Thread Formula

Folk's,

I am trying to understand the whereabouts of a particular factor in a calculation.

The normal Shear calc'n would be 0.5 x Pi x D x L, but the one I have here shows 0.577 for Stub Acme and 0.608 for Acme. Where do these factors come from...?

Also, I cannot quite get my head round why we use 0.5 anyway... ok the thinking is that in Shear, only half the thread engagement is under load, but why... the 'gap' between thread flanks is tiny compared to the thread wall, so removing all those 'gaps' surely does not amount to half the thread engagement...?

Appreciate your comments.

Cheers,

E. Wood