Axial Alignment and Thermal Growth Effects on Turbomachinery Trains with Double-helical Gearing

Turbomachinery shafting and casings typically experience axial growth during thermal transients occurring at startup, shutdown, and changes of load. This growth does not usually present difficulties during operation. However, this paper discusses four recent turbomachinery trains that experienced radial subsynchronous vibration resulting from axial misalignment. In each example, the train included a gearbox with a double-helical gearset that presented a lateral vibration response at system torsional critical frequencies while operating at low load. Thermal growth resulted in axial misalignment that produced a “zero-backlash” condition when one helix contacted on the back side of the teeth. Data collected with an axial displacement probe verified the appearance of lateral subsynchronous vibration was associated with excessive axial misalignment of the gearset. Data are presented from these experiences, including a discussion of other vibration phenomena that could be mistaken for this behavior. Finally, design and analysis recommendations are provided for preventing such occurrences. 93 AXIAL ALIGNMENT AND THERMAL GROWTH EFFECTS ON TURBOMACHINERY TRAINS WITH DOUBLE-HELICAL GEARING