Steel Wire Ropes
Steel Wire Ropes
Lifting operations are performed in every area of the iron and steel plant. In these operations, safety is a critical factor, which requires reliable materials. Steel wire ropes are a key element of the lifting operations, since everything depends on their performance.
Steel wire rope is also known as steel cable. It is a type of rope which consists of several strands of steel wire laid (twisted) into a helix. It is a preferred lifting device for several reasons. Its unique design consists of multiple steel wires which form individual strands laid in a helical pattern around a core. This structure provides strength, flexibility, and the ability to handle bending stresses. Different configurations of the material, wire, and strand structure provide different benefits for the specific lifting applications. These benefits include strength, flexibility, abrasion resistance, crushing resistance, fatigue resistance, corrosion resistance, and rotation resistance.
Wires are the basic building blocks of a wire rope. They lay around a ‘centre’ in a specified pattern in one or more layers to form a strand. The strands are helically laid together around a centre, which is typically some type of core, to form a wire rope. The strands provide all the tensile strength to the wire rope. Properties like fatigue resistance and resistance to abrasion are directly affected by the design of strands. Selection of the proper wire rope for a lifting application needs some careful considerations.
Modern wire rope was invented by the German mining engineer Wilhelm Albert in the years between 1831 and 1834 for use in the mining operations. It was quickly accepted because it proved superior to metal chains and ropes made of hemp which were used earlier. Wilhelm Albert’s first ropes consisted of three strands with each strand having four wires. With the change in the needs, the designs of the wire ropes have also undergone major changes with respect to the core, overlay, and the weight requirement etc.
A wire rope is, in reality, a very complicated machine. It consists of a number of precise moving parts, designed and manufactured to bear a definite relation to one another. In fact, some wire ropes contain more moving parts than many complicated mechanisms. For example, a six strand wire rope laid around and independent wire rope core, each strand and core with 49 wires, contains a total of 343 individual wires. All these wires are to work together and move with respect to one another if the rope is to have the flexibility necessary for successful operation. The wires in a wire rope move independently and together in a very complicated pattern around the core as the rope bends. Clearances between wires and strands are balanced when a rope is designed so that proper bearing clearances exist to permit internal movement and adjustment of wires and strands when the rope has to bend. These clearances vary as bending occurs, but are of the same range as the clearances found in the automobile engine bearings.
The primary factor in wire rope performance is selecting a wire rope with the best combination of properties for the job. The service life of that rope can be greatly extended by following a planned program of installation, operation, maintenance, and inspection to avoid its failure. The appropriate time to replace a wire rope in service is frequently determined by counting the number of broken wires in the length of one rope lay.
The terms which help to define the construction and properties of the wire rope are length, size, pre-formed or non pre-formed, direction and type of lay, finish of wires, grade of rope, and type of core. The length of the wire rope is the total number of meters (cut to size) when wrapped around the spool and the size is the specified nominal diameter of the wire rope and is specified in millimeters.
There are three basic components which make up the design of a steel wire rope. These are (i) wires made from steel which form a singular strand, (ii) multi- wire strands laid around a core in a helical pattern, and (iii) the core.
Wires – Wires are the basic and smallest component of the wire rope and they make up the individual strands in the rope. Wires can be made from a variety of metal materials including steel, iron, stainless steel, Monel, and bronze but in the steel wire rope they are made from steel. The wires can be manufactured to predetermined physical properties and sizes and in a variety of grades which relate to the strength, resistance to wear, fatigue resistance, corrosion resistance, and curve of the wire. A pre-determined number of finished wires are helically laid together in a uniform geometric pattern to form a strand. The process is carried out with precision and exactness to form a strand of correct size and characteristics. The wires themselves can be coated but are most commonly available in a ‘bright’ or uncoated finish.
Strands – Strands of the wire rope consist of two or more wires arranged and twisted in a specific arrangement. The individual strands are then laid in a helical pattern around the core of the wire rope. Strands made of larger diameter wires are more resistant to abrasion, while strands made of smaller diameter wires are more flexible. Strands are designed with various combinations of wires and wire sizes to produce the desired resistance to fatigue and abrasion. Normally, a small number of large wires are more abrasion resistant and less fatigue resistant than a large number of small wires. The required numbers of suitably fabricated strands are laid symmetrical with a definite length of lay around a core to form the finished wire rope.
