CVA Rotor Dynamics & Balancing 2 — Questions and Answers
Question 1: What is the purpose of a Keyphasor® signal in rotor dynamics analysis?
- To measure radial bearing clearance
- To provide a once-per-revolution phase reference for synchronous vibration analysis (Correct answer)
- To trigger the anti-aliasing filter in the data acquisition system
- To measure shaft temperature during operation
Correct 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.
Question 2: What is the difference between a rigid rotor and a flexible rotor in balancing?
- Rigid rotors operate below the first critical speed; flexible rotors operate at or above it, requiring multi-plane balancing (Correct answer)
- Rigid rotors require only static balancing; flexible rotors require dynamic balancing in two planes
- Rigid rotors are balanced at operating speed; flexible rotors are balanced at rest
- Rigid rotors use proximity probes; flexible rotors use accelerometers for measurement
Correct 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.
Question 3: What is 'residual unbalance' in rotor balancing?
- The unbalance remaining in the rotor after the balancing procedure is complete (Correct answer)
- The component of unbalance that cannot be measured by standard instrumentation
- The unbalance created by thermal growth during machine operation
- The unbalance correction applied during the final trim balance
Correct 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).
Question 4: Oil whirl is a self-excited vibration that occurs in journal bearings at approximately what frequency relative to running speed?
- 1× running speed
- 0.43–0.48× running speed (Correct answer)
- 2× running speed
- Bearing natural frequency
Correct 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.
Question 5: What is a Bode plot used for in rotor dynamics?
- Displaying unbalance response (amplitude and phase) as a function of rotational speed (Correct answer)
- Plotting bearing stiffness vs. load for hydrodynamic bearings
- Showing the Campbell diagram of rotor natural frequencies
- Comparing vibration spectra from two different measurement planes
Correct 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.
Question 6: What is the polar (Nyquist) plot in rotor vibration analysis?
- A plot of spectral amplitude in polar frequency coordinates
- A plot of the 1× vibration vector (amplitude and phase) traced as shaft speed changes, shown on a polar coordinate graph (Correct answer)
- A map of bearing clearances measured at multiple angular positions
- A frequency response function plotted in magnitude and phase format
Correct 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.
CVA Rotor Dynamics & Balancing 2
What is the purpose of a Keyphasor® signal in rotor dynamics analysis?