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Author Topic: A Figure of Merit for Twirler Spin Time  (Read 12200 times)

Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #195 on: January 30, 2018, 03:09:29 PM »

I leveled the flat side of a mirror that I'd installed thumbscrews on.  A ball helped as a level.  Even more sensitive is a spinning top. 

I too tried this, yesterday.  I used sheets of paper to level my flat mirror, (but your thumbscrews are certainly more practical).
I had the same result, the top seems too sensitive, even with a horizontal mirror the method seems good for starting balancing a top but not for for fine tuning the balance.


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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #196 on: February 11, 2018, 12:53:26 AM »

Why Can't I Scale Up Stability?

The top on the left was made a few years ago.  1" diameter, 48 grams, 3/8" ball.  I noticed recently that it's more stable than any top I've ever twirled.  Knock it while spinning and it rights in a few seconds.

So I thought it would be nice to scale it up.  I made one 50% larger with the same proportions.  But it doesn't even begin to show stability.  Knock it and it remains tilted and orbiting.

Why?
« Last Edit: February 11, 2018, 01:43:45 AM by Aerobie »
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #197 on: February 11, 2018, 02:26:11 AM »

Have you scaled up the dimensions of the ball tip too ?
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #198 on: February 11, 2018, 05:38:14 AM »

Yes.  Correct scale up is 9/16" ball.  I tried that and 1/2" too.  Strangely, the 9/16" (larger) ball has lower decay rates (~10%), while the smaller ball decay is about 11% to 12%.  Both balls are brand new.  Perhaps the larger ball has better finish.

But decay is a diversion in this case.  I was aiming for stability.  The original 1" top is impressive in how rapidly it rights when disturbed.

This has led me to also want to experiment with Cg height and stability.  At this time, it appears that higher Cg rights quicker when disturbed, while lower Cg tends to remain tilted and orbit longer.

Alan
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #199 on: February 11, 2018, 08:15:17 AM »

As for what I know, a top should raise faster when Cg is lower, and when the ball tip is larger.
Maybe without lubricant it raises faster.
I noticed that the material of the contact points has an influence; my top Nr. 25 uses replaceable tips; with the ruby tip, the top raises in vertical position in 5 - 10 minutes.  But if I use the teflon ball instead of the ruby, (same diameter), the top raises much faster, one minute or less.  I don't know why.
« Last Edit: February 11, 2018, 03:47:40 PM by Iacopo »
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #200 on: February 11, 2018, 12:38:49 PM »

The physics of rising are described in a book I've owned for many decades, Classical Mechanics: A Modern Perspective, by Barger and Olsson.  Here is their diagram.

The diagram shows friction, offset from the axis of rotation, being converted to rising torque by gyroscopic precession.  Big balls rub the mirror farther from the axis of rotation for a given tilt angle.

In your case, I expect your teflon ball is flattened where it touches the mirror because it is soft.  Although it's a low friction material, it's only moderately lower than your ruby (sapphire) which is hard and not flattened.  So the flattened teflon rubs the mirror farther from the axis of rotation. 

I'm sure that you've noticed that a point has no righting effect.

However, to the degree that a tilted top orbits, friction is reduced.  Friction requires slip.

Best,

Alan
« Last Edit: February 11, 2018, 02:06:36 PM by Aerobie »
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #201 on: February 11, 2018, 04:31:39 PM »

This is something I have to think about.
I can confirm that pointed tips have little righting effect, compared to ball tips.  Still there is some righting effect in them because even when they are sharp, their contact points are never exactly an infinitesimal point but they are always at least a bit rounded, so they behave like tiny balls. 
When the Cg is very low, as in my tops with a recessed tip, the top rises relatively fast, (a few minutes), even if the tip is spiked.
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #202 on: February 11, 2018, 06:49:59 PM »

"When the Cg is very low, as in my tops with a recessed tip, the top rises relatively fast, (a few minutes), even if the tip is spiked."   In this case, is the Cg above or below the tip?

Regards,
Alan
« Last Edit: February 14, 2018, 07:38:22 AM by Aerobie »
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #203 on: February 11, 2018, 09:12:51 PM »

I made a 1.75" diameter wheel with setscrews to move it up and down a shaft.  With very high Cg, it recovers from a deliberate disturbance to vertical, stable spinning more quickly. 

The high Cg makes it harder to twirl and shortens the duration due to increased minimum RPM.  But it does recover more quickly.

Alan
« Last Edit: February 11, 2018, 11:50:04 PM by Aerobie »
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #204 on: February 12, 2018, 02:13:13 AM »

"When the Cg is very low, as in my tops with a recessed tip, the top rises relatively fast, (a few minutes), even if the tip is spiked."   In this case, it the Cg above or below the tip?

The Cg is above the tip, by maybe 1 mm.
I also tried with the Cg slightly below the tip: at high speed the precession behaviour is similar, but precession happens in the opposite direction of that of a top with Cg above the tip.  In both cases the top rises generally in a few minutes.
At very slow speed, with Cg below the tip, the top tends to rise spontaneously.

Your observation of tops with higher Cg that rise faster is interesting.  I still have much to learn.
« Last Edit: February 12, 2018, 02:15:22 AM by Iacopo »
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #205 on: February 12, 2018, 10:38:42 PM »

Regarding a higher Cg.  I do not like the resulting increase in minimum RPM.  So I strive for low Cg.    But the shortened rise time of higher Cg is interesting.  The effect is not great, but enough to see.  You should try it too.

I've also sometimes gone too low in Cg, resulting in too many scrapes during launch.

Your long stems are great.  They permit lower scrape angles.  But they too have issues.  The stems on my small 50 gram tops aren't any lighter than your wood stems.  But they have a greater Cg effect on my small, light wheels -- when compared to your larger and heavier wheels.  Also they complicate pocketability, which I often desire.   I'm experimenting with removable stems and with twirl, then lift-off stems, leaving the stem-less wheel to spin.

Alan
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #206 on: February 13, 2018, 02:31:32 AM »

I found this video with a very high Cg top, it rises quickly, in a few swconds.
The top was started at very high speed.

https://youtu.be/5TqxF3jGKlQ?t=15
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the Earl of Whirl

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Re: A Figure of Merit for Twirler Spin Time
« Reply #207 on: February 13, 2018, 05:06:56 AM »

Fun video.  That is quite an assortment of high Cg tops.  Some of the comments are hilarious like "is this some sort of fettish or something" and "those should be enemies in dark souls."
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Happiness runs in a circular motion!

Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #208 on: February 13, 2018, 08:53:39 PM »

That guy is spinning his high Cg tops at 12k RPM.   I'll experiment with some string launches too.  But expect more like 4k to 5k RPM.

Alan
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #209 on: February 14, 2018, 07:35:54 AM »

I realized today that my very stable 1" diameter top will orbit for a long time if disturbed at high RPM (1,000 to 2,000). 

It is most stable between 700 and 800 RPM.  When disturbed while in this range, it orbits for only a few seconds.

Alan
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