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# Ravigneaux Gearset Ratio Analysis

## Ravigneaux Gearset Ratio Analysis

(OP)
I am looking at design alternatives for a gearbox section that will need to offer one reduction ratio forward and one reverse. I know I can use a single planetary set to get the ratios (and directions) I need with a sun gear input, locking the carrier and using the ring for output (reverse) and locking the ring and outputting on the carrier (for forward), but this will require 4 clutches including 2 on the output shaft!  Not pretty from a control point of view.

Going to 2 simple planetaries is relatively easy, as I could simply shift from one sun to the other, but I would need to anchor the ring of one set and the carrier of the other to the case.  This leads me to think I could use a single ravigneaux set and just anchor the ring to the case, always outputting the carrier. One sun would drive through a second planet, giving me a reverse. Simple and maybe cheaper.

Question: When calculating ratios in a ravigneaux gearset, presuming the planets have an equal number of teeth, are ratios calculated exactly as they are with simple planetaries, that is 1 + ring/sun = ratio?   Any other references, hints and comments on the subtleties of ravigneaux geartrains would be appreciated.
Replies continue below

### RE: Ravigneaux Gearset Ratio Analysis

(OP)
I'm sorry, even after reading through the post 3 times, I see I stated my design incorrectly.

For the ravigneaux set, I plan to output from the ring and anchor the carrier!  That is the way I have it drawn.

### RE: Ravigneaux Gearset Ratio Analysis

Woohoo!  My fist forum post anywhere!

Sorry, to answer your question, the ratio of the ravigneaux would be just (ring/sun) and also negative (-ring/sun).

Hope that helps

### RE: Ravigneaux Gearset Ratio Analysis

ops!  I think I mispoke about the negative...  what I meant was that one ratio is negative and one is positive

I think the ratios would be
-(1+ring/sun1)
ring/sun2

Also, the planets do not need to have the same number of teeth-- the ratios are still valid.

### RE: Ravigneaux Gearset Ratio Analysis

Ops!  I assumed that one of the ratios would be (1+ring/sun) but that's only if you anchor the ring!  Sorry, it's been a while since I've thought about this stuff (that's why I have no references for you).

I guess I should be more thorough for accuracy-- that way I know I'm not making any more silly mistakes!

For a single planetary gear set:
if you hold the ring, ratio = (1+ring/sun1)
if you hold the carrier, ratio = -ring/sun1

For a planetary gear set with two sets of planets (sun meshes with one set, and ring meshes with the other).  Like a ravigneaux without one of the sun gears:
if you hold the ring, ratio = -(ring/sun2-1) = (1-ring/sun2)
if you hold the carrier, ratio = ring/sun2

So, the ratios you'd get are by anchoring the carrier and outputing the ring would be
-ring/sun1
ring/sun2
(where sun2 is the one that is connected the ring through two sets of planets.  Usually sun2 is smaller than sun1, but I don't think it has to be)

Sorry for the silly mistakes.  This time I've been thorough, so you can see where I made the little mistakes.

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