GTA
All Springer/NP/PCP Air Gun Discussion General => PCP/CO2/HPA Air Gun Gates "The Darkside" => Topic started by: bstaley on October 20, 2012, 04:11:59 PM
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I originally posted this on the green, and Bob asked me to repost it here:
There's been a lot of discussion recently on this forum and a couple of others about hammer bounce and HDDs. It seems that quite often when this subject is discussed, people state that the o-ring buffer mod that I wrote about is a type of HDD. I know in the past that some have touted similar arrangements where items such as rubber washers, felt pats, etc. are placed around the valve stem as an effective HDD. I consider a true HDD to be a device that is somehow attached to the hammer and eliminates hammer bounce by dissipating the kinetic energy that is imparted back to the hammer from the valve stem into heat energy via friction, ideally without impeding the forward motion of the hammer in any way. A one way dampener if you will, such as Steve's fine devices. I think you will all agree that the o-ring buffer and any similar items placed around the valve stem do no such thing.
As some of you have also mentioned, it's not necessarily that the hammer bouncing is a bad thing, but if that bounce results in the valve opening a second (OR MORE) time, then air is wasted and efficiency suffers. So really when we discuss hammer bounce we are really interested in improving the air usage or efficiency of our PCP toys.
As I see it there are at least a couple of 'shortcomings' of the common self- regulating poppet valve system that result in inefficient air usage. As rsterne and others have pointed out, PCPs are generally very efficient during the first part of the shot curve, but become much less efficient on the 'downside' of this curve.
So, what is different about the first part of the curve and the last part of the curve that makes this difference so apparent?
During the early part of the shot curve, the pressure behind the valve is at its highest. The hammer hits the valve with less energy than is needed to obtain maximum flow. The valve only opens a relatively small amount (very little lift). The valve shuts quickly (short dwell). One could say that at this point we have too little hammer energy for the pressure in the reservoir. Some have also surmised, perhaps correctly, perhaps not, that the effects of hammer bounce are minimal, if not non-existent, during this part of the shot string. I tend to more or less agree with this view. The valve is harder to open at this point. The energy being returned to the hammer by the valve stem happens very quickly and over a short distance. Both of these factors probably mean that the energy will not be transferred back to the hammer efficiently due to its large inertia. So at this point, most of that energy probably goes to heat and vibration and the hammer probably does not bounce too far, and probably does not re-open the valve, or if it does, less air is probably wasted.
During the last part of the shot string, this whole dynamic is completely different. At this end we have too much hammer energy for the pressure. The valve is easier to open. It opens further (more lift). It stays open longer (more dwell). As for hammer bounce, the energy being imparted back to the hammer happens at a slower rate(like the dwell), and over a greater distance (the lift). Both factors that will likely cause the hammer to bounce back further. That coupled with the fact that the valve now only has about 2/3 the pressure behind it as when the first shots were fired probably means that hammer bounce will be more pronounced and more likely to result in the valve reopening and wasting air.
All of the other shots between the first and last, fall somewhere between these two extremes, with everything being in optimal balance right in the middle of the curve.where the peak of the sweet spot lies. It also seems to me that in addition to hammer bounce, there is another big factor in air usage/efficiency that is happening as the shot string progresses. As noted above, as we move though the shot string, both lift and dwell increase. At some point in the string, this will probably result in air continuing to flow through the valve long after (relatively speaking) it can have any influence on the final velocity of the pellet. So this becomes another source of inefficiency that is probably also most pronounced in the latter part of the shot string.
So, how can we overcome these shortcomings? The first thing that comes to mind is varying the hammer energy. If we could control the hammer is such a way to make it hit the valve harder for the early shots and slightly less hard with each subsequent shot, we could accomplish two things. In the early part of the string, we could increase the velocity of the shots on the rising side of the curve, resulting in a much flatter early part of the shot sting. In the latter part of the string we could limit or eliminate some of the wasted air by shortening lift and dwell so that less air is released beyond the point at which it can affect the velocity of the pellet. This would also flatten/lengthen our shot string since pressure would change would be less with each shot.
Great in theory but not too easy in practice. This is where we get into the realm of the electronic guns. Lots of potential there for efficiency and consistency improvements, but at a cost in dollars, complexity, and perhaps reliability.
So, back to the o-ring buffer. Even though I do not consider it to be an HDD, I do consider it a component that can be used to improve the efficiency off the self regulating poppet valve system. Let's look at how the o-ring buffer changes the behavior of the system discussed above.
Once the buffer is installed and everything is properly adjusted, during the first part of the shot string, where the system is already very efficient, the buffer does just about .... nothing! The lift of the valve is very small and most, if not all of the hammer's energy is used just to open the valve. Now, as the shot string progresses (towards the inefficient part), the valve opens a little more with each shot and at some point, the body of the hammer starts to come into contact with the o-rings. At first, most of the hammer energy is still being used to open the valve and very little energy is transferred to the o-rings, but with each shot, a little bit more of the hammer energy goes to the buffer and a little less to the valve.
So how does this improve efficiency? I believe that it helps alleviate both of the causes of inefficiency mentioned above. In the latter part of the shot string, the buffer absorbs more and more of the hammer's energy with each shot, limiting both lift and dwell, so that less air is sent down the barrel that has no effect on the pellets velocity.
I also believe that it does have some effect on hammer bounce. Because the valve lift and dwell are shortened in the latter part of the string, and because the hammer energy is now being imparted to two separate items (valve stem and buffer), that have very different rebound properties, the net bounce of the hammer is likely lessened, also hopefully resulting in less wasted air.
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Thanks for the re-post.... Elegant in both understanding of PCPs and explanation of your buffer system, now only how it works, but more importantly, why....
Bob
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Outstanding ... very eloquent writing easy to follow and understand ;)
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Sometimes, it doesn’t work as simply as just adding o-rings.
Been working with getting the shot count up from a little 2300s converted to HiPac.
One o-ring, and the shot count stayed the same.
