Why Do Machines Vibrate? The Science of Resonance and Fault Diagnosis

Have you ever seen a washing machine shake violently during the spin cycle? Or heard the story of an opera singer shattering a glass with a high-pitched note? These phenomena are caused by a key physics concept: resonance. Today, we’ll explore machine vibrations, how resonance works, and how engineers use this knowledge to diagnose faults.

1. Symptoms ≠ Faults: The Complexity of Vibration Diagnosis

Machine vibration symptoms (like unusual noise or shaking) don’t always directly point to a specific fault—just as a cough could mean a cold, allergies, or even pneumonia. Similarly, machine vibrations can stem from:

•Worn-out parts

•Misalignment

•Resonance (our main focus!)

Engineers must act like detectives, analyzing vibration frequencies and machine structures to pinpoint the real issue.

2. Forced Frequencies vs. Natural Frequencies: What’s Making the Machine Shake?

(1) Forced Frequencies

Source: Rotating parts (motors, gears, fan blades) generate periodic forces, creating predictable vibration frequencies.

Key Features:

Scale with machine speed (double the RPM, double the frequency).

Disappear when the machine is turned off.

Can be calculated from design specs (e.g., gear teeth × RPM).

(2) Natural Frequency (Resonant Frequency)

Definition: Every structure (machine base, rotor, pipe) has a "favorite" vibration frequency, like a swing’s natural rhythm.

Formula:

Fn=(1/2π)∙√(K/m)

K = Stiffness (higher stiffness → higher frequency).

m = Mass (higher mass → lower frequency).

Danger Zone: When forced frequency ≈ natural frequency, resonance occurs, amplifying vibrations and risking damage!

3. Resonance: When Vibration Becomes a Disaster

What is Resonance?

Resonance happens when forced vibrations match a system’s natural frequency, causing extreme oscillations. Famous examples:

Shattering a glass with sound: Sound waves match the glass’s natural frequency, accumulating energy until it breaks.(Fig.2)

Tacoma Narrows Bridge collapse (1940): Wind vortices matched the bridge’s natural frequency, causing violent swaying.(Fig.3)

Two Types of Machine Resonance

1.Structural Resonance

Affects: Non-rotating parts (frames, pipes, foundations).

Causes: External vibrations (nearby machines, earthquakes) or internal forces.

2.Rotor Resonance

Affects: Rotating shafts (turbines, motor rotors).

Critical Speed: When rotor RPM ≈ natural frequency, vibrations spike.

Solution: Quickly pass through critical speed or redesign to avoid matching.

4. How to Prevent Resonance? An Engineer’s Anti-Vibration Guide

1.Adjust RPM: Keep forced frequencies away from natural frequencies.

2.Modify Stiffness/Mass: Change design parameters (recall Fn∝√K/m).

3.Add Damping: Use shock absorbers, rubber mounts, or dampers.

4.Vibration Monitoring: Install sensors to detect resonance risks early.

Conclusion

Resonance is both a fascinating physics phenomenon and a potential engineering hazard. By understanding forced frequencies, natural frequencies, and resonance, engineers can design better machines and prevent catastrophic failures.