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rebuilding

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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rebuilding

Unread postAuthor: gabeb123 » Sun Aug 16, 2009 6:08 pm

hi someone gave me a pneumatic tator cannon but the chamber is cellular core and it leaks so i am rebuilding it.

it has a electric sprinkler valve (i will eventually modify it)

the barrel is 1.5"pvc and it is about 5 feet long(is this too long?)

The chamber was 3" pvc and it was almost 5 feet long too.

first question is how long should the barrel be?

second question is how long should the chamber be?(i already have the pvc)

third question what is the best way to make the shrader valve make a good seal with the pvc?

thanks

p.s. i tried that pneumatic cannon simulator but it doesnt make much sence to me i guess im not smart enuf
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Unread postAuthor: explsvcookie257 » Sun Aug 16, 2009 6:41 pm

Well from what I have heard a 2:1 chamber to barrel ratio for pneumatics is good so that would be a 33" long chamber. The schrader valve can be tapped into the pvc, you can find a tap at home depot or you can just drill a hole slightly smaller then the schrader valve and then screw it in and epoxy the threads the best place to do this is a place with more then one layer of pvc like on top a coupling. hope this helps.

EDIT: the length of the barrel is fine if you want a 2 to 1 chamber to barrel ratio then keep it the same length and have 33" of chamber.
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Unread postAuthor: FORE!!!! » Sun Aug 16, 2009 8:09 pm

A longer barrel will provide you with better accuracy, and a slight increase in power.
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Unread postAuthor: geardog32 » Sun Aug 16, 2009 10:05 pm

use ggdt.

these specs should work for your valve, if its a one inch.

Valve Type Chamber Seal
# of Valves 1
Flow Coef 23%
Seat Diam 1.2 in
Dead Vol 3.1 in^3
Piston Diam 2.0 in
Piston Mass 22 gm
Vent Diam .15 in (1)
Pilot Vol 1.1 in^3 (2)

the rest is self explanatory.
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Unread postAuthor: gabeb123 » Sun Aug 16, 2009 11:19 pm

well i finished building it and i made the chamber 33" long and i had to trim a little off of the barrel but ill post it in the showcase forum tomorrow.
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Unread postAuthor: MrCrowley » Sun Aug 16, 2009 11:34 pm

If you cut the chamber down to only 12", you'll lose almost hardly any performance (about 30fps in projectile velocity) and 90' in distance.

Not really worth it considering a 36" chamber is wasting a lot of precious compressed air. I'd cut it down to 12" or 15" personally.

Did you also get a new chamber? I wouldn't recommend cellular core in a combustion let alone a pneumatic. Cellular core is very dangerous stuff, especially cell core PVC.

Well from what I have heard a 2:1 chamber to barrel ratio for pneumatics is good so that would be a 33" long chamber

Shows you shouldn't always trust one member when asking for help, especially one with a low post count (though in some cases they are some of the smarter spudders (freefall, neospud etc)).

If you need help on buying pressure rated PVC, read this:
http://www.spudfiles.com/spud_wiki/inde ... sure_rated
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Unread postAuthor: explsvcookie257 » Mon Aug 17, 2009 12:53 am

Dude Mcrowley chill you really think 21 extra inches is waisted air? If he wanted to he can get a longer barrel and if you feel so confident my suggestion was wrong then why don't you post sooner. And that doesn't show anything or that im not smart its not like my suggestion is wrong its what I've read so lay off it doesn't have to be perfect. And maybe he wants that extra 90 feet of distance and 30fps.
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Unread postAuthor: noname » Mon Aug 17, 2009 1:07 am

Well hey man, I certainly wouldn't want to be pumping for triple the time to get a shot that's barely better anyway. I mean, maybe you would. I guess it's just a difference in preferences. Or as I say, whatever tickles your pickle.
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Unread postAuthor: MrCrowley » Mon Aug 17, 2009 1:51 am

explsvcookie257 wrote:Dude Mcrowley chill you really think 21 extra inches is waisted air? If he wanted to he can get a longer barrel and if you feel so confident my suggestion was wrong then why don't you post sooner. And that doesn't show anything or that im not smart its not like my suggestion is wrong its what I've read so lay off it doesn't have to be perfect. And maybe he wants that extra 90 feet of distance and 30fps.

Not having a go at you, trusting one poster shouldn't be a recommended thing anyway.

And yes I do think an extra 21 inches is a waste of air for this cannon. You made the calculations based on a C:B ratio, which means you calculated a fixed barrel length not one that can suddenly change to 10' so he can use that extra 21".

I didn't post sooner because I was at school, Spudfiles isn't blocked there but I don't usually go on the computers to go on Spudfiles (rather watch episodes of TV programs, damn I love high-speed internet).

I know it doesn't show you're not smart, it shows you have recieved the wrong information. God knows where, I don't know many people who'd recommend a 2:1 chamber:barrel ratio. Maybe he does want that extra 30fps, just pointing out that it really is inefficient and somewhat pointless.

