CHAPTER 4 FIBER LINE, WIRE ROPE, AND SCAFFOLDING

This chapter presents information on how to use fiber line, wire rope, and timber in rigging and erecting hoisting devices (such as shear legs, tripods, blocks and tackles), and different types of scaffolds and ladders. Formulas are given on how to determine or find the safe working load of these materials.

FIBER LINE

LEARNING OBJECTIVE: Upon completing this section, you should be able to determine the use, breaking strength, and care officer lines and rope used for rigging.

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.

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.

Figure 4-1.-Fiber groupings in a three-strand line.

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.

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.

Figure 4-2.-Time type of fiber line.

Size Designation

Line that is 13/4 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 marline-a 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.

Line larger than 13/4 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

Figure 4-3.-Some commonly used sizes of manila line.

has outlived its usefulness. Full your yarn from the middle, away from the ends, or it will get fouled.

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