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Planetary spinners
I made these smooth-running 1-, 2-, and 3-stage inline planetary spinners to spin up planets, tops, shorts, panties, or anything else in need of a good twist.
About this creation
Please feel free to look over the images and skip the verbiage.

I originally designed the 1-, 2-, and 3-stage "planetary spinners" presented here to spin up LEGOŽ tops with cross-axles for stems, but any cross-axle crying out for a good spin is fair game. All are essentially handheld inline planetary transmissions, each with a 1:4 overdrive per stage.



And yes, they really do spin planets.



Oddly, these spinners seem to have a good bit of appeal as mechanical objects, and not just to geeks and gear-heads like me. I mention this only because I never saw it coming.

For example, visitors at shows not infrequently go straight to a spinner just sitting there on a table crowded with my MOCs. "What's that?!" They watch the action intently as I demonstrate. When handed one, they can't seem to put it down, and many comment on its unexpected smoothness and pleasing mechanical sound and feel.

Frankly, I still marvel at how well those things turned out. Sure, I was going for smooth, but I never imagined that smooth.

The "group video" below shows all 3 spinners in use with an assortment of tops hand-picked to cover a wide range of axial moments of inertia (AMIs) and rotor ground clearances. (See my AMI primer for details.)



The chosen tops make their MOCpages and YouTube debuts on this page. Photos of related tops are here.



The selection criteria reflect the fact that high-AMI, low-clearance tops like "UFO" below take the most muscle and skill to "twirl" (spin by hand).



Tops like this can pack a lot of play value, but for a very young or rusty twirler with no spin-up tool to fall back on, it may never be realized. Top spinners of various kinds have been around for millenia for a reason.

For aerodynamically clean tops with lesser AMIs like Cheeseburger (here sleeping with the 2-stage), the multi-stage spinners deliver release speeds far beyond anything possible by hand with any amount of practice.



The video tries to give a sense of the kind of tops best suited to the maximum torque each spinner can deliver without skipping.

And tops and planets aren't the only things you can spin with planetary gizmos.





The boat getting its rubber band in a twist is X-ray.

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. Really.




Two-stage spinner (1:16 overdrive)

The black 2-stage 1:16 spinner is the workhorse of the bunch. If you could have only one planetary spinner, this would be it.









I'll have more to say about the nuts and bolts of the planetary gear trains here and here.

The 2-stage works with all of my tops but strains with those having the greatest AMIs, like "Cyclotron" here.



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One-stage spinner (1:4 overdrive)

Compared to the 2-stage, the 1-stage 1:4 spinner has a much easier time with "heavy" (high-AMI) tops, but release speeds suffer accordingly.1







The 1-stage chassis is my stiffest and narrowest yet for a planetary transmission.





All 3 spinners use the same splined "chuck" (a rapid shooter magazine and trigger assembly, 18588c02). This part is unique in its ability to transfer torque to a cross-axle with reasonable efficiency while releasing the axle cleanly and without resistance when the spinner pulls away.

"Crossfire" below could be the poster child for high-AMI, low-clearance tops everywhere. As seen in the group video at 3:16, the 1-stage spinner takes it in stride, the 2-stage struggles with its AMI but eventually prevails, and the 3-stage throws in the towel.




The little "landing gears" under the spoke tips smooth out what would otherwise be very bumpy touch-downs but take a big bite out of rotor ground clearance.

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Three-stage spinner (1:64 overdrive)

The bulky 3-stage spinner delivers a whopping 1:64 speed ratio, but output torque suffers accordingly. It's best suited to tops with lower AMIs and fairly clean aerodynamics, but many of my tops fit that description.









If you're after very high release speeds, this is your spinner -- but only if the top's air resistance at high speed isn't too great.

More importantly -- and I'm dead serious about this for safety reasons -- the top itself must be up to the centrifugal stresses that come with high speed. With release speeds approaching 2,500 RPM, turning a LEGOŽ top into into dangerous shrapnel with the 3-stage isn't just a theoretical possibility. My walls and ceilings have the scars to prove it.

The tops below get up to speed nicely with the 3-stage spinner and manage to stay in one piece in the process.