Two o-rings and the shot count went DOWN and the velocity dropped 14fps. Now understand, I only count the shots within 3% as the true “sweet spot”.
Three o-rings and it acted like a machine gun when it got to 1600psi (brappppppppp).
Four o-rings and it acted like a machine gun when it got to 2000psi (brapppppppppppppppp).
Just too much "spring" to the o-rings would be my guess in this case.
On the other hand, adding one o-ring to a QB79 HPA moved it from 105 shots at 12.2 foot pounds (call it 1281 foot pounds per fill) to 200 shots at 11.5 foot pounds (call that 2357 foot pounds per fill). AMAZING amount of gain.
IMHO, it is not a cure-all. BUT it is definitely worth a try on e very PCP/HPA/CO2 that I am working with.
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Definitely not a cure all, just another tool in the designer toolbox.
The length of the buffer relative to the valve stem is VERY critical. That's why this mod is at it's best when coupled with the adjustable stroke hammers in the m-rod and p-rod. And why I only really advocate it's use in those. Adjusting the striker faceplate changes the apparent buffer length in very precise amounts. Lacking this you really need to use some other material and cut it to a very precise length based mostly on trial and error. The thickness of an o-ring is pretty significant compared to the differnence in valve lift between the first and last shots in a shot string.
Just based on the little information you provided above, the 2 o-rings was probably closest to the correct length, but just slightly too much. you could probably get back the velocity and shot count by increasing the spring tension and making the gun breath better (if you haven't already) with valve mods and a bigger transfer port.
The mod does what it does by limiting valve dwell and lift, so it works best when tuning down a gun that is capable of much more power.
.
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If we come up with new design to a power valve where the cylinder pressure does not have any significant role in how much force is needed to open it, would it be easier to control its rebound in comparison to a traditional "pop-open" style valve where the pressure is always acting on the valve seal to oppose any unneeded hammer rebound momentum?
I think it would present new challenges as far as rebound control is concerned since you would then have a valve that could be easily opened with lower strike forces.
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There are "blow open" valves which require minimum force to operate, and once cracked dump virtually all the air in the valve before closing.... In order to produce constant velocity, they need constant pressure.... If we were to build a valve as you suggest, regardless of how that affected bounce, I would suggest that it would be most useful in a regulated PCP.... In an unregulated PCP, we rely on the varying force to cause the gun to "self-regulate" and produce that bell curve we strive for....
One way it could be done would be to have a very easily opened valve that stayed open to dump all the air and have a shock-absorbing material (or a brake) to stop the hammer after the valve opens.... Since there is no closing force opposing the hammer, there would be no bounce.... and even if there was, the valve would stay open until the gun was either recocked or the valve reset in some way....
Bob
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The mod does what it does by limiting valve dwell and lift, so it works best when tuning down a gun that is capable of much more power.
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I think I am still within that definition, even though my 2263 is putting out 37-39 FPE across the string. The rifle is sporting a 2563 valve and transfer sleeve, as well as some porting to the inlet and a closing spring shimmed .146" tighter.
I saw a post by Ed Canoles on lubing his springer piston o-rings with moly powder. I suspect they'd be some slippery little bits...and for something working to effect hammer energy delivered to the valve across a defined/constrained stroke it looks like a good idea to have them slippery. Have you seen any particular benefit to moly v. graphite or some other lube? or does slippery o-rings make this mod go to heck in handbasket?
Thanks for the description of your Buffer invention, it covered questions I didn't even know I had...:)
cheers,
Douglas
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Yep, should work just fine with all your mods...assuming you aren't trying to squeeze every last FP out of it.
Lubricating the o-rings is a good thing. Some have reported inconsistent velocity with dry o-rings, presumably due to inconsistent compression due to the friction from the tube wall......although not really sure what effect the dry molly might have. That's some slippery stuff. Mostly I've just burnished molly paste int the tube wall and on the hammer with good results.
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If there was lighter spring in front of the hammer where the hammer would have enough power to compress the lighter spring and at the same time have enough power to strike the valve. Then on the rebound or the bounce the lighter spring could hold the hammer from hitting the valve. Would this work in theory?
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hey Ribbonstone, I've been thinking on your issue; I wonder if a thin spacer and one o-ring might do the trick. The thing would behave like it had a longer but stiffer o-ring.
cheers,
Douglas
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I reversed the simplest of rules: its the SQUEEKY wheel that needs the grease.
Without the o-ring mod, was getting an efficeny of about 11.6 bar-cc/fpe (or about 1.4 fpe/cuin). Changing something as major as the dwell/stroke in isolation was not a wise idea.
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It is an HDD still because it is a Hammer Dampening Device rather than a Hammer DRAG device. They are both effective Debouncing devices but one causes innacuracy which to me is intolerable. Dragging on ahammer has only recently been the solution and it has a lot to do with the fact that it is a money maker for the Maker & Distributor.
I applaud your simple and effective solution. Elegant in it's simplicity. Hard to commercially gain from something so easy to duplicate but more important it has no drag on the hammer and is a better way to solve the Bounce/Duration issue.
Well done!
Getting the hammer weight/momentum just right is paramount to getting the great effceintcy. If you have the good efficiency that is when you are just filing the barrel up behind the pellet for the power you need. Some tunes will hit that sweet spot as the duration lengthens and will shoot better groups at the low end. This can be tuner induced with a little fine tuning of the stroke and hammer weight.
I use hammer weight as a primary adjuster and hammer spring as a fine adjust of valve duration.
I use the valve head size as the primary adjuster of power via volume and the Port to sink with that for flow during the duration.
The same system that gives 45 fpe in .25 cal can give sub 12 FPE with a super light hammer tiny port and huge valve head. All power levels being with the same hammer spring within a few turns of adjust in tension as a fine adjust.
Accuracy is the key and when people take their eye off that objective Science can come up with stuf we don't need or want, but they sell, so the sellers will push. It is called commerce.
Widgits sell even if they are a heap of junk. CASE IN POINT! HDD might mean Heaps of dollars for dung.
