Figure 4 – Mill bearing technologies In addition to load capacity, several other factors must be considered when evaluating bearing technology: • Self-alignment capability • Load sharing capability (relevant for pad bearings) • Ability to tolerate operational upsets, such as lubrication system faults • Required feed end opening size • Lubrication system requirements For example, reliability issues have been observed when sleeve bearings are used at large diameters. This is partially due to their sensitivity to misalignment and journal runout as journal diameter increases. In these applications pad bearings are preferred as they operate with greater oil film thickness and can be designed with excellent selfaligning capabilities. Self-aligning capability refers to the ability of a bearing to align with the surface of the trunnion freely and actively as mill load changes, and trunnion deformation varies. Sleeve bearings typically use an unlubricated rocker support that permits limited alignment within a single plane. By comparison, most pad bearings include a curved or spherical internal geometry that allows the bearing upper body to pivot. Self-aligning mechanisms can be passive or active. In a passive system, the spherical interface supports the full bearing load with nominal lubrication. In an active system, hydraulic pressure and active lubrication to the spherical support reduces load on the spherical interface and enables continuous alignment without maintenance intervention. In isolation a pad bearing system with self-aligning capabilities will not necessarily provide greater reliability than a sleeve bearing. To achieve reliable long-term performance self-alignment must be coupled with an effective mechanism for precise load-sharing between adjacent pads. In a two-pad self-aligning system, load sharing occurs naturally. However, when three or more pads are used, additional design provisions or maintenance adjustments are required to ensure that load is distributed evenly across all pads. As with self-alignment, load sharing may be implemented passively or actively. Passive systems are common and rely on iterative manual adjustment of bearing height during commissioning, ramp up, and routine maintenance to balance pad pressures. Active systems are designed to adjust automatically as operating conditions change, maintaining appropriate load distribution without intervention. Figure 5 shows a Metso four-pad bearing system with active self-aligning and
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