GTA
All Springer/NP/PCP Air Gun Discussion General => Air Gun Gate => Topic started by: MartyMcFly on October 26, 2018, 09:49:49 PM
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Does anyone have reliable information about the amount of pressure that is generated by a typical springer at the end of the compression stroke? I've seen estimates that are as high as 18,000 psi to as low as 1,200 psi (Tom Gaylord).
Better yet, does anyone know how to calculate it based on swept volume and spring force? I've been thinking about a two stage compression chamber design and I want figure out if it is is worth making into a prototype. The goal is to maximize the pressure at the end of the stroke, while keeping the spring energy the same.
Thanks,
Marty
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According to the Cardew book on spring guns, with a tube diameter of 1 inch and piston stroke of 2.5 inches, the pressure would rise to 1366psi
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Marty,
This is a tricky calculation because it is not a simple closed system: If the pellet were "glued" in place you would generate the highest pressure. That said, I doubt a "magnum" springer would ever get to 18 kPSI, even then.
Because the pellet breaks free at a given force, the peak pressure can't be less than that equivalent pressure. For instance, if a .177 pellet breaks free at 12 lb, the minimum pressure is 500 PSI. Pellet inertia will also contribute to peak pressure, but only once acceleration starts. As in, while and after the pellet has broken loose.
If you could capture pellets fired from springers gently, and compare the extent of skirt flaring with those shot from PCPs, that may help. This borrowed image shows the skirt bulged from air pressure, compared to the un-fired pellet:
(http://www.ctcustomairguns.com/uploads/2/3/8/4/23849268/7735557_orig.jpg)
Indirect springer peak pressure "measurement":
Charge an unregulated PCP to 500 PSI. Adjust the hammer such that it dumps more air than normal (to be sure the pellet sees a full 500 PSI). Then compare the pellet skirt flare at 500 PSI to that of a springer, firing the same pellet. Then the PCP can be filled to 1000, 2000 and 3000 PSI, and the same examination performed again. Find the PCP pellet with skirt flaring that most resembles the springer pellet; and Bob's your uncle...
I suspect that typical springers have peak pressures in the 1000 to 4000+ PSI range (depending on power and pellet weight), when not dieseling in an overt manner. The above image source article contains pertinent information that would seem to bolster my guess: https://www.ctcustomairguns.com/hectors-airgun-blog/at-the-moment-of-firing-and-fit-of-pellet-to-the-rifling (https://www.ctcustomairguns.com/hectors-airgun-blog/at-the-moment-of-firing-and-fit-of-pellet-to-the-rifling)
Do you want to use the spring to charge a chamber that is then dumped. Would this design have a piston that "sticks" or is captured at the end of the stroke to stop it from rebounding? Or, you could have a reed valve that traps the air so it can't follow a rebounding piston.
The pressure does not just depend on the spring force, swept and residual volumes. It depends mostly on the piston speed near the end of the stroke. This is an acceleration/velocity/compression/deceleration equation, and not at all trivial...
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subscriber, thank you for the well informed reply and the link to Hector's article. I figured that the calculations would be tricky due to the non-constant resistance offered by the pellet, which as you pointed out is acting like a plug before it's over-come by air pressure. I was hoping there are approximations I could make. I take it that you have some experience in this field, can you point me to a technical source that describes the math needed to solve it? It probably requires some differential equations or linear algebra, no?
The Design:
The piston I have in mind would be machined as one piece, with a large base acting as the low pressure piston. In the center of this piston base is a protruding smaller solid cylinder, which acts as the 2nd stage high pressure piston (This is the same piston design that is present in some two stage compressors). At the end of the compression tube the chamber narrows to accept the second stage piston, like a single male-to-female plug. Put simply, its like a finger going through an elongated donut at the end of the compression stroke, the finger being the second stage piston and the inside of the donut the 2nd stage compression chamber ahead of the transfer port.
Clearly, an issue with this design is that air is still present in the larger compression chamber as the head of the 2nd stage piston enters the smaller chamber, trapping air behind it. If the air isn't bled it will create back pressure and prevent the 2nd stage from traveling very far into the next chamber. I don't want to waste this left over air volume by bleeding it. I think I've solved this issue by drilling holes into the outer circle or front face of the "donut". These holes (maybe 4 of them) would be at a 45 degree angle channeling the air into the forward space of the smaller compression chamber, right before the transfer port. These "relief" channels would have a simple backstop valve to prevent back flow as the 2nd stage piston travels forward and builds up more pressure behind the pellet. A variation of this design can have the relief channels drilled at varying angles, that way each one empties into a different section of the smaller chamber and each is sealed by the body of the piston as it moves past them sequentially, eliminating the need for backstop valves.
I am not an industrial engineer, so there might be some fundamental conservation of energy effect that I am missing. When I came up with this idea my thinking was that concentrating the same spring force over a smaller area (the 2nd piston head) would result in a faster build-up of pressure. I thought this would result in the air being converted into plasma quicker and therefore increase the overall amount of pressure available to act on the pellet. Alternatively, if there is no change in absolute pressure then perhaps a quicker build-up allows the pellet to start traveling down the barrel earlier, reducing the time for hand movement or recoil to have an impact on the pellet's trajectory.
The goal of all this fanciful thinking is to increase efficiency over a traditional design, while not introducing more moving parts or increasing weight. Maybe the 2nd stage pressure build-up slows down the piston so much that it negates any improvement? I don't know, but I needed to get it out of my head. I'd upload some diagrams, but I don't have enough posts to do it yet. Hopefully you can visualize it.
-Marty
PS: I thought that doing a sticking piston would require some kind of capture mechanism, so I'd like to avoid it. But if you have a simple solution I am all ears.
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Came across some recent and very interesting articles on spring guns including this one. The analysis was done on a Beeman RS2 using various JSB pellets. In paragraph 6.2, Firing Cycle, it says that peak pressure and temperature are achieved after the pellet has proceeded down the barrel and that the peak pressure was over 4,200psi
https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
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Thanks, nice find!
Came across some recent and very interesting articles on spring guns including this one. The analysis was done on a Beeman RS2 using various JSB pellets. In paragraph 6.2, Firing Cycle, it says that peak pressure and temperature are achieved after the pellet has proceeded down the barrel and that the peak pressure was over 4,200psi
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Hi Marty,
The first step in finding a solution is in correctly defining the problem. From the sounds of it, you have identified the challenging aspect of a concentric piston on piston design, as it relates to your goal.
Your goal may be described as using coaxial two stage compression; where the low pressure stage uses a large area piston to maximize swept volume. This; while the second stage uses a small diameter piston to boost air pressure; and to reduce the area that high pressure air acts on, thereby minimizing the force that results in piston bounce. Does that sound accurate?
I suggest you look at how coaxial two stage air compressors work. What is the air flow path and what valves are involved. My gut instinct is that such compressors do not have the residual large piston air volume problem you describe, because the sequence of compression is actually broken into two steps; like with a multi stage hand pump.
Here Jorg Sprave takes apart a two stage coaxial piston compressor and explains how it works: https://www.youtube.com/watch?v=7NdnItyJs-U&feature=youtu.be&t=844 (https://www.youtube.com/watch?v=7NdnItyJs-U&feature=youtu.be&t=844)
As you might know, there is a reed valve that controls air from the large piston compression space and delivers it to feed the small piston space. As such, the two compression events happen one full crank revolution apart. This is not what you want. You want the two compression events to happen as one continuous event, yet without valves.
As I am typing this, I can think of only one way to achieve this. More precisely, one idea popped into my head and no other ideas have joined it :) :
Rather than use too many words, I created this simplified sketch:
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=149983.0;attach=246766;image)
It has the smaller coaxial piston slide in the larger one, that is propelled under its own inertia (probably needs to be longer or made from tungsten to have sufficient weight).
Obviously there are sliding seals required between the two pistons; and between the smaller piston and its cylinder bore. Also, between the large piston and the main cylinder.
I have not shown the main spring; that would energize both the larger and smaller piston together. Nor have I shown a soft return spring that hold the smaller piston all the way back inside the larger one, until the sudden stop at the front of the cylinder under firing.
I think with this arrangement the residual volume problem of the larger piston and apparent need for flow paths or valving goes away. The pistons themselves become the valves that act at just the right stages of operation.
It won't matter if the larger piston bounces back a little, because the smaller one would have already trapped and sealed in the high pressure air volume.
The challenge is going to be the ratio of piston areas and masses. Specifically to prevent the larger one from crashing into the end of the cylinder. The smaller piston will need to be "heavy" to work, but will at least have more of a high pressure air cushion in front of it.
You will also need reasonable alignment of the small piston to its bore, with generous lead-ins to prevent crashing against the edge.
The question will be if the small piston carries the cylinder seal near its nose; or if the small cylinder bore holds the seal near its entrance? Obviously, the small piston also requires a seal with the large piston bore; placed so as to minimize wasted air volume.
What am I missing....
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Came across some recent and very interesting articles on spring guns including this one.
https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
This is an impressive body of work, with a lot of info that is also easily digested.
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I was about to tell Eric that he found the holy grail! A lot of great information and the equations in that paper can be used to build a simulator in software. I hope the gentlemen who wrote it visits these forums, because it will take me a year to review all the concepts he discusses.
Subscriber, you captured my thoughts perfectly. My initial idea was a design very similar to yours, either having the interior piston “fall out” of the main piston via inertia or having a secondary spring fire it out. My first concern was that it would require more moving parts and increase production complexity. The other concern was that it can create two piston slaps, which can’t be good for accuracy.
I’ll watch the video you suggested to get a better idea of what is going on in a compressor, but my ultimate goal would be to have the piston body act as a valve and avoid reed valves if possible.
It might be wishful thinking, but given that only 28% of the springs energy is imparted to the pellet there has to be some way to get more efficiency out of airguns without making them as complex as a Swiss watch.
Thank you both for the intellectual stimulation.
