The reasons for the downward deflection of the main girder of overheadcranes are multifaceted and should be analyzed according to specific circumstances. Generally speaking, issues can arise from design, manufacturing, transportation, installation, and use.
(1) The Impact of Unreasonable Design In the past, China followed the Soviet standards. The static stiffness of the main girder was uniformly designed according to S/700, and fatigue calculations were not carried out. There was a one-sided pursuit of lightweight design, resulting in small cross-sectional dimensions of the main girder, thin webs, and poor rigidity, causing the main girder to experience downward deflection deformation prematurely. In China’s new design codes, the static stiffness of the main girder for class A6 is S/800, while for classes A7 and A8, it is S/1000, which has reached the level of advanced international standards.
The design philosophy back then was rather simplistic. Designers focused too much on reducing the weight of thecraneto cut costs and save materials. However, this led to insufficient consideration of the long-term performance and load-bearing capacity of the main girder. The thin webs, for instance, couldn’t effectively resist the forces exerted during operation. With the development of industrial technology and a deeper understanding of crane mechanics, modern design standards take into account various factors such as fatigue life, dynamic loads, and long-term structural integrity. By increasing the stiffness requirements, the new standards ensure that the main girder can withstand repeated use over many years without excessive deflection.
(2) The Influence of Welding Residual Stress in the Main Girder The box-shaped main girder of a double-girder overheadcranecommonly produced is a welded structure. During the welding process, local heating causes the metal in the weld and the nearby heating area to contract, generating residual stress and resulting in the deformation of the main girder. The distribution of the welding residual stress caused by the four fillet welds of the box-shaped main girder is approximately as shown in Figure 4 – 20. That is, there is tensile stress near the welds of the upper and lower cover plates, with compressive stress in the middle; near the web welds, there is tensile stress, and compressive stress in the middle. Moreover, due to the superposition of the stress from the stiffener plate welds inside the main girder, the center of the compressive stress area of the web moves downward. In fact, the distribution of internal stress in the main girder before loading is very complex. Besides the influence of the welding process, there are other influencing factors. For example, the internal stress of the steel itself and the arch-forming manufacturing process of the main girder can all affect the stress distribution. Some main girder webs are not cut according to the arch-forming requirements. The camber of the main girder is obtained through flame correction or by making the beam deform forcibly by controlling the assembly and welding sequence. All of these will increase the internal stress. Practice has shown that riveted girders or welded truss girders rarely have downward deflection deformation and have good performance.
When welding the main girder, heat is applied unevenly. The areas close to the weld pool experience rapid heating and cooling, which causes the metal to expand and contract differently from the surrounding areas. This non-uniform volumetric change is the root cause of residual stress. The internal stress doesn’t just exist in isolation; it interacts with the external loads the crane will face during operation. If the residual stress is too large, even a normal working load can trigger additional deformation in the main girder. Riveted girders, on the other hand, assemble components through mechanical connections like rivets. This method avoids the heat-induced stress issues of welding, resulting in more stable structures with less likelihood of deflection.
(3) The Impact of the Main Girder Manufacturing Process The method of forming the camber of the main girder has a certain influence on the disappearance of the main girder camber. With the continuous improvement of the manufacturing process methods and the increase in production and operation levels in the factory, this influence is gradually decreasing. It can be summarized into the following three camber-forming methods:
The main girder web is cut straight. After welding the main girder, a pneumatic hammer is used to tap near the joint weld between the upper cover plate and the web, so that the internal stress of this part of the weld is released, generating a certain plastic deformation and forming a certain camber. A heavy hammer is used to press on the lower cover plate or local flame heating is carried out, using the plastic deformation of the material to give the main girder the required camber. Although this method releases the internal stress of the upper cover plate weld, the internal stress of the lower cover plate still remains. Under the action of the load, the lower cover plate weld is subjected to external tensile force, causing tensile plastic deformation and reducing the camber. Therefore, the camber formed by this method is unstable. At the same time, the camber formed by the heavy hammer makes the material hardened and reduces its plasticity.
