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1 2008

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Simulation of a rapid nip pressure strike and its effect on press felt samples

Hakala T., Harlin A.

Autex Research Journal

2008

Vol. 8, no. 3

84--91

Special technical textiles are used in papermaking to support, transfer, and dewater the paper web. These textiles, paper machine cloths, have many essential functions connected to their position on the paper or board machine. Mechanical wet pressing uses press felts, whose porosity and resiliency are important for effective dewatering. Water is squeezed out by two overlapping paper machine rolls, which form a nip. After squeezing, the porous felt should be void and return to its original thickness to ensure efficient dewatering. Friction forces are also present at the nip, since abrasive interfaces occur between the cloths and the rolls by pressure, heat, and movement. Thus in time, the harsh papermaking process wears out the press felt, deteriorating its quality. At high machine speed, defects in press felts, rolls, or other parts of the nip environment can cause runnability problems such as noisy run, that is, vibration in the pressing section, decreasing paper quality and output capacity. This study sought for a new way to simulate the ambiguous nip phenomenon on laboratory scale and to find out a way to predict this noisy run problem. A dynamic test method, the Hopkinson Split Bar, was used to define the ease with which strike energy passed through from the upper roll to the lower roll and the damping of strikes by new and worn felt samples. In our study, the elasticity of the press felt was strongly linked with the ageing time. Decreased elasticity lets a nip impulse more easily through the press felt.