Why Is The Main Girder Of The Crane Deformed?

Why is the Main Girder of the Crane Deformed?
 

There are many reasons for the deflection of the main girder of the bridge crane, which should be analyzed according to the specific situation. Generally speaking, there are a series of problems in design, manufacture, transportation, installation and use.
 

1. Influence of welding internal stress of main girder.
 

The box-shaped main girder of the 50 ton double-girder bridge crane generally produced is a welded structure. Due to the local heating during the welding process, the metal in the weld seam and the nearby heating zone shrinks, resulting in residual stress and deformation of the main beam.
 

The internal stress distribution of the main girder before loading is very complicated. In addition to the influence of the welding process, there are some other influencing factors. For example, the internal stress of the steel itself and the arching manufacturing process of the main girder can all affect the distribution of internal stress. Some main girder webs are not cut according to the arching requirements, and the camber of the main girder is obtained by flame straightening or forced deformation of the beam by controlling the assembly and welding sequence, which will increase the internal stress. Practice has shown that riveted beams or welded truss beams have little deflection deformation and good performance.
 

2. Influence of main beam manufacturing process
 

The arching method of the main girder camber of the bridge crane has a certain influence on the disappearance of the main girder camber. With the continuous improvement of manufacturing process methods and the improvement of production and operation levels, this impact is gradually decreasing. It can be summarized into the following three arching methods:
 

1> The blanking of the main girder web is straight. After the main girder is welded, use a pneumatic hammer to knock near the welding seam connecting the upper cover plate and the web to release the internal stress of this part of the weld and produce a certain plastic deformation. , forming a certain upward arch. And the lower cover plate is pressed by a heavy hammer or heated by a local flame, and the plastic deformation of the material is used to make the main girder have the required camber. Although this method releases the internal stress of the welding seam of the upper cover plate, the internal stress of the lower cover plate has not disappeared. Under the action of load, the weld seam of the lower cover plate is subjected to external tensile force, causing tensile plastic deformation and reducing the camber , so the camber formed by this method is unstable. At the same time, relying on the camber formed by the heavy hammer hardens the material and reduces the plasticity.
 

2> The blanking of the web of the main girder is straight, using the welding sequence of the four joint welds between the cover plate and the web and the method of local flame heating at the lower part of the lower cover plate and the web, so that the main girder produces thermoplastic deformation to achieve the design camber . This method relies on thermoplastic deformation to form the upper arch, and the lower cover plate has high tensile residual stress. When the external load acts, tensile plastic deformation is formed, the crown is reduced, and the crown is also unstable.
 

3> The web of the main girder of the bridge crane is blanked into an arched shape. Since there are more ribs arranged on the upper part of the main girder, the upper part of the main girder shrinks and deforms more than the lower part after welding, so the camber of the web must be higher than that of the finished main girder. The beam arch is large. Since the web is cut into an arch shape, the camber disappearance is much smaller than the former after being loaded. It can be seen that the degree of camber disappearance is related to the method of arching.
 

3. The impact of overload use and bad use conditions
 

The selection of the crane is based on the production capacity of the workshop, the weight of the equipment, and the conditions of use. , Suddenly hoisting without tightening the wire rope, improper adjustment of the brake, excessive braking, sudden braking of the falling heavy object; improper command of the heavy object turning over during the hoisting process, causing impact, etc., causing the main beam to deflect.
 

4. Influence of web wave deformation
 

During the loading process of the main girder of the crane, the web mainly bears the shear stress. In general, the shear deformation has little effect on the deflection of the main beam after loading. However, when there is a large wave deformation in the web, the influence will be significantly increased (the deflection produced by the shear deformation is proportional to the square of the wave value), and the increase in the shear deformation will cause a decrease in the bending stress in the compression zone of the web Increase. When the crane trolley reciprocates on the main girder, each part of the web will be pulled alternately in the 45-degree diagonal direction, and sometimes compressed. In this way, the unevenness of the web itself The uneven wave deformation and the superposition of the wave shape after the main beam is loaded may lead to residual plastic deformation of the web. Therefore, the greater the original wave deformation of the web, the greater the downward deflection of the main beam; and The greater the deflection, the greater the bending stress in the compression zone, and the consequent wave deformation of the web and the resulting deflection will be—maybe more and more serious.
 

5. Effect of high temperature
 

There are more crane deflections in the hot processing workshop than in the cold processing workshop. The crane working above the furnace for a long time and lifting the parts just out of the furnace caused the temperature difference between the upper and lower cover plates of the girder due to radiant heat, and the lower part received a large amount of radiant heat and elongated, resulting in a natural decrease in the camber of the main girder.
 

Therefore, for cranes used in environments with radiant heat, heat insulation measures should be taken on the lower cover of the main girder. In order to prevent the influence of the temperature difference between the upper and lower cover plates.
 

6. The impact of unreasonable storage, handling, lifting and installation methods
 

The crane bridge is a slender metal structure, and there is already a large internal stress in the manufacturing process. Therefore, unreasonable storage, handling, lifting and installation methods will have a more serious impact on the deflection of the main beam.
 

7. The impact of unreasonable maintenance
 

During maintenance, unreasonable heating such as gas cutting and welding on the bridge frame will cause the main girder to deflect. In the past, the installation of trolley rails used the adjustment method of directly welding the pressure plate to the cover plate of the main girder. The process is simple, but the crane with small wheels gnawing on the rails requires many times to replace the rails, cutting the old and welding new ones, and the main girder is easy to scratch.
 

Therefore, when removing and replacing the trolley track, try to avoid random heating on the main beam, and use arc gouging or wind shovel instead of gas cutting track pressure plate. The pressure plate should lift up the middle of the main girder during gas cutting or welding, which can greatly reduce the downward deflection caused by heating. If the trolley track adopts bolts to fix the pressure plate, the influence of cutting and welding heating is eliminated.