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How long does a pneumatic cylinder stroke take?

A cylinder needs a volume of free air per stroke — the swept volume expanded to atmospheric pressure — and the valve, tubing and supply deliver it at a limited rate. That sets the time for a lightly loaded cylinder; as the load takes up the cylinder's force the stroke slows by 1/(1 − load ratio), and every stroke starts with the valve's dead time. Above a load ratio of 0.8 the cylinder crawls and can stall.

What you need

The math: Pneumatic cylinder — stroke timeopen — every formula, so anyone can check it
A        = π·d²/4                (extend)      π·(d² − d_rod²)/4   (retract)
V_free   = A · stroke · (p_gauge + p_amb) / p_amb          free air per stroke
t_flow   = V_free / (Q_system/60 · k_flowcontrol)         time the supply needs to deliver it

F_available = p_gauge · A · η_cylinder (0.88)
F_required  = μ·m·g (horizontal)   or   m·g (lifting)
load ratio  = F_required / F_available

t_stroke = t_flow / (1 − load ratio) + t_dead              status: ok ≤ 0.5 < warn ≤ 0.8 < fail

Under a load ratio of 0.5 the air supply sets the time, not the load; above 0.8 the cylinder is at the edge of its force and crawls. A load ratio of 1 or more never completes the stroke. The rod-diameter table for ISO bores is used when no rod diameter is given.

Package pneumatic-stroke-time (calcpkg/1, open format) — pure functions with tests; the browser version is held against it number for number.

Questions people ask

What is the load ratio?

The force the load needs divided by the force the cylinder makes at the working pressure (with an efficiency of 0.88). Under 0.5 the air supply sets the time; 0.5–0.8 slows the stroke; above 0.8 it crawls.

Why is the retract stroke faster?

The rod takes up part of the piston area, so the rod side needs less air per stroke — but it also makes less force, so a heavy load can slow it more.

How do I make a cylinder faster?

Raise the flow: a bigger valve, shorter and wider tubing, an open flow control. Raising pressure helps a loaded cylinder; a bigger bore helps the load ratio but needs more air.