Machinery and Equipment Troubleshooting Flashcards

Which test is used to find a structure's natural frequencies with the machine shut down?

Answer: Impact (bump) test

An impact test excites the structure with a hammer, and the response spectrum shows its natural frequencies.

Which plot is most useful for spotting a resonance from a coastdown test?

Answer: Bode plot of amplitude and phase versus speed

A Bode plot shows the amplitude peak and phase shift as speed passes through a critical speed.

High 1X vibration persists after balancing, and amplitude changes sharply when speed changes slightly. What should the analyst suspect?

Answer: Operating at or near a resonance

Large amplitude changes from small speed changes mean a natural frequency is amplifying the response near running speed.

Comparing vertical phase readings on a machine foot, its base plate, and the foundation, a large phase difference between foot and base most likely indicates what?

Answer: Looseness or a cracked joint between foot and base

Components bolted solidly together move in phase, so a large phase difference across a joint points to looseness there.

Vibration on a motor-pump set is acceptable at cold startup but rises steadily as the machine reaches operating temperature. What is a likely cause?

Answer: Thermal growth causing misalignment

Unequal thermal growth changes shaft alignment as temperatures stabilize, raising misalignment vibration.

A centrifugal pump operating well below its best efficiency point shows random low-frequency vibration and pulsation. What is the most likely cause?

Answer: Internal recirculation

Operating far below BEP causes flow recirculation, which produces random, unsteady low-frequency vibration.

A truncated or clipped time waveform with subharmonic peaks (such as 1/2X or 1/3X) most likely indicates what?

Answer: A rotor rub

Rotor-to-stator rub creates truncated waveforms and subharmonics as contact limits motion.