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Author Topic: Any Interest in 6mm ? - Build Thread  (Read 2119 times)

rsterne

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Re: Any Interest in 6mm ? - Build Thread
« Reply #60 on: January 03, 2018, 08:41:14 AM »
I have been discussing it with Lloyd and we have been trying to analyze the mode of failure and the forces (did you get the photo I emailed you).... When we calculate the load at 4200 psi, and compare it to the shear strength of the Delrin at the bottom of the O-ring gland, we still have about a 2:1 safety margin to shear.... What Lloyd thinks is happening (and I agree) is that the load from the pressure on the O-ring is mostly near the outer edge of the gland.... Since the gland is only 0.060" from the edge of the poppet, that force is "levering" the gland, ie bending it at the base, and a tear starts at the bottom front of the gland, which then rips across the gland at that point.... The solution we came up with (I just need the time to make one) is to move the O-ring 1/16" further towards the throat (providing I have the room, which I think I do).... This will double the thickness of the side of the gland, greatly reducing its tendency to bend, and hopefully allowing it to live at over 4000 psi.... and increase the safety margin in shear to 4 or 5:1....

I looked at extending the brass front portion of the poppet aft and making the gland in that, and facing the back of it with Delrin as a seal.... The problem is that you have to have a 100% seal between the brass and Delrin, or the HPA would simply slide the Delrin back on the stem and it would leak.... I don't see how any method of "inserting" the Delrin would help.... The other solution is to make the entire poppet of steel (or brass on a steel stem) and then install an O-ringed Delrin seat in the valve.... something I would sooner avoid if possible....

My concern, of course, is that if this happened with a bullet loaded, the gun would fire it on its own.... It might not be lethal, because the air is venting through the tiny groove in the stem.... but I don't want to chance it.... I wish I had the ability to test the new poppet at about 8,000-10,000 psi, just to make sure it will stand that pressure without failure....

Bob
« Last Edit: January 03, 2018, 08:45:27 AM by rsterne »
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