Ball mill trunnion bearings are heavy-duty sliding bearings that support the rotating hollow shaft of a grinding mill. In many mills, the bearing surface is a Babbitt or white metal lining fitted into a split bearing shell. When this interface overheats, the consequences can be immediate: loss of the oil film, scoring, wiping of the lining, unplanned shutdown, and expensive mechanical work. Temperature control is therefore not just a monitoring task; it is a core reliability requirement for plant maintenance teams.
Why ball mill bearing pad temperature rises
The temperature of a bearing pad is influenced by two broad groups of factors: the outside environment and the heat coming from the mill itself. Ambient temperature can raise the pad temperature directly, especially in hot plants or during summer operation. The hollow shaft, trunnion, or journal that runs against the pad also carries heat from the mill interior. That heat may come from the ground material, hot gas inside the drum, friction, and the condition of the lubricating oil film.
When the lubrication state changes, the thermal picture changes as well. A healthy hydrodynamic film separates the shaft from the pad and limits friction heat. If oil viscosity is wrong, oil supply is too low, load is excessive, alignment is poor, or contamination is present, the film may thin and the pad can heat quickly. In other words, a rising temperature is often a symptom of a system problem, not only a cooling problem.
How to measure pad temperature when the sensor is local
Embedded pad sensors, such as thermocouples or RTDs, are useful, but they have a limitation. The sensor point is usually small and fixed, so it reflects a local temperature rather than the full temperature across the width of the bearing. A pad can show a normal reading at one point while another area is already under thermal stress.
For that reason, maintenance teams often combine fixed sensors with field checks. An infrared thermometer can measure the surface temperature of the hollow shaft near the bearing, while the return oil temperature from the bearing housing gives another useful reference. These readings do not replace pad sensors, but they help create a comparison set. As a field rule of thumb, the outer surface of the hollow shaft may read about 10°C to 15°C higher than the pad face, while the bearing return oil may read about 5°C lower than the pad temperature. These relationships can vary with design, speed, lubricant, and measurement location, so they should be validated on the specific mill.
Practical cooling actions when the pad gets hot
If a ball mill bearing pad begins to run hot, operators may take several immediate steps. Increasing the oil spray flow can improve heat removal and support film formation. Increasing cooling water to the bearing pad, bearing housing, or oil station can reduce the operating temperature. In some layouts, shell spray cooling helps lower the heat load coming from the drum. These actions can be effective, but they should be controlled carefully.
Cooling water must not be added so aggressively that it causes thermal shock, condensation, or contamination of the lubrication system. Sudden changes in oil flow or temperature can also disturb viscosity and load-carrying capacity. The better approach is to adjust cooling in steps, monitor trend data, and investigate why the pad is hotter than normal.
Alarm and trip temperatures should be learned, not guessed
Ball mill bearing pads are commonly made with Babbitt alloy. Many plants use an alarm point around 60°C and a shutdown point around 65°C. Some machines run with trip values near 70°C or even higher, depending on the alloy, lubrication system, speed, load, and mill design. The important point is that there is no universal safe number.
Reliability teams should build a baseline for each mill. Track normal start-up temperature, steady-state temperature, and return oil temperature. Compare that baseline with pad sensor data, shaft surface readings, and return oil readings. When the trend begins to rise, check lubrication, cooling, contamination, and mechanical condition before the temperature reaches the shutdown threshold.
Keeping the bearing system reliable
Temperature management is easier when the whole sliding bearing system is maintained properly. That includes the pad surface, oil supply, filtration, seals, housing, cooling circuits, and alignment. For replacement and repair, a reliable supply of industrial plain bearings, bushings, and compatible bearing housings helps restore the machine to its intended condition.
Sealing also matters. Oil contamination can reduce lubrication performance and increase pad temperature, so using the right oil seals and keeping the lubrication system clean is part of bearing protection. General MRO industrial supplies and accessories support this type of maintenance work, from fittings and fasteners to replacement components used during shutdowns.
In some redesign cases, mills that use plain bearing pads may be converted to or supported by different bearing arrangements. Before making that decision, however, it is important to understand load, speed, shaft deflection, lubrication, sealing, and cost. A bearing specialist should review the application before specifying roller bearings as an alternative.
Ball mill bearing temperature control is ultimately a discipline of observation and prevention. Measure the right points, compare trends, cool the system in controlled steps, and never allow a pad to burn. With the correct plain bearing components, lubrication supplies, and maintenance practices, plant teams can protect the mill, reduce downtime, and keep the bearing system operating reliably.
For help selecting replacement sliding bearing components, bearing housings, oil seals, or other MRO supplies, contact us at info@bearingshop.org.
