Design Considerations For Thrust Bearing Applications.

Tutorialpg. 223-242The hydrodynamic thrust bearing load capacity-speed envelope is divided into three major regions by speed. Each region has its own set of limiting physical phenomenon that must be included in the analysis and presents a design challenge. For instance, the high speed region has high lubricant shear rates possibly including turbulence that change pad thermal crowning and local babbitt temperature and pressure, thus limiting the bearing performance in terms of load and speed. Various design tools are used to set the bearing geometry such that the phenomenon limits are not exceeded, and an appropriate design safety factor is present. Several applications are presented that illustrate how the various design modifications such as pivot type, pivot radial and circumferential offset, pad spacing, and pad thickness may be varied to design a thrust bearing for a specific region. Three dimensional finite element models for the fluid film generate film thickness, pressure and temperature distributions, which when coupled with a pad finite element model for thermal conduction, convection, thermal deflection, and elastic deflection show thrust bearing pad performance. Turbulence is included along with hot oil carryover from the previous pad. Methods are shown that reduce vibratory motion by using the squeeze film damping attributes of double thrust bearings. The benefits of offset pivot, chrome copper pads, and controlled lubrication in a load-speed region are reviewed so that the user has an appreciation of how each feature is affecting his thrust bearing's performance. Of primary interest is an example of a high speed application where controlled lubrication increased machine efficiency by minimizing parasitic losses. Three dimensional maps of velocity, fluid mixing, temperature, and viscous heating in the pocket result in two dimensional temperature fields at the pad entrance. Accurate determination of the pad entrance temperature distribution allows for calculation of the pad maximum temperatures or bearing capacity. The examples and results presented will be of use to rotating equipment manufacturers and machinery users in applying thrust bearings to their machines