Whole-body vibrations are mechanical vibrations that enter the human body in the frequency range of 0.1 to 80 Hz. A typical source of whole-body vibrations is vehicles, where vibrations are transferred to the human body via seats or floors. The whole-body vibration is a possible health hazard, especially after prolonged exposure.
Whole body vibration (WBV) is caused by workplace machines and vehicles and is transmitted to the human body through the seat or floor. Whole-body vibration exposure is common in professions like transportation, construction, quarrying, or agriculture. The most prominent examples of sources are off-road vehicles, jet boats, helicopters, and jets. Often the standing operators are also affected by whole-body vibrations transmitted from heavy machinery via floors.
The graph below shows typical vibration acceleration values that are generated by moving vehicles:
Exposure to whole-body vibration may cause health problems. For example, exposure to high shocks may cause back injuries for a seating person or knee injuries for a standing one. The risks rise with higher vibration magnitudes and prolonged exposure times. The effective way to prevent whole-body vibration risk at work is the identification of vibration sources by measurements.
Vibration is an oscillating motion around a reference position. Usually, vibration is expressed in units of magnitude (m/s2) and frequency (Hz). In machines, vibrations are generated by numerous internal components that move or rotate at various frequencies at the same time.
The whole-body vibration measurement identifies sources by plotting the vibration amplitude vs. its frequency. In practice, a frequency spectrogram is used to display the vibration level as a function of frequency. Identification of dominant frequencies enables finding the cause of unwanted vibration.
Whole-body vibration exposure assessment is based on the European “Vibration Directive” (2002/44/EC) that sets limits for daily vibration exposures to reduce the risks of whole-body vibration disease. Based on the directive, all EU countries passed laws to protect workers from the dangers of whole-body vibrations.
The EU Vibration Directive whole-body vibration daily exposure A(8) limit value is 1.15 m/s² (or, a VDV of 21 m/s1.75). In addition, the 0.5 m/s² is a daily exposure action value limit (or, VDV vibration dose value of 9.1 m/s1.75).
To calculate the daily whole-body vibration exposure A(8) (or VDV) two values are needed: the vibration magnitude and the duration of time in which the person is exposed to vibration. Unless the vibration measurement covers the entire working day, the exposure duration time is usually assessed in interviews with workers and their supervisors.
The formula to obtain the whole-body vibration exposure A(8) takes the maximum weighted vibration magnitude of three axes and multiplies it by the square root of the exposure duration time divided by the reference time of 8 hours:
A(8)= awmax*√(Te/T8)
Weighted whole-body vibration is a vibration with applied frequency-weighting filters that emphasize frequencies that are particularly harmful to the human body.
The whole-body vibration magnitude (aw) is obtained with measurements. The aw value is the frequency-weighted acceleration measured in three directions (axes X, Y, Z). As the horizontal vibrations are more harmful, two lateral axes X and Y directions are multiplied by 1.4. For the vertical, z-axis vibration the factor is 1.0. To obtain the A(8) vibration exposure the highest aw of three orthogonal axes (1.4awx, 1.4awy, or awz) is selected.
The whole-body vibration measurement is described by international standard ISO 2631-1. During the measurement, the vibration acceleration aw (in m/s2) is assessed. The aw is a root-mean-square (r.m.s) vibration magnitude expressed in terms of the frequency-weighted acceleration at the seat of a seated person or the feet of a standing person. An alternative method of measuring vibration exposure is the vibration dose value (or VDV).
The VDV vibration dose value in m/s1.75 is a measure of vibration acceleration that provides a more accurate evaluation of the risks associated with vibrations, which include shocks. By ISO, the VDV is used to asses the A(8) for a signal with shocks. The evaluation uses the highest of the three orthogonal axis values (1.4VDVwx, 1.4VDVwy, or VDVwz).
Following ISO, the measurement time of whole-body vibration should cover the representative working period. In practice, the measurement lasts the total time of each task in which the worker is exposed to vibrations. The British standard BS EN 14253:2003 suggests making at least 2 hours of measurements but prefers half or full-working-day measurements, if possible. In case longer measurements are not possible, 20-minute intervals are suggested as a minimum.
Human vibration meter is a dedicated instrument to measure daily exposure to whole-body vibration, using either Daily vibration exposure A(8), or Vibration dose value (VDV). Human vibration meters for whole-body vibration should comply with the ISO 8041:2005 specifications for whole-body vibration measuring instruments.
A personal whole-body vibration exposure meter (PVEM) is a vibration measurement system meeting the relevant requirements of ISO 8041-1 with the additional capability to monitor personal exposures up to a full working day. The PVEM is designed to measure and log personal vibration exposures by detecting occurrences of exposure and measuring associated human vibration together with the exposure time. The goal is to monitor human exposure to vibration over long time periods, e.g. whole working shift according with ISO 2631-1.
The role of calibration in whole-body vibration is to minimize the measurement uncertainty. In order to minimize uncertainty, it is important to perform the in-situ calibration of a human vibration meter with a vibration calibrator.
SVANTEK SV 106D – Human Vibration Meter & Analyser
An authorized SVANTEK consultant will help You with the details such as the required accessories for your noise & vibration monitoring task.
