Safety technical requirements for crane overload time limiters

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Safety technical requirements for crane overload time limiters. Overloading during operations poses a serious threat to the structure of cranes; it can cause the main beam to bend downward, and the upper cover plates and web members of the beam may become unstable, develop cracks, or have their welds fail. It can also lead to serious accidents such as the breakage of the crane’s boom or tower. Since overload undermines the overall stability of the crane, serious accidents such as the overturning of the entire machine can occur. 　　Bridge cranes with a rated lifting capacity of more than 20 tons, gantry cranes, loading/unloading bridges, railway cranes, and portal cranes with a capacity of more than 10 tons must be equipped with overload limiters in accordance with the provisions of GB6067-85 \"Safety Regulations for Lifting Machinery\". Tower cranes with a rated lifting torque of less than 250 KN·m, as well as lifting equipment such as elevators and electric hoists, should also be equipped with overload limiters when necessary, as required by the user. 　　(1) Types of overload limiters. Overload protection devices can be classified into automatic stop type, alarm type, and comprehensive type based on their functions. 　　An automatic-stop type overload limiter can prevent the crane from continuing to move in an unsafe direction when the lifting weight exceeds the rated value, while allowing the crane to move in a safe direction. The safe direction refers to lowering the load, retracting the boom, reducing the amplitude, and combinations of these actions. The automatic stop type is generally a mechanical overload limiter, which is often used on tower cranes. 　　An alarm-type overload limiter can display the lifting capacity, and it emits audible and visual alarm signals when the lifting capacity reaches 95% to 100% of the rated lifting capacity. 　　A comprehensive overload limiter can emit audible and visual alarm signals when the crane reaches 95% to 100% of its rated lifting capacity; it can also prevent the crane from continuing to move in an unsafe direction once the lifting weight exceeds the rated value. 　　Based on their structural design, overload limiters can be divided into three types: mechanical, hydraulic, and electronic. Mechanical overload limiters include lever-type and spring-type ones. 　　The overload protection device is designed with the influence of the lifting load in mind; when the load is being lifted, braked, or subjected to vibrations, changes in speed generate additional loads, and the dynamic loading during lifting can reach 110% to 130% of the rated load. Dynamic load is a dynamic phenomenon inherent in lifting operations and a characteristic of crane operation. Therefore, the overload protection device is equipped with a timing circuit designed based on this characteristic, enabling it to identify and handle such false loads, while also preventing the actual load from exceeding the specified value and thus avoiding false activations. 　　(II) Safety technical requirements for overload limiters ① The comprehensive error of mechanical overload limiters should not exceed 8%, while that of electronic overload limiters should not exceed 5%. 　　②A warning alarm signal should be issued when the load reaches 90% of the rated lifting capacity. 　　③After installing an overload limiter on a lifting machine, it should be adjusted or calibrated based on its performance and accuracy. When the lifting load exceeds the rated capacity, the lifting power source is automatically cut off, and a warning signal is emitted to indicate that use is prohibited. 　　④Crane types such as bridge cranes, railway cranes, and gantry cranes should be equipped with overload limiters; tower cranes, elevators, and electric hoists can also be fitted with such devices as needed.