One of the problems in metal rolling products is forced vibrations in mill stands. These vibrations can degrade the quality of the strip surface and can trigger self-excited vibration (chatter). These vibrations can be due to different components of equipment. Some of these defects are eccentricity and ovality of rollers, misalignment of coupling, wear in gears and defects in bearing. Rolling equipment of mill stands produces sounds which will be increased when chatter happens. It is supposed that the sounds are due to separation of the roller from strip or between the rollers themselves. To analyze the vibrations, a dynamic model of rolling stand has been developed in SIMULINK space of MATLAB software. Rolling mills vibrations are produced as a result of interaction between the structure and the process, therefore, this model consists of two parts: the model of process, and the model of structure. The model of structure is considered to possess two degrees of freedom which are vertical. The model of the process is also considered to be dynamic. Possible defects are considered as external forces. Considering the fact that each defect produces a force with the specified frequency the defects of the system can be identified using frequency analysis of forced vibrations of the system. For model verification data from two stand reversing cold mill of Foolad Mobarake Steel Company (MSC) have been used. This unit consist of two four-high (F-H) rolling mills. Thickness of the strip is reduced in three reversing passes. The direction of rolling in second pass is reverse. All the condition of rolling are measured using tens sensors and saved as digital and analogue signals. Comparing FFT of the data with simulation result, the defects of the equipment can be identified. After investigation into data of two-stand cold rolling unit, it has been understand that the usual defect in this unit is eccentricity of the back-up rollers. In addition, wear in one side of the tooth-gear in the gear box of the first stand has been observed. To investigate the separation, this phenomena has been modeled with kinematical model. Due to the fact that the effects of the inertial forces and spring-back not considered, a more complete model has been proposed using FE analysis. Considering the fact that the amplitude of vibrations reaches its maximum amount during chatter, the possibility of separation has been investigated in chatter. Experiments have been done to observe the vibrations of rollers. Considering simulation and experiment results, it has been proved that there is no separation between strip and work roll also between rolls themselves when close to threshold speed of the chatter. Keywords: Rolling mills, Forced vibration, Separation, Frequency analysis
