If the components within hydraulic systems never had to be removed, connections could be brazed or welded to maximize reliability. However, it is inevitable that connections must be broken to allow servicing or replacing components, so removable fittings are a necessity for all but the most specialized hydraulic systems. To this end, fitting designs have advanced considerably over the years to improve performance and installation convenience, but the overall function of these components remains relatively unchanged.

Fittings seal fluid within the hydraulic system by one of two techniques: all-metal fittings rely on metal-to-metal contact, while O-ring type fittings contain pressurized fluid by compressing an elastomeric seal. In either case, tightening threads between mating halves of the fitting (or fitting and component port) forces two mating surfaces together to form a high-pressure seal.

All-Metal Fittings
Threads on pipe fittings are tapered and rely on the stress generated by forcing the tapered threads of the male half of the fitting into the female half or component port (Fig. 1). Pipe threads are prone to leakage because they are torque-sensitive—over-tightening distorts the threads too much and creates a path for leakage around the threads. Moreover, pipe threads are prone to loosening when exposed to vibration and wide temperature variations—certainly no strangers to hydraulic systems.

Seepage around threads should be expected when pipe fittings are used in high-pressure hydraulic systems. Because pipe threads are tapered, repeated assembly and disassembly only aggravates the leakage problem by distorting threads, especially if a forged fitting is used in a cast-iron port. Thread sealant compound, a potential contaminant, is recommended for pipe fittings, which is still another reason why most designers consider them to be obsolete for use in hydraulic systems.

Flare-type fittings (Fig. 2) were developed as an improvement over pipe fittings many years ago and probably remain the design used most often in hydraulic systems. Tightening the assembly’s nut draws the fitting into the flared end of the tubing, resulting in a positive seal between the flared tube face and the fitting body. The 37-deg. flare fittings are designed for use with thin-wall to medium-thickness tubing in systems with operating pressures to 3,000 psi. Because thick-wall tubing is difficult to form to produce the flare, it is not recommended for use with flare fittings. The 37-deg. flare fitting is suitable for hydraulic systems operating at temperatures from −65° to 400° F. It is more compact than most other fittings and can easily be adapted to metric tubing. It is readily available and one of the most economical.

Read more: Hydraulic Fittings and Flanges