Quick answer
Match the repair to the failure. A kinked or crushed pneumatic tube can restrict airflow. A cracked, abraded or otherwise damaged wall can leak. A tube that pulls out of a push-in fitting needs checks on its size, insertion, condition and connection, as well as the load pulling on it.
Straightening a damaged tube, pushing a loose end back in, or raising regulator pressure may leave the cause untouched. Inspect the complete run between the air-preparation unit and the equipment, then correct the routing, replace damaged tubing or repair the connection as appropriate. Never cut, disconnect or reposition a connection while it is pressurized.
Start with the symptom, not a replacement part
A slow cylinder and a leaking tube call for different investigations. Locate the symptom before deciding whether the problem is the tubing, the fitting or another part of the circuit.
| What you observe | Possible causes | First place to check |
|---|---|---|
| Movement becomes slow or inconsistent | A kink, crushed tube, inadequate flow or another restriction | Tight turns, clamp points and the supply path during the relevant duty cycle |
| Leakage at a fixed point on the tube body | A crack, abrasion or other wall damage | The damaged area and nearby heat, chemicals, edges or moving parts |
| Leakage where tubing enters a fitting | A damaged tube surface, incompatible size, incomplete insertion or a damaged seal | Tube dimensions, the sealing area and the fitting instructions |
| Tubing separates from a fitting | Incomplete engagement, unsuitable tubing, damaged gripping parts or external loading | Insertion, tube compatibility, connection condition and routing |
These are diagnostic leads, not proof of a single cause. For example, a workstation set to 0.4-0.6 MPa may show a normal pressure reading at rest but receive insufficient airflow during a stroke. This is an example operating range, not a recommended setting for every machine. A clogged filter, valve restriction or insufficient supply can produce similar symptoms.
Kinks and crushing: restore the flow path, not just the shape
Why a kink can cause a large flow loss
A kink narrows and distorts the tube's internal passage. This can increase resistance and reduce the air delivered during a cylinder stroke, even when the pressure gauge looks normal at rest. The size of the crease is not a reliable measure of the resulting flow loss.
An idealized circular-tube model explains why bore size matters. For steady, fully developed laminar flow of a Newtonian fluid treated as incompressible through a straight, constant-diameter tube, Poiseuille's law gives:
Q ∝ (ΔP × d4) / (μ × L)
For that model, the full equation is Q = (π × ΔP × d4) / (128 × μ × L). Here, Q is volumetric flow rate, ΔP is the pressure difference across the modeled tube, d is its internal diameter, μ is dynamic viscosity, and L is tube length. Using pascals, meters and pascal-seconds gives Q in cubic meters per second.
For illustration, hold pressure difference, viscosity and length constant. If the circular bore diameter falls to 70% of its original value, Qnew / Qoriginal = 0.74 ≈ 0.24: about 24% of the original flow, or a 76% reduction. This is a calculation for the idealized model, not a prediction for a kinked pneumatic tube.
Do not apply the fourth-power relationship directly to an actual kink. Compressed air changes density, and a flattened, localized restriction does not satisfy the straight circular-tube assumptions. Turbulence and operating conditions can also change the relationship. In practice, check routing and airflow under the relevant duty cycle, including pressure at the point of use where safely measurable. A slow or stuttering cylinder can also reflect a clogged filter, valve restriction or insufficient supply; it does not prove that the tube is the cause.
Check the route through the complete movement
Look for a sharp turn at a regulator outlet, a clamp squeezing the tube, a closed cover pressing against it, or a moving component pulling it tight. A bend that looks acceptable with the machine stopped may become tighter at the end of travel.
A flattened section reduces the available flow area. Its effect depends on the remaining opening, airflow and the rest of the circuit; the crease alone cannot tell you the pressure loss. Do not assign a universal flow-loss percentage from its appearance.
