GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: Motorhead on November 19, 2016, 02:48:46 PM
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This needs to be somewhat generic in that the results talked about below are not a specific gun noticed effects.
Tho in this case prompting this post it is based upon further R&D tuning with my WAR WarP carbine.
To set the stage some baseline info so we can structure whats in place and then talk about the why ?
* We have in place a mechanical SSG that is a Rear spring tension/travel screw anchored unit so ALL the weight of spring, stop rod and required parts stay put as hammer is thrown at valves poppet.
Entirety of test the OAL / spring pre-load of the assembled SSG remained unchanged.
Spring allowed sufficient travel from short stroke to long stroke where it just went into coil bind.
* We have a hammer of @ 31 grams with an adjustable striker nose that allows Stroke changes ( Like an M-rod P-rod etc ...)
* We have a Regulator set at @ 1900 psi and well over 40cc's of plenum volume.
* This is with .177 caliber pellets from 8.4 to 13.4 grains weight & same basic effect was constant.
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So what I am trying to wrap my head around is this ...
In testing we place the hammers stroke adjuster out far as practical " Short Stroking" hammer.
Set the SSG to have @ .050" gap (* ALL Stroke changes SSG SETTINGS RETURNED TO THIS SPEC )
Take a few shots and note ( same pellet weight for now ) velocity of @ 650ish.
Now we start INCREASING stroke incrementally @ .100" per velocity check & SSG reset *
Note that we are getting a rise in velocity as stroke is increased.
In the @ mid range range of adjusted stroke velocity stabilizes and stays +/- 20-30 fps within about a +/- .100" stroke change either way. ( @ 960 fps )
As we continued to INCREASE stroke the speed starts falling away and continues this trend all the way to full stroke settings :P
:o :o :o What is baffling me the SPRING RATE / PRELOAD and SSG gap has remained a constant.
The increasing Momentum with greater stroke should be increasing valves dwell and increase velocity to a point where were just dumping air not accelerating pellet ??? .... but this is NOT what I am witnessing happening.
As stroke was increased and loss of velocity noted the muzzle report while coming up some just as velocity started to fall ... it became quieter as velocity fell and stroke on hammer increased.
Talk about counter intuitive :o :o :o
My feeling are the WEIGH of hammer and the available momentum at a given spring energy found a sweet spot & the gains of stroking it more finds no further speed under the given spring rate driving it.
But WHY the loss in velocity as hammer stroke was made longer with the same spring rate driving it ?
What am I not seeing or understanding here ?
Scott
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The hammer is most likely hitting the valve body. The stroke adjuster recesses into the hammer nose lengthening the travel distance before hitting the valve pin. Once past the sweet spot you start getting a Bstaley effect.
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My first thought was that the spring is pushing the same weight farther, but do like the valve poppet bottoming out theory too.
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The hammer is most likely hitting the valve body. The stroke adjuster recesses into the hammer nose lengthening the travel distance before hitting the valve pin. Once past the sweet spot you start getting a Bstaley effect.
This is NOT the case here being the Striker nose on hammer is always out in front of the hammers body / face surface.
Granted if the poppet stem was hit hard enough the @ .375" diameter face of striker would bottom out on the valve body, but that would require @ .180" of poppet lift to get it flush with valve body.
Good input tho ;)
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This is what happens when your TP or valve throat chokes off the flow past a certain threshold. Very common in Evanix bigbores. Not knowing your specs. it just more fodder.
Tom
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Scott,
I have an idea of what your observing (just an idea). Spring rate "lbs per in". Every time you adjust the gap, your adjusting the spring rate on the SSG/hammer assembly due to the cocking distance.
I have an idea that might help figure it out though.
Using three of your settings that you observe the low to peak to low again, see if you can measure the force it takes to cock the gun. This may explain what your experiencing.
Fish scale on the bolt or put the butt of the stock on a bathroom scale and cock the gun. Might work for some rough numbers.
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If the gap between the striker face and valve stem remains constant and the distance the spring has been compressed is the same then the force exerted on the hammer should also be the same. If i'm picturing the ssg setup correctly the captured spring is resting on the striker inside the hammer. Extending the striker past the hammer face reduces the distance the hammer face has to travel to engage the sear compressing the spring less and therefor lower velocity.
If im picturing it wrong disregard this post :P
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If the gap between the striker face and valve stem remains constant and the distance the spring has been compressed is the same then the force exerted on the hammer should also be the same. If i'm picturing the ssg setup correctly the captured spring is resting on the striker inside the hammer. Extending the striker past the hammer face reduces the distance the hammer face has to travel to engage the sear compressing the spring less and therefor lower velocity.
If im picturing it wrong disregard this post :P
Lol ... Captive SSG only pushs hammer from the backside. Basic configuration like this ;)
(http://i1320.photobucket.com/albums/u532/scott_schneider1/DSCF9928_zpsdwlpxaou.jpg)
Tho fitted to diffent proportion within a Warp rear cap. Stop rod go's out the back side when cocking.