The core – The core of a wire rope is the foundation of a wire rope. It runs through the centre of the rope. Its primary function is to support the wire strands in the rope and to maintain them in their correct relative positions during the operating life of the wire rope. The core supports the strands and helps to maintain their relative position under loading and bending stresses. Cores can be made from a number of different materials including natural or synthetic fibres and steel but in the steel wire rope, the core is made from steel. Steel core provides more support than the fibre core. Steel cores resist crushing, are more resistant to heat, reduce the amount of stretch, and increase the strength of the wire rope.
Steel cores are normally of two types. The first type is wire strand core (WSC). This type of the core is used in case of small diameter ropes and in some rotation resistant wire ropes. The second type of steel core is independent wire rope core (IWRC). The IWRC can be made in a separate operation or during the closing operation of the wire rope. IWRC normally provides increased strength to the rope, greater resistance to crushing, and resistance to excessive heat. IWRC increases the strength of the wire rope by 7 %, increases its weight by 10 %, and decreases the flexibility slightly. These ropes are recommended for use on installations where severe loads are placed on ropes running over sheaves or wound on drums. The wire core can also have a plastic coating.
Cores made of compacted strands have the additional designation (K). An independent wire core made of compacted strands is hence called IWRC (K). A rope closed in a single operation and made out of compacted strands both in the core and the outer strands is called PWRC (K).
Wire ropes and their free rope end rotate to a greater or lesser extent around its longitudinal axis under the influence of tension. Wire ropes having a core lay direction opposite to the lay direction of the outer strands and 3-strand or 4-strand regular lay wire ropes rotate considerably less than wire ropes with the same lay direction of the wire core and the outer strands and wire ropes with fibre cores.
According to VDI 2358, a wire rope is semi rotation-resistant when ‘the wire rope which turns around its longitudinal axis when subjected to unguided load and / or hardly transmits a torque to the attachment at the end in the event of guided rope ends’. According to ISO 21669 and DIN EN 12385-3 ‘a rope is considered to be semi rotation resistant if it rotates at least once and at most four times around its axis at a length of 1,000 x d under a load of 20 % of the minimum breaking force. In terms of rotation angle, the defined limits are between 360 deg and 1,440 deg’.
According to the regulation of VDI 2358, a wire rope is rotation-resistant, when ‘the wire rope, which hardly turns around its longitudinal axis when subjected to unguided load and / or hardly transmits a torque to the attachment at the end in the event of guided rope ends’. According to ISO 21669 and DIN EN 12385-3 ‘a rope is considered to be rotation resistant if it rotates around its axis at most once at a length of 1,000 x d under a load of 20 % of the minimum breaking force. The rotation can be exhibited here in rope closing or rope opening sense. For the rotation angle, this implies between -360 deg and 360 deg’.
A distinction is made between the nominal rope diameter and the actual rope diameter. The nominal wire rope diameter is an agreed theoretical value for the diameter of the smallest circle enclosing the outer strands. The effective diameter of the wire rope, also called actual rope diameter, is the diameter of the smallest circle enclosing all outer strands, as measured on the rope itself. The tolerance range for the effective rope diameter is specified in related national and international standards. In order to define the correct effective rope diameter, the correct measuring device has to be used. The measurement is to be strictly done over the round ends (circumscribed circle of the rope). If the measurement is done in the strand valleys, the result is inaccurate. For ropes with an uneven number of outer strands, it is important that the measuring surface covers several strands. Fig 1 shows components of a wire rope and measurement of its diameter.
Fig 1 Components of a wire rope and measurement of its diameter
Wire ropes are identified by a nomenclature which is referenced to (i) the number of strands in the rope, (ii) the number (nominal or exact) and arrangement of wires in each strand and (iii) a descriptive word or letter indicating the type of construction i.e. the geometric arrangement of wires.
