Single-Rod Differential Cylinder Selection
A single-rod differential cylinder has different effective areas on the piston and rod sides. This produces unequal forces and speeds in the two directions of travel that you need to understand. This guide shows how to size the area ratio, force, speed, buckling length and seals cleanly.
View hydraulic cylindersWhat does the area ratio phi mean?
In a differential cylinder the full piston area acts on the cap side, while on the rod side the rod cross-section is subtracted. The ratio of these two areas is the area ratio phi, and it decides how force and speed differ between extending and retracting. A typical value is 2:1, where the extend force is twice the retract force while the retract speed is twice as high.
How do I calculate force and speed?
Force equals pressure times effective area, and speed equals flow rate divided by the same area. Because the rod side has a smaller area, it produces less force at the same pressure but more speed at the same flow rate. Work out both directions separately, or the weaker retract force is easily overlooked.
- Extend force: pressure times full piston area (area from bore diameter).
- Retract force: pressure times annulus area (piston area minus rod area).
- Speed: flow rate divided by the relevant effective area.
- Pick the rod diameter so the retract force still covers your load.
How do I secure buckling load and guidance?
With long strokes under compression the rod can buckle before the plain material strength is reached. The free buckling length, the rod diameter and the mounting arrangement are decisive. Check the permissible buckling load by Euler with a sound safety margin and avoid side loads through clean alignment and suitable mounting.
Free length and mounting case set the allowable compressive force by Euler.
Compare cylindersWhich seals and oils fit?
Seal material and oil decide tightness, friction and service life. For standard hydraulics with mineral oil, NBR seals are common; at higher temperatures or with synthetic media, FKM or polyurethane are used. Keep the oil clean by filtration and choose the right viscosity so guidance and seals last.
Frequently asked questions
Why is the retract force smaller than the extend force?
On the rod side the rod cross-section is subtracted from the piston area, so only the annulus area stays effective. At the same pressure this gives less force but more speed at the same flow rate.
What does an area ratio of 2:1 tell me?
The piston area is twice the annulus area. Extending delivers double the force at half the speed, and retracting is the reverse. The ratio follows from the rod diameter you select.
How do I prevent the rod from buckling?
Choose a sufficient rod diameter for the free buckling length, check the allowable compressive force by Euler and avoid side loads through clean alignment and a suitable mounting arrangement.
Find the right differential cylinder
Our hydraulics specialists help with area ratio, rod diameter and seals for your application.
Right ratio
Area ratio matched to force and speed.
Buckling-safe design
Rod diameter and buckling length clearly explained.
Vetted selection
Recommendations from maker data and field values.
Expert advice
Hydraulics specialists support your sizing.