Later
Tim
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I reversed the simplest of rules: its the SQUEEKY wheel that needs the grease.
Without the o-ring mod, was getting an efficeny of about 11.6 bar-cc/fpe (or about 1.4 fpe/cuin). Changing something as major as the dwell/stroke in isolation was not a wise idea.
I'd be happy with that efficiency...but I assume it was not on a 40 FPE Marauder 22 cal...:) I will break the shot count into two bits, one absolute efficiency and the other being able to work across a larger pressure variation. The first is slightly preferable, but it is only a tic ahead of the latter.
cheers,
Douglas
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Thanks Tim! That means a lot coming from someone with your expertise and reputation. I can only wish I had the resources to modify hammer and valve parts so I could play with all the variables. But for those of us who don't, its a simple mod, that in my experience yields pretty good improvement to the m-rod and p-rod shot strings.
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The ugliest gun I own (OK, that’s an opinion, but one I think many will agree with) is this 5mm Talon SS.
But it is nearly indestructible and nearly infinitely adjustable.
Wanting 12 foot pounds, went with HPA (1200psi output) screwed into the CO2 adapter. Kept fiddling with the adjustments. Was getting 105 12-foot pound shots per tank fill, consistent velocity, and good accuracy.
(http://i157.photobucket.com/albums/t50/ribbonstone/5mm/DSCF2436-1.jpg)
The top hat is adjustable, but rather than adjust it, slipped two o-rings under the top hat. Was a no-go, vel. was way too low and consistency suffered.
Went to one o-ring under the top hat. Consistent, but a little slow for 12 foot-pounds. Dialed it up (increased striker spring tension) and got back to 12 foot-pounds.
Went from 105 shots to just at 180 shots (if I run it all the way past regulator pressure_. Normally, will stop at 1500psi, which is worth a solid, small variation, 160shots.
(http://i157.photobucket.com/albums/t50/ribbonstone/5mm/DSCF2437.jpg)
Think you could adjust the top hat to get the same effect. But if you do, the bolt is rattle-loose when closed. I kind of like it this way; bolt closes without “rattle” but the dwell is short/fast enough to make efficient use of the short barrel.
Does this work the same as the BStaley o-ring mod? Am thinking it does, just on the far side of the valve stem rather the near side.
Afraid this was only at 22 yards, as that’s all I have. Certainly good enough for whatever I need this little gun to do.
(http://i157.photobucket.com/albums/t50/ribbonstone/5mm/DSCF2432-1-1.jpg)
DOWN SIDES:
Its’ got the ergonomics of a broomstick.
With the sights so far up, makes those occasional ultra-short shots a bit of a challenge.
It’s grip-pressure twitchy. Probably due to balance and having the running parts (striker/spring/top hat) sitting right over the balance point.
Really needs to be taken apart and detail cleaned more often than other PCPs. For whatever reason(s), the striker, bolt, and striker spring seem to get fouled faster than other PCPs (which usually only need yearly cleaning). So long as you have it apart, change the bolt o-rings to keep it quiet.
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Yep, pretty much the same thing. I've just started playing with this on my .25 Talon SS. As it turns out, putting an o-ring under the tophat has been a popular mod for years in the AF crowd....I only found out about it later after I got my first Talon.
Yes, the adjustable tophat gives you much that same control that you have with the striker in the m-rod and p-rod.
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One more question: harder or softer O-rings? I got a set of squooshy black and a set of slightly harder brown viton.
cheers,
Douglas
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Hard to say. Sounds like a good experiment. ;D
My guess would be harder would give a flatter shorter string. Softer would be longer but more ES. But in an airhog of a .25, probably not a noticeable difference either way.
I've always just used 70 duro.
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Hard to say. Sounds like a good experiment. ;D
My guess would be harder would give a flatter shorter string. Softer would be longer but more ES. But in an airhog of a .25, probably not a noticeable difference either way.
I've always just used 70 duro.
It is a 22 cal that currently makes nearly 40 FPE for 22 shots... 8)
cheers,
Douglas
will begin tomorrow; 3-day weekend!!!
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Res Shot vol closing Recorded developed Energy
SHOT # PSI cubic in force lbs Stroke psi used velocity Friction fpe
1 3200.00 0.2120 144.76 0.1143 52.185 883.40 6.948 43.971
2 3147.82 0.2177 142.40 0.1162 52.714 885.50 6.835 44.180
3 3095.10 0.2239 140.01 0.1182 53.295 887.50 6.721 44.380
4 3041.81 0.2301 137.60 0.1203 53.840 890.60 6.605 44.690
5 2987.97 0.2350 135.17 0.1225 54.013 889.40 6.488 44.570
6 2933.95 0.2420 132.72 0.1247 54.617 891.00 6.371 44.731
7 2879.34 0.2515 130.25 0.1271 55.704 897.40 6.252 45.376
8 2823.63 0.2575 127.73 0.1296 55.930 896.10 6.131 45.244
9 2767.70 0.2718 125.20 0.1322 57.866 908.00 6.010 46.454
10 2709.84 0.2762 122.59 0.1350 57.574 902.00 5.884 45.842
11 2652.26 0.2846 119.98 0.1380 58.064 901.90 5.759 45.832
12 2594.20 0.2918 117.36 0.1410 58.230 899.10 5.633 45.548
13 2535.97 0.3000 114.72 0.1443 58.522 897.20 5.507 45.355
14 2477.45 0.3098 112.07 0.1477 59.039 896.30 5.380 45.264
15 2418.41 0.3187 109.40 0.1513 59.039 895.00 5.251 45.133
16 2359.37 0.3276 106.73 0.1551 59.456 890.00 5.123 44.630
17 2299.91
This is a bit of data from my old tune before I enlarged the porting in my .25 madauder. Efficiency is only .89 as per Bobs method of calculating efficiency. I was using the B-staly method to achieve this string. I had also developed a tune using this method that produced 825-835-825fps over 10shots. It makes ballancing a tune very easy. In fact, even though I am able to achieve the same tune without the o-rings, I can do it much faster with the o-rings.