Wire Rope

Wire rope is wound from high-strength metal strands for structural, mechanical actuation and motion control applications. Suppliers typically list the number of wires per strand followed by the number of strands per rope or cable. For example, products with a 7 x 19 designation have seven strands per cable and 19 wires per strand. Some wire rope products are enclosed in a metal wrap or jacket to provide additional surface protection and abrasion resistance. Examples include swaged aluminum casing (lockclad), armored cable, flat wrap, round wrap, wire wrap or braiding, and solid metal conduits or tubing. Other products are encased in a plastic jacket, coating or conduit, or feature a protective plastic filling that is infused into the finished cable. Wire rope assemblies, mechanical control cables, and wire rope slings with attached clips, eyes, handles or other fittings are also commonly available.

Physical specifications for wire rope include diameter, length, breaking strength, and core type. The size of the pulley, sheave or drum determines the maximum diameter of the rope or cable that can be fed through the transit or fitting. With control cables, diameter usually refers to the overall, conduit or outer casing dimension. With bare wire rope, the largest outer diameter (OD) is listed because the rope’s diameter is not uniform in size. Diameter and length are measured in inches (in). Breaking strength is the maximum tensile load or force in pounds (lbs) that a rope or cable will hold before breaking. It is multiplied by a safety factor to determine the actual operating or working load. There are several core types for wire rope. Plastic cores contain a solid, polyvinyl chloride (PVC) rod or a multi-filament rope made of polypropylene (PP), nylon or other synthetic material. Fiber core (FC) and hybrid products with metal and fiber strands are also available. Stranded metal wire core (SWC or SC) and independent wire rope core (IWRC) varieties are the strongest core types.

Wire Rope Structure

Wires are the basic components of wire rope. They are wrapped around a center wire to form strands. The strands are then wrapped around a core to form the rope. The core of the rope can be fiber, a wire strand, or a wire rope (IWRC - Independent Wire Rope Core). Wrapping just the wire strands together (without a core) creates a hollow core wire rope.

The correct way to measure wire rope is with the faces of the caliper in contact with the crowns of two opposing strands.

Selecting the Proper Type of Wire Rope Core

An important point to consider is the selection of the proper type of core needed in the rope. Wire ropes are made with either fiber core or steel wire core.

• Strand Core (SC) - The strand core is usually confined to use in stationary ropes such as guys, suspension bridge cables, and in ropes of small diameter such as aircraft cable. It is also occasionally specified on installations where severe crushing may be experienced.
• Fiber Core (FC) - This core is made of either prelubricated Java sisal fibers or plastic fibers, usually polypropylene. These fibers are made into an extremely hard laid rope which will stand up under the high pressures of rope service. These cores are used only when normal operating loads do not rupture the fibers. The polypropylene core is generally recommended when operating conditions other than crushing destroy the sisal. One example would be the presence of acid.
• Independent Wire Rope Core (IWRC) - An independent wire rope center is usually specified to provide for one or more of three particular requirements, as follows:
  • Increased strength
  • Greater resistance to crushing
  • Resistance to excessive heat

  An "IWRC" increases the strength by 7% and weight of a wire rope by 10%, and decreases the flexibility slightly. It greatly increases the resistance of the wire rope to crushing and is especially recommended on installations where severe loads are placed on ropes running over sheaves or wound on drums. Unless required for one or more of the above properties, the use of an "IWRC" should be avoided.

Strand Construction

Wires are the basic building blocks of a wire rope. They lay around a "center" in a specified pattern in one or more layers to form a strand. The strands lay around a core to form a wire rope. Wire rope classifications and features the strands provide all the tensile strength of a fiber core rope and over 90% of the strength of a typical 6-strand wire rope with an independent wire rope core.

Characteristics like fatigue resistance and resistance to abrasion are directly affected by the design of strands. In most strands with two or more layers of wires, inner layers support outer layers in such a manner that all wires may slide and adjust freely when the rope bends.

As a general rule, a rope that has strands made up of a few large wires will be more abrasion resistant and less fatigue resistant than a rope of the same size made up of strands with many smaller wires. The basic strand constructions are illustrated as follow.

	Single Layer Strand - The most common example of the single layer construction is a 7 wire strand. It has a single-wire center with six wires of the same diameter around it.
	Seale Strand - This construction has two layers of wires around a center with the same number of wires in each layer. All wires in each layer are the same diameter. The strand is designed so that the large outer wires rest in the valleys between the smaller inner wires. Example: 19 Seale (1-9-9) strand.
	Filler Wire Strand - This construction has two layers of uniform-size wire around a center with the inner layer having half the number of wires as the outer layer. Small filler wires, equal in number to the inner layer, are laid in valleys of the inner layer. Example: 25 Filler Wire (1-6-6f -12) strand.
	Warrington Strand - This construction has two layers of wires around a center with one diameter of wire in the inner layer, and two diameters of wire alternating large and small in the outer layer. The larger outer layer wires rest in the valleys, and the smaller ones on the crowns, of the inner layer. Example: 19 Warrington [1-6-(6+6)].
	Combined Pattern Strand - When a strand is formed in a single operation using two or more of the above constructions, it is referred to as a "combined pattern". This example is a seale construction in its first two layers. The third layer utilizes the Warrington construction, and the outer layer is a seale construction. It's described as: 49 Seale Warrington Seale [1-8-8-(8+8)-16].

