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NASA KC-Fitting

History of the KC Fittings 37-Degree Fitting History Arthur L. Parker at Parker Appliance Company 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 the US Army Air Corps (before the USAF became a separate branch of the military) as AC-810 and AC-811. The inverted flare did not perform under high pressures as well as the Triple Fitting. The Triple Fitting was a patented three-piece design similar to current Parker-Hannifin Triple-Lok, except it had a 30° flare angle instead of the 37°. Parker-Hannifin was formed in 1957 when Parker Appliance Company bought the Hannifin Corporation which manufactured valves. The “Triple Fitting” design was the forerunner of the AN, MS, JIC, and KC fittings. Wright-Patterson Field (now called 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 class-3 threads. The AN fittings was adopted by the other branches of U.S. Military, and commercial aviation companies. “AN” fittings and “MS” fittings for “Army/Aeronautical-Navy” and “Military Specification” respectively. After the war, several versions of 37° flare fittings entered the market. Some of these are incompatible with others. There is some confusion about the difference between AN and its industrial counterpart, the Joint Industry Conference (JIC) 37° fitting. The JIC, an association of manufacturers, decided to standardize on the AN design, except with class-2 threads for lower manufacturing cost. These fittings came to be known, throughout the world, as "JIC" fittings. SAE developed the SAE standard 37° flare fitting in SAE J514 in 1950. The fitting became an ISO standard, ISO 8434, in 1986, replaced by ISO 8434-2 in 1996. AN-labeled fittings are still common today. However, the U.S. Government is canceling many AN/MIL/MS/KSC/NASA standards and replacing them with SAE AS (Aerospace Standards) standards. 37° flared tube fittings were widely used in the aerospace industry and in automobile racing. Carbon steel and stainless steel are used in hydraulic systems and aluminum is used in automotive racing fuel systems. These fittings are sold under a variety of specs. They are colloquially called “aircraft fittings” in the automotive genre. KC Fitting Development The Problem: The AN/MS fittings require high torque (clamping force) to generate a metal-to-metal seal. Re-torquing after temperature changes or vibration was often required to eliminate leaks. The 300-series stainless steel has a very low yield strength (compared with its tensile strength). This limits the additional torque available to seal a leaking joint. Engineers and technicians used “Flare Savers” (also called “conical nose seals” which are soft metal inserts made from copper, aluminum, stainless steel, or nickel) to create a better seal. Flare savers don’t really work. They add a softer material to accommodate surface irregularities but add another potential leak path. The fitting needed a compliant seal material to accommodate some motion and surface irregularities and also to lower the required torque to generate a seal. Trivia: The 37-degree angle comes from a 3-4-5 triangle; the smallest angle is 36.87 degrees. It is one of the easiest, most accurate angles to set up, cut, and measure since it can be established with linear measurements and no angular measurements.

The Idea: Bill Cartland had the idea and took the initiative to make the first prototype. He worked for Lockheed and had access to the machine shop in Hanger XX on the Cape Canaveral Air Force Station side of the river. Cartland cut the groove on a lathe and made a Teflon seal ring to fit the groove. Teflon has the advantage of compatibility with virtually all fluid commodities used on the Space Center. Once the fitting worked with Teflon, no other materials were tested. The size of the groove was selected so Teflon could not deform into the path of the fluid.

Developing the Idea: Jack Herron and John “Speedy” Bainbridge worked out seal ring dimensions and produced the drawings for Jack Kunkle. They were non-degreed designers from Birmingham and Kunkle was a Yankee engineer, so they had a natural source of friction to keep them on their toes. The original fittings were made from type 316 stainless steel. It has excellent corrosion resistance and is easier than type 304 to thread. Straight fittings are made from bar stock while Tee’s, elbows, and crosses are made from forgings. Aluminum fittings were made in 2014-T6 (forgings) and 2024-T6 (bar stock). Aluminum fittings are anodized blue.

The Name: Many people believed that the KC fittings were named for “Kennedy Center” or some variant. The “KC” name actually came from Kunkle-Cartland, the names of the two most instrumental men in the creation of the fittings.

Testing the Idea (Pressure Ratings): Original qualification testing was done in Birmingham, Alabama by Lockheed Industrial Products in May 1966. The test report is KSC-TR-324-D. No KC fittings suffered a structural failure during the pressure test. The connecting tubing burst on the ½”, ¾”, and 1 ½” sizes. The seal rings blew out on the other sizes. Under high pressures the tube flare can rotate slightly and the seal ring loses its support and the pressure blows out the seal ring. Fitting design pressures were set at the maximum pressure achieved in the test divided by four. This follows the prevalent fluid system design approach at the time of 4:1 on burst (even though the fittings never actually burst).

Implementation: Fifteen machine shops tried to make the fittings; one succeeded. Allen Aircraft was the only one who could accurately locate the seal ring groove on the 37-degree surface of the fitting.

