During our day-by-day we are exposed to vibration, one way or another, in cars, buses, train, etc. Many people are also exposed to other vibrations, during your work, for example those produced by hand tools, machines or heavy vehicles.
Here are the undesirable vibrations: the effect of overexposure to vibration; the various factors that should be taken into account when it is measured, How is measured and evaluated.
2 – LEGAL AND REGULATORY REFERENCES
Legal and regulatory references (in Portugal) are the following:
- Decree-Law No. 46/2006, from 24 of February
- NP ISO 2631-1:2007 -Mechanical vibration and shock; Evaluation of exposure to whole body vibrations; Part 1: General requirements;
- Relacre Guide 21, Edition September 2008 -Exposure of workers to Vibration – notes on estimation of Uncertainty of Measurement;
- Relacre Guide 23, June edition 2014 -Good practice guide for measurement of Vibration-exposure of workers to Vibration.
3 – EFFECTS OF VIBRATIONS
There are two main types of vibrations in the human body: whole body vibrations and vibration on the hand-arm set.
In the whole body vibration is transmitted to the body in the your globality, generally through the support surface (This is, the feet, back, buttocks, etc.). A person driving a vehicle, for example, is subject to vibrations in the whole body through the buttocks, and if there are lies, also through the back.
The hand-arm vibration is transmitted to the hands and arms. It is mostly supported by operators using hand tools.
The system of the whole body and the hand-arm system are "mechanically different" and are, therefore, evaluated separately.
What follows refers relates to the effects of periodic vibration, random and transient in healthy people exposed to vibrations in the human body during travel, at work and leisure activities.
– Mostly people sitting, Since there are known the effects of vibration on health of people standing, inclined or disposed.
– The people in health and that are regularly exposed to vibration.
– Rectilinear vibration along the axes x-basicêntricos, y- and z- of the human body.
Does not apply:
– The simple high amplitude transient as those that may result from a road accident and traumatic effects.
– The referred applies to vibration in the frequency range 0.5 Hz and 80 Hz which is transmitted, through the seat, the body sitting as a whole that cause nausea..
The respect is based on available data resulting from research on human response to vibrations along the z axis- in people. The existing experience is limited with regard to the application of this part of the standard vibration along the x-axis- and y- in people and for all directions (x-, y- and z-) in people standing, recumbent or disposed.
Investigations in the area of biodynamics and the epidemiological studies conducted to date, provide evidence of the existence of a high risk of degradation of the health effects of exposure of the human body, for long periods, the vibrations of high intensity. It should be noted that the lumbar region (vertebral column) and associated nervous system may be the affected areas with greater importance. Metabolic phenomena and other factors with internal source may produce additional degenerative effects. Sometimes it is assumed that environmental factors such as body posture, the low temperature and air currents can contribute to the onset of muscular pains. However, It is unknown whether these factors can contribute to the degradation of vertebrae and disks.
The relevant literature on the effects of vibration of long duration and high intensity on the human body indicates an increased health risk of dorsal column and nervous system of the affected segments. This may be due to the biodynamic behavior of column: horizontal displacement and torsion of the segments of the vertebral column. Excessive mechanical tension and/or nutrition disorder and diffusion to the fabric of the disk can contribute to degenerative processes in the lumbar segments (spondylosis deformans, intervertebral osteocandrose, deforming arthrosis). The exposure of the human body vibration can also worsen certain pathological endogenous disorders of the spine. Although they admit it usually a dose-effect relationship there is currently no quantitative relation.
With a less likely, also admits that the digestive system, the genital/urinary system, and the female reproductive organs are affected.
Usually, changes in health caused by vibration on the human body take years to process-if. Becomes, therefore, important that the measures of exposure are representative of the whole period of exhibition.
On the one hand, increasing periods of exposure to vibration (throughout the day, or daily over the years) and the increase in mean intensity increased dose of exposure and are considered to increase the risk; On the other hand, It is considered that rest periods can reduce that risk.
4 PRINCIPLES OF RISK ASSESSMENT
The risk assessment shall:
• identify any health or safety risks that cause or aggravating factor the whole body vibration;
• measure/calculate the exposures of workers and to compare them with the exposure action value and the exposure limit value;
• identify risk checks available;
• identify the steps you intend to take to control and monitor the risks due to whole body vibration; e
• record the assessment, the steps that have been taken and their effectiveness.
Associated with whole body vibration, other ergonomic factors can contribute to cause back pain, namely:
• awkward postures while driving or the handling of machines;
• sitting for long periods without being able to change positions;
• improper disposal of control devices, forcing the driver/operator stretching or twisting;
• poor visibility of the operation, What requires the operator to twist and stretch to be able to view properly;
• lifting and manual transport of heavy loads
• have systematically to climb to a high cabin or difficult to access, or jump out of her.
All these factors may, by itself, cause back pain. However, the risk will be greater if the worker is exposed to one or more of these factors and, at the same time, the whole body vibration. For example:
• be exposed to transmission of vibrations to the whole body during long periods without being able to change positions;
• be exposed to transmission of vibrations to the whole body and sitting in a posture of stretching or twisting (p. ex., looking over my shoulder to control the operation of the coupled machines);
• be exposed to transmission of vibrations to the whole body and, at the same time, perform tasks that involve lifting and manual transport of heavy loads.
