Machinery's Handbook, 31st Edition
2776 Tube Fittings Tube Fittings.— Tube fittings are used to connect tubing to tubing, or tubing to pipes or other objects. Tube fittings are generally non- permanent connections, although they may permanently deform the end of the tube. Materials: Fitting materials should be chosen for compatibility with the tubing and the fluid being transported. Combining dissimilar metals should be avoided to prevent gal vanic corrosion. Metal fittings and plastic fittings are widely available. Some plastic fit tings have metal ferrules, and some metal fittings can have elastomeric seals. It is critical to check material compatibility with all components of a fitting. Plastic fittings are used with plastic tubing that is softer than the fitting material. Steel fittings are used with steel or plastic tubing. Brass fittings are used with copper, aluminum, and plastic tubing. Types: There are many types of tube fittings, including proprietary and name-brand fittings. The most common general types are barbed, compression, flare, and push-in. Barbed fittings are used with soft plastic tubing which can expand enough to fit over the barb end but have elastic memory enough to seal over the barb or barbs. A clamp should be employed to hold the tube on if pressures are moderate. Pressure in the system should not exceed 150 psi when using barbed connections. To specify a barbed fitting connection, one must know the tube inner diameter, process pressure, material compatibility, and whether the tubing is suitable for use with barbed fittings. Compression fittings are used on metal tubing and stiff plastic tubing. Metal and plastic compression fittings are available. They are suitable for high pressures when constructed out of high strength material and combined with a high pressure rated tube. There are nu- merous designs available, but all include at least one ferrule, a nut, and a fitting body. When the nut is tightened, the ferrule swages the tube to form a pressure tight seal. The tube does not need to rotate during tightening. Compression fittings can be re-used if care is taken not to damage the ferrule or tube end. To specify a compression fitting, one must know the tube outer diameter, wall thickness, process pressure, and material compatibility requirements. To ensure proper sealing, the tube should be clean and cut square without burrs. The tube material must always be softer than the fitting material. Plastic compression fittings are used with softer plastic tubing, and stainless compression fittings are generally used with stainless tubing. Brass fittings can be used with firmer plastic, aluminum, and copper tub - ing. Carbon steel compression fittings are used with carbon steel tubing. Flare fittings are typically used with metal tubing which is forced over a flared seat and secured with a nut to create a leak-tight seal. They are similar to compression fittings but have a sleeve instead of a ferrule. A flaring tool is used on the end of the tubing to create the proper flare for the fitting being used. These fittings are suitable for high pressures, even greater than with compression fittings. The most common flare fitting standards in use today are the 45° SAE fitting, and the 37° JIC fitting. The 37° style generally has a higher pressure rating for a given size tubing. The 45° style is generally used for low pressure refrigeration applications, while the 37° style is used for high pressure hydraulic applica tions. SAE and JIC fittings are incompatible due to the different flare angles. Flare fittings can be reused if care is taken not to damage the flared end of the tubing. Stainless steel fittings are used with flareable seamless stainless tubing. Brass fittings are used with flare able seamless copper. Carbon steel fittings are used with flareable welded and seamless carbon steel tubing. Aluminum fittings are used with seamless aluminum tubing. Push-in fittings are popular for pneumatic connections, but are also made for liquid ap- plications. These fittings have an internal gripping ring and usually an o-ring. The tube must be cut square and be burr-free. A leak-free seal is created when the end of the tube is pushed into the fitting. To release the tubing, the end of the fitting must be compressed axially to release the gripping mechanism. These fittings normally have elastomeric, plastic, and stainless steel components, regardless of the body material, so material com- patibility can become an issue. These fittings can be selected to accept plastic or metal tubing, and pressure ratings vary. Pressure capability of push-in fittings is generally less than compression fittings but greater than or equivalent to barbed fittings.
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