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

fig. 1. pipe fittings have given way to newer fitting designs that simplify assembly, maintenance, and reduce or eliminate leakage. shown is a 90° adapter elbow with pipe threads at one end that mount permanently into the component port. other end of fitting uses straight-thread flare fitting for tubing connection.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, Figure 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.

fig. 2. flare-type fittings offer several design and performance improvements over pipe fittings and are used with thin-walled and medium-thickness tubing.Flare-type fittings, Figure 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° 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° 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.

fig. 3. flareless fittings offer advantages similar to those of flare fittings and are used with medium- to thick-walled tubing. The flareless fitting, Figure 3, gradually is gaining wider acceptance in the U.S. because it requires minimal tube preparation. It handles average fluid working pressures to 3,000 psi and is more tolerant of vibration than other types of all-metal fittings. Tightening the fitting's nut onto the body draws a ferrule into the body. This compresses the ferrule around the tube, causing the ferrule to contact, then penetrate the outer circumference of the tube, creating a positive seal. Because of this, flareless fittings must be used with medium- or thick-walled tubing.

O-ring-type fittings

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.

fig. 4. non-adjustable, left, and adjustable sae straight-thread o-ring fittings offer ease of assembly and high potential for leak-tight connections.O-ring boss fittings seat an O-ring between threads and wrench flats around the OD of the male half of the connector, Figure 4. A leak-tight seal is formed against a machined seat on the female port. O-ring boss fittings fall into two general groups: adjustable and non-adjustable. Non-adjustable (or non-orientable) fittings include plugs and connectors. These are simply screwed into a port, and no alignment is needed. Adjustable fittings, such as elbows and tees, need to be oriented in a specific direction.

The basic design difference between the two types is that plugs and connectors have no locknuts and require no back-up washer to effectively seal a joint. They depend on their flanged annular area to push the O-ring into the port's tapered seal cavity and squeeze the O-ring to seal the connection. Adjustable fittings are screwed into the mating member, oriented in the required direction, and locked in place when a locknut is tightened. Tightening the locknut also forces a captive backup washer onto the O-ring, which forms the leak-tight seal. Assembly is always predictable, because technicians need only make sure that the backup washer is firmly seated on the port's spot face surface when the assembly is completed and that it is tightened properly.

fig. 5. flat-face o-ring fitting uses o-ring in recessed groove in male half that mates with flat, smooth surface on female half.The FFOR fitting forms a seal between a flat, finished surface on the female half and an O-ring held in a recessed circular groove in the male half, Figure 5. Turning a captive threaded nut on the female half draws the two halves together and compresses the O-ring.

Fittings with O-ring seals offer a number of advantages over metal-to-metal fittings. While under- or over-tightening any fitting can allow leakage, all-metal fittings are more susceptible to leakage because they must be tightened to within a higher, yet narrower torque range. This makes it easier to strip threads or crack or distort fitting components, which prevents proper sealing. The rubber-to-metal seal in O-ring fittings does not distort any metal parts and provides a tangible "feel" when the connection is tight. All-metal fittings tighten more gradually, so technicians may have trouble detecting when a connection is tight enough but not too tight.

On the other hand, O-ring fittings are more expensive than their all-metal counterparts, and care must be exercised during installation to ensure that the O-ring doesn't fall out or get damaged when the assemblies are connected. In addition, O-rings are not interchangeable among all couplings. Selecting the wrong O-ring or reusing one that has been deformed or damaged can invite leakage. Once an O-ring has been used in a fitting, it is not reusable, even though it may appear free of distortions.

Some manufacturers offer specially designed, high-pressure fittings that are equal in leak and weep resistance to FFOR fittings and interchangeable with a number of international fittings. Testing has shown these new designs to surpass all requirements with no evidence of leakage when exposed to vibrations up to 15 times more severe than those experienced on a typical hydrostatic drive. These designs may appear similar to standard fittings, but should not be mated with fittings from different manufacturers.