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Author Topic: A Figure of Merit for Twirler Spin Time  (Read 2476 times)

Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #150 on: January 09, 2018, 03:19:48 PM »

Hello Iacopo,

Let's talk about balancing. 

I'm beginning to think that my machining method, which places the ball within about .0002" of center may not justify further balancing.  With your lighter tops, how close to perfect centering must you get before further improvements are of limited value?

For example if my minimum RPM is 200, and perfect balance would reduce that to 195, that would only extend spin time by about 15 seconds or 1.3% of total.  Perhaps that's not enough to justify dynamic balancing and movable weights, etc.

Also, there are alternative ways to reduce, minimum RPM, such as lowering the CG.

Best regards,
Alan
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Jeremy McCreary

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Re: A Figure of Merit for Twirler Spin Time
« Reply #151 on: January 09, 2018, 07:17:52 PM »

Also, there are alternative ways to reduce, minimum RPM, such as lowering the CG.

Or increasing specific AMI, decreasing specific TMI about the CG, or reducing the acceleration of gravity -- as we would have seen if the Apollo astronauts had had the good sense to take tops to the moon instead of golf clubs. (Here, "specific" means "per unit mass".)

Of these, lowering the CG and increasing the specific AMI would have the greatest effect.
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #152 on: January 10, 2018, 12:52:56 AM »

AMI?
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #153 on: January 10, 2018, 03:51:19 AM »

my machining method, which places the ball within about .0002" of center may not justify further balancing. 

Not sure but I think all the big makers do not balance their tops but they use accurate machining methods, as you do, so that their tops come out from the lathe already balanced.

I prefer a more accurate balance for my tops so I always fine tune it.  I do so mainly because I don't like the wobbling due to unbalance which comes out at slow speed.

I check for static balance, wich is more than sufficient, and only rarely dynamic balance;
because of the way I make my tops, I can have the tip slightly off centered, which causes static unbalance, but not couple unbalance, so practically I only need to correct the static one.

I use the balancing grub screws only in tops with replaceable tips; each time a tip is replaced, the top needs to be balanced again, and the screws are very practical for this aim.
With a fixed tip instead, if needed, I prefer to correct the position of the tip and/or add a permanent weight in the top.   

« Last Edit: January 10, 2018, 04:01:22 AM by Iacopo »
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #154 on: January 10, 2018, 03:57:07 AM »

AMI?

AMI is axial moment of inertia, TMI transverse moment of inertia.
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #155 on: January 10, 2018, 06:25:22 AM »

Thank you Iacopo.  Do you have a guess at the increase (if any) of minimum RPM which would result from moving the tip from perfectly centered to .0002" off center?

Best,

Alan
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #156 on: January 10, 2018, 02:31:03 PM »

Do you have a guess at the increase (if any) of minimum RPM which would result from moving the tip from perfectly centered to .0002" off center?

This is half a hundredth of millimeter..  not a lot..
Maybe, it would cause a 1 - 3 % reduction in minimum RPM ?
I never observed carefully this aspect.
I think there is always at least a bit of loss in the spin time, anyway, also because, apart from probable reduced minimum RPM, there should be more friction in the tip especially at high speed, when the tip is not well centered.
 
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #157 on: January 10, 2018, 02:43:08 PM »

Thank you Iacopo.

I asked that question because I have recently noticed that extra efforts to precisely center the tip have not yielded significant reductions in minimum RPM.  So I'm thinking the tip should be well centered, but perhaps it's not worth the effort to get it better than about .0002".  I'll continue to monitor this and refine my data.  Perhaps you will too.

Also, the above number relates to small tops.  Your larger tops may tolerate greater eccentricity.

Best regards,
Alan
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #158 on: January 13, 2018, 02:48:05 AM »

My tops with long stems have strong intermediate wobble a few hundred RPM above the minimum RPM.  Shortening the stem usually greatly reduces this, but doesn't eliminate it.  The decay rate is higher during wobble.  Wobble is also more likely as height of CG increases.

But even my tops with short stems have slight intermediate wobble, perhaps at an RPM much closer to the minimum RPM.

Today, I made my most precise 1.5" top ever.  It weighs 62 grams and looks like recent photos that I've posted of grooved brass tops.  Stem is only 0.7" long.  But even this top has slight intermediate wobble.  I've twirled it for 20:52 resulting in a record merit = 7.6.  To achieve this time, I tilted the mirror (to move the top to fresh lube) continuously.  Right after that, a run with no tilt has 19% shorter duration.  So lube is very important.
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #159 on: January 14, 2018, 09:32:52 AM »

a run with no tilt has 19% shorter duration.  So lube is very important.

I agree, I too see quite shorter durations, without lube.
As for the intermediate wobble, I still can't understand what it could be; based on my experience, it should be nutation triggered by large contact points, otherwise it is something I don't know.
If it is nutation, a good lubrication, (which you achieve also tilting often the mirror), should reduce it. 
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #160 on: January 14, 2018, 03:37:21 PM »

For a long time, I was using 3/8" and 1/2" balls.  Lately, I'm using mostly 3/16" balls and have started to notice the intermediate wobble.  I define this as a wobble which peaks (well above minimum RPM), then smooths.

I watch my tachometer, which declines in one RPM steps.  Then, near the end, the top wobbles and when the wobble is severe the tach can't read properly and declines in a larger step.  I'm defining minimum RPM as the last reading before the larger step.  Normally the top falls within a second or two of this last reading.

Alan
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Jeremy McCreary

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Re: A Figure of Merit for Twirler Spin Time
« Reply #161 on: January 14, 2018, 04:12:29 PM »

I think Alan's intermediate wobble is unlikely to be pure gyroscopic nutation. Perhaps he could use Iacopo's nutation diagnostic to test that hypothesis.

Iacopo's original description of the nutation test
« Last Edit: January 15, 2018, 03:33:18 PM by Jeremy McCreary »
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Iacopo

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Re: A Figure of Merit for Twirler Spin Time
« Reply #162 on: January 15, 2018, 03:36:18 PM »

Alan, could you try to mark the stem of your top during that intermediate wobbling ?
If yes, the marks appear all on the same side of the stem, or they are randomly distributed around it ?

I know it is a bit difficult to do this with light tops;  for having more marks, it could help to spin the top more than one time.
« Last Edit: January 15, 2018, 03:51:29 PM by Iacopo »
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Jeremy McCreary

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Re: A Figure of Merit for Twirler Spin Time
« Reply #163 on: January 15, 2018, 03:54:34 PM »

Alan, could you try to mark the stem of your top during that intermediate wobbling ?
If yes, the marks appear all on the same side of the stem, or they are randomly distributed around it ?

I know it is a bit difficult to do this with light tops;  for having more marks, it could help to spin the top more than one time.

Ah yes, a much simpler test if feasible. I've had good luck marking light tops with a slack chalk line. To use the rate-comparison test I was thinking of, you'd need to know the top's AMI to at least 2 significant digits.
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Aerobie

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Re: A Figure of Merit for Twirler Spin Time
« Reply #164 on: January 16, 2018, 09:53:10 PM »

I can do this with my strobe light, and with slow-motion video.  I'll post some stuff in several days.

Alan
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