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U-cup seals that are used for pneumatic cylinders seal in one direction. I thought about using them for a hybrid piston. I could fill the pilot and it would (hopefully) push the piston against the barrel, and then the chamber and pilot would equalize due to air being able to move past the unidirectional seal. When the cannon is fired, the seal activates and pushes the piston back. When the chamber neutralizes, the compressed air in the pilot will escape around the seal, leaving the pilot neutral and atmospheric again. What do you think?

This would eliminate the need for any other method of dumping the pilot pressure after each shot. The seal acts as a check valve for the pilot.

Should work...of course, a floating o-ring will also work.

I have minor doubts as to whether it'll bleed the pilot completely, but it should be nearly so if not.

Have used both in pneumatics with success.

Edit...added pic

I attempted to use one of these in a previous piston valve and can tell you there are a few issues I had with this.
1) You need a very robust material to make the valve with just a u-cup shape
2) It is very difficult to make the u-cup seal to inside a cylinder
3) It is very difficult to make the u-cup open in the first place

The design that I came up with to avoid these issues (but failed to implement) would use a rod with a flexible, almost diaphragm-like seal on it, and a large-diameter stop on one side. Gases, in theory, only flow from the side with the stop on it. That designs should be less prone to failure.

If you can make it work, more power to ya!

Gippeto wrote:Should work...of course, a floating o-ring will also work.

I have minor doubts as to whether it'll bleed the pilot completely, but it should be nearly so if not.

Have used both in pneumatics with success.

A floating o-ring seals from both sides doesn't it?

Nope...works like a check valve...at least the way I build 'em.

Groove as usual, and then drill perpendicular into the piston such that the produced hole is half in...half out of the groove and slightly deeper than the groove...on the chamber side.

Easy stuff.

Edited for shpallunk

Wouldn't you have to spring-load it to make it shut again? Or would you just rely on the o-ring deforming to allow air to pass?

Moonbogg wrote:
A floating o-ring seals from both sides doesn't it?

Unless one side of the o ring gland is grooved to prevent a seal.

As far as u cup seals go, the traditional material used for a century has been leather.

saefroch wrote:Wouldn't you have to spring-load it to make it shut again? Or would you just rely on the o-ring deforming to allow air to pass?

I was thinking that by filling the pilot, the volume of incoming air would be sufficient to close the piston against the barrel. once that happens, the barrel will seal with the building chamber pressure. I think I answered your question...not sure though.

@Gippeto

Thats a good idea. I like it. I think I understand it anyway. The hole can be small so that only a little will pass through, allowing the piston to still shut.

I was referring to the floating o-ring design, not the u-cup. I understand how the u-cup works, just not how a floating o-ring could open and close just under pressure changes without the o-ring deforming.

Floating O-ring is starting to sound much more appealing to me. It just seems much simpler and more reliable. Thanks guys for the input. I would hate to build this thing and then realize there was an easier and better way (wouldn't be the first time).

EDIT: @saefroch

I think I get what you're saying. I wonder the same thing about the floating o-ring. It seems that some pressure would have to sneak past the under side of the o-ring as it pushes the o-ring against the walls, forming the seal.

Do you need seals at all? Assuming your piston is just a tight fit, wouldn't the sudden pressure rise push the piston back anyway before it had time to equalise? Unless of course the mix behind the piston ignites too, hmmm...

Moonbogg wrote:Floating O-ring is starting to sound much more appealing to me. It just seems much simpler and more reliable. Thanks guys for the input. I would hate to build this thing and then realize there was an easier and better way (wouldn't be the first time).

It is simple and reliable, but if you have doubts, it really doesn't get any simpler than a ucup seal does it? Just a groove. The ucup in the coax has given me zero problems even at 1500psi...but then the only heat I'm dealing with is from compression heating of the chamber and pilot volumes while filling. It's a buna ucup if it matters, other materials are readily available from your local purveyor of seals. Only real downside with a ucup, is that it does take SOME differential pressure for gases to get past the seal. Much like a spring loaded check valve.

The floating o-ring will be open with ANY differential that overcomes the friction of the o-ring on the cylinder wall...not a heck of a lot.

EDIT: @saefroch

I think I get what you're saying. I wonder the same thing about the floating o-ring. It seems that some pressure would have to sneak past the under side of the o-ring as it pushes the o-ring against the walls, forming the seal.

The o-ring should be making "good" contact with the cylinder wall when things are "at rest". it doesn't take much for a lubricated o-ring to seal. As pressure differential increases, the pressure does force the o-ring against both the cylinder wall and the low pressure side of the groove.

Watch your clearances, or you WILL have extrusion resulting in jamming/high friction. This will show up on your o-rings over time as obvious damage. Done with a degree of care, it presents no problems, and works REALLY well.

Gippeto wrote: The o-ring should be making "good" contact with the cylinder wall when things are "at rest". it doesn't take much for a lubricated o-ring to seal. As pressure differential increases, the pressure does force the o-ring against both the cylinder wall and the low pressure side of the groove.

A video on this subject is here. With no pressure the piston moves very easy and is shown in the video. The ability to seal and hold air works well. It is important that the o ring seals against a smooth surface. In the cannons in the video the inside of the pipe used for the valve was sanded smooth and shiny.
[youtube][/youtube]

Edit;

Adding a link to a thread on using o rings.
http://www.spudfiles.com/forums/how-to- ... rt,15.html

Do you need seals at all? Assuming your piston is just a tight fit, wouldn't the sudden pressure rise push the piston back anyway before it had time to equalise? Unless of course the mix behind the piston ignites too, hmmm...

Actually it can make a difference. When I built the first version of a hybrid (that was never posted here), I used a 1.6 x 2" rubber cylinder that I had slowly worked to size with a belt sander. Fit was pretty near airtight with grease.

When I did the first few test fires at 4x nothing happened. I figured the mix was off until I noticed the piston bulging scarily out of the 1" barrel port for a few seconds. I cut a groove for a floating oring and it never happened again.