Loosely held rotating object tendency to stay with CG

[buchacho]

I am trying to think about this scenario, but not sure what to think. Let's say you have a loosely held (low friction) disk object held between two clamps and spinning on a hub at a certain RPM. As the disk-shaped object is spinning, it is perfectly balanced on its X-Y CG. Does the rotating disk have a tendency to remain balanced with a rotational inertial force (ignoring friction)? If so, how would I calculate what the "self-balancing" force would be?

 

[ivymike]

Does the rotating disk have a tendency to remain balanced with a rotational inertial force (ignoring friction)?

I can't figure out what your question means ... could you paraphrase it?

 

[TheTick]

http://en.wikipedia.org/wiki/Gyroscope

 

[IRstuff]

I think the answer is no, assuming you're talking about placing a round object on a flat thing that spinning. The only thing that keeps a gyroscope from going out of whack is the bearing structure that constrains its movements.

what you describe might be called a "metastable" state in electronics. Any minor perturbation will unbalance the system, and it will get progressively worse, since there is no compensating force.

TTFN
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[buchacho]

To paraphrase: What force would be required to overcome the balancing of the disk when it is rotating at its CG (or central axis)?

The axis of rotation for the disk is fixed by say a rotating motor hub, and the disk is loosely held by a top and bottom clamp. The motor hub rotating axis is parallel with gravity.

Thanks TheTick, I was thinking of gyroscopes, but still not sure how to apply the concept to the scenario I described.

 

[GregLocock]

That's the spin dryer effect. The drum of a spin drier is loosely suspended quite deliberately, and while it wobbles initially, it eventually tends to rotate around the cg, or else it hits a limit switch, stops, and has another go.

I shall await erudite answers.

I suspect damping has something to do with it.




Cheers

Greg Locock


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[Tmoose]

When the CG of the floating disk moves off center far enough so the various centri-whatever forces exceed the "low" clamping force then something resembling this may happen.

http://www.youtube.com/watch?v=hW57tLMcqt4

 

[IRstuff]

Spin drying in the washer tends to be stable only if the the tub is balanced. Any slight imbalance is never corrected, and a large imbalance can cause the washer to dance across the laundry room floor.

http://www.hometips.com/how-it-works/washing-machine-parts-diagram.html

http://en.wikipedia.org/wiki/Washing_machine#Spinning

suggests that the apparent balanced spinning is due to a dynamic balancer.

Does the rotating disk have a tendency to remain balanced with a rotational inertial force (ignoring friction)? If so, how would I calculate what the "self-balancing" force would be?

There is rotational inertia, but no active force.

TTFN
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[GregLocock]

Never post in the middle of the night! Sorry, I misread the question completely.

Cheers

Greg Locock


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[buchacho]

Thanks for the responses, so there is no difference in pushing or sliding the disk when it is spinning or if it were static. The only thing holding it in place is the friction from the clamp, which would also be the same for both scenarios.

 

[IFRs]

A loosely held disk is unstable if acted upon by the smallest of forces. The smallest of forces will lead to an increasingly unstable situation. I don't think it has any tendency to rotate about the CG unless it is perfectly balanced, in a vacuum where gravity and temperature is exactly equal in all directions.

 

[GregLocock]

A couple of thought experiments.

Imagine a uniform disc lying on an ice rink. using a speedo drive, apply a torque about the vertical axis.

The point of application of the torque doesn't matter, the disc will spin up around the cg, as no translational force is applied.

Now clamp it to earth via a vertical axis bearing, again through the cg.

Apply torque, same result.

Now move the axis of the bearing to the rim of the disc. The disc rotates around the bearing, a large rotating restraining force vector is generated in the bearing. Now release the bearing, and the disc will shoot off in its last tangential direction, still rotating.






Cheers

Greg Locock


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[tbuelna]

A rotating disc that is only dynamically balanced in a single (X,Y) plane could obviously become unstable if unconstrained. Even a rotating disc dynamically balanced about all axes may become unstable if the rotational frequency couples with a structural mode of the disc.

 

[GregLocock]

I missed a rather important last sentence out. what axis des the disc now rotate about?

Cheers

Greg Locock


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[tomwalz]

From my narrow little world of Saw blades.

They do tend to be self balancing but it doesn't take much to unbalance them.

The big ones are tensioned and they lose tension with use so they tend to fold over and are thus sort of self unbalancing.

Maybe how much unbalancing (and recovery) gets done by how much force such s knows in logs, loose collars, bumps in a shaft, machine vibration, etc.





Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.