Question

Which one of the following reduces the ratio of vibratory response amplitude to the forcing amplitude, in large stationary engine shaft design?

• A. Reduction in axial vibrations of the rotating shaft
• B. Increase in the fundamental frequency of the rotating shaft
• C. Decrease in the rotational speed of the shaft
• D. Operating the shaft at a speed exceeding the critical speed

Hint:

For systems prone to resonance, operating in the supercritical speed range often minimizes vibratory response amplitude, provided the design accounts for stability beyond the critical speed.

Answer 1

The Correct Option is D

Step 1: Understand the concept of critical speed and vibratory response. 
The critical speed of a shaft is the rotational speed at which resonance occurs, leading to maximum vibratory response. When operating close to this speed, the response amplitude is significantly amplified due to resonance. However, if the shaft operates at a speed well above the critical speed, the system moves into a supercritical operating region where the vibratory response amplitude is reduced. 

Step 2: Analyze the reduction of vibratory response amplitude. 
Operating the shaft at a speed exceeding the critical speed reduces the ratio of vibratory response amplitude to the forcing amplitude due to the dynamic characteristics of the system in the supercritical range. Beyond the critical speed, the shaft's response stabilizes as it passes through resonance. 

Step 3: Analyze other options. 
Reduction in axial vibrations (Option A): This affects the axial stability but does not directly reduce the vibratory response amplitude caused by resonance.
Increase in the fundamental frequency (Option B): While increasing the fundamental frequency shifts the resonance point, it does not directly address the behavior in the supercritical region.
Decrease in rotational speed (Option C): Slowing down the rotational speed may avoid resonance, but it does not specifically target the reduction of vibratory response amplitude at high speeds.

Conclusion: Operating the shaft at a speed exceeding the critical speed reduces the ratio of vibratory response amplitude to the forcing amplitude.