Maximum Solid Height – The Caveat

Spring design is a balancing act-an attempt to obtain needed forces at needed heights, and do whatever it takes in the processing to see that the spring behaves predictably and consistently after it leaves the dock.

To make all that happen, the requirements need to not only be plausible by physics, but pertinent to the application.  Last issue, I mentioned the need for tolerances that make sense.  Over-tolerancing dimensions as an attempt to control rates or loads adds unnecessary cost. Knowing what to reference on a spring design is its own science that will allow the least amount of time and cost, while gaining exactly what is needed to make the part function successfully, part after part and time after time.

It is very common for spring blueprints to have mathematical errors. It is the function of engineers to review all designs and be sure those errors do not translate to the shop-floor documentation. One of the more stifling requirements present on many spring blueprints is a maximum solid height. The requirement itself is relevant as it pertains to the total deflection of a spring.  No one wants the fully deflected height of a spring to be greater than a working deflection-that is a recipe for failure.  But solid heights should never be called out as a maximum requirement unless that need is truly pertinent.  My reason for stating that is due to the practice of applying a “max” designation to a “calculated” solid height. This is quite common, especially for companies that use a lot of springs and have blueprints whose solid height designation has a “max” as a default, whether it’s viable or not.

Here’s an example. A compression spring has a calculated solid height of 1.450″. The blueprint also calls out a 1.450″ maximum solid height. But, the second loaded height is at 1.625″. This means there is some distance between the loaded height and solid height. This spring also requires a toleranced spring rate. If the wire size is a bit large (because spring wire is purchased to within given tolerances and larger wire will automatically create a higher spring rate) the spring maker may need to add just a bit of partial coil to lower the rate into conformance. But, the addition of the added material breaches the “max” solid height requirement and forces the coiler to find solutions for rate adjustment, when the addition of a partial coil would solve the problem quickly and successfully. In so many cases, the maximum solid height is not a true requirement-it’s simply a result of years or decades of doing things by drafting conventions, whether they make sense or not.

This one simple callout with three tiny letters (M-A-X) can create a situation where a spring maker spends much more time trying to rob Peter to pay Paul, and shortchanging them both, yielding a just-conforming, skin-of-your teeth result that is completely unnecessary because the “max” solid height callout is bogus.  This creates increased time in production and associated delays in shipping.  When designs are understood and optimized, processing times can be exponentially decreased and everyone wins.  It is to everyone’s benefit for the spring engineer to take the time to verify a maximum requirement.  It could have a real payback in the role of a consistently performing product on each and every order.

 

Spring Fundamentals … Challenges With Ground Compression Springs

The formula for calculating the solid height of a ground compression spring is nothing more than the wire size multiplied by the total coils.  So a spring with around 125″ material size with 13.4 total coils would have a calculated solid height of 1.675″.

However, there are factors that make this calculation more of a reference than bankable data. Spring coiling has some inherent variations in parameters due to a number of factors. The material itself can have variations from the start of a bundle to the end, which means adjustments may be needed during the process. The capability of the coiling machine itself, which is critically dependent on good maintenance, may produce slight variations in both diameter and free length. Free length anomalies at coiling are then passed down to the grinding department. So many times the grinding of the ends is looked upon as the great equalizer by removing some of the variation from coiling.  However, grinders can also induce their own variation, which is more a function of the spring design than machine capability. Since springs can have spring rates from ultra-fine to thousands of pounds per inch, this wide range of product being forced between two stones is both art and science.

And finally, the all-important spring index (mean diameter divided by material size) also plays a role in the grinding of ends. This is a whole discussion unto itself, but if a compression spring is tightly wound, a lot of material must be removed to yield a 270° bearing surface. Large indices are the opposite-very little material needs to be removed to obtain a decent degree of bearing surface. When indices are very small or very large, the calculated solid height will be incorrect and must be taken into consideration if maximum solid heights are truly critical to function.

 

By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping