A pneumatic cylinder that took 0.8 seconds to extend over 100 mm. The customer wanted 0.3 seconds. The cylinder was fine. The tube from the valve to the cylinder was 6 mm ID, 5 meters long. The air had to fill the tube before the cylinder moved. This is how I size pneumatic tubing for response time.
The tube volume
When the valve energizes, air flows through the tube to the cylinder. The tube has volume. The air must fill that volume at supply pressure before the cylinder sees pressure. For a 6 mm ID tube, 5 m long: V = π × 3² × 5000 = 141,372 mm³ = 0.141 L. At 6 bar, the free air equivalent is 0.141 × 6 = 0.85 L. The valve’s flow is about 500 NL/min (8.3 L/s). Fill time = 0.85 / 8.3 = 0.10 seconds. That’s small. But that’s just filling the tube. The cylinder itself also needs air.
The cylinder volume (Ø32 bore, 100 mm stroke): V = π × 16² × 100 = 80,425 mm³ = 0.080 L. At 6 bar: 0.48 L free air. At 8.3 L/s, that’s 0.058 seconds. Total fill time: 0.10 + 0.058 = 0.16 seconds. But the customer measured 0.8 seconds. Why?
Because the flow through a long, narrow tube isn’t the valve’s rated flow. The tube itself restricts the flow. The effective flow (Cv) of a 6 mm ID, 5 m long tube is about 0.5. The valve’s Cv is 1.2. The tube is the bottleneck, not the valve. The actual flow through the tube is 500 × (0.5/1.2) = 208 L/min = 3.5 L/s. Fill time: 0.85 / 3.5 = 0.24 seconds (tube) + 0.48/3.5 = 0.14 seconds (cylinder) = 0.38 seconds. That’s closer to what the customer saw. The rest of the delay is the cylinder’s cushioning and the valve response.
The tube sizing rule
| Tube length | Recommended ID | Cylinder speed |
|---|---|---|
| < 1 m | 4 mm | Fast (direct mount) |
| 1-3 m | 6 mm | Good |
| 3-5 m | 8 mm | Good |
| 5-10 m | 10 mm | Acceptable |
| > 10 m | 12 mm or valve near cylinder | Use valve terminal |
What I changed
1. Enlarged the tube. I changed from 6 mm to 8 mm ID. The cross-sectional area increased by (8/6)² = 1.78x. The tube flow restriction dropped. The effective Cv of the 8 mm tube is 0.9. Flow increased from 208 to 375 L/min. Fill time dropped to 0.13 + 0.08 = 0.21 seconds. The customer wanted 0.3 seconds. We met it.
2. Moved the valve closer. The real fix is to mount the valve near the cylinder. We moved the valve from the control cabinet (5 m away) to a valve terminal 200 mm from the cylinder. The tube length dropped to 0.2 m. The tube volume is negligible. The fill time is just the cylinder: 0.48 L at 8.3 L/s = 0.058 seconds. The cylinder now extends in 0.15 seconds. The customer got the 0.3-second cycle with margin.
3. Added flow controls. The cylinder was too fast after these changes (0.15 seconds). We adjusted the flow control valves to slow it to 0.25 seconds. The cushioning handles the end-of-stroke. No slamming.
The pressure drop
Long tubes also have pressure drop. At high flow (cylinder extending fast), the pressure at the cylinder end is lower than at the valve. For a 6 mm tube at 5 m length, the pressure drop at 500 L/min flow is about 1 bar. The cylinder sees 5 bar instead of 6 bar. The force drops by 17%. If the cylinder is sized for 6 bar operation, it stalls at 5 bar. This is the same problem — long, narrow tubes. Enlarging the tube or moving the valve closer fixes both the response time and the pressure drop.
The number I check: tube length vs ID. If the tube is over 3 m, use 8 mm or larger. If the cylinder is slow to start after the valve clicks, it’s filling the tube. The cheapest fix is to mount the valve near the cylinder. The cylinder that took 0.8 seconds wasn’t weak — it was waiting for air to travel 5 meters. Pneumatics are fast at the valve, not at the end of a long straw.
