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Author Topic: Top Nr.15 (38 minute spin duration!)  (Read 1035 times)

Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #15 on: June 13, 2019, 03:57:59 PM »

James, Iacopo: At what time, if any, in the graphs above did your tops start precessing? Wobbling?

Are videos of these tops available? Of their entire spins?

Even ballpark times would be helpful. Thanks!

I am sorry but I don't have those data, nor a video.

I can say that, in tops like these, there is generally some nutation and precession at the start, which disappear in a few minutes, until the top spins in sleeping position.  The wobble that appears at the end of the spin is "unbalance wobbling".
There is not nutation nor precession at the end of the spin, in these recessed tip tops, or, at least, I never noticed them, (unless the top was already nutating/precessing before, never having reached the sleeping position).

One curious fact about efficiency and wobbling is that not always a wobbling makes the top less efficient;
in my tops with recessed conical tip, this is still true, and efficiency is better when the top does not precess and does not nutate.
So I strive to spin the top as vertically as possible when I want a very long spin with one of my tops.
Anyway I noticed that the countrary happens with tops with a ball tip spinning on a flat surface:
efficiency is better when the top precesses instead of sleeping, (at parity of RPM).
Curious, isn't it ?
This is something you can experiment by yourself.
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #16 on: June 13, 2019, 06:00:07 PM »

I don't have footage of the 38 minute spin, but i do have a time lapse of a 32 minute spin of this top. In this video, it takes 9 minutes for precession to completely disappear and wobble starts to appear at about 22 minutes in. This wobbling starts very subtly, then quickly increases until the top falls. I believe I started the 38 minute spin a bit faster, and the balance was a bit better. So, I think in the 38 minute spin, wobbling would also have started about 8 minutes from the end (30 minutes in) because the better balance would allow it to spin slower before wobbling starts.

Ignore the smoothed red curve in your efficiency vs. time plot and look just at the data points. Allowing for noise, you can make out a rising segment on the left, a falling segment on the right, and a more or less flat segment in between.

Based on your observations of spin-down behaviors, sounds like the flat segment might have occurred during sleep. If so, the upward and downward segments could reflect changing tip resistances during behaviors other than sleep.

Iacopo's efficiency curve for his Nr. 20 has the same structure. It must mean something.

Next time I do a spin duration test, I will look out for when wobbling starts.

That would be helpful. Getting a good spin decay curve (SDC) is the first step toward understanding the air and tip resistances in play during spin-down. I think we'd get some valuable insights from annotated SDCs showing when sleep, precession, wobble of any kind, and combinations of these behaviors were present. Wish I could collect good SDCs on my LEGO tops, but air resistance and low material densities bring nearly all of them down in under 2 minutes.

Collecting SDC points every minute on the minute gives you a feel for how the spin decay rate is evolving without plotting the data. But the most important thing in collecting a good SDC is to record the right times for the speeds actually measured -- even if the times aren't exactly on the minute.

Since the speed measurements take time, there will always be some slop in time-speed registration, but the misregistration should be minimized as much as possible. The big dips in your efficiencies at 19:00, 33:00, and 38:26 could be real if top was bumped or the tip hit an irregularity on the table. Otherwise, time-speed misregistration is a good suspect.

This might help: Calculate lifetime from successive measurements instead of efficiency (formula above). Then you can collect data points whenever convenient. The lifetimes will automatically correct for differing time steps between speed measurements.
« Last Edit: June 13, 2019, 11:24:14 PM by Jeremy McCreary »
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #17 on: June 14, 2019, 04:38:05 AM »

Based on your observations of spin-down behaviors, sounds like the flat segment might have occurred during sleep. If so, the upward and downward segments could reflect changing tip resistances during behaviors other than sleep.

I believe that the downward segment at the left is mainly due to high air drag.
Then this way "efficiency" is calculated may be misleading because it may lead to believe that there is an increase of the total friction at low speed but this is not true, because we have the lowest total friction at the lowest speeds.   

Like James, I too observe continuous changes of the efficiency of the top during the spin.
my tachometer is good enough and I can say that these continuous changes are real.
These continuous and unpredictable changes are due to the tip friction.  Air drag too changes, at the decreasing of speed, but gradually, and always in the same way. 

