Edited by Kenneth J. Korane
Tube fittings are essential to hydraulic systems, connecting tubing and hose to pumps, valves, cylinders, and other components. Modern connectors deliver years of leak-free performance despite high operating pressures, pressure spikes, corrosion, vibration, and other abuse. And they come in a near-endless variety of sizes, shapes, and styles to suit most any application. Here are some key considerations to match a fitting to the job.
Selecting the proper fitting for a given application is an important part of system design. This selection process is driven by the following fluid system parameters: size, temperature, application, media, and pressure, often referred to as the acronym STAMP.
Size: Consider the tube outside diameter (OD) and wall thickness for all tube fittings. Most types are limited due to tube wall thickness. Flare-type fittings typically have a maximum tube-wall limitation, while flareless bite-type fittings have minimum wall limitations. Select tube OD and wall thickness based on pressure, flow rate, temperature, and other ambient conditions.
Temperature: Temperature range for fittings depends on material, plating, and type of seal. The accompanying table lists temperature ranges for the most common fitting and seal materials.
Application: System environment obviously influences fitting selection. Steel fittings usually have protective coatings to resist corrosion. For more-corrosive environments, stainless steel and brass are viable options. Also consider the severity of the application, such as whether operating conditions are static or dynamic, and potential consequences of failure when selecting fittings.
Media: The fluid media must be considered when selecting fitting and seal materials. Consult fluid-compatibility tables for recommendations.
Pressure: The fitting’s pressure rating must equal or exceed system pressure. Ratings are based on a ratio of the ultimate strength of the fitting material and the pressure-holding capacity of the particular joint design. This provides a measure of safety against unknowns in material and operating conditions. Tube fittings are customarily rated with a 4:1 design factor for normal operating conditions and moderate mechanical and hydraulic shocks.
Fittings can be classed as straights or shapes. Straights are generally machined from hexagonal bar stock while shapes (such as elbows) are typically forged or brazed. Because forged fittings are of a single-piece construction, they have fewer potential leak paths compared with multipiece brazed fittings. Also, forging adds strength, so these fittings typically have higher assembly torque capacities, longer service lives, and higher pressure capabilities.
Tube fittings are also available in flange and push-toconnect styles, but those two classes are beyond the scope of this discussion. Threaded connections are the most common style of industrial tube fittings. Here’s a closer look at the most important designs.
Tube end selection
The tube end of a fitting connects to a tube or hose. The port end, in contrast, connects to a component, such as a pump. Tube ends generally conform to industrial standards, but some are proprietary. The most common styles meet SAE, ISO, DIN, or other standards. These include:
Flare type: The world’s most widely used flare fitting is the 37° flare. It consists of a body, sleeve, and nut. The tube end is flared at a 37° angle (from centerline) and held captive between the fitting nose and sleeve with a tube nut. When tightened, a metal-to-metal sealing line forms between the fitting nose and tube end. This sealing method can be used across a wide range of media and temperatures. However, metal-to-metal seals are not as reliable as elastomeric seals found, for example, on O-ring seal fittings.
Though generally considered to be 3,000-psi fittings, 37° flare fittings have working pressure ranging from 1,500 to 7,000 psi, depending on size, configuration, and material. They are suitable for thin to medium-wall-thickness tubing. Installation requires inserting a small length of tube into the fitting. The amount of tube in the fitting, or “tube entry,” is usually less than required for flareless bite-type fittings but more than that for O-ring face-seal fittings.
Of all tube connectors, 37° flare fittings are available in the broadest range of sizes, configurations, and materials. They conform to SAE J514 and ISO 8434-2, and may also meet MIL-F 18866.
Other common flare-type fittings are 30° and 45° flares. 30° fittings are generally used only for hose connections and may have either BSPP or metric threads. 45° flares connect to hard tube or hose, are typically brass, and used in air conditioning, refrigeration, and water systems.
24° flareless (bite type): North American and European bite-type connections have similar constructions. Each has a flareless fitting body, tube nut, and ferrule or bite ring. Some newer versions add an elastomeric seal to the bite ring to improve reliability.
All designs function on the same principle. As the tube nut tightens, the bite ring (ferrule) wedges between the fitting body and tube. This forces the front end to cut, or bite, into the tube OD while the rear end grips the tube and isolates the bite area from vibration and flexural stresses. Together, the ring’s biting and gripping action provides the holding power needed to withstand system pressures.
Bite-type fittings remain leak-free under service conditions ranging from high-vacuum and small-molecule gases to high-pressure hydraulic fluids. And, because the sealing technique differs from 37° flare fittings, bite-type fittings tend to have a wider pressure range for the same envelope size. Pressures typically range from 1,450 to 9,100 psi.
24° flareless connections are suitable for medium to heavy-wall-thickness tubes. But they generally require more tube entry than other styles, making it more difficult to assemble and disassemble when using rigid lines. Hose end connections, even though not popular, are also available for this style fitting.
O-ring face seal: This is one of the fastest growing categories because O-ring face seal fittings can eliminate leaks in high-pressure hydraulic systems despite severe mechanical shocks and vibration.