( less pellets :D) Here's the part that may interest you. If you observe shot 16, you will see the psi at that shot. It corrisponds with what is on the gauges. The volume for that shot is .328ci @2359psi. At this shot(16), the amount of air that could flow through this factory valve is .346ci @ 2359psi. If I shoot the next shot at the posted 2300psi, the velocity is 887.7fps using .337ci of a possible .343ci. Next shot would render 877fps using .342ci of .342ci. Here's the part that really tells the tale. The hammer stroke adjustment is 3 turns. with the adjusters screw pitch, that is .150" of stroke/ lift of the valve. The valve stem stands .250" proud and the o-ring stack-up is .312" high. when I account for the thickness of the sriker face that gives .250". In theory the valve shouldn't open, but it does. This is due to the compressability of the o-ring material, i'm sure. The amount of air that is released in this state is very small. In fact, I doubt that it would be enough to get the pellet to exit the bore. Why is this realivant? Look at the data above, Shot 16 stroke exactly. The subsiquent shots to #16 mentioned earlier, are calculated. My spread sheet generated that data (#17&18). So what do I do, increase the stroke by about 1/8 turn? Yep and the results were within 2% of the calculated outcome. The coincidence of the max possible flow at the given psi is equal to what did flow, indicates the end of the string. Also the coincidence of the adjusted stroke and the calculated stroke indicates no further gains in performance. These are by no means dump shots. At shot #18, the calculated valve duration is 1.5ms and the lock time of the pellet is a bit under twice that. The valve closes when the pellet is about 13" down the barrle. Any further shot get slower and efficiency gets lower. I have theory about the spike at shot #9. you'll notice that it used air at a gain not consistent to the trend. I think that is the point where the valve bounced dramaticly. I believe this is where the force closing the valve and the force opening were very close to equal and the consevation of momentum came into play as a fine example. (Newton's craddle). Once the ballance was upset, the apparent bounce subsided or atleast became deminished.
Kick it around
Bill
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Sorry about the offset of the heading to the numbers. Didn't look like that until I hit post, go figure.
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Forgot to mention. When I model the velocity figures using a regression, shot#9 should be more like 903-905 depending which regression I use. This would also make the volume line come in to a smoother trend.
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I am going to take this on in just a little bit. I have some questions about getting it running again. I saw the instruction to adjust hammer preload to deliver a shot string centered around 2600 psi with minimal hammer adjustment from full CCW and then adjust throw to set final velocity. I then see that on the rising velocity bit of the curve the buffer is doing not so much...and so see contradiction. It looks like y'all are telling me to shorten stroke( stick the striker further out ) to increase velocity in a regime where the o-rings are not playing.
Also, an observation: I test fit the o-rings and due to casting flash they don't rest in the tube flat. I took some 600 grit and polished it off gently and they now fit in a plane.
cheers,
Douglas
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Once the o-rings are in place, adjusting the stroke setting then determines at what point in the shot string the buffer will begin having an effect on the resultant velocity and efficiency. With it fully CCW the buffer is pretty much fully engaged and you will get little or no velocity at all, As you turn it CW you are adding to the maximum amount of left/dwell that can occur, thereby increasing velocity.
The earlier in the shot string that the buffer is engaged, the more effect it can have on the shot string, BUT, the more it limits velocity. That's one reason that I say this mod is most effective when tuning down a powerful gun.
You can tune anywhere from full power, down to almost nothing with this mod in place, just realize that the more you tune down, the more the effective it will be at flattening the shot string. In an m-rod, this range is from 0 CW to about 5-6 turns CW. If you go beyond this, the buffer basically gets tuned completely out of the picture.
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That makes sense Sir. WIth travel 3.5 turns CW from stopped here is 20 shots:
881.7
883.6
890.3
900.0
897.0--2k8 psi
903.2
909.1
908.7
905.7
907.3
916.0--2k6 psi
918.0
913.7
902.5
898.7
895.2
889.0
884.6
884.2
878.3--2k2 psi
Not too bad at all from a 3k fill. At this power, I suspect the buffer is low in effectiveness. I am just out of 4% with the 918 shot. With 1/4 turn less preload thestarting shot was 850 with the same peak at 916 fps just after 2600 psi. If I go to slightly lower power by dropping preload and CCW on the hammer travel( say an 860-895-860 tune ) I bet the shot count goes up, say -1/4 preload and 1/8 turn CCW on travel.
cheers,
Douglas
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I haven't done a lot of tinkering with this at the high end of the power spectrum. I'd be tempted to increase (CW) spring tension and increase stroke (CCW) in small increments to flatten the string but maintain a high velocity.
CCW on the stroke tends to lower the sweet spot pressure a bit too, so you can compensate by raising spring tension to compensate for the earlier engagement of the buffer. Not sure how much, if anything you'd gain, but it's worth a try.
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I haven't done a lot of tinkering with this at the high end of the power spectrum. I'd be tempted to increase (CW) spring tension and increase stroke (CCW) in small increments to flatten the string but maintain a high velocity.
CCW on the stroke tends to lower the sweet spot pressure a bit too, so you can compensate by raising spring tension to compensate for the earlier engagement of the buffer. Not sure how much, if anything you'd gain, but it's worth a try.
I got the understanding that this was a set preload and vary travel to set velocity. I would say that the same hammer energy behaviours exist at the high pressure fill but the buffer offers some change to the low, and its interaction with high. For a given set of settings( at high power I find it as you suggest, minimal effect...but for tuning a slightly lower power from the same fill it can flatten it out. An increase in travel slows down the lower 2/3 of the shot string( 'cause the striker sticks out less ), and increases power in the first third( due to increasing hammer energy where o-rings have least effect ).
The 20 shots I get now have nearly the same sort of report sound, there is no drastic change at the lower end as there was before. Slow change in report tone, but it is far more subtle...all this is a good indicator IMO/IME.
cheers,
Douglas
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I got the understanding that this was a set preload and vary travel to set velocity.
