In a vibration spectrum analysis, which fault is commonly identified by a characteristic frequency related to rotor speed?

• A. Bearing defect
• B. Oil contamination
• C. Unbalance
• D. Misalignment

Answer: (C)

When a rotor is unbalanced, an uneven mass distribution creates a centrifugal force that repeats once every revolution. That regular, rotating force drives vibration at the same frequency as the shaft speed (1x). In a vibration spectrum, this shows up as a dominant line at the rotor-speed frequency, and its amplitude tends to increase with speed because the centrifugal force grows with how fast the shaft is spinning. This clean, single-frequency signature at 1x is the hallmark of unbalance.

Other faults produce different spectral patterns. Bearing defects generate lines at specific frequencies tied to bearing geometry (such as cage or race frequencies), not just the shaft speed. Misalignment can also show 1x but usually accompanied by additional energy at harmonics like 2x and other characteristic patterns. Oil contamination doesn’t produce a crisp rotor-speed line in the spectrum.