Geoscience Reference
In-Depth Information
requirements of ASME B30.9, and OSHA 1910.184(h) must be followed. All types of slings must
have, as a minimum, the rated capacity clearly and permanently marked on each sling. Each sling
must receive a documented inspection at least annually, more frequently if recommended by the
manufacturer or made necessary by service conditions.
Note: Slings are commonly used between cranes, derricks, or hoists and the load, so the load may
be lifted and moved to a desired location. For the safety engineer, the properties and limita-
tions of the sling, the type and condition of material being lifted, the weight and shape of the
object being lifted, the angle of the lifting sling to the load being lifted, and the environment
in which the lift is to be made are all important considerations to be evaluated—before the
transfer of material can take place safely.
Let's take a look at a few example problems involving forces that the environmental engineer
might be called upon to calculate. In our examples, we use lifting slings under different conditions
of loading.
EXAMPLE 11.3
Problem: Let us assume a load of 2000 lb supported by a two-leg sling; the legs of the sling make
an angle of 60° with the load. What force is exerted on each leg of the sling?
Solution: When solving this type of problem, always draw a rough diagram as shown in Figure
11.2. A resolution of forces provides the answer. We will use the trigonometric method to solve this
problem, but remember that it may also be solved using the graphic method. Using the trigonometric
method with the parallelogram law, the problem could be solved as described below. Again, make a
drawing to show a resolution of forces similar to that shown in Figure 11.3.
We could consider the load (2000 lb) as being concentrated and acting vertically, which can be
indicated by a vertical line. The legs of the slings are at a 60° angle, which can be shown as ab and
ac . The parallelogram can now be constructed by drawing lines parallel to ab and ac , intersecting
at d . The point where cb and ad intersect can be indicated as e . The force on each leg of the sling
( ab , for example) is the resultant of two forces, one acting vertically ( ae ), the other horizontally ( be ),
as shown in the force diagram. Force ae is equal to one-half of ad (the total force acting vertically,
2000 lb), so ae = 1000. This value remains constant regardless of the angle ab makes with bd,
because as the angle increases or decreases, ae also increases or decreases. But ae is always ad /2.
The force ab can be calculated by trigonometry using the right triangle abe :
Opposite side
Hypotenuse
Sine of an angle
=
60°
60°
2000 #
FIGURE 11.2
Illustration for Example 11.3.
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