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Two gears are spinning is opposite directions, I would like a third gear to spin in one direction (the direction is not important), and this third gear must be the sum of the motions of the two gears. The speeds of the two gears can vary. If they were spinning at constant speed, this would be easy for me, however they are always changing.

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  • $\begingroup$ What do you mean by "sum of the motions of the two gears"? That doesn't seem to make sense when talking about gears. $\endgroup$
    – hazzey
    May 24, 2016 at 13:02
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    $\begingroup$ I think what you're looking for is a differential. en.wikipedia.org/wiki/Differential_(mechanical_device) It spins a third gear at the average speed of two input gears. With the addition of other gears, more complex operations are possible. $\endgroup$
    – Ethan48
    May 24, 2016 at 13:16
  • $\begingroup$ @hazzey - Sorry, I am not mechanically trained, I meant that the speeds of both gears are transferred onto the third gear. If the two gears were spinning at 1 radian per second, the third gear should spin at 2 radians per second, or there about; the first two gears drives the third, basicallly. $\endgroup$
    – abzullah
    May 24, 2016 at 14:29
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    $\begingroup$ @maou the output will be $\frac{in_1+in_2}{2}$ but it's easy to add a 2:1 gear to double the output again. $\endgroup$ May 24, 2016 at 16:13

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The device you are looking for is called a differential like @Ethan48 commented. There are two ways of using the differential the typical use is to split one powered shafts power into two separate shafts. This is used in cars and that is the typical place people have heard of the concept.

Given the nature of mechanical couplings it can also be used in reverse to sum the movement of two axes into one. So you can use it to connect two driving shafts into one output.

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