STRENGTH OF FIBER LINE

Overloading a line poses a serious safety threat to personnel It is also likely to result in heavy losses through damage to material and equipment. 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, and safety factor.

Breaking Strength

The term breaking strengfh refers to the tension at which the line will break apart when an additional load is applied. The breaking strength of the various lines has been determined through tests made by line manufacturers, and tables have been established to provide this information. In the absence of a manufacturer's table, a rule of thumb for finding the breaking strength of manila line is as follows:

C squared x 900 = BS

In this rule, C = circumference in inches and BS = breaking strength in pounds. The circumference is squared and the figure obtained is then multiplied by 900 to find BS. With a 3-inch line, for example, you will get a BS of 8,100 pounds. This was figured as follows:

3 x 3 x 900 = 8,100 lb

When the line is measured in centimeters, the breaking strength can be figured in kilograms. The same equation is used with only the constant being changed to 64.8 (vice 900). The breaking strength in kilograms is figured as follows:

7.5 cm x 7.5 cm x 64.8 = 3,645 kg

The breaking strength of manila line is higher than that of an equal-size sisal line. This is because of 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.

Safe Working Load

Briefly defined, the safe working load of a line is the load that can be applied without causing damage to the line. Remember that the safe working load of a line is considerably less than the breaking strength. A wide margin of difference between breaking strength and safe working load is necessary to allow 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. Fortunately, there is a rule of thumb with which you can determine the safe working load (SWL).

SWL = C squared x 150

SWL = the safe working load in pounds

C = the circumference of the line in inches To determine the SWL, simply take the circumference of the line, square it, and then multiply by 150. For example, for a 3-inch line, here is how the rule works:

3 x 3 x 150= 1,350 lb

Thus the safe working load of a 3-inch line is 1,350 pounds.

In the metric system, the rule is as follows: SWL = C squared x 10.8

SWL = the safe working load in kilograms

C = the circumference of the line in centimeters Substituting in the equation for a 3-inch line the centimeter equivalent of 3 inches (3 inches = 7.5 cm), the formula becomes the following:

7.5 cm x7.5 cm x10.8=607.5 kg

Thus the safe working load of a line 7.5 cm in circumference is equal to 607.5 kg.

NOTE: 10.8 is the metric constant equivalent to 150 in the decimal system.

If the line is in good shape, add 30 percent to the calculated SWL. 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 would give you a safe working load of 1,755 pounds. On the other hand, subtracting 30 percent from the 1,350 pounds would leave you a safe working load of 945 pounds.

Remember that the strength of a line will decrease 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 the breaking strength and safe working load capacity of the line. Manufacturers of line provide tables to show the breaking strength and safe working load capacity of line. You will find such tables useful in your work; however, you must remember that the values given in manufacturers' tables apply to NEW LINE used under favorable conditions. For that reason, you must PROGRESSIVELY REDUCE the values given in the manufacturers' tables as the line ages or deteriorates with use.

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 will vary, depending on such things as the condition of the line and circumstances under which it is to be used. While the safety factor should never be less than 3, it often must be well above 4 (possibly as high as 8 or 10). For best, average, or unfavorable conditions, the safety factors indicated below are usually suitable.

Best condition (new line): 4.

Average condition (line used, but in good condition): 6.

Unfavorable condition (frequently used line, such as running rigging): 8.

Breaking Strength of Nylon Line

The breaking strength of nylon line is almost three times that of manila line of the same size. The rule of thumb for the breaking strength of nylon line is as follows:

BS = C squared x 2,400

NOTE: The symbols in this rule are the same as those for fiber line in both the English and metric systems.

Application of the formula: determine the BS for a 2 1/2-inch nylon line in both pounds and kilograms:

Solution: BS = 2.5 x 2.5 x 2,400 = 15,000 pounds or BS = 6.35 cm x 6.35 cm x 172.8 = 6,967 kilograms

NOTE: The constant for the metric system is 172.8.

Nylon line can withstand repeated stretching to this point with no serious effects. When nylon line is underload, it thins out. Under normal safe working loads, nylon line will stretch about one third of its length. When free of tension, it returns to its normal size.

When nylon line is stretched more than 40 percent, it is likely to part. The stretch is immediately recovered with a snapback that sounds like a pistol shot.

WARNING

The snapback of a nylon line can be as deadly as a bullet. This feature is also true for other types of lines, but overconfidence in the strength of nylon may lead one to underestimate its backlash; therefore, ensure that no one stands in the direct line of pull when a heavy strain is applied to a line.

The critical point of loading is 40-percent extension of length; for example, a 10-foot length of nylon line would stretch to 14 feet when underload. Should the stretch exceed 40 percent, the line will be in danger of parting.

Nylon line will hold a load even though a considerable number of strands are abraded. Ordinarily, when abrasion is localized the line maybe made satisfactory for reuse by cutting away the chafed section and splicing the ends.

Privacy Statement - Press Release - Copyright Information. - Contact Us - Support Integrated Publishing