In the stranding process, initially straight wires are forced into a helical or double-helical form. Hence, the wires in a rope are always under tension, even in an unloaded rope. Such a rope is to be sealed very tightly left and right of the joint before cutting the rope since otherwise the free ends of the wires spring open.
By using a ‘pre-forming tool’, the wires and strands can be heavily plastically deformed during the stranding, so they are laying nearly without tension in the rope, the rope now is pre-formed. The rope producers consider such ropes to be ‘dead’. Pre-formed ropes can be cut much easier, also secured by seizings of course, than non pre-formed ropes.
Direction and types of wire rope lay
The wire rope lay is the helix or spiral of the wires and strands. The word ‘lay’ has got three meanings in the rope design. The first two meanings are descriptive of the wire and strand position in the rope. The first meaning describes the direction in which strands rotate around in the wire rope i.e. right lay or left lay. If the strands rotate around the wire rope in a clock wise direction, the rope is said to be right lay. When the strands rotate in the counter-clockwise direction, the wire rope is left lay. The second meaning shows the relationship between the direction strands lay in the wire rope and the direction wire lay in the strands. The third meaning is a length measurement used in manufacturing and inspection. In the third meaning it is the linear length along the rope that a strand makes one complete spiral around the rope core. Lay length is measured in straight line parallel to the centre line of the rope, not by following the path of the strand.
Direction and type of lay refer to the way the wires are laid to form a strand (either right or left) and how the strands are laid around the core (regular lay, lang lay, or alternate lay). Fig 2 shows direction and types of wire rope lay.
Fig 2 Direction and types of wire rope lay
Regular lay in the wire rope denotes that the wires are twisted in one direction, and the strand in opposite direction to form the rope. The wires in regular lay line up with the axis of the rope. The direction of the wire lay in the strand is opposite to the direction of the strand lay. Regular lay wire ropes are distinguished between right hand ordinary lay (RHOL) and left hand ordinary lay (LHOL). Due to the difference in direction between the wires and strand, regular lay ropes are less likely to untwist or kink. Regular lay roes are also less subject to failure from crushing and distortion because of shorter length of exposed outer wires. Regular lay ropes are naturally more rotation-resistant, and also spool better in a drum than lang lay ropes. The advantages of regular lay ropes are (i) better structural stability, (ii) higher number of broken wires is allowed, and (iii) identification of broken wires is easier.
Lang lay in the wire rope is the opposite. The wires form an angle with the axis of the rope. The wire lay and strand lay around the core in the same direction. The wires and strands appear to run at a diagonal to the centre line of the rope. Lang lay wire ropes are distinguished between left hand lang lay (LHLL) and right hand lang lay (RHLL). Due to the longer length of the exposed outer wires, lang lay ropes have greater flexibility. These ropes are more likely to twist, kink, and crush. Lang lay ropes have a greater fatigue-resistance and are more resistant to abrasion. The advantages of lang lay ropes are (i) better contact in the groove of the sheaves, (ii) superior resistance to wear, (iii) longer lifetime in case of high dead loads, and (iv) considerably better spooling behaviour on a multi-layer drum
In case of alternate lay, the wire rope consists of alternating regular lay and lang lay strands. This lay is used mainly for special applications.
As regards lay direction of a wire rope, a distinction is made between right hand and left hand lay ropes. The lay direction is left hand, when the strands (moving away from the beholder) are rotated counter-clockwise. The lay direction of a rope is right hand, when its strands (moving away from the beholder) are rotated clockwise. The lay direction of a rope is frequently given by a capital S for the left hand lay rope and by a capital Z for the right hand lay rope. Others frequently use abbreviations are RH for right hand lay ropes and LH for left hand lay ropes.
One strand is normally made up of seven to several tens of wires with similar, or differing, diameters in single or multi-layers. In the method where the wires are positioned to form more than two layers, there is the cross lay where the wires of each layer are in the same lay angle, and the parallel lay where one process is used to lay the wires so that the wires of each layer are of the same pitch. For strands of the same diameter, the more is the number of wires, the smaller is the diameter of each wire and the greater is the flexibility of the strand. However, conversely, the rope becomes inferior in its wear resistance nature and its shape deformation nature.
