This ball caster was originally released as a replacement part for the 3pi robot, but the 1/2″ ball can also be used a replacement part for the newer 3pi+ robot, which has the housing for the ball built directly into the chassis. This ball caster can also be used as a general-purpose ball caster: when mounted through a hole in a chassis that uses our micro metal gearmotors and our 32×7mm wheels, as on the 3pi, this ball caster is just the right height to keep the chassis level.