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  • Gear reduction

    Discussion in 'Calculations' started by SRM, Nov 24, 2014.

    1. SRM

      SRM New Member

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      Not really sure if this is the right forum to post this, but here goes... Recently I was talking about planetary gear reduction with a friend and how you could make a reduction to the point where you could almost fasten the output shaft stationary and yet be able to turn the input shaft. It seems to me there was a display at the Museum of Science and Industry in Chicago that had something like that. I looked high and low in the internet for pictures and info on this and came up empty handed. Any input would be appreciated.
      Steve
       
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    3. Lochnagar

      Lochnagar Well-Known Member

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      I haven't been to the museum of Science in Chicago - but I am going to guess that all you saw was nothing more than a very large reduction ratio of planetary reduction - which is not uncommon.

      Maxon Motors make both motors and gearboxes - and in the example below you will see a reduction of 1024:1 is obtained over a 5 stage planetary reduction gearbox.

      So with a 1024:1 reduction - if you turn the input shaft by 360 degrees (or 1 revolution) - the output shaft will only turn by 0.35 degrees. So in effect the output shaft looks as though it has hardly moved.
      You can get planetary reduction gearboxes with reductions of 10000:1 - which would mean for 1 full turn of the input shaft - the output shaft will only turn by 0.036 degrees.

      http://www.maxonmotor.com/medias/sys_master/8813565476894/14-245-EN.pdf

      Hope this helps.
       
    4. SRM

      SRM New Member

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      Thanks for the response, Lochnagar! I wish I could remember the stats on that display I saw... I may have to take a trip there again sometime.
       
    5. s.weinberg

      s.weinberg Well-Known Member

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      If you're curious you might want to look up something called 'differential planetary gear', especially combined with what NASA calls, 'phase tuning' (see, for example https://technology.nasa.gov/patent/GSC-TOPS-12). It's a set of arrangements that can give exceptionally high reductions in a small space.
      The most common is a fairly standard planetary arrangement, except the planets span two ring gears, with slightly different number of teeth (down to 1, with phase tuning). One is fixed, and the other is the output. Every full rotation of the planets (which can be many rotations of the sun) moves the output by the differential in teeth (i.e. down to one tooth per revolution of the planets).

      I built one once with a worm gear as the input (the worm wheel was the carrier for the planets), and a differential arrangement of the planets and ring gears, for a pretty impressive reduction in a small space.
       

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