Cross lay – The cross lay is referred to as the point contact lay, as each wire is in contact with each other. The laying of the wires is carried out in such a way that the lay angle is almost equal for each layer of wire of the same diameter. The length of the wires in each layer is also to be the same and the wires of each layer are in contact with each other. Hence, the tension stress which works on the wire becomes uniform, but the bending stress due to the contact points is added and so the fatigue resistance is not as great.
Parallel lay – Parallel lay is also referred to as equal lay. It is also called one operation lay from the number of stranding processes and also as linear contact lay as each wire is in contact with each other. In the parallel lay, the wires of each layer are positioned in such a way that there is no space between them so that the upper layer wires fit neatly into the groove of the lower wires of the strand. For this, wires of differing diameters are positioned at the same time so that each wire layer has the same pitch and is in contact with each other. Hence, parallel lay rope differs from the cross lay rope, although the lay angle of each wire layer and the length of the wires are not uniform, as each wire is in contact with each other, it is superior in its fatigue resistance nature.
The fill factor of a rope is defined as the ratio of the metallic cross section of the rope (or a simplified calculation of the sum of the single wire cross sections) related to the nominal rope diameter. The fill factor specifies the amount of space the wires and strands take in the rope. The fill factors of the most common ropes are between 0.46 and 0.75. This means, that the amount of steel in the rope volume is around 46 % to 75 %. Wire ropes with a wire rope steel core have higher fill factors than ropes with a fibre core. The fill factor of the strand is the proportion of the metallic cross sections at the metal cross section area of the minimum circumscribed circle. Wire ropes which are made of compacted strands have higher fill factors than ropes of un-compacted strands. By compacting and rotary swaging of the rope itself the fill factor can further be increased.
The basic parallel lay are basically of five types namely (i) single layer type, (ii) Seale type, (iii) Warrington type, (iv) filler type with filler wire, and (v) combined pattern type. Fig 3 shows types of basic parallel lay wire rope.
Fig 3 Types of basic parallel lay wire rope
The single layer type wire rope has the basic strand construction of having wires of the same size wound around a centre. The most common example of the single layer construction is a 7 wire strand. It has a single-wire centre with six wires of the same diameter around it.
The Seale type wire rope construction has two layers of wires around a centre with the same number of wires in each layer. All wires in each layer are of the same diameter. The strand is designed so that the large outer wires rest in the valleys between the smaller inner wires. In Seale type, the number of wires of each layer is shown as 1+n+n and the number of wires of the inner and outer layers is the same. The wires of the outer layer fit completely into the grooves of the inner layer wires. The outer layer wires of the Seale type rope is thicker when compared to other parallel lays and so it is superior, particular in its wear resistance and is mainly used for elevators.
The Warrington type wire rope construction has two layers of wires around a centre 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. In the Warrington type, the number of wires of each layer is shown as 1+n+(n+n) and there are two types of wires for the outer layers, one being large and the other being small. The number of wires of the outer layer is double that of the inner layer and through a combination of the inner and outer layers the spaces between the wires are kept small. The Warrington type rope is not being used to a great extent these days.
The filler wire type wire rope construction has two layers of uniform-size wire around a centre 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. In the filler type (with filler wire), the number of wires of each layer is shown as 1+n+(n)+2n and the number of wires of the outer layers is double that of the inner layer. The inner wires and the same number of thin filler wires are used to fill the spaces in the inner and outer layers. This filler type rope has a good balance between the flexibility, fatigue resistance, and wear resistance and has the widest range of use among parallel lay ropes.
The combined patterns type wire rope construction has strand which is formed in a single operation using two or more of the above constructions. As an example, the wire rope can have a Seale construction in its first two layers and a Warrington construction in the third layer, and a Seale construction in the outer layer. The combined pattern type of wire rope construction is very superior in its fatigue resistance nature. It also has high flexibility and is superior in its wear resistance and hence has a wide range of uses.
Flat type – In flat type wire rope, the strands are combined in such a way that the outer circumference of the rope is flat in shape. This rope has a smooth surface and hence the surface pressure due to coming into contact with the groove of the drum and the sheave is smaller than that of ordinary ropes. It is also superior in its wear resistance nature. In general, the triangular strand and the shell strand are used the most. The flat strand is also being used at certain places.
