While most of us are aware of the basics of wire ropes, there’s more to them than meets the eye. We may know about some of the basic wire rope components as well as construction. However, it is crucial to know about the lesser-known factors that give these ropes their unmatched strength.
It’s no secret that wire ropes are considered some of the most useful and commonly-used tools that mobilize safe and secure movement of heavy objects or loads. In order to choose the most suitable wire rope and understand how it will react under specific conditions, you need to have thorough knowledge of its composition.
Further, to meet the varied industry requirements, wire ropes are manufactured in various configurations, which allow them to withstand crushing, increase their flexibility, reduce the chances of rotation, and so on.
In other words, you need to choose the right wire rope for each project to ensure a job done right. Failure to do so is sure to lead to damage and safety hazards.
A clear understanding of the structure, characteristics, and applications of wire rope will help you use it with higher efficacy and safety. This post will offer some insight into these crucial aspects of wire ropes.
A. Structure of a Wire Rope
A wire rope has several moving parts attached to it. All wire rope components work in tandem to support and move an object or load. The wire rope is attached to a crane or hoist and fitted with swivels, shackles, or hooks.
The efficacy and load-bearing abilities of wire ropes make it the best choice for lifting and lowering elevators or as a means of support for suspension bridges or towers.
Understanding wire rope construction can be a bit intriguing, but it’s essential if you want to have a detailed insight into what is wire rope. There are four basic components that make up a wire rope.
- Multi-wire strands
- A fiber or steel core
Let’s understand its structure.
Wires are the smallest parts of a wire rope. They form the individual strands and play an integral role in determining the wire rope capacity to lift loads. The wires are manufactured using various materials, such as steel, iron, stainless steel, Monel, and bronze. They are also made in various grades that signify their strength, curvature, and resistance to wear, fatigue, and corrosion.
The rope comprises multiple strands of wire arranged in a helical pattern surrounding the core. The steel wire’s structure provides it with strength, flexibility, and the ability to handle bending stresses.
The different wire rope components provide various benefits in specific lifting applications. This includes strength, flexibility, and resistance to abrasion, crushing, fatigue, corrosion, and rotation.
Wire rope strands comprise two or more wires twisted in a particular arrangement. The individual strands are then laid helically, surrounding the core of the rope. The strands with a large diameter are more effective in countering abrasion, while those with a smaller diameter have higher flexibility.
The core runs through the center of the rope and supports the strands to maintain their relative position when it undergoes loading and bending stress. The core is manufactured from various materials, including natural or synthetic fibers and steel.
Lubrication is applied to the core during every step of the manufacturing process to ensure maximum potency of the rope. The two significant benefits of lubricating wire ropes are:
- Reduced friction as individual wires and strands slide over each other
- Better protection (of the core, and interior as well as exterior surface of the wires) from corrosion
The space between the wires and strands are appropriately balanced to facilitate proper bearing clearance. This ensures seamless internal movement and adjustment of wires and strands even if the rope is bent under stress.
B. Important Characteristics of Wire Ropes
Every type of wire rope is uniquely designed to meet operational requirements of the lifting industry. However, there are a few essential characteristics that you should be aware of if you are to make smart choices when purchasing it.
The strength of a wire rope is calculated in tons and is marked 2-1/2% lower than the minimum breaking force. The minimum breaking force is the measure above which a new rope will break when placed under tension.
2. Reserve Strength
The reserve strength refers to the relationship between the strength represented by all the wires in the outer strands and the wires remaining in the outer strands without the outer layers. The reserve strength is calculated using the metallic areas of the individual wires. Since metallic area and strength are interrelated, the reserve strength is expressed as a certain percentage of the rope’s minimum breaking force.
3. Resistance Offered Loss of Metal and Deformation
Metal loss refers to the gradual wearing of metals from the outer wires of a rope. On the other hand, deformation is the changing of the shape of the outer wires of a rope. The resistance to metal loss by abrasion is the ability of the rope to withstand the loss of metal along its exterior.
4. Crushing Resistance
Crushing denotes the extent to which a rope is affected due to external pressure. It takes into account the damage on the cross-section shape of the rope, its strands, or the core due to the force. Crushing resistance is, therefore, its ability to withstand or resist the effect of external forces.
5. Fatigue Resistance
The term ‘fatigue resistance’ refers to the ability of wire ropes to withstand gradual metal fatigue. A wire having high fatigue resistance also has high bendability under stress. The fatigue resistance of a wire can be enhanced by using large quantities of high-quality wires.
Bendability of a wire refers to how easily a wire can bend without causing any additional stress. The diameter of the wires, rope and strand construction, the metal used, and the type of core are the major factors that affect the bendability of wire ropes.
C. Applications of Wire Rope
Wire ropes have brought about a massive change in the lifting and rigging industry. They are now extensively used in:
- Underground mining
- Oil and gas extraction
- Electrical constructions
- Mooring lines
- Drill lines
- Hoist ropes
- Drag lines
- Boom lines
D. How to Choose an Appropriate Wire Rope?
Selecting a proper wire rope isn’t easy. Here are a few essential factors to take into consideration:
1. Resistance to Bending
Rope fatigue is the result of repeated bending of ropes around the sheaves and drums. The ropes constructed with small wires offer better resistance to fatigue than those that have long wires. So, always remember that wires with shorter wires lower the strength of a cable to resist abrasions.
