Structural Design and Performance Analysis of Flexible Pivot for Porous Tilting Pad Thrust Bearings

Due to the existing porous gas thrust bearings whose optimal working gas film clearance is usually in the micron order of magnitude, the machining and assembly error requirements for the bearings are extremely high in order to ensure the normal operation of the machine. The flexible pivot porous tilting pad gas thrust bearings use the adaptive ability of the flexible structure can reduce the machining and assembly accuracy requirements in some degree, as well as to compensate for the porous gas bearings of poor high-speed stability problems. However, its structure is single, and unable to meet more complex working conditions and a higher level of requirements. 【Objective】 In order to solve the above problems. The structure of porous tilting pad gas thrust bearing is optimized to improve its performance further. Three flexible pivot structures of bearings, which are the straight beam pivot, bow pivot and hybrid pivot, are proposed and designed for different applications. The bearing is made of integral aluminum alloy base and porous graphite material, adhered by epoxy resin. In order to ensure uniform air supply, a pressure equalizing groove structure is opened on the tilting pad. Choosing the right type of structure for different working conditions realizes a more stable operation of the rotor. 【Method】 Based on Darcy's law, N-S equation, flow balance equation, tile equation of motion and so on, the static characteristic analysis model of porous tilting pad gas thrust bearings was established. The equations of motion of the tilting pads considered the rotational stiffness calculation equations for different flexible pivot structures, as well as the model for calculating the tilt angle of the tilting pad. Finite element analysis was used to further examine the stresses as well as deformations of the bearings under different flexible pivot structures. The finite difference method and Newton's iterative method were used to solve the air film pressure, bearing capacity, and so on. Finally, comparing the theoretical calculation results of the model with the experimental results, the theoretical and experimental results were in good agreement, which verified the rationality of the theoretical model to a certain extent. 【Results】In the theoretical calculation of the rotational stiffness of three flexible pivot structures, the bow pivot structure had the smallest rotational stiffness of 7.08×10⁴ Nmm/rad and the straight beam pivot structure had the largest of 1.13×10⁶ Nmm/rad. And the finite element analysis results showed that the bow pivot structure had the maximum deformation and the maximum stress occurs at the minimum thickness of the pivot. When the external conditions were changed, the static characteristic parameters of the bearings under the bow pivot structure changed most significantly, followed by the hybrid pivot structure, and the straight beam pivot structure was the smallest. Theoretical and experimental results showed that the theoretical and experimental errors were within 10% of the levitation heights of the levitated materials under different structures when the external loads were 4.8 N and 9.8 N, respectively, and the air supply pressure was high. 【Conclusion】 The results showed that the bow pivot had the lowest rotational stiffness, the highest sensitivity and the largest deformation of pad under the same conditions, which was suitable for the occasions of lower load carrying capacity and higher sensitivity; the straight beam pivot was suitable for the occasions with large load capacity; the hybrid pivot combined the characteristics of these two structures, which was suitable for the occasions with high load and high sensitivity; the reasonable degree of pivot offset to a certain extent could also improve the performance of the bearings. Three different flexible pivot structures of bearings theoretical and experimental comparison results had proved the accuracy of the theoretical calculations.