The slewing bearing is one of the important components for the load-bearing and slewing functions of a crane. If it fails, it will not only affect the normal operation of the crane, but also cause extremely great safety risks. Taking the operating principle of crane slewing bearings as the starting point, this paper briefly analyzes crane slewing bearing failures from aspects such as unstable system pressure, failure of the slewing brake valve to close, and looseness of the slewing bearing. Based on the factors causing crane slewing bearing failures, several preventive and control measures for crane slewing bearing failures are proposed.

Keywords: crane; slewing bearing; failure prevention; failure control

Preface

The crane used in a certain metal structure factory is a rope-luffing portal crane, with a maximum rated load of 6×10^4 kg, a maximum lifting height of 40 m, and a maximum lifting radius of 50 m. The portal crane has a long operating time and mostly works in open-air sites. In order to reduce the operational risk of the crane and ensure the safety of the overall work site, it is necessary to carry out regular inspection and maintenance of the key modules of the crane. Taking the slewing bearing failure of this crane as an example, this paper briefly analyzes its prevention and control, as follows:

1. Operating Mechanism of Crane Slewing Bearings

Crane slewing bearings have various operating modes. In the specific design process, the raceway diameter is mostly taken as the main parameter and increased step by step according to load-carrying capacity requirements. At the same time, in order to meet the diverse requirements of lifting equipment for axial dimensions and load-bearing weight, several types can also be selected in the actual rolling element design process, such as single-row four-point contact ball type, three-row ball type, double-row ball type, and single-row crossed roller type. Taking the three-row ball type as an example, it mainly includes evenly arranged balls on the upper, middle, and lower three levels and inner and outer double-layer rings. According to the specific crane load, different rolling diameters can be set in actual design to ensure that the crane can bear various forces such as radial force and axial force in combination.

Type of slewing drive device for M10-50 portal crane

In the figure above, 1 and 2 are the elastic pin coupling and vertical motor; 3 is the coupling with limit torque limiter; 4 is the brake; and 5 and 6 are the vertical planetary reducer and drive gear, respectively.

2. Common Failures of Crane Slewing Bearings

2.1 Failure of the Slewing Bearing to Operate Smoothly

The failure of the slewing bearing to operate smoothly may not only be caused by overload problems, but may also be due to factors such as hydraulic motor failure, reducer manual control valve failure, and relief valve failure. In addition, if there are conditions such as fractured steel columns, deformation and jamming, or collapsed rolling tracks inside the crane slewing bearing, the normal operation of the slewing bearing will also be affected.

2.2 Excessive Clearance and Looseness of the Turntable Bearing

The looseness of the turntable bearing is mainly caused by the aging or damage of related internal components of the turntable bearing, that is, the slewing bearing, resulting in the clearance of the turntable bearing exceeding the standard value.

2.3 Abnormal Noise in the Slewing Bearing

The abnormal noise in the slewing bearing is mainly caused by breakage of rolling elements, damage to the hardened layer of rolling elements, or spalling.

2.4 Pressure of the Slewing Bearing System Lower than the Standard Value

The pressure of the slewing bearing system being lower than the standard value mainly occurs when the crane is under full load or heavy load. Under no-load or light-load conditions, it performs normally, and may even exhibit problems such as inability to slew or excessive slewing resistance.

3. Prevention and Control of Crane Slewing Bearing Failures

3.1 Prevention of Crane Slewing Bearing Failures

Most crane slewing bearing failures are mechanical or electrical faults. On the one hand, in order to minimize the probability of crane slewing bearing failures, maintenance personnel should strengthen the inspection of motor protection components such as thermal relays, fuses, automatic air switches, and overcurrent relays, so as to avoid slewing bearing failures caused by their long-term abnormal working conditions. On the other hand, since crane electrical equipment is mostly in environments with high air humidity, more attention should be paid to the maintenance of electrical lines, so as to avoid slewing bearing failures caused by the insulation value of the terminal block to ground being lower than the standard value of 0.50 MΩ.

3.2 Control of Abnormal Noise Faults in Crane Slewing Bearings

Taking a certain crane slewing bearing failure as an example, during a ship-loading operation, abnormal noise occurred during the slewing stage of the crane boom. Later inspection found that the position of abnormal noise in the crane slewing bearing was mainly friction noise and mechanical impact noise from a driving device on one side of the crane. During the reverse slewing stage of the crane, the abnormal noise increased and then weakened. In view of the above fault condition, the elastic pin coupling of the crane should first be inspected. It was found through inspection that the spacing distance between the elastic washer and the pin position of the crane exceeded the standard value. After replacing the elastic washer, the abnormal noise of the slewing bearing still existed. Subsequently, a megohmmeter was used to test the insulation of the motor rotor and electronic winding to ground one by one, and all were within the standard limit. After disassembling the motor, there were no obvious faults in its internal components, and the taper matching degree of each component was relatively high.

