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Catastrophic failure

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Catastrophic failure

Unread postAuthor: Mountain Storm » Sun Oct 24, 2010 5:30 pm

I wondered what part of the Spud Chucker would fail and it was the 1.5" to 3 flush fit adapter that blew out finally. Right when I was testing a finned projectile that worked amazingly well. Guess that near perfect seal in the bore and the fresh paint on the projectile made for some maximum pressure :oops:

Nice to know that it didn't explode into shards fragging me out.

Anybody know a better PVC plumbing part than the slip fit bushing for adapting 3" to 1.5"?
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Unread postAuthor: Technician1002 » Sun Oct 24, 2010 5:38 pm

I presume it is this launcher;
Image

It should have held unless it was not glued properly. Was fresh primer and the right glue used?

Post photos of the failed connection.
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Unread postAuthor: Gun Freak » Sun Oct 24, 2010 6:32 pm

This should not have happened! A properly glued joint will not fail before the pipe. That is a given.
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Unread postAuthor: jrrdw » Sun Oct 24, 2010 6:46 pm

Is some 'Solvent Welding' 101 needed? A really good how to in the Wiki, button at the top of the page...

A perfect example to pratice spudding safty! :wink:
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Unread postAuthor: ThornsofTime » Sun Oct 24, 2010 6:55 pm

Yeah shouldn't have happened... but lucky that it failed at a fitting and didn't burst a pipe... pvc doesn't show up on an x-ray...
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Unread postAuthor: Gun Freak » Sun Oct 24, 2010 6:58 pm

ThornsofTime wrote:pvc doesn't show up on an x-ray...

Yeah it does :D
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Unread postAuthor: Mountain Storm » Sun Oct 24, 2010 8:35 pm

The glue joint held. The Reducer failed. It split. The 1.5" pipe is still glued to it just fine.

I just spent a couple of hours browsing McMaster and the price to build a 3" x 1.5" of the same proportions out of Schedule 80 is almost $100 :cry:

I think I will just get another schedule 40 reducer from Lowes and reinforce it with epoxy. Note in the picture posted above the 4 webs in the reducer bushing. I could just fill the 4 voids between the webs with epoxy and that would probably hold fine. Beats dropping a Franklin on all new parts. I'll just lop off the straight coupler on the front and have a 2" shorter chamber no biggy.

I think I will go for a longer barrel though.

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Last edited by Mountain Storm on Sun Oct 24, 2010 8:57 pm, edited 1 time in total.

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Unread postAuthor: Lockednloaded » Sun Oct 24, 2010 8:56 pm

I just did a quick search on mcmaster, and for the same setup you had before, in sch 80, it'd cost 26.30. 4881K77 and 4881K662
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Unread postAuthor: Mountain Storm » Sun Oct 24, 2010 9:02 pm

Thanks. I ran across those parts in my searching. But I can probably repair the cannon for less with some casting resin. I happen to have it in the shop...just mix it up and pour it in the voids and that would reinforce the weak spot in the fitting.
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Unread postAuthor: Technician1002 » Sun Oct 24, 2010 9:24 pm

This is a glue failure. One side of the glue joint let go. This allowed barrel wobble which cracked the other side.

The reducer has only 4 webs inside that are very thin. This part might be DWV and not sched 40. The bevel inside the chamber shows this may be a DWV part as most sched 40 parts do not have a bell reducer shape inside.

For added strength for barrel support, it may be advisable to use two reducers instead of just one. Go from 3 inch to 2 inch then down to the barrel size. This will spread the strain of barrel whip over a larger area.
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Unread postAuthor: Mountain Storm » Sun Oct 24, 2010 9:46 pm

Where do you see a glue failure? The crack is on the bevel. The glue joint shows no scorching. Just to clarify the failure I detected (by hearing escaping combustion) was a small split. I broke the barrel off later by hand, pulling apart the bits still holding it together so I could better ram the projectile out of the bore. The failure did not actually send the barrel flying off the cannon. It just split the reducer where you see the scorch marks.

Anyway, I agree that the part is not pressure rated. One of these days when I step up to MAPP I'll build a stouter cannon with schedule 80 parts. For now I'll just repair it. I have not found a different type of reducer locally. I like your idea of using two reducers. That would be in my budget. I'll see what I can find at Lowes tomorrow. Thanks.
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Unread postAuthor: MrCrowley » Sun Oct 24, 2010 10:23 pm

This part might be DWV and not sched 40

I'm assuming that's a slip up because Sch.40 can be DWV :wink:

One of these days when I step up to MAPP I'll build a stouter cannon with schedule 80 parts

You can buy Sch.40 pressure rated parts and they are dirt cheap for you guys in America. I can't tell whether that is DWV or pressure rated (pressure rated will have NSF-PW written on it), but with a properly glued joint and pressure rated parts, the fittings shouldn't fail even with MAPP metered cannons.

Read this if you haven't already.
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Unread postAuthor: motorfixer1 » Mon Oct 25, 2010 5:20 pm

Flex PVC .com sells a 3"x2" reducer bell sch 40 p.r. with a 2"x1.5" bushing you would be good to go and safe to boot. I used this exact arrangement on my metered mapp
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Unread postAuthor: Mountain Storm » Mon Oct 25, 2010 9:35 pm

Nice looking cannon there. Thanks for the info.
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Unread postAuthor: Moonbogg » Tue Oct 26, 2010 1:23 am

Sorry to see your failure. Glad you weren't hurt. Fact is, people rarely get hurt when these things break from what I understand.
I read that you were wondering what part would break first. I share this same curiosity about my own favorite launcher, my venom cannon. I would love to have enough money to replace it easily so that I could test the thing to failure. Would it be the cleanout plug? The welded joints? Who knows.
Not trying to hijack you here by the way. I have seen several videos and photos of broken PVC cannons and it could just be so many things. Tech is likely on to something regarding an area not being glued. But maybe it was just a crack in the plastic. Who knows. Get back on that horse baby! Fix her up and wear eye protection.
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