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"Valveless" piloted o-ring detent concept

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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"Valveless" piloted o-ring detent concept

Unread postAuthor: jackssmirkingrevenge » Mon Jan 08, 2018 5:26 am

Image

Just an idea I wanted to commit to pixels, basically the projectile blocks the chamber, retained by an o-ring that simultaneously prevents it from moving while sealing it. The o-ring is deformed by the force from an unbalanced piston.

When the pilot area is emptied through the pilot valve, the piston is forced back, removing the pressure on the o-ring and allowing the projectile to slip free.

Here's another variation, replacing the o-ring with a ball detent with a tight fitting lead pellet providing the seal:

Image
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: Cthulhu » Mon Jan 08, 2018 7:41 am

This looks like it would be great for a cartridge ejecting airsoft gun, but I don't fully understand how it works.
Is it like a coaxial piston where the vented air from the fill valve moves the entire chamber back?
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: jackssmirkingrevenge » Mon Jan 08, 2018 7:49 am

The air is already pushing on the base of the projectile but it is prevented from moving. Exhausting the pilot moves the piston back which then unlocks the projectile, allowing the pressure to force it out.
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: hectmarr » Mon Jan 08, 2018 12:17 pm

This good idea, you just have to build a prototype and try to see the small adjustment problems that are never missing, and should work perfectly.
In the diagram first, you should make sure that the force of the piston that deforms the o'ring is always greater than the force that the air gathers against the ammunition, especially during the loading of the cartridge.
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: Ragnarok » Mon Jan 08, 2018 12:45 pm

While potentially viable in theory, I suspect that in practice, this would be leak-prone (sealing that will be a bugger), likely to eat through o-rings (the wear as the projectile pushes through), difficult to load (unless the projectile is going to be held in exactly the right place as the pressure is brought up, I suspect that the O-ring won't deform and seal enough at low pressures), pressure limited (at a certain point, the projectile will just push through the o-ring), picky about ammo (it has to have a perfect seal), I'm not sure the flow through the porting is going to be great, and I wouldn't be surprised if it was prone to accidental discharge.

I'm not sure I see any advantage to this valveless concept over a miniature co-axial piston valve, which is going to be a lot less temperamental, as well as less demanding about what it's loaded with.
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: jackssmirkingrevenge » Tue Jan 09, 2018 3:54 am

Ragnarok wrote:I suspect that in practice, this would be leak-prone (sealing that will be a bugger)


It would be limited to projectiles with a polished nose, yes.

likely to eat through o-rings (the wear as the projectile pushes through)


The idea is that once the o-ring is uncompressed, the projectiles needs no effort to go through - though of course the compression itself will take its toll on the component.

difficult to load (unless the projectile is going to be held in exactly the right place as the pressure is brought up, I suspect that the O-ring won't deform and seal enough at low pressures)


Follow the pressure path if you fill through the pilot - compression of the o-ring is instant and a tight fitting projectile would be forced into the right place.

pressure limited (at a certain point, the projectile will just push through the o-ring)


The compression of the o-ring is proportional to the pressure, and my experience with hybrids indicates that an o-ring in compression is capable of retaining a projectile even with many hundreds of psi behind it.

picky about ammo (it has to have a perfect seal)


This is true of any valveless design.

I'm not sure the flow through the porting is going to be great


With big enough porting it should be OK.

and I wouldn't be surprised if it was prone to accidental discharge.


That's dišgusting.

I'm not sure I see any advantage to this valveless concept over a miniature co-axial piston valve, which is going to be a lot less temperamental, as well as less demanding about what it's loaded with.


Not sure I'll build it, but for a pneumatic cartridge type device it would maximize the internal volume while as well as offering burst disk levels of performance while being re-settable.
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: hectmarr » Tue Jan 09, 2018 7:02 am

With the necessary material you can retain the ammunition. The air line that you see is the one that you have to decompress to shoot. A thick silicone tube could be used.
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Re: "Valveless" piloted o-ring detent concept

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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: jackssmirkingrevenge » Tue Jan 09, 2018 7:23 am

hectmarr wrote:With the necessary material you can retain the ammunition. The air line that you see is the one that you have to decompress to shoot. A thick silicone tube could be used.


This is another way the principle could be applied, with a small gap to reduce the stress on the tubing. jimmy101 had proposed it some time ago here:

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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: Ragnarok » Tue Jan 09, 2018 7:37 am

jackssmirkingrevenge wrote:The idea is that once the o-ring is uncompressed, the projectiles needs no effort to go through - though of course the compression itself will take its toll on the component.

More than that - it's not a binary thing, so at some point there's going to be a partially compressed O-ring that the projectile will have enough force to push through.

This is true of any valveless design.

True, but that doesn't mean it's not an argument to be taken into account. I suppose as a technical "can we do this" experiment, valveless is one thing, but in practical terms the question is if any performance benefit is worth the drawbacks of the design. If every projectile needs to be precision engineered, it does need to have a tangible pay-off.

With big enough porting it should be OK.

It's more than just raw area. It's about the smoothness of the flow (ultimately your flow area is limited to that of the barrel) and flow through several small ports is inevitably going to be more turbulent than through one large one.

Not sure I'll build it, but for a pneumatic cartridge type device it would maximize the internal volume while as well as offering burst disk levels of performance while being re-settable.

"Burst disk levels of performance" is a fairly hypothetical bar though. To many people that means "instant", but valve opening time is something of a red herring when it comes to performance, because the fixed variable in the projectile's acceleration is distance, not time.

A one millisecond valve opening might seem slow when you consider a projectile being in the barrel for 10 ms, but when you consider that (treating it as constant acceleration for the case of simplifying the maths) means the projectile has only swept the first 1% of the barrel (as per s = ut + 1/2*at^2) by the time the valve is open, valve speed is clearly a case of diminishing returns as far as muzzle energy (being the integral of net force against distance). It doesn't matter if the valve is infinitely fast, that only means an additional 1% performance (so small it'd be lost in shot-to-shot variance).

Flow becomes more important here - and like I mused, I'm not convinced you're going to get clean flow through those ports.
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Re: "Valveless" piloted o-ring detent concept

Unread postAuthor: jackssmirkingrevenge » Tue Jan 09, 2018 8:03 am

I'll grant you that compared to something like this in most situations you probably would see very similar performance, a few milliseconds gain for a reasonably heavy projectile in a long enough barrel doesn't result in too much performance increase:

Image

That being said there are however other benefits to consider, for example you do not need a lot of piston movement, and assuming your piston has an o-ring seal, the pilot valve opening speed and flow is irrelevant to the performance.
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