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Here you can see some examples of measurements performed on different vibration sources.
1. Use of a continually functioning vibrating instrument
This is the simplest situation. The measurement can be performed on very long or short integration times, with very repeatable results. The exposure time coincides with the tool usage time.
Let's suppose the worker is exposed to a total level of vibrations equal to 7.5 m/s2 for a time Ti of 5 hours.
The daily exposure to vibrations A(8), is obtained according to this expression (1):
2. Using multiple vibrating instruments at different times during the day
Let's suppose the worker uses more than one tool or the same tool for different operations.
In this case, you should calculate the partial exposures concerning each working phase.Let's suppose the worker is subject to the following vibration levels:
Tool 1: ahv1=12m/s2 for a time T1=1.5h
Tool 2: ahv2=4m/s2 for a time T2=3h
Tool 3: ahv3=7m/s2 for a time T3=2hPer each phase, you should apply this expression (1):
(Tool 1)
(Tool 2) (Tool 3) By applying (2), the total daily exposure is:
3. Exposure to an intermittently functioning source
In this case, the vibration source is cyclically turned on.
Each operation has a similar acceleration profile.
To calculate the average level of vibrations transmitted with sufficient accuracy, the integration time should include a complete series of functioning cycles or be sufficiently long so as not to influence the acceleration average value.
If possible, it is better to detect the acceleration average value during one of the source functioning phases.
An effective way to calculate the acceleration average value of a single functioning cycle is the use of parameter A(1) that gives the exposure referred to 1 second. Unlike the acceleration average value, A(1) does not depend on the duration of the measurement, provided that it includes an entire working cycle. By measuring A(1) for different working cycles, you can calculate the average value.
This expression is valid:
(5) |
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| where | ahv |
= average acceleration of the working cycle |
| T | = duration in seconds of the working cycle |
Then, from the measured exposure values referred to 1 second A(1) and the duration of the cycles, you can calculate the average acceleration.
The average value of the exposures referred to 1 second is equal to 39.43 m/s2 and each working cycle takes an average of 2 minutes.
The average acceleration during work is calculated using this expression:
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If you perform 100 work cycles per day, the total exposure time is 200 minutes (3.33 hours). The daily exposure is obtained from the expression:
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An alternative method is measuring the acceleration level during the working phase and using the working time to calculate the daily exposure. This method allows calculating the exposure in different situations, with different working times.
Let's suppose you manufacture metal articles in three phases using the same machine. The duration of the manufacturing phases is 20s, 40s and 40s. Therefore, a single piece needs 100s. If the transmitted acceleration is 3.9m/s2 and you produce 100 articles per day, the daily exposure is:
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4. Exposure to a source that generates single impulses or sequences of impulses
This occurs when the sources generates impacts producing impulses or sequences of impulses transmitted to the operator. In this case, you should evaluate the average exposure level corresponding to each impulse: the measurement is done over a time interval with a sufficient number of impulses to give a stable average value.
For example, suppose you are measuring the exposure to vibrations of an operator working on metal plates. The processing of each plate produces a sequence of 5 impulses and lasts 20 seconds.
To calculate the daily exposure, you should measure the average exposure to vibrations for at least 20 seconds. Let's suppose you get 14.6m/s2
The daily total exposure time should be compared with the reference time of 8 hours, 28800 seconds.
If you process 200 plates per day, you have a total of 1000 impulses.
The daily exposure is obtained from this expression:
End of Quick Guide |
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