Barrel Piston attempt with questions

[jackssmirkingrevenge]

You know, one of these costs next to nothing and will save you a lot of heartache and elbow grease: http://www.ebay.com/itm/Pneumatic-3-4-S ... 35c3c58507

[air_dannon]

That may well be the course of action, JSR...
I had a different idea for the piston system and did a drawing. Rubber diaphragm on the back side sealing in the pressure and outside the pressure zone a spring-loaded piston that pushes the diaphragm against seal? Good idea, bad idea?

[jackssmirkingrevenge]

have a look: http://www.spudfiles.com/pneumatic-cann ... c7457.html

[Alanstone]

Dart guy I commend you on the effort you put into your post. Your idea was cool and it is awesome to gain from the experience from the others on the site. Keep on thinking and keep on trying. Its the quickest way to learn. At 14 it must be obvious to all here that you are passionate about this hobby irrespective of your ability or financial status. Think about what Cammy has said and make your idea better. I am rooting for you son. (To all: sorry for the hijack)

[air_dannon]

have a look: http://www.spudfiles.com/pneumatic-cann ... c7457.html

Awesomeness, JSR! I had a feeling you'd have a link for me. So, the pressure on the top and bottom areas of the piston (noted with arrows on your drawing) slam the piston back once trigger is pulled, yes? How does this release fare in power over the QEV style?

Thanks!

[jackssmirkingrevenge]

That was one of my first pneumatics, it's still an exhaust valve but since it has no pilot valve, it opens much quicker than a normal QEV.

It is poorly configured as I built it (I was a misguided teenager...), the way the trigger levers needs a lot of effort to operate as it needs to move the piston forward a little before releasing, and over 100 psi or so it would leak.

With the benefit of hindsight, these days I would recommend a QDV style valve over this, as championed by Tech:

[youtube][/youtube]

http://www.spudfiles.com/spud_wiki/inde ... Dump_Valve

Here's a pengun design I had imagined using such a valve:

[PeteS]

I pump it up to around 100psi, use a nail to push in the Schrader valve, but ALL the air comes out the Schrader... no piston movement whatsoever.

You probably need a quicker dump than the schrader can provide. As a test, if you have a compressor, try taking the schrader valve out and inflating with a blow gun with a rubber tip. To trigger just pull the blow gun off of the opening. I bet it will fire just fine, then from there figure out what you need to do to fix the problem in a more satisfactory manner.

[air_dannon]

ok, I'm thinking about this idea for the piston now...

(1) For the piston, I already have this nice fitting pvc tube rated to 160psi, could 300-400 psi crack this pvc's wall (when in closed position) from the pressure trying to get out below?

(2) Instead of a hammer slamming the piston over, I'm thinking of using an extension spring that will be held back by a trigger mech. Thoughts, links?

(3) I'm not sure how to calculate an ideal chamber/barrel ratio. The air chamber at the moment is a 6" long, 1" diameter steel pipe and the barrel will be 1/2" copper. The pressure will be 300 to 400. Thoughts on how long the copper barrel should be?

Thanks!

[jackssmirkingrevenge]

(1) For the piston, I already have this nice fitting pvc tube rated to 160psi, could 300-400 psi crack this pvc's wall (when in closed position) from the pressure trying to get out below?

Pipes are typically much stronger in compression.

(2) Instead of a hammer slamming the piston over, I'm thinking of using an extension spring that will be held back by a trigger mech. Thoughts, links?

The way its configured, pressure will try and reclose the piston... you might want to follow the more "conventional" QDV configuration.

(3) I'm not sure how to calculate an ideal chamber/barrel ratio. The air chamber at the moment is a 6" long, 1" diameter steel pipe and the barrel will be 1/2" copper. The pressure will be 300 to 400. Thoughts on how long the copper barrel should be?

GGDT is your friend

[air_dannon]

Watched Technician's video some more and think I see where you were going with more conventional... instead of pushing the piston forward: (1.) chop that unnecessary back section off my piston, keep it solid, float the o-rings (2.) make a mech to pull the piston back just enough to let the pressure slam it back against a bumper.

[jackssmirkingrevenge]

Pretty much... also, since you will be pulling the piston as opposed to pushing it, the design and fabrication of a trigger system is made easier.

[air_dannon]

Based on Technician's QDV I have designed this idea for a sawed-off look. The chamber is a 1" x 6" steel nipple, and the piston sleeve, running through the center of the steel, is a 1/2" copper repair coupling. The sleeve will have two "vents" cut on opposite sides of the piston to equalize the pressure on the piston.

I've played around with the GGDT, but I'm not understanding many of the needed variables it asks for... I'm not even sure what type of piston Technicians would be in the drop-down?

My main question is with the chamber to barrel ratio in the design. With that copper coupling running through the middle of air chamber, it cuts that chamber volume from 1 inch to 1/2 inch essentially, correct? So, a ratio of 2:1 would mean the chamber is really 1/2" x 6", so a 1/2" copper barrel would need to be 12"?

[jackssmirkingrevenge]

The first thing that annoyed me was your trigger geometry ;P

The way you drew it, pulling the trigger means you have to first push the piston slightly forward against the firing spring, meaning it will be hard to open.

Fantastic rendering otherwise

With regards to GGDT, I took the liberty of modelling the valve for you. I used the generic valve setting, with a maximum dwell time (as the valve won't reclose) and an estimated opening time of 5 milliseconds.

This is a piloted piston valve opening filmed at 1000 frames per second:

[youtube][/youtube]

It takes about 15 milliseconds to fully open. As QDVs effectively have zero piloting time, 5 ms is a good estimate.

I used 300 psi and a 10 gram projectile.

With regards to chamber volume and barrel length, plug the values into GGDT and play with the variables, see how a longer chamber or barrel affects performance until you get something you're happy with.

Just my two Canadian cents, if you want real power for less hassle, go hybrid

It doesn't have to be complicated, this was my first successful one: http://www.spudfiles.com/hybrid-cannons/topic21655.html

[air_dannon]

Good point, I need to add a sear (I know what a sear is now... progress) to that trigger system. Thanks for all the info, JSR.

[bravootome]

Needed to work it out visually for myself. This is also going with a larger sealing side 1/2" copper (which is also the planned barrel size).

Are the spaces the piston needs to travel back in these 1/2 ID and 3/4 ID versions of my setup correct? And which is the better choice?

Will using the 1/2" copper for the sealing side help or hinder the explosive air power? If so, what would be a better fitting setup to a 1/2" barrel?

Also, the compression spring... Am I right in thinking it needs to be long enough to at least push that piston against the sealing side, but also loose enough to compress to the amounts shown in my diagrams? Obviously I don't want the spring pushing the piston shut before the full flow is evacuated, but taking this gun to 350-400 psi like I want, is that concern not even an issue?

Thanks!

well, the spring doesnt stop the piston travelling back at all ( at 20 bar meaning 20 kg/cm2 the piston is able to compress most of springs) and is not so important the volume behind of the piston at all. what is imprtant is the pilot( and inside volume of it ) and the 3rd multishoot valve if you're talking about a multishoot