There is a trend in the mining industry towards larger mills of all types. This trend is being driven by the need to reduce both capital and operating costs in the exploitation of large, low grade deposits, which has led to construction of high capacity plants with fewer lines and, in many cases, single lines of production. This, in turn, has led to a requirement for greater reliability and, therefore, much more rigorous design, manufacturing and quality assurance procedures. Whilst the increasing size of AG, SAG and ball mill presents certain design issues common to all these mill types, certain issues are more relevant to the design of Ball Mills since: Ball mills are generally smaller in diameter relative to their lengths (i.e. low aspect ratio) when compared with AG and SAG units; Ball mills have comparatively high charge densities when compared to AG and SAG mills; and Ball mills are rubber lined more often than AG and SAG mills. The progressive increase in the size of Ball Mills has led to an ongoing requirement for more accurate and reliable analysis of mill structure for a number of reasons: Higher levels of reliability have been called for because of the higher capital costs are involved, higher levels of availability required and reduced levels of plant redundancy; Increased sizes have put more pressure on manufacturing constraints. As a result, sizes cannot be indefinitely scaled up, leading to greater pressure to limit section sizes and thicknesses, and hence leading to higher general stress levels, which of necessity are close to design criteria; The increasing sizes have required that mill structures be manufactured from a larger number of components bolted together. The bolted joints now include radial head joints and longitudinal flange joints, as well as the circumferential head and shell joints. Those very large ball mills with geared drives typically use large outstand gears, which can have a significant effect on the stresses in the main mill structure. Consequently, these gears need to be model as part of the main mill analysis, in sufficient detail to establish the effect of the gear and gear loading on the mill structure. Separate analysis of the gear structure is usually carried out.
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