All structures have particular frequencies at which they will vibrate with large amplitudes when subjected to relatively small dynamic loads. These frequencies are referred to as natural frequencies, and the shape which the structure takes up at each frequency is referred to as the mode shape.
For a structure which will be subjected to dynamic loads, it is important to know the natural frequencies in order that it can be confirmed that the applied loads will not excite any modes, but this is frequently overlooked.
Modal analysis involves determining the natural frequencies and corresponding mode shapes of a structure.
When a structure is not of a simple geometry, finite element analysis (FEA) can be used to perform a modal analysis and calculate the natural frequencies and mode shapes of the structure.
As the first part of a dynamic analysis, the natural frequencies can then be compared to the frequencies of any excitation loads to ensure they are sufficiently separated.
When performing this stage of a dynamic analysis on rotating equipment, it is important to consider that imperfections (e.g. misalignment of a shaft, or a bent shaft) can result in forces acting at frequencies which are a multiple of the rotational speed of the shaft.
It is common for specifications for equipment which have significant dynamic loads to state that the equipment must have no structural modes within specified frequency ranges.
Eatec Engineering Analysis has performed many modal analyses, including:
- Skid mounted electrical generator sets
- Vibrating screens for use in process plants
- Floors of buildings
In some projects, the modal analysis identifies that the design has natural frequencies which are too close to potential excitation frequencies, and hence needs to be revised. In these instances, where required, Eatec Engineering Analysis works with the client to optimise the design in order that it complies with the requirements.
Once the modal analysis has been performed, a vibration response analysis can be performed to determine the magnitudes of displacements, velocities and accelerations as well as stresses resulting from the excitation loads (see Vibration Analysis page).