Balanced Spool Valve

[FighterAce]

Looking very nice!   Any chance to turn this into a semi auto?

[Forum Admin]

I think this design lends itself well to semi auto. With a good recoil spring and slightly smaller port, the piston could act as a bolt and feed round ammo quite well. In a earlier experiment with this type of valve, I discovered that the valve can reset even with flow still going through it. Once the piston is reset, the chamber pressure rises quickly, readying itself for another shot. I'll probably build one in the future with a .375" barrel and use ball bearings as ammo.
The one I'm building now is going to be used with discarding sabot darts and hopefully go up against some larger game. 

[Forum Admin]

A month later, and I've got a working valve.  I ended up scrapping the parachute seal, it seals fine without it. Here's a video that shows it: http://www.youtube.com/watch?v=FsRl05rtWt0. I still have a lot of work to get it where I want it to be, but I'm encouraged by its early success. A trigger mechanism is a must before I can fire ammo with it, as the push against the piston will be sufficient to push the allthread through my hand. 

[Forum Admin]

I rigged the piston where I could trigger it with a screwdriver so I could shoot some ammo without having to worry about the piston shooting my way. I used a nail dart made from a 3.5" nail and 12 gauge wad (which has tons of blowby). Much to my surprise, the piston did not fly out. Turns out, air gets trapped between the two seals, acting as a brake. I'll have to see if it does the same thing with better sealing ammo and higher pressure. At 140 psi, it will bury that nail to the head in 3/4" plywood, in spite of all the blowby. Looks a bit more powerful than most other things I've made. 

[BoyntonStu]

I rigged the piston where I could trigger it with a screwdriver so I could shoot some ammo without having to worry about the piston shooting my way. I used a nail dart made from a 3.5" nail and 12 gauge wad (which has tons of blowby). Much to my surprise, the piston did not fly out. Turns out, air gets trapped between the two seals, acting as a brake. I'll have to see if it does the same thing with better sealing ammo and higher pressure. At 140 psi, it will bury that nail to the head in 3/4" plywood, in spite of all the blowby. Looks a bit more powerful than most other things I've made. 

Interesting find.

Be careful and do not rush it.

Would a lighter weight plastic or wood dowel be possible as a substitute for the threaded steel rod?

Should the spacing between the front and the rear washer elements be just a little bit wider than the hole?

[Forum Admin]

A lighter material would work better from an efficiency standpoint. In reality, you would have to use a bolt and nut to secure the seals to the lighter piston, so you'd probably end up with more weight. I'm going to cut the threaded rod way down once I get the trigger system worked out. That will lighten things up a lot.

Seal spacing is something else I'm working on as well as the number and type of washers I'm using. There's quite a bit of friction with my design, I'm hoping to substitute a steel washer for two rubber ones on either side to reduce the friction, but hopefully retaining the seal. If it does, my pneumatic brake might not work, but that's how things go. Since I'm reseating by hand, having the seals spaced farther apart makes it easier to be sure the piston is in a sealing position. It too will be tightened up in the final design as it will be in perfect position when it engages the trigger mechanism. One thing I'm going to try before tightening it is seeing if giving the piston some momentum before it opens helps projectile speed out. I'm anticipating a good amount of friction at higher pressures. 

[BoyntonStu]

My oak piston with O rings is very light.

No metal screws, washers, or nuts.

I can visualize a spool shaped piston with an extended rod.

You are on a good path.

I eagerly await your chrony  results.

Great breakthrough copper design!

[Forum Admin]

I got a new piston up and running and no leaks provided the pressure is 75 psi or higher. That's fine with me, I plan to go much higher than that. It is much easier to open the valve now. I chronographed a 230 grain nail dart at 264 fps - that's 36 fpe. I found the dart embedded in a tree 75 yards away, sunk in about .5" to .75". The seal blew off, so the shot was likely lower powered than it could have been. I'm still quite happy, 36 fpe on a 100 psi fill in a 12" chamber with a 34" barrel aint too bad. Here's some pictures showing this afternoon's progress:

[BoyntonStu]

Looks good.

