1   The problem to be solved

The vertical mill roller lubricant leakage always occurs inside the vertical mill in many cement plants, and it is the big headache for the vertical mill manufacturers. The lubricant leakage will not only pollute the environment but also increase the production cost. Many seasons lead to the lubricant leakage, the two main reasons as below:

1.1  The mill roller seals are faulty At present, the vertical mill roller mainly applied 3 pieces of skeleton lip packing, one lip packing is for dust resistance, another 2 is for oil resistance.However the working condition of vertical mill is very harsh, it is very dusty,the temperature is high and the impact and vibration is high,too. So the wear rate is high. It is difficult to replace the seals,the roller needs to be turned out and roller needs to be dismantled from the rock arm, the task is time- consuming, the workload is big and the maintenance cost is very high. 1.2  The lubricant return flow of the roller lubrication system is in poor state The bearing generates some heat when the roller works, it also works in high temperature environment, so the grease circulating lubrication is used to ensure the bearing work in normal temperature range,but in actual work practice, if the lubricant return flow is in poor state, the lubricant level in the roller will rise, eventually the lubricant will spill from the seals and air cap. The reasons cause the lubricant return flow of the roller lubrication system is in poor state as below: 1.2.1  The temperature When the mill begins operation after it shuts down or the ambient temperature is too low, in general, the lubricant whose viscosity is 320mm²/s is used for the vertical mill, the lubricant has poor fluidity and the lubricant return flow is in poor state. 1.2.2  Large pipe resistance The centralized lubrication method is widely used for the vertical mill lubricating, all the oil stations include the roller lubrication station are located in the same room, the room is not nearby the vertical mill lubrication points,so the resistance strength of lubrication pipes is high and the lubricant return flow is in poor state. 1.2.3  Air into the lubricant pipe In actual practice the lubricant supply volume isn’t equal to that of lubricant return ,in order to ensure the lubricant level isn’t too high, the return volume is always more than supply volume.When the actual lubricant level is lower than the required lowest level,the lubricant return pump will suck some air and the pump capability will drop ,thus the lubricant return flow will be in poor state. 2    Solution It is very difficult for us to significantly improve the mill roller seals and the roller lubrication system, but we can work out a solution to control the lubricant level inside the mill roller: a high temperature-resistant sensor is added and good result is achieved,see Graph 1. The lubricant level sensor is installed on the roller bearing cover, the sensor wire is extended through the roller shaft,a 3-way electromagnetic ball valve is added to the lubricant supply pipes of the lubricant station, the normally open point is connected to the lubricant supply pipe , the normally close point is connected to the lubricant return pipe.

Graph 1  A high-temperature sensor is added inside the roller

  When the lubricant level is higher than the upper limit, the sensor detects it and the electromagnetic ball valve will take action to transfer the pumped lubricant to return pipe, when the lubricant level reaches the bottom limit, the valve will reverse the action, so the lubricant level inside the roller is under control. When the vertical mill is in operation the roller turns and moves up and down, the lubricant level fluctuates, we just take the minimum value in 5 seconds period from the sensor as the actual lubricant value, therefore the real time lubricant level can be displayed. By adding the lubricant level sensor, under conditions such as the seals wear out and the lubricant return flow is in poor state, we can ensure that the lubricant will not leak from the roller , we can also monitor the real time lubricant level change state inside the roller, so the lubricant level can be detected and controlled, furthermore the control of vertical mill takes a step forwards in smart control direction.