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Q4. A valve is fitted with two close coiled helical springs, each of the same material and with the same free length. The outer spring has 14 coils, mean diameter 120 mm and wire diameter 8 mm. The inner spring has 20 coils of wire diameter 5 mm.

Each spring is to have the same shear stress when the springs are compressed.

Calculate EACH of the following:

(a) The mean coil diameter of the inner spring;

(b) The stiffness of the combined springs.

Note: Modulus of Rigidity for spring material = 70 GN/m²