SiteIt's halfway through the night shift. The pusher on the case packer keeps faulting with "cylinder not extended". It goes out, but slowly. Sometimes it stops short and the sensor never sees it. Reset, run, fault. Reset, run, fault.
Someone's already turned the regulator up. It helped for ten minutes.
Most slow or sticky cylinders are being starved, choked or fooled. The cylinder itself is often the last thing wrong. Work through the circuit in order and you'll usually find it before you need a spanner on the rod end.
Watch the gauge while it cycles
Start at the machine's filter-regulator. Read the gauge with the machine stood still. Then watch it while the machine cycles. If the pressure dips hard every time the pusher fires, the cylinder isn't getting the air it needs.
That points upstream. A clogged filter element, a regulator that can't flow enough, a kinked or crushed tube, or a supply line that's too small for what's been added to it. HSE's compressed air guidance says each machine should have its own filter, regulator, gauge and lubricator set. Kept in good order, that limits wear on seals in actuators and valves (HSG39, paragraph 109)*. Our guide to air preparation covers what to check on the set itself.
Don't fix it by winding the regulator up. If the cylinder needed more pressure on Tuesday than it did on Monday, something has changed. Find out what.
Then the speed controls and exhausts
Next, look at the flow controls on the cylinder ports. They get knocked, fiddled with and wound in by someone chasing a different fault. If they're not marked, you can't tell what the right setting was.
Most cylinders are set up with meter-out control, so the speed depends on how fast the air can leave. That makes the exhaust the next suspect. A silencer clogged with oil and dirt will slow a cylinder down just as well as a closed flow control. Take it off and try it. If the cylinder suddenly runs at full speed, you've found it. Replace the silencer and find out where the oil and dirt are coming from.
Is the valve shifting properly?
A valve that's sticking or only half shifting will give you a slow, weak stroke. Dirty or wet air is the usual cause. HSE lists dirty or "wet" air as something that can lead to corrosion and blocked valves (HSG39, paragraph 2)*.
Listen at the valve exhaust ports with the cylinder held at the end of its stroke. A steady hiss means air is getting past a seal somewhere, in the valve or the cylinder. Our guide to solenoid valve faults shows how to tell which, and how to test the coil and the spool.
Is it really slow, or is the sensor lying?
Before you blame the air, check the cylinder isn't doing its job and the sensor's missing it. Reed switches and magnetic sensors on the cylinder body creep along the slot with vibration. Move one a few millimetres and the PLC never sees "extended", even though the rod is out.
HSE's guidance notes that reed switches may malfunction where there's a lot of vibration or strong electromagnetic fields (HSG39, paragraph 121)*. Watch the sensor LED as the rod comes out. If it flickers or comes on late, the fault is the sensor or its position. Our guide to proximity sensors covers the rest.
Now look at the cylinder
If the air, the valve and the sensor all check out, look at the cylinder.
Feel for side load. A rod that's pushing at an angle, a guide that's out of line or a mounting that's moved will wear the rod bearing and seals. HSG39 warns that excessive side loading can lead to piston rod bearing and seal wear (HSG39, paragraph 117)*. If the rod shows a shiny wear mark on one side, fix the alignment as well as the seals, or the new ones will go the same way.
Look for a scored or bent rod, and check whether the bore has dried out. Washdown, heat and the wrong lubricant all strip grease from the seals. A cylinder that's fine first thing and slow by mid-shift can be heat or a dry bore.
Bypass across the piston seal is the other cause. The cylinder holds its stroke on a light load and creeps or stalls on a heavy one. Testing for it means opening up the circuit, and that's where the danger starts.
Vent it before you break it
Compressed air is springy, so it stores energy. Isolating the supply doesn't always empty the cylinder. HSE's guidance warns that if connections to a held cylinder are broken, air under pressure on the other side of the piston can expand and cause movement (HSG39, paragraph 139)*.
Before you dismantle anything, the guidance says it should be isolated from all pressure sources and completely vented to atmosphere (HSG39, paragraph 169)*. Use the machine's three-port dump valve if it has one, and lock it in the vented position. The PUWER code of practice makes the same point: closed isn't always safe, and some vents need securing open (L22, paragraph 196)*.
Then deal with gravity. A vertical cylinder holding a load will drop when the air goes. Prop or pin it first. Our guide to safe isolation has the full sequence.
One more trap is the restart. A double-acting cylinder that's been fully exhausted can shoot out when air comes back. HSG39 describes this rapid acceleration and suggests a soft-start valve where it's a hazard (HSG39, paragraph 120)*. Keep everyone clear for the first stroke.
The cost of the slow fault
A slow cylinder rarely stops a line dead. It throws timeouts, micro-stops and rejects all shift. Each one is a reset and a few minutes lost. By the morning the line has lost more than a proper breakdown would have cost. We cover the bigger picture in pneumatic faults: leaks, valves and cylinders.
Here's our view. Whoever is on shift should be able to work through this list. If they can't find it in half an hour, get someone in who can. Turning the pressure up and hoping for the best just moves the fault somewhere else.
What to do on Monday
- Read the gauge on each machine's filter-regulator while it cycles, and note any that dip hard.
- Paint-mark every flow control setting on your problem cylinders.
- Check the exhaust silencers on your busiest machines, and replace any that are clogged.
- Make sure every machine has a lockable dump valve, and that nights know where it is.
* Paragraph numbers correct as of 27 October 2026 and refer to HSG39, second edition (1998), and L22, fourth edition (2014, amended 2018). Check HSE's HSG39 page for the latest.