58734 - Mechanical and Thermal Deformation Analysis of a Large Polymer Lined Tilting Pad Journal Bearing 

Tilting pad journal bearings (TPJBs) are used in a wide range of applications in turbomachinery. Rapid changes in the energy sector and a therefore increasing demand for more efficiency and flexibility in power generation lead to new requirements regarding load carrying capacity, safety and durability for TPJBs. Since the bearing diameter is usually determined by the geometry of the rotor or other constraints, the research focus is on increasing the specific bearing load. However, higher specific loads can cause axial and circumferential deflection of the pads and thus limit the load carrying capacity. These pad deformations consist of a mechanical and a thermal component. The mechanically induced pad deformation results from the pressure distribution on the pad surface and can therefore not be prevented. The pad thermally induced deformation is due to large temperature gradients within the pads, which result from the inhomogeneous heat input from the oil film. To reduce the heat input, pad linings with insulating properties can be applied.

The application of thermoplastic polymers as pad lining material in hydrodynamic bearings has proven to be promising due to their higher temperature limits compared to white metal in conventional bearings. In terms of their good slide characteristics, polymers show advantages at start and stop. Due to their low thermal conductivity, they serve as an insulating layer between oil film and pad. Therefore, the heat transfer into the pad is significantly lower. In this context, a reduction of temperature gradients and thus the thermal deformation can be achieved. Apart from that, a polymer liner undergoes a higher mechanical deformation than a white metal liner due to its lower Young’s modulus.

In this paper we present a theoretical approach to determine deformation of large TPJBs with PEEK (polyether ether ketone) polymer lined pads by means of a thermo-elasto-hydrodynamic (TEHD) model. The objective of this investigation is a quantitative analysis of the deformation of a five-pad TPJB with a shaft diameter of 500 mm and a length to diameter ratio of L/D=0.7. The deformation in both the steel backing and the polymer lining are considered separately and the respective mechanically induced and thermally induced deformations are determined. The deformation characteristics calculated with the numerical model are compared to simulation results of an experimentally validated model with a white metal lining. When comparing the results, in particular the influence of the PEEK layer on the temperature distribution in the pad lining and backing is analysed.

To determine the deformation, we developed a 3D structural-mechanics finite element model of a tilting pad with a polymer lining. For a precise estimation of the PEEK layer deformation, the plastic deformation and the temperature-dependent creep behaviour are approximated. The model is coupled with a journal bearing calculation programme (COMBROS-R), which performs a thermo-hydrodynamic calculation based on the simplified Reynolds equation and calculates the 2D oil film thickness, the 2D oil film pressure distribution on the pad surface and a 3D temperature field in the shaft, pad and oil film.

When comparing the deformation of the PEEK polymer lined pads with white metal lined pads, the backing of the PEEK pads show a significantly lower thermal deformation. This results in higher minimum film thickness or, with the same film thickness, a higher load carrying capacity of the polymer lined pads can be achieved. Due to the heat insulating effect of the PEEK layer, however, the oil film temperatures are higher than with the white metal lined pads. Neither plasticity nor creep have a significant influence on the deformation of the PEEK liner.