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# How to determine the length of a constant force spring

Quelle:Qianye-Präzision Zeit:2022-9-14

Constant force springs are mechanical devices used in applications that typically require some kind of balance or retraction mechanism. In design and function, constant force springs differ from the more common types of coil springs. They apply a consistent linear force or torque that remains constant over a wide range of motion.

When a coil spring is under force, whether it is stretched, compressed, or rotated, it produces a force proportional to its stretch. How a spring responds when a force is applied or released is called spring deflection. Years of experience tell us what happens when a spring is released from its temporary state. We know that the spring will return to its original position, but it will be proportional to the force exerted on it before releasing. A spring has a restoring force that allows the elastic material to return to its original state – the equilibrium position after the applied force is removed.

Stretching, squeezing or twisting, what makes a coil spring a spring is its proportional restoring force. It’s named after the 17th-century British physicist Robert Hooke who made the discovery, according to Hooke’s Law. Essentially, this law describes a linear relationship where the amount of stress applied to a spring is proportional to the force applied to it.

Determine the length of the constant force spring
Constant force springs differ from traditional helical wound springs. First, they are made from high-volume, thin pre-stretched stainless steel strips that are coiled into flat spirals. In contrast, coil springs are not flat, thin steel strips, but are made of spring wire. The flat helical design of the constant force spring allows for a large angular deflection of multiple rotations, allowing only a small portion of the entire stretch to be used.
Although classified as extension springs, the most common type of constant force spring is the helically wound torsion spring, which can be designed to produce a consistent force in linear motion or manufactured to apply a constant torque. A constant force spring does not have the same proportional restoring force as a spring governed by Hooke’s law. Instead of a proportional force, a spring exerts a consistent linear force over its range of motion.

As the flattened strip unfolds, its energy is stored within the strip. When a force is required for linear motion, the strip extends along a linear path. If torque is needed to provide rotational motion, the strip can either coil itself or move in the opposite direction onto another spool. How designers and engineers need to determine the overall length of a constant force spring when working on a design project. The general rule states that once the full load rating of the spring extends to a length equal to 1.25 times its diameter, a near-constant force is developed. From then on, it maintains a relatively constant force regardless of the extension length. The load is determined by the width and thickness of the material and the diameter of the coil.