Designing with Pneumatics: Force, Stroke, and Air Usage

Step 1: Identify the Stroke

Design your mechanism or linkage first and measure the travel you need — that is your stroke. The fixed cylinder body is longer than the stroke (the 'dead length'); the Pneumatics Manual ballparks double-acting dead length from roughly 2.5 in for a small bore up to around 7 in for a 2 in bore. Make sure the cylinder physically fits at full extension.

Step 2: Calculate Force from Bore

Force comes from pressure acting on the piston area. Area of a circular bore is pi x r^2, where r is half the bore diameter. So:

Extend force (lb) = pi x (bore/2)^2 x pressure (psi)

Example: a 2 in bore at 60 psi gives pi x (1)^2 x 60 = ~188 lb — matching the figure quoted in the Pneumatics Manual. A 1 in bore at 60 psi gives pi x (0.5)^2 x 60 = ~47 lb.

For a double-acting cylinder, the retract force is lower because the rod occupies part of the piston area on the rod side:

Retract force (lb) = pi x [(bore/2)^2 - (rod/2)^2] x pressure (psi)

Size your bore for the worst-case direction so the cylinder never stalls when you need it.

Step 3: Evaluate Air Usage

Each actuation consumes a volume of air; the compressor refills slowly (and faster at low pressure). Teams pre-charge the tanks toward 120 psi before a match. As cylinders fire, tank pressure drops; the compressor kicks on (around the pressure switch's lower threshold) and tries to keep up. The danger point is when pressure falls below the working pressure your cylinders need (often ~60 psi) — then actuation slows or stops.

Hand-calculating this is tedious because of the compressor's variable refill, so use the ReCalc pneumatics calculator. It takes tank volume, cylinder size/pressure, and actuation frequency, then graphs system pressure over a simulated match. If the curve dips below your working pressure, you have three fixes the Manual highlights:

1. Reduce air usage — smaller bore/stroke, or actuate less often.
2. Lower pressure — run the regulator below 60 psi if cylinders still work, cutting consumption per stroke.
3. Add storage — another tank keeps pressure above 60 psi longer.

Putting It Together

Good pneumatic design is a loop: pick a bore for the force, check the stroke fits, simulate air usage, and adjust tank count or pressure until the match-long curve stays in the safe zone. Do this in CAD/spreadsheet before you cut tubing.