LAY LENGTH OF WIRE ROPE

The length of a rope lay is the distance measured parallel to the center line of a wire rope in which a strand makes one complete spiral or turn around the rope. The length of a strand lay is the distance measured parallel to the center line of the strand in which one wire makes one complete spiral or turnaround the strand. Lay length measurement is shown in figure 13-5.

Figure 13-5.-Lay length of wire rope.

CHARACTERISTICS OF WIRE ROPE

The main types of wire rope used by the NCF consist of 6, 7, 12, 19, 24, or 37 wires in each 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 13-6. The 6 x 19 type (having six strands with 19 wires in each strand) is 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) is very flexible, making it suitable for cranes and similar equipment where sheaves are smaller than usual. The wires in the 6 x 37 are smaller than the wires in the 6 x 19 wire rope and, consequently, will not stand as much abrasive wear.

Several factors must be considered whenever a wire rope is selected for use in a particular kind of operation. The manufacture of a 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.

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,

Figure 13-6.-A. 6 x 19 wire rope; B. 6 x 37 wire rope.

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 the 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 smaller wires around the outside of their strands also have greater fatigue 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 the harder steels, such as improved plow steel, have considerable resistance to abrasion. Ropes that have larger wires forming the outside of their strands are more resistant to wear than ropes 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 paint or other special dressings. Wire rope may also be galvanized for corrosion protection. Most wire ropes used in crane operations must rely on their lubricating dressing to double as a corrosion preventive.