Pneumatic Flow Control: How to Set Cylinder Speed

Cylinder speed is set by restricting the exhaust, not the supply. Throttle the air going in and the cylinder moves in jerks; throttle the air coming out and it moves smoothly, because the trapped exhaust air acts as a cushion the piston has to push against. This one decision separates a machine that runs and a machine that judders.

This guide covers meter-out and meter-in control, where the throttle check valve belongs, how to commission it under real load, and why a cylinder that was smooth in January is jerky in July.

Meter-out or meter-in?

Meter-out is the default for double-acting cylinders. The throttle restricts the exhaust port. The supply side fills at full pressure, the exhaust side empties slowly, and the pressure difference across the piston stays under control throughout the stroke. The load cannot run away from the piston.

Meter-in restricts the supply. The cylinder starts moving before the supply side is up to pressure, so it lurches, then stalls, then lurches again. On a vertical cylinder with a falling load it is worse: the load pulls the piston faster than air can fill behind it, and the cylinder drops.

Meter-in has two legitimate uses:

• Single-acting cylinders, where there is no exhaust side to meter on the working stroke.
• Very small cylinders with almost no load, where the trapped-air cushion of meter-out would make them sluggish.

Everywhere else, meter out.

What a throttle check valve actually does

A throttle check valve is two parts in one body: an adjustable restriction and a check valve that bypasses it in the opposite direction. Air flows freely one way and through the restriction the other.

Fitted directly into the cylinder port with the check valve facing so that incoming air bypasses the restriction and outgoing air passes through it, this gives meter-out control on one side of the cylinder. Fit one at each port and both directions are controlled independently.

Orientation is the thing people get wrong. The same valve fitted the other way round gives meter-in and the jerky behaviour that comes with it. If a newly built machine judders on one direction only, check the orientation of that port's valve before anything else.

Fit the valve at the cylinder, not at the manifold. A throttle at the valve island still has to empty the whole length of tubing before it starts controlling the cylinder, so control is vague and the response to adjustment is delayed.

Commissioning: how to actually set it

Set the speed under real load, in the real orientation, at the real duty pressure. A cylinder set on the bench will be wrong on the machine.

1. Close both throttles almost fully, then open each about one turn. Never start with the throttles wide open on a machine that can hit something.
2. Run the cycle at reduced pressure if the machine allows it, and watch for a full, clean stroke in both directions.
3. Open the throttle on the working direction gradually until the stroke completes in the time the cycle needs, without the cylinder arriving hard at end of stroke.
4. Repeat for the return direction. The two rarely end up at the same setting, because extend and retract forces differ.
5. Lock the adjustment. A throttle that vibrates open over a few thousand cycles will speed the machine up until something breaks.
6. Record the settings. When a cylinder is replaced, the fitter needs to know where the throttles were.

If the cylinder will not move smoothly at any setting, the problem is not the throttle. Look for undersized tubing, a valve with insufficient flow, side load on the rod, or a load that varies through the stroke.

Why speed drifts with the seasons

Air viscosity and, more importantly, the volume of water in the air both change with temperature. A plant that runs 6 bar in a cold January morning and 6 bar in a humid July afternoon is not delivering the same air.

Three seasonal effects show up as speed drift:

• Condensate collecting in low points of the tubing reduces effective bore. The cylinder slows, someone opens the throttle, and then in winter it runs too fast.
• Compressor duty rises in summer, so system pressure may sag more under peak demand.
• Seal friction falls as temperature rises, so the same air pressure moves the piston faster.

The fix is not to readjust the throttles twice a year. It is to get the water out: correct drainage falls on the mains, drop legs taken from the top of the main rather than the bottom, and a dryer sized for the plant.

Quick exhaust valves: when speed is the point

Where a cylinder needs to move fast, particularly on the return of an ejector or a blow-off, a quick exhaust valve vents the cylinder port straight to atmosphere instead of back through the tubing and the valve. The air does not have to travel the whole return path, so the cylinder empties far faster.

Fit it at the cylinder port. Fitted anywhere else it does nothing useful, because the tubing between cylinder and valve still has to empty.

Note that a quick exhaust valve and meter-out control work against each other on the same port. Decide which one that direction needs.

Sizing: the throttle is not the only restriction

A throttle check valve has a maximum flow of its own, set by its port size and internal geometry. Fitting a G1/8 throttle to a 63 mm cylinder that needs to move quickly turns the throttle itself into the limit, and opening it fully will still not give the speed required.

Work through the chain in order when a cylinder is too slow at full open throttle:

1. Valve flow rate (often the real limit on larger cylinders)
2. Tubing bore and length
3. Fittings, counting elbows as significant restrictions
4. Throttle check valve size
5. Silencer on the valve exhaust, which clogs with oil and dust and is the most commonly overlooked restriction of all

A blocked silencer produces exactly the symptom of an undersized everything: a machine that gradually slows over months. It costs almost nothing to replace and is worth checking first.

Selection checklist

1. Meter-out unless the cylinder is single-acting or unloaded.
2. Throttle check valves at the cylinder ports, not at the manifold.
3. Orientation verified: free flow in, restricted flow out.
4. Port size matched to the flow the cylinder needs, not just to the thread available.
5. Lockable adjustment, and the setting recorded.
6. Quick exhaust valve where a direction needs to be fast, and not combined with meter-out on the same port.
7. Silencers on the valve exhausts, and a plan for replacing them.

FAQ