Foster Style Check Valve?

[SeanMP]

what kind of pressure would you guess is inside this valve body?

Even at 1500psi I'm only getting 18lb/f on the stem area. Because your right the area of the stem is the only area you can count. because the area of the poppet head has force on all sides

[rsterne]

HI Al....

Those articles are what sent me down this path.... Is the reason you say there will be some bleed back is that the stem of the check must move first?.... If so, then why not fasten it in place with, for example, a set screw.... so that only the O-ring is moving?.... I realize that the dialating O-ring check valve design has only the O-ring moving.... it is expanding on the compression stroke and then contracting and being forced against the stem and the end of the valve on the intake stroke.... Why do you feel that the Foster style check will bleed back before sealing?.... Would you still feel the same way if there was a very light spring holding it seated?....

I noticed your floating O-ring design for the pump head.... I can understand how that bypasses on the intake stroke.... but wouldn't it require a rather slow return stroke or you would be pulling against a fair resistance.... Or, is it in fact better than a conventional flat-topped design which pulls against a vacuum for part of the stroke?.... Why do you think you could get increased stroke length over a flat-topped piston with O-ring?.... The length of slot is basically governed by the pump linkage clearing the tube, is it not?....

Bob

[SeanMP]

HI Al....
.... Why do you think you could get increased stroke length over a flat-topped piston with O-ring?.... The length of slot is basically governed by the pump linkage clearing the tube, is it not?....
Bob

Just jumped in to make sure I understand correctly...
Your floating Oring variable design allows you to take half a stroke or a quarter stroke ...for when the pumping has gotten real tough and your trying to squeeze in the last little bit?

Bob
I think your springless poppet should work fine. especially if the poppet stem was finely fitted. I would think leaded brass for the poppet for it's finely machined finish.

[SeanMP]

Possibly maybe

If the stem is free to move up and down the the force is also acting on the end of the stem. Which would move the stem up and break the seal?

[Gippeto]

Have found that foster checks will bleed back when the orientation allows and flow rate is low...as it would be when pumping a pumper...They always seem to be pointing at the sky when I pump 'em anyway....spring loaded would take care of this.

The usual FTP setup has to come out past the slot and then return into the slot before compression begins..as I recall, the slot is actually a little longer than really needed for the linkages...it is possible I'm mistaken, but I don't think so. If you're intention is to use a "factory" tube, then the slot isn't going to change and no benefit there...would definitely allow the half strokes Sean mentions. There is little resistance when "reloading" the pump cylinder with air...in any event, the effort required to "reload" the pump cylinder should be lower than if it were attempting to pull a vacuum.

Al

Edit: Changes to the picture I posted? Certainly. Out of time atm...will get back to this tonight.

[rsterne]

1500 would hopefully be the max.... That would put a load of 650 lbs. on the piston at max. compression....

I just did the math on the area available for "leakage" using Steve in NC's Dialating O-ring Check Valve design.... With a standard 4-40 screw, using the UNF Diagram for thread dimensions, I get an area of only 0.00006 sq.in. total, for both the inner and outer gaps in the thread.... That is the equivalent of a hole just 0.0043" in diameter (and 3.4" long with only 1/4" thread engagement).... Now I know there is additional room between the "contact" faces of the threads.... but that seems like a VERY small hole to try and force the air through.... In addition, I have a concern about the pressure differential required to dialate the O-ring.... Depending on the actual contact points, the inner area where the pump pressure can push on might be only half as much as the outer area where the pressure inside the valve is forcing the O-ring into the space between the tapered screw head and the end of the valve body (look at the diagram in the thread Al posted).... Without taking into account the pressure required to expand the O-ring away from the screw head, that would be unworkable, because at 1000 psi in the valve, it would take 2000 psi in the pump just to move the O-ring.... I'm thinking that even if the inner and outer sealing areas are the same (eg. the O-ring "floats" in a gap under the screw head).... the pressure to dialate the O-ring might be significant?.... The way I see it, it either has to force into a taper formed by the screw head.... or if not, be a "snug" fit on the stem of the screw to seal.... Either way, it relies on pressure differential to stretch and/or unseat the O-ring....

Is there something I'm missing?....

Bob

[rsterne]

Sean....

I was just planning on using a chromed Foster check valve poppet, I have a spare.... just shorten it by about half.... Good point about the finish, though, I should try and get the hole ASAP (as smooth as possible).... I wonder how much the O-ring will crush under pressure (ie how far I have to leave the stem short of the end of the valve so the piston doesn't hit it).... Gotta have minimum headspace, ya'know....

Al, any changes you would make to your piston design to reduce the headspace losses?....

Bob

[SeanMP]

Bob

Just for fun I just put a 007 under a spring tester. It seems to squish to 50% easily(150lb/f) then the force required to to squish it more jumps exponentially

[rsterne]

The problem with a wide groove is the excessive headspace created.... bad news in a pumper....

Bob

[rsterne]

I'll make sure my design can accept a small spring for the check valve.... although I might try it without.... Maybe if I set it up with just a few thou travel it might grab right away without bleeding back....

I will be custom cutting the slot, so I'll look at making it just barely long enough to clear the linkage with the thought to incorporating your self-valving O-ring design.... Give a though to any headspace saving changes and let me know....

Bob