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What am I doing wrong here?

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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What am I doing wrong here?

Unread postAuthor: maverik94 » Sun May 09, 2010 4:25 pm

Ok, my latest project is building an in-line (non-coaxial) piston valve. I have it completely built, except for one problem...it won't seal. It's essentially this design.
Here is a picture of the sealing face of the piston:
Image
(Yes, i used the macro setting for this picture, but it still didn't turn out too well, sorry.
And here is a picture of the area the piston is sealing against:
Image
My question is, why isn't it sealing? Any ideas?
Thanks!
If you need any more info or pics, please don't hesitate to ask!
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Unread postAuthor: Gun Freak » Sun May 09, 2010 4:35 pm

Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?
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Unread postAuthor: maverik94 » Sun May 09, 2010 4:46 pm

Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?
That's just Vaseline on the rubber, and the seat is prefabricated. It's a brakeline fitting actually. i mean, i suppose can take a look at that seat and see it's nice and clean.
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Unread postAuthor: Gun Freak » Sun May 09, 2010 5:21 pm

yeah try cleaning averything off.
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Unread postAuthor: reedfe » Sun May 09, 2010 6:23 pm

Gun Freak wrote:Well the rubber looks like it has something on it, and maybe the seat isn't cut evenly?


yes it looks like glue but then again you said Vaseline and i recommend the same thing. also your sealing face looks almost as if it would slide into that silver busing in there. but i can't tell from the pictures.
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Unread postAuthor: jrrdw » Sun May 09, 2010 6:59 pm

The seal looks a little off center, witch can allow the piston to slip in a little to the side thus loosing the sealing area. That's my guess. If the center bolt where big enough around to keep the seal centered it might seal...
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Unread postAuthor: Technician1002 » Sun May 09, 2010 7:37 pm

Any distortion of the rubber may wrinkle the surface preventing an initial seal. I found wetting the rubber with a water based marker (coloring marker) and quickly assembling it will show spots that are not touching. Proper use of a valve lapping compound can often fix some surface flaws. Low speed on an electric drill and time can wear the surfaces together in a perfect seal. I hand lapped the valves on my Mouse Musket and Dragon cannons to get them to seal.
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Unread postAuthor: mark.f » Sun May 09, 2010 9:37 pm

How close does the piece holding the rubber seal on fit inside of that sealing face? Can't really tell what it is (by the way, your camera was working fine, it just focused on the top of the bolt. :wink: )

If it's a close fit the piston can turn slightly askew in it's track and the washer/nut/whatever holding the rubber on can bind on the sealing face.
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Unread postAuthor: Crna Legija » Mon May 10, 2010 12:54 am

maybe the air is going between the bolt and rubber
how did you attach the nut to the barss fitting mabe going that way
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Unread postAuthor: Technician1002 » Mon May 10, 2010 1:23 am

Just a quick note on Macro photography with autofocus digital cameras, use the half press to hold the focus (locks) and then re-adjust the distance to focus on the portion you want sharp and finish taking the picture. I took pictures of my valve inside pipe that way.

I moved back and let it focus on the end of the pipe, then locked focus (shutter 1/2 press) and moved down to focus the seat and took the picture.

Image
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Unread postAuthor: maverik94 » Mon May 10, 2010 6:00 pm

also your sealing face looks almost as if it would slide into that silver busing in there.

No it doesn't, it sits on that..it's just not to scale
The seal looks a little off center, witch can allow the piston to slip in a little to the side thus loosing the sealing area. That's my guess. If the center bolt where big enough around to keep the seal centered it might seal...

Nope, it's on center, and it's tightly on there
Any distortion of the rubber may wrinkle the surface preventing an initial seal. I found wetting the rubber with a water based marker (coloring marker) and quickly assembling it will show spots that are not touching. Proper use of a valve lapping compound can often fix some surface flaws. Low speed on an electric drill and time can wear the surfaces together in a perfect seal. I hand lapped the valves on my Mouse Musket and Dragon cannons to get them to seal.

Ok, I'll give that a try.
How close does the piece holding the rubber seal on fit inside of that sealing face? Can't really tell what it is (by the way, your camera was working fine, it just focused on the top of the bolt. )

The rubber sits on the sealing face, not inside it.
maybe the air is going between the bolt and rubber
how did you attach the nut to the barss fitting mabe going that way

It's pretty tight, and it is attached by a ground-down nut.
Just a quick note on Macro photography with autofocus digital cameras, use the half press to hold the focus (locks) and then re-adjust the distance to focus on the portion you want sharp and finish taking the picture.

Yeah...I did that, but it was still giving me crappy results. oh well.
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Unread postAuthor: maverik94 » Mon May 10, 2010 6:01 pm

Edit: Dammit. Sorry, didn't mean to double post. Sorry :?
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Unread postAuthor: mark.f » Mon May 10, 2010 7:25 pm

maverik94 wrote:The rubber sits on the sealing face, not inside it.


I know that, I was asking about the part that holds the rubber on your piston. How close of a fit is it to your sealing face/barrel port?
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Unread postAuthor: maverik94 » Mon May 10, 2010 8:00 pm

Its a ground- down nut. It fits well..not tightly, but not overly loose.
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Unread postAuthor: mark.f » Mon May 10, 2010 8:06 pm

And how is the fit of the piston in the pipe it slides inside of? A loose fit or short piston (the part that contacts the pipe walls) could cause the piston to "wobble" and that ground-down nut to bind on the sealing face.

If this seems to be the case, you could improve the fit of the piston or increase the length that contacts the pipe it slides inside of. Another potential solution it to grind the nut into a cone shape so that it won't bind on the sealing face and will instead center the piston on it.
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