Measuring resonances on machines

Measuring resonances on machines


An increased vibration level can be measured on an exhaust system. To solve this problem, the operator carried out an operational balancing, but this did not bring any improvement.



The investigation was caused by increased operating noises and vibrations at an exhaust system. The fan was driven by a belt (motor speed 2988 rpm, fan speed 1902 rpm). The vibration measurement showed the following result:

    Bearing fan side   Bearing drive side
Wideband characteristic (10 .. 1000 Hz)   9,2 mm/s   10,2 mm/s
Amount of 1. Order (unblance)   7,7 mm/s   0,7mm/s


The increased vibrations cannot be assigned exclusively to an existing unbalance, especially on the drive side. It was therefore necessary to check the extent to which the natural frequencies of the installation are excited and thus an unacceptable resonance case exists.

The bearing supports are mounted on a steel support structure. The support structure was excited by means of a hammer impact. A construction-related natural frequency at 47 Hz is clearly visible. This is located in the close range of the drive speed (49.8 Hz).

In addition, the motor mounting bracket bolted firmly to the fan base was examined. Here it was shown that there is a natural frequency at 37.4 Hz. This is very close to the fan speed (31.7 Hz).

The situation can be explained by certain natural frequencies: The present imbalance of the fan excites the natural frequency of the motor bracket, which leads to an excessive vibration level. It can also be recorded: The residual unbalance of the motor excites the natural frequency of the fan support structure.

This is also the reason why the balancing failed. Due to the amplitude increase caused by the natural frequencies, the vibration response of the construction (exhaust system) no longer behaves linearly. As a result, the balance weights cannot be calculated correctly and balancing does not lead to the required result.

This can be solved by changing the design of the substructure accordingly. By introducing additional cross bars, the natural frequency is shifted to such an extent that interferences between the speeds (unbalances) and natural frequencies are avoided.

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