Compression Spring Force MSA

Dave Cx

Involved In Discussions
I have a PPAP from a supplier where the compression spring force MSA is not for the actual spring, the supplier said they used a 'stable spring' as this validates the force measuring system rather than the variation in using the actual part number being PPAP'd.

I have requested the MSA on the actual spring, but can anyone testify to the suppliers argument that 'some' spring designs are not initially repeatable / stable. I'm told a spring is set by compressing to close the coils then released, initially when compressed it returns to 95% of its original free length. When compressed a 2nd time 97% of free length, then after being compressed 5 or more times it recovers to the same length, more compressions = more consistent free length becomes. As the spring length stabilises so does force measurement.

The actual production process only requires 1 set (that's what we are paying for), so to stabilise (compress x10) the spring before the GR&R is not production representative. What would you do, I expect the new MSA on the actual spring will show the variation initially but then improve throughout the study?
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I would treat it like "destructive testing." So you get one crack at the spring. Not sure how to do MSA in that situation.

Bev D

Heretical Statistician
Staff member
Super Moderator
The intent of MSA is to separate part variation from measurement variation. this situation requires a conversion of science, logic and data. It’s not ‘cookbook’. First we have to realize that an initial measurement of an unconditioned spring is in essence a ‘proxy’ measurement. So the first step is to prove out the basic repeatability of the measurement system adn doing this with a set of stable springs is appropriate. Then teh supplier should measure several (30) springs from initial compression to stability. A correlation can then be made to the initial acceptable range of results to the stable period. At this point you know that if the system measures the first compression to be in the range proven by the study, then the final range that you will experience is also acceptable...this is waht MSA is about as opposed to a standard “Guage R&R”. It’s not about checking a box...


Involved In Discussions
Agree with above. I would not treat it as destructive initially, but would opt to assess if the dynamic property can be stabilized. If it can be stabilized, then I´d proceed with a GRR as normal.

If it cannot be stabilized, I would have done what you supplier did and use stabilized samples, arguing the same logic: the intention is to evaluate the measurement system, with some supporting description of the measurement setup, stabilized samples, and so on.

Am I your supplier? Just kidding, not in the spring business. But I have had to deal with forces in vehicle interior components.

The phenomenon of reaching stability and the principle of correlation described by Bev D is also what I would apply in an injection molding shop floor to allow measuring parts before they have fully contracted, say 10 mins after leaving the tool, to validate the start of production.

Dave Cx

Involved In Discussions
Thanks for the replies, this illustrates that (me) being so conditioned that you cannot deviate from the rules that you miss the point of what the study sets out to achieve. Just when you think you have seen it all........


Starting to get Involved
I was a Quality Manager in the spring industry for 20 years and gauge r&r studies were a nightmare. You have to take the material used into consideration. For example SST 613 it did not matter if you pressed it solid 20 times you would not get all the set out of it. Whereas if use chrome silicon and heat set it at a set height, those will work for a gauge study. Music wire will not get you there either. And it may be your pressing to solid height only one time may not be enough. If you want stability and the study used on your part, there are a lot of variables, and as you know only two things change load, and that the O/D of the spring and the length. We were tier one and actually packed up a load tester and went to the customer with virgin springs and had to put on a show, measuring springs after pressing one time, two, three............ and on and on. The finally realized their spring did not have the ability to pass a study so it decided we would press every spring 3 times before shipping and the customer would press them once because springs will relax sitting on a shelf.
Your spring supplier is telling you the truth. You may have to belly up to the bar and either change material or get a print change. If you are using any SST, music, chrome silicon (without heat set), vanadium, you will never get there. I will glad to offer help but it would be to much to type here. You can message me.

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