-Marty
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Subscriber, I’ll sketch the second stage chamber (hopefully tomorrow) showing how the relief channels work and how they are sealed as the 2nd piston moves forward.
-Marty
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The piston I have in mind would be machined as one piece, with a large base acting as the low pressure piston. In the center of this piston base is a protruding smaller solid cylinder, which acts as the 2nd stage high pressure piston (This is the same piston design that is present in some two stage compressors). At the end of the compression tube the chamber narrows to accept the second stage piston, like a single male-to-female plug. Put simply, its like a finger going through an elongated donut at the end of the compression stroke, the finger being the second stage piston and the inside of the donut the 2nd stage compression chamber ahead of the transfer port.
Clearly, an issue with this design is that air is still present in the larger compression chamber as the head of the 2nd stage piston enters the smaller chamber, trapping air behind it. If the air isn't bled it will create back pressure and prevent the 2nd stage from traveling very far into the next chamber. I don't want to waste this left over air volume by bleeding it. I think I've solved this issue by drilling holes into the outer circle or front face of the "donut". These holes (maybe 4 of them) would be at a 45 degree angle channeling the air into the forward space of the smaller compression chamber, right before the transfer port. These "relief" channels would have a simple backstop valve to prevent back flow as the 2nd stage piston travels forward and builds up more pressure behind the pellet. A variation of this design can have the relief channels drilled at varying angles, that way each one empties into a different section of the smaller chamber and each is sealed by the body of the piston as it moves past them sequentially, eliminating the need for backstop valves.
I am not an industrial engineer, so there might be some fundamental conservation of energy effect that I am missing. When I came up with this idea my thinking was that concentrating the same spring force over a smaller area (the 2nd piston head) would result in a faster build-up of pressure. I thought this would result in the air being converted into plasma quicker and therefore increase the overall amount of pressure available to act on the pellet. Alternatively, if there is no change in absolute pressure then perhaps a quicker build-up allows the pellet to start traveling down the barrel earlier, reducing the time for hand movement or recoil to have an impact on the pellet's trajectory.
The goal of all this fanciful thinking is to increase efficiency over a traditional design, while not introducing more moving parts or increasing weight. Maybe the 2nd stage pressure build-up slows down the piston so much that it negates any improvement? I don't know, but I needed to get it out of my head. I'd upload some diagrams, but I don't have enough posts to do it yet. Hopefully you can visualize it.
-Marty
PS: I thought that doing a sticking piston would require some kind of capture mechanism, so I'd like to avoid it. But if you have a simple solution I am all ears.
Sounds like the design would need to be cocked 2 - 3 times to multiply the pressures ?
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Subscriber, I’ll sketch the second stage chamber (hopefully tomorrow) showing how the relief channels work and how they are sealed as the 2nd piston moves forward.
That would help me understand the concept. Right now it seems like a blend between hard to make, and magic.
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With enough weigh / mass you may get the secondary piston to compress the air required to launch said pellet ... But "Newton" and his proven 3rd law of physics ( equal & opposite reaction ) would have the CYCLIC motion of the Machine / mechanics so reactive to the actually process as it happens. While in application it very well could work, tho having the air gun stable in motion enough to get any kind of accuracy from ... ???
Just sharing thoughts
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Probably why PCP was re-invented ???
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While we are conjuring up springer variants; how about this one:
You cock the airgun by moving the piston back by means of barrel leverage, and capturing it with the spring compressed - just like any break-barrel springer.
Before you want to shoot; you trip the "compression sear" to precharge a dump chamber. If you don't like that; trip it immediately after cocking and allow the temperature to equilibrate to ambient.
When you actually want to launch the pellet, you have a small spring drive a small hammer to open a dump valve; just like a single stroke pneumatic. As vibration free as any PCP.
What might the advantage be over a SSP? the cocking stoke is more user friendly. You can capture the air at a higher pressure, without ridiculously peaky effort. The higher the pressure, the more energy you can get out of the air and into the pellet.
You could even make such a setup "multi-pump" if you want to, but with usable performance with only one spring dump.
I don't think you want to start conventional diabolo pellets at over 4000 PSI, due to the distortion to their skirts...
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One of the main ways that "silly ideas" add value is in spurring other thoughts that had not occurred to anyone before: "This won't work; but if you turn it upside down and cut that off, you may really have something"...
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I also came across some other very interesting articles on air guns. Tons of other articles on this site. Very interesting to note that peak compression is over 4,000psi and temperature over 2,000F in your typical spring gun.
https://www.researchgate.net/publication/277953420_Spring_Buzz_and_Failure_in_Spring_Piston_Airguns (https://www.researchgate.net/publication/277953420_Spring_Buzz_and_Failure_in_Spring_Piston_Airguns)
https://www.researchgate.net/publication/272168018_Dieseling_in_Spring_Piston_Airguns_-_A_Conceptual_Analysis (https://www.researchgate.net/publication/272168018_Dieseling_in_Spring_Piston_Airguns_-_A_Conceptual_Analysis)
https://www.researchgate.net/publication/326191398_Internal_Ballistics_of_PCP_Airguns (https://www.researchgate.net/publication/326191398_Internal_Ballistics_of_PCP_Airguns)
https://www.researchgate.net/publication/312199345_Energy-dependent_expansion_of_177_caliber_hollow-point_air_gun_projectiles (https://www.researchgate.net/publication/312199345_Energy-dependent_expansion_of_177_caliber_hollow-point_air_gun_projectiles)
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Some fascinating information- much appreciated for the reading! Yet there's a lot of other considerations. a "typical springer" such as my CZ-634 will have a wildly different max pressure than my run-of-the-mill gas-piston Hatsan 95QE or even the B-3 Chinese underlever. The CZ has less than 10 FPE, the 95QE hits about 22 FPE, and the B-3 was about 14 FPE. All three of these are "typical" airguns in every respect. MY point is such generic statements are worthless without providing more insight into the caliber, chamber volume, preload on the spring (or gas-piston) and stroke as well as the expected power level; all must all be taken into consideration even when considering generic airgun pressures at the end of the stroke- and that's before involving the pellet and pellet's movement past the leade into the barrel proper.
While the mathematics and dynamics of this is fascinating, in the end, if you want a manageable high-powered airgun, springers are NOT viable. By high-power I do not mean something putting 20 to 30 FPE, I'm talking 30+ FPE, 40+, 50 and well past that. Springers pushing more power than 20-ish FPE tend to be more hold sensitive as power levels increase in my experiences. Though I also desire a Hatsan 135 in .30 since lobbing "mortar"-like pellets onto squirrel heads is very appealing to me.
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Some fascinating information- much appreciated for the reading! Yet there's a lot of other considerations. a "typical springer"
Point taken. The gun used in the study was a Beeman RS2 and pellet was JSB 4.5mm 10.34gr. Muzzle velocity of 812fps for 15fpe
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if you want a manageable high-powered airgun, springers are NOT viable. By high-power I do not mean something putting 20 to 30 FPE, I'm talking 30+ FPE, 40+, 50 and well past that.
You could make a 40 ft.lb springer, but you are not going to like cocking it, aiming it offhand, shooting it or toting it.
To reduce cocking effort to the point that makes 40 ft.lb possible, the barrel or cocking lever would have to be super long. The rest of the hardware super heavy. This would increase the moment of the gun to the point that no one would want to shoot it offhand. And no one would carry it hunting.
The fun limit for me is the RWS 350; and those are just over 20 ft.lb guns. I much prefer the handling qualities of my HW50S and R9. The R9 is harder to cock than the 350, because the R9 has such a short barrel. I have a Webley Patriot, but it seems like a monster, for so little gain.
Anyway, I think the OP is trying to increase efficiency of spring air guns. I doubt his goal is to double their power. What if he succeeds at making a light compact springer that produces 12 ft.lb, but has only a 20 lb peak cocking force? That would be something.
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Here is a rough sketch of what I had in mind. As the 2nd stage piston enters the smaller compression chamber the offset relief channels are closed off sequentially by the body of the smaller piston as it travels forward through the chamber. This allows the larger compression chamber to empty without being bled to the outside. The number, exit locations and diameter of the relief channels will be important in order to maintain appropriate pressure in the smaller chamber.
An alternative, but more complex design can have reed valves at the relief channel entrance holes to prevent back-flow into the main compression chamber.
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I'm pretty sure that design would work as if the small bore was only the length from the front of the deepest bypass port to the front end.... As I see it, both chambers would be the same pressure at that point....
Bob
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Two compression chambers. Bottom one under lever, top one side lever and both dump thru the same transfer port.
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So basically without an intercooler (see picture) and another stroke there is no way to increase the compression? What about the time it takes to reach peak compression, probably the same unless we increase the piston speed?
Back to the drawing board...
I'm pretty sure that design would work as if the small bore was only the length from the front of the deepest bypass port to the front end.... As I see it, both chambers would be the same pressure at that point....
Bob
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subscriber, I like the idea. But I think the paper that Eric lined to mentioned that a lot of the energy comes from the expansion of the gas as it is quickly compressed. Dumping it in a storage chamber for later use would bring down the temperature, which is probably why multi-pump PCPs need so much pumping; to make up for the fact that the compressed gas has no thermal energy (just my guess).
While we are conjuring up springer variants; how about this one:
You cock the airgun by moving the piston back by means of barrel leverage, and capturing it with the spring compressed - just like any break-barrel springer.
Before you want to shoot; you trip the "compression sear" to precharge a dump chamber. If you don't like that; trip it immediately after cocking and allow the temperature to equilibrate to ambient.
When you actually want to launch the pellet, you have a small spring drive a small hammer to open a dump valve; just like a single stroke pneumatic. As vibration free as any PCP.
What might the advantage be over a SSP? the cocking stoke is more user friendly. You can capture the air at a higher pressure, without ridiculously peaky effort. The higher the pressure, the more energy you can get out of the air and into the pellet.
You could even make such a setup "multi-pump" if you want to, but with usable performance with only one spring dump.
