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All Springer/NP/PCP Air Gun Discussion General > PCP/CO2/HPA Air Gun Gates "The Darkside"
Pinning the valve for bulk/pcp. When, why and how
Gippeto:
Thought I might take a little time and put together something to help those folks wanting to do bulk or pcp conversions do so safely.
Going to go over when securing the valve is required, why it's required and how to do the deed. Will mostly be relevant to the Crosman 22xx conversions, but the math component is applicable to any conversion or pcp build regardless of platform. Will try to keep it simple and in some sort of order such that it makes sense.
When do you need to beef up valve retention.
Short answer is whenever you increase the surface area that is exposed to pressure OR increase the pressure.
For Hipac use with the product being used as the manufacturer INTENDED, the valve retention screw needs to be changed to an 8-32 low head socket head cap screw...that's it. :D
You can find the math and reasoning here;
http://www.network54.com/Forum/275684/thread/1328765973/Examining+the+hipac....the
Bulk/pcp conversions that change sealing from the cartridge seal to the tube must be beefed up for safety.
Why is that?
Let's take a look at a stock 2240/2250/2260.
Force is pressure in psi multiplied by area upon which that pressure is acting.
The seal takes place at the end of the cartridge which is .290" in diameter. A circle .290" in diameter has an area of .066 sqin. Multiply this by 1000 psi, and we get a force of 66 lbs. This plus the force required to compress the seal itself (10-20 lbs force) is what is being retained by the stock 6-40 screw.
If we convert that same 2240 to a tube bulked configuration, the seal now takes place at the tube wall, which has a diameter of .748". This works out to an area of .439sqin, and a force at 1000 psi of 439 lbs.
We didn't change the operating pressure, just where the seal takes place, and the force on the valve retention screw took a BIG jump.
On a hot day, the pressure (vapor pressure) of co2 can go MUCH higher...with a corresponding increase in force.
Change this to a pcp configuration, and the pressure can get a lot higher still. @ 3000psi, that .748" tube puts 1317 lbs of force on the valve. An "oopsy" during filling could see 4500psi, resulting in 1976 lbs force.
It should seem obvious that the stock screw which Crosmans engineers intended to hold back less than 100 lbs force is going to be woefully inadequate at these higher force levels and that beefing up valve retention is very much required.
So we need to beef up valve retention. What do we need to do?
There's no stock answer, but the math to figure this out isn't complicated, and should not be frightening to anyone. We'll use a pcp with a tube id of .745" and a maximum operating pressure of 3000 psi as our example.
Area is .436sqin, pressure is 3000psi....force is 1308 lbs.
There are two areas we're going to look at. Shear strength of the screws used to retain the valve, and the bearing stress those screws place on the tube when under load.
Shear strength is calculated by multiplying the fastener tensile strength by .6, I prefer to use .6 x yield strength.
The point at which the screw will fail in shear is calculated by multiplying it's shear strength by the area calculated from the screws minor diameter. Two common screws used to secure valves are 8-32 and 10-32. We'll have a look at the 8-32.
Handy little chart;
https://www.carbidedepot.com/formulas-tap-standard.htm
Minor diameter of an 8-32 screw is .1257"....we'll just use .125" which puts the area at .0123sqin.
All screws are NOT the same. Depending upon which screws you choose, the tensile strength can vary from 55000 psi all the way up to 180 000 psi for socket head cap screws graded ASTM A574.
Lets compare the shear strength of the hardware store variety with the "good stuff".
Hardware store has shear failure at 406 lbs force.
Good stuff has shear failure at 1328 lbs force. Substantial difference yes?
One of the "good" 8-32 shcs (socket head cap screw) would seem to JUST BARELY handle the 1308 lbs force, but leaves nothing, zip, nada for a safety factor. I like a MINIMUM safety factor of 3X MWP (maximum working pressure), so three of the "good" 8-32 screws gives us that with our shear load..and we're happy.
For kicks...the "good" 10-32 has an area of .018sqin and a yield shear failure point of 1944 lbs
So the screws won't shear off...which is good and gives us a warm and fuzzy feeling. What about the tube?
If the fasteners place too much stress on the tube wall, the tube wall will deform...ie...suffer a yield failure. Lets have a look.
This is bearing stress. It can be calculated by multiplying the diameter of the hole in question by the tubes wall thickness to get the bearing area, then multiply bearing area by the yield strength of the tube.
We are going to use the low head variety of shcs, which has a head diameter of .270". Our tube has a wall thickness of .065" and a yield strength of 63100 psi.
