Hoist Ropes: The most complex component in an installation More than static cables, a hoist rope can be thought of as a machine comprised of numerous moving parts that must work in harmony with each other, and with surrounding components, to create an efficiently performing elevator system. In addition to being strong a hoist rope must also be extremely flexible. This creates many design and manufacturing challenges. The fact that hoist ropes are able to provide this balance of toughness and elasticity is vital, for without this quality modern elevators simply could not function. To better acquaint you with wire rope we offer a listing of basic terms. For further information we encourage you to consult international standards and guidelines that apply your own region, specifically: Europe : EN, ISO or DIN Great Britain : BS Japan : JIS USA: ASME Wire Inner Wire Wire Core Center Wire Wire positioned at the center of a rope strand. Core Wire Inner wire of the core of a stranded rope. Crown Wires/Outer Wires Wires in the exterior layers in the outer strands of a rope that make contact with the sheave. Also known as crown wires. Filler Wire Thin wires used in filler constructions to fill voids within layers of wires. Inner Wire Any rope wire other than its center, filler, core or outer wires. Seizing Wire (also known as Serving Wire) Wire used for making close-wound helical wrapping around a hoist rope prior to cutting used to retain rope elements in their assembled position. Strand with built-up center (referred to as U in rope classifications) refers to an uncoated wire, while Galvanized (B) refers to a wire that has been coated in zinc, a zinc alloy, or other protective coating. Valley Wire Wires located in the valleys of two adjacent strands and do not make contact with the sheave. Valley wires do not contact wires of adjacent strands. Valley breaks are attributed to rope fatigue. Wire A single metal element that is the basic structural element of all hoist ropes. Strand Strand A basic component of a hoist rope consisting of an assembly of wires laid helically in the same direction, in layers, around a central element. Round Strand A strand with a perpendicular cross section which is in the shape of a circle. Strand Lay Parallel Lay (Equal Lay) Strand) A strand made in one operation where the lay length of all the wire layers is equal and the wires of any two superimposed layers are parallel, resulting in linear contact (see figure 1). The wire of the outer layer is supported by two wires of the inner layer. Parallel lay strands with two wire layers have Filler, Seale or Warrington (or a combination of these) constructions. figure 1 Strand Wire Rope Components Strand with one center wire Wire Finish The condition of the surface finish of the wires used in a rope construction. The term Bright Parallel Lay Strand Brugg Lifting_0513
Point Contact (Cross Lay) Strand The strands are made in a number of operations where the wire of different layers cross over the crowns of underlying wires (see figure 2). figure 2 a b a. Strand prior to compacting b. Compacted Strand figure 4 Seale A parallel lay pattern with two adjacent layers laid in one operation with any number of uniform sized wires in the outer layer, and with the same number of uniform but smaller wires in the inner layer (see Figure 5). Warrington A parallel lay strand construction with a wire layer (usually the outer) made up of a pattern of alternating large and small wires. (see Figure 6). Point Contact Lay Strand Strand Designs and Terms Combined Parallel Lay A parallel lay strand design with three or more layers laid in one operation and formed from a combination of strand types Warrington and Seale. (see figure 3). figure 3 Seale Rope Construction figure 5 figure 6 Core and Core Components Core The central element of a wire rope around which the strands are laid helically around. Fiber Core A core made from either Natural fibers (NFC) such as Sisal, or Synthetic fibers (SFC) like polypropylene. (See figure 8 and 9). figure 8 Warrington Rope Construction Fiber Core (8x19 Seale) Cross Section Warrington Seale (example of combined parallel lay strand design) Compacted Strand A strand where, due to compacting processes such as drawing, rolling or swagging, the shape of the wires and the dimensions of the strand are modified, while the metallic cross section area remains unaltered. (see figure 4). Filler A parallel lay strand design with an outer layer that contains twice the number of wires than the inner layer. Filler wires are placed in the interstices between layers. (see figure 7). Filler Construction figure 7 Grade (Strand or Rope) Classification determined by its strength or type of material used. It does not imply a strength of the basic wire used to meet the rope s nominal strength. figure 9 Natural Fiber Core
