Customized bearings
A trunnion bearing is designed by the engineers keeping in mind the dimensions and operational parameters of the ball mill. Smaller and medium-sized ball mills utilized for mineral processing are equipped with smaller trunnion bearings while the bigger ones used in cement manufacturing utilize a trunnion with a larger diameter. The calculation has to be done considering the size of the ball mill, its speed, and feed rate. If an inappropriately sized trunnion is issued on a ball mill, it may malfunction and fall apart due to the inability of the trunnion to withstand enough force. The bearings are also different depending upon whether an overflow or grate discharge mill is being used. The overflow mills have larger trunnions.
Material
The trunnions are usually made of SG iron or steel. The SG iron is ductile cast iron with high tensile strength. Such material is strong enough to design a trunnion that can withstand heavy forces generated in processes such as crushing. There are some small-sized ball mills, which use a trunnion made of grey iron which is sufficiently durable and strong for the type of operation to be performed.
Lubrication
For the bearings to operate without friction, lubrication is vital. During the operation, the oil must be continuously supplied to the bearings, else the equipment malfunction would occur. Oil is pumped from the reservoir to the bearing at low pressure through the filters, so that clean oil can be supplied. The trunnion bearing has monitors to detect if flow to the bearing is low. In turn, the bearings would alarm the system and trip the mill to avoid any damage.
On the other hand, there is also a mechanism to provide high-pressure oil to lift the trunnion at the beginning of the operation which is regulated by the pressure transmitters in the supply line. This setup is required to provide breakaway pressure to the ball mill so that the operation can be started. The amount of oil stored in the trunnion bearing is what determines the breakaway pressure. Excessive oil in the bearing will be squeezed out by the weight of the ball mill to gain enough pressure for the startup.
Temperature Regulation
The trunnion bearing also safeguards the equipment from overheating which may result in equipment failure. The control system of the ball mill is preset at a specified temperature beyond which the system will command the mill to shut down. The trunnion bearing consists of a temperature sensor. The reading from this is sent directly to the control system which upon detection of an overall high temperature would trip the mill off. The trunnion bearings are known to stabilize the temperature up to between 32°C to 52°C. However, if the temperature is not regulated, the mill would be closed. The entire assembly would be recalibrated to ensure a working temperature within acceptable limits.
Conclusion