Constant-force spring

A ideal 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. In reality, "constant-force springs" do not provide a truly constant force and are constructed from materials which do obey Hooke's law. Generally constant-force springs are constructed as a rolled ribbon of spring steel such that the spring is in a rolled up form when relaxed. Because the geometry of that region remains nearly constant as the spring unrolls, the resulting force is nearly constant.

The approximation of "constant force" comes from a long travel and a pre-loaded rest position, so that the initial force does not start from zero but a finite value. For relatively small variations around this position the force is approximately constant.

References

• Ansel C. Ugural, Mechanical Design of Machine Components, Second Edition, page 683, CRC Press, 2015 ISBN 1439887810.