Steel Core A core constructed of steel wires and arranged as a Wire Strand Core (WSC). An Independent Wire Rope Core (IWRC) (see figure 10 and 11 ) rope may have either an all steel strand core or a Mixed core (see figure 12 and 13) featuring both polypropylene and steel strands. IWRC ropes are normally made as a separate unit and formed using a different lay when compared to the direction of the outer rope wires. A Parallel with Wire Rope Core (PWRC) is a steel or mixed core that has been formed in parallel with the outer strands of the rope. figure 10 figure 12 Mixed Core Rope Cross Section (MCX 9 x 25 construction) figure 13 C. Strand Construction (Seale, Warrington, Filler, Warrington Seale strand arrangement) D. Core Design (NFC, SFC, WSC, IWRC, PWRC) E. Rope Grade: Tensile strength of wires (N/mm 2 ) F. Wire finish (Bright or Galvanized) G. Lay Type and Direction (Regular/Lang Lay, Right/Left) A in. mm B C D E F G 9/32 7,0 6 x 19 W -IWRC 1960 B sz 7/16 11.0 8 x 19 S -NFC 1370/1770 B sz 1/2 12,7 8 x 19 S -PWRC 1570 U sz 3/4 19,0 9 x 19 S -IWRC 1570 U sz Steel Core (8x19 Seale) Cross Section Lubricant Steel Core figure 11 Any material applied during the manufacture of a strand, core or rope to reduce internal friction and assisting in providing protection against corrosion. Mixed Core A core consisting of a polypropylene center surrounded by stranded steel wires. Mixed Core ropes are classified as IWRC. A Parallel Wire Rope Center uses the symbol PWRC to indicate its presence in a rope design. (See figure 12 and 13). Polymer Core (also called PolyCore) A solid core composed of polypropylene material having a round shape or a round shape with grooves. Polycore ropes may also contain internal elements such as metal, cable, steel chain or fibre. When used in an elevator rope, polycore ropes are also called Mixed Core ropes and are classified as IWRC. MIxed Core Rope and Rope Terms Dimension of Round Rope Diameter (d) which circumscribes the rope cross section (see figure 14). Dimension of Round Strand Diameter of round rope: d (above), Diameter of strand: d s (below) The diameter (d s ) of the perpendicular cross section of the strand (see figure 14). Rope Designations: Format figure 14 In North American installations only Right Regular Lay (RRL) or sz, and Right Lang Lay (RLL), zz ropes are used. When describing a rope construction one lists the factors defining a rope in this order: A. Rope diameter (size imperial or metric units) B. Rope construction (number of strands/wires It is generally understood that if direction and type of lay of a rope are left unspecified then one is referring to RRL. If wire finish is not noted then an uncoated or bright finish is to be used. Additionally, if no reference is made otherwise then a preformed rope is the preferred choice. As an example (Figure 11 of Brugg SCX 9), a rope of 1/2 in. diameter, featuring 9 x 25 Seale construction, Independent Wire Rope Core, 1570 tensile wires, bright finish, in which the direction of the lay of the wires of the outer strands is in the opposite direction of the lay of the outer strands of the rope, would be written as: 1/2 in. 9 x 25 S-IWRC 1570 U sz or (12.7 mm 9 x 25S-IWRC 1570 UsZ) Rotation Resistant Rope A wire rope composed of an inner layer of strand laid in one direction covered by a strand layer running in the opposite direction. This design counteracts torque by reducing the tendency of the rope to rotate. Rotation resistant ropes are used with lifting machinery requiring belts, slings and cranes for lifting industrial loads but not in elevators. Elevators do not employ rotation resistant ropes as they are terminated with shackles, babbit sockets and other devices that arrest rotation. Abrasion Frictional surface wear on rope wires. Wire abrasion can be caused by unequal tensioning; worn or deformed sheave grooves; rope elongation as it passes through sheaves; high torsion on the rope s axis; misaligned sheaves or pulleys; inappropriate rope selection; vibration on the rope; insufficient or infrequently applied lubrication. External wear Brugg Lifting_0513
