WIRE ROPE

During the course of a project, SEABEEs often need to hoist or move heavy objects. Wire rope is used for heavy-duty work. The characteristics, construction, and usage of many types of wire rope are discussed in the following paragraphs. We will also discuss the safe working load, use of attachments and fittings, and procedures for the care and handling of wire rope.

CONSTRUCTION

Wire rope consists of three parts: wires, strands, and core (fig. 6-21). In the manufacture of rope, a number of WIRES are laid together to form the STRAND. Then a number of STRANDS are laid together around a CORE to form the ROPE.

The basic unit of wire rope construction is the individual wire, which may be made of steel, iron, or other metal in various sizes. The number of wires to a strand will vary, depending on the purpose for which the rope is intended. Wire rope is designated by the number of strands per rope and the number of wires per strand. Thus, a 1/2-inch, 6 by 19 wire rope will have 6 strands with 19 wires per strand; but it will have the

Figure 6-21.\Parts of a wire rope.

same outside diameter as a 1/2-inch, 6 by 37 wire rope, which will have 6 strands with 37 wires of much smaller size per strand.

Wire rope that is made up of a large number of small wires is flexible. The small wires are, however, easily broken, so the wire rope does not resist external abrasion. Wire rope that is made up of a smaller number of larger wires is more resistant to external abrasion but is less flexible.

The CORE-the element around which the strands are laid to form the rope-may be a hard fiber (such as manila, hemp, plastic, paper, asbestos, or sisal), a wire strand, or an independent wire rope. Each type of core serves the same basic purpose\to support the strands laid around it.

A FIBER CORE offers the advantage of increased flexibility. Also, it serves as a cushion to reduce the effects of sudden strain and acts as a reservoir for the oil to lubricate the wires and strands to reduce friction between them. Wire rope with a fiber core is used in places where flexibility of the rope is important.

A WIRE STRAND CORE not only resists heat better than a fiber core, but it also adds about 15 percent to the strength of the rope. On the other hand, the wire strand makes the rope less flexible than a fiber core would.

An INDEPENDENT WIRE ROPE CORE is a separate wire rope over which the main strands of the rope are laid. It usually consists of six 7-wire strands laid around either a fiber core or a wire strand core. This core strengthens the rope more, provides support against crushing, and supplies maximum resistance to heat.

Wire rope may be made by either of two methods. If the strands or wires are shaped to conform to the curvature of the finished rope before laying up, the rope is termed preformed.

If they are not shaped before fabrication, the rope is termed nonpreformed. When cut, preformed wire rope tends not to unlay, and it is more flexible than nonpreformed wire rope. With nonpreformed wire rope, twisting produces a stress in the wires; and, when it is cut or broken, the stress causes the strands to unlay. In nonpreformed wire, unlaying is rapid and almost instantaneous, which could cause serious injury to someone not familiar with it.

The main types of wire rope used by the Navy have 6, 7, 12, 19, 24, or 37 wires in each strand. Usually, the rope has six strands laid around a fiber or steel center.

Two common types of wire rope, 6 by 19 and 6 by 37 rope, are shown in figure 6-22. The 6 by 19 type of rope, having 6 strands with 19 wires in each strand, is commonly used for rough hoisting and skidding work where abrasion is likely to occur. The 6 by 37 wire rope, having 6 strands with 37 wires in each strand, is the most flexible of the standard six-strand ropes. For that reason, it is particularly suitable when small sheaves and drums are to be used, such as on cranes and similar machinery.