My Condor Builds

[Scott Endler]

then you have a net OPENING force.... That would sure explain the tendency to dump the tank....

Until the pellet leaves the barrel. The AirForce airguns are not noted for their efficiency. But they do have a big tank. And there aren't too many off the shelf .25 cals that you have to reduce the fill and turn down the tune to keep them under 70 fpe with pellets as you do with a new high flow Condor.

[rsterne]

I still don't think you would have much of a pressure difference across the top hat.... Maybe my suggestion is wrong, but I still think it is possible.... I notice that one of the top hats linked to has the sides milled away, and possibly a flat as well.... maybe to avoid this?....

Bob

[Scott Endler]

Even the stock high flow valves will apparently dump the tank if the stroke stop is set too high. This is from a review on Pyramyd.
 " The tophat is held in place by 2 small screws. If it is too far forward when you engage the cocking lever the whole tank can discharge. Just loosen the screws and screw the top hat in the direction that moves it closer to the tank. Solved the problem."

[Scott Endler]

I notice that one of the top hats linked to has the sides milled away, and possibly a flat as well.... maybe to avoid this?....

I don't think the large diameter flat of the top hat is under barrel pressure. It is at atmospheric, sealed off by the oring of the breech closing slide. Which is actually a giant, sliding, transfer port. Transferring the strike of the hammer to the valve. And transferring the air to the barrel. But there is some area and volume of material at the smaller diameter of the tophat inside the slide for the barrel pressure to turn and build at the base of the barrel and pellet and work against. And some further opening force could develop from the volume of material of the barrel which is sealed inside the slide at the other end which would want to continue to force the slide back toward the valve. Now that I think of it, this would be a much greater volume than end of the tophat. To increase efficiency you might want to cut the oring onto the barrel as far back as possible instead of  into the id of the slide. Or make sure the tolerances are set so the oring in the slide is very near to falling off the end of the barrel when the valve is fully opened.

[rsterne]

Volume doesn't matter for force, only the area exposed to the pressure ( ID of the O-ring) at both ends.... If the front of the top hat is bigger than the back, you have a net opening force, IMO....

Bob

[Big Bore Bart]

   I can see what Bob is getting at, and I can see a possible solution.    Make the "brim"of the top hat longer, recess the valve body for it and put the O-ring in the recess.    That would expose the back of the "brim" to tank pressure while the valve is open.  Causing a net closing force to the valve.

[Scott Endler]

Volume is just area in 3D. Same thing. When the air pressure that is past the poppet in a common airgun valve is trying to spit the stem back out to the atmospheric side of the valve body toward the striker, it is trying to reduce the matter that is inside. The pressure pushes on the surface area but it is trying to get rid of the additional volume.
Edit:
I guess the simpler way to look at it is that there is a cross section of piston in a bore that can move, so when something pushes on it, it moves. And the volume aspect of the piston would only make the force progressive if it were a significant fraction of the air space volume.

[Scott Endler]

  expose the back of the "brim" to tank pressure while the valve is open.  Causing a net closing force to the valve.

I think this would be the same as increasing the stiffness of the steel spring.

[Big Bore Bart]

  You could actually reduce the springs stiffness.  This added area would only be active for the time the valve is open and would make the valve close sooner, increasing efficiency.

[Scott Endler]

Make the "brim"of the top hat longer, recess the valve body for it and put the O-ring in the recess.    That would expose the back of the "brim" to tank pressure while the valve is open.  Causing a net closing force to the valve.

The air that is past the poppet is flowing inside the tube of the top hat. You would have to cross drill the tube again just under the top hat to fill a new chamber in the valve body. Or slot some channels in the bore of the valve down into the seat. This would create a barrel pressure dependent closing force to balance the opening forces in the slide but is getting to be some fancy footwork.

[Big Bore Bart]

Make the "brim"of the top hat longer, recess the valve body for it and put the O-ring in the recess.    That would expose the back of the "brim" to tank pressure while the valve is open.  Causing a net closing force to the valve.

