Personnel safety is priority one when it comes to lifting and moving heavy equipment and parts. OSHA reports that, "slings are the most commonly used materials handling apparatus," and "the mishandling of materials is the single largest cause of accidents and injuries in the workplace." [1] Whenever heavy lifting takes place, precautions must be taken to avoid the risk of accidents. Nylon, chain and wire rope slings are the most common slings, and a basic understanding of their qualities, techniques and common uses will help prevent accidents or injuries.
Nylon Sling Qualities
Nylon slings provide great strength combined with elasticity. Under a heavy load, nylon slings will absorb shock and return to their original shape after release. This combination of stretch and strength is beneficial for repeated lifting situations and tie-downs.
Should your job involve moisture or chemical exposure (alkalis not acids), nylon will not rot or deteriorate, even under repeated contact or long-term storage. Being impervious to UV light, nylon slings are safely used indoors or outdoors, in rain or shine and can withstand temperatures up to 180°F. [2]
Other benefits of nylon slings are their light weight and smooth surface. Rigging situations themselves may involve heavy and cumbersome preparation. The light weight of nylon slings make adjustments, transportation and storage of rigging equipment a simple. Certain loads may be scratched or damaged by chain or wire slings, but nylon slings have no undesired effects on the load itself. The flexibility of nylon gives these slings many advantages over chains or wires which are rigid and may kink when bent at sharp angles.
Extra features are built into the nylon slings themselves for enhanced safety and performance. As the nylon sling wears over time, red indicator yarns at the core of the sling will warn the user of any cuts or damage. Chemical compounds are also infused into these slings to seal out dirt and foreign particles enhancing the sling's grip. Any signs of scorching, cuts or wear should caution the user of potential dangers.
Chain Slings
Chain slings are made of an alloy steel and provide a powerful lifting option. Unlike nylon slings, chain slings have no shock absorbancy and can snap. These slings should be inspected for nicks, gouges, stretching or other signs of wear before being used. [3] Different metal endings can be fixed to chain slings allowing for a variety of hitching options. Because of the rough metallic surface, chain slings should not be used for basket or choker hitches with a delicate or aesthetic load. Any situation which might cause abrasion or crush the sling itself should be prevented as this may compromise the integrity of the sling.
The properties in a chain sling allow it to lift loads of 1000° F or repeatedly lift loads of 600° F according to manufacturers specifications. [4] Chain slings are also impervious to any UV damage making it safe to consistently use in the outdoors.
Wire Rope Slings
Wire rope slings are made from twisting wire fibers over a core of fiber (flexible but susceptible to damage) or wire (less flexible but resistant to damage). Because of the intricate nature of these slings, they are susceptible to fatigue from repeated bending, abrasion or abuse. Any fraying, cuts, or unwinding should be recognized as signs of this fatigue. Wire rope slings should not be exposed to moist working conditions as they may rust and corrode. During storage these slings should be kept in ventilated areas and off of the floor. Wire rope slings are lubricated when manufactured but should also be lubricated at the worksite. The frequency and thoroughness of lubrication varies and depends upon the weight of the load and bending imposed on the sling. OSHA advises that less flexible ropes, such as wire rope, should be used when a load needs to be "moved through or over abrasive materials." [5]
Sling Hitches
Heavy lifting situations rely primarily upon the quality of the sling material itself. Even with a superior nylon sling, poor sling techniques can cause damage to the loads or cause accidents. Because of the various demands of rigging situations, different hitching and lifting techniques have been developed.
The vertical hitch is the simplest lifting technique. What tow-lines are to pulling, vertical hitches are to lifting. Vertical hitches may use any of the endless, eye and eye, or triangle endings. One end of the line is attached to the load via hook or link and the other end is attached to the lifting rig (e.g. crane). In this simple rigging, the one vertical line bears the entire weight of the load making the strength of the line an important factor.
Bridal hitches are formed with two to four separate lines. While it should never be assumed, this option allows for even distribution of weight and strength with careful analysis and application. The angle of the sling in respect to its load becomes important with any non-vertical rig. Because "the load stress on each leg increases as the angle
decreases," [6] rigging lines should rarely dive below 30°. At this angle each line will bear the full weight of the load itself. Any non-vertical hitch should at least maintain a 45° angle.
Basket hitches are a single continuous sling which is wrapped around a load. This option requires attention to the angles of the line. As with other hitches, sling strength is the greatest when at a 90° angle and has decreasing strength with a decreasing angle. To provide greater stability, two slings can be used to form a double basket hitch. Double basket hitches should be kept at angles greater than 60° to prevent slippage. When security of the load is needed, the sling(s) can be wrapped around the object(s) twice.
A choker hitch is made by wrapping the sling around the load and re-attaching the end of the sling to the line itself. This hitch allows for multiple slings for stability and/or double wrapping for security. This hitch is commonly applied with cranes and allows for a single weight-bearing line. As with other hitches, twist in the line can cause the load to rotate. Nylon slings with certain endings can prevent this potential hazard.
The use of slings is not restricted to lifting heavy loads. Many jobs require objects to be securely tied down and slings provide excellent tie-down abilities. Nylon and chain slings are commonly used to tie down loads on trucks, boats and pallets. When fixed with a ratchet, the tightness of the nylon sling can be adjusted to provide heightened security. The environmental endurance, inherent strength, smooth surface and elastic properties make nylon slings the preferred choice not only for heavy lifting, but also for tie-downs. Chain slings provide a strong option that will not stretch when strained and that is also resistant to UV light and moisture.
Remember, equipment and parts can be replaced, workers cannot. Always err on the side of caution!
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