Maintenance Guides 1 min read

Diagnosis and Troubleshooting of Slow or Uneven Operation of Pneumatic Cylinders

Technical analysis: Troubleshooting pneumatic cylinder slow or inconsistent operation: flow control adjustment, seal wea

Diagnosis and Troubleshooting of Slow or Uneven Operation of Pneumatic Cylinders

1. Problem Description and Area of Application

Slow operation or piston shocks of pneumatic cylinders are a common problem in automated production lines of Ukrainian industrial enterprises (automotive industry, food equipment, chemical production and packaging lines). This malfunction causes disruption of the equipment operation cycle, conveyor stoppages and reduction of overall production efficiency (OEE).

This manual covers standard double-acting cylinders according to standards ISO 15552, ISO 6432 and compact cylinders according to ISO 21287.

Severity Class (Severity):

  • Critical: Full cylinder stop under load, mechanism locking in intermediate position, risk of tool or workpiece damage.
  • Major: Significant reduction in service speed (more than 40% from nominal), uneven movement (jumps), sticking at the end of travel.
  • Minor: Minor variation in cycle time, slight air hiss through the seal without loss of working speed.

2. Safety Measures

WARNING:

1. Energy Isolation (LOTO): Perform the Lockout/Tagout procedure before any disassembly or adjustment of pneumatic equipment. Block the compressed air supply main and close the shut-off valve.

2. Exhausting residual pressure: Open the manual or automatic exhaust valve (3/2-way), to fully exhaust the residual pressure in the cylinder chambers and pipelines to 0 bar. Residual pressure may cause uncontrolled rod movement and injury.

3. Energy of the compressed spring: For single-acting cylinders with spring return, remember that energy of the compressed spring is present inside the cylinder body.

4. Personal protective equipment (PPE): Mandatory use of safety goggles (EN 166), protective footwear (EN ISO 20345) and gloves for protection against mechanical injuries.

5. Risk of Scalding: Used oil and components of the pneumatic network may be hot after prolonged operation of the equipment.

3. Required diagnostic tools

Tool Name Specification / Model Measurement range Purpose
Digital Manometer Sub-class 0.5 with two ports 0 – 16 bar Measurement of working and dynamic pressure directly near the cylinder ports
Air flow meter Thermoanemometric flow sensor 0 – 1000 l/min Control of compressed air consumption, detection of internal leaks
Stopwatch / Tachometer Digital or optical measuring device 0.01 – 999 s Stroke extension and retraction time measurement
Ultrasonic Flow Meter Acoustic detector with directional microphone 20 – 100 kHz Leak detection in pneumatic distributors and connections
Set of feeler gauges and vernier caliper DIN 862, DIN 2275 0.05 – 1.0 mm / 0 – 150 mm Gap Check of Guides and Mounting Parallelism
Multimeter True RMS (CAT III 600V) 0 – 24 V DC / 0 – 500 Ω Voltage and Coil Resistance Check of Electromagnetic Valves

4. Initial Condition Assessment Map

What to observe / record Normal indicators Alarm Indicators Method of inspection
Static pressure in the system 6.0 – 8.0 bar < 5.0 bar Pressure gauge on the air preparation block (FRL)
Dynamic pressure during movement Drop not more than 0.5 bar from static Drop more than 1.5 bar Pressure gauge directly on cylinder port
Condensation present Missing in the tubes Water droplets in the pipeline or filter bowl Visual inspection of the filter bowl of the moisture separator
Lubricant condition in lubricators 1 drop per 300–500 cycles (if applicable) Dry lubricator or excess oil Visual inspection of the lubricator window
Full stroke time According to the equipment passport (e.g. 0.3 s) Increase time over 30% of nominal Timer or PLC position sensor

