How do constant force springs work

This is in contrast to typical springs and elastic materials, which produce a force proportional to their extension. Constant force springs are those designed to exert a constant force or torque over a large range of motion.

The most common constant force spring is a spiral wound torsion spring, which is sometimes referred to as a steel strip spring. It can be manufactured to produce a consistent force over a linear motion. Or, it can be designed to exert a torque resisting twisting. Spiral wound torsion springs are formed from a thin strip, typically of sprung steel, coiled into a flat spiral. This design allows for large angular deflections of many revolutions, so it is possible to use just a small part of the full extension.

The act of coiling a ribbon of steel around itself, or wrapping it around a drum, puts the structure in a state of tension. If one were to measure the force of the spring in this state of tension, it would not be zero. In other words, constant force springs have a measurable force in the resting position. 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 small decrease in diameter.

If we compare the springs force value in the loaded state versus its force value in the resting state, we find the difference in force to be relatively small. The width and thickness of the spring material and the diameter of the spring's outer coil generally determine load rating.

In most cases, constant force springs are designed to provide force by turning in the same direction as their initial wind. Unless specifically designed to do so, reverse winding a constant force spring usually causes deformation.

Constant force springs can operate effectively for long periods of time. Most can function from thousands to millions of cycles before material fatigue reduces their effectiveness. Constant force springs are ideal for applications such as commercial door closers where tens of thousands of people may pass through a building's doorway each year. There are numerous, essential engineering parameters to consider when designing or specifying a constant force spring.

Materials, fatigue life, tensile load, torque and friction, speed and acceleration, mounting, safety considerations, and application environment are all critical factors. After that, the door slides down easily, with nearly constant force. In addition to U-Haul doors, constant force springs are used in a wide variety of products. Some of the largest consumers are medical device manufacturers.

MRI machines, dental X-ray equipment—and just about anything articulated and heavy—is equipped with constant force springs. Smaller constant force springs for medical devices are also part of syringes and drug infusers. Constant force springs can be manufactured using the same spring coiling machines used for helical springs.

The coiler will have to be modified to accommodate the flat strip of metal that makes up the constant force spring.