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It's a little complicated. I was working on the problem of efficiency in spring powered guns and the fact that, as is often the case, I should expect around 10% efficiency.
This means that if I want to get the at least 4 joules I want from my rifle, I need 40J of spring energy. If we're talking linear springs, thats a freakin big spring boys.
So then I found out about gas springs. They're really very similar- and I was thinking I would use them instead just because they seem easier to use.
Then I stumbled on this.
http://www.megafileupload.com/en/file/1 ... 7-pdf.html
Its a US patent for a special kind of gas spring rifle. (I know, its patented, shouldnt really be copying it, but I'm not going to make money off it or anything)
It claims to have a fairly uniform force, which would mean a much better amount of energy for a given stroke length and amount of effort required to cock it. Problem is, I don't quite understand it. Any of you brainiacs up to figuring it out?
Can't see your file but is it this one?
If so it's the same system used in many Theoben rifles - you're still left with the problem of coil spring rifles, you need fairly massive construction to get decent power, it's not worth the effort compared to building a pneumatic.
It is Theoben's patent.
I wanna make one. Hehe. I will, but it would help if someone could divine one or two of the finer points of the thing. I get that it's basically air that's already compressed giving a more constant force for obvious reasons, but I don't quite get why they need the dummy piston and such.
The design has very poor efficiency between the air chamber and the barrel. The step plate transition is a known high loss area. A transition area shaped like the toulie valve would be much better. A toulie shaped cylinder with a mating bullet shape piston could provide excellent energy transfer.
The hard part is figuring the piston mass, speed and energy transfer through the air spring between the piston and projectile. Ideally you want the piston to move at very high speed and compress the air into the barrel. While the projectile is accelerating in the barrel, the piston is losing energy and comes to a stop right at the end without hitting it with much energy. Good luck.
Is info on the toulie anywhere on Spud?
So you're saying just a straight cylinder won't cut it? because the large volume of air is trying to get into that small gap so it's losing energy?
But the actual air spring is ok right? like the way the piston actually travels forward
Edit: ok I get it now, having an orifice that suddenly becomes smaller is crapola. The end of my piston is a half inch pvc endcap, and it's rounded, would that improve it? Along with moulding something over it and putting it in the end of the chamber, so it's a good fit, and the end of the chamber is then a little rounded. Im not sure just how gradual the decrease in diameter should be to get better efficiency.
PS. I'm aiming at 15-20% efficiency, actual energy to cock it vs. output energy of bb. Possible here?
1 more thing the nature of this design is that we are using gas already compressed so even when its uncocked its pushing against the end of the chamber with, lets say, 20kg force. My change in volume of the piston after being cocked is in the ratio of about 1.15:1, before:after. So the force exerted at the max compression is only a few kg more. If we integrate the foce vs. distance graph over 0.12 meters or 12cm, we get in the region of 25J of energy transferred throughout the stroke. I realise the end of the cylinder will have to be tough. I calculated that with my piston size the pre-charged air in the piston will only have to be around 9 Bar.
My piston mass is going to be around 40g once its all fabricated. I assumed it'd be alright because thats about how much your average airsoft piston weighs?
This just sparked a thought. I have no Idea if it would work, but I figure it's worth mentioning. Anyway, have the gas spring, say pressurized at 400 lbs, but instead of the spring compressing air, the spring would push forward a bar that would hit a bb at a very high velocity. I have absolutley no idea if it would even be powerfull at all. Also, cocking teh gun would be VERY hard, you'd need a lever to get the mechanical advantage. But anyway, thats my probobly bad idea.
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Current Project: None, I'm in Spudremission.
Its an interesting point, people always told me it's a terrible idea, and I think it probably is, maybe because of the fact that the bb isn't going to take on much energy from the collision between the bar and bb just because of the nature of collisions between small and big things.
Anyway, The system I've got now looks like Theoben's system, except a rounded cylinder head, so that air flows nicer. The stroke pull is 25cm, and starts at 117 Newtons force and ends up around 134 N at the end. So its a much more constant force which is nice. The total energy of that is about 31J, and I'm hoping for 10% efficiency, which gives me 3.1J. I think whith my design 10% efficiency is pretty attainable.
Because the cylinder has about 15 times the volume of the barrel if it is 25cm long, I'm going to port the cylinder up to about 7cm to go, which gives about 3:1 cylinder to barrel air ratio.
Im not sure what would be the best cylinder:barrel ratio for it though, or where to end the porting, because I dont know much about the relationship between how fast the piston is moving and how much air it puts out, and how long it takes for the bb to accelerate.
I was thinking, because the porting is going to have to be done based on trial and error kind of approach, maybe its possible to have an adjustable port? that way I wouldn't have to build a new cylinder to try a different port size each time.
Oh and, do ports work well with o-rings? I know airsoft pistons use o-rings and some of their cylinders have ports, so they must get it to work somehow. I was thinking maybe the o ring would get damaged by the edge of the port or something.
This is true, the relationship of energy transfer from a high mass item to a low mass item is well known. A baseball bat retains most of the energy after hitting the home run and continues the swing. A Newton's cradle transfers nearly 100% from one mass to the nest with little energy loss. Good point.
Properly done high efficiencies can be achieved.
A drawing would help here. The cylinder is both 15 times the volume of the barrel and 3 times air ratio. This is not too clear.
The best cylinder barrel ratio is fully dependent on the projectile mass, piston mass and energy transfer. Ideally the piston will take potential energy from the spring and turn it to kinetic energy while converting piston kinetic energy into potential energy in an air spring while transferring potential energy to kinetic energy in the projectile. Ideally the piston will transfer all it's kinetic energy to compression and come to a stop at the same time the air finishes transferring it's potential energy to the projectile which contains all the system energy leaving at high speed while everything else comes to a silent stop.
