Self cocking front exhaust valve

[sb327]

The shuttle doesn’t seem damaged. Poppet must have been degraded. What gun is this in?

Dave

[jackssmirkingrevenge]

It's the Korean Hunting Master I mentioned here, all the synthetic material on it was in a very poor state so I'm not surprised.

I made another poppet out of acetal but now the thing won't seal...

[Rob M]

It's the Korean Hunting Master I mentioned here, all the synthetic material on it was in a very poor state so I'm not surprised.

I made another poppet out of acetal but now the thing won't seal...

i put the valve in the lathe chuck , with the stem facing the tailstock.. then lock the tailstock to the stem , and lock the tailstock into place.. spin up to 600, then i retract the tailstock till theres a constant pressure between the delrin poppet and the seat of the valve .. this is generally .2 mm or so , not much to make friction..then i let itt run 30 seconds .. I forgot to mention , i oil the seat before doing this.

[sb327]

Hey Bob,

I was reading through old posts and ran across this    https://www.gatewaytoairguns.org/GTA/index.php?topic=73506.0

I see a lot of similarity between your idea in that thread and what I’ve got going on here. Although your idea was for different intent than mine initially, they are similar.

Dave

[CableStop]

Hey Bob,

I was reading through old posts and ran across this    https://www.gatewaytoairguns.org/GTA/index.php?topic=73506.0

This is basically what I'm working on actually. Mine is a stand alone plenum for now though, not integrated into the valve. I don't see how the piston here resets, mine uses a double ended piston with seals at each. It's also proving difficult to make the rear seal reliably and is cumbersome to regulate. If there's a better way to do it I'd like to know.

The regulated side of this valve must have a lower pressure than the reservoir so I don't see what pushes it back. I could easily be missing something I'm more of a visual learner as they say.

[CableStop]

Sorry about kind of hijacking the thread but here's a basic drawing of the design I was talking about just for reference. It's completely air driven and you can see how the different piston diameters allow for the more force on the plenum side than the reservoir side when the valve is closed.

[rsterne]

Yes, David, the concept is similar.... and I abandoned it because of the likelihood of the extreme deceleration force on the piston destroying things.... 

I think the poppet Jack destroyed may be an example?....

Bob

[sb327]

Bob,
Maybe since I’m not running a conventional poppet is why I’m not having that issue. The only destruction problem I had was at the blow open piston end and have since got that under control.

Cablestop,
I’m not sure on your last drawing where the flow occurs to really speak to it. I’m not clear on intake, exhaust, etc.

And I don’t mind thread derail, it’s all just something to discuss.

Dave

[CableStop]

Basically the vent allows the plenum area in front of the valve to fill to reservoir pressure pushing the piston away from the valve. The space between the two o-rings is ambient and needs a vent to ATM somewhere. Then when the valve opens and pressure drops the reservoir pressure on the smaller rear piston drives it towards the valve again to shrink the plenum and drive out the air. It's an evolution of the collapsing plenum idea from my other thread.

[rsterne]

As long as what you are doing works, carry on.... I was merely suggesting that could have been the cause of Jack's exploded poppet.... The shuttle ends up being a hammer (albeit a light one), driven at extremely high speed.... If it travels 1/2" in 1 mSec., then it has a terminal velocity of about 1000 fps.... Multiply that by the mass, and you can see how much that is.... That energy/momentum must be dissipated when it stops at the end of its travel.... The more sudden the deceleration, the higher the resulting force required....

Bob

[jackssmirkingrevenge]

I was merely suggesting that could have been the cause of Jack's exploded poppet

It was definitely the cause, but the caveat is that it was an old part and like the rest of the synthetic parts on this particular rifle, particularly prone to failure.

In spite of several attempts I still haven't managed to make a new poppet that seals properly, I never seem to have luck with the tapered variety.  The flat on a raised lip seat seem to work better.

I tried "lapping" it as Rob suggested further up but no joy.  Perhaps I'm not taking it to a high enough pressure, but I figured if it doesn't seal completely at 1000 psi then there's no point in pumping further.

[sb327]

As long as what you are doing works, carry on.... I was merely suggesting that could have been the cause of Jack's exploded poppet.... The shuttle ends up being a hammer (albeit a light one), driven at extremely high speed.... If it travels 1/2" in 1 mSec., then it has a terminal velocity of about 1000 fps.... Multiply that by the mass, and you can see how much that is.... That energy/momentum must be dissipated when it stops at the end of its travel.... The more sudden the deceleration, the higher the resulting force required....

