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Physical properties of constant force spring

2023-02-24
Ideally, a constant force spring will exert a consistent force over a given range of motion, repeatedly without change. In fact, like all elastomers, constant-force springs are limited by the stress limits of their material. If the limit of the material's stress is exceeded, they will deform, or the material will tire and begin to weaken over time. So Hooke's law does apply to constant force springs. However, it is the way constant force springs are made and their particular physical properties that have given rise to the term "constant force". By studying the physical properties of constant force springs, we can better understand the origin of the name constant force.

Usually made of steel, constant force springs are made by wrapping a flat piece of steel around the drum or by winding a steel strip around itself (with a nearly constant radius). A spring is installed in its application and one end of the spring is attached to the load. A spring exhibits torque forces when the movement of a load causes it to wind against its rest state. The spring wants to return to its resting state, so it applies a force to the load.

In application, the constant force spring is in a preloaded state. The act of winding the steel strip around itself or around the drum puts the structure in a state of tension. If you want to measure the force of the spring in this state of tension, it will not be zero. In other words, a constant force spring has a measurable force at rest. When the load moves against the spring, the spring moves a relatively short distance in a circular direction (most commonly in the direction of the initial wind, resulting in a slight reduction in diameter).


constant force spring


If we compare the force value of the spring in the loaded state to the force value in the rest state, we find that the difference in force is relatively small. More generally, outside the realm of physics, small force differences between the resting state and the loading state of a spring are considered insignificant. Therefore, the spring force is said to be "constant". In fact, the constant force spring exerts a force close to constant force.

Constant force springs can operate efficiently for a long time. Most can run thousands to millions of cycles before material fatigue reduces their effectiveness. Constant force springs are ideal for applications such as commercial door closers.

Many important engineering parameters need to be considered when designing or specifying constant force springs. Material, fatigue life, tensile load, torque and friction, speed and acceleration, installation, safety considerations and application environment are all key factors.

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