wire rope breaking strength calculation quotation

Rope strength is a misunderstood metric. One boater will talk about tensile strength, while the other will talk about working load. Both of these are important measurements, and it’s worth learning how to measure and understand them. Each of these measurements has different uses, and here we’re going to give a brief overview of what’s what. Here’s all you need to know about rope strength.

Each type of line, natural fiber, synthetic and wire rope, have different breaking strengths and safe working loads. Natural breaking strength of manila line is the standard against which other lines are compared. Synthetic lines have been assigned “comparison factors” against which they are compared to manila line. The basic breaking strength factor for manila line is found by multiplying the square of the circumference of the line by 900 lbs.

As an example, if you had a piece of ½” manila line and wanted to find the breaking strength, you would first calculate the circumference. (.5 X 3.14 = 1.57) Then using the formula above:

To calculate the breaking strength of synthetic lines you need to add one more factor. As mentioned above, a comparison factor has been developed to compare the breaking strength of synthetics over manila. Since synthetics are stronger than manila an additional multiplication step is added to the formula above.

Using the example above, letÂ’s find the breaking strength of a piece of ½” nylon line. First, convert the diameter to the circumference as we did above and then write the formula including the extra comparison factor step.

Knots and splices will reduce the breaking strength of a line by as much as 50 to 60 percent. The weakest point in the line is the knot or slice. However, a splice is stronger than a knot.

Just being able to calculate breaking strength doesn’t give one a safety margin. The breaking strength formula was developed on the average breaking strength of a new line under laboratory conditions. Without straining the line until it parts, you don’t know if that particular piece of line was above average or below average. For more information, we have discussed the safe working load of ropes made of different materials in this article here.

It’s very important to understand the fundamental differences between the tensile strength of a rope, and a rope’s working load. Both terms refer to rope strength but they’re not the same measurement.

A rope’s tensile strength is the measure of a brand-new rope’s breaking point tested under strict laboratory-controlled conditions. These tests are done by incrementally increasing the load that a rope is expected to carry, until the rope breaks. Rather than adding weight to a line, the test is performed by wrapping the rope around two capstans that slowly turn the rope, adding increasing tension until the rope fails. This test will be repeated on numerous ropes, and an average will be taken. Note that all of these tests will use the ASTM test method D-6268.

The average number will be quoted as the rope’s tensile strength. However, a manufacturer may also test a rope’s minimum tensile strength. This number is often used instead. A rope’s minimum tensile strength is calculated in the same way, but it takes the average strength rating and reduces it by 20%.

A rope’s working load is a different measurement altogether. It’s determined by taking the tensile strength rating and dividing it accordingly, making a figure that’s more in-line with an appropriate maximum load, taking factors such as construction, weave, and rope longevity into the mix as well. A large number of variables will determine the maximum working load of a rope, including the age and condition of the rope too. It’s a complicated equation (as demonstrated above) and if math isn’t your strong point, it’s best left to professionals.

However, if you want to make an educated guess at the recommended working load of a rope, it usually falls between 15% and 25% of the line’s tensile strength rating. It’s a lotlower than you’d think. There are some exceptions, and different construction methods yield different results. For example, a Nylon rope braided with certain fibers may have a stronger working load than a rope twisted out of natural fibers.

For safety purposes, always refer to the information issued by your rope’s manufacturer, and pay close attention to the working load and don’t exceed it. Safety first! Always.

If you’re a regular sailor, climber, or arborist, or just have a keen interest in knot-tying, be warned! Every knot that you tie will reduce your rope’s overall tensile strength. Some knots aren’t particularly damaging, while others can be devastating. A good rule of thumb is to accept the fact that a tied knot will reduce your rope’s tensile strength by around 50%. That’s an extreme figure, sure, but when it comes to hauling critical loads, why take chances?

Knots are unavoidable: they’re useful, practical, and strong. Splices are the same. They both degrade a rope’s strength. They do this because a slight distortion of a rope will cause certain parts of the rope (namely the outer strands) to carry more weight than others (the inner strand). In some cases, the outer strands end up carrying all the weight while the inner strands carry none of it! This isn’t ideal, as you can imagine.

Some knots cause certain fibers to become compressed, and others stretched. When combined together, all of these issues can have a substantial effect on a rope’s ability to carry loads.

