ArticleWhirl Motion of A Seal Test Rig With Squeeze-Film Dampers
by Edgar J. Gunter, Ph.D. and Margaret P. Proctor
This paper presents the experimental behavior and dynamic analysis of a high speed test rig with rolling element bearings mounted in squeeze film oil damper bearings. The test rig design was intended for operation at over 40,000 RPM. High vibration was experienced during attempts to bring the test rig to speed. The paper discusses vibration analysis, critical speed analysis, dynamic analysis with the original squeeze film damper design, non-linear synchronous unbalance response with the original squeeze film design, time transient analysis with the original squeeze film damper and dynamic analysis with the modified squeeze film damper.
The NASA high temperature, high speed seal rig is designed to test seals over a range of conditions including conditions expected in advanced gas turbine engines. The double overhung rotor has an 8.5 inch seal test disk and is supported by rolling element bearings in squeeze film dampers. The maximum design speed of 43,140 rpm could not originally be achieved due to the occurrences of whirling at the seal test disk and high vibrations at the spline connection to the drive shaft. There were indications of both sub- and super-harmonic whirl motion at the seal test disk and high synchronous response at the balance piston and drive spline. Experimental data indicated both a critical speed and whirling problem with the rig. A nonlinear jump response region was observed between 10,000 to 15,000 RPM with superharmonic excitation. This region is referred to as dead band whirling and occurs even with a well balanced system.
The precise design of squeeze film dampers is complicated by the highly nonlinear nature of the dampers. Initial linear analysis of critical speeds and optimum damping for various assumed bearing stiffness values serve as a useful guide. The original damper was designed with the feature that the damper length could be easily extended if needed. The critical speed analysis of the complete system including the drive train showed that the first two critical speeds are associated with the seal test rotor. Nonlinear synchronous unbalance and time transient whirl simulations were computed for the seal test rotor with the original dampers to simulate the whirling and high synchronous response observed. With the original damper design, the nonlinear synchronous response showed that unbalance could cause damper lockup at 33,000 rpm. Alford cross-coupling forces were also included at the overhung seal test disk for the whirl analysis. Subsynchronous whirling at the seal test disk was observed in the nonlinear time transient analysis. With the extended damper length of 0.50 inch, the subsynchronous motion was eliminated and the rotor unbalance response was acceptable to 45,000 rpm with moderate rotor unbalance. However, the nonlinear analysis shows that with high rotor unbalances, damper lockup could also be obtained at 33,000 rpm, even with the extended squeeze-film dampers. Therefore, the test rotor must be reasonably balanced in order for the uncentered dampers to be effective.”
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