Machinery and Equipment Troubleshooting Flashcards

On a US motor, a strong 120 Hz peak vanishes the instant power is cut, while running-speed peaks decay gradually. What does this indicate?

Answer: An electrical source of vibration

Electrically induced vibration at 2× line frequency (120 Hz in the US) disappears the moment power is removed.

A 4-pole, 60 Hz induction motor runs at 1,780 RPM. What is the pole pass frequency?

Answer: About 1.33 Hz

Slip frequency = (1,800 − 1,780)/60 = 0.333 Hz, and pole pass frequency = slip × poles = 1.33 Hz.

Pole pass frequency sidebands around 1X and its harmonics on an induction motor most commonly indicate what?

Answer: Broken or cracked rotor bars

Rotor bar problems modulate the 1X response at pole pass frequency, producing characteristic sidebands.

A centrifugal pump with a 6-vane impeller runs at 3,550 RPM. What is the vane pass frequency?

Answer: About 355 Hz

VPF = vanes × running speed = 6 × 59.2 Hz ≈ 355 Hz.

Which spectral feature is most characteristic of pump cavitation?

Answer: Random broadband high-frequency energy, often with a raised noise floor

Collapsing vapor bubbles create random, broadband high-frequency energy rather than discrete peaks.

Belt frequency in a belt-driven machine typically appears where in the spectrum?

Answer: Below the running speed of both sheaves

Belt frequency = π × sheave diameter × RPM / belt length, which is subsynchronous to both shafts.

An eccentric sheave on a belt drive typically produces what vibration signature?

Answer: High 1X of the eccentric sheave, highest in line with the belts

An eccentric sheave creates a once-per-revolution belt tension variation, so 1X is strongest in the direction of the belts.