Pneumatic Cylinder issues

Hey everyone i know this is somewhat unrelated to PLC discussion but I couldnt find anything online to answer my question. My cylinders are atleast controlled by a PLC (lol) so i imagine some of you have came across this same issue and i'd like to understand why it happens, so heres my problem:

I got called to fix an issue where 3 out of 4 of my cylinders would not actuate. The weird thing is the first three are upstream (closer to the air compressor) than the fourth one (the fourth was working fine). I checked over the electrical and they were getting the signal to fire accordingly. The solution was simple: Turn the air off and back on again. I've come across the same type of issue where cycling the air gets rid of the 'air lock'. My question is , what causes this to happen.

-I'd like to be able to answer the question myself, 'why' did that happen
Thanks guys and gals

First of all, are these cylinders driven from solenoids i.e. when you say that the cylinders are upstream (near the compressors ) do you actually mean the solenoids that drive them. If we are talking about the solenoids rather than cylinders then is common that on dry air systems (not lubricated) for the shuttles to stick so the pilot does not trigger the opening of the valve. Another possibility is water in the air supply this is common and any compressed air system should have a drier, these dryers need regular maintenance i.e. medium change etc. You are not clear on the process as I cannot see cylinders being driven from a compressor they will need some sort of air control solenoid.

Well, I said they were controlled by a PLC and were electrically being told to fire, i assumed it was implied that a solenoid was involved and by upstream i meant pneumatically. where as the solenoid itself would just be an electrical component paralleled to the pneumatics. All four solenoids, each operating their own valve, worked fine and are placed within 2 feet of their respective cylinders. All 3 of them stopped working at once so i dont see there being an electrical issue(all 3 are fed independently). I believe its more of a pneumatic issue which is why i didnt bother going too into depth with the solenoids.

It depends on the type of solenoid, most work by energising a coil that in turn opens a pilot from the main air to force the shuttle across from one port to the other, so what was meant by the solenoids was not an electrical problem but mechanical. The shuttle inside has "O" rings fitted that give the seal between the ports often in un lubricated systems these can stick (have had this many times, water (condensation) can also cause this problem. It usually happens when the system has been standing idle for some time. We used to get this often as the machines were in a production area that was only 4-5 degrees C, the main air from the above void (which was usually very warm) would suddenly be running in an area with a considerable temperature drop. The company policy was to use filters & water traps feeding each machine and the incoming air was dried, however, company policy was not to use oilers as it was food production even though all our lubricants were food safe. The answer was to spray a little lube into the solenoid valve, operate it a few times to lube and remove the excess lube and off it would go for quite some time.
This sounds like your scenario as the distance from source does not have much effect, only the state of the solenoid valve (I am talking about the pneumatics). We had a number of systems using MAC valves, these were so reliable I cannot remember changing one but on occasions a faulty one would burst into life on the application of lube.
One other thing, the cylinders have a number of rubber seals, one for the main seal for the actuation piston (isolates air from one stroke to the other) and a neck seal for the piston shaft. if these dry out they can either stick or not seal so leaks between the push/pull action can occur, if the leak is close to the input volume the compressed air will leak from the pressure side to the exhaust side either rendering not enough pressure to drive the cylinder or slow it down and reduce the effective force.
Yes I agree that length & diameter of air lines will have an effect on the cylinders but what you describe is not the case, I therefore suggest that as more than one indicated the same fault that it is the seals either in the solenoid valves or cylinders coupled with a long non operational time is the real culprit. This is just my observation as I do not know your system & it would be impossible to diagnose properly without being there.

What type of solenoid are you using, closed center, open center? Is this a manifold block or are they individual? External Pilot, internal pilot? Without all that info its all purely speculation. Everything that parky said could be the culprit, this could be narrowed down more if there was more info.

In my experience it is one of these....

If externally piloted, check your pilot pressure. If the pilot pressure is to low or the volume is restricted the spool will not operate even if energized.

If internally piloted most likely you have debris in the spool that is causing the spool to hang. If you are getting cylinder drift, then this is the most likely culprit.

Dirty air on spool valves is a big problem, especially on something that has high cycle.

Obviously there could be many more things at play but for most basic pneumatic systems operating in a normal environment it is one of those two things.

