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Proposed piston housing design - please comment. Thanks!

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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Proposed piston housing design - please comment. Thanks!

Unread postAuthor: limbeh » Sun Jan 20, 2008 2:01 am

Hi,

Would like to thank all you people at spudfiles for all this info on spudguns. Spudguns are extremely rare in my country (Singapore), and those that are are pretty simple (chamber + un-modded ball valve + barrel & pretty inaccurate / inefficient), so coming to this site was indeed a big eye opener.

I'm thinking of building a piston valve for a cannon with a 3" (89mm outer diameter, 82mm inner diameter) barrel. The diagram of the proposed cannon should be in the attached file. Hope it is clear enough to be understood, though it isn't drawn to scale.

Please leave any comments on the design and such, just dont turn it into a mud-slinging flight. I would have gotten a sprinkler valve if I could, but they are practically non existent in my country as practically nobody has a lawn (yes, my entire country is an urban jungle).

Thanks.
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Attachments
proposed piston housing.JPG
Proposed piston housing. In theory, when the pilot opens, the pressure acting on the back of the piston should push the piston back - or will it?
proposed piston housing.JPG (36.8 KiB) Viewed 429 times
Last edited by limbeh on Tue Jan 29, 2008 9:18 am, edited 1 time in total.
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Unread postAuthor: Novacastrian » Sun Jan 20, 2008 4:16 am

It seems as though it would work, however why do you want to use this design and not a "solid" piston? It would seem that this design could constrict flow. Having said that i have never made a piston for a tee.
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Unread postAuthor: ALIHISGREAT » Sun Jan 20, 2008 5:21 am

i think it would have better performance if the barrel protrudes into the tee, i'm not sure it would even work if it didn't. Also this is quite an interesting design because when the pilot is piloted there will be more surface area pushing the piston back than trying to hold it foreward which could give quicker opening times however the rear of the sealing face could get 'pulled' foreward by the air trying to exit the barrel which could cause slow opening times...
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Unread postAuthor: limbeh » Sun Jan 20, 2008 5:31 am

Actually, I was thinking of this design because I'm not sure if the impact of the piston on the back end (when the pilot is triggered) is enough to send the back part (pilot valve, schrader, endcap / plug etc) flying out, so I was thinking of playing it safe.

Was hoping if those of you with piston cannons could share a bit on this, since I don't know of anybody in my country with a piston cannon in my country.

By the way, does anyone know what a valve seat and flow coefficient is, as well as how to calculate friction? I'm having problems with GGDT because I can't igure those 3 values out.

Thanks.
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Unread postAuthor: ALIHISGREAT » Sun Jan 20, 2008 5:37 am

limbeh wrote:Actually, I was thinking of this design because I'm not sure if the impact of the piston on the back end (when the pilot is triggered) is enough to send the back part (pilot valve, schrader, endcap / plug etc) flying out, so I was thinking of playing it safe.

Was hoping if those of you with piston cannons could share a bit on this, since I don't know of anybody in my country with a piston cannon in my country.

By the way, does anyone know what a valve seat and flow coefficient is, as well as how to calculate friction? I'm having problems with GGDT because I can't igure those 3 values out.

Thanks.


ok well the seat is what the piston seals on with in this case will be the 3" barrel (don't forget wall thickness) which needs to be protruding into the tee (probable about half way) the flow coefficeint is how much can pass through it in a minute or something like that (but don't take my word for it, use google) and you just have to guess the friction but it will be very little.[/i]
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Unread postAuthor: jrrdw » Sun Jan 20, 2008 7:54 am

I believe this to be a 'Burke Lake' style piston. Yes, this valve has the abilty to open, but also the real chance of closing back up during the shot, as mentioned above. Installing check valve/s in your sealing face will help keep reclosour from happening.

The drag will happen when the piston assemble tilts directly after opening. To stop that you could make the sealing assembly the same inside diameter as the tee. I believe these changes to be nessisary or you will have problems.

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Unread postAuthor: limbeh » Sun Jan 20, 2008 8:08 am

Thanks guys. Finally managed to learn what seat and & flow coefficient means in 1 day when 2 months of searching got me nothing. As for the design, I'll probably revert to a normal design, to play it safe.

I'll try to build the device ASAP, but this would probably take up several months due to difficulties in sourcing parts (the local manufacturer doesn't go by Sch 40 dimensions and Home Depot / mcmaster / lowes / rainbird / etc do not exist here)

As for damage pics, this cannon isn't really meant to fire them. Trying to damage stuff could well end up with me in serious trouble (read: court, max-security jail, etc) and a real bad impression of spudgunners by the community. It's meant mainly for trajectory research, but can be used for lots of other things (e.g. special effects, long range waterbombs)

Thanks a lot!
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Unread postAuthor: Carlman » Sun Jan 20, 2008 8:21 am

limbeh wrote:...meant mainly for trajectory research...


al my cannons here in Oz are meant for 'trajectory research' aswell :wink:
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Unread postAuthor: MaxuS the 2nd » Sun Jan 20, 2008 8:40 am

http://www.spudfiles.com/forums/75mm-piston-with-2-porting-t11388.html

There yar, it's just basically a large bolt piston.
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 20, 2008 8:45 am

as it is it will be unstable, you need some full diameter supports on the red portion, or the yellow disk needs to be much thicker.
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Unread postAuthor: LikimysCrotchus5 » Sun Jan 20, 2008 11:49 am

Man i really feel bad for you.

You guys know that gum is illegal there? Ya its crazy.
And if you wanna know why, well its because people kept putting gum on the subway door, and then that screwed it up and caused a major backup so then that had to be fixed and then it was decided that gum should be illegal.

That sucks though that there is no where for you to shoot.

The design looks fairly good to me just fix what JSR told you to.

Also what do you plan for as operating pressures and fill method?
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Unread postAuthor: SpudMonster » Mon Jan 21, 2008 2:33 am

ALIHISGREAT wrote: Also this is quite an interesting design because when the pilot is piloted there will be more surface area pushing the piston back than trying to hold it foreward which could give quicker opening times


Uhhh... Isn't that how piston valves work? I fail to see why that is such a special feature to this valve.


The biggest problem I see with this valve is that the sealing face sticks way out beyond the front of the piston. You will need to either support the sealing face or lengthen the piston skirt to prevent the face from returning to the seat at weird angles. I'd recommend potting the barrel end closer to to the center of the T, as even if you manage to not constrict the flow any, the added turns the air has to make will decrease performance.
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Unread postAuthor: ALIHISGREAT » Mon Jan 21, 2008 2:39 am

Uhhh... Isn't that how piston valves work? I fail to see why that is such a special feature to this valve.


well the front of the red end of the piston will be pushing back when the pilot is piloted and the front of the yellow piston but only the beack of the yellow piston will be resisting.
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Unread postAuthor: SpudMonster » Mon Jan 21, 2008 2:48 am

ALIHISGREAT wrote:
Uhhh... Isn't that how piston valves work? I fail to see why that is such a special feature to this valve.


well the front of the red end of the piston will be pushing back when the pilot is piloted and the front of the yellow piston but only the beack of the yellow piston will be resisting.


You failed entirely to consider the back of the sealing face of the piston, and the fact that it would entirely negate the force exerted by the front of the sealing face.
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Unread postAuthor: ALIHISGREAT » Mon Jan 21, 2008 2:50 am

however the rear of the sealing face could get 'pulled' foreward by the air trying to exit the barrel which could cause slow opening times...


please read my posts before saying i'm wrong :roll:
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