threads in shear plane , grade 8 8-32

[Rob M]

Need a bit of help with some math.. Tube is crosman steel 750 o.d 625 i.d .. stock valve pinned opposite ends with 8-32 grade 8.. Not cap heads .. Ill add picture.. Fill pressure will be 2kpsi even.. The retention tab in the tube is gone so all 611 lbs is resting on these 2 screws with threads in shear plane.
any help appreciated .thanks in advance. Im thinking i may need a third.

20190624_200050 by murphyrobert9, on Flickr

[rsterne]

I can't tell from the photo, are those screw heads down against the valve body, so that the outside of the head is in the hole in the tube?....

For 611 lbs. of force, it would appear you are running 2000 psi, correct?.... It would appear that two Grade 8, 8-32 screws, with the threaded portion in shear, should have a 4:1 safety margin, assuming they are both taking equal loads....

Bob

[Rob M]

they were done simultaneously .. so they should be sharing the load perfectly.. no the heads are not down in the tube .. theyre resting externally on the tube under some torque.. so the threads are in the tube and the valve body... 2000 is correct.. im not sure i wanna go back and open the tube wall holes to 8mm, the odds of me locating the holes that well all over aagain might not be perfect like the holes are now.

[rsterne]

OK, got it.... so the hole in the above photo without a screw (facing the camera) is NOT like the two on the sides, correct?.... Basically you just have two holes in the tube (about 0.160") for the threaded portion of the bolts to pass through and they are tightened into the valve body....

The shear calculation I gave you above should be correct.... but the loading of the screw threads on the tube wall will likely cause the peaks of the threads to press grooves into the rear (loaded) side of the holes, because the compression loads will probably exceed the yield point of the steel tube wall.... If there was a full diameter shank on the screw where it bears against the tube wall, it would have about a 2.5:1 safety margin to yield, which would be fine.... Having the screws tightened against the tube may well keep them from denting matching grooves in the tube wall.... In any case, this would not likely be a safety issue....

Bob

[Rob M]

OK, got it.... so the hole in the above photo without a screw (facing the camera) is NOT like the two on the sides, correct?.... Basically you just have two holes in the tube (about 0.160") for the threaded portion of the bolts to pass through and they are tightened into the valve body....

The shear calculation I gave you above should be correct.... but the loading of the screw threads on the tube wall will likely cause the peaks of the threads to press grooves into the rear (loaded) side of the holes, because the compression loads will probably exceed the yield point of the steel tube wall.... If there was a full diameter shank on the screw where it bears against the tube wall, it would have about a 2.5:1 safety margin to yield, which would be fine.... Having the screws tightened against the tube may well keep them from denting matching grooves in the tube wall.... In any case, this would not likely be a safety issue....

Bob

thanks Bob.. the hole seen on camera is the transfer port.. yes the 2 side pin  holes for set screws are 160 ish threaded 8-32 top to bottom.. I see what you mean about solid shanks, but i would have to forego the torque on the screws since there would be less threaded length to really grab the valve. All compromises..