Design a steel helical compression spring to satisfy the following requirements:
Spring stiffness $(k) \geq 8000 \mathrm{~N} / \mathrm{mm}$
Fundamental natural frequency of vibration $\left(f_{1}\right) \geq 0.4 \mathrm{~Hz}$ Spring index $(D / d) \geq 6$ Number of active turns $(N) \geq 10$
The stiffness and fundamental natural frequency of the spring are given by [1.43]:
$$
k=\frac{G d^{4}}{8 D^{3} N} \text { and } f_{1}=\frac{1}{2} \sqrt{\frac{k g}{W}}
$$
where $G=$ shear modulus, $d=$ wire diameter, $D=$ coil diameter, $W=$ weight of the spring, and $g=$ acceleration due to gravity.