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Constant torque spring motors adapt the principle of the constant force spring by attaching the free end of the spring to another larger drum and winding it around that drum in either the same or the reverse direction to that on the storage drum. The program uses the latter arrangement to produce higher torque and smoother operation. When the output drum is wound up and then released, the storage drum will rotate to retract the spring, thereby rotating the output drum. As the tension in the spring is virtually constant and the radius of the active coil on the output drum varies only slightly, a relatively constant torque is available at the output drum shaft.
The data input form is used for either design or analysis by appropriate selection of mode. Only the relevant text boxes will be available in either case. In the design mode, the inputs include the aspect ratio (width/thickness) of the material, the output torque, a material selection and the total number of revolutions

The result screen for Constant Torque Spring Motor design reproduces the input data values and the proposed spring dimensions. The calculated properties include the material width and thickness, the storage and output drum diameters, the natural coil inside diameter, the minimum center distance and the active and total material lengths.

When the Analysis button is clicked the text boxes switch to that mode and the form can be used for calculation of the properties of an existing spring or spring design. Input values include the storage and output drum diameters, the natural inside coil diameter (if not known, a value will be assigned), the material width and thickness, the active material length and a material selection.

The Constant Torque Analysis Results screen records the input values and the calculated properties including the rated torque, the number of revolutions, the minimum center distance and total material length, and stress considerations.