Piston Speed Coil and Gas Spring Airguns

[Ronno6]

At what speed does a coil spring or gas spring drive a piston in an air rifle??

Inquiring minds want to know.........

[jackssmirkingrevenge]

In the Cardew book they recorded a time of seven milliseconds for a piston stroke of about 2.5 inches, so an average velocity of around 30 feet per second.

[Ronno6]

Would that not be an average speed?
Starting from zero and accelerating to top speed..........dunno how long that would take, but at 7 milliseconds for total travel, I 'd say not too long...
Does the ram ever reach top speed, or is the acceleration continuous til the piston hits home??

[Motorhead]

As I understood how Gas Rams work ... the pressure exerted is based off the differential of a fully exposed piston on one end and the opposing side having less exposed area due to the rod taking away area. As such the bias to have ram motion one direction over the other the force is linear having the Air Rifle compression piston still decelerating on an air cushion same as a spring piston does.

Advantage to a ram is this force is linear being the same threw out the stroke, where a spring packs up being a tad stiffer at full compression and a tad softer near full extension.

Please correct me if wrong 

[Ronno6]

That's all gonna require some serious ponderence........

I was surprised the first time I opened up a gas spring that there is NO piston beyond the area of the back of the rod.....
And the total force of both types declines with extension.
But.....
The piston starts at 0 fps and accelerates to max velocity..somewhere along its travel.
Then, as the pressure ahead of the piston seal builds up, resistance increases.
The force/inertia of the spring/piston overcomes this.........and the resistance begins to abate as the pellet begins its travel down the bore ??

GEEEEEEEEEEEEEEEEEEZZZZZZZZZZ these are complicates little dudes...........

[Duckfish]

Really it’s a miracle they even work at all.
    Little bit of science, little bit of magic. 

I have forgotten who said that

[wchilton]

Resistance due to compressed air in front of the piston is relatively low until near the end of the stroke.  Only 1 atm (14.7 psig) at the half-way point of piston travel and 3atm (44 psig) at 3/4 of piston travel.  I'd be surprised if max velocity is more than 2x the average velocity.  I've shot many spring guns and several gas-ram guns.  Gas ram is inherently smoother but proper tune on a spring powerplant gives very similar shot cycle.  Gas rams require about 10% more cocking effort (compared to spring) for equal power output in my experience.

[mikeyb]

At what speed does a coil spring or gas spring drive a piston in an air rifle??

Inquiring minds want to know.........

Spring piston air rifle "physics" is a lot more complicated than most people think.

I used a piston travel distance of ~4" or 100mm for simulation. I don't claim my simulation is accurate as there are MANY variables and I make several ASSUMPTIONS (simplifications) regarding friction and adiabatic heating. However the simulation produces numbers that are near realistic.

Piston will accelerate from spring force but drag and building air pressure will slow the acceleration near the end of the stroke.

At some point between 95%-100% of the stroke the piston may stop and rebound a few mm while the pellet is accelerating through the bore. The piston may or may not be at final rest when the pellet exits the bore.

My numbers suggest the piston is at or near rest by ~12ms after trigger pull and the pellet is exiting the muzzle around the same time.

I get about 13 meters/second peak piston velocity in some of my simulations.

Assuming piston velocity increases linearly for most of the distance we can use a simplified math quick-check:
x = 1/2 * Vp * t
x = 1/2 * 13(m/s) * 0.012(s)
x = 0.078 (m) = 78 (mm)

Maximum piston stroke is ~100 (mm) so this check is consistent.

These are "ballpark numbers" derived from simulations and measured data by GTA members here...

https://www.gatewaytoairguns.org/GTA/index.php?topic=176445.msg155995664#msg155995664


and D.Tavellas paper here...

https://www.researchgate.net/publication/274638905_Internal_Ballistics_of_Spring_Piston_Airguns

I wanted to set up measurement experiments myself but never had enough time and/or energy to do so. Digging deeper into springer physics has dropped off my to-do-list :-(

What little free time I can get to spend on air guns right now is for repairing, tuning, and shooting them.

[Ronno6]

Excellent!
Thanks, mikeyb.........

