Half-Speed Bearing Whirl Excited by a Single Propagating Stall Cell in a Multi-Stage Axial-Flow Compressor

A plain journal bearing is spontaneously unstable above a critical speed and loses much of its load carrying capacity when subjected to a load rotating at about half the speed of the shaft. The almost axially symmetric flow in a compressor can break down at low flow into a rotating pattern of stall cells but only the special case of a single propagating stall cell gives rise to a mechanical couple about an axis normal to and rotating about the axis of the machine. It was observed in a multi-stage axial-flow compressor that the propagational speed of a single stall cell coincided with the dynamic weakness of the two plain bearings supporting the rotor and resulted in severe vibration at about half the shaft speed. This was interpreted as an excited half-speed bearing whirl and is believed to be a unique observation.