Naturally, it’s not always as drastic as strength loss of 50% or more. Some knots aren’t that damaging, some loads aren’t significant enough to cause stress, and some rope materials, such as polypropylene, Dyneema, and other modern fibers, are more resilient than others. Just keep in mind that any knots or splices will reduce your rope’s operations life span. And that’s before we talk about other factors such as the weather or your rope care regime…

It is because of the versatility of wire rope that engineers must be deeply educated on tensile strength, as well as the impact that wire rope diameter and strand constructions can have on tensile strength, along with other critical characteristics such as cycle count and flexibility.

All wire ropes are tested for breaking strength. Motion cables have a minimum breaking strength requirement, which is determined by the material, diameter and construction of the wire rope. At Carl Stahl Sava Industries, wire rope is tested on tensile strength equipment. Either or both the wire rope itself can be tested against the minimum load requirement, or the fittings swaged to the wire rope can be similarly tested for their holding strength on the cable.

To ensure wire rope stays within its specified tensile limits, engineers will derate the minimum cable tensile strength by a safety factor. This is how engineers arrive at what is known as the Working Load Limit or WLL. The WLL safety factor of each cable being tested is application specific, but often times a factor of 3 to 5 is utilized. Divide the minimum tensile strength by the safety factor to calculate the safe working load limit for a specific cable.

Steel cables are made more durable in part due to a manufacturing process known as cold working or cold forming, where the material is shaped below its recrystallization temperature. Cold worked steel distorts the steel’s crystal grains in the flow of the metal, resulting in hardening of the material, thus strengthening it. Due to steel’s natural strength, combined with its well-known resistance to corrosion, engineers turn to steel cables for applications where exposure to harsh environments does not compromise wire rope integrity.

Although commonly mistaken for stranded cable, braided cable refers to the braided wrapping or insulation found in conductive cables, such as those transmitting power or data. This braided “jacket” insulates and strengthens the conductive, data or electric cable, making it more tolerant to time in the field, along with the twisting, turning and rolling to which braided cable is commonly exposed. Sava does not manufacture braided cable or other conductive cable, like coaxial and network cable, as none of these cables, commonly braided in nature, actuate motion.

Interestingly, even Google machine learning algorithms struggle to understand the differences between motion cable, such as steel wire rope or tungsten mechanical cable, and cables that transmit electricity or information. When a searcher uses Google or other common search engines like Bing or Yahoo! to find steel cable manufacturers like Sava, or for that matter, a maker of braided cable, the search engine confuses the two and often serves the wrong cable to the searcher.

Wire rope manufacturers offer a wide range of what is known as “cable constructions,” which characterizes the number of individual wires used to stranded the wire rope. Consequently, most wire rope comprises a center or “core” wire, that is then wrapped in more wires, thus completing the stand’s construction. In the case of a 1x7 steel cable construction, the core wire represents wire #1, wrapped in six additional wires. Hence 1x7 steel cable.

In many cases, that same 1x7 steel cable is then used as the core in the manufacture of larger, stronger and more durable steel cable. For instance, 7x19 steel cable strand comprises the original seven wires, or 1x7 strand, and is wrapped in 12 more wires, thus making a 7x19 construction. Said another way, the 7x19 wire rope consists of the original 1x7 strand, plus 12 additional wires, making for a total of 19 wires.

Less seldom are wire rope constructions that do not include a core wire. In these cases, take 3x7 cable construction for instance, three, 1x7 stranded wire ropes are wrapped around one another, forming a triangular-shaped wire rope construction.

Safe Working Load (SWL) sometimes stated as the Normal Working Load (NWL) is the mass or force that a piece of lifting equipment, lifting device or accessory can safely utilize to lift, suspend, or lower a mass without fear of breaking. Usually marked on the equipment by the manufacturer and is often 1/5 of the Minimum Breaking Strength (MBS) although other fractions may be used such as 1/4, 1/6 and 1/10.[1][2][3]

Other synonyms include Working Load Limit (WLL), which is the maximum working load designed by the manufacturer. This load represents a force that is much less than that required to make the lifting equipment fail or yield, also known as the Minimum Breaking Load (MBL). SWL or WLL are calculated by dividing MBL by a safety factor (SF). An example of this would be a chain that has a MBL of 2000 lbf (8.89 kN) would have a SWL or WLL of 400 lbf (1.78 kN) if a safety factor of 5 (5:1, 5 to 1, or 1/5) is used.

Here at Industrial Rope Supply, we are not only committed to providing you with a quality product, but also with all the information needed to insure safety comes first on every job. Have safety questions on a product purchased from us? Contact us today and we’ll be happy to talk you through and/or provide you with the safety materials needed.

