Zero-length spring
From Wikipedia, the free encyclopedia
A zero-length spring has a physical length equal to its stretched length. Its force is proportional to its entire length, not just the stretched length, and its force is therefore constant over the range of flexures in which the spring is elastic (that is, it does not follow Hooke's Law).
It was invented in 1932 by Lucien LaCoste, and almost immediately applied to the design of instruments with vertical pendulums, such gravimeters and seismographs.
Theoretically, with the correct mass, a pendulum using such a spring as a return can have an infinite natural period. Long-period pendulums enable seismometers to sense the slowest, most penetrating waves of distant earthquakes. Zero-length springs also find use in gravimeters, which need them to have linear sense-pendulums. Some door springs, especially for screen doors, are zero-length springs to reduce the energy of a slammed door. Zero-length springs sometimes smooth auto suspensions.
Physically, one common form of a practical zero-length spring is a leaf spring curled almost in a circle, with the ends mounted to flexible restraints. A convenient form is a helical spring whose wire is twisted while it is being wound (common in screen-door springs). Another common design is a torque spring or bar. Zero-length springs usually require special compliant mountings, sometimes require precise adjustments to enter zero-length mode, and often have a limited range of motion.
