‘Designing-in’ failures and redesign of bucket wheel excavator undercarriage

The traveling mechanism of the C-700S bucket wheel excavator is of the two-crawler type. Rigid connections between the undercarriage and the crawler beams offer a very high factor of safety against tipping but at the same time create unfavorable conditions for ground surface adaptation. Connections between the crawler beams and the undercarriage cylindrical girder were realized by means of connection girders with open cross sections (I sections). In order to eliminate the occurrence of cracks, the original undercarriage structure (variant I) has been redesigned (variant II). The key idea was to strengthen the already mentioned connection girders by building in vertical plates. In this way the cross-sections were closed and their torsional rigidity became considerably greater, which led to the adaptation of the traveling mechanism to the ground surface becoming practically impossible. Because of that alone, and within a very short period after the reconstruction, new cracks occurred on elements of the original structure and even on added structural elements. Based on the FEA results, it was concluded that cracks occurrence for variants I and II of the undercarriage structure is caused by highly pronounced stress concentration. Subtle load and stress analyses in various working regimes, presented in the paper, led to the achievement of a re-redesigned undercarriage structure (variant III) with more uniform stress distribution. Stress level in the critical zone for variant III of the undercarriage structure is considerably lower than stress levels for both variant I (≈2.1 times) and variant II (≈2.5 times). It is very important to note that the weight of the new built-in elements for variant II is six times greater than that for variant III. During ensuing exploitation in extremely heavy duty conditions, no damages for variant III of the considered structure were observed.

► Concept of leaning and badly shaped junctions were the cause of cracks appearance.
► The intuitive attempt to reinforce the girders was unsuccessful.
► The solution to the problem was found in the redesign of the junctions.
► After the presented reconstruction no damages of the structure were observed.