Lateral Dynamics

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Small radial clearances between rotating and stationary components, as well as the health of surrounding equipment, demand that a machine achieve excellent lateral vibration performance.

The influences of a shaft's surrounding components are often critical in defining a machine's overall lateral dynamics. BRG includes dynamics from the following:

  • Casing, substructure, & pedestal effects

  • Fluid film, active magnetic, and rolling element bearings

  • Electromechanical devices

  • Squeeze film dampers

  • Annular seals

BRG conducts several different types of analysis to determine a machine’s lateral vibration performance:

  • Undamped Critical Speed Analysis

  • Forced Response Analysis

  • Stability Analysis

  • Train & Multilevel Analysis

Lateral Dynamics Analyses

Undamped Critical Speed Analysis

Provides a preliminary estimate of the critical speeds and their sensitivity to support dynamics. In addition, the influence of unbalance distribution as well as component damping can be assessed using the resulting mode shapes.

Undamped mode shapes of steam turbine generator

Undamped critical speed map for an offshore gas injection compressor

Forced Response Analysis

Unbalance response analysis provides the best estimate of critical speed locations and vibration amplitudes. It is crucial for the assessment of separation margins, vibration limits and rubbing risks. Response due to nonsynchronous forcing phenomena can also be determined.

Measured vs. predicted unbalance response of a machine going through its first critical speed

Stability Analysis

A vital analysis for avoiding self-excited instabilities, such as oil whirl and shaft whip. Damped eigenvalues are calculated to determine individual modes' log decrement which indicates their stability level. Influences of operation and components' condition are examined to assess the machine's stability robustness. Synchronous instability due to the Morton effect phenomenon within oil lubricated bearings and seals can also be diagnosed and mitigated using a different type of stability analysis.

Base stability measurements versus predictions for a machine supported by tilting pad journal bearings

API stability sensitivity analysis of an offshore gas injection compressor

Stability spectrum of a subsea compressor supported by active magnetic bearings

Train & Multilevel Analysis

Dynamic interactions between different rotors or rotors with their stators can be significant in some machinery. Long coupling spacers, rigidly coupled trains and flexible casing structures demand these multilevel analyses.

Damped mode shape of a propylene refrigeration compressor train with long coupling spacer

Transient & Nonlinear Analysis

Events such as blade-loss, rubs, rotor drops, and deceleration through critical speeds require analysis in the time domain. Such events and their resulting motions can also be nonlinear. Transient analysis provides a useful design and diagnostic tool for these complex vibrations.

Orbits from a seal rub simulation showing the generation of half frequency whirl