2014

Insurability of Large Gearless Mill Drives (GMDs)

ABB’s and Zurich Risk Services’ concept of a 24 h repair cycle in ring motors for GMDs is reviewed. The application of this concept to winding design is shown to be counterproductive.

Mill Castings – QC – Consultants

It is a terrible waste in terms of energy, cost, human effort, damage to the environment, etc., to scrap a mill casting. However sometimes, this is necessary and it does not take an expert in quality control to see why. The assessment of flaws in castings using fracture mechanics seems to be too conservative but there is no credible alternative proposed by mill vendors at present. There is a need for independent, openly published research into a less conservative approach for casting flaw assessment and this would be useful to Owners and vendors alike.

Grinding Mill Foundations

Mill system analysis is just as important, if not more important, for gear driven mills than for gearless driven mills. The aim of the two analyses is different. For one mill type, the aim is to stop the foundation from vibrating or deflecting excessively, For the other mill type, the aim is to prevent the ring motor from doing so. Some failures and issues relevant to mill system design are presented.

Cadia Girth Gear Failures

Three investigations into the Cadia gear failures are reviewed. The first and third investigations showed that the location of the failures can only be explained by the thermal expansion of the gear. The second investigation shows that pinion diameter was a primary contributor to the failure and that T-shaped gears have better load distribution than Y-shaped gears. In any case, the cause of the gear failures was a design fault.

Literature Review: Gearless Motor Failures – A Mill Designer’s Viewpoint

This post presents a review of a paper by a mill designer on the state-of-the-art of ring motor design in gearless drives. The conclusions in the paper are not justifiable. Ring motors are much more complex than grinding mills and the simple remedies offered in the paper would not have avoided most of the past failures. It is shown that external influences and design culture were the root causes of most of the ring motor problems since 1996 and the problems in design culture also exist in some mill vendor design groups. The conclusion in the reviewed paper that the ring motor failures are not diameter related is incorrect.

Shell vs Trunnion-Mounted Mills

Shell and trunnion-mounted mills are reviewed. A literature review is presented. Mill vendors’ papers are contradictory and in some cases, it is apparent that the papers or articles highlight the advantages of a particular mill design just because the author works for a company that makes a certain type of mill. Shell-mounted mills are not inherently better or worse than trunnion-mounted mills. It is all matter of proper design choices.

Literature Review – Zen and the Art of Specification Writing

A paper on how to write mill specifications is reviewed. The paper is light on detail required to determine what should and what should not be included in a specification. It also incorrectly assumes that the interests of the owner, the engineering contractor and mill vendor are aligned throughout the procurement process without understanding that only one of these three entities will suffer major losses that can result in loss of the entire company when things go wrong. Owners should invest in training their engineers in the details of mill design and manufacture so they can specify what they want and ensure they get it.

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