Hydraulic fittings connect tubes, pipes, or hoses to components (pumps, valves, cylinders, and so on) or together to create a system through which hydraulic fluid can flow without leaking. Fittings are meant to withstand high pressures and be leak resistant. Fitting variations allow designers to change the direction of flow, split flows, and change the elevation of lines. Fitting geometries include plugs, unions, elbows, cross, and others. Care needs to be taken in identifying the threads of the connections. Some connectors from one standard appear to thread into another standard port but do not have enough thread engagement to be safe. Documents from the organizations SAE (sae.org), NFPA (nfpa.com), and ISO (iso.org) help identify problems. Hose fittings are designed to work best with only one brand of hose and may not work with others. It is also critical to remember that hoses will eventually fail and need to be replaced—preferably before failure.
Connection types include: welded (socket weld, butt weld, slip on, etc.), threaded (NPTF [not recommended but used], SAE straight thread, ISO 6149, BSPP, BSPT, etc.), flanged, barbed, quick-disconnect, push-to-connect, 37˚ flare, 24˚ cone, inverted flare, staple lock, face seal, push-fit and 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 shift to fittings with an elastomeric seal, generally O-rings, to prevent leakage. These fittings include the SAE straight thread, face seal, ISO 6149, SAE J518 (Code 61 and Code 62) flanges, and others. The elastomeric seal must be compatible with the fluid in use. Although leakage is sometimes needed for lubrication, there is no reason for unplanned leakage.
When selecting the fitting, several items are important dimensionally. Most hydraulic fittings have two types, 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). Overall dimensions can vary greatly based on fitting type even for the same size conductor.
Additionally, most fitting types are available in a multitude of materials, including plastic, brass, steel, stainless, Monel, etc. Each is used in different environments and has different performance characteristics that allow a lot of customization within a fitting type. Often the first choice is to match the fitting to a similar material to the conductor or component that it is connecting plastic to plastic, steel to steel, and stainless to stainless.
Geometry is an important consideration. Fitting geometry is identified by letters that the fitting diagrams resemble. Fittings are available in-line [–]; to change the angle of flow in various increments (37˚, 45˚, or 90˚ elbows [L]), or swivel either through a range or all around. They can also split or combine flows with: Run and Branch Tees [T], Wyes [Y], and Crosses [+]. Fittings, particularly elbows, are offered in a variety of drop lengths. The drop length is the distance from the centerline of one opening—called a port—to the end of the other port.
The sizes of the ports can vary to compensate for changes in flow. Connection size often is expressed in terms of 1/16th of an inch in the U.S. A dash 32 size is about a 2-in. (32/16) connection. A wye flange may split a 2-in. flow into two 1-1/2-in., instead of two 2-in. flows, to more closely match the cross-sectional area.
O-ring face seal, SAE straight thread, and ISO 6149 fittings have an elastomeric 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 temperature range. An elastomeric seal greatly reduces the possibility of leakage cause by vibration, thermal cycling, and pressure cycling.
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 flat-face fitting or a pad on a pump, valve, or cylinder, by a clamp with 4 bolt holes in it. The clamp can be whole but is often split. The split is designed 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 37˚ flare, are fittings with a conical end called a seat. The seal is formed along a line when this seat is forced against a mating surface generally by a swivel nut on one fitting engaging with a threaded portion of the mating fitting. The angle of the cone is used in the callout, 37˚ being popular, but existing in 24˚ and 45˚ that are not interchangeable. The fittings can be designed to clamp on to a tube by means of a sleeve or ferrule. Care needs to be taken in that the correct size is used because of inch and metric tubing.
It is different from the quick disconnect fittings. The quick disconnect allows multiple reconnections of the connections. Some fittings allow the disconnection to be under pressure, others not. Some safety quick disconnects allow the pressure from one side of the line to bleed off eliminating one source of energy.
A newer fitting is called a push-to-connect fitting. It differs from the quick disconnect in that it is a connect one time fitting. It helps eliminate the use of wrenches in assembly.
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 is only for very 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.
