Two identical three-stage Integrally Geared Centrifugal Compressors, installed as Main Air Compressors in an Air Separation Unit, suffered a first-stage rotor failure just months after start-up, with both twin units later found to share the same incipient damage: metal-to-metal rubbing marks between the thrust collar and the bull gear. Axial vibration is a phenomenon rarely addressed in compressor design standards, and this case shows how gear-tooth geometry, collar design, and impeller-induced axial thrust can combine to excite an unforeseen axial natural frequency.
The root cause analysis combined imprint-periodicity calculations, fractographic and micrographic examination, finite element analysis of the thrust ring, and site vibration measurements to trace the failure back to a resonance between the running speed and the axial natural frequency of the pinion–collar–bull gear system. Loss of hydrodynamic oil-film support due to ring deformation, combined with unbalance-driven synchronous excitation coupled through the helical gear mesh, drove alternating stresses high enough to trigger fatigue cracking and eventual collar fracture.
The issue was resolved by replacing the original pinions with a redesigned configuration that eliminates the thrust collar entirely in favor of an external thrust bearing, a modification that fully prevented recurrence. The study offers practical guidance for future integrally geared compressor designs, recommending that thrust collar sizing be driven by deformation criteria and that fatigue assessments explicitly account for axial resonance risk.
