Pneumatic Tubing

What is Pneumatic Tubing

Pneumatic tubing is small-bore flexible thermoplastic line, typically 3 to 16 mm outer diameter, used to carry compressed air, control signals, and vacuum between the components of a pneumatic system. It runs from the compressor and air preparation set (filter, regulator, lubricator) to directional control valves, and from those valves to the cylinders, grippers, and tools that do the work. It is the circulatory system of factory automation: a single packaging line or robot cell can contain hundreds of meters of tubing in several sizes and colors.

What distinguishes pneumatic tubing from rigid pipe is its connection method. Modern installations use push-in (push-to-connect) fittings: the tube is simply pushed into a fitting where a stainless collet grips the outside wall and an O-ring seals against it, with no threads, ferrules, or tools required to make the joint. This is why pneumatic tubing is specified almost universally by outer diameter rather than inner diameter, and why the OD tolerance is held tight, on the order of plus or minus 0.1 mm. The collet and seal are validated against a tube of known outside dimension and roundness, so an out-of-round or oversize tube leaks and an undersize tube pulls out.

A length of tubing is far more than a hollow plastic tube. Engineering-grade pneumatic tubing must hold its working pressure continuously without creep, survive thousands of bend cycles in moving machine elements without kinking or fatigue cracking, resist the oil mist carried in lubricated air, tolerate the ambient and media temperature of its installation, and keep a clean round sealing surface so the push-in fitting stays leak tight. A tube that meets none of these at low cost will fail at the fitting, and a single pinhole leak across a plant of thousands of joints wastes meaningful compressed-air energy, which is one of the most expensive utilities in a factory.

Historically, pneumatic systems were plumbed in copper and rigid steel, then in rubber hose. The shift to flexible thermoplastic tubing accompanied the rise of modular valve terminals and push-in fittings from the 1970s onward, which let assemblers route air by hand and reconfigure machines quickly. Polyurethane and nylon became the dominant plastics because they combine the flexibility needed for hand routing with the burst strength needed for typical 6 to 10 bar shop-air pressure. Today the global market is served by valve and fitting makers (SMC, Festo, Parker, Camozzi, Aventics) and specialist extruders, with tube color coding standardized within most plants so a maintenance technician can trace a circuit at a glance.

Four parameters dominate tubing quality and selection: material class, outer diameter, working pressure derated for temperature, and minimum bend radius. These four interact. A soft polyurethane tube bends tightly but tops out near 10 bar; a nylon tube holds higher pressure and heat but kinks if bent too sharply. There is no universal tube. Engineering selection is the act of mapping each circuit segment, its pressure, temperature, flexing, and chemical exposure, onto the material and size that fits, and then onto a fitting family that the tube is dimensionally compatible with.

Tubing Material Classes

Four polymer families cover the overwhelming majority of pneumatic tubing: polyurethane (PU/PUR/TPU), nylon (polyamide, PA), polyethylene (PE), and fluoropolymer (PTFE, FEP, PFA). Each occupies a distinct corner of the pressure, temperature, flexibility, and chemical-resistance envelope, and choosing the wrong class is the most common and most expensive specification mistake. The table below compares the four families on the engineering metrics that decide selection. All figures are representative of general-purpose grades at room temperature and must be confirmed against the specific datasheet, since values vary by maker, size, and grade.

Polyurethane is the default for general machine pneumatics where the tube must flex. It is soft, highly elastic, kink resistant, and recovers its shape after bending, making it ideal for tubing that travels with a moving axis, a robot wrist, or a drag-chain. Ether-based polyurethane (as used in the NITRA PUR line) adds hydrolysis, oil, and cold resistance over ester-based grades. The trade-off is a relatively low pressure ceiling, commonly 0.8 to 1.0 MPa at 20 degrees Celsius, and a maximum continuous temperature near +60 to +71 degrees Celsius, above which it softens and can extrude past a fitting collet.

