An ideal constant-force spring is a
spring
Spring(s) may refer to:
Common uses
* Spring (season)
Spring, also known as springtime, is one of the four temperate seasons, succeeding winter and preceding summer. There are various technical definitions of spring, but local usage of ...
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 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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