Constant Force Springs

29th March 2021

What is a Constant Force Spring?

Those of you familiar with Hooke’s law will be aware that the force a material exerts is proportional to the energy used to deform it.

In other words. The more you try and bend or deform a material, the harder it is to deform it further.

From Wikipedia;

Hooke’s law is a law of physics that states that the force (F) needed to extend or compress a spring by some distance (x) scales linearly with respect to that distance—that is, Fs = kx, where k is a constant factor characteristic of the spring (i.e., its stiffness), and x is small compared to the total possible deformation of the spring. The law is named after 17th-century British physicist Robert Hooke. He first stated the law in 1676 as a Latin anagram.

He published the solution of his anagram in 1678 as: ut tensio, sic vis (“as the extension, so the force” or “the extension is proportional to the force”). Hooke states in the 1678 work that he was aware of the law since 1660.

Mounting Requirements for Constant Force Springs

  • A constant force spring would be tightly wound.
  • Any housing used should provide between 10% and 20% greater diameter than the spring in its fully wound state.
  • A constant force spring may deform or kink at long extensions. The choice of material is important here. Guides may be used to ensure the smooth flow.
  • Constant Force Spring is usually mounted by first tightly wrapping it on a drum, then attaching the free end to the loading force such as in a counterbalance application. This relationship can also be reversed.


Uses for Constant Force Springs

Constant force springs are best used as a counterbalance. In everyday application these springs are used most commonly to supply a retracting force for items such as self-closing doors, seatbelts and interior blinds.