The Cylinder That Hammered at Full Stroke
We mounted a pneumatic cylinder (Ø50 mm, 200 mm stroke) for a push station. On the bench, it extended and retracted fine. On the machine, the piston hit the end cap hard every cycle — a loud “clang” at full extension. After a week, the end cap seals leaked. The problem: the cylinder had no cushioning adjustment, and the flow control was wide open. The piston reached full speed and slammed into the end cap. We adjusted the end-of-stroke cushioning screws (the needle valves at the cylinder ends). The piston decelerated in the last 10 mm. The slamming stopped. The mistake was ignoring the cylinder’s internal cushioning adjustment.
Pneumatic cylinder cushioning and speed control isn’t just opening the valve. The cylinder needs to decelerate at the ends to avoid hammering. This article covers the adjustment.
Why Cylinders Slam
A pneumatic cylinder accelerates through the stroke. At the end (near the cap), it’s at full speed. Without cushioning, the piston hits the end cap at full velocity. The impact:
- Is noisy (hammering).
- Wears the end cap and seals.
- Can move the load (the part shifts).
- Shortens the cylinder life.
Most pneumatic cylinders have internal cushioning (needle valves at both ends). It’s adjustable. If it’s not set, the cylinder slams.
Internal Cushioning (End-of-Stroke)
A standard double-acting cylinder has adjustable pneumatic cushioning at both ends. Near the end of the stroke, a bushing on the piston enters the exhaust port. The exhaust air is forced through a small gap (controlled by a needle screw). The air is compressed as a pneumatic spring, decelerating the piston.
How to Adjust
- Run the cylinder at operating pressure.
- Turn the cushioning screw (a small slot at the end of the cylinder) fully in (closed).
- Cycle the cylinder. It will slow too much (or stop before the end).
- Back the screw out 1/4 turn at a time. The piston should reach the end but decelerate smoothly (no clang).
- Set it so the piston arrives softly at both ends.
If the cushioning screw is fully open (backed out), there’s no cushioning — the piston slams. If it’s fully closed, the piston doesn’t reach the end. Find the middle.
Flow Control Valves (Speed Control)
The cylinder’s speed is set by flow control valves on the exhaust ports (or the valve’s exhaust). Two types:
Exhaust Flow Control (Meter-Out)
The flow control valve restricts the air exhausting from the cylinder. This controls the speed (the incoming air is free; the outgoing air is metered). This is the standard method. It gives stable speed under varying loads.
Supply Flow Control (Meter-In)
Restricts the incoming air. This is less stable (the cylinder can stall or jump under load). Not recommended for most applications.
The cylinder adjustment rule: Use exhaust flow control (meter-out) on both ports. Adjust the needle valve to set the speed. Then adjust the internal cushioning screws so the piston decelerates softly at the ends. The cylinder that hammered had the cushioning screws wide open (no adjustment). Set them.
Speed Control: How Fast Is Too Fast?
Pneumatic cylinders can move fast (1 m/s or more). But fast motion:
- Slams at the ends (even with cushioning).
- Causes the load to jerk (parts shift).
- Increases wear.
Rule of thumb: keep the piston speed under 0.5 m/s for most applications. For a 200 mm stroke, that’s 0.4 seconds per stroke. If you need faster, use a bigger valve (article 67) and add external shock absorbers (see below).
External Shock Absorbers (For Heavy/Fast Loads)
When the internal cushioning isn’t enough (heavy load, high speed), add external hydraulic shock absorbers.
The shock absorber is mounted at the end of travel. The cylinder (or the load) hits the shock absorber, which decelerates it hydraulically. The internal cushioning handles light loads; external shock absorbers handle heavy or fast loads.
For a 50 kg load moving at 1 m/s, the kinetic energy (0.5 × 50 × 1² = 25 J) exceeds the internal cushioning (about 2–5 J). Add an external shock absorber rated for 25 J.
| Load × Speed | Kinetic Energy | Cushioning Needed |
|---|---|---|
| 5 kg at 0.3 m/s | 0.2 J | Internal (factory setting) |
| 20 kg at 0.5 m/s | 2.5 J | Internal (adjust cushioning) |
| 50 kg at 1.0 m/s | 25 J | External shock absorber |
| 100 kg at 1.5 m/s | 112 J | Two external shock absorbers |
Pneumatic vs. Hydraulic (For Fast/Heavy)
Pneumatic cylinders are fast but hard to control precisely. For slow, controlled motion (pressing, feeding), a hydraulic cylinder (or a servo pneumatic) gives better speed regulation.
For most automation (gripping, pushing, lifting), pneumatic is fine. For a precision press or a controlled feed, use hydraulic or servo.
Mounting Side Load: Don’t Bend the Rod
When the cylinder extends, the rod must not take a side load (a force perpendicular to the rod). Side load bends the rod and wears the front bushing.
- Use a guide rod (or linear bearing) alongside the cylinder. The cylinder only provides the force; the guide takes the side load.
- Don’t mount the cylinder so the load hangs off the rod (cantilevered). Support the load on a linear guide.
A Cylinder Adjustment Checklist
- Is the cylinder double-acting? (Internal cushioning at both ends?)
- Are exhaust flow control valves installed? (Meter-out.)
- What is the piston speed? (Under 0.5 m/s?)
- Is the piston slamming at the ends? (Adjust cushioning screws.)
- What is the load weight? (kg)
- What is the end speed? (m/s)
- Kinetic energy: E = 0.5 m v². Is it within internal cushioning?
- If not: external shock absorbers? (Size for E.)
- Is there a side load on the rod? (Add a guide.)
- Is the cylinder mounted so the rod doesn’t bind? (Alignment.)
- Are the flow control valves accessible? (For adjustment.)
- Is the cycle time consistent? (No drift in speed?)
The Bottom Line
Pneumatic cylinder cushioning and speed control is two adjustments: flow control (speed) and cushioning screws (end deceleration). The cylinder that hammered at full stroke had the cushioning screws wide open. Adjust them so the piston decelerates softly at both ends. Use exhaust flow control (meter-out) for stable speed. For heavy/fast loads, add external shock absorbers. The cylinder that runs quietly and lasts wasn’t the biggest one — it was adjusted right.
