Looking for a kind of differential gearbox.
September 7, 2015 10:08 AM   Subscribe

Is there a gearbox design with two motors and two shafts that works like this...

There are two, variable speed reversible, motors: motor A and motor B. They attach to a gear box that turn two shafts, shaft A and shaft B.

Is there a gear mechanism such that motor A turns shaft A, but shaft B by default turns at the same speed as A, and the speed of motor B is added to, or subtracted from, the rotational speed of motor A before being applied to shaft B.

So if the motors turn at the same speed in opposite directions, then shaft A turns normally, and shaft B is stationary.

If they go in the same direction at the same speed, shaft B turns twice as fast as shaft A.

A manufactured product like this would be great, by otherwise how would that work in theory? Some kind of variation on a differential?
posted by StickyCarpet to Technology (3 answers total)
In theory what you're describing is just a differential, with the motors connected to what would normally be the "output" shafts in the more conventional use of one, and the "input" shaft used as the output. A 2:1 ratio between pinion and ring gear will give you the output shaft speed that is the sum of the inputs. You would also then need an additional output shaft that is driven directly by shaft A.

It's not what you're looking for in terms of a product you could just go and buy, but have a look at how the drivetrain in the Prius and similar Toyota hybrids works. You'll find a differential used in the way you're describing (where the engine and one of the electric motors are summed).
posted by FishBike at 10:21 AM on September 7, 2015 [2 favorites]

You can make a model that works like this out of Lego (they have a differential piece with gears). Here is a post about the topic.
posted by spacewrench at 11:30 AM on September 7, 2015

That is pretty much how the Prius Power Split Device works. It is a standard sun and planet gear set.

There are three shafts. The electric motor/generator, the gas engine, and the driveshaft to the wheels.

By varying the speed of any two inputs you can control the speed and gear ratio of the third. Typically you want at least one of the motor inputs to be reversible as shown here to handle all cases.
posted by JackFlash at 11:48 AM on September 7, 2015

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