Machine Design

Hose Fittings

Wrong fittings can render even the best hose useless. The best systems require the right type of fitting, using the right attachment method, and made of the right material. Fittings and hoses can be assembled in-house or purchased as assemblies from manufacturers.

Fittings types: Most fittings can be categorized as either reusable or permanently attached. Reusable fittings are screwed onto a hose or are bolted together. They can be removed from a failed hose and installed on a new one. Thus, they can save money, and time as well. Permanent fittings, such as crimped or swaged fittings, are squeezed onto the hose at assembly. As the name implies, these are only for onetime use, and must be discarded when the hose assembly fails. However, these fittings generally cost less than reusable fittings, and require less time to make a hose assembly.

Materials: Fittings are available in a wide variety of materials; material selection is usually based on compatibility with the fluid and the environment. Carbon-steel fittings are most widely used for both high and low-pressure hydraulic systems. Stainless steel, brass, or Monel can be used for systems carrying corrosive fluids or systems used in corrosive environment. Fitting suppliers can provide a detailed list of materials available and the fluids that they can handle.

Connections: Hose fittings terminate in a wide variety of connections that can be used to mate with the components in the system. Most common connections are pipe thread, split-flange, straight thread, O-ring seal, and 37° joints. Pipe threads have limited use on high-pressure systems; otherwise, all of the systems work for their appointed tasks.

High-pressure permanently attached fittings are usually factory assembled on to a standard hose that has been stripped to the bare metal ("skived"). If crimped, the fitting is attached by squeezing the socket to compress the hose between the inner nipple and the radially squeezed socket. Swaged fittings are assembled by drawing a fitting through a tapered die to reduce the outside diameter. These fittings tolerate working pressure to 12,500 psi.

Medium-pressure screw-assembled fittings are commonly used for SAE 100R5 series hose plus some air and refrigerant hoses. With these fittings, no special preparation of the hose (other than lubrication) is required before assembly. Such fittings are suitable for hydraulic and air applications to 5,750 psi.

Field-assembled permanently attached fittings are similar to the swaged or crimped hoses, but are assembled on special portable crimping or swaging machines. Field-assembled units are typically not skived; instead, SAE 100R hoses with thin covers are used. They can handle pressures to 5,000 psi.

Bolt-on and segmented reusable fittings are typically used on large hoses. They are suitable for both air and hydraulic applications, as well as a variety of other fluids, at working pressures to 4,000 psi.

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