Which Wire Rope is best for your Project?

A number of different types of wire rope (also known as aircraft cables) are currently on the market, which might make it difficult for you to decide on the option that’s best for your construction project. Wire rope uses include its original applications in the aerospace industry to hoisting cranes. This has forced engineers to develop numerous sizes and styles, which in turn has helped to further over complicate things.

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Fortunately, you’ll be in a better position to make the right choice once you know a few pieces of insider information. Perhaps the most important thing to know before making a purchasing decision is how to rate the various types of wire rope by their classification.

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Comparing 1×19, 7×7 & 7×19 Wire Rope Designs

Wire rope specifications usually classify a piece of cable by the number of wires in each strand as well as the number of strands in the whole rope. This has lead to a system where cables are identified by a pair of numbers, such as 7×19. The first number, 7 in this case, represents the total number of strands in the rope. If you unravel a 7×19 cable, then you’d be able to see 19 wires in each individual strand.

While the second number does technically represent the number of wires in each of these strands, it might be more accurate to say that it identifies a class or range of values and isn’t an exact measurement.

Even though this might sound confusing, there isn’t much you’ll have to remember when shopping for wire rope. All cables of the same size, grade and core offers the similar breaking strength characteristics. They also exhibit a similar weight per each foot of cable.

Three of the more common classifications you’re likely to come into contact with are 1×19, 7×7 and 7×19. A 1×19 construction gives engineers the freedom to design a stiff cable that won’t flex or bend, so you might find it anywhere you have to run ropes in a straight line such as when putting up guy wire. And go here if you want to know more about OTHER structural classifications.

Construction crews that have to deal with rigging or want to tow and maneu

ver heavy objects often turn to 7×7 cables. Heavy applications might call for 7×19 rope, which can handle a healthy amount of force before it breaks. Overall, 7×7 is used when flexible pieces are required and 7×19 is employed when an even greater level of flexibility is preferred. If you have to make slight bends, then 7×7 ropes may be preferred. Mechanical assemblies that call for sheaves and pulleys work well with 7×19 pieces. And please note that 7×19 is NOT like 19×7… which is explained here.

Construction & Stiffness & Stretch

Since stiffness is such an important consideration when selecting a piece of wire rope, technicians have developed more than one way of measuring it. In most cases, you’ll see people talk about axial and bending stiffness. Stretch is a whole different world – and if that is relevant to your project, please find an engineer.

Axial stiffness measures the elastic deformation of a piece of wire rope under load. It’s normally expressed as a ratio of load to deflection. Since the relationship between these two things aren’t linear, most manufacturers are only ever able to apply guidelines values. As a rule, though, tightly wound wires with many strands won’t experience as much axial gyration as weaker ropes.

Bending stiffness is perhaps more self-explanatory. This metric tracks how likely a particular type of wire rope is to start to sag when put under a load. In most strands that feature multiple layers of wires, the inner layers will start to support the outer layers once a load gets applied. This allows all of the wires to slide and adjust freely to provide additional support against these bending forces.

There’s a general rule of thumb that you might want to follow when trying to decide between different types of wire rope. Those that feature strands made up of a few large wires tend to be more resistant to abrasion but less resistant to fatigue. Pieces of wire rope that use strands made up of many smaller wires tend to suffer more from abrasion but stand up well against fatigue.

Wire Rope Cable Applications

Considering that different applications require different types of cable, you’ll want to think carefully about your company’s particular use case. While fly-by-wire technology has allowed many pilots to use electronic controls, smaller aircraft often still rely on good strong wire ropes.

Nautical uses for wire rope include securing cargo to the decks of ships and securing boats to their moorings. Sailboat rigging and lines going to and from fishing boats are often made from wire rope as well. Most maritime companies prefer to go with wire rope that won’t corrode when exposed to salt water. And here both Galvanised Wire Rope Grades and Stainless Steel Wire Rope Grades are important considertions.

Even if you’re used to seeing wire rope around the job site, there are a number of applications you might not have thought of. For instance, a majority of live theaters feature backdrops that move. Galvanized cable fits the bill in these cases. Recreational zip lines are generally made from wire rope as well, because it’s sturdy enough to handle the elements.

Pay close attention the next time you’re working out in the gym, because you’ll see wire ropes there too. They’re often used to rig weight machines. Some companies have even developed novel designs that use strong coated wire ropes to provide resistance for workout warriors.

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Engineers are constantly looking for new solutions to problems, so you can expect to see additional wire rope uses in the future. Depending on how you intend to use them, though, you might need wire ropes made from a specific type of material.

