A steel helical spring must carry a load that varies continuously from 0 to 900 lb. The steel has a shear yield strength of Ssyp 70,000 psi and an endurance strength of Sse = 40,000 psi in one-way shear. The spring is to have squared and ground ends, a 20% clash allowance, a spring index of C =5, a spring rate of k= 500 lb/in, and a factor of safety of Nfs = 1.5. Determine the wire diameter d, the helix diameter D, the total number of coils Nt, and the free height hf.