It’s rare for a week to go by here at Industrial Wire Rope without discussions about tensile strength or working load limit. We take it for granted that most people in our industry know there is a difference in the meaning of these terms. Yet, these terms and others are highly interrelated, and we thought an overview of them on one page might be a helpful reference. For those who want to dive deeper into the definitions and how they apply on the job, we’re also providing links to sources with additional information.

Let’s start with Tensile Strength. As we described in a post from 2017, “Tensile strength is a measurement of the force required to pull something such as rope, wire, or a structural beam to the point where it breaks. The tensile strength of a material is the maximum amount of tensile stress that it can take before failure, for example breaking.”

In our immediate world, tensile strength is the force required to break the ropes we offer. Tensile strength is determined by testing. Obviously, it is different for every type of rope, being a function of the material and construction of each type.

Although tensile strength is a definitive quantity measuring the force required to break a rope, working load limit is a measure that takes a wide range of variables into account. And always, the tensile strength of a material is greater than the recommended working load limit.

The working load limit provides consideration for factors such as the abrasion, friction and rubbing the rope is subjected to, the variance in temperature extremes it is exposed to, harmful substances that may come into contact with it, age, and even knots in the rope. Working load limit is defined as “the maximum load which should ever be applied to the product, even when the product is new and when the load is uniformly applied”.

Working load limit is always a fraction of tensile strength, allowing for a generous margin of safety. For wire ropes, it’s common for the working load limit to be set at 20% of tensile strength. However, you generally don’t have to be concerned about doing the math on your own. Rope manufacturers typically mark the working load limit on the products, so the information should be readily available to you.

Wire rope is also known by many other names, such as: wire, multi-strand wire, flexible wire, cable, cord, steelcord, etc. but it is essentially a collection of small filaments wound around each other in a manner that largely retains its shape when bent, crushed and/or tensioned.

It is a system for significantly increasing the strength and flexibility of steel wire and is used in almost every important application we see around us. For example: suspension bridges, tyres, brake and accelerator cables (in cars), high-pressure flexible pipes, lifting and rigging cables, electrical conductors, etc. and it comes in many different forms. Fig 2 shows just a very small sample of available designs.

With minor variations, the generally accepted method for designating a wire rope construction in the industry is by describing it numerically. For example:

Whilst "IWRC" wire ropes offer a slightly greater tensile capacity (≈7%) than those with fabric or polymer fillers, the additional strength does not come from the tensile capacity of the core filaments but from improved dimensional stability under load. And whilst they are also much more resistant to crushing, they are stiffer than fibre core ropes and therefore not recommended for applications where tension occurs under bending.

Warrington (Fig 1) is a parallel lay construction with an outer layer comprising wires of alternating large and small diameters, each outer layer having twice the number of wires as the layer immediately beneath. The benefit of this design is to increase packing and therefore strength density, however, unless the different diameter filaments are of the same strength (unlikely), this construction is limited by the strength of the weakest filaments.

Seale (Figs 1 & 2 6x36) is also a parallel lay construction but with the same number of wires in each wire layer. All the wires in any layer are the same diameter. This is an alternative to the Warrington construction, with similar benefits and disadvantages.

Regular lay constructions are used much more widely (than Lang lay) because they have excellent structural stability and less tendency to unwrap under tension (see Rotating vs Non-Rotating below). However, because it has a knobbly (undulating) surface it will wear both itself and any surface over which it is run much more quickly than Lang lay wire rope.

Lang lay constructions have a flatter surface than regular lay constructions giving them better resistance to wear and bending fatigue, especially when made from flattened (elliptical) filaments. They are, however, much less structurally stable and subject to birdcaging if the wire rope is over-bent or twisted against its wrapped direction.

"Regular Lay", multi-strand constructions are normally subject to slightly less rotation under tension (than Lang lay) due to the opposite helical direction of the filaments (within the strands) and the strands (within the rope), however, you can improve their rotation characteristics still further by;

Fillers (Fig 2) may be fabric, polymer or even smaller diameter filaments (e.g. 6x36). Whilst they contribute little to the tensile strength of wire rope, they can significantly; improve performance under bending (fabric and polymer cores only), reduce axial growth, reduce rotation in rotation-resistant constructions, improve structural stability and increase fatigue life.

This filler material should not be included in strength (tensile capacity) calculations, but must be included in those for axial stiffness (extension). If it is ignored, your calculations will reveal excessive extension as the wire rope collapses.

