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I have an irregular shaped object about 100g in weight and 3x3x3 cm in size that I'd like to mount on a shaft (eg of a small motor) and spin. The trick is, I'd like it to be pretty close to perfectly balanced.

Sure, I can 3D print a rotor and attach the object to it, but (considering tolerances, and inaccuracy of my model) this will almost certainly place the center of mass a few millimeters away from the axis of rotation. Then, I could have some small adjustable weights (eg a couple of screws that get closer/further from the shaft) which I could use to try to balance it better; but I really don't know how to do that without a lot of trial and error.

Is there a setup where a slightly off-balance rotor "auto-balances" itself?

I'm thinking of something similar to a spinning top (or just an object spinning in free space) basically something where any imbalance generates a force that brings the center of mass closer to the axis of rotation, not further as centrifugal force normally would. It could do that by moving the axis of rotation itself. I imagine a lot of devices with really fast rotors (turbomolecular pumps, etc) almost have to have something like this.

Note: in my case this is not spinning fast, under 1000 rpm, so it doesn't have huge forces acting on the rotor. I'd just really like it to be vibration-free.

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    $\begingroup$ Do your first approach. And no, turbomolecular pumps are manually balanced. As are jet engines. $\endgroup$
    – DKNguyen
    Dec 4, 2023 at 2:54
  • $\begingroup$ @DKNguyen Huh, I wouldn’t have guessed that. Any tips about exactly how to manually balance if I go that route? (other than trial and error) $\endgroup$
    – Alex I
    Dec 5, 2023 at 6:35
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    $\begingroup$ You put the rotor in between low friction spindles and adjust it until it stays where it is rather than rotating to drop the heavy side low. Look at videos on RC air plane propeller balancing. $\endgroup$
    – DKNguyen
    Dec 5, 2023 at 15:29

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The high school science method is to hang your object from two random points and mark the vertical plumb lines, the intersection of the two lines is the CG.

There are a vast number of different types and different precision levels of automatic balancing depending on the critical utility demand of the part. A car tire can be balanced with a simple machine. An airplane propeller is more precise. A several-ton steam power generator turbine, a jet engine, or a windmill with 80-foot-long blades all require specific ways. Som worth may be a Ph.D. level theses.

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