Shock Absorber Selection for Pneumatic Cylinder End Stops
How to review cylinder-driven stops where thrust force continues during absorber compression.
Direct answer
For pneumatic cylinder end stops, calculate the moving mass energy and add the work done by cylinder thrust over the absorber stroke. The absorber must handle both the impact energy and the continuing drive force while staying within hourly duty and force limits.
Questions this page answers
- How do I select a shock absorber for a pneumatic cylinder?
- Should cylinder thrust force be included in shock absorber sizing?
- Why does a cylinder end stop need more than mass and velocity?
Required inputs
Formula logic
Cylinder thrust work
F is cylinder thrust and s is absorber stroke in meters. Add this when pressure continues pushing during the stop.
W = F x sUnit: N m
Review steps
- 1
Recognize the problem and its limits
A pneumatic axis hits its end stop or rebounds. Residual air pressure can continue doing work while the absorber compresses. Confirm whether the cylinder is extending or retracting and whether pressure is vented. Do not apply a free-motion calculation to a still-driven cylinder.
- 2
Collect the operating data
Weigh the complete moving assembly. Obtain speed at first contact from a trace or a safely recorded movement, rather than using the machine’s nominal speed. Count impacts at this individual stop during peak production. Measure usable travel and record whether a belt, queued pallets or an actuator keep pushing. Mark estimates and unknowns; send photos or a drawing when measurements are unavailable.
- 3
Worked engineering example — not a customer case
Assume a horizontal 50 kg load contacts one absorber at 1 m/s. A measured constant net thrust of 500 N continues over 25 mm (0.025 m), at 30 impacts/min. Kinetic energy = 25 J; drive work = 500 × 0.025 = 12.5 J; total = 37.5 J/impact; hourly energy = 37.5 × 1,800 = 67,500 J/h. Thrust is an assumed net force here, not a value inferred from bore alone.
- 4
Why one model is excluded
At 25 mm stopping travel, 37.5 J exceeds EK20x12’s 31 J rating, and the model has only 12 mm stroke. Even recalculating drive work over 12 mm gives (25 + 500 × 0.012) × 1,800 = 55,800 J/h, above its 35,200 J/h rating. It is excluded in either check.
- 5
What may enter the shortlist
EK25x25 offers the example’s 25 mm travel and catalog ratings of 81 J/cycle and 77,000 J/h. It clears those energy checks only. Confirm permissible continuous thrust, velocity, peak force and adjustment before selection; an energy margin does not prove that the cylinder force is acceptable.
- 6
Validate before ordering
Confirm effective mass and velocity range, peak transmitted force, allowable structural load, adjustment, full return before the next impact, alignment and thermal conditions. Record initial and stabilized temperature, cycle rate, stroke use and observed rebound during a controlled trial. Define acceptance limits with the machine designer; missing model limits require engineering confirmation.
Common mistakes
- Sizing the absorber as if the moving load were free motion while the cylinder is still pushing.
- Ignoring pressure variation between commissioning and production settings.
- Mounting the absorber off-axis and creating side load on the piston rod.
Technical notes
- Illustrative calculation; EK20x12 and EK25x25 ratings checked against published EKD model records on 4 October 2026. J and Nm are equivalent energy units. Catalog ratings do not establish service life or application suitability.
Move from answer to model shortlist.
Use the sizing tool when you have the inputs, or send the application data for engineering review.