Couldn’t help it. Had to get some specs on these retractable keychain gadgets.
So, I got my grandson involved in the formal test. He’s 12 now, going on impatient, but I managed to throw in a digital scale that I’d been hiding from him which allowed his current psyche to control him just a bit. (The scale has a small piece counter function that I’ll hold back on for a while.)
What we found was interesting at least. On the two that I bought, they consistently held magnetically at over 2 lbs. (35 ounces).
Then to test the spring capacity to “balance” the load, we obtained a closely consistent 1.4 lbs. (22 ounces). This weight was a little bit of an average, probably because the spring retracting mechanism has a variable torque curve. It pulls less close to maximum extension than minimum extension, but there does seem to be a middle ground where it just holds the weight.
Anyway, I was able to explain / demonstrate a basic engineering process and the difference between raw engineering versus technical engineering, theory versus practicality and Engineer versus Technician.
I’m sure the folks that created this retractor went through many iterations of this process to develop a viable product, but so many people have access to so many technical products that they rarely consider all that went into them so they could find some use for the product (even a use that the original team never considered. 
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