A composite spring has two close coiled helical springs connected in series; each spring has twelve coils at a mean diameter of \(24 \mathrm{~mm}\). Find the wire diameter in one if the other is \(3 \mathrm{~mm}\) and the stiffness of the composite spring is \(640 \mathrm{~N} / \mathrm{m}\). Estimate the greatest load can be carried out by the composite spring, and the corresponding extension, for a maximum shearing stress of 160 \(\mathrm{N} / \mathrm{mm}^{2} . \mathrm{G}=75,000 \mathrm{~N} / \mathrm{mm}^{2}\).