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, so 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 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 checked are each roll in the press and each felt for that press section. Synchronous time averaging is used to determine which roll or felt, if any, is the source of the vibration. It is also possible that resonance is causing the vibration problem.

If this 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 harmonic of any of the rolls, indicates the existence of a resonance problem. Depending upon the type of resonance problem, an operating deflection shape analysis or modal analysis is performed. The speed sweep gives detailed information about the resonance and the mode shapes (shape of the structure while vibrating at a specific frequency). There are certain types of vibration problems that are self-excited. In this type of problem, vibration occurs at a resonance, engraving a wear pattern into a component of the structure.

This wear pattern will then act as the vibration source. In a press section the component usually affected is the press fabric. The fabric takes a caliper set, which then acts as the vibration source. Alternatively, a barring pattern can wear into the roll. Reducing the resonance in the system best rectifies these problems.