GTA
All Springer/NP/PCP Air Gun Discussion General => Machine Shop Talk & AG Parts Machining => Engineering- Research & Development => Topic started by: jackssmirkingrevenge on February 08, 2021, 09:44:11 PM
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Something I've been pondering, a .30 cal pump pistol based on the Crosman 1322, so I put a couple of potential variations to pixels approximately to scale (10 pixels = 1mm)
In both cases the hammer valve is replaced by a piston that is slightly biased forward.
The second one has a bit less volume but has less o-rings... and in any case, still vastly more volume than the original, pumping would be a chore... in practice I would aim for around 4 cc.
(https://i.imgur.com/fRVB63X.png) (https://i.imgur.com/fRVB63X.png)
(https://i.imgur.com/SlYItUi.png) (https://i.imgur.com/SlYItUi.png)
Some very rough calculations suggest around 450 fps with a 45 grain pellet from a 12 inch barrel would be achievable with something below 25 pumps.
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pretty cool. 2 suggestions.. use a larger tube and piston , maybe in the .750 id range , this would equate to more swept volume per pump , but the pumps would be harder. other suggestion is to make it a siidlever , this will allow a longer pump arm bringing the effort needed bback down to factory ( or less.
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How do you physically make the valve body without having a joint in it?.... It looks like you have moved the transfer port way back to gain volume?....
Bob
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2 suggestions.. use a larger tube and piston , maybe in the .750 id range , this would equate to more swept volume per pump , but the pumps would be harder. other suggestion is to make it a sidelever , this will allow a longer pump arm bringing the effort needed back down to factory ( or less.
Your suggestions make a lot of sense but I really want to keep the 1322 "look", I like the idea of having something that doesn't look too different from the host but more than doubling the standard FPE.
How do you physically make the valve body without having a joint in it?....
These are more or less concept drawings with some construction details omitted, in practice the valve body would be made in at least two parts.
It looks like you have moved the transfer port way back to gain volume?....
Well spotted, you can make out the original transfer port above the trigger pin. Tranfer port centerline is shifted about 1.1 inches to the rear.
One of my concerns is the o-ring sealing the chamber, I didn't have much luck with this setup when playing with the 1400 (https://www.gatewaytoairguns.org/GTA/index.php?topic=157736.msg155747192#msg155747192) with the o-ring jamming on the transfer port:
(https://i.imgur.com/MzRZu1J.gif)
(that 1400 is still there and if this goes well might be a candidate for a similar bore embiggening)
The difference here however is that the spool enters the chamber as opposed to moving into the flow, a different dynamic that I maybe optimistically hope will prevent it from blowing off.
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I figured the valve body would have to be made in 2 pieces.... Won't that drastically reduce it's volume, however?.... Perhaps not, if the check valve end was threaded in, but even then, wall thickness could be an issue, and require boring out significantly larger than the threads.... ???
One thing you might consider is a "moving O-ring" style check valve, where the stub it sits on (which normally moves as part of the check valve) is threaded into the front of the valve, using a flat-head screw with a tiny hole through it.... Only the O-ring moves on pumping....
(https://hosting.photobucket.com/albums/oo221/rsterne/Parts%20for%20Sale/.highres/O-ring%20Check%20Valve_zpsvdevebgt.jpg) (https://app.photobucket.com/u/rsterne/a/ec7c3593-4578-4151-a54b-04e884d967df/p/4e830093-9cc6-43c4-a6fa-54856e66e61c)
The center hole through the screw has a cross-hole at the left end of it, feeding the space under the O-ring.... The O-ring expands when pressure is applied, and gets squeezed back into the corner under the screw head on the pump stroke.... Not my idea (Steve in NC I think).... but works well.... Instead of the drilled hole, you can grind a shallow notch through the threads....
Bob
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Won't that drastically reduce it's volume, however?.... Perhaps not, if the check valve end was threaded in, but even then, wall thickness could be an issue, and require boring out significantly larger than the threads.... ???