Here are the data of the tip friction, (obtained making the top spin in deep vacuum conditions), of 19 spins of  my Nr. 27, (156 grams):
the top has an external carbide tip and was spun always vertically so there was nearly no precession at the start of each spin.
 






The 19 spins are numbered chronologically, the spin Nr. 1 was the first spin performed, when the tip was new and very sharp.
The first two spins are the ones with the lowest friction, telling that a sharp tip has lower friction that a dull tip.

The spins with a red number were performed without lubricant, the ones with green, violet and blue numbers are spins with some different lubricants.  It is evident that the tip friction is lower with the lubricants.

There is a continuous and random variability of the tip friction. 
There are more important and frequent changes in heavy tops than in light tops, especially without lubricant.

This is the tip friction of a light, (77 grams), top on lubricated spinning surface;
With light tops spinning on a lubricated surface there are not significative up and downs but the tip friction decreases gradually with speed.


 
I believe that the unpredictable changes of the tip friction are related to wear. 
In fact there is wear in the contact points;
this is the photo of a dimple in the carbide spinning surface caused by wear, at the microscope; the dimple is about 0.1 mm large and 0.03 mm deep, and is the result of 50 hours of spinning of a top with a conical tip.
These dimples are responsible for making tops with a recessed tip to wobble spontaneously, (nutate), when they spin on them.

 
« Last Edit: June 14, 2019, 05:00:55 AM by Iacopo »
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #18 on: June 14, 2019, 01:43:48 PM »

Love discussions like this -- especially when they bring in real data on real tops!

I believe that the downward segment at the left is mainly due to high air drag.

Could well be. I'll have more to say about this in a coming post. The spin decay curve (SDC) data you provided some time ago on your Nr.25 under normal air pressure will figure prominently.

Like James, I too observe continuous changes of the efficiency of the top during the spin.
my tachometer is good enough and I can say that these continuous changes are real.
These continuous and unpredictable changes are due to the tip friction.  Air drag too changes, at the decreasing of speed, but gradually, and always in the same way.

Totally agree (i) that air resistance isn't to blame for the sudden variations in efficiency, and (ii) that varying tip resistance (sliding friction + rolling resistance + active wear processes + ??) is likely the main culprit. But still worry about artifacts due to time-speed misregistration. The efficiency calculation could be quite sensitive to such errors.

Q1: How exactly do you assign times to your speed measurements? (For both Iacopo and James.)

My tachometer readings can take many seconds to stabilize. Certainly couldn't take them every minute on the minute and expect the speeds I get to correspond to exact minute marks.

That's why I keep pushing lifetimes (formula several posts back) instead of minute efficiencies to track the evolution of total resistance. Lifetimes correct for variations in measurement intervals automatically.

There is a continuous and random variability of the tip friction.
There are more important and frequent changes in heavy tops than in light tops, especially without lubricant.

Interesting and potentially important observation.

I believe that the unpredictable changes of the tip friction are related to wear.
In fact there is wear in the contact points;
this is the photo of a dimple in the carbide spinning surface caused by wear, at the microscope; the dimple is about 0.1 mm large and 0.03 mm deep, and is the result of 50 hours of spinning of a top with a conical tip.
These dimples are responsible for making tops with a recessed tip to wobble spontaneously, (nutate), when they spin on them.



That's quite a hole! Totally agree that active wear processes are likely contributors and perhaps even the main contributors.

Imagine all the "contact shenanighans" -- stick-slip, transient jamming, hopping, rapid variations in contact area and normal force, etc. -- that might go on with a conical tip spinning in a conical hole -- especially in the presence of precession or wobble!

Here are the data of the tip friction, (obtained making the top spin in deep vacuum conditions), of 19 spins of  my Nr. 27, (156 grams): the top has an external carbide tip and was spun always vertically so there was nearly no precession at the start of each spin.



The 19 spins are numbered chronologically, the spin Nr. 1 was the first spin performed, when the tip was new and very sharp.
The first two spins are the ones with the lowest friction, telling that a sharp tip has lower friction that a dull tip.