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Using the spinners with tops

These spinners deliver their highest release speeds with tops with enough AMI to keep the gears spinning for a moment after you let go of the finger wheel. Every top on this page fits that description.

You can then build up speed in steps before releasing the top. You'll see this technique used throughout the group video. Whether you reach your target release speed depends on the choice of spinner and the top's AMI, tip friction, and rotor drag. It also depends on your patience.



A high release speed becomes a necessity when dealing with a top like "Blue Marble" or "Zargon IV" with (i) large transverse to axial moment of inertia ratio or (ii) large center of mass (CM) height -- and especially with both. Both conditions increase "critical speed" (the speed at which the top loses stability and falls over), and spin time suffers accordingly. The quickest remedy is an offsetting boost in release speed, and that's where the spinner comes in.

A spinner also makes it much easier to keep a vertical spin axis during spin-up. That's the most the reliable way to get a sleeper and the simplest way to start a top with a broad, ground-hugging rotor.



If Asteroid above strays more than 15° from the vertical, its rotor will strike the ground.



Broader "Three-Lime Pie" allows only 10° of wiggle room.

That kind of twirling precision may well come with practice, but the greater the AMI, and the broader the rotor, the harder it will be to avoid a rotor strike during a strenuous twirl. It never hurts to have a back-up spinner with tops like these.



Given enough patience, all 3 spinners can handle tops with AMIs as large as that of "Connect the Dots" here without skipping, but you'll have to take it really slowly with the 3-stage.

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Design challenges

Granted, there are simpler ways to spin things (e.g., here), but what could I do? I was interested in planetary gearing at the time and eager to find an experimental LEGOŽ version.



First off, all 3 spinners could have been a lot more compact and ergonomic if we had a 48-tooth "annulus" (aka "ring gear", "internal gear") in a space-efficient housing with decent attachment points. Such an annulus could be housed in a studless 8x8 or 9x9 Technic frame. The frame alone would be useful.

But alas, the only worthwhile annulus in all of LEGOdom at the moment is the 48-tooth annulus inside the awkward and very bulky "hard plastic wheel with small cleats and flanges" (64712) seen in DBG above. The current mounting options are equally problematic.2

One of the tougher challenges with this awkward 48-tooth annulus lies in providing sufficiently rigid supports for each stage's annulus and input and output shafts.

The support goals here are pretty much same as always: (i) To keep all shafts properly aligned with the annulus, (ii) to keep the carrier and annulus at the optimal separation, and (iii) to minimize friction and unwanted motions throughout the system.



The clunky supports shown on this page represent my best stab at the goals to date. The supports still aren't not rigid enough for my taste, but they are rigid enough to allow very smooth operation. Working on the supports is unlikely to pay prior to a stubstantial improvement in annulus bulk and mounting.

As seen in the group video, the resulting ergonomics are tolerable at best, with the 3-stage being particularly awkward.

Finally, most real planetary transmissions use 3 planets per stage, but after much trial and error, I found 4 planets a better choice WRT torque capacity, efficiency, and smoothness.3 This increases frictional losses and the AMI of the transmission itself, but the net gain in performance is dramatic.

Despite these shortcomings, the planetary approach earns its keep with its inline input and output shafts, minimal torque reactions, and high rotational stiffness, efficiency, and torque capacity. That these generic planetary benefits can be realized in a smoothly running LEGOŽ incarnation built up from many parts is a tribute to the extreme precision of TLG’s proprietary molding process and the carefully engineered tooth profiles of its gears.

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The planetary gear trains

Each planetary stage here consists of 4 main components not counting shafts and supports:
  • A black or DBG 48-tooth annulus fixed to the spinner chassis
  • A 16-tooth LBG "sun gear" or "sun" with axle hole at the center of the annulus
  • Four 16-tooth DBG "planet gears" or "planets" with pin holes
  • A black or mostly LBG 4-arm "planet carrier" or "carrier" to hold the planets in place
In our case, the stage's carrier shaft is the input, and the sun shaft, the output to the next stage or the chuck.





