There are two types of optical axes, one is called hollow axis and the other is called solid axis. If the material and cross-sectional area are in the same state, then the hollow shaft compared to the solid shaft, its torsion resistance is much stronger than the solid shaft, can bear a larger external force, and the hollow shaft can be used to save material. And if from a circle on the surface of the load stress distribution, solid shaft is the biggest shear stress on the circumference of a circle and the hollow shaft shear stress is very similar, only the middle part of the shear stress and the hollow shaft shear stress is far from your poor by comparison, and solid shaft in the middle of the materials will not be able to play their own role completely, the cost is much higher than for the hollow shaft, at the time of processing program is more difficult. But these two are used in many, very wide range, especially for mechanical packaging is particularly ideal, but if you have to say which is more widely used, it depends on their own needs to make a judgment of the actual situation.