I am trying to design a gear that has a 1:2 speed ratio (input:output). This design is very limited in space and for simplicity and space savings requires the input and output shafts to be coaxial.

This leads me to think of using a planetary gearset, but in order to achieve a 1:2 gear ratio, we must use a fixed planet carrier gear, which reverses the direction of the output, which is not desired. Using a fixed ring or sun is not possible when targeting a 1:2 ratio because the ring and sun would have to be the same size. Am I missing something, or is there no simple way to make a 1:2 gear ratio with coaxial shafts, or do I require a reverse gear attached to the output of a planetary gear with fixed planet carrier?

  • $\begingroup$ If the space is so limited, just use a motor with double the speed... $\endgroup$ – Solar Mike Apr 18 at 6:01
  • $\begingroup$ This design does not use a motor but uses existing mechanical motion for the input drive. I have shaft that is turning and not driven by a motor, and want it to drive another coaxial shaft at 2x the speed, in the same direction. $\endgroup$ – Alex Rosenberry Apr 18 at 14:12
  • $\begingroup$ Can you stand a ratio of 25:49? 20-tooth and 28 tooth gears should be findable, and a pair of 20:28 stages could be arranged with a coaxial output. $\endgroup$ – TimWescott Apr 19 at 0:37

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