Hi,
I was searching for high ratio gearing to use in my DIY telescope drive.
The first idea was a strain wave gearing, but 3D-printed wave is not reliable. I could use timing belt, but then gearing is noticeably bigger, and the belt has to much flexibility.
Finally, I created this: 3D-printed planetary compound, 354:1 ratio. Animation was done with Assembly3, Spreadsheet and a small python script. I uploaded everything on github.
There are two sets of ring gears:
-fixed, 60 teeth, module = 1
-moving, output, 59 teeth, module = 1.029
Planet gears are shared but modules of inside and outside teeth rows are different. Number of teeth in both is same.
I did not update the telescope repository yet, since gearing ratio will be changed probably. Also there are some teething issues (pun not intended) that have to be fixed, like too small output ring gear rigidity.
The new features in FreeCAD Link Branch were very useful. Eg. when I needed second very similar part but without holes, I could just suppress them instead of removing them. Supressing does not remove references, and does not break later features.
I used FCGear to external gears, because it has more features (like shift) than Part Desing Involute Gear. Unfortunately FCGear does not support internal gears, so I made them in Part Design.
High ratio (354:1) planetary compound and telescope drive (3D-printed)
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High ratio (354:1) planetary compound and telescope drive (3D-printed)
Last edited by kwahoo on Sun Jan 03, 2021 10:29 pm, edited 1 time in total.
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Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
Awesome work! Interesting how the ring gear has an external gearing. Did you have inspirations for this choice, I have not seen that before.
Why did you go with the different modules on the same wheel? It is to minimize backlash, to retain the planets without having a planet carrier? Do you print it all in place?
Why did you go with the different modules on the same wheel? It is to minimize backlash, to retain the planets without having a planet carrier? Do you print it all in place?
Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
I never printed one, but the Cycloidal drive looks as if it was well printable.kwahoo wrote: ↑Sun Jan 03, 2021 2:15 pm I was searching for high ratio gearing to use in my DIY telescope drive.
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Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
Really fascinating work!
This looks similar to a "Wolfrom" gearset, which combines elements from two planetary sets and omits the carriers and sun #2, Ring #2 is the output. With that, to achieve maximum reduction the planets of set #2 are smaller by 1 tooth than those of #1,
Here the trick seems to be that rings differ by 1 tooth and the module is adjusted so that the planets have the same number of teeth. Or something along these lines. Here, ring #2 is the output too, and then it's just fair to give it teeth on the outside
This looks similar to a "Wolfrom" gearset, which combines elements from two planetary sets and omits the carriers and sun #2, Ring #2 is the output. With that, to achieve maximum reduction the planets of set #2 are smaller by 1 tooth than those of #1,
Here the trick seems to be that rings differ by 1 tooth and the module is adjusted so that the planets have the same number of teeth. Or something along these lines. Here, ring #2 is the output too, and then it's just fair to give it teeth on the outside
Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
Thank you for kind words!davidosterberg wrote: ↑Sun Jan 03, 2021 3:18 pm Awesome work! Interesting how the ring gear has an external gearing. Did you have inspirations for this choice, I have not seen that before.
Why did you go with the different modules on the same wheel?
Since planet gears are shared, and ring gears are different, I have to compensate this. :
In an earlier prototype I used involute shift for this:
External gearing on ring gear is only to get output torque. I printed parts one by one.
About inspirations:
1. Strain wave,
2. Ok, a single planet planetary makes the same thing as strain wave, but have to be eccentric
3. Make involute shift to compensate eccentricity
4. Use different modules instead of shift
It is printable, but I'm affraid that friction would be simply to high.chrisb wrote: ↑Sun Jan 03, 2021 3:46 pm I never printed one, but the Cycloidal drive looks as if it was well printable.
Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
Am I right that the output gear is only guided by the 3 planetary gears ?
If I understand the principle correctly, you have 1 true planetary gear with fixed outer ring (60 - 24 - 12) , and a virtual planetary gear (59 - 24 - 11) where all 3 components rotate, including the non-existent internal 11-teeth one.
Where did you get this from ? This seems better than the harmonic drive, since it has probably low friction. Actually ... I'll need something like that for work to replace a harmonic drive gear, might as well give it a try.
Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
so what you'd really want is an internal gear with 59 teeth and module 1, but shifted inside. Then the 3 satellite gears could be straight 24 teeth module 1. A pity FCgear doesn't do that.
EDIT: erm no, that wouldn't do
Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)
Yes. This is not perfect from mechanical POV ("real", steel planetary gearings don't work that way), but it seems to be good enough for 3D-printing.
EDIT: After some testing I'm thinking about single fixed ring design and output ring guided by a bearing. This should give me more rigidity and less friction. Also, maybe planets should be guided by a carrier and bearings?
Exactly.
This is my original idea, but... as 99.9% ideas this was probably invented earlier by someone. Who knows.
Re: High ratio (354:1) planetary compound and telescope drive (3D-printed)
Updated design with ball bearings:
https://imgur.com/a/vhsvhnI
https://github.com/kwahoo2/Mortargear/tree/main/v2
I had to increase teeth module to 1.25 due lack of 3d printing precision.
https://imgur.com/a/vhsvhnI
https://github.com/kwahoo2/Mortargear/tree/main/v2
I had to increase teeth module to 1.25 due lack of 3d printing precision.
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Re: High ratio (354:1) planetary compound and telescope drive (3D-printed)
Looks great. Did you mount the gearbox to the telescope?