That is a very good point if one imagines two threaded sections like the original valve is made. In that case I would likely need to move the TP back even further if I'm to get the 4cc I'm aiming for.
I think it would also be possible however to have the body as a single closed tube then close it off with an endcap pinned in place.
One thing you might consider is a "moving O-ring" style check valve, where the stub it sits on (which normally moves as part of the check valve) is threaded into the front of the valve, using a flat-head screw with a tiny hole through it.... Only the O-ring moves on pumping....
Very neat solution, thanks for sharing!
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Alternately, can you make the valve in two pieces.... a check valve section and a rear section.... and use the steel tube in between as the reservoir?.... I used that on my .25 cal Disco build, and also on my Millennium Pumper.... The 0.62" ID tube would hold about 5 cc of HPA per inch of length.... 8)
A 12" barrel that is .30 cal would be about 14 cc.... so 4-5 cc would seem about right, I would think....
Bob
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That also makes sense, though I would have to take the original TP port and breech screw port into account when positioning the o-ring.
The new breech screw would also have to be more substantial if it was anchoring the endcap.
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Yes, you might have to use 2 or 3 larger screws for each half.... Through the sides of the tube would make sense, not in the way then....
Bob
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A 12" barrel that is .30 cal would be about 14 cc.... so 4-5 cc would seem about right, I would think...
Gratifying to hear my estimate concurs with your thoughts :) I'm hoping to get decent power (along the lines of your big bore pistol build, within non-restricted Canadian limits) with 20 pumps or so.
Through the sides of the tube would make sense, not in the way then....
Yep, I figure one on each side extra will do nicely. I doubt pressure will exceed 2000 psi anyway.
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Just a quick sketch of how you might make the 2-piece valve.... no details of the valve innards....
(https://hosting.photobucket.com/images/oo221/rsterne/2_piece_valve.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/2b6b9a8f-bf4c-41f4-8426-985eaf5ed6aa/p/a169e19a-950c-47f3-9620-d94c8db0cf4c)
The O-ring glands do not need much wall thickness around them in the rear half, as the force is towards the outer tube and the end of the valve body.... The O-rings squeeze up into the outer back corners.... and you could use a 1-1.2 mm CS O-ring to minimize the volume loss from the wall thickness....
Bob
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Valve body detail drawn to scale, I put a 2mm wall thickness and still have 4.5cc chamber volume:
(https://i.imgur.com/0tl5HVTh.png) (https://i.imgur.com/0tl5HVT.png)
The central portion that balances the spool is technically free to move but in practice would be forced to the rear of the pistol by pressure differential.
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looks like you could haave it biased torward the front , and have a rear cocker stem coming out the back of the gun to reset the valve .. maybe not , since on the last pump thered still be a small charge ahead of the forward piston
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I think if I made it like this, with a lip stopping the spool from going too far back and a return spring, it would reset automatically:
(https://i.imgur.com/xKO4WN8h.png) (https://i.imgur.com/xKO4WN8.png)
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Are you gonna have the area in front of the forward oring vented to atm? The area where the spring is?
Dave
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As it's a dump valve I don't think it would be necessary, the trapped air should act as a sort of bumper too.
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I would be concerned that a slow (or fast) leak past the O-ring into that section would increase the pressure and change the dynamics of the valve action.... therefore it would be better vented, IMO.... Just like the middle (spring) section of a regulator....
Bob
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You're right that a leak would be impossible to detect, however at worst it will simply lock the spool shut so you'd need to take it apart anyway.
That being said, suddenly compressing the air in that small space might have some undesirable effects so perhaps venting would be worthwhile after all.
I wonder if it's worth venting into the transfer port... maybe it would encourage the spool to re-close on firing?
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It would and if tuned properly could make a nice ‘air saver’.
Dave
ETA: as long as it doesn’t rob too much
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Be tough to drill a small hole through the stem that far.... but a good choice if you can manage it....
Bob
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Have not paid attention to this, but hey that looks neat concept.
Making the stem with pipe and tig welding it in place, bigger pistons to the ends and then turning after welding.
Marko