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need advice on floatin o-ring

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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need advice on floatin o-ring

Unread postAuthor: JDP12 » Wed Mar 28, 2012 4:50 pm

So as I was talkin to ramses about machining my bolt, he proposed that I should use a floating o-ring, which i had not thought about. I've never used floatin o-rings so I could use some advice.

The barrel ID is .527". Currently I have .5625" OD o-rings, and thats the closest I can get without going under the barrel ID. my worry is this wouldn't seal properly as it doesn't just slide through the barrel, it compresses some. I know you want some compression, but Im worried this may be too much.

Second question... how wide should the o-ring groove be? The o-rings cross-sectional diameter is 1/16", so I was thinking something like 3/32", which is 1.5 times wider than the cross sectional diameter, but should i go wider to maybe 1/8"?
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Unread postAuthor: Fnord » Wed Mar 28, 2012 5:23 pm

It'll most likely work. When you have nice smoothly machined surfaces and thick grease, orings are quite forgiving. I made a floating oring piston recently, and I had to file the little equalization notch out deeper, twice, and finally go at it with a drill to get to *not* seal perfectly.

If you're careful, you can cut up an o-ring and superglue it back together to make any size you want, but use some sort of magnifying device to be sure you butt the ends together precisely.

The groove dimensions you gave will work. I think mine is about 1.5x the ring width.

It's also ok to go a little under the actual barrel ID, but you'll need to machine the grooves so they stretch the o-ring out.
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Unread postAuthor: dewey-1 » Wed Mar 28, 2012 5:42 pm

page 3483 of McMaster Carr

7/16 O-ring see the actual OD of .566 inch OD.
AS568A-013

Woops .527 barrel not .557 barrel

Possibly look for metric size that will work.

10 mm ID 1.5 mm thick total OD 13 mm = .512 inch
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Unread postAuthor: JDP12 » Wed Mar 28, 2012 6:31 pm

Ok, so right now I drew it out so that the diameter of the o-ring groove is .3775", which is .06" less than the ID of the o-ring. Is this too small? Should I make it just barely smaller than the o-ring or will this suffice?
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Unread postAuthor: Gippeto » Wed Mar 28, 2012 11:22 pm

Your trying to seal the bolt into the barrel?

Do you need the o-ring to function as check valve?

If not, I'd suggest using a 1/2" od o-ring (012) in an oversized groove. For a 1/16" cross section o-ring, I'd likely try a groove diameter ~ .420"

Would think a groove width of .066" would be close as well.

Will give it a go on a piece of scrap tomorrow...see how it measures up. :)
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Unread postAuthor: JDP12 » Wed Mar 28, 2012 11:33 pm

yea i need the bolt to seal into the barrel. It sort of does have to act as a check valve i suppose. Since the air is actually coming into the barrel ahead of the o-ring it has to keep air from going backwards.. dunno if that makes it qualify as a check valve or not.

That was my original plan, but there would probably be a fair amount of friction. And I want the bolt to be extremely smooth in cycling....

Either way, I need to move fast on this. I'm going to take Fnords advice and try to make a couple o-rings of different sizes with superglue.

My main concern is the groove diameter in the bolt. Can anyone think of a good way of increasing the groove diameter if need be? Only thing I can think of is maybe spread a thin layer of epoxy or a layer of tape. I only wanna have to pay to get one bolt machined, and right now i'm leaning towards a groove diameter to allow for floating o-rings.

and that .420" groove diameter was approximately what i had calculated for a normal o-ring setup (non floating)
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Last edited by JDP12 on Wed Mar 28, 2012 11:47 pm, edited 2 times in total.
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Unread postAuthor: Gippeto » Wed Mar 28, 2012 11:44 pm

Uh huh...you don't need a check valve...only a seal.

Floating o-rings are not going to live if you're constantly moving them in and out of the barrel anyway...gravity will drop them down into the "sloppy" groove...and they'll bind and get nicked when you close the bolt.

Will do a test groove on a piece of scrap tomorrow and post the resulting dimensions for you....not exactly a major undertaking. :wink:

Thinking a final od on the fitted o-ring of barrel id + .010" will get the job done with minimal friction if lubricated with silicon grease.

Let me know your thoughts on the matter.
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Unread postAuthor: JDP12 » Wed Mar 28, 2012 11:51 pm

I told ramses (who is machining this for me) that I need 20 hours to figure this out because he pretty much has to get it done this weekend and I don't wanna delay him any more than necessary.


EDIT: my calculations were relying on o-ring compression of 30-40%, which is a significant amount. looks like i'll have to make a custom o-ring, and calculate the groove for say... 10-20% compression? What do you think the minimum amount of compression that I can have and still maintain a seal?

your initial guess of bore id plus .010" seems spot on for a 10-20% compression of the o-ring.
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Unread postAuthor: Technician1002 » Thu Mar 29, 2012 12:42 am

In my air cannons, minimal compression is needed. Simply full contact against a smooth surface is all that is required. Addition of air pressure seats the ring.

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Just enough compression so it does not fall is enough.

My video on the subject....
[youtube]http://www.youtube.com/watch?v=BbDiZiRVrnk[/youtube]
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Unread postAuthor: JDP12 » Thu Mar 29, 2012 10:24 am

Thanks Tech, I'll have to custom make an o-ring now I guess.

Just waiting to hear from Gippeto.
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Unread postAuthor: JDP12 » Thu Mar 29, 2012 12:36 pm

WAIT... ok, so on a whim i decided to measure the diameter of my barrel again.

Turns out its not .527" like mcmaster said, but .500". So.. if I just get an o-ring with OD of .5", should be able to easily make the groove.

Seems like problems solved... guess I should follow the measure twice rule a bit more rigorously..
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Unread postAuthor: Gippeto » Thu Mar 29, 2012 1:26 pm

Measure twice?? Why on earth would you want to do that? :lol:

Might want to measure it in a few places in the event it's actually out of round.

012 o-ring is 1/16" cross section and 1/2" od.


Aside...and on account of "I said I was gonna do it"...

Put that 012 o-ring into a groove with a .420" diameter and near as I can measure without actually starting to compress the o-ring, it measures .542"...still pretty good for a guess "on the fly" I think. :)

See...no custom o-ring required. :wink:
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Unread postAuthor: JDP12 » Thu Mar 29, 2012 1:34 pm

lol yea I have some .5" OD o-rings on the way, time to update the drawings for the umpteenth time for ramses. calculating about 10% o-ring compression.
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Unread postAuthor: JDP12 » Thu Mar 29, 2012 2:21 pm

Sorry for the bump.. but i measured in a couple different spots.. and I've gotten .50, .51, and .52.. lol
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Unread postAuthor: Gippeto » Thu Mar 29, 2012 4:17 pm

Have a vice?

Go slow. :)
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