greater than the SWL of the wire rope, the test weight
will be computed from the SWL of the wire rope or if
the winch capacity is less than the SWL of the wire
rope, the test weight will be computed from the
capacity of the winch. The winch capacity is either
documented on the load charts or in the
The formula for computing the SWL for a hoist
rope is the diameter of the rope squared multiplied by
8 or (D x D x 8 = SWL in tons).
Example: The wire rope on a crane is 1/2 inch in
diameter. Compute the SWL for the rope.
The first step is to convert the 1/2 into a decimal
number by dividing the bottom number of the fraction
into the top number of the fraction: (1 divided by
2 = .5). Next, compute the SWL formula: (.5 x .5 x 8
= 2 tons). The SWL of the l/2-inch wire rope is
The next factor to compute is the breaking
strength of the wire rope. On some wire rope spools,
the nominal breaking strength of the wire rope is
published; however, if the breaking strength is
unknown, a break test can be performed on the wire
rope. This is accomplished by cutting off sections of
the wire rope and placing each section of the rope on
a wire rope break test machine. The machine pulls the
wire rope apart and computes the breaking strength.
By testing several sections of the wire rope, you can
determine the average breaking strength for that type
of wire rope.
Overseas, Public Works Centers
normally have wire rope break test machines that can
be used by the NCF.
If the break test cannot be
performed, the rule of thumb used for finding the
breaking strength is to multiply the SWL by 5
(SWL x 5 = B.S.). For example, a l/2-inch wire rope
with a SWL of 2 tons has a breaking strength of 10
tons (2 x 5 = 10 tons).
REMEMBER: When the
single line wire rope end connection is assembled
with a wedge socket, the wedge socket only develops
70 percent of the breaking strength. Example: The
crane is rigged with l/2-inch wire rope with a wedge
socket end connection. The wedge socket only
develops seventy percent of the l/2-inch wire rope
B.S. of 10 tons, which gives the wire rope a B.S.
determined by an end connection of 7 tons. Swaged
socket, cappel socket, and the zinc (spelter) socket all
provide 100 percent of the breaking strength when
The next factor to compute is the AWL by using
the factor of safety (F.S.). To compute the allowable
working load (AWL) of a wire rope, you must first
understand the following wire rope safety factors:
1. Rigging rope
a. 5 to 1 under operating conditions
b. 10 to 1 when used to lift personnel
2. Pendants or standing rope
a. 3.0 to 1 under operating conditions
b. 2.5 to 1 when erecting the boom
3. Ropes that wind on drums or pass over
a. 3.5 to 1 under operating conditions
b. 3.0 to 1 when erecting the boom
For the auxiliary line, use the F.S. of 3.5 for wire
rope that winds on drums or passes over sheaves. The
formula for the F.S. is the breaking strength (B.S.),
determined by the type of end connection divided by
F.S. (Example: B.S. = 7 divided by 3.5 = AWL of
The next factor to compute is the test weight.
This is done by multiplying the AWL of 2 tons by 110
percent (2 x 110% or 2 x 1.10 = 2.2 tons). Your test
weight for the l/2-inch wire rope is 2.2 tons. After the
test weight is figured, you must remember that the
hook blocks and rigging gear are weight that are part
of the test weight.
Leveling a crane cannot be overemphasized.
Cranes must be set up as per manufacturers
instruction, with the outriggers fully extended and the
crane leveled. Crane capacity is lost when the crane
is out of level by a few degrees (fig. 3-14). Most
Figure 3-14.-Crane capacity lost by crane out of level.