Sling’s lifting capacity calculation

Sling’s carrying capacity calculation

Calculation of load acting on sling legs is based on trigonometrical functions of angles created by the sling legs and a vertical or a horizontal.

Foreign standards use the angle made by a horizontal and the sling legs.

The basic angle, recommended by the Regulations of hoisting cranes installation and operation safety, is the angle between a vertical and the sling legs. This angle is called a slinging angle. We will use this angle hereinafter. When using slings with different length of legs, the slinging angle will be the largest one.

As it can be seen in the Table, extension of the slinging angle causes increase of load on the sling.

When calculating the sling legs load, it is necessary to take into account its dependence on the slinging angle. According to the Table when the angle is over 45 degrees, the sling load increases rapidly. Extension of the slinging angle to more than 60 degrees is not recommended, unless the clearance is limited. In this case, make sure that the used sling’s specification corresponds to potential loads.

Angular coefficient (AC)

According to the Regulations of hoisting cranes installation and operation safety, “when calculating carrying capacity of general-purpose slings with several legs, it is assumed that the angle between them is 90 degrees (slinging angle – 45 degrees). When calculating carrying capacity of slings intended for specific cargo, the actual angle can be assumed.

Example: 12-ton load is to be carried by a 4-leg sling. The slinging angle is 30 degrees; the sling legs are equal; the center of gravity is in the load mid-point.
Each sling leg load is calculated as follows:

In our case 4-ton legs are enough for hoisting the load. When using a general-purpose sling, we would have to take 12,5-ton sling with 5-ton legs. Thus, for occasional hoisting of heavy loads we recommend that the carrying capacity should be calculated with the minimum possible slinging angle. It will enable you to use cheaper and more serviceable sling legs of less carrying capacity as compared to a general-purpose sling.

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