I have dealt with the meaning of spring rate in past articles; but let me explain it again so everyone understands.
Rate is the amount of force produced when a spring moves an inch. That’s the standard and it’s not too hard to understand. So don’t make it difficult on yourself by believing it’s more complex than that.
For example, a spring with a spring rate of 200#/ inch will produce 200# for each inch of travel. However, sometimes the movement of spring steel as it moves can throw a wrench in the works.
Rate is a direct result of how much wire is active. As the active material is reduced in a spring as it moves, the rate will increase and more load will be produced. But, sometimes the obvious is not so obvious.
Several years ago I worked with a spring maker on a suspension spring. He told me the spring rate was decreasing as the spring deflected. This is not possible because there is simply no way a spring will move, lose active coil and then produce less rate. So, the next step was to put the spring in the load tester and watch the travel of the spring.
The spring in question was a less than desirable design. It had just over one active coil and the pitch was high. With so little active material in motion and with the material moving a great distance as force is applied, strange things can happen.
This was one of those occasions where all my knowledge and experience got watered down to a moment of confusion. You grasp for the reasoning behind how this anomaly could happen. The fact was, the spring maker was right — as the spring deflected, it would reach a certain point in the deflection and the resultant loads verified the rate was actually going down, not up.
We took the spring up and down several times. At least it was starting to trend, which was a good thing (and nothing can put the whammy to both logic and common sense as well as a good case of randomness). And then the moment of truth arrived. On one of the tests, I noticed the active material was slipping outside the coil diameter. This meant that active material was actually being increased, opposite to conventional movement. As the spring would travel farther, this slipping coil would then line back up into place and start to dampen into dead/inactive material. But, for a short period of travel it was actually adding active material, which then explained the rate decrease.
This was a good experience for me and I learned more than one lesson. The first was to listen to the floor operator. He came to me with a puzzle and hoped I could solve it without thinking he’d lost all reason. Second, it taught me to look closer and closer to see why the theory was not working. In this case, the theory was working perfectly — the design was outside of good practice and this forced the movement of the spring to be unconventional. Springs are design dependent, meaning that a good design behaves, whereas a design on the edge will nearly always produce some kind of abnormal result. It asks the spring to often deliver beyond its physical limitations.
So, what was the solution? With compression spring coiling machines, it is quite typical for the body of the spring to be slightly smaller in diameter than the ends. If a spring has the one-two combination of very little coil and high pitch, this may produce the condition I just described. It is then necessary for the coiler to find a method, by way of camming for a mechanical coiler, or programming for a CNC coiler, to tuck in the ends coil(s). This was done, and the problem went away.
So, that funny little thing called “rate” can sometimes throw you a curve. Believe the theory, but look at the real world and see why they are colliding. The experience can serve as a lesson for saving time when you face similar challenges down the road.
By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping