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Author Topic: Top Nr.15 (38 minute spin duration!)  (Read 466 times)

James

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Top Nr.15 (38 minute spin duration!)
« on: June 10, 2019, 04:13:14 PM »

Hey ;D!

I recently finished making my 15th top. It has a brass flywheel, walnut core/spindle and a conical tungsten carbide tip (in a tapered brass holder *credit to Iacopo Simonelli for the great idea*) which is internal. It weighs 252g, 58mm diameter. Current best spin duration: 38 minutes 26 seconds (Personal best!).

Here are some pictures:





Above is Top Nr.15 ^





Above is Top Nr.15 (right) and Top Nr.11 (left) for scale ^





Above is the underside. You can see the conical tungsten carbide tip. ^

I tried to attach a time lapse of Nr.15 spinning for 30 minutes (lasts 30 seconds). In it, you could see how it takes a while for the precession to stop, and it wobbles significantly just before falling over at a very slow RPM. (It was too large of an attachment, could you tell me how to post a .mov file? thanks)

I will conduct some efficiency experiments etc in the near future hopefully, and will upload the results. It has a very narrow spindle at the top (it is tapered to 3mm and is hand-knurled), allowing me to start it at a very high speed. I believe I can start it at about 2,500 rpm (and it falls at about 120 rpm or maybe less) but I will check this with my tachometer when I measure efficiency.

Please give me your feedback, comments or ideas for improvement! Thanks for reading.
« Last Edit: June 11, 2019, 07:43:35 PM by James »
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topper777

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Re: Top Nr.15 (35 minute spin duration!)
« Reply #1 on: June 10, 2019, 08:12:20 PM »

Great work!
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the Earl of Whirl

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Re: Top Nr.15 (35 minute spin duration!)
« Reply #2 on: June 10, 2019, 10:24:53 PM »

Once again, just so amazing!!!
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James

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Top Nr.15 (38 minute spin duration!) Efficiency
« Reply #3 on: June 11, 2019, 08:20:24 PM »

I used my tachometer to take RPM readings every minute during the 38 minute spin. I started the top at 2650 RPM (though I think I could start it a bit faster), and it fell at 169 RPM (I think i can get the balance a bit better too). I calculated the efficiency as: RPM at end of minute / RPM at start of minute. Using that, I made this graph:





I hope that isn't cropped. If it is, could you remind me how to fix it? I tried altering the size on the imb page, but it corrected itself to the automatic size each time.

Anyway, the graph should show that efficiency increases from 92%, peaking at about 94% during the middle of the spin, and then decreases to 90% just as it falls. Due to my cheap tachometer, there are some anomalous results, however the trend line in red shows the curve of best fit.

Please feel free to give me your thoughts or questions. Thanks for reading!
« Last Edit: June 12, 2019, 05:59:25 AM by James »
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!) Efficiency
« Reply #4 on: June 11, 2019, 09:59:31 PM »

Strong work on both the top and the efficiency measurements!

If aerodynamic drag and simple sliding friction at the tip were the only braking torques acting on your top, your efficiency vs. time curve wouldn't have a peak. So something else is going on -- probably at the tip.

Q1: Do you see any evidence that your tip is wearing down or digging into its supporting surface in the course of a single spin?

Q2: Would you be willing to post your raw (time, RPM) data points? If so, just paste them into a message as text if that's easiest for you.

I'd like to compare your spin decay and efficiency curves to the curves predicted by some simple models of air and tip resistance. The latter can involve contact processes beyond simple friction. Tip resistance might then have a weak non-linear speed dependence, as your efficiency curve suggests.
« Last Edit: June 11, 2019, 10:01:46 PM by Jeremy McCreary »
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ta0

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #5 on: June 11, 2019, 10:26:20 PM »

That's a very impressive time! Congratulations!

Can you tell us again how you define efficiency and how you measure it?
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #6 on: June 12, 2019, 12:42:37 AM »

Can you tell us again how you define efficiency and how you measure it?

I used my tachometer to take RPM readings every minute during the 38 minute spin. I started the top at 2650 RPM (though I think I could start it a bit faster), and it fell at 169 RPM (I think i can get the balance a bit better too). I calculated the efficiency as: RPM at end of minute / RPM at start of minute.

James and Iacopo calculate this figure of merit the same way now. It's basically a minute-by-minute spin retention rate and hence a rough measure of the average braking torque exerted on the top over a particular minute of spin-down.