Suspension bridges tend to be constructed from densely packed, single strand plain "Wire Rope" constructions using large diameter galvanised filaments. Little heed is paid to rotational resistance as strength is paramount and once tensioned, they should remain in that loading condition for their design life.

Lifting & winching normally require wire ropes of good flexibility and fatigue resistance. Therefore they tend to be similar to 6x36 but with fibre core instead of the IWRC in Fig 2

Hosecord is suitable for HPHT flexible pipes as lateral flexibility is generally considered less important than minimal longitudinal growth or maximum tensile strength (per unit cross-sectional area).

Remote operating cables such as hand-brakes and accelerators on cars normally only work in tension so they need to be strong but not necessarily stiff (as they are fully contained in reinforced outer sheaths). These tend to be manufactured from large diameter "TyreCord" or small diameter single-strand "Wire Rope".

Wire rope does not obey Hooke"s law. Therefore, you cannot accurately predict how much it will stretch for any specified force. This unpredictability applies to any section removed from the same manufactured length of cord and even between cords produced to the same specification but by different manufacturers.

CalQlata has decided that the accuracy of axial stiffness (EA) of wire rope falls outside its own levels of acceptability and therefore does not include it in the wire rope calculator. The extension calculated in the Wire Rope calculator (δLᵀ) is based upon the effect of axial tension on packing density. It is therefore important that core material is not ignored when using the calculator to evaluate this characteristic.

Wire rope does not obey Hooke"s law. Therefore, you cannot accurately predict how much it will twist for any specified torque. This unpredictability applies to any section removed from the same manufactured length of cord and even between cords produced to the same specification but by different manufacturers.

CalQlata has decided that the accuracy of torsional stiffness (GJ) of wire rope falls outside its own levels of acceptability and therefore does not include it in the wire rope calculator.

1) No wire rope calculator, whether dedicated or generic, will accurately predict the properties of any single construction under a wide range of loading conditions

2) No wire rope calculator, whether dedicated or generic, will accurately predict any single property for a range of constructions under a wide range of loading conditions

The only wire rope that can be reliably analysed is that which is used for suspension bridges, because; it comprises a single strand, is very densely packed, has negligible twist, contains filaments of only one diameter, is never subjected to minimum bending and every filament is individually tensioned.

There is a very good reason why manufacturers do not present calculated performance data for construction or design proposals, because even they cannot accurately predict such properties and quite rightly rely on, and publish, test data.

During his time working in the industry, the wire rope calculator"s creator has seen, created and abandoned numerous mathematical models both simple and complex. He has gradually developed his own simplified calculation principle based upon his own experience that still provides him with consistently reliable results of reasonable accuracy.

The purpose of CalQlata"s wire rope calculator is to provide its user with the ability to obtain a reasonable approximation for a generic construction, after which, accurate test data should be sought from the manufacturer for the user"s preferred construction.

The calculation principle in the wire rope calculator is based upon changes in the properties of the wire rope that occur with variations in packing density under tension

Bearing in mind the above limitations CalQlata can provide the following assistance when generating (manipulating) the wire rope calculator"s input data and interpreting its output

Alternatively, for wire rope with multiple filament diameters, you need to find an equivalent diameter with the following proviso; you must enter the minimum filament yield stress (SMYS)

It is expected that apart from fillers, all the material in the wire rope will be identical and therefore have the same density, i.e. using different materials will result in less than "best" performance. However, if such a construction is proposed, you can calculate an equivalent density as follows:

It is expected that apart from fillers, all the material in the wire rope will be identical and therefore have the same tensile modulus, i.e. using different materials will result in less than "best" performance. However, if such a construction is proposed, you should enter the highest tensile modulus.

The wire rope calculator simply adds together the total area of all the filaments and multiplies them by the SMYS entered, which represents a theoretical maximum breaking load that would exist if this load is equally shared across all of the filaments and the lay angles have been arranged to eliminate localised (point) loads between adjacent filaments.

If the wire rope has been properly constructed it is likely that its actual break load will be greater than 80% of this theoretical value. However, given the vagaries of wire rope construction, the actual break load can vary considerably dependent upon a number of factors. CalQlata suggest that the following factors may be used to define the anticipated break load of any given construction:

The axial stiffness and strain under load will be affected by this value, hence the reason why the most reliable (predictable) constructions tend to be minimum [number of] strands and single filament diameter. The Warrington and Seale constructions and combinations thereof tend to provide the highest packing density (but lowest flexibility) and there is little to be gained from using these constructions in more than single stranded wire rope as the benefit of high-packing density will be lost with no gain in flexibility.

