Double Acting Pneumatic Cylinders

Air supplied to port A, in the rear cap, acts on the full piston area and pushes the rod out while the air in front of the piston escapes through port B. Reversing the valve feeds port B: the air now acts on the annulus around the rod and pulls the piston back while port A exhausts.

Because the rod occupies part of the front face, the pull force is always a little lower than the push force. The difference depends on the rod-to-bore ratio — from about 16 % on a Ø40 mm cylinder with a Ø16 mm rod to about 4 % on the large bores — and it matters when the return stroke has to lift or drag a load.

Formula: push F = p × A; pull F = p × (A − a), where A is the piston area and a the rod area. The Fenitsa pneumatic catalogue lists the same forces in kilograms using 1 kg ≈ 10 N — for example 471 kg push and 441 kg pull for Ø100 mm, or 4,823 kg and 4,638 kg for Ø320 mm.

For dynamic motion size the bore so that the load needs no more than 50–70 % of the theoretical force. The air cylinder calculator gives push and pull force for any pressure in bar or psi.

A pusher has to move cartons with a force of 800 N, and the factory network supplies 6 bar. For a moving load, plan on using no more than about 60 % of the theoretical force, so the cylinder should deliver 800 / 0.6 ≈ 1,333 N.

Required piston area: A = F / p = 1,333 N / 0.6 N/mm² ≈ 2,222 mm², which corresponds to a bore of √(4A / π) ≈ 53 mm. The next standard bore is Ø63 mm, giving about 1,870 N push and 1,682 N pull at 6 bar — enough margin in both directions. Ø50 mm (1,178 N) would be too small.

Each series is also available as a double rod cylinder (code C), a tandem cylinder (code T) that doubles the force from the same bore, and a two-position tandem (code D) with two strokes in one unit. See tandem cylinders and double rod cylinders.

Adjustable end cushioning slows the piston during the last millimetres of travel: a cushion sleeve enters a seal in the end cap, traps a small volume of air and lets it out through an adjustable needle. It protects the cylinder and the machine from impact at the end of each stroke, and it is set once on the machine with a screwdriver.

FMY, PMB and FMC cylinders have a magnetic piston. Position sensors mounted on the outside of the barrel detect the magnet, so the control system knows when the cylinder has reached its end positions without mechanical limit switches.

Because ISO 15552 fixes these interface dimensions, an FMY cylinder fits the same mountings, rod ends and space as other ISO 15552 cylinders of the same bore — useful when replacing a cylinder on an existing machine. Mountings (rear clevis, flange, foot, trunnion, rod clevis and spherical rod end) are available from Ø32 to Ø320 mm.

Cylinder speed is set with flow control valves on the exhaust side (meter-out), which gives smoother motion than restricting the supply. The achievable speed depends on the valve and hose size, the load and the pressure, so it is best confirmed on the machine. For very slow, even motion, a pneumatic–hydraulic speed-adjusting tank is available as a special accessory.

Choose double acting when the return stroke has to do work, when the stroke is long or when the speed must be controlled in both directions. A single acting (spring return) cylinder is simpler and uses about half the air, but only for short strokes and light return loads.