Lubrication of wire ropes
Lubrication is applied during the manufacturing process of the steel wire rope and penetrates all the way to the core. Wire rope lubrication has two primary benefits namely (i) it reduces friction as the individual wires and strands move over each other, and (ii) it provides corrosion protection and lubrication in the core, inside wires, and outside surface.
Lubrication of wire ropes is a difficult proposition, regardless of the construction and composition. Wire rope lubricants have two principal functions namely (i) to reduce friction as the individual wires move over each other, and (ii) to provide corrosion protection and lubrication in the core and inside wires and on the exterior surfaces.
There are two types of wire rope lubricants namely (i) penetrating and (ii) coating. Penetrating lubricants contain a petroleum solvent which carries the lubricant into the core of the wire rope then evaporates, leaving behind a heavy lubricating film to protect and lubricate each strand. Coating lubricants penetrate slightly, sealing the outside of the cable from moisture and reducing wear and fretting corrosion from contact with external bodies. Both types of wire rope lubricants are used. But because most wire ropes fail from the inside, it is important to make sure that the centre core receives sufficient lubricant.
A combination approach in which a penetrating lubricant is used to saturate the core, followed with a coating to seal and protect the outer surface, is normally recommended. Wire rope lubricants can be petrolatum, asphaltic, grease, petroleum oils or vegetable oil-based. Petrolatum compounds, with the proper additives, provide excellent corrosion and water resistance. In addition, petrolatum compounds are translucent, allowing the technician to perform visible inspection. Petrolatum lubricants can drip-off at higher temperatures but maintain their consistency well under cold temperature conditions. Asphaltic compounds normally dry to a very dark hardened surface, which makes inspection difficult. They adhere well for extended long-term storage but crack and become brittle in cold climates. Asphaltic compounds are of coating type.
Various types of greases are used for wire rope lubrication. These are the coating types which penetrate partially but normally do not saturate the rope core. Common grease thickeners include sodium, lithium, lithium complex, and aluminum complex soaps. Greases used for this application normally have a soft semi-fluid consistency. They coat and achieve partial penetration if applied with pressure lubricators. Petroleum and vegetable oils penetrate best and are the easiest to apply since proper additive design of these penetrating types gives them excellent wear and corrosion resistance. The fluid property of oil type lubricants helps to wash the rope to remove abrasive external contaminants.
Wire ropes are lubricated during the manufacturing process. In case of wire rope with a steel core, the lubricant (both oil and grease type) is pumped in a stream just ahead of the die which twists the wires into a strand. This allows complete coverage of all the wires.
After the cable is put into service, re-lubrication is required due to loss of the original lubricant from loading, bending, and stretching of the cable. Field re-lubrication is necessary to minimize corrosion, and to protect and preserve the rope core and wires, and thus extend the service life of the wire rope.
Wire rope characteristics
The term ‘bright’ refers to a wire rope manufactured with no protective coating or finish other than lubricant can provide. These wire ropes are normally manufactured from high carbon steel. The chemistry of the steel used and the practice employed in drawing the wire are varied to achieve the ultimate combination of tensile strength, fatigue resistance, and wear resistance in the finished wire rope.
Bright finish is suitable for most of the applications. Galvanized finish is done for corrosive environments. Galvanized finished wire ropes have improved corrosion resistance. These wire ropes are produced from the drawn wires which have been galvanized. Wire ropes are normally produced in three grades as given below.
- Improved plow steel (IPS) -This steel is strong, tough, durable steel which combines good strength with high resistance to fatigue. Its minimum tensile strength varies from 154 kg/sq mm to 178 kg/sq mm depending upon wire diameter.
- Extra improved plow steel (EIP) – In this steel minimum tensile strength varies from 169 kg/sq mm to 196 kg/sq mm depending on the wire diameter.
- Extra-extra improved plow steel (EEIP) – It is a grade where a high breaking strength is needed. This grade typically provides a breaking strength which is a minimum of 10 % higher than the EIP grade.
