Wire Rope Slings
Properties of Wire Rope Slings
Wire rope slings consist of separate wires that have been twisted into strands. These strands are then twisted together to produce a structurally sound rope. To give them more flexibility, wire rope slings sometimes have a fiber core. Wire rope slings are
- Strong and durable
- Able to withstand abrasion
- Pliable or able to bed without distortion
The strength of a wire rope sling and its other properties depend on its size, how it has been constructed, its grade, and the numbers of wires used in its creation.
Sling Inspection
Slings must be regularly and properly inspected. Even seemingly “minor” damage to a Wire Rope Sling can significantly reduce its capacity to hold or lift objects and increases the chance that the sling will fail during use. If you are not sure whether a sling is damaged, DO NOT USE IT.
Frequency of Inspection
A three-stage procedure is recommended to help ensure that web slings are inspected with appropriate frequency.
- Initial Inspection – Slings must be inspected by a designated person as soon as they are received. This ensures that the correct web sling has been received, is undamaged, and meets the requirements for its intended use.
- Frequent Inspection – The entire sling must be inspected before each shift or day in Normal service and before each use in Severe service applications.
- Periodic Inspection – Every sling must be inspected “periodically” by a qualified and designated person. The frequency of periodic inspections is based on the sling’s actual or expected frequency of use, severity of service conditions, and the nature of the work performed with the sling.
Inspection Guide
The following procedures are offered as a guide for conducting inspections:
- Place the sling in a position that enables the inspector to access and see every part of the sling.
- Clean off all dirt and grease with a wire brush or rags to reveal wires and fittings.
- Examine the entire length of the sling thoroughly, especially the parts showing the most wear.
- Pay special attention to fittings and end attachments and areas of the sling next to these fittings.
- Find the most worn or damaged section of the sling and carefully check it against removal criteria.
- Label or identify all slings you have inspected.
- Keep records of all inspections, including dates and conditions of slings.
- Immediately destroy all slings you have rejected.
- Store slings you want to reuse in a safe place away from damaging weather, heat and dirt.
Removal Criteria
A sling shall be removed from service if any defects such as the following are visible:
- Rated capacity tag – Missing or illegible sling identification tag.
- Broken wires – For single-part body slings and strand-laid grommets: 5 broken wires in one strand in one rope lay or 10 broken wires in all strands in one rope lay. For cable-laid grommets and multi-part slings: cable-laid grommet 20 per lay, less than 8-part braid 20 per braid, 8-part braid or more 40 per braid.
- Metal loss – Wear or scraping of one-third the original diameter of the outside individual wires.
- Distortion – Such as kinking, crushing, or bird-caging. Look closely for wires or strands that may have been pushed out of their original positions in the rope.
- Heat damage – Any metallic discoloration or loss of internal lubricant caused by heat exposure.
- Damaged end attachments – Cracked, bent, or broken fittings. Also, any evidence that eye splices have slipped or tucked strands have moved.
- Bent hooks – Any visible apparent bend or twist from the plane of the unbent hook. Throat opening: any distortion causing an increase in throat opening of 5% not to exceed 1/4 in. (66 mm) (or as recommended by the manufacturer).
- Metal corrosion – Severe corrosion of the rope or end attachments that has caused pitting or binding of wires. Light rusting doesn’t normally affect a sling’s strength.
Wire Rope Sling Disposal
Once the qualified person has determined a sling is no longer usable, he should tag it immediately, “Do Not Use.” The sling should then be destroyed as soon as possible by cutting the eye and fittings from the rope. This will prevent accidental reuse of the sling.
Sling Care and Maintenance
Wire Rope is lubricated during manufacturing so that the strands – as well as the individual wires in the strands – may move and adjust as the rope moves and bends. No wire rope can be lubricated sufficiently during manufacturing to last its entire life. That’s why it’s important to lubricate periodically through the life of the rope.
The surface of some ropes may be covered with dirt, rock dust or other material during their operation. This can prevent field-applied lubricants from properly penetrating into the rope, so it’s a good practice to clean these ropes before you lubricate them.
The lubricant you apply should be light-bodied enough to penetrate to the rope’s core. You can normally apply lubricant by dripping, spraying, or brushing it on the rope. Your rope’s service life will be directly proportional to the effectiveness of the method you use and the amount of lubricant that reaches the ropes working parts.
A proper lubricant must reduce friction, protect against corrosion and adhere to every wire. It should also be pliable and not crack or separate when cold – yet not drip when warm. Never apply heavy grease to the rope because it can trap excessive grit which can damage the rope. Nor should you apply used “engine oil” because it contains materials that can damage the wire rope.
Wire Rope Sling Storage and Handling
- Wire Rope slings should be kept in an area where exposure to water, extreme heat or corrosive fumes, liquids and sprays are non-existent.
