Over the years, we have received quite a few inquiries regarding "AN" fittings. AN (Air Force - Navy Aeronautical Standard) and AND (Air Force - Navy Aeronautical Design Standard) are standards used by the U.S. Military in aviation applications. AN fittings are manufactured to meet above standards. These fittings include the 37° flare, flareless, hose connections, "banjo" type fittings, specialized flange fittings, pipe fittings and other types of connections.There are many frequently asked questions about AN 37° flare fittings and how they relate to their industrial 37° flare counterpart, the SAE/ISO 37° flare fittings, so I thought I would share the background and differences between these fittings.
Parker actually pioneered the flare fitting technology in the 1920's with the introduction of the inverted flare fitting followed by the Parker Triple Fitting in early 1930's. They were adopted by Air Corps, a part of the U.S. Army at that time, as AC-810 and AC-811. As the operating pressures increased, inverted flare did not perform as well as the Triple Fitting, and its use started declining. The ease of manufacture of Triple Fitting provided additional advantage that resulted in quick acceptance of it in various industrial and military applications. The Triple Fitting was a patented three-piece design similar to current Parker Triple-Lok 37° flare fitting, except it had 30° flare angle instead of the 37°. This fitting design was the forerunner of the current AN and SAE 37° flare fittings.
The U.S. Air Force, with help from Wright Patterson Air Force base, developed a fitting with 37° flare angle, before WWII, which became known as the "AN" fitting. This fitting had precision 3A/3B threads. The use of "AN" fittings proliferated from the 1930's through the 1960's to include most branches of U.S. Military, Military Contractors, General Aviation and Commercial Aviation. These fittings were even adopted for use in many land and sea applications of the U.S. Military as well, leading to confusion between AN and its industrial counterpart, the SAE 37° fitting. After the war, several versions of 37° flare fittings flooded the industrial market, creating a nightmare for the users. The Joint Industry Conference (JIC), an organization of manufacturers, decided to standardize on the "AN" design, except with 2A/2B thread class for ease in manufacturing. These fittings came to be known, throughout the world, as "JIC" fittings. The JIC wanted the prestige of SAE for the fitting standard. They convinced SAE to take on the task and helped in the development of the standard. Thus, the SAE standard 37° flare fitting became part of SAE J514 in 1950. The fitting became an ISO standard, ISO 8434, in 1986, replaced by ISO 8434-2 in 1996.
AN Fittings Today
AN fittings remain prevalent today. However, the U.S. Military is canceling many of the true AN/AND standards and replacing them with AS (Aerospace Standards) standards developed, again, by SAE. Many aircraft/aerospace applications are moving towards aluminum and titanium for optimized weight-to-strength parameters.
Differences Between AN 37° Flare and Industrial 37° Flare Fittings
There are several differences between "comparable" industrial 37° flare (SAE/ISO) and AN 37° flare style fittings. Some include:
- AN 37° Flare: Male and female, Class 3A/3B UNJ/UNJF (radiused root threads)
- SAE/ISO 37° Flare: Male and female, class 2A/2B, UN/UNF series threads
- Reason: Tighter tolerances and better fatigue life for aircraft, aerospace, military applications
Military Conformance Standards:
- AN 37° Flare: AN flare fittings conform to MIL-F-5509 specifications, and also AS4841
- SAE/ISO 37° Flare: Some fittings conform to MIL-F-18866 as shown on MS51500 through MS51534
Industrial Conformance Standards:
- AN 37° Flare: Meets SAE Aerospace (AS) standards
- SAE/ISO 37° Flare: Meet the applicable dimensional and performance requirements of SAE J514/ISO 8434-2
- AN 37° Flare: Available commonly in carbon steel, stainless steel (CRES), aluminum, titanium and copper-nickel
- SAE/ISO 37° Flare: Available commonly in carbon steel, stainless steel, and brass
- In addition to the above noted differences, drop lengths, hex sizes, hex widths may also vary between "comparable" AN 37° flare and industrial 37° flare fittings in some sizes.
AN 37° flare and industrial 37° flare fittings function identically. In many cases they appear to be functionally interchangeable, but they are not. What this means is that while the products may look similar, you must not use an industrial 37° flare fitting design as a direct substitution.
Note: Parker Triple-Lok 37° flare fittings (or other TFD products) are not for use in Aerospace, General Aviation, Commercial Aviation or Military Aviation applications.
If you have any questions or comments, please post them and we will respond if warranted. To talk to our techConnect engineer team directly, they can be reached at Parker Tube Fittings Division, 614.279.7070.
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Contributed by Burleigh Bailey, senior project engineer, Parker Tube Fittings Division
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