Abstract
Opencast mining machinery represents a group of large-scale individually manufactured technical objects operated with long-life requests. Since their manufacturers are obliged to provide product that will reach declared time of life, fatigue strength and durability conditions have to be taken into account for superstructures to meet the requirements. The paper highlights main problems occurring while assessing fatigue lifetime during design. Firstly, the short survey of current state of the art regarding the approach to this problem is presented. Secondly, the most important reasons of unsatisfactory accuracy of the assessments are discussed. As a main objective of the study, the authors introduce the unique method of continuous fatigue lifetime correction for the welded superstructures during the machine lifecycle, as a remedy for this group of machinery. Furthermore, results and experience from adapting the approach in real object are presented, including fatigue lifetime correction due to the real intensity of loading acquired from a bucket-wheel excavator during its longlasting operation. It is expected that proposed procedure can help to improve credibility of fatigue lifetime assessment of heavy earthmoving machinery.
Highlights
Opencast mining machines that are commonly used for earthmoving work such as surface mining, transportation and dumping of material, are one of the largest and heaviest machines ever produced and their operational efficiency is beyond any comparison – these affects their design and the way of operating
In order to reduce the problem of analyzing fatigue lifetime of structural components of heavy construction equipment, it seems to be reasonable to correct the pre-designed lifetime at the stage of machinery operating
As well as other large-scale machines manufactured one-off or semi one-off, with similar life cycles and working conditions, the fatigue degradation of welded structural components is crucial for a machine to reach the designed time of life and to ensure safe and economical operating
Summary
Opencast mining machines (wheel-bucket excavators, spreaders, etc. – Fig. 1) that are commonly used for earthmoving work such as surface mining, transportation and dumping of material, are one of the largest and heaviest machines ever produced and their operational efficiency is beyond any comparison – these affects their design and the way of operating. Further complication of the problem is caused by gradual degradation of machines components (increasing clearances of knuckle joints, which tends to alternate the structure’s response to external impulses) Such a missed load spectrum, substantially increases the differences between designed and real fatigue lifetime consumption ratio (the time necessary to obtain a critical damage). They are, in the considered case, harmonized with Machinery Directive [13], which requires designer to evaluate fatigue lifetime In this case, the only possible solution is to predict FL, according to the manufacturer’s experience referring to operating similar objects of previous generations and bearing in mind economical factors, which usually are just expectations. In order to avoid misunderstanding, fatigue design completed on the design stage of life (as it is commonly understood), is mainly called the original design (or pre-design) in the latter part of the paper, and the one being performed periodically during service life – the corrected fatigue design
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