Breaking strength – The calculated breaking strength of a steel wire rope is defined as the metallic cross-section of a steel wire rope (the sum of the individual cross sections of all the wires making up the rope) multiplied by the nominal tensile strength of the steel wire rope. The minimum breaking strength of the steel wire rope is the calculated breaking strength of the rope multiplied by the spin factor. The actual breaking strength of a steel wire rope is the breaking strength of the rope as determined in a breaking test. A new steel wire rope is required to achieve an actual breaking strength equal to or higher than the minimum breaking strength.
The breaking strength of a steel wire rope can be increased by increasing the metallic area of the rope (e.g. by using strands with higher fill factors, by compacting the strands or by swaging the rope), by increasing the tensile strengths of the individual wires, or by increasing the spin factor of the rope. This can also be achieved by improving the contact conditions between the rope elements by using a plastic infill.
Bending fatigue resistance – The bending fatigue resistance of steel wire rope is defined as the number of bending cycles a rope can achieve in a bending fatigue test under defined parameters (e.g. running over sheaves with a defined diameter and a pre-determined line pull corresponding to the minimum braking load of the steel wire rope). The bending fatigue resistance of the steel wire rope increases with increasing D/d ratio (sheave diameter (D) / nominal rope diameter (d)), and by reducing the line pull.
The bending fatigue resistance of a steel wire rope can be increased by increasing the contact area between the steel wire rope and the sheave and by increasing the contact conditions between the rope elements, by adding a plastic layer between the IWRC, and the outer strands. Due to the larger contact area between the ropes and the sheaves and due to the increased flexibility, 8- strand wire ropes are more resistant to bending fatigue than 6- strand wire ropes of a similar design.
Flexibility – The flexibility of a wire rope is a measure of how easily the rope allows itself to bend around a given diameter. The flexibility of the wire rope is among other things dependent on the line pull. The flexibility of an unloaded rope can be measured by the sag of a rope under its own weight.
The flexibility of a steel wire rope typically increases with an increasing number of strands and wires in the rope. The flexibility is also influenced by the lay lengths of the strands, of the rope core and the rope, as well as by the gaps between wires and strands.
If a rope is not flexible enough, it is forced to bend around a sheave of a given diameter, which reduces the bending fatigue life of the rope. It also forced to bend around a drum of a given diameter. Spooling problems can be a consequence.
Efficiency factor – When running over a sheave, a wire rope has to be converted from a straight condition into a bent condition at the point when the rope runs onto the sheave and has to be converted again from the bent into the straight condition when it runs off the sheave. Also the bearing has to be turned. In doing so, the friction forces in the rope as well as the friction forces in the bearing have to be overcome. This leads to a change of the rope force.
One describes the relationship of the rope force on both sides of the sheave as the efficiency factor and accepts that this numerical value also takes into account the friction losses of the bearing. When measuring the efficiency factor of a wire rope, the loss of the line pull while the rope is running over the sheave is measured. An efficiency factor of 0.98, or alternatively a strength loss of 2 %, is normally assumed for wire ropes.
Wear resistance – Changes in line pull cause changes in the rope length. Rope sections lying on a sheave or on the first wraps of a drum can only adapt to the changing line pull by sliding over the groove surface of the sheave or the drum when the length change occurs. This relative motion causes abrasion (both in the grooves and on the special wire rope). Using less and hence larger outer wires can increase the wear resistance of the rope. The pressure between the sheave and the rope can be minimized due to the optimized contact areas and hence also the wear of the rope can be minimized. The wear resistance can also be influenced by the metallurgy of the outer strands.
Modulus of elasticity – The modulus of elasticity of a material is defined as the proportional factor between load and elongation. The modulus of elasticity is a material property. Besides the elastic properties of the wire material used, the modulus of elasticity of wire ropes is dependent on the rope geometry and the load history of the rope. Since this is not a material property, ISO 12076 recommends calling this factor as the ‘rope modulus’. Fig 4 shows the deformation behaviour of wire rope.
