O-ring boss seal

O-Ring boss seal is a technique for joining two fluid-carrying pipes, hoses or tubing. In an o-ring boss (abbreviated ORB) system, a male threaded part is inserted into a female threaded part, providing a mechanical seal. This system differs from others in that an additional nut is tightened over an o-ring into a beveled area that creates an excellent seal.

This system is used most frequently in hydraulics, although it has been applied to many other systems including vacuum pumps, such as many Robinair pumps, in which the intake tee has an o-ring boss seal on the bottom. Being a relatively rare system, it is often confused with others, most commonly NPT. While the threads sometimes fit (although very inexactly), o-ring boss seal systems should never be used with any other as leaks are common when using such an improper method. Under high fluid pressures seen with hydraulic use, a failure of the threads could be deadly!

Because ORB adapters are relatively rare, one can be fashioned using a 37 degree AN (Army/Navy) flare fitting because the threads are compatible in diameter, pitch and spacing. To do this, a suitable o-ring, washer and nut are obtained separately. The flared end of the male AN flare is machined off. Optionally, a chamfer tool (or, if one cannot be obtained, a drill bit) can then be used to knock the inside shoulder off and make a reverse or inward angle that will smooth the flow of the fluid into the fitting bore. The last step is often unnecessary because most applications don't require a smooth flow of fluid unless operating at an extremely high flow rate.

This system has the advantage of being able to be tightened mechanically before being sealed. Most threaded systems, such as NPT, have a seal provided by a taper in the thread, so it is difficult to orient both ends of the hose, pipe or tube so that it is not twisted. In the o-ring boss system, this problem is eliminated because the threads do not seal the connection and therefore can be rotated at least a full revolution before they are sealed while maintaining a proper mechanical connection.