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Calculating loads and lengths of slings is vital when specifying a spreader beam. This becomes even more important when the center of gravity (CoG or CG) is offset, as it changes the load in the slings, and the length of each sling.

The key components to know when conducting this calculation are as follows:
- Sling to Horizontal Angle
- Length of Base
- Distance of the CoG from centre Essentially this is a trigonometry problem. Lets consider for example a central CoG lifting a 2t (2000kg) load and the base distance is 1m (1000mm) and the sling to horizontal angle is 45 degrees [typical angles in the lifting industry are 45 or 60 degrees]. Each sling will be taking a load of 1000kg / sin(45) = 1414kg, each sling length will be 500mm / cos(45) = 707mm. For purchasing purposes, I would buy 2 off 2t slings at length of 710mm. Dont forget if using shackles, the length of the sling will change according to the bow of the shackle you are using

Now consider an offset CoG - assume our product above has a CoG offset by 300mm from the center point. It is critical that our slings lift over the CoG, so we must account for this in our calculations. Now the two sling lengths will be sling length will be 800mm / cos(45) = 1132mm (500mm + 300mm = 800mm). To calculate the other side I will find the height of the slings using 800mm x tan(45) = 800mm. To find the other sling length I will use Pythagoras' Theorem square root(800^2 + 200^2) = 825mm [200mm as 1000-800]

This now allows me to find the sling to horizontal angle in the opposite corner using arccos(200 / 825) = 76 degrees

This allows me to calculate the greatest load in a sling. The load is offset by 300mm from center, so 500 + 300 = 800mm. This is 80% offset as 800 / 1000. 2000kg x 0.8 = 1600kg and therefore the sling load is 1600 / sin(76) = 1650kg

When buying slings, shackles and lifting equipment, ensure that you purchase from a reputable supplier such as Pfeifer Rope and Tackle or Britlift, and ensure they double check your calculations and your lift plan, as they may be able to suggest improved methods and better equipment. Ensure that you follow the suppliers instructions to ensure you conduct a safe lift.

Please use the calculator below to experiment with different angles and offsets.