Hose fittings connect runs of hose to other hose sections, equipment, or other components. Systems are typically configured with straight sections connected by fittings or specially formed connections and joints.

Hoses vs. Tubes and Pipes

To begin, it may be important to distinguish some terminology. In industry, "tube" and "pipe" are nearly interchangeable terms describing rigid hollow cylinders composed primarily of one solid material throughout. "Hose" however, is typically more flexible and portable than piping or tubing, and is made up of multiple layers of different materials depending on the application and transported media. 

In addition, the ends of a hose length typically have integral (attached, mostly permanent) connectors and fitting ends. Tubing and piping typically have separate fittings which are applied at the time of use or construction.

Applications

Fitting construction and material specifications are application-dependent; consulting the fitting supplier is typically a wise route for optimizing component selection. In general, however, hose fittings correspond to either hydraulic or pneumatic systems. Defining this system is the first step to determining the appropriate fittings for the application.

Hydraulic applications involve the transfer of liquid fluids such as water and other chemical solvents. Hydraulic fittings must have seals which prevent the leakage of liquid and often must be resistant to rust or other types of chemical corrosion.

Pneumatic applications involve the transfer of gases. Pneumatic fittings must have very tight seals to prevent gas leakage and must be resistant to chemical corrosion.

Types of Fittings

Hose fittings are distinguished based on their connection type and function it performs.

Connection

Fittings are attached to tubes via a number of different connection methods, each with its own conveniences and advantages.

Ball and sleeve fittings connect an outer sleeve to an inner (ball) fitting. The sleeve retracts to connect and disconnect the two ends of the fitting. Some ball and sleeve fittings function as push-to-connect fittings which are convenient for applications requiring frequent disconnection and reconnection of the hose section.

Image Credit: MISUMI USA | Staubli Corporation

Barbed fittings connect hoses via a barbed tube with a tapered stub and ridges inserted into the hose. They are best suited for low pressure applications, since they do not provide a strong seal.

Image Credit: Clippard Instrument Laboratory, Inc.

Cam-lock fittings are connected using tabs which fold down after insertion into the receiver to lock the fitting in place. They are used in many heavy-duty hose applications such as fire hose and sludge/sewage pumping.

Image Credit: Parker Hannifin

This video provides an introduction to cam-lock fittings:

Video Credit: TrunkPump

Crimp fittings involve placing hose over a tubular end and crimping against it with a sleeve or crimp socket. These fittings typically require crimping tools to make the connections.

Image Credit: Parker Hannifin

Compression fittings are designed to mate two hoses together by compressing the hose using a compression nut and ferrule. Compression fittings are commonly used for tubing and piping applications, but can be designed to connect two hose sections or to connect a hose to a tube.

Image Credit: U.S. Plastic Corporation

Flare fittings are a type of compression fitting consisting of a body with a tapered (flared) or coned end. Special flaring tools are used to install the hose to fit inside the flared end, providing a deep seal. Flare fittings are the best connections for hoses handling high pressures. can handle higher pressures and a wider range of operating parameters than standard compression fittings

Image Credit: Parker Hannifin

End fittings provide specific surfaces for making hose connections:

• Clamp ends are fittings which allow hose to be clamped to the connection.
• Plain ends are fittings which allow hose to be connected by adhesive, solder, or other forms.

Flange fittings are ports with flush surfaces perpendicular to the attached hose. These surfaces are joined and sealed via clamps, bolts, and/or welding. Flange connections in hose are often built-in rather than joined separately as with pipes and tubes. For more information on industrial flanges, visit the Pipe Flanges Selection Guide on IEEE GlobalSpec.

Image Credit: Kee Safety Inc.

Threaded fittings have screw threads (built-in grooves) on their inner (female) or outer (male) surfaces designed to accept connections with matching threads. Threads which provide a simple connection but no seal on their own are called straight threads. Tapered threads are designed to provide a tight seal for gases or fluids under pressure. Seal reliability can be improved by adding a coating or seal tape (Teflon). Especially precise threads are called "dry fit", meaning they seal without the need for an additional sealant, which is important in applications where sealant addition could cause contamination or corrosion.

Image Credit: Beswick Engineering Co., Inc.

