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Author Topic: Scratch build big bore  (Read 681 times))

Offline NccWarp9

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Scratch build big bore
« on: March 12, 2021, 09:55:35 AM »
Hello,

Im new here, Hello  ;D
Im trying to design a air gun for myself. The guiding restrictions ( in order of importance):
  • Big bore (.38 < .50 cal)
  • Regulated, with plenum
  • Ease of production
  • Ease of maintenance ( springs instead of belleville, avoiding O-rings if possible)
  • Flexible ( plenum can be made as big as needed without too much hassle, and power regulated )

I looked at a lot of documentations and examples but they (at a start) were confusing as work with pressure was never up there in my interest zone but I think I have the gist of it now and wanted to start small, as simple as I can make it.  I found a simple idea in Airenlaces mechanism.

Attached is a image of that fist start, I would much appreciate any input and comments anyone has.


« Last Edit: March 12, 2021, 10:00:23 AM by NccWarp9 »
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Offline Gippeto

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Re: Scratch build big bore
« Reply #1 on: March 12, 2021, 10:16:03 AM »
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Offline Rob M

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Re: Scratch build big bore
« Reply #2 on: March 12, 2021, 10:36:13 AM »
you may be suprised at the cracking force neeeded to get a larger poppet off its seat on a larger throat .. Which also means cocking effort will be fairly high shy of a sidelever  mechanism or balanced valve. Regulator will reduce the force needed , but also reduce the  fpe potential.
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Offline NccWarp9

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Re: Scratch build big bore
« Reply #3 on: March 12, 2021, 12:14:19 PM »
Im having trouble differentiating between check valve and regulator. Isn't the pressure in plenum regulated by the spring on the tank side?
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Offline JuryRigger

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Re: Scratch build big bore
« Reply #4 on: March 12, 2021, 12:18:45 PM »
Im having trouble differentiating between check valve and regulator. Isn't the pressure in plenum regulated by the spring on the tank side?
The part that you're missing; is a balance chamber vented to the atmosphere (a control, if you will)... Let me see if I can find a graphic; unless someone else posts one first...
Edit-Gippeto posted a good search link; if you get the same GIF from zdspb.com as the first result as I have; then I'll try to explain:
The difference between your design; and the one shown in the GIF; is the vent to atmospheric pressure. In the GIF; the high pressure air comes to the valve-which; in the absence of spring pressure; would be held closed by the HPA. We now crank down on that spring, increasing the force that it exerts on the inlet valve stem. Eventually the spring force will exceed the total force on the poppet; (pressure times area), cracking the valve open; allowing high pressure air to fill the plenum. As the pressure builds; it will push back against atmospheric pressure (again a function of pressure vs area); forcing the spring to retract; closing the valve. Increasing the spring pressure will now change the "set point" of the regulator; as it will require more pressure in the plenum to force the spring back to the atmosphere.
If that makes any sense; you may now be able to see how your initial design would not work-there is no "control" through an atmospheric vent, it will function only as a restrictive check valve. You will have some PSI drop through it; due to cracking the spring; but it will not function as a regulator; giving a set output. If it takes a 300 PSI differential to crack that check valve open; then you will have a constant drop of that value from reservoir pressure... That is; if you have 3000 PSI in the tank; then you will get 2700 PSI in the plenum... 2700 PSI in the tank; 2400 in the plenum... Not a regulator; since there is no actual control over a set point, only a constant pressure drop...
Hope that I made sense to you there; and welcome to the forum.  :D
Jesse
« Last Edit: March 12, 2021, 12:43:37 PM by JuryRigger »
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Offline rsterne

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Re: Scratch build big bore
« Reply #5 on: March 12, 2021, 02:33:00 PM »
Quote
Isn't the pressure in plenum regulated by the spring on the tank side?

The pressure in the valve chamber would equal the air pressure in the bottle, minus the pressure required to open the check valve.... That is the area of the check valve seat divided into the force of the spring.... For example, if the spring force (with the check valve closed) was 10 lbs. and the check valve seat was 1/100 sq.in., it would take (10 / 0.01) = 1000 psi. to open the check valve.... Using this example, if the bottle was at 3000 psi, the valve chamber would fill to 2000 psi, at which point the check valve would close.... When the bottle pressure drops to 2000 psi, the valve chamber would only fill to 1000 psi.... When the bottle pressure dropped to 1000 psi, the check valve would no longer open.... The pressure in the valve chamber, while less than the bottle pressure, is not constant....

