Fittings and flanges make the connection:

If the components within hydraulic systems never had to be removed, connections could be brazed or welded to maximize reliability. Inevitably 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 part) forces two mating surfaces together to form a high-pressure seal.

All-metal fittings:

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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 part. 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-things common to many 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 part. 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 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 3000psi. Because thick-wall tubing is difficult to form to produce the flare, it is not recommended for use with flare fittings. The 37’ flare fittings is suitable for hydraulic systems operating at temperatures from 165’ 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. Some manufacturers certify stainless steel 37’flare fittings to 6000 psi. on common sizes and configurations, while specialty versions can be rated up to 9000 psi.

The flareless fitting requires minimal tube preparation. It handles average fluid working pressures to 3000 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:

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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 2500 and 3000 psi-which certainly are not uncommon today. (In fact, the O-ring face seal style is currently approved to 6000 psi by SAE J1453 in all material types.)

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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. There are two basic types of O-ring sealing fittings for tubing, pipe, and hose: face seal or flat-face O-ring (FFOR) fittings, and O-ring flange fittings. Closely related are the SAE straight-thread O-ring boss adapters for threaded parts.

Flange connections generally are used with tubing that has an OD greater than 7/8in. or for applications involving extremely high pressures.
O-ring boss fittings seat an O-ring between threads and wrench flats around the OD of the m ale half of the connector. A leak tight seal is formed against a machined seat on the female part. 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 part, 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 part’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 part’s spot face surface when the assembly is completed and that it is tightened properly.

The FFOR fitting forms a seal between a flat, finished surface attached to the hose, tube, or pipe, and an O-ring held in a recessed circular groove in the male half. 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 fittings 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, 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. This is a significant O-ring fitting advantage, as there are practically no limits to the number of breaks and remakes possible. By contrast, metal-to-metal fittings increasingly and rapidly fatigue themselves (as well as the tubing or part to which they are connected) with each subsequent re-assembly. All metal fittings tighten more gradually, so technicians may have trouble detecting when a connection is tight enough but not too tight.

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FFOR fittings, unlike metal-to-metal flare and flareless fittings, are zero-clearance systems. This means that after the nut has been loosened the body drops freely away. Flare and flareless fittings need to be seated in the tubing or hose to which they are connected, requiring the installer to spring or pull on the hose or tube. This zero-clearance advantage is important in lines which must be repeatedly broken for cleaning, transporting, and removal.
On the other hand, O-ring fittings are
more expensive than their all-metal counterparts, and they 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. A common limitation of O-ring fittings has to do with system temperature. Most feature a 90-durometer buna-N O-ring, which is rated to 250’ F. Viton O-rings are commonly available, which increase the operating temperature to 400’F.

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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.

Flanges:

Fittings for tubing larger than 1-in. OD have to be tightened with large hexnuts which, in turn, require larger wrenches to enable workers to apply sufficient torque to tighten the fittings properly. To install such large fittings, system designers must provide the necessary space to give workers enough room to swing large wrenches. In addition, worker strength and fatigue could be factors affecting proper assembly. Extensions might be needed for some workers to exert an applicable amount of torque.

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Fittings manufacturers have designed
split-flange fittings so that they overcome both of these problems. Split flange fittings, use an O-ring to seal a joint and contain pressurized fluid. An elastomeric O-ring rests in a groove on a flange and mates with a flat surface on a part-an arrangement later used on the FFOR fitting. The O-ring flange is attached to the part using four mounting bolts that tighten down onto flange clamps, thus eliminating the need for a large wrench when connecting large-diameter components. When installing flange connections, it is important to apply even torque on the four flange bolts to avoid creating a gap through which the O-ring can extrude under high pressure.

The basic split-flange fitting consists of four elements: a flanged head connected permanently (generally welded or brazed) to the tube, an O-ring that fits into a groove machined into the end face of the flange, and two mating clamp halves with appropriate bolts to connect the split-flange assembly to a mating surface.

All mating surfaces must be clean and smooth. Joints are more likely to leak if either of the surfaces is scratched, scored, or gouged. Additionally, wear tends to accelerate on O-rings that are assembled against rough surfaces. Where perpendicular relationships are critical, all parts must meet appropriate tolerances. While the SAE J518 and ISO 6162 standards allow a 3-u (125-uin) finish, most flange manufacturers recommend 1.6 u (64-uin) to 0.8 u (32 uin) on the mating surface to assure leak-free connections.
When all the bolts are fingertight, the flange head protrudes about 0.25 mm (0.010 in.) to 0.75 mm (0.030 in.) beyond the clamp face, to ensure adequate contact and seal squeeze with the mating face. The most critical operation during the assembly of a split-flange fitting to its mating surface is the proper tightening of the bolts. Gradually and evenly tighten the bolts in a cross-pattern. Make sure that the proper bolt torque is achieved.

Fully tightening one of the bolts while the others are still loose will tend to cause the flange to tip upward. This action can pinch the O-ring, and the joint may leak eventually. When the bolts are fully tightened, the flange will bend downward and may or may not bottom on the part face.

A variation of the basic split flange is a solid flange that combines the flange head and clamp in one piece. This design reduces or eliminates the flange tipping problem and may be stronger, but may create some flange orientation problems.

Reprinted with the permission of Hydraulics & Pneumatics magazine.

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