Frequency Modulation of High-Speed Mill Chatter

Mill chatter is a common phenomenon in the metal strip rolling process. Product defects caused by mill vibration were reported worldwide during last two decades, which is usually classified as torque vibration of the driving system with low frequencies and vertical vibration of the mill stand with comparative higher frequencies. The frequency range of the vertical vibration is wide (in general from more than 100 Hz to more than 1 000 Hz), and the vibration phenomena are very complex, even it is very difficult to determine the vibration causes, which is followed the name "ghost vibration". During a fieldwork to measure the vibration in high-speed rolling mills, a frequency modulation phenomenon was identified, which had not been reported in strip rolling community. This article makes a theoretical investigation to the newly encountered phenomenon in strip rolling process. According to an analysis of the mill roller bearings, the periodic change of stiffness of roller bearings at the fixed load orientation, together with the periodic change of the effective errors of the bearings, may cause amplitude and frequency modulation,based on the theories of Fourier series and Bessel function. The comparison between test results and theoretical analysis implies that the frequency modulation is caused by the effective errors of the mill roller bearings that have 12 ripples on the internal ring of the bearings.