I have been thinking:

What if the barrel air hole was at the top of the T so that gravity would keep the dart sliding along the bottom of the barrel.

It  could be a breech loader when the piston comes way to a rear.

The piston becomes the 'bolt' and it shoves the dart in and seals.

Understand?

[Forum Admin]

You are thinking along the same lines as me. The piston makes a perfect bolt action for breech loading. The hole at the bottom wont matter, my sabot dart was designed with this valve in mind. The sabot is too big to drop down the barrel and the dart is kept perfectly centered by the sabot and fins. I just hope work stays at 40 hours, I havent had much time for airguns lately. 

[Forum Admin]

Here's a video of my airgun, getting the trigger system worked out. http://www.youtube.com/watch?v=UWKWrAfN4tU

[Forum Admin]

Here's a video of a close range indoor shot: http://www.youtube.com/watch?v=4i7v4Q9dSl4

[FighterAce]

Pretty nice! But the piston appears to be stiff and hard to move... is it? Have you considered Ultra-high-molecular-weight polyethylene (UHMWPE) with floating O rings instead of the all thread/nuts/washers/rubber gasket setup? it would reduce the friction a lot and lower the weight.

I've been working on my firing mechanism for a few weeks now and it seems you need, quite literally, the reverse of what I need. It involves a charging handle separate of the hammer/spring and the sear.
This is how it works...

pull charging handle back,
charging handle pulls hammer/pin
sear catches the hammer/pin,
let go,
spring pulls charging handle back.
pull trigger,
hammer moves forward,
hits the schrader - pilots the piston valve

If you literally turn it 180, it would pull the piston back for you hehe    I'm not sure what you meant by linking the piston with the breech loading mechanism. Can you clarify?

[Forum Admin]

I originally was going to go with floating o-rings, but I'm trying to make this something that can be built without a lathe. It is a bit stiff to move when under pressure, but once the front seal passes the port, the air does the rest of the work for you. I've actually fired a couple guns with stiffer triggers.  If a spring is used to open the valve, it will be even more efficient than it is now. I'm currently working on using a repair coupling to cover the rear of the valve as well as trigger the piston by spring loading it. I plan to cut the end of it and bend it upward to engage the piston. By increasing the length and width of the slot I will have a loading port. When the repair coupling is pulled to the rear, it brings the piston with it, allowing me to load a dart in the slot. When the repair coupling is pushed forward, it will push the piston to firing position (that is where I'm a bit stumped) and compress a spring that will be held by a trigger. When the trigger is released, the coupling flies backward, bringing the piston with it and firing the shot. The details are a bit sketchy in my head, but are slowly taking shape as I play with the idea. 

[BoyntonStu]

I originally was going to go with floating o-rings, but I'm trying to make this something that can be built without a lathe. It is a bit stiff to move when under pressure, but once the front seal passes the port, the air does the rest of the work for you. I've actually fired a couple guns with stiffer triggers.  If a spring is used to open the valve, it will be even more efficient than it is now. I'm currently working on using a repair coupling to cover the rear of the valve as well as trigger the piston by spring loading it. I plan to cut the end of it and bend it upward to engage the piston. By increasing the length and width of the slot I will have a loading port. When the repair coupling is pulled to the rear, it brings the piston with it, allowing me to load a dart in the slot. When the repair coupling is pushed forward, it will push the piston to firing position (that is where I'm a bit stumped) and compress a spring that will be held by a trigger. When the trigger is released, the coupling flies backward, bringing the piston with it and firing the shot. The details are a bit sketchy in my head, but are slowly taking shape as I play with the idea. 

The repair coupling idea is probably better than moving the piston/bolt further back to another loading slot for breech loading.

The more that I think about the present setup, the more I like it.

I was concerned about having the chamber to the barrel hole at the bottom because of the possibility of a dart falling into the hole.

However a shotshell wad tail would probably keep the nail off the bottom.

Perhaps a few side slots or a top slot would be better because they would allow a ball projectile to pass the opening.

Great work!