Pneumatic hose & tubing

System designers use pneumatic hose and tubing to convey pressurized air to actuators, valves, tools and other devices.

Tubing manufactured for pneumatic applications may be extruded of a single material or reinforced internally, typically with textile fibers, for higher strength. Air hose generally consists of an inner tube, one or more layers of reinforcing braided or spiral-wound fiber, and an outer protective cover. In broad terms, hose is more rugged than tubing — but it tends to cost more.

Air supply and application set a baseline for product performance. Flow requirements help determine hose or tubing size. Tubing is generally specified by OD and wall thickness, while hose is specified by ID. Regardless, choosing too small an inner diameter “chokes” flow and results in pressure losses, inefficiency and excessive fluid velocity that can shorten service life. Too large a diameter, on the other hand, results in higher than necessary weight, size and cost.

Also ensure that products operate below the stated maximum working pressure. Manufacturers generally rate tubing by measuring the burst pressure at 75° F, and then divide it by an appropriate safety factor (typically 3:1 or 4:1) to determine the maximum working pressure. Keep in mind that published burst-pressure ratings are only for manufacturing test purposes, and in no way indicate that a product can safely handle pressure spikes or otherwise operate above maximum working pressure. Also note that some products handle vacuum to approximately 28 in.-Hg without collapse.

Image courtesy of Kuriyama

Thermoplastic tubing is made from several common materials. Typical tubing materials used in pneumatic applications include:

• Polyurethane tubing is strong, flexible, kink and abrasion resistant, and it withstands contact with fuels and oils. It’s commonly used in pneumatic actuation and logic systems, robotics and vacuum equipment, and semiconductor manufacturing, medical and laboratory applications.

• Nylon tubing is tough, light and dimensionally stable. It can be formulated for higher-pressure pneumatics, flexibility for routing in tight spaces, high flexuralfatigue resistance and low water absorption.

• Polyethylene tubing is often used in low-pressure pneumatics and pneumatic controls. It has wide resistance to chemicals and solvents, good flexibility and relatively low cost. HDPE tubing comes in semi-rigid versions that resist cuts and physical damage and has a higher burst pressure than polyethylene tubing.

• Polyvinyl chloride (PVC) tubing is light and generally more flexible than nylon and polyethylene, offers good chemical resistance and can be repeatedly sterilized. It is suitable for low-pressure medical applications and can be formulated to meet FDA specifications. It is typically clear, and thus well-suited where visible indication of flow is necessary.

• Polypropylene tubing can be formulated for food-contact applications, resists chemical attack and withstands UV radiation in outdoor applications.

Image courtesy of Kuriyama

Consider fluid compatibility. For instance, oil from air lubricators, as well as fumes or other substances ingested by the compressor, could affect the inner tube. Also, remember exterior environmental exposure. Hose and tubing assemblies can be attacked by chemicals, ozone, UV radiation, salt water, air pollutants and other substances that lead to degradation and premature failure.

External mechanical influences can also hasten hose and tubing failure. Protect against excessive flexing, twisting, kinking, tensile and side loading, and vibration as well as abrasive wear, snagging or bending beyond the minimum bend radius. Replace and discard any hose or tube that is cut, worn or otherwise damaged.

Image courtesy of Eisele Connectors

Don’t overlook the effect of temperature and heat on tubing materials — inside and outside the assembly. Always operate within minimum and maximum temperature limits.

Tubing is extruded in straight lengths and stored on reels, but can be molded into spring-like coils. This lets the tubing extend considerably as needed, then retract to a compact configuration for storage. Tails — short, straight lengths of tubing that extend from each end of the coiled section — facilitate coupling attachment. Coil diameter, tubing diameter, wall thickness and the type of material affect retractability. Smaller and tighter coils generate more retraction force than do larger coils; polyurethane and nylon generally offer better material-memory characteristics. These tend to let the product collapse more easily.

Tubing variations can include products made for special attributes like high strength, abrasion resistance or compatibility with a specific chemical; characteristics like flame resistance, weld-spatter resistance, and electrical conductivity or nonconductivity; nductivity; coextruded products that combine the properties es of two materials in a single tube; and multiple tubes bonded together in a single assembly or tubes formed into elbows and bends.

PNEUMATIC FITTINGS AND COUPLINGS

When looking for connectors for your pneumatic systems, it is important to understand what type of connection is being made. Fittings can be used to connect pneumatic tube, pipe or hose and each connection requires specific fittings.

Pipe fittings, push-in fittings, barb adapters, compression fittings, and quick-connect fittings are some of the most common types available. They can be manufactured in plastic, brass, steel, stainless steel and other materials. It is critical the fitting and tubing or hose are compatible with each other. Other important considerations for fittings include pressureholding capacity, ease of installation, size and weight, corrosion resistance and, of course, cost.

With hose barb terminations, the tubing is pushed over a barb slightly larger than the tube.

Compression fittings use a ferrule. This ferrule is slid over the tube to make the connection and then is secured by compressing the assembly together.

In push-to-connect fittings, the tubing is inserted into the fitting end. Push-to-connect are often quick-connect fittings, which offer easy connections and safe, dry-break release. They are available in plastic and all-metal, stainless steel designs for pneumatic applications.