A frequently asked question concerns the life cycle rating of a Variable Force Spring. In this blog, we will focus on this discussion and how the life cycles are analyzed. Let’s first give a brief description of what a variable force spring is and what it looks like.
A V spring is a gradient positive spring. This means that the spring increases in force as it is extended. The amount of increase over the length of the spring can vary widely. Depending on the design, the spring force can remain nearly constant or may undergo an increase as great as 500%. The easiest way to identify a “V” spring is by the opening between the coils as shown to the right:
The opening between the coils is caused by the changing diameter of the material. The tighter the material is coiled, the greater the force required to extend the spring becomes. Thus, the force required to begin extension is small and as extension continues this force increases.
This brings us to life cycle calculation. In an earlier blog on constant force spring life cycles, we discussed the life cycles of a constant force spring. The variable force spring utilizes the same basic concept at each varying coil diameter. In effect, the material comprising the outer coils is much less stressed than that of the inner coils. Therefore, the outside coil will have a much higher life cycle expectation than the inner coil.
At Vulcan, we usually determine the life cycle rating of the smallest diameter coil to be used in a design and rate the part based on that number. It is possible that some short cycling could occur during which the smallest coils of the spring are not active. In this case the cycle life would be greater than estimated.
Let’s use a quick example:
A Variable Force “V” Spring is used in a track to dispense a number of products on a shelf. When the shelf is fully loaded there is 10 lbs. of product that needs to be pushed and the shelf is 20” deep. When all of the products are removed from the shelf the spring force has dropped to coincide with the reduction of the weight of product. That is one full cycle per Vulcan’s rating method.
Now if the shelf is always fully loaded each time product is added it is safe to claim that the life cycle determination can be made based on the diameter of the spring at 20” of extension. However, if the shelf is only filled halfway, the cycle life can be considered at 10” of extension. At this extension the spring force will be much less, the active spring material is less stressed, and the life cycle expectation will be much greater.
In real life, it is usually not known how the loading process will occur each time. Therefore, the conservative approach is to calculate the lowest possible cycle life of the spring design and work with that number. The buyer must be careful to understand how the life cycle rating was quoted and how the usage of the spring will affect the cycle life. Understanding life cycle ratings is an important part of choosing an optimal spring design.