Constant-force spring

A constant-force spring is a spring for which the force it exerts over its range of motion is a constant. That is, it does not obey Hooke's law. Generally constant-force springs are constructed as a rolled ribbon of spring steel such that the spring is relaxed when it is fully rolled up. As it is unrolled, the restoring force comes primarily from the portion of the ribbon near the roll. Because the geometry of that region remains nearly constant as the spring unrolls, the resulting force is nearly constant.

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