In general, valves, cylinders and pumps have female threaded bosses for the fluid connection, and hoses have female ends with captive nuts. A male-male fitting is chosen to connect the two. Many standardized systems are in use.
Fittings serve several purposes:
-To bridge different standards; O-ring boss to JIC (hydraulic), or pipe threads to face seal, for example.
-To allow proper orientation of components, a 90°, 45°, straight, or swivel fitting is chosen as needed. They are designed to be positioned in the correct orientation and then tightened.
-To incorporate bulkhead hardware.
-A quick disconnect fitting may be added to a machine without modification of hoses or valves
A typical piece of heavy equipment may have thousands of sealed connection points and several different types:
-Pipe fittings, the fitting is screwed in until tight, difficult to orient an angled fitting correctly without over or under tightening.
-O-ring boss, the fitting is screwed into a boss and orientated as needed, an additional nut tightens the fitting, washer and o-ring in place.
-Flare seal, a metal to metal compression seal with a cone and flare mating.
-Face seal, metal flanges with a groove and o-ring are fastened together.
-Beam seal, a costly metal to metal seal used primarily in aircraft.
-Swaged seals, tubes are connected with fittings that are swaged permanently in place. Primarily used in aircraft.
Elastomeric seals (O-ring boss and face seal) are the most common types of seals in heavy equipment and are capable of reliably sealing 6000+ psi (40+ MPa) of fluid pressure.
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