This traditional method might seem simple and straightforward at first glance, but it has significant drawbacks. By only addressing the stress in the upper part of the girder, it leaves the lower part vulnerable. When the crane starts to carry loads, the unrelieved stress in the lower cover plate weld begins to act. The external load exacerbates the internal stress imbalance, leading to plastic deformation that eats away at the carefully crafted camber. And the hardening of the material from the heavy hammer pressing further reduces its ability to adapt to future stress changes, making the camber even more unstable over time.
The main girder web is cut straight. By using the welding sequence of the four joint welds between the cover plate and the web and carrying out local flame heating at the lower part of the lower cover plate and the web, the main girder is made to produce thermoplastic deformation to reach the designed camber. This method relies on thermoplastic deformation to form the camber, and there is relatively high tensile residual stress in the lower cover plate. When an external load acts, tensile plastic deformation occurs, the camber decreases, and the camber is also unstable.
The use of thermal energy to create the camber seems like a clever approach, but it comes with its own set of problems. The heat applied causes the metal to expand and deform plastically. However, once the heat dissipates and the metal cools, it leaves behind a large amount of residual tensile stress in the lower cover plate. When the crane is in operation, this residual stress combines with the external load-induced stress. The combined effect often leads to a reduction in the camber, as the material in the lower cover plate is already in a stressed state and more prone to further plastic deformation under load.
The main girder web is cut into an arch shape. Since there are more stiffener plates arranged on the upper part of the main girder, the upper part of the main girder shrinks more than the lower part after welding. Therefore, the camber of the web must be larger than that of the finished main girder. The web cutting should increase the camber amount to F = (2.5 – 3.5)S/1000. After a single main girder is welded, it maintains an upward camber of 1.8S/1000. After the bridge is assembled and the track is welded, it maintains the factory-set upward camber of S/1000. With this method, because the web is cut into an arch shape, the degree of camber disappearance after loading is much smaller than the former. It can be seen that the degree of camber disappearance is related to the camber-forming method.
This method takes into account the natural deformation tendencies during welding. By pre-forming the web into an arch, it compensates for the future shrinkage and stress changes. When the upper part of the girder with more stiffeners shrinks during welding, the pre-formed arch in the web helps maintain the overall camber. This approach is more in line with the mechanical behavior of the structure during welding and subsequent loading, resulting in a more stable camber over the life of the crane.
(4) The Impact of Overloading and Poor Operating Conditions The selection of a crane is based on factors such as the production capacity of the workshop, the weight of the equipment, and the operating conditions. However, some units do not pay attention to the operating conditions and use the crane in an overloaded state for a long time (such as changing from light-duty to medium-duty, medium-duty to heavy-duty use, etc.); they lift and pull out buried objects; they suddenly lift without tightening the wire rope; the brakes are improperly adjusted, braking too harshly, and suddenly stopping the descending heavy object; improper command during the turning of the heavy object during hoisting causes impacts, etc., all of which cause the main girder to deflect downward.
Overloading is a common yet extremely harmful practice. Cranes are designed with specific load ratings. When these limits are exceeded, the main girder has to bear forces far beyond what it was engineered for. Each additional ton of load puts extra stress on the girder structure, accelerating fatigue and leading to plastic deformation. Uncontrolled starting and stopping, like sudden braking, also generate large impact forces. These dynamic loads are much more damaging than static loads of the same magnitude, as they can cause immediate and severe stress spikes in the main girder, further contributing to downward deflection.
(5) The Impact of Web Wave Deformation During the load-bearing process of the crane main girder, the web mainly bears shear stress. Under normal circumstances, the shear deformation has little effect on the deflection of the main girder after loading. However, when there is a large wave deformation in the web, the impact is significantly increased (the deflection caused by shear deformation is proportional to the square of the wave value), and the increase in shear deformation also causes an increase in the bending stress in the compressive area of the web. When the crane trolley runs back and forth on the main girder, each part of the web will alternately change from being under tension to being under compression in the 45-degree diagonal direction. As a result, the original uneven wave deformation of the web itself, combined with the wave shape after the main girder is loaded, may cause the web to produce residual plastic deformation. Therefore, the larger the original wave deformation of the web, the greater the resulting downward deflection of the main girder; and the greater the downward deflection, the larger the bending stress in its compressive area, and the web wave deformation and the resulting downward deflection may become more and more serious.
