Design criteria require any tube fitting to achieve two functions. It must both seal against leaks and mechanically hold the tubing against system pressure. Flared fittings achieve both functions in the simplest manner possible. The seal is a "coined" line of contact between the tube flare and the 37° nose on the fitting – the high and low points of each surface "coining" together to affect a seal. Mechanical holding power is achieved by the clamping action that traps the tube flare between the sleeve and the nose of the fitting when the nut is tightened. Tube twist does not occur because the bearing area between the nut and sleeve is larger than that of the flare to nose contact line.
The flared tube fitting is the safest of all tube fittings in that it is "blowout proof” up to the point at which extreme pressure causes the tube to cold flow and extrude from the fitting. Even an incorrectly flared tube will "hold", even though it may leak. Compare this with "flareless" fittings where preparatory steps are often neglected resulting in improper "setting" of the ferrule(s) – such action causing irreparable leaks and blowouts.
Complete Reusability & Remakeability
Unlike flareless fittings which have limited remakeability and require at least a new ferrule when tubing is replaced, flared fittings have almost unlimited remakeability and all parts are completely reusable. Note that soft metal seal washers are available which slip over the nose of the fitting and serve as a gasket to perfect a seal between damaged flare parts.
More Shapes, Sizes & Manufacturers
Because 37° flared fitting dimensions are controlled by the Society of Automotive Engineers (SAE) specification J514, all manufacturers' parts are interchangeable. This interchangeability and wide usage as both tube fittings and hose adapters make the 37° flare fitting the most universally available tube fitting.
Only Parker has all of the most important features: Forged shapes, nuts and sleeves; stronger rolled male threads and burnished seal surfaces; brass, steel, and stainless as standard.
The most obvious drawback of flared fittings is that the tube must be flared. New hydraulic flaring equipment makes this task easy and will flare up to 0.120 wall, 2" OD stainless tubing. While flaring might be considered an extra step, 5/8'' OD and larger flareless fittings must be preset using hydraulic tooling.
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