Fittings: Flared vs. Flareless

Flared and flareless fittings are both commonly used throughout the aerospace industry in many applications. For example, the Army-Navy (AN) military standards require tighter tolerances and define all fittings types, wherein the joint intelligence center utilizes AN specifications as a basis to form their own standards covering fewer sizes. This blog will discuss both flared and flareless fittings and their specific types.

When compared to standard pipe fittings, flared fittings provide unique design and performance benefits and can be used in thin to medium wall tubing. In the majority of hydraulic design applications, flared fittings have long since replaced pipe fittings, especially within military and aerospace equipment. In low to medium pressure applications, the flared fitting is the most common connection type. In most hydraulic systems, the single flare SAE (Society of Automotive Engineers) 37° is the standard. SAE Standards pertaining to the maximum tube wall thickness must be adhered to ahead of flaring the tube to match the fitting connection.

The primary type of flared fitting is the AN flared fitting. Like all flared fittings, the tubing is flared in preparation for installation to secure the seal. The fitting connection is made with a sleeve and nut. The nut is fitted and tightened over the sleeve and draws the sleeve and flared tubing securely to a cone shaped fitting to form a seal. In a male fitting, the cone is of the same angle as the inside of the flare (i.e. 24°, 37°, etc). The sleeve serves as a support to minimize vibration at the flare and distribute the energy over a wider area. AN fittings meet requirements for tighter tolerances, enhanced exposure to fatigue, and service life in aerospace and military applications. These specifications also make them desirable in commercial and industrial uses where high performance is needed.

Flareless fittings, also known as compression fittings, are used in medium and high pressure applications. They consist of a nut, single or double ferrule, and the fitting body. The nut and ferrule/ferrules slide over a tube with an outer diameter that matches the inner diameter of the fitting receiving the nut and ferrule. When tightened to the proper torque, the nut compresses the ferrule to the tube to provide a tight, leak-free seal. The higher pressure a system has, the thicker the tube wall must be, making them more suited to flareless fittings. Thick-walled tubing is not possible to flare.

The most common type of flareless fitting is the MS (military standard) fitting. These are used in high pressure hydraulic systems in aircarft (where pressure is greater than 3,000 psi) in areas that will experience significant vibration and inconsistent pressure levels. MS fittings, which are made of a body, sleeve, and nut, replace the need for flaring and still provide a safe and dependable connection.

The internal design of the body causes the sleeve to crimp into the tube’s outer diameter as the nut and body are joined. For steel connections, there is a counterbore with a reverse angle of 15° in the shoulder body of the fitting. This is designed to prevent the tubing from collapsing inward while tightening and act as a sealing force against the counterbore’s body.

 
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