[mikeyb]

As I understood how Gas Rams work ... the pressure exerted is based off the differential of a fully exposed piston on one end and the opposing side having less exposed area due to the rod taking away area. As such the bias to have ram motion one direction over the other the force is linear having the Air Rifle compression piston still decelerating on an air cushion same as a spring piston does.

Advantage to a ram is this force is linear being the same threw out the stroke, where a spring packs up being a tad stiffer at full compression and a tad softer near full extension.

Please correct me if wrong 

That's all gonna require some serious ponderence........

I was surprised the first time I opened up a gas spring that there is NO piston beyond the area of the back of the rod.....
And the total force of both types declines with extension.
But.....
The piston starts at 0 fps and accelerates to max velocity..somewhere along its travel.
Then, as the pressure ahead of the piston seal builds up, resistance increases.
The force/inertia of the spring/piston overcomes this.........and the resistance begins to abate as the pellet begins its travel down the bore ??

GEEEEEEEEEEEEEEEEEEZZZZZZZZZZ these are complicates little dudes...........

Air and gas springs as used in airguns are very simple and don't have an actual "piston". They are basically the internal pressure acting on a piston ROD (the cross sectional area of the rod) with no piston installed. There is something at the end of the piston rod ( a bump, washer, screw, etc. ) that STOPS the rod from flying out of the body but it has no other function related to spring force.

Just to clarify, both air/gas spring and coil spring forces vary over distance.

A coil spring force varies linearly with distance and can be described by the spring constant k with units of pound-force/inch.

An air/gas spring force over distance is actually a curve (not a straight line) BUT it is a VERY SHALLOW curve where we use them in air guns than can be considered a straight line just like the spring constant k for coil springs. The straight line "approximation" is useful and practical in MANY cases because it has error of 1% or less.

A big difference between the two springs is that the force for the air/gas spring varies MUCH LESS over distance than a similar coil spring. In some cases (with larger tube volumes) the change in force for an air/gas spring can be VERY SMALL which is why they are sometimes referred to as constant force springs. In those cases k ~ 0 and only the constant preload force remains. Again, not really constant but so close we treat is as constant for all practical applications.

Here is a comparison of a coil spring and an air/gas spring that could be commonly used in a B19 style spring piston air rifle:



As a semi-retired engineer I see little to no "practical benefit" of using air/gas springs in spring piston air rifles. I personally have had "0" nitrogen gas spring failures and also "0" coil spring failures since restarting my airgun adventure in 2017. I'm now up to 8 air spring failures (Hatsan Vortex models specifically) so those leakers have left me UNIMPRESSED with their design and construction.

Has anyone here ever done a FMEA (Failure Mode and Effect Analysis) or a MTBF (Mean Time Between Failure) analysis/calculation?
I did on every security sensor product I helped design.

Steel Coil Spring
1 steel coil spring

= 1 (one!) part


Vortex Air Spring
5 o-rings
1 polished shaft
1 shaft retaining washer
1 shaft retaining screw
1 pressure tube
2 threaded endcaps
1 bleed-port screw
1 bleed-port sealing washer

= 13 (thirteen!) 11 (eleven!) parts


Does anyone see a problem here?

I see one.

>>>  KISS  <<<

:-)

[Poor Ballistics]

I am really enjoying all of the input here.  I have made a few attempts to get my head around the physics / engineering of  spring piston air guns off and on for decades. 

Maybe someone here can definitively clear up my confusion about  gas springs.

Someone talked about  Air filled springs vs  Nitrogen filled springs.  What?

I was reading an article by Tom Gaylord recently about the history of Theoben.   The spring that was described was a tightly sealed piston at one end with high pressure air (or nitrogen?) filling up the cylinder behind it, and that air or gas getting compressed when cocking the gun.  On one side high pressure and on the other no pressure, like a bike pump with the outlet plugged.  What about heat generated and lost due to compression?

Searching online, I could find no information about gas  springs  engineered or made as Tom Gaylord described. 
 
So was that actually the way the Theoben springs work?   

