Characteristics of Low Frequency Non-Synchronous Vibrations Induced by an Epicyclic Gearbox in Gas Turbogenerator Applications

Abstract

Epicyclic gearboxes of star configuration running at partial loads were found to induce non-synchronous (not related to speed) low-frequency vibrations, besides low level sub-synchronous (speed related) which were transmitted to other parts of a turbogenerator power train. At certain loads, the amplitudes of the non-synchronous vibrations were high enough to cause potential damage to sleeve bearings used in the power train system if a generator set would run for any considerable length of time at these loads. It was also observed that a very small increase in load above a certain limit (about 18% of full load) resulted in almost total elimination of these vibrations. Analysis of test data showed the non-synchronous vibrations were due to ‘backward whirl’ motion of gearbox output shaft in its sleeve bearings. Higher damping in the bearings was considered to be one of the most effective methods to suppress backward whirl of a shaft and hence, the non-synchronous vibrations. Accordingly, a new set of gearbox output shaft sleeve bearings was designed for higher damping that would allow these types of generator sets to run at partial and full loads without any detrimental vibration.