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Barrel Piston attempt with questions

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Sun Oct 13, 2013 2:55 am

Working on second gun, in the picture you can see the setup I'm attempting...

I've watched wyz2285's YouTube video on the QEV/piston valve and Clide's helpful flash animation for the barrel seal. I want to make sure I have the concept down before moving on...

I believe I have the tapered copper fitting and piston sealing together in the right place above the air chamber opening in the picture, yes?

How far does the piston have to travel backwards with release? Just past the lip of the air chamber opening so the HPA can travel through the barrel and when equalized the spring, behind the piston, can once again push it against the tapered piece?

Thanks.
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Re: Barrel Piston attempt with questions

Unread postAuthor: jackssmirkingrevenge » Sun Oct 13, 2013 6:47 am

air_dannon wrote:I believe I have the tapered copper fitting and piston sealing together in the right place above the air chamber opening in the picture, yes?


Yep - the problem is that the copper fitting is going to go through that rubber seal like a cookie cutter.

You need a wider area for the piston to push against to spread the load.

How far does the piston have to travel backwards with release? Just past the lip of the air chamber opening so the HPA can travel through the barrel and when equalized the spring, behind the piston, can once again push it against the tapered piece?


Approximately 1/4 for the barrel diameter to achieve full flow, but typically one is advised to up that to 1/2-3/4 barrel diameter to avoid the piston bouncing back.
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Re: Barrel Piston attempt with questions

Unread postAuthor: PeteS » Sun Oct 13, 2013 8:39 am

Based on my fumbling with piston designs...
You may have issues with the pilot not triggering the piston valve. There are two reasons I say that. First there is a good bit of volume behind the valve, and second the trigger valve looks like the opening may be kind of restricted. I am only basing that on a glance at your image and I may be wrong, but if it doesn't fire you might look into those two areas.

On the sealing surface cutting through the rubber piston face... That may be an issue as well. It will depend on how hard the rubber is, how smooth and rounded the sealing area is, and how much pressure you use. I used a solid teflon piston in my mini gun to resolve that problem, but it seems to only seal with a rapid application of high pressure. In that case it works great when charged with a CO2 cartridge, but not when charged with a bike shock pump. When I used a rubber faced piston it worked with the shock pump, but the rubber seal didn't last long at all. The good news is that it is pretty easy to tinker with different pistons and piston faces. Still, best to avoid the whole problem by having more sealing surface as Jackssmirkingrevenge suggested if possible.
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Re: Barrel Piston attempt with questions

Unread postAuthor: dart guy » Sun Oct 13, 2013 10:13 am

i think that if you put a peice of 1/4 inch pipe and put it inside so that it wont cut your piston.
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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Sun Oct 13, 2013 2:01 pm

Thanks, guys. I was wondering about the volume on the backside, PeteS... the Schraeder valve will have the lip cut down to expose the pin for the trigger in the end, but does anyone else think that much volume will be an issue?

Also, the little testing I've done so far, the piston is hard to seal when using my handpump, which is going to be the air source. I guess I'll have to play with compression springs.

I'll make the sealing barrel bigger and look into a different piston material.
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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Sun Oct 13, 2013 4:13 pm

Approximately 1/4 for the barrel diameter to achieve full flow, but typically one is advised to up that to 1/2-3/4 barrel diameter to avoid the piston bouncing back.


I'm not getting this pictured in my head, JSR. In Clide's animation http://www.spudfiles.com/pneumatic-cannons/topic8157.html (BTW, it seems the new website doesn't like to show swf files) the piston travels back almost the length of itself before hitting the wall. Is that an accurate representation of the room needed behind the piston?
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Re: Barrel Piston attempt with questions

Unread postAuthor: dart guy » Sun Oct 13, 2013 7:25 pm

Behind the qev you should be fine because it will work about the same, but do make a sutable trigger for the shrater intead of cutting it down.
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Re: Barrel Piston attempt with questions

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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Sun Oct 13, 2013 8:13 pm

Thanks, DG. The valve needle wasn't going to be the actual trigger. I was going to cut down an eighth inch so the needle would be exposed to a sliding trigger yet to be made. Is exposing the needle like that a known problem-in-the-making?
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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Mon Oct 14, 2013 12:50 am

Needed to work it out visually for myself. This is also going with a larger sealing side 1/2" copper (which is also the planned barrel size).

