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Need help on a co-axial piston

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Need help on a co-axial piston

Unread postAuthor: behnt » Sat Jan 28, 2017 1:32 pm

I am building my first co-axial piston using a 3D printer and making a small one before I go big but things are just not working out. I can pressure up to 60-80 psi and hit the initiator valve but the piston does not move and dump air to the barrel. So here is my report:

1. Nice sealed piston was tried no dice
2. vented piston was tried no dice (small groove vents along the side. I was worried the chamber was not pressuring up)
3. PTFE tape on the outside of a smaller diameter piston was used to ensure piston moved smoothly with no binding yet a sung fit in the chamber
4. I went from a rubber face seal to a molded silicone putty head on the piston thinking the barrel seal was not adequate
5. Barrel is 3/4 pvc 12" long
6. chamber is 1.5" abs 12" long
7. Even though not a conventional build it can hold 60PSI for a hour so it is sealed

Now I am stuck.

Am I not using enough PSI?
Does the piston need to be more or less leaky to the chamber?
Is my initiator valve not big enough?
Is my barrel to chamber ratio inadequate?
Am I missing something important?
Could a micro leak on the chamber cause it to malfunction

You can see my design in 3D CAD here:

https://cad.onshape.com/documents/73c1c66cd575fcedf1c33d95/w/a956b8b810ecfe18d0bcc683/e/eb2aa884f028e4a656f9e2d8

Pictures of As built attached
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Re: Need help on a co-axial piston

Unread postAuthor: jrrdw » Sun Jan 29, 2017 10:52 am

Maybe piston tipping, from your drawing it looks like there is a lot of room to get jammed and they are short pistons. Most successful co-axils have small pilot chambers. Might want to try reconfiguring your fill and dump valves into one and center it in the end cap.
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Re: Need help on a co-axial piston

Unread postAuthor: behnt » Sun Jan 29, 2017 3:11 pm

Cool that is easy to test. Ill make a longer piston and see if it is tipping, or too big a pilot chamber. Printer will be fired up in a hour and should have some test results tomorrow. Thanks!
Fingers crossed
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Re: Need help on a co-axial piston

Unread postAuthor: behnt » Mon Jan 30, 2017 9:53 am

Update:

Longer piston does not help unfortunately. That rules out the piston jamming due to being too short and the pilot chamber being too big.

I tried a air compressor relief valve thinking the pilot trigger was not venting fast enough. Again no dice. I am going to re-configure the pilot/fill area to be all inline next.
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Re: Need help on a co-axial piston

Unread postAuthor: Gippeto » Mon Jan 30, 2017 10:25 pm

Make a piston without those grooves in the sides. It's leaking past into the pilot volume as fast as you can vent it off.

Large diameter piston or large pilot volume along with a smallish pilot valve makes things difficult enough...adding the grooves is just making it impossible.

Piston travel should be no more than 1/2 the barrel id. 1/4 of the barrel id is all that's really needed in most cases. This will help limit the pilot volume. Piston should be a pretty close sliding fit on the od...closer is better....o-ringed (floating o-ring check) is better yet.
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Re: Need help on a co-axial piston

Unread postAuthor: Anatine Duo » Tue Jan 31, 2017 8:31 am

I agree with Gippeto. It's rare that pistons need venting.

I like your flanges! Did you fabricate them yourself?
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Re: Need help on a co-axial piston

Unread postAuthor: behnt » Tue Jan 31, 2017 10:17 am

Anatine, Yes they are 3D printed. I know 3D prints leak air but if you seal the ABS tube surfaces and put in rubber gasket or Silicone the hell out of it, it seals up nicely. You can see the 3D models in the link in the original post. https://cad.onshape.com/documents/73c1c ... a656f9e2d8

The latest iterations have the vents removed from the side. But maybe a snug sliding fit with the 3D printed piston is to leaky? So I am going to try the below...

-Try a larger piston yet (depth wise to make a smaller pilot chamber to take it to 1/4 barrel I.D.)
-Try the same piston but with a better seal.
-Design up a tight fitting piston with one way valves printed in the piston venting to the chamber.

I am starting to think that my concept of the piston is flawed in the sense that I am making it WAY to loose. But I question the ability of the air to get to the pressure chamber if the piston is sealed too well. Currently the piston slides easily in the barrel. Maybe I need to get to a press fit barely slides scenario.

Would vaseline on the piston to get a better seal help? Current gap on the diameter is about 0.002"
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Re: Need help on a co-axial piston

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Re: Need help on a co-axial piston

Unread postAuthor: Anatine Duo » Tue Jan 31, 2017 11:42 pm

.002" gap around a 1.5" piston is like a 7/64" hole... I think...

that would probably be your problem.

The pilot on my last 1.25" piston valve is smaller (3/32") than your gap.

The low tech way to fix this and fire your launcher in a couple minutes is a wrap of masking tape around the piston. It is compressible for a tight fit but also does not really stop the filling of the reservoir.
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Re: Need help on a co-axial piston

Unread postAuthor: behnt » Thu Feb 02, 2017 12:21 am

It fired! Not the best and it needs a revamp on the build but now I have something functioning to work with. Thank you all for the help. FWIW the floating O-ring was the special sauce. Thank you Gipetto. Now to fix some issues with the flanges, better sealing faces and work on optimizing 3D printed pistons.

Thank you again everyone I was on the verge of quitting. But your help got me back going and excited.
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Re: Need help on a co-axial piston

Unread postAuthor: jrrdw » Thu Feb 02, 2017 8:25 am

Congrats! That's the way it normally happens with piston valves. The next one will probably work 1st time. That right, there will be more, there can never be enough you are officially hooked and there are no cures for the addiction other then building more...
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Re: Need help on a co-axial piston

Unread postAuthor: behnt » Thu Feb 02, 2017 11:51 am

LOL, yup bigger better is on its way :) I was building the tiny one to learn lessons like the one I just sorted out. Now one more tiny to sort out geometry and efficiency, next a midsize and then I am going to go big. Stay tuned.
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Re: Need help on a co-axial piston

Unread postAuthor: Gippeto » Thu Feb 02, 2017 5:33 pm

behnt wrote:It fired!


First of many, well done! :thumbright: :thumbright:
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