How do you calculate Hertzian contact stresses between curved surfaces?

Hertzian theory gives contact stress between elastic bodies: for sphere-sphere contact, maximum pressure pmax = (3P)/(2πa²), where contact radius a = ∛(3PR/(4E*)). E* is equivalent modulus and R is equivalent radius. Maximum shear stress occurs below surface at 0.48a depth, explaining subsurface fatigue failures in bearings and gears. Contact area is elliptical for general curved surfaces.