Are the spaces the piston needs to travel back in these 1/2 ID and 3/4 ID versions of my setup correct? And which is the better choice?

Will using the 1/2" copper for the sealing side help or hinder the explosive air power? If so, what would be a better fitting setup to a 1/2" barrel?

Also, the compression spring... Am I right in thinking it needs to be long enough to at least push that piston against the sealing side, but also loose enough to compress to the amounts shown in my diagrams? Obviously I don't want the spring pushing the piston shut before the full flow is evacuated, but taking this gun to 350-400 psi like I want, is that concern not even an issue?

Thanks!
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Re: Barrel Piston attempt with questions

Unread postAuthor: jackssmirkingrevenge » Mon Oct 14, 2013 8:58 am

You might find a read through this useful: pneumatic-cannons/topic11354.html

The use of a tee means that the dynamic changes a little.

There is no need for a spring.

Does the rubber seal exactly against the sides of the pipe?
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Re: Barrel Piston attempt with questions

Unread postAuthor: dart guy » Mon Oct 14, 2013 3:12 pm

Exosing the needle could be a huge problem because you could melt the seal if you get it to hot or you could break the seal by hiting the needle.just make a wooden block with a hole drilled in the back to fit the outside of the valve and place a bbinside the trigger. also use the edit or hold off on posting untill someone answers a double post is actualy against the rules.(i was repremanded for that)
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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Mon Oct 14, 2013 6:16 pm

@DG - Those are good points for not exposing the pin, I'll make the "sleeve" version you described. Double posting, I get it... I should have just added (edited) my second post to the prior. Thanks.

Does the rubber seal exactly against the sides of the pipe?

@JSR - Yes, it does seal against the sides. I've removed the thick, softer rubber washer and instead cut a piece of bike inner tube and epoxied it to the piston head, then put it on my drill/apartment living lathe and sanded the rubber tight to the lip. Also, for the inner 1/2" copper sealing edge, I put extra pressure on the tube cutter to bend it inward and make the cut have a sight bevel that is then sanded smooth. I read the link, and another one the search feature offered at the page bottom,... starting to get it, thanks.
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Re: Barrel Piston attempt with questions

Unread postAuthor: dart guy » Mon Oct 14, 2013 6:49 pm

Image
visual representation, green is a space filler (epoxy) orange is a bb
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Re: Barrel Piston attempt with questions

Unread postAuthor: air_dannon » Tue Oct 15, 2013 1:22 am

This is driving me nutso... I can't get the piston to travel back at all. The setup is still quite close to the initial workbench photo. I was afraid the barrel wasn't sealing with the (later posted) 1/2" sealing side, so I soldered in the reducer and ditched the softer rubber washer for bike tube rubber and cut/epoxied it to the piston.

I had to throw in a super weak compression spring because filling the chamber with my handpump was giving me problems getting that initial seal going.

So, I know it's sealing, I've checked the piston is covering the air chamber hole fully. The piston is nice n' snug in there and travels freely (couldn't even put one layer of thin tape around it when I thought, maybe it needed a bit more diameter).

I pump it up to around 100psi, use a nail to push in the Schrader valve, but ALL the air comes out the Schrader... no piston movement whatsoever. :?
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Re: Barrel Piston attempt with questions

Unread postAuthor: cammyd32 » Tue Oct 15, 2013 3:02 am

air_dannon wrote:Thanks, DG. The valve needle wasn't going to be the actual trigger. I was going to cut down an eighth inch so the needle would be exposed to a sliding trigger yet to be made. Is exposing the needle like that a known problem-in-the-making?


No, not in the slightest. In fact cutting down the end of schrader valves actually increases the flow you can get out of them, reduces constrictions in the gasses path.
If you're worried about damaging the core then you can just use a core removal tool or pliers to get the core out before you cut, iv'e used cut down schraders on many occasions in the past, most recently with a very fast cycling bolt hitting them, and from experience unless you're hitting them with a sledgehammer they are not gonna break.
Alternately jamming something into the valve to push the pin will ultimately cause a much lower flow of the valve, constricts airflow and also it very often means that you never actually push the valve all the way open, all of which might well be contributing reasons why all your gas is escaping through the valve.
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