wire rope

Wire Ropes Wire rope is a vital machine element for transmitting tensile forces and motion. Describing wire rope or cable as a machine is generally accepted, as it has multiple moving parts that transfer force and dynamically distribute the applied loading to perform useful work. These versatile constructs are used in a wide variety of industries and in very severe applications. The purpose of this article is to explain the complicated selection, use, care, inspection, and failure analysis of wire rope.

1. Development and Applications The archaeological record shows that Stone Age man invented natural fiber ropes. The use of metal wires to manufacture much stronger ropes began over 2500 years ago. Modern stranded wire rope was primarily developed and refined in the last 200 years. Many of the advances were application oriented, for silver mine hoists, railways, and cable cars. Foremost among the primary advantages of wire rope is that it can transmit very high forces and remain flexible. Rope can withstand multiaxis bending that is not always possible in other flexible tensile members, such as chain. Standard wire rope consists of many individual wires, precisely arranged into strands that are assembled into a rope, as shown in Fig. 1. It is the continuous realignment of the individual wires and strands that permits the assembly to endure the tension, torsion, bending, and compression forces applied in service. Wire rope service is typically categorized as static or dynamic. These categorizations are significant, as the concerns accompanying each are substantially different. Static or stationary applications include tower supports, guy wires, suspension bridge supports, and electrical power transmission lines. Dynamic applications are usually for pulling or lifting, and include elevators, te! le! fe! riques (aerial cable cars), cranes, hoists, dredges, and control cables. Dynamically stressed ropes require flexibility to pass over sheaves and onto drums.

Figure 1 Diagram of wire rope components.

2. Wire Rope Configuration The basic element of a wire rope is metal wire. Wire is manufactured from rod by successive cold drawing processes until the final diameter and strength level are attained. Interim annealing processes are required to restore the requisite ductility between successive drawing steps. The high strength of rope wires is due to cold work rather than heat treatment operations. The wires are then fabricated into rope by automatic stranding machinery. All of the properties of wire ropes are a result of the wire manufacturing, wire sizes, and the manner in which the wires are arranged. The descriptions of wire rope for design or selection purposes have been standardized. A normal description contains the following attributes: length, diameter, construction, lay, grade, finish, core, and lubrication. These characteristics are described below and the appropriate designations are summarized in Table 1. Many standard organizations have prepared detailed specifications for wire rope, including ASTM A 1023 and ISO 2408. Length. The length of a wire rope in meters or feet. Diameter. A rope’s nominal or rated size is measured across the circumscribed diameter, rather than across the flat sides of the geometric shape that is formed (e.g., such as a hexagon or octagon). Individual wire diameters are not usually specified, but will be dependent upon the rope size and construction. Construction. The design configuration of a wire rope is called the construction. The number of strands and wires is the class of the rope and is included in the construction. The most widely used class is 6
25, for six strands of 25 wires. Rope strands were originally made with a single wire diameter in single-layer construction. As wires get larger, more unused space exists between the wires, reducing both the load-bearing cross-sectional area and the crushing resistance. Several mixed wire size strand constructions were developed in the 1800s to optimize properties as more severe applications were envisioned for wire ropes. Various cross-sections of ropes are shown in Fig. 2. The Warrington (W) construction contains alternating wire sizes to form a more compact, dense arrangement. The Seale (S) strand arrangement contains alternating layers of wire sizes, with larger diameter wires on the exterior. Filler wire (FW) constructions contain auxiliary interior wires that serve primarily to support the rope’s geometrical configuration under loading. Small filler wires also provide some cushioning as the outer wires seat better on the intermediate or inner wires. Hybrid strand constructions of numerous layers are used, often requiring complex multiple stranding operations. Additional constructions contain nonround wires, plastic-coated strands, and other features for very specialized service characteristics. For example, the locked coil tramway cable shown in Fig. 2 was 1

Wire Ropes Table 1 Standard steel wire rope identifications. Characteristic

Description

Designation

Length

Linear length

meters (feet)

Diameter

Nominal diameter or size

centimeters (inches)

Construction

Number of strands by number of wires per strand Single layer—uniform wire diameter in strand Warrington—alternating wire sizes in a single layer Seale—alternate layers of different wire sizes Filler wire—fine wires between layer wires

e.g., 6
25 None W S FW or F

Lay

Right regular lay—strands laid right and strand wires laid left Left regular lay—strands laid left and strand wires laid right Right lang lay—strands laid right and wires laid right Left lang lay—strands laid left and wires laid left Alternate lay—regular and lang lay strands alternate

