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The hook, hook nut, thrust bearing, hook beam, pulley, pulley bearing, hook pull plate and other parts form a hook set. It is the most widely used retrieval device on cranes. The head of the hook has a straight handle and is threaded. Through the thread and hook The nut is connected to the hook beam) (It is considered that the main curved section of the hook body of the straight-shank single hook is the most dangerous section and is most prone to breakage. However, during the lifting process, a 10t short hook of a certain company suddenly moved from the transition point between the handle and the thread tail, that is, the journal, as shown in Figure 1(a) The part pointed by the middle arrow broke. The diameter of the journal is Φ55mm, the major diameter of the external thread is Φ64mm, the small neck is Φ56mm, and the diameter of the handle is Φ70mm. The macroscopic appearance of the broken hook is shown in Figure 1. The hook is made of 20 steel and is forged. After processing, it is normalized and then assembled and used after machining. The designed lifting weight is 10t, and the normal use can reach 10a (years). However, the hook broke and was used for less than 2 years. The following is the author’s inspection results and analysis of the hook.

The chemical composition of the hook meets the standard requirements, the microstructure is normal, the mechanical properties are good, and the safety margin is large, indicating that there are no problems with the design, materials, production process and heat treatment of the hook. Judging from the location of the fracture, the handle journal is exactly where the cross-sectional area suddenly changes, which is where the stress is concentrated. The design, processing and strengthening process of the transition fillet of this step are extremely important. However, because the R angle at the transition point between the hook handle and the thread tail is too small, and there are obvious machining marks, the stress is seriously concentrated, which promotes the initiation of cracks here. During the operation of the hook, due to the lifting tilt angle and the back and forth swing during lifting, both sides of the hook handle bear large tensile and compressive stresses, which promotes the continuous expansion of cracks and eventually causes the hook to undergo bidirectional bending fatigue fracture.

The hook, hook nut, thrust bearing, hook beam, pulley, pulley bearing, hook pull plate and other parts form a hook set. It is the most widely used retrieval device on cranes. The head of the hook has a straight handle and is threaded. Through the thread and hook The nut is connected to the hook beam) (It is considered that the main curved section of the hook body of the straight-shank single hook is the most dangerous section and is most prone to breakage. However, during the lifting process, a 10t short hook of a certain company suddenly moved from the transition point between the handle and the thread tail, that is, the journal, as shown in Figure 1(a) The part pointed by the middle arrow broke. The diameter of the journal is Φ55mm, the major diameter of the external thread is Φ64mm, the small neck is Φ56mm, and the diameter of the handle is Φ70mm. The macroscopic appearance of the broken hook is shown in Figure 1. The hook is made of 20 steel and is forged. After processing, it is normalized and then assembled and used after machining. The designed lifting weight is 10t, and the normal use can reach 10a (years). However, the hook broke and was used for less than 2 years. The following is the author’s inspection results and analysis of the hook.

The chemical composition of the hook meets the standard requirements, the microstructure is normal, the mechanical properties are good, and the safety margin is large, indicating that there are no problems with the design, materials, production process and heat treatment of the hook. Judging from the location of the fracture, the handle journal is exactly where the cross-sectional area suddenly changes, which is where the stress is concentrated. The design, processing and strengthening process of the transition fillet of this step are extremely important. However, because the R angle at the transition point between the hook handle and the thread tail is too small, and there are obvious machining marks, the stress is seriously concentrated, which promotes the initiation of cracks here. During the operation of the hook, due to the lifting tilt angle and the back and forth swing during lifting, both sides of the hook handle bear large tensile and compressive stresses, which promotes the continuous expansion of cracks and eventually causes the hook to undergo bidirectional bending fatigue fracture.