Cothran PowerHouse Valve and TP.

[Voltar1]

Closing force does not require movement
The pressure tries to push the red part off the piston hence closing force

[miksatx]

and you didn't read my post.... I bolded where I covered that....

Bob

Sure I read it. thats why I mentioned the drawing is wrong. Not real sure what respecting his invention is supposed to mean in explaing how the system works. There's a link to a airguns mag that has a article on some of the valve design. the valve is tore apart and each part named. Mr Cothran don't seem to worried about his invention that I can tell. But leaving out that stabilizer piston in the valve stem in real life the valve doesn't work right.

[rsterne]

Michael.... Since you seem to know exactly how the valve works, I suggest you draw it "properly", with all the parts, in several positions; closed, just open, and closing, and show the position of the rod (stabilizer piston) in each, and the airflow that occurs.... It only took me an hour to "whip up" that drawing, and I feel that it 100% represents the situation when the valve is CLOSED, since the stabilizer piston is a loose fit inside the valve stem, with atmospheric air pressure surround it, which is why I omitted it from the drawing.... because it makes NO difference to the valve when closed....

Walter.... Yes, the air pressure tries to push the red part off the piston.... but can only act on the area OUTSIDE the diameter of the piston.... Thank you for making my point.... If the piston diameter equalled the seat diameter, the only force holding the valve close would be the spring.... Draw it out and see....

Bob

[Monkeydad1969]

http://www.cothranmachine.com/WhatNew.HTML

Hey all, just go to this link and you can find the link to actual story in Airgun Hobbyist Magazine.

And lets all just relax.  It's just a valve, even though it is BAD @R$E!

[miksatx]

because it makes NO difference to the valve when closed

Bob

if the gun is charged with air and you have left the stabilizer piston out of the valve stem. Tell me Bob what keeps the air from leaking out the hole in the valve stem?

[rsterne]

Last attempt.... Here is a drawing with the piston diameter equal to the seat diameter....



With the valve closed, as shown, there is NO force holding the valve closed except the spring.... Have a nice day !!!

Michael.... The hole in the stem and the pocket in the head of the poppet are on the OUTSIDE of the valve seat, and are at atmospheric pressure when the valve is closed.... there is no pressure there to leak.... I repeat, the rod (stabilizer piston) is a LOOSE FIT and rattles around inside the valve stem and does NOT seal anything.... It simply slows down the pressure changes during the firing cycle to prevent valve bounce.... I would imagine it was added later because the valve opened TOO easily and became a machine-gun from hammer bounce.... It was, I must admit, a VERY clever way to deal with that problem.... but it has NOTHING to do with the "Force Reduction System" which is the key to why the Cothran valve opens so easily.... Like I said, take the time to draw it out and show me where I am wrong.... Same thing goes for anyone....

Bob

[Voltar1]

Michael.... Since you seem to know exactly how the valve works, I suggest you draw it "properly", with all the parts, in several positions; closed, just open, and closing, and show the position of the rod (stabilizer piston) in each, and the airflow that occurs.... It only took me an hour to "whip up" that drawing, and I feel that it 100% represents the situation when the valve is CLOSED, since the stabilizer piston is a loose fit inside the valve stem, with atmospheric air pressure surround it, which is why I omitted it from the drawing.... because it makes NO difference to the valve when closed....

Walter.... Yes, the air pressure tries to push the red part off the piston.... but can only act on the area OUTSIDE the diameter of the piston.... Thank you for making my point.... If the piston diameter equalled the seat diameter, the only force holding the valve close would be the spring.... Draw it out and see....

Bob

Sorry Bob I dont follow the outside the piston
I see it as closing forces being the sum of the areas with pressure differential in the same direction
The valve seal force is leftward in the sketch
The piston tophat force is leftward as well.

[rsterne]

OK, Walter, maybe one more try.... If you have a sleeve sliding on a rod, and surround it by HPA, is there an end force on it?.... Rhetorical question, so I'll answer it.... NO of course not.... On the second drawing I just posted, where the piston diameter is the same as the seat diameter.... there are two shoulders on the ends of that red part exposed to the HPA.... one on the right (where the HPA will push to the left) and one on the left, just above the seat (where the HPA will push to the right).... Those forces will completely cancel out, leaving no net force on the red part, just as if it was a sleeve surrounded by HPA....

