16
Dec
Working Load Limits
The lifting of any freely suspended load, is a high risk activity requiring a range of controls to ensure work is performed safely. Lifting capacity labelling is an important component of a safe system of work, and the most significant change to the assessment of lifting capacity occurred over a decade ago with the replacement of ‘Safe Working Load’ to ‘Rated Capacity’.
The Australian Standard 1418.1 Cranes, Hoists and Winches was revised in 2002 and one of the key changes related to this terminology. Any reference to the acronym ‘SWL’ was removed from the Standard and replaced with “rated capacity”.
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.”
SO WHAT DOES THAT DEFINITION REALLY MEAN?
The definition of 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. MRC is often called the Manufacturers Rated Capacity or Maximum Rated Capacity (MRC) to avoid any confusion with maximum gross load.
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. For example, the rated capacity for an excavator, is maximum load that can be lifted over side at full reach on the maximum allowable slope.
HOW IS THE RATED CAPACITY CALCULATED?
It’s the responsibility of the manufacturer to determine the right or approximate MRC value for each hoisting device. To come up with a WLL or MRC value, there are many factors to consider. This includes the speed of operation, the applied load, the length of each rope or line, size, number, and etc. However when calculating the rated capacity there are two key machine features that must be considered – stability and hydraulic capacity. The rated capacity must not be greater than 87% of the hydraulic capacity at maximum reach/radius. There must be no more than 75% of the tipping load for stationary lifting and no more than 66% of tipping load for pick & carry lifting. If the machine is articulated this reduces to 50% (Plant Assessor, 2018) .
WHAT STICKER SHOULD WE BE USING?
The decal (sticker) should include the words RATED CAPACITY, a mass in kilograms followed by the letters kg. – eg. “Rated Capacity 3500kg”.
MY MACHINE CAN LIFT 5000KG BUT THE RATED CAPACITY LABEL SAYS 900KG?
Since the change to the Australian Standard, there is only one figure for rated capacity. Previously machines were supplied with a chart that detailed the maximum load that could be lifted at varying distances. Some machines are still supplied with this chart. Since 2008, AS1418.8 only requires a lifting chart to specify one weight in kilograms and one distance in metres, namely the maximum reach of the machine.
TO SUMMARISE
- Safe Working Limit (SWL) has been phased out and should no longer be used, and all reasonable practicable efforts should be made to replace SWL with MRC.
- Maximum Rated Capacity (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.
- Working Load Limit (WLL) should be used for all lifting devices below the crane hook.
- As always, allowance must be taken into consideration for the arrangement of the hoisting devices by derating the WLL following an assessment by a competent person of the maximum load the item can sustain under the conditions in which the item is going to be used.