Semidefinite modal analysis of paper finishing unit with variable stiffness roll pressing mechanism

It has been observed serious roll bending vibrations in paper finishing systems, where the web is driven through the contact zone of two rolls pressed together. The upper roll is mounted rigidly on to the machine frame while the lower roll is moved by a hydraulic loading arm. The support conditions for the lower roll are orthotropic because the horizontal bearing stiffness differs from the equivalent vertical hydraulic stiffness. Continuous discussion among the machine builders and end-users is subjected to the question whether the hydraulic spring should be stiff or soft in order to reduce the oscillating component of the line load between the rolls. It has been introduced a new variable stiffness loading circuit, whose vibration characteristics are investigated here. A dynamic model of the system consists of semidefinite eigenvector expansions developed for the machine frame and rolls. The lifting circuit is considered as stiff, medium stiff and soft joint between the frame and lower roll. Frequence response analysis is carried out to find out the dynamic amplification of line load with respect to harmonic paper thickness variation entering the nip under sliding running speed conditions.