Abstract: An axial air movement fan having a plurality of variable pitch fan blades (2), extending radially from a hub (4) rotatable about a main axis (3), each blade (2) further being pivotable about its radial axis to vary the blade pitch. The blades (2) each are mounted on an associated bearing (5) mounted on said hub, the bearing comprising a ball bearing having a first radially inner race (25) fast the blade (2) and a second radially outer race (26) rotationally fast to the hub (4). The balls (14) in the bearing are separated by separators (15), each separator being generally cylindrical and having its cylindrical axis coaxial with the orbital axis (22) of the orbit of the balls (14) and being adapted to rotate about the said orbital axis (22), the side faces of the separator each having a part spherical recess (16) adapted to receive a ball (14).

Abstract: A method of detecting stall in an axial fan through acoustical measurements in air the air flow, includes measuring the sound emanating from the flow adjacent the fan, preparing a visual representation of the sound, comparing the visual representation with a library of fixed visual representations derived from a plurality of tests representative of the performance of the fan under a range of operating parameters, selecting the fixed visual representation most closely matching the visual representation of the said sound, and deriving a control signal from the selected fixed visual representation. The visual representations are formed by a symmetrised dot pattern (SDP) technique.