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Behaviour of self-drilling fasteners under shear (specifically fastener unwinding)

Behaviour of self-drilling fasteners under shear (specifically fastener unwinding)

Behaviour of self-drilling fasteners under shear (specifically fastener unwinding)

Hi all,

This is more of an academic exercise at the moment but I'm looking into the mechanisms of loosening of fasteners under temperature actions for an upcoming project and came across a paper by Auburn University for NASA (https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa....) which explains that the fastener + nut assembly can be idealized as a bolt sitting on an inclined plane. Given some room for sideways movement, any side shear would only need to overcome the friction resistance for the bolt to start unwinding.

I've assumed that the same theory can also be applied to self-drilling fasteners where nuts are absent. (Do correct me if I'm wrong tho !)

My question is, could the direction of the threads in relation to the applied shear affect the overall outcome ? That is, if the direction of the shear force is against that of the thread, and if shear force is still greater than the friction, could the fastener then begin to tighten instead ? This is, of course, assuming that other effects (such as ratcheting) are ignored.

I've no previous experience with bolt/fastener design so all opinions are welcome.

RE: Behaviour of self-drilling fasteners under shear (specifically fastener unwinding)

Self-drilling self-tappers are normally only for sheet-metal or similar applications. That type of connection puts an upper bound on how much torque (i.e. preload) that you can apply, and it also limits the amount of force that could be applied in the direction of pulling the joint apart. (The sheet metal parts will just bend)

If you have a single self-tapper holding two pieces of sheet metal together and forces lead to the two sheets twisting relative to each other, that's going to get you in trouble ... but real world applications of this type of screw never rely on just one to hold everything together!

I've seen self-tapping machine screws, but their design is such that it becomes a plain ordinary bolted connection with a bolt screwed into a tapped hole. The part of the screw that cuts the threads ends up sticking out past the tapped hole, but is irrelevant in terms of looking at the loading.

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