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Schrader question

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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Unread postAuthor: Gippeto » Tue Sep 09, 2008 11:15 am

I can't say for sure if anyone else has actually tested the cracking pressure of a schrader with reasonably accurate equipment.

I have. :) (6" test gauge with parallax dial, calibrated within .25 of 1%)

It took 70psi to open the schrader using pressure alone.

This would mean that there would be 70psi less in the chamber than in the pilot area. :(

It also means that you have to vent a lot more pressure (volume) before the piston will actuate. ie. you will need a larger pilot valve.
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Unread postAuthor: jackssmirkingrevenge » Tue Sep 09, 2008 12:26 pm

Interesting bit of R&D, particualrly on a personal level considering that my shock pump is set in such a way not to physically open the shrader valve.

Perhaps this makes the case to use presta valves in the case of mini-piston launchers? That, or tampering with a schrader's return spring.

*reads through post before pressing "submit"* I'm either getting more eloquent at PUI, or I could use another beer :)
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Unread postAuthor: PotatoNick » Thu Sep 11, 2008 12:59 am

Gippeto wrote:I can't say for sure if anyone else has actually tested the cracking pressure of a schrader with reasonably accurate equipment.

I have. :) (6" test gauge with parallax dial, calibrated within .25 of 1%)

It took 70psi to open the schrader using pressure alone.

This would mean that there would be 70psi less in the chamber than in the pilot area. :(

It also means that you have to vent a lot more pressure (volume) before the piston will actuate. ie. you will need a larger pilot valve.


I don't think you would need a larger pilot valve. I'd think that you would use the same size pilot valve, more pilot volume pressure would just mean that you dump more air before you get to the actuation pressure (the pressure on the pilot surface area (back of the piston) that generates the same force as the chamber pressure on the exposed area of the front of the piston (outer area on a barrel sealer, inner area on a chamber sealer), which is the pressure when the piston begins to move). Because you have to dump more air before you reach the actuation pressure, you have more time to get to the actuation pressure or more time to open your valve. This would mean that, assuming you can't fully open your valve in less than a ms or 2, the effective cross-sectional flow area of your valve would be greater by the time that it matters. So you could really even use a slightly smaller valve...

So, higher pilot volume pressures should make more effective use of your pilot valve and result in faster actuation.
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Unread postAuthor: CasinoVanart » Thu Sep 11, 2008 1:46 am

Gippeto wrote:I can't say for sure if anyone else has actually tested the cracking pressure of a schrader with reasonably accurate equipment.

I have. :) (6" test gauge with parallax dial, calibrated within .25 of 1%)

It took 70psi to open the schrader using pressure alone.

This would mean that there would be 70psi less in the chamber than in the pilot area. :(

It also means that you have to vent a lot more pressure (volume) before the piston will actuate. ie. you will need a larger pilot valve.


Hey Gippeto, whilst i have great respect for your talents and skills i think you have missed a small detail about the valve in question.
It can be adjusted! I simply took some torque off the screw so that the valve would open easier, at around 5psi. It could be adjusted even further but you risk leakage or coarse.

Again you do great work man, i love your stuff:)

edit: Spling
edit edit: Sorry JSR, i missed the part in your post about messing with the return spring :wink:
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Unread postAuthor: Gippeto » Thu Sep 11, 2008 9:19 am

PotatoNick;

Perhaps my issue is that I can't stand to hear anything go "sssboom"?


CasinoVanart;

Right back at ya!

At times, I get tunnel vision and have difficulty thinking "outside the box". (Loosen the screw :idea:. Slaps forehead with palm. SMMAAACK!) :)
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Unread postAuthor: CasinoVanart » Thu Sep 11, 2008 9:46 pm

Gippeto wrote:

CasinoVanart;

Right back at ya!

At times, I get tunnel vision and have difficulty thinking "outside the box". (Loosen the screw :idea:. Slaps forehead with palm. SMMAAACK!) :)



Thats nothing mate, at work a while ago i was stupid enough to think out loud, this is the trash that came out of my gob "Duh three doesn't go into twelve very good" what a dope, my workmate just looked and said "you f#$%ing idiot" :D Still living that one down!
Have a great day Gippeto.
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Unread postAuthor: Gippeto » Thu Sep 11, 2008 11:30 pm

Been there bud, more than I care to admit. Brain farts are an unfortunate fact of life. :lol:

So, we'll just deal with it and have another cold one. I am. :wink:
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Unread postAuthor: CasinoVanart » Thu Sep 11, 2008 11:38 pm

Gippeto wrote:Been there bud, more than I care to admit. Brain farts are an unfortunate fact of life. :lol:

So, we'll just deal with it and have another cold one. I am. :wink:


I am 19 minutes away from Lager, wait 18.
Friday afternoon grogs always go down a treat :D

Oh yeah, scrhaders are cool and i am still on topic...
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Unread postAuthor: Rob Thornton » Thu Sep 18, 2008 9:01 am

It's been a while since my last post so I might be a little rusty...

I think for the most part we need to look at the materials and dimensions of the design here.I mean no offense to anyone so please take my pointers form a constructive rather than destructive point of view:

A piston needs to be as lightweight as possible, as weight affects the performance negatively - unless its made out of really light balsa wood or something similar, I would not use wood because it is HEAVY. There are a number of platics that can be milled on a wood turning lathe.

Adding a schrader valve to the mix only makes it heavier. And by looking at your pic, I think you barrel diameter is too large compared to your piston chamber. YOu need to get more air behind the piston (barrel side) to force it back open. An easy fix to this is just a smaller barrel...

Lastly The piston diameter may be too loose (small) it needs to fit tightly into the valve seat to prevent air from rushing past.

Please dont take offence at my suggestions- only trying to help.
Your idea is a good one....

Rob
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Unread postAuthor: daxspudder » Thu Sep 18, 2008 1:03 pm

the piston weighs very little, seals very snuggly, and opens half the diameter of the T, which is more than enough for effective "venting" aka firing of the chamber... and its basswood, which is only twice as heavy as balsa, but has a crush resistance nearly 15 times as much as balsa... FYI
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