Case – Modal analysis 

A client in the energy sector has 4 frequency-controlled vertical pumps. During a periodic measurement the company noticed high vibrations on 1x rpm. Vibration values up to 22 mm/s may significantly decrease the life span of the components and the structure.

Bumptest on running machine: 

This could indicate a resonance problem, so a bump test was conducted. By means of ‘negative averaging’ this excitation measurement could be conducted on a running machine. It led to the following spectrum:

We notice a (first) resonance area, at 1306 cpm, right next to the number of revolutions. 

The bump test confirms the suspicion that this is a resonance problem.

Modal analysis 

To gain more insight in the resonance area, a modal analysis was conducted. The idle machine was bumped and measured on 48 points and in 3 different directions, according to the following overview:

These measurements resulted in an arithmetic model. This model shows resonance at 1300 cpm, with a motion according to the following figure or movie clip:

Structural adaptations 

The desired modulation range is between 1050 and 1500 rpm. The data from the modal analysis were used to create several possible scenarios. The best results were achieved by using a rear plate (purple in the overview) that was 6 mm thicker. This will shift the natural frequency up to +/- 1805 cpm. This adaptation will make sure that the structural resonance will cause no further hindrance. The adaptation can be effected without removing the pump/engine, thus minimizing the down time.

Conclusion 

By means of a modal analysis we have been able to demonstrate the cause of the high vibrations. Additionally, we have managed to make a proposal for a structural adaptation, only causing a minimal increase in production time. This measurement and analysis results in a maximally deployable machine over the entire modulation range, without excessive vibrations.