That's the KISS description for the average user who has little or no experience with tuning or the physics involved. It gets you to a good tune with minimal work and gives you good velocity control with a single adjustment. I didn't want to scare people off making the explanations sound too complex.
If you understand what's really happening, there is always room for tweaking and fine tuning to optimize a specific tune you are looking for. Especially when working at the margins of the devices effectiveness as you are.
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I got the understanding that this was a set preload and vary travel to set velocity.
That's the KISS description for the average user who has little or no experience with tuning or the physics involved. It gets you to a good tune with minimal work and gives you good velocity control with a single adjustment. I didn't want to scare people off making the explanations sound too complex.
If you understand what's really happening, there is always room for tweaking and fine tuning to optimize a specific tune you are looking for. Especially when working at the margins of the devices effectiveness as you are.
That was good thinking on your part...no question about it. In any case, your support and explanation does you credit, and thanks for the consultation, it was most useful.
cheers,
Douglas
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Glad to help.
BTW What pellets were you shooting for that string and which o-rings did you decide to go with....hard or soft?
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Glad to help.
BTW What pellets were you shooting for that string and which o-rings did you decide to go with....hard or soft?
Pellets were the 21.3 gr Barracuda Match with 5.53 head. First try is with the soft O-rings( the black ones ).
One thing to note, rubber at this sort of compression speed is not a good spring; the hysteresis is doing some energy consumption where it is most required to prevent valve-opening hammer bounce.
cheers,
Douglas
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Res Shot vol closing Recorded developed Energy
SHOT # PSI cubic in force lbs Stroke psi used velocity Friction fpe
1 3200.00 0.2120 144.76 0.1143 52.185 883.40 6.948 43.971
2 3147.82 0.2177 142.40 0.1162 52.714 885.50 6.835 44.180
3 3095.10 0.2239 140.01 0.1182 53.295 887.50 6.721 44.380
4 3041.81 0.2301 137.60 0.1203 53.840 890.60 6.605 44.690
5 2987.97 0.2350 135.17 0.1225 54.013 889.40 6.488 44.570
6 2933.95 0.2420 132.72 0.1247 54.617 891.00 6.371 44.731
7 2879.34 0.2515 130.25 0.1271 55.704 897.40 6.252 45.376
8 2823.63 0.2575 127.73 0.1296 55.930 896.10 6.131 45.244
9 2767.70 0.2718 125.20 0.1322 57.866 908.00 6.010 46.454
10 2709.84 0.2762 122.59 0.1350 57.574 902.00 5.884 45.842
11 2652.26 0.2846 119.98 0.1380 58.064 901.90 5.759 45.832
12 2594.20 0.2918 117.36 0.1410 58.230 899.10 5.633 45.548
13 2535.97 0.3000 114.72 0.1443 58.522 897.20 5.507 45.355
14 2477.45 0.3098 112.07 0.1477 59.039 896.30 5.380 45.264
15 2418.41 0.3187 109.40 0.1513 59.039 895.00 5.251 45.133
16 2359.37 0.3276 106.73 0.1551 59.456 890.00 5.123 44.630
17 2299.91
This is a bit of data from my old tune before I enlarged the porting in my .25 madauder. Efficiency is only .89 as per Bobs method of calculating efficiency. I was using the B-staly method to achieve this string. I had also developed a tune using this method that produced 825-835-825fps over 10shots. It makes ballancing a tune very easy. In fact, even though I am able to achieve the same tune without the o-rings, I can do it much faster with the o-rings.( less pellets :D) Here's the part that may interest you. If you observe shot 16, you will see the psi at that shot. It corrisponds with what is on the gauges. The volume for that shot is .328ci @2359psi. At this shot(16), the amount of air that could flow through this factory valve is .346ci @ 2359psi. If I shoot the next shot at the posted 2300psi, the velocity is 887.7fps using .337ci of a possible .343ci. Next shot would render 877fps using .342ci of .342ci. Here's the part that really tells the tale. The hammer stroke adjustment is 3 turns. with the adjusters screw pitch, that is .150" of stroke/ lift of the valve. The valve stem stands .250" proud and the o-ring stack-up is .312" high. when I account for the thickness of the sriker face that gives .250". In theory the valve shouldn't open, but it does. This is due to the compressability of the o-ring material, i'm sure. The amount of air that is released in this state is very small. In fact, I doubt that it would be enough to get the pellet to exit the bore. Why is this realivant? Look at the data above, Shot 16 stroke exactly. The subsiquent shots to #16 mentioned earlier, are calculated. My spread sheet generated that data (#17&18). So what do I do, increase the stroke by about 1/8 turn? Yep and the results were within 2% of the calculated outcome. The coincidence of the max possible flow at the given psi is equal to what did flow, indicates the end of the string. Also the coincidence of the adjusted stroke and the calculated stroke indicates no further gains in performance. These are by no means dump shots. At shot #18, the calculated valve duration is 1.5ms and the lock time of the pellet is a bit under twice that. The valve closes when the pellet is about 13" down the barrle. Any further shot get slower and efficiency gets lower. I have theory about the spike at shot #9. you'll notice that it used air at a gain not consistent to the trend. I think that is the point where the valve bounced dramaticly. I believe this is where the force closing the valve and the force opening were very close to equal and the consevation of momentum came into play as a fine example. (Newton's craddle). Once the ballance was upset, the apparent bounce subsided or atleast became deminished.
Kick it around
Bill
Bill, thanks for that data.....very interesting information. Much more detailed than I have ever gathered. Gives me new insight into how this all works.
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My pleasure. This stuff keeps my face out of the TV when I'm not hunting, shooting, fishing,hiking, camping, working or training with by 10 and 6 yr old sons. Allows me to use the grey matter in a way that doesn't leave me wanting for more, like most days at work.
