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Oh boy, I think I'm gonna blow my cannon apart!

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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Oh boy, I think I'm gonna blow my cannon apart!

Unread postAuthor: jmccalip » Fri Apr 17, 2009 9:36 pm

1lb 11oz aluminum piston! :confused2:

That's what I get for being an impulse buyer and not calculating the weight before hand...


I left the grooves like that in the back for a giant spring to use as a bumper. Disregard the horrid finish of the middle of the piston, I was lazy and using a dull insert and not overlapping, ect. That part doesn't matter anyways.



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Anyone want to buy it for a steel cannon? lol
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Unread postAuthor: Gippeto » Fri Apr 17, 2009 9:48 pm

No thanks.

I like the shape though. :wink:

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Unread postAuthor: c11man » Fri Apr 17, 2009 9:52 pm

to cut down on the weight you could make it more hollow and reduce the diameter of thecenter section
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Unread postAuthor: jackssmirkingrevenge » Fri Apr 17, 2009 10:27 pm

Weight watchers indeed :roll:

Remember, you don't need you piston to travel more than 1/4 of the barrel diameter for maximum theoretical performance, don't give it space to accelerate too much ;)
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Unread postAuthor: Technician1002 » Fri Apr 17, 2009 11:17 pm

c11man wrote:to cut down on the weight you could make it more hollow and reduce the diameter of thecenter section


To cut down on weight I use HDPE. Gippeto used Nylon. The density is less than 1/2 that of Aluminum.
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Unread postAuthor: jmccalip » Sat Apr 18, 2009 12:17 am

So, with one inch of travel, would ya'll think it's safe at say 50PSI? It couldn't gain that much momentum in one inch, could it? I need to find a good bumper in the mean time...

The only thing that worries me is if the tee breaks. All the fittings are glued to that and have a combined cost of $20.
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Unread postAuthor: rna_duelers » Sat Apr 18, 2009 2:48 am

Well make sure you have a bumper stop that's upto the challenge and I'm calling it a challenge for a reason.

I'm with Technician on this one,get yourself some HDPE and get lathing,half the weight and it's got a very low friction.
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Unread postAuthor: sniper hero » Sat Apr 18, 2009 3:43 am

WOW
this is like pilot............................................fire :lol:
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Unread postAuthor: psycix » Sat Apr 18, 2009 5:56 pm

You should definitely take off some weight there!

jackssmirkingrevenge wrote:Remember, you don't need you piston to travel more than 1/4 of the barrel diameter for maximum theoretical performance, don't give it space to accelerate too much ;)

Not in a barrel sealer built inside a T and the porting being not very far into the T, especially with a larger seat diameter compared to piston diameter.
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Unread postAuthor: Technician1002 » Sat Apr 18, 2009 6:09 pm

psycix wrote:You should definitely take off some weight there!

jackssmirkingrevenge wrote:Remember, you don't need you piston to travel more than 1/4 of the barrel diameter for maximum theoretical performance, don't give it space to accelerate too much ;)

Not in a barrel sealer built inside a T and the porting being not very far into the T, especially with a larger seat diameter compared to piston diameter.


I try to have the piston free to travel that distance at least before hitting the bumper and slowing. I tend to design so the full piston stroke is about 2/3 the valve seat diameter by the time it stops in the bumper and begins the recoil.
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Unread postAuthor: jmccalip » Sun Apr 19, 2009 12:34 am

Ok, I made a bumper out of an 5" 80lb compression spring.


But then I did some calculations and the surface area of the end of the piston is approx 7". This at 50psi equals 350lbs of force. I don't know how the weight of the piston itself affects things...


Will the spring pancake?
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Unread postAuthor: rcman50166 » Sun Apr 19, 2009 1:10 am

Well I can only calculate it if I know how long 350lbs of force would be acting on the piston, if it is a continuous pressure I can do that now though....350>80 so...yes it would
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Unread postAuthor: jmccalip » Sun Apr 19, 2009 1:44 am

rcman50166 wrote:Well I can only calculate it if I know how long 350lbs of force would be acting on the piston, if it is a continuous pressure I can do that now though....350>80 so...yes it would


Why does time matter. Wouldn't the force still hit a maximum of 350lbs(for example) no matter what, constant or split second?

Here's a pic BTW.

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Unread postAuthor: Technician1002 » Sun Apr 19, 2009 1:59 am

jmccalip wrote:
rcman50166 wrote:Well I can only calculate it if I know how long 350lbs of force would be acting on the piston, if it is a continuous pressure I can do that now though....350>80 so...yes it would


Why does time matter. Wouldn't the force still hit a maximum of 350lbs(for example) no matter what, constant or split second?

Here's a pic BTW.

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How long the pressure is present affects the duration of the pulse accellerating the piston. In my t shirt cannon, the recoil is fairly mild unless I launch a full Gaterad bottle, then it leaves bruises.

Nice pics. It would be a good idea to fill the end cap with someting not comperessable. If the bumper materal compresses enough so the piston hits the male plug, it may shatter the female adaptor. I learned this the hard way with my apple cannon. It had a PVC adaptor, but I had to change it after it was shattered once. Stay clear of the breech. Sometimes they do blow out.
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Unread postAuthor: Carlman » Sun Apr 19, 2009 3:26 am

isnt that a dwv T?
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