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Ok, tomorrow, I'll buy some steel parts for my pneumatic, and I was
wondering, how much psi will the cannon take?
(Tell me if i can improve something..)

Steel is overkill for this design since the performance will be limited by the ball valve. For this setup, just use PVC and save yourself a bunch-o money.

With the money you saved get a better valve. Even a $15 sprinkler valve will be better than the ball valve.

Do you have a source of compressed air that is greater than ~150 PSI? Most shop compressors are limited to about 120~150 which is well within the range of PVC so steel isn't needed. If you have a source of higher pressures (CO2 tank, SCUBA air tank etc.) then steel is needed. But with the ball valve the performance will still suck, considering how much $$$ it'll cost for the rest of the gun.

elad311 wrote:Ok, tomorrow, I'll buy some steel parts for my pneumatic, and I was
wondering, how much psi will the cannon take?
(Tell me if i can improve something..)

Are you going to simply thread this together using all pre-cut (factory) threads?

If so, what is the rating on the lowest rated part? That is your answer.

I wouldn't worry about it up to around 500psi.(depending on your valve of course.) (but that's me, not you )

Suggestions;

You could go over/under if you wanted to shorten things up.

Use a couple of 90 degree elbows from the chamber and into the ball valve.

Leave the barrel attached to the ball valve, or use a union to mount the barrel for a simple breech loader.

jimmy101 wrote:Steel is overkill for this design since the performance will be limited by the ball valve. For this setup, just use PVC and save yourself a bunch-o money.

With the money you saved get a better valve. Even a $15 sprinkler valve will be better than the ball valve.

Do you have a source of compressed air that is greater than ~150 PSI? Most shop compressors are limited to about 120~150 which is well within the range of PVC so steel isn't needed. If you have a source of higher pressures (CO2 tank, SCUBA air tank etc.) then steel is needed. But with the ball valve the performance will still suck, considering how much $$$ it'll cost for the rest of the gun.

But i'm gonna make a trigger system for it, and it should open it very fast, right?

i would highly reccomend a QEV valve or a Piston valve, they outperform ball valves massively.

But i'm gonna make a trigger system for it, and it should open it very fast, right?

A modded sprinkler valve open a lot faster then a "modded"ball valve...


Edit: Alihisgreat posted 10 sec before me

elad311 wrote:But i'm gonna make a trigger system for it, and it should open it very fast, right?

Perhaps 50 milliseconds, possibly a little less with a strong spring mechanism
Even an unmodded sprinkler valve is down at 10 or 15 milliseconds.

Given that projectiles are usually in the barrel for only a couple of dozen milliseconds at the outside, a spring loaded ball valve cannot open fast enough to provide enough power.

To elaborate on what the people above have said, seriously, a ball valve is a waste - and if you're asking about how much pressure it can take... why waste what pressure you put in by using an inefficient valve?

I have a cannon that uses 300 psi, and it has a piston valve.
If I were to give it a sprung loaded ball valve instead with no other changes, I'd be looking at less than a quarter of the power, at best.

Put it another way, with the piston valve, it performs better at 60 psi than if it had a ball valve and 300 psi. (It does with 15 psi what my first cannon couldn't do with 120psi!)
Given I can get 4 or 5 times the energy by using a better valve, and for no more pressure input, the ball valve seems laughable as a concept.

I don't really know about piston valve, i need to buy it, or i need
to make it?
and, can you give me a picture?

If I were you, (assuming you live in the US) I'd go for a sprinkler valve. In the UK, go for a QEV. The mechanisms of both are fairly complicated, but are explained here. I wouldn't expect you to understand straight off.

Of course if you can't get either of these valve types, by all means go for a ball valve. If you have it non-handheld, so that you can hit the valve open with your fist it will be much more powerful.

The ball valve will connect fine to PVC, just by screwing it onto the threads on the plastic (so long as they are the same size)

Sprinkler valves are the best balance of simplicity and power, imo.

search for the part #6646K23 at McCaster-Carr. It says it is rated to 125psi, but I have used it at 600psi without any signs of deterioration or failure.


edit:
Here it is. Believe me, the QEV really helps. I shot a piece of wood at 5/6 the speed of sound...
http://www.spudfiles.com/forums/pics-up ... 14547.html