The sling’s the thing

Slings play a vital role on construction sites and manufacturing plant floors. Selecting the right sling for the task can be a confusing choice for job foremen or plant managers. Here’s some basic advice compiled from sling manufacturers and wire rope and rigging consultants on the types of material handling products available to help you help your customers make the right choice for their application.

Slings tend to fall into three groups: chain, wire rope and mesh, and fiber rope web. Each type has its own advantages and disadvantages. Factors to consider when choosing the best sling for the job include size, weight, shape, temperature and sensitivity of the material being moved, and the environmental conditions under which the sling will be used.

While it’s always best to check with the manufacturer before making a recommendation to a customer, here’s a brief guide for selecting the appropriate sling.

Chains
Alloy steel chains are popular because they’re strong and can adapt to the shape of the load. Take care, however, when using alloy chain slings because sudden shocks can damage them. This could lead to sling failure and possible injuries or damage to the load.

Chain slings are the best choice for lifting hot materials. They can be heated to temperatures of up to 1000 F; however, if chain slings are consistently exposed to service temperatures in excess of 600 F, operators must reduce the working load limits in accordance with the manufacturer's recommendations.

Always visually inspect chain slings prior to use. Pay close attention to any stretching, nicks, gouges, and wear in excess of the allowances made by the manufacturer. These signs indicate the sling may be unsafe and must be removed from service immediately. Operators should never use  a sling with cracked or deformed links or other components. Get rid of alloy steel if hooks are cracked, opened more than 15 percent of their normal narrowest opening, or twisted more than 10 degrees.

Wire Rope
A second type of sling is made of wire rope, which is composed of individual wires twisted to form strands. The strands are then twisted to form a wire rope. When wire rope has a fiber core, it is usually more flexible but less resistant to environmental damage.

There are four characteristics to consider when recommending a wire rope sling: strength, ability to bend without distortion, ability to withstand abrasive wear, and ability to withstand abuse.

1) Strength. The strength of a wire rope is a function of its size, grade and construction. It must be capable of accommodating the maximum load that will be applied. The maximum load limit is determined by an appropriate multiplier (the number by which the ultimate strength of a wire rope is divided to determine the working load limit).

A wire rope sling with a strength of 10,000 pounds and a total working load of 2,000 pounds has a design factor (multiplier) of 5. New wire rope slings have a design factor of 5. As a sling suffers from the rigors of continued service, however, both the design factor and the sling's ultimate strength are proportionately reduced. If a sling is loaded beyond its ultimate strength, it will fail. That is why it is crucial for your customers to regularly inspect older slings.

2) Fatigue. A wire rope must withstand repeated bending without the wires failing from fatigue. Fatigue failure results from small cracks developing under repeated bending. It occurs when ropes make small radius bends. The best way to prevent fatigue failure of wire rope slings is to use blocking or padding to increase the radius of the bend.

3) Abrasive Wear. Size, number of wires, and construction of the rope determines the ability of a wire rope to withstand abrasion. Smaller wires bend more readily and therefore offer greater flexibility but are less able to withstand abrasive wear.

4) Abuse. All other factors being equal, misuse or abuse of wire rope will cause a wire rope sling to become unsafe long before any other factor. Abusing a wire rope sling can cause serious structural damage to the wire rope, such as kinking or bird caging which reduces the strength of the wire rope. (In bird caging, the wire rope strands are forcibly untwisted and become spread outward.)

Like chain slings, it’s wise to visually inspect wire rope slings before use. Slings with excessive broken wires, severe corrosion, localized wear, damage to end-fittings (hooks, rings, links, or collars), or damage to the rope structure (kinks, bird caging, distortion) must be removed from service and discarded.

Wire rope slings are especially sensitive to misuse or abuse. Don’t use one if you spot severe corrosion; shiny worn spots on the outside; a loss of one-third in outer wire diameter; damage or displacement of hooks, rings, or other end fittings; distortion, kinking, or other damage to wire rope structure; or excessive broken wires. Also remove a sling from use if it’s exposed to temperatures of 200 F or higher or if there’s evidence of heat or wire rope structure damage.