RRL or sZ LRL or zS RLL or zZ LLL or sS RAL/LAL

Grade

Traction steel Plow steel Improved plow steel Extra-improved plow steel Extra-extra-improved plow steel

TS PS IPS EIPS or XIPS EEIPS or XXIPS

Finish

Bright–uncoated, bare wires Galvanized—zinc or zinc alloy coated wires

B G

Core

Fiber core Wire strand core Independent wire rope core

FC WSC IWRC

Lubrication

Special requirements

Adapted from Wire Rope Technical Board (1993) and ASTM A 1023 (2002).

developed for high strength and abrasion resistance. The interlocking construction prevents broken wires from protruding from the rope. High-strength straight wires evince spring behavior; therefore, the spiral stranding into a rope will result in residual stresses within the individual wires and strands. These stresses are superimposed onto applied stresses, thereby reducing the safe working load permissible. Preforming is a process where wires and strands are mechanically formed into the nested helices they assume in the rope, minimizing inherent residual stresses. The reduction in internal friction is also manifested as better flexibility and fatigue resistance. Lay. The strand lay, or lay direction, of a rope is the direction strands are laid around the core, and the direction wires are laid around the strands. Five standard lays are shown in Fig. 3 with their designations included in Table 1. Regular lay and lang lay ropes exhibit substantially different characteristics. Regular lay ropes are typically easier to handle and are not prone to untwisting in hoisting applications with suspended loads. The axial lay of the wires in lang lay ropes provide better flexibility and fatigue resistance, but they are less resistant to crushing under heavy loading. 2

Alternate lays are special-purpose constructions of alternating regular and lang lay strands. Rotationresistant ropes are available, using greater numbers of strands or strands with successive layers laid in opposite directions. The term rope lay or pitch is used to signify the distance in which one exterior strand makes a complete revolution about the core. The lay length is of particular importance in visual inspection, as described in Sect. 6. Grade. Most wire ropes are made from steel. Steel rope wires are classified by a number of historical names, but these are somewhat imprecise. The strength grades include traction steel, plow steel, and various grades of improved plow steel. Ropes and cables are also made from stainless steel, aluminum, copper alloys and other specialty materials. These materials are discussed in greater depth in Sect. 4. Finish. The finish of a steel wire rope indicates whether it is coated. Most ropes have a bright finish, indicative of uncoated steel. Galvanized (zinc-coated) ropes provide better corrosion resistance and are usually used for static service, such as ship rigging, guy wires, and suspension bridge supports. These ropes are not for heavy hoisting and they abrade

Wire Ropes

Figure 3 Diagrams of several standard wire rope lays: (a) right regular lay, (b) left regular lay, (c) right lang lay, (d) left lang lay, and (e) right alternate lay. Each depiction is a single rope lay. Figure 2 Typical wire rope constructions: (a) 7
7 WSC, (b) 6
19 Warrington construction with a fiber core (W FC), (c) 8
19 Seale construction with a fiber core (S FC), (d) 6
21 Filler wire construction with an IWRC (FW IWRC), (e) 6
26 Warrington–Seale construction with an IWRC (WS IWRC), and (f) locked coil tramway cable. Shading is representative of a fiber core.

easily, removing the protective zinc. Polymeric coatings are also available. Core. The outer strands of a wire rope are laid about a core. The core acts primarily as a foundation for the outer strands, which carry most of the load. The type of core has a substantial influence on the properties of a wire rope. Cores are identified as fiber core (FC), wire strand core (WSC), or independent wire rope core (IWRC). Fiber cores do not add any mechanical strength to wire ropes, only modest support for the outer strands to prevent crushing. The natural materials used for these cores include manila, sisal, cotton, hemp, and jute. Synthetic fiber cores from extruded petrochemical resin are also used, predominantly polypropylene

