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  • Ravigneaux Gear planning

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

    1. Philip_Il

      Philip_Il New 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.

      I tried using the formulas from this page: https://www.mathworks.com/help/physmod/sdl/ref/ravigneauxgear.html
      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!
       
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    3. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks 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.weinberg

      s.weinberg Well-Known Member EngineeringClicks 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_Il

      Philip_Il New Member

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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?
       
    6. s.weinberg

      s.weinberg Well-Known Member EngineeringClicks Expert

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      I'm not sure what you can lock, where you can connect your input and where you can connect your output. That defines how you can use any particular planetary set
       

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