Wrapping Up Know-how for Safe Rigging
It’s tough to imagine worksites without machinery and systems that are designed to lift materials. Without hoists, cranes, pulley systems, and other devices, all work would have to take place at ground level, and the only objects that could be lifted would be those light enough for humans to handle.
From the tower cranes used to bring materials to the tops of multi-story structures, to the familiar overhead cranes built into many industrial facilities, to the hoists mounted in many work trucks, the basic principles of lifting materials are generally similar. Some type of cable is connected to a drum, run across a boom through a series of pulleys, and then terminates in a hook or similar device that’s used to attach it to whatever is being lifted. When it comes to safety, many workers focus on the lifting device itself.
What’s just as important is how the material that’s going to be raised and moved is prepared for that lift. The process of preparing the materials and equipment is known as rigging, and knowing proper rigging techniques is a critical part of preventing damage to the equipment that’s being lifted and the machinery performing the lift, as well as preventing serous injuries and fatalities among workers in the area. That’s why OSHA requires specialized training for rigging operations.
Choosing the right sling
In most cases when objects are being lifted or moved by cranes or hoists, workers will use a sling to hold the load. There are four different types of slings, and it’s important not only to select the right type for the task, but to choose a sling that has the proper length, thickness, and strength for the situation. The four primary types of slings include:
Wire rope slings. Made of wires twisted into strands (sometimes around a fiber core for extra flexibility), with the strands then twisted into ropes, this is the most widely used type of sling, thanks to its strength and heat resistance. The strength of a wire rope sling is largely determined by its size and the quality of wires in its construction. Generally, the larger the wire rope, the less the sling will be affected by abrasion.
Over time, wire rope slings that are forced into tight bends may develop tiny cracks that can lead to failure. Padding and blocking can be used to make the bends larger and reduce the potential for fatigue and cracking. If the slings are misused or subjected to excessive loads, damage can occur. Two of the most common types of damage affecting wire rope slings are kinks and bird-caging, in which the wire strands spread out from the core. Because they’re made from metal wires, these slings may also be subject to corrosion.
Chain slings. As the name implies, these slings are created from lengths of metal chains, typically from alloys selected to provide extra strength. Chain slings are usually able to accommodate heavier loads and offer the added advantage of conforming to the shape of the load. However, as the old saying claims, a chain is only as strong as its weakest link. Any damage to a single link, whether that’s a gouge or signs of stretching, reduces the entire chain’s durability and capacity. That’s why it’s important to inspect every link for damage before using a chain sling.
Mesh slings. Most often made from chain and wire, mesh slings tend to have larger areas for holding large loads. You’ll often see them used in metalworking and machine-shop settings, especially where objects need to be carefully balanced or may be very hot.
Synthetic slings. These slings, made from materials such as polypropylene, polyester, and nylon, are lighter in weight and are often chosen when there’s a potential to scratch or otherwise damage the load. That’s why they’re used with fragile parts and finished equipment. The materials usually conform to the shape of the load and grip it tightly, reducing the potential for slippage, and they may be able to absorb more physical shocks than other materials. However, synthetic materials may degrade when exposed to chemicals that are highly acidic or caustic.
Preparing and moving the load
All types of rigging equipment have a safe working load, and as that term would imply, it’s unsafe to use the equipment to lift anything heavier. Regardless of the type of sling selected for the task, it must be inspected before every use to ensure that it has been properly attached and positioned, and that there aren’t any flaws such as cuts or tears that could lead to failure.
The load should be placed in the sling in such a way that it can be lifted from directly above its center of gravity. If the crane or hoist tries to lift a load with a sling that’s off center, the load is likely to tilt, increasing the stress on the legs of the sling. Centering the load also evenly distributes the weight across all of the sling legs. No matter which type of sling is being used, it’s not acceptable to tie it in a knot in an effort to shorten it or snug it up against the load.
Before the lift begins, workers need to check for overhead power lines, materials stacked in the area, and any workers who are close to the area where it will take place. It’s a good idea to have spotters watch the area, so they can provide a warning if necessary. As the load is lifted, workers need to keep their fingers, hands, and feet away from any potential pinch points. After raising the load a few inches, visually inspect the sling to ensure that it is positioned properly and not being damaged. If the sling needs to be repositioned, lower the load completely before doing so.
Move the load slowly and carefully. Before lowering it into place, the spotters should verify that the area is clear of workers and physical obstructions. The hook from the crane or hoist should be lowered further to make it easier to remove, and only when the hook is completely out of the way should the workers remove the sling.