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• # Ravigneaux Gear planning

Discussion in 'Calculations' started by Philip_Il, Jul 17, 2021.

1. ### Philip_IlNew Member

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Hi hope someone here can help me or at least guide me to a sensible approach to calculating this problem I have.

I need to calculate a Ravigneaux Gear I'm aiming to get 4 gears + reverse with the following ratios:
1st gear: 3.00 ratio
2nd gear: 1.67 ratio
3rd gear: 1.00 ratio
4th gear: 0.75 ratio
Reverse gear: (-3.00) ratio.

but since it is based on radius and not teeth count I'm getting really confused.
Anyone has experience with it and can help with a "best practice" for how to approach this issue?

Thanks!

2.
3. ### s.weinbergWell-Known MemberEngineeringClicks Expert

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Do you know the method of superposition for gears?

It's a nice universal tool for evaluating planetary gear sets of all sorts. I cover it here: https://www.engineeringclicks.com/planetary-gears/ but it's not the best explanation, so you might have better luck Googling it.

For a simple Ravigneaux Gear (where the 'planetary gear 2', which sits between the large sun and ring gear is of a single size, and not differently sized where it encounters the other planet vs where it bridges the large sun and ring gear), the equations come out as follows:

Where the teeth of each gear are:
Small sun: Z1
Large sun: Z2
Planet 1 (meshes with small sun): Z3
Planet 2: Z4
Ring Gear: Z5

Small sun: x+y
Large Sun: x-(Z1/Z2)y
Planet 1: x- (Z1/Z3)y
Planet 2: x+(Z1/Z4)y
Carrier: x
Ring Gear: x+(Z1/Z5)y

For torque ratio, divide output by input, for speed, input by output

The fixed member will have 0 speed, so you can use it to get an equation between x and y and solve for your gear ratio.

For example, in first gear, the carrier is fixed. Carrier = x. So x=0
Speed ratio is: Small sun/Ring Gear or:
0+y/((0+Z1/Z5)y
or just Z5/Z1

4. ### s.weinbergWell-Known MemberEngineeringClicks Expert

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Did some calculations, and you're going to have a real tough time making a Ravigneaux Gear with the ratios you want, unless you pull some tricks (like using different modules for the two halves of your gear train, requiring planet 2 to be 2 gears of different modules stuck together).

To get 3:1 gear ratio in 1st gear, the Ring gear has to be 3 times the small sun gear.
To get -3:1 gear ratio in reverse, the Ring gear likewise has to be 3 times the large sun gear.
Your 'large' and 'small' sun gears being of the same size makes this gear arrangement awkward to realize, at least simply

5. ### Philip_IlNew Member

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Jul 2021
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Hi s.weinberg,

I must thank you for your organized and easy to understand method of calculating the gear ratios! It's amazing, made it so much easier to understand.
2nd I see what you mean, I did get to that dead-end of Z1/Z2=1.

If I'd aim for an alternative design of 2 stages planetary gear, how would you recommend connecting those? The carrier as the output of the 1st set to the input sun of the 2nd set or is there a smarter way considering the predefined gear ratios?

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