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Author Topic: Some unusual construction methods  (Read 622 times)

Jeremy McCreary

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Some unusual construction methods
« on: December 06, 2019, 12:30:06 AM »

This LEGO top does 8-fold rotational symmetry the hard way -- and it's not an easy symmetry to begin with. I like this original rotor color scheme the best...



The 140 g, 160 mm top easily tolerates 1,000 RPM starts with a best spin time of 75 s. Since the chassis allows a bit of (stiff) relative motion between adjacent rotor panels, it takes some fiddling to get the rotor balanced. But wobble's minimal when you do.

As a color-mixing experiment, replaced the rotor's inner yellow ring with red, green, and blue parts adjusted to mix to dark gray at speed, with the goal of matching the adjoining dark gray ring already there...



In person, the color-mixing version looks like this at speed...



But in a photo or video with automatic cameras settings, it looks more like this...



The flat 8-fold rotor is a simplified version of a SNOT (studs not on top) building technique some call the "magic circle". Nothing magic, though. Since 360° / 8 = 45°, you just have to get the 8 identical wedge-shaped panels to meet at 45° angles at every step from the center outward.



The good news: The rotor has a very smooth, low-drag upper surface.

The bad news: You still get a lot of drag from the chassis underneath the rotor. But so be it. Pointing studs inward or outward is a recipe for catastropic centrifugal failure in any LEGO top, and the chassis has to prevent that.



« Last Edit: December 06, 2019, 10:51:32 AM by Jeremy McCreary »
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Playing with the physical world through LEGO

ta0

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Re: Some unusual construction methods
« Reply #1 on: December 07, 2019, 07:17:11 PM »

Yeah, that's not a trivial assembly.
It has rotational symmetry but not mirror symmetry.
I like it!
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Jeremy McCreary

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Re: Some unusual construction methods
« Reply #2 on: December 24, 2019, 06:30:09 PM »

An unusual 6-fold structure made almost entirely of Technic connectors...



The crinkled hexagonal ring holding it all together came within 1 mm of closing naturally. And if that weren't miraculous enough, the bent spokes mated perfectly to the ring thereafter.



Severe drag limits spin time to ~10 s. With a good twirl, you can hear it hiss.


« Last Edit: December 24, 2019, 08:23:20 PM by Jeremy McCreary »
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Jeremy McCreary

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Re: Some unusual construction methods
« Reply #3 on: March 19, 2020, 01:34:14 PM »

A recent attempt to get artistic with yellow, lime, and white decor on an unobtrusive 6-spoke black chassis...



The visual's better at rest than at speed. The color pattern breaks the structure's 6-fold rotational symmetry. No mirror symmetry.

A mass of  121 g and maximum diameter of 240 mm give this top a large axial moment of inertia. Combine that with a relatively low CM, and you get both a low critical speed and a small scrape angle. Spin time (to first scrape) is at best 10 s by hand and no better with available starters.

The small scrape angle doesn't help, but severe drag is the main culprit, as it limits release speed while greatly increasing decay rate. Can't get this one going fast enough to hiss, but you can feel the stirred air currents on the back of your hand a full top diameter from the edge. That's a big induced flow field even by LEGO top standards.

Wobble isn't bad considering all the opportunities for structural vibration.
« Last Edit: March 20, 2020, 01:05:16 PM by Jeremy McCreary »
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Jeremy McCreary

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Re: Some unusual construction methods
« Reply #4 on: March 19, 2020, 03:09:38 PM »

Sturdy structues with 5-fold rotational symmetry are very rare in the LEGO realm. The strong 106.3° vertex used in these 2 examples is as close as you'll get to the 108° ideal. The discrepancy makes for a slightly strained but visually convincing pentagonal rotor. The strain turns out to be a plus, as it reduces vibration wobble by adding some stiffness to the structure.





Drag (aerodynamic braking torque) limits spin time to ~15 s in this airy red/black example. A little wobble on most spin-ups by hand, but occasionally you get a very smooth one. And that's surprising, given the balancing challenges and rather bendy chassis. The very small scrape angle takes some getting used to.





This blue/black "pentatop" spins twice as long as the red/black above. Most of the credit probably goes to reduced drag. Spins without visible wobble are common -- in part, I think, because the structure's much stiffer everywhere.

Center of mass heights are about the same in these two, but not sure yet how their moments of inertia and critical speeds compare. Both stay together nicely at well over 1,000 RPM, and the blue one can probably go much higher.

PS: The circle through the vertices of the 66 g red/black top is 170 mm in diameter. That of the 43 g blue/black top is 88 mm.
« Last Edit: May 05, 2020, 10:46:37 AM by Jeremy McCreary »
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Jeremy McCreary

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Re: Some unusual construction methods
« Reply #5 on: May 05, 2020, 10:21:02 PM »

Since I seem to have this topic all to myself now, nothing to stop me from blathering on about LEGO top design...

Flowers are big fans of five-fold rotational symmetry. This abstract flower top carries most of its inertia and visual impact in a dark red/gray 5-fold "ring" of 10 square "links". Connecting the ring to the stem and tip, and keeping them all coaxial, is a "hub" based on an partially hidden 8-sided dark gray plate.



The top's a chorus of symmetries. The ring's outer margin forms a 10-fold star with outward vertices on a circle 144 mm in diameter. The inner ring margin makes a smaller concentric 10-fold star 18° out of phase...



Each square 4x4 ring link is 4-fold in outline but only 2-fold in detail. Structural and ornamental differences divide the links into 2 equal groups. Alternating links reduces the ring's overall symmetry from 10- to 5-fold.



The 10x10 hub plate (above right) approximates an 8-fold regular octagon in outline but is really only 4-fold in detail. The 2 dark gray ring attachments and 8 light gray ring rests on the hub plate reduce overall hub symmetry to bilateral with no surviving rotational symmetry. And at the center is a dark gray cone on a dark red base, each with a mix of 4-fold and continuous (aka circular) symmetry.

The first draft of the ring seemed to build itself from a pile of parts I was putting away. When it closed with minimal play and no strain, its fate as a top rotor was sealed. The trick would be to come up with a perfectly coaxial LEGO hub with secure ring mounts. I started with the hub plate above for its stiffness but seriously doubted that I'd be able to center a 5-fold ring on a 4-fold plate with the necessary strength. Surprise!



The top's total mass is 66 g. Spins are generally very smooth, but severe drag limits spin times to ~25 s by hand. The oblique view above gives a sense of the extreme surface roughness behind the drag.
« Last Edit: May 05, 2020, 10:35:44 PM by Jeremy McCreary »
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ta0

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Re: Some unusual construction methods
« Reply #6 on: May 05, 2020, 11:22:54 PM »

To get 5 fold symmetry when you start with even-sided parts is not trivial. Without doubt a challenge but you solved it pretty well.

I like symmetry  ;)
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Jeremy McCreary

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Re: Some unusual construction methods
« Reply #7 on: May 06, 2020, 04:00:40 AM »

To get 5 fold symmetry when you start with even-sided parts is not trivial. Without doubt a challenge but you solved it pretty well.
I like symmetry  ;)

Me, too. LEGO tops are commonly built up from coaxial substructures of differing symmetries, principal moments of inertia, and CM heights. Interesting how the various combinations affect top behavior -- especially spin time and tendency to sleep, precess, or wobble.
« Last Edit: May 06, 2020, 04:07:58 AM by Jeremy McCreary »
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