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Simple centrifugal tops
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These mechanically simple tops open and close in response to a speed-dependent tug-of-war between centrifugal force and rubber band tension.
About this creation
Please feel free to look over the images and skip the verbiage.

The idea behind a "centrifugal top" is to turn the centrifugal force exerted on certain top parts into interesting mechanical behaviors and visual effects during spin-down.

Here I present a pair of simple centrifugal tops that open and close in response to a speed-dependent tug-of-war between centrifugal force and rubber band tension.



The larger top is "Tomcat" (after the US Navy's iconic variable-sweep wing F-14 fighter jet). I call the smaller one "Cranky". (I can't repeat what it calls me.)



This first video shows the dynamic duo in action.



These are not long-spinning tops due to their lack of rigidity. (Tomcat stays up only 11 sec; Cranky, a mere 9 sec.) They spin more smoothly than many of my centrifugal tops but not as smoothly as the rest of my tops.

On this page:Warning! Always wear eye protection when working or playing with high-speed LEGOŽ rotating machinery and keep valuables and bystanders (including pets) a safe distance away -- especially when testing new designs.




Cranky

The trick to getting a simple centrifugal top that opens gradually during spin-up by hand and closes gradually during spin-down is to strike the right balance between centrifugal force and the restoring force exerted by the elastics.

The top's "centrifugal mechanism" ("mechanism" for short) needs to give centrifugal force just the right mechanical advantage over the elastics: Too little, and it never opens; too much, and it flies open at the slightest twirl. No fun in that.



Twirling Cranky by hand would never open up its centrifugal mechanism if I'd used a LEGOŽ rubber band. The thin office rubber band is just right.



Dialing in the centrifugal force was a matter of shaping and weighting the lime and black moving parts just so. The greater the mass of a moving part, and the farther its mass lies from the spin axis on average, the greater the force.

Cranky and Tomcat both use my preferred tip. A purist tip would necessarily be broader, and smoothness and spin time (via tip friction) would suffer accordingly.





The white motorcycle wheel is part of Cranky's "rigid body" -- the set of all parts that never move relative to one another. It makes a great foundation for a centrifugal top for several reasons.

First, the 6 peripheral pin holes provide well-placed mounting points for the mechanism. Secondly, the wheel's own high axial moment of inertia per unit mass stabilizes the top to some extent and allows it to spin longer in the face of the aerodynamic drag generated by the rather dirty mechanism.

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Tomcat

Tomcat's rigid body is also built around a white motorcycle wheel, and for all the same reasons. LEGOŽ rubber bands work here only because the wings see a lot of centrifugal force during spin-up.

Wings closed.





Wings propped open about half way. The black stopped 4L axles are normally removed during use.





The inner black and lime ornaments make the opening and closing more conspicuous -- especially from above.





As befitting a top named after a fighter jet, Tomcat's mechanism is much more aerodynamic than Cranky's. However, a comment from favorite builder Oliver Becker now makes me think that I should named this one "VegeMatic" instead.

An older version of Tomcat in better light: Different colors, same mechanism.







A good look at the mechanism.

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Engineering notes (optional)

Surgeon General's Warning: Excessive consumption of acronyms may cause headaches, dizziness, confusion, and violent thoughts.

Centrifugal tops are a most interesting genre from an engineering standpoint. Unfortunately, we'll need some jargon to proceed.

A simple centrifugal top bears a mechanism akin to that in a centrifugal clutch or the centrifugal tachometer below.



The mechanism contains moving parts driven outward by centrifugal force during spin-up and reeled back in by elastic elements ("elastics" for short) during spin-down. Cranky's mechanism appears below.



The moving parts in the mechanism move relative to each other and to parts outside the mechanism under the influence of centrifugal and elastic forces. Conversely, the parts that never move relative to each other make up its "rigid body". Thus, anything rigidly attached to the white motorcycle wheel here belongs to Cranky's rigid body.

The rigid body is just as important as the mechanism. In addition to providing rigid attachments for the mechanism, it lowers the top's overall center of mass (CM) and supplements the top's overall axial moment of inertia (AMI) as needed to prolong spin time and suppress wobbling. (See my AMI primer for details.)

The elastics are typically rubber bands or springs. A top with elastics stiff enough to immobilize its moving parts at all attainable speeds isn't really a centrifugal top at all.



Alas, LEGOŽ rubber bands and springs are usually too stiff in this sense, but they work well in Tomcat here.

So just what makes a simple centrifugal top "simple"? For me, it all comes down to the mechanical complexity of the mechanism and the range of motions its moving parts are allowed. The distinction will become clearer as we go along.

Note that AMI grows with speed in all true centrifugal tops. In contrast, CM height and transverse moment of inertia (TMI) change with speed only if the mechanism forces moving parts move "axially" (in a direction parallel to the spin axis) in order to move transversely under centrifugal force.

Of all the properties a top might have, AMI, TMI, and CM height are by far the most important dynamically. In a perfectly rigid top, spin time grows sharply with increasing AMI, falls even more sharply with increasing CM height, and also falls with increasing TMI.

