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Author Topic: introducing myself  (Read 2506 times)

ortwin

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Re: introducing myself
« Reply #60 on: February 23, 2021, 02:35:06 PM »

Curtain-Ring-Top Nr. 1.5 - colon top
This is the symmetric version of Nr. 1 :

 

It has some more air drag then Nr.1, but it has the advantage that tip positioning and top balancing can be performed independent of each other:

After the tip of the lower pin is adjusted by radial shifting and by tilting of the spoke so that is on the axis of the ring, the upper pin can radially shifted and tilted until the top is balanced.

https://youtu.be/Lgjgga5xaw8

Besides the commercially available "exclamation mark top"and the PERIOD top (curtain rin top nr. 1) this "colon top" is the third punctuation top I know of, can you think of any other ones?


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ta0

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Re: introducing myself
« Reply #61 on: February 23, 2021, 09:01:02 PM »

You have created a family of curtain-ring tops!  ;D

Besides the commercially available "exclamation mark top"and the PERIOD top (curtain rin top nr. 1) this "colon top" is the third punctuation top I know of, can you think of any other ones?

I cannot find the post, but we once discussed this top offered on the Shapeway marketplace, the Less is More spinning top:

https://www.youtube.com/watch?v=TScaKArj0RA

I like its looks, but the dynamic balance cannot be perfect along the axis. It still manages over one minute and a half with less than 2 grams of weight.

They also have a medium size and an extra large version.

When we originally discussed it, I tried to order one but the order was later canceled because they could not make it in the material I had chosen ??? I don't recall the material I selected but it definitely was not solid gold  ;D
Edit: I checked my Shapeways history and the material was polished bronze steel. It seems that the problem was the "polished" part, so the creator removed that option. I ordered again but it will take a month to deliver.
« Last Edit: February 24, 2021, 10:27:14 AM by ta0 »
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Jeremy McCreary

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Re: introducing myself
« Reply #62 on: February 23, 2021, 11:25:24 PM »

Curtain-Ring-Top Nr. 1.5 - colon top
Besides the commercially available "exclamation mark top"and the PERIOD top (curtain rin top nr. 1) this "colon top" is the third punctuation top I know of, can you think of any other ones?

I like the colon top. Here's the asterisk top...


« Last Edit: February 23, 2021, 11:27:56 PM by Jeremy McCreary »
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Playing with the physical world through LEGO

Jeremy McCreary

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Re: introducing myself
« Reply #63 on: February 24, 2021, 09:49:49 AM »

@ta0: The elegance of that top design and the efficiency of its mass distribution are hard to beat.

The rendering you showed handles its inherent couple unbalance well. My much larger LEGO knock-off is a favorite despite the minor wobble.

@ortwin: I'm calling the curved stem and tip arms a pair of commas.
« Last Edit: February 24, 2021, 12:54:42 PM by Jeremy McCreary »
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ortwin

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Re: introducing myself
« Reply #64 on: February 24, 2021, 02:40:34 PM »

The nice top that ta0 showed us, reminded me of the paper clip  top in the (German) paper I attach here.




Probably well known around here, but I can't know for sure.

While I was searching to find that paper again I came across this also very special carousel top:
https://www.instagram.com/p/BxAZA-7FwGh/ 
@Jeremy: how fast can you make a Lego version of this one? Or have you done it already?


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Jeremy McCreary

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Re: introducing myself
« Reply #65 on: February 24, 2021, 03:49:56 PM »

@Jeremy: how fast can you make a Lego version of this one? Or have you done it already?

A forum post back in 2017 inspired this attempt at a Sakai top with the usual 60⁰ ring gap...

https://youtu.be/X859epET9Fg

Dynamic balance turned out OK but not great. Truly awful aerodynamics.

Probably 3 hours of work -- most of which was spent in stiffening the structure and fiddling with small trim weights.
« Last Edit: February 24, 2021, 03:56:36 PM by Jeremy McCreary »
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ortwin

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Re: introducing myself
« Reply #66 on: March 01, 2021, 03:29:42 PM »

Curtain-Ring-Top Nr. 6 - three double spokes, interchangeable tip




In principle the tip can be positioned precisely in the center by tensioning  the three double spokes accordingly. In this simple plastic version though it is not so easy once the top is completed.
 If the single lines are arranged properly next to each other, a double spoke can have the appearance of wide spoke which we said could possibly deliver an aerodynamic advantage. 
Even with only three (double) spokes, there are quite some different possibilities one  could arrange the strings/spokes: more complicated then the one shown here and also more simple possibilities.