I don't think you want to start conventional diabolo pellets at over 4000 PSI, due to the distortion to their skirts...
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Allowing the air to cool between compression and firing would make the gun act like a PCP.... instead of taking advantage of the adiabatic compression of a springer firing, where the compression causes heat which increases the pressure even more....
Bob
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I'm pretty sure that design would work as if the small bore was only the length from the front of the deepest bypass port to the front end.... As I see it, both chambers would be the same pressure at that point....
Whereas, the inertial inner piston concept I proposed won't have that problem. It probably will require a much larger combined piston weight, with the jerky shot cycle that goes with that, but the reverse air flow shut off, would be handled automatically.
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Dumping it in a storage chamber for later use would bring down the temperature, which is probably why multi-pump PCPs need so much pumping; to make up for the fact that the compressed gas has no thermal
Exactly right. By pre-compressing the air and letting it cool you get a vibration free shot. But at the cost of probably half the energy in that air... Hence, the follow up that you might "fire" a number of pumping shots for each pellet fired.
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Marty,
How about a dual opposing (equal diameter) piston design, where the piston inertia does not jerk the gun around at all? There are a few such designs in airgun history. Even on GTA, a respected airgun manufacturer started such a development; ultimately giving up and continuing with a line of PCPs.
As I recall, the problem was piston bounce when using pistons that were too light, with a stroke that was too short for the large bore diameter. I suggested the design could benefit from reed valves, but by the time I saw the thread the originator had moved on to PCPs. Will see if I can find the thread, as it makes for fascinating reading.
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While searching old posts, I found The Air Gun From Trigger To Muzzle:
https://www.airrifle.co.za/library/The_Air_Gun_From_Trigger_To_Muzzle.pdf (https://www.airrifle.co.za/library/The_Air_Gun_From_Trigger_To_Muzzle.pdf)
This is old and not nearly as incitement as the airgun ballistics study posted in this thread earlier. Still, a short read that covers some basics rather well.
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Found it! This most fascinating thread about a dual opposing piston airgun build starts here:
https://www.gatewaytoairguns.org/GTA/index.php?topic=25115.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=25115.0)
It ended here, with me late to the party:
My conclusion is.............. Large piston face/short stroke is unworkable. The large piston face area makes them prone to bounce and it is impossible to make them heavy enough to overcome the bounce.
Yes, I know it has been years, but, to quell piston bounce, ponder this idea:
Place a reed-valve inside your breech bolt (as used on some two-stroke bike engine intakes) . The valve would snap shut due to the pressure differential, just as the pistons start to bounce back. That would trap the bulk of the air behind the pellet, regardless of what the pistons felt like doing.
I'm sure there are many ways to subdue piston bounce but the increase in complexity at some point becomes untenable. Slide weights, reed valves, etc. are all good ideas but they need to be applied to a design that is not basically flawed. The piston face area/stroke ratio must be reasonable from the start.
The Whiscombe with its two pistons mechanically connected has a distinct advantage in the area/stroke department; each piston shares each other's mass.
Thanks for the interest!
Tom
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More food for thought on telescoping piston designs. Not for a springer, but some aspects may be transferable:
https://www.tapatalk.com/groups/thegreencrosmanforum/telescoping-piston-pics-t40137.html#p240081 (https://www.tapatalk.com/groups/thegreencrosmanforum/telescoping-piston-pics-t40137.html#p240081)
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Marty,
If you take a step back and define the problem you are trying to solve as, "how can the efficiency of a spring airgun be improved", then some reasonable, simple feasible ideas present themselves.
For instance; the fact that the piston bounces back while the pellet is still in the barrel, reduces average air pressure and therefore reduces pellet muzzle velocity; especially with longer barrels: https://www.researchgate.net/figure/Typical-firing-cycle-for-the-Beeman-RS2-air-rifle-with-the-0670-g-JSB-pellet-03556-m_fig3_274638905 (https://www.researchgate.net/figure/Typical-firing-cycle-for-the-Beeman-RS2-air-rifle-with-the-0670-g-JSB-pellet-03556-m_fig3_274638905)
Rather than just using a heavier piston to try to minimize the bounce, the proposal is that you convert 1/3 to 1/2 of the standard piston weight to steel shot. The shot pellets would need to be able to move freely, front-to-back by perhaps 1/4". The "deadblow hammer" effect should hold the piston at the end of the compression cylinder longer, so transferring more air pressure energy to the pellet, without increasing airgun "recoil".
The steel shot would be housed in a thin walled steel tube that also acts as a spring guide; and is attached to the front inside end of the piston. There would be no need for a sub-diameter coaxial piston or cylinder...
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Interesting, but how about the start of the stroke, will there be any issues with shifting of the piston’s center of gravity as the steel shot is accelerated? Enough to change the balance of the gun during the shot cycle? I guess it depends on the amount of mass in the steel shot relative to the gun’s total mass. You’ll just have to make sure that the steel shot is rubber coated so it doesn’t sound like you have rocks in the rifle :D
I’ll read up on your links. I’ve heard of the Whiscombe opposing piston rifle from Tom’s blog. I wish they were still available.
Marty,
If you take a step back and define the problem you are trying to solve as, "how can the efficiency of a spring airgun be improved", then some reasonable, simple feasible ideas present themselves.
For instance; the fact that the piston bounces back while the pellet is still in the barrel, reduces average air pressure and therefore reduces pellet muzzle velocity; especially with longer barrels: https://www.researchgate.net/figure/Typical-firing-cycle-for-the-Beeman-RS2-air-rifle-with-the-0670-g-JSB-pellet-03556-m_fig3_274638905 (https://www.researchgate.net/figure/Typical-firing-cycle-for-the-Beeman-RS2-air-rifle-with-the-0670-g-JSB-pellet-03556-m_fig3_274638905)
Rather than just using a heavier piston to try to minimize the bounce, the proposal is that you convert 1/3 to 1/2 of the standard piston weight to steel shot. The shot pellets would need to be able to move freely, front-to-back by perhaps 1/4". The "deadblow hammer" effect should hold the piston at the end of the compression cylinder longer, so transferring more air pressure energy to the pellet, without increasing airgun "recoil".
The steel shot would be housed in a thin walled steel tube that also acts as a spring guide; and is attached to the front inside end of the piston. There would be no need for a sub-diameter coaxial piston or cylinder...
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When you start to swing a deadblow hammer, the shot inside settles against the back wall of the tube housing; regardless of the shot position before the swing started. You are not aware of the shot moving as you start the swing. Neither are you aware of the shot hitting the front of the housing when the blow lands. The only difference is that the hammer does not bounce.
I think that the acceleration of the piston in such an air rifle will settle the shot towards the rear just as the piston starts off. As long as the free play of the shot is not too long, I don't think it will have any negative effects.
As the shot will be inside a steel tube, even shaking that tube will make hardly any noise: Try shaking a deadblow hammer. That is not loud; and you can't shake a 7 lb air rifle like you can shake a 2 lb hammer.
Rather than rubber coat steel shot, use lead shot; although that might change too much with use. I think steel will last better "bare", without the balls changing impulse characteristics, or getting chewed up.
You might also consider tungsten shot:
https://www.amazon.com/Tungsten-Carbide-Bearings-0-096-VXB/dp/B00J5OPDH2 (https://www.amazon.com/Tungsten-Carbide-Bearings-0-096-VXB/dp/B00J5OPDH2)
https://www.amazon.com/Pinewood-Weights-Pro-Tungsten-Spheres/dp/B003TY3CAY (https://www.amazon.com/Pinewood-Weights-Pro-Tungsten-Spheres/dp/B003TY3CAY)
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You can experiment with the volume of shot and the free travel distance until your airgun shott the way you want it to.
https://youtu.be/4RlcZVqDhFg (https://youtu.be/4RlcZVqDhFg)
https://youtu.be/to-D7KM-fWg (https://youtu.be/to-D7KM-fWg)
Cheap source of steel shot is BBs made for airguns: https://www.pyramydair.com/product/copperhead-177-cal-5-1-grains-bbs-1500ct?p=106 (https://www.pyramydair.com/product/copperhead-177-cal-5-1-grains-bbs-1500ct?p=106)
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You convinced me with the hammer demo, seems like a better design than a piston capture mechanism and easy to implement in a standard piston body.
I have another idea, which I have been giving some thought. I’d like to figure out if a Redux Wave spring can be substituted for a conventional spring, but I haven’t found a supplier that has the right dimensions.
The nice thing about a redux wave spring is that it takes up a lot less space for the same load. I think this would allow for either a shorter stroke or an increase in the swept volume. Some questions I have about a redux spring are whether it takes a set faster than a conventional spring and how to make it easier to compress it. Since it takes less space I also assume that the compression motion requires more energy per distance of compression.
You can experiment with the volume of shot and the free travel distance until your airgun shott the way you want it to.
https://youtu.be/4RlcZVqDhFg (https://youtu.be/4RlcZVqDhFg)
https://youtu.be/to-D7KM-fWg (https://youtu.be/to-D7KM-fWg)
Cheap source of steel shot is BBs made for airguns: https://www.pyramydair.com/product/copperhead-177-cal-5-1-grains-bbs-1500ct?p=106 (https://www.pyramydair.com/product/copperhead-177-cal-5-1-grains-bbs-1500ct?p=106)
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Here is a link to the redux wave spring video. It does tend to become less stable as length increases, but I think that can be fixed using guides.
And a link to technical characteristics
https://www.designworldonline.com/best-springs-havent-tried-yet/ (https://www.designworldonline.com/best-springs-havent-tried-yet/)
https://m.youtube.com/watch?v=e3CY3v8KBZg (https://m.youtube.com/watch?v=e3CY3v8KBZg)
-Marty
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The lack of torsional windup makes that redux wave spring interesting.
