Line manufacturers have determined breaking strength through tests and have set up tables to provide this information 

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. 

SAFETY FACTOR.- 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

Table 1-1 lists some of the properties of manila and sisal line, including strength.

The table shows that the minimum breaking strength is considerably greater than the safe working capacity. The difference is caused by the application of a safety factor. The safe working load (SWL) of line is obtained by dividing the breaking strength (BS) by a factor of safety (FS). A new 1-inch-diameter No. 1 manila line has a breaking strength of 9,000 pounds, as indicated in table l-l. To determine the safe working load of the line, you would divide its breaking strength by a minimum standard safety factor of 4. The result is an SWL of 2,250 pounds. A safety factor is always used because the breaking strength of line be comes reduced after use and exposure to weather conditions. In addition, a safety factor is required because of shock loading, knots, sharp bends, and other stresses that the line may encounter during its use.

If tables are not available, the SWL may be closely approximated by a rule of thumb. The rule of thumb for the SWL, in tons, for fiber line is equal to the square of the line diameter in inches (SWL = D 2 ). The SWL, in tons, of a 1/ 2-inch-diameter fiber line would be 1/ 2 inch squared or 1/ 4 ton. The rule of thumb allows a safety factor of approximately 4.

SYNTHETIC-FIBER LINES Synthetic-fiber lines, such as nylon and polyester, have rapidly gamed wide use by the Navy. They are lighter in weight, more flexible, less bulky, and easier to handle and stow than manila lines. Also, they are highly resistant to mildew, rot, and fungus. Synthetic lines are stronger than natural-fiber line; for example, nylon is about three times stronger than manila. When nylon line is wet or frozen, the loss of strength is relatively small. Nylon line will hold a load, even though several strands may be frayed. Ordinarily, the line can be made reusable by cutting away the chafed or frayed section and splicing the good line together.

Table 1-1.- Properties of Manila and Sisal Line

Nominal Circum-Lb per ft No. 1 Manila Sisal diameter ference (inches) (inches) Breaking Safe Breaking Safe strength load strength load

Tables 1-2 and 1-3 list the approximate safe work-ing loads of new fiber lines with a safety factor of 5. NOTE: These are for reference only. Check the manufacturer's ratings before determining the safe working loads, as they may differ from the tables. You may not always have a chart available to tell you the safe working load for a particular size of line.

There is a rule of thumb that will adequately serve your needs on such an occasion. The formulas below are examples of synthetic fiber lines used for rigging, not slings.

For nylon or polyester line, change the line diameter into eighths of an inch, square the numerator,

and multiply by 60. Example: 1/ 2-inch nylon line = 4/ 8-inch diameter

SWL = 4 x 4 x 60 = 960 lb

For polypropylene line, multiply the diameter by 40, and for polyethylene, by 35.

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, you should give these and other factors affecting strength careful consideration and make proper adjustment in the breaking strength and SWL of the line.