Truss cranes are widely used in the bridge erection construction sites of construction enterprises due to their characteristics such as flexible lifting, simple operation, and convenient installation and disassembly. Therefore, ensuring the safe operation of truss cranes plays a crucial role in improving work efficiency, reducing production costs, preventing casualties, and establishing a good corporate reputation. However, due to various reasons, truss cranes may encounter various accidents during their use, causing unnecessary losses to construction enterprises.
The largest proportion of truss crane accidents is the fall of the load. The damage of the wire rope, the damage of the components of the transmission and bearing system, the serious overweight of the load, operation errors, and improper maintenance are the main reasons for the fall of the load. Therefore, in order to prevent crane accidents, it is necessary to focus on solving the problems in several aspects such as design, manufacturing, installation, inspection, and maintenance. Based on years of practical experience and through the analysis and summary of multiple potential accident hazards of truss cranes, the author believes that the main causes of crane accidents are as follows:
1. Damage to Components of the Bearing System
Wire Rope Damage
Squeezing or Breaking Caused by the Collision of the Movable and Fixed Pulley Blocks When the hook lifts an object and rises, if the upward limit switch of the hoisting mechanism malfunctions and the crane operator fails to notice and continues the upward operation, it will cause the movable pulley block to collide with the fixed pulley block or the trolley frame. This will result in the wire rope being squeezed and cut by the edge of the structural components or the pulley rim. Alternatively, because all the torque of the motor and the energy of the transmission system are absorbed by the wire rope, exceeding the load-bearing limit of the wire rope, it will eventually break. For example, in a busy construction site, if the limit switch has not been properly maintained and fails during operation, and the operator is not vigilant, this kind of accident is very likely to occur.
Knotting or Breaking Caused by the Chaotic Winding of the Wire Rope During the lifting process, if the wire rope leaves the drum rope groove or the pulley groove, or when the hook is lowered and then continues to descend, causing the wire rope to loosen, it is likely to cause the wire rope to be chaotically wound. In some cases, due to the hasty operation of the operator, the wire rope is not properly arranged, which also increases the risk of chaotic winding.
Knotting or Breaking of the Wire Rope Caused by the Malfunction of the Lower Limit Switch When the hook releases the load, if the lower limit switch of the hoisting mechanism malfunctions and the crane operator continues the lowering operation, and the drum continues to rotate after all the wire rope is paid out, the wire rope will be chaotically wound in the opposite direction, causing the wire rope to knot or break. This situation often occurs when the operator does not pay attention to the operation status of the equipment and blindly operates according to the usual habits.
Damage to the Wire Rope Joint The wire rope joint is an important part connecting the wire rope. If the joint is not properly manufactured or installed, or is damaged due to long-term use and lack of maintenance, it will pose a great threat to the safety of the wire rope. For example, if the joint is not tightly connected, it may gradually loosen during the operation of the crane, and finally lead to the disconnection of the wire rope.
Breaking Caused by Severe Wear and Tear without Regular Replacement After the wire rope is severely worn, if the user does not replace it regularly, it will eventually be pulled apart. The wear and tear of the wire rope is a gradual process. If the user ignores the inspection and maintenance of the wire rope and does not replace it in time when it reaches the wear limit, it will greatly increase the risk of accidents.
Hook Damage
Most hooks are forged, and large-tonnage cranes use plate hooks. If non-destructive inspection is not carried out during the manufacturing process and forging defects are not discovered in time, accidents such as fracture may occur during the use of the user. If the material of the forged hook is not strictly controlled and the material is misused, hook damage accidents will also occur. For example, if the forging process is not up to standard, there may be internal cracks or inclusions in the hook, which will make the hook brittle and prone to breakage under load.
Pulley Damage
If the design is not appropriate or the maintenance is improper, the rim of the ductile iron pulley is often damaged during transportation. There are strict restrictions on the maximum deflection angle of the wire rope of the pulley block of the hoisting mechanism. Otherwise, the friction between the wire rope and the pulley will also cause the wear and tear of the rim and even damage. In some cases, due to the lack of regular inspection and adjustment of the pulley, the deflection angle of the wire rope exceeds the standard, resulting in accelerated wear of the pulley rim.
