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.

Critical speed occurs when the rotational frequency of a shaft matches its natural resonant frequency. This harmonic synchronism causes amplification of vibration amplitudes, potentially leading to catastrophic fatigue failure. Marine engineering standards require machinery to operate outside these resonant bands to maintain structural integrity and prevent excessive bearing wear.