Spring Index

When designing and engineering a spring there is a mindset that is engraved into Southern Precision culture.  Forces on paper do not move physical components only manufactured springs move physical components.  In other words, why would you design a spring that you can’t make? 

Today we are going to talk about Spring Index.  The relationship between the springs mean coil diameter and the wire diameter is your Spring Index.  The rule of thumb for our engineers is between 4 and 12 the index is very attainable.  If you get outside these limits more questions need to be answered before moving forward.  Let’s walk through some examples of different springs and why they become challenging.

We have two compression springs illustrated above.  One is below 4 index (spring A) the other is above 12 index (spring B).  Spring A has a wire diameter of .125 and a mean diameter of .375.  Spring B has a wire diameter of .060 and a mean diameter of .900, keep in mind to find the mean diameter I could not measure accurately from the middle of the wire.  Therefore, I measured the Outside Diameter and subtracted one wire size—half a wire size from each side.

Spring A--   .375 divided by .125 is 3 which is obviously below 4.  Now let’s talk about why this spring is difficult to make.  In order to coil this spring you are wrapping it very aggressively around your arbor.  Your arbor is small enough to accomplish the desired diameter but not large enough to handle the stresses, especially when you cut the part after coiling.  In order to cut the part you are pinching the heavy wire below the arbor using the cutting tool.  This part is an arbor killer, and if you were to move forward with the design the production because of broken tooling would be a nightmare.

Spring B--   .900 divided by .060 is a spring index of 15.  The problems that arise from a spring with a high index are very different than those that arise from an index below 4.  Above we noticed the stresses of a low index, with a high index it is reversed so we have much less stress and less control over the coil.  What I mean by less control is in a low index situation the coiling point is very close to your arbor giving the production line a great deal of tension, control and wire guidance.  On the contrary, high indexes require the coiling point to be further and further away from the arbor, giving the wire freedom to wiggle, dive, jump, and do all the wrong things.  Spring makers label this problem as a tension problem.  Spring manufacturers love to be in control!  Like most of us at our jobs the main cause of stress is directly correlated to lack of control in any given situation.   High index springs are no different; outside diameter, pitch, number or coils, rate, and even free length can vary rather drastically the higher an index becomes.  A general rule of thumb is the higher an index the larger the need for more and more tolerance deviation because of the lack of control.


 Just remember low index = high stress, and high index = less control.