I've begun working on a Mass Relay model from Mass Effect and one of the things I've been stuck on is whether it is possible to get the concentric rings in the MR to spin.
Obviously you can motorize the outside ring to spin on the x-axis, but I'm unsure how you could get the inner ring to spin on the y-axis without a second motor and a slip-ring setup on the spindle holding the inner ring to the outer ring. One other thought is to mount magnets on each ring so that they will oppose each other, causing the inner ring to spin.
You can see this same 'design' with a 3-axis setup in a few other spots. Event Horizon's Gravity Drive or the machine that's built in Contact (pictured below).
One final note, the scale I'm working on is very small, meaning my MR rings are approximately 1inch at their largest dimension, so that may further limit my options, but nevertheless, I would love to hear any ideas!