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Thread: Gear Train Questions

  1. #1
    Associate Engineer
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    Apr 2015
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    Gear Train Questions

    Hi, folks.

    Not a credentialed engineer, but I invent/build stuff and have managed to teach myself enough to be pretty useful. Unfortunately I've run into the limits of my knowledge, and Google is not proving to be super helpful in providing edumacation on this one. I'm hoping someone here can help:

    I want to put a gear on the output shaft of an electric motor, connect it to a reduction gear on a layshaft, and then further reduce it through another pair of gears to a driven shaft colinear with the motor's output shaft that will drive a propeller. (Building an underwater propulsion unit.)

    To get the reduction where I want it, my default choice would be a 15T gear on the motor driving a 23T gear on the layshaft, and then a 15T gear on the layshaft driving a 23T gear on the driven shaft. (About a 60% overall reduction.)

    The motor puts out about 750W at 2500RPM.

    For size / weight / cost / environmental reasons, my preference would be to go with 20DP, 14.5PA gears made of Delrin.

    What I donít know how to calculate is the safe power transmission capability of that gear train. Am I going to strip teeth if I put that much power through them? How do I calculate the safe load for gears of different tooth parameters / materials?

    Usually I can google around and find the math for these kinds of calcs, but Iím finding this one tricky. Any guidance would be MUCH appreciated.

    -Ben

  2. #2
    Lead Engineer
    Join Date
    Aug 2013
    Location
    Houston TX USA
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    I apologize if this a route you have attempted but I would suggest directly contacting a gear manufacturer of plastic gears from delrin (a pretty common material for polmer gears) to see if they can provide you with a torque or HP limit on their products. The torque rating of a each gear can be determined by classical gear tooth design formulas but this requires knowing the exact tooth design profile, width and diameter of the gear, as well as the shaft mounting connection limit; as a result, would suggest that even an extended period of chasing the information you need from gear suppliers is well worth the effort. You may also possibly find a manufactured planetary gear assembly that can compactly meet your speed reduction requirements.

    Just an additional note, in your planned application the actual maximum torque loading on the gear train will be a function of the propeller loading; as a result, your gear requirements may not need to be sufficient to handle the full rating load of your selected drive motor.

  3. #3
    Associate Engineer
    Join Date
    Apr 2015
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    2
    Good advice. I especially appreciate the second part about prop loading vs. motor load capacity. Hadn't thought about that one.

    I was planning to machine the gears myself so I hadn't started down the "finding a mfgr's guidance" path. I've got a bunch of delrin round bar left over from another project and am always looking for an excuse to make myself what can be purchased cheaper and better from someone else! ;-)

    But I could ping those guys for data and see what they've got. Or just build it and hope it doesn't blow up.

    More than anything I was a bit surprised I couldn't figure out how to calculate this for myself. Seemed like it should be pretty straightforward, though I'm gathering now there are more variables than I'd anticipated.

  4. #4
    Lead Engineer
    Join Date
    Aug 2013
    Location
    Houston TX USA
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    In fact, as I learded the hard way years ago, on a gearbox design project, the gear design alone is not the only critical issue. The design of an adequately rigid gearbox that insures correct gear engagement and alignment under load; as well as, lubrication of the gears and sealing and lubrication of the shaft bearings are also very critical for a long lived and reliable gear drive assembly.

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