A pneumatic cylinder that moved slowly at 6 bar. It was a 50 mm bore cylinder, 200 mm stroke, pushing a 30 kg slide on linear bearings. The customer expected it to take 0.5 seconds. It took 2 seconds. The compressor was fine (8 bar, 200 L tank). The valves were new. The problem: the cylinder was sized for force, not for speed. And the tubing and valve were too small. This is about pneumatic circuit sizing — not just the cylinder, but everything in the air path.

The force calculation

The theoretical push force of a 50 mm bore cylinder at 6 bar:

F = P × A = P × π × D²/4

Where P is pressure in N/mm² (6 bar = 0.6 N/mm²) and D is bore in mm. A = π × 50² / 4 = 1963 mm². F = 0.6 × 1963 = 1178 N. The 30 kg slide on bearings has friction of about 30 N (1% of weight). The cylinder has 1148 N of push. That’s 38x more than needed. The force is fine. The speed is the issue.

The speed bottleneck

The cylinder speed depends on how fast air can fill the cylinder. At 6 bar, the air consumption per stroke:

V = A × L = 1963 × 200 = 392,600 mm³ = 0.393 L (at 6 bar absolute)

At atmospheric pressure (1 bar): V_atm = 0.393 × 6 = 2.36 L per stroke. At 2 seconds per stroke: flow = 2.36 / 2 × 60 = 71 L/min. The valve had a Cv of 0.6. The flow through a 0.6 Cv valve at 6 bar supply is about 80 L/min. That’s just enough. But the tubing was 6 mm ID, 3 meters long. The pressure drop in the tubing at 71 L/min is about 1.5 bar. The cylinder actually saw 4.5 bar, not 6 bar. The force dropped to 884 N. Still plenty. But the flow into the cylinder was limited by the small tubing.

What I changed

1. Up-sized the tubing to 8 mm ID. The 8 mm tubing has twice the area. The pressure drop dropped from 1.5 bar to 0.4 bar. The cylinder saw 5.6 bar. The speed increased by 30%.

2. Up-sized the valve to Cv=1.2. A 1.2 Cv valve flows 160 L/min at 6 bar. The bottleneck moved from the valve to the cylinder itself. The speed dropped from 2 seconds to 0.8 seconds.

3. Added a quick-exhaust valve near the cylinder. Instead of exhausting air back through the 3-meter tubing and valve, I mounted a quick-exhaust valve directly on the cylinder cap. The exhaust air dumps to atmosphere within 100 mm. The return stroke dropped to 0.4 seconds. The extend stroke was still 0.8 seconds (through the valve and tubing). The cycle time is now 1.2 seconds instead of 4 seconds.

The air consumption

For a multi-station machine, the air consumption matters for compressor sizing. Each stroke: 2.36 L at atmospheric. At 30 cycles per minute: 71 L/min per cylinder. For 10 cylinders: 710 L/min. The compressor must supply this plus leakage. I size the compressor 20% above calculated consumption. A 10-cylinder machine needs a 1000 L/min compressor with a 200 L tank.

The force margin rule

I always size the cylinder for 3x the required force. At 3x margin, the cylinder moves at rated speed even with pressure drops in the tubing and valve. A 50 mm bore pushing 30 kg has 38x margin — overkill but cheap. For a 300 kg load needing 3000 N: I’d use an 80 mm bore at 6 bar (F = 3016 N), then upsize to 100 mm for 3x margin (F = 4712 N). The 100 mm cylinder costs $100 more but moves at the designed speed.

The cylinder I size: 3x force margin, then check the valve Cv and tubing ID. The slow cylinder wasn’t undersized — the 6 mm tubing and 0.6 Cv valve choked the airflow. Up-size tubing to 8 mm, valve to 1.2 Cv, add a quick-exhaust valve. Force is rarely the bottleneck; flow rate always is.