Vibration monitoring and analysis in tri-nip press, case study

Tending side press vibration…

A paper machine experienced vibration in its tri-nip press on the tending side only. Sensodec (now called Valmet condition monitoring and vibration analysis) synchronous time averaging (STA) determined that the vibration was 12 times the rotational frequency of the center roll. The machine had changed felts about two weeks before the vibration became a problem.

STA circle pictures and STA signals in frequency range of 50-500 Hz and STA spectra from center roll back and front sides.

…and top felt barring

It was also noticed that the 3rd top wet felt was showing signs of barring at the same frequency. If this vibration were to run until the next felt change, there was a chance that the center roll would become permanently barred along with the felt.

From top 3rd nip's nip analysis front and back side signals and spectra synchronized to center roll, then front and back side signals and spectra synchronized to 3rd felt.

Solution – skew felt roll

Rather than changing the wet felt or center roll, the felt stretch roll was cocked. This skewed the wet felt barring enough to change the resonating affect between the center roll and wet felt and the vibration disappeared.

This example illustrates that roll vibration alone does not tell the whole story. In this case, changing the center roll would not have solved the roll vibration problem and precious downtime would be needed to change it out.

If you are interested in finding out more online condition monitoring and vibration analysis, contact your Valmet representative.