Limit Cycles for Pad Bearings Under Fluid Excitation

Abstract

Pad bearings for turbine rotors are known to exhibit unstable limit cycles due to steam flow excitation in the power generation stages. To simulate the rotor-bearing system, static and dynamic behavior of a tilting pad have been modeled by means of phenomenological laws expressed as nonlinear function of pivot distance and the time derivative. These functions have been obtained experimentally for some common pad geometries.

The pad functions are further assembled to express the static and dynamic behavior of tilting pad bearings in functional form. Popular linear properties, such as spring and damping constants can be obtained also by differentiation. The functional description of individual pads offers: a) a great reduction in the variety of pad bearing geometries and properties which need to be tabulated, b) feasibility of inexpensive experimental determination of the essential data, and c) a convenient utilization in large numerical rotor dynamic analysis programs, as compared with the on-line numerical methods for tilting pads bearing analysis.

This description of the bearings is used in the system simulation under rotor excitation due to load and displacement dependent forces at the power generation stages due to steam flow. Jump phenomena, of the Duffing Equation type, have been demonstrated. Limit cycles exist which at a certain load jump to much higher amplitudes.

Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987

Notes

Presented at the 42nd Annual Meeting in Anaheim, California May 11–14, 1987