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Homemade Pump- Floating 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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Homemade Pump- Floating O-ring

Unread postAuthor: saefroch » Wed Jun 09, 2010 9:07 pm

Well I made a thread a while back on my "homemade HPA pump" and now I've made some serious headway on actually building it. In fact, it's almost completed. I've just one problem with the build (the first so far actually, yay!), and that's with the fit of the o-ring. I made the pump head and polished the o-ring seat to ensure that it slides (even lubed the whole thing), but when I put the pump head into the pump cylinder I'm using, it sticks and refuses to move, so much so that I can apply enough force to jam the pump head through the o-ring and separate the two... So I've got a dilemma. The whole pump is built out of steel or brass schedule 40, NPT threads. The pump cylinder I'm using has an ID of .495", while the o-rings I'm using are .5" OD. Is that .005" what's causing the o-rings to stick? Or is it that I need to take a bit more off the seat and allow more space for the o-ring? I'll post pictures in the morning, they'll probably help.

I may also use this thread as a build log/ use log.
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Unread postAuthor: Technician1002 » Wed Jun 09, 2010 11:54 pm

Look up my video on using O rings in air cannons. If the o ring can get between the piston and wall of the cylinder, you have the piston way too small. The o ring should never attempt to squeeze through. The side of the o ring should seal on the flat wall of the groove.

A proper fit piston should have little space for an o ring to escape the grove and go between the piston and cylinder wall. See any space in this piston for an o ring to slide by?
Image
Disclaimer, that piston does not have an o ring, My other valves do.
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Unread postAuthor: saefroch » Thu Jun 10, 2010 5:35 am

Okay, so that fixes half the problem, but why was the o-ring jamming in the shaft? To actually force the head through the o-ring I had to put most of my weight on it... I will replace the pump head with something of a larger diameter though, thanks.
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Unread postAuthor: Technician1002 » Thu Jun 10, 2010 10:47 am

If there is space the o ring being round will tend to roll off off the piston and get caught in the space. Both air pressure and friction tend to start that if there is room.

It is important to ensure the o ring can't wedge between the piston and cylinder wall. Make sure your cylinder is very smooth inside and lubricated so friction is low.
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Unread postAuthor: saefroch » Thu Jun 10, 2010 11:58 am

Ahhhhhh so the o-ring is just jamming between the cylinder wall and the head. Gotcha. Gonna get some 1/2" steel and sand that down, replace my 7/16" head I'm currently trying to use. Thanks. Hope to have the pump working (since everything else works) with pictures up by the end of the weekend (somewhere (I love parentheses)).
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Unread postAuthor: Technician1002 » Thu Jun 10, 2010 12:31 pm

When the o ring doesn't get pinched a 1 inch piston with an o ring has very little friction. I have video of my preschooler grandson pulling the 1 inch QDV at 100 PSI while shooting marshmallows. In a pump, the piston will be pushing air pressure.

This video shows just how little 100 PSI adds friction to a pair of o rings on a 1 inch cylinder. The o rings are floating. The shot is about 100 PSI. A preschooler can easily pull it.

[youtube]http://www.youtube.com/watch?v=FusGJKJ4EL0[/youtube]

My older grandson has better aim and is able to hit the pop can with a marshmallow and send it flying.
[youtube]http://www.youtube.com/watch?v=wsRsVh2lUr0[/youtube]
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Unread postAuthor: saefroch » Thu Jun 10, 2010 1:05 pm

Wow. So I take it that by that response, my tiny diameter difference isn't a problem at all. As soon as I get a piston head of about .2 sq. in. :twisted: up and working, I'll be sure to evaluate the internal friction of the pump... and see how far I can push this thing. I got a gauge that goes to 1,000 psi, and all my parts are steel or brass, so even a measly 131 lb. adolescent should be able to get some really high-pressure shots (at least 600 psi).
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Unread postAuthor: Technician1002 » Thu Jun 10, 2010 1:11 pm

You have to watch the amount of volume in the cylinder at the end of the piton stroke. The volume of air compressed between the piston and check valve will re-expand to it's original size when the piston comes back. If the volume is large, the compressed air may fill most of your stroke distance with re-expanded air and you will pump very little. This is often overlooked on high pressure single stage pumps.
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Unread postAuthor: saefroch » Thu Jun 10, 2010 1:26 pm

And that's called dead volume, right? I have gotten mine to be pretty small, but a possible addition of even some modeling clay could cut that down pretty small. I'll measure that by water displacement if I get my hands on a gram balance or a mini graduated cylinder. Thanks for the advice, that may actually be my pressure limitation right there.
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Unread postAuthor: Technician1002 » Thu Jun 10, 2010 1:45 pm

Yes, dead volume. I'm glad you are understanding it. This is why most compressors for efficiency are limited to about 10X per stage. 1 ATM (15 PSI absolute) provides a practical limit of about 150 PSI for a single stage shop compressor. Multi stage with successively smaller displacements take that up about 10X per stage max.

1 stage - 100-150 PSI
2 stage 500 - 1200 PSI
3 stage 2,000 - 5,000 PSI

Fridge pumps with small displacement and dead volume for efficiency are sometimes pressed to high pressures.
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Unread postAuthor: ramses » Thu Jun 10, 2010 3:12 pm

Here is the pump calculator I wrote a while ago. It makes some epic assumptions(constant temperature), and no one has ever actually checked it. You need to mash the "recalculate" button to get accurate-ish results, since the spreadsheet was made using circular references, which is pretty poor form.
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