(PP). Fiber materials can be severely degraded by drying or charring. Fiber cores are not suitable for service over 82 1C (180 1F) (Wire Rope Technical Board 1993). Wire strand cores and independent wire rope cores add from 7% to 10% to the strength of a wire rope, but do not provide some FC benefits, such as greater flexibility and lubricant retention. Metal core ropes exhibit better crushing resistance than fiber core ropes. Lubrication. Like most machines with moving parts, wire ropes and cables require lubrication. Lubrication reduces friction between individual wires, between strands, between coils of rope, and between the rope and other surfaces, such as sheaves and drums. The wires and strands must slide in relation to each other to permit stress distribution and equalization. Fiber cores act as an effective reservoir for a continuous supply of lubricant. A variety of different lubricants are used in wire ropes, dependent upon the specific service conditions. Lubricants include natural and manmade substances, such as boiled linseed oil and graphite greases, but are usually petroleum oil based. Additives may be included in these compounds to provide better adherence to the wires, increase water repellence, 3

Wire Ropes improve heat degradation resistance, prevent drying, and other properties. The most important lubrication is added during rope assembly, when all strands and individual wires are accessible. Ropes are frequently relubricated to replace the material that physically exudes out due to flexure, or is affected by thermal degradation or chemical reaction. Some types of service are not conducive to adequate, perpetual lubrication. Service in soil and rock, such as dredging, will accumulate dirt and wear particles that enter the rope and cause wear. Ropes in this type of service must be replaced frequently. An example of wire rope specification is as follows: 100 m 25 mm 6
25 RRL EIPS Galv IWRC. This would indicate 100 m of 25 mm rope, 6
25 construction, right regular lay, extra improved plow steel, galvanized wires, with an independent wire rope core. Rope does not always lend itself to manufacturer identification markings; however, some manufacturers use color-coded fiber cores or strands to identify their product.

3. Properties and Selection Many aspects of intended service and their relative importance must be considered in the selection of a wire rope. These characteristics include loading magnitude, loading type (constant or variable), abrasion, acceleration, sheaves and attachments, environment, economy, safety, etc. The primary wire rope selection factors are strength, fatigue resistance, damage resistance, crushing resistance, and reserve strength. The properties of interest in the selection of wire ropes are listed and explained individually, but it must be kept in mind that these properties cannot be considered separately. All attribute choices can affect

other performance characteristics, which is the classic design compromise in engineering decisions. 3.1

Strength

The only mechanical property of wire ropes that is specified is the minimum breaking force (MBF) or nominal strength. Minimum wire strengths are also specified in some cases. Strength ratings are listed in many specifications such as ASTM A 1023, US Federal Specification RR-W-410E, and ISO 2408. Strength ratings are specified for types and classes of constructions, such as 6
19 and 6
37. For example, as shown in Table 2, 6
19 S, 6
21 FW, 6
26 WS, and 6
25 FW are all considered 6
19 constructions and would have an equivalent strength requirement. Galvanized rope is rated 10% lower. Unfortunately, there is no reliable way to measure rope strength without excision of a test length, perhaps making the rope unusable. During strength testing, rope cannot be gripped in normal vise jaws, as the crushing and nonuniform stress distribution will produce arbitrarily low results. Testing specifications suggest proper socketing of the wire ends (ASTM A 931 2002, ISO 3108 1974). The strengths of wire ropes are usually 80–95% of the aggregate wire strengths, dependent upon construction. A portion of the applied axial stress is accommodated as shear, due to the helical geometry. Wire cross-sections are usually depicted as circular for illustrative convenience, but they are elliptical. The actual metallic areas of ropes are published in tables and some are included in Table 2. Filler wires are traditionally excluded from metallic area determinations. Strength is the only really quantifiable property that can be used by a designer. The remaining rope

Table 2 X-chart showing the general relationship between abrasion and bending fatigue resistances.

Construction 6×7 6 × 19 S 6 × 21 FW 6 × 26 WS 6 × 25 FW 6 × 31 WS 6 × 36 WS 6 × 41 SFW 6 × 46 SFW

Relative comparisons Least bending fatigue resistance

Greatest abrasion resistance

Least abrasion resistance

Greatest bending fatigue resistance

Adapted from Wire Rope Technical Board (1993). a Metallic area assumes 1 in rope diameter, and IWRC.

4

Metallic area (in2a)

Outside wires per strand

Reserve strength (%)

Minimum sheave ratio (D/d b)

0.451 0.470 0.478 0.476 0.483 0.481 0.485 0.491 0.492

6 9 10 10 12 12 14 16 18

8 32 36 36 43 43 49 54 58

42 34 26 30 26 30 20 21 18

b D/d ratio is the sheave diameter divided by the rope diameter.