The planets are basically idlers. They mesh with both the annulus and the sun but not with each other, whereas the annulus and sun mesh only with the planets. The carrier shaft is coaxial with and butts up against the sun shaft of the same stage, but the two are free to turn relative to each other and the annulus.

This early version of the 1-stage spinner shows the DBG planets and their meshing relations with the black annulus and LBG sun.



The carrier and sun are keyed to their shafts, but the planets spin freely on their carrier axles. The finger wheel turns the 1st stage carrier, which transfers the torque, first to the 1st stage planets, and from there to the 1st stage sun via the 1st stage annulus.

In the 2-stage, the 1st stage sun drives the 2nd stage carrier via a common shaft. And in the 3-stage spinner, the 2nd stage sun drives the 3rd stage carrier in the same manner. In all cases, the final sun shaft drives the chuck.

For a single planetary stage with this "carrier in, sun out" topology, the speed ratio per stage qi is given by

qi = (Zannulus / Zsun) + 1,

where Zannulus is the number of teeth inside the annulus, and Zsun, the teeth on the sun.

The planets don't enter here directly, but their sizes have to satisfy the "fit constraint" below for all the meshes to work:

Zannulus = Zsun + 2 Zplanet,

where all planets in the stage have the same number of teeth, Zplanet.

Since Zannulus = 48, and Zsun = 16, the fit constraint demands that Zplanet = 16 as well. The speed ratio for each stage then comes out to qi = 4, which denotes a 1:4 overdrive.4

And since each stage turns the next, spinner's final overdrive speed ratio is qfinal = 1:4n, where n is the number of stages.

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The rest of the tops in the video








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Specifications
(Spinners only -- [n] indicates the number of stages.)
Overall dimensions:[1] 130x60x84, [2] 208x86x70, [3] 270x86x70 mm (LxWxH)
Mass:[1] 94, [2] 182, [3] 222 g
Chuck:Rapid shooter magazine and trigger assembly (18588c02)
Modified LEGOŽ parts:None
Non-LEGOŽ parts:None
Credits:Original MOCs

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Footnotes

1 The term "heavy top" here refers to a top with large axial moment of inertia (AMI), not a large total mass. (See my primer on AMI for details.)

A top's AMI is proportional to its total mass, but 2 tops with identical masses but very different mass distributions will have very different AMIs. In general, the farther a top carries its mass, on average, from the spin axis, the greater the AMI. And the greater the AMI, the greater the torque needed to accelerate the top from rest. This torque will correlate loosely at best with the top's total mass.

2 Yes, there's a 24-tooth annulus inside the large Technic turntable, but its inner diameter is just too small for most applications. If you can turn it into a smooth-running, low-friction planetary transmission with much in the way of torque capacity, you're a better LEGOŽ engineer than I.

3 Following the lead of real planetary transmissions, I started out with 3 planets. The problem with that in the LEGOŽ realm comes down to simple arithmetic: 48 is divisible by 3, but 16 isn't.

Three evenly spaced planets can mesh perfectly with a 48-tooth annulus, but only 1 of them would be able to mesh perfectly with a 16-tooth sun at the same time given a rigid carrier. Since the other 2 planets would always be slightly out of phase with the sun, the gear train would bind constantly, and that's exactly what I've observed with every attempt at a 3-planet solution.

So why not use a sun with a tooth count divisible by 3? The 48-tooth annulus, planetary fit constraint, and available LEGOŽ gears limit workable suns to 12, 16, and 24 teeth. The 12- and 24-tooth options are certainly divisible by 3, but there's a catch: Available LEGOŽ parts limit possible carriers for them to just 2 planets.

So why not a 2-planet stage? Using only 2 planets leaves the annulus with an unwanted degree of freedom WRT the rest of the transmission. The result is an annulus that deflects under load, and that cuts into torque capacity, efficiency, and smoothness. In practice, the 2-planet solutions aren't any better than the 3-planet.

It took me along time to sort through all these gotchas and arrive at the 4-planet solution, which works perfectly.

4 As noted in my gearing tutorial, this is in fact the only way to get a 1:4 or 4:1 final ratio with a reasonable number of LEGOŽ gears.

