An ideal constant-force spring is a
spring for which the
force
In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a p ...
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.
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 initial position the force is approximately constant.
References
* Ansel C. Ugural, ''Mechanical Design of Machine Components, Second Edition''
page 683 CRC Press, 2015 .
Springs (mechanical)
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