# keep cylinder pistons from rotating



## bikerglen (Feb 1, 2017)

I want to build a popup skull in a flower pot using a spare 10" pneumatic cylinder. The skull would be concealed in the flower pot until it’s tripped by a motion sensor then the skull would pop straight out of the pot and some sounds would play and some light would flash.

The problem I’m having is that the piston is free to rotate within the body of the pneumatic cylinder so, depending on weight balance, the skull could possibly rotate as it’s moving up and down where the skull’s face is no longer pointed toward the crowd.

I’m trying to think of a cheap creative solution to keep the piston from rotating. One thought was to use a guided cylinder. These are expensive, heavy, and bulky though. Another thought was to find some small drawer slides and mount these to the base and skull in parallel with the pneumatic cylinder. Yet another thought was to use two or more cylinders in parallel—with only one plumbed to air. These would probably need to fit within the body of the skull though which could get tight.

Or maybe just a piece of wire cut to the extended length of the cylinder and mounted to the back of the prop so when the cylinder is extended, the wire pulls the prop back into the correct orientation.

How have others solved this problem?

Thanks,
Glen


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## DarkOne (Oct 16, 2012)

Maybe finding some small nesting tubes, round should be fine. Attach the larger one to the body of the pneumatic cylinder, and the smaller one to the mount of the cylinder on the skull. This should do the same thing as using two parallel cylinders, but cost less. Seems like a waste to use a second cylinder just as a guide. The only thing i see as a possible hiccup is binding, but there shouldn't really be any pressure on the tubes. Good luck with whatever you figure out. I hope you'll share your solution.


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## 69-cat (Jan 18, 2014)

All of the cylinders have threads on both ends just for this reason. Each end has a plate with 2 holes, one to mount to the cylinder and the other has a 5/16 id tube welded to the rid end of the cylinder. The tube is longer than the cylinder and the plate on the other end of the cylinder is a clearance hole for the 5/16 tube. I have a plate mounted to the rod with a 1/4 rod that slips into the 5/16 tube and runs along side the cylinder. As the cylinder extends, the 1/4 rod will keep the cylinder rod from rotating. FYI, the rod should be 6 inches longer than the stroke so the rod does not bind up as it retracts from a full extension. Keep in mind what you are operating with the cylinder, you might have to beef up the anti rotating assembly to match the load.
Dave


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## bikerglen (Feb 1, 2017)

Thanks, 69-cat. I don’t weld (yet), but this gives me an idea:

On the nose of the cylinder, I can mount a plate with two holes. One hole for the cylinder nose mount and a second hole that is about 5/16".

On the rod of the cylinder, I can mount a 2nd plate with two holes. One hole for the rod and a second hole that is about 1/4". Through the 1/4" hole, I can mount a ~10-11" length of threaded rod.

As the rod moves up and down, the threaded rod will slide through the 5/16" hole in the nose-mounted plate and keep things aligned.

This shouldn’t have the binding issues that a tube would have and will let me postpone learning to weld another few months. 

Sound plausible to you?

Thanks,
Glen


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## 69-cat (Jan 18, 2014)

Since you don't have the option for welding, you might be able to attach the tube to the side of the cylinder with tape. You do want to use a tube as long as the cylinder, if you just use a single plate, it could still have a twisting action as it moves. Again, this depends on the load of the prop.


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## wickedbeernut (Dec 14, 2016)

They make air cylinders with non-rotating rods. Appearance-wise, they're indistinguishable from air cylinders with rotating rods (no more heavy or bulky). Of course, they're a little more expensive. They can be found surplus (new) on eBay.

I like your wire idea. I'd probably use a spring or a bungee cord.

You could always use a mechanical linkage (e.g., four-bar linkage). By varying the relative length of the bars, you can get the skull to lunge not only up, but out.

wbn


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## bikerglen (Feb 1, 2017)

Hmm, I'll start watching eBay for a non-rotating cylinder with a useful stroke length. Looks like lots of possibilities to solve this problem. I still need to find a flower pot and a hollow skull too.


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