1.1 Torsional Vibration

Torsional vibration involves angular oscillations of the rotors of a machine. For large rotating machinery the mechanical system often consists of several rotors that are connected by relatively slender shafts and couplings. For example, Fig. 1.1 is a photograph of the generator rotor of a large steam turbine-generator. It has a large-diameter body section and relatively flexible shaft extensions.

Figure 1.1: Generator rotor photograph. (Courtesy of General Electric.)

Each rotor in the system will oscillate following a torsional disturbance to the machine about its rotational axis, resulting in twisting in the shafts and to a lesser extent in the large-diameter rotor bodies themselves. For some machines involving geared rotor connections, for example, there may be several rotor axes of rotation. The twisting oscillations following severe torsional disturbances to a machine may be sufficient to cause fatigue damage to the machine s shafts and other components.1

In the design of rotating machinery, torsional vibration analysis is vital for ensuring reliable machine operation due to machine stimuli that range from rarely occurring, high-level transients to continuous, relatively low levels of excitation.

If shaft and rotating component failures occur on these large machines as a result of shaft torsional oscillations, the consequences can be catastrophic. In the worst case, an entire machine can be wrecked as a result of the large unbalancing forces that can arise following shaft separation and turbine blade failures, and this has actually occurred. There is also potential for loss of human life. For these...