Can a big valve be frugal with air ?

[bstaley]

The problem with restricting flow with a reduced transfer port is that it reduces flow at all pressures.  It only lengthens shot strings by conserving air...it won't widen the usable pressure differential for a given power level.  The buffer limits flow only beyond the point of engagement.  it in effect chops the top off the curve instead of pulling the ENTIRE curve down.

[lloyd-ss]

Bob,
It may be hard to really analyze the data because it is rather disconnected by the use of different springs.  The last spreadsheet uses .052 wire springs and the graph uses .047 springs.  Plus I was doing tweaking during those sessions so there will be some anomalies. 
I agree about the plateau, but in this case it is pretty high up there. I think any additional gains will involve playing with hammer weight and stroke, and valve limits and spring. And of course, more air.

Marvin, I like the name PB25, it is fitting.  I think it is funny that you say you rushed me a little at the end.  With my nature, I have a tough time saying that I am really "finished" with a project. Sometimes I feel like it is more a matter of having stopped on it temporarily, knowing that there is always more to be done.  But overall, I think it came out well, and it is your turn to play now.

Lloyd-ss

[1377x]

i will be following any future posts closely
please keep us informed on hows performing

[lloyd-ss]

This is another series of shots I did while looking for the right spring to use in Marvin's Part's Bin 25.
Most of the choices seem to have their strengths, but overall, it seems like the shorter spring (.047 wire, 19 coils) won out over the long spring (.047 wire 26 coils). This setup had a 76 gram hammer (yes, yet another variation) and a .90 hammer stroke (about 45% longer than stock).

24" barrel, 51.5 gn bullet three different pressures, and 2 different springs, gives 6 different combinations.  These are not shot strings, they are experiments with the springs.


The lines on the bottom are the efficiency lines, and their values are read off of the right hand axis. 
Efficiency improves as the spring pre-load is reduced.

The lines on the top are the FPE and are read off of the left hand axis.
As pre-load is reduced, efficiency improves and velocity and power goes down.

It is worth noting, that with the shorter spring, at 2 and 3 turns out, the spring is not touching the hammer
when it hits the valve stem, and is barely touching at one turn out.

The colors of the lines are matched from the upper FPE line to the lower efficiency line.



I am fairly pleased to get some of the better efficiency numbers at such high power levels.

[rsterne]

Dang I wish I wasn't (partiallly) colour blind.... I'm finding it very confusing, unfortunately....

I'm wondering if the short spring is a lot less prone to hammer bounce opening the valve a second time?.... With two springs of the same OD and wire size, the longer spring has a lower rate (lb./in.), although at coil bind it can deliver a higher force (lb.).... I'm wondering if when the valve is throwing the hammer back, if it is compressing the shorter spring less and then it's being thrown back against the stem with insufficient force to reopen the valve.... That could account for the difference in efficiency....

There is no doubt you have discovered an interesting FACT here.... Now I want to know WHY....

Bob

[melloroadman]

This is another series of shots I did while looking for the right spring to use in Marvin's Part's Bin 25.
Most of the choices seem to have their strengths, but overall, it seems like the shorter spring (.047 wire, 19 coils) won out over the long spring (.047 wire 26 coils). This setup had a 76 gram hammer (yes, yet another variation) and a .90 hammer stroke (about 45% longer than stock).

24" barrel, 51.5 gn bullet three different pressures, and 2 different springs, gives 6 different combinations.  These are not shot strings, they are experiments with the springs.


The lines on the bottom are the efficiency lines, and their values are read off of the right hand axis. 
Efficiency improves as the spring pre-load is reduced.

The lines on the top are the FPE and are read off of the left hand axis.
As pre-load is reduced, efficiency improves and velocity and power goes down.

It is worth noting, that with the shorter spring, at 2 and 3 turns out, the spring is not touching the hammer
when it hits the valve stem, and is barely touching at one turn out.

The colors of the lines are matched from the upper FPE line to the lower efficiency line.



I am fairly pleased to get some of the better efficiency numbers at such high power levels.

But if I am correct I now have a 68 grain hammer and this might have a effect on the out come .Marvin

[lloyd-ss]

Marvin, Yes, you are correct.  The hammer got a little lighter when I drilled it out to accept the .052 wire spring. The last set of numbers that you have, dated 11/26, utilize the lighter hammer, which is still heavier than the stock hammer.  It seems that when the hammer is of a "moderate" weight for the set-up, it takes a substantial amount of weight change to have a large impact on the velocity.

Bob, When I was looking at that chart before uploading it, I was thinking about just how confusing it was.  But I really couldn't think of how to get all the data onto one chart.
I think the fact that this gun is operating with a .90 stroke vs a .65 stroke, changes the behavior of the spring and hammer significantly, and again, the short spring doesn't touch the hammer at impact .

[rsterne]

the short spring doesn't touch the hammer at impact .

which makes me wonder if it is virtually impossible for it to re-open the valve after it bounces?....

Bob

[Motorhead]

the short spring doesn't touch the hammer at impact .

which makes me wonder if it is virtually impossible for it to re-open the valve after it bounces?....

Bob

Exactly my thoughts ... as with also a spring with very minimal preload

[PakProtector]

the short spring doesn't touch the hammer at impact .

which makes me wonder if it is virtually impossible for it to re-open the valve after it bounces?....

Bob

Exactly my thoughts ... as with also a spring with very minimal preload

Both my un-reg'd Marauders got more efficient with shorter travel and more pre-load( energy up by 20%, count at same spread up by 50% ). That this beastie shoots a pellet twice the mass of a King at the same velocity as my 2563 Marauder on nearly the same air is just awesome.
cheers,
Douglas

[rsterne]

Just goes to show we ALL have a LOT to learn about WHY things happen the way they do.... There is no one "right" answer.... but it would sure be nice to understand what is the reason behind results that seem to be polar opposites, wouldn't it?....

