Shifting Critical Speeds Out Of The Operating Range By Changing From Tilting Pad To Sleeve Bearings.

Abstract

One property of heavily to moderately loaded sleeve bearings is the resulting asymmetric stiffness and damping coefficients. These asymmetric properties often result in split first critical speeds. Since the horizontal stiffness is much softer than the vertical, a horizontal first critical speed may appear from several hundred to several thousand revolutions per minute lower than the vertical first critical. Tilting pad bearings produce more symmetric bearing properties. This symmetry usually results in a single unsplit first critical speed that is located approximately midway between the sleeve bearing split peaks. Two different case histories are examined. An induction motor and a steam turbine were both initially designed with tilting pad bearings. Both rotor bearing systems resulted in first critical speeds in the operating speed range. Numerous attempts to shift the critical out of the operating range were explored without success. As a last attempt short of a complete rotor redesign, switching to sleeve bearings was successful. Analytical results that support the decision to switch to sleeve bearings are shown along with actual test stand response plots. Additionally, for the steam turbine, results from a high speed balance are presented along with actual field speed-amplitude plots. While a tilting pad to sleeve bearing change is not recommended for all classes of turbomachinery to shift critical speeds, it is a powerful design tool as it may produce critical speed changes of up to 1000 rpm.