Pneumatic Pressures: The Safe, The Possible, And The Crazy

[mako]

Hey, guys; I've been playing around with pipes, o-rings, solder, the Internet, and the GGDT trying to figure out how to get maximum ft/lbs from pressurized-gas propellants. As far as I can tell, with standard pressures (800 to 2,000 PSI), the velocity is limited by the expansion speed of the gas in atmosphere. (I'm ignoring vacuum-barreled guns for practicality) In practical terms, it's also limited by the volume of pressurized gas available.

With standard pressures, it seems you have to choose between high-velocity/low-mass shooting, or low-velocity/high-mass shooting. In both cases, it's a trade off. The low-mass rounds have limited ft/lbs of force, but the high-mass rounds don't go fast enough to carry far. Of course, the high-mass rounds could be moved up to higher velocity by a greater volume of propellant...

So, mostly out of curiosity, I'm wondering what you guys' thoughts on pressures are. Assuming a machine shop, machinist's knowledge, and good materials, what kind of pressures are achievable? What's the limit of safety with pressures? And at what point does higher pressure require insanity?

[jackssmirkingrevenge]

Most commercially available pneumatic rifles operate in the 3000-4500 psi range, this is also the fill pressure of diving tanks and paintball bottles and is a sensible range to work with.

Have a look at Gippeto's project to see what's possible in terms of high ft/lbs in a rifle-sized package: http://www.spudfiles.com/pneumatic-cann ... 12803.html

[mako]

Thanks for the link! Damn, that thing looks terrifying.

Anyhow, in YOUR opinion, given the right materials and tools, what's the limit on sane pressures?

[jackssmirkingrevenge]

Hydraulic fittings are commonly rated to 8,000-10,000 psi, that would be the "sane" limit to me - but then you're going to have to figure out how to pressurize gas to those levels - unless of course you do the sensible thing and go hybrid

[mako]

Really? That's an impressive amount of pressure to handle. Do you think a hammer valve be used with those levels of compression?

I'm mostly just exploring the theoretical limits of pressurized-gas propulsion. I'll probably venture out into hybrids at some point.

[Gippeto]

Do you think a hammer valve be used with those levels of compression?

Yes.

Just need to get it open...smallish diameter at the effective seal, heavy strikers, big springs, counter balanced valve...combination of all. Imagination is the major limitation in airgun design.

[mako]

It'd have to be a pretty small valve, right? At 8,000 PSI... that's what, over 1,500 pounds of force on a 0.25 inch diameter valve face? You'd need a heck of a hammer, not to mention a pretty sturdy valve stem.

Counter balanced? How do you counter balance a hammer valve?

[jackssmirkingrevenge]

It'd have to be a pretty small valve, right? At 8,000 PSI... that's what, over 1,500 pounds of force on a 0.25 inch diameter valve face?

That's why you counter balance the valve, to negate this pressure - have a look at Brian's CBHV:



With a perfectly balanced valve all you would have to overcome is the friction from the o-rings.

[mako]

Oh. Well, this is one of those "duh" moments...
I've seen that schematic somewhere before, but didn't realize that's what it did. As usual, Brian's solution is elegant and simple.

Question: at 10,000 PSI pressures, would delrin on metal (or brass on steel) sealing faces be necessary? It seems like that kind of pressure would destroy o-rings. Or are they tougher than I think?

[jackssmirkingrevenge]

o-ring bible, you can certainly have dynamic o-rings at those pressures but you have to pay attention to the tolerances.

When you're dealing with 800-4500 psi pressures on airguns though, metal on delrin seals are certainly the most common.

[mako]

Well,THAT page is certainly going in my bookmarks! I much prefer to use o-rings where possible.

What kinds of pressure do you need for a metal on delrin seal to function properly? I'm operating on the assumption that the delrin deforms just enough to make a perfect seal against the metal, right?

[Gippeto]

I use an undercut on the delrin sealing face. Have had delrin seal to brass (flat faced) valve bodies as low as 100psi.

Actual radius of the undercut is unimportant. I face off to leave the flat sealing edge ~.020" wide.

[jackssmirkingrevenge]

As Gippeto noted it's important to have a relatively small contact area between the delrin and metal faces in order to maximize pressure and seal at lower pressures. The t-shirt launcher I posted recently has an aluminum piston sealing on a delrin face and it does not seal completely until it reaches 200 psi or so.

[mako]

I thought high pressures (relatively) might be needed for a good seal. I'm impressed by the 100 PSI sealing, Gippeto; that's lower than I expected possible. The undercut idea is genius; less surface area on the actual sealing face, but more surface area to present to the pressure chamber, right?
Not quite sure how to interpret your schematic, though. Which lines are which part? Or is it all one part (a delrin block
attached to the valve stem)?

jackssmirkingrevenge wrote:
aluminum piston sealing on a delrin face and it does not seal completely until it reaches 200 psi or so.

How does that affect re-sealing after firing, JSR? Does the 'slam' of the piston into the delrin face as it closes allow it to compress the delrin and seal properly? It seems like would be hard on the delrin. Or is the stuff tougher than that? I confess, I've not had a chance to use any, yet.

[jackssmirkingrevenge]

Delrin's great stuff, it's easy to work but very strong. It doesn't need to "slam" to seal, but a rapid increase in pressure does help it to reseal better.