When designing a Constant Force Spring one must begin by taking a hard look at life cycle requirements. This decision will have an affect on every other decision made and ultimately could lead the design down the wrong path. If done correctly, the design can fall into place and allow for a successful project. First, let’s discuss the definition of life cycles as it applies to the Constant Force Spring.
Life Cycle Explanation:
A life cycle is one extension AND retraction of any given area of the spring.
Since the Constant Force Spring derives its power from the coiled curvature of the material, the only area being used at any given time is the part of the spring coming off of the coil. In other words, a design that has 25” of total spring length, but only uses 6” of extension, will have the same life cycles as a design using the same spring with 18” of travel.
Life Cycle Determination:
In the early stages of design it is important to focus on the life cycle requirement. While it is normal to want very high life cycles for a design it may lead to a much larger spring than necessary. Of course, underestimating the life cycle requirement will lead to early failure. However, most estimates usually are overestimated and lead to larger, more expensive spring designs.
In closing, it is very important to have a clearly defined life cycle requirement prior to designing and testing a Constant Force Spring.