Based on the overall vehicle force and the flange connection method, the lower connecting bolts of the electric wheel are subjected to tensile forces, while the upper bolts experience pre-tightening tension under normal operating conditions. Additionally, since the bolts are mounted in an upward direction, the tensile force gradually decreases, theoretically reaching zero at a higher axial position. Although shear stress is also present, the pre-tightening force ensures that there is no relative motion between the electric wheel and the rear axle housing during truck operation, making the effect of shear stress negligible. So, why do these coupling bolts fail? In the case of the 630E electric haul truck, the self-weight is 108.8 tonnes, and the total weight at full load is 281.1 tonnes. According to the design, each rear wheel carries approximately 93.7 tonnes.
From a static load perspective, these forces should not be enough to break the bolts. However, during actual operation, the truck experiences significant impact loads due to road conditions, speed, and loading. Using the momentum formula (mv = Ft), we can infer that the impact load can be several times or even ten times greater than the static load. The coupling bolt, due to its structure and manufacturing precision, is prone to stress concentration at the thread root. Based on damage characteristics and force analysis, the movement of the truck and road bumps generate repeated impacts. While oil and gas shock absorbers help reduce this impact, they cannot fully eliminate it. These repeated impacts lead to stress concentration in the coupling bolts. Over time, this stress concentration causes fatigue cracks to develop at the bottom of the joint bolt. Once these cracks form and propagate, the connection between the electric wheel and the vehicle becomes less reliable. Meanwhile, the point of maximum force moves upward, worsening the stress on the joint bolt and accelerating the crack propagation. When the fatigue cracks reach a critical level, especially when most of the seven bolts at the lowest level are cracked, and under strong vibrations, the coupling bolt can no longer withstand the impact, leading to a sudden failure. This results in the electric wheel separating from the truck, with the lower connecting bolt failing due to fatigue and the upper bolt experiencing tensile fracture, causing the top edge of the electric wheel to tilt outward. In summary, the main cause of the coupling bolt failure is insufficient fatigue strength combined with vibration during truck operation.
To prevent such failures, it is crucial to strengthen the management and regular inspection of high-stress bolts, as they are key components in ensuring equipment safety. Bolts should be purchased and stored strictly according to technical specifications to support safe operation of the electric wheels. Before installation, maintenance personnel must carefully inspect each connecting bolt. Any bolt showing deformation, cracks, or damaged threads must be discarded immediately to ensure the quality of the coupling. During installation, the pre-tightening torque must be set to 2007 Nm. It's also important to determine the service life of the coupling bolts. Even if the surface appears intact after some usage, internal micro-cracks or plastic deformation may have occurred due to repeated stress. These issues cannot be detected by visual inspection alone. Continued use without replacement poses serious safety risks. Therefore, setting a clear service life for the bolts is essential for ensuring the safe and reliable operation of the truck.
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