Pneumatic Cylinder Sizing: A Practical Guide for Plant Engineers
Bore size, stroke length and force calculation basics for pneumatic actuator selection.
A pneumatic cylinder that is too small stalls under load; one that is too large wastes air and moves too fast. Sizing starts from the force you need and the air pressure you have.
Step 1: Work out the force needed
Calculate the load the cylinder must move or hold. For a vertical lift, this includes the weight. For a horizontal push, include friction. Add extra for acceleration and for any process force, such as clamping or pressing.
Step 2: Know your working pressure
Use the pressure that will actually be available at the cylinder, which is lower than the compressor setting once the pipe and valve losses are taken off. Many plants size for around 6 bar supply, but use your own measured figure.
Step 3: Calculate the bore
Force equals pressure times piston area. For a piston of diameter D, the theoretical push force is pressure x π x D² / 4. The pull (retract) force is lower, because the rod takes up part of the piston area.
Example: at 6 bar (0.6 N/mm²), a 50 mm piston has an area of about 1,963 mm², giving a theoretical push force of about 1,178 N.
Step 4: Add a safety margin
Real force is lower than theoretical because of seal friction. A common practice is to choose a bore giving a theoretical force of at least 25 to 50% more than the load, and more for fast moving or high-friction loads. Choose the next standard bore up.
Step 5: Stroke, speed and cushioning
Set the stroke from the movement needed. Check the speed: it depends on the valve flow and the pipe size as well as the cylinder. For heavy or fast loads, specify end-of-stroke cushioning or an external shock absorber.
Other points
Check the rod for buckling on long strokes with a push load, and avoid side loads, which wear the seals and bearings. Use a guided cylinder or external guide if side load cannot be avoided.
Key Takeaways
- Force = pressure x piston area; use the pressure at the cylinder.
- Add a margin of 25 to 50% or more for friction and acceleration.
- Check the speed, cushioning and rod buckling.
This article is general guidance. For a recommendation for your own application, contact our engineering team. Pneumatic cylinders and valves →