What are constant force springs and how are they used?

Constant force springs are specialized springs that are used to provide a constant force over a range of motion. This makes them ideal for use in a variety of applications, such as automotive or robotic systems.under we will take a closer look at what constant force springs are and how they can be used.

What are constant force springs?

A constant force spring is a type of spring that produces a constant force over its entire range of deflection. This type of spring is often used in applications where a stable force is required, such as in the actuation of a valve.

Constant force springs don’t have much in common, either in form or function, with standard helically wound springs. For starters, constant force springs are formed from pre-tensioned metal strips, rather than wire.

At their most basic level, however, constant force springs do exactly what other springs do. That is, they conform to Hooke’s Law. Hooke’s Law essentially states that when an elastic object is deformed, the force it exerts is directly proportional to the amount of the deformation. In other words, the farther you extend an extension spring, the more force it takes to pull it even farther; the more you compress a compression spring, the more force it takes to push it down even more.

Determining the Length for Constant Force Springs
Constant-force springs differ from traditional helical-wound springs. To begin with, they are made from high yield, thin strips of pre-tensioned stainless steel coiled into a flat spiral. In contrast, rather than a flat, thin strip of steel, coiled springs are made from spring wire. The flat spiral design of constant-force springs permits large angular deflections of numerous revolutions, making it possible to use only a small part of the full extension.

Though classified as an extension spring, the most common constant-force spring is a spiral wound torsion spring, which can be designed to either produce a consistent force over a linear motion or manufactured to exert a constant torque. The properties of constant-force springs do not exhibit the same proportional restorative force as springs governed by Hooke’s Law. Rather than proportional force, the spring exerts a consistent linear force over its range of motion.

As the flattened strip is uncoiled, its energy is stored within the strip. The strip is extended along a straight path when a force is required over a linear motion. If torque is necessary to provide a rotary motion, the strip can be coiled either in on itself or moved in the opposite direction onto another spool.

How the overall length is determined for a constant-force spring is necessary for designers and engineers when working on design projects. The general rule states that once the full rated load of the spring is extended to a length equal to 1.25 times its diameter, a near-constant force is created. From that point forward, it maintains a relatively constant force regardless of extension length. Loads are factored by the width and thickness of the material combined with the diameter of the coil.

Benefits of Constant Force Springs
Constant-force springs are exceptionally durable with a fatigue life cycle ranging from 2,500 cycles up to 1,000,000 cycles depending on the size of the spring and the load it’s designed to support. They are found in mechanical devices that have long-motion functions.