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, 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, and tables have been set up to provide this information. In the absence of manufacturers' tables, a rule of thumb for finding the breaking strength of manila line is as follows:

In the rule, C equals the circumference in inches, and BS equals the breaking strength in pounds. To find BS, square the circumference and then multiply the figure obtained by 900. With a 3-inch line, for example, you will get a BS of 8,100 pounds, figuring as follows:

3 3 900 = 8,100 lb

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 as follows:

The symbols in the rule are the same as those for fiber line.

For 2 1/2-inch nylon line,

BS = 2.5 2.5 2,400 = 15,000 lb

SAFE WORKING LOAD

Briefly defined, the "safe working load" (SWL) of a line is the load that can be applied without causing any kind of damage to 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 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-so what do you do then? Fortunately, there is a rule of thumb that will adequately serve your needs on such an occasion.

Where SWL equals the safe working load in pounds and C equals the circumference of the line in inches, you simply take the circumference of the line, square it, and then multiply by 150. For a 3-inch line, here is how the rule works.

3 3 150 = 1,350 lb

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

If the line is in good shape, add 30 percent to the SWL determined by means of the rule; or 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 lb would give you a safe working load of 1,755 lb. On the other hand, subtracting 30 percent from the 1,350 lb would leave you a safe working load of 945 lb.

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 should be 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. You must remember, however, that the values given in manufacturers' tables apply only 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.

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 should be well above 4 (possibly as high as 8 or 10). For best, average, or unfavorable conditions, the safety factor indicated below may often be suitable.

BEST conditions (new line): 4

AVERAGE conditions (line used, but in good condition): 6

UNFAVORABLE conditions (frequently used line, such as running rigging): 8