Torsion Spring Design Guide: Legs, Wind Direction, and Torque Specification

Torsion springs deliver rotational force and appear in hinges, clothespins, garage doors, and counterbalance mechanisms. They are the spring type where a missing detail — wind direction — most often causes wrong parts.

Key parameters

1. Wire diameter and material
2. Body (coil) outer or inner diameter — note that ID shrinks when the spring is wound
3. Number of body coils
4. Leg length, leg style (straight, bent, hooked), and the free angle between legs
5. Wind direction: left-hand or right-hand
6. Torque at one or two working angles
7. The shaft or tube the spring works over/in

Three rules that prevent failures

• Load in the wind direction. A torsion spring should be rotated so the coils tighten. Loading it open raises stress sharply and risks unpredictable behavior.
• Account for ID reduction. When wound, the body ID decreases roughly by (free ID × free coils)/(free coils + degrees of rotation/360). Leave clearance over the shaft — typically 10% of ID.
• Support the spring. Torsion springs work best over a shaft or inside a tube; unsupported springs distort under load.

Specifying torque correctly

Give torque (N·mm or in-lb) at a defined angular deflection from free position — for example "150 N·mm at 90° deflection." Two torque points define the rate. Avoid specifying only the free angle between legs: it is the least controllable dimension in production and rarely functional on its own.

Legs are features, not afterthoughts

Leg bends, hooks, and offsets are formed in secondary operations and drive much of the cost. Simplify leg geometry where possible, and dimension legs from the body tangent point. For complex legs, send a 3D model or physical sample through our RFQ form.