Self Build Orrery

[macmangan]

mmmmm.... couple hundred thousand gears.

[MorrisHeaton]

I'd be interested to find out how you figured your gear ratios. I'm looking for a project for my engineering group next year (we already plan on a stirling engine, imagine a combination of the two!). We've got access to mills, with indexing heads, which we can never find enough excuses to use.

[Mechanism Man]

I'd be interested to find out how you figured your gear ratios. I'm looking for a project for my engineering group next year (we already plan on a stirling engine, imagine a combination of the two!). We've got access to mills, with indexing heads, which we can never find enough excuses to use.

To build a simple orrery isn't really too much of a problem, really, honest....
 
You've got 2 pyramid shaped stacks of gears, one stack driving the other. The driving (or input) stack has the largest gear at the bottom (driving Mercury), and smallest at the top (driving whichever planet is farthest from the sun in your orrery - Saturn in my case). One revolution of this first stack of gears will equal one year on Earth. The second stack, which is driven by the gears in the first, is the other way up, so you've got a big gear at the top and a small one at the bottom. But each gear in this driven stack each drives its own seperate nested tube, as all the driven gears will obviously be going at different speeds to drive their own planets. The tops of the tubes are all at different hights (the innermost tube being the tallest) to enable you to fit the planet support arms.

The trick is to remember that all the ratios relate to the speed of Earth - i.e. one orbit of the Sun per year. So I started from the bottom, Mercury, whos orbit period is just 0.24 of Earths, so a fair bit quicker. As my maths is rubbish, I went for the easiest way of achieving this first ratio - a 100 tooth gear driving a 24 tooth. Hey prest, an input speed that's 0.24 of the ouput. This pair also had 124 teeth in total. So I now also knew that for all the planets, whatever their ratio turned out to be, each pair of gears had to have a total of 124 teeth - why? Because that would keep the 2 gears driving each planet on exactly the same centres as the pairs driving all the rest, which was crucial.

As one rotation of the first stack of gears equals one Earth year, it stood to reason that the two Earth gears were going to be identical. So that pair was also then easy to work out. They had to have 124 teeth total (the same as Mercurys pair), so that had to mean 62 teeth each, which would give exactly one year for each full rotation of both the input and output gears.
 
Two down, four to go!

With a bit of trial and error, I worked out that Venus would have a 77 driving a 47 (making it quite quick at 0.61 of Earths year), and Mars was a 43 driving 81 (making a slightly slower 1.88 times Earths year).

So far, so good, unfortunately, Jupiter and Saturn were a more complicated affair due to the fact that Jupiter runs at just 1 orbit of the sun for 11.86 years on Earth, and Saturn is 1 orbit for 29.5 years on Earth! If I were to use just 2 gears for each of these 2 planets, one of the gears would have been very, very small, and the other very, very big, and the meshing of wooden gears of such different sizes would have been terrible. So I did these two planets by driving them in two steps, using a second pair of gears located on seperate centres.
 
After much experimenting, for Jupiter I ended up with a 27 driving a 90, and mounted on that 90 was another 27 driving another 90. The first and last gears in this train were on the original centres. Saturn had a 19 driving a 103, and another 19 on top of that driving another 103. This meant cutting more gears, but the end result worked smoothly and without any noticeable snatch through the gear trains.

Hope that helps Mr. Heaton.... 

Sorry if all that was really dull and hard to follow, I almost fell asleep writing it. I'm better at building than I am explaining. Hopefully, somebody out there will understand what I was talking about. I 'spose, in a nutshell, it's really not as hard as it sounds, so sit down with a pen and paper and have a go at working through it yourself. And if you've got access to milling machines and dividing plates, then you're sorted for accurately producing any gear size you want.

So what are you waiting for - give it a go!                   

[Hieronimous Stonebender]

You make it sound surprisingly simple, and almost doable... I feel my spirits rising... I might give this a go...

[Mechanism Man]

You make it sound surprisingly simple, and almost doable... I feel my spirits rising... I might give this a go...

Building it was easier than explaining it...  did about 10 edits to make it make sense!
Any questions anyone, throw them my way and I'll do my best to answer.

[Brass Monkey]

Nicely explained

How does one link it to the real solar system for planetary billiards?

[Mercury Wells]

Nicely explained

How does one link it to the real solar system for planetary billiards?

Difficult, very difficult!. 

[Mechanism Man]

Nicely explained

How does one link it to the real solar system for planetary billiards?

Difficult, very difficult!. 

Anyone out there got Steven Hawkings number....?

[Mr. Fish]

Ah-ha,
Very nice explaination there, looking back at the rest of the photos in the thread I can finally get my head fully around what's going on.

Also, I've just ordered a copy of "Making a tellurian/orrery" by John Locke.
The benefits of working in a really big bookshop show true once more.

Miniature orreries in tiny belljars here we come!
(maybe.......if I don't go insane first)

[Mercury Wells]

Nicely explained

How does one link it to the real solar system for planetary billiards?

Difficult, very difficult!. 

Anyone out there got Steven Hawkings number....?

Nope, but try here.

[Mr. Fish]

Hah!
possible customer/sponsor right there Mech!
Don't forget Patrick Moore either,
Sic 'em!
 

[Mechanism Man]

I think that the good Mr. Hawkin, intellectually speaking, is a little beyond my little creation. I think that maybe he would laugh in the face of my cogs.
   

[rogue_designer]

He doesn't strike me as the type that would laugh at anyone sincere efforts. Particularly not those displaying such skill.

He'd probably be tickled pink to own one. Maybe you could rig up a motor to turn it that he can actuate from his chair.

