Function
Tilting Pad Thrust Bearings are used to balance the axial thrust of the rotor and establish a fixed axial reference point for rotor expansion. This ensures that the axial clearance between the rotating and stationary parts remains within the design range.
Materials
Tilting Pad Thrust Bearings are typically composed of forged steel backing combined with Babbitt alloy. The Babbitt alloy is designed to begin melting at around 110°C, which provides additional axial displacement space for the rotor, thus protecting it from dry friction damage caused by excessive axial movement. Other common materials include tin bronze and steel backing with PTFE (polytetrafluoroethylene). PTFE-based pads have a low friction coefficient, are suitable for higher temperatures, do not require scraping during assembly, and are less likely to fail due to lubrication loss.
Application
In steam turbines, Tilting Pad Thrust Bearings are used to determine the axial position of the rotor within the cylinder. Under normal operating conditions, the active thrust pads bear the load to prevent the rotor from shifting toward the generator side. However, during load reduction or sudden load rejection, the rotor may tend to shift toward the turbine inlet due to inertia. In such cases, the non-active thrust pads take the load, maintaining the rotor's axial position within the cylinder. These thrust pads are categorized into forward and reverse thrust pads.
In hydro-generators, Tilting Pad Thrust Bearings support the entire weight of the rotating assembly as well as the axial hydraulic forces, transferring these loads to the bearing frame. The thrust bearing assembly includes a thrust collar, mirror plate, thrust pads, bearing housing, oil bath, and cooler. The thrust collar and mirror plate rotate with the shaft, while the thrust pads-typically sector-shaped-are supported by the bearing housing. The oil bath is filled with turbine oil, which serves both as a lubricant and a heat transfer medium. Heat generated by friction between the thrust pads and mirror plate is absorbed by the oil and carried away by water-cooled heat exchangers.
Manufacturing and Precision Requirements
Tilting Pad Thrust Bearings must operate under reliable oil lubrication conditions. The minimum oil film thickness on the trailing edge must meet design specifications (typically 0.03 to 0.07 mm for large units). Therefore, the mirror plate must have high dimensional accuracy and low surface roughness. Any scratches, rust, or defects on the mirror plate can disrupt the oil film and lead to bearing damage or failure.
Precision grinding of the mirror plate and scraping of the thrust pads are critical procedures. The flatness of the mirror plate is typically required to be within 0.02 mm per meter, and the height difference between thrust pads is usually kept within 0.02 mm. The flatness of both the thrust pads and the mirror plate must be closely matched. If either surface deviates beyond the minimum oil film thickness, the oil film may break down, resulting in mixed or dry friction, which could cause overheating or surface damage.
The thrust pad's load-bearing capacity is also closely related to its flatness and surface contact. A larger contact area allows the pad to withstand higher pressures. Uneven surfaces with local high spots may lead to stress concentration and eventual damage such as pad burnout or severe wear.
Grinding the mirror plate and scraping the thrust pads are essential operations to ensure proper contact, rapid oil film formation at startup, and stable, reliable operation during service.
