Design a handgrip strengthener of the type shown in Figure P12.65D. The force required to rotate the handle from the initial unstressed spring position to the retained position is Fi, and to the final position is Fmax. The initial handle rotation is u, and the handle rotates 45° from the retained position to the final position. The steel spring wire diameter is d, and the spring mean coil diameter D = 25 mm. Consider using four turns in the spring coil. The grip handle is molded from an engineering thermoplastic. Calculate the nominal bending stress for the maximum force configuration, select a specific material, and calculate the safety factor for a torsion spring design providing infinite fatigue life.
