The Critical Speed Analysis calculates the undamped critical speeds, mode shapes and associated kinetic and potential energy distribution, and modal stress. The isotropic system is assumed and the users can select the bearing stiffness direction (Kxx, Kyy, or average) be used in the analysis, in case if the bearing/support is not isotropic. For multiple speed dependent bearing coefficients, the bearing stiffness at the highest speed is used. The flexible support effect is also included in the Critical Speed Analysis.
The undamped critical speeds are determined directly by solving a reduced eigenvalue problem associated with the system equations expressed in a rotating reference frame. These undamped modes are circular relative to the fixed reference frame but are constant relative to the rotating reference frame. Therefore, it is convenient to consider only one of the two planes of motion. This procedure will simplify the calculation and reduce computational time. Due to the simplifying assumptions applied in the undamped critical speed calculation, extreme care must be taken in the preparation and interpretation of these results.
A brief description of the associated run time data for the Critical Speed Analysis is included below:
Spin/Whirl Ratio
This parameter determines the type of critical speeds under calculation. The typical values for the Spin/Whirl Ratio are:
1 = Forward synchronous critical speeds
-1 = Backward synchronous critical speeds
0 = Planar critical speeds for non-rotating systems
2 = Half frequency whirl (Subsynchronous criticals), etc.
The forward synchronous critical speeds and modes are the most commonly calculated due to unbalance excitation.
Number of Modes
This parameter is used to calculate the selected mode shapes. The value of zero indicates that only the critical speeds are calculated in the analysis and no mode output. In practice, only the lowest 3-5 critical speed modes are of importance. For large systems, more mode shapes may be required in the analysis.
Stiffness
The isotropic system is assumed in the undamped critical speed calculation. However, the bearing stiffness, Kxx and Kyy, in the bearing data may not be the same. This option allows you to select the stiffness to be used in the calculation.
For sample outputs, click Critical Speed Analysis PostProcessor.
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