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Determine o-ring diAmeter?

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Determine o-ring diAmeter?

Unread postAuthor: JDP12 » Sun Jan 17, 2010 11:32 pm

Hey--

I'm working on improving my combustion gun vastly and one thing I need is a truly gastight projectile- which means o-rings similAr to jagerbonds UHMW rounds.

I'd like to not say why at the moment but basically say I have 2" diameter PVC how would I determine what o ring OD would give a seal- how much "squeeze" should the o- ring undergo in essence.

I know some of you might me wondering what gun I'm referencing, but you won't find it anywhere, mostly because I don't post most of my projects up here-- because i'm not satisfied and they get stripped down again- currently i've built 6 or 7 guns but only have one still operational-- and if I can figure this one out it will be posted.


So sorry for a super long probably superfluous explanation.

Thanks for the help-

JDP
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Unread postAuthor: inonickname » Sun Jan 17, 2010 11:56 pm

Very little. The friction you will suffer will probably hurt your performance and you'll be better off with a high tolerance projectile, or machine a small lip of plastic to help seal (at the back of the projectile).
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Unread postAuthor: JDP12 » Sun Jan 17, 2010 11:59 pm

Ok-- wellthe reason I need it to be gastight isn't so much for performance of the projectile, but I can see how it would hurt it.

May have to rethink my design a bit
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Unread postAuthor: Gippeto » Mon Jan 18, 2010 12:13 am

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Unread postAuthor: kenbo0422 » Sat Jan 23, 2010 12:40 pm

Standard 'rule of thumb' for o-ring compression:

30% compression for moving parts, 40% compression for non-moving parts.

Stretch:

1-5%, ideally at 2%

Or you can free float an o-ring by letting the OD just hug the cylinder wall and let pressure force it shut against the wall and the face of the projectile and the ring it is slotted into.
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