1. Start from the actual required force
A = F / pStart from the useful machine force, not from cylinder bore. Divide that force by available pressure, then add a margin for friction, pressure loss, acceleration and load variation.
The theoretical bore follows from D = √(4A/π). Final selection is then made from a manufacturer series.
2. Distinguish extension and retraction
During extension, pressure acts on the full piston area. During retraction, the rod removes part of the effective area, so retraction force is lower for a single-rod cylinder.
Back pressure in the opposite chamber further reduces net force.
3. Relate flow rate and speed
v = Q / AFor the same flow rate, a larger area moves more slowly. Because annular retraction area is smaller, retraction is usually faster.
Actual cylinder flow can be below nominal pump flow because of leakage, valves and flow controls.
4. Check stroke and travel time
V = A · L ; t = V / QRequired oil volume depends on active area and stroke. This volume gives the minimum travel time for a given flow rate.
Actual time can be longer because of acceleration ramps, end cushioning and losses.
5. Essential checks before final selection
A force calculation alone is not enough to declare a cylinder suitable.
- Rod buckling in compression.
- Side loads, guidance and misalignment.
- Maximum pressure, pressure spikes and back pressure.
- Permissible speed and end cushioning.
- Mounting, stroke, environment and cycle frequency.
Numerical application
Worked example: 20 kN at 100 bar
A machine must push with 20 kN. Estimated useful cylinder pressure is 100 bar. Losses are initially neglected to obtain a theoretical bore.
- Convert: F = 20,000 N and p = 100 bar = 10,000,000 Pa.
- Calculate A = F/p = 0.002 m².
- Calculate D = √(4A/π) = 0.0505 m.
- The theoretical bore is therefore about 50.5 mm.
- Then select the next suitable catalogue size and recalculate net force with efficiency and back pressure.
Result: A theoretical 50.5 mm bore is the ideal minimum. In practice, selection must be larger and validated against manufacturer data and safety requirements.
Example built from the basic relations and framed by the general requirements of ISO 4413. Open exact source ↗
For checking and further study
Direct references
Each link points to the exact course, standard or publication page used, rather than a generic homepage.
- ISO 4413:2010 — Hydraulic fluid powerInternational Organization for StandardizationGeneral rules and safety requirements for hydraulic systems and components.↗
- The International System of Units, 9th editionInternational Bureau of Weights and Measures (BIPM)Official reference for coherent units, symbols and conversions.↗
- NIST Special Publication 811 — Guide to the SINational Institute of Standards and TechnologyPractical rules for writing and converting physical quantities correctly.↗