I am sure that a spring manufacturer could also wind you a custom Confined-Space Conical Compression Spring. This one is a light weight, but I show it just in case you are not familiar with the type: https://www.mcmaster.com/1692k61 (https://www.mcmaster.com/1692k61)
(https://images1.mcmaster.com/mvA/contents/gfx/large/1692k38c1-d03d-digitall.png?ver=1503396502)
The advantage of cone springs is their short stacked height. I would want a much shallower cone angle for airgun applications...
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Just a note of encouragement. This is captivatingly interesting and I hope the thoughts develop into experiments.
The engineering required seems to be within the capability of an adventurous amateur machinist so the next step seems to be to give it a go.
Sure problems will arise.
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Unless I am mistaken, a piston with a complex shape, as the stepped one in Reply #23 above.... with valving the allows the air to pass in either direction between the small and large diameters.... will not provide any boost in pressure over a flat, single diameter piston of the same bore and stroke.... The shape of the front of the piston should not matter, if the compression chamber in the front of the tube matches that profile, and the headspace is the same.... at least that is what I see happening....
Think of a flat piston and flat end on the compression tube.... compared to a domed piston and matching concave end on the front of the tube.... If both have (for example) a 0.050" space, and a 5" stoke, they should have a compression ratio of 100:1.... and produce about 1500 psi (assuming a strong enough spring to get there)…. I think the same principle applies to a stepped piston and reverse step in the front of the compression tube, and any valving would only serve to equalize the pressure difference, if there was any.... That's what I see, anyways....
Now subscriber's progressive piston with the inner (or outer) portion moving separately through its own momentum may be a different story.... but if the portions of the piston are rigid (ie one piece), I can't see a way to boost the pressure....
Bob
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Getting back to the "deadblow hammer" piston design. The question is, how much would one want to get the piston to dwell at the end of its stroke, that now represents lost performance? This would effect the optimal open space in the shot tube (linear distance is the important factor, at nominal piston deceleration). Also, the ratio of piston mass that is made up of shot would factor into holding peak pressure longer, without reducing the peak pressure significantly.
Looking at figure number 14 of that spring air rifle study, the answer appears to be just under 2/1000 second:
https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
Obviously, this is a very specific case, but it gives us some idea. Using some very fuzzy math where I "divide by the number I first thought of", I think that free play in the shot column (with all shot against the back wall, then all shot against the front wall) of 1/4 to 1/2" would be about right...
I still think that the shot weight should be 1/3 to 1/2 the total piston weight, with less than 1/4 probably not being very useful. Considering the space inside the spring for shot, tungsten is starting to look like the best solution. That, after perhaps machining off 1/3 to 1/2 of the original piston weight...
Based on the quote below from section 6.4 of the linked study, simply adding piston weight may help boost pellet velocity too. So, just adding weight in the form of shot, without reducing the original piston weight may be beneficial. However, the gain from a simple mass increase is small:
Computations show that, at the margin, an increment of one gram in piston weight leads to an increment of about 0.047 Joules in power output for the 0.547 g pellet
There are reasons why simply adding lots of mass to the piston may not be ideal in practice, such as a slower harsher shot cycle that increases hold sensitivity; boosting velocity at the expense of practical accuracy. As such, your preferred embodiment would be to keep the piston mass almost original, except that a large portion of that mass is comprised of movably contained metal shot.
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Generally speaking, with no other changes, increasing the piston mass optimizes a springer for a heavier pellet.... The heavier pellet takes longer to accelerate, giving more time for the air cushion to arrest the higher momentum of the heavier piston.... Heavy pistons and light pellets tend to make for a very "harsh" shot cycle, with the gun jumping forward as the piston collides with the end of the compression chamber instead of bouncing off a cushion of air.... More power, maybe, but often at the expense of less accuracy.... The same can apply if you drill out the transfer port, it tends to make the gun harsher with light pellets.... Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....
Bob
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Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....
So, if one is going to try to improve over standard springer design, start with a cheap airgun that won't make you cry if you turn it into scrap...
I wonder if a piston of the same weight, but with deadblow debouncing would be any more likely to crash into the end of the cylinder. My gut says, no. I predict it would behave a little better, regardless of pellet weight (within reason). Assuming the execution is appropriate to work with the rest of the system...
Damping piston bounce may even increase spring life as an added bonus.
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I've been scouring website deals for just such a springer. It will likely be a Hatsan or a Gamo, one of those magnum ones.
Thanks for that link to QVTom's post, I wish I had his skills!
As for piston slam, I agree that as long as you keep the entire piston assembly and BB shot filler at the same weight as the unmodified piston then it shouldn't be a problem. Dialing it in will take some tries, but it should be ok.
Well made springers have a delicate balance between all these factors, in many cases quite well worked out by the manufacturer.... particularly in the more expensive, higher quality springers....
So, if one is going to try to improve over standard springer design, start with a cheap airgun that won't make you cry if you turn it into scrap...
I wonder if a piston of the same weight, but with deadblow debouncing would be any more likely to crash into the end of the cylinder. My gut says, no. I predict it would behave a little better, regardless of pellet weight (within reason). Assuming the execution is appropriate to work with the rest of the system...
Damping piston bounce may even increase spring life as an added bonus.
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subscriber, have you thought about the variability in performance of the dead hammer design depending on the angle that the rifle is pointed? Would the initial position of the steel shot, for example when the rifle is pointed downward, create large variation in either recoil or velocity?
Also, would placement of the steel shot compartment at the end versus at the front of the piston assembly have different performance characteristics? My gut tells me that the forces involved would be the same, but then again pulling something forward versus pushing it may have an impact on recoil characteristics. Would having the steel shot near the back of the piston have less tendency for the rifle to “tip” forward as its center of mass changes?
-Marty
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In theory, pointing the rifle down, up or level would change the way the shot is "collected" as the piston accelerates initially. Exactly how much variability that might introduce is something to be determined experimentally.
Unless the "free space" in the shot column is "huge", I don't think the starting position of the shot is going to cause any problems. Here is why:
Practically speaking, most shots are fired with the barrel not many degrees off from horizontal; either up or down. Also, the shot would end up "finding its own level" like a liquid; such that at rest the free space is at the top of the tube, rather than at the front or rear.
If you are shooting down from a tree stand, then all your shots are from that position. So, even if it shifts the POI from that when shooting near level, a little, you have to account for range and angle anyway when you shoot steeply "down" or "up".
When selecting a candidate rifle to modify, I suggest one without an integral spring guide that is part of the piston, that also contains the sear notch. This style of piston occupies the space you want to place the shot tube...
Choose piston #2 below; not #1 below:
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I think the tendency for the rifle to "tip forward", if the shot is concentrated near the front of the piston will be hard to measure, let alone feel:
Can you tell if your spring air rifle is cocked, versus fired; based on a change in its center of gravity? That involves a 4" shift in the position of 100 % of the piston mass and half that of the spring. If the steel shot makes up half the piston mass, and it has shifted 1/2" forward, I don't think anyone will notice...
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Yes, but depending on the location of the pellet in the barrel, as the piston comes to the end of the stroke, it may have an impact on accuracy. Probably minimal, but something that is worth testing.
Another thing I’d like to test is if the diameter of the steel shot is important. I dont’t have any Mercury on hand to make a comparison to steel shot, but I would guess that the more it acts like a fluid the faster it would self level against the back of the piston at the start of the stroke. Now I’m just nitpicking, but as they say the whole is sometimes greater than the sum of its parts.
-Marty
I think the tendency for the rifle to "tip forward", if the shot is concentrated near the front of the piston will be hard to measure, let alone feel:
Can you tell if your spring air rifle is cocked, versus fired; based on a change in its center of gravity? That involves a 4" shift in the position of 100 % of the piston mass and half that of the spring. If the steel shot makes up half the piston mass, and it has shifted 1/2" forward, I don't think anyone will notice...
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I may have to buy two identical rifles, just so I can get an objective comparison of before and after. ;D
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Due to the peak acceleration at max spring compression, the steel shot would collect completely at the rear of the piston within a gnat's whisker of initial piston travel. Only when the piston starts to see negative acceleration does the shot column start moving forwards. By the time the piston stops, there would be no difference in shot position or density to affect the pellet leaving the barrel.
In fact; if one could write a spec for the shot string settling inside the piston; it needs to completely stack towards the rear and front of the piston, at the start and end of the piston travel respectively; within 0.002 second - the same time interval for both events.
The reason to use shot is because each little pellet transmits its momentum in a continuous stream (for 0.002 seconds) of small sequential incremental impacts. Also, the sliding of pellets over each other as they "pack" into peak density causes the action to be damped, as if the shot column is a viscous fluid.
Using an actual "thin" liquid such as mercury would probably result in the equivalent of water hammer; where the entire slug of fluid crashes into the front of the piston almost simultaneously. Yes, the air in front of the mercury would have to travel through the liquid, and that would break up the stream to some degree.
This venting of air would not be as predictable or repeatable through mercury, compared to steel shot; because of all the discreet spaces between shot pellets that never collapse completely.
If you made some experimental hammers where you varied the size of the shot and the size of the free space, then whacked the hammers into a steel plate and observed the lack of bounce, that would be useful. While you are at it, try a mercury or water hammer.
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Start with the same size shot as is used in dead blow hammers....
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Any good reason to consider a conical shape for the back-end where the steel shot gathers during acceleration?
Start with the same size shot as is used in dead blow hammers....
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Any good reason to consider a conical shape for the back-end where the steel shot gathers during acceleration?
If some deadblow hammers have cones at the back (how does one tell the back end when the hammer head is symmetrical on the outside?), this may be to soften the reaction on you hand, as the pellets collect at that end of the hammer tube; just after you start the swing. Another reason may be to reduce pellet bounce and separation on the very gentle acceleration humans are capable of at the start of the swing; compared to the abrupt stop at the end when you hit something stiff with the hammer.
