Why pneumatic cylinders cannot stop accurately
Why a compressible fluid makes mid-stroke positioning unreliable.
A pneumatic cylinder stops reliably at the two hard mechanical limits of its travel, where a physical stop defines the position, and struggles to stop anywhere in between, because holding a piston mid-stroke means balancing two compressible air springs against each other at an exact equilibrium that leaks, temperature and friction all keep quietly disturbing.
Two air springs holding one piston
Closing the valve feeding a pneumatic cylinder mid-stroke traps a fixed mass of air on each side of the piston, and the piston sits wherever the pressures from those two trapped volumes happen to balance. That balance is a live equilibrium between two compressible cushions, with none of the permanence of a mechanical stop. A slow leak past a seal, air warming and expanding as the cylinder sits in a warm workshop, or a shift in the load on the piston rod each moves the point where the equilibrium settles, and the piston goes with it.
The disturbances need not be large. A leak far too small to hiss can still let a mid-stroke piston creep by more than a precision task would tolerate within minutes, because the trapped volume is small, so even a tiny loss is a real share of its pressure, and pressure is the only thing holding the piston where it sits. A hydraulic cylinder held at the same mid-stroke point has no equivalent problem, since the incompressible liquid trapped on either side has nowhere to go and nothing to settle into, which is why hydraulics are trusted with mid-stroke holding in ways pneumatics rarely are.
Friction in the seals adds a further complication when the piston is moving. A seal at rest grips the bore harder than a seal already sliding, so a piston nearing its target tends to stick, pressure builds behind it, and it then breaks free and lurches past the point where it was meant to stop. The air behind it is a compressed spring by that point, and it releases its stored energy the moment the seal lets go, which a rigid liquid would never have stored in the first place. Any attempt to stop the piston by timing the valve therefore has to contend with a stroke that moves in small jerks instead of a smooth glide, and the final position depends on where the last jerk happened to end.
A balloon against a tap
Squeezing a balloon to let a little air escape and trying to stop at exactly half its original size, by feel, rarely lands on the size wanted. The balloon keeps settling for a moment after the fingers stop, the rubber's tension and the escaping air still finding a new equilibrium. Letting water out of a container to a marked half-full line is a different experience, because the water stops the instant the tap closes, with no drift left to happen afterwards. A pneumatic cylinder asked to stop mid-stroke behaves like the balloon, its resting position never as fixed as intuition trained on water expects.
The ends of the bore do the holding
At full extension or full retraction, the piston is held by a genuine mechanical limit, the physical end of the cylinder's own bore, which does not drift, does not leak, and does not care how warm the workshop gets. This is why pneumatic cylinders are so overwhelmingly used to move between exactly two positions, fully in and fully out. Only those two positions borrow the reliability of a real stop instead of depending on the equilibrium of trapped air.
The cheapest fix for needing some other position is to add a physical, adjustable stop, such as a collar clamped onto the piston rod or a bracket bolted at a chosen point along the travel. The cylinder still only ever runs fully against a hard limit, one that has simply been relocated to wherever the job needs it. The drift problem disappears because no air-pressure equilibrium is being relied on, at the cost of serving only a small, fixed number of positions decided in advance.
The price of true mid-stroke control
Real mid-stroke positioning with air is possible, but only by abandoning simple on-off valve control and adding the machinery a precise electric or hydraulic system would need anyway: a proportional valve capable of fine, continuous adjustment, a position sensor reporting where the piston actually is, and a control loop constantly correcting for the drift described above. That complexity costs parts, tuning effort and maintenance, so most pneumatic systems are designed from the outset to need only the two hard stops, letting the air do the job it does reliably, moving fast between two fixed points.
A designer reaching for compressed air should ask, from the first sketch, whether the task needs only two positions. The answer decides whether pneumatics is the simple, cheap choice or an expensive, over-engineered one dressed up to look simple.