Question

Spring index is given as 12.5 and diameter of wire given is 5 mm. Mean diameter of coil is _______

(a) 60 mm

(b) 62.5 mm

(c) 6 cm

(d) 56.2 mm

The question was posed to me in an international level competition.

My question is based upon Construction of Helical Springs in section Curves used in Engineering Practice of Engineering Drawing

Answer 1

Correct answer is (b) 62.5 mm

To explain I would say: Spring index is the ratio of the mean diameter of coil to diameter of wire. 12.5 = mean diameter of coil / 5mm, mean diameter of coil = 12.5 x 5 mm = 62.5 mm. We need to use same units while substituting in formulae.