What is meant by a critical speed for rotating machinery?

Correct Answer: Option C — The critical speed of a shaft is the speed of rotation that is in harmonic synchronism with the shaft's natural vibration frequency which produces excessive vibration.

Under USCG National Maritime Center examination standards, the accepted correct answer is: The critical speed of a shaft is the speed of rotation that is in harmonic synchronism with the shaft's natural vibration frequency which produces excessive vibration..