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DIAGNOSTICS OF THE RESIDUAL SERVICE LIFE OF STEEL FRAMES OF LARGE-SCALE EXCAVATORS

Daniela Marasová

Abstract

The diagnostics of residual service life of key industrial products is essential in real technical operations. The diagnostics facilitates reliable monitoring of real-time state of the equipment and thus ensures its safety. Steel structures in heavy machinery are used for brown coal mining and require high purchase investments. That is why the diagnostics of these structures is so important. Each equipment failure represents a considerably high loss of mined volumes and thus also the financial loss. It is therefore extremely necessary to determine certain rules to be applied for the purpose of avoiding such failures and thus reduce the loss to a minimum. This applies especially to the present times when mining activities are in depression and, accordingly, the amounts of investments significantly decrease. The term ?residual service life? means a period of exploitation of a particular machine within which is it still cost-effective to invest in repair or replacement of damaged parts while considering also additional costs of machinery decommissioning. With regard to the residual service life, it is necessary to bear in mind that such service life may be determined for a predefined stress to which a structure is exposed. An overload caused by a failure to adhere to a prescribed technological procedure imposes a risk of service life miscalculation which may result in undesirable accidents prior to a scheduled inspection. The identification of critical locations in a heavy machine is based on the consequences of failures, considering the following three factors: endangering human life, repair duration, and loss caused by a machine idle time. The present article analyses the problems associated with the operation of heavy machinery. It deals with basic preconditions for the diagnostics of steel structures. It points out the need for the basic knowledge required for the diagnostics of steel frames in heavy machinery, in particular the stress magnitude, stress time development, baseline mechanical properties of materials, determination of intervals for periodic inspections, as well as critical length of a crack. The article also discusses the issues regarding a combination of material fatigue and corrosion and its impact on the service life of steel structures. It describes the possibilities of extending the service life of steel structures and continuous monitoring of growing damage to steel structures while applying: - tensometric monitoring of crack formation and propagation at critical locations; - indications of growing plastic deformation or cracks based on the elastic stress wave emission (AET); and - indications of damage progression using ultrasonic defectoscopy. The final part of the article presents the basic criteria for the service life of steel structures and the measures facilitating significant extension of service life of steel structures in heavy machinery without imposing a risk of unexpected accidents of a particular machine.

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