CVA Resonance & Structural Analysis 2 β Questions and Answers
Question 1: What is operational deflection shape (ODS) analysis, and how does it differ from modal analysis?
- ODS shows actual deformation under real operating forces at specific frequencies; modal analysis shows theoretical mode shapes from controlled excitation tests (Correct answer)
- ODS is performed on rotating shafts; modal analysis is only applicable to stationary structures
- ODS provides damping ratios; modal analysis only provides natural frequencies
- ODS requires force measurement; modal analysis does not
Correct answer: ODS shows actual deformation under real operating forces at specific frequencies; modal analysis shows theoretical mode shapes from controlled excitation tests
ODS shows how a structure actually moves under its real operating loads at specific frequencies or time instants, while modal analysis isolates the intrinsic dynamic properties (natural frequencies, mode shapes, damping) from controlled excitation tests.
Question 2: What is structural resonance a common cause of in rotating machinery?
- Bearing defect frequency amplification leading to false positive diagnostics
- Excessive vibration at a running speed that excites a structural natural frequency, resulting in damage or fatigue (Correct answer)
- Rotor unbalance appearing at multiple harmonics simultaneously
- Misalignment producing high axial vibration
Correct answer: Excessive vibration at a running speed that excites a structural natural frequency, resulting in damage or fatigue
When a machine's running speed or a harmonic of it coincides with a structural natural frequency (of the baseplate, piping, or casing), resonance amplifies vibration to levels that can cause fatigue cracking or accelerated bearing wear.
Question 3: What technique is commonly used to excite structural resonances for impact testing?
- Continuous sinusoidal excitation with an electrodynamic shaker at fixed frequency
- Impact from an instrumented hammer with a force transducer built into the tip (Correct answer)
- Rotor unbalance at a specific trial weight location
- Broadband random noise from a loudspeaker mounted near the structure
Correct answer: Impact from an instrumented hammer with a force transducer built into the tip
An instrumented impact hammer with a built-in force transducer simultaneously measures the applied force and triggers the response measurement, providing the force spectrum needed to calculate the FRF.
Question 4: What does the half-power (3 dB) bandwidth method measure?
- The frequency range over which an accelerometer responds within 3 dB of its reference sensitivity
- The bandwidth around a resonant peak used to estimate the damping ratio of the system (Correct answer)
- The frequency span of bearing defect sidebands around the carrier frequency
- The usable frequency range of an anti-aliasing filter
Correct answer: The bandwidth around a resonant peak used to estimate the damping ratio of the system
The half-power bandwidth method measures the frequency width at 0.707 (β3 dB) of the peak amplitude in the FRF; the damping ratio is approximately ΞΆ = Ξf/(2fn), where Ξf is the bandwidth and fn is the natural frequency.
Question 5: What is transmissibility, and why is it important for vibration isolation design?
- The ratio of output vibration to input vibration across a vibration isolator as a function of frequency (Correct answer)
- The efficiency with which energy is transmitted from a bearing defect to the measurement transducer
- The ratio of structural damping to viscous damping in a support structure
- The fraction of rotor unbalance force transmitted to the bearing housing
Correct answer: The ratio of output vibration to input vibration across a vibration isolator as a function of frequency
Transmissibility (T = Xout/Xin) describes how much vibration passes through an isolator; it is greater than 1 near isolator resonance and less than 1 above β2 Γ the isolator's natural frequency, defining the effective isolation region.
Question 6: What is the primary purpose of adding mass or stiffness to a structure to address a resonance problem?
- To increase the amplitude at resonance, making diagnostics easier
- To shift the natural frequency away from the excitation frequency to detune the resonance (Correct answer)
- To add damping to the structure, reducing the resonant peak height
- To decrease the operating speed of the machine to avoid resonance
Correct answer: To shift the natural frequency away from the excitation frequency to detune the resonance
Adding stiffness increases the natural frequency (fn β βk) while adding mass decreases it (fn β 1/βm); either change shifts the natural frequency away from the troublesome excitation frequency.
CVA Resonance & Structural Analysis 2
What is operational deflection shape (ODS) analysis, and how does it differ from modal analysis?