Differential (or is it?) question

I need a gearset like a differential, except that I want each axle to rotate in the opposite direction. Right wheel rotate clockwise, left
wheel rotate counterclockwise. Always. The device will be mounted with the axles vertical, not horizontal, so that the counter-rotation will offset rotational torque.
Does this gearset have a name? Do I have to build my own? I have looked at helicopters, but haven't found one with two counter-rotating blades, and such a gearbox as that would be ideal.
Any help/advice greatly appreciated.
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Ramon wrote:

In a car differential each axle already rotates in opposite directions, because the wheels are opposite one another. Driving down the road one turns clockwise and the other counter-clockwise. Of course, all this depends what side of the wheel you're looking at. I'm talking end-on.
Making an axle with two parallel shafts that rotate in opposite directions takes exactly one gear on each shaft. Seems to me a very simple gearbox. How difficult it would be to build it depends on the application.
-- Gordon
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Gordon McComb wrote:

Depends on whether one wants them to be coaxial or not. If they're coaxial then you need at least three. Basically drive one of the pinion gears instead of the ring gear and you've got it.

--
--John
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