Surprising as it may seem, leakage in hydraulic systems could have been eliminated more than a generation ago. Although leak-free hydraulic operation had always been desirable, the need became more acute with higher operating pressures that became necessary during World War II, primarily in the hydraulic systems of military aircraft. Until then, common operating pressures had hovered around 800 to 1000 psi. The post-war era ushered in systems designed to operate at pressures to 1500 psi and higher on applications where rapid cycling and high shock pressures were present. It was not long until pressures climbed to 2,500 and 3,000 psi - which certainly are not uncommon today.
Faced with increased hydraulic fluid leakage brought on by higher pressures, a consortium of fittings manufacturers - working under the umbrella of SAE's Committee on Tubing, Piping, Hoses, Lubrication, and Fittings - undertook solving the problem. Their joint effort in the early 1950s culminated in the straight-thread design, which ultimately became known as the SAE straight-thread O-ring boss.
Fittings that use O-rings for leak-tight connections continue to gain acceptance by equipment designers around the world. Three basic types now are available: SAE straight-thread O-ring boss fittings, face seal or flat-face O-ring (FFOR) fittings, and O-ring flange fittings. The choice between O-ring boss and FFOR fittings usually depends on such factors as fitting location, wrench clearance, or individual preference. Flange connections generally are used with tubing that has an OD greater than 7/8-in. or for applications involving extremely high pressures.