On every stroke, a pneumatic cylinder uses the volume of its chamber filled at working pressure. Its consumption therefore comes from three figures: the swept volume, the number of cycles per minute and the absolute pressure. A cylinder with a 50 mm bore and a 300 mm stroke, at 6 bar and 8 cycles per minute, uses about 60 l/min of free air.
This calculation is how you size the compressor for an automated line, for example in sawmills and the timber industry, where dozens of cylinders and clamps work all day long.
The formula, step by step
Step 1, piston area. Area (cm²) = π × (bore in cm)² ÷ 4. On a double-acting cylinder, the return stroke happens on the rod side, so subtract the rod area.
Step 2, volume per cycle. Volume (cm³) = area × stroke (cm). Single-acting: outstroke only. Double-acting: outstroke plus return stroke (rod side).
Step 3, free air per cycle. The compressor has to supply that volume filled at working pressure. Multiply the volume in litres by the absolute pressure, which is the gauge pressure plus 1 bar.
Step 4, consumption per minute. Multiply by the number of cycles per minute.
Three worked examples
All examples are at 6 bar gauge, so 7 bar absolute, and double-acting.
Positioning cylinder, 50 mm bore, 20 mm rod, 300 mm stroke, 8 cycles per minute.
- Piston area: 19.6 cm². Rod area: 3.1 cm².
- Outstroke: 19.6 × 30 = 589 cm³. Return: (19.6 - 3.1) × 30 = 495 cm³. Total: 1.08 litres per cycle.
- Free air: 1.08 × 7 = 7.6 litres per cycle.
- Consumption: 7.6 × 8 = about 61 l/min.
Clamp, 32 mm bore, 12 mm rod, 50 mm stroke, 20 cycles per minute. Volume per cycle: 0.075 litres. Free air: 0.52 litres per cycle. Consumption: about 10 l/min. A clamp moves often, but its small size keeps it frugal.
Turning cylinder, 100 mm bore, 25 mm rod, 500 mm stroke, 4 cycles per minute. Volume per cycle: 7.6 litres. Free air: 53 litres per cycle. Consumption: about 213 l/min. The bore counts squared: doubling it multiplies consumption by four.
For a single-acting cylinder, count the outstroke only. The 50 mm cylinder above would drop from 61 to 33 l/min.
From one cylinder to a whole line
Add up the cylinders on the line, then add a margin. Example of a board sorting and stacking line, as a guide:
| Component | Quantity | Each | Total |
|---|---|---|---|
| Turning cylinder 100 mm | 2 | 213 l/min | 426 l/min |
| Positioning cylinder 50 mm | 4 | 61 l/min | 244 l/min |
| Clamp 32 mm | 6 | 10 l/min | 60 l/min |
| Total | 730 l/min |
With a 25% margin, aim for about 915 l/min at working pressure. The cycle-based figure is an average: the margin and an air receiver absorb the peaks when several large cylinders move at once.
Which compressor for this line?
- A line like the example: the S 75. 1.1 m³/min at 8 bar, or 1,100 l/min, with room for a few more cylinders.
- Two lines, or one line plus blow-off points: the S 110. 1.7 m³/min at 8 bar.
- Multi-shift site with several machines: the S 150. 2.2 m³/min at 8 bar.
Cylinders work in waves: the line speeds up, slows down, stops. The variable speed S series follows that demand instead of running unloaded. Our flow and pressure converter turns m³/min into l/min if your machine's data sheet uses another unit.
Calculation traps
- Hoses count too. The hose between the valve and the cylinder empties on every stroke. On long runs, add its volume to the cylinder's.
- Pressure set too high. The same 100 mm cylinder uses 213 l/min at 6 bar and about 183 l/min at 5 bar. Set each regulator to the pressure the cylinder really needs. Our article on how to calculate compressor pressure explains the method.
- Leaks. A worn rod seal or a leaking valve uses air even when the line is stopped. Listen to the line in the evening, still under pressure: see detecting and repairing leaks.
- Water. Wet air rusts piston rods and makes sawdust stick inside valves. Fit a dryer after the compressor.
Line builders often state the air consumption of their machines: ask for it and compare it with your own figure. Then enter your equipment in the configurator, or send us your list of cylinders with a quote request.
For your trade: Compressor for sawmills and the timber industry.