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The downside of using single-acting return spring pneumatic cylinders is the decreasing output force through a full stroke due to the opposing spring force. I'm wondering how would you design a spring return pneumatic cylinder which produces an increasing output force? Would you simply need to increase the input pressure?

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I'm wondering how would you design a spring return pneumatic cylinder which produces an increasing output force?

I think you mean "produces an increased output force".

The counter-force exerted by the spring at maximum stroke (or any point during the stroke) is fixed and determined by the spring parameters such as spring set, and spring rate. (See Lee Spring's A guide to spring design for some terminology.) So the actual force at the shaft will be Fshaft = Fair - Fspring.

It stands, therefore, that by increasing the air pressure that the shaft force can be increased.

Would you simply need to increase the input pressure?

Yes.

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  • $\begingroup$ @Phil, when I read the question originally the meaning wasn't clear to me. The OP seemed to suggest a requirement for force not proportional to pressure, hence the clarification. I don't see how back-pressure regulation is an option on a single-acting cylinder. Bellcrank linkage is a good idea if there is sufficient cylinder stroke. $\endgroup$
    – Transistor
    Mar 29 '20 at 13:25
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You cannot use a return spring system without reducing the output force as the spring gets energy put into it. You could design an equivalent return system, however, using something other than a spring:

  • a pulley that moves a weight vertically against gravity. The force of the weight would be constant regardless of the stroke position.
  • a reservoir fitted to the non-pressurized side that has liquid that would rise in level and be vented to atmosphere.

I'm sure you could come up with other similar ideas.

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there are a couple of ways to increase the output:

  • Because the force of the spring is proportional to its deformation, you could use a cylinder with additional room for the spring so that even when the piston is fully deployed the spring is only partially pushed back.

  • You can use a softer spring with a sealed bladder of air on the back assisting it.

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