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Enough piston travel??

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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Enough piston travel??

Unread postAuthor: dongfang » Thu Aug 02, 2007 1:53 pm

Hi,

I got a little surprised when I test assembled the big brother of the Schweizer-Valve.
Piston travel is very short! Need a little feedback from someone with experience (preferably - yes I have heard about the much repeated d/4 rule, but I don´t know whether to believe in it - and it´s a tee valve not a coax).

Piston travels 20 mm full stroke. Seat diam. is 63 mm. Optimistically, the area of the arced rectangle uncovered by the opening piston is 75 mm (tee housing size) * 20 mm * pi (it´s a semi circle) - pessimistically that area cannot all be used effectively (one lower limit is just 63 mm * 20 mm).

Barrel is 57 or 69 mm ID, depending on what I fit.

So the flow area through the open valve is somewhere between 1260 mm^2 and 3960 mm^2. Seat area is 3120 mm^2.

I got this idea of moving the o-ring on the end plug back to the other side of the screws - and accept having to seal those. That will also save me the damage on the ring every time I push it past the screw holes ..... Moving the o-ring could improve piston travel to 23-24 mm.

What do you think; OK as is, or move o-ring (yes I will do both versions on a chrony if I find the time...)

Regards
Soren
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Attachments
lukket.JPG
Closed. There is a tiny gap behind the piston, making a vent from pilot to main chamber. Feature, not bug.
loebside.JPG
View from barrel port. A 75 mm pipe could go straight in, or a reducer or a threaded something could be installed. The valve seat is epoxied in - NOT solvent welded.
aaben2.JPG
Fully open.
spilt.JPG
Exploded view. There is really nothing to or in this valve. Not even a spring. The piston is lightweight and monolithic - an end cap.
ventil_oven.JPG
General view. It´s like my Schweizer-Valve, just bigger. I guess 10 screws could have done the job but now there are 12.
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Unread postAuthor: hi » Thu Aug 02, 2007 3:00 pm

it is just right. if it opened any more it wouldnt matter.

if you really think that there is a problem you can take a cm or so off the piston, but it is unnecessary .
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Unread postAuthor: dongfang » Thu Aug 02, 2007 3:30 pm

Hi,

OK. If I shortened the piston, there would be a laaaarge gap behind it when closed...

In fact, the end caps become stubbier with larger size; that´s one thing preventing me from scaling the design to a 110 mm (4 1/2") giga version. A 110 mm end cap is flat like a pan... I´m surprised they do that: I´d think the adhesion area necessary should be proportional to the force .. and the force in proportional to the square of the diameter. So the adhesion area should be proportional to the square of the diameter -- its width is proportional to the diameter, so should the length not also be that? Well OK that with the adhesion area was just an assumption.

Regards
Soren
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Unread postAuthor: clide » Thu Aug 02, 2007 5:21 pm

Personally I try to get at least 1/2 the outlet diameter on tee valves. I have no mathematical backing for this, but it makes sense to me. Air can only get to the outlet from one side. Which is half if not less than where air can flow into the outlet of a coaxial, so at the very least you should double the the piston travel over the 1/4 needed in a coaxial IMO.

That is one of several reasons why I haven't built any piston valves with the bolt-in pilot plug style construction. The piston travel is often limited to less than I feel is needed.
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Unread postAuthor: dongfang » Thu Aug 02, 2007 5:28 pm

Hi Clide,

So how do you plug the pilot end then? A socket going over the tee, extending it?

Regards
Soren
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Unread postAuthor: Gepard » Thu Aug 02, 2007 5:39 pm

Did you ever get your last one to work?

Michael
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Unread postAuthor: clide » Thu Aug 02, 2007 5:41 pm

In previous valves I have a piece of pipe in the tee and attach a female adapter where I screw in reducing bushings to attach the pilot valve. Piston travels in the pipe connecting the tee and female adapter. This makes it a bit more bulky, but it is easy and strong.

The next time I build a tee valve I plan on gluing a plug into the tee for the piston to travel on and bolting the front fitting/piston seat into the valve for piston access. This will avoid the piston slamming into bolted on fitting (something I've never been fond of) and will allow the valve to be compact. I've done something like this before, but it was for a specialized launcher. I have yet to build it into a compact interchangeable valve.
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Unread postAuthor: Gepard » Thu Aug 02, 2007 5:43 pm

That's what I'm doing now with my one (bolting the front fitting on).

I'm thinking that with that method you could crank up the pressure above the usual 125psi max.....

Michael
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Unread postAuthor: Modderxtrordanare » Thu Aug 02, 2007 6:36 pm

If the caps get stubbier as they get bigger, why not just move the sealing face further towards the back of the tee to compensate for the bigger gap they'd leave inbetween the pilot and chamber?
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Unread postAuthor: dongfang » Fri Aug 03, 2007 4:31 am

Hi

Did you ever get your last one to work?


Oh yes. It seals and re-seals perfectly now, and it seems very powerful and fast. The only thing missing it filling that hollow piston. And the main reason I don´t shoot so much with it here (see the "ammo just disintegrating" thread in ammo and materials) is that it goes too far..

OK Clide I see it.. that would definitely also be a solution for me, if the "pistons" I use would fit inside the one size larger pipe, but the never do..

I´m a little squeezed in the lengthwise dimension. Of course I have the piston go as far forward as possible when closed. And as far back (from there) as possible when open. If I glued in the pilot end plug, it would leave the piston with very very little space to move on. So, since I´d have to shorten it anyway, I decided (standard practise I guess) to bolt it in the pilot end.

I´m not all that worried about the piston slam on it, because at least there is no pilot pressure left at the time the piston does that..

If I have time I will try make a shorter end plug AND some precision comparison. The piston travel question seems half unresolved, so some empirical results could be interesting...

And:
If the caps get stubbier as they get bigger, why not just move the sealing face further towards the back of the tee to compensate for the bigger gap they'd leave in between the pilot and chamber?


Of course that´s what I would do. But for the largest sizes, it would not work very well: A large part of the opening would be hidden under the pilot branch of the tee, if you know what I mean.
OK I guess if I really wanted to make a giga-Schweizer, I could extend the piston by making it of: An end cap, a piece of pipe, and a female female socket cut in half.

Regards
Soren[/quote]
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Unread postAuthor: SnowFlox » Fri Aug 03, 2007 5:42 pm

looks very impressive.. but whats the use of this valve ? i see the other topic, and you used an sprinkler valve allready. :?:
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Unread postAuthor: wannabie » Fri Aug 03, 2007 5:45 pm

Its a piston valve.... they have more flow and open faster he used a sprinkler valve to actuate the piston.
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Unread postAuthor: SnowFlox » Fri Aug 03, 2007 5:50 pm

aaaah ok !! now i get it :) thanks !
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Unread postAuthor: CannonCreator » Wed Sep 05, 2007 1:12 am

Whats the Tee size? And What size is that end cap that you are using for the piston?
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