On the Causes of Rolling Instability and Chatter

Dynamic rolling instability known as fifth-octave chatter is studied using a nonlinear model developed to describe the responses of work rolls subjected to the exertion of sheet forces of various spectral characteristics. The model enables the dynamic instability of sheet rolling to be correlated with work roll stiffiness, rolling speed, inter-stand tension, roll-bite entry and exit thickness, and the sheet force resulted from the interactive action of the work roll with the plastic deformation of the rolled strip. It is found that roll-bit elastic-plastic deformation sheet force is nonlinearly coupled with the rollstack stiffness and also plays a dominant role in affecting rolling instability. In addition, whenever the frequency spectrum of the normal sheet forces containing spectral components that match the fundamental modes of the roll stand, resonance-like beating would occur. This highly linear physical phenomenon, which is accompanied by large vibration amplitudes in the active work rolls, is recognized as chatter. Although many different dynamic modes of instability including mode excitation and beating are induced in response to sheet forces of nonlinear and non-stationary in nature, however, the rolling system never responds chaotically.