Plunger and Ram Cylinder Selection
A plunger cylinder (ram cylinder) is single-acting: oil pushes the rod out and the return comes from load or a spring. The design is robust and low-cost because only one rod seal is needed and no piston runs inside the tube. This guide shows when a plunger pays off and how to size pressure, stroke and return correctly.
View hydraulic cylindersHow does a plunger or ram cylinder work?
A plunger cylinder has no piston in the classic sense: the solid rod (the plunger) dips into the tube itself and seals only at the guide. Pressurised oil pushes the rod out, so force is created only on extension. The return needs an external force - the load's own weight, a return spring or a second cylinder. That makes the design simple, high-pressure capable and low-maintenance.
How do I size pressure, force and rod diameter?
Extension force equals working pressure times rod cross-section. As the plunger uses the full tube bore, it delivers high force for a given diameter. Check the required force first, choose the pump pressure and derive the rod diameter from it. For long strokes, also check buckling by Euler's formula.
- Apply the required force with a safety factor of 1.5 to 2.
- For strokes over 10 times the diameter, check buckling load by Euler.
- Verify rod guide and length against side loads and deflection.
- Choose the oil grade to suit temperature, usually ISO VG 32 to 68.
How do I secure return and sealing?
Since the plunger only extends, the return must work reliably. In vertical mounting the load pulls it back; horizontal mounting needs a return spring or an opposing cylinder. The rod seal and wiper decide leakage and service life: a good wiper keeps dirt out of the oil and protects the guide.
Keep cleanliness to ISO 4406, otherwise the rod seal wears out fast.
View cylinder guideFrequently asked questions
What is the difference between a plunger and a differential cylinder?
The plunger cylinder is single-acting and creates force only on extension, with the return coming from load or a spring. The differential cylinder is double-acting and moves in both directions under oil pressure. The plunger has just one sealing point and is especially pressure resistant.
Why does my ram cylinder not retract on its own?
A plunger has no return force from the oil. Without enough load or a return spring the rod stays extended. In horizontal mounting an external return force is always required.
How high can the working pressure be on a plunger cylinder?
Plunger cylinders suit high pressures well, commonly 160 to 700 bar depending on the build. Tube, rod guide and seal are decisive. Always stay within the maximum pressure stated by the manufacturer.
Find the right plunger cylinder
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Return, sealing and buckling clearly explained.
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