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Where does it contact the hammer the end of the adjustment screw or bottom of hammer like a stock spring?
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Where does it contact the hammer the end of the adjustment screw or bottom of hammer like a stock spring?
Bottom of the spring cavity like a conventional spring set up.
The head on the stop rod within spring cavity is @ 3/4" long and has a counter bore drilled in it so as the strike / stroke screw is adjusted for more stroke & drawn in having it enter spring cavity it simply go's up into this counter bore having the SSG / spring come to rest in hammer spring cavity the same position irregardless of stroke setting.
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Something people don't think about with hammers with an adjustable nose, is that as they recess the striker (or move the nose closer to the hammer face), they are not only increasing the hammer stroke, but DECREASING the preload on the spring (note, not the spring RATE, but the force decreases).... Scott says he is changing the SSG adjustment to maintain the same 0.050" gap, however, so that eliminates (or should eliminate) that variable.... I assume, Scott, that you mean when you lengthen the stroke 0.100" (moving the hammer body forward that far) that you more the SSG gap adjuster 0.100" forward as well?....
Assuming that is the case, then as you increase the stroke, since you are starting with the same preload and gap, you compress the spring further, so the force when cocked increases.... Therefore, you increase both the hammer energy and momentum.... If there was no preload on the spring, doubling the stroke would double the momentum, but increase the energy by a factor of four.... Since dwell is proportional to momentum, and lift is proportional to energy, the lift of the valve would increase rapidly as you increase the stroke....
The first part of the hammer energy is used up to crack the valve off the seat.... That force is constant, at constant pressure (the case here), but it robs a portion of the energy and momentum before any useful lift and dwell can take place.... This means that the energy available to produce lift will be even MORE than 4 times as much if you double the stoke.... It could be that when you double the stroke, you may be getting, say, 5 times the lift.... If the valve was opening 1 mm at minimum stroke, and you double the stroke, it might now be opening 5mm (just an example, not hard numbers)....
I think it MAY be possible that after you go past the mid-point of your stroke, the valve stem may be being driven flush with the back of the valve, and the hammer starts rebounding off the valve in an elastic collision.... The valve can't be driven open any more than flush (0.180"), but the hammer keeps bouncing away from it faster with increased stroke, and so the dwell gets progressively less.... I don't know for sure if that is what is happening in your gun, but I have definitely seen that happen before on a gun with adjustable stroke.... In that case, I didn't have an SSG, so I could fit a lift measuring rod that stuck out the back of the gun, and had proof that the hammer was hitting the valve.... Once that point was reached, increased stroke REDUCED the velocity.... I have seen similar things before with other guns, and any time I plot velocity vs preload and I see that plateau, and then decline again with increased preload, I always suspect that is what is happening....
Increasing hammer strike by increasing stroke instead of preload has virtually the same effect.... If the pressure is a constant, the velocity will increase rapidly (from low numbers), then more slowly (on the knee), and eventually plateau (which you are experiencing).... If it declines after that, I think the most likely suspect is the hammer hitting the back of the valve and bouncing off it.... The point is, however, that you are already up on the plateau, so you are past the knee where most efficient operation takes place.... So even if I'm wrong, I don't think it really matters, as you shouldn't be there.... :o
Bob
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Scott could it be you have the reg set at 1900 psi and a 40cc plenum so that after a certain amount of energy in the hammer strike is reached you are releasing all of the 40cc volume of air so that the pressure behind the pellet actually drops faster than it can leave the barrel so the friction slows it before it exits the barrel. It may be you are at the peak of the plenum volume of air so after a certain amount of energy in the hammer strike any more is depleting the plenum to fast to increase the pellets velocity any further and actually causes it to slow down.
That could explain the sound you are hearing going from a loud report to a slightly lower report since the air has lost some of its velocity before the pellet is out of the barrel.
Mike
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The barrel volume is only 5 cc, there is no way you can dump 40 cc of air at 1900 psi into it.... That is 320 std. CI, and we're talking about 20 FPE, which would produce an efficiency of just 0.06 FPE/CI....
Bob
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striker face = surface that contacts the valve stem
hammer face = surface where the sear engages hammer
with the striker flush with the hammer face lets say the hammer face has to move back 1" to engage sear
if the striker is sticking out 0.25" past the hammer face, it will only move back 0.75" to engage sear
you compress a 2.5" 10 lb spring down to 2" (pretension) and have same gap between striker face and valve stem.
when you cock the flush striker the spring is compressed an additional 1" (total 1.5")
the extended striker cocked will compress the spring an additional 0.75" (total 1.25")
10 lb is required to compress the spring every inch so it will take 2.5 lb to compress it 0.25"
the flush striker have 15 lb of force ( 2.5 x 1.5 x 4 ) behind the hammer
and extended striker would have 12.5 lb of force ( 2.5 x 1.25 x 4 ).