Bill
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Well, now that Bob (rsterne) has started gathering some of the cold hard statistics on how this mod works, I suppose it's as good a time as any to put forth my theory on how and why this mod works. The original post in this thread suggests some ways in which this mod can make a pcp more efficient, but I think there is something much more important going on here.
I have long maintained, and Bob's data confirms, that the more valve lift is limited by the buffer, the greater the efficiency gains. I have also observed, as I'm sure Bob will when he gets there, that the better your valve porting breathes (bigger throat, exhaust port, transfer port, and barrel porting), the more significant the efficiency gains, and the higher power that can be achieved while still observing efficiency benefits from the buffer.
So, this mod works best while tuning down a gun to some percentage below the peak power that the gun could otherwise achieve. Prior to this, the preferred way to tune down a gun was with restrictive porting downstream of the poppet valve. So let's compare the effects of these two methods of tuning.
Let’s consider first what happens when you downtune using port restriction. Adding restriction attenuates flow and increases pressure drop. So the pressure pulse, as seen by the pellet, will rise more slowly than it does with a less restrictive port. This slower rise is less efficient than a faster rise in pressure, since the pellet will start moving immediately and air released later in the shot cycle will have less effect on the final velocity of the pellet. It is also important to note that port restriction reduces velocity at ALL reservoir pressures, not just at the peak of the curve. This has the effect of narrowing the pressure range over which a given velocity can be achieved. The shot sting is still extended, because less air is being used, but the pressure range is not extended much if at all for the restricted shot string.
Let's also consider what the effect will be on valve lift and dwell. Again, adding restriction, reduces flow, and increases the pressure differential across the length of the restriction. Reduced flow around the poppet, reduces drag, and also reduces any pressure differential that builds up due to any restriction between the poppet and the valve wall. Also the increased pressure drop across the restriction, increases "backpressure", which also reduces the pressure differential across the poppet. The net effect is that the closing forces on the vavle poppet are reduced. What will reduced Closing forces on the valve due to lift and dwell? It would seem obvious to me that both lift and dwell will be increased!. So, not only is the pressure waveform being attenuated by the restriction, it is also being elongated (decreased frequency), and as we know, the later the air leaves the valve, the less effective it is in adding velocity to the pellet.
Now let's look at what happens when we tune down by limiting valve travel. Since we haven't introduced any additional restriction downstream of the valve, the pressure pulse, again as seen by the pellet, will rise as fast as it did without the buffer or restriction, so we don't lose any efficiency there. Only once the buffer becomes engaged does the pressure stop rising (clipping rather than attenuation). Also since flow is not being restricted, we are not reducing the pressure range between which a given velocity can be achieved. So as we decrease velocity with valve limitation, the operating pressure range can actually increase.
Now let's also consider how this affects valve lift and dwell. We've established that the buffer limits lift, so the valve closing forces have less effect on lift than they did with restriction, but what about dwell. Fist of, since the valve doesn't open as far, it's pretty much a given that dwell will decrease as well, but there's more to it. Since we haven't reduced flow (relative to the valve's lift at any point) and we haven't introduced any additional pressure drop across the porting, closing forces are just as strong as in the un-restricted configuration. So closing forces remain higher, lift is decreased, so dwell will decrease.
Now, the final difference. If we limit lift enough, we are also increasing the amount of time that the valve is either not fully open, or we may in fact limit it to the point that it never fully opens. This increases the overall average closing forces even more, by increasing the pressure differential across the poppet even more, effectively slamming the valve shut much faster that in the restrictive setup. Again, a more efficient situation since a larger percentage of the total air released by the valve is released very early in the shot cycle.
So in summary, adding restriction results in a slower rising, attenuated, elongated pressure pulse, whereas using a buffer to limit lift results in a quickly rising, still attenuated at it’s peak (clipped), but significantly shortened, more impulse like, pressure wave. And the more valve lift is limited, the greater this effect becomes.
Thoughts? :o
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I think you have some pretty valid points.... One thing you neglected (or I missed it) is that the majority of the closing force on the valve, IMO, is the air pressure working on the diameter of the valve STEM.... In a Disco, the seat of the poppet is about 0.28" diameter (0.062 sq.in.) and the force to open the valve at 2000 psi is about 124 lbs (plus another 7 for the spring).... Once the valve is open, the pressure downstream of the seat (ie in the exhaust port), and in the barrel, is going to be very close to the reservoir pressure (at least that is how we treat it in Lloyd's spreadsheet, and it seems to give very good predictions).... That pressure now acts on the 0.156" diameter valve stem (0.019 sq.in.) and pushes the valve closed with 38 lbs. force at 2000 psi (again plus the 7 for the spring).... If indeed, your supposition that the pressure across the poppet is less in a gun with a restricted transfer port is correct, then there would be a greater percentage of that 38 lbs. closing force available, not less.... Again, referring to the modeling done in Lloyd's spreadsheet, if there was very much pressure differential across the poppet, the pellet wouldn't see enough acceleration to acheive what it does.... If all the port areas (throat, exhaust port, transfer port, and barrel port) are about the same size, I think the pellet sees pretty much reservoir pressure.... Stick a restricted transfer port in the middle, particularly a really tiny one.... not so much....
However, I agree with your basic premise that AT THE PELLET, using a restricted transfer port (or, as Hatsan does, a thick valve stem plugging the throat) is likely causing the pellet to see a slower pressure rise than what it sees using the buffer.... It is pretty much a given that the higher the pressure of the initial pulse, the more efficiently the pellet is accelerated.... Higher pressures tend to be more efficient.... Heck, an unregulated PCP is nearly twice as efficient at the beginning of the shot string than at the end.... It might be fairer to compare a gun with a buffer to a gun with open porting but a very short dwell, because that is what you are really achieving in a severely detuned bstaley gun.... Then the problem becomes how do we detune a gun to be like that?.... Well, if we don't change the transfer port, we have to reduce the hammer strike.... That invariably reduces the fill pressure, and hence the pressure at the pellet.... Once again, the efficiency drops.... OK, there might be one other way to achieve a low power, high efficiency tune.... I'm pretty sure Mac1 uses it in the 12 FPE USFT.... You could use a very small valve poppet and stem, and a very light hammer strike, and balance the forces to work at the higher pressures used in a low powered bstaley gun.... You would be generating a short, sharp, high pressure pulse, and providing it unrestricted passage to the pellet.... Sounds like the way to go for efficiency....
