Ergonomics - Atlas Copco

80
Safety factor a 2
Frequency of the operation
The capacity of a muscle group to gener-
ate force is related to the degree of muscle
fatigue. The more fatigued the muscle, the
lower the maximum force generating cap-
acity. The fatigue effect depends on the
frequency and duration of each operation.
When considering the effect of operation
frequency on the maximum allowable force,
a safety factor may be selected according to
Table 3.7.
Operation
duration(s):
Table 3.7 Safety factor a 2 to reduce MVC due to
frequency of operation.
If frequency is not known, the total
duration of use, Table 3.10, can be of help.
The total duration of use for a screwdriver,
for example, is two hours per day.
Let’s assume that each tightening cycle
lasts two seconds. This gives 3,600 tighten-
ing cycles per day, 450 per hour, on average
7.5 tightenings per minute.
In this case a 2 = 0.6.
Frequency of
operation ( times/min.):
6
Safety factor a2 10 seconds 0.6 -
Safety factor a 3
The total duration of the operation
The maximum force generating capacity is
reduced as the total duration of the work
task increases and muscles become fatigued.
Total duration per day (a 3 ) should also
take into consideration other tasks per-
formed during the day where the same mus-
cle groups are used. For instance, if there is
time for other tasks, and they involve using
other hand-held tools which also need to be
evaluated, the evaluation tends to become
rather complicated.
If the total duration of operation is less
than one hour in a working day, the influence
of duration on muscle fatigue can be ignored.
However, if the total duration of the operation
is longer than one hour, a safety factor must
be taken into consideration when calculating
the maximum allowable force limit. When the
total duration of operation is 1 to 2 hours, a
safety factor of 0.8 may be included and when
the duration is longer than 2 hours in one
working day, the safety factor should be 0.5.
Total duration per day Safety factor a 3
Less than 1 hour 1
Between 1 and 2 hours 0.8
More than 2 hours 0.5
Table 3.8 Safety factor a 3 to reduce MVC due to
total duration per day of operation.