11
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
hertz is potentially damaging and is most hazardous in the range from about 5 to 20 hertz.
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/s2 are considered hazardous when used for prolonged periods.
A tool with a vibration value of 2.8 m/s2 may be used for up to 8 hours (cumulative) per
day, whereas a tool with a value of 11.2 m/s2 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 abnor mal, 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.
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