Figure 6-36. - Lays 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
Figure 6-37. - A. 6 x 19 wire rope; B. 6 x 37 wire rope.
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, 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.Continue Reading