Modal Analysis Of Roller System In Zinc Pot Based On Fluid Structure Interaction

Chatter marks on surface of galvanized steel strips and uniformity of coating thickness often occur in productive process of galvanizing lines. Meanwhile, sleeves and bushings of rolls in zinc pot wear fast. Rolls need to be replaced frequently and their life is only 7-10 days. Aiming at the above problems, roller system(three-roll and six-arm system and one-strip) and subsystems(roll and strip) in zinc pot of a continuous hot-dip galvanizing line are chosen as the study objects in this paper. Dry and wet modal analysis of systems are conducted in order to lay a theoretical foundation for proposing a reasonable control program and provide theoretical reference for the adjustment of structure design and technical parameters of the production line. The main work is organized as follows:Firstly, by combining the structure features and working principles of the three-roll and six-arm system, finite element models for the roll frame, rolls and steel strip are built. The steel strip is treated as line-spring. Local cylindrical coordinates are created at nodes on the surface of rolls' sleeve and bushing. Radial and axial degrees of freedom of adjacent nodes are coupled while the circular degree of freedom is not, so that rolls can rotate freely with respect to arms. Dry and wet modes up to 10 orders are calculated for the roller system. It is found that the modes of the roller system mostly present the local modes of sink roll frame or correcting roll frame. This is because the weak connection of the strip threading through the rolls. So, the flexibility of roll frames can be concerned individually when preliminary modelings are built for a complicated strip system.Secondly, ignoring the influence of zinc liquid, dry modes of three subsystems are calculated under two conditions, i.e., free-free(when rolls jump off the bush because of large amplitude vibration) and simply-supported(when rollers run normally). It is found that the ring modes of the sink roll appear in the both two boundary conditions, and their natural frequencies are the same, about 330 Hz. The lower order modes of the slender stabilizing roll and correcting roll which are simply supported are mainly bending vibrations. The first bending natural frequency of slender rolls is about 180 Hz. Influence of steel strip on dynamic characteristics of the rolls is mainly reflected in rigid modes of rolls.Finally, fluid-structure coupling modal analysis of three subsystems are calculated, and the influence of zinc liquid's density is analysed. It is found that the zinc liquid makes natural frequencies decrease both roller system and subsystems. However, the degrees of decrement are not equal, which is related to vibration modes. If a certain vibration mode vibrates obviously in the normal direction of interface of solid and liquid domain, the natural frequency is significantly reduced due to the inhibition of the liquid zinc. The modal orders change also. The first order natural frequency of the bending vibration in wet mode of slender rolls which are simply supported at two ends is about 120 Hz. The natural frequency of the sink roll in four-node-ring-mode is about 180 Hz, and about 350 Hz in six-node-ring-mode. The chatter marks in the production line are probably related to the above two ring modes excited by disturbances.