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Comments

 I made it 
  November 23, 2015
Quoting Stephan Niehoff Very impressive, I must admit. I should experiment in the future in this direction.
Thanks, Stephan! Please do! I'm sure there are better solutions out there somewhere.
 I like it 
  November 23, 2015
Very impressive, I must admit. I should experiment in the future in this direction.
Jeremy McCreary
 I like it 
Matt Bace
  November 20, 2015
This is really cool! It certainly takes a little time to wrap your head around how planetary gears work (I had to look it up on Wikipedia so that I could visualize the math involved in the speed and torque ratios). My understanding is that planetary gearing systems are incredibly robust, which is one of the reasons they are used in the transmissions of 8,000+ HP top-fuel dragsters.
 I made it 
  November 20, 2015
Quoting Matt Bace This is really cool! It certainly takes a little time to wrap your head around how planetary gears work (I had to look it up on Wikipedia so that I could visualize the math involved in the speed and torque ratios). My understanding is that planetary gearing systems are incredibly robust, which is one of the reasons they are used in the transmissions of 8,000+ HP top-fuel dragsters.
Many thanks, Matt! Have to admit that I'm still having a hard time wrapping my head around planetary transmissions. I got as far as I did with this particular configuration only by forcing myself to build it, but there are so many more configurations out there, and some of them are totally counter-intuitive. Yes, they're very tough. In the real world, they're also very compact for the amount of power they can transmit and the torque they can handle, but alas, that's far from the case in the LEGO realm.
 I made it 
  November 19, 2015
Quoting Nerds forprez Wonderful work Jeremy. I love the notion of these planetary gear systems, and have even fiddled with using them in my own creations. As you already know, some of my MOCs contain ridiculous amounts of gear reduction. Planetary gear system are an excellent way to reduce gearing and a confined amount of space. Well done.
Many thanks, NFP! The good news: You can get a 6:1 reduction in a single stage with a fixed carrier, 48z annulus as input, 8z sun as output, and three 20z planets. The bad news: It runs a little rough (footnote 3) and is still too bulky for many if not most Technic applications. There's gotta be a better way!
 I made it 
  November 19, 2015
Quoting Chris Maxter It is really interesting ;)
Thanks, Chris!
 I like it 
  November 19, 2015
It is really interesting ;)
 I made it 
  November 19, 2015
Quoting David Roberts When I was recently experimenting with Lego gyroscopes, my gearboxes were getting taller and taller and less and less stable. Planetary gearboxes were the obvious way ahead but I couldn't see how to do it from Lego's parts. Inspired vision on your part to see that that mining drill/wheel could be actually used as a gear in a gearbox! I'm not surprised that they attracted so much attention on your display: they look interesting and must produce spectacular results.
Too kind, David! Will we get to see your gyroscope? Realized today that the best spinner to use on a particular top often depends on the top's air resistance at high speed more than on its axial moment of inertia (AMI). The 3-stage can push the low-drag "Cyclops" top (see dish top page) to over 3,000 RPM, but the 2-stage outperforms the 3-stage with much dirtier tops of similar AMI. In such cases, the best the 2-stage can do is often 500-1,000 RPM.
 I like it 
  November 19, 2015
When I was recently experimenting with Lego gyroscopes, my gearboxes were getting taller and taller and less and less stable. Planetary gearboxes were the obvious way ahead but I couldn't see how to do it from Lego's parts. Inspired vision on your part to see that that mining drill/wheel could be actually used as a gear in a gearbox! I'm not surprised that they attracted so much attention on your display: they look interesting and must produce spectacular results.
 I like it 
  November 18, 2015
Wonderful work Jeremy. I love the notion of these planetary gear systems, and have even fiddled with using them in my own creations. As you already know, some of my MOCs contain ridiculous amounts of gear reduction. Planetary gear system are an excellent way to reduce gearing and a confined amount of space. Well done.
 I made it 
  November 18, 2015
