The 7 Best Things about Wire Rope
Every wire rope has its own “personality” that comes from the way it was made. Each rope’s construction is based on a desired combination of operating characteristics that will best meet the performance requirements of the work or application for which that design is intended. Because of this, each rope’s construction is a design compromise.
The relationship between abrasion resistance and fatigue resistance is the best example of a design compromise, or the best way to get what you want.
Using many wires in the strands gives a rope the ability to bend over and over again when it is under stress. Abrasion resistance is mostly achieved with a rope design that uses fewer and, as a result, larger wires in the outer layer to reduce the effects of surface wear.
So, from a design point of view, if you do anything to change either abrasion resistance or fatigue resistance, you will change both of these properties.
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Strength
Most of the time, wire rope strength is measured in tons. In written materials, the minimum breaking force is used to show how strong a wire rope is. The term “minimum breaking force” refers to the calculated strength numbers that the wire rope industry has accepted.
When put under tension on a test device, a new rope should break at a number equal to or higher than the minimum breaking force shown for that rope.
When these kinds of tests are done to find out how strong a new wire rope is, a “acceptance” strength may be used to account for things that could go wrong. The minimum breaking force is 2-12% less than the acceptance strength, and ropes must meet or be stronger than this strength.
The minimum breaking force is for a rope that has never been used before. A rope should never be used at or close to the least force that can break it. During its useful life, a rope gradually loses strength because of things like surface wear and metal fatigue that happen naturally.
- Reserve Strength
The reserve strength of a standard rope is based on how the strength of all the wires in the outer strands compares to the strength of the wires left in the outer strands after the outer layer of wires is taken away. Reserve strength is calculated by taking into account the actual metal areas of each wire. Since the amount of metal and its strength are directly related, reserve strength is usually given as a percentage of the minimum breaking force of the rope. Reserve strength is used to compare how well different rope constructions can support loads on their internal wires.
Reserve strength is an important thing to think about when choosing, inspecting, and judging a rope for use in situations where it would be very bad if it broke. The idea behind using reserve strength is that the outer wires of the strands are the first to get damaged or worn down. So, the reserve strength numbers don’t mean as much when the rope has internal wear, damage, abuse, corrosion, or a change in shape.
The reserve strength of a stranded rope will be higher the more wires there are in the outer layer. Since there are more wires in the outer layer of a strand, the wires in the outer layer must be smaller in diameter. This means that there is more metal space left for the inner wires to fill. Standard fiber core and independent wire rope core (IWRC) ropes are shown in their own columns. For ropes with a fiber core, the reserve strength is about how much of the metal area of the rope is made up by the inner wires of the outer strands.
- Can’t break or bend when metal loses shape
Metal loss is when metal wears away from the outside wires of a rope, and metal deformation is when the outside wires of a rope change shape.
In general, “abrasion resistance,” which is another name for “resistance to metal loss by abrasion,” means a rope’s ability to keep metal from being worn away from its outside. This makes a rope less strong.
The most common way to change the shape of metal is called “peening.” This is because the wires on the outside of a peened rope look like they were “hammered” along their exposed surface. Peening usually happens on drums because the ropes touch each other when the rope is put on the drum. It could also happen on the sheaves.
Peening wears down metal, which can lead to wire failure. The “hammering” causes the metal of the wire to flow into a new shape. This realigns the metal’s grain structure, which changes its resistance to wear and tear. The wire can’t move as well when the rope bends because the shape isn’t round.
- Resistance to Crushing
Crushing is what happens when pressure from the outside is put on a rope. This damages the rope by changing the shape of its cross section, its strands, or its core—or all three.
Crushing resistance, then, is the ability to resist or stand up to outside forces, and the term is usually used to compare ropes.
When a rope is damaged by being crushed, the wires, strands, and core can’t move and adjust like they should. In general, IWRC ropes are stronger than fiber core ropes when it comes to being crushed. Lang lay ropes aren’t as strong as regular lay ropes when it comes to being crushed, and 6-strand ropes are stronger than 8-strand ropes when it comes to being crushed.
- Fatigue Resistance
For a rope to be resistant to wear and tear, the wires that make it up must not rust. Wires must be able to bend repeatedly under stress, like when a rope goes over a sheave, in order to have a high resistance to fatigue.
Using a lot of wires in a rope design makes it more resistant to wear and tear. It has to do with both the basic science of metals and the sizes of wires.
In general, a rope with a lot of small wires will last longer than a rope of the same size with a few large wires. This is because smaller wires can bend more when the rope goes over sheaves or around drums. To avoid getting tired, ropes must never bend over sheaves or drums with such a small diameter that they kink wires or bend them too much. There are specific rules for how big the sheave and drum should be so that they can fit all sizes and types of ropes.
As a rope is used, it bends and twists, which causes the metal to wear out. As a result, the rope’s strength gradually decreases as it is used.
- Ability to bend
Bend-ability means how easy it is for a rope to curve into an arc. This ability is affected by four main things:
- The size of the wires in the rope
- Building with ropes and strands
- The type of metal and finish of the wires, such as galvanizing
- Type of core for the rope: fiber or IWRC
Some types of rope are naturally more flexible than others. Smaller ropes are easier to bend than larger ones. Fiber core ropes are easier to bend than IWRC ropes of the same size. As a general rule, ropes with many smaller wires are easier to bend than ropes with fewer larger wires but the same size.
- Stability
Most of the time, the word “stable” is used to describe how easy a rope is to handle and how well it works. It is not a precise term because the idea it describes is somewhat subjective and has more in common with a rope’s “personality” than with any other feature.
For example, a rope is stable if it goes on and off a drum without getting tangled or if it doesn’t tend to get tangled when a multi-part reeving system is let go.
Most of the stability comes from the way strands and ropes are made. Most of the time, preformed rope is more stable than regular lay rope, and lang lay rope is less stable than regular lay rope. Most of the time, a rope with simple 7-wire strands will be more stable than a rope with many wires per strand. There is no one way to measure how stable a rope is.