CVA Resonance & Structural Analysis 2

Question 1

What is operational deflection shape (ODS) analysis, and how does it differ from modal analysis?

Accepted Answer

ODS shows actual deformation under real operating forces at specific frequencies; modal analysis shows theoretical mode shapes from controlled excitation tests

Answer

ODS is performed on rotating shafts; modal analysis is only applicable to stationary structures

Answer

ODS provides damping ratios; modal analysis only provides natural frequencies

Answer

ODS requires force measurement; modal analysis does not

Question 2

What is structural resonance a common cause of in rotating machinery?

Accepted Answer

Excessive vibration at a running speed that excites a structural natural frequency, resulting in damage or fatigue

Answer

Bearing defect frequency amplification leading to false positive diagnostics

Answer

Rotor unbalance appearing at multiple harmonics simultaneously

Answer

Misalignment producing high axial vibration

Question 3

What technique is commonly used to excite structural resonances for impact testing?

Accepted Answer

Impact from an instrumented hammer with a force transducer built into the tip

Answer

Continuous sinusoidal excitation with an electrodynamic shaker at fixed frequency

Answer

Rotor unbalance at a specific trial weight location

Answer

Broadband random noise from a loudspeaker mounted near the structure

Question 4

What does the half-power (3 dB) bandwidth method measure?

Accepted Answer

The bandwidth around a resonant peak used to estimate the damping ratio of the system

Answer

The frequency range over which an accelerometer responds within 3 dB of its reference sensitivity

Answer

The frequency span of bearing defect sidebands around the carrier frequency

Answer

The usable frequency range of an anti-aliasing filter

Question 5

What is transmissibility, and why is it important for vibration isolation design?

Accepted Answer

The ratio of output vibration to input vibration across a vibration isolator as a function of frequency

Answer

The efficiency with which energy is transmitted from a bearing defect to the measurement transducer

Answer

The ratio of structural damping to viscous damping in a support structure

Answer

The fraction of rotor unbalance force transmitted to the bearing housing

Question 6

What is the primary purpose of adding mass or stiffness to a structure to address a resonance problem?

Accepted Answer

To shift the natural frequency away from the excitation frequency to detune the resonance

Answer

To increase the amplitude at resonance, making diagnostics easier

Answer

To add damping to the structure, reducing the resonant peak height

Answer

To decrease the operating speed of the machine to avoid resonance