Springer pressure at end of stroke

[subscriber]

Eliminator user manual found, with info about dual weight piston:  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.

[Theoben HE]

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.

[MartyMcFly]

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.

[Theoben HE]

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

[subscriber]

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

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

[subscriber]

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.

[Rob M]

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

[subscriber]

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

[rsterne]

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.

Somebody asked me what it might take, so I scaled up the important numbers (swept volume and spring force) to provide some guidance....

Bob

[subscriber]

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.

[Theoben HE]

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.

[MartyMcFly]

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

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.

[subscriber]

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...

[MartyMcFly]

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...

[MartyMcFly]

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.

[Novagun]

weight to prevent

[subscriber]

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...

[Theoben HE]

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

[subscriber]

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: 

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

[Theoben HE]

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.