appears on outer rope strands due to rubbing contact under pressure with the grooves in the sheaves and drums. The condition is particularly evident on moving ropes at points of sheave contact when the load is being accelerated or decelerated (see figure 15). figure 15 Amount of Stretch Stretch During Rope Life Span figure 18 c b a External wear due to abrasion Bending Cycles a b c* One of the ultimate factors used to determine actual rope life expectancy (as opposed to service time). A bend cycle may be counted each time a hoist rope passes over a sheave or deflection sheave, bends to accommodate the curved surface, and then flexes back into its original shape as it emerges from the sheave. a b c* Rope Life Number of Cycles/Trips figure 16 figure 17 A 1 Stages of Rope Life a: Settling Phase b: Nominal Life Span c *: Caution (approaching failure) Simple Bend Reverse Bend A Simple Bend (see figure 16) is an arrangement where the rope departs on a travel path from one sheave directly to another, and the rope bends in the same direction over each sheave. The Reverse Bend (found in many modern installations, see figure 17) is a instance where sheave and deflector sheaves are arranged either below or above the actual travel path of the rope. In this sort of arrangement the rope bends over a sheave in one direction and then under another in the opposite direction within a comparatively short distance (see A 1 and A 2 ). This means that the rope will be flexed in two directions. This is often used where additional deflection is required to increase the wrapping angle of the traction sheave and thus improve traction performance. For ropes running over sheaves, a distinction is made between simple bending and reverse bends. In roping arrangements with reverse bends especially A 2 in situations where sheaves, or sheaves and drum are spaced very closely the rope operates at a disadvantage which greatly impacts rope longevity. When trying to estimate rope longevity it is necessary to note both the number and type of bends to be used in an installation. Breaking Force The ultimate load at which a tensile failure occurs in a wire rope sample being tested. Constructional Stretch When a load is applied to the helically set wires and strands of an elevator rope the rope constricts. This contraction squeezes the core, causing a slight reduction in overall rope diameter as all the elements within the rope s construction come closer together. Simultaneously, as core diameter shrinks, rope length increases as well. It is difficult to define a finite value for the amount of constructional stretch due to the fact that one must consider a wide range of factors including: core type, rope design, lay length, rope steel used, rope length, and the degree of preforming utilized. In addition one must also consider the multiplicative effects of car weight, rope speed, roping configurations, acceleration and braking speeds, shaft height and sheave conditions. During a typical lifetime elevator rope can be said to go through three phases of constructional stretch (consult figure 18 for details): Settling Phase (a): After installation as a new rope adjusts to the system and achieves equilibrium within it (this is sometimes referred to as the run- in period). Nominal Life Span (b): The service segment of the rope s life. The rope shows only slight overall increase in stretch due to wear and the effects of fatigue. Tertiary (c): Constructional stretch increases rapidly due to prolonged subjection to abrasion and fatigue. The rope rapidly begins to degrade (which is evidence by wire breaks). At this point the rope should be replaced to eliminate the potential for catastrophic rope failures.