Wire Ropes characteristics are qualitatively comparable through service evaluation and historical experience. For specification use, a design or safety factor is always applied. This factor is the ratio of nominal rope strength to the service load, and is rarely less than 5 for normal service.

3.2

Fatigue Resistance

Bending fatigue and vibration fatigue resistance are probably the most important nonquantifiable properties of a wire rope. Bending fatigue resistance is related to flexibility, but these terms are not synonymous. Fatigue resistance is a measure of endurance, while flexibility is the relative ease of bending. Flexibility is of greater importance in dynamic ropes; static ropes can be more rigid. Construction and core type contribute to greater cracking resistance. As shown in Table 2, greater numbers of smaller strand wires provide better fatigue resistance. This is the greatest benefit of the Warrington construction. Single operation stranding provides better exterior and interior wire alignment and increases fatigue resistance. Lang lay ropes are superior to regular lay ropes. Bending diameter and attachment variables directly affect the fatigue performance of dynamically loaded ropes. Reversed bending should be avoided because it has been shown to reduce rope life as much as 50%. Both static and dynamic service can apply vibration loading to wire ropes that can lead to fracture. Seemingly innocuous small vibrations can be amplified harmonically to levels above the endurance limit, the stress level below which service life is theoretically infinite. Static ropes and quiescent dynamic ropes often fail by vibration if no active damping is provided, or if assumptions of isolation from vibration are erroneous.

3.3

Damage Resistance

Wire ropes require substantial resistance to abrasion and physical damage in dynamic service. Contact with sheaves, drums, or pulleys, and overwinding can cause severe degradation to rope wires. The effect of abrasion is to wear the outside wires, reducing the mechanical strength. Abrasion is a function of many characteristics including relative hardness, pressure, lubrication, and work-hardening characteristics. Abrasion resistance can be maximized by using larger outside, or cover strands, as in a Seale construction. Lang lay is also somewhat better than regular lay, as individual wires may need to abrade more prior to bending fracture. Abrasion can also cause microstructural alteration, work hardening, and can facilitate corrosion. Other damage such as peening of outer wires can occur.

3.4

Crushing Resistance

Crushing occurs when applied stresses result in permanent collapsing damage in a rope. The axial and bending forces collapse and compress the core along with displacement of the outer strands. Crushed spots result in stress concentration. Excessive pressure and improper sheave geometry often cause crushing. Smaller rope diameters and metallic cores provide better crushing resistance than larger diameters and fiber cores. Larger strand core wires in flexible ropes have reduced metallic area, providing less crushing resistance. Overwinding, where multiple layers of rope are applied to a drum, is not advisable for highly loaded ropes with low inherent crushing resistance. 3.5

Reserve Strength

Reserve strength is the percentage of cross-sectional area of interior strand wires, those that would be unaffected by normal abrasion of the outer wires. This is a conservative comparison attribute used to estimate how much of the strength likely remains after significant service, making the assumption that all surface wires in each strand are fractured. Reserve strengths for different rope configurations can be as high as 60% for finer wire strands. Representative published reserve strengths are included in Table 2. High reserve strengths are required in applications where the potential consequences of rope failure are severe. 3.6

Additional Considerations

(a) Stretching. Wire ropes stretch in service. Some initial stretching is permanent and is known as constructional stretch. This results from seating of the wires and also results in slight diameter constriction. This may occur quickly in highly stressed ropes and gradually in moderately stressed ropes. The constructional stretch of wire rope is dependent on construction and is usually between 0.25% and 1.00%. A breaking-in period with the loading gradually increasing to the operating load will ensure that the rope is properly stretched. This stretching could be disastrous in many applications, especially in static service where the rope length is a fundamental design characteristic. Prestressed ropes can be ordered to avoid constructional stretch. One method prescribed for prestretching or prestressing rope is to load it three times to 40% of the rated strength for 5 min, reducing the load to 5% between cycles, followed by release of the load (Federal Specification RR-W-410E 2002). Additional reversible stretching occurs throughout the life of a dynamically stressed rope, as a function of stress magnitude. Elastic strain recovery behavior permits the wires to elongate under load and revert to their former shape when the load is released. Equations for estimating elastic stretching during service have been developed. 5

Wire Ropes The Young’s (elastic) modulus of metals is microstructure insensitive, meaning that the modulus is similar, regardless of mechanical and thermal processing. As an assembly, however, wire rope can exhibit a varying modulus, dependent upon the construction, grade, and loading. The modulus will gradually increase in heavily loaded ropes. Ropes that are overloaded in service can exhibit permanent elongation and an accelerated reduction in useful life. The elastic or proportional limit of wire rope, the point at which permanent deformation takes place, is B55–65% of the breaking strength.

