A rotor assembly comprising: an inner rotor and an outer rotor for rotating in a magnetic field of a motor, the inner rotor comprising a main inner rotor cavity, and a plurality of first holes being formed between the inner rotor and the outer rotor; two side boards, each side board having a main side board cavity, and being perforated by a plurality of second holes; two damping elements each damping element comprising a main damping element cavity; a plurality of rubber sleeves, each rubber sleeve having a rubber sleeve lip; and a plurality of third holes; an axle; and a plurality of positioning pins.