[BillTodd]

Another thought:

What about a traction drive? i.e. a tooth-less gear-train. It's very easy to get high reduction ratios in a compact planetary drive design.

As long as you have a positional feed-back from the output (high resolution encoder?) Your servo can compensate for the slightly unpredictable ratio.

[Zoidberg]

What about a traction drive? i.e. a tooth-less gear-train. It's very easy to get high reduction ratios in a compact planetary drive design.

Hmm that sounds interesting! I got a bunch of 3600 ppr encoders, if i use them in quadrature with a gearbelt i should be able to get pretty good resolution! The question is how much torque you could get from the traction drive!?

[sparkness]

Great information, in my situation I have a 9" troyke rotary table that I am converting to cnc for gear cutting. My plan is to advance in only one direction and use a separate motor to actuate the table lock. this seems simple and easy. Any other Ideas on how to do it?

[BillTodd]

how much torque you could get from the traction drive!?

That's down to the material technology you use. It would be interesting to see if you could persuade a someone to let you have some their magic traction fluid for a home made device

[Zoidberg]

http://www.gears-manufacturers.com/traction-drives.html
"Dry Traction Drive - These drives eliminate the need for lubricant and allow nearly 100% efficiency in power transmission. Slippage between driving and driven members is presented by a spring-loaded system."

As it is used for low speed and not as often as a mill spindle i think dry with feedback from the output is acceptable

[BillTodd]

As it is used for low speed and not as often as a mill spindle i think dry with feedback from the output is acceptable

Sounds do-able

I await your drawing with interest

[Zoidberg]

I'm playing around in 2D now to find suitable dimensions/ratios now...
20mm sun, 40mm planet and 100mm ring should give 5:1!?
Then i'm thinking of using 3 stages for 125:1, and with increased witdh of each stage to handle more torque at the output end. How does that sound?

I found this page! Check a bit down under "Advanced rotational speed reduction traction drive device. 3D."
http://www.rotaryeng.net/turbo-compound.html

[BillTodd]

I found this page! Check a bit down under "Advanced rotational speed reduction traction drive device. 3D."

Ah yes - small world...

I occasionally make engine animations for Doug Self's 'Museum of Retrotech'. Somewhere on his site is page about compound IC engines. Reading that prompted research that lead me to turbo compound engines and thus to the site you linked to. It was that site where I first saw a home-brew traction gearbox and what I had in mind when I suggested it to you

A complete circle :LOL:

[RICHARD ZASTROW]

FYI Cone Drive uses a belleville washer type spring to preload a two piece worm against a worm-wheel in their "AccuDrive" zero backlash applications.

There is also a near zero backlash planetary gear set developed for NASA called a gear-bearing. This is a combination of planetary gears that also functions as a roller bearing in an individual element. It's one of those "Why didn't I think of that?" type things.

It's available on NASA's technology sharing site.

Dick Z

[BillTodd]

A flight of fancy

This gives about 56:1 using two stages of cones.

BTW. there is considerable aliasing* visible in the gif animation, which is why the input shaft appears to be turning slowly.

*( the same thing that causes wagon wheels to go backwards when being chased by indians )

[Zoidberg]

I like that gear-bearing thing! Started to draw one, but have a little problem with the simulation!

The planets rotation around the Ground ring gear is a little off. Is this correct?
For a planet to rotate 1 revolution around the ring gear, it have to rotate 66/21 turns around its own axis.
For a planet to rotate 66/21 turns around its own axis, the input have to rotate (66/21)*(24/21) = 3,1429*1,1429 = 3,5920 turns?

I found a PDF with these numbers:
Sun: 24
Bottom Planet: 21
Ground Ring: 66
Top Planet:21
Output Ring: 67
For 251,25:1 reduction

The secondary planets and the output ring is missing until i get the first stage done correctly!

[BillTodd]

With your numbers;
Sun: 24
Bottom Planet: 21
Ground Ring: 66

the sun has to turn 66/24 times for the planet to make one orbit.

The planet will revolve 24/21 times for each revolution of the sun.

Does that help?

[Zoidberg]

I made a simple test to try out some ratios.
Sun: 18
Front Planet: 24
Ground Ring: 66
Back Planet: 30
Output Ring: 72

Now i got the simulation to run as it should!
I belive the Sun to Orbit ratio is 4,889:1
But i have no clue how to calculate the Sun to Output ratio! But would belive it's around 30:1

[BillTodd]

But i have no clue how to calculate the Sun to Output ratio! But would believe it's around 30:1

The planet set orbits forward 18/66 teeth for each revolution of the sun

The planet revolves (backwards) 66/24 times every orbit

The output ring moves forward 72 teeth every orbit (imagine if the output gear was directly connected to the planet) and backwards 30 teeth each revolution of the planet (imagine the planet stationary).

So, the output rings moves 72-30 * (66/24) teeth each orbit.

The output rings moves 72-30 * (66/24) * (18/66) for each revolution of the sun

Simplifying we get : 42*18/24 = 31.5:1

[Zoidberg]

Nice! Thanks Back to the drawingboard then

[raymondg]

As an alternative, "google" Howimat. This concept is ball driven like a ballscrew in the round.

Dick Z

I poked around the DetlevHofmann site, and while they mention the Howimat drives, there is almost no info. They do mention a patent but I couldn't find it. I was wondering if anyone has seen the patent, or knows where it is?

Regards,
Raymond

[Zoidberg]

Did just draw a 105:1 gearbox!
The housing is 100mm diameter and 45 mm long.

[BillTodd]

Very nice

[BillTodd]

The planet set orbits forward 18/66 teeth for each revolution of the sun

The planet revolves (backwards) 66/24 times every orbit

The output ring moves forward 72 teeth every orbit (imagine if the output gear was directly connected to the planet) and backwards 30 teeth each revolution of the planet (imagine the planet stationary).

So, the output rings moves 72-30 * (66/24) teeth each orbit.

The output rings moves 72-30 * (66/24) * (18/66) for each revolution of the sun

Simplifying we get : 42*18/24 = 31.5:1

Hang on. I've made a cock-up here(parenthesis missing).



[edit] still wrong

[Zoidberg]

In my last model i have:
Ground: 72
Sun: 12
Planet: 30
Output Planet: 24
Output Ring: 66

I just checked, it seem to move about 6,5 degrees in the same direction as the sun!