The factor gives the relation between the torsion stress of a spring in the fully loaded state and the allowable torsion stress, such as uS»t8 / tA. If a greater value is selected, less material is needed for spring production, the spring dimensions, and the space for mounting are less, but the securing of spring stability during its function is lower, and vice versa. Therefore this factor is a reciprocal value of the safety rate. For common operational conditions, the value recommended for the utilization factor of the material is in the range of uS = 0.75 ... 0.95. You can use lower values for springs working in aggressive surroundings, at high temperatures, or loaded with impacts.