All Things Springs: The Quick-Fix Spring Rate Compensation Formula for Coilers Only
One of the daily challenges for springmakers/coilers is the adjustment needed to yield nominal rate. As all spring manufacturers know, the only bill of material for a spring is the material used to make the part. That very material has a list of properties that vary from wire lot to wire lot. These properties are modulus of elasticity, tensile, material size and out-of-roundness.
Any or all of these properties will have slight variations and the end user has no control over the chemistry of the material that is brought to his/her spring coiler for processing. Much like stack-up tolerances, a variation in each property will have a noted effect on the elastic reaction of the resulting spring. And since a spring is a rate-producing mechanical device, its very purpose is to produce a force, based on the spring rale. That spring rate is born from the amount of active material in the spring… period.
So… what does a springmaker do if a spring is set up identically to the previous order, but produces a different rate, even though ail physical dimensions are identical? Well, I have an answer for that question. If you review the spring rate formula, you’ll find that rate and active coils are inversely proportional. This means that if a coiler sets up the coiling machine and the sample spring is a bit high on rate, he/she will need to add some material length to lower the spring rate to the acceptable tolerance. Many times this is a tweak based on past experience with no math involved. However, there is a way to calculate the change required with a minute of investment.
By eliminating all the algebraic variables in spring formulas and stripping down to just the coils and rate, this formula appears.
N2 = (N1 * R1) / R2
Or… the rate you seek (N2) is equal to the number of active coils of your sample (N1), times the rate of your sample (R2), all divided by the rate you need (R2). This math will yield the adjusted active coils needed to get rate back to nominal.
Here is an example. The spring coiler coils a spring that calls for 12.0 total coils (10.0 active coils) and a rate of 56.0 #/inch. He/she makes the first set of samples for testing, only to find the rate is actually 52.4 #/in. What to do?
Using the above formula, the answer is:
New active coils needed = (10 * 52.4) / 56
(Remember to use the active coils, not the total coils)
New active coils needed = 9.36….or 9.4
This simple formula should serve the springmaker well and take away the guess work associated with coil change for rate adjustment.
As useful as it sounds, there are a couple of issues (as seen below) to keep in mind when amending the active coils.
CAVEAT 1— If the coil change requires coil to be removed, that change will increase the pitch and that will raise the stresses, Be sure the change is not excessive or that increase in pitch could allow the spring to take set sooner in its deflection and this could be an issue. See an engineer if the change is excessive.
CAVEAT 2— If the coil change requires coil to be added, that change will decrease the pitch and this will add to the total solid height. Compression springs are quite commonly held to maximum solid heights to be certain they can deflect the total travel required. Be sure the change does not add so much coil that the maximum solid height is breached.