Rotor Dynamics & Balancing Flashcards

What is the purpose of a Keyphasor® signal in rotor dynamics analysis?

Answer: To provide a once-per-revolution phase reference for synchronous vibration analysis

A Keyphasor (once-per-revolution trigger) provides a phase reference that allows analysts to measure the phase angle of vibration components relative to a fixed shaft position, essential for balancing and diagnostics.

What is the difference between a rigid rotor and a flexible rotor in balancing?

Answer: Rigid rotors operate below the first critical speed; flexible rotors operate at or above it, requiring multi-plane balancing

A rigid rotor operates well below its first critical speed and maintains its shape during operation; a flexible rotor operates near or above critical speeds and bends into mode shapes that require mode-based multi-plane balancing.

What is 'residual unbalance' in rotor balancing?

Answer: The unbalance remaining in the rotor after the balancing procedure is complete

Residual unbalance is the amount of unbalance that remains in a rotor after balancing; it must be within the tolerance specified by the applicable balance quality standard (e.g., ISO 1940).

Oil whirl is a self-excited vibration that occurs in journal bearings at approximately what frequency relative to running speed?

Answer: 0.43–0.48× running speed

Oil whirl is a hydrodynamic instability in journal bearings where the oil film wedge circulates at approximately 43–48% of shaft rotational speed, producing a sub-synchronous vibration component.

What is a Bode plot used for in rotor dynamics?

Answer: Displaying unbalance response (amplitude and phase) as a function of rotational speed

A Bode plot simultaneously displays the amplitude and phase of a synchronous (1×) vibration component versus shaft speed, making critical speeds and phase shifts clearly visible during run-up or coast-down tests.

What is the polar (Nyquist) plot in rotor vibration analysis?

Answer: A plot of the 1× vibration vector (amplitude and phase) traced as shaft speed changes, shown on a polar coordinate graph

A polar plot traces the tip of the 1× vibration vector on polar coordinates as shaft speed varies, creating a loop whose shape reveals critical speeds, modal behavior, and the quality of the balance correction.