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Clockwork Solar System
Orcman posted a topic in LEGO Technic, Mindstorms, Model Team and Scale Modeling
I'd like to share our MOC and my experience building it, that I (Chris Orchard) and Brent Waller (yes, that Brent of Lego Ideas sets 21108 Ghostbusters and 21328 Seinfeld fame) have been collaborating on. Background During one of our Covid lock-down skype Brisbricks (our LUG) social nights. Brent was talking about something he would have liked to have made, but found he just couldn't get it to work. He elaborated that it was an geared model of the solar system or orrery (which I had always wanted to try, but never had), and he lacked the Technic know-how to complete, so I offered my help. We decided on dividing the work between the aesthetics and the technical, which worked well. Brent was responsible for the overall idea and look, as well as the base and planets, and I was responsible for the gearing and arms. We each gave feedback on the others work and I found that really helpful I had written a number of Technic articles for BrisBricks to help members get a grip on how to integrate and use Technic in their builds. So I was sure I could make it work, especially as I just had that "A-Ha moment" of using the big ring gear driven internally by smaller gears (that would also keep the ring centered). At first I thought it would be easy as the ring gear is a nice quarter circle, right? That piece turns out to have a little quirk - it is pinched in at the end cross-holes (or bowed out in the middle depending on how you look at it)! Check this video out to see what I mean. This meant that the standard Lego spacing would create far too much friction - it would slow down as the ring "tightened up", then it would speed up as it became looser. This wasn't what I wanted - I needed steady, constant speed. It took me around 90 hours of constant fiddling with various techniques to get a geometry that was slightly under a standard spacing. I found that a distance of less than 0.5mm would be the difference between it reliably turning, and gear slippage. Gearing Getting the gear ratios (planet orbit times) accurate was a key goal of ours. This is a small snippet of the design spreadsheet that I used. The original data is from https://space-facts.com/orbital-periods-planets/ and double checked with various NASA pages. Most solar system transit times are based on earth. Which would mean "gearing up" to make Venus and Mercury turn faster than Earth. I decided to turn that on its head, and base all the transit times on Mercury. So every planet would be "geared down" from Mercury, the idea was to make the movement as smooth as possible. In the spreadsheet this is represented by line 2. In the end every planet was less than 0.15% out, most being less than 0.09% It's really the introduction of the new 28 tooth gear that really enabled me to get the desired accuracy. Having a 7 in the ratios really helped. In the end I wrote a quick Perl program to help sift through the gear combinations. That program could handle up searches to 5 gears deep Early proof of concept video The physical model was created first, to ensure that it all fit correctly. The digital model was then created from that physical Moc. The planets are driven by a single M motor and turns Mercury at about 2s an orbit. We should have a video of that soon. Unfortunately for the physical model, I lacked the exact parts to make the planets just like the digital design, so an approximation of size and weight was used. Compromises Our goal was to create as accurate version of the planet's timings as possible, while still looking like attractive, but certain things had to be dropped. By introducing another ring, I did get a Moon to orbit Earth too as a prototype, but it was unwieldy and really threw the aesthetics out, so it was dropped. Pluto didn't make it, sorry. Although adding a ninth face in the base would be easy (as would adding another ring), constructing a nice looking, strong nonagon(?) on the base top just wasn't possible. MOC Stats: 2996 pieces 153 gears 15 months of collaboration 8 major revisions 2 completely different designs 3 gearing spreadsheets 1 custom written program to help work out the closest gear ratios Overall I am proud of what Brent and I have created, and we hope you like it too. More details, videos and images at our Lego Ideas Page.