Fiber Rope and Synthetic Web
Fiber rope and synthetic web slings are used primarily for temporary work, such as construction and painting jobs, and in marine operations. They are also the best choice for use on expensive loads, highly finished parts, fragile parts, and delicate equipment.

Fiber Rope
Fiber rope slings are preferred for some applications because they are pliant, grip loads well and don’t mar the surface of the load. They should be used only on light loads, however, and shouldn’t be used on objects with sharp edges that can cut the rope, or where the sling will be exposed to high temperatures, severe abrasion or acids.

The choice of rope type and size will depend upon the application, the weight to be lifted and the sling angle. Before lifting any load with a fiber rope sling, be sure to inspect the sling carefully because they deteriorate far more rapidly than wire rope slings and their actual strength is very difficult to estimate.

When inspecting a fiber rope sling prior to using it, look first at its surface. Look for dry, brittle, scorched, or discolored fibers. If any of these conditions are found, notify the job supervisor to determine the safety of the sling. If the sling is found to be unsafe, it must be discarded.

Next, check the interior of the sling. It should be as clean as when the rope was new. A build-up of powder-like sawdust on the inside of the fiber rope indicates excessive internal wear and is an indication that the sling is unsafe.

Finally, scratch the fibers with a fingernail. If the fibers come apart easily, the fiber sling has suffered some kind of chemical damage and must be discarded.

Synthetic Web Slings
Synthetic web slings offer a number of advantages for rigging purposes. The most commonly used synthetic web slings are made of nylon, dacron and polyester. They have the following properties in common:
• Strength – can handle load of up to 300,000 lbs.
• Convenience – can conform to any shape.
• Safety – will adjust to the load contour and hold it with a tight, non-slip grip.
• Load protection – will not mar, deface, or scratch highly polished or delicate surfaces.
• Long life – are unaffected by mildew, rot or bacteria; resist some chemical action; and have excellent abrasion resistance.
• Economy – have low initial cost plus long service life.
• Shock absorbency – can absorb heavy shocks without damage.
• Temperature resistance – are unaffected by temperatures up to 180 F.

Each synthetic material has its own unique properties. Nylon must be used wherever alkaline or greasy conditions exist. It is also preferable when neutral conditions prevail and when resistance to chemicals and solvents is important. Dacron must be used where high concentrations of acid solutions — such as sulfuric, hydrochloric, nitric, and formic acids — and where high-temperature bleach solutions are prevalent. (Nylon will deteriorate under these conditions.) Do not use dacron in alkaline conditions because it will deteriorate; use nylon or polypropylene instead. Polyester must be used where acids or bleaching agents are present and is also ideal for applications where a minimum of stretching is important.

When you inspect natural and synthetic fiber rope slings, look for abnormal wear. Dry, brittle, scorched or discolored fibers could be signs of trouble. So can powdered fiber between inside strands, broken or cut fibers, variations in strand size or roundness, or distortion of hardware.

OSHA states firmly that you may not use a repaired or reconditioned fiber rope sling. Don’t use a synthetic web sling that’s damaged by acid or caustic burns. Other fatal problems include melting or charring, snags, punctures, tears or cuts, broken or worn stitches, or distorted fittings.

Your customers should be aware of a number of good practices to follow to protect them while using slings to move materials. First, learn as much as you can about the materials with which you will be working. Slings come in many different types, one of which is right for your purpose. Second, analyze the load to be moved — in terms of size, weight, shape, temperature, and sensitivity — then choose the sling which best meets those needs. Third, always inspect all the equipment before and after a move. Always be sure to give equipment whatever “in service” maintenance it may need. Fourth, use safe lifting practices. Use the proper lifting technique for the type of sling and the type of load.

This article appeared in the January/February 2005 issue of Progressive Distributor. Copyright 2005.

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