Bit of help with hammer valve

[mako]

Okay, guys. I'm trying to build an automatic airsoft gun. I decided to use a hammer valve, paintball gun style. The hammer opens the valve, then the pressure fires the BB, closes the valve, and resets the hammer. Pretty much like this:



Now, I know this has been done a million times, but I'm hoping for some insight on the how large the valve opening should be. It needs to be small enough that the pressure on it doesn't prevent the hammer from opening. This is easy enough, except that it the problem cannot be solved by a powerful hammer spring. This is because the hammer still has to be pushed back after it's impetus has been expended.

JSR, help on this one? I think this may be right up your alley.

Thanks guys

[jackssmirkingrevenge]

What calibre are you planning?

[mako]

6mm actually. Airsoft. Though that can mean either 6mm or 8mm.

[cammyd32]

Most 6mm hammer valve launchers on the site, use blowgun valves as their main valve. With a valve stem of 4-5mm at the rear that's around 2/5ths of what a paintball valve stem is.
Using a clever design you can usually tailor the spring force to obtain the muzzle velocity you want, so increasing compression of the spring for more power and less blowback, and visa versa, it's a fine art balancing the two, and usually can be helped by changing input pressure.
What pressures are you thinking of running at?

[jackssmirkingrevenge]

You need the cross sectional area to be as close as possible to the barrel.

An airsoft barrel has a cross section of 0.283 square cm. A 6.2mm diameter port with a 1.5mm vavle stem would give you a free cross section of 0.284 square cm

Have a look at Brian's balanced hammer valve: http://www.spudfiles.com/pneumatic-cann ... 23276.html



You get very high flow without the need for a very strong hammer.

[mako]

Well, cammyd32, I was planning around 150 PSI or so for the experimental version, just to get off the ground. After that, I should have enough experience to walk on my own.

@JSR. That's and interesting design. Thanks. Why the correlation between port and barrel size? I can understand the problem with high-flow, but this is an airsoft gun. The flow really isn't that important, at least not for velocities of 400- FPS. The automatic function is the important part. The hammer has to open the valve, while still being weak enough to be pushed back by the released air.

P.S. Oh, wait. On reading through Brian's design notes, this does answer the question. The valve should have plenty of air flow and pressure, while needing only a very light hammer.

[jackssmirkingrevenge]

If your port size is too big, too much dead space. If your port size is too small, restricted flow which will seriously harm performance.

I took the opportunity to take some measurements of a typical airsoft gas valve:



The seat diameter is 5.5mm and the valve stem is 2mm in diameter, with 4 x 3mm diameter ports for the gas to flow through once the valve is opened. Valve travel is 2.5mm.

The maximum cross-sectional area is therefore 0.206 square cm, which is about 73% that of the barrel. If it were 100%, with all other factors being equal there would be a significant increase in performance. I will make a GGDT model to get some numbers.

For 150 psi I don't think there's any need to resort to a balanced hammer valve.

edit: Did some GGDT modelling, in one instance the valve seat was 6mm diameter, equivalent to the barrel, while in the second instance I reduced it to 5.1mm to give the equivalent cross section of the airsoft valve I measured. Note that the tiny (0.9mm) difference in diameter led to a 16% loss of projectile energy. Not enormous, but worth thinking about.

[mako]

Thanks, JSR. I'm not much of a numbers man, prefer experimentation. It's worth noting that the goal here isn't sheer power.
I'm trying to design this for my sisters airsoft weapon. She's kind of limited in mobility, so it needs to be able to supply massive amounts of downrange projectiles, hence the full automatic. So, it's more of a support weapon than a precision instrument.
What do you think my chances are of building something like this without access to a lathe?

[jackssmirkingrevenge]

Thanks, JSR. I'm not much of a numbers man, prefer experimentation.

I hear you, but not punching in numbers before grabbing your hacksaw means a lot of experimentation before you get it right

What do you think my chances are of building something like this without access to a lathe?

It depends on how well you can use the resources available to you. Wyz certainly did a good job before he got his lathe: http://www.spudfiles.com/how-to-databas ... 23202.html - that however was not an automatic.

If you want a support weapon that can be made with limited resources, I would go for a BBMG - a hammer valve is a bit of a waste of effort in this case.

[cammyd32]

If you're just looking for massive amounts of downrange projectiles, rather than attempting a fully automatic hammer valve system I would look into cloud or vortex systems. Attached to a source of regged hpa it would have the power and volume of fire you need and you could easily make it without lathe tools, while not exactly being gas efficient, it would be a cheap and easy build.

Edit:
Beaten to it by JSR...

[mako]

I've looked into them and built them. I appreciate the thought, but I'm looking into controlled blowback for a reason. Several of them in fact.

1. She can't haul a lot of weight, due to a leg problem. This limits the amount of air. If I use a vortex gun, she'd have to either be restricted to a very low amount of air, or a fixed position with a large tank. This won't work.
2. Airsoft. Anyone who has played knows that you CANNOT use vortex guns, due to the jackhammer effect. I once tested a breath powered vortex gun on a friend at 15 ft, and the hammer just about made him cry. Even low RPS from a regular AEG hurts like blazes.
3. With vortex guns, you have the problem of inconsistent fire. In an airsoft war this will get you taken out, very fast.

I'm just looking for idea's before I start.

[dart guy]

What kind of leg problem, like she cant bend her knee?. Also are you thinking of a semi auto sniper idea?

[cammyd32]

Also are you thinking of a semi auto sniper idea?

Okay, guys. I'm trying to build an automatic airsoft gun.

[mako]

@ dart guy. Yeah, sort of. She can bend it, but she has a bit of a mental block about it. Most of the problem, though, is it gets sore easily and very fast. That means extra weight compounds the problem.

@cammyd32. Thanks.

[mako]

So, I'm thinking something like this. The small exit port should allow enough time for the compressed air to push the hammer back to it's appropriate point. A sear would catch it, readying it for the next shot, or if the trigger was held, the hammer would simply fly back down to repeat the cycle.