Angles and Tensions

Anyone using lifting gear must understand the effects of angles in lifting suspended loads. The principles of tension are very important. It can be said, that if a suspended load is hanging vertically, with no other movement, then the tension in the supporting gear is equal to the weight of the load. If the angle of the suspension is not 0 degrees, then the tension in the suspension of the load must increase. There are two forces acting on the load, one vertical and one horizontal, of course, when more than one sling is used on the load there is usually an angle between the legs, this means that the tension in each sling is more than the weight of its proportion of the load.

When lifting with multi-leg slings, they are rated at a certain tonnage from 0 to 90 degrees and this should NOT be exceeded even if the angle is less than 90 degrees. When using single slings in pairs however, you must always be aware of the loadings in the slings when lifting at an angle. Thus if a sling is to be used safely, allowance must be made for this included angle and this is achieved by rating the sling in one of two ways. The two ways are the „uniform load method‟ and the „trigonometric method‟.

The uniform load method is the simpler option, having inherent safety advantages, permitting only one working load limit/safe working load up to an included angle of 90 degrees. This is the method which should be used for all multipurpose slings. Safe working loads/working load limits are derived from the following.