Damage to Structural Components
The bridge frame and the trolley frame of the truss crane are the two major load-bearing components of the crane. The design of the bridge frame of small-tonnage truss cranes is mainly limited by the stiffness conditions. When the two major components are damaged due to reasons such as improper design and poor manufacturing quality, the fall of the load may be caused. For example, if the welding quality of the bridge frame is not good, there may be cracks in the welds during the operation of the crane, which will gradually expand and eventually lead to the damage of the bridge frame.
2. Damage to Components of the Hoisting Mechanism Transmission System
The components of the hoisting mechanical transmission system of the truss crane are composed of drums, shafts, bearings, couplings, reduction boxes, brakes, etc. Due to design errors, material defects, manufacturing tolerances, improper assembly, etc., the components of the transmission system may be damaged, resulting in the fall of the load. For example, if the material of the shaft has internal defects, it may break under the action of load, causing the hoisting mechanism to fail. Or if the coupling is not properly installed, it may cause vibration and noise during operation, and eventually lead to the loosening and damage of the coupling.
3. Improper Use and Maintenance
Misoperation and Improper Maintenance
The design of truss cranes is usually conservative, and there are also mature design experiences. Therefore, misoperation and improper maintenance generally do not cause damage to the crane. However, if it is frequently abused, crane accidents will occur. For example, due to the misjudgment of the crane operator or lack of experience, operation errors occur; the lack of regular lubrication of wire ropes, bearings, gears, pulleys, etc. often accelerates wear; the crane commander does not pay attention to the load binding method, and the load center of gravity is not centered, causing unbalanced hoisting, which is likely to cause damage. In actual operation, some operators may ignore the operating procedures and operate the crane at will, which increases the probability of accidents.
Severe Overloading Causes Damage to the Weakest Components
Severe overloading of the crane will cause damage to the weakest components. There are many reasons for overloading. For example, due to the operator’s underestimation of the load weight, the object lifted by the crane has not been completely separated from the transport vehicle or other equipment, and the driver blindly lifts it; or during the hoisting process, the load is accidentally caught by a fixed obstacle. At this time, the maximum torque of the motor is all transmitted to the wire rope and the hoisting mechanism, causing severe overloading. In some construction sites, in order to catch up with the construction progress, the crane is overloaded for operation, which is a very dangerous behavior.
4. Environmental Factors
In addition to the above internal factors of the crane itself, environmental factors also have an impact on the safe operation of the crane. For example, in bad weather conditions such as strong winds, heavy rain, and snow, the stability of the crane will be affected. Strong winds may cause the crane to shake violently, and if the wind speed exceeds the design limit of the crane, it may even cause the crane to tip over. Heavy rain and snow may cause the working surface to be slippery, increasing the difficulty of the crane’s movement and operation. Moreover, in a corrosive environment, the metal components of the crane will be corroded, reducing their strength and service life. If the crane is used in a chemical plant or a coastal area for a long time, the salt spray and chemical substances in the air will gradually corrode the crane, making it more prone to failure.
5. Lack of Regular Inspection and Maintenance
Regular inspection and maintenance are essential for ensuring the safe operation of the crane. However, in some cases, due to the lack of awareness of the importance of inspection and maintenance or the pursuit of cost savings, the crane is not inspected and maintained regularly. This may lead to the failure to detect potential problems in time, such as the wear and tear of components, the loosening of bolts, and the malfunction of electrical systems. For example, if the brake system of the crane is not inspected regularly, the brake pads may be severely worn, and the braking effect will be reduced, which will pose a great threat to the safety of the crane operation.
6. Operator Training and Management Issues
The quality and skills of the crane operator are crucial for the safe operation of the crane. If the operator does not receive professional training, lacks the necessary operating skills and safety awareness, it is easy to cause accidents. In addition, the management of the crane operation also needs to be strengthened. For example, the establishment of a sound operating procedure and safety management system, the implementation of regular safety inspections and training, and the clear division of responsibilities of operators and managers. If the management is not in place, it may lead to chaos in the operation process and increase the risk of accidents.
In conclusion, the accidents of truss cranes are caused by a variety of factors, including the damage of components, improper use and maintenance, environmental factors, lack of regular inspection and maintenance, and operator training and management issues. In order to prevent crane accidents, it is necessary to comprehensively consider these factors, strengthen the design, manufacturing, installation, inspection, and maintenance of the crane, improve the quality and skills of the operator, and strengthen the management of the crane operation. Only in this way can the safe operation of the crane be ensured and the occurrence of accidents be reduced.
