Experimental study of coupled shaft bending - blade umbrella mode vibration of a turbine rotor model

Abstract

This paper describes an experimental study on the characteristics of blade-shaft coupled bending vibration of a rotor system with a bladed disk. In a rotor with relatively long blades such as low-pressure steam turbines, the coupled vibration of shaft torsional and blade bending vibration with nodal diameter of zero (umbrella mode) must be considered in terms of reliability. The bending-torsion coupled resonance of rotor systems occurs under specific conditions, when the rotational speed is equal to the sum and/or difference of the bending natural frequency and torsional natural frequency. In this study, a test apparatus with a flexible rotor equipped with a bladed disk which simulates actual turbine blade structure was developed to study the coupled vibration characteristics of shaft bending and blade (κ=0). The rotor was excited with an active magnetic bearing for lateral direction and a servomotor for torsional direction. Resonance of a bladed disk with nodal diameter (κ) of zero, which was coupled with the rotor’s torsional vibration, occurred under the above specific condition with lateral excitation force where frequency was equal to the rotor’s bending natural frequency. In addition, resonance of rotor bending vibration was found when the rotor was excited in torsional direction of its natural frequency.

Keywords

• rotating machinery
• vibration of rotating body
• flexible rotor
• torsional vibration
• coupled vibration