The wave deformation of the web is often overlooked in the initial inspection of cranes. However, it plays a crucial role in the long-term performance of the main girder. The irregular shape of the web due to waves changes the stress distribution within the girder. When loads are applied, the areas of tension and compression are no longer evenly distributed as in an ideal flat-web situation. This uneven stress distribution accelerates the degradation of the web material, leading to more plastic deformation over time. And as the deflection increases, it forms a vicious cycle, with the changing stress further exacerbating the wave deformation, which in turn causes more deflection.
In addition to the above factors, there are several other important aspects that contribute to the downward deflection of the main girder of overhead cranes:
(6) The Influence of Material Quality The quality of the steel used in the main girder is of utmost importance. Inferior steel may have inconsistent mechanical properties, such as varying hardness, tensile strength, and ductility across different batches or even within the same batch. If the steel has low tensile strength, it will be more prone to stretching under load, leading to the main girder’s downward deflection. Additionally, impurities in the steel can act as stress concentrators. These microscopic defects can initiate cracks during cyclic loading, weakening the overall structure of the main girder and accelerating deflection. Some steel may also have poor corrosion resistance. Over time, corrosion can eat away at the metal, reducing the cross-sectional area of the girder and impairing its load-bearing capacity, ultimately resulting in deflection.
Modern manufacturing requires strict quality control of steel materials. Advanced testing techniques, such as ultrasonic testing and spectroscopy, are used to detect internal defects and ensure the proper chemical composition of the steel. High-quality steel suppliers have standardized production processes to guarantee consistent mechanical properties, which is essential for the long-term stability of the crane’s main girder.
(7) The Impact of Environmental Factors The environment where the crane operates can have a significant impact. High humidity and corrosive atmospheres, such as those found in coastal areas or chemical plants, can accelerate the corrosion of the main girder. Corrosion not only reduces the thickness of the metal but also changes its mechanical properties. In cold environments, the low temperature can make the steel more brittle. When the crane is operating under cold conditions, sudden impacts or vibrations can cause small cracks to form more easily in the main girder. Extreme heat can also be a problem. High temperatures can cause thermal expansion of the metal, and if the crane structure is not designed to accommodate such expansion properly, it can lead to internal stress changes and subsequent deflection.
To mitigate the effects of the environment, protective coatings are often applied to the crane. These coatings act as a barrier against moisture, chemicals, and oxygen, reducing the rate of corrosion. In cold environments, preheating the crane before operation can help reduce the brittleness of the steel. And in hot areas, proper ventilation and heat dissipation design in the crane structure can accommodate thermal expansion.
(8) The Influence of Inadequate Maintenance Regular maintenance is crucial for the proper functioning of overhead cranes. Failure to lubricate the moving parts, such as the wheels and bearings, can increase friction. This extra friction not only consumes more energy but also generates additional vibrations. These vibrations are transmitted to the main girder, causing cyclic stress that can contribute to fatigue and deflection over time. Neglecting to inspect and tighten bolts and connections can also lead to loose parts. Loose connections can change the load distribution within the crane structure, putting uneven stress on the main girder and accelerating its downward deflection.
A comprehensive maintenance schedule should include routine inspections of all components, lubrication of key moving parts, and timely replacement of worn-out elements. By keeping the crane in optimal condition, the lifespan of the main girder can be extended, and the risk of downward deflection can be minimized.
Overall, understanding these multiple factors – from design and manufacturing to operation, environment, and maintenance – is essential for preventing and addressing the issue of main girder downward deflection in overhead cranes. Each factor interacts with the others, and only by comprehensively managing all aspects can we ensure the long-term safety and efficient operation of these important industrial machines.
