When you walk into your office building on your way to work every day, or into your local bank, or any commercial facility, you may open a heavy glass door. When you cross the threshold, the door closes behind you, and you don’t think about it. However, the heavy glass door was probably closed by a constant force spring. Constant force springs are widely used in daily applications. From door closers to rope and hose retractors, to sports equipment, toys, motors, etc., constant force springs exert force at an amazing high frequency and consistency. Let’s take a closer look at the basic physics behind the constant force spring.
In the 17th century, the British physicist Robert Hooke proposed a formula for force and elasticity, which is now called Hooke’s law. Hooke’s law states that the force (f) required to stretch or compress a spring for a distance (x) varies linearly in proportion to that distance. The formula is f=kx, where k is a constant and represents the stiffness of the spring.
Constant force springs in terms of actual spring performance, Hooke’s law is nominal. In the equation, the distance X that the spring moves is a relatively small number. Finally, any elastomer that is stretched or compressed beyond the stress limit of its material composition will show signs of deformation. Therefore, the force properties that depend on the consistent shape and tension state will change in the deformation state.
Ideally,constant force springs will exert a consistent force over a given range of motion, repeating without changing. In fact, constant force springs, like all elastomers, are limited by their material stress limits. If the limit of material stress is exceeded, they will deform, or the material will fatigue and begin to weaken over time. Therefore, Hooke’s law does apply to constant force springs. However, the manufacturing method of constant force spring and its special physical characteristics lead to the term “constant force”. By carefully studying the physical properties of constant force springs, we can better understand how the name constant force is derived.
In application, the constant force springs are in the pre tightening state. The act of winding a steel strip around itself or on a drum puts the structure in tension. If the force of the spring is to be measured in this tension state, it will not be zero. In other words, the constant force springs have a measurable force in the rest position. When the load moves against the spring, the spring moves a relatively short distance in the circumferential direction (most commonly in the direction of the initial wind, resulting in a small reduction in diameter). If we compare the spring force value in the loaded state with the force value in the static state, we will find that the force difference is relatively small. More generally, outside the field of physics, the small force difference between the static state and the loaded state of a spring is considered insignificant. Therefore, the force of the spring is called “constant”. In fact, constant force springs exert a nearly constant force.
Constant force springs can work effectively for a long time. Most can run thousands to millions of cycles before material fatigue reduces its effectiveness. Constant force springs are very suitable for commercial door closers and other applications. Thousands of people may pass through the doors of buildings every year. When designing or specifying a constant force spring, many basic engineering parameters need to be considered. Material, fatigue life, tensile load, torque and friction, speed and acceleration, installation, safety considerations and application environment are all key factors.
Shenzhen Qianye Precision Metal Co., Ltd.‘s knowledgeable spring engineering staff can help improve constant force and other spring designs to provide optimum performance in any application. Feel free to contact us to learn more about our spring manufacturing capabilities.
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