Finally, through communication with crane maintenance personnel, it was found that there was a lack of an effective isolation device between the tail bearing and the end cover of the crane slewing bearing, causing the slewing bearing to fail to achieve normal axial positioning. In order to verify the correctness of the above assumption, after the motor was disassembled, the appearance of the stator and rotor coils was inspected, and it was found that the insulating paint at one turn of the rotor coil was burnt and discolored. After replacement with standby motor equipment, the fault problem was effectively solved.

3.3 Control of Looseness Faults in Crane Slewing Bearings

According to the assembly requirements of crane slewing bearings, the assembly clearance between the raceway and rolling elements of the crane slewing bearing should be between 0.30 and 0.49 mm. However, in the actual crane assembly process, due to the influence of various factors such as process parameters and manufacturing techniques, the distance between the raceway and rolling elements of crane slewing bearings is mostly below the standard value. Under such circumstances, the probability of looseness in crane slewing bearings is greatly increased. Therefore, in order to fundamentally reduce the looseness problem of crane slewing bearings, maintenance personnel may calculate the load of the slewing bearing and draw a specific load-carrying capacity curve according to the operating standards of the crane slewing bearing. Taking the portal crane slewing bearing as an example, the maximum static load coefficient for a 360° slewing life is 1.450, while its safe use coefficient is 1.69. Considering that fatigue failure may occur within a certain range in crane slewing bearings, in the specific calculation process, repeated slewing calculation should be carried out twice according to the actual slewing angle value. After the load-carrying capacity curve of the crane slewing bearing is completed, materials such as 50Mn or 42CrMo may be strictly selected as replacement devices according to the slewing bearing manufacturing standard.

In order to reduce the probability of loosening and fracture during the operation of crane slewing bearings, first, after the crane slewing bearing has been in operation for more than 4 days, bolt preload should be checked, and after its working cycle reaches 20 days, the preload should be rechecked. When the operating time of the slewing bearing exceeds 600 days, new bolts should be replaced. Secondly, during the operation of the crane slewing bearing, regular lubrication maintenance can be carried out by comprehensively considering factors such as grease, lubrication cycle, and smoothness of the lubrication oil circuit. If there is already good lubrication in the slewing bearing operating track, lubricating oil may be appropriately added according to specific work requirements. According to different operating mechanisms, the interval for adding lubricating fluid also has different requirements. For example, in the lubrication of ball-type slewing bearings, it needs to be carried out once every 4 days; while the lubrication cycle of roller-type slewing bearings is 2 days. Finally, during the operation of the crane slewing bearing, a comprehensive cleaning should be carried out every 10 days to avoid the adverse effect of the accumulation of hard foreign matter on the slewing bearing.

3.4 Control of Jamming Faults in Crane Slewing Bearings

During crane slewing operation, the driving force is mainly generated by the slewing motor and reaches the large gear ring of the slewing bearing through the coupling, reducer, and slewing pinion. In the whole process, the meshing of the gear and large gear ring is very important for the smooth progress of slewing operation. Therefore, after the jamming problem of the crane slewing bearing occurs, maintenance personnel may first inspect the external structure of the slewing bearing, and then gradually inspect and repair the raceway, steel columns, hydraulic motor columns, bearings, reducer gears, worm gear, etc. Targeted solutions should be carried out according to the specific problem factors. Especially during the replacement of the large gear on the outer ring of the crane slewing bearing, the upper slewing platform and boom system of the crane should be completely dismantled. Then, during the selection of slewing bearing gear materials, the gear modulus and life safety coefficient should be appropriately adjusted. Combined with the reasonable selection of the transition fillet radius at the root of the gear, the bending resistance performance of the gear can be effectively improved.

Finally, during the gear reassembly process, on the basis of adjusting the backlash of gear meshing teeth and connection stiffness, reasonable damping elements can be added to minimize the adverse impact of external loads on the crane slewing bearing.

In addition, in daily inspection and maintenance, attention should be paid to checking the flexibility of gear rotation, surface finish, and integrity of the gear edge. If the wear amount at the gear groove position exceeds 3.0 mm, or the reduction in gear groove diameter reaches 49% of the diameter of the driving rope, gear replacement measures are required.

Conclusion

In summary, the occurrence of slewing bearing failures poses a serious threat to the smooth and safe operation of the overall crane. Therefore, in the actual application of cranes, maintenance personnel should regularly maintain and manage the slewing bearing. For problems such as abnormal noise, looseness, excessively low pressure, and jamming of crane slewing bearings, centralized analysis and treatment should be carried out. The fault control mechanism should be gradually improved to ensure the timely and effective resolution of crane slewing bearing failures.