of a single mill often results in a shutdown of the entire plant. Consequently, any failure can have an immediate and severe impact on plant performance and profitability. This paper examines several common mill failure modes, with emphasis on those that lead to significant downtime or long-term reductions in equipment operability. The topics covered include: - Mill rotating structure: Specification, design, manufacturing, and operational considerations required to prevent fatigue related failures. - Main mill bearings: Selection of appropriate bearing technology to ensure reliable longterm performance. - Drive train: Specification and selection of the most suitable drive technology for the duty. - System design: Consideration of the combined behavior of the foundation, mill, and drive system to achieve optimal performance. For each of the topics outlined above, strategies are presented to prevent or minimise the likelihood of failure. These include the application of modern design tools, appropriate technology selection, robust manufacturing and installation practices, and effective maintenance approaches. 2. Rotating Structure As illustrated in Figure 1 below, a grinding mill may be either shell supported or trunnion supported. The more common trunnion-supported arrangement consists of trunnions, heads, and shell segments. A shell-supported mill uses the shell itself as the journal surface and incorporates vertical end plates instead of trunnions and heads. In both configurations, the mill can be driven by either an open gear or a gearless motor. The rotating structure is composed of heavy steel fabrications and castings, with cast components typically more prevalent in trunnion-supported designs. These components are joined by welded or bolted connections. Figure 1 - Trunnion supported mill (top) and shell supported mill (bottom) Failure of any component within the rotating structure is catastrophic, as spare parts are often not held and repairs or replacements require extended shutdowns. The most common cause of damage or failure in the rotating structure is fatigue cracking.
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