Check the specified minimum bend radius for the actual tube size and construction. Routing assumptions such as 3-4 times outside diameter for a PU line or 5-6 times outside diameter for a PA line are examples, not installation rules. For a 6 mm tube, those assumptions would mean 18-24 mm and 30-36 mm respectively. The manufacturer's specified radius takes priority; it can fall outside either range.
Remove the condition that made the kink
Give the run enough space to curve without collapsing, reposition restrictive supports, and protect it from moving covers or sharp edges. Leave the movement allowance required by the approved layout without creating a loose loop that catches on equipment.
Replace a permanently creased or damaged section rather than relying on repeated straightening. If a revised route still pulls on the connection, the repair is incomplete. The compressed-air installation and airflow guide provides related point-of-use layout context.
Cracks, abrasion and swelling: inspect the exposure as well as the tube
Separate mechanical damage from material deterioration
A groove beside a bracket suggests rubbing; a split at a repeatedly moving bend suggests local mechanical stress. Crazing, hardening, softening or swelling gives a reason to investigate the material and its exposure. None of these observations alone identifies the cause.
Check contact with hot surfaces, sunlight, lubricant, cleaners and other process chemicals. Identify the actual substance rather than assuming all oils or cleaning agents behave alike. Different PU formulations and PA grades do not have identical compatibility.
Temperatures of 50-60°C can be near the upper working range of some pneumatic PU tubing. Some PA tubing designs allow operation in the 80-100°C range under their specified conditions. These are product-dependent operating ranges, not temperatures at which every tube cracks. Pressure capability may decrease as temperature rises, and the attached fitting can impose a lower limit than the tube.
Replace damage and correct the environment
Replace tubing with wall damage; tape or an improvised sleeve is not a substitute for a specified pressure-containing repair. Inspect adjacent lengths so that replacing one visible split does not leave another damaged section in service.
Then address the exposure: remove rubbing contact, revise routing near a heat source, or use tubing verified for the actual fluid and environment. Replacing a damaged section with the same material in unchanged conditions may reproduce the problem. However, a crack does not automatically justify changing every tube in the machine.
Leakage and tube pull-out at a fitting: check two different functions
Sealing is not the same as holding the tube
In a typical push-in connection, an elastomeric seal contacts the tube's outside surface, while a gripping mechanism holds the tube in position. Leakage can occur without pull-out, and a retention problem can exist even when the connection initially seals.
Check the specified outside diameter and permitted tube construction. Metric and inch sizes are not interchangeable merely because they look similar. Also inspect the cut end, the surface passing through the seal, and any previous gripping damage. If the fitting is damaged, replacing only the tube may not restore the connection.
For 6-12 mm outside-diameter tubing, insertion depths within 15-25 mm occur in some fitting designs. That range is not a universal target. Find the actual model's insertion dimension, prepare the end as specified, and insert it fully according to the instructions. Where permitted, use a reference mark to make incomplete insertion easier to spot.
Relieve the load on the connection
Check whether the line pulls sideways at full travel, twists during movement, or transfers vibration directly to the fitting. A tube that pulls out without an obvious crack can still have incompatible dimensions or properties; an apparently intact end does not prove the fitting alone is faulty.
In an example circuit operating at 0.4-0.7 MPa, the joint must still tolerate its actual service conditions within the complete assembly's ratings. This range is not a pull-out threshold. Investigate pressure excursions and connection loading rather than assuming normal gauge pressure makes the joint suitable.
Re-terminate only where the tube and fitting instructions permit it. Otherwise use new specified tubing or replacement parts. For the broader connection checks, see how to choose pneumatic fittings to reduce air leaks.
