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[Jeremy McCreary]

But then again an inverted ball is not as  impressive as a top with stem that reverses and stays balanced on that stem even without spinning. 

Agree. You really need a visual that makes the viewer's inner physicist think, "That'll never stay up."

How about your flipping disks you mention here Jeremy? Any chance they stay balanced after inversion?

In theory, maybe, but in practice, no.

It takes a really strenuous twirl to get them to invert. And even then, never for long, and usually not all the way. In the process, you usually end up exciting extraneous motions besides spin.

Not a recipe for coming to rest inverted. Even with a small flat at at top dead center. And in this case, there's no real flat to speak of.

[Iacopo]

This thread inspired me a new system for to balance tops that could be used instead of the screws.

These are two eccentric rings.  They have the same thickness, and the littler one can be inserted into the larger one.
The two rings would stay in the flywheel of a top, and they would be free to rotate the one in the other and relatively to the top too.



When the heavy sides of the two rings, (the red dots), point in opposite directions, (A), they have no effect on the balance of the top.
Rotating the two rings so that their heavy sides point more and more in the same direction, gradually increases the amount of weight shifted to that side of the top, (B, C, D).  Of course, the weight can be shifted towards whatever sector of the top.
This makes this system suitable for to balance spinning tops.



The two rings should be placed at the same height of the center of mass of the top. 
Since the two rings are at the same height, one into the other, they would not introduce couple unbalance when they are rotated;
they only change the static balance of the top.

The design is not too difficult, and the eccentric rings could be a bit easier and more intuitive to use than the screws, so I will try to make them.   

[Jeremy McCreary]

Ingenious! Looking forward to ypur prototype.

[ortwin]

This thread inspired me a new system for to balance tops that could be used instead of the screws.

Another good example for independent rediscovery Iacopo!


Exactly that system that you propose, I came across about half a year ago when looking into ways to balance a top. The one I found at the time was actually offered for balancing a lathe.
My suggestion would be not to make them eccentric, but perfectly round and from a lightweight material like aluminum. Then you make in both  one radial thread where you can insert some heavy weight metal. I would guess this way it is easier to build as well as to work with.


https://news.thomasnet.com/fullstory/balancing-rings-benefit-machining-processes-516423
An example

[Iacopo]

My suggestion would be not to make them eccentric, but perfectly round and from a lightweight material like aluminum.

Thank you, Ortwin, it is a good idea to make them round instead of eccentric;
even if it is not difficult to make eccentric rings, (I have a chuck with independent jaws which would make the task easy), the design will be more beautiful making them round. 
One problem I will have to solve is that the rings must be steady, (not loose), the joints between the moving parts tight fitting, and at the same time the rings must rotate with the correct amount of resistance, not too easily, but also without difficulty;
in my experience aluminium is a material that tends to grip, it doesn't slip, and in this sense I don't like it for this application.
For these delicate precision parts I would prefer to use a more robust material like inox steel; then I could make holes or dimples at one side to make the rings unbalanced, it would work fine in this way too.     
 

[ortwin]

...
One problem I will have to solve is that the rings must be steady, (not loose), the joints between the moving parts tight fitting, and at the same time the rings must rotate with the correct amount of resistance, not too easily, but also without difficulty;
...
For these delicate precision parts I would prefer to use a more robust material like inox steel; then I could make holes or dimples at one side to make the rings unbalanced, it would work fine in this way too.       

My idea to solve this problem are again grub screws. This time not as balancing weights but as fastening measure. In the (heavy) inox steel you could make not only holes but threads. In those threads you insert light weight plastic/nylon grub screws.  It is a bit inconvenient that you would have to fasten the inner ring before you slide on the outer ring, but I guess one could live with that. The other little problem with this, would be the case where you need the two holes to be at exactly the same radial position. But I feel that this is a rather theoretical problem that should not arise in practice if your pre-balance is good enough.

[Bill Wells]

When the heavy sides of the two rings, (the red dots), point in opposite directions, (A), they have no effect on the balance of the top.

Very nice idea. Since the rings have different diameters, you would make the mass of the smaller slightly greater than the larger? So that in position "A" the rings are balanced so as to be neutral.
I have often wished for a system where the balancing weights could be moved while the top is spinning. No  stopping to re-balance. I am dreaming.

[ortwin]

...
I have often wished for a system where the balancing weights could be moved while the top is spinning. No  stopping to re-balance. I am dreaming.

It seems you are dreaming of some self balancing system we just started discussing here.

[Iacopo]

grub screws.... as fastening measure.

This is also a good idea, I will think to it.
My idea was to place the rings in the inner part of the flywheel of a top with recessed tip, with the rings in recessed position, where, with a bit of care, they would not be easily rotated by error. 

I would like to have bearing rings for to hold the rotating parts, but it could be difficult to find bearing rings of a suitable size.
Maybe I should use some teflon in contact with the rotating rings, or maybe I could use phosphor bronze... 
My primary concern is that, making the rings with sufficiently tight fitting joints, the rings could be difficult to rotate.

[ortwin]

But what advantage do you expect over set screws by using the rings?

[Iacopo]

But what advantage do you expect over set screws by using the rings?

Mainly it's the pleasure to experiment making something different.
Then I don't know what it could come out. Maybe the system will be too much labour intensive to make it work properly.  But I will not know if I don't try.  Aesthetically I think that it could be more elegant than the screws.  Also the rings could be a bit more intuitive to use than the screws.  So I want at least to try.   

[Iacopo]

Since the rings have different diameters, you would make the mass of the smaller slightly greater than the larger? So that in position "A" the rings are balanced so as to be neutral.

Not necessarily.
Think to a washer, for example, it is a symmetrical body, with the center of mass in its geometrical center.
If you cut the washer along the dotted line, (supposing a thickless cut), you obtain two eccentric rings, but if you leave them in the same position as before to cut them, together, they still have the center of mass in the same position as before, even if the outer ring is the heavier one.



But if I use round rings with holes at one side, in this case I agree that I will have to remove more weight from the littler ring, for the rings being able to be neutrally balanced.

[Jeremy McCreary]

A triangular offset top in that it spins with surprisingly little wobble about the chosen spin axis:

[ortwin]

Well done Jeremy!
The offset is not gigantic but clearly visible.  Did you receive that spiral for the Lego pump top already?

[Jeremy McCreary]

Well done Jeremy!
The offset is not gigantic but clearly visible.  Did you receive that spiral for the Lego pump top already?

Thanks! Not yet. Have been putting off buying new LEGO for 2 years now, as I no longer have room for the stuff I already own. But with the lead screw, the number of newly introduced parts with juicy top-making potential has finally reached critical mass. Heaven help me when I go back on BrickLink.