PNEUMATIC TUBING
Thermoplastic tubing is made from several common materials. But manufacturers offer countless variations of polymer formulations to suit specific needs. Typical tubing materials used in pneumatic applications include: •
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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 in a variety of 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 flexural-fatigue 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 semirigid 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 for contact with food and drugs. PVC tubing 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. www.fluidpowerworld.com
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IMAGE COURTESY OF NEW AGE INDUSTRIES
Engineers should weigh the features, benefits and drawbacks of each material. One important factor is compatibility of fluids conveyed through the conduit. For instance in the case of air systems, oil from air lubricators, as well as fumes or other substances ingested by the compressor, could affect the inner tube. Likewise, 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. Loads to keep in mind include excessive flexing, twisting, kinking, tensile and side loading, and vibration. Also protect against abrasive wear, snagging or bending beyond the minimum bend radius, all of which can lead to premature failure. Replace and discard any hose or tube that is cut, worn or otherwise damaged. Two common physical characteristics to consider when selecting tubing are flexibility and kink resistance. These are often subjective, and it’s best to compare samples rather than rely on manufacturer’s literature. Another physical attribute is color. Tubing comes in a wide range of colors as well as clear, depending on the compound. And don’t overlook the effect of temperature and heat on tubing materials— both inside and outside the assembly. Always operate within minimum and maximum temperature limits. In particular, be aware that elevated temperatures will impact working pressure, especially as tubing materials reach their upper temperature limit. For example, one version of nylon tubing with a 0.25-in. ID has a rated burst pressure of 1,250 psi at 75° F, but only 600 psi at 200° F. 6 • 2016
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