Environmental Engineering Reference
In-Depth Information
The variables in equations (1) and (2) were subject to the following limits
(NIOSH 1981):
H
is between 15 cm (6 in.) and 80 cm (32 in.). Objects cannot, in general,
be closer than 15 cm (6 in.) without interference with the body. Objects
further than 80 cm (32 in.) cannot be reached by many people.
V
is assumed between 0 cm and 175 cm (70 in.), representing the range of
vertical reach for most people.
D
is assumed between 25 cm (10 in.) and (200
−
V
) cm [(80
−
V
) in.]. For
25.
F
is assumed between 0.2 (one lift every 5 minutes) and
F
max
(see Table 2.1).
For lifting less frequently than once every 5 minutes, set
F
travel less than 25 cm, set
D
=
0.
Most common errors in calculating the AL from the 1981 NIOSH guide arose
from not using the limitations for each of the factors. One way to self-check for
calculation errors is to consider the four factors in the equation (the calculations
in parentheses) as modifiers to an initial load of 40 kg or 90 lbs. The factors can
only have values of 1 or less. If a factor has a value greater than 1, there has
been an error (most likely one of the limitations was not followed). For example,
suppose a load is lifted only 4 inches onto a platform. The vertical travel distance
factor would be (0
.
7
=
+
3
/D
). If
D
=
4 were substituted into the equation, the
value of the factor would be
(
0
.
7
3
/
4
)
, or 1.45. Since this value exceeds 1.0,
there has been an error. In checking the limits, it is seen that for
D
less than 10
inches, set
D
+
10. Now, going back to our vertical travel distance factor, the
proper calculation would be
(
0
.
7
=
3
/
10
)
,or1.0.
For the more graphical individuals, NIOSH (1981) presented the four factors in
graphical form in nomograms. One advantage of the nomograms is that no factor
can be greater than 1.0 when using all four nonograms (see Figures 2.1-2.4). In
+
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
20
10
40
60
80
(cm)
0.1
20
30
(in.)
Horizontal Location
Figure 2.1
Horizontal factor graph
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