Making the front end of the airgun piston shot tube conical may be a way of increasing the dwell, if that is required. Somehow, I think that might "oversolve" the bouncing problem, and reduce the force of the blow. You don't want to reduce the force of the piston slamming into the air column (if you can help it); you just want to damp the tendency of the piston to bounce on the air spring in front of it, long enough to clear the pellet from the muzzle.
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I spent a part of today looking at suitable platforms for this dead blow hammer experiment. Aside from the Stoeger X20, any other suggestions for suitable cheap springers to experiment on? Easy disassembly are a plus, larger piston diameter is probably good too because it would make placing a another cylinder inside it easier. Since we are trying to see if piston bounce is reduced any known models with this problem would be good candidates as well.
Thanks,
Marty
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All spring airguns "suffer" from piston bounce. One might even call it a feature, as it reduces muzzle pressure and quiets the report. Yes, at the expense of velocity...
Probably any airgun sold with a "gas ram" would have the piston ID open. As many spring guns have identical versions with gas rams, converting back to a spring would seem feasible. To get appropriate spring, shop here: https://www.pyramydair.com/search-results-ext?keyword=main+spring&sid=1375A617A415&N=0&Ntk=primary&q=main+spring&cx=002970863286801882398:jlcminxfwdw&cof=FORID:11;NB:1&saSearch (https://www.pyramydair.com/search-results-ext?keyword=main+spring&sid=1375A617A415&N=0&Ntk=primary&q=main+spring&cx=002970863286801882398:jlcminxfwdw&cof=FORID:11;NB:1&saSearch)
This Benajamin Trail would seem a candidate with an open piston ID. : http://www.airguntoys.com/NP-GRS_files/GRS-NP_PrintPage1.htm (http://www.airguntoys.com/NP-GRS_files/GRS-NP_PrintPage1.htm)
(http://www.airguntoys.com/NP-GRS_files/Piston_Ram_Washer-2.jpg)
https://www.pyramydair.com/blog/2013/01/ive-got-gas-part-1/ (https://www.pyramydair.com/blog/2013/01/ive-got-gas-part-1/)
(https://www.pyramydair.com/blog/wp-content/uploads/2013/01/01-14-13-08-pistons.jpg)
https://www.airgununiverse.net/wp/gamo-shadow-teardown-and-tune/ (https://www.airgununiverse.net/wp/gamo-shadow-teardown-and-tune/)
(http://www.airgununiverse.net/wp/wp-content/uploads/2017/11/1109170946-e1510260164779.jpg)
RWS Diana 350 with appropriate piston:
https://www.youtube.com/watch?v=rL4ukRx1Omo (https://www.youtube.com/watch?v=rL4ukRx1Omo)
Found these piston images and articles by searching google images for gamo piston seal replacement. If you have instructions on how to get the piston out, that would seem logically to include images of the piston: https://www.google.com/search?rlz=1C1NDCM_enUS793US793&biw=1524&bih=712&tbm=isch&sa=1&ei=fPLYW5nULsSS0wLLhJSICg&q=gamo+piston+seal+replacement&oq=gamo+piston+seal+replacement&gs_l=img.3..0i24k1.234970.237256.0.238127.6.5.1.0.0.0.112.412.4j1.5.0....0...1c.1.64.img..0.1.112....0.RhDgFVeusWI#imgrc=jmyoXa9r1ydffM: (https://www.google.com/search?rlz=1C1NDCM_enUS793US793&biw=1524&bih=712&tbm=isch&sa=1&ei=fPLYW5nULsSS0wLLhJSICg&q=gamo+piston+seal+replacement&oq=gamo+piston+seal+replacement&gs_l=img.3..0i24k1.234970.237256.0.238127.6.5.1.0.0.0.112.412.4j1.5.0....0...1c.1.64.img..0.1.112....0.RhDgFVeusWI#imgrc=jmyoXa9r1ydffM:)
Change "gamo" for any brand name. Using "airgun" or "air rifle" yielded too many unrelated images...
You may want to invest in something like this too: https://www.pyramydair.com/product/air-venturi-rail-lock-spring-compressor?a=7764 (https://www.pyramydair.com/product/air-venturi-rail-lock-spring-compressor?a=7764)
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Another way of looking at the piston design of any air rifle sold by Pyramyd is to download the user manual or schematics.
For instance, here is the Trail: https://www.pyramydair.com/product/benjamin-trail-np-xl-air-rifle?m=2052 (https://www.pyramydair.com/product/benjamin-trail-np-xl-air-rifle?m=2052)
Here is the schematic, found by scrolling down on the right side of the "description" tab of the above page: https://www.pyramydair.com/air-gunsairsoft-schematics (https://www.pyramydair.com/air-gunsairsoft-schematics)
(https://www.pyramydair.com/images/schematics/cr/Trail-XL-EVP-2.png)
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Schematics for a bunch of airgun brands: https://www.pyramydair.com/air-gunsairsoft-schematics (https://www.pyramydair.com/air-gunsairsoft-schematics)
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Low cost gas ram rifles: https://www.pyramydair.com/air-guns?orderBy=price_asc (https://www.pyramydair.com/air-guns?orderBy=price_asc)
Pick one with a high customer rating, because that probably means the rest of the parts are not complete rubbish. Also, find discussions about taking them apart and choose one that is easy to work on...
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I've ordered one of the Hatsan models with a gas ram and I'll be ordering a spring to go with it soon. I've heard that converting the gas ram to spring is easier in the Hatsan rifles than some others. If not, oh well, I'll just have another rifle ;D
In the meantime I've also done some more reading on the subject and came upon the Theoben line of rifles. These sweet English beauties make some of the German Fräulein look quaint. A few were sold in the US under the Beeman name as the Crowmagnum models. Getting back on topic, one of Tom Gaylord's articles mentions that the Theoben Eliminator had a floating inertia piston design coupled with some kind of value. Has anyone seen the internals of these guns or a diagram?
-Marty
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Eliminator user manual found, with info about dual weight piston: https://www.diamanco.gr/Upload/OWNERS_MANUAL_ENGLISH.pdf (https://www.diamanco.gr/Upload/OWNERS_MANUAL_ENGLISH.pdf)
Eliminator has adjustable pressure gas ram. My understanding is that the valve was to couple to that.
Floating two piece piston would work like shot, except the collision event would be more abrupt. Changing the mass of the ballast would require machining a new weight (or just reducing the one you have). Shot is more flexible and probably produces less shock and noise on firing.
There must be a reason why Theoben went with a single weight. Perhaps a list of them. Maybe the behavior of the mass was more predictable...
The attached image is from another source than the user manual, so check both.
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Hi guys nice thread about spring piston peak pressure.
Im have a theoben Eliminator its a fine made air gun.
mine have the HE system ( 68 bar ).
it have a 31.4mm bore and 100mm stroke
the tp is 4mm and 6mm long
the static compression ratio is around 1000:1 !!
the piston seal is 2 O rings.
mine make 27 fpe with .22 around 65 bar ram pressure
its very nice to shoot better than mine 37 fpe hatsan 135 .30
the hatsan I modified the piston to use a vortek hw80 seal, the oem seals go bust after few shot
they damage around the tp hole.
the hatsan also have 125mm stroke give it at 88.5 cc swept volume 30x125mm.
I have it with 110kg gasram aftermarket the piston weight around 460 grams.
it shot a 46.3 grain pellet at 605 fps.
the tp hole is 4.2mm and 13mm long static cr around 531:1 to low
im have found a 31.4mm cutter so Im go cutting the cylinder tube from 30mm to 31.4mm make a costum piston for it with old type theoben eli piston zypher seal. grooves front seal to channel to air to tp hole.
the swept volume increasse to 96.8 cc and the stcr to 582:1.
hope to hit 40 fpe.
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Theoben HE, thanks for the useful information. Its good to know that the HW80 Vortek seal is an improvement over the Hatsan original. I will order it too.
subscriber, I've managed to find the patent that Theoben had on the design. It looks to be expired, so we are on safe ground and using steel shot is still different enough that it shouldn't pose a problem. Here is a link if you are interested in seeing the diagram, it is toward the end of the application.
https://patentimages.storage.googleapis.com/6c/8d/d6/e5c6e68e8b7330/EP0460961A2.pdf
Thanks to everyone for contributing your ideas. Just thinking about this stuff is lots of fun.
-Marty
Hi guys nice thread about spring piston peak pressure.
Im have a theoben Eliminator its a fine made air gun.
mine have the HE system ( 68 bar ).
it have a 31.4mm bore and 100mm stroke
the tp is 4mm and 6mm long
the static compression ratio is around 1000:1 !!
the piston seal is 2 O rings.
mine make 27 fpe with .22 around 65 bar ram pressure
its very nice to shoot better than mine 37 fpe hatsan 135 .30
the hatsan I modified the piston to use a vortek hw80 seal, the oem seals go bust after few shot
they damage around the tp hole.
the hatsan also have 125mm stroke give it at 88.5 cc swept volume 30x125mm.
I have it with 110kg gasram aftermarket the piston weight around 460 grams.
it shot a 46.3 grain pellet at 605 fps.
the tp hole is 4.2mm and 13mm long static cr around 531:1 to low
im have found a 31.4mm cutter so Im go cutting the cylinder tube from 30mm to 31.4mm make a costum piston for it with old type theoben eli piston zypher seal. grooves front seal to channel to air to tp hole.
the swept volume increasse to 96.8 cc and the stcr to 582:1.
hope to hit 40 fpe.
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Hi Marty
yes nice to thinking about airguns, the vortek seal holds up very well you need to modified the piston to fit the seal ore buy the hw35 adapter and weld it.
nice info about theoben gasram system, the sliding dummy prevents piston bounch it slids forward and give the piston extra mass to the end.
What youre thoughts about static compression ratio ? ( sweptvolume vs lost volume from transferport)
theoben eli. 4mm hole 6mm long = 1000:1
short port = more effincie
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Bernard,
The higher the static compression ratio the more efficient the airgun. Dynamically, the piston will slow down long before it reaches infinite (or even 1000:1) compression. Where a lower static or nominal compression ratio may help is to reduce piston bounce. Now the object of the exercise is to reduce piston bounce by means of a dual mass piston, so favoring higher compression.