Smoothness also tends to improve with increasing AMI and deteriorate with increasing CM height. Things are much more complicated in a centrifugal top, but the same general trends apply at various times during spin-down.



One thing that makes a centrifugal top "simple" in my mind is a mechanism that allows only "transverse" motions of the top's moving parts -- i.e., motions in a direction perpendicular to the spin axis. In a top with a vertical spin axis, for example, these motions would be strictly horizontal.

In practice, that means that a simple centrifugal top's CM height and TMI don't change with speed, and that in turn alters the mechanism's impact on spin time and smoothness -- though not always in a predictable way.



You can tell that Cranky's mechanism disallows axial motions just by looking at its pivots on the rigid body.

This next video features a pair of expanding ring tops. The black and white one does well with the weakest of all LEGOŽ rubber bands (installed after the video), but the red and yellow one requires an elastic much weaker than that -- hence, the thin office rubber band.



The mechanisms in these fancy centrifugal tops allow only transverse motions but hardly qualify as simple from a mechanical standpoint. Hence I don't view them as simple centrifugal tops.

Note that nearly all of the rotor mass and AMI in an expanding ring top resides in the mechanism itself. Such a mass distribution only amplifies the mechanism's negative impact on spin time and smoothness. In contrast, my simple centrifugal tops have a good bit of mass and AMI tied up in their rigid bodies and indeed would perform rather poorly otherwise.

The final video features a batch of "flyball tops" -- so called because their mechanisms bring to mind a flyball or centrifugal governor.



The red, orange, and yellow flyball tops here are my longest- and smoothest-spinning centrifugal tops by a good margin.

Not-so-simple centrifugal tops: My favorite expanding ring top (9 sec) on the left and my favorite flyball (21 sec) on the right. The latter spins twice as long as Tomcat and Cranky.



The 3x3 Technic disks above and below the central part of the flyball mechanism here can be used to lock the mechanism in any desired position. Not surprisingly, spin times are longest when the flyballs are held at maximum distance from the spin axis and shortest when kept at minimum distance. Intermediate spin times result when the mechanism is free to move.

Since the flyballs have to move axially in order to move transversely under centrifugal force, and since the latter depends on the square of angular speed, AMI, TMI, and CM height are now all speed-dependent. For that reason, I don't view flyball tops as simple centrifugal tops.

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Specifications (simple centrifugal tops only)
NB: Specs that vary from top to top are given in "Cranky,Tomcat" format.

Overall dimensions:43x80,72x114 mm (DxH) closed
Mass:32,46 g
Best spin times:9,11 sec (by hand)
Typical release speeds:800,900 RPM (by hand)
Modified LEGOŽ parts:Tips cut from 4L round-tipped antennas
Non-LEGOŽ parts:Cranky's rubber band
Credits:Original MOCs
See also:Expanding ring tops, Centrifugal tachometer, Hartnell steam governor

