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Planetary fidgets
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Why would a device that does nothing and goes nowhere have working planetary gears? Because they're fun, of course!
About this creation
Please feel free to look over the images and skip the verbiage.

Recently saw a beautifully machined stainless steel fidget spinner that reminded me of a basic planetary gear set without the enclosing ring gear (annulus).

Having worked with LEGOŽ planetary transmissions before, I just couldn't resist trying some "planetary fidgets" of my own.



The planetary fidgets featured here...





The smallest and largest for scale. Their masses are 14 and 39 g, and their maximum diameters are 51 and 101 mm, respectively.

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Planetary spinner anatomy

Every fidget spinner has 2 main components -- the "stator" held between the fingers, and the "rotor" one spins around the stator.



Separated orange 3-planet spinner with stator on left and rotor on right.



At minimum, the stator includes a pair of opposing grips or buttons connected by a central "stator axle". In a planetary fidget, it also includes a central sun gear (here, a black 12-tooth double-bevel) keyed to the stator axle.

In this simple 2-planet example, the black part of each grip consists of a black 2x2 dome thickened with a black 2x2 round plate. When these grips are fully seated on the ends of a 6L stator axle, the irreducible gap between them is ~24.7 mm -- perfect for the 3-wide (24.0 mm) rotor shown!



The rotor here consists of 2 black 36-tooth planets between a pair of opposing lime planet carriers designed to hold the planets in proper mesh with the sun. In operation, the carriers must spin freely on the stator axle, and the planets must spin freely in their carrier bearings -- all with the least friction possible.

Orange 3-planet spinner with one grip removed.



Now with one grip and planet carrier removed.



Orange 3-planet spinner rotor (right) with the entire stator (left) and one planet carrier removed. In practice, the need for freely spinning carriers severely limits the parts suitable for carrier hubs.



In the LEGOŽ realm, you can have 2, 3, 4, or 6 planets evenly spaced around the sun. Just make sure that the number of planets divides evenly into the number of teeth on the sun.



All of these possibilities are represented here.

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Eight examples

Frictional losses in the gear trains and lousy aerodynamics make short spin times par for the course in LEGOŽ planetary fidgets. But they work quite well otherwise and still manage to entertain. All of the ones here have comfortable hand clearances.

∨ Spinner 1: In this 15 g spinner, two 36-tooth planets orbit a 12-tooth sun on a pair of parallel 2-spoke carriers. Spin time is ~6 sec.





As seen in the video at 0:58, you can also roll this spinner across the table like a bicycle.

∨ Spinner 2: This 2-planet example uses carriers based on Technic 5x7 frames. I see the 40-tooth LBG spur gears as the planets here, but they could also be viewed as idlers.








This large spinner may look awkward, but it's a pleasure to use and goes up to 9 sec -- the longest spin time on this page. Relatively loose sun-planet meshes and a high axial moment of inertia (AMI) contribute to this success.

∨ Spinner 3: Why stop at 2 planets when you can just as easily have 3?





This basic 22 g 3-planet spinner, my favorite, has 36-tooth planets orbiting a 12-tooth sun. It goes for ~6 sec.



Unlike the other spinners on this page, this one functions reasonably well as a spinning top when fitted with the fine-tipped grips on my best non-planetary fidget on the left.

∨ Spinner 4: This more elaborate 31 g 3-planet spinner also goes for ~6 sec.



Starting from Spinner 3, I added 6 non-meshing 20-tooth gears near the spoke ends to increase rotor AMI. This also increased aerodynamic drag, but the opposing effects on spin time relative to Spinner 3 nicely cancelled.



Widely spaced stator grips provide adequate hand clearance despite the bulk.

∨ Spinner 5: This 32 g 3-planet spinner has its 36-tooth planets on one carrier and the 12-tooth idlers between them on the other. I enjoy the action added by the idlers, but the added friction limits spin time to ~3 sec.



The meshing planets and idlers enforce the 60° offset between carriers all by themselves with no structural help.



I love dark red and black!

∨ Spinner 6: This 4-planet spinner with 16-tooth sun and planets is the smallest of the bunch by a good margin. Spins last only 3 sec or so but are very smooth.







Studless 4-planet rotors turn out to be a real challenge, mainly for lack of suitable parts. This is the only workable solution I've found, but my gut tells me that better solutions await.

∨ Spinner 7: This mostly studless 4-planet spinner with 36-tooth sun and planets is the largest of the bunch by a good margin. Studded 4x4 round plates with center holes serve as carrier hubs.







Despite the size, it's surprisingly comfortable in the hand with ample clearance. Spin time is ~4 sec.

∨ Spinner 8: Finally, a 6-planet spinner with 12-tooth planets orbiting a 36-tooth sun.







This one spins for only 1 sec, but no wonder: It has the greatest number of gear meshes and bearings, the least rigid structure, and hence the greatest frictional and flexural losses by far. The non-meshing 20-tooth gears are mainly just for looks but in theory serve the same purposes as in Spinner 4.

