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An intricate system of slings, chains and straps, crane rigging utilizes the natural hoisting properties of mechanical angles to safely distribute weight. The term “load angle” is used in crane rigging to refer to the angle between any straps or chains and the surface of the load being held up. For instance, if one sling strap has been positioned at each corner of a rectangular crate, the load angle will be the angle between the crate lid and the sling of each corner, once suspended by the crane.
Determine the weight of your desired load by consulting the paperwork that accompanies the cargo, sometimes referred to as the “cargo manifest.” Add this weight to the weight of the cargo container, a standardized number generally available at the shipyard’s registrar.
Decide on the number of sling legs utilized to transport the cargo. Divide the total load weight by this number, for instance: if the total cargo and container weight equals 1,200 pounds, a load using four sling legs will divide that into 300 pounds distributed to each sling leg. This number is called the vertical load.
Consult the manufacturer’s paperwork for each of your slings and determine the maximum rating, or “weight limit” of each sling. Divide the vertical load distributed to each sling leg by that sling’s maximum rating, for example: with 300 pounds of vertical load and a 600 pound maximum rating, the resulting 300/600 division will result in the number two. Using a calculator, press the “arcsine” button and “two” in order to receive the maximum load angle of your specific load and sling angle – in this case, 30 degrees.
Always double-check your numbers when calculating the maximum load angle for your load. As a rule of thumb, 30-degree angles are very unsafe and are generally not used for their unnecessary strain placed on the sling straps. Angles between 45 and 90 degrees are the most common and most safe for normal operation.
- cargo image by Jorge Casais from Fotolia.com