By Dr. Jody Muelaner
A constant force spring is engineered to exert a consistent force over its range of motion. This is in contrast to typical springs and elastic materials, which produce a force proportional to their extension.
The most common constant force spring is a spiral wound torsion spring, which is sometimes referred to as a steel strip spring. It can be manufactured to produce a consistent force over a linear motion. Or, it can be designed to exert a torque resisting twisting. In the latter case it’s more accurately described as a constant torque spring (although the term power spring is generally used).
Spiral wound torsion springs are formed from a thin strip, typically of sprung steel, coiled into a flat spiral. This design allows for large angular deflections of many revolutions, so it is possible to use just a small part of the full extension. Combined with a significant pre-tension in the spring, this can result in a relatively flat force or torque profile.
As the strip is unwound, energy is stored in the flattened strip. When a force is required over a linear motion, the strip is extended along a straight path. However, if torque is required for a rotary motion, the strip may be wound in on itself or onto another spool in the opposite direction. Constant force linear motion has many uses, including feed mechanisms for shop floor dispensers and automated machinery, and for counterbalances. Constant force rotary motion is useful for retracting cords, hoses, seatbelts and the like onto spools.
A special use for spiral wound torsion springs is to power the mechanism in mechanical watches and clocks. In this application, the springs are called “mainsprings” and constant force is particularly critical because it will affect timekeeping. Other clockwork machines also use springs in a similar way and may also use the term mainspring.
When the mechanism is “wound,” energy is stored in the spring. It is, then, released as a torque that powers the mechanism. For example, turning the “hands” of a watch.