But maybe, if that is indeed the way the Theoben springs worked,  if the pressure is so high to begin with, and the volume is not reduced to much (say a 8 inch cylinder and the piston only goes back 4 inches, not much energy lost to heat transfer to the metal walls of the cylinder?  Theoretically,  Nitrogen will heat up less than air, but not that much less:

From a google search:
"When compressed under the same conditions, nitrogen will generally experience a slightly lower temperature rise compared to air because air contains moisture which can contribute to additional heat[ generation during compression, while pure nitrogen is dry and does not have this factor; therefore, the temperature increase will be more consistent and predictable with nitrogen"

And then we have the nitrogen filled gas strut or spring, witch has been mentioned, and it works like was said, same high pressure in the cylinder, one side of the piston has allot of the diameter filled with the shaft, the other nothing, and and the difference between the two surface areas times the pressure = the force.  The only loss of energy is in the friction in the system, and both types would probably have about the same friction losses.

By the way, gas springs are used in industry when a coil spring does not fill the bill, even  though for the same force and travel, coil springs cost less. 

And, I am pretty sure the gas springs used in air guns  use the type of  gas springs  engineered for  use in large die presses.  They are designed to expand as fast a possible.

 

[Ronno6]

I can furnish a limited amount of insight into the workings of the air spring.

First, air as opposed ti nitrogen....it is my understanding that N2 is a larger molecule than an air molecule.
Hence N2 is less likely to leak out. This is why aircraft and new car mfrs. use it.

As for the dynamics of the air spring...
I was surprised when I dissected my first that there is NO piston.
The air inside the cylinder is trying to escape thru the gland end, but the rod is in the way...so the air pressure pushes it to extension.
Only the end of the rod, and an small ring to retain the rod at the end of its stroke.
The initial pressure inside the cylinder is high....1800 to 2400 psi or so.
The pressure increases some due to the decrease in internal volume due to that which
is taken up by the intrusion of the rod. I did the math once but am too lazy to do so now.
But, ie the good ols P1 X V1 = P2 X V2 gas law.
As the ram is maybe 5/16" dia and the cylinder is maybe 5/8" the reduction in volume resulting in higher internal pressure is appreciable, but really not as drastic as you would think. In use, the stroke of the rod is very near it's maximum travel.
So, as the ram extends once released, the pressure begins to decrease, but not by a huge amount.
This contributes to the near linear performance of the system.

I tire of typing, and I have already said more than I know, so.............take with a grain of salt...

[jackssmirkingrevenge]

So was that actually the way the Theoben springs work?

Here is the Theoben patent for their gas spring.

Of peripheral relevance is their patent for an inertia system to reduce piston bounce

[wchilton]

Air is 79% NITROGEN and 20% OXYGEN with the balance being CO2, Water, and other gasses.  A nitrogen-filled piston will act only slightly different from an air-filled piston.  I own both spring-powered and gas-spring-powered airguns.  Gas springs provide 5-10% less power for the amount of cocking effort compared to coiled springs, probably a lot of that loss is from friction due to the seals.  A coil spring with fitted guides can provide a shot cycle every bit as "linear" and vibration-free as a gas spring.  I've tuned a number of my own spring guns (BSA, Weihrauch, FWB, Webley, and others) and all of them can be made to shoot with solid "thunk" shot cycle that you'll experience with a gas ram gun.  The gas-spring guns I've shot have all been Theobens, which have great build quality.  Comparable build quality would be Air Arms and Brit-made Webleys like the Longbow and Tomahawk.  I've never shot one of the lower-cost gas-spring guns, mainly because by the time those became generally available I was finished acquiring gas-spring guns and had decided I actually prefer coil spring powerplants.  Just a personal preference, others may disagree.

[mikeyb]

Air is 79% NITROGEN and 20% OXYGEN with the balance being CO2, Water, and other gasses.  A nitrogen-filled piston will act only slightly different from an air-filled piston.  I own both spring-powered and gas-spring-powered airguns.  Gas springs provide 5-10% less power for the amount of cocking effort compared to coiled springs, probably a lot of that loss is from friction due to the seals.  A coil spring with fitted guides can provide a shot cycle every bit as "linear" and vibration-free as a gas spring.  I've tuned a number of my own spring guns (BSA, Weihrauch, FWB, Webley, and others) and all of them can be made to shoot with solid "thunk" shot cycle that you'll experience with a gas ram gun.  The gas-spring guns I've shot have all been Theobens, which have great build quality.  Comparable build quality would be Air Arms and Brit-made Webleys like the Longbow and Tomahawk.  I've never shot one of the lower-cost gas-spring guns, mainly because by the time those became generally available I was finished acquiring gas-spring guns and had decided I actually prefer coil spring powerplants.  Just a personal preference, others may disagree.

