4.1. Numerical Analysis Results
When the deflection angle is 4°, the deflection instability of the swivel bridge occurs based on FEM, which indicates that the deflection angle of the upper structure should be controlled within 4° during the actual rotation process. Von Mises stress nephograms of the swivel bridge with 0°, 1°, 2°, and 3° deflection angles are shown in
Figure 6.
From
Figure 6, it is evident that the maximum Von Mises stress of the swivel bridge with different deflection angles was generated on the spherical hinge, and it increased significantly with the increase in the deflection angle. This showed that the spherical hinge structure could perceive the deflection state of the swivel bridge. When the deflection angle was 0°, the mechanical characteristics of the spherical hinge structure were uniform and distributed in layers. As shown in
Figure 7, the maximum Von Mises stress of the spherical hinge structure was 17.84 MPa.
Von Mises stress nephograms of the spherical hinge with different deflection angles are shown in
Figure 7.
When the swivel bridge deflected, the mechanical characteristics of the spherical hinge structure changed, and the maximum Von Mises stress was generated on the side of the rotation central axis of the spherical hinge structure. At deflection angles of 1°, 2°, and 3°, the maximum Von Mises stress of the swivel spherical hinge structure was 22.87 MPa, 29.64 MPa, and 39.73 MPa, respectively, which was 28.2%, 66.1%, and 122.7% higher than before deflection. The maximum stress changed greatly when the deflection angle shifted from 2° to 3°. At a deflection angle of 3°, the maximum stress of the upper spherical hinge structure’s concrete was 39.76 MPa, slightly exceeding the allowable stress of the concrete compressive strength of 50 × 0.75 = 37.5 MPa. In this scenario, the structure was already in a dangerous state, and the deflection angle should be controlled within 3° during the rotation process. Due to the center of gravity shift caused by the upper structure’s deflection, eccentric pressure was generated on the surface of the upper turntable, and gradually increased with the deflection angle’s increase.
Contact pressure nephograms of a lower spherical hinge with different deflection angles are shown in
Figure 8.
From
Figure 8, it is observed that the maximum contact pressure of a spherical hinge with different deflection angles was generated at the edge of the contact between the upper spherical hinge and the lower spherical hinge, and gradually increased with the deflection angle’s increase. At the deflection angles of 1°, 2°, and 3°, the maximum contact force of the spherical hinge interface was 32.95 MPa, 40.01 MPa, and 53.55 MPa, respectively, representing a 54.9%, 88.1%, and 151.8% increase compared to before deflection. As the deflection angle increased, the contact pressure on the right side of the lower spherical hinge gradually increased, while the contact pressure on the left side gradually decreased, potentially indicating a center of gravity shift of the upper structure.
Through the analysis of the friction force of the lower spherical hinge in both the longitudinal and the transverse direction at different deflection angles, it can be found that the longitudinal friction force was negative, opposing the deflection direction, serving as the anti-overturning force of the swivel bridge. The transverse bridge friction force of the lower spherical hinge exhibited symmetrical distribution, equal in magnitude and opposite in direction, indicating that the solution model did not exhibit deflection in the transverse bridge direction.
In order to prevent such deflection accidents, supporting feet are set on the outer ring of the spherical hinge. They can contact the base and transfer the eccentric pressure of the deflection bridge. In this manuscript, the simplified mechanical analysis diagram of the deflection swivel bridge is drawn in
Figure 9.
When the deflection angle was θ, the supporting feet just contacted the lower pile cap. As the deflection angle exceeded θ, the supporting feet contacted the lower file cap, and also provided a force to resist the overturning of the upper structure. The value of θ, approximately 0.616°, was calculated based on the spatial position of different components of actual engineering. At a deflection angle of 0.616°, the supporting feet contacted the lower pile cap, and the friction moment of the lower spherical hinge with different friction coefficients was obtained based on the FEM, as shown in
Table 6.
