Causes and improvements for the fracture of the upper crossbeam of the loading boom

The upper crossbeam of a ZL40 loader boom of Linyi Construction Machinery Co., Ltd. broke. The original structure of the boom and the location of the fracture are shown in Figure 1. As a result, the cause of the fracture was identified, and improvements were made to the structure of the upper crossbeam and the processing technology of the boom.

1. Cause Analysis

After ruling out the possibility of the upper crossbeam breaking due to improper use or damaged connecting parts, the boom structure was analyzed. The two ends of the upper crossbeam were welded between the boom plates and rigorously fixed. Support 3 was welded to the upper crossbeam and pasted to the boom plate. During use, when the first cylinder is flipped to promote the movement of the bucket, the force on the rocker arm is transmitted to the upper crossbeam and the boom plate through the support. The upper crossbeam is subjected to shear force and bending moment force, and its magnitude and direction often change. It can be seen that the upper crossbeam itself should have sufficient bending strength. Meanwhile, we aligned the upper crossbeam break and measured the boom width dimension L at the section shown in Figure 1A-A. See Figure 2a. The actual upper dimension L1 was measured to be 1208mm, and the lower dimension L2 was 1105mm, which is quite different from the designed dimension 1210mm. This indicates that the boom section is in a "bell mouth" shape. Measurements were also taken on the undamaged boom, and it was found that the "bell mouth" phenomenon was also presented to varying degrees. Therefore, in practical use, the force on the crossbeam of the power arm becomes more complicated, which aggravates the damage of the crossbeam. The cross-section deformation of the "bell mouth" boom is simplified as shown in Figure 2a, and the force analysis of the boom lifting is conducted. At this point, the force acting on the upper crossbeam is the pressure G exerted by the heavy object on the boom plate and the lifting force F exerted by the hydraulic cylinder on the boom.

During the lifting process of the bell-shaped boom, due to the non-overlap of the pressure G and the lifting force F, a bending moment is generated within the upper crossbeam, causing internal stress. The upper part of the crossbeam is subjected to tensile stress, while the lower part is subjected to compressive stress, as shown in Figure 2b. The maximum stress is located at the upper edge of the upper crossbeam.

When the internal stress exceeds the allowable bending stress of the crossbeam, fracture will occur. Moreover, the internal stress is a constantly changing dynamic load as the boom rises and falls. During use, it will also cause the bell mouth shape of the boom to gradually intensify, resulting in fatigue cracks at the weak parts of the crossbeam, which will rapidly expand and lead to fracture. In addition, as the weld seam on the crossbeam is over 1 meter long, defects such as incomplete penetration, incomplete fusion, and slag inclusion that may occur during manual welding are all causes of weld cracking.

2. Structural process improvement

During use, the upper crossbeam of the boom is subjected to complex forces and has relatively large internal stress. Therefore, it is necessary to ensure that its structure has sufficient bending strength. Due to the fact that the cross-sectional area of the crossbeam is relatively small compared to the side of the boom, and its position is slightly above the boom plate, with a large welding corner height, the deformation of the upper and lower parts of the boom plate is different after welding, resulting in a "bell mouth" cross-section of the boom. Therefore, we improved the structural shape of the crossbeam to ensure the structural dimensions of L1 and L2, avoid the "bell mouth" shape, change the force condition, and keep the weld position away from the maximum stress area. At the same time, we also increased its cross-sectional area to enhance its bending strength. The cross-sections of the crossbeam before and after the improvement are shown in Figure 3 (b). Meanwhile, The position of the upper crossbeam section relative to the boom has also been adjusted.

In terms of technology, manual welding has been changed to automatic welding by robots, which has improved the quality of weld seams. Adjust the welding sequence of the boom to reduce post-welding deformation; Add post-weld inspection and correction procedures.

After the above improvements, the ZL40 loading boom has been enhanced in quality and extended in service life. It no longer experiences breakage or welding cracking during use.