Spudtech Forum

Can't get a seal::cries for help::pistonvavle Pics
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Author:  Spudtech [ Wed Jul 23, 2003 10:00 am ]
Post subject:  Can't get a seal::cries for help::pistonvavle Pics

Originaly posted by GisMo,

The piston will not seal against the chamber..I have a heavy rubber with a metal plate backing it..when I pressurize you see the piston push tight against the chamber side(3") then aight just squeeze all out of the sealing area...i dont get it...i tried silicon, I tried leveling the surface(again), nothing seals...Also, as soon as I stop filling, the pressure just drops...i have been at this for a week or so now..please help..thanks

Author:  Spudtech [ Wed Jul 23, 2003 12:00 pm ]
Post subject:  a few ideas about what might be the problem...

Originaly posted by corncob909,
is air leaking from those holes you drilled through the piston side of the tee and the piston bushing? that might be allowing air to leak outside the piston body (into the main part of the tee or out of the entire tee itself) this pressure leak might cause the piston to not opperate correctly and would deffinitely cause that drop in pressure. However if this was the problem and there was a leak there im sure you would have noticed it but just a thought. Also, I imagine these holes are for bolts so you can easily remove the piston?, a suggestion- im not sure if your able to do this but put an o ring in a groove right behind where the bolts go. And one more thing- i have been working on a very similar design so keep us posted on how things go. OH, i just noticed another thing, is there more surface area on the back side of the piston face (inside the piston body) then there is inside the 3 inch pipe when the piston pushes against it? You need a greater surface area on the backside of the piston so that it pushes against the 3 inch port but also stays there with 100 pounds per SQUARE INCH, the more surface area there is the more the pressure has to push against as you push more air in creating a greater force on that side then on the 3" pipe side once it has completely equalized, this should keep the piston face tight against the 3 inch pipe but im not quite sure so correct me if im wrong- anyway i hope these toughts will help and nice work with the piston :-)

Author:  Spudtech [ Wed Jul 23, 2003 1:00 pm ]
Post subject:  Somethin to check

Originaly posted by FoxxLord,
Is the center of the rubber disk sealed air tight? Pressure might be slowly leaking right through the middle of the piston where the hardware is. Feel for air coming out the muzzle.


Author:  Spudtech [ Wed Jul 23, 2003 4:00 pm ]
Post subject:  Thoughts and more PICS

Originaly posted by GisMo,
The holes are tapped for screws to hold th piston assembly in. The piston is a 2" sch80 coupler with the 0-ring grooves and a sold metal plate cold welded to it. The rubber was glued and the there is a brass nipple with a rubber washer on each side to seal it off. Any other questions? comments? help? thanks

Author:  Spudtech [ Wed Jul 23, 2003 4:00 pm ]
Post subject:  I'm flattered GisMo....that looks like my design!!

Originaly posted by spudsonfire,

#1 - Compare your piston diameter to your sealing face diameter. The sealing face diameter MUST be smaller or you will never seal. (I have recommendations on how to fix, if it is not.)

#2 - I machined a round groove into the face of my piston and placed a O-ring in it to ensure a proper air seal between the PVC piston and the sealing material. A complete seal here is required.

#3 - When you get it all working, MAKE DAM SURE the back of your valve is secure. I would recommend placing double the amount of screws and in addition some safety fabric. (I have that shown on some other post.) If you take the area of your piston in sqin x the psi you take the gun to, that is the force trying to push the valve apart. Once you do that calculation, you'll never stand directly behind your gun.

Author:  Spudtech [ Wed Jul 23, 2003 4:00 pm ]
Post subject:  You should be flattered :) Thanks..

Originaly posted by GisMo,
My 2nd vavle is based on your original design :)..anyways..Here is a pic of the piston against the place where is it to seal. The piston is definately larger, yet it still leaks I am almost positive it does not leak from between the metal plate the and sealing material. I do plan on putting more screws in to hold in the piston assembly..I am going to try to get a video of what happens..any other thoughts?

Author:  Spudtech [ Wed Jul 23, 2003 5:00 pm ]
Post subject:  Lemme get some info

Originaly posted by spudsonfire,
Could you give me:
PVC piston outter diameter =
Pipe inner diameter of what the rubber face is sealing to =

I just want to confirm. I had this problem initially. The pipe inner diameter coming from the pressure chamber was smaller, but not by enough to make a positive seal. I can plug those diameters into a simple spreadsheet I made for mine to view the forces acting on the valve over a range of pressures.

Back in a few hours.

Author:  Spudtech [ Wed Jul 23, 2003 5:00 pm ]
Post subject:  Dimesions...

Originaly posted by GisMo,
piston dia= 4"
Pipe Inner dia.=3"


Author:  Spudtech [ Wed Jul 23, 2003 7:00 pm ]
Post subject:  Piston?

Originaly posted by spudsonfire,

I gotta ask on the piston...I was looking on the "Thoughts and more PICs" post and you mentioned the piston is a 2" sch 80 coupling. With the OD of 2" pipe at 2.375 and the wall thickness of a sch 80 coupling at .36 (I miked a 4" sch 80 coupling wall, didn't have a 2" on hand) I'm getting 2.375+.36+.36 = 3.09" diameter for a piston diameter. What am I missing here?

Incidentally, for a 4" piston in a gun at 80 psi, that will be a force of 1005 lbs pushing on the piston back (and thus the back cylinder casing.)

Author:  Spudtech [ Wed Jul 23, 2003 7:00 pm ]
Post subject:  not sure if i misunderstandood or if this a typo

Originaly posted by corncob909,
but you said "The piston is a 2" sch80 coupler with the 0-ring grooves and a solid metal plate cold welded to it" and you also previously said something about the chamber inlet being a 3" pipe- well there ya go- two inch is less then 3 and your the back side of your piston has to be bigger, so enless there is something else i dont know or if this was a typo then i dont see how the piston side would ever have more surface area then the chamber outlet when pushed against the piston face. Im pretty sure im just misunderstanding so plz correct me if im completely wrong, but also take a look at this pic from the top outlet in the tee, where it shows the piston face coming together with the chamber outlet- you can clearly see the piston is smaller then the chamber outlet pipe, and also i believe that since the piston is sch 80 those walls are even thicker then the Sch 40 3" outlet making it impossible to say that the piston has more surface area on the back side of the piston face then when it is pushed against the 3" outlet- just a few thoughts, and again, plz correct me if i missed something or i am completely wrong.

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