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Comments

 I made it 
  February 26, 2016
Quoting Nick Barrett Interesting, as always; it's got to take some fine calibration to get the right rubber band tension.
Thanks, Nick! Man, tell me about it! Luckily, though perhaps not surprisingly, the small white LEGO elastics used on Tomcat seem to be pretty uniform from one to the next. The big challenge is evening out the tension on the office rubber band wrapped around the mechanism on the other top.
 I like it 
  February 26, 2016
Interesting, as always; it's got to take some fine calibration to get the right rubber band tension.
 I made it 
  February 25, 2016
Quoting Topsy Creatori Nice lesson. I really think a course like this for high schoolers would be great... ha, ha... no messy grease, motor fluids, or lubricants! ;) (Talking about lubricants reminded me... long ago I had a job analyzing lubricants, hydrocarbon/synthetic used in a special type of de-spin bearing.) Sorry for the regression... onward. One of the excuses for schools terminating classes like auto/wood shop is that it's expensive to maintain the machines. But after the initial set-up cost for a LEGO based class, one would just have to buy elastic bands, ha, ha! :)
Yeah, LEGO science demonstrations are low-maintenance, involve no fabrication, and can be modified or recycled at will. What's not to like? Tomorrow I'm going to the K-8 up the street to see about putting on something akin to a "simple machines" mini-course for interested students. Really hope it works out.
 I made it 
  February 25, 2016
Quoting Topsy Creatori Hey, hey, hey... I finally got over here. I always like to leave more time to read your write-ups. As I write this comment, I have only viewed/read on the first 2 tops. The first question that came into my mind while watching their youtube was... whether you used the same 'rubber' bands for both tops. My patience paid off as after viewing, tah, dah, you told us. (My guess is that LEGO 'rubber' bands are polyether type polymers, which would have different propterties than standard rubber bands.) Okay, I'm going to return now to further viewing. :)
Thanks, Topsy! I read somewhere that the LEGO elastics with round sections are silicone, and they kinda feel like it. Anyway, they return to shape much better and are much stiffer per unit length than standard office elastics.
 I like it 
  February 25, 2016
Nice lesson. I really think a course like this for high schoolers would be great... ha, ha... no messy grease, motor fluids, or lubricants! ;) (Talking about lubricants reminded me... long ago I had a job analyzing lubricants, hydrocarbon/synthetic used in a special type of de-spin bearing.) Sorry for the regression... onward. One of the excuses for schools terminating classes like auto/wood shop is that it's expensive to maintain the machines. But after the initial set-up cost for a LEGO based class, one would just have to buy elastic bands, ha, ha! :)
 I like it 
  February 25, 2016
Okay, digested next stuff. As I watched the tachometer/clutch in motion my thoughts... mechanical complexity! Ha, ha... once I get past simple mechanics and into the realm of dynamics, I'm challenged! Oh yes, from your commentary I just realized LEGO does have 2 kinds of elastic bands. The less elastic, more plastic-like ones are the ones I think might be polyether. :)
 I like it 
  February 25, 2016
Hey, hey, hey... I finally got over here. I always like to leave more time to read your write-ups. As I write this comment, I have only viewed/read on the first 2 tops. The first question that came into my mind while watching their youtube was... whether you used the same 'rubber' bands for both tops. My patience paid off as after viewing, tah, dah, you told us. (My guess is that LEGO 'rubber' bands are polyether type polymers, which would have different propterties than standard rubber bands.) Okay, I'm going to return now to further viewing. :)
 I made it 
  February 15, 2016
Quoting Firas Abu-Jaber I actually enjoyed the MOC, the preparations, the video everything, good job! but please, let someone hold the camera for you next time :-D
Thanks, Firas! Yes, a lot of fumbling around on the main video, if that's what you mean. Should have used my SLR and a tripod, I suppose, but the Canon 7D won't follow focus automatically in video mode like my cell phone does, and that creates problems of its own. (Canon deems this a feature, not a bug.) Ah, a LEGO tripod for the cell phone! On it!
 I made it 
  February 15, 2016
Quoting Nils O. Very cool and very clever, I really like the idea. Great job! :-))
Thanks, Nils!
 I like it 
  February 15, 2016
I actually enjoyed the MOC, the preparations, the video everything, good job! but please, let someone hold the camera for you next time :-D
 I like it 
  February 15, 2016
Very cool and very clever, I really like the idea. Great job! :-))
 I made it 
  February 14, 2016
Quoting Sam Sanister I wasn't complaining, just making a semi-serious observation. ;) (I wasn't clear on that before)
No worries, Sam! I lost touch with my inner Ms. Monteith (11th grade English teacher), you made a valid observation, the page is now better for it, and I really am grateful. :)
 I like it 
  February 14, 2016
I wasn't complaining, just making a semi-serious observation. ;) (I wasn't clear on that before)
 I made it 
  February 14, 2016
Quoting Sam Sanister SO many acronyms... How long does it take you to write this stuff down?
Sam, thanks for bringing me to my senses regarding all the acronyms. I went back and removed a lot of them.
 I made it 
  February 14, 2016
Quoting David Roberts Fascinating, finger chopping, fun! The blades remind me of the mechanism used in this German motorglider: https://en.wikipedia.org/wiki/Stemme_S10 Do visitors to your house automatically get goggles and gloves? (plus a warning not to sit on the boats in the bath!)
Hi, David -- glad you're back! Yes, I see the resemblance -- what a gorgeous machine! Yes, goggles, gloves, hard hats, and body armor, too. (You never know where the shrapnel from a disintegrating rotor will end up.) Signs all over the bathroom, but does anybody ever bother to read them? Nooooo.
 I like it 
  February 14, 2016
Fascinating, finger chopping, fun! The blades remind me of the mechanism used in this German motorglider: https://en.wikipedia.org/wiki/Stemme_S10 Do visitors to your house automatically get goggles and gloves? (plus a warning not to sit on the boats in the bath!)
 I made it 
  February 13, 2016
Quoting Sam Sanister SO many acronyms... How long does it take you to write this stuff down?
Based on your comment, thought it best to add an SGW (Surgeon General's Warning) regarding ECA (excessive consumption of acronyms).
 I made it 
  February 13, 2016
Quoting Sam Sanister SO many acronyms... How long does it take you to write this stuff down?
Thanks, Sam! I wrote this one out in a long day, but I've been thinking and writing about tops for a long time now, and the physics is familiar. The photography took longer. Agree, too many acronyms, but it would have taken a lot longer to type without them. Sorry to say that after decades in science, that's the way I think now, for better or worse.
 I like it 
  February 13, 2016
SO many acronyms... How long does it take you to write this stuff down?
 I made it 
  February 13, 2016
Quoting Oliver Becker First one maybe able to slice my turnips in the kitchen! Sorry Jeremy, my comment is a contribution to dinner time here and I have to be quick! Great TOPs again from you, my friend! Don't forget to leave your votes at the HPC, not for me but at all of course... ! ;)
Thanks, as always, Oliver! Darn, should have called it VegeMatic instead of Tomcat! Thanks also for the reminder.
 I like it 
  February 13, 2016
First one maybe able to slice my turnips in the kitchen! Sorry Jeremy, my comment is a contribution to dinner time here and I have to be quick! Great TOPs again from you, my friend! Don't forget to leave your votes at the HPC, not for me but at all of course... ! ;)
 
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