Fiber line is made from either natural or synthetic fiber. Natural fibers, which come from plants, include manila, sisal, and hemp. The synthetic fibers include nylon, polyester, and polypropylene.
NATURAL FIBER ROPES
The two most commonly used natural fiber ropes are manila and sisal, but the only type suitable for construction rigging is a good grade of manila. High-quality manila is light cream in color, smooth, clean, and pliable. The quality of the line can be distinguished by varying shades of brown: Number 1 grade is very light in color; Number 2 grade is slightly darker; Number 3 grade is considerably darker. The next best line-making fiber is sisal. The sisal fiber is similar to manila, but it is lighter in color. This type of fiber is only about 80 percent as strong as manila fiber.
SYNTHETIC FIBER ROPES
Synthetic fiber rope, such as nylon and polyester, has rapidly gained wide use by the Navy. It is lighter in weight, more flexible, less bulky, and easier to handle and store than manila line. It is also highly resistant to mildew, rot, and fungus. Synthetic rope is stronger than natural fiber rope. 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 rope will hold a load even though several stands may be frayed. Ordinarily, the line can be made reusable by cutting away the chafed or frayed section and splicing the good line together.
FABRICATION OF LINE
The fabrication of line consists essentially of three twisting operations. First, the fibers are twisted to the right to form the yarns. Next, the yarns are twisted to the left to form the strands. Finally, the strands are twisted to the right to form the line. Figure 4-1 shows you how the fibers are grouped to form a three-strand line.
Figure 4-1.Fiber groupings in a three-strand line.
The operation just described is the standard procedure, and the resulting product is known as a right-laid line. When the process is reversed, the result is a left-laid line. In either instance, the principle of opposite twists must al ways be observed. The two main reasons for the principle of opposite twists are to keep the line tight to prevent the fibers from unlaying with a load suspended on it and to prevent moisture penetration.
Types of Line Lays
There are three types of fiber line lays: hawser-laid, shroud-laid, and cable-laid lines. Each type is illustrated in figure 4-2.
Figure 4-2.Time type of fiber line.
Hawser-laid line generally consists of three strands twisted together, usually in a right-hand direction. A shroud-laid line ordinarily is composed of four strands twisted together in a right-hand direction around a center strand, or core, which usually is of the same material, but smaller in diameter than the four strands. You will find that shroud-laid line is more pliable and stronger than hawser-laid line, but it has a strong tendency toward kinking. In most instances, it is used on sheaves and drums. This not only prevents kinking, but also makes use of its pliability and strength. Cable-laid line usually consists of three right-hand, hawser-laid lines twisted together in a left-hand direction. It is especially safe to use in heavy construction work; if cable laid line untwists, it will tend to tighten any regular right-hand screw connection to which it is attached.
Line that is 1 3/4 inches or less in circumference is called small stuff this size is usually designated by the number of threads (or yarns) that make up each strand. You may use from 6- to 24-thread strands, but the most commonly used are 9- to 21-thread strands (figure 4-3). You may hear some small stuff designated by name without reference to size. One such type is marlinea tarred, two-strand, left-laid hemp. Marline is the small stuff you will use most for seizing. When you need something stronger than marline, you will use a tarred, three-strand, left-laid hemp called houseline.
Figure 4-3.Some commonly used sizes of manila line.
Line larger than 1 3/4 inches in circumference is generally size designated by its circumference in inches. A 6-inch manila line, for instance, is constructed of manila fibers and measures 6 inches in circumference. Line is available in sizes ranging up to 16 inches in circumference, but 12 inches is about the largest carried in stock. Anything larger is used only on special jobs.
If you have occasion to order line, you may find that in the catalogs, it is designated and ordered by diameter. The catalog may also use the term "rope" rather than "line."
Rope yarns for temporary seizing, whippings, and lashings are pulled from large strands of old line that 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 same size.
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 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.
Figure 4-4.Results of a strong strain on a tine with a kink in it.
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 10), For best, average, or unfavorable conditions, the following safety factors may often be suitable:
HANDLING AND CARE OF LINES
If you expect the fiber line you work with to give safe and dependable service, make sure it is handled and cared for properly. Study the precautions and procedures given here and carry them out properly.
Cleanliness is part of the care of fiber line. Never drag a line over the deck or ground, or over rough or dirty surfaces. The line can easily pick up sand and grit, which will work into the strands and wear the fibers. If a line does get dirty, use only water to clean it. Do not use soap because it will remove oil from the line, thereby weakening it.
Avoid pulling a line over sharp edges because the strands may break. When you encounter a sharp edge, place chafing gear, such as a board, folded cardboard or canvas, or part of a rubber tire between the line and the sharp edge to prevent damaging the line.
Never cut a line unless you have to. When possible, always use knots that can be untied easily.
Fiber line contracts, or shrinks, when it gets wet. If there is not enough slack in a wet line to permit shrinkage, the line is likely to become overstrained and weakened. If a taut line is exposed to rain or dampness, make sure the line, while still dry, is slackened to allow for the shrinkage.
Line should be inspected carefully at regular intervals to determine whether it is safe. The outside of a line does not show the condition of the line on the inside. Untwisting the strands slightly allows you to check the condition of the line on the inside. Mildewed line gives off a musty odor. Broken strands or yarns usually can be spotted immediately by a trained observer. You will want to look carefully to ensure there is not dirt or sawdust-like material inside the line. Dirt or other foreign matter inside reveals possible damage to the internal structure of the line. A smaller circumference of the line is usually a sure sign that too much strain has been applied to the line.
For a thorough inspection, a line should be examined at several places along its length. Only one weak spotanywhere in a line-makes the entire line weak. As a final check, pull out a couple of fibers from the line and try to break them. Sound fibers show a strong resistance to breakage.
If an inspection discloses any unsatisfactory conditions in a line, make sure the line is destroyed or cut up in small pieces as soon as possible. This precaution prevents the defective line from being used for hoisting.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015