The anticipated second moment of area of the wire rope at tension "T" due to deformation but insignificant flattening as it is assumed the wire rope will be bent over a formed (shaped) sheave or roller.

The anticipated tensile modulus of the wire rope at tension "T" due to deformation but insignificant flattening as it is assumed the wire rope will be bent over a formed (shaped) sheave or roller.

It is not advisable to induce this bend radius in operation due to uncertainties associated with wire rope construction, especially for dynamic applications. CalQlata suggests that a similar approach to that used for the break load (Fb) above also be applied here, i.e.:

A change in diameter will occur in all wire rope, irrespective of construction, until packing density has reached a limiting value. The value provided in the wire rope calculator is that which would be expected if the construction remains intact at the applied tension "T"

Unreliability of this value increases with complexity in wire rope due to its longitudinal variability and the increased likelihood of premature failure.

The accuracy of this data will range from about ±1% for wire rope with a single strand and a single filament diameter, up to about ±15% for constructions of similar complexity to OTR cord

A change in length of any wire rope will occur due to the fact that the packing density increases with tension. This is not, however, a linear relationship.

This can be an unreliable value as illustrated by tests carried out (by the author) on two pieces of wire rope supplied by the same well-known manufacturer both of which were cut from the same length, varied in tensile capacity by only 1.5%, but the tensile modulus (and strain at break) varied by 34%. Whilst this was an extreme case, significant variations have been seen in wire rope manufactured by a number of manufacturers.

Whilst the wire rope calculator does not calculate axial stiffness (see Calculation Limitations 9) above), CalQlata can suggest the following rule-of-thumb that will provide reasonable results for most constructions at the applied tension "T":

Whilst the wire rope calculator does not calculate bending stiffness (see Calculation Limitations 8) above), CalQlata can suggest the following rule-of-thumb that will provide reasonable results for most constructions at the applied tension "T":

Low complexity means single strand and single wire diameter. Medium complexity means multi-strand and single wire diameter. High complexity means multi-strand and multiple wire diameters.

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Breaking strength Test is one the most common testing requirements. Breaking Strength is generally the tensile or compressive load required to fracture or to cause the sample to fail. In a tensile test, the tensile strength at break is the breaking strength. For an ampoule bottles test, the max force of the bottles breaking is the break strength.

Any assembly of steel wires spun into a helical formation, either as a strand or wire rope, when subjected to a tensile load, can extend in three separate phases, depending on the magnitude of the applied load. There are also other factors which produce rope extension which are very small and can normally be ignored.

At the commencement of loading a new rope, extension is created by the bedding down of the assembled wires with a corresponding reduction in overall diameter. This reduction in diameter is accommodated by a lengthening of the helical lay. When sufficiently large bearing areas have been generated on adjacent wires to withstand the circumferential compressive loads, this mechanically created extension ceases and the extension in Phase 2 commences. The Initial Extension of any rope cannot be accurately determined by calculation and has no elastic properties.

The practical value of this characteristic depends upon many factors, the most important being the type and construction of rope, the range of loads and the number and frequency of the cycles of operation. It is not possible to quote exact values for the various constructions of rope in use, but the following approximate values may be employed to give reasonably accurate results.

Following Phase 1, the rope extends in a manner which complies approximately with Hookes Law (stress is proportional to strain) until the Limit of Proportionality or Elastic Limit is reached. It is important to note that wire ropes do not possess a Young’s Modulus of Elasticity, but an ‘apparent’ Modulus of Elasticity can be determined between two fixed loads.

The Modulus of Elasticity also varies with different rope constructions, but generally increases as the cross-sectional area of steel increases. By using the values given, it is possible to make a reasonable estimate of elastic extension, but if greater accuracy is required it is advisable to carry out a modulus test on an actual sample of the rope. As rope users will find it difficult to calculate the actual metallic steel area , the values can be found in the Wire Rope Users Manual or obtained from Bridon Engineering.

The permanent, non-elastic extension of the steel caused by tensile loads exceeding the yield point of the material. If the load exceeds the Limit of Proportionality, the rate of extension will accelerate as the load is increased, until a loading is reached at which continuous extension will commence, causing the wire rope to fracture without any further increase of load.