Elastic Stretch Any hoist rope will stretch to a degree under load due to lay of the rope which resembles a coiled spring and the natural elastic deformation of the individual steel wires themselves. When that load is removed the rope will return to its original length, hence the term elastic stretch. It can be difficult to distinguish between the effects of constructional stretch and elastic stretch when installing a new rope. To determine Elastic Stretch one must consider rope construction, lay length (the longer the lay length, the less elastic stretch) and type, the steel used and the tensile strength of wires. Fatigue The progressive fracture of individual wires over time as bend over a sheave and then flex back. These fractures normally occur at bending stresses well below the ultimate strength of the material. Fatigue can be an indication of high rope bending stresses, or excessive stress being placed upon loaded wires as they bend over a sheave. Rope fatigue can be a problem in system designs that feature reverse bends, or in installations where the sheaves are undersized, closely placed, used in multiple sheave configurations, or utilize aggressive sheave groove designs with wide undercuts. Nicked wires or other kinds of mishandling (such as kinking) during installation or reroping can greatly impact fatigue conditions. Inadequate sheave maintenance (infrequent review of sheave groove deviations), improper rope tensioning and poor lubrication can also seriously impact rope fatigue. For further details on rope deformation consult our section on Rope Defects. Fatigue Resistance The ability of wire rope to withstand repeated bending over drive and deflector sheaves (cycles). The term describes rope life based on the maximum mechanical fatigue resistance of the wire material. and does not describe the ability of a rope to withstand mechanical damages or its crush resistance. Fleet Angle The angle of deviation (see figure 19) at which a rope leaves the center of a sheave groove and connects to another. For optimum performance this angle must be kept relatively shallow (usually less than 2 ). Too wide an approach angle and rope will rub against the flanges of a groove, resulting in rope and sheave wear. Kink A kink (figure 20) is the result of unreeling the rope incorrectly during installation, of a certain amount of spin left over from the manufacturing process, or is due to a lack care during installation. A kinked rope can only be returned to its original shape by twisting at one of its ends. Twisting the actual area of the kink itself, or loading a rope in an effort to remove a kink, can result in permanent rope damage and require its replacement. Preforming The process in which the wires and strands are permanently formed during manufacturing into the helical shape they will assume. In preformed ropes, the inner tensions of wires inside the strands, and the strands within rope, are reduced. The end result is a rope that will not spring open when a binder has been removed prior to cutting and installation. Preformed elevator ropes have become the standard in the industry today. Prestretching Kink figure 19 Fleet Angle figure 20 Applying tension to a rope by the manufacturer in an effort to remove constructional stretch. The procedure tries to induce core compaction (and the subsequent rope elongation) that normally occurs after a rope has been installed on site. Due to inherent system limitations in their rope closing processes some manufacturers find that prestretching is the only method they have to lower rope elongation. Some offer the process in an effort to reduce the need, or frequency of rope shortenings over a rope s lifetime. Many agree that prestretching is only of limited effectiveness, except when dealing with 8-strand ropes featuring NFC (and even then impact upon permanent rope elongation is negligible). No single internationally approved process for prestretching exists at present. Some find the process to be counterproductive, as it reduces the rope diameter to that of being either nominal or even slightly less than nominal diameter. This can adversely impact expected rope longevity. While a few professionals believe prestretching creates a rope that fits more snuggly into a worn groove (and thus lessens the opportunity for rope vibration), this can also serve to disguise a far greater problem in regards to system performance, such as in recognizing and addressing sheave wear and sheave groove deviations. Strand Lay Length The distance measured parallel to the axis of a strand in which it makes one complete helical rotation about the center (see figure U). Rope Lay Length The distance measured parallel to the axis of a rope in which the outer wires make one complete helical rotation about the center (see figure 22). Sheave l 1 2 3 4 5 6 1 Lay Length (strand) L 1 2 3 4 5 6 1 Lay Length (rope) figure 21 figure 22 A grooved pulley for hoist ropes. Most roping arrangements feature a main drive sheave and either one or multiple deflector sheaves (see figure 23). While most ropes feature outer wires that are harder than the typical hardness of a sheave, if the original metal casting is made correctly, the rope properly matched installed and maintained, then the chance Brugg Lifting_0513
