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summary: This paper introduces the appearance characteristics and causes of the fractured part of the concrete pump truck boom. Based on the observation and analysis of the boom fracture, a reasonable repair plan is proposed to improve product quality, extend service life, use safety and reliability. keywords: Analysis and Repair of Fracture Characteristics of Concrete Pump Truck Boom The concrete pump truck is a professional vehicle that is increasingly used in current modern construction. It is used in construction to continuously and evenly pump the concrete delivered by the concrete mixer truck to the concrete pouring site to ensure the quality of concrete pouring, improve construction efficiency, and reduce labor intensity. However, once there is a problem with the pump truck, it will bring a lot of inconvenience to the construction. For example, when we participated in the repair of a 37m imported concrete pump truck purchased from Italy, we observed the characteristics of the broken arm and analyzed the cause of the damage, and implemented a repair plan based on the analysis. The specific location and appearance characteristics of the boom breakage. Based on the on-site inspection of the entire vehicle and the fracture site, it is not difficult to find that the fracture (crack) site has the following characteristics:: The lateral ears at the hinge points of the two jointed arm tails were completely broken, and most of the fracture traces showed old cracks (the jointed arm tail: The connection end between each arm and the head of the previous arm ； Head of arm joint: The connecting end of each section arm and the tail section of the next section arm. The same below) The ear plate on the other side of the tail of the second-section arm is completely twisted, and the large-bent connecting rod, small-bent connecting rod and the ear plate of the first-section arm head are partially squeezed and deformed due to the breakage of the lug at the tail of the second-section arm. Except for the partial deformation of the first-section arm head, the rest are new cracks, which are caused by the breakage of the ear plate on one side of the tail end of the second-section arm. Due to the breakage of the ear plate on one side of the root of the two-section boom, defects that are difficult to detect with the naked eye and need to be measured, such as deformation of the boom and damage to the cylinder and connecting rod, etc. Analysis of fracture formation and propagation According to the location and shape of the fracture, most of the fracture cracks are old cracks. It can be inferred that micro-cracks first form, and then diffuse to produce macro-cracks after a certain period of cycles of alternating stress. Finally, the base material suddenly breaks due to crack expansion and extension. As for the formation of cracks, there are several reasons:: (1) The weld seam and the base metal are not fused. This defect is relatively common in welding defects, especially when the plate of the pump truck is made of high-strength steel plate. It can be clearly seen from the fracture surface that there is a section of the weld seam that is not well fused. ； (2) From the perspective of foreign and domestic designs, the pump truck boom, especially the plates of the first few arms connected to the turntable, generally use high-strength steel plates. When high-strength steel plates ensure yield strength, some indicators such as plasticity, ductility, and weldability are reduced, especially in the thermal stress affected area formed during welding, which causes the strength of the weld and surrounding areas to decrease. ； (3) When the concrete pump truck is in construction, the distribution arm expands, each section of the distribution arm and the leg hydraulic cylinder are locked, and the entire distribution arm system forms a cantilever beam structure. The boom of the concrete pump truck adopts an R-shaped structure. After the two-section arm, there are three or four-section arms. The three-section arm also adopts a bending structure, and the two-section arm has to bear a certain torsional load. At the same time, when the concrete pump truck is working, the two pumping hydraulic cylinders alternately circulate to continuously press the concrete to the pouring position. Due to the alternating action of the two pumping cylinders, the concrete pump truck is subjected to a forced impact with a certain frequency. Under the impact of the pumping cylinder, the vibration of the cloth arm is amplified, so the cloth arm is subjected to alternating loads, causing fatigue damage. ； (4) The wear of the pins and copper sleeves caused the impact force at the tail of the cloth arm. During the inspection, it was found that three pins and six copper sleeves were worn, and the two pins and copper sleeves in the same group had inconsistent wear levels. ； (5) There is stress concentration at the bushings at the roots of the two-section arms. According to a qualitative analysis of the root of this arm on a previous 37m similar pump truck, there is obvious stress concentration on the lower side of the bushing. The high-strength plate used as the boom plate has good toughness and plasticity, and the initial crack tip yields locally in a small area under the action of high stress concentration area and residual force. In this case, the action of alternating loads causes tiny cracks to gradually expand ； (6) Pump trucks basically operate in an open-air environment. Due to long-term erosion by corrosive media such as rainwater and hydrogen sulfide in the air, oxidative corrosion is caused and local strength is reduced. ； (7)us * * Concrete construction machinery has heavy construction tasks and harsh working environment ； Not paying attention to equipment maintenance ； Operators are poorly trained and of low quality. like us * * It is normal for concrete pump truck rental companies to pump more than 8,000 to 10,000 cubic meters a month, and the pumping volume in a year is at least 100,000 to 130,000 cubic meters. Correspondingly, in foreign countries, there is an 8-hour day and no overtime