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4" piston

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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Unread postAuthor: Technician1002 » Tue Jun 30, 2009 4:15 am

mark.f wrote:
spudtyrrant wrote:@ mobile general rule for barrel sealers is 1/2d thats what mr.crowley uses and thats i have used. i have never actually seen the math that proves this rule nor have i seen any comparisons between 1/2d piston travel and 1/4d travel would like to know how you guys know whether it is overkill or not and if it is overkill what does it matter this cannon is going to be large enough where if i add another inch to the travel it shouldn't make too noticeable a difference
@spudamine yeah my piston is not likely to come to those kind of forces it also looks like their was a lot of bubbles in the glue i will be casting mine into a mold all at once so it will have much less bubbles also look at how the pvc shattered it looks like the pvc didn't come through much better than the hot glue and that is what most people have suggested for my piston. with the glue i wouldnt have to go to the trouble of cutting peices of plywood to cover each end of a fitting plus it will take a lot less time


http://www.spudfiles.com/forums/how-far-your-piston-should-travel-t12772.html

I generally use a little longer than 1/4 of the port diameter for my valves.


I just read your link and for the most part the math is right on, but there is a slight flaw in the theory.

The orifice limits the flow. It's the COF of the valve. Having the piston opening the bare minimum, there would be two stacked orifices. One to get into the barrel and one to get past the piston. Opening the piston a little further reduces or makes insignificant the effect of one restriction in the flow. I don't consider a piston fully open until it is 1/3 to 1/2 the diameter of the piston. As an example, the valve orifice on my 2 inch valve is 3.14 square inches. The ports into the valve body has 6 square inches of area.

Some thoughts on the piston.. It has to do with the math again.

In going from a 2 inch to 4 inch (double size) the volume and mass increase by a cube where the cross section increases by only a square.

In simple terms a 4 inch piston has 4 times the area of a 2 inch piston but 8 times the mass. Think about it carefully. In opening it moves twice as far so at the end of the stroke, it is traveling the same speed as the smaller piston, but with 8X the mass. Care must be given on a bumper for this and the forces.
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Unread postAuthor: spudtyrrant » Tue Jun 30, 2009 7:26 am

@ tech cleanout plugs only come in dwv and it looks like that is what to used for the back i only said that because i have had cleanouts fail for my piston gun when i first made it thats why i switched to screws also i am including my bumper in the opening so it will be opening 1 1/2inches but only when it fully compresses the bumper my bumper will be 1/2 inch neoprene sheet which i'm sure will do well at shock absorbtion.

@everyone i will take what people say here into account when making my cannon but i also want to incorporate some of my own ideas into they design i'm starting the build today so wish me luck
and if i'm wrong i'm not the only one scroll down in this thread http://www.spudfiles.com/forums/viewtopic.php?t=15456&.
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Unread postAuthor: Technician1002 » Tue Jun 30, 2009 9:05 am

spudtyrrant wrote:@ tech cleanout plugs only come in dwv and it looks like that is what to used for the back i only said that because i have had cleanouts fail for my piston gun when i first made it thats why i switched to screws also i am including my bumper in the opening so it will be opening 1 1/2inches but only when it fully compresses the bumper my bumper will be 1/2 inch neoprene sheet which i'm sure will do well at shock absorbtion.


It is possible the plug on the Mouse Musket is a clean out. It's so old, I don't remember what the part was labeled, but it appears to be as heavy as the rest of the Sched 40 parts. The rest of the cannon is Sched 40. It never had a plug failure, only 4 piston failures. The female adapter is double layer, inserted fully inside 2 inch chamber.

The plug on the ABS cannon is a clean out plug. The female adapter and plug was originally PVC. They were replaced. It was originally PVC for strength. The female adapter shattered and the piston launched onto my foot. The current configuration of all ABS has not failed.

Edit; Google is your friend. SCHEDULE 40 PVC pipe plugs are for sale

This is the part I have. The clean out plugs have a square end for the wrench. The Sched 40 parts are this shape. The Mouse Musket does have a Schedule 40 plug.
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Attachments
Plug.jpg
Schedule 40 PVC Plug
Plug.jpg (6.74 KiB) Viewed 249 times
Cleanout.jpg
PVC Clean out. I don't use these in PVC.
Plug1.jpg
Another style Sched 40 PVC plug. The Mouse Musket uses this one.
Plug1.jpg (3.52 KiB) Viewed 248 times
Last edited by Technician1002 on Wed Jul 08, 2009 2:06 am, edited 1 time in total.
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Unread postAuthor: mobile chernobyl » Wed Jul 01, 2009 10:33 pm

spudtyrrant wrote:@ mobile general rule for barrel sealers is 1/2d thats what mr.crowley uses and thats i have used. i have never actually seen the math that proves this rule nor have i seen any comparisons between 1/2d piston travel and 1/4d travel would like to know how you guys know whether it is overkill or not and if it is overkill what does it matter this cannon is going to be large enough where if i add another inch to the travel it shouldn't make too noticeable a difference


A little late on this one, but from my "10 gallon" thread, I was wondering where Dia/4 came from, and it is proven in the math I did here:

mobile chernobyl wrote:I'll do this mathematically instead of random he-said equations.

Area of porting hole = PI * (Radius of porting hole, or diameter/2)
Area = PI * (3.0625/2)
Area = 7.367 sq inches.

So I want that same area from the "gap" formed by the piston opening, in order to not "bottleneck" it.

Area of "gap" created by piston = PI * (Diamter or porting area) * (Distance piston moved back)

But we already know the area we WANT, so lets figure out the distance it must open to equal that...

Area, or 7.367 sq in = PI * PortDia * PistonDist

Area / (PI * PortDia) = Distance

7.367 / (PI * 3.0625) = .77"

OK so it basically is Dia/4, but I proved it so I'm a little more confident now lol.


It's cool to just rely on other peoples opinions, but it's also cool to have them backed up with math you can trust ;)
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Unread postAuthor: glass_05 » Tue Jul 07, 2009 11:17 pm

very informative thread guys! i ordered all my parts yesterday to make a 4inch piston. of course pics will come when i start putting it together
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