Also, if these are double solenoid valves make sure there is interlocking in your PLC code to prevent more than one port from being energized at the same time.

One more potential cause of non-moving pneumatic cylinder:
If you use flow-control valves on the cylinder itself (to control the speed of the cylinder), and the flow-control is adjust to almost shut (so as to achieve very slow speed), then dirt can relatively easily block the flow-control valve thereby trapping the air pressure on the side that should otherwise be vented when the solenoid activates.

N.B. The trapped air inside can cause the cylinder to move unexpectedly if you for troubleshooting remove the supply hose to the other side of the piston.

JesperMP said:

One more potential cause of non-moving pneumatic cylinder:
If you use flow-control valves on the cylinder itself (to control the speed of the cylinder), and the flow-control is adjust to almost shut (so as to achieve very slow speed), then dirt can relatively easily block the flow-control valve thereby trapping the air pressure on the side that should otherwise be vented when the solenoid activates.

N.B. The trapped air inside can cause the cylinder to move unexpectedly if you for troubleshooting remove the supply hose to the other side of the piston.

Click to expand...

And to expand on Jespers post even further when troubleshooting the flow controls a tech needs to have a good understanding of meter in/meter out and how they operate based on their physical location on the cylinder.

How old are the cylinders? Failure mode here at times is when the piston seal wears and lets air flow around it. A little oil in a port will keep it going until I put a new piston kit in it. I'm talking 2" cylinders with 8mm tubing a long way from the air valves. Flow controls on the cylinders.

there are lots of things to discuss regarding this issue.
this is some of the things that I have run across.


air compressor oil. some valves do not work well with some air compressor oils, the oil will gum up the seals, swell the seals, cause the valve to stick.


air contaminents, water, moisture, rust, dirt. use a coalesent filter to remove the contaminates. you will have to check the filter more often.


air pressure. valves have a pressure operating range. a 120 psi valve may not operate correctly or at all on a 200 psi air line. you will need a bigger size valve for this type of application.


my personal preference is the "Numatics" brand, they use a spool with no seals. if they stop, use the manual over ride to shift the valve several times to get rid of the water, gum, grit, trash. very rugged.


regards,
james

Thanks for the information guys, one thing that still gets me though is that it happened on all three of the cylinders at the same time, so I think its reasonable to assume its an issue with the air, or the air compressor itself and not the particular cylinders or valves controlling the cylinders. the main hose that all three of these cylinders get fed from is nearly impossible to trace back, I'm fairly certain that the fourth cylinder is fed from that same line and worked fine (ill double check when I'm back to work).

-It's a 24/7 food process plant as well like parky's reply, but there was no downtime or temperature drops at the time.

Again, thanks guys. I'm only trying to learn, not meaning to be stubborn

I would try moving the cylinders manually to make sure they aren't stuck. Air is often not clean and gums up the cylinders after a while so they stick.
You should be able to hear or feel the solenoids open and close.
If they don't then check for continuity.

I am familiar with food processing plants. Starch can gum up the works quickly.

When you operate the cylinders, how is the opposite side of the cylinder being exhausted?
I assume the solenoids are 2 position / 3 way (ports), so you can vent when the solenoid is off. One to each side of cylinder for up and down directions....or in and out, depending (or 2 position 4 way/5 port if one solenoid operates cylinder both ways.
AND the solenoids are plumbed to the correct ports...
Could be air lock because the opposite side of the cylinder is building pressure.
Whatever position the cylinders are in, loosen the air fitting on the opposite side of cylinder...should be no pressure because it is vented.

A slow start valve on the incoming line tuned to a minimum could be one of the cause, which doesn't build up the pressure, if there are air leakages or air blow nozzles active.

Very often the exhaust port(s) are common so a semi blocked exhaust could be the culprit, check to see if there is any common points that may affect those 3 cylinders.

parky said:

Very often the exhaust port(s) are common so a semi blocked exhaust could be the culprit, check to see if there is any common points that may affect those 3 cylinders.

Click to expand...

I concur, especially with valve manifolds that share a common exhaust port, often going through a muffler. I have fixed many a pneumatic problem in a former life by installing a new, clean muffler on a valve stack.