Certainly, If you could generate peak pressures of 25,000 PSI in a spring airgun, all parts except the walls of the air cylinder near or at its front should be strong enough (due to the large area for the gas to act on). I say this with apparent confidence, because .22 rimfire parts are not particularly heavy and operate in this pressure range. Even the thin crushable brass wall of a .22 rimfire cartridge case can withstand the pressure, unsupported (over a small distance), right at the rim.
The shorter the transfer port the better. The optimal TP diameter is surprisingly small, unless it is very short. See images below, borrowed from the PDF linked below.
Transfer port length depends on the end wall thickness of the compression cylinder. I would calculate how thick the compression cylinder wall needs to be, to handle the pressure it will see; then at least double that. The snag here is "what is the peak pressure in a springer?". This is a huge variable; and the very aspect that we are trying to increase.
Somehow, I don't think we are going to get the peak pressure much over 5000 PSI; unless we jam ten pellets into the breech or apply a blob of epoxy to block off the chamber. Now, depending on the diameter, it does not take that much steel (not iron) to hold 10,000 PSI safely. If you are concerned, design a place to install a 5000 PSI burst disk; so that breaks before any other parts are damaged.
If the volume of air at 10,000 PSI were larger than that, holding perhaps 100 ft.lb of potential pressure energy, then I would increase the factor of safety to at least 3 based on the material yield strength. A PCP air cylinder can hold potential pressure energy of 5000 ft.lb at 3000 PSI, depending on the air volume; so it must be treated with much more caution. Obviously, anything that throws even tiny shrapnel at your eyes would be very bad...
You mention the desire to bore out the Hatsan for more swept volume. Now, the effect of compression cylinder diameter is going to depend on the piston mass, spring force and energy, TP dimension etc; and I would not dare to assume that you are not changing any or all of these factors. Yet, I want to caution you that the bore to stroke ratio is another important parameter to make spring airguns work well. See graph on bore diameter VS power below (also found at PDF link).
This link is a long thread about a dual opposing piston gas ram air gun development. The project died because the bore diameter was too large for the stroke, resulting in so much piston bounce that efficiency and power were unimpressive. The design had no room for heavier pistons or a longer stroke, and so was abandoned: https://www.gatewaytoairguns.org/GTA/index.php?topic=25115.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=25115.0)
The reason I mention the above, is that increasing your swept volume by means of a larger piston may actually cost you power. Now, my gut tells me that if you increased the piston area by 10%, the piston mass should be increased by at least that percentage to maintain the same level of piston bounce. To take advantage of the 10% extra swept volume and the increased compression ratio, the piston mass may need to be increased 20%...
You will never know, unless you try; or find an article where someone published their results (obtained under similar conditions with similar equipment):
Images below about TP dimensions and piston diameter were obtained from this very useful spring airgun study:
https://www.researchgate.net/profile/Domingo_Tavella/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns/links/552473050cf2b123c51740aa/Internal-Ballistics-of-Spring-Piston-Airguns.pdf (https://www.researchgate.net/profile/Domingo_Tavella/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns/links/552473050cf2b123c51740aa/Internal-Ballistics-of-Spring-Piston-Airguns.pdf)
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Marty,
The patent you linked, at: https://patentimages.storage.googleapis.com/6c/8d/d6/e5c6e68e8b7330/EP0460961A2.pdf
appears to cover much more than a dual mass piston. It seems to have variable compression due to twin diameter telescoping piston; perhaps closer to your original intent when you started this thread.
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if you want a manageable high-powered airgun, springers are NOT viable. By high-power I do not mean something putting 20 to 30 FPE, I'm talking 30+ FPE, 40+, 50 and well past that.
You could make a 40 ft.lb springer, but you are not going to like cocking it, aiming it offhand, shooting it or toting it.
To reduce cocking effort to the point that makes 40 ft.lb possible, the barrel or cocking lever would have to be super long. The rest of the hardware super heavy. This would increase the moment of the gun to the point that no one would want to shoot it offhand. And no one would carry it hunting.
The fun limit for me is the RWS 350; and those are just over 20 ft.lb guns. I much prefer the handling qualities of my HW50S and R9. The R9 is harder to cock than the 350, because the R9 has such a short barrel. I have a Webley Patriot, but it seems like a monster, for so little gain.
Anyway, I think the OP is trying to increase efficiency of spring air guns. I doubt his goal is to double their power. What if he succeeds at making a light compact springer that produces 12 ft.lb, but has only a 20 lb peak cocking force? That would be something.
bob sterne had an idea for a 75 fpe springer , it was very interesting but a lot of work to bring to life , not to mention money
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Bob sterne had an idea for a 75 fpe springer , it was very interesting but a lot of work to bring to life , not to mention money
When it comes to airguns; Bob blends vast theoretical knowledge with more real experimental results than many full time engineers see over decades. The fact that Bob dreamed up such a springer, then chose not to build it should not be taken lightly. I would like to read Bob's thread and understand his reasoning fully before trying to emulate that.
I think that if you are going to make a monster springer, it needs dual opposing pistons to cancel out what would otherwise prove to be a rather harsh airgun to shoot. That is, unless you make it weigh 45 lb (a 7 lb 12 ft.lb springer scaled up to 75 ft.lb).
Look on the bright side, if you make a monster single piston springer heavy enough, you can use the weight to help cock it. At least once :)
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It was simply a "design study".... ie what would be required to build a .30 cal springer.... Yes, it would be a beast....
https://www.gatewaytoairguns.org/GTA/index.php?topic=94485. (https://www.gatewaytoairguns.org/GTA/index.php?topic=94485.)
Somebody asked me what it might take, so I scaled up the important numbers (swept volume and spring force) to provide some guidance....
Bob
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Thanks for finding and posting the link, Bob.
From that thread, the quote below comes closest to eventually hinting at dual opposed pistons - the key to taming the monster:
I think it's feasible to make one, though the only way I would guess it would work is using two or three gas pistons and using individual levers to cock each piston then the trigger mechanism would release them at once.
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thanks for youre information, im going to bore mine h135 to 31.4mm same as theoben eli the stroke lenght 125mm + 25mm than the eliminator.
the transfer port 4.2 mm and 13mm long im try reduce that to 8mm the stcrs go up to 874:1
so a high stcr is more effi. thats why a eliminator with 76cc swept volume and 1000:1 stcr can make 30 fpe in .25.
yes mine piston im make form stainless steel weight around 500 550 grams ( 460 oem).
now im calulate the stcr with 30x125 bore stroke, with 13mm long port give me:
3mm port = 1:961
3.2mm = 1:850
3.5mm = 1:707
wich port youre advise? im make interchange tp.
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Thanks, I didnt read it in detail when I posted it. Here is another link showing one of the Theoben rams taken apart. There is a counter weight hidden inside.
http://dursley.org.uk/air/gasramsstrip.pdf (http://dursley.org.uk/air/gasramsstrip.pdf)
Marty,
The patent you linked, at: https://patentimages.storage.googleapis.com/6c/8d/d6/e5c6e68e8b7330/EP0460961A2.pdf (https://patentimages.storage.googleapis.com/6c/8d/d6/e5c6e68e8b7330/EP0460961A2.pdf)
appears to cover much more than a dual mass piston. It seems to have variable compression due to twin diameter telescoping piston; perhaps closer to your original intent when you started this thread.
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Marty,
I am struck by three things after reading that article:
The Theoben looks extremely well made.
The pictures and tutorial are extremely clear.
The apparent mass of the secondary piston weight seems lower than I anticipated:
If 75% of the piston's kinetic energy is converted to air pressure behind the pellet, then piston bounce may only be "wasting" 25%. That might point to a floating weight of only 25% (or even less) of the total moving piston mass. I figured anything less than 25% would be "useless"; but these images suggests that my mental model was not accurate.
Anyway, by using steel shot, you would be able to vary the secondary piston mass easily to measure the effect. I assume that the highest pellet velocity, combined with the smoothest shooting cycle and best accuracy will point the way.
I wonder how much free travel there is in the Theoben solid secondary piston weight? That will affect the timing of action, "sharpness" and magnitude of the secondary piston momentum transfer.
It looks like the secondary piston slides on O-rings. I presume the O-rings are to stop it from rattling around (not to seal anything).
I can also imagine that the floating weight sits at (or near) the front of the piston due to O-ring friction (from the last shot), until you fire a shot. Then the weight "stays put" until it impacts the rear of the piston proper. This set the weight up to be carried along with the piston. Then, when the piston decelerates against increasing air pressure, the weight slams into the front of the piston.
The advantage of the O-rings might be that the floating weight is always in the same position at the start of the shot cycle, regardless of the shooting angle or manner of carrying the rifle. You alluded to this by asking if steel shot sloshing around might matter. I am still not convinced that it will. You should find out soon enough...
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I've been told that in the Theoben design the o-rings on the weight are meant to create a seal in order to trap a small amount of air behind the weight. As the ram is cocked the weight is pushed to the front of the ram chamber creating another pressurized pocket. There were apparently two designs, one that created a complete seal and one that had a hole in the middle allowing the inertia weight to move more freely. I'm trying to play out the dynamics in my head, but it's been a long day and my brain isn't working...
Marty,
I am struck by three things after reading that article:
The Theoben looks extremely well made.
The pictures and tutorial are extremely clear.
The apparent mass of the secondary piston weight seems lower than I anticipated:
If 75% of the piston's kinetic energy is converted to air pressure behind the pellet, then piston bounce may only be "wasting" 25%. That might point to a floating weight of only 25% (or even less) of the total moving piston mass. I figured anything less than 25% would be "useless"; but these images suggests that my mental model was not accurate.
Anyway, by using steel shot, you would be able to vary the secondary piston mass easily to measure the effect. I assume that the highest pellet velocity, combined with the smoothest shooting cycle and best accuracy will point the way.