A regulator uses a spring (often Belleville washers) to open a piston that has an opposing force, created by the air pressure on the output side (in this case your valve chamber pressure), to close the piston.... There is a seal at the end of the piston that seals off a tiny hole that is at bottle pressure on the inlet side.... Any time the air pressure on the outlet side is above the setpoint of the regulator, the piston moves to compress the Bellevilles and seal off the inlet hole.... This occurs virtually independant of the bottle pressure.... Therefore the outlet pressure is "regulated" (held constant) at all times, provided the bottle pressure exceeds the setpoint....

Bob
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Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline CableStop

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Re: Scratch build big bore
« Reply #6 on: March 14, 2021, 09:47:54 PM »
A regulators basically works by a difference in surface area being acted upon on each end of a stepped piston.  I would recommend looking up Robert Lanes's video on this as he really does a very good job explaining how a regulator works. 

Pressure fills the plenum and pushes on the big end of the piston to compress the spring, the small end of the piston presses against a seal which closes the airflow when the plenum pressure gets high enough to overcome the spring tension.  The pressure on the small end is insignificant compared to the large end so the spring bias is the only thing holding the regulator valve open.  In your design the pressure from the spring will be overcome until the pressure on each side equalizes in which case it will close.  This changes depending on reservoir pressure, plenum pressure won't be regulated, it will just be offset by the amount of force the spring is exerting on the valve.

This diagram shows a basic fixed regulator set up.  Side A is the small end of the piston, side B is the large end, the spring in between is what sets the regulated pressure. 

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Offline NccWarp9

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Re: Scratch build big bore
« Reply #7 on: March 23, 2021, 06:41:44 AM »
Thank you for your input.
I redesigned it and ended up with this. Regulator is, I think, as standard as I understand it while the valve itself is a modification of balanced valve.

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Offline sb327

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Re: Scratch build big bore
« Reply #8 on: March 23, 2021, 08:35:23 AM »
The reg design looks correct, it does need the Belleville stack vented to atmosphere. This is to release any seepage of oring.

I think I understand the valve. What sealing method are you using?

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Offline NccWarp9

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Re: Scratch build big bore
« Reply #9 on: March 23, 2021, 09:59:21 AM »
The Belleville stack is vented to the atm its just not visible in the image, new image is attached. The position of it is not ideal but that can be adjusted once I did all the force calculations and design is refined. As a first step I just wanted to get to grips with regulators and valves operations.
Regarding sealing, well, thats kinda the issue, would like to remove as much o-rings as possible. Regulator has o-rings but the valve does not and Im worried about tolerances being to lose when trying to make it. I can add them to the valve but would need to increase the overall length of the valve and different spring with larger travel. Looking at it the valve assembly is the dodgiest one and will most certainly need an redesign with o-rings or something different. Would love to use a loose fit with high tolerance. Something like shown in High precision air bearing CNC lathe and grinder video by Dan Gelbart (cant post link but it should be easy to find it on youtube)
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Offline CableStop

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Re: Scratch build big bore
« Reply #10 on: March 23, 2021, 01:38:58 PM »
I think I understand your valve design however I don't think it will work, I don't think you can make a piston fit so tightly that it's airtight and still have it move freely enough.  Basically you're talking about an AirForce style top hat/hollow valve stem.  Instead of terminating it in a poppet valve you want to have the end of the valve stem sealed with holes drilled radially which retracts into a tube that's so tightly fit that it cuts off airflow. 

I don't think this will work for a few reasons.  The tolerances will have to be so tight that it will almost certainly bind up after a few cycles unless you used something self lubricating.  Even if you get it working reliably at first, once the piston and tube wear a little the tolerances will no longer be tight enough to seal the air out.  Finally, a traditional poppet has the force of the air pressure holding it closed (Area of the poppet in inches square x PSI), that means it seals better as pressure rises.  In fact harder valve materials like PEEK usually need a fair bit of pressure to seal completely. Your design will not benefit from higher pressure, as it increases the valve will see no change in sealing force so all of the sealing ability will have to come from close tolerances.   
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Offline CableStop

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Re: Scratch build big bore
« Reply #11 on: March 23, 2021, 01:58:39 PM »
Just for reference a stock Marauder poppet (.34" diameter) at 3000 PSI will have about 270 pounds pushing it against the seat.  Sometimes people run unregulated guns without valve springs since they're not actually needed once the gun is pressurized. 
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Offline rsterne

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Re: Scratch build big bore
« Reply #12 on: March 23, 2021, 05:58:39 PM »
Close tolerances will not work for sealing a valve, IMO.... Even if you could hit some magic combination that didn't leak and yet would move easily, it wouldn't last long.... You need to either seal it with an O-ring (which would tend to get sliced sliding across the port) or change to some other design more like a conventional poppet valve.... The way it is drawn currently you would need 3 O-rings, one either side of the fixed port, and one outboard of the sliding port to prevent HPA from leaking out around the stem on firing....

