The Mohr-Coulomb model is based on plotting Mohr's circle for states of stress at failure in the plane of the maximum and minimum principal stresses.

This paper has given a pithy analytic Proof of "principal diameter circle"Theorem. And on the basis of the thought of the theorem. We obtain two theorem about ellipse normal.

All the tools include principal of the drill tools, overshot assembly, drill rods, holding dog, circle wrench and etc.