# Two cylinders at same speed!



## idir (Oct 21, 2011)

Hello all,

I am in a predicament at work, and I thought that some of you might have some clever ideas about how to make my day better!
Here is the situation: in a current telecom project, I am using two 25mm heavy duty (with two pins in addition to the rod) cylinders, distant from about 500 mm from each other. I could not use a single cylinder since I have a lot of connectors to connect (the cylinders pulls a device towards test connectors).
I have clearly underrated the speed difference problem between two cylinders. I thought that, considering the forces at stake, and the guiding pins helping, that any mismatch would be "eaten" by the cylinders themselves. Well, it works in most cases, but it does become problem at some point, randomly. One goes quicker than the other one, and both cylinders stop due to the side load they are subject to.
I could redesign quite a lot, to better link mechanically these cylinders, using PLC, or by having some kind of dampener that could take care of the non parallelism, but my project is in a very critical phase with virtually no time at all. Only a few days to have a stable enough solution.

It is pretty obvious that the problem disappears when the cylinders work at high speed. But since I test connectors in place, I must be "gentle" with them. But my first idea is still to make the cylinders go quickly enough to prevent the current trouble, but not so quick that the connectors are worn out quickly.

But thinking about the speed of the cylinders, I thought of the other way around. Since the pins in the cylinders allow quite a lot of side load, it might just work to make the cylinders move very slowly. In that case, one will not "have the time" to go first all the way until getting stuck. It would so that the other one has enough time to move before the first one get stuck, and this during the whole pulling process.
Am I totally dreaming?

Another obvious thing is that some preloading on the cylinders help them to go down well. I haven't tested with my device on (it is a 15 pound device) so it might just work directly, but I was wondering if any of you guys had similar "preloading" experience.

And at last, I cannot be the first one to encounter this problem. A few of my colleagues have had it before, but none of them really solved it (they all redesigned heavily). I cannot believe that there is not one "somehow quick" fix considering that the whole thing is working already, and it feels as if I am so close to something stable.

Oh, and for those wondering, I use separate speed cylinders and valves for each cylinder, that I tune until the cylinders go at the very same time. But it is only an eye tuning, those tens of seconds that I lose might be the very reason why the whole thing is not stable after a few minutes.

Any idea / support would be greatly appreciated! Pneumatics rock, but this has got to be one of their major drawbacks.
Thanks a lot to everybody reading this message!

ps: I did search for a similar topic on this forum but could not find anything. My apologies if I missed something already existing, please let me know in that case!


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## bfjou812 (Aug 15, 2008)

Are the cylinders the same ? Bore ,Stroke, Manfacturer? Is there any reason you can't use one solenoid and flow control valves?


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## idir (Oct 21, 2011)

My apologies for the lack of precision in my previous message.
The cylinders have a 25mm bore diameter and are both the same (MGP series from SMC, stroke 25mm). 
As I understand, flow control valves work with hydraulics but not with pneumatics, am I wrong?
And the reason why I did not consider a solenoid (or anything electrically controlled) is the test equipment design, purely mechanical (the only thing to use it is to plug the air input and you are good to go). 

With a solenoid, and the need to power it, a signal is needed for every test equipment, and such a signal comes from the test station which I cannot change at all. But I might be able to use a power signal from that station if no "programming signal" is needed, simply the same power signal all the time ( I guess in that case it would mean that the solenoid must be configured only one time, and then behaves the same as long as it is powered up).

I have never used such a solenoid with pneumatics. Is the solenoid there to ensure that the air is properly divided 50% / 50%?
Would you happen to have an example of such a device for use with two pneumatic cylinders?

Thanks for your help so far!


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## Spooky Dave (May 12, 2011)

Hi Idir,

I'm afraid I'm not following a lot of what you're discussing, and that's mainly because it's such a different application.

I can at least answer one piece of your question, though, and that's a big YES to flow control valves for pneumatics. They're used all the time in props. Most of us want our monsters to jump up really quick, and then to reset slowly. That difference in speed comes, almost always, through a flow control valve. 

The only other application I can mention is that I have one part of my haunted house where I run two different monsters (bats that fly out toward people) on one solenoid valve. There are two cylinders, one per bat, but I split the air between the two cylinders. Seems to work pretty well in unison, but they're not attached to the same object, so I can't say if that would be a problem or not.

Good luck!

Dave


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## idir (Oct 21, 2011)

Hi Dave,
Thanks a lot for your answer. Thanks for the confirmation about the flow control valve. I will keep this in mind when I troubleshoot the test fixture next week, trying to find a stable solution to the problem.
The "at the same time when attached together" is tricky! I underestimated the time delay effect. We are probably talking about 0.1 or 0.2s here, and I was certain that by having speed controllers well adjusted, and steady (for side loads) cylinders, the whole thing would just go down nicely. It does in most cases (if I open the speed fylly, it sure does but comes with a rather big bang!), but does also stop in a very few cases (in such a case, putting a little force on the slower cylinder helps it to go down and everything works again). I will probably end up adjusting to either quite quick or quite slow, depending on my application, and what is the best to overcome this getting stuck effect. My hope is that by going very slowly, the slower cylinder will have time to start pulling before the other one has been stuck by its own force and the angle difference of the plate attached to the cylinders.
But for now, thanks again for taking the time to answer! Your job does seem like a lot of fun by the way


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## corey872 (Jan 10, 2010)

Sounds like the problem may be you are trying to run two cylinders off a single (regulated?) air supply? Maybe you have the two cylinders with a 'T" fitting between, or one small air line with a branch to both? 

With two cylinders on the same supply, the load would have to be absolutely perfectly balanced for them to act the same. This isn't likely to happen in reality. So one cylinder would start moving until it hits a higher load, then the other would start to move until it loaded higher, then back to the first, etc.

The simplest thing I can think of is to remove this 'sharing' of air. You might try putting a reservoir in the line between the two cylinders, or ideally, run each off it's own regulator - which would be a bit more complicated, but have a better chance of success. I don't know that this would absolutely work, but it's where I'd start.

In the end, air is squishy stuff - very hard to control precisely. Linear actuators, on the other hand....


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## walterb (Jul 27, 2010)

Have u thought of moving to one cylinder with a connective plate between the two things u want to move?


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## idir (Oct 21, 2011)

@corey872
You got the issue perfectly I must say  Actually, I do have a single air supply, but each cylinder has its own speed control. Having the "same" air going directly to the cylinders with the T-fitting is something I tried early on and it really does not work. Now with separate speed controllers (and separate valve), it "almost" works, but still not perfectly.
I am trying to figure out if this could work with very accurate adjustments at a certain speed which would guarantee good testing and "overcome" the getting stuck effect.

Your idea of a reservoir could be a good one, it will give it a thought and try with a slight system redesign. I also will try a different speed controller. Some seem to have more precis adjustment, it might just be what I need.

Thanks a lot for your ideas, it really helps to read others' comments and suggestions!

@walterb
I must have two cylinders as they are, unfortunately, as the whole test system has a very precise height limitation, and that no single cylinder can develop the force that I need with that height... 
But thanks for your suggestion, next time I will make sure to request more height and go for a single cylinder!


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