The straight thread ports and fitting were developed by a consortium of fitting manufacturers, working under the umbrella of the SEA Committee on Tubing, Piping, Hoses, Lubrication and Fittings. Their joint effort, in the early 1950s, culminated in the design of a new port and fitting configuration (Figure 6.4). The basic design concept was surprisingly simple: instead of relying on metal interference and crushing of threads to effect a hydraulic seal, as in the case with any tapered pipe threads, the committee came up with a straight thread design in which an elastomeric seal, such as an O-ring, does the sealing.

SAE straight thread fittings can be divided into two general groups: positionable (Figure 6.5) and nonpositionable (Figure 6.4). Nonpositionable fittings retain the 0-ring with a shoulder machined on the body of the connector. Positionable fitting can be machined in many different configurations with a lock nut and backup washer (Figure 6.6).

Two alternate designs for a port entry plug to be used with the SAE straight thread port are illustrated in Figure 6.7 (NWD International, Inc.; patented). The tapered metal-to- metal seal supports and retains the elastomeric seal.

Figure 6.8 illustrates an alternate design for a positionable fitting that employs the SAE J514 standard straight thread 0-ring boss port. The metallic intrusion in the taper near the surface of the boss provides high integrity metal-to-metal seal, even without the O-ring.

This fitting design also provides an elastomeric seal between the inner metal surface of the flared tubing and the matching nose of the fitting (NWD International, Inc.; patented). Figure 6.9 illustrates the tube just prior to assembly. Figure 6.8 illustrates the unit assem- bled with the elastomer providing the fluid barrier in conjunction with the two metal-to- metal sealing surfaces.

Figure 6.5: Infinite positioning is possible with use of the backup washer and lock nut.