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 will be only referred to 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, and what actions can be developed to reduce sources of harmful and/or dangerous vibrations.
2 – NORMATIVE REFERENCES
Legal and regulatory references (in Portugal) are the following:
- Decree-Law No. 46/2006, from 24 of February
- NP EN ISO 5349-1:2009 -Mechanical vibration-measurement and evaluation of the exposure of individuals to vibrations transmitted by the hand-arm system; Part 1: General requirements;
- EN ISO 5349-2:2001 -Mechanical Vibration-Measurement and evalution of human exposure to hand-transmitted vibration – Part 2: Pratical guidance for measurement at workplace;
- 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, studied separately.
The daily exposure to hand-arm vibration, over a period of years, can cause permanent physical damage normally resulting in designated "white finger syndrome", or can damage the joints and muscles of the wrist or elbow.
The white finger syndrome, in its advanced stages, It is characterized by a whitening of the extremities of the fingers, What is caused by damage to the arteries and nerves in the soft tissue of hand. The syndrome typically affects, first one finger, but it will also affect the other, If exposure to hand-arm vibrations continue. In the initial stages of the "white finger syndrome" symptoms are, entre outros, loss of control and sensitivity of the affected fingers. These symptoms are serious because they affect not only the professional activities, but also leisure activities, and are largely irreversible.
The loss of sensitivity and control of fingers, even for short periods of time, can pose a direct and immediate danger. For example when periods of exposure (use of vibrating portable tools) are switched with precision manual work. This situation is often found, for example in slaughterhouses, where the butchers use circular saws and sharp knives.
Damage to the wrist and elbow joints are often caused by long-term exposure to vibration produced by percussion tools (pneumatic hammers and rock drills). This damage causes pain in the joints and muscles of the forearm and is accompanied by a reduction of muscle strength and control of the same.
4 PRINCIPLES OF RISK ASSESSMENT
The risk assessment shall:
• identify where there may be a risk derived from hand-arm vibration;
• measure 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 it intends to take to control and monitor the risks of transmission of hand-arm vibration;
• record the assessment, the steps that have been taken and their effectiveness.
A starting point is to consider the work that is to be executed, the processes involved and the tools and equipment used, and ask: «The company uses manual equipment, guided by hand or manually powered?» If applicable, may need to manage exposures to vibration.
The figure shows examples of amplitudes of vibrations of some tools and machines that create risks.
It is important to keep the workers and their representatives involved and informed in the process of evaluation of the risk of vibrations. An efficient partnership with employees will help to ensure that the information used for the risk assessment is based on realistic assessments of the work being performed and the time spent to do this job.
The factors that govern a person's daily exposure to vibrations are the amplitude (level) weighted vibration frequency and the time period during which the person is exposed. The higher the amplitude or the more long exposure, so much the greater that person's exposure to vibrations.
The RMS values of the weighted acceleration in frequency (AEQ) associated with occupational exposure to hand-arm vibration are typically between 2-50 m/s2 , While those who are in the range of the entire body are in the range of 0,1- 40 m/s2 .
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 vibration exposure for each tool or process used. One should only count the time when the worker is exposed to vibration; one should not count on a period in which a worker has landed the equipment or that is holding it without being in operation.
The contact time is the time during which the hands are effectively exposed to vibration from the tool or workpiece. Often, This period is much lower than the ' working time ' and is usually overrated by operators. The method used to estimate the run times, often depends on the use of the tool to be continuous or intermittent:
Continuous operation of the tool:
Example: use a grinder to eliminate large quantities of a material during several hours.
Observe work during a representative part of the working day and record how much of that time during the tool is in operation. for such, It can be useful a stopwatch or video recording.
Intermittent operation of the tools:
Example: Use torque wrench to tighten nuts by percussion of the vehicle wheels.
Maybe you have access to information on the number of operations carried out during the working day (for example, the number of finished components per day). If you estimate the average duration of an operation, observing the work rate during a period chosen as sample, one can then calculate the total daily duration.
In the example of the wrench for percussion, Maybe it's known the number of wheels and replaced a day and the number of nuts per wheel, and you also need to know how long does it take usually to get or replace a pig.
Labour standards must also be carefully taken into account. You can, for example, that some workers only use vibrating tools during certain periods of a day or a week. Standards should be established for regular use, because it will be an important factor in the calculation of the likely exposure of the worker to vibrations.
6 -BASICS OF VIBRATION MEASUREMENT
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7 THE HAND-ARM MEASURING POINTS
When a vibrating object is safe in hand, transmits the vibrations to the hand and arm through palma. The transducer can, therefore, be mounted between the contact surface between the Palm and the vibrating object or fixed in the tool.
Even a small accelerometer placed on the palm tends to disturb the form of the operator grasping the tool and leads, therefore, to incorrect measurements. A practical solution is to mount the accelerometer on a bracket that is then held in the hand tool interface. Both the accelerometer and the adapter should be light so as not to introduce the risk of resonances.
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) no two people respond to vibrations 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, By measuring the machine, machine or job, being the final daily vibration value per worker for the exposure time for each operation, from Eq formula. 1:
T – Total daily duration of exposure;
T0 – Duration of reference 8 hours (28800s).
being akwx, akwy akwz and effective values of the weighted acceleration for the axes x, y and z, respectively.
When the employee performs more than one operation, exposing themselves to vibrations resulting from the same, the form to use is the following: Eq.3 e Eq.4;
Like this, the daily exposure value is given by the following equation:
Where i represents an operation of the entire set of operations performed by the worker.
10 – – DECREE-LAW LIMIT VALUES 46/2006
Exposure limit values and exposure values that trigger the action:
For the vibrations transmitted to the hand-arm system:
a) The daily exposure limit value standardised, corresponding to a reference period of 8 hours, is set at 5 m/s2;
b) The daily exposure value normalised, corresponding to a reference period of 8 hours, that triggers the action is set at 2,5 m/s2.
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