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[ta0]

Almost two hours spin in vacuum with a 100 gr finger top? Wow! 

Great data you collected! Congratulations!
I don't have time now to totally absorb it, but it will be very useful in the future.

I'm guessing you calculated the torque at each rotational speed by calculating the deceleration at that interval, times the moment of inertia that you experimentally measured for the top.

If the vacuum is not perfect, that only means that the real difference between air drag and tip friction would be even greater. There is no doubt that only at very low spinning rates close to topple spin would the tip friction matter.

[Jeremy McCreary]

Iacopo and ta0: This is an exciting day! I think an experimental tour de force like this deserves its own topic -- especially given its importance. If both of you are agreeable, maybe ta0 can make that happen.

These are the first coming data.  The top is my Nr. 26b, which is similar to that sent me from Alan, but a bit smaller and more thick, and with a carbide spiked tip, spinning on a carbide base, with a thin film of oil.  Weight 103.8 grams. 
Spin time from 1250 to 150 RPM,  28'07" with air and 116'49" in the vacuum.

Wow, you nearly quadrupled spin time with the vacuum! Sounds like topple speed was the same at both pressures, as expected.

How does Nr. 26b differ from Nr. 26? Do the specs and atmospheric pressure spin-down data you recently gave for Nr. 26 still hold?

If the rotor is still cylindrical rather than toroidal, the rotor's size and shape allow theoretical calculation of the aerodynamic braking torque (ABT) coming from the rotor faces using the highly accurate von Karman swirling flow model. The ABT coming from the rotor edge is harder to predict but is likely to be a small fraction of the facial ABT given the rotor's small aspect (length/radius) ratio.

If you'd be willing to supply your hard-won raw time-speed data points (either here or by PM), I'd be happy to post a thorough spreadsheet analysis with appropriate curve-fittings and graphs.

Then I found ethilene glycol doesn't boil so I tried it and it works much better in fact.... But ... at times there is some bubbles formation even in the ethilene glycol; the bubbles form always in the same spots of the plastic surface in contact with the liquid, so it is not the ethilene glycol by itself, but some interation of the liquid with the plastic surface in these spots; maybe there is some pollution there, maybe a fingerprint, I don't know.  I tried to clean but it is difficult to eliminate completely these bubbles formations.

Guessing that you're getting close to ethylene glycol's boiling point at your lowest pressures. Any small irregularity in the plastic surface could then trigger premature cavitation (vapor bubble formation) -- which would look just as you described. (For similar reasons, ship propeller blades are finished to a high polish to delay suction-induced cavitation as long as possible.) Can you borrow/rent a more accurate vacuum gauge just for the calibration?

[Aerobie]

Magnificent!   A beautiful experiment.  We spinning top aficionados are fortunate to have Iacopo!

Congratulations.

Alan

[Jeremy McCreary]

Then I found ethilene glycol doesn't boil so I tried it and it works much better in fact.... But ... at times there is some bubbles formation even in the ethilene glycol; the bubbles form always in the same spots of the plastic surface in contact with the liquid, so it is not the ethilene glycol by itself, but some interation of the liquid with the plastic surface in these spots; maybe there is some pollution there, maybe a fingerprint, I don't know.  I tried to clean but it is difficult to eliminate completely these bubbles formations.

Wait, we might be able to get a ballpark calibration with a little thermodynamics...
Q1: When the bubbles start to form, what's the ethylene glycol's concentration and temperature?
Q2: If not 100% ethylene glycol, is water the solvent?
Q3: When the bubbles start to form, what's your pressure reading?
Q4: Can you see any surface defects where the bubbles tend to show up first?

[Iacopo]

I'm guessing you calculated the torque at each rotational speed by calculating the deceleration at that interval, times the moment of inertia that you experimentally measured for the top.

Yes, this is exactly how I did. 
Now I want to know better what is the real ultimate pressure, then I will collect all the data, and start a new thread where I give them, raw data included, graphs, calculations... for various tops.
It will take some weeks, (I am very busy), but I will do it, a step at a time.

[Iacopo]

Wait, we might be able to get a ballpark calibration with a little thermodynamics...
Q1: When the bubbles start to form, what's the ethylene glycol's concentration and temperature?
Q2: If not 100% ethylene glycol, is water the solvent?
Q3: When the bubbles start to form, what's your pressure reading?
Q4: Can you see any surface defects where the bubbles tend to show up first?

