The transfer of electricity across a continuously rotating joint is especially important in robot joints and brush-type motors. In this paper, the shortfalls of present technologies, such as electric brushes, are discussed. Flex-gears are presented as an alternative method of transferring electricity across a continuously rotating joint. A new type gear, called a pitch- folling-gear, is developed from involute gear technology for use as a flex-gear. Finally, the design of planetary flex-gear devices using pitch-rolling-gears is studied.