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: C'2 x 900 = 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 same size.

The best rule of thumb for the breaking strength of nylon is BS = C2x 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 = C2 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 with use.

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 it.

10), For best, average, or unfavorable conditions, the following safety factors may often be suitable:

Best conditions (new line): 4; Average conditions (line used, but in good condition): 6; and Unfavorable conditions (frequently used line, such as running rigging): 8.

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