The term Safe Working Load, (SWL) has been the cornerstone of engineering, particularly with respect to load carrying equipment, for many years. It was generally considered to be the minimum breaking load of a component divided by an appropriate factor of safety giving a ‘safe’ load that could be lifted or be carried.
In Australia the use of Safe Working Load (SWL) for cranes, hoists and winches was universally used throughout all industries and referenced in legislation and the Australian Standards.
The Wikipedia definition of SWL is:
Safe Working Load (SWL) sometimes stated as the Normal Working Load (NWL) is the mass or force that a piece of lifting equipment, lifting device or accessory can safely use to lift, suspend, or lower a mass without fear of breaking.
About 20 years ago, the USA ceased using this term, because of legal implications and the European and ISO Standards followed suit a few years later.
Australian Standard 1418.1 Cranes, Hoists and Winches details the requirements for cranes. It was revised in 2002, and part of this revision included replacing the term “safe working load” with “rated capacity”.
Any reference to the acronym SWL was also removed in this version of the standard. In the preface of AS1418.1 the following insight is provided into the change”
“The term ‘safe working load’ has been changed to ‘rated capacity’ and other uses of the word ‘safe’ have been avoided due to the legal significance placed on the word.”
The term SWL was changed to Rated Capacity for cranes, hoists and winches and Working Load Limit (WLL) should replace Safe Working Load (SWL) in describing the capacity of items such as hooks, slings and shackles etc.; that is for lifting devices below the crane hook, as referenced in AS4991:2004 Lifting devices.
From Clause 1.4.8 of AS1418.1:2002 the definition of Rated Capacity is:
Rated Capacity is the maximum gross load which may be applied to the crane or hoist or lifting attachment while in a particular working configuration and under a particular condition of use.
When used on a crane, hoist or winch the Rated Capacity includes the weight of any attachments, spreader beams or lifting devices below the crane hook and is the maximum allowable lifting capacity of the crane, hoist or winch when the lift is a straight line pull.
MRC is often called the Manufacturers Rated Capacity or Maximum Rated Capacity (MRC) to avoid any confusion with maximum gross load.
A definition of WLL is:
Working Load Limit (WLL) is the maximum working load specified by the manufacturer. This load represents a mass or force that is much less than that required to make the lifting equipment fail or yield, (known as the Minimum Breaking Load – MBL). SWL or WLL are calculated by dividing the MBL by a safety factor ( SF).
Whilst the definition for Working Load Limit was originally used for the manufacturers’ specified maximum load that the item could lift, the use of WLL has generally been extended to include the maximum load that an item can lift in a particular configuration or application.
For example, the WLL of a single leg wire rope or chain sling in a straight pull or lift is the WLL specified by the manufacturer, however if a two leg sling consisting of two single legs is used, then the WLL is reduced depending on the included angle between the slings.
The WLL is only as good as its weakest, or lowest WLL components. That is why it is essential to ensure that all components in a particular configuration match the duty required for the job.
When determining the WLL needed, all components, including shackles, turnbuckles and hook must be made from the same strength materials with equal WLL ratings.
The use of WLL on overhead bridge and gantry cranes and monorails is still evident, however technically MRC should be used.
- SWL has been phased out and should no longer be used, and all reasonable practicable efforts should be made to replace SWL with MRC.
- MRC should be used for all cranes, hoists and winches. The MRC must be clearly labelled on both sides of the crane beam or boom.
- WLL should be used for all for all lifting devices below the crane hook. Allowance must be taken into consideration for the arrangement of the lifting devices by derating the WLL.