Tuesday, September 8, 2009

Orrery Instructions

Here is a guide on how I designed and built the orrery. At a later stage I will put together a pdf containing all of these. Feel free to use these as you would, I ask only that if you do use these instructions and make an orrery that you send me some photos.

These instructions aren't entirely comprehensive. I don't, for instance, tell you exactly how long to make the tubes or how much to space the gears. What I present in this guide is a list of parts, especially the gears, and a description of how they fit together. It's enough to produce the innards of the orrery, but what exact size and shape you want is up to you.

Description

This is a Copernican Orrery, which means it has the planets from Mercury out to Saturn. The other features of the orrery are:
  • The Moon revolves around the earth
  • The Earth remains at a constant axial tilt (of around 23 degrees)
  • The Earth rotates on its axis
A final feature of the orrery is that it can be taken entirely apart (as you can see in the previous photo of the orrery compenents). This means if anything breaks, it can be fixed without destroying other pieces.

Tools

I don't have much in the way of a workshop, actually I have a table on the deck out the back and that's about it. Also, I am not much of a handyman. Therefore, the orrery was constructed using:
  • A hacksaw
  • A dremel tool
  • A cordless drill
  • Sandpaper
  • and lots of Superglue
There are a couple of reasons why I used superglue rather than solder. First, I don't have a soldering iron, and I figured I'd probably burn down the house if I tried using one. Second, given I was making some of this up as I went along, the ability to drop a piece of work into acetone, dissolve the glue and start again seemed like a good idea (and was). Finally, I like the idea of kids being able to make this (like mine in a few years time), and superglue just seems a bit safer. The particular glue I used was Selleys Quick Fix Shock Proof, it gave me a bit of time to set the pieces and so far seems strong enough.

Materials

You will need
  • Wood for the box
  • Brass tubing and rods of different diameters, available from K and S Metals. There was a stand from this group at my local hobby store. Specifically you will need a solid rods of 1/16" and 3/32" diameter, and tubes ranging from 1/8" to 11/32". They come in 1 foot lengths and I would advise getting two of each up to about 3/16".
  • Brass gears, which I got from www.sdp-si.com. Following is an exact list of the part numbers I ordered, along with a description of what they are used for.


Part NumberQuantity
Main gearsA 1B 1-N320164
A 1B 1-N320323
A 1B 1-N320402
A 1B 1-N320483
A 1B 1-N320503
A 1B 1-N320643
Moon rotation gearsA 1B 1-N321281
A 1B 1-N320101
Earth rotation gearsA 1B 1-N480082
A 1B 1-N480321
Worm Screw and gearA 1C55-5N321
A 1B 6-N320301
Sprocket gearA 6B 8-19322
Ladder ChainA 6B 8-191

Note: The last three digits in each of the gear wheels indicates how many teeth they have. This is the expensive part, with these costing just over USD$300.

Glossary

I'll use the following nomenclature throughout:
Planetary tubes – the metal tubes, one for each planet, with a gear to drive the planet.
Gear trees – Collections of gears on a single tube, apart from the planetary rods.
Earth arm – The collection of gears, rods and sprocket with the earth and moon attached.
Earth large gear – The 128 tooth gear and sprocket that the Earth/moon arm revolves around.
Earth large gear bracket – A bracket where the earth large gear sits.
Planet stalks and collars – The little arms and collar that the planets are attached to.

Diagram

The following picture shows the main gear trees and planetary tubes, with the number of teeth on each gear wheel shown next to the wheels.


A solid black line connecting the gears indicates that they are attached to the same rod and revolve at the same pace, a hollow tube indicates they spin independently. The worm screw and gear aren't shown, but should spin one of the four lowest gears on the left most gear tree. You can see that these bottom four gears are all attached and revolve together. They drive the planetary tubes, to the right, of Mercury through Mars, which spin independently. Mars then spins the next gear tree which in turns spins the final gear tree, far right, which in turn passes back to a free gear on the previous gear tree. This drives the Jupiter planetary tube and has the effect of stepping down from Mars by a factor of 9. This in turn drives the top two gears on the left hand gear tree, which are independent from the others and which finally drive Saturn.

