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Top Nr.15 (38 minute spin duration!)

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James:
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.

topper777:
Great work!

the Earl of Whirl:
Once again, just so amazing!!!

James:
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!

Jeremy McCreary:
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.

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