When the spring is twisted, the material is in tension or shear state, such as the calculation of torsion degree mentioned above. Therefore, the torsion angle is limited by Z due to the failure of the material. When the material is plastically deformed, the original state cannot be restored. Therefore, Z is summed up as the tensile strength and shear strength of the material. Of course, the von Mises stress calculated by the fourth strength theory is the maximum Z strength that the material can bear. Combined with the length of the spring, the allowable torsion angle of Z can be calculated.
