Glossary term
Rotor Dynamics
The study of vibration, stability, critical speeds, bearing forces, and dynamic response in rotating machinery.
Definition
termRotor dynamics is the study of vibration, stability, and dynamic forces in shafts, disks, bearings, seals, and other rotating machinery.
Rotor dynamics predicts how rotating assemblies respond to unbalance, bearing stiffness, damping, gyroscopic effects, fluid-film forces, rubs, misalignment, thermal growth, and speed changes. It is essential for turbines, compressors, pumps, motors, generators, spindles, turbochargers, flywheels, and aircraft engines.
Rotor dynamics treats a rotating machine as a coupled system of rotor mass and stiffness, bearings, supports, seals, casings, foundations, and fluid forces. The same shaft can behave differently at different speeds because gyroscopic effects, bearing coefficients, damping, and mode shapes change with rotation.
Key phenomena include critical speeds, synchronous vibration, subsynchronous whirl, oil whirl, oil whip, forward and backward modes, shaft bow, rub, instability, and response to residual unbalance. A critical speed occurs when a running speed intersects a rotor mode strongly enough to create high vibration. Machines may be designed to operate below the first critical speed, between critical speeds, or above one or more critical speeds if acceleration through resonance is controlled.
Engineering analysis
Rotor-dynamic models often use beam elements, lumped disks, bearing stiffness and damping coefficients, seal coefficients, and unbalance distributions. Campbell diagrams show how natural frequencies vary with rotational speed and where they intersect excitation orders. Unbalance-response analysis predicts vibration amplitude and phase across run-up and coast-down.
Measurements commonly include shaft proximity probes, casing accelerometers, tachometer phase reference, orbit plots, Bode plots, waterfall spectra, and order tracking. Interpreting the data requires distinguishing imbalance from misalignment, looseness, rub, bearing defects, aerodynamic excitation, electrical forcing, and structural resonance.
Design and operation
Rotor dynamics affects bearing selection, balance quality, seal design, coupling alignment, operating-speed range, startup procedure, trip limits, foundation stiffness, and maintenance strategy. High-speed machines need margins between operating speed and critical speeds, stable bearing behaviour across temperature and load, and clear acceptance criteria for vibration amplitude and phase.
Common mistakes
A common mistake is assuming that static balance alone guarantees smooth operation. Another is treating bearing supports as rigid when their stiffness and damping dominate the mode shape. A strong rotor-dynamics review states rotor geometry, bearing data, support stiffness, speed range, damping assumptions, excitation orders, critical-speed margins, balance condition, thermal growth allowance, and measured vibration evidence.