I'm not even kidding.

[Mechanism Man]

Hmm, maybe I should punt one in his general direction and see what happens.... That would be beyond good to have Steven Hawkin own something of mine!
 

[MorrisHeaton]

Thanks for the gear info. It should help me muchly. Going to try a few cardboard experiments (with Elmer's reinforced teeth!) over the next little bit.

[mordred888]

I would really like to see the design and gear rotios for the orrey
Ive been thinking of making one in brass ( got lots of brass sheet and a gear cutting machine ) but no matter how many spreadsheets I set up I can never get a really acurate 9 planet gearing worked out.
The best accuracy I have managed is about 10%.
 

[Mechanism Man]

I have failed you - and just when I thought that I'd cracked it.
I tried, I experimented and I fiddled. I cut gears of varying sizes, in a multitude of combinations. I played around with several different frame configurations to hold it all together. I tried different bearing tube sizes and materials. I even built a shrine to the Great Earth Cog to see if that would help. But alas, it was all in vein.
Unfortunately, despite my best efforts and much cursing, the world will be motionless, static, free from all independant rotational movement as it orbits the Sun. That part of my mechanism which was designed to turn the world was a leap too far and just won't work properly. Bugger.
I have basically reached the practical limit of my wooden gears. Although they all mesh really well, the power that needs to be transmitted to the final gear train (the one that spins merrily around The Great Eath Cog) was just too much, and the backlash down through the system of gears inside the box was just too great. The upshot was that as I wound the handle, the machine tries to A) strip teeth from the gears and B) jam solid. I'm not saying that this won't feature in a later version, but at the moment I don't have the time to put into finding a practical solution - and I'm sure that there is one, and it will probably be develishly clever. Just not yet. Never mind.
So, for the time being at least, I shall have to content myself with the 6 planets orbiting the Sun, and the Moon orbiting our tiny little blue green planet.
But no matter, on the plus side, there will ultimately be less gears to cut, and the big plus is that I can now speed the whole thing up a bit to show a much quicker movement of all the planets and gears. Before, I had to keep the speed down to approx one earth day per turn of the handle - really slow in other words, but now I've ditched all the really spinny bits for the Earth that demanded so much power to get them moving, I can now do about 2 weeks to one handle turn which will make the whole thing much more dynamic and full of movement.
Now that I've made the desicion to ditch that part of the machine, hopefully progress should now pick up again. More photos and updates very soon. Fingers crossed for no more disasters please!
Right, back to the cog shop. 

[Ampère Volt]

Keeping fingers crossed!!

[Arcturon the hobo]

Man, nobody on this site will ever call any part of your project a failure. You remain an inspiration to us all.

[rogue_designer]

Man, nobody on this site will ever call any part of your project a failure. You remain an inspiration to us all.

Quoted for truth.

The earth spinning was really icing on the cake, but by no means critical to the "success" of this wonderful project.

[Mr. Fish]

Not to worry Mech,
it will still look amazing by the time it's finished.
How about this? instead of having the earth direcly driven by interfacing with the Great Earth Cog, why not have it friction driven?
sort of like a slip drive?
That way it could take some of the pressure off the drive mechanism. Sure, it won't be accurate at all but at least you might be able to get some movement into the planet.....


OR..... *crazy idea alert*

How about a wind up motor up near the end of the final gear train, thereby providing motive power from that end as well?
In this case the mass of the rest of the orrery (as well as its inertia) would act as an escapement, and the power coming from the far end might act to reduce the backlash down through the system...?

As I said, a crazy idea, but might be worth thinking about?

Sorry to hear that you hit a wall, but it will be great to see in the end no matter what.

[Mechanism Man]

I think that saying I was going to give up on a spinning Earth may have been a little premature.... I had one last ditch attempt at getting it to run smoothly last night.... and I think that I may have fixed it.... possibly....
The culprit causing the backlash problem may have been simply a bent tube. Having replaced it, and had a quick fettle of some of the gear teeth that were minutely binding, it now seems to work without any problems. But we shall see.
So I think that now may be a good time to throw it all back together (it's in 101 bits at the moment whilst I grappled with it's problems) and post a film of it doing it's thing. Still no planets yet, but at least you can see all the spinny bits spinning. I'm waiting for a tool to come in the post (hopefully today) to enable me to cut a thread into a piece of half inch dowel. This will make up half of the worm drive system that will power the thing when the handle is turned. So todays film will be powered by a well placed thumb in the mechanism!
Jobs to do first though - I have to go to the tip and dump some rubbish and also pay a visit to the inlaws - and then I shall set to it.
Thanks for all the encouragement by the way, this was half the reason for making this a public project I guess, it stops it being a lonely affair.
Mr. Fish, top ideas my good man - the friction drive I had actually toyed with just to get some rotation. If the mech that I think I've fixed doesn't actually work, then I may well give that a go.
See you all in a bit.
            

[Hieronimous Stonebender]

Ooooh... I can not wait to see the spinny bits spinning. Inlaws be damned I tell you... Onwards!

[Ampère Volt]

Will love to see the spinning!!

[Commander catastrophe]

It looks really great, maybe you could stain it with tea instead of paint to prevent the gears getting bulky.

  Not a good plan, have used both tea and coffee before as a stopgap. Looks well initially but doesn't seal the piece and introduces water borne organics that quickly lead to mildew and separation of the ply if you're not using marine. Not something that should happen to this magnificent design. I'm taking on the same project with an engineering class (though we might cheat and laser cut the cogs) I'll post you the result.