The regulator design is basically correct as far as the piston goes.... but your sealing surface is MUCH too large.... The setpoint will have a drastic change as the HPA pressure in the reservoir drops.... You need a TINY inlet hole, so that the force from the HPA on the piston is a TINY percentage of the total forces.... You will also need an O-ring seal between the movable seat and the regulator body so that the HPA cannot leak through the threads where you adjust the seat position.... Also, there is nothing to prevent the piston from flying out into the valve chamber on firing, you need something to retain it, a snap ring, or whatever....

One other issue is that your plenum volume, which provides the air for the shot, is tiny, just the volume inside the valve.... While this might work for a small caliber target gun with only a few FPE, it won't for any Big Bore....

Bob
« Last Edit: March 23, 2021, 06:12:19 PM by rsterne »
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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsenal:
1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 2260 PCP 8-shot Carbine, 2260 HPA (37 FPE), 2560 HPA (52 FPE), XS-60c HPA in .30 cal (90 FPE), .22 cal QB79 HPA, Disco Doubles in .22, .25 & .30 cal, "Hayabusa" Custom PCP Project (Mk.I is .22 & .25 cal regulated; Mk.II is .224, .257, 7mm, .308 & .357; Mk.III is .410 shotgun and .458 cal), .257 "Monocoque" Benchrest PCP, .172/6mm Regulated PCP and .224/.257 Unregulated, Three regulated BRods in .25 cal (70 FPE), .30 cal (100 FPE) & .35 cal (145 FPE), .257 Condor (180 FPE).

Offline CableStop

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Re: Scratch build big bore
« Reply #13 on: March 23, 2021, 06:49:42 PM »
Also, on the regulator piston, you don't want the air inlet on the face since it will generally always be in contact with the sealing material, having an L shaped passage that enters from the side of the piston just in front of the o-ring is how I've seen it done before. 

All in all it's an interesting design and has it's merits, and I commend your efforts, but I think you need to do a lot more research about airguns before starting on this.  I didn't get into airguns that long ago compared to a lot of people on the forum but I read everything I could get my hands on and it helped immensely.  There are lots of people thinking about these things and asking questions, Bob Sterne has answered a lot of them and basically everything he's posted on this forum is worth a read if you want to design an airgun. 

Even if we wanted to, there is no way to compile all the information about airgun valves into one thread, there are just too many variables.  I'm not telling you to stop asking questions, we like answering questions here. However it will probably be easier and faster for you to do your own research and come back with specific questions rather than figuring things out solely based on design feedback. 

That being said you're on the right track, there's just a lot of unforeseen issues that emerge when dealing with high pressure, you have to remember we're talking about 33x the working pressure of most air tools.  Good luck on your endeavor and hopefully you stick around to show us what you come up with because it does sound interesting.
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Offline MJP

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Re: Scratch build big bore
« Reply #14 on: April 29, 2021, 11:25:05 AM »
The Belleville stack is vented to the atm its just not visible in the image, new image is attached. The position of it is not ideal but that can be adjusted once I did all the force calculations and design is refined. As a first step I just wanted to get to grips with regulators and valves operations.
Regarding sealing, well, thats kinda the issue, would like to remove as much o-rings as possible. Regulator has o-rings but the valve does not and Im worried about tolerances being to lose when trying to make it. I can add them to the valve but would need to increase the overall length of the valve and different spring with larger travel. Looking at it the valve assembly is the dodgiest one and will most certainly need an redesign with o-rings or something different. Would love to use a loose fit with high tolerance. Something like shown in High precision air bearing CNC lathe and grinder video by Dan Gelbart (cant post link but it should be easy to find it on youtube)

If that stem moves in the valve it will leak no matter what, you need a shoulder that it seals to.
To make it work you need to turn the stem diameter smaller and make a poppet valve out of it. Or make a bigger valve head for it and valve seat.

Marko
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