To get it to work efficiently is the balancing act of force volume mass volume and mass.
I'm thinking it can be done with a spring loaded piston launching into the chamber with just a barrel on the other end. Why would a port be needed?
Any use of o rings should have low friction and low leakage to conserve energy for the transfer. Without ports, it should not have an area for damage.
The reason I wanted to use porting is because, I wanted the piston to start compressing when it's already up to a pretty high speed, I thought this would give better compression and things.
What I meant about the cylinder volume and air volume:
Without porting, the cylinder would be too big- Im going for a long stroke to get as much kinetic energy behind the piston as possible
So I'm thinking I would port it so that the piston starts compressing air in the cylinder after a fair bit of the kinetic energy has been given to it.
What you said about the piston coming to a halt and such, thats a pretty perfect scenario, I would like to be able to do that, but I dont have the necessary mathematical equations to figure out the correct piston weight, spring force and stroke pull.
I always assumed it was inevitable that the piston was going to slam into the end of the cylinder because its sort of impossible for the piston to give ALL its energy away, into the bb and the vibrations and such. but I see that with your information it is possible. But like I said, I dont know where to start mathematically for that. I do have the following known facts though:
I am using a system similar to Theoben's patent- main piston and dummy piston gas spring
The cylinder's diameter is 3.02 cm.
The main piston will fit into this, and is hollow. Its internal diameter is 1.73cm, +/- 0.2mm.
The piston head I think will be an epoxy mould in a bullet-like shape as technician spoke of for improved efficiency. The internal end of the cylinder that the piston head will reach will be epoxy, moulded around the piston head for a snug fit.
I think the seal between the piston head and cylinder will be epoxy, lapped with the cylinder's inside and greased. If that fails then I'll have to use an o-ring.
The dummy piston is 0.96cm in diameter, its wall thickness is 0.12cm, and it is this wall thickness that causes a change in pressure upon the stroke. It is stainless steel with a 20-30 micron diamond paste used to lap its outside and make it quite smooth with little friction.
Seal between dummy and main piston is two o-rings, the o-rings are attached to the main piston, the steel pipe slides through their internal diameter. They are 11.8 cm internal diameter, and I think 2.5mm corss section, giving about 12.5% squeeze, which is right for dynamic o rings, I think.
The main piston is pn18 PVC, and so has an upper limit of 1800 kpa in pressure, or close to 260 psi. The weight of this piston is going to be hard to keep below 50 grams, (perhaps I can replace it with thin-walled aluminium?)
With the current setup, the max force the gas spring can exert is about 130 newtons, as the effective area the gas is working on is only the dummy piston's internal diameter. Would flaring the end of the steel dummy piston give it a bigger effective area here?
Barrel length: 50cm
Barrel bore: 6.04 mm
Stroke length: Unsure of now
Pressure to be used: Unsure of, but I assume as much as it can handle is best
As I dont know how much efficiency to expect from the setup, I dont know what force will be needed, over what distance, and whether my piston is sufficiently light (50 grams)
I'd rather build an electromagnetic "disc launcher" the type that typically shoots hard drive rings and quite impressively through watermelons and such...'nuff (too much) said...
I'm really interested in a combustion or hybrid that isolates the chamber by use of a piston in a cylinder, then converts it's energy pneumatically into a barrel also,...future projects...I don't mind giving away ideas because a poofy pattent is not my objective, the world however is and I figure if I'm thinking something, probably someone else is too...
It's always a contest for me.
edit: no way I'm gonna download file...nope.
Im doing it this way because, once I build it, all I'll need is an arm and a mag and I can shoot it whenever, wherever with the same energy.
With a larger area the energy transfer from the gas to the mass can happen faster (shorter distance) to build KE in the piston. The drawing shows little energy transfer to KE as the gas retains most of the potential energy at the end of piston travel.
you mean the gas itself in the hollow piston keeps the energy? If thats the case then I'm at a loss as to how this is gonna work. How the hell do Theoben do it?
But hang on, how can the gas in the piston NOT be giving the piston the expected amount of kinetic energy, if the gas is exerting a known force on the piston mass? Or are you basically just saying that the dummy piston is too small and therefore the force is much less than if the force was over a large area. If this is what you mean, I looked in the Theoben patent and it said the dummy piston's internal diameter should be as close to 50% of the main pistons's ID as possible, as the force loss is weighed up with the friction that is added when you need larger o-rings to seal a bigger dummy piston.
The problem for me is figuring out my forces and masses. And it's all to do with the fact that I COULD figure out the average force required on the bb, but then I wouldn't know how the piston is giving it that force. Like I know how the piston gives kinetic energy to the bb, but I have no clue about how it gives a force to it, because
A) as the piston moves, it is compressing the air, and as the pressure is changing over time, and also the bb is moving over time, which in turn is changing the pressure itself, its all too complicated for me.
I'm looking at the volume change and resulting pressure change from piston movement. Assume a low pressure say 15 psi. That is double 1 atm.
Now use the piston and volume compressed and expanded. With a 2:1 diameter ratio, that is a 4:1 area ratio. The difference inside the chamber (not counting additional volume inside the pistion) the maximum expansion is from 4/4 compressed to subtract 1/4 the gas pressure with the piston out is 3/4 the pressure remains. Our 2 atm pressure above would expand dropping pressure to 1.5 atm or 7.5 psi gauge. At high pressure the remaining pressure is simply potential energy not used to convert into KE. In simpler terms, after full piston travel, the gas is still under high pressure as little of the pressure was used due to small volumetric expansion.
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