The air that is past the poppet is flowing inside the tube of the top hat. You would have to cross drill the tube again just under the top hat to fill a new chamber in the valve body. Or slot some channels in the bore of the valve down into the seat. This would create a barrel pressure dependent closing force to balance the opening forces in the slide but is getting to be some fancy footwork.

   And the air is deadheaded against the O-ring that seals the OD of the top hat in the valve body.    It is this O-ring I'm suggesting relocating.   

[Scott Endler]

Good concept.
This is the more in line with the discussion Bob was trying to get started over here:
.
http://www.gatewaytoairguns.org/GTA/index.php?topic=62679.msg600686#msg600686
.
Maybe the moderators could move some of this valve theory over there so the OP could have his thread back.

[rsterne]

OK, after watching the video showing the quick-change top hat being installed I realize that the flats on the quick change top hat are only there to accept a 3/8" wrench.... It appears to me that the top hat must be sealed at both ends with an O-ring, correct?.... one in the valve body, and one in the sliding breech?.... There is a hole drilled through the top hat, so once the poppet is open, there is very little, if any, pressure difference between the ends of the top hat.... even if the pellet has left the barrel.... and the larger the hole through it, the less the pressure drop across it....

Therefore, as I see it, there are two opposing forces on the top hat.... From the front, the air pressure is pushing against the top hat at the ID of the O-ring inside the sliding breech, and therefore at the OD of the front portion of the top hat.... From the rear, the air pressure is pushing against the top hat at the ID of the O-ring inside the valve body, and therefore at the OD of the rear portion of the top hat.... then you subtract from that area the (smallest) diameter of the hole through the top hat (from both ends).... Here is an example:

Bore of top hat 0.250" ID: area is 0.0491 sq.in.
Front stem of top hat 0.350" OD: area is 0.0962 sq.in.
Back stem of top hat 0.300" OD: area is 0.0707 sq.in.

Effective area of front = 0.0962 - 0.0491 = 0.0471 sq.in.... at 3000 psi that is a force of 141.3 lbs.
Effective area of rear = 0.0707 - 0.0491 = 0.0216 sq.in.... at 3000 psi that is a force of 64.8 lbs.
This becomes an net force pushing the top hat back into the valve body of 141.3 - 64.8 = 76.5 lbs.

If the rear stem was 0.350" and the front stem was 0.300", then that force would be forwards towards the breech.... If the stems were equal in size, there would be no net force.... If the rear O-ring was on the OD of the top hat, which let's say was 0.500" in diameter, the force towards the front (breech) would be much greater, in fact so great that the valve would tend to slam shut immediately.... It might even blow the top hat out of the valve body.... To me it seems clear that the relationship between the front and rear diameters of the top hat have a huge effect on the opening or closing forces on the valve.... If I am not understanding something, somebody please give me a smack....

Bob

[Big Bore Bart]

   No smacks from me.    You saw what I saw.   You just put the numbers to it.   A stepped top hat may be the right answer.

[rsterne]

or simply increase the diameter of the back stem or decrease that of the front stem until you get the forces you need.... If you don't want ANY net force, so that the spring does all the work, then just make them the same size....

Bob

[Scott Endler]

Is there also some force generated from the pressure working against the  cross section area of the base of the barrel?. The barrel is the piston and the sleeve is it's cylinder. If there is force across the area of the piston (barrel and pellet), but the piston is held fixed (the pellet can start to move but not the barrel) and the cylinder is free to move, won't the  cylinder (sleeve) try to move back against the top hat adding to the opening force?

[rsterne]

Only if the O-ring ID at the back of the sleeve is larger than the O-ring ID at the front.... If they are the same size, it works just like a fill probe, there is no net force trying to push it out of the gun....

Bob

[Scott Endler]

I see. The area of the slide that the barrel pressure could act on to move it is occupied by the top hat which is getting the force instead.

[Big Bore Bart]

I see. The area of the slide that the barrel pressure could act on to move it is occupied by the top hat which is getting the force instead.

   I see your misconception.    Pressure acts in all directions.  Force can be seen as "pressure given a specific direction".    The forces on a valve system act only in a linear direction.   Therefore only the ends of the top hat will "see" the pressure as a force.    Whichever end has the larger area will generate the larger force.