Both flared and flareless fittings are commonly used in the aerospace industry, depending on the application. The AN (Army-Navy) military standard calls for tighter tolerances and defines all fitting types, where the JIC uses AN specifications as the base line for their own standards and covers fewer sizes.
Flared fittings provide significant design and performance characteristics as compared to pipe fittings and can be used with thin to medium wall tubing. In most hydraulic design applications, pipe fittings have long been replaced with flared fittings, particularly in military and aerospace equipment. In low to medium pressure applications, the most common connection type is a flared fitting. Single flare SAE 37° is the standard in most hydraulic systems. SAE standards for the maximum tube wall thickness must be followed before flaring the tube to match the fitting connection.
AN (Army-Navy) Flared Fittings
As with all flared fittings, the tubing is flared in preparation to installation to secure the seal. The flared tube fitting connection is made with a sleeve and a nut. With the nut fitted and tightened over the sleeve, it draws the sleeve and flared tubing securely to a cone shaped male fitting to create the seal. The cone on the male fitting is of same angle as the inside of the flare (i.e. 24°, 37°, etc). The sleeve will serve as a support to alleviate vibration at the flare, distributing the energy over a wider area. Male and female AN 37° flared fittings meet Class 3A/3B UNJ/UNJF allowing for tighter tolerances, enhanced exposure to fatigue and longer service life for aerospace and military applications. The tighter specifications of AN fittings also make them ideal for commercial and industrial applications where higher performance is desired.
Also commonly known as compression fittings, flareless fittings are used in medium to high pressure applications. They are comprised of a nut, single or double ferrule and the fitting body. The nut and ferrule(s) slide over a tube with an OD that matches the ID of the receiving fitting. As the nut is tightened with the proper torque, the nut compresses the ferrule to the tube, thus providing a tight and leak-free seal. The higher the system pressure the greater the required wall thickness of the tube and therefore more applicable for flareless fittings. Thicker walled tubing is not practical or even possible to flare.
MS FLARELESS FITTINGS
MS (Military Standard) flareless fittings are used predominantly for high pressure (≥ 3,000 psi) hydraulic systems in areas that could experience rigorous vibration or inconsistent pressure. An MS type fitting replaces the need for flaring the tube and still provides a safe and dependable connection. An MS fitting is made up of a body, a sleeve and a nut. The internal design of the body causes the sleeve to crimp into the OD of the tube as the body and nut are joined. There is a counter bore shoulder in the body of the fitting with a reverse angle of 15° for steel connections, designed to prevent the tubing from an inward collapse when tightening and serves as a sealing force against the body of the counter bore.
This overview of flared and flareless fittings is an excerpt from our Aerospace Whitepaper, if you would like to learn more please click below: