Red Seal Industrial Mechanic (Millwright) Practice Exam

What does 'critical speed' refer to in rotating machinery?

1. The speed that minimizes vibration in machinery.

2. The speed at which a motor operates most efficiently.

3. The speed at which the shaft's rotational frequency matches its natural frequency, leading to excessive vibrations.

4. The maximum sustainable speed before machinery fails.

The correct answer is: The speed at which the shaft's rotational frequency matches its natural frequency, leading to excessive vibrations.

Critical speed in rotating machinery specifically refers to the condition where the rotational speed of the shaft aligns with its natural frequency. This alignment can cause resonance, which leads to excessive vibrations that can compromise both the performance and integrity of the machinery. When a machine operates at this critical speed, the vibrational amplitudes can increase significantly, potentially causing damage to bearings, support structures, and other components over time. Understanding critical speed is essential for the design and operation of rotating machinery, as it helps in establishing operational limits and designing systems to avoid running at those specific speeds. This is why option C accurately captures the concept of critical speed in the context of rotating machinery.