[asid61]

Originally Posted by GeeTwo

On this, I was thinking something a bit different. That is, there would be two concentric shafts, counterrotating at the same speed. The outer shaft would end above the wheel axle, with a bevel gear with downward-facing teeth. The inner shaft would end the same distance below the wheel axle, with the teeth facing up. The output bevel gear would engage both bevel gears on the shaft.
Actually, to make this a proper caster and to allow for two wheels, one on each side of the drive shaft, the gears would not be true bevels, but would allow an offset angle so that the wheel axle is behind that shaft.
If you have the capability to make or buy such gears, it might work.

Interesting. You could use two hypoid bevel gears for that, at huge expense though.