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Rope Breaking
Strength
Each type of
line, natural fiber, synthetic and wire rope, have different
breaking strengths and safe working loads. Natural breaking
strength of manila line is the standard against which other
lines are compared. Synthetic lines have been assigned
"comparison factors" against which they are compared
to manila line. The basic breaking strength factor for manila
line is found by multiplying the square of the circumference
of the line by 900 lbs.
(900
lbs. X circumference2 = breaking strength)
When you
purchase line you will buy it by its diameter. However, for
purposes of the USCG license exams, all lines must be measured
by circumference. To convert use the following formula.
Circumference
= p PI (3.14) X diameter
As an example,
if you had a piece of ½" manila line and wanted to find
the breaking strength, you would first calculate the
circumference. (.5 X 3.14 = 1.57) Then using the formula
above:
1.572
X 900 = 2,218 pounds of breaking strength
To calculate the
breaking strength of synthetic lines you need to add one more
factor. As mentioned above, a comparison factor has been
developed to compare the breaking strength of synthetics over
manila. Since synthetics are stronger than manila an
additional multiplication step is added to the formula above.
(comparison
factor X 900 lbs. X circumference2 = breaking
strength)
Following is a
comparison factor chart for synthetic lines.
|
Line
Material
|
Comparison
Factor (greater than manila)
|
| Nylon |
2.5 |
| Dacron |
2.0 |
| Polypropylene |
1.4 |
Using the
example above, let’s find the breaking strength of a piece
of ½" nylon line. First convert the diameter to
circumference as we did above and then write the formula
including the extra comparison factor step.
2.5
X 1.572 X 900 = 5, 546 pounds of breaking strength
Knots and
splices will reduce the breaking strength of a line by as much
as 50 to 60 percent. The weakest point in the line is the knot
or slice. However, a splice is stronger than a knot.
Just being able
to calculate breaking strength doesn’t give one a safety
margin. The breaking strength formula was developed on the
average breaking strength of new line under laboratory
conditions. Without straining the line until it parts, you
don’t know if that particular piece of line was above
average or below average. Next we will discuss safe working
load.
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