to channel air to axially opposite sides of an annular obstruction in the flow path of air being exhausted from the fan thereby further increasing the fan 39 s efficiency application_number US 75996601 A description the preferred embodiment of the fan 20 of the invention is shown in its entirety in fig1 3 the fan 20 preferably comprises a central hub 22 a plurality of blades 24 and an annular ring 26 the fan 20 of the preferred embodiment is configured to cool an electric motor 28 of the type having a rotor that rotates in only one direction and is enclosed by a cover 30 once it is assembled on the motor as shown in fig6 and 7 additionally the fan 20 is preferably formed of a polymeric resin as a single monolithic part the hub 22 of the preferred embodiment of the fan 20 is a generally cylindrical body having an outer periphal surface 32 and a center bore or opening 34 that extends through the hub 22 along its axis the opening 34 of the hub 22 preferably has opposite arcuate surfaces 36 configured to slidably engage with the rotor shaft 38 of the motor 28 as shown in fig7 an opposite pair of flats 40 separate the arcuate surfaces 36 of the opening 34 and are configured to engage with a pair of opposite notches 42 formed in the end of the rotor shaft 38 of.