Study on Relations Between Abnormal Vibration and Friction Characteristics of a Roller Depending on Water Content for Three Types of Fined Coal

In coal-fired power generation, it is necessary to pulverize coal, and a vertical roller mill with excellent crushing efficiency has become the mainstream. However, In the process of pulverization by compression and shear, abnormal vibrations, which are considered to be self-excited vibration, occur rarely. This will cause equipment damage and mechanical trouble. In order to ameliorate these problems, we have conducted roller experiments, considering that the powder characteristics of coal and the friction characteristics between the roller and the powder. In previous research, we conducted experiments using three types of coal with different powder characteristics. At this time, the RMS of the horizontal acceleration and the RMS of the Slip ratio, which indicates the degree of slippage of the rollers, were investigated. In both cases, as the speed of the test machine increases, the vibration acceleration has a local maximum and then decreases. The speed condition that reaches the local maximum value differs depending on the type of coal. Investigation of the relationship between the dynamic friction characteristics generated during rolling and the slip rate revealed that the slip rate and the friction coefficient could be divided into four sections. We found that the Slip-Stick phenomenon appears remarkably in the change of the coefficient of friction with respect to the slip ratio, which is considered to be caused by abnormal vibration. However, it is necessary to investigate how the vibration characteristics and slip characteristics change when water is added because the state of the slip ratio changes due to water addition. In this paper, we examined the effects of adding water to artificial coal, sub-bituminous coal A, and sub-bituminous coal B with different powder characteristics on the dynamic friction characteristics and dynamic response generated between the roller and the powder. Investigating the relationship between the RMS value of the horizontal acceleration response and the slip ratio that indicates the degree of slippage of the roller, we found that artificial coal reduced the slip ratio and increased the vibration acceleration as the test machine's running speed increased by adding water. Although the sub-bituminous coals A and B increased the slip ratio and vibration acceleration by adding water, their characteristics did not change depending on the test machine running speed. Besides, the relationship between the dynamic friction characteristics generated during rolling and the slip ratio was clarified. The variation in slip ratio and coefficient of friction of artificial coal was reduced by adding water. However, the fluctuation of sub-bituminous coal A was confirmed to increase. While the fluctuation of sub-bituminous coal B did not change much. At this time, we found that the Slip-Stick phenomenon appeared remarkably at the test machine speed where the vibration acceleration was maximum even when water was added. Also, a frequency analysis showed that when the vibration acceleration increased, the resonance frequency decreased, and the phenomenon of uniting was confirmed for all types of coal, suggesting that the abnormal vibration that occurred was self-excited vibration.