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This is going to seem kind of ironic...help with o-ring size

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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This is going to seem kind of ironic...help with o-ring size

Unread postAuthor: JDP12 » Fri Dec 09, 2011 4:49 pm

So despite having written an o-ring calculator... that is for "stretched" o-rings, and not floating o-rings, as far as I know itd be hard to determine a size for floating o-rings via calculation..

I need to make a piston to seal in a .957" diameter pipe.. This is for a floating o-ring, barrel sealing piston. I was thinking of using 1/8" cross sectional diameter nominal, .139" actual width..

Problem is in determing the OD of the o-ring. Right now its either 1.012", 1.074", or 1.137"..

Which should I use for proper squeeze? I have to order them, so I can't size them up by hand..

Thoughts are appreciated, thanks.
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Unread postAuthor: Hawkeye » Fri Dec 09, 2011 5:39 pm

Why do you have to order them? Any home improvement or Auto supply store has all kinds of o-rings.
I will say that a floating o-ring on a piston valve seems to largely be a waste of time.
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 5:41 pm

a floating o-ring is not a waste of time at all... why would you say that?
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Re: This is going to seem kind of ironic...help with o-ring

Unread postAuthor: velocity3x » Fri Dec 09, 2011 6:07 pm

JDP12 wrote:Problem is in determing the OD of the o-ring. Right now its either 1.012", 1.074", or 1.137"..

Which should I use for proper squeeze?


I think they're all much to large. Without consulting the Machinery Handbook, I'd guess that you should have no more than .010 squeeze. Just a guess on my part.

EDIT....Maybe this will help:
http://www.allorings.com/gland_dynamic_reciprocating.htm
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Last edited by velocity3x on Fri Dec 09, 2011 6:10 pm, edited 1 time in total.
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Unread postAuthor: Gun Freak » Fri Dec 09, 2011 6:09 pm

I agree with velocity, I'd say they're all too big.
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 6:14 pm

hm thats helpful! May have to create a program to calculate size of floating o-rings...
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Unread postAuthor: Hawkeye » Fri Dec 09, 2011 6:58 pm

The o-ring really needs to be the actual id of the tube. A floating o-ring in the piston seems to be a waste of time because whatever gain is had by the o-ring sealing the pliot area upon venting seems offset by friction.
Real world comparison between two pistons, one with a leak and the other with a floating o-ring seems to be a splitting hairs comparison.
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Unread postAuthor: Gun Freak » Fri Dec 09, 2011 7:00 pm

Floating o-rings, I always thought, were very low friction, compared to tight rings?
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 7:03 pm

floating pistons allow less friction as you aren't forcing the o-ring into the pipe prestretched for the seal, the seal is achieved when air rushes in and forces the o-ring up against the tubing..which is why I want to use floating o-ring, and to be honest I don't know why they aren't in more prolific use nowadays.. although they do seem to be gaining popularity

see here.. http://www.spudfiles.com/spud_wiki/inde ... matic1.jpg
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Unread postAuthor: Hawkeye » Fri Dec 09, 2011 7:04 pm

Under pressure they balloon out though. An o-ring that will slowly fall through a tube on its own will seal tightly under pressure.
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 7:05 pm

you still want to have an o-ring that is a semitight fit in the tube though for floating o-rings.

I'll prolly just wait til I get to go home and try and find an o-ring there that will do the job instead of trying to find one online.
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Unread postAuthor: Hawkeye » Fri Dec 09, 2011 7:12 pm

No, it only has to brush the sides. It does not have to be semi-tight. Have you actually used them or are you spouting theory? It doesn't really matter whether the seal it from a stretched o-ring or a floating o-ring under pressure when it comes to the seal.
The only difference is that the floating o-ring is designed to be able to move back and forth in the groove.
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 7:21 pm

I'm going off of experience from one valve I've built and theory from Technician. At first I tried semi-loose o-rings but they didn't seal.

It could have been because they were too hard however.

So Hawkeye it seems you have a bit more experience with floating o-rings.. just get one that brushes the sides of the tube is your recommendation?
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Unread postAuthor: CS » Fri Dec 09, 2011 7:32 pm

In my experience, at the hardware store, the dimensions given for o-rings are relative - not spot on. You would need to order a variety of sizes, perhaps one that is stated a few thousandths under will actually be the one to work. One other option is getting the o-ring with the smallest tolerance over what you need, and then sanding... not fun.

To test fitting, the o-ring should fit in there with a small amount of pressure and no deformation from a perfect circle. For instance if it is to large it will be wavy, etc.

edit:emphasis on no deformation and small
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Unread postAuthor: JDP12 » Fri Dec 09, 2011 7:43 pm

Yea.... i wish mcmaster sold o-rings in packs of less than 100.... sucks.

I think I'll try my local hardware store first before trying to order some online.
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