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[Qianye precision]Physical design and recoil force of constant force spring

source：Qianye Precision time：2022-9-15

In most types of springs, when the spring is compressed and then relaxed, the force applied is proportional to the distance the spring is stretched. Unlike ordinary springs, constant force springs do not comply with this rule. Due to the physical design of constant force spring, its return force remains constant. Therefore, constant force springs are useful in many applications that require consistent recoil.

The behavior of ordinary springs can be described by a mathematical equation called Hooke’s law. The law points out that the force of the recoil spring in the recoil direction is equal to the distance between the spring displacement and the equilibrium point. In other words, it is how much the spring is elongated and multiplied by the mathematical constant, that is, the spring stiffness. The spring recoils in the direction opposite to the stretching direction. Because many elastic materials work in this way, Hooke’s law has a wide range of applications in the field of physics.

Hooke’s law does not apply to constant force springs because their recoil force remains constant over the entire range of motion. This is achieved through its unique physical design, which consists of thin and curly steel strip. The constant force spring is in a relaxed state when it is fully rolled up, and is loaded when it is partially or fully unfolded. After release, the spring rebounds by pulling the extended carbon belt back into the tight coil. Since the recoil force mainly comes from the part closest to the coiling roll, a constant force can be maintained.

Common applications of constant force springs can be seen in tape measures used in home applications and buildings. A typical tape measure mechanism consists of a thin metal strip wound around the inner cylinder inside the plastic housing. When the tape measure is pulled out and locked, the spring will be stressed. When the lock is released, the ribbon will automatically retract into the plastic box. The retraction is realized due to the recoil of the roll, which is used as a constant force spring.

Many other devices with automatic retraction also use constant force springs. Some examples include car seat belts, window counterweights, seat backs, and staplers. In all these applications, the force generated by the equipment must be consistent and behave the same every time when working in a small space. Constant force springs can meet these two conditions at the same time because they can exert a constant force and can be wound tightly.

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