Unstable vibration of roller mills

The purpose of this study is to reveal the mechanism of unstable vibration occurring in the grinding operation of roller mills and to show the design guidelines for reducing the vibration. To study the basic cause of the unstable vibration, we first investigated the dynamic characteristics of the mill vibration in its stable state and its unstable state, using a small laboratory roller mill. This showed that the unstable vibration was due to the stick–slip motion of the rollers. Further, the modal analysis showed that the natural frequency of the torsional driving system of the mill corresponded with the unstable vibration frequency. Next, we researched the frictional and compressive characteristics of the ground materials using a simplified test apparatus. This showed that the ground material has a negative-damping property known to be the cause of the self-excited vibration. Furthermore, we proposed the analytical vibration model for the simplified test apparatus considering the negative-damping property, as well as the corresponding equations of motion. By integrating them numerically, fluctuating phenomena similar to the experimental results were obtained.