Hydraulic tube fittings are used to connect hydraulic system components with hydraulic tubing or hoses, ensuring a secure connection and reliable sealing within the hydraulic circuit. Common types of hydraulic tube fittings include flare fittings, welded fittings, compression fittings, crimp fittings, detachable fittings, and quick-connect fittings, as shown in Figure 1.
1. Flare Fittings
As shown in Figure 1(a), flare fittings achieve sealing by clamping the flared end of the tube (1) against the fitting sleeve. These fittings feature a simple construction and are suitable for connecting copper tubing, thin-wall steel tubing, nylon tubing, and plastic tubing. They are generally used in low- and medium-pressure piping systems and can also be used in water and pneumatic systems.
2. Welded Fittings
As shown in Figure 1(b), welded fittings consist of a tube welded to the fitting insert (1). The spherical sealing surface of the fitting insert mates tightly with the conical bore of the fitting body, providing excellent sealing performance, a simple construction, and high pressure resistance.
The main disadvantage is that welding is relatively time-consuming and requires high welding quality. These fittings are mainly used for connecting high-pressure, thick-wall steel tubing and are suitable for hydraulic piping systems using oil as the working medium. Installation involves considerable welding work, and disassembly is relatively inconvenient.
3. Compression Fittings
As shown in Figure 1(c), compression fittings achieve sealing by means of a highly elastic ferrule (2), which securely grips the tube (1). They feature a simple construction and easy installation, but require high dimensional accuracy and surface quality on the outside diameter of the tube.
Compression fittings are suitable for connecting high-pressure cold-drawn seamless steel tubing, and the ferrule requires high manufacturing precision. They are commonly used in piping systems conveying hydraulic oil and gas.
4. Crimp Fittings
As shown in Figure 1(d), crimp fittings consist mainly of a fitting sleeve (1) and a fitting insert (2). They are primarily designed for connecting flexible hydraulic hoses. The sleeve is mechanically crimped onto the hose and fitting insert to provide a secure connection and reliable sealing.
5. Detachable Fittings
As shown in Figure 1(e), detachable fittings are designed with hexagonal sections on both the fitting sleeve (1) and fitting insert (2), making them convenient for frequent installation and removal of hoses. They are suitable for connecting small-diameter, high-pressure hydraulic hoses where frequent disconnection is required.
6. Quick-Connect Fittings
As shown in Figure 1(f), quick-connect fittings have a relatively complex internal structure and may result in comparatively higher pressure drop and flow resistance. They are designed for quick connection and disconnection and are commonly used in piping systems conveying hydraulic oil and gas.
1. NPTF Tapered Thread Fittings
NPTF is a dryseal tapered pipe thread primarily used for fuel and fluid service applications. It can be used for both male and female threaded connections.
An NPTF male thread can mate with NPTF, NPSF, or NPSM female threads, depending on the specific application and sealing requirements.
NPTF fittings are similar to BSPT fittings in that both use tapered threads; however, NPTF and BSPT are not interchangeable. For most sizes, the thread pitch is different. In addition, NPTF threads have a 60° thread angle, while BSPT threads have a 55° thread angle.
2. JIC 37° Flare Female Thread Fittings
JIC 37° Flare - The 37° flare angle (JIC) is specified by the Society of Automotive Engineers (SAE) and is commonly used for high-pressure hydraulic piping systems. Fittings of this type are generally referred to as JIC fittings.
JIC fittings feature straight threads and a 37° flare seat. The JIC male fitting has a straight thread and a 37° flare seat, while the JIC female fitting also has a straight thread and a 37° flare seat.
Sealing is achieved at the 37° flare seating surface. For certain sizes, the thread dimensions are the same as those of SAE 45° flare fittings. Therefore, the flare angle should be carefully measured to distinguish between 37° and 45° flare fittings.
3. SAE 45° Flare Male Thread Fittings
SAE 45° Flare refers to fittings featuring a 45° flare angle or flare seat. This type of fitting is commonly used with soft copper tubing, as copper can be easily flared to a 45° angle.
These fittings are primarily designed for low-pressure applications, such as fuel lines and refrigeration systems.
SAE 45° flare male fittings can only be properly mated with SAE 45° flare female fittings. The SAE male fitting has a straight thread and a 45° flare seat, while the SAE female fitting also has a straight thread and a 45° flare seat.
Sealing is achieved at the 45° flare seating surface. For certain sizes, the thread dimensions are identical to those of SAE 37° flare fittings. Therefore, the flare angle must be carefully measured to distinguish between the two types.
4. O-Ring Face Seal Male Fittings
O-Ring Face Seal (ORFS) male fittings can only be properly mated with ORFS female fittings. The male fitting features a straight thread with an O-ring, while the female fitting has a straight thread with a sealing face.
Sealing is achieved between the O-ring on the male fitting and the flat sealing face of the female fitting. This design provides reliable sealing and is widely used in high-pressure hydraulic systems.
5. O-Ring Flange – SAE J518
SAE J518 O-Ring Flanges, including SAE Code 61 and Code 62 four-bolt split flanges, are widely used worldwide for connecting hydraulic pumps, motors, valves, and other hydraulic components.
The four-bolt split flange design provides a reliable high-pressure connection and is commonly used in demanding hydraulic applications.
Three Exceptions
There are three exceptions:
1. Multiplier ×10
This is not an SAE standard size, but it is widely used in regions outside North America.
2. Caterpillar Flange
The Caterpillar-style flange features a thicker flange head (dimension "C" in the table). Its outside diameter is the same as that of the SAE Code 62 flange.
3. Poclain Flange
The Poclain flange is fundamentally different from the SAE flange and does not conform to the SAE flange dimensions.
6. O-Ring Flat-Face Seal Captive Male Fittings
A seal is formed when the O-ring on the male fitting comes into contact with the flat face of the female fitting. In hydraulic systems, elastomeric seals can be used to control fluid leakage. This type of fitting is suitable for applications where leak prevention and sealing reliability are critical.
7. Cutting Ring Fittings
Captive male cutting ring fittings are designed to be used with the corresponding female nut and cutting ring.
The male fitting features a straight thread and a 24° cone seat. The female connection consists of a straight-thread nut with a cutting ring, which serves as part of the sealing system.
Sealing is achieved at the interface between the cutting ring and the 24° cone seat of the male fitting, as well as between the cutting ring and the tube.
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