It can be seen from
Table 6 that the average friction force of the spherical hinge slider gradually increased with the increasing friction coefficient, resulting in an increase in the friction moment. When the friction coefficient reached 0.01 or 0.02, the upper structure of the swivel bridge was overturned. This indicated that the friction force of the lower spherical hinge was not sufficient to support the stability of the upper structure when the friction coefficient was 0.01 or 0.02. At a deflection angle of 0.616° for the upper structure, the center of mass had shifted, with coordinates X: 61.00701, Y: 3.060855 × 10
−4, Z: 1.448855 × 10
4, and the gravitational deflection moment of the upper structure could be calculated as
Mp = G × L = 7.28 × 103 KN·m. The ratio of the friction moment of the spherical hinge structure to the gravitational deflection moment of the upper structure under different friction coefficients is shown in
Figure 10.
It can be seen from
Figure 10 that the ratio of the deflection moment to the friction moment changed with the variation in the friction coefficient. When this ratio equaled 1, the corresponding friction coefficient was 0.031. It demonstrated that the minimum friction coefficient required to maintain the stability of the deflected bridge is 0.031.
As the friction coefficient increased, the friction moment of the rotational spherical hinge also increased, leading to improved stability of the bridge but requiring a higher rotation traction. However, an excessive friction coefficient could lead to unbalanced traction during the rotation process, resulting in rotational angular acceleration of the swivel bridge and potentially causing the beam balance to break more easily. Therefore, it is recommended to keep the friction coefficient of the spherical hinge structure within a reasonable range. Based on the above discussion, the range of the friction coefficient is defined as 0.1, so the optimum theoretical rotational friction coefficient of the spherical hinge structure is 0.031–1.131.
4.3. Four-Ball Machine Test Results
The friction coefficient test results of four spherical hinge lubrication coatings samples are shown in
Figure 12 and
Table 7.
It can be seen from
Figure 12 and
Table 7 that the friction coefficient of GPG was smallest at 0.065, while PTG exhibited the highest friction coefficient among the several lubrication coatings at 0.072, marking a 9.1% increase compared to GPG. Moreover, the friction coefficients of CNG and TDG were relatively close, both around 0.068, which was 5.6% lower than that of PTG.
Regarding the wear scar diameter, PTFE showed the largest value at 0.848 mm, while GPG had the smallest wear scar diameter among the materials at 0.79 mm, marking a 16% reduction compared to PTG. Furthermore, the wear scar diameters of TDG and CNG were 0.775 mm and 0.790 mm, which were 9.4% and 7.3% smaller than PTG, respectively.
In summary, the GPG, TDG, and CNG spherical hinge lubrication coatings exhibited lower friction coefficients and wear scar diameters compared to PTG, indicating lubrication performance. Particularly, GPG demonstrated the best wear resistance among the coatings. The standard error of the five sets of parallel test results is within 6.8%, ensuring the accuracy and reliability of the findings.
Analysis from
Figure 13 revealed several observations about the lubrication coatings. CNG lubrication coating exhibited discoloration compared with other coatings, attributed to the generation of friction heat on its surface. PTG, on the other hand, displayed relatively high surface roughness and numerous surface scratches, likely resulting from the movement of friction particles during the friction process. This also indicates that the PTG lubrication coating is relatively rough. There are many small scratches on the friction surface of TDG, and a part of the coating surface is peeled off, indicating that the viscosity of TDG is relatively high.
The maximum non-seize load of four spherical hinge lubrication coating samples is shown in
Figure 14 and
Table 8.
