active load-sharing capabilities. In this design load-sharing is achieved via master-slave pairs. Figure 5 – Metso’s four pad bearing system Taken together, active self-aligning and load-sharing capabilities produces a bearing system that is highly insensitive to trunnion deflection and variations in operating load. This significantly reduces the likelihood of bearing failures that are typically associated with large sleeve bearing arrangements. Another cause of significant unplanned downtime is trunnion damage associated with bearing failure. During periods of normal operation, a mill with pad or sleeve bearings floats on a hydrostatic or hydrodynamic oil film, such that there is no bearing to journal contact. Although not regularly anticipated, bearing or lubrication system faults, commissioning incidents and control system errors can lead to contact between bearings and journal surfaces. In these instances, bearing material selection will drastically impact the mill’s ability to run through the event without damage. A variety of bearing materials are available for modern multi-pad bearings. Traditional options include bronze and babbitt bearing surfaces, whereas recent developments employ engineered polymers to provide a more resilient solution. When a metal bearing surface contacts the mill journal, even for a few seconds, the bearing is typically damaged beyond repair. More critically, this contact often results in damage to the mill trunnion itself. In some cases, the trunnion can be restored through linishing or similar processes, but in more severe cases replacement is the only viable option. Either scenario results in extended downtime that may last from several weeks to several months. By contrast, a polymer bearing is relatively insensitive to brief periods of unintended journal contact. The mechanical durability and inherent self-lubricating properties of Polymer Hydrostatic Shoe Bearings (HSBs) allows the pad to withstand periodic surface-to-journal contact associated with a range of upset conditions without causing notable damage to the bearing or journal surfaces (Bordi et al., 2023) o Drive Train Figure 6With advances in structural design methods and sophisticated bearing systems, mill drivetrain reliability becomes an increasingly significant contributor to unplanned downtime. Typical mill drive arrangements, along with their recommended
RkJQdWJsaXNoZXIy MTM0Mzk2