PU or PA: change material only when it addresses the cause
The useful comparison is between specific tubing designs, not color or material initials alone.
| Selection factor | PU tubing | PA tubing |
|---|---|---|
| Routing and flexibility | Often selected for flexible routing; check the size-specific bend radius and any dynamic-use approval | Often stiffer; verify the required radius and whether the product permits repeated movement |
| Temperature and pressure | Some designs reach an upper air-service temperature of 60°C; check pressure derating | Some nylon designs reach 100°C under specified conditions; the fitting may limit the assembly |
| Oils and chemicals | Compatibility depends on the formulation and the substance | Compatibility depends on the grade and additives; nylon is not immune to deterioration |
For example, a tube rubbing against a bracket needs the contact corrected, not simply a change from PU to PA. A tube exposed to an incompatible cleaner needs a verified compatibility solution. Incomplete insertion needs the connection corrected; changing material does not make an under-inserted end acceptable.
Repair safely, then test under the relevant conditions
Use the machine's documented energy-control procedure and appropriately trained personnel. In U.S. workplaces where it applies, 29 CFR 1910.147 on hazardous-energy control covers isolation, stored energy and verification before servicing.
- Make the affected equipment safe. Isolate the required energy sources, prevent unintended repressurization, and release or restrain stored energy as the procedure requires. One gauge reading zero does not prove every trapped section is depressurized.
- Inspect beyond the visible failure. Check the tube, fittings, supports and nearby exposures. Confirm whether reuse or re-termination is permitted.
- Prepare and replace correctly. Use the specified cutting and connection method. Keep contamination out of the open line and replace damaged parts rather than forcing them back together.
- Correct routing and support. Check clearances and movement allowance before returning the equipment to operation. Do not adjust tubing inside a hazardous operating area.
- Restore and verify under the approved procedure. Keep personnel clear. Check for leakage with an appropriate, compatible method, then check the relevant operating cycle from a safe position. If more servicing is needed, reapply the energy-control procedure first.
A stationary leak check does not test every movement-related fault. Record the repaired location, replacement specification and observed result. The U.S. Department of Energy's leak-maintenance guidance likewise emphasizes identification, repair, tracking and verification rather than a one-time repair alone.
Repeated failures: widen the check to point-of-use air preparation
Several failures on one branch justify looking beyond individual tubes. Check the actual temperature and pressure conditions, the lubricant used where lubrication is required, and changes to machine movement or supports. Repeated pull-out does not by itself prove a regulator fault; establish the pressure behavior before blaming it.
If movement remains slow, check the supply path and filter condition as well as the tubing. The air-filter micron-rating guide explains filtration and pressure-drop considerations. For routine checks on nearby air-preparation components, use the FRL maintenance checklist.
Filtering, regulating and lubricating serve different tasks. None repairs a cracked wall, corrects incompatible tubing or replaces proper connection engagement.
AIRZOE air-preparation products to consider
Explore the AIRZOE AF2000-02 air filter, AR2000-02 pressure regulator and AIRZOE filter-regulator-lubricator combination for the air-preparation stage. Visit their product pages for specifications and ordering details.
Frequently asked questions
Can I keep using a kinked tube after straightening it?
Not simply because its shape looks better. Replace tubing with a persistent crease, flattening or wall damage, and correct the routing so the replacement meets its bend-radius requirements throughout the machine's movement.
Should I replace the whole tube or only the damaged section?
Consider the damage extent, remaining length, permitted repair method and surrounding conditions. Broad deterioration favors replacement of the affected run. Do not add an unapproved splice just to preserve a short section.
Why does new tubing still leak or pull out?
New tubing does not correct the wrong outside diameter, incomplete insertion, damaged fitting parts or continued pulling at the joint. Check the complete connection and its routing, not just the new tube.
Will replacing PU with PA stop cracking?
Only if the selected PA product addresses the verified cause and meets the application's pressure, temperature, chemical and movement requirements. Changing material will not remove a sharp edge or make unsuitable routing acceptable.
Why is the cylinder slow when the pressure gauge looks normal?
A normal pressure reading at rest does not show whether enough air reaches the cylinder during operation. Check the supply path for restrictions in the tubing, filter and valves before increasing the pressure setting.