High ratio (354:1) planetary compound and telescope drive (3D-printed)

Show off your FreeCAD projects here!
kwahoo
Posts: 401
Joined: Fri Nov 29, 2013 3:09 pm
Contact:

High ratio (354:1) planetary compound and telescope drive (3D-printed)

Postby kwahoo » Sun Jan 03, 2021 2:15 pm

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.

phpBB [video]


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.

Image

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.

Image
Image

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.
Last edited by kwahoo on Sun Jan 03, 2021 10:29 pm, edited 1 time in total.
davidosterberg
Posts: 482
Joined: Fri Sep 18, 2020 5:40 pm

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby davidosterberg » 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? It is to minimize backlash, to retain the planets without having a planet carrier? Do you print it all in place?
chrisb
Posts: 33721
Joined: Tue Mar 17, 2015 9:14 am

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby chrisb » Sun Jan 03, 2021 3:46 pm

kwahoo wrote:
Sun Jan 03, 2021 2:15 pm
I was searching for high ratio gearing to use in my DIY telescope drive.
I never printed one, but the Cycloidal drive looks as if it was well printable.
A Sketcher Lecture with in-depth information is available in English, auf Deutsch, en français, en español.
Frickelandian
Posts: 1
Joined: Sun Jan 03, 2021 4:05 pm

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby Frickelandian » Sun Jan 03, 2021 4:22 pm

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 ;)
kwahoo
Posts: 401
Joined: Fri Nov 29, 2013 3:09 pm
Contact:

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby kwahoo » Sun Jan 03, 2021 4:38 pm

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?
Thank you for kind words!
Since planet gears are shared, and ring gears are different, I have to compensate this. :

Image

In an earlier prototype I used involute shift for this:
Image

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
chrisb wrote:
Sun Jan 03, 2021 3:46 pm
I never printed one, but the Cycloidal drive looks as if it was well printable.
It is printable, but I'm affraid that friction would be simply to high.
User avatar
Zolko
Posts: 1335
Joined: Mon Dec 17, 2018 10:02 am

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby Zolko » Sun Jan 03, 2021 5:25 pm

kwahoo wrote:
Sun Jan 03, 2021 2:15 pm
There are two sets of ring gears:
-fixed, 60 teeth, module = 1
-moving, output, 59 teeth, module = 1.029
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.
try the Assembly4 workbench for FreCAD v0.19
install with Tools > Addon Manager > Assembly4 — tutorials here and here
User avatar
Zolko
Posts: 1335
Joined: Mon Dec 17, 2018 10:02 am

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby Zolko » Sun Jan 03, 2021 5:48 pm

kwahoo wrote:
Sun Jan 03, 2021 2:15 pm
-moving, output, 59 teeth, module = 1.029

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.
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
try the Assembly4 workbench for FreCAD v0.19
install with Tools > Addon Manager > Assembly4 — tutorials here and here
kwahoo
Posts: 401
Joined: Fri Nov 29, 2013 3:09 pm
Contact:

Re: High ratio (1:354) planetary compound and telescope drive (3D-printed)

Postby kwahoo » Sun Jan 03, 2021 5:55 pm

Zolko wrote:
Sun Jan 03, 2021 5:25 pm
Am I right that the output gear is only guided by the 3 planetary gears ?
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?
Zolko wrote:
Sun Jan 03, 2021 5:25 pm
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.
Exactly.
Zolko wrote:
Sun Jan 03, 2021 5:25 pm
Where did you get this from ?
This is my original idea, but... as 99.9% ideas this was probably invented earlier by someone. Who knows.
kwahoo
Posts: 401
Joined: Fri Nov 29, 2013 3:09 pm
Contact:

Re: High ratio (354:1) planetary compound and telescope drive (3D-printed)

Postby kwahoo » Sun Feb 28, 2021 1:46 pm

Updated design with ball bearings:
phpBB [video]

Image
Image

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.
User avatar
arturromarr
Posts: 282
Joined: Sat Jun 24, 2017 9:32 pm

Re: High ratio (354:1) planetary compound and telescope drive (3D-printed)

Postby arturromarr » Tue Apr 06, 2021 10:33 pm

Looks great. Did you mount the gearbox to the telescope?