BIZOMAT MAKEOS · Pneumatics
Pneumatic Cylinder Force and Bore Selection
What force does this air cylinder give, and is the bore large enough for the load?
Extend and retract forces of a double-acting pneumatic cylinder, the load ratio against a recommended limit, and the smallest bore that meets it.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Bore | mm | Cylinder bore. |
| Rod diameter | mm | Piston rod diameter. |
| Supply pressure (gauge) | bar | Minimum supply pressure at the valve. |
| Efficiency (seal friction) | % | Allowance for seal friction (about 85–95%). |
| Load force to move | N | Force needed to move the load (friction, weight, process force). |
| Maximum load ratio | % | Highest share of theoretical force to use; makers often recommend about 50–70% for dynamic motion and higher for slow clamping. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Bore
- What it is
- The inside diameter of the cylinder barrel, which equals the piston diameter.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Force rises with the square of the bore.
- Get it right
- Use the bore, not the outside diameter of the barrel.
What this means: Rod diameter
- What it is
- The diameter of the piston rod, which reduces the area the air acts on when the cylinder retracts.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Affects only the retract force; the extend force uses the full bore.
What this means: Supply pressure (gauge)
- What it is
- The lowest gauge pressure available at the valve feeding the cylinder while it is working.
- Units and format
- Enter a number in bar: bar (1 bar = 100 kPa). The form accepts 0.000001 bar to 50 bar. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 bar as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Extend and retract forces rise in direct proportion to pressure.
- Get it right
- Use the minimum pressure at the valve, not the compressor setting; line losses reduce it.
What this means: Efficiency (seal friction)
- What it is
- The share of the theoretical force that remains after seal and guide friction inside the cylinder.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The extend and retract forces reported rise in direct proportion.
- Get it right
- Type the percentage; take it from the cylinder maker for the seal type.
What this means: Load force to move
- What it is
- The force needed to move the load, including friction, weight lifted and any process force.
- Units and format
- Enter a number in N: newtons. The form accepts 0 N to 10000000 N. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 600 N as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger load raises the load ratio and the minimum bore.
- Get it right
- Enter newtons; for a vertical lift include the weight, which is the mass times g.
What this means: Maximum load ratio
- What it is
- The highest share of the theoretical extend force you are willing to use for the load, from the cylinder maker’s guidance for the duty.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 10% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 70% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A lower limit requires a larger minimum bore.
- Get it right
- Type the percentage. Lower ratios give faster, smoother motion; check the maker’s advice for the duty.
Method and formulas
- Extend F = p × π D² ÷ 4 × η; retract F = p × π(D² − d²) ÷ 4 × η (p in N/mm² = bar ÷ 10).
- Load ratio = load ÷ theoretical extend force.
- Minimum bore = √(4 × load ÷ (π p × ratio)).
- Example: a 50 mm bore at 6 bar pushes 1,060.3 N; a 600 N load needs at least 42.649 mm bore.
Limitations
- Static force; acceleration, cushioning and air supply flow are not included.
- Side loads on the rod need guides.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Select a cylinder bore for a load
- Check an existing cylinder at a lower pressure
- Compare push and pull forces
Who it is for
- Automation engineers
- Maintenance engineers
- Machine builders
What you get
- Extend and retract forces
- Load ratio
- Smallest bore meeting the load ratio
Interpreting the result
- Choose the next standard bore (32, 40, 50, 63, 80, 100 mm…) above the minimum.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3032–3041. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Pneumatics by the Numbers
What it covers: Size a pneumatic cylinder, time and cushion its stroke, choose valves and a safe fail state, count air and leak costs, supply clean air and commission and maintain the system.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
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