I have to check it, but it should be pure, no water in it, no solvents.  I will let you know tomorrow.
Temperature 18 °C.
Pressure reading...  as said, the gauges are rough, (they are cheap), the full scale is 1065 millibar, they are not reliable for a 10 millibar reading.  I can check it better, but it seems like the needle doesn't move anymore when bubbles start to form.
Also I will try to look for a more accurate gauge.
I don't see defects on the plastic surface where bubbles form.  But if I put the ethylene glycol in a glass, I see it doesn't form bubbles.  Maybe if I can find a U glass tube, it could solve the problem.
 

[Iacopo]

If you'd be willing to supply your hard-won raw time-speed data points (either here or by PM), I'd be happy to post a thorough spreadsheet analysis with appropriate curve-fittings and graphs.

First I would want to know better the ultimate pressure, then I will give all the data, the raw time-speed ones, and the data of each tested top. 

Can you borrow/rent a more accurate vacuum gauge just for the calibration?

I have ordered this one, I should receive it in the first days of January;
the full scale is 20 mm hg, about 27 millibar.  Much better than my actual vacuum gauges.
https://www.ebay.it/itm/132427555661
With my actual gauges the problem is not calibration, but the fact that their needles nearly don't move with 1, or even 5 millibars differences of pressure; it's a bit like trying to weigh a feather with a big scale. 


Data of the ethylene glycol:
http://www.ebay.it/itm/262314816445
Also I will try again with the ethylene glycol, using glass containers, in some way. I have an idea how to do it, even without a U glass pipe.

[ta0]

The vapor pressure of Ethylen Glycol at 20 degrees C is 0.05 mm Hg. If it doesn't boil that tells you that the pressure should be higher. This is 20 times the pressure nominally achievable with your pump (0.0022 mm Hg).
A gauge with a 20 mm Hg full scale should have an absolute error of not more than 0.2 to 0.5 mm Hg.
It seems you should be able to pin your pressure pretty accurately.

[Iacopo]

The vapor pressure of Ethylen Glycol at 20 degrees C is 0.05 mm Hg. If it doesn't boil that tells you that the pressure should be higher. This is 20 times the pressure nominally achievable with your pump (0.0022 mm Hg).
A gauge with a 20 mm Hg full scale should have an absolute error of not more than 0.2 to 0.5 mm Hg.
It seems you should be able to pin your pressure pretty accurately.

This is another useful piece of information, thank you.
Since a glass of ethylene glycol doesn't boil at all, then the pressure should be higher than 0.05 mm Hg.
But maybe it is not far from it, since there is bubble formation in this liquid when it is contained in the transparent plastic tube, maybe a premature cavitation triggered by small irregularities in the plastic surface, as Jeremy noted.
 

[Jeremy McCreary]

The vapor pressure of Ethylen Glycol at 20 degrees C is 0.05 mm Hg. If it doesn't boil that tells you that the pressure should be higher. This is 20 times the pressure nominally achievable with your pump (0.0022 mm Hg).

That's where I was headed, too, but you found a vapor pressure first. I just found 2 sources quoting 0.06 mm Hg at 20°C, but let's go with yours. Adjusting it to Iacopo's 18°C (291 K) with the Clausius-Clapeyron equation reduces it by ~17%, from 6.7 to 5.5 Pa. The latter is still some 18 times the claimed ultimate pressure of 0.3 Pa.

That said, a very thorough MEGlobal Product Guide for pure ethylene glycol gives a 6-parameter Antoine equation for the vapor pressure valid for 260-720 K. This equation gives 1.8 Pa at 18°C, which is only 6 times the claimed ultimate pressure.

[Aerobie]

Inspired by Iacopo, I've been experimenting with longer stems.  This photo shows a 2" long thin-wall aluminum tube on a 1.75" 52g top.  The tube weighs 1 gram.  I enjoy twirling it with the extension, and my percent of twirl-crashes is greatly diminished.

The extension is delicate, and it has bent a few times.  I true it by setting the wheel in my lathe and bending the stem with gentle pushes of my fingers until it indicates true to about .001".

