What is the purpose of a vibration spectrum (FFT) analysis in machinery maintenance?

• A. To identify the color of surfaces.
• B. To measure temperature change over time.
• C. To measure oil viscosity.
• D. To identify characteristic frequencies of faults (unbalance, misalignment, bearing defects) by transforming time-domain vibration data into frequency domain.

Answer: (D)

Vibration spectrum analysis uses the FFT to turn time-based vibration data into a frequency-domain picture. By watching how much energy appears at different frequencies, you can spot patterns tied to specific faults. For example, unbalance shows up as a strong vibration at the rotor’s running speed, misalignment can create harmonics of that speed, and bearing defects produce characteristic frequencies determined by the bearing geometry. Identifying these peaks lets you diagnose likely issues and plan maintenance before a failure.

Color of surfaces, temperature changes, or oil viscosity relate to different aspects of machinery health and don’t reveal the dynamic fault patterns that vibration data can.