The coefficient of linear expansion (∝) of steel wire rope is (6.94 x10-6 per oF) and therefore the change in length of 1 foot of rope produced by a temperature change of t (oF) would be:

Example: What will be the total elongation of a 200 Ft length of 11/8" diameter Blue Strand 6x41 IWRC wire rope at a tension of 20,000 Ibs and with an increase in temperature of 20oF.

In addition to bending stresses experienced by wire ropes operating over sheaves or pulleys, ropes are also subjected to radial pressure as they make contact with the sheave. This pressure sets up shearing stresses in the wires, distorts the rope’s structure and affects the rate of wear of the sheave grooves. When a rope passes over a sheave, the load on the sheave bearing results from the tension in the rope and the angle of rope contact. It is independent of the diameter of the sheave.

Assuming that the rope is supported in a well fitting groove, then the pressure between the rope and the groove is dependent upon the rope tension and diameter but is independent of the arc of contact.

It must be realized that this method of estimation of pressure assumes that the area of contact of the rope in the groove is on the full rope diameter, whereas in fact only the crowns of the outer wires are actually in contact with the groove. It is estimated that the local pressures at these contact points may be as high as five times those calculated.

If the pressure is high, the compressive strength of the material in the groove may be insufficient to prevent excessive wear and indentation and this in turn will damage the outer wires of the rope and effect its working life. As with bending stresses, stresses due to radial pressure increase as the diameter of the sheave decreases.

Although high bending stresses generally call for the use of flexible rope constructions having relatively small diameter outer wires, these have less ability to withstand heavy pressures than do the larger wires in the less flexible constructions. If the calculated pressures are too high for the particular material chosen for the sheaves or drums or indentations are being experienced, consideration should be given to an increase in sheave or drum diameter. Such a modification would not only reduce the groove pressure, but would also improve the fatigue life of the rope.

The pressure of the rope against the sheave also causes distortion and flattening of the rope structure. This can be controlled by using sheaves with the correct groove profile which, for general purposes, suggests a recommended groove diameter of nominal rope diameter +6%. The profile at the bottom of the groove should be circular over an angle of approximately 120o, and the angle of flare between the sides of the sheave should be approximately 52o.

Bend fatigue testing of ropes usually consists of cycling a length of rope over a sheave while the rope is under a constant tension. As part of its ongoing development program BRIDON has tested literally thousands of ropes in this manner over the years on its in-house own design bend testing equipment.

Through this work, BRIDON has been able to compare the effects of rope construction, tensile strength, lay direction, sheave size, groove profile and tensile loading on bend fatigue performance under ideal operating conditions. At the same time it has been possible to compare rope life to discard criteria (e.g. as laid down in ISO 4309) with that to complete failure of the rope, i.e. to the point where the rope has been unable to sustain the load any longer. As part of the exercise, it has also been possible to establish the residual breaking strength of the rope at discard level of deterioration.

What needs to be recognized, however, is that very few ropes operate under these controlled operating conditions, making it very difficult to use this base information when attempting to predict rope life under other conditions. Other influencing factors, such as dynamic loading, differential loads in the cycle, fleet angle, reeving arrangement, type of spooling on the drum, change in rope direction, sheave alignment, sheave size and groove profile, can have an equally dramatic effect on rope performance.

However, the benefit of such testing can be particularly helpful to the rope manufacturer when developing new or improving existing products. If designers or operators of equipment are seeking optimum rope performance or regard bending fatigue life as a key factor in the operation of equipment, such information can be provided by BRIDON for guidance purposes.

Under certain circumstances it may be necessary to use a swivel in a lifting system to prevent rotation of the load. This is typically done for employee safety considerations. It is possible however, that the use of a swivel will have an adverse affect on rope performance and may in some cases damage the wire rope.

There are many types of accessories available that incorporate different types and degrees of rotation preventing swivels. The swivel may be either an independent accessory or an integral part of a lifting device such as a crane block with a swivel hook. A typical independent accessory is a ball bearing anti- friction swivel. There are also headache balls with swivel hooks. The type of swivel that causes the most concern from the standpoint of the wire rope is the independent anti-friction swivel that attaches directly to the rope. The purpose of using a swivel in a lifting system is to prevent rotation of the load. This then allows the wire rope to rotate. Excessive rope rotation can damage a wire rope.

To assist in determining whether or not a swivel should be used in the lifting system, the following recommendations should be considered. It must also be recognized that the rotation characteristics of different types and constructions of wire rope vary considerably. The following types and constructions of wire rope are grouped according to their rotation characteristics.

