Flare fittings are used on hard lines like brake lines and are made by flaring the hard line to fit snugly over a fitting and nut. They can withstand high line pressure and eliminate the need for gaskets. A double flare can be used for heavy-duty connections. SAE flare fittings have a 45-degree angle, while AN fittings have a 37-degree angle and are used in military and high-performance applications. Flare fittings are made using a flare tool and lubrication to prevent wear or cracking.
A flare fitting is a type of fitting used on hard lines, such as steel and aluminum brake lines. Commonly made from hard line that has been flared to mesh against a fitting and nut, the flared fitting fits snugly over the fitting and is held securely in place with the nut. The flared end of the hardline matches the angle on the end of the fitting so precisely that no gaskets or sealants are required to prevent leaks. Most flare fitting connections are created by home mechanics with a flare tool. The key to making a good flare fitting is not to break the tubing and to avoid overtightening the connection.
One benefit to using a flare fitting is the amount of line pressure the connection can withstand. By eliminating the gasket, the connection is virtually leak-proof. In heavy-duty connections a double flare will be used which effectively doubles the material in the flared line, making an even greater clamp possible; this squeezes the flare into the steel fitting even tighter. The action virtually eliminates any possibility of fluid passage into the fitting.
The angle of the flare in a Society of Automotive Engineers (SAE) flare fitting is 45 degrees. All related fittings and connections used with a flared line are also 45 degree angles. While the flare fitting provides a high pressure rating and is used on critical systems such as brake lines and fuel lines, the system does not provide the same level of strength and cannot be combined with the AN fitting. Navy (AN) fittings use a 37 degree flare and undergo a much more stringent quality control and safety testing procedure. While commonly used on aircraft and ultra-high performance racing applications, the AN fitting is overkill for most non-military or road-going applications.
When creating a flare fitting, a flare nut is placed on the hard line before the line is fed into the flare tool. The hard line is then placed into an appropriately sized collar in the flaring tool, with the end of the line flush with the tool before it is tightened into place. A tapered mandrel is then placed on the flaring tool directly centered on the exposed hardline. As the mandrel is tightened, it begins to penetrate the pipe and the flared shape of the mandrel forms the flare. A little lubrication will usually prevent any wear or cracking of the tube as it is stretched into shape against the walls of the flaring tool.
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