Force opening valve will determine pellet velocity. The more force the higher the velocity. Would it be safe to assume the more the striker extends past the hammer face the lower the velocity if spring pretension is the same?
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Bob,
Yes in that as Stroke increased in @ .100" increments and hammer as it sits against poppet deeper, the SSG as a "cartilage" assembly is simply moved further into the rear the same .100" incrementally maintaining the GAP as a constant.
Guess i need to remove hammer and look carefully at face of valve body abd see if the 3/8" striker is leaving a mark being telling of a collision and subsequent rebound effects in play ???
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The barrel volume is only 5 cc, there is no way you can dump 40 cc of air at 1900 psi into it.... That is 320 std. CI, and we're talking about 20 FPE, which would produce an efficiency of just 0.06 FPE/CI....
Bob
Bob
I was not sure if that was happening so was just throwing it out there. But yes if the barrel is only 5cc then it would be difficult if not impossible to empty 40cc of 1900 psi air at one time. I knew you would chime in before to long.
Thanks Mike
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Scott, get some bearing clearance putty (small threads of malleable plastic) that crush to show clearance.... Even a small dab of putty on the back of the valve, adjacent to the stem.... I'm betting it will get flattened....
Bob
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striker face = surface that contacts the valve stem
hammer face = surface where the sear engages hammer
with the striker flush with the hammer face lets say the hammer face has to move back 1" to engage sear
if the striker is sticking out 0.25" past the hammer face, it will only move back 0.75" to engage sear
you compress a 2.5" 10 lb spring down to 2" (pretension) and have same gap between striker face and valve stem.
when you cock the flush striker the spring is compressed an additional 1" (total 1.5")
the extended striker cocked will compress the spring an additional 0.75" (total 1.25")
10 lb is required to compress the spring every inch so it will take 2.5 lb to compress it 0.25"
the flush striker have 15 lb of force ( 2.5 x 1.5 x 4 ) behind the hammer
and extended striker would have 12.5 lb of force ( 2.5 x 1.25 x 4 ).
Force opening valve will determine pellet velocity. The more force the higher the velocity. Would it be safe to assume the more the striker extends past the hammer face the lower the velocity if spring pretension is the same?
Yes ... that is the correct read.
Being GAP is a constant, the amount of stroke gained becomes the same INCREASE in spring compression when cocking.
Energy delivered to valves poppet should Exponentially increase with stroke and decrease as stroke is shortened.
* That's the point of the post ... we are not getting that past a point velocity peaks and adding more stroke to hammer and more spring tension the velocity starts dropping away.
Never figured .180" of lift would happen, especially with 1900 psi on a .312" poppet head over a .250" throat struck with just 31 grams of hammer with a fairly light hammer spring ???
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Could be something else happening.... but once you are on the plateau, additional stroke piles up the lift quickly.... If you find out the striker isn't hitting the back of the valve then we have a mystery indeed....
Bob
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The hammer is most likely hitting the valve body. The stroke adjuster recesses into the hammer nose lengthening the travel distance before hitting the valve pin. Once past the sweet spot you start getting a Bstaley effect.
This is NOT the case here being the Striker nose on hammer is always out in front of the hammers body / face surface.
Granted if the poppet stem was hit hard enough the @ .375" diameter face of striker would bottom out on the valve body, but that would require @ .180" of poppet lift to get it flush with valve body.
Good input tho ;)
Just catching up on this thread, and thought that was a good match of the symptoms, too. :(
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I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).
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I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).
Agree ... in a typical purpose built gun, My FT rigged 19 ft lb M-rod ran for years quite successfully on a 14 gram hammer. Tad more weight required with an SSG tho.
PCP's are odd little beasts that no sooner than you think you got em figured out they throw you a curve ball.
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I couldn't agree more Scott.
This thread is going to help a lot of members out with their tuning.
After all the hammer weight, stroke, and spring rate is what it's mostly about.....(just have to play a little with that balancing act between them).
Once you do.....it's a sweet thing!
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I have had excellent results with even much lower hammer weights....15g-19g.
Running really weak poppet spring and a pretty strong hammer spring.
I think my hammer being so lightweight compared to anyone else's....married to the really light poppet spring (my valve opens/shuts very quick).
Your results with that light of hammer have surprised me. Especially given this is a 25 cal reg'ed at 2200psi.
So, it seems to be keyed off of the very light poppet spring. That LW hammer weight seems more inline with a 177 reg setup, especially with ssg. And that is with a much less than stock poppet spring. I don't recall seeing much experimentation with the poppet spring strength/preload, probably because it is such a pita to change/adj.
