Question for the brain trust.....

Tony Wells

Tony Wells

President Emeritus (Retired)
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Working on a small project, and today a slight modification to the mechanism came to mind. So I pose this question: What is available (short of making from scratch, which I can, but rather not do) in the way of a low crack pressure check valve, on the order of working on a 6mm line, with a crack pressure or ~1 lb. While at the local Parker distributor, I asked about one, and they only offer a 5 PSI crack and I'm quite certain that is too high.

For a general idea of what I am building, it is a peristaltic pump driving a hydraulic cylinder. At both ends of the stroke, I want a safety in the way of a bypass so I don't dead-head the pump. I want to avoid complications such as an electronic solution where the current rise when the pump does dead-head. I could do that, but it's just added components to deal with, and more chance of breakdown. Plus, the possibility exists of accidental pressure being placed on the system by manual manipulation, so there should be a "safety" to prevent any damage from someone forcing the cylinder. The electronic solution wouldn't protect against that. There is not room for limit switches. Or if anyone has another idea, that would be welcome too.
 
When I was a kid, my Dad had a friend who made check valves in his garage on a turret lathe and I went over there once and his friend showed me how he made them. It was a neat idea and I recall it as if it was yesterday, but it had to have been 50 years ago. I just looked on the net and could not find anything. It was a shaft that looked like it was 3 1/2 to 4" long with 6 sized holes step drilled in it. I re-call (aprox sizes) it had a 5/16 at the bottom then 3/8 hole with ball then a 7/16 hole to 1/2 hole and ball in the middle, then a 5/8 hole and ball. Each step was a beveled. He would mount the shaft vertically and would load the shaft with ball bearings with no springs like an ordinary check valve. Thinking about it now the 5/8 hole must have had a tapped hole too for a seat that the bigger ball could not shoot out. But the weight of the balls was all that created the check pressure.
I hope this makes sense? I will keep looking for it now...bugs me now...lol.. :-) Rich

PS: Must have had tapped pipe thread on the ends too....
 
What is the fluid, temperature, pressure? A simple swing check could work but the common ones have fluid/pressure/temp limitations.
 
As I was reading your goals I was thinking right along Richard's idea. Dead weight. I'd calc the area needed on the bottom of a poppet valve disk, make the seat and valve disk to suit (size and material wise). It'd make a simple vertical inline solution. Balls would work but you'd need a range of them to get it right and they are harder to make.:))

I do not know of a commercial answer at ~1 psi and i suspect if one was found it'd be a safety valve vice simple relief and pricey.

Steve
 
... I wonder if it's possible to buy small springs with a calibrated constant. If so, F=pA (Force = pressure X Area) and with springs, F = kD (Force = SpringConstant(k) X Distance(D)). If you use a ball bearing, you could calculate the exposed surface area or just approximate it with Pi^^2 x radius with radius calculated from the hole drilled in the seating surface. You could load the spring with a small set screw and fine tune it to give-way at 1lb. Of course, you'd need to make a small cylinder to house the BB and some relief holes drilled in the side to vent to atmosphere. -Fun little project.

BTW: Area of a sphere is 4/3 X Pi X Radius^^3.

Ray

EDIT: Upon thinking about this, you wouldn't need a BB... -maybe just a small flat piece of metal suitably shaped to fit in a hole. -Possibly a small rod with a shoulder on it... -You get the picture.
 
I just love "crowd-sourcing"! OK, first of all, thanks for the responses. As I said, I'd rather not make one, but of course, I can and very well may end up doing that. One thing I didn't mention is that it probably needs to be adjustable.

Richard, a design like that would have to be vertical to work, and each stage vented to flow. I might have room for something like that, and use a single stage and just add weights on top of the ball to tune it. Worth considering if I build.

steve, the fluid is either 99.5% anhydrous glycerin or a slightly aqueous solution. Temp is ambient room, approximately 65-75° f.

Ray, as I said, I could build one, but was really looking for an off-the-shelf solution.

Bill, thanks for the link. I'll be exploring that site/source more.

Ken, thanks for that link. The guys at my local distributor didn't find that one in their system. P-H does have a lot of smaller parts though. I have bought few pieces for my GC at one, so I guess I should have pushed the guy a little harder, but I was in a hurry at the time.

All in all, If I build one, it will be adjustable, either with a spring an screw, much like a pressure regulator, or weights. I've used hundreds of check valves of larger sizes and I don't believe a flapper/swing type will do what I want, unless I were to also build it, with a soft (buna or something) face to seal, and a spring assist. Or it too would need to have a vertical mount. I'll use a ball rather than a cone, or a disc, I believe. Again I'm going to shop around a little more.

Thanks again, guys. When I get this project finished, I'll show it here. It's not really much of a machining project, but there is a little in it.
 
Thanks, Ken. The price isn't bad on that one. If I can find one, I'd rather have an adjustable in the range of 1 PSI. I don't have the total weight of the elevated portion of the mechanism, so it's just a guess at this point that it will be near 1 lb.
 
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