An ideal constant-force spring is a
spring
Spring(s) may refer to:
Common uses
* Spring (season), a season of the year
* Spring (device), a mechanical device that stores energy
* Spring (hydrology), a natural source of water
* Spring (mathematics), a geometric surface in the shape of a h ...
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 ...
it exerts over its range of motion is a constant, that is, it does not obey
Hooke's law
In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of ...
. 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
Spring steel is a name given to a wide range of steels used in the manufacture of different products, including swords, saw blades, springs and many more. These steels are generally low-alloy manganese, medium-carbon steel or high-carbon stee ...
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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