Causes, hazards and treatment measures of main bea

The causes, hazards and treatment measures of the main beam deflection of general bridge cranes

our factory is a large state-owned enterprise producing small and medium-sized steam turbine generators and large and medium-sized asynchronous motors, with 40 general bridge cranes of all kinds. Whether the bridge crane can operate normally directly affects and restricts the successful completion of production tasks. In order to ensure the safe and normal operation of hoisting machinery, our factory will organize relevant professional and technical personnel to conduct a comprehensive safety inspection of the hoisting machinery of the whole factory every year, and timely rectify the problems found, so as to eliminate potential accidents. According to the relevant national technical standards that the continuous speed regulating loading system and load measuring device constitute a closed-loop standard, the main beam of the bridge crane must have sufficient camber (Note 1). However, we found in the safety inspection that some main beams of the crane not only do not have camber, but also have downward deflection, which has become a major disaster threatening the safe operation of the crane. This paper only talks about the causes, hazards and treatment measures of the downward deflection of the main beam of the general bridge crane

I. causes of main beam deflection

from the annual safety inspection results, our factory has found 16 bridge cranes with main beam deflection, accounting for 40% of the total number of cranes in our factory. We investigated the working environment, service life, main beam structure, origin, etc. of these 16 cranes, visited the crane users and operators, and learned about the installation, commissioning, use and maintenance of the crane. In terms of the distribution of the use environment, there are 5 casting workshops, 6 machining workshops, 3 assembly workshops and 2 riveting and welding workshops; In terms of service life, 0 set for less than 5 years, 2 sets for 5-10 years, 6 sets for 10-20 years, and 8 sets for more than 20 years; From the perspective of the main beam structure, there are 9 box type double beams, 2 four truss types and 5 single web plates; From the point of origin, 10 sets are purchased and 6 sets are self-made by our factory. Through the analysis of the above aspects, we believe that the main reasons for the downward deflection of the main beam of the bridge crane are as follows:

1, the impact of high temperature working environment. From the above analysis, it can be seen that the casting workshop accounts for 5 of the 16 main beam deflection cranes. Therefore, the high temperature working environment has a great impact on the main beam of the crane. This is because the main beam of the bridge crane used in the hot processing workshop is in a high-temperature baking state for a long time, which reduces the yield limit of metal materials and generates temperature stress. On the one hand, the superposition of temperature stress and other stresses may exceed the yield limit of materials; On the other hand, due to the uneven heating of the upper and lower cover plates of the main beam, the temperature of the lower cover plate is much higher than that of the upper cover plate, and the lower cover plate extends more, which finally leads to the downward deflection of the main beam

2. Effects of overload and unreasonable use. Overloading, overtime and unreasonable use of cranes are another important reason for the downward deflection of the main beam. In our investigation, we found that the weight of some lifting objects in some workshops exceeded the rated load of the crane, causing the crane to be overloaded for a long time; In order to catch up with the work period and seize the time, some workshops adopt the method of "stop people and machine" and use cranes beyond the working level (Note 2), so that the cranes are in a long-term fatigue state. The above two unscientific methods will make the local stress of the main beam at or even exceed the yield limit, resulting in the deformation and downward deflection of the main beam. Moreover, some operators use cranes to drag heavy objects, which is an important reason for the downward deflection of the main beam

3. Impact of unreasonable storage, lifting and installation of cranes. As the crane bridge is a long structural member with large elasticity, unreasonable storage, hoisting and installation will cause the deformation of the bridge. During the investigation, we learned that some new cranes were not installed in time due to various reasons, and were let go everywhere before installation, and no measures were taken to prevent the deformation of the bridge; During the hoisting and installation of some new cranes, the hoisting scheme was not designed according to the center of gravity and stress of the bridge, and the bridge was deformed due to rough hoisting. This is also one of the reasons for the downward deflection of the crane girder

4. Due to the influence of design and manufacturing process, the design and manufacturing quality of cranes in our factory are different due to many varieties and miscellaneous sources. The inherent deficiency of crane design and manufacture is also one of the reasons for the downward deflection of the main beam. First, because the design and manufacturing quality of the crane can not meet the standard requirements, the rigidity of its main beam has not been fully guaranteed, and the main beam has downward deflection when in use; Second, due to the tensile and compressive stresses in different directions caused by forced assembly in the manufacturing process and the residual internal stresses caused by welding, these stresses gradually homogenize and even disappear in the use process, resulting in permanent deformation of the main beam and downward deflection of the main beam

second, the harm of the main beam deflection

the main beam deflection will seriously affect the safe operation of the crane. If it is not repaired or scrapped in time, it may cause serious equipment and personal accidents. The harm can be summarized as follows

1. Impact on the operation of the trolley. When the main beam has downward deflection and permanent deformation, the downward deflection after bearing will focus on the goal of building a 100 billion ceramic aluminum peak in Huaibei City, which will make the trolley track slope and the trolley can't work normally. When the trolley runs from the center to one end, it is difficult to run due to climbing, and in serious cases, the motor will be burned out due to increased resistance. 4.1 place it safely. When the car runs from the ladder to the middle, there will be "sliding", the speed will be accelerated, and the car will not stop accurately, and even lead to accidents

2. Impact on crane operation. Especially for the bridge crane with centralized drive type, the downward deflection of the main beam will cause the transmission shaft support and structure of the crane operating mechanism to move down together, and the transmission shaft will be bent. When the transmission shaft is seriously bent, the journal will be damaged, and even the teeth of the coupling will be broken, and the connecting bolts will be broken. In serious cases, the shaft will be cut off, and even the motor will be burned

3. Impact on the trolley. When the downward deflection of the two main beams is different, the four wheels of the trolley cannot contact the track at the same time, forming the phenomenon of "three legs" of the trolley. At the same time, with the downward deflection of the main beam, the horizontal bending of the main beam is caused. The main beam is bent inward to reduce the trolley gauge. When the gauge is reduced to a certain value, the double rim trolley will produce running rail clamping, and the outer single rim trolley will cause derailment

III. treatment measures

from the above analysis, it can be seen that the downward deflection of the main beam has a significant impact on the safe operation of the crane. Our factory attaches great importance to it, and carries out special analysis and research every year to formulate a treatment plan. At present, the state has no clear regulations on the extent to which the crane girder is not allowed to be used. If all cranes with downward deflection are repaired, it is bound to cause waste. Therefore, according to the relevant provisions of GB6067-85 "safety regulations for cranes", we take the deflection s (s is the crane span) of the main beam under the rated load as the reference value. For the crane with small lower deflection and far below the reference value, it will not be repaired temporarily, and the safety precautions of degrading and limited time use, strengthening safety technology management and increasing the number of rechecks will be taken; For the cranes whose deflection under the two cranes far exceeds the reference value, the service life is long, and there is no maintenance value, the measures of resolutely scrapping and updating have been taken. For the cranes whose deflection under the nine cranes is close to or reaches the reference value, two methods of prestress or flame heating have been adopted to correct and repair the main beam as appropriate. The camber of the main beam of the crane after maintenance meets the requirements of the national standard. After long-term use and testing, the camber is stable and the effect is good, which effectively ensures the safe operation of the crane

note (1): camber - the crane main beam is an elastic structure, which will produce elastic downward deflection under the action of lifting objects. The camber prefabricated to compensate for this downward deflection is called the camber of the main beam

note (2): working level - GB3811-83 "crane design code" divides the working level of bridge crane into light level, medium level, heavy level and extra heavy level according to load characteristics and working busy degree