3. A helical compression spring with plain ends is to be designed so that a force of 80 N leads to a deflection of 3 mm. It is made from wire with a diameter of 10 mm = 480 MPa. and spring index of 7. The maximum allowable shear stress is Ssy The modulus of rigidity is 80 GPa. How many coils should this spring have? Note: this does not have to be an integer. N = 4. For the spring of the preceding problem, if the factor of safety at solidity is n, = 1.2, what is shear stress at solidity? MPa. 5. For the preceding problem, what is the free length? Lo mm.