SpudFiles

Hi,
I've built a few pneumatic guns in the past, but on my most recent one two really bad things happened.
1. I use a bike pump with a schraeder valve, and when i pumped it up, for some reason the chamber exploded straight off it's joints. I was aware how much psi was in it, around 30 psi, and i still don't know why it did that.
2. After repairing where it popped, i heard a weird rattling noise from my chamber, so while the cement was still drying, i pulled out the chamber to look inside, and found a few shards of pvc in there.
If anyone could please explain how this happened, or what i did wrong, please help.

(p.s., when the chamber blew off, it was so loud, i couldn't hear for a while.)

First off this is in the wrong forum.


Your problem is probably caused by a couple things.

1). Not using pressure rated pipe/ fittings. Sch 40 is NOT a pressure rating.

2). You didn't screw in the schrader through a double layer-- pipe and coupling

3). You didn't let the PVC cement fully cure for 24 hours.


4). All of the above

I bet, in fact i know that it was a bad solvent weld. be generous with both the primer and cement and apply the cement while the primer is still wet.

i know i am always anxious to try out a new cannon, but i force myself to wait until its properly dried and i make sure i use pressure rated materials. a good proper solvent weld will not fail before the pipe fails.

Thanks for the help, but I did use pressure rated pipe, which i made sure of. Also the schrader( thnx for correct spelling ) i used was the type that pops in on an indent, so no air leaks out. Oh, and also i made sure i let the pvc cement cure before i used it. I waited a week. ( i know that was alot, but it was because i wanted to make sure and because i was busy). I probobly didn't due a very good weld, and the primer i was using i found in my shed, so it was probably that.

Thnx for the help.

spudkid10 wrote:Oh, and where should have i put this post, just wondering.

Edited by jrrdw.

Please edit posts instead of double posting.

Imthe pneumatic discussion.

Ok. Well the fittings have to be pressure rated as well, not just the pipe.

Also, if you let it dry a week, it wouldn't be wet like you said.

Third, again you have to attach the schrader through a double layer, not through the side of a pipe or in an endcap.

These things don't just blow up on their own, it's usually an operator error.

Also, PVC guns really shouldn't be used under, my personal safety preferance, 45 degrees Fahrenheit

I didn't double post, and the cement was still wet because it was after it blew off and i was gluing it back on

i dont think that "blew up" is the correct term. i just sounds like you have a solvent weld fail. so it more like it popped off.

solvent welding just takes a little practice. like i said, use plenty and dont skimp out. you dont want to over do it and make a mess, but dont try to get a way and save a little.

and also, like you said, its not the best idea to use old pvc cement/primer that you are unsure of.


edit- (not trying to argue, but notice the edit button in the upper right?) i would recommend that you hack off the part that was previously cemented and get a new fitting. pvc is so cheap there is no reason to try and not do a good job.

thanks for the help. Next time I'll make sure to use new cement and do a better job gluing.

Yes you did double post.


Well.. Without pctures it's kind of hard to help you, if you could post some pictures of the damaged section it would be easier.


But it definitely sounds like a poor weld job, old glue can contribute as well, make sure you cleanthe parts really well and use primer/ cement. LIBERALLY

EDIT: dammit hi beat me

Oh.. i didn't clean anything. didn't know thanks a bunch. So should i just use some hot soapy water?

spudkid10 wrote:Oh.. i didn't clean anything. didn't know thanks a bunch. So should i just use some hot soapy water?

like i said, get rid of the old glue and primer (on the pipe, and buy new cement/primer while you are there)

i usually use water and a rag. they sell pvc cleaner, but i have never used it. clean pipe makes a huge difference.

Thanks a bunch i'll make sure to do a better job next time i make a cannon (which is very soon) . I'm making a metered propane combustion soon, i think it will be alot of fun, and i'll make sure to cement is properly. Thank you for all the advice.

PVC cement and primer does expire so be sure to check the dates before using it

And double posting is when you post twice in a row in a topic without someone posting between your two posts. So it's two posts, by the same member, one after another in the same topic. This isn't allowed here at Spudfiles, instead edit your posts or wait several hours before making your second post if no one has replied yet.

Ok. Thanks for explaining that. It won't happen again.

hey, also keep in mind if you are gluing large diameter pipe, some hardware stores only sell glue for up to 2" diameter pipe, i didn't realize that on a cannon with a 4" chamber, the whole chamber blew out and flew about 20 feet into a fence. anyway, read the side of the container, if might have some size limitations