Question

A Spring is made of wire whose cross-section is an equilateral triangle of 8 mm side. Inner diameter of spring is 40 mm then outer diameter will be _________

(a) 57.88 mm

(b) 54.88 mm

(c) 60 mm

(d) 56 mm

I got this question in an interview for job.

This interesting question is from Construction of Helical Springs topic in section Curves used in Engineering Practice of Engineering Drawing

Answer 1

Right option is (a) 57.88 mm

The explanation is: Outer diameter = inner diameter + (2 x diameter of wire), Here wire has cross section of equilateral triangle of side 8 mm so it covers a length of 8.94 (square root (82 + 42 )) mm from inner to outer end of spring. Outer diameter = 40 + 2 x 8.94 = 57.88 mm.