Springer pressure at end of stroke

[MartyMcFly]

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.

[rsterne]

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

[lefteyeshot]

Two compression chambers. Bottom one under lever, top one side lever and both dump thru the same transfer port.

[MartyMcFly]

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

[MartyMcFly]

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...

[rsterne]

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

[subscriber]

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.

[subscriber]

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.

[subscriber]

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.

[subscriber]

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

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.

[subscriber]

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

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

[subscriber]

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

[subscriber]

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

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...

[MartyMcFly]

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

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

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...

[subscriber]

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:

[subscriber]

You can experiment with the volume of shot and the free travel distance until your airgun shott the way you want it to. 







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

[MartyMcFly]

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. 







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

[MartyMcFly]

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/



-Marty

[subscriber]

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



The advantage of cone springs is their short stacked height.  I would want a much shallower cone angle for airgun applications...

[Novagun]

 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.