Wire rope is identified by its construction, or the number of strands per rope, and number of wires in each strand. For example, the construction 6x25 denotes a 6-strand rope, with each strand having 25 wires. Constructions having similar weights and breaking strengths are grouped into wire rope classifications, such as the 6x19 and 6x37 Classes.
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The term bright refers to a wire rope manufactured with no protective coating or finish other than lubricant. Some applications do require more corrosion protection than lubricant can provide. In these instances, a galvanized finish is provided. Contact Assembly Specialty for more information on galvanized wire rope.
Improved plow (IPS) steel is strong, tough, durable steel that combines great strength with high resistance to fatigue. Its minimum tensile strength varies from 223 to 258 ksi, depending upon wire diameter. Extra Improved Plow (EIP) steel has a minimum tensile strength that varies from 245 to 284 ksi, depending upon wire diameter. Extra Extra Improved Plow (EEIP) steel, is a grade used where a high breaking strength is required. This grade typically provides a breaking strength a minimum of 10% higher than (EIP) and is found primarily as a standard grade for specialized wire rope and often is available for standard wire ropes upon request. Developed for the federal government, Royal Purple Plus is Wirerope Works’ highest strength grade available to Bethlehem Wire Rope customers. Royal Purple Plus provides a breaking strength 35% higher than Purple Plus, and is available in Wirerope Works’ Triple-Pac hoist rope. It is the grade of wire which determines the nominal breaking strength for each diameter and construction. Note the acceptance strength listed in the various tables for wire rope products is 2-1/2% below the nominal strengths listed. Other grades are available to meet specific requirements. Some grades are covered by wire rope standards while others may be specially tailored. Contact Assembly Specialty for more information.
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 varies, 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 rope will have 6 strands with 19 wires per strand; but it will have the 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 made up of a large number of small wires is flexible, but the small wires are easily broken, so the wire rope does not resist external abrasion. Wire rope made up of a smaller number of larger wires is more resistant to external abrasion but is less flexible.
The core is the element around which the strands are laid to form the rope. It 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 more than a fiber core, but 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.
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An independent wire rope core is a separate wire rope over which the main strands of the row are laid. It usually consists of six, seven-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 maybe 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. Wire 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 consist of 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 illustrated in views A and B of figure 4-6, respectively. 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 6-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.
Wire rope is made in a number of different grades. Three of the most common are mild plow steel, plow steel, and improved plow steel.
Mild plow steel rope is tough and pliable. It can stand up under repeated strain and stress, and it has a tensile strength of from 200,000 to 220,000 pounds per square inch (psi). Plow steel wire rope is unusually tough and strong. It has a tensile strength (resistance to lengthwise stress) of 220,000 to 240,000 psi. This rope is suitable for hauling, hoisting, and logging. Improved plow steel rope is one of the best grades of rope available, and most, if not all, of the wire rope in your work will probably be made of this material. It is stronger, tougher, and more resistant to wear than either plow steel or mild plow steel. Each square inch of improved plow steel can withstand a strain of 240,000 to 260,000 psi.
Figure 4-6. - Two common types of wire rope.
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