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Committee meeting test

If ever a situation called for fidget spinners, it was last week's DENLUG Bylaws Committee meeting. So I put Spinners 1, 3, and 4 and the older non-planetary fidgets here and here on the table and watched what people did with them over the course of the meeting.

Results: The spinners saw little rest. Heaviest use occurred during the driest parts of the discussion and also the most contentious. Despite their relatively short spin times, the planetaries were the most popular, and Spinners 3 and 4 most of all.

I'm pleased to report that the committee approved all 6 spinners unanimously.

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Optional: Design tips and tricks

Fidget spinners may lack socially redeeming qualities, but they do present some interesting design trade-offs in the LEGOŽ realm, where one has little control over densities and friction coefficients. At stake are spin time, hand clearance, and flicking finger comfort in prolonged use. The last 2 go to general ease of use.

All spinners: Adult fingers are strong enough, and LEGOŽ parts are light enough, that increasing the rotor's axial moment of inertia (AMI) is usually a good way to increase spin time -- at least in non-planetary spinners. This often works in planetaries as well, but only if you keep aerodynamic drag and friction in check.

In practice, you increase AMI by adding mass to or relocating mass toward the rotor's periphery. But in the LEGOŽ realm, you'll have to increase rotor bulk in the process, and that will usually increase drag along with the AMI. If you can add the AMI without cutting into hand clearance, so much the better.

To go from the upper to lower spinner here, I simply added mass at the spoke ends in hand clearance-neutral fashion. This increased the lower spinner's rotor AMI and drag with no impact on friction.





The AMI/drag trade-off paid royally this time, as spin time doubled from 7 to 14 sec.

Drag and rotor-stator friction reduce spin time in 2 ways: They (i) limit initial rotor speed and (ii) cause the speed to decay in roughly exponential fashion from there. Hand clearance also enters here, as rotor strikes against your hand during spin-up and spin-down have a similar effect.

Lubrication (try forehead oil!) and avoidance of translucent (polycarbonate) bearing parts are the only ways to reduce friction coefficients in the LEGOŽ realm. Otherwise, minimizing rotor-stator friction comes down to the tried and true strategy embodied in ball bearings: Minimize bearing contact areas and the pressures across them.

Above all, don't let grips contact the rotor when the axle is horizontal. Design your stators to keep finger pressure on your grips from closing key rotor-stator clearances inadvertently. The 24.7 mm minimum gap between grips maintained by this stator (left, with 6L axle) is ideal for rotors that are 3-wide (24.0 mm) at the stator.



As for comfort during prolonged play, pay attention to the surfaces your flicking fingers actually hit during use. Avoid pain and bloodshed by eliminating sharp corners, and your flicks will be more effective.

Planetary spinners: Every gear mesh and every axle bearing in a planetary fidget adds friction to the works above and beyond that already present at the rotor-stator interface. Worse yet, every gear adds a good bit of drag.

Stiff, well-aligned axle supports and avoidance of tight meshes are the best defenses against the frictional losses. You generally won't have room for all the extra plastic needed for really stiff supports, so you'll just have to eliminate as much of the slop as you can without running afoul of hand clearance.

This 6-planet spinner is a case in point. I barely had room for the wiggly axle supports shown. Stiffer ones would have taken up all remaining hand clearance and then some. Spin time = 1 sec.



NB: For smooth operation, the number of planets must divide evenly into the number of teeth on the sun. This is more than a friction issue, as it will otherwise be impossible for all the planets to mesh cleanly with the sun at the same time.

Note also that when you spin up a planetary fidget's rotor, you're spinning up all the planets and any idlers present at the same time. This has potentially important implications for initial speed and spin time. Whether it helps or hurts spin time in the general case is hard to say.

Finally, keep in mind that gears are very dirty aerodynamically. The more gears you add, the larger they are, and the faster the relative air flows around them, the more drag and the shorter the spin time on aerodynamic grounds alone.

To go from the orange to the lime planetary here, I switched to longer planet axles and added masses to their exposed ends in the form of non-meshing 20-tooth gears.



This change increased the lime spinner's rotor AMI and drag with completely offsetting effects on spin time (~6 sec for both). There was no significant impact on hand clearance or friction.

But here, friction and drag totally outweigh AMI. The orange planetary on the right has a much higher rotor AMI but spins only 6 sec. The simple lime spinner, on the other hand, goes 14 sec thanks to cleaner aerodynamics and much lower total friction.



In summary, the spin times reported above support a valuable rule of thumb with significant exceptions: The greater the number of planets and idlers, the greater the overall friction and drag, and the shorter the spin time.