The fitting has a body, tube nut, sleeve, and O-ring. The O-ring seats in a groove on the fitting’s flat face. The sleeve permanently attaches to the tube either mechanically (by flanging) or by brazing.
As the nut tightens onto the fitting body, it compresses the O-ring between the body and flat face of the tube flange (or braze sleeve) to form a tight seal. O-rings provide a more-reliable seal than traditional metal-to-metal connections, though it comes with more-restrictive media and temperature limitations.
Maximum operating pressure is generally 6,000 psi for sizes to 1 in. and 4,000 psi for 1.25 and 1.50-in. sizes. Temperature range depends on the seal material. Nitrile is most common and typically has a range of –30 to 250°F. Fluorocarbon O-rings handle temperatures to 400°F, while other materials suit various fluids and temperatures.
The range of tube-wall thicknesses for O-ring face seal fittings exceed that of other types. Brazed fittings are recommended for thin to heavy-wall tubing, while mechanical-flange construction has a more-limited tube wall range.
ORFS fittings require no tube entry during installation and conform to performance and dimensional requirements of SAE J1453 and ISO 8434-3.
60° Cone: These fittings typically connect to hose or another fitting. They are seldom used with tube. It forms a metal-to-metal seal between the surface of the 60° cone and mating seat of the hose fitting or adapter. Threads provide a clamping load, not sealing. Four types commonly used in hydraulic applications are SAE J516/J514, JIS B8363 Code C and R, BS B5200, and DIN 7631.
SAE J516/J514 fittings’ NPT/NPTF male threads mate with NPSM female threads usually found on swivel-hose ends and other adapters. Prior to assembly, ensure that the NPT/NPTF thread has a 60° cone seat (30° chamfer). This is critical because most NPT/NPTF threaded male ends do not have the required 30° chamfer.
JIS B8363 (Code C and R) versions typically serve as hose adapters on equipment designed or manufactured in Japan and Korea.
BS B5200 styles typically handle the same applications, but mostly on equipment from England and other European countries.
Even though JIS and BS fittings have the same BSPP thread and seat angle, other dimensions are not identical. Hence, in some instances these fittings will not function properly if installers interchange components. For example, interchanging JIS and BS components can cause leaks due to different thread lengths and chamfer diameters that cause the swivel nut to contact the body before the fitting seals. The BS B5200 is also designed as a dual-function end connection (hose and port end connection). With the appropriate bonded seal, it attaches to DIN 3852-2 type ports (BSPP ports with flat face seals).
DIN 7631 versions are the least widely used of all 60° cone fittings. Its distinguishing characteristic is metric parallel threads instead of BSPP threads.
Generally, 60° cone fittings are for applications with pressures ranging from 1,100 to 5,000 psi.
Two general types of hydraulic port connections are tapered thread and straight (parallel) thread. Base selection on factors such as pressure- holding capacity, seal reliability, available space, and government, industry, or corporate standards. An important aspect is how threaded port connections seal.
Tapered thread: These are the oldest style and still widely used. Tapered threads can be NPT/NPTF (American pipe), BSPT (British pipe), or metric taper. The threads serve two functions: holding the fitting in place and forming the primary seal. Metal-tometal contact creates a seal between the mating roots, crests, and flanks of the male fitting thread and female port thread.
And as the fitting tightens and threads wedge against each other, the taper provides additional sealing and holding. Even so, thread sealant is highly recommended to reduce the risk of leaks.
While tapered port connections withstand relatively high static pressures, hydraulic systems are rarely static. And, although still widely accepted, inherent limitations tend to reduce their reliability. In general, experts do not recommend tapered-thread ports and connectors in hydraulic applications. OEMs and users can avoid leaks by instead using straight-thread and flange-type connections with O-ring seals.
Straight (parallel) thread: Parallel threads only hold fittings in place, so they also need an elastomeric or metal seal. The most reliable sealing method uses an O-ring in a conical cavity at the top of the port. Three different types of ports use this principle. However, while they may appear the same, threads and O-ring sizes differ. ISO 6149 uses metric parallel threads and the male end includes a special identification groove. Most experts recommend this port for new designs, and it is considered the world standard.
SAE J1926 uses UN/UNF threads. The female port is often referred to as an ORB or Oring boss. This style is widely used in North America.
JIS B2351 uses JIS PF parallel pipe threads (similar to BSPP). This style is usually found only on equipment manufactured or designed in Japan or Korea.
Other varieties of parallel thread ports provide a spotface (flat surface surrounding the port thread) for sealing. These ports typically have either metric parallel threads (according to DIN 3852-1 and ISO 9974-1) or BSPP threads (DIN 3852-2 and ISO 1179-1). A wide variety of male ends with elastomers, metal washers, or just metal-to-metal contact are used with these ports. Flat-face parallel threaded ports are usually found on equipment manufactured or designed in Asia and Europe.
Parallel thread ports offer another advantage — users can orient or position shaped fittings during installation by adding a locknut and backup washer to the parallel port end. The installer holds the fitting in the proper orientation and tightens the locknut, not the fitting body, to the recommended torque during final installation.