800 included in most mills operating with motor powers less than 8 MW. Beyond risk mitigation, it is possible to eliminate all gearbox related downtime by removing the gearbox from the mill drivetrain and opting for a drive comprising of a low-speed motor directly coupled to the pinion. Although slightly more intensive from a capital expenditure perspective, the inherent reliability and efficiency of this technology make it increasingly attractive as gear driven mill power continues to grow. For example, Metso is currently installing a 22 MW dual pinion ball mill in Africa with low-speed motors. This will be the highest power gear drive mill on record. In the author’s experience there has also been an increase in interest in low-speed motors at lower power levels. 5. SYSTEM DESIGN System performance, defined as the combined behaviour of the foundation, mill, and mill drive, has a major influence on overall equipment availability. A system that is poorly designed or constructed may exhibit large deflections, differential settlement, excessive vibration, or even concrete failure. Any of these conditions adversely affect mill operability and compromise long-term reliability. The following discussion focuses on design considerations that can eliminate or minimise reliability issues associated with mill foundations. This area warrants particular attention, as foundation-related problems can often be mitigated relatively easily during the design phase, yet are extremely difficult and costly to address once the mill is installed. Mill foundations are typically the largest concrete structures within a processing plant. For large mills they can consist of thousands of tonnes of concrete and their design requires close collaboration among several independent parties, including the owner, mill supplier, motor supplier, foundation designer, and the plant’s geotechnical engineer. To ensure a robust foundation design, the roles, responsibilities, and scope of each party should be clearly defined before the project begins. Likewise, clear and prescriptive communication pathways should be established as early as possible to support effective coordination throughout the design process. Ensuring adequate foundation performance starts with the mill supplier. It is their responsibility to specify the foundation requirements for the grinding mill. This includes prescribing the design loading, the allowable vibration response, the allowable differential settlement and the allowable differential movement between bearing plinths. This information is typically communicated by a foundation loading drawing, as well as accompanying specifications and documentation. As far as practicable, the mill supplier should make the loading drawing concise and unambiguous, while ensuring all loads are presented. In the author’s experience, there are two aspects of foundation design that are occasionally overlooked. The first is the definition of the mill to foundation interface, and the battery limit of the associated foundation drawings prepared by the mill supplier. The second is the need for dynamic foundation analysis. The mill to foundation interface is prescribed by the mill supplier and includes the foundation bolting details, shear key geometry, baseplate requirements, and the dimensional constraints necessary to avoid clashes and ensure adequate space for operation and maintenance. Unfortunately, there have been several cases where the mill supplier’s foundation drawings have been taken as a complete foundation design and used directly for the installation arrangement (Belke & Berger, 2014). The dimensions and drawings provided by the mill supplier are intended only to communicate interface requirements and constraints; they do not constitute a full foundation design. Instead, they should be treated as an envelope within which the civil designer develops the surrounding structure, adding concrete volume and detailing as required to achieve the desired foundation performance. In addition to static strength checks, dynamic loading passing from the mill into its foundation must be considered as part of the mill foundation design. Although not all mill suppliers proactively provide dynamic foundation loads, it is a critical aspect that should not be overlooked. There are two primary sources of excitation passing from the mill into its foundation. The first is charge motion, which acts at lifter passing frequency. The second is drive dynamic loading, which can act at gear mesh frequency, pole passing frequency or converter excitation frequency depending on the drivetrain configuration.
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