Tube Fittings

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. 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