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shrader as check valve?

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: SubsonicSpud » Tue Apr 07, 2009 10:16 pm

hubb017 wrote:You know, I got to thinking about this: If you build an airtight piston and use a schrader valve for a check valve, and the schrader opens at around 70 psi, there would be no doubt that the piston would be seated before the schrader is opened. If you plan on using this setup on something that is going to handle more than the cracking pressure of a scrader, and you are intent on using the schrader as a check valve, then why do you need to remove the spring?

As far as your above statement, I've never removed a spring from a schrader, as I've never needed to. Now, a presta valve is essentially a schrader valve without a spring, so that might be something that you could look into.

Also of note, the last piston I built had a homemade check valve in it about the same size as a schrader. It consisted of a 1/8" compression coupling (the kind used on copper pipe and stuff). A small o-ring was obtained to seat inside it, then an airsoft bb used to create the "check." If this interests you any, I can make a diagram with more detail if needed. Otherwise, I don't know.


If you use the schrader with the spring as a check valve and it has a crack pressure of 70psi then the pressure in you're chamber is going to be 70psi less than the air supply you fill it up with, which would mean a big reduction in power. There are about 3 or 4 different styles of schrader valves, the type you want are the long ones about 1-1/4" in length. They have a long spring on the internal side of the schrader, and this can be removed easily. The other option is just to get a Presta style valve as they do not have a return spring.

EDIT:

Sorry I just quoted the wrong person, lol
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Unread postAuthor: Hubb » Wed Apr 08, 2009 8:08 am

SubsonicSpud wrote:If you use the schrader with the spring as a check valve and it has a crack pressure of 70psi then the pressure in you're chamber is going to be 70psi less than the air supply you fill it up with, which would mean a big reduction in power. There are about 3 or 4 different styles of schrader valves, the type you want are the long ones about 1-1/4" in length. They have a long spring on the internal side of the schrader, and this can be removed easily. The other option is just to get a Presta style valve as they do not have a return spring.


I don't honestly see how there would be a reduction in power here. Maybe there would be more pressure in the pilot area than the chamber area, but why would he stop filling the chamber before it reaches his goal pressure? In other words, if he wants 100psi, he stops filling when the chamber is at 100psi. Who knows, the slightly higher pressure in the pilot area may even help it open faster...but I've never tried it, so it may flop.

As far as the diagram, well, here is my check valve...
Image

I've built three of these, and each one works great. I'm not going into any building details, as the image should be self explanatory. I will, however, answer any questions that anyone has about it.

Notes:
- This entire setup was cheaper than a schrader valve, costing around $2.

- On one of these, the bb would roll out of the compression nut, due to the hole being slightly too large. To fix this, I cut a thin piece of small tubing and inserted it between the compression coupling and the nut, having the nut hold it in place.

- I used a rubber airsoft bb on one and this eliminated the o-ring, although the o-ring tends to work better.
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