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Author Topic: The O-Ring buffer and Air Usage Efficiency  (Read 15847 times - 1 votes) 
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Offline bstaley

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The O-Ring buffer and Air Usage Efficiency
« on: October 20, 2012, 04:11:59 PM »
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.
« Last Edit: October 21, 2012, 04:51:17 PM by bstaley »

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #1 on: October 20, 2012, 04:22:14 PM »
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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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #2 on: October 20, 2012, 04:29:12 PM »
Outstanding ... very eloquent writing easy to follow and understand   ;)
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Offline Ribbonstone

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« Reply #3 on: October 20, 2012, 05:11:03 PM »
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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Offline bstaley

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #4 on: October 20, 2012, 05:35:03 PM »
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. 
.
« Last Edit: October 20, 2012, 06:04:54 PM by bstaley »

Offline rangerman

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #5 on: October 20, 2012, 05:46:30 PM »
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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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #6 on: October 20, 2012, 08:02:28 PM »
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....

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Offline PakProtector

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #7 on: October 20, 2012, 11:17:00 PM »

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. 
.

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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Offline bstaley

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #8 on: October 21, 2012, 01:05:46 AM »
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.

Offline Kailua

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #9 on: October 21, 2012, 05:19:20 AM »
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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Offline PakProtector

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #10 on: October 21, 2012, 08:05:04 AM »
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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Offline Ribbonstone

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #11 on: October 21, 2012, 10:24:03 AM »
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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Offline TimmyMac1

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« Reply #12 on: October 21, 2012, 11:30:35 AM »
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

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Offline PakProtector

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #13 on: October 21, 2012, 01:27:40 PM »
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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Offline bstaley

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #14 on: October 21, 2012, 01:28:20 PM »
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.


 

Offline Ribbonstone

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #15 on: October 21, 2012, 03:34:27 PM »
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.


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.


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.


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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Offline bstaley

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #16 on: October 21, 2012, 03:49:48 PM »
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.

Offline PakProtector

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #17 on: October 21, 2012, 08:17:07 PM »
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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Offline bstaley

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #18 on: October 21, 2012, 08:49:52 PM »
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.

Offline PakProtector

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Re: The O-Ring buffer and Air Usage Efficiency
« Reply #19 on: October 21, 2012, 09:03:09 PM »
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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