Meanwhile, the Earth large gear is kept stationary by its bracket. You can see that represented in the picture below, but it's probably easiest to see in some of the earlier photos and video.

The Earth tube spins the earth arm, which rotates around the large stationary 128 tooth gear. This spins a smaller gear which is attached to the moon, causing it to rotate around the earth. The Earth is attached to the sprocket system, which keeps its tilt in the same direction as it rotates around the sun. Finally, the gear that is spinning the moon is also used to spin the earth through a series of 3 other gears.

The following diagram shows the bore diameters of each of the gears along with the diameters of the corresponding tubes that are driven by them. In terms of following the instructions here, this is really the only critical part. Unless you have the equipment to increase the bore diameter of these gears, you need to ensure that any changes you make to the plans still leave you with a series of tubes that will fit into your gears.


Some of the gears have bores larger than the tube they are meant to be attached to. My solution to this is to cut a small section of the next size up tube and glue it around the first tube. I may need to do this several times until the tube is finally the same size as the gear bore.

Planetary Stalks and Collars

The collars are just a small piece of tube the next size up, so they fit snuggly over the planetary tube. Then I simply crimp the end slightly, so the collar sits on the tube. This means when the tube spins, friction will ensure the collar spins, but when I want to reset the planetary positions, I can just move them easily.

Fixing the Earth Large Gear in its Bracket

I wanted to ensure that I could take the entire orrery apart and put it back together. This meant that I needed to keep the earth large gear stationary in the bracket, but also remove it from the bracket. To do this, I cut half the tube off from the end to about 1 cm along. I then made a collar, one tube size up, and into this glued a small piece roughly the size of the one I had cut off. The earth large gear tube fits into this collar and won't turn. This has been a particularly poor description, and I'll upload some pictures later.

The Case

This can really be any shape you like. The main point is that it have two levels. The gear trees need contact points above and below to keep them in place, preferably as frictionless as possible. I used two pieces of tube for each contact point, one the size of the gear trees central tube, the other a smaller tube just inside that. It would rest on the larger brass tube and rotate around the smaller one. The main planetary tree only needs on contact point and a hole to exit the case.


Accuracy

This isn't perfectly accurate. To do that you'd really need to cut your own gears. However, with the available gears, this is pretty close. Following is a table of actual and orrery orbital periods.

















































ActualOrrery
Mercury0.240.25
Venus0.620.64
Earth11
Mars1.881.56
Jupiter11.8714.04
Saturn29.4535.94
Moon (around Earth)0.070.08
Earth (rotations on axis per year)36551


It was pointed out to me that my original calculation for Jupiter and Saturn was wrong. It would actually be slightly closer to reality if the three 48 tooth gears used on the step down arms were actually 40 tooth gears. This would actually make Jupiter and Saturn slightly too fast, but given how slowly they move that probably isn't a bad thing.

Hopefully these instructions are relatively clear, or at least not totally confusing. I assume if you're serious about doing this you're going to be able to fill in some blanks yourself, and that the main things I've provided are the sources of material and some ideas. If you do have any questions, feel free to contact me at Murray Keir at gmail (I'm sure you can figure it out if you're not a bot). Good luck.

4 comments:

  1. hi, your orrery truly inspire me, may i have the complete plan in a pdf

    ReplyDelete
  2. I wonder if we could find inexpensive, plastic gears? Wouldn't be as pretty but would be an affordable (accessible) project for kids. Have you seen any plastic gears?

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  3. Hi! Thank you for giving the world this awesome information! I have looked everywhere to find plans to build a working orrery. I have no experience with mechanics or building but I am determined to do this. And I want to do it right no matter how hard or complicated it is. I admire you for the work you've done to put all this together, it's truly awesome!! I just wanted to say thank you for helping me in my quest to build an orrery! ~ Kitty

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  4. Thanks for the design, your model is beautiful.
    I used it to create a CG version, you can view it here:
    https://www.youtube.com/watch?v=G98cUGuzzm0

    ReplyDelete