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Vacuum actuated QDV

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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Vacuum actuated QDV

Unread postAuthor: auxiliary » Tue Oct 05, 2010 2:43 pm

Here's how it works: First open the 3-way valve so that the pressure pump is open to the chamber behind the piston. Use the pump untill the piston shoots forward and the sear catches onto the middle set of washers. Pressurize the chamber through the inlet at the bottom forward end of the gun (I plan on hooking up an onboard stirrup pump to this area) Switch the 3-way valve to the vacuum pump (some air should hiss out of the check valve and shoot the pump handle out). Start pumping and pull a vacuum on the chamber behind the piston. Load a gold ball using the sliding breech. Pull the trigger!

I plan on building this hand held pneumatic golf ball launcher using 2" sch 40 aluminum pipe and fittings. I thought about what kind of valve I wanted to use and figured that I might as well make a QDV because I have made two before and it is very efficient. I tried to improve the original QDV design by doing away with the rod that serves two important purposes. (1) to actuated the piston by manually pulling it back and (2) resetting the piston by pushing it back forward.

I want to use pneumatics to do the same job that the pull rod does. A benefit is that there is less dead space between the piston and the projectile because there is no pull rod. Another benefit is that the piston may accelerate even faster becuase the pressure on the barrel side of the piston is pushing against nothing but vacuum. The last major benefit I can think of is that it is easier to fire the cannon because all it takes is a pull of a small trigger to disengage the sear.

Does this design seems theoretically and practically sound?

The drawing at the bottom left of the schematic is a top-down view of the valve housing. I couldn't have the pumps in the overall diagram because they are positioned horizontally and would cover up everything
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Re: Vacuum actuated QDV

Unread postAuthor: jackssmirkingrevenge » Tue Oct 05, 2010 2:51 pm

auxiliary wrote:Does this design seems theoretically and practically sound?


Theoretically? yes.

Practically? Well...
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Unread postAuthor: POLAND_SPUD » Tue Oct 05, 2010 3:37 pm

or just reduce the diameter of the front disk... and voila you've got a negative pressure piloting piston valve....
a.k.a. piston valve (yeah just a regular piston valve... the same design that most people use)

if you really insist on mechanical trigger then again reduce the diameter of the front disk hte pressure differential will do the rest for you - no need to use vacuum...
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 05, 2010 3:52 pm

Poland makes a good point, vaguely what I did here.
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Unread postAuthor: velocity3x » Tue Oct 05, 2010 6:09 pm

I think the o-ring on the center piston will be destroyed (quickly) from retracting across the trigger / piston latch.
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Re: Vacuum actuated QDV

Unread postAuthor: Ragnarok » Tue Oct 05, 2010 6:45 pm

auxiliary wrote:Another benefit is that the piston may accelerate even faster because the pressure on the barrel side of the piston is pushing against nothing but vacuum.

As I have said several times now, the difference in a QDV and Piston Valve opening time is not a good reason to choose between the two. Not only is the difference very small, the effect on muzzle energy is practically imperceptible.
Once a valve is that fast, any further improvement will barely affect the end velocity. For that reason, vacuum piloting a QDV will not do anything worthwhile to velocity.

I have to agree with Poland_Spud - it looks like you're trying to shoehorn a QDV into doing a job that a piston valve has been doing just fine for years.
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Unread postAuthor: Technician1002 » Tue Oct 05, 2010 8:00 pm

There is also the difficulty in adding piston mass and friction in adding a third set of washers and o ring. Minor leakage would cause loss of the vacuum in the pilot. I have to agree with Ragnarok that the benefit is slim and the added o ring decreases performance compared to a 1 o ring piston. Have you looked at the piston in the Mouse Musket. That worked very well and does not require a spring or vacuum. In small diameters with a close tolerance, it does not require an o ring for very low friction operation..

I am working on a zero dead space version where the rod does not extend through the piston face. The rod and nut would fit inside the piston and push it closed and open from inside the piston. I'll sketch it up later for you.
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