Galvanized vs. Stainless Steel Wire Rope

Many wire rope applications expose the metal to harsh conditions. Galvanized steel wire rope features a layer of compressed zinc to help reduce the risk of corrosion. While they’re still not suitable for marine use, they should stand up to quite a bit of abuse. Choosing a higer grade increases both strength and reistance to corrosion – in fact G is considered Marine Grade Galvanised Wire Rope

If you plan to install cable anywhere that it could be exposed to salt water spray or other forms of moisture, then you’ll want to go with stainless steel wire ropes, such as these. While they cost more, they’re made of genuine 304  or 316 steel. This helps them stand up against the corrosive influence of seawater.

Nylon, Vinyl & Bare Wire Ropes

Nylon coated ropes can hold up well in high-friction applications that generate a fair amount of heat. Since the exterior coating protects the inner cable from fraying, these designs are perfect for conveyor belts and push-push control actuators. Smaller diameter nylon ropes are often used on exercise machines for this reason.

Vinyl (PVC) coatings are often applied in the form of a poly vinyl chloride sheath. PVC-coated cables are more resilient to the kind of damage done by sunlight and water. Construction sites and mines often opt for bright vinyl coatings to increase visibility for safety reasons – like our Red Coated Galvanised Wire Rope.

Some wire rope applications don’t require anything special on the exterior. Cost-conscious technicians often specify bare aircraft cable whenever it’s safe to do so in order to save a little cash. You can find a nice selection of quality coated and uncoated wire rope  here.

Strengths, Loads & Capacities of Wire Rope

You’ll likely see two different ways to measure the breaking strength of wire rope. Minimum breaking strength (MBS) refers to the smallest load that will pull a piece of wire rope apart. Aggregate strength refers to the collective breaking strength of all of the wires in a single cable when a manufacturer tests them individually.

An overwhelming majority of suppliers define tensile failures a bit differently than users might. When suppliers run wire rope capacity tests, they consider the slightest problem to be a failure and thus will rate the rope for whatever value they found caused the issue. On a work site, technicians might not normally consider a small problem to constitute failure.

That being said, don’t test your luck and don’t overload your cables. Hardware vendors are cautious for a reason. You must always stay within the working load limit. It’s easy to calculate wire rope capacity, so you’ll never be without guidelines.

Safe Working Load is related to the Working Load Limit (WLL) This should be determined by an Engineer based on the risk profile of the application – however if in doubt, then divide the tensile strength by 5 (or more). This is the design safety factor (typically 5, but determined the engineer designing the use of the wire rope).

The number you get is the maximum working load or Working Load Limit WLL.  This is the load that you can apply without risking tensile failure or metal fatigue. This value is often expressed in kgf (Kilograms Force) or kN (kilo Newtons).

While wire rope is noted for its high tensile strength, you don’t want to continuously load a piece close to capacity. As a general rule, you shouldn’t ever exceed the working load limit (WLL) you have calculated for your wire on your project. Regardless, over time this will eventually wear it out even if you’re not exceeding any of the manufacturer’s printed limitations. A simple wire rope strength chart can help you keep a close eye on wire rope strengths if find this to be an issue:

All ropes of the same size, grade and core offer somewhat similar minimum breaking force characteristics and weight per foot, though they do differ depending on the construction type and materials used. That makes a wire rope guide useful even if you only know the diameter of the aircraft cable you’re working with.

Consulting a Wire Rope Guide

Finding the maximum /achievable safe load for  slings of all sorts in certain esoteric applications or configurations is extremely important, which is why you’ll find specific guides made just for this reason. Riggers use them all the time and its a fast way to access ‘answers’ that would otherwise require exercising your High School Triginometry. Consider the following wire rope capacity chart if you find yourself dealing with any installation that’s perhaps a little unusual:

Choosing the Right Wire Cable for Your Job Site

Wire rope is a complex piece of device. Few people ever stop to consider how each piece of wire rope is a machine unto itself. Remember to think about how strong and flexible you need your new cable to be and consider whether it’ll get exposed to harsh conditions or have to weather the elements. You’ll be rewarded by your research with a piece of cable that works as hard as you do.

Cable 101

Wire rope and cable are each considered a “machine”. The configuration and method of manufacture combined with the proper selection of material when designed for a specific purpose enables a wire rope or cable to transmit forces, motion and energy in some predetermined manner and to some desired end. The term cable is often used interchangeably with wire rope. However, in general, wire rope refers to diameters larger than 3/8 inch. Sizes smaller than this are designated as cable or cords. Two or more wires concentrically laid around a center wire is called a strand. It may consist of one or more layers. Typically, the number of wires in a strand is 7, 19 or 37. A group of strands laid around a core would be called a cable or wire rope. In terms of product designation, 7 strands with 19 wires in each strand would be a 7×19 cable: 7 strands with 7 wires in each strand would be a 7×7 cable.

