has outlived its usefulness. Full your yarn from the
middle, away from the ends, or it will get fouled.
STRENGTH OF FIBER LINE
Overloading a line poses a serious threat to the
safety of personnel, not to mention the heavy losses
likely to result through damage to material. To avoid
overloading, you must know the strength of the line
with which you are working. This involves three
factors: breaking strength, safe working load (swl),
and safety factor.
Breaking strength refers to the tension at which
the line will part when a load is applied. Breaking
strength has been determined through tests made by
rope manufacturers, who provide tables with this
information. In the absence of manufacturers tables,
a rule of thumb for finding the breaking strength of
manila line using the formula:
= BS. C
equals the circumference in inches, and BS equals the
breaking strength in pounds. To find BS, first square
the circumference; you then multiply the value
obtained by 900. With a 3-inch line, for example, you
will get a BS of 8,100, or 3 x 3 x 900= 8,100 pounds.
The breaking strength of manila line is higher
than that of sisal line. This is caused by the difference
in strength of the two fibers. The fiber from which a
particular line is constructed has a definite bearing on
its breaking strength. The breaking strength of nylon
line is almost three times that of manila line of the
The best rule of thumb for the breaking strength
of nylon is BS = C2 x 2,400. The symbols in the rule
are the same as those for fiber line. For 2 1/2-inch
nylon line, BS = 2.5 x 2.5 x 2,400= 15,000 pounds.
Briefly defined, the safe working load of a line is
the load that can be applied without damaging the
line. Note that the safe working load is considerably
less than the breaking strength. A wide margin of
difference between breaking strength and safe
working load is necessary. This difference allows for
such factors as additional strain imposed on the line
by jerky movements in hoisting or bending over
sheaves in a pulley block.
You may not always have a chart available to tell
you the safe working load for a particular size line.
Here is a rule of thumb that will adequately serve your
needs on such an occasion: swl =
x 150. In this
equation, swl equals the safe working load in pounds,
and C equals the circumference of the line in inches.
Simply take the circumference of the line, square it,
then multiply by 150. For a 3-inch line, 3 x 3 x 150=
1,350 pounds. Thus, the safe working load of a 3-inch
line is equal to 1,350 pounds.
If line is in good shape, add 30 percent to the swl
arrived at by means of the preceding rule; if it is in
bad shape, subtract 30 percent from the swl. In the
example given above for the 3-inch line, adding 30
percent to the 1,350 pounds gives you a safe working
load of 1,755 pounds. On the other hand, subtracting
30 percent from the 1,350 pounds leaves you with a
safe working load of 945 pounds.
Remember that the strength of a line decreases
with age, use, and exposure to excessive heat, boiling
water, or sharp bends.
Especially with used line,
these and other factors affecting strength should be
given careful consideration and proper adjustment
made in determining the breaking strength and safe
working load capacity of the line. Manufacturers of
line provide tables that show the breaking strength
and safe working load capacity of line. You will find
such tables very useful in your work. You must
remember, however, that the values given in
manufacturers tables only apply to new line being
used under favorable conditions. For that reason, you
must progressively reduce the values given in
manufacturers tables as the line ages or deteriorates
Keep in mind that a strong strain on a kinked or
twisted line will put a permanent distortion in the line.
Figure 4-4 shows what frequently happens when
pressure is applied to a line with a kink in it. The kink
that could have been worked out is now permanent,
and the line is ruined.
The safety factor of a line is the ratio between the
breaking strength and the safe working load. Usually,
a safety factor of 4 is acceptable, but this is not always
the case. In other words, the safety factor varies
depending on such things as the condition of the line
and circumstances under which it is to be used.
Although the safety factor should never be less than 3,
it often must be well above 4 (possibly as high as 8 or
Figure 4-4.Results of a strong strain on a tine with a kink in