I have though about another way of approaching this hole deal.

Different types of screw can take different amounts of force before they bind or die. Sqaure thread can take more than triangualar thread. Ball threads much more than square threads. Engineering supplies list different types of screws and the forces that they can cope with.

So how are these limits calculated and can we make the same calculations with wood threads. The problem for me is that I suspect that the calculations will come down to shear again.