GAS TURBINE THRUST BEARING DESIGN FAILURE BASIC INFORMATION AND TUTORIALS

Factors Affecting Thrust-Bearing Design

The principal function of a thrust beating is to resist the thrust unbalance developed within the working elements of a turbomachine and to maintain the rotor position within tolerable limits.

After an accurate analysis has been made of the thrust load, the thrust bearing should be sized to support this load in the most efficient method possible. Many tests have proven that thrust bearings are limited in load capacity by the strength of the babbitt surface in the high load and temperature zone of the beating.

In normal steel-backed babbitted tilting-pad thrust bearings, this capacity is limited to between 250 and 500 psi (17 and 35 Bar) average pressure. It is the temperature accumulation at the surface and pad crowning that cause this limit.

The thrust-carrying capacity can be greatly improved by maintaining pad flatness and by removing heat from the loaded zone. By the use of high thermal conductivity backing materials with proper thickness and proper support, the maximum continuous thrust limit can be increased to 1000 psi or more.

This new limit can be used to increase either the factor of safety and improve the surge capacity of a given size bearing or reduce the thrust beating size and consequently the losses generated for a given load.

Since the higher thermal conductivity material (copper or bronze) is a much better beating material than the conventional steel backing, it is possible to reduce the babbitt thickness to .010-.030 of an inch (.254-.762 mm). Embedded thermocouples and RTDs will signal distress in the beating if properly positioned.

Temperature monitoring systems have been found to be more accurate than axial position indicators, which tend to have linearity problems at high temperatures. In a change from steel-backing to copper-backing a different set of temperature limiting criteria should be used..