightly against the crest of the other, causing an interference fit across the entire length engaged. Since dry-fit metal-to-metal contact seldom can be sealed perfectly even at high pressures of hydraulics, NPT fittings are nearly always used with a thread sealant or tape to fill the tiny gaps caused by the fit. NPT fittings are commonplace in medium- to low-pressure pipelines and general plumbing, but they are not recommended in high-pressure systems because threads with tapered threads may loosen or break under the force of vibration.

It is BSPT (British normal pipe taper).

BSPT is based on the same tapering concept as NPTA 1:16, which creates interference when the joint is tightened, but it utilizes a different angle for threads (55 degrees as opposed to NPT's 60 degrees) as well as pitch. This means that BSPT and NPT fittings will be non-interchangeable, although they may be pushed to join with just a few turns of engagement prior to leaks or binding. Similar to NPT, BSPT relies on a sealing agent to fill in the small spaces in the interference fitting tapered.

BSPP (British Standard Pipe Parallel)

BSPP threads are straight, rather than tapered. Therefore, they are not able to create the pressure-tight seal. However, BSPP fittings seal using a bonded seal washer or a captive O-ring pressed against a flat surface underneath the fitting's hex or against a 30-degree seat in certain ports. The only function of the straight thread is to pull the sealing surface down using the same amount of clamping force. This allows BSPP joints to be more resistant to repetition than tapered threads because the sealing element—and not the thread—is the one that absorbs wear.

JIC 37 deg Flare

JIC fittings employ straight threads to pull two parts that are mating. However, the actual seal occurs at a 37-degree conical flare, which is machined into the hose or tube end, and presses against a 37-degree seat on the body of the fitting. The metal-to-metal cone connection caused by the force of clamping axial generated by the straight thread is what prevents leakage. There's not an O-ring on a typical JIC joint. This makes the correct flare angle and finish crucial. A damaged or poorly flared cone can leak regardless of how tightly the thread has been tightened.

ORFS (o-ring sealing the face)

ORFS fittings incorporate straight threads and an O-ring that is captured and set in a groove on the flat side of the fitting. When the straight thread draws the two faces together, the O-ring is compressed evenly over its entire circumference. This creates the seal, which is completely independent of the torque's variation once it reaches a minimum compression. Since ORFS does not depend on a cone-tocone metal fitting or interference between threads, it's thought of as one of the leak-proof designs for high-pressure and high-vibration hydraulic systems.

Threads of Metric DIN/ISO

Metric hydraulic threads (commonly DIN 3852 or ISO 6149) generally adhere to the same principles as BSPP, which is that straight metric threads pull the fitting downwards onto the sealing element, typically an O-ring placed on cone seats as well as a washer bonded to it instead of sealing the thread. Like BSPP, the correct identification is crucial because straight threads made of metric can appear very similar to straight threads from the imperial style (like SAE O-ring bosses), but they are not compatible in dimensions and pitch.

What is the significance of thread identification? important?

Because a lot of these standards appear almost similar at a glance, straight 3/4-inch threads could be BSPP or metric, for instance, straight-thread from SAE; for instance, the misidentification of a thread is among the most frequently occurring assembly errors during maintenance of hydraulics. When a fitting is not properly matched, it could cause metal to fall off the threads, ruin the sealing surface, or result in an assembly that appears to be tight but leaks during pressure cycling. The best practice is to check the thread's standard with a gauge for threads or a pitch caliper instead of assuming that based on appearance. This is especially true for fittings that are being replaced on a piece of equipment whose source (domestic, European, or Asian manufacturing) isn't clearly stated.

Sealant, torque, and assembly procedures

Even with the proper thread standard being identified, the correct method of assembly is still important:

• Threads that are tapered (NPT/BSPT) Make sure you use the proper thread sealant or PTFE tape to prevent contamination of the hydraulic fluid. Tighten up to the manufacturer's recommended number of turns beyond the hand-tight, rather than using torque as the sole factor.
• O-rings or flare fittings (BSPP, ORFS, JIC (metric, JIC, BSPP, ORFS, JIC)) torque according to the specifications set by the manufacturer of the fitting. The seal will be compressed if you under-torque it, while over-torquing could cause damage or extrusion to an O-ring or alter the shape of the flare seat.
• Reused fittings: Check flare cones, O-ring seal washers, and flare cones for damage prior to disassembling, since they are wear objects even when threads appear fine.

The threads for hydraulic fittings aren't interchangeable, as the mechanism that they use to seal tapered thread interlocking an O-ring with a flat face, a conical flare of metal, or a captured sealing face -- decides the way it is installed and the reason it fails when it is assembled incorrectly. It is important to determine the thread normal, employ the correct sealing method, and then follow the correct torque procedures. This is one of the easiest and most effective methods to stop leaks from the hydraulic systems of any kind.

1. What is the distinction between NPT and BSPP threads?

NPT is a tapered, bonded thread that can be sealed by removing thread interference. It requires sealing agents, while BSPP is a straight thread, which seals by a bonded or O-ring washer with a smooth surface, while the thread is responsible for creating the force needed to clamp.

2. Can I combine fittings from BSPT and NPT?

No. Despite a similar appearance, NPT and BSPT use different thread pitches and angles, and putting them together usually results in damage to the threads and in the possibility of a leak.

3. What is the reason why JIC fittings leak despite being properly tightened?

JIC seals are based on a precise 37-degree metal-to-metal flare. Most leaks are the result of a scratched, damaged, or poorly formed flare cone, rather than a lack of torque.

4. Are ORFS fittings superior to JIC fittings?

ORFS is usually regarded to be more leak-proof because its O-ring seal face is less prone to variation in torque and surface imperfections as opposed to the cone seal made of metal that is used to seal JIC fittings.

5. How do I determine whether a fitting is a straight thread that is imperial or metric?

Make use of a thread pitch gauge or caliper for measuring the thread's pitch and diameter, as imperial and metric straight threads may appear similar visually, but they aren't dimensionally compatible.