The empirical spin decay curves (SDCs) posted here in the past by ta0 and Iacopo are nearly exponential over roughly the first 2/3 of spin time. In that approximation, the efficiency between time t1 and some later time t2 is

E12 = N2 / N1 = e-(t2 - t1)/T,

where Ni is the speed in RPM at time ti, and T is the top's "lifetime" in the same time unit used for t. No matter where you start on an exponential decay curve, the decaying quantity will drop by ~63% over the next lifetime T.

When t2 - t1 = 1 minute, and T is in minutes, the efficiency reduces to

E12 = e-1/T,

Hence, the efficiency is constant in a truly exponential decay.

For an approximately exponential SDC, you can use the 1st equation to find the lifetime T from any two data points (t1, N1) and (t2, N2), like so...

T = (t2 - t1) / ln(N1 / N2),

where ln() is the natural log. If the SDC were truly exponential, you'd get the same lifetime from every pair of points on the curve.

Oops, forgot the best part: The lifetime T would be a great figure of merit for comparing tops WRT spin decay rate. Even if an actual spin decay curve is only roughly exponential, the   calculated lifetime should correlate pretty well with actual spin time once release and topple speed differences are figured in.
« Last Edit: June 13, 2019, 12:22:35 AM by Jeremy McCreary »
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #7 on: June 12, 2019, 04:54:36 AM »


Hi, James !

Another nice top, congratulations !

My most similar top to this your new one is my Nr. 20, and your spinning data are not very different from the mine.

This is the efficiency graph of a spin of my Nr. 20, the top was started at 2962 RPM and toppled down at less than 30 RPM.
I calculate efficiency as percentage of RPM retained relatively to one minute before.



38 minutes is a very good spin already, I offer you some suggestions if you want to further improve it;

-  Your spindle is quite tapered in the last upper part; you have 3 mm at the end of the spindle, but the most significant diameter is not that at the end of the spindle, but that at the height where you grasp it with your fingers, at about 10 mm from the top of the spindle.
I have about 4 mm diameter at that height in my tops.
Because of the quite tapered shape at the end of your spindle, probably you have more than 4 mm at that height, and, if so, I believe you can improve a bit the starting speed making it a bit narrower, at that height.
You say that your spindle is knurled but it seems smooth, looking at the photos, (maybe you knurled it later ?)

-  Lubricants are not all the same. I use a thin layer of WD40, which allows for 1-3 minutes longer spins compared to other lubricants.

-  Accurate balance also is important for longest spins, as you already know.

-  The tip of your top seems slightly dull, not sharp.  If you can make it sharper, the top will spin slightly longer.

-  The tip in your top is not deeply recessed.  The more you recess it, the more stable the top, and the top will spin longer.  You can recess it up to about half the thickness of the flywheel;  if you recess it more than so, your top will become a "spindulum", which will never topple down but it will stay upright even when not spinning.  If you recess the tip enough to make the top to topple down at about 30 RPM, instead of 169, you will have about additional 6 minutes of spinning.   

     
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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #8 on: June 12, 2019, 05:58:57 AM »

Thank you all for the comments and advice. I will try to incorporate them into my next top.
I believe I did knurl the spindle after those photos were taken. Some photos:





Above is a photo with magnification, revealing the imperfections in my hand-filing.





Above is without magnification - what you would see normally.

As Iacopo thought, the diameter of the spindle where I hold it is about 4mm diameter. It is true, the tip could be more recessed. I was worried the hole I was drilling for the brass holder would break through the wood on the other side if I went to deep. I might try a non - replaceable tip for my next top, allowing me to have a deeper recession. The tip could be sharper. I was originally more conical, but as I balanced it with diamond filers, it became more rounded. I will also try to fix this next time, and I will eventually try with this top as well using diamond burrs. I believe I am experiencing some wear during the spin, both in the tip and base. Both tip and base are the exact same material. Supposedly, they are tungsten carbide gauge pins (I bought from amazon). However, over time, brown spots appear. This only occurs when I do not sharpen/grind them for some time, and they have no oil coating.

A photo:





Above shows the spots on 'tungsten carbide'. I am not sure if this is meant to happen, or if it is an indicator that the material is not tungsten carbide. I also calculated it has a density of 7.6g/cm^3. This is very different to the density of carbide which, according to google, should be around 15 g/cm^3. Thoughts?

