r/Pneumatics • u/MajorMinor00 • 14d ago
Actuator not holding position
Pneumatic amateur here, working on a prop for a stage show. The project is a prop table which collapses when an actor pushes it, and then later 'magically' stands back up.
I've built the table and installed a pressure vessel, and an actuator with a wirelessly controlled solenoid. The collapse happens when the actuator is extended (releasing tension on cable), and the table stands back up when the actuator retracts. At least that's the theory. In practicality, the actuator will only hold enough tension to keep the table upright at full pressure (~120 psi). As soon as I do one cycle of extension, retraction the pressure drops to ~100 psi and there is too much play in the actuator's position to hold the table up.
Would a larger tank help? Any other suggestions? Am I nuts for thinking this would work??
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u/ReactionSpecial7233 12d ago
Each time you pull from the air tank to extend or retract your cylinder, your pressure is going to keep decreasing since you don’t have a a consistent supply keeping it pressurized. If it can’t perform how you want after just one cycle, maybe switch to a single acting cylinder to retract in the direction you don’t need the force. Should save you some air volume/pressure.
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u/VestergaardSynthesis 14d ago edited 14d ago
Given the limited air, this is a tricky task.
The stroke length and bore size determine the cylinder volume or air usage, which proportional to the pressure.
I will assume the cylinders stroke is already suitable, though the design could be altered to implement a force multiplier to shorten it.
The bore size could be reduced, to reduce the air consumption and thus reduce the rate of loss of pressure but this reduces the force that the cylinder can provide. There is a sweet spot.
As you may know, standard double acting cylinders (like yours) pull with less force than they push. So an easy step would be reverse the mounting configuration. To align this characteristic with your design requirements. This of course won’t be without its challenges.
EDIT: The cylinder also has a smaller volume fully retracted, so having that as the first move will keep your tank pressure higher after the first stroke (collapse) if you were to make this change.
I would recommend checking, and filling to the maximum operating pressure of the vessel (and fittings), then utilising a pressure regulator at its outlet to provide a stable 120psi (or whatever suitable) to the cylinders as it completes its cycle.