Engineering & Mining Journal

OCT 2018

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74 E&MJ • OCTOBER 2018 OPERATING STRATEGIES Cranes and other types of lifting gear are essential for constructing and maintaining mining facilities and equipment, and it's almost impossible to visit a mine without seeing at least one at work, whether it's on a service truck, moving a heavy load inside a plant, or being operated by a con- tractor somewhere on site. In fact, they're so common that their maintenance and operation occasionally becomes some- thing taken for granted, but "crane com- placency," for lack of a better term, can lead to serious problems. Two industry sources recently commented on the topic. Lifting gear failure can have dramat- ic consequences for both those directly affected and those held responsible. To ensure safe practice, it is crucial that us- ers are fully aware of issues surrounding "design working period" and maintenance responsibilities, wrote Ben Dobbs, techni- cal manager at the Lifting Equipment En- gineers Association (LEEA), a U.K.-based organization representing lifting-equip- ment industry stakeholders worldwide. Dobbs said, "The sad litany of in-ser- vice failure of cranes and other lifting gear has many causes, from criminal negli- gence downward. One recurrent, and to the uninitiated rather baffl ing, theme is the failure of equipment that appears to be op- erating well within its "safe working load" and "design life." Such incidents are seen both in ageing and in relatively new equip- ment, and suggest that those responsible for safe maintenance and operation do not always fully understand the "life cycle" of the crane and its implications for safety. It is worth noting that recent crane-re- lated fatalities have resulted in multi- million-pound fi nes, corporate failure, disbarment from being a director, and in some cases, jail sentences. Age-related failures are entirely avoidable provided everyone in the chain of responsibili- ty performs their role, and understands that unlike their Victorian forebears, the modern fi nely engineered crane does not necessarily last forever. They have to be looked after. Part of the problem may be that the simple concepts of "design life" and "safe working load" are not really valid in mod- ern practice, he continued. The whole ap- proach to the design of cranes has changed in recent times. Greater understanding, combined with the ability of computer-aid- ed engineering to enable a less conserva- tive evaluation of stress and strain calcu- lation, has enabled engineers to design close to the "limit state," not just in terms of static loads, but in terms of cyclical or fatigue loading, and in the effects of wear and tear. That has yielded great benefi ts in initial cost, ease of transport and assembly. However, this also means that cranes are designed, and classifi ed, for particular patterns of duty. Structures are designed for a lifetime measured not in calendar years but in working cycles (and mecha- nisms similarly for a life in running hours). Working cycles are related to the load spectrum — the average load handled by comparison with the nominal rated load. So, for example, a crane rated at 10 tons and intended to perform occasional maintenance tasks will be designed differ- ently, and have different vulnerabilities, from a 10-ton crane intended for contin- ual use. If the inspection, maintenance and use of the crane takes due account of what is now a quite complicated specifi ca- tion — not a "design life" but a "Design Working Period" (DWP) — all should be well, but there is little margin for error. This does not just apply to structures, there are similar considerations around motors, brakes, wire ropes and other el- ements. Various components and assem- blies may have DWPs, which are not the same as that of the crane as a whole. It is easy to see how things can go wrong, he observed. The intended usage may have been inadequately defi ned when it was bought or hired; usage may change, perhaps because production increases; it may be used for purposes it wasn't in- tended for. It becomes very important to maintain a history of usage and to relate that to the design parameters. That may not be easy, for example with a hired crane, or when a new owner takes over a site with cranes already installed. It isn't enough to depend on the periodic inspections and ex- aminations specifi ed under LOLER (Lifting Operations and Lifting Equipment Regula- tions), incipient failure, whether in struc- tural members or for example in wire rope cores, may not be readily visible. Industry Experts: Crane Safety is an Ongoing Process Advances in design technologies allow modern cranes to be designed for particular patterns of duty, according to a lifting-industry expert. That brings benefi ts in cost and other factors, but also requires adherence to specifi c working cycles and attention to the machine's load spectrum. (Photo: Liebherr)

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