One of the key features of a constant force or variable force spring is its overall length. How this overall length is determined and why it is important is a discussion that will aid designers and engineers when working on a project.
To begin let’s discuss the attributes of the two springs mentioned above. Both constant force and variable force springs are made to exert a linear force. Both springs are extended by overcoming the force manufactured into the spring and both will retract when the spring force is greater than that opposing it. The difference in the two designs is indicated by their respective names: A constant force spring will exert a relatively constant force through its extension while a variable force spring will change in force throughout the travel length. Usually variable force springs are stronger when extended and weaker as they retract. In order to maintain a consistent retraction of the spring, minimize space, and obtain the lowest possible cost, the overall 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. A formula for approximation could then look like this:
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 (spool greater than the free ID of the spring) the spool diameter should replace the inside diameter in the above equation.
Before completing this exercise it is 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 formula.
A variable force spring has a spiral form without a constant diameter. However, it is the final inside diameter that should be used to determine the overall length. The extra length of steel required to keep 1.5 wraps will be formed to this final inside diameter. The expansion of the coils for a weaker force will only affect the outside diameter.
Finally, we will briefly discuss the pitfalls of designing a spring that is 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 an important step in designing for large volume production.