Abstract
The article presents design faults related to the carrying structure of the bucket wheel of the SchRs4600 excavator working in the brown coal mine. The authors analyzed cases of damages of this type of structure occurred in the past in such machines. In order to determine the damage of the carrying structure of the bucket-wheel excavator, non-destructive examinations were carried out using visual and magnetic-particle methods. The real loads occurring during operation of the machine in the case of fatigue were also determined. These measurements were used to verify the numerical model. A strength analysis was carried out using the Finite Element Method. The cause of fatigue cracks was determined by measurements and numerical calculations.
Keywords
1 Introduction
One of the basic methods of exploiting mineral raw materials such as brown coal is the opencast method, and its systems have been described in detail in [1]. In order to conduct brown coal mining, specialized machines operating in appropriate systems are used [1].
One of these machines used in open cast mining are bucket-wheel excavators (Fig. 1) and they are a part of a group of machines called basic mining machines. These machines are working in a continuous manner and equipped with a mining head with a number of elements, which are e.g. buckets with teeth. These machines are an integral part of the basic technological system of the open cast mine, where they constitute its first and one of the most important links. The construction of various types of bucket-wheel excavators is practically the same and is based on very similar or even identical functional systems. The construction of various types of bucket-wheel excavators is practically the same and is based on very similar or even identical functional systems. Small differences are described in [2]. One of the basic functional system of a bucket-wheel excavator is a mining system consisting of a carrying structure, which is a bucket wheel and a mining boom.
Currently, in the design of carrying structures of basic mining machines, in this case bucket-wheel excavators, the available standards are used. They have been developed based on many years of experience of the manufacturers of these machines. These standards were created at the beginning of the 20th century, and some machines working in brown coal opencast mines were designed according to older standards, which were less precise. The standards have some differences depending on which parts of the world they are used in. Differences between the assumptions occurring in individual standards concerning the design of basic mining machinery are presented in paper [3], where the authors presented differences in terms of static, dynamic and fatigue loads.
Due to the age of these machines, and consequently their large repair history, which is not always correctly carried out, the size of these machines, the complexity of the technological process and the existence of high variable loads that cannot be clearly predicted, these machines are exposed to occurrence of various types of failures [4]. One of the first and at the same time one of the most key elements that is in contact with the material is the bucket wheel of the machine. It is exposed to extreme dynamic loads resulting both from the mining technique, the properties of the material being mined and the hardly-abrasive or non-abrasive materials contained therein. Due to the high variability of loads, sometimes this part of the structure is exposed to various types of failures.
Research articles that have started to appear recently are the result of these failures, and the authors present their individual approaches to determining their causes. One of such failures of the mining system, which was the drive shaft of the bucket wheel in the SRs 2000.32/5.0 + VR92 bucket-wheel excavator, was described in [5], where the authors showed that by using an additional split sleeve, the shaft was broken. Another example of failure of the bucket-wheel excavator working system is paper [6], in which the authors give incorrectly made welding technology as a cause of bucket-wheel failure. The abovementioned possibilities of degradation of bucket-wheel excavators do not only apply to the machine mining system, but also to other components, as shown in [7,8,9,10].
The problems described above were also visible on the SchRs4600 bucket-wheel excavator after about two years. In the bucket wheel, the regions of its damage were identified. A view of the bucket-wheel excavator with the marked areas of the identified defects are shown in Fig. 2.
Figure 3 shows the technical documentation of the critical area of the bucket-wheel with the location of the occurrence of cracks. Figure 4a and b show examples of damage, in the form of fatigue circumferential cracks, located in a bent bar made of the HEA240 profile in the area of intermittent fillet welds fixing the wheel cladding. They occur in a repetitive manner, in the area of joining the divisions wall with the cylindrical part of the bucket-wheel.
2 Investigation of the Problem
In order to identify the problem, non-destructive testing (NDT) [11] were carried out using visual and magnetic-particle methods near the connection of the divisions wall and sidewall of the bucket wheel. The research area included selected structural elements and welds together with the heat affected zone (20 mm on both sides). During the preliminary assessment of joints, the visual method was used in accordance with the standard [12]. On the basis of the conducted investigations, the detected geometric welding non-conformities according to the standard [13] were classified, and their permissible values were compared with the standard [14].
In order to fully illustrate the magnitude of detected defects, additional testing of joints was carried out using the wet magnetic-particle method with permanent magnetization according to the standard [15]. Figure 5 shows the location and designation of test areas on the bucket-wheel. The places in which welding incompatibilities were detected are highlighted in red. The Figure also indicates the location of a strain gauge measuring point, which was used to measure actual loads in the vicinity of the localized crack.
As a result of the conducted tests, numerous material discontinuities in the structure were found at the connecting points of the divisions wall and sidewall of the bucket-wheel from the conveyor side. The lengths of detected welding non-conformities for individual test areas, according to Fig. 5, are summarized in Table 1, in case of material discontinuity, this was marked as MD (material discontinuity).
Figures 6 and 7 show, together with the description, two representative areas with the greatest length of material discontinuity.
3 Investigation of the Cause
In order to determine the actual state of strain of the carrying structure of the bucket-wheel, in the area of cracks, strain gauge experimental tests were used. The results of the conducted research served as a validation of the calculation model of the bucket-wheel in order to determine the causes of the occurrence of fatigue cracks.
