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. o 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
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