Orrery With 5 Planets and 4 Moons (page 4) at MROB

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H: Finishing the Inner Planets

20080101

After a bit of a break for the holidays I brought the 8273+8287 project up to version H:

H, Showing Base and Counterweights

The Planet Arms

The following aspects were improved:

2^nd planet arm is more elegant.
Complete moon gearing for the 3^rd planet.
4^th planet has a moon and its arm is more sturdy.
Minor refinements to 5^th planet.
Central axle is easier to assemble and disassemble.

Here is a closeup of the new attachment for the 2^nd planet:

H 2nd planet arm
H 2^nd planet arm

I rebuilt part of the 3^rd planet arm to complete the moon gearing:

3rd planet gears
3^rd planet gears

The flimsy arm for the 4^th planet was replaced with the recently developed robust differential attachment, and a moon was added.

4th Planet, Inner End
4^th Planet, Inner End

4th Planet, Outer End
4^th Planet, Outer End

A few minor changes were made to the 5^th planet arm, and I did extensive testing to ensure I was ready to freeze the design and begin documenting it.

20080102

I disassembled the model and photographed everything necessary to create building instructions — 347 images averaging 27K each.

20080106

I explored another way of making use of multiple levels of stacked turntables. This one places a geartrain at each level, which has whatever gears are needed to generate an arbitrary rate. Each level turns the base of the level above it. In addition there is a radial axle that goes out to the planet to make its moon turn.

Stacked Turntable Module

20080108

It would be nice to be able to add Callisto to model 2B. This could be done with another turntable — but if you just stick model 2B on top of a rotating platform, the direction in which the three inner moons align would rotate with it. If the simple gearing in model 2B is to be kept, it has to be supported without rotating — but Callisto has to be below it.

The solution to this is a fixed support that extends through the center of the two turntables, and leaves the lower one free to rotate.

Jovilabe Core

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20080109

What is the most number of planets you can get in an orrery that has all of its gears in the same plane? That means: one gear per axle, all axles are parallel, and no bevel, worm or crown gears.

The answer seems to be three, using this surprising arrangement based on the Hailfire Droid wheel, a part that was only ever used for one set. Here is the beginning of it:

Two Concentric Planetary Gear Systems

The turntable in the center would be made to turn by the same method used in model A above. If the carrier for the z36 and z40 gears remains fixed, the large ring gear moves at 1/3 the rate of the turntable. The first planet would be attached to the yellow sun gear, the second to the turntable and the third to the large ring gear.

20080111

While still working on the building instructions for model H (and ideas on how to make a huge PDF file available to others), I continued investigating alternate ways of gearing. Today I developed the "inside-out differential" described here.

Using it as the core mechanism, I created the orrery shown here:

2B: Triple Star with Planet and Moon

The gearing in this model is quite diverse. Here is a closeup:

Before I Added the Base

I'll describe it starting at the bottom.

The crank turns a 16-tooth gear, which drives a 20 attached to the central axle of the orrery. Also attached to the central axle is the sideways 36-tooth gear, a 24-tooth gear inside the chain, and a 40-tooth gear at the top.

The crank also turns a 20-tooth gear, that drives a pair of dark-gray gears. All the dark gray gears in the photo are part 6542, the 16-tooth gear with a round center, and they are free to turn independently of the axle.

The sideways gear and the two dark gray gears it touches act as a differential. Their effect is to make the upper dark gray gear turn a lot slower than the lower one, but still in the same direction.

Using the little teeth in its hub, the dark gray gear is attached to a 1x8 beam (part 4442) which also happens to have the little teeth and fits perfectly. The 1x8 beam can turn freely on the axle, so the result is that it "orbits" at the rate the dark gray gear is turning, without regard for the speed of the central axle. Another identical beam is positioned two studs higher, and upside down. This allows another dark gray 16-tooth gear to be attached, and also allows both beams to be stabilized by connecting them with pin joiners (part 75535).

At the outer end of the 1x8 beams, the planet axle is mounted. This axle is secured with old-style half bushings, but as you can see, I have faced the teeth the other way so the axle is free to turn. This entire arrangement of the beams, the pin joiners and second axle allows me to mount gears linked by a chain, and the planet axle turns with it. A z24 drives a z16, so the planet axle will rotate a bit faster than the central axle. This rotation makes the moon go around the planet.

The topmost of the dark gray gears turns with the planet, and the large 40-tooth gear above it turns with the central axle. There is a little axle mounted through one of the round holes in the 40-tooth gear (I've always wanted to use one of those holes for something!). On this small axle is a little 8-tooth gear that meshes with the dark gray gear. As the 40-tooth gear turns, the little 8-tooth gear turns too, and at a fairly high rate. It makes the close pair of stars (represented by the red and yellow bits at the top) revolve around each other. The other "star" (the white bit) is attached to another of the holes in the 40-tooth gear. It is a bit off-center to illustrate the idea that it is counterbalancing the red and yellow stars.

20080113

The inside-out differential is nice, but it takes up a lot of vertical space. I also saw that the 40-tooth gear wasn't being used for anything aside from being a platform for the stuff on top of it. So, I decided to redesign it to save vertical space.

Here is the result:

2C: Optimized and Regeared

The elimination of the differential meant that the bottom 16-tooth gear now has to be driven a lot slower than in model 2B. So, this model also uses different gearing inside the box.

20080117

I have finished the first draft of building instructions for model H. You can see me testing them here (actual elapsed time: 91 minutes)

20080124

Today I thought a bit more about an old challenge I set myself when first considering LEGO orreries — multiple concentric axles. I realized that if the gears were 8:16, 12:12 and 16:8, the Jovian Moons ratio of 1:2:4 would result — so I started building, determined to find a way to secure the flex cable (LEGO's only option for the center in triple-concentric motion) to a gear.

The breakthrough was realizing a tiny rubber band (one of the black ones, and preferably the smallest size LEGO makes) would work quite nicely to hold the flex cable inside the gear, by filling the gaps in the gear's center.

Just a couple hours later I had this!

3A: Downsized Jovilabe

When I grow up, I want to have 5 planets and 4 moons! What's that sonny? When I was yer age, all our bricks faced up — and the only piece that turned was 7039...

The base uses fairly standard building techniques to produce 4 vertical axle-holes each spaced 1.5 studs from the center. This makes great use of parts 4070 and 3794.

Parts referred to in the discussion above:

z8 gear	z16 gear	worm gear
3-unit half-thick liftarm	axle joiner perpendicular
Flex system hose	Ribbed hose

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