Press Section Analysis- Low Press Vibration is Key to Long Life and Uniform Paper

The press section is crucial to give the sheet uniform moisture content, such that the dryers will not affect the sheet stiffness and shrinkage properties.

Not only does the sheet require a uniform cross direction moisture content, it also requires uniform moisture in the machine direction. Non-uniform moisture content can cause cockling on the sheet resulting in register problems in the pressroom or converting operation. Vibration in the press section is the chief reason for a machine direction moisture variation. Vibration in the press section has many sources. The normal vibration sources are each roll and felt in the press. Synchronous time averaging is used to determine which roll or felt is the source of the vibration. It is also possible that resonance is the problem.

If resonance is suspected a vibration speed sweep is performed. This shows the vibration as a function of the machine speed. A vibration increase followed by a decrease at the rotational speed or harmonic of any of the rolls, indicates the existence of a resonance problem. If required, an operating deflection shape analysis or modal analysis is performed. There are certain types of vibration problems that are self-excited. In this type of problem, vibration occurs near a resonance at an integer multiple of a rotating component, engraving a wear pattern into that component.

This wear pattern will then act as the vibration source. In a press section the most normal scenario is that fabric takes a caliper set, which then acts as the vibration source. Alternatively, a barring pattern can wear into one of the press rolls. Reducing the resonance in the system, or reducing the rate of wear are the best solutions for these problems.