Wire rope constructions having very high rotation characteristics should not be used with a swivel under any circumstances. These rope constructions will rotate excessively with one end free to rotate and the rope will unlay and distort and be easily damaged with a loss of rope breaking force.

Wire rope constructions having high rotation characteristics when used in single part reeving may require a swivel in the system to prevent rotation in certain operating conditions. However, this should be done only when employee safety is the issue.

These rope constructions when used in a reeving system with one end free to rotate will have a high level of rotation. This will cause the rope to unlay to some degree and distortion of the rope will occur.

The ropes in this Group are designed with an inner rope that is laid in the opposite direction to the outer strands to provide a medium resistance to rotation. Ropes with medium rotation characteristics are used with a swivel in single part reeving applications. However, a swivel is not recommended for multiple part hoisting applications or in any application where the swivel is not necessary for safety reasons. If it is necessary to use a swivel the rope must be operating within the design factor of 5, must not be shock loaded and must be inspected daily by a qualified person for distortion.

It should be noted that if a swivel is used in conjunction with Group 3a ropes, rope service life might be reduced due to increased internal wear between the outer strands and the inner rope.

Wire ropes having low rotation characteristics used in either single or multiple part reeving may be used with a swivel. The reason for this is that the ropes will exhibit very little, if any, rotation when used at the proper design factor. Application parameters such as a fleet angle may induce turn into a wire rope that can be relieved by the use of a swivel. However, if the application does not induce any turn into the rope or if a swivel is not beneficial to the performance of the rope the swivel may not be necessary.

Of all the factors which have some influence on the winding of a rope on a smooth drum, the fleet angle, arguably, has the greatest effect. Fleet angle is usually defined as the included angle between two lines, one which extends from a fixed sheave to the flange of a drum and the other which extends from the same fixed sheave to the drum in a line perpendicular to the axis of the drum. (See illustration).

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

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

If the rope is allowed to pile up it will eventually roll away from the flange creating a shock load in both the rope and the structure of the mechanism, an undesirable and unsafe operating condition. Excessively high fleet angles will return the rope across the drum prematurely, creating gaps between wraps of rope close to the flanges as well as increasing the pressure on the rope at the cross-over positions.

Even where helical grooving is provided, large fleet angles will inevitably result in localized areas of mechanical damage as the wires ‘pluck’ against each other. This is often referred to as ‘interference’ but the amount can be reduced by selecting a Langs lay rope if the reeving allows.

The “interference” effect can also be reduced by employing a Dyform rope which offers a much smoother exterior surface than conventional rope constructions. Floating sheaves or specially designed fleet angle compensating devices may also be employed to reduce the fleet angle effect.

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

To reduce the amount of twist to an acceptable level the fleet angle should be limited to 2.5O for grooved drums and 1.5O for plain drums and when using Rotation Resistant, ropes the fleet angle should be limited to 1.5O. However, for some crane and hoist applications it is recognized that for practical reasons it is not always possible to comply with these general recommendations, in which case the rope life could be affected.

The problem of torsional instability in crane hoist ropes would not exist if the ropes could be perfectly torque balanced under load. The torque generated in a wire rope under load is usually directly related to the applied load by a constant ‘torque factor’. For a given rope construction the torque factor can be expressed as a proportion of the rope diameter and this has been done below.

Variation with rope construction is relatively small and hence the scope for dramatically changing the stability of a hoisting system is limited. Nevertheless the choice of the correct rope can have a deciding influence, especially in systems which are operating close to the critical limit. It should be noted that the rope torque referred to here is purely that due to tensile loading. No account is taken of the possible residual torque due, for example, to rope manufacture or installation procedures.

Torsional Stability and the Cabling Graph (see page 78) are two methods which can be used to determine torsional stability or the tendency of the rope to cable. The torque factors quoted on page 79 are approximate maximum values for the particular constructions. To calculate the torque value for a particular rope size multiply by the nominal rope diameter. Example: for 20mm dia. Dyform 34LR at 20% of minimum breaking force:

The torsional characteristics of wire rope will have the effect of causing angular displacement of a sheave block when used in multi-fall reeving arrangements. The formula below gives a good approximation under such arrangements.

The preceding equations are all relative to a simple two part reeving. For more complex systems a similar approach may be used if account is taken of the different spacing of the ropes.