Nylon (polyamide) is the choice when pressure or temperature exceeds what polyurethane handles. Grades PA11 and PA12 are semi-rigid, hold higher pressure (typically 1.0 to 1.5 MPa), tolerate more heat (NITRA nylon 12 rates -51 to +93 degrees Celsius), and resist hydrocarbons and many chemicals better than PU. Nylon 12 in particular has low moisture absorption, so it stays dimensionally stable and flexible over a long life. The penalty is that nylon is stiffer, needs a larger bend radius, and, unlike polyurethane, can take a permanent kink that never recovers if bent too sharply or crushed.

Polyethylene is an economy material with good chemical resistance and decent flexibility, often used for low-pressure air, water, and instrument signal lines where the higher strength and heat tolerance of nylon and the elasticity of polyurethane are not required. Fluoropolymer tubing (PTFE, FEP, PFA) is the specialist option for extreme temperature and aggressive chemistry: PTFE runs continuously from roughly -70 to +260 degrees Celsius and FEP from about -200 to +200 degrees Celsius, both with near-universal chemical resistance. Fluoropolymer is stiff, expensive, and reserved for analyzer lines, semiconductor gas delivery, and hot zones rather than general plumbing.

Polymer Grades Decoded

Within each material family, hardness and chemistry create distinct grades with different pressure, flexibility, and approval profiles. Datasheets express hardness on the Shore scale: Shore A for soft elastomers such as polyurethane and Shore D for harder semi-rigid plastics such as nylon. A higher number means a harder, stiffer, higher-pressure tube. The table below pairs representative published grades with their measured specs so the relationship between hardness, pressure, and temperature is concrete rather than abstract.

Polyurethane grades are graded mainly by Shore A hardness and chemistry. A Shore A 98 grade like NITRA PUR is firm enough to hold a respectable burst rating (540 psi at 1/4 inch) yet soft enough to bend tightly and resist kinking. Within polyurethane, the ether-versus-ester distinction matters for durability: ether-based grades resist hydrolysis (water and humidity attack), oil, and cold better, which is why they are preferred for outdoor, washdown, and cold-store duty. UV-stabilized grades (often colored dark green) add outdoor weathering resistance. Note that pressure ratings drop with both temperature and size: the same NITRA PUR family falls from 240 psi at 1/8 inch to 150 psi at 3/8 inch because the larger bore carries more force on a proportionally thinner wall.

Nylon grades are typically PA11 or PA12 at around Shore D 70, with soft-nylon variants engineered to flex more like polyurethane while keeping nylon's chemical and thermal stability. SMC, for example, offers hard nylon (T series) and soft nylon (TS series) precisely to bridge this gap. Nylon 12 is favored for its low moisture absorption and long flex life. Within nylon, the burst-to-working ratio is generous, the NITRA nylon 12 6 mm tube lists 340 psi working against 1020 psi burst, roughly a 3:1 factor, consistent with the safety margin that ISO 14743 test practice and reputable makers target.

Reinforced and specialty grades exist where single-wall extrusion is not enough. SAE J844 air brake tubing comes as Type A single-wall nylon and Type B nylon core with fiber reinforcement and a nylon cover, both DOT compliant to FMVSS 106 (49 CFR 571.106), rated for 150 psi working with a minimum 950 psi burst and a wider -40 to +93 degrees Celsius range. Flame-resistant double-layer tubing, such as SMC's TRB series, uses a flame-retardant resin equivalent to UL 94 V-0 for environments where welding sparks or grinding debris would otherwise melt standard tube. These grades cost more and are specified only where the regulation or hazard demands them.

OD Sizing and Fitting Compatibility

The single most consequential dimension on a pneumatic tube is its outer diameter, because the push-in fitting grips and seals on the outside wall. Metric tubing is called out by OD with a stated wall, written as OD by ID, for example 6 by 4 (6 mm OD, 4 mm ID, 1 mm wall). Inch tubing in North America is also normally called out by OD, for example 1/4 inch OD by 0.156 inch ID. This is the opposite convention to hydraulic hose, which is sized by inner diameter, so the very first selection check is to confirm whether the system is metric or inch and whether it is OD-based or ID-based. The table below lists the common metric and inch sizes and their typical bores.