I'm still wrestling with a very steep learning curve here.... However, what I have seen so far, that I really like, is the ability to change the velocity over a very wide range without having to drastically change the pressure range used.... The number of variables (O-ring stack height, durometer, hammer stroke, percent of buffer engagement, etc.etc.etc.) has my mind reeling.... I can see that if you are primarily interested in low to medium power tunes, and are prepared to play with all these variable, you can apparently acheive some pretty remarkable results.... I'm more interested in how I can increase the shot count in, for example, my .30 cal Disco Double when tuned for 100 FPE.... If I can go from 12 shots to 14, that would be a major accomplishment.... However, it's difficult just to get to those kinds of power levels.... and even more difficult to figure out how to get 25% beyond that so that I can then give up 20% from there to get those extra couple of shots.... One other limiting factor is that you need a LOT of hammer strike available so that you can give up a large percentage of it to the buffer.... With high powered PCPs getting harder and harder to cock, that is another thing it will be hard to justify....
Bob
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Ya, it's not going to do much for you in max power applications.
I think you may be underestimating the pressure drop induced by all the porting and restrictions though, and the effect of flow on closing forces. Even if they are not great compared to other forces it still results in tendencies towards what I describe above. We are of course talking in generalities and specifics of any given design would dictate how much effect these tendencies might have.
After all, we're only talking about percentages of increased efficiency until we get to the case where the valve never fully opens and dwell is more significantly reduced.
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I'm not sure what you mean by "where the valve never fully opens".... By "fully" do you mean when the lift is 1/4 of the diameter, which is when the flow rate peaks?.... It is at that point (lift = 1/4 D) that the "curtain area" (diameter times lift times PI) equals the "throat area" (diameter squared over 4 times PI), and the flow rate becomes limited by the throat area.... Basically, with a stock Disco valve, where the throat is 0.219", once the lift is 0.055" or more, the flow RATE won't increase, only the dwell.... The VOLUME of air flowing through the valve continues to increase, however, because the additional lift causes additional dwell.... You can consider the flow rate as being "clipped" once the lift hits 0.055", which has the effect of squaring off the profile of the pulse of air....
Somewhere in there may be a clue as to what's happening with the buffer.... Maybe (when it's working really well) it's preventing the valve from lifting past that 1/4 D point, and so not really affecting the flow RATE at all?.... However, it is affecting the dwell, shortening it.... and eventually of course it will start preventing the valve from opening to the 1/4 D point and dropping the power even more....
Bob
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Maybe I'm not saying it quite right. durring the transition from closed to 1/4 d, the closing forces are greater than when lift passes 1/4 d. likewise as the valve is closing from 1/4 to fully closed. By severely limiting lift, and thus reducing dwell, a proportionally greater percentage of the total dwell will be spent under the influence of this transitional state. So the buffer allows us to limit dwell, whereas restriction tends to have the opposite effect.
Taken to the extreme, where lift is limited to 1/4 d, this effect would be the greatest......whether it would produce useful velocities is another question altogether...The seting of the stroke adjustment (apparent buffer length) determines how much effect this will have between unbuffered and almost fully buffered.
Another thing to consider is that the buffer absorbs some of the hammer's excess energy which it may then return to the hammer, pushing back and countering it's momentum even futher, reducing dwell even more. Adding to the forces countering the hammer.
This effect would also be more pronounced later in the shot string.
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mmmmmmmmm.... don't see how the buffer can be absorbing energy and at the same time releasing it.... I would think it progressively absorbs energy until the hammer stops... and then returns a portion of it to the hammer during rebound.... That would accelerate the hammer backwards, and thereby reduce the dwell.... but nowhere as quickly as the valve stem, propelled by the spring and air pressure would do, at least IMO.... I think that is why the buffer may help to eliminate, or at least reduce, hammer bounce, because of the loss of energy that goes into distorting the rubber (ie heat).... rather than the greater lift, giving a greater push to the hammer from the spring and air pressure....
So many variables, my brain hurts.... *LOL*....
Bob
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LOL....yes many variables.
Not saying that it's absorbing and returning energy at the same time. As the vavle is opening, the buffer absorbs some of the excess hammer energy that would otherwise go into increased lift and dwell. Then once lift peaks, and the valve starts closing, some of that energy may then be returned to the hammer, increasing the closing speed. Again, perhaps not a huge factor, but when combined with everything else results in significant net changes in dwell.
I think we basically agree. Several things seem to be happening, all of which are tending to reduce dwell.
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As Bstaley can confirm, my thoughts on how this works, has been all over the map. ;) Anyway, my latest theory, given Bob's recent data, is that the o-ring compression range is what drives the best tune with the o-rings. For it to work well requires a narrow valve throw range. With a narrow valve throw range, the only way to get higher velocities, is by opening the transfer port and valve exhaust. I can't say that I found any better efficiencies by increasing the transfer port, and detuning further. ie, using 0.110" transfer port and tuning to 24 shots @ 16fpe, vs 0.125" transfer port and trying to that same level. Actually, might not have tried that. :-[ But, it does sound like something worth trying, just to see if it can extend shot count or flatten string further.
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As Bstaley can confirm, my thoughts on how this works, has been all over the map. ;) Anyway, my latest theory, given Bob's recent data, is that the o-ring compression range is what drives the best tune with the o-rings. For it to work well requires a narrow valve throw range. With a narrow valve throw range, the only way to get higher velocities, is by opening the transfer port and valve exhaust. I can't say that I found any better efficiencies by increasing the transfer port, and detuning further. ie, using 0.110" transfer port and tuning to 24 shots @ 16fpe, vs 0.125" transfer port and trying to that same level. Actually, might not have tried that. :-[ But, it does sound like something worth trying, just to see if it can extend shot count or flatten string further.