I wonder how much free travel there is in the Theoben solid secondary piston weight? That will affect the timing of action, "sharpness" and magnitude of the secondary piston momentum transfer.
It looks like the secondary piston slides on O-rings. I presume the O-rings are to stop it from rattling around (not to seal anything).
I can also imagine that the floating weight sits at (or near) the front of the piston due to O-ring friction (from the last shot), until you fire a shot. Then the weight "stays put" until it impacts the rear of the piston proper. This set the weight up to be carried along with the piston. Then, when the piston decelerates against increasing air pressure, the weight slams into the front of the piston.
The advantage of the O-rings might be that the floating weight is always in the same position at the start of the shot cycle, regardless of the shooting angle or manner of carrying the rifle. You alluded to this by asking if steel shot sloshing around might matter. I am still not convinced that it will. You should find out soon enough...
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After Thinking about it some more, I believe the completely sealed version of the weight provides a small anti-recoil effect initially, then as the piston is about to bounce the weight's momentum drives it into the back of the piston for the extra push. If the pressure behind and after the weight are equalized at the end of the stroke it shouldn't move much at all as you suggested. Well, that's what my tired brain is telling me.
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weight to prevent
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Marty,
A floating piston with "air tight" O-rings will end up in unpredictable positions over time. Any O-rings leak; especially at low pressure. So, the air in front and behind the weight would not be as constant as that gas (or air) inside the gas spring.
I can see an O-ringed piston weight with a small vent hole. The vent would enable a damping effect on the weight's motion, but not build residual air pressure to determine where the weight settles. It is the uncertainly of that residual air that makes me skeptical about a floating piston within a sealed system.
Of course; if you could guarantee that the O-rings don't leak, then placing the floating weight in the middle of the piston (front to rear) might allow the weight bounce off its own air cushion; first at the rear, then the front. I just don't see the piston staying in the nominal middle of the piston (front to rear) over time.
Better to allow a small controlled vent that would still allow the weight to reach hard contact with both the front and rear of the inside of the piston. Just cushion it a bit so the slamming is not so binary and violent.
Hence the beauty of steel shot...
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Yes theoben internia weight have small vent hole.
the reason theoben have a realtive low piston weight to have a fast lock time, the internia weight helps to prevent piston bounch.
the fast lock time is needed to get the air pressure quick to max pressure, that is neede because the static compression ratio is 1000:1 !!.
the question im have how fast is a gasram vs spring
hatsan 135 update
tested the 3.2mm tp = same fpe as 4.2mm maybe little more because with 4.2mm it diesel very little with 3.2mm port 100% no dieseling !
46.3 grain pellet 182.6 m/s = 37 fpe
piston weight 460 grams
110kg gasram
30x110mm bore / stroke
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the question im have how fast is a gasram vs spring
In this video, at this time-stamp, Tom Gaylord of Pyramyd Air says 80 and 100 FPS for spring and gas ram respectively: https://www.youtube.com/watch?v=bIllYUzBr_Q&feature=youtu.be&t=170 (https://www.youtube.com/watch?v=bIllYUzBr_Q&feature=youtu.be&t=170)
Obviously, these figures should be treated only as a general indicator. Even so, I presume they were actual measurements taken off particular airguns at some point. Something tells me that these figures represent "magnum springers". Below is specific data for a medium powered spring air rifle:
From the attached graph below: The steel spring powered Beeman RS2 air rifle study showed a peak piston velocity of 56 feet per second.
Source: www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (http://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
(https://www.gatewaytoairguns.org/GTA/index.php?action=dlattach;topic=149983.0;attach=248117;image)
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Thanks alot so around 100 fps , is that the fixed speed a gasram mechnical can do ?
Also a gasram cocking force is higher, and almost same force true the hole cocking part.
so gasram is not progressive like most coil springs are.
im use a 110kg gasram in mine 135 its a heavy one 80 lbs.
can a gasram have less piston bounce ? because its have always max force on piston.
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A gas ram accelerates a given piston faster than a spring, at the same peak load. One reason is because the average force provided by the gas ram is higher: The force of a steel spring drops off much more steeply.
Also; the gas in a gas ram does not have the same inertia as a steel spring. For that same reason, I think a gas ram has more of a tendency towards piston bounce. The bounce may be quickly reversed because of the high force at the end of a gas spring stroke; but that would be too late; after the pellet is very near the muzzle.
Hence; this is why Theoben added the secondary piston mass. Mass resists changes more than the relatively low force (even of the gas spring). The difference is like that, between the acceleration seen when a car with a powerful engine crashes into a concrete block. No way any engine can match the instantaneous force that a moving (or stationary) mass can apply.
Gas rams should have more "area under the curve" when it comes to average force x stroke. Yet, my impression is that they do not always deliver that energy efficiently to the pellet. Perhaps a "stronger" effective spring is a detriment, unless you also increase the piston mass. That combination in a light airgun can become harsh to shoot. (although no springer recoil ever bothers me when I apply a light hold)
If you have too much "spring" and "too heavy a piston" for the swept volume, I think the piston just crashes into the front of the cylinder, rather than adding energy to the pellet. This is not good for power or accuracy, as the crash surely produces massive vibration down the barrel.
Increasing the peak air pressure behind the pellet too much probably distorts it to the point of making it inaccurate. A harder and heavier pellet would probably be better then.
If you want to emulate PCP pressure delivery, perhaps springer compression ratio should actually be reduced. This reduces efficiency only if there is zero piston bounce, yet high compression with a light piston will cause more bounce. In other words; sacrifice peak pressure to achieve more area under the pressure curve, before the piston starts reversing direction. The reverse acceleration would also be reduced if the peak pressure were reduced.
What I am suggesting, is how small displacement turbocharged racing car engines make huge power: The static compression ratios are made very low, so that the boost can be increased dramatically without causing knock. This means that the engine flows a lot of air and burns a lot of fuel. That makes huge power; even if very inefficiently compared to the engine intended for a commuter car.
What you want in your gas spring air rifle, is a screw adjustable counter piston; so that you can vary the volume of the compression space to achieve the best overall performance... Such a compression chamber might have a volume that is adjustable from zero to 1 CC; assuming a swept volume of 100 CC. With it set to .5 CC, it should produce roughly 3000 PSI peak pressure (a good target value, I think). More because of adiabatic heating; less due to some residual piston bounce.
If you produce such a variable compression springer; start testing with it set to "zero" and gradually increase the secondary volume. If you start with too low a compression ratio, your piston will crash right into the end of the cylinder...
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so a gasram is maybe to fast to compressing the air to peak pressure, not give air the time to build and heat up when compression is done.
mine ram sits with the body inside the piston, piston weight 460 grams.
a spring looks to me its little faster in first part and when air is compressed it holds little back and so peak pressure holds up little longer than a gasram, that give no room to hold the peak pressure at little.
about the theoben gasrams, what I know people how have them theoben gasram system removed to spring there fpe out put drop a good amound to.
what I under stand that you can make 2 ways to make power:
1 high compression ratio high pressure wave ( high effi )
2 go for peak pressure with alot of swept volume behind it to compensate to low compression ratio. ( low effi )
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In Adiabatic compression, the heating is instantaneous.... The longer it takes to compress the air, the more heat can transfer out into the metal, lowering the peak pressure.... However, the faster the piston is moving, the more momentum it has, so the more likely it is to crash into the end of the compression chamber.... which is very bad for accuracy, and probably longevity as well.... not to mention hard on scopes.... A lighter piston would reduce the momentum, but increase the velocity even more, which is why oversprung Springers have such a harsh shot cycle....
Bob
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Bernard; zoals ze zeggen, de duivel is in de details...
If we consider peak cocking forces; it makes sense that replacing a gas spring with steel spring would generally drop the pellet velocity. However, how do we know that in each instance where such a velocity drop was reported, that the most appropriate steel spring for that airgun had been used?
All I can suggest is that you explore the changes you want to make and measure what happens.
Generally, higher compression means better efficiency; thus greater pellet energy. Just to confound matters; consider this:
A lower compression ratio allows the piston to travel further down the cylinder. Thus, the fact that the gas spring is still pushing hard near the end of the stroke adds more energy; that would not have been added, if the piston had bounced sooner off high pressure air...
What you are modeling is a person jumping off a 10 story building onto an airbag. You are asking what effect the pressure inside the air bag has on how deep the person sinks into it. Also, how far and how quickly the person might bounce up. If the airbag is 5 foot deep, we tend to ignore that detail when calculating the gravitation potential energy, and multiply by 10 the floor to floor height.
However, if the airbag is 9 stories high, then the gravitational energy released is only one story; or about 10 feet of free fall; rather than 100 feet. An extreme example, but my point is that one has to consider the whole system in detail; otherwise our predictions are only educated guesses.
If we can't calculate or model exactly what happens inside an airgun, we can still test a system with known characteristic. Provided such a system does not have a leaky piston seal, or excess friction it should provide valid results.
Remember; just because you increase your airgun's swept volume by using a larger diameter piston (and cylinder), does not mean it will produce the increase in pellet energy you expected. This may be because the piston also needs to weigh a lot more to offset the increased "bouncing force" the now higher air pressure applies to the larger frontal area...
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As I twiddle my thumbs waiting for my packages to arrive, I’ve been trying to come up with objective ways to measure the changes in efficiency as well as recoil characteristics that may result from these experiments.
For testing energy transfer efficiency I will measure the before and after velocity of three different grain pellets (heavy to light) over an average of 30 shots per pellet type and then calculate FPE for each. My goal is to maintain the same piston weight from the unmodified to the modified versions. If the velocity improves I think we can say that efficiency has improved.
I have not come up with a good way to measure recoil or to estimate the actual piston bounce that might be already present in the rifle. The first thing that came to mind was using a camera with graphing paper in the background of the rifle. This should capture how much the rifle moves from start to finish, but it seems like a crude approach.
