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1. Vibration Control:

2. Reasoning for controlling
vibration:
We control vibration because it can lead to:
–Fatigue failure
–Fretting wear
–Loosening of fastenings
And can also cause:
–Whole body vibration effects
–Localized physiological effects
–Passenger discomfort

3. Strategies for vibration
control:
The main strategies of controlling vibration
are:
–Detuning
–Damping/energy dissipation
–Isolation
–Absorbing

4. Vibration management
strategies:
1.Control the natural frequencies of the system and avoiding
resonance under external excitations.
2.Prevent an excessive response of the system, even at
resonance, by introducing a damping or energy-dissipating
mechanism.
3.Reduce the transmission of the excitation forces from one part
of a machine to another by use of vibration isolators.
4.Reduce the response of the system by adding an auxiliary
mass neutraliser or vibration absorber.

5. 1. Controlling natural
frequencies:

6. 2. Damping
Dampen resulting vibrations
A small amount of damping has a big effect on
amplitudes near resonance (See W3 p8 Graph)
Structure- bolting and riveting
Material (interlayers)
–Tapes
–Gaskets
–Viscoelastic materials

7. 2. Damping
• Damping requires a large Loss factor h
• –A measure of the energy transformed (‘lost’)
per load/unload cycle
• –Much higher in viscoelastic materials like:
• §Polymers- esp. elastomers (rubbers)
• §Foams
• §Naturals- cork, leather

8. 3. Vibration oscillators:
Passive
–an isolator or resilient member (spring and damper
equivalent)
–metal, pneumatic, elastomer springs, cork, felt
–eg Engine mounts
Active
–Sensor, signal processor and actuator
–Continuously trying to reposition the system

9. 3. Vibration oscillators:
Amplification region
–forcing frequencies < than 1.414 wn
–the force transmitted to the foundation is actually greater
than the force applied by the machine
–here the damping does help to reduce the force transmitted.
Isolation region
–forcing frequencies > 1.414 wn
–isolation of the vibration occurs so that the transmitted
force is less than the exciting force.
–amplitude of vibration is low.
–damping actually increases the force transmitted.
–an adequate level of damping is however desirable to protect
against excessive vibration when passing through resonance

10. 3. Vibration oscillators:
Use of vibration oscillators:
• To keep the force transmitted (Ft) small ‘w/wn’ should
be large.
• The isolator should have a natural frequency much
smaller than the operating speed of the machine.
So THEORETICALLY- flexibly mounted engine should be
supported on very weak springs and the damping kept to a
minimum.
REALISTICALLY
–the dead weight alone of a large static mass on weak
springs since would give impossibly large static
deflections.
–such a system would be laterally unstable
§A compromise solution must therefore be adopted which
will depend on the actual problem.

11. 4. Vibration Absorbers:
These are attached spring-mass systems
–‘vibration neutralizer’
–‘dynamic vibration absorber’
Undamped or Damped - They create a different
resonance frequency away from the operating or
problem frequency.

12. 4. Vibration Absorbers: