Temperature and pressure decide which type of tube fitting is needed. Metal fittings are used for high temperature and pressures, plastic fittings for low. Typically, metal fittings are used with metal tubes, but plastic fittings are sometimes used with metal tubes to reduce galvanic corrosion.
The fittings used on hydraulic tubing for high-pressure applications are categorized as either permanent or separable. Most separable fittings are threaded.
Threaded flare fittings press the flared end of the tubing against the mating surface of the fitting. As the fitting pieces are drawn up tight, a conical seal is formed. Two flare configurations are standard with the Society of Automotive Engineers (SAE). The 45° flare is used extensively in nonhydraulic automotive, refrigerant, and marine systems. The 37° flare (formerly known as the JIC flare) is extensively used in hydraulic applications. The 45° fittings are commonly made of brass, but 37° fittings are commercially available in brass, steel, and stainless steel.
Several types of flare fittings are available. Two-piece fittings place pressure against the flare with a long-necked nut that aligns and supports the tubing. Application of tightening torque tends to twist the tubing. Friction between nut and flare may produce unequal distribution of the compression force on the flare. The long-necked nut requires its full length of tubing before a bend can start.
A three-piece fitting (nut and sleeve) creates tightening force with a short nut which is transmitted by a sleeve to the flared tube. No twisting action is transmitted to the flare and, because the nut is short, tubing bends can be placed closer to the fitting.
The 60° cone fittings seal by metal-to-metal contact, with a tapered cone seating onto the tube (or nose in the hose swivel) in the most widely used version. The fittings exhibit good sealing reliability, but are highly torque sensitive in small sizes.
O-ring face-seal fittings compress an O-ring against a flat face, providing a positive, leak-free seal. They provide near-instantaneous torque rise when the connection is tight, ensuring proper installation. And once hand tight, they require less rotation to seal properly, making them suitable for plumbing in tight spaces. O-ring materials must be compatible with the sealed fluid.
Threaded self-flaring fittings do not require a special flaring operation. During nut tightening, a special wedge-shaped sleeve presses against the tubing end to create a flare. The fitting works best on thin-walled tubing, where high torque is not required to produce a flare. This joint is mechanically strong and vibration resistant, with good reusability.
Threaded flareless fittings do not require a flaring operation and are usually used with thick-wall tube. Ferrule type flareless fittings seal by forcing the ferrule or sleeve cutting edge into the tubing wall to create an interference seal. The resulting spring-action mechanical joint offers a leakproof seal and resistance to vibration.
The 24° cone flareless fitting is second only to the 37° flared fitting in popularity, and is the most widely used fitting in Europe. Sometimes called an EO style fitting, the design incorporates a 24° tapered throat on the fitting body, and a nut that drives a ferrule into the tube as it is tightened. Metric threads are specified by DIN 2353 (the SAE J514 version of the 24° cone has UN and UNF threads). Three different series are offered, LL for low-pressure, L for high-pressure, and S for high-pressure, severe-service applications. Sealing is by metal-to-metal contact, but there is a welded option for L and S-series fittings that seal with an O-ring as well. These fittings offer excellent sealing reliability.
The ferrule-fitting principle is also used in an inverted flareless arrangement, wherein the internal design configuration is machined in the boss part. The tubing is inserted directly into the boss and connected with a male threaded nut which eliminates one seal and permits closer bends. Compression fittings seal when both ends of the sleeve are compressed during assembly and deflect into the tubing diameter to form two parallel grooves. Since sealing results from nondestructive surface contact between sleeve and tubing, longitudinal defects can affect the seal. Application of this fitting is limited to thin-walled soft tubing, usually copper, and low-pressure vibration-free systems.
Permanent fittings may be welded, brazed, swaged, or adhesively bonded. These fittings were developed by the aerospace industry, where high reliability, low installation cost, and light weight are prime factors. Though developed for aerospace, many of these fittings are now used in industry. Obviously, they cannot be reused.
Plastic fittings cannot tolerate extreme heat or high pressures. But they cost less than metal, stop galvanic corrosion, deaden noise, and withstand assault by a wide range of corrosive fluids.
Generally, plastic fittings are well suited to low-temperature fluid-transfer applications. At pressures below 500 psi and temperatures below 250°F, plastic fittings are highly competitive with steel, stainless-steel, or brass fittings.
Plastic fittings have a pullout resistance ranging from 20 to 300 lb, depending on size, tubing material, and locking devices. Plastic or metal grippers are used to increase pullout resistance. Plastic grippers are used only with plastic tubes, and are so strong that the tubing tears before it pulls out of the fitting. Metal grippers are used with metal tubes, and can resist pullout forces over 300 lb.
Although plastic fittings are usually used at pressures of 500 psi or less, some fittings have been tested at much higher pressures without failure. With polyethylene tubing, the tubing always fails before the fitting. With nylon, copper, and steel tubing, failure pressures range from 250 to 3,000 psi.
Because plastic fittings are molded with smooth internal surfaces, they have inherently low resistance to flow. Furthermore, plastic resists scale buildup and does not rust, so flow passages tend to remain free of obstructions.