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rsterne · Moderator

I have been asked to post the safety mods which can be done to a QB79 tank block when it is used for an HPA conversion on either a QB or 22XX.... Here are the pics:

The two screw holes closest to the drop portion of the block are the original setup.... but the screws supplied are phillips pan head and very soft material.... I have replaced these with 4mm high tensile socket head cap screws that are longer.... Part of the problem is that these screws are only 3mm from the end of the tube (1.5xD = 6mm is the recommended design minimum, 2xD = 8mm is better).... and hence prone to tearing out of the end of the tube.... The two accepted methods for securing the block are two (or three) 8-32 socket head set screws set into countersunk holes in the block so that the heads are in shear.... or a shear pin through the entire assembly.... Either option is located between the O-ring grooves.... I did a combination of both.... Shocked

The problem in using 8-32 screws on the sides in large holes is that by the time they are far enough from the rear O-ring groove, they are too close to the original holes in the tube.... That is why I used a 3/16" D stainless steel shear pin instead.... The flat in the middle of the pin is so that the screw in the top can retain it (it's a push fit, not a press fit).... Since there is no original hole in the top of the tube, I put a 1/4" long 8-32 SHCS there counterbored 0.080" (half the head thickness) so that the head is in shear.... I used a 1/4" end mill to make the counterbore, and will use a 1/4" hole in the tube.... I turned the head down slightly (from 0.265" TO 0.250") so that it is a snug fit in the holes.... The shear pin and the bolt hole centerlines are 0.380" from the center of the original bolt holes, which puts them 0.577" from the end of the tube.... It was necessary to shorten the 8-32 bolt slightly so that it barely touches the shear pin when tightened.... Wink

When pressurized to 3000 psi (as would happen in a regulator failure on a 3000 psi tank), the end force on the tank block is 1325 lbs.... In theory, either of these modifications should be strong enough, but together they have a significant safety margin.... I'm not going to guarantee that this method is safe.... but I feel confident in it.... Note that the O-ring closest to the end of the tube is no longer used.... Only one O-ring remains available for sealing, so obviously everything must be perfect.... Mr. Green

High Pressure Air is dangerous.... proceed at your own risk !!!

Bob