Condition Monitoring & Diagnostics Flashcards

On a center-hung fan, horizontal 1x phase readings at the inboard and outboard bearings are nearly identical (within about 30°). Which fault does this most strongly suggest?

Answer: Static (force) imbalance

Static imbalance produces in-phase 1x radial motion at both bearings because the heavy spot pulls both ends in the same direction.

Which spectral and phase pattern is most characteristic of angular misalignment across a flexible coupling?

Answer: High 1x axial with roughly 180° axial phase difference across the coupling

Angular misalignment creates a bending moment that drives strong 1x axial vibration out of phase across the coupling.

A pump base shows many running-speed harmonics plus 0.5x subharmonics, and the time waveform is erratic and non-repeating. What is the most likely cause?

Answer: Rotating looseness (Type C) such as an oversized bearing bore fit

Type C rotating looseness produces truncated, non-linear response with many harmonics and half-order components.

A machine with sleeve bearings shows a peak at 0.44x running speed that tracks speed as RPM changes. Which condition is indicated?

Answer: Oil whirl

Oil whirl occurs at roughly 0.40-0.48x shaft speed in fluid-film bearings and follows running speed.

On an AC induction motor fed at 60 Hz, a dominant peak at 120 Hz that disappears instantly when power is cut points to what type of problem?

Answer: Electrical problem such as stator eccentricity or soft-foot-induced air gap variation

Vibration at twice line frequency that vanishes the instant power is removed is electromagnetic in origin.

A bent shaft is suspected on an overhung rotor. Which observation best supports this diagnosis over simple imbalance?

Answer: High 1x axial vibration with about 180° axial phase between the two bearings

A bent shaft causes strong axial 1x motion with opposing axial phase, which simple imbalance does not produce.

In motor current signature analysis, broken rotor bars are indicated by which feature?

Answer: Sidebands around line frequency spaced at pole pass frequency

Broken bars modulate current, producing sidebands around line frequency at the pole pass frequency (slip × number of poles).