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Specifications

Construction: Mostly studless
Planet gears per sun: 2, 3, 4, 6
Sun-planet gear pairs: 12-36, 16-40, 36-36
Masses: 14-39 g
Maximum diameters: 51-101 mm
Widths at grips: 58-61 mm
Best spin times: 1-9 sec
Modified LEGOŽ parts: None
Non-LEGOŽ parts: None
Credits: Original MOCs based on existing 2-planet metal design
See also: Planetary top starters, color-mixing fidget spinners, and Improved fidget spinner and top

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Comments

 I made it 
  May 21, 2017
Quoting adam thelegofan rutland what are these again?:D
Thanks, adam! That's probably not a serious question, but just in case, the very first link on this page goes to the Wikipedia page on fidget spinners.
 I made it 
  May 21, 2017
Quoting Philippe Moisan My kids love your spinners (and so do I).
Glad to hear it, Philippe! If you built some, any suggestions for improvement?
 I like it 
  May 21, 2017
what are these again?:D
 I like it 
  May 21, 2017
My kids love your spinners (and so do I).
 I made it 
  May 19, 2017
Quoting Clayton Marchetti Lego just released directions to build the official lego spinner. Not as cool as yours though.
Well, theirs isn't bad, but I like mine better, too. Interesting that they chose to go with almost 100% studded construction when studless makes so much more sense mechanically. Could reflect what they think people have on hand.
 I made it 
  May 19, 2017
Quoting Clayton Marchetti Lego just released directions to build the official lego spinner. Not as cool as yours though.
Thanks, Clayton! Guess they're getting in on the fad while it lasts. I'll have to check out those instructions.
 I like it 
  May 19, 2017
Lego just released directions to build the official lego spinner. Not as cool as yours though.
 I made it 
  May 19, 2017
Quoting Doug Hughes Very cool! Makes me sad that I have so few technic gears...
Thanks, Doug! You know what they say: Just add money! The black 36-tooth double-bevels used in most of the spinners here would normally be too expensive for this sort of thing, but I lucked into a large, cheap lot on BrickLink.
 I like it 
  May 19, 2017
Very cool! Makes me sad that I have so few technic gears...
 I made it 
  May 18, 2017
Quoting André Fontes I haven't really understood this new fad neither this sudden appeal for fidget spinners, but gotta say yours are very creative and actually interesting! Great work.
Thanks, Andre! I don't really get the craze part, either, but they're fun to have around and even more fun to design and build in LEGO.
 I like it 
  May 18, 2017
I haven't really understood this new fad neither this sudden appeal for fidget spinners, but gotta say yours are very creative and actually interesting! Great work.
 I made it 
  May 18, 2017
Quoting Sven ;o) Your creativity is awesome and your new spinners look really good. Also like that you always explain what you did. Very very nice my friend!
Thank you, Sven! Writing the explanations forces me to think about what I'm doing and often leads to major improvements (which then necessitate more photography). Glad you like that part of it, my friend.
 I made it 
  May 18, 2017
Quoting Daniel H. Daaaaang, those are freaking awesome! Keep up the great work!
Many thanks, Daniel!
 I made it 
  May 18, 2017
Quoting gene 3S You are the man!!! These are brilliant and not moving away from lego parts!
Thanks, gene! Really wish there were more weighted parts. Unfortunately, the few that exist (e.g., boat weights and Spinjitzu turntables) are way too bulky and too hard to secure for use in moving assemblies.
 I like it 
  May 18, 2017
Daaaaang, those are freaking awesome! Keep up the great work!
 I like it 
  May 18, 2017
You are the man!!! These are brilliant and not moving away from lego parts!
 I like it 
  May 18, 2017
Your creativity is awesome and your new spinners look really good. Also like that you always explain what you did. Very very nice my friend!
 I made it 
  May 17, 2017
Quoting Gabor Pauler Mmmm. About powered version, see http://gicl.cs.drexel.edu/index.php/Independent_Study_for_Biobots,_A_Continuation Mindstorms student project
Very interesting page, Gabor! I'll be studying it carefully. Looks like their wheels are like the ones on my dolly. Never thought of them as drive wheels (torque applied to carrier, no sun), but they sure did.
 I like it 
  May 17, 2017
Mmmm. About powered version, see http://gicl.cs.drexel.edu/index.php/Independent_Study_for_Biobots,_A_Continuation Mindstorms student project
 I made it 
  May 17, 2017
Quoting Toa Whero Wow! These are fantastic! Great work!
Many thanks!
 I made it 
  May 17, 2017
Quoting Gabor Pauler The triple geared mechanism is interesting. Something similar is used in the "wheels" of some steps-crawler mechanism for disabled persons.
Thanks, Gabor! Guessing you're referring to my favorite, Spinner 3. The folding dolly my wife got me for lugging MOCs back and forth to LEGO shows also has triple wheels for steps. The wheels in each wheel set share a common carrier on the main dolly axle, but they turn independently. That said, vaguely recall seeing a LEGO crawler with triple wheels linked by planetary gears.
 I made it 
  May 17, 2017
Quoting Clayton Marchetti Wow! Spinner 8 blew my mind. These are really awesome. Brilliant work!
Too kind, Clayton! Wish #8 spun longer, but it's still fun. So was getting it work at all.
 I like it 
  May 17, 2017
Wow! These are fantastic! Great work!
 I like it 
  May 17, 2017
The triple geared mechanism is interesting. Something similar is used in the "wheels" of some steps-crawler mechanism for disabled persons.
 I like it 
  May 17, 2017
Wow! Spinner 8 blew my mind. These are really awesome. Brilliant work!
 
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LEGO models my own creation MOCpages toys shop Planetary fidgets


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