Oh and if you double the length of the barrel to 10', I get a 42fps increase in performance using a 36" long chamber over a 12" chamber and an extra 100' in distance.

Like I said, not looking for an argument here, just be sure to read all the facts before stating them. I can say for sure that most members at Spudfiles would not endorse a 2:1 C:B ratio for this cannon, so for future reference, don't get all your information from a single source.
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Unread postAuthor: jackssmirkingrevenge » Mon Aug 17, 2009 2:02 am

MrCrowley wrote:God knows where, I don't know many people who'd recommend a 2:1 chamber:barrel ratio. Maybe he does want that extra 30fps, just pointing out that it really is inefficient and somewhat pointless


Here here, unless you're operating at ridiculously low pressures even a 1:1 ratio is very wasteful and only useful for those who want to put in a lot of effort pressurising their chamber for very little real gain in performance, not to mention the excess noise that comes with it.

If it's power you're after, lengthning the barrel will give you a lot more benefit than increasing chamber size plus noise levels actually go down.
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Unread postAuthor: MrCrowley » Mon Aug 17, 2009 2:15 am

jackssmirkingrevenge wrote:
MrCrowley wrote:God knows where, I don't know many people who'd recommend a 2:1 chamber:barrel ratio. Maybe he does want that extra 30fps, just pointing out that it really is inefficient and somewhat pointless


Here here, unless you're operating at ridiculously low pressures even a 1:1 ratio is very wasteful and only useful for those who want to put in a lot of effort pressurising their chamber for very little real gain in performance, not to mention the excess noise that comes with it.

Yeah, I know a 1:1 ratio isn't perfect either, but it gets to the stage where you end up with a barrel like SWAT. I mean half the launchers on this website with 4" chambers could benefit from a 20' barrel. :P

But in this case I was mainly referring to this cannon, where a 2:1 C:B ratio is pretty wasteful, even with an extra 5' of barrel.
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Unread postAuthor: explsvcookie257 » Mon Aug 17, 2009 9:49 am

So then what is a good chamber to barrel ratio for a pneumatic?
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Unread postAuthor: Technician1002 » Mon Aug 17, 2009 10:01 am

explsvcookie257 wrote:So then what is a good chamber to barrel ratio for a pneumatic?


You are going to hate my answer, but officially it varies.

Lower pressure uses larger chambers. Higher pressure uses smaller chambers.

Valve size is a factor. Use GGDT to find an optimum.
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Unread postAuthor: jimmy101 » Mon Aug 17, 2009 10:57 am

explsvcookie257 wrote:So then what is a good chamber to barrel ratio for a pneumatic?

Like tech implied, there isn't one.

Depends on what you are doing, what you have in the way of equipment, whether you'll be firing the gun from the shoulder, hip or from a mount. And a bunch of other things.

If you are pressurizing from a shop compressor than you might as well go with a large CB since the incremental cost is basically zero. If you are using a bike pump, or some other manual pump, then go with a smaller chamber (unless you're into spudding for the excercise benefit :D)

I would start with how you are going to hold the gun while firing. From the shoulder you are probably limited to a total length of 5' or less. From the hip a foot or two longer, from a mount it can be very long.

Once you've chosen an overall length decide on the barrel ID. What do you want to use as ammo?

How will you pressurize the gun? As I said, if you use a shop compressor than that limits your maximum pressure but the chamber can be any size. A hand pump may get to higher pressures but a big chamber is a heck of a lot of work, especially at high pressures.

Now check what pressure rated materials your local hardware store has. Choose the largest ID pressure rated pipe they have for the chamber. The required pressure rating depends on your pressurization method. With a typical shop compressor that limits your pressure to probably less than 150 PSI and any pressure rated pipe should be OK. If you are using a high pressure system (shock pump, HPA, CO2 etc.) then your pipe's pressure rating needs to meet, and preferably exceed by a large amount, the sources pressure (which may be regulated down to a manageable pressure).

Now, do you want an over-under or an inline design? Inline is simpler but over-under will reduce the overall length of the gun. The over-under setup requires more parts and you must pay close attention to bracing the barrel to the chamber.

All that is left is partitioning the length of the gun into barrel versus chamber for an inline. For an over-under the overall length is basically the barrel length and the chamber can be whatever length you feel comfortable holding (and filling).

Fire up GGDT and play with different CB ratios given your valve, choosen barrel length and ID, chamber ID, typical ammo, etc. You will probably find that the only real thing you have to play with is the chamber length and beyond a certain point it takes a very large increases in chamber length to get a small increase in performance.
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Unread postAuthor: gabeb123 » Mon Aug 17, 2009 3:12 pm

hey you guys are all so hostile chill out.

i replaced the chamber because it was cellular core(i didn't want to have it burst) now it is pressure rated sch 40.

i had to trim the barrel about 8 inches because i had a little accident with a chop saw (oops)

i don't know if ill have time to put it in the showcase today but ill get it on their eventualy.
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