Materials

Different applications for wire rope present varying demands for strength, abrasion and corrosion resistance. In order to meet these requirements, wire rope is produced in a number of different materials.

Stainless Steel

This is used where corrosion is a prime factor and the cost increase warrants its use. The 18% chromium, 8% nickel alloy known as type 302 is the most common grade accepted due to both corrosion resistance and high strength. Other types frequently used in wire rope are 304, 305, 316 and 321, each having its specific advantage over the other. Type 305 is used where non-magnetic properties are required, however, there is a slight loss of strength.

Galvanized Carbon Steel

This is used where strength is a prime factor and corrosion resistance is not great enough to require the use of stainless steel. The lower cost is usually a consideration in the selection of galvanized carbon steel. Wires used in these wire ropes are individually coated with a layer of zinc which offers a good measure of protection from corrosive elements.

Cable Construction

The greater the number of wires in a strand or cable of a given diameter, the more flexibility it has. A 1×7 or a 1×19 strand, having 7 and 19 wires respectively, is used principally as a fixed member, as a straight linkage, or where flexing is minimal.

Cables designed with 3×7, 7×7 and 7×19 construction provide for increasing degrees of flexibility but decreased abrasion resistance. These designs would be incorporated where continuous flexing is a requirement.ConstructionDescriptionBasic strand for all concentric cable, relatively stiff in larger diameters, offers the least stretch. Stiffest construction in small diameters.Smooth outside, fairly flexible, resists compressive forces, strongest construction in sizes above 3/32-inch diameter.Durable, higher flexibility and abrasion resistance. Good general purpose construction for strength and flexibility. Can be used over pulleys.The strongest and most flexible of cables with the greatest stretch. Recommended for use over pulleys.

Selecting Wire Rope

When selecting a wire rope to give the best service, there are four requirements which should be given consideration. A proper choice is made by correctly estimating the relative importance of these requirements and selecting a rope which has the qualities best suited to withstand the effects of continued use. The rope should possess:

1. Strength sufficient to take care of the maximum load that may be applied, with a proper safety factor.
2. Ability to withstand repeated bending without failure of the wire from fatigue.
3. Ability to withstand abrasive wear.
4. Ability to withstand distortion and crushing, otherwise known as abuse.

Strength

Wire rope in service is subjected to several kinds of stresses. The stresses most frequently encountered are direct tension, stress due to acceleration, stress due to sudden or shock loads, stress due to bending, and stress resulting from several forces acting at one time. For the most part, these stresses can be converted into terms of simple tension, and a rope of approximately the correct strength can be chosen. As the strength of a wire rope is determined by its, size, grade and construction, these three factors should be considered.

Safety Factors

The safety factor is the ratio of the strength of the rope to the working load. A wire rope with a strength of 10,000 pounds and a total working load of 2,000 pounds would be operating with a safety factor of five.

It is not possible to set safety factors for the various types of wire rope using equipment, as this factor can vary with conditions on individual units of equipment.

The proper safety factor depends not only on the loads applied, but also on the speed of operation, shock load applied, the type of fittings used for securing the rope ends, the acceleration and deceleration, the length of rope, the number, size and location of sheaves and drums, the factors causing abrasion and corrosion and the facilities for inspection.

Fatigue

Fatigue failure of the wires in a wire rope is the result of the propagation of small cracks under repeated applications of bending loads. It occurs when ropes operate over comparatively small sheaves or drums. The repeated bending of the individual wires, as the rope bends when passing over the sheaves or drums, and the straightening of the individual wires, as the rope leaves the sheaves or drums, causing fatigue. The effect of fatigue on wires is illustrated by bending a wire repeatedly back and forth until it breaks.

The best means of preventing early fatigue of wire ropes is to use sheaves and drums of adequate size. To increase the resistance to fatigue, a rope of more flexible construction should be used, as increased flexibility is secured through the use of smaller wires.

Abrasive Wear

The ability of a wire rope to withstand abrasion is determined by the size, the carbon and manganese content, the heat treatment of the outer wires and the construction of the rope. The larger outer wires of the less flexible constructions are better able to withstand abrasion than the finer outer wires of the more flexible ropes. The higher carbon and manganese content and the heat treatment used in producing wire for the stronger ropes, make the higher grade ropes better able to withstand abrasive wear than the lower grade ropes.

