Short Answer:

A dynamic vibration absorber is a mechanical device that reduces or eliminates unwanted vibrations in machines or structures by using a secondary mass and spring system. It is attached to the main vibrating body and designed so that its natural frequency matches the excitation frequency, allowing it to counteract vibrations through opposite motion.

The dynamic vibration absorber works by transferring the vibration energy from the main system to the absorber, where it is dissipated or balanced out. It is commonly used in rotating machinery, vehicles, bridges, and tall buildings to prevent resonance and maintain stable performance.

Detailed Explanation :

Dynamic Vibration Absorber

A dynamic vibration absorber is a device used to control and minimize vibrations in mechanical systems. It is an auxiliary system consisting of a mass-spring or mass-spring-damper arrangement attached to the main vibrating system. The purpose of the absorber is to reduce the amplitude of vibration of the main system by introducing an additional degree of freedom that redistributes and absorbs the vibrational energy.

In simple terms, when the main system vibrates due to a periodic force, the absorber oscillates out of phase with the main system. This generates a counteracting force that reduces the vibration amplitude of the primary structure, effectively neutralizing resonance.

Dynamic vibration absorbers are widely used in engineering applications where unwanted vibrations could cause discomfort, noise, fatigue failure, or structural damage.

1. Principle of Dynamic Vibration Absorber

The dynamic vibration absorber works on the principle of resonance control. When a system is subjected to a periodic external force, large vibrations occur if the forcing frequency matches the natural frequency of the system. To counter this, a dynamic absorber is attached to the system.

The absorber is designed such that its natural frequency equals the excitation frequency. When tuned correctly, the absorber vibrates out of phase with the main system, creating a destructive interference that cancels the vibration effect.

At this point, the total amplitude of vibration of the main system becomes nearly zero, and the vibration energy is transferred to the absorber, where it is either stored elastically or dissipated through damping.

Mathematically, the absorber is tuned when:

Where,

•  = Natural frequency of the absorber,
•  = Excitation frequency.

2. Construction of Dynamic Vibration Absorber

A dynamic vibration absorber typically consists of:

1. Auxiliary Mass (m₂):
   A smaller secondary mass connected to the main system. It moves in the opposite direction of the main vibration.
2. Spring (k₂):
   Provides the restoring force to the auxiliary mass and helps determine the absorber’s natural frequency.
3. Damper (c₂) (optional):
   Sometimes included to dissipate energy and prevent continuous oscillation.
4. Mounting Mechanism:
   The absorber is firmly attached to the main vibrating system (primary mass).

Together, these components form a two-degree-of-freedom system consisting of the main structure and the attached absorber.

3. Working of Dynamic Vibration Absorber

When an external periodic force acts on the main vibrating system, both the main system and the absorber begin to vibrate.

If the absorber is properly tuned:

• It vibrates out of phase with the main system.
• The motion of the absorber generates an equal and opposite force that counteracts the original vibration force.
• This results in a significant reduction in vibration amplitude of the main system.

The energy of vibration is transferred from the main structure to the absorber, where it is either dissipated through damping or converted into harmless elastic motion.

In practical applications, the absorber must be precisely tuned to the excitation frequency for maximum effectiveness. Even a small deviation can reduce performance.

4. Mathematical Model

For a system with a main mass  and absorber mass , the equations of motion are:

 

Where,

• , : mass and stiffness of the main system,
• , : mass and stiffness of the absorber,
• : amplitude of the external force,
• : excitation frequency.

Solving these equations gives the frequency response of the combined system, showing two natural frequencies — one for the main system and one for the absorber.

At the tuned frequency, the main system’s amplitude becomes nearly zero, while the absorber vibrates with high amplitude.

5. Advantages of Dynamic Vibration Absorbers

1. Effective Resonance Control:
   Can completely eliminate vibrations at a specific frequency.
2. Simple and Compact Design:
   Easy to install without complex electronics or control systems.
3. Cost-Effective:
   Cheaper compared to active vibration control systems.
4. Low Maintenance:
   No external power supply or continuous adjustment needed.
5. Improves Structural Safety and Comfort:
   Prevents excessive vibration, noise, and fatigue damage.

6. Limitations of Dynamic Vibration Absorbers

1. Tuning Requirement:
   Must be tuned accurately to the excitation frequency for effective performance.
2. Narrow Frequency Range:
   Works efficiently only at or near the tuned frequency.
3. Added Weight:
   Increases the total mass of the system.
4. Reduced Efficiency if Detuned:
   Performance decreases if vibration frequency changes during operation.
5. Maintenance of Components:
   Springs and damping materials may require replacement over time.

7. Applications of Dynamic Vibration Absorbers

1. Rotating Machinery:
   Used in turbines, fans, and engines to control unbalanced vibration forces.
2. Automobiles:
   Used in engines, suspensions, and vehicle frames to reduce road-induced and engine vibrations.
3. Buildings and Bridges:
   Used to minimize oscillations due to wind, traffic, or seismic activity.
4. Aerospace Structures:
   Applied in aircraft wings and spacecraft to counter aerodynamic vibration.
5. Precision Equipment:
   Installed in sensitive machines and instruments to reduce vibration-induced measurement errors.

8. Examples of Dynamic Vibration Absorbers

• The Taipei 101 skyscraper uses a large mass absorber weighing 660 tons to control wind and earthquake vibrations.
• Automotive engines use small tuned absorbers to control torsional vibrations in crankshafts.
• Bridge structures, such as the Millennium Bridge in London, employ dynamic absorbers to counteract pedestrian-induced vibrations.

These examples show that dynamic vibration absorbers are crucial for both structural safety and operational efficiency in various fields.

Conclusion:

A dynamic vibration absorber is a mechanical device that controls and reduces unwanted vibrations by using a secondary mass-spring system tuned to the vibration frequency of the main structure. It works on the principle of destructive interference, where the absorber vibrates out of phase with the main system, reducing its amplitude. Though effective only within a narrow frequency range, dynamic vibration absorbers are simple, reliable, and widely used in mechanical and structural engineering applications for preventing resonance and ensuring safe, smooth operation.