THIS! ^ (except I've never had the privilege of shooting a Theoban)

I'd add that:
1) Industrial Gas Springs designed to replace coil Die Springs are built to an entirely different MUCH HIGHER level of quality (>10^6 operations) than common airgun Gas Springs.
2) Airgun gas springs are designed to be functional (~10^4 operations) with LOW COST as the primary design metric.
3) Airgun gas springs likely use polymer cup seals similar to their EXPENSIVE Die Spring siblings for the shaft seal which can handle the high shaft velocity and wear over time.
4) Airgun AIR springs (Vortex specifically) use common o-rings for the shaft seal which are NOT designed for long life under high speed conditions.

I have some surplus industrial gas springs and the quality of construction is far superior to NP, TNT, etc air rifle gas springs. Sadly, their construction is SO ROBUST they will not fit into any common air gun platform I own.
Air IS mostly Nitrogen and many industrial gas springs can be re-filled/adjusted with AIR when customers need to vary the spring force. The manufactures of these springs recommend Dry Nitrogen to keep out the water vapor and oxygen components of ambient air.
IMO the MAIN reason airgun Gas Springs seem to far outlast (Vortex) Air Springs is purely the shaft seal design.

Side Note: Gas LIFT SPRINGS last a long time because they use a cup seal on the shaft AND have a SPEED LIMITING DAMPER on the internal end of the shaft. That damper is why we can't use lift springs in airguns.

:-)

[Poor Ballistics]

Hey all,

Lots of really knowledgeable folks on this thread.   

I remember hearing about the Theoben inertia system  back in the day,  did that actually get produced,  and if so,  did it eliminate or reduce the piston bounce?

I could not  figure out from the patent pictures how the Theoben spring worked,  can anyone here explain the difference if any from the gas springs we use now?

[mikeyb]

Ronno6, sorry we are drifting a bit off topic here ;-)

Hey all,

Lots of really knowledgeable folks on this thread.   

I remember hearing about the Theoben inertia system  back in the day,  did that actually get produced,  and if so,  did it eliminate or reduce the piston bounce?

I could not  figure out from the patent pictures how the Theoben spring worked,  can anyone here explain the difference if any from the gas springs we use now?

Piston BOUNCE and Inertial Balancing are 2 separate issues.

Inertial Balancing adds moving masses in the OPPOSITE direction of main piston travel. The acceleration forces CANCEL so the rifle feels like is has NO recoils. Sometimes the cancel mass is another active piston which ALSO contributes to the pellet energy. Sometimes the cancel mass is just there to null out recoils and does NOTHING to add to pellet energy.

Theoben and others (GISS) have used this counterbalance in springers to make recoil-less rifles. They apparently work well but the added complexity leads to ADDED COST and most (all?) of the designs are simply no longer economically feasible. If I'm wrong please add link to any in current production... INTERNAL counterbalance designs, NOT sled designs like Norica and Diana ;-)

https://patents.google.com/patent/US2938513A/en
https://www.gatewaytoairguns.org/GTA/index.php?topic=153844.msg155693663#msg155693663

https://www.pyramydair.com/blog/2021/03/diana-75-beeman-400-recoilless-target-air-rifle-part-5/

********

Piston Bounce occurs in MOST springers as the piston rebounds a few mm on the high pressure air pocket behind the pellet. Piston bounce is highly dependent on MANY factors and it is likely most springers will shoot with some bounce using certain pellets and no bounce using OTHER pellets.

IMO piston bounce is OVERHYPED. An IDEAL system (doesn't exist) to stop piston bounce, a piston brake, is only likely to increase rifle performance maybe 2%. Systems that reduce piston bounce are mostly MARKETING TOOLS designed to increase sales.

The speed, timing, and instantaneous forces involved make "piston brakes" very impractical. IMO the floating polymer bushing just behind the piston seal seems like it MIGHT work... a little? As the piston decelerates HARD on the trapped air pocket the bushing gets SQUASHED. It expands to grab the side of the compression tube and THAT brakes any rearward motion or piston BOUNCE. Trouble is it ALSO reduces/brakes FORWARD motion!