Table 3

(b) Sheave design. Proper sheave selection is essential for maximizing rope life. Suitable rope diameters for existing sheaves and suitable sheaves for selected ropes have been studied at great depth. Sizing recommendations are usually expressed as D=d, or the ratio of sheave to rope diameter, and some are included in Table 2. Bending fatigue failure is very often a direct result of undersized sheaves and drums. When small radii of curvature are used, the foreshortened underside strands cannot move sufficiently to accommodate the compressive forces, resulting in buckling. In addition, improper reeving design may not effectively distribute loading between multiple rope sections. The radial pressure between rope and sheave is another variable considered in sizing decisions. Equations for radial pressure have been published by many sources. Drums for highly stressed ropes have grooves machined to match the rope diameter, for better construction support. The contours of sheave grooves are abraded in service, requiring periodic sheave inspection to prevent accelerated rope wear.

Source: Wire Rope Technical Board (1993). n.r. ¼ not recommended.

(c) Attachments. Wire ropes perform useful work through attachment by fittings, clamps, and connectors. These components are necessary to fabricate hoists, slings, and controls. The efficiency of the connection will influence the permissible load on the assembly in order to retain the same safety factor. A table of attachment efficiencies is shown in Table 3. Sockets and swaged fittings are typically the strongest attachments. Even when correctly affixed, fittings are highly stressed and can be a preferential failure location. Poorly attached clamps and fittings cause disproportionate loading. Efficiencies can drop dramatically if the rope ends are free to rotate. Splicing, which is the interweaving of rope ends, can create endless rope lengths for service such as cable cars.

Efficiencies of some standard rope attachments and fittings. Approximate efficiency (%) Types of termination Socket (spelter or resin) Swaged socket Spliced sleeve Loop thimble/hand splice Wedge sockets Clips

FC

IWRC

100 n.r. 90–92 80–90 75–80 80

100 100 90–95 80–90 75–80 80

as follows: iron rope 0.05% to 0.15%, traction steel 0.20% to 0.50%, mild plow steel 0.40% to 0.65%, plow steel 0.65% to 0.80%, and improved plow steel 0.70% to 0.85% (American Society for Metals 1948). These typical ranges are not an industryaccepted requirement, but the carbon content must be sufficiently high to achieve the necessary strength by cold working. Low-strength ropes are in very limited use. Ropes for static and dynamic service are not manufactured from different wire grades. The stronger grades evolved from better steel making, cleaner steels, better wire drawing practice, and other advancements. The strongest steel wires are drawn to strengths greater than 1700 MPa (247 ksi). Zinc or zinc alloy galvanizing, which can be hotdipped or electrodeposited, reduces the strength rating of a steel rope since the steel cross-sectional area is reduced. Specially drawn galvanized ropes can be ordered with full steel wire diameters to avoid this strength reduction. Alternate materials with superior corrosion resistance were developed for special applications. Stainless steel is the predominant alternate material available in rope and cable. The alloys typically used are X10CrNi18-8 or X5CrNi18-10 (US Types 302 or 304). High-strength stainless steel cables are in substantial use in aircraft controls. Stainless steel ropes have lower strengths and greater constructional stretch than regular steel, but can be used in severe environments such as pickling lines. Wire ropes are also made from phosphor bronze, Monel, and other specialty materials. Plastic coating and/or impregnation are used to achieve additional corrosion resistance. Stranded aluminum rope with a reinforcing steel IWRC is used for power transmission lines.

4. Rope Materials of Construction Carbon steel wire ropes are by far the most abundant, due to their high strength and relatively low cost. The standard grades have typical carbon content ranges 6

5. Degradation and Fracture Like any machine, factors of installation, use, and maintenance will affect the useful life of a wire rope.