Being delicate, and rather long, it eliminates this top as a contender for EDC (every day carry).

Working with this stem got me thinking about a question for Iacopo that I may have posed a year ago.
How do you know that your stem is true with your flywheel when you use it (with paintbrush) for balancing?

Best regards,
Alan

[Iacopo]

How do you know that your stem is true with your flywheel when you use it (with paintbrush) for balancing?

I always turn the flywheel and the stem together, on the lathe, so I know that they are precisely concentric and aligned.  I don't have problems of this kind.  I use only stable and well seasoned wood for my tops, and I varnish it with epoxy resin, which is an excellent barrier against humidity, to assure dimensional stability.  Also, it never happend to me that one of my stems becomes distorted or broken because of a too hard spin, (and I can spin quite hard), even when the stem is long, narrow and made of a very light wood like obeche.
In brief, I don't have this problem.   

Some times I still check it anyway;
first I balance the top as usual, using the paintbrush on the upper part of the stem;
the result is that the balanced top will spin with the stem perfectly steady, no wobbling at all in the stem, at least at the height where the stem receives the marks of the brush.
 
At this point I observe the outline of the flywheel of the top while it is spinning, with a 20x lens;
the littlest wobbling of the flywheel will be easily noticed.

If the flywheel spins smoothly without any wobbling, as the stem does, it is ok.
If there is some wobbling of the flywheel, in vertical direction, it means that the stem is distorted.
If there is some wobbling of the flywheel, in horizontal direction, it means that the tip is off centered.

[Jeremy McCreary]

Inspired by Iacopo, I've been experimenting with longer stems.  This photo shows a 2" long thin-wall aluminum tube on a 1.75" 52g top.  The tube weighs 1 gram.  I enjoy twirling it with the extension, and my percent of twirl-crashes is greatly diminished. The extension is delicate, and it has bent a few times.

What about a solid, tapered, knurled extension of something even less dense than aluminum -- say, Delrin or ABS or titanium alloy or even wood? Guessing that this top has a high-enough axial moment of inertia to benefit from a stem taper with a starting diameter of ~8 mm below and a final diameter of 3-4 mm above. In my experience, such a taper should cover an axial length of ~32 mm for best results in single-twirl play.

[Jeremy McCreary]

At this point I observe the outline of the flywheel of the top while it is spinning, with a 20x lens; the littlest wobbling of the flywheel will be easily noticed.
If the flywheel spins smoothly without any wobbling, as the stem does, it is ok. If there is some wobbling of the flywheel, in vertical direction, it means that the stem is distorted. If there is some wobbling of the flywheel, in horizontal direction, it means that the tip is off centered.

Brilliant! Never thought to examine my wobbles under magnification.

Most of my LEGO tops are built around through-going central plastic axles 4.8 mm in outside diameter and 16-96 mm in length.




The good news: LEGO remains a world leader in precision plastic molding with dimensional tolerances on the order of 1 part in 10,000, and the dimensional stability of their proprietary ABS plastic is phenomenal. Hence, any axially symmetric arrangement of fully seated parts is guaranteed at least static balance. The bad news: Not sure why, but well over 50% of the axles I use most (64 and 80 mm lengths) are out of true fresh out of the box, and used axles are even worse.

Despite my best screening and engineering efforts, undetected axle bends and inadequate structural rigidity of the top as a whole are my main sources of wobble by far. Maximizing rigidity within LEGO constraints is a challenge I enjoy, but a new approach to bent axles is clearly in order. In the photo below, I'm looking for wiggle at the free end while I turn a 64 mm axle manually at various speeds...



Anyone: So, looking for suggestions on (i) axle screening and (ii) straightening splined plastic axles with bends I can't see. NB: All axles have 4 deep full-length splines as at upper left and hence are not free to roll under gravity alone. An axle can be bent anywhere along its length.

[Aerobie]

Long Stems:
I forgot to mention that I'm experimenting with lift-off stem extensions.  My first try only works occasionally, but I hope to improve that.

Jeremy.  One way to check for straightness of a shaft it to chuck it in an electric drill or Dremel and run it.  You'll see bend - especially with magnification.

One thing to keep in mind with plastic pieces is that all thermoplastics have huge coefficients of thermal expansion. 

Regards,

Alan