Minute   Rpm      Efficiency
0   2650   0.923   92.3
1   2445   0.924   92.4
2   2260   0.926   92.6
3   2093   0.929   92.9
4   1945   0.931   93.1
5   1810   0.932   93.2
6   1687   0.932   93.2
7   1572   0.934   93.4
8   1468   0.933   93.3
9   1370   0.938   93.8
10   1285   0.935   93.5
11   1202   0.939   93.9
12   1129   0.942   94.2
13   1063   0.940   94.0
14   999   0.937   93.7
15   936   0.940   94.0
16   880   0.942   94.2
17   829   0.941   94.1
18   780   0.919   91.9
19   717   0.934   93.4
20   670   0.934   93.4
21   626   0.942   94.2
22   590   0.941   94.1
23   555   0.939   93.9
24   521   0.939   93.9
25   489   0.941   94.1
26   460   0.937   93.7
27   431   0.933   93.3
28   402   0.935   93.5
29   376   0.931   93.1
30   350   0.934   93.4
31   327   0.927   92.7
32   303   0.908   90.8
33   275   0.931   93.1
34   256   0.918   91.8
35   235   0.911   91.1
36   214   0.907   90.7
37   194   0.897   89.7
38   174   0.000   
            
38:26:00   169   (Falls over)

Above is my raw data for the 38 minute spin (efficiency graph)   

I have tried taking screenshots of the graphs so less is cropped:





Above is spin decay (RPM vs time)





Above is efficiency of Top Nr.15

Thanks for reading!
« Last Edit: June 12, 2019, 06:01:20 AM by James »
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #9 on: June 12, 2019, 08:29:17 AM »

I also calculated it has a density of 7.6g/cm^3.

In fact this is far too low a density for tungsten carbide.
I calculated the density of my tungsten carbide rod and I found it is 14.3 g/cm3.
Also the carbide I have does not rust.
The your seems more like HSS steel, or carbon steel.
But you can find easily tungsten carbide rods on Amazon and they are not expensive..

This also could explain the difference between our spin times, because with carbide contact points the top spins longer than with steels.

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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #10 on: June 12, 2019, 03:16:58 PM »

Thanks for the reply. I will look on amazon for some real tungsten carbide. Since the tip of this top is replaceable, I could make a new tip with a carbide point for use in this top Nr.15 as well as those in the future. Also, Iacopo, I watched one of your videos a while ago about tip and base materials. I remember you saying that HSS and tungsten carbide together had the least wear. Do you think a tungsten carbide tip on an HSS base would be good? Also, have you ever tried using a base made of ruby/sapphire? If you could use this material, do you think it would be good in combination with a sharp carbide point?
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Iacopo

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #11 on: June 12, 2019, 04:59:30 PM »

Do you think a tungsten carbide tip on an HSS base would be good? Also, have you ever tried using a base made of ruby/sapphire? If you could use this material, do you think it would be good in combination with a sharp carbide point?

No, it doesn't work well in that way.
To do things right the spinning surface should be made of tungsten carbide, in any case;
the tip instead can be made of carbide, or HHS, if you prefer it;
with the HSS tip the contact points will last longer.
With the carbide tip instead the spin times will be about 10-15 % longer than with the HSS tip.

Tops with a recessed tip wobble without mercy, (nutate), when the spinning surface is weared out, so it is important to reduce to a minimum the wear of the base, (unless you are ok seeing your top to wobble persistently while spinning).
If your base is made of steel and not carbide, I believe that it will take not many spins before you will see your top to wobble spontaneously because of the worn out base.

I made and tried a ruby base, long time ago, and it behaved poorly.
If it worked as well as I hoped, I would be using ruby for my bases, but I don't;
certainly ruby is hard, and harder than tungsten carbide, but I read somewhere that wear is related not only to hardness, but to toughness too.  So I suppose ruby fails on the side of toughness, (it's hard but brittle).
Tungsten carbide is the most wear resistant material I know at present, for our needs.

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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #12 on: June 12, 2019, 06:33:54 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!
« Last Edit: June 12, 2019, 07:05:05 PM by Jeremy McCreary »
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Jeremy McCreary

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #13 on: June 13, 2019, 01:16:40 AM »

Above is my raw data for the 38 minute spin (efficiency graph)

Thanks for the data. Some prelim results...

Based on surrounding trends, there may be problems with the times or speeds reported at times 19, 33, and especially 38:26 minutes. So I ignored the last. The rest of your empirical spin decay curve is very close to the exponential

N(t) = N0 e-t / T,

where first measured speed N0 = 2,650 RPM and lifetime T =  14.52 min. The correlation coefficient R² = 0.99800 indicates a near-perfect exponential fit.

But if the measured spin decay curve had been exactly exponential, the efficiency would have been constant (a flat 93.3% for the N0 and T values above). Instead, you and Iacopo both got convex-upward efficiency vs. time curves. Given the differences in your tops, that's probably trying to tell us something about the braking processes involved. Hard to imagine that it's an aerodynamic signal, though.
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James

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Re: Top Nr.15 (38 minute spin duration!)
« Reply #14 on: June 13, 2019, 01:30:01 PM »

Jeremy,

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