TLDR; If you access to an Instron and can run a test on the Peg spring, it will help us discover if there is a way to make a more robust spring that behaves literally the same in every way that FIRST teams care about.
Chief Delphi Community,
I am on a quest to see if there is any room to improve the spring on used for the Peg so that it is less prone to being deformed in use while keeping everything that is essential to teams for design purposes (e.g. spring dimensions, spring stiffness rate, initial tension in the coils, …).
It may not be possible… …but after talking with some spring experts (see this thread Looking for Pics of Pristine & Damaged/Broken Peg) there is a reasonable chance that there might well be.
Let me start by saying that I am not FIRST, I am not working with FIRST, I am just a freelancer trying to give FIRST options if they decide they need it.
Okay so here is the deal. There is a reasonably good chance that the spring that is sold by McMaster (9664k68) is not the made from “music wire” (the steel that results in a spring with the highest yield point) but another type of steel.
There are many ways to discover this** but most involve lab tests and take time and equipment FIRST folk don’t have hanging around (typically).
I think that in this case, the fastest way to discover the answer is for someone reading this messaging who also has access to an “Instron” to do a test for me.
Here is the procedure:
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Take a 9" length of the 9664k68 spring (the length of the peg), bend hooks into each end (snap a picture next to a scale so we can estimate the “active coils” of the spring – not 100% needed but helpful)
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Install the spring into an Instron.
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Pull slowly from unloaded to loaded state.
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Zero the travel when the coils first start to separate (should be ~25lbs)
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Move to .5". Note force
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Move back to 0.0" travel. Note force
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Move to 1.0". Note force
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Move back to 0.0" travel. Note force
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Move to 1.5". Note force
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Move back to 0.0" travel. Note force
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continue going up by 0.5" increments until the force at the Zero Position has reduced by significant amount (see example data chart below)
A music wire spring should fail to return to the 25lbs* somewhere near 5" of extension. A spring made with steel with a lower yield will fail to return to 25lbs* at after smaller extension.
So… This is not an easy thing to do but it isn’t that hard for a person with the right equipment.
With these data, we can infer what the type of steel that the manufacture of the spring is using to make these springs**.
Once we know that, we can then discover if there is significant room for improvement to make the peg less susceptible to damage (not impervious).
If there is a way to do this without being unfair to the teams that have depended on the “normal” properties of the spring (i.e. those before the spring starts to permanently deform) then maybe it will make sense for FIRST to change to a new, more robust spring design.
Time will tell, but not if a volunteer doesn’t step up and offer to do these tests.
Cheers,
Dr. Joe J.
*or whatever the initial force you get from your batch of springs. Typically these vary 15% from batch to batch.
**Yes, I have a request into McMaster – they are typically quite fast with responses. We will see. But even if I knew the material, I still want the data to see if there is any margin for improvement.
