A differential drive assembly contributed by Bryan Bonahoom (with credit also to Steve Hassenplug for the differential idea). This assembly is a working differential from a single NXT set parts.

Power is applied to the differential through the drive gear (36 tooth double conical gear in the middle of the assembly). As the 36 tooth gear spins, the whole assembly spins. If the same amount of resistance is applied to each axle, they turn at the same speed. If lower resistance is applied to one axle, the smaller gears allow that axle to turn slower than the other axle (one axle speeds up and the other slows down). The operation is very similar to the stock Lego differential that uses 3 small beveled gears. Mechanically, there is no advantage to using this homemade differential, except that it looks cooler and it is constructed out of parts available in every NXT kit.Note that the axles need to be well supported. An example shown here is Bryan’s CAS robot viewed from below. It is recommended to drive the differential with a 24 tooth gear that is mounted 2 studs forward and 3 studs above the differential axle. And, a 24 tooth gear should be mounted 3 studs above and 2 studs to the rear of the axle to support the drive gear triangularly. If this isn’t done, then the differential will flex too much and come apart over time.

Building instructions produced by Guy Ziv