Mild plow steel wire rope is tough and pliable. It can stand repeated strain and stress and has a tensile strength (resistance to lengthwise stress) from 200,000 to 220,000 pounds per square inch (psi). These characteristics make it desirable for cable tool drilling and other purposes where abrasion is encountered.

Figure 6-35.- Core construction.

Plow steel wire rope is usually tough and strong. This steel has a tensile strength of 220,000 to 240,000 psi. Plow steel wire rope is suitable for hauling, hoisting, and logging.

Improved plow steel wire rope is one of the best grades of rope available and is the most common rope used in the NCF. This type of rope is stronger, tougher, and more resistant to wear than the others. Each square inch of improved plow steel can stand a strain of 240,000 to 260,000 psi. This makes it especially useful for heavy-duty service, such as on cranes with excavating and weight-handling equipment.

Lays of Wire Rope
The term lay refers to the direction of the twist of the wires in a strand and to the direction that the strands are laid in the rope. In some instances, both the wires in the strand and the strands in the rope are laid in the same direction; and, in other instances, the wires are laid in one direction and the strands are laid in the opposite direction, depending on the intended use of the rope. Most manufacturers specify the types and lays of wire rope to be used on their piece of equipment. Be sure and consult the operator's manual for proper application.

The five different lays used in wire rope are as follows (fig. 6-36):

RIGHT REGULAR LAY has the wires in the strands laid to the left, while the strands are laid to the right to form the wire rope.

LEFT REGULAR LAY has the wires in the strands laid to the right, while the strands are laid to the left to form the wire rope. In this lay, each step of fabrication is exactly opposite from the right regular lay.

RIGHT LANG LAY has the wires in the strands and the strands in the rope laid to the right.

LEFT LANG LAY has the wire in the strands and the strands in the rope laid to the left.

REVERSE LAY has the wires in one strand laid to the right, the wire in the nearby strand are laid to the left, the wire in the next strand are to the right, and so forth, alternating direction from one strand to the other. Then all strands are laid to the right.

Figure 6-36.- Lays of wire rope.

Characteristics of Wire Rope
The main types of wire rope used consist of 6, 7, 12, 19, 24, or 37 wires per strand. Usually, the wire rope has six strands laid around the core.

The two most common types of wire rope, 6 x 19 and 6 x 37, are shown in figure 6-37. The 6 x 19 type (having six strands with 19 wires in each strand) are the stiffest and strongest construction of the types of wire rope suitable for general hoisting operations. The 6 x 37 wire rope (six strands with 37 wires in each strand) are very flexible, making it suitable for cranes and similar equipment.

Several factors must be considered whenever a wire rope is selected for use in a particular kind of operation. The manufacture of wire rope which can withstand equally well all kinds of wear and stress, it may be subjected to, is not possible. Because of this, selecting a rope is often a matter of compromise- sacrificing one quality to have some other more urgently needed characteristic.

Figure 6-37.- A. 6 x 19 wire rope; B. 6 x 37 wire rope.

TENSILE STRENGTH.- Tensile strength is the strength necessary to withstand a certain maximum load applied to the rope. It includes a reserve of strength measured in a so-called factor of safety.

CRUSHING STRENGTH.- Crushing strength is the strength necessary to resist the compressive and squeezing forces that distort the cross section of a wire rope, as it runs over sheaves, rollers, and hoist drums when under a heavy load. Regular lay rope distorts less in these situations than lang lay.

FATIGUE RESISTANCE.- Fatigue resistance is the ability to withstand the constant bending and flexing of wire rope that runs continuously on sheaves and hoist drums. Fatigue resistance is important when wire rope must run at high speeds. Such constant and rapid bending of the rope can break individual wires in the strands. Lang lay ropes are best for service requiring high fatigue resistance. Ropes with similar wires around the outside of their strands also have a greater resistance, since these strands are more flexible.

ABRASION RESISTANCE.- Abrasion resistance is the ability to withstand the gradual wearing away of the outer metal, as the rope runs across sheaves and hoist drums. The rate of abrasion depends mainly on the load carried by the rope and its running speed. Generally, abrasion resistance in a rope depends on the type of metal of which the rope is made and the size of the individual outer wires. Wire rope made of harder steels, such as improved plow steel, has a considerable resistance to abrasion. Ropes that have larger wires forming the outside of their strands are more resistant to wear than rope having smaller wires which wear away more quickly.

CORROSION RESISTANCE.- Corrosion resistance is the ability to withstand the dissolution of the wire metal that results from chemical attack by moisture in the atmosphere or elsewhere in the working environment. Ropes that are put to static work, such as guy wires, may be protected from corrosive elements by paints or other special dressings. Wire rope may be galvanized for corrosion protection. Most wire rope used in crane operations must rely on their lubricating dressing to double as a corrosion preventive.