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Employers are advised to refer to the HSE publication “Guide for Employers”.
All hand held power tools vibrate to some extent, and this vibration is transmitted to the operator
via the handle, or hand used to steady the tool. Vibration from about 2 to 1500 Herz is potentially
damaging and is most hazardous in the range from about 5 to 20 Herz.
Operators who are regularly exposed to vibration may suffer from Hand Arm Vibration Syndrome
(HAVS), which includes ‘dead hand’, ‘dead finger’, and ‘white finger’. These are painful conditions
and are widespread in industries where vibrating tools are used.
The health risk depends upon the vibration level and the length of time of exposure to it……in
effect, a daily vibration dose.
Tools are tested using specialised equipment, to approximate the vibration level generated under
normal, acceptable operating conditions for the tool in question. For example, a grinder used at
45° on mild steel plate, or a sander on softwood in a horizontal plane etc.
These tests produce a value‘
a’
, expressed in metres per second per second, which represents the
average vibration level of all tests taken, in three axes where necessary, and a second figure ‘K’,
which represents the uncertainty factor, i.e. a value in excess of ‘a’, to which the tool could
vibrate under normal conditions. These values appear in the declaration on page 2.
HAND-ARM VIBRATION
You will note that a third value is given in the specification - the highest measured reading in a
single plane. This is the maximum level of vibration measured during testing in one of the axes,
and this should also be taken into account when making a risk assessment.
‘
a
’ values in excess of 2.5 m/s
2
are considered hazardous when used for prolonged periods. A tool
with a vibration value of 2.8 m/s
2
may be used for up to 8 hours (cumulative) per day, whereas a
tool with a value of 11.2 m/s
2
may be used for ½ hour per day only.
The graph below shows the vibration value against the maximum time the respective tool may be
used, per day.
The uncertainty factor should also be taken into account when assessing a risk. The two figures
‘
a
’ and ‘
K
’may be added together and the resultant value used to assess the risk.
It should be noted that if a tool is used under abnormal, or unusual conditions, then the vibration
level could possibly increase significantly. Users must always take this into account and make
their own risk assessment, using the
graph as a reference.
Some tools with a high vibration value,
such as impact wrenches, are
generally used for a few seconds at a
time, therefore the cumulative time
may only be in the order of a few
minutes per day. Nevertheless, the
cumulative effect, particularly when
added to that of other hand held
power tools that may be used, must
always be taken into account when
the total daily dose rate is
determined.