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 Post subject: Airgun design.
PostPosted: Fri Oct 27, 2017 7:44 am 
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Location: Central West River Nova Scotia
For some time now I have postulated that a large piston in a springer of a large caliber, say 357 as an example, might be possible ,,,,,,,IF,,,,?
I know some people have tried using larger dia piston and springs, but have run into issues with cocking effort or spring binding before sufficient volume and piston travel could be reached. The idea I have been tossing around in my head, is this.
Using compressed air to drive the piston. Not like the nitro piston guns that have a pre pressurized piston, but one that uses a pcp reservoir and valve to charge the piston per shot.
So you may ask,, what would be the advantage ? Why not just use the pcp and skip the piston ? Well,, my thinking is this. A standard pcp would require a large volume per shot in 357 cal, also a heavy hammer spring, and high pressure. A pcp cools as the air expands so depends on volume and pressure to be effective. Whereas a spring or piston powered gun uses less air but due to the rapid compression it generates intense heat which increases pressure exponentially, and is the reason springers work.
So the reason I am asking for some feedback on this is because I will never be able to test the theory, and I need some reasons why it can’t work to get this out of my head. :cry:

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 9:04 am 
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I'm having a hard time visualizing. Are you saying using hpa to replace the spring portion of a spring powered ag. The piston would then act normally like any other spring piston ag?

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 10:13 am 
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jckstrthmghty wrote:
I'm having a hard time visualizing. Are you saying using hpa to replace the spring portion of a spring powered ag. The piston would then act normally like any other spring piston ag?

Not exactly. If the piston was already pressurized, then the same problem would apply. IE difficult cocking. As far as I know, a nitro piston has around 230 psi in them. Correct me if im wrong. What im thinking is an empty piston suddenly injected with 1000 or more psi. When it flies forward it compresses the air and heats it up same as a normal spring piston.
The air in the piston would have to be bled off after each shot and the piston reset, but thats only a minor design issue to overcome.
Also im not talking about modding an existing breakbarrel. Im talking about a large piston , 1 1/4” dia, or 1 1/2” . aAnd 6” of travel or 7” or 8”, whatever is necessary.

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 10:37 am 
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Your numbers would suggest a better than medium volume requirement, just off the top of my head.
My guess would be the straight PCP gun would be the efficiency winner over such a piston driven system....
Lower that operating pressure to 850 psi, and you may come a shade closer to something that would be efficient enough to be usable.

Just waiting for Bob S. to weigh in here. I could be wrong as heck on this~ but I'm sure workable volume is going to be the sticking point..

-D.S.

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 10:38 am 
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Let me make sure I understand, Joe....

You have a Gas Ram type Springer which is cocked by applying air to the front side of the piston using a regulated HPA source?.... ie all the HPA does is cock the gun, and it is then exhausted, returning the firing side of the piston to atmospheric pressure before loading the pellet and firing?....

OR....

You have a free sliding piston that is cocked mechanically but no spring behind it, so a small amount of compression of the air behind the piston occurs on cocking.... Once cocked, then you use regulated HPA to increase the pressure behind the piston to the desired value.... On firing the gun acts like a Gas Ram style Springer, except at the end of the firing stroke the HPA behind it is exhausted so the cycle can be repeated....

OR....

Something else?

You are correct that the efficiency of a Springer is higher than a PCP, based on the volume of air compressed to produce a given FPE.... However, in either scenario above, you would be using MUCH more air because the HPA you use to cock the piston (or pressurize the air behind it) is pressurized already.... For example, let's say the pressure required to cock the piston was 145 psi (10 bar).... You have used 10 times the swept volume of the piston to cock or pressurize it.... This now means your efficiency on firing is 1/10th, based on the amount of air you used.... You had to pressurized the HPA source somehow.... maybe with a compressor, maybe with a hand pump.... If you did it with a hand pump, you would have done much more work to pressurize the air than had you just cocked a spring.... IMO....

I would say that yes, it could be done.... but based on your work input it would be less efficient.... In addition, you still have the mass of a huge piston, with a long stroke, to deal with on firing.... The forward recoil when that piston stops might rip a light gun from your hands.... or at the very least make it impossible to hit anything unless the gun weighed a ton.... again, JMO....

