A closely coiled helical spring is subjected to an axial load. Predominantly, the wire of the spring is under which type of action?

Introduction / Context:
Helical compression and tension springs are ubiquitous in machines. Understanding how an axial load translates into stresses within the spring wire is vital for sizing and fatigue analysis.

Given Data / Assumptions:

• Closely-coiled helical spring (helix angle small).
• Axial load applied along the spring axis.
• Standard thin-wire spring theory (Wahl, curvature effects ignored for first order).

Concept / Approach:
In a closely-coiled spring, the axial load primarily produces a twisting moment in the wire. Hence, the wire experiences torsion, which induces shear stress. There is a small direct shear component, but torsion dominates the stress state and governs design.

Step-by-Step Solution:

Verification / Alternative check:
Energy methods and Castigliano’s theorem yield the same deflection when torsional strain energy dominates, confirming torsion as the primary action.

Why Other Options Are Wrong:
Bending and crushing do not represent the principal action in closely-coiled springs; “direct shear only” ignores the dominant torsion.

Common Pitfalls:
Neglecting curvature correction (Wahl factor) and stress concentration at the inner fibre in detailed design.

Final Answer:

Torsion (resulting in shear stress in the wire)