The thread size is measured and based on the inside of the hose fitting. Standard thread size standards include NPT (National Pipe Thread) and BSP (British Standard Pipe), though other standards exist and usage varies by country. Each standard corresponds to a particular number of threads per inch (TPI). www.plumbingsupply.com provides an excellent overview on pipe thread sizing. Below is a chart depicting nominal thread sizes and their corresponding values.

Function

There are a vast number of types of fittings installed in hose systems which perform different functions. Most fittings can be grouped into one of four categories:

Specifications

Sizing

Once the type of fitting required has been determined, the most fundamental aspect of hose fittings can be addressed: proper sizing. Proper sizing is essential to successful fitting selection, as oversized or undersized parts will either be completely incompatible or will seal or connect inadequately.

The sizing of a fitting is defined by the inside diameter (ID) and outside diameter (OD) of its corresponding connections, measured in inches (in) or millimeters (mm). In other words, a fitting designed to connect to a hose with a 2" OD is rated as a 2" OD fitting. Inside diameter measures the diameter of the empty portion of the cylinder, while outside diameter includes the thickness of the hose wall, as pictured below:

ID and OD measurements. Image Credit: Engineering Toolbox

Operation

The operating specifications that are most important for fittings are pressure and temperature.

• Operating pressure range is the working range of pressures or the pressure ratings at which the fitting was designed to operate, typically measured in pounds per square inch (psi). Operating above or below this rating could cause the fitting to fail (i.e. break, leak, lose its seal).
• Operating temperature range is the working range of temperatures or the temperature ratings at which the fitting was designed to operate, measured in degrees Fahrenheit (°F) or degrees Celsius (°C). Operating above or below this rating could cause the fitting to fail.

Materials

The materials for hose fittings often correspond to the hose materials or hose application. Selection depends on factors such as cost, flexibility, media, environmental conditions, and required pressure ratings. Material choices include different types of plastic or metal.

Common metals include:

Aluminum - lightweight and corrosion resistant. Aluminum is commonly used for plumbing and is the preferred fitting material for aluminum tubing. By itself, aluminum has low tensile strength and is used when high corrosion resistance is needed. It is alloyed with zinc, copper, silicon, manganese, and/or other metals to improve its strength and hardness.

Brass - strong, durable, and corrosion resistant, with high temperature ductility and good conductivity. Brass is an alloy of copper and zinc, and is the most common tube fitting metal used in industry because of its machinability and its excellent performance properties. Brass fittings can have various protective or decorative finishes which should match the finish of the tubing.

Steel - durable and strong, with a high resistance to heat. Steel is an alloy of iron and carbon - it is commonly alloyed with other metals to improve its corrosion resistance and durability. It is used in both commercial and industrial applications for carrying water, flammable gases, and other fluids. Galvanized steel is coated with zinc for rust and chemical corrosion resistance. Carbon steel is alloyed with higher levels of carbon for increased durability and strength.

Stainless steel- relatively strong with excellent chemical and corrosion resistance. Stainless steel is an alloy of steel that contains over 10.5% chromium, providing excellent corrosion resistance for sanitary applications and systems handling aggressive fluids and materials.

Common plastics include:

Fluororesin (PFA/PTFE/PVDF) - used for temperature requirements of 500°F (250°C). These materials exhibit very good chemical resistance and dielectric properties.

Polyethylene (PE) - the most preferred plastic for hot and cold temperature applications. PE is grey or black, semi-flexible plastic. Fittings are used commonly for supply lines to sprinkler systems and underground geothermal heating loops.

Polypropylene (PP) - thermoplastic material that exhibits excellent cold flow, bi-axial strength, and yield elongation properties. It is similar to PVC, but can be used in exposed applications because of its resistance to UV, weathering, and ozone. 

Selection Tip: It is essential to check the compatibility of the system fluid with any proposed fitting materials. Incompatibility could result in corrosion causing leakage or system damage.

References

hoseandittings.com - Hose and Fitting Shop

Legacy - Selection Guide: Quick-Disconnect Couplers & Plugs

Parker - Hydraulic Hose, Fittings and Equipment: Technical Handbook

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Parker Hannifin | CEJN Industrial Corporation | DME Company