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What's the Advantage of a Sun-and Planet Gear Setup over a Traditional Setup?

  1. Nov 12, 2011 #1
    Sorry about the less-than articulate title, but I was defeated by the thread-title character limit. The title of this three was to have been: "What's the Advantage of a Sun-and Planet Gear Setup over an Eccentric Rod and Driver Wheel Setup?"

    The eccentric rod-driver wheel setup seems to minimize the mechanical backlash inherent in the sun-and-panet setup, while maximizing torque with the more eccentric placement of the joint, which, in turn, allows for the efficient functioning of the counterweight opposite the joint.
  2. jcsd
  3. Nov 12, 2011 #2


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    advantage - concentric, compact
  4. Nov 12, 2011 #3
    Got it. But how is concentricity an advantage when an eccentric rod/driving wheel converts reciprocal hertz into rotational torque, when I want torsional power?
  5. Nov 13, 2011 #4
    It's a bit of an odd question, and I don't think I understand what you are getting at.
    Could you clarify?

    The way the question is worded at the moment, one is a gearbox the other is a crank slider. So comparing them just seems odd.
  6. Nov 13, 2011 #5

    Here's a sun and planet gear:


    And here's a traditional connecting rod:

    Last edited by a moderator: Nov 13, 2011
  7. Nov 13, 2011 #6


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    I can barely make it out. Can you send a larger picture?
  8. Nov 13, 2011 #7


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    Staff: Mentor

  9. Nov 14, 2011 #8
    For one thing, a crank & rod design is typically used for converting linear motion to rotary motion. The only rotary-to-rotary configuration that comes to my mind is in the case of old locomotives, where a link connects successive wheels, and that is at 1:1. It also has an efficiency problem -- power is transferred over limited degrees of rotation, and at a constantly varying ratio.

    The crank & rod design also relies on momentum to function. From a stopped position, if the rod as at top-dead-center or bottom-dead-center, no motion can occur until the crank is rotated by an external force.
  10. Nov 14, 2011 #9
    Add I see, so you are talking about the driven wheel.

    They seem to have totally diffent applications, the sun and planet gear has several ratios depending if you lock off the sun, carrier or everything. The input and output shafts are on the same axis.

    I'm not seeing the reason for the comparison.
  11. Nov 14, 2011 #10
    Thanks. I don't know how to resize a photo. Next time that happens I'll just link to the image, rather than inserting it.
  12. Nov 16, 2011 #11
    With the "traditional connecting rod", you get one rotation of the driven wheel per full cycle, but with the sun-and-planet gears pictured, you get 2 rotations per push-pull cycle.

    This point is easy to overlook because a pivot (as in the traditional rod) appears to be doing substantially the same thing as the planet gear -- but it isn't. The gear-gear interaction includes a single active point of contact which moves, while the pivot bears symmetrically over its entire circumference. The point of contact completes one revolution per cycle, which is added to the one revolution directly introduced by the (pivoted) connecting rod.

    The animation shows this better than I can possibly explain. I only attempted words because the moving point of contact might elude some people (it eluded me at first)

    I'd also *guess* that the outer race (in which the planet gear rides) may be easier to manufacture with greater strength and wear tolerance than a corresponding pivot. A pivot can become unusably loose with relatively little wear, while the race (and the depth of the gear teeth) would seem to provide a much larger tolerable margin.
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