Modal analysis of rotating machinery

Abstract

Modal analysis of rotating machinery and its supporting structure is discussed. This paper presents the natural frequencies, damping ratios, and mode shapes of various modes of rotating machinery systems such as fans and radar antenna foundations as examples. For a rotating machinery, it is necessary to have a dynamic correction factor for the dynamic natural frequency of a rotor (critical speed). The dynamic correction factor for ventilation fans is presented with examples. The overall natural frequency of an entire system depends on the dynamic combination of the rotor and its supporting structure. If a heavy duty rotor bearing is used, the mode for the rotor having a rigid support becomes significant, which simplifies design, and the dynamic sensitivity of the system is reduced. For a large industrial fan, an early foundation design is compared with an improved foundation design in later stage from a mode shape viewpoint. Based on suggestions developed from modal analyses, a supporting structure was improved, gear noise resulting from torsional vibration was eliminated, and a rotor resonant problem was resolved.