Safe working loads (SWLs) are weight limit ratings for a crane's rigging equipment. When dealing with the amount of weight that cranes typically lift, taking every factor into consideration is essential. Adding more lifting points helps distribute the weight and lessen the amount that each point has to bear. However, that also brings angles into play which increase the stress on individual members. The closer a sling's angle approaches being horizontal, the greater the tension on the sling.
Divide the weight of the load by the number of lifting points to find the share of the load that each point has to bear. For example, if the weight is 14,000 pounds and there are four lifting points, the load shared by each is 3,500 pounds.
Calculate the sling angle factor for each sling with the formula F = 1/sin(A), where A is the angle of the sling with the horizontal. Use a scientific calculator to find the sine of the angle. For example, if two of the slings have an angle of 40 degrees and the other two have 70 degrees, the sling angle factor is F = sin(40) = 1.56 for the first two and F = sin(70) = 1.06 for the other two.
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Multiply the sling angle factor times the sling's shared load to find the tension in each sling. For example, the tension in the slings at 40 degrees is 3,500 x 1.56 = 5,460 pounds. The tension in the slings at 70 degrees is 3,500 x 1.06 = 3,710 pounds.
Multiply the rated weight limit for the slings and shackles by 0.8 to allow an extra margin of safety. For example, if a sling is rated for 6,000 pounds, make sure that the tension is less than 6,000 x 0.8 = 4,800 pounds.
You can also calculate the sling angle factor with the formula F = L/H, where H is the vertical height to the top of the sling from the level of the lifting point, and L is the diagonal length of the sling from the lifting point to the top of the sling.
Do not attempt to rig a crane's load without proper training.