Cooling fan with balance structure

A cooling fan, comprising:

• a fan housing having a central tube extending upwardly therefrom, the central tube having an annular protrusion extending inwardly from a bottom end thereof;
• a bearing received in the central tube and mounted on the protrusion of the central tube, the bearing defining a plurality of channels in an outer circumferential surface thereof;
• a stator mounted around the central tube;
• a rotor comprising a shaft having a free end extending through the bearing, the free end defining a notch therein;
• a locking washer arranged between the protrusion of the central tube and the bearing, the locking washer engaging into the notch of the shaft to limit movement of the shaft along an axial direction thereof;
• and
• a balance structure being arranged between the locking washer and the bearing, the balance structure having a portion facing the notch of the shaft, the balance structure exerting a downwardly attractive, magnetic force on the shaft, wherein the balance structure is column shaped, a cross section of the balance structure is polygonal-shaped, and a gap is defined between each side of the balance structure and an inner circumferential surface of the central tube, the gap communicating with the channels of the bearing.