Speed Sweeps - Transient Rotating Machinery Analysis

Normally on rotating equipment the dominant vibration is at the rotational speed. A very powerful method of determining the vibration characteristics of the structure is transient vibration analysis (speed sweep analysis). In this procedure the machine speed and vibration are measured during a speed run-up or run-down. This procedure maps the vibration amplitude and frequency to the machine speed. From the results the problem can be tracked to a rotating component or to a resonant condition.

The advantage of this method, as compared to modal analysis and finite element analysis, is that the dynamics of the structure is measured at operating conditions. This will include thermal effects as well a gyroscopic effect of the operating machine.

The vibration data acquired under the speed sweep is processed in a variety of ways to produce time or speed based spectral maps, order ratio maps, order tracking and order spectrums and other modes of display. For specialized systems the machine speed parameter can be replaced with time or a calculated parameter such as a roll diameter that is continuously changing. Normally the frequency or orders are determined from spectral analysis but specialize algorithms such at the  TVDFT's (time varying discrete fourier transform) are also used

Much insight into the characteristics of the structure can be gained from these measurements. In particular, the resonant characteristics will be much better known. Speeds at which operation should be avoided can also be determined.