Strand is two or more wires wound concentrically in a helix. They are usually wound around a center wire. Strand is normally referred to as 1 by the total number of wires in the given strand. Such as 1 x 7 (one group of seven wires) or 1 x 19 (one group of nineteen wires).
CABLE / WIRE ROPE
Cable is the generic term that refers to constructions of wire rope that fall into the category of aircraft cable. These constructions include 7 x 7 (seven groups of 1 x 7 strand) and 7 x 19 (seven groups of 1 x 19 strand) respectively. “Aircraft cable” ranges in size from 1/32″ to 3/8″.
WIRE ROPE is technically the correct term for groups of strand wrapped in a uniform helix around a core. Sizes over 3/8″ are not considered aircraft cable, they are wire rope. The constructions included in wire rope are numerous, but the most common are 6 x 19 and 6 x 37 class wire ropes. These descriptions of the construction fail to include the core as a part of the primary number. Part of the reason for this is that the composition and the construction of the cores in these wire ropes is so numerous, that it is necessary to call out the outer construction of the wire rope and then describe the core. Such as 6 x 19 fiber core, or 6 x 37 IWRC (Independent Wire Rope Core). The terms CABLE and WIRE ROPE are synonymous for all intents and purposes.
The core can be a variety of things, including: Strand in many constructions or even a cable or wire rope. The core can also be of a composition other than metal, such as polypropylene rope. Whatever the construction or the composition of the core, it is the center member of the cable or wire rope.
Construction of strand, wire rope or cable is the nomenclature for describing the number of wires contained in and their relationships to each other in the particular product being described. 1 x 19 describes one group of nineteen wires; 7 x 19 describes seven groups of nineteen wires (or seven groups of 1 x 19); 6 x 37 IWRC describes six groups of thirty-seven wires wound around a core that might actually be 7 x 7 construction itself. Thus the term Independent Wire Rope Core, since the core of 7 x 7 is actually a piece of wire rope. The construction of the core is partially determined by the diameter of the wire rope being described.
Composition of the wire rope or strand refers to the material used to manufacture the product being described. Strand, wire rope and cable are made from various grades of both stainless steel and carbon steel.
The STAINLESS STEEL grades that are the most common are Type 302/304, Type 305 (nonmagnetic), and Type 316.
Type 302/304 is 18-8 (18 parts chromium and 8 parts nickel) stainless steel and is most commonly used in the manufacture of wire rope. This type of stainless steel is commonly used in applications that require more corrosion protection than is available from galvanized carbon steel cable. Contrary to popular belief, stainless steel is not stronger than galvanized carbon steel cable. The fact is, it usually has a lower breaking strength than galvanized carbon steel cable of the same diameter and construction.
Type 305 S/S cable (nonmagnetic) is commonly available in 1/16″ and 1/32″ diameters. This type of S/S is generally used in applications where sensitive instrumentation or other systems might be affected by magnetism.
Type 316 S/S is increasingly popular in many outdoor salt air environments. It has excellent corrosion protection and is the preferred choice of the marine industry. The increase in popularity of Type 316 S/S has spurred the availability of many wire rope terminals also becoming available in Type 316 S/S.
Monel is an uncommon grade of stainless and is used in applications where added resistance to corrosive substances and liquids are required. The main drawbacks to Monel are that its minimum breaking strength is about 30% less than that of Type 302/304 S/S, and that it is not readily available.
CARBON STEEL WIRE is available in different grades, sometimes referred to as Plow Steel, Improved Plow Steel, or even Extra Improved Plow Steel. Wire rope manufactured from uncoated wire is commonly referred to as Bright. Bright wire rope is available in sizes 1/4 inch and over.
GALVANIZED carbon steel wire is frequently used to manufacture wire rope. Galvanized wire rope provides good corrosion protection in mild environments. In smaller sizes (less than 1/4 inch) it is almost automatically used when stainless steel is not specified.
The lay of the strand, wire rope or cable, is the direction in which the helix of the wires orbit the core. An easy way to determine the lay is to hold the specimen vertically in front of you and observe whether the strands or wires travel up and to the right or up and to the left. Just as you might visually determine the direction of a thread on a bolt. There are other types of lay or wire rope, such as Lang and Herringbone. These types of lays in wire rope are for special purposes, so we will not go into them here.
Preforming of the wires refers to the forming of the individual wires of a strand into a helix so as to enable them to all be “closed” into a uniform cylinder capable of retaining its shape when cut. Furthermore, when wire rope or cable is made (using preformed strand) the strand (group of preformed wires) is also preformed into a helix so that the 6 or 7 groups of strand all lay together in a cylindrical form and remain that way when cut. Miniature diameters (smaller than 3/64 inch) often are not preformed and must be specified to be “stress relieved” at the time of ordering. Without preforming or stress relieving, cable will typically “Broom” or fray when cut or disturbed. Miniature cable not withstanding, it is not usually necessary to specify preforming when ordering cable or wire rope since almost all wire rope and cable is preformed.
The coating of a cable is optional, but can serve many useful purposes. The most common types of coatings are PVC Polyvinylchloride, various Nylon compounds, and Teflon. These materials are referred to as compounds. These compounds are applied to the cable via the use of a plastic extrusion machine.
PVC is relatively inexpensive and is available in almost any color. PVC lends itself well to applications that are going to be exposed to sunlight. PVC would also be a good choice for an application that requires an increase in mass, or a particular color. PVC is relatively soft and does not have good abrasion resistance.
- NYLON AND THERMOPLASTIC ELASTOMERS
Nylon and thermoplastic elastomers are numerous in types of compounds, each with its own special properties that may be more or less suitable for a particular application. Most are suitable for cycling over pulleys. The differences vary from high temperature environments to specific chemical exposures. This is one reason to discuss your application with a KNOWLEDGEABLE salesperson.
Teflon is used in certain applications that may include high temperature, chemical exposure, or applications that require a low coefficient of friction. Teflon is expensive, but commonly used and available.