Crunching those numbers we get a yield failure in the tube wall at 1107 lbs force.
Three such fasteners bearing on the tube wall will handle 3321 lbs, but this is still less than our minimum 3x safety factor, so something has to be changed.
We can add a fourth 8-32 low head shcs....that will give us 4428 lbs and restore both our safety factor...and the warm and fuzzy feeling.
That covers when and why, but leaves out how...which IS kind of important hmm?
For most folks, it's going to come down to locating the holes for a couple new screws. For others doing a full on custom tube...you've likely got this covered.
One method that I have used in the past to locate the screw holes is with masking tape.
Wrap the tape around the tube such that the edges line up and the ends overlap.
Cut through both layers where they overlap with a razor blade.
Remove the tape and lay it flat. Measure with a ruler and divide by the number of screws. Mark the tape at those divisions.
Re-apply the tape to the tube, centring one of the marks on the existing screw hole and wrap around lining up the edges. Mark the tube and centre punch for the holes.
Now it's drill and tap time. I HIGHLY recommend at least a drill press and vice for this.
When I do this, I do it in the mill. I use a centre drill first, then drill to depth for the screw. Next I mill out for the head of the screw, and finally tap the hole. Cleaning swarf/chips out as I go.
Drill the hole to depth, I prefer to drill for 100% thread engagement. For 8-32, this is a 1/8" drill bit. Don't need to get technical, if you've the equipment to do so, that's great, but can get "good nuff" depth measurement by measuring the one stock hole and marking drill bit with masking tape.
Next we need to bore out for the screw head. A letter "I" drill bit has an od of .272"...close enough for our .270" 8-32. Need a grinder to flatten the cutting end and re-profile the cutting edge. Use this to cut through the tube wall and just skim off enough of the valve to create a flat. The shorter this drill bit is the better...if you can find an "I" starter drill...use that. If you want to spend more, there are cutters designed JUST for this task, and they are the best tool for the job, they're called counterbores and resemble a piloted reamer.
Time to tap the hole. You'll need to use a starter/taper tap to begin, and switch to a bottoming tap. I hold taps in the drill chuck and turn the chuck by hand...unplug the machine FIRST.
1/4 turn of the tap inwards, then back out 1/3 turn to break the chips and help prevent breaking the tap. Can use wd-40 as a tap lube in aluminium.
Break the little chip at the top of the tapped hole, insert a screw to check everything. Adjust depths if needed. I like to install the screw at this point, then move on to the next hole.
Foot note;
I hope this answers some questions folks might have or at least they might find it somewhat helpful. At the time I'm writing this, the airgun community has had two valves depart the pressure tube within the last couple weeks...one fellow was badly injured...it could easily have been two fatalities.
Al
Ribbonstone:
One trick to the HiPac used as it was intended to be uses, is that it only pressurizes the INSIDE of the valve. That's a lot less airea than when the whole tube is pressurized and it acts on the OUSIDE diameter of the valve.
Still, I believe that there is no such thing as "too strong" or "too safe", so the only Crosman's still in use with HiPAcs have BOTH the issue screw replaced with a countersunk 8-32 AND two 8-32's from each side of the tube. Did find that using just the one 8-32/counter sunk screw worked with a 2240 running at lower pressure (2K) air and bulk co2, but I'm in the process of transforming that one into a 5mm (which will get thr 3 screw treatment and run at higher pressure).
sshewins:
Al, well done. Good read. Seeing as how I am really not nearly educated enough in this area, would it be possible to use set screws that are run from the outer tube into the valve, instead of the screws you mentioned? Unless the shear strength of the set/grub screw is pathetically weak, that is. Thanks again for hopefully opening up the eyes of lesser educated airgunners, myself included. Thanks again.
rsterne:
For those of you who wish to see a test of the Disco valve screws, and some better alternatives, here is a video.... http://www.gatewaytoairguns.org/GTA/index.php?topic=72672
There is a summary of the tests on page 2 of that thread.... The OEM Disco 8-32 valve screws failed at about 1000 lb.f. each, which puts them in between Al's numbers for cheap screws from the hardware store and the high-tensile low-profile screws that are preferred.... Three of them, assuming they are all taking equal loads (which pretty much happens as they settle into the pockets in the tube before they fail), would have a 3:1 safety margin at just shy of 2300 psi.... One of those screws is NOT recommended for a Bulk-Fill CO2 setup, let alone HPA.... it would have a MSWP of about 750 psi....
Bob
HappyHunter:
Excellent post, Al....well said. :)
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