The hook is an important part of the jib crane. It is subjected to complex stress conditions. In addition to bearing the weight of heavy objects, it also needs to bear the impact load generated during lifting and braking. The hook will inevitably crack. Once the hook cracks during use, it will cause serious personal and equipment damage accidents.
“Safety Regulations for Hoisting Machinery” GB6067-85 and related standards stipulate that cracks are strictly prohibited on the hook. Most enterprises and testing departments are restricted by many objective factors such as the working environment. In actual testing, rough macroscopic testing is generally adopted. The hook does not disintegrate and is only observed with the naked eye without any measures. Hidden danger of missing detection.
First of all, at present, the hooks of domestic cantilever cranes use forged hooks, especially those below 75t. The commonly used 20# high-quality carbon steel is subjected to forging, stamping, heat treatment, and machining of the hook tail. According to the characteristics and stress conditions of the hook, the hook has three main stress areas A, B, and C (as shown in the figure), and these three stress areas are also areas where the hook is prone to breakage.
At present, the conventional on-site macroscopic inspection method is adopted, and only the A and B areas can be inspected, but the C area cannot be inspected because it is covered by the hook bearing and the beam. Area C is located at the hook neck relief groove where the cross section suddenly becomes smaller. It is a section where the hook tail bearing part bears a large tensile stress but is weak in strength. According to years of inspection experience and analysis of the causes of accidents, most of the fractures occur here.
Then there are hooks with a long service life and poor use conditions. In the hidden part, there are often defects such as corrosion damage to the thread of the hook tail and circumferential wear of the tail shaft due to unavoidable installation and use. Therefore, the disintegration test of the C stress area is very necessary and must not be ignored.
Secondly, the cracks on the surface of the hook of the cantilever crane are generally very small, while the surface condition of the hook in use is generally very poor, the surface is very rough, and there are defects such as scale oil, impurities, burrs and main section pressure pits, such as lifting molten steel There are different degrees of molten steel splashes on the surface of the hook. At the job site, due to the limitations of inspection time, conditions, and equipment usage, it is impossible to remove the sundries on the surface of the hook, so it is difficult to find microscopic cracks by macroscopic inspection. If cracks can be found, it means that the cracks are very serious.
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