Protecting the Idea: S&P sued NASA over the fitting design. They lost the law suit. NASA never patented the fittings. Allen Aircraft sells the design commercially. Parker Hannifin sells a similar design under the trade name, “Triple-Lok” referring to the metal-Teflon-metal seal pattern as three (i.e. “Triple”) seals.

Swivel Fittings (KC137-type) Swivel fittings are cut at a different angle (39.5-degrees) than all the other fittings. The swivel fitting end is unique in the way it supports (or doesn’t support) the sealing surface. If you look at conical end (KC141) or a tube (KC154) backed up by a sleeve (KC143) those two configurations are quite stiff. As the load is applied, the 37-degree male end is compressed straight (relatively) against a 37-degree female end. This creates something resembling even compression on the sealing surface. If you look at the KC137, the load path is from the stainless wire pressed into the hole created by the swivel nut-KC137 interface. Each part has half a circle to accommodate the round wire (really it is half a torus shape in three dimensions). With that relatively flexible (unsupported) end, the inner part of the sealing surface comes into contact first. Then the force which is applied to the outer edge of the KC137 rotates the outer part of the sealing surface into contact. Part of the reason there is extra rotation in the KC137 end is that the line-of-action is where the wire centerline contacts the outside edge of the groove. This offset in the line-of-action vs. the resultant force (at the center of the sealing surface) creates a moment which bends the 39.5-degree angle to a 37-degree angle. The extra two and a half degrees came from trial-and-error. It would be quite difficult to predict, especially in the 1960’s. The larger sizes (one inch and larger) fittings are sufficiently stiff so they don’t need the extra two and a half degrees. The actual problem is three dimensional, but it is easier to see and explain in 2D. The ‘rotation’ is really the KC137 going from a trumpet shape to a less-pronounced trumpet shape. New Pressure Ratings Code changes: In 1988, KSC was beginning to adopt commercial standards in place of the NASA and KSC specs and standards started in the 1960’s. Roger Hall in KSC Propellants and Gasses Design started the research into applicable industry standards for the design of pressure systems. ASME B31.3 (then called “Chemical Plant and Petroleum Refinery Piping”) was selected as the most appropriate for KSC design work. Hall had a young engineer, Eric Thaxton, start an analysis project to re-evaluate the KC fittings for the B31.3 criteria. This criteria would replace the 4:1 burst factor the fitting theoretically used previously. Finite Element Modeling: Thaxton created a series of finite element models of the KC fittings to establish ASME B31.3 design pressures. Hand calculations determined the design pressures for the standard tube sizes. Each fitting was represented by the KC105 fitting end because that included the thinnest/weakest part of each fitting. The exceptions are the KC106 reducer which had to be examined individually and the swivel nut fittings (several KC numbers) which had unique a weakness in the inserted wire which allows the swiveling. Aluminum fittings were evaluated using the same models changing the material card from the modulus of stainless steel to aluminum. The finite element models showed that some sizes would fail by bursting (if the tubing were sufficiently strong) and some would fail by deformation until the seal ring lost contact with the flare. This second failure mode perplexed Thaxton until he saw the 1966 test report. Lack of Documentation (no update to KSC-SPEC-Z-0008): The new pressure ratings were sent to the NASA and contractor design organizations on a letter signed drafted by Hall and Thaxton and signed by the DE Director, Walt Murphy. The one-page letter directed the designers to use the new working and test pressures for tubing and fittings. The letter was intended to be a temporary measure until the upcoming release of the revised KSC-SPEC-Z-0008. 0008 contains a table of the design pressures. It was last updated in 1978 and was due for a new release. The new revision was started by Thaxton under Hall’s direction. Thaxton left for the Analysis group and the revision past through two more engineers unreleased. Hall moved on to the Space Station Directorate and the revision was forgotten. The new data sat in the files of a Fluids Design Engineer for the following twenty years until the “new” revision work was old. As of 2010, the revision of 0008 is still delinquent. SAE Standard: SAE has a standard for 37-degree flared tube fittings so KSC approached the G-3 Committee to add the KC-style fittings to their standards. Allen Aircraft was the sponsor (since they provide most of the KC fittings to KSC. SAE adopted most of the fittings in GP-425.  They did not adopt the butt-weld fittings or the Grayloc fittings (those belong to Gray Tool Company) or the orifice fittings (union or bulkhead). AL-6XN: The addition of AL-6XN (UNS N08367) to the KC fitting family didn’t change almost anything.  Allegany-Ludlum’s alloy (the “AL”) which contains 6% Molybdenum to combat pitting corrosion (the “6”) strengthened with added Nitrogen (“XN”) is stronger than 316 CRES so the pressure ratings are higher.  Since the AL-6XN fittings look identical to 316 (except for the “L” in place of the “K” material code) we kept the pressure ratings the same for both.  A 316 fitting could be substituted without being noticed.  Form a corrosion standpoint, there is no problem with 316 fittings and AL-6XN tubing or vice versa.  The Materials Science Labs tested the galvanic action between AL-6XN and 300-series stainless steel and found little effect. (Compiled by Eric Thaxton from discussions with Jack Kunkle and Roger Hall)