5. Systematic Diagnostic Block Diagram

  • Symptom: Cylinder operates slowly or in jerks
    • Step 1. Check the air pressure in the pneumatic network at the inlet of the equipment.
      • IF pressure < 5.0 bar → Check the central compressor, main filters and regulator on the FRL block. Adjust the regulator to 6.0–7.0 bar.
      • If pressure is within normal range (6.0–8.0 bar) → Proceed to Step 2.
    • Step 2. Check the position of the check valve flow control valves (Flow Control Valves).
      • IF the valves are over-tightened → Slowly loosen counter-clockwise and measure the actuation time.
      • If regulation does not help or air does not pass → Proceed to Step 3.
    • Step 3. Check the condition of the main tubes and fittings for cracks or restrictions.
      • IF a mechanical jamming of the tube or internal clogging of the distributor silencer is detected → Replace the tube or clean/replace the pneumatic silencer.
      • If tubes and mufflers are clean → Proceed to Step 4.
    • Step 4. Perform a test for internal and external air leaks through the piston seal.
      • IF air is detected escaping from the exhaust port of the opposite chamber when the rod is stopped → Worn piston rings. Replacement of the piston kit or cylinder is required.
      • If turns are absent → Proceed to Step 5.
    • Step 5. Check the mechanical load and guide mechanism.
      • IF there is lateral load or piston rod tilt (deflection) → Level the cylinder mounting brackets according to ISO 15552, check the lubrication of the guides.
      • IF mechanics are normal → Replace the distributor or diagnose the coil.

6. Symptom - Cause - Test Matrix

Symptom Likely causes (by likelihood) Diagnostic test Expected result upon confirmation of the cause
Slow movement throughout the entire stroke 1. Incorrect regulation of throttles
2. Clogged muffler of the distributor
3. Pressure drop in the network		 Measure the inlet pressure; remove the muffler; check the throttle screw position Pressure drops under load; without silencer, speed recovers
Fasteners and stick-slip 1. Lack of lubrication / dry sleeve walls
2. Wear of guide rings
3. Misalignment during installation		 Apply 3–5 drops of special lubricant; check parallelism with a feeler gauge Temporary improvement of movement after lubrication; elimination of misalignment restores smoothness
Slow movement only in one direction 1. Piston ring damage in one direction
2. Incomplete throttle opening on exhaust
3. Distributor section failure		 Check for leakage at the exhaust port; swap the pilot tubes Air leakage from the exhaust silencer; change in behavior during tube switching
End cushioning (damping) 1. Over-tightened buffer adjustment screw
2. Wear of the damper pin seal		 Turn the adjustment screw of the damper 1–2 turns The piston passes through the damping zone without stopping

7. Root Cause Analysis

Reason 1: Wear or damage to the piston seal (Internal Bypass)

Why it occurs: Natural abrasive wear of the polyurethane (PU) or nitrile rubber (NBR) seals due to operation in an environment with abrasive dust or in the absence of proper cleaning of compressed air (contamination class higher than ISO 8573-1:2010 [7:4:4]). Also, the cause is chemical degradation due to incompatible synthetic oils in the compressor.

Як підтвердити: Зупиніть шток у середньому положенні. Від'єднайте трубку з протилежного боку циліндра від розподільника або перевірте порт на наявність постійного витоку повітря під час подачі тиску в робочу порожнину. Використовуйте ультразвуковий течешукач або мильний розчин на вихлопному порті.

Damage caused by: Drop in effective cylinder force to 50%, excessive compressed air consumption, overheating of the distributor due to constant leakage, inability to hold position under load.

Reason 2: Problems with air preparation and lubrication (Stick-Slip Effect)

Why it occurs: Absence or washing out of factory lubricant due to entry of water from unsaturated air (dew point above +3 °C). Dryness between the piston seal and the inner surface of the aluminum sleeve with anodized coating causes a stick-slip effect.

How to confirm: Visual inspection of the filter bowl (presence of water). Disassembly of the supply tube and introduction of 2 ml of special pneumatic oil (ISO VG 32). If the movement becomes smooth — the cause is confirmed.

What damage is caused: Wear of the internal mirror surface of the cylinder sleeve, formation of scoring, complete wearing of the working edges of the seals, seizing of the unit.

Reason 3: Incorrect throttle settings and line flow capacity

Why it occurs: Using throttles for flow control (In-port metering) instead of exhaust control (Out-port metering) for double-acting cylinders, or using too narrow fittings and tubes (for example, 4 mm tube for a 63 mm diameter cylinder).

Як підтвердити: Виміряти час спрацьовування та порівняти з номограмами виробника. Перевірити схему підключення дроселів (дроселювання повинно відбуватися на вихлопі повітря з порожнини циліндра).

What damage is caused by: Uneven movement due to air compression, increased load on the piston during pressure surges, premature wear of dampers.

