Vibration is defined as oscillatory motions of particles (molecules) around their equilibrium in a solid body, liquid, or gas in the areas of infrasound (i.e.< 20 Hz) and partially in the audible sound frequency ranges (upto 1500 Hz), Since in industrial situation vibration usually occurs in air and in the audible range it is normally experienced as sound. Understanding the source and mode of transmission of vibration and so it's appropriate control measures can be complex.
There are two forms of exposure that can be distinguished:
The directive defines the terms as follows:
In simple terms, vibration is defined by its magnitude (traditionally described using acceleration, expressed in m/s2)
|ACGIH TLVs (Threshold Limit Value)|
|Hand-arm vibration||Whole body vibration|
|Four hours and less than eight hours |
|Two hours and less than four hours|
|One hour and less than two hours |
|Less than one hour |
The ISO Vibration Occupational Guidelines establishes exposure action values above which employers must control vibration hazards to their employees, as well as exposure limit values below which workers must not be exposed:
A typical vibration measuring system consists of a device that detects vibration (accelerometer) & vibration level along with a frequency-weighting network, & a display such as a metre, printer, or recorder, are included in this equipment.
An electrical signal is generated by the accelerometer, this signal's magnitude is proportional to the amount of acceleration applied to it. The frequency-weighting network simulates human sensitivity to various frequencies of vibration.The application of weighting networks results in a single number that is represented as the frequency-weighted vibration exposure in metres per second squared (m/s2) units of acceleration.
The International Organization for Standardization (ISO) standard ISO 5349 specifies the frequency-weighting network for hand-arm vibration. The human hand is not equally responsive to all frequencies of vibration energy. The sensitivity is at its peak around 8-16 Hz, (Hertz or cycles per second). A weighting network is used in measuring equipment to account for this phenomenon. The gain is set to 1 for vibration frequencies to which the hand-arm system has the highest sensitivity.
|Rank||Class IV large equipment||Description|
|A||0 to 2.8||Good|
|B||2.8 to 7.1||Satisfying|
|C||7.1 to 18||Not Satisfying|
|D||18 to 45||Forbidden|