work is allowed. The operating personnel are well trained, highly qualified, and know how to use, maintain, and maintain the equipment. A foreign pump truck only pumps 100,000 cubic meters in 3 to 5 years. Basically, one year of use in our country is equivalent to more than three years of use abroad. Under such a high construction intensity, the maintenance work of the operator and maintenance personnel is much worse. This pump truck is imported from Europe. When designing, the construction intensity and conditions in China were taken into consideration according to European and American working conditions. In this way, it is not surprising that local fatigue cracking occurred after this concrete pump truck was nearly 4 years old (the label showed it was delivered to customers in June 2002), which is equivalent to more than ten years of use abroad. ； (8) Due to the highly comprehensive equipment of pump trucks, local cracks and fatigue cracks cannot be completely avoided. Europe requires 500 hours of maintenance as a system, but our country does not do well in this regard. Sometimes even the minimum maintenance, such as oiling between the pin shaft and the copper sleeve, is not performed, let alone the overall maintenance. Sometimes a crack is not terrible. It can be repaired or strengthened with a small amount. If it is not discovered, or if measures are not taken in time, the crack will expand, which may eventually lead to structural damage or even an accident. The damage to this pump truck illustrates this problem. Boom repair method: Based on the analysis of the accident, the boom repair method is summarized. First, hang down the first, second, third and fourth sections of the boom (from the upper turntable upward). Inspect each component, focusing on damage to the first and second-section arms and whether the second-section arm cylinder is damaged. After inspection, the results are as follows: The turntable, chassis, three- and four-section arms, and each section arm cylinder are in good condition. ； The first and second-section arms are connected to the pin shaft and copper sleeve. The third and fourth-section arms are connected to the pin shaft and copper sleeve. The copper sleeve connecting the big arm and the upper turntable is damaged and needs to be measured and manufactured. ； Except for a small area on the head of the first-section arm that is squeezed and deformed by the second-section arm, the other deformations are very small and do not affect the assembly performance and use. ； One side of the ear plate at the root of the two-section arm has been completely pulled apart, and the other side is also severely distorted due to cracking of the opposite side ear plate. ； The large curved connecting rod and the large straight connecting rod connecting the first and second-section arms were also broken due to the ear plate of the second-section arm. Since this pump truck is not a domestic pump truck and does not have drawings and measurement data, the following measurements need to be made before repair.: (1) Dimensions of the pins and copper sleeves connecting the first and second arms, dimensions of the pins and copper sleeves connecting the third and fourth arms, and dimensions of the copper sleeves connecting the big arm and the upper turntable. ； (2) After trimming and restoring the large curved connecting rod and the large straight connecting rod, measure part of the dimensions. The remaining dimensions, such as the connection dimensions with the first and second joint arms and the dimensions between the hinge points, need to be calculated from the components connected to them. ； (3) Head size of one-section arm (to the cylinder support seat of two-section arm) ； (4) The retraction size, stroke and earring thickness of the two-section arm cylinder ； (5) After trimming and welding the ear plate at the root of the two-section arm, measure part of the dimensions. The remaining dimensions, such as the connection dimensions with one-section arm, need to be calculated from the one-section arm connected to it. ； (6) The limit dimensions of the first and third-section arms and the second and third-section arms are used to check whether the repaired boom can be reasonably limited. After the measurement results are obtained, the linkage mechanism needs to be analyzed to determine the shape and size. Due to the breakage of the ear plate of the second-section arm, the root of the second-section arm, the large curved connecting rod and the large straight connecting rod were seriously distorted and deformed, and the head of the first-section arm was slightly deformed. The measured dimensions were inconsistent with the original objects due to component deformation. Therefore, the measured shape and size of the large curved connecting rod, large straight connecting rod and the root of the two-section arm must be fine-tuned using diagrams to ensure that both opening and retracting can be in place without interference. After completing the above work, it is necessary to carry out process specifications. Before welding, the paint and stains should be polished off and preheated before welding. The flame is required to move back and forth, left and right during heating to heat the metal evenly and prevent local overheating. ； All welding should use high-strength welding rods, or use carbon dioxide gas shielded welding, and use high-strength welding wires. If high-strength welding rods are used, the welding rods should be dried before welding. ； All welds should be inspected in detail after welding and should be free of cracks, undercuts, pores and other welding defects. ； The above operations are generally performed by welders with experience in welding high-strength plates. Conclusion: The pump truck repaired by this repair method meets various requirements after debugging, and after more than a year of construction site operations, no abnormal conditions such as cracking, fracture, deformation, etc. have occurred. Practice has proved that this repair method is reasonable and easy to implement, which can not only ensure the performance of the boom, but also extend the service life of the boom.