A pneumatic cylinder moves too fast on extension and slams into its end cap, or creeps on retraction. The operator turns a flow control on the supply, and one direction improves while the other gets worse. Air speed control is not just putting a restriction somewhere in the line. Where the restriction goes decides whether the cylinder is actually controlled or only the supply pressure is throttled.
Why air is hard to control
Compressed air is compressible. Unlike hydraulic oil, it does not transmit flow rigidly; it springs. A cylinder can accelerate unevenly because the air volume behind it compresses and expands, and the load and friction vary. This is why simple needle valves on the supply often produce jerky motion rather than smooth speed. The control must regulate air leaving the chamber so the cylinder does not run away.
The two basic methods are meter in, throttling air entering the cylinder, and meter out, throttling air leaving. The choice controls whether the cylinder moves smoothly under real loads.
Meter out is the default for most cylinders
Meter out places the flow control on the exhaust side, so the air leaving the cylinder is restricted. The incoming air fills the chamber freely, while the exhaust backs up and creates a controlled resistance. The piston cannot run away because the air leaving governs how fast it moves. This is the reliable arrangement for most double-acting cylinders, especially with driven or over-running loads, and it produces steadier motion.
Meter out works because it controls the air that is actually pushing the load. The cylinder moves at the rate at which it can expel air, which resists acceleration and runaway.
Meter in suits specific cases
Meter in throttles the supply air entering the cylinder while exhaust leaves freely. It is used where the load is always retracting or pushing against gravity, so uncontrolled extension would overrun, and where the cylinder extends under low load. Meter in also works for single-acting spring-return cylinders, where the exhaust side is simply a spring. For general double-acting work, meter in is the source of jerky movement, because the compressible supply air has no back pressure to oppose runaway on the return.
Install flow controls at the cylinder
Mount the flow control directly at the cylinder port, not at the valve or far up the line. A long tube between the valve and cylinder adds volume and delays response; throttling there makes the cylinder jump as air fills the line. Regulated speed depends on the control acting on the chamber itself. Install both exhaust flow controls, one per port, so extension and retraction are adjusted independently.
Check the fitting: a flow control that only restricts in one direction, with free reverse flow, behaves differently on extension and retraction. Use the correct adjustment direction for the side being controlled.
Single-acting cylinders
A single-acting cylinder pushes on air and returns by spring or gravity. Speed control on the supply, meter in, governs extension; the return is governed by the spring or load. Trying to meter exhaust on a spring return adds little, because the spring controls return. Size the spring for the return load and control extension. If the spring is too weak, return is slow regardless of flow control.
Over-running and falling loads
A vertical or driven load can pull the piston faster than the air supply fills the chamber, causing vacuum, choking, and end-cap banging. Meter out on the driven side resists this, but for heavy vertical loads a flow control or check valve may not be enough; add a pilot-operated check or a flow control that locks the load. The rule is to control the air leaving the side that would otherwise runaway.
Cushions at the end of stroke
Even with good speed control, a fast cylinder slams at the end. Built-in end cushions trap a small amount of air near the end of travel and throttle it, decelerating the piston before it reaches the cap. Adjust them with the screw on the end cap. Cushions are not a substitute for overall speed control; they handle the last few millimeters. If banging persists after setting the cushions, slow the whole cycle rather than tightening the cushion too far, which stalls the piston.
Regulator pressure and flow
Speed is not pressure. Raising supply pressure changes force and, indirectly, speed through better control, but the flow control sets speed. A regulator set low can starve the cylinder and make it sluggish; a flow control set nearly closed can overheat the valve. Set pressure for the force the load needs, then adjust flow controls for the speed. Don’t use pressure to tune speed unless the application genuinely needs less force.
A worked comparison
A horizontal clamp extends against a light part and retracts empty. Metering the supply on extension lets the empty return accelerate freely because the exhaust is unrestricted, so it slams. Metering the exhaust on both ports instead: extension fills the cap end while rod-end exhaust is throttled, and retraction fills the rod end while cap-end exhaust is throttled. Both directions now move at the rate air can leave, giving a controlled clamp and a controlled return. This is the usual fix when one direction slams after the operator adjusted only the supply side.
Effect of load and friction
Air cylinders are not stiff positioners. Friction in the seals and guides causes stick-slip at low speeds: the piston sticks, then jumps, rather than creeping smoothly. At very low speeds, increase lubrication, reduce side load, or use a hydraulic damper; air alone cannot hold a slow, steady feed. If the application needs precise low speed, a pneumatic cylinder with flow control is the wrong actuator, and a hydraulic or electric option is more reliable.
Line size and valve capacity
Even correct flow controls cannot deliver speed the valve and tubing cannot supply. An undersized valve or long small-bore tube chokes the system regardless of the adjustment. Size the valve and lines for the required flow, and keep tubing short between valve and cylinder. If the cylinder cannot reach the desired speed even with the flow control nearly open, the supply path is the limit, not the adjustment.
Shock absorbers and external stops
Where the cylinder must move fast and stop accurately, built-in cushions may not absorb the energy. External shock absorbers at the end of travel decelerate the load, while hard stops take the force. Set cushions for gentle approach and shock absorbers for the actual stop. Don’t rely on cushions to stop a high-inertia load; they are for light deceleration, not energy absorption.
Lubrication and dry air
Modern cylinders with pre-lubricated seals run on dry air, but unlubricated systems can still suffer seal drag that changes speed over time. Water in the line rusts internals and alters friction. Keep the air dry and clean; a cylinder that ran smoothly and now hesitates often has contaminated or dried seals rather than a misadjusted flow control.
Common mistakes
Metering supply instead of exhaust, mounting flow controls at the valve, using pressure to tune speed, expecting cushions to stop heavy loads, and ignoring line size are the recurring errors. Treat each port as independently controlled and remember that air is compressible: the goal is to regulate the exhaust so the piston cannot runaway. With that arrangement, most speed problems disappear.
Setting speed on a real machine
Start with both flow controls nearly open, then turn them in while cycling the cylinder under the real load. Adjust one direction at a time and let the cycle repeat several times, because air lines and friction take a few strokes to settle. Note the setting on the adjustment screw and the resulting speed, so later changes can be reproduced. Test the slowest and fastest expected loads; a cylinder tuned empty may be too slow or too fast when loaded. If the motion is smooth but the end still bangs, adjust the end cushion rather than closing the flow control further, which would only slow the whole stroke. Document the final settings with the machine so maintenance can restore them after a valve or cylinder change.
Recording these settings also prevents the common habit of re-adjusting a cylinder after a routine service, when the real cause was dirty air or a worn seal. The adjustment is the last thing to check, not the first.
Recording these settings also prevents the common habit of re-adjusting a cylinder after a routine service, when the real cause was dirty air, a worn seal, or a leaking fitting. The adjustment is the last thing to check, not the first.
Bottom line
Use meter-out flow controls mounted at each cylinder port for smooth double-acting motion, and reserve meter-in for single-acting or specific loaded cases. Control the air leaving the chamber that would otherwise runaway, adjust end cushions for the last stroke, and set pressure for force rather than speed. Jerky or slamming cylinders usually come from throttling the supply instead of the exhaust. Set the flow controls at the cylinder, and the air behaves predictably despite being compressible.
