Torsion spring: how it works and how to calculate the torque

A torsion spring doesn't push or pull: it rotates. The legs apply torque about the spring axis, and the wire works in bending, not shear like axial helical springs. It's the spring of clothespins, hinges and counterbalances.

Angular rate and torque

A torsion spring's stiffness is expressed as torque per degree of rotation. It depends on the material's elastic modulus E, the wire diameter and the coil count. Torque grows with the applied angular deflection.

Wind direction matters

A torsion spring should only be loaded in the direction that closes the coils (winds tighter). Loading it in the opening direction reduces life and capacity. That's why the wind direction (right or left hand) must match the application's working direction.

Design cautions

• Always work in the coil-closing direction.
• The inner diameter shrinks under load — leave clearance on the arbor.
• Longer legs increase the lever arm and lower the required torque.
• The bending Wahl factor (Ki) concentrates stress on the inner fibre.

Tool

In the designer you set wire diameter, outer diameter, coil count, leg length and leg angle — and get the angular rate, max safe torque and quote instantly.