1) For the case of a floating connection between the screw tail and the main shaft of the gearbox, due to the screw floating in the barrel, the bending stress caused by the weight of the screw is equal to zero, so only the compressive stress and shear stress of the screw are calculated. 2) Due to the small bending stress caused by the weight of the screw (which is filled with material around the screw), even in the former case, it can be ignored, so these two methods are actually the same. 3) There are also estimates of screw diameter based on torque.