Quoting Topsy Creatori Thanks for viewing. I'm guessing that action is due to the slight imbalance in mass in the x-y plane of the 10x10 octahedral plate due to my use of those 2 trans purple snowflakes along 1 axis and the pink decorated 2x2 dishes and associated 1x1 rounds along the other axis? Also, I had to film it spinning on my quilt cutting board which has a rubbery texture and allowed less, what seemed to me, orbital traveling, so that the top would remain spinning in the camera's field of view. I can make that top spin much faster on a smoother surface, but then it also travels more.
Ah, yes! The righting behavior is known to be due to tip friction, so it was probably the rubbery surface that allowed the top to save itself more than once. Irregularities in the cutting board could also have triggered at least some of the attempted falls. Your observations connecting traveling to supporting surface properties match my own, but tip properties are also big players. Didn't get a look at your tip, but you could eliminate traveling almost completely with a smooth fine tip like the ones used above. The only useable purist-approved fine tip I know of is on the pointy side of a train wheel (photo on dish top page). If unwilling to slaughter innocent round-ended 4L antennas for their tips, you might also try a Technic ball. In my experience, tops don't tolerate much mass imbalance about the spin axis, but they =do= tolerate a little 2-fold rotational symmetry about the spin axis if =most= of their axial moment of inertia (AMI) lay in structures with at least 3-fold symmetry (in your case, 4-fold). The 2-fold symmetries you noted might have induced a little wobbling, but the relatively small amount of AMI involved suggests to me that the main causes of all these behaviors lay elsewhere. PLEASE e-mail me at "mocpages@cliffshade.com" if you post more top stuff on Flickr. I know you'll explore valuable approaches that might never occur to me.
 I like it 
  November 18, 2015
Thanks for viewing. I'm guessing that action is due to the slight imbalance in mass in the x-y plane of the 10x10 octahedral plate due to my use of those 2 trans purple snowflakes along 1 axis and the pink decorated 2x2 dishes and associated 1x1 rounds along the other axis? Also, I had to film it spinning on my quilt cutting board which has a rubbery texture and allowed less, what seemed to me, orbital traveling, so that the top would remain spinning in the camera's field of view. I can make that top spin much faster on a smoother surface, but then it also travels more.
 I made it 
  November 18, 2015
Quoting Henrik Jensen Another Top cool post, showing the beauty of both your creative tops and the brilliant planetary geartrains. Very fascinating how simple, and yet ingenious, this Works. Also very fascinating to see when your tops reach a speed equal to, or or a multiple of the framerate of your camera, it`s like it stops rotating, and suddenly starts spinning Again.
Many thanks, Henrik! The frame-rate effects with tops get pretty wild at times. The smaller tops here start out at speeds approaching 1.7 times the frame rate (30 fps) and spin down from there, passing through various harmonics along the way, I think. Soon, I'll post a video featuring tops with the best video effects, shot once at 30 fps and again at 60 fps.
 I like it 
  November 18, 2015
Another Top cool post, showing the beauty of both your creative tops and the brilliant planetary geartrains. Very fascinating how simple, and yet ingenious, this Works. Also very fascinating to see when your tops reach a speed equal to, or or a multiple of the framerate of your camera, it`s like it stops rotating, and suddenly starts spinning Again.
 I made it 
  November 18, 2015
Quoting Gabor Pauler Its a doctoral dissertation about planetary gears molded in Lego.
Thanks, Gabor! As I know you know, planetary gears are really cool and have many practical benefits. That's why they're everywhere in real life -- automatic transmissions in cars and trucks, rotor and propeller drivetrains in helicopters and turboprops, wind turbine generator drivelines. TLG clearly understands this, having used planetaries inside their motors for decades. They put a useful number of teeth (48) inside the only LEGO annulus we have to work with, but alas, the current form factor's way too bulky (not to mention ugly) to see much use in Technic MOCs.
 I made it 
  November 18, 2015
Quoting Topsy Creatori Okay, she's up. Here is the link to my page... https://www.flickr.com/photos/76542 You won't be able to miss it! Also, I gave your MOCpage site a plug as thanks to your referencing me for that idea. You really should place some of your videos on Flickr now that they allow ones up to 3-minutes. There are a lot of talented LEGO builders there. :)
