How to Calculate Pneumatic Cylinder Bore Size
Choosing the correct pneumatic cylinder bore size is essential for ensuring your machinery operates safely, reliably and efficiently.
If the cylinder bore is too small, it may not generate enough force to move the load. If it is unnecessarily large, it can consume more compressed air than required and increase operating costs.
The correct bore size depends on several factors, including the force required, operating air pressure, cylinder type, load characteristics and application.
In this guide, we explain how to calculate pneumatic cylinder bore size, the formulas involved and what to consider when selecting a cylinder.
What Is Pneumatic Cylinder Bore Size?
The bore size of a pneumatic cylinder is the internal diameter of its cylinder barrel. It determines the surface area of the piston that compressed air acts against.
Generally, a larger bore provides a greater piston surface area and therefore produces more force at the same operating pressure.
Common industrial pneumatic cylinder bore sizes include: 32mm, 40mm, 50mm, 63mm, 80mm, 100mm, 125mm.
The appropriate size depends on the force and movement required by the application.
What Information Do You Need Before Calculating Bore Size?
Before calculating the required cylinder bore, establish the following:
1. Required force
Determine how much force the cylinder needs to produce, usually measured in Newtons (N).
2. Operating air pressure
Establish the pressure available at the cylinder, measured in bar. This may be lower than the compressor’s stated delivery pressure due to losses through pipework, filters, regulators and valves.
3. Cylinder type
Determine whether you need a single-acting or double-acting cylinder. For a double-acting cylinder, consider both extension and retraction forces.
4. Stroke length
The stroke is the distance the piston rod needs to travel. Although stroke length does not directly determine the theoretical force, it is important when selecting the correct cylinder.
5. Application and load conditions
Consider the direction of movement, friction, acceleration, external forces and any additional resistance the cylinder must overcome.
Pneumatic Cylinder Bore Size Formula
To calculate the bore size of a pneumatic cylinder, you need to know two things:
- The force required: How much force the cylinder needs to produce, measured in Newtons (N).
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The operating air pressure: The pressure available to the cylinder, measured in bar.
The formula is: Bore size (mm) = √ [(40 × Force (N)) ÷ (3.142 × Pressure (bar))]
Where:
- Force is measured in Newtons (N).
- Pressure is measured in bar.
- 3.142 is the value of pi.
-
The result is the required bore diameter in millimetres.
Example calculation
Suppose you need a pneumatic cylinder to produce 500N of force at an operating pressure of 6 bar.
Put these values into the formula:
Bore size = √ [(40 × 500) ÷ (3.142 × 6)]
Bore size = √ (2,000 ÷ 18.852)
Bore size = √ 1,061
Bore size = 32.6mm
The calculation gives a minimum theoretical bore diameter of approximately 32.6mm.
You would normally select the next suitable standard cylinder size. In this example, that would be a 40mm bore cylinder.
Pneumatic Cylinder Extension Force Formula
Extension force is the pushing force produced when the piston moves out of the cylinder.
To calculate the extension force, you need to know the cylinder bore size and the operating air pressure.
Extension force (N) = Pressure (bar) × Bore area (mm²) ÷ 10
The bore area is calculated using:
Bore area (mm²) = 3.142 × Bore diameter² ÷ 4
Example calculation
Suppose you have a pneumatic cylinder with:
- Bore diameter: 40mm
-
Operating pressure: 6 bar
First, calculate the bore area:
Bore area = (3.142 × 40 × 40) ÷ 4
Bore area = 1,256.8mm²
Next, calculate the extension force:
Extension force = (6 × 1,256.8) ÷ 10
Extension force = 754N
The cylinder produces a theoretical extension force of approximately 754N at 6 bar.
What Factors Affect Pneumatic Cylinder Force?
The formula provides a theoretical force, but several factors can affect the actual force available.
Operating Pressure
The pressure available at the cylinder directly affects the force produced. Always use the expected operating pressure at the cylinder rather than relying solely on the compressor pressure setting.
Piston Rod Diameter
The piston rod reduces the effective piston area during retraction. A larger rod diameter results in a smaller annular area and lower retraction force.
Friction and Mechanical Resistance
Internal seals, guides and external mechanical resistance can reduce the force available to move the load.
Stroke Length and Buckling
Longer strokes can introduce additional considerations, particularly where the piston rod operates under compression. Rod diameter, unsupported length, mounting arrangement and load alignment should be checked to reduce the risk of buckling.
Safety and Design Margin
A cylinder should not normally be selected solely because its theoretical force exactly matches the calculated load.
Allow an appropriate design margin for friction, variations in operating pressure, acceleration and changes in load. The margin required will depend on the application and should be assessed as part of the design.
Choosing the Right Pneumatic Cylinder
Once you have calculated the required bore size, there are other important specifications to consider when selecting a pneumatic cylinder.
These include:
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Bore diameter: Determines the piston area and theoretical force.
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Stroke length: Determines how far the piston rod needs to travel.
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Operating pressure: Must be compatible with your compressed air supply.
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Mounting arrangement: Must suit the machine and direction of movement.
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Piston rod diameter: Affects retraction force and resistance to buckling.
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Cushioning: Helps control piston movement and reduce impact at the end of the stroke.
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Seals and materials: Must be suitable for the operating environment and temperature.
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Magnetic piston: May be required where cylinder position sensing is needed.
For industrial applications, ISO 15552 cylinders are a common choice, offering standardised dimensions and mounting arrangements.
Need Help Selecting or Manufacturing a Pneumatic Cylinder?
Air System Controls specialises in pneumatic cylinder supply, manufacture and repair, helping businesses find suitable cylinder solutions for their machinery and equipment.
Our in-house cylinder manufacturing service includes ISO 15552 pneumatic cylinders with bore sizes from 32mm to 125mm and custom strokes up to 2,000mm.
We can also offer a range of mounting options and cylinder specifications to suit individual requirements.
If you are replacing an existing cylinder, experiencing a breakdown or specifying a new application, our team can help identify the right solution.
Telephone: 01952 290959
Email: sales@airsystemcontrols.co.uk