Proper mill operation is crucial for efficient grinding and material processing. Deviations from the optimal load, whether in excess (overload) or deficiency (underload), can lead to decreased productivity, equipment damage, and inconsistent product quality. Recognizing the signs of these imbalances is key to maintaining a smooth and effective milling process.
Overload indicators: a mill’s cry for help
A mill is overloaded when the volume of material fed into it exceeds its grinding capacity [1]. This condition manifests through a variety of sensory and data-driven cues.
Auditory and visual clues: a primary indicator of an overloaded mill is a significant change in its operational sound. The distinct, sharp sounds of the grinding media (such as steel balls) impacting each other and the liner are replaced by a dull, muffled thudding noise. This is because the excessive material dampens the impacts. Visually, one of the most apparent signs is the discharge of coarse, unground material. In severe cases, a phenomenon known as a “swollen belly” can occur, where the mill shell appears distended due to the excessive internal load [2]. Another visual cue can be slurry or material spilling out from the feed and discharge chutes.
Data-driven and mechanical signs: monitoring the mill’s power consumption provides a critical data point. in an overload situation, the motor’s current or power draw will initially increase as it struggles to turn the heavier load. however, as the overload becomes severe and the grinding media’s movement is restricted, the power draw may paradoxically decrease. this is because the charge begins to slide rather than tumble, reducing the energy required to rotate the mill.
furthermore, an overloaded mill can experience increased vibrations and may even lead to structural stress, potentially causing cracks in the gears, pinions, or the mill shell itself over time.
Underload indicators: an inefficient rumble
An underloaded mill, conversely, does not have enough material to efficiently utilize the grinding media. This condition also presents a unique set of signs.
Auditory and visual clues: in contrast to the muffled sound of an overloaded mill, an underloaded mill will produce a louder, sharper, and more metallic grinding noise. this is due to the increased metal-on-metal contact between the grinding media and the mill liners, as there is insufficient material to cushion the blows. visually, the product discharged from an underloaded mill will be excessively fine. while this may seem desirable, it is an indication of inefficient energy use, as the grinding media is expending energy over-grinding the material rather than breaking down new feed.
Data-driven and mechanical signs: from a power perspective, an underloaded mill will exhibit a lower-than-normal power draw. This is a direct result of the reduced load that the motor needs to turn. Prolonged operation in an underloaded state can lead to accelerated wear of the grinding media and liners. The constant, un-cushioned impact can cause chipping, cracking, and premature failure of these components.
Vibration analysis can also be indicative of an underload condition. The increased metal-on-metal impact can generate a distinct high-frequency vibration signature. Monitoring product quality will reveal an overly fine grind size, which can negatively impact downstream processes and represents a waste of energy and grinding capacity.
In summary, paying close attention to the sounds, sights, and operational data of a grinding mill can provide early warnings of both overload and underload conditions, allowing for timely adjustments to maintain optimal and efficient operation.
Reference
[1] K. S. McClure and R. B. Gopaluni, “Overload Detection in Semi-Autogenous Grinding: A Nonlinear Process Monitoring Approach,” IFAC-PapersOnLine, vol. 48, no. 8, pp. 960–965, 2015, doi: 10.1016/j.ifacol.2015.09.094.
[2] F. Machinery, “17 Signs of Problems with Ball Mills: Quickly Remove Hidden Troubles | Fote Machinery.” Accessed: Sept. 30, 2025. [Online]. Available: https://www.ftmmachinery.com/blog/17-signs-of-problems-with-ball-mills-quickly-remove-hidden-troubles.html
