793 • 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 shellsupported 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. Fatigue issues may arise from insufficient fatigue strength in the original design, material washout, poor installation practices, or inadequate maintenance. The mill body undergoes one full stress cycle with every revolution. With typical operating speeds of around 10 to 12 rpm, and with mills expected to run continuously for the life of the mine, the rotating structure rapidly enters the realm of high-cycle fatigue. To ensure long-term durability, the structural components and their connections must be designed carefully for fatigue resistance. If the design is inadequate or if manufacturing quality is poor, the likelihood of bolt failures or cracking in the mill body increases significantly. To that end, mill longevity can be maximized by utilizing a combination of applied learnings from decades of grinding mill design and operation, along with intelligent adoption of modern design tools. This includes leveraging advances in computing that facilitated significant development in engineering best practice. These developments allow for more-accurate drafting, improved collaboration globally, more
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