- Slings should also be kept out of the way where they may get ran over by vehicles or kinked.
- Slings should never be left beneath loads or laying where they could be possibly damaged.
Wire Rope Sling Temperature
- Wire rope should be protected from extreme temperatures. Steel core (IWRC) slings should never be used at 400° F or more and never below -40°F.
- It is not always easy to spot when wire rope has been damaged by heat. The most visible signs are loss of lubrication, discoloration and fusing of the wires.
- If there is even the slightest suspicion that a sling was exposed to heat then it should be removed from service immediately.
Capacities and Hitches
The listed capacities of wire rope slings & assemblies are based on the industry standard of a 5 to 1 design factor. This is the method used to determine the working load limit (WLL) of a sling: minimum breaking strength of the wire rope (MBL) multiplied by the efficiency of the splice or end fittings divided by the design factor 5.
The factors listed below affect the capacity of a wire rope sling:
- Efficiency of the end termination or eye splice.
- Type of hitch being used when lifting the load.
- Diameter of the item being lifted where the sling is attached.
- Diameter of the hook or shackle where the sling attaches to the lifting device.
- Efficiency of the End Termination or Eye Splice
Common Hitches for Web Slings
Vertical Pull
Vertical Pull
A method of rigging in which the load is attached to one end of the synthetic web sling, usually being attached by means of a hook or shackle, and the other end of the sling is attached to the lifting device. A tag line should be used to prevent load rotation.
WARNING: Any single sling hitch must never be used to transport a load that is not balanced.
Basket Hitch
Basket Hitch
A method of rigging in which the synthetic web sling is passed around the load and both ends are attached overhead.
WARNING: Rated capacities are affected by the sling to load angle when used in multi-leg bridles or basket hitches. Sling angles of less than 30 degrees are not recommended.
Choker Hitch
Choker Hitch
| Angle of Choke Degree | Rated Capacity |
|---|---|
| Over 120 | 100% |
| 90–120 | 87% |
| 60–89 | 74% |
| 30–59 | 62% |
| 0–29 | 49% |
Increased Tension on Slings Used at Angles
Another important consideration is the sling-to-load angle, which is the angle formed between a horizontal line and the sling leg or body. This angle is very important and can have a dramatic effect on the rated capacity of the sling. When the sling-to-load angle decreases, the load on each leg increases. Sling angles less than 30 degrees are not recommended.
| Degrees from Horizontal | Tension Factor |
|---|---|
| 90 | 1.000 |
| 85 | 1.004 |
| 80 | 1.015 |
| 75 | 1.035 |
| 70 | 1.064 |
| 65 | 1.104 |
| 60 | 1.155 |
| 55 | 1.221 |
| 50 | 1.305 |
| 45 | 1.414 |
| 40 | 1.555 |
| 35 | 1.742 |
| 30 | 2.000 |
Efficiency of the End Termination or Eye Splice
Hand Spliced Eyes
| Rope Diameter | Efficiency |
|---|---|
| 1/4″ | 90% |
| 5/16″ | 89% |
| 3/8″ | 88% |
| 7/16″ | 87% |
| 1/2″ | 86% |
| 9/16″ | 85% |
| 5/8″ | 84% |
| 3/4″ | 82% |
| 7/8″ to 2-1/2″ | 80% |
Mechanical Spliced Eyes
| Rope Diameter (IWRC) | Efficiency |
|---|---|
| 1/4″ to 1″ | 95% |
| 1-1/8″ to 2″ | 92.5% |
| 2-1/4″ to 4-1/2″ | 90% |
Swage and Spelter Sockets
| Rope Diameter | Efficiency |
|---|---|
| 1/4″ to 4-1/2″ | 100% |
Efficiency of Various Wire Rope Sling Constructions
When a sling is rigged as a basket, the diameter of the bend where the sling contacts the load can affect the sling’s lifting capacity. How much the lifting capacity is affected can be calculated by dividing the diameter of the bend where the rope contacts the load (represented by “D”) by the diameter of the rope or the component rope diameter in a multipart sling (represented by “d”).
For example, if the diameter of the bend (“D”) is 10 and the component rope diameter (“d”) is 1/2, the D/d ratio is 10 ÷ 1/2 or 20.
When using D/d ratios that are smaller than those shown in the table, the rated capacity of the sling must be decreased.
| Sling Construction | Efficiency |
|---|---|
| Mechanically spliced, single-part slings | 25 times rope diameter |
| Hand-spliced, single-part slings | 15 times rope diameter |
| Braided multi-part slings of 3 parts | 10 times component rope diameter |
| Braided multi-part slings of 6 parts | 25 times component rope diameter |
| Braided multi-part slings of 8 parts | 25 times component rope diameter |
| Braided multi-part slings of 9 parts | 20 times component rope diameter |
| Mechanically spliced grommets | 5 times sling body diameter |
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