The Cylinder That Wouldn’t Reach the Speed
We sized a cylinder for a 500 mm stroke. The spec said “extend in 0.5 s.” On the floor, the cylinder took 1.2 s (too slow). The bore and pressure were right (the force was enough). The problem: the air flow (into the cylinder) was limited by the tubing (and the valve). The cylinder’s speed depends on the air flow, not the force. We upsized the tubing (8 mm instead of 6 mm) and the valve (higher flow). The speed dropped to 0.5 s. The mistake was sizing the cylinder by force (bore × pressure) without checking the flow (which determines the speed).
Pneumatic cylinder speed calculation is about the air flow, not just the force. This article covers the estimation.
Speed vs Flow
A cylinder’s speed (extend time) depends on how fast air fills the bore. The flow (L/min) into the cylinder determines the speed. For a given bore (area) and stroke, the flow needed is:
Q = A × S / t
Where Q is the flow (L/s), A is the bore area (m²), S is the stroke (m), and t is the time (s). For Ø50 mm bore (A = 0.00196 m²), S = 0.5 m, t = 0.5 s: Q = 0.00196 × 0.5 / 0.5 = 0.00196 m³/s = 1.96 L/s = 118 L/min. The valve and tubing must pass 118 L/min (at 6 bar). If the valve (or tubing) only passes 50 L/min, the cylinder takes 1.2 s (not 0.5).
The speed rule: Calculate Q = A × S / t. The cylinder that was too slow had a small valve/tubing (50 L/min) on a demand of 118 L/min. Upsize the valve and tubing (to pass 118 L/min). The cylinder’s speed is flow-limited, not force-limited.
Step 1: The Valve Flow (CV)
A valve’s flow rating (CV, or KV) is the air it passes. A small valve (CV = 0.5) passes little. A big valve (CV = 2) passes more. Pick a valve with a CV that passes Q (118 L/min). For a 118 L/min demand, pick a valve with CV ≥ 1.5 (or per the manufacturer’s flow curve).
Step 2: The Tubing (Article 117, 149)
The tubing (hose) restricts the flow. A 6 mm ID tube (long run) drops the pressure (article 149). For a fast cylinder, use a larger ID tube (8 mm) and a short run. The tubing’s restriction adds to the valve’s restriction.
Step 3: The Flow Control (Article 124)
The flow control (meter-in/meter-out, article 124) limits the speed (on purpose). If the cylinder is too fast, the flow control slows it. If the cylinder is too slow (and the flow control is fully open), the valve/tubing is too small.
Step 4: Cylinder Cushioning (Article 110)
At the end of the stroke, the cylinder cushions (article 110) (slows at the end). The cushioning adds time (the last 50 mm is slow). For a fast cycle, the cushioning shortens the effective speed. Adjust the cushioning (article 110) for the load (not too tight).
| Bore | Stroke | Time | Flow (Q) |
|---|---|---|---|
| Ø32 | 100 mm | 0.2 s | 40 L/min |
| Ø50 | 500 mm | 0.5 s | 118 L/min |
| Ø80 | 500 mm | 0.5 s | 300 L/min |
A Speed Calculation Checklist
- What is the bore? (Area A?)
- What is the stroke? (S?)
- What is the target time? (t?)
- Calculate Q = A × S / t?
- Is the valve CV big enough? (Passes Q?)
- Is the tubing ID big enough? (Article 117?)
- Is the flow control open? (Article 124?)
- Is the cushioning adjusted? (Article 110?)
- Is the supply pressure right? (Article 85?)
- Does the cylinder reach the speed? (Test?)
The Bottom Line
Pneumatic cylinder speed calculation is about flow. The cylinder that was too slow had a small valve/tubing. Calculate Q = A × S / t. Pick a valve (CV) and tubing that pass Q. The cylinder that reached the speed wasn’t the biggest bore — it had enough flow.