Wire Ropes Wire ropes degrade in service, through the oftensynergistic processes of wear, corrosion, and fracture. The rate of degradation is dependent upon the severity of the service environment and the loading conditions. Degradation and fracture result in a loss in breaking strength (LBS) up to the point of catastrophic failure. Degradation can be loosely categorized as rope damage, corrosion, wear, and fracture.

electropositive to steel. Polymeric coatings are also available for wire ropes. Corrosion resistant ropes, such as stainless steel, are suitable for a broader range of industrial environments. Fiber cores store lubricant, but they can also act as a trap for moisture and corrosive compounds. Fiber cores can also be degraded by fungus.

5.1

Wear damage is an expected occurrence in most dynamic rope applications. Abrasive wear damage to the outer wires is very common, and is usually due to overloading, winding mistakes, and abrasive materials. Minor wear consists of polishing wear and flattening of the outer wires. Abraded wires can exhibit microstructural alteration and severe surface roughening.

Rope Damage and Defects

A large variety of damage and defects can occur in wire ropes, through normal service, improper use, and abuse. Permanent bends in rope are identified as kinks and doglegs and are usually related to improper handling. Strand separation known as ‘‘bird-caging’’ and exposure of the fiber core known as a ‘‘popped core’’ are acceleration- or deceleration-related defects. Damage can also include pinching, crushing, hightemperature exposure, electrical arcing, lightning strikes, and contamination. All of these alterations can be of a severity to suggest immediate removal from service, as they tend to concentrate applied stresses. Individual wire damage can also be very severe. 5.2

Corrosion

Corrosion can occur to wire ropes used in nearly all environments. Unfortunately, it is not possible to accurately predict the remaining mechanical strength of a corroded wire rope. General corrosion results in a uniform attack, degrading the rope at a somewhat predictable rate. Localized pitting attack is more severe, as it is usually more rapid and unpredictable. Wire cracking can also result from corrosion, in the forms of stress corrosion cracking (SCC) in static service, and corrosion fatigue in dynamic service. In some cases active ropes corrode at a slower rate than static ones, as bending may dislodge brittle corrosion products. Corrosion products, which are primarily oxides, act as abrasive particles, increasing internal wear. Severely corroded or ‘‘rust-bound’’ ropes creak upon bending and have reduced flexibility due to the volumetric expansion that accompanies corrosion processes. In steel ropes the lubricant inhibits corrosion by coating the wires and plastically filling the interwire voids, physically preventing the ingress of moisture and corrodants. However, steel wires are prone to corrosion due to moisture, salts, and acids, even when properly lubricated. Galvanized steel ropes afford better resistance to corrosion. In normal environments, zinc forms a protective film of corrosion product which thereafter corrodes more slowly than steel. Additionally, when disruptions or holidays in the plating occur, zinc provides galvanic protection and acts as a sacrificial anode since it is

5.3

5.4

Wear

Fracture

Fracture in wire ropes is usually due to overload failure or fatigue. Overload fracture in wire ropes is almost exclusively ductile, with microvoid coalescence as the fracture mechanism. Even in cases of shock loading, normal steel wires will fracture in a cup-and-cone manner instead of the more brittle behavior encouraged by faster strain rates. Due to the homogeneous distribution of forces in a wire rope, ductile rupture of individual wires is uncommon. Ductile fracture typically occurs to complete strands at different locations or to the entire rope at a single location. Fatigue fracture is very prevalent in wire ropes and it most often occurs at stress levels below the yield strength. Conservative design using safety factors will usually prevent predictable overload failure, but fatigue resistant design is more problematic. Distributed or localized wire fractures increase the proportion of the load on the remaining wires. Ropes with smaller wires are less affected by the fracture of a single wire. Larger wires are weaker and are more likely to have detrimental flaws. Wire breaks from fatigue occur at the outer wires, in damaged valleys between strands and also within wire cores, depending on the nature of service. Occasional wire breaks, in general, do not significantly affect the performance of long wire ropes. The inherent friction between wires enables a broken wire to reaccommodate its proportion of the total load in a relatively short distance from the break. The effectively weakened length of rope surrounding a wire break can be mathematically estimated, and this length may only be slightly greater than the pitch or lay length (Costello 1997). It should be noted that it is not unusual to have evidence of damage, corrosion, wear, fatigue, and overload on a single failed wire rope. 7