Does anyone have a better way to measure piston bounce without creating a see through compression chamber?
-Marty
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To track piston position over time, get a voltage source and oscilloscope that can measure voltage or electrical resistance at 10 kHz+ resolution:
Make a linear potentiometer out of a graphite pencil lead. Use a lightweight wiper attached to the exposed underside of the piston at the cocking slot. The graphite rod would be attached to the outside of the air cylinder (electrically isolated). The electrical resistance would be proportional to the effective length of the current path through the graphite; thus would represent piston position captureable externally...
Here alternative methods to make potentiameters:
www.youtube.com/watch?v=aElHVTv-75M (http://www.youtube.com/watch?v=aElHVTv-75M#)
https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer (https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer)
www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/ (http://www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/)
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Subscriber, that is a very cool idea! I’m glad I asked, its not something I would have thought of. Here I was wondering where to get plexiglass rods and whether they could be milled into a makeshift compression tube. ;D
To track piston position over time, get a voltage source and oscilloscope that can measure voltage or electrical resistance at 10 kHz+ resolution:
Make a linear potentiometer out of a graphite pencil lead. Use a lightweight wiper attached to the exposed underside of the piston at the cocking slot. The graphite rod would be attached to the outside of the air cylinder (electrically isolated). The electrical resistance would be proportional to the effective length of the current path through the graphite; thus would represent piston position captureable externally...
Here alternative methods to make potentiameters:
www.youtube.com/watch?v=aElHVTv-75M (http://www.youtube.com/watch?v=aElHVTv-75M#)
https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer (https://electronics.stackexchange.com/questions/24420/how-can-i-make-my-own-potentiometer)
www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/ (http://www.instructables.com/id/Make-a-Pencils-Lead-Potentiometer-Experimentatio/)
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Thanks lads for the information, Yes its all in the detail.
what I have tested so far:
with the vortek high picht spring it make less power : 150 ms 46.3 grain pellet
with the 85kg gasram make 170 ms
the 110kg gasram does 183 ms.
The thing is when I bought the gun new the QE .25 barrel is only 27cm 10/6 " long it piston slams hard with 20grain pellets.
So I replaced the QE barrel to h 125 .25 42cm barrel, The piston slam stops and the vo up 20 ms !
Thats why Im think the barrel lenght does matters, the .30 barrel is also 27cm maybe to short
for the power plant of the h135.
now with the long .25 barrel the gun is loud
the piston seal is little stiff inside bore.
the bigger the piston there more force needed to get the air to compress thats why a 25 mm piston is such good preformer on mid fpe guns. low piston weight not to hard to cock and accurate. because its needs less mass and spring force.
I try a 31.4mm hatsan 135 with 120-125mm stroke, I have order a h135 cylinder to cutting to 31.4mm
I have a good arms shop here he can almost order everthing of hatsan.
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The difference in velocity and piston slam could be due to the bore diameter, rather than the length.... If the longer barrel is tighter, that would delay the pellet release, increasing the peak pressure, which would reduce or eliminate piston slam, and could also increase the velocity....
Bob
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the .25 barrel is not 100% tight bore its alright but I really see a better bore than now.
the .30 bore is better fitment of pellet also a choke.
now with the 3.5mm tp it lost 10 ms
so back to the 3.2 mm tp.
the pellet is also the relaese valve what I know, so a .30 46 grain pellet needs some air to get moving.
But the air throws to a bigger surface so less pressure is needed ? than a .25 ?
im working on difference in piston weights i can increase weight by 10 grams a time.
maybe the hole system is already over max fpe and im chasing a ghost.
only thing to try is a longer .30 barrel now 27cm 10.6 " so to a 35- 40 cm is to try.
Im can't get hold on such barrel with 15-16mm diameter.
If there a .30 barrel to try please contact me.
what choice in pellets .30 im looking for a tight fit .30 and .25 pellet
what size does the .25 .30 FX pellets have ?
and the h&n baracuda ,30 ,25
how do they fit ?
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Hi Bernard,
The only way to study the effect of barrel length as a single factor, is to start with a long barrel and cut off an inch at a time (or cm, if you prefer); without changing anything else (other than de-burring the muzzle). Then to measure the pellet velocity at each discreet length. Comparing two barrels and expecting the difference in length to be the driving factor for observed velocity differences, does not make it so.
The PDF link posted earlier in this thread showed the effect of barrel length as a function of pellet weight. Image of velocity graph shown below. Springers can work with surprisingly short barrels (if you can stand the cocking force).
For a springer powerplant with a given swept volume, the larger the caliber, the shorter the barrel required to reach peak pellet velocity. This is simply because with a large bore, the expansion ratio is so much greater. A .25 caliber has twice the bore area of a .177. So, for the same powerplant, the .25 barrel length that would produce peak pellet velocity would tend to be half that of the .177.
Yes, the exact pellet weight and "tightness" in the breech matter a lot. However; the fundamental principle is the same as for efficiency due to "compression ratio"; just in reverse. Once the air pressure drops below about 100 PSI, the pellet will actually slow down due to barrel friction. This is a general value that works for all calibers with diabolo pellets (discussed in a few other threads on GTA).
.25 caliber 31 grain H&N Barracuda pellets I have measure:
Head: .2505" Very consistent.
Skirt: .254 to .259" around the same pellet skirt. I checked a number from the same can, and none of them have round skirts. May assume average effective diameter of .256 to .257".
The attached image compares measured VS calculated "springer" pellet velocity VS barrel length.
From: www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns (http://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns)
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Once the air pressure drops below about 100 PSI, the pellet will actually slow down due to barrel friction.]
I don't think so but I don't really know. I've shot a .22 pellet out of a barrel like a blowgun (lung pressure @ 2 psi). I've shot a .22 pellet out of a barrel with shop air (@ 100 psi) using a rubber tipped nozzle and it penetrated the drywall :o
The dynamic friction will increase with pressure as the force normal increases (due to pellet obturation). But... as pressure declines, so does the force normal, and so does the dynamic friction. kind of a paradox. More pressure creates more friction. Difficult to measure.
My empirical results have shown that longer barrels on low powered springers (i.e. HW45s) will increase the fpe consistent with the increase in barrel volume. Does anyone really know? Interesting discussion, thanks-
Wyo
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I've shot a .22 pellet out of a barrel like a blowgun (lung pressure @ 2 psi).
WyoMan,
Let me guess; you pushed the pellet in via the muzzle, past the choke. Thereby setting it up for loose, practically friction free fit; before blowing it out the chamber end...
A .22 caliber bore has an area of 0.038 square inches. At two PSI that equates to 0.76 lb of force. So, either your pellets were undersized, or your barrel oversized; or you pushed the pellets through multiple times before blowing them through. The only way you are blowing a pellet through the choke with your mouth, is if it has been through before...
Now the driving force on a .22 pellet would be 3.8 lb at a steady 100 PSI. What is the friction of a .22 pellet going down the barrel? If you can overcome that, and maintain perhaps 2 lb over 2 feet of travel, then the pellet is going to accelerate "fast enough to penetrate drywall".
A PCP and springer work by providing a limited pulse of high pressure air (not a nearly endless supply of low pressure air, like a shop compressor tank). As such, with these airguns air the pressure keeps dropping as it expands down the barrel. In the case of a springer, the pressure drops much faster due to piston bounce, and the fact that the air will be cooling rapidly after its initial adiabatic compression heating.
Now; the question that was also asked the context of how much is typical pellet friction; was how long should a barrel be to achieve full velocity, practically speaking? The answer that came back from multiple practitioners of the art, was that making the barrel long enough in a PCP, to the point where the air pressure drops below 100 PSI, made for a ludicrously long barrel, that gained hardly any practical velocity.
Now, the friction break even point may have been 50 PSI (rather than the 100 PSI my statement implied). That said, the added FPE from a delta of 50 PSI (100 - 50) acting on a .22 pellet over another 6" of barrel would gain only 0.95 ft.lb.
By the way; the HW45 is a pistol, rather than a rifle. Just because you can gain a few FPS by increasing the barrel length of a springer pistol does not mean you will achieve the same gains with a rifle. There are many threads on GTA of springer owners cutting several inches off their rifle barrels; then reporting "no loss in velocity"...
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A quick update, lest anyone think the project was abandoned. The patient... ahem, I mean rifle, has arrived! It came in a beat-up box and was bleeding badly, oil was oozing out of her everywhere. I have cleaned her up (externally) and put her through the chrony. Its a Hatsan 125 .22 Vortex Sniper and after a 100 shot break-in she is shooting 885 fps with a standard deviation of 8 using 14.3g Crosman Domed Premiers. The rifle is making a honking noise when its cocked and a ping (like a tiny vibrating spring) when fired, so she needs to be opened up anyway.
Since she's a gas piston/ram gun I will be removing her heart and replacing it with a spring. For that I need to order a new longer backstop, a spring guide and of course the spring. I will reengage this thread once I have the parts and ready to operate.
-MartyMcFly
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That sounds like a good candidate, Marty. More expensive than I thought you were going for...
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I've got an old Benjamin Trail coming from family, but it won't arrive till past Thanksgiving, so when I saw the Hatsan new at a refurb price and free shipping I just had to have it to experiment with ;D Besides, I wanted to see the build quality of these rifles. So far I don't think I got ripped off.
That sounds like a good candidate, Marty. More expensive than I thought you were going for...
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I always dream of owning Theoben Eliminator or Beemen Crow Magnum at the time I was 14yr old I couldn't afford $1300. Hopefully Rapid Air Worx (RAW) well bring it back to the market. I'm willing to pay 2-2.5K
New Improvement Design
*Reduce weight maybe like TI tube cylinder or other ways
*increase piston stroke from 100mm to 125mm Get over 30FPE
*Improve HE Gas System if possible or Coat the inside with DLC help reduce fiction.
*Hard Wood Ambidextrous Stock
*.22 cal .25 cal only - Walther or 4130 CM