Introduction to common safety devices for cranes

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To ensure the safety and reliability of lifting operations, cranes are generally required to be equipped with comprehensive and effective safety devices. These devices serve to protect the machinery in case of an accident, alert operators to potential hazards, and, when necessary, stop any movements in dangerous directions, thereby providing a safety safeguard. I. Height limit device, also known as over-winding protection device. Once the hook is raised to the specified height, it touches the limit weight, which activates the travel switch; the \"over-winding\" indicator light comes on, and at the same time the operations of raising the hook, extending the boom, and lowering the boom are stopped to ensure safety. At this point, simply operating levers such as those for lowering the hook, retracting the boom, or raising the boom (that is, operating it in a safe direction) will release the restraint of the limit weight, and normal operation will be restored. On special occasions, when a slight amount of over-winding is still required, the forced release button on the instrument panel can be pressed; this will deactivate the limit function. However, such operations must be carried out with great caution to prevent accidents. Once the over-winding protection device fails, it must be restored immediately; carelessness is strictly prohibited. There are countless cases in which serious accidents have occurred due to \"hooking\" resulting from the failure of the overwind protection device. II. Relief valves in the hydraulic system: These relief valves, which function as safety valves, can prevent abnormal high pressures in the circuit, thereby protecting the components of the hydraulic pump and motor from damage and preventing them from being overloaded. Most of these relief valves are adjustable, and at the time of vehicle manufacture, these adjustment parts are usually sealed with lead seals. The author advises that adjustments should not be made by non-professionals. III. Boom luffing safety device (luffing balance valve): From a hydraulic perspective, a balance valve is a combined valve consisting of a check valve and a relief valve placed side by side. When an accident occurs, such as the bursting or rupture of a high-pressure hose or tubing in the boom lifting cylinder circuit, the balance valve in the hydraulic circuit comes into action by locking in the working fluid from the lower chamber of the cylinder, preventing the boom from dropping and thus ensuring the safety of the operation. In the event of a failure, untrained personnel must not disassemble or repair it, especially the balance valve. IV. Boom extension safety device (extension cylinder balance valve): In the event of an accident, if the high-pressure hoses or pipes in the boom extension cylinder circuit burst or get severed, the balance valve in the hydraulic circuit comes into action by blocking the working fluid coming from the lower chamber of the cylinder, preventing the boom from retracting on its own due to gravity and thus ensuring the safety of operations. V. Leg locking device (hydraulic lock): A typical hydraulic lock consists of two fluid-controlled check valves that control each other. In the event of an accident in which the high-pressure hose or tubing leading to the vertical leg cylinders is damaged or severed, the two-way hydraulic lock in the hydraulic system can prevent the pressure fluid in both chambers of the leg locking cylinders from flowing, thereby preventing the legs from retracting or falling away and ensuring the safety of lifting operations. The bidirectional hydraulic lock also ensures that the vertical leg cylinder does not drop during operation. VI. Weight indicator: The lifting capacity indicator is located on the side of the main arm (that is, on the right side of the control cabin); it can be clearly seen by the operator sitting inside the control cabin, and it accurately indicates the elevation angle of the boom as well as the rated lifting capacity permitted by the crane under the corresponding operating conditions. Vehicles produced in recent years are equipped with torque limiters, so weight indicators are no longer installed. VII. The lifting characteristics table, also known as the performance table, is located on the lower front wall panel inside the hoisting cab; it lists the rated lifting capacity and lifting height for various arm lengths and working ranges, for easy reference during operation. When performing lifting operations, it is absolutely forbidden to exceed the values specified in the table. Of course, due to the complex operating conditions, large cranes have numerous performance specifications; before carrying out lifting operations, it is necessary to refer to the documentation provided with the crane. VIII. Torque Limiter (ACS): An electronic device that collects and displays seven types of information, including safety level, boom angle, working radius, boom length, maximum load capacity, actual load, and maximum hook lift height, thereby providing comprehensive safety control for the crane. IX. Level: Helps to keep the vehicle level and stable. Many vehicle leg systems are equipped with automatic leveling devices, which further enhance safe operation. With the advancement of technology, more advanced, systematic, and scientific safety control and operating systems are being used in cranes, making them safer and more reliable in use. Tan Jingwei, Equipment Lifting Branch, China National Petroleum First Construction Company