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Cylinder force is pressure times effective area. On extension the full bore area acts, but on retraction the rod area is subtracted, so a 40 mm bore with a 16 mm rod retracts with only about 84% of its extend force — the asymmetry that catches out clamping and pressing applications.
Cylinder force
extend = P x pi D^2/4; retract = P x pi (D^2 - d^2)/4, both x efficiency
The rod occupies part of the piston face on the annulus side, so less area is available for the pressure to act on.
85-95% for a well-lubricated pneumatic cylinder. Seal friction and side loading account for the loss, and it worsens at low pressure.