Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[Iacopo]

I made a couple of new tops, using pure silver for the flywheels.
Silver is very dense, (10.5), which helps for longest spins.  These tops can spin for more than one hour.








Video:

[ta0]

Wow! Soon we will see Iacopo's tops at Tiffany & Co.!
Another incredible work of art combined with high-tech!

We can see where this is heading, flywheels of even higher density metals: gold and platinum!
If I ever win the lottery I will be able to afford a second Simonelli top! 

From the graph I see that, with a starting speed of about 2550 RPMs, it spins over 66 minutes.
Also, about 1900 RPMs are needed to reach 1 hour.
How fast can you spin it with just one finger whirl?
Do you think that a 1-hour top with 1 finger snap is possible (perhaps made of platinum)?

[Jeremy McCreary]

I made a couple of new tops, using pure silver for the flywheels.
Silver is very dense, (10.5), which helps for longest spins.  These tops can spin for more than one hour.

Absolutely stunning! I see the shroud is still working for you.

Wow! Soon we will see Iacopo's tops at Tiffany & Co.!
Another incredible work of art combined with high-tech!

My first thought as well. Or a museum of fine arts. But tops are meant to spin and entertain, and it would be a shame for any Simonelli to just sit in display case. Hope the buyer actually plays with it from time to time.

[Jeremy McCreary]

From the graph I see that, with a starting speed of about 2550 RPMs, it spins over 66 minutes.
Also, about 1900 RPMs are needed to reach 1 hour.
How fast can you spin it with just one finger whirl?
Do you think that a 1-hour top with 1 finger snap is possible (perhaps made of platinum)?

A very interesting engineering question. Increasing launch speed or rotor density may not be the best approaches.

There are 5 basic but somewhat conflicting lines of attack for any top. For a single-twirl finger top on a pedestal, from launch to fall...

1. Increase launch speed:
a. Practice, practice, practice!
b. Optimize stem diameter and taper.
c. Reduce air resistance.
d. Reduce AMI.

2. Reduce high-speed spin decay rate — mainly but not solely by attacking air resistance:
a. Increase AMI.
b. Streamline the shape with special attention to colliding flows.
c. Reduce max radius.
d. Reduce surface area.
e. Reduce surface roughness.

3. Reduce low-speed spin decay rate— mainly but not solely by attacking tip resistance:
a. Increase AMI.
b. Optimize tip and base materials — possibly at the expense of wear rate.
c. Reduce total weight.
d. Reduce tip radius of curvature.
e. Increase base radius of curvature if concave.

4. Reduce critical speed (remember, mass and density cancel out when uniform):
a. Reduce CM height above contact.
b. Increase AMI per unit mass.
b. Reduce TMI per unit mass about the CM.

5. Eliminate precession and especially wobble:
a. Practice clean vertical launches at max effort.
b. Eliminate unbalance.
c. Insure that tip contact is exactly on spin axis.

Iacopo already does nearly all these things, and arguably better than anybody. But there are crucial trade-offs to be made with single twirls — especially around AMI, max radius, and critical speed — and they'll have to be played just so to max out single-twirl spin times.

Note the nearly exponential shape of Iacopo's spin decay curve (SDC). At every point, the slope is the deceleration rate, and in quiet sleep, it's proportional to total resistance and inversely proportional to AMI.

That's the good news about AMI: From launch to fall, the greater the AMI, the less the top slows down for a given total resistance.

The bad news: At some point, AMI starts to limit single-twirl launch speed. I say this from long experience. And the more you struggle against it, the harder it is to get a clean vertical release with no wobble or precession. Practice definitely helps, but only to a point.

Note also that the SDC slopes go from very steep at launch to very shallow at fall. Hence, you add significantly more spin time by shaving an RPM off critical speed than you do by adding an RPM to launch speed, regardless of the air and tip resistances involved.

To me, that makes critical speed reduction (item 4 above) a prime target, as it extends the tail of your SDC. Iacopo already does the most important thing here: Reduce CM height above contact. That leaves size and shape.

It's easy to show that for any shape with known AMI and TMI  formulas and uniform density, you can always scale the absolute size by max radius R and the shape by one or more key proportions. You then find that critical speed is inversely proportional to sqrt(R). Probably true, at least roughly, for any realistic top shape.

So bigger is better at the arguably more important low-speed tail of the SDC but generally worse at the high-speed head, where the braking is mostly aerodynamic. (I take the von Karman disk as good guidance here.)

So where's the sweet spot in size — considering that bigger also means greater AMI, density and proportions being equal? I think you'd have to find that sweet spot to max out single-twirl spin time, and that would require lots and lots of guess-and-check testing.

Note that critical speed has nothing to do with dissipation. And that absolute AMI and TMI play no role, as mass and density completely cancel out in a uniform flywheel. Only AMI and TMI per unit mass count here, and these properties depend only on size and shape, not on rotor density.

All of which leaves me wondering, what combo of mass, size, and mass distribution optimizes a single-twirl top for spin time? Not sure we've seen it yet.

[Iacopo]

Thank you, Ta0 and Jeremy !

I would like to make a golden flywheel, but gold is incredibly expensive, there is too much responsibility, so probably I will never make it.
That silver ingot costed me about € 1000, not cheap but still affordable for my pockets.