Lots of information out there, also lots of marketing HYPE. Good luck wading through that swamp :-)

*********

To me the Theoben patent drawing looks the same as a modern air/gas spring design. EXCEPT, the air/gas spring is an INTEGRAL PART of the Theoben rifle. All modern air/gas springers I'm familiar with use a SEPARATE air/gas spring that can be easily swapped out for repair, replacement, or even conversion BACK to using a steel coil spring.

*********

My opinion is that springer rifles are WONDERFULLY SIMPLE machines. They ARE jumpy and can be VERY DIFFICULT to shoot accurately. Springers have made me a MUCH BETTER shooter in general AND I really enjoy the challenge. Those are the main reasons I shoot my springers about 10x MORE than my PCPs.

I personally have no use for a "recoil-less springer" (oxymoron? = jumbo shrimp), especially if the added complexity and cost becomes 2x-4x or more the cost of a nice entry level PCP rifle.

[wchilton]

I think there may have been a few Theoben guns with the inertia dampening system.  It only reduced piston bounce, not recoil.  None of the ones I tried had it.  There were other old match rifles (Anschutz 250, 280?) that had recoil dampening systems but they didn't ELIMINATE recoil and those systems were prone to leakage and failure (they were oil-filled).  These days with better seal materials they can be repaired and last longer. 

I don't believe  Theoben ever made a GISS system.  GISS systems have the opposing pistons with enough inertia to balance out recoil forces.  Diana made a few of those for match guns and Whiscombe guns also utilize a GISS system.

Gas spring airguns may have a "snappier" (ie. faster) shot cycle.  But that can be harder on scopes (spring guns already challenge scope integrity) and is probably why Theoben sold a lot of their rifles with integral dampa mounts.  Dampa mounts have a polymer/rubber bushing that absorbs some of the harshness of recoil with respect to the scope.

[mikeyb]

I think there may have been a few Theoben guns with the inertia dampening system.  It only reduced piston bounce, not recoil.  None of the ones I tried had it.  There were other old match rifles (Anschutz 250, 280?) that had recoil dampening systems but they didn't ELIMINATE recoil and those systems were prone to leakage and failure (they were oil-filled).  These days with better seal materials they can be repaired and last longer. 

I don't believe  Theoben ever made a GISS system.  GISS systems have the opposing pistons with enough inertia to balance out recoil forces.  Diana made a few of those for match guns and Whiscombe guns also utilize a GISS system.

Gas spring airguns may have a "snappier" (ie. faster) shot cycle.  But that can be harder on scopes (spring guns already challenge scope integrity) and is probably why Theoben sold a lot of their rifles with integral dampa mounts.  Dampa mounts have a polymer/rubber bushing that absorbs some of the harshness of recoil with respect to the scope.

I don't know if Theoben actually made an opposed piston GISS-like rifle for sale, but they certainly listed that as a variant in their patent. Pistons coming together Fig6 and pistons flying apart Fig7.

I can't feel any difference between a gas spring rifle and a coil spring rifle with a proper spring guide. Pellet muzzle energy can be made identical. Spring force profile IS DIFFERENT and gas springs "finish" with a higher force. That means second recoil will be larger in gas spring guns. Second recoil is usually blamed for most scope failures on high energy springers.

[wchilton]

When looking at gas-spring gun cost/value, you have to consider all the factors.  When I was comparing options (a decade or so ago...) a gas-spring powerplant was approximately a $100 premium compared to a coiled spring.  Then you had to also consider that many Theoben rifles also came with integral dampa mounts worth about another $100 premium.  The rest of the cost had to do with build quality, and by my estimation the Theoben airguns were comparable build quality to Air Arms TX200 or PE (have had both).  Considering all factors, the Theoben gas-spring guns were right in line price-wise with other guns of similar quality.  As far as gas-spring vs coil spring I actually prefer coil spring so the premium for a low-cost gas spring gun is something I'm not willing to pay for.  There may be some advantage of gas spring powerplants when shooting heavy pellets since that shouldn't put undue stress on the "spring" like with a coil spring.  Another "advantage" is if you are dealing with very high power spring guns since a gas spring can provide more power for the size of the "package" so might give a bit of flexibility in design even though you still have to have enough air volume to utilize the powerplant output.