GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: rkr on October 19, 2021, 08:01:31 AM
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Since my 2mm stem valve for .257 Evanix was a great success I started thinking of small diameter stems for other guns as well. Starting with the material and hardness, is HSS good material what HC hardness it should have? Aliexpress has lot's of HSS rods with different diameters and stated HC varies from 61 to 67. Then comes the stem diameter, 2mm seems to be fine for a 7.5mm valve tunnel. My .45 has 10mm tunnel so I guess it would need larger stem, 2.5mm or 3mm - what do you think? How about small diameter valve tunnel, could we use 1.5mm stem in a 6mm tunnel? I'm trying to figure out the limits of this small diameter stem approach, feel free to contribute.
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I usually use O1 steel rod for my valve stems.... I have never bothered to harden it, and I have never seen it bend or peen over.... I don't know what the limits are on how small you can go.... You already found one problem, with your small stem penetrating the PEEK poppet head.... I often thread the end of my stems into the poppet to avoid that.... When you talk about using very small stems, remember that the resistance of the stem to bending changes with the 4th power of the diameter.... Half the diameter is 16 times easier to bend, for the same length....
Bob
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While there are some balancing ratios where the stem plays a part ... that alone does not dictate stem size in my tuning.
Driving the stem ( No matter its size ) threw the poppet head becomes an issue when poppet head over the throat increases and stem is kept small. There is so much effort to open said bigger poppets the stem becomes a nail and can punch its way threw the poppet head. With smaller throats and poppet heads, small stems can and do work just fine. * Pressure being run at contributes greatly to the above paragraph.
Stem Hardness .... What are you striking it with ? Peek face strikers can operate against milder / softer steels. When hammer strike face is metal / steel, the stem needs to be harder to prevent mushrooming.
JMO .. sharing thoughts.
Scott S
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Is the smaller stem diameter wanted for more flow in throat or less closing force?
Some of the driving through large poppets issue can be resolved by making a steel cup that sets in poppet.
HSS would work but can be a pain if you want to turn it down. A simple cheap source for longer pieces is extended length drill bits. I’m not sure of the hardness but they do resist bending much better than most alloys. It is pretty tough and forgiving. I think it would work well.
Dave
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A smaller stem reduces the closing force.... If the throat is marginal to allow full flow around the larger stem, then you could also get an increase in flow.... Using both ideas together, using a smaller stem allows a smaller throat and poppet, and hence also a reduction in opening force.... This can lead to a further reduction in closing force from reducing flow friction past the smaller poppet head....
Bob
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I was just curious what the op’s intent was. Maybe he has a link to his thread on it.
Dave
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Ive used a 2mm stem where the application allowed it .. The obvious reason was to reduce opening force by proportionately reducing the throat , stem , and poppet.. it worked well , the valves flow was the same as before with less cracking force.. I think the logevity relies on how far the stem protrudes from the rear of the valve , the tolerance of the hole the stem passes through at the rear, and as mentioned above , the stem not punching through the poppet.
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I was just curious what the op’s intent was. Maybe he has a link to his thread on it.
Dave
Sorry, I should have linked the thread https://www.gatewaytoairguns.org/GTA/index.php?topic=147066.180 (https://www.gatewaytoairguns.org/GTA/index.php?topic=147066.180)
The purpose was to reduce the hammer spring force without going to a balanced valve. With the set screw the hammer penetrating the poppet problem goes away leaving me wonder how low I can go and other things mentioned in the first post. Current stem is 2mm HSS bar/rod and it does bend a bit so I was wondering if I should use something better for the stem. A cup is indeed another option that I used in my first 2mm stem BSA valve that didn't give me the results I was hoping for, most likely due to a heavy duty valve spring.
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I'd imagine that for optimum bend/mushroom resistance, that S7 shock-resistant tool steel would be best, as it is used for punches meant to be beaten on in that fashion...
Jesse
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Whether it was truly necessary or not, I got into the habit of hardening and tempering my stems when making new poppets. It's easy in the home shop with W1 steel with nothing more than a propane torch. Heat the stem to glowing red and drop it into water. Then retrieve it and reheat it to a straw color and set it aside to let it cool slowly back down to room temperature. The result is a very hard and bend resistant stem for a couple of minutes effort.
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I can't really find a source for those O1, S7 or W1 steels here locally and e-bay gives ridiculous prices. Aliexpress has HC67 HSS rods or I could get extra long HSS drill bits, which one would be better option for a valve stem? IIRC my current 2mm stem is just HSS bar so probably high fifties in HC as it was flexing a bit when I pushed it into the the poppet (200mm piece that I was pushing).
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Music wire becomes another source to look at if you are having difficulty getting materials. As long as it comes straight and not rolled up.
I believe you may be overly focused on the hardness. As nervous mentioned, it’s good practice to harden and temper which gives a bit softer material but allows it to spring back to shape if it experiences slight bending. It’s a trade off.
I really think the HSS is the way to go.
One suggestion would be to keep the actual weight of poppet to an absolute minimum. Not only does the stem have to crack the seal it has to accelerate the mass of the poppet. The set screw weight may be working against you. Just a suggestion.
Dave
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Music wire becomes another source to look at if you are having difficulty getting materials. As long as it comes straight and not rolled up.
I believe you may be overly focused on the hardness. As nervous mentioned, it’s good practice to harden and temper which gives a bit softer material but allows it to spring back to shape if it experiences slight bending. It’s a trade off.
I really think the HSS is the way to go.
One suggestion would be to keep the actual weight of poppet to an absolute minimum. Not only does the stem have to crack the seal it has to accelerate the mass of the poppet. The set screw weight may be working against you. Just a suggestion.
Dave
I guess you are right, I should get shooting it instead of worrying about the stem material. After all springy steel at around 55-58HC works great for swords so why not for valve stems. I also minimized the free length of the stem by making the brass "screw" as long as possible without limiting the flow so that should help as well. Time to cast some bullets before our next range session.