The PB index refers to the maximum load at which a steel ball does not seize under a lubrication environment at a certain temperature and speed. The PB value represented the level of load prior to a sudden change in the diameter of the steel ball’s wear spot. The higher the PB value, the better the lubrication performance of the lubrication coatings. As is shown in
Figure 14 and
Table 8, the GPG, TDG, and CNG spherical hinge lubrication coatings had higher maximum non-seize loads than PTG. The maximum non-seize load of PTG and GPG is 421.5 N and 455.5 N, respectively. The maximum non-seize load of GPG is 8.1% higher than that of PTG. The PB value of TDG is the largest among several coatings, 480 N, which is 13.9% higher than PTG. The PB value of CNG is 460.5 N, which is 9.3% higher than PTG. The test results indicate that the load-bearing capacity of GPG, TDG, and CNG spherical hinge lubrication coatings are improved in different degrees compared with PTG, and the TDG spherical hinge lubrication coating has the most outstanding load-bearing capacity.
4.4. Rheological Performance Test Results
The thixotropic test results of four spherical hinge lubrication coatings samples are shown in
Figure 15.
The ability of lubrication coating to flow or deform under external forces is termed its rheological properties. As a non-Newtonian fluid, lubrication coating exhibits complex rheological properties. Under low temperatures and small loads, the lubrication coating can maintain a fixed form without flowing. However, at high temperatures or when shear stress reaches the yield stress, the lubrication coating transforms from semi-solid to semi-fluid, initiating flow.
The rheological properties of lubrication coating are crucial as they determine load-bearing capacity and lubrication performance. As can be seen from
Figure 15, the thixotropic ring of PTG is the largest, followed by GPG, TDG, and CNG spherical hinge lubrication coatings, with CNG exhibiting the smallest thixotropic ring. The test results indicate that PTG has poor thixotropic performance, while GPG, TDG, and CNG show better thixotropic properties, with CNG displaying the best thixotropic properties.
When subjected to shear stress, the highly intertwined soap fibers forming the surface microstructure of the material may experience knot failure or fiber fracture, leading to a significant decrease in the lubrication coating’s viscosity. Upon removing the shear stress, the original damaged structure can recover to some extent through recombination, but this recoverability is limited and depends on the speed of soap fiber recombination within the microscopic skeleton. Test results demonstrate that CNG lubrication coating has the best recovery speed.
Dynamic shear rheological test results of four spherical hinge lubrication coatings samples are shown in
Figure 16.
The storage modulus (G’) and loss modulus (G”), respectively, represent the viscoelastic properties of the lubrication coating. The storage modulus signifies the elastic performance, while the loss modulus indicates the viscosity performance. When G’ > G”, the lubrication coating primarily exhibits recoverable elastic characteristics and lubrication performance. When G’ = G”, the dynamic phase transition point of lubrication coating is reached, transitioning gradually from a semi-fluid state to a fluid state. The shear stress at the intersection can represent the structural strength of the lubrication coating. When G’ < G”, the lubrication coating mainly displays irreversible viscosity properties.
Figure 16 illustrates that the storage modulus (G’) and loss modulus (G”) of four spherical hinge lubrication coatings decrease gradually with increasing strain. After passing through a linear viscoelastic region, all four spherical hinge lubrication coatings enter a nonlinear viscoelastic region. The storage modulus and loss modulus are completely intersected, indicating a phase transition point where the semi-fluid state of lubrication coating shifts to a fluid state. Prior to this point, the storage modulus of the lubrication coating was generally higher than the loss modulus, emphasizing elasticity. After the phase transition point, the loss modulus surpasses the storage modulus significantly, emphasizing viscous properties.
Analyzing the structural strength of the lubrication coatings at the same temperature reveals that PTG has the lowest structural strength compared to both CNG and TDG, while GPG exhibits the highest structural strength. When the strain rates for PTG, GPG, CNG, and TDG are 8.8%, 12.7%, 13.5%, and 12.2%, respectively, the storage modulus curve and the loss modulus curve intersect. This indicates that the phase transition point of PTG among these lubrication coatings is closest to the left, implying its elastic recovery is the poorest.
The apparent viscosity of four spherical hinge lubrication coatings samples under shear conditions is shown in
Figure 17.
