MEASURING WIRE ROPE
The size of wire rope is designated by its
diameter. The true diameter of a wire rope is the
diameter of a circle that will just enclose all of its
strands. Correct and incorrect methods of measuring
wire rope are illustrated in figure 4-7. In particular,
note that the correct way is to measure from the top of
one strand to the top of the strand directly opposite it.
The wrong way is to measure across two strands side
by side. Use calipers to take the measurement. If
calipers are not available, an adjustable wrench will
To ensure an accurate measurement of the
diameter of a wire rope, always measure the rope at
three places, at least 5 feet apart. Use the average of
the three measurements as the diameter of the rope.
SAFE WORKING LOAD
The term safe working load (swl), as used in
reference to wire rope, means the load that can be
applied and still obtain the most efficient service and
also prolong the life of the rope. Most manufacturers
provide tables that show the safe working load for
their rope under various conditions. In the absence of
these tables, you must apply a thumb rule formula to
obtain the swl. There are rules of thumb that may be
used to compute the strength of wire rope. The one
recommended by the Naval Facilities Engineering
Command (NAVFAC) is swl (in tons)= D2 x 8. This
particular formula provides an ample safety margin to
account for such variables as the number, size, and
location of sheaves and drums on which the rope runs.
Also included are dynamic stresses, such as the speed
of operation and the acceleration and deceleration of
the load. All can affect the endurance and breaking
strength of the rope.
Lets work an example. In the above formula, D
represents the diameter of the rope in inches.
Suppose you want to find the swl of a 2-inch rope.
Using the formula above, your figures would be:
swl = 22 x 8, or 4 x 8 = 32. The answer is 32,
meaning that the rope has a swl of 32 tons.
It is very important to remember that any formula
for determining swl is only a rule of thumb. In
computing the swl of old rope, worn rope, or rope that
is otherwise in poor condition, you should reduce the
swl as much as 50 percent, depending on the
condition of the rope.
The manufacturers data
concerning the breaking strength (BS) of wire rope
should be used if available. But if you do not have
that information, one rule of thumb recommended is
BS = C2 x 8,000 pounds.
As you recall, wire rope is measured by the
diameter (D). To obtain the circumference (C)
required in the formula, multiply D by pi (usually
shown by the Greek letter
which is approximately
3.1416. Thus, the formula to find the circumference
is C =
WIRE ROPE FAILURE
Wire can fail due to any number of causes. Here
is a list of some of the common causes of wire rope
Using the incorrect size, construction, or grade
of wire rope;
Dragging rope over obstacles;
Having improper lubrication;
Operating over sheaves and drums of
Overriding or crosswinding on drums;
Operating over sheaves and drums with
improperly fitted grooves or broken flanges;
Jumping off sheaves;
Subjecting it to acid fumes;
Attaching fittings improperly;
Promoting internal wear by allowing grit to
penetrate between the strands; and
Subjecting it to severe or continuing overload.
Figure 4-7.Correct and incorrect methods of measuring