Wire Ropes 6. Inspection Due to the myriad potential degradation phenomena, wire ropes cannot be considered permanent pieces of machinery. Economic factors dictate that prudent and frequent inspection of wire ropes be performed so that maximum service life may be attained prior to costly, and possibly inconvenient, replacement. It is often recommended that retired wire ropes be destroyed or cut into unusable short lengths to prevent inadvertent reuse. Critical rope locations, such as attachments, regular sheave stopping points, and drum crossovers, may require special scrutiny. Sometimes inactive portions of rope lengths are not subjected to inspection. The inspection frequency and formality of documentation is dictated by the severity of service. Records can be evaluated to discern any changes in the rate of degradation that may suggest that the inspection frequency should be changed. 6.1

Visual Inspection

Visual inspection is the simplest and most often performed nondestructive examination (NDE) method employed for wire ropes. With training, operators or inspectors can identify damage and make judgments about rope replacement. Visual examination can detect abrasion, crushing, corrosion, broken wires, kinks, pinching, and strand nicking. Operators are sometimes required to perform visual examination daily or at a scheduled frequency. Unfortunately, visual inspection can only include the exterior strands. Core damage may be invisible, including IWRC or WSC fatigue cracking, internal corrosion attack, insufficient lubrication, and other potentially serious types of degradation. Excess lubricant may obfuscate strand nicking or other damage. Wire breaks inside fittings are often undetectable. The visual inspection of uniform wear is problematic, and the extent of material removal is a subjective determination. 6.2

Other NDE Methods

Various electromagnetic examination methods have been developed specifically for in situ assessment of wire ropes, such as in mine hoists, at speeds up to 122 m (400 ft) min1 (Poffenroth 1996). The characteristics measured by these methods are a loss in metallic area (LMA) and local flaws (LF). Loss in metallic area is distributed damage, such as corrosion and abrasion, whereas local flaws or faults are broken wires, damaged wires, or corrosion pits. These tests are only suitable for steel wire ropes, due to the ferromagnetic character required. The methods are similar in the necessity to create a suitable magnetic field in the rope, as residual fields are insufficiently 8

strong or homogeneous. The methods have been utilized successfully for many years, and are specified in standards such as ASTM E 1571. The three principal NDE equipment types are: (1) electromagnetic instruments, (2) direct current and permanent magnet method instruments, and (3) magnetic flux leakage instruments. Electromagnetic instruments use the suspect wire rope as the core of a transformer. The rope exhibits changes in magnetic character when acted upon by an encircling primary (exciter) coil. A secondary (search) coil measures the variation in the metallic area by voltage changes. Direct current and permanent magnet method instruments induce a constant flux in the tested wire rope within a test head using direct current or permanent magnets. Another device measures the absolute axial magnitude of the magnetic flux, estimating the local cross-sectional area of the rope. Magnetic flux leakage instruments also use a direct current or permanent magnet instrument to create a constant flux in the test rope. The detector sensor or coil quantifies the flux leakage to identify the number of LF such as wire discontinuities. Magnetic flux leakage instruments are often dual function, obtaining simultaneous LMA and LF data to provide a better estimate of rope condition. Electromagnetic equipment is generally standardized on a known good section of the same rope grade, or the rope in question. Calibration may also require a reference standard with artificial flaws, to verify sensitivity. These methods have some disadvantages, however, including the expense of the equipment and substantial operator training. Electromagnetic methods are somewhat insensitive to interior flaws, metallurgical alteration, and fatigue cracks without separation (ASTM E 1571 2001). 6.3

Inspection Criteria

The results of inspection can be used to prepare an estimate of the LBS, using LMA, LF, and visual inspection results. When the LBS estimate exceeds specifications, the rope is retired. In most applications, however, the user does not have sufficient experience and training to estimate the LBS. Alternatively, direct visual or electromagnetic rejection criteria are usually applied according to specific industry standards, without actual LBS calculation. Visual inspection criteria are typically based upon the number of broken wires in the worst rope lay or a length equivalent to some multiple of the rope diameter. Abrasive wear criteria very often include a maximum of 13 outer wire removal. LMA rejection criteria are typically expressed as percentages of crosssectional area that has been subtracted by degradation. In some cases this criterion is 10% maximum reduction (Wire Rope Technical Board 1993). Rejection criteria and inspection frequency are generally specified by application, as historical

Wire Ropes experience has shown varying damage tolerance levels for specific rope constructions and service conditions. Proper inspection is often a governmental mandate in dangerous applications where fatalities may result from wire rope failure.

