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The S-DSTV/32 Trigger Valve and Mini Gun

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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The S-DSTV/32 Trigger Valve and Mini Gun

Unread postAuthor: dongfang » Fri Sep 07, 2007 5:30 pm

Hi,

I made the DSTV(Darn Simple Trigger Valve) recently. It was a bolt through a tee with a piston on each end, that you could pull half way out to dump air (for triggering a piston valve).

It works much better for triggering than my sprinkler valve does, but the pull cord triggering was not so elegant.

Looking at the design, it it a DFTV (the old Darn Fast Turbo-Valve, not my design), minus the opener disk and the turbo disk.

For those who don´t know about the DFTV, it is a very interesting alternative to a piston valve, supposed to be faster because it has three pistons that cooperate to boost the valve opening: One that seals off the exit port, another one that speeds up dumping the remaining air in pilot chamber after initial blowgun triggering, and a third "opener" piston between the other two to pull/push them. There is actually a fourth disk in the DFTV, the "turbo disk" which is supposed to boost the opening speed further, but in (my) static analysis, it does noting at all, and I know of no really well-founded, non-speculative dynamic analysis of it.

I have now made a version 2 of the DSTV, the S-DSTV (Servo-). It is now a Darn Fast Turbo Valve only missing the turbo disk (that I have decided to drop since I can´t understand how it works anyway :lol: ).

It is supposed to trigger my 75 mm piston valve some day, when the political (terrorism) climate gets a little better here in Europe 8) .

Anyway here are the pictures.. I think this is a pretty good trigger valve for piston valves. It will cope ok with lots of dead space, long hoses and crappy blow guns. My sprinkler valve would only open reluctantly and under such circumstances, and the piston valve underperformed.

Regards
Soren
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Attachments
DSC_0656.JPG
Hooked to a test chamber, it makes a mini gun of its own. I just had to try that, so I plugged a 80 cm 32 mm pipe in the exit port, and put a spare end cap in a plastic bag as a projectile. I fired it at a bucket at only 2 or 3 bar. I now have no spare end cap.
DSC_0651.JPG
Looking down the interface to the piston valve controlled. This kind of pluggable modular design costs some dead space sometimes. Exit port open.
DSC_0650.JPG
Exit port, open.
DSC_0648.JPG
Pilot port, open.
DSC_0647.JPG
Pilot port, closed.
DSC_0646.JPG
Exit port, closed.
DSC_0645.JPG
Almost finished now... I tried to put a check valve inside the 20 mm bolt pipe, but of course it shifted while the epoxy was curing. Now it´s just 2 holes as eq. port.
DSC_0640.JPG
From a different angle.
DSC_0635.JPG
The line-up. Threaded 1" coupler, tee, coupler to go over tee to extend it, and (bottom to top) pilot valve seat, opener (central) piston, and exit port piston and seat. To the right is the uncompleted bolt.
The o-ring is for the opener piston.
DSC_0632.JPG
Spudder´s Paradise? My parts shelf.
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Unread postAuthor: e1337 » Fri Sep 07, 2007 5:33 pm

Nice design, thats pretty weird fittings.

-e1337
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Unread postAuthor: dongfang » Fri Sep 07, 2007 5:59 pm

Hi,

Those are European PN16 fittings. About the same strength as schedule 80, as far as I know. Dimensions are all metric. The S-DSTV tee is a 32 mm one.

Regards
Soren
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Unread postAuthor: clide » Sat Sep 08, 2007 12:48 am

The theory behind the turbo disk is that basically as soon as the bottom(in the original drawing) o-ring unseals the pressure will act on the turbo disk and act as if the pilot pressure instantly dropped to nearly zero. However the exhaust is so big that it wouldn't really add much, the valve is going to open damn fast with or without the turbo disk.

Nice work on the valve. Too bad you couldn't get the check valve to work, those are the best improvement to any o-ringed piston in my opinion. Although the good news is that they are less vital on a DFTV design because it will require less of a drop in pilot pressure to fire.
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Unread postAuthor: dongfang » Sat Sep 08, 2007 1:06 am

Hi, Clide,

Statically, I see the turbo disk as just another opener disk. It will create some back pressure, which will reduce the effect of the main opener disk by the same amount (x * p across one of them, (1-x) * p across the other, why not just have p across a single piston?)
And dynamically, when timing is involved, I don´t understand it ;)

Anyway, the problem is more (for me) to find a way to slow down that bolt so it doesn´t shatter at higher pressures... I think I have found a solution though.

Regards
Soren
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Unread postAuthor: clide » Sat Sep 08, 2007 1:29 am

I'm not really taking anything dynamically, just statically at different points in time. Imagine that the valve has just started to open. There will still be pressure in the pilot, but the turbo disk will also be exposed to that pressure and there will be a net effect of 0 force on the piston due to pilot pressure and the only force will be the force of the pressure in the chamber slamming the piston open.

Now without the disk you will still have the pressure in the pilot area as the piston begins to open, but without the turbo disk it will be pushing the piston forward with some force which will create an overall smaller net force trying to open the piston.

Of course this all relies on the assumption that it is faster to pressurize the small area of the turbo disk than to vent the pressure that remains in the pilot. I can't say whether that is a good assumption or not, but that is the theory behind the disk.

Still, like I said, it won't really matter at all in the end. I wouldn't put a turbo disk on a DFTV if i built one.
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Unread postAuthor: spud yeti » Sat Sep 08, 2007 3:21 am

That looks really great, well done dongfang. Other than the nice triggering method, is this much better in performance to the original DSTV?

btw, I love the pieces collection :D
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really good quote/phrase here
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Unread postAuthor: dongfang » Sat Sep 08, 2007 3:57 am

Hi Spud Yeti,

I don´t know the difference in triggering performance, neither do I think I will ever be able to find any difference. Theoretically the DSTV has some more valve seat area. This thing has boosted bolt extraction.

They are both just fine, and with so little competition on the sprinkler valve market here (nobody has a sprinkler system) they are way cheaper than a sprinkler valve, too.

Regards
Soren
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