Vibrations occur in any rotating system, this must be considered in the operation of fan wheels, as they represent an additional load. The vibrations arise from various causes:
- Excitation in a natural frequency . Natural frequency is the frequency at which the wheel or the system goes into resonance and starts to oscillate. Depending on the frequency, the vibration modes vary, which can be represented, for example, by a finite element analysis (FEA). The natural frequency can shift or new frequencies are added when the wheel is installed in the system.
- Excitation by the unbalance or residual unbalance of the wheel
- Excitation by the rotating pressure field
- Drive speed and speed fluctuations from the drive
- etc.
It is necessary for every device manufacturer to make sure in the test before release that no impermissibly high resonance vibrations are present in connection with the structure or the installation/mounting location or that the values of DIN ISO 20816 are not exceeded in the process. Since these phenomena are not a property of the fan impeller alone, they are therefore not known prior to completion of the unit development. The vibrations resulting from the impeller and partly also from the structure (depending on the circumstances) can be determined, for example, with a Campbell measurement. For this purpose, the impeller and, if possible, the superstructure with motor are clamped to the test rig and operated at varying speeds up to the intended maximum operating speed. The sound or vibrations are recorded. Subsequently, a diagram with the speed plotted against the frequency can be generated. The intensity is shown by means of coloring. Resonance frequencies can then be read from this diagram. Natural frequencies can also be determined statically, e.g. from a fan wheel as such. For this purpose, the impeller is either mounted in a fixed way or decoupled as far as possible. Subsequently, the impeller is excited with an impulse hammer or a shaker (excitation at a defined force and frequency) and the reaction of the impeller is measured, e.g. by means of accelerometers. The result of the acceleration can then be plotted in diagrams versus frequency and resulting natural frequencies, which are represented in peaks, can be evaluated.