Originally Posted by
lysander
It all depends on how you see them fail.
- Screw head breaks just under the head. These are usually the result of over aggressive staking that winds up side loading the screw head (FM 1). This is why the diameter of the head needs to be controlled very tightly so there is little play between the head in the counter bore. The other possibility is the screw was over-torqued during assembly (FM 2). If you look at the fracture surface, you can tell which failure happened, An over-torque will result in the 45 degree shear plane, side loading in a 90 degree plane.
- Screw is loose. This can have several causes: (FM 3) Insufficient torque to start with. (FM 4) Dirty mating surfaces during assembly giving a false clamping at specified torque. (FM 5) Over-torque, but instead of cracking the screw, the threads yields. (FM 6) threads yield under service loads (in this application, highly unlikely as the forces in the axial direction are rather modest). (FM 7) Screw un-screws itself under service conditions. (FM 7) Tampering, or unauthorized removal of the screws.
In the AR design, only the last the last two failure modes are mitigated by staking, so the argument to not stake does have merit. The primary reason the Army wanted staked screws was to prevent Private Snuffy from thinking it was okay to remove and clean under the carrier key (or, maybe Sergeant White-Gloves, and Lieutenant Inspection).
From what I have seen in broken screw heads: side loading is the biggest problem.
The biggest advantage I see with Ned's OCKS knurling, is that it gives excessive material displacement most people seem to feel is necessary a place to go before it starts to bear in the head of the screw and side-load it. (And, keeping the head dimension consistent, that itself is worth a good bit.)
If you read the drawing requirements, staking is only supposed to add 5 - 45 in-lbs to the removal torque. That does not require much in the way of material displacement.
Oh, and if you use a MOACKS, or similar tool - Tighten opposing screws at the same time, do not tighten down one side, then tighten down the other.
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