The hazard at the boundary is that 5/32 inch (3.97 mm) and 4 mm look nearly identical but are not interchangeable. A 4 mm metric fitting will not reliably seal a 5/32 inch tube and vice versa, and the 1/4 inch (6.35 mm) versus 6 mm mismatch is even larger. Mixing inch and metric in the same fitting is a leading cause of blow-outs and slow leaks. Standardizing a plant on one measurement system and a short list of OD sizes (often 6 mm and 8 mm metric, or 1/4 inch and 3/8 inch) eliminates this whole class of error and simplifies spares.

Push-in fitting compatibility is governed by ISO 14743, Pneumatic fluid power, Push-in connectors for thermoplastic tubes. The standard specifies general requirements and test methods (pull-out, burst, leakage, repeated assembly) for connector assemblies used with thermoplastic tubes from 3 mm to 16 mm OD, including inch sizes. Crucially, it tests the tube and fitting as a complete assembly, which is why the tube must hold a tight OD tolerance (about plus or minus 0.1 mm) and good roundness: the fitting maker validates the collet and O-ring against tubes meeting these dimensions. Using a tube that is out-of-round, scratched on the sealing surface, or from an unverified extruder defeats the validated seal even if the nominal OD is correct.

ISO 14743 explicitly does not apply to air braking systems. Vehicle air brake tube is a separate world governed by SAE J844 and US federal regulation FMVSS 106 (49 CFR 571.106), with stricter burst, impulse, and environmental requirements and its own DOT marking. Do not substitute general pneumatic tubing into a brake line, and do not assume a brake-rated tube drops into a standard push-in fitting without checking. For correct field assembly, cut the tube square with a tube cutter (never a diagonal side cutter, which ovalizes the end), push it fully home until it bottoms, tug to confirm the collet grips, and trim a fresh end on every reconnection because collet teeth marks ruin the old seal.

Key Specification Parameters

Tubing datasheets list many numbers, but only a handful drive a sound selection: outer diameter and wall, working pressure, burst pressure and safety factor, temperature derating, minimum bend radius, vacuum rating, and material approvals. Each is explained below so a buyer can read any maker's sheet and compare like for like.

Working pressure (maximum operating pressure) is the continuous rating at a stated reference temperature, normally 20 to 23 degrees Celsius. It is the number you size to, but only after derating for temperature (below). Burst pressure is the laboratory failure point of a new tube. The ratio of burst to working is the safety factor: ISO 14743 test practice and reputable makers target roughly 3:1 to 4:1. The NITRA nylon 12 6 mm tube, for example, lists 340 psi working against 1020 psi burst, a clean 3:1. Never run continuous pressure near the burst figure, and reserve headroom for spikes from fast valve switching, sudden load drops, and water hammer in mixed air-water lines.

Temperature derating is the parameter most often forgotten. Pressure rating falls steeply with temperature, and you must derate to the hottest point the tube sees, not the ambient at the gauge. SMC TU polyurethane is rated 0.8 MPa at 20 degrees Celsius, 0.65 MPa at 40 degrees Celsius, and only 0.5 MPa at 60 degrees Celsius, a 38 percent loss of capacity across a 40-degree rise. A tube routed near a hot motor, an oven, or a sun-exposed outdoor run can sit well above shop ambient. If the duty exceeds the polyurethane and nylon ceiling (roughly +71 to +93 degrees Celsius), move to fluoropolymer.

Minimum bend radius is the tightest curve a tube can take without kinking. Below it, flow throttles, the wall stresses, and on nylon a permanent kink can form. Polyurethane bends far tighter than the same-OD nylon: as published examples, 5/32 inch (4 mm) PU lists about 0.25 inch (6.4 mm) minimum radius while 6 mm nylon lists about 32 mm. For continuously flexing routes, use at least twice the static minimum and prefer a flex-rated polyurethane or energy-chain grade. Vacuum rating matters on the suction side: general-purpose tube is rated to a vacuum such as 28 inches of mercury, but thin-wall tube can collapse under strong vacuum, so confirm the rating before plumbing vacuum grippers or generators.

