Increase of Operational Safety of Tilting-Pad Journal Bearings by Extraction of Hot Oil at the Trailing Edge

Abstract

A theoretical study is presented with the main objective on the operational safety parameters (minimum film thickness and maximum pad temperature) and thermomechanical deformations of a ø500 mm rocker pad tilting-pad journal bearing (TPJB) for application in large turbo machinery. It can be described by the following specifications: Five pads, 0.23 nominal preload, 60% offset, 56° pad arc angle, 350 mm pad length and 1.28‰ relative bearing clearance. Theoretical investigations are carried out for circumferential speeds up to 78 m/s and static loads up to 3.60 MPa. The simulation tool simultaneously solves both Reynolds and energy equations for the oil film (3D temperature distribution) on the one hand and computes thermomechanical deformations of the pad on the other hand. The simulations are conducted for a single pad and are supported by boundary conditions taken from experiments. The results with regard to static bearing characteristics and pad deformation show good agreement with experiments. The impact of axial pad arching on operational safety parameters and load-carrying capacity are shown and compared to experimental results. It is shown that the axial deviation in film thickness Δh can be even higher than the minimum film thickness hmin. This leads to reduced hydrodynamic pressure build-up towards the axial edges and therefore significantly decreased safety parameters or load-carrying capacity. In order to reduce pad crowning, radial bores through the pad body are modelled to simulate the extraction of hot oil from the trailing edge. In the simulation, the hot oil is used to heat up the back of the pad for a decrease of radial temperature gradients and thus pad arching. It is shown that by extracting 0.4 l/s of hot oil, a decrease in axial pad crowning from Δh = 47μm to Δh = 26μm can be achieved and that this leads to a decrease of 7.8 K in maximum temperature and an increase of 5 μm in minimum film thickness respectively a gain of load-carrying capacity of 0.4–0.6 MPa.