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Bob,
Yes in that as Stroke increased in @ .100" increments and hammer as it sits against poppet deeper, the SSG as a "cartilage" assembly is simply moved further into the rear the same .100" incrementally maintaining the GAP as a constant.
Guess i need to remove hammer and look carefully at face of valve body abd see if the 3/8" striker is leaving a mark being telling of a collision and subsequent rebound effects in play ???
Put a tiny ball of modeling clay on the valve body face. Once you fire the gun it will show you how close the hammer gets.
Edit: Bob already suggested that. Please remove.
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Scott
As Bob suggested you can use modeling clay or even some plastigage that is used to measure bearing clearance in engines when building as I believe he stated that also just used a different name for it.
Mike
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Wouldn't it be silly if it was just the transfer port leaking through between the valve and the pressure tube. :D
On the SSG for it, I didn't see any pass through holes to allow for air compressing between the face of the hammer and the face of the valve.
Just another possibility ? ;)
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Agree ... in a typical purpose built gun, My FT rigged 19 ft lb M-rod ran for years quite successfully on a 14 gram hammer. Tad more weight required with an SSG tho.
PCP's are odd little beasts that no sooner than you think you got em figured out they throw you a curve ball.
These PCPs is keep us enthused and motivated and for some, eager to share their findings with others. I for one enjoy tinkering with mine and sharing what I've found regardless of others shortfalls towards me.
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That's a really good thought, Tim.... If the space between the hammer and the back of the valve does not have sufficient venting, it can get pressurized from a leak around the valve stem, blowby aft from the transfer port, or just the movement of the hammer itself.... If Scott finds that the striker isn't hitting the back of the valve, he can look for those things while he's in there.... I've had that problem in a few PCPs year ago, just forgot about it.... I know Scott had it happen on his B-50/51 as well.... I actually had an early version of my Hayabusa go "machine-gun" on me because of a leaky valve stem, a good fit on the hammer, and no vent hole.... A 3/16" hole in the side of the tube right behind the valve cured it completely....
Bob
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I would think the trigger slot in the tube should be capable of venting the air in front of the hammer fairly easy but I may be wrong depending on how well the trigger housing is sealed when assembled. If the valve stem is loose enough to allow pressurized air past it in any great amount that certainly could be what's happening. its worth looking at indeed.
Mike
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Well,
While not having any clay handy, did remove hammer and place some Prussian blue onto strikers face.
Maxed out stroke and fired a few shots .... CONTACT :o with face of valve body.
So the kinetic rebound theory likely my culprit here.
Now whats interesting and likely a LIGHT HAMMER cause & effect is that we're getting .180" of lift !!! WOW !!
But due to hammer weight being so low the valve is able to close super fast ( Bob S did some math on this within another thread )
The DWELL is so low and valves opening and closing so fast were not really getting to a point where DUMP shots are happening ... not even close it would seem ???
My read on this ( Past the reason for the velocity decrease we likely have a handle on ) is that we are operating at the extreme side of Low Dwell / High Lift where as at the other side we could use a HEAVY hammer with a light strike and get Low lift / High dwell
It appears most PCP's likely operate somewhere in the middle +/- having modest lift and modest dwell.
** BTW,
Hammer used is well vented and has 4 lengthwise flutes of 1/2" radius @ .075" deep end to end. @ 50% of the contact area hammer has to the tube is air clearance. This not only gives air a path to vent front to back, but also reduces frictional losses.
(http://i1320.photobucket.com/albums/u532/scott_schneider1/DSCF0182_zpsnhmbcpov.jpg)
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So you have been giving that light hammer a real workout bouncing it back and forth off the valve and hammer spring.
Makes sense and another thing to keep in mind with custom tuning.
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Thanks for the confirmation, Scott.... always nice when the suggested answer turns out to be the correct one.... You are absolutely correct, the relationship between lift and dwell depends on the hammer weight, for a given spring and hammer stroke.... The average spring force (average of preload and cocked) and the travel are the governing factors in hammer energy, which affects the lift.... The weight of the hammer doesn't affect it.... however, changing the hammer weight changes the momentum, which changes the dwell.... Using a heavy hammer, for a given spring and travel, therefore increases the potential amount of air released, and hence the potential FPE....
Since the area under the lift/dwell curve governs how much air the valve allows to get to the pellet, and that is what governs the potential FPE, there are many combinations of lift and dwell that will produce the same FPE.... If you do it with a very light hammer, you get lots of lift and not much dwell.... If you do it with a heavy hammer, you gets lots of dwell and not much lift.... A light hammer will generally require a bit more spring force to make up in dwell what it loses in lift to produce the same FPE.... It appears that in this case, Scott's light hammer was running into the back of the valve, limiting the lift and dwell, and then rebounding faster as he continued to increase the hammer stroke, reducing the dwell and hence the velocity....
Bob