One of the key features of a constant force spring or variable force spring is overall length. The importance of spring length calculation is a discussion that aids designers and engineers when working on a project.
Variable Force & Constant Force Spring Design
To begin, let’s discuss the attributes of these two springs. Both constant and variable force springs exert a linear force. Both springs are extended by overcoming the force manufactured into the spring, and both retract when the spring force is greater than that of the opposing force. The difference in the two spring designs is indicated by their respective names. A constant force spring exerts a relatively constant force through its extension, while a variable force spring changes in force throughout the travel length. Usually, variable force springs are stronger when extended and weaker as they retract. To maintain consistent spring retraction, minimize space, and obtain the lowest possible cost, the overall spring length should be customized for the specific application.
The general rule for constant and variable force springs is that the overall length should include at least 1.5 wraps of material to remain when fully extended. Consideration must be made for the initial pickup and any pre-load positioning as well as the actual travel length when in use.
Spring Length Calculation
Overall length = Pickup length + Pre-load length + Working extension + (Inside diameter x 3.14 x 1.5). If the spring is mounted on an oversized spool, which is greater than the free inside diameter of the spring, the spool diameter should replace the inside diameter in the above spring length calculation.
Before completing this exercise, it’s important to keep tolerances in mind. The inside diameter of a constant force or variable force spring has a standard ± 10% tolerance. Specialized designs may have a tighter or looser tolerance. The maximum possible ID should be used in the spring length calculation.
A variable force spring has a spiral form without a constant diameter. However, it’s the final inside diameter determines the overall spring length. The extra length of steel required to keep 1.5 wraps will form to this final inside diameter. Expanding the coils for a weaker force will only affect the outside diameter.
Spring Length Calculation Errors
Finally, let’s discuss the pitfalls of designing a spring that’s too short or too long for a given application. The most obvious problem with a design that is too short is that the spring will escape from its mounting position. For instance, a short spring could release off a spool once fully extended. This would cause product failure as the spring would not retract to the intended position and could become a hazard. If a design uses slightly less than the 1.5 wraps the ID could deform into an oblong shape when mounted in a cavity. This could cause the spring to jam or destroy the force profile of the spring.
Designing a constant force spring that is too long will cause a design to be larger than required. The outside diameter of the spring will be larger due to the layering of additional length and therefore require more space. For both variable and constant force springs, the additional length will increase the weight of steel used and the overall price of the spring. Carefully optimizing spring length is important in designing for large volume production.
