Short Answer:

Methods to control resonance are the techniques used to prevent or reduce excessive vibrations that occur when the frequency of an external force equals the natural frequency of a system. These methods aim to lower the vibration amplitude and maintain stable operation of machinery and structures.

In simple words, resonance can be controlled by changing the system’s natural frequency, adding damping, avoiding operating speeds near resonance, using vibration isolators, and maintaining proper balance. These methods help protect machines from damage, increase their lifespan, and ensure smooth and safe operation.

Detailed Explanation :

Methods to Control Resonance

Resonance occurs when the frequency of an external periodic force becomes equal to the natural frequency of a mechanical or structural system. Under this condition, the amplitude of vibration becomes maximum because the system absorbs energy continuously from the external source. This high amplitude can cause mechanical failure, fatigue, noise, and instability in machines and structures.

To avoid these harmful effects, certain methods are applied to control resonance. These methods either change the conditions that cause resonance or reduce the resulting amplitude. The aim is to ensure that machines and structures operate safely without reaching dangerous vibration levels.

The main methods to control resonance include:

1. Changing the natural frequency.
2. Increasing damping in the system.
3. Avoiding operating speeds near resonant frequency.
4. Using vibration isolators or absorbers.
5. Proper balancing and maintenance.

Let us explain each method in detail.

1. Changing the Natural Frequency

The most direct way to control resonance is by changing the natural frequency of the system so that it does not coincide with the frequency of the external excitation.

The natural frequency (ωₙ) of a system is given by:

Where:

•  = stiffness of the system
•  = mass of the system

Hence, the natural frequency depends on the mass and stiffness of the system.
Resonance can be avoided by adjusting these two parameters:

• Increasing the stiffness (k):
  A stiffer system has a higher natural frequency. This can be done by using stronger materials or adding supports.
• Increasing the mass (m):
  A system with higher mass has a lower natural frequency. This method is effective in some structures where adding weight shifts the resonance condition away from the operating frequency.

By properly modifying stiffness or mass, the system’s natural frequency can be shifted away from the excitation frequency, thereby controlling resonance.

Example:
In machine foundations, adding extra concrete mass lowers the natural frequency, preventing resonance with the machine’s operating speed.

2. Increasing Damping

Damping is one of the most effective ways to control resonance. It helps reduce the amplitude of vibration by dissipating energy in the form of heat or other forms.

The amplitude at resonance is inversely proportional to the damping ratio :

Hence, when damping increases, the amplitude of vibration at resonance decreases significantly.

Methods to increase damping:

• Using viscous dampers filled with oil or fluid.
• Applying rubber mounts or elastomer pads that provide both elasticity and damping.
• Adding friction dampers in joints or connections.

Increasing damping not only reduces the amplitude during resonance but also smoothens the vibration response, preventing sudden increases in amplitude.

Example:
Automobile shock absorbers are designed to provide damping to control resonance and maintain a comfortable ride.

3. Avoiding Operating Near Resonant Frequency

In rotating machines and equipment, the operating speed often corresponds to the forcing frequency. Resonance can be avoided by ensuring that the operating frequency (ω) does not match the natural frequency (ωₙ) of the system.

This can be achieved by:

• Designing the machine to operate well below or above the resonant speed.
• Gradually passing through the resonant speed during startup and shutdown so that the machine does not stay in resonance for long.
• Monitoring vibrations to detect resonance conditions early.

Example:
In turbines or engines, the critical speed (resonant speed) is identified and marked. The machine is either operated below or above that speed range to avoid dangerous vibrations.

4. Using Vibration Isolators or Absorbers

Vibration isolators and absorbers are devices that reduce vibration transmission and control resonance by altering the vibration path.

• Vibration Isolators:
  These are flexible supports such as springs, rubber mounts, or air cushions that reduce the transmission of vibration from a machine to its foundation. By introducing isolation, the system’s natural frequency is changed and resonance is minimized.
• Vibration Absorbers:
  These are secondary systems attached to the main system to absorb vibration energy at a particular frequency. They are designed so that their own natural frequency matches the disturbing frequency, thereby reducing the vibration of the main system.

Example:

• In buildings, base isolators are used to prevent resonance caused by earthquakes.
• In engines, rubber mounts isolate vibration from being transmitted to the vehicle body.

5. Proper Balancing and Maintenance

Unbalanced or misaligned rotating components can create periodic forces that may lead to resonance. Proper balancing and maintenance are important to avoid such conditions.

Key practices include:

• Dynamic balancing of rotating parts like fans, rotors, and flywheels.
• Ensuring correct alignment of shafts and couplings.
• Regular tightening of bolts and joints to prevent looseness.
• Replacing worn-out components that may cause unbalanced forces.

By maintaining balance, the excitation frequency remains steady, preventing the system from approaching resonance conditions.

Example:
Balancing of turbine rotors and generator shafts is done during installation to avoid resonance and ensure smooth operation.

6. Structural Modifications

In some cases, resonance can be controlled by changing the geometry or structural configuration of the system.
This may involve:

• Adding supports or braces to increase stiffness.
• Using different materials with better damping properties.
• Changing the boundary conditions (fixed, hinged, or free ends).

These design modifications help in shifting the natural frequency or increasing damping to control resonance.

Example:
Tall chimneys or towers are often provided with dampers or tuned absorbers to prevent resonance due to wind forces.

Conclusion

In conclusion, the methods to control resonance aim to prevent the dangerous effects of excessive vibrations in machines and structures. Resonance can be controlled by changing the natural frequency (by adjusting mass or stiffness), increasing damping, avoiding operation near resonant frequency, using vibration isolators or absorbers, and maintaining proper balance. These methods ensure the system operates safely, efficiently, and without damage. Proper design, damping, and maintenance are key to preventing resonance-related failures and achieving long-term machine reliability.