The ceramic ball is glued to the inner small tube, the ball pen tip is just stuck into it. I have no idea if it has a steel ball or a tungsten carbide ball, but at this level  of refinement it will not matter at all. 

https://youtu.be/5mA4SOTyE2o
 The centering of the tip was only roughly done by eye judgement, no further balancing. Both versions Nr. 6a and 6b can spin for about a minute. Does that mean a brass version will spin for 10 minutes ? I could probably  spin it to the same starting RPM, that means almost ten times as much energy due to density difference. But then the loss of energy over time due to aerodynamics is roughly the same as in the plastic case, so ............I know this is not so easy, and also not completely correct, but 5 minutes should be possible with a brass Curtain_Ring_Top of similar (but  a bit more careful) build .
 As "spokes", thinner and stronger strings can be found in fishing supply stores.



0.03 mm sounds thin, the ones I used here measure 0.3 mm.There is even a fishing line called "Platinum". I am afraid it's only the name, otherwise we  would just take a reel of that stuff as flywheel....

This is the first of the Curtain-Ring-Tops that I am thinking of taking it up a step and build as a metal version. I really consider investing in a brass curtain ring! Well, maybe  after I make one or two other plastic Curtain-Ring-Tops. When I start with the brass ring tops, I will call it the "golden series". Just for the heck of it!;-)

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ta0

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Re: introducing myself
« Reply #67 on: March 01, 2021, 05:42:48 PM »

Your ring tops look pretty good considering the materials you use.

Yeah, I think you can hit 5+ minutes with the brass ring if you build it carefully.
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Jeremy McCreary

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Re: introducing myself
« Reply #68 on: March 01, 2021, 06:55:03 PM »

Curtain-Ring-Top Nr. 6 - three double spokes, interchangeable tip
Both versions Nr. 6a and 6b can spin for about a minute. Does that mean a brass version will spin for 10 minutes ? I could probably spin it to the same starting RPM, that means almost ten times as much energy due to density difference. But then the loss of energy over time due to aerodynamics is roughly the same as in the plastic case, so ............I know this is not so easy...

When I start with the brass ring tops, I will call it the "golden series". Just for the heck of it!;-)

Love this curtain-ring series -- especially the last one with the pen tip! My gut tells me that the spin time isn't going to end up proportional to ring density, but I'll think about it.
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ta0

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Re: introducing myself
« Reply #69 on: March 01, 2021, 10:33:05 PM »

Love this curtain-ring series -- especially the last one with the pen tip! My gut tells me that the spin time isn't going to end up proportional to ring density, but I'll think about it.

My gut also told me that it should be something more complicated than linear, but that's not what I got.

If we assume the air drag is proportional to the spin w then we previously got an exponential decay of the spin:

w = w0 exp(-k/I t)

where I is the moment of inertia, w0 the initial spin and the drag torque = k w.
If we set w to the critical spin at which the top falls, wc, and we solve for the time tc to get there:

tc= I 1/k ln(w0/wc)

So the time it takes to fall is indeed proportional to the moment of inertia and therefore to the density, if you keep the dimensions constant and you also manage to keep the initial spin constant.   :)

Edit: This seems to be a general result, not just for an exponential decay. We found before: I dw/dt = - T, where T is the torque produced by air drag and tip friction and may be an arbitrary function of the spin, T(w), but does not depend on I. It can be re-written as  - 1/T(w) dw = 1/I dt.  Integrating both sides: F(w) = 1/I t, where F(w) can be a complicated function of w, but does not depend on I or t. Then tc = I F(wc).


« Last Edit: March 01, 2021, 11:57:14 PM by ta0 »
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Iacopo

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Re: introducing myself
« Reply #70 on: Today at 03:28:46 AM »

Using brass instead of plastic with the same shape/dimensions will not change the air drag but will change the tip friction, being brass denser, so the advantage as for spin time should be not proportional to the flywheel density, but a bit less.
If brass is about 7 times denser than plastic, the brass top could spin maybe 4-6 times longer than the plastic one, at parity of all other factors.
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ta0

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Re: introducing myself
« Reply #71 on: Today at 09:17:07 AM »

Good point on the dependence of tip friction on weight, what I did not consider. Tip friction torque is proportional to weight and independent of rotational speed. The question is how big it is with respect to air drag. Given that the tip and mirror are hard surfaces and even the brass ring would be relatively light, plus the critical spinning speed not very low, I expect the air drag to be dominant.
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