Polymer cover induced self-excited vibrations of nipped rolls

As a result of increased speeds, the dynamic instability of rotatory machines including polymer-covered nipped rolls has grown. The instability originates from the viscoelastic behavior of the covers and leads to strong barring vibrations, which limit the operating speed of many machines. In this work, the self-excited vibrations of a nipped two-roll system with a polymer cover on the other roll are investigated using an analytical model developed for the roll system. The viscoelastic properties of the cover are accounted for by the standard linear solid (SLS) model. The numerical results display wave-like roll cover deformation patterns, separate instability regions of the system and moving wave patterns near the resonances. The roll system is unstable when the excitation frequency of the polygonal cover deformation lies in the vicinity of the higher eigenfrequency of the system. By using a speed-up ramp, it is shown that at high speeds the instability regions may become too wide and unstable to be crossed in industrial machines. An experiment was carried out, and a good agreement is found between the numerical and experimental results.