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This paper examines the effect of squeeze-film damper bearings on the steady state and transient unbalance response of aircraft engine rotors. The nonlinear effects of the damper are examined, and the variance of the motion due to unbalance, static pressurization, retainer springs, and rotor preload is shown. The nonlinear analysis is performed using a time-transient method incorporating a solution of the Reynolds equation at each instant in time. The analysis shows that excessive stiffness in the damper results in large journal amplitudes and transmission of bearing forces to the engine casing which greatly exceed the unbalance forces. Reduction of the total effective bearing stiffness through static pressurization and rotor preload is considered. The reduction in stiffness allows the damping generated by the bearing to be more effective in attenuating rotor forces. It is observed that in an unpressurized damper, the dynamic transmissibility will exceed unity when the unbalance eccentricity exceeds approximately 50 percent of the damper clearance for the relatively wide range of conditions examined in this study.

This paper presents a rapid approximate method for calculating the optimum bearing or support damping for multimass flexible rotors to minimize unbalance response and to maximize stability in the vicinity of the rotor first critical speed. A multimass rotor is represented by an equivalent single-mass model for purposes of the analysis. The optimum bearing damping is expressed as a function of the bearing stiffness and rotor modal stiffness at the rigid bearing critical speed. Stability limits for aerodynamic cross coupling and viscous internal rotor friction damping are also presented. Comparison of the optimum damping obtained by this approximate method with that obtained by full scale linearized transfer matrix methods for several rotor-bearing configurations shows good agreement. The method has the advantage of being quickly and easily applied and can reduce analysis time by eliminating a time consuming search for the approximate optimum damping using more exact methods.

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An experimental study of the effects of bearing support flexibility on rotor stability and unbalance response is presented. A flexible rotor supported by fluid film bearings on flexible supports was used with fifteen support configurations. The horizontal support stiffness was varied systematically while the vertical stiffness was kept constant. The support characteristics were determined experimentally by measuring the frequency response functions of the support structure at the bearing locations. These frequency response functions were used to calculate polynomial transfer functions that represented the support structure. Stability predictions were compared with measured stability thresholds. The predicted stability thresholds agree with the experimental data within a confidence bound for the logarithmic decrement of ±0.01. For unbalance response, the second critical speed of the rotor varied from 3690 rpm to 5200 rpm, depending on the support configuration. The predicted first critical speeds agree with the experimental data within −1.7 percent. The predicted second critical speeds agree with the experimental data within 3.4 percent. Predictions for the rotor on rigid supports are included for comparison.

The predicted performunce of a three-lobe journal bearing with a and slope of the coeflcients vs. Sommerfeld number match well. D k preload factor of 0.75 is compared with the measured performance. agreement in coeflcient magnitude by as much as 100% is seen at Operating eccentricity and dynamic coeflcients versus Sommerfeld high Sommerfeld numbers, although there is general agreement in the number are compared for three shaft speeds and various steady loads. trend for predicted coeflcients vs. Sommerfeld number. Numerical results are based on a model which solves the Rtynoldc equation and allows for a variety of thermal effects including cir-WORDS cumferential and mss-~ilm viscosity and temperature variation. The linearized coeflciats are experimentally determined by an average Hydrodynamic Bearings, Journal Bearings, ~u l t i -k b e magnitude and phase method using synchronous, sinusoidal exciBearings, Hydrodynamic Lubrication tations. Comparisons of journal position show good agreement (within 9.5 %) for Somme$eld numbers below 0.7 and increasing disagreement (30 % or greater) at higher Sommerfeld numbers. Agreement in journal position improves as load and shaft speed increase. At Sommerfeld numbers less than 0.4 to 0.7, agreement between the numm'cal and experimental dynamic coeflcients is very good, typically within the uncertainty of the measured data. The magnitude