Spring powered internal air compressor

Hello everyone,
I've come here for some help on designing a PVC-based air tank. I'm planning on building a pneumatic cannon that I can charge once and use to fire multiple short bursts of air. To do I decided to have two tanks -one large tank acting as the main tank and a local tank with a much smaller volume fed by the main.

My problem is that I want to squeeze out all the air I can from the main tank, and I'm not entirely sure what the best way to do that would be. My current design uses a spring behind a wooden plunger so that as the volume of compressed air drops, the spring can extend and restore the pressure. (Image attached for clarity)

Can anyone spot potential problem areas or places that could be improved?

Picture of Spring powered internal air compressor
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Toga_Dan3 years ago

i recommend using steel rather than pvc. theres a reason pvc isnt rated for air, especially when you introduce moving parts which just might jam. I've been around for 1 catastrophic failure of pvc, and know that another occured in my club before i was around. You've come up with an interesting idea which might have merit, but the safety issue oughta be addressed 1st.

Ninja Togo (author)  Toga_Dan3 years ago

I want to use steel; I've never used metal in a project like this because I found it much easier to cut and glue the PVC. For my current cannon, the PVC has held up just fine for almost six years now. However, I now realize, in this design with the spring focusing the opposing air pressure on one wall of the tank this is currently a recipe for disaster. Will make sure to go with steel this time.

Indeed, PVC is cheap, quick, easy. But I've been at 1 catastrophic failure, folks I know did another one, (luckily no- one hurt) One can also find stories of folk injured by PVC fails w/ air.

How big are you planning to do this? Have u thought of using an off-the-shelf bike-pump for the spring-loaded bit?

Ninja Togo (author)  Toga_Dan3 years ago

I was planning on making it at about 18 inches or so. The plan was to have something small and light enough that it could be tucked into a small backpack. I like the bike pump idea; it'll handle the high pressure better than PVC and I'll actually have some figures for the maximum pressure it can take. I'm still a little confused on how it would fit together in this case and store a large volume of air.

Toga_Dan3 years ago

https://www.instructables.com/id/MONKEY-HUNTER-PHYSICS/ metal gun construction.

Hi, Have you considered a rubber diaphragm? That is used in most of the little fish tank air bubbler thingys to compress air. I have modified your drawing to show how it might be.

diaphragm.LARGE.jpg
Ninja Togo (author)  gaiatechnician3 years ago

Thanks for the reply, as for the rubber diaphragm, I'm not really sure how I would get it to work. My thinking is that it would bend the outer ring of the diaphragm back due to whatever friction builds when it's moving back and forth in the tank.

As you basically want a main storage tank and a seperate firing tank you already have pressure problems.

Each time your firing tank fills the result is a massive drop in the main tank.

A much better approch would be to use proper sized ball valves instead of the tyre valves for everything else but filling the main tank.

You get free spinning ball valves so if you take a full turn from the "off" position you get one quick burst of air coming through.
Depending on the air volume you require you might want to check the firing mechanism on a paint ball gun and make something in a bigger diameter to allow for the amout of air you need.

Ninja Togo (author)  Downunder35m3 years ago

Sorry I should have labelled my diagram a bit better, but I'm not actually using tyre valves for everything just for the input to the tanks. I have one on each tank in case I want to charge the main tank or for a single shot from the local tank.