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Thread: newbie question: two pneumatic cylinders in series?

  1. #1
    Associate Engineer
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    newbie question: two pneumatic cylinders in series?

    Is it possible to run two double acting air cylinders in series?

    I know it works on hydraulics, but with compressible air I'm uncertain.

    Useful info:
    1. My goal is reducing air needed so the movement is faster and the air compressor runs less frequently
    2. The mechanical design of the system keeps the two cylinders synchronized


    Here is a schematic of what I'm thinking:
    Air-cylinders-in-series.PNG

    Will this work?

  2. #2
    Administrator Kelly Bramble's Avatar
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    Absolutely - depending on application, think hard about the line length and flow friction.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
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    Thanks Kelly, I will give it a whirl. fingers crossed

    You mentioned
    think hard about the line length and flow friction.
    . Does this matter? The two air cylinders are mechanically synchronized by a heavy frame. I'm thinking the air hose lengths doesn't affect things. no?

  4. #4
    Administrator Kelly Bramble's Avatar
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    Absolutely it does.. Even with mechanical linkage, if one line is 300 m and the other is 20 M the friction will cause the long line cylinder build upto to force slower.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
    Technical Fellow jboggs's Avatar
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    Your diagram confuses me. I don't see where it will cause any motion at all. Because of that tee connection at the top of the diagram you effectively have line pressure fed to the rod end of one cylinder and the cap end of the other cylinder at all times. I don't see where switching the valve will make any difference. If you leave that line in I don't think you will see any motion. If you do see any motion it will be unpredictable.

    You asked about running cylinders in series. That means the exhaust from one cylinder is fed to the pressure side of the next cylinder. If you remove that line I asked about in the above paragraph that is what you would have, a series circuit.

    (Sorry to disagree with you Kelly.) But that won't work either, at least not as shown. If you made both cylinders double rod types (where the pressure areas on both sides of the pistons is the same) it would theoretically work. But I've never seen it work very well in practice.

    Think about it. Every inch of piston travel in a single rod cylinder represents a certain volume change on the cap end. However the volume change on the rod end is different because of the presence of the rod itself.

    Do the math. Assume a 2" bore cylinder with a 3/4" diameter rod. The pressure area on the cap end is just the area of the bore: 3.14 sq in. The pressure area on the rod end is the area of the bore MINUS the area of the rod: 3.14 - 0.44 = 2.70 sq in. That means that for a given amount of travel the fluid volume fed in is different from the fluid volume exhausted out.

    If your two cylinders are indeed tied to a common frame then the best way to feed them is with a PARALLEL circuit rather than a series circuit. If you have two cylinders in your current setup I would bet that is the way they are currently piped.

  6. #6
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    Thanks jboggs,

    You are right. They are currently running in parallel perfectly and the diagram I put above will never work or provide motion.

    My challenge is I need to reduce the cubic feet/minute of air needed.

    Is there any way to run compressed air in series reliably?

  7. #7
    Technical Fellow jboggs's Avatar
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    The best way to reduce CFM is to reduce bore or stroke. What is the actual force required of those cylinders? Is it possible they are a little oversized now? If you attach them to the frame at a different location could you reduce the required stroke? How much are you trying to reduce the CFM, and why? You could also replace them with electric actuators.

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