The molecules of lubrication coating form a stable spatial structure through hydrogen bonding and Van der Waals force connection. Lubrication coatings prepared with different molecular groups form different intermolecular forces and spatial structures. It can be observed in
Figure 17 that the apparent viscosity of the four spherical hinge lubrication coatings decreases as the shear time gradually increases. TDG exhibited the highest apparent viscosity among the lubrication coatings, while PTG had the lowest apparent viscosity. Specifically, when the shear time was 300 s, the apparent viscosity of PTG and TDG was 1.67 Pa·s and 1.85 Pa·s, respectively. The apparent viscosity of TDG was 10.8% higher than that of PTG. The apparent viscosity of GPG was 1.78 Pa·s, which was 6.6% higher than PTG. The apparent viscosity of CNG was 1.76 Pa·s, which was 5.4% higher than PTG. Test results indicate that the viscosity performance of GPG, CNG, and TDG is superior to that of PTG.
4.5. Mechanical Property Test Results
Quasi-static compression test results of four spherical hinge slider samples are shown in
Figure 18.
As observed from compressive failure morphology, the four spherical hinge slider samples underwent significant deformation and exhibited a flattened cake-like form after compression failure. PTFE displayed several obvious and large cracks after the compression failure, appearing in a scattered state, indicating poor viscosity under ultimate pressure and insufficient plasticity, and potentially hindering the smooth rotation of the swivel bridge after the compressive failure of sliders. Conversely, PPS and UWHPEF showed no obvious cracks after compression failure, maintaining a relatively smooth surface, suggesting good adhesive properties and strong plasticity. Even after enduring ultimate pressure loads, failed PPS and UWHPEF sliders may still retain some shape and structural integrity.
PEEK also exhibited cracks after compression, but its overall shape remained intact with a relatively smooth surface. Compared with the compressive failure morphology, PPS, UWHPEF, and PEEK are more conducive to the safe and smooth rotation of bridges than PTFE sliders.
Examining the stress-strain curves of four spherical hinge slider samples, it is evident that the maximum compressive stress of PTFE was 91.l MPa. The maximum compressive stress of UWHPEF is 120.3 MPa, which is 32.1% higher than PTFE. PPS demonstrated a maximum compressive stress of 127.8 MPa, marking a 40.3% increase over PTFE. PEEK showcased a maximum compressive stress of 171 MPa, an 87.7% surge over PTFE. Notably, all four materials met the specification for maximum compressive stress.
The pressure resistance properties of PPS, UWHPEF, and PEEK were improved compared with conventional PTFE sliders. However, considering the failure form and compression deformation, UWHPEF’s smoother surface comes with significant deformation post-compression, necessitating very high UWHPEF sliders, which is not conducive to construction and rotation. On the other hand, PEEK, being the hardest material, maintains good surface morphology after compression, meeting the rotation requirements of large tonnage swivel bridges. PEEK slider can remain in the linear elastic working stage under maximum stress, which is an ideal compressive slider for swivel bridges.
Quasi-static shear property test results of four spherical hinge slider samples are shown in
Figure 19.
As shown in
Figure 19, it can be seen that the shear resistance of the PEEK slider was better than that of PTFE. The shear strength of PEEK was 215 MPa, which was 6.07 times higher than that of PTFE. PEEK had a hard texture and a smooth fracture, making it an ideal slider material for extreme shear requirements of large tonnage swivel bridges. Compared with PTFE, the shear strength of UWHPEF also had a certain improvement, reaching 139 MPa, which was 3.93 times the shear strength of PTFE. However, from the perspective of damage form, the texture of UWHPEF was soft, and it could be pushed out during the shear process, damaging the overall structure of the slider and hindering the normal and orderly rotation of the swivel bridge. The shear resistance of PPS was higher than PTFE, up to 132 MPa, which was 3.39 times that of PTFE. The shear deformation of PPS material was better than that of PTFE, but there was still a possibility of warping, hindering the rotation procedure. It could be concluded that the shear strength of PPS, UWHPEF, and PEEK was much higher than that of PEEK, and PEEK was more suitable for large tonnage bridges to improve the shear resistance of sliders.