Even Number of Falls Note: For hoisting arrangements in which the rope falls are not parallel an average rope spacing should be used. Uneven Number of Falls

The equations assume that rope is torque-free in the noload condition, therefore, induced torque during or immediately after installation will adversely influence the calculated effect.

The above data assumes a constant torque value which is a valid assumption for a new rope. Wear and usage can have a significant effect on the torque value but practical work shows that under such circumstances the torque value will diminish, thus improving the stability of the arrangement.

Assuming a pedestal crane working on two falls is roped with 20mm diameter DYFORM 34LR and the bottom block carries a sheave of 360mm diameter with the falls parallel:

If the rope is new (worst condition) and no account is taken of block weight and friction then angular displacement for a height of lift of 30 meters is given by sin è = (4 000 . 30 . 0.152)

In the search for the right steel wire rope, you will soon come across the terms: breaking load, usage rate and workload. Find out what these terms mean and take advantage of our tips.

The breaking load is the maximum load before the steel wire rope breaks. We make a distinction between the “calculated breaking load” and “minimum breaking load”.

This is the theoretical breaking load of the component wires in the steel wire rope. For the calculated breaking load, the material cross section is considered in combination with the average tensile strength of the material.

The minimum or actual breaking load is always lower than the calculated breaking load. This is due to the helical positioning of the wires (double helix): the so-called spinning loss. The difference between the calculated and actual breaking load is determined by the wire construction and the lay length of the strands and wire rope.

The ratio between the actual breaking load and the workload is called the usage rate. With a usage rate of 10, the maximum load is one tenth of the minimum breaking load of the wire rope. With an actual breaking load of 50 tons, the wire rope is allowed to pull 5 tons when used.

The usage rate depends on the rope’s intended purpose. For hoisting purposes, the usage rate for conventional wire ropes is a 5 and a 6 for rotation-resistant ropes.

The assumption is often: the higher the breaking load, the better the wire rope. But this doesn’t have to be the case. For example, a high breaking load says nothing about destruction or wear caused by external influences. The course of the steel wire rope and the construction in which the wire rope is applied can strongly influence the life span of a steel wire rope. Frits van Boetzelaer, a steel wire rope expert, discusses this topic in depth.

The Minimum Breaking Strain is also known as the Maximum Working Load and is the stated minimum ‘strength’ of the wire rope. This load represents a mass or force that is required to make the wire rope fail or yield, also known as the Minimum Breaking Load (MBL). SWL or WLL are calculated by dividing the MBL by a safety factor (SF).

Low Cost Wire recommends a 10x SF if the lifting element (wire in this case) holds an object above ‘people’ and a 5x SF if the object is above ‘things’ and could never injure a person in the event of a catastrophic failure. The table below presents the WWL of the wire with a 10x SF when the suspended weight is held by wire in a Vertical Orientation or in a Horizontal Orientation.

If load specification is critical, Low Cost Wire recommend that you confirm all relevant details before ordering as information may change without prior notice. Diameter Structure Grade MBS* (kN) MBS (kg)

As a wire rope is used, the outer wires wear through abrasion and so the rope suffers loss of cross-sectional area – this obviously reduces the breaking strength of the wire rope. Resistance to abrasive wear is therefore an important property of a wire rope.

Abrasion resistance is directly related to the design of the rope, in particular the design of the strands of the rope. In general, ropes with fewer larger wires will be more abrasion resistant than a similar rope made up of smaller wires – a 6 x 19 rope will therefore be more abrasion resistant than a 6 x 36 rope.

In a drilling operation, wire ropes run through sheaves constantly and so are constantly subjected to alternating tensile and compressive loads – i.e. cyclic stress reversals.

Figure 1 illustrates a wire rope bending over a sheave. It is clear that the outer parts of the rope running over the sheave are in tension and the inner parts of the rope running over the sheave are in compression and as the rope moves over the sheave these stresses reverse.

It should be obvious that the smaller the diameter of the sheave the greater the magnitude (amplitude) of the stress reversal and so the more rapidly fatigue will occur in the rope.

Wire ropes experience external forces that will tend to alter or distort the shape of the rope. Crushing prevents wires and strands moving easily over one another during normal operation and this can lead to accelerated wear and reduced rope life.

Actual operating loads may vary. NEVER exceed the recommended design factor of 20% of catalogue strength. Wire Rope must have the strength required to handle the maximum load plus a design factor. The design factor is the ratio of the breaking strength of the rope to the maximum working load.