Fabulous top there, and oh so Topsy! Love the way it starts to fall and then picks itself right back up several times. Aren't those 10x10 octagonal plates great top foundations? Per your suggestion, tried to upload some images to Flickr just now but got an error.
 I like it 
  November 18, 2015
Its a doctoral dissertation about planetary gears molded in Lego.
 I like it 
  November 18, 2015
Okay, she's up. Here is the link to my page... https://www.flickr.com/photos/76542 You won't be able to miss it! Also, I gave your MOCpage site a plug as thanks to your referencing me for that idea. You really should place some of your videos on Flickr now that they allow ones up to 3-minutes. There are a lot of talented LEGO builders there. :)
 I made it 
  November 18, 2015
Quoting Sam Sanister Not only did you figure all this stuff out, you typed it all out and explained how it works! That must take a lot of patience!
Thanks, Sam! Hope the explanation wasn't too muddy. I find that best way to see if I really understand something is to try to explain it to someone else, especially in writing. That's part of the motivation for a page like this. I also like to share the results of the experimentation and trial and error behind an MOC (in this case, quite a bit) so that others can build on it rather than just repeat it. Hopefully, other LEGO engineers will try planetary gears and come up with even better solutions.
 I like it 
  November 18, 2015
Not only did you figure all this stuff out, you typed it all out and explained how it works! That must take a lot of patience!
 I made it 
  November 18, 2015
Quoting Didier B Jeremy, let me bet that you are a trained engineer ! Don't you ? Or maybe you're just totally crazy... ;-) Didier
Put your money on totally crazy, Didier. Just ask my wife! (Not an engineer, but I've spent decades in medicine and science, have always loved machines, and enjoy studying the real counterparts of the things I build.)
 I like it 
  November 18, 2015
Jeremy, let me bet that you are a trained engineer ! Don't you ? Or maybe you're just totally crazy... ;-) Didier
Jeremy McCreary
 I like it 
Kain .
  November 17, 2015
Pure Genius, This is just amazing
 I made it 
  November 17, 2015
Quoting Clayton Marchetti Wow 4000 rpms! That's amazing.
That's the beauty of the original 9V Technic motor. It has no internal gearing and not much torque by PF standards, but boy is it fast! (No load speed ~4,400 RPM at 9V.)
 I like it 
  November 17, 2015
Wow 4000 rpms! That's amazing.
 I made it 
  November 17, 2015
Quoting Clayton Marchetti That's positively brilliant! I could watch the video for hours! The speeds your reaching are incredible. Awesome!
Too kind, Clayton! The motorized spinner I posted a few months ago can get really clean tops to speeds approaching 4,000 RPM, but these spinners aren't that far behind, and they're a lot more fun.
 I like it 
  November 17, 2015
That's positively brilliant! I could watch the video for hours! The speeds your reaching are incredible. Awesome!
 I made it 
  November 17, 2015
Quoting Topsy Creatori Like everyone else, I find the spinners almost more fascinating than the tops, even the cyclotron which is one of my favorites! Seeing all your tops made be go and build one out of some of the accessory pieces I hope to use in a group project build that I am invovled with. I might make a video of it spinning and post it to my Flickr account. I say might as I have found that tops are not the easiest things to film and mine travels a bit. Anyhoo, thanks again for sharing your orbital studies with us all! :)
Thanks, Topsy! Yes, these gizmos seem to have quite an appeal in their own right -- why, I'm still not sure. PLEASE post a link to the Flickr images when they're up. No telling what a Topsy top will look like, but it's sure to be fun!
 I like it 
  November 17, 2015
Like everyone else, I find the spinners almost more fascinating than the tops, even the cyclotron which is one of my favorites! Seeing all your tops made be go and build one out of some of the accessory pieces I hope to use in a group project build that I am invovled with. I might make a video of it spinning and post it to my Flickr account. I say might as I have found that tops are not the easiest things to film and mine travels a bit. Anyhoo, thanks again for sharing your orbital studies with us all! :)
 I made it 
  November 17, 2015
Quoting Kain . Pure Genius, This is just amazing
Too kind, Kain. Thank you.
 
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