A Computational Model for the Analysis of the Static Forced Performance of Self-Equalizing Tilting Pad Thrust Bearings

While failure analyses in the archival literature report thrust collar misalignment as a major cause of collapse in oil lubricated thrust bearings, a self-equalizing tilting pad thrust bearing (TPTB) improves operation reliability by adjusting its pads to account for thrust collar tilt. A self-equalizing system includes a series of upper leveling plates supporting the bearing pads, and themselves carried on the shoulders of lower leveling plates. An uneven load distribution across the pads due to collar misalignment, and/or elastic deformation of the bearing elements, or manufacturing tolerances can induce a moment on the lower plates thus tilting them. Hence, the upper plates supporting pads with a larger load displace by pushing up the other pad under lighter load until the moments on the lower plates balance. The paper describes a model for the pads leveling kinematics mechanism integrated into an existing thermo-elasto-hydrodynamic (TEHD) analysis tool to deliver static and dynamic load performance predictions for self-equalizing TPTBs. The analysis uses the actual leveling plates geometry model as obtained from a solids modeling commercial software. A static load analysis determines the forces acting at the contact lines of the leveling plates as well as the moments acting on them as a function of the applied load on each of the bearing pads. Friction models estimate the sliding friction forces acting at the contact points of the leveling plates and the rolling friction at their pivots. Further, a Hertz contact analysis model uses the predicted forces to deliver a peak pressure and deformation over the contact area between the leveling plates. Next, the paper presents predictions for an example self-equalizing TPTB operating with 0.01⁰ thrust collar static misalignment. The bearing has six pads with 126 mm in outer diameter, operates at 4krpm (maximum surface speed = 26 m/s) and under a specific load/pad ranging from 0.5 to 3.5 MPa. The maximum axial displacement of the thrust collar due to the misalignment is 0.011 mm and which equals to 75% of the nominal minimum film thickness when the bearing operates under 2 MPa/pad load and with a perfectly aligned thrust collar. Compared to a regular (non-equalizing) TPTB, a self-equalizing TPTB operates with up to 50% larger minimum film thickness a roughly 1/2 of peak elastic deformation. Variations of the pads peak temperature are insignificant (max of 9 ⁰C) for both the regular and self-equalizing TPTBs. Friction forces acting at the contact points of the leveling plates show a significant effect on the performance of the pad leveling system as they reduce the film clearance and increase a pad peak pressure. Predictions from the Hertz contact analysis agree with those from a commercial finite element analysis tool and show a significantly large peak pressure at the contact points of the leveling plates (> 0.9 GPa) when the bearings operate under a 3 MPa/pad specific load. The present work thus shows the importance of performing a comprehensive multiple-pad analysis to accurately evaluate the performance of self-equalizing TPTBs hence assuring their safe operation.