Fig 4 Deformation behaviour of wire rope
Fig 4(a) shows a load-elongation diagram of a wire. Here the modulus of elasticity can be determined as the gradient of the curve in the linear area. Fig 4(b) shows a load-elongation diagram of a strand. As the strand consists of several wires of different lengths and different lay lengths or different lay angles, here the shorter and less elastic elements get loaded first. For this reason the curve is not linear in the lower area. The graph only gets linear, when all the wires in the strand bear the load together.
Fig 4(c) shows the load-elongation diagram of a rope. Here also a non-linear correlation is found in the lower area between load and elongation. Here again the non-linearity can be explained by the overload of the shorter and the less elastic rope elements. The load-elongation diagram is linear in the area in which all elements share the load and plastically deform. As a result of settling effects, the modulus of elasticity of wire ropes increases over the life time. The biggest part of this change happens with the first loading of the rope. Later the modulus of elasticity varies only very slightly. For this reason a new wire rope is always to be loaded and relived multiple times before measuring the modulus of elasticity. The determination of the modulus of elasticity is described in ISO 12076.
Radial stability – The radial stability of a wire rope is a function of the rope geometry and the line pull. The radial stability of a steel wire rope typically reduces with an increasing number of rope elements. It also increases with the increase of the line pull. Ropes with insufficient radial stability are not suitable for multi-layer spooling.
Structural stability – It is necessary that a rope maintains its structure during its working life. Adding a plastic layer between the IWRC and the outer strands can increase the stability of the rope structure. The plastic fixes the position of the rope elements relative to each other.
Diameter reduction of a special wire rope – With increasing line pull, a special wire rope not only gets longer, but it also reduces in diameter. A great part of that diameter reduction is reversible which means that the rope diameter increases again after unloading. Part of the diameter reduction, however, is permanent. If the diameter reduction of a steel wire rope under load is too high, in multi-layer spooling the rope can pull into deeper layers of the drum. Hence, the diameter reduction of steel wire ropes is to be considered when designing ropes for multi-layer applications as shown in Fig 4(d).
Causes of wire rope failure
The most common causes of wire rope failure are kinking, overloading, dragging, improper spooling, and whipping.
Kinking – It results in the permanent rope deformation and localized wear. It is normally caused by allowing a loop to form in a slack line and then pulling the loop down to a tight permanent set.
Overloading – It results in accelerated wear, abrasion, rope crushing, and distortion on drums and sheaves, and can result in complete rope failure.
Dragging – Dragging of wire rope over a bank or some other object results in localized wear, which means shorter life.
Improper spooling – It results in crushed and distorted ropes. It comes from careless installation and operation of the rope.
Whipping – Whipping a line, which results in many squared off broken wires, comes from jerking or running the line loose.
Usage of wire ropes
Steel wire ropes are widely used in many applications such as crane, tower crane, surface and underground mining, excavation, logging of any type of terrain, tramway, elevator, oil and gas, drilling, marine and electrical constructions. They are used dynamically for lifting and hoisting in cranes and elevators, and for transmission of mechanical power. Wire rope is also used to transmit force in mechanisms, such as a Bowden cable or the control surfaces of an automobile or an airplane connected to levers and pedals. Wire ropes have to fulfill different requirements depending on where they are used. Some of the uses of the wire ropes are as follows.
Running ropes – They are also called stranded ropes. These wire ropes are bent over sheaves and drums. They are hence stressed mainly by bending and secondly by tension.
Stationary ropes – These ropes are stay ropes (spiral ropes, mostly full-locked) and have to carry tensile forces. These ropes are therefore mainly loaded by static and fluctuating tensile stresses. Ropes used for suspension are frequently called cable.
Track ropes – Track ropes (full locked ropes) have to act as rails for the rollers of cabins or other loads in aerial ropeways and cable cranes. In contrast to running ropes, track ropes do not take on the curvature of the rollers. Under the roller force, a so called free bending radius of the rope occurs. This radius increases (and the bending stresses decrease) with the tensile force and decreases with the roller force.
Wire rope slings – Wire (stranded ropes) are used to harness various kinds of goods. These slings are stressed by the tensile forces but first of all by bending stresses when bent over the more or less sharp edges of the goods.