# Failure Analysis of Gantry Crane Slewing Bearing Based on Gear Position Accuracy Error
## 4 Discussion and Conclusions  
### Discussion  
#### 4.1.1 Characteristics of Gear Position Accuracy Error Method  
The gantry crane is a typical slewing-type crane widely used in ports, shipyards, and other places, mainly used to perform the handling of goods and loading and unloading operations. The slewing bearing is the key structure connecting the slewing and non-slewing parts of the gantry crane, and its importance is self-evident. After research found that China's ports large gantry cranes' slewing bearing in the design life of the large gear ring before the phenomenon of universal broken teeth. The broken teeth rate of up to approximately 30% results in the failure of the slewing bearing, of which the maintenance costs and economic losses are huge. The main parameters of the gantry crane with such failure problems are a rated lifting weight of 40-60 t, a maximum amplitude of 38-45 m, and a lifting height of 40 m or more. The similarity failure phenomenon of this type of gantry crane in large quantities can be ruled out to be caused by factors of bad working conditions of the equipment, and the causes of failure should be assessed from the aspects of design, manufacture, and installation.  
In this study, the gear position accuracy error method is proposed for the failure analysis of gantry cranes' slewing bearings. Through the analysis of the gantry crane design process, it is found that the traditional method concerns the load on the slewing bearing, which is divided into the equivalent axial load and equivalent overturning moment and is then based on the bearing capacity curve for the selection of the slewing bearing. Under normal meshing conditions, the meshing line of the slewing ring gear is parallel to its axis, and in the gear transmission process, the gear teeth will be disengaged or make contact at the same time in the direction of the tooth width, as shown in Figure 21.  
The gear position accuracy error method considers the relative displacement between the teeth of the slewing ring, and this displacement will result in the drive pinion of the slewing ring and the axis of the large gear ring not being parallel, as shown in Figure 22. The non-parallelism of the axis affects the meshing relationship between the teeth, thus causing the large gear ring to fail with broken teeth. By comparing the finite element analysis results before and after assembly of the slewing bearing model with the slewing table structure and the cylinder structure, it was found that the forced nature of the gears in the two cases was different. Before assembly, the pinion is in good mesh with the large gear ring of the slewing bearing due to its rolling seat shift limitation [19; 20]. The tooth root stress of the slewing ring is uniform, and the gear teeth are in a loaded state. After assembly,  
Figure 21: Pinion gear meshing process with large gear ring.  
the displacement of the cylinder structure and the internal structure of the slewing ring itself cause a certain angle between the gear axes, which affects the meshing relationship between the gear teeth and leads to a concentration of stress in the upper gear root at the engagement of the large gear ring. This stress value is much larger than the lower gear root stress. Its undesirable meshing situation leads to broken tooth failure.