The spins with a red number were performed without lubricant, the ones with green, violet and blue numbers are spins with some different lubricants.  It is evident that the tip friction is lower with the lubricants.

Early on, your lubricated vacuum runs showed minor, long-period variations that could well be within measurement error. The big short-period spikes and dips didn't show up until lubricated run 13, when a hole had presumably already been started. But without lubrication (which would accelerate hole formation and also promote stick-slip behavior), they showed up in much earlier runs.

All of this points to the kinds of contact shenanighans I mentioned earlier.

This is the tip friction of a light, (77 grams), top on lubricated spinning surface;
With light tops spinning on a lubricated surface there are not significative up and downs but the tip friction decreases gradually with speed.



A linear tip resistance (braking torque) vs. speed curve with a negative slope implies an exponential SDC (speed vs. time curve). These tip resistance vs. time data points are nearly linear with a clear negative slope over a wide range of speeds. If simple sliding friction were the only contact process involved, you'd have a tip resistance vs. speed curve that's both linear and flat -- i.e., with zero slope.

But you would expect this kind of tip resistance vs. speed curve from speed-dependent viscous friction, and that points to the lubrication. Active tip wear and other contact shenanighans might also add some speed dependence.

Q2: How do you calculate your braking torques again?


« Last Edit: June 14, 2019, 01:58:53 PM by Jeremy McCreary »
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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #19 on: June 14, 2019, 02:31:03 PM »

Thanks for all of this really interesting information! For such a seemingly simple motion - spinning - there is a lot more going on than you would expect.

Q1) I usually use my tachometer starting about 5 seconds before the minute mark, allowing it some time to stabilise.
Q2) I divide the RPM reading of 'minute X' by the PM reading of 'minute X-1'. For example if at minute 8 rpm is 1000, and at minute 9 rpm is at 800, I divide 800 by 1000 to get an 80% efficiency for that minute.

I hope that answers your questions. If not, could you elaborate slightly?
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #20 on: June 14, 2019, 04:04:19 PM »

Imagine all the "contact shenanighans" -- stick-slip, transient jamming, hopping, rapid variations in contact area and normal force, etc. -- that might go on with a conical tip spinning in a conical hole -- especially in the presence of precession or wobble!

The shape of the dimple is not conical, you can se it better here:

https://youtu.be/kXDKc8vOz3M?t=157

I will answer Q1 and Q2 later when I have a bit more time.


James, how many hours did it take to make your last beauty ?
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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #21 on: June 14, 2019, 04:23:12 PM »

If I were to take a guess, I would think It took about 10 hours. The base probably took another 8 hours. However, due to me being in school, I only have access to a workshop for about 3 hours a week. So It took me around 6 weeks. If I wasn't rushing to finish the top before my school ends, I would have spent longer on it.
By the way, I finished my GCSEs today! I am taking A-level DT for the next 2 years. As well as this, I have been working on a model rocket car with a group of friends in an attempt to break the world record for the fastest model rocket car. In addition, I will be working on a safe making competition and a can sized meteorological-engineering project. So I will be very busy. I will try to work on tops in my spare time, however, I will have less of it so production may slow down a bit.

How long does it normally take you?
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #22 on: June 14, 2019, 07:48:16 PM »

Imagine all the "contact shenanighans" -- stick-slip, transient jamming, hopping, rapid variations in contact area and normal force, etc. -- that might go on with a conical tip spinning in a conical hole -- especially in the presence of precession or wobble!
The shape of the dimple is not conical...

Still, imagine all the shenanighans!

Like your diagrams showing how tip radius of curvature affects the size and shape of the lubricant film.

Your tops clearly benefit from lubricant, and I see that you prefer WD40. My heavier tops benefit, too, but the only lubricant I ever use now is skin oil from my face (per Alan's recommendation). Works pretty well.

Tried silicone lube once. Took days to get that *$^#%@! stuff off my hands.
« Last Edit: June 14, 2019, 09:52:15 PM by Jeremy McCreary »
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #23 on: June 14, 2019, 11:18:18 PM »

For such a seemingly simple motion - spinning - there is a lot more going on than you would expect.

