Unless I have misread your post, it is not clear that you have documented 'failure' of the bolts. If I have misread your post, my apologies. You say that the 'bolts were gone'. Are you saying that the threaded bolts were completely missing from the STA? That would be extraordinary - for the bolts to be completely missing, both bolts would have to shear right beneath the bolt head, so that the STA would cleanly separate, the bolt heads would drop out from under the cam band threaded through the slots in the STA, AND the bolts, with wing nuts attached, would then have to drop out of the backplate. Not impossible. But, improbable. And, the fact that, per your description, the wing was still attached to the STA, suggests that the bolts were still in place on the STA.
In my experience, it would be equally probable that the wings nuts simply came unscrewed.
1) Two stainless steel bolts with wing nuts (how they failed is beyond me this was dive 30 on this setup)
I am going to presume something here - that you had kept your rig assembled the whole time, rather than breaking it down after each dive trip.
IF that was the case, there is no surprise. The wing nuts get loose over time. Yes, there is - or there certainly SHOULD BE - a lock washer (hopefully present, in your case) between the wing nut and the flat washer. Nonetheless, as a ROUTINE part of your gear set-up, you should check the wing nuts for tightness.
IF you routinely disassembled your rig between dive trips, then the culprit is most likely a failure to appropriately tighten the wing nuts when you assembled your rig.
apivonka:
2) All the load was on the two bolts. Failure was the tank separated from the back-plate.
Actually, as I understand your post, the failure was that the STA separated from the BP, and the cylinder was attached (by cam band) to the STA.
aviponka:
This will put all the weight of the tank on the back-plate + cam-bands and not on two bolts.
Yes, that is true. But, the issue is most likely NOT the weight on the bolts. It is whether the wing nuts were sufficiently tight.
apivonka:
Certainly, if you wish to thread your cam bands through the plate, and avoid use of a STA, that is fine. If you wish to use a STA, then you need to regularly check the tightness of the wing nuts.
I routinely use a STA. I have had wings nuts come loose several times - when
I failed to check their tightness before a dive. In each case, that was user error on my part, not equipment failure. In each case, I sensed there was a problem simply because the cylinder seemed to 'swing' from side to side during the dive. On one occasion, I even executed that OW skill that some divers suggest is completely unnecessary - I removed the scuba unit underwater (at 110 feet), tightened the wing nuts, re-donned the scuba unit, and continued the (otherwise uneventful) dive.