If anyone is wondering if there are any good uses for the drilled out 393 gears, besides building differential transmissions, the answer is yes.
You can use them anytime where you need two independent coaxial mechanisms and don’t want the same axle to support both.
Here is an example from Team 91A of the adjustable hood, which rotates around the same axis, but without adding extra friction to the high speed flywheel axle. This is the mount point, where flywheel axle could freely pass through the 393 gear:
And this is the video of the robot in action:
In the following video, in addition to adjustable hood, two more 393 gear bearings are used to support a movable bracket with the flywheel motor:
This allows a single motor to power the flywheel (when it spins forward) and to act as an indexer (when it pulses backward).
Mounting motors such that they are not fixed to the chassis opens up a lot of exciting possibilities. Passing axles through the drilled out 393 gears allows to avoid extra friction that otherwise would have resulted from supporting the motor assemblies.
Another example where coaxial motor mounting could add efficiency is this clever motor sharing setup between the intake and indexer from Team 10E:
When motor runs:
Clockwise: intake runs at 1000 rpm, indexer 0 rpm
Counterclockwise: intake 333 rpm, indexer 1000 rpm
Do you see how it works?