During service introduction of an uprated turbojet engine, which has a three-bearing rotor support system with an overhung turbine, several problems related to system vibration were encountered — cabin noise and high engine vibration levels. These problems led to a factory and field investigation which showed that the source of the problem was a nonlinear interaction between rotor and casing modes coupled through bearing clearance. This interaction led to a bistable vibration of the system. This paper documents the results of this investigation and demonstrates that the use of a plain, uncentralized, squeeze film damper to support the turbine rotor solves all vibration problems by reducing the turbine critical speed and separating it from the casing mode. Also included are effects of exhaust system weight on engine vibration and cabin noise levels.

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