Bob

PS.... Based on your last post, it appears you may be thinking of injecting the HPA behind the piston to fire it, compressing the air in front of it?.... First of all, the pressure in gas rams is MUCH greater than 230 psi.... N-forcer rams run 500 - 2560 psi depending on the force required.... http://www.n-forcer.com/content/pdf/Mini.pdf .... the "blue" rams are similar to airgun gas rams, and they run 1280 psi.... Secondly, to power the piston forward by putting HPA suddenly in behind it, would require the in-rush of a HUGE amount of air in a fraction of a second, requiring HUGE ports.... Using your example of a 1.5" piston with an 8" stroke, that is 14 CI of air.... Even at only 230 psi (giving a force of about 400 lbs) that is over 220 CI of air at 1 bar being used.... Most PCPs using that amount of air could easily deliver over 200 FPE.... This method is using expanding air to compress air to produce the shot.... Basically it is a PCP with a piston added between the power source and the pellet.... I don't see any gain in efficiency possible by adding a component....

Bob

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 10:53 am 
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Joe, here is a thread on the GTA about the design criteria for a .30 cal Springer.... https://www.gatewaytoairguns.org/GTA/in ... ic=94485.0

You will see that there is a relationship between the compression chamber volume and the FPE output.... and that most Springers use a stroke 3-4 times the bore of the chamber.... The idea is to start with your required FPE and then work backwards to figure out how big the compression chamber has to be.... Somewhere about 3 cc of swept volume per FPE will put you in the ballpark.... JSB .357 cal pellets are about 80 gr., so at 950 fps that is 160 FPE.... and at a more moderate 820 fps about 120 FPE.... You would need roughly a 360 cc compression chamber.... so something around a 2" bore and a 7" stroke, ballpark.... with the 2" piston driven by about 400 lbs. of force to look after the roughly 50% losses typical in a Springer.... The good news is that is only about 130 psi "shop air" pressure on that 2" piston.... but you would have to dump 360 cc (22 CI) of it in about a millisecond (from a huge reservoir to prevent pressure drop).... Using shop air to cock a Gas Ram with a 400 lb. force, on the other hand, should be do-able.... although not very efficient....

If you want an even wilder thread, check out this one, for a 1.5" bore Springer Howitzer, lobbing 1 lb. projectiles at 10,000+ FPE.... https://www.gatewaytoairguns.org/GTA/in ... pic=104558

HTHs....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


Last edited by rsterne on Fri Oct 27, 2017 12:15 pm, edited 1 time in total.

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 12:14 pm 
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Thank you for clearing that up. Now I can sleep,,,,finally.

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 2:08 pm 
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There is one other variable to consider. In a normal NP air rifle. What is the cross section of the piston inside like. How much surface area is the piston plunger inside the piston. The reason for this particular inquiry is,,,if the plunger is only 1/2 square inch in dia, ( and I suspect its much less ),then the pressure is reduced. Even at 1200 psi, if only 1/2 square inch is used then only half the psi is bieng applied. Where as if the plunger is 2 square inch,s then 1200 psi is applied then the pressure acting on the plunger is doubled.
Just to clarify the setup, the piston would be like a syringe, with a seal on both ends. on the needle end, the TP is connected. When the trigger is pulled it pushes the plunger down the guns tube the same as a spring would. The idea bieng to add (heat) to the equation, to increase pressure.. All that said, Im pleased with the response and am satisfied its not practical.

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 Post subject: Re: Airgun design.
PostPosted: Fri Oct 27, 2017 4:00 pm 
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Joe, in a typical gas ram (like the N-Forcer), there is pressure on both sides of the piston, so the EFFECTIVE area is only the area of the piston ROD.... For example, for the 19mm mini-rams, the rod is 0.315" diameter, and at 2560 psi that only develops 200 lbs. of force.... When the ram is depressed, only the additional volume of the rod is displaced inside the ram, which is why the pressure only increases by a small amount.... Gas rams are therefore nearly a constant force spring, compared to a metal coil spring which starts off with only a small force and gets much harder as the spring is compressed....

