Rigid fluid lines are used throughout aircraft for fuel, oil, coolant, oxygen, instrument, and hydraulic lines. They are used in stationary applications where long and relatively straight runs are possible. To function properly, rigid fluid lines utilize a wide range of fittings. These include AN flared fittings, MS flareless fittings, swaged fittings, and cryofit fittings. In this blog, we will discuss each type of fitting and their unique characteristics.
AN flared tube fittings feature a sleeve and nut. The nut fits over the sleeve and draws the sleeve and tubing flare tightly against a male fitting to form a seal when tightened. When used with this type of fitting, tubing must be flared prior to installation. The male fitting consists of a cone-shaped surface with the same angle as the inside of the flare. The sleeve supports the tube such that vibration does not concentrate at the edge of the flare, instead distributing the shearing action across a wide area. To prevent dissimilar metal corrosion, fitting combinations of different alloys should be avoided. AN fittings are characterized by their black or blue color - AN steel fittings are black, while AN aluminum fittings are blue.
MS flareless fittings are designed for high pressure hydraulic systems (over 3,000 psi) that may experience severe vibration or fluctuating pressure. This type of fittings negates the use of tube flaring, but still provides a safe, strong, and dependable tube connection. MS flareless fittings comprise three parts: a body, sleeve, and nut. The internal design of the body forces the sleeve to cut into the outside of the tube when the body and nut are joined. The counterbore shoulder in the body is designed with a reverse angle of 15° for steel connectors and 45° for aluminum fittings. The reverse angle helps prevent inward collapse of the tubing when tightened and provides a partial sealing force
Swaged fittings are a popular solution for repairing and connecting hydraulic lines on transport category aircraft. Fittings of this type create a permanent connection that is effectively maintenance-free. Swaged fittings are most commonly used to join hydraulic lines in areas where disconnections are not routinely required. They are frequently used with titanium and corrosion resistant steel tubing. The fittings are installed with portable, hydraulically-powered tooling, compact enough to be used in tight spaces. Should the fittings need to be disconnected, this is done with a tube cutter. Special installation tooling is also available, and manufacturer’s instructions should always be followed when installing swaged fittings.
Finally, cryofit fittings are used in many transport category aircraft to join hydraulic lines in areas where routing disconnections are not required. They are standard fittings with a cryogenic sleeve made of Tinel, a shape memory alloy. The sleeve is manufactured 3% smaller, frozen in liquid nitrogen, and then expanded to 5% larger than the fluid line. During installation, the fitting is removed from liquid nitrogen and inserted onto the tube. At this point, the fitting warms up and contracts to its original smaller size, clamping down on the tube to form a permanent seal. They can only be removed by cutting the tube at the sleeve.
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