Bob

Is that milli or micro seconds?

Dave

[sb327]

Cablestop,

As drawn, res to right, the pressure at valve will have to drop extremely low before piston starts to move. You can achieve the same ‘self reset’ feature by making the res end of piston just slightly smaller than valve side. See the top drawing. This would cause piston to move with less pressure drop.

If you are fine with spring reset, it will move with the least amount of pressure drop. Bottom drawing.

Unless this piston interacts with valve some how or limits amount of air somehow, it would AT BEST only act as an open plenum.

Dave

[rsterne]

Velocity = distance over time.... 0.5" / 0.001 sec. = 500 in/sec.... Oops, divide by 12 in/ft.... Average velocity = 42 fps, so the peak velocity is about twice that (84 fps).... However, at one point you mentioned a travel of over 2", which would multiply that by 4.... My point is that you have a LOT of energy in that shuttle, and it has a lot of momentum.... If you stop that in 0.010", the forces is astronomical.... Even if it takes 0.050" to stop, that means 10 X the force required to slow it than to accelerate it in 1/2"....

Not much wonder Jack's poppet exploded.... and exactly why I never pursued my idea from that thread from 6 years ago....

Bob

[sb327]

Not trying to argue, but

6061 aluminum weighs .0975 lbs. per ci

My piston is .800” od. Spool shaped. Let’s guess at equivalent to .5” long solid.

So, that’s 0.2531 ci x .0975= 0.0245 lbs.

.0245lbs. = 171.5 grains

That’s equivalent to a 171.5gr bullet traveling @ 85 FPS. When I put that into the calculator, I get 2.8 fpe.

That doesn’t seem astronomical, unless I missed something. I had to guesstimate at the piston weight because I’m away from home and can’t measure directly. I cant get 2” of stroke on mine because the cylinder is only 2” long and piston is approximately .8” long.

I’m really trying to understand why I had no issues with mine. I did run a small oring between my spool and where it pushes on valve. And I had a spring pushing back on the spool.

If I needed to release more air (increase power) and keep max shuttle speed slower, I could move up a size on the cylinder id.

Dave

[rsterne]

Take that 2.8 FPE and stop it in 0.010" and see what force that requires.... 2.8 / (0.010/12) = 3360 lbs....  Now apply that force to the contact area of whatever the shuttle/piston hits and see what the pressure is.... 

This is the same reason that a bolt handle that is perfectly capable of retaining a bolt when a PCP is fired providing it is locked down, can shear off if the bolt is not locked down when the gun is fired.... There is a thread about that in "The Workshop", with a Video by Lloyd.... Think driving a nail into a board with a hammer, compared to trying to push it in with the same hammer.... Impacts have a way of destroying things very quickly.... and the less "give" (or compression) there is in what the object in motion hits, the higher the force....

Like I said, if it is working for you, great.... It didn't work for Jack.... and may not even with "new" parts of the same dimensions and materials.... Put a Belleville washer stack in there to absorb the momentum of the shuttle, stopping it over a larger distance, and you lower the impact force.... Put an O-ring bumper on the end of the poppet, where the shuttle hits, and it might survive.... Having a metal shuttle crashing into a plastic poppet CAN shatter the poppet.... I'm just trying to explain why.... Taken to extremes, with metal to metal contact, it might even shear valve screws or threads.... something to think about.... 

Bob

[jackssmirkingrevenge]

Taken to extremes, with metal to metal contact, it might even shear valve screws or threads.... something to think about.

A very valid point, it's the same dynamic we see in a Quick Exhaust Valve.  In high performance applications, you definitely need some sort of bumper to mitigate the impact from the piston when it reaches the end of its travel.  I will try and give my setup another go this evening to see what sort of damage one can expect.

At this point it might be worth trying it out as a single component as first suggested, now that I need to make another poppet anyway:

[CableStop]

In Dave's design, could the rear piston that provides opening force also be working as a damper to slow the valve/shuttle as it closes? 

[sb327]

I have things going on that definitely must be absorbing this energy. I think the fact that the shuttle picks up the valve and seats it must use a lot of it. And the oring. And the spring.

I may try making my spool from delrin to drop some weight from it though just for safety sake.

Dave

[rsterne]

If there is some mechanism occurring that slows the shuttle/piston slowly, then the force is much reduced.... If instead of 0.010", it slows over 0.100", the force required drops to 336 lbs....

Bob