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Design Considerations for Torsion Springs

Torsion Springs whose ends are rotated in angular deflection, offer resistance to an applied torque. The wire itself is subjected to a bending stress rather than a torsional stress. Springs of this type are usually close wound and reduce in coil diameter and increase in body length as they are deflected. The designer must consider the effects of friction and arm deflection on torque.

illustration: torsion spring design parameters

Spring Dynamics

As the load is applied to the moment arm of the Torsion Spring, the spring will wind tighter, thereby reducing the inside and outside diameter of the spring. The spring will also elongate from its body length to the minimum axial size listed in the spring specifications. Care should be taken not to rotate the spring past it's design parameters, because of the effects on mandrel size and axial length. In most cases Torsion Springs should be used in the direction that winds the coils. In the unwinding direction the maximum load is lower and the spring dynamics are reversed.

Type of Ends

The type of ends on Torsion Springs should be considered carefully. The load should be applied at the radius indicated. Using other radii may appreciably alter the active length of wire and the resulting torque values.

Mandrel Size

The mandrel is the shaft over which the Torsion Spring is mounted. Suggested mandrel sizes will allow 10% clearance at the maximum deflection.

Spring Load

The torque values listed for a spring can be translated into a direct load by using the formula;

P = M / R where;

P = is the load applied, lbs.
R = is the design radius for the spring, inches
M = moment or torque, in·lbs.

The torque values listed will be attained at the deflection listed for the spring. Torque values at intermediate deflections can be computed by direct prorating.