Vibration

Vibrations are mechanical oscillations, produced by regular or irregular period movements of a member or body about its rest position..

Vibration can be a source problems at an engineering level because they can result in damage to equipment, loss of control of equipment, and reduction in the efficiency of operation of machines..

Vibration effects on people
Vibration can also be at a low level a source of discomfort and at a high level can be a safety risk to personnel affected

In the context of the effect of vibration on people important factors include:

1. Point of application of the vibration -
2. Frequency of the vibration -
3. Acceleration of the vibration -
4. Duration of the vibration -

Vibration is most normally a problem experienced in driving vehicles and in operating tools.

Vibration can affect visual perception, muscles, concentration, circulation and the respiratory system and at certain levels can even result in physical harm to the body.

The effect of vibration on the human body is related to the natural frequency of parts of the human body affected. Frequencies of above 30 Hz are heavily damped by the human tissue.

The body of a sitting person responds to vertical vibration as follows:

• 3- 4 Hz ....Strong resonance in the cervical vertebrae
• 4Hz.........Peak of resonance in the lumber vertebrae
• 5 Hz........High resonance in the shoulder girdle
• 20-30 Hz....High resonance between the head and shoulders
• 60-90 Hz....Resonance in the eyeballs
• 100-200 Hz..Resonance in the lower jaw<

Higher frequency vibrating machines would have a significant effect on the body components but this is largely mitigated by the damping effects of the tissue which tends to confine the vibration to the point of application e.g the hands

Control Of Vibrations
The primary method of eliminating vibration is at source by design the equipment and ensuring control over the manufacturing tolerances. Other methods of reducing the vibrations generated by machinery are listed below

• Modify the system such that the natural frequencies are not close to the operating speed or any generated frequencies.
• Prevent large responses by including damping
• Install vibration isolating devices between adjacent sub-systems
• Install equipment on dense heavy rigid vibration absorbent floors
• Include auxiliary mass into the equipment to reduce the response and absorb vibration
• Base design on engineering materials which have high inherent damping, such as cast iron and laminated sandwich materials

There are a number of proprietary machine resilient support pads which provide engineering isolation of vibration from the foundations.