The predominant effect of an axial tensile force on a (close-coiled) helical spring is:
Try it first: pick an option, then read the explanation.
Correct answer: D. Twisting
Asked in SSC JE 2012
Detailed Explanation
Cut the spring across one coil. To keep the lower part in equilibrium, the wire section must carry a force W and a moment W·R about the wire's own axis, because the load line (the spring axis) is a distance R away.
Why “Twisting” Is Correct
In a close-coiled spring the helix angle is tiny, so that moment acts almost exactly about the wire axis: it is a torque. The predominant effect is twisting; the direct shear W is small in comparison.
Why the Other Options Are Wrong
- A. Bending: Bending is negligible in a close-coiled spring under axial load (it appears under an axial torque, or in open-coiled springs).
- B. Tension: The wire does not carry axial tension; the force on each section is a shear W.
- C. Compression: There is no axial compression in the wire.
Key Point
Close-coiled spring under axial load → wire in torsion: τ = 16WR/πd³.
Key Concepts for Students
- Spring deflection comes from wire twist.
- Direct shear 4W/πd² is a small correction.
- Wahl's factor adds curvature and direct-shear effects.
Practice more: this is question 3 in Spring MCQs – Part 1. Try the whole set and check every answer instantly.