2. Strength-Resistance to Breaking
A wire rope has to be strong enough to be able to lift the maximum designated load. Wire rope cables incapable of bearing high tensile force will break when lifting heavy loads.
3. Resistance to Crushing
Wire ropes encounter high crushing forces when in use. This flattens and distorts the rope leading to premature breaking. It is crucial to ensure that the ropes have sufficient lateral stability to resist the crushing pressures that they will undergo daily.
- How to Inspect a Wire Rope?
Wire rope inspection ensures that it won’t break due to the tension force when in use. Replacing any wire rope becomes a must if there are:
- Variations in the diameter
- Broken wires
- Deformed strands
- Changes in its usual appearance
As a precautionary measure, it’s always better to replace the wire ropes if there’s any doubt regarding their working efficacy. In your basic wire rope inspection, you should look for:
- Broken Wires
- External Wear
- Rope Lay
- Rope Diameter Reduction
- Internal Wear
Remember that some sections of a wire rope may break without warning. The damage that occurs near the end terminations and the parts where the ropes enter and leave sheaves (such as boom hoists, semi-operational systems, and suspension systems) can be difficult to notice. The core of the rope fails in these sections, eventually resulting in adjacent strands undergoing abrasion
The best way to ensure that a wire rope performs at its maximum efficiency is to inspect the wire rope immediately after receiving it from the manufacturer. Double-check the sling tag to ensure that the right product is delivered and the rated capacity is sufficient to meet your requirements.
Designating someone knowledgeable from your team to perform a daily visual inspection of slings and attachments also helps detect defects and deformities in time. Of course, the necessary parameters should be kept in mind when working with wire ropes.
It is important that the wire ropes undergo a thorough examination at least once a month. Any rope that has not been used for more than a month should be inspected in greater detail before reusing. After the inspection, care must be taken to record the date of inspection and the conclusions drawn. These records will help identify the efficacy of the existing system and the improvements needed.
F. Specification of Wire Ropes
The specifications of the wire ropes refer to the standard pattern determined by wire size, the wire strand, and the total number of strands. Here are the standard wire rope specifications:
1. Single Layer
A single layer consists of a single wire center surrounded by six wires having the same diameter.
2. Filler Wire
The core is surrounded by two layers of wire of different sizes. The number of wires in the inner layer is half that of the outer layer. The filler wires are placed in the valleys between the inner and outer layers.
It comprises two layers made of the same number of wires surrounding the core. The wires in the core and the outer layer have the same diameter, with smaller wires in the inner layer.
The core is surrounded by an inner layer with wires having the same diameter. The outer layer has two wire diameters: alternating small and large.
5. Wire Finishes
Wire finishes include uncoated or bright high-carbon steel, zinc-galvanized, zinc-aluminum coated, and stainless steel.
6. Wire Grades
The wire grades determine the strength of the wire rope. This includes the percentage of plow steel, improved plow steel, or extra improved plow steel.
7. Wire Lay
Wire lay refers to the directions of the strands in the rope. It determines the orientation of the rope running up and down. The wire lay offers necessary stability and crush resistance. On the other hand, the lang lay (with strands inclined at a specific angle) provides more resistance to fatigue and abrasion.
Wire cores are made of either steel or fiber. Steel cores comprise an independent rope or a strand. These are preferred for additional support or where the average temperature in the cores is higher than 180° F.
G. When Does a Wire Rope Break?
Though wire ropes are usually stable and reliable, they can break under certain circumstances. It’s necessary to conduct periodic inspections and keep an eye on unsafe wire ropes. Here are the most common reasons that may cause wire ropes to break:
- Wear in sections where hoist drums and sheave come in contact with the rope
- The lack of lubrication for extended periods
- Prolonged exposure to heat and moisture
- Repeated bending
- Neglecting manufacturers’ weight charts and the overloading system
- Crushing, dragging, or cutting rope because of mechanical abuse
- Spooling onto multi-layered drums
- Using frozen or stiff ropes
- Improper installation, leading to kinks
The above problems can be easily resolved through proper inspections and regular maintenance.
H. Classification of Wire Ropes According to Usage
Wire ropes serve a myriad of purposes, and are classified accordingly.
- Running ropes (stranded ropes) are bent over sheaves and drums. The ropes are stressed mainly due to bending and tension.
- Stationary ropes, stay ropes are responsible for bearing tensile forces and primarily undergo static and varying tensile stresses.
- Track ropes act as rails for the rollers for loads in aerial ropeways and cable cranes. These ropes don’t take on the curvature of the rollers, unlike running ropes.
- Wire rope slings are used to lift a range of goods. These slings are stressed by the tensile forces and bending stresses when these bent over sharp edges of the loads it lifts.
Unarguably, wire ropes are complex machinery that play an integral role in the lifting industry. Judging by their utility and their importance in the lifting industry, it’s always best to buy the best-quality wire ropes. It is also essential to understand the type of load that is required to be lifted or moved as this is the key to buying the most appropriate wire rope.