In the case of the Cothran valve, the piston PREVENTS the HPA from pushing on the end of the red part over its whole diameter.... Only the DIFFERENCE in area between the piston and the valve seat can cause a force to hold the valve closed once it is on the seat....

Bob

[Gippeto]

Kind of a hard one to put in simple terms, but will take a swing at it...

Force generated is pressure multiplied by the area upon which it acts, think we can all agree on that.

For an example, lets say seal diameter of the "hat" part of stem is .300", and the piston diameter is .250"...works out to areas of .071sqin and .049sqin respectively.

No disagreement about valve stem diameter generating closing force during the shot cycle, so we'll call that part a given.

Will break the valve stem into a few parts...the shaft, what I'm going to refer to as the "hat"...usually a piece of delrin, the soft seal, and the little rod which moves inside the stem of the Cothran valve. Common terminology is important when trying to share an idea.

In our case, to generate force with a direction, one side of a contained object is exposed to a higher pressure than the other.

In the case of a "conventional" poppet valve, the throat side of the "hat" is at atmospheric pressure, while the other side is exposed to reservoir pressure. This generates a force holding the valve closed. If we take our example of .300" dia....that's 213 lbs force at 3000psi holding the valve closed.

With the Cothran valve, we have the added factor of the piston assembly...which, being vented to the valve throat is at atmospheric pressure before the shot cycle begins. This area, with atmospheric pressure on both sides, generates a net zero force.

Before shot cycle begins, effective area upon which pressure in the reservoir can act is .071sqin - .049 sqin.... .022sqin...@ 3000psi, that's 66 lbs force holding valve closed.

If it helps, think of the "hat" in a Cothran valve as a doughnut without a center (riding on a tube and all mounted within a larger tube) ...having no center, it has less surface area than if it were solid the full diameter.

Once things get pressurized by the shot cycle (pressure from the throat area enters via the small hole in the side of the shaft), the area of the piston does generate closing force...the little rod within the shaft helps to maintain that closing force after throat pressure has fallen to atmospheric...and helps keep the valve closed while the hammer is bouncing.

Hope that makes some sense...

Al

[Voltar1]

I see it as pressure across seat = closing force
Plus pressure across piston tophat seal = more closing force
Inside valve is hp and atm on other side of piston seal
That produces a closing force
I think we need to determine the seal barrier locations and then assign the force direction

[Monkeydad1969]

Thanks, Gippeto.

Joe

[miksatx]

I have one a part right now seems that the little rod got stuck in the valve stem. Velocity dropped 250 ft per sec. Waiting on replacement parts. Didn't leave it aired up when I was waiting on a new breech.

[rsterne]

Thanks, Al.... I guess Walter is going to need a demonstration....

Bob

[Gippeto]

Not sure what you mean by that Bob..

Al

[rsterne]

Just that he doesn't agree, so perhaps showing him that the valve does, in fact, require less force to open than a conventional valve might change his mind.... That is, after all, the whole point behind the "Force Reduction System"....

Bob

[Voltar1]

Yup, a demo is definitely overdue. My brain is hurting real bad.
Will be making a test fixture to sort this out. Cannot get my head around d this one lol

[tkerrigan]

Darn it, you guys have made more work for me, I bought a used Cothran .357 about a month ago. It does shoot hard. Now I have to disassemble it to satisfy my curiosity about what valve is in it. I know, relieve the air pressure first.   Regards, Tom

[rsterne]

Don't feel bad, Walter, I know what you mean about brain burn.... I looked at the photo forever with no joy until Al explained it.... then I went DUH!.... *lol*....

Bob

[miksatx]

my valve. i'll try to get a pix of the hole in the valve stem when the valve is together. As it's not visible while the valve is closed.

Not a real clear pix but you can make out the hole in the stem this is valve wide open.

[miksatx]

if you take the valve a part be sure the hole is facing up in the exhaust port. i have had this valve stem hole rotate out of the exhaust port and lose performance. I use long slim needle nose to grab the stem push in and line the hole back center in the exhaust port.
 
Leaks I've had them on both sides caused by debris under the valve seal and a nick in the o ring on the piston leaked right out the vent hole in the valve stem. I believe I'll be changing out my fill valve with a disco one since it has a metal filter.