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Sling selection and working load limit
Selection and use of slings
There's a lot to consider when choosing a sling. This page has all the information you need on selection and use of slings.
There are a few variables to consider when working out the safe method of slinging a load and placing it into position. These variables may include:
- the weight of the load
- shape
- dimensions
- surface finish
- balance
- designated lifting points
- environmental and weather conditions
- positioning of load.
A competent dogger or rigger should discuss and agree with the crane operator on the following factors:
- how they will select the appropriate sling
- attachment method to the load (direct, basket or choke hitch)
- placement of the load
- capacity of the crane
- operating radius.
These all determine the working load limit of rigging equipment.
Lifting a load
What is a Working Load Limit?
A Working Load Limit (WLL) is the maximum you can lift in a straight configuration. It must never be exceeded.
A 'design factor' is the ratio between the minimum breaking strength of a rope or component and the WLL. A design factor is a proactive measure to reduce risk. It is not extra capacity to use, as the WLL must never be exceeded.
A proof load (normally 2x WLL) is a load that is applied by a competent person during a proof test. This makes sure the sling or component is safe for use.
Geometry of multi leg slings
The geometry of the sling includes:
- the number of sling legs
- the angle of each leg to the included angle.
Impact of angles on multi leg slings
All multi leg slings exert a horizontal component of force, which increases as the included angle becomes greater. No sling should be used if the included angle exceeds 120 degrees. Beyond this point, the forces in the legs increases rapidly.
Selection and use of lifting slings
A competent person shall be able to choose the correct lifting sling or component and know how to inspect them.
Here's how to choose the correct sling or component:
- assess the mass of the load to be lifted as accurately as possible
- make sure the load, including any lifting points and durations, will withstand the lifting operation
- check that the lifting machine or appliance is rated to lift the load
- check the condition of the sling.
Listed below are factors to consider when selecting a suitable sling.
Joining device
Make sure each joining device has no more than one load-bearing component in use at any time.
Load attachment
Check the manner in which the load is attached. This could be straight, reeved or basket configuration. This includes the angle in the reeving and the de-rating effect due to the higher stresses caused by reeving on the sling.
Load shape, centre of gravity and distribution
The shape of the load, centre of gravity and load distribution on the sling legs. Uneven loading may cause the lifting capacity of the sling to be reduced. It may also cause instability and tilting of the load.
Working load limit
The determination of working load limit (WLL).
Protecting the sling from sharp corners
The lifting sling needs to be protected from sharp corners by using packing material. This prevents bending or damage to the lifting sling and/or the load. Use wherever practical.
Prepare landing site
The site where the load is to be landed should be prepared before the lifting operation. Make sure is it secured from movement. Make sure the landing site is:
- planned
- accessible
- clear of obstructions
- stable
- capable of supporting the load.
Environmental conditions
Take into account environmental conditions such as temperature and corrosive atmospheres.
Identify the hazards
Identify the hazards associated with the task. Examples of hazards include:
- inaccurately assessing the weight of the load
- selecting incorrect slings
- overloading the slings
- instability of the load.
Apply controls that will mitigate risk.