Vibration Audit - Paper Machine Speed Increase

Paper mills depend upon a well running, low maintenance paper machine for high efficiency, low cost operation. When a paper machine’s speed is increased the efficiency must not drop. The new speed must not excite a resonance resulting in high vibration levels with increased sheet breaks and poor bearing life.

Excessive vibration is always the result of dynamic forces, which are usually related to the rotation of a roll exciting the structure to move. The dynamic forces often result from an unbalanced or mis-aligned roll. The amount of vibration is a combination of the dynamic force of the rotating elements and the dynamic stiffness of the structure. When the dynamic stiffness is low the structure is in resonance, resulting in a much higher vibration.

Excessive vibration at current operating speeds is analyzed using an Operating Deflection Shape (ODS) analysis. With this analysis, the vibration of the structure is measured during operation. The amplitude ratios and phase difference of the vibration at each location are compared on a stick figure of the structure, showing an animation of its vibrational shape during operation. This information is then used to determine possible solutions to the problem.

Potential resonance problems with a machine speed increase are assessed using two main procedures. The first is a speed sweep test, and the second utilizes modal analysis.

In the speed sweep, the machine speed is measured with an optical tachometer. The vibration is also measured at a variety of locations. The results are processed into a number of spectra, each at a different operating speed.

These spectra show the vibration severity as a function of frequency and machine speed. In a modal test the structure is forced to vibrate, either by an instrumented impact hammer or a shaker. The vibration is measured at a variety of locations. The results are displayed as Frequency Response Functions (FRFs) or as mode shapes.

When the vibration characteristics are known, plans can be made to modify the structure so that no resonances are present when the speed is increased. This will ensure that the machine speed increase will go smoothly, and the expected production increase will be present.