[Dick Linn]

This might be useful: www.eng.auburn.edu/users/marghitu/Screws.pdf

For other than a square thread, "The angle increases the frictional force by the wedging action of the threads".

Also see "Friction coefficient Bolted Joints" partway down here: http://www.roymech.co.uk/Useful_Tabl...o_of_frict.htm