Yes, one of my favorite things about tops! Articles about them still appear in physics journals. And a model of top behavior that includes air resistance (usually the main contributor to spin decay) has yet to be published.

Q1) I usually use my tachometer starting about 5 seconds before the minute mark, allowing it some time to stabilise.

Thanks! Sounds like your times and speeds are likely to be well-aligned. Which means that the sudden efficiency spikes and dips you and Iacopo see are likely to be real. All the more interesting, no?

Q2) I divide the RPM reading of 'minute X' by the PM reading of 'minute X-1'. For example if at minute 8 rpm is 1000, and at minute 9 rpm is at 800, I divide 800 by 1000 to get an 80% efficiency for that minute.

Sorry, Q2 was for Iacopo. His tip friction graphs are really braking torque vs. speed curves, and I was wondering how he estimated the torques from his SDC data. There's more than 1 way. Efficiency and braking torque aren't the same thing, but the greater the torque over a given time interval, the lower the efficiency.
« Last Edit: June 14, 2019, 11:23:59 PM by Jeremy McCreary »
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #24 on: June 15, 2019, 01:42:32 AM »

Q1: How exactly do you assign times to your speed measurements?

I timed the lapses every 50 RPM lost in the first graph and every 200 RPM lost in the second graph.

Q2: How do you calculate your braking torques again?

Here is a sample:

I timed 1'36"2 for my top Nr. 27 to go from 1000 to 900 RPM.
100 RPM lost in 96.2 seconds.

Deceleration is:
100 RPM = 10.47 rad/sec, (lost in 96.2 seconds).
10.47 : 96.2  = 0.109 rad/sec, (lost in 1 second).
Angular deceleration is  0.109 rad/sec2.

Torque is:
Moment of inertia x angular deceleration.
The moment of inertia of this top is 0.0000643 kg-m2.
0.0000643 x 0.109 = 0.00000701 Newtonmeters = 7.01 millionths of Newtonmeter.

« Last Edit: June 15, 2019, 01:48:18 AM by Iacopo »
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #25 on: June 15, 2019, 08:24:11 AM »

I have been working on a model rocket car with a group of friends in an attempt to break the world record for the fastest model rocket car...

How long does it normally take you?

I didn't know about these model rocket cars, I looked for them in the web and.. wow.. they are really fast !!
You are involved in very interesting projects.

For the same top as the your I think it would take me about the same time like you.
But for more complex tops it takes me more time.
The most complex one I made at present is the Nr. 39 and it took me about 150 hours to make the full set, (top, base, case and accesories).
62 hours for to make the Nr. 20, (top and base).
About 4 hours for to make the Nr. 27, which is very simple, (top alone).
Some things I do are quite time consuming; varnishing, and polishing, for example.
Doing things with accuracy in every detail generally takes time.

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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #26 on: July 27, 2019, 10:35:36 AM »

For anyone interested in this top, there is now a video about it on my youtube channel: HiPer Spinning Tops. It has better shots of the spinning top than I have been able to post here and other pieces of information.

A link: https://www.youtube.com/watch?v=THPeeyesEIM
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ta0

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #27 on: July 27, 2019, 11:33:07 AM »

Wow! It was difficult not to think I was watching a Iacopo video!
You have learned well from the Master!
Excellent work!
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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #28 on: July 28, 2019, 02:49:36 AM »

Thank you very much! I owe a lot to Iacopo as he helped me when I started making them. As well as that, as you can see, my tops are inspired mainly by his.
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #29 on: July 28, 2019, 09:25:45 AM »

For anyone interested in this top, there is now a video about it on my youtube channel: HiPer Spinning Tops. It has better shots of the spinning top than I have been able to post here and other pieces of information.

A link: https://www.youtube.com/watch?v=THPeeyesEIM

Just out of curiosity;  did you make something particular for driving the motion of the camcorder in the sequence starting at 0:19 ?
Or you simply moved it by hand, along the edge of the table ? The motion seems very fluid.
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