Rope Construction

The following common wire rope constructions are known as class constructions: 6 x 7, 6 x 19, 6 x 37, 7 x 19, and 8 x 19. Within a given class construction, the number of wires is allowed to vary within established industry guidelines. For example, a 6 x 37 class fiber core rope may have 27 to 49 wires in one strand.

When wires and strands are shaped into the form they will take on as a wire rope, the wire rope is called preformed. Preformed wire rope is easier to handle, resists kinking, and won't unravel when cut. All of our wire rope is preformed (unless noted). Lubrication is provided on many ropes to increase service life.

• Abrasion - Surface wear on the wires of a wire rope occurs as the wire rope moves over any surface. A wire rope constructed of strands with fewer wires will be more abrasion resistant than wire rope made with more wires. For example, 6 x 19 rope construction is more abrasion resistant than 6 x 37 construction.
• Fatigue - Caused when a wire rope is repeatedly bent around drums and pulleys. A wire rope constructed of strands with more wires is more fatigue resistant than one made of strands with fewer wires. For example, 7 x 19 construction is more fatigue resistant than 7 x 7 construction.
• Crushing - Any external force that causes a wire rope to become flattened or distorted and lead to breakage. This is especially true when a wire rope is used on drums and pulleys. In general, strand core and IWRC wire ropes are more crush resistant (less susceptible to flattening and distortion) than fiber and hollow core wire ropes.
• Flexibility - A measure of the wire rope's ability to stand up to bending stresses such as repeated movement over drums and pulleys. Typically, small diameter fiber core and hollow core wire ropes are more flexible than large diameter strand and IWRC wire ropes. For wire ropes of the same diameter, flexibility increases as the number of wires per strand increases. For example, a 1/4" diameter 7 x 19 construction wire rope is more flexible than a 1/4" diameter 7 x 7 construction wire rope.

Fleet Angle

Fleet angle is usually defined as the included angle between two lines, one which extends from a fixed sheave to the flange of a drum and the other which extends from the same fixed sheave to the drum in a line perpendicular to the axis of the drum.

If the drum incorporates helical grooving, the helix angle of the groove needs to be added or subtracted from the fleet angle as described above to determine the actual fleet angle experienced by the rope.

• At the drum

  When spooling rope onto a drum it is generally recommended that the fleet angle is limited to between 0.5° and 2.5°. If the fleet angle is too small, i.e. less than 0.5°, the rope will tend to pile up at the drum flange and fail to return across the drum. In this situation, the problem may be alleviated by introducing a 'kicker' device or by increasing the fleet angle through the introduction of a sheave or spooling mechanism.

  If the rope is allowed to pile up it will eventually roll away from the flange creating a shock load in both the rope and the structure of the mechanism, an undesirable and unsafe operating condition.

  Excessively high fleet angles will return the rope across the drum prematurely, creating gaps between wraps of rope close to the flanges as well as increasing the pressure on the rope at the cross-over positions.

  Even where helical grooving is provided, large fleet angles will inevitably result in localised areas of mechanical damage as the wires 'pluck' against each other. This is often referred to as "interference" but the amount can be reduced by selecting a Lang’s lay rope if the reeving allows. The "interference" effect can also be reduced by employing a dyform rope which offers a much smoother exterior surface than conventional rope constructions.

  Floating sheaves or specially designed fleet angle compensating devices may also be employed to reduce the fleet angle effect.

• At the sheave

  Where a fleet angle exists as the rope enters a sheave, it initially makes contact with the sheave flange. As the rope continues to pass through the sheave it moves down the flange until it sits in the bottom of the groove. In doing so, even when under tension, the rope will actually roll as well as slide. As a result of the rolling action the rope is twisted, i.e. turn is induced into or out of the rope, either shortening or lengthening the lay length of the outer layer of strands.

  As the fleet angle increases so does the amount of twist. To reduce the amount of twist to an acceptable level the fleet angle should be limited to 2.5° for grooved drums and 1.5° for plain drums and when using rotation-resistant low rotation and parallel-closed ropes the fleet angle should be limited to 1.5°.

  However, for some applications it is recognised that for practical reasons it is not always possible to comply with these general recommendations, in which case the rope life could be affected.