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All Springer/NP/PCP Air Gun Discussion General > "Bob and Lloyds Workshop"

Poppet/check valve diameters?

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Nvreloader:
Is there a perfect size dimensions for Peek poppets and check seals for
 .058" - .102" - .140" and .156" holes used in valves,
with .180" throat dia?

I have to build new check and poppet seats with a .375" hole, flat faced,
to get rid of the OEM tapered seats, with the original check/poppets of .320" dia.

rsterne:
Sorry, I don't understand how the poppet can be smaller than the throat?....

Bob

Nvreloader:
Bob
This (PUMPER) valve has a .400" dia tapered cone shaped sealing holes that are .500" long,
the sealing check and poppets are also coned tapered shaped, that are .320" dia
and fit inside these tapered holes for sealing. They jamb and stick very tight,
I am finding out.

See photo here: https://ibb.co/8b8nZD7, click to enlarge, page#17, post# 332 for all the photo's, https://www.gatewaytoairguns.org/GTA/index.php?topic=195725.320.

My plan is to use a .375" counterbore with a piloted shaft .118" dia to fit the valve stem hole,
by going deep enough to make a new flat faced seats .375" dia, (in the OEM tapered holes),
and then make new Peek poppets/check seal to fit with/in this new .375" hole,
that leads into a .180" throat.

nervoustrigger:
AFAIK, the main consideration is having sufficient margin / overlap so the material does not risk extruding and failing due to the compressive load carried by the thin rim.  The property that describes this factor is the material’s compressive strength.  For PEEK, it is generally about 15,000psi. 

A previous write up I drafted...

While we are on the topic of making a reliable, long-lasting poppet, the other thing I wanted to comment on is the sealing margin on the poppet's OD. In other words, how much overlap onto the valve throat do you need to ensure the material will not extrude over time? This will take a couple of paragraphs to explain and work through an example but after you've done it once, you'll realize it's actually quite simple.

The compressive strength of PEEK is typically about 15,000psi which means we want to stay well under that. I like to expose a poppet to only about half that much on account of there being a dynamic element to the poppet snapping closed [see footnote 1 below].

But how how do we figure how much load it's seeing? Well, we first look at the force holding the poppet closed. That force is calculated as the throat area times the operating pressure. So let's put some numbers to it...let's say we have a 0.230" throat and running at a regulated pressure of 2000psi. The area of a circle is pi * radius squared. So a 0.230" dia circle has an area of pi * 0.115^2 = 0.0415in^2. 2000psi operating against 0.0415 sq. in. is 2000*0.0415 = 83lbs.

So we have 83lbs of force squeezing the poppet closed. Our poppet is a circle that overlaps the throat by some small amount. Meaning this 83lbs is supported by just the rim of the poppet. Let's take a stab at it and guess that maybe a 0.020" overlap might be adequate. Twenty thou ain't much but let's see. This 0.020" overlap on the radius doubles to 0.040" in terms of the poppet's diameter, meaning our poppet is the 0.230" throat plus 0.040" = 0.270" dia. The area of this poppet is therefore pi * 0.135^2 = 0.0573in^2. Subtract the throat area of 0.0415in^2 and we have a rim of 0.0157in^2 supporting the 83lbs of force. So the load on the PEEK material is 83 / 0.0157 = 5300psi. 5300psi is well under PEEK's 15000psi rating and also under my preferred "half of 15000psi". So you might consider going a little more aggressive and reduce the poppet a few thousandths but unless you have good reason to think it will help with the valve's flow characteristics, that would probably represent a high risk/low reward scenario.

All of this was for a flat valve seat so you can think of it as a worst-case. Meaning if you go with a conical seat, I think there is an angled wedging component to the force acting on the poppet that would tolerate a slightly smaller sealing margin. But again, in most cases there is no compelling reason to shave down the sealing margin to the razor's edge of what is reliable.

[1] some discussion on that topic https://www.gatewaytoairguns.org/GTA/index.php?topic=180328

Nvreloader:
Jason
Thank you  for that information,
I'll read it a couple times to make sure it sinks in,   8)
It is greatly appreciated.

Don

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