Question 58 QMED01 - Junior Engineer

The Correct Answer is B **Explanation for Option B (Critical speed):** "Critical speed" refers to an operational rotational speed (RPM) of an engine or rotating machinery where the natural vibrational frequency of the rotating component matches the frequency of the rotational speed. When these frequencies synchronize, resonance occurs. Resonance causes a significant amplification of vibration, often resulting in excessive, potentially destructive, engine vibration. Therefore, the critical speed is the speed at which excessive engine vibration is created due to resonance. **Explanation of Incorrect Options:** * **A) Non-harmonic speed:** This term is not standard terminology for engine operational limitations. It vaguely implies a speed that is not a multiple of some fundamental frequency, but it does not specifically define the resonance point where vibration becomes excessive. * **C) Maximum speed:** This generally refers to the highest speed the engine is designed to operate safely, often limited by mechanical stress, thermal limits, or operational requirements. While excessive vibration might occur near or at maximum speed due to other factors, "critical speed" specifically denotes the resonant speed where vibration peaks dramatically. The maximum operating speed and the critical speed are not necessarily the same. * **D) Design maximum speed:** Similar to "Maximum speed," this refers to the engineering limit or tested top speed of the engine. It defines the operating envelope but does not specifically identify the resonant point (critical speed) where vibration becomes excessive due to sympathetic frequencies.