8. Step-by-Step Resolution (Step-by-Step Resolution)

Procedure A: Seal Replacement and Cylinder Sleeve Maintenance (ISO 15552)

  1. Perform the LOTO procedure: block the pressure, bleed down to 0 bar.
  2. Remove the cylinder from the equipment, disconnecting the pneumatic tubes and the rod end swivel fittings.
  3. Grip the cylinder body in pliers with soft pads (aluminum/copper).
  4. Remove the cylinder covers (depending on the construction: tie rods or threaded covers).
  5. Remove the piston together with the rod from the aluminum sleeve.
  6. Clean the inner surface of the sleeve and piston with industrial alcohol or part cleaner. Check the sleeve for the absence of burrs and scratches.
  7. Sealing kit lubricate with a consistent pneumatic grease (for example, based on silicone or polytetrafluoroethylene — PTFE).
  8. Install new piston rings, guide rings, and rod seals in the front cover. Avoid twisting of the seals.
  9. Carefully assemble the unit by inserting the piston into the bore using a conical mandrel to avoid cutting the seal edges.
  10. Tighten the fastening screws / covers with even force crosswise (torque for M6 thread: 9.0 Nm, for M8: 22.0 Nm).
  11. Install the cylinder on the equipment, connect the piping, and perform a leak test with soap solution under a pressure of 6.0 bar.
  12. Check smooth operation without load, then under load.

Procedure B: Regulation and Cleaning of Throttles and Pneumatic Networks

  1. Check the condition of the mufflers on the distributors. If they are contaminated with oil or sludge — unscrew and replace with new ones.
  2. Make sure that the speed is controlled by the exhaust throttles (according to the exhaust air control principle).
  3. Fully tighten the throttle screws (turn clockwise until they are seated), then loosen them by 1.5–2 turns for initial adjustment.
  4. Apply pressure to the system (6.0 bar). Perform 5 test cycles of cylinder activation.
  5. Adjust the throttle screws so as to achieve smooth stop in the end positions without striking the covers (using built-in pneumatic damping).
  6. Fix the position of the adjustment screws with locking nuts (torque for the locking nut: 2.5 – 4.0 Nm).

9. Preventive Measures

Primary Cause Preventive strategy Monitoring method Recommended interval
Contamination and humidity in the air Air Preparation Unit (FRL) Servicing, Replacement of 5 µm Filtering Elements Pressure drop indicator check on filters, condensate drain Every 500 hours of operation / once a month
Shaft seal wear due to dry running Use of dried air according to ISO 8573-1 (class 4.4.3) or installation of lubricant doser Control of air dew point and oil level in lubricators Weekly visual inspection
Loosening of fastenings and misalignment Torque check of mounting bolts and hinge connections Dynamic vibration control and torque wrench check Quarterly (2000 hours)

10. Spare Parts and Components

Description of Parts Specification When to replace Category UNITEC
Cylinder seal repair kit ISO 15552 NBR / PUR, for diameters Ø32 – Ø100 mm Every 2 years or upon appearance of internal leaks Pneumatic cylinders / Seals
Throttle with check valve Thread G1/8", G1/4", G3/8" for pipes 6–10 mm If the threads are damaged or the needle is not hermetic Pneumatic Actuators / Valves
Air Preparation Block (FRL) Filter-regulator + lubricator, port G1/4" When the flow capacity drops or the membrane is damaged Air Preparation
Pneumatic muffler Spun bronze or plastic, G1/8" – G1/2" When contaminated with oil or increase in exhaust resistance Pneumatic accessories

Full range of certified pneumatic components, rebuilds and cylinders of European brands is available in our electronic catalog: https://www.unitecd.com/e-catalog/

11. References

  • ISO 15552:2018 — Pneumatic hydraulic cylinders with removable covers, series of nominal diameters from 32 to 320 mm.
  • ISO 8573-1:2010 — Compressed air. Part 1: Contamination and cleanliness classes.
  • ISO 4414:2010 — Pneumatic Fluid. General rules and safety requirements for systems and their components.
  • OEM Technical Guides — Installation and Maintenance Instructions for Pneumatic Cylinders from Festo, SMC, Parker and Aventics.
  • Internal Lubricants UNITEC-D — Oil and Seal Selection Guide for Industrial Pneumatics (UG-PN-04).

Related Articles