Environmental factors, as the temperature, may increase even more the risk of back pain or spinal trauma.
All these factors must be considered together in the plans which are intended to minimize the risk of back pain. Rules and guidelines should be laid down concerning the manual transportation if this factor is important in labour activity of workers.
As a starting point for the assessment of risks might want to analyze the work done, as well as the processes, the machines and equipment used. All types of moving vehicle are susceptible to transmit vibrations to the whole body of the driver. Health risks increase when workers are exposed to high levels of whole-body vibration on a regular basis and lasting.
Example of vibration levels for common vehicles.
5 DURATION OF EXPOSURE
To evaluate the daily exposure to vibration, It is necessary to estimate the time when the tools are exposed to vibration. Experience has shown that this is often over-estimated during the risk assessment. Then we analyze what the information on the need for exposure time and determine that.
Before you can estimate the daily exposure to vibration [A(8)], need to know the total daily duration of exposure to vibration of each vehicle or machine used. You might want to be careful to use data that are compatible with the data available regarding the extent of vibrations, in other words, If the data concerning the vibration values are based on measurements made with the machine in operation, then mede‑se the time during which the worker is exposed to vibrations. When questioned about the usual daily duration of exposure, the operators of machines or vehicles indicate generally a value which includes periods without exposure, as for example the truck load times and waiting. Usually, the vibration of a vehicle in circulation has a preponderant role in exposure.
However, some exhibitions verificam‑se especially in operations performed while the vehicle is stopped, as in the case of excavators and forestry mowers. Labour standards must also be examined carefully. For example, can dar‑se the case of some workers could only work during certain periods of the day. Standards should be established for regular use, because it will be an important factor in the calculation of the likely exposure of a worker to mechanical vibrations.
6 -BASICS OF VIBRATION MEASUREMENT
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7 – THE MEASUREMENT POINTS
It is essential that the vibrations in the human body are accurately measures in order to carry out an evaluation of: (a) discomfort produced by vibration, e (b) the possible danger involved in exposure, in order to take appropriate measures for the reduction of these factors.
It is extremely important, When you measure vibrations in the human body, the vibrations are measures as close as possible to the point where the vibrations are transmitted to the body.
For whole body vibration, the vibration enters the body in ground interfaces/feet, seat/back and seat/buttocks and, therefore, the measures must be carried out in these points. For this purpose Triaxial accelerometers were developed inserted in a rubber pad, that can be placed in the excitement of the original position of the disturbance-free person or reducing your comfort. To measure the vibrations transmitted to the driver of a vehicle, This should sit on top of it or put it between the back and the seat. To measure vibrations transmitted by the ground, the transducer is placed on the ground with a weight on top to ensure adequate contact between the transducer and ground to vibrate.
The accelerometers seat triaxias contain three independent accelerometers which measure vibrations simultaneously in the three orthogonal axes.
8- FREQUENCY RESPONSE OF THE HUMAN BODY AND WEIGHTING
To understand the reason why humans are more sensitive to some frequencies than others is useful to consider the human body as a mechanical system. The system is complicated by the fact that: (a) each body part has your greater sensitivity in different frequency ranges; (b) the human body is not symmetrical, e (c) There are no two people respond to vibrations in exactly the same way. Anyway, biomechanical models were developed suitable to simulate human response to vibration.
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9 – THE CALCULATION OF THE(8) -Daily exposure to vibrations
In accordance with the methodology defined for the test, are quantified the vibrations associated with each operation, for measurement at the workplace, being the final daily vibration value per worker for the exposure time for each operation, from the Eq. 1, 2 or 3:
When the worker is exposed to vibrations arising solely from an operation, the daily exposure to vibrations is obtained using the: EQ. 1, EQ. 2 and EQ. 3;
T – Total daily duration of exposure;
T0 – Duration of reference 8 hours (28800s).
Being awdx, awdy and awkz and the effective values of the weighted accelerations and multiplicative factors affected Kx= Ky= 1.4 and Kz= 1.0 for os x, y and z, respectively.
Finally the daily vibration exposure value is given by the following expression:
When the employee performs more than one operation, exposing themselves to vibrations resulting from the same, the form to use is the following:
A(8)XI, A(8)Yi and the(8)Zi represent the values of daily exposure to vibrations resulting from the operation, for the x-axis, y and z, respectively.
It is the daily duration of exposure as a result of the operation i;
T0 is the reference length 8 hours (28 800s);
Being , awdxi, awdyi and awkzi the effective values of the weighted acceleration of the operation i and affected the multiplicative factors K-X = Ky = 1.4 and Kz = 1.0 for os x, y and z, respectively.
The daily exposure to vibrations is obtained using the following equation:
Where i represents an operation of the entire set of operations performed by the worker.
The value of daily exposure to vibrations(8) is given by Eq. 4, along with the equations Eq. 8, EQ. 9 and Eq. 10
10 – – DECREE-LAW LIMIT VALUES 46/2006
For whole-body vibration:
a) The daily exposure limit value standardised, corresponding to a reference period of 8 hours, is set at 1,15 m/s2
b) The daily exposure value normalised, corresponding to a reference period of 8 hours, that triggers the action shall be 0,5 m/s2
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