Material approvals and ratings close the spec sheet. The ones that recur:

• UL 94 flammability: V-0 grades (such as SMC TRB flame-resistant double-layer) are required near welding, grinding, or hot-work zones.
• RoHS / REACH: restriction of hazardous substances compliance is now standard for most reputable tube and is often required by OEM purchasing.
• Food and pharma contact: grades compliant with food-contact regulations (for example FDA or EU 10/2011) are needed for clean lines in food, beverage, and pharma machinery.
• Hydrolysis / UV resistance: ether-based polyurethane resists humidity and oil; UV-stabilized grades resist outdoor weathering.
• SAE J844 / FMVSS 106: the mandatory designation for on-vehicle air brake tube, distinct from general pneumatic tube.

Finally, confirm color coding against the plant convention. Tube color carries no pressure meaning but is used to map a tube to its circuit function (for example one color for supply air, another for vacuum, another for a specific machine zone). Most makers stock five to seven colors precisely so installers can color-code at a glance, which dramatically speeds fault tracing during maintenance.

Selection Decision Factors

To turn the preceding chapters into a specific tube and fitting order, follow the decision sequence below. Most selection errors come not from a single wrong number but from skipping a step, choosing OD before confirming the measurement system, or sizing pressure before derating for temperature. These eight steps double as a fixed RFQ template.

1. Confirm the measurement system: Metric (mm OD) or inch (fractional OD), and confirm the system is OD-based, as nearly all push-in pneumatics are. This single decision prevents the 4 mm versus 5/32 inch and 6 mm versus 1/4 inch mismatch that causes most field leaks.
2. Set pressure and derate for temperature: Take the system working pressure, find the hottest point the tube will see, and confirm the tube's derated rating at that temperature still exceeds the duty with margin. Verify a burst-to-working safety factor of at least 3:1.
3. Choose the material class: Polyurethane for flexing and tight routing, nylon for higher pressure and heat or hydrocarbon contact, polyethylene for low-cost low-pressure lines, fluoropolymer (PTFE/FEP/PFA) for extreme heat or aggressive chemicals above the PU and nylon ceiling.
4. Size the OD and bore for flow: Larger OD and ID reduce pressure drop and speed actuator fill, but cost more and bend less tightly. Match the bore to the valve and actuator ports, and standardize on a short list of sizes (commonly 6 mm and 8 mm, or 1/4 inch and 3/8 inch).
5. Check bend radius at every flex point: Compare the routed radius to the published minimum, and for moving elements use at least twice the static minimum or a flex-rated grade. A tube kinked at the actuator is the most common throttling fault.
6. Verify media and environment compatibility: Oil mist (ether-based PU or nylon), outdoor UV (UV-stabilized grade), washdown and humidity (ether-based PU), vacuum (confirm collapse rating), and hot-work zones (UL 94 V-0 flame-resistant grade).
7. Confirm fitting and standard compatibility: Tube OD tolerance and roundness must meet the fitting maker's specification, validated to ISO 14743 for general pneumatics. For on-vehicle air brakes, specify SAE J844 / FMVSS 106 tube and matching DOT fittings, never general pneumatic tube.
8. Specify approvals and color code: RoHS / REACH as standard, food or pharma contact grades where required, and a tube color mapped to the plant circuit-coding convention so maintenance can trace lines at a glance.

One last commonly overlooked dimension is serviceability and standardization across the plant: keeping fittings and tubing from compatible families, holding a minimal set of OD sizes and colors in spares, and using one tube cutter so every reconnection gets a clean square end. SMC (TU polyurethane, T and TS nylon), Festo (PUN and PUN-H polyurethane, PAN polyamide), Parker (Parflex, including SAE J844 air brake lines), Camozzi, and distributor house brands such as AutomationDirect NITRA all publish full dimension and pressure tables and maintain regional stock, which is what keeps a line running after the original machine builder has moved on. Buying tube and fittings as a validated pair from a single family is the cheapest way to avoid the leaks that come from OD or roundness mismatch.