Whether you're building a workshop press, a log splitter, or perhaps designing a new loader arm, selecting a hydraulic cylinder by eye or relying solely on its appearance is the quickest path to disaster.
The actuator may be too weak to lift the load, or worse, its thin piston rod may bend into an arc at the first maximum stroke.
For the machine to operate reliably, you need to combine the pressure generated by the pump with the surface area of the piston. BRhydraulic's team of experts has prepared an easy-to-follow guide that shows you how to calculate the actual force of any cylinder in just 3 minutes.
1. Basic mistake: A piston is not the same as a piston rod!
Before we start counting, we need to clear up the biggest misconception in plumbing:
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Cylinder (outer tube): This is the outer casing. Its diameter depends on the wall thickness and is not included in the calculation.
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Piston (inner diameter): This is the part that sits inside the pipe and is pressed against by the oil. It's the piston's inner diameter that generates the force!
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Piston rod: This is the chrome-plated rod that extends from the cylinder. Its thickness determines its resistance to bending, not its pushing force.
When purchasing a cylinder, you'll typically see something like an 80/40 cylinder . This means the internal piston is 80mm in diameter and the chrome rod is 40mm.
2. How to calculate the actuator's PUSH force? (Extension)
When oil is forced to the rear of the cylinder, the machine does the heaviest work (e.g. pressing a press or pushing a load).
To calculate the extension force in kilograms (kg), we use a simple formula based on the working system pressure (in bar) and the piston surface area (in cm²).
Simplified step by step:
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Convert the piston diameter from millimeters to centimeters (e.g. an 80 mm piston is 8 cm).
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Calculate the surface area of the piston using the formula for the area of a circle:
Pole = 3.14 * Promień * Promień. (The radius is half the diameter, so for an 8 cm piston, the radius is 4 cm).-
Pole = 3.14 * 4 * 4 = 50.24 cm².
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Multiply the surface area by the system pressure (e.g. the pump gives 160 bar) and by a factor of 1.02 to get the result in kilograms.
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Pushing force =
50.24 cm² * 160 bar * 1.02= approx. 8200 kg (i.e. 8.2 tons) .
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Conclusion: A cylinder with an 80 mm piston at a pressure of 160 bar will push a load weighing over 8 tons.
3. Why is the PULL (return) force always smaller?
A double-acting cylinder has both a pushing and a pulling force. However, remember that the cylinder always pulls less than it pushes!
Why does this happen? When you apply oil to the front of the cylinder to retract it, the oil pushes against the piston, but it's hindered by a chrome rod (piston rod), which takes up valuable surface area inside the cylinder. To calculate the retraction force, you subtract the rod's area from the main piston's area and then multiply that result by the pressure. Typically, the pulling force is 20% to 40% less than the pushing force.
4. Buckling – when does an actuator fold in half?
You've calculated that you need an 80 mm cylinder for a press that applies 8 tons of force. The stroke (extension) must be a full 1 meter. If you buy a cylinder with a rod (piston rod) just 30 mm thick, at full extension and under load, 8 tons of force will bend it like a matchstick! This phenomenon is called buckling .
The golden rule: The longer the stroke and the greater the weight, the thicker the rod (piston rod) must be. For pull cylinders (e.g., Tur) and long splitters, look for cylinders with a reinforced rod (e.g., an 80/50 cylinder instead of an 80/40).
Summary
Selecting a cylinder is pure physics. Knowing your pump's pressure and required force, you can calculate the required piston diameter in just a few minutes.
Ready for your project? At BRhydraulic, you'll find hundreds of high-quality single- and double-acting hydraulic cylinder options, including eyebolts, swivel bearings, and threaded options.
👉 Go to category Hydraulic Cylinders and choose a cylinder with a guaranteed durability for your machine!
