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Last edited 23 Nov 2018
Constant force springs
Constant-force springs differ from other springs, such as helically wound springs (although they do obey Hooke’s Law). This is mainly because constant-force springs are created from pre-tensioned metal strips instead of wire. They fall under the ‘extension spring’ category and are designed to provide tension in a linear direction.
Despite their name, these springs don’t provide true constant force; the initial force starts from a finite value. Constant-force springs resemble a wound coil and, because the force to restore the unrolled spring is nearest the roll, there is almost no restriction when it comes to speed of extension or acceleration.
After the spring is deflected to a length equal to 1.25 times its diameter, it reaches full load and maintains a relatively constant force independent of the length of the extension. The fatigue life of these springs can range anywhere from 2,500 cycles to more than one million, depending on the size and load of the spring.
The unique qualities of these springs make them valuable as a counterbalance for heavy moving parts as well. An example is a van door, which requires an extra ‘push’ before closing. Once that initial force is overcome, the door slides with ease and with near constant force.
The material used in making the spring (as well as its width and thickness) is one of them. Usually, in order to create high-quality constant-force springs, stainless steel grade 301 is chosen, although high-carbon steel, Inconel and other materials can also be suitable.
The initial load or force that the springs will have to overcome to reach ‘constant force’ also matters when it comes to designing the spring, as does its internal and external diameters. Equally important is the end detail of the spring, which means the outside ends that will attach to other parts and components to coil or uncoil it.
Some of their most popular uses include:
- Door closers
- Cable retractors
- Hose retrievers
- Gym equipment
- Electric motors
- Space vehicles
- Furniture components
- MRI and x-ray machines
- Retractable dog leashes
- Vacuum cleaner cords
Just like other types of springs, external factors can contribute to a reduced lifespan of a constant-force spring. This can include anything from exposure to corrosive elements, which can also affect its performance, to extreme temperatures.
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