Login    Register
User Information
Username:
Password:
We are a free and open
community, all are welcome.
Click here to Register
Sponsored
Who is online

In total there are 41 users online :: 5 registered, 0 hidden and 36 guests


Most users ever online was 218 on Wed Dec 07, 2016 6:58 pm

Registered users: Bing [Bot], Exabot [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot] based on users active over the past 5 minutes

The Team
Administrators
Global Moderators
global_moderators.png CS

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

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.
Sponsored 
  • Author
    Message

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

Unread postAuthor: mako » Wed Nov 18, 2015 1:16 pm

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?
  • 0

Last edited by mako on Wed Nov 18, 2015 2:05 pm, edited 1 time in total.
“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pnuematic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Wed Nov 18, 2015 1:55 pm

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: pneumatic-cannons/topic12803.html
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Re: Pnuematic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Wed Nov 18, 2015 2:03 pm

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?
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Wed Nov 18, 2015 2:30 pm

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 ;)
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Wed Nov 18, 2015 2:36 pm

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. ;)
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: Gippeto » Wed Nov 18, 2015 9:33 pm

mako wrote: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. :)
  • 0

"It could be that the purpose of your life is to serve as a warning to others" – unknown

Liberalism is a mental disorder, reality is it's cure.
User avatar
Gippeto
Donating Member
Donating Member
 
Posts: 2394
Joined: Sat Jan 19, 2008 10:14 am
Location: The Great White North...Canada eh!
Reputation: 11

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Thu Nov 19, 2015 9:49 pm

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?
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Sponsored

Sponsor
 


Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Fri Nov 20, 2015 5:01 am

mako wrote: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:

Image

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

Image
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Sat Nov 21, 2015 3:06 pm

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?
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Sat Nov 21, 2015 3:57 pm

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.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Sat Nov 21, 2015 8:44 pm

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?
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: Gippeto » Sun Nov 22, 2015 1:50 am

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.
  • 0

Attachments
valve stem typical.png
valve stem typical.png (3.28 KiB) Viewed 1005 times
"It could be that the purpose of your life is to serve as a warning to others" – unknown

Liberalism is a mental disorder, reality is it's cure.
User avatar
Gippeto
Donating Member
Donating Member
 
Posts: 2394
Joined: Sat Jan 19, 2008 10:14 am
Location: The Great White North...Canada eh!
Reputation: 11

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Sun Nov 22, 2015 6:23 am

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.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: mako » Sun Nov 22, 2015 12:58 pm

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.
  • 0

“Whenever you find yourself on the side of the majority, it is time to pause and reflect.”

Mark Twain
User avatar
mako
Master Sergeant
Master Sergeant
 
Posts: 163
Joined: Sun Apr 14, 2013 10:43 pm
Reputation: 2

Re: Pneumatic Pressures: The Safe, The Possible, And The Cra

Unread postAuthor: jackssmirkingrevenge » Sun Nov 22, 2015 6:05 pm

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.
  • 0

User avatar
jackssmirkingrevenge
Donating Member
Donating Member
 
Posts: 24225
Joined: Thu Mar 15, 2007 11:28 pm
Country: Holy See (Vatican City State) (va)
Reputation: 66

Next

Return to Pneumatic Cannon Discussion

Who is online

Registered users: Bing [Bot], Exabot [Bot], Google [Bot], Google Adsense [Bot], Yahoo [Bot]

Reputation System ©'