3.1 Experimental Research Using Strain Gauges
As part of the research, a measuring point consisting of three strain gauges was installed in the region marked in Fig. 5 (individual strain gauges were marked as C1, C2, C3). Figure 8a and b present the exact positioning of individual measurement locations. The strain gauges were placed from the inside of the bucket-wheel at the height of the divisions wall on the T-shaped profile (made of the HEA240 beam), which is subject to fatigue failure. The tests were carried out within one week of continuous registration during the normal operation of the excavator.
Based on the recorded data, the path of stress changes at the measurement points C1, C2 and C3 were determined. Examples of stress alternation diagrams during operation are shown in Fig. 9.
The maximum range of stress changes was registered at measurement point C1 and was:
At the symmetrical point C3 slightly lower values of stresses were recorded:
However, at point C2 (at the height of the center of the division wall), the maximum range of stress changes by a value of:
The recorded values of changes in the stress range were used to calibrate the numerical model.
3.2 Numerical Calculation
Thanks to the previously prepared geometrical and discrete model, fatigue calculations in accordance with the standard [16] were carried out, considering the results of experimental studies. These works were carried out using Finite Element Method.
The numerical model of the bucket-wheel was built as shell model, while in the area of fatigue cracks the geometric features of the T-profile and cladding were modeled in a detailed manner. In order to distribute the load on the bucket-wheel, beam elements were used to simulate buckets. This model is shown in Figs. 10 and 11.
The scheme of the load application on the bucket-wheel is shown in Fig. 12. The calculations consider the following load cases of the bucket-wheel:
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digging forces included in Fig. 12 - applied to three buckets, and for fatigue strength calculations:
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transverse force U = 560 kN,
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lateral force S = 160 kN.
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According to [16] the calculations were carried out for the following combinations of load:
H1b – fatigue strength according to the following algorithm:
The loads were applied in the places where the corners of the buckets were located. Two cases of mining were adopted:
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mining to the left – L case,
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mining to the right - R case.
The bucket-wheel is mainly made of S355J2 steel according to [17] with the following strength parameters (Table 2) (for sheets up to a thickness of t = 40 mm):
The results of numerical calculations are presented in the form of contours of the range of stresses. For the case of fatigue strength, the permissible stress range values according to [16] are:
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base material outside the area of connections (typical W2 assessment group):
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welded joints (assumed for the region of the occurrence of fillet welds in the combination of the cladding with the T-profile, in the place of cracks) - the lowest strength parameters:
The stress range contours for the above case are shown in Figs. 13 and 14. The model compliance with experimental studies was also assessed. To this end, the stress values obtained from the tests were compared with the results obtained from the numerical model in analogous points. In point C1, the maximum range of stress changes recorded during experimental studies, comparison of the results of the numerical model obtained is as follows:
The value determined from the numerical model is determined for the maximum normal mining force U and S and is slightly higher than the measured values. The comparison shows that the numerical model correctly describes the critical strength of the region and clearly identifies the causes of fatigue cracks.
Due to the occurrence of high stress values in the vicinity of the place of joining the claddings with the T-profile of the bucket-wheel and in order to thoroughly understand the impact of their use, a numerical analysis was carried out using a model without claddings. This model is shown in Fig. 15.
The same boundary conditions as in the previous simulation were used. The stress range contours for the above case are shown in Figs. 16 and 17.
4 Conclusion
In the article, authors have presented a method for determining the cause of the bucket-wheel cracks in critical jointing areas of claddings with an integral part of the bucket-wheel structure which is the HEA240 profile.
The numerical and experimental approach was presented to determine the cause of fatigue cracks in the area of joining claddings with the HEA240 profile. The use of non-destructive testing using visual and defectoscopic methods made it possible to determine the exact length of cracks. The total length of all cracks was 3662 mm. Strain gauge experimental tests allowed to determine the real effort of the bucket-wheel in the vicinity of the connection of the claddings with the HEA240 profile.
Due to the measured values of actual loads and combining of loads in accordance with the standard [16], the correctness of the numerical model was verified. After numerical calculations, for the numerical model with claddings, in the region of maximal effort (134 MPa - welded joints), the significant exceeding of the acceptable range of fatigue stress was found, which is the reason for fatigue crack of the SchRs4600 bucket-wheel excavator. In order to analyze the influence of the welded claddings, calculations of a bucket-wheel without claddings were made. In the case of removing claddings (and thus welded joints) from the sensitive region, we obtain a similar maximum stress range of 132 MPa. For this variant, there are no welded joints in the area of maximum effort, therefore the condition of fatigue strength is met because:
The difference between the model using claddings and the model without claddings is shown in Fig. 18.
It was found that the cause of occurrence of fatigue cracks in the construction of the bucket-wheel is the concentration of stress in the fillet weld of attaching claddings to the T-profile. In the case of a model without cladding around the radius of the T-profile, i.e. around the geometric notch, no welds are present, so despite the occurrence of stresses of similar magnitudes, the fatigue strength condition is retained.
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Andruszko, J., Moczko, P., Pietrusiak, D., Przybyłek, G., Rusiński, E. (2019). Analysis of the Causes of Fatigue Cracks in the Carrying Structure of the Bucket Wheel in the SchRs4600 Excavator Using Experimental-Numerical Techniques. In: Rusiński, E., Pietrusiak, D. (eds) Proceedings of the 14th International Scientific Conference: Computer Aided Engineering. CAE 2018. Lecture Notes in Mechanical Engineering. Springer, Cham. https://doi.org/10.1007/978-3-030-04975-1_3
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