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Schrader Valve thread size?

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Schrader Valve thread size?

Unread postAuthor: simonr » Mon Aug 07, 2006 2:54 pm

Anyone know that the thread size is for the bore the schrader insert screws in to is?

I found a reference to the size of the outer stem of a cycle/car valve, but I would prefer to make the whole thing!

I have a plan for a small CoAx gun, using full pressure CO2. It uses a piston type valve with the schrader to allow gas to flow in one direction through the piston to fill the reservoir.

Another schrader will be used to depressurise the void behind the piston for firing.

Any ideas much appreciated.

Si
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Unread postAuthor: djt » Mon Aug 07, 2006 2:57 pm

you do know you have to push the tiny bar in the middle of the schrader down to allow air to pass through it dont you? you need a small check valve.
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Unread postAuthor: simonr » Mon Aug 07, 2006 3:12 pm

I had hoped, with enough pressure, it would act as a check valve without pressing the pin?

Si
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Unread postAuthor: djt » Mon Aug 07, 2006 3:57 pm

im not sure if it would work that way. i guess it could with enough pressure but it would take alot.

why dont you just make a sort of check valve? drill a an equilization hole in the piston then cover the side you dont want air coming back out of with a flap of rubber or something strong. that way when you fill it the air will easilly push the flap up and get into the chamber through the hole, but when it tries to get back out when you open the other valve to fire it, the flap will stop the air from coming through
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Unread postAuthor: simonr » Mon Aug 07, 2006 4:59 pm

I've had a reasonable amount of experience building piston & similar type valves.
Image
What I'm planning is something which operates using full pressure (750 psi or so) Carbon Dioxide.

Although it is certainly possible, it's hard to prevent liquid CO2 entering the reservoir / valve.

That presents two problems for a rubber (and a lot of metal) flap valves. They are hard pressed to take the pressure and they tend to freeze. Once frozen, they very often snap.

Schrader valves (for whatever reason) seem to operate pretty well at high pressures and rarely fail at low temperatures.

Si
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Unread postAuthor: djt » Mon Aug 07, 2006 5:08 pm

ohh. i forgot about the whole 750 psi thing. im not sure about the valve opening though. you will just have to test it i guess. you could figure it out if you knew the surface area of part in the schrader valve you have to push down to let air through and how many pounds of force it took to open the valve. it would be hard to find the surface area of the middle part of the schrader though.

i like the looks of that cannon by the way!
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Unread postAuthor: chosen_way » Wed Aug 09, 2006 2:29 am

couldnt you just get the end off one of the thinggs you screw into the schrader
valve and use that? just buy a check valve seems to be the best idea.
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Unread postAuthor: simonr » Wed Aug 09, 2006 5:20 am

I tested such a valve - and it took 60psi to force it to open - which is probably too much.

I'll instead just use a ball-bearing in a counter-bored hole in the piston.

However, I still want to use a pair of schrader valves, one to hold the gas pressure inside the container the sparklet bulb screw in to, such that the reservoir can be charged by screwing that container in to the base of the reservoir, which in turn pishes the pin on the valve. The second be used to discharge the pressure behind the piston.

Either way, I need to know the thread size!

I have thread gauges (and a full commervial machine shop), but there are only 2 complete threads on the valve insert and I do not think this is enough to measure accuratly - particularly on such a small thread.

Worst case, I'll just use the whole valve stem.

Si
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Unread postAuthor: Shrimphead » Wed Aug 09, 2006 3:07 pm

Co2 is more than 750 psi I believe. I've heard somebody say that in the right conditions it can get to over 1000 psi.

Are you sure that a schrader valve can handle those pressures? Because the ones for tires only need to hold 50 psi to do their job. You have to worry about the strength of all of the gun components, not just the pipe. Also, what kind of pipe are you planning on using.
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Unread postAuthor: simonr » Wed Aug 09, 2006 6:09 pm

At 20 deg C, the vapour pressure of CO2 is about 750 psi. You can only get it to go higher if you heat it.

The rest of the gun will be machined out of Aluminium with sufficient wall thickness to survive 2000psi. Prior to use it will be proofed by pressurising it with hydraulic oil (less exciting if it bursts!)

Schrader valves are easy good for 1000psi - because the valve is so tiny, the amount of force acting on it is also tiny.

Si
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Unread postAuthor: mark.f » Thu Aug 10, 2006 6:52 am

I hope it isn't like the gun above, you know... sealed with hot glue...

It's called <a HREF="http://www.mcmaster.com">M-C-M-A-S-T-E-R</A>... J/K :D

But seriously, McMaster isn't as expensive as you think. You can get Schrader valves there with both 1/4" mnpt and 1/8" mnpt threads, and even hose barb ends, (not useful for your cannon, but I though they were neat). Give them a try.

Hydrotesting is DEFINITELY a good thing to do if you're planning on using unregulated CO<sub>2</sub>. Go find somebody who can do it and they can probably do it through the Schrader you're installing on there.
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Unread postAuthor: djt » Thu Aug 10, 2006 2:57 pm

sealed with hot glue? what are smoking?!
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Unread postAuthor: simonr » Thu Aug 10, 2006 3:07 pm

Hot glue is marvelous!

[for sealing tiny holes at least]

It was surprising with the one above with all the parts accuratly machined and properly secured, just how much deformation there was at higher pressures and how much air can leak out of a gap as small as a couple of thou.

Trouble with it was, it was already glued and bolted at the point the holes were evident - so all I could do is add a load on the outside.

It is almost perfectly sealed now though!

I stand by it - hot glue is marvelous!

Si
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Unread postAuthor: djt » Thu Aug 10, 2006 3:15 pm

wow it holds all that pressure? well i guess with that small of a leak its not much pressure but still. be careful though hot glue can melt in the sun.
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Unread postAuthor: simonr » Thu Aug 10, 2006 6:15 pm

Sun? It barely shines in England!

It's only holding back 170psi or so.

The 750psi version will be turned Al with screw threads & neoprene O ring seals (made properly!). It will also be a lot smaller - about the right size for paint-ball pellets & peporami sausages :wink:

Si
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