Analysis of Super-Synchronous, Sub-mesh Frequency Vibrations in a Speed Increaser Gearbox

Abstract

During full-load, full-speed string testing of a compressor train, its gearbox pinion experienced high radial vibrations at a particular frequency that was well below the gear mesh frequency, but which was excited by the pinion’s harmonics at 9× , 10× or 11× running speed. To identify the vibrations’ root cause, a coupled lateral, torsional, and axial analysis was performed on the gearbox’s two testing configurations: its no-load, uncoupled mechanical run test, and the string test. Results indicate that the vibrations likely were caused by a resonance situation involving the alignment of two highly amplified natural frequencies: the pinion’s fifth lateral mode, and a torsional-lateral mode of the pinion and its adjacent coupling. Contrary to all known published discussions of similar problems, the vibrations were not likely associated with the pinion’s heavily damped, fourth lateral mode. Modeling and analysis recommendations include aspects not typically considered in gearbox applications, such as the need for higher fidelity rotor and coupling models and the inclusion of pad/pivot radial dynamics.