of a rope proving to be too hard for a sheave remote. To achieve the microstructure necessary for a good sheave, great attention must be paid to the chemistry and the cooling rate of the drive sheave blank during casting. figure 23 Lay Direction and Types of Lay Lay Direction of Strand (described as z or s) Strand lay is described as z (lower case) for Right Lay and Left Lay as s. The terms correspond to the direction of lay of the outer wires in relation to the axis of the strand s center. (See below). figure 25 (s or z) denotes strand direction, while the second (S or Z) indicates rope direction. (Figure 26 and 27.) figure 27 Rouging A fine, red iron oxide crust or powder that appears indicating abrasion where the rope wires strands contact the core, or between adjacent strands themselves. It is a sign of internal rope degradation and not an indication of rust (which is evidence of moisture being introduced). When frictional forces cause steel particles to rub off wires they are deposited on wire strands and naturally oxidize. Field lubrication does not counteract rouging. Tensioning 2:1 Sheave Assembly Right Lay (z) Left Lay (s) Lay Direction of Rope (described as Z or S) Right (Z) or Left (S) descriptive terms (upper case) that identify the direction of lay of the outer strands in a stranded rope. The majority of ropes are supplied as right hand lay. Left hand lay ropes are supplied only to meet specific requirements Right Regular/Ordinary Lay (sz) Lang Lay: zz, ss A rope in which the direction of lay of the wires in the outer strands is in the same direction as the lay of the outer strands in the rope. Note: the first symbol (s or z) denotes strand direction, while the second (S or Z) indicates rope direction. (See Figure 28 and 29 for more detail.) Lang Lay figure 28 Regular Lay/Ordinary Lay figure 26 F 1 F2 F Unequal 3 F4 Tension F 1F 2F 3F 4 Equal Tension Also called load equalization. The process whereby rope loads are measured and tensions balanced by mechanically adjusting termination devices. When combined with efficient lubrication, proper tensioning can promote rope life and overall system performance. Unbalanced ropes stretch unevenly as they try to achieve equilibrium and accommodate load discrepancies. Unequal tensions can lead to: unequal sheave wear short rope life system vibrations rope slippage a change in safety factors figure 24 Right (sz) Left (zs) Regular Lay: sz, zs (also called Ordinary Lay) A rope in which the strand wires are laid in one direction and the completed strands are laid in the rope in the opposite direction. Note: the first symbol Right (zz) Left (ss) figure 29 Right Lang Lay (zz)
COMMON CONSTRUCTIONS Commonly Used Rope Classifications In North American elevator installations only Right Regular Lay (RRL) or sz, and Right Lang Lay (RLL), zz ropes are used. However there is a wide range of rope constructions available to the industry ranging from 6 -strand (for hydraulic and governor applications), 8-strand (by far the most used choice in the hoisting industry today) and 9 -strand designs for advanced installations. For a detailed listing of the more popular rope designs used today review Rope Selection. 6 Strand Constructions 6 x 36 Warrington Seale + IWRC 6 x 36 Warrington Seale + NFC 8 Strand Constructions 6 x 19 Seale + NFC 8 x 19 Seale + NFC/SFC 8 x 19 Warrington + SFC IWRC (Brugg MCX8) 6 x 19 Warrington + NFC 8 x 19 Seale + IWRC 8 x 19 Warrington + IWRC (Brugg SCX8) 6 x 19 Warrington + IWRC 8 x 19 Seale + PWRC (Brugg SC8) 8 x 25 Filler + SFC 6 x 25 Filler + NFC 8 x 19 Warrington +NFC/SFC 8 x 25 Filler + IWRC Brugg Lifting_0513
COMMON CONSTRUCTIONS 9 Strand Constructions 9 x 19 Seale + IWRC (Brugg SCX9) 9 x 21 + IWRC (Brugg SCX9) 9 x 25 + IWRC (Brugg SCX9) 9 x 19 Seale + IWRC (Brugg MCX9) 9 x 21 + IWRC (Brugg MCX9) 9 x 21 + IWRC (Brugg MCX9) 9 x 19 Seale + PWRC (Brugg HRS) 9 x 21 + PWRC (Brugg HRS) 9 x 25 + PWRC (Brugg HRS) 9 x 19 Seale + PWRC (Brugg DP9) 9 x 21 + PWRC (Brugg DP9) 9 x 25 + PWRC (Brugg DP9)