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.
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.
Surprising as it may seem, leakage in hydraulic systems could have been eliminated more than a couple generations 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 1,000 psi. The post-war era ushered in systems designed to operate at pressures to 1,500 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.
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 outside diameter (OD) greater than 7/8-in. 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 male half of the connector (Fig. 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.
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 (Fig. 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.
Fittings for tubing larger than 1-in. OD have to be tightened with large hex nuts 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. Wrench extensions (cheater bars) might be needed for some workers to exert an applicable amount of torque.
Standard hydraulic flanges (Fig. 6) overcome both of these problems. Flanges 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 port—an arrangement similar to the FFOR fitting. The O-ring flange is attached to the port using 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.
Manufacturers also offer split flanges, which can be installed into existing systems. 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.
For more information, please visit MASTER.
All mating surfaces must be clean and smooth. Joints are more likely to leak if either of the mating surfaces are scratched, scored, or gouged. Additionally, wear tends to accelerate on O-rings which are assembled against rough surfaces. Where perpendicular relationships are critical, all parts must meet appropriate tolerances. While 64-µin. surface finishes are acceptable, most flange manufacturers prefer and recommend 32-µin. finishes on mating surfaces to ensure leak-free connections.
In a properly designed split-flange assembly, the flange shoulder protrudes approximately 0.010 to 0.030 in. beyond the clamp face to ensure adequate contact and seal squeeze with the mating face (Fig. 7). However, the clamp halves do not actually contact the mating surface. The most critical operation during assembly of a split-flange fitting to its mating surface is to make certain that the four fastening bolts are tightened gradually and evenly in a cross pattern. Air wrenches should not be used because they are difficult to control and can easily over-tighten a bolt.
Hydraulic hose fittings are parts used to connect hoses, pipes and tubes on hydraulic systems. It is important to choose the correct hydraulic hose fitting to maintain seal quality, hose pressure and reduce the risk of torqueing. There is an extensive range of hydraulic hose fittings available online and we have developed this guide to help you choose the correct fittings for your requirements.
Sizing – Choosing the correct size is essential for optimum functionality. The sizing of a hydraulic fitting is defined by the size of vessels it is connected to. Tubes, hoses, and pipes are sized based on inside diameter (ID) and outside diameter (OD), measured in inches (in) or millimetres (mm). If the fitting is over-sized or too small it will not connect or seal properly and effect the performance of the hose.
Material – Hose fittings are manufactured using a range of materials including, brass, aluminium, cast iron, steel and plastics. Metals are more commonly used than plastics as the high pressures necessitate higher strength materials. Choosing the correct material is essential for maintaining seal quality and hose pressure. Our material breakdown is as follows:
Brass is strong, durable and corrosion resistant. The material is often used for smaller compression due to its machinability and excellent performance abilities.
Aluminium has low tensile strength and is used for its corrosion resistance and low-density benefits. However, this material shouldn’t be favoured over brass for high pressure hydraulic systems because of it lack of strength.
Steel is strong and durable with a high resistance to heat. It is typically alloyed with other materials to improve its low corrosion resistance. Stainless steel provides a higher tolerance to corrosion but is typically more expensive than materials such as brass and aluminium.
Choosing the correct material for a hose fitting is essential factor for hydraulic systems to maintain the performance of the hose. Although the metals have varying benefits, brass would be the most advisable material for hose fittings as it is an economical, affordable fitting that has a strong and durable density. Brass fittings are suitable for a wide range of media, including but not exclusive to water, oil and air mainly in the hydraulic and pneumatic industries. Browse our diverse range of brass fittings and brass hose tails.
Pressure rating is important when choosing a hydraulic hose fitting as hydraulic systems produce a high pressure that needs to be contained by the hose fitting in order to reduce the risk of torqueing. A hose assembly is rated at the maximum working pressure of the hose and the fitting component.
Malfunctioning of hose fittings can cause potential hazards to machine operators. Even though they may seem like small, insignificant components to a hose system for some, they are an absolutely essential fluid power transmitting component. Consider the reliability of the product before purchasing. JIC fittings have been the industry’s standard for years and O rings are highly rated due to their seal fittings which reduce the risk of torqueing.
BSP – (British Standard Pipe) The BSP connection is similar to National Pipe Straight Mechanical (NPSM), except the thread pitches are different in most sizes. The BSP female swivel has a tapered nose/flareless swivel which seals on the cone seat of the male. These fittings are measured in inches.
JIC- JIC (Joint Industry Council) fittings are commonly used in the Fluid Power industry in a diagnostic and test-point setting, especially where high pressure is concerned.
Metric L Series and Metric S Series are measured in millimetres. Unless otherwise specified, the size refers to the outside diameter.
ORFS – (O Ring Face Seal). The male fitting has a straight thread and O-ring in the face. The female has a straight thread and a machined flat face. The seal takes place by compressing the O-ring onto the flat face of the female, similar to a split flange fitting. The threads hold the connection mechanically.
Are you interested in learning more about hydraulic metric flat seat fitting? Contact us today to secure an expert consultation!