7. Failure Analysis The investigation of failed wire ropes is an important part of future failure prevention. These engineering investigations are not always straightforward, as many service factors can be contributory and many postfracture conditions can be confusing (Miller 2000). If the nature and cause of a failure are not determined, decisions on rope replacement or substitution may be arbitrary or potentially dangerous. The engineering investigation of a failed wire rope includes evaluation of the rope service. The loading, sheaves and attachments, environment, and all other potentially contributory extrinsic variables must be qualified, or quantified wherever possible. Computer simulation and failure recreation may confirm a mechanical failure hypothesis. Systematic metallurgical failure investigation is often necessary to identify the causes of a wire rope failure. To a large extent, failure analysis is a reverse analog of the material selection process (Miller 2002). Destructive physical analysis typically includes visual examination, dimensional evaluation, chemical analysis, mechanical testing, scanning electron microscope (SEM) fractography, microhardness testing, and metallography. Thorough visual examination should assess the state of the rope, at the failure location and surrounding regions. The fractured ends of a wire rope often exhibit important telltale features from overloading, fatigue, or abuse. Individual wires that failed via fatigue are characteristically flat, whereas ductile overload results in necked, cup-and-cone fractures. Abrasion fractures are usually angular and shear fractures are usually flat. It is quite common for wire breaks of many types in a single failure: abrasive wear followed by fatigue, corrosive thinning followed by ductile overload, and so on. It is also not unusual for outer strands and outer strand wires to separate by differing mechanism(s) than the core or internal wires.

8. Concluding Remarks This article is an overview and is not intended to be exhaustive. It does not provide the level of requisite

information to select, maintain, inspect, or analyze wire ropes. It is essential that additional information be referenced regarding legal requirements for regulated and nonregulated types of service. Wire rope manufacturers, suppliers, and their organizations are often good sources for information on properties and can provide historical data on which ropes are recommended for different types of service.

Bibliography American Society for Metals 1948 Metals Handbook, 1948 edn. ASM, Cleveland, OH ASTM A 931-02 2002 Standard Test Method for Tension Testing of Wire Ropes and Strand. ASTM International, West Conshohocken, PA, USA ASTM A 1023-02 2002 Standard Specification for Stranded Carbon Steel Wire Ropes for General Purposes. ASTM International, West Conshohocken, PA, USA ASTM E 1571-01 2001 Standard Practice for Electromagnetic Examination of Ferromagnetic Steel Wire Rope. ASTM International, West Conshohocken, PA, USA Chaplin C R 1995 Failure mechanisms in wire rope. J. Eng. Failure Analysis 2 Mar. 1995, 45–57 Costello G 1997 Theory of Wire Rope, 2nd edn. Springer, New York Federal Specification RR-W-410E (USA) 2002 Wire Rope and Strand. Defense Supply Center, Richmond, VA, USA ISO 3108 1974 Steel Wire Ropes for General Purposes— Determination of Actual Breaking Load. International Organization for Standardization, Geneva, Switzerland ISO 2408 1985 Steel Wire Ropes for General Purposes— Characteristics. International Organization for Standardization, Geneva, Switzerland Jamieson F L 1987 Failures of lifting equipment. In: Failure Analysis and Prevention, ASM Handbook 9th edn. ASM International, Metals Park, OH, Vol. 11, pp. 514–28 Miller B A 2000 Failure analysis of wire rope. Advanced Materials and Processes. ASM International, Metals Park, OH, USA Vol. 157, pp. 43–6 Miller B A 2002 Materials selection for failure prevention. In: Becker W T, Shipley R J (eds.) Failure Analysis and Prevention, ASM Handbook. ASM International, Metals Park, OH, Vol. 11, pp. 24–39 Naumann F K 1983 Failure Analysis Case Histories and Methodology. Dr. Riederer-Verlag GmbH, Stuttgart Poffenroth D 1996 Nondestructive testing of elevator suspension and governor ropes, Elevator World, pp. 73–5 Wire Rope Technical Board (USA) 1993 Wire Rope Users Manual, 3rd edn.

B. A. Miller

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