Hydraulic fittings & flanges

Hydraulic fittings connect conductors such as hoses, pipes and tubesto the components in a hydraulic system. Theyallow the pressurized fluid to move through thesystem without leakage. Available in a varietyof styles to change and direct, change or splitflow, most fittings have a male and femalecomponent that join to form a connection.They can be manufactured as unions, plugs,crosses and elbows.

It is critical to carefully identify threads on the fitting connections, as these threads can look almost identical from one standard to another. However, because they are not mates, they will not properly engage. In addition to reviewing documents from SAE, NFPA and ISO to help identify each standard to confirm diameter and thread type, keeping thread identification kits on hand can help with this concern.

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Fitting connection types include: welded (socket weld, butt weld, slip on); threaded (NPTF, BSPT [both not recommended but used], SAE straight thread, ISO 6149; BSPP); flanged; barbed; quick-disconnect; push-to-connect; 37˚ flare; 24˚ cone; and inverted flare, among others. When selecting a type of fitting, some important considerations are working pressure, vibration, type of fitting, desired attachment, size of piping, flow, material of the conductor or component, and price. The fluid power industry is trying to transition to fittings with an elastomeric seal — generally O-rings — to prevent leakage. These include, but are not limited to, the SAE straight thread, face seal, ISO 6149, and SAE J518 (Code 61 and Code 62) flanges. Seal construction must be compatible with the type of fluid being used in the system, although very few applications require anything other than Buna Nitrile or Viton.

When selecting a fitting, several considerations are important. Most non-flanged fittings have a gender — called male and female — that are joined together to form a union. Most fittings are sized based on the size of the conductor (size of hose, pipe or tube), and overall dimensions can vary greatly based on fitting type, even for the same size conductors.

Additionally, most fitting types are available in a multitude of materials, including plastic, brass, steel, stainless or specialty metals like Monel. Each are applied in applications based on the fluid medium and ambient conditions, and each has different performance characteristics that allow customization within a fitting type. Often the first choice is to match the fitting to a similar material of the conductor or component that it is connecting to: plastic to plastic, steel to steel and stainless to stainless.

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Geometry is also an important consideration, and geometry is typically identified by alphabet letters the fittings resemble. Fittings are available inline to change the direction of flow in various increments (45˚ or 90˚ elbows [L]), or a swivel to allow two joined sections to rotate. They can also split or combine flows with run and branch tees [T], “wahys” [Y] and crosses [+]. Fittings, particularly elbows, are offered in a variety of drop lengths, which is the distance from the centerline of one opening— called a port—to the end of the other port. Fittings are available in various sizes to suit differing flow demands, and connection size is often expressed in dimensionless terms representing 1 ⁄16 of an inch. For example, a -06 thread is 3 ⁄8 (6/16), and a “dash” 32 size is a 2-in. (32/16) thread. A Y-flange may split a 2-in. flow into two reduced 1 1 ⁄2-in., instead of creating three 2-in. connections to more closely match the cross-sectional area.

O-ring face seal, SAE straight thread and ISO 6149 fittings have a seal, normally Buna N, contained within a groove to seal the fluid. It is important for the seal to be compatible with the fluid and the operating temperature range. An elastomeric seal greatly reduces the possibility of leakage caused by vibration, thermal cycling and pressure cycling.

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SAE J518 split flange fittings are used on larger line sizes, starting at 1 ⁄2 in. (-8) but coming into predominance at 2 in. (-32) and above. A flange head with an O-ring groove on its face is attached to a conductor (hose, tube or pipe) and is secured to the port, which could be a flatface fitting or a pad on a pump, valve or cylinder, by a clamp with four bolt holes. The clamp can be whole, but is often split so that a quarter of the diameter of the flange head is on either side of the centerline of the bolt holes to help minimize torque on the clamp. The screws used are tightened to a high torque value to avoid problems with fatigue. In many cases, using pipe or tubing, the flange connections have operated within their specified working pressure for decades.

Flare fittings, such as the JIC 37˚, are fittings with a conical end face and the seal is formed when this seat is forced against a mating seat, generally by torquing a swivel nut on one fitting, engaging with a threaded portion of the mating fitting. The angle of the seat and face for most JIC fittings in the North American market is 37°, and it is popular enough that the 24° and 45° versions are rarely used. The fittings can be designed to clamp onto a tube by means of a sleeve or ferrule, and care needs to be taken so that the correct size is used because inch and metric tubing sometimes have sizes that are close to overlapping.

The quick disconnect allows multiple reconnections of the assembly without causing excess wear or concern for thread damage. Some fittings allow disconnection and reconnection under pressure; others do not. Disconnects hold fluid pressure by way of a ball or poppet, which is spring offset to remain closed when the lines are unattached. Upon reattachment, the balls or poppets push against each other, lifting themselves from their seats and allowing fluid flow. Standard plug and socket configurations, such as the Pioneer coupling, are prone to trapping contamination, which was addressed with the advent of flat-face couplers, which have no recess to collect contamination.

Staple and band fittings are low-pressure fittings. Band fittings are attached to the hose by a barbed or beaded end being inserted into a hose and a band clamp securing the connection. This method is only for extremely low pressures. Staple fittings have a cylinder with an O-ring and a bead further up on it that slides into a socket. The connection is secured by a staple that goes through both sides of the connection behind the bead, although it is still typically used for low pressure or suction lines.