SpudFiles

Heres my design for an upcoming pneumatic rifle, I tried to keep things as simple as possible building wise whilst keeping high performance. It is a coaxial design with single shot chamber, shrader filling valve and a shrader for the trigger.

A pipe is sleaved over the trigger shrader, this pipe contains a small push rod that is connected to the firing lever. when the trigger is pulled towards the user, the rod pushes down on the shrader pin releasing air from the pilot volume allowing the piston to be pushed back by the chamber air allowing the gun to fire. Air that is released from the shrader exits though gaps in the mechanism, as well as holes drilled into the pipe covering the shrader valve.

The piston is a piece of steel pipe with a close fit to the ID of the chamber, which has a rubber sealant at the front, with epoxy filling the rest the space.

The piston backstop is a piece of steel pipe (same pipe as used with piston) which has a firm rubber epoxied onto the edge of the pipe. The pipe has thick 3mm walls so a good amount of surface area is there for the rubber backstop.

Hope you like it, suggestions/ thoughts appreciated.

A schrader pilot needs an o-ring piston at this diameter or a custom machined piston for tight tolerance. How tight is your piston?

depends, im still waiting on an email for the exact ID of 22mm pipe from my supplier. But hopefully 0.25mm gap max. But if its what I think it is then 0.01mm. If the pilot volume is kept to a minimum, will that require less tolerance on how tight the piston is because it can drain the pilot of air faster?

oh and the drawing isn't to scale, im using 22mm pipe. The drawing makes out that im using 28mm or more. Sorry about that.

Careful.

22mm pipe cross sectional area = 3.801 cm²

21.5mm piston cross sectional area = 3.631 cm²

Gap cross sectional area = 0.170 cm²

Schrader cross sectional area = 0.024 cm²

With those sort of tolerances, a 0.5mm gap is far too much - more air can flow from the main chamber to the pilot than out of the pilot through the schrader, so it will never fire.

edit: a 0.25mm difference between piston and chamber diameters still leaves you with a 0.086 cm² gap, four times the flow of a schrader.

I suggest using a cast epoxy piston directly in order to guarantee a good fit.

A smaller pilot chamber will improve efficiency and performance, but it will not make it more likely to fire if the gap is too big.

I might just cast an epoxy piston, but if the ID is 20mm then I can get some precision 20mm steel pipe for a super tight fit. And I believe metal on metal will slide better?

You can also have a solid epoxy piston with the rubber seal on the barrel, here's one such launcher I had made:



Epoxy poured into cut down syringe



Piston popped out before finishing



Components before final construction (20mm cannon cartridge is just there for scale ;D)



Actual final construction

I thought epoxy didn't slide very well even with grease?

it slides well actually in my cases

Probably just my stuff, has friction like rubber when you touch it.

This is proper marine epoxy, hardens like acrylic when cured.

You could always ask someone to machine it for you

I was going to do that but moneys tight these days . Hot wired doesn't live too far from me, does he have a lathe?

So are you saying I should go with an O ring piston?

Alster370 wrote:I was going to do that but moneys tight these days

It's a small job, in terms of time and material. I think most people will charge you more for postage than the actual piston! No harm in asking for a quote it's important to give the exact internal diameter of the chamber though, you'll need a vernier caliper.

Hot wired doesn't live too far from me, does he have a lathe?

Not to my knowledge.

So are you saying I should go with an O ring piston?

No, a tight fit should suffice.

Well I think Il wait for some info from the supplier on the exact dimensions of their steel pipes. Should they not be a tight enough fit, Il get a piston machined. Have you any thoughts on the other parts of my design?

Alster370 wrote: Have you any thoughts on the other parts of my design?

It reminds me of some of my early coax designs, what's not clear is calibre, dimensions, pressure and how exactly you intend to put everything together - looks more like a vague concept than an actual blueprint.

Well im not as graced in using paint as some of the users on this forum are. It is more of a concept anyway.

Pressure: 600 psi
Calibre: 6 or 10mm haven't decided yet (same fps anyway according to GGDT, probably better flow.)

Im assembling it will epoxy as last time but using jb weld instead of permatex permapoxy.

I haven't decided on final dimensions as of yet.