The free end of a torsional spring deflects through 90° when subjected to a torque of 4 N-m. The spring index is 6. Determine the coil wire diameter and number of turns with the following data : Modulus of rigidity = 80 GPa ; Modulus of elasticity = 200 GPa; Allowable stress = 500 MPa. this question is of design of torsion spring from the book machine design RS khurmi ... answer is diameter of sring wire 5mm, and 26 number of turns... no more data is given other than this..........