What are the failure analysis and prevention of wedge joint connection of steel wire rope?

A 25t truck crane with rated lifting capacity was used at a building site, and the auxiliary hook installed the tower crane for the construction site. When the balance arm of the tower crane was being assembled with the pull rod part of the balance arm, the balance arm of the tower crane suddenly fell out of balance, and four installers fell, resulting in a major accident of three deaths and one serious injury.

1. Inspection and investigation

The scene of the accident was basically intact. There were many steel wire rope heads scattered on the site. The auxiliary hook of the truck crane had fallen and was linked with the steel wire rope used for lifting on the balance arm and hung on the main frame of the tower crane. Collect and identify all the steel wire rope ends on site, clean up the site, and find that the metal cherry shaped joint connecting the auxiliary hook of the truck crane and the lifting steel wire rope is disassembled. It is preliminarily inferred that the broken part of the lifting chain is at the mold joint.

The truck crane driver confirmed that the main and auxiliary hook wire ropes had been replaced in an unauthorized truck crane repair shop a few days before the accident, and the mold joint wire rope had no rope clamp.

The site extract certificate shows that the steel wire rope of the auxiliary hook of the truck crane, starting from the rope head, is 3M long, and the metal wedge sleeve, locking module and other physical evidences have been lost.

2 Physical Evidence Inspection

2.1 steel wire rope

The steel wire in the area about 150mm away from the rope head is divergent, the rest is wrapped completely, and the whole steel wire rope is well lubricated; There is no horizontal trace left by the installation of rope clamps in the area of wire rope head; There are obvious hard object crushing damage grooves along the spiral direction on the surface of the wire rope head

2.2 metal wedge sleeve

There is no defect in the appearance of the metal wedge sleeve, the surface is rusted, and there are light old mechanical bruises on the external surface locally, without dimensional deformation. The macro morphology of the front and side of the metal mold sleeve is shown in Figure 3.

There are obvious traces of steel wire rope on the inner surface of the mold sleeve. The two U-shaped parts of the inner surface of the mold sleeve have obvious traces left by the steel wire rope, and the traces are linear along the U-shape. The trace morphology is shown in Figure 4.

3.1 truck crane lifting weight transfer chain the lifting chain of this accident is the target lifting weight (tower crane balance arm) tower crane balance arm shackle (4) Four hoisting wire rope slings - auxiliary hook of truck crane - wedge sleeve of auxiliary hook of truck crane + wedge + rope head of wire rope for auxiliary hook of truck crane - wire rope for auxiliary hook of truck crane - wire rope drum of auxiliary hook of truck crane - truck crane body, and the lifting weight transmission chain of truck crane is shown in Figure 6.

During the lifting process, the failure of any component of the chain will lead to the falling accident of the target lifting object. The on-site inspection confirmed that the wedge sleeve and wedge block of the auxiliary hook of the truck crane were disconnected from the connection of the steel wire rope head of the auxiliary hook, resulting in the breakage of the lifting chain and the accident.

3.2 quality analysis of steel wire rope

The specification and model of steel wire rope used on site are:

4v*39s+5fc-14.00, which meets the specification and model of steel wire rope required by the factory regulations. Divergence, surface damage and deformation of the connecting part between the steel wire rope head and the auxiliary hook mold block; The steel wire rope at the rear of the rope head has passed the appearance, structure, size inspection, spectrum test and steel wire rope flaw detection. The physical index, chemical property and wear rate of the steel wire rope are qualified, and meet the steel wire rope requirements specified in 10.5.1.1 of gb/t3811-2008 code for design of cranes].

The morphological characteristics of the steel wire rope end section show that the end of the steel wire rope is a one-time cutting trace of mechanical tools, not the tensile fracture of the steel wire rope caused by quality defects, and the macroscopic morphological characteristics of other steel wire rope sections collected on site are not consistent with the morphological characteristics of the steel wire rope, so it can be inferred that the steel wire rope of the auxiliary hook of the truck crane has not broken due to its own quality problems.

3.3 wedge sleeve connection quality

The working principle of the cherry shaped joint is self tensioning design. When the wedge-shaped joint has good locking quality and the direction of the force on the steel wire rope remains unchanged, the steel wire rope will not slide out of the joint. The conditions to ensure locking are that the locked surface of the steel wire rope is free of defects, the steel wire rope is not divergent, and the installation position of the steel wire rope is correct: the contact surface of the wedge sleeve and the module lock is free of defects, and the installation position is correct; After the installation and assembly are firm, the stress direction of the bearing steel wire rope remains unchanged.

