Performance Impacts of Practical Fabrication Tradeoffs for a Radial Flux Coaxial Magnetic Gear with Halbach Arrays and Air Cores

This paper evaluates the impacts of various means of securing the magnetically active materials in a radial flux coaxial magnetic gear with Halbach arrays and air cores. Rods through holes in the modulators and spacers between adjacent modulators can be used to attach the modulators to their end caps with a minimal reduction of the design's slip torque. A bridge between adjacent modulators simplifies assembly but significantly reduces the slip torque. Retention sleeves can secure the magnets, but, due to an increase in the effective air gaps, these sleeves drastically reduce the slip torque, especially if the effective outer air gap is increased. Nonmagnetic walls between adjacent magnets facilitate positioning and holding the magnets but significantly reduce the slip torque. However, extending the magnets axially beyond the modulators provides additional space for structural support while increasing the slip torque. A prototype with a 4.67:1 gear ratio was fabricated and tested. Its active material torque density of 25.2 N·m/kg is competitive with commercially available mechanical planetary gear examples. However, when the structural mass of the prototype is considered, its torque density is 7.1 N·m/kg. It achieves 99% efficiency, which is higher than the efficiency of the example mechanical gears.