Regardless of the idea of heat being added on the compression side of the piston.... the opposite must occur on the other side of the piston as the air expands.... the air must cool, and hence lose energy.... Since there is no such thing a perpetual motion, there must be some losses occurring.... which would make the work output less than the work input.... The bottom line is that driving the piston with HPA will be less efficient than simply using the HPA to fire the pellet....

Using my idea of using HPA only to COCK the piston which in turn compresses a spring, or better yet a gas ram.... separates the energy input (air expanding to cock the gun) which takes place over a (relatively) long time.... from the energy output of firing the gun which releases the spring or gas ram in the usual manner.... This would not require huge ports for the HPA to flow through, like it would to fire the gun in a millisecond.... Just as in a conventional Springer you store your cocking effort to be suddenly released, so would you store the work done by the HPA.... although some the work done in expanding the air to cock the gun is still lost....

HTHs....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Airgun design.
PostPosted: Sun Oct 29, 2017 6:52 am 
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This sounds like an idea I had for running high velocity from low pressure shop air. I propose to run 100 psi through a large port quick exhaust valve acting on an air cylinder through large porting. That cylinder houses the large diameter piston which is connected to a small diameter piston (such as a piston and seal in a magnum springer) to compress and heat air and push out the pellet. It's a high speed pressure booster... or a model of a light gas gun. Efficiency is lost but when running on a shop air compressor that is not relevant.

It's really just for fun or demonstration purposes :)


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 Post subject: Re: Airgun design.
PostPosted: Sun Oct 29, 2017 2:44 pm 
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As I said, providing the porting is large enough to move the piston quickly enough, and with enough force (pressure), to compress the air in the secondary chamber.... it will work.... Remember, once the pressure in the secondary chamber reaches the pressure in the primary chamber (eg. 100 psi) the piston will no longer accelerate.... The pressure in the secondary chamber can never exceed that in the first chamber.... unless they are of different diameters, as you state.... then the maximum pressure would be the ratio of the areas, times the driving pressure....

If you were driving with 100 psi air, and wanted 1600 psi (including heating), you need 4 times the diameter (16X the area) for the driving chamber.... In our .357 cal. example, we need about 360 cc swept volume for the driven piston.... so that would be 5,760 cc = 351 CI for the driving cylinder.... With our 7" stroke, that would need a driving chamber of 8" in diameter.... You need to fill that with 100 psi air in about a millisecond.... Big ports, indeed, and a large enough tank to maintain that 100 psi after you remove 1.5 gallons of 100 psi air from it....

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject: Re: Airgun design.
PostPosted: Sun Oct 29, 2017 6:44 pm 
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The way I understood the question is to have what is basically a much larger gun that would be too hard to cock. The fix being a gas spring, a gas spring that is vented after firing so cocking is basically zero. Once cocked you would then charge the spring with say 200psi and you're ready. That seems pretty straight forward... If you can find a diagram for the Theoben gas spring like used in the HW90 you'll find it's already pretty much what you want, which is a larger diameter gas piston so less pressure is needed.

A generic B18 gun makes excellent power for its displacement, which is ~46cc and makes 18+ftlbs. Lets assume 50cc = 20. To get a generic 81gr 357 pellet to 900 you'd need ~145ftlbs, so a bore a bore/stroke of 42 x 260mm would be ~ what you need. The B18 receiver tube is ~3x the stroke but you could do it with less so I est 28". A barrel in .357 would need to be ~24" long? Unless you subscribe to 6-10" is all you need theory, if so then good luck with that...
Not sure on the transfer port dia/length but you could estimate based on a B18 port.
So my est for 145ftlbs in 357 would be a 65" long gun, which would no doubt weigh a lot as well. Plus having to drag a tank around with you.
How much air you'd need per shot would depend on the springs volume. I est 200psi but that's just a guess, too many factors to think about but basically it doesn't matter if tank fed. The question would be how many shots per tank...

I like the idea, I just think it's easier and better to simply use a pcp since imo the biggest bennie for a springer is it doesn't need a tank...
Now you have a tank and a massive gun. Imo the trick would be to make said gun but with multi-cocking to get it cocked, so either a gear or hydraulic setup so it takes say 6 cocks/pumps to do it. Then you'd have a self contained super magnum springer :)


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