There is slight old mechanical damage and corrosion on the outer surface of the die sleeve, and there are normal traces of steel wire rope on the two U-shaped parts of the inner surface of the wedge sleeve. The wedge sleeve has no dimensional deformation as a whole, and the quality of the wedge sleeve meets the requirements. There are traces of steel wire pressed on the flat part of the inner surface of the wedge sleeve, showing an arc state. There are obvious grooves and hard object extrusion damage defects on the surface of the steel wire rope, and the rope hair is scattered, indicating that there are divergent steel wires pressed on the flat part during the installation of the mold sleeve + steel wire rope + wedge combination or in the process of use, which destroys the self-locking friction. After the disintegration of the wedge joint, the physical evidence of the wedge block was lost, and the installation direction of the steel wire rope and the quality state of the wedge block cannot be inferred.

3.4 discussion and prevention of wedge joint connection standard gb/t5973-2006 wedge joint for steel wire rope stipulates that the connection method of steel wire rope with mold joint is shown in Figure 7 and figure 8. The working principle of the wedge-shaped joint is self tensioning design. When the direction of the force on the steel wire rope remains unchanged, the steel wire rope will not slide out of the joint, which can fully meet the requirements of use.

However, when the stress direction of the steel wire rope changes, such as sudden unloading, the wedge block has made requirements for the anti loosening measures of the wedge according to the standard gb/t5973-2006 wedge joint for steel wire rope. Due to the accumulation of manufacturing errors and other factors, it is difficult to install the anti release pin. Similar to gb/t5976-2006 steel wire rope clamp, it is also common in domestic actual installation and use to tie the steel wire rope at the end of the mold to prevent it from falling off. See Figure 91 for the installation method. According to the author's many years of inspection experience, the long-term use of the rope clamp fastening and anti stripping method shown in Figure 9 will lead to wear, wire breakage, distortion, rope core extrusion and other defects affecting the safe hoisting at the positions of the load-bearing steel wire rope and the free end rope clamp buckle.

Refer to relevant foreign standards, American Standard asmeb30.3-

2004 constructiontowercrane (tower crane) stipulates the connection and locking mode of mold joint, as shown in Figure 10; European standard en13411-6:2004 "asymmetricwidgesocket" (asymmetric wedge connecting sleeve) also stipulates the rope segment rope clamp locking structure, as shown in Figure 114.

1) Correctly purchase the mold joint parts produced according to the requirements of gb/t5973-2006 wedge joint for steel wire rope, and do not arbitrarily change the mold sleeve and the appearance shape and size of the mold during use;

2) The specification and model of steel wire rope shall be selected in strict accordance with the requirements of gb/t5973-2006 wedge joint for steel wire rope. It is recommended to use steel core steel wire rope, and hemp core or fiber core is not recommended. After long-term use of hemp core or fiber core steel wire rope, the rope core is easy to deform, causing the module and rope to pull out of the mold sleeve;

3) When installing the wedge sleeve + steel wire rope + wedge combination, the connection requirements of gb/t5973-2006 wedge joint for steel wire rope shall be strictly observed. The installation direction of the bearing end and the free end of the steel wire rope shall not be inverted. The binding of the free end of the rope head shall meet the requirements to ensure the good installation quality of the cherry joint combination;

4) Correctly select the type and install the anti pin reliably in strict accordance with the requirements of gb/t5973-2006 wedge joint for steel wire rope;

5) It is forbidden to change the normal stress direction of the steel wire rope at the bearing end during use;

6) In order to further improve the safety and reliability of the wedge-shaped joint, the free end of the wedge-shaped joint steel wire rope can be locked according to the European and American standards. If the European and American standard mold joint is used, it must be selected and installed in strict accordance with its standard requirements.

4 Conclusion

The connection quality between the steel wire rope of the auxiliary hook of the truck crane and the wedge joint is poor. The direct cause of this accident is that the steel wire rope slips from the wedge joint under the action of alternating lifting load.

Comparing with domestic and foreign wedge-shaped joint standards, it can be seen that in order to further improve the safety and reliability of the national standard mold joint, on the basis of meeting the standard connection of China's mold joint, it can also be recommended to add the locking treatment method of the free end of the cherry shaped joint steel wire rope stipulated by Europe and the United States, so as to promote the safe connection standard of China's wedge-shaped joint to move towards a more reasonable, safer and more reliable goal.