Spindle design experience shows that the spindle is most durable when actually designed. Just imitate this shape and make it thick in the middle and thin on both sides of the stairs.

From the point of view of saving material and reducing mass, the cross sections of the shaft are preferably of equal strength. But from the point of view of processing, the shape of the shaft is indeed the simpler the better. Simple shaft fabrication saves time, heat treatment prevents deformation, and may reduce stress concentration.

When determining the shape of the shaft, on the premise of ensuring the assembly accuracy, it is necessary to consider both material saving and convenient processing and assembly.

Therefore, in order to facilitate the assembly and disassembly and fixation of the parts on the shaft, and to make each section on the shaft close to the same strength, the shaft is usually designed as a thick middle stepped with fine ends.