Foster Style Check Valve?

[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

[rsterne]

It seems to squish to 50% easily

Hmmmmmmm.... not good, that would require about 0.035" stem end clearance.... or maybe not, perhaps the O-ring will just try and fill the gap without the head of the stem moving much.... dunno....

Bob

[AirGunEric]

If you use the o-ring normally found on the valve poppet from a QD connector, it is polyurethane and should not squish that much.

[rsterne]

Thinking about how the O-ring should seal.... it should not be an issue anyway.... The O-ring should seal against the end of the valve body and the stem, there should not be any force on the metal head of the stem....

Bob

[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]

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

[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]

Some thoughts on the pump head:

Somewhere to start....

Starting with a piece of rod the correct diameter (with tube clearance in mind) and faced off nicely, drill a hole (1/16") running length wise starting at the face of the rod. This hole will be off center such as to intersect the bottom of the o-ring groove yet to be made. (will depend on series of o-ring) This hole should end where the inlet side of the o-ring groove will wind up.

O-ring groove should be as far toward the valve side of things as practical... .010" max. O-ring groove should be o-ring cross section + .005". Could likely get away with less...this is ~ 3x the area of the drilled passage.


That's where I'd start anyways....piece of cake to make the groove or hole larger...somewhat more challenging to shrink them.


Hope that makes some sense...it's been a friggin long day and I'm about done in...goin to bed damn soon.

Al

[SeanMP]

O-ring groove should be o-ring cross section + .005". Could likely get away with less...this is ~ 3x the area of the drilled passage.
Al

My experience is that you'll get better O ring life and a better seal with a wider groove than this. ASTM/SAE calls for this particular O ring universal #111 (0.103"CS) to have a groove width of 0.141 to 0.149 in a moving piston application.

Because the O ring seals against the side wall and bore wall by pressure extrusion in this application having a wider groove and the ID right on minus spec allows the Oring a space to conform into during the non pressure stroke.

Assuming
Piston OD 0.613 to .615
Groove ID .431 to .437"
Groove width .125 to .149"

Going towards the wider groove will allow leak past on the non pressure stroke.

[rsterne]

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

Bob