Bob

[lloyd-ss]

Just goes to show we ALL have a LOT to learn about WHY things happen the way they do.... There is no one "right" answer.... but it would sure be nice to understand what is the reason behind results that seem to be polar opposites, wouldn't it?....

Bob

So true.  Give me  a pile of dirt and a shovel.  I can figure that out ok. 
There are so many variables in the valve train in a PCP, and many of them behave in very unusual, not at all linear, with steps and reverses. It is definitely a challenge.   Need a way to record the pressure trace during a firing event.
Lloyd

[Motorhead]

Nothing to quantify thoughts ... But thinking about a hammers motion as valve is closing one may view than when no or little spring pressure is present the hammer would be like a "Pogo Stick"
If hammer has no tension it will travel further away from valve stem from the shove of closing valve gaining momentum until spring gets compressed enough to stop it and then fires it back against valve again.

** So it may be safe to assume SOME pre-tension required as nothing more than a damper to lessen valve closing energy exerted against hammer

Always a balance someplace ... it just seems because of all the Variables this balance has few constants   

[PakProtector]

Nothing to quantify thoughts ... But thinking about a hammers motion as valve is closing one may view than when no or little spring pressure is present the hammer would be like a "Pogo Stick"
If hammer has no tension it will travel further away from valve stem from the shove of closing valve gaining momentum until spring gets compressed enough to stop it and then fires it back against valve again.

** So it may be safe to assume SOME pre-tension required as nothing more than a damper to lessen valve closing energy exerted against hammer

Always a balance someplace ... it just seems because of all the Variables this balance has few constants   

Differential Matrix Algebra...

Now the spring is no damper. Well...steel spring is about the worst damper. Rubber spring slightly less bad( I suspect one reason for bstaley success ).
cheers,
Douglas

[lloyd-ss]

mutter, mutter, mutter.  Maybe quantum physics would help.

[PakProtector]

mutter, mutter, mutter.  Maybe quantum physics would help.

Oh no...Newton will do just fine. From Feynman's Lectures on Physics, vol. ii there is an index card-sized box with a few equations on it. Maxwell's we can ignore... All we need is right there.
cheers,
Douglas

[Cal]

I was doing some internet searching on an entirely different interest, and came across this reference

http://www.gatewaytoairguns.org/GTA/index.php/topic,31067.15/wap2.html

I took the time to read the thread, and the part that struck me was Bob's comment that the energy extraction potential was "somewhere between adiabatic and isothermal.

Considering the time involved, I wonder about this aspect.

The calculator returns a much lower energy potential for adiabatic expansion.  With consideration of the gas volume expended with any single shot, there is not much "heat" moving one way or the other regardless of temperature difference.

Would anyone care to conjecture on the pressure behind the pell at the moment the muzzle is cleared for one of these "high powered" examples?

I'm happy not to be a cat with such curiosity.

TIA

Cal

[Cal]

Nothing to quantify thoughts ... But thinking about a hammers motion as valve is closing one may view than when no or little spring pressure is present the hammer would be like a "Pogo Stick"
If hammer has no tension it will travel further away from valve stem from the shove of closing valve gaining momentum until spring gets compressed enough to stop it and then fires it back against valve again.

** So it may be safe to assume SOME pre-tension required as nothing more than a damper to lessen valve closing energy exerted against hammer

Always a balance someplace ... it just seems because of all the Variables this balance has few constants   

It should be noted that in the case of two springs opposed, (vis striker spring and reservoir pressure acting on the valve stem) the rates are Summed.

It would be easy to have a very high rate "hammer spring" during the valve actuation displacement.

How this may fold in to "cause and effect"  for an airgun...I have not considered ;-)

[PakProtector]

On strike, mV^2=k'x^2, now k'=k_valve return spring, k_air pressure. k_air pressure is a f'n of opening, chamber pressure and valve pocket pressure...likley with interdependancies that are not linear. Say valve all the way open, and chamber pressure is high, and valve pocket area comparable pressure...k' dominated by valve return spring. I smell path integral to determine energy returned to hammer...grrrr
cheers,
Douglas

[rsterne]

So it may be safe to assume SOME pre-tension required as nothing more than a damper to lessen valve closing energy exerted against hammer

I don't think that is safe to assume at all.... pre-tension on the hammer spring raises the resonant frequency somewhat, and because of the preload the hammer is still being returned to the valve stem "actively" by the hammer spring....

I have no idea if having the hammer NOT preloaded when in contact with the valve stem reduces the tendency for the hammer to reopen the valve or not.... but take it to the extreme.... Put the hammer a foot away from the valve when it hits the hammer spring on rebound.... Wouldn't friction rob the hammer of a lot of it's velocity in both directions while it's not in contact with the spring?.... At some point, wouldn't the hammer stop before even reaching the point where it compresses the hammer spring on rebound?.... However, if the hammer spring was strong enough to launch the hammer with enough velocity.... it would still hit the valve hard enough to open it.... ONCE....

I'm just assuming the same thing MAY happen over a much shorter scale when the hammer spring has no static preload.... Can't prove it, but it sure seems logical.... and it just MIGHT help explain why two relatively equal hammer strikes (they have to be, they are resulting in the same velocity).... are resulting in such large differences in efficiency.... I think an audio recording of the muzzle blast to see if you can detect a second "blast" from hammer bounce with the long spring could prove very interesting....

Bob