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Hydraulic fittings and flanges in mobile equipment

- Feb 01, 2018 -

Fittings and flanges are required in nearly every installation job as an intermediate joint between the pump, valve or actuator port to another component in the system. A simple cylinder circuit might require SAE Code 61 flanges at the pump to join its ports to the suction and pressure hoses. The pressure hose might then terminate at the directional valve, where it uses a JIC to NPT adapter to join the hose to said valve. Finally, NPT to JIC adapters at the work ports might feed hoses to a cylinder employing SAE O-Ring boss ports fitted with 90° JIC to ORB adapters. Hydraulic fittings and flanges are simply unavoidable.

The granddaddy to most of today’s fittings was, of course, tapered-thread fittings, such as NPT. Still in use today— even in the high-pressure tool market reaching up to 10,000 psi or higher—this venerable design won’t go away any time soon. A taper from base-to-tip on the male fitting’s threads match the opposing female fitting’s profile exactly, and as the fitting is torqued, the clearances reduce and a seal is created. Because of manufacturing imperfections and the nature of the thread, some sort of sealant is required, such as pipe dope or Teflon tape.

Now the popular standard, O-ring fittings such as SAE or BSPP, use a seal between the thread and nut of the male fittings which is cradled by a chamfer on the female thread, allowing for a superior seal, especially at higher pressure. This fitting style is popular because of its reliability and ease of installation, requiring no thread locking paste or tape. Most pumps, valves and actuators now offer O-ring ports as standard.

Also gaining fame are flange style connections—especially in high pressure and large diameter applications. Flanges mount flat against a port’s surrounding surface and seal with an O-ring on the flange itself. They are attached by four bolts torqued on a solid flange adapter, such as a Code 61 to SAE female fitting, for example. A flange is often a type of hose end as well, requiring a split flange kit consisting of two brackets to surround the flange from either side, torqued down and sealing the flange against the port face. Flanges are most often used in high-pressure, high-flow applications, such as piston pumps or bent-axis piston motors. Flanges are sometimes metric, but most often SAE Code 61 and Code 62, which differ from each other based on their pressure rating (Code 62 being the higher pressure brother, with a unique bolt pattern so the two can’t be confused).

Also worth mentioning is the JIC standard fitting. It uses a bevel and taper configuration that seals something like a poppet valve; the threads are required to simply torque the connection rather than seal it. Because most female JIC fittings are a swivel, they allow two fittings or hose ends to connect to each other without rotating the hose, fitting or component, something impossible with pipe fittings.

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