Yes, you should try opening up the TP port more, as well as the exhaust port on the valve. Opening these both up I'm now able to get 38 shots at an average FPE of just over 15 FPE within 4% from my p-rod.
See teh 4th post down in this thread:
www.network54.com/Forum/581291/thread/1356374854/ (http://www.network54.com/Forum/581291/thread/1356374854/)
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At some point, this trend to increasing efficiency with opening the ports and reducing the dwell HAS to stop.... It is the same situation as putting a large wasted volume between the valve and the pellet.... In Lloyd's spreadsheet we call that the "transfer port volume", but it's really a combination of all the volumes between the seat of the poppet and the base of the pellet.... and it's a negative in terms of efficiency.... What happens is that the pressure (and hence the force) available to accelerate the pellet drops as the air expands into that volume.... In a .22 cal Disco, you have a reservoir of 135 cc, and a TP volume of 0.34 cc, so the pressure drop is small, using those numbers about 0.25%, ie from 2000 psi down to 1995 psi.... However, in my .25 cal Pumper, with a valve of only 6.5 cc and a TP volume of 0.46 cc, and now the volume is 7%.... The pressure drops from 1600 psi down to 1494.... no longer insignificant.... Boring out the ports larger and larger, and then filling that volume with a smaller and smaller amount of HPA, obviously has to have it's limits.... Going too big on the ports just wastes volume, and hence pressure and efficiency....
Bob
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Yep, as with all components of a system, changing certain properties will provide benefits, until some point where other factors become more of a detriment. This point is usually identified by the knee in many of your graphs....the point of diminishing returns.
At some point the pressure drop due to the increased volume will start to overcome the decrease in pressure drop from the reduced restriction......now to just figure out where that is :)
The pumper's limited valve volume probably makes this point much lower than it would be in an unregulated PCP.
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You can't just generalize all unregulated PCPs, it depends on how much of the reservoir air is available to the shot cycle.... In the gun I'm using, with a Disco valve but a PRod gauge port, virtually all of the reservoir air is available to keep the pressure up in the valve during the shot.... The smallest restriction to flow on the inlet side is the 1/4" hole in the front of the valve, which is still about 3 times the area of the transfer port.... In a stock Disco, however, the hole in the gauge block is only 1/8", smaller than the barrel port, although between that and the valve seat there is "almost enough" air for stock power levels.... Start modding the Disco for twice the power output, and that is no longer the case, you need to allow more air INTO the valve by using a PRod gauge port.... I've never had one, but I understand that possibly Air Arms rifles have a "firing pot" of fixed volume that feeds the valve, and could possibly cause a similar starvation at very high power levels.... In regulated PCPs it's VERY common to not have enough air in the "plenum" to support high FPE levels, and it's something I always address in my regulated designs....
Incidently, it is possible that the O-ring buffer may have a use in regulated PCPs as well.... It just may be that using a buffer to limit the dwell and "shape" the pulse could provide increased efficiency.... if not directly, through control of hammer bounce.... It would also allow easy detuning, although with a regulated gun just backing off the preload is usually enough.... That does cause one weird thing though.... if you go too far, the velocity INCREASES below the setpoint as the valve tries to self-regulate over the lower pressures.... Using a rigid (or nearly rigid) buffer might prevent that....
Bob
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Yep, I understand that everything is dependent on everything else. Every gun will be different.
I actually have tried tuning with this in my regulated .177 m-rod, and as it turned out there was a LOT of hammer bounce. Could easily hear the burrrrping from the bounce......efficiency wasn't very good at all! I think motorhead may have found the same thing when he first started tuning with his regulator.
Will have to try a rigid buffer....that might just help!
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Here is some food for thought....
In some ways, the buffer is working like a very stiff valve spring, in that it is limiting the lift and making the difference between the lift at high and low pressure smaller.... The major difference is that it is easily adjustable without tearing the valve out of the gun.... The more I think about it, the more I like that analogy.... The difference between a 70D and a 90D buffer is like the difference in using a stiffer spring (90D) but with less preload.... The major difference between the rubber buffer and a coil spring is that the O-rings are (I think) more progressive than most steel springs are.... and of course they can "absorb" some energy by converting it to heat instead of sending (nearly) all of it back to the hammer as rebound force....
Anyone?....
Bob
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The spring rate on a stacked o-ring buffer changes remarkably quickly. The more o-rings, the slower the change to go along with a lower rate. I suspect that this spring is comparable to the closing force from the valve, perhaps a bit less...but a singnificant fraction at the bottom of the string, and therefore at its maximum compression. I am going to see just what this looks like; I have available #211 o-rings and a spring rate checker. Compression range oto of up to .1"...then get a curve fit function...better thanthen paper and pencil and geometry...LOL too late on Friday afternoon, will take it up on Monday...heh-heh-heh
cheers,
Douglas
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Here is some food for thought....
In some ways, the buffer is working like a very stiff valve spring, in that it is limiting the lift and making the difference between the lift at high and low pressure smaller.... The major difference is that it is easily adjustable without tearing the valve out of the gun.... The more I think about it, the more I like that analogy.... The difference between a 70D and a 90D buffer is like the difference in using a stiffer spring (90D) but with less preload.... The major difference between the rubber buffer and a coil spring is that the O-rings are (I think) more progressive than most steel springs are.... and of course they can "absorb" some energy by converting it to heat instead of sending (nearly) all of it back to the hammer as rebound force....
Anyone?....
Bob
Yup, with the advantage that it adds little or nothing to the force required to open th valve, but then can counter the excessive momentum that remains.
Just looked back at my original writeup on the mod, and that was the exact analogy I used.
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I'm currently thinking that the buffer works best when engaged even at the beginning of the shot string, or nearly so.... Of course it's hard to tell, as the initial "spring rate" is so low for the first few thou of movement, especially with a tall stack of 70Ds....
Bob