Effects of Bending

All wire ropes, except stationary ropes used as guys or supports, are subjected to bending around sheaves or drums. The service obtained from wire ropes is, to a large extent, dependent upon the proper choice and location of the sheaves and drums about which it operates.

A wire rope may be considered a machine in which the individual elements (wires and strands) slide upon each other when the rope is bent. Therefore, as a prerequisite to the satisfactory operation of wire rope over sheaves and drums, the rope must be properly lubricated.

With this in mind, the effects of bending may be classified as:

• Loss of strength due to bending.
• Fatigue effect of bending.

Loss of strength due to bending is caused by the inability of the individual strands and wires to adjust themselves to their changed position when the rope is bent. Tests made by the National Institute of Standards and Technology show that the rope strength decreases in a marked degree as the sheave diameter grows smaller with respect to the diameter of the rope. The loss of strength due to bending wire ropes over the sheaves found in common use will not exceed 6% and will usually be about 4%.

The bending of a wire rope is accompanied by readjustment in the positions of the strands and wires and results in actual bending of the wires. Repetitive flexing of the wires develops bending loads which, even though well within the elastic limit of the wires, set up points of stress concentration.

The fatigue effect of bending appears in the form of small cracks in the wires at these over-stressed foci. These cracks propagate under repeated stress cycles, until the remaining sound metal is inadequate to withstand the bending load. This results in broken wires showing no apparent contraction of cross section.

Experience has established the fact that from the service view-point, a very definite relationship exists between the size of the individual outer wires of a wire rope and the size of the sheave or drum about which it operates. Sheaves and drums smaller than 200 times the diameter of the outer wires will cause permanent set in a heavily loaded rope. Good practice requires the use of sheaves and drums with diameters 800 times the diameter of the outer wires in the rope for heavily loaded fast-moving ropes.

It is impossible to give a definite minimum size of sheave or drum about which a wire rope will operate with satisfactory results, because of the other factors affecting the useful life of the rope. If the loads are light or the speed slow, smaller sheaves and drums can be used without causing early fatigue of the wires than if the loads are heavy or the speed is fast. Reverse bends, where a rope is bent in one direction and then in the opposite direction, cause excessive fatigue and should be avoided whenever possible. When a reverse bend is necessary larger sheaves are required than would be the case if the rope were bent in one direction only.

Stretch of Wire Rope

The stretch of a wire rope under load is the result of two components: the structural stretch and the elastic stretch. Structural stretch of wire rope is caused by the lengthening of the rope lay, compression of the core and adjustment of the wires and strands to the load placed upon the wire rope. The elastic stretch is caused by elongation of the wires.

The structural stretch varies with the size of core, the lengths of lays and the construction of the rope. This stretch also varies with the loads imposed and the amount of bending to which the rope is subjected. For estimating this stretch the value of one-half percent, or .005 times the length of the rope under load, gives an approximate figure. If loads are light, one-quarter percent or . times the rope length may be used. With heavy loads, this stretch may approach one percent, or .01 times the rope length.

The elastic stretch of a wire rope is directly proportional to the load and the length of rope under load, and inversely proportional to the metallic area and modulus of elasticity. This applies only to loads that do not exceed the elastic limit of a wire rope. The elastic limit of stainless steel wire rope is approximately 60% of its breaking strength and for galvanized ropes it is approximately 50%.

This may be expressed as:

Preformed Wire Ropes

Preformed ropes differ from the standard, or non-preformed ropes, in that the individual wires in the strands and the strands in the rope are preformed, or pre-shaped to their proper shape before they are assembled in the finished rope.


The performing operation removes the natural tendency of the wires and strands to straighten, and causes them to retain their proper positions.


This, in turn, results in preformed wire ropes having the following characteristics:

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1. They can be cut without the seizing necessary to retain the rope structure of non-preformed ropes.
2. Broken rope ends do not untwist, as do the ends of the non-preformed ropes. This increases the salvage value of broken ropes.
3. They are substantially free from liveliness and twisting tendencies. This makes installation and handling easier and lessens the likelihood of damage to the rope from kinking or fouling. Preforming permits the more general use of Lang lay and wire core constructions.
4. Removal of internal stresses increase resistance to fatigue from bending. This results in increased service where ability to withstand bending is the important requirement. It also permits the use of ropes with larger outer wires, when increased wear resistance is desired.
5. Outer wires will wear thinner before breaking, and broken wire ends will not protrude from the rope to injure worker’s hands, to nick and distort adjacent wires, or to wear sheaves and drums. Because of the fact that broken wire ends do not porcupine, they are not as noticeable as they are in non-preformed ropes. This necessitates the use of greater care when inspecting worn preformed ropes, to determine their true condition.

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