With one single twirl I can reach 800-900 RPM, then the top would spin for about 40 minutes.
To reach one hour with a single twirl seems very difficult, maybe impossible, even with a top optimized for single twirls, at least with the technicalities I am using generally with my tops at present.   

[Jeremy McCreary]

Thank you, Ta0 and Jeremy !

Your artistry and performance never cease to amaze!

Would you be willing to share the spin decay curve's (time,RPM) data points for this top? I'd like to see how exponential the decay is. And if not closely exponential all the way through, how the best exponential fits to the post-launch and pre-fall segments differ in decay constants.

Thanks!

[ortwin]

Congratulations Iacopo! Excellent!! Formidabile!!! (Notice the last word was "Italicized")

...
With one single twirl I can reach 800-900 RPM, then the top would spin for about 40 minutes.
...

Did you check what speed you can reach with a single two handed palm shear?

[Iacopo]

Would you be willing to share the spin decay curve's (time,RPM) data points for this top?

Here it is.  The blue line is that of the silver top, with the shroud.

I added the curves of the top Nr. 55, (with and without the shroud), which has the same design of the silver top, but a brass flywheel, with a littler diameter, (mm 60 for the brass top vs. mm 67 for the silver top).
With the shroud, the Nr. 55 spins about 5 minutes longer than without it; the advantage is especially at high speed.

[Iacopo]

Did you check what speed you can reach with a single two handed palm shear?

Thank you, Ortwin.  No, I didn't, the stem is too narrow for the palms, also I feel there is less control with the palms and more risk for the top to exit from the base.  But I think that the idea is good, and with a more suitable stem, and some practice, the starting speed could be interesting.

[Jeremy McCreary]

Would you be willing to share the spin decay curve's (time,RPM) data points for this top?

Here it is.  The blue line is that of the silver top, with the shroud.

I added the curves of the top Nr. 55, (with and without the shroud), which has the same design of the silver top, but a brass flywheel, with a littler diameter, (mm 60 for the brass top vs. mm 67 for the silver top).
With the shroud, the Nr. 55 spins about 5 minutes longer than without it; the advantage is especially at high speed.

Thanks! So the silver top won out in the end despite a significantly slower launch. With similar rotor shapes and presumably similar tips — i.e., with similar total resistance as a function of speed — the silver top's consistently shallower slope could reflect its greater AMI.

Would you be willing to share the raw numbers behind the silver top's spin decay curve?

EDIT: No, the stricken part is wrong. The tops have nearly identical low-speed slopes despite the silver top's greater AMI. That suggests less total low-speed resistance for Nr. 55. Its lesser weight could be part of that.

[Iacopo]

Would you be willing to share the raw numbers behind the silver top's spin decay curve?

Yes. The following numbers are the RPM measurements, taken every 4 minutes;

2550 - 2065 - 1707 - 1423 - 1196 - 1011 - 857 - 728 - 616 - ? - 432 - ? - 293 - 235 - 181 - 123 - 72.
Spin time, 65 m 13 s.

[Jeremy McCreary]

Thanks! That's a low topple speed. Have you seen lower in your tops?

[Iacopo]

Thanks! That's a low topple speed. Have you seen lower in your tops?

50-70 RPM topple speed is normal with my tops with the recessed tip.
The lowest topple speed that I remember is 30 RPM in my top Nr. 15; in that top the tip is mounted on a screw, which makes possible to fine tune the height of the tip and have it very near to the center of mass.
But I don't use anymore the screw joint, from a long time, because the taper joint is more accurate for to have the tip centered after every tip substitution, which is a more important issue to me. 

[Jeremy McCreary]

Been thinking about how to increase spin time in a Simonelli-like single-twirl top. One possibility, based on this old post, would be to try a disc-like rotor of radius R greater than a typical Simonelli with a thickness (axial length) of under R/10.

The aerodynamic braking torque on the rotor would then be given by von Karman's formula. And the central TMI / AMI ratio would then be fixed at ½ — the lowest possible value for an axisymmeyric body. Result: The deceleration rate and critical speed would then both decrease with increasing R.

The single-twirl requirement would limit how much you could increase R, as absolute AMI would grow as R² at constant density. Otherwise, the bigger, the better!

[Iacopo]

Been thinking about how to increase spin time in a Simonelli-like single-twirl top. One possibility, based on this old post, would be to try a disc-like rotor of radius R greater than a typical Simonelli with a thickness (axial length) of under R/10.

Something like this ?



The diameter is 76 mm and thickness 6 mm.  0.0000802 kg m².
The problem is that when the thickness is little, the AMI too is little, relatively to the air drag.
The top can spin for 20 minutes or little more.
Spin decay:  starting speed 2000 RPM, spin time every 200 RPM lost: 27.9 s at 1800 RPM - 61.3 s - 102.0 s - 152.3 s - 216.7 s - 304.1 s - 430.6 s - 638.4 s - 1060.8 s at 200 RPM - 1257.0 at 150 RPM. Topple down at 150 RPM, spin time 20 m 57 s.
Starting speed by multiple twirls up to 2200 RPM. Starting speed by a single twirl up to 1000 RPM.