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Screws for Additonal Support

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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Screws for Additonal Support

Unread postAuthor: alkapwn1 » Tue Oct 07, 2008 7:07 pm

Now I've seen some combustion cannons that, Including the basic PVC primer/cement, use small 2 cm (about an inch) wood screws every couple of cm around the piping. Its like a ring around the pipe of screws. They are drilled through the couplers into the piping underneath.

My question is would that increase safety all that much for the entire gun? It would seem that that could possibly let out pressure through the holes that have been drilled into the pipe.
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Unread postAuthor: Floyd » Tue Oct 07, 2008 7:12 pm

It would not be worth it. If anything it would just make it leak easier. (but since it is a combustion it won't affect it all that much.)

PVC Cement is the strongest thing, it basically welds the two together so they are one peice, so screws might help if you really messed up with the cement, but if you took your time with it and was careful when priming and cementing(?) it won't be worth it.
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Unread postAuthor: alkapwn1 » Tue Oct 07, 2008 7:28 pm

I haven't built it yet, so I was just inquiring ahead of time
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Unread postAuthor: Lentamentalisk » Tue Oct 07, 2008 8:34 pm

I have not ever seen what you are talking about. Can you link me to it?

Now it is very common to have screws in a pneumatic with a home made piston for easy servicing, but that is a completely different thing entirely.
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Unread postAuthor: starman » Tue Oct 07, 2008 11:48 pm

I've never seen a combustion (that I can remember) that reinforced a cemented joint with screws.

There are tee piston valves for pneumatics that connect the rear pieces together with several screws instead of cementing for purposes of piston maintenance.

Totally unnecessary and possibly even a weakening feature on properly cemented joints.
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Unread postAuthor: alkapwn1 » Wed Oct 08, 2008 8:58 am

I've seen it once and I'm not sure if they even cemented it... that seems dangerous eh?
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Unread postAuthor: starman » Wed Oct 08, 2008 9:51 am

alkapwn1 wrote:I've seen it once and I'm not sure if they even cemented it... that seems dangerous eh?


Not necessarily. However, you want to avoid drilling holes in your pipe whenever possible. Nothing beats a properly cemented PVC joint.
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Unread postAuthor: psycix » Wed Oct 08, 2008 10:38 am

An o-ring and a set of bolts can already make airtight joints and hold pressure.
Bolts + primer/cement is somewhat overkill, though if the glueing was done so bad that the fittings want to shove out of eachother, the bolts can hold it in, though not stop it from leaking.

Note that I used the word "bolts" instead of "screws". I would suspect screws to leak earlier then bolts.
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Unread postAuthor: jimmy101 » Wed Oct 08, 2008 10:47 am

The screws really won't do anything. Like others have said;

1. The glued joint is more than strong enough for a combustion and screws will not increase the joint's strength.

2. Screws are sometimes used in pneumatics, they seal fine and can handle the much greater forces and length of time that pneumatics use.

3. If done properly, screws will not leak, otherwise they would never be used in a pneumatic. In a combustion, minor leaks are really not significant. (A 3" cleanout plug generally leaks like crazy so minor leaks around screws would be of no consequence.)

It's always good to be thinking safety when designing a gun. In this case, the screws would not make the gun safer. A properly done PVC primed and glued joint is as stong as the pipe itself so there is really no reason to make it stronger.
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