Constant-force spring
Encyclopedia
A constant-force spring is a spring
for which the force
it exerts over its range of motion is a constant. That is, it does not obey Hooke's law
.
Generally constant-force springs are constructed as a rolled ribbon of spring steel
such that the spring is relaxed when it is fully rolled up. As it is unrolled, the restoring force comes primarily from the portion of the ribbon near the roll. Because the geometry of that region remains nearly constant as the spring unrolls, the resulting force is nearly constant.
A conical spring can be made to have a constant rate by creating the spring with a variable pitch. Putting a larger pitch in the larger OD coils and a smaller pitch in the smaller OD coils will force the spring to collapse all the coils at the same rate when compressed.
Spring (device)
A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
for which the force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
it exerts over its range of motion is a constant. That is, it does not obey Hooke's law
Hooke's law
In mechanics, and physics, Hooke's law of elasticity is an approximation that states that the extension of a spring is in direct proportion with the load applied to it. Many materials obey this law as long as the load does not exceed the material's elastic limit. Materials for which Hooke's law...
.
Generally constant-force springs are constructed as a rolled ribbon of spring steel
Spring steel
Spring steel is a low alloy, medium carbon steel or high carbon steel with a very high yield strength. This allows objects made of spring steel to return to their original shape despite significant bending or twisting.-Grades:...
such that the spring is relaxed when it is fully rolled up. As it is unrolled, the restoring force comes primarily from the portion of the ribbon near the roll. Because the geometry of that region remains nearly constant as the spring unrolls, the resulting force is nearly constant.
A conical spring can be made to have a constant rate by creating the spring with a variable pitch. Putting a larger pitch in the larger OD coils and a smaller pitch in the smaller OD coils will force the spring to collapse all the coils at the same rate when compressed.